Ultra-high strength hot-rolled Q&P steel with excellent hole expansion performance and production method thereof

Abstract

The present invention relates to an ultra-high strength hot-rolled Q&P steel with excellent hole expansion performance and a production method thereof. The chemical components in the steel are calculated by weight percentage as follows: C 0.25%-0.28%, Si≤0.12%, Mn 1.80%-2.0%, P 0.06%-0.075%, S≤0.005%, Al 0.02%-0.05%, Nb 0.03%-0.07%, V 0.03%-0.06%, Ti 0.010%-0.040%, B0.002%-0.004%, rare earth elements La+Ce 0.030%-0.050%, N≤0.005%, and the balance is Fe and inevitable impurities. The steel plate strength of the present invention reaches more than 1550MPa, has good steel plate surface quality, and has good strength, plasticity and hole expansion performance matching.

Description

Technical Field

[0001] The present invention belongs to the technical field of steel rolling, and specifically relates to an ultra-high strength hot-rolled Q&P steel with excellent hole expansion performance and a production method thereof. The ultra-high strength hot-rolled Q&P steel with excellent hole expansion performance of the present invention is mainly suitable for manufacturing automotive parts with high safety requirements. Background Art

[0002] Q&P steel is a kind of third-generation advanced high-strength steel for automobiles. It is mainly produced by quenching and partitioning process, and the volume fraction of carbon-rich residual austenite and martensite at room temperature is controlled to produce high-strength steel with TRIP effect, high strength and high plasticity (toughness). It is mainly used in automobile safety parts, structural parts, etc., and has good formability and collision safety. At present, Q&P steel is mainly produced by cold rolling heat treatment method, that is, the hot-rolled raw material is cold-rolled and then reheated to the austenite zone for insulation, and then quenched and partitioned to obtain Q&P steel plate. The advantage of this method is that the production process is stable and easy to control, but the production efficiency is low and the cost is high. At the same time, the thickness of the steel plate produced is generally less than 2mm, which has great limitations. With the development of technology, the method of producing Q&P steel by hot rolling has gradually begun to increase.

[0003] The patent document with publication number CN103805851B proposes an ultra-high strength and low-cost hot-rolled Q&P steel and a manufacturing method thereof. The hot-rolled steel plate proposed has an ultra-high strength of more than 1400MPa and an elongation of more than 10%, and the thickness can reach 12mm. At the same time, it has a low alloy cost, but the Si content in the composition is 0.8%-2.0%, and the surface quality of the produced steel plate is poor, which affects the subsequent processing and use; the patent document with publication number CN104532126B proposes a low yield ratio ultra-high strength hot-rolled Q&P steel and a manufacturing method thereof. The tensile strength of the steel plate can reach more than 1300MPa, the yield strength ratio is less than 0.50, and the structure of the steel plate is composed of ferrite, martensite and residual austenite, which has good strength-plasticity matching, but the Si content in the steel plate is 1.0-2.0%, the surface quality of the steel plate is poor, and the steel contains 0.5%-1.0% Al, which is difficult and costly to smelt; the patent document with publication number CN105177415A proposes an ultra-high strength hot-rolled Q&P steel and its production method, in which the composition has C: 0.14-0.45%, Si: 0.12-2. 0%, Mn: 1.0-5.0%, S≤0.010%, P≤0.015%, H≤40ppm, Al: 0.02-1.5%, Nb: 0.01-0.08%, the ingot is heated, hot rolled and cooled to obtain the organizational structure of martensite + residual austenite, the tensile strength of the steel plate is 1220-1520MPa, the elongation is 14.5%-21.2%, and it has a high strength-ductility product, but the steel plate has a high content of Si, Mn and Al elements, which deteriorates the surface quality of the steel plate, increases the difficulty of smelting, and has a high cost; the publication number is C The patent document N108504925A proposes a short-process hot-rolled Q&P steel plate and its preparation method. The chemical composition of the steel plate is C: 0.15-0.23%, Si: 0.50-1.0%, Mn: 1.0-2.0%, Al: 0.03-0.06%, Nb: 0.03-0.06%, S≤0.005%, P≤0.02%, N≤0.006%. The ingot is heated and hot rolled and cooled. After the final rolling, it is quickly cooled to 330-420℃ for partitioning. The partitioning time is 10-60s, and then quenched to room temperature. The structure of the steel plate is martensite + residual austenite, the tensile strength can reach more than 1000MPa, and the elongation is higher than 23.5%. The overall process of the patent is short and the alloy cost is low, but the Si content in the composition is high, which affects the surface quality. At the same time, the steel plate needs to be quenched after the partitioning is completed, and the conventional industrial production line cannot meet the manufacturing requirements.

[0004] The above-mentioned prior arts all adopt hot rolling method to produce high-strength Q&P steel, but the composition contains relatively high Si, Al, Mn and other elements, which deteriorates the surface quality of the steel plate, increases the difficulty of smelting, has high cost, and limits its production and application scope. At the same time, the strength of the proposed steel plate is basically below 1500MPa, and the hole expansion performance of the steel plate is not proposed. In view of the above situation, the development of ultra-high strength hot-rolled Q&P steel with excellent hole expansion performance is conducive to solving the above problems and meeting the production and use requirements of ultra-high strength automotive parts with hole expansion process in the production process. Summary of the invention

[0005] The present invention provides an ultra-high strength hot-rolled Q&P steel with excellent hole expansion performance and a production method thereof. The strength of the steel plate reaches above 1550MPa. At the same time, the steel plate has good matching of strength, plasticity and hole expansion performance, and the steel plate has good surface quality.

[0006] In order to achieve the above object, the present invention adopts the following technical solutions:

[0007] The invention discloses an ultra-high strength hot-rolled Q&P steel with excellent hole expansion performance. The chemical composition of the steel is as follows by weight percentage: C 0.25%-0.28%, Si≤0.12%, Mn 1.80%-2.0%, P 0.06%-0.075%, S≤0.005%, Al 0.02%-0.05%, Nb 0.03%-0.07%, V 0.03%-0.06%, Ti 0.010%-0.040%, B 0.002%-0.004%, rare earth elements La+Ce 0.030%-0.050%, N≤0.005%, and the balance is Fe and unavoidable impurities.

[0008] The steel plate structure consists of bainite, martensite and retained austenite, wherein the volume fraction of bainite is 20% to 30%, the volume fraction of martensite is 55% to 70%, and the volume fraction of retained austenite is 10% to 15%.

[0009] The tensile strength of the steel plate is greater than 1550MPa, the yield strength is greater than 1350MPa, the elongation is ≥14.0%, the yield strength ratio is ≥0.85, and the hole expansion rate is ≥50%.

[0010] The thickness of the finished steel plate is 2.0 to 5.0 mm, and there are no defects such as iron oxide stripes and color difference on the surface of the steel plate.

[0011] The main functions of the steel plate components of the present invention are:

[0012] C: Carbon is the guarantee of steel plate strength. At the same time, after coiling, C in martensite diffuses into the retained austenite, improving the stability of austenite and increasing the percentage of retained austenite, thereby improving the plasticity of the steel plate. If the C content is too high, the ferrite content generated in the air cooling process in the structure is insufficient, the strength of the steel plate is too high, and the elongation is low; if the C content is too low, the martensite content formed is too low, and the residual austenite stability is poor, which is easy to reduce the strength and plasticity of the steel plate. The optimal range of C in the present invention is 0.25%-0.28%.

[0013] Si: Silicon can expand the α+γ region of the Fe-C phase diagram, which is beneficial to promote the transformation of austenite to ferrite during air cooling, so that a certain content of ferrite can be obtained in the steel plate structure. At the same time, Si can inhibit the precipitation of cementite during the coiling air cooling process, ensure the distribution of carbon from martensite to residual austenite, and improve the stability of austenite. At present, a large amount of Si element is added to most Q&P steels, but a large Si content will greatly deteriorate the surface quality of the steel plate, affecting production and use. The steel plate composition proposed in the present invention adopts a low Si component design, and the Si content is ≤0.12%.

[0014] Mn: Manganese is an austenite stabilizing element in the present invention, which significantly improves the hardenability of steel, plays a role in solid solution strengthening and grain refinement, and can significantly delay pearlite transformation and bainite transformation. However, if the Mn content is too high, the precipitation of ferrite will be delayed and banded structure will easily appear in the steel plate. Therefore, the Mn content is selected to be 1.80%-2.0% in the present invention.

[0015] P: Phosphorus plays two main roles in the present invention. One is to inhibit the formation of cementite and increase the stability of austenite; the other is to make the size of martensite islands smaller and more evenly distributed. The P content cannot be too high. If the content is too high, grain boundary segregation will occur easily, resulting in a sharp decrease in the toughness of the steel plate. The C and Nb elements in the composition can effectively reduce the grain boundary segregation of the P element. The phosphorus content in the present invention is 0.06%-0.075%.

[0016] S: Sulfur is an impurity element in the present invention, and is easy to form sulfide inclusions such as MnS in steel, thereby becoming a crack source and deteriorating the processing performance. Therefore, the lower the content, the better, but too low a content will increase the smelting cost. In the present invention, the sulfur content is ≤0.005%.

[0017] Al: Aluminum plays a role in nitrogen and deoxidation in the present invention, but too much aluminum will lead to a large number of aluminum inclusions. The range of Al in the present invention is 0.02%-0.05%.

[0018] Nb, V: Nb and V play the role of grain refinement and precipitation strengthening in steel. In the present invention, the two can effectively pin the original austenite grain boundary through the dispersion and precipitation of nano-scale fine carbonitrides, thereby refining the various phase structures in the steel plate and improving the comprehensive properties such as strength and toughness. In the present invention, the Nb content is 0.030-0.070%, and the V content is 0.03-0.06%.

[0019] Ti: Ti is mainly used to fix nitrogen in the present invention to ensure that the hardening effect of boron can be exerted. In addition, Ti can also form fine carbides with C elements to precipitate, reduce the hardness and strength of martensite in the structure, and help improve the plasticity and toughness of the steel plate. The Ti content in the present invention is 0.010-0.040%.

[0020] B: Adding boron in the present invention can significantly improve the hardenability of steel and ensure the strength of the steel plate. When the B content is too low, the effect is not significant, while when the B content is too high, it is easy to form B compounds with N in the steel, reducing the performance of the steel plate. In the present invention, the B content is 0.002-0.0040%.

[0021] RE: Rare earth has strong deoxidation and desulfurization capabilities, and can change the morphology of sulfide inclusions, improve the plasticity and hole expansion performance of steel plates, and reduce anisotropy; rare earth can improve the fatigue performance of steel plates, improve the welding performance of steel plates, and improve the cold forming performance of steel plates. The content of rare earth (La+Ce) in the present invention is 0.03-0.05%.

[0022] N: The lower the N content, the better. However, too low a content of N will lead to production difficulties and increase costs. Therefore, in the present invention, the N content is ≤ 0.005%.

[0023] A method for producing ultra-high strength hot-rolled Q&P steel with excellent hole expansion performance, comprising the following steps:

[0024] 1) Smelting process: The RH+LF double process is used in the refining process during smelting, and the H and O contents are strictly controlled, H≤0.0002%, O≤0.0015%;

[0025] 2) Heating process: heat the ingot with a thickness of 150-230 mm to 1210-1250°C and keep it warm for 1-2 hours to ensure uniform distribution of alloy elements and control the original austenite grain size;

[0026] 3) Rolling and cooling process: two-stage controlled rolling is adopted, and the finishing rolling temperature is 900-940℃; after the final rolling, the cooling mode of rapid cooling + air cooling + rapid cooling is adopted, and the rapid cooling rate is greater than 40℃ / s. The steel plate is cooled to 480-520℃ and then air-cooled for 7-10s, and then rapidly cooled, and the cooling rate is greater than 60℃ / s. The steel plate is cooled to 220-280℃, then coiled and air-cooled to room temperature.

[0027] In the present invention, the steel plate adopts a low Si component design, which improves the surface quality of the steel plate, and can be applied to automobile parts with high surface quality requirements. The risk of cementite precipitation in the coiling and matching stage caused by low Si is eliminated by elements such as P and Mn; the composite addition of Nb, V and Ti elements in the structure is beneficial to refining the steel plate structure, improving plasticity, hole expansion performance and toughness, etc.; the addition of B element improves the hardenability of the steel plate and ensures the martensite content in the structure; the addition of RE changes the morphology of sulfide inclusions, which is beneficial to improving the hole expansion performance; the refining process of the casting is subjected to a double process treatment during the smelting process, and the H and O contents are strictly controlled to ensure the purity of the steel; the hot rolling process adopts a segmented cooling system of rapid cooling + air cooling + rapid cooling, and the steel plate is air-cooled at 480-520°C to produce a certain amount of bainite in the structure, and finally obtains a three-phase structure of bainite + martensite + residual austenite, and the steel plate has good strength, plasticity and hole expansion performance matching.

[0028] Compared with the prior art, the present invention has the following beneficial effects:

[0029] 1) Low Si content design is adopted, and the Si content is ≤0.12%, which is much lower than the percentage content of >1.0% in traditional processes, which can improve the surface quality of steel plates and is suitable for the use requirements of high surface quality automotive parts under existing process conditions;

[0030] 2) A cooling process of rapid cooling + air cooling + rapid cooling is adopted to obtain a three-phase structure of bainite + martensite + retained austenite, so that the strength of the steel plate reaches more than 1550MPa, while the elongation is ≥14%, the yield strength ratio is ≥0.85, and the hole expansion rate is ≥50%. DETAILED DESCRIPTION

[0031] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0032] The embodiments of the invention are as follows: the casting billet adopts the RH+LF double process in the refining process, and the H and O contents are strictly controlled, H≤0.0002%, O≤0.0015%, and a continuous casting billet with a thickness of 150mm-230mm is formed after continuous casting. The continuous casting billet is heated to 1210-1250°C, kept warm for 1-2 hours, and then rolled. The final rolling temperature of the finishing rolling is 900-940°C. After the final rolling, a cooling mode of rapid cooling+air cooling+rapid cooling is adopted, and the rapid cooling cooling rate is greater than 40°C / s. The steel plate is cooled to 480-520°C, air-cooled for 7-10s, and then rapidly cooled, and the cooling rate is greater than 60°C / s. The steel plate is cooled to 220-280°C, coiled and air-cooled to room temperature. The thickness of the finished steel plate is 2-5mm.

[0033] The specific composition, temperature regime, structure and properties of the steel plates of the six embodiments of the present invention are shown in Tables 1-3.

[0034] Table 1 Chemical composition of the present invention (wt, %)

[0035]

[0036] Table 2 Temperature system of the embodiment of the present invention

[0037]

[0038]

[0039] Table 3 Organization and mechanical properties parameters of the embodiments of the present invention

[0040]

Claims

1. An ultra-high strength hot-rolled Q&P steel with excellent hole expansion performance, characterized in that: The chemical composition of the steel is calculated by weight percentage: C 0.25%~0.28%, Si≤0.12%, Mn 1.80%~2.0%, P 0.06%~0.075%, S≤0.005%, Al 0.02%~0.05%, Nb 0.03%~0.07%, V 0.03%~0.06%, Ti 0.010%~0.040%, B 0.002%~0.004%, rare earth elements La+Ce 0.030%~0.050%, N≤0.005%, the balance is Fe and unavoidable impurities; The steel plate structure is composed of bainite, martensite and retained austenite, wherein the volume fraction of bainite is 20% to 30%, the volume fraction of martensite is 55% to 70%, and the volume fraction of retained austenite is 10% to 15%.

2. The ultra-high strength hot-rolled Q&P steel with excellent hole expansion performance according to claim 1, characterized in that: The tensile strength of the steel plate is greater than 1550MPa, the yield strength is greater than 1350MPa, the elongation is ≥14.0%, the yield strength ratio is ≥0.85, and the hole expansion rate is ≥50%.

3. The ultra-high strength hot-rolled Q&P steel with excellent hole expansion performance according to claim 1, characterized in that: The thickness of the finished steel plate is 2.0 to 5.0 mm.

4. A method for producing ultra-high strength hot-rolled Q&P steel with excellent hole expansion performance as claimed in any one of claims 1 to 3, characterized in that: The method comprises the following steps: 1) Smelting process: The refining process adopts RH+LF double process during smelting to control the H and O contents, H≤0.0002%, O≤0.0015%; 2) Heating process: heat the ingot to 1210-1250℃ and keep it warm for 1-2 hours; 3) Rolling and cooling process: adopt two-stage controlled rolling, and the finishing rolling temperature is 900-940℃; after the final rolling, adopt the cooling mode of rapid cooling + air cooling + rapid cooling, the rapid cooling cooling rate is greater than 40℃ / s, the steel plate is cooled to 480-520℃, air cooled for 7-10s, and then rapidly cooled, the cooling rate is greater than 60℃ / s, the steel plate is cooled to 220-280℃, coiled and air cooled to room temperature; in the above step 2), the thickness of the ingot is 150-230mm.

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