High strength and toughness metastable austenitic stainless steel for cryogenic structural materials and method of manufacturing the same

Abstract

The application discloses a kind of high strength and toughness metastable austenitic stainless steel for ultra-low temperature structural material and a manufacturing method thereof, the chemical composition of steel C0.01~0.03, Si 0.25~0.45, Mn 2.00~3.00, P≤0.015, S≤0.005, Cr 20.0~25.0, N 0.3~0.5.The billet preheating section is 750~850 ℃, the heating section is 1180~1220 ℃, the soaking section is 1150~1170 ℃;Opening 1050~1080 ℃, final rolling 970~1020 ℃, deformation rate 10%~30% per pass, the hot-rolled steel plate is subjected to 900~940 ℃ isothermal heat treatment, then heated to 1080~1120 ℃, after 16~21min isothermal water quenching.Under-269 ℃, σ0.2≥1500MPa, σb≥1930MPa, A≥28.50%, K1C≥150MPa·m 1 / 2 , solve the problem that austenitic stainless steel solid solution state ultra-low temperature strength and toughness is difficult to meet engineering needs.

Description

Technical Field

[0001] This invention belongs to the field of stainless steel technology, specifically relating to a high-strength and high-toughness metastable austenitic stainless steel for use in ultra-low temperature structural materials and its manufacturing method. Background Technology

[0002] Currently, for structural materials operating in low-temperature environments, in addition to meeting certain strength and ductility requirements, these materials must also possess sufficiently high toughness at low temperatures. Austenitic stainless steel, with its high impact toughness, excellent plasticity, and good corrosion resistance, is a relatively ideal material for low-temperature applications. Furthermore, austenitic stainless steel readily undergoes strain hardening, and high strength levels can be easily achieved through cold working. However, this strengthening is often achieved through strain-induced martensitic phase transformation, which requires relatively low deformation temperatures. For thicker austenitic stainless steel, only medium-thick plate mills or free forging hydraulic presses can be used to hot-work continuously cast billets or ingots. However, these processes all involve plastic deformation at high temperatures, making it difficult to meet the low-temperature deformation conditions. This results in the strain effect not being fully realized, and the strength indicators failing to meet the engineering requirements of structural materials, significantly limiting the application of austenitic stainless steel in low-temperature structural applications. Summary of the Invention

[0003] The purpose of this invention is to provide a high-strength and high-toughness metastable austenitic stainless steel for ultra-low temperature structural materials and its manufacturing method, thereby solving the problem that the strength and toughness of austenitic stainless steel in the solution-treated state at ultra-low temperatures cannot meet engineering requirements.

[0004] According to the dislocation pile-up model, the high yield stress of fine-grained materials is achieved by reducing the number of dislocations piled up on the slip surface, thereby reducing stress concentration at grain boundaries. Grain refinement significantly reduces stress concentration at grain boundaries, thus shifting the occurrence of intergranular fracture towards directions of high strain and high stress. This invention achieves metastable austenitic microstructure in the rolled state through compositional design, and refines the grains of metastable austenitic stainless steel through a two-step phase transformation, reducing stress concentration at grain boundaries during deformation, thereby improving the strength and fracture toughness of the material at an ultra-low temperature of -269℃.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0006] One of the technical solutions of this invention is to provide a high-strength and high-toughness metastable austenitic stainless steel for ultra-low temperature structural materials, the chemical composition (weight percentage) of which is: C: 0.01%~0.03%, Si: 0.25%~0.45%, Mn: 2.00%~3.00%, P≤0.015%, S≤0.005%, Cr: 20.0%~25.0%, N: 0.3%~0.5%, with the remainder being Fe and unavoidable inclusions.

[0007] The above chemical elements and their parameters were chosen because:

[0008] The carbon content is controlled at 0.01%~0.03% primarily to ensure the stability of austenite, preventing the induction of martensitic phase transformation during ultra-low temperature plastic deformation. The carbon atoms at the above content are dissolved in austenite, and solid solution strengthening can significantly improve the low-temperature strength of the steel plate. The preferred carbon content is 0.016%~0.028%.

[0009] The Si content is controlled at 0.25%~0.45%, primarily to balance the dissolved oxygen content in the steel and ensure its cleanliness. The above-mentioned silicon content can effectively improve the low-temperature fatigue performance of the steel plate, with Si being preferably 0.30%~0.40%.

[0010] The Mn content is controlled at 2%~3% to work together with Cr to increase the solubility of nitrogen in steel. In addition, excessive Mn will promote the segregation of Cr in the liquid phase. Limiting Mn to below 3% can avoid Cr segregation. The preferred Mn content is 2.30%~2.60%.

[0011] The Cr content is controlled at 20.0%~25.0% to take advantage of chromium's strong ferrite-forming properties. By controlling its content within this range, it can be fully decomposed into a "ferrite + nitride" dual-phase structure during isothermal heat treatment in the medium temperature range of 900~940℃.

[0012] The nitrogen content is controlled at 0.3%~0.5%. Considering that the Hall-Petch coefficient is strongly affected by the nitrogen content in austenitic steel, the fine grain strengthening effect can only be reflected within this nitrogen content range.

[0013] P and S are harmful impurity elements that easily form defects such as segregation and inclusions, and excessive content can lead to cracking of steel during hot working. Therefore, in this invention, the P content should be controlled below 0.015% and the S content should be controlled below 0.005%.

[0014] The second technical solution of this invention provides a method for manufacturing high-strength and high-toughness metastable austenitic stainless steel for ultra-low temperature structural materials, including billet heating, rolling, and heat treatment, wherein:

[0015] The billet heating process consists of three stages: preheating, heating, and soaking. The preheating temperature is 750~850℃, the heating temperature is 1180~1220℃, and the soaking temperature is 1150~1170℃, with a total furnace time of 3~4 hours. Considering the high deformation resistance of stainless steel, higher heating temperatures can soften the billet, providing a sufficiently high initial temperature to ensure that the slab deformation process is completed within the full austenitizing temperature range.

[0016] The rolling deformation process adopts high temperature and high reduction rate rolling, with an initial rolling temperature of 1050~1080℃ and a final rolling temperature of 970~1020℃. The deformation rate of each pass is controlled at 10%~30%, with the rolling speed of the first 6 passes controlled at 0.5~1.4m / s, and the rolling speed controlled at 1.5~3.2m / s from the 7th pass onwards. Considering the high-temperature, low-speed deformation, the above parameters were selected primarily because the third brittle zone temperature of this high-nitrogen steel composition system is 800~950℃, therefore the minimum final rolling temperature is controlled at 970℃ to avoid the third brittle zone; within the optimal plasticity range of 1080~1020℃, dynamic recovery and dynamic recrystallization are more likely to occur, making grain boundary migration easier; the rolling speed of 0.5~1.4m / s for the first 6 passes ensures that the rolling force fully penetrates to the core, improving the recrystallization degree of the core of the workpiece; after the 7th pass, the rolling speed is increased to 1.5~3.2m / s, considering that after the workpiece temperature decreases in the later stages of rolling, the deformation energy storage of the {111} component of the high-nitrogen austenitic stainless steel will increase with the increase of the rolling speed, providing sufficient driving force for the decomposition of austenite during the subsequent isothermal heat treatment in the medium temperature range.

[0017] Grain refinement of metastable austenitic stainless steel is achieved through a two-step heat treatment method:

[0018] Step 1: Perform isothermal heat treatment on hot-rolled steel plates in the medium temperature range of 900~940℃, and determine the critical holding time t for the precipitation and aging of Cr2N during the medium temperature stage. s (Unit: min) Calculated according to formula (1):

[0019] Equation (1)

[0020] In the formula, w is the mass fraction of the alloying element, and T is the absolute temperature in K.

[0021] The actual aging time of the steel plate is extended by 1-2 min based on the calculation in formula (1), and water quenching is performed immediately after aging. The rolled steel plate is subjected to isothermal heat treatment in the medium temperature range of 900-940℃ because the austenitic structure of nickel-free high-nitrogen steel is thermally unstable. During the heat preservation process, the austenite decomposes into a "ferrite + nitride" dual-phase structure. For the alloy under the composition system of this invention, the austenite can be fully decomposed into a "ferrite + Cr2N" eutectoid structure, in which the large austenite grains are divided into small eutectoid blocks.

[0022] Step 2: After isothermal aging and water quenching in the medium-temperature zone, the steel plate is reheated to 1080~1120℃, isothermal for 16~21 minutes, and then immediately water quenched. The purpose is that after the austenite decomposes, 1080~1120℃ can ensure that the steel plate re-enters the austenite single-phase region and can reverse ferrite into austenite, obtaining a fine-grained austenite single-phase microstructure.

[0023] Conventional high-nitrogen austenitic stainless steel has relatively coarse grains, and brittle fracture occurs during the uniform deformation stage before local necking deformation, causing typical intergranular fracture in most areas of the austenitic grain boundaries. On the one hand, coarse grains easily lead to dislocation accumulation at grain boundaries, resulting in stress concentration; on the other hand, intergranular fracture tends to occur at large-angle random grain boundaries perpendicular to the tensile axis. Coarse-grained tensile specimens have long and straight austenitic grain boundaries from the surface to the center, and these grain boundaries are perpendicular to the tensile axis. This arrangement easily causes intergranular fracture. Finally, after solution treatment, impurities such as phosphorus and sulfur in conventional austenitic stainless steel tend to segregate at the grain boundaries, which is also a factor causing intergranular fracture.

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

[0025] This invention differs from conventional high-nitrogen austenitic stainless steel by reducing manganese and eliminating nickel in its chemical composition. The aim is to obtain a metastable austenitic structure in the rolled state, which readily decomposes into an α+Cr2N eutectoid structure during subsequent medium-temperature aging. In this process, the originally coarse austenitic grains are divided into fine eutectoid blocks. The steel plate is reheated to the austenitic single-phase region (1080~1120℃), and the ferrite inversion transformation continues during the heating process and the high-temperature solution treatment stage, ultimately resulting in a fine-grained austenitic single-phase structure. The grains resulting from the two-step phase transformation exhibit equiaxed grain characteristics, leading to a significant reduction in the number of dislocations accumulated on the principal slip plane. Furthermore, the serrated grain boundaries of the fine-grained structure can prevent crack propagation. Segregated impurities are released into the austenitic matrix through the two phase transformations, forming "fresh grain boundaries" and improving grain boundary strength. The above characteristics significantly suppress intergranular fracture, greatly reduce the intergranular fracture area fraction, and significantly improve the strength and fracture toughness of the steel plate. At -269℃, the steel plate exhibits a yield strength σ0.2 ≥ 1500 MPa, tensile strength σb ≥ 1930 MPa, elongation A ≥ 28.50%, and fracture toughness K1C ≥ 150 MPa·m. 1 / 2 This solves the problem that the strength and toughness of austenitic stainless steel in the solution-treated state at ultra-low temperatures are difficult to meet engineering requirements. Attached Figure Description

[0026] Figure 1 The microstructure is that of conventional austenitic stainless steel.

[0027] Figure 2 The microstructure is that of conventional austenitic stainless steel.

[0028] Figure 3 The fracture structure is that of conventional austenitic stainless steel.

[0029] Figure 4 The metallographic structure is that of Example 1;

[0030] Figure 5 This is the transmissive tissue of Example 1;

[0031] Figure 6 The fracture surface structure is as shown in Example 1;

[0032] Figure 7 This is a schematic diagram of the microstructure evolution during the heat treatment process of the present invention. Detailed Implementation

[0033] The following embodiments are merely some preferred implementations of the present invention and do not limit the scope and technical means of the invention in any way.

[0034] A high-strength, high-toughness metastable austenitic stainless steel for ultra-low temperature structural materials has the following chemical composition by weight percentage: C: 0.01%~0.03%, Si: 0.25%~0.45%, Mn: 2.00%~3.00%, P≤0.015%, S≤0.005%, Cr: 20.0%~25.0%, N: 0.3%~0.5%, with the remainder being Fe and unavoidable inclusions. Table 1 shows the composition involved in each embodiment.

[0035]

[0036] A method for manufacturing high-strength and high-toughness metastable austenitic stainless steel for ultra-low temperature structural materials includes billet heating, rolling, and heat treatment, wherein:

[0037] Billet heating: preheating section temperature 750~850℃, heating section temperature 1180~1220℃, soaking section temperature 1150~1170℃, total furnace time 3~4 hours; the billet can be a continuously cast billet or a die-cast billet.

[0038] Rolling: Initial rolling temperature 1050~1080℃, final rolling temperature 970~1020℃, deformation rate controlled at 10%~30% per pass, with rolling speed controlled at 0.5~1.4m / s for the first 6 passes, and at 1.5~3.2m / s from the 7th pass onwards. Table 2 shows the heating and rolling process parameters for each embodiment.

[0039]

[0040] Heat treatment: Hot-rolled steel plates are subjected to isothermal heat treatment in the medium temperature range of 900~940℃. After isothermal aging in the medium temperature range, the steel plates are reheated to 1080~1120℃ and isothermal for 16~21 min before water quenching. Table 3 shows the heat treatment process parameters for each embodiment. The isothermal holding time is obtained by adding 1~2 min to the critical holding time for Cr2N precipitation during the medium temperature stage, based on formula (1).

[0041] Figure 7 This is a schematic diagram of the microstructure evolution during the heat treatment process of this invention. Unlike conventional high-nitrogen austenitic stainless steel, this invention first reduces manganese and eliminates nickel in its chemical composition design. The aim is to obtain a metastable austenitic structure in the rolled state, which easily decomposes into an α+Cr2N eutectoid structure during subsequent medium-temperature aging. (a) Stage: Equiaxed rolled grains; (b) Stage: After isothermal aging in the medium-temperature zone, water quenching, solvent extraction, and precipitation aging result in Cr2N precipitation. The austenite decomposes into a "ferrite + Cr2N" eutectoid structure. During this process, the originally coarse austenite grains are divided into fine eutectoid blocks; (c) Stage: During reheating of the steel plate, the high-density dislocations retained after medium-temperature aging promote the nucleation of reverse austenite, resulting in a ferrite-to-austenite transformation; (d) Stage: The reverse austenite grains continuously grow, consuming dislocations. Ultimately, the grains, after two phase transformations, exhibit a fine-grained structure with equiaxed grain characteristics, leading to a significant reduction in the number of accumulated dislocations on the principal slip plane.

[0042]

[0043] Table 4 shows the mechanical properties of each embodiment.

[0044]

[0045] As shown in Table 4, for steel plates with a thickness of 50–100 mm, at a temperature of -269℃, the yield strength σ0.2 ≥ 1500 MPa, tensile strength σb ≥ 1930 MPa, elongation A ≥ 28.50%, and fracture toughness K1C ≥ 150 MPa·m 1 / 2 .

[0046] Figures 1-3 Grain size, dislocation substructure, and fracture morphology of conventional austenitic stainless steel tensile specimens at -269℃ are shown. The average grain size in Example 1 is approximately 10 μm, while the grain size of conventional austenitic stainless steel is approximately 150 μm. It is evident that the grain size of Example 1 is significantly refined after two heat treatments, less than one-tenth the grain size of conventional solution-treated austenitic stainless steel; conventional austenitic stainless steel ( Figure 3 This causes typical intergranular fracture in most regions of the austenite grain boundaries, while in Example 1 ( Figure 6 A tough surface with dimples can be clearly observed; in both materials, the dislocation substructure exhibits a planar dislocation array that is strongly developed on the principal slip plane. Figure 2 and Figure 5 Multiple dislocation slips occurred near the grain boundaries, leading to complex dislocation reactions and entanglements. A comparison of the two showed that the grain refinement in Example 1 resulted in a significant reduction in the number of accumulated dislocations on the main slip surface. As a result, deformation at a temperature of -269°C could suppress stress concentration at the grain boundaries and greatly improve tensile strength.

[0047] It is hereby noted that the above embodiments are only for illustrating the technical concept and features of the present invention, and are not intended to limit the present invention. Any equivalent substitutions or modifications made without departing from the essence of the present invention fall within the protection scope of the present invention.

Claims

1. A high-strength, high-toughness metastable austenitic stainless steel for ultra-low temperature structural materials, characterized in that, The chemical composition of the steel, by weight percentage, is as follows: C: 0.01%~0.03%, Si: 0.25%~0.45%, Mn: 2.00%~3.00%, P≤0.015%, S≤0.005%, Cr: 20.0%~25.0%, N: 0.3%~0.5%, with the remainder being Fe and unavoidable inclusions. The manufacturing method of metastable austenitic stainless steel includes billet heating, rolling, and heat treatment, wherein: Billet heating: preheating section temperature 750~850℃, heating section temperature 1180~1220℃, soaking section temperature 1150~1170℃, total furnace time 3~4 hours; Rolling: The initial rolling temperature is 1050~1080℃, the final rolling temperature is 970~1020℃, and the deformation rate of each pass is controlled at 10%~30%. The rolling speed of the first 6 passes is controlled at 0.5~1.4m / s, and the rolling speed from the 7th pass onwards is controlled at 1.5~3.2m / s. Heat treatment: Hot-rolled steel plates are subjected to isothermal heat treatment in the medium temperature zone of 900~940℃, followed by water quenching after holding at the temperature; after isothermal aging in the medium temperature zone, the steel plates are reheated to 1080~1120℃, isothermal for 16~21 minutes, and then water quenched.

2. The high-strength, high-toughness metastable austenitic stainless steel for ultra-low temperature structural materials according to claim 1, characterized in that, C: 0.016%~0.028%, Si: 0.30%~0.40%, Mn: 2.30%~2.60%.

3. The high-strength, high-toughness metastable austenitic stainless steel for ultra-low temperature structural materials according to claim 1, characterized in that, At -269℃, the grain size is 10~15μm.

4. The high-strength, high-toughness metastable austenitic stainless steel for ultra-low temperature structural materials according to claim 1, characterized in that, For steel plates with a thickness of 50–100 mm, the yield strength σ of the steel plate at -269℃ is... 0.2 ≥1500MPa, tensile strength σ b ≥1930MPa, elongation A≥28.50%, fracture toughness K1C≥150MPa·m 1 / 2 .

5. A method for manufacturing high-strength and high-toughness metastable austenitic stainless steel for ultra-low temperature structural materials as described in any one of claims 1 to 4, characterized in that, This includes billet heating, rolling, and heat treatment, among which: Billet heating: preheating section temperature 750~850℃, heating section temperature 1180~1220℃, soaking section temperature 1150~1170℃, total furnace time 3~4 hours; Rolling: The initial rolling temperature is 1050~1080℃, the final rolling temperature is 970~1020℃, and the deformation rate of each pass is controlled at 10%~30%. The rolling speed of the first 6 passes is controlled at 0.5~1.4m / s, and the rolling speed from the 7th pass onwards is controlled at 1.5~3.2m / s. Heat treatment: Hot-rolled steel plates are subjected to isothermal heat treatment in the medium temperature zone of 900~940℃, followed by water quenching after holding at the temperature; after isothermal aging in the medium temperature zone, the steel plates are reheated to 1080~1120℃, isothermal for 16~21 minutes, and then water quenched.

6. The method for manufacturing a high-strength, high-toughness metastable austenitic stainless steel for ultra-low temperature structural materials according to claim 5, characterized in that, The critical holding time ts for the precipitation of Cr2N during the intermediate temperature stage is calculated according to formula (1): Equation (1) In the formula, w is the mass fraction of the alloying element; T is the absolute temperature, in K.

7. The method for manufacturing a high-strength, high-toughness metastable austenitic stainless steel for ultra-low temperature structural materials according to claim 6, characterized in that, The isothermal aging time in the medium temperature zone is extended by 1 to 2 minutes based on the calculation in formula (1).

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