A 1.5GPa grade Q&P steel with stable mechanical properties and its preparation method

Abstract

This invention relates to the field of high-strength steel technology for automobiles, specifically to a 1.5GPa grade Q&P steel with stable mechanical properties and its preparation method. The steps are as follows: S1: Heat and hold the steel billet at a certain temperature, then forge; S2: Heat and hold the forged billet at a certain temperature, then hot-roll; S3: Perform a first annealing, pickling, and first cold rolling on the hot-rolled steel sheet; S4: Perform a second annealing and second cold rolling on the chilled hardened strip; S5: Perform continuous annealing on the chilled hardened strip; cool to a slow cooling temperature and then rapidly cool to 335±15℃, then heat to the distribution temperature using resistance induction heating; finally, quench the steel sheet to room temperature. The 1.5GPa grade Q&P steel prepared by this invention has a stable yield strength ratio of 0.63±0.015, a stable yield strength of 990±25MPa, a stable tensile strength of 1550±30MPa, an elongation after fracture >12.5%, a porosity λ≥30%, and a strength-ductility product >20GPa·%.

Description

Technical Field

[0001] This invention relates to the field of high-strength steel technology for automobiles, specifically to a 1.5GPa grade Q&P steel with stable mechanical properties and its preparation method. Background Technology

[0002] Q&P steel (quenched-partitioned steel) has been a hot topic in the research of high-strength steel for automobiles in the past decade. It has been recognized by steel companies and the automotive industry as a typical representative of the third generation of advanced high-strength steel. Numerous research groups and researchers from well-known steel companies around the world have conducted detailed and in-depth research on this type of steel, from theoretical research to process simulation, small-batch industrial trial production, and user application.

[0003] When the strength of ultra-high strength Q&P steel increases to 1.5 GPa, the proportion of hard bainite in the microstructure increases significantly, and the various performance indicators of the material become extremely unstable. This makes it difficult to meet the formability and plasticity requirements of automobile manufacturers for ultra-high strength steel. Therefore, how to improve the performance stability of the produced ultra-high strength Q&P steel products is a pressing technical problem that needs to be solved.

[0004] Based on currently available patents, no technology has yet been proposed that can improve the performance stability of Q&P steel at the 1.5GPa level. Summary of the Invention

[0005] To address the technical problem of unstable material performance indicators when the strength of ultra-high strength Q&P steel is increased to 1.5 GPa, this invention provides a 1.5 GPa level Q&P steel with stable mechanical properties and its preparation method.

[0006] In a first aspect, the present invention provides a method for preparing 1.5GPa grade Q&P steel with stable mechanical properties, comprising the following steps:

[0007] S1: Heat and hold the steel billet at a certain temperature, then forge it;

[0008] S2: The forging billet is heated and held at a certain temperature, and then hot-rolled to obtain a steel plate with a thickness of 3.5~6.5mm. The hot-rolled steel plate is cooled to 540±20℃ and held at that temperature for 120±10min, and then air-cooled to room temperature.

[0009] S3: The hot-rolled steel plate is held at 690℃ for 6~8h for one annealing; then pickled and cold-rolled once to obtain cold-hardened strip steel with a thickness of 1.8~3.4mm;

[0010] S4: The cold-hardened strip steel is held at 690℃ for 6~8h for secondary annealing; then it is cold-rolled again to obtain a cold-hardened strip steel with a thickness of 1.0~2.0mm;

[0011] S5: Continuously anneal the chilled strip steel. The annealing soaking temperature is 900 ± 10 °C, and the annealing time is 240 ± 30 s; cool it at a cooling rate of 5 - 8 °C / s to the slow cooling temperature of 690 ± 10 °C; then quickly cool it at a cooling rate > 50 °C / s to 335 ± 15 °C, and immediately heat it to the partitioning temperature of 415 ± 15 °C by resistance induction heating, with the partitioning time of 360 ± 30 s; finally, quench the steel plate to room temperature to obtain the product.

[0012] Furthermore, the steel billet adopts a C-Mn-Si composition system.

[0013] Furthermore, S1 specifically means heating the steel billet to 1230 ± 15 °C, holding it for 45 ± 15 min, and then forging the steel billet, controlling the final forging temperature ≥ 920 °C.

[0014] Furthermore, in S2, heat the forged billet to the soaking temperature of 1235 ± 15 °C, control the soaking section holding time to be 120 ± 20 min, the hot rolling starting temperature to be 1170 ± 20 °C, the hot rolling finishing temperature to be 880 ± 15 °C, and control the hot rolling reduction ratios of the last 3 passes to be > 40%, > 35% and > 30% respectively.

[0015] Furthermore, in S2, first cool the hot rolled steel plate to 540 ± 20 °C at a cooling rate of 30 °C / s, place it in a holding furnace at 550 °C for holding, with the holding time of 120 ± 10 min, and then air cool it to room temperature.

[0016] Furthermore, in S3, control the cumulative reduction ratio of the first cold rolling to be 45% - 55%, and control the cold rolling reduction ratios of the last 2 passes to be 14% - 16%.

[0017] Furthermore, in S4, control the cumulative reduction ratio of the second cold rolling to be 40% - 45%, and control the cold rolling reduction ratios of the last 2 passes to be 12% - 14%.

[0018] In the second aspect, the present invention provides a 1.5 GPa grade Q&P steel produced by the above preparation method. In terms of mass percentage, the chemical composition of the Q&P steel is C: 0.20% - 0.24%, Mn: 2.50% - 2.90%, Si: 0.60% - 0.80%, Alt: 0.80% - 1.20%, Cr: 0.50% - 0.80%, Mo: 0.25% - 0.30%, Nb: 0.025% - 0.035%, Cu: 0.7% - 0.9%, Ni: 0.25% - ০.60%, B: 0.0025% - 0.0040%. The four elements of P, S, O, and N are all not more than 0.005%, and satisfy 3.5 < Mn / Cr < 5.5, 0.50 ≤ Si / Alt ≤ 0.95, 7 < Mo / Nb < ११, 1.2 < Cu / Ni < 3.5, and the balance is Fe and other inevitable impurities.

[0019] Furthermore, the microstructure of the 1.5 GPa grade Q&P steel includes ferrite, bainite and retained austenite.

[0020] Furthermore, the 1.5 GPa grade Q&P steel has high performance stability. Its yield ratio is stably controlled at 0.63 ± 0.015, the yield strength is stably controlled at 990 ± 25 MPa, the tensile strength is stably controlled at 1550 ± 30 MPa, the elongation after fracture is > 12.5%, the hole expansion rate λ ≥ 30%, and the product of strength and plasticity is > 20 GPa·%.

[0021] The beneficial effects of the present invention are as follows:

[0022] (1) Based on the composition design of the C-Si-Mn series low-carbon steel, the present invention satisfies 3.5 < Mn / Cr < 5.5, 0.50 ≤ Si / Alt ≤ 0.95, 7 < Mo / Nb < 11, 1.2 < Cu / Ni < 3.5. Such composition regulation is more conducive to improving the performance stability of the 1.5 GPa grade Q&P steel.

[0023] (2) The present invention adds a small amount of Cu and Ni micro-alloying elements and regulates 1.2 < Cu / Ni < 3.5. On the one hand, it can significantly improve the corrosion resistance of the 1.5 GPa grade Q&P steel while controlling the alloy cost; on the other hand, it is beneficial for the TRIP effect to occur in 5% - 15% retained austenite existing in the matrix structure in the high-strain area. This not only improves the plasticity stability of the ultra-high strength Q&P steel, making the elongation after fracture > 12.5%; but also optimizes the strength-plasticity matching of the ultra-high strength Q&P steel, increases the stability of the product of strength and plasticity, and the product of strength and plasticity is > 20 GPa·%, thereby significantly enhancing the anti-collision ability of the ultra-high strength Q&P steel.

[0024] (3) The present invention adopts the preparation method of secondary cold rolling + annealing in the secondary non-recrystallization zone, which is particularly suitable for producing the 1.5 GPa grade Q&P steel with thinner specifications. The finished product thickness of this embodiment is 1.0 - 2.0 mm, but the present invention is not limited thereto. This process flow is more suitable for the batch, continuous and industrial production of ultra-high strength steel.

[0025] (4) The present invention ensures that the cold rolling reduction rates of the last two passes of the first cold rolling and the second cold rolling are respectively controlled at 14% - 16% and 12% - 14%, which refines the initial medium-temperature mixed grain structure before the continuous annealing process, eliminates the work hardening and residual stress of the strip steel to a certain extent, and at the same time helps to store the distortion energy in the grains, all of which are beneficial to improving the performance stability of the 1.5 GPa grade Q&P steel. Description of the Drawings

[0026] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the attached drawings required in the description of the embodiments or the prior art. Obviously, for those of ordinary skill in the art, without creative efforts, other attached drawings can also be obtained based on these drawings.

[0027] Figure 1 It is a typical microscopic scanning microstructure diagram of the 1.5GPa grade Q&P steel prepared in Example 1. Detailed implementation manners

[0028] In order to enable those skilled in the art to better understand the technical solutions in the present invention, the following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the attached drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the scope of protection of the present invention.

[0029] Embodiment

[0030] A 1.5GPa grade Q&P steel with stable mechanical properties, adopting a C-Mn-Si component system. In terms of mass percentage, the chemical components are C: 0.20% - 0.24%, Mn: 2.50% - 2.90%, Si: 0.60% - 0.80%, Alt: 0.80% - 1.20%, Cr: 0.50% - 0.80%, Mo: 0.25% - 0.30%, Nb: 0.025% - 0.035%, Cu: 0.7% - 0.9%, Ni: 0.25% - 0.60%, B: 0.0025% - 0.0040%. The four elements of P, S, O, and N are all not more than 0.005%, and 3.5 < Mn / Cr < 5.5, 0.50 ≤ Si / Alt ≤ 0.95, 7 < Mo / Nb < 11, 1.2 < Cu / Ni < 3.5 are satisfied. The balance is Fe and other inevitable impurities.

[0031] It is obtained according to the following preparation method:

[0032] S1: Heat the steel billet to 1230 ± 15 °C, keep it warm for 45 ± 15 min, and then forge the steel billet, controlling the final forging temperature ≥ 920 °C;

[0033] S2: Heat the forging billet to a homogenization temperature of 1235±15℃, control the holding time in the homogenization section to 120±20min, and then perform hot rolling. The initial hot rolling temperature is 1170±20℃, and the final hot rolling temperature is 880±15℃. Control the hot rolling reduction rate of the last three passes to >40%, >35%, and >30%, respectively, to obtain a steel plate with a thickness of 3.5~6.5mm. The hot-rolled steel plate is first cooled to 540±20℃ at a cooling rate of 30℃ / s, and then placed in a holding furnace at 550℃ for holding time of 120±10min, and then air-cooled to room temperature.

[0034] S3: The hot-rolled steel plate is placed in an annealing furnace at 690℃ and held for 6~8 hours for one annealing; then pickling and one cold rolling are performed. The cumulative reduction rate of the first cold rolling is controlled at 45%~55%, and the reduction rate of the last two cold rolling passes is controlled at 14%~16%, to obtain cold-hardened strip steel with a thickness of 1.8~3.4mm.

[0035] S4: The cold-hardened strip steel is placed in an annealing furnace at 690℃ and held for 6~8 hours for secondary annealing; then it is subjected to secondary cold rolling, with the cumulative reduction rate of the secondary cold rolling controlled at 40%~45%, and the reduction rate of the last two cold rolling passes controlled at 12%~14%, to obtain cold-hardened strip steel with a thickness of 1.0~2.0mm.

[0036] S5: The cold-hardened strip steel is subjected to continuous annealing treatment. The annealing temperature is 900±10℃ and the annealing time is 240±30s. It is then cooled to a slow cooling temperature of 690±10℃ at a cooling rate of 5~8℃ / s. Then it is rapidly cooled to 335±15℃ at a cooling rate of >50℃ / s. Subsequently, it is heated to a partitioning temperature of 415±15℃ by resistance induction heating for a partitioning time of 360±30s. Finally, the steel plate is quenched to room temperature to obtain the Q&P steel of the present invention.

[0037] In specific embodiments 1 to 8, 1.5 GPa grade Q&P steel was prepared using the above method. The chemical composition is shown in Table 1, and the specific parameters for each step are shown in Tables 2 to 4.

[0038] Table 1. Composition of Examples 1-8 (balance is Fe and other unavoidable impurities, unit: wt%)

[0039]

[0040] Table 2 Specific parameters of S1 and S2 in Examples 1-8

[0041]

[0042] Table 3 Specific parameters of S3 and S4 in Examples 1-8

[0043]

[0044] Table 4 Specific parameters of S5 in Examples 1-8

[0045]

[0046] Mechanical properties were tested on samples of the 1.5 GPa grade Q&P steel prepared in Examples 1-8. 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.

[0047] Table 5. Mechanical property test results of products in Examples 1-8

[0048]

[0049] As shown in Table 5, the 1.5GPa grade Q&P steel prepared in Examples 1-8 of this invention has high performance stability. Its yield strength ratio is stably controlled at 0.63±0.015, its yield strength is stably controlled at 990±25MPa, its tensile strength is stably controlled at 1550±30MPa, its elongation after fracture is >12.5%, its porosity λ≥30%, and its strength-ductility product is >20GPa·%.

[0050] Microstructure analysis was performed on the 1.5 GPa grade Q&P steels prepared in Examples 1-8, and the volume fraction of each phase was calculated. The results showed that the microstructure of the 1.5 GPa grade Q&P steel products prepared in Examples 1-8 consisted of ferrite, bainite and a small amount of retained austenite, with the volume fraction of bainite being about 80%-90%, the volume fraction of ferrite being about 5%-15%, and the volume fraction of retained austenite being about 5%-15%.

[0051] 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 1.5GPa grade Q&P steel with stable mechanical properties, characterized in that, It includes the following steps: S1: Heat and hold the steel billet, and then forge it. S2: Heat and hold the forged billet, perform hot rolling to obtain a steel plate with a thickness of 3.5 - 6.5 mm. Cool the hot-rolled steel plate to 540 ± 20 °C and hold for 120 ± 10 min, and then air-cool to room temperature. S3: Hold the hot-rolled steel plate at 690 °C for 6 - 8 h for primary annealing; then perform pickling and primary cold rolling to obtain a cold-rolled strip steel with a thickness of 1.8 - 3.4 mm. S4: Hold the cold-rolled strip steel at 690 °C for 6 - 8 h for secondary annealing; then perform secondary cold rolling to obtain a cold-rolled strip steel with a thickness of 1.0 - 2.0 mm. S5: Perform continuous annealing treatment on the cold-rolled strip steel. The annealing soaking temperature is 900 ± 10 °C, and the annealing time is 240 ± 30 s; cool at a cooling rate of 5 - 8 °C / s to the slow cooling temperature of 690 ± 10 °C; then quickly cool at a cooling rate of >50 °C / s to 335 ± 15 °C, and immediately heat by resistance induction to the partitioning temperature of 415 ± 15 °C, and the partitioning time is 360 ± 30 s; finally quench the steel plate to room temperature to obtain it. The cold rolling reduction rate of the last 2 passes of the primary cold rolling is controlled at 14% - 16%. The cold rolling reduction rate of the last 2 passes of the secondary cold rolling is controlled at 12% - 14%. By mass percentage, the chemical composition of the Q&P steel is C: 0.20% - 0.24%, Mn: 2.50% - 2.90%, Si: 0.60% - 0.80%, Alt: 0.80% - 1.20%, Cr: 0.50% - 0.80%, Mo: 0.25% - 0.30%, Nb: 0.025% - 0.035%, Cu: 0.7% - 0.9%, Ni: 0.25% - 0.60%, B: 0.0025% - 0.0040%. The four elements of P, S, O, and N are all not more than 0.005%, and it satisfies 3.5 < Mn / Cr < 5.5, 0.50 ≤ Si / Alt ≤ 0.95, 7 < Mo / Nb < 11, 1.2 < Cu / Ni < 3.5, and the balance is Fe and other inevitable impurities.

2. The preparation method according to claim 1, characterized in that, Specifically, in S1, heat the steel billet to 1230 ± 15 °C, hold for 45 ± 15 min, and then forge the steel billet, controlling the final forging temperature ≥920 °C.

3. The preparation method according to claim 1, characterized in that, In S2, heat the forged billet to the soaking temperature of 1235 ± 15 °C, control the soaking section holding time to be 120 ± 20 min, the hot rolling starting rolling temperature is 1170 ± 20 °C, the hot rolling final rolling temperature is 880 ± 15 °C, and control the hot rolling reduction rates of the last 3 passes to be >40%, >35% and >30% respectively.

4. The preparation method according to claim 1, characterized in that, In S2, first cool the hot-rolled steel plate to 540 ± 20 °C at a cooling rate of 30 °C / s, place it in a holding furnace at 550 °C for holding, the holding time is 120 ± 10 min, and then air-cool to room temperature.

5. The preparation method according to claim 1, characterized in that, In S3, the cumulative cold rolling reduction rate of the primary cold rolling is controlled at 45% - 55%.

6. The preparation method according to claim 1, characterized in that, In S4, the cumulative cold rolling reduction rate of the secondary cold rolling is controlled at 40% - 45%.

7. The 1.5 GPa grade Q&P steel produced by the preparation method according to any one of claims 1 to 6, characterized in that, The microstructure of the Q&P steel includes ferrite, bainite and retained austenite.

8. The 1.5GPa grade Q&P steel as described in claim 7, characterized in that, The yield strength ratio of Q&P steel is stably controlled at 0.63±0.015, the yield strength is stably controlled at 990±25MPa, the tensile strength is stably controlled at 1550±30MPa, the elongation after fracture is >12.5%, the expansion rate λ≥30%, and the strength-ductility product is >20GPa.

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