Production method of 1500mpa high corrosion-resistant hot stamping stainless steel cold-rolled strip steel

Abstract

This invention relates to a method for producing 1500MPa high corrosion-resistant hot-stamped stainless steel cold-rolled strip. The chemical composition of the strip 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%, with the balance being Fe and impurities. The production method includes hot metal pretreatment, smelting, refining, continuous casting, hot rolling, spheroidizing annealing, pickling, cold rolling, and hot stamping forming processes. This invention is suitable for large-scale production operations, and the products have the characteristics of high strength, toughness, and good stamping performance. The thickness of the finished product is 1.0 to 3.0 mm. The thin strip steel produced by the method described in this invention has high quality stability and is suitable for manufacturing hot-formed parts.

Description

Technical Field

[0001] This invention relates to the field of martensitic stainless steel cold-rolled strip production technology, and in particular to an industrial manufacturing method for hot-formed stainless steel sheet cold-rolled products with a tensile strength of up to 1500 MPa, a pitting potential ≥0.05 VSCE, and 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. Currently, high-strength automotive parts with a strength of 1500MPa are typically produced using 22MnB5 carbon steel through hot stamping. However, this process presents an oxidation problem during the austenitization of hot-formed steel. Approximately half of the hot-formed steel sheets used globally are coated with Al-Si to prevent oxidation and decarburization at high temperatures, increasing production costs and causing scaling on the die surfaces during hot stamping, affecting die life and increasing cleaning costs. Furthermore, due to the alloy composition of carbon steel, it requires painting to protect it from corrosion throughout its service life, adding to production processes and costs, while also contributing to environmental pollution.

[0003] Stainless steel is gaining increasing attention in the automotive industry due to its low life-cycle cost of materials (LCA). Stainless steel car bodies and ultra-high-strength stainless steel structural components, serving as the car body frame, offer advantages such as high strength, corrosion resistance, and paint-free operation. Applying it to hot stamping production not only solves problems like high-temperature oxidation and mold scaling associated with carbon steel during hot stamping, but also reduces the need for pickling, degreasing, phosphating, and painting processes in car body manufacturing, significantly lowering production costs and meeting environmental protection requirements. Furthermore, stainless steel is 100% recyclable without degradation, resulting in minimal metal loss during its use and reducing environmental pollution caused by metal corrosion and deterioration. This makes its life-cycle application cost lower and more environmentally friendly.

[0004] Hot-formed steel, due to its high strength, is commonly used in automotive safety components such as front and rear bumpers, door anti-collision beams, and A-pillars, B-pillars, C-pillars, and center consoles. Due to the need for lightweighting, hot-formed steel parts in passenger vehicles have relatively small thicknesses and are typically made from cold-rolled hot-formed steel strip.

[0005] PCT patent application No. 201880011193.7 discloses a "steel for manufacturing parts by hot forming and the use of such parts." It describes the use of stainless steel in manufacturing parts and the applications of such parts. The chemical composition of the specified steel is as follows: 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%. The product can be used in vehicle transport components, high-strength collision safety rollover protection bars, or complex structural components such as pillars or fairings; even in anti-graffiti solutions, such as park benches and railway surfaces; and in tableware. However, the limitations on its composition and processing are quite broad, and it does not provide a cold-rolled sheet preparation process suitable for large-scale production operations.

[0006] Based on this, the present invention proposes a high-strength, corrosion-resistant hot-stamped stainless steel and designs a process route for cold-rolled strip steel suitable for large-scale industrial production of this steel. Summary of the Invention

[0007] This invention provides a method for producing 1500MPa high corrosion-resistant hot-stamping stainless steel cold-rolled strip, which is suitable for large-scale production operations. The product has the characteristics of high strength, toughness and good stamping performance, and the finished product thickness is 1.0 to 3.0 mm. The thin strip produced by the method of this invention has high quality stability and is suitable for manufacturing hot-formed parts.

[0008] To achieve the above objectives, the present invention employs the following technical solution:

[0009] A method for producing 1500MPa high corrosion-resistant hot-stamped stainless steel cold-rolled strip includes hot metal pretreatment, smelting, refining, continuous casting, hot rolling, spheroidizing annealing, pickling, cold rolling, and hot stamping forming processes; the specific process control is as follows:

[0010] 1) Steelmaking: After pretreatment, molten iron is smelted in a converter or electric furnace, and then refined into refined steel through ladle refining.

[0011] 2) Continuous casting: The target temperature of the tundish in continuous casting is controlled at 20-35℃ above the liquidus temperature. The secondary cooling adopts a weak cooling regime with a secondary cooling water flow rate of 220-230L / min. Dynamic light reduction is adopted with a reduction of 4-12mm. The thickness of the continuous casting billet is 70-200mm.

[0012] 3) Hot rolling: The billet heating temperature is 1180~1280℃; after descaling by high pressure water, the rough rolling adopts multi-pass rolling; the thickness of the intermediate billet is 30~50mm, the total reduction rate of the finishing rolling is ≥85%, the hot rolling final rolling temperature is 840℃~950℃; the coiling temperature is 650℃~750℃, and the thickness of the hot rolled plate is 4~10mm;

[0013] 4) Spheroidizing annealing: The hot-rolled plate is subjected to annealing in a hood. The annealing temperature of the hood is 850-870℃, the holding time is 6-12h, and the plate is held in the hood until it reaches below 400℃ and then air-cooled to room temperature.

[0014] 5) Pickling: Pickling removes the oxide scale from the surface of hot-rolled plates;

[0015] 6) Cold rolling: After pickling, cold rolling is carried out with a cumulative reduction rate of 30% to 60%; the leveling elongation rate is controlled at 0.5% to 1.2%;

[0016] 7) Hot stamping: After the cold-rolled coil is uncoiled and cut, it is hot stamped and formed, as follows:

[0017] (a) Austenitization of steel: Heat the cold-rolled sheet to 900-1050°C and hold for 3-10 minutes;

[0018] (b) Steel transfer: The heated cold-rolled sheet is quickly transferred to the hot stamping forming die, and the transfer time is controlled within 5 to 15 seconds; the hot stamping forming die is preheated to 50 to 200°C, and the cooling rate after hot stamping is 0.1 to 0.3°C / s;

[0019] (c) Hot stamping: The holding time for hot stamping is controlled at 5 to 10 seconds;

[0020] (d) Demolding: Demolding temperature ≤250℃.

[0021] Furthermore, the chemical composition of the strip 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%, with the balance being Fe and unavoidable impurities.

[0022] Furthermore, the thickness of the finished steel strip is 1.0 to 3.0 mm.

[0023] Furthermore, the tensile strength Rm of the finished strip steel product is ≥1500MPa, and the elongation A is... 50 ≥20%, pitting potential ≥0.05V SCE .

[0024] Compared with the prior art, the beneficial effects of the present invention are:

[0025] 1) By controlling the entire production process conditions of hot-stamped stainless steel cold-rolled strip, a uniform martensitic structure is obtained, preventing the occurrence of hot-rolling cracking.

[0026] 2) By subjecting cold-rolled steel sheets to spheroidizing annealing, cold-rolled steel sheets with tensile strength Rm≥1500MPa and elongation A50mm≥20% were obtained. These were then hot-stamped to obtain highly corrosion-resistant sheets (pitting potential ≥0.05V). SCE Finished stainless steel cold-rolled strip;

[0027] 3) The product has suitable mechanical properties and excellent processing performance, which is conducive to users to perform blanking and hot stamping processing. Attached Figure Description

[0028] Figure 1 This is a SEM micrograph of the stainless steel cold-rolled strip prepared in Example 1 of the present invention after being hot-stamped at 1000℃. Detailed Implementation

[0029] The present invention discloses a method for producing 1500MPa high corrosion-resistant hot-stamped stainless steel cold-rolled strip, comprising hot metal pretreatment, smelting, refining, continuous casting, hot rolling, spheroidizing annealing, pickling, cold rolling, and hot stamping forming processes; the specific control of the process is as follows:

[0030] 1) Steelmaking: After pretreatment, molten iron is smelted in a converter or electric furnace, and then refined into refined steel through ladle refining.

[0031] 2) Continuous casting: The target temperature of the tundish in continuous casting is controlled at 20-35℃ above the liquidus temperature. The secondary cooling adopts a weak cooling regime with a secondary cooling water flow rate of 220-230L / min. Dynamic light reduction is adopted with a reduction of 4-12mm. The thickness of the continuous casting billet is 70-200mm.

[0032] 3) Hot rolling: The billet heating temperature is 1180~1280℃; after descaling by high pressure water, the rough rolling adopts multi-pass rolling; the thickness of the intermediate billet is 30~50mm, the total reduction rate of the finishing rolling is ≥85%, the hot rolling final rolling temperature is 840℃~950℃; the coiling temperature is 650℃~750℃, and the thickness of the hot rolled plate is 4~10mm;

[0033] 4) Spheroidizing annealing: The hot-rolled plate is subjected to annealing in a hood. The annealing temperature of the hood is 850-870℃, the holding time is 6-12h, and the plate is held in the hood until it reaches below 400℃ and then air-cooled to room temperature.

[0034] 5) Pickling: Pickling removes the oxide scale from the surface of hot-rolled plates;

[0035] 6) Cold rolling: After pickling, cold rolling is carried out with a cumulative reduction rate of 30% to 60%; the leveling elongation rate is controlled at 0.5% to 1.2%;

[0036] 7) Hot stamping: After the cold-rolled coil is uncoiled and cut, it is hot stamped and formed, as follows:

[0037] (a) Austenitization of steel: Heat the cold-rolled sheet to 900-1050°C and hold for 3-10 minutes;

[0038] (b) Steel transfer: The heated cold-rolled sheet is quickly transferred to the hot stamping forming die, and the transfer time is controlled within 5 to 15 seconds; the hot stamping forming die is preheated to 50 to 200°C, and the cooling rate after hot stamping is 0.1 to 0.3°C / s;

[0039] (c) Hot stamping: The holding time for hot stamping is controlled at 5 to 10 seconds;

[0040] (d) Demolding: Demolding temperature ≤250℃.

[0041] Furthermore, the chemical composition of the strip 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%, with the balance being Fe and unavoidable impurities.

[0042] Furthermore, the thickness of the finished steel strip is 1.0 to 3.0 mm.

[0043] Furthermore, the tensile strength Rm of the finished strip steel product is ≥1500MPa, and the elongation A is... 50 ≥20%, pitting potential ≥0.05V SCE .

[0044] The rationale for the design of the above process and parameters is as follows:

[0045] This invention involves placing a 70-200mm thick cast billet into a heating furnace at a high temperature or after cooling to room temperature, and heating it at 1180-1280℃, followed by hot rolling. The final hot rolling temperature is between 840-950℃. When the final hot rolling temperature is below 840℃, the grains tend to form a mixed state of coarse and fine grains, which reduces the processing performance of the steel plate. When the final hot rolling temperature is above 950℃, the resulting grains are too coarse, which reduces the mechanical properties of the steel plate.

[0046] High-temperature coiling is beneficial to the forming performance of steel plates after hot rolling. Therefore, the present invention selects a coiling temperature of 650℃~750℃.

[0047] Because the critical cooling rate of hot rolling in this invention is extremely low, the steel plate after hot rolling has high strength, making it difficult to proceed with the next step of cold rolling. Therefore, this invention is designed to perform a bell-type annealing treatment on the hot-rolled plate before cold rolling. The annealing temperature of the bell-type annealing is 850-870℃. When the bell-type annealing temperature is below 850℃, the yield strength cannot be sufficiently reduced. When the bell-type annealing temperature is above 870℃, it will cause coarse grains and reduce the final mechanical properties of the steel plate.

[0048] Hot-rolled steel sheets are cold-rolled after pickling. Considering the rolling load and material properties, the cumulative reduction rate of cold rolling is set to 30% to 60%.

[0049] The hot stamping process is a key factor determining the final properties of steel sheets. This invention heats the cold-rolled sheet to between 900℃ and 980℃, which is beneficial for obtaining a uniform austenitic structure. When the heating temperature is below 900℃, Cr... 23 C6 cannot be dissolved in the matrix, resulting in poor corrosion resistance. When the heating temperature exceeds 980℃, it leads to rapid grain growth, reducing the mechanical properties of the steel plate.

[0050] The tensile strength Rm of the finished strip steel product is ≥1500MPa, and the elongation A is ≥1500MPa. 50 ≥20%, pitting potential ≥0.05V SCE .

[0051] The following embodiments are implemented based on the technical solution of the present invention, and provide detailed implementation methods and specific operation processes. However, the scope of protection of the present invention is not limited to the following embodiments.

[0052]

Example

[0053] In this embodiment, molten steel with the chemical composition shown in Table 1 is used for smelting, hot rolling is performed using the hot rolling process parameters shown in Table 2, spheroidizing annealing is performed using the hood annealing method with the process parameters shown in Table 3, and then the strip is kept at 400°C under a hood and air-cooled to room temperature. After hot stamping forming with the process parameters shown in Table 4, the mechanical properties of the finished strip are shown in Table 5. Figure 1 The image shows the SEM microstructure of the stainless steel cold-rolled strip prepared in Example 1 after hot stamping at 1000℃.

[0054] Table 1. Main chemical components of steel (wt%)

[0055] Example C Si Mn Cr N Als V Nb 1 0.084 0.083 0.815 13.07 0.044 0.033 0.117 0.032 2 0.072 0.117 0.954 13.21 0.069 0.024 0.095 0.054 3 0.097 0.094 1.176 12.92 0.053 0.028 0.086 0.049 4 0.085 0.105 1.088 13.48 0.075 0.036 0.108 0.039 5 0.066 0.089 0.886 12.87 0.061 0.038 0.113 0.058

[0056] Note: In each embodiment, P≤0.012%, S≤0.005%, and the balance is Fe and impurities.

[0057] Table 2 Hot Rolling Process Parameters

[0058]

[0059] Table 3 Cold rolling and spheroidizing annealing process parameters

[0060]

[0061] Table 4 Hot stamping forming process parameters

[0062]

[0063] Table 5 Performance of Finished Strip Steel

[0064]

[0065] As can be seen from Examples 1-5, the hot-stamped stainless steel cold-rolled strip prepared by the method described in this invention has a tensile strength ≥1500MPa, a pitting potential ≥0.05VSCE, and a tensile strength reaching the 1500MPa level, and also has good corrosion resistance.

[0066] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for producing 1500MPa high corrosion-resistant hot-stamped stainless steel cold-rolled strip, characterized in that, The chemical composition of the strip steel, by weight percentage, is: C: 0.084%–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%–0.028%, V: 0.08%–0.12%, Nb: 0.03%–0.06%, with the balance being Fe and unavoidable impurities. The production process includes hot metal pretreatment, smelting, refining, continuous casting, hot rolling, spheroidizing annealing, pickling, cold rolling, and hot stamping. Specific process controls are as follows: 1) Steelmaking: After pretreatment, molten iron is smelted in a converter or electric furnace, and then refined into refined steel through ladle refining; 2) Continuous casting: The target temperature of the tundish for continuous casting is controlled at 20-35°C above the liquidus temperature. The secondary cooling adopts a weak cooling regime with a secondary cooling water flow rate of 220-230 L / min. Dynamic light reduction is adopted with a reduction of 4-12 mm. The thickness of the continuous casting billet is 70-200 mm. 3) Hot rolling: The billet heating temperature is 1180~1280℃; after descaling by high pressure water, the rough rolling adopts multi-pass rolling; the thickness of the intermediate billet is 30~50mm, the total reduction rate of the finishing rolling is ≥85%, the hot rolling final rolling temperature is 900℃~950℃; the coiling temperature is 670℃~750℃, and the thickness of the hot rolled plate is 4~10mm; 4) Spheroidizing annealing: The hot-rolled plate is subjected to annealing in a hood. The annealing temperature of the hood is 850-870℃, the holding time is 6-12h, and the plate is held in the hood until it reaches below 400℃, and then air-cooled to room temperature. 5) Pickling: Pickling removes the oxide scale from the surface of hot-rolled plates; 6) Cold rolling: After pickling, cold rolling is carried out with a cumulative reduction rate of 30% to 48.5%; the leveling elongation rate is controlled at 0.5% to 1.2%. 7) Hot stamping: After the cold-rolled coil is uncoiled and cut, it is hot stamped and formed, as follows: (a) Austenitization of steel: Heat the cold-rolled sheet to 980-1050℃ and hold for 3-10 minutes; (b) Steel transfer: The heated cold-rolled sheet is quickly transferred to the hot stamping forming die, and the transfer time is controlled within 5 to 15 seconds; the hot stamping forming die is preheated to 50 to 200°C, and the cooling rate after hot stamping is 0.1 to 0.3°C / s; (c) Hot stamping: The holding time for hot stamping is controlled at 5 to 10 seconds; (d) Demolding: Demolding temperature ≤ 250℃; The tensile strength Rm of the finished steel strip is ≥1500MPa, and the pitting potential is ≥0.05V. SCE .

2. The method for producing 1500MPa high corrosion-resistant hot-stamped stainless steel cold-rolled strip according to claim 1, characterized in that, The thickness of the finished steel strip is 1.0 to 3.0 mm.

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