An economical 1180MPa grade hot-rolled Q&P steel and its production method

Abstract

The invention relates to an economical 1180MPa hot-rolled Q&P steel and a production method thereof, wherein the chemical components of the economical 1180MPa hot-rolled Q&P steel are as follows by weight percentage: C 0.18%-0.21%, Si≤0.12%, Mn 1.70%-1.90%, P 0.050%-0.065%, S≤0.005%, Al 0.02%-0.05%, Nb 0.03%-0.05%, V 0.025%-0.045%, Ti 0.010%-0.030%, B 0.0015%-0.003%, N≤0.006%, and the balance is Fe and unavoidable impurities. The surface quality of the steel plate is optimized, the smelting difficulty is reduced, and the production cost is reduced.

Description

Technical Field

[0001] The present invention belongs to the technical field of steel rolling, and specifically relates to an economical 1180MPa hot-rolled Q&P steel and a production method thereof. The 1180MPa hot-rolled Q&P steel 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 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 makes the smelting difficulty and cost high; the patent document with publication number CN105177415A proposes an ultra-high strength hot-rolled Q&P steel and its production method, with 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%. After heating, hot rolling and cooling, the ingot obtains the organizational structure of martensite + residual austenite. The tensile strength of the steel plate is 1220-1520MPa, and the elongation is 14.5%-21.2%. It has a high strength-ductility product. However, 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 CN The patent document 108504925A 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 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 flow of this method 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 public 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 an economical 1180MPa grade hot-rolled Q&P steel. Summary of the invention

[0005] The present invention provides an economical 1180MPa grade hot-rolled Q&P steel and a production method thereof, which optimizes the surface quality of steel plates, reduces smelting difficulty, and reduces production costs.

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

[0007] The invention discloses an economical 1180MPa grade hot-rolled Q&P steel. The chemical components of the steel are as follows by weight percentage: C 0.18%-0.21%, Si≤0.12%, Mn 1.70%-1.90%, P 0.050%-0.065%, S≤0.005%, Al 0.02%-0.05%, Nb 0.03%-0.05%, V 0.025%-0.045%, Ti 0.010%-0.030%, B 0.0015%-0.003%, N≤0.006%, and the balance is Fe and unavoidable impurities.

[0008] The steel plate structure is composed of bainite, martensite and retained austenite, wherein the volume fraction of bainite is 8% to 12%, the volume fraction of martensite is 75% to 83%, and the volume fraction of retained austenite is 9% to 13%;

[0009] The tensile strength of the steel plate is 1180-1300MPa, the yield strength is 780-880MPa, the elongation is ≥20.0%, and the hole expansion rate is ≥40%;

[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 0.18%-0.21%.

[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 1.70%-1.9% in the present invention.

[0015] P: Phosphorus plays two main roles in this patent. 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 0.05%-0.065%.

[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 main roles 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 comprehensive properties such as strength and toughness. In the present invention, the Nb content is 0.030-0.050%, and the V content is 0.025-0.045%.

[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 content of Ti in the present invention is 0.010-0.030%.

[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 0.0015-0.0030%.

[0021] 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%.

[0022] A method for producing an economical 1180MPa grade hot-rolled Q&P steel comprises the following steps:

[0023] 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%;

[0024] 2) Heating process: heat the ingot with a thickness of 150-230 mm to 1220±20°C and keep it warm for 1-2 hours to ensure uniform distribution of alloy elements and control the original austenite grain size;

[0025] 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 500-530℃ and then air-cooled for 6-8s, and then rapidly cooled, and the cooling rate is greater than 70℃ / s. The steel plate is cooled to 230-280℃, then coiled and air-cooled to room temperature.

[0026] 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 and Mn 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 ingot 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; the hot rolling process adopts a segmented cooling system of rapid cooling + air cooling + rapid cooling, and the steel plate is air-cooled at 500-530°C to produce a certain amount of bainite in the structure, and finally a three-phase structure of bainite + martensite + residual austenite is obtained, and the steel plate has good strength, plasticity and hole expansion performance matching.

[0027] Compared with the prior art, the present invention has the following beneficial effects:

[0028] 1) The steel plate composition does not contain precious metal elements such as Mo, and does not contain a large amount of elements such as Al and Mn that increase the difficulty and cost of smelting; compared with the production of cold-rolled Q&P steel, the production process used is simple, without cold rolling, annealing and rapid heating processes. Therefore, compared with the existing production method, the hot-rolled Q&P steel and its production method proposed in the present invention have low alloy cost and production cost, and are easy to produce;

[0029] 2) Low Si content design is adopted, with Si content ≤ 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] 3) 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 1180MPa, while the elongation is ≥20% and the hole expansion rate is ≥40%. 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 production method of the present invention is as follows: the casting billet adopts the RH+LF double process in the refining process, strictly controls the H and O contents, H≤0.0002%, O≤0.0015%, forms a continuous casting billet with a thickness of 150mm-230mm after continuous casting, heats the continuous casting billet to 1200-1240°C, keeps the temperature for 1-2 hours and then rolls it, the final rolling temperature of the finishing rolling is 900-940°C, adopts a cooling mode of rapid cooling+air cooling+rapid cooling after the final rolling, the rapid cooling cooling rate is greater than 50°C / s, cools the steel plate to 500-530°C, air cools it for 6-8s, then rapidly cools it, the cooling rate is greater than 70°C / s, cools the steel plate to 230-280°C, coils it and air cools it to room temperature, and 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]

Claims

1. An economical 1180MPa grade hot-rolled Q&P steel, characterized in that: The chemical composition of the steel is calculated by weight percentage: C0.18%~0.21%, Si≤0.12%, Mn 1.70%~1.90%, P 0.050%~0.065%, S≤0.005%, Al 0.02%~0.05%, Nb 0.03%~0.05%, V 0.025%~0.045%, Ti 0.010%~0.030%, B 0.0015%~0.003%, N≤0.006%, the balance is Fe and unavoidable impurities; The method for producing the economical 1180MPa grade hot-rolled Q&P steel comprises the following steps: 1) Smelting process: The refining process adopts RH+LF double process during smelting, and controls H≤0.0002%, O≤0.0015%; 2) Heating process: Heat the ingot to 1220±20℃ 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 500-530℃, air cooled for 6-8s, and then rapid cooling is carried out, the cooling rate is greater than 70℃ / s, the steel plate is cooled to 230-280℃, then coiled and air cooled to room temperature; The steel plate structure is composed of bainite, martensite and retained austenite, of which the volume fraction of bainite is 8% to 12%, the volume fraction of martensite is 75% to 83%, and the volume fraction of retained austenite is 9% to 13%; The tensile strength of the steel plate is 1180-1300MPa, the yield strength is 780-880MPa, the elongation is ≥20.0%, and the hole expansion rate is ≥40%.

2. The economical 1180MPa grade hot-rolled Q&P steel according to claim 1, characterized in that: The thickness of the finished steel plate is 2.0~5.0mm.

3. A method for producing an economical 1180 MPa grade hot-rolled Q&P steel as claimed in claim 1 or 2, characterized in that: The following steps are involved: 1) Smelting process: The refining process adopts RH+LF double process during smelting, and controls H≤0.0002%, O≤0.0015%; 2) Heating process: Heat the ingot to 1220±20℃ and keep it warm for 1-2 hours; 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 500-530℃ and then air-cooled for 6-8s, and then rapidly cooled, and the cooling rate is greater than 70℃ / s. The steel plate is cooled to 230-280℃, then coiled and air-cooled to room temperature.

4. The method for producing an economical 1180MPa grade hot-rolled Q&P steel according to claim 3, characterized in that: In the above step 2), the thickness of the ingot is 150-230 mm.

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