A kind of 800MPa grade hot-rolled pickled Q&P steel and its production method

Abstract

The present invention relates to a 800MPa grade hot-rolled pickled Q&P steel and a production method thereof. The chemical components of the steel are as follows by weight percentage: C 0.14% to 0.16%, Si≤0.10%, Mn 1.40% to 1.60%, P 0.045% to 0.065%, S≤0.005%, Al 0.02% to 0.05%, Nb 0.02% to 0.04%, V 0.015% to 0.035%, Mo 0.15% to 0.30%, N≤0.006%, and the balance is Fe and unavoidable impurities. The present invention aims to improve the plasticity of the steel plate and improve the surface quality of the steel plate under the premise of ensuring the mechanical properties of the steel plate.

Description

Technical Field

[0001] The invention belongs to the technical field of steel rolling, and specifically relates to an 800MPa-grade hot-rolled pickled Q&P steel and a production method thereof. The 800MPa-grade hot-rolled pickled Q&P steel of the 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.

[0004] The above existing technologies all use hot rolling to produce high-strength Q&P steel, but because the composition contains a high Si element, the surface quality of the steel plate is deteriorated, limiting its application range. At the same time, the composition contains a high Al, Mn and other elements, which increases the difficulty of smelting. In view of the above situation, it is imperative to develop easy-to-produce hot-rolled pickled surface high-strength 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, aiming to provide a steel plate with high strength while having high plasticity and good surface quality.

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

[0007] The invention discloses an 800MPa grade hot-rolled pickled Q&P steel, wherein the chemical components of the steel are as follows by weight percentage: C 0.14%-0.16%, Si≤0.10%, Mn 1.40%-1.60%, P 0.045%-0.065%, S≤0.005%, Al 0.02%-0.05%, Nb0.02%-0.04%, V 0.015%-0.035%, Mo 0.15%-0.30%, N≤0.006%, and the balance is Fe and unavoidable impurities.

[0008] The steel plate structure is composed of ferrite, martensite and retained austenite, wherein the volume fraction of ferrite is 13% to 17%, the volume fraction of martensite is 69% to 76%, and the volume fraction of retained austenite is 11% to 14%.

[0009] The tensile strength of the steel plate is 800-880 MPa, the yield strength is 500-580 MPa, and the elongation is ≥30.0%.

[0010] The thickness of the finished steel plate is 2.0-6.0mm.

[0011] The surface of the steel plate is a pickling surface without defects such as iron oxide stripes, color difference and cracks. The surface roughness Ra of the steel plate is 0.8~1.5μm.

[0012] The main functions of the steel plate components of the present invention are:

[0013] 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 0.14%-0.16%.

[0014] 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%.

[0015] 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 1.40%-1.6% in the present invention.

[0016] 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 0.045%-0.065%.

[0017] 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%.

[0018] 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%.

[0019] 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 0.020-0.040%, and the V content is 0.015-0.035%.

[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 0.15-0.30%.

[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 800MPa grade hot-rolled pickled Q&P steel comprises the following steps:

[0023] 1) Heating process: heat the ingot with a thickness of 150-230 mm to 1210-1230°C and keep it warm for 1-2 hours to ensure that the alloy elements of the ingot are evenly distributed and have a smaller original austenite grain size;

[0024] 2) Rolling and cooling process: two-stage controlled rolling is adopted, and the finishing rolling temperature is 780-800℃; after the final rolling, the cooling mode of air cooling + ultra-fast cooling is adopted: air cooling to A r3 After reaching the temperature, continue air cooling for 4-6s, and then carry out the second stage of ultra-fast cooling, with a cooling rate of ≥100℃ / s. After cooling the steel plate to 280-330℃, coil it and air cool it to room temperature;

[0025] 3) Pickling process: The steel plate is pickled after being cooled to room temperature. The pickling tensile elongation is 0.5% to 0.8%. The steel plate travel speed is 40 to 60 m / min. The pickling liquid is hydrochloric acid with a concentration of 140 to 180 g / l. The pickling process is tension pickling with a tension of 10 to 13 kN and a pickling temperature of 80°C to 85°C.

[0026] In the present invention, the steel plate adopts a low Si component design, which improves the surface quality of the steel plate, creates favorable conditions for the subsequent pickling process, and can be applied to automobile parts with high surface quality requirements. The risk of cementite precipitation in the coiling and partitioning stage caused by low Si is eliminated by P, Mn and Mo elements; the composite addition of Nb and V elements in the structure is beneficial to refining the steel plate structure, improving the delayed fracture resistance and toughness of the steel plate, etc.; the steel plate hot rolling process adopts low-temperature rolling, which is beneficial to refining the structure; the cooling mode adopts a two-stage cooling system of air cooling + ultra-fast cooling, and a certain amount of ferrite is generated in the structure during the air cooling stage, thereby achieving the purpose of improving the plasticity of the steel plate; at the same time, the steel plate adopts reasonable process system parameters in the pickling stage to ensure the mechanical properties of the steel plate while obtaining excellent surface quality.

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

[0028] 1) The present invention adopts a low Si component design, with a Si content of ≤0.10%, which is much lower than the percentage content of >1.0% in the traditional process, and can improve the surface quality of the steel plate. The surface roughness Ra of the steel plate after pickling is 0.8-1.5μm, which is suitable for the use requirements of automotive parts under existing process conditions;

[0029] 2) The present invention adopts the cooling process of air cooling + ultra-fast cooling to obtain a three-phase structure of ferrite + martensite + residual austenite, so that the strength of the steel plate reaches more than 800MPa and the elongation is ≥30%. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 This is the microstructure of the steel plate according to Example 2 of the present invention. 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 embodiment of the invention is as follows: a continuous casting billet with a thickness of 150 mm to 230 mm is heated to 1210 to 1230 ° C, kept at this temperature for 1 to 2 hours, and then rolled. The final rolling temperature of the finishing rolling is 780 to 800 ° C. After the final rolling, an air cooling + ultra-fast cooling mode is adopted, and the air cooling is cooled to A. r3After reaching the temperature, continue air cooling for 4-6s, and then carry out the second stage of ultra-fast cooling, with a cooling rate of ≥100℃ / s. After cooling the steel plate to 280-330℃, coil it and air cool it to room temperature. After the steel plate is cooled to room temperature, it is pickled. The pickling straightening elongation is 0.5-0.8%. The pickling liquid is hydrochloric acid with a concentration of 140-180g / l. The steel plate travel speed is 40-60m / min. The pickling process is pickling with tension, with a tension of 10-13KN, and the pickling temperature is 80℃-85℃. The finished steel plate thickness is 2.0-6.0mm.

[0033] The specific composition, temperature regime, pickling parameters, and microstructure and properties of the steel plates of the six embodiments of the present invention are shown in Tables 1-4.

[0034] The microstructure of the steel plate of Example 4 is shown in Figure 1 Table 1 Chemical composition of the embodiments of the present invention (wt, %)

[0035]

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

[0037]

[0038] Table 3 Pickling system of the embodiment of the present invention

[0039]

[0040] Table 4 Organization and mechanical properties parameters of the embodiments of the present invention

[0041]

[0042]

Claims

1. An 800MPa grade hot-rolled pickled Q&P steel, characterized in that: The chemical composition of the steel is calculated by weight percentage: C0.14%~0.16%, Si ≤0.10%, Mn 1.40%~1.60%, P 0.045%~0.065%, S≤0.005%, Al 0.02%~0.05%, Nb 0.02%~0.04%, V 0.015%~0.035%, Mo 0.15%~0.30%, N≤0.006%, the balance is Fe and unavoidable impurities; The production method of the 800MPa grade hot-rolled pickled Q&P steel comprises the following steps: 1) Heating process: heat the ingot to 1210-1230℃ and keep it warm for 1-2 hours; 2) Rolling and cooling process: two-stage controlled rolling is adopted, and the finishing rolling temperature is 780-795℃; after the final rolling, the cooling mode of air cooling + ultra-fast cooling is adopted: air cooling to A r3 After reaching the temperature, continue air cooling for 4-6s, and then carry out the second stage of ultra-fast cooling, with a cooling rate of ≥100℃ / s. After cooling the steel plate to 280-330℃, coil it and air cool it to room temperature; 3) Pickling process: The steel plate is pickled after cooling to room temperature. The pickling tensile elongation is 0.5% to 0.8%. The steel plate travel speed is 40 to 60 m / min. The pickling process is tension pickling with a tension of 10 to 13 kN and a pickling temperature of 80°C to 85°C.

2. The 800MPa grade hot-rolled pickled Q&P steel according to claim 1, characterized in that: The steel plate structure is composed of ferrite, martensite and retained austenite, in which the volume fraction of ferrite is 13% to 17%, the volume fraction of martensite is 69% to 76%, and the volume fraction of retained austenite is 11% to 14%.

3. The 800MPa grade hot-rolled pickled Q&P steel according to claim 1, characterized in that: The tensile strength of the steel plate is 800-880MPa, the yield strength is 500-580MPa, and the elongation is ≥30.0%.

4. The 800MPa grade hot-rolled pickled Q&P steel according to claim 1, characterized in that: The thickness of the finished steel plate is 2.0-6.0mm.

5. The 800MPa grade hot-rolled pickled Q&P steel according to claim 1, characterized in that: The surface roughness Ra of the steel plate is 0.8 to 1.5 μm.

6. A method for producing 800MPa grade hot-rolled pickled Q&P steel according to any one of claims 1 to 5, characterized in that: The method comprises the following steps: 1) Heating process: heat the ingot to 1210-1230℃ and keep it warm for 1-2 hours; 2) Rolling and cooling process: two-stage controlled rolling is adopted, and the finishing rolling temperature is 780-795℃; after the final rolling, the cooling mode of air cooling + ultra-fast cooling is adopted: air cooling to A r3 After reaching the temperature, continue air cooling for 4-6s, and then carry out the second stage of ultra-fast cooling, with a cooling rate of ≥100℃ / s. After cooling the steel plate to 280-330℃, coil it and air cool it to room temperature; 3) Pickling process: The steel plate is pickled after cooling to room temperature. The pickling tensile elongation is 0.5% to 0.8%. The steel plate travel speed is 40 to 60 m / min. The pickling process is tension pickling with a tension of 10 to 13 kN and a pickling temperature of 80°C to 85°C.

7. The method for producing 800MPa grade hot-rolled pickled Q&P steel according to claim 6, characterized in that: In the above step 1), the thickness of the ingot is 150 to 230 mm.

8. The method for producing 800MPa grade hot-rolled pickled Q&P steel according to claim 6, characterized in that: In the above step 3), the pickling solution is hydrochloric acid with a concentration of 140-180 g / l.

Images