1500MPa-grade high-strength high-corrosion-resistance martensitic stainless steel hot stamping process

Abstract

The invention relates to a 1500MPa-grade high-strength and high-corrosion-resistance martensitic stainless steel hot stamping process which is characterized in that 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 minutes after reaching the temperature; or the heating temperature is 930-1050 DEG C, and the temperature is kept for 3-5 min after the temperature is reached; the cooling rate is larger than or equal to 0.1 DEG C / s, and when the temperature of the steel plate reaches 720-870 DEG C, die assembly is conducted, and the pressure is larger than or equal to 20 MPa; the cooling rate after die assembly is greater than or equal to 0.1 DEG C / s; opening the mold when the temperature is less than or equal to 180 DEG C; for a stainless steel plate produced by a cold rolling process, the heating temperature is 890-945 DEG C, and heat preservation is carried out for 15-20 min; or the heating temperature ranges from 950 DEG C to 1100 DEG C, and heat preservation is conducted for 3 The cooling rate is larger than or equal to 0.1 DEG C / s, when the temperature reaches 720-870 DEG C, die assembly is conducted, and the pressure is larger than or equal to 20 MPa; the cooling rate after die assembly is greater than or equal to 0.1 DEG C / s; opening the mold when the temperature is less than or equal to 180 DEG C; precipitation of carbides, nitrides and microalloy elements of Cr is controlled through a hot stamping process technology, and phase composition of Cr23C6, Cr2N, (Nb, V) (C, N) and the like and precipitated phase microstructure regulation and control are carried out.

Description

Technical Field

[0001] The present application relates to the field of heat treatment technology for martensitic stainless steel in the manufacture of high-strength steel plates for automobiles, and in particular to a method for producing a high-strength steel plate with a tensile strength of 1500 MPa and a pitting potential of ≥0.05 V. SCE Hot stamping process of hot stamping stainless steel plate with good corrosion resistance. Background Art

[0002] Lightweighting of automobiles can effectively improve fuel efficiency, driving performance and safety, and is one of the important concerns of 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] At present, stainless steel is being tried in the production process of automobile bodies for its green and environmentally friendly characteristics throughout its life cycle. Usually, carbon steel has a huge application as steel for automobile parts. In recent years, automobile manufacturers have tried to use stainless steel bodies and ultra-high-strength stainless steel structural parts as body frames to make them have the advantages of high strength, corrosion resistance, and no painting. Not only can it reduce the steps of pickling, degreasing, phosphating, and painting in the body manufacturing process, but it can also greatly reduce costs in the production and manufacturing process, and has the advantages of green environmental protection. Stainless steel adds more precious metals such as Cr, Ni, and Mo to the composition system, but stainless steel can be 100% recycled without downgrading and recycling. There is no metal loss during use, which reduces environmental pollution caused by metal corrosion and deterioration. The application cost is lower and more environmentally friendly throughout the life cycle.

[0004] In the existing technology, carbon steel 22MnB5 is usually used to produce 1500MPa-level high-strength parts through hot stamping process. There are also technologies that try to use martensitic stainless steel to produce 1500MPa-level high-strength parts through hot stamping technology to solve the problems of coating and high-temperature oxidation and decarburization in the production process of carbon steel. Stainless steel has its own problems when used for hot stamping. If the heat treatment process is unreasonable, it will lead to low strength, insufficient corrosion resistance and other problems. For example, if the alloy element Cr is added more, the critical cooling rate is low. Due to the increase of Cr content in hot stamping stainless steel, Cr 23 The tendency of C6 and Cr2N intermetallic compounds to precipitate increases. Excessive Cr precipitates cause local Cr depletion in the matrix, which becomes an area prone to pitting corrosion, thereby reducing the corrosion resistance of stainless steel. At the same time, improper heat treatment process will cause the coarsening of precipitates, significantly reducing the plasticity and toughness of steel. Compared with traditional carbon steel 22MnB5, the hot stamping process of martensitic stainless steel has its own characteristics. According to the characteristics of stainless steel materials, the hot stamping process for this material should be designed to obtain hot stamping materials with high strength and high corrosion resistance.

[0005] The PCT patent application with application number 201880011193.7 discloses a "Steel for manufacturing parts by hot forming and the use of the parts". The method describes the use of stainless steel to manufacture parts and describes the use of the parts. The invention stipulates that C≤0.2%, Si≤3.5%, Mn1.5%~16%, Cr8%~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%. Due to the wide scope of application of the patent, it can be used for transportation components of vehicles, high-strength collision safety rollover protection bars, or complex structural components such as columns or fairings; even for anti-graffiti solutions, such as park benches, railway surfaces; and for tableware. As a result, the composition and process regulations in the patent are broad, and the heat treatment process is not studied in detail. The corrosion resistance is optimized by regulating the heat treatment process.

[0006] The patent application with publication number CN 116144895 A provides a heat treatment process for preparing high-strength stainless steel formed components, which are widely used in cutting tools, turbine blades, bearings, valves, structural parts and wear-resistant parts. This method obtains high-strength and high-elongation products (tensile strength 1600MPa~1900MPa, elongation 9.0%~15%) through high-temperature solution aging and low-temperature tempering treatment, and stainless steel products with good corrosion resistance (pitting potential reaches more than 270mV in a 3.5% mass fraction NaCl solution), and high corrosion resistance, high toughness and strong plasticity are achieved at the same time. This method improves the corrosion resistance of steel by increasing the solid solution of Cr carbides, and through the partitioning during the low-temperature tempering process, a proper amount of austenite structure is present in the organization to obtain good elongation. Due to the high solid solution temperature and long time of this patent, the green environmental protection performance is poor. At the same time, due to the two-stage heating, the process is complicated and the heat treatment cost is high. Summary of the invention

[0007] The present invention provides a 1500MPa grade high-strength and high-corrosion-resistant martensitic stainless steel hot stamping process, which is suitable for martensitic stainless steel whose substrate is hot-rolled plate and cold-rolled plate, and is produced by heat treatment method to manufacture parts with high strength and high corrosion resistance. 23 The precipitation phases such as C6, Cr2N, (Nb, V) (C, N) and microstructure regulation make the material after hot stamping have the characteristics of high strength and high corrosion resistance.

[0008] In order to achieve the above object, the present invention adopts the following technical solutions:

[0009] The hot stamping process of 1500MPa grade high-strength and high-corrosion-resistant martensitic stainless steel adopts different hot forming processes for different stainless steel plates under hot rolling and cold rolling processes. The specific contents include:

[0010] For stainless steel plates produced by hot rolling process: heat and keep the steel plates warm; heating temperature is 870-925℃, keep warm for 15-20min after reaching temperature; or heating temperature is 930-1050℃, keep warm for 3-5min after reaching temperature; transferring from heating furnace to hot stamping die will cause temperature drop, it is necessary to ensure that the mold closing temperature is between 720℃ and 870℃, and the mold closing pressure is ≥20MPa; cooling rate after mold closing is ≥0.1℃ / s. Based on the operability of production, the cooling rate is preferably 5-20℃ / s; open the mold when the temperature of steel is ≤180℃;

[0011] For stainless steel plates produced by cold rolling process: heat and keep the steel plates warm; heating temperature is 890-945℃, keep warm for 15-20min; or heating temperature is 950-1100℃, keep warm for 3-5min; transfer from heating furnace to hot stamping die will cause temperature drop, it is necessary to ensure that the mold temperature is between 720℃ and 870℃, and the mold pressure is ≥20MPa; cooling rate after mold closing is ≥0.1℃ / s. Based on the operability of production, the cooling rate is preferably 5-20℃ / s; open the mold when the temperature of steel is ≤180℃;

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

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

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

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

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

[0017] The cold rolling process comprises: heating the ingot at a temperature of 1180-1280°C, the thickness of the intermediate ingot before finishing rolling is 30-60 mm, the total finishing reduction rate is ≥85%, the hot rolling final rolling temperature is 840-950°C, cooling to the coiling temperature after rolling, the coiling temperature is 650-750°C, and a 3.0-8 mm hot-rolled steel plate is obtained; then the hot-rolled steel plate is subjected to a hood annealing treatment, the annealing temperature of the hood annealing is 830-870°C, and the holding time is 6-12 hours; then pickling is performed, and after pickling, cold rolling is performed with a cumulative reduction of 30%-60%.

[0018] The stainless steel hot-formed products produced by the hot stamping process of 1500MPa grade high-strength and high-corrosion-resistant martensitic stainless steel, the chemical composition of the steel is as follows by weight percentage: 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%, the balance is Fe and unavoidable impurities.

[0019] The martensitic stainless steel mentioned in the present invention has sufficient hardenability and belongs to air-quenched steel. When the composition design satisfies the cooling rate ≥ 0.1°C / s, there is no ferrite and pearlite phase transformation in the microstructure. Cr 23 The precipitation temperature of C6 is 830~885℃, and the precipitation temperature of Cr2N is 820~870℃. The starting temperature of martensitic transformation is 270~320℃, and the ending temperature is 180~225℃. In the composition design, in order to achieve the required strength, there must be a full austenite area and avoid the formation of high-temperature ferrite. The carbon content of the alloy should be in a reasonable phase area, and some nitrogen should be added to avoid the formation of high-temperature ferrite. Microalloying with N, Nb, V and other elements to improve strength and corrosion resistance, Cr 23 By regulating the phase composition of C6, (Nb, V) (C, N) and the microstructure of the precipitated phase, a high-strength and corrosion-resistant hot stamping stainless steel composition system is obtained.

[0020] After the cold rolling process and hot rolling process, the cold rolled plates and hot rolled plates still need to go through a hot stamping process to obtain the desired products.

[0021] The technical solution of the present invention specifically includes a 1500MPa grade high-strength and high-corrosion-resistant martensitic stainless steel hot stamping process, which is similar to traditional hot stamping steel and includes multiple links such as heating, forming, and quenching. The hot forming process includes the following steps:

[0022] 1. Heating and insulation: Heat the steel plate to an appropriate temperature and keep it warm for a certain period of time to make it completely austenitized and maintain good plasticity.

[0023] 2. Transfer and mold closing: Take the heated steel plate out of the heating furnace and place it in the hot forming mold. This requires ensuring that the steel plate can be quickly transferred to the mold, reducing the degree of oxidation of the steel plate at high temperature, and ensuring that it still maintains a high temperature during forming, reducing deformation resistance and preventing cracking during stamping.

[0024] 3. Stamping and quenching: After the steel plate is placed in the mold, it is immediately stamped and formed. After the mold is kept under pressure for a period of time, the steel plate is quenched using a water channel to achieve a uniform martensitic structure and obtain qualified size and performance.

[0025] 4. Die opening: When the temperature of the hot stamping steel drops below the martensitic transformation point, the die is opened to obtain high-strength and corrosion-resistant hot stamping stainless steel with the above properties.

[0026] Reasons for the process design of the present invention:

[0027] The mechanical properties and corrosion resistance of 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 carbides and nitrides, which are related to the heating temperature, holding time and mold closing temperature during austenitization. Therefore, reasonable control of the temperature of each process section is the key to obtaining an ideal organization. Different heating temperatures affect the size of austenite and the dissolution of Cr carbides and nitrides. As the austenitizing temperature increases, the chromium carbides and nitrides can be fully dissolved to avoid the decrease in corrosion resistance caused by local chromium deficiency. At the same time, too high a heating temperature will cause the austenite grain size to increase and reduce the mechanical properties. Therefore, the austenitizing temperature for hot-rolled products of hot stamping stainless steel is 930-1050℃, and the holding time is 3-5min, and 950-1000℃ is the best choice. At the same time, when the heating temperature is low (but higher than Cr 23 C6 precipitation temperature) Due to the kinetics of carbonitride dissolution, in order to achieve a sufficient dissolution effect, the low-temperature and long-time austenitization method can be used to obtain hot stamping stainless steel with qualified strength and corrosion resistance. The optimal process system of low temperature and long heating time is 875-925℃, and the insulation time is 15-20min.

[0028] At the same time, the martensitic stainless steel cold-rolled sheet will cause Cr to be annealed due to the addition of a bell-type furnace process in the process. 23 The large amount of precipitation of C6 causes the insufficiency of mechanical properties and corrosion resistance. Therefore, the hot stamping temperature and holding time of martensitic cold rolled sheet are higher than those of hot rolled sheet during hot stamping. The heating process is 950-1100℃, holding for 3-5min, and the best choice is heating temperature of 980-1000℃; or heating temperature of 890-945℃, holding for 15-20min.

[0029] At the same time, Cr 23C6 will precipitate again, so the mold temperature of hot stamping stainless steel should be between 720℃ and 870℃.

[0030] Compared with the prior art, the present invention has the following beneficial effects:

[0031] A hot stamping process suitable for martensitic stainless steel is provided. The hot stamping process technology is used to control the precipitation of Cr carbides, nitrides and micro-alloy elements to carry out Cr 23 By regulating the phase composition of C6, Cr2N, (Nb, V) (C, N) and the microstructure of the precipitated phase, a 1500MPa high-strength and corrosion-resistant hot stamping stainless steel product is obtained. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 This is the dynamic CCT curve of the 1500MPa grade hot stamping stainless steel according to the embodiment of the present invention. DETAILED DESCRIPTION

[0033] The specific implementation modes of the present invention are further described below in conjunction with examples. The following examples are used to specifically illustrate the contents of the present invention. These examples are only general descriptions of the contents of the present invention and do not limit the contents of the present invention.

[0034] The chemical composition and weight percentage of the 1500MPa grade hot-formed steel in all the following embodiments are: 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 through different hot stamping processes.

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

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

[0037]

[0038] Table 2 Hot forming process parameters of 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] Example Rp0.2 / MPa Rm / MPa A50 / ％ <![CDATA[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 Tables 1 to 3, it can be seen that the technical solution adopted by the present invention can produce a hot-rolled hot stamped steel plate with a tensile strength of ≥1500MPa and a pitting potential of ≥0.05V. SCE It is a hot stamped stainless steel with a tensile strength of 1500MPa and good corrosion resistance.

[0044] When the thickness of hot-formed stainless steel is cold-rolled plate ranging from 1.5 to 3 mm, the stamping heat treatment process is shown in Table 4 to Table 6.

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

[0046]

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

[0048]

[0049]

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

[0051] Example Rp0.2 / MPa Rm / MPa A50 / ％ <![CDATA[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 Tables 4 to 6, it can be seen that the technical solution adopted by the present invention can produce a steel plate with a tensile strength of ≥1500MPa and a pitting potential of ≥0.05V. SCE It is a hot stamped stainless steel with a tensile strength of 1500MPa and good corrosion resistance.

Claims

1. 1500MPa grade high strength and high corrosion resistance martensitic stainless steel hot stamping process, characterized by: Different hot forming processes are used for different stainless steel plates under hot rolling and cold rolling processes. The specific contents include: For stainless steel plates produced by hot rolling process: heat and keep the steel plates warm; heating temperature is 870-925℃, keep warm for 15-20min after reaching temperature; or heating temperature is 930-1050℃, keep warm for 3-5min after reaching temperature; the mold closing temperature of the steel plate is 720-870℃, and the mold closing pressure is ≥20MPa; the cooling rate after mold closing is ≥0.1℃ / s; open the mold when the temperature of the steel is ≤180℃; For stainless steel plates produced by cold rolling process: heat and keep the steel plates warm; heating temperature is 890-945℃, keep warm for 15-20min; or heating temperature is 950-1100℃, keep warm for 3-5min; the mold closing temperature of the steel plate is 720-870℃, and the mold closing pressure is ≥20MPa; the cooling rate after mold closing is ≥0.1℃ / s; open the mold when the temperature of the steel is ≤180℃.

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

3. The hot stamping process for 1500MPa grade high strength and high corrosion resistance martensitic stainless steel according to claim 1, characterized in that: For the stainless steel plate produced by hot rolling process, the heating temperature is 950-1000°C and the insulation time is 3-5 minutes.

4. The hot stamping process for 1500MPa high-strength and high-corrosion-resistant martensitic stainless steel according to any one of claims 1 to 3, characterized in that: The hot rolling process includes: the heating temperature of the casting is 1180-1280°C, the thickness of the intermediate billet before finishing rolling is 30-60mm, the total reduction rate of finishing rolling is ≥85%, the hot rolling final rolling temperature is 840-950°C, and the hot rolling is cooled to the coiling temperature after rolling, and the coiling temperature is 650-750°C to obtain a hot-rolled stainless steel plate.

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

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

7. The 1500MPa grade high strength and high corrosion resistance martensitic stainless steel hot stamping process according to claim 1, 5 or 6, characterized in that: The cold rolling process includes: heating the ingot at a temperature of 1180-1280°C, a total finishing reduction rate of ≥85%, a hot rolling final rolling temperature of 840-950°C, cooling to a coiling temperature after rolling, a coiling temperature of 650-750°C, and obtaining a 3.0-8 mm hot-rolled steel plate; then subjecting the hot-rolled steel plate to a hood annealing treatment, the annealing temperature of the hood annealing being 830-870°C, and the holding time being 6-12 hours; then pickling, and cold rolling with a cumulative reduction of 30%-60% after pickling.

8. A stainless steel hot-formed product produced by the 1500MPa grade high-strength and high-corrosion-resistant martensitic stainless steel hot stamping process as claimed in any one of claims 1 to 8, characterized in that: The chemical composition of the steel by weight percentage is: 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 unavoidable impurities.

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