1.4gpa grade trip steel with stable mechanical properties and preparation method thereof

Abstract

This invention relates to the field of advanced high-strength cold-rolled steel materials for automobiles, specifically to a 1.4 GPa grade TRIP steel with stable mechanical properties and its preparation method. The preparation method includes the following steps: forging, hot rolling, first-stage annealing cold rolling, second-stage annealing cold rolling, and continuous annealing treatment. This preparation method, through second-stage annealing cold rolling and optimized control of process parameters in each step, yields a 1.4 GPa grade TRIP steel with stable performance. This product has advantages such as high strength, high toughness, low cost, and good porosity. Its yield strength ratio is stably controlled at 0.67±0.01, yield strength at 975±20 MPa, tensile strength at 1455±30 MPa, elongation after fracture at 17.0±1.0%, porosity λ at 30.5±5.5%, and strength-ductility product >23.6 GPa·%, stably controlled at 24.6±1.1 GPa·%.

Description

Technical Field

[0001] This invention relates to the field of advanced high-strength cold-rolled steel materials for automobiles, specifically to a 1.4GPa grade TRIP steel with stable mechanical properties and its preparation method. Background Technology

[0002] Transformation-induced plasticity steel (TRIP steel) is a low-alloy high-strength steel with a microstructure containing various morphologies such as ferrite, bainite, and austenite. It possesses the characteristic of exhibiting high plasticity through solid-state phase transformation under stress. The interaction between bainite and austenite induces a phase transformation from ferrite to bainite, thereby achieving high plasticity. TRIP steel is a low-alloy high-strength steel with excellent mechanical properties and good formability, widely used in automotive, construction, aerospace, and other fields.

[0003] The use of ultra-high-strength steel plates can effectively reduce vehicle weight and improve safety. Furthermore, it can reduce energy consumption and emissions, making it an essential requirement for the high-quality development and transformation of the automotive industry. Therefore, it has always been a focus of research in both the automotive and steel industries. TRIP steel offers a new direction for resolving the contradiction between strength and plasticity, becoming a research hotspot in the field of ultra-high-strength steel for automobiles.

[0004] Cold-rolled TRIP steel is produced by heating steel strip to the ferrite-austenite two-phase region through continuous annealing. In the subsequent aging stage (bainitic treatment), some austenite transforms into bainite, and carbon is enriched in the remaining austenite. When the strength of ultra-high-strength TRIP steel increases to the 1.4 GPa level, the proportion of hard bainite in the microstructure increases significantly. This leads to large fluctuations in the material's yield strength, tensile strength, elongation after fracture, and porosity, resulting in highly unstable mechanical properties. This makes it difficult to meet the formability and plasticity requirements of OEMs for ultra-high-strength steel. Therefore, there is an urgent technical challenge to improve the performance stability of ultra-high-strength TRIP steel. Summary of the Invention

[0005] To address the technical problem of unstable mechanical properties in existing ultra-high strength TRIP steels, this invention provides a 1.4 GPa grade TRIP steel with stable mechanical properties and its preparation method.

[0006] In a first aspect, the present invention provides a method for preparing 1.4GPa grade TRIP steel with stable mechanical properties, comprising the following steps: forging, hot rolling, first annealing cold rolling, second annealing cold rolling, and continuous annealing treatment.

[0007] Furthermore, the specific steps include: (1) Forging: heating a steel billet to 1220±15°C, holding the temperature for 60±15min, then forging the steel billet to obtain a forged billet; (2) Hot rolling: heating the forged billet to a soaking temperature of 1215±15°C, holding the temperature for 180±20min, then rolling the forged billet into a steel plate with a thickness of 4.0~7.0mm, cooling the steel plate to 550±25°C, then placing the steel plate into a holding furnace at 560°C for heat preservation, holding the temperature for 150±30min, then air-cooling the steel plate to room temperature; (3) Primary annealing and cold rolling: performing primary annealing on the steel plate, placing the steel plate into an annealing furnace at 720°C and holding the temperature for 8~12h; performing primary cold rolling after pickling to obtain a hard cold-rolled strip steel with a thickness of 1.8~3.5mm; (4) Secondary annealing and cold rolling: performing secondary annealing on the hard cold-rolled strip steel, placing the strip steel into an annealing furnace at 720°C and holding the temperature for 8~12h; performing secondary cold rolling to obtain a hard cold-rolled strip steel with a thickness of 1.00~2.40mm; (5) Continuous annealing treatment.

[0008] Further, in step (1), the steel billet is based on C-Mn-Si as basic components, and its chemical composition by mass percentage is: C 0.19%~0.23%, Mn 2.40%~2.70%, Si 0.40%~0.80%, Alt 0.80%~1.20%, Cr 0.50%~0.80%, Mo 0.24%~0.30%, V 0.18%~0.23%, B 0.0025%~0.0040%, P, S, O and N are all not more than 0.005%; and the composition satisfies 1.45%≤Si+Alt≤1.95%, 0.18%<Cr / Mn<0.34%, 0.42%≤Mo+V≤0.50%, with the balance being Fe and other unavoidable impurities.

[0009] In order to improve the surface quality of TRIP steel, the present invention adopts a low-Si composition design and adds 0.8%~1.2% Al element. This can not only reduce the diffusion coefficient of C atoms, greatly increase the stacking fault energy of the material to inhibit the phase transformation from γ to δ, but also inhibit the delayed fracture of the material, and effectively improve the plasticity and formability of the material. The effect of microalloy element V in steel, which improves hardenability, inhibits grain growth and provides precipitation strengthening, is utilized to further improve the strength of TRIP steel. In addition, to ensure that TRIP steel has good hardenability, a small amount of B element is added. And based on tests and big data analysis, the addition amounts of various elements are determined according to the optimal and most economical proportioning principle.

[0010] Further, in step (1), the final forging temperature is >950°C.

[0011] Furthermore, in step (2), during the rolling process, the initial rolling temperature is 1150±25℃, the final rolling temperature is 890±15℃, and the hot rolling reduction rate of the last two passes is controlled to be >45% and >35% respectively; after rolling, laminar flow cooling is adopted, and the cooling rate is 25~35℃ / s.

[0012] Furthermore, in step (3), the cumulative reduction rate of the first cold rolling is controlled at 50%~60%, and the reduction rate of the last two cold rolling passes is controlled at 14%~16%. Furthermore, in step (4), the cumulative reduction rate of the second cold rolling is controlled at 30%~45%, and the reduction rate of the last two cold rolling passes is controlled at 14%~16%. Further, step (5) specifically involves: continuously annealing the cold-hardened strip steel after secondary cold rolling, with a annealing temperature of 880±10℃ and an annealing time of 90±20s; cooling to a slow cooling temperature of 700±10℃ at a cooling rate of 5~8℃ / s; then rapidly cooling to an over-aging temperature of 400±15℃ at a cooling rate of 45~55℃ / s, and isothermal for 150±30s; subsequently cooling to room temperature at a cooling rate of 5~8℃ / s to obtain 1.4GPa grade TRIP steel with stable mechanical properties.

[0013] Secondly, the present invention provides a 1.4 GPa grade TRIP steel with stable mechanical properties prepared by the above-mentioned preparation method.

[0014] Furthermore, the aforementioned 1.4 GPa grade TRIP steel with stable mechanical properties has a yield strength ratio of 0.67 ± 0.01, a yield strength of 975 ± 20 MPa, a tensile strength of 1455 ± 30 MPa, an elongation after fracture of 17.0% ± 1.0%, a hole expansion rate of 30.5% ± 5.5%, and a strength-ductility product of 24.6 ± 1.1 GPa·.

[0015] The beneficial effects of this invention are as follows: This invention provides a method for preparing 1.4 GPa grade TRIP steel with stable mechanical properties. Through secondary annealing and cold rolling, and optimized control of process parameters in each step, a stable 1.4 GPa grade TRIP steel is obtained. This product has advantages such as high strength, high toughness, low cost, and good porosity. Its yield strength ratio is stably controlled at 0.67±0.01, yield strength at 975±20 MPa, tensile strength at 1455±30 MPa, elongation after fracture ≥16.0%, and stably controlled at 17.0±1.0%; porosity λ ≥25%, and stably controlled at 30.5±5.5%; strength-ductility product >23.6 GPa·%, and stably controlled at 24.6±1.1 GPa·%. Attached Figure Description

[0016] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following briefly introduces the accompanying drawings required for describing the embodiments or the prior art. Obviously, for those skilled in the art, other drawings can be obtained from these drawings without creative efforts.

[0017] Figure 1 is a typical microscopic scanning structure diagram of the 1.4 GPa grade TRIP steel product produced in Example 1-I of the present invention. DETAILED DESCRIPTION

[0018] In order that those skilled in the art can better understand the technical solutions in the present invention, the technical solutions in the embodiments of the present invention will be described clearly and completely with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative efforts shall fall within the scope of protection of the present invention.

[0019] Example 1.4 GPa Grade TRIP Steel with Stable Mechanical Properties and Preparation Method Thereof The steel billet used takes C-Mn-Si as the basic composition, and its chemical composition by mass percentage is: C 0.19%~0.23%, Mn 2.40%~2.70%, Si 0.40%~0.80%, Alt 0.80%~1.20%, Cr 0.50%~0.80%, Mo 0.24%~0.30%, V 0.18%~0.23%, B 0.0025%~0.0040%, P, S, O and N are all not more than 0.005%; and it satisfies 1.45% ≤ Si+Alt ≤ 1.95%; 0.18% < Cr / Mn < 0.34%; 0.42% ≤ Mo+V ≤ 0.50%, with the balance being Fe and other unavoidable impurities.

[0020] The chemical compositions of the steel billets used in Examples 1 to 8 of the present invention are shown in Table 1.

[0021] Table 1 Chemical compositions of steel billets used in Examples 1 to 8 (mass percentage, unit: %)

[0022] The preparation methods of Examples 1 to 8 of the present invention specifically include the following steps: (1) Forging: heating the steel billet to 1220±15°C, holding the temperature for 60±15 min, then forging the steel billet, with a final forging temperature > 950°C.

[0023] (2) Hot rolling: The forging billet is heated to a uniform heating temperature of 1215±15℃, held for 180±20min, with an initial rolling temperature of 1150±25℃ and a final rolling temperature of 890±15℃. The reduction rates of the last two hot rolling passes are controlled at >45% and >35% respectively, and the forging billet is rolled into a steel plate of 4.0~7.0mm. After rolling, laminar flow cooling is adopted to cool the steel plate to 550±25℃ at a cooling rate of 25~35℃ / s. Then, it is placed in a holding furnace at 560℃ for holding for 150±30min. Finally, the steel plate is air-cooled to room temperature.

[0024] (3) First annealing and cold rolling: The steel plate is annealed once and placed in an annealing furnace at 720℃ for 8~12h; after pickling, it is cold rolled once, and the cumulative reduction rate of cold rolling is controlled at 50%~60%, and the reduction rate of the last two cold rolling passes is controlled at 14%~16%, to obtain cold hardened strip steel of 1.8~3.5mm.

[0025] (4) Secondary annealing and cold rolling: The above-mentioned cold-hardened strip steel is annealed for a second time and placed in an annealing furnace at 720℃ for 8~12h; then it is cold-rolled for a second time, with the cumulative reduction rate controlled at 30%~45% and the reduction rate of the last two cold rolling passes controlled at 14%~16%, to obtain cold-hardened strip steel with a diameter of 1.00~2.40mm.

[0026] The specific process parameters for the hot rolling, primary cold rolling, and secondary cold rolling processes in Examples 1-8 of this invention are shown in Table 2.

[0027] Table 2. Specific process parameters for hot rolling, single cold rolling, and double cold rolling processes in Examples 1-8.

[0028] (5) The cold-rolled strip steel after the second cold rolling is subjected to continuous annealing treatment. The annealing temperature is 880±10℃ and the annealing time is 90±20s. It is cooled to a slow cooling temperature of 700±10℃ at a cooling rate of 5~8℃ / s. Then it is rapidly cooled to an over-aging temperature of 400±15℃ at a cooling rate of 45~55℃ / s and isothermal for 150±30s. Then it is cooled to room temperature at a cooling rate of 5~8℃ / s to obtain a 1.4GPa grade TRIP steel with stable mechanical properties.

[0029] The specific process parameters of the continuous annealing process in Examples 1-8 of the present invention are shown in Table 3.

[0030] Table 3. Specific process parameters for the continuous annealing process in Examples 1-8

[0031] Microstructure analysis and mechanical property testing were performed on samples of the 1.4 GPa grade TRIP steel prepared in Examples 1-8. The specific test and analysis results are shown in Table 4. The mechanical property testing methods were prepared according to GB / T 228.1-2021 "Metallic materials, tensile testing—Part 1: Test at room temperature". A 50 Tensile specimens were prepared, and yield strength, tensile strength, and elongation after fracture were measured on a Zwick / Roell Z100 tensile testing machine. A 50 The test was conducted according to GB / T 15825.4-2008 "Forming Properties and Test Methods of Metal Sheets - Part 4: Hole Enlargement Test". Samples of 150×150 mm were prepared, holes were drilled, and then an enlargement test was performed to determine the enlargement rate. λ .

[0032] Table 4. Mechanical property test results of Examples 1-8

[0033] Analysis of the mechanical property test results shows that the 1.4GPa grade TRIP steel prepared in Examples 1-8 of this invention has high performance stability and advantages such as low cost and good hole expansion properties. Its yield strength ratio is stably controlled at 0.67±0.01; yield strength is stably controlled at 975±20MPa; tensile strength is stably controlled at 1455±30MPa; elongation after fracture is ≥16.0%, and stably controlled at 17.0±1.0%; hole expansion rate λ≥25%, and stably controlled at 30.5±5.5%; strength-ductility product is >23.6GPa•%, and stably controlled at 24.6±1.1 GPa•%.

[0034] Based on the microstructure analysis and the volume fraction measurements of each phase, the matrix structure of the 1.4 GPa grade TRIP steel products prepared in Examples 1-8 of this invention consists of ferrite, bainite, and a small amount of retained austenite, with a bainite volume fraction >75% and a retained austenite volume fraction >7%. Typical microstructure images of the 1.4 GPa grade TRIP steel products produced in Examples 1-II are shown below. Figure 1 As shown.

[0035] Although the present invention has been described in detail with reference to the accompanying drawings and preferred embodiments, the present invention is not limited thereto. Various equivalent modifications or substitutions can be made to the embodiments of the present invention by those skilled in the art without departing from the spirit and essence of the invention, and such modifications or substitutions should all be within the scope of the present invention. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should also be covered within the protection scope of the present invention.

Claims

1. A method for preparing 1.4 GPa grade TRIP steel with stable mechanical properties, characterized in that, It specifically comprises the following steps: (1) Forging: heating a steel billet to 1220±15°C, holding the temperature for 60±15min, then forging the steel billet to obtain a forged billet; the steel billet is based on C-Mn-Si, and its chemical composition by mass percentage is C 0.19%~0.23%, Mn 2.40%~2.70%, Si 0.40%~0.80%, Alt 0.80%~1.20%, Cr 0.50%~0.80%, Mo 0.24%~0.30%, V 0.18%~0.23%, B 0.0025%~0.0040%, P, S, O, N are all not more than 0.005%; and the composition satisfies 1.45%≤Si+Alt≤1.95%, 0.18<Cr / Mn<0.34, 0.42%≤Mo+V≤0.50%, with the balance being Fe and other unavoidable impurities; (2) Hot rolling: heating the forged billet to a soaking temperature of 1215±15°C, holding the temperature for 180±20min, then rolling the forged billet into a steel plate of 4.0~7.0mm, controlling the hot rolling reduction rates of the last two passes to be >45% and >35% respectively; after rolling, laminar cooling is adopted, cooling to 550±25°C at a cooling rate of 25~35°C / s, then placing the steel plate into a holding furnace at 560°C for holding, holding for 150±30min, then air-cooling the steel plate to room temperature; (3) Primary annealing and cold rolling: subjecting the steel plate to primary annealing, placing the steel plate into an annealing furnace at 720°C and holding for 8~12h; after pickling, performing primary cold rolling to obtain a hard cold-rolled strip steel of 1.8~3.5mm; (4) Secondary annealing and cold rolling: subjecting the hard cold-rolled strip steel to secondary annealing, placing the strip steel into an annealing furnace at 720°C and holding for 8~12h; performing secondary cold rolling to obtain a hard cold-rolled strip steel of 1.00~2.40mm; (5) Continuous annealing treatment: subjecting the hard cold-rolled strip steel after secondary cold rolling to continuous annealing treatment, wherein the soaking annealing temperature is 880±10°C, and the annealing time is 90±20s.

2. The method for preparing 1.4 GPa grade TRIP steel with stable mechanical properties as described in claim 1, characterized in that, In step (1), the final forging temperature is >950°C.

3. The method for preparing a mechanically stable 1.4 GPa grade TRIP steel as described in claim 1, characterized in that, In step (2), during the rolling process, the starting rolling temperature is 1150±25°C, and the final rolling temperature is 890±15°C.

4. The method for preparing a mechanically stable 1.4 GPa grade TRIP steel as described in claim 1, characterized in that, In step (3), the cumulative reduction rate of primary cold rolling is controlled at 50%~60%, and the cold rolling reduction rates of the last two passes are controlled at 14%~16%.

5. The method for preparing a mechanically stable 1.4 GPa grade TRIP steel as described in claim 1, characterized in that, In step (4), the cumulative reduction rate of secondary cold rolling is controlled at 30%~45%, and the cold rolling reduction rates of the last two passes are controlled at 14%~16%.

6. The method for preparing a mechanically stable 1.4 GPa grade TRIP steel as described in claim 1, characterized in that, Step (5) further comprises: cooling to a slow cooling temperature of 700±10°C at a cooling rate of 5~8°C / s; then rapidly cooling to an overaging temperature of 400±15°C at a cooling rate of 45~55°C / s, and holding isothermally for 150±30s; then cooling to room temperature at a cooling rate of 5~8°C / s, to obtain 1.4GPa grade TRIP steel with stable mechanical properties.

7. A 1.4GPa grade TRIP steel with stable mechanical properties prepared by the preparation method according to any one of claims 1 to 6.

8. The 1.4 GPa grade TRIP steel with stable mechanical properties as described in claim 7, characterized in that, The yield strength ratio is 0.67±0.01, the yield strength is 975±20MPa, the tensile strength is 1455±30MPa, the elongation after fracture is 17.0%±1.0%, the porosity is 30.5%±5.5%, and the strength-ductility product is 24.6±1.1GPa·.

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