Steel sheet and method for manufacturing same

Abstract

This steel sheet has a prescribed component composition, contains a tempered martensite structure having a metal structure constituting at least 95% thereof by area, and has a number density of carbides having an aspect ratio of at least 2.0 of 0.60 per μm2 or less.

Description

Steel plate and its manufacturing method

[0001] The present disclosure relates to steel sheets and methods for manufacturing the same.

[0002] There has been a demand for improved passenger safety in vehicles, and for this purpose, the strength of vehicle bodies has been improved. On the other hand, against the backdrop of the worsening problem of global warming, efforts to improve the fuel efficiency of automobiles are accelerating. It is known that reducing the weight of vehicle bodies is an effective way to improve fuel efficiency.

[0003] To improve the safety and reduce the weight of automobile bodies, there is a demand for even higher strength steel sheets for automobiles, and the application of steel sheets containing tempered martensite (e.g., 95% by area or more) (hereinafter also referred to as "martensitic steel sheets") is being considered. Martensitic steel sheets have the advantage that high strength can be easily achieved with small amounts of added elements.

[0004] Patent Document 1 discloses a martensitic steel sheet having improved delayed fracture resistance, and Patent Document 2 discloses a martensitic steel sheet having improved crash resistance.

[0005] Patent No. 7140301 Patent No. 7120461

[0006] While martensitic steel sheets can easily achieve high strength, their poor ductility makes them difficult to apply to automotive parts. Furthermore, when considering application to automotive parts, martensitic steel sheets must have a low yield ratio to make them less susceptible to fracture.

[0007] In order to improve ductility, it is common to introduce ferrite and / or residual γ, which are soft phases, and little research has been done on improving the ductility of martensitic steel sheets, including in Patent Document 1. Furthermore, little research has been done on lowering the yield ratio, including in Patent Document 1.

[0008] The present disclosure has been made in view of the above circumstances, and one of its objectives is to provide a martensitic steel sheet having sufficient ductility and a sufficiently low yield ratio, and a method for manufacturing the same.

[0009] Aspect 1 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 (including 0 mass%), P: 0.020 mass% or less (including 0 mass%), S: 0.010 mass% or less (including 0 mass%), and N: 0.010 mass% or less (including 0 mass%), with the balance being Fe and unavoidable impurities, the metallographic structure including a tempered martensite structure of 95 area% or more, and the number density of carbides having an aspect ratio of 2.0 or more being 0.60 / μm ２ Below is the steel plate.

[0010] A second aspect of the present invention is the steel sheet according to the first aspect, further comprising at least one selected from the group consisting of Ti: 0.010 to 0.070 mass%, B: 0.0100 mass% or less (excluding 0 mass%), and Cr: 2.00 mass% or less (excluding 0 mass%).

[0011] A third aspect of the present invention is the steel sheet according to the first or second aspect, further comprising at least one selected from the group consisting of Cu: 0.50 mass % or less (excluding 0 mass %) and Ni: 0.50 mass % or less (excluding 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 comprising at least one selected from the group consisting of V: ​​0.10 mass % or less (excluding 0 mass %) and Nb: 0.10 mass % or less (excluding 0 mass %).

[0013] In a fifth aspect of the present invention, the number density of carbides having an aspect ratio of 2.0 to 2.5 is 0.50 pieces / μm ２ The steel sheet according to any one of aspects 1 to 4, which is as follows:

[0014] Aspect 6 of the present invention is a method for producing a steel sheet according to any one of Aspects 1 to 5, comprising: a step of heating to a first temperature range of 840°C to 950°C and holding the temperature therein for 30 seconds or more; a step of quenching, after the holding step, from a second temperature range of 450°C to the first temperature range to less than 100°C, wherein the average cooling rate from 420°C to 250°C is 300°C / second or more; and a step of tempering, after the quenching step, in a third temperature range of 100 to 300°C for 30 seconds or more.

[0015] According to the embodiments of the present invention, it is possible to provide a martensitic steel sheet having sufficient ductility and a method for manufacturing the same.

[0016] Fig. 1 is a schematic top view of a test piece for measuring the number density of carbides having each aspect ratio, as viewed from the plate thickness direction. Fig. 2 is a schematic top view of a steel plate showing the positions of thermocouples during quenching in the example.

[0017] The present inventors have conducted various investigations to realize a martensitic steel sheet having sufficient ductility and a sufficiently low yield ratio. As a result, they have found that a martensitic steel sheet having sufficient ductility and a sufficiently low yield ratio can be realized by adjusting the chemical composition to a predetermined value and controlling the number density of carbides having an aspect ratio of 2.0 or more to a predetermined value or less. Details of each requirement specified in the embodiments of the present invention are described below.

[0018] <1. Elemental Composition> The elemental composition of the steel sheet according to an embodiment of the present invention preferably includes 0.15 to 0.25 mass% C, 0.70 mass% or less (including 0 mass%) Si, 1.10 to 2.50 mass% Mn, 0.150 mass% or less (including 0 mass%) Al, 0.020 mass% or less (including 0 mass%) P, 0.010 mass% or less (including 0 mass%) S, and 0.010 mass% or less (including 0 mass%) N, with the balance being Fe and unavoidable impurities. Each element will be described in detail below.

[0019] (C: 0.15 to 0.25 mass%) C is an element necessary for ensuring strength. Therefore, the C content is set to 0.15 mass% or more, preferably 0.16 mass% or more, more preferably 0.17 mass% or more, and even more preferably 0.18 mass% or more. However, if the C content is excessive, weldability deteriorates. Therefore, the C content is set to 0.25 mass% or less, preferably 0.24 mass% or less, more preferably 0.23 mass% or less, and even more preferably 0.22 mass% or less.

[0020] (Si: 0.70 mass% or less (including 0 mass%)) Si is an element that may be optionally contained. Si can be an element effective for improving temper softening resistance and strength through solid solution strengthening. It can also be an element effective for suppressing the growth of carbides during the quenching process. To exert 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, even more preferably 0.15 mass% or more, and even more preferably 0.20 mass% or more. However, since Si is a ferrite-forming element, if it is contained in large amounts, hardenability is impaired, making it difficult to ensure high strength and also deteriorating weldability. Therefore, the Si content is set to 0.70 mass% or less, preferably 0.60 mass% or less, more preferably 0.50 mass% or less, and even more preferably 0.40 mass% or less. Note that in this specification, the term "including 0 mass%" means that it is not intentionally added, i.e., it includes cases where the content is below the unavoidable impurity level (it does not exclude cases where it is intentionally added).

[0021] (Mn: 1.10 to 2.50 mass%) Mn is an element effective in improving hardenability and increasing strength. It is also an element effective in suppressing the growth of carbides during the quenching process. To achieve these effects, the Mn content is set to 1.10 mass% or more, preferably 1.50 mass% or more, more preferably 1.60 mass% or more, even more preferably 1.70 mass% or more, and even more preferably 1.80 mass% or more. However, an excessive Mn content deteriorates delayed fracture resistance and weldability. Therefore, the Mn content is set to 2.50 mass% or less, preferably 2.40 mass% or less, more preferably 2.30 mass% or less, and even more preferably 2.20 mass% or less.

[0022] (Al: 0.150 mass% or less (excluding 0 mass%)) Al is an element added as a deoxidizer and also has the effect of improving the corrosion resistance of steel. To fully exert these effects, the Al content is set to more than 0 mass%, preferably 0.010 mass% or more, more preferably 0.020 mass% or more, even more preferably 0.030 mass% or more, and even more preferably 0.040 mass% or more. However, if Al is contained in excess, a large amount of C-based inclusions will be formed, causing surface defects. Therefore, the Al content is set to 0.150 mass% or less, preferably 0.140 mass% or less, more preferably 0.130 mass% or less, and even more preferably 0.120 mass% or less.

[0023] (P: 0.020% by mass or less (including 0% by mass)) P (phosphorus) is an inevitable impurity that reduces ductility due to brittleness. Therefore, the P content is set to 0.020% by mass or less, preferably 0.015% by mass or less, and more preferably 0.010% by mass or less.

[0024] (S: 0.010% by mass or less (including 0% by mass)) S is an inevitable impurity that generates sulfide-based inclusions and deteriorates the workability and weldability of the base metal. Therefore, the S content is set to 0.010% by mass or less, preferably 0.005% by mass or less, and more preferably 0.003% by mass or less.

[0025] (N: 0.010 mass% or less (including 0 mass%)) N is an inevitable impurity that increases the amount of nitride precipitation and adversely affects toughness. Therefore, the N content is set to 0.010 mass% or less, preferably 0.008 mass% or less, and more preferably 0.006 mass% or less. On the other hand, in consideration of steelmaking costs and the like, the N content may be 0.001 mass% or more.

[0026] The steel sheet according to the embodiment of the present invention preferably contains the above-described chemical composition, with the balance being iron and inevitable impurities in one embodiment of the present invention. The inevitable impurities include trace elements that are introduced depending on the conditions of raw materials, materials, manufacturing equipment, etc. Note that, for example, elements such as P, S, and N are usually preferable when their content is low, and therefore are considered inevitable impurities, but their composition ranges are separately specified as above. Therefore, in this specification, the term "unavoidable impurities" is used to refer to elements that exclude elements whose composition ranges are separately specified.

[0027] In a preferred embodiment of the present invention, in addition to the above elements, it is also effective to further contain, as necessary, (a) at least one selected from the group consisting of Ti: 0.010 to 0.070 mass%, B: 0.0100 mass% or less (excluding 0 mass%), and Cr: 2.00 mass% or less (excluding 0 mass%); (b) at least one selected from the group consisting of Cu: 0.50 mass% or less (excluding 0 mass%) and Ni: 0.50 mass% or less (excluding 0 mass%); (c) at least one selected from the group consisting of V: ​​0.10 mass% or less (excluding 0 mass%) and Nb: 0.10 mass% or less (excluding 0 mass%); and the like, and the properties of the steel sheet are further improved depending on the type of element contained.

[0028] (a) At least one element selected from the group consisting of Ti: 0.010 to 0.070 mass%, B: 0.0100 mass% or less (excluding 0 mass%), and Cr: 2.00 mass% or less (excluding 0 mass%). Ti is an element that is effective in improving delayed fracture resistance by refining the prior γ grain size through carbide precipitation. Therefore, the Ti content is preferably 0.010 mass% or more, more preferably 0.015 mass% or more, even more preferably 0.020 mass% or more, and even more preferably 0.025 mass% or more. However, excessive Ti content increases the precipitation of carbonitrides, deteriorating the ductility and workability of the base metal. Therefore, when Ti is contained, its content is 0.070 mass% or less, preferably 0.060 mass% or less, more preferably 0.050 mass% or less, and even more preferably 0.040 mass% or less. B is an element that improves hardenability when added in small amounts. To fully exert these effects, the B content may be greater than 0% by mass, preferably 0.0001% by mass or more, more preferably 0.0003% by mass or more, even more preferably 0.0005% by mass or more, and even more preferably 0.0007% by mass or more. However, excessive B content reduces ductility. Therefore, when B is contained, its content is set to 0.0100% by mass or less, preferably 0.0090% by mass or less, more preferably 0.0080% by mass or less, and even more preferably 0.0070% by mass or less. Cr is an element effective for increasing strength by improving hardenability. Cr is also an element effective for increasing the temper softening resistance of the martensite phase. To exert these effects, the Cr content may be greater than 0% by mass, preferably 0.10% by mass or more, more preferably 0.20% by mass or more, even more preferably 0.30% by mass or more, and even more preferably 0.40% by mass or more. However, excessive Cr content deteriorates delayed fracture resistance. Therefore, when Cr is contained, the content is set to 2.00 mass % or less, preferably 1.80 mass % or less, more preferably 1.60 mass % or less, and further preferably 1.40 mass % or less.

[0029] (b) At least one selected from the group consisting of Cu: 0.50% by mass or less (excluding 0% by mass) and Ni: 0.50% by mass or less (excluding 0% by mass) Cu is an element effective for improving corrosion resistance and thereby improving delayed fracture resistance. To achieve this effect, the Cu content may be more than 0% by mass, and is preferably 0.01% by mass or more, more preferably 0.03% by mass or more, even more preferably 0.05% by mass or more, and even more preferably 0.07% by mass or more. However, excessive Cu content reduces ductility and workability. Therefore, when Cu is contained, its content is set to 0.50% by mass or less, preferably 0.40% by mass or less, more preferably 0.30% by mass or less, and even more preferably 0.20% by mass or less. Ni is an element effective for improving corrosion resistance and thereby improving delayed fracture resistance. To achieve this effect, the Ni content may be more than 0 mass%, preferably 0.01 mass% or more, more preferably 0.03 mass% or more, even more preferably 0.05 mass% or more, and even more preferably 0.07 mass% or more. However, excessive Ni content reduces ductility and workability. Therefore, when Ni is contained, its content is set to 0.50 mass% or less, preferably 0.40 mass% or less, more preferably 0.30 mass% or less, and even more preferably 0.20 mass% or less.

[0030] (c) At least one selected from the group consisting of V: ​​0.10% by mass or less (excluding 0% by mass) and Nb: 0.10% by mass or less (excluding 0% by mass)) V is an element effective in improving strength by precipitating carbides. To achieve this effect, the V content may be more than 0% by mass, and is preferably 0.01% by mass or more, more preferably 0.02% by mass or more, even more preferably 0.03% by mass or more, and even more preferably 0.04% by mass or more. However, excessive V content increases the precipitation of carbonitrides, reducing the ductility and workability of the base material. Therefore, when V is contained, its content is set to 0.10% by mass or less, preferably 0.09% by mass or less, more preferably 0.08% by mass or less, and even more preferably 0.07% by mass or less. Nb is an element effective in improving strength by precipitating carbides. To achieve this 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, even more preferably 0.03 mass% or more, and even more preferably 0.04 mass% or more. However, excessive Nb content increases the precipitation of carbonitrides, reducing the ductility and workability of the base metal. Therefore, when Nb is contained, its content is set to 0.10 mass% or less, preferably 0.09 mass% or less, more preferably 0.08 mass% or less, and even more preferably 0.07 mass% or less.

[0031] Furthermore, as other elements, for example, 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 (these may be unavoidable impurities) may be contained in a total amount of 0.01 mass % or less (including 0 mass %) as necessary in order to improve the corrosion resistance and the delayed fracture resistance of the base metal.

[0032] <2. Metallographic Structure> In the steel sheet according to the embodiment of the present invention, the metallographic structure includes a tempered martensite structure of 95 area % or more. Preferably, the tempered martensite structure includes 97 area % or more. The upper limit of the area ratio of the tempered martensite structure is not particularly limited, and may be 100 area %. In the steel sheet according to the embodiment of the present invention, the metallographic structure may include, in addition to the tempered martensite structure, one or more structures that are inevitably included in the manufacturing process, such as a ferrite structure, a bainite structure, a retained austenite (γ) structure, and as-quenched martensite. The area ratio of the tempered martensite structure can be determined by the method described in the Examples below.

[0033] <3. Carbides> The present inventors have found that carbides with a high aspect ratio, which may be present mainly at grain boundaries in martensitic steel sheets, embrittle the grain boundaries and reduce ductility. They have found that by adjusting the composition to a predetermined level, the number density of carbides with an aspect ratio of 2.0 or more is reduced to 0.60 particles / μm ２ Furthermore, when considering application to automobile parts and the like, it is preferable that the yield ratio is not too high, since it is less likely to break. ２ It has been found that the yield ratio can be sufficiently reduced by setting the aspect ratio to 0.50 or less. Here, the "aspect ratio" is the value obtained by dividing the length in the longitudinal direction by the minor axis (the maximum length in the direction perpendicular to the longitudinal direction) when the carbide is viewed in plan view using an SEM or the like (longitudinal length / minor axis). The number density of carbides with an aspect ratio of 2.0 or more is preferably 0.50 pieces / μm ２ Less than or equal to 0.40 particles / μm, more preferably 0.40 particles / μm ２ More preferably, 0.30 particles / μm or less ２or less. The upper limit of the aspect ratio is not particularly limited, but considering the range in which it is likely to exist in the manufacturing method described later, it may be, for example, 5.5 or less. The number density of carbides having an aspect ratio of 2.0 or more can be determined by the method described in the Examples below. When determining the number density of the carbides, carbides having an equivalent circle diameter of 0.0245 μm or more can be counted. If the equivalent circle diameter of the carbides is less than 0.0245 μm, the carbides will not have grown sufficiently, and it is thought that carbides with a high aspect ratio that would affect ductility are unlikely to exist.

[0034] The steel sheet according to the embodiment of the present invention has a number density of carbides having an aspect ratio of 2.0 to 2.5 of 0.50 particles / μm ２ It is believed that carbides with an aspect ratio of 2.0 to 2.5 are likely to precipitate in large quantities during the quenching process, and by suppressing the precipitation of these carbides, ductility can be further improved, and the yield ratio can also be more easily controlled to a predetermined value or less. The number density of carbides with an aspect ratio of 2.0 to 2.5 is more preferably 0.45 particles / μm ２ More preferably, 0.40 particles / μm or less ２ More preferably, 0.35 particles / μm or less ２ The number density of carbides having an aspect ratio of 2.0 to 2.5 can be determined by the method described in the Examples below. When determining the number density of the carbides, carbides having a circle-equivalent diameter of 0.0245 μm or more can be counted.

[0035] The steel sheet according to the embodiment of the present invention has sufficient ductility, specifically an elongation of 6.7% or more. Furthermore, the steel sheet according to the embodiment of the present invention has a sufficiently low yield ratio, specifically a yield ratio of 0.860 or less. The yield ratio is preferably 0.700 or more, which makes it easier to ensure the yield strength required for automotive parts. Furthermore, the steel sheet according to the embodiment of the present invention is a martensitic steel sheet and has high strength, specifically a tensile strength of 1270 MPa or more.

[0036] <4. Manufacturing Method> A manufacturing method of a steel sheet according to an embodiment of the present invention includes: (A) a step of heating to a first temperature range of 840°C to 950°C and holding the temperature therefor for 30 seconds or more; (B) a step of quenching, after the holding step, from a second temperature range of 450°C to the first temperature range to less than 100°C, wherein the average cooling rate from 420°C to 250°C is 300°C / second or more; and (C) a step of tempering, after the quenching step, in a third temperature range of 100 to 300°C for 30 seconds or more. The above method allows the manufacturing of a steel sheet according to an embodiment of the present invention. Each step will be described in detail below.

[0037] (A) Step of heating to a first temperature range and holding for 30 seconds or more First, a steel sheet is heated to 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-mentioned composition. The method for preparing a steel sheet satisfying the above-mentioned composition is not particularly limited. For example, the steel sheet may be melted and continuously cast according to a conventional method to obtain a steel billet such as a slab satisfying the above-mentioned composition, and then heated to about 1100°C to 1250°C, hot-rolled, coiled, pickled, and cold-rolled to prepare the steel sheet.

[0038] By setting the first temperature range to 840°C or higher, it becomes possible to form an austenite single-phase structure (to later obtain a tempered martensite structure of 95% or more by area). Furthermore, by setting the first temperature range to 950°C or lower, it is possible to reduce the equipment load and reduce manufacturing costs. The heating and holding time in the first temperature range needs to be 30 seconds or longer to complete the austenite transformation. There is no particular upper limit to the heating and holding time, but it is preferably 400 seconds or shorter from the viewpoint of productivity.

[0039] (B) Step of Quenching from the Second Temperature Range After step (A), quenching is performed from the second temperature range (450°C to the first temperature range) to less than 100°C (e.g., room temperature). If the second temperature range is less than 450°C, a large amount of bainite and / or autotempered martensite may be formed, which may result in a decrease in strength. The upper limit of the second temperature range is the first temperature range, i.e., quenching may be performed immediately after step (A). On the other hand, the second temperature range may be less than the first temperature range (450°C or higher). In this case, the cooling rate from the first temperature range to the second temperature range is not particularly limited, but a cooling rate of 5°C / second or higher is preferable, for example, from the viewpoint of preventing ferrite formation. The quenching method is not particularly limited, and known methods such as water cooling and oil cooling may be used.

[0040] Furthermore, in step (B), the average cooling rate from 420°C to 250°C is set to 300°C / second or higher. The temperature range from 420°C to 250°C corresponds to the temperature range in which martensitic transformation begins and ends, and when the martensitic structure transformed in this temperature range is self-tempered, carbides with high aspect ratios (e.g., aspect ratios of 2.0 or higher, or 2.0 to 2.5) tend to precipitate. Therefore, by increasing the cooling rate in this temperature range, the formation of carbides with high aspect ratios can be suppressed. The average cooling rate from 420°C to 250°C is more preferably 400°C / second or higher, even more preferably 500°C / second or higher, and even more preferably 600°C / second or higher.

[0041] (C) Tempering Step After step (B), tempering is performed in a third temperature range of 100 to 300°C for 30 seconds or more. If the third temperature range exceeds 300°C, it becomes difficult to ensure strength. The third temperature range is preferably 280°C or less, more preferably 260°C or less. On the other hand, if the third temperature range is less than 100°C, it becomes difficult to improve toughness.

[0042] The method for manufacturing a steel sheet according to an embodiment of the present invention may include other steps as long as the object of the present disclosure is achieved.

[0043] The following examples are provided to more specifically describe the embodiments of the present invention. The embodiments of the present invention are not limited to the following examples, and may be modified as appropriate within the scope of the above-described and below-described aims, and all such modifications are within the technical scope of the embodiments of the present invention.

[0044] Steel billets satisfying the composition of steel types A to H shown in Table 1 (the balance being iron and inevitable impurities) were obtained by conventional melting and continuous casting. The billets were then heated to 1200°C, hot-rolled (finishing temperature: 870°C, finishing thickness: 2.6 mm), coiled at 620°C, pickled, and cold-rolled to obtain steel plates with a thickness of 1.4 mm. The resulting steel plates were cut to dimensions of 150 mm x 160 mm, subjected to the heat treatment described in step (A) in Table 2, and then quenched under the conditions described in step (B) in Table 2. For 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 step (A) to the quenching start temperature in step (B) was 5°C / sec or faster. The quenching conditions were as follows: the water volume was 70 L to ensure the cooling rate, and the water temperature was room temperature for Tests Nos. 1, 2, 4, 5, 7, 8, 10, 11, 13, 14, 16, 17, 19, 20, 22, and 23; and the water temperature was heated to 60°C by a heater for Tests Nos. 3, 6, 9, 12, 15, 18, 21, and 24 to intentionally slow the cooling rate. Furthermore, for Tests Nos. 1, 4, 7, 10, 13, 16, 19, and 22, cooling water was circulated at 1.0 m / min and 0.2 MPa using a water pump to intentionally increase the cooling rate. Figure 2 is a top view schematic diagram of a 150 mm x 160 mm steel plate cut in the thickness direction, showing the positions of thermocouples during quenching. As shown in Figure 2, thermocouples 2 (shown by black circles) were attached to five locations on steel plate 1, and the temperature was measured at a sampling period of 0.01 seconds. The cooling rate from 420 to 250°C was measured at the five locations, and the average value was taken as the average cooling rate from 420 to 250°C. After quenching, tempering was performed under the conditions described in step (C) of Table 2. In Table 1, "-" in the "Si," "Cu," and "Cr" columns indicates that the content was at the level of an unavoidable impurity that was not intentionally added. Furthermore, although P, S, and N were not intentionally added but were present at the level of an unavoidable impurity, their contents are specified because their content must be strictly controlled as described above.

[0045]

[0046]

[0047] The steel plates of Test Nos. 1 to 24 were evaluated as follows.

[0048] <Area ratio of tempered martensite structure> A cross section of a 1.4 mm (thickness direction) × 20 mm (rolling direction) × 20 mm (width direction) test piece parallel to the rolling direction and thickness direction (i.e., perpendicular to the width direction) was polished and subjected to nital etching, after which an SEM image was obtained at a magnification of 1000 times for a ¼t portion (t is the thickness). ２ The area ratio of tempered martensite was calculated by dividing the number of intersections where the tempered martensite structure was present by the total number of intersections. The results are shown in Table 3 below.

[0049] <Tensile Properties> For the tensile properties, JIS No. 5 tensile test specimens were taken from the steel plates of Test Nos. 1 to 24 so that the plate width direction (i.e., the direction perpendicular to the rolling direction and plate thickness direction) was the longitudinal direction, and tensile strength TS (MPa), yield stress YS (MPa), and elongation EL (%) as ductility were determined according to the method specified in JIS Z 2241. The yield ratio YR was taken as YS / TS. For each physical property value, n = 2 tests were performed, and the average value was used. The results are shown in Table 3 below.

[0050] <Number Density of Carbides with Each Aspect Ratio> Figure 1 is a schematic top view of a test piece 1 for measuring the number density of carbides with each aspect ratio, as viewed from the sheet thickness direction. As shown in Figure 1, a test piece 1 was cut out from the remaining material after the tensile test so that a cross section S2 2 mm away from the fracture surface S1 could be observed. After polishing and picral etching, the test piece 1 was observed from the observation direction OD at the position t / 2 (t is the sheet thickness) and near the center of the test piece in the rolling direction LD (indicated by the broken line CL), and an FE-SEM image was obtained at a magnification of 5000x. An arbitrary field of view (field area: 416 μm) of the obtained Fe-SEM image was taken. ２), binarization was performed, and the longitudinal length and minor axis (maximum length perpendicular to the longitudinal direction) of the carbides displayed in white were measured to determine the aspect ratio (longitudinal length / minor axis) of each carbide. The number of carbides with aspect ratios of 2.0 or greater and the number of carbides with aspect ratios of 2.0 to 2.5 were then determined and divided by the field area to calculate the number density. When determining the number density, carbides with a circle-equivalent diameter of 0.0245 μm or greater were counted. The maximum circle-equivalent diameter of the counted carbides was 0.227 μm. Furthermore, for the number density with an aspect ratio of 2.0 or greater, all of the counted carbides had an aspect ratio of 5.5 or less, and no carbides exceeding 5.5 were confirmed. The results are shown in Table 3.

[0051]

[0052] The following can be seen from Table 3. Test Nos. 1 to 3 and 7 to 24 all satisfied the requirements defined in the embodiment of the present invention, and had sufficient ductility (elongation of 6.7% or more) and a sufficiently low yield ratio (0.860 or less). On the other hand, Test Nos. 4 to 6 did not satisfy the requirements defined in the embodiment of the present invention, and had insufficient ductility and a high yield ratio. Specifically, Test Nos. 4 to 6 all had an insufficient Mn content, and the number density of carbides with an aspect ratio of 2 or more was 0.60 / μm ２ Therefore, the elongation was less than 6.7% and the yield ratio was also more than 0.860.

[0053] This application claims priority from Japanese Patent Application No. 2023-108449, filed June 30, 2023, and Japanese Patent Application No. 2024-062940, filed April 9, 2024. Japanese Patent Application Nos. 2023-108449 and 2024-062940 are incorporated herein by reference.

[0054] 1 Steel plate 2 Thermocouple

Claims

1. The composition of the alloy is as follows: 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 (including 0 mass%), P: 0.020 mass% or less (including 0 mass%), S: 0.010 mass% or less (including 0 mass%), and N: 0.010 mass% or less (including 0 mass%), with the balance being Fe and unavoidable impurities; the metal structure is a tempered martensite structure that accounts for 95 area% or more; and the number density of carbides having an aspect ratio of 2.0 or more is 0.60 pieces / μm ２ Below is a steel plate.

2. The steel sheet according to claim 1, further comprising one or more of the following (a) to (c): (a) at least one selected from the group consisting of Ti: 0.010 to 0.070 mass%, B: 0.0100 mass% or less (excluding 0 mass%), and Cr: 2.00 mass% or less (excluding 0 mass%), (b) at least one selected from the group consisting of Cu: 0.50 mass% or less (excluding 0 mass%) and Ni: 0.50 mass% or less (excluding 0 mass%), and (c) at least one selected from the group consisting of V: ​​0.10 mass% or less (excluding 0 mass%) and Nb: 0.10 mass% or less (excluding 0 mass%).

3. The density of carbides with an aspect ratio of 2.0 to 2.5 is 0.50 pieces / μm ２ The steel sheet according to claim 1 or 2, wherein:

4. A method for producing a steel plate according to claim 1 or 2, comprising the steps of: heating to a first temperature range of 840°C to 950°C and holding for 30 seconds or more; and, after said holding step, quenching from a second temperature range of 450°C to said first temperature range to less than 100°C, wherein the average cooling rate of 420°C to 250°C is 300°C / sec or more; and, after said quenching step, tempering in a third temperature range of 100 to 300°C for 30 seconds or more.

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