Technical field
[0001]
The present invention is an automobile, building materials, of a preferred high strength steel sheet household appliances.
BACKGROUND
[0002]
For weight reduction and improvement in collision safety of automobiles, the tensile strength applied to the mobile member of the above high strength steel sheet 980MPa is rapidly expanding. Further, as a high strength steel sheet excellent ductility is obtained, transformation induced plasticity: TRIP steel utilizing (transformation induced plasticity TRIP) is known.
[0003]
　However, in the conventional TRIP steel sheet, in addition to the tensile strength and ductility, it is impossible to achieve both hole expansion, hydrogen embrittlement resistance and toughness.
CITATION
Patent Document
[0004]
Patent Document 1: JP-A-11-293383 Patent Publication
Patent Document 2: JP-A 1-230715 Patent Publication
Patent Document 3: JP-A 2-217425 Patent Publication
Patent Document 4: JP 2010-90475 JP
Patent Document 5: International Publication No. 2013/051238
Patent Document 6: JP 2013-227653 Patent Publication
Patent Document 7: WO 2012/133563
Patent Document 8: JP 2014-34716 JP
Patent Document 9: WO 2012/144567
Summary of the Invention
Problems that the Invention is to Solve
[0005]
　The present invention aims tensile strength, ductility, hole expansion, to provide a steel sheet can achieve both hydrogen embrittlement resistance and toughness.
Means for Solving the Problems
[0006]
　The present inventors have conducted extensive studies to solve the above problems. Consequently, in TRIP steel sheet, the main phase is a tempered martensite or bainite, or both of these with a predetermined effective crystal grain size, the inclusion of the iron-based carbides predetermined number density in the tempered martensite and lower bainite , tensile strength, ductility, hole expansion was found that it is possible to achieve both hydrogen embrittlement resistance and toughness.
[0007]
　The present inventors, as a result of further intensive studies based on these findings, and conceived to aspects of the invention described below.
[0008]
　(1)
　in
　mass%,
　C:
　0.15% ~ 0.45%, Si: 1.0% ~ 2.5%, Mn:
　1.2% ~ 3.5%, Al: 0.001% ~
　% 2.0, P: 0.02% or
　less, S: 0.02% or
　less, N: 0.007% or
　less, O: 0.01% or less,
　Mo: 0.0% ~
　1.0%, Cr :
　0.0%
　~ 2.0%, Ni: 0.0% ~
　2.0%, Cu: 0.0% ~ 2.0%, Nb: 0.0% ~
　0.3%, Ti: 0
　%
　~ 0.3 .0%, V:
　0.0% ~ 0.3%, B: 0.00% ~ 0.01%, Ca: 0.00%
　~ 0.01%, Mg: 0.00
　~ 0.01%%, REM: 0.00% ~ 0.01%, and
　balance: Fe and impurities,
have in a chemical composition represented,
　by volume fraction,
　tempered martensite and bainite: 70 in total % or more but less than 92%,
　residual austenite : Less than 8% to 30%
　ferrite: less than 10%,
　fresh martensite: less than 10%, and
　perlite: less than 10%,
in having a steel structure represented,
　tempered martensite and iron-based carbides in the lower bainite the number density of 1.0 × 10 6 (pieces / mm 2 and a) above,
　steel sheets effective crystal grain size of tempered martensite and bainite is equal to or is 5μm or less.
[0009]
　(2)
　In the above chemical composition, in
　mass%,
　Mo: 0.01% ~ 1.0%,
　Cr: 0.05% ~ 2.0%, Ni: 0.05% ~ 2.0%, or
　Cu : 0.05% to 2.0%,
　the steel sheet according to or, characterized in that any combination of these is true (1).
[0010]
　(3)
　In the above chemical composition, by
　mass%, Nb:
　0.005% ~ 0.3%, Ti: 0.005% ~ 0.3%, or
　V: 0.005% ~ 0.3%,
　or steel sheet according to, characterized in that any combination of these is true (1) or (2).
[0011]
　(4)
　In the above chemical composition, by
　mass%, B: 0.0001% - 0.01%,
the steel sheet according to any one of wherein the holds (1) to (3).
[0012]
　(5)
　In the above chemical composition, by
　mass%,
　Ca: 0.0005% ~ 0.01%, Mg: 0.0005% ~ 0.01%, or
　REM: 0.0005% ~ 0.01%,
　or steel sheet according to any one of wherein the any combination of these is true (1) to (4).
Effect of the invention
[0013]
　According to the present invention, since the steel structure and tempered martensite and bainite effective grain size, etc. it is appropriate, tensile strength, ductility, it is possible to achieve both the hole expansion, hydrogen embrittlement resistance and toughness.
DESCRIPTION OF THE INVENTION
[0014]
　Hereinafter, embodiments of the present invention will be described.
[0015]
　First, it will be described steel sheet steel structure according to the embodiment of the present invention. Steel sheet according to the present embodiment, the volume fraction of tempered martensite and bainite: less than 92% 70% in total, the residual austenite: less than 8% to 30% ferrite: less than 10%, fresh martensite: 10% below, and perlite: has less than 10%, in the steel structure represented.
[0016]
　(Tempered martensite and bainite: 70% or more but less than 92% in total)
　tempered martensite and bainite are low temperature transformation structure containing iron-base carbides, contributing to both of hole expansion and hydrogen embrittlement resistance. If it is less than tempered martensite and 70% volume fraction total bainite, it is difficult to sufficiently satisfy both hole expansion and hydrogen embrittlement resistance. Therefore, the volume fraction of tempered martensite and bainite are a total of 70% or more. Meanwhile, tempered martensite and the volume fraction of bainite is at 92% or more, is insufficient residual austenite which will be described later. Therefore, the volume fraction of tempered martensite and bainite is less than 92%.
[0017]
　Tempered martensite is a set of lath-like grains, diameter therein contains more iron-based carbides 5 nm. Iron-based carbides contained in the tempered martensite has a plurality of variants, iron-based carbides present in one of the crystal grains are extended in multiple directions.
[0018]
　The bainite include upper bainite and lower bainite. Lower bainite is a set of lath-like grains, diameter therein contains more iron-based carbides 5 nm. However, unlike the tempered martensite, iron-base carbide contained in the lower bainite has a single variant, iron-based carbides present in one of the crystal grains is substantially elongated in a single direction . The term "substantially single direction", angular difference means directions within 5 °. Upper bainite is a set of lath-shaped crystal grains containing no iron-based carbides therein.
[0019]
　Tempering the martensite and lower bainite can be a direction that the iron-based carbide is extended it is determined depending on whether multiple or single. If tempered martensite and the volume fraction of bainite is 70% or more in total, the breakdown is not limited. This will be described in detail later, the iron-based carbides variants because not affect the balance of hole expansion and hydrogen embrittlement resistance. However, the formation of bainite, since the relatively long retention in 300 ° C. ~ 500 ° C. are required, from the viewpoint of productivity, it ratio of tempered martensite is high is preferable.
[0020]
　(Retained austenite: less than 8% to 30%)
　residual austenite transformation induced plasticity (transformation induced plasticity: TRIP) contributes to the improvement of ductility through. The volume fraction of retained austenite is less than 8%, no sufficient ductility can not be obtained. Therefore, the volume fraction of retained austenite is set to 8% or more, desirably 10% or more. On the other hand, the volume fraction of retained austenite in 30% or more, is insufficient tempered martensite and bainite. Therefore, the volume fraction of the retained austenite is less than 30%.
[0021]
　(Ferrite: 10% less)
　ferrite is a tissue of soft without the infrastructure lath or the like inside, tempered martensite and cracking due to the intensity difference at the interface between the bainite tends to occur a hard tissue. That is, the ferrite easy to degrade the toughness and hole expansion. In addition, the ferrite leads to a deterioration of the low temperature toughness. Therefore, the volume fraction of the ferrite is better as low as possible. In particular, the volume fraction of the ferrite is at least 10%, decrease in toughness and hole expansion is remarkable. Therefore, the volume fraction of ferrite is less than 10%.
[0022]
　(Fresh martensite: less than 10%)
　fresh martensite is martensite left quenching free of iron-based carbides, but contributes to the improvement of strength, greatly deteriorates hydrogen embrittlement resistance. Also, fresh martensite, resulting in deterioration of low-temperature toughness due to the difference in hardness between the tempered martensite and bainite. Therefore, the volume fraction of fresh martensite good as low as possible. In particular, the volume fraction of fresh martensite is 10% or more, significant deterioration of hydrogen embrittlement resistance. Therefore, the volume fraction of fresh martensite is less than 10%.
[0023]
　(Perlite: less than 10%)
　perlite, like ferrite, degrades the toughness and hole expansion. Therefore, the volume fraction of pearlite good The lower. In particular, the volume fraction of pearlite in more than 10%, decrease in toughness and hole expansion is remarkable. Therefore, the volume fraction of pearlite is less than 10%.
[0024]
　Will now be described iron-based carbide tempered martensite and in the lower bainite. Between the tempered martensite and iron-based carbide and the mother phase in the lower bainite contains matching interface, there is a coherent strain the alignment interface. The coherent strain exerts hydrogen trapping ability, improve hydrogen embrittlement resistance, improving the delayed fracture resistance. Such number density of iron-based carbide 1.0 × 10 6 (pieces / mm 2 is less than), Accordingly, tempering the number density of the iron-based carbide martensite and in the lower bainite 1.0 × 10 6 (pieces / mm 2 and) or more, preferably 2.0 × 10 6 (pieces / mm 2 and) above, and more desirably 3.0 × 10 6 (pieces / mm 2 and) above.
[0025]
　The iron-based carbides mainly is a general term for carbides consisting Fe and C, for example, different ε carbide crystal structure, chi carbides, cementite (theta carbide) belongs to the iron-based carbide. Iron-based carbides are present with a specific orientation relationship martensite and in the lower bainite as a matrix phase. Some of Fe contained in the iron-based carbides may be substituted by other elements Mn, Si and Cr. In this case, the number density of the iron-based carbide length of more than 5nm long axis 1.0 × 10 6 (pieces / mm 2 if) above, obtained excellent hydrogen embrittlement resistance.
[0026]
　Number density counting target, the size of the long axis is equal to or greater than the iron-based carbides 5 nm. Although the observable size by scanning electron microscopy and transmission electron microscopy is limited, generally the long axis ferrous carbide size is more than 5nm in is observable. Major axis size may contain iron-based carbide having smaller than 5nm in tempered martensite and lower bainite. Excellent hydrogen embrittlement resistance as iron-based carbides are fine is obtained. Therefore, it is desirable iron-based carbides are fine, for example, the average length of the major axis is desirably at 350nm or less, more desirably at 250nm or less, more preferably is 200nm or less.
[0027]
　Previously, the iron-based carbides contributes to the improvement of hydrogen embrittlement resistance is not finding. This is generally due to the utilization of residual austenite and improves formability with this is the suppression of the precipitation of iron-based carbide is particularly important, presumably because precipitation of iron-based carbides have been suppressed . In other words, heretofore, not been investigated steel sheet containing retained austenite and fine iron-based carbide is believed that the effect of improving hydrogen embrittlement resistance by ferrous carbide in TRIP steel was found.
[0028]
　Next, a description will be given effective crystal grain size of tempered martensite and bainite. Although will be described later method of measuring the effective crystal grain size of tempered martensite and bainite, the effective crystal grain size of tempered martensite and bainite in the 5μm greater, sufficient toughness can not be obtained. Therefore, the effective crystal grain size of tempered martensite and bainite and 5μm or less, preferably to 3μm or less.
[0029]
　Next, an example of a method of measuring the volume fraction of the tissue.
[0030]
　Ferrite, pearlite, upper bainite, the measurement of the volume fraction of lower bainite and tempered martensite, a sample is taken as an observation plane section parallel to the parallel and the thickness direction to the rolling direction of a steel plate. Then, by polishing the observation surface, and nital etching, electrolytic emission at 5000 × magnification range of the thickness from the surface of the steel sheet when the t from the depth of t / 8 to a depth of 3t / 8 of the steel sheet scanning electron microscope observation at (field emission scanning electron microscope FE-SEM). By this method, it is possible to identify ferrite, pearlite, bainite and tempered martensite. Tempered martensite, upper bainite and lower bainite can be distinguished from each other by the presence or absence and the extension direction of the iron-based carbides within lath-like grains. Such observation performed on 10 fields, ferrite from the average value of 10 fields, pearlite, upper bainite, the area fraction of lower bainite and tempered martensite is obtained. Since the area fraction is equivalent to the volume fraction, it can be directly used as the volume fraction. In this observation, the number density of the iron-based carbide tempered martensite and in the lower bainite can also be specified.
[0031]
　In the measurement of the volume fraction of retained austenite, a sample was taken from the steel sheet, a portion to a depth of t / 4 from the surface of the steel sheet was chemically polished, the depth from the parallel surface of the steel sheet in the rolling surface of the t / 4 measuring the X-ray diffraction intensity in a plane. For example, the volume fraction Vγ of the retained austenite is represented by the following equation.
　V? = (I 200f + I 220f + I 311f ) / (I 200b + I 211b ) ×
100 (I 200f , I 220f , I 311f are respectively face-centered cubic (fcc) lattice phase (200), (220), (311 intensity of the diffraction peaks of), I 200b , I 211b has a body-centered cubic lattice, respectively (bcc) phase (200), indicating the intensity of the diffraction peak of (211).)
[0032]
　Fresh martensite and retained austenite, because they are not sufficiently corrosion by nital etching, can be distinguished ferrite, pearlite, bainite and tempered martensite. Therefore, it is possible to identify the volume fraction of fresh martensite by reducing the volume fraction Vγ of retained austenite from the volume fraction of the remainder of the FE-SEM observation.
[0033]
　Tempering the martensite and measurement of the effective crystal grain size of the bainite, electron backscatter diffraction (electron back-scatter diffraction: EBSD) performing the crystal orientation analysis by. In this analysis, it is possible to calculate the orientation difference between two adjacent measurement points. While thinking about the effective crystal grain size of tempered martensite and bainite There are various, the present inventors have found that the block boundary is a valid crystal units for crack propagation governing the toughness. Block boundary is generally because the heading difference can be determined in a region surrounded by the boundaries of more than 10 °, the crystal orientation map on measurements by EBSD, can be reflected by illustrating the boundaries with misorientation than 10 °. The circle equivalent diameter of the area surrounded by boundaries having a misorientation of such 10 ° or more and the effective crystal grain size. According to the verification by the present inventors, if the heading difference is regarded as the effective crystal grain boundaries between 10 ° or more measurement points are present, a significant correlation between the effective crystal grain boundary and the toughness is confirmed ing.
[0034]
　Next, a description will be given chemical composition of the slab used in the steel plate and its production according to the embodiment of the present invention. As described above, the steel sheet according to the embodiment of the present invention, hot rolling of the slab, cold rolling, is manufactured through a continuous annealing and tempering treatment. Therefore, the chemical composition of the steel sheet and slab, not only the properties of the steel sheet, is taken into consideration these processes. In the following description, a unit of content of each element contained in the steel sheet and slab "%" is especially meant to "mass%" unless otherwise specified. Steel sheet according to the present embodiment, by mass%, C: 0.15% ~ 0.45%, Si: 1.0% ~ 2.5%, Mn: 1.2% ~ 3.5%, Al: 0.001% ~ 2.0%, P: 0.02% or less, S: 0.02% or less, N: 0.007% or less, O: 0.01% or less, Mo: 0.0% ~ 1 .0%, Cr: 0.0% ~ 2.0%, Ni: 0.0% ~ 2.0%, Cu: 0.0% ~ 2.0%, Nb: 0.0% ~ 0.3 %, Ti: 0.0% ~ 0.3%, V: 0.0% ~ 0.3%, B: 0.00% ~ 0.01%, Ca: 0.00% ~ 0.01%, mg: 0.00% ~ 0.01%, REM: 0.00% ~ 0.01%, and the balance has Fe and impurities, in a chemical composition represented. As the impurity, those included in raw materials such as ores and scrap, intended to be included in the manufacturing process, is exemplified.
[0035]
　(C: 0.15% ~ 0.45%)
　C is or contribute to the improvement of the strength, or to contribute to the improvement of hydrogen embrittlement resistance through the formation of iron-based carbide. The C content is less than 0.15%, no sufficient tensile strength, for example, 980MPa or more tensile strength. Therefore, C content is 0.15% or more, desirably 0.18% or more. On the other hand, C in the amount of 0.45 percent, the martensitic transformation start temperature becomes extremely low, it can not be secured martensite sufficient volume fraction, the tempered martensite and the volume fraction of bainite of 70% or more It can not be with. Further, there is also insufficient strength of the weld. Therefore, C content is not more than 0.45%, preferably to 0.35% or less.
[0036]
　(Si: 1.0% ~ 2.5%) Si
　is or contribute to the improvement of strength, and suppress the precipitation of coarse iron-based carbides in the austenite, contributes to the creation of a stable residual austenite at room temperature to or. The Si content is less than 1.0%, can not be sufficiently suppressed precipitation of coarse iron-based carbide. Therefore, Si content is set to 1.0% or more, desirably 1.2% or more. On the other hand, Si content is 2.5 percent, formability embrittlement of the steel sheet is lowered. Therefore, Si content is set to 2.5% or less, desirably 2.0% or less.
[0037]
　(Mn: 1.2% ~
　3.5%) Mn is or contribute to the improvement of the strength, or to suppress ferrite transformation during the cooling after annealing. The Mn content is less than 1.2%, ferrite is excessively formed, is difficult to secure a sufficient tensile strength, for example, 980MPa or more tensile strength. Therefore, Mn content is 1.2% or more, desirably 2.2% or more. On the other hand, Mn content is 3.5%, the slab and hot-rolled steel sheet is excessively high strength, manufacturability is lowered. Therefore, Mn content is not more than 3.5%, preferably at most 2.8%. From the standpoint of manufacturability, Mn is preferably at most 3.00%.
[0038]
　(Al: 0.001% ~ 2.0%) Al
　is contained in unavoidably steel, to suppress the precipitation of coarse iron-based carbides in the austenite, the generation of stable retained austenite at room temperature contribute to. Al functions as a deoxidizer. Therefore, Al may be contained. On the other hand, Al content is 2.0%, the productivity is lowered. Thus, Al is 2.0% or less, desirably 1.5% or less. Costly in reducing the Al content, an attempt to reduce to less than 0.001%, the cost increases considerably. Therefore, Al content is 0.001% or more.
[0039]
　(P: 0.02% or less)
　P is not an essential element, is contained as an impurity, for example, in the steel. P is liable to segregate in the center in the thickness direction of the steel sheet, thereby embrittling the weld. Therefore, P content is better as low as possible. In particular, P content is 0.02 percent, a significant decrease in weldability. Accordingly, P content is 0.02% or less, preferably 0.015% or less. Costly in reducing the P content, an attempt to reduce to less than 0.0001%, cost increases considerably. Therefore, P content may be 0.0001% or more.
[0040]
　(S: 0.02% or less)
　S is not an essential element, is contained as an impurity, for example, in the steel. S lowers the hole expansion to form a coarse MnS. S is or reduces the weldability sometimes or reduce the production of casting and hot rolling. Therefore, S content is preferably as low as possible. In particular, S content is 0.02 percent, reduction of hole expansion is remarkable. Thus, S content is 0.02% or less, preferably to 0.005% or less. Costly in reducing S content, an attempt to reduce to less than 0.0001%, the cost remarkably increases, an attempt to reduce to less than 0.0001%, the cost is increased further significantly. Therefore, S content may be 0.0001% or more.
[0041]
　(N: 0.007% or less)
　N is not an essential element, is contained as an impurity, for example, in the steel. N is to form a coarse nitrides and deteriorates the bendability and hole expansibility. N is also the cause of the occurrence of a blow hole during welding. Therefore, N content is better as low as possible. In particular, an N content of 0.007 percent, a significant bending resistance and hole expansion drop in. Therefore, N content is set to 0.007% or less, preferably to 0.004% or less. Costly in reducing the N content, an attempt to reduce to less than 0.0005%, cost increases considerably. Therefore, N content may be 0.0005% or more.
[0042]
　(O: 0.01% or less)
　O is not an essential element, is contained as an impurity, for example, in the steel. O deteriorates the formability by forming an oxide. Therefore, O content is preferably as low as possible. In particular, O content is 0.01 percent, a decrease in moldability becomes remarkable. Therefore, O content is 0.01% or less, preferably to 0.005% or less. The reduction of the O content is costly, an attempt to reduce to less than 0.0001%, cost increases considerably. Therefore, O content may be 0.0001% or more.
[0043]
　Mo, Cr, Ni, Cu, Nb, Ti, V, B, Ca, Mg and REM is not an essential element, a steel plate and may optionally elements be appropriately contained in the limits of the predetermined amount to the slab.
[0044]
　(Mo: 0.0% ~ 1.0%, Cr: 0.0% ~ 2.0%, Ni: 0.0% ~ 2.0%, Cu: 0.0% ~
　2.0%) Mo , Cr, Ni and Cu, or contribute to the improvement of the strength, or to suppress ferrite transformation during the cooling after annealing. Therefore, Mo, Cr, Ni or Cu, or any combination thereof may be contained. To obtain this effect sufficiently, it is preferable that the Mo content is not less than 0.01%, Cr content is preferably 0.05% or more, Ni content is 0.05% or more it is preferred, Cu content is preferably 0.05% or more. On the other hand, if the Mo content is 1.0 percent, or the Cr content is 2.0 percent, or Ni content is 2.0 percent, or Cu content of 2.0% If it is super, production of hot rolling is lowered. Therefore, Mo content is set to 1.0%, Cr content is set to 2.0% or less, Ni content is 2.0% or less, Cu content is 2.0% or less. That, Mo: 0.01% ~ 1.0% , Cr: 0.05% ~ 2.0%, Ni: 0.05% ~ 2.0%, or Cu: 0.05% ~ 2.0% , or it is preferable that any combination of these holds.
[0045]
　(Nb: 0.0% ~ 0.3%, Ti: 0.0% ~ 0.3%, V: 0.0% ~
　0.3%) Nb, Ti and V, generates an alloy carbonitrides and contributes to improvement in strength by strengthening precipitation strengthening and grain refining. Therefore, Nb, Ti or V, or any combination thereof may be contained. To obtain this effect sufficiently, it is preferable that the Nb content is 0.005% or more, Ti content is preferably 0.005% or more, V content is 0.005% or more it is preferable. On the other hand, if the Nb content is 0.3 percent, when either the Ti content of 0.3 percent, or V content is 0.3%, the excess precipitation alloy carbonitrides formability is degraded by. Therefore, 0.3% or less of Nb content, Ti content is not more than 0.3%, V content is 0.3% or less. That, Nb: 0.005% ~ 0.3% , Ti: 0.005% ~ 0.3%, or V: 0.005% ~ 0.3%, or that any combination of these holds preferred .
[0046]
　(B: 0.00 The Pasento ～ 0.01 Pasento)
　B is, or to strengthen the grain boundaries, or to suppress the ferrite transformation in the cooling after annealing. Thus, B may be contained. To obtain this effect sufficiently, B content is preferably 0.0001% or more. On the other hand, B content is 0.01 percent, production of hot rolling is lowered. Therefore, B content is 0.01% or less. That, B: it is preferable to hold 0.0001% to 0.01%.
[0047]
　(Ca: 0.00% ~ 0.01%, Mg: 0.00% ~ 0.01%, REM: 0.00% ~
　0.01%) Ca, Mg and REM are the oxides and sulfides It contributes to the improvement of hole expansion by controlling the form. Therefore, Ca, Mg or REM, or any combination thereof may be contained. To obtain this effect sufficiently, it is preferable that the Ca content is 0.0005% or more, Mg content is preferably 0.0005% or more, REM content is 0.0005% or more it is preferable. On the other hand, if the Ca content is 0.01 percent, or Mg content is 0.01 percent, or the REM content is 0.01 percent, the deterioration manufacture of such castability to. Therefore, Ca content is 0.01% or less, Mg content is 0.01% or less, REM content is set to 0.01% or less. That, Ca: 0.0005% ~ 0.01% , Mg: 0.0005% ~ 0.01%, or REM: 0.0005% ~ 0.01%, or that any combination of these holds preferred .
[0048]
　REM (rare earth metals) refers Sc, Y, and total 17 kinds of lanthanoid elements, "REM content" means total content of these 17 kinds of elements. REM, for example, is added at mischmetal, mischmetal sometimes contain lanthanoid other La and Ce. The addition of REM, the metal La, may be used single metal such as metal Ce.
[0049]
　According to this embodiment, high tensile strength, for example 980MPa or more, preferably while obtaining the above tensile strength 1180 MPa, excellent ductility, hole expansion, hydrogen embrittlement resistance and toughness can be obtained.
[0050]
　Next, a method for manufacturing a steel sheet according to the embodiment of the present invention. In the method of manufacturing the steel sheet according to the embodiment of the present invention, it performs steel hot rolling having the above chemical composition, cold rolling, continuous annealing and tempering treatment and the like in this order.
[0051]
　(Hot rolling)
　The hot-rolling, performing rough rolling and finish rolling. Method for producing a slab subjected to hot rolling is not limited, may be used continuously cast slab may be used those produced by thin slab caster or the like. May also be carried out immediately hot rolled after continuous casting. Cast slab, after casting, without cooling, or once after cooling, heating above 1150 ° C.. The heating temperature is less than 1150 ° C., the finish rolling temperature tends to be lower than 850 ° C., the rolling load becomes higher. From the viewpoint of cost, the heating temperature is desirably set to less than 1350 ° C..
[0052]
　In rough rolling, rolling reduction at 1000 ° C. or higher 1150 ° C. or less performs at least once a rolling 40% or more, to fine the austenite before the finish rolling.
[0053]
　In the finish rolling, the continuous rolling using five-seven finish rolling mill are arranged at intervals of about 5 m. Then, a rolling of the final three stages at 1020 ° C. or less, the total reduction ratio in the rolling of the final three stages was 40% or more, the passing time of the rolling of the final three stages to 2.0 seconds or less. Further, to start water cooling by the elapsed time rolled from the 1.5 seconds of the final stage. Here, the rolling of the final three stages, means a rolled using the last three rolling mills. For example, when performing continuous rolling with six rolling mills, means rolling at six th rolling mills from four eyes total reduction ratio in the rolling of the final three stages, the four second rolling mill when the plate thickness t6 when the plate thickness coming out t4,6 units th rolling mill on entry, is calculated by "(t4-t6) / t4 × 100 (%)". Transit time of rolling of the final three stages, refers to the time from coming out of the steel plate from the four eyes of the rolling mill to come out of the six eyes of the rolling mill, the elapsed time from the rolling of the final stage, It refers to the time from coming out of the steel sheet from six eyes of the rolling mill until the water cooling is started. Between the rolling mill and a water cooling facility in the final stage, there may be sections that measure the properties of the steel sheet, such as temperature and thickness.
[0054]
　The grain refining of tissue after the finish rolling, rolling reduction in finishing rolling, between temperature and path time is important.
[0055]
　When the temperature of the steel sheet during the rolling of the final three stages is 1020 ° C. greater than can not be sufficiently fine austenite grains. Accordingly, rolling of the final three stages are carried out at 1020 ° C. or less. When performing continuous rolling with six rolling mills, in order to perform rolling of the final three stages at 1020 ° C. or less, the inlet side temperature of the four th rolling mill and 1020 ° C. or less, even by subsequent processing heat generated during rolling , the temperature of the steel sheet so as not to become a 1020 ℃ than.
[0056]
　If the total reduction ratio in the rolling of the final three stages is less than 40%, the cumulative rolling strain becomes insufficient, it can not be sufficiently fine austenite grains. Therefore, the total reduction ratio in the rolling of the final three stages is 40% or more.
[0057]
　The final three stages transit time rolling is dependent on the path between the time, the transit time is longer between longer paths time, recrystallization and grain growth of austenite grains is likely to progress between two consecutive rolling mills. Then, recrystallization and grain growth of the austenite grains the transit time of 2.0 seconds greater tends to be remarkable. Accordingly, the final three stages transit time rolling is less 2.0 seconds. From the viewpoint of suppressing recrystallization and grain growth of austenite grains, the elapsed time from the rolling of the final stage to the water-cooling start good the shorter. The elapsed time is the austenite grains recrystallization and grain growth at 1.5 seconds than likely noticeable. Thus, the elapsed time from the rolling of the final stage to the water-cooling start is less 1.5 seconds. Between the rolling mill and a water cooling facility in the final stage, if there is the section to measure the properties of the steel sheet, such as temperature and thickness, even if it is not possible to start the water cooling immediately elapsed time is less than 1.5 seconds if it is possible to suppress the recrystallization and grain growth of austenite grains.
[0058]
　In a range that does not inhibit the ability of the finish rolling, it cooled at a water cooling nozzle or the like immediately after the finish rolling, it is also possible to refine the austenite grains. After rough rolling, by joining a plurality of rough rolling plate obtained in rough rolling may be subjected to continuous finish rolling them. Further, the rough rolled sheet once wound up may be subjected to finish rolling while unwinding it.
[0059]
　Finish rolling temperature (completion temperature of finish rolling) is the 850 ° C. or higher 950 ° C. or less. When the finish rolling temperature is 2-phase region of austenite and ferrite, tissue of the steel sheet becomes uneven, it can not be obtained excellent formability. Further, the finish rolling temperature is less than 850 ° C., the rolling load becomes higher. From the viewpoint of fine austenite grains, the finishing rolling temperature is desirably to 930 ° C. or less.
[0060]
　Coiling temperature after hot rolling is set to 730 ° C. or less. The coiling temperature is 730 ° C. greater, it can not be an effective crystal grain size of tempered martensite and bainite in the steel sheet 5μm or less. Further, the coiling temperature is 730 ° C. greater, is formed a thick oxide on the surface of the steel sheet, pickling properties sometimes decrease. The effective crystal grain size in the fine and improve the toughness, from the viewpoint of improving the hole expansion by uniformly dispersing the residual austenite, the coiling temperature is desirably between 680 ° C. or less. The lower limit of the coiling temperature is not limited, the winding of below room temperature because technically challenging, the coiling temperature is desirably higher than room temperature.
[0061]
　After hot rolling, carried out once or two or more pickling hot-rolled steel sheets obtained by hot rolling. The pickling, oxides of the generated surface in the hot rolling is eliminated. Pickling contributes to the improvement of plating Improved and plated steel sheet of the chemical conversion of the cold rolled steel sheet.
[0062]
　Between the hot rolling to cold rolling, may be heated hot-rolled steel sheet to 300 ℃ ~ 730 ℃. By the heat treatment (tempering), hot-rolled steel sheet is softened, easily perform cold rolling. The heating temperature is 730 ° C. greater, since the microstructure during heating becomes two phases of ferrite and austenite, despite performed tempering treatment for the purpose of softening, strength of the hot-rolled steel sheet after cooling is it is likely to rise. Therefore, the temperature of the heat treatment (tempering) is set to 730 ° C. or less, preferably 650 ° C. or less. On the other hand, the heating temperature is lower than 300 ° C., is insufficient hot-rolled steel sheet is not sufficiently softened tempering effect. Therefore, the temperature of the heat treatment (tempering) is a 300 ° C. or higher, preferably 400 ° C. or higher. In the case of performing a long-time heat treatment at 600 ° C. or higher, various alloys carbides precipitated in the heat treatment, remelting of alloys carbide becomes difficult in the subsequent continuous annealing, the desired mechanical properties are obtained there is a possibility that the no.
[0063]
　(Cold rolling)
　after pickling is carried out cold rolling the hot-rolled steel sheet. Rolling reduction in cold rolling is 30% to 90%. The rolling reduction is less than 30%, the austenite grains become coarse in annealing, it can not be an effective crystal grain size of tempered martensite and bainite in the steel sheet 5μm or less. Therefore, the rolling reduction was 30% or more, desirably 40% or more. On the other hand, the reduction ratio is 90%, operation becomes difficult rolling load is too high. Therefore, the rolling reduction is 90% or less, desirably 70% or less. Rolling reduction in the number of rolling passes and each path is not limited.
[0064]
　(Continuous annealing)
　after cold rolling, performing a continuous annealing of the cold rolled steel sheet obtained by cold rolling. Continuous annealing is carried out, for example, in a continuous annealing line or continuous galvanizing line. Maximum heating temperature in the continuous annealing to 760 ℃ ~ 900 ℃. Is less than the maximum heating temperature is 760 ° C., tempered martensite and the volume fraction of bainite becomes less than 70% in total, it is impossible to achieve both hole expansion and hydrogen embrittlement resistance. On the other hand, the maximum heating temperature of 900 ° C. greater, austenite grains are coarsened, or not be an effective crystal grain size of tempered martensite and bainite in the steel sheet to 5μm or less, or to unnecessarily increase the cost.
[0065]
　In the continuous annealing, holding 760 ° C. ~ 900 to a temperature range of ° C. 20 seconds or more. The retention time is less than 20 seconds, continuously during annealing can not be sufficiently dissolved iron carbide, it will be less than 70% volume fraction of tempered martensite and bainite in total, hole expansion and resistance to hydrogen embrittlement not only it is impossible to achieve both properties, because residual carbides are coarse, degrading the hole expansion and toughness. From a cost perspective, the retention time is desirably to 1000 seconds or less. May be kept isothermally at the maximum heating temperature, it performs heat ramped, after reaching the maximum heating temperature immediately may initiate cooling.
[0066]
　In the continuous annealing, and the average heating rate from room temperature to a maximum heating temperature of 2 ° C. / sec or more. Is less than the average heating rate is 2 ° C. / sec, strain introduced by the cold rolling is released during heating, austenite grains are coarsened and can not the effective crystal grain size of tempered martensite and bainite in the steel sheet to 5μm or less .
[0067]
　After holding for 20 seconds or more to a temperature region of 760 ° C. ~ 900 ° C. is cooled to 0.99 ° C. ~ 300 ° C., in that case, the average cooling rate from the holding temperature to 300 ° C. and 5 ° C. / sec or more. The cooling stop temperature at this time is 300 ° C. greater than or cooling stop temperature is higher than the martensitic transformation starting temperature, even as the cooling stop temperature is equal to or less than the martensitic transformation start temperature, or not produced sufficient martensite to. Consequently, tempered martensite and the volume fraction of bainite becomes less than 70% in total, it is impossible to achieve both hole expansion and hydrogen embrittlement resistance. The cooling stop temperature is lower than 0.99 ° C., martensite excessively generated, the volume fraction of residual austenite is less than 8%. The average cooling rate from the holding temperature to 300 ° C. of less than 5 ° C. / sec, ferrite is excessively formed during cooling, sufficient martensite is not generated. From a cost standpoint, the average cooling rate is preferably to 300 ° C. / sec or less. Cooling method is not limited, for example, hydrogen gas cooling, roll cooling, it is possible to perform air cooling or water cooling, or any combination thereof. During this cooling, nucleation sites for precipitating fine iron-based carbide in tempered after is introduced into martensite. This cooling is important cooling stop temperature, holding time after stopping is not limited. This is despite tempered martensite and the volume fraction of bainite depends on the cooling stop temperature, the holding time is not dependent.
[0068]
　(Tempering)
　after up to 0.99 ° C. ~ 300 ° C. cooling, reheated to 300 ° C. ~ 500 ° C., held at this temperature range for more than 10 seconds. Hydrogen embrittlement resistance of the generated by the continuous annealing cooling quenching martensite of the low. The reheating to 300 ° C. ~ 500 ° C., martensite tempered returned, the number density of the iron-based carbide 1.0 × 10 6 (pieces / mm 2 a) above. Further, when the reheating, or to generate bainite, since the austenite F C martensite and bainite or diffuse the austenite is stabilized.
[0069]
　In the 500 ° C. greater reheating temperature (holding temperature), martensite is excessively tempered, sufficient tensile strength, for example, 980MPa or more tensile strength. Further, the precipitated iron-based carbide is coarsened may not sufficient hydrogen embrittlement resistance can not be obtained. Furthermore, also contain Si, carbides generated during the austenite, since the austenite is decomposed, the volume fraction of residual austenite is less than 8%, no sufficient formability can not be obtained. With decreasing volume fraction of retained austenite volume fraction of fresh martensite also be more than 10%. If it is less than the temperature of the reheating 300 ° C., tempering is insufficient, the number density of the iron-based carbide 1.0 × 10 6 (pieces / mm 2 does not become) The retention time is less than 10 seconds, the tempering is insufficient, the number density of the iron-based carbide 1.0 × 10 6 (pieces / mm 2 does not become) Further, enrichment of the austenite F C is insufficient, the volume fraction of residual austenite is less than 8%, may not be obtained sufficient formability. From a cost perspective, the retention time is desirably to 1000 seconds or less. 300 ° C. ~ may be kept isothermally at a temperature range of 500 ° C., it may be carried out cooling and heating in this temperature range.
[0070]
　In this way, it is possible to produce a steel sheet according to the embodiment of the present invention.
[0071]
　After tempering, Ni, Cu, Co, or Fe or may be subjected to plating treatment any combination thereof. By performing such plating treatment, thereby improving the chemical conversion treatability and coating properties. Further, the dew point is heated steel sheet in an atmosphere of -50 ° C. ~ 20 ° C., or may be by controlling the form of an oxide formed on the surface of the steel sheet to improve further chemical properties. Temporarily increasing the dew point in the furnace, chemical conversion treatability to adversely affect Si, it oxidizes Mn or the like in steel interior, may improve the chemical conversion treatability thereafter to perform the reduction processing. Further, it may be subjected to electroplating the steel sheet. Tensile strength of the steel sheet, ductility, hole expansion, hydrogen embrittlement resistance and toughness is not affected by the electroplating process. Steel sheet according to the present embodiment is also suitable as a material for electroplating.
[0072]
　It may also be carried out galvanizing treatment to the steel sheet. When performing galvanizing treatment, the continuous annealing and tempering treatment of the in a continuous galvanizing line, subsequently, the steel sheet is immersed in a plating bath temperature of the steel sheet as 400 ℃ ~ 500 ℃. The temperature of the steel sheet is less than 400 ° C., heat removal of the plating bath during immersion intrusion is large, part of the molten zinc to solidify, there is the appearance of the plating may be impaired. On the other hand, the temperature of the steel sheet at the 500 ° C. greater, there is a fear that operation troubles with increasing temperature of the plating bath may occur. If the temperature is lower than 400 ° C. of the steel sheet after the tempering process, it may be heated to 400 ° C. ~ 500 ° C. prior to immersion. Plating bath can be a pure zinc plating bath, in addition to Fe zinc, Al, Mg, Mn, Si, or Cr or may contain any combination thereof.
[0073]
　In this way, it is possible to obtain a galvanized steel sheet having a plating layer mainly composed of Zn. Fe content of the plating layer of the galvanized steel sheet is generally less than 7%.
[0074]
　It may be carried out alloying treatment with respect to hot-dip galvanized steel sheet. Temperature of the alloying process to 450 ℃ ~ 550 ℃. Is lower than the temperature of the alloying treatment is 450 ° C., slow progress of alloying, the productivity is low. The temperature of alloying is 550 ° C. greater than or longer austenite provides excellent moldability by decomposing, sufficient tensile strength tempered martensite is excessively softened to may become impossible to obtain.
[0075]
　In this way, it is possible to obtain a galvannealed steel sheet. Fe content of the coating layer of the galvannealed steel sheet is generally at least 7%. The melting point of the coating layer of the galvannealed steel sheet is higher than the melting point of the plating layer of the galvanized steel sheet, galvannealed steel sheet is excellent in spot weldability.
[0076]
　In plating treatment, Sendzimir method, the total reduction furnace method may employ any of the flux method. The Sendzimir method, degreased pickled, heated in a non-oxidizing atmosphere, H 2 and N 2 after annealing at reducing atmosphere containing, cooled to the plating bath temperature near, is immersed in the plating bath. The total reduction furnace method, to adjust the atmosphere during annealing, first, after oxidizing the surface of the steel sheet is dipped in a plating bath after the cleaning before plating by subsequent reduction. The flux method, was degreased pickled steel sheet by performing a flux treatment using such ammonium chloride, are immersed in the plating bath.
[0077]
　After tempering, after plating or after alloying treatment may be carried out skin pass rolling. Reduction ratio of the skin pass rolling is 1.0% or less. The reduction rate of 1.0 percent, the volume fraction of residual austenite is significantly lowered during skin pass rolling. The reduction ratio is less than 0.1%, less effect of the skin pass rolling, control is also difficult. Skin pass rolling may be performed in-line in a continuous annealing line, after completion of the continuous annealing in the continuous annealing line may be performed off-line. Skin pass rolling may be performed in one, so that the total rolling reduction of 1.0% or less, may be divided into a plurality of times.
[0078]
　The above embodiments are all merely illustrate concrete examples of implementing the present invention, in which technical scope of the present invention should not be limitedly interpreted. That is, the present invention is its technical idea or without departing from the essential characteristics thereof, can be implemented in various forms.
Example
[0079]
　Next, a description will be given of an embodiment of the present invention. Conditions in examples are an example of conditions adopted for confirming the workability and effects of the present invention, the present invention is not limited to this single example of conditions. The present invention does not depart from the gist of the present invention, as long as they achieve the object of the present invention, it is capable of adopting various conditions.
[0080]
　Slabs having the chemical compositions shown in Table 1 was heated to 1230 ° C., the thickness of performing hot rolling to obtain a hot rolled steel sheet of 2.5mm under the conditions shown in Table 2 and Table 3. In hot rolling, a rough rolling and six rolling mill performs water cooling after finish rolling using, taken subsequently winding the hot-rolled steel sheet. Table "CR" 2, and steels in Table 3 shows the cold-rolled steel sheet, "GI" represents a galvanized steel sheet, "GA" represents an alloyed hot-dip galvanized steel sheet. Table 2 and "extraction temperature" in Table 3 is the temperature of the slab when extracted from the heating furnace in the slab heating before the rough rolling. "Pass number" is the number of paths of the rolling reduction ratio of 40% or more at 1000 ° C. or higher 1150 ° C. or less. "First pass between the time" is the time from when the steel plate is coming out of the four eyes of the rolling mill until it enters in five eyes of the rolling mill, "between the first and second pass time" steel plate is five eyes is the time from coming out of the rolling mill until it enters the six eyes of the rolling mill. "Elapsed time" is the time from when the come out steel plate from six eyes of the rolling mill until the water cooling is started, "passing time", from coming out of the steel plate from the four eyes of the rolling mill 6 is the time to come out from the base eyes of the rolling mill. "Total reduction rate", when the plate thickness of the case came out the plate thickness at the time of entering the four eyes of the rolling mill from t4,6 single day of the rolling mill and t6, "(t4-t6) / t4 is calculated by × 100 (%) ". The remainder of the chemical compositions shown in Table 1 is Fe and impurities. Underlined in Table 1 indicates that the value is out of range of the present invention. It underlined in Table 2 and in Table 3 indicates that the numerical value is outside the range suitable for the manufacture of steel sheet according to the present invention.
[0081]
[Table 1]

[0082]
[Table 2]

[0083]
[table 3]

[0084]
　Then, the hot-rolled steel sheet was pickled, thickness was obtained cold-rolled steel sheet of 1.2mm by performing cold rolling. Thereafter, continuous annealing and tempering treatment of cold-rolled steel sheet under the conditions shown in Table 4 and Table 5, the rolling rate was 0.1% skin pass rolling. The continuous annealing was the holding temperature shown in Table 4 and Table 5 and the maximum heating temperature. The cooling rate is an average cooling rate from the holding temperature to 300 ° C.. For some samples were galvanizing treatment between the tempering and the skin pass rolling. Basis weight of this time, both surfaces of about 50 g / m 2 was. For some it was galvanizing treatment samples were alloying treatment under the conditions shown in Tables 4 and 5 between the galvanizing treatment and skin pass rolling. Using a continuous galvanizing line for galvanizing treatment was performed continuously continuous annealing, tempering and galvanizing treatment. Table 4 and underlined in Table 5 indicates that the numerical value is outside the range suitable for the manufacture of steel sheet according to the present invention.
[0085]
[Table 4]

[0086]
[table 5]

[0087]
　Then, by observing the steel structure of the steel sheet after skin pass rolling was measured number density and the average size of the volume fraction and iron carbide of each organization. The results are shown in Tables 6 and 7. It underlined in Table 6 and in Table 7 indicates that the value is out of range of the present invention. Table 6 and the "average length" in Table 7 means a mean length of the long axis of the iron-based carbides, that the blank, since the number density of the iron-based carbide is too low, which could not be measured show.
[0088]
[Table 6]

[0089]
[Table 7]

[0090]
　Furthermore, the strength of the steel sheet after skin pass rolling, the ductility, hole expansion, the evaluation of hydrogen embrittlement resistance and toughness was carried out.
[0091]
　In the evaluation of strength and ductility, a JIS5 No. 5 test pieces with the longitudinal direction and the direction perpendicular to the rolling direction were taken from the steel sheet, subjected to a tensile test according to JISZ2242, it was measured tensile strength TS and total elongation El. In the evaluation of the hole expansion, it performs a spread hole in compliance test in Japan Iron and Steel Federation standard JFST1001, to measure the hole expanding ratio λ. The results are shown in Tables 8 and 9. It underlined in Table 8 and in Table 9 indicates that the numerical value is outside the desired range. The desired range here, the tensile strength TS at least 980 MPa, an indication of ductility (TS × El) is 15,000 MPa% or more, hole expansion index (TS 1.7 × lambda) is 5000000MPa 1.7 is% or more .
[0092]
　In the evaluation of hydrogen embrittlement resistance, a strip-shaped test piece of 100 mm × 30 mm whose longitudinal direction and a direction perpendicular to the rolling direction were taken from the steel plate, to form a hole for stressing at both ends thereof. Then, the test piece was bent at a radius 10 mm, a strain gauge is attached to the surface of the bending apex of the specimen, through the bolt holes at both ends, fitted with a nut to the tip of the bolt. Then, by applying a stress to the test piece by tightening the bolts and nuts. Stress applied to the the maximum tensile 60% strength TS and 90% as measured in a separate tensile test, when the applied stress was converted to stress strain which can be read from the strain gauges in Young's modulus. Then immersed in ammonium thiocyanate solution, the current density of 0.1 mA / cm 2 electrolyzed hydrogen charging, the evolution was observed cracks after 2 hours. Then, the maximum without break at 60% of the load stress of the tensile strength TS, the maximum "possible" what was broken at 90% of the load stress of the tensile strength TS, "bad" what was broken in both conditions, any of the conditions But were the ones that did not break and determined to be "good". The results are shown in Tables 8 and 9. In the Tables 8 and 9, represents the "good" in the "○", represents a "possible" in the "△", represents the "bad" in the "×". It underlined in Table 8 and in Table 9 indicates that the numerical value is outside the desired range.
[0093]
　In the evaluation of the toughness, it was Charpy impact test. Test level, a constant plate thickness at 1.2 mm, three times at -40 ℃ test temperature was measured absorbed energy at -40 ℃. The results are shown in Tables 8 and 9. It underlined in Table 8 and in Table 9 indicates that the numerical value is outside the desired range. The desired range here, the absorption energy is 40 J / cm 2 is not less than.
[0094]
[Table 8]

[0095]
[Table 9]

[0096]
　Table 8 and Table 9, Samples A-1 are within the scope of the invention, A-6, A-8, B-1, C-1, D-1, E-1, F-1, G -1, G-3, G-4, G-7, H-1, I-1, J-1, K-1, L-1, M-1, N-1, O-1, P-1 , Q-1, the R-1, S-1, S-7, T-1, U-1, V-1, W-1, W-3, X-1 and Y-1, excellent tensile strength , it was possible to obtain ductility, hole expansion, the hydrogen embrittlement resistance and toughness.
[0097]
　On the other hand, in Sample A-2, the volume fraction of residual austenite is too low, too high volume fraction of fresh martensite, tempered martensite and the total volume fraction of bainite is too low, the number density of the iron-based carbide too low, ductility, hole expansion, hydrogen embrittlement characteristics and toughness was low.
　Sample A-3, the volume fraction of residual austenite is too low, and tempered martensite and the total volume fraction of bainite is too high, ductility is low.
　Sample A-4, the volume fraction of residual austenite is too low, the volume fraction of fresh martensite is too high, the number density of the iron-based carbide is too low, ductility, hole expansion, and toughness was low.
　Sample A-5, the volume fraction of residual austenite is too low, tempered effective crystal grain size of the martensite and bainite is too large, ductility, hole expansion and toughness was low.
　Sample A-7, too low volume fraction of retained austenite, the ductility and toughness was low.
　Sample A-9, too the volume fraction of residual austenite is low, ductility, hole expansion, and toughness was low.
　Sample A-10, the volume fraction of the ferrite is too high, the volume fraction of residual austenite is too low, the effective crystal grain size of tempered martensite and bainite is too large, hole expansion and toughness was low.
　Sample A-11, too low volume fraction of retained austenite, a volume fraction of fresh martensite is too high, and the number density of the iron-based carbide is too low, hole expansion, hydrogen embrittlement lower characteristics and toughness It was.
[0098]
　Sample G-2, the volume fraction of the ferrite is too high, too low volume fraction of retained austenite, tempered martensite and the total volume fraction of bainite is too low, the effective crystal grain size of tempered martensite and bainite too large, hole expansion and toughness was low.
　Sample G-5, the volume fraction of residual austenite is too low, and the number density of the iron-based carbide is too low, ductility, hole expansion and toughness was low.
　Sample G-6, and the volume fraction of residual austenite is too low, ductility is low.
　Sample G-8, the volume fraction of the ferrite is too high, the volume fraction of residual austenite is too low, the volume fraction of fresh martensite is too high, the effective crystal grain size of tempered martensite and bainite is too large, iron number density of carbides is too low, ductility, hole expansion, hydrogen embrittlement characteristics and toughness was low.
　Sample G-9, the volume fraction of residual austenite is too low, and tempered martensite and the total volume fraction of bainite is too high, ductility is low.
[0099]
　Sample S-2, too large effective crystal grain size of tempered martensite and bainite, hole expansion, the hydrogen embrittlement resistance and toughness was low.
　Sample S-3, and the effective crystal grain size of tempered martensite and bainite is too large, hole expansion and toughness was low.
　In the sample S-4, too large effective crystal grain size of tempered martensite and bainite, toughness is low.
　Sample S-5, too low volume fraction of retained austenite, fresh volume fraction of martensite is too high, tempered martensite and the total volume fraction of bainite is too low, tempered martensite and bainite effective grain diameter is too large, the number density of the iron-based carbide is too low, ductility, hole expansion, hydrogen embrittlement characteristics and toughness was low.
　Sample S-6, and the effective crystal grain size of tempered martensite and bainite is too large, hole expansion and toughness was low.
　Sample S-8, so large is the effective crystal grain size of tempered martensite and bainite, toughness is low.
　In the sample S-9, and the number density of the iron-based carbide is too low, hole expansion, the hydrogen embrittlement resistance and toughness was low.
　Sample S-10, the volume fraction of the ferrite is too high, too low volume fraction of retained austenite, tempered martensite and the total volume fraction of bainite is too low, the effective crystal grain size of tempered martensite and bainite too large, hole expansion, the hydrogen embrittlement resistance and toughness was low.
　In the sample S-11, the volume fraction of residual austenite is too low, too high, the volume fraction of fresh martensite, hole expansion, the hydrogen embrittlement resistance and toughness was low.
　In the sample S-12, the volume fraction of residual austenite is too low, the volume fraction of pearlite is too high, the effective crystal grain size of tempered martensite and bainite becomes too large, hole expansion, hydrogen embrittlement characteristics and toughness was low.
　Sample S-13, too low volume fraction of retained austenite, too high volume fraction of fresh martensite, ductility and resistance to hydrogen embrittlement is low.
　In the sample S-14, too low volume fraction of retained austenite, hole expansion, hydrogen embrittlement characteristics and toughness was low.
　Sample W-2, fresh too the volume fraction of martensite is high, is too low volume fraction of residual austenite, the ductility was low.
[0100]
　Sample a-1, C content is too low, the volume fraction of the ferrite is too high, the volume fraction of residual austenite is too low, the volume fraction of fresh martensite is too high, the sum of tempered martensite and bainite the volume fraction is too low, ductility, hole expansion, and toughness was low.
　Sample b-1, C content is too high, too the volume fraction of residual austenite is low, ductility, hole expansion, the hydrogen embrittlement resistance and toughness was low.
　Sample c-1, Si content is too low, the volume fraction of the ferrite is too high, the volume fraction of residual austenite is too low, the volume fraction of fresh martensite is too high, the sum of tempered martensite and bainite and the volume fraction is too low, ductility was low.
　Sample d-1, Mn content is too low, the volume fraction of the ferrite is too high, too low volume fraction of retained austenite, tempered martensite and the total volume fraction of bainite is too low, ductility, hole expansibility, hydrogen embrittlement resistance and toughness was low.
　Sample e-1, and P content is too high, hole expansion, the hydrogen embrittlement resistance and toughness was low.
　Sample f-1, and the S content is too high, hole expansion, the hydrogen embrittlement resistance and toughness was low.
　Sample g-1, Al content is too high, the volume fraction of the ferrite is too high, the volume fraction of residual austenite is too low, the volume fraction of fresh martensite is too high, the sum of tempered martensite and bainite and the volume fraction is too low, hole expansion, the hydrogen embrittlement resistance and toughness was low.
　Sample h-1, the effective crystal grain size of tempered martensite and bainite is too large. Therefore, hole expansion, and toughness was low.
　Sample i-1, the effective crystal grain size of tempered martensite and bainite is too large. For this reason, the toughness was low.
　Sample j-1, the effective crystal grain size of tempered martensite and bainite is too large. For this reason, the toughness was low.
　Sample k-1, the effective crystal grain size of tempered martensite and bainite is too large. For this reason, the toughness was low.
[0101]
　Focusing on the production method, in Sample A-2, cooling stop temperature is too high in the continuous annealing. Therefore, excessively high volume fraction of fresh martensite, too low volume fraction of residual austenite becomes too low tempered martensite and the total volume fraction of bainite, the number density of the iron-based carbide is low It was too.
　Sample A-3, cooling stop temperature is too low in the continuous annealing. Thus, too low volume fraction of the retained austenite, tempered martensite and the total volume fraction of bainite becomes too high.
　Sample A-4, the holding temperature in the tempering treatment was too low. Therefore, excessively high volume fraction of fresh martensite volume fraction of residual austenite becomes too low, the number density of the iron-based carbide is too low.
　Sample A-5, the holding temperature in the tempering treatment was too high. Therefore, the volume fraction of residual austenite becomes too low, the effective crystal grain size of tempered martensite and bainite are too large.
　Sample A-7, retention time in the tempering treatment was too short. Therefore, the volume fraction of residual austenite becomes too low.
　Sample A-9, the temperature of the alloying treatment was too high. The volume fraction of the residual austenite is too low.
　Sample A-10, was holding temperature in the continuous annealing is too low. Therefore, excessively high volume fraction of ferrite, the volume fraction of residual austenite becomes too low, the effective crystal grain size of tempered martensite and bainite are too large.
　Sample A-11, cooling stop temperature is too high in the continuous annealing. Therefore, excessively high volume fraction of fresh martensite volume fraction of residual austenite becomes too low, the number density of the iron-based carbide is too low.

The scope of the claims
[Requested item 1]
　By
　mass%,
　C:
　0.15% ~ 0.45%, Si: 1.0% ~ 2.5%,
　Mn: 1.2% ~ 3.5%, Al: 0.001% ~ 2.0
　%, P: 0.02% or
　less, S: 0.02% or
　less, N: 0.007% or
　less, O: 0.01% or
　less, Mo: 0.0% ~ 1.0%,
　Cr: 0.
　%
　~ 2.0 0%, Ni: 0.0% ~
　2.0%, Cu: 0.0% ~ 2.0%, Nb: 0.0%
　~ 0.3%, Ti: 0.0%
　0.3%

　0.0%~, ~ 0.3%, B: 0.00% ~ 0.01%, Ca:
　0.00% ~ 0.01%, Mg: 0.00% ~ 0
　% .01, REM: 0.00% ~ 0.01%, and
　balance: Fe and impurities,
have in chemical composition represented,
　by volume fraction,
　tempered martensite and bainite: 70% or more in total 92 % less than,
　retained austenite: 8% Or less than 30%,
　ferrite: less than 10%,
　fresh martensite: less than 10%, and
　perlite: less than 10%,
in having a steel structure represented,
　the number density of the iron-based carbide tempered martensite and in the lower bainite There 1.0 × 10 6 (pieces / mm 2 and a) above,
　steel sheets effective crystal grain size of tempered martensite and bainite is equal to or is 5μm or less.
[Requested item 2]
　In the chemical composition, by
　mass%,
　Mo: 0.01% ~ 1.0%,
　Cr: 0.05% ~ 2.0%, Ni: 0.05% ~ 2.0%, or
　Cu: 0. 0.05% to 2.0%
　or steel sheet according to claim 1, characterized in that any combination of these holds.
[Requested item 3]
　In the chemical composition, by
　mass%, Nb:
　0.005% ~ 0.3%, Ti: 0.005% ~ 0.3%, or
　V: 0.005% ~ 0.3%,
　or any steel sheet according to claim 1 or 2, characterized in that the combination of holds.
[Requested item 4]
　In the chemical composition, by
　mass%, B: 0.0001% ~ 0.01%,
the steel sheet according to any one of claims 1 to 3, characterized in that holds to.
[Requested item 5]
　In the chemical composition, by
　mass%,
　Ca: 0.0005% ~ 0.01%, Mg: 0.0005% ~ 0.01%, or
　REM: 0.0005% ~ 0.01%,
　or any steel sheet according to any one of claims 1 to 4, characterized in that the combination of holds.