Preparation method of high-efficiency short-process subcritical quenching anti-oxidation hot forming steel

Abstract

The invention belongs to the technical field of preparation of hot-formed steel, and particularly relates to a preparation method of efficient short-process subcritical quenching anti-oxidation hot-formed steel. The forming steel comprises the following components in percentage by mass except Fe: 0.20 to 0.30 percent of C, 0.5 to 1.5 percent of Mn, 1.0 to 1.5 percent of Cr, 1.2 to 1.4 percent of Si, 0.4 to 0.6 percent of Al, 0.15 to 0.30 percent of Nb, 0.15 to 0.30 percent of V and the balance of inevitable impurities. In the preparation process, a rapid heating strategy is adopted when hot-rolled thin strip steel is heated and subjected to heat preservation, the temperature is set to be slightly lower than the complete austenitizing temperature of steel, a small amount of fine ferrite is introduced, a dispersed precipitated phase is reserved, and on the basis of fine grain strong plasticization, the strong plasticity of the antioxidant hot-formed steel is further improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of hot forming steel preparation, and more specifically relates to a method for preparing a high-efficiency short-process sub-temperature quenching anti-oxidation hot forming steel. Background Art

[0002] In the context of lightweight automobiles, the design and use of steel for automobile structural parts have higher requirements. As the main high-strength safety structural steel in automobile bodies, hot-formed steel requires the steel plate to be heated to a high temperature and kept warm before being stamped. The main steel types and process flows currently used will produce a large amount of oxide scale in the process, which usually requires shot peening or coating before painting, increasing production costs and energy consumption.

[0003] In order to solve the surface oxidation problem in the hot forming process, reduce the production process and reduce the production cost, domestic and foreign scientific researchers have done a lot of research and exploration in the new materials and new processes of hot forming steel. Some existing technologies have proposed a method for removing the oxide scale of high Cr-Si alloyed 1000-1800MPa hot forming steel, which uses a combination of hydrochloric acid pickling and EPS treatment to remove the oxide scale of hot forming steel. Under the premise of no Al-Si coating, it meets the control requirements of the oxide scale thickness of subsequent hot forming, but this process is similar to shot peening, which makes the process flow complicated. Some existing technologies have proposed a high-plasticity hot forming steel and a rapid heating preparation method thereof. The multiphase structure of martensitic matrix, submicron residual austenite and nano-carbide is obtained by a process of rapid heating and long-term heat preservation, which improves the strength of the sample, but the heat treatment process is relatively complicated and the process flow is long, which is not conducive to energy conservation and emission reduction. Some existing technologies have proposed a 1500MPa grade high plasticity, high fracture strain, oxidation-resistant hot-formed steel. By introducing a mixed structure of lath-shaped ferrite, twinned martensite and residual austenite, a hot stamping steel with a yield strength of 830-870MPa, a tensile strength of 1520-1570MPa and an elongation after fracture of 13%-18% is obtained. However, while improving the plasticity, part of the yield strength is sacrificed. Summary of the invention

[0004] The purpose of the present invention is to provide a method for preparing an efficient short-process sub-temperature quenching anti-oxidation hot-formed steel, which utilizes a rapid heating process to obtain high-strength, high-toughness, anti-oxidation hot-formed steel, reduce the degree of surface oxidation during the hot forming process, reduce the time required for the hot forming process, and at the same time improve the strength and plasticity of the hot-formed steel, thereby improving production efficiency and solving the problems existing in the above-mentioned prior art.

[0005] To achieve the above object, the present invention provides the following solutions:

[0006] One of the technical solutions of the present invention is to provide a high-efficiency short-process sub-temperature quenching and oxidation-resistant hot-forming steel, which, in addition to Fe, comprises, by mass percentage, C 0.20-0.30%, Mn 0.5-1.5%, Cr 1.0-1.5%, Si 1.2-1.4%, Al 0.4-0.6%, Nb 0.15-0.30% and V 0.15-0.30%, as well as inevitable impurities.

[0007] Furthermore, the high-efficiency short-process sub-temperature quenching and oxidation-resistant hot-formed steel has the following components, calculated by mass percentage: C 0.20-0.30%, Mn 0.5-1.5%, Cr 1.0-1.5%, Si 1.2-1.4%, Al 0.4-0.6%, Nb 0.15-0.30% and V 0.15-0.30%, with the remainder being Fe and unavoidable impurities.

[0008] The reasons for limiting the chemical composition of the high-strength, high-toughness, oxidation-resistant hot-formed steel of the present invention are as follows:

[0009] As a strengthening element in steel, C has a significant effect on the martensite content and tensile strength of the hot-formed steel plate. In order to ensure the martensite structure and sufficiently high tensile strength of the hot-formed steel, and to ensure good plasticity, toughness and weldability of the hot-formed steel, the C content is controlled to 0.20-0.30%.

[0010] Mn plays a role in solid solution strengthening and improving toughness in steel. However, when the Mn content is too high, it is easy to cause segregation of the ingot and affect the uniformity of the structure. The Mn content is controlled to 1.00-1.80%.

[0011] The addition of Si can play a role in solid solution strengthening, improve the stability of austenite, and promote the subsequent martensite transformation. At the same time, Si can form a dense oxide film during hot forming, inhibiting the continued growth of the oxide film on the surface of the steel plate, thereby improving the high-temperature oxidation resistance of the steel. However, too high Si content will lead to deterioration of weldability and toughness of the welding heat-affected zone. The Si content is controlled to 0.60-1.20%;

[0012] Cr can improve the hardenability of steel and promote martensitic transformation. At the same time, Cr can improve the high temperature oxidation resistance of steel. The Cr content is controlled to 2.00-2.5%.

[0013] The Al element can improve the oxidation resistance and hydrogen embrittlement resistance of hot-formed steel at the same time, and can also refine the grains. However, when the Al content is too high, non-metallic inclusions will be produced, causing adverse effects. Therefore, the Al content is controlled to 0.60-1.20%.

[0014] Nb and V are strong carbide-forming elements, which precipitate to form composite carbides during the preparation process. The Nb and V composite carbides can act as hydrogen traps, which can reduce the concentration of diffusible hydrogen, thereby reducing the hydrogen embrittlement sensitivity of hot-formed steel and improving the safety of automotive hot stamping parts. In addition, the Nb and V composite carbides dispersed in ferrite can improve the strength of ferrite and resist the strength reduction caused by the presence of ferrite. Taking all factors into consideration, the mass percentage of Nb is 0.05-0.20%, and the mass percentage of V is 0.05-0.20%. The above-mentioned element composition cooperates synergistically, combined with the mechanical metallurgical preparation method, to obtain a high-strength, high-toughness, and oxidation-resistant hot-formed steel composed of 2-3.5% ferrite and 96.5-98% martensite.

[0015] The second technical solution of the present invention is to provide a method for preparing a highly efficient short-process sub-temperature quenching anti-oxidation hot-formed steel, the steps comprising:

[0016] Prepare raw materials according to the above components, melt them into molten steel, and prepare slabs;

[0017] The slab is heat treated, dephosphorized before rolling before the heat treatment is completed, and then hot rolled for multiple times, and then shaped, cooled, and curled to obtain a hot-rolled thin strip steel;

[0018] The hot-rolled thin strip steel is subjected to rapid heating and short-time heat preservation (ultra-fast heating process) and then press-quenched to obtain the high-efficiency short-process sub-temperature quenching and oxidation-resistant hot-formed steel.

[0019] Furthermore, the heat treatment temperature is 1150-1200° C., and the insulation time is 2-4 hours.

[0020] Furthermore, the starting rolling temperature of the multiple hot rolling is not less than 1180°C, and the final rolling temperature is Ac of the steel. 3 -20℃~Ac 3 , the final rolling deformation is not less than 20%.

[0021] Furthermore, the shaping is performed by using a rolling mill after multiple hot rolling passes.

[0022] Furthermore, the cooling is cooling to 650-750°C.

[0023] The cooling here is carried out by air cooling, and the cooling rate is between 10-15℃ / s.

[0024] Furthermore, the heating and heat preservation is to heat the hot-rolled thin strip steel from room temperature to Ac in 10-20s. 3 -20℃~Ac 3 , keep warm for 10 to 60 seconds.

[0025] Ultra-fast heating methods such as induction heating are used during hot forming to refine the grains and simultaneously improve strength and plasticity. Rapid heating can reduce the high-temperature holding time, reduce high-temperature oxidation, and reduce the content of antioxidant elements such as Cr and Si in steel, thereby reducing production costs.

[0026] Press quenching can be adaptively adjusted according to the required automotive structural parts.

[0027] The third technical solution of the present invention is to provide an application of the above-mentioned high-efficiency short-process sub-temperature quenching and oxidation-resistant hot-formed steel or the above-mentioned preparation method in the preparation of automobile safety structural parts.

[0028] The present invention discloses the following technical effects:

[0029] The present invention adopts a rapid heating and short-time insulation process when heating and insulating the hot-rolled thin strip steel, which shortens the time required for the process flow and improves production efficiency. Hot forming is performed at a two-phase region temperature with a lower temperature, which can save energy, reduce carbon dioxide emissions, and reduce costs.

[0030] The present invention adopts a rapid heating strategy when heating and insulating the hot-rolled thin strip steel, and the temperature is set slightly lower than the complete austenitization temperature of the steel, introducing a small amount of fine ferrite and retaining the dispersed precipitate phase, thereby further improving the strength and plasticity of the oxidation-resistant hot-formed steel on the basis of fine grain strengthening and plasticization.

[0031] In addition to improving the oxidation resistance and hardenability, the Si element in the hot-formed steel prepared by the present invention can also promote the segregation of the V element at the grain boundary, reduce the grain boundary energy, further improve the stability and hardenability of austenite, inhibit the phase transformation in the high temperature zone, obtain a finer martensite structure during press quenching, and improve the strength of the steel; while the V element plays a role in precipitation strengthening, the interaction between Si and V can make the solid solution V play a role in significantly improving the hardenability, replace the B element, and reduce the difficulty of steel smelting. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] The accompanying drawings constituting a part of the present invention are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the accompanying drawings:

[0033] Figure 1 Schematic diagram comparing ultrafast heating process and conventional heating process.

[0034] Figure 2 Schematic diagram of the specifications and dimensions of the plate tensile specimen.

[0035] Figure 3 This is a microstructure characteristic diagram of Example 1. DETAILED DESCRIPTION

[0036] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as limiting the present invention, but should be understood as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0037] It should be understood that the terms described in the present invention are only for describing a particular embodiment and are not intended to limit the present invention. In addition, for the numerical range in the present invention, it should be understood that each intermediate value between the upper and lower limits of the scope is also specifically disclosed. The intermediate value in any stated value or stated range, and each smaller range between any other stated value or intermediate value in the described range is also included in the present invention. The upper and lower limits of these smaller ranges can be independently included or excluded in the scope.

[0038] Unless otherwise indicated, all technical and scientific terms used herein have the same meanings as those generally understood by those skilled in the art. Although the present invention describes only preferred methods and materials, any methods and materials similar or equivalent to those described herein may also be used in the implementation or testing of the present invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials associated with the documents. In the event of a conflict with any incorporated document, the content of this specification shall prevail.

[0039] It will be apparent to those skilled in the art that various modifications and variations may be made to the specific embodiments of the present invention description without departing from the scope or spirit of the present invention. Other embodiments derived from the present invention description will be apparent to those skilled in the art. The present invention description and examples are exemplary only.

[0040] The words “include,” “including,” “have,” “contain,” etc. used in this document are open-ended terms, meaning including but not limited to.

[0041] The raw materials and reagents used in the specific implementation scheme of the present invention are all commercially available products; the "room temperature" and "normal temperature" involved both refer to 20-30°C.

[0042] The schematic diagram of the comparison between the ultrafast heating process and the conventional heating process in step S4 of the specific implementation scheme of the present invention is as follows: Figure 1 shown.

[0043] Example 1

[0044] The steps of efficient short process sub-temperature quenching and oxidation-resistant hot forming steel include:

[0045] S1. According to the chemical composition of hot-formed steel: C 0.20-0.30%, Mn 0.5-1.5%, Cr 1.0-1.5%, Si1.2-1.4%, Al 0.4-0.6%, Nb 0.15-0.30%, V 0.15-0.30% and balance Fe, prepare raw materials for smelting to obtain molten steel;

[0046] S2. Prepare slabs from molten steel through continuous casting equipment, heat the slabs to 1160° C. in a heating furnace, keep them warm for 4 hours, remove the slabs from the furnace and dephosphorize them before rolling, then perform multiple hot rolling on the slabs, with a starting rolling temperature of 1150° C., a final rolling temperature of 870° C., and a final rolling deformation of 25%, to obtain 2 mm thick steel plates;

[0047] S3, reshaping the steel plate obtained in step S2 by a rolling mill, and then rapidly cooling the steel plate to 650° C. (cooling rate is between 10° C. / s and 15° C. / s), and coiling the steel plate to obtain a hot-rolled thin strip steel;

[0048] S4. Use induction heating equipment to quickly heat the hot-rolled thin plate (ultra-fast heating process), heat to 870°C within 10 seconds, keep warm for 20 seconds, and then immediately perform press quenching to obtain efficient short-process sub-temperature quenching and oxidation-resistant hot-formed steel. The chemical composition of the hot-formed steel is: C 0.21wt.%, Mn 0.65wt.%, Cr1.38wt.%, Si 1.31wt.%, Al0.42wt.%, Nb 0.18wt.%, V 0.26wt.% and the balance Fe.

[0049] Example 2

[0050] The steps of efficient short process sub-temperature quenching and oxidation-resistant hot forming steel include:

[0051] S1. According to the chemical composition of hot-formed steel: C 0.20-0.30%, Mn 0.5-1.5%, Cr 1.0-1.5%, Si1.2-1.4%, Al 0.4-0.6%, Nb 0.15-0.30%, V 0.15-0.30% and balance Fe, prepare raw materials for smelting to obtain molten steel;

[0052] S2. Prepare slabs from molten steel through continuous casting equipment, heat the slabs to 1190° C. in a heating furnace, keep the temperature for 4 hours, remove the slabs from the furnace and perform pre-rolling dephosphorization, then perform multiple hot rolling on the slabs, with a starting rolling temperature of 1170° C., a final rolling temperature of 865° C., and a final rolling deformation of 30%, to obtain 2.5 mm thick steel plates;

[0053] S3, reshaping the steel plate obtained in step S2 by a rolling mill, and then rapidly cooling the steel plate to 700° C. (cooling rate is between 10° C. / s and 15° C. / s), and coiling the steel plate to obtain a hot-rolled thin strip steel;

[0054] S4. Use induction heating equipment to quickly heat the hot-rolled thin plate (ultra-fast heating process), heat to 865°C within 15 seconds, keep warm for 30 seconds, and then immediately perform press quenching to obtain high-efficiency short-process sub-temperature quenching and oxidation-resistant hot-formed steel. The chemical composition of the hot-formed steel is: C 0.28wt.%, Mn 1.49wt.%, Cr1.12wt.%, Si 1.38wt.%, Al0.44wt.%, Nb 0.15wt.%, V 0.19wt.% and the balance Fe.

[0055] Example 3

[0056] The steps of efficient short process sub-temperature quenching and oxidation-resistant hot forming steel include:

[0057] S1. According to the chemical composition of hot-formed steel: C 0.20-0.30%, Mn 0.5-1.5%, Cr 1.0-1.5%, Si1.2-1.4%, Al 0.4-0.6%, Nb 0.15-0.30%, V 0.15-0.30% and balance Fe, prepare raw materials for smelting to obtain molten steel;

[0058] S2. The molten steel is prepared into a slab through a continuous casting device, and the slab is heated to 1200° C. in a heating furnace, kept warm for 3 hours, and the slab is taken out of the furnace and dephosphorized before rolling, and then the slab is subjected to multiple hot rolling, with a starting rolling temperature of 1175° C., a final rolling temperature of 850° C., and a final rolling deformation of 25%, to obtain a 3 mm thick steel plate;

[0059] S3, reshaping the steel plate obtained in step S2 by a rolling mill, and then rapidly cooling the steel plate to 750° C. (cooling rate is between 10° C. / s and 15° C. / s), and coiling the steel plate to obtain a hot-rolled thin strip steel;

[0060] S4. Use induction heating equipment to quickly heat the hot-rolled thin plate (ultra-fast heating process), heat to 850°C within 20 seconds, keep warm for 45 seconds, and then immediately perform press quenching to obtain efficient short-process sub-temperature quenching and oxidation-resistant hot-formed steel. The chemical composition of the hot-formed steel is: C 0.30wt.%, Mn 0.96wt.%, Cr1.32wt.%, Si 1.24wt.%, Al0.50wt.%, Nb 0.22wt.%, V 0.26wt.% and the balance Fe.

[0061] Comparative Example 1

[0062] The steps of preparing hot-formed steel by conventional heating process include:

[0063] S1. According to the chemical composition of hot-formed steel: C 0.20-0.30%, Mn 0.5-1.5%, Cr 1.0-1.5%, Si1.2-1.4%, Al 0.4-0.6%, Nb 0.15-0.30%, V 0.15-0.30% and balance Fe, prepare raw materials for smelting to obtain molten steel;

[0064] S2, preparing slabs from molten steel through continuous casting equipment, heating the slabs to 1200° C. in a heating furnace, keeping the temperature for 3 hours, taking the slabs out of the furnace and dephosphorizing before rolling, then performing multiple hot rolling on the slabs, with a starting rolling temperature of 1175° C., a final rolling temperature of 950° C., and a final rolling deformation of 25%, to obtain 3 mm thick steel plates;

[0065] S3, reshaping the steel plate obtained in step S2 by a rolling mill, and then rapidly cooling the steel plate to 750° C. (cooling rate is between 10° C. / s and 15° C. / s), and coiling the steel plate to obtain a hot-rolled thin strip steel;

[0066] S4. The hot rolled sheet (conventional heating process) is heated to 930°C within 10 minutes, kept at this temperature for 5 minutes, and then immediately subjected to press quenching to obtain hot-formed steel. The chemical composition of the hot-formed steel is as follows: C 0.30wt.%, Mn 0.96wt.%, Cr 1.32wt.%, Si 1.24wt.%, Al 0.50wt.%, Nb 0.22wt.%, V 0.26wt.% and the balance Fe.

[0067] Comparative Example 2

[0068] The steps of efficient short process sub-temperature quenching and oxidation-resistant hot forming steel include:

[0069] S1. According to the chemical composition of hot-formed steel: C 0.30wt.%, Mn 0.96wt.%, Cr 2.46wt.%, Si0.1wt.%, Al 0.50wt.%, Nb 0.25wt.%, V 0.24wt.% and the balance Fe, smelting to obtain molten steel;

[0070] S2, preparing slabs from molten steel through continuous casting equipment, heating the slabs to 1200° C. in a heating furnace, keeping the temperature for 3 hours, taking the slabs out of the furnace and dephosphorizing before rolling, then performing multiple hot rolling on the slabs, with a starting rolling temperature of 1180° C., a final rolling temperature of 830° C., and a final rolling deformation of 25%, to obtain 3 mm thick steel plates;

[0071] S3, reshaping the steel plate obtained in step S2 by a rolling mill, and then rapidly cooling the steel plate to 750° C., and coiling the steel plate to obtain a hot-rolled thin strip steel;

[0072] S4. Use induction heating equipment to quickly heat the hot-rolled thin strip steel (ultra-fast heating process), heat it to 830°C within 20 seconds, keep it warm for 45 seconds, and then immediately perform press quenching to obtain efficient short-process sub-temperature quenching and oxidation-resistant hot-formed steel.

[0073] Test example

[0074] The plate-shaped tensile specimens were prepared by the methods of Examples 1-3 and Comparative Examples 1-2, with specifications as follows: Figure 2 As shown, the mechanical properties of the obtained plate-shaped tensile specimens were tested and their microstructures were observed. The results are shown in Table 1.

[0075] Mechanical properties test methods and standards: According to the national standard "Metallic material tensile test Part 1 Room temperature test method GB / T228.1-2010". The tensile properties test was carried out using an MTS810 universal hydraulic servo testing machine with a tensile rate of 3mm / min. Sandpaper of different sizes from small to large was used to polish the surface and both sides of the sample until smooth.

[0076] Figure 2 Schematic diagram of the specifications and dimensions of the plate tensile specimen.

[0077] Figure 3 This is a microstructure characteristic diagram of Example 1.

[0078] Table 1. Conventional mechanical properties and microstructural characteristics

[0079]

[0080] It can be seen from the data in Table 1 that the conventional heating process is adopted in Comparative Example 1, so that the microstructure of the prepared hot-formed steel is a completely martensitic microstructure, and the tensile strength reaches 1800 MPa, but has a lower elongation; in Comparative Example 2, the addition of Si element is reduced, so that the martensitic microstructure of the prepared hot-formed steel is increased, and the tensile strength and elongation are reduced.

[0081] The various embodiments in this specification are described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The same or similar parts between the various embodiments can be referenced to each other.

[0082] The above description of the disclosed embodiments enables one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but rather to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A high-efficiency short-process sub-temperature quenching and oxidation-resistant hot-formed steel, characterized in that: In terms of mass percentage, in addition to Fe, the components include: C 0.20-0.30%, Mn 0.5-1.5%, Cr 1.0-1.5%, Si 1.2-1.4%, Al 0.4-0.6%, Nb 0.15-0.30% and V 0.15-0.30%, and inevitable impurities.

2. The high-efficiency short-process sub-temperature quenching and oxidation-resistant hot-formed steel according to claim 1, characterized in that: Calculated by mass percentage, the components are: C 0.20-0.30%, Mn 0.5-1.5%, Cr 1.0-1.5%, Si 1.2-1.4%, Al 0.4-0.6%, Nb 0.15-0.30% and V 0.15-0.30%, and the balance is Fe and inevitable impurities.

3. A method for preparing a highly efficient short-process sub-temperature quenching anti-oxidation hot-formed steel, characterized in that the steps include: Prepare raw materials according to the components of the high-efficiency short-process sub-temperature quenching and oxidation-resistant hot-formed steel according to claim 1 or 2, melt them into molten steel, and prepare them into slabs; The slab is heat treated, dephosphorized before rolling before the heat treatment is completed, and then hot rolled for multiple times, and then shaped, cooled, and curled to obtain a hot-rolled thin strip steel; The hot-rolled thin strip steel is subjected to press quenching after rapid heating and short-time heat preservation to obtain the high-efficiency short-process sub-temperature quenching and oxidation-resistant hot-formed steel; The rapid heating and short-time heat preservation is to heat the hot-rolled thin strip steel from room temperature to Ac3-20°C~Ac3 within 10-20s, and keep it warm for 10~60s.

4. The preparation method according to claim 3, characterized in that: The heat treatment temperature is 1150-1200° C., and the heat treatment time is 2-4 hours.

5. The preparation method according to claim 3, characterized in that: The starting rolling temperature of the multi-pass hot rolling is not less than 1140°C, the final rolling temperature is Ac3-20°C to Ac3 of the steel, and the final rolling deformation is not less than 20%.

6. The preparation method according to claim 3, characterized in that: The cooling is cooling to 650-750°C.

7. Use of the high-efficiency short-process sub-temperature quenching anti-oxidation hot-formed steel according to any one of claims 1 to 2 or the preparation method according to any one of claims 3 to 6 in the preparation of automobile safety structural parts.

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