Steel sheet and method for manufacturing same

Abstract

Provided are: a steel sheet which has a prescribed chemical composition and has a metallographic structure containing, in area%, 60.0-85.0% of martensite, 10.0-30.0% of granular bainite having a maximum orientation difference of 3.5 DEG or less and an intragranular orientation difference of 10 DEG or more at an intragranular 0.1 [mu] m interval surrounded by grain boundaries having an orientation difference of 15 DEG or more, and 20.0% or less of ferrite; the average interval between the granular bainite grains is 50.0 [mu] m or less.

Description

TECHNICAL FIELD

[0001] The present application relates to a steel sheet and a method for manufacturing the same. BACKGROUND

[0002] In recent years, in order to cope with environmental problems, weight reduction of automobile parts is desired for the purpose of reducing the discharge of CO2 gas and improving fuel economy. On the other hand, social demands for improvement in collision safety are also increasing. In order to achieve both weight reduction and improvement in collision safety, high-strengthening of steel materials is an effective means. However, generally, if steel materials are high-strengthened, workability is reduced, and thus steel materials in which both strength and workability are improved are required.

[0003] In relation to improvement in strength and workability, for example, in Patent Literature 1, a high-strength hot-rolled steel sheet is described, which is characterized by having a prescribed chemical composition, having a structure in which bainite phase is more than 95% in area ratio in the entire region in the sheet thickness direction, and the average grain diameter of the bainite phase in a region from the surface up to a position of 1 / 4 of the sheet thickness in the sheet thickness direction is 5 μm or less in a sheet thickness section parallel to the rolling direction, and 4 μm or less in a sheet thickness section in a direction perpendicular to the rolling direction, and in a region centered on the central position of the sheet thickness and having a width of 1 / 10 of the sheet thickness in the sheet thickness direction, grains extending in the rolling direction having an aspect ratio of 5 or more are 7 or less, and the steel sheet has a tensile strength TS of 780 MPa or more. In addition, in Patent Literature 1, it is taught that according to the above constitution, a high-strength hot-rolled steel sheet having a tensile strength TS of 780 MPa or more and having remarkably improved blanking workability can be easily and inexpensively manufactured.

[0004] In Patent Literature 2, a cold-rolled annealed steel sheet is described, which has a prescribed chemical composition, has a microstructure composed of martensite and / or lower bainite in the surface existence ratio, the martensite includes fresh martensite and / or self-tempered martensite, the martensite and lower bainite in total in the surface existence ratio is in the range of 60 to 95%, low-carbide-containing bainite is in the range of 4 to 35%, ferrite is in the range of 0 to 5%, and island-shaped residual austenite is less than 5%. In addition, in Patent Literature 2, it is taught that according to the above constitution, a yield strength in the range of 800 to 970 MPa before skin pass rolling operation can be achieved, and a tensile strength in the range of 1180 to 1320 MPa, an elongation at break of at least 5%, and a hole expansion ratio Ac% of 30% or more can be achieved.

[0005] In Patent Document 3, a hot-rolled steel sheet is described, which is characterized by having a prescribed chemical composition, at a position of 1 / 4 of the sheet thickness from the surface, a total area ratio of a martensite phase and a lower bainite structure is 85% or more, an average grain size of grains surrounded by a boundary having a crystal orientation difference of 15° or more is 20 μm or less, grains having an aspect ratio of 0.30 or less are 50% or less in terms of area ratio, at a center position of the sheet thickness, an average value of X-ray random intensity ratios of {100} <011> to {211} <011> orientation groups is 6.0 or less, and a maximum value is 8.0 or less. In addition, in Patent Document 3, it is taught that, according to the above-described configuration, a high-strength hot-rolled steel sheet having high strength and also excellent in hole expandability and low-temperature toughness can be stably manufactured.

[0006] In Patent Document 4, a high-strength hot-rolled steel sheet is described, which has a prescribed chemical composition, a steel structure has, as main phases, a martensite and a bainite at a total area ratio of 80 to 100%, a total area ratio of the martensite in the bainite is 2 to 20%, among the martensite in the bainite, an area ratio of the martensite having an orientation difference of less than 15° between the crystal orientation of the martensite and the crystal orientation of at least one bainite adjacent to the martensite is 50% or more with respect to the entire martensite. In addition, in Patent Document 4, it is taught that, according to the above-described configuration, a high-strength hot-rolled steel sheet excellent in ductility, end-face crack resistance, and hole expandability suitable as a blank for an automobile member can be provided.

[0007] In Patent Document 5, a high-strength hot-rolled steel sheet is described, which has a prescribed chemical composition, a steel structure has, as main phases, a martensite and a bainite at a total area ratio of 80 to 100%, a total area ratio of the martensite in the bainite is 2 to 20%, among the martensite in the bainite, an area ratio of the martensite having an orientation difference of 15° or more between the crystal orientation of the martensite and the crystal orientation of at least one bainite adjacent to the martensite is more than 50% with respect to the entire martensite, when a region surrounded by a boundary having an orientation difference of 15° or more of adjacent crystals is taken as a grain, an average aspect ratio of the grains present in a region from the surface of the steel sheet to a depth of 5 μm is 2.0 or less. In addition, in Patent Document 5, it is taught that, according to the above-described configuration, a high-strength hot-rolled steel sheet excellent in ductility and bend-back bendability suitable as a blank for an automobile member can be provided.

[0008] Prior Art Documents

[0009] Patent Documents

[0010] Patent Document 1: Japanese Patent Application Publication No. 2012-062562

[0011] Patent Document 2: Japanese Patent Application Publication No. 2017-507241

[0012] Patent Literature 3: Japanese Patent Application Laid-Open No. 2017-057472

[0013] Patent Literature 4: International Publication No. 2022 / 244706

[0014] Patent Literature 5: International Publication No. 2022 / 244707 SUMMARY

[0015] As described above, it is known that the workability of steel material decreases with high-strengthening, and characteristics such as hole expandability described in Patent Literatures 2 to 4 decrease. If the hole expandability decreases, it is not always possible to process into a desired shape in, for example, a walking member of an automobile or the like. Therefore, in the development of high-strength steel sheets such as high-strength hot-rolled steel sheets, it is important to ensure characteristics such as uniform elongation at or above a certain level in addition to characteristics corresponding to the use, for example, the hole expandability described above, and to achieve high-strengthening. For example, in a member having a complex shape such as a lower arm and a rear swing arm among walking members of an automobile or the like, the workability decreases with high-strengthening, and thus the shaped member sometimes necks, which reduces the function.

[0016] In addition, for a member requiring impact resistance, plastic deformation occurs if an impact exceeding the yield strength is received, and thus from the viewpoint of ensuring the collision safety of an automobile, it is required not only to increase the tensile strength but also to increase the yield strength, and thus it is required to increase the ratio of the yield strength to the tensile strength, that is, the yield ratio.

[0017] The present application was completed in view of such actual circumstances, and aims to provide a steel sheet having high strength and high uniform elongation, hole expandability, and yield ratio by a new configuration, and capable of suppressing the occurrence of necking at the time of forming, and a method for manufacturing the same.

[0018] The present inventors have conducted research focusing on the metal structure of a steel sheet, particularly a hot-rolled steel sheet, in order to achieve the above-mentioned object. As a result, the present inventors have found that, by adopting a metal structure of a hot-rolled steel sheet having a prescribed chemical composition with a structure configuration in which martensite is the main body and is controlled within a prescribed range, high-strengthening and an increase in uniform elongation can be achieved, in addition, by containing a prescribed amount of a specific granular bainite in the metal structure, the yield ratio and the hole expandability can be improved, and the occurrence of necking at the time of forming can be significantly suppressed, furthermore, by utilizing precipitation strengthening due to the addition of Ti, the yield ratio can be further increased, and the hardness difference of each phase in the metal structure can be reduced, by the combination of this reduction in hardness difference and the improvement in hole expandability due to the above-mentioned specific granular bainite, the hole expandability can be more significantly improved, and thus the present application has been completed.

[0019] The present application capable of achieving the above-mentioned object is as follows.

[0020] (1) A steel sheet characterized in that a chemical composition is, in mass %,

[0021] C: 0.060 to 0.200%,

[0022] Si: 0.30 to 2.00%,

[0023] Mn: 1.20 to 2.70%,

[0024] P: 0.100% or less,

[0025] S: 0.0300% or less,

[0026] sol. Al: 0.001 to 0.500%,

[0027] Nb: 0.001 to 1.000%,

[0028] O: 0.0100% or less,

[0029] N: 0.0070% or less,

[0030] Ti: 0.070 to 0.200%,

[0031] B: 0 to 0.0030%,

[0032] Cr: 0 to 0.90%,

[0033] Mo: 0 to 0.12%,

[0034] Cu: 0 to 0.40%,

[0035] Ni: 0 to 0.30%,

[0036] V: 0 to 0.300%,

[0037] Sn: 0 to 0.040%,

[0038] As: 0 to 0.100%,

[0039] Zr: 0 to 0.050%,

[0040] Ca: 0 to 0.0010%,

[0041] Mg: 0 to 0.0010%,

[0042] Bi: 0 to 0.010%,

[0043] Co: 0 to 0.010%,

[0044] W: 0 to 0.100%,

[0045] Zn: 0 to 0.010%,

[0046] REM: 0 to 0.0100%, and

[0047] balance: Fe and impurities,

[0048] the metal structure contains, in area %,

[0049] martensite: 60.0 to 85.0%;

[0050] granular bainite having a maximum misorientation of 3.5° or less at intervals of 0.1 μm in a grain surrounded by grain boundaries having a misorientation of 15° or more, and having a misorientation of 10° or more within the grain: 10.0 to 30.0%; and

[0051] ferrite: 20.0% or less,

[0052] the average interval of the granular bainite grains is 50.0 μm or less.

[0053] (2) The steel sheet according to the above (1), characterized in that the chemical composition contains, in mass %,

[0054] B: 0.0001 to 0.0030%,

[0055] Cr: 0.001 to 0.90%,

[0056] Mo: 0.001 to 0.12%,

[0057] Cu: 0.001 to 0.40%,

[0058] Ni: 0.001 to 0.30%,

[0059] V: 0.001 to 0.300%,

[0060] Sn: 0.001 to 0.040%,

[0061] As: 0.001 to 0.100%,

[0062] Zr: 0.001 to 0.050%,

[0063] Ca: 0.0001 to 0.0010%,

[0064] Mg: 0.0001 to 0.0010%,

[0065] Bi: 0.001 to 0.010%,

[0066] Co: 0.001 to 0.010%,

[0067] W: 0.001 to 0.100%,

[0068] Zn: 0.001 to 0.010%, and

[0069] REM: 0.0001 to 0.0100%

[0070] at least one of bainite, pearlite and residual austenite.

[0071] (3) The steel sheet according to any one of (1) or (2), characterized in that the metal structure further contains, in area %, at least one of bainite, pearlite and residual austenite in a total of 20.0% or less.

[0072] (4) The steel sheet according to any one of (1) to (3), characterized in that the average grain diameter of the granular bainite grains is 5.0 to 30.0 μm.

[0073] (5) A member characterized by comprising the steel sheet according to any one of (1) to (4).

[0074] (6) A method of manufacturing a steel sheet, comprising a heating step, a hot rolling step and a cooling step,

[0075] the heating step comprising a step of heating a slab having the chemical composition described in (1) or (2) and holding at a temperature of 1180 to 1320°C for 6000 seconds or more,

[0076] the hot rolling step comprising a step of finish rolling the slab using a tandem rolling mill composed of four or more rolling stands, and satisfying the following conditions (a) to (c),

[0077] (a) the rolling temperature in each of the rolling passes of the immediately preceding two stands of the last two stands is 960 to 1080°C, and the reduction ratio in each of the rolling passes is 30 to 40%;

[0078] (b) the rolled material is cooled to 910°C or less at an average cooling rate of 400°C / sec or more within 0.20 seconds after the rolling passes of the immediately preceding two stands of the last two stands; and

[0079] (c) the reduction ratio in each of the rolling passes of the last two stands is 20 to 30%,

[0080] the cooling step comprising a step of water cooling the steel sheet after finish rolling, cooling to a temperature range of 500 to 650°C within 4.0 seconds from the start of water cooling, then performing air cooling for 2.0 to 6.0 seconds in the temperature range, and water cooling the steel sheet to 50°C or less within 13 seconds after air cooling.

[0081] According to the present application, it is possible to provide a steel sheet, particularly a hot-rolled steel sheet, having high strength and high uniform elongation, hole expansibility, and yield ratio, and in which occurrence of necking at the time of forming can be suppressed, and a method for manufacturing the same. DETAILED DESCRIPTION

[0082] <STEEL SHEET>

[0083] The steel sheet, particularly a hot-rolled steel sheet, according to an embodiment of the present application has a chemical composition, in terms of mass %, of

[0084] C: 0.060 to 0.200%,

[0085] Si: 0.30 to 2.00%,

[0086] Mn: 1.20 to 2.70%,

[0087] P: 0.100% or less,

[0088] S: 0.0300% or less,

[0089] sol. Al: 0.001 to 0.500%,

[0090] Nb: 0.001 to 1.000%,

[0091] O: 0.0100% or less,

[0092] N: 0.0070% or less,

[0093] Ti: 0.070 to 0.200%,

[0094] B: 0 to 0.0030%,

[0095] Cr: 0 to 0.90%,

[0096] Mo: 0 to 0.12%,

[0097] Cu: 0 to 0.40%,

[0098] Ni: 0 to 0.30%,

[0099] V: 0 to 0.300%,

[0100] Sn: 0 to 0.040%,

[0101] As: 0 to 0.100%,

[0102] Zr: 0 to 0.050%,

[0103] Ca: 0 to 0.0010%,

[0104] Mg: 0 to 0.0010%,

[0105] Bi: 0 to 0.010%,

[0106] Co: 0 to 0.010%,

[0107] W: 0 to 0.100%,

[0108] Zn: 0 to 0.010%,

[0109] REM: 0 to 0.0100%, and

[0110] balance: Fe and impurities,

[0111] the metal structure contains, in area%,

[0112] martensite: 60.0 to 85.0%;

[0113] granular bainite having a maximum misorientation of 3.5° or less at intervals of 0.1 μm in a grain surrounded by a grain boundary having a misorientation of 15° or more, and having a misorientation of 10° or more within the grain: 10.0 to 30.0%; and

[0114] ferrite: 20.0% or less,

[0115] the average interval of the granular bainite grains is 50.0 μm or less.

[0116] As described above, it is known that as steel materials are high-strengthed, properties such as hole expandability decrease. For example, in order to manufacture a member having a complex shape such as a lower arm, a rear swing arm, and the like among walking members of an automobile, a steel sheet having high strength, for example, a tensile strength of 1180 MPa or more which enables weight reduction, and excellent hole expandability is required. From the viewpoint of high-strength, the metal structure of the steel sheet is preferably constituted of a structure in which martensite is the main body. However, a martensite steel, although having excellent strength, has a problem in that workability is low because of excessively containing martensite, which decreases properties such as uniform elongation. In addition, for a member having a complex shape such as a lower arm, a rear swing arm, and the like, workability decreases as high-strength is accompanied, and thus, sometimes, a formed member is necked, and its function decreases. Therefore, from the viewpoint of improving properties such as hole expandability, uniform elongation, and the like, and also suppressing occurrence of necking in forming of a member having a complex shape, and ensuring collision safety of an automobile and the like, a high-strength steel sheet having a high yield ratio is required.

[0117] Therefore, the present inventors have conducted research with particular focus on the metal structure of the hot-rolled steel sheet in addition to making the chemical composition of the steel sheet, particularly the hot-rolled steel sheet, an appropriate chemical composition. First, the present inventors have found that by adopting a structure in which a hard martensite is the main body, more specifically a structure in which 60.0 to 85.0% of the area is occupied by martensite, as the metal structure of a hot-rolled steel sheet having a prescribed chemical composition, it is possible to achieve high strength, for example, a tensile strength of 1180 MPa or more, and significantly improve the uniform elongation of the resulting hot-rolled steel sheet.

[0118] Second, the present inventors have found that by including a prescribed amount of a specific granular bainite in the metal structure, more specifically, by including 10.0 to 30.0% of the area occupied by granular bainite in which the maximum orientation difference in 0.1 μm intervals within a grain surrounded by a grain boundary having an orientation difference of 15° or more is 3.5° or less, and the orientation difference within the grain is 10° or more, and the average interval of adjacent grains is 50.0 μm or less, it is possible to improve the yield ratio and the hole expandability, and significantly suppress the occurrence of necking during forming. Without intending to be bound by any particular theory, it is believed that the characteristic orientation change of the granular bainite is particularly effective in suppressing the occurrence of necking. To explain in more detail, the characteristic "in which the maximum orientation difference in 0.1 μm intervals within a grain surrounded by a grain boundary having an orientation difference of 15° or more is 3.5° or less, and the orientation difference within the grain is 10° or more" means that, although the orientation change within the grain of the granular bainite is relatively gentle and continuous, the orientation difference in the entire grain is large. For example, bainite has the characteristic of having many various interfaces within the grain, and as a result, exhibits a discontinuous and steep orientation change. On the other hand, ferrite has the characteristic of having a small orientation change within the grain, and thus, although it is continuous, the orientation difference in the entire grain is also small. Therefore, the granular bainite can be considered to have the characteristic between bainite and ferrite in terms of the orientation change. Bainite structures are prone to necking due to the discontinuous orientation change, but the granular bainite, although exhibiting a large orientation difference in the entire grain, exhibits a continuous orientation change as described above, unlike bainite and martensite, which also have many interfaces within the grain. Therefore, it is believed that, due to this characteristic orientation change of the granular bainite, it is possible to significantly suppress the occurrence of necking during forming. Furthermore, the present inventors have found that by including 10.0% or more of the granular bainite in terms of area%, it is also possible to improve the hole expandability. Although not intending to be bound by any particular theory, it is believed that by having a prescribed amount of the granular bainite having the properties intermediate between martensite or bainite and ferrite in the steel, it is possible to suppress the occurrence of pores from the phase boundary during hole expansion processing, and thus, the hole expandability is improved.

[0119] However, as described above, the granular bainite has a characteristic similar to that of ferrite. Therefore, it is considered that, in a metal structure in which the martensite is the main component, if the amount of the granular bainite is too large, the metal structure becomes similar to a so-called DP steel (duplex structure steel) composed of the martensite and the ferrite, and thus the yield ratio is reduced. Even in the case where the amount of the granular bainite is appropriate, if the amount of the ferrite is excessively large, or if the amount of the martensite is reduced and the total amount of the granular bainite and the ferrite becomes large, the metal structure becomes similar to the DP steel, and thus the yield ratio is reduced. Therefore, from the viewpoint of sufficiently suppressing the occurrence of the necking and maintaining a high yield ratio, it is necessary to cause the granular bainite to exist in the metal structure in an appropriate amount, and on the other hand, to maintain the area fraction of the martensite at 60.0% or more and control the total amount of the granular bainite and the ferrite within an appropriate range. Furthermore, the present inventors have further conducted research and found that, although the reason is not necessarily clear, by arranging the granular bainite grains at an appropriate interval, more specifically, by controlling the average interval of the granular bainite grains to be 50.0 μm or less, it is possible to improve the hole expandability of the steel sheet, and furthermore, the control of the average interval of such granular bainite grains is also important in suppressing the occurrence of the necking during forming. From these insights, the steel sheet according to the embodiment of the present application involves a steel sheet which, by containing 10.0 to 30.0% of granular bainite in terms of area% in a metal structure, the granular bainite being surrounded by a grain boundary having an orientation difference of 15° or more and having a maximum orientation difference of 3.5° or less in a 0.1 μm interval within the grain and an orientation difference of 10° or more within the grain, and controlling the average interval of the granular bainite grains to be 50.0 μm or less, it is possible to improve the yield ratio and the hole expandability and significantly suppress the occurrence of the necking during forming.

[0120] Furthermore, the inventors found that by utilizing precipitation strengthening due to the addition of Ti, the yield ratio can be further improved, and by combining this with the improvement in the hole expansibility due to the above-described specific granular bainite, the hole expansibility can be more significantly improved. While not intending to be bound by any particular theory, it is thought that the improvement in the hole expansibility due to such precipitation strengthening is due to a reduction in the hardness difference of the phases in the metal structure. In more detail, in the steel sheet according to the embodiments of the present application, as described above, the metal structure is constituted of a structure in which martensite is the main body, but other structures that are softer than the martensite are also included, for example, ferrite as a soft structure can be included even at a maximum of 20.0% in area %. In this case, the hardness difference of the phases in the metal structure becomes high, and the hole expansibility decreases. However, it is thought that in the steel sheet according to the embodiments of the present application, by controlling the Ti content in the steel to be 0.070 mass% or more, the soft structure such as ferrite is precipitatively strengthened by Ti precipitates, thereby reducing the hardness difference of the phases in the metal structure, and by combining this with the improvement in the hole expansibility due to the above-described specific granular bainite, the hole expansibility can be more significantly improved.

[0121] Generally, a steel sheet for a car is processed into a target component shape by press forming. Usually, press forming is performed in a plurality of steps, and thus, for example, a portion that receives another deformation in a state where a strain is accumulated inside the steel sheet due to a primary deformation is present more. However, if a strain is introduced into the steel sheet, work hardening occurs to thereby increase the strength, and thus, the workability in a later step generally decreases, and necking occurs in a formed portion. In the present inventor's research, it was found that, in order to suppress the occurrence of such necking in the formed portion at the time of forming, it is effective to improve the bendability after pre-strain, and more specifically, by applying a pre-strain of 10% to a test piece of a steel sheet in a certain direction by uniaxial stretching, and then performing a 90° bend test in a direction intersecting the direction, the occurrence of necking can be reproduced. In particular, in the C direction (a direction perpendicular to the rolling direction) of the steel sheet, ductility is lacking, and in connection therewith, a tensile test is performed in the C direction, and then a bend test is performed in the L direction (the rolling direction), and as a result, it was found that, in a case where necking does not occur in the bent test piece, necking can also be improved in the forming of an actual component. The steel sheet according to the embodiment of the present application can reliably suppress the occurrence of necking even in such a bend test after pre-strain by containing the above-described specific granular bainite in an area% of 10.0 to 30.0% in the metal structure. The fact that the occurrence of necking in the forming of an actual component can be reproduced by such a bend test after pre-strain, and the fact that the occurrence of necking in the bend test after pre-strain can be significantly suppressed by containing the granular bainite that exhibits the above-described characteristic orientation change in an area% of 10.0% or more were not known in the past, and were first clarified by the present inventor. Therefore, according to the embodiment of the present application, even if it is high-strength, for example, with a tensile strength of 1180 MPa or more, it has a high uniform elongation, a hole expansion property, and a yield ratio, and the occurrence of necking can also be reliably suppressed in the forming of an actual component, and thus, the steel sheet according to the embodiment of the present application is particularly useful in the use in the automobile field.

[0122] Hereinafter, the steel sheet according to the embodiment of the present application will be described in more detail. In the following description, unless otherwise specified, "%" as a unit of content of each element means "mass%". In addition, in the present specification, "~" indicating a numerical range is used with the meaning that the lower limit value and the upper limit value are included in the numerical values recited before and after it, unless otherwise specified.

[0123] [C: 0.060 to 0.200%]

[0124] C is an element effective for increasing the strength of the steel sheet. In addition, C forms carbides and / or carbonitrides with Nb in the steel, and also contributes to the refinement of the structure through the pinning effect of the precipitates formed. In order to sufficiently obtain these effects, the C content is set to 0.060% or more. The C content can be 0.070% or more, 0.080% or more, 0.100% or more, or 0.120% or more. On the other hand, if C is excessively contained, the hole expandability is sometimes reduced. 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.

[0125] [Si: 0.30 to 2.00%]

[0126] Si is an element that suppresses the generation of iron carbides and contributes to the increase in strength and formability. In order to sufficiently obtain such effects, 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, the ferrite fraction becomes high, and the hole expandability is sometimes reduced. In addition, since the ferrite fraction becomes high, the total amount of the granular bainite and the ferrite becomes large, and a metal structure similar to that of a DP steel is obtained, so the yield ratio is sometimes reduced. 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.

[0127] [Mn: 1.20 to 2.70%]

[0128] Mn is an element effective for increasing the strength as a quenching element and a solid solution strengthening element. In order to sufficiently obtain these effects, the Mn content is set to 1.20% or more. The Mn content can be 1.30% or more, 1.50% or more, 1.60% or more, 1.80% or more, or 2.00% or more. On the other hand, if Mn is excessively contained, the granular bainite fraction is reduced, the hole expandability is sometimes reduced, and the occurrence of necking at the time of forming cannot be sufficiently suppressed. Therefore, the Mn content is set to 2.70% or less. The Mn content can be 2.60% or less, 2.50% or less, 2.40% or less, 2.30% or less, or 2.20% or less.

[0129] [P: 0.100% or less]

[0130] If P is excessively contained, workability is sometimes reduced due to grain boundary segregation or the like. Therefore, the P content is set to 0.100% or less. The P content can 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 can be 0%, but excessive reduction leads to an increase in cost. Therefore, the P content can be 0.0001% or more, 0.001% or more, or 0.005% or more.

[0131] [S: 0.0300% or less]

[0132] If S is excessively contained, sulfides such as MnS are generated more, and workability is sometimes reduced. 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 leads to an increase in cost. Therefore, the S content can be 0.0001% or more, 0.0010% or more, or 0.0030% or more.

[0133] [sol. Al: 0.001 to 0.500%]

[0134] sol. Al is an element that functions as a deoxidizer of molten steel. In addition, sol. Al is also an element effective for increasing the fraction of granular bainite. 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, if sol. Al is excessively contained, the fraction of ferrite becomes high, and workability is sometimes reduced. In addition, since the fraction of ferrite becomes high, the total amount of granular bainite and ferrite becomes large, and a metal structure similar to that of DP steel is obtained, and therefore the yield ratio is sometimes reduced. 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 and indicates solid solution Al present in steel in a solid solution state.

[0135] [Nb: 0.001 to 1.000%]

[0136] Nb is an element that forms carbides, nitrides, and / or carbonitrides in the steel and contributes to the refinement of the original austenite grains through a pinning effect, and in turn, to the high-strength of the steel sheet. In addition, Nb is also an effective element for increasing the fraction of granular bainite and controlling its morphology. In order to sufficiently obtain 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 and the like are sometimes generated in the steel, thereby reducing the workability of the steel sheet. Therefore, the Nb content is set to 1.000% or less. The Nb content can be 0.800% or less, 0.600% or less, 0.500% or less, or 0.400% or less.

[0137] [O: 0.0100% or less]

[0138] O is an element that is mixed in during the manufacturing process. If O is excessively contained, coarse inclusions are sometimes formed, thereby reducing the workability of the steel sheet. 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 in order to reduce it to less than 0.0001%, a refining time is required, resulting in a reduction in productivity. Therefore, the O content can be 0.0001% or more or 0.0005% or more.

[0139] [N: 0.0070% or less]

[0140] If N is excessively contained, coarse nitrides are formed, and slab cracks sometimes occur in hot rolling. 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 results in an increase in cost. Therefore, the N content can be 0.0001% or more or 0.0005% or more.

[0141] [Ti: 0.070~0.200%]

[0142] Ti is an element that precipitates in the steel as Ti carbide or the like, strengthens soft structures such as ferrite through precipitation strengthening, and contributes to an increase in strength and yield ratio. Furthermore, Ti can reduce the hardness difference among phases in the metal structure due to precipitation strengthening, and is thus also effective in improving the expandability. In order to sufficiently obtain these effects, the Ti content is set to 0.070% or more. The Ti content can be 0.080% or more, 0.090% or more, 0.100% or more, or 0.120% or more. On the other hand, if Ti is excessively contained, coarse carbides or the like are generated in the steel, and in some cases, slab cracks occur in hot rolling or the workability of the steel sheet is reduced. Therefore, the Ti content is set to 0.200% or less. The Ti content can be 0.180% or less, 0.170% or less, 0.160% or less, or 0.150% or less.

[0143] The basic chemical composition of the steel sheet according to the embodiment of the present application is as described above. In addition, the steel sheet can also contain at least one of the following elements in place of a part of Fe in the balance as needed.

[0144] [B: 0 to 0.0030%]

[0145] B is an element that improves the quenchability of the steel and contributes to an increase in strength. The B content can be 0%, but in order to obtain such an effect, the B content is preferably 0.0001% or more. The B content can be 0.0002% or more, 0.0003% or more, or 0.0005% or more. On the other hand, even if B is excessively contained, the effect is saturated, and it can lead to an increase in manufacturing cost. Therefore, the B content is preferably 0.0030% or less. The B content can be 0.0025% or less, 0.0020% or less, 0.0015% or less, or 0.0010% or less.

[0146] [Cr: 0 to 0.90%]

[0147] Cr is an element that improves the quenchability of the steel and contributes to an increase in strength and / or corrosion resistance. The Cr content can be 0%, but in order to obtain these effects, the Cr content is preferably 0.001% or more, can be 0.01% or more, 0.05% or more, or 0.10% or more. On the other hand, even if Cr is excessively contained, the effect is saturated, and it can lead to an increase in manufacturing cost. Therefore, the Cr content is preferably 0.90% or less, can be 0.70% or less, 0.50% or less, 0.40% or less, or 0.30% or less.

[0148] [Mo: 0 to 0.12%]

[0149] Mo is an element that improves the hardenability of the steel and contributes to an increase in strength. The Mo content can be 0%, but in order to obtain such an effect, the Mo content is preferably 0.001% or more. The Mo content can be 0.01% or more, 0.02% or more, or 0.03% or more. On the other hand, if Mo is excessively contained, the deformation resistance during hot working increases, and sometimes the equipment load becomes large. Therefore, the Mo content is preferably 0.12% or less. The Mo content can be 0.10% or less, 0.08% or less, 0.06% or less, or 0.05% or less.

[0150] [Cu: 0 ~ 0.40%]

[0151] Cu is an element that contributes to an increase in strength through precipitation strengthening or solid solution strengthening. The Cu content can be 0%, but in order to obtain such an effect, the Cu content is preferably 0.001% or more. The Cu content can be 0.01% or more, 0.03% or more, or 0.05% or more. On the other hand, even if this element is excessively contained, the effect saturates, and it is possible to incur an increase in manufacturing cost. Therefore, the Cu content is preferably 0.40% or less. The Cu content can be 0.30% or less, 0.20% or less, 0.10% or less, or 0.08% or less.

[0152] [Ni: 0 ~ 0.30%]

[0153] Ni is an element that contributes to an increase in strength through precipitation strengthening or solid solution strengthening. The Ni content can be 0%, but in order to obtain such an effect, the Ni content is preferably 0.001% or more. The Ni content can be 0.01% or more, 0.03% or more, or 0.05% or more. On the other hand, even if this element is excessively contained, the effect saturates, and it is possible to incur an increase in manufacturing cost. Therefore, the Ni content is preferably 0.30% or less. The Ni content can be 0.20% or less, 0.15% or less, 0.10% or less, or 0.08% or less.

[0154] [V: 0 ~ 0.300%]

[0155] V is an element that contributes to an increase in strength through precipitation strengthening or the like. The V content can be 0%, but in order to obtain such an effect, the V content is preferably 0.001% or more. The V content can be 0.010% or more, 0.030% or more, or 0.050% or more. On the other hand, even if V is excessively contained, the effect saturates, and it is possible to incur an increase in manufacturing cost. Therefore, the V content is preferably 0.300% or less. The V content can be 0.200% or less, 0.100% or less, or 0.080% or less.

[0156] [Sn: 0 to 0.040%, As: 0 to 0.100%, Zr: 0 to 0.050%, Ca: 0 to 0.0010%, Mg: 0 to 0.0010%, Bi: 0 to 0.010%, Co: 0 to 0.010%, W: 0 to 0.100%, Zn: 0 to 0.010%, and REM: 0 to 0.0100%]

[0157] Sn, As, Zr, Ca, Mg, Bi, Co, W, Zn, and REM can be contained in the steel sheet as optional elements, or, at times, exist in the steel sheet as mixed-in elements. The contents of these elements can be Sn: 0 to 0.040% or 0.020%, As: 0 to 0.100% or 0.050%, Zr: 0 to 0.050% or 0.030%, Ca: 0 to 0.0010% or 0.0008%, Mg: 0 to 0.0010% or 0.0008%, Bi: 0 to 0.010%, Co: 0 to 0.010%, W: 0 to 0.100% or 0.050%, Zn: 0 to 0.010%, and REM: 0 to 0.0100% or 0.0050%. As to the lower limit values of these elements, for example, the Sn, As, Zr, Bi, Co, W, and Zn contents can be 0.001% or more, 0.005% or more, or 0.008% or more, respectively. Likewise, the Ca, Mg, and REM contents can be 0.0001% or more, 0.0002% or more, or 0.0005% or more.

[0158] In the steel sheet according to the embodiment of the present application, the remainder (balance) other than the above-described elements is composed of Fe and impurities. The so-called impurities are, for example, components and the like mixed in due to various factors of manufacturing processes, starting with raw materials such as ores, scrap, and the like, when the steel sheet is industrially manufactured. It is also permissible to contain them within a range not affecting the effects of the present application.

[0159] The chemical composition of the steel sheet according to the embodiment of the present application can be measured using a general analysis method. 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 a combustion-infrared absorption method, N can be measured using a non-active gas melting-thermal conductivity method, and O can be measured using a non-active gas melting-non-dispersive infrared absorption method.

[0160] [metal structure]

[0161] [martensite: 60.0 to 85.0%]

[0162] The steel sheet according to the embodiment of the present application contains, in terms of area, 60.0 to 85.0% of martensite. By adopting a steel sheet having a microstructure containing hard martensite in such a range, high strength, for example, a tensile strength of 1180 MPa or more, can be achieved, and the uniform elongation of the resulting steel sheet is significantly improved. From the viewpoint of further increasing the strength, the higher the area ratio of the martensite is, the more preferable it is, for example, it can be 62.0% or more, 65.0% or more, 68.0% or more, or 70.0% or more. From the viewpoint of further improving the uniform elongation, the lower the area ratio of the martensite is, the more preferable it is, for example, it can be 82.0% or less, 80.0% or less, 78.0% or less, or 75.0% or less. In the present application, the term "martensite" includes not only quenched martensite (so-called fresh martensite) but also tempered martensite.

[0163] [Granular bainite in which the maximum misorientation in 0.1 μm intervals in a grain surrounded by grain boundaries having an orientation difference of 15° or more is 3.5° or less, and the misorientation within the grain is 10° or more: 10.0 to 30.0%]

[0164] The steel sheet according to the embodiment of the present application contains, in the metal structure, 10.0 to 30.0% by area of granular bainite having a characteristic that the maximum orientation difference within a 0.1 μm interval in a grain surrounded by a grain boundary having an orientation difference of 15° or more is 3.5° or less and the orientation difference within the grain is 10° or more. In the prior art, the structure called granular bainite does not necessarily have the characteristic described above. Therefore, the structure called granular bainite does not necessarily mean the same structure as the granular bainite according to the embodiment of the present application. In the embodiment of the present application, it is extremely important that the steel sheet contains, in the metal structure, 10.0 to 30.0% by area of the granular bainite having the characteristic described above, that is, the granular bainite having the characteristic that the orientation difference is large in the whole of the grain although the orientation change within the grain is gradual and continuous. Such a technical matter and the effects resulting therefrom are discovered for the first time by the present inventors. As described above, by containing 10.0% or more by area of the granular bainite having the characteristic that the orientation difference is large in the whole of the grain although the orientation change within the grain is gradual and continuous, the occurrence of necking at the time of forming can be significantly suppressed due to the orientation change having the characteristic. Further, as described above, by containing 10.0% or more by area of the granular bainite, the occurrence of pores from the phase boundary at the time of hole expansion can be suppressed, and the hole expandability can also be improved due to this. From the viewpoint of further suppressing the occurrence of necking and / or further improving the hole expandability, it is more preferable that the area ratio of the granular bainite is higher, for example, 12.0% or more, 15.0% or more, or 18.0% or more. On the other hand, as described above, the granular bainite has a characteristic similar to ferrite, and therefore, in a metal structure in which martensite is the main component, when the area ratio of the granular bainite is too high, the metal structure becomes similar to so-called DP steel, and the yield ratio can be reduced. Therefore, from the viewpoint of maintaining a higher yield ratio, it is more preferable that the area ratio of the granular bainite is lower, for example, 28.0% or less, 25.0% or less, or 22.0% or less.

[0165] In the present inventor's research, after 10% pre-strain was applied to a steel sheet in the C direction (a direction perpendicular to the rolling direction) by uniaxial stretching, a 90° bend test was performed in the L direction (the rolling direction), and as a result, it was found that necking did not occur in the bent test piece, and necking could also be improved in actual component forming. The steel sheet according to the embodiment of the present invention, by containing 10.0 to 30.0% of the above-described specific granular bainite in area%, necking can be reliably suppressed even in the bend test after such pre-strain. Therefore, according to the embodiment of the present invention, necking can be reliably suppressed even in the late stage of deformation in a forming operation that is performed in multiple steps as in actual press forming of a steel sheet for automobiles, and thus the steel sheet according to the embodiment of the present invention is particularly useful in use in the automobile field.

[0166] [Ferrite: 20.0% or less]

[0167] The metal structure of the steel sheet according to the embodiment of the present invention contains 20.0% or less of ferrite in area%. If the ferrite, which is a soft structure, can be limited to 20.0% or less in area%, by utilizing the precipitation strengthening of the Ti precipitates to include the soft structure of the ferrite, the hardness difference of each phase in the metal structure can be sufficiently reduced. Therefore, by the combination of such reduction in hardness difference and improvement in hole expansibility resulting from control of the average interval of the granular bainite described later, the hole expansibility can be more significantly improved. When the area ratio of the ferrite exceeds 20.0%, sometimes the hole expansibility cannot be sufficiently improved even by the combination of the precipitation strengthening of the Ti precipitates and the control of the average interval of the granular bainite. Furthermore, if the area ratio of the ferrite exceeds 20.0%, the total amount of the granular bainite and the ferrite becomes large, and the metal structure becomes similar to that of a DP steel, and thus sometimes the yield ratio decreases. From the viewpoint of further improving the hole expansibility and / or the yield ratio, the area ratio of the ferrite is more preferably lower, and can be, for example, 18.0% or less, 15.0% or less, 12.0% or less, 10.0% or less, 8.0% or less, 5.0% or less, or 3.0% or less. The lower limit of the area ratio of the ferrite is not particularly limited, and can be 0%, and can be, for example, 0.5% or more or 1.0% or more.

[0168] [Remaining structure]

[0169] The balance of the microstructure other than the martensite, the above-described specific granular bainite, and the ferrite can be 0% by area, but in the presence of the balance, the balance can include at least one of bainite, pearlite, and retained austenite in a total of 20.0% or less by area. When the area ratio of at least one of the bainite, the pearlite, and the retained austenite exceeds 20.0% in total, sometimes the uniform elongation and the like are reduced, or other microstructures such as the martensite and the granular bainite cannot be controlled within the desired range. Therefore, the smaller the area ratio of the balance, the more preferable, and for example, the area ratio of the total of at least one of the bainite, the pearlite, and the retained austenite can be 15.0% or less, 10.0% or less, 8.0% or less, 5.0% or less, or 3.0% or less. On the other hand, the lower limit is not particularly limited, and the area ratio of the total of at least one of the bainite, the pearlite, and the retained austenite can be 0%, and can be, for example, 0.1% or more, 0.5% or more, or 1.0% or more.

[0170] [average interval of granular bainite particles: 50.0 μm or less]

[0171] In the steel sheet according to the embodiment of the present application, the average interval of the granular bainite particles is controlled to be 50.0 μm or less. Here, the so-called granular bainite particle refers to a particle (grain) of granular bainite in which the maximum misorientation in an interval of 0.1 μm within the particle surrounded by a grain boundary having a misorientation of 15° or more is 3.5° or less, and the misorientation within the particle is 10° or more. By controlling the average interval of the particles of the granular bainite showing the above-described characteristic change in orientation to be 50.0 μm or less, the hole expandability of the steel sheet can be significantly improved by the combination with the above-described 20% or less by area of ferrite and the precipitation strengthening by the Ti precipitates. Further, the average interval of the granular bainite particles is also a factor that determines the arrangement of the granular bainite microstructure, and therefore, if there is a deviation in the interval of the granular bainite particles, even if the granular bainite showing the above-described characteristic change in orientation is included in an amount of 10.0% or more by area, sometimes the occurrence of necking during forming cannot be reliably suppressed. From the viewpoint of further improving the hole expandability and more reliably suppressing the occurrence of necking, the smaller the average interval of the granular bainite particles, the more preferable, and for example, can be 35.0 μm or less, 30.0 μm or less, 28.0 μm or less, 25.0 μm or less, or 23.0 μm or less. The lower limit is not particularly limited, and for example, the average interval of the granular bainite particles can be 5.0 μm or more, 7.0 μm or more, 10.0 μm or more, or 15.0 μm or more.

[0172] [average particle diameter of granular bainite particles: 5.0 to 30.0 μm]

[0173] In the steel sheet according to the embodiment of the present application, the average grain size of the granular bainite grains is preferably 5.0 to 30.0 μm. By controlling the average grain size of the granular bainite grains to be within the range of 5.0 to 30.0 μm, a fine and uniform granular bainite structure can be obtained, and thus the bendability after pre-straining can be further improved. For example, the average grain size of the granular bainite grains can be 6.0 μm or more, 8.0 μm or more, or 10.0 μm or more. Also, the average grain size of the granular bainite grains can be 25.0 μm or less, 22.0 μm or less, 20.0 μm or less, or 18.0 μm or less.

[0174] [Identification of martensite, bainite, pearlite, and retained austenite and calculation of area ratio]

[0175] The identification of martensite, bainite, pearlite, and retained austenite and the calculation of area ratio are performed by optical microscope observation after etching using a nitric acid ethanol etchant (Nital) reagent or Lepera solution and X-ray diffraction. The observation of the structure by the optical microscope is performed on a plate thickness cross section in a direction perpendicular to the plate surface. The plate thickness cross section is preferably parallel to the rolling direction. Specifically, first, a sample is prepared from the steel sheet, and the observation surface of the sample is etched with the nitric acid ethanol etchant. Next, a structure photograph is obtained at a 1 / 4 depth position of the plate thickness under a field of view of 300 μm x 300 μm using the optical microscope, and the structure photograph is subjected to image analysis, whereby the total area ratio of martensite and bainite and the area ratio of pearlite are calculated. Next, a sample subjected to Lepera etching of the observation surface is used, and likewise a structure photograph is obtained at a 1 / 4 depth position of the plate thickness under a field of view of 300 μm x 300 μm using the optical microscope, and the structure photograph is subjected to image analysis, whereby the total area ratio of martensite and retained austenite is calculated. Next, a sample subjected to surface cutting from the normal direction of the rolling surface to a 1 / 4 depth of the plate thickness is used, and the volume ratio of retained austenite is calculated by X-ray diffraction measurement. Since the volume ratio of retained austenite is equal to the area ratio, it is used as the area ratio of retained austenite. The area ratio of martensite is calculated by subtracting the obtained area ratio of retained austenite from the previously calculated total area ratio of martensite and retained austenite. Finally, the area ratio of bainite is calculated by likewise subtracting the obtained area ratio of martensite from the previously calculated total area ratio of martensite and bainite.

[0176] [Identification of ferrite and calculation of area ratio]

[0177] The ferrite was identified and the area ratio was calculated using electron backscattered diffraction (EBSD: Electron Back Scattered Diffraction) as follows. Specifically, first, a sample was prepared from the steel sheet in such a manner that a plate thickness cross section in a direction perpendicular to the plate surface becomes an observation surface. The plate thickness cross section is preferably parallel to the rolling direction. Next, EBSD analysis was performed on a rectangular region of 200 pm in the plate thickness direction and 400 pm in a direction perpendicular to the plate thickness direction with a measurement interval of 0.2 pm, with a position 1 / 4 of the plate thickness from the steel sheet surface as the center, to obtain crystal orientation information of the rectangular region. The EBSD analysis was performed at an analysis speed of 50 to 300 points / second using an apparatus composed of a hot field emission type scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (Hikari detector manufactured by TSL). Next, using the crystal orientation information of the rectangular region, the orientation difference in the grain (GAM value: Grain Average Misorientation) was calculated using software "OIM Analysis (registered trademark)" attached to the EBSD analysis apparatus. Finally, the region in which the GAM value was 0.5° or less was identified as ferrite, and the area ratio was calculated. Here, the "GAM value" is a value obtained by averaging the orientation difference between adjacent pixels in a region surrounded by grain boundaries having an orientation difference of 15° or more.

[0178] [Identification of granular bainite and calculation of area ratio]

[0179] The identification of the granular bainite and the calculation of the area ratio are performed by EBSD as follows. Specifically, first, a sample is prepared from the steel sheet in such a manner that a plate-thickness cross section in a direction perpendicular to the plate surface becomes an observation surface. The plate-thickness cross section is preferably parallel to the rolling direction. Next, EBSD analysis is performed on a rectangular region of 200 μm in the plate-thickness direction and 400 μm in the direction perpendicular to the plate-thickness direction with the center at a position 1 / 4 of the plate thickness from the steel sheet surface at a measurement interval of 0.1 μm to obtain crystal orientation information of the rectangular region. The EBSD analysis is performed at an analysis speed of 50 to 300 points / second using an apparatus composed of a hot field emission type scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (HIKARI detector manufactured by TSL). Next, using the software "OIM Analysis (registered trademark)" attached to the EBSD analysis apparatus, the region surrounded by grain boundaries having an orientation difference of 15° or more is defined as a grain based on the crystal orientation information of the rectangular region, the intra-grain orientation difference of the grain is calculated, and the grain having an intra-grain orientation difference of 3.5° or less and, more specifically, an intra-grain maximum orientation difference of 10° or more is identified as the granular bainite, and the area ratio thereof is calculated. The average value of the area ratios obtained for any three intra-grain lines is determined as the area ratio of the granular bainite. The "intra-grain maximum orientation difference" of the granular bainite is found by "Grain Reference Orientation Deviation (GROD)". The value of the intra-grain maximum orientation difference is found as the orientation difference from the orientation of the pixel having the minimum value of the KAM value (Karnel Average Misorientation) in the same grain. In the embodiment of the present application, the crystal orientation serving as the reference is the orientation having the minimum value of the KAM value in the same grain. The values of GROD and KAM can be calculated using the software "OIM Analysis (registered trademark) Version 7.0.1" attached to the EBSD analysis apparatus.

[0180] [Method for determining average interval and average particle diameter of granular bainite particles]

[0181] As for the average interval of the granular bainite particles, the intervals of the centers of gravity of the grains of the granular bainite identified in the EBSD and the centers of gravity of the nearest grains of the granular bainite are measured, and the average value obtained by measuring the intervals of 100 points or more is determined as the average interval of the granular bainite particles. In addition, the average value of the equivalent circle diameters of all the grains of the granular bainite for which the intervals of 100 points or more are measured is determined as the average particle diameter of the grains of the granular bainite.

[0182] [Plate thickness]

[0183] The steel sheet according to the embodiment of the present application is not particularly limited in thickness, but generally has a thickness of 1.0 to 8.0 mm. For example, the thickness can be 1.2 mm or more, 1.6 mm or more, or 2.0 mm or more, and / or can 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.

[0184] The steel sheet according to the embodiment of the present application can suppress occurrence of necking even in forming of a member having a complex shape, and thus can reliably satisfy both high strength and excellent workability at a high level. Therefore, the steel sheet according to the embodiment of the present application is useful for use in a member or the like in a technical field in which both of these characteristics are required, and particularly useful for use in a member or the like in the automobile field. Therefore, in a preferred embodiment, an automobile member, particularly a running member of an automobile, is provided, which contains the steel sheet according to the embodiment of the present application. As an example of the running member of an automobile, a lower arm, a rear swing arm, or the like can be given. These automobile members, particularly the running member of an automobile, contain the steel sheet according to the embodiment of the present application in at least a part thereof, and thus satisfy the above-described features of chemical composition and metal structure in at least a part thereof. In a portion of the steel sheet which is less processed in forming such as press forming, the features of the steel sheet do not change particularly before and after forming. The portion of the steel sheet which is less processed is determined based on the features that the shape is smooth without being deformed by bending or the like, and the rate of increase and decrease in thickness is small. In a member having a complex shape such as a lower arm, a rear swing arm, or the like, necking can occur due to a plurality of forming operations, and thus the rigidity of a specific portion can be reduced. Therefore, it is sometimes impossible to manufacture such a member from one steel sheet, and a specific portion becomes separate, resulting in an increase in cost of the member. However, according to the steel sheet according to the embodiment of the present application, even in a member having a complex shape such as a lower arm, a rear swing arm, or the like, one steel sheet can be used to perform a plurality of forming operations without occurrence of necking, and this is economically advantageous.

[0185] [mechanical properties]

[0186] [tensile strength (TS) and uniform elongation (u-El)]

[0187] According to the steel sheet, particularly hot-rolled steel sheet, having the above-described chemical composition and metal structure, a high tensile strength, specifically a tensile strength of 1180 MPa or more can be achieved. The tensile strength is preferably 1200 MPa or more, 1220 MPa or more, or 1240 MPa or more. According to the steel sheet, particularly hot-rolled steel sheet, related to the embodiments of the present application, although having such a very high tensile strength, by the specific combination of the chemical composition and the metal structure described above, the uniform elongation and the hole expandability can be improved, and the occurrence of necking at the time of forming can be significantly suppressed. The upper limit of the tensile strength is not particularly limited, for example, the tensile strength of the steel sheet can be 1780 MPa or less, 1470 MPa or less, or 1400 MPa or less. In addition, according to the steel sheet, particularly hot-rolled steel sheet, related to the embodiments of the present application, a high uniform elongation, specifically a uniform elongation of 5.0% or more can be achieved. The uniform elongation is preferably 5.2% or more, 5.5% or more, 5.8% or more, or 6.0% or more. The upper limit of the uniform elongation is not particularly limited, for example, the uniform elongation of the steel sheet can be 15.0% or less, 10.0% or less, or 8.0% or less. The tensile strength and the uniform elongation are measured by performing a tensile test based on JIS Z 2241:2011 by taking a JIS No. 5 test piece in a direction (C direction) in which the length direction of the test piece is parallel to the rolling right angle direction (direction perpendicular to the rolling direction) of the steel sheet. For example, in the case where it is difficult to take a JIS No. 5 test piece due to dimensional constraints, other test pieces described in JIS Z2241:2011 can be used. However, in the case where the sheet thickness is less than 0.5 mm, in order to perform appropriate evaluation, 0.5 mm is set as the lower limit. For example, in the case where it is difficult to take a JIS No. 5 test piece due to dimensional constraints, and it is also difficult to use other test pieces described in JIS Z2241:2011, a micro Vickers hardness test based on JIS Z 2244-1:2020 can be performed and the value obtained by converting the hardness (HV) thereof into the tensile strength can be used. The test piece for the micro Vickers hardness test can be made as follows. First, a sample is cut out from an arbitrary position of 50 mm or more from the end surface of the steel sheet (in the case where it is not possible to take a sample from this position, a position avoiding the end portion) in a manner that enables observation of a sheet thickness cross section perpendicular to the sheet surface. The sheet thickness cross section is preferably parallel to the rolling direction. The size of the sample also depends on the measuring device, but is set to a size that enables observation of about 10 mm in a direction perpendicular to the sheet thickness direction. After the cross section of the above-described sample is ground using a silicon carbide paper of #600 to #1500, the observation surface is finished to a mirror surface using a liquid obtained by dispersing diamond powder having a particle size of 1 to 6 pm in a diluent such as alcohol and / or pure water. Next, the observation surface is finished by electrolytic polishing. In the micro Vickers hardness test, 30 points are measured at the 1 / 4 position of the sheet thickness with a load of 500 gf, and the average value thereof can be used. The conversion can be performed using the following formula.

[0188] Tensile strength [MPa] = 3.12 × Vickers hardness [HV] + 16

[0189] [Porosity (λ)]

[0190] Based on the steel sheet having the above-mentioned chemical composition and metallic structure, high hole expansion performance can be achieved, specifically, a hole expansion rate of 40% or more can be achieved. The hole expansion rate is preferably 42% or more, and more preferably 45% or more or 50% or more. There is no particular upper limit to the hole expansion rate; for example, it can be 150% or less, 100% or less, or 70% or less. The hole expansion rate is determined as follows: First, a test piece with a width of 100 mm and a length of 100 mm is prepared from the steel sheet. A punching tool with a punch diameter of 10 mm and a die diameter of 10.25~11.5 mm (12.5% ​​clearance) is used to make a punched hole (initial hole: hole diameter d0 = 10 mm). Next, the initial hole is expanded with a conical punch with a 60° apex angle so that the burr is on the die side until a crack penetrating the sheet thickness occurs. The hole diameter d1 mm at the time of crack occurrence is measured, and the hole expansion rate λ (%) of each test piece is calculated using the following formula. The hole expansion test was conducted three times, and their average value was determined as the hole expansion rate λ.

[0191] λ = 100 × {(d1 - d0) / d0}

[0192] [Yield Ratio (YR)]

[0193] Based on the steel sheet having the aforementioned chemical composition and metallic structure, in addition to high tensile strength, the yield ratio can also be improved; more specifically, a yield ratio of 80% or higher can be achieved. The yield ratio is preferably 82% or higher, more preferably 85% or higher. There is no particular upper limit; for example, the yield ratio can be 95% or lower, or 92% or lower. The yield ratio is based on the tensile strength and the conditional yield strength σ. 0.2 The tensile strength and conditional yield strength σ are determined using the following formula. 0.2 The test is performed by taking a JIS 5 specimen from a direction (C direction) that makes the length of the specimen parallel to the rolling right angle of the steel plate, and conducting a tensile test based on JIS Z2241:2011.

[0194] Yield ratio YR = Conditional yield strength σ 0.2 Tensile strength TS×100

[0195] <Methods for manufacturing steel plates>

[0196] Next, a preferred manufacturing method of the steel sheet according to the embodiment of the present application will be described. The following description is intended to exemplify a characteristic method for manufacturing the steel sheet according to the embodiment of the present application, particularly a steel sheet having desired properties, and is not intended to limit the steel sheet to one manufactured by the manufacturing method described below. More specifically, the following description is specifically shown for the manufacturing of a hot-rolled steel sheet, but the steel sheet according to the embodiment of the present application is any steel sheet having the chemical composition and the metal structure described above, i.e., not only a hot-rolled steel sheet, but also a steel sheet including a cold-rolled steel sheet, a plated steel sheet, and the like. Therefore, the following description is merely a simple description of an example of a preferred manufacturing method in the case where the steel sheet according to the embodiment of the present application is a hot-rolled steel sheet.

[0197] The manufacturing method of the steel sheet according to the embodiment of the present application is characterized by comprising a heating step, a hot-rolling step, and a cooling step,

[0198] The heating step comprises a step of heating a slab having the chemical composition described above with respect to the steel sheet, at a temperature of 1180 to 1320°C, for 6000 seconds or more,

[0199] The hot-rolling step comprises a step of finish-rolling the slab using a tandem rolling mill composed of four or more rolling stands, and satisfies the following conditions (a) to (c),

[0200] (a) the rolling temperature in each rolling pass of the immediately preceding two stands to the last two stands (the two stands immediately preceding the last two stands and adjacent to the last two stands) is 960 to 1080°C, and the reduction ratio in each rolling pass is 30 to 40%;

[0201] (b) the rolled material is cooled to 910°C or less at an average cooling rate of 400°C / sec or more within 0.20 seconds after the rolling pass of the immediately preceding two stands to the last two stands; and

[0202] (c) the reduction ratio in each rolling pass of the last two stands is 20 to 30%,

[0203] The cooling step comprises a step of water-cooling the steel sheet after finish-rolling, cooling to a temperature range of 500 to 650°C within 4.0 seconds from the start of water-cooling, then performing air-cooling for 2.0 to 6.0 seconds in the temperature range, and water-cooling the steel sheet to 50°C or less within 13 seconds after air-cooling.

[0204] In the above manufacturing method, the temperatures described with respect to the slab and the steel sheet each refer to the surface temperature of the slab and the surface temperature of the steel sheet. The following describes each step in detail.

[0205] [Heating Step]

[0206] First, a slab having the chemical composition described above for the steel sheet is heated and held at a temperature range of 1180 to 1320°C for 6000 seconds or more. From the viewpoint of productivity, the slab is preferably a slab obtained by continuous casting, but a slab obtained by casting and blooming can also be used, and a slab subjected to hot working or cold working as needed can also be used. In the present production method, the holding at a temperature range of 1180 to 1320°C includes not only the case where the temperature of the slab is held at a certain temperature within the range of 1180 to 1320°C, but also the case where the temperature of the slab is varied within the range of 1180 to 1320°C. By holding the slab at a temperature range of 1180 to 1320°C for 6000 seconds or more, coarse carbides present in the structure can be completely dissolved, and the starting point of cracks can be eliminated. If the holding temperature is lower than 1180°C or the holding time is shorter than 6000 seconds, the dissolution of coarse carbides becomes incomplete. If the dissolution of coarse carbides is incomplete, ferrite and bainite transformation starting from such carbides occurs in the cooling step described later, and thus the area fraction of martensite becomes lower than 60.0%, and as a result, the desired strength cannot be obtained in some cases. From the viewpoint of the capacity of the heating equipment and productivity, the upper limit of the heating temperature of the slab is set to 1320°C or lower. The upper limit of the holding time at a temperature range of 1180 to 1320°C is preferably 10000 seconds or less.

[0207] [hot rolling step]

[0208] [coarse rolling]

[0209] In the present production method, for example, coarse rolling can be performed on the heated slab before finish rolling in order to adjust the sheet thickness or the like. The coarse rolling is not particularly limited as long as the desired thin slab size can be ensured.

[0210] [finish rolling]

[0211] [rolling temperature in each of the two immediately preceding rolling passes: 960 to 1080°C, and reduction ratio in each of the two immediately preceding rolling passes: 30 to 40%]

[0212] The heated slab or the slab subjected to rough rolling as needed is subjected to finish rolling next. In the present production method, the finish rolling is performed using a tandem rolling mill composed of four or more rolling stands. In the present production method, in the finish rolling of the heated slab, the rolling temperature and the reduction rate in each rolling pass of the immediately preceding two stands of the last two stands are appropriately controlled, and specifically, the rolling temperature in each rolling pass of the immediately preceding two stands of the last two stands is controlled to 960 to 1080°C, and likewise, the reduction rate in each rolling pass of the immediately preceding two stands of the last two stands is controlled to 30 to 40%. By performing rolling at a high reduction rate under a high temperature condition in each rolling pass of the immediately preceding two stands of the last two stands, recrystallization is promoted, and thus austenite grains are refined. In association therewith, in the finally obtained metal structure, the average interval of granular bainite can be reduced to a desired range.

[0213] On the other hand, if the reduction rate in each rolling pass of the immediately preceding two stands of the last two stands exceeds 40%, flat austenite grains are formed due to the introduction of excessive strain, and in the finally obtained metal structure, likewise, the average interval of granular bainite cannot be reduced to a desired range. In addition, if the rolling temperature in each rolling pass of the immediately preceding two stands of the last two stands exceeds 1080°C, the austenite grains are coarsened, and even if subsequent rolling and cooling control are performed, a desired structure fraction cannot be obtained, or in addition thereto, the average interval and / or average grain diameter of granular bainite cannot be controlled to a desired range.

[0214] [(b) cooling to 910°C or lower at an average cooling rate of 400°C / sec or more within 0.20 seconds after the rolling pass of the immediately preceding two stands of the last two stands]

[0215] In the present production method, the rolled material is cooled to 910°C or lower at an average cooling rate of 400°C / sec or more within 0.20 seconds after the rolling pass of the immediately preceding two stands of the last two stands. By cooling the rolled material to 910°C or lower as early as this after the rolling pass of the immediately preceding two stands of the last two stands, post-recrystallization grain growth is suppressed, and thus, in the finally obtained metal structure, the average interval of granular bainite can be reduced to a desired range. If the time to cool to 910°C or lower after the rolling pass of the immediately preceding two stands of the last two stands exceeds 0.20 seconds, post-recrystallization grain growth cannot be sufficiently suppressed, and even if appropriate cooling is performed in a subsequent cooling step, the average interval and / or average grain diameter of granular bainite cannot be controlled to a desired range.

[0216] Further, the average cooling rate between the rolling passes of the latter two stages and the rolling passes of the immediately preceding two stages is very important in terms of generating granular bainite having a desired morphology within a prescribed range. More specifically, if the average cooling rate therebetween is less than 400°C / sec, then sometimes the maximum orientation difference in a 0.1 μm interval within a grain surrounded by grain boundaries having an orientation difference of 15° or more exceeds 3.5°, and thus it becomes impossible to generate granular bainite having a maximum orientation difference of 3.5° or less and a grain-internal orientation difference of 10° or more in 10.0% or more of the grains. The average cooling rate between the rolling passes of the latter two stages and the rolling passes of the immediately preceding two stages is preferably 500°C / sec or more. Further, if the cooling stop temperature is higher than 910°C, then likewise it sometimes becomes impossible to generate granular bainite having a maximum orientation difference of 3.5° or less in a 0.1 μm interval within a grain surrounded by grain boundaries having an orientation difference of 15° or more, and a grain-internal orientation difference of 10° or more in 10.0% or more of the grains.

[0217] [ (c) Reduction ratio in each of the rolling passes of the latter two stages: 20 to 30%]

[0218] In the present production method, the reduction ratio in each of the rolling passes of the latter two stages of the finishing rolling is controlled to be 20 to 30%. By introducing strain at such a moderate reduction ratio in each of the rolling passes of the latter two stages, it is possible to increase the nucleation sites for generating granular bainite in the subsequent cooling process. If the reduction ratio in each of the rolling passes of the latter two stages is less than 20%, then the nucleation sites for generating granular bainite cannot be formed sufficiently, and in the finally obtained metal structure, it is not possible to obtain the desired area ratio of granular bainite. On the other hand, if the reduction ratio in each of the rolling passes of the latter two stages exceeds 30%, then flattened austenite grains are formed due to the introduction of excessive strain, and in the finally obtained metal structure, it is not possible to reduce the average interval of granular bainite to the desired range.

[0219] [cooling process]

[0220] [cooling to a temperature range of 500 to 650°C within 4.0 seconds from the start of water cooling, followed by air cooling for 2.0 to 6.0 seconds]

[0221] The finish-rolled steel sheet is water-cooled in the subsequent cooling step, and is cooled to a temperature range of 500 to 650°C within 4.0 seconds from the start of water-cooling, and then is air-cooled in the temperature range for 2.0 to 6.0 seconds. First, by cooling to the temperature range of 500 to 650°C within 4.0 seconds from the start of water-cooling, particularly, the generation of pearlite can be reliably suppressed, and thus, in the finally obtained steel sheet, the area fraction of the desired metal structure can be realized. On the contrary, if the time from the start of water-cooling until the temperature range of 500 to 650°C exceeds 4.0 seconds, pearlite is generated more, and in the metal structure of the finally obtained steel sheet, the desired amount of martensite and / or granular bainite cannot be obtained.

[0222] Further, by air-cooling in the temperature range of 500 to 650°C for 2.0 to 6.0 seconds after water-cooling, the phase transformation to granular bainite can be promoted, and Ti precipitates are appropriately precipitated. Thus, the air-cooling operation of 2.0 to 6.0 seconds in the temperature range of 500 to 650°C after water-cooling is extremely important not only in the effect of suppressing the occurrence of necking by granular bainite, but also in the effect of improving the hole expandability and the like by precipitation strengthening due to the precipitation of Ti precipitates. For example, if the air-cooling temperature is lower than 500°C, the phase transformation to granular bainite cannot be sufficiently promoted, on the other hand, bainite is sometimes generated more. In such a case, not only the effect of suppressing the occurrence of necking is reduced, but also the uniform elongation is reduced due to the generation of more bainite, and in relation to the generation of bainite, the generation of martensite is less, and sometimes, sufficient strength cannot be obtained.

[0223] In addition, if the air cooling temperature exceeds 650°C, the phase transformation to granular bainite cannot be sufficiently promoted, on the other hand, the ferrite phase transformation is promoted, and sometimes a large amount of ferrite is generated. In addition, the Ti precipitates cannot be sufficiently precipitated. In such a case, in addition to the effect of suppressing the occurrence of necking being reduced, the hole expandability and the yield ratio of the obtained steel sheet are also reduced due to the large amount of generation of ferrite and the insufficient precipitation strengthening by the Ti precipitates. In addition, if the air cooling time is less than 2.0 seconds, the phase transformation to granular bainite cannot be sufficiently promoted, and sometimes martensite is generated in a large amount by subsequent cooling. In such a case, in addition to the hole expandability being reduced and / or the effect of suppressing the occurrence of necking being reduced, the uniform elongation is also reduced due to the excessive generation of martensite. On the other hand, if the air cooling time exceeds 6.0 seconds, sometimes granular bainite is generated in a large amount. In such a case, the amount of martensite is reduced, and the total amount of granular bainite and ferrite becomes high. As described above, granular bainite has characteristics similar to those of ferrite, and therefore in a metal structure in which martensite is the main component, the total of the area ratios of granular bainite and ferrite becomes high, a metal structure similar to so-called DP steel is formed, and the yield ratio is reduced. The air cooling temperature is preferably 525 to 625°C, and the air cooling time is preferably 3.0 to 5.0 seconds.

[0224] [water-cooled to 50°C or lower within 13 seconds after air cooling]

[0225] After air cooling for 2.0 to 6.0 seconds in the temperature range of 500 to 650°C, the steel sheet is water-cooled to 50°C or lower within 13 seconds. By performing such rapid cooling, it is possible to generate martensite in the desired area ratio. If the water cooling to 50°C or lower takes more than 13 seconds or the cooling stop temperature is higher than 50°C, sometimes it is not possible to achieve a martensite area ratio of 60.0% or more. In such a case, it is not possible to achieve the desired strength of the steel sheet. The lower limit of the water cooling time is not particularly limited, and for example, the water cooling time to 50°C or lower after air cooling can be 4 seconds or more or 5 seconds or more. In addition, the lower limit of the water cooling stop temperature is not particularly limited, and for example, the water cooling stop temperature can be 20°C or more or 25°C or more. The steel sheet that has been water-cooled can be finally coiled in the form of a hot-rolled coil. The conditions of the coiling are not particularly limited, and can be performed under any appropriate temperature conditions.

[0226] According to the steel sheet manufactured by the manufacturing method described above, by adopting a metal structure constituted of a structure containing 60.0 to 85.0% of martensite in area%, high strength, for example, a tensile strength of 1180 MPa or more, can be achieved, and the uniform elongation is significantly improved. Also, by containing 10.0 to 30.0% of granular bainite surrounded by grain boundaries having an orientation difference of 15° or more, in which the maximum orientation difference in 0.1 μm intervals within the grains is 3.5° or less, and the orientation difference within the grains is 10° or more, in area% in the metal structure, and controlling the average interval of the granular bainite grains to be 50.0 μm or less, the yield ratio and the hole expansibility can be improved, and the occurrence of necking at the time of forming can be significantly suppressed. Furthermore, by controlling the Ti content in the steel to be 0.070 mass% or more, a soft structure such as a precipitated ferrite is precipitated by Ti precipitates, thereby reducing the hardness difference of each phase in the metal structure, and by the combination of such reduction of the hardness difference and the improvement of the hole expansibility due to the specific granular bainite described above, the hole expansibility can be more significantly improved. Therefore, the steel sheet manufactured by the manufacturing method described above can suppress the occurrence of necking even in the forming of a member having a complex shape, and thus, can reliably have high strength and excellent workability, which are contradictory characteristics, at a high level, and thus is particularly useful in the automobile field where these characteristics are required.

[0227] Hereinafter, the present application will be described in more detail using examples, but the present application is by no means limited to these examples.

[0228] Example

[0229] In the following examples, steel sheets, particularly hot-rolled steel sheets, relating to the embodiments of the present application were manufactured under various conditions, and the tensile strength (TS), the yield ratio (YR), the uniform elongation (u-El), the hole expansibility (λ), and the occurrence of necking in the bending test after pre-strain were investigated for the obtained steel sheets.

[0230] First, molten steel was cast by a continuous casting method to form slabs having various chemical compositions shown in Tables 1 and 2, and these slabs were heated to a temperature of 1180 to 1320°C and held for a time of 6000 to 10000 seconds, and then, hot-rolled. The hot-rolling was performed by performing rough rolling and finish rolling. More specifically, as for the rough rolling, the same conditions were used in all of the examples and comparative examples, and as for the finish rolling, a tandem rolling mill composed of 5 rolling stands was used to perform finish rolling under the conditions shown in Table 3. Then, the steel sheets that were finish-rolled were subjected to water cooling, air cooling, and water cooling under the conditions shown in Table 3, and coiled to obtain steel sheets having a sheet thickness of 2.4 to 3.4 mm.

[0231]

[0232]

[0233]

[0234] The properties of the obtained steel plates were measured and evaluated using the following methods.

[0235] [Tensile strength (TS) and uniform elongation (u-El)]

[0236] Tensile strength (TS) and uniform elongation (u-El) were determined by tensile testing based on JIS Z 2241:2011, using JIS No. 5 specimens prepared in a direction (C direction) parallel to the rolling right angle of the steel plate.

[0237] [Porosity (λ)]

[0238] The expansion ratio (λ) is determined as follows. First, a specimen with a width of 100 mm and a length of 100 mm is prepared from a steel plate. A punching tool with a punch diameter of 10 mm and a die diameter of 10.25~11.5 mm (12.5% ​​clearance) is used to make a punched hole (initial hole: hole diameter d0 = 10 mm). Next, the initial hole is expanded with a conical punch with a 60° apex angle so that the burr is on the die side until a crack penetrating the plate thickness occurs. The hole diameter d1 mm at the time of crack occurrence is measured, and the expansion ratio λ (%) of each specimen is calculated using the following formula. This expansion test is performed 3 times, and their average value is determined as the expansion ratio λ.

[0239] λ = 100 × {(d1 - d0) / d0}

[0240] [Yield Ratio (YR)]

[0241] Yield ratio (YR), based on tensile strength (TS) and conditional yield strength σ 0.2 The tensile strength (TS) and conditional yield strength σ are determined using the following formulas. 0.2 The test is performed by taking a JIS 5 specimen from a direction (C direction) that makes the length of the specimen parallel to the rolling right angle of the steel plate, and conducting a tensile test based on JIS Z 2241:2011.

[0242] Yield ratio YR = Conditional yield strength σ 0.2 Tensile strength TS×100

[0243] [Presence or absence of necking in the bending test after pre-strain]

[0244] First, a tensile test piece having a parallel portion width of 36 mm, a parallel portion length of 86 mm, an R of 36 mm, a clamping portion width of 50 mm, and a total length of 372 mm was prepared from the steel sheet. A 10% pre-strain was applied to the test piece by uniaxial stretching in the C direction. Next, a test piece having a size of 60 mm [C direction] x 30 mm [L direction] was prepared from the center of the test piece, and a 90° bending test was performed in the L direction, whereby it was confirmed whether or not necking occurred. The case where necking was not confirmed to have occurred in the test piece was evaluated as acceptable, and the case where necking was confirmed to have occurred was evaluated as unacceptable.

[0245] A steel sheet having a tensile strength (TS) of 1180 MPa or more, a uniform elongation (u-El) of 5.0% or more, a hole expansion ratio (λ) of 40% or more, a yield ratio (YR) of 80% or more, and no occurrence of necking in the bending test after pre-straining was evaluated as a steel sheet having high strength and high uniform elongation, hole expandability, and yield ratio, and capable of suppressing the occurrence of necking during forming. The results are shown in Table 4. "GB particle" in Table 4 means a granular bainite particle.

[0246]

[0247] With reference to Tables 1 to 4, it is considered that in Comparative Example 4, recrystallization is not sufficiently promoted because the rolling temperatures in the respective rolling passes of the two immediately preceding passes of the two final passes are low. As a result thereof, in the finally obtained metal structure, the average interval of granular bainite grains exceeds 50.0 μm, λ is reduced, and necking occurs in the bending test after pre-strain. It is considered that in Comparative Example 5, austenite grains are coarsened because the rolling temperatures in the respective rolling passes of the two immediately preceding passes of the two final passes are high. As a result thereof, the area fraction of granular bainite is less than 10.0% and the average interval of granular bainite grains exceeds 50.0 μm, λ is reduced, and necking occurs in the bending test after pre-strain. It is considered that in Comparative Example 6, recrystallization is not sufficiently promoted because the reduction in the rolling pass in the second of the two immediately preceding passes of the two final passes is low. As a result thereof, the average interval of granular bainite grains exceeds 50.0 μm, λ is reduced, and necking occurs in the bending test after pre-strain. It is considered that in Comparative Example 7, flattened austenite grains are formed due to the introduction of excessive strain because the reduction in the rolling pass in the first of the two immediately preceding passes of the two final passes is high. As a result thereof, the average interval of granular bainite grains exceeds 50.0 μm, λ is reduced, and necking occurs in the bending test after pre-strain. It is considered that in Comparative Example 8, post- recrystallization grain growth is not sufficiently suppressed because the time for cooling to 910°C or lower after the rolling passes of the two immediately preceding passes of the two final passes exceeds 0.20 seconds. As a result thereof, the average interval of granular bainite grains exceeds 50.0 μm, λ is reduced, and necking occurs in the bending test after pre-strain. In Comparative Example 9, the average cooling rate between the rolling passes of the two final passes and the rolling passes of the two immediately preceding passes thereof is slow, and as a result, the area fraction of granular bainite showing the prescribed orientation change becomes less than 10.0%, and in association therewith, the area fraction of martensite becomes higher than 85.0%. As a result thereof, u-El and λ are reduced, and necking occurs in the bending test after pre-strain. In Comparative Example 10, the cooling stop temperature in the cooling between the rolling passes of the two final passes and the rolling passes of the two immediately preceding passes thereof is high, and as a result, similarly, the area fraction of granular bainite showing the prescribed orientation change becomes less than 10.0%, and in association therewith, the area fraction of martensite becomes higher than 85.0%. As a result thereof, u-El and λ are reduced, and necking occurs in the bending test after pre-strain.

[0248] It is considered that Comparative Examples 11 and 12 could not sufficiently form nucleation sites for generating granular bainite because of low reduction rates in the rolling passes of the first and second passes in the latter two. As a result thereof, the area fraction of granular bainite became less than 10.0%, in association with which the area fraction of martensite became higher than 85.0%, u-El and λ decreased, and necking occurred in the bending test after pre-straining. It is considered that Comparative Examples 13 and 14 formed flattened austenite grains due to excessive introduction of strain because of high reduction rates in the rolling passes of the first and second passes in the latter two, respectively. As a result thereof, the average interval of granular bainite grains exceeded 50.0 μm, λ decreased, and necking occurred in the bending test after pre-straining. Comparative Example 15 generated more pearlite because of long water cooling time until air cooling in the cooling step. As a result thereof, the area fraction of granular bainite became less than 10.0%, λ decreased, and necking occurred in the bending test after pre-straining. Comparative Example 16 could not sufficiently promote phase transition to granular bainite because of low air cooling temperature, in association with which more bainite was generated. As a result thereof, u-El and λ decreased, and necking occurred in the bending test after pre-straining. Comparative Example 17 could not sufficiently promote phase transition to granular bainite because of high air cooling temperature, in association with which more ferrite was generated. In addition, it is considered that Ti precipitates could not be sufficiently precipitated. As a result thereof, λ and YR decreased, and necking occurred in the bending test after pre-straining. Comparative Example 18 could not sufficiently promote phase transition to granular bainite because of short air cooling time, and in addition, more martensite was generated by subsequent cooling. As a result thereof, u-El and λ decreased, and necking occurred in the bending test after pre-straining. Comparative Example 19 generated more granular bainite because of long air cooling time, in association with which generation of martensite became less, and the total amount of granular bainite and ferrite became more. As a result thereof, TS and YR decreased. Comparative Example 20 became less than 60.0% because of long water cooling time until 50°C or lower after air cooling, and TS decreased.

[0249] Comparative Example 46, TS decreased because of low C content. Comparative Example 47, λ decreased because of high C content. Comparative Example 48, u-El decreased because of low Si content. Comparative Example 49, ferrite was generated more because of high Si content, and in association therewith, the total amount of granular bainite and ferrite also became high. As a result thereof, λ and YR decreased. Comparative Example 50, because of low Mn content, the quenching property decreased, as a result thereof, the area fraction of martensite became low, and in association therewith, the total amount of granular bainite and ferrite became high. As a result thereof, TS and YR decreased. Comparative Example 51, because of high Mn content, the area fraction of granular bainite became low, λ decreased, and necking occurred in the bending test after pre-strain. Comparative Example 52, because of high sol. Al content, ferrite was generated more, and in association therewith, the total amount of granular bainite and ferrite also became high. As a result thereof, λ and YR decreased. It is considered that Comparative Example 53, because of high Nb content, coarse carbides and the like were generated. As a result thereof, the workability of the steel sheet decreased, u-El and λ decreased, and necking occurred in the bending test after pre-strain. It is considered that Comparative Example 54, because of low Ti content, the precipitation strengthening by Ti precipitates could not function sufficiently. As a result thereof, TS and λ decreased. It is considered that Comparative Example 55, because of high Ti content, coarse carbides and the like were generated. As a result thereof, the workability of the steel sheet decreased, and λ decreased.

[0250] In contrast thereto, in the steel sheets of all of the inventive examples, by having the prescribed chemical composition, and by appropriately controlling each condition in the manufacturing method, steel sheets having a metal structure containing, in area %, martensite: 60.0 to 85.0 %, granular bainite having a maximum orientation difference of 3.5° or less at an interval of 0.1 μm in a grain surrounded by a grain boundary having an orientation difference of 15° or more and having an orientation difference of 10° or more within the grain: 10.0 to 30.0 %, and ferrite: 20.0 % or less, and an average interval of the granular bainite grains being 50.0 μm or less were obtained. In addition, as a result thereof, although being high-strength having a tensile strength of 1180 MPa or more, a high uniform elongation, a hole expansibility, and a yield ratio were possessed, and the occurrence of necking in the bending test after pre-strain could also be reliably suppressed.

Claims

1. A steel plate, characterized in that, Chemical composition in mass% is C：0.060~0.200%、 Si: 0.30~2.00% Mn: 1.20~2.70%, P: Below 0.100% S: Below 0.0300% sol.Al: 0.001~0.500% Nb: 0.001~1.000% O: Below 0.0100% N: below 0.0070% Ti: 0.070~0.200% B：0~0.0030%、 Cr：0~0.90%、 Mo: 0~0.12%, Cu: 0~0.40%, Ni: 0~0.30% V：0~0.300%、 Sn: 0~0.040% As: 0~0.100% Zr：0~0.050%、 Ca: 0~0.0010%, Mg: 0~0.0010%, Bi: 0~0.010% Co: 0~0.010%, W：0~0.100%、 Zn: 0~0.010%, REM: 0~0.0100%, and Balance: Fe and impurities, Metallic structure, expressed as a percentage of area, contains Martensite: 60.0~85.0%; Granular bainite with a maximum orientation difference of less than 3.5° and an intragranular orientation difference of more than 10° within a 0.1 μm spacing, surrounded by grain boundaries with an orientation difference of more than 15°: 10.0%~30.0%; and Ferrite: below 20.0%, The average spacing between granular bainite particles is less than 50.0 μm.

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

3. The steel plate according to claim 1 or 2, characterized in that, The metal microstructure further contains, by area percentage, at least one of bainite, pearlite and retained austenite, totaling less than 20.0%.

4. The steel plate according to any one of claims 1 to 3, characterized in that, The average particle size of the granular bainite particles is 5.0~30.0 μm.

5. A component, characterized in that, It includes the steel plate described in any one of claims 1 to 4.

6. A method for manufacturing a steel plate, comprising a heating process, a hot rolling process, and a cooling process. The heating process includes heating a slab having the chemical composition described in claim 1 or 2 and holding it at a temperature of 1180~1320°C for more than 6000 seconds. The hot rolling process includes a process of finishing the slab using a tandem rolling mill consisting of four or more rolling mills, and satisfies the following conditions (a) to (c). (a) The rolling temperature in each rolling pass of the two immediately preceding sections of the last two sections is 960~1080°C, and the rolling rate in each rolling pass is 30~40%; (b) Within 0.20 seconds after the rolling passes immediately preceding the last two sections, the rolled material is cooled to below 910°C at an average cooling rate of 400°C / second or higher; and (c) The reduction rate in each rolling pass of the last two sections is 20-30%. The cooling process includes the following steps: water cooling the precision-rolled steel plate to a temperature range of 500~650°C within 4.0 seconds from the start of water cooling; then air cooling for 2.0~6.0 seconds in the temperature range; and water cooling the steel plate to below 50°C within 13 seconds after air cooling.

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