Steel plates, components and their manufacturing methods

Abstract

This invention provides a high-strength steel plate with excellent formability and resistance to delayed fracture, and a method for manufacturing the same. A steel plate having, by mass percent, a composition comprising: C: 0.20% or more and 0.40% or less; Si: greater than 1.0% and 3.0% or less; Mn: 1.5% or more and 3.5% or less; P: 0.002% or more and 0.010% or less; S: 0.0002% or more and 0.0020% or less; sol.Al: 0.40% or less (excluding 0%); N: 0.0100% or less; and the balance being Fe and unavoidable impurities; and having, by area percent, a steel structure comprising: tempered martensite: 45% or more and 83% or less; bainite: 15% or more and 53% or less; retained austenite: 2% or more; the average grain size of carbides in the tempered martensite is 0.40 μm or less; the average C content in the retained austenite is 0.5% or more by mass; and a tensile strength of 1470 MPa or more.

Description

Technical Field

[0001] This invention relates to high-strength steel sheets suitable for use in cold stamping processes in automobiles, home appliances, etc., components made from such high-strength steel sheets, and methods for manufacturing them. Background Technology

[0002] In recent years, due to the increasing demand for lightweight automotive bodies, the application of high-strength steel sheets with a tensile strength (TS) of 1320–1470 MPa (hereinafter sometimes simply referred to as TS) in body frame components such as center pillars and r / f (reinforcement members), bumpers, and anti-collision beams is being promoted. Furthermore, from the perspective of further lightweighting, research is also underway to achieve strengths of 1.8 GPa or higher. Previously, there has been active research on high-strength steel based on hot stamping, but recently, from the perspective of cost and productivity, the application of high-strength steel in cold stamping has been re-examined.

[0003] However, when cold stamping high-strength steel sheets of TS: 1470MPa and above, stamping fracture is easily caused by reduced ductility, limiting its application to parts with relatively simple shapes. Therefore, the formability requirements for high-strength steel sheets are now much better than before. Furthermore, when cold stamping high-strength steel sheets of TS: 1470MPa and above to manufacture parts, delayed fracture becomes significant due to increased residual stress within the part and deterioration of the material's inherent resistance to delayed fracture. Delayed fracture is a phenomenon where, under high stress, when a part is placed in a hydrogen-infiltrating environment, hydrogen penetrates the steel sheet, reducing interatomic bonding or causing localized deformation, resulting in microcracks that eventually lead to fracture.

[0004] To date, TRIP steel, in which retained austenite is dispersed in the microstructure of steel plates, has been developed as a technique to improve the ductility of steel plates.

[0005] In addition, Patent Document 1 discloses a high-strength steel plate with excellent workability, characterized in that the alloy composition is specified such that the area ratio of martensite relative to the overall steel plate structure is set to be 10% or more and 90% or less, the amount of retained austenite is set to be 5% or more and 50% or less, the area ratio of bainitic ferrite in the upper bainite relative to the overall steel plate structure is set to be 5% or more, the total area ratio of the above-mentioned martensite relative to the overall steel plate structure, the above-mentioned retained austenite amount, and the above-mentioned area ratio of bainitic ferrite in the upper bainite relative to the overall steel plate structure satisfies 60% or more, and the area ratio of polygonal ferrite relative to the overall steel plate structure satisfies 10% or less (including 0%).

[0006] Patent document 2 discloses a high-strength steel sheet with excellent ductility by dispersing fine residual austenite in the steel structure.

[0007] Patent document 3 discloses a technique for improving the hydrogen embrittlement resistance of TRIP steel plates containing retained austenite by suppressing fresh martensite to less than 5% by appropriately controlling quenching conditions and post-quenching holding conditions.

[0008] Patent document 4 discloses a high-strength steel sheet with excellent formability.

[0009] Patent document 5 discloses a high-strength steel sheet with bainite and tempered martensite as the main phases in the steel structure, which results in a steel sheet with excellent shape.

[0010] Patent document 6 discloses a high-strength steel sheet that combines high ductility with excellent elongation flange formability.

[0011] Prior art literature

[0012] Patent documents

[0013] Patent Document 1: Japanese Patent Application Publication No. 2010-65272

[0014] Patent Document 2: Japanese Patent No. 6540162

[0015] Patent Document 3: Japanese Patent No. 5412182

[0016] Patent Document 4: Japanese Patent No. 6288394

[0017] Patent Document 5: Japanese Patent No. 6291289

[0018] Patent Document 6: International Publication No. 2020 / 080402 Summary of the Invention

[0019] The problem that the invention aims to solve

[0020] However, in the technology of Patent Document 1, tempering was not performed after quenching, resulting in the presence of fresh martensite in the steel plate microstructure. This fresh martensite becomes the initiation point for delayed fracture, potentially deteriorating the resistance to delayed fracture. Furthermore, due to the low cooling rate and the presence of self-tempered martensite, a strength of TS: 1470 MPa may not be obtained. Additionally, the coarsening of carbides in the self-tempered martensite may further deteriorate the resistance to delayed fracture.

[0021] In the technologies described in Patent Documents 2-4, the tempering temperature of the martensite is high, which may not yield a strength of TS: 1470 MPa. Furthermore, the coarsening of carbides in the tempered martensite may deteriorate its resistance to delayed fracture.

[0022] In the technology of Patent Document 5, the low carbon content may prevent the achievement of a strength of TS: 1470 MPa. Furthermore, the low average carbon content in the retained austenite may also degrade ductility.

[0023] In the technology of Patent Document 6, the cooling rate after obtaining bainite is about 30°C / s. During the cooling, the martensite undergoes self-tempering, and further reheating coarsens the carbides, making it impossible to obtain fine carbides. The delayed fracture resistance may be deteriorated.

[0024] The present invention was made to solve such problems, and its purpose is to provide steel plates, components and methods thereof having tensile strength of 1470 MPa or more (TS≥1470 MPa), excellent formability and excellent resistance to delayed fracture.

[0025] It should be noted that the tensile strength is the tensile strength TS obtained by cutting JIS No. 5 tensile test pieces with the rolling direction as the length direction and by conducting a tensile test according to JIS Z2241.

[0026] In addition, excellent formability means that the elongation E1 obtained by cutting JIS No. 5 tensile test piece with the rolling direction as the length direction and by tensile test according to JIS Z2241 is 11% or more.

[0027] In addition, excellent resistance to delayed fracture means that no fracture occurred as evaluated below.

[0028] (1) Cut a strip test piece with a rolling right angle of 100 mm and a rolling direction of 30 mm from 1 / 4 of the width of the steel plate coil.

[0029] (2) The cutting process of the 100mm long end face is set as shearing process. While maintaining the shearing process (without performing deburring machining), the bending process is performed in such a way that the burr is bent on the outer periphery. The test piece shape is maintained when it is bent into shape, and the test piece is fixed with bolts.

[0030] The clearance rate for shearing is set to 15%, and the rake angle is set to 0 degrees.

[0031] The bending process is carried out in such a way that when the top bending radius R and the plate thickness t are given, the bending radius R / t = 4 (for example, when the plate thickness t: 2.0 mm, the bending is performed using a punch with a top radius of 8.0 mm) and the bottom inner angle is 90 degrees (V bending).

[0032] The punch uses a U-shaped punch with the above-mentioned radius at the top (the top R part is semi-circular, and the thickness of the punch body is 2R), and the corner R of the die is 30mm.

[0033] The depth of the punch pressed into the steel plate is adjusted so that the top bends at a 90-degree angle (V-shape) to form the shape.

[0034] During bolt tightening, the test piece is clamped and locked in such a way that the distance between the flange ends of the straight piece during bending is the same as the distance during bending (in order to counteract the opening of the straight piece based on springback). The bolt is tightened in this state.

[0035] Bolts are used to secure the test piece by passing through elliptical holes (10 mm short axis, 15 mm long axis) pre-set 10 mm inside the short edge of the long test piece.

[0036] (3) The obtained bolt-tightened test pieces are immersed in hydrochloric acid (hydrogen chloride aqueous solution) with a pH of more than 1L per piece at 3, and the pH is kept constant under the condition that the aqueous solution temperature is 25℃ to carry out the test.

[0037] The presence or absence of microcracks (the initial state of delayed fracture) that can be visually confirmed at any time by visual inspection or by camera is determined as the delayed fracture time from the start of impregnation to the beginning of the formation of microcracks.

[0038] (4) Even after 10 days of soaking, the soaking process will continue. (-0.0055×(TS-1760)+0.3) If no fracture occurs within 10 to the power of (-0.0055×(TS-1760)+0.3) hours, it is considered as no fracture.

[0039] Methods for solving problems

[0040] In order to solve the above-mentioned problems, the inventors have conducted repeated in-depth research and obtained the following insights.

[0041] i) It was discovered that high strength can be achieved by using tempered martensite, and by increasing the C content to more than 0.20% by mass and then using bainitic phase transformation, the retained austenite can be ensured. Thus, high-strength steel plates with excellent formability and tensile strength of more than 1470 MPa (TS≥1470 MPa) can be obtained.

[0042] ii) It was determined that by suppressing the coarsening of carbides in tempered martensite, the resistance to delayed fracture in high-strength steel plates with TS≥1470MPa can be improved.

[0043] iii) It was determined that in order to properly control the particle size of carbides, it is necessary to optimize the content of C, Si, and Mn, the cooling rate after bainitic phase transformation, and the tempering temperature.

[0044] The present invention is based on the above insights, and specifically, the following invention is provided.

[0045] [1] A steel plate having, by mass percent, a composition comprising: C: 0.20% or more and 0.40% or less, Si: greater than 1.0% and 3.0% or less, Mn: 1.5% or more and 3.5% or less, P: 0.002% or more and 0.010% or less, S: 0.0002% or more and 0.0020% or less, sol.Al: 0.40% or less (excluding 0%), N: 0.0100% or less, and the balance being Fe and unavoidable impurities.

[0046] Furthermore, it possesses a steel microstructure containing, by area ratio, 45% to 83% tempered martensite, 15% to 53% bainite, and 2% or more retained austenite.

[0047] The average particle size of the carbides in the above-mentioned tempered martensite is less than 0.40 μm.

[0048] The average carbon content in the aforementioned retained austenite is 0.5% by mass or more.

[0049] The tensile strength is above 1470 MPa.

[0050] [2] According to the steel plate described in [1] above, the hardness difference between the microhardness of the bainite and the tempered martensite is 1.5 GPa or more.

[0051] [3] The steel plate according to [1] or [2] above, wherein, as a component of the above composition, it further contains, by mass %, one or more of the following: Nb: less than 0.1%, Ti: less than 0.10%, B: less than 0.0050%, Cu: less than 1% and Ni: less than 1%.

[0052] [4] The steel plate according to any one of [1] to [3] above, wherein, as a component of the above composition, it further contains, by mass %, one or more of the following: Cr: less than 1.0%, Mo: less than 0.3%, V: less than 0.45%, Zr: less than 0.2% and W: less than 0.2%.

[0053] [5] The steel plate according to any one of [1] to [4] above, wherein, as a component of the above composition, it further contains, by mass %, one or both selected from Sb: less than 0.1% and Sn: less than 0.1%.

[0054] [6] The steel plate according to any one of [1] to [5] above, wherein, as a component of the above composition, it further contains, by mass %, one or more of the following: Ca: less than 0.0050%, Mg: less than 0.01%, and REM: less than 0.01%.

[0055] [7] The steel plate according to any one of [1] to [6] above, wherein the steel plate has a coating on its surface.

[0056] [8] A component obtained by performing at least one of forming and joining processes on a steel plate as described in any one of [1] to [7].

[0057] [9] A method for manufacturing a steel plate, comprising manufacturing a steel plate according to any one of [1] to [6] above, comprising:

[0058] The hot rolling process involves hot rolling a steel billet to obtain a hot-rolled steel plate.

[0059] The cold rolling process involves cold rolling the hot-rolled steel sheet after the hot rolling process to obtain a cold-rolled steel sheet.

[0060] In the continuous annealing process, after the aforementioned cold rolling process, the cold-rolled steel sheet is annealed at an annealing temperature above Ac3 and for a soaking time of 15 seconds or more, then cooled at a first average cooling rate of 5°C / second or more to a holding temperature above Ms and below 600°C, held at the aforementioned holding temperature for a holding time of 1 second or more and below 1000 seconds, and then cooled at a second average cooling rate of 100°C / second or more to a cooling stop temperature below 150°C; and

[0061] After the aging process, the temperature is maintained at a range of 150°C to 250°C for 30 seconds to 1500 seconds following the continuous annealing process.

[0062]

[10] The steel plate manufacturing method according to [9] above includes the following plating process: plating treatment is performed on the surface of the steel plate before or after the above-mentioned aging treatment process.

[0063]

[11] A method for manufacturing a component, comprising the following steps: performing at least one of forming and joining processes on a steel plate manufactured by the steel plate manufacturing method described in [9] or

[10] above.

[0064] Invention Effects

[0065] According to the present invention, a high-strength steel sheet with excellent formability and resistance to delayed fracture can be obtained. This improved property enables the application of high-strength steel sheets in difficult-to-form parts for cold stamping applications, contributing to increased part strength and weight reduction. Detailed Implementation

[0066] The embodiments of the present invention will be described below. It should be noted that the present invention is not limited to the following embodiments.

[0067] The steel plate of the present invention comprises, by mass percent, C: 0.20% or more and 0.40% or less, Si: greater than 1.0% and 3.0% or less, Mn: 1.5% or more and 3.5% or less, P: 0.002% or more and 0.010% or less, S: 0.0002% or more and 0.0020% or less, sol.Al: 0.40% or less (excluding 0%), N: 0.0100% or less, and the balance being Fe and unavoidable impurities. It also comprises, by area percent, tempered martensite: 45% or more and 83% or less, bainite: 15% or more and 53% or less, and retained austenite: 2% or more. The average grain size of the carbides in the tempered martensite is 0.40 μm or less, the average C content in the retained austenite is 0.5% by mass or more, and the tensile strength is 1470 MPa or more.

[0068] Composition

[0069] First, the content of each component will be explained. Unless otherwise specified, the "%" indicating the content of a component refers to "mass %".

[0070] C: Above 0.20% and below 0.40%

[0071] Carbon (C) increases the strength of tempered martensite or bainite, and is included from the viewpoint of ensuring a fracture strength (TS) ≥ 1470 MPa. Furthermore, C is included from the viewpoint of forming fine carbides within tempered martensite or bainite that serve as hydrogen trapping sites. When the C content is less than 0.20%, it is impossible to maintain excellent resistance to delayed fracture and obtain the required strength. From the viewpoint of maintaining excellent resistance to delayed fracture and obtaining a TS ≥ 1470 MPa, the C content is set to 0.20% or more. On the other hand, when the C content exceeds 0.40%, the strength becomes too high, making it difficult to obtain sufficient resistance to delayed fracture. Therefore, the C content is set to 0.20% or more and 0.40% or less. The C content is preferably 0.21% or more, more preferably 0.22% or more, and even more preferably 0.24% or more. Furthermore, the C content is preferably 0.35% or less, more preferably 0.33% or less, and even more preferably 0.30% or less.

[0072] Si: greater than 1.0% and less than 3.0%

[0073] Si is included as a strengthening element based on solid solution strengthening, and it is also included to suppress the formation of film carbides during tempering at temperatures above 200°C, thus improving resistance to delayed fracture. From the viewpoint of achieving the above effects, a Si content greater than 1.0% is considered. Furthermore, when the Si content is less than 1.0%, the formability is poor.

[0074] On the other hand, when the Si content exceeds 3.0%, its segregation increases, leading to a deterioration in resistance to delayed fracture. Furthermore, it results in a significant increase in rolling load during hot and cold rolling, and a decrease in toughness. Therefore, the Si content is set to be greater than 1.0% and less than 3.0%. The Si content is preferably 1.1% or more, more preferably 1.2% or more. Additionally, the Si content is preferably 2.8% or less, more preferably 2.6% or less.

[0075] Mn: 1.5% or more and 3.5% or less

[0076] Mn improves the hardenability of steel and is contained to maintain the total area ratio of tempered martensite and bainite within a specified range. Additionally, Mn is contained to fix sulfur in the steel as MnS and reduce hot brittleness. Mn is an element that particularly promotes the formation and coarsening of MnS in the central part of the plate thickness, and it precipitates in combination with inclusions such as Al2O3, (Nb,Ti)(C,N), TiN, and TiS. However, this can be avoided by controlling the segregation state of Mn. However, considering the stability of weldability, the upper limit of Mn content is set to 3.5%. Furthermore, from the viewpoint of obtaining sufficient resistance to delayed fracture, the upper limit of Mn content is also set to 3.5%. Preferably, it is 3.3% or less, more preferably 3.1% or less. On the other hand, when the Mn content is less than 1.5%, ferrite is formed during cooling before the bainitic phase transformation, resulting in a significant decrease in strength. Therefore, the Mn content is set to 1.5% or more and 3.5% or less. The Mn content is preferably 1.6% or more, more preferably 1.7% or more, and even more preferably 1.9% or more.

[0077] P: Above 0.002% and below 0.010%

[0078] Polymer (P) is an element that strengthens steel, but high P content significantly degrades its resistance to delayed fracture and its weldability. Therefore, the P content is set to 0.010% or less. Furthermore, considering the above points, the P content is preferably set to 0.006% or less. On the other hand, the currently industrially feasible lower limit for P content is 0.002%. Therefore, the P content is set to 0.002% or more and 0.010% or less.

[0079] S: ≥0.0002% and ≤0.0020%

[0080] Sulfur (S) significantly influences delayed fracture resistance through the formation of MnS, TiS, and Ti(C,S), thus requiring precise control. To mitigate the drawbacks caused by inclusion clusters, the S content needs to be set at least 0.0020% or less. From the viewpoint of improving delayed fracture resistance, the S content is preferably set at 0.0010% or less. On the other hand, the lower limit of S content currently feasible in industry is 0.0002%. Therefore, the S content is set at 0.0002% or more and 0.0020% or less.

[0081] sol.Al: Below 0.40% (excluding 0%)

[0082] Al is included to ensure sufficient deoxidation and reduce inclusions in the steel. For stable deoxidation, the sol.Al content is preferably set to 0.005% or more, more preferably 0.01% or more. On the other hand, when the sol.Al content exceeds 0.40%, the cementite formed during coiling is difficult to dissolve during annealing, resulting in deterioration of the delayed fracture resistance. Therefore, the sol.Al content is set to 0.40% or less, preferably 0.35% or less, more preferably 0.30% or less.

[0083] N: below 0.0100%

[0084] Nitrogen (N) is an element that forms inclusions in steel such as TiN, (Nb,Ti)(C,N), and AlN, as well as carbonitride systems, thereby deteriorating the resistance to delayed fracture. These inclusions hinder the formation of the steel microstructure specified in this invention, adversely affecting the resistance to delayed fracture. To reduce such adverse effects, the N content needs to be set to 0.0100% or less. Furthermore, a N content of 0.0055% or less is preferred. On the other hand, the lower limit of the N content currently feasible in industry is 0.0006%.

[0085] The balance other than those described above has a composition containing Fe (iron) and unavoidable impurities. Here, the steel sheet of the present invention preferably has a composition containing the above-described basic components, with the balance consisting of Fe and unavoidable impurities. It should be noted that the following optional elements may be included. When the following optional elements are included in amounts less than the preferred lower limit, the optional elements may be included as unavoidable impurities.

[0086] Nb: below 0.1%

[0087] Nb contributes to increased strength by refining the internal structure of martensite and bainite, and improves resistance to delayed fracture as mentioned above. From this perspective, a content of 0.001% or more Nb is preferred, and 0.005% or more is more preferable. On the other hand, when the Nb content is in excess of 0.1%, a large number of Nb-based inclusions are generated in a dotted pattern along the rolling direction, which is considered to have an adverse effect on resistance to delayed fracture. To mitigate this adverse effect, when Nb is present, the Nb content is set to 0.1% or less. A content of 0.08% or less is preferred, and 0.06% or less is more preferable.

[0088] Ti: below 0.10%

[0089] Ti contributes to increased strength by refining the internal structure of martensite and bainite. Furthermore, it improves resistance to delayed fracture by forming fine Ti-based carbides and carbonitrides that act as hydrogen trapping sites. Additionally, it improves castability. From this perspective, the Ti content is preferably set to 0.002% or more, more preferably 0.005% or more. On the other hand, when the Ti content becomes excessive, a large number of Ti-based inclusion particles are generated in a dotted pattern along the rolling direction, which is considered to have a negative impact on resistance to delayed fracture. To mitigate this negative impact, when Ti is present, the Ti content is set to 0.10% or less. Preferably, the Ti content is 0.07% or less, more preferably 0.05% or less.

[0090] B: Below 0.0050%

[0091] Boron (B) is an element that improves the hardenability of steel, possessing the advantage of generating martensite and bainite with a specified area ratio even with low Mn content. To achieve this effect, the B content is preferably set to 0.0001% or more, more preferably 0.0005% or more. From the viewpoint of fixing N, it is preferable to contain B in combination with 0.002% or more Ti. On the other hand, when the B content exceeds 0.0050%, not only is its effect saturated, but the solid solution rate of cementite during annealing is also delayed, leaving undissolved cementite, thereby deteriorating the resistance to delayed fracture. Therefore, when B is present, the B content is set to 0.0050% or less, preferably less than 0.0035%.

[0092] Cu: less than 1%

[0093] Cu improves corrosion resistance in the automotive operating environment. Furthermore, the presence of Cu allows corrosion products to coat the steel surface, inhibiting hydrogen penetration into the steel. Additionally, Cu is an element often incorporated when using waste materials as raw materials; allowing Cu inclusion allows for the utilization of recycled materials as raw materials, reducing manufacturing costs. From the above perspectives, a Cu content of 0.01% or more is preferred, and further from the viewpoint of improving resistance to delayed fracture, a Cu content of 0.05% or more is preferred. However, when the Cu content exceeds 1%, it becomes a cause of surface defects. Therefore, when Cu is present, the Cu content is set to 1% or less. A Cu content of 0.40% or less is preferred, and 0.30% or less is more preferred.

[0094] Ni: less than 1%

[0095] Ni also improves corrosion resistance. Furthermore, Ni reduces surface defects that are easily generated when Cu is present. Therefore, from the above perspective, it is preferable to contain 0.01% or more Ni, more preferably 0.02% or more. However, when the Ni content exceeds 1%, the formation of oxide scale in the furnace becomes uneven, leading to surface defects and a significant increase in cost. Therefore, when Ni is present, the Ni content is set to 1% or less. Preferably, the Ni content is 0.20% or less, more preferably 0.10% or less.

[0096] Cr: less than 1.0%

[0097] Cr can be added to improve the hardenability of steel. To achieve this effect, a Cr content of 0.01% or more is preferred, and 0.02% or more is more preferable. However, when the Cr content exceeds 1.0%, the solid solution rate of cementite during annealing is delayed, leaving undissolved cementite residue, thereby deteriorating the resistance to delayed fracture. Furthermore, resistance to pitting corrosion also deteriorates. In addition, chemical conversion treatability also deteriorates. Therefore, when Cr is present, the Cr content is set to 1.0% or less. Resistance to delayed fracture, pitting corrosion, and chemical conversion treatability all tend to deteriorate when the Cr content exceeds 0.8%. Therefore, from the viewpoint of preventing these phenomena, the Cr content is preferably 0.8% or less. More preferably, the Cr content is 0.6% or less, and even more preferably 0.4% or less.

[0098] Mo: less than 0.3%

[0099] Mo can be added to improve the hardenability of steel, to generate fine Mo-containing carbides that serve as hydrogen trapping sites, and to improve resistance to delayed fracture by refining martensite. When large amounts of Nb and Ti are added, coarse precipitates are formed, which deteriorates resistance to delayed fracture; however, the solid solution limit of Mo is greater than that of Nb and Ti. When added in combination with Nb and Ti, fine precipitates formed by these compounds with Mo are formed, which has the effect of refining the microstructure. Therefore, by adding Mo in combination with small amounts of Nb and Ti, it is possible to improve resistance to delayed fracture without leaving coarse precipitates, refine the microstructure, and disperse fine carbides in large quantities. To achieve this effect, a Mo content of 0.01% or more is preferred, and 0.02% or more is more preferred. However, when the Mo content is 0.3% or more, the chemical conversion treatment properties deteriorate. Therefore, when Mo is present, the Mo content is set to be less than 0.3%. A Mo content of 0.2% or less is preferred, and 0.1% or less is more preferred.

[0100] V: below 0.45%

[0101] Vitamin V can be added to improve the hardenability of steel, to promote the formation of fine V-containing carbides that serve as hydrogen trapping sites, and to refine martensite and improve resistance to delayed fracture. To achieve these effects, the V content is preferably set to 0.003% or more, more preferably 0.005% or more. However, when the V content exceeds 0.45%, castability deteriorates significantly. Therefore, when V is present, the V content is set to 0.45% or less. Preferably, the V content is 0.2% or less, more preferably 0.1% or less.

[0102] Zr: below 0.2%

[0103] Zr contributes to increased strength and improved resistance to delayed fracture by refining the original γ grain size and reducing the size of the internal structural units of martensite and bainite, such as lath block size and bainite grain size. Furthermore, it increases strength and improves resistance to delayed fracture by forming fine Zr-based carbides and carbonitrides that serve as hydrogen trapping sites. It also improves castability. From this perspective, the Zr content is preferably set to 0.001% or more, more preferably 0.005% or more. However, when a large amount of Zr is added, the amount of coarse ZrN and ZrS precipitates that remain undissolved during the hot rolling process increases, deteriorating the resistance to delayed fracture. Therefore, when Zr is present, the Zr content is set to 0.2% or less. Preferably, the Zr content is 0.05% or less, more preferably 0.01% or less.

[0104] W: below 0.2%

[0105] W contributes to increased strength and improved resistance to delayed fracture by forming fine W-based carbides and carbonitrides that act as hydrogen trapping sites. From this perspective, a W content of 0.005% or more is preferred, and 0.01% or more is more preferable. However, when the W content exceeds 0.2%, the amount of coarse precipitates remaining undissolved during the hot-rolling process increases, leading to a deterioration in resistance to delayed fracture. Therefore, when W is present, the W content is set to 0.2% or less. A W content of 0.1% or less is preferred, and 0.05% or less is more preferable.

[0106] Sb: below 0.1%

[0107] Sb inhibits surface oxidation and nitriding, thereby suppressing the resulting reduction of carbon (C) and boron (B). Suppressing the reduction of C and B and thus inhibiting ferrite formation on the surface contributes to increased strength and improved resistance to delayed fracture. From this perspective, the Sb content is preferably set to 0.002% or more, more preferably 0.005% or more. However, when the Sb content exceeds 0.1%, castability deteriorates, and Sb segregates at the original γ grain boundaries, further deteriorating resistance to delayed fracture. Therefore, when Sb is present, the Sb content is set to 0.1% or less. Preferably, the Sb content is 0.06% or less, more preferably 0.04% or less.

[0108] Sn: less than 0.1%

[0109] Sn inhibits surface oxidation and nitriding, thereby suppressing the resulting reduction in the content of carbon (C) and boron (B) in the surface layer. Suppressing the reduction of C and B and thus inhibiting the formation of ferrite in the surface layer contributes to increased strength and improved resistance to delayed fracture. From this perspective, the Sn content is preferably 0.002% or more, more preferably 0.004% or more. However, when the Sn content exceeds 0.1%, castability deteriorates, and Sn segregates at the original γ grain boundaries, further deteriorating resistance to delayed fracture. Therefore, when Sn is present, the Sn content is set to 0.1% or less. Preferably, the Sn content is 0.04% or less, more preferably 0.02% or less.

[0110] Ca: below 0.0050%

[0111] Ca fixes S as CaS, improving resistance to delayed fracture. To achieve this effect, it is preferable to contain 0.0001% or more of Ca. More preferably, the Ca content is 0.0005% or more. However, when the Ca content exceeds 0.0050%, the surface quality and flexibility deteriorate. Therefore, when Ca is present, the Ca content is set to 0.0050% or less.

[0112] Mg: less than 0.01%

[0113] Mg fixes O to MgO, improving resistance to delayed fracture. To achieve this effect, it is preferable to contain 0.0001% or more of Mg. However, when Mg content exceeds 0.01%, surface quality and flexibility deteriorate. Therefore, when Mg is present, the Mg content is set to 0.01% or less. Preferably, the Mg content is 0.005% or less, and more preferably 0.001% or less.

[0114] REM: below 0.01%

[0115] REM (reactive iron) refines inclusions, reducing the number of fracture initiation points and thereby improving flexural properties and resistance to delayed fracture. To achieve this effect, a REM content of 0.0001% or more is preferred. However, a REM content exceeding 0.01% actually coarsens the inclusions, deteriorating flexural properties and resistance to delayed fracture. Therefore, when REM is present, the REM content is set to 0.01% or less. Preferably, the REM content is 0.004% or less, more preferably 0.002% or less.

[0116] steel structure

[0117] The steel plate of the present invention has the following steel structure.

[0118] (Composition 1) Contains tempered martensite of 45% to 83% or less, bainite of 15% to 53% or less, and retained austenite of 2% or more, by area ratio.

[0119] (Composition 2) The average carbon content in the retained austenite is 0.5% by mass or more.

[0120] (Component 3) The average particle size of the carbides in the tempered martensite is less than 0.40 μm.

[0121] (Constitution 4 (Preferred Condition)) The difference in microhardness between bainite and tempered martensite is greater than 1.5 GPa.

[0122] (Composition 1) Contains tempered martensite of 45% to 83% or less, bainite of 15% to 53% or less, and retained austenite of 2% or more, by area ratio.

[0123] To obtain a high strength (TS) ≥ 1470 MPa, the area fraction of tempered martensite in the steel microstructure is set to 45% or more. When it is less than 45%, bainite and retained austenite increase, leading to a decrease in strength. To obtain an even higher TS, the area fraction of tempered martensite is preferably 50% or more, more preferably 55% or more. On the other hand, when the area fraction of tempered martensite is greater than 83%, bainite and retained austenite are insufficient, reducing formability. Therefore, the area fraction of tempered martensite is set to 83% or less. To obtain even better formability, the area fraction of tempered martensite is preferably 80% or less, more preferably 75% or less.

[0124] Bainite is a microstructure with excellent strength and formability. To obtain high formability, the area fraction of bainite is set to 15% or more. When it is less than 15%, tempered martensite increases, and formability decreases. Retained austenite increases along with the formation of bainite; therefore, the area fraction of bainite is preferably 20% or more, more preferably 25% or more. On the other hand, when the area fraction of bainite is greater than 53%, tempered martensite decreases, and strength decreases. Therefore, the bainite content is 53% or less, and to obtain even better strength, it is preferably 50% or less, more preferably 45% or less.

[0125] Retained austenite improves the balance between strength and ductility. A good balance between strength and ductility cannot be achieved when the retained austenite content is less than 2%. Therefore, the area ratio of retained austenite is 2% or more. To obtain a better balance between strength and ductility, the area ratio of retained austenite is preferably 3% or more, more preferably 4% or more. No upper limit is specified, but when there is an excess of retained austenite, martensitic transformation occurs during forming, increasing the initiation point of delayed fracture. The area ratio of retained austenite is preferably 20% or less, more preferably 15% or less. Furthermore, in this invention, the retained austenite satisfies the following condition (Construction 2).

[0126] When ferrite is present, it is very soft, thus reducing strength. Furthermore, the large hardness difference between ferrite and tempered martensite causes strain to concentrate at the martensite-ferrite interface during deformation, becoming the initiation point of fracture and potentially deteriorating resistance to delayed fracture. Therefore, the area fraction of ferrite is preferably 3% or less, more preferably 0%. Pearlite is a microstructure composed of layered ferrite and cementite; its formation reduces the carbon content in martensite, potentially decreasing strength. The area fraction of pearlite is preferably 3% or less, more preferably 0%. In this invention, the combined area fraction of ferrite and pearlite is preferably 6% or less, more preferably 2%, and even more preferably 0%.

[0127] (Composition 2) The average carbon content in the retained austenite is 0.5% by mass or more.

[0128] In this invention, the average carbon content in the retained austenite is 0.5% by mass or more. A higher average carbon content in the retained austenite results in higher stability and a better balance between strength and ductility. When the average carbon content in the retained austenite is less than 0.5% by mass, a good balance between strength and ductility is not achieved. Furthermore, low stability leads to an increase in the amount of retained austenite undergoing martensitic transformation during forming, which can become the starting point for delayed fracture, potentially deteriorating the resistance to delayed fracture. Therefore, the average carbon content in the retained austenite is 0.5% by mass or more, preferably 0.6% by mass or more, and more preferably 0.7% by mass or more. While there is no upper limit specified for the average carbon content in the retained austenite, if the average carbon content is too high, the phase transformation from retained austenite to martensite during tensile deformation will not proceed sufficiently, resulting in insufficient work hardening ability. The average carbon content in the retained austenite is preferably 2.0% by mass or less, more preferably 1.8% by mass or less.

[0129] (Component 3) The average particle size of the carbides in the tempered martensite is less than 0.40 μm.

[0130] In the steel microstructure of the present invention, component 3 is important for improving the delayed fracture resistance of the steel sheet. In the present invention, the average grain size of the carbides in the tempered martensite is 0.40 μm or less. By making the average grain size of the carbides 0.40 μm or less, the delayed fracture resistance can be improved. When the carbides are coarser than this, the delayed fracture resistance may deteriorate. The average grain size is preferably 0.38 μm or less, more preferably 0.36 μm or less. There is no particular limitation on the lower limit, but to improve toughness, the average grain size of the carbides is preferably set to 0.001 μm or more, more preferably 0.01 μm or more.

[0131] (Constitution 4 (Preferred Condition)) The difference in microhardness between bainite and tempered martensite is greater than 1.5 GPa.

[0132] In the steel microstructure of this invention, configuration 4 is important for obtaining high formability. A greater hardness difference between bainite and tempered martensite results in a larger plastic deformation gradient during plastic deformation, leading to a higher density of GN dislocations accumulated in the bainite. Therefore, a greater hardness difference results in greater work hardening based on GN dislocations, leading to higher elongation. Thus, the hardness difference between bainite and tempered martensite is preferably 1.5 GPa or more. While no specific upper limit is defined, excessively large hardness differences tend to degrade porosity. Therefore, the hardness difference between bainite and tempered martensite is preferably set to 15 GPa or less, more preferably 13 GPa or less.

[0133] (Tissue assay conditions)

[0134] In the quantification of metal microstructure, the L-section (parallel to the rolling direction and perpendicular to the steel plate surface) of the steel plate is ground and etched with nitric acid-ethanol solution. At a position 1 / 4 thickness from the steel plate surface, four fields of view are observed using a SEM at 2000x magnification. The microstructure images are then analyzed and measured. Here, tempered martensite refers to the microstructure appearing gray in the SEM. On the other hand, bainite and ferrite refer to regions appearing with black contrast in the SEM. It should be noted that tempered martensite and bainite contain trace amounts of carbides, nitrides, sulfides, and oxides, but these are difficult to exclude; therefore, the area ratio of the region including these is set as its area ratio. Furthermore, ferrite is a microstructure composed of bcc lattice grains formed through a phase transformation from austenite at relatively high temperatures. Bainite is a microstructure formed from austenite at relatively low temperatures (above the martensitic transformation point), consisting of spherical carbides dispersed within acicular or plate-like ferrite. Pearlite appears within ferrite as a layered precipitation of cementite.

[0135] In addition, in the determination of retained austenite, the surface 200 μm of the steel plate was chemically polished with oxalic acid, and the austenite was determined by X-ray diffraction intensity method using the plate surface as the object. This was achieved using Mo-K... α The calculation is based on the integrated intensity of the diffraction peaks (200)α, (211)α, (220)α, (200)γ, (220)γ, and (311)γ obtained from X-ray diffraction measurements.

[0136] The area ratio of retained austenite obtained in the above manner is subtracted from 100%. For the remaining area ratio, the ratio of tempered martensite, bainite, ferrite, and pearlite obtained by point counting method through observation of the microstructure based on SEM is used to determine the area ratio of tempered martensite, bainite, ferrite, pearlite, and retained austenite.

[0137] Regarding the average carbon content in the retained austenite, Co-K was used. α The lattice constant (α) of γ is calculated from the {220} peak angle of γ. γ Substitute the amounts of the alloying elements into the following formula to obtain the result.

[0138] α γ =3.578+0.00095(%Mn)+0.022(%N)+0.0056(%Al)+0.033(%C)

[0139] In the formula, (%Mn), (%N), and (%Al) represent the contents (mass%) of Mn, N, and Al, respectively. Additionally, (%C) represents the average carbon content (mass%) in the retained austenite.

[0140] In quantifying the size of carbides within tempered martensite, the L-section (a section perpendicular to the rolling direction) of a steel plate was ground and etched with a nitric acid-ethanol solution. Two fields of view were observed using SEM at 10,000x magnification at a distance of 1 / 4 thickness from the steel plate surface. The finely dispersed white microstructure within the tempered martensite in the photographs was then considered as carbides. The long axis length of the carbides was measured under appropriate magnification. The carbides were assumed to be elliptical or needle-shaped to determine their long axis. Specifically, the long axis lengths of five carbides from each of three martensite laths were measured and averaged to obtain the average particle size of the carbides in the tempered martensite.

[0141] Regarding microhardness, the nanoindentation method was used, with a load of 100 μN applied to the surface of the plate at one-quarter of its thickness after grinding. Five points were measured in each of the bainitic and tempered martensite microstructures, and the average value was calculated. The difference between these average values ​​was taken as the hardness difference. The maximum load was set to 500 μN, and a Berkovich type indenter with a 115-degree angle was used.

[0142] The steel sheet of the present invention described above may have a coating on its surface. The type of coating is not particularly limited and can be any of a Zn coating (zinc coating) or a coating of metals other than Zn. Furthermore, the coating may contain components other than Zn as the main component. For example, a zinc coating may be a hot-dip galvanized layer or an electro-galvanized layer.

[0143] Next, the method for manufacturing the steel plate of the present invention will be described.

[0144] The steel plate manufacturing method of the present invention includes the following steps: a hot rolling process of hot rolling a steel billet having the above-mentioned composition to obtain a hot-rolled steel plate; a cold rolling process of cold rolling the hot-rolled steel plate after the hot rolling process to obtain a cold-rolled steel plate; a continuous annealing process of annealing the cold-rolled steel plate at an annealing temperature of Ac3 or higher for a soaking time of 15 seconds or more after the cold rolling process, cooling it at a first average cooling rate of 5°C / second or higher to a holding temperature of Ms or higher and 600°C or lower, holding it at the holding temperature for a holding time of 1 second or higher and 1000 seconds or lower, and cooling it at a second average cooling rate of 100°C / second or higher to a cooling stop temperature of 150°C or lower after the continuous annealing process; and an over-aging treatment process of holding the steel plate at a temperature range of 150°C or higher and 250°C or lower for 30 seconds or higher and 1500 seconds or lower after the continuous annealing process.

[0145] Hot rolling process

[0146] Methods for hot rolling steel billets include heating the billet before rolling, rolling the continuously cast billet directly without heating, and subjecting the continuously cast billet to short-time heat treatment before rolling. In hot rolling, as in conventional methods, the average heating rate of the billet is set to 5–15°C / min, the finishing rolling temperature (FT) is set to 840–950°C, and the coiling temperature (CT) is set to 400–700°C.

[0147] To remove the primary and secondary oxide scale formed on the surface of the steel sheet, descaling can be performed appropriately. It is preferable to thoroughly pickle the hot-rolled coil before cold rolling to reduce residual oxide scale. Furthermore, from the viewpoint of reducing cold rolling load, the hot-rolled steel sheet can be annealed as needed.

[0148] cold rolling process

[0149] In cold rolling, setting the reduction rate (cold rolling ratio) to 40% or higher can stabilize the recrystallization behavior and texture orientation during subsequent continuous annealing. When it is less than 40%, some of the austenite grains become coarse during annealing, which may reduce the strength.

[0150] Continuous annealing process

[0151] For cold-rolled steel sheets, annealing and tempering are carried out in a continuous annealing line (CAL).

[0152] In this invention, to obtain the specified tempered martensite and bainite, the annealing temperature is set to above Ac3 and the soaking time is set to above 15 seconds. When the annealing temperature is below Ac3 or the soaking time is less than 15 seconds, sufficient austenite (γ) will not be generated during annealing, and the specified tempered martensite and / or bainite will not be obtained in the final product, resulting in a tensile strength of 1470 MPa or higher. There is no particular upper limit to the annealing temperature and soaking time. However, when the annealing temperature and soaking time reach a certain level, the austenite grain size becomes coarse, and the resistance to delayed fracture may deteriorate. Therefore, it is preferable that the annealing temperature is below 950°C and the soaking time is below 900 seconds.

[0153] Then, in order to reduce ferrite, it is necessary to cool to a holding temperature above the Ms point and below 600°C at a first average cooling rate of 5°C / second or higher. When the first average cooling rate is less than 5°C / second, a large amount of ferrite is generated, and carbon is enriched in γ, causing the Ms point to decrease. As a result, the amount of untempered martensite (fresh martensite) increases. Therefore, the first average cooling rate is preferably 7°C / second or higher, and more preferably 10°C / second or higher.

[0154] Then, in order to obtain the specified bainite, the isothermal holding at a holding temperature above Ms point and below 600°C needs to be 1 second to 1000 seconds. When the holding temperature is above 600°C, a large amount of ferrite is formed. On the other hand, when the holding temperature is below Ms point, martensite (self-tempered martensite) that has been tempered at a temperature higher than 250°C is formed during isothermal holding, and the coarsening of carbides within the grains and at the lath grain boundaries becomes significant, which may deteriorate the resistance to delayed fracture. The lower the holding temperature, the easier it is to obtain retained austenite, so 550°C or below is preferred, and 500°C or below is more preferred.

[0155] When the holding time is less than 1 second, there is less bainite and retained austenite, resulting in deterioration of formability. The holding time is preferably 10 seconds or more, more preferably 50 seconds or more. On the other hand, when the holding time is more than 1000 seconds, there is less tempered martensite, resulting in a failure to obtain a tensile strength of 1470 MPa or more. The holding time is preferably 500 seconds or less, more preferably 300 seconds or less.

[0156] Then, in order to obtain tempered martensite with excellent resistance to delayed fracture, it is necessary to cool to a cooling stop temperature below 150°C at a second average cooling rate of 100°C / second or higher. When the second average cooling rate is less than 100°C / second, or the cooling stop temperature exceeds 150°C, coarse self-tempered martensite is formed from carbides within the grains, and the resistance to delayed fracture may deteriorate.

[0157] Over-aging process

[0158] The carbides distributed within the tempered martensite are formed during the low-temperature holding period after quenching. To ensure resistance to delayed fracture and TS≥1470MPa, appropriate control is required. Specifically, the temperature for reheating after quenching to below 150℃ needs to be set above 150℃ and below 250℃, and the holding time needs to be controlled to be above 30 seconds and below 1500 seconds.

[0159] Below 150°C or for less than 30 seconds, the carbide distribution density becomes insufficient, and the toughness may deteriorate. On the other hand, above 250°C or above 1500°C, the coarsening of carbides within the grains and at the lath grain boundaries becomes significant, and the resistance to delayed fracture may deteriorate.

[0160] Plating process

[0161] Furthermore, a plating treatment can be performed on the surface of the obtained steel sheet before or after the aforementioned aging process. By performing the plating treatment, a steel sheet with a coating on its surface can be obtained. There are no particular limitations on the type of plating treatment; plating using spray-based coating techniques or electroplating are both acceptable. It should be noted that when performing the plating treatment, the surface finishing rolling is performed after the plating treatment.

[0162] The Ac3 and Ms points mentioned above are calculated according to the following formulas (1) and (2) as described in "Leslie Steel Materials Science" (Maruzen Co., Ltd., published in 1985, pp. 273, 231). It should be noted that [M%] is set as the content (mass%) of each element M.

[0163] Ac3 (°C) = 910 - 203 × [C%] 1 / 2 +44.7×[Si%]-30×[Mn%]+700×[P%]+130×[Al%]-15.2×[Ni%]-11×[Cr%]-20×[Cu%]+31.5×[Mo%]+104×[V%]+400×[Ti%]…(1)

[0164] Ms(℃)=561-474×[C%]-33×[Mn%]-17×[Ni%]-17×[Cr%]-21×[Mo%]…(2)

[0165] The steel plate of the present invention obtained by the above manufacturing method preferably has a thickness of 0.5 mm or more. Furthermore, the thickness of the steel plate is preferably 2.5 mm or less.

[0166] Next, the components of the present invention and their manufacturing method will be described.

[0167] The component of the present invention is obtained by performing at least one of forming and joining processes on the steel plate of the present invention. Furthermore, the manufacturing method of the component of the present invention includes a step of performing at least one of forming and joining processes on a steel plate manufactured by the steel plate manufacturing method of the present invention.

[0168] The steel sheet of the present invention has a tensile strength of 1470 MPa or higher and exhibits excellent formability and resistance to delayed fracture. Therefore, components obtained using the steel sheet of the present invention also possess high strength, with superior formability and resistance to delayed fracture compared to conventional high-strength components. Furthermore, lightweighting can be achieved when using components of the present invention. Therefore, components of the present invention are suitable, for example, for complex-shaped components used in the automotive field such as body frame parts.

[0169] Forming processes can utilize any conventional methods, such as stamping, without restriction. Joining methods are also not particularly limited; common welding methods such as spot welding, laser welding, and arc welding, as well as riveting and fastening, can be used. It should be noted that forming and joining conditions are not particularly limited; conventional methods can be followed.

[0170] Example

[0171] [Example 1]

[0172] The embodiments of the present invention will be described below.

[0173] Cold-rolled steel sheets with a thickness of 1.2 mm and the composition shown in Table 1 were subjected to heat treatment under the annealing and over-aging conditions shown in Table 2.

[0174] The soaking time at the annealing temperature was set to 300 seconds. Additionally, regarding No. 19, after aging treatment, the steel sheet surface underwent a plating process. The plating conditions were set as electroplating zinc.

[0175] It should be noted that cold-rolled steel sheets are obtained by hot rolling (average heating rate during heating: 10℃ / min, finishing temperature FT: 900℃, coiling temperature CT: 500℃) on a steel billet with the composition shown in Table 1, followed by cold rolling (reduction rate: 55%).

[0176]

[0177] [Table 2]

[0178]

[0179] The obtained steel plate is then subjected to quantitative analysis of its metal structure, followed by tensile testing and evaluation of its resistance to delayed fracture.

[0180] In the quantification of metal microstructure, the L-section of the steel plate (parallel to the rolling direction and perpendicular to the steel plate surface) is ground and etched with nitric acid-ethanol solution. At a position 1 / 4 thickness from the steel plate surface, four fields of view are observed using SEM at 2000x magnification. The microstructure images are then analyzed and measured. Here, tempered martensite refers to the gray microstructure observed in SEM. On the other hand, bainite and ferrite refer to the regions exhibiting black contrast in SEM. It should be noted that tempered martensite and bainite contain trace amounts of carbides, nitrides, sulfides, and oxides, but these are difficult to exclude; therefore, the area ratio of the region including these substances is defined as its area ratio.

[0181] It should be noted that in the observed metallic structures, ferrite is a bcc lattice structure formed at relatively high temperatures through a phase transformation originating from austenite. Bainite is a structure formed at relatively low temperatures (above the martensitic transformation point) from austenite, with spherical carbides dispersed within acicular or plate-like ferrite. Pearlite appears within ferrite as a layered precipitation of cementite.

[0182] In addition, in the determination of retained austenite, the surface 200 μm of the steel plate was chemically polished with oxalic acid, and the austenite was determined by X-ray diffraction intensity method using the plate surface as the object. This was achieved using Mo-K... α The calculation is based on the integrated intensity of the diffraction peaks (200)α, (211)α, (220)α, (200)γ, (220)γ, and (311)γ obtained from X-ray diffraction measurements.

[0183] The area ratio of retained austenite obtained in the above manner is subtracted from 100%. For the remaining area ratio, the ratio of tempered martensite, bainite, ferrite, and pearlite obtained by point counting method based on the above SEM-based microstructure observation is used to determine the area ratio of tempered martensite, bainite, ferrite, pearlite, and retained austenite.

[0184] Regarding the average carbon content in the retained austenite, Co-K was used. α The lattice constant (α) of γ is calculated from the {220} peak angle of γ. γ Substitute the amounts of the alloying elements into the following formula to obtain the result.

[0185] α γ =3.578+0.00095(%Mn)+0.022(%N)+0.0056(%Al)+0.033(%C)

[0186] In the formula, (%Mn), (%N), and (%Al) represent the contents of Mn, N, and Al, respectively. Additionally, (%C) represents the average carbon content in the retained austenite.

[0187] In quantifying the size of carbides within tempered martensite, the L-section (a section perpendicular to the rolling direction) of a steel plate was ground and etched with a nitric acid-ethanol solution. Two fields of view were observed using SEM at 10,000x magnification at a distance of 1 / 4 thickness from the steel plate surface. The finely dispersed white microstructure within the tempered martensite in the photographs was then considered as carbides. The long axis length of the carbides was measured under appropriate magnification. The carbides were assumed to be elliptical or needle-shaped to determine their long axis. Specifically, the long axis lengths of five carbides from each of three martensite laths were measured and averaged to obtain the average particle size of the carbides in the tempered martensite.

[0188] In the tensile test, JIS No. 5 tensile test pieces are cut with the rolling direction as the length direction, and the tensile test is carried out (according to JIS Z2241) to evaluate TS and E1.

[0189] Regarding microhardness, nanoindentation was used, with a load of 100 μN applied to the surface of the plate at one-quarter of its thickness after grinding. Five points were measured in each of the bainitic and tempered martensite microstructures, and the average value was calculated. The difference between these average values ​​was taken as the hardness difference. A Triboindenter manufactured by Hysitron was used as the nanoindentation device, with a maximum load set to 500 μN, and a Berkovich type indenter with a 115-degree angle was used.

[0190] The evaluation of the delayed fracture resistance of steel plates involves evaluating the delayed fracture of the steel plate base material.

[0191] The delayed fracture evaluation of the steel sheet base material was conducted by cutting a long test piece with a rolling right-angle direction of 100 mm and a rolling direction of 30 mm from 1 / 4 of the width of the obtained steel sheet coil. The 100 mm long end face cutting was set as a shearing process. While maintaining the shearing process (without deburring), bending was performed with the burrs on the outer periphery of the bend, maintaining the shape of the test piece after bending, and securing the test piece with bolts. The shearing clearance rate was set to 15%, and the rake angle was set to 0 degrees. Bending was performed with a bending radius of R / t = 4 (plate thickness t: 1.2 mm, bending using a punch with a tip radius of 5.0 mm) and a tip inner angle of 90 degrees (V-bending). A U-shaped punch with the aforementioned tip radius (the tip R portion is semi-circular, and the thickness of the punch body is 2R) was used, and the corner radius of the die was 30 mm. The depth of the punch pressed into the steel plate is adjusted to form a 90-degree bend at the tip (V-shape). During bolt tightening, the test piece is clamped and locked with a hydraulic jack so that the distance between the flange ends of the straight sections during bending is the same as during bending (to counteract the opening of the straight sections based on springback). The bolt is then secured by passing it through an elliptical hole (10mm minor axis, 15mm major axis) pre-set 10mm inside the short edge of the long strip test piece. The bolted test piece is then immersed in at least 1L of hydrochloric acid (hydrogen chloride aqueous solution) at pH 3, and the pH is maintained constant at an aqueous solution temperature of 25°C. The presence of visually detectable microcracks (at least 1mm in length) at any time (the initial state of delayed fracture) is checked visually or with a camera, and the time from the start of immersion to the onset of microcracks is measured as the delayed fracture time.

[0192] With the increasing strength of steel plates, concerns about delayed fracture have grown. Therefore, even at high strength, differences in delayed fracture characteristics can occur. Thus, a fracture time of 10... (-0.0055×(TS-1760)+0.3) Cases with a fracture time of 10 to the power of (-0.0055×(TS-1760)+0.3) hours or more are considered to have excellent delayed fracture characteristics and are marked as "○" (qualified). Cases with a fracture time less than the above time are marked as "×" (unqualified).

[0193] [Table 3]

[0194]

[0195] (*1): Average carbon content (mass%) in the retained austenite

[0196] (*2): Average particle size (μm) of carbides in tempered martensite.

[0197] (*3): The difference in microhardness (GPa) between bainite and the tempered martensite.

[0198] For steels with optimized composition, hot rolling conditions, and annealing conditions, a tensile strength (TS) of 1470 MPa or higher was obtained. Furthermore, with an average carbon content of 0.5% by mass or higher in the retained austenite and an area fraction of 2% or higher, an elongation of 11% or higher was achieved.

[0199] Regarding the delayed fracture resistance characteristics of the steel plate base material, the fracture time is 10... (-0.0055×(TS-1760)+0.3) Excellent properties regarding delayed fracture were obtained after more than 10 hours. The steel plate of the present invention has a tensile strength of more than 1470 MPa, an elongation of more than 11%, and a fracture time of more than 10 hours in the delayed fracture resistance evaluation test. (-0.0055×(TS-1760)+0.3) More than 24 hours. For the steel plate of the comparative example, none of these conditions were met.

[0200] [Example 2]

[0201] The galvanized steel sheet treated with manufacturing condition No. 1 (suitable steel) in Table 2 of Example 1 was stamped to manufacture the component of the present invention. Furthermore, the galvanized steel sheet treated with manufacturing condition No. 1 (suitable steel) in Table 2 of Example 1 and the galvanized steel sheet treated with manufacturing condition No. 2 (suitable steel) in Table 2 of Example 1 were joined by spot welding to manufacture the component of the present invention.

[0202] The components of these inventions have a tensile strength TS of 1470 MPa or higher, and excellent formability and resistance to delayed fracture. Therefore, these components are suitable for use in automotive parts, etc.

[0203] Similarly, the steel sheet based on manufacturing conditions No. 1 (suitable example) in Table 2 of Example 1 was stamped to manufacture the component of the present invention. Furthermore, the steel sheet based on manufacturing conditions No. 1 (suitable example) in Table 2 of Example 1 and the steel sheet based on manufacturing conditions No. 2 (suitable example) in Table 2 of Example 1 were joined by spot welding to manufacture the component of the present invention. These components of the present invention have a tensile strength TS of 1470 MPa or higher, and exhibit excellent formability and resistance to delayed fracture; therefore, these components are suitable for use in automotive parts, etc.

[0204] Industrial availability

[0205] According to the present invention, a high-strength steel sheet with excellent formability and resistance to delayed fracture can be obtained. This improved property enables the application of high-strength steel sheets in difficult-to-form parts for cold stamping applications, contributing to increased part strength and weight reduction.

Claims

1. A type of steel plate, It contains, by mass percent, C: ≥0.20% and ≤0.40%, Si: ≥1.0% and ≤3.0%, Mn: ≥1.5% and ≤3.5%, P: ≥0.002% and ≤0.010%, S: ≥0.0002% and ≤0.0020%, sol.Al: ≤0.40% and excluding 0%, N: ≤0.0100%, and the balance is composed of Fe and unavoidable impurities. Furthermore, it possesses a steel microstructure containing, by area ratio, 45% to 83% tempered martensite, 15% to 53% bainite, and 2% or more retained austenite. The average particle size of the carbides in the tempered martensite is less than 0.40 μm. The average carbon content in the retained austenite is 0.5% by mass or more. The tensile strength of the steel plate is above 1470 MPa.

2. The steel plate according to claim 1, wherein, The difference in microhardness between the bainite and the tempered martensite is greater than 1.5 GPa.

3. The steel plate according to claim 1 or 2, wherein, As a component, the ingredient also contains, by mass%, one or more selected from groups A to D below: Group A: Selected from one or more of the following: Nb: less than 0.1%, Ti: less than 0.10%, B: less than 0.0050%, Cu: less than 1%, and Ni: less than 1%. Group B: Selected from one or more of the following: Cr: less than 1.0%, Mo: less than 0.3%, V: less than 0.45%, Zr: less than 0.2%, and W: less than 0.2%; Group C: Selected from one or both of Sb: less than 0.1% and Sn: less than 0.1%; Group D: Selected from one or more of the following: Ca: less than 0.0050%, Mg: less than 0.01%, and REM: less than 0.01%.

4. The steel plate according to claim 1 or 2, wherein, The steel plate has a coating.

5. The steel plate according to claim 3, wherein, The steel plate has a coating.

6. A component obtained by performing at least one of forming and joining processes on the steel plate according to any one of claims 1 to 5.

7. A method for manufacturing a steel plate, comprising the method for manufacturing the steel plate according to any one of claims 1 to 3, comprising: The hot rolling process involves hot rolling a steel billet to obtain a hot-rolled steel plate. The cold rolling process involves cold rolling the hot-rolled steel sheet after the hot rolling process to obtain a cold-rolled steel sheet. In the continuous annealing process, after the cold rolling process, for the cold-rolled steel sheet, annealing is performed at an annealing temperature above Ac3 and with a soaking time of 15 seconds or more, followed by cooling at a first average cooling rate of 5°C / second or more to a holding temperature above Ms and below 600°C, holding at the holding temperature for a holding time of 1 second or more and below 1000 seconds, and then cooling at a second average cooling rate of 100°C / second or more to a cooling stop temperature below 150°C; and After the aging process, the temperature is maintained at a range of 150°C to 250°C for 30 seconds to 1500 seconds.

8. The method for manufacturing a steel plate according to claim 7, wherein, The process includes the following plating steps: plating the surface of the steel plate before or after the over-aging treatment step.

9. A method for manufacturing a component, comprising the steps of performing at least one of forming and joining processes on a steel plate manufactured by the steel plate manufacturing method of claim 7 or 8.

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