[0001]The present invention relates to a hot-dip galvanized steel sheet and a method for manufacturing the same, and is mainly related to a high-strength hot-dip galvanized steel sheet and a method for manufacturing the same, which are formed into various shapes by press working or the like as a steel sheet for automobiles.
Background technology
[0002]
　In recent years, there has been a demand for improved fuel efficiency of automobiles from the viewpoint of greenhouse gas emission regulations associated with global warming countermeasures, and the application of high-strength steel sheets is expanding more and more in order to reduce the weight of the vehicle body and ensure collision safety. be. In particular, recently, there is an increasing need for ultra-high strength steel sheets having a tensile strength of 980 MPa or more. Further, a high-strength hot-dip galvanized steel sheet having a hot-dip galvanized surface is required for a portion of the vehicle body where rust prevention is required.
[0003]
　Hot-dip galvanized steel sheets used for automobile parts are required to have various workability required for component molding, such as press formability and weldability as well as strength. Specifically, from the viewpoint of press formability, the steel sheet is required to have excellent elongation (total elongation in the tensile test: El), elongation flangeability (hole expansion ratio: λ), and bendability.
[0004]
　Generally, as the strength of a steel sheet increases, the press formability deteriorates. A TRIP steel sheet (TRansformation Induced Plasticity) utilizing the transformation-induced plasticity of retained austenite is known as a means for achieving both high strength of steel and press formability.
[0005]
　Patent Documents 1 to 3 disclose a technique relating to a high-strength TRIP steel sheet in which the structure composition fraction is controlled within a predetermined range to improve the elongation and the hole expansion ratio.
[0006]
　Further, a TRIP type high-strength hot-dip galvanized steel sheet is also disclosed in some documents.
[0007]
　Normally, in order to produce a hot-dip galvanized steel sheet in a continuous annealing furnace, the steel sheet is heated and leveled in the reverse transformation temperature range (> Ac1), and then cooled to room temperature at about 460 ° C. It is necessary to immerse in a hot dip galvanizing bath. Alternatively, after heating and soaking heat treatment and cooling to room temperature, it is necessary to reheat the steel sheet to the hot-dip galvanizing bath temperature and immerse it in the bath. Further, in order to produce an alloyed hot-dip galvanized steel sheet, it is usually necessary to reheat the steel sheet to a temperature range of 460 ° C. or higher because the alloying treatment is performed after immersion in the plating bath. For example, in Patent Document 4, austenite is stabilized by heating the steel plate to Ac1 or higher, rapidly cooling it to the martensitic transformation start temperature (Ms) or lower, reheating it to the bainite transformation temperature range, and holding it in the temperature range. It is described that after advancing the austenite), it is reheated to the plating bath temperature or the alloying treatment temperature for the plating alloying treatment. However, in such a manufacturing method, martensite and bainite are excessively tempered in the plating alloying treatment step, so that there is a problem that the material is deteriorated.
[0008]
　Patent Documents 5 to 9 disclose a method for producing a hot-dip galvanized steel sheet, which comprises reheating martensite by cooling and reheating the steel sheet after the plating alloying treatment.
[0009]
　As a technique for improving the bending workability of a high-strength steel sheet, for example, Patent Document 10 describes a high-strength cold-rolled steel sheet whose surface layer portion is mainly made of ferrite, which is produced by decarburizing the steel sheet. There is. Further, Patent Document 11 describes an ultra-high-strength cold-rolled steel sheet having a soft layer on the surface layer, which is produced by decarburizing and annealing a steel sheet.
Prior art literature
Patent documents
[0010]
Patent Document 1: International Publication No. 2013/051238
Patent Document 2: Japanese Patent Application Laid-Open No. 2006-104532
Patent Document 3: Japanese Patent Application Laid-Open No. 2011-184757
Patent Document 4: International Publication No. 2014/20640
Patent Document 5: Japanese Patent Application Laid-Open No. 2013-144830 JP
Patent Document 6: WO 2016/113789 Patent
Patent Document 7: WO 2016/113788
Patent Document 8: WO 2016/171237 Patent
Patent Document 9: JP 2017-48412 JP
Patent Document 10: Japanese Patent Application Laid-Open No. 10-130782
Patent Document 11: Japanese Patent Application Laid-Open No. 5-195149
Outline of the invention
Problems to be solved by the invention
[0011]
　However, when the bending workability of the steel sheet is improved by softening the surface layer of the steel sheet as described above, the bending deformation load of the member is originally expected from the strength of the steel sheet depending on the deformation mode of the member at the time of collision deformation. It may be lower than the deformation load (that is, the deformation load when the surface layer of the steel sheet is not softened). Generally, when a steel sheet undergoes bending deformation, the generated plastic strain increases toward the surface of the steel sheet. That is, the degree of contribution to the deformation load is greater on the surface of the steel sheet than on the inside of the steel sheet. Therefore, when the deformation of the member at the time of collision deformation becomes bending deformation, the deformation load of the member may decrease due to the softening of the steel plate surface.
[0012]
　The present invention has been made in view of the above background, and an object of the present invention is to provide a hot-dip galvanized steel sheet which is excellent in press formability and suppresses a load reduction during bending deformation and a method for producing the same.
Means to solve problems
[0013]
　As a result of diligent studies to solve the above problems, the present inventors have obtained the following findings.
　(I) In the continuous hot-dip galvanizing heat treatment step, martensite is generated by cooling to Ms or less after the plating treatment or the plating alloying treatment. After that, martensite can be appropriately tempered by reheating and isothermal maintenance, and in the case of a steel sheet containing retained austenite, the retained austenite can be further stabilized. By such a heat treatment, martensite is not excessively tempered by the plating treatment or the plating alloying treatment, so that the balance between strength and ductility is improved.
　(Ii) It is well known that it is effective to perform decarburization treatment to soften the surface layer portion in order to improve the bendability of the high-strength steel sheet. However, when the surface layer portion is softened, the bending deformation load may be lower than the deformation load expected from the strength of the steel sheet in some cases. In order to solve this problem, the present inventors may limit the rate of change (increase rate) in the thickness direction of the area ratio of martensite, which is a hard structure, from the surface of the steel sheet to the inside of the steel sheet to a predetermined value or less. We found that we could overcome the above problems. Further, in order to realize such metallographic structure control, in the continuous hot-dip galvanizing heat treatment step, first, the steel sheet is heated to a high temperature range of 650 ° C. or higher, and the atmosphere in the furnace is removed to the surface layer as a high oxygen potential. Form a coal region. After that, the steel sheet is cooled to a low temperature range of 600 ° C. or lower, and the atmosphere in the furnace is set to a low oxygen potential to maintain an isothermal temperature for a certain period of time or longer. By maintaining the isothermal temperature, carbon atoms inside the steel sheet are appropriately diffused in the decarburized region of the surface layer. As a result, it was found that the rate of change in the area ratio of the finally formed martensite in the plate thickness direction becomes gradual as compared with the case where the isothermal maintenance is not performed. However, this isothermal holding step needs to be carried out before the step of cooling to Ms or less described in (i). This is because when austenite is transformed into martensite, solute carbon is precipitated in martensite as carbide, so that carbon atoms do not re-diffuse from the inside of the steel sheet to the surface layer of the steel sheet.
　(Iii) Furthermore, it has been found that the effect of (ii) is manifested when the cold rolling conditions before the continuous hot-dip galvanizing heat treatment step are within a predetermined range. Although the details are not clear, it is considered that the shear strain applied to the surface layer of the steel sheet is increased by limiting the cold rolling conditions to a predetermined range. When a steel sheet having such surface strain is annealed in a continuous hot-dip galvanizing heat treatment step, the surface structure of the steel sheet becomes finer. That is, the area of ​​the crystal grain boundaries increases at the surface layer of the steel sheet. Since the grain boundaries act as diffusion paths for carbon atoms, it is considered that as a result of the increase in the area of ​​the crystal grain boundaries, carbon atoms are likely to re-diffuse into the surface layer when the temperature is maintained at an isothermal temperature of 600 ° C. or lower.
[0014]
　The present invention has been made based on the above findings, and the specifics are as follows.
　(1) A hot-dip zinc-plated steel sheet having a hot-dip zinc-plated layer on at least one surface of the base steel sheet, wherein the base steel sheet has a mass% of
　C: 0.050% to 0.350%,
　Si: 0.10% to 2.50%,
　Mn: 1.00% to 3.50%,
　P: 0.050% or less,
　S: 0.0100% or less,
　Al: 0.001% to 1.500%,
　N: 0.0100% or less,
　O: 0.0100% or less,
　Ti: 0% to 0.200%,
　B: 0% to 0.0100%,
　V: 0% to 1.00%,
　Nb: 0% ~ 0.100%,
　Cr: 0% to 2.00%,
　Ni: 0% to 1.00%,
　Cu: 0% to 1.00%,
　Co: 0% to 1.00%,
　Mo: 0% ~ 1.00%,
　W: 0% to 1.00%,
　Sn: 0% to 1.00%,
　Sb: 0% to 1.00%,
　Ca: 0% to 0.0100%,
　Mg: 0% to 0.0100%,
　Ce: 0% to 0.0100%,
　Zr: 0% to 0.0100%,
　La: 0% to 0.0100%,
　Hf: 0% to 0.0100%,
　Bi: 0% to 0.0100%, and
　REM other than Ce and La: 0% to 0.0100%
, the balance has a chemical composition of Fe and impurities, and
　1 from the surface of the base steel sheet. Steel structure in the range of 1/8 to 3/8 thickness centered on the position of / 4 thickness is area%,
　ferrite: 0% to 50%,
　retained austenite: 0% to 30%,
　tempered martensite. : 5% or more,
　fresh martensite: 0% to 10%, and
　total of pearlite and cementite: 0% to 5%
, and if residual tissue is present, the residual tissue is composed of baynite and the
　mother. When a region having a hardness of 90% or less with respect to the hardness at a position 1/4 thickness on the base steel plate side from the interface between the material steel plate and the molten zinc plating layer is defined as a soft layer, the base material is formed from the interface. A soft layer having a thickness of 10 μm or more exists on the steel plate side, and the soft layer
　contains tempered martensite and has a thickness of 10 μm or more .
　A hot-dip galvanized steel sheet, characterized in that the rate of increase in the area% of the area% of the tempered martensite from the interface in the soft layer to the inside of the base steel sheet in the plate thickness direction is 5.0% / μm or less.
　(2) The hot-dip galvanized steel sheet according to (1) above, wherein the steel structure further contains retained austenite: 6% to 30% in an area%.
　(3) A hot-rolling step of hot-rolling a slab having the chemical composition described in (1) above, a cold-rolling step of cold-rolling the obtained hot-rolled steel sheet, and melting into the obtained cold-rolled steel sheet.
　A method for producing a hot-dip zinc-plated steel sheet including a hot-dip zinc-plating step of performing zinc plating, wherein (A) the cold rolling step is the following conditions (A1) and (A2):
　　(A1) Performing cold rolling at least once while satisfying the formula (1) and having a rolling reduction ratio of 6% or more,
　　　13 ≤ A / B ≤ 35 ... (1)
(In the formula, A is the rolling wire load. (Kgf / mm), where B is the tensile strength (kgf / mm 2 ) of the hot-rolled steel sheet .)
　　(A2)
Satisfied with the total cold reduction rate of 30 to 80% ,
　(B) In the hot-dip zinc plating process, the steel plate is heated to perform the first leveling heat treatment, the first leveling heat-treated steel plate is first cooled and then the second leveling heat treatment is performed, and the second leveling heat-treated steel plate is subjected to a hot-dip zinc plating bath. The following conditions (B1) to (B6) are further included, including immersion in, second cooling of the plated steel sheet, and heating of the second cooled steel sheet and then third leveling heat treatment. :
　　(B1) When heating a steel plate before the first leveling heat treatment, an average heating rate from 650 ° C. to Ac1 ° C. + 30 ° C. or higher and a maximum heating temperature of 950 ° C. or lower under an atmosphere satisfying the following formulas (2) and (3). Is 0.5 ° C./sec to 10.0 ° C./sec.
　　(B2) Hold the steel plate at the maximum heating temperature for 1 second to 1000 seconds (first leveling heat treatment),
　　(B3) first cooling.
　　The first cooled steel plate is placed under an atmosphere in which the average cooling rate in the temperature range from 700 to 600 ° C. is 10 to 100 ° C./sec, and (B4) the following equations (4) and (5) are satisfied. Hold in the range of 300 to 600 ° C for 80 to 500 seconds (second leveling heat treatment),
　　(B5) second cooling is performed to Ms-50 ° C or lower,
　　(B6) 200 second cooled steel plates.
The molten zinc according to (1) or (2) above, which satisfies the condition that it is heated to a temperature range of about 420 ° C. and then held in the temperature range for 5 to 500 seconds (third leveling heat treatment). Manufacturing method of plated steel plate.
　　　-1.10 ≤ log (PH 2 O / PH 2 ) ≤ -0.07 ... (2)
　　　0.010 ≤ PH 2 ≤ 0.150 ... (3)
　　　log (PH 2 O / PH 2)) < -1.10 ・・ ・ (4)
　　　0.0010 ≦ PH 2 ≦ 0.1500 ・ ・ ・ (5)
(In the formula, PH 2 O indicates the partial pressure of water vapor, and PH 2 indicates the partial pressure of hydrogen. Shows.)
The invention's effect
[0015]
　INDUSTRIAL APPLICABILITY According to the present invention, it is possible to obtain a hot-dip galvanized steel sheet which is excellent in press formability, specifically, ductility, hole widening property and bendability, and further suppresses load reduction during bending deformation.
A brief description of the drawing
[0016]
FIG. 1 shows a reference diagram of an SEM secondary electron image.
FIG. 2 is a temperature-thermal expansion curve when a heat cycle corresponding to the hot-dip galvanizing treatment according to the embodiment of the present invention is simulated by a thermal expansion measuring device.
[Fig. 3] Fig. 3 is a diagram schematically showing a test method for evaluating a bending deformation load.
Mode for carrying out the invention
[0017]

　The hot-dip galvanized steel sheet according to the embodiment of the present invention has a hot-dip galvanized layer on at least one surface of the base steel sheet, and the base steel sheet has a mass% of
　C: 0. 050% to 0.350%,
　Si: 0.10% to 2.50%,
　Mn: 1.00% to 3.50%,
　P: 0.050% or less,
　S: 0.0100% or less,
　Al: 0.001% to 1.500%,
　N: 0.0100% or less,
　O: 0.0100% or less,
　Ti: 0% to 0.200%,
　B: 0% to 0.0100%,
　V: 0% ~ 1.00%,
　Nb: 0% to 0.100%,
　Cr: 0% to 2.00%,
　Ni: 0% to 1.00%,
　Cu: 0% to 1.00%,
　Co: 0% ~ 1.00%,
　Mo: 0% to 1.00%,
　W: 0% to 1.00%,
　Sn: 0% to 1.00%,
　Sb: 0% to 1.00%,
　Ca: 0% ~ 0.0100%,
　Mg: 0% to 0.0100%,
　Ce: 0% to 0.0100%,
　Zr: 0% to 0.0100%,
　La: 0% to 0.0100%,
　Hf: 0% to 0.0100%,
　Bi: 0% to 0.0100%, and
　REM other than Ce and La: 0% to 0.0100%
, the balance has a chemical composition of Fe and impurities, and
　1 from the surface of the base steel sheet. Steel structure in the range of 1/8 to 3/8 thickness centered on the position of / 4 thickness is area%,
　ferrite: 0% to 50%,
　retained austenite: 0% to 30%,
　tempered martensite. : 5% or more,
　fresh martensite: 0% to 10%, and
　total of pearlite and cementite: 0% to 5%
, and if residual tissue is present, the residual tissue is composed of baynite and the
　mother. When a region having a hardness of 90% or less with respect to the hardness at a position 1/4 thickness on the base steel plate side from the interface between the material steel plate and the molten zinc plating layer is defined as a soft layer, the base material is formed from the interface. A soft layer having a thickness of 10 μm or more exists on the steel plate side, and the soft layer
　contains tempered martensite and has a thickness of 10 μm or more .
　It is characterized in that the rate of increase in the area% of the area% of the tempered martensite from the interface in the soft layer to the inside of the base steel sheet in the plate thickness direction is 5.0% / μm or less.
[0018]
"Chemical composition"
　First, the reason why the chemical composition of the base steel sheet (hereinafter, also simply referred to as a steel sheet) according to the embodiment of the present invention is limited as described above will be described. Unless otherwise specified, all "%" that define the chemical composition in this specification are "mass%". Further, in the present specification, "-" indicating a numerical range is used to mean that the numerical values ​​described before and after the numerical range are included as the lower limit value and the upper limit value unless otherwise specified.
[0019]
[C: 0.050% to 0.350%]
　C is an essential element for ensuring the strength of the steel sheet. Since the required high strength cannot be obtained if it is less than 0.050%, the C content is set to 0.050% or more. The C content may be 0.070% or more, 0.080% or more, or 0.100% or more. On the other hand, if it exceeds 0.350%, the workability and weldability are lowered, so the C content is set to 0.350% or less. The C content may be 0.340% or less, 0.320% or less, or 0.300% or less.
[0020]
[Si: 0.10% to 2.50%]
　Si is an element that suppresses the formation of iron carbide and contributes to the improvement of strength and moldability, but excessive addition deteriorates the weldability of the steel sheet. Therefore, the content is set to 0.10 to 2.50%. The Si content may be 0.20% or more, 0.30% or more, 0.40% or more or 0.50% or more, and / or 2.20% or less, 2.00% or less or 1. It may be 90% or less.
[0021]
[Mn: 1.00% to 3.50%]
　Mn (manganese) is a strong austenite stabilizing element and is an element effective for increasing the strength of steel sheets. Excessive addition deteriorates weldability and low temperature toughness. Therefore, the content is set to 1.00 to 3.50%. The Mn content may be 1.10% or more, 1.30% or more or 1.50% or more, and / or 3.30% or less, 3.10% or less or 3.00% or less. May be good.
[0022]
[P: 0.050% or less]
　P (phosphorus) is a solid solution strengthening element and is an element effective for increasing the strength of steel sheets, but excessive addition deteriorates weldability and toughness. Therefore, the P content is limited to 0.050% or less. It is preferably 0.045% or less, 0.035% or less, or 0.020% or less. However, in order to extremely reduce the P content, the cost of removing P is high, so the lower limit is preferably 0.001% from the viewpoint of economy.
[0023]
[S: 0.0100% or less]
　S (sulfur) is an element contained as an impurity and forms MnS in steel to deteriorate toughness and hole-expanding property. Therefore, the S content is limited to 0.0100% or less as a range in which the deterioration of toughness and hole expanding property is not remarkable. It is preferably 0.0050% or less, 0.0040% or less, or 0.0030% or less. However, in order to extremely reduce the S content, the desulfurization cost becomes high, so that the lower limit is preferably 0.0001% from the viewpoint of economy.
[0024]
[Al: 0.001% to 1.500%]
　Al (aluminum) is added at least 0.001% for deoxidation of steel. However, even if it is added excessively, the effect is saturated and not only the cost is increased, but also the transformation temperature of the steel is increased and the load during hot rolling is increased. Therefore, the amount of Al is limited to 1.500%. It is preferably 1.200% or less, 1.000% or less, or 0.800% or less.
[0025]
[N: 0.0100% or less]
　N (nitrogen) is an element contained as an impurity, and if the content exceeds 0.0100%, coarse nitrides are formed in the steel for bendability and hole expansion. Deteriorates sex. Therefore, the N content is limited to 0.0100% or less. It is preferably 0.0080% or less, 0.0060% or less, or 0.0050% or less. However, in order to extremely reduce the N content, the cost of removing N is high, so the lower limit is preferably 0.0001% from the viewpoint of economy.
[0026]
[O: 0.0100% or less]
　O (oxygen) is an element contained as an impurity, and when the content exceeds 0.0100%, a coarse oxide is formed in the steel to make it bendable and expand holes. Let me. Therefore, the O content is limited to 0.0100% or less. It is preferably 0.0080% or less, 0.0060% or less, or 0.0050% or less. However, from the viewpoint of manufacturing cost, the lower limit is preferably 0.0001%.
[0027]
　The basic chemical composition of the base steel sheet according to the embodiment of the present invention is as described above. Further, the base steel sheet may contain the following elements, if necessary.
[0028]
[V: 0% to 1.00%, Nb: 0% to 0.100%, Ti: 0% to 0.200%, B: 0% to 0.0100%, Cr: 0% to 2.00% , Ni: 0% to 1.00%, Cu: 0% to 1.00%, Co: 0% to 1.00%, Mo: 0% to 1.00%, W: 0% to 1.00% , Sn: 0% to 1.00% and Sb: 0% to 1.00%]
　V (vanadium), Nb (niobium), Ti (titanium), B (boron), Cr (chromium), Ni (nickel) , Cu (copper), Co (cobalt), Mo (molybdenum), W (tungsten), Sn (tin) and Sb (antimon) are all effective elements for increasing the strength of steel sheets. Therefore, one or more of these elements may be added as needed. However, excessive addition of these elements saturates the effect and causes an increase in cost. Therefore, the content thereof is V: 0% to 1.00%, Nb: 0% to 0.100%, Ti: 0% to 0.200%, B: 0% to 0.0100%, Cr: 0%. ~ 2.00%, Ni: 0% to 1.00%, Cu: 0% to 1.00%, Co: 0% to 1.00%, Mo: 0% to 1.00%, W: 0% It is set to 1.00%, Sn: 0% to 1.00%, and Sb: 0% to 1.00%. Each element may be 0.005% or more or 0.010% or more. In particular, the B content may be 0.0001% or more or 0.0005% or more.
[0029]
[Ca: 0% to 0.0100%, Mg: 0% to 0.0100%, Ce: 0% to 0.0100%, Zr: 0% to 0.0100%, La: 0% to 0.0100% , Hf: 0% to 0.0100%, Bi: 0% to 0.0100% and REM other than Ce and La: 0% to 0.0100%]
　Ca (calcium), Mg (magnesium), Ce (cerium) , Zr (zirconium), La (lanthanum), Hf (hafnium) and REM (rare earth element) other than Ce and La are elements that contribute to the fine dispersion of inclusions in steel, and Bi (bismus) is in steel. It is an element that reduces microsegregation of substitutional alloy elements such as Mn and Si. Since each contributes to the improvement of workability of the steel sheet, one or more of these elements may be added as needed. However, excessive addition causes deterioration of ductility. Therefore, the content is limited to 0.0100%. Moreover, each element may be 0.0005% or more or 0.0010% or more.
[0030]
　In the base steel sheet according to the embodiment of the present invention, the balance other than the above-mentioned elements is composed of Fe and impurities. The impurity is a component that is mixed due to various factors in the manufacturing process, including raw materials such as ore and scrap, when the base steel sheet is industrially manufactured, and is a mother according to an embodiment of the present invention. It includes components that are not intentionally added to the steel sheet. Impurities are elements other than the components described above, and are contained in the base steel sheet at a level at which the action and effect peculiar to the element do not affect the characteristics of the hot-dip galvanized steel sheet according to the embodiment of the present invention. It also includes elements.
[0031]
"Steel Structure Inside Steel Sheet"
　Next, the reason for limiting the internal structure of the base steel sheet according to the embodiment of the present invention will be described.
[0032]
[Ferrite: 0 to 50%]
　Ferrite has excellent ductility but a soft structure. In order to improve the elongation of the steel sheet, it may be contained depending on the required strength or ductility. However, if it is contained in an excessive amount, it becomes difficult to secure the desired steel sheet strength. Therefore, the content thereof is up to 50% in area%, and may be 45% or less, 40% or less, or 35% or less. The ferrite content may be 0% in area%, for example, 3% or more, 5% or more, or 10% or more.
[0033]
[Tempering martensite: 5% or more]
　Tempering martensite is a high-strength and tough structure, and is an essential metal structure in the present invention. At least 5% or more in area% is contained in order to balance strength, ductility and hole widening property at a high level. The area% is preferably 10% or more, and may be 15% or more or 20% or more. For example, the tempered martensite content may be 95% or less, 90% or less, 85% or less, 80% or less or 70% or less in area%.
[0034]
[Fresh martensite: 0 to 10%] In the
　present invention, fresh martensite refers to untempered martensite, that is, martensite containing no carbide. Since this fresh martensite has a brittle structure, it becomes a starting point of fracture during plastic deformation and deteriorates the local ductility of the steel sheet. Therefore, the content is 0 to 10% in area%. More preferably, it is 0 to 8% or 0 to 5%. The fresh martensite content may be 1% or more or 2% or more in area%.
[0035]
[Residual austenite: 0% to 30%]
　Retained austenite improves the ductility of a steel sheet by the TRIP effect of transforming it into martensite by work-induced transformation during deformation of the steel sheet. On the other hand, in order to obtain a large amount of retained austenite, it is necessary to contain a large amount of alloying elements such as C. Therefore, the upper limit of retained austenite is 30% in area%, and may be 25% or less or 20% or less. However, when it is desired to improve the ductility of the steel sheet, the content thereof is preferably 6% or more in area%, and may be 8% or more or 10% or more. When the content of retained austenite is 6% or more, the Si content in the base steel sheet is preferably 0.50% or more in mass%.
[0036]
[Total of pearlite and cementite: 0 to 5%] Since
　pearlite contains hard and coarse cementite and becomes a starting point of fracture during plastic deformation, it deteriorates the local ductility of the steel sheet. Therefore, the content thereof, together with cementite, may be 0 to 5% in area%, and may be 0 to 3% or 0 to 2%.
[0037]
　The remaining tissue other than the above tissue may be 0%, but if it is present, it is bainite. The bainite of the residual structure may be either upper bainite or lower bainite, or a mixed structure thereof.
[0038]
[A soft layer having a thickness of 10 μm or more exists on the base steel plate side from the interface between the base steel plate and the hot-dip galvanized layer]
　The base steel plate according to the present embodiment has a soft layer on its surface. In the present invention, the soft layer is a region in the base steel sheet having a hardness of 90% or less with respect to the hardness at a position 1/4 thickness on the base steel plate side from the interface between the base steel plate and the hot-dip galvanized layer. Is to say. The thickness of the soft layer is 10 μm or more. If the thickness of the soft layer is less than 10 μm, the bendability deteriorates. The thickness of the soft layer may be, for example, 15 μm or more, 18 μm or more, 20 μm or more or 30 μm or more, and / or 120 μm or less, 100 μm or less or 80 μm or less. The hardness (Vickers hardness) at a position 1/4 thick from the interface between the base steel plate and the hot-dip galvanized layer to the base steel plate side is generally 200 to 600 HV, for example, 250 HV or more or 300 HV or more. And / or 550 HV or less or 500 HV or less. Normally, the Vickers hardness (HV) is about 1 / 3.2 of the tensile strength (MPa).
[0039]
[The rate of increase in the thickness direction of the area% of the area% of the tempered martensite from the interface in the soft layer to the inside of the base steel sheet is 5.0% / μm or less] In the
　hot-dip galvanized steel sheet according to the embodiment of the present invention, the soft layer Contains tempered martensite, and the rate of increase in the area% of the area% of the tempered martensite from the interface between the base steel sheet and the hot-dip galvanized layer to the inside of the base steel sheet is 5.0% / μm or less. If it exceeds 5.0% / μm, the load decrease at the time of bending deformation becomes apparent. For example, the rate of increase in the plate thickness direction is 4.5% / μm or less, 4.0% / μm or less, 3.0% / μm or less, 2.0% / μm or less, or 1.0% / μm or less. It may be. The lower limit of the rate of increase in the plate thickness direction is not particularly limited, but may be, for example, 0.1% / μm or 0.2% / μm.
[0040]
　The steel structure fraction of the hot-dip galvanized steel sheet is evaluated by the SEM-EBSD method (electron backscatter diffraction method) and the SEM secondary electron image observation.
[0041]
　First, a sample is taken with the thickness cross section parallel to the rolling direction of the steel sheet as the observation surface at the center position in the width direction, and the observation surface is mechanically polished to a mirror surface, and then electrolytic polishing is performed. .. Next, in one or more observation fields in the range of 1/8 to 3/8 thickness centered on 1/4 thickness from the surface of the base steel plate on the observation surface, a total of 2.0 × 10 -9 m Crystal structure and orientation analysis of 2 or more areas is performed by the SEM-EBSD method. "OIM Analysis 6.0" manufactured by TSL Co., Ltd. is used for the analysis of the data obtained by the EBSD method. The distance between scores (step) is 0.03 to 0.20 μm. The region judged to be FCC iron from the observation results is defined as retained austenite. Further, a crystal grain boundary map is obtained with the boundary where the crystal orientation difference is 15 degrees or more as the grain boundary.
[0042]
　Next, nital corrosion is performed on the same sample in which the EBSD observation is performed, and the secondary electron image observation is performed in the same field of view as the EBSD observation. In order to observe the same field of view as in the EBSD measurement, it is advisable to add a mark such as a Vickers indentation in advance. From the obtained secondary electron images, the area fractions of ferrite, retained austenite, bainite, tempered martensite, fresh martensite, and pearlite are measured. A region having a substructure in the grain and in which cementite is precipitated with a plurality of variants, more specifically, two or more variants is judged to be tempered martensite (for example, reference in FIG. 1). See figure). The region where cementite is deposited in a lamellar manner is judged to be pearlite (or the sum of pearlite and cementite). The region where the brightness is low and the substructure is not recognized is determined to be ferrite (see, for example, the reference diagram of FIG. 1). Regions with high brightness and no underlying structure exposed by etching are judged to be fresh martensite and retained austenite (see, for example, the reference diagram of FIG. 1). A region that does not correspond to any of the above regions is judged to be bainite. Each area ratio is calculated by the point counting method to obtain the area ratio of each tissue. The area ratio of fresh martensite can be obtained by subtracting the area ratio of retained austenite obtained by the X-ray diffraction method.
[0043]
　The area ratio of retained austenite is measured by X-ray diffraction. In the range of 1/8 to 3/8 thickness centered on 1/4 thickness from the surface of the base steel sheet, the surface parallel to the plate surface is finished as a mirror surface, and the area ratio of FCC iron is measured by X-ray diffraction method. Then, it is used as the area ratio of retained austenite.
[0044]
　The rate of increase in the area% of the tempered martensite according to the embodiment of the present invention in the plate thickness direction is determined by the following method. First, with respect to the microstructure observation sample subjected to the nital corrosion, a microstructure photograph is taken for a region including a soft layer. For the microstructure photograph, the area fraction of tempered martensite was calculated every 10 μm from the interface between the base steel plate and the hot-dip galvanized layer to the inside of the steel sheet by the point counting method for the region of thickness 10 μm × width 100 μm or more. The rate of increase in the area% of the tempered martensite in the plate thickness direction is determined based on the value that has the maximum inclination in the soft layer by plotting the area fractions obtained in. For example, the slope between two points obtained by plotting the area fraction obtained in one region in the soft layer and the area fraction obtained in the region including the outside of the soft layer adjacent to the region is the maximum slope. If so, the inclination is determined as "the rate of increase in the area% of the area% of the tempered martensite from the interface in the soft layer to the inside of the base steel plate" in the plate thickness direction.
[0045]
　The hardness from the surface layer of the steel sheet to the inside of the steel sheet is measured by the following method. A sample is taken with the cross section parallel to the rolling direction of the steel sheet as the observation surface at the center position in the width direction, and the observation surface is polished to a mirror surface, and colloidal silica is used to remove the processed layer on the surface layer. Perform chemical polishing using. 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 1/4 of the plate thickness, a pitch of 10 μm in the thickness direction of the steel sheet. Then, a quadrangular pyramid-shaped Vickers indenter with an apex angle of 136 ° is pushed in with a load of 2 g. At this time, the Vickers indentations may interfere with each other depending on the size of the Vickers indentations. In such a case, the Vickers indenters are pushed in a staggered pattern to avoid mutual interference. The Vickers hardness is measured at 5 points for each thickness position, and the average value is taken as the hardness at that thickness position. A hardness profile in the depth direction is obtained by interpolating between each data with a straight line. The thickness of the soft layer is obtained by reading the depth position where the hardness is 90% or less of the hardness at the position where the hardness is 1/4 thickness from the hardness profile.
[0046]
(Hot-dip galvanized layer)
　The base steel sheet according to the embodiment of the present invention has a hot-dip galvanized layer on at least one surface, preferably both surfaces. The plating layer may be a hot-dip galvanizing layer or an alloyed hot-dip galvanizing layer having an arbitrary composition known to those skilled in the art, and may contain an additive element such as Al in addition to Zn. Further, the amount of adhesion of the plating layer is not particularly limited and may be a general amount of adhesion.
[0047]

　Next, a method for manufacturing a hot-dip galvanized steel sheet according to the embodiment of the present invention will be described. The following description is intended to illustrate a characteristic method for manufacturing a hot-dip galvanized steel sheet according to an embodiment of the present invention, and the hot-dip galvanized steel sheet is manufactured by a manufacturing method as described below. It is not intended to be limited to what is manufactured.
[0048]
　The method for producing a hot-dip zinc-plated steel sheet is a hot-rolling step of hot-rolling a slab having the same chemical composition as the chemical composition described above for the base steel sheet, and cold-rolling of the obtained hot-rolled steel sheet. Including the step and the hot-dip zinc plating step of applying hot-dip zinc plating to the obtained cold-rolled steel sheet,
　(A) the conditions of (A1) and (A2) that the cold rolling step is as follows:
　　(A1) The rolling wire load is Performing cold rolling at least once while satisfying the following formula (1) and having a rolling reduction of 6% or more,
　　　13 ≤ A / B ≤ 35 ... (1)
(In the formula, A is a rolling wire. It is a load (kgf / mm), and B is the tensile strength (kgf / mm 2 ) of the hot-rolled steel sheet .)
　　(A2)
Satisfied with the total cold reduction rate of 30 to 80% ,
　(B) In the hot-dip zinc plating step, the steel plate is heated to perform the first leveling heat treatment, the first leveling heat-treated steel plate is first cooled and then the second leveling heat treatment is performed, and the second leveling heat-treated steel plate is subjected to hot-dip zinc plating. It includes immersing in a bath, second-cooling the plated steel sheet, and heating the second-cooled steel sheet and then performing a third soaking heat treatment, and further comprising the following (B1) to (B6). Conditions:
　　(B1) When the steel sheet is heated before the first leveling heat treatment, the average up to the maximum heating temperature of 650 ° C. to Ac1 ° C. + 30 ° C. or higher and 950 ° C. or lower under an atmosphere satisfying the following formulas (2) and (3). The heating rate is 0.5 ° C./sec to 10.0 ° C./sec.
　　(B2) The steel plate is held at the maximum heating temperature for 1 second to 1000 seconds (first leveling heat treatment).
　　(B3) The average cooling rate in the temperature range from 700 to 600 ° C. in the first cooling is 10 to 100 ° C./sec, and
　　(B4) in an atmosphere satisfying the following equations (4) and (5). (1) Hold the cooled steel plate in the range of 300 to 600 ° C. for 80 to 500 seconds (second leveling heat treatment),
　　(B5) perform the second cooling to Ms-50 ° C. or lower, and
　　(B6) second. It is characterized in that the cooled steel plate is heated to a temperature range of 200 to 420 ° C. and then held in the temperature range for 5 to 500 seconds (third leveling heat treatment)
.
　　　-1.10 ≤ log (PH 2 O / PH 2 ) ≤ -0.07 ... (2)
　　　0.010 ≤ PH 2 ≤ 0.150 ... (3)
　　　log (PH 2 O / PH 2 ) <-1.10 ・ ・ ・ (4)
　　　0.0010 ≦ PH 2 ≦ 0.1500 ・ ・ ・ (5)
(In the formula, PH 2 O indicates the partial pressure of water vapor, and PH 2Indicates the partial pressure of hydrogen. )
[0049]
　Hereinafter, a method for manufacturing the hot-dip galvanized steel sheet will be described in detail.
[0050]
"Hot rolling process" In
　this method, the hot rolling process is not particularly limited and can be carried out under any suitable conditions. Therefore, the following description of the hot rolling process is intended to be merely exemplary and is intended to limit the hot rolling process in the method to those performed under specific conditions as described below. It's not something to do.
[0051]
　First, in the hot rolling step, a slab having the same chemical composition as that described above for the base steel sheet is heated before hot rolling. The heating temperature of the slab is not particularly limited, but is generally preferably 1150 ° C. or higher in order to sufficiently dissolve boride and carbides. The steel slab to be used is preferably cast by a continuous casting method from the viewpoint of manufacturability, but may be manufactured by an ingot forming method or a thin slab casting method.
[0052]
[Rough rolling] In
　this method, for example, a heated slab may be roughly rolled before finish rolling in order to adjust the plate thickness or the like. Such rough rolling is not particularly limited, but it is preferable to carry out such rough rolling so that the total rolling reduction at 1050 ° C. or higher is 60% or higher. If the total reduction ratio is less than 60%, recrystallization during hot rolling becomes insufficient, which may lead to inhomogeneization of the hot-rolled plate structure. The total reduction rate may be, for example, 90% or less.
[0053]
[Finish rolling inlet temperature: 900 to 1050 ° C, finish rolling outlet temperature: 850 ° C to 1000 ° C, and total
　rolling reduction: 70 to 95%] Finish rolling has a finish rolling inlet temperature of 900 to 1050 ° C. It is preferable that the rolling out side temperature is 850 ° C. to 1000 ° C. and the total reduction rate is 70 to 95%. If the finish-rolled inlet temperature is below 900 ° C, the finish-rolled exit temperature is below 850 ° C, or the total reduction rate is above 95%, the texture of the hot-rolled steel sheet develops, resulting in the final product plate. Anisotropy may become apparent. On the other hand, when the finish-rolled inlet-side temperature exceeds 1050 ° C., the finish-rolled exit-side temperature exceeds 1000 ° C., or the total reduction rate falls below 70%, the crystal grain size of the hot-rolled steel sheet becomes coarse and the final product It may lead to coarsening of the plate structure and deterioration of workability. For example, the finish rolling inlet temperature may be 950 ° C. or higher. The finish rolling output side temperature may be 900 ° C. or higher. The total reduction rate may be 75% or more or 80% or more.
[0054]
[Taking temperature: 450 to 680 ° C] The
　winding temperature is 450 to 680 ° C. If the take-up temperature is lower than 450 ° C., the hot-rolled plate strength becomes excessive and the cold rollability may be impaired. On the other hand, when the winding temperature exceeds 680 ° C., cementite becomes coarse and undissolved cementite remains, which may impair processability. The take-up temperature may be 500 ° C. or higher and / or 650 ° C. or lower.
[0055]
　In this method, the obtained hot-rolled steel sheet (hot-rolled coil) may be subjected to a treatment such as pickling, if necessary. The pickling method of the hot-rolled coil may follow a conventional method. Further, skin pass rolling may be performed in order to correct the shape of the hot-rolled coil and improve the pickling property.
[0056]
"(A) Cold rolling process"
[ Cold rolling with a rolling line load satisfying the formula (1) and a reduction rate of 6% or more is performed one or more times]
　Hot rolling obtained by this method. The steel sheet is subjected to a cold rolling process, in which the cold rolling is performed at least once when the rolling wire load satisfies the following formula (1) and the rolling reduction ratio is 6% or more. include.
　　　13 ≦ A / B ≦ 35 ... In the
　formula (1) , A is the rolled wire load (kgf / mm), and B is the tensile strength of the hot-rolled steel sheet (kgf / mm 2 ).
[0057]
　The cold rolling may be any of a tandem method in which a plurality of rolling stands are connected in series and a reverse mill method in which one rolling stand is reciprocated. In addition to the strength of the steel sheet before cold rolling, the rolled wire load includes the roughness of the steel sheet before cold rolling, the diameter of the work roll, the surface roughness of the work roll, the rotation speed of the work roll, the tension, the supply amount / temperature of the emulsion, and so on. It varies depending on various factors such as viscosity. However, an increase in the rolling wire load means an increase in the frictional force generated at the interface between the steel sheet and the work roll. As the frictional force increases, the shear strain applied to the surface layer of the steel sheet increases, recrystallization on the surface layer of the steel sheet is promoted during heating in the subsequent hot-dip galvanizing step, and the structure of the surface layer of the steel sheet becomes finer. The miniaturization of the structure means that the area of ​​the grain boundaries, which is the diffusion path of carbon, becomes large. As a result, the rediffusion of carbon atoms from the inside of the steel sheet to the surface layer is promoted during the second leveling heat treatment. In order to obtain this effect, it is necessary to control the rolling wire load so that the A / B is 13 or more and the rolling reduction ratio is 6% or more. On the other hand, if the rolling wire load becomes excessively large, the load on the cold rolling mill may increase and cause equipment damage. Therefore, the upper limit of A / B is set to 35. A / B may be 20 or more and / or 30 or less. Further, the reduction rate may be 10% or more and / or 25% or less. In the prior art, for example, in order to miniaturize the structure of the surface layer of a steel sheet, A (rolling line load) / B (tensile strength of hot-rolled steel sheet) is not controlled within a predetermined range. It has not been conventionally known that the structure of the surface layer of a steel sheet can be miniaturized by such control. This is because the rolled wire load changes depending on the capacity of the cold-rolled mill, and the tensile strength of the hot-rolled steel sheet also changes depending on the chemical composition, steel structure, etc., so these ratios, that is, the rolled wire load / heat This is because it is not easy to control the tensile strength of the rolled steel sheet within a desired range.
　The tensile strength of the hot-rolled steel sheet is measured by collecting JIS No. 5 tensile test pieces from the vicinity of the center of the width of the hot-rolled steel sheet with the plate width direction as the test piece longitudinal direction, and performing a tensile test in accordance with JIS Z2241: 2011. do. The rolling line load is usually measured constantly as an operation management index, but for example, a measuring meter such as a load cell equipped in the rolling mill may be used.
[0058]
[Total cold reduction rate: 30 to 80%] The
　total cold reduction rate is limited to 30 to 80%. If it is less than 30%, the accumulation of strain becomes insufficient and the above-mentioned surface layer structure miniaturization effect cannot be obtained. On the other hand, excessive rolling causes an excessive rolling load and increases the load on the cold rolling mill. Therefore, the upper limit thereof is preferably 80%. For example, the total cold reduction rate may be 40% or more and / or 70% or less or 60% or less.
[0059]
"(B) Hot-dip galvanizing step"
[In an atmosphere satisfying the formulas (2) and (3), the average heating rate from 650 ° C. to Ac1 + 30 ° C. or higher and the maximum heating temperature of 950 ° C. or lower: 0.5 to 10. 0 ° C./sec] In
　this method, after the cold rolling step, the obtained steel sheet is plated in the hot dip galvanizing step. In the hot-dip galvanizing step, the steel sheet is first heated and exposed to the first leveling heat treatment in an atmosphere satisfying the following formulas (2) and (3). When the steel sheet is heated, the average heating rate up to the maximum heating temperature of 650 ° C. to Ac1 + 30 ° C. or higher and 950 ° C. or lower is limited to 0.5 to 10.0 ° C./sec. If the heating rate exceeds 10.0 ° C./sec, recrystallization of ferrite does not proceed sufficiently, and the elongation of the steel sheet may deteriorate. On the other hand, if the average heating rate is less than 0.5 ° C./sec, the austenite becomes coarse, so that the finally obtained steel structure may become coarse. This average heating rate may be 1.0 ° C./sec or higher and / or 8.0 ° C./sec or lower or 5.0 ° C./sec or lower. In the present invention, the "average heating rate" refers to a value obtained by dividing the difference between 650 ° C. and the maximum heating temperature by the elapsed time from 650 ° C. to the maximum heating temperature.
[0060]
　The atmosphere in the furnace during heating satisfies the following equations (2) and (3). Here, the log (PH 2 O / PH 2 ) in the equation (2) is the logarithm of the ratio of the partial pressure of water vapor (PH 2 O) and the partial pressure of hydrogen (PH 2 ) in the atmosphere , and is also called oxygen potential. .. When the log (PH 2 O / PH 2 ) is less than -1.10, a soft layer of 10 μm or more is not formed on the surface layer of the steel sheet in the final structure. On the other hand, when the log (PH 2 O / PH 2 ) exceeds −0.07, the decarburization reaction proceeds excessively and the strength is lowered. In addition, the wettability with the plating may deteriorate, causing defects such as non-plating. When PH 2 is less than 0.010, oxides are formed on the outside of the steel sheet, the wettability with plating deteriorates, and defects such as non-plating may occur. The upper limit of PH 2 is 0.150 from the viewpoint of the risk of hydrogen explosion. For example, the log (PH 2 O / PH 2 ) may be greater than or equal to −1.00 and / or less than or equal to −0.10. Also, PH 2May be 0.020 or more and / or 0.120 or less.
　　　-1.10 ≤ log (PH 2 O / PH 2 ) ≤ -0.07 ... (2)
　　　0.010 ≤ PH 2 ≤ 0.150 ... (3)
[0061]
[First leveling heat treatment: Hold at a maximum heating temperature of Ac1 + 30 ° C. or higher and 950 ° C. or lower for 1 to 1000 seconds] In order to
　sufficiently promote austenitization, the steel sheet is heated to at least Ac1 + 30 ° C. or higher, and the temperature (maximum heating temperature). Perform soaking heat treatment with. However, if the heating temperature is raised excessively, not only the toughness deteriorates due to the coarsening of the austenite particle size, but also the annealing equipment is damaged. Therefore, the upper limit is 950 ° C, preferably 900 ° C. If the soaking time is short, austenitization does not proceed sufficiently, so it should be at least 1 second or longer. It is preferably 30 seconds or longer or 60 seconds or longer. On the other hand, if the soaking time is too long, the productivity is hindered, so the upper limit is 1000 seconds, preferably 500 seconds. The steel sheet does not necessarily have to be kept at a constant temperature during soaking, and may fluctuate within a range that satisfies the above conditions. "Retention" in the first leveling heat treatment and the second leveling heat treatment and the third leveling heat treatment described later means that the temperature is set to a predetermined temperature ± 20 ° C., preferably ±, within a range not exceeding the upper and lower limit values ​​specified in each leveling heat treatment. It means that the temperature is maintained within the range of 10 ° C. Therefore, for example, a heating or cooling operation that fluctuates within a temperature range specified in each soaking heat treatment by exceeding 40 ° C., preferably 20 ° C. by gradually heating or gradually cooling is an embodiment of the present invention. It is not included in the first, second and third leveling heat treatments according to the above.
[0062]
[First cooling: Average cooling rate in the temperature range of 700 to 600 ° C.: 10 to 100 ° C./sec]
　After holding at the maximum heating temperature, the first cooling is performed. The cooling shutdown temperature is 300 ° C. to 600 ° C., which is the temperature of the subsequent second leveling heat treatment. The average cooling rate in the temperature range of 700 ° C. to 600 ° C. is 10 to 100 ° C./sec. If the average cooling rate is less than 10 ° C./sec, the desired ferrite fraction may not be obtained. The average cooling rate may be 15 ° C./sec or higher or 20 ° C./sec or higher. Further, the average cooling rate may be 80 ° C./sec or less or 60 ° C./sec or less. In the present invention, the "average cooling rate" refers to a value obtained by dividing 100 ° C., which is the difference between 700 ° C. and 600 ° C., by the elapsed time from 700 ° C. to 600 ° C.
[0063]
[Second leveling heat treatment: Holds in the range of 300 to 600 ° C. for 80 to 500 seconds in an atmosphere satisfying the formulas (4) and (5)] Holds in
　the range of 300 to 600 ° C. for 80 to 500 seconds. The heat treatment is performed in order to make the atmosphere in the furnace have a low oxygen potential and appropriately re-diffuse the carbon atoms inside the steel sheet toward the decarburized region formed during the previous heating. If the temperature of the second leveling heat treatment is lower than 300 ° C. or the holding time is lower than 80 seconds, the rediffusion of carbon atoms becomes insufficient and the desired surface structure cannot be obtained. On the other hand, if the temperature of the second leveling heat treatment exceeds 600 ° C., the ferrite transformation proceeds and the desired ferrite fraction cannot be obtained. If the holding time exceeds 500 seconds, the bainite transformation proceeds excessively, so that the metal structure according to the embodiment of the present invention cannot be obtained. When the log (PH 2 O / PH 2 ) exceeds -1.10, decarburization proceeds and the desired surface structure cannot be obtained. Further, when PH 2 is less than 0.0010, oxides are formed on the outside of the steel sheet, and the wettability with plating deteriorates, which may cause defects such as non-plating. The upper limit of PH 2 is 0.1500 from the viewpoint of the risk of hydrogen explosion. For example, the log (PH 2 O / PH 2 ) may be −1.00 or less. Further, PH 2 may be 0.0050 or more and / or 0.1000 or less.
　　　log (PH 2 O / PH 2 ) < -1.10 ・ ・・ (4)
　　　0.0010 ≦ PH 2 ≦ 0.1500 ・ ・ ・ (5)
[0064]
　After the second leveling heat treatment, the steel sheet is immersed in hot dip galvanizing. The effect of the steel sheet temperature at this time on the steel sheet performance is small, but if the difference between the steel sheet temperature and the plating bath temperature is too large, the plating bath temperature may change and hinder the operation. It is desirable to provide a step of cooling the steel sheet in the range of 20 ° C. to the plating bath temperature + 20 ° C. Hot-dip galvanizing may be performed according to a conventional method. For example, the plating bath temperature may be 440 to 460 ° 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 Fe, Si, Mg, Mn, Cr, Ti and Pb as impurities. Further, it is preferable to control the basis weight of plating by a known method such as gas wiping. The basis weight is preferably 25 to 75 g / m 2 per side .
[0065]
[Alloying treatment]
　For example, a hot-dip galvanized steel sheet on which a hot-dip galvanized layer is formed may be alloyed, if necessary. In that case, if the alloying treatment temperature is less than 460 ° C., not only the alloying rate becomes slow and the productivity is impaired, but also the alloying treatment unevenness occurs. Therefore, the alloying treatment temperature is set to 460 ° C. or higher. On the other hand, if the alloying treatment temperature exceeds 600 ° C., alloying may proceed excessively and the plating adhesion of the steel sheet may deteriorate. In addition, pearlite transformation may proceed and a desired metal structure may not be obtained. Therefore, the alloying treatment temperature is set to 600 ° C. or lower.
[0066]
[Second cooling: Cooling to Ms-50 ° C or lower]
　Martensitic transformation start temperature (Ms) -50 ° C to transform part or most of austenite into martensite on the steel sheet after plating or plating alloying. Perform the second cooling to cool to the following. The martensite produced here is tempered by the subsequent reheating and the third soaking heat treatment to become tempered martensite. If the cooling shutdown temperature exceeds Ms-50 ° C., tempered martensite is not sufficiently formed, so that a desired metallographic structure cannot be obtained. If it is desired to utilize retained austenite to improve the ductility of the steel sheet, it is desirable to set a lower limit for the cooling shutdown temperature. Specifically, it is desirable to control the cooling shutdown temperature in the range of Ms-50 ° C to Ms-130 ° C.
[0067]
　The martensitic transformation in the present invention occurs after the ferrite transformation and the bainite transformation. C is distributed to austenite with the ferrite transformation and the bainite transformation. Therefore, it does not match the Ms when heated to austenite single phase and rapidly cooled. Ms in the present invention is obtained by measuring the thermal expansion temperature in the second cooling. For example, Ms in the present invention is hot-dip zinc from the start of hot-dip galvanizing heat treatment (equivalent to room temperature) to the second cooling using a device such as a Formaster tester that can measure the amount of thermal expansion during continuous heat treatment. It can be obtained by reproducing the heat cycle of the plating line and measuring the thermal expansion temperature in the second cooling. However, in the actual hot-dip galvanizing heat treatment, cooling may be stopped between Ms and room temperature, but when measuring thermal expansion, cooling is performed to room temperature. FIG. 2 is a temperature-thermal expansion curve when the heat cycle corresponding to the hot dip galvanizing treatment according to the embodiment of the present invention is simulated by a thermal expansion measuring device. The steel sheet undergoes linear heat shrinkage in the second cooling step, but deviates from the linear relationship at a certain temperature. The temperature at this time is Ms in the present invention.
[0068]
[Third leveling heat treatment: Hold for 5 to 500 seconds in a temperature range of 200 ° C. to 420 ° C.] After
　the second cooling, reheat to the range of 200 ° C. to 420 ° C. to perform the third leveling heat treatment. In this step, the martensite produced during the second cooling is burned back. If the holding temperature is less than 200 ° C. or the holding time is less than 5 seconds, tempering does not proceed sufficiently. On the other hand, since the bainite transformation does not proceed sufficiently, it becomes difficult to obtain a desired residual austenite amount. On the other hand, when the holding temperature exceeds 420 ° C. or the holding time exceeds 500 seconds, martensite is excessively tempered and the bainite transformation proceeds excessively, so that the desired strength and metallographic structure can be obtained. It will be difficult. The temperature of the third leveling heat treatment may be 240 ° C. or higher, or 400 ° C. or lower. Further, the holding time may be 15 seconds or more, 100 seconds or more, or 400 seconds or less.
[0069]
　After the third soaking heat treatment, it is cooled to room temperature to make a final product. Temper rolling may be performed for flattening of the steel sheet and adjustment of the surface roughness. In this case, the elongation rate is preferably 2% or less in order to avoid deterioration of ductility.
Example
[0070]
　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.
[0071]
　Steels having the chemical compositions shown in Table 1 were cast to prepare slabs. The rest other than the components shown in Table 1 are Fe and impurities. These slabs were hot-rolled under the conditions shown in Table 2 to produce hot-rolled steel sheets. Then, the hot-rolled steel sheet was pickled to remove the scale on the surface. Then, it was cold-rolled. The plate thickness after cold spreading was 1.4 mm. Further, the obtained steel sheet was subjected to continuous hot-dip galvanizing treatment under the conditions shown in Table 2 and alloying treatment was appropriately carried out. In each leveling heat treatment shown in Table 2, the temperature was maintained within the temperature range of ± 10 ° C. shown in Table 2. The composition of the base steel sheet obtained by analyzing the sample collected from the produced hot-dip galvanized steel sheet was equivalent to the composition of the steel shown in Table 1.
[0072]
[Table 1-1]

[0073]
[Table 1-2]

[0074]
[Table 2-1]

[0075]
[Table 2-2]

[0076]
[Table 2-3]

[0077]
[Table 2-4]

[0078]
[Table 2-5]

[0079]
[Table 2-6]

[0080]
　From the steel sheet thus obtained, a JIS No. 5 tensile test piece is collected from a direction perpendicular to the rolling direction, a tensile test is performed in accordance with JIS Z2241: 2011, and the tensile strength (TS) and total elongation (El) are measured. bottom. In addition, the "JFS T 1001 hole expansion test method" of the Japan Iron and Steel Federation standard was performed, and the hole expansion rate (λ) was measured. Those having TS of 980 MPa or more, TS × El × λ 0.5 / 1000 of 80 or more, and passing the following bending test are judged to have good mechanical properties and preferable press formability for use as automobile members. bottom.
[0081]
　In addition, a bending test was performed by the method specified in 238-100 of the German Association of the Automotive Industry (VDA) standard, and the maximum bending angle was measured. Good bendability with a bending angle of 90 degrees or more for those with a tensile strength of less than 1180 MPa, a bending angle of 80 degrees or more for those with a tensile strength of 1180 MPa or more and less than 1470 MPa, and a bending angle of 70 degrees or more for those with a tensile strength of more than 1470 MPa. It was judged that the result was acceptable (“◎” in Table 3).
[0082]
　In addition, a hat-shaped member having a closed cross-sectional shape as shown in FIG. 2 was produced, and a static three-point bending test was carried out. The maximum load at that time was measured. A value obtained by dividing the maximum load [kN] by the tensile strength [MPa] of 0.015 or more was regarded as acceptable (“⊚” in Table 3) as the load decrease during bending deformation was sufficiently suppressed.
[0083]
　The results are shown in Table 3. GA in Table 3 means hot-dip galvanized alloyed, and GI means hot-dip galvanized without alloying treatment.
[0084]
[Table 3-1]

[0085]
[Table 3-2]

[0086]
[Table 3-3]

[0087]
　In Comparative Example 4, the atmosphere in the furnace during the second leveling heat treatment in the hot-dip galvanizing step did not satisfy the formula (4). As a result, the desired surface structure was not obtained, and the maximum load during the three-point bending test was inferior. In Comparative Example 5, the atmosphere during heating in the hot-dip galvanizing step did not satisfy the formula (2). As a result, a soft layer was not formed and the bendability was inferior. In Comparative Example 7, the stop temperature of the second cooling in the hot-dip galvanizing step was more than Ms-50 ° C. As a result, tempered martensite was not obtained, and the tensile strength was less than 980 MPa. In addition, the maximum load during the three-point bending test was also inferior. In Comparative Example 8, the temperature of the third leveling heat treatment in the hot-dip galvanizing step was less than 200 ° C. As a result, the desired metal structure was not obtained, and the press moldability was inferior. In Comparative Example 13, the A / B (rolling line load / tensile strength) in the cold rolling step was less than 13. Further, in Comparative Example 32, the rolling reduction in the cold rolling step was less than 6%. As a result, the rate of increase in the area% of tempered martensite in the surface layer structure in the plate thickness direction was more than 5.0% / μm, and the maximum load during the three-point bending test was inferior. In Comparative Example 14, the temperature of the first leveling heat treatment in the hot-dip galvanizing step was less than Ac1 ° C. + 30 ° C., and the stop temperature of the second cooling was more than Ms-50 ° C. As a result, the desired metallographic structure was not obtained, and the press formability and the maximum load during the three-point bending test were inferior. In Comparative Example 15, the average cooling rate of the first cooling was less than 10 ° C./sec. As a result, ferrite was more than 50%, and the total of pearlite and cementite was more than 5%, and the press formability was inferior.
[0088]
　In Comparative Example 18, the holding time of the second leveling heat treatment was more than 500 seconds, and the stop temperature of the second cooling was more than Ms-50 ° C. As a result, the desired metal structure was not obtained, and the press moldability was inferior. In Comparative Example 22, the temperature of the second leveling heat treatment was over 600 ° C. As a result, ferrite was more than 50%, and the total of pearlite and cementite was more than 5%, and the press formability was inferior. In Comparative Example 23, the temperature of the second leveling heat treatment in the hot-dip galvanizing step was less than 300 ° C. As a result, the desired surface structure was not obtained, and the maximum load during the three-point bending test was inferior. In Comparative Example 27, the stop temperature of the second cooling in the hot-dip galvanizing step was more than Ms-50 ° C. As a result, the desired metallographic structure was not obtained, and the press formability and the maximum load during the three-point bending test were inferior. In Comparative Example 28, the holding time of the second leveling heat treatment was less than 80 seconds. As a result, the rate of increase in the area% of tempered martensite in the surface layer structure in the plate thickness direction was more than 5.0% / μm, and the maximum load during the three-point bending test was inferior. In Comparative Example 29, the holding time of the third leveling heat treatment in the hot-dip galvanizing step was less than 5 seconds. As a result, the amount of fresh martensite was more than 10%, and the press formability was inferior. In Comparative Example 33, the atmosphere during heating in the hot-dip galvanizing step did not satisfy the formula (2). Further, in Comparative Example 34, the hydrogen partial pressure at the time of heating did not satisfy the formula (3). Further, in Comparative Example 35, the hydrogen partial pressure at the time of the second leveling heat treatment did not satisfy the formula (5). As a result, non-plating occurred in these comparative examples. In Comparative Examples 57 to 62, since the chemical composition was not controlled within a predetermined range, a desired metal structure could not be obtained, and the press moldability was inferior. Further, in Comparative Examples 59 to 61, the toughness of the steel sheet was insufficient because the C, Si and Mn contents were excessive, and the test piece was brittlely fractured during the three-point bending test.
[0089]
　In contrast, the hot-dip galvanized steel sheet of the example had a tensile strength of 980 MPa or more, TS × El × λ 0.5 / 1000 of 80 or more, and the result of the three-point bending test was good. From this, it can be seen that the press formability is excellent, and the load reduction during bending deformation is suppressed. Further, when the hardness of the hot-dip galvanized steel sheets of Examples 10, 24, 31 and 39 was examined at a position of 1/4 thickness from the interface between the base steel sheet and the hot-dip galvanized layer to the base steel sheet side, the hardness was 315 HV, respectively. It was 394 HV, 390 HV and 487 HV.

WE CLAIMS

[Claim 1]A hot-dip galvanized steel sheet having a hot-dip galvanized layer on at least one surface of the base steel sheet, wherein the base steel sheet has a mass% of
　C: 0.050% to 0.350% and
　Si: 0.10. % To 2.50%,
　Mn: 1.00% to 3.50%,
　P: 0.050% or less,
　S: 0.0100% or less,
　Al: 0.001% to 1.500%,
　N: 0 .0100% or less,
　O: 0.0100% or less,
　Ti: 0% to 0.200%,
　B: 0% to 0.0100%,
　V: 0% to 1.00%,
　Nb: 0% to 0. 100%,
　Cr: 0% to 2.00%,
　Ni: 0% to 1.00%,
　Cu: 0% to 1.00%,
　Co: 0% to 1.00%,
　Mo: 0% to 1. 00%,
　W: 0% to 1.00%,
　Sn: 0% to 1.00%,
　Sb: 0% to 1.00%,
　Ca: 0% to 0.0100%,
　Mg: 0% to 0. 0100%,
　Ce: 0% to 0.0100%,
　Zr: 0% to 0.0100%,
　La: 0% to 0.0100%,
　Hf: 0% to 0.0100%,
　Bi: 0% to 0.0100%, and
　Ce, other than La REM: 0% ~ 0.0100%
contained, has a chemical composition the balance being Fe and impurities,
　with a focus on the position of 1/4 thickness from the surface of the base steel sheet 1 Steel structure in the range of / 8 to 3/8 thickness is, in area%,
　ferrite: 0% to 50%,
　retained austenite: 0% to 30%,
　tempered martensite: 5% or more,
　fresh martensite: 0. % To 10%, and
　total of pearlite and cementite: 0% to 5%,
and if there is a residual structure, the residual structure is
　composed of baynite, and the interface between the base steel plate and the hot martensite layer. When a region having a hardness of 90% or less with respect to the hardness at a position of 1/4 thickness on the base steel plate side is used as a soft layer, a soft layer having a thickness of 10 μm or more from the interface to the base steel plate side. Exists, the
　soft layer contains tempered martensite, and
　A hot-dip galvanized steel sheet, characterized in that the rate of increase in the area% of the area% of the tempered martensite from the interface in the soft layer to the inside of the base steel sheet in the plate thickness direction is 5.0% / μm or less.
[Claim 2]
　The hot-dip galvanized steel sheet according to claim 1, wherein the steel structure further contains retained austenite: 6% to 30% in an area%.
[Claim 3]
　A hot-rolling step of hot-rolling a slab having the chemical composition according to claim 1, a cold-rolling step of cold-rolling the obtained hot-rolled steel sheet, and hot-rolling zinc plating on the obtained cold-rolled steel sheet.
　A method for producing a hot-dip zinc-plated steel sheet including a hot-dip zinc-plating step, wherein (A) the cold rolling step is the following conditions (A1) and (A2):
　　(A1) satisfied, and applying cold rolling reduction ratio is 6% or more than
　　　once, 13 ≦ a / B ≦ 35 · · · (1)
(in the formula, a rolling line load (kgf / mm ), and, B is the tensile strength of the hot rolled steel sheet (kgf / mm 2 is).)
　　(A2) the total cold reduction ratio to be 30-80%
satisfied,
　(B) the hot-dip galvanizing process However, heating the steel plate to perform the first leveling heat treatment, first cooling the first leveling heat-treated steel plate and then performing the second leveling heat treatment, and immersing the second leveling heat-treated steel plate in a hot-dip zinc plating bath. The second cooling of the plated steel sheet and the heating of the second cooled steel sheet and then the third leveling heat treatment are included, and the following conditions (B1) to (B6):
　　(B1) When heating the steel sheet before the first leveling heat treatment, the average heating rate from 650 ° C. to Ac1 ° C. + 30 ° C. or higher and 950 ° C. or lower is 0. 5 ° C./sec to 10.0 ° C./sec,
　　(B2) Holding the steel plate at the maximum heating temperature for 1 second to 1000 seconds (first leveling heat treatment).
　　(B3) The average cooling rate in the temperature range from 700 to 600 ° C. in the first cooling is 10 to 100 ° C./sec, and
　　(B4) in an atmosphere satisfying the following equations (4) and (5). (1) Hold the cooled steel plate in the range of 300 to 600 ° C. for 80 to 500 seconds (second leveling heat treatment),
　　(B5) perform the second cooling to Ms-50 ° C. or lower, and
　　(B6) second. The invention according
to claim 1 or 2, wherein the cooled steel plate is heated to a temperature range of 200 to 420 ° C., and then held in the temperature range for 5 to 500 seconds (third leveling heat treatment). How to manufacture hot-dip zinc-plated steel sheet.
　　　-1.10 ≤ log (PH 2 O / PH 2 ) ≤ -0.07 ... (2)
　　　0.010 ≤ PH 2 ≤ 0.150 ... (3)
　　　log (PH 2 O / PH 2 ) <-1.10 ・ ・ ・ (4)
　　　0.0010 ≦ PH 2 ≦ 0.1500 ・ ・ ・ (5)
(In the formula, PH 2 O indicates the partial pressure of water vapor, and PH 2 indicates the partial pressure of hydrogen. )