Hot stamping process of 1500mpa grade high-strength and high-corrosion-resistance martensitic stainless steel

Abstract

The application relates to a 1500MPa-grade high-strength and high-corrosion-resistance martensitic stainless steel hot stamping process, which is suitable for a stainless steel plate produced by a hot rolling process: the heating temperature is 870-925 DEG C, and the temperature is kept for 15-20 min after being heated; or the heating temperature is 930-1050 DEG C, and the temperature is kept for 3-5 min after being heated; the cooling rate is greater than or equal to 0.1 DEG C / s; when the temperature of the steel plate reaches 720-870 DEG C, die closing is carried out, and the pressure is greater than or equal to 20 MPa; the cooling rate is greater than or equal to 0.1 DEG C / s after die closing; the die is opened when the temperature is less than or equal to 180 DEG C; the stainless steel plate produced by a cold rolling process: the heating temperature is 890-945 DEG C, and the temperature is kept for 15-20 min; or the heating temperature is 950-1100 DEG C, and the temperature is kept for 3-5 min; the cooling rate is greater than or equal to 0.1 DEG C / s; when the temperature reaches 720-870 DEG C, die closing is carried out, and the pressure is greater than or equal to 20 MPa; the cooling rate is greater than or equal to 0.1 DEG C / s after die closing; the die is opened when the temperature is less than or equal to 180 DEG C. The application controls the precipitation of Cr carbide, Cr nitride and micro-alloying elements through the hot stamping process technology, and the microstructure of Cr 23 C6, Cr2N, (Nb, V)(C, N) and other phase compositions and precipitated phase microstructure is regulated.

Description

Technical Field

[0001] This application relates to the field of heat treatment technology for manufacturing high-strength steel sheets for automobiles using martensitic stainless steel, and particularly to a type of stainless steel with a tensile strength of 1500 MPa and a pitting potential ≥0.05V. SCE The hot stamping process of hot-stamped stainless steel sheets with good corrosion resistance. Background Technology

[0002] Lightweighting of automobiles can effectively improve fuel efficiency, driving performance, and safety, making it a key focus in the automotive industry. In recent years, with the implementation of the Carbon Border Adjustment Mechanism (CBAM), green and low-carbon automotive steel products have become another important research direction in the automotive industry.

[0003] Currently, stainless steel is being explored for use in automobile body production due to its green and environmentally friendly characteristics throughout its entire life cycle. While carbon steel is commonly used in automotive parts, in recent years, automakers have been experimenting with stainless steel body panels and ultra-high-strength stainless steel structural components as the body frame, offering advantages such as high strength, corrosion resistance, and paint-free operation. This not only reduces the need for pickling, degreasing, phosphating, and painting processes in body manufacturing, but also significantly lowers production costs, demonstrating its green and environmentally friendly advantages. Although stainless steel contains a significant amount of precious metals such as Cr, Ni, and Mo in its composition, it is 100% recyclable without degradation, resulting in no metal loss during its use and reducing environmental pollution caused by metal corrosion and deterioration. Its cost-effectiveness throughout its entire life cycle makes it even more environmentally friendly.

[0004] Current technologies typically use 22MnB5 carbon steel to produce 1500MPa-level high-strength parts through hot stamping. Some technologies also explore using martensitic stainless steel for this purpose, addressing the coating and high-temperature oxidation decarburization issues inherent in carbon steel production. However, using stainless steel for hot stamping presents its own challenges. Improper heat treatment can lead to lower strength and insufficient corrosion resistance, for example, a higher chromium (Cr) content results in a lower critical cooling rate. The increased Cr content in hot-stamped stainless steel further exacerbates these problems. 23 The increased tendency for C6 and Cr2N intermetallic compounds to precipitate leads to localized Cr depletion in the matrix, creating areas prone to pitting corrosion and thus reducing the corrosion resistance of stainless steel. Simultaneously, improper heat treatment processes can cause coarsening of the precipitates, significantly reducing the steel's plasticity and toughness. Compared to traditional carbon steel 22MnB5, the hot stamping process for martensitic stainless steel has its own characteristics. Therefore, a hot stamping process specifically designed for stainless steel should be developed to obtain hot-stamped materials with high strength and high corrosion resistance.

[0005] PCT patent application No. 201880011193.7 discloses a "Steel for manufacturing a component by hot forming and use of the component". The method describes a steel for manufacturing a component from stainless steel and describes the use of the component. In the invention, C≤0.2%, Si≤3.5%, Mn 1.5%~16%, Cr 8%~14%, Ni≤6%, N≤1%, Nb≤1.2%, Ti≤1.2%, Mo≤2.0%, V≤0.15%, Cu≤2.0%, Al≤0.2%, B≤0.05% are defined. Since the scope of the patent is wide, it can be used for transportation components of vehicles and also for high-strength crash safety roll bars, or complex structural components such as columns or fairings; even for anti-graffiti solutions, such as surfaces of park benches, railways; and for tableware. The composition and process specified in the patent are broad, and the heat treatment process is not studied in detail, and by controlling the heat treatment process, the corrosion resistance is optimized.

[0006] Patent application No. CN 116144895 A provides a heat treatment process for preparing high-strength stainless steel formed components, which is widely used in cutters, turbine blades, bearings, valves, structural parts and wear-resistant parts, etc. The method obtains a high-strength high-elongation product (tensile strength 1600MPa~1900MPa, elongation 9.0%~15%) through high-temperature solid solution aging and low-temperature tempering, and a stainless steel product with good corrosion resistance (pitting potential in 3.5% NaCl solution reaches 270mV or more), high corrosion resistance, high toughness and high plasticity are achieved at the same time. The method improves the corrosion resistance of the steel grade by increasing the carbide solid solution of Cr, and the appropriate amount of austenite structure in the organization is obtained by partitioning during low-temperature tempering to obtain good elongation. Since the patent has high solid solution temperature and long time, the green environmental protection performance is poor. At the same time, due to the use of two-stage heating, the process is complex, and the heat treatment cost is high. SUMMARY

[0007] The present application provides a 1500MPa level high-strength high-corrosion-resistant martensitic stainless steel hot stamping process, which is suitable for martensitic stainless steel with hot-rolled plate and cold-rolled plate as base material, and is produced by heat treatment method to manufacture components with high strength and high corrosion resistance. Through heat treatment, the effective control of Cr 23 C6, Cr2N, (Nb, V)(C, N) and other precipitates and microstructure regulation make the material after hot stamping forming have the characteristics of high strength and high corrosion resistance.

[0008] In order to achieve the above purpose, the technical scheme adopted by the present application is as follows:

[0009] 1500MPa grade high-strength and high-corrosion-resistance martensitic stainless steel hot stamping process, different hot stamping processes are adopted for different stainless steel plates produced by hot rolling and cold rolling processes, and the specific contents include:

[0010] For the stainless steel plate produced by the hot rolling process: the steel plate is heated and held; the heating temperature is 870-925℃, and the holding time is 15-20min after reaching the temperature; or the heating temperature is 930-1050℃, and the holding time is 3-5min after reaching the temperature; the temperature will drop when transferring from the heating furnace to the hot stamping die, and the closing temperature needs to be ensured between 720℃-870℃, and the closing pressure is ≥20MPa; the cooling rate after closing is ≥0.1℃ / s, and the cooling rate is preferably 5-20℃ / s based on the operability of production; the die is opened when the temperature of the steel is ≤180℃;

[0011] For the stainless steel plate produced by the cold rolling process: the steel plate is heated and held; the heating temperature is 890-945℃, and the holding time is 15-20min; or the heating temperature is 950-1100℃, and the holding time is 3-5min; the temperature will drop when transferring from the heating furnace to the hot stamping die, and the closing temperature needs to be ensured between 720℃-870℃, and the closing pressure is ≥20MPa; the cooling rate after closing is ≥0.1℃ / s, and the cooling rate is preferably 5-20℃ / s based on the operability of production; the die is opened when the temperature of the steel is ≤180℃;

[0012] The thickness of the stainless steel plate produced by the hot rolling process is 3-8mm.

[0013] Preferably, for the stainless steel plate produced by the hot rolling process, the heating temperature is 950-1000℃, and the holding time is 3-5min.

[0014] The hot rolling process includes: the billet heating temperature is 1180-1280℃, the intermediate billet thickness before finish rolling is 30-60mm, the total reduction rate of finish rolling is ≥85%, the hot rolling final rolling temperature is 840-950℃, the rolling is cooled to the coiling temperature, and the coiling temperature is 650-750℃, to obtain the hot rolled stainless steel plate.

[0015] The thickness of the stainless steel plate produced by the cold rolling process is 1.5-3mm.

[0016] Preferably, for the stainless steel plate produced by the cold rolling process, the heating temperature is 980-1000℃, and the holding time is 3-5min.

[0017] The cold rolling process comprises: the billet heating temperature is 1180-1280 DEG C, the intermediate billet thickness before finish rolling is 30-60mm, the total reduction of finish rolling is greater than or equal to 85%, the hot rolling final rolling temperature is 840-950 DEG C, the hot rolling is cooled to the coiling temperature, and the coiling temperature is 650-750 DEG C, so that the 3.0-8mm hot-rolled steel plate is obtained; then the hot-rolled steel plate is subjected to the bell annealing treatment, the annealing temperature of the bell annealing is 830-870 DEG C, and the holding time is 6-12h; then the hot-rolled steel plate is subjected to the pickling, and then the cold rolling is carried out with the cumulative reduction of 30%-60%.

[0018] The stainless steel hot forming product produced by the hot stamping process of the 1500MPa high-strength and high-corrosion-resistance martensitic stainless steel has the following chemical components in percentage by weight: C: 0.06%-0.10%, Si: 0.08%-0.12%, Mn: 0.8%-1.2%, P≤0.012%, S≤0.005%, Cr: 12.8%-13.5%, N: 0.04%-0.08%, Als: 0.02%-0.04%, V: 0.08%-0.12%, Nb: 0.03%-0.06%, and the balance of Fe and inevitable impurities.

[0019] The martensitic stainless steel mentioned in the application has sufficient hardenability and belongs to air-hardening steel. 23 The C6 precipitation temperature is 830-885 DEG C, the Cr2N precipitation temperature is 820-870 DEG C, the martensitic transformation starting temperature is 270-320 DEG C, and the ending temperature is 180-225 DEG C. 23 The C6, (Nb, V)(C, N) and other phase compositions and the precipitation phase microstructure are controlled to obtain the high-strength and high-corrosion-resistance hot stamping stainless steel composition system.

[0020] The cold-rolled plate and the hot-rolled plate need to be subjected to the hot stamping forming process after the cold rolling process and the hot rolling process, so that the required product can be obtained.

[0021] The technical scheme of the application specifically comprises a 1500MPa high-strength and high-corrosion-resistance martensitic stainless steel hot stamping process.

[0022] 1. heating and holding: the steel plate is heated to an appropriate temperature and held for a certain time to be completely austenitized and keep good plasticity.

[0023] 2. Transfer and die closing: the heated steel plate is taken out of the heating furnace and placed in the hot forming die, which needs to ensure that the steel plate can be quickly transferred to the die, reduce the oxidation degree of the steel plate at high temperature, and ensure that it still maintains a high temperature during forming, reduces the deformation resistance and prevents cracking during stamping.

[0024] 3. Stamping and quenching: after the steel plate is placed in the die, it is immediately stamped and formed, and after the die is kept for a period of time, the steel plate is quenched using a water channel to achieve uniform martensite organization and obtain qualified size and performance.

[0025] 4. Die opening: when the temperature of the hot stamped steel is reduced below the martensite phase transition point, the die is opened to obtain the high-strength corrosion-resistant hot stamped stainless steel with the performance.

[0026] Reason for process design:

[0027] The mechanical properties and corrosion resistance of the ultra-high strength corrosion-resistant hot-formed martensitic stainless steel after hot stamping mainly depend on the chemical composition, martensite morphology and the dispersion distribution of chromium carbide and nitride, that is, the heating temperature, holding time and die closing temperature during austenitizing. Therefore, reasonable control of the temperature of each process section is the key to obtaining the ideal organization. Different heating temperatures affect the size of austenite and the resolubility of chromium carbide and nitride. With the increase of austenitizing temperature, chromium carbide and nitride can be fully resolubilized to avoid the decrease of corrosion resistance caused by local chromium depletion. At the same time, if the heating temperature is too high, the austenite grain size will increase, which will reduce the mechanical properties. Therefore, the austenitizing temperature of the hot stamped stainless steel hot-rolled product is 930-1050℃, the holding time is 3-5min, and 950-1000℃ is the optimal choice. At the same time, when the heating temperature is relatively low (but higher than the Cr 23 C6 precipitation temperature), in order to achieve sufficient resolubility, low-temperature long-time austenitizing method can also be used to obtain hot stamped stainless steel with qualified strength and corrosion resistance. The optimal process system for low-temperature long-time heating is 875-925℃, holding for 15-20min.

[0028] At the same time, the cold-rolled martensitic stainless steel will cause a large amount of Cr 23 C6 precipitation due to the addition of softening annealing process under the cover furnace process in the process, which will cause insufficient mechanical properties and corrosion resistance. Therefore, the hot stamping temperature and holding time of the martensitic cold-rolled plate during hot stamping are higher than those of the hot-rolled plate. The heating process is 950-1100℃, the holding time is 3-5min, and the optimal choice is that the heating temperature is 980-1000℃; or the heating temperature is 890-945℃, and the holding time is 15-20min.

[0029] At the same time, in the two transfer processes, Cr 23The C6 is precipitated again, so the hot stamping stainless steel should ensure that the die temperature of the steel is between 720℃ and 870℃.

[0030] Compared with the prior art, the present application has the beneficial effects that:

[0031] A hot stamping process suitable for martensitic stainless steel is provided, and the precipitation of Cr carbide, nitride and micro-alloying elements is controlled by the hot stamping process technology, and the Cr 23 C6, Cr2N, (Nb, V)(C, N) and other phase compositions and precipitated phase microstructure are regulated to obtain 1500MPa high-strength corrosion-resistant hot stamping stainless steel products. BRIEF DESCRIPTION OF DRAWINGS

[0032] Figure 1 The dynamic CCT curve of the 1500MPa grade hot stamping stainless steel of the present application is shown in the following table. DETAILED DESCRIPTION

[0033] The specific embodiments of the present application will be further described below in combination with examples, and the following examples are used to specifically illustrate the content of the present application, which are only general descriptions of the content of the present application, and do not limit the content of the present application.

[0034] The chemical composition and weight percentage of the 1500MPa grade hot forming steel in all the following examples are as follows: C: 0.09%, Si: 0.12%, Mn: 1.15%, P: 0.010%, S: 0.004%, Cr: 12.87%, N: 0.042%, Als: 0.035%, V: 0.10%, Nb: 0.057%, and the balance is Fe and unavoidable impurities. Different mechanical properties and corrosion resistance can be obtained by different hot stamping processes.

[0035] When the hot forming stainless steel is a hot-rolled plate with a thickness ranging from 3 to 8mm, the stamping heat treatment process is shown in Tables 1-3.

[0036] Table 1 Hot rolling process parameters of the steel in examples (A) 1-5

[0037]

[0038] Table 2 Hot forming process parameters of the steel in examples (A) 1-5

[0039]

[0040]

[0041] Table 3 Mechanical properties and corrosion resistance of the steel finally obtained in examples (A) 1-5

[0042] Examples Rp0.2 / MPa Rm / MPa A50 / ％ Pitting potential / V SCE ]] A-1-1 1222 1570 4.0 0.05 A-1-2 1143 1645 5.0 0.08 A-2-1 1220 1708 3.8 0.08 A-2-2 1205 1728 4.5 0.08 A-3-1 1270 1674 6.5 0.06 A-3-2 1145 1709 6.0 0.05 A-4-1 1235 1538 4.8 0.07 A-4-2 1098 1636 6.0 0.08 A-5-1 1277 1601 5.0 0.1 A-5-2 1176 1672 6.5 0.07

[0043] From the data in Table 1-Table 3, it can be seen that the technical scheme adopted in the present application, the prepared hot stamping steel plate has a tensile strength ≥ 1500MPa, and a pitting potential ≥ 0.05V SCE , and has good corrosion resistance.

[0044] When the cold-rolled plate of the hot forming stainless steel has a thickness in the range of 1.5-3mm, the stamping heat treatment process is shown in Table 4-Table 6.

[0045] Table 4 Cold rolling process parameters of the steel in Examples (B) 1-5

[0046]

[0047] Table 5 Hot forming process parameters of the steel in Examples (B) 1-5

[0048]

[0049]

[0050] Table 6 Mechanical properties and corrosion resistance of the steel finally obtained in Examples (B) 1-5

[0051] Examples Rp0.2 / MPa Rm / MPa A50 / ％ Pitting potential / V SCE ]] B-1-1 1115 1564 5 0.06 B-1-2 1100 1654 5.5 0.08 B-2-1 1123 1640 6.5 0.07 B-2-2 1079 1724 7.0 0.09 B-3-1 1285 1665 7.2 0.08 B-3-2 1158 1704 7.0 0.08 B-4-1 1317 1719 5.9 0.09 B-4-2 1254 1763 5.0 0.05 B-5-1 1293 1698 5.5 0.1 B-5-2 1240 1756 6.5 0.08

[0052] From the data in Table 4-Table 6, it can be seen that the technical scheme adopted in the present application, the prepared steel plate has a tensile strength ≥ 1500MPa, and a pitting potential ≥ 0.05V SCE , and has good corrosion resistance.

Claims

1. Hot stamping process of a high-strength and high-corrosion-resistant martensitic stainless steel of the 1500 MPa class, characterized in that, The different hot forming processes are used for different stainless steel plates produced by hot rolling and cold rolling processes, and the specific contents include: For the stainless steel plate produced by the hot rolling process: the plate is heated and kept warm; the heating temperature is 870-925℃, and the temperature is kept for 15-20min after heating; or the heating temperature is 930-1050℃, and the temperature is kept for 3-5min after heating; the die temperature of the plate is 720-870℃, and the die pressure is greater than or equal to 20MPa; the cooling rate after die closing is greater than or equal to 0.1℃ / s; the die is opened when the temperature of the steel is less than or equal to 180℃; For the stainless steel plate produced by the cold rolling process: the plate is heated and kept warm; the heating temperature is 890-945℃, and the temperature is kept for 15-20min; or the heating temperature is 980-1000℃, and the temperature is kept for 3-5min; the die temperature of the plate is 720-870℃, and the die pressure is greater than or equal to 20MPa; the cooling rate after die closing is greater than or equal to 0.1℃ / s; the die is opened when the temperature of the steel is less than or equal to 180℃; The stainless steel hot forming product produced by the hot stamping process of the 1500MPa grade high-strength and high-corrosion-resistance martensitic stainless steel has the following chemical components in terms of percentage by weight: C: 0.06%-0.10%, Si: 0.08%-0.12%, Mn: 0.8%-1.2%, P≤0.012%, S≤0.005%, Cr: 12.8%-13.5%, N: 0.04%-0.08%, Als: 0.02%-0.04%, V: 0.08%-0.12%, Nb: 0.03%-0.06%, and the balance is Fe and inevitable impurities.

2. The 1500 MPa grade high strength high corrosion resistant martensitic stainless steel hot stamping process according to claim 1, characterized in that, The thickness of the stainless steel plate produced by the hot rolling process is 3-8mm.

3. The 1500 MPa grade high strength high corrosion resistant martensitic stainless steel hot stamping process according to claim 1, characterized in that, For the stainless steel plate produced by the hot rolling process, the heating temperature is 950-1000℃, and the temperature keeping time is 3-5min.

4. The 1500 MPa grade high-strength high-corrosion-resistance martensitic stainless steel hot stamping process according to any one of claims 1-3, characterized in that, The hot rolling process includes: the heating temperature of the casting blank is 1180-1280℃, the thickness of the intermediate blank before finish rolling is 30-60mm, the total reduction rate of finish rolling is greater than or equal to 85%, the final rolling temperature of hot rolling is 840-950℃, the plate is cooled to the coiling temperature after rolling, the coiling temperature is 650-750℃, and the hot rolled stainless steel plate is obtained.

5. The 1500 MPa grade high strength high corrosion resistant martensitic stainless steel hot stamping process according to claim 1, characterized in that, The thickness of the stainless steel plate produced by the cold rolling process is 1.5-3mm.

6. The 1500 MPa grade high strength high corrosion resistant martensitic stainless steel hot stamping process according to claim 1, characterized in that, For the stainless steel plate produced by the cold rolling process, the heating temperature is 980-1000℃, and the temperature keeping time is 3-5min.

7. The 1500 MPa grade high strength high corrosion resistant martensitic stainless steel hot stamping process according to claim 1 or 5 or 6, characterized in that, The cold rolling process includes: the heating temperature of the casting blank is 1180-1280℃, the total reduction rate of finish rolling is greater than or equal to 85%, the final rolling temperature of hot rolling is 840-950℃, the plate is cooled to the coiling temperature after rolling, the coiling temperature is 650-750℃, and the 3.0-8mm hot rolled plate is obtained; then the hot rolled plate is subjected to cover annealing treatment, the annealing temperature of the cover annealing is 830-870℃, and the temperature keeping time is 6-12h; then the plate is pickled, and the cold rolling is carried out with a cumulative reduction of 30%-60% after pickling.

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