A 1.5GPa grade high-strength cold-rolled stainless steel and a preparation method thereof

Abstract

The present invention discloses a 1.5GPa grade high-strength cold-rolled stainless steel. The chemical composition of the stainless steel includes Ni: 4.0-8.0%; Mn: ≤2.0%; Cr: 16.0-19.0%; W≥0.5%; C: 0-0.4%; N: 0-0.4%, and the total mass fraction of Ni and Mn in the alloy is Ni+Mn≥5.0%, C+N≥0.1%, and the rest is iron and inevitable impurities. The present invention also discloses a preparation method of 1.5GPa grade high-strength cold-rolled stainless steel. The stainless steel of the present invention has a recrystallized state of austenite or contains a large amount of austenite, has high strength and toughness, does not contain precious elements and can use raw materials containing C and N, does not need to use high-purity raw materials, and therefore has low cost.

Description

Technical Field

[0001] The invention belongs to the field of metal material preparation, and in particular relates to a 1.5 GPa-level high-strength cold-rolled stainless steel and a preparation method thereof. Background Art

[0002] The development of high-end manufacturing, energy and power and other fields has put forward higher requirements for lightweight and low-cost metal parts. High strength can reduce the wall thickness of parts under the same stress conditions, reduce the amount of metal structural materials, and achieve the purpose of lightweight and cost reduction. Therefore, the industry has been developing high-strength stainless steel.

[0003] The most widely used stainless steels are austenitic stainless steels such as 304, 316, and 310s. However, these 3 series stainless steels often have high plasticity but low strength, and the Ni content of these steels is often greater than 8wt.%, and the raw material cost is high, making it difficult to meet the requirements of high strength and low cost in industrial applications. The high-strength stainless steels currently under development mainly include martensitic stainless steels, cold-rolled stainless steels, etc.

[0004] Martensitic stainless steel [such as S280 (Cr12Ni4Co14V0.3WMo2), Custom475 (Cr10.8Ni8.1Co8.5Al1.2), 0Cr17Ni7Ti0.7Al0.2, 0Cr17Ni4Cu4Nb, 0Cr15Ni5Cu2Ti, etc.] has high strength (tensile strength is often greater than 1.4GPa), but contains more alloy elements, has a complex system, and often requires the addition of expensive alloy elements such as Co, Mo, and Nb, or the addition of elements such as Al and Ti that are easy to introduce harmful phases, making smelting and processing more difficult. Therefore, high-strength martensitic stainless steel is often more expensive.

[0005] Cold-rolled stainless steel is achieved by cold working the steel and introducing deformation strengthening to increase strength. However, since deformation strengthening often comes at the expense of toughness, it is difficult to balance the strength and toughness of cold-rolled stainless steel. For example, U.S. Patent WO2023 / 023083A1 discloses a cold-rolled stainless steel with Cr: 15-18wt.%, Ni: 4-8wt.%, Mn: 1.5-6wt.%, Fe: Bal. The annealed alloy has a yield strength of about 350MPa, a tensile strength of about 791MPa, and a uniform deformation of about 43%. When the cold deformation reaches 56%, although the tensile strength of the alloy is increased to 1458MPa, the uniform deformation is only 2%.

[0006] Therefore, there is an urgent need to develop a low-cost stainless steel with a tensile strength higher than 1.4 GPa after cold deformation while maintaining good toughness. Summary of the invention

[0007] In order to overcome the deficiencies of the prior art, the present invention provides a 1.5 GPa grade high-strength cold-rolled stainless steel and a preparation method thereof.

[0008] The technical solution adopted by the present invention to solve the technical problem is: a 1.5GPa grade high-strength cold-rolled stainless steel, the chemical composition of the stainless steel includes Ni: 4.0-8.0%; Mn: ≤2.0%; Cr: 16.0-19.0%; W≥0.5%; C: 0-0.4%; N: 0-0.4%, and the total mass fraction of Ni and Mn in the alloy is Ni+Mn≥5.0%, C+N≥0.1%, and the rest is iron and unavoidable impurities.

[0009] Furthermore, the chemical composition of the stainless steel includes, by weight percentage: Ni: 5.0-7.0%; Mn: ≤1.5%; Cr: 17.0-19.0%; W≥0.5%; C: 0.2-0.3%; N: 0.1-0.3%, and the total mass fraction of Ni and Mn in the alloy is Ni+Mn≥6.0%, C+N≥0.2%, and the rest is iron and unavoidable impurities.

[0010] Furthermore, the strength of the stainless steel is above 1.5 GPa, while the uniform deformation exceeds 10% and the elongation at break exceeds 15%.

[0011] The present invention also discloses a method for preparing 1.5 GPa grade high-strength cold-rolled stainless steel, comprising the following steps:

[0012] 1) Smelting and casting

[0013] The chemical composition according to any one of claims 1 or 2 is smelted in a smelting furnace and cast into a billet;

[0014] 2) Hot and cold processing + annealing

[0015] The hot working temperature is above 800℃; after deformation, the annealing temperature is above 800℃ and the time is greater than 10 minutes;

[0016] 3) Cold working

[0017] The cold deformation amount should be greater than 20% to obtain a high-strength cold-rolled stainless steel product. The cold deformation amount should be greater than 20%, preferably about 40%, to obtain a high-strength cold-rolled stainless steel product.

[0018] Furthermore, the smelting furnace in step 1) includes but is not limited to a converter, an electric furnace, an induction furnace, a magnetic suspension furnace or an electric arc furnace.

[0019] Furthermore, in the step 1), the nitrogen element can be added through the chromium-nitrogen alloy raw material and by blowing nitrogen during the smelting process.

[0020] Furthermore, the amount of cold deformation in step 3) is about 40%.

[0021] The reasons for limiting the content range of each chemical component in the stainless steel are explained below.

[0022] Ni is an austenite stabilizing element. However, too much Ni will increase the cost of steel. Therefore, the Ni content should be controlled to 4.0-8.0%. Mn is also an austenite stabilizing element and can replace part of the Ni element, but too much Mn will increase the difficulty of smelting and processing. Therefore, the Mn content should be controlled to ≤2.0%, and the total mass fraction of Ni and Mn should be ≥5%.

[0023] Cr is a key element to ensure the corrosion resistance of the alloy, but too high Cr will increase the cost of steel. Therefore, the Cr content should be controlled at 16.0-19.0%.

[0024] As a solid solution strengthening element, W can improve the strength of the alloy, but excessive W will increase the cost and density of the steel. Therefore, the W content should be controlled to ≥ 0.5%.

[0025] C and N can provide solid solution strengthening and are also austenite stabilizing elements. If the C and N content is too low, the alloy strength will be low and the toughness will be poor, but if the C and N content is too high, the smelting and processing will be difficult. Therefore, the C and N element content should be controlled to 0-0.4%, and C+N≥0.1%.

[0026] The innovation of the present invention lies in that austenite is stabilized by a small amount of Mn, C, and N elements, the Ni content is low, the alloy cost is low, and C and N can simultaneously improve the strength and toughness of the alloy, and a certain amount of Cr is added to ensure the corrosion resistance of the steel, thereby obtaining a low-cost high-strength, high-toughness stainless steel, the strength of which can reach 1.5GPa after cold rolling, and the uniform deformation exceeds 10%, the elongation at break exceeds 15%, and the comprehensive mechanical properties far exceed stainless steel of similar cost. The production process of the present invention is relatively simple, the alloy does not contain expensive elements such as Co, Mo, and Nb, the cost is controllable, and it can meet the needs of structural parts in the fields of transportation, energy and electricity, and high-end equipment manufacturing.

[0027] The advantages and beneficial effects of the present invention are:

[0028] (1) The recrystallized state of the stainless steel of the present invention is austenite or contains a large amount of austenite, and the strength and toughness are relatively high;

[0029] (2) The present invention can significantly improve the strength of steel through simple cold working while maintaining good toughness;

[0030] (3) The preparation and heat treatment process of the stainless steel of the present invention is simple and suitable for large-scale production;

[0031] (4) Since the stainless steel of the present invention does not contain precious elements and can use raw materials containing C and N, there is no need to use high-purity raw materials, so the cost is relatively low. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 This is the EBSD result of the annealed stainless steel in Example 1. The left figure is a phase distribution diagram, indicating that the annealed state is austenitic steel; the right figure is an IPF diagram, which shows that recrystallization has occurred in the annealed steel.

[0033] Figure 2 It is the room temperature tensile stress-strain curve of the annealed stainless steel in Examples 1-3 and Comparative Examples 1-2.

[0034] Figure 3 1 is the room temperature tensile stress-strain curve of the cold-rolled stainless steel with different cold deformation amounts in Example 1. DETAILED DESCRIPTION

[0035] In order to enable those skilled in the art to better understand the solutions of the present invention, the following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work should fall within the scope of protection of the present invention.

[0036] Example 1

[0037] Use metal materials with a purity of ≥99.5% to prepare metal raw materials according to the following alloy composition (mass percentage). Cr: 18.0%, Ni: 6.0%, Mn: 2.0%, W: 0.5%, C: 0.2%, N: 0.3%, and the rest is iron and unavoidable impurities. At this time, Ni+Mn is 8.0%, and C+N is 0.5%.

[0038] The raw materials of each element are weighed according to the raw material ratio, melted and mixed evenly in a protective atmosphere using an arc melting furnace, and cast into alloy ingots. Hot rolling is performed at 1100°C, and the deformation is about 82%; then the hot-rolled steel plate is annealed at 1000°C for 30 minutes; cold rolling is performed, and the deformation is 30%-40% to obtain cold-rolled stainless steel.

[0039] The EBSD image of the annealed stainless steel of Example 1 is as follows: Figure 1 As shown in FIG. 1 , the phase distribution diagram shows that the annealed state is austenitic steel, and its microstructure is almost entirely austenite, and the stainless steel has undergone recrystallization. The room temperature tensile stress-strain curve and data of the annealed stainless steel in Example 1 are shown in FIG. Figure 2As shown, the yield strength of annealed stainless steel is 560MPa, the tensile strength exceeds 1000MPa, and the elongation at break exceeds 60%, and it has high strength and plasticity.

[0040] Figure 3 The stress-strain tensile curve of the stainless steel of Example 1 when the cold rolling deformation is 30%-40% is shown. When the cold rolling deformation increases, the strength of the stainless steel increases and the plasticity decreases. When the cold rolling deformation of the stainless steel is 30%, the yield strength and tensile strength are 1262 and 1501 MPa respectively, and the elongation at break is 18.6%; when the cold rolling deformation is 40%, its yield strength exceeds 1500 MPa, the tensile strength exceeds 1700 MPa, and there is still an elongation at break of more than 15%, and the uniform elongation is 10%.

[0041] Example 2

[0042] Use metal materials with a purity of ≥99.5% to prepare metal raw materials according to the following alloy composition (mass percentage). Cr: 18.0%, Ni: 8.0%, Mn: 0%, W: 0.5%, C: 0.2%, N: 0.3%, and the rest is iron and unavoidable impurities. At this time, Ni+Mn is 8.0%, and C+N is 0.5%.

[0043] The raw materials of each element are weighed according to the raw material ratio, melted and mixed evenly in a protective atmosphere using an arc melting furnace, and cast into alloy ingots. Hot rolling is performed at 1100°C, and the deformation is about 86%; then the hot-rolled steel plate is annealed at 1000°C for 30 minutes; cold rolling is performed, and the deformation is 40% to obtain cold-rolled stainless steel.

[0044] The room temperature tensile stress-strain curve and data of the annealed stainless steel in Example 2 are as follows: Figure 2 As shown, at room temperature, the yield strength of annealed stainless steel is 430MPa, the tensile strength is 950MPa, and the elongation at break is 74.9%, which shows high strength and plasticity.

[0045] The room temperature tensile data of the cold rolled stainless steel in Example 3 are shown in Table 1. When the cold rolling deformation of the stainless steel is 40%, the yield strength reaches 1440 MPa, the tensile strength is 1570 MPa, the elongation at break is 15%, and the uniform elongation is 10%.

[0046] Example 3

[0047] Use metal materials with a purity of ≥99.5% to prepare metal raw materials according to the following alloy composition (mass percentage). Cr: 18.0%, Ni: 6.0%, Mn: 2.0%, W: 0.5%, C: 0.2%, N: 0%, and the rest is iron and unavoidable impurities. At this time, Ni+Mn is 8.0%, and C+N is 0.2%.

[0048] The raw materials of each element are weighed according to the raw material ratio, melted and mixed evenly in a protective atmosphere using an arc melting furnace, and cast into alloy ingots. Hot rolling is performed at 1100°C, and the deformation is about 86%; then the hot-rolled steel plate is annealed at 1000°C for 30 minutes; cold rolling is performed, and the deformation is 20% to obtain cold-rolled stainless steel.

[0049] The room temperature tensile stress-strain curve and data of the annealed stainless steel in Example 3 are as follows: Figure 2 As shown, at room temperature, the yield strength of annealed stainless steel is 360MPa, the tensile strength is 1280MPa, and the elongation at break is 38.1%, which shows high strength and plasticity.

[0050] The room temperature tensile data of the cold rolled stainless steel in Example 3 are shown in Table 1. When the cold rolling deformation of the stainless steel is 20%, the yield strength reaches 1140 MPa, the tensile strength is 1510 MPa, the elongation at break is 18.4%, and the uniform deformation is 15.3%.

[0051] Comparative Example 1

[0052] Metal raw materials with a purity of ≥99.5% are used to prepare the metal raw materials according to the following alloy composition (mass percentage): Cr: 18.0%, Ni: 6.0%, Mn: 2.0%, W: 0.5%, and the rest are iron and inevitable impurities.

[0053] The raw materials of each element are weighed according to the raw material ratio, melted and mixed evenly in a protective atmosphere using an arc melting furnace, and cast into alloy ingots. Hot rolling is performed at 1100°C, and the deformation is about 79%; then the hot-rolled steel plate is annealed at 1000°C for 30 minutes.

[0054] Comparative Example 1: The room temperature tensile stress-strain curve and data of the annealed stainless steel are as follows: Figure 1 As shown in the figure, when no C and N are added to the stainless steel, the plasticity is poor, the yield strength and tensile strength are 470 and 880 MPa respectively, and the elongation at break is only 17%. Under the premise of similar strength, the elongation at break is far less than that of Examples 1-3 with C / N added, and it is difficult to continue to improve its strength by cold rolling while ensuring its plastic toughness (for example: elongation at break is greater than 10%).

[0055] Comparative Example 2

[0056] Metal raw materials with a purity of ≥99.5% are used to prepare the metal raw materials according to the following alloy composition (mass percentage): Cr: 18.0%, Ni: 2.0%, Mn: 2.0%, W: 0.5%, C: 0.2%, N: 0.3%, and the rest are iron and inevitable impurities.

[0057] The raw materials of each element are weighed according to the raw material ratio, melted and mixed evenly in a protective atmosphere using an arc melting furnace, and cast into alloy ingots. Hot rolling is performed at 1100°C, and the deformation is about 86%; then the hot-rolled steel plate is annealed at 1000°C for 30 minutes.

[0058] Comparative Example 2 annealed stainless steel room temperature tensile stress strain curve and data as shown Figure 1 As shown, when the Ni content is reduced to 2%, although the yield strength and tensile strength of the stainless steel exceed 1000 MPa and 1500 MPa respectively, its plasticity is greatly weakened, and the elongation at break is only 15.4%, which is much lower than that of Examples 1-3.

[0059] Table 1 Room temperature tensile properties data of cold rolled stainless steel prepared in Examples 1-3

[0060]

[0061] The above specific implementation modes are used to explain the present invention rather than to limit the present invention. Any modification and change made to the present invention within the spirit of the present invention and the protection scope of the claims shall fall within the protection scope of the present invention.

Claims

1. A 1.5 GPa grade high-strength cold-rolled stainless steel, characterized in that: Calculated by weight percentage, the chemical composition of the stainless steel includes Ni: 4.0-8.0%; Mn: ≤2.0%; Cr: 16.0-19.0%; W≥0.5％； C：0.2～0.4％； N: 0-0.4%, and the total mass fraction of Ni and Mn in the alloy is Ni+Mn≥5.0%, C+N≥0.2%, and the rest is iron and unavoidable impurities; The preparation method of the 1.5 GPa grade high-strength cold-rolled stainless steel comprises the following steps: 1) Smelting and casting According to the above chemical composition, the product is smelted in a smelting furnace and cast into a billet; 2) Hot and cold processing + annealing The hot working temperature is above 800℃; after deformation, the annealing temperature is above 800℃ and the time is greater than 10 minutes; 3) Cold working The cold deformation should be greater than 20% to obtain high-strength cold-rolled stainless steel products; The 1.5 GPa grade high-strength cold-rolled stainless steel has a strength of more than 1.5 GPa, a uniform deformation of more than 10%, and a fracture elongation of more than 15%.

2. The 1.5 GPa grade high strength cold rolled stainless steel according to claim 1, characterized in that: The chemical composition of the stainless steel includes, by weight percentage: Ni: 5.0-7.0%; Mn: ≤1.5%; Cr: 17.0~19.0%; W≥0.5%; C: 0.2~0.3%; N: 0.1~0.3%, and the total mass fraction of Ni and Mn in the alloy is Ni+Mn≥6.0%, C+N≥0.2%, and the rest is iron and unavoidable impurities.

3. A method for preparing 1.5GPa grade high-strength cold-rolled stainless steel, characterized in that: The following steps are involved: 1) Smelting and casting The chemical composition according to any one of claims 1 or 2 is smelted in a smelting furnace and cast into a billet; 2) Hot and cold processing + annealing The hot working temperature is above 800℃; after deformation, the annealing temperature is above 800℃ and the time is greater than 10 minutes; 3) Cold working The cold deformation should be greater than 20% to obtain high-strength cold-rolled stainless steel products.

4. The method for preparing 1.5 GPa grade high-strength cold-rolled stainless steel according to claim 3, characterized in that: The smelting furnace in step 1) includes a converter, an electric furnace, an induction furnace or an electric arc furnace.

5. The method for preparing 1.5 GPa grade high-strength cold-rolled stainless steel according to claim 3, characterized in that: In the step 1), the nitrogen element is added through the chromium-nitrogen alloy raw material and by blowing nitrogen during the smelting process.

6. The method for preparing 1.5 GPa grade high-strength cold-rolled stainless steel according to claim 3, characterized in that: The cold deformation amount in step 3) is 40%.

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