Steel plate

Abstract

Provided is a steel sheet which has a prescribed chemical composition, and which is characterized in that the metallographic structure contains, in area%, 90.0% or more of martensite and 3.0% or less of retained austenite, the average grain diameter of the original austenite grains is 30.0 [mu] m or less, and the average grain diameter of the original austenite grains is 30.0 [mu] m or less when a region surrounded by grain boundaries having a difference in orientation of 15 degrees or more in the martensite is defined as crystal grains. And the proportion of the crystal grains having an intragranular orientation difference of 4 DEG or more is 45.0-70.0% in area%.

Description

Technical Field

[0001] The present invention relates to steel plates. Background Art

[0002] In recent years, to address environmental issues, there has been a growing desire to reduce CO₂ emissions and improve fuel economy in automotive components. At the same time, social demands for improved crash safety are also increasing. To achieve both lightweighting and improved crash safety, increasing the strength of steel is an effective approach. However, increasing the strength of steel typically reduces workability, necessitating a need for steel that simultaneously improves both strength and workability.

[0003] Regarding the improvement of strength and workability, for example, Patent Document 1 describes a hot-rolled steel sheet characterized in that the chemical composition, in mass %, contains C: 0.020-0.070%, Si: 0.10-1.70%, Mn: 0.60-2.50%, Al: 0.01-1.00%, Ti: 0.015-0.170%, Nb: 0.005-0.050%, etc., and P: 0.05% or less, S: 0.000% or less, and Nb: 0.000% or less. 0.010% or less, N: 0.0060% or less, the balance consisting of Fe and impurities, and a structure comprising, by area ratio, 5-60% ferrite and 30-95% bainite. In this structure, when boundaries with an orientation difference of 15° or more are defined as grain boundaries, and regions surrounded by these grain boundaries and having an equivalent circle diameter of 0.3 μm or more are defined as grains, the proportion of grains within the grains with an orientation difference of 5-14° is 20-100% by area ratio. Furthermore, Patent Document 1 teaches that by increasing the proportion of grains within the grains with an orientation difference of 5-14° to 20% or more by area ratio, it is possible to maintain the desired steel sheet strength while improving the edge expansion performance (hole expandability).

[0004] Patent Document 2 describes a high-strength steel material having spherical primary gamma grains, characterized in that the steel material contains, by mass, 0.06-0.19% C, 0.15-0.60% Si, 0.60-1.80% Mn, 0.05-1.20% Cr, and 0.05-1.00% Mo, and further contains at least one of 0.005-0.10% Nb, 0.005-0.10% V, and 0.005-0.10% Ti. The steel material also contains, by volume, 0.01-0.8% of carbonitrides of Nb, Ti, or V having a particle size of 100 nm or less, and the grain size number of the primary gamma grains is 7 or greater, and the primary gamma grains have a martensite structure or a mixed structure of martensite and bainite. Patent Document 2 teaches that the above-described configuration can provide a high-strength steel material that is excellent in toughness, crack arrestability, and weldability, has a large uniform elongation exceeding 10%, and is easy to mass-produce.

[0005] Patent document 3 describes a high-carbon steel plate component containing C: 0.80 mass% to 1.10 mass%, Si: 0.05 mass% to 0.40 mass%, Mn: 0.05 mass% to 0.50 mass%, Cr: 0.01 mass% to 0.35 mass%, P: 0.03 mass% or less, S: 0.03 mass% or less, the balance being Fe and inevitable impurities, the equivalent circle diameter of a martensite block defined by an orientation difference of 15° or more is 3.2 μm or less, 0.6 to 5.0 area % of carbides with an equivalent circle diameter of 0.2 μm or more are present, the average particle size of the carbides is 0.3 μm or more and 2.0 μm or less, the carbide occupancy on the prior austenite grain boundaries is 0.35% or less, and the average KAM value measured by EBSD is 0.690 to 0.710. Patent Document 3 teaches that, according to the above-mentioned configuration, a high-carbon steel plate member having high hardness and high hardness-toughness balance can be obtained.

[0006] Prior art literature

[0007] Patent Literature

[0008] Patent Document 1: International Publication No. 2016 / 136672

[0009] Patent Document 2: Japanese Patent Application Laid-Open No. 2002-088440

[0010] Patent Document 3: Japanese Patent Application Publication No. 2018-048375 Summary of the Invention

[0011] As mentioned above, it is known that as steel is strengthened, its workability decreases, and properties such as hole expandability described in Patent Document 1 are reduced. If hole expandability decreases, for example, it may not be possible to form the desired shape in automotive chassis parts. Therefore, in the development of high-strength steel sheets, such as high-strength hot-rolled steel sheets, it is important to ensure that the properties appropriate to the intended application are at least certain levels while also achieving high strength. Furthermore, automotive steel sheets are often formed into the desired component shape through press forming. Press forming is typically performed in multiple steps, resulting in a high number of areas that, for example, undergo additional deformation (such as side deformation) after initially deforming and accumulating strain within the steel sheet. However, when strain is introduced into the steel sheet, it undergoes work hardening, resulting in increased strength, which generally reduces workability in subsequent steps. Therefore, steel sheets are required to exhibit high formability even when subjected to some degree of strain, for example, by exhibiting excellent work hardening ability (the ability to continue hardening).

[0012] The present invention has been made in view of such actual circumstances, and an object of the present invention is to provide a steel plate having improved hole expandability and work hardening ability despite high strength due to a novel structure.

[0013] To achieve the above-mentioned objectives, the present inventors conducted research focusing on the metallurgical structure of steel sheets, particularly hot-rolled steel sheets. As a result, the present inventors discovered that by constructing the metallurgical structure of a hot-rolled steel sheet having a predetermined chemical composition with a structure primarily composed of martensite, it is possible to achieve increased strength and improved hole expandability. Furthermore, by limiting the average grain size of prior austenite grains in the metallurgical structure to a predetermined range and appropriately controlling the proportion of grains with a predetermined intragranular orientation difference in the martensite structure, the work hardening ability can be significantly improved. This led to the completion of the present invention.

[0014] The present invention that can achieve the above-mentioned object is as follows.

[0015] (1) A steel plate characterized in that the chemical composition is expressed in mass %.

[0016] C: 0.040~0.200%,

[0017] Si: 0.30~2.00%,

[0018] Mn: 1.00~4.00%,

[0019] sol.Al: 0.001~0.500%,

[0020] P: 0.100% or less,

[0021] S: 0.0300% or less,

[0022] N: 0.0070% or less,

[0023] O: 0.0100% or less,

[0024] Nb: 0.001~1.000%,

[0025] Ti: 0~0.200%,

[0026] V: 0~0.300%,

[0027] Cu: 0~0.40%,

[0028] Cr: 0~0.90%,

[0029] Mo: 0~0.12%,

[0030] Ni: 0~0.30%,

[0031] B: 0~0.0030%,

[0032] Ca: 0~0.0010%,

[0033] Mg: 0~0.0010%,

[0034] Bi: 0~0.010%,

[0035] Zr: 0~0.050%,

[0036] Co: 0~0.010%,

[0037] Zn: 0~0.010%,

[0038] W: 0~0.100%,

[0039] Sn: 0~0.040%,

[0040] As:0~0.100%,

[0041] REM: 0~0.0100%, and

[0042] Balance: Fe and impurities,

[0043] The metal structure contains

[0044] Martensite: more than 90.0%, and

[0045] Retained austenite: less than 3.0%,

[0046] The average particle size of the original austenite grains is less than 30.0 μm.

[0047] In the martensite, when a region surrounded by grain boundaries having an orientation difference of 15° or more is defined as a grain, the ratio of the grains having an orientation difference of 4° or more within the grain is 45.0 to 70.0% in terms of area %.

[0048] (2) The steel plate according to (1) above, wherein the chemical composition comprises, in mass %,

[0049] Ti: 0.001~0.200%,

[0050] V: 0.001~0.300%,

[0051] Cu: 0.001~0.40%,

[0052] Cr: 0.001~0.90%,

[0053] Mo: 0.001~0.12%,

[0054] Ni: 0.001~0.30%,

[0055] B: 0.0001~0.0030%,

[0056] Ca: 0.0001~0.0010%,

[0057] Mg: 0.0001~0.0010%,

[0058] Bi: 0.001~0.010%,

[0059] Zr: 0.001~0.050%,

[0060] Co: 0.001~0.010%,

[0061] Zn: 0.001~0.010%,

[0062] W: 0.001~0.100%,

[0063] Sn: 0.001~0.040%,

[0064] As: 0.001~0.100%, and

[0065] REM: 0.0001~0.0100%

[0066] At least one of the following.

[0067] (3) The steel plate according to (1) or (2) above, wherein the metal structure further comprises, in terms of area %,

[0068] Ferrite: less than 10.0%,

[0069] Bainite: less than 10.0%, and

[0070] Pearlite: less than 10.0%

[0071] At least one of the following.

[0072] (4) The steel sheet according to any one of (1) to (3) above, wherein the proportion of prior austenite grains having an aspect ratio of 2.0 or less in all prior austenite grains is 90.0% or more in terms of area %.

[0073] (5) The steel plate according to any one of (1) to (4) above, wherein the plate thickness is 1.0 to 7.0 mm.

[0074] (6) A component comprising the steel plate according to any one of (1) to (5) above.

[0075] According to the present invention, it is possible to provide a steel sheet, particularly a hot-rolled steel sheet, which has improved hole expandability and work hardening ability despite high strength. DETAILED DESCRIPTION

[0076] <Steel Plate>

[0077] The steel sheet according to the embodiment of the present invention, in particular the hot rolled steel sheet, has a chemical composition in mass %.

[0078] C: 0.040~0.200%,

[0079] Si: 0.30~2.00%,

[0080] Mn: 1.00~4.00%,

[0081] sol.Al: 0.001~0.500%,

[0082] P: 0.100% or less,

[0083] S: 0.0300% or less,

[0084] N: 0.0070% or less,

[0085] O: 0.0100% or less,

[0086] Nb: 0.001~1.000%,

[0087] Ti: 0~0.200%,

[0088] V: 0~0.300%,

[0089] Cu: 0~0.40%,

[0090] Cr: 0~0.90%,

[0091] Mo: 0~0.12%,

[0092] Ni: 0~0.30%,

[0093] B: 0~0.0030%,

[0094] Ca: 0~0.0010%,

[0095] Mg: 0~0.0010%,

[0096] Bi: 0~0.010%,

[0097] Zr: 0~0.050%,

[0098] Co: 0~0.010%,

[0099] Zn: 0~0.010%,

[0100] W: 0~0.100%,

[0101] Sn: 0~0.040%,

[0102] As:0~0.100%,

[0103] REM: 0~0.0100%, and

[0104] Balance: Fe and impurities,

[0105] The metal structure contains

[0106] Martensite: more than 90.0%, and

[0107] Retained austenite: less than 3.0%,

[0108] The average particle size of the original austenite grains is less than 30.0 μm.

[0109] In the martensite, when a region surrounded by grain boundaries having an orientation difference of 15° or more is defined as a grain, the ratio of the grains having an orientation difference of 4° or more within the grain is 45.0 to 70.0% in terms of area %.

[0110] As previously mentioned, it is known that as steel becomes stronger, properties such as hole expandability decrease. For example, in order to manufacture complex-shaped components such as lower arms and rear swing arms in automotive chassis, it is necessary to ensure high strength, especially high strength with a tensile strength of 980 MPa or more that can be lightweight, and excellent hole expandability. From the perspective of high strength, the metallurgical structure of the steel plate preferably adopts a structure composed mainly of martensite. However, martensitic steel has a layered structure that further includes lath bundles, lath blocks, and laths within the original austenite grains. Although it has excellent strength, it generally suffers from low machinability. Therefore, in forming operations that are divided into multiple steps, such as press forming, machinability generally decreases in the later stages of deformation due to work hardening caused by the strain introduced in the early stages of deformation. Therefore, there is a demand for steel plates that can achieve both high strength and machinability by utilizing high work hardening capabilities even in the later stages of deformation in press forming.

[0111] Therefore, the present inventors not only optimized the chemical composition of steel sheets, particularly hot-rolled steel sheets, but also conducted research focusing specifically on the metallurgical structure of the hot-rolled steel sheets. First, the present inventors discovered that by constructing the metallurgical structure of a hot-rolled steel sheet having a predetermined chemical composition with a structure primarily composed of martensite, more specifically, a structure comprising at least 90.0% martensite and at most 3.0% retained austenite, high strength, such as a tensile strength of 980 MPa or more, can be achieved, while significantly improving the hole expandability of the resulting hot-rolled steel sheet. While not intending to be bound by any particular theory, it is believed that by achieving a more uniform structure with at least 90.0% martensite by area, the hardness difference within the metallurgical structure can be reduced compared to a structure containing a higher proportion of other structures softer than martensite, such as ferrite. This reduction in hardness difference can lead to improved hole expandability. Furthermore, retained austenite can serve as a starting point for fracture during deformation such as press forming. Therefore, by limiting the retained austenite to 3.0% or less in addition to controlling the martensite to 90.0% by area, the hole expandability can be more significantly improved.

[0112] Secondly, the inventors believe that the original austenite grain boundaries act as a resistance to the movement of dislocations, which is effective for improving the work hardening ability. Therefore, from the perspective of making the particle size of the original austenite grains in the metal structure mainly composed of martensite appropriate, the improvement of the work hardening ability has been studied. More specifically, by refining the original austenite grains, the density of the original austenite grain boundaries can be increased. Therefore, by refining the original austenite grains, the obstacles of dislocations can be increased, and therefore, the work hardening ability can be improved. However, if the original austenite grains are only refined, sometimes in the late stage of deformation of the forming operation, which is divided into multiple steps such as press forming, it is not possible to exert sufficient work hardening ability. Therefore, in addition to controlling the particle size of the original austenite grains, the inventors also conducted research with an eye on the morphology of the martensitic structure. As a result, the present inventors discovered that by refining the prior austenite grains within a specified range, more specifically, controlling the average grain size of the prior austenite grains to 30.0 μm or less, the work hardening ability of the hot-rolled steel sheet as a whole can be improved, and by appropriately controlling the proportion of specific grains in the martensite structure, more specifically, in the martensite, when defining the region surrounded by grain boundaries with an orientation difference of 15° or more as grains, the proportion of grains with an orientation difference of 4° or more within the grains is controlled to be within the range of 45.0 to 70.0% in terms of area %. As a result, a high work hardening rate can be achieved even in a state where a certain degree of strain has been introduced, such as in the late stage of deformation during press forming.

[0113] It is not intended to be bound by any particular theory, but it is believed that by controlling the proportion of grains with an intragranular orientation difference of 4° or more in the range of 45.0% to 70.0% by area, a martensitic structure with unevenly dispersed dislocations can be formed, and such a martensitic structure contributes to a high work hardening rate in the late stage of deformation such as press forming. More specifically, the intragranular orientation difference represents the distribution state of dislocations. The more unevenly the dislocations are dispersed, the greater the intragranular orientation difference is. Therefore, by containing grains with increased intragranular orientation difference in an appropriate proportion in martensite so that a certain amount of structure with unevenly dispersed dislocations exists, uneven deformation develops in processes such as press forming. As a result, it is believed that sufficient work hardening capacity can be maintained even in the late stage of deformation, and therefore a high work hardening rate can be achieved. In particular, the intragranular orientation difference of 4° or more is a grain with an orientation difference sufficient to develop uneven deformation. By controlling such grains by area % in the range of 45.0% to 70.0%, the desired work hardening rate can be achieved. Therefore, for example, in press forming, even if a portion of the steel sheet undergoes additional deformation (such as side-releasing deformation) after initial deformation, which accumulates strain within the steel sheet, the steel sheet according to embodiments of the present invention maintains high work hardening capacity, enabling stable forming. The inventors have now clarified for the first time that the work hardening capacity of a steel sheet can be improved by controlling the proportion of grains with increased intragranular orientation misalignment within a predetermined range within a metal structure primarily composed of martensite, thereby forming a structure with unevenly dispersed dislocations. This fact was previously unknown and has been clarified by the present inventors. As a result, the steel sheet according to embodiments of the present invention can significantly improve hole expandability and work hardening capacity despite having a high tensile strength of, for example, 980 MPa or more. Therefore, the steel sheet according to embodiments of the present invention can reliably achieve both high strength and excellent workability, which are mutually exclusive properties, making it particularly useful in the automotive industry, where these properties are required to be combined.

[0114] The steel sheet according to the embodiment of the present invention is described in more detail below. In the following description, "%" as a unit of the content of each element means "mass %" unless otherwise specified. Furthermore, in this specification, "to" indicating a numerical range is used to indicate that the lower and upper limits are inclusive of the values ​​described before and after it, unless otherwise specified.

[0115] [C: 0.040~0.200%]

[0116] C is an element that is effective in improving the strength of steel plates. Furthermore, C forms carbides and / or carbonitrides with Nb in steel, and the pinning effect of the formed precipitates also contributes to the refinement of the structure. To fully achieve these effects, the C content is set to 0.040% or more. The C content can be 0.060% or more, 0.080% or more, 0.100% or more, or 0.120% or more. On the other hand, excessive C content can sometimes reduce workability. Therefore, the C content is set to 0.200% or less. The C content can be 0.180% or less, 0.160% or less, 0.150% or less, or 0.140% or less.

[0117] [Si: 0.30~2.00%]

[0118] Si is an element that is effective in increasing strength as a solid solution strengthening element. In order to fully obtain such an effect, the Si content is set to 0.30% or more. The Si content can be 0.40% or more, 0.50% or more, 0.60% or more, 0.70% or more, 0.85% or more, 1.00% or more, or 1.20% or more. On the other hand, if Si is excessively contained, chemical conversion treatability and workability are reduced, and slab cracks are sometimes generated during hot rolling. Therefore, the Si content is set to 2.00% or less. The Si content can be 1.80% or less, 1.60% or less, 1.50% or less, or 1.40% or less.

[0119] [Mn: 1.00~4.00%]

[0120] Mn is an element that is effective for the rise of hardenability and the rise of strength as a solid solution strengthening element. In order to fully obtain these effects, the Mn content is set to more than 1.00%. The Mn content can be more than 1.20%, more than 1.50%, more than 1.80%, more than 2.00% or more than 2.20%. On the other hand, if excessively containing Mn, sometimes workability is reduced. Therefore, the Mn content is set to less than 4.00%. The Mn content can be less than 3.80%, less than 3.50%, less than 3.20%, less than 3.00% or less than 2.80%.

[0121] [sol.Al: 0.001~0.500%]

[0122] Sol.Al is an element that acts as a deoxidizer for molten steel. In addition, sol.Al is also an element that suppresses the precipitation of cementite that is harmful to hole expandability. In order to obtain these effects, the sol.Al content is set to 0.001% or more. The sol.Al content can be 0.010% or more, 0.020% or more, 0.030% or more, 0.050% or more, or 0.100% or more. On the other hand, even if sol.Al is excessively contained, the effect is saturated, which may lead to an increase in manufacturing costs. Therefore, the sol.Al content is set to 0.500% or less. The sol.Al content can be 0.400% or less, 0.300% or less, or 0.200% or less. Sol.Al means acid-soluble Al, which means solid-solution Al present in the steel in a solid solution state.

[0123] [P: 0.100% or less]

[0124] If P is contained excessively, workability may be reduced due to grain boundary segregation and the like. Therefore, the P content is set to 0.100% or less. The P content may be 0.050% or less, 0.030% or less, 0.020% or less, or 0.015% or less. The lower limit of the P content is not particularly limited and may be 0%, but excessive reduction may lead to increased costs. Therefore, the P content may be 0.0001% or more, 0.001% or more, or 0.005% or more.

[0125] [S: 0.0300% or less]

[0126] If S is contained excessively, a large amount of sulfides such as MnS will be generated, which may reduce workability. Therefore, the S content is set to 0.0300% or less. The S content can be 0.0200% or less, 0.0100% or less, or 0.0050% or less. The lower limit of the S content is not particularly limited and can be 0%, but excessive reduction will lead to increased costs. Therefore, the S content can be 0.0001% or more, 0.0010% or more, or 0.0030% or more.

[0127] [N: 0.0070% or less]

[0128] Excessive N content may form coarse nitrides, reducing workability. Therefore, the N content is set to 0.0070% or less. The N content can be 0.0050% or less, 0.0040% or less, or 0.0030% or less. The lower limit of the N content is not particularly limited and can be 0%, but excessive reduction will lead to increased costs. Therefore, the N content can be 0.0001% or more or 0.0005% or more.

[0129] [O: 0.0100% or less]

[0130] O is an element that is mixed in during the manufacturing process. Excessive O content may form coarse inclusions, reducing the workability of the steel plate. Therefore, the O content is set to 0.0100% or less. The O content can be 0.0080% or less, 0.0060% or less, or 0.0040% or less. The lower limit of the O content is not particularly limited and can be 0%, but reducing it to less than 0.0001% requires time for refining, resulting in reduced productivity. Therefore, the O content can be 0.0001% or more, or 0.0005% or more.

[0131] [Nb: 0.001~1.000%]

[0132] Nb is an element that forms carbides, nitrides, and / or carbonitrides in steel, thereby contributing to the refinement of prior austenite grains through the pinning effect, and thus to the high strength of the steel plate. In order to fully achieve these effects, the Nb content is set to 0.001% or more. The Nb content can be 0.005% or more, 0.010% or more, 0.050% or more, 0.100% or more, 0.200% or more, or 0.300% or more. On the other hand, if Nb is excessively contained, coarse carbides are sometimes generated in the steel, thereby reducing the workability of the steel plate. Therefore, the Nb content is set to 1.000% or less. The Nb content can be 0.800% or less, 0.600% or less, or 0.500% or less.

[0133] The basic chemical composition of the steel sheet according to the embodiment of the present invention is as described above. In addition, the steel sheet may contain at least one of the following elements as needed to replace a portion of Fe in the balance.

[0134] [Cr: 0~0.90%]

[0135] Cr is an element that improves the hardenability of steel and contributes to increased strength and / or corrosion resistance. The Cr content can be 0%, but to achieve these effects, the Cr content is preferably 0.001% or more, and can be 0.01%, 0.05%, or 0.10%. On the other hand, even if excessive Cr is contained, the effect is saturated, which may lead to an increase in manufacturing costs. Therefore, the Cr content is preferably 0.90% or less, and can be 0.70%, 0.50%, 0.40%, or 0.30% or less.

[0136] [Ti: 0~0.200%, V: 0~0.300%, Cu: 0~0.40%, Mo: 0~0.12%, Ni: 0~0.30%, B: 0~0.0030%, Ca: 0~0.0010%, Mg: 0~0.0010%, B i: 0~0.010%, Zr: 0~0.050%, Co: 0~0.010%, Zn: 0~0.010%, W: 0~0.100%, Sn: 0~0.040%, As: 0~0.100% and REM: 0~0.0100%]

[0137] Ti, V, Cu, Mo, Ni, B, Ca, Mg, Bi, Zr, Co, Zn, W, Sn, As, and REM may be contained in the steel sheet as optional elements or may be present in the steel sheet as mixed elements. The contents of these elements may be Ti: 0 to 0.200% or 0.100%, V: 0 to 0.300% or 0.200%, Cu: 0 to 0.40% or 0.20%, Mo: 0 to 0.12% or 0.08%, Ni: 0 to 0.30% or 0.15%, B: 0 to 0.0030% or 0.0015%, Ca: 0 to 0.0010% or 0.0008%, Mg: 0 to 0.0 The following elements are present in the following compositions: 0.010% or 0.0008%, Bi: 0-0.010%, Zr: 0-0.050% or 0.030%, Co: 0-0.010%, Zn: 0-0.010%, W: 0-0.100% or 0.050%, Sn: 0-0.040% or 0.020%, As: 0-0.100% or 0.050%, and REM: 0-0.0100% or 0.0050%. Regarding the lower limits of these elements, for example, the contents of Ti, V, Cu, Mo, Ni, Bi, Zr, Co, Zn, W, Sn, and As can be 0.001% or more, 0.005% or more, or 0.008% or more, respectively. Similarly, the contents of B, Ca, Mg, and REM can be 0.0001% or more, 0.0002% or more, or 0.0005% or more.

[0138] In the steel sheet according to the embodiment of the present invention, the balance other than the above-mentioned elements is composed of Fe and impurities. Impurities are, for example, components that are introduced into the steel sheet during industrial production, including raw materials such as ore and scrap, due to various factors in the manufacturing process. Impurities are permitted to be present within a range that does not affect the effects of the present invention.

[0139] The chemical composition of the steel sheet according to the embodiments of the present invention can be measured using common analytical methods. For example, the chemical composition of the steel sheet can be measured using inductively coupled plasma-atomic emission spectrometry (ICP-AES). C and S can be measured using the combustion-infrared absorption method, N can be measured using the inert gas fusion-thermal conductivity method, and O can be measured using the inert gas fusion-non-dispersive infrared absorption method.

[0140] [Metallic Structure]

[0141] [Martensite: 90.0% or more, and retained austenite: 3.0% or less]

[0142] The metal structure of the steel sheet according to the embodiment of the present invention contains, by area %, 90.0% or more of martensite and 3.0% or less of retained austenite. By forming the metal structure of the steel sheet in such a manner as to include these structures, high strength, for example, a tensile strength of 980 MPa or more, can be achieved, and the hole expandability of the resulting steel sheet can be significantly improved. More specifically, by controlling the hard martensite to be within a range of 90.0% or more by area %, a more uniform structure is formed, which not only contributes to high strength but also reduces the hardness difference in the metal structure. Due to such a reduction in hardness difference, hole expandability can be improved. If the area ratio of martensite is less than 90.0%, the desired strength and hole expandability cannot be achieved. From the perspective of further increasing strength and improving hole expandability, the higher the area ratio of martensite, the more preferred it is, and for example, it can be 92.0% or more, 94.0% or more, 96.0% or more, or 98.0% or more. The upper limit of the area ratio of martensite is not particularly limited and can be 100.0%, for example, can be 99.0% or less. On the other hand, retained austenite can become the starting point of destruction in the deformation in press forming, etc., so in addition to controlling martensite to be more than 90.0% by area%, the retained austenite is also limited to less than 3.0% by area, thereby being able to more significantly improve the hole expansion. If the area ratio of retained austenite exceeds 3.0%, they become the starting point of destruction in deformation, and hole expansion is reduced. From the viewpoint of further improving hole expansion, the lower the area ratio of retained austenite, the more preferably, for example, can be less than 2.5%, less than 2.0%, less than 1.5% or less than 1.0%. The lower limit of the area ratio of retained austenite is not particularly limited and can be 0%, for example, can be more than 0.5%.

[0143] [Other organizations]

[0144] The remaining structure other than martensite and retained austenite can be 0% by area percentage. However, if other structure exists, the remaining structure can be a structure containing at least one of ferrite: 10.0% or less, bainite: 10.0% or less, and pearlite: 10.0% or less. If the area ratio of at least one of ferrite, bainite, and pearlite exceeds 10.0% in total, the area ratio of martensite becomes less than 90.0%, and as a result, the desired strength and hole expandability cannot be achieved. The lower limits of ferrite, bainite, and pearlite can each be 0%, for example, 0.1% or more, 0.5% or more, 1.0% or more, 2.0% or more, or 3.0% or more. Similarly, the upper limits of ferrite, bainite, and pearlite can each be 8.0% or less, 6.0% or less, 5.0% or less, or 4.0% or less.

[0145] [Identification of Metallic Structure and Calculation of Area Ratio]

[0146] The metallographic structure of the steel sheet and the calculation of its area ratio are determined by optical microscopic observation and X-ray diffraction after etching with Nital or Lepera solution. The optical microscopic observation is performed on a cross-section perpendicular to the sheet surface. The cross-section is preferably parallel to the rolling direction. Specifically, a sample is first prepared from the steel sheet and the observation surface is etched with Nital. Next, an optical microscopic micrograph is obtained at a depth of 1 / 4 the sheet thickness within a 300 μm × 300 μm field of view. Image analysis of this micrograph is performed to calculate the combined area ratio of martensite and bainite, as well as the individual area ratios of ferrite and pearlite. Next, using a sample whose observation surface has been Lepera-etched, an optical microscopic micrograph is similarly obtained at a depth of 1 / 4 the sheet thickness within a 300 μm × 300 μm field of view. Image analysis of this micrograph is performed to calculate the combined area ratio of martensite and retained austenite. Next, the volume fraction of retained austenite was calculated by X-ray diffraction measurement using a sample face-cut to a depth of 1 / 4 the plate thickness from the direction normal to the rolling surface. Since the volume fraction of retained austenite is equal to the area fraction, this is taken as the area fraction of retained austenite. The area fraction of martensite was calculated by subtracting the area fraction of retained austenite obtained from the previously calculated total area fraction of martensite and retained austenite. Finally, the area fraction of bainite was calculated by similarly subtracting the area fraction of martensite obtained from the previously calculated total area fraction of martensite and bainite.

[0147] [Average grain size of prior austenite grains: 30.0 μm or less]

[0148] In the steel plate involved in the embodiment of the present invention, the average particle size of the original austenite grains is 30.0 μm or less. As mentioned above, it is believed that the original austenite grain boundaries act as a resistance relative to the movement of dislocations, which is effective in improving the work hardening ability. In this regard, by refining the original austenite grains, the density of the original austenite grain boundaries can be increased. Therefore, by refining the original austenite grains to 30.0 μm or less, the obstacles of dislocations can be increased, and thus the work hardening ability of the resulting steel plate can be improved. From the viewpoint of further improving the work hardening ability of the steel plate, the smaller the average particle size of the original austenite grains, the more preferred, for example, it can be 28.0 μm or less, 25.0 μm or less, 22.0 μm or less, 20.0 μm or less, 18.0 μm or less, or 15.0 μm or less. The lower limit is not particularly limited, but the average grain size of the prior austenite grains may be, for example, 3.0 μm or more, 5.0 μm or more, 8.0 μm or more, 10.0 μm or more, or 12.0 μm or more.

[0149] [The ratio of prior austenite grains having an aspect ratio of 2.0 or less to all prior austenite grains: 90.0% or more in terms of area %]

[0150] In the metal structure of the steel plate involved in the embodiment of the present invention, although not particularly limited, the proportion of the original austenite grains with an aspect ratio of 2.0 or less in the total original austenite grains can be, for example, 90.0% or more, 92.0% or more, 94.0% or more, or 96.0% or more in terms of area %. By increasing the proportion of original austenite grains with such a small aspect ratio, the anisotropy of the metal structure can be reduced. The upper limit is not particularly limited, but, for example, the proportion of original austenite grains with an aspect ratio of 2.0 or less in the total original austenite grains can be 100.0%, 99.0% or less, or 98.0% or less. As described above, the purpose of the present invention is to provide a steel plate having improved hole expandability and work hardening ability despite high strength, and the above purpose is achieved by constituting the metal structure of the steel plate having a prescribed chemical composition with a structure mainly composed of martensite, limiting the average particle size of the original austenite grains in the metal structure to a prescribed range, and appropriately controlling the proportion of grains with a prescribed intragranular orientation difference in the martensite structure. Therefore, it is clear that the proportion of prior austenite grains having an aspect ratio of 2.0 or less in all prior austenite grains is not a technical feature essential for achieving the object of the present invention.

[0151] [Method for determining the average grain size of prior austenite grains and the ratio of prior austenite grains having an aspect ratio of 2.0 or less to all prior austenite grains]

[0152] The average particle size of the original austenite grains and the proportion of the original austenite grains with an aspect ratio of 2.0 or less in the total original austenite grains are determined as follows. First, a sample is cut out from any position more than 50 mm from the end face of the steel plate (in the case where the sample cannot be prepared from this position, the position of the end is avoided) in such a way that a plate thickness section perpendicular to the plate surface can be observed. The plate thickness section is preferably parallel to the rolling direction. The size of the sample also depends on the measuring device, but it is set to a size of about 10 mm that can be observed in a direction perpendicular to the plate thickness direction. After the cross section of the above sample is polished using silicon carbide paper of #600 to #1500, it is finished into a mirror surface using a liquid obtained by dispersing diamond powder with a particle size of 1 to 6 μm in a diluent such as alcohol and / or pure water. Then, the observation surface is finished by electrolytic polishing. At any position in the longitudinal direction of the sample cross section and at a depth of 1 / 4 of the plate thickness, an area of ​​50 μm in length and 50 μm in the plate thickness direction is measured using electron backscatter diffraction at a measurement interval of 0.1 μm to obtain crystal orientation information. In the measurement, an EBSD analysis device consisting of a thermal field emission scanning electron microscope and an EBSD detector can be used. For example, an EBSD analysis device consisting of a JEOL JSM-7001F and a TSL DVC5 detector can be used. At this time, the vacuum degree in the EBSD analysis device can be set to 9.6×10 -5 Pa or less, the acceleration voltage can be set to 15 kV, and the irradiation current level can be set to 13. Using the obtained crystal orientation information, the crystal orientation of the prior austenite grains is calculated based on the crystal orientation relationship between general prior austenite grains and grains with a body-centered structure after phase transformation. The method for calculating the crystal orientation of the prior austenite grains uses the following method. First, a crystal orientation map of the prior austenite grains is created using the method described in Acta Materialia, 58 (2010), 6393-6403. For one of the prior austenite grains contained in the observation field, the average value of the shortest diameter and the longest diameter is calculated, and this average value is used as the grain size of the prior austenite grain. The above operation is performed on all prior austenite grains, excluding the prior austenite grains at the ends of the imaging field of view, where the entire grain is not included in the imaging field of view, to determine the grain size of all prior austenite grains in the imaging field of view. The average grain size of the prior austenite grains is determined by calculating the average grain size from the obtained grain sizes of all prior austenite grains.

[0153] Next, for each prior austenite grain within the observation field, the ratio of its diameter in the plate thickness direction to its diameter in the rolling direction (rolling direction diameter / plate thickness diameter) is calculated, and this value is used as the aspect ratio of the prior austenite grain. If the rolling direction is unknown, cross-sections are observed at arbitrary angles of 0°, 45°, 90°, and 135°. The cross-section with the highest aspect ratio is defined as the cross-section parallel to the rolling direction, and the ratio of its diameter in the plate thickness direction to its diameter in the rolling direction (rolling direction diameter / plate thickness diameter) is calculated. This operation is repeated for all prior austenite grains, excluding those at the ends of the field of view where the entire grain is not contained within the field of view, to determine the aspect ratio of all prior austenite grains within the field of view. The proportion of prior austenite grains with an aspect ratio of 2.0 or less among all prior austenite grains is determined by dividing the total number of prior austenite grains with an aspect ratio of 2.0 or less by the total number of prior austenite grains.

[0154] [Ratio of grains with intra-grain orientation difference of 4° or more in martensite: 45.0-70.0% by area]

[0155] In the martensitic structure of the steel plate according to the embodiment of the present invention, when the region surrounded by grain boundaries with an orientation difference of 15° or more is defined as a grain, the proportion of grains with an orientation difference of 4° or more within the grain is controlled to be within the range of 45.0% to 70.0% in terms of area %. As previously mentioned, the more unevenly the dislocations are dispersed, the greater the orientation difference within the grain is generally. Grains with an orientation difference of 4° or more within the grain are grains with an orientation difference sufficient to form a structure with unevenly dispersed dislocations, thereby developing uneven deformation. In this regard, in the embodiment of the present invention, by making such grains exist in the martensite within the range of 45.0% to 70.0% in terms of area %, a certain amount of structure with unevenly dispersed dislocations exists, thereby developing uneven deformation during processing such as press forming. As a result, sufficient work hardening ability can be maintained even in the later stages of deformation, thereby achieving a high work hardening rate. When the ratio of crystal grains with an intragranular orientation difference of 4° or more is less than 45.0% or exceeds 70.0% in area %, uneven deformation is difficult to develop in processes such as press forming, and therefore the desired work hardening rate cannot be achieved. The ratio of crystal grains with an intragranular orientation difference of 4° or more in a martensitic structure can be, for example, 48.0% or more, 50.0% or more, or 55.0% or more in area %. Similarly, the ratio of crystal grains with an intragranular orientation difference of 4° or more in a martensitic structure can be, for example, 65.0% or less, 62.0% or less, or 60.0% or less in area %.

[0156] [Method for determining the ratio of grains having an intra-grain orientation difference of 4° or more in martensite]

[0157] The proportion of grains with an orientation difference of 4° or more within the martensite is measured by electron backscatter diffraction (EBSD). More specifically, first, a sample is prepared from the steel plate in such a way that the cross section of the plate thickness in the direction perpendicular to the plate surface becomes the observation surface. Then, at a depth of 1 / 4 of the plate thickness from the surface of the steel plate, an EBSD analysis is performed on an area of ​​200 μm in the direction perpendicular to the plate thickness direction and 100 μm in the plate thickness direction at a measurement interval of 0.2 μm to obtain crystal orientation information. Here, the EBSD analysis is performed using an EBSD analysis device consisting of a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (HIKARI detector manufactured by TSL) at an analysis speed of 50 to 300 points / second. At this time, the vacuum degree in the EBSD analysis device can be set to 9.6×10 -5 Pa or less, the acceleration voltage can be set to 15kV, and the irradiation current level can be set to 13. Then, for the obtained crystal orientation information, the "Phase Map" function installed in the software "OIM Analysis (registered trademark)" attached to the EBSD analysis device is used to determine the martensitic structure. Then, in the determined martensitic structure, the area surrounded by grain boundaries with an orientation difference of 15° or more is defined as a grain, the average orientation difference within the grain of the grain is calculated, and the proportion of grains with an orientation difference of 4° or more within the grain is obtained. The grains defined as described above and the average orientation difference within the grain can also be calculated using the software "OIM Analysis (registered trademark)" attached to the EBSD analysis device. In the present invention, "orientation difference within the grain" means the orientation dispersion within the grain, namely "Grain Orientation Spread (GOS)". The intra-grain misorientation value is determined as the average of the misorientations between the reference crystal orientation and all measurement points within the same grain, as described in "Analysis of misorientation during plastic deformation of stainless steel based on EBSD and X-ray diffraction," by Hideho Kimura et al., Proceedings of the Japan Society of Mechanical Engineers (Edition A), Vol. 71, No. 712, 2005, pp. 1722-1728. In the embodiment of the present invention, the reference crystal orientation is the orientation averaged across all measurement points within the same grain. The GOS value can be calculated using the software "OIM Analysis (registered trademark) Version 7.0.1" included with the EBSD analyzer.

[0158] [Plate thickness]

[0159] The steel plates according to the embodiments of the present invention are not particularly limited, but generally have a thickness of 1.0 to 8.0 mm. For example, the thickness may be 1.2 mm or greater, 1.6 mm or greater, or 2.0 mm or greater, and / or may be 7.0 mm or less, 6.0 mm or less, 5.5 mm or less, 5.0 mm or less, 4.4 mm or less, 4.2 mm or less, or 4.0 mm or less.

[0160] The steel sheets according to the embodiments of the present invention can effectively combine the conflicting characteristics of high strength and excellent workability, and are useful for use in parts and the like in technical fields requiring both of these characteristics, and are particularly useful for use in parts and the like in the automotive field. Therefore, in a preferred embodiment, an automobile part, particularly a running part of an automobile, is provided, comprising the steel sheets according to the embodiments of the present invention. Examples of running parts of an automobile include lower arms and rear swing arms. These automobile parts, particularly running parts of an automobile, only need to include the steel sheets according to the embodiments of the present invention in at least a portion of these parts, and therefore, at least a portion of these parts satisfies the above-mentioned chemical composition and structural characteristics. In areas of the steel sheet that are processed to a relatively low degree during press forming, the characteristics of the steel sheet do not change significantly before and after forming. Areas of the steel sheet that are processed to a relatively low degree are judged by characteristics such as a smooth shape without deformation such as bending, and a small rate of increase or decrease in the plate thickness.

[0161] [Mechanical properties]

[0162] [Tensile strength: TS]

[0163] According to the steel plate having the above-mentioned chemical composition and metal structure, in particular the hot-rolled steel plate, a high tensile strength can be achieved, specifically, a tensile strength of 980 MPa or more can be achieved. The tensile strength is preferably 1000 MPa or more, 1080 MPa or more, or 1180 MPa or more. The steel plate according to the embodiment of the present invention, despite having such a very high tensile strength, can achieve excellent hole expandability and work hardening ability through the specific combination of the chemical composition and metal structure described above. The upper limit of the tensile strength is not particularly limited, but for example, the tensile strength of the steel plate can be 1780 MPa or less, 1700 MPa or less, or 1600 MPa or less. The tensile strength is measured by preparing a JIS No. 5 test specimen in a direction (C direction) such that the longitudinal direction of the specimen is parallel to the rolling direction (direction perpendicular to the rolling direction) of the steel plate, and performing a tensile test based on JIS Z 2241:2011. For example, if it's difficult to produce a JIS 5 specimen due to dimensional constraints, other specimens listed in JIS Z 2241:2011 can be used. However, if the plate thickness is less than 0.5 mm, 0.5 mm is used as the lower limit for proper evaluation. For example, if it's difficult to produce a JIS 5 specimen due to dimensional constraints, and it's also difficult to use other specimens listed in JIS Z 2241:2011, a micro Vickers hardness test based on JIS 2244-1:2020 can be performed, and the hardness (HV) converted to tensile strength can be used. Specimens for the micro Vickers hardness test can be prepared using the same method used to prepare specimens for evaluating the average grain size and aspect ratio of prior austenite grains. In the micro Vickers hardness test, 30 points are measured at 1 / 4 of the plate thickness with a load of 500 gf, and the average value is used. Conversion can be performed using the following formula.

[0164] Tensile strength [MPa] = 3.12 × Vickers hardness [HV] + 16

[0165] [Hole expansion ratio: λ]

[0166] The steel sheet having the above-described chemical composition and metallographic structure can achieve high hole expandability, specifically, a hole expansion ratio of 45% or greater. The hole expansion ratio can preferably be 50% or greater, and more preferably 60% or greater or 70% or greater. The upper limit of the hole expansion ratio is not particularly limited; for example, the hole expansion ratio can be 150% or less, 120% or less, or 100% or less. The hole expansion ratio is determined as follows. First, a test piece measuring 100 mm wide by 100 mm long is prepared from the steel sheet. A punching tool with a punch diameter of 10 mm and a die diameter of 10.25 to 11.5 mm (12.5% ​​clearance) is used to create a punched hole (initial hole: hole diameter d0 = 10 mm). Next, the initial hole is expanded using a conical punch with a 60° apex angle, with the burr facing the die side, until a through-thickness crack is generated. The hole diameter d1 mm at the time of crack initiation is measured, and the hole expansion ratio λ (%) of each test piece is calculated using the following formula. This hole expansion test was performed three times, and the average value thereof was determined as the hole expansion ratio λ.

[0167] λ=100×{(d1-d0) / d0}

[0168] <Method for Manufacturing Steel Sheet>

[0169] Next, a preferred method for manufacturing the steel sheet according to the embodiment of the present invention will be described. The following description is intended to illustrate the characteristic method for manufacturing the steel sheet according to the embodiment of the present invention, in particular, the steel sheet having preferred characteristics, and is not intended to limit the steel sheet to the steel sheet manufactured using the manufacturing method described below. More specifically, the following specifically illustrates the manufacture of hot-rolled steel sheet, but the steel sheet according to the embodiment of the present invention is any steel sheet having the chemical composition and metal structure described above, that is, not only hot-rolled steel sheet, but also cold-rolled steel sheet, plated steel sheet, etc. Therefore, the following description is merely an example of a preferred manufacturing method when the steel sheet according to the embodiment of the present invention is a hot-rolled steel sheet.

[0170] A method for manufacturing a steel plate according to an embodiment of the present invention is characterized in that it includes a heating step, a hot rolling step, a cooling step, a coiling step, and a strain imparting step.

[0171] In the heating step, the slab having the chemical composition described above with respect to the steel plate is heated and maintained at a temperature range of 1100° C. or higher for 6000 seconds or longer.

[0172] The hot rolling process includes the step of performing finish rolling on the slab, wherein the finish rolling satisfies the following conditions (a) to (c).

[0173] (a) The reduction rate in the rolling pass before the final section is 30-50%, and the reduction rate in the rolling pass of the final section is 20-50%;

[0174] (b) a total reduction of 90% or more; and

[0175] (c) The rolling temperature in the rolling pass before the final section is 970~1100℃, and the final rolling temperature is 960~1050℃,

[0176] In the cooling step, cooling of the finish-rolled steel sheet begins within 0.5 to 10.0 seconds after completion of the hot rolling step, and then the steel sheet is cooled to a temperature of 400° C. or less within 20.0 seconds from the start of cooling.

[0177] In the coiling process, the cooled steel sheet is coiled at a temperature of 400°C or below.

[0178] In the strain imparting step, a strain of 0.16 to 1.40% in absolute value is imparted to the obtained steel plate at a position 1 / 4 of the plate thickness while repeatedly switching between positive and negative strains three or more times.

[0179] In the above-mentioned manufacturing method, the temperatures described for the slab and the steel plate refer to the surface temperature of the slab and the surface temperature of the steel plate, respectively.

[0180] [Heating process]

[0181] First, a slab having the chemical composition described above for the steel plate is heated and held in a temperature range of 1100°C or higher for 6000 seconds or longer. From a productivity perspective, slabs obtained by continuous casting are preferred, but slabs obtained by casting or slab-opening can also be used, or slabs obtained by hot working or cold working as needed can also be used. In this manufacturing method, holding the slab in a temperature range of 1100°C or higher does not necessarily mean holding the slab at a constant temperature of 1100°C or higher, but also includes holding the slab while varying the temperature within a temperature range of 1100°C or higher. By holding the slab in a temperature range of 1100°C or higher for 6000 seconds or longer, the coarse carbides present in the structure can be completely dissolved, eliminating the starting point of cracks. If the holding temperature is lower than 1100°C or the holding time is lower than 6000 seconds, the dissolution of the coarse carbides becomes incomplete. If the coarse carbides are not fully dissolved, a ferrite-bainite transformation starting from these carbides will occur during the cooling process described later, causing the martensite area ratio to fall below 90.0%. As a result, the desired strength and / or hole expandability cannot be achieved. The upper limit of the slab heating temperature is preferably 1300°C or lower, or 1200°C or lower. Similarly, the upper limit of the holding time in the temperature range of 1100°C or higher is preferably 10,000 seconds or lower.

[0182] [Hot rolling process]

[0183] [Rough rolling]

[0184] In the present production method, for example, the heated slab may be subjected to rough rolling before finish rolling in order to adjust the thickness, etc. The conditions for rough rolling are not particularly limited as long as the desired thin slab dimensions can be ensured.

[0185] [(a) Reduction rate in the rolling pass before the final section: 30-50%, and reduction rate in the rolling pass of the final section: 20-50%]

[0186] The heated slab, or a slab that has been rough-rolled as needed, is then subjected to finish rolling. In this manufacturing method, finish rolling is performed using a tandem rolling mill consisting of multiple rolling stands, for example, five or more. In this manufacturing method, during the finish rolling of the heated slab, the reduction ratio in each of the two subsequent rolling passes must be appropriately controlled. Specifically, the reduction ratio in the rolling pass preceding the final stage is controlled to 30-50%, and the reduction ratio in the final stage is controlled to 20-50%. By performing such high reduction ratio rolling in each of the rolling passes preceding the final stage and the final stage, recrystallization is promoted, resulting in a finer metal structure. Furthermore, the proportion of prior austenite grains with an aspect ratio of 2.0 or less within the total prior austenite grains can be increased. If the reduction rate in the rolling pass of the first section of the final section is less than 30%, and / or the reduction rate in the rolling pass of the final section is less than 20%, the recrystallization is not completed or is not fully promoted, and sometimes the desired average particle size of the original austenite grains cannot be achieved in the metal structure of the steel plate finally obtained, and / or sometimes the proportion of original austenite grains with an aspect ratio of less than 2.0 becomes a smaller value. In the case where the desired average particle size of the original austenite grains cannot be achieved, sufficient work hardening ability cannot be obtained. On the other hand, if the reduction rate in each rolling pass of the first section of the final section and / or the final section is too high, the rolling load becomes too large, and the load on equipment such as the rolling mill becomes higher. Therefore, the reduction rate in each rolling pass of the first section of the final section and the final section is set to less than 50%. Preferably, the reduction rate in each rolling pass of the first section of the final section and the final section is less than 45%.

[0187] [(b) Total reduction rate: more than 90%]

[0188] In this manufacturing method, the total reduction rate in the finishing rolling is controlled to be 90% or more. Since the Mn contained in the steel is an element that reduces the destruction energy of the grain boundary, if there is an area where Mn is locally concentrated, it sometimes promotes the generation of cracks during plastic deformation in press forming, etc. Therefore, from the perspective of further improving the hole expandability, it is effective to suppress or reduce the local concentration of Mn. By controlling the total reduction rate in the finishing rolling to be 90% or more, Mn can be diffused in the steel, and in connection with this, the deviation of the Mn concentration in the steel can be suppressed or reduced, that is, the local concentration of Mn can be suppressed or reduced. If the total reduction rate in the finishing rolling is lower than 90%, the deviation of the Mn concentration becomes higher, Mn is locally concentrated, and sometimes it is not possible to fully suppress the development of the area with reduced destruction energy. Furthermore, if the total reduction ratio during finish rolling is less than 90%, the accumulated strain during rolling becomes insufficient, resulting in incomplete recrystallization or insufficient promotion. This can sometimes prevent the desired average particle size of the prior austenite grains from being achieved in the metallurgical structure of the resulting steel sheet, and / or can sometimes reduce the proportion of prior austenite grains with an aspect ratio of 2.0 or less. The upper limit of the total reduction ratio during finish rolling can be, for example, 99% or less or 98% or less. The total reduction ratio during finish rolling is calculated using the following formula.

[0189] Total reduction (%) = (plate thickness before finishing rolling - plate thickness after finishing rolling) / plate thickness before finishing rolling × 100

[0190] [(c) Rolling temperature in the rolling pass before the final stage: 970-1100°C, final rolling temperature: 960-1050°C]

[0191] In this manufacturing method, in addition to controlling the reduction ratio in each of the two subsequent rolling passes of finish rolling, the rolling temperature in the rolling pass preceding the final stage (entry temperature in the rolling pass preceding the final stage) and the final rolling temperature (finishing temperature) are also crucial for controlling the metallurgical structure of the steel sheet. If the rolling temperature in the rolling pass preceding the final stage is lower than 970°C and / or the final rolling temperature is lower than 960°C, recrystallization is not completed or sufficiently promoted, and the desired average grain size of prior austenite grains may not be achieved in the final steel sheet metallurgy, and / or the proportion of prior austenite grains with an aspect ratio of 2.0 or less may be low. If the desired average grain size of prior austenite grains cannot be achieved, sufficient work hardening performance cannot be achieved. On the other hand, if the rolling temperature before the final stage exceeds 1100°C and / or the final rolling temperature exceeds 1050°C, the prior austenite grains become coarse and the desired average grain size of the prior austenite grains may not be achieved. In this case, sufficient work hardening ability cannot be obtained.

[0192] [Cooling process]

[0193] [Time from completion of hot rolling to start of cooling: 0.5 to 10.0 seconds]

[0194] [Time from the start of cooling until the temperature reaches 400°C or lower: 20.0 seconds or less]

[0195] The finish-rolled steel sheet begins cooling in the subsequent cooling step within 0.5 to 10 seconds after the completion of the hot rolling step, and is then cooled to a temperature of 400°C or less within 20 seconds from the start of cooling. This cooling control achieves a desired average grain size of prior austenite grains and a desired ratio of prior austenite grains with an aspect ratio of 2.0 or less in the metallurgical structure of the resulting steel sheet.

[0196] If the time from the completion of the hot rolling process to the start of cooling is less than 0.5 seconds, recrystallization is not completed or is not sufficiently promoted, and the desired average grain size of the prior austenite grains and / or the desired proportion of prior austenite grains with an aspect ratio of 2.0 or less may not be achieved in the metallurgical structure of the final steel plate. Furthermore, if the time from the completion of the hot rolling process to the start of cooling exceeds 10.0 seconds, grain growth proceeds excessively, and the desired average grain size of the prior austenite grains cannot be achieved. As a result, sufficient work hardening ability cannot be achieved in the steel plate in any case. On the other hand, if the cooling time from the start of cooling to a temperature below 400°C exceeds 20.0 seconds or the cooling stop temperature exceeds 400°C, ferrite, bainite, and / or pearlite are generated during cooling, and the area fraction of martensite becomes less than 90.0%. As a result, the desired strength and / or hole expandability cannot be achieved.

[0197] [Coiling process]

[0198] Next, the cooled steel sheet is coiled in a temperature range of 400°C or lower. If the coiling temperature exceeds 400°C, ferrite, bainite, and / or pearlite are generated during coiling, similar to the cooling process, and the area ratio of martensite becomes less than 90.0%. As a result, the desired strength and / or hole expandability cannot be achieved.

[0199] [Strain imparting step]

[0200] Finally, the resulting steel sheet is subjected to a strain of 0.16 to 1.40% in absolute value at a position 1 / 4 of the thickness of the steel sheet, while repeatedly applying positive and negative strains three or more times, to produce the steel sheet. By repeatedly applying such strain to the steel sheet, dislocations can be unevenly introduced into the grains of the martensite structure. As a result, the proportion of grains with an orientation difference of 4° or more within the grains can be controlled within the range of 45.0 to 70.0% by area. If the absolute value of the applied strain is less than 0.16% or the number of times the strain is applied is less than three times, the uneven introduction of dislocations within the grains becomes insufficient, and the proportion of grains with an orientation difference of 4° or more within the grains becomes less than 45.0% by area, failing to achieve the desired work hardening rate. On the other hand, if the absolute value of the applied strain exceeds 1.40%, the accumulation of dislocations in the grains becomes excessive, and the proportion of grains with an orientation difference of 4° or more within the grains exceeds 70.0% by area, and similarly, the desired work hardening rate cannot be achieved. The upper limit of the number of times the strain is applied is not particularly limited, but from the viewpoint of manufacturability, it is preferably 10 times or less, 7 times or less, or 5 times or less.

[0201] Because the martensite structure inherits the dislocations within the austenite grains before the transformation, the intragranular orientation difference of the martensite grains formed from unrecrystallized austenite grains is generally high, while the intragranular orientation difference of the martensite grains formed from recrystallized austenite grains is low. In this production method, as described above, the structure is refined by promoting recrystallization during the hot rolling process, thereby controlling the average grain size of the prior austenite grains to 30.0 μm or less. However, in this case, since martensite is generated from the recrystallized austenite grains, the intragranular orientation difference of the martensite grains is reduced, making it impossible to achieve a ratio of grains with an intragranular orientation difference of 4° or greater within the range of 45.0 to 70.0% by area. Therefore, in the present manufacturing method, the original austenite structure is formed into a structure in which recrystallization is roughly or completely completed by appropriately controlling the conditions of the hot rolling process, thereby making the structure finer and improving the work hardening ability of the steel plate as a whole. In addition, in the strain-imparting process, orientation differences are appropriately generated by unevenly introducing dislocations into the grains of a portion of the martensite structure, thereby achieving a high work hardening rate even in a state where strain is introduced to a certain extent, such as in the late stage of deformation during press forming.

[0202] The strain-imparting process can be performed using any appropriate method known to those skilled in the art. While not particularly limited, examples of such methods include bending and rebending using a tension leveler. In this case, the absolute value of the applied strain can be easily varied by adjusting the size of the tension leveler's rollers, their relative positional relationship, and the thickness of the steel sheet. Therefore, by appropriately controlling these parameters, the desired strain can be easily imparted to the steel sheet at a position ¼ of its thickness.

[0203] The steel sheet produced using the above-described manufacturing method achieves high strength, for example, a tensile strength of 980 MPa or higher, by adopting a more uniform structure composed of at least 90.0% martensite and at most 3.0% retained austenite, based on area percentage. This structure also significantly improves hole expandability due to, for example, reduced hardness gradients. Furthermore, by controlling the average grain size of the prior austenite grains in the metallographic structure to 30.0 μm or less, the work hardening capability of the steel sheet as a whole is enhanced. Furthermore, by controlling the proportion of grains with an intra-grain orientation difference of 4° or greater within the martensite to within a range of 45.0% to 70.0% by area percentage, a high work hardening rate can be achieved even in conditions where a certain degree of strain is introduced, such as in the late stages of press forming. Therefore, the steel sheet produced using the above-described manufacturing method reliably combines the conflicting properties of high strength and excellent workability, making it particularly useful in the automotive industry, where these properties are required to be combined.

[0204] Hereinafter, the present invention will be described in more detail using examples, but the present invention is not limited to these examples at all.

[0205] Example

[0206] In the following examples, steel sheets according to embodiments of the present invention, particularly hot-rolled steel sheets, were produced under various conditions, and the tensile strength (TS), hole expansion ratio (λ), and work hardening ratio (WHR) of the obtained steel sheets were examined.

[0207] First, molten steel was cast using a continuous casting method to form slabs having the various chemical compositions shown in Tables 1 and 2. These slabs were then heated to a temperature of 1100-1200°C and held for the time shown in Table 3. Hot rolling was then performed. Hot rolling was performed by performing rough rolling and finish rolling. More specifically, rough rolling was performed under the same conditions in all Examples and Comparative Examples, while finish rolling was performed using a tandem mill consisting of five rolling stands under the conditions shown in Table 3. The finish-rolled steel plates were then cooled and coiled under the conditions shown in Table 3. Finally, the absolute values ​​of the strain shown in Table 3 were applied a predetermined number of times at a position 1 / 4 of the thickness of the steel plates while repeatedly alternating between positive and negative strains, resulting in steel plates having the thickness shown in Table 4.

[0208]

[0209]

[0210]

[0211] The properties of the obtained steel sheets were measured and evaluated by the following methods.

[0212] [Tensile Strength (TS)]

[0213] The tensile strength (TS) was measured by preparing a JIS No. 5 test specimen in a direction (C direction) such that the longitudinal direction of the test specimen was parallel to the rolling direction of the steel sheet at right angles, and performing a tensile test in accordance with JIS Z 2241:2011.

[0214] [Hole Expansion Ratio (λ)]

[0215] The hole expansion ratio is determined as follows. First, a test piece measuring 100 mm wide by 100 mm long is prepared from a steel plate. Using a punching tool with a punch diameter of 10 mm and a die diameter of 10.25 to 11.5 mm (12.5% ​​clearance), a punched hole (initial hole diameter d0 = 10 mm) is created. Next, the initial hole is expanded using a conical punch with a 60° apex angle, with the burr facing the die side, until a through-the-plate crack forms. The hole diameter d1 mm at the time of crack initiation is measured, and the hole expansion ratio λ (%) for each test piece is calculated using the following formula. This hole expansion test is performed three times, and the average value is determined as the hole expansion ratio λ.

[0216] λ=100×{(d1-d0) / d0}

[0217] [Work hardening ability]

[0218] Work hardening capacity is evaluated by determining the work hardening rate (WHR) of the steel plate through a tensile test. Specifically, the region where the strain (true strain) during tensile deformation is 0.04 or greater during the same tensile test as the TS measurement is simulated as the late stage of press forming. The maximum work hardening rate (WHR) in this region is determined using the following formula.

[0219] WHR (MPa) = dσ / dε

[0220] Where σ is the true stress and ε is the true strain.

[0221] Steel plates with a tensile strength (TS) of 980 MPa or greater, a hole expansion ratio (λ) of 45% or greater, and a maximum work hardening rate (WHR) of 1000 MPa or greater in the true strain range of 0.04 or greater were evaluated as having improved hole expandability and work hardening capability despite high strength. The results are shown in Table 4.

[0222]

[0223] Referring to Tables 1-4, it is believed that in Comparative Example 2, due to the short holding time in the temperature range above 1100°C during the heating process, the solid solution of coarse carbides was incomplete. In the subsequent cooling process, these carbides served as the starting point for ferrite and bainite transformations. As a result, the area fraction of martensite became less than 90.0%, and TS and λ decreased. In Comparative Examples 3 and 6, it is believed that due to the low reduction ratios in the preceding and final rolling passes of the finishing rolling, respectively, recrystallization was incomplete or insufficiently promoted. As a result, the average grain size of the prior austenite grains in the resulting metal structure increased, reducing the work hardening ability of the steel sheet. In Comparative Examples 4 and 7, it is believed that due to the low rolling temperatures in the preceding and final rolling passes of the finishing rolling, respectively, recrystallization was incomplete or insufficiently promoted. As a result, the average grain size of the prior austenite grains in the resulting metal structure increased, reducing the work hardening ability of the steel sheet. In Comparative Examples 5 and 8, it is believed that the prior austenite grains became coarser overall due to the high rolling temperatures in the rolling pass preceding the final stage of finish rolling and the high final rolling temperatures, respectively. As a result, the average particle size of the prior austenite grains in the resulting metal structure increased, reducing the work hardening ability of the steel sheet. In Comparative Example 9, it is believed that the time from the completion of the hot rolling process to the start of the cooling process was short, so recrystallization was not completed or was not sufficiently promoted. As a result, the average particle size of the prior austenite grains in the resulting metal structure increased, reducing the work hardening ability of the steel sheet. In Comparative Example 10, it is believed that the time from the completion of the hot rolling process to the start of the cooling process was long, resulting in excessive grain growth overall. As a result, the desired average particle size of the prior austenite grains could not be achieved, reducing the work hardening ability of the steel sheet. In Comparative Example 11, due to the long time from the start of cooling to the temperature reaching below 400°C in the cooling process, the area fraction of martensite became less than 90.0%, resulting in reduced TS and λ. In Comparative Example 12, due to the high coiling temperature, the area ratio of martensite is similarly less than 90.0%, and TS and λ are reduced. It is believed that in Comparative Example 13, the absolute value of the strain applied in the strain-imparting step is small, resulting in insufficient uneven introduction of dislocations within the grains. As a result, the proportion of grains with an orientation difference of 4° or more within the martensite grains decreases, and the work hardening rate of the steel sheet decreases. It is believed that in Comparative Example 14, the absolute value of the strain applied in the strain-imparting step is large, resulting in excessive accumulation of dislocations within the grains. As a result, the proportion of grains with an orientation difference of 4° or more within the martensite grains increases, and the work hardening rate of the steel sheet decreases. It is believed that in Comparative Example 15, the number of times strain is applied in the strain-imparting step is small, resulting in insufficient uneven introduction of dislocations within the grains. As a result, the proportion of grains with an orientation difference of 4° or more within the martensite grains decreases, and the work hardening rate of the steel sheet decreases.In Comparative Example 16, the strain-imparting step was omitted, so uneven dislocations were not introduced into the grains. As a result, the proportion of grains with an orientation difference of 4° or more within the martensite grains decreased, and the work hardening rate of the steel sheet decreased.

[0224] Comparative Examples 37 and 39, respectively, have low C and Si contents, resulting in reduced TS. On the other hand, Comparative Examples 38 and 40, respectively, have high C and Si contents, resulting in a high amount of retained austenite, which reduces λ. Comparative Example 41, due to its low Mn content, has reduced hardenability, resulting in a lower area ratio of martensite and a reduced TS. Comparative Example 42, due to its high Mn content, has reduced λ. It is believed that Comparative Example 43, due to its low sol.Al content, was unable to adequately suppress cementite precipitation. As a result, λ decreased. It is believed that Comparative Example 44, due to its low Nb content, was unable to adequately promote the refinement of prior austenite grains due to the pinning effect. As a result, the average particle size of the prior austenite grains in the resulting metal structure increased, reducing the work hardening ability of the steel plate. It is believed that due to its high Nb content, coarse carbides and the like were generated in the steel in Comparative Example 45. As a result, λ decreased.

[0225] In contrast, all of the inventive examples' steel sheets, by having a predetermined chemical composition and appropriately controlling various manufacturing process conditions, yielded a metallographic structure comprising, by area%, 90.0% or more of martensite and 3.0% or less of retained austenite, an average prior-austenite grain size of 30.0 μm or less, and a martensite grain ratio of 45.0% to 70.0% with an intra-grain orientation difference of 4° or greater. Furthermore, despite a high tensile strength of 980 MPa or more, significantly improved hole expandability and work hardening capability were achieved.

Claims

1. A steel plate, characterized in that: Chemical composition is expressed in mass %. C：0.040~0.200%、 Si: 0.30~2.00%, Mn: 1.00~4.00%, sol.Al: 0.001~0.500%, P: 0.100% or less, S: 0.0300% or less, N: 0.0070% or less, O: 0.0100% or less, Nb: 0.001~1.000%, Ti: 0~0.200%, V：0~0.300%、 Cu: 0~0.40%, Cr：0~0.90%、 Mo: 0~0.12%, Ni: 0~0.30%, B：0~0.0030%、 Ca: 0~0.0010%, Mg: 0~0.0010%, Bi: 0~0.010%, Zr：0~0.050%、 Co: 0~0.010%, Zn: 0~0.010%, W：0~0.100%、 Sn: 0~0.040%, As:0~0.100% REM: 0~0.0100%, and Balance: Fe and impurities, The metal structure contains Martensite: more than 90.0%, and Retained austenite: less than 3.0%, The average particle size of the original austenite grains is less than 30.0 μm. In the martensite, when regions surrounded by grain boundaries having an orientation difference of 15° or more are defined as grains, the ratio of the grains having an orientation difference of 4° or more within the grains is 45.0 to 70.0% in terms of area %.

2. The steel plate according to claim 1, wherein The chemical composition includes by mass % Ti: 0.001~0.200%, V：0.001~0.300%、 Cu: 0.001~0.40%, Cr：0.001~0.90%、 Mo: 0.001~0.12%, Ni: 0.001~0.30%, B：0.0001~0.0030%、 Ca: 0.0001~0.0010%, Mg: 0.0001~0.0010%, Bi: 0.001~0.010%, Zr：0.001~0.050%、 Co: 0.001~0.010%, Zn: 0.001~0.010%, W：0.001~0.100%、 Sn: 0.001~0.040%, As: 0.001~0.100%, and REM: 0.0001~0.0100% At least one of the following.

3. The steel plate according to claim 1 or 2, characterized in that: The metal structure further comprises, in terms of area %, Ferrite: less than 10.0%, Bainite: less than 10.0%, and Pearlite: less than 10.0% At least one of the following.

4. The steel plate according to any one of claims 1 to 3, characterized in that The ratio of prior austenite grains having an aspect ratio of 2.0 or less to all prior austenite grains is 90.0% or more in terms of area %.

5. The steel plate according to any one of claims 1 to 4, characterized in that The plate thickness is 1.0~8.0mm.

6. A component, characterized in that A steel plate comprising the steel plate according to any one of claims 1 to 5.

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