Title of the invention: Galvanized steel sheet and its manufacturing method
Technical field
[0001]
　The present invention relates to a galvanized steel sheet and a method for producing the same.
　The present application claims priority based on Japanese Patent Application No. 2018-088418 filed in Japan on May 1, 2018, the contents of which are incorporated herein by reference.
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 measures against global warming, and the application of high-strength steel sheets is expanding more and more to reduce the weight of automobile bodies and ensure collision safety. is there. In particular, recently, there is an increasing need for ultra-high strength steel sheets having a tensile strength of 1470 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]
　When such a high-strength hot-dip galvanized steel sheet is used as a steel sheet for automobiles, it is formed into various shapes by press working or the like. However, when an ultra-high strength steel sheet having a tensile strength exceeding 1470 MPa is applied as an automobile member, it is necessary to solve hydrogen embrittlement cracking of the steel sheet as well as its press formability.
[0004]
　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.
[0005]
　Various attempts have been made to improve the hydrogen embrittlement resistance of steel sheets. An example of the study is shown below.
[0006]
　According to Patent Document 1 and Patent Document 2, a cold-rolled steel sheet having a predetermined chemical composition is heated to 3 points or more of Ac, and then hardened and tempered to obtain a martensite-based steel structure, which has hydrogen embrittlement resistance. The technology related to ultra-high-strength cold-rolled steel sheets that attempts to improve is described.
[0007]
　Patent Document 3 describes 120 kg / mm 2 or more , which attempts to improve hydrogen embrittlement resistance by containing a trace amount of Cu, Cr, Nb, Ni, etc. as a chemical composition and forming a bainite-based steel structure. A technique relating to a high-strength cold-rolled steel sheet having the tensile strength of is described.
[0008]
　In Patent Document 4, a steel sheet having a predetermined chemical composition is annealed on the surface layer and then heated to 3 points or more of Ac , and then quenched and tempered to obtain a tempered martensite-based structure. However, there is described a technique relating to a cold-rolled steel sheet having a tensile strength of 1270 MPa class or higher, which aims to improve bendability and delayed fracture resistance by softening the surface layer.
[0009]
　Patent Document 5 describes a high-strength cold-rolled steel sheet that attempts to improve hydrogen embrittlement resistance by controlling the amount and dispersion form of retained austenite contained in the steel structure and utilizing the hydrogen trapping effect of retained austenite. The technology related to is described.
[0010]
　When a hot-dip galvanized steel sheet is manufactured on a continuous hot-dip galvanized line, heat treatment is performed in a hydrogen-containing atmosphere for the purpose of reducing the surface of the steel sheet and ensuring wettability with hot-dip galvanized steel. At this time, hydrogen contained in the atmosphere penetrates into the steel sheet being heat-treated.
[0011]
　Normally, hydrogen atoms have a sufficiently high diffusion rate even at room temperature, and hydrogen in the steel sheet is released into the atmosphere in a short time. Therefore, in non-plated steel sheets, hydrogen that penetrates into the steel sheet during the manufacturing process is practically not a problem. .. However, in the case of a hot-dip galvanized steel sheet, hydrogen is hardly released to the atmosphere at room temperature because the hot-dip galvanized layer inhibits the release of hydrogen from the steel sheet to the atmosphere. Therefore, the hot-dip galvanized steel sheet is subjected to processing such as blanking and pressing while containing hydrogen that has entered during the steel sheet manufacturing, and is used as an automobile member. Hydrogen in steel is practically not a problem for low-strength steel sheets with a tensile strength of 780 MPa or less, but for hot-dip galvanized steel sheets with a tensile strength of 1470 MPa or more, it is caused by hydrogen in steel depending on the processing conditions and the stress applied. There is a risk of hydrogen embrittlement cracking.
[0012]
　However, from the viewpoint of suppressing hydrogen embrittlement cracking, there are few cases in which the amount of invading hydrogen in the hot-dip galvanized steel sheet is suppressed. Further, the present inventors have found that the hydrogen embrittlement resistance cannot be sufficiently improved by simply reducing the amount of invading hydrogen in the hot-dip galvanized steel sheet.
[0013]
　Patent Document 6 describes a technique relating to a hot-dip galvanized steel sheet in which the amount of hydrogen entering the steel sheet is reduced by controlling the atmosphere during heat treatment from the viewpoint of suppressing blisters. However, Patent Document 6 does not consider the mechanical properties and hydrogen embrittlement resistance of the steel sheet.
[0014]
　Patent Document 7 describes a technique relating to a high-strength galvanized steel sheet in which the amount of diffusible hydrogen in the base steel sheet is 0.00008% or less (0.8 ppm or less) in mass%. However, Patent Document 7 does not consider hydrogen embrittlement resistance.
Prior art literature
Patent documents
[0015]
Patent Document 1: Japanese Patent Application Laid-Open No. 10-001740
Patent Document 2: Japanese Patent Application Laid-Open No. 9-111398
Patent Document 3: Japanese Patent Application Laid-Open No. 6-145891
Patent Document 4: International Publication No. 2011/105385
Patent Document 5: Japanese Patent Application Laid-Open No. 2007-197819
Patent Document 6: International Publication No. 2015/029404
Patent Document 7: International Publication No. 2018/124157
Outline of the invention
Problems to be solved by the invention
[0016]
　In this way, although attempts have been made to improve the hydrogen embrittlement resistance of hot-dip galvanized steel sheets by various methods, efforts to reduce hydrogen invading during steel sheet manufacturing from the viewpoint of suppressing hydrogen embrittlement cracking have been made. Not done at all.
[0017]
　The present invention has been made in view of the above circumstances, and is a zinc-based plating preferably used for automobile members, which has excellent mechanical properties, reduces the amount of hydrogen invading during manufacturing, and has excellent hydrogen embrittlement resistance. An object of the present invention is to provide a steel plate and a method for producing the same. Another object of the present invention is to provide a zinc-based plated steel sheet having the above-mentioned various properties and having excellent plating adhesion, which is a characteristic generally required for a zinc-based plated steel sheet, and a method for producing the same. And. The plating adhesion refers to the adhesion between the steel sheet and the hot-dip galvanized layer or the alloyed hot-dip galvanized layer.
Means to solve problems
[0018]
　The gist of the present invention is as follows.
[1] The zinc-based plated steel sheet according to one aspect of the present invention includes a steel sheet and a zinc-based plated layer arranged on the surface of the steel sheet, and the steel
　sheet has a mass% of
C: 0.200% to 0.500%,
Si: 1.00% to 2.50%,
Mn: 1.50% to 5.00%,
P: 0.100% or less,
S: 0.0100% or less,
Al: 0.001 % To 1.000%,
N: 0.0100% or less,
O: 0.0100% or less,
Cr: 0% to 2.00%,
Mo: 0% to 1.00%,
B: 0% to 0. 010%,
Cu: 0% to 1.00%,
Ni: 0% to 1.00%,
Co: 0% to 1.00%,
W: 0% to 1.00%,
Sn: 0% to 1. 00%,
Sb: 0% to 0.50%,
Ti: 0% to 0.30%,
Nb: 0% to 0.30%,
V: 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%,
It
contains Bi: 0% to 0.0100% and REM: 0% to 0.0100%
, has a chemical composition with the balance consisting of Fe and impurities, and
　is centered on a thickness of 1/4 from the surface of the steel plate. Steel structure in the range of 1/8 to 3/8 thickness is by volume%,
ferrite: 0 to 10%,
baynite: 0 to 30%,
tempered martensite: 50% or more,
fresh martensite: 0 to 10%. ,
Residual austenite: more than 10% and 30% or less, and
pearlite: 0 to 5%, and the
　amount of hydrogen released when the steel sheet is heated from room temperature to 200 ° C. after removing the zinc-based plating layer is the steel sheet. It is 0.40 ppm or less per mass, the
　tensile strength is 1470
　MPa or more, and cracks do not occur in the U-shaped bending test in which a stress equivalent to 1000 MPa is applied for 24 hours.
[2] The zinc-based plated steel sheet according to the above [1] has a chemical composition of
Cr: 0.001% to 2.00%.
Mo: 0.001% to 1.00%,
B: 0.0001% to 0.010%,
Cu: 0.001% to 1.00%,
Ni: 0.001% to 1.00%,
Co:
Pasento ～ 1.00 0.001 Pasento,
W: 0.001 Pasento ～ 1.00 Pasento, Sn: 0.001 Pasento ～ 1.00 Pasento, and
Sb: 0.001% ~ 0.50%,
one of the Alternatively, it may contain two or more kinds.
[3] The zinc-based plated steel sheet according to the above [1] or [2] has a chemical composition of
Ti: 0.001% to 0.30% and
Nb: 0.001% to 0.30. %, And
V: 0.001% to 1.00%,
which may contain one or more.
[4] The zinc-based plated steel sheet according to any one of the above [1] to [3] has a chemical composition of
Ca: 0.0001% to 0.0100% and
Mg: 0.0001. % To 0.0100%,
Ce: 0.0001% to 0.0100%,
Zr: 0.0001% to 0.0100%,
La: 0.0001% to 0.0100%,
Hf: 0.0001% to 0.0100%,
Bi: 0.0001% to 0.0100% and
REM: 0.0001% to 0.0100%
may be contained in one or more.
[5] The zinc-based plated steel sheet according to any one of the above [1] to [4] may have a ductility-brittle transition temperature of −40 ° C. or lower.
[6] The method for producing a zinc-based plated steel sheet according to another aspect of the present invention is the method for producing a zinc-based plated steel sheet according to any one of the above [1] to [5]. ] To [4], the following steps (I) to (IV) are sequentially performed on the steel sheet having the chemical composition according to any one of the following items:
(I) Heating temperature: Ac 3 points to 950 ° C. , Ac Retention time in the temperature range of 3 points to 950 ° C: Annealing under the condition of 1 to 500 s, and when the steel sheet temperature reaches 600 ° C, retention in the temperature range of Ac 3 points to 950 ° C is completed. Annealing step in which the hydrogen concentration in the furnace is always maintained at 1.0 to 15.0% by volume;
(II) The temperature range of Ms point to 600 ° C. is maintained for 20 to 500 s. The first holding step of always maintaining the hydrogen concentration in the furnace at 1.0 to 10.0% by volume during the holding; (III) After immersing the steel sheet in the hot-dip galvanizing bath, the steel sheet temperature is Ms point-. Plating step of cooling to 150 ° C. to Ms point-30 ° C.; and
(IV) after holding for 50 to 1000 s in a temperature range of 330 to 430 ° C. in an atmosphere where the hydrogen concentration is less than 0.50% by volume. The second holding process of winding into a coil.
[7] In the method for producing a zinc-based plated steel sheet according to the above [6], in
　the step (III), the steel sheet is immersed in a hot-dip galvanized bath and then alloyed in a temperature range of 460 to 600 ° C. The step may be a step of cooling until the temperature of the steel sheet reaches the temperature range of Ms point −150 ° C. to Ms point −30 ° C.
Effect of the invention
[0019]
　According to the above aspect according to the present invention, zinc-based plating which is preferably used as an automobile member, has excellent mechanical properties, reduces the amount of invading hydrogen during manufacturing, and has excellent hydrogen embrittlement resistance and plating adhesion. A steel plate and a method for producing the same can be provided. According to a preferred embodiment of the present invention, it is possible to provide a galvanized steel sheet having the above-mentioned various properties and further excellent in low temperature toughness, and a method for producing the same.
A brief description of the drawing
[0020]
FIG. 1 is a schematic view illustrating a U-shaped bending test method for a steel plate.
Mode for carrying out the invention
[0021]
　The zinc-based plated steel sheet according to the present embodiment includes a steel sheet and a zinc-based plated layer arranged on the surface of the steel sheet. In the present embodiment, the zinc-based plated steel sheet refers to a hot-dip galvanized steel sheet or an alloyed hot-dip galvanized steel sheet, and the zinc-based plated layer is a hot-dip galvanized layer or an alloyed hot-dip galvanized steel sheet. Say that. Further, in the present embodiment, the steel sheet refers to a base steel sheet on which a zinc-based plating layer is arranged on the surface.
　The steel plate according to the present embodiment has C: 0.200% to 0.500%, Si: 1.00% to 2.50%, Mn: 1.50% to 5.00%, P: 0.100% or less, S: 0.0100% or less, Al: 0.001% to 1.000%, N: 0.0100% or less, O: 0.0100% or less, Cr: 0% to 2.00 %, Mo: 0% to 1.00%, B: 0% to 0.010%, Cu: 0% to 1.00%, Ni: 0% to 1.00%, Co: 0% to 1.00. %, W: 0% to 1.00%, Sn: 0% to 1.00%, Sb: 0% to 0.50%, Ti: 0% to 0.30%, Nb: 0% to 0.30 %, V: 0% to 1.00%, Ca: 0% to 0.0100%, Mg: 0% to 0.0100%, Ce: 0% to 0.0100%, Zr: 0% to 0.0100 %, La: 0% to 0.0100%, Hf: 0% to 0.0100%, Bi: 0% to 0.0100% and REM: 0% to 0.0100%, and the balance is Fe and impurities. It has a chemical composition consisting of.
　The steel sheet according to the present embodiment has a steel structure in the range of 1/8 to 3/8 thickness centered on 1/4 thickness from the surface, in terms of volume%, ferrite: 0 to 10%, bainite: 0 to 30. %, Tempered martensite: 50% or more, fresh martensite: 0 to 10%, retained austenite: more than 10% and 30% or less, and pearlite: 0 to 5%.
　In the zinc-based plated steel sheet according to the present embodiment, the amount of hydrogen released when the steel sheet is heated from room temperature to 200 ° C. after removing the zinc-based plating layer is 0.40 ppm or less per steel sheet mass.
　The galvanized steel sheet according to this embodiment has a tensile strength of 1470 MPa or more, and cracks do not occur in a U-shaped bending test in which a stress equivalent to 1000 MPa is applied for 24 hours.
　The zinc-based plated steel sheet according to the present embodiment has a chemical composition of Cr: 0.001% to 2.00%, Mo: 0.001% to 1.00%, B: 0.0001% to 0. 010%, Cu: 0.001% to 1.00%, Ni: 0.001% to 1.00%, Co: 0.001% to 1.00%, W: 0.001% to 1.00% , Sn: 0.001% to 1.00% and Sb: 0.001% to 0.50%, which may contain one or more.
　The zinc-based plated steel sheet according to the present embodiment has a chemical composition of Ti: 0.001% to 0.30%, Nb: 0.001% to 0.30%, and V: 0.001% to 1. It may contain one or more of 00%.
　The zinc-based plated steel sheet according to the present embodiment has a chemical composition of Ca: 0.0001% to 0.0100%, Mg: 0.0001% to 0.0100%, and Ce: 0.0001% to 0. 0100%, Zr: 0.0001% to 0.0100%, La: 0.0001% to 0.0100%, Hf: 0.0001% to 0.0100%, Bi: 0.0001% to 0.0100% And REM: One or more of 0.0001% to 0.0100% may be contained.
　Hereinafter, the galvanized steel sheet according to the present embodiment will be described in detail.
[0022]
"Chemical composition"
　First, the reason why the chemical composition of the steel sheet according to the present embodiment is limited as described above will be described. Unless otherwise specified, all "%" that specify the chemical composition in this specification are "mass%". The numerical limitation range described below includes the lower limit value and the upper limit value. Numerical values ​​indicating "super" and "less than" do not include the value in the numerical range.
[0023]
[C: 0.200% to 0.500%]
　C (carbon) is an essential element for obtaining the desired strength of the galvanized steel sheet. If the C content is less than 0.200%, the desired high strength cannot be obtained and the desired residual γ content cannot be obtained. Therefore, the C content is set to 0.200% or more. It is preferably 0.250% or more, or 0.270% or more. On the other hand, if the C content exceeds 0.500%, the weldability of the galvanized steel sheet deteriorates, so the C content is set to 0.500% or less. From the viewpoint of suppressing deterioration of weldability, the C content is preferably 0.430% or less, 0.400% or less, or 0.380% or less.
[0024]
[Si: 1.00% to 2.50%]
　Si (silicon) is an essential element for suppressing the formation of iron carbide and obtaining a desired residual γ amount. In order to obtain the desired residual γ content, the Si content is set to 1.00% or more. Preferably, it is 1.20% or more, 1.30% or more, or 1.50% or more. On the other hand, excessive content deteriorates the weldability of galvanized steel sheets. Therefore, the Si content is 2.50% or less. Preferably, it is 2.00% or less, or 1.80% or less.
[0025]
[Mn: 1.50% to 5.00%]
　Mn (manganese) is a strong austenite stabilizing element and is an element effective for increasing the strength of galvanized steel sheets. In order to obtain the desired strength, the Mn content is 1.50% or more. Preferably, it is 2.00% or more and 2.40% or more. Excessive content deteriorates the weldability and low temperature toughness of galvanized steel sheets. Therefore, the Mn content is set to 5.00% or less. It is preferably 4.10% or less, 3.50% or less, or 3.10% or less.
[0026]
[P: 0.100% or less]
　P (phosphorus) is a solid solution strengthening element and is an element effective for increasing the strength of galvanized steel sheets, but excessive content of P is the weldability of galvanized steel sheets. And deteriorates toughness. Therefore, the P content is limited to 0.100% or less. It is preferably 0.050% or less, more preferably 0.020% or less. However, in order to extremely reduce the P content, the cost of removing P is high. Therefore, from the viewpoint of economy, the P content is preferably 0.0010% or more, or 0.012% or more.
[0027]
[S: 0.0100% or less]
　S (sulfur) is an element contained as an impurity, and forming MnS in steel deteriorates the toughness and hole-expanding property of galvanized steel sheets. Therefore, the S content is limited to 0.0100% or less so as not to significantly deteriorate the toughness and hole-spreading property of the galvanized steel sheet. It is preferably 0.0050% or less, or 0.0035% or less. However, in order to extremely reduce the S content, the desulfurization cost becomes high. Therefore, from the viewpoint of economy, the S content is preferably 0.0005% or more, or 0.0010% or more.
[0028]
[Al: 0.001% to 1.000%]
　Al (aluminum) contains at least 0.001% or more for deoxidation of steel. It is preferably 0.005% or more, or 0.015% or more. However, even if Al is excessively contained, not only the above effect is saturated and the cost is increased, but also the transformation temperature of the steel is increased to increase the load during hot rolling. Therefore, the Al content is set to 1.000% or less. It is preferably 0.500% or less, 0.200% or less, or 0.100% or less.
[0029]
[N: 0.0100% or less]
　N (nitrogen) is an element contained in steel as an impurity, and when the N content exceeds 0.0100%, coarse nitrides are formed in the steel to form zinc. Deteriorates the bendability and hole widening property of galvanized steel sheets. Therefore, the N content is limited to 0.0100% or less. It is preferably 0.0050% or less, or 0.0040% or less. However, in order to extremely reduce the N content, the cost of removing N is high. Therefore, from the viewpoint of economy, the N content is preferably 0.0005% or more, or 0.0020% or more.
[0030]
[O: 0.0100% or less]
　O (oxygen) is an element contained in steel as an impurity, and when the O content exceeds 0.0100%, a coarse oxide is formed in the steel and zinc is formed. Deteriorates bendability and hole expansion of galvanized steel. Therefore, the O content is limited to 0.0100% or less. It is preferably 0.0050% or less, or 0.0030% or less. However, from the viewpoint of manufacturing cost, the O content is preferably 0.0001% or more, 0.0005% or more, or 0.0010% or more.
[0031]
[Amount of hydrogen released when the steel sheet is heated from room temperature to 200 ° C. after removing the zinc-based plating layer: 0.40 ppm or less per steel sheet mass]
　To prevent hydrogen embrittlement cracking of the zinc-based plated steel sheet, the steel sheet The amount of hydrogen released when the steel sheet is heated from room temperature to 200 ° C. is 0.40 ppm or less per steel sheet mass. The smaller the amount of hydrogen released, the better, and more preferably 0.20 ppm or less, or 0.15 ppm or less. Hydrogen that affects hydrogen embrittlement is hydrogen released when the steel sheet is heated at a relatively low temperature, and hydrogen released by heating at a relatively high temperature does not affect hydrogen embrittlement. In the present embodiment, the amount of hydrogen released when the steel sheet is heated from room temperature to 200 ° C. is regarded as one of the factors affecting hydrogen embrittlement cracking, and this amount of hydrogen is limited to 0.40 ppm or less. The room temperature range is 15 to 25 ° C.
[0032]
　The amount of hydrogen released when the steel sheet is heated from room temperature to 200 ° C. is measured by the following method. First, in order to remove the zinc-based plating layer (hot-dip galvanizing layer or alloyed hot-dip galvanizing layer) of the zinc-based plated steel sheet, the front and back surfaces of the zinc-based plated steel sheet are mechanically ground by 0.1 mm. After that, the cumulative amount of the mass of hydrogen released from the steel sheet when heated from room temperature to 200 ° C. by the temperature-raising hydrogen analysis method (heating rate: 100 ° C./hour, measured up to 300 ° C.) by gas chromatography (gas chromatograph). (Measured value of) is obtained. By dividing the cumulative mass of the obtained hydrogen (measured value of the gas chromatograph) by the mass of the steel sheet after removing the zinc-based plating layer used for measurement, it is released when the steel sheet is heated from room temperature to 200 ° C. The amount of hydrogen to be produced (mass ppm) is obtained. The mass of the steel sheet (steel sheet after removing the zinc-based plating layer) to be measured is preferably at least 0.5 g or more, preferably 1.0 g or more. In order to prevent hydrogen from being released to the atmosphere before measurement, it is necessary to start the analysis within 1 hour after removing the zinc-based plating layer. Alternatively, the steel sheet after removing the zinc-based plating layer must be stored in liquid nitrogen until the start of analysis.
[0033]
　The steel sheet according to this embodiment has the above chemical composition, and the balance is composed of Fe and impurities. "Impurities" are components that are mixed in when steel sheets are industrially manufactured due to various factors in the raw materials such as ores and scraps and the manufacturing process.
　The steel sheet according to the present embodiment may contain the following optional elements, if necessary, in place of a part of the remaining Fe. However, since the galvanized steel sheet according to the present embodiment can solve the problem without containing the optional elements shown below, the lower limit of the content when the optional elements are not contained is 0%.
[0034]
[Cr: 0% to 2.00%, Mo: 0% to 1.00%, B: 0% to 0.010%, Ni: 0% to 1.00%, Cu: 0% to 1.00% , Co: 0% to 1.00%, W: 0% to 1.00%, Sn: 0% to 1.00% and Sb: 0% to 0.50%, one or more]
　Cr ( Chromium), Mo (molybdenum), B (boron), Ni (nickel), Cu (copper), Co (cobalt), W (tungsten), Sn (tin) and Sb (antimony) are all zinc-based plated steel sheets. Since it is an element effective for increasing strength, it may be contained as necessary. However, if the element is excessively contained, the effect is saturated and the cost is increased. Therefore, the contents of the above elements are Cr: 0% to 2.00%, Mo: 0% to 1.00%, B: 0% to 0.010%, Ni: 0% to 1.00%, respectively. Cu: 0% to 1.00%, Co: 0% to 1.00%, W: 0% to 1.00%, Sn: 0% to 1.00% and Sb: 0% to 0.50%. To do. In order to further improve the strength of the galvanized steel sheet, the content of any one of Cr, Mo, Ni, Cu, Co, W, Sn and Sb should be 0.001% or more, or B content. The amount is preferably 0.0001% or more.
[0035]
[Ti: 0% to 0.30%, Nb: 0% to 0.30% and V: 0% to 1.00% of one or more]
　Ti (titanium), Nb (niobium) and V ( Vanadium) is a carbide-forming element, and since it is an element effective for increasing the strength of zinc-based plated steel sheets, it may be contained as necessary. However, even if the above elements are excessively contained, the above effects are saturated and the cost is increased. Therefore, the contents of the above elements are Ti: 0% to 0.30%, Nb: 0% to 0.30%, and V: 0% to 1.00%, respectively. In order to further improve the strength of the galvanized steel sheet, it is preferable that the content of even one of the above elements is 0.001% or more.
[0036]
[Ca: 0% to 0.0100%, Mg: 0% to 0.0100%, Ce: 0% to 0.0100%, Zr: 0% to 0.0100%, La: 0% to 0.0100% , Hf: 0% to 0.0100%, Bi: 0% to 0.0100% and REM: 0% to 0.0100%, one or more]
　Ca (calcium), Mg (magnesium), Ce (cerium), Zr (zirconium), La (lanthanum), Hf (hafnium) and REM are elements that contribute to the fine dispersion of inclusions in steel. Bi (bismuth) is an element that reduces microsegregation of substituted alloy elements such as Mn and Si in steel.
　Since Ca, Mg, Ce, Zr, La, Hf, Bi and REM each contribute to improving the workability of the steel sheet, it is preferable to include them as necessary. In order to obtain the effect of improving workability, the content of even one of the above elements needs to be more than 0%. It is preferably 0.0001% or more. On the other hand, if even one of the above elements is excessively contained, the ductility of the galvanized steel sheet is deteriorated. Therefore, the content of each of the above elements is 0.0100% or less.
　The REM in the present embodiment is a rare earth element having an atomic number of 59 to 71, and the REM content is the total content of these elements. When two or more kinds of rare earth elements are contained, it is preferable to add misch metal.
[0037]
　The steel sheet according to the present embodiment is composed of Fe and impurities other than the elements described above, but can be contained in addition to the elements described above as long as the effects of the present invention are not impaired.
[0038]
"Steel structure"
　Next, the reason for limiting the steel structure of the steel sheet according to the present embodiment will be described. Unless otherwise specified, all "%" that define the steel structure are "volume%". The steel structure described below is a steel structure in the range of 1/8 thickness to 3/8 thickness centered on 1/4 thickness of the steel sheet from the surface. The steel structure in this range is defined because the steel structure in this range represents the steel structure of the entire steel sheet.
[0039]
[Ferrite: 0 to 10%]
　Ferrite is a structure that is soft but has excellent ductility. The larger the volume% of ferrite, the better the elongation of the galvanized steel sheet, but the lower the strength. Therefore, the volume% of ferrite is 0 to 10%. It is preferably 0 to 8%, more preferably 0 to 5%. Since the galvanized steel sheet according to the present embodiment can solve the problem even if ferrite is not contained, the volume% of ferrite may be 0%.
[0040]
[Residual austenite: more than 10% and 30% or less]
　Residual austenite improves the ductility of galvanized steel sheets by the TRIP effect, which transforms into martensite by work-induced transformation during deformation of the steel sheet. To obtain the desired ductility, the volume% of retained austenite should be greater than 10%. Preferably, it is 12% or more, 15% or more, or 18% or more. On the other hand, in order to generate a large amount of retained austenite, it is necessary to contain a large amount of alloying elements such as C and Si. Therefore, the volume% of retained austenite is 30% or less. Preferably, it is 25% or less, or 22% or less.
[0041]
[Pearlite: 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 galvanized steel sheets. Therefore, the volume% of pearlite is 5% or less. It is preferably 4% or less. Since the galvanized steel sheet according to the present embodiment can solve the problem even if pearlite is not contained, the volume% of pearlite may be 0%, but may be 1% or more.
[0042]
[Bainite: 0 to 30%]
　Bainite is a structure formed in an intermediate temperature zone between the ferrite transformation temperature and the martensite transformation temperature, and has intermediate characteristics between ferrite and fresh martensite. Although it has higher strength than ferrite, it has lower strength than fresh martensite, so if bainite is excessively produced, the desired strength cannot be obtained. Therefore, the volume% of bainite is 30% or less. It is preferably 20% or less, or 15% or less. Since the galvanized steel sheet according to the present embodiment can solve the problem even if bainite is not contained, the volume% of bainite may be 0%. From the viewpoint of ensuring strength and ductility, the volume% of bainite may be 5% or more, 7% or more, or 10% or more.
[0043]
[Fresh martensite: 0 to 10%] Since
　fresh martensite has high strength, it is an effective structure for ensuring strength. However, since fresh martensite is also a brittle structure, it becomes a starting point of fracture during plastic deformation and deteriorates local ductility such as hole expandability of galvanized steel sheets. Therefore, the volume% of fresh martensite is 10% or less. Preferably, it is 7% or less. Since the galvanized steel sheet according to the present embodiment can solve the problem even if it does not contain fresh martensite, the volume% of the fresh martensite may be 0%, but from the viewpoint of ensuring strength, it is 1%. The above may be 2% or more or 3% or more.
[0044]
[Tempering martensite: 50% or more]
　Tempering martensite is a structure that achieves both high strength and high toughness of galvanized steel sheets. The steel sheet according to this embodiment is mainly composed of tempered martensite. The volume% of tempered martensite shall be 50% or more. Preferably, it is 55% or more, 60% or more, or 65% or more. The tempered martensite may be 100% or less, but the tempered martensite may be 90% or less, 85% or less, or 80% or less. Tempered martensite is produced by tempering a part of fresh martensite in the second holding step described later.
[0045]
　The total volume fraction of ferrite, bainite, tempered martensite, fresh martensite, retained austenite and pearlite is 100%. In this embodiment, inclusions and precipitates are not included in the steel structure.
[0046]
　The method for calculating the volume fraction of the steel structure of the steel sheet according to the present embodiment is as follows.
　The volume fraction of retained austenite is measured by X-ray diffraction. First, a test piece is collected from the soaking portion of the galvanized steel sheet. The soaking portion is a portion that has been sufficiently heat-treated. The portion that has not been sufficiently heat-treated may not have the metal structure of the steel sheet according to the present embodiment. Chemically using hydrofluoric acid and hydrogen peroxide solution so that the sampled test piece can be observed in the range of 1/8 to 3/8 thickness centered on 1/4 thickness from the surface of the plate thickness. Polish it to reveal a surface parallel to the plate surface, and finish it to a mirror surface to make it a measurement surface. RINT2000 manufactured by Rigaku is used as the X-ray diffractometer, and Co-Kα 1 line is used as the light source . The scanning range is 2θ and the measurement is performed in the range of 45 ° to 105 °. The area ratio of the X-ray diffraction pattern of the one having a crystal structure of fcc (retained austenite) is measured by the X-ray diffraction method, and the area ratio is defined as the volume fraction of retained austenite.
[0047]
　Regarding the volume ratios of ferrite, tempered martensite, fresh martensite, pearlite and bainite, a cross section in the plate thickness direction orthogonal to the rolling direction of the steel sheet is cut out, mirror-polished, and then the steel structure is exposed with a nital solution, and an electric field radiation type. A secondary electron image is taken using a scanning electron microscope. The observation position is in the range of 1/8 to 3/8 thickness centered on 1/4 thickness from the surface of the plate thickness, and a total area of 6000 μm 2 is observed in a plurality of fields of view (photographing magnification: 3000 times). For the obtained tissue photograph, the fraction of each tissue is calculated by the point counting method. First, draw an evenly spaced grid on the tissue photograph. Next, it is determined whether the structure at each lattice point corresponds to ferrite, tempered martensite, fresh martensite or retained austenite, pearlite, or bainite. The fraction of each tissue can be measured by obtaining the number of grid points corresponding to each tissue and dividing by the total number of grid points. The larger the total number of grid points, the more accurately the volume fraction can be obtained. In the present embodiment, the grid spacing is 2 μm × 2 μm, and the total number of grid points is 1500 points. Since the steel structure of the steel sheet according to the present embodiment is an isotropic structure, the fraction of each structure obtained by the point counting method of the cross section can be regarded as the volume fraction.
[0048]
　A region having a substructure (lath boundary, block boundary) in the grain and in which carbides are precipitated with a plurality of variants is judged to be tempered martensite. Further, the region where cementite is precipitated in a lamellar shape is judged to be pearlite. The region where the brightness is low and the substructure is not recognized is judged as ferrite. The region where the brightness is high and the substructure is not exposed by etching is judged as fresh martensite or retained austenite. Areas that do not fall under any of the above are judged to be bainite. The volume fraction of fresh martensite can be obtained by subtracting the volume fraction of retained austenite obtained by the X-ray diffraction method from the volume fraction of fresh martensite and retained austenite obtained by the point counting method.
[0049]
"Mechanical characteristics"
[Tensile strength is 1470 MPa or more]
　The tensile strength of the galvanized steel sheet according to this embodiment is 1470 MPa or more. The tensile strength is measured by collecting a JIS No. 5 tensile test piece whose longitudinal direction is perpendicular to the rolling direction and performing a tensile test in accordance with JIS Z 2241: 2011. The crosshead speed is 2 mm / min up to 2% strain and 20 mm / min after 2% strain.
[0050]
[No cracks occur in the U-shaped bending test in which a stress equivalent to 1000 MPa is applied for 24 hours] When the
　amount of hydrogen embrittlement in the steel sheet is reduced, that is, after removing the zinc-based plating layer, the steel sheet is kept at room temperature. It was found that the hydrogen embrittlement resistance does not necessarily improve even when the amount of hydrogen released when heated to 200 ° C. is 0.40 ppm or less per steel sheet mass. The present inventors have found that the amount of hydrogen in the steel sheet can be reduced and the hydrogen embrittlement resistance can be improved by performing the second holding step described later.
[0051]
　In the present embodiment, excellent hydrogen embrittlement resistance means that cracks do not occur in a U-shaped bending test in which a stress equivalent to 1000 MPa is applied for 24 hours. The U-shaped bending test will be described with reference to FIG.
　First, a strip-shaped test piece having a size of 30 mm × 120 mm is collected from the soaking portion of the galvanized steel sheet so that the longitudinal direction of the test piece and the rolling direction of the steel sheet are perpendicular to each other. Both ends of the strip-shaped test piece are drilled for bolt fastening. Next, a punch with a radius of 10 mm is used to bend 180 ° ((1) in FIG. 1). After that, the spring-backed U-shaped bending test piece ((2) in FIG. 1) is stressed by fastening with bolts and nuts ((3) in FIG. 1). At this time, a strain gauge of GL 5 mm is attached to the top of the U-shaped bending test piece, and a stress equivalent to 1000 MPa is applied by controlling the amount of strain. At this time, the strain is converted into stress from the stress-strain curve obtained by the tensile test conducted in advance. The end face of the U-shaped bending test piece is left as shear-cut. After 24 hours have passed since the stress was applied, the presence or absence of cracks is visually observed. The test temperature is room temperature. The room temperature range is 15 to 25 ° C. If it deviates from this, the temperature of the test room is adjusted to the range of 15 to 25 ° C.
　In the U-shaped bending test, a stress equivalent to 1200 MPa may be applied, and even in this case, if cracks do not occur, the hydrogen embrittlement resistance is excellent, which is preferable.
[0052]
　The ductility-brittle transition temperature (Trs) of the galvanized steel sheet according to the present embodiment is preferably −40 ° C. or lower. A ductility-brittle transition temperature of −40 ° C. or lower is preferable because it has excellent low temperature toughness.
　The ductility-brittle transition temperature is measured by the Charpy impact test. The Charpy test piece used for the Charpy impact test is collected so that the longitudinal direction of the test piece is parallel to the rolling direction of the galvanized steel sheet, and a V notch is introduced in the plate width direction. In addition, for the Charpy test piece, in order to avoid out-of-plane deformation, multiple zinc-based plated steel sheets are stacked and fastened with bolts, and after confirming that there is no gap between the zinc-based plated steel sheets, a V with a depth of 2 mm Make a notched test piece. The number of zinc-based plated steel sheets to be stacked is set so that the thickness of the test piece after lamination is closest to 10 mm. For example, when the plate thickness is 1.6 mm, six sheets are laminated so that the test piece thickness is 9.6 mm. The test temperature is -40 ° C to 60 ° C and measured at 20 ° C intervals. Let Tr be the maximum temperature at which the absorbed energy is less than 1/2 of the absorbed energy of the upper shelf. Conditions other than the above are in accordance with JIS Z 2242: 2005.
[0053]
"Zinc-based plating layer" The
　zinc-based plating layer may be a plating layer mainly composed of zinc, and may contain chemical components other than zinc. The zinc-based plating layer may be any element as long as the element having the maximum content among the elements constituting the plating layer is Zn. For example, Al, Mg, Si, Mn, Fe, Ni, etc. in the balance other than Zn. Cu, Sn, Sb, Pb, Cr, Ti and the like may be contained in the plating layer. Further, the zinc-based plating layer may be a hot-dip galvanizing layer or an alloyed hot-dip galvanizing layer obtained by alloying the hot-dip galvanizing layer.
　When the zinc-based plating layer is a hot-dip galvanizing layer, the iron content in the hot-dip galvanizing layer is preferably less than 7.0% by mass.
　When the zinc-based plating layer is an alloyed hot-dip galvanized layer, the iron content in the alloyed hot-dip galvanized layer is preferably 6.0% by mass or more. When the zinc-based plating layer is an alloyed hot-dip galvanized layer, the weldability can be improved as compared with the case where the zinc-based plating layer is a hot-dip galvanized layer.
　The amount of plating adhered to the zinc-based plating layer is not particularly limited , but is preferably 5 g / m 2 or more per side, and more preferably 25 to 75 g / m 2 from the viewpoint of corrosion resistance .
[0054]
　Next, a method for manufacturing a galvanized steel sheet according to the present embodiment will be described. The zinc-based plated steel sheet according to the present embodiment is produced by hot-rolling the cast slab and then cold-rolling to produce a steel sheet having the above chemical components, and then continuously melted. Manufactured by forming a zinc-based plating layer on the surface of a steel sheet using a zinc plating line.
　In the method for producing a galvanized steel sheet according to the present embodiment, hot-rolled sheet may be annealed between hot rolling and cold rolling. Moreover, you may perform pickling.
　Cold rolling may be omitted and the hot-rolled steel sheet may be introduced into the continuous hot-dip galvanizing line. When cold rolling is omitted, hot-rolled sheet annealing and pickling may or may not be omitted.
　Further, in the plating step, alloying treatment may or may not be performed.
[0055]
　In the continuous hot dip galvanizing line, the annealing step, the first holding step, the plating step and the second holding step are sequentially performed. In addition, all the temperatures in the following description are the surface temperature (steel plate temperature) of the steel sheet, and may be measured with a radiation thermometer or the like.
　In the annealing step, annealing is performed under the conditions of heating temperature: Ac 3 points to 950 ° C. and holding time in the temperature range of Ac 3 points to 950 ° C.: 1 to 500 s. Further, in the annealing process, since the steel sheet temperature reaches 600 ° C., Ac 3 until when held in a temperature range of point-950 ° C. is completed, the hydrogen concentration in the furnace always 1.0 ~ Maintain at 15.0% by volume.
　In the first holding step, holding is performed for 20 to 500 s in a temperature range of Ms point to 600 ° C. During this holding, the hydrogen concentration in the furnace is always maintained at 1.0 to 10.0% by volume.
　In the plating step, the steel sheet is immersed in a hot-dip galvanizing bath and then cooled until the temperature of the steel sheet reaches the temperature range of Ms point −150 ° C. to Ms point −30 ° C. Further, after immersion in the hot-dip galvanizing bath, alloying treatment may be performed in a temperature range of 460 to 600 ° C., and then cooling may be performed until the temperature of the steel sheet reaches a temperature range of Ms point −150 ° C. to Ms point −30 ° C.
　In the second holding step, the hydrogen concentration is held in an atmosphere of less than 0.50% by volume in a temperature range of 330 ° C. to 430 ° C. for 50 to 1000 s, and then wound into a coil.
[0056]
　For the Ac 3 points and the Ms points, the values ​​calculated by the following formulas are used. [Element symbol] in each formula indicates the content of each element in mass%. If no element is contained, substitute 0%.
　Ac 3 (° C.) = 912-230.5 x [C] + 31.6 x [Si] -20.4 x [Mn] -39.8 x [Cu] -18.1 x [Ni] -14.8 × [Cr] + 16.8 × [Mo] + 100.0 × [Al]
　Ms (° C.) = 550-361 × [C] -39 × [Mn] -35 × [V] -20 × [Cr] -17 × [Ni] -10 × [Cu] -5 × [Mo] +30 × [Al]
[0057]
　Hereinafter, each step will be described in detail.
(I) Annealing step
[Heating temperature: Ac 3 points to 950 ° C. Holding time in the temperature range of Ac 3 points to 950 ° C.: Annealing under the condition of 1 to 500 s] Steel
　sheet after cold rolling or hot rolling After that, the steel sheet once cooled to room temperature is annealed. The term "annealing" as used herein means that the steel sheet is heated to 3 points or more of Ac , held in a temperature range of 3 points to 950 ° C., and then cooled to 3 points or less of Ac . In order to sufficiently promote austenitization, the heating temperature at the time of annealing shall be Ac 3 points or more. It is preferably Ac 3 points + 20 ° C. or higher. On the other hand, if the heating temperature during annealing is excessively increased, not only the toughness deteriorates due to the coarsening of the austenite particle size, but also the annealing equipment is damaged. Therefore, the heating temperature at the time of annealing is set to 950 ° C. or lower. Preferably, it is 900 ° C. or lower.
　If the holding time (annealing time) in the temperature range of Ac 3 points to 950 ° C is short, austenitization does not proceed sufficiently . Therefore, the holding time in the temperature range of Ac 3 points to 950 ° C is set to 1 s or more. It is preferably 30 s or more, or 50 s or more. On the other hand, Ac If the holding time in the temperature range of 3 points to 950 ° C is too long, productivity will be hindered. Therefore , the holding time in the temperature range of 3 points to 950 ° C is set to 500 s or less.
　At the time of annealing, Ac 3 may be varied steel sheet temperature in a temperature range of point ~ 950 ° C., Ac 3 may be kept steel temperature constant in the temperature range of point ~ 950 ° C..
[0058]
[From the time when the temperature of the steel sheet reaches 600 ° C to the time when the holding in the temperature range of Ac 3 points to 950 ° C is completed, the hydrogen concentration in the furnace is always 1.0 to 15.0% by volume. ] In
　order to ensure the wettability between the steel sheet and hot-dip galvanizing , hydrogen in the furnace from the time when the steel sheet temperature reaches 600 ° C to the time when the holding in the temperature range of Ac 3 points to 950 ° C is completed. The concentration is always 1.0% by volume or more. In other words, a steel plate temperature 600 ° C. Ac 3 rises to the heating temperature of the point ~ 950 ° C., Ac 3 and between the hydrogen concentration in the furnace is always 1.0% by volume or more which is held at point ~ 950 ° C. .. Further in other words, the steel sheet is heated in the furnace, since the steel sheet temperature reaches 600 ° C., Ac 3 is heated to a temperature range of point ~ 950 ° C., Ac 3 1 ~ in the temperature range of point ~ 950 ° C. It is held for 500 seconds, and the hydrogen concentration in the furnace is always 1.0% by volume or more until the steel sheet is taken out of the furnace. Preferably, it is 2.0% by volume or more. On the other hand, if the hydrogen concentration is too high, the amount of hydrogen that penetrates into the steel sheet increases and the risk of hydrogen embrittlement cracking increases. Therefore, the hydrogen concentration in the furnace is set to 15.0% by volume or less. It is preferably 10.0% by volume or less, or 5.0% by volume or less.
[0059]
The average heating rate until the 　steel sheet temperature reaches the Ac 3 point does not need to be particularly limited, but a preferable range is 0.5 to 10 ° C./s. If the average heating rate exceeds 10 ° C./s, recrystallization of ferrite does not proceed sufficiently, and the elongation of the galvanized steel sheet may deteriorate. On the other hand, if the average heating rate is less than 0.5 ° C./s, the austenite becomes coarse, so that the finally obtained steel structure may become coarse. The average heating rate is the temperature difference between the steel plate temperature at the time of introduction of the annealing furnace ( a furnace that holds in the temperature range of 3 points to 950 ° C.) and 3 points of Ac, and the steel plate temperature is Ac from the time of introduction of the annealing furnace. The value is divided by the time difference until reaching 3 points.
[0060]
(II) First holding step
[Holding is performed for 20 to 500 s in a temperature range of Ms point to 600 ° C., and during the holding, the hydrogen concentration in the furnace is always 1.0 to 10.0% by volume]
　annealing. After the step, the steel sheet is cooled to a temperature range of Ms point or more and 600 ° C. or less, and held for 20 to 500 s in a temperature range of Ms point to 600 ° C. This is called the first holding step. The average cooling rate when cooling the steel sheet after the annealing step in the temperature range of Ms point to 600 ° C. is preferably, for example, 5 ° C./s or more. The average cooling rate referred to here is a value obtained by dividing the temperature difference between the steel sheet temperature at the start of cooling and 600 ° C. by the time difference from the start of cooling to the time when the steel sheet temperature reaches 600 ° C.
　By holding the steel sheet in a temperature range of Ms to 600 ° C. before immersion in the hot-dip galvanizing bath, hydrogen that has entered the steel sheet in the annealing step is dissipated to the outside air, and the amount of hydrogen that has entered the steel sheet can be reduced. From the viewpoint of hydrogen dissipation, it is preferable that the hydrogen concentration in the furnace is low, but if it is too low, the surface of the steel sheet is oxidized and the wettability with hot dip galvanizing deteriorates. Therefore, the hydrogen concentration in the furnace is set to 1.0% by volume or more. Preferably, it is 2.0% by volume or more. On the other hand, if the hydrogen concentration in the furnace exceeds 10.0% by volume, the hydrogen in the steel sheet does not sufficiently dissipate to the outside air. Therefore, the hydrogen concentration in the furnace is set to 10.0% by volume or less. It is preferably 5.0% by volume or less.
[0061]
　If the holding temperature in the first holding step is less than the Ms point, the produced martensite is excessively tempered by the subsequent plating and alloying treatment, and the desired strength cannot be obtained. Therefore, the holding temperature is set to Ms point or higher. Preferably, the Ms point is + 100 ° C. or higher. On the other hand, if the holding temperature in the first holding step exceeds 600 ° C., ferrite is excessively generated, and a desired steel structure cannot be obtained. Therefore, the holding temperature is set to 600 ° C. or lower. It is preferably 550 ° C. or lower.
　Further, from the viewpoint of hydrogen dissipation, the longer the holding time in the first holding step is, the more preferable it is, but if it is too long, the bainite transformation proceeds excessively and the desired tissue fraction cannot be obtained. Therefore, the holding time is set to 20 to 500 s. The preferred lower limit is 100 s and the preferred upper limit is 300 s. The holding time here means the time from when the steel sheet temperature reaches 600 ° C. to the time when the steel sheet is immersed in the hot-dip galvanizing bath.
　In the first holding step, the temperature of the steel sheet may be changed in the temperature range of Ms point to 600 ° C., or the temperature of the steel sheet may be kept constant in the temperature range of Ms point to 600 ° C.
[0062]
(III) Plating Step After
　the first holding step, the steel sheet is immersed in a hot-dip galvanizing bath. Hot-dip galvanizing may be performed according to a conventional method. For example, the plating bath temperature may be 440 to 480 ° C., and the immersion time may be 5 s or less. At this time, if the holding temperature in the first holding step is significantly different from the plating bath temperature, the steel sheet after the first holding step is reheated or cooled to bring the steel sheet temperature closer to the plating bath temperature, so that even during continuous manufacturing. The plating bath temperature can be maintained stably. The hot-dip galvanizing bath preferably contains 0.08 to 0.2% by mass of Al as a component other than zinc, but other impurities such as Fe, Si, Mg, Mn, Cr, Ti, Ni and Cu, It may contain Sn, Sb and Pb. Further, it is preferable to control the basis weight of the zinc-based plating layer (hot-dip galvanizing layer) by a known method such as gas wiping. The basis weight is preferably 25 to 75 g / m 2 per side .
[0063]
[Alloying temperature: 460 to 600 ° C.] The
　zinc-based plated steel sheet on which the hot-dip galvanized layer is formed may be alloyed, if necessary. In that case, if the alloying 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, so the alloying temperature is set to 460 ° C. or higher. It is preferably 480 ° C. or higher. On the other hand, if the alloying temperature exceeds 600 ° C., the alloying proceeds excessively and the plating adhesion of the galvanized steel sheet deteriorates. Therefore, the alloying temperature is set to 600 ° C. or lower. It is preferably 580 ° C. or lower. The alloying treatment time (holding time in a temperature range of 460 to 600 ° C.) may be, for example, 10 to 60 s.
　When the hot-dip galvanized layer is not alloyed, the alloying treatment may be omitted and cooling may be started after plating.
[0064]
[Cooling to Ms point -150 ° C to Ms point -30 ° C after hot dip galvanizing or alloying treatment] After
　hot dip galvanizing (after pulling up from hot dip galvanizing bath) or after alloying treatment, Ms point -150 ° C or higher By cooling to a temperature range of Ms point −30 ° C. or lower, a part of austenite is transformed into martensite to form martensite. The martensite produced at this time is tempered in the subsequent second holding step to become tempered martensite. When the cooling stop temperature exceeds the Ms point −30 ° C., the amount of tempered martensite produced becomes insufficient. Therefore, the cooling stop temperature is set to Ms point −30 ° C. or lower. Preferably, the Ms point is −60 ° C. or lower. On the other hand, when the cooling stop temperature becomes less than the Ms point −150 ° C., the amount of untransformed austenite decreases, and a desired amount of retained austenite cannot be obtained. Therefore, the cooling stop temperature is set to Ms point −150 ° C. or higher. The Ms point is preferably −120 ° C. or higher.
　After hot-dip galvanizing or alloying treatment, the average cooling rate when cooling to a temperature range of Ms point −150 ° C. or higher and Ms point −30 ° C. or lower is preferably 5 ° C./s or higher. The average cooling rate here means the temperature difference between the steel sheet temperature and the cooling stop temperature at the end of hot-dip galvanizing (when pulled up from the hot-dip galvanizing bath) or at the end of alloying treatment, or when pulling up from the hot-dip galvanizing bath. The value is divided by the time difference from the end of alloying treatment to the stop of cooling.
[0065]
(IV) Second holding step
[ Holding for 50 to 1000 s in a temperature range of 330 to 430 ° C. in an atmosphere where the hydrogen concentration is less than 0.50% by volume, and then winding it into a coil]
　The second holding step is as follows. It is carried out for the purpose of achieving (1) to (3) of. (1) Martensite produced after hot-dip galvanizing or alloying treatment is tempered to form tempered martensite. (2) Stabilizes austenite and produces retained austenite (austemper). (3) Hydrogen existing inside the steel sheet, at the interface between the steel sheet and the zinc-based plating layer, and in the zinc-based plating layer is dissipated to the outside air. In the present embodiment, the amount of hydrogen in the steel sheet can be reduced and the hydrogen embrittlement resistance can be improved by performing the second holding step under the conditions described later.
[0066]
　If the holding temperature in the second holding step is less than 330 ° C. or the holding time is less than 50 s, the austemper does not proceed sufficiently and a desired amount of residual γ cannot be obtained. Further, when the holding time is less than 50 s, the emission of hydrogen becomes insufficient. Therefore, the holding temperature is set to 330 ° C. or higher, and the holding time is set to 50 s or higher. Preferably, the holding temperature is 350 ° C. or higher, and the holding time is 100 s or higher.
　If the holding temperature in the second holding step exceeds 430 ° C. or the holding time exceeds 1000 s, martensite is excessively tempered, and it becomes difficult to obtain the desired strength. Therefore, the holding temperature is set to 430 ° C. or lower, and the holding time is set to 1000 s or less. Preferably, the holding temperature is 400 ° C. or lower and the holding time is 500 s or less. The holding time here means the time from when the steel sheet temperature reaches 430 ° C to when it reaches 330 ° C. However, the time before reaching the Ms point -30 ° C is not included.
　In the second holding step, the steel sheet temperature may be changed in the temperature range of 330 ° C. to 430 ° C., or the steel sheet temperature may be kept constant in the temperature range of 330 ° C. to 430 ° C.
[0067]
　The hydrogen concentration in the furnace in the second holding step shall be less than 0.50% by volume. If the hydrogen concentration in the furnace is 0.50% by volume or more, hydrogen is not sufficiently released into the atmosphere. The lower the hydrogen concentration in the furnace, the more preferable, and it is preferably 0.30% by volume or less, 0.20% by volume or less, or less than 0.10% by volume.
[0068]
　The second holding step must be performed after hot dip galvanizing or alloying treatment, and before winding into a coil. This is because hydrogen is dissipated only in the outermost peripheral portion of the coil even if the coil is held in a coiled state, and hydrogen is not sufficiently dissipated inside the coil.
　After hot-dip galvanizing and cooling to an Ms point of less than −150 ° C., or after alloying treatment and cooling to an Ms point of less than −150 ° C., a second holding step may be performed on the zinc-based plated steel sheet. After reheating, the second holding step may be performed.
[0069]
　In the present embodiment, the production conditions up to the continuous hot-dip galvanizing line need not be particularly limited, but a preferable example will be described below.
[0070]
"Manufacturing conditions in the hot rolling process"
[Slab heating process, slab heating temperature: 1150 ° C. or higher] The
　slab heating temperature is preferably 1150 ° C. or higher in order to sufficiently dissolve boride and carbides. The slab to be used is preferably cast by a continuous casting method from the viewpoint of manufacturability, but an ingot forming method or a thin slab casting method may also be used. Further, the cast slab may be once cooled to room temperature, or may be sent directly to the heating furnace without being cooled to room temperature.
[0071]
[Rough rolling step: Total reduction rate at 1050 ° C or higher: 60% or more]
　Rough rolling is preferable so that the total reduction rate at 1050 ° C or higher is 60% or more. 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.
[0072]
[Finish rolling process, finish rolling inlet temperature: 950 to 1050 ° C, finish rolling exit temperature: 850 ° C to 1000 ° C, total
　rolling reduction: 70 to 95%] Finish rolling inlet temperature ranges from 950 to 1050 ° C. preferable.
　Further, when the finish-rolled output side temperature is less than 850 ° C. or the total reduction ratio exceeds 95%, the texture of the hot-rolled steel sheet develops, so that anisotropy in the final product sheet may become apparent. If the finish-rolled output side temperature exceeds 1000 ° C or the total reduction ratio is less than 70%, the crystal grain size of the hot-rolled steel sheet may become coarse, leading to coarsening of the final product plate structure and deterioration of workability. is there.
[0073]
[Taking process, winding temperature: 450 to 700 ° C] The
　winding temperature is 450 to 700 ° C. If the take-up temperature is less than 450 ° C., the hot-rolled plate strength becomes excessive and the cold rollability may be impaired. On the other hand, if the winding temperature exceeds 700 ° C., cementite becomes coarse and undissolved cementite remains, which may impair processability.
[0074]
　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. Further, softening annealing (hot rolling sheet annealing) may be performed in order to improve the cold rollability. In that case, it is desirable to perform heat treatment for about 0.5 to 10 hours in a temperature range of 500 to 650 ° C.
[0075]
"Manufacturing conditions in the cold rolling process"
[Cold rolling rate: 20 to 80%] After
　hot rolling and pickling, heat treatment may be performed as it is on a continuous hot-dip galvanizing line, or cold rolling is performed. After that, heat treatment may be performed on a continuous hot-dip galvanizing line. When cold rolling is performed, the cold rolling rate (cumulative rolling reduction rate) is preferably 20% or more. On the other hand, excessive rolling causes an excessive rolling load and an increase in the load of the cold rolling mill. Therefore, the cold rolling ratio is preferably 80% or less.
Example
[0076]
　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.
[0077]
　Steels having the chemical compositions shown in Tables 1A and 1B were cast to prepare slabs. Table 1B shows the Ac 3 points and Ms points of each steel obtained by the following relational expression . These slabs were hot-rolled under the conditions shown in Tables 2A and 2B to produce hot-rolled steel sheets. Then, the hot-rolled steel sheet was pickled to remove the scale on the surface. Then, cold rolling was performed to obtain a steel sheet. The obtained steel sheets were subjected to continuous hot-dip galvanizing treatment under the conditions shown in Tables 3A and 3B, and some zinc-based plated steel sheets were alloyed. For the plating types in Tables 3A to 4B, "GA" indicates an alloyed hot-dip galvanized layer, and "GI" indicates a hot-dip galvanized layer.
[0078]
　Ac 3 (° C.) = 912-230.5 x [C] + 31.6 x [Si] -20.4 x [Mn] -39.8 x [Cu] -18.1 x [Ni] -14.8 × [Cr] + 16.8 × [Mo] + 100.0 × [Al]
　Ms (° C.) = 550-361 × [C] -39 × [Mn] -35 × [V] -20 × [Cr] -17 × [Ni] -10 × [Cu] -5 × [Mo] + 30 × [Al]
　However, the element symbol in each formula indicates the content of each element in mass%. When no element was contained, 0% was substituted.
[0079]
　The average cooling rate from the start of cooling to the time when the temperature reaches 600 ° C. (from the end of the annealing process to the start of the first holding process), and from the end of hot dip galvanizing or the end of alloying treatment to the cooling stop temperature. The average cooling rate was 5 ° C./s or higher. The plating bath temperature in the plating step is 440 to 480 ° C., the immersion time is 5 s or less, and the hot dip galvanizing bath is a hot dip galvanizing bath containing 0.08 to 0.2% by mass of Al in addition to zinc. And said. The alloyed hot-dip galvanized layer after the alloying treatment had an iron content of 6.0% by mass or more, and the hot-dip galvanized layer had an iron content of less than 7.0% by mass.
　The hydrogen concentration in the annealing steps of Tables 3A and 3B is from the time when the steel sheet temperature reaches 600 ° C. to the time when the holding in the temperature range of Ac 3 points to 950 ° C. is completed (in other words, the steel sheet is in the furnace). in is heated, since the steel sheet temperature reaches 600 ° C., Ac 3 is heated to a temperature range of point ~ 950 ° C., Ac 3 is held in a temperature range of point ~ 950 ° C., until the steel sheet is issued from the furnace The hydrogen concentration in the furnace in the first holding step is the hydrogen concentration in the furnace during holding in the temperature range of Ms point to 600 ° C.
　In addition, No. of Table 3A. No. 21 was not held in the temperature range of Acc 3 points to 950 ° C. in the annealing step, but was held at 820 ° C. for 100 s. Therefore, the holding time was described as “−”.
[0080]
　From the obtained galvanized steel sheet, JIS No. 5 tensile test pieces having a direction perpendicular to the rolling direction as the longitudinal direction were collected and subjected to a tensile test in accordance with JIS Z 2241: 2011, and the tensile strength (TS) and the total were obtained. Elongation (El) was measured. In the tensile test, the crosshead speed was 2 mm / min up to 2% strain and 20 mm / min after 2% strain. The total elongation was measured by matching the samples after fracture.
　In addition, the "JFS T 1001-1996 hole expansion test method" of the Japan Iron and Steel Federation standard was performed, and the hole expansion rate (λ) was measured. The blank size was 150 mm. The punching conditions were set so that the punch diameter was 10 mm, the die diameter was 0.1 mm pitch, and the one-sided clearance was closest to 12%. The hole expansion test was carried out under the condition outside the burr, that is, the surface of the steel plate that was in contact with the die at the time of punching was on the opposite side of the punch during the hole expansion test, and the punch was performed at a 60-degree conical punch and a punch speed of 1 mm / s. The wrinkle pressing pressure was 60 tons, the die shoulder R was 5 mm, and the inner diameter of the die was φ95 mm. The number of tests was N = 3, and the hole expansion rate λ was obtained by calculating the average value thereof.
　The tensile strength is 1470 MPa or more, and the combined value of the tensile strength, total elongation and hole expansion ratio (TS [MPa] × EL [%] × λ [%] 0.5 × 10 -3 ) is 95 or more. Was judged to be acceptable as having good mechanical properties. If one or more conditions are not met, it is judged to be inferior in mechanical properties and rejected.
[0081]
　The amount of hydrogen per mass of the steel sheet released when the steel sheet was heated from room temperature to 200 ° C. was determined by the following method. In order to remove the zinc-based plating layer (hot-dip galvanizing layer or alloyed hot-dip galvanizing layer), the front and back surfaces of the zinc-based plated steel sheet are mechanically ground by 0.1 mm, and the hydrogen in the steel sheet after plating removal is measured by a gas chromatograph. Cumulative amount of the mass of hydrogen released from the steel sheet during heating from room temperature to 200 ° C (gas chromatograph) as measured by the heating hydrogen analysis method (heating rate: 100 ° C / hour, measured from room temperature to 300 ° C). (Measured value of) was obtained. By dividing the cumulative amount of the obtained mass of hydrogen (measured value of the gas chromatograph) by the mass of the steel sheet, the amount of hydrogen per mass of the steel sheet (mass ppm) released when the steel sheet is heated from room temperature to 200 ° C. Got
[0082]
　The hydrogen embrittlement resistance test was evaluated by a U-shaped bending test. The U-shaped bending test will be described with reference to FIG.
　First, a strip-shaped test piece having a size of 30 mm × 120 mm was collected from the soaking portion of the galvanized steel sheet so that the longitudinal direction of the test piece and the rolling direction of the steel sheet were perpendicular to each other. Holes were drilled at both ends of this strip-shaped test piece for fastening bolts. Next, 180 ° bending was performed with a punch having a radius of 10 mm ((1) in FIG. 1). After that, the spring-backed U-shaped bending test piece ((2) in FIG. 1) was stressed by fastening with bolts and nuts ((3) in FIG. 1). At this time, a strain gauge of GL 5 mm was attached to the top of the U-shaped bending test piece, and a stress equivalent to 1000 MPa and 1200 MPa was applied by controlling the amount of strain. At this time, the strain was converted into stress from the stress-strain curve obtained by performing the tensile test in advance. The end face of the U-shaped bending test piece was left as shear-cut. The test temperature was room temperature (15 to 25 ° C.).
[0083]
　Twenty-four hours after the stress was applied, the presence or absence of cracks was visually observed. "<1000" for those with cracks at 1000 MPa, "1000-1200" for those with no cracks at 1000 MPa and "> 1200" for those with no cracks at 1200 MPa ", Described in the table. Those with no cracks at 1000 MPa were judged to be excellent in hydrogen embrittlement resistance, and those with cracks at 1000 MPa were judged to be inferior in hydrogen embrittlement and were judged to be unacceptable.
[0084]
　The low temperature toughness of the galvanized steel sheet was evaluated by measuring the ductility-brittle transition temperature by the Charpy impact test.
　The Charpy test piece used for the Charpy impact test was sampled so that the longitudinal direction of the test piece was parallel to the rolling direction of the galvanized steel sheet, and a V notch was introduced in the plate width direction. In addition, for the Charpy test piece, in order to avoid out-of-plane deformation, multiple zinc-based plated steel sheets are stacked and fastened with bolts, and after confirming that there is no gap between the zinc-based plated steel sheets, a V with a depth of 2 mm A notched test piece was prepared. The number of zinc-based plated steel sheets to be laminated was set so that the thickness of the test piece after lamination was closest to 10 mm. The test temperature was -40 ° C to 60 ° C and measured at 20 ° C intervals. The maximum temperature at which the absorbed energy is less than 1/2 of the absorbed energy of the upper shelf is defined as the ductile-brittle transition temperature (Trs). Conditions other than the above were in accordance with JIS Z 2242: 2005.
　When the ductility-brittleness transition temperature was −40 ° C. or lower, it was evaluated as having excellent low temperature toughness, and was described as “<-40” in the table. When the ductile-brittle transition temperature exceeds -40 ° C, the ductile-brittle transition temperature is shown in the table.
[0085]
　The plating adhesion was evaluated by a tape peeling test. A 30 mm × 100 mm test piece was taken from the soaking portion of the galvanized steel sheet and subjected to a 90 ° V bending test. The tip radius of the punch was 5 mm. 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.
[0086]
　G (Good): Small plating peeling or peeling to the extent that there is no problem in practical use (peeling width 0 to less than 10 mm)
　B (Bad): Severe peeling (peeling width 10 mm or more)
[0087]
　The steel structure of the steel sheet was measured by the method described above.
　The above measurement results and test results are shown in Tables 4A and 4B.
[0088]
　Looking at Tables 4A and 4B, it can be seen that all of the examples of the present invention were excellent in mechanical properties, hydrogen embrittlement resistance and plating adhesion, and were able to reduce the amount of invading hydrogen during production. On the other hand, in the comparative example in which one or more of the chemical composition and the steel structure are outside the scope of the present invention, it can be seen that at least one of the above characteristics did not reach the acceptance criteria.
[0089]
　No. In No. 2, the H 2 concentration in the first holding step exceeded the specified upper limit, so that the hydrogen concentration in the steel became high and the hydrogen embrittlement resistance deteriorated.
[0090]
　No. In No. 3, since the holding time in the first holding step was less than the specified lower limit, the hydrogen concentration in the steel became high and the hydrogen embrittlement resistance deteriorated.
[0091]
　No. In No. 4, since the holding temperature in the first holding step exceeded the specified upper limit, the amount of ferrite increased and the TS became less than 1470 MPa.
[0092]
　No. In No. 5, the cooling stop temperature after the alloying treatment exceeded the specified upper limit, so that the amount of fresh martensite increased and the mechanical properties became inferior.
[0093]
　No. In No. 6, since the holding time in the second holding step was less than the specified lower limit, the stabilization of austenite was insufficient, the desired amount of retained austenite could not be obtained, and the mechanical properties were inferior.
[0094]
　No. In No. 7, since the H 2 concentration in the second holding step exceeded the specified upper limit, the hydrogen concentration in the steel became high and the hydrogen embrittlement resistance deteriorated.
[0095]
　No. In No. 9, since the cooling stop temperature after the alloying treatment was below the specified lower limit, the desired amount of retained austenite could not be obtained, and the mechanical properties were inferior.
[0096]
　No. In No. 10, the holding temperature in the second holding step was high, and the tempered martensite was excessively tempered, so that the tensile strength TS was less than 1470 MPa.
[0097]
　No. In No. 11, since the holding temperature in the second holding step was lower than the lower limit, the amount of fresh martensite became excessive, the mechanical properties became inferior, the hydrogen concentration in the steel became high, and the hydrogen embrittlement resistance deteriorated.
[0098]
　No. In No. 13, the holding time in the second holding step was long, and the tempered martensite was excessively tempered, so that the tensile strength TS was less than 1470 MPa.
[0099]
　No. Since the second holding step was not performed in No. 16, the amount of tempered martensite was insufficient, the mechanical properties were inferior, the hydrogen concentration in the steel was high, and the hydrogen embrittlement resistance was deteriorated.
[0100]
　No. In No. 19, the holding temperature in the first holding step was low, and the martensite produced was excessively tempered by the subsequent plating / alloying treatment, so that the tensile strength TS was less than 1470 MPa.
[0101]
　No. In No. 20, since the H 2 concentration in the annealing step exceeded the specified upper limit, the hydrogen concentration in the steel became high and the hydrogen embrittlement resistance deteriorated.
[0102]
　No. In No. 21, since the heating temperature in the annealing step was below the specified lower limit, the amount of ferrite increased and the tensile strength TS became less than 1470 MPa.
[0103]
　No. In No. 22, the holding time in the first holding step exceeded the specified upper limit, so that the amount of bainite increased and the tensile strength TS became less than 1470 MPa.
[0104]
　No. Since the chemical composition of 37 to 41 was out of the range specified by the present invention, the mechanical properties were inferior.
[0105]
　No. Since No. 42 did not undergo the second holding step, the amount of fresh martensite increased and the hydrogen embrittlement resistance deteriorated.
[0106]
　No. In No. 43, since the holding temperature in the second holding step was below the specified lower limit, the amount of fresh martensite increased and the hydrogen embrittlement resistance deteriorated.
[0107]
　No. Since the holding time of 44 was less than the specified lower limit in the second holding step, the amount of fresh martensite increased and the hydrogen embrittlement resistance deteriorated.
[0108]
　No. In No. 46, the H 2 concentration in the second holding step exceeded the specified upper limit, so that the amount of hydrogen in the steel increased and the hydrogen embrittlement resistance deteriorated.
[0109]
[Table 1A]

[0110]
[Table 1B]

[0111]
[Table 2A]

[0112]
[Table 2B]

[0113]
[Table 3A]

[0114]
[Table 3B]

[0115]
[Table 4A]

[0116]
[Table 4B]

Industrial applicability
[0117]
　According to the above aspect according to the present invention, zinc-based plating which is preferably used as an automobile member, has excellent mechanical properties, reduces the amount of invading hydrogen during manufacturing, and has excellent hydrogen embrittlement resistance and plating adhesion. A steel plate and a method for producing the same can be provided. According to a preferred embodiment of the present invention, it is possible to provide a galvanized steel sheet having the above-mentioned various properties and further excellent in low temperature toughness, and a method for producing the same.
The scope of the claims
[Claim 1]
　A steel sheet and a zinc-based plating layer arranged on the surface of the
　steel sheet are provided, and the steel sheet has a mass% of
C: 0.200% to 0.500% and
Si: 1.00% to 2.50%. ,
Mn: 1.50% to 5.00%,
P: 0.100% or less,
S: 0.0100% or less,
Al: 0.001% to 1.000%,
N: 0.0100% or less,
O : 0.0100% or less,
Cr: 0% to 2.00%,
Mo: 0% to 1.00%,
B: 0% to 0.010%,
Cu: 0% to 1.00%,
Ni: 0 % To 1.00%,
Co: 0% to 1.00%,
W: 0% to 1.00%,
Sn: 0% to 1.00%,
Sb: 0% to 0.50%,
Ti: 0 % To 0.30%,
Nb: 0% to 0.30%,
V: 0% to 1.00%,
Ca: 0% to 0.0100%,
Mg: 0% to 0.0100%,
Ce: 0 % ~ 0.0100%,
Zr: 0% to 0.0100%,
La: 0% to 0.0100%,
Hf: 0% to 0.0100%,
Bi: 0% to 0.0100%, and
REM: 0% to 0.0100% 　The steel structure in the range of 1/8 to 3/8 thickness centered on 1/4 thickness from the surface of the steel plate
has a chemical composition of Fe and impurities in the balance
.
Ferrite: 0-10%,
baynite: 0-30%,
tempered martensite: 50% or more,
fresh maltensite: 0-10%,
retained austenite: more than 10% and 30% or less, and
pearlite: 0-5% The
　amount of hydrogen released when the steel sheet is heated from room temperature to 200 ° C. after removing the zinc-based plating layer is 0.40 ppm or less per steel sheet mass, and the
　tensile strength is 1470
　MPa or more, which is equivalent to 1000 MPa. A zinc-based plated steel sheet characterized in that cracks do not occur in a U-shaped bending test in which stress is applied for 24 hours.
[Claim 2]
　The chemical composition of the steel sheet is
Cr: 0.001% to 2.00%,
Mo: 0.001% to 1.00%,
B: 0.0001% to 0.010%,
Cu: 0.001%. ~ 1.00%,
Ni: 0.001% to 1.00%,
Co: 0.001% to 1.00%,
W: 0.001% to 1.00%,
Sn: 0.001% to 1 The zinc-based plated steel sheet according to claim 1, which contains one or more of
0.001% and Sb: 0.001% to 0.50%
.
[Claim 3]
　The chemical composition of the steel sheet is one or two of
Ti: 0.001% to 0.30%,
Nb: 0.001% to 0.30%, and
V: 0.001% to 1.00%.
The zinc-based plated steel sheet according to claim 1 or 2, characterized in that it contains seeds or more.
[Claim 4]
　The chemical composition of the steel sheet 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%, and
REM: 0.0001% ~
The zinc-based plated steel sheet according to any one of claims 1 to 3, characterized in that it contains one or more of 0.0100% .
[Claim 5]
　The galvanized steel sheet according to any one of claims 1 to 4, wherein the ductility-brittle transition temperature is −40 ° C. or lower.
[Claim 6]
　Any of claims 1 to 5, wherein each of the following steps (I) to (IV) is sequentially performed on a steel plate having the chemical composition according to any one of claims 1 to 4. Method for producing zinc-based plated steel sheet according to item
(1) : (I) Heating temperature: Ac 3 points to 950 ° C. Holding time in the temperature range of Ac 3 points to 950 ° C. Retention time: Annealing under the conditions of 1 to 500 s. From the time when the steel plate temperature reaches 600 ° C to the time when the holding in the temperature range of Ac 3 points to 950 ° C is completed, the hydrogen concentration in the furnace is always 1.0 to 15.0% by volume. Annealing step to maintain
the temperature ; (II) Hold for 20 to 500 s in the temperature range of Ms point to 600 ° C., and keep the hydrogen concentration in the furnace at 1.0 to 10.0% by volume during the holding. First holding step to maintain;
(III) A plating step in which the steel plate is immersed in a hot-dip zinc plating bath and then cooled until the temperature of the steel plate reaches the temperature range of Ms point −150 ° C. to Ms point-30 ° C.; and
(IV) Hydrogen A second holding step in which the mixture is held in a temperature range of 330 to 430 ° C. for 50 to 1000 s in an atmosphere having a concentration of less than 0.50% by volume, and then wound into a coil.
[Claim 7]
　In the step (III), the steel sheet is immersed in a hot-dip galvanizing bath and then alloyed in a temperature range of 460 to 600 ° C., and then the steel sheet temperature is brought into a temperature range of Ms point-150 ° C. to Ms point-30 ° C. The
method for producing a zinc-based plated steel sheet according to claim 6, wherein the step is to cool the steel sheet until it becomes .