Heat treatment process for controlling austenitic stainless steel grain size and microstructure uniformity

Abstract

This invention discloses a heat treatment process for controlling grain size and microstructure uniformity in austenitic stainless steel. It is applied to 00Cr20Ni15Mn5Mo3N austenitic stainless steel with diameters of 100mm-400mm and lengths of 2000mm-5000mm. Through a limited-rate heating design combined with segmented medium-temperature holding, long-term holding at three key temperature points before the final solution temperature of 1065℃ allows for the dissolution and diffusion of carbides, nitrides, and intermetallic compounds such as the σ phase in the original microstructure, preventing damage during rapid heating. During the process, local overheating leads to grain growth, resulting in austenite grain size that is 2-3 grades higher than that of traditional processes. The staged holding provides sufficient time and energy for the diffusion of alloying elements, ensuring their uniform distribution in the austenite matrix, eliminating microscale compositional segregation, and improving material uniformity. The stepped heating process is an efficient stress relaxation process. Holding at 450℃ and 650℃ for extended periods can eliminate residual stress, improve workpiece dimensional stability, and effectively improve the phenomena of grain coarsening, uneven microstructure, and compositional segregation in the material.

Description

Technical Field

[0001] This invention belongs to the field of heat treatment technology for metallic materials, specifically relating to a heat treatment process for controlling the grain size and microstructure uniformity of austenitic stainless steel for marine applications. This heat treatment process is mainly applied to austenitic stainless steel products with strict requirements on grain size level and grade difference within the size range of 100mm-400mm in diameter and 2000mm-5000mm in length. Background Technology

[0002] 00Cr20Ni15Mn5Mo3N is a high-performance austenitic stainless steel widely used in harsh environments such as shipbuilding, marine engineering, and chemical equipment due to its excellent resistance to pitting and crevice corrosion, as well as its high strength. This steel is strengthened by adding a high content of nitrogen (N), and its performance largely depends on its microstructure, particularly the dissolution state of carbides and nitrides, and the homogenization of alloying elements. Grain size directly determines the material's mechanical and performance characteristics; smaller grains result in higher yield strength and hardness, and finer grains significantly improve toughness and plasticity. Conversely, coarser grains increase the material's susceptibility to stress corrosion cracking.

[0003] Currently, when 00Cr20Ni15Mn5Mo3N austenitic stainless steel for shipbuilding is produced using conventional heat treatment processes (heating temperature of 1100-1150℃ followed by rapid water cooling), the microstructure exhibits significant grain coarsening and inhomogeneity (grain size below level 4, with Cr precipitates present). To address the issues of grain coarsening and inhomogeneity, reduce product magnetism, and improve overall product performance, there is an urgent need to develop a heat treatment process that can effectively control the grain size of 00Cr20Ni15Mn5Mo3N austenitic stainless steel, enabling the product to achieve a grain size level of 4 or higher, a single austenitic microstructure, and no precipitates. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of the existing technology and provide a heat treatment process for controlling the grain size and microstructure uniformity of austenitic stainless steel. This process can effectively solve the problems of grain coarsening and microstructure inhomogeneity, enabling the product to achieve a grain size level of 4 or above, with a single austenitic microstructure and no precipitated phases.

[0005] To achieve the above-mentioned objectives, the present invention adopts the following technical solution: a heat treatment process for controlling the grain size and microstructure uniformity of austenitic stainless steel, the process steps of which are as follows: Step 1) Stepped heating and segmented heat preservation solution treatment: Increase the furnace temperature from room temperature to 450℃±10℃ at a heating rate not exceeding 80℃ / hour, and hold at this temperature for 1 hour per 100mm of workpiece diameter. After holding, continue heating at a rate not exceeding 80℃ / hour to 650℃±10℃, and hold at this temperature for 1 hour per 100mm of workpiece diameter. After holding, continue heating at a rate not exceeding 80℃ / hour to 850℃±10℃, and hold at this temperature for 1 hour per 100mm of workpiece diameter. After holding, continue heating at a rate not exceeding 100℃ / hour to the final solution temperature of 1065℃±10℃, and hold at this temperature for 2 hours per 100mm of workpiece diameter. Step 2) Variable speed cooling: Within 30 seconds after the heat preservation is completed, the workpiece is lifted into the water tank for water cooling. The water temperature is controlled below 25℃. The water cooling time is calculated based on the diameter of the workpiece, which is 0.3min / mm. During the first 10 minutes of water cooling, the workpiece should be repeatedly removed from the water to break the water film on the workpiece. The water tank should be thoroughly stirred. When the workpiece is removed from the water tank, the surface temperature should be within the range of 100℃~200℃.

[0006] It is applicable to 00Cr20Ni15Mn5Mo3N austenitic stainless steel with diameters of 100mm-400mm and lengths of 2000mm-5000mm, thereby improving the quality of 00Cr20Ni15Mn5Mo3N austenitic stainless steel.

[0007] The 00Cr20Ni15Mn5Mo3N austenitic stainless steel has the following chemical composition controlled by mass percentage: C≤0.03%, Si≤1.00%, Mn: 4.00~6.00%, Cr: 19.00~20.00%, Mo: 2.00~3.00%, Ni: 13.50~16.50%, S≤0.005%, P≤0.020%, Nb≤0.35%, N0.10~0.25%, with the balance being iron.

[0008] Compared with the prior art, the process of this invention has the following advantages: 1. By employing a process design of "limited-rate heating + segmented medium-temperature holding," the material is held at three key temperatures—450℃, 650℃, and 850℃—before reaching the final solution temperature of 1065℃. This process allows for a more complete and phased dissolution and diffusion of carbides, nitrides, and intermetallic compounds such as σ and χ phases present in the original microstructure. This avoids the melting of individual compounds and abnormal grain growth due to localized overheating during rapid heating, resulting in an austenite grain size that is on average 2-3 grades higher than that obtained using traditional processes.

[0009] 2. The staged heat preservation provides sufficient time and energy for the diffusion of alloying elements (especially Mo and Cr), resulting in a more uniform distribution of them in the austenitic matrix and greatly eliminating compositional segregation at the microscale. This directly leads to a more uniform and denser passivation film, significantly improving the material's uniformity and reducing its magnetic properties.

[0010] 3. The stepped heating process itself is a highly efficient stress relaxation process. Prolonged holding at 450℃ and 650℃ effectively eliminates macroscopic and microscopic residual stresses generated during cold working, improving the dimensional stability of the workpiece. The 00Cr20Ni15Mn5Mo3N austenitic stainless steel produced according to this invention effectively improves the phenomena of grain coarsening, uneven microstructure, and compositional segregation, thus enhancing the overall performance of the product. Detailed Implementation

[0011] Example 1: A heat treatment process for controlling grain size and microstructure uniformity of austenitic stainless steel. Steel grade: 00Cr20Ni15Mn5Mo3N, chemical composition by mass percentage: C=0.02%, Si=0.56%, Mn=4.85%, Cr=19.50%, Mo=2.40%, Ni=14.50%, S=0.002%, P=0.015%, Nb=0.13%, N=0.20%, balance is iron, specifications: Φ150mm*5000mm, heat treatment process is as follows: Step 1) Stepped heating and segmented heat preservation solution treatment: The furnace temperature is raised from room temperature to 450℃±10℃ at a heating rate of 80℃ / hour, and held at this temperature for 1.5 hours (150mm*1h / 100mm); after the heat preservation is completed, the furnace temperature is raised to 650℃±10℃ at a heating rate of 80℃ / hour, and held at this temperature for 1.5 hours (150mm*1h / 100mm); after the heat preservation is completed, the furnace temperature is raised to 850℃±10℃ at a heating rate of 80℃ / hour, and held at this temperature for 1.5 hours (150mm*1h / 100mm); after the heat preservation is completed, the furnace temperature is raised to the final solution temperature of 1065℃±10℃ at a heating rate of 100℃ / hour, and held at this temperature for 3 hours.

[0012] Step 2) Variable speed cooling: After the heat preservation is completed, the workpiece is hoisted into the water tank for water cooling for 20 seconds (the water temperature starts at 23℃). Six pumps are turned on in the water tank to stir the water thoroughly. The water cooling time is 45 minutes (150mm*0.3min / mm). During the first 10 minutes of water cooling, the workpiece moves up and down once per minute to break the water film (i.e., water is poured back and forth to break the water film on the workpiece). The surface temperature of the workpiece when it comes out of the water tank is 150℃.

[0013] After production according to the above heat treatment process, the test results according to GB / T6394 standard are shown in Table 1: Table 1 Test Results

[0014] After production using the heat treatment process of the present invention to control the grain size and microstructure uniformity of austenitic stainless steel, the grain size and microstructure test results meet the requirements.

[0015] Example 2: A heat treatment process for controlling grain size and microstructure uniformity of austenitic stainless steel. Steel grade: 00Cr20Ni15Mn5Mo3N, chemical composition by mass percentage: C=0.02%, Si=0.58%, Mn=4.90%, Cr=19.45%, Mo=2.35%, Ni=14.85%, S=0.003%, P=0.016%, Nb=0.14%, N=0.19%, balance is iron, specifications: Φ300mm*4000mm, heat treatment process is as follows: Step 1) Stepped heating and segmented heat preservation solution treatment: The furnace temperature is raised from room temperature to 450℃±10℃ at a heating rate of 80℃ / hour, and held at this temperature for 3 hours (300mm*100h / mm); after the heat preservation, the furnace temperature is raised to 650℃±10℃ at a heating rate of 80℃ / hour, and held at this temperature for 3 hours; after the heat preservation, the furnace temperature is raised to 850℃±10℃ at a heating rate of 80℃ / hour, and held at this temperature for 3 hours (300mm*100h / mm); after the heat preservation, the furnace temperature is raised to the final solution temperature of 1065℃±10℃ at a heating rate of 100℃ / hour, and held at this temperature for 6 hours (300mm*2h / 100mm); after exiting the furnace, step 3) is performed for variable speed cooling; Step 2) Variable speed cooling: After the heat preservation is completed, the workpiece is hoisted into the water tank for water cooling 30 seconds later (initially the water temperature is 25℃). Six pumps are turned on in the water tank to stir the water thoroughly. The water cooling time is 90 minutes (300mm*0.3min / mm). During the water cooling, the workpiece is moved up and down once in the first 10 minutes to break the water film (i.e., water is poured back and forth to break the water film on the workpiece). The surface temperature of the workpiece when it comes out of the water tank is 170℃.

[0016] After production according to the above heat treatment process, the test results according to GB / T6394 and GB / T18254 standards are shown in Table 2: Table 2 Test Results

[0017] After production using a heat treatment process that improves the grain size and microstructure uniformity of austenitic stainless steel according to the present invention, the grain size and microstructure test results meet the requirements.

Claims

1. A heat treatment process for controlling the grain size and microstructure uniformity of austenitic stainless steel, characterized in that: The process steps are as follows: Step 1) Stepped heating and segmented heat preservation solution treatment: Increase the furnace temperature from room temperature to 450℃±10℃ at a heating rate not exceeding 80℃ / hour, and hold at this temperature for 1 hour per 100mm of workpiece diameter. After holding, continue heating at a rate not exceeding 80℃ / hour to 650℃±10℃, and hold at this temperature for 1 hour per 100mm of workpiece diameter. After holding, continue heating at a rate not exceeding 80℃ / hour to 850℃±10℃, and hold at this temperature for 1 hour per 100mm of workpiece diameter. After holding, continue heating at a rate not exceeding 100℃ / hour to the final solution temperature of 1065℃±10℃, and hold at this temperature for 2 hours per 100mm of workpiece diameter. Step 2) Variable speed cooling: Within 30 seconds after the heat preservation is completed, the workpiece is lifted into the water tank for water cooling. The water temperature is controlled below 25℃. The water cooling time is calculated based on the diameter of the workpiece, which is 0.3min / mm. During the first 10 minutes of water cooling, the workpiece should be repeatedly removed from the water to break the water film on the workpiece. The water tank should be thoroughly stirred. When the workpiece is removed from the water tank, the surface temperature should be within the range of 100℃~200℃.

2. The heat treatment process for controlling grain size and microstructure uniformity of austenitic stainless steel according to claim 1, characterized in that: It is applicable to 00Cr20Ni15Mn5Mo3N austenitic stainless steel with diameters of 100mm-400mm and lengths of 2000mm-5000mm, thereby improving the quality of 00Cr20Ni15Mn5Mo3N austenitic stainless steel.

3. The heat treatment process for controlling grain size and microstructure uniformity of austenitic stainless steel according to claim 2, characterized in that: The chemical composition of the 00Cr20Ni15Mn5Mo3N austenitic stainless steel is set according to the following mass percentages: C≤0.03%, Si≤1.00%, Mn: 4.00~6.00%, Cr: 19.00~20.00%, Mo: 2.00~3.00%, Ni: 13.50~16.50%, S≤0.005%, P≤0.020%, Nb≤0.35%, N: 0.10~0.25%, with the balance being iron.

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