Title of the invention: Steel sheet and its manufacturing method
Technical field
[0001]
　The present invention relates to a steel sheet and a method for producing the same.
Background technology
[0002]
　In recent years, further improvement in fuel efficiency of automobiles has been required from the viewpoint of greenhouse gas emission regulation accompanying global warming countermeasures. The application of high-strength steel sheets in automobile parts is expanding more and more in order to reduce the weight of the vehicle body and ensure collision safety.
　Needless to say, steel sheets used for automobile parts are required to have various workability required at the time of forming parts, such as press workability and weldability, as well as strength. Specifically, from the viewpoint of press workability, the steel sheet is often required to have excellent elongation (total elongation in a tensile test; El) and elongation flangeability (hole expansion ratio; λ).
[0003]
　As a method for improving press workability in a high-strength steel sheet, a DP steel (Dual Phase steel) having a ferrite phase and a martensite phase is known (see, for example, Patent Document 1). DP steel has excellent ductility. However, DP steel is inferior in hole expandability because the hard phase is the starting point for void formation.
[0004]
　Further, as a technique for improving the ductility of a high-strength steel sheet, there is a TRIP steel in which the austenite phase remains in the steel structure and the TRIP (transformation-induced plasticity) effect is utilized (see, for example, Patent Document 2). TRIP steel has higher ductility than DP steel. However, TRIP steel is inferior in hole expandability. In addition, in TRIP steel, it is necessary to add a large amount of alloy such as Si in order to leave austenite. Therefore, TRIP steel is inferior in chemical conversion treatment property and plating adhesion.
[0005]
　Further, Patent Document 3 describes a high-strength steel sheet having a microstructure as bainite or bainitic ferrite having an area ratio of 70% or more and having a tensile strength of 800 MPa or more and excellent hole expandability. In Patent Document 4, the microstructure has bainite or bainitic ferrite as the main phase, austenite as the second phase, and ferrite or martensite as the balance, and has excellent hole expandability and ductility with a tensile strength of 800 MPa or more. High-strength steel plates are listed.
[0006]
　As a technique for improving the bending workability of a high-strength steel sheet, for example, Patent Document 5 describes a high-strength cold-rolled steel sheet whose surface layer portion is mainly made of ferrite, which is produced by decarburizing the steel sheet. .. Further, Patent Document 6 describes an ultra-high-strength cold-rolled steel sheet having a soft layer on the surface layer, which is manufactured by decarburizing and annealing a steel sheet. However, the techniques described in Patent Documents 5 and 6 have insufficient fatigue resistance.
[0007]
　Further, Non-Patent Document 1 discloses that the elongation and hole expandability of a steel sheet are improved by using the double annealing method in which the steel sheet is annealed twice. However, the technique described in Non-Patent Document 1 has insufficient bendability.
Prior art literature
Patent documents
[0008]
Patent Document 1: Japanese Patent Application Laid-Open No. 6-128688
Patent Document 2: Japanese Patent Application Laid-Open No. 2006-274418
Patent Document 3: Japanese Patent Application Laid-Open No. 2003-193194
Patent Document 4: Japanese Patent Application Laid-Open No. 2003-193193 JP
Patent Document 5: Japanese Patent Laid-Open 10-130782 discloses
Patent Document 6: Japanese Patent Laid-Open 5-195149 discloses
Non-patent literature
[0009]
Non-Patent Document 1: K.K. Sugimoto et al. : ISIJ int. , (1993), 775.
Outline of the invention
Problems to be solved by the invention
[0010]
　The conventional high-strength steel sheet has excellent bendability and does not have good fatigue resistance.
　The present invention has been made in view of the above circumstances, and is a steel sheet having good ductility and hole expansion property, and excellent fatigue resistance, bendability, and plating adhesion, a hot-dip galvanized steel sheet, and its manufacture. The challenge is to provide a method.
Means to solve problems
[0011]
　The present inventor has made extensive studies in order to solve the above problems.
　As a result, the hot-rolled steel sheet or the cold-rolled steel sheet having a predetermined chemical composition is heat-treated (annealed) twice under different conditions to form a predetermined steel structure inside the steel sheet, and to have a predetermined thickness and a surface layer of the steel structure. It was found that an internal oxide layer containing a Si oxide should be formed at a predetermined depth.
[0012]
　Specifically, by the first heat treatment, the inside of the steel sheet is made into a steel structure mainly composed of lath-like structure such as martensite, and the surface layer is made into a steel structure mainly composed of soft ferrite. Then, in the second heat treatment, the maximum heating temperature is set to the two-phase region of α (ferrite) and γ (austenite). As a result, in the steel sheet obtained after two heat treatments and arbitrary hot-dip galvanizing, the inside of the steel sheet has a steel structure in which needle-like retained austenite is dispersed, and the surface layer is mainly composed of soft ferrite, with a small amount of martensite and residual. It becomes a composite structure of a predetermined thickness in which austenite is dispersed. Such steel sheets and hot-dip galvanized steel sheets have excellent ductility and hole-expanding properties, and have a good balance between bendability and fatigue resistance.
[0013]
　Further, in the first and second heat treatments described above, the internal oxide layer containing Si oxide at a predetermined depth suppresses the oxidation of alloying elements such as Si contained in the steel outside the steel sheet. By forming the above, excellent chemical conversion treatment property and plating adhesion can be obtained.
　The present invention has been made based on the above findings. The gist of the present invention is as follows.
[0014]
(1) The steel plate according to one aspect of the present invention is, in mass%, C: 0.050% to 0.500%, Si: 0.01% to 3.00%, Mn: 0.50% to 5.00. %, P: 0.0001% to 0.1000%, S: 0.0001% to 0.0100%, Al: 0.001% to 2.500%, N: 0.0001% to 0.0100%, O: 0.0001% to 0.0100%, Ti: 0% to 0.300%, V: 0% to 1.00%, Nb: 0% to 0.100%, Cr: 0% to 2.00 %, Ni: 0% to 2.00%, Cu: 0% to 2.00%, Co: 0% to 2.00%, Mo: 0% to 1.00%, W: 0% to 1.00. %, B: 0% to 0.0100%, Sn: 0% to 1.00%, Sb: 0% to 1.00%, Ca: 0% to 0.0100%, Mg: 0% to 0.0100 %, Ce: 0% to 0.0100%, Zr: 0% to 0.0100%, La: 0% to 0.0100%, Hf: 0% to 0.0100%, Bi: 0% to 0.0100 % And REM: 0% to 0.0100%, the balance has a chemical composition consisting of Fe and impurities, and 1/8 to 3/8 thickness centered on the position 1/4 thickness from the surface. The steel structure in the range of is, in terms of body integration ratio, soft ferrite: 0% to 30%, retained austenite: 3% to 40%, fresh martensite: 0% to 30%, total of pearlite and cementite: 0% to 10 Percentage of the retained austenite in the range of 1/8 to 3/8 thickness centered on the position 1/4 thickness from the surface, with the balance containing hard ferrite. When the number ratio of the retained austenite of 2.0 or more is 50% or more and the region having a hardness of 80% or less of the hardness in the above range of 1/8 thickness to 3/8 thickness is defined as a soft layer, A soft layer having a thickness of 1 to 100 μm exists in the plate thickness direction from the surface, and among the ferrite crystal grains contained in the soft layer, the body integration ratio of the crystal grains having an aspect ratio of less than 3.0 is 50% or more. The body integration rate of retained austenite in the soft layer is 1/8 to 3/8 of the thickness of the retained aus.When the emission intensity of a wavelength that is 50% or more of the volume fraction of tenite and indicates Si from the surface in the plate thickness direction is analyzed by a high-frequency glow discharge analysis method, it is more than 0.2 μm from the surface and the surface. The peak of the emission intensity of the wavelength indicating the Si appears in the range of 5 μm or less.
(2) The steel sheet according to (1) above has the chemical composition of Ti: 0.001% to 0.300%, V: 0.001% to 1.00%, and Nb: 0.001% to. It may contain one or more selected from the group consisting of 0.100%.
(3) The steel sheet according to (1) or (2) has a chemical composition of Cr: 0.001% to 2.00%, Ni: 0.001% to 2.00%, Cu: 0. 001% to 2.00%, Co: 0.001% to 2.00%, Mo: 0.001% to 1.00%, W: 0.001% to 1.00%, and B: 0.0001 It may contain one or more selected from the group consisting of% to 0.0100%.
(4) The steel sheet according to any one of (1) to (3) above has a chemical composition of Sn: 0.001% to 1.00% and Sb: 0.001% to 1.00. It may contain one or two selected from the group consisting of%.
(5) The steel sheet according to any one of (1) to (4) above has a chemical composition of Ca: 0.0001% to 0.0100% and Mg: 0.0001% to 0.0100%. , Ce: 0.0001% to 0.0100%, Zr: 0.0001% to 0.0100%, La: 0.0001% to 0.0100%, Hf: 0.0001% to 0.0100%, Bi It may contain one or more selected from the group consisting of: 0.0001% to 0.0100% and REM: 0.0001% to 0.0100%.
(6) The steel sheet according to any one of (1) to (5) above may have the chemical composition satisfying the following formula (1).
　Si + 0.1 × Mn + 0.6 × Al ≧ 0.35 ... (1)
　Si, Mn and Al in the formula (1) are the contents of each element in mass%.
(7) The steel sheet according to any one of (1) to (6) above has a thickness of 1/8 to 3/8 centered on the position of 1/4 thickness from the surface in the above range. The volume fraction of tempered martensite may be 0% to 50%.
(8) The steel sheet according to any one of (1) to (7) above may have a hot-dip galvanized layer on its surface.
(9) The steel sheet according to any one of (1) to (7) above may have an electrogalvanized layer on its surface.
(10) The method for producing a steel sheet according to another aspect of the present invention is the method for producing a steel sheet according to any one of the above (1) to (9), and the above (1) to (6). The following (a) to (e) are applied to a hot-rolled steel sheet obtained by hot-rolling and pickling a slab having the chemical composition according to any one of the above items, or a cold-rolled steel sheet obtained by cold-rolling the hot-rolled steel sheet. After performing the satisfactory first heat treatment, the second heat treatment satisfying the following (A) to (E) is performed.
(A) During the period from 650 ° C. to heating to the maximum heating temperature, the atmosphere around the hot-rolled steel sheet or the cold-rolled steel sheet contains H 2 of 0.1% by volume or more, and the following formula (2) Create an atmosphere that meets the requirements.
(B) Ac3 Hold at the maximum heating temperature of -30 ° C to 1000 ° C for 1 second to 1000 seconds.
(C) Heat from 650 ° C. to the maximum heating temperature at an average heating rate of 0.5 ° C./sec to 500 ° C./sec.
(D) After holding at the maximum heating temperature, the mixture is cooled from 700 ° C. to Ms at an average cooling rate of 5 ° C./sec or more.
(E) The cooling at an average cooling rate of 5 ° C./sec or more is performed up to a cooling stop temperature of Ms or less.
(A) From 650 ° C. to heating to the maximum heating temperature, the atmosphere around the hot-rolled steel sheet or the cold-rolled steel sheet contains H 2 of 0.1% by volume or more, and the following formula (3) Create an atmosphere that meets the requirements.
(B) Hold at the maximum heating temperature of Ac1 + 25 ° C. to Ac3-10 ° C. for 1 second to 1000 seconds.
(C) Heating is performed from 650 ° C. to the maximum heating temperature at an average heating rate of 0.5 ° C./sec to 500 ° C./sec.
(D) Cool from the maximum heating temperature to 480 ° C. or lower so that the average cooling rate between 600 and 700 ° C. is 3 ° C./sec or more.
(E) After cooling at an average cooling rate of 3 ° C./sec or higher, the temperature is maintained between 300 ° C. and 480 ° C. for 10 seconds or longer.
　-1.1 ≤ log (PH 2 O / PH 2 ) ≤ -0.07 ... (2) In
　equation (2), PH 2 O indicates the partial pressure of water vapor, and PH 2 indicates the partial pressure of hydrogen. Shown.
　log (PH 2 O / PH 2 ) <-1.1 ... (3) In
　equation (3), PH 2O indicates the partial pressure of water vapor, and PH 2 indicates the partial pressure of hydrogen.
(11) The method for producing a steel sheet according to (10) above is the method for producing a steel sheet according to (8) above, from 650 ° C. to reaching the maximum heating temperature in the second heat treatment. Always, the atmosphere contains H 2 of 0.1% by volume or more , O 2 is 0.020% by volume or less, satisfies the formula (3), and in the second heat treatment, of the above (D). Hot-dip galvanizing treatment may be performed at a stage after the cooling process.
Effect of the invention
[0015]
　According to the present invention, it is possible to provide a steel sheet having good ductility and hole expandability, excellent fatigue resistance, bendability, and excellent plating adhesion, a hot-dip galvanized steel sheet, and a method for producing the same.
　The high-strength steel sheet and the high-strength hot-dip galvanized steel sheet of the present invention have good ductility and hole-spreading properties, and have excellent fatigue resistance, bendability, and plating adhesion, so that they can be made into various shapes by press working or the like. It is suitable as a steel sheet for automobiles to be molded.
A brief description of the drawing
[0016]
FIG. 1 is a cross-sectional view of the steel plate of the present embodiment parallel to the rolling direction and the plate thickness direction.
FIG. 2 shows the depth from the surface and the emission intensity (Intensity) of a wavelength indicating Si when the steel sheet of the present embodiment is analyzed from the surface in the depth direction (plate thickness direction) by a high-frequency glow discharge analysis method. It is a graph which shows the relationship.
[Fig. 3] When a steel sheet different from the present embodiment is analyzed from the surface in the depth direction (plate thickness direction) by a high-frequency glow discharge analysis method, the emission intensity (Intensity) of a wavelength indicating the depth from the surface and Si. It is a graph which shows the relationship with.
FIG. 4 is a flowchart of a steel sheet manufacturing method of the present embodiment.
FIG. 5 is a diagram showing a first example of a temperature / time pattern of a second heat treatment to a hot-dip galvanizing / alloying treatment in the method for manufacturing a hot-dip galvanized steel sheet of the present embodiment.
FIG. 6 is a diagram showing a second example of a temperature / time pattern of a second heat treatment to a hot-dip galvanizing / alloying treatment in the method for manufacturing a hot-dip galvanized steel sheet of the present embodiment.
FIG. 7 is a diagram showing a third example of a temperature / time pattern of the second heat treatment to the hot-dip galvanizing / alloying treatment in the method for producing a hot-dip galvanized steel sheet of the present embodiment.
Mode for carrying out the invention
[0017]
"Steel plate" The steel plate
　1 of the present embodiment shown in FIG. 1 is formed on the inside 11 of the steel plate having a thickness in the range of 1/8 to 3/8 centering on the position of 1/4 thickness from the surface and on the surface of the steel plate. It has an arranged soft layer 12. The 1/4 thickness position is a position at a depth of 1/4 of the thickness t of the steel plate from the surface of the steel plate, and corresponds to a region marked with a reference numeral 1/4 t in FIG. The range of 1/8 thickness to 3/8 thickness is a range from the surface of the steel sheet to a region having a depth of 1/8 of the thickness t of the steel plate and a region having a depth of 3/8. The 1/8 thickness position and the 3/8 thickness position correspond to the positions marked with reference numerals 1/8 t and 3/8 t in FIG. As will be described later, the soft layer 12 is a region having a hardness of 80% or less of the hardness of the inside 11 of the steel sheet. The steel sheet 1 may further include a hot-dip galvanized layer, an electrogalvanized layer, and the like on its surface (that is, the surface of the soft layer 12). Hereinafter, the steel sheet of the present embodiment will be described in detail.
　First, the chemical composition of the steel sheet will be described. In the following description, [%] indicating the content of the element means [mass%].
[0018]
"C: 0.050% to 0.500%"
　C is an element that greatly enhances strength. C is effective in achieving both strength and moldability by stabilizing austenite and obtaining retained austenite. However, if the C content exceeds 0.500%, the weldability is significantly deteriorated. Therefore, the C content is set to 0.500% or less. From the viewpoint of spot weldability, the C content is preferably 0.350% or less, more preferably 0.300% or less, 0.250% or less, or 0.200% or less. On the other hand, if the C content is less than 0.050%, sufficient retained austenite cannot be obtained, and it becomes difficult to secure sufficient strength and moldability. Therefore, the C content is set to 0.050% or more. In order to further increase the strength and moldability, the C content is preferably 0.075% or more, more preferably 0.100% or more, or 0.200%.
[0019]
"Si: 0.01% to 3.00%"
　Si is an element that stabilizes retained austenite by suppressing the formation of iron-based carbides in steel sheets, and enhances strength and moldability. However, Si is an element that embrittles steel. If the Si content exceeds 3.00%, the hole expandability of the steel sheet becomes insufficient. Further, if the Si content exceeds 3.00%, troubles such as cracking of the cast slab are likely to occur. Therefore, the Si content is set to 3.00% or less. Further, since Si impairs the impact resistance characteristics of the steel sheet, the Si content is preferably 2.50% or less, more preferably 2.00% or less, or 1.80% or less. On the other hand, if the Si content is less than 0.01%, a large amount of coarse iron-based carbides are generated, and the strength and moldability deteriorate. Therefore, the Si content is set to 0.01% or more. From this viewpoint, the lower limit of Si is preferably 0.10%, more preferably 0.25%, 0.30%, or 0.50%.
[0020]
"Mn: 0.50% to 5.00%"
　Mn is added to enhance the hardenability of the steel sheet and increase the strength. However, if the Mn content exceeds 5.00%, the elongation and hole expandability of the steel sheet become insufficient. Further, when the Mn content exceeds 5.00%, a coarse Mn-concentrated portion is generated in the central portion of the steel plate thickness, embrittlement is likely to occur, and troubles such as cracking of the cast slab are likely to occur. Become. Therefore, the Mn content is set to 5.00% or less. Further, since the spot weldability deteriorates as the Mn content increases, the Mn content is preferably 3.50% or less, preferably 3.00% or less, or 2.80% or less. More preferred. On the other hand, if the Mn content is less than 0.50%, a large amount of soft structure is formed during cooling after annealing, so that it is difficult to secure a sufficiently high maximum tensile strength. Therefore, the Mn content needs to be 0.50% or more. In order to further increase the strength, the Mn content is preferably 0.80% or more, more preferably 1.00% or more, or 1.50% or more.
[0021]
"P: 0.0001% to 0.1000%"
　P is an element that embrittles a steel material. If the P content exceeds 0.1000%, the elongation and hole expandability of the steel sheet become insufficient. Further, if the P content exceeds 0.1000%, troubles such as cracking of the cast slab are likely to occur. Therefore, the content of P is set to 0.1000% or less. Further, P is an element that embrittles the molten portion generated by spot welding. In order to obtain sufficient welded joint strength, the P content is preferably 0.0400% or less, more preferably 0.3000% or less, or 0.0200% or less. On the other hand, setting the P content to less than 0.0001% is accompanied by a significant increase in manufacturing cost. From this, the P content is set to 0.0001% or more. The P content is preferably 0.0010% or more, 0.0012%, or 0.0015% or more.
[0022]
"S: 0.0001% to 0.0100%"
　S combines with Mn to form coarse MnS, which reduces moldability such as ductility, hole expandability (stretchable flangeability) and bendability. Therefore, the upper limit of S is set to 0.0100% or less. Further, since S deteriorates spot weldability, it is preferably 0.0070% or less, and more preferably 0.0050% or less, or 0.0030% or less. On the other hand, setting the S content to less than 0.0001% is accompanied by a significant increase in manufacturing cost. Therefore, the S content is set to 0.0001% or more. The S content is preferably 0.0003% or more, more preferably 0.0006% or more, or 0.0010% or more.
[0023]
"Al: 0.001% to 2.500%"
　Al is an element that embrittles steel materials. If the Al content exceeds 2.500%, troubles such as cracking of the cast slab are likely to occur. Therefore, the Al content is set to 2.500% or less. Further, as the Al content increases, the spot weldability deteriorates. Therefore, the Al content is more preferably 2.000% or less, and further preferably 1.500% or less, or 1.000% or less. On the other hand, although the effect of the steel sheet according to this embodiment is exhibited without setting the lower limit of the Al content, Al is an impurity present in a trace amount in the raw material, and the content thereof is less than 0.001%. Is accompanied by a significant increase in manufacturing costs. Therefore, the Al content was set to 0.001% or more. Al is an element that is also effective as a deoxidizing material, but in order to obtain a sufficient deoxidizing effect, the Al content is preferably 0.010% or more. Further, Al is an element that suppresses the formation of coarse carbides, and may be added for the purpose of stabilizing retained austenite. For stabilization of retained austenite, the Al content is preferably 0.100% or more, and more preferably 0.250% or more.
[0024]
"N: 0.0001% to 0.0100%"
　N forms coarse nitrides and deteriorates moldability such as ductility, hole expansion (stretch flangeability) and bendability. It needs to be suppressed. When the content of N exceeds 0.0100%, the deterioration of moldability becomes remarkable. For this reason, the upper limit of the N content was set to 0.0100%. Further, since N causes blow holes during welding, it is preferable that the content is small. The N content is preferably 0.0075% or less, more preferably 0.0060% or less, or 0.0050% or less. Although the effect of the steel sheet according to the present embodiment is exhibited even if the lower limit of the N content is not particularly specified, setting the N content to less than 0.0001% causes a significant increase in manufacturing cost. .. For this reason, the lower limit of the N content was set to 0.0001% or more. The N content is preferably 0.0003% or more, more preferably 0.0005% or more, or 0.0010% or more.
[0025]
"O: 0.0001% to 0.0100%"
　O forms an oxide and deteriorates moldability such as ductility, hole expansion property (stretch flange property) and bendability, so it is necessary to suppress the content. is there. When the O content exceeds 0.0100%, the moldability is significantly deteriorated. Therefore, the upper limit of the O content is set to 0.0100%. Further, the content of O is preferably 0.0050% or less, more preferably 0.0030% or less, or 0.0020% or less. Although the effect of the steel sheet according to this embodiment is exhibited even if the lower limit of the O content is not particularly specified, setting the O content to less than 0.0001% is accompanied by a significant increase in manufacturing cost. Therefore, 0.0001% was set as the lower limit. The O content may be 0.0005% or more, 0.0010% or more, or 0.0012% or more.
[0026]
“Si + 0.1 × Mn + 0.6 × Al ≧ 0.35”
　Retained austenite may be decomposed into bainite, pearlite or coarse cementite during heat treatment. Si, Mn and Al are particularly important elements for suppressing the decomposition of retained austenite and improving the moldability, and it is preferable to satisfy the following formula (1). The value on the left side of the formula (1) is more preferably 0.60 or more, and further preferably 0.80 or more, or 1.00% or more.
　Si + 0.1 × Mn + 0.6 × Al ≧ 0.35 ・ ・ ・ (1)
　(Si, Mn and Al in the formula (1) are the contents of each element in mass%).
[0027]
　The steel sheet of the present embodiment may further contain one or more of the following optional elements, if necessary. However, since the steel sheet according to the present embodiment can solve the problem without containing the following optional elements, the content of the following optional elements may be 0%.
[0028]
"Ti: 0% to 0.300%"
　Ti is an element that contributes to increasing the strength of steel sheets by strengthening with precipitates, strengthening fine grains by suppressing the growth of ferrite crystal grains, and strengthening dislocations by suppressing recrystallization. .. However, when the Ti content exceeds 0.300%, the precipitation of carbonitride increases and the moldability deteriorates. Therefore, the Ti content is preferably 0.300% or less. Further, from the viewpoint of moldability, the Ti content is more preferably 0.150% or less. Although the effect of the steel sheet according to this embodiment is exhibited even if the lower limit of the Ti content is not particularly determined, the Ti content should be 0.001% or more in order to sufficiently obtain the strength increasing effect of Ti. It is preferable to have. In order to further increase the strength of the steel sheet, the Ti content is more preferably 0.010% or more.
[0029]
"V: 0% to 1.00%"
　V is an element that contributes to the increase in the strength of the steel sheet by strengthening the precipitate, strengthening the fine grains by suppressing the growth of ferrite crystal grains, and strengthening the dislocations by suppressing recrystallization. However, if the V content exceeds 1.00%, the carbonitride is excessively precipitated and the moldability is deteriorated. Therefore, the V content is preferably 1.00% or less, and more preferably 0.50% or less. Although the effect of the steel sheet according to this embodiment is exhibited even if the lower limit of the V content is not particularly defined, the V content should be 0.001% or more in order to sufficiently obtain the strength increasing effect of V. It is preferably present, and more preferably 0.010% or more.
[0030]
"Nb: 0% to 0.100%"
　Nb is an element that contributes to the increase in the strength of the steel sheet by strengthening the precipitate, strengthening the fine grains by suppressing the growth of ferrite crystal grains, and strengthening the dislocations by suppressing recrystallization. However, if the Nb content exceeds 0.100%, the precipitation of carbonitride increases and the moldability deteriorates. Therefore, the Nb content is preferably 0.100% or less. From the viewpoint of moldability, the Nb content is more preferably 0.060% or less. Although the effect of the steel sheet according to the present embodiment is exhibited even if the lower limit of the Nb content is not particularly determined, the Nb content is 0.001% or more in order to sufficiently obtain the strength increasing effect of Nb. Is preferable. In order to further increase the strength of the steel sheet, the Nb content is more preferably 0.005% or more.
[0031]
"Cr: 0% to 2.00%"
　Cr is an element effective for enhancing hardenability and increasing strength, and may be added in place of a part of C and / or Mn. If the Cr content exceeds 2.00%, the workability in hot water is impaired and the productivity is lowered. From this, the Cr content is preferably 2.00% or less, and more preferably 1.20% or less. Although the effect of the steel sheet according to the present embodiment is exhibited even if the lower limit of the Cr content is not particularly determined, the Cr content is 0.001% in order to sufficiently obtain the effect of increasing the strength by Cr. The above is preferable, and 0.010% or more is more preferable.
[0032]
"Ni: 0% to 2.00%"
　Ni is an element effective for suppressing phase transformation at high temperature and increasing the strength, and may be added in place of a part of C and / or Mn. If the Ni content exceeds 2.00%, the weldability is impaired. From this, the Ni content is preferably 2.00% or less, and more preferably 1.20% or less. Although the effect of the steel sheet according to this embodiment is exhibited even if the lower limit of the Ni content is not particularly determined, the Ni content is 0.001% in order to sufficiently obtain the effect of increasing the strength by adding Ni. The above is preferable, and 0.010% or more is more preferable.
[0033]
"Cu: 0% to 2.00%"
　Cu is an element that increases the strength by being present in steel as fine particles, and can be added in place of a part of C and / or Mn. If the Cu content exceeds 2.00%, the weldability is impaired. Therefore, the Cu content is preferably 2.00% or less, and more preferably 1.20% or less. Although the effect of the steel sheet according to this embodiment is exhibited even if the lower limit of the Cu content is not particularly determined, the Cu content is 0.001% in order to sufficiently obtain the effect of increasing the strength by adding Cu. It is preferably 0.010% or more, and more preferably 0.010% or more.
[0034]
"Co: 0% to 2.00%"
　Co is an element effective for enhancing hardenability and increasing strength, and may be added in place of a part of C and / or Mn. If the Co content exceeds 2.00%, the workability in hot water is impaired and the productivity is lowered. From this, the Co content is preferably 2.00% or less, and more preferably 1.20% or less. Although the effect of the steel sheet according to the present embodiment is exhibited even if the lower limit of the Co content is not particularly determined, the Co content is 0.001% in order to sufficiently obtain the effect of increasing the strength by adding Co. The above is preferable, and 0.010% or more is more preferable.
[0035]
"Mo: 0% to 1.00%"
　Mo is an element effective for suppressing phase transformation at high temperature and increasing the strength, and may be added in place of a part of C and / or Mn. If the Mo content exceeds 1.00%, the workability in hot water is impaired and the productivity is lowered. From this, the Mo content is preferably 1.00% or less, and more preferably 0.50% or less. Although the effect of the steel sheet according to this embodiment is exhibited even if the lower limit of the Mo content is not particularly defined, the Mo content is 0.001 in order to sufficiently obtain the effect of increasing the strength by adding Mo. It is preferably% or more, and more preferably 0.005% or more.
[0036]
"W: 0% to 1.00%"
　W is an element effective for suppressing phase transformation at high temperature and increasing the strength, and may be added in place of a part of C and / or Mn. If the W content exceeds 1.00%, the workability in hot water is impaired and the productivity is lowered. From this, the W content is preferably 1.00% or less, and more preferably 0.50% or less. The lower limit of the W content is not particularly specified, and the effect of the steel sheet according to the present embodiment is exhibited. However, in order to sufficiently obtain high strength by W, the W content is 0.001% or more. It is preferably present, and more preferably 0.010% or more.
[0037]
"B: 0% to 0.0100%"
　B is an element effective for suppressing phase transformation at high temperature and increasing the strength, and may be added in place of a part of C and / or Mn. If the content of B exceeds 0.0100%, the workability in hot water is impaired and the productivity is lowered. For this reason, the B content is preferably 0.0100% or less. From the viewpoint of productivity, the content of B is more preferably 0.0050% or less. Although the effect of the steel sheet according to this embodiment is exhibited even if the lower limit of the B content is not particularly determined, the B content is 0.0001 in order to sufficiently obtain the effect of increasing the strength by adding B. It is preferably% or more. The B content is more preferably 0.0005% or more in order to further increase the strength.
[0038]
"Sn: 0% to 1.00%"
　Sn is an element effective for suppressing the coarsening of the structure and increasing the strength, and 1.00% may be added as an upper limit. If the amount of Sn added exceeds 1.00%, the steel sheet may become excessively brittle and the steel sheet may break during rolling. Therefore, the Sn content is preferably 1.00% or less. The lower limit of the Sn content is not particularly specified, and the effect of the steel sheet according to the present embodiment is exhibited, but in order to sufficiently obtain the effect of increasing the strength by Sn, the Sn content is 0.001% or more. It is preferably 0.010% or more, and more preferably 0.010% or more.
[0039]
"Sb: 0% to 1.00%"
　Sb is an element effective for suppressing the coarsening of the structure and increasing the strength, and may be added up to 1.00%. If the amount of Sb added exceeds 1.00%, the steel sheet may become excessively brittle and the steel sheet may break during rolling. Therefore, the Sb content is preferably 1.00% or less. The lower limit of the Sb content is not particularly specified, and the effect of the steel sheet according to the present embodiment is exhibited, but in order to sufficiently obtain the high strength effect of Sb, the Sb content is 0.001% or more. It is preferably 0.005% or more, and more preferably 0.005% or more.
[0040]
"One or more selected from the group consisting of Ca, Mg, Ce, Zr, La, Hf, Bi, REM: 0% to 0.0100%, respectively"
　REM is an abbreviation for Rare Earth Metal. Usually refers to an element belonging to the lanthanoid series. However, in this embodiment, REM excludes Ce and La. In the present embodiment, La and / or Ce is often added as mischmetal, and may contain a lanthanoid series element in combination with La and / or Ce. The effect of the steel sheet according to the present embodiment is exhibited even if an element of the lanthanoid series other than La and / or Ce is contained as an impurity. Further, even if the metal La and / or Ce is added, the effect of the steel sheet according to the present embodiment is exhibited. In this embodiment, the REM content is the total content of the elements belonging to the lanthanoid series.
[0041]
　The effects of these elements are as follows.
　Ca, Mg, Ce, Zr, La, Hf, Bi, and REM are elements effective for improving moldability, and one or more of them can be contained in an amount of 0.0001% to 0.0100%, respectively. If the content of one or more of Ca, Mg, Ce, Zr, La, Hf, Bi, and REM exceeds 0.0100%, ductility may be impaired. Therefore, the content of each of the above elements is preferably 0.0100% or less, and more preferably 0.0070% or less. When two or more of the above elements are contained, the total content of Ca, Mg, Ce, Zr, La, Hf, Bi and REM is preferably 0.0100% or less. Although the lower limit of the content of each element is not particularly specified, the effect of the steel sheet according to the present embodiment is exhibited, but in order to sufficiently obtain the effect of improving the formability of the steel sheet, the content of each element is obtained. Is preferably 0.0001% or more. From the viewpoint of moldability, the content of one or more of Ca, Mg, Ce, Zr, La, Hf, Bi and REM is more preferably 0.0010% or more.
[0042]
　The rest of each of the above elements is Fe and impurities. It is permissible that all of the above-mentioned Ti, V, Nb, Cr, Ni, Cu, Co, Mo, W, B, Sn, and Sb contain a trace amount less than the above-mentioned preferable lower limit value as an impurity. To.
　Further, it is permissible that Ca, Mg, Ce, Zr, La, Hf, Bi and REM also contain a very small amount less than the preferable lower limit value as an impurity.
　As impurities, H, Na, Cl, Sc, Zn, Ga, Ge, As, Se, Y, Tc, Ru, Rh, Pd, Ag, Cd, In, Te, Cs, Ta, Re, Os, Ir. , Pt, Au and Pb are allowed to be contained in a total amount of 0.0100% or less.
[0043]
　Next, the steel structure (microstructure) of the steel sheet inside 11 of the steel sheet according to the present embodiment will be described. In addition, [%] in the description of the content of each tissue is [volume%].
[0044]
(Microstructure of 11 inside the steel sheet) In
　the steel sheet of the present embodiment, the steel structure in the range of 1/8 to 3/8 thickness centered on the position of 1/4 thickness from the surface (hereinafter, "steel structure inside the steel sheet"). ”) Contains 30% or less of soft ferrite, 3% to 40% of retained austenite, 30% or less of fresh martensite, and 10% or less of the total of pearlite and cementite, and accounts for the total retained austenite. The number ratio of retained austenite having an aspect ratio of 2.0 or more is 50% or more.
[0045]
"Soft ferrite: 0% to 30%"
　Ferrite is a structure having excellent ductility. However, ferrite has a low strength and is difficult to utilize in high-strength steel sheets. In the steel sheet of the present embodiment, the steel structure inside the steel sheet contains 0% to 30% of soft ferrite. The "soft ferrite" in the present embodiment means that the ferrite does not contain retained austenite in the grains. Soft ferrite has low strength, strain is more likely to be concentrated than the peripheral part, and fracture is more likely to occur. When the volume fraction of soft ferrite exceeds 30%, the balance between strength and moldability is significantly deteriorated. Therefore, the soft ferrite is limited to 30% or less. The soft ferrite is more preferably limited to 15% or less, and may be 0%.
[0046]
"Residual austenite: 3% -40%"
　Retained austenite is a tissue that enhances the strength-ductility balance. In the steel sheet of the present embodiment, the steel structure inside the steel sheet contains 3% to 40% of retained austenite. From the viewpoint of formability, the volume fraction of retained austenite inside the steel sheet is preferably 3% or more, preferably 5% or more, and more preferably 7% or more. On the other hand, in order to make the volume fraction of retained austenite more than 40%, it is necessary to add a large amount of C, Mn and / or Ni, and the weldability is significantly impaired. Therefore, the volume fraction of retained austenite is set to 40% or less. In order to improve the weldability and convenience of the steel sheet, the volume fraction of retained austenite is preferably 30% or less, and more preferably 20% or less.
[0047]
"Fresh martensite: 0% to 30%"
　Fresh martensite greatly improves tensile strength. On the other hand, fresh martensite becomes a starting point of fracture and significantly deteriorates the impact resistance characteristics. Therefore, the volume fraction of fresh martensite is set to 30% or less. In particular, in order to improve the impact resistance characteristics, the volume fraction of fresh martensite is preferably 15% or less, and more preferably 7% or less. The fresh martensite may be 0%, but is preferably 2% or more in order to secure the strength of the steel sheet.
[0048]
"Total of pearlite and cementite: 0% to 10%"
　The microstructure inside the steel sheet of the steel sheet may contain pearlite and / or cementite. However, high volume fractions of pearlite and / or cementite deteriorate ductility. Therefore, the volume fractions of pearlite and cementite are limited to 10% or less in total. The volume fraction of pearlite and cementite is preferably 5% or less, and may be 0%.
[0049]
"The number ratio of retained austenite having an aspect ratio of 2.0 or more is 50% or more of the total retained austenite." In the
　present embodiment, the aspect ratio of the retained austenite grains inside the steel sheet is important. The elongated retained austenite having a large aspect ratio is stable at the initial stage of deformation of the steel sheet due to processing. However, in the case of retained austenite having a large aspect ratio, strain is concentrated at the tip portion as the processing progresses, and the strain is appropriately transformed to produce a TRIP (transformation-induced plasticity) effect. Therefore, since the steel structure inside the steel sheet contains retained austenite having a large aspect ratio, ductility can be improved without impairing toughness, hydrogen embrittlement resistance, hole expandability, and the like. From the above viewpoint, in the present embodiment, the ratio of the number of retained austenites having an aspect ratio of 2.0 or more to the total retained austenite is 50% or more. The number ratio of retained austenite having an aspect ratio of 2.0 or more is preferably 60% or more, more preferably 70% or more, and particularly preferably 80% or more.
[0050]
"Tempering martensite"
　Tempering martensite is a structure that greatly improves the tensile strength of a steel sheet without impairing the impact resistance characteristics, and may be contained in the steel structure inside the steel sheet. However, if a large amount of tempered martensite is generated inside the steel sheet, retained austenite may not be sufficiently obtained. Therefore, the volume fraction of tempered martensite is preferably limited to 50% or less or 30% or less. Since the content of tempered martensite is not essential for the steel sheet according to the present embodiment, the lower limit of tempered martensite is 0%.
[0051]
　In the steel sheet of the present embodiment, the residual structure in the steel structure inside the steel sheet is mainly "hard ferrite" containing retained austenite in the grains. For hard ferrite, a second heat treatment described later is performed on a heat treatment steel sheet having a steel structure containing one or more lath-like structures of upper bainite, bainite ferrite, tempered martensite, and fresh martensite. It is formed by. Hard ferrite has high strength because it contains retained austenite in the grains. Further, hard ferrite has good moldability because interfacial peeling between ferrite and retained austenite is less likely to occur as compared with the case where retained austenite is present at the ferrite grain boundaries.
[0052]
　Further, bainite may be contained in the remaining structure in the steel structure inside the steel sheet. The bainite in the present embodiment includes granular bainite composed of fine BCC crystals and coarse iron-based carbides, upper bainite composed of lath-shaped BCC crystals and coarse iron-based carbides, and plate-shaped BCC crystals and their interiors. Includes lower bainite consisting of fine iron-based carbides lined up parallel to. In the steel sheet according to the present embodiment, the remaining structure in the steel structure inside the steel sheet is mainly hard ferrite. That is, the residual structure in the steel structure inside the steel sheet contains more hard ferrite than bainite.
[0053]
(Surface microstructure)
"1/8 thickness 1-3 / 8 when a region having 80% or less of the hardness of the hardness in the range of thickness is defined as a soft layer, there are soft layer of 1 - 100 [mu] m in the surface layer"
　steel In order to improve the bendability, it is one of the necessary requirements to soften the surface layer of the steel sheet. In the steel sheet according to the present embodiment, when a region where the hardness is 80% or less of the hardness (average hardness) inside the steel sheet is defined as a soft layer, a soft layer of 1 to 100 μm exists in the plate thickness direction from the surface of the steel sheet. .. In other words, a soft layer having a hardness of 80% or less of the average hardness inside the steel sheet is present on the surface layer portion of the steel sheet, and the thickness of the soft layer is 1 to 100 μm.
[0054]
　If the thickness of the soft layer is less than 1 μm in the depth direction (plate thickness direction) from the surface, sufficient bendability cannot be obtained. The thickness of the soft layer (depth range from the surface) is preferably 5 μm or more, and more preferably 10 μm or more.
[0055]
　On the other hand, when the thickness of the soft layer exceeds 100 μm, the strength of the steel sheet is significantly reduced. Therefore, the thickness of the soft layer is set to 100 μm or less. The thickness of the soft layer is preferably 70 μm or less.
[0056]
"The volume fraction of the
　ferrite crystal grains contained in the soft layer having an aspect ratio of less than 3.0 is 50% or more." Among the ferrite crystal grains contained in the soft layer, crystals having an aspect ratio of less than 3.0 If the volume fraction of the grains (the ratio of ferrite crystal grains having an aspect ratio of less than 3.0 to the volume fraction of all ferrite grains in the soft layer) is less than 50%, the bendability deteriorates. Therefore, among the ferrites contained in the soft layer, the volume fraction of crystal grains having an aspect ratio of less than 3.0 is set to 50% or more. This volume fraction is preferably 60% or more, more preferably 70% or more. The ferrite contained in the soft layer includes both the above-mentioned hard ferrite and soft ferrite.
[0057]
"The volume fraction of retained austenite in the soft layer is 50% or more of the volume fraction of retained austenite inside the steel sheet." The
　retained austenite contained in the soft layer suppresses the growth of fatigue cracks to improve the fatigue strength of the steel sheet. Improve. Therefore, the volume fraction of retained austenite contained in the soft layer is set to 50% or more of the volume fraction of retained austenite inside the steel sheet. More preferably, the volume fraction of retained austenite contained in the soft layer is 60% or more, 70% or more, or 80% or more of the volume fraction of retained austenite inside the steel sheet. The area ratio of retained austenite inside the steel sheet refers to the area ratio of retained austenite contained in the range of 1/8 to 3/8 thickness centered on the position of 1/4 thickness of the steel sheet from the surface.
[0058]
"Internal oxide layer containing Si oxide"
　The steel sheet of the present embodiment is more than 0.2 μm from the surface when analyzed by a high-frequency glow discharge (high-frequency GDS) analysis method in the depth direction (plate thickness direction) from the surface. A peak of emission intensity having a wavelength indicating Si appears in a range of 5 μm or less from the surface. This indicates that the steel sheet is internally oxidized and has an internal oxide layer containing a Si oxide in a range of more than 0.2 μm from the surface of the steel sheet and 5 μm or less from the surface. A steel sheet having such an internal oxide layer suppresses the formation of an oxide film such as Si oxide on the surface of the steel sheet due to heat treatment during production. Therefore, the steel sheet having such an internal oxide layer has excellent chemical conversion treatment property and plating adhesion.
[0059]
　The steel sheet of the present embodiment has a range of more than 0.2 μm and 5 μm or less from the surface and a range of 0 μm to 0.2 μm (depth 0.) from the surface when analyzed in the depth direction from the surface by the high-frequency glow discharge analysis method. It may have a peak of emission intensity at a wavelength indicating Si in both the region shallower than 2 μm). The presence of peaks in both of these ranges indicates that the steel sheet has an internal oxide layer and an external oxide layer containing Si oxide on the surface.
[0060]
　FIG. 2 is a graph showing the relationship between the depth from the surface and the emission intensity (Intensity) of the wavelength indicating Si when the steel sheet of the present embodiment is analyzed in the depth direction from the surface by the high-frequency glow discharge analysis method. is there. In the steel sheet of the present embodiment shown in FIG. 2, a peak of emission intensity (derived from the internal oxide layer) having a wavelength indicating Si appears in a range of more than 0.2 μm and 5 μm or less from the surface. Further, a peak of emission intensity (derived from the external oxide layer ( IMAX )) having a wavelength indicating Si also appears in the range of 0 (outermost surface) to 0.2 μm from the surface . Therefore, it can be seen that the steel sheet shown in FIG. 2 has an internal oxide layer and also has an external oxide layer.
[0061]
　FIG. 3 shows the relationship between the depth from the surface and the emission intensity (Intensity) of the wavelength indicating Si when the steel sheet different from the present embodiment is analyzed by the high-frequency glow discharge analysis method in the depth direction from the surface. It is a graph. In the steel sheet shown in FIG. 3, the peak of the emission intensity at the wavelength indicating Si appears in the range of 0 (outermost surface) to 0.2 μm from the surface, but does not appear in the range of more than 0.2 μm and 5 μm or less. .. This indicates that the steel sheet does not have an internal oxide layer but has only an external oxide layer.
[0062]
"Hot-dip galvanized layer"
　The steel sheet of the present embodiment may have a hot-dip galvanized layer formed on its surface (both sides or one side). It is called "hot-dip galvanized steel sheet"). The hot-dip galvanized layer may be an alloyed hot-dip galvanized layer obtained by alloying the hot-dip galvanized layer.
　When the hot-dip galvanized layer is not alloyed, the iron content in the hot-dip galvanized layer is preferably less than 7.0% by mass. When the hot-dip galvanized layer is an alloyed hot-dip galvanized layer, the iron content is preferably 6.0% by mass or more. The alloyed hot-dip galvanized steel sheet has better weldability than the hot-dip galvanized steel sheet.
[0063]
　The amount of the hot-dip galvanized layer plated is not particularly limited , but is preferably 5 g / m 2 or more per side from the viewpoint of corrosion resistance, and is 20 to 120 g / m 2 and further 25 to 75 g / m 2 . It is more preferable that it is within the range.
[0064]
　In the hot-dip galvanized steel sheet of the present embodiment, an upper-layer plating layer may be further provided on the hot-dip galvanized layer for the purpose of improving coatability, weldability, and the like. Further, in the hot-dip galvanized steel sheet of the present embodiment, various treatments such as chromate treatment, phosphate treatment, lubricity improvement treatment, weldability improvement treatment and the like may be performed on the hot-dip galvanized layer.
[0065]
"Electrogalvanized layer"
　An electrogalvanized layer may be formed on the surface of the steel sheet of the present embodiment. The electrogalvanized layer can be formed by a conventionally known method.
[0066]
"Heat treatment steel sheet"
　A heat treatment steel sheet (referred to as "heat treatment steel sheet of the present embodiment ") used as a material for the steel sheet of the present embodiment will be described below.
　Specifically, the heat-treated steel sheet of the present embodiment has any of the chemical compositions of the above steel sheets and has the steel structure (microstructure) shown below. In addition, [%] in the description of the content of each tissue is [volume%].
[0067]
(Microstructure inside the heat-treated steel sheet)
"A total of 70% or more of the lath-shaped structure in terms of body
　integration rate" The heat-treated steel sheet of this embodiment has a thickness of 1/8 to 3 centered on a position 1/4 thick from the surface. The steel structure in the range of / 8 thickness (the steel structure inside the steel sheet for heat treatment) has a lath-like structure consisting of one or more of upper bainite, bainite ferrite, tempered martensite, and fresh martensite. It contains 70% or more in total in fractions.
[0068]
　When the heat-treated steel sheet contains the above lath-shaped structure in a total body integration ratio of 70% or more, the steel sheet obtained by subjecting the heat-treated steel sheet to the second heat treatment described later has a hard ferrite structure inside the heat-treated steel sheet. Will be the main subject. When the total volume fraction of the lath-shaped structure in the heat-treated steel sheet is less than 70%, the steel sheet obtained by subjecting the heat-treated steel sheet to the second heat treatment contains a large amount of soft ferrite in the steel structure inside the steel sheet. As a result, the steel sheet of the present embodiment cannot be obtained. The steel structure inside the heat-treated steel sheet in the heat-treated steel sheet preferably contains the above lath-like structure in a volume fraction of 80% or more in total, more preferably 90% or more in total, and even if it is 100%. I do not care.
[0069]
"Number density of residual austenite grains having an aspect ratio of less than 1.3 and a major axis of more than 2.5 μm in
　the heat-treated steel sheet " The steel structure inside the heat-treated steel sheet of the present embodiment remains in addition to the lath-like structure described above. includes austenite, the aspect ratio and less than 1.3 long diameter and the number density of the residual austenite grains 2.5μm than 1.0 × 10 -2 cells / [mu] m 2 is obtained by limiting below.
[0070]
　If the retained austenite present in the steel structure inside the heat-treated steel sheet is in the form of coarse lumps, coarse lump-shaped retained austenite grains are present inside the steel sheet of the steel sheet obtained by subjecting the heat-treated steel sheet to the second heat treatment. However, it may not be possible to sufficiently secure retained austenite having an aspect ratio of 2.0 or more. Therefore, in the heat treatment for steel, the aspect ratio is the number density of the residual austenite grains of coarse massive long diameter is 2.5μm greater less than 1.3 1.0 × 10 -2 cells / [mu] m 2 or less. The lower the number density of the coarse lumpy residual austenite grains in the heat-treated steel sheet is, the more preferable, and the density is preferably 0.5 × 10-2 pieces / μm 2 or less.
[0071]
　Further, if the retained austenite is excessively present inside the heat-treated steel sheet, a part of the retained austenite is isotropic by performing the second heat treatment described later on the heat-treated steel sheet. As a result, it may not be possible to sufficiently secure retained austenite having an aspect ratio of 2.0 or more inside the steel sheet of the steel sheet obtained after the second heat treatment. Therefore, the volume fraction of retained austenite contained in the steel structure inside the heat-treated steel sheet is preferably 10% or less.
[0072]
(Microstructure of the surface layer of the heat-treated steel sheet)
"Soft layer containing 80% or more of soft ferrite in volume fraction"
　The heat-treated steel sheet used as the material of the steel sheet according to the present embodiment has a volume fraction of 80% or more. A surface layer composed of a soft layer containing ferrite is formed. The thickness of the soft layer of the heat-treated steel sheet is 1 μm to 50 μm. When the thickness of the soft layer in the heat-treated steel sheet is less than 1 μm in the depth direction from the surface, the thickness of the soft layer formed on the steel sheet obtained by subjecting the heat-treated steel sheet to the second heat treatment (depth range from the surface). ) Is insufficient. On the other hand, when the thickness of the soft layer of the heat-treated steel sheet exceeds 50 μm in the depth direction from the surface, the thickness of the soft layer formed on the steel sheet obtained by subjecting the heat-treated steel sheet to the second heat treatment (depth from the surface). Since the range) becomes excessive, the decrease in strength of the steel sheet due to having the soft layer becomes apparent. Therefore, in the heat treatment steel sheet, the thickness of the soft layer is preferably 50 μm or less, and preferably 10 μm or less.
[0073]
"Internal oxide layer containing Si oxide"
　The heat-treated steel sheet of the present embodiment has a range of more than 0.2 μm and less than 5 μm from the surface when analyzed by a high-frequency glow discharge (high-frequency GDS) analysis method in the depth direction from the surface. The peak of the emission intensity of the wavelength indicating Si appears. This indicates that the heat-treated steel sheet is internally oxidized and has an internal oxide layer containing a Si oxide in a range of more than 0.2 μm and 5 μm or less from the surface. The heat treatment steel sheet having an internal oxide layer in the above range is one in which the formation of an oxide film such as Si oxide on the surface of the steel sheet due to the heat treatment during production is suppressed.
[0074]
　The heat treatment steel sheet of the present embodiment has a range of more than 0.2 μm and 5 μm or less from the surface and a range of 0 μm to 0.2 μm (depth) from the surface when analyzed by a high-frequency glow discharge analysis method in the depth direction from the surface. It may have a peak of emission intensity at a wavelength indicating Si in both (a region shallower than 0.2 μm). This indicates that the heat-treated steel sheet has an internal oxide layer and an external oxide layer containing Si oxide on the surface.
[0075]
"
　Method of manufacturing a steel sheet according to the present embodiment " Next, a method of manufacturing a steel sheet of the present embodiment will be described.
[0076]
　In the method for producing a steel sheet of the present embodiment, as shown in FIG. 4, a slab having the above chemical composition is hot-rolled to obtain a pickled hot-rolled steel sheet or a cold-rolled hot-rolled steel sheet. , A steel sheet for heat treatment is manufactured by performing the first heat treatment shown below. Then, the heat-treated steel sheet is subjected to the second heat treatment shown below. The first heat treatment and / or the second heat treatment may be carried out using a dedicated heat treatment line or may be carried out using an existing annealing line.
[0077]
(Casting Step) In order
　to manufacture the steel sheet of the present embodiment, first, a slab having the above chemical composition (composition) is cast. As the slab to be used for hot rolling, a slab manufactured by a continuously cast slab, a thin slab caster, or the like can be used. The slab after casting may be hot-rolled after being cooled to room temperature, or may be directly hot-rolled at a high temperature. It is preferable that the slab after casting is directly subjected to hot rolling at a high temperature because the energy required for heating the hot rolling can be reduced.
[0078]
(Slab heating) The
　slab is heated prior to hot rolling. When producing the steel sheet of the present embodiment, it is preferable to select slab heating conditions that satisfy the following formula (4).
[0079]
[

Equation 1] (In the formula (4), fγ is a value represented by the following formula (5), WMnγ is a value represented by the following formula (6), and D is a value represented by the following formula (7). and a, a c1 is the value represented by the following formula (8), a c3 is a value represented by the following formula (9), ts (T) is the residence time of the slab in the slab heating temperature T (sec) Is.)
[0080]
[Number 2]

In (Equation (5), T is a slab heating temperature (° C.), C the amount of WC is in the steel (mass%), A c1 is the value represented by the following formula (8), A c3 is It is a value represented by the following formula (9).)
[0081]
[Expression 3]

In (Equation (6), T is a slab heating temperature (° C.), WMn the Mn content in steel (mass%), A c1 is the value represented by the following formula (8), A c3 is It is a value represented by the following formula (9).)
[0082]
[

Equation 4] (In equation (7), T is the slab heating temperature (° C.) and R is the gas constant; 8.314 J / mol.)
[0083]
A c1 = 723-10.7 × Mn-16.9 × Ni + 29.1 × Si + 16.9 × Cr · · (8)
(element symbol of the formula (8) where is the mass% in the steel of the element there.)
a c3 = 879-346 × C + 65 × Si-18 × Mn + 54 × Al · · (9) (formula (9) element symbol in the formula is the mass% in the steel of the element.)
[0084]
　The molecule of formula (4) represents the degree of Mn content distributed from α to γ ​​during the stay in the two-phase region of α (ferrite) and γ (austenite). The larger the molecule of the formula (4), the more inhomogeneous the Mn concentration distribution in the steel. The denominator of the formula (4) is a term corresponding to the distance of Mn atoms diffused in γ during stay in the γ single-phase region. The larger the denominator of the formula (4), the more homogenized the Mn concentration distribution. In order to sufficiently homogenize the Mn concentration distribution in the steel, it is preferable to select the slab heating conditions so that the value of the formula (4) is 1.0 or less. The smaller the value of the formula (4), the smaller the number density of the heat-treated steel sheet and the coarse austenite grains inside the steel sheet of the steel sheet.
[0085]
(Hot rolling)
　After heating the slab, hot rolling is performed. If the completion temperature (finishing temperature) of hot rolling is less than 850 ° C., the rolling reaction force increases, and it becomes difficult to stably obtain a specified plate thickness. Therefore, the completion temperature of hot rolling is preferably 850 ° C. or higher. From the viewpoint of rolling reaction force, the completion temperature of hot rolling is preferably 870 ° C. or higher. On the other hand, in order to make the completion temperature of hot rolling over 1050 ° C, it is necessary to heat the steel sheet using a heating device or the like in the process from the end of heating of the slab to the completion of hot rolling, which requires a high cost. It becomes. Therefore, it is preferable that the completion temperature of hot rolling is 1050 ° C. or lower. In order to make it easier to secure the temperature of the steel sheet during hot rolling, the completion temperature of hot rolling is preferably 1000 ° C. or lower, and more preferably 980 ° C. or lower.
[0086]
(Pickling)
　Next, the hot-rolled steel sheet produced in this manner is pickled . Pickling is a step of removing oxides on the surface of a hot-rolled steel sheet, and is important for improving the chemical conversion treatment property of the steel sheet. The pickling of the hot-rolled steel sheet may be performed once or may be performed in a plurality of times.
[0087]
(Cold rolling) The
　pickled hot-rolled steel sheet may be cold-rolled to obtain a cold-rolled steel sheet. By cold-rolling a hot-rolled steel sheet, it is possible to manufacture a steel sheet having a predetermined thickness with high accuracy. In cold rolling, if the total rolling ratio exceeds 85%, the ductility of the steel sheet is lost and the risk of the steel sheet breaking during cold rolling increases. Therefore, the total reduction rate is preferably 85% or less, and more preferably 75% or less. The lower limit of the total rolling reduction in the cold rolling process is not particularly set, and cold rolling may not be performed. In order to improve the shape homogeneity of the steel sheet to obtain a good appearance and to make the temperature of the steel sheet during the first heat treatment and the second heat treatment uniform to obtain good ductility, the rolling reduction ratio of cold rolling is 0 in total. It is preferably 5.5% or more, and more preferably 1.0% or more.
[0088]
(First heat treatment)
　Next, a heat-treated steel sheet is produced by subjecting a pickled hot-rolled steel sheet or a cold-rolled cold-rolled hot-rolled steel sheet to a first heat treatment. The first heat treatment is performed under conditions that satisfy the following (a) to (e).
[0089]
(A) During the period from 650 ° C. to heating to the maximum heating temperature, the atmosphere around the hot-rolled steel sheet or the cold-rolled steel sheet contains H 2 of 0.1% by volume or more and satisfies the following formula (2). Make it an atmosphere.
　-1.1 ≤ log (PH 2 O / PH 2 ) ≤ -0.07 ... (2)
(In formula (2), PH 2 O indicates the partial pressure of water vapor, and PH 2 indicates the partial pressure of hydrogen. In
　the first heat treatment, by satisfying the above (a), the oxidation reaction outside the steel plate is suppressed and the decarburization reaction is promoted. In the first heat treatment, in a part of the temperature range from 650 ° C. to heating to the maximum heating temperature, it is necessary to set the atmosphere around the steel sheet as described in (a) above, and the temperature is changed from 650 ° C. to the maximum heating temperature. It is preferable that the atmosphere around the steel sheet is the atmosphere described in (a) above in the entire temperature range until heating.
[0090]
　If H 2 in the atmosphere is less than 0.1% by volume, the oxide film existing on the surface of the steel sheet cannot be sufficiently reduced, and an oxide film is formed on the steel sheet. Therefore, the chemical conversion treatment property and the plating adhesion of the steel sheet obtained after the second heat treatment are lowered. On the other hand, if the H 2 content in the atmosphere is more than 20% by volume, the effect is saturated. Further, if the H 2 content in the atmosphere is more than 20% by volume, the risk of hydrogen explosion increases in operation. Therefore, it is preferable that the H 2 content in the atmosphere is 20% by volume or less.
[0091]
　When the log (PH 2 O / PH 2 ) is less than −1.1, external oxidation of Si and Mn occurs on the surface layer of the steel sheet, and the decarburization reaction becomes insufficient to form the surface layer of the steel sheet for heat treatment. The thickness of the soft layer becomes thin. On the other hand, when the log (PH 2 O / PH 2 ) exceeds −0.07, the decarburization reaction proceeds excessively, so that the strength of the steel sheet after the second heat treatment is insufficient.
[0092]
(B) Hold at a maximum heating temperature of (A c3-30 ) ° C. to 1000 ° C. for 1 second to 1000 seconds.
　In the first heat treatment, the maximum heating temperature is set to ( Ac3-30 ) ° C. or higher. When the maximum heating temperature is less than ( Ac3-30 ) ° C., lumpy coarse ferrite remains in the steel sheet structure inside the steel sheet in the heat treatment steel sheet. As a result, the soft ferrite phase of the steel sheet obtained after the second heat treatment of the heat-treated steel sheet becomes excessive, and the number ratio of the retained austenite having an aspect ratio of 2.0 or more is insufficient, resulting in deterioration of the characteristics. Maximum heating temperature (A c3 preferably -15) ° C. or higher, (A c3 more preferably be +5) ° C. or higher. On the other hand, when heated to an excessively high temperature, decarburization of the surface layer may proceed excessively and the fatigue resistance characteristics may be insufficient, the fuel cost required for heating may increase, and the furnace body may be damaged. Therefore, it is desirable that the maximum heating temperature is 1000 ° C. or lower.
[0093]
　In the first heat treatment, the holding time at the maximum heating temperature is set to 1 second to 1000 seconds. If the holding time is less than 1 second, lumpy coarse ferrite remains in the steel sheet structure inside the steel sheet in the heat treatment steel sheet. As a result, the volume fraction of the soft ferrite of the steel sheet obtained after the second heat treatment becomes excessive, and the characteristics deteriorate. The holding time is preferably 10 seconds or longer, and more preferably 50 seconds or longer. On the other hand, if the holding time is too long, not only the effect of heating to the maximum heating temperature is saturated, but also the productivity is impaired. Therefore, the holding time is set to 1000 seconds or less.
[0094]
(C) Heating is performed so that the average heating rate from 650 ° C. to the maximum heating temperature is 0.5 ° C./sec to 500 ° C./sec.
　In the first heat treatment, if the average heating rate from 650 ° C. to the maximum heating temperature is less than 0.5 ° C./sec, Mn segregation proceeds during the heat treatment, and a coarse massive Mn concentrated region is formed, and the second heat treatment is performed. The characteristics of the steel sheet obtained after the heat treatment deteriorate. The average heating rate is preferably 1.5 ° C./sec or higher in order to suppress the formation of austenite in the form of agglomerates. On the other hand, if the average heating rate exceeds 500 ° C./sec, the decarburization reaction does not proceed sufficiently. Therefore, the average heating rate is set to 500 ° C./sec or less. The average heating rate from 650 ° C to the maximum heating temperature is a value obtained by dividing the difference between 650 ° C and the maximum heating temperature by the elapsed time from the steel sheet surface temperature of 650 ° C to the maximum heating temperature. is there.
[0095]
(D) After holding at the maximum heating temperature, cooling is performed so that the average cooling rate in the temperature range from 700 ° C. to Ms is 5 ° C./sec or more.
　In the first heat treatment, in order to make the steel sheet structure inside the steel sheet in the heat treatment steel sheet mainly composed of lath-like structure, after holding at the maximum heating temperature, cooling in the temperature range from 700 ° C. to Ms represented by the following formula (10) Cool so that the average cooling rate is 5 ° C./sec or higher. If the average cooling rate is less than 5 ° C./sec, massive ferrite may form. The average cooling rate is preferably 10 ° C./sec or higher, and more preferably 30 ° C./sec or higher. Further, the upper limit of the average cooling rate does not need to be set in particular, but special equipment is required to cool at an average cooling rate of more than 500 ° C./sec. Therefore, the average cooling rate is preferably 500 ° C./sec or less. The average cooling rate in the temperature range from 700 ° C. to Ms is a value obtained by dividing the difference between 700 ° C. and Ms by the elapsed time from 700 ° C. to Ms on the surface temperature of the steel sheet. Ms is calculated by the following formula.
[0096]
Ms = 561-407 x C-7.3 x Si-37.8 x Mn-20.5 x Cu-19.5 x Ni-19.8 x Cr-4.5 x Mo ... (10)
(Equation ) The element symbol in the formula (10) is the mass% of the element in steel.)
[0097]
(E) Cooling at an average cooling rate of 5 ° C./sec or more is performed up to a cooling stop temperature of Ms or less.
　In the first heat treatment, cooling is performed so that the average cooling rate in the temperature range from 700 ° C. to Ms is 5 ° C./sec or more until the cooling stop temperature is Ms or less. The cooling stop temperature may be room temperature (25 ° C.). By setting the cooling stop temperature to Ms or less, the steel sheet structure inside the steel sheet in the heat treatment steel sheet obtained after the first heat treatment becomes mainly lath-shaped. The cooling stop temperature is the surface temperature of the steel sheet at the time when the injection of the refrigerant (cooling water, atmosphere, etc.) that causes the temperature drop of the steel sheet is completed.
[0098]
　In the manufacturing method of the present embodiment, the steel sheet cooled to a cooling stop temperature of Ms or less and room temperature or more in the first heat treatment may be continuously subjected to the second heat treatment shown below. In addition, the second heat treatment shown below may be performed after cooling to room temperature in the first heat treatment and winding.
[0099]
　The steel sheet cooled to room temperature in the first heat treatment is the heat treatment steel sheet of the present embodiment described above. The heat treatment steel sheet becomes the steel sheet of the present embodiment by performing the second heat treatment shown below. Further, by performing hot-dip galvanizing (further alloying treatment if necessary) on this, the hot-dip galvanized steel sheet of the present embodiment can be obtained.
　In the present embodiment, the heat-treated steel sheet before the second heat treatment may be subjected to various treatments. For example, in order to correct the shape of the heat-treated steel sheet, the heat-treated steel sheet may be subjected to a temper rolling process. Further, in order to remove the oxide present on the surface of the heat-treated steel sheet, the heat-treated steel sheet may be pickled.
[0100]
(Second heat treatment) The second heat
　treatment is applied to the steel sheet (heat treatment steel sheet) that has been subjected to the first heat treatment. The second heat treatment satisfies the following (A) to (E).
(A) The atmosphere around the steel sheet from 650 ° C. to the maximum heating temperature is such that it contains 0.1% by volume or more of H 2 and satisfies the above formula (3).
　log (PH 2 O / PH 2 ) <-1.1 ... (3)
(In formula (3), PH 2 O indicates the partial pressure of water vapor and PH 2 indicates the partial pressure of hydrogen.)
[0101]
　In the second heat treatment, the atmosphere around the hot-rolled steel sheet or the cold-rolled steel sheet needs to be the atmosphere described in (A) above in a part of the temperature range from 650 ° C. to heating to the maximum heating temperature. , It is preferable that the atmosphere around the steel sheet is the atmosphere described in (A) above in the entire temperature range from 650 ° C. to heating to the maximum heating temperature. When hot-dip galvanizing a steel sheet, in the second heat treatment, it is necessary to create the atmosphere described in (A) above in the entire temperature range from 650 ° C. to the maximum heating temperature. is there. Further, when the steel sheet is hot-dip galvanized, in the second heat treatment, the atmosphere around the steel sheet contains H 2 of 0.1 volume or more , O 2 is 0.020% by volume or less, and the above formula ( 3) needs to be satisfied.
　In the second heat treatment, in order to satisfy the above (A), the decarburization reaction on the surface of the steel sheet is suppressed, and carbon atoms are supplied from the inside of the steel sheet to the surface layer portion decarburized during the first heat treatment. As a result, a composite structure having a predetermined thickness in which a small amount of martensite and retained austenite are dispersed is formed on the surface of the steel sheet after the second heat treatment.
[0102]
　If H 2 in the atmosphere is less than 0.1% by volume, the oxide film existing on the surface of the steel sheet cannot be sufficiently reduced, and an oxide film is formed on the steel sheet. Therefore, the chemical conversion treatment property of the steel sheet obtained after the second heat treatment is lowered. Further, when the steel sheet is hot-dip galvanized and the H 2 in the atmosphere is less than 0.1% by volume or the O 2 in the atmosphere is more than 0.020% by volume, the plating adhesion of the steel sheet is lowered. Further, when the H 2 content in the atmosphere is more than 20% by volume, the effect is saturated. Further, if the H 2 content in the atmosphere is more than 20% by volume, the risk of hydrogen explosion increases in operation. Therefore, it is preferable that the H 2 content in the atmosphere is 20% by volume or less. The preferred range of H 2 is 2.0% by volume or more, more preferably 3.0% by volume or more. The preferred range of O 2 is 0.010% by volume or less, more preferably 0.005% by volume or less.
[0103]
　Further, when the log (PH 2 O / PH 2 ) is −1.1 or more, the decarburization reaction on the surface of the steel sheet proceeds excessively, so that the thickness of the soft layer forming the surface layer of the steel sheet obtained after the second heat treatment is thick. The thickness becomes thicker and the strength of the steel sheet becomes insufficient. The lower the log (PH 2 O / PH 2 ) value, the more preferable it is. Therefore, it is not necessary to set a lower limit on the value. However, in order to make the value of log (PH 2 O / PH 2 ) less than -2.2, special equipment is required, so the lower limit of the value of log (PH 2 O / PH 2 ) is set to -2. It is preferably 2.
[0104]
(B) (A c1 +25) ° C. - (A c3 -10) to hold 1 second to 1000 seconds at a maximum heating temperature ° C..
　In the second heat treatment, the maximum heating temperature (A c1 +25) ° C. ~ (A c3 and -10) ° C.. Maximum heating temperature (A c1 If it is +25) In order to increase the hard structure fraction in the steel sheet obtained after the second heat treatment and obtain a steel sheet having higher strength, it is preferable to set the maximum heating temperature to ( Ac1 +40) ° C. or higher.
[0105]
　On the other hand, when the maximum heating temperature exceeds ( Ac3-10 ) ° C., almost or all of the microstructure becomes austenite, the lath-like structure of the steel sheet before the second heat treatment (heat treatment steel sheet) disappears, and the steel sheet takes over. It disappears. As a result, the retained austenite fraction in the internal structure of the steel sheet obtained after the second heat treatment is insufficient, and the number ratio of the retained austenite having an aspect ratio of 2.0 or more is insufficient, so that the characteristics are significantly deteriorated. From this, the maximum heating temperature is set to Acc3-10 ° C. or lower. Fully take over the steel sheet lath-shaped structure in the second heat treatment before the steel sheet, in order to further improve the properties of the steel sheet, the maximum heating temperature (A c3 preferably set to -20) ° C. or less, (A c3 -30) It is more preferable to keep the temperature below ° C.
[0106]
　In the second heat treatment, the holding time at the maximum heating temperature is set to 1 second to 1000 seconds. If the holding time is less than 1 second, the diffusion of carbon atoms from the inside of the steel sheet to the surface layer becomes insufficient, and there is a concern that cementite in the steel remains undissolved and the characteristics of the steel sheet deteriorate. The holding time is preferably 30 seconds or more. On the other hand, if the holding time is too long, the diffusion of carbon atoms from the inside of the steel sheet to the surface layer proceeds excessively, and the effect of decarburizing the surface layer by the first heat treatment disappears. Therefore, the holding time is limited to 1000 seconds.
[0107]
(C) Heating is performed so that the average heating rate from 650 ° C. to the maximum heating temperature is 0.5 ° C./sec to 500 ° C./sec.
　If the average heating rate from 650 ° C. to the maximum heating temperature in the second heat treatment is less than 0.5 ° C./sec, the recovery of the lath-like structure created in the first heat treatment proceeds, so that austenite grains are formed in the grains. The volume fraction of soft ferrite that does not exist increases. On the other hand, if the average heating rate exceeds 500 ° C./sec, the decarburization reaction does not proceed sufficiently. The average heating rate from 650 ° C to the maximum heating rate is a value obtained by dividing the difference between 650 ° C and the maximum heating rate by the elapsed time from the steel sheet surface temperature of 650 ° C to the maximum heating rate. is there.
[0108]
(D) Cooling from the maximum heating temperature to 480 ° C. or lower so that the average cooling rate between 600 and 700 ° C. is 3 ° C./sec or more In
　the second heat treatment, the average cooling rate between 600 and 700 ° C. is 3 Cool from the maximum heating temperature to 480 ° C so that the temperature is equal to or higher than ° C./sec. When the average cooling rate in the temperature range is less than 3 ° C./sec, coarse carbides are generated and the characteristics of the steel sheet are impaired. The average cooling rate in the temperature range is preferably 10 ° C./sec or higher. The upper limit of the average cooling rate in the temperature range may not be set in particular, but it is preferably 200 ° C./sec or less because a special cooling device is required to exceed 200 ° C./sec. The average cooling rate in the temperature range is a value obtained by dividing the temperature difference between 600 and 700 ° C. (that is, 100 ° C.) by the time required for cooling from 700 ° C. to 600 ° C.
[0109]
(E) After cooling at an average cooling rate of 3 ° C./sec or higher, the temperature is maintained between 300 ° C. and 480 ° C. for 10 seconds or longer.
　In the second heat treatment, if the holding time between 300 ° C. and 480 ° C. is less than 10 seconds, carbon is not sufficiently concentrated in the untransformed austenite, so that the lath-shaped ferrite does not grow sufficiently and becomes austenite. C enrichment does not progress. As a result, fresh martensite is generated, and the characteristics of the steel sheet are greatly deteriorated. The holding time is preferably 100 seconds or more in order to sufficiently promote carbon concentration in austenite, reduce the amount of martensite produced, and improve the characteristics of the steel sheet. In addition, holding for N seconds or more between 300 ° C. and 480 ° C. means that the temperature of the steel sheet is kept within the temperature range of 300 ° C. to 480 ° C. for N seconds or more.
[0110]
　By performing the second heat treatment described above, the steel sheet of the present embodiment described above can be obtained.
　In the present embodiment, the steel sheet may be cold-rolled for the purpose of shape correction. The cold rolling may be performed after the first heat treatment or after the second heat treatment. Further, it may be applied both after the first heat treatment and after the second heat treatment. The reduction rate of cold rolling is preferably 3.0% or less, and more preferably 1.2% or less. If the rolling reduction of cold rolling exceeds 3.0%, some austenite may be lost due to work-induced transformation, and the characteristics may be impaired. On the other hand, the lower limit of the rolling ratio of cold rolling is not particularly defined, and the characteristics of the steel sheet according to the present embodiment can be exhibited even if cold rolling is not performed.
[0111]
(Hot-dip galvanizing) In the
　method for producing a steel sheet according to the present embodiment, a hot-dip galvanizing step of forming a hot-dip galvanizing layer on the surface of the base steel sheet after the second heat treatment may be performed. Following the formation of the hot-dip galvanized layer, the alloying treatment of the plated layer may be performed.
[0112]
　The hot-dip galvanizing and alloying treatment may be performed at any timing after the completion of the cooling step (D) in the second heat treatment as long as the conditions specified by the manufacturing method according to the present embodiment are satisfied. For example, as shown as a pattern [1] in FIG. 5, a plating treatment (further, an alloying treatment if necessary) may be performed after the cooling step (D) and the isothermal holding step (E). , As shown in the pattern [2] in FIG. 6, after the cooling step (D), a plating treatment (further, an alloying treatment if necessary) may be performed, and then an isothermal maintenance (E) may be performed. .. Alternatively, as shown as a pattern [3] in FIG. 7, after the cooling step (D) and the isothermal holding step (E), the mixture is once cooled to room temperature and then plated (and alloyed if necessary). ) May be applied.
[0113]
　General conditions can be used as the plating conditions such as the galvanizing bath temperature and the galvanizing bath composition in the hot-dip galvanizing step, and there is no particular limitation. For example, the plating bath temperature may be 420 to 500 ° C., the penetration plate temperature of the steel sheet may be 420 to 500 ° C., and the immersion time may be 5 seconds or less. The plating bath is preferably a plating bath containing 0.08 to 0.2% of Al, but may also contain impurities such as Fe, Si, Mg, Mn, Cr, Ti, and Pb. Further, it is preferable to control the basis weight of hot-dip galvanizing by a known method such as gas wiping. The basis weight is usually 5 g / m 2 or more per side , but is preferably 25 to 75 g / m 2 , and more preferably 20 to 120 g / m 2 .
[0114]
　As described above, the high-strength hot-dip galvanized steel sheet on which the hot-dip galvanized layer is formed may be alloyed, if necessary.
　The alloying treatment may be carried out according to a conventional method, but the alloying treatment temperature is preferably 460 to 600 ° C. If the alloying treatment is less than 460 ° C., not only the alloying rate becomes slow and the productivity is impaired, but also the alloying treatment unevenness occurs. Therefore, the alloying treatment temperature is preferably 460 ° C. or higher. On the other hand, if the alloying treatment temperature exceeds 600 ° C., alloying proceeds excessively and the plating adhesion of the steel sheet deteriorates. Therefore, the alloying treatment temperature is preferably 600 ° C. or lower. The alloying treatment temperature is more preferably 480 to 580 ° C. or lower. The heating time for the alloying treatment is preferably 5 to 60 seconds.
　Further, the alloying treatment is preferably performed under the condition that the iron concentration in the hot-dip galvanized layer is 6.0% by mass or more.
[0115]
　Further, an electrogalvanized layer may be formed on the surface of the steel sheet of the present embodiment. The electrogalvanized layer can be formed by a conventionally known method.
[0116]
　Next, a method for measuring each configuration of the steel sheet of the present embodiment and the steel sheet for heat treatment will be described.
"Measurement of steel structure" In steel
　sheets and heat treatment steel sheets, ferrite (soft ferrite and hard ferrite), bainite, tempered martensite, fresh martensite, pearlite, cementite, upper bainite, contained in the steel structure inside and on the surface of the steel sheet, The body integration rate of bainitic ferrite can be measured by using the method shown below.
[0117]
　A sample is taken with the cross section parallel to the rolling direction and the thickness direction of the steel sheet as the observation surface, and the observation surface is polished and nightal-etched. Next, in one or more observation fields in the range of 1/8 to 3/8 thickness centered on the position of 1/4 thickness from the surface on the observation surface, a total of 2.0 × 10-9 m 2 or more. The area of ​​the field is observed with a field emission scanning electron microscope (FE-SEM: Field Emission Scanning Electron Microscope). Then, the area fractions of ferrite, bainite, tempered martensite, fresh martensite, pearlite, and cementite are measured, respectively, and these are regarded as volume fractions. Here, a region having a substructure in the grain and in which carbides are precipitated with a plurality of variants is determined to be tempered martensite. Further, the region where cementite is precipitated in a lamellar shape is judged to be pearlite. The region where the brightness is low and the substructure is not recognized is judged as ferrite (soft ferrite or hard ferrite). The region where the brightness is high and the substructure is not exposed by etching is judged as fresh martensite or retained austenite. The volume fraction of each tissue is calculated by the point counting method to obtain the volume fraction of each tissue. The volume fraction of fresh martensite can be determined by subtracting the volume fraction of retained austenite obtained by the X-ray diffraction method.
　The volume fractions of hard ferrite and soft ferrite are obtained by the method described later based on the measured volume fractions of ferrite.
　The volume fraction of fresh martensite can be obtained by subtracting the volume fraction of retained austenite obtained by the X-ray diffraction method described later from the volume fraction of fresh martensite or retained austenite.
[0118]
　In the steel sheet and the steel sheet for heat treatment, the volume fraction of retained austenite contained in the steel sheet is evaluated by an X-ray diffraction method. In the range of 1/8 to 3/8 thickness centered on the position of 1/4 thickness from the surface of the plate thickness, the surface parallel to the plate surface is finished as a mirror surface, and the area fraction of FCC iron is divided by X-ray diffraction. Is measured and used as the volume fraction of retained austenite.
[0119]
"Ratio of
　retained austenite volume fraction contained in the soft layer and retained austenite volume fraction contained inside the steel plate " In the steel plate according to the present embodiment, the volume fraction of retained austenite contained in the soft layer and the volume fraction inside the steel plate. The ratio of the retained austenite to the volume fraction is evaluated by performing a high-resolution crystal structure analysis by the EBSD method (electron backscatter diffraction method). Specifically, a sample is taken with a sheet thickness cross section parallel to the rolling direction of the steel sheet as an observation surface, and the observation surface is polished to a mirror surface. Further, electrolytic polishing or mechanical polishing using colloidal silica is performed to remove the processed layer on the surface layer. Next, the total area of ​​the observation field of view is 2 in total for the surface layer portion of the steel sheet including the soft layer and the inside of the steel sheet (range of 1/8 to 3/8 thickness centered on the position of 1/4 thickness from the surface). Crystal structure analysis is performed by the EBSD method so that the size is 0 × 10 -9 m 2 or more (multiple fields of view or the same field of view is possible). For the analysis of the data obtained by the EBSD method in the measurement, "OIM Analysys 6.0" manufactured by TSL Co., Ltd. is used. The distance between scores (step) is 0.01 to 0.20 μm. From the observation results, the region determined to be FCC iron is determined to be retained austenite, and the volume fractions of retained austenite inside the soft layer and the steel sheet are calculated respectively.
[0120]
"Measurement of aspect ratio and major axis of retained austenite grains" In
　steel sheets and heat-treated steel sheets, the aspect ratio and major axis of retained austenite grains contained in the steel structure inside the steel sheet are determined by observing the crystal grains using FE-SEM and EBSD. High-resolution crystal orientation analysis is performed by the method (electron backscatter diffraction method) and evaluated.
[0121]
　First, a sample is taken with a cross section parallel to the rolling direction and the thickness direction of the steel sheet as an observation surface, and the observation surface is polished to a mirror surface. Next, in one or more observation fields in the range of 1/8 to 3/8 thickness centered on the position of 1/4 thickness from the surface on the observation surface, a total of 2.0 × 10-9 m 2 or more. Crystal structure analysis is performed by the EBSD method for the area (either a plurality of fields of view or the same field of view is possible). Next, in order to avoid measurement errors, only austenite having a major axis length of 0.1 μm or more is extracted from the crystal orientation of the retained austenite grains measured by the above method, and a crystal orientation map is drawn. The boundary that causes a crystal orientation difference of 10 ° or more is regarded as the grain boundary of the retained austenite grains. The aspect ratio is a value obtained by dividing the major axis length of the retained austenite grains by the minor axis length. The major axis is the major axis length of the retained austenite grains. For the analysis of the data obtained by the EBSD method in the measurement, "OIM Analysys 6.0" manufactured by TSL Co., Ltd. is used. The distance between scores (step) is 0.01 to 0.20 μm. The region determined to be FCC iron from the observation results is defined as retained austenite. From this result, the ratio of the number of retained austenites having an aspect ratio of 2.0 or more to the total retained austenites is determined.
　The aspect ratio of ferrite is evaluated by observing crystal grains using FE-SEM and performing high-resolution crystal orientation analysis by the EBSD method (electron backscatter diffraction method). For the analysis of the data obtained by the EBSD method, "OIM Analysys 6.0" manufactured by TSL Co., Ltd. is used. The distance between scores (step) is 0.01 to 0.20 μm. From the observation results, the region judged to be BCC iron is set to ferrite, and a crystal orientation map is drawn. Then, the boundary that causes a crystal orientation difference of 15 ° or more is regarded as a crystal grain boundary. The aspect ratio is a value obtained by dividing the major axis length of each ferrite grain by the minor axis length.
　As a ferrite having a large aspect ratio, there is an unrecrystallized ferrite elongated in the rolling direction by cold rolling, which is clearly distinguished from the ferrite having a large aspect ratio in the steel sheet according to the present embodiment. The unrecrystallized ferrite has a larger directional gradient in the crystal grains than the ferrite in the steel sheet according to the present embodiment. Specifically, both can be distinguished by the GAM value (Grain Average Missionation) obtained by the EBSD (Electron Backscatter Diffraction Patterns) method. In general, unrecrystallized ferrite has a GAM value of 0.5 ° or more, and ferrite having a large aspect ratio in the steel sheet according to the present embodiment has a GAM value of 0.5 ° or less.
[0122]
"Ferrite grains containing austenite grains (hard ferrite) / ferrite grains not containing austenite grains (soft ferrite)"
　A method for separating grains containing austenite grains from grains not containing austenite grains will be described. First, crystal grains are observed using FE-SEM, and high-resolution crystal orientation analysis is performed by the EBSD method. Specifically, a sample is taken with a sheet thickness cross section parallel to the rolling direction of the steel sheet as an observation surface, and the observation surface is polished to a mirror surface. Further, electrolytic polishing or mechanical polishing using colloidal silica is performed to remove the processed layer on the surface layer. Next, about the inside of the steel sheet (the range of 1/8 to 3/8 thickness centered on the position of 1/4 thickness from the surface), a total of 2.0 × 10-9 m 2 or more (even in multiple visual fields or the same visual field). Crystal structure analysis is performed by the EBSD method for the area of ​​(possible). Next, with respect to the data obtained from BCC iron, the boundary that causes a crystal orientation difference of 15 ° or more is set as the grain boundary, and a grain boundary map of ferrite is drawn. Next, from the data obtained from FCC iron, in order to avoid measurement errors, draw a grain distribution map using only austenite grains with a major axis length of 0.1 μm or more, and use a grain boundary map of ferrite grains. Stack.
　In one ferrite grain, if there is one or more austenite grains completely incorporated therein, it is regarded as "ferrite grain containing austenite grains". Further, the case where the austenite grains are not adjacent to each other or are adjacent to the austenite grains only at the boundary with other grains is defined as "ferrite grains not containing austenite grains".
[0123]
"Thickness of soft layer"
　The hardness distribution from the surface layer to the inside of the steel sheet can be obtained by, for example, the following method. A sample is taken with the thickness cross section parallel to the rolling direction of the steel sheet as the observation surface, the observation surface is polished to a mirror surface, and chemical polishing is performed using colloidal silica to remove the processed layer on the surface layer. Regarding the observation surface of the obtained sample, using a micro-hardness measuring device, starting from a position 5 μm deep from the outermost layer, from the surface to a position 1/8 of the plate thickness, 10 μm in the thickness direction of the steel sheet. At a pitch, a quadrangular pyramid-shaped Vickers indenter with an apex angle of 136 ° is pushed in with a load of 2 g. At this time, the pushing load is set so that the Vickers indentations do not interfere with each other. Then, the diagonal length of the indentation is measured using an optical microscope, a scanning electron microscope, or the like, and converted into Vickers hardness (Hv).
　Next, the measurement position is moved by 10 μm or more in the rolling direction, and the same measurement is performed from the outermost layer to a position having a depth of 10 μm and a plate thickness of 1/8. By doing so, hardness measurement data having a pitch of 5 μm in the depth direction can be effectively obtained. The reason why the measurement interval is not simply set to 5 μm pitch is to avoid interference between indentations. The Vickers hardness is measured at 5 points for each thickness position, and the average value is taken as the hardness at that thickness position. A hardness profile in the depth direction is obtained by interpolating between each data with a straight line. The thickness of the soft layer is obtained by reading the depth position where the hardness is 80% or less of the hardness of the base material from the hardness profile.
[0124]
"High Frequency Glow Discharge (High Frequency GDS) Analysis" When the
　steel sheet and the steel sheet for heat treatment are analyzed by the high frequency glow discharge analysis method, a known high frequency GDS analysis method can be used.
　Specifically, a method is used in which the surface of the steel sheet is made into an Ar atmosphere, a voltage is applied to generate glow plasma, and the surface of the steel sheet is sputtered to analyze in the depth direction. Then, the element contained in the material (steel plate) is identified from the emission spectrum wavelength peculiar to the element emitted by exciting the atom in the glow plasma, and the amount of the element contained in the material is estimated from the emission intensity of the identified element. The data in the depth direction can be estimated from the sputtering time. Specifically, the sputtering time can be converted into the sputtering depth by obtaining the relationship between the sputtering time and the sputtering depth in advance using a standard sample. Therefore, the sputtering depth converted from the sputtering time can be defined as the depth from the surface of the material.
　Further, in the high frequency GDS analysis of the steel sheet and the steel sheet for heat treatment in the present embodiment, a commercially available analyzer can be used. In this embodiment, a high-frequency glow discharge emission analyzer GD-Profiler 2 manufactured by HORIBA, Ltd. is used.
Example
[0125]
　Next, examples of the present invention will be described. The conditions in the examples are one condition example adopted for confirming the feasibility and effect of the present invention. The present invention is not limited to this one-condition example. The present invention may adopt various conditions as long as the gist of the present invention is not deviated and the object of the present invention is achieved.
[0126]
(Example 1)
　Steels having the chemical compositions shown in Table 1 were melted to prepare slabs. This slab is heated under the slab heating temperature shown in Tables 2 and 3 and the slab heating conditions having the numerical values ​​of the formula (4) shown in Tables 2 and 3, and the rolling completion temperature is set to the temperature shown in Tables 2 and 3. Hot rolling was performed to produce a hot-rolled steel sheet. Then, the hot-rolled steel sheet was pickled to remove the scale on the surface. Then, some hot-rolled steel sheets were cold-rolled to obtain cold-rolled steel sheets.
[0127]
[table 1]

[0128]
[Table 2]

[0129]
[Table 3]

[0130]
　The hot-rolled steel sheet having a plate thickness of 1.2 mm or the cold-rolled steel sheet having a plate thickness of 1.2 mm thus obtained was subjected to the first heat treatment and / or the second heat treatment shown below. For some of the steel sheets, the cold-rolled steel sheets cooled to the cooling stop temperatures shown in Tables 4 and 5 in the first heat treatment were continuously subjected to the second heat treatment without being cooled to room temperature. For other examples, after cooling to the cooling stop temperature in the first heat treatment, the second heat treatment was performed after cooling to room temperature. Moreover, some steel sheets were not subjected to the first heat treatment.
[0131]
(First heat treatment)
　The average heating rate from 650 ° C. to the maximum heating temperature shown in Tables 4 and 5 is used to heat to the maximum heating temperature shown in Tables 4 and 5, and the maximum heating temperature is shown in Tables 4 and 5. Retention time was retained. Then, 700 ° C. to Ms was cooled at the average cooling rate shown in Tables 4 and 5, and cooled to the cooling stop temperature shown in Tables 4 and 5. In the first heat treatment, H 2 was contained at the concentrations shown in Tables 4 and 5, and the maximum heating was performed from 650 ° C. in an atmosphere in which the log (PH 2 O / PH 2 ) was the numerical value shown in Tables 4 and 5. Heated until the temperature was reached.
[0132]
Ac3 　shown in Tables 4 and 5 was calculated by the following formula (9), and Ms was calculated by the following formula (10).
A c3 = 879-346 x C + 65 x Si-18 x Mn + 54 x Al ... (9)
(The element symbol in formula (9) is the mass% of the element in steel.)
Ms = 561-407 × C-7.3 × Si-37.8 × Mn-20.5 × Cu-19.5 × Ni-19.8 × Cr-4.5 × Mo ... (10)
(in equation (10)) The element symbol of is the mass% of the element in steel.)
[0133]
[Table 4]

[0134]
[Table 5]

[0135]
(Second heat treatment)
　The average heating rate from 650 ° C. to the maximum heating temperature shown in Tables 6 and 7 is used to heat to the maximum heating temperature shown in Tables 6 and 7, and the maximum heating temperature is shown in Tables 6 and 7. Retention time was retained. Then, it was cooled at the average cooling rate shown in Tables 6 and 7, and cooled to the cooling stop temperature shown in Tables 6 and 7. Then, the holding time shown in Tables 6 and 7 was maintained between 300 ° C. and 480 ° C., and the mixture was cooled to room temperature to obtain a steel sheet. In the second heat treatment, H 2 was contained at the concentrations shown in Tables 6 and 7, and the maximum heating was performed from 650 ° C. in an atmosphere in which the log (PH 2 O / PH 2 ) was the numerical value shown in Tables 6 and 7. Heated until the temperature was reached.
　Next, an electrogalvanized steel sheet was subjected to an electrogalvanizing step on a part of the steel sheet after the second heat treatment, and an electrogalvanized layer was formed on both surfaces of the steel sheet to obtain an electrogalvanized steel sheet (EG).
[0136]
Ac1 　shown in Tables 6 and 7 was calculated by the following formula (8).
A c1 = 723-10.7 × Mn-16.9 × Ni + 29.1 × Si + 16.9 × Cr · · (8)
(element symbol of the formula (8) where is the mass% in the steel of the element is there.)
[0137]
[Table 6]

[0138]
[Table 7]

[0139]
　Next, with respect to each steel sheet thus obtained, the steel structure (steel inside the steel sheet) in the range of 1/8 to 3/8 thickness centered on the position of 1/4 thickness from the surface by the above-mentioned method. The structure) was measured, and the body integration ratios of soft ferrite, retained austenite, tempered martensite, fresh martensite, and the sum of pearlite and cementite (pearlite + cementite) were examined. Furthermore, the volume fractions of bainite and hard ferrite were also investigated.
[0140]
　Further, inside each steel sheet, the ratio of the number of retained austenites having an aspect ratio of 2.0 or more to the total retained austenite was examined by the above-mentioned method.
　These results are shown in Tables 8 and 9.
[0141]
[Table 8]

[0142]
[Table 9]

[0143]
　Next, the steel structure of each steel sheet was measured by the method described above, and the thickness of the soft layer (depth range from the surface) and the aspect ratio of the ferrite crystal grains contained in the soft layer were less than 3.0. The number ratio of the crystal grains of the above was examined.
　The steel structure of each steel sheet was measured by the method described above, and the ratio of the volume fraction of retained austenite in the soft layer to the volume fraction of retained austenite in the range of 1/8 to 3/8 thickness (soft). The residual γ volume fraction in the layer / residual γ volume fraction inside the steel sheet) was investigated.
　The results are shown in Tables 10 and 11.
[0144]
　Further, each steel sheet is analyzed by a high-frequency glow discharge analysis method in the depth direction from the surface by the above-mentioned method, and the emission intensity peak (Si) having a wavelength indicating Si is obtained during the depth of more than 0.2 μm and less than 5 μm. It was investigated whether or not a peak indicating that an internal oxide layer containing an oxide appears). Then, in each steel sheet, the peak of the emission intensity of the wavelength indicating Si appears between the depth of more than 0.2 μm and less than 5 μm in the depth direction from the surface, and it is evaluated as “presence” of the internal oxidation peak. Those in which did not appear were evaluated as having no internal oxidation peak. The results are shown in Tables 10 and 11.
[0145]
　“EG” on the surface in Tables 10 and 11 indicates that it is an electrogalvanized steel sheet.
[0146]
[Table 10]

[0147]
[Table 11]

[0148]
　Further, for each steel sheet, the maximum tensile stress (TS), elongation (El), hole expandability (hole expansion rate), bendability (minimum bending radius), fatigue resistance characteristics (fatigue limit / TS) are obtained by the methods shown below. I checked. The results are shown in Tables 12 and 13.
[0149]
　JIS No. 5 tensile test pieces were sampled so that the direction perpendicular to the rolling direction was the tensile direction, the maximum tensile stress and elongation were measured according to JIS Z2241, and the hole expandability was measured according to JIS Z2256. .. Then, those having a maximum tensile stress of 700 MPa or more were evaluated as good.
[0150]
　In addition, in order to evaluate the balance between strength, elongation and hole expandability, the results of maximum tensile stress (TS), elongation (El), and hole expandability (hole expansion rate) measured by the above method are used as described below. The value represented by the formula (11) was calculated. When the value represented by the formula (11) was 80 × 10 -7 or more, it was evaluated that the balance between strength, elongation and hole expandability was good.
　TS 2 x El x λ ... (11)
　(In equation (11), TS indicates the maximum tensile stress (MPa), El indicates elongation (%), and λ indicates hole expandability (%). )
　the results are shown in tables 12 and 13.
[0151]
　Based on JIS Z 2248, a steel plate was cut out in a direction perpendicular to the rolling direction, and the end face was mechanically ground to prepare a test piece of 35 mm × 100 mm. Then, a 90 ° V bending test was performed on the prepared test piece using a 90 ° die and a punch having a tip radius of 0.5 to 6 mm. The bending ridgeline of the test piece after the bending test was observed with a magnifying glass, and the minimum bending radius without cracks was defined as the limit bending radius. A steel sheet having a critical bending radius of less than 3.0 mm was evaluated as having good bendability.
[0152]
　Fatigue resistance was evaluated by a plane bending fatigue test. A JIS No. 1 test piece was used as the test piece, and the stress ratio was set to -1. Repetition frequency is set to 25Hz, repeating the number 10 7 was the maximum of the stress that was not broken at the time and the fatigue limit. Then, a steel sheet having a ratio (fatigue limit / TS) of the fatigue limit to the maximum tensile stress (TS) of 0.45 or more was evaluated as having good fatigue resistance.
[0153]
　In addition, the chemical conversion processability of each steel sheet was measured by the method shown below.
　Each steel sheet was cut into 70 mm × 150 mm, and an 18 g / l aqueous solution of a degreasing agent (trade name: Fine Cleaner E2083) manufactured by Nihon Parkerizing Co., Ltd. was sprayed and applied at 40 ° C. for 120 seconds. Next, the steel sheet coated with the degreasing agent was washed with water to degreas it, and then immersed in a 0.5 g / l aqueous solution of a surface conditioner (trade name: Preparen XG) manufactured by Nihon Parkerizing Co., Ltd. at room temperature for 60 seconds. Then, the steel sheet coated with the surface conditioner was immersed in a zinc phosphate treatment agent (trade name: Palbond L3065) manufactured by Nihon Parkerizing Co., Ltd. for 120 seconds, washed with water, and dried. As a result, a chemical conversion-treated film made of a zinc phosphate film was formed on the surface of the steel sheet.
[0154]
　A test piece having a width of 70 mm and a length of 150 mm was collected from the steel plate on which the chemical conversion treatment film was formed. Then, three places (central part and both ends) along the length direction of the test piece were observed at a magnification of 1000 times using a scanning electron microscope (SEM). Then, for each test piece, the degree of adhesion of crystal grains in the chemical conversion treatment film was evaluated according to the following criteria.
[0155]
No part of the "G" (GOOD) surface is clearly not covered with the chemical conversion coating.
The surface of "B" (BAD) is clearly not covered with the chemical conversion coating.
[0156]
[Table 12]

[0157]
[Table 13]

[0158]
　The steel sheet of the example of the present invention had high strength, had a good balance between strength, elongation and hole widening property, and had good fatigue resistance, bendability and chemical conversion treatment property.
[0159]
　Experimental Example No. Since the first heat treatment was not applied to the steel plates of 11, 16, 27, 45, and 46, the metal structure does not contain hard ferrite, and the balance of strength, elongation, and hole expansion ratio becomes poor.
　Experimental Example No. Since the maximum heating temperature of the steel sheet 2 is low in the first heat treatment, the number ratio of retained austenite having an aspect ratio of 2.0 or more is insufficient, and the balance between strength, elongation, and hole expansion ratio becomes poor.
　Experimental Example No. Since the maximum heating temperature of the steel sheet No. 3 was high in the first heat treatment, the thickness of the soft layer in the heat treatment steel sheet and the steel sheet became thick, and the fatigue resistance property became low.
[0160]
　Experimental Example No. Since the average heating rate of the steel sheet 5 from 650 ° C. to the maximum heating temperature in the first heat treatment is slow, the number ratio of retained austenite having an aspect ratio of 2.0 or more is insufficient, and the balance between strength, elongation, and hole expansion ratio is insufficient. Get worse.
　Experimental Example No. Since the steel sheets 6, 15 and 23 have a low log (PH 2 O / PH 2 ) in the first heat treatment, the soft layer thickness of the heat treatment steel sheet and the steel sheet is insufficient, and the bendability is deteriorated.
[0161]
　Experimental Example No. Since the cooling rate of the steel sheet No. 8 was slow in the first heat treatment, the lath-like structure of the heat-treated steel sheet was insufficient, and the fraction of soft ferrite in the internal structure of the steel sheet was increased. Therefore, Experimental Example No. The steel plate of No. 8 has a poor balance of strength, elongation, and hole expansion ratio.
　Experimental Example No. Since the steel sheets 9, 10, 19, 22, and 48 have a high log (PH 2 O / PH 2 ) in the second heat treatment, the ratio of the residual γ volume fraction in the soft layer to the residual γ volume fraction inside the steel sheet is high. If it is insufficient, the fatigue resistance characteristics will deteriorate.
[0162]
　Experimental Example No. For the steel sheets 6, 15 and 23, since the log (PH 2 O / PH 2 ) was low in both the first heat treatment and the second heat treatment , an internal oxide layer was not formed, and the evaluation of chemical conversion processability was “B”. It became. Experimental Example No. For the steel sheets 11, 16 and 46, since the first heat treatment was not performed and the log (PH 2 O / PH 2 ) of the second heat treatment was low, an internal oxide layer was not formed, and the evaluation of chemical conversion treatment was " It became "B".
[0163]
　Experimental Example No. Since the maximum temperature reached in the second heat treatment of the 24 steel sheets is high, the metal structure does not contain hard ferrite, and the balance between strength, elongation, and hole expansion ratio becomes poor.
　Experimental Example No. Since the holding time of the steel sheet 33 in the second heat treatment between 300 ° C. and 480 ° C. is insufficient, the fraction of fresh martensite in the internal structure increases, and the balance between strength, elongation, and hole expansion ratio becomes poor.
[0164]
　Experimental Example No. Since the cooling stop temperature of the 36 steel sheets in the first heat treatment is high, the number ratio of retained austenite having an aspect ratio of 2.0 or more is insufficient, and the balance between strength, elongation, and hole expansion ratio becomes poor.
　Experimental Example No. Since the cooling rate of the steel sheet 41 is slow in the second heat treatment, the total fraction of pearlite and cementite in the internal structure of the steel sheet becomes large, and the balance between strength, elongation, and hole expansion ratio becomes poor.
[0165]
　Experimental Example No. Since the maximum heating temperature of the steel sheet 62 in the second heat treatment is low, the retained austenite fraction in the internal structure of the steel sheet is insufficient, and the balance between strength, elongation, and hole expansion ratio becomes poor.
[0166]
　Experimental Example No. The chemical composition of the steel sheets 68 to 72 is outside the scope of the present invention. Experimental Example No. The maximum tensile stress (TS) of the 68 steel sheet was insufficient due to the insufficient C content. Experimental Example No. Since the steel sheet of 69 had a high Nb content, the bendability was deteriorated. Experimental Example No. The steel sheet of No. 70 had an insufficient maximum tensile stress (TS) due to the insufficient Mn content. Experimental Example No. Since the steel plate of 71 has a high Si content, the hole expanding property is deteriorated. Experimental Example No. Since the steel sheet of 72 had a large Mn content and P content, the elongation and hole expansion properties were deteriorated.
[0167]
(Example 2)
　Steel having the chemical composition shown in Table 14 was melted to prepare a slab. This slab is heated under the slab heating temperature shown in Tables 15 and 16 and the slab heating conditions having the numerical values ​​of the formula (4) shown in Tables 15 and 16, and the rolling completion temperature is set to the temperature shown in Tables 15 and 16. Hot rolling was performed to produce a hot-rolled steel sheet. Then, the hot-rolled steel sheet was pickled to remove the scale on the surface. Then, some hot-rolled steel sheets were cold-rolled to obtain cold-rolled steel sheets.
[0168]
[Table 14]

[0169]
[Table 15]

[0170]
[Table 16]

[0171]
　The hot-rolled steel sheet having a plate thickness of 1.2 mm or the cold-rolled steel sheet having a plate thickness of 1.2 mm thus obtained was subjected to the first heat treatment and / or the second heat treatment shown below. For some experimental examples, the cold-rolled steel sheets cooled to the cooling stop temperatures shown in Tables 17 and 18 in the first heat treatment were continuously subjected to the second heat treatment without being cooled to room temperature. For other examples, after cooling to the cooling stop temperature in the first heat treatment, the second heat treatment was performed after cooling to room temperature.
[0172]
(First heat treatment)
　The average heating rate from 650 ° C. to the maximum heating temperature shown in Tables 17 and 18 is heated to the maximum heating temperature shown in Tables 17 and 18, and the maximum heating temperature is shown in Tables 17 and 18. Retention time was retained. Then, 700 ° C. to Ms was cooled at the average cooling rate shown in Tables 17 and 18, and cooled to the cooling stop temperature shown in Tables 17 and 18. In the first heat treatment, H 2 was contained at the concentrations shown in Tables 17 and 18, and the maximum heating was performed from 650 ° C. in an atmosphere in which the log (PH 2 O / PH 2 ) was the numerical value shown in Tables 17 and 18. Heated until the temperature was reached.
[0173]
Ac3 　shown in Tables 17 and 18 was calculated by the following formula (9), and Ms was calculated by the following formula (10).
A c3 = 879-346 x C + 65 x Si-18 x Mn + 54 x Al ... (9)
(The element symbol in formula (9) is the mass% of the element in steel.)
[0174]
Ms = 561-407 x C-7.3 x Si-37.8 x Mn-20.5 x Cu-19.5 x Ni-19.8 x Cr-4.5 x Mo ... (10)
(Equation ) The element symbol in the formula (10) is the mass% of the element in steel.)
[0175]
[Table 17]

[0176]
[Table 18]

[0177]
(Second heat treatment-hot-dip galvanizing)
Of the experimental examples, Experimental Example No. For 1'to 76', heat under the conditions shown in Tables 19 and 20, cool to the cooling stop temperature at the cooling rates shown in Tables 19 and 20, and maintain isothermal under the conditions shown in Tables 19 and 20. After that, it was immersed in a hot-dip galvanizing bath and alloyed. That is, the hot-dip galvanizing treatment was performed at the timing shown in the pattern [1] of FIG. However, the alloying treatment was not applied to Experimental Example 76'.
[0178]
　Experimental Example No. For 77'-84', 86'and 87', after heating under the conditions shown in Table 20, the temperature is cooled to the hot-dip galvanizing bath temperature at the cooling rate shown in Table 20, and then the alloy is immersed in the hot-dip galvanizing bath. Galvanized. Then, after further cooling to the cooling stop temperature shown in Table 20, isothermal maintenance was performed under the conditions shown in Table 20. That is, the hot-dip galvanizing treatment was performed at the timing shown in the pattern [2] of FIG. However, the alloying treatment was not performed on Experimental Example 82'.
[0179]
　In addition, Experimental Example No. About 85', it was heated under the conditions shown in Table 20, cooled to the cooling stop temperature at the cooling rate shown in Table 20, maintained at an isothermal temperature under the conditions shown in Table 20, and then once cooled to room temperature. Then, the steel sheet was heated to the hot-dip galvanizing bath temperature again, and then immersed in the hot-dip galvanizing bath to perform alloying treatment. That is, the hot-dip galvanizing treatment was performed according to the pattern [3] shown in FIG.
[0180]
　In each example, hot-dip galvanizing was carried out on both sides of the steel sheet at a basis weight of 50 g / m 2 by immersing the steel sheet in a hot-dip zinc bath at 460 ° C.
　Further, in the second heat treatment, H 2 is contained at the concentrations shown in Tables 19 and 20, and the log (PH 2 O / PH 2 ) is the numerical value shown in Tables 19 and 20. It was heated until it reached the heating temperature.
[0181]
Ac1 　shown in Tables 6 and 7 was calculated by the following formula (8).
A c1 = 723-10.7 × Mn-16.9 × Ni + 29.1 × Si + 16.9 × Cr · · (8)
(element symbol of the formula (8) where is the mass% in the steel of the element is there.)
[0182]
[Table 19]

[0183]
[Table 20]

[0184]
　Next, with respect to each hot-dip zinc-plated steel sheet thus obtained, the steel structure (steel plate) in the range of 1/8 thickness to 3/4 thickness centered on the position of 1/4 thickness from the surface by the above-mentioned method. The internal steel structure) was measured, and the body integration ratios of soft ferrite, retained austenite, tempered martensite, fresh martensite, and the sum of pearlite and cementite (pearlite + cementite) were examined. Furthermore, the volume fractions of bainite and hard ferrite were also investigated.
[0185]
　Further, in the inside of each hot-dip galvanized steel sheet, the ratio of the number of retained austenites having an aspect ratio of 2.0 or more to the total retained austenite was investigated by the method described above.
　These results are shown in Tables 21 and 22.
[0186]
[Table 21]

[0187]
[Table 22]

[0188]
　Next, the steel structure of each hot-dip galvanized steel sheet was measured by the method described above, and the thickness of the soft layer (depth range from the surface) and the aspect ratio of the soft ferrite crystal grains contained in the soft layer. The number ratio of crystal grains less than 3.0 was examined.
[0189]
　The steel structure of each hot-dip galvanized steel sheet was measured by the method described above, and the ratio of the residual γ volume fraction in the soft layer to the residual γ volume fraction inside the steel sheet (residual γ volume fraction in the soft layer / steel sheet). The internal residual γ volume fraction) was examined.
　The results are shown in Tables 23 and 24.
[0190]
　Further, each hot-dip galvanized steel sheet was analyzed by a high-frequency glow discharge analysis method in the depth direction from the surface by the above-mentioned method, and the emission intensity of the wavelength indicating Si was analyzed during the depth of more than 0.2 μm and less than 5 μm. It was investigated whether or not a peak (a peak indicating that an internal oxide layer containing Si oxide was present) appeared.
[0191]
　Then, in each hot-dip galvanized steel sheet, the one in which the peak of the emission intensity of the wavelength indicating Si appears between the depth of more than 0.2 μm and less than 5 μm in the depth direction from the surface is evaluated as “presence” of the internal oxidation peak. However, those in which no peak appeared were evaluated as having no internal oxidation peak. The results are shown in Tables 23 and 24.
[0192]
[Table 23]

[0193]
[Table 24]

[0194]
　Further, for each hot-dip galvanized steel sheet, the maximum tensile stress (TS), elongation (El), hole expandability (hole expansion rate), bendability (minimum bending radius), and fatigue resistance (fatigue limit) are applied by the methods shown below. / TS) was examined. The results are shown in Tables 25 and 26.
　JIS No. 5 tensile test pieces were sampled so that the direction perpendicular to the rolling direction was the tensile direction, the maximum tensile stress and elongation were measured according to JIS Z2241, and the hole expandability was measured according to JIS Z2256. .. Then, those having a maximum tensile stress of 700 MPa or more were evaluated as good.
[0195]
　In addition, in order to evaluate the balance between strength, elongation and hole expandability, the results of maximum tensile stress (TS), elongation (El), and hole expandability (hole expansion rate) measured by the above method are used as described below. The value represented by the formula (11) was calculated. When the value represented by the formula (11) was 80 × 10 -7 or more, it was evaluated that the balance between strength, elongation and hole expandability was good.
[0196]
　TS 2 x El x λ ... (11)
　(In equation (11), TS indicates the maximum tensile stress (MPa), El indicates elongation (%), and λ indicates hole expandability (%). )
　and the results are shown in Table 25 and Table 26.
[0197]
　Based on JIS Z 2248, a steel plate was cut out in a direction perpendicular to the rolling direction, and the end face was mechanically ground to prepare a test piece of 35 mm × 100 mm. Then, a 90 ° V bending test was performed on the prepared test piece using a 90 ° die and a punch having a tip radius of 0.5 to 6 mm. The bending ridgeline of the test piece after the bending test was observed with a magnifying glass, and the minimum bending radius without cracks was defined as the limit bending radius. A steel sheet having a critical bending radius of less than 3.0 mm was evaluated as having good bendability.
[0198]
　Fatigue resistance was evaluated by a plane bending fatigue test. A JIS No. 1 test piece was used as the test piece, and the stress ratio was set to -1. The repetition frequency was 25 Hz, and the maximum stress that did not break after 107 repetitions was set as the fatigue limit. Then, a steel sheet having a ratio (fatigue limit / TS) of the fatigue limit to the maximum tensile stress (TS) of 0.45 or more was evaluated as having good fatigue resistance.
[0199]
　In addition, the plating adhesion of each hot-dip galvanized steel sheet was measured by the method shown below.
[0200]
　A 30 mm × 100 mm test piece was collected from each hot-dip galvanized steel sheet and subjected to a 90 ° V bending test. Then, a commercially available cellophane tape (registered trademark) was attached along the bending ridge line, and the width of the plating adhering to the tape was measured as the peeling width. The evaluation was as follows.
[0201]
"G" (GOOD): Small plating peeling or peeling to the extent 　that there is no problem in practical use (peeling width 0 to less than 10 mm)
"B" (BAD): Severe peeling (peeling width 10 mm or more)
Plating adhesion is evaluated. A sample in which is G was judged to be acceptable.
[0202]
[Table 25]

[0203]
[Table 26]

[0204]
　The evaluation results for each experimental example will be described below.
[0205]
　The hot-dip galvanized steel sheet of the example of the present invention had high strength, had a good balance between strength, elongation and hole expandability, and had good fatigue resistance, bendability, and plating adhesion.
[0206]
　As for the steel sheets of Experimental Examples No. 14', 19', 30', 48', and 49', since the first heat treatment was not performed, the metal structure does not contain hard ferrite, so that the balance of strength, elongation, and hole expansion ratio is balanced. Get worse.
[0207]
　Since the maximum heating temperature of the steel sheet of Experimental Example No. 2'is low in the first heat treatment, the number ratio of retained austenite having an aspect ratio of 2.0 or more is insufficient, and the balance between strength, elongation, and hole expansion ratio becomes poor. ..
[0208]
　Since the steel sheet of Experimental Example No. 3'has a high maximum heating temperature in the first heat treatment, the soft layer thickness of the heat treatment steel sheet and the hot-dip galvanized steel sheet is thick, and the fatigue resistance is low.
[0209]
　Since the average heating rate of the steel sheet of Experimental Example No. 5'from 650 ° C. to the maximum heating temperature in the first heat treatment is slow, the number ratio of retained austenite having an aspect ratio of 2.0 or more is insufficient, and the strength, elongation, and strength are increased. The balance of the hole expansion rate becomes unbalanced.
[0210]
　Since the steel sheets of Experimental Examples No. 6', 18', and 26' have a low log (PH 2 O / PH 2 ) in the first heat treatment , their bendability and plating adhesion are deteriorated.
[0211]
　Since the cooling rate of the steel sheet of Experimental Example No. 8'was slow in the first heat treatment, the lath-like structure of the heat-treated steel sheet was insufficient, and the fraction of soft ferrite in the internal structure of the hot-dip galvanized steel sheet increased. Therefore, the steel sheet of Experimental Example No. 8'has a poor balance of strength, elongation, and hole expansion ratio.
[0212]
　The steel sheets of Experimental Example No. 9', 10', 22', 25', 30', 48', and 51' have a high log (PH 2 O / PH 2 ) in the second heat treatment, so that they remain in the soft layer. The ratio of the γ volume fraction to the residual γ volume fraction inside the steel sheet is insufficient, and the fatigue resistance characteristics deteriorate.
[0213]
　Since the steel sheet of Experimental Example No. 27'has a high maximum temperature reached in the second heat treatment, the metal structure does not contain hard ferrite, so that the balance between strength, elongation, and hole expansion ratio becomes poor.
[0214]
　In the steel sheet of Experimental Example No. 36', the retention time between 300 ° C. and 480 ° C. in the second heat treatment was insufficient, so that the fraction of fresh martensite in the internal structure increased, and the strength, elongation, and hole expansion ratio increased. The balance gets worse.
[0215]
　Since the steel sheet of Experimental Example No. 39'has a high cooling stop temperature in the first heat treatment, the number ratio of retained austenite having an aspect ratio of 2.0 or more is insufficient, and the balance between strength, elongation, and hole expansion ratio becomes poor. ..
[0216]
　Since the steel sheet of Experimental Example No. 44'has a slow cooling rate in the second heat treatment, the total fraction of pearlite and cementite in the internal structure of the hot-dip galvanized steel sheet becomes large, and the balance of strength, elongation, and hole expansion ratio is balanced. become worse.
[0217]
　Since the steel sheet of Experimental Example No. 65'has a low maximum temperature reached in the second heat treatment, the residual austenite fraction in the internal structure of the hot-dip galvanized steel sheet is insufficient, and the balance between strength, elongation, and hole expansion rate becomes poor. ..
[0218]
　The chemical composition of the steel sheets of Experimental Examples No. 71'to 75'is outside the scope of the present invention. The steel sheet of Experimental Example No. 71'had insufficient maximum tensile stress (TS) due to insufficient C content. Since the steel sheet of Experimental Example No. 72'has a large Nb content, the bendability is deteriorated. The steel sheet of Experimental Example No. 73'had insufficient maximum tensile stress (TS) due to insufficient Mn content. Since the steel sheet of Experimental Example No. 74'has a large Si content, the hole expandability is deteriorated. Since the steel sheet of Experimental Example No. 75'has a large Mn content and P content, the elongation and hole expandability are deteriorated.
[0219]
　Although the preferred embodiments and examples of the present invention have been described above, these embodiments and examples are merely examples within the scope of the gist of the present invention and do not deviate from the gist of the present invention. Allows you to add, omit, replace, and make other changes to the configuration. That is, the present invention is not limited by the above description, but is limited only by the scope of the appended claims, and of course, it can be appropriately modified within the scope.
Description of the sign
[0220]
1 Steel plate
11 Range of 1/8 to 3/8 thickness centered on the position of 1/4 thickness from the surface of the steel plate (inside the steel plate)
12 Soft layer
The scope of the claims
[Claim 1]
　By mass%,
　C: 0.050% to 0.500%,
　Si: 0.01% to 3.00%,
　Mn: 0.50% to 5.00%,
　P: 0.0001% to 0.1000 %,
　S: 0.0001% to 0.0100%,
　Al: 0.001% to 2.500%,
　N: 0.0001% to 0.0100%,
　O: 0.0001% to 0.0100%,
　Ti: 0% to 0.300%,
　V: 0% to 1.00%,
　Nb: 0% to 0.100%,
　Cr: 0% to 2.00%,
　Ni: 0% to 2.00%,
　Cu: 0% to 2.00%,
　Co: 0% to 2.00%,
　Mo: 0% to 1.00%,
　W: 0% to 1.00%,
　B: 0% to 0.0100%,
　Sn: 0% to 1.00%,
　Sb: 0% to 1.00%,
　Ca: 0% to 0.0100%,
　Mg: 0% to 0.0100%,
　Ce: 0% to 0.0100%,
　Zr: 0% to 0.0100%,
　La: 0% to 0.0100%,
　Hf: 0% to 0.0100%,
　Bi: 0% to 0.0100%, and
　REM: 0% to 0.0100% 　The steel structure in the range of 1/8 to 3/8 thickness centered on the position of 1/4 thickness from
　the
surface has a chemical composition consisting of Fe and impurities in the balance .
　Soft ferrite: 0% to 30%,
　retained austenite: 3% to 40%,
　fresh martensite: 0% to 30%,
　total of pearlite and cementite: 0% to 10%
, the balance containing hard ferrite.
　In the range of 1/8 thickness to 3/4 thickness centered on the position 1/4 thickness from the surface, the number ratio of the retained austenite having an aspect ratio of 2.0 or more to all the retained austenite is When
　a region having a hardness of 50% or more and 80% or less of the hardness in the above range of 1/8 to 3/8 thickness is defined as a soft layer, the thickness is 1 to 1 to 3 in the plate thickness direction from the surface. A soft layer of 100 μm exists, and
　among the ferrite crystal grains contained in the soft layer, the body integration ratio of the crystal grains having an aspect ratio of less than 3.0 is 50% or more.
　The volume fraction of retained austenite in the soft layer is 50% or more of the volume fraction of the retained austenite in the range of 1/8 to 3/8 thickness, and a
　high frequency is generated from the surface in the plate thickness direction. When the emission intensity of the wavelength indicating Si appears by the glow discharge analysis method, the peak of the emission intensity of the wavelength indicating Si appears in the range of more than 0.2 μm from the surface and 5 μm or less from the surface. A steel plate characterized by.
[Claim 2]
　One selected from the group whose chemical composition is
　Ti: 0.001% to 0.300%,
　V: 0.001% to 1.00%, and
　Nb: 0.001% to 0.100%.
The steel sheet according to claim 1, wherein the steel sheet contains two or more kinds.
[Claim 3]
　The chemical composition is
　Cr: 0.001% to 2.00%,
　Ni: 0.001% to 2.00%,
　Cu: 0.001% to 2.00%,
　Co: 0.001% to 2. One or two selected from the group consisting of 00%,
　Mo: 0.001% to 1.00%,
　W: 0.001% to 1.00%, and
　B: 0.0001% to 0.0100%.
The steel sheet according to claim 1 or 2, wherein the steel sheet contains the above.
[Claim 4]

　Claimed 　, wherein the chemical composition contains one or two selected from the group consisting of Sn: 0.001% to 1.00% and
　Sb: 0.001% to 1.00%.
The steel plate according to any one of Items 1 to 3.
[Claim 5]
　The chemical composition is
　Ca: 0.0001% to 0.0100%,
　Mg: 0.0001% to 0.0100%,
　Ce: 0.0001% to 0.0100%,
　Zr: 0.0001% to 0. 0100%,
　La: 0.0001% to 0.0100%,
　Hf: 0.0001% to 0.0100%,
　Bi: 0.0001% to 0.0100%, and
　REM: 0.0001% to 0.0100.
The steel sheet according to any one of claims 1 to 4, which contains one kind or two or more kinds selected from the group consisting of % .
[Claim 6]
　The steel sheet according to any one of claims 1 to 5, wherein the chemical composition satisfies the following formula (1).
　Si + 0.1 × Mn + 0.6 × Al ≧ 0.35 ... (1)
　Si, Mn and Al in the formula (1) are the contents of each element in mass%.
[Claim 7]
　A claim characterized in that the volume fraction of tempered martensite is 0% to 50% in the range of 1/8 to 3/8 thickness centered on the position of 1/4 thickness from the surface. The steel plate according to any one of Items 1 to 6.
[Claim 8]
　The steel sheet according to any one of claims 1 to 7, which has a hot-dip galvanized layer on its surface.
[Claim 9]
　The steel sheet according to any one of claims 1 to 7, wherein the steel sheet has an electrogalvanized layer on its surface.
[Claim 10]
　A method for producing a steel sheet according to any one of claims 1 to 9, wherein
　a slab having the chemical composition according to any one of claims 1 to 6 is hot-rolled and pickled. The rolled steel sheet or the cold rolled steel sheet obtained by cold-rolling the hot-rolled steel sheet is subjected to the first heat treatment satisfying the following (a) to (e), and then the second one satisfying the following (A) to (E). A method for manufacturing a steel sheet, which comprises applying heat treatment.
(A) During the period from 650 ° C. to heating to the maximum heating temperature, the atmosphere around the hot-rolled steel sheet or the cold-rolled steel sheet contains H 2 of 0.1% by volume or more, and the following formula (2) Create an atmosphere that meets the requirements.
(B) Ac3 Hold at the maximum heating temperature of -30 ° C to 1000 ° C for 1 second to 1000 seconds.
(C) Heat from 650 ° C. to the maximum heating temperature at an average heating rate of 0.5 ° C./sec to 500 ° C./sec.
(D) After holding at the maximum heating temperature, the mixture is cooled from 700 ° C. to Ms at an average cooling rate of 5 ° C./sec or more.
(E) The cooling at an average cooling rate of 5 ° C./sec or more is performed up to a cooling stop temperature of Ms or less.
(A) From 650 ° C. to heating to the maximum heating temperature, the atmosphere around the hot-rolled steel sheet or the cold-rolled steel sheet contains H 2 of 0.1% by volume or more, and the following formula (3) Create an atmosphere that meets the requirements.
(B) A c1 + 25 ° C to A c3Hold for 1 to 1000 seconds at a maximum heating temperature of -10 ° C.
(C) Heating is performed from 650 ° C. to the maximum heating temperature at an average heating rate of 0.5 ° C./sec to 500 ° C./sec.
(D) Cool from the maximum heating temperature to 480 ° C. or lower so that the average cooling rate between 600 and 700 ° C. is 3 ° C./sec or more.
(E) After cooling at an average cooling rate of 3 ° C./sec or higher, the temperature is maintained between 300 ° C. and 480 ° C. for 10 seconds or longer.
　-1.1 ≤ log (PH 2 O / PH 2 ) ≤ -0.07 ... (2) In
　equation (2), PH 2 O indicates the partial pressure of water vapor, and PH 2 indicates the partial pressure of hydrogen. Shown.
　log (PH 2 O / PH 2 ) <-1.1 ... (3) In
　equation (3), PH 2 O indicates the partial pressure of water vapor, and PH 2 indicates the partial pressure of hydrogen.
[Claim 11]
　The method for producing a steel sheet according to claim 8,
　wherein in the second heat treatment, the atmosphere always contains H 2 of 0.1% by volume or more from 650 ° C. until the maximum heating temperature is reached. The O 2 is 0.020% by volume or less, satisfies the above formula (3),
　and is characterized in that the hot dip galvanizing treatment is performed in the second heat treatment after the cooling process of the above (D). The method for manufacturing a steel sheet according to claim 10.