Technical field
[0001]
　The present invention relates to a suitable high strength steel sheet machine structural parts and the like, including a body structure component of a motor vehicle.
BACKGROUND
[0002]
　To suppress the emission of carbon dioxide gas from automobiles, the vehicle body weight reduction of automobiles using high strength steel sheets have been developed. Further, in order to ensure passenger safety also have come into high-strength steel sheet is widely used in vehicle body. In order to promote further lightening of the vehicle body, a further improvement in strength is important. On the other hand, depending on the body part are required to have excellent formability. For example, the high strength steel plate for skeletal system component is required to have excellent elongation and hole expansion. In particular, the high-strength steel sheet used for a member is skeletal member of a motor vehicle (subframe) and reinforcement (reinforcement member), not good ductility only obtained excellent hole expansion.
[0003]
　However, both of the improvement of strength and improvement in moldability is difficult. Although techniques aimed at both of improving the strength and improving the moldability is proposed, it is impossible to obtain sufficient characteristics thereby. In recent years, has been required further improvement in strength, a technique for the purpose of compatibility between improvement in moldability is proposed, formability, particularly, stretch flange improvement of is difficult. Meanwhile, with the improvement of the productivity of the steel sheet, but excellent hole expansion under conditions having improved test speed in quality survey of the steel sheet is desired, the conventional steel sheet, spread well when machining speed is high sex improvement of it is difficult.
CITATION
Patent Document
[0004]
Patent Document 1: JP 2009-13488 JP
Patent Document 2: JP 2012-36497 JP
Patent Document 3: JP 2002-88447 JP
Patent Document 4: JP 2009-249669 Patent Publication
Patent Document 5: JP 2010-65307 JP
Patent Document 6: JP 2002-66601 JP
Patent Document 7: JP 2014-34716 JP
Patent Document 8: WO 2014/171427 Patent
Patent Document 9: JP 56-6704 JP
Patent Document 10: JP 2006-207016 JP
Patent Document 11: JP 2009-256773 JP
Patent Document 12: JP 2010-121175 JP
Summary of the Invention
Problems that the Invention is to Solve
[0005]
　The present invention has excellent strength and can be obtained moldability, and to provide a particularly excellent steel sheet formability during high speed machining.
Means for Solving the Problems
[0006]
　The present inventors have conducted extensive studies to solve the above problems. As a result, the conventional steel sheet, bainite, the rigid structure consisting of martensite or residual austenite, or any combination thereof are present continuous band-like tissue in the band shape, a band shape tissue becomes stress concentration portions, voids generating revealed to be promoted. The martensite include fresh martensite and tempered martensite. Moreover, since the generation point of the voids due to the band-like structure is present densely, also revealed that the connection of the voids is facilitated. That is, the band-like structure was found to have affected the properties widened hole. Then, the present inventors have found that the improvement in hole expansion, has found that it is important to suppress the band-like structure. Furthermore, the present inventors have found that by suppressing a band-like structure, was also found to improve the surface quality at the time of molding.
[0007]
　Band-like tissue, and alloying elements segregation of Mn or the like in the melting step, in hot rolling and cold rolling, is formed in a region where the alloy elements are segregated is stretched in the rolling direction. Therefore, the inhibition of the band-like tissue, it is important to suppress the segregation of alloying elements. Further, the present inventors have found that the inhibition of the band-like tissue, prior to finish rolling, to cause recrystallization of austenite by introducing lattice defects at a high temperature, and to increase the Si concentration in the alloy segregation It was found to be very effective. That is, by recrystallization, diffusion of alloying elements along the grain boundary of recrystallized austenite grains is accelerated, now alloying elements are distributed in a network form is the segregation of alloying elements can be suppressed. Furthermore, the present inventors have found that by containing Si by increasing the Si concentration of the Mn segregation, was homogeneously found that ferrite is formed band tissue is effectively removed by cooling. According to this method, it is possible to conventional prolonged heating and, without the addition of expensive alloying elements, to eliminate the effective band tissue.
[0008]
　Hole expansion is evaluated by the method specified in JIS T 1001, JIS Z 2256 or JFS T 1001,. Generally, test speed of hole expansion test is a 0.2 mm / sec. However, the inventors are that are test results obtained are different depending test speed, the results obtained in the tests of the test speed of about 0.2 mm / sec, can adequately reflect the hole expansion during high speed machining It was found to be not. This strain rate is also believed to be due to increased with increasing machining speed. Therefore, it can be said that the evaluation of hole expansion during high speed machining, the results obtained in hole expansion tests 1 mm / sec approximately and is the upper limit which is defined a test speed is important. The inventors have found that in steel sheet band structure is eliminated as described above, the results of the test speed was obtained by hole expansion test of 1 mm / sec was also found to be good.
[0009]
　The present inventors, as a result of further intensive studies based on these findings, and conceived to aspects of the invention described below.
[0010]
　(1)
　in
　mass%,
　C: 0.05% ~
　0.40%, Si: 0.05% ~ 6.00%, Mn: 1.50% ~ 10.00%,
　acid-soluble Al: 0.01 ~
　1.00%%, P: 0.10% or
　less, S: 0.01% or
　less, N: 0.01% or
　less, Ti: 0.0%
　~ 0.2%, Nb: 0.0% ~
　% 0.2,
　V: 0.0%
　~ 0.2%, Cr: 0.0% ~ 1.0%, Mo:
　0.0% ~ 1.0%, Cu: 0.0% ~ 1.
　%
　0,
　Ni: 0.0% ~ 1.0%, Ca: 0.00% ~ 0.01%,
　Mg: 0.00% ~ 0.01%, REM: 0.00% ~ 0.01% ,
　Zr: 0.00% - 0.01% and
　the balance: Fe and impurities,
have in a chemical composition represented,
　by area ratio,
　ferrite: 5% to 80%,
　Bainite, martensite or retained austenite or hard tissue consisting of any combination: 20% to 95%, and
　the hard tissue segment rate in line in the thickness direction in a plane perpendicular standard deviation: the steel plate the depth of the thickness from the surface when the t is 0.050 or less, in the depth range of 3t / 8 to t / 2
steel sheet characterized by having a in steel structure represented.
[0011]
　(2)
　In the steel structure, an area ratio,
　the retained austenite: 5.0% or more,
　the steel sheet according to (1) that holds.
[0012]
　(3)
　In the above chemical composition, by
　mass%, Ti:
　0.003% ~ 0.2%, Nb: 0.003% ~ 0.2%, or
　V: 0.003% ~ 0.2%,
　or steel sheet according to, characterized in that any combination of these is true (1) or (2).
[0013]
　(4)
　In the above chemical composition, in
　mass%,
　Cr: 0.005% ~ 1.0%,
　Mo: 0.005% ~ 1.0%, Cu: 0.005% ~ 1.0%, or
　Ni : 0.005% to 1.0%,
　the steel sheet according to any one of or characterized by any combination of these is true (1) to (3).
[0014]
　(5)
　In the above chemical composition, by
　mass%,
　Ca: 0.0003% ~
　0.01%, Mg: 0.0003% ~ 0.01%, REM: 0.0003% ~ 0.01%, or
　Zr 0.0003% - 0.01%,
　the steel sheet according to any one of or characterized by any combination of these is true (1) to (4).
Effect of the invention
[0015]
　According to the present invention, since the steel structure is appropriate, excellent strength and could be obtained formability, it can also be obtained formability during high speed machining. Further, according to the present invention, by suppressing the band-like tissue, suppressing striped surface defects that occur during the molding of ultrahigh-tensile, it is possible to obtain an excellent appearance.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016]
[1] Figure 1 is a diagram illustrating a method for determining the line rate of the hard tissue.
DESCRIPTION OF THE INVENTION
[0017]
　Hereinafter, embodiments of the present invention will be described.
[0018]
　First, a description will be given chemical composition of the slab used in the steel plate and its production according to the embodiment of the present invention. As described later, the steel sheet according to the embodiment of the present invention, the multi-axial compression working of the slab, hot rolling is manufactured by the cold rolling and annealing or the like. Therefore, the chemical composition of the steel sheet and slab, not only the properties of the steel sheet, is taken into consideration these processes. In the following description, a unit of content of each element contained in the steel sheet and slab "%" is especially meant to "mass%" unless otherwise specified. Steel sheet according to the present embodiment, by mass%, in mass%, C: 0.05% ~ 0.40%, Si: 0.05% ~ 6.00%, Mn: 1.50% ~ 10.00 %, acid-soluble Al: 0.01% ~ 1.00%, P: 0.10% or less, S: 0.01% or less, N: 0.01% or less, Ti: 0.0% ~ 0.2 %, Nb: 0.0% ~ 0.2%, V: 0.0% ~ 0.2%, Cr: 0.0% ~ 1.0%, Mo: 0.0% ~ 1.0%, Cu: 0.0% ~ 1.0%, Ni: 0.0% ~ 1.0%, Ca: 0.00% ~ 0.01%, Mg: 0.00% ~ 0.01%, REM ( rare earth metals: rare earth metal): 0.00% ~ 0.01%, Zr: 0.00% ~ 0.01%, and the balance has Fe and impurities, in a chemical composition represented. As the impurity, those included in raw materials such as ores and scrap, intended to be included in the manufacturing process, is exemplified.
[0019]
　(C: 0.05% ~ 0.40%)
　C contributes to the improvement of the tensile strength. The C content is less than 0.05%, no sufficient tensile strength, for example, 780MPa or more tensile strength. Therefore, C content is set to 0.05% or more, preferably 0.07% or more. On the other hand, the C content of 0.40 percent, martensite becomes hard, weldability is degraded. Therefore, the C content 0.40% or less, preferably not more than 0.35%, and more and preferably from 0.30% or less, more preferably to 0.20% or less.
[0020]
　: (Si 0.05% ~ 6.00% )
　is Si by solid solution strengthening, enhancing the tensile strength without degrading hole expansibility. The Si content is less than 0.05%, no sufficient tensile strength, for example, 780MPa or more tensile strength. Therefore, Si content is set to 0.05% or more, preferably 0.20% or more, more preferably 0.50% or more. Si is enriched in Mn segregation, and promote the formation of ferrite, also has the effect of suppressing the band-like distribution of the hard tissue. This effect Si content is particularly pronounced in the case of more than 2.00%. Therefore, Si content is preferably 2.00% or more, more preferably 2.50% or more. On the other hand, Si content is 6.00 percent, the ferrite phase stabilizing effect of alloy segregation exceeds the austenite phase stabilizing effect of Mn, the formation of the band-like tissue is promoted. Therefore, Si content is not more than 6.00%, preferably not more than 5.00%. Further, it is possible to more effectively suppress the band-like distribution by Si is contained in accordance with the Mn content. In this respect, Si content is preferably not more than 1.3 times 1.0 times the Mn content. From the viewpoint of the surface properties of the steel sheet, good Si content be 2.00% or less, may be less 1.50%, may be less 1.20%.
[0021]
　(Mn: 1.50 Pasento ～ 10.00
　Pasento) Mn contributes to the improvement of the tensile strength. The Mn content is less than 1.50%, no sufficient tensile strength, for example, 780MPa or more tensile strength. Therefore, Mn content is 1.50% or more. Mn is without the addition of expensive alloying elements, it is possible to increase the residual austenite fraction. From this point of view, Mn content is preferably 1.70% or more, more preferably 2.00% or more. On the other hand, Mn content is 10.00%, the increased amount of precipitation of MnS, low temperature toughness is degraded. Therefore, Mn content is at most 10.00%. From the viewpoint of productivity in hot rolling and cold rolling, Mn content is preferably not more than 4.00%, more preferably at most 3.00%.
[0022]
　(Acid-soluble Al: 0.01% ~ 1.00%)
　acid-soluble Al has the effect of consolidation of the steel sheet by deoxidation of steel. The acid-soluble Al content is less than 0.01%, the effect can not be sufficiently obtained by this action. Thus, acid-soluble Al content is 0.01% or more, preferably 0.02% or more. On the other hand, acid-soluble Al content is 1.00 percent, or decreased weldability, oxide inclusions are or surface texture with deterioration increases. Thus, acid-soluble Al content is not more than 1.00%, preferably 0.80%. Incidentally, the acid-soluble Al is, Al 2 O 3 does not become a compound which is not soluble in acid such as, for soluble in acid.
[0023]
　(P: 0.10% or less)
　P is not an essential element, is contained as an impurity, for example, in the steel. From the standpoint of weldability, P content is better as low as possible. In particular, P content is 0.10 percent, a significant decrease in weldability. Accordingly, P content is not more than 0.10%, preferably not more than 0.03%. Costly in reducing the P content, an attempt to reduce to less than 0.0001%, cost increases considerably. Therefore, P content may be 0.0001% or more. P is to contribute to the improvement of the strength, P content may be 0.01% or more.
[0024]
　(S: 0.01% or less)
　S is not an essential element, is contained as an impurity, for example, in the steel. From the standpoint of weldability, S content is preferably as low as possible. The higher the S content increases the amount of precipitation of MnS, the low temperature toughness is reduced. In particular, S content is 0.01 percent, a significant decrease in the reduction and low temperature toughness of the weld properties. Thus, S content is 0.01% or less, preferably 0.003% or less, more preferably 0.0015% or less. Costly in reducing S content, an attempt to reduce to less than 0.001%, the cost remarkably increases, an attempt to reduce to less than 0.0001%, the cost is increased further significantly. Therefore, S content should be 0.0001% or more, may be more than 0.001%.
[0025]
　(N: 0.01% or less)
　N is not an essential element, is contained as an impurity, for example, in the steel. From the standpoint of weldability, N content, the better the lower the. In particular, N content 0.01 percent, a significant decrease in weldability. Therefore, N content is 0.01% or less, preferably 0.006% or less. Costly in reducing the N content, an attempt to reduce to less than 0.0001%, cost increases considerably. Therefore, N content may be 0.0001% or more.
[0026]
　Ti, Nb, V, Cr, Mo, Cu, Ni, Ca, Mg, REM and Zr is not an essential element, a steel plate and may optionally elements be appropriately contained in the limits of the predetermined amount to the steel.
[0027]
　(Ti: 0.0% ~ 0.2%, Nb: 0.0% ~ 0.2%, V: 0.0% ~
　0.2%) Ti, Nb and V contribute to improvement of strength. Thus, Ti, Nb or V, or any combination thereof may be contained. To obtain this effect sufficiently, Ti content, Nb content or the V content or any combination thereof, preferably 0.003% or more. On the other hand, Ti content, the Nb content or the V content or any combination thereof of 0.2 percent, hot rolling and cold rolling becomes difficult. Therefore, Ti content, Nb content or the V content or any combination thereof is 0.2% or less. That, Ti: 0.003% ~ 0.2% , Nb: 0.003% ~ 0.2%, or V: 0.003% ~ 0.2%, or that any combination of these are met preferable.
[0028]
　(Cr: 0.0% ~ 1.0%, Mo: 0.0% ~ 1.0%, Cu: 0.0% ~ 1.0%, Ni: 0.0% ~
　1.0%) Cr , Mo, Cu and Ni contributes to improvement in strength. Therefore, Cr, Mo, Cu, or Ni or any combination of these may be contained. To obtain this effect sufficiently, Cr content, Mo content, Cu content or Ni content or any combination thereof, preferably 0.005% or more. On the other hand, Cr content, Mo content, the Cu content or Ni content or any combination thereof of 1.0 percent, unnecessarily cost becomes higher effect by the working becomes saturated. Therefore, Cr content, Mo content, Cu content or Ni content or any combination thereof is 1.0% or less. That, Cr: 0.005% ~ 1.0% , Mo: 0.005% ~ 1.0%, Cu: 0.005% ~ 1.0%, or Ni: 0.005% ~ 1.0% , or it is preferable that any combination of these are met.
[0029]
　(Ca: 0.00% ~ 0.01%, Mg: 0.00% ~ 0.01%, REM: 0.00% ~ 0.01%, Zr: 0.00% ~
　0.01%) Ca , Mg, REM and Zr contributes to fine dispersion of inclusions, increase the toughness. Therefore, Ca, Mg, REM or Zr, or any combination thereof may be contained. To obtain this effect sufficiently, Ca content, Mg content, REM content or the Zr content, or any combination thereof, preferably 0.0003% or more. On the other hand, Ca content, Mg content, the REM content or the Zr content, or any combination thereof 0.01 percent, surface properties are deteriorated. Therefore, Ca content, Mg content, REM content or the Zr content, or any combination thereof is not more than 0.01%. That, Ca: 0.0003% ~ 0.01% , Mg: 0.0003% ~ 0.01%, REM: 0.0003% ~ 0.01%, or Zr: 0.0003% ~ 0.01% , or it is preferable that any combination of these are met.
[0030]
　REM (rare earth metals) refers Sc, Y, and total 17 kinds of lanthanoid elements, "REM content" means total content of these 17 kinds of elements. Lanthanide is industrially, is added in the form of, for example, misch metal.
[0031]
　It will now be described steel sheet steel structure according to the embodiment of the present invention. Steel sheet according to the present embodiment, an area ratio, ferrite: 5% to 80%, bainite, martensite or retained austenite or hard tissue consisting of any combination: 20% to 95%, and a thickness direction perpendicular such a standard deviation of the line rate of the hard tissue in line in a plane: 0.050 depth of the thickness of the steel sheet from the surface when the t is within the depth range from 3t / 8 to t / 2 hereinafter, in which a steel structure represented. The martensite include fresh martensite and tempered martensite.
[0032]
　(Ferrite: 5% to 80%)
　in the area ratio of ferrite is less than 5%, it is difficult to secure 10% or more of elongation at break (EL). Therefore, the area ratio of ferrite is 5% or more, preferably 10% or more, more preferably 20% or more. On the other hand, the area ratio of ferrite is 80%, no sufficient tensile strength, for example, 780MPa or more tensile strength. Therefore, the area ratio of ferrite is 80% or less, preferably 70% or less.
[0033]
　(Hard tissue: 20% to 95%)
　hard tissue area ratio of is less than 20%, no sufficient tensile strength, for example, 780MPa or more tensile strength. Therefore, the area ratio of the hard tissues were 20% or more, preferably 30% or more. On the other hand, the area ratio of the hard tissues in 95%, no sufficient ductility can not be obtained. Therefore, the area ratio of the hard tissue is 95% or less, preferably 90% or less, more preferably 80% or less.
[0034]
　(Residual austenite (residual gamma): 5.0% or more)
　when the area ratio of residual austenite is 5.0% or more, easily obtained breaking elongation of 12% or more. Therefore, the area ratio of residual austenite is preferably set to 5.0% or more, more preferably 10.0% or more. The upper limit of the area ratio of residual austenite is not limited, in the present state of the art, it is not easy to area ratio of residual austenite is produced 30.0% of the steel sheet.
[0035]
　The area ratio of the area ratio and the hard structure of ferrite can be measured as follows. First, a sample was taken as a cross section perpendicular to the width direction of the 1/4 position of the width of the steel sheet is exposed to corrode the cross-section by Les expeller etchant. Then, the depth from the surface of the steel sheet to shoot an optical micrograph of the region from 3t / 8 to t / 2. In this case, for example, the magnification is 200 times. Observation plane due to corrosion with Les expeller etchant can generally distinguish the black portion and the white portion. Then, the black portion, ferrite, bainite, may include carbides and pearlite. Portion including lamellar tissue in grains among the black portion corresponds to pearlite. Free of lamellar tissue in grains among black portion, the portion not including the infrastructure corresponds to the ferrite. Particularly low brightness among the black parts, diameter spherical portion of about 1 [mu] m ~ 5 [mu] m corresponds to a carbide. Portion including a lower tissue in grains among the black portion corresponds to bainite. Thus, free of lamellar tissue in grains among black portion, the area ratio of ferrite can be obtained by measuring the area ratio of the portion not including the infrastructure, infrastructure in grains among the black portion the area ratio of bainite is obtained by measuring the area ratio of the portion including the. The area of ​​the white part is the total area fraction of martensite and residual austenite. Therefore, the area ratio of bainite and an area ratio of the hard tissue from the total area fraction of martensite and residual austenite is obtained. This optical micrograph, it is possible to measure the circle-equivalent mean diameter r of the hard tissue for use in measurement of the standard deviation of the line segment ratio of the hard tissue below.
[0036]
　Area fraction of retained austenite, for example, can be identified by X-ray measurements. In this case, it is possible to convert the volume fraction of retained austenite was determined by X-ray measurement, the area fraction of retained austenite in terms of quantitative metallography. In this way, for example, the portion from the surface of the steel sheet to 1/4 of the thickness of the steel sheet is removed by mechanical polishing and chemical polishing, using MoKα rays as characteristic X-rays. The body-centered cubic (bcc) phase (200) and (211), as well as face centered cubic (fcc) lattice phase (200), from the integrated intensity ratio of the diffraction peaks of (220) and (311), the following calculating the area fraction of retained austenite using the equation.
　S [gamma] = (I 200f + I 220f + I 311f ) / (I 200b + I 211b ) × 100
(S [gamma] is the area fraction of retained austenite, I 200f , I 220f , I 311f are the fcc phase, respectively (200), (220) , (311) intensity of the diffraction peak, I 200b , I 211b are respectively bcc phase (200), indicating the intensity of the diffraction peak of (211).)
[0037]
　(Line segment ratio of the hard tissue in line in the plane perpendicular to the thickness direction standard deviation: the depth of the depth of the thickness of the steel sheet from the surface when the t is from 3t / 8 to t / 2 0.050 or less) within
　the steel sheet, in the processing of adding the local large deformation processing such as spread holes, to fracture through the voids and ligation of in necking or the steel structure. The tensile deformation when a steel sheet is constricted, the center portion of the steel sheet is stress concentration point, usually voids primarily from the surface of the steel sheet at the position of t / 2. The voids are connected, the void until t / 8 or more size becomes coarse, breaking occurs the coarsened void as a starting point. Generating sites of voids serving as a starting point for breaking as described above, it is a hard tissue depth from the surface is present in the range of 3t / 8 to t / 2. Therefore, greatly affects the distribution of hole expansion of the hard tissue at a depth range of depth from the surface from 3t / 8 to t / 2.
[0038]
　Then, the standard deviation of the line segment ratio of the hard tissue within the depth range is large, the variation of the percentage of hard tissue in the thickness direction is large, i.e., the steel structure is in a band like structure It means. Especially in the standard deviation of the line segment ratio of the hard tissue 0.050 greater than the band-like structure is remarkable, stress density of concentration portions is locally high, no sufficient hole expansion is obtained. Therefore, the standard deviation of the line segment ratio of the hard tissue, depth from the surface is 0.050 or less in the depth region of from 3t / 8 to t / 2, preferably 0.040 or less.
[0039]
　Here, a method of measuring the standard deviation of the line segment ratio of the hard tissue.
[0040]
　First, the image processing on the optical photomicrographs taken in the same manner as the area ratio measurement and binarization to the black portion and the white portion. Figure 1 shows an example of an image after the binarization. Then, over the depth 3t / 8 parts from the depth t / 2 portion of the image of the observed object, sets the starting point of the line segment for each r / 30 (r is the circle-equivalent mean diameter of the hard tissue) . Since the depth range of the observation target is a region having a thickness of t / 8 from 3t / 8 to t / 2, the number of the starting point is the 15 t / 4r. Then, the direction perpendicular to the thickness direction from the starting point, for example, a length extending in the rolling direction to set the line segment 50r, measuring the line rate of the hard tissue on this line. Then, to calculate the standard deviation of the line segment ratio between 15 t / 4r book segment.
[0041]
　Circular thickness t of the corresponding average diameter r and steel is not limited. For example, circle-equivalent mean diameter r is 5 [mu] m ~ 15 [mu] m, the thickness t of the steel sheet is 1mm ~ 2mm (1000μm ~ 2000μm). Interval to set the starting point of the line segment is not limited, the resolution of the image of interest, may be changed according to the number of pixels and the measured operation time and the like. For example, even intervals as about r / 10, when set to r / 30 and equivalent results are obtained.
[0042]
　The depth range of depth from the surface from 3t / 8 to t / 2 can infinitely subdivided theoretically, also there are an infinite plane perpendicular to the thickness direction. However, it is impossible to measure the line rate for all of these. On the other hand, according to the measuring method described above, the above depth range is subdivided in a sufficiently small interval, it is possible to obtain an equivalent result to the case of infinitely subdivided. For example, in FIG. 1, the X-X line, high line rate of the hard tissue, the Y-Y line, a low line rate of the hard tissue.
[0043]
　According to this embodiment, for example, tensile strength can be obtained than 780 MPa, 30% or more hole expansion rate when measuring hole expanding test speed of 1 mm / sec in the method specified in JIS Z 2256 (hole expansion ratio: HER) is obtained. Further, a steel plate, a tensile direction is taken JIS5 No. Tensile test piece such that the direction orthogonal to the rolling direction, resulting elongation at break of more than 10% when measured by the method specified in JIS Z 2241 .
[0044]
　Next, a method for manufacturing a steel sheet according to the embodiment of the present invention. In the method of manufacturing the steel sheet according to the embodiment of the present invention, it performs multi-axis compression processing of the slab having the above chemical composition, hot rolling, cold rolling and annealing in this order.
[0045]
　(Multi-axis compression processing)
　slab, for example, using a converter furnace or an electric furnace or the like was melted the molten steel of the chemical composition can be produced by continuous casting. Instead of the continuous casting, Zokatamariho may be employed a thin slab casting method and the like.
[0046]
　Slabs, before subjecting the multi-axial compression working, heating to 950 ℃ ~ 1300 ℃. The holding time after the heating is not limited, and is preferably from the viewpoint of hole expansion and more than 30 minutes, preferably not more than 10 hours from the viewpoint of the suppression of excessive scale loss, more preferably 5 hours or less. When performing direct rolling or direct rolling without heating the slab may be subjected to multi-axial compression working as it is.
[0047]
　The multi-axis temperature of the slab to be subjected to compression processing is lower than 950 ° C., it is impossible to diffuse alloy elements is significantly delayed, to suppress the formation of the band-like structure. Therefore, the temperature of the slab and 950 ° C. or higher, preferably 1020 ° C. or higher. On the other hand, when the temperature is 1300 ° C. than the slab to be subjected to multi-axial compression working, unnecessarily manufacturing cost or increased yield scale loss is increased or decreased. Therefore, the temperature of the slab and 1300 ° C. or less, preferably 1250 ° C. or less.
[0048]
　The multi-axis compression processing, performs compression processing of the compression processing and the thickness direction of the widthwise direction 950 ° C. ~ 1300 ° C. slab. The multi-axis compression processing, or the portion alloying elements are enriched in Mn and the like in the slab is subdivided, lattice defects or introduced. Therefore, alloying elements are evenly spread during processing multi-axis compression, the formation of the band-like structure in a later step is suppressed, extremely homogeneous structure is obtained. In particular, the compression processing in the width direction is effective. That is, by processing the multi-axial compression, is concentrated portion is finely divided alloy elements present linked to the width direction, so that the alloy elements are uniformly dispersed. As a result, the homogenization of the tissue that can not be achieved by diffusion of alloying elements by simply prolonged heating can be realized in a short time.
[0049]
　In the compression processing once per deformation ratio in the width direction is less than 3%, the amount of lattice defects introduced by plastic deformation is insufficient, the diffusion of alloy elements not promoted, it inhibits the formation of the band-like tissue it can not be. Therefore, deformation ratio per compression forming one widthwise direction is 3% or more, preferably 10% or more. Meanwhile, the 50 percent compression processing once per deformation ratio in the width direction, or cause slab cracking, or the shape of the slab is not uniform and decreases the dimensional accuracy of the hot-rolled steel sheet obtained by hot rolling to. Therefore, the deformation ratio per compression forming one widthwise 50% or less, preferably 40% or less.
[0050]
　The deformation ratio per compression processing once in the thickness direction is less than 3%, the amount of lattice defects introduced by plastic deformation is insufficient, the diffusion of alloy elements is not promoted, inhibited the formation of the band-like tissue Can not do it. Further, the shape defect at the time of hot rolling biting into slabs reduction roll may become poor. Therefore, deformation ratio per compression processing once in the thickness direction is 3% or more, preferably 10% or more. On the other hand, in the thickness direction of the compression process once per deformation of 50 percent, or cause slab cracking, the shape of the slab is not uniform dimensional precision of hot-rolled steel sheet obtained by hot rolling decreases or. Therefore, deformation ratio per compression processing once in the thickness direction is 50% or less, preferably 40% or less.
[0051]
　If the difference between the rolling amount and the thickness direction of the rolling of the width direction is excessively large, alloy elements such as Mn in the direction perpendicular to the direction rolling amount is small is not sufficiently diffused, the formation of the band-like structure sufficient there can not be suppressed. Particularly easy band-like tissue is formed when the difference between the amount of rolling is 20 percent. Therefore, the difference in the amount of rolling between the width direction and the thickness direction is 20% or less.
[0052]
　By performing at least once the multi-axis compression processing, it is possible to suppress the formation of the band-like structure. The effect of suppressing the formation of band-like structure becomes remarkable by repeating a multi-axial compression working. Therefore, the number of multi-axial compression working is not less than one, preferably more than once. When performing multi-axis compression processing of two or more times, it may be reheated slab between the multi-axial compression working. On the other hand, in the multi-number of axial compression working is 5 times greater, unnecessarily manufacturing cost or increased yield scale loss is increased or decreased. Further, there is a case where the thickness of the slab hot rolling becomes difficult becomes nonuniform. Therefore, the number of multi-axial compression working is preferably not more than 5 times, more preferably at most 4 times.
[0053]
　(Hot rolling)
　The hot-rolling, carried out rough rolling of the slab after working polyaxial compression, performs subsequent finish rolling. Temperature of the slab to be subjected to finish rolling is set to 1050 ℃ ~ 1150 ℃, in the finish rolling, performing a first rolling, and then to perform a second rolling, winding up at 650 ℃ or less. In the first rolling, reduction ratio in the temperature range of 1050 ° C. ~ 1150 ° C. (first reduction ratio) is 70% or more, in the second rolling, reduction ratio in the temperature range of 850 ° C. ~ 950 ° C. ( the second reduction ratio) is 50% or less.
[0054]
　Is less than the temperature 1050 ° C. slab subjected to the first rolling, high deformation resistance during finish rolling, operation becomes difficult. Therefore, the temperature of the slab subjected to the first rolling and 1050 ° C. or higher, preferably 1070 ° C. or higher. On the other hand, the temperature of the slab to be subjected to finish rolling at the 1150 ° C. greater than the yield scale loss is increased is reduced. Therefore, the temperature of the slab subjected to the first rolling and 1150 ° C. or less, preferably 1130 ° C. or less.
[0055]
　In the first rolling, recrystallization occurs at a temperature range of 1050 ℃ ~ 1150 ℃ (austenite single phase region). The reduction ratio in the temperature range in the (first reduction ratio) is less than 70%, it can not be crystal grains to stably obtain the single phase of austenite of fine and spherical, easily followed banded structure is formed . Therefore, the first reduction ratio is 70% or more, preferably 75% or more. The first rolling may be carried out in a single stand may be carried out in a plurality of stands.
[0056]
　Reduction ratio in the temperature range of 850 ° C. ~ 950 ° C. of the second rolling (Second rolling reduction) of 50 percent, due to non-recrystallized austenite during winding, flat band-like tissue It is formed, not the desired standard deviation obtained. Therefore, the second reduction ratio is 50% or less. The second rolling may be carried out in a single stand may be carried out in a plurality of stands.
[0057]
　In the second of less than completion temperature of rolling is 850 ° C., recrystallization does not sufficiently occur, easy band-like tissue is formed. Therefore, completion temperature is set to 850 ° C. or higher, preferably 870 ° C. or higher. On the other hand, in the completion temperature of 1000 ° C. greater than the crystal grains are likely to grow, it is difficult to obtain a fine structure. Therefore, completion temperature is set to 1000 ° C. or less, preferably 950 ° C. or less.
[0058]
　The coiling temperature 650 ° C. greater than the internal oxidation, surface properties are deteriorated. Therefore, the coiling temperature is set to 650 ° C. or less, preferably a 450 ° C. or less, and more preferably 50 ° C. or less. The cooling rate to coiling temperature from the temperature of the finish rolling is less than 5 ° C. / s, difficult to obtain a homogeneous structure, it is difficult to obtain a homogeneous steel structure in the annealing after. Therefore, cooling rate to winding the finish rolling was set to 5 ° C. / s or higher, preferably 30 ° C. / s or higher. 5 ° C. / s or more cooling rate, may be implemented by water cooling.
[0059]
　(Cold rolling)
　cold rolling is performed, for example after pickling hot-rolled steel sheet. Homogenizing tissue cold-rolled steel sheet, from the viewpoint of miniaturization, the reduction ratio of cold rolling is preferably 40% or more, more preferably 50% or more.
[0060]
　(Annealing)
　The annealing, for example, the continuous annealing. Annealing temperature (Ac 1 is less than +10) Therefore, the annealing temperature (Ac 1 and +10) ° C. or higher, preferably (Ac 1 to +20) ° C. or higher. On the other hand, the annealing temperature is (Ac 3 in the +100) ° C. greater than or productivity decreases, austenite becomes coarse, the area ratio is 5% or more of ferrite or not obtained. Therefore, the annealing temperature (Ac 3 and +100) ° C. or less, preferably (Ac 3 and +50) ° C. or less. Here, Ac 1 and Ac 3 is a temperature defined from the components of the steel, the content of the element to "% element" (mass%), for example, Mn content to "% Mn" (wt%) When, equation 1 below, respectively, of the formula 2.
Ac 1 (° C.) = 723-10.7 (% Mn) -16.9 (% Ni) Tasu29.1 (% Si) Tasu16.9 (% Cr) (Equation 1)
Ac 3 (° C.) = 910-203√% C-15.2 ( % Ni) +44.7 (% Si) +104 (% V) +31.5 (% Mo) ( equation 2)
[0061]
　Annealing time is not limited, but preferably be at least 60 seconds. The non-recrystallized structure is significantly reduced, in order to ensure stable homogeneous steel structure. After annealing, the steel sheet, (Ac 1 cooled at +10) ° C. until the first cooling stop temperature of less temperature range, 1 ° C. / sec or higher 15 ° C. / sec of average cooling rate (first average cooling rate) it is preferable to. It is to secure a ferrite sufficient area ratio. First average cooling rate is more preferably from 2 ° C. / sec 10 ° C. / sec or less. (Ac 1 from +10) ° C. below the temperature range, 200 to the second cooling stop temperature of ° C. or higher 350 ° C. or less of the temperature range, cooled at an average cooling rate of more than 35 ° C. / sec (second average cooling rate) , it is preferable to maintain 200 ° C. or higher 350 ° C. 200 seconds or more at a temperature region of the holding temperature. It is to increase the resistance spread holes by ensuring the ductility of the hard tissue.
[0062]
　In this way, it is possible to produce a steel sheet according to the embodiment of the present invention.
[0063]
　The above embodiments are all merely illustrate concrete examples of implementing the present invention, in which technical scope of the present invention should not be limitedly interpreted. That is, the present invention is its technical idea or without departing from the essential characteristics thereof, can be implemented in various forms.
Example
[0064]
　Next, a description will be given of an embodiment of the present invention. Conditions in examples are an example of conditions adopted for confirming the workability and effects of the present invention, the present invention is not limited to this single example of conditions. The present invention does not depart from the gist of the present invention, as long as they achieve the object of the present invention, it is capable of adopting various conditions.
[0065]
　(First embodiment)
　the slab having the chemical composition shown in Table 1 was prepared, after 1 hour of heating slab to 1250 ° C., was multi-axis compression processing under the conditions shown in Table 2. Then reheating the slab to 1250 ° C., to obtain a rough rolling to rough rolled sheet. Thereafter, the roughly rolled sheet was 1 hour reheated at 1250 ° C., to obtain a hot rolled steel sheet by performing a finish rolling under the conditions shown in Table 2. In this experiment, for the convenience of the experiment facility, is performed a reheating for had to lower the temperature of the slab, if it sent directly without lowering the temperature of the slab may not be performed reheating. The finish rolling, performing a first rolling in four stages, performing a second rolling in two stages, and after winding, and held for 1 hour to coiling temperature. Thereafter, pickling hot-rolled steel sheet, thickness by performing cold rolling to obtain a cold-rolled steel sheet of 1.0mm at a reduction ratio shown in Table 2. This was followed by a continuous annealing at a temperature shown in Table 3. The continuous annealing, the heating rate and 2 ° C. / sec, and the annealing time was 200 seconds. After 200 seconds hold, cooled in the first average cooling rate of 2.3 ° C. / sec to a first cooling stop temperature of temperature range of 720 ℃ ~ 600 ℃, 300 ℃ ( second cooling stop temperature) until further cooled in the second average cooling rate of 40 ° C. / sec, 300 ° C. and held for 60 seconds (holding temperature) and cooled to room temperature, about 30 ° C. at an average cooling rate of 0.75 ° C. / sec. The remainder of the chemical compositions shown in Table 1 is Fe and impurities. Underlined in Table 1 indicates that the value is out of range of the present invention. Table 2 and underlined in Table 3 indicates that the numerical value is outside the range suitable for the manufacture of steel sheet of the present invention.
[0066]
[Table 1]

[0067]
[Table 2]

[0068]
[table 3]

[0069]
　Then, it was observed steel microstructure of the resulting cold rolled steel sheet. In the steel structure observation, by the above method, the area ratio of ferrite, the area ratio of the hard tissue (bainite, the total area fraction of martensite and residual austenite), the total area ratio of pearlite and carbides, and the line segment ratio of the hard tissue the standard deviation of the measured. The results are shown in Table 4. Underlined in Table 4 indicates that the numerical value is outside the scope of the present invention.
[0070]
　Furthermore, the tensile strength of the resulting cold rolled steel sheet TS, was measured elongation at break EL and hole expanding ratio HER. In the measurement of the tensile strength TS and elongation at break EL, a JIS5 No. Tensile test specimen to a direction perpendicular to the longitudinal direction to the rolling direction were taken and subjected to tensile test in accordance with JIS Z 2241. In the measurement of the hole expanding ratio HER, cold-rolled steel sheet, a test piece of 90mm square was taken and subjected to hole expansion test to comply with the provisions of JIS Z 2256 (or JIS T 1001). At this time, the hole expansion test speed was 1 mm / sec. These results are shown in Table 4. Underlined in Table 4 indicates that the numerical value is outside the desired range. The desired range here, the tensile strength TS is above 780 MPa, elongation at break EL of 10% or more, the hole expanding ratio HER is 30% or more.
[0071]
　In addition, it visually inspected at the time of molding by visual observation. Visual inspection was carried out by the following methods. First, the steel sheet was cut to a width of 40 mm × length 100 mm, it was polished test piece until the surface metallic luster is observed. The test piece, the ratio of the radius R bending plate thickness t (R / t) is in two levels of 2.0, 2.5, the bending ridge line was 90 ° V bending test under the condition that the rolling direction. After the test was to observe the surface properties of the bent portion was visually. The ratio (R / t) is the case where the uneven pattern or crack on the surface was observed in the test of 2.5 was determined to be defective. The ratio (R / t) but is not uneven pattern and cracks observed in the test of 2.5, when the ratio (R / t) is the rugged pattern or cracks on the surface in the test of the test 2.0 was observed It was judged as good. In any ratio (R / t) test and a ratio of 2.5 (R / t) of the test of 2.0 even if not observed uneven pattern and cracks on the surface it was determined to Yu. The results are also shown in Table 4.
[0072]
[Table 4]

[0073]
　As shown in Table 4, the samples are within the scope of the present invention No. 2 ~ No. 4, No. 16, No. 19, No. 21 ~ No. 30, No. 33, No. 36, and No. In 37, it was possible to obtain excellent tensile strength, elongation at break and hole expansion. Among these, sample No. In 23 or the like, since the area ratio of residual austenite (residual gamma) is 5.0% or more, Sample No. Excellent elongation at break than 16 were obtained.
[0074]
　On the other hand, sample No. In 1, C content is too low, the area ratio of ferrite is excessively high, since the area ratio of the hard tissue is too low, the tensile strength was low. Sample No. In 18, Si content is too low, since the area ratio of ferrite is too low, the tensile strength was low. Sample No. In 20, Mn content is too low, since the area ratio of ferrite is too low, the tensile strength was low.
[0075]
　Sample No. 5 ~ No. 8, No. 10 ~ No. 14, No. 31, and No. In 35, since the standard deviation of the line segment ratio of the hard tissue is too large, the hole expansion ratio was low. Sample No. In 9, the area ratio of ferrite is excessively high, since the area ratio of the hard tissue is too low, the tensile strength and the hole expansion ratio was low. Sample No. In 15, the width direction of the deformation ratio in a multi-axis compression processing is too low, can not be performed subsequent hot rolling. Sample No. At 17, since the area ratio of ferrite it was too low, elongation at break was low. Sample No. In 32, since the area ratio of the hard tissue is too low, the tensile strength was low. Sample No. At 33, for the area ratio of the hard tissue it was too high, elongation at break was low.
[0076]
　(Second embodiment)
　the slab having the chemical compositions shown in Table 5 was prepared, after 1 hour of heating slab to 1250 ° C., was multi-axis compression processing under the conditions shown in Table 6. Then reheating the slab to 1250 ° C., to obtain a rough rolling to rough rolled sheet. Thereafter, the roughly rolled sheet was 1 hour reheated at 1250 ° C., to obtain a hot rolled steel sheet by performing a finish rolling under the conditions shown in Table 6. In this experiment, for the convenience of the experiment facility, is performed a reheating for had to lower the temperature of the slab, if it sent directly without lowering the temperature of the slab may not be performed reheating. The finish rolling, performing a first rolling in four stages, performing a second rolling in two stages, and after winding, and held for 1 hour to coiling temperature. Thereafter, pickling hot-rolled steel sheet, thickness by performing cold rolling to obtain a cold-rolled steel sheet of 1.0mm at a reduction ratio shown in Table 6. This was followed by a continuous annealing at a temperature shown in Table 7. The continuous annealing, the heating rate and the rate shown in Table 7, and the annealing time was 100 seconds. After 100 seconds hold, until the first cooling stop temperature shown in Table 7 was cooled in the first average cooling rate shown in Table 7, to a second cooling stop temperature shown in Table 7 40 ° C. / sec second further cooled at an average cooling rate of, and held for 300 seconds in the holding temperature shown in Table 7, were cooled at an average cooling rate of 10 ° C. / sec to room temperature, about 30 ° C.. The remainder of the chemical compositions shown in Table 5 is Fe and impurities. Underlined in Table 5 indicates that the number is out of range of the present invention. It underlined in Table 6 and in Table 7 indicates that the numerical value is outside the range suitable for the manufacture of steel sheet of the present invention.
[0077]
[table 5]

[0078]
[Table 6]

[0079]
[Table 7]

[0080]
　Then, it was observed steel microstructure of the resulting cold rolled steel sheet. In the steel structure observation, by the above method, the area ratio of ferrite, the area ratio of the hard tissue (bainite, martensite, tempered total area fraction of martensite and residual austenite), the total area ratio of pearlite and carbides, and hard to measure the standard deviation of the line segment of the tissue. The results are shown in Table 8. Underlined in Table 8 indicates that the value is out of range of the present invention.
[0081]
　Furthermore, the tensile strength of the resulting cold rolled steel sheet TS, was measured elongation at break EL and hole expanding ratio HER. In the measurement of the tensile strength TS and elongation at break EL, a JIS5 No. Tensile test specimen to a direction perpendicular to the longitudinal direction to the rolling direction were taken and subjected to tensile test in accordance with JIS Z 2241. In the measurement of the hole expanding ratio HER, cold-rolled steel sheet, a test piece of 90mm square was taken and subjected to hole expansion test to comply with the provisions of JIS Z 2256 (or JIS T 1001). At this time, the hole expansion test speed was 1 mm / sec. These results are shown in Table 8. Underlined in Table 8 indicates that the numerical value is outside the desired range. The desired range here, the tensile strength TS is above 780 MPa, elongation at break EL of 10% or more, the hole expanding ratio HER is 30% or more.
[0082]
　In addition, it visually inspected at the time of molding by visual observation. Visual inspection was carried out by the following methods. First, the steel sheet was cut to a width of 40 mm × length 100 mm, it was polished test piece until the surface metallic luster is observed. The test piece, the ratio of the radius R bending plate thickness t (R / t) is in two levels of 2.0, 2.5, the bending ridge line was 90 ° V bending test under the condition that the rolling direction. After the test was to observe the surface properties of the bent portion was visually. The ratio (R / t) is the case where the uneven pattern or crack on the surface was observed in the test of 2.5 was determined to be defective. The ratio (R / t) but is not uneven pattern and cracks observed in the test of 2.5, when the ratio (R / t) is the rugged pattern or cracks on the surface in the test of the test 2.0 was observed It was judged as good. In any ratio (R / t) test and a ratio of 2.5 (R / t) of the test of 2.0 even if not observed uneven pattern and cracks on the surface it was determined to Yu. The results are also shown in Table 8.
[0083]
[Table 8]

[0084]
　As shown in Table 8, the samples are within the scope of the present invention No. 42, No. 43, No. 49, No. 54, No. 56, No. 58 ~ No. 62, and No. 64 ~ No. In 72, it was possible to obtain excellent tensile strength, elongation at break and hole expansion. Among these, sample No. In 58 or the like, since the area ratio of residual austenite (residual gamma) is 5.0% or more, Sample No. Excellent elongation at break than 69 were obtained. Moreover, when compared with the invention of the first experimental example, the value of TS × HER was large. This while ensuring excellent hole expansion shows that higher tensile strength can be obtained. One value is larger reason for the invention examples than TS × HER the first experimental example in the present invention example of the second experimental example is that the Si content is high.
[0085]
　On the other hand, sample No. In 41, C content is too low, the area ratio of ferrite is excessively high, since the area ratio of the hard tissue is too low, the tensile strength was low. Sample No. In 51, Si content is too low, because the standard deviation of the line segment ratio of the hard tissue is too large, the hole expansion ratio was low. Sample No. In 52, Si content is too high, because the standard deviation of the line segment ratio of the hard tissue is too large, the hole expansion ratio was low. Sample No. In 53, since the Mn content is too low, the tensile strength was low.
[0086]
　Sample No. 44, No. 45, No. 48, No. 50, No. 57, and No. In 63, since the standard deviation of the line segment ratio of the hard tissue is too large, the hole expansion ratio was low. Sample No. In 46, the area ratio of ferrite is excessively high, the area ratio of the hard tissue is too low, because the standard deviation of the line segment ratio of the hard tissue is too large, the tensile strength and the hole expansion ratio was low. Sample No. In 47, since the thickness direction of the deformation ratio in a multi-axis compression processing is too low, it can not be performed subsequent hot rolling. Sample No. In 55, the area ratio of the ferrite is too low, because the area ratio of the hard tissue is too high, breaking elongation was low.
Industrial Applicability
[0087]
　The present invention is, for example, can be used in industry in which a suitable steel automobile parts.

claims
[Requested item 1]
　By
　mass%,
　C: 0.05% ~
　0.40%, Si: 0.05% ~ 6.00%, Mn: 1.50% ~ 10.00%,
　acid-soluble Al: 0.01% ~ 1
　% .00, P: 0.10% or
　less, S: 0.01% or
　less, N: 0.01% or
　less, Ti:
　0.0% ~ 0.2%, Nb: 0.0% ~ 0.2
　%,
　V:
　0.0% ~ 0.2%, Cr: 0.0% ~ 1.0%,
　Mo: 0.0% ~ 1.0%, Cu: 0.0% ~ 1.0%, 　Ni:
　0.0% ~ 1.0%, Ca: 0.00% ~ 　0.01%, Mg: 0.00% ~ 0.01%, REM: 0.00% ~ 　0.01%, Zr: 0.00% to 0.01% and 　the balance: Fe and impurities, have in a chemical composition represented, 　by area ratio, 　ferrite: 5% to 80%,

　Bainite, martensite or retained austenite or hard tissue consisting of any combination: 20% to 95%, and
　the hard tissue segment rate in line in the thickness direction in a plane perpendicular standard deviation: the steel plate the depth of the thickness from the surface when the t is 0.050 or less, in the depth range of 3t / 8 to t / 2
steel sheet characterized by having a in steel structure represented.
[Requested item 2]
　In the steel structure, an area ratio,
　the retained austenite: 5.0% or more,
　the steel sheet according to claim 1, characterized in that hold.
[Requested item 3]
　In the chemical composition, by
　mass%, Ti:
　0.003% ~ 0.2%, Nb: 0.003% ~ 0.2%, or
　V: 0.003% ~ 0.2%,
　or any steel sheet according to claim 1 or 2, characterized in that the combination of holds.
[Requested item 4]
　In the chemical composition, in
　mass%,
　Cr: 0.005% ~ 1.0%,
　Mo: 0.005% ~ 1.0%, Cu: 0.005% ~ 1.0%, or
　Ni: 0. 005% to 1.0%
　or steel sheet according to any one of claims 1 to 3, characterized in that a combination of any of these is true.
[Requested item 5]
　In the chemical composition, by
　mass%,
　Ca: 0.0003% ~
　0.01%, Mg: 0.0003% ~ 0.01%, REM: 0.0003% ~ 0.01%, or
　Zr: 0. 0003% to 0.01%
　or steel sheet according to any one of claims 1 to 4, characterized in that a combination of any of these is true.