0001]The present invention relates to a hot-rolled steel sheet.
BACKGROUND
[0002]The steel sheet used in the vehicle body structure, from the viewpoint of improvement and weight reduction of safety, and a high strength and high press workability is required. In particular, in order to improve the press formability, during machining while maintaining the ductility, high-strength steel sheet to ensure collision resistant is when mounted on the automobile is demanded.
[0003]
　Such steel plates, strain-induced transformation type steel sheet having a mixed structure containing retained austenite is known (for example, see Patent Document 1). In the following description, the strain-induced transformation type steel sheet may be referred to as TRIP (Transformation Induced Plasticity) steel.
[0004]
　Furthermore, in order to cope with requirements of complicated shapes in recent years the automobile weight and parts, good mixed structure steel sheet and a high elongation and local elongation than conventionally been proposed. For example, Patent Document 2, a ferrite phase and a hard second phase (martensite, residual austenite) in tissue consisting of, at the time of cooling after hot rolling, by precipitating alloy carbides in the ferrite phase, the ferrite phase reinforced steel plate has been proposed. Incidentally, in the following description, there are cases where the soft tissue and hard tissue balance well dispersed steel martensite or the like, such ferrite as in Patent Document 2, referred to as DP (Dupal Phase) steel.
[0005]
　In Patent Document 3, in the phase interface during transformation from austenite to ferrite, mainly using a mixed structure of precipitation distribution and precipitation strengthening ferrite controlled residual austenite by precipitation phenomena at grain boundary diffusion , high-strength steel sheet has been proposed which is excellent in elongation and local elongation.
[0006]
　Patent Document 4 discloses excellent tensile strength 540MPa or more processing-induced transformation type composite structure steel sheet in burring workability. Patent Document 5, high formability hot-rolled high strength steel sheet excellent in a small hot-rolled TRIP steels That material homogeneity of the coil in the material change is disclosed. Patent Document 6, the occurrence of cracks at the time when an impact load is applied is suppressed, and further can provide the steel material disclosed high impact absorbing member for effective flow stress. Patent Document 7, stretch flangeability, DP steel sheet that excellent high strength composite structure hot-rolled steel sheet in corrosion resistance after painting and notch fatigue characteristics is disclosed. Then, Patent Document 8, a high Young's modulus steel sheet excellent in hole expandability are disclosed.
CITATION
Patent Document
[0007]
Patent Document 1: JP-A-10-158735 Patent Publication
Patent Document 2: JP 2009-84648 JP
Patent Document 3: JP 2011-225941 Patent Publication
Patent Document 4: JP 2002-129286 Patent Publication
Patent Document 5: JP 2001-152254 JP
Patent Document 6: JP 2015-124411 Patent Publication
Patent Document 7: WO 2016/133222 Patent
Patent Document 8: JP 2009-19265 JP
Summary of the Invention
Problems that the Invention is to Solve
[0008]
　Complexity of vehicle body structure, with the complexity of the part shape, machining of automotive steel sheets, not only the elements of the conventional press working, a new processing element to conventional pressing elements such as plate forging it has been combined in a complex manner. The conventional stamping elements, for example deep drawing, hole expansion, stretch forming process, bending, was elements such ironing.
[0009]
　However, pressing typified by recent plate forging, a conventional pressing element of the, by further dispersing the press load, by applying a partial compressive load, processing elements of the forging, for example, laid write processing , thickening (thickening) processing, processing elements also have been added, such as. That is, the plate forging, the other processing elements at the time of pressing a steel plate as in the prior art, a press working with complex processing elements including forging specific processing elements.
[0010]
　By performing such a plate forging by conventional press working, or while the plate thickness of the steel sheet of the original thickness, while the molded part is performed to deform the steel sheet with the thickness reduction (thinning), the partially portion receiving the forging depends compressive force, by the thickness of the steel sheet is thickened (thickening), be efficiently deformed so as to be the thickness of the steel sheet of the functions necessary for points can be, it is possible to secure the strength of the part.
[0011]
　Conventional TRIP steels, the conventional press working are known to exhibit a good moldability. However, the plate-forging a molding method including element of forging the conventional press working, cracks on the steel sheet even with a small working ratio has been found that sometimes occurs fracture.
[0012]
　That is, in the conventional press working, but pressing crack occurs in a portion where the plate thickness constriction (thickness reduction of the sheet thickness of the steel sheet) is generated, even in the processing without constriction thickness as the plate forging, cracks on the material there it was found that there is a case that can not be obtained finished products to generate break.
[0013]
　Limitation of such plate forging crack occurrence, have been dominated by what the nature of the steel plate, little is known about how can be improved if. Therefore, a function of the conventional TRIP steel, deep drawability, hole expandability, stretch forming processability, such while effectively utilizing the function, TRIP steel does not break even if the plate forging has been demanded.
[0014]
　The present invention has been made to solve the above problems, while maintaining the basic functions of a TRIP steel, improve the cracking limit of the received partial forging partially compressive force takes and to provide a good hot-rolled steel sheet into a plate forgeability capable of.
Means for Solving the Problems
[0015]
　The present invention has been made to solve the above problems, and the gist of the hot rolled steel sheet below.
[0016]
　(1) Chemical Composition ga,で
　mass%,
　C: 0.07
　~ 0.22%, Si: 1.00 ~ 3.20%, Mn: 0.80
　~ 2.20%, of Al: 0.010 ~
　% 1.000, N: 0.0060% or
　less, P: 0.050% or
　less, S: 0.005% or
　less,
　of Ti: 0 ~ 0.150%, of Nb: 0 ~ 0.100%,
　V: 0 　0.300%
~, 　a Cu: 0 ~ 2.00%, of Ni: 　0 ~ 2.00%, of Cr: 0 ~ 2.00%, of 　Mo: 0 ~ 1.00%, B: 0.0100 ~ 0% , 　of 　Mg: 　0.0100 ~ 0%, of 　Ca: 0.0100 ~ 0%, the REM: 0 ~ 　0.1000%, Zr: 0 ~ 1.000%, of Co: 0 ~ 　1.000%, a Zn: 0 ~ 　% 1.000, W 　is: 0 ~ 1.000%, of Sn: 0 ~ 0.050%, yoびお, 　remnants: Fe impuritiesおyoびでthou ri,

　In the rolling direction and the cross section perpendicular of the steel sheet, the width and thickness of the steel sheet is W and t, respectively, at the end face of the steel plate 1 / 4W or 3 / 4W, and, 1 / 4t from the surface of the steel plate or metal structure in the position of the 3 / 4t is in area%,
　residual austenite: 10% more than 2% or less,
　martensite: 2% or less,
　bainite: 10% to 70%,
　perlite 2% or less,
　the balance being ferrite , and the
　average circle equivalent diameter of the metal phase consisting retained austenite and / or martensite is the 1.0 ~ 5.0 .mu.m,
　the mean value of the shortest distance of the metal phase adjacent is at 3μm or more,
　the nano-hardness standard deviation is less 2.5 GPa,
　hot-rolled steel sheet.
[0017]
　(2) a tensile strength of not less than 780 MPa,
　the thickness is 1.0 ~ 4.0 mm,
　hot-rolled steel sheet according to (1).
The invention's effect
[0018]
　According to the present invention, deep drawability, while maintaining the basic functions of a TRIP steel such bulging formability, it becomes possible to obtain a good hot-rolled steel sheet into a plate forgeability.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019]
FIG. 1 is a schematic diagram for explaining the simple shear test. 1 (a) is a diagram showing a test piece of a simple shear test. Figure 1 (b) is a diagram showing a test piece after simple shear test.
DESCRIPTION OF THE INVENTION
[0020]
　The present inventors have conducted intensive investigations to solve the above problems, the following findings were obtained.
[0021]
　(A) the equivalent plastic strain
　plate forging includes deformation at strain range exceeding the breaking strain of the conventional tensile test (high strain region). The plate forging for complex processing, can not be evaluated by simply tensile test and the shear test data. Accordingly, the present invention have found that, by introducing the "equivalent plastic strain" as an indicator, has established a new evaluation method.
[0022]
　By using the equivalent plastic strain as an index, tensile test tensile and stress and tensile strain at break when the, the shear stress and shear strain at break when the shear test, to be able to evaluate the complex heading was.
[0023]
　Equivalent plastic strain, the relationship between the shear stress σs and shear plastic strain εsp in simple shear test, different variations and converts the relationship between the tensile stress σ and the tensile strain ε in uniaxial tensile test . Then, assuming isotropic hardening law and the plastic work conjugate relationship, by using the conversion factor is a constant (kappa), it can be converted by the following equation. By the method described below, after calculating the transform coefficients (kappa), corresponding to derive the plastic strain.
　Shear stress [sigma] s × kappa in tensile stress sigma = simple shear test at uniaxial tensile testing
　shear plastic strain εsp / κ in tensile strain epsilon = simple shear test at a tensile uniaxial test
[0024]
　(B) multi-stage shear test
　to determine the equivalent plastic strain, tensile and stress and tensile strain relationship tensile by the test, it is necessary to obtain the relationship between shear stress and shear strain due to shear test. However, the plate forging includes a deformation at high strain region. Therefore, when tested in once with shear test apparatus normally used, cracks will proceeds to the test strip from the portion that holds the test piece. As a result, often it can not be tested deformation up to high strain region. Therefore, a method of reproducing a machining plate thickness reduction of the thickness of the steel plate, such as plate-forging (thinning and constriction) does not occur is necessary.
[0025]
　Accordingly, we performed separately shear test in multiple stages, after each shear test of each step, and machining the origin of crack of the specimen occurring in a portion that holds the specimen, crack specimens There should not proceed, it was decided to evaluate the test results by connecting these shear test results in series. By applying this test method, it is possible to obtain a shear test results up to high strain region, it is possible to obtain the relationship between shear stress and shear strain to high strain region.
[0026]
　On the other hand, the tensile stress and tensile strain can be applied a conventional tensile test method. For example, it is possible to use a JIS5 No. specimen based on JIS Z2241 (2011).
[0027]
　(C) the mechanism of crack initiation
　and multistage shear test described above, the evaluation method using the equivalent plastic strain, by adopting the micro survey of the steel sheet before and after the plate forging, the cracking mechanism, the following findings Obtained.
[0028]
　Hard phase (martensite, residual austenite) and, from the difference between the deformability of the soft phase (ferrite, bainite), voids (microvoids) are generated at the interface of both phases. Thereafter, the distortion of the plate forging is increased, voids are grown, it will crack combined with adjacent voids, to fracture. Therefore, to prevent the occurrence of voids, and, even if voids are grown, if inhibit the binding of the adjacent voids, the cracks can be suppressed. However, it is also important not to impair the inherent function of the TRIP steel at that time. In the following description, it referred to as a hard phase are collectively retained austenite and martensite. Hard phase is exactly the same as the "metal phase composed of residual austenite and / or martensite" described in "claims".
[0029]
　And we found the following items from these findings.
[0030]
　(I) limiting the average diameter of the hard phase.
　In other words, the voids to occur at the boundary between the hard phase and (non-hard phase) metal phase, by limiting the average diameter of the hard phase, voids can be reduced.
[0031]
　(Ii) reducing the nano hardness variation.
　That is, by reducing as much as possible difference in hardness between the hard and soft phases, the generation of voids can be reduced.
[0032]
　And (iii) limiting the distance of the hard phase with each other.
　Nachi Suwa, voids for generating at the boundary between the hard phase and the other metal phase (soft phase), by placing away hard phase can be difficult to bind even voids are grown.
[0033]
　(Iv) that the equivalent plastic strain at break is 0.50 (50%) or more.
　By satisfying the condition of said (i) ~ (iii), the equivalent plastic strain at break is 0.50 (50%) or more, also in the complex processing such as a leaf forging, certain processability it was confirmed that it is possible to collateral.
[0034]
　(D) effective accumulated strain
　in order to obtain the tissue of the (i) ~ (iv), in a multistage finish rolling carried out in continuous rolling of 3 or more stages in hot rolling (e.g. six-stage or seven-stage), final as three stages cumulative strain in rolling (hereinafter sometimes described as "effective accumulated strain") is 0.10-0.40, it is necessary to perform a final finish rolling.
[0035]
　Effective accumulated strain, temperature during rolling, recovery of the crystal grains due to rolling reduction of the steel sheet by rolling, is an index that takes into account the recrystallization and grain growth. Therefore, when determining the effective cumulative strain was used constitutive law representing the static recovery phenomenon with time after rolling. The crystal grains are considered to be static recover with time after rolling, the release of stored energy as a distortion in the crystal grains after rolling, the static recovery by annihilation of dislocations of thermal grain This is because occur. Then, the disappearance of thermal rearrangement is to be affected to the time elapsed after the rolling and the rolling temperature. Accordingly, this static recovery be considered, the temperature during rolling, the rolling reduction of steel by rolling (logarithmic strain), introducing the index describing the time course after rolling as a parameter, which the "effective accumulated strain" defined.
[0036]
　Thus, by limiting the effective cumulative strain, the average circle equivalent diameter of the hard phase is limited, is limited distance between adjacent hard phase, the variation in nano-hardness is reduced. The effect is to suppress void growth occurring at the interface of the hard and soft phases can be difficult to bind voids grow. Thus, a crack does not occur even if the plate forging, it is possible to obtain an excellent steel sheet plate forgeability.
[0037]
　The present invention has been made based on the above findings. It will be described in detail below each requirement of the present invention.
[0038]
　(A) Chemical composition
　reasons for limiting each element are as follows. Incidentally, "%" for the content in the following description means "mass%".
[0039]
　C: 0.07 ~ 0.22%
　C is an element effective for securing retained austenite to enhance the strength. Can not be increased when the C content is too low the strength enough, also can not be secured retained austenite. On the other hand, the amount of residual austenite content thereof to be excessive (area ratio) of the breaking strain in many becomes plate forging decreases. Therefore, C content is made 0.07 to 0.22%. C content is 0.08% or more, preferably 0.10% or more, or 0.12% or more, 0.14% or more, more preferably at least 0.15%, or 0.16%. Also, C content is preferably 0.20% or less or 0.18% or less, more preferably 0.17% or less.
[0040]
　Si: 1.00 ~
　3.20% Si has a deoxidizing effect, is an effective element to produce ferrite suppress generation of harmful carbides. Also has an action to suppress the decomposition of the residual austenite. Meanwhile, in addition to the content of ductility and it is excessive drops, also chemical conversion treatability deteriorates the reduced corrosion resistance after coating. Therefore, Si content is made 1.00 to 3.20%. Si content 1.20% or more, preferably 1.30% or more, or 1.40% or more, more preferably 1.50% or more, or 1.60% or more. Further, Si content is 3.00% or less, preferably 2.80% or less or 2.60% or less, 2.50% or less, more preferably 2.40% or less or 2.30% or less.
[0041]
　Mn: 0.80 ~ 2.20% Mn
　is an austenite region temperature by expanding to the low temperature side to expand the temperature range of two-phase region of ferrite and austenite, is an element effective in stabilizing the retained austenite. On the other hand, the ferrite increases its content is more than necessary hardenability to be excessive will not be sufficient, also the slab cracking occurs during the casting. Therefore, Mn content is made 0.80 to 2.20%. Mn content 0.90% or more, 1.00% or more, preferably 1.20% or more, or 1.40% or more, more preferably at least 1.50%. Further, Mn content is preferably 2.00% or less or 1.90% or less, more preferably 1.80% or less or 1.70% or less.
[0042]
　Al: 0.010 ~
　1.000% Al has the effect of generating a deoxidation effect and ferrite as with Si. On the other hand, the lead to embrittlement and its content is excessive, to facilitate closing the tundish nozzle during casting. Therefore, Al content is made 0.010 to 1.000%. Al content is preferably 0.015% or more, or 0.020% or more, more preferably 0.025% or more, or 0.030% or more. Further, Al content is less 0.800%, preferably 0.700% or less, or 0.600% or less, more preferably 0.500% or less, or 0.400% or less.
[0043]
　N: 0.0060% or less
　N is an element effective for refining crystal grains by precipitating AlN or the like. On the other hand, not only the ductility is reduced by solid-solution nitrogen residue content thereof to be excessive, aging deterioration becomes severe. Therefore, N content is made 0.0060% or less. Is not particularly necessary to determine the lower limit of the N content, the lower limit is 0%. N content is preferably less or 0.0040% 0.0050%. In addition, lowering the content too, because it leads to increased costs during refining, the lower limit may be used as 0.0010%.
[0044]
　P: 0.050% or less
　P is an impurity contained in the hot metal, with degrading the local ductility to grain boundary segregation, so degrades the weldability, it is as small as possible. Therefore, P content is limited to 0.050% or less. P content is preferably 0.030% or less, or 0.020% or less. There is no particular need to define the lower limit, the lower limit is 0%. However, since unduly reducing the content of the cost increase during refining, the lower limit may be 0.001%.
[0045]
　S: 0.005% or less
　S is an impurity which is also contained in the hot metal, so degrades the local ductility and weldability by forming MnS, it is as small as possible. Therefore, S content is limited to 0.005% or less. For ductility or weldability improving the S content may be less or 0.002% 0.003% or less. There is no particular need to define the lower limit, the lower limit is 0%. However, since unduly reducing the content of the cost increase during refining, the lower limit may be used as 0.0005%.
[0046]
　Ti: 0 ~ 0.150%
　Ti, by carbonitrides, or solid solution Ti delays the grain growth during hot rolling, the grain size of the hot-rolled sheet finer, the effect of improving the low temperature toughness a. Moreover, the presence as TiC, contributes to increasing the strength of a steel sheet through precipitation strengthening. Therefore, it may be contained if necessary. However, when the content is excessive, the effect is in addition to saturated, causing nozzle clogging during casting. Therefore, Ti content is at most 0.150%. If necessary, the upper limit of 0.100%, may be 0.060% or 0.020% or. The lower limit of the Ti content is 0%, in order to obtain a sufficient effect of precipitation strengthening, the lower limit may be used as 0.001% or 0.010%.
[0047]
　Nb: 0 ~ 0.100%
　Nb, by carbonitrides, or solute Nb delays the grain growth during hot rolling, the grain size of the hot-rolled sheet finer, the effect of improving the low temperature toughness a. Moreover, the presence as NbC, contributes to increasing the strength of a steel sheet through precipitation strengthening. Therefore, it may be contained if necessary. However, when the content is excessive, the effect is lowered economical efficiency saturated. Therefore, Nb content is 0.100% or less. Its lower limit is 0%, in order to sufficiently obtain the above effects, the lower limit may be 0.001% or 0.010%.
[0048]
　V: 0 ~ 0.300% V
　is an element having an effect of improving the strength of the steel sheet by precipitation strengthening or solid solution strengthening. Therefore, it may be contained if necessary. However, when the content is excessive, the effect is lowered economical efficiency saturated. Therefore, V content is at most 0.300%. Optionally V, a V content of 0.200% or less, may be 0.100% or less, or 0.060% or less. Its lower limit is 0%, in order to sufficiently obtain the above effects, the lower limit may be 0.001% or 0.010%.
[0049]
　Cu: 0 ~
　2.00% Cu is an element having an effect of improving the strength of the steel sheet by precipitation strengthening or solid solution strengthening. Therefore, it may be contained if necessary. However, when the content is excessive, the effect is lowered economical efficiency saturated. Therefore, Cu content is made 2.00% or less. Further, there is the Cu content of the scale caused scratches are generated on the surface of the contains a large amount of steel plate. Therefore, Cu content 1.20% below 0.80% or less, may be 0.50% or less or 0.25% or less. Its lower limit is 0%, in order to sufficiently obtain the above effects, the lower limit of the Cu content may be 0.01%.
[0050]
　Ni: 0 ~
　2.00% Ni is an element having an effect of improving the strength of the steel sheet by solid solution strengthening. Therefore, it may be contained if necessary. However, when the content is excessive, the effect is lowered economical efficiency saturated. Therefore, Ni content is at most 2.00%. Further, the Ni content is contained in a large amount ductility may be deteriorated. Therefore, the Ni content of 0.60% or less, may be 0.35% or less or 0.20% or less. Its lower limit is 0%, in order to sufficiently obtain the above effects, the lower limit of the Ni content may be 0.01%.
[0051]
　Cr: 0 ~
　2.00% Cr is an element having an effect of improving the strength of the steel sheet by solid solution strengthening. Therefore, it may be contained if necessary. However, when the content is excessive, the effect is lowered economical efficiency saturated. Therefore, Cr content is at most 2.00%. For even greater economy, the upper limit 1.00%, may be 0.60% or 0.30%. Its lower limit is 0%, in order to sufficiently obtain the above effects, the lower limit of the Cr content may be 0.01%.
[0052]
　Mo: 0 ~
　1.00% Mo is an element which is effective in improving the strength of the steel sheet by precipitation strengthening or solid solution strengthening. Therefore, it may be contained if necessary. However, when the content is excessive, the effect is lowered economical efficiency saturated. Therefore, Mo content is at most 1.00%. For even greater economy, the upper limit 0.60%, or 0.30%, or 0.10%. Its lower limit is 0%, in order to sufficiently obtain the above effects, the lower limit of the Mo content may be 0.005% or 0.01%.
[0053]
　B: 0 ~ 0.0100%
　B segregates in grain boundaries, to improve the low temperature toughness by increasing the grain boundary strength. Therefore, it may be contained if necessary. However, when the content is excessive, the effect is lowered economical efficiency saturated. Therefore, B content is made 0.0100% or less. Moreover, B is a strong quenching elements, ferrite transformation does not proceed sufficiently during cooling and the content contains a large amount, may not sufficient residual austenite is obtained. Therefore, B content 0.0050% or less, may be 0.0020% or less, or 0.0015%. Its lower limit is 0%, in order to sufficiently obtain the above effects, the lower limit of the B content may be 0.0001% or 0.0002%.
[0054]
　Mg: 0 ~ 0.0100% Mg
　becomes a starting point of fracture, by controlling the form of nonmetallic inclusions causing degradation of workability, an element for improving the workability. Therefore, it may be contained if necessary. However, when the content is excessive, the effect is lowered economical efficiency saturated. Therefore, Mg content is made 0.0100% or less. Its lower limit is 0%, in order to sufficiently obtain the above effects, the lower limit of the Mg content may be 0.0001% or 0.0005%.
[0055]
　Ca: 0 ~ 0.0100% Ca
　becomes a starting point of fracture, by controlling the form of nonmetallic inclusions causing degradation of workability, an element for improving the workability. Therefore, it may be contained if necessary. However, when the content is excessive, the effect is lowered economical efficiency saturated. Therefore, Ca content is made 0.0100% or less. Its lower limit is 0%, in order to sufficiently obtain the above effects, Ca content is preferably 0.0005% or more.
[0056]
　REM: 0 ~ 0.1000% REM
　(rare earth element) becomes a starting point of fracture, by controlling the form of nonmetallic inclusions causing degradation of workability, an element for improving the workability. Therefore, it may be contained if necessary. However, when the content is excessive, the effect is lowered economical efficiency saturated. Therefore, REM content is at most 0.1000%. It may optionally be the upper limit as 0.0100% or 0.0060%. Its lower limit is 0%, in order to sufficiently obtain the above effects, the lower limit of the REM content may be 0.0001% or 0.0005%.
[0057]
　In the present invention, REM refers to a total of 17 elements Sc, Y and lanthanoid, and the content of the REM means the total content of these elements. It should be noted that the lanthanide is industrially, it is added in the form of misch metal.
[0058]

　Zr:
　0 ~ 1.000% Co: 0 ~
　1.000% Zn: 0 ~ 1.000% W: 0
　~ 1.000% Zr, Co, Zn and W in the range of less than 1.000%, respectively It is contained if it has been confirmed that the effect of the present invention is not impaired. These limits may be 0.300% or 0.10%. Zr, Co, it is preferable that the total content of Zn and W is 1.000% or less, or 0.100%. These content is not essential, but the lower limit is 0%, if necessary, the lower limit may be used as 0.0001%.
[0059]
　Sn: 0 ~
　0.050% Sn has confirmed that the effect of the present invention is not impaired also contain, if small amounts. However, there is a possibility that defects may occur during hot rolling exceeds 0.05%. Therefore, Sn content is made 0.050% or less. Containing Sn is not essential, but the lower limit is 0%, if necessary, the lower limit may be 0.001%.
[0060]
　In the chemical composition of the steel sheet of the present invention, the balance being Fe and impurities.
[0061]
　Here, the "impurities", in manufacturing the steel sheet industrially, ores, raw material scraps, a component mixed by various factors of the manufacturing process, is allowed to the extent that the present invention does not adversely affect means shall.
[0062]
　(B) metal structure
　will be described steel sheet metal structure of the present invention. The metal structure in the present invention, in the rolling direction and the cross section perpendicular of the steel sheet, the width and thickness of the steel sheet is W and t, respectively, at the end face of the steel plate 1 / 4W or 3 / 4W, and We shall refer to tissue at the position of the surface from 1 / 4t or 3 / 4t of the steel plate. Further, "%" in the following description means "area%".
[0063]
　It retained austenite: 10% or less than 2%
　residual austenite is necessary tissue for obtaining a strain-induced transformation (so-called TRIP phenomenon). Residual austenite, processed by martensitic transformation, by present as martensite after processing, while ensuring the workability, in which it is possible to ensure the strength in the component after processing. To obtain the original functions of the TRIP steel sheet, the area ratio of retained austenite is set to a value greater than 2%.
[0064]
　On the other hand, when the residual austenite becomes excessive, deformation-induced transformation martensite is hard phase often present in, the difference in deformability between the ferrite is soft phase, voids are generated at the interface of both phases, the plate forging with the distortion increase of steel sheet by, to grow into the void are bonded crack. Therefore, the area ratio of residual austenite is 10% or less. Area ratio of retained austenite is preferably 2.5% or more, and more preferably 3% or more, or more than 4%. The area ratio of residual austenite is preferably 9% or less, and more preferably 8% or less.
[0065]
　Martensite: 2% or less
　TRIP steels, while ensuring workability, is characterized by the martensite retained austenite by strain induced transformation during processing. Therefore, in order to ensure the workability of martensite is a hard phase is better as low as possible. Therefore, the area ratio of martensite is 2% or less. The area ratio of martensite 1.5% or less, preferably less than 1% or 0.5%. However, it is not particularly necessary to define the lower limit, the lower limit is 0%.
[0066]
　Bainite: 10% to 70%
　bainite is soft phase is an important organization in order to ensure a balance between the strength and elongation, the effect of suppressing the propagation of cracks. From this viewpoint, the area fraction of bainite is 10% or more. Improve the strength, the lower limit of 20%, 30%, may be 35% or 40%. On the other hand, when the area ratio of bainite becomes excessive, it can not be secured retained austenite, since the original operation of the TRIP steel is secured, and 70% or less. If necessary, 65% upper limit, 60% may be 55% or 50%.
[0067]
　Perlite: 2% or less
　because the pearlite large amount there strength is reduced, the area ratio is 2% or less. It may optionally be the upper limit as a 1% or 0.5%. Area ratio of pearlite as much as possible it is preferable to reduce, and preferably 0%.
[0068]
　Remainder: ferrite
　ferrite is a soft phase also ensures a balance between the strength and elongation are important tissues from the viewpoint of improving the workability. Thus, residual austenite, martensite, bainite, other than the pearlite structure is ferrite. It is not particularly necessary to limit the area ratio of ferrite is the remaining structure. However, 10% the lower limit of the area ratio, the upper limit may be set as 88%. If necessary, the lower limit of 20%, 30%, 35% or 40%, the upper limit of 80%, 70%, may be 60% or 55%.
[0069]
　In the present invention, the area ratio of the metal structure is obtained as follows. As described above, first, from the end face of the steel plate 1 / 4W or 3 / 4W, and samples are taken from the position of 1 / 4t or 3 / 4t from the surface of the steel sheet. Then, observing the rolling direction cross-section of the sample (the so-called L cross section).
[0070]
　Specifically, the sample was nital etching, to observe in the field of view of 300 [mu] m × 300 [mu] m using an optical microscope after etching. Then the obtained structure photograph, obtained by performing image analysis, the ferrite area ratio A, pearlite area ratio B, and bainite, the total area ratio C of martensite and residual austenite.
[0071]
　Then Le Pera etching moiety nital etching, to observe in the field of view of 300 [mu] m × 300 [mu] m using an optical microscope. Then the obtained structure photograph, by performing image analysis to calculate the total area ratio D of the retained austenite and martensite. Further rolling surface normal direction using samples cut surfaces to 1/4 the depth of the sheet thickness from the X-ray diffraction measurement determining the volume fraction of retained austenite. Since the volume ratio is approximately equal to the area ratio, the volume ratio and the area ratio E of residual austenite. The area ratio of bainite from the difference between the area ratio C and the area ratio D, obtains the area ratio of martensite from the difference between the area ratio E and the area ratio D. By this method, it is possible to obtain ferrite, bainite, martensite, residual austenite, the respective area ratios perlite.
[0072]
　In the present invention, the metal phase consisting of residual austenite and / or martensite (hereinafter, simply referred to as "metal phase".) Also for the presence states of that defined as follows. Note that the metal phase (hard phase) is mainly residual austenite, that is, that the area ratio of residual austenite is greater than the area ratio of martensite, preferred.
[0073]
　The average circle equivalent diameter of the metal phase: 1.0 ~ 5.0 .mu.m
　to ensure intrinsic function of the TRIP steel, since the area of the metal phase is required more than a certain, average circle-equivalent diameter of the metal phase and 1.0μm or more. On the other hand, when the metal phase is too large, with the increase in distortion of the steel sheet by the plate forging, since the easily bonded voids existing in the grain boundary, an average equivalent circle diameter ratio of the metal phase is less 5.0 .mu.m. The average circle equivalent diameter of the metal phase is preferably at least 1.5 [mu] m, more than or 2.0 .mu.m 1.8 .mu.m is more preferable. The average circle equivalent diameter of the metal phase is 4.8μm or less, or less preferably 4.2 .mu.m 4.4 [mu] m, 4 [mu] m or less, more preferably 3.5μm or less, or 3 [mu] m.
[0074]
　The average circle equivalent diameter of the metal phase (diameter) is obtained as follows. First, according to the method of measuring the area ratio D, than a structure photograph after Les Pera etching, obtaining the equivalent circle diameter from the individual metal phase area. Then, the measured equivalent circle diameter (simple) average, and the average circle equivalent diameter.
[0075]
　The average value of the shortest distance between adjacent metallic phase: 3 [mu] m or more
　voids generated at the interface between the hard and soft phases will grow, in order to avoid further and large voids attached voids between the distance between the hard phase a certain amount is necessary to secure. Therefore, the average value of the distance between adjacent metal phase be at least 3 [mu] m.
[0076]
　From the viewpoint of suppressing crack generation due to the growth of voids is preferably the average value is 4μm or more, more preferably 5μm or more. The upper limit is not specifically set, in order to ensure the intrinsic function of the TRIP steel, the average value is preferably set to 10μm or less.
[0077]
　The average value of the shortest distance between adjacent metallic phase is obtained as follows. Any metal layer was 20 selects therewith a distance to the nearest metal phase were measured, to calculate the average value. Incidentally, the shortest distance between the metallic phase, according to the method of measuring the area ratio D, obtains an optical microscope observation image after Le Pera etching by image analysis.
[0078]
　(C) mechanical properties
　nanohardness standard deviation: 2.5 GPa or less
　to reduce the voids generated at the interface of the two phases by reducing the difference in deformability between the hard and soft phases, by further opening the void space , it is possible to suppress the growth voids can couple to the crack. Accordingly, by reducing as much as possible nano hardness difference corresponding to the difference between the deformability of the hard phase and the soft phase, generation of voids can be suppressed. In the present invention, as an index of the hardness difference between the soft and hard phases, adopting the standard deviation of nano-hardness in the sample section.
[0079]
　Nano hardness, for example, can be measured using a Hysitron Inc. TriboScope / TriboIndenter. Optionally measuring the nano hardness of more than 100 at a load of 1 mN, it is possible to calculate the standard deviation of nano-hardness from the result.
[0080]
　Reducing the difference in hardness between the soft and hard phases, in order to suppress the generation of voids, the standard deviation of nano-hardness preferably small well, or less 2.5 GPa. Preferably, or equal to or less 2.4GPa or less or 2.3 GPa.
[0081]
　Tensile strength: more than 780MPa
　steel sheet according to the present invention preferably has a conventional TRIP steel equivalent 780MPa or more tensile strength. We need not determine the upper limit of the tensile strength in particular, 1200 MPa, may be 1150MPa or 1000 MPa. However, tensile strength, shows a tensile strength of JIS Z 2241 (2011).
[0082]
　Uniform elongation and tensile strength and the product: more than 9500MPa%
　and uniform elongation is less liable to occur thickness reduction due to necking at the time of press molding causes the press cracking. To ensure press formability, product of uniform elongation (u-EL) and tensile strength (TS): preferably satisfies the TS × u-EL ≧ 9500MPa% . However, uniform elongation, in the test specified in JIS Z 2241 (2011), in relation to the nominal stress σn nominal strain .epsilon.n, nominal point value when differentiating the nominal stress σn at a nominal strain .epsilon.n is zero when the εn0 distortion is expressed by the following equation.
　Uniform elongation (u-EL) = ln ( εn0 + 1)
[0083]
　Equivalent plastic strain of 0.50 or more
　equivalent plastic strain, the relationship between the shear stress σs and shear plastic strain εsp in simple shear test, different variations, the tensile stress σ in uniaxial tensile test and a tensile strain ε it is intended to convert the relationship, assuming the relation between the isotropic hardening law and plastic work conjugate is obtained by conversion using the conversion factor is a constant (kappa).
[0084]
　Here, the isotropic hardening law, the shape of the yield curve, distortion does not change even if the progress (i.e., expanding in a similar shape) is a work hardening law was assumed. The relationship plastic work conjugate, work hardening is described as a function of only the plastic work, when given the same plastic work regardless of the variation (σ × ε), a relationship that shows the same work hardening amount.
[0085]
　This makes it possible to convert the shear plastic strain and shear stress of the simple shear test, the tensile strain and the respective uniaxial tensile test tensile stress. This relationship is shown in the following.
　Tensile stress at uniaxial tensile testing sigma (converted) = shear stress [sigma] s × kappa in simple shear test
　tensile strain epsilon (transformation) of a tensile uniaxial test = shear plastic strain εsp / κ in simple shear test
[0086]
　Next, the relationship between the shear plastic strain and shear stress, becomes similar to the relation between tensile strain and the tensile stress to determine a conversion factor kappa. For example, transform coefficients κ can be determined by the following procedure. First, previously obtained relation tensile strain at uniaxial tensile testing epsilon (measured value) of the tensile stress sigma (measured value). Then, determine the relationship between the shear strain in uniaxial shear test .epsilon.s (measured value) and the shear stress [sigma] s (measured value).
[0087]
　Next, by changing the kappa, shear strain εs tensile strain obtained from (found) epsilon (the conversion), to previously obtain the shear stress σs tensile obtained from (found) stress sigma (conversion), tensile strain ε is (conversion), when between 0.2% to uniform elongation (u-EL), a tensile stress σ Request (conversion). At this time, a tensile stress calculated an error between the sigma (conversion) and the tensile stress sigma (measured value), the κ error is minimized, determined using the least squares method.
[0088]
　Equivalent plastic strain εeq, using κ determined, shear plastic strain at break of a simple shear test Ipushironsp (break) is defined as converted to tensile strain ε in a simple tensile test.
[0089]
　Steel sheet according to the present invention, good processability characteristics in the high strain region to be represented in a plate forging is characterized, equivalent plastic strain εeq meets 0.50 or more. Since the equivalent plastic strain in the conventional TRIP steel is at most about 0.30, it was confirmed plate forgeability of the steel sheet according to the present invention is good.
[0090]
　(D) Dimensions
　thickness: 1.0 ~ 4.0 mm
　steel sheet according to the present invention, mainly a like main applications automotive, its thickness range is mainly 1.0 ~ 4.0 mm. Therefore, the thickness range may be 1.0 ~ 4.0 mm, as required, the lower limit 1.2 mm, a 1.4mm or 1.6 mm, 3.6 mm and the upper limit, 3.2 mm or 2. it may be used as 8mm.
[0091]
　(E) the production method
　inventors, studies so far, the manufacturing process is shown in the following (a) to (l), it was confirmed that it is possible to produce hot-rolled steel sheet of the present invention . It will be described in detail below each manufacturing step.
[0092]
　(A) melting step
　manufacturing method preceding the hot rolling is not particularly limited. That is, by performing the continuing various secondary smelting melting by blast furnace or electric furnace or the like is adjusted so that the above-mentioned composition of ingredients. Then, ordinary continuous casting, may be manufactured slabs by a method such as thin slab casting. At that time, as long as it can control the component range of the present invention, the raw material may be used scraps.
[0093]
　(B) hot-rolling process
　the manufactured slab is heated by applying hot rolling, the hot rolled steel sheet. Although not provided any special restriction on the conditions in the hot rolling process, for example, the heating temperature before hot rolling preferably in the 1050 ~ 1260 ° C.. After once cooled to a low temperature in the case of continuous casting, it may be hot rolled after re-heating may be hot rolled by heating subsequent to continuous casting without any particular cooling.
[0094]
　After heating, subjected to rough rolling and subsequent finish rolling the extracted slabs from the furnace. As described above, the finish rolling is multi finish rolling carried out in continuous rolling of 3 or more stages (e.g. six stages or 7 stages). The final three stages cumulative strain in the rolling of the (effective cumulative distortion), for final finish rolling so as to 0.10-0.40.
[0095]
　As described above, the effective accumulated strain, temperature during rolling, and the change in grain size due to reduction rate of the steel sheet by rolling, a change in crystal grain size crystal grains are statically recover with time after rolling is a consideration to the index. Effective accumulated strain (εeff) can be obtained by the following equation.
[0096]
　Effective accumulated strain (Ipushironeff) = Shigumaipushiron'ai (ti, Ti) · · · (1)
　sigma in the formula (1) indicates the sum of i = 1 ~ 3.
　However, i = 1, the rolling of the first stage from the last in a multistage finishing rolling (that is, the final stage rolling) a, i = 2 is the last from the second stage rolling, i = 3 is rolled from the end of the third stage a, respectively.
[0097]
　Here, in the rolling represented by i, .epsilon.i is expressed by the following equation.
　.epsilon.i (ti, Ti) = ei / exp ((ti / .tau.R) 2/3 ) · · ·
　(2) ti: primary cooling start to the time after the last stage rolling from the end of the i-th stage rolling (s)
　Ti: rolling temperature of the last from the i-th stage rolling
　(K) ei: logarithmic distortion when the pressure at the end of the i-th stage rolling
　ei = | ln {1- (i-th thickness at entrance side -i stage delivery side thickness of the) / (thickness at entrance side of the i-th
　　　stage)} | = | ln {(delivery side thickness of the i-th stage) / (thickness at entrance side of the i-th stage)} | · · · (3)
　.tau.R = · exp .tau.0 (Q / (R · Ti)) · · · (4)
　.tau.0 = 8.46 × 10 -9
　(s) Q: constant activation energy on the movement of dislocations Fe = 183200 (J / mol)
　R: gas constant = 8.314 (J / (K · mol))
[0098]
　By defining the effective accumulated strain derived in this manner, the distance between the average circle equivalent diameter of the metal phase composed mainly of residual austenite and adjacent metal phase is restricted, it is further reduced variation in nano-hardness. As a result, to suppress the void growth that occur at the interface between the hard and soft phases, even if voids are grown can be difficult coupling, cracking does not occur even if the plate forging, excellent in plate forgeability it is possible to obtain a steel sheet.
[0099]
　Termination temperature of finish rolling, i.e. the finishing temperature of the continuous hot rolling process, Ar 3 (° C.) or higher, Ar 3 (° C.) + 30 may be a temperature below ° C.. Thus, while limiting the amount of residual austenite, it is because it is possible to complete the rolling in the two-phase region. Incidentally, it is possible to calculate the value of Ar3 by the following equation.
　Ar 3 = 970-325 × C + 33 × Si + 287 × P + 40 × Al-92 × (Mn + Mo + Cu) -46 × (Cr + Ni)
　where the element symbol in the above formula, the content of the hot-rolled steel sheet of each element (mass% ) represents, if not contained shall substituting 0.
[0100]
　(C) first (accelerating) the cooling step
　after the completion of finish rolling, cooling is started in the hot-rolled steel sheet obtained within 0.5 s. Then, 650 ~ cooled at an average cooling rate of temperature up to 10 ~ 40 ° C. / s of 750 ° C., for 3 ~ 10s then cooled in the air (air cooling process). Ferrite transformation promotes the cooling in the atmosphere that leads this step, the distribution C of necessary residual austenite in the winding process after. Average cooling rate in the first cooling step is easily generated pearlite is less than 10 ° C. / s. On the other hand, the 40 ° C. / s greater will happening bainite transformation at a relatively high temperature rather than a ferrite transformation prevents residual austenite after.
[0101]
　Further, if the cooling rate in air there is 8 ° C. / s greater or air time is 10s, more than likely to produce bainite, bainite area ratio increases. On the other hand, if the cooling rate is 4 ° C. / s or less than air cooling time in the air it is less than 3s, pearlite is likely generated. Note that the cooling in the atmosphere as referred to herein means that the steel sheet is cooled at a cooling rate 4 ~ 8 ℃ / s in air.
[0102]
　(D) second (acceleration) cooling step
　is cooled by air cooling step after the immediately 350 ~ 450 ° C. temperature to 30 ° C. / s or more average cooling rate. This upper limit of the average cooling rate is not particularly limited, since the steel sheet by thermal distortion due to thermal deviation that warpage is concerned, may be less than or equal to 1000 ° C. / s.
[0103]
　(E) winding process
　then winding the cooled hot rolled steel sheet. Conditions in the winding process is not particularly limited, after winding, the average cooling rate to the coil surface temperature becomes 200 ° C. better to 30 ~ 100 ℃ / h. After the second (acceleration) cooling step and before the winding process, may be carried out air-cooling in the air. If air cooling of the atmosphere is not particularly necessary to limit the cooling rate.
[0104]
　The following examples illustrate the present invention more specifically, the present invention is not limited to these examples.
Example 1
[0105]
　Steels having the chemical compositions shown in Table 1 were melted to prepare a slab, the slab was wound was cooled after hot rolling under the conditions shown in Table 2, to produce a hot-rolled steel sheet. Thickness of the resultant hot-rolled steel sheet shown in Table 3.
[0106]
[Table 1]

[0107]
[Table 2]

[0108]
[table 3]

[0109]
　[Metallographic]
　performs metallographic observation of the obtained hot-rolled steel sheet was measured for area ratio of each organization. Specifically, in the rolling direction and the cross section perpendicular first steel sheet, the width and thickness of the steel sheet is W and t, respectively, in 1 / 4W from the end surface of the steel plate, and, from the surface of the steel plate 1 / cut test pieces for metallographic observation from the position of 4t.
[0110]
　The rolling direction cross-section of the test piece (the so-called L cross section) to nital etching, was observed by the field of view of 300 [mu] m × 300 [mu] m using an optical microscope after etching. Then the obtained structure photograph, by performing image analysis, the ferrite area ratio A, pearlite area ratio B, and bainite, the total area ratio C of martensite and residual austenite was determined.
[0111]
　Then Le Pera etching moiety nital etched, was observed by the field of view of 300 [mu] m × 300 [mu] m using an optical microscope. Then, the obtained structure photograph, by performing image analysis to calculate the total area ratio D of the retained austenite and martensite. Further using the rolling surface normal direction was cut surface to 1/4 depth of thickness sample, was determined volume fraction of residual austenite by X-ray diffraction measurement. Since the volume ratio is approximately equal to the area ratio and the volume ratio and the area ratio E of residual austenite. The area ratio of bainite from the difference between the area ratio C and the area ratio D, was determined the area ratio of martensite from the difference between the area ratio E and the area ratio D. In this way, ferrite, bainite, martensite, residual austenite, pearlite respective area ratios were calculated.
[0112]
　Furthermore, from the structural photograph after the aforementioned Le Pera etching, we obtain the number and area of ​​the metal phase, measuring the circle equivalent diameter (diameter), to obtain an average circle equivalent diameter of this number on average. Similarly, from the structural photograph after Les Pera etching any metal layer was 20 selects therewith a distance to the nearest metal phase were measured, and the average value was calculated.
[0113]
　[Mechanical characteristics]
　of which tensile strength properties of the mechanical properties (tensile strength (TS), uniform elongation (u-EL)), when the plate width is W, 1 / 4W or 3 from one end of the plate in the plate width direction in any position / 4W, using No. 5 test pieces of JIS Z 2241 were taken direction (width direction) as the lengthwise direction perpendicular to the rolling direction (2011), in conformity with JIS Z 2241 (2011) evaluated.
[0114]
　Moreover, subjected to simple shear test by the following procedure to determine the equivalent plastic strain based on the result.
[0115]
　Specimen simple shear test, a sheet width of the steel sheet is W, in any position of the 1 / 4W or 3 / 4W from one end of the plate in the plate width direction and the direction perpendicular to the rolling direction (width direction) It is harvested as the longitudinal direction. Shows an example of the test piece in FIG. 1 (a). Specimen simple shear test shown in FIG. 1 will snap thickness evenly grinding the both surfaces so that the plate thickness is 2.0 mm, 23 mm in the width direction of the steel sheet, of rectangular 38mm in the rolling direction of the steel sheet was processed so that the test piece.
[0116]
　Long side (the rolling direction) of the specimen, toward a short one-way (width direction) chucked on both sides of the chucking portion 2 by 10 mm, in the center of the test piece, 3 mm shear width (shear deformation generating section 1) It was to be provided. In addition, the plate thickness is less than 2.0 mm, without grinding, the plate thickness was tested as is. At the center of the test piece was marked linear with a pen or the like to a short one-way (width direction).
[0117]
　Then, the long side of the chucking, the long one-way (rolling direction), by moving so as to be opposite to each other, a shearing stress σs was loaded, and the shear deformation is added to the specimen. In FIG. 1 (b), shows an example of a shear deformation test specimens. Shear stress σs is the nominal stress determined by the following formula.
　Shear stress [sigma] s = shear / (thickness length × specimens in the rolling direction of the test piece of steel sheet)
[0118]
　Since the length and thickness of the specimen in shear test does not change, it may be considered that shear nominal stress ≒ shear true stress. During shear test, a straight line drawn on the test strip middle photographed by a CCD camera, and measuring the inclination theta (see Figure 1 (b)). This inclination theta, using the following equation, generated by the shear deformation was determined shear strain .epsilon.s.
　Shear strain εs = tan (θ)
[0119]
　Note that the simple shear test was used simple shear tester (maximum displacement 8 mm). Therefore, there is a limit of the stroke (displacement) of the testing machine. Further, the occurrence of cracks eaves at the end or chuck portion of the test piece, in a single shear test, there is a case where the test piece can not be tested until breakage. Therefore, adoption as described above, the load of the shear test load, unloading of the load, straight lines cut the chuck portion end of the test piece, rechallenge of the load and repeats the series of operations such as, the "multi-stage shear test method" did.
[0120]
　By connecting shear test results of these multi-stage in series, considered to evaluate the resulting simple shear test one consecutive, from shear strain obtained in shear test of each stage (.epsilon.s), the shear modulus by subtracting the elastic shear strain (εse), shear plastic strain (εsp) and obtained as follows, by connecting to one collectively shear strain plasticity of each stage (.epsilon.s).
　Shear plastic strain Ipushironsp = shear strain εs- shear elastic strain Ipushironse
　shear elastic strain Ipushironse = [sigma] s / G
　[sigma] s: shear stress
　G: shear modulus
　herein, was G = E / 2 (1 + ν) ≒ 78000 (MPa).
　E (Young's modulus (longitudinal elastic modulus)) = 206000 (MPa)
　Poisson's ratio ([nu) = 0.3
[0121]
　In simple shear test specimen perform the test until breakage. In this way, we tracked relationship of shear stress σs and shear plastic strain Ipushironsp. Then, a shear plastic strain εspf when the test piece is broken.
[0122]
　It said a simple shear test shear resulting in stress [sigma] s, the relationship between the shear plastic strain εspf when the test piece is broken, by the above-described method, using a conversion factor kappa, was determined equivalent plastic strain Ipushironeq.
[0123]
　Then, measurements were made of the standard deviation of nano-hardness. Polished specimens of the metal microstructure observation again, 1 mN load (loading 10s, unloading 10s) in, in a cross section parallel to the rolling direction, 1/4 depth position of the sheet thickness t from the surface of the steel sheet (1 / 4t part) for, and the measurement area of ​​25 [mu] m × 25 [mu] m measured at 5μm intervals. The results were calculated the standard deviation of the mean and nano hardness of the nano hardness. Measurements of the nano hardness was carried out using a Hysitron Inc. TriboScope / TriboIndenter.
[0124]
　These measurement results are shown in Table 3.
[0125]
　Table 3 As is apparent from, if hot-rolled steel sheet according to the present invention, tensile strength (TS) is more than 780 MPa, the product of the uniform elongation u-EL and tensile strength TS (TS × u-EL ) has a higher ·% 9500MPa, showing the balance of properties. Further, hot rolled steel sheet according to the present invention, the equivalent plastic strain becomes 0.50 or more, it was confirmed that the steel plate to withstand the high strain region processing such as plate forging.
Industrial Applicability
[0126]
　According to the present invention, deep drawability, while maintaining the basic functions of a TRIP steel such bulging formability, it becomes possible to obtain a good hot-rolled steel sheet into a plate forgeability. Therefore, hot-rolled steel sheet according to the present invention broadly, it can be utilized, such as machine parts. In particular, by applying the processing of steel sheet having a processing at high strain region or substrate forging, it is possible to obtain the remarkable effects.
DESCRIPTION OF SYMBOLS
[0127]
　1 shear deformation generating unit
　2 chucking unit

WE CLAIM

[Requested item 1] Ga chemical composition,で
　mass%,
　C:
　0.07 ~ 0.22%, Si: 1.00 ~
　3.20%, Mn: 0.80 ~ 2.20%, of Al: 0.010 ~ 1.000
　%, N: 0.0060% or
　less, P: 0.050% or
　less, S: 0.005% or
　less,
　of Ti: 0 ~ 0.150%, of Nb: 0 ~ 0.100%,
　V: 0 ~ 0.
　%
　300,
　a Cu: 0 ~
　2.00%, of Ni: 0 ~ 2.00%, of Cr: 0 ~
　2.00%, of Mo: 0 ~ 1.00%, B: 0.0100 ~
　0%, of Mg:
　~
　0.0100%
0,
　0 ~ 0.1000%, Zr: 0 ~ 1.000%, of
　Co: 0 ~ 1.000%, a Zn: 0 ~ 1.000
　%, W
　is: 0 ~ 1.000%, of Sn: 0 ~ 0.050%, yoびお,
　remnants: Fe impuritiesおyoびでthou ri,
　In the rolling direction and the cross section perpendicular of the steel sheet, the width and thickness of the steel sheet is W and t, respectively, at the end face of the steel plate 1 / 4W or 3 / 4W, and, 1 / 4t from the surface of the steel plate or metal structure in the position of the 3 / 4t is in area%,
　residual austenite: 10% more than 2% or less,
　martensite: 2% or less,
　bainite: 10% to 70%,
　perlite 2% or less,
　the balance being ferrite , and the
　average circle equivalent diameter of the metal phase consisting retained austenite and / or martensite is the 1.0 ~ 5.0 .mu.m,
　the mean value of the shortest distance of the metal phase adjacent is at 3μm or more,
　the nano-hardness standard deviation is less 2.5 GPa,
　hot-rolled steel sheet.
[Requested item 2]
　A tensile strength of not less than 780 MPa,
　the thickness is 1.0 ~ 4.0 mm,
　hot-rolled steel sheet according to claim 1.