Production method of 1500MPa high-corrosion-resistance hot stamping stainless steel cold-rolled strip steel

Abstract

The invention relates to a production method of 1500MPa high-corrosion-resistance hot stamping stainless steel cold-rolled strip steel. The strip steel comprises the following chemical components: 0.06%-0.10% of C, 0.08%-0.12% of Si, 0.8%-1.2% of Mn, less than or equal to 0.012% of P, less than or equal to 0.005% of S, 12.8%-13.5% of Cr, 0.04%-0.08% of N, 0.02%-0.04% of Als, 0.08%-0.12% of V, 0.03%-0.06% of Nb and the balance of Fe and impurities. The production method comprises the working procedures of molten iron pretreatment, smelting, refining, continuous casting, hot rolling, spheroidizing annealing, acid pickling, cold rolling and hot stamping forming. The method is suitable for large-scale production operation, the product has the characteristics of high strength, good toughness and good stamping performance, and the thickness specification of the finished product is 1.0-3.0 mm; the thin-gauge strip steel produced through the method is high in quality stability and suitable for preparing hot forming parts.

Description

Technical Field

[0001] The present invention relates to the technical field of martensitic stainless steel cold-rolled steel strip production, and in particular to an industrial manufacturing method for a hot-formed stainless steel plate cold-rolled product with a tensile strength of 1500 MPa, a pitting potential of ≥0.05 VSCE and 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. At present, carbon steel 22MnB5 is usually used to produce 1500MPa-level high-strength parts for automobiles through hot stamping process, but there is an oxidation problem of hot-formed steel during austenitization. About half of the hot-formed steel plates used in the world are coated with Al-Si to prevent oxidation and decarburization at high temperatures, which increases the production cost of hot-formed steel and causes scaling on the surface of the mold during the hot stamping process, affecting the life of the mold and increasing the cleaning cost. In addition, due to the alloy composition characteristics of carbon steel, paint protection is required to protect the material from corrosion during its service life, which increases the production process and cost. At the same time, the production process further causes environmental pollution.

[0003] Stainless steel is gaining more and more attention from the automotive industry due to its low LCA over its entire life cycle. Stainless steel bodies and ultra-high-strength stainless steel structural parts serve as body frames, and have the advantages of high strength, corrosion resistance, and no painting. Applying them to hot stamping production can not only solve the problems of high-temperature oxidation and mold scaling of ordinary carbon steel during hot stamping, but also reduce the steps of pickling, degreasing, phosphating, and painting in the body manufacturing process, greatly reducing the production and manufacturing costs and meeting the requirements of green environmental protection; at the same time, stainless steel can be 100% recycled without downgrading and recycling, and the metal loss during use is extremely low, reducing the environmental pollution caused by metal rust and deterioration, and the application cost over the entire life cycle is lower and more environmentally friendly.

[0004] Due to its high strength, hot-formed steel is usually used in front and rear bumpers, door anti-collision beams, and body safety parts such as A-pillars, B-pillars, C-pillars, and center channels. Due to the demand for lightweight, the thickness specifications of hot-formed steel parts on passenger cars are relatively small, and they are usually made of hot-formed steel cold-rolled strip.

[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". It records the use of stainless steel for manufacturing parts and the use of parts. The chemical composition of the steel is defined 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%. Its products can be used for transportation components of vehicles, as well as for 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. However, the definition of its composition and process is relatively broad, and no cold-rolled sheet preparation process suitable for large-scale production operations and production is given.

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

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

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

[0009] A production method of 1500MPa high corrosion-resistant hot stamping stainless steel cold-rolled strip, including molten iron pretreatment, smelting, refining, continuous casting, hot rolling, spheroidizing annealing, pickling, cold rolling and hot stamping forming processes; specifically controlling the following processes:

[0010] 1) Steelmaking: After pretreatment of molten iron, it is smelted in a converter or electric furnace, and then refined by refining outside the furnace to obtain refined molten steel;

[0011] 2) Continuous casting: The target temperature of the continuous casting tundish is controlled at 20-35°C above the liquidus temperature. The secondary cooling adopts a weak cooling system with a secondary cooling water volume 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 heating temperature of the casting is 1180-1280℃; after high-pressure water descaling, the rough rolling adopts multi-pass rolling; the thickness of the intermediate billet is 30-50mm, the total reduction rate of 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 annealed at a temperature of 850-870°C for 6-12 hours. After being kept below 400°C with a hood, it is air-cooled to room temperature.

[0014] 5) Pickling: Remove the oxide scale on the surface of the hot-rolled plate by pickling;

[0015] 6) Cold rolling: cold rolling after pickling, the cumulative reduction rate of cold rolling is 30% to 60%; the flat elongation is controlled at 0.5% to 1.2%;

[0016] 7) Hot stamping: After the cold rolled coil is unrolled and cut into pieces, hot stamping is performed as follows:

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

[0018] (b) Steel transfer: The heated cold-rolled sheet is quickly transferred to the hot stamping die, and the transfer time is controlled within 5 to 15 seconds; the hot stamping 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 of hot stamping is controlled within 5 to 10 seconds;

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

[0021] Furthermore, the chemical composition of the strip steel is, by weight percentage, C: 0.06% to 0.10%, Si: 0.08% to 0.12%, Mn: 0.8% to 1.2%, P≤0.012%, S≤0.005%, Cr: 12.8% to 13.5%, N: 0.04% to 0.08%, Als: 0.02% to 0.04%, V: 0.08% to 0.12%, Nb: 0.03% to 0.06%, and the balance is Fe and unavoidable impurities.

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

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

[0024] Compared with the prior art, the present invention has the following beneficial effects:

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

[0026] 2) By spheroidizing annealing the cold-rolled sheet, a cold-hardened sheet with a tensile strength of Rm ≥ 1500MPa and an elongation of A50mm ≥ 20% is obtained, and then hot stamping is performed to obtain a high corrosion resistance (pitting potential ≥ 0.05V SCE ) of stainless steel cold-rolled strip products;

[0027] 3) The product has suitable mechanical properties and excellent processing performance, which is conducive to users' drop-part and hot stamping processing. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 This is a SEM microstructure photograph of the stainless steel cold-rolled strip prepared in Example 1 of the present invention after being hot stamped at 1000°C. DETAILED DESCRIPTION

[0029] The method for producing a 1500MPa high corrosion-resistant hot stamping stainless steel cold-rolled strip of the present invention comprises the steps of molten iron pretreatment, smelting, refining, continuous casting, hot rolling, spheroidizing annealing, pickling, cold rolling and hot stamping forming; the following processes are specifically controlled:

[0030] 1) Steelmaking: After pretreatment of molten iron, it is smelted in a converter or electric furnace, and then refined by refining outside the furnace to obtain refined molten steel;

[0031] 2) Continuous casting: The target temperature of the continuous casting tundish is controlled at 20-35°C above the liquidus temperature. The secondary cooling adopts a weak cooling system with a secondary cooling water volume 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 heating temperature of the casting is 1180-1280℃; after high-pressure water descaling, the rough rolling adopts multi-pass rolling; the thickness of the intermediate billet is 30-50mm, the total reduction rate of 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 annealed at a temperature of 850-870°C for 6-12 hours. After being kept below 400°C with a hood, it is air-cooled to room temperature.

[0034] 5) Pickling: Remove the oxide scale on the surface of the hot-rolled plate by pickling;

[0035] 6) Cold rolling: cold rolling after pickling, the cumulative reduction rate of cold rolling is 30% to 60%; the flat elongation is controlled at 0.5% to 1.2%;

[0036] 7) Hot stamping: After the cold rolled coil is unrolled and cut into pieces, hot stamping is performed as follows:

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

[0038] (b) Steel transfer: The heated cold-rolled sheet is quickly transferred to the hot stamping die, and the transfer time is controlled within 5 to 15 seconds; the hot stamping 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 of hot stamping is controlled within 5 to 10 seconds;

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

[0041] Furthermore, the chemical composition of the strip steel is, by weight percentage, C: 0.06% to 0.10%, Si: 0.08% to 0.12%, Mn: 0.8% to 1.2%, P≤0.012%, S≤0.005%, Cr: 12.8% to 13.5%, N: 0.04% to 0.08%, Als: 0.02% to 0.04%, V: 0.08% to 0.12%, Nb: 0.03% to 0.06%, and the balance is Fe and unavoidable impurities.

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

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

[0044] The design reasons for the above process and parameters are as follows:

[0045] The present invention loads a 70-200 mm thick ingot into a heating furnace at a high temperature or after cooling to room temperature, heats it at 1180-1280° C., and then performs hot rolling, wherein the hot rolling final rolling temperature is between 840 and 950° C. When the hot rolling final rolling temperature is lower than 840° C., the grains are likely to form a mixed state of coarse and fine grains, which reduces the processing performance of the steel plate; when the hot rolling final rolling temperature is higher than 950° C., the obtained grains are too coarse, which reduces the mechanical properties of the steel plate.

[0046] High temperature coiling is beneficial to the formability of the steel plate after hot rolling, so the coiling temperature of the present invention is selected to be 650°C to 750°C.

[0047] Since the critical cooling rate of hot rolling of the present invention is extremely low, the strength of the steel plate after hot rolling is relatively high, and it will be difficult to carry out the next step of cold rolling. Therefore, the present invention is designed to perform hood annealing on the hot rolled plate before cold rolling; the annealing temperature of the hood annealing is 850-870°C. When the hood annealing temperature is lower than 850°C, the yield strength cannot be fully reduced. When the hood annealing temperature is higher than 870°C, it will cause coarse grains and reduce the final mechanical properties of the steel plate.

[0048] The hot-rolled steel sheet is pickled and then cold-rolled. Considering the rolling load and material properties, the cumulative cold-rolling reduction ratio is set to 30% to 60%.

[0049] The hot stamping process is a key factor in determining the final performance of the steel plate. The present invention heats the cold-rolled plate to between 900°C and 980°C, which is conducive to obtaining a uniform austenite structure. When the heating temperature is lower than 900°C, Cr 23 C6 cannot be dissolved in the matrix, resulting in poor corrosion resistance. When the heating temperature is higher than 980℃, it will cause the grains to grow rapidly and reduce the mechanical properties of the steel plate.

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

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

[0052] [Example]

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

[0054] Table 1 Main chemical components of steel (weight percentage, 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 example, P≤0.012%, S≤0.005%, and the remainder 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 process parameters

[0062]

[0063] Table 5 Strip finished product properties

[0064]

[0065] It can be seen from Examples 1-5 that the hot stamped stainless steel cold rolled strip prepared by the method of the present invention has a tensile strength of ≥1500 MPa, a pitting potential of ≥0.05VSCE, a tensile strength of up to 1500 MPa, and good corrosion resistance.

[0066] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can make equivalent replacements or changes according to the technical scheme and inventive concept of the present invention within the technical scope disclosed by the present invention, which should be covered by the protection scope of the present invention.

Claims

1. A method for producing 1500MPa high corrosion resistant hot stamping stainless steel cold rolled strip, characterized in that: It includes hot metal pretreatment, smelting, refining, continuous casting, hot rolling, spheroidizing annealing, pickling, cold rolling and hot stamping forming processes; specifically controls the following processes: 1) Steelmaking: After pretreatment of molten iron, it is smelted in a converter or electric furnace, and then refined out of the furnace to obtain refined molten steel; 2) Continuous casting: The target temperature of the continuous casting tundish is controlled at 20-35°C above the liquidus temperature. The secondary cooling adopts a weak cooling system with a secondary cooling water volume 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. 3) Hot rolling: the heating temperature of the casting is 1180-1280℃; after high-pressure water descaling, the rough rolling adopts multi-pass rolling; the thickness of the intermediate billet is 30-50mm, the total reduction rate of 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; 4) Spheroidizing annealing: The hot-rolled plate is annealed at a temperature of 850-870°C for 6-12 hours. After being kept below 400°C with a hood, it is air-cooled to room temperature. 5) Pickling: Remove the oxide scale on the surface of the hot-rolled plate by pickling; 6) Cold rolling: cold rolling after pickling, the cumulative reduction rate of cold rolling is 30% to 60%; the flat elongation is controlled at 0.5% to 1.2%; 7) Hot stamping: After the cold rolled coil is unrolled and cut into pieces, hot stamping is performed as follows: (a) Austenitization of steel: Heat the cold-rolled sheet to 900-1050°C and keep it at this temperature for 3-10 minutes; (b) Steel transfer: The heated cold-rolled sheet is quickly transferred to the hot stamping die, and the transfer time is controlled within 5 to 15 seconds; the hot stamping 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 of hot stamping is controlled within 5 to 10 seconds; (d) Demolding: Demolding temperature ≤ 250℃.

2. The method for producing a 1500MPa high corrosion resistant hot stamping stainless steel cold rolled strip according to claim 1, characterized in that: The chemical composition of the strip steel is calculated by weight percentage as follows: 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.

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

4. The method for producing a 1500MPa high corrosion resistant hot stamping stainless steel cold rolled strip according to claim 1, characterized in that: The tensile strength of the finished strip steel is Rm≥1500MPa, and the elongation is A 50 ≥20%, pitting potential ≥0.05VSCE.

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