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, awareness of environmental issues has increased. Therefore, in the automobile industry, it is important to reduce the weight of the vehicle body for the purpose of improving fuel efficiency. On the other hand, it is also necessary to increase the strength of the vehicle body to ensure collision safety. In order to achieve both weight reduction of the vehicle body and collision safety, a high-strength steel plate may be used as the material of the vehicle body. However, the higher the strength of the steel sheet, the more difficult it is to press form. This is because, in general, the higher the strength of the steel sheet, the lower the formability such as ductility, bendability, and hole expandability.
[0003]
　Further, in the case of an ultra-high strength steel sheet having a tensile strength of more than 980 MPa, it is necessary to solve not only the formability but also the problem of hydrogen embrittlement cracking of the steel sheet. Hydrogen embrittlement cracking is a phenomenon in which a steel member under high stress under usage conditions suddenly breaks due to hydrogen that has entered the steel from the environment. This phenomenon is also called delayed fracture because of the mode of fracture. In general, it is known that hydrogen embrittlement cracking of a steel sheet is more likely to occur as the tensile strength of the steel sheet increases. It is considered that this is because the higher the tensile strength of the steel sheet, the greater the stress remaining on the steel sheet after forming the part. The sensitivity to hydrogen embrittlement cracking (delayed fracture) is called hydrogen embrittlement resistance. In the case of steel sheets for automobiles, hydrogen embrittlement cracking is particularly likely to occur in a bent portion where a large plastic strain is applied. Therefore, when a high-strength steel sheet is used for an automobile member, it is required to improve not only formability such as ductility, bendability, and hole widening property but also hydrogen embrittlement resistance of the bent portion.
[0004]
　Conventionally, as a high-strength steel sheet having high press workability, 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 and bendability because the hard phase is the starting point for void formation.
[0005]
　Further, as a high-strength steel plate having excellent ductility, 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 Documents 2 and 3). TRIP steel has higher ductility than DP steel. However, TRIP steel is inferior in hole expandability. In addition, TRIP steel needs to contain a large amount of alloy such as Si in order to leave austenite. Therefore, TRIP steel is inferior in plating adhesion and chemical conversion treatment property.
[0006]
　Further, Patent Document 4 describes a high-strength steel sheet having a microstructure containing bainite or bainitic ferrite in an area ratio of 70% or more and a tensile strength of 800 MPa or more and having excellent drilling properties. .. In Patent Document 5, 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.
[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, it has been difficult to improve the ductility and hole expansion properties of conventional high-strength steel sheets and the hydrogen embrittlement resistance of bent portions at the same time.
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. 2008-56993
Patent Document 4: Japanese Patent Application Laid-Open No. 2003-193194 No.
Patent Document 5: Japanese Patent Application Laid-Open No. 2003-193193
Non-patent literature
[0009]
Non-Patent Document 1: K. Sugimoto et al ,, ISIJ International, Vol.33 (1993), No.7, pp775-782
Outline of the invention
Problems to be solved by the invention
[0010]
　The present invention has been made in view of the above circumstances. An object of the present invention is to provide a high-strength steel plate having excellent formability, fatigue characteristics, and hydrogen embrittlement resistance of a bent portion, and a method for producing the same.
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 obtain a predetermined steel structure inside the steel sheet and to have a predetermined thickness and steel structure. It was found that forming a surface layer is effective.
　Further, it has been found that by forming an internal oxide layer containing a Si oxide at a predetermined depth, it is possible to secure the plating adhesion and chemical conversion treatment property required for a steel sheet for automobiles.
[0012]
　Specifically, by the first heat treatment, the metal structure inside the steel sheet and the surface layer of the steel sheet is made mainly of a lath-like structure such as martensite. Then, in the second heat treatment, the maximum heating temperature is set to a two-phase region of α (ferrite) and γ (austenite), and decarburization treatment is performed at the same time. As a result, the steel sheet obtained after the two heat treatments has a steel structure in which needle-shaped retained austenite is dispersed inside the steel sheet, and the surface layer is mainly composed of lath-shaped ferrite having a large aspect ratio and has a predetermined thickness. It becomes. It has been found that such a steel sheet is excellent in all of excellent formability, fatigue characteristics, and hydrogen embrittlement resistance of bent portions.
[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 processability can be obtained. Further, when a plating layer is formed on the surface of the steel sheet, excellent 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 has 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. It contains 0100% and REM: 0% to 0.0100%, has a chemical composition in which the balance is composed of Fe and impurities, and has a thickness of 1/8 to 3/8 centered on a position 1/4 thick from the surface. Steel structure in the thickness range, soft ferrite: 0% to 30%, retained austenite: 3% to 40%, fresh martensite: 0% to 30%, total of pearlite and cementite: 0% to In the above range of 1/8 to 3/8 thickness, which contains 10% and the balance contains hard ferrite, the number ratio of retained austenite having an aspect ratio of 2.0 or more to the total retained austenite is 50% or more. When a region having a hardness of 80% or less of the hardness in the above range of 1/8 to 3/8 thickness is defined as a soft layer, a soft layer having a thickness of 1 to 100 μm in the plate thickness direction from the surface. Is present, and among the ferrites contained in the soft layer, the body integration rate of crystal grains having an aspect ratio of 3.0 or more is 50% or more, and the body integration rate of retained austenite in the soft layer is 1/8 of the above. It is 80% or less of the body integration ratio of retained austenite in the above range of thickness to 3/8 thickness, and is higher in the plate thickness direction from the surface.When the emission intensity of the wavelength indicating Si is analyzed by the frequency 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 and not more than 10.0 μm from the surface.
[0015]
(2) The chemical composition is one or more of Ti: 0.001% to 0.300%, V: 0.001% to 1.00%, and Nb: 0.001% to 0.100%. The steel sheet according to the above (1), which is characterized by containing.
[0016]
(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%. Contains one or more of 2.00%, Mo: 0.001% to 1.00%, W: 0.001% to 1.00%, B: 0.0001% to 0.0100% The steel sheet according to (1) or (2) above, which is characterized by the above.
[0017]
(4) The above (1), wherein the chemical composition contains one or two of Sn: 0.001% to 1.00% and Sb: 0.001% to 1.00%. The steel sheet according to any one of (3).
[0018]
(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%. ~ 0.0100%, La: 0.0001% ~ 0.0100%, Hf: 0.0001% ~ 0.0100%, Bi: 0.0001% ~ 0.0100%, REM: 0.0001% ~ 0
The steel sheet according to any one of (1) to (4) above, which contains one or more of 0.0100% .
[0019]
(6) The steel sheet according to any one of (1) to (5) above, wherein the chemical composition satisfies the following formula (i).
　Si + 0.1 × Mn + 0.6 × Al ≧ 0.35 ... (i)
(Si, Mn and Al in the formula (i) are the contents of each element in mass%).
[0020]
(7) The steel sheet according to any one of (1) to (6) above, which has a hot-dip galvanized layer or an electrogalvanized layer on the surface.
[0021]
(8) 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 (6), 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, 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) From 650 ° C. to reaching the maximum heating temperature, the atmosphere is such that it contains 0.1% by volume or more of H2 and satisfies the following formula (ii).
(B) Hold at the maximum heating temperature of Ac3-30 ° C. to 1000 ° C. for 1 second to 1000 seconds.
(C) Heating is performed so that the average heating rate in the temperature range from 650 ° C. to the maximum heating temperature is 0.5 ° C./sec to 500 ° C./sec.
(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.
(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.
(A) An atmosphere in which H2 is 0.1% by volume or more, O2 is 0.020% by volume or less, and log (PH2O / PH2) satisfies the following formula (iii) between 650 ° C. and reaching the maximum heating temperature. And.
(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 so that the average heating rate from 650 ° C. to the maximum heating temperature is 0.5 ° C./sec to 500 ° C./sec.
(D) Cool so that the average cooling rate in the temperature range of 700 to 600 ° 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.
　log (PH2O / PH2) <
　-1.1 ... (ii) -1.1 ≤ log (PH2O / PH2) ≤ -0.07 ... (iii)
(in equations (ii) and (iii) , PH2O indicates the partial pressure of water vapor, and PH2 indicates the partial pressure of hydrogen.)
[0022]
(9) The method for producing a steel sheet according to (8) above, wherein the hot-dip galvanizing treatment is performed at a stage after the cooling process of (D).
Effect of the invention
[0023]
　According to the above aspect of the present invention, a high-strength steel plate having excellent ductility and hole expanding property, excellent chemical conversion treatment property, plating adhesion property, and good fatigue property and hydrogen embrittlement resistance of bent portion, and a method for producing the same. Can be provided.
A brief description of the drawing
[0024]
FIG. 1 is a cross-sectional view of the steel plate according to the present embodiment parallel to the rolling direction and the plate thickness direction.
FIG. 2 shows the intensity of the wavelength indicating the depth from the surface and Si when the steel sheet according to the present embodiment is analyzed by the high-frequency glow discharge analysis method from the surface to the depth direction (plate thickness direction). It is a graph which shows the relationship of.
[Fig. 3] Emission of a wavelength indicating the depth from the surface and Si when a steel plate (comparative steel plate) 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. It is a graph which shows the relationship with the strength (Intensity).
FIG. 4 is a diagram showing a first example of a temperature / time pattern of a second heat treatment to hot dip galvanizing / alloying treatment in the method for manufacturing a steel sheet according to the present embodiment.
FIG. 5 is a diagram showing a second example of a temperature / time pattern of a second heat treatment to hot dip galvanizing / alloying treatment in the method for manufacturing a steel sheet according to the present embodiment.
FIG. 6 is a diagram showing a third example of a temperature / time pattern of a second heat treatment to hot dip galvanizing / alloying treatment in the method for manufacturing a steel sheet according to the present embodiment.
FIG. 7 is a schematic view showing an example of hardness measurement of a steel plate according to the present embodiment.
Mode for carrying out the invention
[0025]
"Steel plate"
　Hereinafter, a steel plate according to an embodiment of the present invention (steel plate according to the present embodiment) will be described in detail.
　First, the chemical composition of the steel sheet according to the present embodiment will be described. In the following description, [%] indicating the content of the element means [mass%].
[0026]
"C: 0.050 to 0.500%"
　C is an element that greatly enhances the strength of the steel sheet. Further, C is an element necessary for obtaining retained austenite that contributes to the improvement of ductility because it stabilizes austenite. Therefore, C is effective in achieving both strength and moldability. If the C content is less than 0.050%, retained austenite cannot be sufficiently 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 enhance the strength and moldability, the C content is preferably 0.075% or more, and more preferably 0.100% or more.
　On the other hand, 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, and more preferably 0.250% or less.
[0027]
"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 formability. If the Si content is less than 0.01%, a large amount of coarse iron-based carbide is generated, and the strength and moldability are deteriorated. Therefore, the Si content is set to 0.01% or more. From this viewpoint, the lower limit of Si is preferably 0.10% or more, more preferably 0.25% or more.
　On the other hand, Si is an element that embrittles steel materials. 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, Si impairs the impact resistance of the steel sheet. Therefore, the Si content is preferably 2.50% or less, and more preferably 2.00% or less.
[0028]
"Mn: 0.50 to 5.00%"
　Mn is contained in order to enhance the hardenability of the steel sheet and increase the strength. If the Mn content is less than 0.50%, a large amount of soft structure is formed during cooling after annealing, and it becomes difficult to secure a sufficiently high 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, and more preferably 1.00% or more.
　On the other hand, 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. .. Therefore, the Mn content is set to 5.00% or less. Further, as the Mn content increases, the spot weldability also deteriorates, so the Mn content is preferably 3.50% or less, and more preferably 3.00% or less.
[0029]
"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 P content 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, and more preferably 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.
[0030]
"S: 0.0001 to 0.0100%"
　S is an element that combines with Mn to form coarse MnS and reduces moldability such as ductility, hole expandability (stretchable flangeability) and bendability. Therefore, the S content is set to 0.0100% or less. Further, S deteriorates the spot weldability. Therefore, the S content is preferably 0.0070% or less, and more preferably 0.0050% 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, and more preferably 0.0006% or more.
[0031]
"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.
　On the other hand, although the effect can be obtained without setting the lower limit of the Al content, Al is an impurity present in a trace amount in the raw material, and if the content is less than 0.001%, the manufacturing cost is significantly increased. Accompanied by. Therefore, the Al content is set to 0.001% or more. Al is an element that is also effective as a deoxidizing material, and 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 contained for the purpose of stabilizing retained austenite. In order to stabilize the retained austenite, the Al content is preferably 0.100% or more, and more preferably 0.250% or more.
[0032]
"N: 0.0001 to 0.0100%"
　N forms coarse nitrides and deteriorates moldability such as ductility, hole expansion (stretch flangeability) and bendability, so it is necessary to suppress the content thereof. There is. When the N content exceeds 0.0100%, the deterioration of moldability becomes remarkable. From this, the N content is set to 0.0100% or less. 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.
　Although the lower limit of the N content can be obtained without any particular setting, setting the N content to less than 0.0001% causes a significant increase in manufacturing cost. From this, the N content is set to 0.0001% or more. The N content is preferably 0.0003% or more, and more preferably 0.0005% or more.
[0033]
"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. If the O content exceeds 0.0100%, the moldability is significantly deteriorated, so the upper limit of the O content is set to 0.0100%. The O content is preferably 0.0050% or less, and more preferably 0.0030% or less.
　Although the lower limit of the O content is not particularly specified, the effect can be obtained, but since setting the O content to less than 0.0001% involves a significant increase in manufacturing cost, the lower limit is 0.0001%. ..
[0034]
“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. In order to suppress the decomposition of retained austenite, 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.
　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%).
[0035]
　The steel sheet according to the present embodiment basically contains the above elements, but further, Ti, V, Nb, Cr, Ni, Cu, Co, Mo, W, B, Sn, Sb, if necessary. , Ca, Mg, Ce, Zr, La, Hf, Bi, REM, or may contain one or more elements selected from. Since these elements are arbitrary elements and do not necessarily have to be contained, the lower limit is 0%.
[0036]
"Ti: 0 to 0.300%"
　Ti is an element that contributes to an increase in the strength of a steel sheet by strengthening precipitation, 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, even when it is contained, 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 can be obtained without setting the lower limit of the Ti content, the Ti content is preferably 0.001% or more in order to sufficiently obtain the strength increasing effect due to the Ti content. The Ti content is more preferably 0.010% or more in order to further increase the strength of the steel sheet.
[0037]
"V: 0 to 1.00%"
　V is an element that contributes to the increase in the strength of the steel sheet by strengthening precipitation, strengthening fine grains by suppressing the growth of ferrite crystal grains, and strengthening dislocations by suppressing recrystallization. However, if the V content exceeds 1.00%, the carbonitride is excessively precipitated and the moldability is deteriorated. Therefore, even when it is contained, the V content is preferably 1.00% or less, and more preferably 0.50% or less. Although the effect can be obtained without setting the lower limit of the V content, the V content is preferably 0.001% or more, preferably 0.010, in order to sufficiently obtain the strength increasing effect due to the V content. More preferably, it is% or more.
[0038]
"Nb: 0 to 0.100%"
　Nb is an element that contributes to an increase in the strength of a steel sheet by strengthening precipitation, strengthening fine grains by suppressing the growth of ferrite crystal grains, and strengthening dislocations by suppressing recrystallization. However, when the Nb content exceeds 0.100%, the precipitation of carbonitride increases and the moldability deteriorates. Therefore, even when it is contained, 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 can be obtained without particularly defining the lower limit of the Nb content, the Nb content is preferably 0.001% or more in order to sufficiently obtain the strength increasing effect due to the Nb content. The Nb content is more preferably 0.005% or more in order to further increase the strength of the steel sheet.
[0039]
"Cr: 0 to 2.00%"
　Cr is an element that enhances the hardenability of steel sheets and is effective in increasing the strength. However, if the Cr content exceeds 2.00%, the workability in hot water is impaired and the productivity is lowered. From this, even when it is contained, the Cr content is preferably 2.00% or less, and more preferably 1.20% or less.
　Although the effect can be obtained without setting the lower limit of the Cr content, the Cr content is preferably 0.001% or more in order to sufficiently obtain the effect of increasing the strength due to the Cr content. More preferably, it is 010% or more.
[0040]
"Ni: 0 to 2.00%"
　Ni is an element that suppresses phase transformation at high temperatures and is effective in increasing the strength of steel sheets. However, if the Ni content exceeds 2.00%, the weldability is impaired. From this, even when it is contained, the Ni content is preferably 2.00% or less, and more preferably 1.20% or less.
　Although the effect can be obtained without setting the lower limit of the Ni content, the Ni content is preferably 0.001% or more, and 0.010 % Or more is more preferable.
[0041]
"Cu: 0 to 2.00%"
　Cu is an element that enhances the strength of a steel sheet by being present in steel as fine particles. However, if the Cu content exceeds 2.00%, the weldability is impaired. Therefore, even when it is contained, the Cu content is preferably 2.00% or less, and more preferably 1.20% or less. The effect can be obtained even if the lower limit of the Cu content is not particularly set, but in order to sufficiently obtain the effect of increasing the strength by the Cu content, the Cu content is preferably 0.001% or more, and 0.010. More preferably, it is% or more.
[0042]
"Co: 0 to 2.00%"
　Co is an element that enhances hardenability and is effective in increasing the strength of steel sheets. However, if the Co content exceeds 2.00%, the workability in hot water is impaired and the productivity is lowered. From this, even when it is contained, the Co content is preferably 2.00% or less, and more preferably 1.20% or less.
　Although the effect can be obtained without setting the lower limit of the Co content, the Co content is preferably 0.001% or more in order to sufficiently obtain the effect of increasing the strength due to the Co content. More preferably, it is 010% or more.
[0043]
"Mo: 0 to 1.00%"
　Mo is an element that suppresses phase transformation at high temperatures and is effective in increasing the strength of steel sheets. However, if the Mo content exceeds 1.00%, the workability in hot water is impaired and the productivity is lowered. From this, even when it is contained, the Mo content is preferably 1.00% or less, and more preferably 0.50% or less.
　Although the effect can be obtained without setting the lower limit of the Mo content, the Mo content is preferably 0.001% or more in order to sufficiently obtain the effect of increasing the strength due to the Mo content. More preferably, it is 005% or more.
[0044]
"W: 0 to 1.00%"
　W is an element that suppresses phase transformation at high temperatures and is effective in increasing the strength of steel sheets. However, if the W content exceeds 1.00%, the workability in hot water is impaired and the productivity is lowered. From this, even when it is contained, the W content is preferably 1.00% or less, and more preferably 0.50% or less.
　The lower limit of the W content can be obtained without any particular setting, but in order to sufficiently obtain the effect of increasing the strength by W, the W content is preferably 0.001% or more, preferably 0.010. More preferably, it is% or more.
[0045]
"B: 0 to 0.0100%"
　B is an element that suppresses phase transformation at high temperatures and is effective in increasing the strength of steel sheets. However, if the B content 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 even when it is contained. From the viewpoint of productivity, the B content is more preferably 0.0050% or less.
　Although the effect can be obtained without setting the lower limit of the B content in particular, the B content is preferably 0.0001% or more in order to sufficiently obtain the effect of increasing the strength due to the B content. The B content is more preferably 0.0005% or more in order to further increase the strength.
[0046]
"Sn: 0 to 1.00%"
　Sn is an element that suppresses the coarsening of the structure and is effective in increasing the strength of the steel sheet. However, if the Sn content exceeds 1.00%, the steel sheet may become excessively brittle and the steel sheet may break during rolling. Therefore, even when it is contained, the Sn content is preferably 1.00% or less.
　The lower limit of the Sn content can be obtained without any particular setting, but in order to sufficiently obtain the effect of increasing the strength by Sn, the Sn content is preferably 0.001% or more, preferably 0.010%. The above is more preferable.
[0047]
"Sb: 0 to 1.00%"
　Sb is an element that suppresses the coarsening of the structure and is effective in increasing the strength of the steel sheet. However, if the Sb content exceeds 1.00%, the steel sheet may become excessively brittle and the steel sheet may break during rolling. Therefore, even when it is contained, the Sb content is preferably 1.00% or less.
　The lower limit of the Sb content can be obtained without any particular setting, but in order to sufficiently obtain the effect of increasing the strength by Sb, the Sb content is preferably 0.001% or more, preferably 0.005%. The above is more preferable.
[0048]
"One or more of Ca, Mg, Ce, Zr, La, Hf, Bi, and REM are 0 to 0.0100%, respectively."
　REM is an abbreviation for Rare Earth Metal, and in this embodiment, Ce, Refers to elements belonging to the lanthanoid series, excluding La. In the present embodiment, REM, Ce, and La are often added as mischmetal, and may contain elements of the lanthanoid series in a complex manner. Even if an element of the lanthanide series other than La and / or Ce is contained as an impurity, the effect can be obtained. Further, even if the metal La and / or Ce is added, the effect can be obtained. In the present embodiment, the REM content is the total content of the elements belonging to the lanthanoid series excluding Ce and La.
[0049]
　The reasons for containing 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 may 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%, the ductility may decrease. Therefore, even when it is contained, 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.
　The lower limit of the content of each element is not particularly specified, but the effect can be obtained, but in order to sufficiently obtain the effect of improving the formability of the steel sheet, the content of each element is 0.0001% or more. Is preferable. From the viewpoint of moldability, it is more preferable that the total content of one or more of Ca, Mg, Ce, Zr, La, Hf, Bi and REM is 0.0010% or more.
[0050]
　The steel sheet according to this embodiment contains the above elements, and the balance is Fe and impurities. Regarding the above-mentioned Ti, V, Nb, Cr, Ni, Cu, Co, Mo, W, B, Sn, and Sb, it is permissible that a trace amount less than the lower limit value is contained as an impurity.
　Further, it is permissible for Ca, Mg, Ce, Zr, La, Hf, Bi and REM to contain a very small amount less than the lower limit 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.
[0051]
　Next, the steel structure (microstructure) of the steel sheet according to the present embodiment will be described. [%] In the description of the content of each tissue is [volume%].
(Steel Structure Inside Steel Sheet) As
　shown in FIG. 1, in the steel sheet 1 according to the present embodiment, a position of 1/4 of the plate thickness from the surface of the steel plate 1 (1/4 of the plate thickness in the plate thickness direction from the surface) The steel structure in the range of 1/8 to 3/8 thickness 11 (hereinafter, may be referred to as "steel structure inside the steel sheet") centered on (position) is 0 to 30% soft ferrite and retained austenite. 3% -40%, fresh martensite 0-30%, pearlite and cementite total 0-10%, 50% of the total retained austenite with an aspect ratio of 2.0 or more. That is all.
[0052]
"Soft ferrite: 0-30%"
　Ferrite is a structure having excellent ductility. However, since ferrite has low strength, it is a structure that is difficult to utilize in high-strength steel sheets. In the steel sheet according to the present embodiment, the steel structure inside the steel sheet (microstructure 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%.
[0053]
"Residual austenite: 3% -40%"
　Retained austenite is a tissue that enhances the strength-ductility balance. In the steel sheet according to 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 for the volume fraction of retained austenite to exceed 40%, it is necessary to contain a large amount of C, Mn and / or Ni. In this case, 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.
[0054]
"Fresh martensite: 0-30%"
　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.
[0055]
"Total of pearlite and cementite: 0-10%"
　The steel structure inside the steel sheet may contain pearlite and / or cementite. However, high volume fractions of pearlite and / or cementite deteriorate ductility. Therefore, the volume fraction of pearlite and / or cementite is limited to 10% or less in total. The volume fraction of pearlite and / or cementite is preferably 5% or less in total, and may be 0%.
[0056]
"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 in the steel structure 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 steel sheet according to 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 set to 50% or more. The number ratio of retained austenite having an aspect ratio of 2.0 or more is preferably 70% or more, and more preferably 80% or more.
[0057]
"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, and more preferably 30% or less.
[0058]
　In the steel sheet according to the present embodiment, the residual structure in the steel structure inside the steel sheet is mainly "hard ferrite" containing retained austenite in the grains. Mainly means that hard ferrite has the largest volume fraction in the residual structure.
　Hard ferrite is formed by performing a second heat treatment described later on a heat treatment steel sheet having a steel structure including one or more lath-like structures of bainite, tempered martensite, and fresh martensite. 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.
[0059]
　Further, the remaining structure in the steel structure inside the steel sheet may contain bainite in addition to the above-mentioned hard ferrite. 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 bainite ferrite free of iron carbide, which consists of fine iron carbides lined up parallel to.
[0060]
(Microstructure
　of surface layer ) Next, the steel structure (microstructure) of the surface layer of the steel sheet will be described.
[0061]
"When the region having 80% or less of the hardness of the hardness of 1/8 thickness 1-3 / 8 thickness in the range (steel inside) was defined as the soft layer, there is a soft layer having a thickness of the surface layer is 1 ~ 100 [mu] m"
　processing It is one of the requirements to soften the surface layer of the steel sheet in order to improve the flexibility afterwards. In the steel sheet according to the present embodiment, when the region where the hardness is 80% or less of the hardness (average hardness) inside the steel sheet is defined as the soft layer, the soft layer having a thickness of 1 to 100 μm in the plate thickness direction from the surface of the steel sheet. Exists. 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.
[0062]
　If the thickness of the soft layer is less than 1 μm in the depth direction (plate thickness direction) from the surface, sufficient bendability after processing 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.
　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.
[0063]
[Of the ferrites contained in the soft layer, the volume fraction of crystal grains having an aspect ratio of 3.0 or more is 50% or more] The volume fraction of the
　ferrite contained in the soft layer having an aspect ratio of 3.0 or more ( If the ratio of ferrite crystal grains having an aspect ratio of less than 3.0 to the volume fraction of all ferrite crystal grains in the soft layer is less than 50%, the hydrogen embrittlement resistance of the bent portion deteriorates. Therefore, among the ferrites contained in the soft layer, the volume fraction of crystal grains having an aspect ratio of 3.0 or more is set to 50% or more. It is preferably 60% or more, more preferably 70% or more. Here, the target ferrite includes soft ferrite and hard ferrite.
　The reason why the aspect ratio of ferrite in the soft layer affects the hydrogen embrittlement resistance of the bent portion is not always clear, but it is presumed as follows. That is, in the steel sheet according to the present embodiment, the steel structure of the soft layer and the steel structure (internal structure) inside the steel sheet are significantly different. However, in the steel sheet according to the present embodiment, since the number ratio of crystal grains having an aspect ratio of 3.0 or more contained in the surface layer is 50% or more, the shape similarity between the surface layer and the internal structure is high. As a result, it is presumed that the local concentration of stress and strain caused by the bending process at the boundary between the surface layer and the inside is suppressed, and the hydrogen embrittlement resistance is improved.
[0064]
[The volume fraction of retained austenite in the soft layer is 80% or less of the volume fraction of retained austenite inside the steel plate]
　The volume fraction of retained austenite contained in the soft layer is 1/4 of the thickness of the steel plate from the surface. When the volume fraction of retained austenite contained in the range of 1/8 to 3/8 thickness centered on the position of 1 is limited to 80% or less, the hydrogen embrittlement resistance property of the bent portion is improved. The volume fraction of retained austenite contained in the soft layer is preferably 50% or less, more preferably 30% or less, with respect to the volume fraction of retained austenite contained in the range of 1/8 to 3/8 thickness.
　The mechanism by which the volume fraction of retained austenite in the soft layer improves the hydrogen embrittlement resistance of the bent portion is not clear, but it is presumed as follows. That is, in the bent portion, a larger plastic strain is generated from the center of the plate thickness toward the outer surface of the bend. Therefore, most of the retained austenite existing near the surface on the outer side of bending is transformed into martensite by work-induced transformation. Martensite obtained by processing-induced transformation of such retained austenite is extremely hard and brittle, and is therefore considered to have an adverse effect on hydrogen embrittlement cracking resistance. Therefore, the smaller the volume fraction of retained austenite contained in the soft layer with respect to the volume fraction of retained austenite contained in the range of 1/8 to 3/8 thickness of the steel sheet, the more hydrogen embrittlement resistance of the bent portion. It is thought that the chemical characteristics will improve.
[0065]
"Internal oxide layer containing Si oxide"
　The steel sheet according to this embodiment was analyzed for emission intensity at a wavelength indicating Si by a high frequency glow discharge (high frequency GDS) analysis method from the surface to the depth direction (plate thickness direction). Occasionally, a peak of emission intensity having a wavelength indicating Si appears in a range of more than 0.2 μm and less than 10.0 μm from the surface. The fact that the peak of the emission intensity of the wavelength indicating Si appears in the range of more than 0.2 μm and less than 10.0 μm from the surface means that the steel sheet is internally oxidized and more than 0.2 μm and less than 10.0 μm from the surface of the steel sheet. It shows that an internal oxide layer containing a Si oxide is provided in the range of. A steel sheet having an internal oxide layer in the above depth range suppresses the formation of an oxide film such as Si oxide on the surface of the steel sheet due to heat treatment during manufacturing, so that it has excellent chemical conversion treatment properties and plating adhesion. Has.
[0066]
　The steel sheet according to the present embodiment has a range of more than 0.2 μm and less than 10.0 μm from the surface and a range of 0 μm to 0.2 μm from the surface when analyzed by a high-frequency glow discharge analysis method from the surface to the depth direction. It may have a peak of emission intensity at a wavelength indicating Si in both the region shallower than 0.2 μm in depth. Having peaks in both ranges indicates that the steel sheet has an internal oxide layer and an external oxide layer containing Si oxide on the surface.
[0067]
　FIG. 2 shows the emission intensity of the wavelength indicating Si and the depth from the surface when the emission intensity of the wavelength indicating Si is analyzed by the high-frequency glow discharge analysis method in the depth direction from the surface of the steel sheet according to the present embodiment. It is a graph which shows the relationship with (Intensity). In the steel sheet according to 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 less than 10.0 μm 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.
[0068]
　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 in the range of more than 0.2 μm and less than 10.0 μm. It has not appeared. This indicates that the steel sheet does not have an internal oxide layer but has only an external oxide layer.
[0069]
"Zinc-plated layer"
　A zinc-plated layer (hot-dip galvanized layer or electrogalvanized layer) may be formed on the surface (both sides or one side) of the steel sheet according to the present embodiment. 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 Fe 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 Fe 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.
[0070]
　The amount of plating adhered to the zinc plating layer is not particularly limited, but is preferably 5 g / m 2 or more per side from the viewpoint of corrosion resistance, within the range of 20 to 120 g / m 2 , and further 25 to 75 g / m. It is more preferably within the range of 2 .
[0071]
　In the steel sheet according to the present embodiment, an upper plating layer may be further provided on the galvanized layer and the galvanized layer for the purpose of improving coatability, weldability, and the like. Further, the galvanized steel sheet may be subjected to various treatments such as chromate treatment, phosphate treatment, lubricity improvement treatment, weldability improvement treatment and the like.
[0072]
　The steel sheet according to the present embodiment is formed by subjecting the following steel sheet (material before the second heat treatment; hereinafter referred to as "heat treatment steel sheet") obtained by the step including the first heat treatment to the second heat treatment described later. Will be done.
[0073]
"Heat treatment steel sheet"
　The heat treatment steel sheet according to the present embodiment is used as a material for the steel sheet according to the present embodiment.
　Specifically, the heat-treated steel sheet, which is the material of the steel sheet according to the present embodiment, has the same chemical composition as the above-mentioned steel sheet according to the present embodiment, and has the steel structure (microstructure) shown below. preferable. In addition, [%] in the description of the content of each tissue indicates [volume%] unless otherwise specified.
[0074]
　That is, the steel structure (steel structure inside the steel plate) in the range of 1/8 to 3/8 thickness centered on the position of 1/4 of the plate thickness from the surface is that of bainite, tempered martensite, and fresh martensite. The number density of retained austenite grains having a volume fraction of 70% or more, containing retained austenite, an aspect ratio of less than 1.3, and a major axis of more than 2.5 μm is 1 It is preferable that the volume fraction of ferrite is less than 50% in the steel structure of the surface layer portion, which is 0.0 × 10-2 pieces / μm 2 or less and is in the range of 0 to 20 μm in the depth direction from the surface. The bainite 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 parallel to the inside. It contains lower bainite composed of fine iron-based carbides lined up and bainite ferrite that does not contain iron-based carbides.
[0075]
　The preferable steel structure (microstructure) of the heat treatment steel sheet which is the material of the steel sheet according to the present embodiment will be described below. [%] In the description of the content of each tissue is [volume%].
[0076]
(Steel structure inside the steel sheet for heat treatment)
"A total of 70% or more of the lath-shaped structure in terms of body
　integration rate" The steel sheet for heat treatment of the present embodiment is centered on a position 1/4 of the thickness of the steel sheet from the surface. The steel structure (steel structure inside the steel sheet) in the range of 1/8 to 3/8 thickness is a lath-like structure consisting of one or more of baynite, tempered martensite, and fresh martensite, with a body integration rate. It is preferable that the total content is 70% or more.
[0077]
　By containing the lath-like structure in a total volume fraction of 70% or more, the steel sheet obtained by subjecting the heat-treated steel sheet to the second heat treatment described later is mainly composed of hard ferrite. When the total volume fraction of the lath-shaped structure is less than 70%, the steel sheet obtained by subjecting the heat-treated steel sheet to the second heat treatment has a steel structure inside the steel sheet containing a large amount of soft ferrite, and this embodiment The steel plate according to the above cannot be obtained. The steel structure inside the 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 may be 100% in total. ..
[0078]
"Number density of retained austenite grains having an aspect ratio of less than 1.3 and a major axis of more than 2.5 μm"
　The steel structure inside the steel sheet in the heat-treated steel sheet may contain retained austenite in addition to the lath-like structure described above. However, if it contains residual austenite, the number density of the residual austenite grains having an aspect ratio of and the major axis is 2.5μm greater less than 1.3, 1.0 × 10 -2 cells / [mu] m 2 is preferably limited to less ..
[0079]
　If the retained austenite present in the steel structure inside the steel sheet is in the form of coarse lumps, coarse lumpy retained austenite grains are present inside the steel sheet obtained by subjecting the steel sheet for heat treatment to the second heat treatment, and the aspect ratio is In some cases, the number ratio of retained austenite with a value of 2.0 or more cannot be sufficiently secured. Therefore, the aspect ratio of 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 number density of the residual austenite grains of coarse massive, as low Preferably, 0.5 × 10 -2 cells / [mu] m 2 is preferably less.
[0080]
　Further, if the retained austenite is excessively present inside the steel sheet of the heat treatment steel sheet, a part of the retained austenite is isotropic by performing the second heat treatment described later on the heat treatment 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 obtained after the second heat treatment. Therefore, the volume fraction of retained austenite contained in the steel structure inside the steel sheet for heat treatment is preferably 10% or less.
[0081]
(Microstructure of the surface layer of the steel sheet for heat treatment)
"The volume fraction of ferrite is less than 20% in the surface layer portion in the range from the surface of the steel sheet to 20 μm in the depth direction." For
　heat treatment as the material of the steel sheet according to the present embodiment. The steel sheet preferably has a volume fraction of less than 20% in the range from the surface of the steel sheet to 20 μm in the depth direction. When the volume fraction of ferrite is 20% or more, the volume fraction of ferrite grains having an aspect ratio of more than 3.0 is predetermined in the soft layer formed on the steel sheet obtained by subjecting the heat treatment steel sheet to the second heat treatment. Does not satisfy the range of. The smaller the volume fraction of ferrite, the more preferable, 10% or less is more preferable, and 0% may be used.
[0082]
"Method of manufacturing a steel sheet according to
　this embodiment " Next, a method of manufacturing a steel sheet according to this embodiment will be described.
[0083]
　In the method for producing a steel sheet according to the present embodiment, the slab having the above chemical composition is hot-rolled and pickled, or the hot-rolled steel sheet is cold-rolled and the first is shown below. A steel sheet for heat treatment is manufactured by subjecting it to heat treatment. 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.
[0084]
(Casting Step) In order
　to manufacture the steel sheet according to 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.
[0085]
(Slab heating) The
　slab is heated prior to hot rolling. When producing the steel sheet according to the present embodiment, it is preferable to select slab heating conditions that satisfy the following formula (4).
[0086]
[

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.)
[0087]
[Number 2]

In (Equation (5), T is a slab heating temperature (° C.), C content of WC is in the steel (mass%), A c1 is the value represented by the following formula (8), A c3 Is the value represented by the following equation (9).)
[0088]
[Expression 3]

In (Equation (6), T is a slab heating temperature (° C.), WMn the Mn content in the steel (mass%), A c1 is the value represented by the following formula (8), A c3 Is the value represented by the following equation (9).)
[0089]
[

Equation 4] (In equation (7), T is the slab heating temperature (° C.) and R is the gas constant; 8.314 J / mol.)
[0090]
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.)
[0091]
　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 coarse lump austenite grains inside the heat-treated steel sheet and the steel sheet obtained by performing the second heat treatment on the heat-treated steel sheet.
[0092]
(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.
[0093]
(Pickling Step)
　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 and plating adhesion 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.
[0094]
(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 reduction (cumulative rolling in cold rolling) 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 defined, 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.
[0095]
(First Heat Treatment)
　Next, a heat-treated steel sheet is produced by subjecting the pickled hot-rolled steel sheet or the cold-rolled steel sheet obtained by cold-rolling the hot-rolled steel sheet to the first heat treatment. The first heat treatment is performed under conditions that satisfy the following (a) to (e).
(A) The atmosphere is such that it contains 0.1% by volume or more of H 2 and satisfies the following formula (3) from 650 ° C. to reaching the maximum heating temperature .
　log (PH 2 O / PH 2 ) <-1.1 ... (3)
(In formula (3), log is the common logarithm, PH 2 O is the partial pressure of water vapor, and PH 2 is the partial pressure of hydrogen. Shows.)
[0096]
In the first heat treatment, by satisfying the above (a), the decarburization reaction in the surface layer portion of the steel sheet is suppressed, and thus the formation of ferrite is suppressed.
[0097]
　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.
　When the log (PH 2 O / PH 2 ) is −1.1 or higher, the decarburization reaction proceeds on the surface layer of the steel sheet, and ferrite is formed on the surface layer. As a result, the proportion of ferrite grains having an aspect ratio of less than 3.0 increases in the steel sheet after the second heat treatment.
[0098]
(B) Ac3 Hold at the maximum heating temperature of -30 ° C to 1000 ° C for 1 second to 1000 seconds.
　In the first heat treatment, the maximum heating temperature is set to Acc3-30 ° C. or higher. Maximum heating temperature A c3 is less than -30 ° C., coarse ferrite massive remains in the steel sheet inside the steel structure in the heat treatment for steel plates. As a result, the volume fraction of 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 becomes insufficient, resulting in deterioration of the characteristics. The maximum heating temperature is preferably Ac3 -15 ° C. or higher, and more preferably Ac3 + 5 ° C. or higher. On the other hand, heating to an excessively high temperature increases the fuel cost required for heating and causes damage to the furnace body. Therefore, the maximum heating temperature is set to 1000 ° C. or lower.
[0099]
　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 structure inside the steel sheet for heat treatment. 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, 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.
[0100]
(C) Heating is performed so that the average heating rate in the temperature range from 650 ° C. to the maximum heating temperature is 0.5 ° C./sec to 500 ° C./sec.
　In the first heat treatment, when the average heating rate is less than 0.5 ° C./sec in the temperature range from 650 ° C. to the maximum heating temperature during heating, Mn segregation proceeds during the heat treatment, resulting in a coarse massive Mn concentration. A chemical region is formed. In this case, the characteristics of the steel sheet obtained after the second heat treatment deteriorate. In order to suppress the formation of austenite in the form of agglomerates, the average heating rate from 650 ° C. to the maximum heating temperature is 0.5 ° C./sec or more. It is preferably 1.5 ° C./sec or higher.
　On the other hand, regarding the upper limit of the average heating rate, it is difficult in actual operation to set it to more than 500 ° C./sec, and it is also difficult to control the temperature. Therefore, the average heating rate is set to 500 ° C./sec as the upper limit. The average heating rate from 650 ° C. to the maximum heating temperature is 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.
[0101]
(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 structure inside the steel sheet for heat treatment 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) is performed. 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 be formed in the heat-treated steel sheet. In this case, the volume fraction of the soft ferrite of the steel sheet obtained after the second heat treatment becomes excessive, and the properties such as tensile strength deteriorate. The average cooling rate is preferably 10 ° C./sec or higher, and more preferably 30 ° C./sec or higher.
　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 or less is 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.
[0102]
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.)
[0103]
(E) The above-mentioned 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 of Ms or less represented by the formula (10). The cooling stop temperature may be room temperature (25 ° C.). By setting the cooling stop temperature to Ms or less, the steel structure inside the steel sheet in the heat treatment steel sheet obtained after the first heat treatment becomes mainly lath-shaped.
[0104]
　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.
[0105]
　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 according to the present embodiment by performing the second heat treatment shown below.
　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.
[0106]
(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 is performed under conditions that satisfy the following (A) to (E).
(A) From 650 ° C. to reaching the maximum heating temperature, H 2 is 0.1% by volume or more, O 2 is 0.020% by volume or less, and the log (PH 2 O / PH 2 ) is the following formula ( PH 2 O / PH 2 ). Create an atmosphere that satisfies 4).
　-1.1 ≤ log (PH 2 O / PH 2 ) ≤ -0.07 ... (4)
(In equation (3), log is the common logarithm, PH 2 O is the partial pressure of water vapor, and PH 2 Indicates the partial pressure of hydrogen.)
　By satisfying the above (A) in the second heat treatment, the oxidation reaction outside the steel plate is suppressed and the decarburization reaction of the surface layer portion is promoted.
[0107]
　If H 2 in the atmosphere is less than 0.1% by volume or O 2 is more than 0.020% by volume, the oxide film existing on the surface of the steel sheet cannot be sufficiently reduced and is placed on the steel sheet. An oxide film is formed. As a result, the chemical conversion treatment property and plating adhesion of the steel sheet obtained after the second heat treatment are lowered. The preferred range of H 2 is 1.0% by volume or more, more preferably 2.0% 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.
　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.
[0108]
　When the log (PH 2 O / PH 2 ) is less than -1.1, external oxidation of Si and Mn in the surface layer of the steel sheet occurs, the decarburization reaction becomes insufficient, and the surface structure of the steel sheet obtained after the second heat treatment becomes It does not satisfy the desired range. Therefore, the log (PH 2 O / PH 2 ) is set to -1.1 or higher. Preferably the log (PH 2 O / PH 2 ) is −0.8 or higher.
　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 obtained after the second heat treatment is insufficient. Therefore, the log (PH 2 O / PH 2 ) is set to −0.07 or less.
[0109]
(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.
[0110]
　On the other hand, when the maximum heating temperature exceeds ( Ac3-10 ) ° C., almost or all of the internal steel structure becomes austenite, so that the lath-like structure of the steel sheet (heat treatment steel sheet) before the second heat treatment disappears. , The lath-like structure of the steel sheet before the second heat treatment is not taken over by the steel sheet after the second heat treatment. 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 ( Ac3-10 ) ° C. or lower. Fully takes over the 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) ° C. or less Is more preferable.
[0111]
　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, cementite in the steel may remain undissolved and the characteristics of the steel sheet may deteriorate. The holding time is preferably 30 seconds or more. 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 lowered. Therefore, the holding time is set to 1000 seconds or less.
[0112]
(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.
　When 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, and austenite grains are present in the grains. Does not increase the volume fraction of soft ferrite. On the other hand, if the average heating rate exceeds 500 ° C./sec, the decarburization reaction does not proceed sufficiently.
[0113]
(D) Cool from the maximum heating temperature to 480 ° C. or lower so that the average cooling rate from 700 to 600 ° C. is 3 ° C./sec or more.
　In the second heat treatment, the temperature is cooled from the maximum heating temperature to 480 ° C. or lower. At this time, the average cooling rate between 700 and 600 ° C. is set to 3 ° C./sec or more. If the above range is cooled at an average cooling rate of less than 3 ° C./sec, coarse carbides are generated and the characteristics of the steel sheet are impaired. The average cooling rate is preferably 10 ° C./sec or higher. The upper limit of the average cooling rate 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.
[0114]
(E) Hold at 300 ° C. to 480 ° C. for 10 seconds or longer.
　Subsequently, the steel sheet is held for 10 seconds or longer in a temperature range between 300 ° C. and 480 ° C. If the retention time is less than 10 seconds, carbon is not sufficiently concentrated in untransformed austenite. In this case, the lath-shaped ferrite does not grow sufficiently, and C concentration to austenite does not proceed. As a result, fresh martensite is generated during the final cooling after the holding, 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. It is not necessary to limit the upper limit of the holding time, but the holding time may be 1000 seconds or less because the productivity is lowered even if the holding time is excessively long.
　If the cooling stop temperature is less than 300 ° C., it may be reheated to 300 to 480 ° C. and then held.
[0115]

　The steel sheet after the second heat treatment may be subjected to hot-dip galvanizing to form a hot-dip galvanizing layer on the surface. Further, the alloying treatment of the plating layer may be performed following the formation of the hot-dip galvanizing layer.
　Further, the steel sheet after the second heat treatment may be subjected to electrogalvanization to form an electrogalvanized layer on the surface.
[0116]
　The hot-dip galvanizing, alloying treatment, and electrogalvanizing 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 in the present invention are satisfied. For example, as shown as a pattern [1] in FIG. 4, 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 as a pattern [2] in FIG. 5, 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. 6, 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.
[0117]
　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 into the plating bath 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 unavoidable impurities 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 20 to 120 g / m 2 is preferable, and 25 to 75 g / m 2 is more preferable .
[0118]
　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.
　For the alloying treatment, the alloying treatment temperature is preferably 460 to 600 ° C. If the alloying treatment is less than 460 ° C., the alloying rate becomes slow, not only the productivity is lowered, but also the alloying treatment unevenness occurs.
　On the other hand, if the alloying treatment temperature exceeds 600 ° C., alloying proceeds excessively and the plating adhesion of the steel sheet deteriorates. The alloying treatment temperature is more preferably 480 to 580 ° C. The heating time of 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.
[0119]
　When electrogalvanizing is performed, the conditions are not particularly limited.
[0120]
　By performing the second heat treatment described above, the steel sheet according to 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 retained austenite is transformed into martensite by work-induced transformation, which may reduce the volume fraction of retained austenite and impair its characteristics. .. 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 obtained without cold rolling.
[0121]
　Next, a method for measuring each configuration of the steel sheet according to the present embodiment and the heat treatment steel sheet according to the present embodiment will be described.
"Measurement of steel structure" The body
　integration ratios of ferrite (soft ferrite, hard ferrite), bainite, tempered martensite, fresh martensite, pearlite, cementite, and retained austenite contained in the steel structure inside the steel plate and in the soft layer are as follows. It can be measured using the method shown.
[0122]
　A sample is taken with the thickness cross section parallel to the rolling direction of the steel sheet as the observation surface, and the observation surface is polished and night-tar-etched. Next, in the case of observing the steel structure inside the steel plate, in one or more observation fields in the range of 1/8 to 3/4 thickness centered on the position of 1/4 thickness from the surface on the observation surface. , when the observation of the steel structure of the soft layer, in one or a plurality of the observation field of view region including the soft layer depth range to the outermost layer, 2.0 × 10 by the sum of the steel plate -9 m 2 or more areas Is observed with an electric field radiation type scanning electron microscope (FE-SEM: Field Emission Scanning Electron Microscope). Then, the area fractions of ferrite, bainite, tempered martensite, fresh martensite, pearlite, cementite, and retained austenite 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 determined to be pearlite or cementite. 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. Judge the rest as bainite. The volume fraction of each tissue is calculated by the point counting method to obtain the volume fraction of each tissue.
　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.
[0123]
　In the steel sheet according to the present embodiment and the heat treatment steel sheet as a material thereof, the volume fraction of retained austenite contained in the steel sheet is evaluated by an X-ray diffraction method. Specifically, 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 FCC is performed by X-ray diffraction method. The area fraction of iron is measured and used as the volume fraction of retained austenite.
[0124]
"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.
[0125]
"Measurement of aspect ratio and major axis of retained austenite grains" The aspect ratio and major axis of
　retained austenite grains contained in the steel structure inside the steel sheet are evaluated by performing high-resolution crystal orientation analysis 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). The region determined to be FCC iron from the observation results is defined as retained austenite.
　Next, in order to avoid measurement errors, only austenite grains having a major axis length of 0.1 μm or more are extracted from the crystal orientations 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. 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 obtained.
　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.
[0126]
"Ferrite grains containing austenite grains (hard ferrite) / ferrite grains not containing austenite grains (soft ferrite)"
　A method for separating grains containing (encapsulating) 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, a boundary that causes a crystal orientation difference of 15 ° or more is set as a grain boundary, and a grain boundary map of ferrite grains 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 overlay it with the grain boundary map of ferrite grains. ..
　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".
[0127]
"Hardness from the surface layer to the inside of the steel sheet"
　The hardness distribution from the surface layer to the inside of the steel sheet for determining the thickness of the soft layer 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. Push in a quadrangular pyramid-shaped Vickers indenter with an apex angle of 136 ° at a pitch. At this time, the pushing load is set so that the Vickers indentations do not interfere with each other. For example, 2 gf. 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. Next, the measurement position is moved by 10 μm or more in the rolling direction, and the same measurement is performed from the surface to the position of 1/8 of the plate thickness, starting from the position at a depth of 5 μm from the outermost layer. 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. As shown in FIG. 7, by repeating this, the Vickers hardness of 5 points each is measured at each thickness position. 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. Let the average value of 5 points be the hardness at the 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.
　On the other hand, the hardness inside the steel sheet is measured at least 5 points in the range of 1/8 to 3/8 thickness centered on the 1/4 thickness position using a micro hardness measuring device in the same manner as above. Then, it is calculated by averaging the values.
　As the micro-hardness measuring device, for example, FISCHERSCOPE (registered trademark) HM2000 XYp can be used.
[0128]
"Aspect ratio of ferrite contained in the soft layer and the ratio of crystal grains having an aspect ratio of 3.0 or more"
　The aspect ratio of ferrite in the soft layer is determined by observing the crystal grains using FE-SEM and using the EBSD method (EBSD method). High-resolution crystal orientation analysis is performed by electron backscatter diffraction method) and evaluated. 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.
[0129]
"High Frequency Glow Discharge (High Frequency GDS) Analysis" When the
　steel sheet and the steel sheet for heat treatment according to the present embodiment 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.
　A commercially available analyzer can be used for high frequency GDS analysis. In this embodiment, a high-frequency glow discharge emission analyzer GD-Profiler 2 manufactured by HORIBA, Ltd. is used.
Example
[0130]
　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.
[0131]
　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 to 5 and the slab heating conditions having the numerical values ​​of the formula (4) shown in Tables 2 to 5, and the rolling completion temperature is set to the temperature shown in Tables 2 to 5. 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.
[0132]
[table 1]

[0133]
[Table 2]

[0134]
[Table 3]

[0135]
[Table 4]

[0136]
[Table 5]

[0137]
　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. In some examples, the cold-rolled steel sheets cooled to the cooling stop temperatures shown in Tables 6 to 9 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. Further, in some examples, the second heat treatment was performed without performing the first heat treatment.
[0138]
(First heat treatment) Under the
　conditions shown in Tables 6 to 9, the mixture was heated to the maximum heating temperature and maintained at the maximum heating temperature. After that, it was cooled to the cooling stop temperature. In the first heat treatment, H 2 is contained at the concentrations shown in Tables 6 to 9, and the log (PH 2 O / PH 2 ) is the numerical value shown in Tables 6 to 9, and the temperature is 650 ° C. to the maximum heating temperature. Heated until reached.
[0139]
　A c3 is calculated by the following equation (9), Ms was determined by the following equation (10).
A c3 = 879-346C + 65Si-18Mn + 54Al ... (9)
(The element symbol in the formula (9) is the mass% of the element in the steel.)
Ms = 561-407 × C-7.3 × Si -37.8 x Mn-20.5 x Cu-19.5 x Ni-19.8 x Cr-4.5 x Mo ... (10)
(The element symbol in formula (10) is that of the element. % By mass in steel.)
[0140]
[Table 6]

[0141]
[Table 7]

[0142]
[Table 8]

[0143]
[Table 9]

[0144]
(Second heat treatment) The
　temperature was raised to the maximum heating temperature and maintained at the maximum heating temperature so that the average heating rate from 650 ° C. to the maximum heating temperature was the conditions shown in Tables 10 to 13. Then, it was cooled to the cooling stop temperature so that the average cooling rate of 700 to 600 ° C. became the average cooling rate shown in Tables 10 to 13. In the second heat treatment, heating was performed in the atmospheres shown in Tables 10 to 13 until the temperature reached 650 ° C. to the maximum heating temperature.
[0145]
　Next, a part of the high-strength steel sheets (Experimental Examples No. 54 and 69) after the second heat treatment were electrogalvanized to form electrogalvanized layers on both surfaces of the high-strength steel sheets, and the electrogalvanized steel sheets were formed. (EG) was obtained.
　In addition, among each experimental example, Experimental Example No. For 1'to 80', alloying hot dip galvanizing was performed at the timing after cooling and isothermal maintenance under the conditions shown in the table (that is, at the timing shown in the pattern [1] of FIG. 4). In addition, these experimental example No. Of 1'to 80', Experimental Examples 1'to 16', 18' to 58', 60' to 73', 75' to 80'were subjected to alloying treatment following hot dip galvanizing. For Experimental Examples 17', 59', and 74', no alloying treatment was performed after hot dip galvanizing.
[0146]
　Experimental Example No. For 81'-88', as shown in the table, according to the pattern [2] shown in FIG. 5, after heating to the maximum heating temperature, the mixture was cooled at the average cooling rate, and then Experimental Example No. Except for 86, alloying hot dip galvanizing and alloying treatment were performed, and cooling and isothermal maintenance were performed under the conditions shown in Tables 10 to 13.
[0147]
　In addition, Experimental Example No. For 89', according to the pattern [3] shown in FIG. 6, under the conditions shown in the table, after heating to the maximum heating temperature, cooling, isothermal holding, cooling to room temperature, and then alloying hot-dip zinc again. It was plated and alloyed.
[0148]
　Hot-dip galvanizing was carried out on both sides of the steel sheet at a basis weight of 50 g / m 2 per side by immersing each example in a hot-dip zinc bath at 460 ° C.
[0149]
　A c1 is determined by the following equation (8), A c3 is determined by the equation (9).
A c1 = 723-10.7 × Mn-16.9 × Ni + 29.1 × Si + 16.9 × Cr ... (8) (The element symbol in the formula (8) is the mass% of the element in steel. is there.)
[0150]
[Table 10]

[0151]
[Table 11]

[0152]
[Table 12]

[0153]
[Table 13]

[0154]
　Next, the steel plates of Experimental Examples No. 1 to No. 78 and Experimental Examples No. 1'to No. 89' obtained in this manner were centered at a position 1/4 thickness from the surface by the above method. The steel structure (steel structure inside the steel plate) in the range of 1/8 to 3/8 thickness was measured, and soft ferrite, retained austenite, tempered martensite, fresh martensite, the total of pearlite and cementite, hard ferrite, The body integration ratio was examined for each bainite.
[0155]
　Further, with respect to the inside of the steel sheets of Experimental Examples No. 1 to No. 78 and Experimental Examples No. 1'to No. 89' by the method described above, retained austenite having an aspect ratio of 2.0 or more in total retained austenite. The number ratio of was examined.
　These results are shown in Tables 14 to 17.
[0156]
[Table 14]

[0157]
[Table 15]

[0158]
[Table 16]

[0159]
[Table 17]

[0160]
　Next, with respect to the steel sheets of Experimental Examples No. 1 to No. 78 and Experimental Examples No. 1'to No. 89', the steel structure and hardness were measured by the above-mentioned method, and the thickness of the soft layer and the soft layer were measured. The volume fraction of ferrite having an aspect ratio of 3.0 or more and the volume fraction of retained austenite in the soft layer and retained austenite inside the steel sheet were investigated. The results are shown in Tables 18 to 21.
[0161]
　Further, with respect to the steel plates of Experimental Examples No. 1 to No. 78 and Experimental Examples No. 1'to No. 89', light emission having a wavelength indicating Si by a high-frequency glow discharge analysis method from the surface to the depth direction by the above method. The intensity peak is analyzed, and a peak of emission intensity at a wavelength indicating Si (a peak indicating that an internal oxide layer containing a Si oxide is present) appears in a depth range of more than 0.2 μm and less than 10.0 μm. I checked whether it was.
　Then, in the steel sheets of Experimental Examples No. 1 to No. 78 and Experimental Examples No. 1'to No. 89', Si is between a depth of more than 0.2 μm and less than 10.0 μm in the depth direction from the surface. The one in which the peak of the emission intensity of the wavelength indicating the above appeared was evaluated as “with” the internal oxidation peak, and the one in which the peak did not appear was evaluated as “without” the internal oxidation peak. The results are shown in Tables 18 to 21.
[0162]
[Table 18]

[0163]
[Table 19]

[0164]
[Table 20]

[0165]
[Table 21]

[0166]
　Further, with respect to the steel plates of Experimental Examples No. 1 to No. 78 and Experimental Examples No. 1'to No. 89', the maximum tensile stress (TS), elongation (El), and hole expandability (holes) were obtained by the methods shown below. Spread ratio), hydrogen embrittlement resistance of the bent part, chemical conversion processability or plating adhesion were investigated. The results are shown in Tables 22 to 25.
[0167]
　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.
[0168]
　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. The larger the value represented by the formula (11), the better the balance between strength, elongation and hole expandability. Those having a value of the formula (11) of 80 × 10-7 or more were evaluated as 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 22 to 25.
[0169]
　The hydrogen embrittlement resistance of the bent portion was evaluated by the following method.
　First, a strip-shaped test piece having a size of 30 mm × 120 mm was collected from the steel sheet so that the longitudinal direction of the test piece and the rolling direction of the steel sheet were perpendicular to each other, and holes were drilled at both ends of the test piece for bolt fastening. Next, the test piece was bent 180 ° with a punch having a radius of 5 mm. After that, the spring-backed U-bending test piece was stressed by fastening with bolts and nuts. At this time, a strain gauge of GL 5 mm was attached to the top of the U-bending test piece, and a stress of 0.8 times the tensile strength was applied by controlling the strain amount. At that time, the stress was set by converting the strain into stress from the stress-strain curve previously collected in the tensile test. The end face of the U-bending test piece was left shear-cut.
　The U-bending test piece after the stress was applied was continuously charged with cathode hydrogen using an electrochemical cell until the test piece broke. As the electrolytic solution, a 3% NaCl aqueous solution to which 3 g / L of ammonium thiocyanate was added was used, and the charge current density was −0.05 mA / cm 2 . The test piece after breaking was immediately stored in liquid nitrogen, and the amount of hydrogen in the steel was measured by a gas chromatograph-based temperature-rising hydrogen analysis method (heating rate: 100 ° C./hour, measured up to 300 ° C.). The amount of hydrogen released from the steel material from room temperature to 200 ° C. was defined as the amount of diffusible hydrogen.
　The same test was performed three times, and the average value was defined as the critical diffusible hydrogen content. For materials with a tensile strength of 1100 MPa or less, those with a critical diffusible hydrogen content of 1.0 ppm or more are "Ex", those with a tensile strength of 0.6 to 1.0 ppm are "G", and those with a tensile strength of less than 0.6 ppm are "B". It was judged. For materials with a tensile strength of more than 1100 MPa and less than 1350 MPa, those with a critical diffusible hydrogen amount of 0.8 ppm or more are "Ex", those with 0.5 to 0.8 ppm are "G", and those with a tensile strength of less than 0.5 ppm. Was determined to be "B". For materials with a tensile strength exceeding 1350 MPa, those with a critical diffusible hydrogen content of 0.6 ppm or more are "Ex", those with a tensile strength of 0.3 to 0.6 ppm are "G", and those with a tensile strength of less than 0.3 ppm are "B". It was judged.
[0170]
　In addition, Experimental Example No. 54, No. The chemical conversion processability of the steel sheets No. 1 to No. 78 excluding 69 was measured by the method shown below.
　A 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.
[0171]
　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.
[0172]
Zinc phosphate crystals of the chemical conversion treatment film are densely attached to the surface of "Ex".
"G" zinc phosphate crystals are sparse, and slight gaps (parts generally called "scale" where the zinc phosphate coating is not attached) can be seen between adjacent crystals.
The surface of "B" is clearly not covered with the chemical conversion coating.
[0173]
　“EG” on the surface in Tables 21 to 25 indicates an electrogalvanized steel sheet, “GI” is a hot-dip galvanized steel sheet, and “GA” is an alloyed hot-dip galvanized steel sheet.
[0174]
　In addition, the plating adhesion of the steel sheets of Experimental Examples No. 54, No. 69, and No. 1'to No. 89' was measured by the method shown below.
[0175]
　A 30 mm × 100 mm test piece was collected from these steel plates 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.
　Ex: Small plating peeling (peeling width less than 5 mm)
　G: Peeling to the extent 　that there is no problem in practical use (peeling width 5 mm or more and less than 10 mm)
　B: Severe peeling (peeling width 10 mm or more)
Plating adhesion passed Ex and G did.
[0176]
　The evaluation results for each experimental example will be described below.
[0177]
[Table 22]

[0178]
[Table 23]

[0179]
[Table 24]

[0180]
[Table 25]

[0181]
　Experimental Example No. which is an example of the present invention. 1, 3, 4, 7, 10, 12-14, 18, 19, 21-23, 27, 28, 30-34, 36, 37, 39-42, 44-46, 49, 50, 52-63, 66-70, 76-78, 1'3', 4', 7', 10'-14', 16'-19', 23', 24'26'-28', 32', 33', 35' ~ 39', 41', 42', 44'-47', 49'-51', 54', 55', 57'-68', 71'-75', 81'-89'have high strength It was excellent in ductility and hole expandability, and had good bendability after processing, chemical conversion treatment property, and plating adhesion.
[0182]
　As for the steel sheets of Experimental Examples Nos. 11, 17, 29, 47, and 48, since the first heat treatment was not performed, the metal structure did not contain hard ferrite, so that the balance between strength, elongation, and hole expansion ratio was poor.
　Since the maximum heating temperature of the steel sheet of Experimental Example No. 2 is low in the first heat treatment, there are many soft ferrites, and the number ratio of retained austenite having an aspect ratio of 2.0 or more is insufficient, resulting in strength, elongation, and hole expansion ratio. The balance was bad.
[0183]
　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 holes The balance of the spread rate was bad.
　The steel sheets of Experimental Examples Nos. 6, 15, 16 and 24 had a high log (PH 2 O / PH 2 ) in the first heat treatment , and the desired surface structure could not be obtained. Therefore, the hydrogen brittleness of the bent portion was poor. It was.
[0184]
　Since the steel sheet of Experimental Example No. 8 had a slow cooling rate in the first heat treatment, the fraction of soft ferrite in the internal structure of the steel sheet increased. Therefore, the steel sheet of Experimental Example No. 8 had a poor balance of strength, elongation, and hole expansion ratio.
　The steel sheets of Experimental Examples No. 9, 15, 20, 25, 48, and 51 had a low log (PH 2 O / PH 2 ) in the second heat treatment , and a desired surface structure could not be obtained. Therefore, the resistance of the bent portion was not obtained. The hydrogen embrittlement property was poor.
[0185]
　As for the steel sheets of Experimental Examples No. 9, 20, 25, 48, and 51, since there was no internal oxidation peak, the evaluation of chemical conversion processability was “B”.
[0186]
　The steel plate of Experimental Example No. 26 did not contain hard ferrite in the metal structure because the maximum heating temperature in the second heat treatment was high, and further, the desired surface layer structure could not be obtained, so that the strength, elongation, and hole expansion ratio were not obtained. The
　steel plate of Experimental Example No. 35 , which had a poor balance and poor hydrogen embrittlement resistance of the bent portion, had a short holding time between 300 ° C. and 480 ° C. in the second heat treatment, so that the internal structure of fresh martensite The division rate of the site increased, and the balance of strength, elongation, and hole expansion rate was poor.
[0187]
　In the steel sheet of Experimental Example No. 38, since the cooling stop temperature in the first heat treatment was high, the number ratio of retained austenite having an aspect ratio of 2.0 or more was insufficient, and the balance between strength, elongation, and hole expansion ratio was poor.
　Since the steel sheet of Experimental Example No. 43 had a slow cooling rate in the second heat treatment, the total fraction of pearlite and cementite in the internal structure of the steel sheet was large, and the balance of strength, elongation, and hole expansion ratio was poor.
[0188]
　Since the maximum heating temperature of the steel sheet of Experimental Example No. 64 was low in the second heat treatment, the retained austenite fraction in the internal structure of the steel sheet was insufficient, and the balance between strength, elongation, and hole expansion ratio was poor.
　Since the steel sheet of Experimental Example No. 65 has a large log (PH 2 O / PH 2 ) in the second heat treatment, the soft layer thickness in the surface layer structure of the steel sheet becomes thick and the maximum tensile stress (TS) becomes insufficient. It was.
[0189]
　The chemical composition of the steel sheets of Experimental Examples Nos. 71 to 75 is outside the scope of the present invention. The steel sheet of Experimental Example No. 71 had an insufficient maximum tensile stress (TS) because the C content was insufficient. Since the steel sheet of Experimental Example No. 72 had a high Nb content, the bendability after processing was deteriorated. The steel sheet of Experimental Example No. 73 had an insufficient maximum tensile stress (TS) because the Mn content was insufficient. Since the steel sheet of Experimental Example No. 74 had a high Si content, the hole-expandability was deteriorated. Since the steel sheet of Experimental Example No. 75 had a large Mn content and P content, the elongation and hole expansion properties were deteriorated.
[0190]
　Since the steel sheets of Experimental Examples No. 15', 22', 34', 52', and 53' did not undergo the first heat treatment and did not contain hard ferrite in the metal structure, the balance between strength, elongation, and hole expansion ratio was balanced. Has gotten worse.
[0191]
　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. It was.
[0192]
　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 became unbalanced.
[0193]
　The steel sheets of Experimental Examples No. 6', 20', 21', and 29' had a high log (PH 2 O / PH 2 ) in the first heat treatment , and the desired surface structure could not be obtained. The hydrogen brittleness resistance was poor.
[0194]
　The steel sheet of Experimental Example No. 8'had a high fraction of soft ferrite because the cooling rate in the first heat treatment was slow. For this reason, the balance between strength, elongation, and hole expansion rate became poor.
[0195]
　The steel sheets of Experimental Examples No. 9', 20', 22', 25', 29', 30', 53', and 56' have a low log (PH 2 O / PH 2 ) in the second heat treatment , and are desired surface layers. Since no structure was obtained, the hydrogen embrittlement resistance of the bent portion was poor.
[0196]
　Regarding the steel sheets of Experimental Examples No. 9', 22', 25', 30', 53', and 56', since no soft layer was formed on the surface layer structure of the steel sheet and there was no internal oxidation peak, the steel sheets adhered to the plating. The evaluation of sex was "B".
[0197]
　Since the steel sheet of Experimental Example No. 31'has a high maximum temperature reached in the second heat treatment, the metal structure does not contain hard ferrite, and the desired surface layer structure cannot be obtained. Therefore, the strength, elongation, and hole expansion ratio are not obtained. The balance was poor, and the hydrogen embrittlement resistance of the bent part was poor.
[0198]
　Since the steel sheet of Experimental Example No. 40'was insufficient in the holding time between 300 ° C. and 480 ° C. in the second heat treatment, the fraction of fresh martensite in the internal structure increased, and the strength, elongation, and hole expansion ratio increased. I'm out of balance.
[0199]
　Since the steel sheet of Experimental Example No. 43'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. It was.
[0200]
　Since the cooling rate of the steel sheet of Experimental Example No. 48'was slow in the second heat treatment, the total fraction of pearlite and cementite in the internal structure of the steel sheet became large, and the balance of strength, elongation, and hole expansion ratio became poor. ..
[0201]
　Since the maximum temperature reached in the second heat treatment of the steel sheet of Experimental Example No. 69'was low, the retained austenite fraction in the internal structure of the steel sheet was insufficient, and the balance between strength, elongation, and hole expansion ratio became poor.
[0202]
　Since the steel sheet of Experimental Example No. 70'has a large log (PH 2 O / PH 2 ) in the second heat treatment, the soft layer thickness in the surface layer structure of the steel sheet becomes thick, and the maximum tensile stress (TS) is insufficient. became.
[0203]
　The chemical composition of the steel sheets of Experimental Examples No. 76'to 80'is outside the scope of the present invention. Of these, the steel sheet of Experimental Example No. 76'had insufficient maximum tensile stress (TS) because the C content was insufficient. Since the steel sheet of Experimental Example No. 77'has a large Nb content, the bendability after processing is deteriorated. The steel sheet of Experimental Example No. 78'had insufficient maximum tensile stress (TS) due to insufficient Mn content. Since the steel sheet of Experimental Example No. 79'has a large Si content, the hole expandability is deteriorated. Since the steel sheet of Experimental Example No. 80'has a large Mn content and P content, the elongation and hole expandability are deteriorated.
[0204]
　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 appended claims, and can be appropriately modified within the scope.
Industrial applicability
[0205]
　According to the present invention, it is possible to provide a high-strength steel sheet having excellent ductility and hole expanding property, excellent chemical conversion processability, plating adhesion, and good bendability after processing, and a method for producing the same.
　Since the steel sheet of the present invention is excellent in ductility and hole expanding property and has good bendability after processing, it is suitable as a steel sheet for automobiles which is formed into various shapes by press working or the like. Further, since the steel sheet of the present invention is excellent in chemical conversion treatment property and plating adhesion, it is suitable for a steel sheet in which a chemical conversion treatment film or a plating layer is formed on the surface.
Description of the sign
[0206]
　1 Steel plate
　11 Range from 1/8 thickness position to 3/8 thickness centered on 1/4 thickness position from the surface of the steel sheet (inside the steel sheet)
　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%,
REM: 0% to 0.0100%,
and the balance is from Fe and impurities.
　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 of, and the body
　　integration ratio is soft ferrite: 0% to 30%,
　　residual. Austenite: 3% to 40%,
　　fresh martensite: 0% to 30%,
　　total of pearlite and cementite: 0% to 10%
, the balance containing hard ferrite,
　1/8 to 3/8 thickness In the above range, the number ratio of retained austenite having an aspect ratio of 2.0 or more to the total retained austenite is 50% or more, and
　80% or less of the hardness in the above range of 1/8 thickness to 3/8 thickness. When the region having hardness is defined as a soft layer, there is a soft layer having a thickness of 1 to 100 μm in the plate thickness direction from the surface
　, and among the ferrites contained in the soft layer, crystals having an aspect ratio of 3.0 or more. The body integration rate of the grains is 50% or more, and the
　body integration rate of retained austenite in the soft layer is 80% or less of the body integration rate of retained austenite in the above range of 1/8 thickness to 3/8 thickness. ,
　When the emission intensity of the wavelength indicating Si is analyzed from the surface in the plate thickness direction by the high-frequency glow discharge analysis method, the emission of the wavelength indicating Si is emitted in the range of more than 0.2 μm and not more than 10.0 μm from the surface.
A steel sheet characterized by the appearance of a peak in strength .
[Claim 2]
　The chemical composition contains one or more of
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 is characterized by the above.
[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. 00%,
Mo: 0.001% to 1.00%,
W: 0.001% to 1.00%,
B: 0.0001% to 0.0100%
of one or more. The steel plate according to claim 1 or 2, which is characterized.
[Claim 4]
Any of claims 1 to 3, 　wherein the chemical composition contains one or two of
Sn: 0.001% to 1.00% and
Sb: 0.001% to 1.00%.
The steel plate described in item 1.
[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%,
REM: 0.0001% to 0.0100%
The
steel sheet according to any one of claims 1 to 4, wherein the steel sheet contains one or more of the above.
[Claim 6]
　The steel sheet according to any one of claims 1 to 5, wherein the chemical composition satisfies the following formula (i).
　Si + 0.1 × Mn + 0.6 × Al ≧ 0.35 ... (i)
(Si, Mn and Al in the formula (i) are the contents of each element in mass%).
[Claim 7]
　The steel sheet according to any one of claims 1 to 6, further comprising a hot-dip galvanized layer or an electrogalvanized layer on the surface.
[Claim 8]
　A method for producing a steel sheet according to any one of
　claims 1 to 6, 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 producing a steel sheet, which comprises applying heat treatment.
(A) From 650 ° C. to reaching the maximum heating temperature, the atmosphere is such that it contains 0.1% by volume or more of H 2 and satisfies the following formula (ii).
(B) Ac3 Hold at the maximum heating temperature of -30 ° C to 1000 ° C for 1 second to 1000 seconds.
(C) Heating is performed so that the average heating rate in the temperature range from 650 ° C. to the maximum heating temperature is 0.5 ° C./sec to 500 ° C./sec.
(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.
(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.
(A) From 650 ° C. to reaching the maximum heating temperature, H 2 is 0.1% by volume or more, O 2 is 0.020% by volume or less, and the log (PH 2 O / PH 2 ) is the following formula ( PH 2 O / PH 2 ). Create an atmosphere that satisfies iii).
(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 so that the average heating rate from 650 ° C. to the maximum heating temperature is 0.5 ° C./sec to 500 ° C./sec.
(D) Cool so that the average cooling rate in the temperature range of 700 to 600 ° 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.
　log (PH 2 O / PH 2 ) <
　-1.1 ... (ii) -1.1 ≤ log (PH 2 O / PH 2 ) ≤ -0.07 ... (iii)
(Equation (ii) And in formula (iii), PH 2 O indicates the partial pressure of water vapor and PH 2 indicates the partial pressure of hydrogen.)
[Claim 9]
　The method for producing a steel sheet according to claim 8, wherein the hot-dip galvanizing treatment is performed at a stage after the cooling process of (D).