Ultra-high strength hot-rolled Q&P steel with good surface quality and production method thereof

Abstract

The present invention relates to an ultra-high strength hot-rolled Q&P steel with good surface quality and a production method thereof. The chemical components of the steel are as follows by weight percentage: C 0.22%-0.25%, Si≤0.10%, Mn 1.80%-2.0%, P 0.055%-0.070%, S≤0.005%, Al 0.02%-0.05%, Nb 0.04%-0.06%, V 0.04%-0.06%, Ti 0.020%-0.040%, Mo 0.25%-0.45%, B 0.0015%-0.003%, N≤0.006%, and the balance is Fe and inevitable impurities. The present invention enables the strength of the steel plate to reach more than 1400MPa, and the steel plate can have good plasticity and hole expansion performance matching, and at the same time, improves the surface quality of the steel plate.

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 good surface quality and a production method thereof. The ultra-high strength hot-rolled Q&P steel with high surface quality of the present invention is mainly suitable for manufacturing automotive parts with high requirements on safety and surface quality. 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 method for manufacturing the same. The hot-rolled steel plate has an ultra-high strength of more than 1400MPa and an elongation of more than 10%, and has a lower alloy cost. However, the Si content in the composition is 0.8%-2.0%, and the surface quality of the produced steel plate is poor, which affects 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 method for manufacturing the same. The tensile strength of the steel plate can reach more than 1300MPa, and the yield ratio is lower than 0.50. The structure of the steel plate consists of ferrite, martensite and residual austenite, and has a good strength-plasticity match. However, the Si content in the steel plate is 1.0-2.0%, and the surface quality of the steel plate is poor. At the same time, the steel contains 0.5%-1.0% Al, which makes smelting difficult. The patent document with publication number CN105177415A proposes an ultra-high strength hot-rolled Q&P steel and its production method. The automotive components include C: 0.14-0.45%, Si: 0.12-2.0%, Mn1.0-5.0%, S≤0.010%, P≤0.015%, H≤40ppm, Al: 0.02-1.5%, Nb: 0.01-0.08%. After heating, hot rolling and cooling, the ingot obtains a martensite + residual austenite structure. The tensile strength of the steel plate is 1220-1520MPa, and the elongation is 14.5%-21.2%. It has a high strength-plasticity product, but the steel plate has a high content of Si, Mn and Al elements, which deteriorates the surface quality of the steel plate and increases the difficulty of smelting. The patent document with the publication number CN108504925A 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 water quenched to room temperature. The structure of the obtained 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 existing technologies all use hot rolling 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 costs, and limits its production and application scope. In view of the above situation, it is imperative to develop high-surface quality ultra-high-strength hot-rolled Q&P steel. Summary of the invention

[0005] The present invention provides an 800MPa grade hot-rolled pickled Q&P steel and a production method thereof, so that the strength of the steel plate can reach above 1400MPa, and the steel plate can have good plasticity and hole expansion performance matching, and at the same time, the surface quality of the steel plate is improved.

[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 good surface quality. The chemical components of the steel are as follows by weight percentage: C 0.22%-0.25%, Si≤0.10%, Mn 1.80%-2.0%, P 0.055%-0.070%, S≤0.005%, Al 0.02%-0.05%, Nb 0.04%-0.06%, V 0.04%-0.06%, Ti 0.020%-0.040%, Mo 0.25%-0.45%, B 0.0015%-0.003%, N≤0.006%, and the balance is Fe and unavoidable impurities.

[0008] The steel plate structure consists of ferrite, martensite and retained austenite, wherein the volume fraction of ferrite is 5% to 10%, the volume fraction of martensite is 80% to 90%, and the volume fraction of retained austenite is 5% to 10%.

[0009] The tensile strength of the steel plate is 1400-1500 MPa, the yield strength is 1200-1300 MPa, the elongation is ≥17.0%, and the hole expansion rate is ≥30%.

[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 0.22%-0.25%.

[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 ≤0.10%.

[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 0.055%-0.070%.

[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 0.04-0.06%, and the V content is 0.04-0.06%.

[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 0.020-0.040%.

[0020] Mo: Mo is a medium-strong carbide-forming element that can improve the strength and toughness of steel plates; Mo can significantly improve the stability of retained austenite after coiling, so that a certain amount of retained austenite exists in the final structure, thereby enabling the steel plate to achieve the expected mechanical performance indicators. The Mo content in the present invention is 0.25%-0.45%.

[0021] 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 0.0015-0.0030%.

[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 ≤ 0.006%.

[0023] A method for producing ultra-high strength hot-rolled Q&P steel with good surface quality, 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 50℃ / s. The steel plate is cooled to 620-650℃ and then air-cooled for 5-8s, and then rapidly cooled, and the cooling rate is greater than 70℃ / s. The steel plate is cooled to 200-250℃, 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 P, Mn and Mo elements; 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 refining process of the cast billet during the smelting process is subjected to a double process treatment, and the H and O contents are strictly controlled to ensure the purity of the steel material; the hot rolling process adopts a segmented cooling system of rapid cooling + air cooling + rapid cooling, and the steel plate is air-cooled at 620-650°C to produce a certain amount of ferrite in the structure, and finally a three-phase structure of ferrite + martensite + residual austenite is obtained, 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 ≤0.10%, 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 ferrite + martensite + retained austenite, so that the strength of the steel plate reaches more than 1400MPa, while the elongation is ≥17.0% and the hole expansion rate is ≥30%. 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℃, kept warm for 1-2 hours, and then rolled. The final rolling temperature of the finishing rolling is 900-940℃. 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 50℃ / s. The steel plate is cooled to 620-650℃ and then air-cooled for 5-8s, and then rapidly cooled, and the cooling rate is greater than 70℃ / s. The steel plate is cooled to 200-250℃, then 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] Table 3 Organization and mechanical properties parameters of the embodiments of the present invention

[0039]

[0040]

Claims

1. An ultra-high strength hot-rolled Q&P steel with good surface quality, characterized in that: The chemical composition of the steel is calculated by weight percentage: C 0.22%~0.25%, Si ≤0.10%, Mn 1.80%~1.93%, P 0.055%~0.070%, S≤0.005%, Al0.02%~0.05%, Nb 0.04%~0.06%, V 0.04%~0.048%, Ti 0.020%~0.040%, Mo 0.31%~0.45%, B 0.0015%~0.003%, N≤0.006%, the balance is Fe and unavoidable impurities; The steel plate structure is composed of ferrite, martensite and retained austenite, of which the volume fraction of ferrite is 5% to 10%, the volume fraction of martensite is 80% to 90%, and the volume fraction of retained austenite is 5% to 10%; A method for producing steel, comprising 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, 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 rate is greater than 50℃ / s, the steel plate is cooled to 620-645℃, air cooled for 5-8s, and then rapid cooling is performed, the cooling rate is greater than 70℃ / s, the steel plate is cooled to 200-250℃, then coiled and air cooled to room temperature; The thickness of the ingot in the above step 2) is 150 to 230 mm.

2. The ultra-high strength hot-rolled Q&P steel with good surface quality according to claim 1, characterized in that: The tensile strength of the steel plate is 1400-1500MPa, the yield strength is 1200-1300MPa, the elongation is ≥17.0%, and the hole expansion rate is ≥30%.

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

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