Thin high-strength steel plate

Abstract

Provided is a high strength steel sheet, which comprises a sheet thickness center section and softened surface layer sections disposed on one or both sides of the sheet thickness center section and the tensile strength of which is at least 800 MPa, the high strength steel sheet being characterized in that: each softened surface layer section has a thickness of greater than 10 µm to not more than 30% of the sheet thickness; the average Vickers hardness of the softened surface layer sections is greater than 0.60 times to not more than 0.90 times the average Vickers hardness at the ½ sheet thickness position; and the standard deviation of the nano-hardness of the softened surface layer sections is 0.8 or less.

Description

High strength steel plate

[0001] The present invention relates to a high-strength steel plate, more particularly to a high-strength steel plate having a tensile strength of 800 MPa or more, preferably 1100 MPa or more.

[0002] In recent years, there has been a strong demand for higher strength steel sheets for automobiles in order to improve fuel efficiency, which contributes to environmental conservation. Ultra-high strength cold-rolled steel sheets generally cannot be formed using forming techniques such as drawing and stretch forming, which are used with mild steel sheets, and bending is the primary forming method. Furthermore, to achieve high strength, good bendability and high bending load are required. Therefore, when using ultra-high strength cold-rolled steel sheets for automotive structural components, good bendability and bending load are important selection criteria.

[0003]

[0003] During bending of a steel sheet, a large tensile stress is applied in the circumferential direction to the outer peripheral surface layer of the bend, while a large compressive stress is applied to the inner peripheral surface layer of the bend. Therefore, the state of the surface layer significantly affects the bendability of an ultra-high strength cold-rolled steel sheet. Therefore, it is known that the inclusion of a soft layer in the surface layer alleviates the tensile stress and compressive stress generated on the steel sheet surface during bending, thereby improving the bendability. Regarding such high strength steel sheets having a soft layer in the surface layer, the following steel sheets and manufacturing methods thereof are disclosed in Patent Documents 1 to 3.

[0004] First, Patent Document 1 describes a high-strength plated steel sheet having, in this order from the interface between the steel sheet and the plated layer toward the steel sheet side, an internal oxide layer containing oxides of Si and / or Mn, a soft layer including the internal oxide layer, and a hard layer composed of a structure mainly composed of martensite and bainite, wherein the average depth T of the soft layer is 20 μm or more and the average depth t of the internal oxide layer is 4 μm or more but less than T, and a method for producing the same.

[0005] Next, Patent Document 2 describes a high-strength hot-dip galvanized steel sheet characterized in that the value (ΔHv) obtained by subtracting the Vickers hardness at a position 20 μm deep from the steel sheet surface from the Vickers hardness at a position 100 μm from the steel sheet surface is 30 or more, and a manufacturing method thereof.

[0006] Next, Patent Document 3 describes a high-strength hot-dip galvanized steel sheet characterized in that the Vickers hardness at a position 5 μm from the surface layer in the sheet thickness direction is 80% or less of the hardness at a position 1 / 2 of the way in the sheet thickness direction, and the hardness at a position 15 μm from the surface layer in the sheet thickness direction is 90% or more of the Vickers hardness at a position 1 / 2 of the way in the sheet thickness direction, and a method for manufacturing the same.

[0007] However, none of Patent Documents 1 to 3 sufficiently considers the variation in hardness of the soft layer. For example, Patent Document 1 describes that the soft layer has an internal oxidation layer, and in this case, it is presumed that variation in hardness occurs between the oxide and other structures within the soft layer. If there is variation in hardness of the soft layer, a steel plate having such a soft layer may not be able to achieve sufficient bendability. Furthermore, none of Patent Documents 1 to 3 mentions controlling the hardness gradient in the transition zone between the surface soft layer and the internal hard layer. Furthermore, although it is presumed that having a soft layer on the surface layer will deteriorate bending load, none of Patent Documents 1 to 3 mentions bending load.

[0008] JP 2015-34334 A JP 2015-117403 A International Publication No. 2016 / 013145

[0009] The present invention advantageously solves the above-mentioned problems associated with the prior art, and aims to provide a high-strength steel sheet having bending workability suitable as a material for automobile parts.

[0010] The present inventors conducted extensive research to solve problems related to the bendability of ultra-high strength steel sheets. First, the present inventors, referring to conventional knowledge, manufactured steel sheets having a soft layer on the surface and investigated the bendability. All steel sheets having a soft layer on the surface showed improved bendability. It was found that lowering the average hardness of the soft layer and increasing the thickness of the soft layer generally improved the bendability and decreased the bending load. However, as a result of further detailed research, the present inventors realized that when softening the surface layer using a variety of methods, simply adjusting the average hardness or thickness of the soft layer on the surface did not sufficiently improve the bendability of the steel sheet or significantly decreased the bending load.

[0011] Therefore, the inventors conducted further detailed studies. As a result, they found that a multi-layer steel sheet obtained by welding a steel sheet having certain characteristics to one or both sides of a base material and hot rolling or annealing the steel sheet under specific conditions can most effectively improve bendability without deteriorating bending load. They also clarified that the main reason for the improvement in bendability achieved by the above method is the suppression of microscopic hardness variations in the soft layer. This effect is very remarkable, and sufficient bendability improvement was achieved even when the average hardness of the soft layer was high and the thickness of the soft layer was small, compared to when the hardness variations in the soft layer were large. This minimized the deterioration of tensile strength due to the soft layer, making it possible to achieve an unprecedented tensile strength, specifically, a tensile strength of 800 MPa or more, preferably 1100 MPa or more, and bendability at the same time. Although the mechanism of this effect is not completely clear, the following is thought to be the case. When the soft layer has hardness variations, it often contains multiple structures (ferrite, pearlite, bainite, martensite, retained austenite) and / or oxides. These second phases (or second structures) with different mechanical properties can cause strain and stress concentration during bending, potentially leading to the formation of voids that can become the origin of cracks. Therefore, it is believed that suppressing the hardness variation in the soft layer improved bendability. The inventors also discovered that bendability can be further improved by simultaneously suppressing the microscopic hardness variation in the surface soft layer and narrowing the hardness gradient in the thickness direction in the transition region from the surface soft layer to the internal hard layer (hereinafter referred to as the transition zone). When the hardness gradient in the transition zone between the soft layer and the hard layer is steep, the plastic deformation amounts of the soft layer and the hard layer differ significantly, increasing the likelihood of fracture in the transition zone. Therefore, it is believed that suppressing the microscopic hardness variation in the soft layer and narrowing the hardness gradient in the thickness direction in the transition zone between the soft layer and the hard layer improved bendability.

[0012] The hardness variation in the area other than the softened surface (hereinafter referred to as the hard layer) did not affect the bendability. Therefore, DP steel and TRIP (Transformation Induced Plasticity) steel, which have excellent ductility and have been considered to be disadvantageous in bendability, can be used for the hard layer. One of the advantages of the present invention is that it is possible to achieve both ductility in addition to tensile strength and bendability.

[0013] The gist of the present invention thus obtained is as follows: (1) A high-strength steel plate having a tensile strength of 800 MPa or more, including a thickness center and softened surface zones located on one or both sides of the thickness center, wherein each softened surface zone has a thickness of more than 10 μm and not more than 30% of the thickness, the average Vickers hardness of the softened surface zones is more than 0.60 times but not more than 0.90 times the average Vickers hardness at half the thickness position, and the standard deviation of the nanohardness of the softened surface zones is 0.8 or less. (2) The high-strength steel plate according to (1) above, further including hardness transition zones formed adjacent to and between the thickness center and each softened surface zone, wherein the average change in hardness in the thickness direction of the hardness transition zones is 5000 (ΔHv / mm) or less. (3) The high-strength steel plate according to (1) or (2) above, wherein the thickness center contains retained austenite at an area fraction of 10% or more. (4) The high-strength steel sheet according to any one of (1) to (3), characterized in that the sheet thickness center portion contains, by mass%, C: 0.05 to 0.8%, Si: 0.01 to 2.50%, Mn: 0.010 to 8.0%, P: 0.1% or less, S: 0.05% or less, Al: 0 to 3%, and N: 0.01% or less, with the balance consisting of iron and inevitable impurities. (5) The high-strength steel sheet according to (4), characterized in that the sheet thickness center portion further contains, by mass%, at least one element selected from the group consisting of Cr: 0.01 to 3%, Mo: 0.01 to 1%, and B: 0.0001% to 0.01%. (6) The high-strength steel sheet according to (4) or (5) above, characterized in that the thickness center portion further contains, by mass%, at least one element selected from the group consisting of Ti: 0.01 to 0.2%, Nb: 0.01 to 0.2%, and V: 0.01 to 0.2%. (7) The high-strength steel sheet according to any one of (4) to (6) above, characterized in that the thickness center portion further contains, by mass%, at least one element selected from the group consisting of Cu: 0.01 to 1%, and Ni: 0.01 to 1%. (8) The high-strength steel sheet according to any one of (4) to (7) above, characterized in that the C content in the softened surface portion is 0.30 to 0.90 times the C content in the thickness center portion.(9) The high-strength steel sheet according to any one of (5) to (8), characterized in that the sum of the Mn amount, Cr amount, and Mo amount in the softened surface portion is 0.3 times or more the sum of the Mn amount, Cr amount, and Mo amount in the center portion of the sheet thickness. (10) The high-strength steel sheet according to any one of (5) to (9), characterized in that the B amount in the softened surface portion is 0.3 times or more the sum of the B amount in the center portion of the sheet thickness. (11) The high-strength steel sheet according to any one of (7) to (10), characterized in that the sum of the Cu amount and Ni amount in the softened surface portion is 0.3 times or more the sum of the Cu amount and Ni amount in the center portion of the sheet thickness. (12) The high-strength steel sheet according to any one of (1) to (11), characterized in that the surface of the softened surface portion further comprises a hot-dip galvanized layer, a galvannealed hot-dip galvanized layer, or an electrogalvanized layer.

[0014] The high-strength steel sheet of the present invention has excellent bending workability suitable for use as a material for automobile parts. Therefore, the high-strength steel sheet of the present invention can be preferably used as a material for automobile parts. In addition, when the high-strength steel sheet includes a hardness transition zone between the thickness center portion and the surface softened portion, where the average hardness change in the thickness direction is 5000 (ΔHv / mm) or less, bending workability can be further improved. Furthermore, when the thickness center portion contains retained austenite at an area fraction of 10% or more, ductility can be improved in addition to bending workability.

[0015] 1 is a graph showing a change in dislocation density after a rolling pass in rough rolling used in a method for producing a high-strength steel plate according to a preferred embodiment of the present invention;

[0016] Hereinafter, embodiments of the present invention will be described, but the present invention is not limited to the following embodiments.

[0017] In the steel sheet according to the present invention, the average Vickers hardness of the surface-softened portion having a thickness of more than 10 μm to 30% or less of the sheet thickness, more specifically, the average Vickers hardness of the entire surface-softened portion, must be more than 0.60 times but not more than 0.90 times the average Vickers hardness at the half-sheet thickness position. If the thickness of the surface-softened portion is 10 μm or less, sufficient improvement in bendability cannot be obtained, and if it is more than 30%, significant deterioration in tensile strength occurs. The thickness of the surface-softened portion is more preferably 20% or less of the sheet thickness, and even more preferably 10% or less. If the average Vickers hardness of the surface-softened portion is more than 0.90 times the average Vickers hardness at the half-sheet thickness position, sufficient improvement in bendability cannot be obtained.

[0018] In the present invention, the "average Vickers hardness of the surface-softened portion" is determined as follows. First, the Vickers hardness is measured at a certain thickness direction position from the 1 / 2 position toward the surface at regular intervals in the thickness direction (for example, every 5% of the thickness, or every 1% or 0.5% as necessary) with an indentation load of 100 g. Next, the Vickers hardness is measured at a total of three or more points, for example, five or ten points, on a line perpendicular to the thickness direction and parallel to the rolling direction from that position with an indentation load of 100 g, and the average value is taken as the average Vickers hardness at that thickness direction position. Note that the distance between each measurement point in the thickness direction and rolling direction is preferably at least four times the distance of the indentation, if possible. In this specification, "a distance of at least four times the distance of the indentation" means a distance of at least four times the length of the diagonal line of the rectangular opening of the indentation made by the diamond indenter during Vickers hardness measurement. When the average Vickers hardness at a certain position in the thickness direction is 0.90 times or less of the average Vickers hardness at a similarly measured position at 1 / 2 the thickness, the surface side of that position is defined as a surface-softened portion. The Vickers hardness is measured at 10 random points within the surface-softened portion defined in this way, and the average value is calculated to determine the average Vickers hardness of the surface-softened portion. The average Vickers hardness of the surface-softened portion is more than 0.60 times and less than 0.90 times the average Vickers hardness at the 1 / 2 the thickness position, which further improves bendability. It is more preferably more than 0.60 times and less than 0.85 times, and even more preferably more than 0.60 times and less than 0.80 times.

[0019] The standard deviation of the nanohardness of the surface-softened portion must be 0.8 or less. This is because, as mentioned above, suppressing the hardness variation in the surface-softened portion significantly improves bendability. If the standard deviation is greater than 0.8, this effect is insufficient. From this perspective, the standard deviation is more preferably 0.6 or less, and even more preferably 0.4 or less. Although no lower limit for the standard deviation is specified, it is technically difficult to achieve a standard deviation of 0.05 or less. What affects bendability is the microscopic hardness variation in the surface-softened portion, particularly in the direction perpendicular to the thickness of the plate. Even if there is a gradual hardness gradient in the thickness direction within the surface-softened portion, the effects of the present invention are not impaired. Therefore, the standard deviation of the nanohardness must be measured at a position perpendicular to the thickness direction at a certain position in the thickness direction. In the present invention, the "standard deviation of the nanohardness of the surface-softened portion" refers to the standard deviation obtained from the histogram of nanohardness obtained by measuring the nanohardness at a total of 100 locations at 3 μm intervals using a Tribo-900 from Hysitron Corporation at a position halfway through the thickness of the surface-softened portion defined above, on a line perpendicular to the plate thickness direction and parallel to the rolling direction, with a Berkovich-shaped diamond indenter at an indentation depth of 80 nm.

[0020] In order to further improve the bendability of high-strength steel sheets, it is preferable that the average change in hardness in the thickness direction of the hardness transition zone is 5,000 (ΔHv / mm) or less. In the present invention, the "hardness transition zone" is defined as follows: First, the Vickers hardness is measured at a certain position in the thickness direction from the half-thickness position toward the surface at regular intervals in the thickness direction (for example, every 5% of the thickness, or every 1% or 0.5% as necessary) with an indentation load of 100 g. Next, the Vickers hardness is measured at three or more points, for example, five or ten points, on a line perpendicular to the thickness direction and parallel to the rolling direction from that position, also with an indentation load of 100 g, and the average value of these measurements is taken as the average Vickers hardness at that position in the thickness direction. It is preferable that the distance between each measurement point in the thickness direction and the rolling direction be at least four times the distance of the indentation, if possible. When the average Vickers hardness at a certain position in the thickness direction is 0.95 times or less of the average Vickers hardness measured similarly at a position halfway through the thickness direction, the region from that position to the previously defined softened surface portion is defined as the hardness transition zone.

[0021] The average hardness change (ΔHv / mm) in the plate thickness direction of the hardness transition zone is defined by the following formula: Average hardness change (ΔHv / mm) = (maximum average Vickers hardness in the hardness transition zone) - (minimum average Vickers hardness in the hardness transition zone) / thickness of the hardness transition zone

[0022] Here, the maximum average Vickers hardness of the hardness transition zone is the largest value among the average Vickers hardnesses at each position in the plate thickness direction within the hardness transition zone, and the minimum average Vickers hardness of the hardness transition zone is the smallest value among the average Vickers hardnesses at each position in the plate thickness direction within the hardness transition zone.

[0023] If the average change in hardness in the thickness direction of the hardness transition zone is greater than 5000 (ΔHv / mm), bendability may decrease. It is preferably 4000 (ΔHv / mm) or less, more preferably 3000 (ΔHv / mm) or less, and most preferably 2000 (ΔHv / mm) or less. The thickness of the hardness transition zone is not specified. However, if the hardness transition zone accounts for a large proportion of the thickness, the tensile strength decreases, so the hardness transition zone is preferably 20% or less of the thickness on one side, more preferably 10% or less.

[0024] In order to prevent deterioration of the bending load of high-strength steel sheets, the average Vickers hardness of the surface-softened portion must be more than 0.60 times the average Vickers hardness at the half-thickness position. If the hardness is less than 0.60 times, the surface-softened portion deforms significantly during bending, causing the center of the sheet thickness to move outward, resulting in early cracking, and the bending load will be significantly reduced. Note that the bending load referred to here refers to the maximum load obtained by taking a 60 mm x 60 mm test piece from the steel sheet and conducting a bending test in accordance with the German Association of the Automotive Industry (VDA) standard 238-100 under the following conditions: a punch curvature of 0.4 mm, a roll diameter of 30 mm, a roll distance of 2 x sheet thickness + 0.5 (mm), and a maximum indentation stroke of 11 mm.

[0025] Figure 1 shows an example of the hardness distribution of a high-strength steel plate according to a preferred embodiment of the present invention. The hardness distribution is shown from the surface to the 1 / 2 position of a 1 mm thick steel plate. The horizontal axis represents the position (mm) in the thickness direction, with 0 mm at the surface and 0.5 mm at the 1 / 2 position. The vertical axis represents the five-point average of the Vickers hardness at each position in the thickness direction. The Vickers hardness at the 1 / 2 position is 430 Hv, and the surface side of the point where the Vickers hardness is 0.90 times or less is the surface softened zone, while the range between the point where the Vickers hardness is 0.95 times or less and the surface softened zone is the hardness transition zone.

[0026] In order to improve the ductility of high-strength steel sheets, the thickness center portion preferably contains retained austenite at an area fraction of 10% or more. This is because the transformation-induced plasticity of the retained austenite improves ductility, and a ductility of 15% or more can be obtained with an area fraction of retained austenite of 10% or more. By utilizing the effect of this retained austenite, a ductility of 15% or more can be ensured even in the absence of soft ferrite, thereby promoting the enhancement of strength in the thickness center portion and achieving both high strength and high ductility. Note that the ductility referred to here refers to the total elongation obtained by taking a Japanese Industrial Standards (JIS) No. 5 test piece from a steel sheet perpendicular to the rolling direction and conducting a tensile test in accordance with JIS Z2241.

[0027] Next, the chemical composition at the center of the plate thickness that is desirable for achieving the effects of the present invention will be described. Note that "%" regarding the content of an element means "mass %" unless otherwise specified. Note that, in the center of the plate thickness, the chemical composition near the boundary with the surface-softened zone may differ from that at a position sufficiently distant from the boundary due to diffusion of alloying elements with the surface-softened zone. For example, when the high-strength steel plate of the present invention includes the above-mentioned hardness transition zone, the chemical composition near the boundary with the hardness transition zone may differ from that at a position sufficiently distant from the boundary in the center of the plate thickness. In such cases, the chemical composition measured near the half-thickness position is defined below.

[0028] "C: 0.05 to 0.8%" C increases the strength of steel sheets and is added to increase the strength of high-strength steel sheets. However, if the C content exceeds 0.8%, the toughness becomes insufficient. Also, if the C content is less than 0.05%, the strength becomes insufficient. The C content is preferably in the range of 0.6% or less, and more preferably in the range of 0.5% or less.

[0029] "Si: 0.01 to 2.50%" Si is a ferrite stabilizing element that increases the Ac3 transformation point, thereby enabling the formation of a large amount of ferrite over a wide annealing temperature range. Therefore, Si is added from the viewpoint of improving structural controllability. To achieve this effect, the Si content needs to be 0.01% or more. On the other hand, from the viewpoint of ensuring ductility, if the Si content is less than 0.30%, a large amount of coarse iron-based carbides is generated, making it impossible to achieve a retained austenite fraction of 10% or more in the internal microstructure, which may result in reduced elongation. From this viewpoint, the lower limit of Si is preferably 0.30% or more, and more preferably 0.50% or more. In addition, Si is an element necessary for suppressing the coarsening of iron-based carbides in the center portion of the sheet thickness and for improving strength and formability. Furthermore, Si needs to be added as a solid-solution strengthening element to contribute to increasing the strength of the steel sheet. From these viewpoints, the lower limit of Si is preferably 1% or more, and more preferably 1.2% or more. However, if the Si content exceeds 2.50%, the center of the plate thickness becomes embrittled and ductility deteriorates, so the upper limit is set to 2.50%. From the viewpoint of ensuring ductility, the Si content is preferably 2.20% or less, and more preferably 2.00% or less.

[0030] "Mn: 0.010 to 8.0%" Mn is added to increase the strength of high-strength steel sheets. To achieve this effect, the Mn content needs to be 0.010% or more. However, if the Mn content exceeds 8.0%, the hardness distribution in the surface layer of the steel sheet due to Mn segregation becomes large. From this perspective, the Mn content is preferably 5.0% or less, more preferably 4.0%, and even more preferably 3.0% or less.

[0031] "P: 0.1% or less" P tends to segregate in the center of the steel plate thickness, embrittling the welded joint. If the P content exceeds 0.1%, the embrittlement of the welded joint becomes significant, so the appropriate range is limited to 0.1% or less. There is no lower limit for the P content, but it is economically disadvantageous to set it to less than 0.001%.

[0032] "S: 0.05% or less" S has an adverse effect on weldability and manufacturability during casting and hot rolling. For this reason, the upper limit is set to 0.05% or less. There is no lower limit for the S content, but setting it to less than 0.0001% is economically disadvantageous.

[0033] "Al: 0 to 3%" Al acts as a deoxidizer and is preferably added in the deoxidation process. To achieve this effect, the Al content must be 0.01% or more. On the other hand, if the Al content exceeds 3%, the risk of slab cracking during continuous casting increases.

[0034] "N: 0.01% or less" N forms coarse nitrides and deteriorates bendability, so the amount added must be kept low. This tendency becomes more pronounced when N exceeds 0.01%, so the N content range is set to 0.01% or less. In addition, N causes blowholes during welding, so a lower N content is better. The effects of the present invention can be achieved without specifying a lower limit for the N content, but setting the N content to less than 0.0005% results in a significant increase in manufacturing costs, so this is the substantial lower limit.

[0035] "At least one selected from the group consisting of Cr: 0.01 to 3%, Mo: 0.01 to 1%, and B: 0.0001 to 0.01%" Cr, Mo, and B are elements that contribute to improving strength and can be used in place of part of Mn. It is preferable that one or more of Cr, Mo, and B are contained in an amount of 0.01% or more, 0.01% or more, and 0.0001% or more, respectively. On the other hand, if the content of each element is too high, pickling property, weldability, hot workability, etc. may be deteriorated, so the contents of Cr, Mo, and B are preferably 3% or less, 1% or less, and 0.01% or less, respectively.

[0036] "At least one selected from the group consisting of Ti: 0.01-0.2%, Nb: 0.01-0.2%, and V: 0.01-0.2%" Ti, Nb, and V are strengthening elements. They contribute to increasing the strength of steel sheet through precipitation strengthening, fine grain strengthening by inhibiting ferrite grain growth, and dislocation strengthening through inhibiting recrystallization. When added for this purpose, it is preferable to add 0.01% or more. However, if the content of each element exceeds 0.2%, the precipitation of carbonitrides increases, deteriorating formability.

[0037] "At least one selected from the group consisting of Cu: 0.01 to 1% and Ni: 0.01 to 1%" Cu and Ni are elements that contribute to improving strength and can be used in place of part of Mn. It is preferable that one or both of Cu and Ni are contained in an amount of 0.01% or more. On the other hand, if the content of each element is too high, pickling properties, weldability, hot workability, etc. may be deteriorated, so the content of Cu and Ni is preferably 1.0% or less.

[0038] Furthermore, the effects of the present invention are not impaired even if the following elements are intentionally or unavoidably added to the center portion of the sheet thickness: O: 0.001 to 0.02%, W: 0.001 to 0.1%, Ta: 0.001 to 0.1%, Sn: 0.001 to 0.05%, Sb: 0.001 to 0.05%, As: 0.001 to 0.05%, Mg: 0.0001 to 0.05%, Ca: 0.001 to 0.05%, Zr: 0.001 to 0.05%, and rare-earth metals (REMs) such as Y: 0.001 to 0.05%, La: 0.001 to 0.05%, and Ce: 0.001 to 0.05%.

[0039] The steel sheet of the present invention may have different chemical compositions between the surface-softened portion and the center portion of the sheet thickness. As will be described later, an important point of the present invention is to make the surface layer almost entirely of a low-temperature transformation structure (bainite, martensite, etc.) and to suppress ferrite and pearlite transformation, thereby reducing hardness variation. In such cases, the preferred chemical composition of the surface-softened portion is as follows.

[0040] "C: 0.30 to 0.90 times, and 0.72% or less, the amount of C in the center of the plate thickness" C increases the strength of the steel plate and is added to increase the strength of high-strength steel plate. The amount of C in the surface-softened portion is preferably 0.90 times or less the amount of C in the center of the plate thickness. This is to make the hardness of the surface-softened portion lower than the hardness of the center of the plate thickness. If it is more than 0.90 times, the average Vickers hardness of the surface-softened portion may not be 0.90 times or less the average Vickers hardness at the half-thickness position. More preferably, the amount of C in the surface-softened portion is 0.80 times or less the amount of C in the center of the plate thickness, and even more preferably, it is 0.70 times or less. The amount of C in the surface-softened portion needs to be 0.30 times or more the amount of C in the center of the plate thickness. If it is less than 0.30 times, the average Vickers hardness of the surface-softened portion may not exceed 0.60 times the average Vickers hardness at the half-thickness position. When the C content in the surface-softened portion is 0.90 times or less the C content in the center portion of the plate thickness, the preferred C content in the center portion of the plate thickness is 0.8% or less, and therefore the preferred C content in the surface-softened portion is 0.72% or less. It is preferably 0.5% or less, more preferably 0.3% or less, and most preferably 0.1% or less. There is no particular lower limit for the C content. When using industrial ultra-low C steel, the substantial lower limit is about 0.001%, but from the viewpoint of the amount of solute C, it is also possible to use interstitial-free steel in which solute C is completely eliminated by using Ti, Nb, etc.

[0041] "Si: 0.01 to 2.5%" Si is an element that suppresses temper softening of martensite, and adding it can suppress the decrease in strength due to tempering. To achieve this effect, the Si content must be 0.01% or more. However, adding more than 2.5% of Si reduces toughness, so the content is set to 2.5% or less.

[0042] "Mn: 0.01 to 8.0%" Mn is added to increase the strength of high-strength steel sheets. To achieve this effect, the Mn content needs to be 0.01% or more. However, if the Mn content exceeds 8.0%, the hardness distribution in the surface layer of the steel sheet due to Mn segregation becomes large. From this perspective, the Mn content is preferably 5% or less, and more preferably 3% or less.

[0043] In addition, the sum of the Mn, Cr, and Mo amounts in the surface-softened portion is preferably 0.3 times or more the sum of the Mn, Cr, and Mo amounts in the center of the sheet thickness. As will be described later, the surface-softened portion has a low-temperature transformation structure (such as bainite and martensite) as the majority of its structure, thereby reducing hardness variation. If the sum of the Mn, Cr, and Mo amounts that improve hardenability is less than 0.3 times the sum of the Mn, Cr, and Mo amounts in the center of the sheet thickness, ferrite transformation is likely to occur, causing hardness variation. The sum is more preferably 0.5 times or more, and even more preferably 0.7 times or more. No upper limit is specified for each.

[0044] "P: 0.1% or less" P embrittles welds. If the content exceeds 0.1%, embrittlement of welds becomes significant, so the appropriate range is limited to 0.1% or less. There is no lower limit for the P content, but it is economically disadvantageous to set it to less than 0.001%.

[0045] "S: 0.05% or less" S has an adverse effect on weldability and manufacturability during casting and hot rolling. For this reason, the upper limit is set to 0.05% or less. There is no lower limit for the S content, but setting it to less than 0.0001% is economically disadvantageous.

[0046] "Al: 0 to 3%" Al acts as a deoxidizer and is preferably added in the deoxidation process. To achieve this effect, the Al content must be 0.01% or more. On the other hand, if the Al content exceeds 3%, the risk of slab cracking during continuous casting increases.

[0047] "N: 0.01% or less" N forms coarse nitrides and deteriorates bendability, so the amount added must be kept low. This tendency becomes more pronounced when N exceeds 0.01%, so the N content range is set to 0.01% or less. In addition, N causes blowholes during welding, so a lower N content is better. The effects of the present invention can be achieved without specifying a lower limit for the N content, but setting the N content to less than 0.0005% results in a significant increase in manufacturing costs, so this is the substantial lower limit.

[0048] "At least one selected from the group consisting of Cr: 0.01 to 3%, Mo: 0.01 to 1%, and B: 0.0001 to 0.01%" Cr, Mo, and B are elements that contribute to improving strength and can be used in place of a portion of Mn. It is preferable that one or more of Cr, Mo, and B are contained in an amount of 0.01% or more, 0.01% or more, and 0.0001% or more, respectively. On the other hand, if the content of each element is too high, pickling properties, weldability, hot workability, etc. may be deteriorated. Therefore, the contents of Cr, Mo, and B are preferably 3% or less, 1% or less, and 0.01% or less, respectively. In addition, there is a preferred range for the total amount of Cr and Mo with Mn, as described above.

[0049] Furthermore, the B content in the softened surface portion is preferably 0.3 times or more the B content in the center of the plate thickness. If the B content that improves hardenability is less than 0.3 times the B content in the center of the plate thickness, ferrite transformation is likely to occur, causing variations in hardness. It is more preferably 0.5 times or more, and even more preferably 0.7 times or more. No upper limit is specified.

[0050] "At least one selected from the group consisting of Ti: 0.01-0.2%, Nb: 0.01-0.2%, and V: 0.01-0.2%" Ti, Nb, and V are strengthening elements. They contribute to increasing the strength of steel sheet through precipitation strengthening, fine grain strengthening by inhibiting ferrite grain growth, and dislocation strengthening through inhibiting recrystallization. When added for this purpose, it is preferable to add 0.01% or more. However, if the content of each element exceeds 0.2%, the precipitation of carbonitrides increases, deteriorating formability.

[0051] "At least one selected from the group consisting of Cu: 0.01 to 1% and Ni: 0.01 to 1%" Cu and Ni are elements that contribute to improving strength and can be used in place of part of Mn. It is preferable that one or both of Cu and Ni are contained in an amount of 0.01% or more. On the other hand, if the content of each element is too high, pickling properties, weldability, hot workability, etc. may be deteriorated, so the content of Cu and Ni is preferably 1.0% or less.

[0052] Furthermore, it is preferable that the sum of the Cu and Ni contents in the surface-softened portion is 0.3 times or more the sum of the Cu and Ni contents in the center portion of the sheet thickness. If the sum of the Cu and Ni contents that improve hardenability is less than 0.3 times the sum of the Cu and Ni contents in the center portion of the sheet thickness, ferrite transformation is likely to occur, causing variations in hardness. It is more preferable that it is 0.5 times or more, and even more preferable that it is 0.7 times or more. There are no upper limits specified for each of these.

[0053] Furthermore, the effects of the present invention are not impaired even if the following elements are intentionally or unavoidably added to the surface-softened portion: O: 0.001 to 0.02%, W: 0.001 to 0.1%, Ta: 0.001 to 0.1%, Sn: 0.001 to 0.05%, Sb: 0.001 to 0.05%, As: 0.001 to 0.05%, Mg: 0.0001 to 0.05%, Ca: 0.001 to 0.05%, Zr: 0.001 to 0.05%, and rare-earth metals (REMs) such as Y: 0.001 to 0.05%, La: 0.001 to 0.05%, and Ce: 0.001 to 0.05%.

[0054] The effects of the present invention, i.e., excellent bending workability and / or ductility, can also be achieved when the surface of the surface-softened portion is subjected to hot-dip galvanizing, alloyed hot-dip galvanizing, or electrogalvanizing.

[0055] Next, a manufacturing method for obtaining the high-strength steel sheet of the present invention will be described. The following description is intended to merely exemplify the manufacturing method for obtaining the high-strength steel sheet of the present invention, and is not intended to limit the high-strength steel sheet of the present invention to a multi-layer steel sheet in which two steel sheets are laminated as described below. For example, it is also possible to produce a high-strength steel sheet consisting of a softened surface layer and a center portion in the thickness direction by decarburizing a single-layer steel sheet to soften its surface layer.

[0056] An important point of the present invention is to reduce the variation in hardness of the surface layer. The variation in hardness of the surface layer increases when the surface layer contains both a relatively soft structure such as ferrite or pearlite and a low-temperature transformed structure (bainite or martensite). In the following manufacturing method, a method for forming the surface layer into a mostly low-temperature transformed structure will be described.

[0057] A surface layer steel sheet is laminated on one or both sides of a base steel sheet whose surface has been degreased and satisfies the above-mentioned composition requirements at the center of the sheet thickness.

[0058] The high-strength steel sheet according to the present invention, more specifically, a hot-rolled steel sheet, a cold-rolled steel sheet, or a plated steel sheet, can be obtained by subjecting the above-mentioned laminate (multilayer steel sheet) to hot rolling, cold rolling, continuous annealing, continuous hot-dip plating, or the like.

[0059] For example, a method for producing a hot-rolled steel sheet among high-strength steel sheets included in the present invention includes the steps of: forming a multi-layer steel sheet by laminating a surface layer steel sheet, which has the above-described chemical composition, on one or both sides of a base steel sheet, which has the above-described chemical composition and constitutes a center portion of the plate thickness; The hot rolling process is characterized by including a hot rolling step in which the multilayer steel plate is heated at a heating temperature of 1100°C or higher and 1350°C or lower, preferably higher than 1150°C and 1350°C or lower, and then hot-rolled, the hot rolling step including rough rolling and finish rolling at a finish temperature of 800 to 980°C, and the rough rolling is carried out two or more times under conditions of a rough rolling temperature of 1100°C or higher, a plate thickness reduction rate per pass of 5% or higher and less than 50%, and an inter-pass time of 3 seconds or longer, and a step in which the hot-rolled multilayer steel plate is cooled to a temperature of 750°C to 550°C at an average cooling rate of 2.5°C / s or higher during a cooling process, and then coiled at a coiling temperature of 550°C or lower.

[0060] When elements are diffused between the base steel sheet and the surface layer steel sheet to form a hardness transition zone between them in which the average hardness change in the sheet thickness direction is 5000 (ΔHv / mm) or less, it is preferable to heat the multilayer steel sheet at a heating temperature of 1100°C or more and 1350°C or less for 2 hours or more in the hot rolling process, and it is more preferable to heat it at a temperature higher than 1150°C and 1350°C or less for 2 hours or more.

[0061] In order to increase the area fraction of retained austenite in the center of the plate thickness of a high-strength steel plate to 10% or more and improve the ductility of the high-strength steel plate, it is preferable to include, instead of the process after hot rolling specified above, a process of holding the hot-rolled multilayer steel plate at a temperature of 700°C to 500°C for 3 seconds or more in the cooling process, and then coiling the plate at a temperature of not less than the martensitic transformation start temperature Ms and not more than the bainite transformation start temperature Bs of the base steel plate. Here, Bs (°C) = 820 - 290C / (1 - Sf) - 37Si - 90Mn - 65Cr - 50Ni + 70Al Ms (°C) = 541 - 474C / (1 - Sf) - 15Si - 35Mn - 17Cr - 17Ni + 19Al Here, C, Si, Mn, Cr, Ni and Al are the contents [mass%] of each element in the base steel plate, and Sf is the area fraction of ferrite in the base steel plate.

[0062] Each step will be explained in more detail. When obtaining a hot-rolled steel sheet, first, the multilayer steel sheet produced by the above method is heated at a heating temperature of 1100 ° C or higher, preferably above 1150 ° C and below 1350 ° C. In order to suppress the anisotropy of the crystal orientation caused by casting, it is preferable to set the slab heating temperature to 1100 ° C or higher. On the other hand, since heating the slab to a temperature above 1350 ° C requires the input of a large amount of energy, which significantly increases the manufacturing cost, the heating temperature is set to 1350 ° C or lower. In addition, in order to control the standard deviation of the nanohardness of the surface softened portion to 0.8 or less, and further, in order to impart a gradual change in hardness to the hardness transition zone if one exists, it is necessary to control the concentration of alloy elements, particularly C atoms, so that they are distributed gradually. The C concentration distribution is obtained by the diffusion of C atoms, and the diffusion frequency of C atoms increases at higher temperatures. Therefore, in order to control the C concentration, control from hot rolling heating to rough rolling is important. In hot rolling, the heating temperature must be increased to promote the diffusion of C atoms, preferably 1100°C or higher and 1350°C or lower, more preferably higher than 1150°C and 1350°C or lower. In hot rolling, the changes (i) and (ii) shown in FIG. 2 occur. (i) is the diffusion of C atoms from the center of the sheet thickness to the softened surface, and (ii) is a decarburization reaction of C desorbed from the softened surface to the outside. The balance between the diffusion and desorption reactions of C atoms (i) and (ii) results in a distribution of C concentration. At temperatures below 1100°C, the reaction (i) is insufficient, making it impossible to obtain a desirable C concentration distribution. On the other hand, at temperatures above 1350°C, the reaction (ii) occurs excessively, making it impossible to obtain a desirable concentration distribution.

[0063] Furthermore, in order to obtain an optimal C concentration distribution after controlling the C concentration distribution to a desirable level by adjusting the hot rolling heating temperature, pass control during rough rolling is extremely important. Rough rolling is performed at least twice under the following conditions: a rough rolling temperature of 1100°C or higher, a thickness reduction rate per pass of 5% to less than 50%, and an inter-pass time of 3 seconds or more. This is because the strain introduced by rough rolling promotes the diffusion of C atoms (i) in Figure 2. If a slab whose C concentration has been controlled to a desirable level by hot rolling heating is rough rolled and finish rolled using a conventional method, the thickness will decrease without the C atoms being able to sufficiently diffuse in the surface-softened portion. Therefore, if a hot-rolled steel sheet having a thickness of several mm is produced from a slab having a thickness of more than 200 mm using conventional hot rolling, the C concentration will change rapidly in the surface-softened portion, making it impossible to obtain a gradual change in hardness. The above-mentioned rough rolling pass control was discovered as a solution to this problem. The diffusion of C atoms is greatly affected not only by temperature but also by strain (dislocation density). In particular, the diffusion frequency in dislocation diffusion is more than 10 times higher than in lattice diffusion. Therefore, it is necessary to devise a way to thin the sheet thickness by rolling while maintaining the dislocation density. Curve 1 in Figure 3 shows the change in dislocation density after a rolling pass when the thickness reduction rate per pass of rough rolling is small, and it can be seen that strain remains for a long time. By leaving strain in the surface-softened portion for such a long time, C atoms in the surface-softened portion can sufficiently diffuse, making it possible to obtain an optimal C concentration distribution. On the other hand, curve 2 shows the change in dislocation density when the thickness reduction rate is large. As the amount of strain introduced by rolling increases, recovery is more likely to be promoted, and the dislocation density rapidly decreases. Therefore, in order to obtain an optimal C concentration distribution, it is necessary to prevent changes in dislocation density such as those shown in curve 2. From this perspective, the upper limit of the thickness reduction rate per pass is less than 50%. In addition, in order to promote the diffusion of C atoms in the surface-softened portion, it is necessary to ensure a certain amount of dislocation density and holding time, so the lower limit of the sheet thickness reduction rate is 5%, and it is necessary to ensure an inter-pass time of 3 seconds or more.

[0064] Furthermore, when forming a hardness transition zone, the heating time of the slab is set to 2 hours or more. This is to allow elements to diffuse between the base steel sheet and the surface layer steel sheet during slab heating, thereby reducing the average hardness change in the hardness transition zone formed between them. If the heating time is shorter than 2 hours, the average hardness change in the hardness transition zone will not be sufficiently small. There is no upper limit to the heating time, but heating for 8 hours or more requires a large amount of heating energy and is not preferable from a cost perspective.

[0065] After heating the slab, hot rolling is performed. If the completion temperature (finishing temperature) of hot rolling is less than 800°C, the rolling reaction force increases, making it difficult to stably obtain the specified plate thickness. For this reason, the completion temperature of hot rolling is set to 800°C or higher. On the other hand, if the completion temperature of hot rolling is set to more than 980°C, a device for heating the steel plate is required in the process from the end of slab heating to the completion of hot rolling, which requires high costs, so the completion temperature of hot rolling is set to 980°C or lower.

[0066] Thereafter, in the cooling process, the temperature is cooled from 750°C to 550°C at an average cooling rate of 2.5°C / s or more. This is an important condition in the present invention, and is a necessary process for converting most of the surface-softened portion into a low-temperature transformed structure and reducing hardness variation. If the average cooling rate is slower than 2.5°C / s, ferrite transformation or pearlite transformation occurs in the surface-softened portion, causing hardness variation. The average cooling rate is preferably 5°C / s or more, more preferably 10°C / s or more. Since ferrite transformation or pearlite transformation is unlikely to occur at temperatures higher than 750°C, the average cooling rate is not specified. Since transformation to a low-temperature transformed structure occurs at temperatures lower than 550°C, the average cooling rate is not specified.

[0067] The coiling temperature is set to 550° C. or less. At temperatures higher than 550° C., ferrite transformation or pearlite transformation occurs in the softened surface portion, causing variations in hardness. The coiling temperature is preferably 500° C. or less, and more preferably 300° C. or less.

[0068] On the other hand, in order to increase the area fraction of retained austenite in the center of the plate thickness of a high-strength steel plate to 10% or more and improve the ductility of the high-strength steel plate, the plate is held at a temperature between 700°C and 500°C for 3 seconds or more during the cooling process after the hot rolling. This is an important condition in the present invention and is a necessary step for transforming only the soft surface layer to ferrite and reducing hardness variation. At temperatures of 700°C or higher, ferrite transformation is delayed, making it impossible to transform the surface layer into ferrite. At temperatures of 500°C or lower, a portion of the surface layer becomes a low-temperature transformed structure. Since the presence of multiple structures of ferrite and low-temperature transformed structures causes hardness variation in the surface layer, the holding temperature is set to 500°C or higher. The holding time is set to 3 seconds or longer. Holding for 3 seconds or longer is necessary to sufficiently promote ferrite transformation in the surface layer. Preferably, the holding time is set to 5 seconds or longer, more preferably 10 seconds or longer.

[0069] The coiling temperature is set to a temperature in the bainite transformation temperature range of the base steel sheet, i.e., a temperature above the martensitic transformation start temperature Ms and below the bainite transformation start temperature Bs of the base steel sheet. This is to produce bainite or martensite in the base steel sheet to produce high-strength steel and further to stabilize the retained austenite. In this way, one of the features of the present invention is that by changing the timing of the transformation of the base steel sheet and the steel sheet for the surface layer, a structure with small hardness variation can be obtained in the surface layer. In the present invention, the martensitic transformation start temperature Ms and the bainite transformation start temperature Bs are calculated by the following formulas. Bs (°C) = 820 - 290C / (1 - Sf) - 37Si - 90Mn - 65Cr - 50Ni + 70Al Ms (°C) = 541 - 474C / (1 - Sf) - 15Si - 35Mn - 17Cr - 17Ni + 19Al Here, C, Si, Mn, Cr, Ni and Al are the contents [mass%] of each element in the base steel plate, and Sf is the area fraction of ferrite in the base steel plate.

[0070] Since it is difficult to determine the area fraction of ferrite during the production of a steel sheet, in the present invention, in calculating Bs and Ms, a cold-rolled sheet before entering the annealing process is sampled and annealed with the same temperature history as in the annealing process, and the determined area fraction of ferrite is used.

[0071] Next, a method for obtaining a cold-rolled steel sheet among the high-strength steel sheets included in the present invention will be described. The method for producing the cold-rolled steel sheet comprises the steps of forming a multi-layer steel sheet by laminating a surface layer steel sheet, which has the above-described chemical composition, on one or both sides of a base steel sheet, which has the above-described chemical composition and constitutes a center portion of the sheet thickness, and which similarly has the above-described chemical composition, to form a surface layer softened portion; The method is characterized by including a step of heating the multilayer steel plate at a heating temperature of 1100°C or higher and 1350°C or lower, preferably higher than 1150°C and 1350°C or lower, and then hot rolling and cold rolling, wherein the hot rolling includes rough rolling and finish rolling at a finishing temperature of 800 to 980°C, and the rough rolling is carried out two or more times under conditions of a rough rolling temperature of 1100°C or higher, a plate thickness reduction rate per pass of 5% or higher and less than 50%, and an inter-pass time of 3 seconds or longer, and a step of holding the rolled multilayer steel plate at a temperature of the Ac3 point of the surface layer steel plate −50°C or higher and the Ac3 point of the base steel plate −50°C or higher and 900°C or lower for 5 seconds or longer, and then cooling from 750°C to 550°C or lower at an average cooling rate of 2.5°C / s or higher. Here, Ac3 = 910 - 203√C + 44.7Si - 30Mn + 700P - 20Cu - 15.2Ni - 11Cr + 31.5Mo + 400Ti + 104V + 400Al (Formula 1), where C, Si, Mn, P, Cu, Ni, Cr, Mo, Ti, V, and Al are the contents [mass%] of each element.

[0072] Furthermore, when elements are diffused between the base steel sheet and the surface layer steel sheet to form a hardness transition zone between them in which the average hardness change in the sheet thickness direction is 5000 (ΔHv / mm) or less, it is preferable to heat the multilayer steel sheet at a heating temperature of 1100°C or more and 1350°C or less, or more than 1150°C and 1350°C or less, for 2 hours or more, and then hot rolled and cold rolled.

[0073] Furthermore, in order to increase the area fraction of retained austenite in the center of the plate thickness of a high-strength steel plate to 10% or more and improve the ductility of the high-strength steel plate, it is preferable to include a step of passing the rolled multilayer steel plate through a continuous annealing line and annealing it, instead of the step after cold rolling specified above, and the annealing in the continuous annealing line first holds the multilayer steel plate at a heating temperature of 700°C or more and 900°C or less for 5 seconds or more, and then, optionally, pre-cooling the multilayer steel plate from the heating temperature to a pre-cooling stop temperature of not less than the Bs point and not more than the Ac3 point - 20°C of the base steel plate so that it stays for not less than 5 seconds and not more than 400 seconds, Next, it is preferable to cool the multilayer steel plate at an average cooling rate of 10°C / s or more to a cooling stop temperature of Ms-100°C or more and less than Bs of the base steel plate, and then to hold the multilayer steel plate in a temperature range of Ms-100°C or more of the base steel plate for 30 seconds to 600 seconds. Ac3 (°C) = 910 - 203√C + 44.7Si - 30Mn + 700P - 20Cu - 15.2Ni - 11Cr + 31.5Mo + 400Ti + 104V + 400Al (Formula 1) Bs (°C) = 820 - 290C / (1 - Sf) - 37Si - 90Mn - 65Cr - 50Ni + 70Al (Formula 2) Ms (°C) = 541 - 474C / (1 - Sf) - 15Si - 35Mn - 17Cr - 17Ni + 19Al (Formula 3) Here, C, Si, Mn, P, Cu, Ni, Cr, Mo, Ti, V, and Al are the contents [mass%] of each element in the base steel plate, and Sf is the area fraction of ferrite in the base steel plate.

[0074] Each step will be described in more detail. First, the multilayer steel sheet produced by the above method is heated at a heating temperature of 1100°C or higher and 1350°C or lower, or higher than 1150°C and lower than 1350°C, as described in the method for producing a hot-rolled steel sheet. It is then hot-rolled and coiled, for example, at a coiling temperature of 20°C or higher and 700°C or lower. Next, the hot-rolled steel sheet produced in this manner is pickled. Pickling removes oxides from the surface of the hot-rolled steel sheet and may be performed once or multiple times. To form a hardness transition zone, it is preferable to first heat the multilayer steel sheet at a heating temperature of 1100°C or higher and 1350°C or lower, or higher than 1150°C and lower than 1350°C, for 2 hours or more. This is to diffuse elements between the base steel sheet and the surface layer steel sheet during heating, thereby reducing the average hardness change in the hardness transition zone formed between them. If the heating time is shorter than 2 hours, the average hardness change in the hardness transition zone will not be sufficiently small. Next, the hot-rolled steel sheet produced in this manner is subjected to pickling. Pickling is for removing oxides on the surface of the hot-rolled steel sheet, and may be performed once or in multiple steps.

[0075] In cold rolling, if the total reduction exceeds 85%, the ductility of the base steel sheet is lost and the risk of the base steel sheet breaking during cold rolling increases, so the total reduction is preferably 85% or less. On the other hand, in order to sufficiently promote recrystallization of the soft layer in the annealing step, the total reduction is preferably 20% or more, and more preferably 30% or more. Annealing may be performed at a temperature of 700°C or less in order to reduce the cold rolling load before cold rolling.

[0076] Next, annealing will be explained. In order to reduce the hardness variation in the surface-softened portion during annealing, it is important that most of the structure of the surface-softened portion is a low-temperature transformed structure and that ferrite transformation and pearlite transformation are suppressed. Note that, if the chemical composition of the steel sheet for the surface layer satisfies the above-mentioned appropriate range, the entire surface-softened portion will be a low-temperature transformed structure, and there is no concern that the average Vickers hardness of the surface-softened portion will be higher than 0.90 times the average Vickers hardness at the half-plate thickness position.

[0077] The steel plate for the surface layer is held at a temperature of at least Ac3 point minus 50°C and at least Ac3 point minus 50°C and 900°C or less for at least 5 seconds. The reason for setting the base steel plate at a temperature of at least Ac3 point minus 50°C is that by heating the base steel plate to the two-phase region of ferrite and austenite or the austenite single-phase region, a transformed structure is obtained by subsequent heat treatment, and the required strength is obtained. At temperatures lower than this, strength is significantly reduced. The reason for setting the steel plate for the surface layer at a temperature of at least Ac3 point minus 50°C is that by heating the surface layer to the two-phase region of ferrite and austenite or the austenite single-phase region, the majority of the surface layer becomes a low-temperature transformed structure by subsequent heat treatment, thereby reducing hardness variation. At temperatures lower than this, hardness variation increases. Heating to 900°C or higher is undesirable because it coarsens the prior γ grain size in the hard layer and deteriorates toughness.

[0078] The steel is then cooled from 750°C to 550°C or less at an average cooling rate of 2.5°C / s or more. This is an important condition in the present invention and is a necessary step for converting most of the surface-softened portion into a low-temperature transformed structure and reducing hardness variation. If the average cooling rate is slower than 2.5°C / s, ferrite transformation or pearlite transformation occurs in the surface-softened portion, causing hardness variation. The average cooling rate is preferably 5°C / s or more, and more preferably 10°C / s or more. Since ferrite transformation or pearlite transformation is unlikely to occur at temperatures higher than 750°C, the average cooling rate is not specified. Since the steel transforms into a low-temperature transformed structure at temperatures lower than 550°C, the average cooling rate is not specified.

[0079] At temperatures below 550°C, the material may be cooled to room temperature at a constant cooling rate, or may be held at a temperature of about 200°C to 550°C to promote bainite transformation or temper martensite. However, if the material is held at 300°C to 550°C for a long period of time, the strength may decrease, so if the material is held at that temperature, the holding time is preferably 600 seconds or less.

[0080] In order to increase the area fraction of retained austenite in the center of the thickness direction of a high-strength steel sheet to 10% or more and improve the ductility of the high-strength steel sheet, it is preferable to perform the following annealing and cooling instead of the annealing and cooling described above. First, in annealing, the steel sheet is heated to 700°C or more and 900°C or less, and held for 5 seconds or more. The reason for setting the temperature at 700°C or more is to sufficiently promote recrystallization in the softened layer, reduce the unrecrystallized fraction, and reduce hardness variation. Temperatures below 700°C increase hardness variation in the softened layer. Heating to 900°C or more is undesirable because it coarsens the prior γ grain size in the hard layer and deteriorates toughness. It is necessary to hold the steel sheet at the heating temperature for 5 seconds or more. If the holding time is 5 seconds or less, the austenite transformation of the base steel sheet will not progress sufficiently, resulting in a significant decrease in strength. Furthermore, the recrystallization of the softened layer will be insufficient, resulting in large hardness variation in the surface layer. From these perspectives, a holding time of 10 seconds or more is preferable. More preferably, it is 20 seconds or more.

[0081] Annealing is performed, for example, by passing the rolled multilayer steel sheet through a continuous annealing line. Here, annealing in the continuous annealing line includes first holding the multilayer steel sheet at a heating temperature of 700°C or higher and 900°C or lower for 5 seconds or longer, and then, optionally, pre-cooling the multilayer steel sheet from the heating temperature to a pre-cooling stop temperature that is equal to or higher than the Bs point of the base steel sheet and lower than the Ac3 point -20°C for 5 seconds or longer and shorter than 400 seconds. Such a pre-cooling step may be performed as needed, and the subsequent cooling step may be performed without the pre-cooling step.

[0082] Following an optional pre-cooling step, annealing in a continuous annealing line involves cooling the multilayer steel sheet at an average cooling rate of 10°C / s or more to a cooling stop temperature of at least Ms-100°C and less than Bs of the base steel sheet, and then dwelling the multilayer steel sheet in a temperature range of at least Ms-100°C of the base steel sheet, more preferably in a temperature range of at least 300°C and less than 500°C, for 30 to 600 seconds. During this dwell, heating and cooling may be performed multiple times as needed. This dwell time is important for stabilizing the retained austenite. If the required dwell time is less than 30 seconds, it is difficult to obtain 10% or more retained austenite. On the other hand, if it is 600 seconds or more, softening of the entire structure progresses, making it difficult to obtain sufficient strength. In the present invention, Ac3, Bs, and Ms are calculated using the following formulas. Ac3 (°C) = 910 - 203√C + 44.7Si - 30Mn + 700P - 20Cu - 15.2Ni - 11Cr + 31.5Mo + 400Ti + 104V + 400Al (Equation 1) Bs (°C) = 820 - 290C / (1 - Sf) - 37Si - 90Mn - 65Cr - 50Ni + 70Al Ms (°C) = 541 - 474C / (1 - Sf) - 15Si - 35Mn - 17Cr - 17Ni + 19Al Here, C, Si, Mn, P, Cu, Ni, Cr, Mo, Ti, V, and Al are the contents [mass%] of each element in the base steel plate, and Sf is the area fraction of ferrite in the base steel plate.

[0083] Since it is difficult to determine the area fraction of ferrite during the production of a steel sheet, in the present invention, in calculating Bs and Ms, a cold-rolled sheet before entering the annealing process is sampled and annealed with the same temperature history as in the annealing process, and the determined area fraction of ferrite is used.

[0084] When hot-dip galvanizing is then performed, the plating bath temperature may be the conventionally applied condition, for example, 440°C to 550°C. Furthermore, when hot-dip galvanizing is performed and a hot-dip galvannealed steel sheet is produced, the alloying heating temperature may be the conventionally applied condition, for example, 400°C to 600°C. The heating method for alloying is not particularly limited, and a heating method suitable for conventional hot-dip galvanizing equipment, such as direct heating by combustion gas, induction heating, or direct electrical heating, may be used.

[0085] After the alloying treatment, the steel sheet is cooled to 200°C or less and, if necessary, subjected to temper rolling.

[0086] In the case of producing an electrogalvanized steel sheet, for example, alkaline degreasing, water washing, pickling, and water washing are performed as pretreatments for plating, and then the pretreated steel sheet is subjected to an electrolytic treatment using a liquid circulation type electroplating apparatus in a plating bath containing zinc sulfate, sodium sulfate, and sulfuric acid at a current density of about 100 A / dm until a predetermined plating thickness is achieved.

[0087] Finally, the preferred chemical composition of the steel sheet for the surface layer will be described. In the steel sheet of the present invention, the chemical composition may differ between the softened surface layer and the center portion of the sheet thickness. In such cases, the preferred chemical composition of the steel sheet for the surface layer that constitutes the softened surface layer is as follows.

[0088] The C content of the surface layer steel plate is preferably 0.30 to 0.90 times the C content of the base steel plate. This is to make the hardness of the surface layer steel plate lower than that of the base steel plate. If it is more than 0.90 times, the average Vickers hardness of the softened surface portion in the finally obtained high-strength steel plate may not be 0.90 times or less the average Vickers hardness at the 1 / 2 position of the plate thickness. More preferably, the C content of the surface layer steel plate is 0.85 times or less, and even more preferably 0.80 times or less, the C content of the base steel plate.

[0089] The sum of the Mn, Cr, and Mo contents of the surface layer steel sheet is preferably 0.3 times or more the sum of the Mn, Cr, and Mo contents of the base steel sheet. If the sum of the Mn, Cr, and Mo contents that improve hardenability is less than 0.3 times the sum of the Mn, Cr, and Mo contents of the base steel sheet, low-temperature transformation structures are unlikely to occur, causing variations in hardness. The sum is more preferably 0.5 times or more, and even more preferably 0.7 times or more.

[0090] The B content of the surface layer steel sheet is preferably 0.3 times or more the B content of the base steel sheet. If the B content for improving hardenability is less than 0.3 times that of the base steel sheet, low-temperature transformation structures are unlikely to occur, which causes variations in hardness. It is more preferably 0.5 times or more, and even more preferably 0.7 times or more.

[0091] It is preferable that the sum of the Cu and Ni contents of the surface layer steel sheet is 0.3 times or more the sum of the Cu and Ni contents of the base steel sheet. If the sum of the Cu and Ni contents that improve hardenability is less than 0.3 times the sum of the Cu and Ni contents of the base steel sheet, low-temperature transformation structures are unlikely to occur, which causes variations in hardness. It is more preferable that the sum is 0.5 times or more, and even more preferable that it is 0.7 times or more.

[0092] In addition to the above elements, the steel sheet for the surface layer may contain Si, P, S, Al, N, Cr, B, Ti, Nb, V, Cu, Ni, O, W, Ta, Sn, Sb, As, Mg, Ca, Y, Zr, La, and Ce. The preferred composition ranges of the above elements are the same as the preferred ranges of the center part of the sheet thickness.

[0093] Next, a method for identifying a steel structure according to the present invention will be described. The steel structure can be identified by observing a cross section of a steel sheet parallel to the rolling direction and thickness direction and / or a cross section perpendicular to the rolling direction at a magnification of 500x to 10,000x. For example, after cutting out a steel sheet, the surface is mechanically polished to a mirror finish, and then a Nital reagent is used to reveal the steel structure. The steel structure in a region from the surface to a depth of approximately half the thickness of the steel sheet is then observed using a scanning electron microscope (SEM). This allows the area fraction of ferrite in the base steel sheet to be measured. Furthermore, in the present invention, the area fraction of retained austenite in the center of the sheet thickness is determined by X-ray measurement as follows. First, the portion from the surface of the steel sheet to half the thickness of the steel sheet is removed by mechanical polishing and chemical polishing, and the chemically polished surface is measured using MoKα rays as characteristic X-rays. Then, the area fraction of retained austenite in the center of the sheet thickness is calculated using the following formula from the integrated intensity ratio of the diffraction peaks of (200) and (211) of the body-centered cubic lattice (bcc) phase and (200), (220) and (311) of the face-centered cubic lattice (fcc) phase. Sγ=(I 200f +I 220f +I 311f ) / (I 200b +I 211b ) × 100 (Sγ is the area fraction of retained austenite in the center of the plate thickness, and I 200f , I 220f and I 311f indicate the intensities of the (200), (220) and (311) diffraction peaks of the fcc phase, respectively, and I 200b and I 211b indicate the intensities of the (200) and (211) diffraction peaks of the bcc phase, respectively.)

[0094] In this example, the obtained products were subjected to a Vickers hardness test, a nano-hardness test, a tensile test, a V-bend test, and a bending load test.

[0095] The average Vickers hardness was determined as follows. First, the Vickers hardness was measured at a certain thickness direction position at 5% intervals of the thickness from the half-thickness position toward the surface with an indentation load of 100 g. Next, the Vickers hardness was measured at a total of five points from that position on a line perpendicular to the thickness direction and parallel to the rolling direction with an indentation load of 100 g. The average value was taken as the average Vickers hardness at that thickness direction position. The distance between each measurement point in the thickness direction and rolling direction was at least four times the distance of the indentation. When the average Vickers hardness at a certain thickness direction position was 0.90 times or less the average Vickers hardness at the half-thickness position measured in the same manner, the surface side of that position was defined as the surface-softened portion. The average Vickers hardness of the entire surface-softened portion was calculated by measuring the Vickers hardness at 10 random points within the surface-softened portion defined in this way and averaging them.

[0096] The thickness of the surface-softened portion was determined by the method specified herein, and the ratio to the plate thickness was determined. Similarly, the average hardness change in the plate thickness direction of the hardness transition zone was determined by the method specified herein.

[0097] The nanohardness of the surface-softened portion was measured at 100 points in the direction perpendicular to the plate thickness at a position halfway down from the surface to the thickness of the surface-softened portion, and the standard deviation of these values ​​was taken as the standard deviation of the nanohardness of the surface-softened portion.

[0098] The tensile strength TS and elongation (%) were measured in accordance with JIS Z 2241 using a No. 5 test piece according to JIS Z 2201, prepared with its major axis perpendicular to the rolling direction.

[0099] The critical bending radius R was determined by preparing a No. 1 test piece as specified in JIS Z2204 so that the direction perpendicular to the rolling direction was the longitudinal direction (the bending ridge line coincided with the rolling direction), and conducting a V-bend test in accordance with JIS Z2248. Samples having a surface-softened portion on only one side were bent so that the side having the surface-softened portion was on the outside of the bend. The angle between the die and punch was set to 60°, and the bending test was performed by changing the punch tip radius in 0.5 mm increments, and the punch tip radius at which the sample could be bent without cracking was determined as the critical bending radius R.

[0100] The bending load test was carried out by taking a 60 mm × 60 mm test piece from the steel plate and performing a bending test in accordance with German Association of the Automotive Industry (VDA) standard 238-100 under the conditions of a punch curvature of 0.4 mm, a roll diameter of 30 mm, a roll distance of 2 × plate thickness + 0.5 (mm), and a maximum push-in stroke of 11 mm, and measuring the maximum load (N) at that time. In this example, a bending load (N) of more than 3000 times the plate thickness (mm) was deemed to have passed.

[0101] Example A A continuously cast slab (base steel plate) having a thickness of 20 mm and a chemical composition shown in Table 1 was ground to remove surface oxides, and then a surface layer steel plate having a chemical composition shown in Table 1 was laminated on one or both sides by arc welding. The ratio of the thickness of the surface layer steel plate to the plate thickness is shown in "Ratio (%) of surface layer steel plate (one side)" in Table 1. This was hot rolled under the heating temperature, finishing temperature, and coiling temperature conditions shown in Table 2 to obtain a laminated hot-rolled steel plate. In the case of test materials for which hot-rolled steel plates were used as products, the holding time at 700°C to 500°C during hot rolling was intentionally controlled to the value shown in Table 2. In the case of test materials for which cold-rolled steel plates were used as products, the slabs were then pickled, cold-rolled to 50%, and annealed under the conditions shown in Table 2.

[0102] Furthermore, when the chemical composition of the obtained product was measured at a position 2% of the plate thickness from the surface and at a position 1 / 2 of the plate thickness, it was found to be almost unchanged from the chemical compositions of the base steel plate and the surface layer steel plate shown in Table 1, respectively.

[0103]

[0104]

[0105]

[0106]

[0107]

[0108]

[0109] Referring to Table 2, for example, in the steel sheets of Comparative Examples 7, 27, and 28, the average Vickers hardness of the surface-softened portion satisfied the requirement of more than 0.60 times and not more than 0.90 times the average Vickers hardness at the half-thickness position, but the standard deviation of the nanohardness of the surface-softened portion was 0.9, i.e., not satisfying the requirement of 0.8 or less. As a result, the critical bending radius R of the steel sheets of these comparative examples was 2.5 mm. In contrast, the critical bending radius R of the steel sheets of the examples of the present invention, which satisfied the above two requirements, was less than 2 mm, particularly 1.5 mm or 1 mm. Therefore, it was found that by keeping the hardness variation of the surface-softened portion within a predetermined range, the bendability of the steel sheet can be significantly improved compared to a steel sheet simply combining a softer surface-softened portion in the center of the sheet thickness.

[0110] Furthermore, with reference to the hot-rolled steel sheet of Comparative Example 4, when the holding time at 750°C to 550°C in the cooling process after hot rolling was 1 second, the average Vickers hardness of the surface-softened portion was 0.57 times the average Vickers hardness at the half-thickness position, the standard deviation of the nano-hardness of the surface-softened portion was 0.9, and the critical bending radius R was 2.5 mm. In contrast to this, in the hot-rolled steel sheet of Example 3, which was produced in the same manner as Comparative Example 4 except that the holding time was 5 seconds and the coiling temperature was 180°C, the average Vickers hardness of the surface-softened portion was 0.86 times the average Vickers hardness at the half-thickness position, the standard deviation of the nano-hardness of the surface-softened portion was 0.5, and the critical bending radius R was 1 mm.

[0111] Further, with reference to the cold-rolled steel sheets of Examples 5 and 8, the surface layer steel sheet is at Ac3 point -50 ° C or higher and the base steel sheet is at Ac3 point -50 ° C or higher, 900 ° C or lower, held for 5 seconds or more, and the average cooling rate from 750 ° C to 550 ° C or lower is 2.5 ° C / s or higher. By appropriately selecting the temperature, holding time and average cooling rate during annealing to meet the requirements, the variation in hardness of the surface softened portion is suppressed (standard deviation of nanohardness of the surface softened portion: 0.4 or 0.5), and as a result, it was found that the bendability of the cold-rolled steel sheet can be significantly improved (limit bending radius R is 1.5 mm). On the other hand, in the cold-rolled steel sheets of Comparative Examples 6, 7 and 9 which do not satisfy the above requirements, the standard deviation of the nanohardness of the surface softened portion was 0.9 and the limit bending radius R was 2.5 mm.

[0112] Furthermore, in the hot rolling, a steel sheet produced without performing rough rolling at least twice under the conditions of a rough rolling temperature of 1100°C or higher, a thickness reduction rate per pass of 5% or higher but less than 50%, and an inter-pass time of 3 seconds or longer had a high limit bending radius R and / or a low bending load, and was unable to achieve sufficient bending workability.

[0113] [Example B: Formation of hardness transition zone] A continuously cast slab (base steel plate) having a thickness of 20 mm and a chemical composition shown in Table 3 was ground to remove surface oxides, and then a surface layer steel plate having a chemical composition shown in Table 1 was laminated on one or both sides by arc welding. The ratio of the thickness of the surface layer steel plate to the plate thickness is shown in "Ratio (%) of surface layer steel plate (one side)" in Table 3. This was hot rolled under the conditions of heating temperature, heating time, finishing temperature, and coiling temperature shown in Table 4 to obtain a laminated hot-rolled steel plate. In the case of test materials for which hot-rolled steel plates were used as products, the average cooling rate during hot rolling from 750°C to 550°C was intentionally controlled to the value shown in Table 4. In the case of test materials for which cold-rolled steel plates were used as products, the slabs were then pickled, cold-rolled to 50%, and annealed under the conditions shown in Table 4.

[0114] Furthermore, when the chemical composition of the obtained product was measured at a position 2% of the plate thickness from the surface layer and at a position 1 / 2 of the plate thickness, it was found to be almost unchanged from the chemical compositions of the base steel plate and the surface layer steel plate shown in Table 3, respectively.

[0115]

[0116]

[0117]

[0118]

[0119]

[0120]

[0121] Referring to Table 4, for example, in the steel plates of Comparative Examples 107, 128, and 129, the average Vickers hardness of the surface-softened portion met the requirement of more than 0.60 times and not more than 0.90 times the average Vickers hardness at the half-thickness position, and further the requirement of the average hardness change in the plate thickness direction in the hardness transition zone being not more than 5000 (ΔHv / mm), but the standard deviation of the nanohardness of the surface-softened portion was 0.9, that is, it can be seen that the requirement of not more than 0.8 was not met. As a result, the critical bending radius R of the steel plates of these Comparative Examples was 2.5 mm. On the other hand, in Example 110, the average Vickers hardness of the surface-softened portion satisfies the requirement that the average Vickers hardness at the half-thickness position is more than 0.60 times and not more than 0.90 times the average Vickers hardness, and furthermore, the standard deviation of the nanohardness of the surface-softened portion is 0.8 or less. However, the average hardness change in the thickness direction of the hardness transition zone is 5015 (ΔHv / mm), that is, more than 5000 (ΔHv / mm). As a result, the critical bending radius R of the steel plate of Example 110 was 1.5 mm. In contrast, in the steel plate in the example in which the two requirements of "the average Vickers hardness of the surface-softened portion is more than 0.60 times and not more than 0.90 times the average Vickers hardness at the half-thickness position" and "the standard deviation of the nanohardness of the surface-softened portion is 0.8 or less" are satisfied, and "the average hardness change in the thickness direction of the hardness transition zone is 5000 (ΔHv / mm) or less" are satisfied, the critical bending radius R was 1 mm. Therefore, it was found that by controlling both the hardness variation in the surface-softened portion and the average hardness change in the thickness direction of the hardness transition zone within a specific range, the bendability of the steel plate can be significantly improved compared to a steel plate that simply combines a softer surface-softened portion in the center of the plate thickness, etc., and in which only one of the hardness variation in the surface-softened portion and the average hardness change in the thickness direction of the hardness transition zone is controlled within a specific range.

[0122] Furthermore, with reference to the hot-rolled steel sheet of Comparative Example 104, when the holding time at 750°C to 550°C in the cooling process after hot rolling was set to 1 second, the standard deviation of the nanohardness in the surface-softened portion was 0.9 and the limit bending radius R was 2.5 mm. In contrast to this, in the hot-rolled steel sheet of Example 103, which was produced in the same manner as Comparative Example 104 except that the holding time was set to 5 seconds and the coiling temperature was set to 180°C, the standard deviation of the nanohardness in the surface-softened portion was 0.5 and the limit bending radius R was 1 mm.

[0123] Further, with reference to the cold-rolled steel sheets of Examples 105 and 108, the surface layer steel sheet was held at a temperature of Ac3 point -50 ° C or higher and Ac3 point -50 ° C or higher and 900 ° C or lower for 5 seconds or more, and the average cooling rate from 750 ° C to 550 ° C or lower was 2.5 ° C / s or higher. By appropriately selecting the annealing temperature, holding time and average cooling rate, the variation in hardness of the surface softened portion was suppressed (standard deviation of nanohardness of the surface softened portion: 0.4 or 0.5), and as a result, it was found that the bendability of the cold-rolled steel sheet could be significantly improved (limit bending radius R is 1 mm). On the other hand, in the cold-rolled steel sheets of Comparative Examples 106, 107 and 109 which did not satisfy the above requirements, the standard deviation of the nanohardness of the surface softened portion was 0.9, and the limit bending radius R was 2.5 mm.

[0124] Furthermore, in the hot rolling, a steel sheet produced without performing rough rolling at least twice under the conditions of a rough rolling temperature of 1100°C or higher, a thickness reduction rate per pass of 5% or higher but less than 50%, and an inter-pass time of 3 seconds or longer had a high limit bending radius R and / or a low bending load, and was unable to achieve sufficient bending workability.

[0125] Example C: Formation of a central portion of a plate thickness containing retained austenite at an area fraction of 10% or more A continuously cast slab (base steel plate) having a thickness of 20 mm and a chemical composition shown in Table 5 was ground to remove surface oxides, and then a surface layer steel plate having a chemical composition shown in Table 5 was laminated on one or both sides by arc welding. This was hot rolled under the heating temperature, finishing temperature, and coiling temperature conditions shown in Table 6 to obtain a laminated hot-rolled steel plate. In the case of test materials for which hot-rolled steel plates were used as products, the holding time at 700°C to 500°C during hot rolling was intentionally controlled to the value shown in Table 6. In the case of cold-rolled steel plates for products, the slabs were then pickled, cold-rolled at the cold reduction rates shown in Table 6, and further annealed under the conditions shown in Table 6.

[0126] Furthermore, when the chemical composition of the obtained product was measured at a position 2% of the plate thickness from the surface layer and at a position 1 / 2 of the plate thickness, it was found to be almost unchanged from the chemical compositions of the base steel plate and the surface layer steel plate shown in Table 5, respectively.

[0127]

[0128]

[0129]

[0130]

[0131]

[0132]

[0133]

[0134]

[0135]

[0136]

[0137] Steel sheets having a tensile strength of 800 MPa or more, a limit bending radius R of less than 2 mm, and a bending load (N) of more than 3000 times the plate thickness (mm) were evaluated as high-strength steel sheets with excellent bendability (Examples in Table 6). Steel sheets having an elongation of 15% or more were also evaluated as high-strength steel sheets with excellent bendability and ductility (Examples 201 to 241 in Table 6). On the other hand, steel sheets that did not satisfy any one of the performances of "tensile strength of 800 MPa or more," "limit bending radius R of less than 2 mm," and "bending load (N) of more than 3000 times the plate thickness (mm)" were classified as comparative examples.

[0138] Furthermore, in the hot rolling, a steel sheet produced without performing rough rolling at least twice under the conditions of a rough rolling temperature of 1100°C or higher, a thickness reduction rate per pass of 5% or higher but less than 50%, and an inter-pass time of 3 seconds or longer had a high limit bending radius R and / or a low bending load, and was unable to achieve sufficient bending workability.

[0139] Example D: Formation of a hardness transition zone and a thickness center portion containing 10% or more of retained austenite by area fraction A continuously cast slab (base steel plate) having a thickness of 20 mm and having the chemical composition shown in Table 7 was ground to remove surface oxides, and then a surface layer steel plate having the chemical composition shown in Table 7 was laminated on one or both sides by arc welding. This was hot rolled under the conditions of the heating temperature, heating time, finishing temperature, and coiling temperature shown in Table 8 to obtain a laminated hot-rolled steel plate. In the case of test materials for which hot-rolled steel plates were used as products, the holding time at 700°C to 500°C during hot rolling was intentionally controlled to the value shown in Table 8. In the case of cold-rolled steel plates for products, the steel plates were then pickled, cold-rolled at the cold reduction rates shown in Table 8, and further annealed under the conditions shown in Table 8.

[0140] Furthermore, when the chemical composition of the obtained product was measured at a position 2% of the plate thickness from the surface layer and at a position 1 / 2 of the plate thickness, it was found to be almost unchanged from the chemical compositions of the base steel plate and the surface layer steel plate shown in Table 7, respectively.

[0141]

[0142]

[0143]

[0144]

[0145]

[0146]

[0147]

[0148]

[0149]

[0150]

[0151] High-strength steel sheets with excellent bendability were evaluated as those with a tensile strength of 800 MPa or more, a limit bending radius R of less than 2 mm, and a bending load (N) of more than 3,000 times the plate thickness (mm) (Examples in Table 8). In particular, in Example 356, the average Vickers hardness of the surface-softened portion met the requirement of being more than 0.60 times and not more than 0.90 times the average Vickers hardness at the half-plate thickness position, and further, the standard deviation of the nanohardness of the surface-softened portion met the requirement of being 0.8 or less, but the average hardness change in the plate thickness direction in the hardness transition zone exceeded 5,000 (ΔHv / mm). As a result, the limit bending radius R of the steel sheet of Example 356 was 1.5 mm. In contrast, in the steel sheets of the examples in which the two requirements of "the average Vickers hardness of the surface-softened portion is more than 0.60 times but not more than 0.90 times the average Vickers hardness at the half-thickness position" and "the standard deviation of the nanohardness of the surface-softened portion is 0.8 or less" were satisfied and "the average hardness change in the thickness direction of the hardness transition zone is 5000 (ΔHv / mm) or less" were met, the critical bending radius R was 1 mm. Furthermore, when the center of the sheet thickness contains 10% or more of retained austenite in area fraction, the elongation was 15% or more, and high-strength steel sheets excellent in both bendability and ductility were obtained (Examples 301 to 341 in Table 8). On the other hand, when any one of the performances of "tensile strength of 800 MPa or more," "critical bending radius R of less than 2 mm," and "bending load (N) of more than 3000 times the sheet thickness (mm)" was not satisfied, it was classified as a comparative example.

[0152] Furthermore, in the hot rolling, a steel sheet produced without performing rough rolling at least twice under the conditions of a rough rolling temperature of 1100°C or higher, a thickness reduction rate per pass of 5% or higher but less than 50%, and an inter-pass time of 3 seconds or longer had a high limit bending radius R and / or a low bending load, and was unable to achieve sufficient bending workability.

Claims

------15 / 11 / 2019------(OCR)1. A high-strength thin steel plate with a tensile strength of 800 MPa or more, comprising a core in the thickness of the plate and soft layers arranged on one or both sides of the core in the thickness, where each soft layer is thicker than 10 micrometers and 30 percent or less of the plate thickness, the soft layer has an average Vickers hardness value greater than 0.60 times and 0.90 times or less of the average Vickers hardness value of the 1 / 2 plate thickness position, and the soft layer has a standard deviation of nano-hardness of 0.8 or less.

2. A high-strength thin steel plate according to claim 1, in which this high-strength thin steel plate is further comprising a hardness transition zone occurring between the core in the thickness and each soft layer adjacent to this core and layer, where the hardness transition zone has a change in average hardness value in the thickness direction of 5000 (delta Vickers / mm) or less.3.

4. High-strength thin steel plates under any one of Reputation 1 to 3, where the central portion in the plate thickness contains (in percentage by area) 10 percent or more of residual austenite. Furthermore, S: 0.05 percent or less, A1: 0 to 3 percent, and N: 0.01 percent or less, and the remainder is Fe and unavoidable impurities.

5. High-strength thin steel plates according to claim 4, in which the central portion in the plate thickness contains (in percentage by mass) at least one element selected from a group consisting of: Cr: 0.01 to 3 percent, Mo: 0.01 to 1 percent, and B: 0.0001 to 0.01 percent. 6.High-strength thin steel plates under claim 4 or 5, in which the central portion in the plate thickness is incorporated (in percentage by mass) at least one element selected from a group consisting of: Ti: 0.01 to 0.2 percent, Nb: 0.01 to 0.2 percent, and V: 0.01 to 0.2 percent.

7. High-strength thin steel plates under one of claim 4 to 6, in which the central portion in the plate thickness is incorporated (in percentage by mass) at least one element selected from a group consisting of: Cu: 0.01 to 0.2 percent.

8. High-strength thin steel plates according to any one of the claims of claims 4 through 7 where the C content of the soft layer is 0.30 times or more and 0.90 times or less of the C content of the middle layer in the plate thickness.

9. High-strength thin steel plates according to any one of the claims of claims 5 through 8 where the total combined Mn, Cr, and Mo content of the soft layer is 0.30 times or more of the total combined Mn, Cr, and Mo content of the middle layer in the plate thickness. 10.

11. High-strength thin steel sheets pursuant to any one of the claims in Case 5 through 9 where the B content of the soft enamel layer is 0.3 times or more of the B content of the middle layer in sheet thickness.

12. High-strength thin steel sheets pursuant to any one of the claims in Case 7 through 10 where the total Cu and Ni content of the soft enamel layer is 0.3 times or more of the total Cu and Ni content of the middle layer in sheet thickness.

13. High-strength thin steel sheets pursuant to any one of the claims in Case 1 through 11 which are further incorporated with a hot-dip galvanized layer, a hot-dip galvanized and annealed layer, or an electro-galvanized layer on the surface of the soft enamel layer.A high-strength steel sheet with a tensile strength of 800 megapascals or more consists of a middle-part in sheet thickness and a soft surface layer placed on one or both sides of the middle-part. Each soft surface layer is thicker than 10 micrometers and 30 percent or less of the sheet thickness. The soft surface layer has an average Vickers hardness value greater than 0.60 times and 0.90 times or less than the average Vickers hardness value of half the sheet thickness, and the soft surface layer has a nanoscale hardness standard deviation of 0.8 or less. High-strength thin steel plates under claim I, in which this high-strength thin steel plate incorporates an additional hardness transition zone occurring between the thickness center and each soft surface layer adjacent to this center and layer, where the hardness transition zone has a change in average hardness value in the thickness direction of 5000 (delta Vickers / mm) or less.

34. High-strength thin steel plates under any one of Requisitions 1 through 3, where the central portion in the plate thickness contains (in percentage by area) 10 percent or more of retained austenite.

5. Thin high-strength steel plates under claim 4, in which the central portion in the plate thickness is additionally composed (in percentage by mass) of at least one element selected from a group consisting of: Cr: 0.01 to 3 percent, Mo: 0.01 to 1 percent, and B: 0.0001 to 0.01 percent.6High-strength thin steel plates under claim 4 or 5 in which the middle layer of the plate thickness contains (in percentage by mass) at least one element selected from a group consisting of: Ti: 0.01 to 0.2 percent, Nb: 0.01 to 0.2 percent, and V: 0.01 to 0.2 percent.

7. High-strength thin steel plates under one of claim 4 to 6 in which the middle layer of the plate thickness contains (in percentage by mass) at least one element selected from a group consisting of: Cu: 0.01 to 1 percent, and Ni: 0.01 to 1 percent.

8. High-strength thin steel plates under one of claim 4 to 7 in which the amount of C of the soft layer is 0.30 times or more and 0.90 times or less of the amount of C of the middle layer of the plate thickness.

9. High-strength thin steel plates according to any one of the claims 5 through 8 where the total combined Mn, Cr, and Mo content of the soft surface layer is 0.30 times or more of the total combined Mn, Cr, and Mo content of the middle layer in a plate thickness of 10 oz.

11. High-strength thin steel sheets pursuant to any one of the claims in Case 5 through 9 where the B content of the soft enamel layer is 0.3 times or more of the B content of the middle layer in sheet thickness.

12. High-strength thin steel sheets pursuant to any one of the claims in Case 7 through 10 where the total Cu and Ni content of the soft enamel layer is 0.3 times or more of the total Cu and Ni content of the middle layer in sheet thickness.

13. High-strength thin steel sheets pursuant to any one of the claims in Case 1 through 11 which are further incorporated with a hot-dip galvanized layer, a hot-dip galvanized and annealed layer, or an electro-galvanized layer on the surface of the soft enamel layer.