Technical field
[0001]The present invention, high strength steel sheet, more particularly the tensile strength is more than 800 MPa, preferably not relate high-strength steel sheet of more than 1100 MPa.
BACKGROUND
[0002]Recently, from the viewpoint of fuel efficiency that leads to environmental conservation, higher strength of automotive steel sheets is strongly required. Generally, the ultra-high-strength cold-rolled steel sheet, molding technique applied in mild steel plate such as drawing and stretch forming is not applicable, bending is mainly as a molding technique. Therefore, when using an ultra-high strength cold rolled steel sheet as a structural component of a motor vehicle, good bending properties are important selection criteria.
[0003]
　Incidentally, in the bending of steel plate, the bending takes large tensile stress in the circumferential direction of the outer peripheral surface portion, on the other hand, since the circumferential surface portion bent such a large compressive stress, bending of the ultra-high strength cold rolled steel sheet the state of the surface layer portion has a great influence on. Therefore, to have a soft layer on the surface layer, the tensile stress generated on the surface of the steel sheet during the bending, compressive stress is alleviated, it is known to improve the bending property. For such high-strength steel sheet surface layer having a soft layer, steel sheet and methods for their preparation as follows in Patent Documents 1 to 3 is disclosed.
[0004]
　First, in Patent Document 1, in order from the interface between the steel sheet and the plated layer on the steel sheet side, and the inner oxide layer containing an oxide of Si and / or Mn, and a soft layer containing the inner oxide layer, and martensite bainite and a hard layer composed of a structure mainly of an average depth T of the soft layer is 20μm or more, and an average depth t of the inner oxide layer is 4μm or more, to satisfy the below the T It describes a high strength plated steel sheet and a manufacturing method thereof, wherein.
[0005]
　Next, Patent Document 2, the Vickers hardness of 100μm position from the surface of the steel sheet, high strength molten zinc, characterized in that the value obtained by subtracting the Vickers hardness depth 20μm position from the surface of the steel sheet (△ Hv) of 30 or more plated steel sheet and a manufacturing method thereof are described.
[0006]
　Next Patent Document 3, a Vickers hardness of 5μm position from the surface layer to the thickness direction is 80% or less of the hardness of the 1/2 position in the plate thickness direction, the hardness of 15μm position from the surface layer to the thickness direction thickness direction describes a high strength galvanized steel sheet and a manufacturing method thereof, characterized in that the at least 90% of the Vickers hardness of 1/2 position.
[0007]
　However, in any of Patent Documents 1 to 3 also sufficient study has not been made for variations in the hardness of the softer layer. For example, Patent Document 1, the soft layer has been described as having an inner oxide layer, in this case, the variation that may occur is estimated to hardness between the oxide and other tissue in the soft layer that. When there is a variation in the hardness of the soft layer, it may not be possible to achieve sufficient bending resistance in the steel sheet having such a soft layer. Further, in any of Patent Documents 1 to 3 also does not refer to control the slope of the stiffness in the transition zone between the surface of the soft layer and the inner hard layer.
CITATION
Patent Document
[0008]
Patent Document 1: JP 2015-34334 JP
Patent Document 2: JP 2015-117403 Patent Publication
Patent Document 3: WO 2016/013145
Summary of the Invention
Problems that the Invention is to Solve
[0009]
　The present invention advantageously solves the prior art problems facing described above, suitable as a material for automobile parts, and to provide a high-strength steel sheet having a bending property.
Means for Solving the Problems
[0010]
　The present inventors have found that in order to solve the problems associated with bending of the ultra-high strength steel plate, of intensive investigation. First, the present inventors as reference the traditional knowledge, to produce a steel sheet having a soft layer on the surface layer was investigated bendability. Steel sheet having a soft layer on the surface layer are both improved bendability was observed. At this time, the thicker the soft layer thickness to lower the average hardness of the softer layer was found to be a direction roughly bent improved. However, the inventors have found that continued a more detailed investigation, when performing surface layer of soft in a wide variety of ways, is merely to adjust the thickness of the average hardness and softer layer of the surface of the soft layer, bending of the steel sheet was noticed that not sufficiently improved.
[0011]
　Therefore, the present inventors have carried out a more detailed investigation. As a result, a steel sheet having a certain characteristic welded to one or both sides of the base material, multi-layer steel sheet obtained by hot rolling or annealing was found to be improved most bendability in certain conditions. The biggest reason of bending by the above method improves revealed that by suppressing micro hardness variations in the soft layer. This effect is very pronounced, compared with the case hardness variation of the soft layer is large, even with a high average hardness of the soft layer, also improved even if the thickness is small enough bending of the soft layer was gotten. Thus, to minimize tensile strength degradation due to the soft layer, unconventional tensile strength, specifically, 800MPa or more, preferably it became possible to achieve both of the bending and tensile strength of more than 1100 MPa. The mechanism of this effect is not entirely clear, but is considered less. If the hardness in the soft layer has a variation, a plurality of tissue in the soft layer (ferrite, pearlite, bainite, martensite, residual austenite) often have and / or oxides. Second phase these mechanical properties are different (or second tissue) is bent cause distortion or stress is concentrated during processing, it may become a starting point of cracks in a void is produced. Therefore, by suppressing the hardness variation of the soft layer is believed that it was possible to improve the bending property. Further, the present inventors have found that in addition to suppressing the micro hardness variation in the surface layer of the soft layer, region of transition from the surface of the soft layer to the inside of the hard layer (hereinafter, referred to as transition zone) found that bendability by simultaneously satisfies to reduce the inclination of the sheet thickness direction stiffness is further improved. When the hardness of the slope of the transition bands of soft layer and the hard layer is steeper, greatly deviate the plastic deformation of the soft layer and the hard layer, it is more likely to rupture occurs in the transition zone. Therefore, in addition to suppressing the micro hardness variations in the soft layer, the soft layer and the transition zone of the hard layer, of bending by simultaneously satisfies to reduce the inclination of the sheet thickness direction stiffness but it is believed that further improved.
[0012]
　Incidentally, other than the surface layer softening unit (hereinafter, referred to as a hard layer) variations in the hardness of the did not affect bendability. Therefore, conventionally, has been disadvantageous in bending resistance, DP steel and TRIP (transformation induced plasticity: Transformation Induced Plasticity) excellent in ductility can be used like a hard layer of steel, in addition to the tensile strength and bending resistance It points further capable both ductility, which is one of the points having excellent present invention.
[0013]
　Also, automobile members, to be processed into complex shapes, which may include a notch. If having a notch, stress is concentrated on the cutout portion at the time of deformation, it is known that to fracture at low stress as compared to the case where there is no notch. Further, it is known to express the notch effect similar pitting formed by corrosion of the steel sheet surface. Such a notch effect, be particularly noticeable in high strain rate under, such as at the time of vehicle collision, it is a phenomenon you want to avoid on the safety design of the car. The present inventors have found that the tissue of the surface layer of the soft layer that is included in the high-strength steel sheet according to the present invention, notches even if there was further found that can inhibit-out effect notch. This is because the micro hardness variation in the surface layer of the soft layer has been reduced, also less of a starting point of fracture, also steel is deformed containing soft layer of such a surface layer as a stress is concentrated on the notch easy, is considered to be due to the slow down a notch.
[0014]
　Gist of the present invention obtained in this way is as follows.
　(1) and the plate thickness center, tensile strength and a surface layer softened portion disposed on one or both sides of the plate thickness center is a high-strength steel sheet of more than 800 MPa, the plate from 10μm than the respective surface layer softened portions has 30% or less of the thickness of the thickness, the average Vickers hardness of the surface layer softened portion is less than 0.60 times the average Vickers hardness of the plate thickness 1/2 position, nano hardness of the surface layer softened portion wherein the standard deviation of 0.8 or less, high-strength steel sheet.
　(2) the thickness center and further includes a hardness transition zone formed adjacent to them between the surface layer softened portion, the average change in hardness in the plate thickness direction of the hardness is a transition zone 5000 ( and characterized in that Delta] HV / mm) or less, high-strength steel sheet according to (1).
　(3) the thickness center portion, characterized in that it comprises a residual austenite an area fraction of 10% or more, high strength steel sheet according to (1) or (2).
　(4) the thickness center portion, in
　mass%, C:
　0.05 ~
　0.8%, Si: 0.01 ~ 2.50%, Mn: 0.010 ~
　8.0%, P: 0 .1% or
　less, S: 0.05% or less,
　Al: 0 ~ 3%, and
　N: to 0.01% or less,
　wherein the balance of iron and inevitable impurities, the (1) the steel plate according to any one of - (3).
　(5) the thickness center portion, further, by mass%,
　Cr:
　0.01 ~ 3%, Mo: 0.01 ~ 1%, and
　B: 0.0001% ~ 0.01%
, characterized in that it contains at least one selected from the group consisting of the above ( high strength steel sheet according to 4).
　(6) the thickness center portion, further, by
　mass%, Ti:
　0.01 ~ 0.2%, Nb: 0.01 ~ 0.2%, and
　V: 0.01 ~ 0.2%
from characterized in that it contains at least one selected from the group consisting, high strength steel sheet according to (4) or (5).
　(7) The plate thickness center is further, by mass%,
　Cu: 0.01 ~ 1%, and
　Ni: 0.01 ~ 1%
and characterized in that it contains at least one selected from the group consisting of to a high-strength steel sheet according to any one of the above (4) to (6).
　(8) C content of the surface layer softened portion is equal to or less than 0.9 times the C content of the plate thickness center, the (4) High according to any one of (1) to (7) strength steel sheet.
　(9) Mn amount of the surface layer softened portion, wherein the total amount of Cr and Mo weight Mn of the plate thickness center is 0.9 times or less of the total amount of Cr and Mo content, the (5) high-strength steel sheet according to any one of - (8).
　(10) the amount of B surface softened portion is equal to or less than 0.9 times the amount of B the thickness center, the (5) High according to any one of - (9) strength steel sheet.
　(11), wherein the sum of the Cu content and Ni content of the surface layer softened portion is less than 0.9 times the sum of the Cu content and Ni content of the plate thickness center, the (7) - (10 high strength steel sheet according to any one of).
　(12) on the surface of the surface layer softened portion, galvanized layer, characterized in that it comprises galvannealed layer, or an electro-galvanized layer further any one of the above (1) to (11) high-strength steel sheet according to.
The invention's effect
[0015]
　High-strength steel sheet of the present invention, suitable as a material for automobile parts, having excellent bending workability. Therefore, high-strength steel sheet of the present invention can be preferably used as a material for automobile parts. In addition, when the average change in hardness in the thickness direction between the center of plate thickness and the surface layer softened portion of the high strength steel sheet contains 5000 (ΔHv / mm) or less of hardness transition zone, bending workability it can be further improved. Further, when the thickness center portion comprises a residual austenite an area fraction of 10% or more, it is possible ductility improving in addition to the improvement of bending workability. Further, according to the high-strength steel sheet of the present invention, it is possible to suppress-out effect notch.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016]
[Figure 1] shows an example of the hardness distribution for high-strength steel sheet according to the preferred embodiment of the present invention.
Is a schematic view for explaining the diffusion of C atoms in the production of high-strength steel sheet of the present invention; FIG.
3 is a graph showing the dislocation density change after rolling pass about rough rolling used in the method for producing a high-strength steel sheet of the present invention.
DESCRIPTION OF THE INVENTION
[0017]
　Hereinafter, embodiments of the present invention will be described. The present invention is not limited to the following embodiments.
[0018]
　Steel sheet according to the present invention, 10 [mu] m average Vickers hardness of the surface layer softened portion having 30% or less of the thickness of the plate thickness from the greater sheet thickness 1/2 position average Vickers hardness of the whole surface layer softened section and more specifically there needs to be equal to or less than 0.60 times the average Vickers hardness. Not the thickness of the surface layer softened portion is obtained a sufficient improvement in bendability is 10μm or less, a remarkable deterioration of the tensile strength greater than 30%. The thickness of the surface layer softened portion is more preferably 20% thickness or less, more preferably 10% or less.
[0019]
　In the present invention, the "average Vickers hardness of the surface layer softened portion" is determined as follows. First, in the thickness of 1/2 position from the constant in the thickness direction toward the surface interval (e.g. thickness 5% each of. 1% and each 0.5% if necessary), there thickness direction position Vickers hardness was measured under a load 100g heavy pushing and then thickness vertically on a line parallel to the rolling direction as well indentation load 100g weight in total of 3 points or more from that position, e.g., 5 points or 10 points Vickers in the hardness was measured and the average value thereof and the average Vickers hardness at the plate thickness direction position. The distance between the measurement points aligned in the thickness direction and the rolling direction is preferably not more than four times the distance of the indentation when possible. And "more than four times the distance of the indentation" as used herein, is intended to mean a rectangular opening in the diagonal of length 4 times the length of the indentation caused by the diamond indenter in the measurement of Vickers hardness is there. The average Vickers hardness of a certain thickness direction position, when it becomes less than 0.6 times the average Vickers hardness at the plate thickness 1/2 position as measured in the same manner, and the surface softened portion of the surface side from that position Define. Thus the Vickers hardness of randomly 10 points at a defined surface softened portion was measured, the average Vickers hardness of the surface layer softened portion is determined by calculating the average value thereof. The average Vickers hardness of the surface layer softened portion is more bend resistant at 0.60 times or less the average Vickers hardness of the plate thickness 1/2 position is improved. More preferably 0.40 times or less, and more preferably not more than 0.30 times.
[0020]
　The standard deviation of the surface layer softened portion of the nano-hardness is required to be 0.8 or less. This, as described above, by suppressing the hardness variation in the surface layer softened portion, because the bending property is remarkably improved. This effect standard deviation greater than 0.8 is insufficient. From that perspective, standard deviation and more preferably 0.6 or less, further desirably 0.4 or less. The lower limit of the standard deviation is not specified, but it is technically difficult to 0.05 or less. To affect the bendability, especially a micro hardness variations in thickness vertical surface softened portion, have an inclination of gentle hardness in the plate thickness direction in the surface layer softened portion effects of the present invention not inhibition. Therefore, the standard deviation of nano-hardness, it is necessary to measure at a position perpendicular to the plate thickness direction at a certain thickness direction position. In the present invention, the "standard deviation of nano-hardness of the surface layer softened portion", the vertical and in the thickness direction at a 1/2 position in the thickness of the surface layer softened portion as defined above, on a line parallel to the rolling direction, using Hysitron Inc. tribo-900, under conditions of a diamond indenter with 80nm indentation depth of Berkovich shape, an interval of 3 [mu] m, measured nano hardness of total 100 positions, resulting nano hardness of it is intended to refer to the standard deviation obtained from the histogram.
[0021]
　In order to improve the bending properties of a high strength steel sheet further preferably has an average hardness change in the thickness direction of the stiffness transition zone is 5000 (ΔHv / mm) or less. "Hardness transition zone" in the present invention are defined as follows. First, in the thickness of 1/2 position from the constant in the thickness direction toward the surface interval (e.g. thickness 5% each of. 1% and each 0.5% if necessary), there thickness direction position Vickers hardness was measured under a load 100g heavy pushing and then thickness vertically on a line parallel to the rolling direction as well indentation load 100g weight in total of 3 points or more from that position, e.g., 5 points or 10 points Vickers in the hardness was measured and the average value thereof and the average Vickers hardness at the plate thickness direction position. The distance between the measurement points aligned in the thickness direction and the rolling direction is preferably not more than four times the distance of the indentation when possible. When in average Vickers hardness at the plate thickness direction position, and the following 0.9 times the average Vickers hardness at the plate thickness 1/2 position as measured in the same manner, the surface layer softened portion previously defined from the position region up is defined as the hardness transition zone.
[0022]
　The thickness direction of the average hardness change in hardness transition zone (ΔHv / mm) is defined by the following equation.
　Mean Hardness change (Delta] HV / mm) = (maximum average hardness of Vickers hardness transition zone) - (Minimum Mean hardness of Vickers hardness transition zone) / hardness the thickness of the transition zone
[0023]
　Here, the maximum average hardness of Vickers hardness transition zone, among the average Vickers hardness of each plate thickness direction position of the hardness transition zone, a largest value, Vickers hardness transition zone the minimum average hardness of hardness, of the average Vickers hardness of each plate thickness direction position of the hardness transition zone, the smallest value.
[0024]
　Mean Hardness change in the thickness direction of the stiffness transition zone may be lowered is greater bending than 5000 (ΔHv / mm). Preferably 4000 (ΔHv / mm) or less, more preferably 3000 (ΔHv / mm) or less, and most preferably not more than 2000 (ΔHv / mm). The thickness of the hardness of the transition zone is not specified. However, the hardness transition zone from the reduced percentage is large, the tensile strength occupied in plate thickness, hardness transition zone 20% of the thickness is preferably not more than one side. More preferably 10% or less.
[0025]
　It shows a preferred embodiment example of the hardness distribution for high-strength steel sheet according to the embodiment of the present invention in FIG. From the surface of the steel sheet having a thickness of 1mm showing the hardness distribution of the to a thickness of 1/2 position. The horizontal axis is a position in the thickness direction (mm), the surface 0 mm, the thickness 1/2 position is 0.5 mm. The vertical axis shows the 5-point average Vickers hardness at KakuitaAtsu direction position. Vickers hardness of the plate thickness 1/2 position is 430Hv, the range between the surface layer softened portion surface side than the point became 0.6 times or less, and the surface softened portion that becomes 0.9 times or less is the hardness of the transition zone.
[0026]
　In order to improve the ductility of high-strength steel sheet, the thickness center portion preferably contains 10% or more of residual austenite in area fraction. This is done to improve ductility by the transformation-induced plasticity of retained austenite, the area fraction of retained austenite of 10% or more, 15% or more of ductility is obtained. With the effect of this residual austenite, even if free of soft ferrite, since it can be secured more than 15% of the ductility, it is possible to advance the high strength of the center of plate thickness, high strength and high ductility compatibility is possible. Note that the ductility here, the Japanese Industrial Standards JIS5 No. 5 test piece was taken at right angles to the rolling direction of a steel plate, it refers to a total elongation obtained subjected to a tensile test according to JIS Z2241.
[0027]
　Next, we describe the chemical composition of the desired thickness center part in order to obtain the effect of the present invention. Incidentally, "%" for the content of element, in particular means "% by mass" unless otherwise specified. Incidentally, in the vicinity of the boundary between the surface layer softened portion at the center of plate thickness may be different from the position at which the chemical composition is well away from the boundary by diffusion of alloying elements between the surface layer softened portion. For example, when a high strength steel sheet of the present invention comprises a hardness transition zone above, if the chemical composition near the boundary between the well away from the boundary between the hardness transition zone are different in the thickness center portion is there. In such a case, determining the chemical composition determined in the vicinity of the sheet thickness 1/2 position below.
[0028]
"C: 0.05 ~ 0.8%"
　C is intended to increase the strength of the steel sheet, are added to improve the strength of high-strength steel sheet. However, the toughness becomes insufficient when the content of C exceeds 0.8%. The intensity and the content of C is less than 0.05% is insufficient. The content of C is preferably in the range of 0.6% or less, and more preferably in a range of 0.5% or less.
[0029]
"Si: 0.01 ~ 2.50%"
　Si is a ferrite stabilizing element, since increasing the Ac3 transformation point, it is possible to form a large amount of ferrite at a wide annealing temperature, microstructure control It is added from the viewpoint of tendency. To achieve such advantages, it is necessary to set the Si content to 0.01% or more. On the other hand, from the viewpoint of ductility secured, the content of Si is coarse iron-based carbide is less than 0.30% and produced in large amounts, without the residual austenite fraction of the internal microstructure can be 10% or more , there is a case in which growth is lowered. From this point of view, the lower limit of Si is preferably 0.30% or more, more preferably at least 0.50%. In addition, Si suppresses the coarsening of iron-based carbides in the sheet thickness center is also a necessary element to improve the strength and formability. Further, as the solid solution strengthening element, it is necessary to add to contribute to strengthening of the steel sheet. From these viewpoints, it is preferable that the lower limit of Si is 1% or more, more preferably 1.2% or more. However, the thickness center portion is brittle when the content of Si exceeds 2.50%, since the ductility is degraded, and 2.50% the upper limit. From the viewpoint of ductility secured, the content of Si is less 2.20%, more preferably at most 2.00%.
[0030]
"Mn: 0.010 ~ 8.0%"
　Mn is added to increase the strength of high-strength steel sheet. To achieve such advantages, it is necessary to set the Mn content to 0.010% or more. However, the hardness distribution of the steel sheet surface layer due to Mn segregation of the Mn content exceeds 8.0% increase. In this viewpoint, preferably 5.0% or less, more preferably 4.0% or less, still more preferably 3.0% or less.
[0031]
"P: 0.1% or less"
　P tends to segregate in the plate thickness center part of the steel sheet, thereby embrittling the weld. Since embrittlement of the weld zone exceeds 0.1% is significant and limit its proper range below 0.1%. The lower limit of the content of P is not specified, but be less than 0.001%, it is economically disadvantageous.
[0032]
"S: 0.05% or less"
　S is weldability and during casting and adversely affect the productivity of the hot rolling. Therefore, to the upper limit value and 0.05% or less. The lower limit of the content of S is not specified, but be less than 0.0001%, it is economically disadvantageous.
[0033]
"Al: 0 ~ 3%"
　Al acts as a deoxidizer, it is preferably added in deoxidation process. To achieve such advantages, it is necessary to set the amount of Al 0.01% or more. On the other hand, when the Al content exceeds 3%, it increases the risk of slab cracking during continuous casting.
[0034]
"N: 0.01% or less"
　N forms coarse nitrides, from degrading the bendability, it is necessary to suppress the addition amount. This is because when N exceeds 0.01%, since this tendency becomes remarkable, and the range of N content is set to 0.01% or less. In addition, N is the, better less since it causes blowhole occurrence during welding. The lower limit of the N content, the effect of the present invention is exhibited without being specifically defined by the content of N to less than 0.0005%, since it leads to a significant increase in manufacturing cost, which is it is a substantial lower limit.
[0035]
"Cr: 0.01 ~ 3%, Mo : 0.01 ~ 1%, and B: at least one selected from the group consisting of 0.0001 to 0.01%"
　Cr, Mo and B, enhancement of strength an element contributing to, can be used instead of a part of Mn. Cr, Mo and B, one or two or more, respectively, 0.01% or more, preferably not less than 0.01% and 0.0001% or more. On the other hand, when the content of each element is too large, pickling and weldability, since hot workability etc. may deteriorate, Cr, respectively the content of Mo and B 3% or less, 1% or less and is preferably 0.01% or less.
[0036]
"Ti: 0.01 ~ 0.2%, Nb : 0.01 ~ 0.2%, and V: at least one selected from the group consisting of 0.01 to 0.2%"
　Ti, Nb and V , it is a strengthening element. Precipitation strengthening, by dislocation strengthening through suppression of fine grain strengthening and recrystallization of a ferrite grain growth suppression, contribute to the strength increase of the steel sheet. When added for this purpose it is preferably added more than 0.01%. However, when each content exceeds 0.2%, a number becomes formability precipitation of carbonitrides degrades.
[0037]
"Cu: 0.01 to 1%, and Ni: at least one selected from the group consisting of 0.01 to 1%"
　Cu and Ni is an element contributing to the improvement of strength, instead of a part of Mn it can be used Te. Cu and Ni, one or two, respectively, preferably not less than 0.01%. On the other hand, when the content of each element is too large, pickling and weldability, since hot workability like may be degraded, the content of Cu and Ni is preferably 1.0% or less.
[0038]
　Furthermore, it does not interfere with the effects of the present invention also has the center of plate thickness by the addition of the following elements intentionally or unavoidably. That, O: 0.001 ~ 0.02%, W: 0.001 ~ 0.1%, Ta: 0.001 ~ 0.1%, Sn: 0.001 ~ 0.05%, Sb: 0. 001 ~ 0.05%, As: 0.001 ~ 0.05%, Mg: 0.0001 ~ 0.05%, Ca: 0.001 ~ 0.05%, Zr: 0.001 ~ 0.05% , and Y: 0.001 ~ 0.05%, La: 0.001 ~ 0.05%, and Ce: 0.001 ~ 0.05%, etc. of REM (rare earth metals: rare-Earth metal) is.
[0039]
　Steel sheet in the present invention may chemical composition is different surface softened portion and the plate thickness center. In such cases, the preferred chemical composition of the surface layer softening unit is as follows.
[0040]
"C: 0.9 times the amount of C thickness center portion less and less 0.72%"
　C is intended to increase the strength of the steel sheet, are added to improve the strength of high-strength steel sheet. C content of the surface layer softened portion is preferably not more than 0.9 times the C content of the thickness center portion. This is to the hardness of the surface layer softened portion lower than the hardness of the center of plate thickness. Larger than 0.9 times, there are cases where the average Vickers hardness of the surface layer softened portion does not become 0.60 times the average Vickers hardness of the plate thickness 1/2 position. More preferably C content of the surface layer softened portion than 0.7 times the C content of the thickness center portion, even more preferably 0.5 times or less, and most preferably not more than 0.3 times. If C content of the surface layer softened portion is 0.9 times the C content of the thickness center portion, since the content of the preferred C of the center of plate thickness is less than 0.8%, the surface layer softened portions preferably C of the content is as follows 0.72%. Preferably 0.5% or less, more preferably 0.3% or less, and most preferably 0.1% or less. The lower limit of the C content is not particularly defined. When using industrial ultra-low C steel, but the order of 0.001% is a substantial lower limit, from the viewpoint of solid solution C amount was completely eliminated solute C by using a Ti or Nb, it may be used Interstitial Free steel.
[0041]
"Si: 0.01 ~ 2.5%"
　Si is a ferrite stabilizing element, since increasing the Ac3 transformation point, it is possible to form a large amount of ferrite at a wide annealing temperature, microstructure control It is added from the viewpoint of tendency. To achieve such advantages, it is necessary to set the Si content to 0.01% or more. However, the addition of more than 2.5% since deteriorates the toughness, and 2.5% or less.
[0042]
"Mn: 0.01 ~ 8.0%"
　Mn is added to increase the strength of high-strength steel sheet. To achieve such advantages, it is necessary to set the amount of Mn in 0.01% or more. However, the hardness distribution of the steel sheet surface layer due to Mn segregation of the Mn content exceeds 8.0% increase. In this viewpoint, preferably 5% or less, more preferably 3% or less.
[0043]
　In addition, the amount of Mn in the surface layer softened portion, the sum of the Cr amount and the Mo amount is the amount of Mn center of plate thickness, preferably 0.9 times or less of the total amount of Cr and Mo amount. This will be described later, the surface layer softened portion is the most tissues recrystallized ferrite, non-recrystallized ferrite, a transformation ferrite, thereby reducing the hardness variation is possible to further reduce the fraction of non-recrystallized ferrite. Mn amount to improve the hardenability, Cr amount and amount of Mo sum Mn amount of the thickness center portion, the low-temperature transformation structure is likely to occur larger than 0.9 times the total amount of Cr and Mo content, hardness made of the cause of the variation. More preferably 0.7 times or less, even more preferably 0.5 times or less, and most preferably not more than 0.3 times. Each lower limit is not specified.
[0044]
"P: 0.1% or less"
　P causes embrittlement of the welded portion. Since embrittlement of the weld zone exceeds 0.1% is significant and limit its proper range below 0.1%. The lower limit of the content of P is not specified, but be less than 0.001%, it is economically disadvantageous.
[0045]
"S: 0.05% or less"
　S is weldability and during casting and adversely affect the productivity of the hot rolling. Therefore, to the upper limit value and 0.05% or less. The lower limit of the content of S is not specified, but be less than 0.0001%, it is economically disadvantageous.
[0046]
"Al: 0 ~ 3%"
　Al acts as a deoxidizer, it is preferably added in deoxidation process. To achieve such advantages, it is necessary to set the amount of Al 0.01% or more. On the other hand, when the Al content exceeds 3%, it increases the risk of slab cracking during continuous casting.
[0047]
"N: 0.01% or less"
　N forms coarse nitrides, from degrading the bendability, it is necessary to suppress the addition amount. This is because when N exceeds 0.01%, since this tendency becomes remarkable, and the range of N content is set to 0.01% or less. In addition, N is the, better less since it causes blowhole occurrence during welding. The lower limit of the N content, the effect of the present invention is exhibited without being specifically defined by the content of N to less than 0.0005%, since it leads to a significant increase in manufacturing cost, which is it is a substantial lower limit.
[0048]
"Cr: 0.01 ~ 3%, Mo : 0.01 ~ 1%, and B: at least one selected from the group consisting of 0.0001 to 0.01%"
　Cr, Mo and B, enhancement of strength an element contributing to, can be used instead of a part of Mn. Cr, Mo and B, one or two or more, respectively, 0.01% or more, preferably not less than 0.01% and 0.0001% or more. On the other hand, when the content of each element is too large, pickling and weldability, since hot workability etc. may deteriorate, Cr, respectively the content of Mo and B 3% or less, 1% or less and is preferably 0.01% or less. Moreover, Cr and Mo is preferably in the range to the sum of the Mn, as described above.
[0049]
　Also, B of the surface layer softened portion is preferably not more than 0.9 times the amount of B thickness center portion. B amount to improve the hardenability low temperature transformed structure is likely to occur larger than 0.9 times the amount of B thickness center portion, causing variations in hardness. More preferably 0.7 times or less, even more preferably 0.5 times or less, and most preferably not more than 0.3 times. Each lower limit is not specified.
[0050]
"Ti: 0.01 ~ 0.2%, Nb : 0.01 ~ 0.2%, and V: at least one selected from the group consisting of 0.01 to 0.2%"
　Ti, Nb and V , it is a strengthening element. Precipitation strengthening, by dislocation strengthening through suppression of fine grain strengthening and recrystallization of a ferrite grain growth suppression, contribute to the strength increase of the steel sheet. When added for this purpose it is preferably added more than 0.01%. However, when each content exceeds 0.2%, a number becomes formability precipitation of carbonitrides degrades.
[0051]
"Cu: 0.01 to 1%, and Ni: at least one selected from the group consisting of 0.01 to 1%"
　Cu and Ni is an element contributing to the improvement of strength, instead of a part of Mn it can be used Te. Cu and Ni, one or two, respectively, preferably not less than 0.01%. On the other hand, when the content of each element is too large, pickling and weldability, since hot workability like may be degraded, the content of Cu and Ni is preferably 1.0% or less.
[0052]
　Further, it is preferable that the sum of the Cu content and Ni content of the surface layer softened portion is less 0.9 times the sum of the Cu content and Ni content of the thickness center portion. The sum of the Cu content and Ni content to improve the hardenability is greater than 0.9 times the sum of the Cu content and Ni content of the plate thickness center low-temperature transformation structure is likely to occur, causing variations in hardness . More preferably 0.7 times or less, even more preferably 0.5 times or less, and most preferably not more than 0.3 times. Each lower limit is not specified.
[0053]
　Furthermore, does not interfere with the effects of the present invention also has a surface layer softened portion by adding the following elements intentionally or unavoidably. That, O: 0.001 ~ 0.02%, W: 0.001 ~ 0.1%, Ta: 0.001 ~ 0.1%, Sn: 0.001 ~ 0.05%, Sb: 0. 001 ~ 0.05%, As: 0.001 ~ 0.05%, Mg: 0.0001 ~ 0.05%, Ca: 0.001 ~ 0.05%, Zr: 0.001 ~ 0.05% , and Y: 0.001 ~ 0.05%, La: 0.001 ~ 0.05%, and Ce: 0.001 ~ 0.05%, etc. of REM (rare earth metals: rare-Earth metal) is.
[0054]
　Effect of the present invention, i.e. excellent bending workability and / or ductility, hot-dip galvanizing on the surface of the surface layer softened portion, galvannealed, it is possible to achieve the same when subjected to electro-galvanized .
[0055]
　Next, the form of preparation for obtaining high-strength steel sheet of the present invention. The following description is for modifications thereof are merely illustrative of the process for obtaining a high-strength steel sheet of the present invention, a high strength steel sheet of the present invention into two multilayer steel sheet steel was laminated as described below it is not intended to be limiting. For example, by softening the surface layer of the single layer steel sheet with decarburization, it is also possible to produce a high strength steel sheet consisting of a surface layer softened portion and the thickness center section.
[0056]
　On one or both sides of defatted base material steel plate surface which satisfies the components of the center of plate thickness, laminated surface layer steel plate.
[0057]
　The above laminate hot rolling-cold rolling to the (multi-layer steel), continuous annealing, by performing such continuous hot dipping, high-strength steel sheet according to the present invention, more specifically, hot-rolled steel sheets, cold-rolled steel sheet, plated steel sheet it is possible to obtain.
[0058]
　For example, a method of manufacturing a hot rolled steel sheet of high-strength steel sheet encompassed by the present invention, on one side or both sides of the base material steel plate which constitutes the center of plate thickness having a chemical composition as described above, likewise the upper in the step of forming the multilayered steel sheet surface layer steel plate laminated constituting the surface layer softened portion having a chemical composition as described,
　the multi-layer steel sheet heating temperature 1100 ° C. or higher 1350 ° C. or less, preferably 1150 ° C. ultra 1350 ° C. in heating, then a hot rolling step of hot rolling comprises finish rolling of the hot rolling step in the roughing and finishing temperature 800 ~ 980 ° C., the rough rolling is rough rolling temperature: 1100 ° C. or higher , thickness reduction rate per pass: 5% or more and less than 50%, and inter-pass time: hot rolling step to be performed more than once under the conditions of more than 3 seconds, and
　a rolled multilayered steel hot cooling 700 in the process Holding ~ 500 ° C. of the temperature at 3 seconds or more, then winding up at coiling temperature 600 ° C. or less
is characterized in that it comprises a.
[0059]
　If the element between the base steel sheet and the surface layer steel plate is diffused, both the average change in hardness in the plate thickness direction during the forming of 5000 (ΔHv / mm) or less of hardness transition zone, between the heat it is preferable to heat at least 2 hours multilayered steel at a heating temperature of 1100 ° C. or higher 1350 ° C. or less in the rolling process, it is more preferable to heat 1150 ° C. ultra 1350 ° C. or less for 2 hours or more.
[0060]
　To be more than 10% area fraction of retained austenite of the center of plate thickness to improve the ductility of the high strength steel sheet in the high-strength steel sheet, the coiling temperature is, the martensitic transformation start temperature Ms or more of the base material steel plate preferably a temperature below the bainite transformation starting temperature Bs.
　Here,
　Bs (° C.) = 820-290C / (1-Sf) -37Si-90 mN-65Cr-50Ni + 70Al
　Ms (° C.) = 541-474C / (1-Sf) -15Si-35 mN-17Cr-17Ni + 19Al
　here, C, Si, Mn, Cr, Ni and Al are the contents of the elements of the base material steel plate [wt%], Sf is the area fraction of ferrite of the base material steel plate.
[0061]
　In more detail for each step, the case of obtaining a hot rolled steel sheet, firstly, the multilayered steel sheet produced by the above method, the heating temperature of 1100 ° C. or higher, preferably heated at 1150 ° C. ultra 1350 ° C. or less. To suppress the anisotropy of crystal orientation due to casting, it is preferable to set the heating temperature of the slab and 1100 ° C. or higher. On the other hand, the heating temperature of the slab from incurring a significant increase in manufacturing cost must be turned a large amount of energy to heat beyond 1350 ° C., and 1350 ° C. or less. Further, in order to control the standard deviation of nano-hardness of the surface layer softened portion 0.8 or less, in order to provide a change in the gradual hardness to it if there are more hardness transition zone, alloying elements, in particular it is necessary to control the concentration of C atoms are distributed slowly. Distribution of C concentration obtained by the diffusion of C atoms, the diffusion frequency of C atoms is increased as the temperature rises. Therefore, in order to control the C concentration is controlled in the rough rolling it is important from the hot rolling heating. The hot rolling heating, in order to promote the diffusion of C atoms, it is necessary to high temperature heating temperature, preferably 1350 ° C. 1100 ° C. or higher, more preferably at most 1150 ° C. Ultra 1350 ° C.. The hot rolling heating, a change shown in FIG. 2 (i) and (ii) occur. (I) is the diffusion of C atoms from the center of plate thickness to the surface softening unit, a decarburization reaction C desorbed to the outside from (ii) a surface layer softened portion. Distribution C concentration caused by balance of the diffusion and elimination reaction of the C atoms of the (i) and (ii). Is less than 1100 ° C., since the reaction of (i) is insufficient, not obtained preferably C concentration distribution. On the other hand, in the 1350 ° C. greater than the reactivity of (ii) it is excessively generated, can not be obtained likewise preferred concentration distribution.
[0062]
　Furthermore, after controlled to a preferred C concentration distribution by adjusting the hot rolling heating temperature, in order to obtain a more optimal C concentration distribution, the path control in the rough rolling is very important. Rough rolling the rough rolling temperature: 1100 ° C. or more, the thickness reduction rate per pass: 5% or more and less than 50%, and inter-pass time: performed more than once under the conditions of more than 3 seconds. This is because the distortion introduced by the rough rolling, in order to promote the diffusion of C atoms of (i) in FIG. Assuming that a slab with a controlled C concentration in a preferable state in hot rolling heating to rough rolling and finish rolling in the usual manner, so that the C atom plate thickness without being able to sufficiently diffuse the surface layer softened portion is reduced. Accordingly, the slab having a thickness of more than 200 mm, when manufacturing a hot rolled steel sheet having a thickness of several mm by hot rolling of a conventional method, C concentration in the surface layer softened portion becomes rapidly changing steel sheet, a moderate hardness change The resulting longer. Found methods to solve this is the path control of the rough rolling. Diffusion of C atoms is greatly influenced by the strain (dislocation density) as well temperature. In particular, in comparison with the lattice diffusion, since the increasing spread frequency more than 10 times in the dislocation diffusion, while leaving the dislocation density, it is necessary to devise to reduce the sheet thickness by rolling. Curve 1 in Figure 3 of the plate thickness reduction ratio per one pass of the rough rolling is small, shows the dislocation density change after rolling pass, it can be seen that the strain remains for a long time. By is left this way the distortion over time in the surface layer softened portion, the diffusion of C atoms in the surface layer softened portion occurs sufficiently, it becomes possible to obtain an optimal C concentration distribution. Meanwhile, the curve 2 is the change of the dislocation density when the sheet thickness reduction rate large, the amount of strain introduced by rolling is increased, the recovery is likely to be promoted, the dislocation density is rapidly lowered. Therefore, in order to obtain an optimal concentration distribution of C, it is necessary not to cause a change of the dislocation density, such as curve 2. From this viewpoint, the upper limit of the sheet thickness reduction rate per one pass is less than 50%. In order to promote the diffusion of C atoms in the surface layer softened portion, since the securing of the dislocation density and the retention time of a quantity required, next to the lower limit of the sheet thickness reduction rate of 5%, as the path between the time 3
[0063]
　Further, in the case of forming the hardness transition zone, the heating time of the slab to 2 hours or more. This is because by diffusing elements between the base steel sheet and the surface layer steel plate during slab heating, thereby reducing the average hardness change in hardness transition zone to form therebetween. Mean hardness change of the heating time is shorter than 2 hours hardness transition zone does not become sufficiently small. The upper limit of the heating time is not specified, over 8 hours of heating requires a lot of heating energy, it is not preferred from the cost.
[0064]
　After heating the slabs, performing hot rolling. The completion temperature of hot rolling (finishing temperature) of less than 800 ° C., it increased rolling reaction force, it is difficult to get a thickness of a specified stable. Therefore, completion temperature of the hot rolling is made 800 ° C. or higher. On the other hand, the completion temperature of the hot rolling in a 980 ° C. greater, because the device for heating the steel sheet in the process from the end of heating of the slab until the completion of hot rolling is required, it requires high costs, hot the completion temperature of the rolled and 980 ° C. or less.
[0065]
　Thereafter, in the cooling process, to hold more than 3 seconds at a temperature between 700 ℃ ~ 500 ℃. This is an important condition in the present invention, only the surface layer of the soft layer is ferrite transformation, a necessary step in order to reduce the variation in hardness. Because at a temperature 700 ° C. or higher to ferrite transformation is delayed, it is impossible to the surface of ferrite. Some of the surface layer is a low temperature transformation structure at 500 ° C. or less. To become as having a plurality of structure of ferrite and low temperature transformation structure cause variations in the surface hardness, the holding temperature is set to 500 ° C. or higher. Retention time shall not be less than 3 seconds. Order to sufficiently proceed the surface of the ferrite transformation, it is necessary to hold more than 3 seconds. Preferably the retention time is at least 5 seconds, more preferably not less than 10 seconds.
[0066]
　Coiling temperature is set to 600 ° C. or less. This is because a high-strength steel to produce a low-temperature transformation structure on the base steel sheet. Preferably 500 ° C. or less, more preferably 400 ° C. or less. In this way, by changing the timing of the transformation of the base material steel plate and the surface layer steel plate, to obtain a tissue hardness variation is small in the surface layer is one of the features of the present invention. In order to improve the ductility of the high strength steel plate with 10% or more residual austenite an area fraction of the thickness center portion in the high-strength steel sheet, the coiling temperature is the temperature of the bainite transformation temperature range of the base material steel plate , i.e. the martensitic transformation start temperature Ms or bainite transformation starting temperature Bs or less in the temperature of the base steel sheet. This is to produce a bainite or martensite base steel sheet and high-strength steel, further, in order to stabilize the residual austenite. In this way, by changing the timing of the transformation of the base material steel plate and the surface layer steel plate, to obtain a tissue hardness variation is small in the surface layer is one of the features of the present invention. In the present invention, the martensitic transformation start temperature Ms and bainite transformation starting temperature Bs is calculated by the following equation.
　Bs (° C.) = 820-290C / (1-Sf) -37Si-90 mN-65Cr-50Ni + 70Al
　Ms (° C.) = 541-474C / (1-Sf) -15Si-35 mN-17Cr-17Ni + 19Al
　Here, C, Si , Mn, Cr, Ni and Al are contents of the elements of the base material steel plate [wt%], Sf is the area fraction of ferrite of the base material steel plate.
[0067]
　Since it is difficult to determine the area fraction of ferrite during manufacture of the steel sheet, the present invention, when calculating the Bs and Ms, the same temperature history as the annealing step was taken cold rolled sheet before it enters the annealing step in annealing, the area fraction of ferrite was determined is used.
[0068]
　Next, a method of obtaining a cold rolled steel sheet of high-strength steel sheet encompassed by the present invention. Method of manufacturing the cold-rolled steel sheet,
　on one side or both sides of the base material steel plate which constitutes the center of plate thickness having a chemical composition as described above, constituting the surface layer softened portion having a chemical composition as described above as well step of surface steel plate are laminated to form a multilayered steel,
　the multilayered steel sheet heating temperature 1100 ° C. or higher 1350 ° C. or less, preferably by heating at 1150 ° C. ultra 1350 ° C. or less, then hot rolling and cold rolling a process, comprising the finish rolling of the hot rolling in the rough rolling and the finishing temperature 800 ~ 980 ° C., the rough rolling is rough rolling temperature: 1100 ° C. or more, the thickness reduction rate per pass: 5% or more less than 50%, and inter-pass time: step is carried out twice or more under the conditions of more than 3 seconds, and
　rolled a multilayer steel of the base material steel plate Ac3 point -50 ° C. or higher and 700 ° C. or higher, 900 ° C. or less in the temperature Holding sec, then a step of cooling below the average cooling rate 100 ° C. / s from 750 ° C. to 550 ° C. or less
is characterized in that it comprises a.
　Here,
　Ac3 = 910-203√C + 44.7Si-30 mN + 700P-20Cu-15.2Ni-11Cr + 31.5Mo + 400Ti + 104V + 400Al · · (Equation 1)
　where, C, Si, Mn, P , Cu, Ni, Cr, Mo, Ti , V and Al are contents (mass%) of the elements.
[0069]
　Further, when the element between the base steel sheet and the surface layer steel plate is diffused, both the average change in hardness in the plate thickness direction during the forming of 5000 (ΔHv / mm) or less of hardness transition zone, the it is preferred that multi layer steel sheet was heated over 2 hours and the heating temperature 1100 ° C. or higher 1350 ° C. or less, or 1150 ° C. ultra 1350 ° C. or less, then hot rolling and cold rolling.
[0070]
　Furthermore, in order to be more than 10% area fraction of retained austenite of the center of plate thickness to improve the ductility of the high strength steel sheet in a high strength steel sheet, instead of the step after the cold rolling as defined above it is preferable that the annealing in Tsuban the continuous annealing line rolled multilayered steel, annealing in the continuous annealing line, first, the double-layer steel sheet 700 ° C. or higher, 5 at 900 ° C. below the heating temperature keeping sec,
　then optionally, the multi-layer steel sheet to less than 400 seconds dwell preliminary cooling stop temperature to more than 5 seconds less than Bs point than Ac3 point -20 ° C. of the base steel sheet from the heating temperature to pre-cooling,
　then cooling said multilayer steel sheet at an average cooling rate Ms-100 of less ° C. or higher Bs cooling stop temperature to above 10 ° C. / s of the base material steel plate, and
　then the multi-layer before the steel plate We are staying in a temperature range of not lower than Ms-100 ° C. of the base material steel plate 30 seconds to 600 seconds or less
preferably contains.
　Ac3 (℃) = 910-203√C + 44.7Si -30Mn + 700P-20Cu-15.2Ni-11Cr + 31.5Mo + 400Ti + 104V + 400Al ·· ( Equation
　1) Bs (℃) = 820-290C / (1-Sf) -37Si-90Mn-65Cr -50Ni + 70Al ·· (equation
　2) Ms (° C.) = 541-474C / (1-Sf) -15Si-35 mN-17Cr-17Ni + 19Al · · (equation 3)
　here, C, Si, Mn, P , Cu, Ni , Cr, Mo, Ti, V and Al are contents of the elements of the base material steel plate [wt%], Sf is the area fraction of ferrite of the base material steel plate.
[0071]
　In more detail for each step, heating the multilayer steel sheet produced by the above method, the following 1350 ° C. The heating temperature 1100 ° C. As described above in the method for producing a hot-rolled steel sheet or 1150 ° C. ultra 1350 ° C. or less and then hot rolled, it is wound, for example, coiling temperature 20 ° C. or higher 700 ° C. or less. Next, the pickling of hot rolled steel sheet manufactured in this way. Pickling is to remove oxides on the surface of the hot-rolled steel sheet, may be the one, it may be performed a plurality of times. In the case of forming the hardness transition zone, first, it is preferred to heat at least 2 hours multilayered steel heating temperature 1100 ° C. or higher 1350 ° C. or less, or 1150 ° C. ultra 1350 ° C. or less. This is to reduce the element is diffused, the average hardness change in hardness transition zone to form between them between the base steel sheet and the surface layer steel sheet during heating. Mean hardness change of the heating time is shorter than 2 hours hardness transition zone does not become sufficiently small. Next, the pickling of hot rolled steel sheet manufactured in this way. Pickling is to remove oxides on the surface of the hot-rolled steel sheet, may be the one, it may be performed a plurality of times.
[0072]
　In cold rolling, the total rolling reduction exceeds 85%, ductility of the base steel sheet is lost, since the base steel sheet during cold rolling increases the risk of fracture, the total reduction rate of 85% or less It is desirable On the other hand, sufficiently advance the recrystallization of the soft layer in the annealing step, it is preferable that the total reduction ratio of 20% or more, and more preferably 30% or more. For the purpose of lowering the cold rolling load before cold rolling, 700 ° C. may be annealed at a temperature of.
[0073]
　Then the annealing be described, but the soft layer as described above is first in the present invention as compared to the hard layer, for hardenability is lower component system, hardly be low-temperature transformation structure is generated during annealing, annealing organization was once austenite transformation part of the later transformation ferrite, the part that did not austenite transformation becomes a recrystallization ferrite or non-recrystallized ferrite. For this to be the main cause of the variation in hardness in is a great non-recrystallized ferrite hardness, reducing the non-recrystallized ferrite is a point of the invention in the following description.
[0074]
　Ac3 point -50 ° C. or more base steel sheet in annealing and 700 ° C. or higher, then heated to 900 ° C. or less, holds more than 5 seconds. The reason for the Ac3 point -50 ° C. or more base steel sheet, by heating the base material steel plate into a 2-phase region or austenite single-phase region of ferrite and austenite, with the transformation structure by subsequent heat treatment, necessary strength in order to obtain a. From this intensity at low temperature is significantly decreased. The reason for the 700 ° C. or more, sufficiently promote recrystallization of the softened layer to reduce the non-recrystallization fraction, in order to reduce the variation in hardness. Hardness variation in softened layer is increased at temperatures below 700 ° C.. When heated to 900 ° C. or higher, the old γ grain diameter of the hard layer is coarsened, undesirable since the toughness is deteriorated. It is necessary to maintain a heating temperature for 5 seconds or more. If the holding time is 5 seconds or less, the progress of the austenitic transformation of the base material steel plate becomes insufficient, reduction in strength becomes remarkable. Further, recrystallization is insufficient softening layer, the greater variation in surface hardness. From these viewpoints, the holding time is preferably 10 seconds or more. More preferably 20 seconds or more.
[0075]
　The subsequently cooled, cooled below the average cooling rate 100 ° C. / s from 750 ° C. to 550 ° C. or less. The lower limit of the average cooling rate is not particularly limited, it may be, for example, 2.5 ° C. / s. The reason why the lower limit of the average cooling rate 2.5 ° C. / s, the ferrite transformation occurs in the base material steel plate, because the base material steel plate to prevent the softening. If 2.5 ° C. / s average cooling rate from the slow, the strength is significantly decreased. More preferably 5 ° C. / s or higher, more preferably 10 ° C. / s or higher, further preferably 20 ° C. / s or higher. Because significantly less likely to occur ferrite transformation at 750 ° C. or higher, the cooling rate is not limited. The 550 ° C. temperature below for low-temperature transformation structure is obtained, it does not limit the cooling rate. Low temperature transformation structure occurs in the surface layer when cooled than at a faster rate 100 ° C. / s, it will cause variations in hardness, it is cooled below 100 ° C. / s. More preferably not more than 50 ° C. / s. More preferably not more than 20 ° C. / s.
[0076]
　The 550 ° C. or less, may be cooled at a constant cooling rate to room temperature, by holding at a temperature of about 200 ° C. ~ 550 ° C., or allowed to proceed for bainite transformation, may or tempered martensite. However, when held for a long time at 300 ° C. ~ 550 ° C., the strength may be reduced, if held at that temperature holding time is desired the following 600 seconds.
[0077]
　In order to improve the ductility of the high strength steel plate with 10% or more residual austenite an area fraction of the thickness center portion in the high-strength steel sheet, instead of the annealing and cooling is described above, the following annealing and it is preferred to carry out the cooling. First, annealing at the 700 ° C. or higher, then heated to 900 ° C. or less, holds more than 5 seconds. The reason for the 700 ° C. or more, sufficiently promote recrystallization of the softened layer to reduce the non-recrystallization fraction, in order to reduce the variation in hardness. Hardness variation in softened layer is increased at temperatures below 700 ° C.. When heated to 900 ° C. or higher, the old γ grain diameter of the hard layer is coarsened, undesirable since the toughness is deteriorated. It is necessary to maintain a heating temperature for 5 seconds or more. If the holding time is 5 seconds or less, the progress of the austenitic transformation of the base material steel plate becomes insufficient, reduction in strength becomes remarkable. Further, recrystallization is insufficient softening layer, the greater variation in surface hardness. From these viewpoints, the holding time is preferably 10 seconds or more. More preferably 20 seconds or more.
[0078]
　Annealing is carried out, for example, by Tsuban a rolled multi-layer steel sheet to a continuous annealing line. Here, the annealing of the continuous annealing line, first, a multi-layer steel sheet 700 ° C. or higher, 900 ° C. to hold more than 5 seconds under the following heating temperature, then, optionally, mother multilayer steel from the heating temperature comprising pre-cooling to stop less than 5 seconds 400 seconds to pre-cooling stop temperature of Ac3 point below -20 ° C. or higher Bs point of wood steel. Such pre-cooling step may be performed if necessary, may be subjected to subsequent cooling step without the pre-cooling process.
[0079]
　Following optional precooling step, annealing in a continuous annealing line, a multi-layer steel sheet is cooled at an average cooling rate Ms-100 of less ° C. or higher Bs cooling stop temperature to above 10 ° C. / s of the base material steel plate it, and then includes a multilayer steel temperature range of not lower than Ms-100 ° C. of the base material steel plate, more preferably to dwell 300 ° C. or higher 500 ° C. 600 seconds or less at the temperature range more than 30 seconds. During this dwell may be performed a plurality of times any heating and cooling as needed. In order to stabilize the residual austenite, the residence time is important. The required residence time is less than 30 seconds, it is difficult to obtain a residual austenite of 10% or more. On the other hand, in 600 seconds or more, it is difficult to soften the entire organization to obtain sufficient strength by progress. In the present invention, Ac3, Bs and Ms is calculated by the following equation.
　Ac3 (℃) = 910-203√C + 44.7Si -30Mn + 700P-20Cu-15.2Ni-11Cr + 31.5Mo + 400Ti + 104V + 400Al ·· ( Equation
　1) Bs (℃) = 820-290C / (1-Sf) -37Si-90Mn-65Cr -50Ni + 70Al
　Ms (° C.) = 541-474C / (1-Sf) -15Si-35 mN-17Cr-17Ni + 19Al
　here, C, Si, Mn, P , Cu, Ni, Cr, Mo, Ti, V and Al mother the content of each element of wood steel was [wt%], Sf is the area fraction of ferrite of the base material steel plate.
[0080]
　Since it is difficult to determine the area fraction of ferrite during manufacture of the steel sheet, the present invention, when calculating the Bs and Ms, the same temperature history as the annealing step was taken cold rolled sheet before it enters the annealing step in annealing, the area fraction of ferrite was determined is used.
[0081]
　Thereafter, the plating bath temperature when subjected to hot-dip galvanizing may be a condition that is conventionally applied, for example, it can be applied conditions such 440 ℃ ~ 550 ℃. Further, after galvanized, treated heating alloying, the heating temperature of the alloying in the case of producing a galvannealed steel sheet well conditions that are conventionally applied, for example, 400 ° C. ~ 600 ℃ conditions can be applied, such as. Heating method of alloying is not limited in particular, direct heating or by combustion gases, induction heating, direct current heating and the like, it can be used heating system in accordance with the hot-dip plating equipment conventionally.
[0082]
　After the alloying treatment, the steel sheet is cooled to 200 ° C. or less, and subjected to more temper rolling necessary.
[0083]
　When manufacturing the electro-galvanized steel sheet, for example, as a pretreatment for plating, alkaline degreasing, water washing, pickling, and conduct water washing, then, to the steel sheet after pretreatment, the electroplating apparatus of the liquid circulation type used, zinc sulfate as a plating bath, sodium sulfate, used as consisting of sulfuric acid, there is a method of electrolytic treatment to a predetermined plating thickness of about a current density of 100A / dm @ 2.
[0084]
　Finally, the surface layer steel sheet, mark the preferred components. Steel sheet in the present invention may chemical composition is different surface softened portion and the plate thickness center. In such cases, the preferred chemical composition of the surface layer steel plate constituting the surface layer softened portion is as follows.
[0085]
　C content of the surface layer steel plate is preferably not more than 0.9 times the C content of the base material steel plate. The hardness of the surface layer steel plate is to be lower than the hardness of the base steel sheet. Larger than 0.9 times, the average Vickers hardness of the surface layer softened portion in the finally obtained high-strength steel sheet may not become a 0.60 times the average Vickers hardness of the plate thickness 1/2 position. More preferably C content of the surface layer for steel sheet 0.7 times or less of the C content of the base material steel plate, even more preferably 0.5 times or less, and most preferably not more than 0.3 times.
[0086]
　Mn amount of the surface layer steel plate, the sum of the Cr content and Mo content is Mn content of the base material steel plate is preferably 0.9 times or less of the total amount of Cr and Mo amount. Mn amount to improve the hardenability, the sum of the Cr content and Mo content is Mn content of the base material steel plate, greater than 0.9 times the total amount of Cr and Mo content tends to occur low-temperature transformation structure, the hardness of cause of variation. More preferably 0.7 times or less, even more preferably 0.5 times or less, and most preferably not more than 0.3 times.
[0087]
　B of the surface layer steel plate is preferably not more than 0.9 times the B amount of the base material steel plate. B amount to improve the hardenability low temperature transformed structure is likely to occur larger than 0.9 times the base steel sheet, causing variations in hardness. More preferably 0.7 times or less, even more preferably 0.5 times or less, and most preferably not more than 0.3 times.
[0088]
　It is preferred that the sum of the Cu content and Ni content of the surface layer steel plate is less 0.9 times the sum of the Cu content and Ni content of the base material steel plate. The sum of the Cu content and Ni content to improve the hardenability is greater than 0.9 times the sum of the Cu content and Ni content of the base material steel plate easily low temperature transformation structure occurs, causing variations in hardness. More preferably 0.7 times or less, even more preferably 0.5 times or less, and most preferably not more than 0.3 times.
[0089]
　For surface layer steel sheet, in addition to the above elements, Si, P, S, Al, N, Cr, B, Ti, Nb, V, Cu, Ni, O, W, Ta, Sn, Sb, As, Mg, Ca , Y, Zr, La, may have a Ce. A preferred composition range of the element is the same as the preferred range of the thickness center portion.
[0090]
　Subsequently, we describe a method for identifying a steel structure according to the present invention. Steel structure is a cross-section perpendicular to the cross section parallel and / or rolling direction to the rolling direction and the thickness direction of the steel sheet can be identified by observing at 500-fold to 10,000-fold magnification. For example, after cutting out the steel sheet after finish of the surface mirror by mechanical polishing, to revealing the steel structure using a nital reagent. Then, about 1/2 of the area of the steel structure using a scanning electron microscope depth from the surface of the steel plate thickness: observation using (SEM scanning electron microscope). This makes it possible to measure the area fraction of ferrite of the base material steel plate. Further, in the present invention, the area fraction of retained austenite of the center of plate thickness is determined as follows by X-ray measurement. First, the portion from the surface of the steel sheet to 1/2 of the thickness of the steel sheet is removed by mechanical polishing and chemical polishing, it performs measurement by using MoKα rays as characteristic X-rays with respect to the chemical polished surface. The body-centered cubic (bcc) phase (200) and (211), and 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 of the center of plate thickness 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 of the thickness center portion, I 200f , I 220f and I 311f are the fcc phase, respectively (200), shows the intensity of diffraction peaks of (220) and (311), I 200b , and I 211b , the diffraction peaks of the respective bcc phase (200) and (211) It shows the strength of the. )
Example
[0091]
　In this embodiment, for each product obtained, Vickers hardness test, nano hardness test, tensile test was carried out V bending test and notched susceptibility testing.
[0092]
　The average Vickers hardness was determined as follows. First, toward the surface from the 1/2 position of the sheet thickness in the sheet thickness direction at 5% intervals thickness was measured under a load 100g heavy pushing a Vickers hardness at a certain thickness direction position, and then the plate from its position the Vickers hardness of similarly indentation load 100g weight in total 5 points on a line parallel to the rolling direction in a thickness direction perpendicular were measured, and the average value thereof and the average Vickers hardness at the plate thickness direction position. The distance between the measurement points aligned in the thickness direction and the rolling direction was 4 times the distance of the indentation. The average Vickers hardness of a certain thickness direction position, when it becomes less than 0.6 times the average Vickers hardness at the plate thickness 1/2 position as measured in the same manner, and the surface softened portion of the surface side from that position defined. The average Vickers hardness of the whole surface layer softened section, this manner by measuring the Vickers hardness of randomly 10 points at the surface softened portion defined, obtained as their average.
[0093]
　Also, determine the thickness of the surface layer softened portion by the method prescribed in the specification, to determine the ratio of plate thickness. Similarly, to determine the value of the thickness direction of the average hardness change in hardness transition zone in a manner prescribed in this specification.
[0094]
　Nano hardness of the surface layer softened portion is a 1/2 position of the surface layer softened portion thickness from the surface, a nano-hardness is measured at 100 points in the thickness direction perpendicular nano hardness of the surface layer softened portion the standard deviation of the values was the difference between the standard deviation.
[0095]
　Tensile strength TS and elongation (%) is to prepare a No. 5 test piece described in JIS Z 2201 taking a major axis in the rolling direction and perpendicular direction was measured in accordance with JIS Z 2241.
[0096]
　Moreover, critical bending radius R creates a No. 1 test piece described in JIS Z2204 as the direction perpendicular the longitudinal direction (bending ridge is coincident with the rolling direction) to the rolling direction, according to JIS Z2248 It went a V-bending test. For samples with surface softening unit only on one side, bent so as to outwardly bend surface having a surface softening unit. The angle of the die and punch and 60 °, perform bending test by changing the tip radius of the punch in 0.5mm increment, cracks were determined as a limit bending radius R of the punch tip radius can be bent without causing.
[0097]
　Notch sensitivity was determined as follows. First, the tensile similar to the test to produce a 6 No. 5 test piece according to JIS Z2201, to introduce a notch depth 50μm in the center of the parallel portion. Performing a tensile test crosshead speed of 100 mm / min and 0.1 mm / min. A tensile strength when pulled cross head speed of 100 mm / min (sigma fast), the ratio of tensile strength when pulled cross head speed of 0.1 mm / min (sigma slow) (sigma Fast / sigma slow) as an indicator of the notch sensitivity. sigma Fast / sigma low speed low above 1.0 and become Invite notch sensitive, it can be determined that can be alleviated-out effect notch. On the other hand, sigma fast / sigma slow high notch sensitivity if 1.0, it can be determined easily broken regardless of the pulling speed.
[0098]
[Example A]
　For Table thickness 20mm continuous casting slab having the chemical composition shown in 1 (the base steel sheet), after removing the surface oxide by grinding the surface chemistry shown in Table 1 on one side or both sides the surface layer steel sheet having a composition layered in arc welding. The ratio of the thickness of the surface layer steel plate for plate thickness are shown in Table 1, "percentage of surface softening unit (one side) (%)". This hot rolled under the conditions of heating temperature, finishing temperature, coiling temperature shown in Table 2, to obtain a laminated hot-rolled steel sheet. For test materials for hot-rolled steel sheet and products, the holding time at 700 ° C. ~ 500 ° C. hot-rolled intentionally controlled to the values shown in Table 2. If the cold-rolled steel sheet product, then, pickling, performed 50% of the cold rolled and subjected to annealing under the conditions shown in Table 2.
[0099]
　Incidentally, the obtained product, was measured the chemical composition of the chemical composition and thickness 1/2 position of the 2% position of the sheet thickness from the surface layer, the base steel sheet are shown in Table 1, respectively, and the surface layer steel plate substantially change the chemical composition was not.
[0100]
[Table 1-1]

[0101]
[Table 1-2]

[0102]
[table 2-1]

[0103]
[Table 2-2]

[0104]
[Table 2-3]

[0105]
[Table 2-4]

[0106]
　Referring to Table 2, for example, in the steel sheets of Comparative Examples 4, 6, 7 and 10, the average Vickers hardness of the surface layer softened portion 0.60 times the requirements of the average Vickers hardness of the plate thickness 1/2 position although satisfied, the standard deviation of nano-hardness of the surface layer softened portion is 0.9, i.e. it can be seen that does not meet the 0.8 requirement. As a result, the steel sheets of comparative examples, critical bending radius R was 2.5mm or 2 mm. In contrast, in the steel plate in the embodiment of the present invention to meet the two requirements described above, the limit bending radius R is less than 2 mm, especially was 1.5mm or 1 mm. Therefore, by suppressing the variations in the hardness of the surface layer softened portion within a predetermined range, just significantly improves the bending of the steel sheet as compared to the steel sheets that combine a soft surface layer softened portion than in the center of plate thickness it has been found that can be.
[0107]
　Referring also to hot-rolled steel sheet of Comparative Example 4, when the holding time at 700 ° C. ~ 500 ° C. and 1 second in the course of cooling after hot rolling, the standard deviation of nano-hardness of the surface layer softened portion 0.9, critical bending radius R was 2 mm. In contrast, the retention time except that the 20 seconds by hot-rolled steel sheets of Example 3 was produced in the same manner as in Comparative Example 4, the standard deviation of nano-hardness of the surface layer softened portion 0.5 , and the critical bending radius R was 1 mm.
[0108]
　Referring also to cold-rolled steel sheet of Example 5, to meet the Ac3 point -50 ° C. or higher and 700 ° C. or higher 900 ° C. or less of the temperature and 5 seconds or more requirements, suitably the temperature and holding time during annealing by choosing to suppress variations in hardness of the surface layer softened portion (nano hardness of the standard deviation of the surface layer softened portion: 0.5), it was found to be significantly improved bendability of cold-rolled steel sheet as a result (critical bending radius R is 1 mm). On the other hand, in the cold-rolled steel sheets of Comparative Examples 6 and 7 do not meet the above requirements, the standard deviation of nano-hardness of the surface layer softened portion is 0.9, the limit bending radius R was 2 mm.
[0109]
　In addition, the cold-rolled steel sheet of Example 9, the average cooling rate during annealing is is 2 ° C. / s, tensile strength was 1500 MPa. On the other hand, in the cold-rolled steel sheet of Example 8 using the same steel type b, by controlling the average cooling rate to 20 ° C. / s, it was possible to achieve a higher tensile strength of 1670MPa.
[0110]
　Further, in the hot rolling rough rolling the rough rolling temperature: 1100 ° C. or more, the thickness reduction rate per pass: 5% or more and less than 50%, and inter-pass time: implementing more than once under the conditions of more than 3 seconds in the steel sheet manufactured without, critical bending radius R is high, it was not possible to achieve sufficient bending workability. Furthermore, the value of sigma fast / sigma slow in all of the steel sheet according to an embodiment of the present invention is greater than 1.0, it was possible to suppress the thus notch effect.
[0111]
: Example B formation hardness transition zone]
　For continuously cast slab having a thickness of 20mm with the chemical compositions shown in Table 3 (the base steel sheet), after removing the surface oxide by grinding the surface, one side or and the surface layer steel sheet having a chemical composition shown in Table 1 on both sides laminated with arc welding. The ratio of the thickness of the surface layer steel plate for plate thickness are shown in "Percentage of surface layer steel plate (one side) (%)" in Table 3. The heating temperature, heating time shown in table 4, hot rolled under the conditions of finishing temperature, coiling temperature, to obtain a laminated hot-rolled steel sheet. For test materials for hot-rolled steel sheet and products, the holding time at 700 ° C. ~ 500 ° C. hot-rolled intentionally controlled to the values shown in Table 4. If the cold-rolled steel sheet product, then, pickling, performed 50% of the cold rolled and subjected to annealing under the conditions shown in Table 4.
[0112]
　Incidentally, the obtained product, the surface layer was measured the chemical composition of the chemical composition and thickness 1/2 position of the 2% position of the sheet thickness, the base material steel plate shown in Tables 3, and the surface layer steel plate substantially change the chemical composition was not.
[0113]
[Table 3-1]

[0114]
[Table 3-2]

[0115]
[Table 4-1]

[0116]
[Table 4-2]

[0117]
[Table 4-3]

[0118]
[Table 4-4]

[0119]
　Referring to Table 4, for example, in the steel sheets of the comparative examples 104, 106, 107 and 111, the average Vickers hardness of the surface layer softened portion 0.60 times the requirements of the average Vickers hardness of the plate thickness 1/2 position filled, although further average hardness change in the thickness direction of the stiffness transition zone satisfies the 5000 (ΔHv / mm) following requirements, the standard deviation of nano-hardness of the surface layer softened portion is 0.9, i.e. 0. it can be seen that 8 does not meet the following requirements. As a result, the steel sheets of comparative examples, critical bending radius R was 2.5mm or 2 mm. On the other hand, in Example 112, the average Vickers hardness of the surface layer softened portion satisfies 0.60 times the requirements of the average Vickers hardness of the plate thickness 1/2 position, further the standard deviation of nano-hardness of the surface layer softened portion although satisfy 0.8 following requirements, the average change in hardness in the plate thickness direction of the stiffness transition zone is 5633 (ΔHv / mm), i.e. it can be seen that beyond 5000 (ΔHv / mm). As a result, the steel sheets of Example 112, critical bending radius R was 1.5 mm. In contrast, the standard deviation of "0.60 times the average Vickers hardness of the average Vickers hardness of the surface layer softened portion thickness 1/2 position" and "the surface softening of the nano-hardness is 0. filled and "hardness thickness direction average hardness change in the transition zone of the two requirements of 8 or less" is 5000 in the steel sheet in an exemplary (ΔHv / mm) or less ", critical bending radius R was 1mm . Therefore, by controlling both the thickness direction of the average hardness change in hardness variation and hardness transition zone of the surface layer softened portion within a specific range, simply soft surface softening than the center of plate thickness a steel sheet that combines a part like steel sheet as compared with the steel plate only one plate thickness direction of the average hardness change in hardness variation and hardness transition zone of the surface softened portion is not controlled within a specific range it was found that the bending of the can remarkably improved.
[0120]
　Referring also to hot-rolled steel sheets of the comparative example 104, if the holding time at 700 ° C. ~ 500 ° C. and 1 second in the course of cooling after hot rolling, the standard deviation of nano-hardness of the surface layer softened portion 0.9, critical bending radius R was 2 mm. In contrast, in the hot-rolled steel sheets of Example 103 which is manufactured in the same manner as in Comparative Example 104 except that the retention time 300 ° C. for 30 seconds and the coiling temperature of the nano hardness of the surface layer softened portion the standard deviation of 0.5, the limit bending radius R was 1 mm.
[0121]
　Referring also to cold-rolled steel sheets of Example 105, to meet the requirements of holding Ac3 point -50 ° C. or higher and 700 ° C. or higher 900 ° C. or less of the temperature and 5 seconds or more, the temperature and holding time during annealing appropriate by choosing to, and suppress variations in hardness of the surface layer softened portion (nano hardness of the standard deviation of the surface layer softened portion: 0.5), found to be significantly improved bendability of cold-rolled steel sheet as a result and (critical bending radius R is 1 mm). On the other hand, in the cold-rolled steel sheets of Comparative Examples 106 and 107 that do not meet the above requirements, the standard deviation of nano-hardness of the surface layer softened portion is 0.9, the limit bending radius R was 2 mm.
[0122]
　In addition, the cold-rolled steel sheets of Example 109, the average cooling rate during annealing is is 2 ° C. / s, tensile strength was 1470 MPa. On the other hand, in the cold-rolled steel sheets of Example 108 using the same steel type b ', by controlling the average cooling rate to 10 ° C. / s, it was possible to achieve a higher tensile strength of 1650 MPa.
[0123]
　Further, in the hot rolling rough rolling the rough rolling temperature: 1100 ° C. or more, the thickness reduction rate per pass: 5% or more and less than 50%, and inter-pass time: implementing more than once under the conditions of more than 3 seconds in the steel sheet manufactured without, critical bending radius R is high, it was not possible to achieve sufficient bending workability. Furthermore, the value of sigma fast / sigma slow in all of the steel sheet according to an embodiment of the present invention is greater than 1.0, it was possible to suppress the thus notch effect.
[0124]
: Example C thickness formation of the central part including the residual austenite an area fraction of 10% or more]
　The continuously cast slab having a thickness of 20mm with the chemical compositions shown in Table 5 (the base steel sheet), by grinding the surface after removing the surface oxide was laminated surface layer steel sheet having a chemical composition shown in Table 5 on one side or both sides in arc welding. This hot rolled under the conditions of heating temperature, finishing temperature, coiling temperature shown in Table 6 to obtain a laminated hot-rolled steel sheet. For test materials for hot-rolled steel sheet and products, the holding time at 700 ° C. ~ 500 ° C. hot-rolled intentionally controlled to the values shown in Table 6. If the cold-rolled steel sheet product, then, pickled, subjected to cold rolling at a cold rolling rate shown in Table 6, were annealed under conditions further shown in Table 6.
[0125]
　Incidentally, the obtained product, the surface layer was measured the chemical composition of the chemical composition and thickness 1/2 position of the 2% position of the sheet thickness, the base material steel plate shown in Tables 5, and the surface layer steel plate substantially change the chemical composition was not.
[0126]
[Table 5-1]

[0127]
[Table 5-2]

[0128]
[Table 5-3]

[0129]
[Table 5-4]

[0130]
[Table 6-1]

[0131]
[Table 6-2]

[0132]
[Table 6-3]

[0133]
[Table 6-4]

[0134]
[Table 6-5]

[0135]
[Table 6-6]

[0136]
　Tensile strength not less than 800 MPa, was evaluated as a high-strength steel sheet critical bending radius R and excellent resistance bending is less than 2 mm (examples in Table 6). Further elongation was evaluated as high-strength steel sheet having excellent bendability and ductility the case where 15% or more (Examples 201 to 241 in Table 6). On the other hand, "tensile strength than 800MPa" in the performance of, and "less than the limit bending radius R is 2mm", if not satisfied with any one sample of Comparative Example.
[0137]
　Further, in the hot rolling rough rolling the rough rolling temperature: 1100 ° C. or more, the thickness reduction rate per pass: 5% or more and less than 50%, and inter-pass time: implementing more than once under the conditions of more than 3 seconds in the steel sheet manufactured without, critical bending radius R is high, it was not possible to achieve sufficient bending workability. Furthermore, the value of sigma fast / sigma slow in all of the steel sheet according to an embodiment of the present invention is greater than 1.0, it was possible to suppress the thus notch effect.
[0138]
: Example D thickness formation of the central part including the hardness transition zone and residual austenite in an area fraction of 10% or more]
　The continuously cast slab having a thickness of 20mm with the chemical compositions shown in Table 7 (the base steel sheet), after removing the surface oxide surface by grinding, by laminating a surface layer for steel sheet having a chemical composition shown in Table 7 on one or both sides in arc welding. The heating temperature, heating time shown in table 8, hot rolled under the conditions of finishing temperature, coiling temperature, to obtain a laminated hot-rolled steel sheet. For test materials for hot-rolled steel sheet and products, the holding time at 700 ° C. ~ 500 ° C. hot-rolled intentionally controlled to the values shown in Table 8. If the cold-rolled steel sheet product, then, pickled, subjected to cold rolling at a cold rolling rate shown in Table 8, was annealed under conditions further shown in Table 8.
[0139]
　Incidentally, the obtained product, the surface layer was measured the chemical composition of the chemical composition and thickness 1/2 position of the 2% position of the sheet thickness, the base material steel plate shown in Tables 7, and the surface layer steel plate substantially change the chemical composition was not.
[0140]
[Table 7-1]

[0141]
[Table 7-2]

[0142]
[Table 7-3]

[0143]
[Table 7-4]

[0144]
[Table 8-1]

[0145]
[Table 8-2]

[0146]
[Table 8-3]

[0147]
[Table 8-4]

[0148]
[Table 8-5]

[0149]
[Table 8-6]

[0150]
　Tensile strength not less than 800 MPa, was evaluated as a high-strength steel sheet critical bending radius R and excellent resistance bending is less than 2 mm (examples in Table 8). In particular, in Examples 353 and 356, the average Vickers hardness of the surface layer softened portion satisfies 0.60 times the requirements of the average Vickers hardness of the plate thickness 1/2 position, further the surface layer softened portion nano hardness standard although deviation satisfies 0.8 following requirements, the average change in hardness in the plate thickness direction of the stiffness transition zone is found to be better than 5000 (ΔHv / mm). As a result, the steel sheets of Examples 353 and 356, limits the bending radius R was 1.5 mm. In contrast, the standard deviation of "0.60 times the average Vickers hardness of the average Vickers hardness of the surface layer softened portion thickness 1/2 position" and "the surface softening of the nano-hardness is 0. filled and "hardness thickness direction average hardness change in the transition zone of the two requirements of 8 or less" is 5000 in the steel sheet in an exemplary (ΔHv / mm) or less ", critical bending radius R was 1mm . Further, the thickness center portion is when the residual austenite containing an area fraction of 10% or more, elongation is 15% or more, in addition to the bending resistance, it was possible to obtain a high strength steel sheet excellent in ductility ( examples 301-341 in Table 8). On the other hand, "tensile strength than 800MPa" in the performance of, and "less than the limit bending radius R is 2mm", if not satisfied with any one sample of Comparative Example.
[0151]
　Further, in the hot rolling rough rolling the rough rolling temperature: 1100 ° C. or more, the thickness reduction rate per pass: 5% or more and less than 50%, and inter-pass time: implementing more than once under the conditions of more than 3 seconds in the steel sheet manufactured without, critical bending radius R is high, it was not possible to achieve sufficient bending workability. Furthermore, the value of sigma fast / sigma slow in all of the steel sheet according to an embodiment of the present invention is greater than 1.0, it was possible to suppress the thus notch effect.

The scope of the claims
[Requested item 1]
　And the plate thickness center, tensile strength and a surface layer softened portion disposed on one or both sides of the plate thickness center is a high-strength steel sheet of more than 800 MPa, 30 of plate thickness the surface softened portion from 10μm greater % has a thickness of less than an average Vickers hardness of the surface layer softened portion is less than 0.60 times the average Vickers hardness of the plate thickness 1/2 position, the standard deviation of nano-hardness of the surface layer softened portion wherein the but 0.8 or less, high-strength steel sheet.
[Requested item 2]
　Further comprising a hardness transition zone formed adjacent to them between the plate thickness center and the respective surface layer softened portion, the average change in hardness in the plate thickness direction of the hardness is a transition zone 5000 (ΔHv / mm ) and wherein the less, high-strength steel sheet according to claim 1.
[Requested item 3]
　Characterized in that it comprises the plate thickness center is 10% or more of residual austenite in an area fraction, the high strength steel sheet according to claim 1 or 2.
[Requested item 4]
　The thickness center portion, in
　mass%, C:
　0.05 ~
　0.8%, Si: 0.01 ~ 2.50%, Mn: 0.010 ~
　8.0%, P: 0.1%
　hereinafter, S: 0.05% or
　less, Al: 0 ~ 3%, and
　N: to 0.01% or less,
　the balance being composed of iron and inevitable impurities, one of the claims 1 to 3, the steel plate according to any one of claims.
[Requested item 5]
　The thickness center portion, further, by
　mass%, Cr:
　0.01 ~ 3%, Mo: 0.01 ~ 1%, and
　0.0001% ~ 0.01%: B
is selected from the group consisting of characterized in that it contains at least one high-strength steel sheet according to claim 4.
[Requested item 6]
　The thickness center portion, further, by
　mass%, Ti:
　0.01 ~ 0.2%, Nb: 0.01 ~ 0.2%, and
　V: 0.01 ~ 0.2%
from the group consisting of characterized in that it contains at least one selected, high-strength steel sheet according to claim 4 or 5.
[Requested item 7]
　The thickness center portion, further, by
　mass%, Cu: 0.01 ~ 1%, and
　Ni: 0.01 ~ 1%
, characterized in that it contains at least one selected from the group consisting of, wherein steel plate according to any one of claim 4-6.
[Requested item 8]
　Wherein the C content of the surface layer softened portion is less than 0.9 times the C content of the plate thickness center, high-strength steel sheet according to any one of claims 4-7.
[Requested item 9]
　Characterized in that the Mn content of the surface layer softened portion, the sum of the Cr amount and the Mo amount Mn of the plate thickness center is 0.9 times or less of the total amount of Cr and Mo content, claims 5 to the steel plate according to any one of 8.
[Requested item 10]
　Wherein the B content of the surface layer softened portion is less than 0.9 times the amount of B the thickness center, high-strength steel sheet according to any one of claims 5-9.
[Requested item 11]
　Wherein the sum of the Cu content and Ni content of the surface layer softened portion is less than 0.9 times the sum of the Cu content and Ni content of the plate thickness center, any one of claims 7-10 high-strength steel sheet according to.
[Requested item 12]
　On the surface of the surface layer softened portion, galvanized layer, and further comprising galvannealed layer, or an electro-galvanized layer, high-strength steel sheet according to any one of claims 1 to 11 .