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

Abstract

The present invention belongs to the technical field of hot-formed steel preparation, and more specifically, relates to a preparation method of an efficient short-process hypothermal quenching antioxidant hot-formed steel. The formed steel, by mass percentage, except for 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%, as well as inevitable impurities. During the preparation process of the present invention, a rapid heating strategy is adopted when heating and holding 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 precipitation phase. On the basis of fine grain strengthening and plasticization, the strength and plasticity of the antioxidant hot-formed steel are further improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of the preparation of hot-formed steel, and more specifically relates to a preparation method for an efficient short-process subcritical quenching antioxidant hot-formed steel. Background Art

[0002] Under the background of automotive lightweighting, higher requirements are imposed on the design and use of steel for automotive structural parts. As the main high-strength safety structural steel used in automobile bodies, hot-formed steel needs to heat the steel plate to a relatively high temperature, keep it warm, and then perform stamping forming during the production process. A large amount of scale will be generated during the production process of the currently mainly used steel grades and process flows. Subsequently, shot peening treatment or coating addition before painting is usually required, which increases production costs and energy consumption.

[0003] In order to solve the surface oxidation problem during the hot-forming process, reduce production processes, and lower production costs, domestic and foreign scientific researchers have conducted a large amount of research and exploration on new materials and new processes of hot-formed steel. Some existing technologies propose a method for removing scale from 1000 - 1800 MPa grade hot-formed steel with high Cr-Si alloying, which combines hydrochloric acid pickling and EPS treatment to remove the scale of hot-formed steel. Without the premise of an Al-Si coating, it meets the control requirements for the scale thickness in subsequent hot forming, but this process is similar to shot peening treatment, making the process flow complex. Some existing technologies propose a high-plasticity hot-formed steel and its rapid heating preparation method, which obtains a complex phase structure of martensite matrix, submicron retained austenite, and nanometer carbides by using a process of rapid heating and long-time heat preservation, improving the strength of the specimen, but the heat treatment process is relatively complex, the process flow time is long, and it is not conducive to energy conservation and emission reduction. Some existing technologies propose a 1500 MPa grade high-plasticity, high fracture strain antioxidant hot-formed steel. By introducing a mixed structure of lath-shaped ferrite, twinned martensite, and retained austenite, a hot stamping forming steel with a yield strength of 830 - 870 MPa, a tensile strength of 1520 - 1570 MPa, and an elongation after fracture of 13% - 18% is obtained, but part of the yield strength is sacrificed while improving the plasticity. Summary of the Invention

[0004] The purpose of the present invention is to provide a preparation method for an efficient short-process subcritical quenching antioxidant hot-formed steel, which uses a rapid heating process to obtain a high-strength, high-toughness antioxidant 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, so as to solve the problems existing in the above-mentioned prior art.

[0005] To achieve the above purpose, the present invention provides the following solution:

[0006] One of the technical solutions of the present invention: Provide a high-efficiency short-process subcritical quenching antioxidant hot-forming steel. By mass percentage, except for 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%, as well as inevitable impurities.

[0007] Further, for the high-efficiency short-process subcritical quenching antioxidant hot-forming steel, 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.

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

[0009] As a strengthening element in the steel, C has a significant impact on the martensite content, tensile strength, etc. of the steel plate after hot forming. In order to ensure the martensite structure and sufficiently high tensile strength of the hot-forming steel, and at the same time to ensure good plasticity, toughness and weldability of the hot-forming steel, the C content is controlled to be 0.20 - 0.30%;

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

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

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

[0013] Element Al can improve the antioxidant performance and anti-hydrogen embrittlement performance of the hot-forming steel at the same time, and can play a role in refining grains. However, when the Al content is too high, it will produce non-metallic inclusions and cause adverse effects. Therefore, the Al content is controlled to be 0.60 - 1.20%;

[0014] Nb and V are strong carbide-forming elements. During the preparation process, they precipitate to form complex carbides. The Nb-V complex 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-stamped parts. In addition, the dispersion of Nb-V complex carbides in ferrite can increase the strength of ferrite and resist the strength reduction caused by the presence of ferrite. Considering comprehensively, the mass percentage of Nb is 0.05-0.20%, and the mass percentage of V is 0.05-0.20%. The above element compositions cooperate synergistically, and combined with the cooperation of the mechanical metallurgy preparation method, a high-strength, high-toughness, and oxidation-resistant hot-formed steel composed of 2-3.5% ferrite and 96.5-98% martensite can be obtained.

[0015] The second technical solution of the present invention: Provide a preparation method for an efficient short-process subcritical quenching oxidation-resistant hot-formed steel, and the steps include:

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

[0017] Perform heat treatment on the slab. Before the end of the heat treatment, dephosphorization before rolling is carried out, and then multi-pass hot rolling is carried out. Subsequently, it is shaped, cooled, and coiled to obtain hot-rolled thin strip steel;

[0018] After the hot-rolled thin strip steel is rapidly heated and short-time held (ultra-rapid heating process), press quenching and forming are carried out to obtain the efficient short-process subcritical quenching oxidation-resistant hot-formed steel.

[0019] Further, the temperature of the heat treatment is 1150-1200 °C, and the holding time is 2-zh4 h.

[0020] Further, the starting rolling temperature of the multi-pass hot rolling is not lower than 1180 °C, the final rolling temperature is Ac3-20 °C to Ac3 of the steel, and the final rolling reduction is not lower than 20%.

[0021] Further, the shaping is carried out by using a rolling mill for shaping after multi-pass hot rolling.

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

[0023] Here, the cooling is carried out by air cooling, and the cooling rate is between 10-15 °C / s.

[0024] Further, the heating and holding are to heat the hot-rolled thin strip steel from room temperature to Ac3-20 °C to Ac3 within 10-20 s and hold for 10-60 s.

[0025] During hot forming, an ultra-fast heating method such as induction heating is used to refine the grains, synchronously improve the strength and plasticity, and the rapid heating can reduce the high-temperature holding time, reduce the high-temperature oxidation, and thus reduce the content of antioxidant elements such as Cr and Si in the steel, reducing the production cost.

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

[0027] The third technical solution of the present invention: Provide an application of the above-mentioned high-efficiency short-process intercritical quenching antioxidant hot forming steel or the above-mentioned preparation method in the preparation of automotive safety structural parts.

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

[0029] When heating and holding the hot-rolled thin strip steel in the present invention, a rapid heating and short-time holding process is adopted, which shortens the time required for the process flow, improves the production efficiency, and performs hot forming at the two-phase region temperature, where the temperature is lower, which can save energy, reduce the carbon dioxide emissions, and at the same time reduce the cost.

[0030] When heating and holding the hot-rolled thin strip steel in the present invention, a rapid heating strategy is adopted, 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 precipitation phase. On the basis of fine grain strengthening and plasticization, the strength and plasticity of the antioxidant hot forming steel are further improved.

[0031] In the hot forming steel prepared by the present invention, in addition to improving the antioxidant performance and hardenability, the Si element can also promote the segregation of the V element at the grain boundary, reduce the grain boundary energy, further improve the austenite stability and hardenability, inhibit the phase transformation in the high-temperature region, 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 significant role in improving the hardenability, replace the B element, and reduce the smelting difficulty of the steel. Brief Description of the Drawings

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

[0033] Figure 1 It is a schematic diagram for comparing the ultra-fast heating process and the conventional heating process.

[0034] Figure 2 It is a schematic diagram of the specification dimensions of the plate-shaped tensile specimen.

[0035] Figure 3 It is a microstructural characteristic diagram of Example 1. Detailed Description of the Invention

[0036] The various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be construed as a limitation on the present invention, but rather as a more detailed description of certain aspects, features, and implementation manners of the present invention.

[0037] It should be understood that the terms described in the present invention are only for describing specific embodiments and are not used to limit the present invention. Additionally, for the numerical ranges in the present invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Any intermediate value within any stated value or stated range, as well as each smaller range between any other stated value or intermediate value within the stated range, is also included in the present invention. The upper and lower limits of these smaller ranges can be independently included or excluded from the range.

[0038] Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the art to which the present invention pertains. Although the present invention only describes preferred methods and materials, any methods and materials similar or equivalent to those described herein can 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 related to the said documents. In case of conflict with any incorporated document, the content of this specification shall prevail.

[0039] Without departing from the scope or spirit of the present invention, various improvements and changes can be made to the specific embodiments of the present invention specification, which are obvious to those skilled in the art. Other embodiments obtained from the specification of the present invention are obvious to those skilled in the art. The specification and examples of the present invention are merely exemplary.

[0040] Regarding the use of "comprising", "including", "having", "containing", etc. herein, they are all open-ended terms, meaning including but not limited to.

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

[0042] A schematic diagram comparing the ultra-fast heating process and the conventional heating process in step S4 of the specific embodiment of the present invention is as Figure 1 shown.

[0043] Example 1

[0044] The steps of the high-efficiency short-process subcritical quenching antioxidant hot-forming steel include:

[0045] S1. Prepare molten steel by smelting raw materials according to the chemical composition of the hot-formed steel: 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%, V 0.15 - 0.30%, and the balance Fe;

[0046] S2. Prepare a slab through a continuous casting device, heat the slab in a heating furnace to 1160°C, hold for 4 h, take the slab out of the furnace and remove phosphorus before rolling, then conduct multi-pass hot rolling on the slab, with the starting rolling temperature of 1150°C, the final rolling temperature of 870°C, and the final rolling reduction of 25% to obtain a 2-mm thick steel plate;

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

[0048] S4. Rapidly heat the hot-rolled thin plate using an induction heating device (ultra-rapid heating process), heat it to 870°C within 10 s, hold for 20 s, and then immediately perform press quenching forming to obtain high-efficiency short-process subcritical quenching antioxidant hot-formed steel. After testing, the chemical composition of the hot-formed steel is: C 0.21 wt.%, Mn 0.65 wt.%, Cr 1.38 wt.%, Si 1.31 wt.%, Al 0.42 wt.%, Nb 0.18 wt.%, V 0.26 wt.%, and the balance Fe.

[0049] Example 2

[0050] The steps for high-efficiency short-process subcritical quenching antioxidant hot-formed steel include:

[0051] S1. Prepare molten steel by smelting raw materials according to the chemical composition of the hot-formed steel: 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%, V 0.15 - 0.30%, and the balance Fe;

[0052] S2. Prepare a slab through a continuous casting device, heat the slab in a heating furnace to 1190°C, hold for 4 h, take the slab out of the furnace and remove phosphorus before rolling, then conduct multi-pass hot rolling on the slab, with the starting rolling temperature of 1170°C, the final rolling temperature of 865°C, and the final rolling reduction of 30% to obtain a 2.5-mm thick steel plate;

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

[0054] S4. Use an induction heating device to rapidly heat the hot-rolled thin plate (ultra-rapid heating process), heat it to 865 °C within 15 s, hold for 30 s, and then immediately perform press quenching forming to obtain a high-efficiency short-process subcritical quenching antioxidant hot-formed steel. After testing, the chemical composition of the hot-formed steel is: C 0.28 wt.%, Mn 1.49 wt.%, Cr 1.12 wt.%, Si 1.38 wt.%, Al 0.44 wt.%, Nb 0.15 wt.%, V 0.19 wt.% and the balance Fe.

[0055] Example 3

[0056] The steps for preparing the high-efficiency short-process subcritical quenching antioxidant hot-formed steel include:

[0057] S1. According to the chemical composition of the hot-formed steel: 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%, V 0.15 - 0.30% and the balance Fe, prepare raw materials for smelting to obtain molten steel;

[0058] S2. Prepare a slab from the molten steel through a continuous casting device, heat the slab to 1200 °C in a heating furnace, hold for 3 h, take the slab out of the furnace and perform descaling before rolling, and then perform multi-pass hot rolling on the slab. The starting rolling temperature is 1175 °C, the finishing rolling temperature is 850 °C, and the finishing rolling reduction is 25% to obtain a 3-mm thick steel plate;

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

[0060] S4. Use an induction heating device to rapidly heat the hot-rolled thin plate (ultra-rapid heating process), heat it to 850 °C within 20 s, hold for 45 s, and then immediately perform press quenching forming to obtain a high-efficiency short-process subcritical quenching antioxidant hot-formed steel. After testing, the chemical composition of the hot-formed steel is: C 0.30 wt.%, Mn 0.96 wt.%, Cr 1.32 wt.%, Si 1.24 wt.%, Al 0.50 wt.%, Nb 0.22 wt.%, V 0.26 wt.% and the balance Fe.

[0061] Comparative Example 1

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

[0063] S1. Prepare molten steel by smelting raw materials according to the chemical composition of hot-formed steel: 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%, V 0.15 - 0.30%, and the balance Fe.

[0064] S2. Prepare a slab through continuous casting equipment for the molten steel. Heat the slab in a heating furnace to 1200°C and hold for 3 hours. After the slab is taken out of the furnace and descaled before rolling, then conduct multi-pass hot rolling on the slab. The starting rolling temperature is 1175°C, the finishing rolling temperature is 950°C, and the finishing rolling reduction is 25% to obtain a 3-mm thick steel plate.

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

[0066] S4. Heat the hot-rolled thin plate (conventional heating process) to 930°C within 10 minutes and hold for 5 minutes, then immediately perform press quenching to form the hot-formed steel. After testing, the chemical composition of the hot-formed steel is: C 0.30 wt.%, Mn 0.96 wt.%, Cr 1.32 wt.%, Si 1.24 wt.%, Al 0.50 wt.%, Nb 0.22 wt.%, V 0.26 wt.%, and the balance Fe.

[0067] Comparative Example 2

[0068] The steps of the high-efficiency short-process subcritical quenching antioxidant hot-formed steel include:

[0069] S1. Smelt molten steel according to the chemical composition of hot-formed steel: C 0.30 wt.%, Mn 0.96 wt.%, Cr 2.46 wt.%, Si 0.1 wt.%, Al 0.50 wt.%, Nb 0.25 wt.%, V 0.24 wt.%, and the balance Fe.

[0070] S2. Prepare a slab through continuous casting equipment for the molten steel. Heat the slab in a heating furnace to 1200°C and hold for 3 hours. After the slab is taken out of the furnace and descaled before rolling, then conduct multi-pass hot rolling on the slab. The starting rolling temperature is 1180°C, the finishing rolling temperature is 830°C, and the finishing rolling reduction is 25% to obtain a 3-mm thick steel plate.

[0071] S3. After reshaping the steel plate obtained in step S2 by a rolling mill, rapidly cool it to 750°C and coil the steel plate to obtain hot-rolled thin strip steel.

[0072] S4. Use an induction heating device to rapidly heat the hot-rolled thin strip steel (ultra-rapid heating process), heat it to 830 °C within 20 s, hold for 45 s, and then immediately perform press quenching to form a high-efficiency short-process subcritical quenching antioxidant hot-formed steel.

[0073] Test example

[0074] Plate-shaped tensile specimens were prepared by the methods of Examples 1-3 and Comparative Examples 1-2, and the specifications are as Figure 2 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 property test method and standard: Conducted in accordance with the national standard "Metallic materials - Tensile testing - Part 1: Method of test at room temperature GB / T 228.1-2010". Use an MTS810 universal hydraulic servo testing machine to conduct tensile property tests, and the tensile rate is 3 mm / min. Polish the surface and both sides of the specimen with sandpapers of different models from small to large until smooth.

[0076] Figure 2 It is a schematic diagram of the specification dimensions of the plate-shaped tensile specimen.

[0077] Figure 3 It is a microstructural 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 in Comparative Example 1, a conventional heating process was adopted, so that the structure of the prepared hot-formed steel was a fully martensitic structure, and the tensile strength reached 1800 MPa, but it had a low elongation; in Comparative Example 2, the addition of Si element was reduced, so that the martensitic structure of the prepared hot-formed steel increased, and the tensile strength and elongation decreased.

[0081] In this specification, each embodiment is described in a progressive manner. The key point of each embodiment is to illustrate the differences from other embodiments. The same or similar parts among the embodiments can be referred to each other.

[0082] The above description of the disclosed embodiments enables those skilled in the art to implement or use the present invention. Various modifications to these embodiments will be obvious to those skilled in the art. The general principles defined herein can 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 these embodiments shown herein, but will conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. An efficient short-process subcritical quenching antioxidant hot-forming steel, characterized in that, 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; The preparation steps of the high-efficiency short-process subcritical quenching antioxidant hot forming steel include: Preparing raw materials according to the components of the high-efficiency short-process subcritical quenching antioxidant hot forming steel, melting them into molten steel, and preparing them into slabs; Performing heat treatment on the slabs, removing phosphorus before rolling before the end of the heat treatment, then performing multi-pass hot rolling, and then obtaining hot-rolled thin strip steel through shaping, cooling, and coiling; Performing press quenching and forming on the hot-rolled thin strip steel after rapid heating and short-time holding to obtain the high-efficiency short-process subcritical quenching antioxidant hot forming steel; The rapid heating and short-time holding is to heat the hot-rolled thin strip steel from room temperature to Ac3 - 20°C to Ac3 within 10 - 20 s and hold for 10 - 60 s; The starting rolling temperature of the multi-pass hot rolling is not lower than 1140°C, the finishing rolling temperature is Ac3 - 20°C to Ac3 of the steel, and the finishing rolling deformation amount is not lower than 20%.

2. A preparation method of an efficient short-process subcritical quenching antioxidant hot-forming steel, characterized in that the steps Including: Preparing raw materials according to the components of the high-efficiency short-process subcritical quenching antioxidant hot forming steel as claimed in claim 1, melting them into molten steel, and preparing them into slabs; Performing heat treatment on the slabs, removing phosphorus before rolling before the end of the heat treatment, then performing multi-pass hot rolling, and then obtaining hot-rolled thin strip steel through shaping, cooling, and coiling; Performing press quenching and forming on the hot-rolled thin strip steel after rapid heating and short-time holding to obtain the high-efficiency short-process subcritical quenching antioxidant hot forming steel; The rapid heating and short-time holding is to heat the hot-rolled thin strip steel from room temperature to Ac3 - 20°C to Ac3 within 10 - 20 s and hold for 10 - 60 s; The starting rolling temperature of the multi-pass hot rolling is not lower than 1140°C, the finishing rolling temperature is Ac3 - 20°C to Ac3 of the steel, and the finishing rolling deformation amount is not lower than 20%.

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

4. The preparation method according to claim 2, characterized in that, The cooling is to cool to 650 - 750°C.

5. Application of the high-efficiency short-process subcritical quenching antioxidant hot forming steel as claimed in claim 1 or the preparation method as claimed in any one of claims 2 - 4 in the preparation of automotive safety structural parts.

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