A 1.5gpa-grade dual-phase steel with stable mechanical properties and a preparation method thereof

Abstract

The present application relates to the technical field of advanced high-strength steel material, and particularly relates to a 1.5GPa-grade dual-phase steel with stable mechanical properties and a preparation method thereof. The method comprises the following steps: (1) heating the steel blank to a soaking temperature, controlling the opening rolling temperature to be 1160±20 DEG C, and the final rolling temperature to be 880±15 DEG C; after rolling, the steel plate is cooled to 570±20 DEG C for heat preservation; (2) performing primary annealing on the hot-rolled steel plate; after pickling, performing primary cold rolling; (3) performing secondary annealing on the cold hard steel strip; after annealing, performing secondary cold rolling; (4) performing continuous annealing treatment on the cold hard steel strip; cooling to a slow cooling temperature; cooling to an overaging temperature; finally, air cooling the steel plate to room temperature. The 1.5GPa-grade dual-phase steel prepared by the method has high performance stability, the stable control of the yield strength ratio is 0.60±0.015, the stable control of the yield strength is 925±25 MPa, and the stable control of the tensile strength is 1565±30 MPa.

Description

Technical Field

[0001] This invention relates to the field of advanced high-strength steel materials technology, specifically to a 1.5GPa level duplex steel with stable mechanical properties and its preparation method. Background Technology

[0002] With increasing awareness of safety and environmental protection, people are placing more stringent demands on the safety and energy efficiency of vehicles such as automobiles. 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 effective way to promote green and low-carbon development. It is also an inevitable requirement for the high-quality development and transformation and upgrading of the automotive industry, and therefore has always been a focus of research in both the automotive and steel industries.

[0003] When the strength of ultra-high strength dual-phase steel increases to 1.5 GPa, the matrix structure becomes extremely refined and the proportion of hard phase martensite increases significantly. As a result, the various performance indicators of the material become extremely unstable, which makes it difficult to meet the requirements of OEMs for the formability and plasticity of ultra-high strength steel.

[0004] Therefore, there is an urgent need to solve the technical challenges of improving the performance stability of ultra-high strength duplex steel. Summary of the Invention

[0005] To address the technical problem of extremely unstable mechanical properties in the production of ultra-high strength duplex steel, this invention provides a 1.5GPa level duplex steel with stable mechanical properties and its preparation method.

[0006] In a first aspect, the present invention provides a method for preparing a 1.5 GPa level dual-phase steel with stable mechanical properties. The chemical composition of the steel billet used, by mass percentage, is C: 0.19%~0.22%, Mn: 2.7%~3.0%, Si: 0.50%~0.70%, Alt: 0.80%~1.20%, Cr: 0.40%~0.80%, Nb: 0.045%~0.065%, Cu: 0.7%~0.9%, Ni: 0.25%~0.60%, B: 0.0025%~0.0040%, P≤0.005%, S≤0.005%, O≤0.005%, N≤0.005%, with the balance being Fe and other unavoidable impurities. The preparation method includes the following steps: (1) Heat the billet to a uniform temperature of 1230±15℃ and hold for 50±10min. Control the initial rolling temperature to 1160±20℃ and the final rolling temperature to 880±15℃. After rolling, cool the steel plate to 570±20℃ and hold for 60±10min. Then cool the steel plate to room temperature with the furnace. (2) Place the hot-rolled steel plate obtained in step (1) into an annealing furnace at 670℃ and keep it at that temperature for 6-8 hours for one annealing; after pickling, perform one cold rolling. (3) Placing the cold-hardened strip steel obtained in step (2) into an annealing furnace at 670°C, holding for 6 to 8 hours for secondary annealing; performing secondary cold rolling after annealing; (4) Subjecting the cold-hardened strip steel obtained in step (3) to continuous annealing treatment, wherein the annealing soaking temperature is 840±10°C, and the annealing time is 120±30s; cooling to a slow cooling temperature of 700±10°C at a cooling rate of 5 to 8°C / s; then rapidly cooling to an overaging temperature of 285±15°C at a cooling rate of >40°C / s, wherein the overaging isothermal time is 240±20s; finally, air-cooling the steel plate to room temperature to obtain the product.

[0007] Further, the chemical composition of the used steel billet satisfies 0.15<Cr / Mn<0.28 and 3.1%<Cr+Mn<3.7%, 1.2<Alt / Si<1.9 and 1.4%<Alt+Si<1.9%, 0.35<Ni / Cu<0.68 and 0.95%<Ni+Cu<1.5%.

[0008] Further, in step (1), the hot rolling reduction ratios of the last three passes are controlled to be >45%, >35% and >30% respectively, and the thickness of the steel plate obtained by hot rolling is 4.0 to 8.0mm.

[0009] Further, in step (1), after hot rolling, the steel plate is cooled to 570±20°C at a cooling rate of 25 to 35°C / s.

[0010] Further, in step (2), the cumulative cold rolling reduction ratio is controlled at 55% to 60%, the cold rolling reduction ratios of the last two passes are controlled to be >18% and >14% respectively, and the thickness of the cold-hardened strip steel obtained by cold rolling is 1.8 to 3.4mm.

[0011] Further, in step (3), the cumulative cold rolling reduction ratio is controlled at 40% to 45%, the cold rolling reduction ratios of the last two passes are controlled to be >15% and >12% respectively, and the thickness of the cold-hardened strip steel obtained by cold rolling is 1.0 to 2.0mm.

[0012] In a second aspect, the present invention provides a 1.5GPa grade dual-phase steel prepared by the above method.

[0013] Further, the 1.5GPa grade dual-phase steel has high performance stability, the yield ratio is stably controlled at 0.60±0.015, the yield strength is stably controlled at 925±25MPa, the tensile strength is stably controlled at 1565±30MPa, the elongation after fracture is >14%, and the hole expansion rate λ≥28%.

[0014] Further, the structure of the 1.5GPa grade dual-phase steel comprises ferrite, martensite and retained austenite.

[0015] Further, the thickness of the 1.5 GPa grade dual-phase steel is 1.0 to 2.0 mm.

[0016] Beneficial effects of the present invention lie in: (1) The component design of the present technical solution is based on the component design of C-Si-Mn series low-carbon steel, satisfying 0.15<Cr / Mn<0.28 and 3.1%<Cr+Mn<3.7%, 1.2<Alt / Si<1.9 and 1.4%<Alt+Si<1.9%, 0.35<Ni / Cu<0.68 and 0.95%<Ni+Cu<1.5%. This component regulation is more conducive to improving the performance stability of 1.5 GPa grade dual-phase steel.

[0017] (2) The present technical solution adds a small amount of Cu and Ni microalloy elements, and regulates 0.35<Ni / Cu<0.68 and 0.95%<Ni+Cu<1.5%. On the basis of controlling alloy cost, on the one hand, the corrosion resistance of 1.5 GPa grade dual-phase steel can be significantly improved; on the other hand, it facilitates 8% to 20% retained austenite in the matrix structure to produce TRIP effect in the high strain region, thereby improving the plastic stability of ultra-high strength dual-phase steel, so that while meeting the strength requirement, the post-fracture elongation is >14%.

[0018] (3) The present technical solution adopts the preparation method of double cold rolling + annealing in secondary non-recrystallization region, which is particularly suitable for producing 1.5 GPa grade dual-phase steel with a thickness specification ≤ 2.0 mm. The finished product thickness in the present embodiment is 1.0~2.0 mm, but is not limited thereto. This process flow is more suitable for mass, continuous and industrial production of thin-specification ultra-high strength dual-phase steel.

[0019] (4) The present technical solution adopts the preparation method of double cold rolling + annealing in secondary non-recrystallization region, ensuring that the cold rolling reduction rates of the last two passes of primary cold rolling are controlled at >18% and >14% respectively, and the cold rolling reduction rates of the last two passes of secondary cold rolling are controlled at >15% and >12% respectively. This enables solid solution C atoms to fully diffuse and refine the grain structure before continuous annealing, meanwhile increases and effectively stores the distortion energy in grains, and significantly improves the recrystallization driving force of γ grains during continuous annealing and heating. This not only improves the uniformity of the phase transformation structure, but also increases the average C content in martensite and retained austenite, significantly improving the performance stability of 1.5 GPa grade dual-phase steel. Description of Drawings

[0020] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the accompanying drawings required for the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative work.

[0021] Figure 1 It is a typical microscopic scanning structure diagram of the 1.5 GPa grade dual-phase steel prepared in Example 1. DETAILED DESCRIPTION OF EMBODIMENTS

[0022] 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 of the 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 protection scope of the present invention.

[0023] Examples A preparation method of a 1.5 GPa grade dual-phase steel with stable mechanical properties, wherein the chemical composition of the steel billet used in mass percentage is: C: 0.19%~0.22%, Mn: 2.7%~3.0%, Si: 0.50%~0.70%, Alt: 0.80%~1.20%, Cr: 0.40%~0.80%, Nb: 0.045%~0.065%, Cu: 0.7%~0.9%, Ni: 0.25%~0.60%, B: 0.0025%~0.0040%, P≤0.005%, S≤0.005%, O≤0.005%, N≤0.005%, with the balance being Fe and other unavoidable impurities; Meanwhile, the chemical composition of the steel billet further satisfies 0.15 < Cr / Mn < 0.28 and 3.1% < Cr+Mn < 3.7%, 1.2 < Alt / Si < 1.9 and 1.4% < Alt+Si < 1.9%, 0.35 < Ni / Cu < 0.68 and 0.95% < Ni+Cu < 1.5%.

[0024] The preparation method comprises the following steps: (1) heating a steel billet to a soaking temperature of 1230±15°C, holding the temperature for 50±10 min, controlling the starting rolling temperature to 1160±20°C and the finishing rolling temperature to 880±15°C, controlling the hot rolling reduction rates of the last three passes to be >45%, >35% and >30% respectively, so that a steel plate with a thickness of 4.0~8.0 mm is obtained by hot rolling; after rolling, cooling the steel plate to 570±20°C at a cooling rate of 25~35°C / s, placing the steel plate into a 580°C holding furnace for heat preservation with a holding time of 60±10 min, and then cooling the steel plate to room temperature along with the furnace; (2) The hot-rolled steel plate obtained in step (1) is placed in an annealing furnace at 670℃ and kept for 6~8h for one annealing; after pickling, it is cold-rolled once, and the cumulative reduction rate of cold rolling is controlled at 55%~60%, and the reduction rates of the last two cold rolling passes are controlled at >18% and >14% respectively. The thickness of the cold-rolled hard strip steel is 1.8~3.4mm. (3) The cold-hardened strip steel obtained in step (2) is placed in an annealing furnace at 670℃ and kept at 6~8h for secondary annealing; after annealing, it is subjected to secondary cold rolling, and the cumulative reduction rate of cold rolling is controlled at 40%~45%, and the reduction rates of the last two cold rolling passes are controlled at >15% and >12% respectively. The thickness of the cold-hardened strip steel obtained by cold rolling is 1.0~2.0mm. (4) The cold-hardened strip steel obtained in step (3) is subjected to continuous annealing process. The annealing temperature is 840±10℃ and the annealing time is 120±30s. 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 285±15℃ at a cooling rate of >40℃ / s. The over-aging isothermal time is 240±20s. Finally, the steel plate is air-cooled to room temperature to obtain the 1.5GPa grade duplex steel of the present invention.

[0025] In specific embodiments 1 to 7, the above method was used to prepare 1.5 GPa grade duplex steel. The chemical composition of the molten steel produced from the billet is shown in Table 1, and the specific parameters of each step are shown in Tables 2 to 4.

[0026] Table 1 Actual smelting composition of Examples 1-7 (balance is Fe and other unavoidable impurities, unit: wt%)

[0027] Table 2 Specific parameters of step (1) in Examples 1-7

[0028] Table 3 Specific parameters of steps (2) and (3) in Examples 1-7

[0029] Table 4 Specific parameters of step (4) in Examples 1-7

[0030] Mechanical properties were tested on samples of the 1.5 GPa grade duplex steels prepared in Examples 1-7. The mechanical property testing methods were prepared in accordance with GB / T 228.1-2021 "Metallic materials, tensile testing - Part 1: Test at room temperature". A 50 Tensile specimens were prepared, and their 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 The test results are detailed in Table 5.

[0031] Table 5. Mechanical property test results of products in Examples 1-7

[0032] As shown in Table 5, the 1.5GPa grade duplex steel prepared in Examples 1 to 7 of this invention has high performance stability. Its yield strength ratio is stably controlled at 0.60±0.015, its yield strength is stably controlled at 925±25MPa, its tensile strength is stably controlled at 1565±30MPa, its elongation after fracture is >14%, and its hole expansion rate λ≥28%.

[0033] Microstructure analysis was performed on the 1.5 GPa grade duplex steels prepared in Examples 1-7, and the volume fraction of each phase was calculated. The results showed that the microstructure of the 1.5 GPa grade duplex steel products prepared in Examples 1-7 consisted of ferrite, martensite, and a small amount of retained austenite, with the volume fraction of ferrite being approximately 10%-32%, the volume fraction of martensite being approximately 60%-82%, and the volume fraction of retained austenite being approximately 8%-20%.

[0034] 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 a mechanically stable 1.5 GPa level duplex steel, characterized in that, The chemical composition of the steel billet used, by mass percentage, is: C: 0.19%~0.22%, Mn: 2.7%~3.0%, Si: 0.50%~0.70%, Alt: 0.80%~1.20%, Cr: 0.40%~0.80%, Nb: 0.045%~0.065%, Cu: 0.7%~0.9%, Ni: 0.25%~0.60%, B: 0.0025%~0.0040%, P≤0.005%, S≤0.005%, O≤0.005%, N≤0.005%, with the balance being Fe and other unavoidable impurities; The preparation method comprises the following steps: (1) Heating the steel billet to a soaking temperature of 1230±15°C, holding for 50±10min, controlling the starting rolling temperature to be 1160±20°C and the finishing rolling temperature to be 880±15°C; after rolling, cooling the steel plate to 570±20°C, holding for 60±10min, then cooling the steel plate to room temperature along with the furnace; (2) Placing the hot-rolled steel plate obtained in step (1) into an annealing furnace at 670°C, holding for 6~8h for primary annealing; subjecting to primary cold rolling after pickling; (3) Placing the work-hardened strip steel obtained in step (2) into an annealing furnace at 670°C, holding for 6~8h for secondary annealing; subjecting to secondary cold rolling after annealing; (4) Subjecting the work-hardened strip steel obtained in step (3) to continuous annealing treatment, wherein the annealing soaking temperature is 840±10°C, and the annealing time is 120±30s; 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 285±15°C at a cooling rate of >40°C / s, wherein the overaging isothermal time is 240±20s; finally air-cooling the steel plate to room temperature, and the target product is obtained; The chemical composition of the steel billet used satisfies 0.15 < Cr / Mn < 0.28 and 3.1% < Cr+Mn < 3.7%, 1.2 < Alt / Si < 1.9 and 1.4% < Alt+Si < 1.9%, 0.35 < Ni / Cu < 0.68 and 0.95% < Ni+Cu < 1.5%; In step (2), the cumulative cold rolling reduction is controlled at 55%~60%, the cold rolling reductions of the last two passes are controlled to be >18% and >14% respectively, and the thickness of the work-hardened strip steel obtained by cold rolling is 1.8~3.4mm; In step (3), the cumulative cold rolling reduction is controlled at 40%~45%, the cold rolling reductions of the last two passes are controlled to be >15% and >12% respectively, and the thickness of the work-hardened strip steel obtained by cold rolling is 1.0~2.0mm.

2. The preparation method according to claim 1, characterized in that, In step (1), the hot rolling reductions of the last three passes are controlled to be >45%, >35% and >30% respectively, and the thickness of the steel plate obtained by hot rolling is 4.0~8.0mm.

3. The preparation method according to claim 1, characterized in that, In step (1), after hot rolling, the steel plate is cooled to 570±20°C at a cooling rate of 25~35°C / s.

4. A 1.5 GPa grade dual-phase steel prepared by the preparation method according to any one of claims 1 to 3.

5. The 1.5GPa level duplex steel as described in claim 4, characterized in that, The yield strength ratio of 1.5GPa grade duplex steel is stably controlled at 0.60±0.015, the yield strength is stably controlled at 925±25MPa, the tensile strength is stably controlled at 1565±30MPa, the elongation after fracture is >14%, and the hole expansion rate λ≥28%.

6. The 1.5GPa level duplex steel as described in claim 4, characterized in that, The microstructure of 1.5GPa grade duplex steel includes ferrite, martensite and retained austenite.

7. The 1.5GPa level duplex steel as described in claim 4, characterized in that, The thickness of 1.5GPa grade duplex steel is 1.0~2.0mm.

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