High formability austenitic stainless steel and method of manufacturing the same

Abstract

This invention discloses a highly formable austenitic stainless steel with the following chemical composition by weight percentage: C: ≤0.03%; Si: 0.3~1.0%; Mn: 0.5~1.5%; Cr: 16.0~18.0%; Ni: 7.0~8.0%; N: ≤0.03%; Cu: 2.5~4.0%; Mo: 0.5~1.0%; P: ≤0.030%; S: ≤0.002%; the remainder being Fe and unavoidable impurity elements; simultaneously, it must satisfy: %(Ni+Cu)+0.5*%Mn+35*%(C+N)≥(%Cr+1.5*%Mo-15.5) 2 / 12+12. The battery casing processed from the blank obtained by the composition design and manufacturing method of this invention can withstand a certain degree of deformation under high temperature conditions of about 600℃, and can cope with the safety test of complex driving conditions and extreme conditions.

Description

Technical Field

[0001] This invention relates to the field of stainless steel smelting, and more particularly to a highly formable austenitic stainless steel and its manufacturing method. Background Technology

[0002] With the rapid development of the new energy vehicle industry, the power battery, as the core energy unit of the vehicle, directly determines the vehicle's range and passenger safety through its safety and reliability. The battery casing, as a key structural component protecting the battery, must possess excellent mechanical strength, corrosion resistance, high-temperature resistance, and lightweight characteristics to withstand the safety challenges of complex driving conditions and extreme environments.

[0003] Currently, the mainstream materials used for power battery casings include aluminum alloys and low-alloy steel. Aluminum alloys are widely used due to their low density and light weight, but they have significant limitations in practical applications: 1. Poor high temperature resistance: Under extreme high temperature conditions such as fire, aluminum alloy materials can only maintain structural integrity for about 5 minutes at around 600℃, which is difficult to meet the fire escape time of more than 10 minutes generally required by the new energy vehicle industry, posing a significant safety hazard.

[0004] 2. Poor thermal management: Aluminum alloys have high thermal conductivity and relatively poor thermal insulation performance, which is not conducive to the battery maintaining an ideal operating range (15℃-35℃) under extreme temperature environments, affecting the battery's charging and discharging efficiency and cycle life.

[0005] Low battery capacity: Aluminum alloys are mostly made with a thickness of 1.0mm or more, resulting in low battery capacity.

[0006] 3. Although low alloy steel is cheaper, it is generally necessary to plate it with nickel to meet the corrosion resistance requirements of battery casings. However, the nickel plating layer does not coordinate with the deformation of the substrate during the stamping process of the battery casing, resulting in unsatisfactory corrosion resistance.

[0007] To address the aforementioned issues, developing a new material that combines high formability and excellent high-temperature performance has become crucial for improving the safety of power batteries and the lightweighting of vehicles.

[0008] Stainless steel, due to its excellent mechanical properties, corrosion resistance, and recyclability, shows broad application prospects in the automotive structural components field. However, in the application of power battery casings, traditional stainless steels such as 304 and 316 exhibit significant work hardening effects, making them prone to aging cracks after stamping, which fails to meet the stamping performance requirements of battery casings. Moreover, it is generally accepted in the industry that formability and high yield strength at 600℃ are mutually restrictive and cannot be simultaneously achieved. It is difficult to simultaneously achieve good formability and excellent high-temperature strength in the same material. Bound by this inherent understanding, traditional designs can only make compromises between high formability and medium-to-high temperature strength, failing to simultaneously achieve excellent room-temperature formability and structural strength, thermal stability, and resistance to thermal deformation under high-temperature conditions. This has become a technical bottleneck restricting the application of high-performance stainless steel and alloy materials. Summary of the Invention

[0009] One of the objectives of this invention is to provide a highly formable austenitic stainless steel.

[0010] The technical solution to achieve the first objective of this invention is: a highly formable austenitic stainless steel, the weight percentage of its chemical composition being as follows: C: ≤0.03%; Si: 0.3–1.0%; Mn: 0.5–1.5%; Cr: 16.0–18.0%; Ni: 7.0–8.0%; N: ≤0.03%; Cu: 2.5–4.0%; Mo: 0.5–1.0%; P: ≤0.030%; S: ≤0.002%; The remainder consists of Fe and unavoidable impurity elements; Simultaneously, the following conditions must be met: %(Ni+Cu)+0.5*%Mn+35*%(C+N)≥(%Cr+1.5*%Mo-15.5) 2 / 12+12.

[0011] Further, the preferred composition is: C: 0.01%–0.03%, Si: 0.3–0.8%, Mn: 0.5–1.2%, Cr: 16.0–18.0%, Ni: 7.0–8.0%, N: ≤0.03%, Cu: 2.5–4.0%, Mo: 0.5–1.0%, P: ≤0.030%, S: ≤0.002%. This further narrowing of the composition design range broadens the subsequent manufacturing process window, better meeting the requirements of continuous large-scale industrial production for process tolerance, product performance consistency, and yield control.

[0012] The compositional design principle of the high-formability austenitic stainless steel is as follows: Carbon (C) is a strong austenite-forming element, which helps maintain the stability of the austenite structure at room temperature. However, carbon is also an interstitial element, reducing the fluidity of metallic materials during the forming process. Therefore, the carbon content is defined as ≤0.03% by mass.

[0013] Si is an important deoxidizing element in the steelmaking process, which helps improve the purity of molten steel. However, it is also a ferrite-forming element. Excessive Si content causes the ferrite formed at high temperatures to remain at room temperature, reducing formability. Therefore, the Si content is defined as 0.3–0.8% by mass.

[0014] Mn is a relatively weak austenite-forming element, but a strong austenite-stabilizing element. To reduce the transformation of austenite into deformed martensite during cold stamping deformation, a certain manganese content is required. Therefore, the Mn content is defined as 0.5–1.2% by mass.

[0015] Cr is a key element for improving corrosion resistance, especially resistance to pitting and intergranular corrosion. However, it is also a ferrite-forming element, requiring a certain amount of chromium content to balance with austenite-forming elements. To achieve the objectives of this invention, Cr is defined as 16.0–18.0% by mass.

[0016] Ni is an element that forms and stabilizes austenitic microstructure, which is beneficial for improving the deformation resistance of austenitic stainless steel. It can effectively prevent austenite from transforming into deformable martensite during cold deformation, thus improving stamping performance. However, Ni is a precious metal element, which has a significant impact on cost. Therefore, Ni is defined as 7.0–8.0% by mass.

[0017] Nitrogen (N) is a strong austenite-forming element, and dissolved N can improve the stability and strength of austenitic stainless steel at room temperature. However, nitrogen is also an interstitial element, reducing formability, and achieving ultra-low nitrogen content in the process is costly. Therefore, in this invention, N is defined as ≤0.03% by mass.

[0018] Cu is an austenite-forming element that can improve the stamping performance of austenitic stainless steel, but higher Cu content will reduce hot working performance. In this invention, Cu is defined as 2.5 to 4.0 by mass.

[0019] Both P and S are unavoidable impurity elements that adversely affect performance and should be less than 0.030% by mass and 0.002% by mass, respectively. Extremely low sulfur content (≤0.002%) can also prevent Ca from preferentially reacting with S to form CaS during the calcium-silicon feed line step, ensuring the effectiveness of calcium treatment.

[0020] In this invention, the left side of the inequality represents the weighted equivalent of austenite-forming and stabilizing elements, while the right side represents the nonlinear threshold function of ferrite-forming elements. When the value on the left side is greater than or equal to the value on the right side, the austenite structure remains highly stable in the solid solution state and during cold deformation, avoiding the formation of deformation martensite. This controls the work hardening index n below 0.35, and the product elongation can reach over 55%, both of which are necessary conditions for achieving deep drawing. Simultaneously, the Cu and Mo content range defined by this inequality ensures a yield strength ≥50MPa at 600℃, allowing the product to withstand certain deformations under high-temperature conditions, thus ensuring the safety of the battery casing. The right side uses a square term, based on the nonlinear accelerating effect of Cr and Mo on ferrite stability. This is a key principle discovered through extensive experiments based on measurements of the tensile strength and magnetic permeability of austenitic stainless steel processed at room temperature.

[0021] The first objective of this invention is to introduce a nonlinear compositional inequality as a formability criterion in industrially manufacturable stainless steel, rather than relying solely on the compositional range of elements. Through Cu+Mo synergistic design, batch fluctuations of "single element qualified, overall poor formability" are avoided. Furthermore, Cu+Mo synergistic design achieves a balance between low work hardening and high yield strength at 600℃, overcoming the technical bias that "high formability materials cannot simultaneously achieve high temperature and high yield strength." The stainless steel prepared by this invention is less prone to aging cracking when stamped into battery casings.

[0022] On the other hand, the yield strength of general aluminum alloys is 120MPa, and the force F that it can withstand per unit area at room temperature is 120*h*b. However, the yield strength of stainless steel obtained by the composition design of this invention can be increased to 200MPa at room temperature. Correspondingly, the thickness h can be reduced to 67% of the original thickness. The reduction in thickness can significantly increase the battery capacity and improve the overall safety performance and market competitiveness of electric vehicles.

[0023] The second objective of this invention is to provide a method for manufacturing the aforementioned highly formable austenitic stainless steel.

[0024] The technical solution to achieve the second objective of this invention is: a method for manufacturing highly formable austenitic stainless steel, comprising the following steps: (1) Smelting: Smelting in a converter according to the chemical composition described in the first objective of this invention; (2) Refining: The composition is finely adjusted in the refining process to meet the range requirements of the inequality in the first objective of this invention; after the composition adjustment is completed, calcium treatment is carried out by feeding calcium wire. The amount of calcium wire fed is calculated according to 1.0 to 1.2 kg / ton of steel, and the Ca / Al ratio is ensured to be in the range of 0.1 to 0.3. The temperature of the molten steel during calcium treatment is 1550 to 1600℃. Then, the bottom blow argon gas is used for soft stirring for 12 to 14 minutes to allow the inclusions to float fully; the total time for calcium treatment and bottom blow argon gas should not exceed 15 minutes. (3) Casting; (4) Hot rolling; (5) Cold rolling, annealing and pickling.

[0025] Cu is sensitive to grain boundary purity. Residual inclusions generated by general calcium treatment can induce Cu segregation, leading to cracking during hot rolling. This invention uses a calcium treatment with an extremely narrow window to avoid secondary oxidation of molten steel and calcium volatilization, thus preventing the formation of large-sized calcium inclusions, ensuring grain boundary purity, and preventing Cu segregation from causing cracking during hot rolling.

[0026] Furthermore, in step 4), the steel extraction temperature in the hot rolling process is 1140–1180°C, the total furnace time is 260–320 min, and the strain rate is ensured to be 30–50 / s during high-temperature hot rolling to obtain the hot-rolled slab.

[0027] Because Cu enrichment at grain boundaries reduces thermoplasticity, high strain rate hot rolling easily leads to hot cracking in high-Cu-content stainless steel. Therefore, those skilled in the art usually avoid using high strain rates in austenitic stainless steel containing ≥2.5% Cu. The inventors discovered through experiments that a hot rolling strain rate of 30–50 s, combined with the high-temperature strengthening effect of molybdenum and the improvement of thermoplasticity by copper, results in an extremely short deformation time. This prevents Cu segregation from diffusing to the grain boundaries to form a liquid film, thereby suppressing hot cracking and achieving a balance between grain refinement and the risk of hot cracking.

[0028] Further, step 5) cold rolling annealing and pickling process includes cold rolling the hot-rolled slab obtained in step 4) with a reduction of 70-80%, followed by annealing and pickling. The annealing temperature is 1080-1120℃, the annealing time of the cold-rolled coil is 1-3 minutes, and the pickling temperature is 50-70℃ to obtain a white-skinned coil.

[0029] Typically, increasing the annealing temperature and holding time can replace the large cold rolling reduction process, but this requires continuous annealing during stamping, increasing both the production process and costs. This invention addresses this by controlling the S content to ≤0.002%, ensuring a low work hardening index (n<0.35) through compositional inequalities, and leveraging the promoting effect of high copper on cross-slip. Furthermore, the refining process precisely controls the Ca / Al ratio at 0.1–0.3 during the silicon-calcium wire feeding stage, while simultaneously performing calcium treatment within an extremely narrow window. This allows for a large cold rolling reduction of 70–80% without the risk of inclusion-induced cracking. The large cold rolling reduction accumulates deformation energy, increasing recrystallization power during subsequent annealing, thereby shortening the process flow and reducing costs. Detailed Implementation

[0030] The preferred embodiments of the present invention will be described in detail below. Example 1

[0031] A highly formable austenitic stainless steel has the following chemical composition by weight percentage: C: 0.025%, Si: 0.5%, Mn: 1%, Cr: 17.5%, Ni: 7.5%, N: 0.02%, Cu: 3.1%, Mo: 0.5%, P: 0.03%, S: 0.001%. %(Ni+Cu)+0.5*%Mn+35*%(C+N)=12.68, (%Cr+1.5*%Mo-15.5) 2 / 12+12=12.63 This embodiment also provides a method for manufacturing the highly formable austenitic stainless steel, which includes the following steps: (1) Smelting: Smelting in a converter according to the aforementioned chemical composition; (2) Refining: The composition is finely adjusted in the refining process and the composition meets the range requirements of the inequality. After the composition adjustment is completed, calcium treatment is carried out by feeding calcium wire. The amount of calcium wire is calculated according to 1.0 kg / ton of steel and the Ca / Al ratio is ensured to be 0.1. The temperature of the molten steel during calcium treatment is 1550℃. Then, bottom blowing argon gas is used for soft stirring for 12 minutes to allow the inclusions to float fully. The total time for calcium treatment and bottom blowing argon is 14 minutes. (3) Casting; (4) Hot rolling: The steel drawing temperature is 1140℃, the total time in the furnace is 260min, and the strain rate is ensured to be 50 / s during high-temperature hot rolling to obtain the hot-rolled slab; (5) Cold rolling, annealing and pickling: After the hot-rolled slab obtained in step 4) is cold-rolled with a reduction of 70%, it is annealed and pickled. The annealing temperature is 1080℃, the cold rolling annealing time is 3 minutes, and the pickling temperature is 60℃ to obtain white sheet roll.

[0032] Examples 2-5 The chemical composition of the slabs in Examples 2-5 is shown in Table 1, and the specific process parameters are shown in Table 2.

[0033] Comparative Example 1 Comparative Example 1 of this invention uses the same chemical composition as Example 1, the difference being that the manufacturing method uses a general reduction amount, while increasing the annealing temperature and extending the annealing time. Its chemical composition is shown in Table 1, and the specific process parameters are shown in Table 2.

[0034] Comparative Example 2 The content of each element in the composition design of Comparative Example 2 meets the requirements of this invention, but does not meet the requirements of the inequality. The manufacturing method adopted is the manufacturing method of this invention, and its chemical composition is shown in Table 1. The specific process parameters are shown in Table 2.

[0035] Comparative Example 3 The content of each element in the composition design of Comparative Example 3 meets the requirements of this invention, but does not meet the requirements of the inequality. The manufacturing method adopts a general reduction amount, while increasing the annealing temperature and extending the annealing time. Its chemical composition is shown in Table 1, and the specific process parameters are shown in Table 2.

[0036] Table 1 Chemical composition (mass%) of Examples 1-5 and Comparative Examples 1-3

[0037] Table 2 Main process parameters

[0038] The present invention also selected 304 stainless steel slab as comparative example 4 and 3003 aluminum alloy slab as comparative example 5 to compare with the slabs prepared in examples 1-5. The properties of the slabs prepared in examples 1-5 and comparative examples 1-5 are shown in Table 3.

[0039] Table 3 Performance of Examples 1-5 and Comparative Examples 1-5

[0040] As shown in Table 3, the slab obtained by the combined composition design and manufacturing method of this invention can achieve a low yield strength, a work hardening index of less than 0.35 after cold deformation, and a high-temperature yield strength of greater than 50 MPa at 600℃. Therefore, the battery casing made of this material can withstand certain deformation under high-temperature conditions of around 600℃, and can cope with the safety tests under complex driving conditions and extreme conditions. The composition design of Comparative Examples 2-3 does not meet the inequality requirements, and the resulting austenitic stainless steel has a yield strength of less than 50 MPa at 600℃, and the material has poor high-temperature resistance. Although 304 stainless steel and the cold rolling and annealing heat treatment process under general cold rolling reduction conditions in Comparative Example 1 can also achieve a high-temperature yield strength of greater than 50 MPa at 600℃, the work hardening index is greater than 0.35, which means that continuous annealing is required during the stamping process to continue stamping, which not only increases the production process but also increases the production cost. 3003 aluminum alloy can achieve continuous stamping without annealing, but the high-temperature strength at 600℃ is almost 0.

[0041] Thus, this invention breaks through the technical prejudice that "high formability materials are difficult to achieve high temperature yield strength", and achieves the unity of low work hardening and high yield strength at 600℃.

[0042] The above description is merely an embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent process transformations made using the content of the present invention specification, or direct or indirect applications in other related technical fields, are similarly included within the patent protection scope of the present invention.

Claims

1. A highly formable austenitic stainless steel, characterized in that: Its chemical composition by weight percentage is as follows: C：≤0.03%； Si: 0.3–1.0%; Mn: 0.5–1.5%; Cr：16.0～18.0%； Ni: 7.0–8.0%; N：≤0.03%； Cu: 2.5–4.0%; Mo: 0.5–1.0%; P：≤0.030%； S：≤0.002%； The remainder consists of Fe and unavoidable impurity elements; Simultaneously, the following conditions must be met: %(Ni+Cu)+0.5*%Mn+35*%(C+N)≥(%Cr+1.5*%Mo-15.5) 2 / 12+12。 2. The high formability austenitic stainless steel as described in claim 1, characterized in that: Its chemical composition by weight percentage is as follows: C: 0.01%~0.03%, Si: 0.3~0.8%, Mn: 0.5~1.2%, Cr: 16.0~18.0%, Ni: 7.0~8.0%, N: ≤0.03%, Cu: 2.5~4.0%, Mo: 0.5~1.0%, P: ≤0.030%, S: ≤0.002%.

3. A method for manufacturing austenitic stainless steel with high formability, characterized in that: It includes the following steps: (1) Smelting: Smelting in a converter according to the chemical composition described in claim 1 of this invention; (2) Refining: The composition is finely adjusted in the refining process to meet the range requirements of the inequality in claim 1; after the composition adjustment is completed, calcium treatment is carried out by feeding calcium wire. The amount of calcium wire is calculated according to 1.0 to 1.2 kg / ton of steel, and the Ca / Al ratio is ensured to be in the range of 0.1 to 0.

3. The temperature of the molten steel during calcium treatment is 1550 to 1600℃. Then, the bottom blow argon gas is used for soft stirring for 12 to 14 minutes to make the inclusions float to the surface. The total time for calcium treatment and bottom blow argon gas should not exceed 15 minutes. (3) Casting; (4) Hot rolling; (5) Cold rolling, annealing and pickling.

4. The method for manufacturing highly formable austenitic stainless steel according to claim 3, characterized in that: In step 4), the steel extraction temperature in the hot rolling process is 1140-1180℃, the total furnace time is 260-320min, and the strain rate is ensured to be 30-50 / s during high-temperature hot rolling to obtain the hot-rolled slab.

5. The method for manufacturing highly formable austenitic stainless steel according to claim 4, characterized in that: Step 5) cold rolling annealing and pickling process includes cold rolling the hot-rolled slab obtained in step 4) with a reduction of 70-80%, followed by annealing and pickling. The annealing temperature is 1080-1120℃, the annealing time of the cold-rolled coil is 1-3 minutes, and the pickling temperature is 50-70℃ to obtain a white sheet coil.

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