Technical field
[0001]The present invention, hot rolled steel sheets and steel forging and methods for their preparation.
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. For such requirements, high-strength steel sheet excellent in than conventional good hole expansion (high burring workability) have been proposed. For example, steel sheet excellent in hole expandability (lambda value), Ti, steel and manufacturing method thereof precipitation strengthened ferrite main phase by fine precipitates such as Nb have been reported.
[0003]
　Patent Document 1, the hot-rolled steel sheet is disclosed having excellent stretch flangeability high strength. Further, Patent Document 2, high formability high-tensile hot-rolled steel sheet excellent in material homogeneity is disclosed. Further, Patent Document 3, high-tensile hot-rolled steel sheet is disclosed which is excellent in elongation and stretch flangeability.
CITATION
Patent Document
[0004]
Patent Document 1: JP 2002-105595 Patent Publication
Patent Document 2: JP 2002-322540 Patent Publication
Patent Document 3: JP 2002-322541 JP
Summary of the Invention
Problems that the Invention is to Solve
[0005]
　However, complexity of the 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 process to the conventional press working elements such as a plate forging elements have been combined in a complex manner. The conventional stamping elements, for example deep drawing, hole expansion, stretch forming process, bending, was elements such ironing.
[0006]
　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.
[0007]
　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.
[0008]
　However, Patent Documents 1-3 No mention is made regarding processing including complex processing element typified by a plate forging. Further, the winding conditions for producing hot-rolled steel sheet described in Patent Document 1 is not very strict real. For further hot rolled steel sheet described in Patent Documents 2 and 3 containing Mo is an expensive alloying element than 0.07%, there is a problem of high manufacturing cost.
[0009]
　High burring 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.
[0010]
　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.
[0011]
　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 high burring steel, deep drawability, hole expansion, bulging formability, such while effectively utilizing the function, high burring steel does not break even if the plate forging has been sought .
[0012]
　The present invention has been made to solve the above problems, while maintaining the basic functions as a high burring steel, the cracking limit of the portion receiving the forging partially compressive force takes and to provide a good hot-rolled steel sheet into a plate forgeability capable of improving.
Means for Solving the Problems
[0013]
　The present invention has been made to solve the above problems, the hot rolled steel sheet and steel forged parts and gist methods for their preparation below.
[0014]
　(1) chemical composition of the steel sheet contains, by
　mass%,
　C: 0.020 ~
　0.070%, Si: 0.05 ~ 1.70%, Mn: 0.60 ~
　2.50%, Al: 0. ~
　1.000% 010, N: 0% ultra-0.0030% or
　less, P: 0.050% or
　less, S: 0.005% or
　less, Ti: 0.015 ~
　0.170%, Nb: 0 -
　Pasento
　0.100,
　V: 0 ～ 0.300 Pasento, Cu:
　0 ～ 2.00 Pasento, Ni: 0 ～ 2.00 Pasento,
　Cr: 0 ～ 2.00 Pasento, Mo: 0 ～ 1.00 Pasento, 　B:
　0 ~ 　0.0100%, Mg: 0 ~ 0.0100%, Ca: 0 ~ 0.0100%, 　REM: 0 ~ 0.1000%, Zr: 0 ~ 　1.000%, Co: 0 ~ 1 　Pasento .000, 　Zn: 0 ～ 1.000 Pasento, 　W: 0 ～ 1.000 Pasento, Sn: 0 ～ 0.050 Pasento, and 　the balance: Fe and impurities der ,

　In the rolling direction and the perpendicular cross section of the steel sheet, the width and thickness of the steel sheet is W and t, respectively, at the end face 1 / 4W or 3 / 4W of the steel plate, and, from the surface of the steel sheet 1 / metal structure in 4t or 3 / 4t positions, in area%,
　ferrite: 5 to 70%
　bainite 30 to 95%,
　residual austenite: 2% or less,
　martensite: 2% or less, and,
　perlite: 1 % or less, a, and
　the sum of ferrite and bainite: at least 95%,
　the ferrite has a deposit containing Ti in the grains,
　the number density of precipitates containing the Ti, 1.0 10 × 16 ~ 50.0 × 10 16 atoms / cm 3 is,
　TiN precipitates are contained in the steel sheet,
　the average circle equivalent diameter of said TiN precipitates is 1.0 ~ 10.0 [mu] m,
　the adjacent said The average value of the shortest distance between TiN precipitates not less than 10.0 [mu] m,
　standard deviation of nano-hardness of less 1.00GPa,
　hot-rolled steel sheet.
[0015]
　(2) Average circle equivalent diameter of precipitates containing the Ti is 1.00 ~ 3.00 nm,
　hot rolled steel sheet according to (1).
[0016]
　(3) a tensile strength of not less than 780 MPa,
　and the product of the tensile uniform elongation strength of 7000 MPa ·% or more,
　the product of the tensile hole expansion rate intensity is 50000 mPa ·% or more,
　the (1) or hot-rolled steel sheet according to (2).
[0017]
　(4) A method of manufacturing a hot rolled steel sheet according to any one of (1) to (3),
　against the slab having the chemical composition described in the above (1), the heating step, continuous subjecting the hot-rolled process, the first cooling step, the second cooling step and the winding step in the order,
　in the heating step, the slab below (i) represented by SRTmin ° C. or higher in formula, by heating to a temperature of 1260 ° C. or less,
　the continuous hot rolling step includes a rough rolling and 3 or more stages finish rolling,
　wherein it is end temperature of the rough rolling is 1100 ° C. or higher,
　cumulative distortions in the rolling of the final three stages in the multistage finish rolling is, 0 0.01 to 0.10,
　rolling end temperature of the multi-stage finishing rolling, Ar is obtained by the following (ii) formula 3 a + 30 ° C. or higher,
　wherein in the first cooling step, the multi-stage finish rolling is terminated after, 1.00 to 5.00 The cooling started after s, from the rolling end temperature to a temperature range of 650 ~ 750 ° C., and cooled at an average cooling rate of more than 10 ° C. / s, then 1 ~ 10s held in the air,
　the second cooling in step, after holding in the atmosphere, the temperature range of 600 ~ 740 ° C., and cooled at an average cooling rate of more than 10 ° C. / s,
　and in the winding process, winding at coiling temperature of 450 ~ 650 ° C. ,
　method for producing hot-rolled steel sheet.
　= 7000 SRTmin / {2.75-log (Ti × C)} - 273 · · ·
　(i) Ar 3 = 970-325 × C + 33 × Si + 287 × P + 40 × Al-92 × (Mn + Mo + Cu) -46 × (Cr + Ni) · · · (ii)
　where each element symbol in the above formula, the hot rolled steel sheet of each element It represents content (mass%), if not contained shall substituting 0.
[0018]
　(5) obtained from hot-rolled steel sheet according to any one of (1) to (3),
　a steel forged parts.
[0019]
　(6) with respect to hot-rolled steel sheet according to any one of (1) to (3), subjected to at least forging,
　manufacturing method of a steel forging.
The invention's effect
[0020]
　According to the present invention, while maintaining good hole expansion is a fundamental function of a high burring steel, it becomes possible to obtain a good hot-rolled steel sheet into a plate forgeability.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021]
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
[0022]
　The present inventors have conducted intensive investigations to solve the above problems, the following findings were obtained.
[0023]
　(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.
[0024]
　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.
[0025]
　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
[0026]
　(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.
[0027]
　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.
[0028]
　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).
[0029]
　(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.
[0030]
　As microstructure of high burring steel, excellent in order to obtain a hole expansion, Ti, ferrite, which is precipitation strengthening by fine precipitates such as Nb (the precipitation hardening ferritic) tissue as a main phase is used. On the other hand, when Ti is added, coarse TiN precipitates unless a special manufacturing method (hereinafter, also referred to as the deposited TiN simply "TiN".). This is a compound which TiN is thermodynamically very stable, during the casting in the steel sheet production process, during heating of hot rolling, or at high temperature of rough rolling initial etc., preferentially relative to other compounds in order to crystallize or precipitate the.
[0031]
　TiN is, cutting tools, machine parts, molds of plastic molding, sports equipment, enough to be used as a coating applications, such as ornaments hard, its hardness is known to be about Hv2000 ~ 2300, very hard precipitation thing is. Therefore, when receiving the deformation at high strain region, such as a leaf forging, voids are easily generated at the interface from the difference in deformability between the matrix structure.
[0032]
　Hard precipitates and (TiN), moderately from the difference in deformability of soft matrix structure (ferrite or 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, preventing the occurrence of voids, and even voids are grown, if inhibit the binding of the adjacent voids found that the cracking can be suppressed. However, it is also important not to impair the inherent function of the high burring steel at that time.
[0033]
　And we found the following items from these findings.
[0034]
　(I) limiting the average diameter of TiN.
　In other words, the voids to generate in the grain boundary of the TiN hard precipitate, by limiting the average diameter of TiN, voids can be reduced.
[0035]
　(Ii) limiting the distance of TiN between.
　Nachi Suwa, voids to generate in the grain boundary of the TiN, by arranging away TiN each other, can be difficult to bind even voids are grown.
[0036]
　And (iii) reducing the nano hardness variation.
　That is, by reducing as much as possible difference in hardness between the hard tissue and soft tissue, generation of voids can be reduced.
[0037]
　(Iv) that the equivalent plastic strain at break is 0.90 (90%) or more.
　By satisfying the condition of said (i) ~ (iii), the equivalent plastic strain at break is 0.90 (90%) or more, also in the complex processing such as a leaf forging, certain processability it was confirmed that it is possible to collateral.
[0038]
　(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 0.01-0.10, it is necessary to perform a final finish rolling.
[0039]
　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.
[0040]
　Thus, by limiting the effective accumulated strain, the variation in nano-hardness with microstructure of interest is obtained is reduced, by suppressing the generation of voids at the interface between the hard tissue and soft tissue, a crack does not occur even if the plate forging, it is possible to obtain an excellent steel sheet plate forgeability.
[0041]
　The present invention has been made based on the above findings. It will be described in detail below each requirement of the present invention.
[0042]
　(A) Chemical composition
　reasons for limiting each element are as follows. Incidentally, "%" for the content in the following description means "mass%".
[0043]
　C: 0.020 ~ 0.070%
　C is, Nb, combines with Ti or the like to form precipitates in the steel sheet, which contributes to improvement of strength by precipitation strengthening. The C content is less than 0.020% or, the effect is not sufficiently obtained due to the action. On the other hand, C content exceeds 0.070%, the increase in the iron-based carbide serving as a starting point for cracking during hole expansion processing, hole expanding value is deteriorated. Therefore, C content is made 0.020 to 0.070%. C content is preferably 0.025% or more, and more preferably 0.030% or more. Further, it is preferable C content is 0.060% or less, more preferably 0.050% or less.
[0044]
　Si: 0.05 ~ 1.70% Si
　is a deoxidizing effect, and to suppress the precipitation of iron-based carbide cementite or the like in the material in the tissue, with the effect contributing to the improvement of ductility and hole expansion. However, if its content is excessive, ferrite transformation occurs easily at high temperatures, carbides tends to deposit containing Ti in a high temperature range accordingly. Precipitation of carbides in a high temperature range is prone to variation in precipitation quantity, resulting in a material change, such as resulting strength and hole expansion. Therefore, Si content is from 0.05 to 1.70%.
[0045]
　Si content scales, from the viewpoint of suppression of generation of scale-based defects such spindles scale, preferably at 0.06% or more, more preferably 0.08% or more. Further, it is preferable Si content is less 1.50%, further chemical conversion treatability, from the viewpoint of improving the corrosion resistance after painting, and more preferably not more than 1.00%.
[0046]
　Mn: 0.60 ~
　2.50% Mn is an element contributing to the improvement of strengthening and hardenability of the ferrite. On the other hand, the inclusion in a large amount, can not be sufficiently ensured ferrite increases to more than necessary hardenability, also slab cracking occurs during the casting. Therefore, Mn content is made 0.60 to 2.50%. Mn content is preferably at least 1.00%, more preferably 1.50% or more. Further, it is preferable Mn content is 2.00% or less, and more preferably not more than 1.80%.
[0047]
　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.
[0048]
　N: 0% ultra-0.0030% or less
　N is to contain a number, not only the ductility is reduced solid solution nitrogen remains, TiN reduces the precipitated hole expansion. Therefore, N content is made 0.0030% or less. N content is preferably at 0.0025% or less.
[0049]
　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%.
[0050]
　S: 0.005% or less,
　is an impurity that S 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%.
[0051]
　Ti: 0.015 ~ 0.170%
　Ti, by carbonitrides, or solid solution Ti delays the grain growth during hot rolling, the grain size of the hot-rolled sheet finer, and improves the low temperature toughness It has an effect. In addition, by finely dispersed ferrite as TiC, contributes to increasing the strength of a steel sheet through precipitation strengthening. However, when the content is excessive, the effect is in addition to saturated, easily precipitate TiN is a hard deposit. Therefore, Ti content is from 0.015 to 0.170%. Ti content is 0.030% or more, preferably in the range 0.045% or more, or 0.060% or more 0.070% to 0.080% or 0.090% or more, or 0.100% or more in is more preferable. Further, Ti content is 0.160% or less 0.150% or less 0.140% or less, is preferably less 0.130% or less, or 0.120%.
[0052]
　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. If necessary, the Nb content 0.080% or less, may be 0.060% or less, or 0.050% 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%.
[0053]
　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%. If necessary, the V content 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%.
[0054]
　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%.
[0055]
　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%.
[0056]
　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%.
[0057]
　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%.
[0058]
　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%.
[0059]
　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%.
[0060]
　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.
[0061]
　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%.
[0062]
　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.
[0063]

　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%.
[0064]
　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%.
[0065]
　In the chemical composition of the steel sheet of the present invention, the balance being Fe and impurities.
[0066]
　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.
[0067]
　(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%".
[0068]
　Precipitation strengthening ferrite: 5-70%
　fine precipitates containing Ti (. And the like carbides of Ti which was fine precipitation, hereinafter also referred to as "fine Ti precipitates") is γ → α transformation in the post-rolling cooling Ti carbide carbide Ti supersaturation as the driving force of Ti with the phase interface precipitate or homogeneous nucleation in ferrite carbides when it is ferrite pro-eutectoid ferrite is strengthened precipitated dispersed finely (below, also referred to as "precipitation strengthening ferrite".). Precipitation strengthening ferrite is tissue required in order to achieve both good uniform elongation and strength.
[0069]
　However, the area ratio of the precipitation strengthening ferrite difficult to achieve both uniform elongation and strength is less than 5%, whereas, the uniform elongation exceeds 70% degraded is local ductility are excellent. Therefore, the area ratio of the precipitation strengthening ferrite is 5 to 70%. In order to ensure a balance between uniform elongation and strength, the area ratio of the precipitation strengthening ferrite is preferably 7% or more, more preferably 10% or more. The area ratio of the precipitation strengthening ferrite is preferably 65% ​​or less, more preferably 60% or less.
[0070]
　Here, in the present invention, the precipitation strengthening ferrite, the number density of fine Ti precipitates contained in the grains is, 1.0 × 10 16 ~ 50.0 × 10 16 atoms / cm 3 means a ferrite is . The number density of the ferrite grains fine Ti precipitates that contain, 1.0 × 10 16 atoms / cm 3 is less than, the effect is not sufficiently obtained due to precipitation strengthening. On the other hand, the number density of fine Ti precipitates, 50.0 × 10 16 atoms / cm 3 by weight, not only the strength is saturated, ductility is lowered.
[0071]
　That is, the area ratio of the precipitation strengthening ferrite is 5 to 70%, the area ratio of 5 to 70% ferrite, and the number density of fine Ti precipitates contained in the ferrite grains, 1.0 10 × 16 ~ 50.0 × 10 16 atoms / cm 3 means that it is.
[0072]
　Furthermore, the average circle equivalent diameter of the fine Ti precipitates contained in grains of precipitation strengthening ferrite is preferably 1.00 ~ 3.00 nm. Is less than the average equivalent circular diameter of the fine Ti precipitates 1.00 nm, the effect is obtained hardly precipitation strengthening, whereas, when the average circle equivalent diameter becomes coarse particles exceeds 3.00 nm, sufficient amount of fine Ti precipitates it becomes impossible secured objects.
[0073]
　Bainite: 30-95%
　bainite is a structure necessary for obtaining a balance between strength and local ductility, the effect of suppressing the propagation of cracks. However, the bainite is too large, a ferrite is reduced, local ductility has uniform elongation of those excellent conspicuously deteriorated. Therefore, the area ratio of bainite is 30 to 95%. Preferably the area ratio of bainite is 80% or less, to further emphasize the uniform elongation is more preferably to less than 70%.
[0074]
　Residual austenite: 2% or less
　high burring steel, while ensuring workability due to the presence of precipitation strengthening ferrite and bainite, high strength ensured, it is a feature to achieve both strength and workability. However, the fact that thermodynamically stable retained austenite did not cause martensitic transformation is present in the steel sheet, the C concentration of the retained austenite is high, martensite residual austenite is produced by processing-induced transformation at the plate forging the hardness of the site is too high, resulting in promoting the generation of voids. Therefore, the residual austenite may have as low as possible, the area ratio is 2% or less. Area ratio of residual austenite 1.5% or less, preferably 1% or less or 0.5% or less. There is no particular need to define the lower limit, the lower limit is 0%, and most preferably 0%.
[0075]
　Martensite: 2% or less
　high burring steel, while ensuring workability due to the presence of precipitation strengthening ferrite and bainite, high strength ensured, it is a feature to achieve both strength and workability. However, when the area ratio of martensite is a hard tissue exceeds 2%, with the increase in distortion of the steel sheet by the plate forging, easily voids are generated in the boundary between martensite and ferrite, it tends to break. 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%. There is no particular need to define the lower limit, the lower limit is 0%.
[0076]
　Perlite: 1% or less
　perlite to become starting points of fracture at the time of hole expansion molding, the area ratio is 1% or less. Area ratio of pearlite is preferably 0.5% or less. Area ratio of pearlite as much as possible it is preferable to reduce, and preferably 0%.
[0077]
　Precipitation strengthening ferrite and bainite Total: 95%
　high burring steel has a bainite to achieve both precipitation strengthening ferrite to achieve both good uniform elongation and strength, and strength and local ductility. Thereby excellent strength, uniform elongation and local elongation are obtained. When precipitation strengthening ferrite and the total area ratio of the bainite is less than 95%, these characteristics deteriorate. Therefore, the total area ratio of the precipitation strengthening ferrite and bainite is 95% or more. Is preferably the total area ratio is 97% or more, more preferably 98% or more.
[0078]
　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).
[0079]
　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.
[0080]
　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.
[0081]
　The area ratio of the precipitation strengthening ferrite, EBSP-OIMTM is equipped to (Electron Back Scatter Diffraction Pattern-Orientation Image Microscopy), the Kernel Average Misorientation (KAM) method, can be obtained.
[0082]
　KAM method, pixels of six adjacent regular hexagonal pixels certain of the measured data (first approximation), further the 12 outer (second approximation), or even 18 (third approximation) of the outer averaging the azimuth difference between, performs calculations for the average value and the value of the center pixel in each pixel. Create a map representing the orientation change in the grains by performing this calculation so as not to exceed the grain boundaries. In other words, this map represents a distribution of strain based on local orientation changes in the grain.
[0083]
　Analysis Conditions precipitation strengthening ferrite in the present invention is the EBSP-OIMTM, misorientation average between adjacent pixels in the third approximation is calculated, the portion where the orientation difference is calculated to be 1 ° or less, precipitation strengthening It was a ferrite.
[0084]
　Generating a temperature range of precipitation strengthening ferrite of the present invention, carbide Ti is phase interfacial precipitate or homogeneous nucleation in ferrite as a driving force supersaturation carbide Ti when transformed gamma → alpha in after rolling cooling It is consistent with the temperature range. Because polygonal a pro-eutectoid ferrite transformed at a high temperature to produce a diffusion transformation, the dislocation density is small, because the strain in the grains is small, intragranular difference in the crystal orientation is reduced. Therefore, precipitation hardening ferritic likewise crystal orientation difference is small. The various findings inventors have conducted so far, a polygonal ferrite area ratio obtained by light microscopy, the area in which the orientation difference between the third approximation measured at KAM method is obtained in 1 ° or less area ratio, in order to substantially coincide.
[0085]
　Measurements of the area fraction of the precipitation strengthening ferrite, in particular was carried out as follows. The sample taken in the same manner as described in structure observation was polished 30-60 minutes colloidal silica abrasive, 400 magnifications, 160 .mu.m × 256 .mu.m area was performed EBSP measurement under measurement conditions of the measurement step 0.5 [mu] m. EBSP-OIMTM method applying an electron beam to a specimen which is a high-tilt in a scanning electron microscope (SEM), the Kikuchi pattern formed by backscattered imaging with high sensitivity camera, the irradiation point by the computer image processing the crystal orientation of which is constituted by devices and software measuring in a short waiting.
[0086]
　The EBSP method can quantitatively analyze the microstructure and crystal orientation of the bulk sample surface, the analysis area is an area that can be observed by SEM, depending on the resolution of the SEM, can be analyzed with minimal 20nm resolution. Analysis over several hours, the area to be analyzed by mapping tens of thousands equally spaced grid. The polycrystalline material can be seen the crystal orientation distribution and size of the crystal grains in the sample.
[0087]
　In this way, the portion where the orientation difference is calculated to be 1 ° or less in the third approximation of the above, the precipitation strengthening ferrite, and measuring the area of ​​precipitation strengthening ferrite, the area ratio of the precipitation hardening ferritic to the measurement area I was determined.
[0088]
　Moreover, the observation of fine Ti precipitates, the three-dimensional atom probe measurement was performed as follows.
[0089]
　First, from a sample to be measured, by cutting and electropolishing method, utilizing a focused ion beam processing method together with the electrolytic polishing method as required to produce the needle-like sample. The three-dimensional atom probe measurement can be obtained by reconstructing the accumulated data as the actual distribution image of atoms in real space. For fine Ti precipitates of Na-Cl structure, the unit cell, since it is 4.33 Å, the distance between atoms of Ti and Ti were to be 4.33 × √2 = 6.1 Å.
[0090]
　Therefore, substantially the same coordinate position (7 angstroms) in the case where Ti atoms are more present, these Ti atoms is judged to be in the same deposit, to be in during the same precipitate counts the number of the determined Ti atom, the number is when there 50 or more was defined the precipitates fine Ti precipitates.
[0091]
　The size of the fine Ti precipitates from the lattice constant of the number of fine Ti precipitates of atoms of Ti which constitute the observed fine Ti precipitates, the equivalent circle diameter was assumed to calculate the spherical fine Ti precipitates.
[0092]
　Using the number of Ti atoms in the obtained fine Ti precipitates in the three-dimensional atom probe measurement, shows a method for determining the equivalent circle diameter (diameter) R of the precipitates below.
[0093]
　Measuring a number N of all the atoms of the target sample in a three-dimensional atom probe measurement method but, in fact, in the three-dimensional atom probe measurement method can not detect the number N of all the atoms of the target sample. Because of the detection rate of each device specific atom alpha (= total number / atoms of the detected atoms), calculates the number N of would have existed from the actual measured values ​​n atoms. That is the total number of atoms N = n / α. The detection rate of the device with measured in the present invention α was 0.35.
[0094]
　Next, the number N of atoms, and eight Ti atoms in the unit cell case of Ti precipitates Na-Cl structure is present, also, the lattice constant a of Na-Cl structure, 4. and is 33 Å to calculate the equivalent circle diameter by the following equation.
　Circle equivalent diameter (diameter) R = {(6/8) · (1 / [pi) · N · a 3 } (1/3)
[0095]
　For example, when the number of Ti was 50, the circle equivalent diameter is approximately, is calculated to be 1 nm. In the present invention, optionally after measurement of the circle equivalent diameter (diameter) of 30 or more fine Ti precipitates and calculate the average.
[0096]
　The number density of fine Ti precipitates, the measurement field as the denominator to determine the fine Ti precipitates the number as molecules. In the measurement of the number density, 10 nm field of view (thickness direction t) × 40 nm (sheet width direction W) × 60 nm (sheet longitudinal direction L), measured 5 or more visual fields, and the number density (number / cm 3 ) of and the average value was obtained.
[0097]
　In the present invention, also defined as follows for the presence states of TiN.
[0098]
　The average circle equivalent diameter of TiN: 1.0 ~ 10.0 [mu] m
　when TiN is 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 of TiN is below to 10.0μm. To ensure these effects more reliably, preferably has an average equivalent circle diameter of TiN is less than 8.0 .mu.m, more preferably at most 5.0 .mu.m.
[0099]
　Since preferably as TiN is small, the average circle equivalent diameter of TiN does not have to inherently provide a lower limit. However, in the observation method of TiN which will be described later, is less than the equivalent circle diameter of TiN is 1.0 .mu.m, it is difficult to determine whether it is TiN. Therefore, in the present invention, it is measured only what circle equivalent diameter of 1.0μm or more as TiN. Therefore, the average circle equivalent diameter of TiN becomes more 1.0 .mu.m.
[0100]
　The average circle equivalent diameter of TiN (diameter) 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, by polishing the rolling direction cross-section of the sample (the so-called L cross section) is observed in a state that does not etch. Specifically, taking a microstructure photograph 1000 times magnification using an optical microscope to observe the microstructure photograph visually or image processing apparatus or the like.
[0101]
　In microstructure photograph, for what can be identified as TiN, the circle equivalent diameter (diameter) determined, the circle equivalent diameter (diameter) is only more than 1.0μm and TiN. Then, 60 [mu] m field of view (the rolling direction L) × 40 [mu] m (thickness direction t), and observed for more than 20 fields, an average of the all the circle equivalent diameter of TiN (diameter), the average circle equivalent diameter of TiN ( the diameter).
[0102]
　The average value of the shortest distance between adjacent TiN: more 10.0μm
　grown voids to interface occurs between the TiN and the ferrite, in order to avoid further and large voids attached voids between the distance between the TiN a certain amount is necessary to secure. Therefore, the average value of the distance between adjacent TiN and more 10.0 [mu] m.
[0103]
　From the viewpoint of suppressing crack generation due to the growth of the voids, the average value is preferably not less than 15.0 .mu.m, and more preferably at least 20.0 .mu.m. The upper limit is not specifically set, since the precipitation of some TiN is unavoidable, the average value of the shortest distance between adjacent TiN is preferably set to 1000μm or less.
[0104]
　The average value of the shortest distance between adjacent TiN is obtained as follows. Any TiN and 20 select therewith a distance to TiN nearest were measured to calculate the average value. The measurement of the minimum distance between TiN, like the measurement of the average equivalent circle diameter determined.
[0105]
　(C) mechanical properties
　nanohardness standard deviation: 1.0 GPa or less
　to reduce the voids generated at the interface of both organizations by reducing the difference in the deformability of the hard tissue and soft tissue, 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 tissue and soft tissue, generation of voids can be suppressed. In the present invention, as an index of the hardness difference between the soft tissue and hard tissue, employing the standard deviation of nano-hardness in the sample section.
[0106]
　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.
[0107]
　Reducing the difference in hardness between the soft tissue and hard tissue, to suppress the generation of voids, the standard deviation of nano-hardness preferably small well, or less 1.0 GPa. The standard deviation of nano-hardness is preferably not more than 0.8 GPa.
[0108]
　Tensile strength: more than 780MPa
　steel sheet according to the present invention preferably has a conventional high burring 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).
[0109]
　Uniform elongation and tensile strength and the product: 7000 MPa ·% or more
　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 ≧ 7000MPa% . 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)
[0110]
　Hole expanding ratio and tensile strength and the product: -% or more 50000MPa
　the hole expansion is poor, which can result in cracking material flow is poor upon the stretch flanging. Therefore, to ensure the hole expansion, the product of the hole expansion ratio (lambda) and tensile strength (TS): (TS) preferably satisfies × (λ) ≧ 50000MPa%. However, the hole expanding ratio (lambda) denotes the JIS Z 2256 (2010) on the hole expansion rate by the test method conforming (lambda).
[0111]
　Equivalent plastic strain: 0.9 or
　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).
[0112]
　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.
[0113]
　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
[0114]
　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).
[0115]
　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.
[0116]
　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.
[0117]
　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.
[0118]
　(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.
[0119]
　(E) the production method
　inventors, studies so far, by performing the process of manufacturing the below (a) to (f) in order to be able to manufacture a hot-rolled steel sheet of the present invention It has been confirmed. It will be described in detail below each manufacturing step.
[0120]
　(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.
[0121]
　(B) heating step
　subjected to hot rolling to manufacture slabs, and hot-rolled steel sheet. In performing hot rolling, first, heating the slab. In the heating step, the slab below (i) represented by SRTmin ° C. or higher in formula, is heated to a temperature of 1260 ° C. or less. After once cooled to a low temperature in the case of continuous casting, may be heated again, it may be heated subsequent to continuous casting without any particular cooling. Here, SRTmin means the solution temperature of TiC.
　SRTmin = 7000 / {2.75-log (Ti × C)} - 273 ··· (i)
　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.
[0122]
　(C) Continuous hot rolling process
　after heating, subjected to rough rolling and subsequent multi-stage finish rolling against the slab extracted from the heating furnace. As precipitates containing Ti does not precipitate, the end temperature of the rough rolling is set to 1100 ° C. or higher. Further, as described above, the multi-stage finish rolling is 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), performs a multistage finish rolling so as to 0.01-0.10.
[0123]
　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.
[0124]
　Effective accumulated strain (εeff) = Σεi (ti, Ti) ··· (iii)
　the Σ in the above formula (iii), shows 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.
[0125]
　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 ) · · ·
　(iv) 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)} | · · · (v)
　.tau.R = · exp .tau.0 (Q / (R · Ti)) · · · (vi)
　.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))
[0126]
　By defining the effective accumulated strain derived in this way, variations in the nano-hardness is decreased with microstructure of interest are obtained. As a result, to suppress the void growth that occur at the interface between the hard tissue and soft tissue, even 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.
[0127]
　Multistage finish rolling end temperature, i.e. the finishing temperature of the continuous hot rolling process, Ar is determined by the following (ii) Formula 3 with, Ar 3 (° C.) + 30 may be greater than or equal to the temperature of ° C.. Thus it is because precipitation strengthening ferrite and bainite of interest in the present invention is obtained.
　Ar 3 = 970-325 × C + 33 × Si + 287 × P + 40 × Al-92 × (Mn + Mo + Cu) -46 × (Cr + Ni) · · · (ii)
　where the element symbol in the above formulas, hot-rolled steel sheet of each element It represents the content (mass%), if not contained shall substituting 0.
[0128]
　(D) first (accelerating) the cooling step
　after the finish multistage rolling end, to start cooling of the resulting hot rolled steel sheet after 1.00 ~ 5.00s. Then, the rolling end temperature, then cooled at a temperature up to 10 ° C. / s or more average cooling rate 650 ~ 750 ° C., then held 1 ~ 10s in the air.
[0129]
　If after the continuous hot rolling process is completed to start the cooling at less than 1.00 s, ferrite transformation is promoted, no bainite area ratio of interest is obtained in the final microstructure only either precipitates present invention coarsens effect of can not be obtained. On the other hand, the ferrite transformation starts the cooling beyond 5.00s can not be obtained area ratio of precipitation strengthening ferrite of interest with a delay.
[0130]
　When the average cooling rate in the first cooling step is less than 10 ° C. / s, easily generated pearlite. On the other hand, the upper limit of the cooling rate is not particularly limited, it may be a 300 ° C. / s or less to avoid supercooling. Further, the holding temperature in air is less than 650 ° C., tends to produce bainite, bainite area ratio increases. On the other hand, if it exceeds the holding temperature is 750 ° C. in air, pearlite is likely generated.
[0131]
　Here, the retention of atmospheric say, the method comprising hot-rolled steel sheet air or cooling is limited to a minimum in the cooling facility, the lower limit of the cooling rate at this time is ideally 0 ° C. / s , and the upper limit is 8 ℃ / s.
[0132]
　(E) second (acceleration) cooling step
　after holding in the air, the temperature range of 600 ~ 740 ° C., cooled at 10 ° C. / s or more average cooling rate. Cooling start temperature is less than 600 ° C., ferrite transformation does not proceed sufficiently, it becomes insufficient precipitation of fine Ti precipitates. On the other hand, if it exceeds ℃ cooling start temperature 740, along with ferrite transformation proceeds excessively, there is a possibility that the hole expansion pearlite is generated to deteriorate. Further, there is a possibility that fine Ti precipitates decreases the strength coarsened.
[0133]
　Also, if the average cooling rate is less than 10 ° C. / s, there is a possibility that the hole expansion pearlite is generated to deteriorate. The average upper limit of the 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.
[0134]
　(F) winding process
　then the cooled hot rolled steel sheet, wound in a coiling temperature of 450 ~ 650 ° C.. Conditions after winding process is not particularly limited.
[0135]
　(F) steel forged parts
　hot rolled steel sheets obtained as described above is excellent in plate forgeability, by forging such as a leaf forging the hot-rolled steel sheet, we could not talk with conventional forging a complicated shape requiring high strength can be obtained.
[0136]
　The following examples illustrate the present invention more specifically, the present invention is not limited to these examples.
Example 1
[0137]
　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.
[0138]
[Table 1]

[0139]
[Table 2]

[0140]
[table 3]

[0141]
　[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.
[0142]
　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.
[0143]
　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.
[0144]
　The area ratio of the precipitation strengthening ferrite, as mentioned above, the test pieces polished with colloidal silica abrasive, and EBSP measuring the visual field of 160 × 256 .mu.m at a magnification of 400 times under measurement conditions of the measurement step 0.5 [mu] m, KAM method at was determined.
[0145]
　Fine Ti precipitates also as described above, the electrolytic polishing the specimen was measured in a three-dimensional atom probe measurement was the equivalent circle diameter, the number density calculated.
[0146]
　TiN also, as described above, the test piece at a magnification of 1000 times, the field of view of 60 × 40 [mu] m to 20 field observation, to obtain an average circle equivalent diameter of TiN by image processing. Further, the shortest distance of TiN each other, the same portion as the tissue investigated was observed with 500 times metallurgical microscope, was determined.
[0147]
　[Mechanical characteristics]
　of which tensile strength properties of the mechanical properties (tensile strength (TS), uniform elongation (u-EL), the hole expanding ratio (lambda)), when the plate width has is W, a plate width from one end of the plate in any position of the 1 / 4W or 3 / 4W direction, using a No. 5 test piece of JIS Z 2241 were taken a direction perpendicular to the rolling direction (width direction) as the longitudinal direction (2011), JIS Z 2241 (2011) were evaluated in accordance with the. Hole expansion rate, the test pieces were taken from the tensile test specimen sampling position similar position, it was evaluated according to the test method JIS Z 2256 2010 forth.
[0148]
　Moreover, subjected to simple shear test by the following procedure to determine the equivalent plastic strain based on the result.
[0149]
　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.
[0150]
　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).
[0151]
　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)
[0152]
　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)
[0153]
　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.
[0154]
　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
[0155]
　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. The shear strain plasticity when the test piece is broken is Ipushironspf.
[0156]
　Said a simple shear shear stress obtained in Test [sigma] s, the shear plastic strain relation εspf when the test piece is broken, by the above-described method, using a conversion factor kappa, was determined equivalent plastic strain Ipushironeq.
[0157]
　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.
[0158]
　These measurement results are shown in Table 3.
[0159]
　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 ) is 7000 MPa ·% or more, the product of the strength TS and tensile hole expansion ratio lambda (TS × lambda) has a higher ·% 50000 mPa, hot rolled steel sheet having balanced properties is obtained. Further, hot-rolled steel sheet according to the present invention, the equivalent plastic strain also exceed 0.90 (90%), it was confirmed that the steel plate to withstand the high strain region processing such as plate forging.
Industrial Applicability
[0160]
　According to the present invention, while maintaining good hole expansion is a fundamental function of a high burring steel, 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
[0161]
　1 shear deformation generating unit
　2 chucking unit

The scope of the claims

[Requested item 1]Chemical composition of the steel sheet contains, by
　mass%,
　C: 0.020
　~ 0.070%, Si: 0.05 ~ 1.70%, Mn:
　0.60 ~ 2.50%, Al: 0.010 ~ 1
　% .000, N: 0% ultra-0.0030% or
　less, P: 0.050% or
　less, S: 0.005% or
　less, Ti: 0.015
　~ 0.170%, Nb: 0 - 0.100
　%,
　V:
　0 ~ 0.300%,
　Cu: 0 ~ 2.00%, Ni: 0 ~
　2.00%, Cr: 0 ~ 2.00%, Mo: 0 ~
　1.00%, B: 0 　0.0100%
~, 　Mg: 0 ~ 0.0100%, Ca: 0 　~ 0.0100%, REM: 0 ~ 0.1000%, 　Zr: 0 ~ 1.000%, Co: 0 ~ 1.000% , 　Zn: 　0 ～ 1.000 Pasento, 　W: 0 ～ 1.000 Pasento, Sn: 0 ～ 0.050 Pasento, and, 　the balance is Fe and impurities,

　In the rolling direction and the perpendicular cross section of the steel sheet, the width and thickness of the steel sheet is W and t, respectively, at the end face 1 / 4W or 3 / 4W of the steel plate, and, from the surface of the steel sheet 1 / metal structure in 4t or 3 / 4t positions, in area%,
　ferrite: 5 to 70%
　bainite 30 to 95%,
　residual austenite: 2% or less,
　martensite: 2% or less, and,
　perlite: 1 % or less, a, and
　the sum of ferrite and bainite: at least 95%,
　the ferrite has a deposit containing Ti in the grains,
　the number density of precipitates containing the Ti, 1.0 10 × 16 ~ 50.0 × 10 16 atoms / cm 3 is,
　TiN precipitates are contained in the steel sheet,
　the average circle equivalent diameter of said TiN precipitates is 1.0 ~ 10.0 [mu] m,
　the adjacent said The average value of the shortest distance between TiN precipitates not less than 10.0 [mu] m,
　standard deviation of nano-hardness of less 1.00GPa,
　hot-rolled steel sheet.
[Requested item 2]
　The average circle equivalent diameter of precipitates containing Ti is 1.00 ~ 3.00 nm,
　hot rolled steel sheet according to claim 1.
[Requested item 3]
　A tensile strength of not less than 780 MPa,
　and the product of the tensile strength and uniform elongation 7000 MPa ·% or more,
　the product of the tensile strength and the hole expansion rate is 50000 mPa ·% or more,
　in claim 1 or claim 2 hot-rolled steel sheet according.
[Requested item 4]
　A method of manufacturing a hot rolled steel sheet according to any one of claims 1 to 3,
　relative to a slab having a chemical composition according to claim 1, the heating step, the continuous hot rolling process, the first cooling step, subjected to a second cooling step and the winding step in the order,
　in the heating step, the slab below (i) represented by SRTmin ° C. or higher in formula, by heating to a temperature of 1260 ° C. or less,
　the continuous hot rolling process includes a rough rolling and 3 or more stages finish rolling,
　the finishing temperature of the rough rolling is at 1100 ° C. or higher,
　cumulative distortions in the rolling of the final three stages in the multistage finish rolling is 0.01 to zero. is 10,
　rolling end temperature of the multi-stage finishing rolling, Ar is obtained by the following (ii) formula 3 a + 30 ° C. or higher,
　wherein in the first cooling step, after the multi-stage finish rolling is completed, 1. 00 to cold after 5.00s Retirement was started from the rolling end temperature to a temperature range of 650 ~ 750 ° C., and cooled at an average cooling rate of more than 10 ° C. / s, then holding 1 ~ 10s in air,
　in the second cooling step , after retention in the atmosphere, the temperature range of 600 ~ 740 ° C., and cooled at an average cooling rate of more than 10 ° C. / s,
　and in the winding process, winding at coiling temperature of 450 ~ 650 ° C.,
　thermal production process between a rolled steel plate.
　= 7000 SRTmin / {2.75-log (Ti × C)} - 273 · · ·
　(i) Ar 3 = 970-325 × C + 33 × Si + 287 × P + 40 × Al-92 × (Mn + Mo + Cu) -46 × (Cr + Ni) · · · (ii)
　where each element symbol in the above formula, the hot rolled steel sheet of each element It represents content (mass%), if not contained shall substituting 0.
[Requested item 5]
　Obtained from hot-rolled steel sheet according to any one of claims 1 to 3,
　a steel forged parts.
[Requested item 6]
　Against hot-rolled steel sheet according to any one of claims 1 to 3, subjected to at least forging,
　manufacturing method of a steel forging.