Steel sheet and method for manufacturing same

Abstract

A steel sheet having: a predetermined chemical composition; and a metal microstructure including 95 area% or more of a tempered martensite microstructure, wherein a number density of carbides having an aspect ratio of 2.0 or more is 0.60 / µm2 or less.

Description

TECHNICAL FIELD

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

[0002] Conventionally, improvement of safety of an occupant in a vehicle has been required, and strength of a vehicle body has been improved for such a purpose. On the other hand, against the background of the growing global warming problem, the movement for improving the fuel efficiency of automobiles is accelerating. It is known that weight reduction of a vehicle body is effective for improving fuel efficiency.

[0003] In order to improve safety and reduce weight for vehicle bodies, steel sheets for automobiles are increasingly required to have higher strength, and the application of a steel sheet mainly including (for example, 95 area% or more of) a tempered martensite microstructure (hereinafter, also referred to as "martensite microstructure steel sheet") has been studied. The martensite microstructure steel sheet has an advantage that higher strength is easily realized with a small amount of additive elements.

[0004] Patent Document 1 describes a martensite microstructure steel sheet having improved delayed fracture resistance. Patent Document 2 describes a martensite microstructure steel sheet having improved collision resistance.CONVENTIONAL ART DOCUMENTSPATENT DOCUMENTS

[0005] Patent Document 1: JP 7140301 B Patent Document 2: JP 7120461 B SUMMARY OF THE INVENTIONPROBLEMS TO BE SOLVED BY THE INVENTION

[0006] The martensite microstructure steel sheet is easy to realize higher strength, but the structure is poor in ductility, and thus difficult to be applied to automobile parts from the viewpoint of processability. In addition, considering application to automobile parts, the martensite microstructure steel sheet needs to have a low yield ratio to some extent for fracture resistance.

[0007] Ductility is generally improved by introducing a soft phase ferrite and / or a residual γ, and it has been hardly studied, either in Patent Document 1 or the like, to improve the ductility of the martensite microstructure steel sheet. In addition, it has been hardly studied, either in Patent Document 1 or the like, to reduce yield ratio.

[0008] The present disclosure has been made in view of such a circumstance, and an object of the present disclosure is to provide a martensite microstructure steel sheet having sufficient ductility and a sufficiently low yield ratio, and a method for manufacturing the same.MEANS FOR SOLVING THE PROBLEMS

[0009] A first aspect of the present invention is a steel sheet having: a chemical composition including: C: 0.15 to 0.25 mass%, Si: 0.70 mass% or less (including 0 mass%), Mn: 1.10 to 2.50 mass%, Al: 0.150 mass% or less (not including 0 mass%), P: 0.020 mass% or less (including 0 mass%), S: 0.010 mass% or less (including 0 mass%), N: 0.010 mass% or less (including 0 mass%), and a balance being Fe and inevitable impurities; and a metal microstructure including 95 area% or more of a tempered martensite microstructure, wherein a number density of carbides having an aspect ratio of 2.0 or more is 0.60 / µm 2< or less.

[0010] A second aspect of the present invention is the steel sheet according to the first aspect, further including at least one selected from the group consisting of Ti: 0.010 to 0.070 mass%, B: 0.0100 mass% or less (not including 0 mass%) and Cr: 2.00 mass% or less (not including 0 mass%).

[0011] A third aspect of the present invention is the steel sheet according to the first or second aspect, further including at least one selected from the group consisting of Cu: 0.50 mass% or less (not including 0 mass%) and Ni: 0.50 mass% or less (not including 0 mass%).

[0012] A fourth aspect of the present invention is the steel sheet according to any one of the first to third aspects, further including at least one selected from the group consisting of V: 0.10 mass% or less (not including 0 mass%) and Nb: 0.10 mass% or less (not including 0 mass%).

[0013] A fifth aspect of the present invention is the steel sheet according to any one of the first to fourth aspects, wherein a number density of carbides having an aspect ratio of 2.0 to 2.5 is 0.50 / µm 2< or less.

[0014] A sixth aspect of the present invention is a method for manufacturing the steel sheet according to any one of the first to fifth aspects, the method including: heating a steel sheet in a first temperature range of 840°C to 950°C and holding for 30 seconds or more; after the holding, quenching the steel sheet from a second temperature range between 450°C and the first temperature range down to below 100°C, wherein an average cooling rate between 420°C and 250°C is 300 °C / sec or higher; and after the quenching, tempering the steel sheet in a third temperature range of 100°C to 300°C for 30 seconds or more. TECHNICAL EFFECTS OF THE INVENTION

[0015] According to the embodiments of the present invention, it is possible to provide a martensite microstructure steel sheet having sufficient ductility, and a method for manufacturing the same.BRIEF DESCRIPTION OF THE DRAWINGS

[0016] [Fig. 1] Fig. 1 shows a schematic top view of a test piece for measuring the number density of carbides having each aspect ratio as viewed in the sheet thickness direction. [Fig. 2] Fig. 2 shows a schematic top view illustrating the position of thermocouples on a steel sheet at the time of quenching in Examples. MODE FOR CARRYING OUT THE INVENTION

[0017] The present inventor has studied from various angles in order to realize a martensite microstructure steel sheet having sufficient ductility and a sufficiently low yield ratio. As a result, the present inventor has found that a martensite microstructure steel sheet having sufficient ductility and a sufficiently low yield ratio can be realized by predeterminedly adjusting the chemical composition and controlling the number density of carbides having an aspect ratio of 2.0 or more to a predetermined value or lower.

[0018] Hereinafter, details of each requirement defined by the embodiments of the present invention will be described.<1. Chemical Composition>

[0019] The steel sheet according to the embodiments of the present invention preferably has a chemical composition including: C: 0.15 to 0.25 mass%, Si: 0.70 mass% or less (including 0 mass%), Mn: 1.10 to 2.50 mass%, Al: 0.150 mass% or less (not including 0 mass%), P: 0.020 mass% or less (including 0 mass%), S: 0.010 mass% or less (including 0 mass%), N: 0.010 mass% or less (including 0 mass%), and a balance being Fe and inevitable impurities.

[0020] Hereinafter, each element will be described in detail.(C: 0.15 to 0.25 mass%)

[0021] C is an element necessary for securing strength. Therefore, the C content is 0.15 mass% or more, preferably 0.16 mass% or more, more preferably 0.17 mass% or more, and still more preferably 0.18 mass% or more. However, when the C content is excessive, weldability is deteriorated. Therefore, the C content is 0.25 mass% or less, preferably 0.24 mass% or less, more preferably 0.23 mass% or less, and still more preferably 0.22 mass% or less.(Si: 0.70 mass% or less (including 0 mass%))

[0022] Si is an element that may be optionally contained. Si may be an element effective for improving tempering softening resistance and strength by solid solution strengthening. In addition, Si may be an element effective for suppressing the growth of carbides in the quenching process. In order to exhibit these effects, the Si content may be more than 0 mass%, and is preferably 0.05 mass% or more, more preferably 0.10 mass% or more, still more preferably 0.15 mass% or more, and even more preferably 0.20 mass% or more. However, since Si is a ferrite-forming element, when Si is contained in a large amount, quenchability is impaired, securing higher strength becomes difficult, and weldability is also deteriorated. Therefore, the Si content is 0.70 mass% or less, preferably 0.60 mass% or less, more preferably 0.50 mass% or less, and still more preferably 0.40 mass% or less.

[0023] In the specification, "including 0 mass%" means that an embodiment where intentional addition thereof is not performed, that is, a case where the content is equal to or lower than the inevitable impurity level is included therein (not excluding a case where intentional addition thereof is performed).(Mn: 1.10 to 2.50 mass%)

[0024] Mn is an element effective for improving quenchability and increasing strength. In addition, Mn is an element effective for suppressing the growth of carbides in the quenching process. In order to exhibit these effects, the Mn content is 1.10 mass% or more, preferably 1.50 mass% or more, more preferably 1.60 mass% or more, still more preferably 1.70 mass% or more, and even more preferably 1.80 mass% or more. However, when the Mn content is excessive, delayed fracture resistance and weldability deteriorate. Therefore, the Mn content is 2.50 mass% or less, preferably 2.40 mass% or less, more preferably 2.30 mass% or less, and still more preferably 2.20 mass% or less.(Al: 0.150 mass% or less (not including 0 mass%))

[0025] Al is an element added as a deoxidizing agent, and also has an effect of improving the corrosion resistance of steel. In order to sufficiently exhibit these effects, the Al content is more than 0 mass%, preferably 0.010 mass% or more, more preferably 0.020 mass% or more, still more preferably 0.030 mass% or more, and even more preferably 0.040 mass% or more. However, when Al is excessively contained, a large amount of C-based inclusions are generated, which causes surface defects. Therefore, the Al content is 0.150 mass% or less, preferably 0.140 mass% or less, more preferably 0.130 mass% or less, and still more preferably 0.120 mass% or less.(P: 0.020 mass% or less (including 0 mass%))

[0026] P (phosphorus) is an inevitable impurity and reduces ductility due to brittleness. Therefore, the P content is 0.020 mass% or less, preferably 0.015 mass% or less, and more preferably 0.010 mass% or less.(S: 0.010 mass% or less (including 0 mass%))

[0027] S is an inevitable impurity, generates sulfide-based inclusions, and deteriorates processability and weldability of the base material. Therefore, the S content is 0.010 mass% or less, preferably 0.005 mass% or less, and more preferably 0.003 mass% or less.(N: 0.010 mass% or less (including 0 mass%))

[0028] N is an inevitable impurity, increases the precipitation amount of nitride, and adversely affects toughness. Therefore, the N content is 0.010 mass% or less, preferably 0.008 mass% or less, and more preferably 0.006 mass% or less. On the other hand, considering the costs for steelmaking or the like, the N content may be 0.001 mass% or more.

[0029] The steel sheet according to the embodiments of the present invention includes the chemical composition described above, and in one embodiment of the present invention, the balance preferably is iron and inevitable impurities. As the inevitable impurities, contamination of elements brought depending on the situation of raw materials, materials, manufacturing facilities, and the like is allowed. For example, there are elements such as P, S and N, which are typically preferable as the content is smaller, and thus are inevitable impurities, but are separately defined as the composition range as described above. Therefore, in the specification, the "inevitable impurities" constituting the residual portion indicates a concept excluding elements with composition range separately defined.

[0030] As preferred embodiments of the present invention, it is also effective to further include as necessary, in addition to the above elements, (a) at least one selected from the group consisting of Ti: 0.010 to 0.070 mass%, B: 0.0100 mass% or less (not including 0 mass%), and Cr: 2.00 mass% or less (not including 0 mass%), (b) at least one selected from the group consisting of Cu: 0.50 mass% or less (not including 0 mass%) and Ni: 0.50 mass% or less (not including 0 mass%), (c) at least one selected from the group consisting of V: 0.10 mass% or less (not including 0 mass%) and Nb: 0.10 mass% or less (not including 0 mass%), and the like, thereby further improving the characteristics of the steel sheet depending on the kinds of the contained elements.((a) At least one selected from the group consisting of Ti: 0.010 to 0.070 mass%, B: 0.0100 mass% or less (not including 0 mass%), and Cr: 2.00 mass% or less (not including 0 mass%))

[0031] Ti is an element effective for improving the delayed fracture characteristics by precipitation of carbides and refinement of the prior γ grain size. Therefore, the Ti content is preferably 0.010 mass% or more, more preferably 0.015 mass% or more, still more preferably 0.020 mass% or more, and even more preferably 0.025 mass% or more. However, when Ti is excessively contained, precipitation of carbonitrides increases, and ductility and processability of the base material deteriorate. Therefore, when Ti is contained, the content thereof is 0.070 mass% or less, preferably 0.060 mass% or less, more preferably 0.050 mass% or less, and still more preferably 0.040 mass% or less.

[0032] B is an element that improves quenchability by adding a small amount. In order to sufficiently exhibit the effect, the B content may be more than 0 mass%, and is preferably 0.0001 mass% or more, more preferably 0.0003 mass% or more, still more preferably 0.0005 mass% or more, and even more preferably 0.0007 mass% or more. However, when B is excessively contained, ductility decreases. Therefore, when B is contained, the content thereof is 0.0100 mass% or less, preferably 0.0090 mass% or less, more preferably 0.0080 mass% or less, and still more preferably 0.0070 mass% or less.

[0033] Cr is an element effective for improving quenchability and increasing strength. Cr is an element effective for increasing the tempering softening resistance of the martensitic phase. In order to exhibit these effects, the Cr content may be more than 0 mass%, and is preferably 0.10 mass% or more, more preferably 0.20 mass% or more, still more preferably 0.30 mass% or more, and even more preferably 0.40 mass% or more. However, when Cr is excessively contained, delayed fracture resistance is deteriorated. Therefore, when Cr is contained, the content thereof is 2.00 mass% or less, preferably 1.80 mass% or less, more preferably 1.60 mass% or less, and still more preferably 1.40 mass% or less.((b) At least one selected from the group consisting of Cu: 0.50 mass% or less (not including 0 mass%) and Ni: 0.50 mass% or less (not including 0 mass%))

[0034] Cu is an element effective for improving corrosion resistance and improving delayed fracture resistance. In order to exhibit the effect, the Cu content may be more than 0 mass%, and is preferably 0.01 mass or more, more preferably 0.03 mass% or more, still more preferably 0.05 mass% or more, and even more preferably 0.07 mass% or more. However, when Cu is excessively contained, ductility and processability are deteriorated. Therefore, when Cu is contained, the content thereof is 0.50 mass% or less, preferably 0.40 mass% or less, more preferably 0.30 mass% or less, and still more preferably 0.20 mass% or less.

[0035] Ni is an element effective for improving corrosion resistance and improving delayed fracture resistance. In order to exhibit the effect, the Ni content may be more than 0 mass%, and is preferably 0.01 mass% or more, more preferably 0.03 mass% or more, still more preferably 0.05 mass% or more, and even more preferably 0.07 mass% or more. However, when Ni is excessively contained, ductility and processability are deteriorated. Therefore, when Ni is contained, the content thereof is 0.50 mass% or less, preferably 0.40 mass% or less, more preferably 0.30 mass% or less, and still more preferably 0.20 mass% or less.((c) At least one selected from the group consisting of V: 0.10 mass% or less (not including 0 mass%) and Nb: 0.10 mass% or less (not including 0 mass%))

[0036] V is an element effective for improving the strength by precipitation of carbides. In order to exhibit the effect, the V content may be more than 0 mass%, and is preferably 0.01 mass% or more, more preferably 0.02 mass% or more, still more preferably 0.03 mass% or more, and even more preferably 0.04 mass% or more. However, when V is excessively contained, precipitation of carbonitrides increases, and ductility and processability of the base material decrease. Therefore, when V is contained, the content thereof is 0.10 mass% or less, preferably 0.09 mass% or less, more preferably 0.08 mass% or less, and still more preferably 0.07 mass% or less.

[0037] Nb is an element effective for improving the strength by precipitation of carbides. In order to exhibit the effect, the Nb content may be more than 0 mass%, and is preferably 0.01 mass% or more, more preferably 0.02 mass% or more, still more preferably 0.03 mass% or more, and even more preferably 0.04 mass% or more. However, when Nb is excessively contained, precipitation of carbonitrides increases, and ductility and processability of the base material decrease. Therefore, when Nb is contained, the content thereof is 0.10 mass% or less, preferably 0.09 mass% or less, more preferably 0.08 mass% or less, and still more preferably 0.07 mass% or less.

[0038] As still another element, for example, any one or more selected from the group consisting of Se, As, Sb, Pb, Sn, Bi, Mg, Zn, Zr, W, Cs, Rb, Ca, Co, La, Tl, Nd, Y, In, Be, Hf, Tc, Ta and O (the elements may be inevitable impurities) may be contained in a total amount of 0.01 mass% or less (including 0 mass%) as necessary in order to improve corrosion resistance and the delayed fracture resistance of the base material part.<2. Metal Microstructure>

[0039] In the steel sheet according to the embodiments of the present invention, the metal microstructure includes 95 area% or more of a tempered martensite structure. The metal microstructure preferably includes 97 area% or more of a tempered martensite microstructure. The upper limit of the area ratio of the tempered martensite microstructure is not particularly limited, and may be 100 area%.

[0040] In the steel sheet according to the embodiments of the present invention, the metal microstructure may include not only the tempered martensite microstructure, but also microstructures inevitably contained due to the manufacturing process, for example, any one or more selected from the group consisting of a ferrite microstructure, a bainite microstructure, a retained austenite (γ) microstructure, and an as-quenched martensite.

[0041] The area ratio of the tempered martensite microstructure can be determined by the method in Examples described later.<3. Carbides>

[0042] The present inventor has found that carbides having a high aspect ratio that may be mainly present at grain boundaries in the martensite microstructure steel sheet embrittle the grain boundaries and reduce ductility. Then, the present inventor has found that a martensite microstructure steel sheet having sufficient ductility can be realized when the chemical composition thereof is predeterminedly adjusted and the number density of carbides having an aspect ratio of 2.0 or more becomes 0.60 / µm 2< or less.

[0043] Furthermore, considering application to automobile parts and the like, those having a moderately high yield ratio are fracture-resistant, and therefore preferable. The present inventor has found that when the number density of carbides having an aspect ratio of 2.0 or more is 0.60 / µm 2< or less, the yield ratio thereof can be sufficiently lowered.

[0044] Here, the "aspect ratio" is a value obtained by dividing the length in the longitudinal direction by the short-axis length (the maximum length in the direction orthogonal to the longitudinal direction) (length in longitudinal direction / short-axis length) when carbides are viewed in a plan view with SEM or the like. The number density of carbides having an aspect ratio of 2.0 or more is preferably 0.50 / µm 2< or less, more preferably 0.40 / µm 2< or less, and still more preferably 0.30 / µm 2< or less. The upper limit of the aspect ratio is not particularly limited, and may be, for example, 5.5 or less, considering the range where they easily exist in the method for manufacturing the same described later.

[0045] The number density of carbides having an aspect ratio of 2.0 or more can be determined by the method in Examples described later. When the number density of the carbides is determined, carbides having an equivalent circle diameter of 0.0245 µm or more can be counted. When the equivalent circle diameter of the carbides is less than 0.0245 µm, it is considered that the carbides is not sufficiently grown, and a carbide having a high aspect ratio that affects ductility is hardly present.

[0046] In the steel sheet according to the embodiments of the present invention, the number density of carbides having an aspect ratio of 2.0 to 2.5 is preferably 0.50 / µm 2< or less. It is considered that a large number of carbides having an aspect ratio of 2.0 to 2.5 is likely to be precipitated during the quenching. When the precipitation of the carbides is suppressed, ductility can be further improved, and yield ratio is easily controlled to a predetermined value or less. The number density of carbides having an aspect ratio of 2.0 to 2.5 is more preferably 0.45 / µm 2< or less, still more preferably 0.40 / µm 2< or less, and even more preferably 0.35 / µm 2< or less.

[0047] The number density of carbides having an aspect ratio of 2.0 to 2.5 can be determined by the method in Examples described later. When the number density of the carbides is determined, carbides having an equivalent circle diameter of 0.0245 µm or more can be counted.

[0048] The steel sheet according to the embodiments of the present invention has sufficient ductility, and specifically has an elongation of 6.7% or more. Further, the steel sheet according to the embodiments of the present invention has a sufficiently low yield ratio, specifically, an yield ratio of 0.860 or less. The yield ratio is preferably 0.700 or more, which makes it easy to secure the yield strength required as automobile parts. Furthermore, the steel sheet according to the embodiments of the present invention is a martensite microstructure steel sheet, and may have high strength, specifically, a tensile strength of 1270 MPa or more.<4. Manufacturing Method>

[0049] The method for manufacturing the steel sheet according to the embodiments of the present invention includes: (A) heating a steel sheet in a first temperature range of 840°C to 950°C and holding for 30 seconds or more; (B) after the holding, quenching the steel sheet from a second temperature range between 450°C and the first temperature range down to below 100°C, wherein an average cooling rate between 420°C and 250°C is 300 °C / sec or higher; and (C) after the quenching, tempering the steel sheet in a third temperature range of 100°C to 300°C for 30 seconds or more.

[0050] The steel sheet according to the embodiments of the present invention can be manufactured by the above method. Hereinafter, each step will be described in detail.(A) Heating steel sheet in first temperature range and holding for 30 seconds or more

[0051] First, a steel sheet is heated in a first temperature range of 840°C to 950°C and held for 30 seconds or more. The steel sheet to be heated may be, for example, a steel sheet satisfying the above-described chemical composition. The method for preparing a steel sheet satisfying the above-described chemical composition is not particularly limited. For example, the steel sheet may be prepared by: performing smelting and continuous casting in accordance with a conventional method to obtain a steel piece such as a slab satisfying the above-described chemical composition; then heating the steel piece to about 1100°C to 1250°C; then hot rolling, winding, pickling, and cold rolling the steel piece.

[0052] When the first temperature range is 840°C or higher, an austenite single phase microstructure can be formed (to subsequently obtain 95 area% or more of a tempered martensite microstructure). When the first temperature range is 950°C or lower, the equipment load can be reduced and the manufacturing cost can be reduced. In the first temperature range, the heating and holding time needs to be 30 seconds or more in order to complete austenite transformation. The upper limit of the heating and holding time is not particularly limited, and is preferably 400 seconds or less from the viewpoint of productivity.(B) Quenching steel sheet from second temperature range

[0053] After the step (A), the steel sheet is quenched from a second temperature range between 450°C and the first temperature range down to below 100°C (for example, room temperature). When the second temperature range is less than 450, bainite and / or auto-tempered martensite can be generated in a large amount, and the strength may be lowered. The upper limit of the second temperature range is the first temperature range. That is, quenching may be performed immediately after the step (A). On the other hand, the second temperature range may be lower than the first temperature range (and 450°C or higher). In this case, the cooling rate from the first temperature range to the second temperature range is not particularly limited, and is preferably, for example, 5 °C / sec or higher from the viewpoint that ferrite is hardly formed. The quenching method is not particularly limited, and may be a known method such as water cooling and oil cooling.

[0054] Further, in the step (B), the average cooling rate between 420°C and 250°C is 300 °C / sec or higher. The temperature range between 420°C and 250°C corresponds to the temperature range in which martensitic transformation starts and ends. When the martensite microstructure transformed in the temperature range is auto-tempered, carbides having a high aspect ratio (for example, an aspect ratio of 2.0 or more, or 2.0 to 2.5) are easily precipitated. Therefore, when the cooling rate is increased in the temperature range, it is possible to suppress the generation of carbides having a high aspect ratio. The average cooling rate between 420°C and 250°C is more preferably 400 °C / sec or more, still more preferably 500 °C / sec or more, and even more preferably 600 °C / sec or more.(C) Tempering steel sheet

[0055] After the step (B), the steel sheet is tempered in a third temperature range of 100 to 300°C for 30 seconds or more. When the third temperature range is higher than 300°C, it is difficult to secure strength. The third temperature range is preferably 280°C or lower, and more preferably 260°C or lower. On the other hand, when the third temperature range is lower than 100°C, toughness is hardly improved.

[0056] The method for manufacturing the steel sheet according to the embodiments of the present invention may include other steps as long as the object of the present disclosure is achieved.EXAMPLES

[0057] Hereinafter, the embodiments of the present invention will be described more specifically with reference to Examples. The embodiments of the present invention are not limited by the following Examples, and can be implemented with appropriate modifications within the scope that can be consistent with the above-described and later-described gist, and all of them are included in the scope of the embodiments of the present invention.

[0058] Steel pieces satisfying the chemical compositions of the steel types A to H shown in Table 1 (the balance being iron and inevitable impurities) were obtained by smelting and continuous casting in accordance with a conventional method. Thereafter, each steel piece was heated to 1200°C, and then subjected to hot rolling (finishing temperature: 870°C, finishing thickness: 2.6 mm), winding at 620°C, pickling, and cold rolling, thereby obtaining a steel sheet having a sheet thickness of 1.4 mm. The obtained steel sheet was cut into a size of 150 mm × 160 mm, subjected to the heat treatment described in the step (A) of Table 2, and then quenched under the conditions described in the step (B) of Table 2. In Test Nos. 1, 2, 4, 5, 7, 8, 10, 11, 13, 14, 16, 17, 19, 20, 22 and 23, the cooling rate from the heating temperature in the step (A) to the quenching start temperature in the step (B) was 5 °C / sec or more. The quenching conditions were as follows. The water amount was 70 L in order to secure the cooling rate. In Test Nos. 1, 2, 4, 5, 7, 8, 10, 11, 13, 14, 16, 17, 19, 20, 22 and 23, the water temperature was room temperature. In Test Nos. 3, 6, 9, 12, 15, 18, 21 and 24, the water temperature was set to 60°C by heating with a heater in order to intentionally lower the cooling rate. In Test Nos. 1, 4, 7, 10, 13, 16, 19 and 22, cooling water was circulated under the conditions of 1.0 m / min × 0.2 MPa by a water flow pump in order to intentionally increase the cooling rate. Fig. 2 shows a schematic top view illustrating the position of thermocouples at the time of quenching on the steel sheet cut into 150 mm × 160 mm, viewed in the sheet thickness direction. As shown in Fig. 2, thermocouples 2 (indicated by black circles) were attached to 5 places of a steel sheet 1, the temperature was measured at a sampling period of 0.01 seconds, the cooling rate between 420°C and 250°C was measured at 5 places, and the average value thereof was defined as the average cooling rate between 420°C and 250°C. After quenching, the steel sheet was tempered under the conditions described in the step (C) in Table 2.

[0059] In Table 1, "-" in the columns of "Si", "Cu" and "Cr" means that the content was at the level of inevitable impurities, which were not intentionally added. In addition, P, S and N are not intentionally added, and the contents thereof are at the level of inevitable impurities. However, the contents thereof are required to be strictly controlled as described above, and therefore the contents thereof are clarified. [Table 1]Steel type No.Chemical Composition (mass%) (the balance being iron and inevitable impurities)CSiMnPSAlCuNiCrTiBNA0.200.201.790.005< 0.0010.039-0.09-0.0520.00250.0044B0.22-1.000.005< 0.0010.0660.100.100.070.0510.00270.0044C0.190.022.130.005< 0.0010.035-0.10-0.0500.00270.0051D0.200.612.170.0050.0020.040-0.09-0.0320.00310.0027E0.210.211.370.0040.00090.041-0.09-0.0530.00250.0052F0.200.202.170.0050.00090.040-0.09-0.0490.00270.0051G0.200.101.400.0050.00120.041-0.10-0.0500.00270.0053H0.200.201.270.0050.00070.0460.07-0.210.0260.00240.0056 [Table 2] Test No.Steel typeStep (A)Step (B)Step (B)Heating temperature (°C)Holding time (s)Quenching start temperature (°C)Water temperature of quench tank (°C)Presence or absence of circulating cooling waterAverage cooling rate (°C / s) between 420°C and 250°CTempering temperature (°C)Tempering time (s)1A90033750Room temperaturePresent33842002432A90033750Room temperatureAbsent15022002433A9003390060°CAbsent4552002434B90033750Room temperaturePresent28332302435B90033750Room temperatureAbsent21912302436B9003390060°CAbsent7952302437C90033750Room temperaturePresent33232002438C90033750Room temperatureAbsent38002002439C9003390060°CAbsent76220024310D90033750Room temperaturePresent480720024311D90033750Room temperatureAbsent308720024312D9003390060°CAbsent44820024313E90033750Room temperaturePresent287020024314E90033750Room temperatureAbsent235620024315E9003390060°CAbsent71420024316F90033750Room temperaturePresent240920024317F90033750Room temperatureAbsent361320024318F9003390060°CAbsent35520024319G90033750Room temperaturePresent242920024320G90033750Room temperatureAbsent183220024321G9003390060°CAbsent50420024322H90033750Room temperaturePresent235820024323H90033750Room temperatureAbsent181120024324H9003390060°CAbsent482200243

[0060] The steel sheets of Test Nos. 1 to 24 were evaluated as follows.<Area Ratio of Tempered Martensite Microstructure>

[0061] For the test piece of 1.4 mm (sheet thickness direction) × 20 mm (rolling direction) × 20 mm (sheet width direction), a cross section parallel to the rolling direction and the sheet thickness direction (that is, perpendicular to the sheet width direction) was polished, and subjected to Nital corrosion, and then a SEM image was obtained for the 1 / 4t part (t is the sheet thickness) at a magnification of 1000 times.

[0062] In one arbitrary visual field (visual field area: 12275 µm 2< ) of the obtained SEM image, 10 lines were drawn at equal intervals vertically and horizontally, respectively, and the number of intersections where the intersection falls on the tempered martensite microstructure was divided by the total number of intersections to determine the area ratio of the tempered martensite microstructure. The results are shown in Table 3 described later.<Tensile Properties>

[0063] For tensile properties, a JIS No. 5 tensile test piece was obtained from each of the steel sheets of Test Nos. 1 to 24 such that the sheet width direction (that is, the direction perpendicular to the rolling direction and the sheet thickness direction) was the longitudinal direction, and tensile strength TS (MPa), yield stress YS (MPa), and elongation EL (%) as ductility were determined in accordance with the method defined in JIS Z 2241. The yield ratio YR was YS / TS. For each physical property value, the test was performed in n = 2, and the average value thereof was adopted. The results are shown in Table 3 described later.<Number Density of Carbides Having Each Aspect Ratio>

[0064] Fig. 1 shows a schematic top view of a test piece 1 for measuring the number density of carbides having each aspect ratio as viewed in the sheet thickness direction. As shown in Fig. 1, from the remaining material after the tensile test, the test piece 1 was cut out so that the cross section S2 that was 2 mm away from the fracture surface S1 could be observed, the test piece 1 was polished and subjected to picral corrosion, and then the vicinity of the t / 2 (t is the sheet thickness) position and the center of the test piece in the rolling direction LD (shown by a broken line CL) was observed in the observation direction OD to obtain an FE-SEM image at a magnification of 5000 times.

[0065] In one arbitrary visual field (visual field area: 416 µm 2< ) of the obtained FE-SEM image, the visual field was binarized, and the carbides displayed in white were measured for the length in the longitudinal direction and the short-axis length (the maximum length in the direction orthogonal to the longitudinal direction) thereof, thereby determining the aspect ratio (length in longitudinal direction / short-axis length) of each carbide. Then, the number of those having an aspect ratio of 2.0 or more and the number of those having an aspect ratio of 2.0 to 2.5 were determined, and divided by the visual field area to calculate each number density. When the number density was determined for each, carbides having an equivalent circle diameter of 0.0245 µm or more were counted. The maximum equivalent circle diameter of the counted carbides was 0.227 µm. As for the number density of those having an aspect ratio of 2.0 or more, all the counted carbides had an aspect ratio of 5.5 or less, and those having an aspect ratio of more than 5.5 were not observed. The results are shown in Table 3. [Table 3]Test No.Steel typeArea ratio of martensite microstructure (%)Number density of carbides having aspect ratio of 2 or more (particles / µm 2< )Number density of carbides having aspect ratio of 2 to 2.5 (particles / µm 2< )YS (MPa)TS (MPa)YR (-)EL (%)1A1000.210.17130115200.8566.92A1000.180.15129215130.8546.73A1000.180.14123014610.8427.44B1000.910.69129114690.8796.65B1000.780.61124614370.8676.06B1000.870.67123414290.8646.57C1000.260.20126914850.8546.98C1000.170.15126614880.8516.89C1000.490.39120514380.8387.310D1000.290.22129515470.8377.811D1000.210.15129515570.8318.212D1000.290.18127415250.8367.113E1000.190.17124114650.8477.414E1000.160.13125814980.8407.215E1000.270.24118714190.8376.716F1000.120.11131515420.8537.517F1000.180.15128915150.8516.818F1000.130.11124814920.8367.619G1000.160.13127315020.8487.320G1000.160.12128415130.8497.121G1000.340.29120814410.8386.722H1000.190.16121914840.8217.023H1000.260.22118714490.8196.724H1000.440.37115614180.8157.2

[0066] Table 3 shows the following. Test Nos. 1 to 3 and 7 to 24 each satisfied the requirements prescribed in the embodiments of the present invention, and had sufficient ductility (elongation: 6.7% or more) and a sufficiently low yield ratio (0.860 or less).

[0067] On the other hand, Test Nos. 4 to 6 did not satisfy the requirements prescribed in the embodiments of the present invention, and had insufficient ductility and a high yield ratio. Specifically, in all of Test Nos. 4 to 6, the Mn content was insufficient, and the number density of carbides having an aspect ratio of 2 or more was more than 0.60 / µm 2< , so that the elongation was less than 6.7%, and the yield ratio was also more than 0.860.

[0068] This application claims priority based on Japanese Patent Application No. 2023-108449 filed on June 30, 2023 and Japanese Patent Application No. 2024-062940 filed on April 9, 2024. Japanese Patent Application No. 2023-108449 and Japanese Patent Application No. 2024-062940 are incorporated herein by reference.EXPLANATION OF REFERENCES

[0069] 1Steel sheet 2Thermocouple

Examples

examples

[0057]Hereinafter, the embodiments of the present invention will be described more specifically with reference to Examples. The embodiments of the present invention are not limited by the following Examples, and can be implemented with appropriate modifications within the scope that can be consistent with the above-described and later-described gist, and all of them are included in the scope of the embodiments of the present invention.

[0058]Steel pieces satisfying the chemical compositions of the steel types A to H shown in Table 1 (the balance being iron and inevitable impurities) were obtained by smelting and continuous casting in accordance with a conventional method. Thereafter, each steel piece was heated to 1200°C, and then subjected to hot rolling (finishing temperature: 870°C, finishing thickness: 2.6 mm), winding at 620°C, pickling, and cold rolling, thereby obtaining a steel sheet having a sheet thickness of 1.4 mm. The obtained steel sheet was cut into a size of 150 mm × ...

TECHNICAL FIELD

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

[0002] Conventionally, improvement of safety of an occupant in a vehicle has been required, and strength of a vehicle body has been improved for such a purpose. On the other hand, against the background of the growing global warming problem, the movement for improving the fuel efficiency of automobiles is accelerating. It is known that weight reduction of a vehicle body is effective for improving fuel efficiency.

[0003] In order to improve safety and reduce weight for vehicle bodies, steel sheets for automobiles are increasingly required to have higher strength, and the application of a steel sheet mainly including (for example, 95 area% or more of) a tempered martensite microstructure (hereinafter, also referred to as "martensite microstructure steel sheet") has been studied. The martensite microstructure steel sheet has an advantage that higher strength is easily realized with a small amount of additive elements.

[0004] Patent Document 1 describes a martensite microstructure steel sheet having improved delayed fracture resistance. Patent Document 2 describes a martensite microstructure steel sheet having improved collision resistance.CONVENTIONAL ART DOCUMENTSPATENT DOCUMENTS

[0005] Patent Document 1: JP 7140301 B Patent Document 2: JP 7120461 B SUMMARY OF THE INVENTIONPROBLEMS TO BE SOLVED BY THE INVENTION

[0006] The martensite microstructure steel sheet is easy to realize higher strength, but the structure is poor in ductility, and thus difficult to be applied to automobile parts from the viewpoint of processability. In addition, considering application to automobile parts, the martensite microstructure steel sheet needs to have a low yield ratio to some extent for fracture resistance.

[0007] Ductility is generally improved by introducing a soft phase ferrite and / or a residual γ, and it has been hardly studied, either in Patent Document 1 or the like, to improve the ductility of the martensite microstructure steel sheet. In addition, it has been hardly studied, either in Patent Document 1 or the like, to reduce yield ratio.

[0008] The present disclosure has been made in view of such a circumstance, and an object of the present disclosure is to provide a martensite microstructure steel sheet having sufficient ductility and a sufficiently low yield ratio, and a method for manufacturing the same.MEANS FOR SOLVING THE PROBLEMS

[0009] A first aspect of the present invention is a steel sheet having: a chemical composition including: C: 0.15 to 0.25 mass%, Si: 0.70 mass% or less (including 0 mass%), Mn: 1.10 to 2.50 mass%, Al: 0.150 mass% or less (not including 0 mass%), P: 0.020 mass% or less (including 0 mass%), S: 0.010 mass% or less (including 0 mass%), N: 0.010 mass% or less (including 0 mass%), and a balance being Fe and inevitable impurities; and a metal microstructure including 95 area% or more of a tempered martensite microstructure, wherein a number density of carbides having an aspect ratio of 2.0 or more is 0.60 / µm 2< or less.

[0010] A second aspect of the present invention is the steel sheet according to the first aspect, further including at least one selected from the group consisting of Ti: 0.010 to 0.070 mass%, B: 0.0100 mass% or less (not including 0 mass%) and Cr: 2.00 mass% or less (not including 0 mass%).

[0011] A third aspect of the present invention is the steel sheet according to the first or second aspect, further including at least one selected from the group consisting of Cu: 0.50 mass% or less (not including 0 mass%) and Ni: 0.50 mass% or less (not including 0 mass%).

[0012] A fourth aspect of the present invention is the steel sheet according to any one of the first to third aspects, further including at least one selected from the group consisting of V: 0.10 mass% or less (not including 0 mass%) and Nb: 0.10 mass% or less (not including 0 mass%).

[0013] A fifth aspect of the present invention is the steel sheet according to any one of the first to fourth aspects, wherein a number density of carbides having an aspect ratio of 2.0 to 2.5 is 0.50 / µm 2< or less.

[0014] A sixth aspect of the present invention is a method for manufacturing the steel sheet according to any one of the first to fifth aspects, the method including: heating a steel sheet in a first temperature range of 840°C to 950°C and holding for 30 seconds or more; after the holding, quenching the steel sheet from a second temperature range between 450°C and the first temperature range down to below 100°C, wherein an average cooling rate between 420°C and 250°C is 300 °C / sec or higher; and after the quenching, tempering the steel sheet in a third temperature range of 100°C to 300°C for 30 seconds or more. TECHNICAL EFFECTS OF THE INVENTION

[0015] According to the embodiments of the present invention, it is possible to provide a martensite microstructure steel sheet having sufficient ductility, and a method for manufacturing the same.BRIEF DESCRIPTION OF THE DRAWINGS

[0016] [Fig. 1] Fig. 1 shows a schematic top view of a test piece for measuring the number density of carbides having each aspect ratio as viewed in the sheet thickness direction. [Fig. 2] Fig. 2 shows a schematic top view illustrating the position of thermocouples on a steel sheet at the time of quenching in Examples. MODE FOR CARRYING OUT THE INVENTION

[0017] The present inventor has studied from various angles in order to realize a martensite microstructure steel sheet having sufficient ductility and a sufficiently low yield ratio. As a result, the present inventor has found that a martensite microstructure steel sheet having sufficient ductility and a sufficiently low yield ratio can be realized by predeterminedly adjusting the chemical composition and controlling the number density of carbides having an aspect ratio of 2.0 or more to a predetermined value or lower.

[0018] Hereinafter, details of each requirement defined by the embodiments of the present invention will be described.<1. Chemical Composition>

[0019] The steel sheet according to the embodiments of the present invention preferably has a chemical composition including: C: 0.15 to 0.25 mass%, Si: 0.70 mass% or less (including 0 mass%), Mn: 1.10 to 2.50 mass%, Al: 0.150 mass% or less (not including 0 mass%), P: 0.020 mass% or less (including 0 mass%), S: 0.010 mass% or less (including 0 mass%), N: 0.010 mass% or less (including 0 mass%), and a balance being Fe and inevitable impurities.

[0020] Hereinafter, each element will be described in detail.(C: 0.15 to 0.25 mass%)

[0021] C is an element necessary for securing strength. Therefore, the C content is 0.15 mass% or more, preferably 0.16 mass% or more, more preferably 0.17 mass% or more, and still more preferably 0.18 mass% or more. However, when the C content is excessive, weldability is deteriorated. Therefore, the C content is 0.25 mass% or less, preferably 0.24 mass% or less, more preferably 0.23 mass% or less, and still more preferably 0.22 mass% or less.(Si: 0.70 mass% or less (including 0 mass%))

[0022] Si is an element that may be optionally contained. Si may be an element effective for improving tempering softening resistance and strength by solid solution strengthening. In addition, Si may be an element effective for suppressing the growth of carbides in the quenching process. In order to exhibit these effects, the Si content may be more than 0 mass%, and is preferably 0.05 mass% or more, more preferably 0.10 mass% or more, still more preferably 0.15 mass% or more, and even more preferably 0.20 mass% or more. However, since Si is a ferrite-forming element, when Si is contained in a large amount, quenchability is impaired, securing higher strength becomes difficult, and weldability is also deteriorated. Therefore, the Si content is 0.70 mass% or less, preferably 0.60 mass% or less, more preferably 0.50 mass% or less, and still more preferably 0.40 mass% or less.

[0023] In the specification, "including 0 mass%" means that an embodiment where intentional addition thereof is not performed, that is, a case where the content is equal to or lower than the inevitable impurity level is included therein (not excluding a case where intentional addition thereof is performed).(Mn: 1.10 to 2.50 mass%)

[0024] Mn is an element effective for improving quenchability and increasing strength. In addition, Mn is an element effective for suppressing the growth of carbides in the quenching process. In order to exhibit these effects, the Mn content is 1.10 mass% or more, preferably 1.50 mass% or more, more preferably 1.60 mass% or more, still more preferably 1.70 mass% or more, and even more preferably 1.80 mass% or more. However, when the Mn content is excessive, delayed fracture resistance and weldability deteriorate. Therefore, the Mn content is 2.50 mass% or less, preferably 2.40 mass% or less, more preferably 2.30 mass% or less, and still more preferably 2.20 mass% or less.(Al: 0.150 mass% or less (not including 0 mass%))

[0025] Al is an element added as a deoxidizing agent, and also has an effect of improving the corrosion resistance of steel. In order to sufficiently exhibit these effects, the Al content is more than 0 mass%, preferably 0.010 mass% or more, more preferably 0.020 mass% or more, still more preferably 0.030 mass% or more, and even more preferably 0.040 mass% or more. However, when Al is excessively contained, a large amount of C-based inclusions are generated, which causes surface defects. Therefore, the Al content is 0.150 mass% or less, preferably 0.140 mass% or less, more preferably 0.130 mass% or less, and still more preferably 0.120 mass% or less.(P: 0.020 mass% or less (including 0 mass%))

[0026] P (phosphorus) is an inevitable impurity and reduces ductility due to brittleness. Therefore, the P content is 0.020 mass% or less, preferably 0.015 mass% or less, and more preferably 0.010 mass% or less.(S: 0.010 mass% or less (including 0 mass%))

[0027] S is an inevitable impurity, generates sulfide-based inclusions, and deteriorates processability and weldability of the base material. Therefore, the S content is 0.010 mass% or less, preferably 0.005 mass% or less, and more preferably 0.003 mass% or less.(N: 0.010 mass% or less (including 0 mass%))

[0028] N is an inevitable impurity, increases the precipitation amount of nitride, and adversely affects toughness. Therefore, the N content is 0.010 mass% or less, preferably 0.008 mass% or less, and more preferably 0.006 mass% or less. On the other hand, considering the costs for steelmaking or the like, the N content may be 0.001 mass% or more.

[0029] The steel sheet according to the embodiments of the present invention includes the chemical composition described above, and in one embodiment of the present invention, the balance preferably is iron and inevitable impurities. As the inevitable impurities, contamination of elements brought depending on the situation of raw materials, materials, manufacturing facilities, and the like is allowed. For example, there are elements such as P, S and N, which are typically preferable as the content is smaller, and thus are inevitable impurities, but are separately defined as the composition range as described above. Therefore, in the specification, the "inevitable impurities" constituting the residual portion indicates a concept excluding elements with composition range separately defined.

[0030] As preferred embodiments of the present invention, it is also effective to further include as necessary, in addition to the above elements, (a) at least one selected from the group consisting of Ti: 0.010 to 0.070 mass%, B: 0.0100 mass% or less (not including 0 mass%), and Cr: 2.00 mass% or less (not including 0 mass%), (b) at least one selected from the group consisting of Cu: 0.50 mass% or less (not including 0 mass%) and Ni: 0.50 mass% or less (not including 0 mass%), (c) at least one selected from the group consisting of V: 0.10 mass% or less (not including 0 mass%) and Nb: 0.10 mass% or less (not including 0 mass%), and the like, thereby further improving the characteristics of the steel sheet depending on the kinds of the contained elements.((a) At least one selected from the group consisting of Ti: 0.010 to 0.070 mass%, B: 0.0100 mass% or less (not including 0 mass%), and Cr: 2.00 mass% or less (not including 0 mass%))

[0031] Ti is an element effective for improving the delayed fracture characteristics by precipitation of carbides and refinement of the prior γ grain size. Therefore, the Ti content is preferably 0.010 mass% or more, more preferably 0.015 mass% or more, still more preferably 0.020 mass% or more, and even more preferably 0.025 mass% or more. However, when Ti is excessively contained, precipitation of carbonitrides increases, and ductility and processability of the base material deteriorate. Therefore, when Ti is contained, the content thereof is 0.070 mass% or less, preferably 0.060 mass% or less, more preferably 0.050 mass% or less, and still more preferably 0.040 mass% or less.

[0032] B is an element that improves quenchability by adding a small amount. In order to sufficiently exhibit the effect, the B content may be more than 0 mass%, and is preferably 0.0001 mass% or more, more preferably 0.0003 mass% or more, still more preferably 0.0005 mass% or more, and even more preferably 0.0007 mass% or more. However, when B is excessively contained, ductility decreases. Therefore, when B is contained, the content thereof is 0.0100 mass% or less, preferably 0.0090 mass% or less, more preferably 0.0080 mass% or less, and still more preferably 0.0070 mass% or less.

[0033] Cr is an element effective for improving quenchability and increasing strength. Cr is an element effective for increasing the tempering softening resistance of the martensitic phase. In order to exhibit these effects, the Cr content may be more than 0 mass%, and is preferably 0.10 mass% or more, more preferably 0.20 mass% or more, still more preferably 0.30 mass% or more, and even more preferably 0.40 mass% or more. However, when Cr is excessively contained, delayed fracture resistance is deteriorated. Therefore, when Cr is contained, the content thereof is 2.00 mass% or less, preferably 1.80 mass% or less, more preferably 1.60 mass% or less, and still more preferably 1.40 mass% or less.((b) At least one selected from the group consisting of Cu: 0.50 mass% or less (not including 0 mass%) and Ni: 0.50 mass% or less (not including 0 mass%))

[0034] Cu is an element effective for improving corrosion resistance and improving delayed fracture resistance. In order to exhibit the effect, the Cu content may be more than 0 mass%, and is preferably 0.01 mass or more, more preferably 0.03 mass% or more, still more preferably 0.05 mass% or more, and even more preferably 0.07 mass% or more. However, when Cu is excessively contained, ductility and processability are deteriorated. Therefore, when Cu is contained, the content thereof is 0.50 mass% or less, preferably 0.40 mass% or less, more preferably 0.30 mass% or less, and still more preferably 0.20 mass% or less.

[0035] Ni is an element effective for improving corrosion resistance and improving delayed fracture resistance. In order to exhibit the effect, the Ni content may be more than 0 mass%, and is preferably 0.01 mass% or more, more preferably 0.03 mass% or more, still more preferably 0.05 mass% or more, and even more preferably 0.07 mass% or more. However, when Ni is excessively contained, ductility and processability are deteriorated. Therefore, when Ni is contained, the content thereof is 0.50 mass% or less, preferably 0.40 mass% or less, more preferably 0.30 mass% or less, and still more preferably 0.20 mass% or less.((c) At least one selected from the group consisting of V: 0.10 mass% or less (not including 0 mass%) and Nb: 0.10 mass% or less (not including 0 mass%))

[0036] V is an element effective for improving the strength by precipitation of carbides. In order to exhibit the effect, the V content may be more than 0 mass%, and is preferably 0.01 mass% or more, more preferably 0.02 mass% or more, still more preferably 0.03 mass% or more, and even more preferably 0.04 mass% or more. However, when V is excessively contained, precipitation of carbonitrides increases, and ductility and processability of the base material decrease. Therefore, when V is contained, the content thereof is 0.10 mass% or less, preferably 0.09 mass% or less, more preferably 0.08 mass% or less, and still more preferably 0.07 mass% or less.

[0037] Nb is an element effective for improving the strength by precipitation of carbides. In order to exhibit the effect, the Nb content may be more than 0 mass%, and is preferably 0.01 mass% or more, more preferably 0.02 mass% or more, still more preferably 0.03 mass% or more, and even more preferably 0.04 mass% or more. However, when Nb is excessively contained, precipitation of carbonitrides increases, and ductility and processability of the base material decrease. Therefore, when Nb is contained, the content thereof is 0.10 mass% or less, preferably 0.09 mass% or less, more preferably 0.08 mass% or less, and still more preferably 0.07 mass% or less.

[0038] As still another element, for example, any one or more selected from the group consisting of Se, As, Sb, Pb, Sn, Bi, Mg, Zn, Zr, W, Cs, Rb, Ca, Co, La, Tl, Nd, Y, In, Be, Hf, Tc, Ta and O (the elements may be inevitable impurities) may be contained in a total amount of 0.01 mass% or less (including 0 mass%) as necessary in order to improve corrosion resistance and the delayed fracture resistance of the base material part.<2. Metal Microstructure>

[0039] In the steel sheet according to the embodiments of the present invention, the metal microstructure includes 95 area% or more of a tempered martensite structure. The metal microstructure preferably includes 97 area% or more of a tempered martensite microstructure. The upper limit of the area ratio of the tempered martensite microstructure is not particularly limited, and may be 100 area%.

[0040] In the steel sheet according to the embodiments of the present invention, the metal microstructure may include not only the tempered martensite microstructure, but also microstructures inevitably contained due to the manufacturing process, for example, any one or more selected from the group consisting of a ferrite microstructure, a bainite microstructure, a retained austenite (γ) microstructure, and an as-quenched martensite.

[0041] The area ratio of the tempered martensite microstructure can be determined by the method in Examples described later.<3. Carbides>

[0042] The present inventor has found that carbides having a high aspect ratio that may be mainly present at grain boundaries in the martensite microstructure steel sheet embrittle the grain boundaries and reduce ductility. Then, the present inventor has found that a martensite microstructure steel sheet having sufficient ductility can be realized when the chemical composition thereof is predeterminedly adjusted and the number density of carbides having an aspect ratio of 2.0 or more becomes 0.60 / µm 2< or less.

[0043] Furthermore, considering application to automobile parts and the like, those having a moderately high yield ratio are fracture-resistant, and therefore preferable. The present inventor has found that when the number density of carbides having an aspect ratio of 2.0 or more is 0.60 / µm 2< or less, the yield ratio thereof can be sufficiently lowered.

[0044] Here, the "aspect ratio" is a value obtained by dividing the length in the longitudinal direction by the short-axis length (the maximum length in the direction orthogonal to the longitudinal direction) (length in longitudinal direction / short-axis length) when carbides are viewed in a plan view with SEM or the like. The number density of carbides having an aspect ratio of 2.0 or more is preferably 0.50 / µm 2< or less, more preferably 0.40 / µm 2< or less, and still more preferably 0.30 / µm 2< or less. The upper limit of the aspect ratio is not particularly limited, and may be, for example, 5.5 or less, considering the range where they easily exist in the method for manufacturing the same described later.

[0045] The number density of carbides having an aspect ratio of 2.0 or more can be determined by the method in Examples described later. When the number density of the carbides is determined, carbides having an equivalent circle diameter of 0.0245 µm or more can be counted. When the equivalent circle diameter of the carbides is less than 0.0245 µm, it is considered that the carbides is not sufficiently grown, and a carbide having a high aspect ratio that affects ductility is hardly present.

[0046] In the steel sheet according to the embodiments of the present invention, the number density of carbides having an aspect ratio of 2.0 to 2.5 is preferably 0.50 / µm 2< or less. It is considered that a large number of carbides having an aspect ratio of 2.0 to 2.5 is likely to be precipitated during the quenching. When the precipitation of the carbides is suppressed, ductility can be further improved, and yield ratio is easily controlled to a predetermined value or less. The number density of carbides having an aspect ratio of 2.0 to 2.5 is more preferably 0.45 / µm 2< or less, still more preferably 0.40 / µm 2< or less, and even more preferably 0.35 / µm 2< or less.

[0047] The number density of carbides having an aspect ratio of 2.0 to 2.5 can be determined by the method in Examples described later. When the number density of the carbides is determined, carbides having an equivalent circle diameter of 0.0245 µm or more can be counted.

[0048] The steel sheet according to the embodiments of the present invention has sufficient ductility, and specifically has an elongation of 6.7% or more. Further, the steel sheet according to the embodiments of the present invention has a sufficiently low yield ratio, specifically, an yield ratio of 0.860 or less. The yield ratio is preferably 0.700 or more, which makes it easy to secure the yield strength required as automobile parts. Furthermore, the steel sheet according to the embodiments of the present invention is a martensite microstructure steel sheet, and may have high strength, specifically, a tensile strength of 1270 MPa or more.<4. Manufacturing Method>

[0049] The method for manufacturing the steel sheet according to the embodiments of the present invention includes: (A) heating a steel sheet in a first temperature range of 840°C to 950°C and holding for 30 seconds or more; (B) after the holding, quenching the steel sheet from a second temperature range between 450°C and the first temperature range down to below 100°C, wherein an average cooling rate between 420°C and 250°C is 300 °C / sec or higher; and (C) after the quenching, tempering the steel sheet in a third temperature range of 100°C to 300°C for 30 seconds or more.

[0050] The steel sheet according to the embodiments of the present invention can be manufactured by the above method. Hereinafter, each step will be described in detail.(A) Heating steel sheet in first temperature range and holding for 30 seconds or more

[0051] First, a steel sheet is heated in a first temperature range of 840°C to 950°C and held for 30 seconds or more. The steel sheet to be heated may be, for example, a steel sheet satisfying the above-described chemical composition. The method for preparing a steel sheet satisfying the above-described chemical composition is not particularly limited. For example, the steel sheet may be prepared by: performing smelting and continuous casting in accordance with a conventional method to obtain a steel piece such as a slab satisfying the above-described chemical composition; then heating the steel piece to about 1100°C to 1250°C; then hot rolling, winding, pickling, and cold rolling the steel piece.

[0052] When the first temperature range is 840°C or higher, an austenite single phase microstructure can be formed (to subsequently obtain 95 area% or more of a tempered martensite microstructure). When the first temperature range is 950°C or lower, the equipment load can be reduced and the manufacturing cost can be reduced. In the first temperature range, the heating and holding time needs to be 30 seconds or more in order to complete austenite transformation. The upper limit of the heating and holding time is not particularly limited, and is preferably 400 seconds or less from the viewpoint of productivity.(B) Quenching steel sheet from second temperature range

[0053] After the step (A), the steel sheet is quenched from a second temperature range between 450°C and the first temperature range down to below 100°C (for example, room temperature). When the second temperature range is less than 450, bainite and / or auto-tempered martensite can be generated in a large amount, and the strength may be lowered. The upper limit of the second temperature range is the first temperature range. That is, quenching may be performed immediately after the step (A). On the other hand, the second temperature range may be lower than the first temperature range (and 450°C or higher). In this case, the cooling rate from the first temperature range to the second temperature range is not particularly limited, and is preferably, for example, 5 °C / sec or higher from the viewpoint that ferrite is hardly formed. The quenching method is not particularly limited, and may be a known method such as water cooling and oil cooling.

[0054] Further, in the step (B), the average cooling rate between 420°C and 250°C is 300 °C / sec or higher. The temperature range between 420°C and 250°C corresponds to the temperature range in which martensitic transformation starts and ends. When the martensite microstructure transformed in the temperature range is auto-tempered, carbides having a high aspect ratio (for example, an aspect ratio of 2.0 or more, or 2.0 to 2.5) are easily precipitated. Therefore, when the cooling rate is increased in the temperature range, it is possible to suppress the generation of carbides having a high aspect ratio. The average cooling rate between 420°C and 250°C is more preferably 400 °C / sec or more, still more preferably 500 °C / sec or more, and even more preferably 600 °C / sec or more.(C) Tempering steel sheet

[0055] After the step (B), the steel sheet is tempered in a third temperature range of 100 to 300°C for 30 seconds or more. When the third temperature range is higher than 300°C, it is difficult to secure strength. The third temperature range is preferably 280°C or lower, and more preferably 260°C or lower. On the other hand, when the third temperature range is lower than 100°C, toughness is hardly improved.

[0056] The method for manufacturing the steel sheet according to the embodiments of the present invention may include other steps as long as the object of the present disclosure is achieved.EXAMPLES

[0057] Hereinafter, the embodiments of the present invention will be described more specifically with reference to Examples. The embodiments of the present invention are not limited by the following Examples, and can be implemented with appropriate modifications within the scope that can be consistent with the above-described and later-described gist, and all of them are included in the scope of the embodiments of the present invention.

[0058] Steel pieces satisfying the chemical compositions of the steel types A to H shown in Table 1 (the balance being iron and inevitable impurities) were obtained by smelting and continuous casting in accordance with a conventional method. Thereafter, each steel piece was heated to 1200°C, and then subjected to hot rolling (finishing temperature: 870°C, finishing thickness: 2.6 mm), winding at 620°C, pickling, and cold rolling, thereby obtaining a steel sheet having a sheet thickness of 1.4 mm. The obtained steel sheet was cut into a size of 150 mm × 160 mm, subjected to the heat treatment described in the step (A) of Table 2, and then quenched under the conditions described in the step (B) of Table 2. In Test Nos. 1, 2, 4, 5, 7, 8, 10, 11, 13, 14, 16, 17, 19, 20, 22 and 23, the cooling rate from the heating temperature in the step (A) to the quenching start temperature in the step (B) was 5 °C / sec or more. The quenching conditions were as follows. The water amount was 70 L in order to secure the cooling rate. In Test Nos. 1, 2, 4, 5, 7, 8, 10, 11, 13, 14, 16, 17, 19, 20, 22 and 23, the water temperature was room temperature. In Test Nos. 3, 6, 9, 12, 15, 18, 21 and 24, the water temperature was set to 60°C by heating with a heater in order to intentionally lower the cooling rate. In Test Nos. 1, 4, 7, 10, 13, 16, 19 and 22, cooling water was circulated under the conditions of 1.0 m / min × 0.2 MPa by a water flow pump in order to intentionally increase the cooling rate. Fig. 2 shows a schematic top view illustrating the position of thermocouples at the time of quenching on the steel sheet cut into 150 mm × 160 mm, viewed in the sheet thickness direction. As shown in Fig. 2, thermocouples 2 (indicated by black circles) were attached to 5 places of a steel sheet 1, the temperature was measured at a sampling period of 0.01 seconds, the cooling rate between 420°C and 250°C was measured at 5 places, and the average value thereof was defined as the average cooling rate between 420°C and 250°C. After quenching, the steel sheet was tempered under the conditions described in the step (C) in Table 2.

[0059] In Table 1, "-" in the columns of "Si", "Cu" and "Cr" means that the content was at the level of inevitable impurities, which were not intentionally added. In addition, P, S and N are not intentionally added, and the contents thereof are at the level of inevitable impurities. However, the contents thereof are required to be strictly controlled as described above, and therefore the contents thereof are clarified. [Table 1]Steel type No.Chemical Composition (mass%) (the balance being iron and inevitable impurities)CSiMnPSAlCuNiCrTiBNA0.200.201.790.005< 0.0010.039-0.09-0.0520.00250.0044B0.22-1.000.005< 0.0010.0660.100.100.070.0510.00270.0044C0.190.022.130.005< 0.0010.035-0.10-0.0500.00270.0051D0.200.612.170.0050.0020.040-0.09-0.0320.00310.0027E0.210.211.370.0040.00090.041-0.09-0.0530.00250.0052F0.200.202.170.0050.00090.040-0.09-0.0490.00270.0051G0.200.101.400.0050.00120.041-0.10-0.0500.00270.0053H0.200.201.270.0050.00070.0460.07-0.210.0260.00240.0056 [Table 2] Test No.Steel typeStep (A)Step (B)Step (B)Heating temperature (°C)Holding time (s)Quenching start temperature (°C)Water temperature of quench tank (°C)Presence or absence of circulating cooling waterAverage cooling rate (°C / s) between 420°C and 250°CTempering temperature (°C)Tempering time (s)1A90033750Room temperaturePresent33842002432A90033750Room temperatureAbsent15022002433A9003390060°CAbsent4552002434B90033750Room temperaturePresent28332302435B90033750Room temperatureAbsent21912302436B9003390060°CAbsent7952302437C90033750Room temperaturePresent33232002438C90033750Room temperatureAbsent38002002439C9003390060°CAbsent76220024310D90033750Room temperaturePresent480720024311D90033750Room temperatureAbsent308720024312D9003390060°CAbsent44820024313E90033750Room temperaturePresent287020024314E90033750Room temperatureAbsent235620024315E9003390060°CAbsent71420024316F90033750Room temperaturePresent240920024317F90033750Room temperatureAbsent361320024318F9003390060°CAbsent35520024319G90033750Room temperaturePresent242920024320G90033750Room temperatureAbsent183220024321G9003390060°CAbsent50420024322H90033750Room temperaturePresent235820024323H90033750Room temperatureAbsent181120024324H9003390060°CAbsent482200243

[0060] The steel sheets of Test Nos. 1 to 24 were evaluated as follows.<Area Ratio of Tempered Martensite Microstructure>

[0061] For the test piece of 1.4 mm (sheet thickness direction) × 20 mm (rolling direction) × 20 mm (sheet width direction), a cross section parallel to the rolling direction and the sheet thickness direction (that is, perpendicular to the sheet width direction) was polished, and subjected to Nital corrosion, and then a SEM image was obtained for the 1 / 4t part (t is the sheet thickness) at a magnification of 1000 times.

[0062] In one arbitrary visual field (visual field area: 12275 µm 2< ) of the obtained SEM image, 10 lines were drawn at equal intervals vertically and horizontally, respectively, and the number of intersections where the intersection falls on the tempered martensite microstructure was divided by the total number of intersections to determine the area ratio of the tempered martensite microstructure. The results are shown in Table 3 described later.<Tensile Properties>

[0063] For tensile properties, a JIS No. 5 tensile test piece was obtained from each of the steel sheets of Test Nos. 1 to 24 such that the sheet width direction (that is, the direction perpendicular to the rolling direction and the sheet thickness direction) was the longitudinal direction, and tensile strength TS (MPa), yield stress YS (MPa), and elongation EL (%) as ductility were determined in accordance with the method defined in JIS Z 2241. The yield ratio YR was YS / TS. For each physical property value, the test was performed in n = 2, and the average value thereof was adopted. The results are shown in Table 3 described later.<Number Density of Carbides Having Each Aspect Ratio>

[0064] Fig. 1 shows a schematic top view of a test piece 1 for measuring the number density of carbides having each aspect ratio as viewed in the sheet thickness direction. As shown in Fig. 1, from the remaining material after the tensile test, the test piece 1 was cut out so that the cross section S2 that was 2 mm away from the fracture surface S1 could be observed, the test piece 1 was polished and subjected to picral corrosion, and then the vicinity of the t / 2 (t is the sheet thickness) position and the center of the test piece in the rolling direction LD (shown by a broken line CL) was observed in the observation direction OD to obtain an FE-SEM image at a magnification of 5000 times.

[0065] In one arbitrary visual field (visual field area: 416 µm 2< ) of the obtained FE-SEM image, the visual field was binarized, and the carbides displayed in white were measured for the length in the longitudinal direction and the short-axis length (the maximum length in the direction orthogonal to the longitudinal direction) thereof, thereby determining the aspect ratio (length in longitudinal direction / short-axis length) of each carbide. Then, the number of those having an aspect ratio of 2.0 or more and the number of those having an aspect ratio of 2.0 to 2.5 were determined, and divided by the visual field area to calculate each number density. When the number density was determined for each, carbides having an equivalent circle diameter of 0.0245 µm or more were counted. The maximum equivalent circle diameter of the counted carbides was 0.227 µm. As for the number density of those having an aspect ratio of 2.0 or more, all the counted carbides had an aspect ratio of 5.5 or less, and those having an aspect ratio of more than 5.5 were not observed. The results are shown in Table 3. [Table 3]Test No.Steel typeArea ratio of martensite microstructure (%)Number density of carbides having aspect ratio of 2 or more (particles / µm 2< )Number density of carbides having aspect ratio of 2 to 2.5 (particles / µm 2< )YS (MPa)TS (MPa)YR (-)EL (%)1A1000.210.17130115200.8566.92A1000.180.15129215130.8546.73A1000.180.14123014610.8427.44B1000.910.69129114690.8796.65B1000.780.61124614370.8676.06B1000.870.67123414290.8646.57C1000.260.20126914850.8546.98C1000.170.15126614880.8516.89C1000.490.39120514380.8387.310D1000.290.22129515470.8377.811D1000.210.15129515570.8318.212D1000.290.18127415250.8367.113E1000.190.17124114650.8477.414E1000.160.13125814980.8407.215E1000.270.24118714190.8376.716F1000.120.11131515420.8537.517F1000.180.15128915150.8516.818F1000.130.11124814920.8367.619G1000.160.13127315020.8487.320G1000.160.12128415130.8497.121G1000.340.29120814410.8386.722H1000.190.16121914840.8217.023H1000.260.22118714490.8196.724H1000.440.37115614180.8157.2

[0066] Table 3 shows the following. Test Nos. 1 to 3 and 7 to 24 each satisfied the requirements prescribed in the embodiments of the present invention, and had sufficient ductility (elongation: 6.7% or more) and a sufficiently low yield ratio (0.860 or less).

[0067] On the other hand, Test Nos. 4 to 6 did not satisfy the requirements prescribed in the embodiments of the present invention, and had insufficient ductility and a high yield ratio. Specifically, in all of Test Nos. 4 to 6, the Mn content was insufficient, and the number density of carbides having an aspect ratio of 2 or more was more than 0.60 / µm 2< , so that the elongation was less than 6.7%, and the yield ratio was also more than 0.860.

[0068] This application claims priority based on Japanese Patent Application No. 2023-108449 filed on June 30, 2023 and Japanese Patent Application No. 2024-062940 filed on April 9, 2024. Japanese Patent Application No. 2023-108449 and Japanese Patent Application No. 2024-062940 are incorporated herein by reference.EXPLANATION OF REFERENCES

[0069] 1Steel sheet 2Thermocouple

Examples

examples

[0057]Hereinafter, the embodiments of the present invention will be described more specifically with reference to Examples. The embodiments of the present invention are not limited by the following Examples, and can be implemented with appropriate modifications within the scope that can be consistent with the above-described and later-described gist, and all of them are included in the scope of the embodiments of the present invention.

[0058]Steel pieces satisfying the chemical compositions of the steel types A to H shown in Table 1 (the balance being iron and inevitable impurities) were obtained by smelting and continuous casting in accordance with a conventional method. Thereafter, each steel piece was heated to 1200°C, and then subjected to hot rolling (finishing temperature: 870°C, finishing thickness: 2.6 mm), winding at 620°C, pickling, and cold rolling, thereby obtaining a steel sheet having a sheet thickness of 1.4 mm. The obtained steel sheet was cut into a size of 150 mm × ...

Claims

1. A steel sheet having: a chemical composition comprising: C: 0.15 to 0.25 mass%, Si: 0.70 mass% or less (including 0 mass%), Mn: 1.10 to 2.50 mass%, Al: 0.150 mass% or less (not including 0 mass%), P: 0.020 mass% or less (including 0 mass%), S: 0.010 mass% or less (including 0 mass%), N: 0.010 mass% or less (including 0 mass%), and a balance being Fe and inevitable impurities; and a metal microstructure including 95 area% or more of a tempered martensite microstructure, wherein a number density of carbides having an aspect ratio of 2.0 or more is 0.60 / µm2 or less.

2. The steel sheet according to claim 1, further comprising one or more from the group consisting of (a) to (c) as follows: (a) at least one selected from the group consisting of Ti: 0.010 to 0.070 mass%, B: 0.0100 mass% or less (not including 0 mass%), and Cr: 2.00 mass% or less (not including 0 mass%), (b) at least one selected from the group consisting of Cu: 0.50 mass% or less (not including 0 mass%) and Ni: 0.50 mass% or less (not including 0 mass%), and (c) at least one selected from the group consisting of V: 0.10 mass% or less (not including 0 mass%) and Nb: 0.10 mass% or less (not including 0 mass%).

3. The steel sheet according to claim 1 or 2, wherein a number density of carbides having an aspect ratio of 2.0 to 2.5 is 0.50 / µm2 or less.

4. A method for manufacturing the steel sheet according to claim 1 or 2, the method comprising: heating a steel sheet in a first temperature range of 840°C to 950°C and holding for 30 seconds or more; after the holding, quenching the steel sheet from a second temperature range between 450°C and the first temperature range down to below 100°C, wherein an average cooling rate between 420°C and 250°C is 300 °C / sec or higher; and after the quenching, tempering the steel sheet in a third temperature range of 100°C to 300°C for 30 seconds or more.

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