A heat treatment method for improving the microstructure stability of high-nitrogen stainless bearing steel

Abstract

The application belongs to the technical field of heat treatment of bearing steel, and particularly discloses a heat treatment method for improving the organization stability of high-nitrogen stainless bearing steel, which comprises the following steps: firstly, high-temperature diffusion annealing is performed to reduce the segregation degree; then, forging is performed after heat preservation at 1140-1160 DEG C for 1-2 hours; and subsequently, normalizing, spheroidizing annealing, quenching, deep cooling, partitioning and tempering are sequentially performed after the forging. Through reasonable design of the heat treatment process, the problems of low impact toughness, too many residual austenite and poor stability of the high-nitrogen stainless bearing steel are effectively solved by the sequential partitioning and tempering, and the high-nitrogen martensitic stainless bearing steel with high strength and toughness and impact resistance is prepared.

Description

Technical Field

[0001] This invention belongs to the field of bearing steel heat treatment technology, specifically relating to a heat treatment method for improving the microstructure stability of high-nitrogen stainless bearing steel. Background Technology

[0002] Bearing steel is widely used in aerospace, transportation machinery, food industry, energy, and many other fields, and is considered a symbol of a country's metallurgical level. Aerospace bearing steel operates in harsh environments such as high temperatures and heavy loads, making it one of the steel types with the most stringent production requirements. Compared to general alloy steel, high-nitrogen steel has extremely strong corrosion resistance and a longer service life. In recent years, with the development of high-end equipment manufacturing industries such as aerospace and rail transportation, the production technology of high-end bearing steel has achieved rapid breakthroughs and improvements. The third-generation aerospace high-nitrogen stainless bearing steel, developed based on the design concept of "reducing carbon and increasing nitrogen," possesses excellent mechanical, corrosion, and service properties, and has gradually become an important development direction for high-quality bearing steel.

[0003] High-nitrogen stainless bearing steel produced by pressurized electroslag remelting currently possesses high cleanliness, uniform microstructure, and excellent strength, hardness, wear resistance, corrosion resistance, and contact fatigue performance. The addition of nitrogen promotes the precipitation of fine dispersed phases, grain refinement, and solid solution strengthening, resulting in superior corrosion resistance and fatigue properties. Heat treatment processes can control the content, morphology, size, and distribution of precipitated phases, martensite, and retained austenite in bearing steel, ultimately determining its comprehensive mechanical properties. After traditional quenching-cryogenic-tempering heat treatment, the microstructure of high-nitrogen stainless bearing steel consists of martensite + carbonitrides + retained austenite. Lowering the quenching temperature, performing multiple cryogenic treatments, and appropriately increasing the tempering temperature can reduce the retained austenite content to some extent. However, due to nitrogen's strong austenite-stabilizing ability, traditional quenching-cryogenic-tempering heat treatment cannot completely solve the problem of a large amount of unstable blocky retained austenite in high-nitrogen stainless bearing steel. Face-centered cubic austenite has higher carbon and nitrogen solubility than body-centered cubic martensite. Quenching-partitioning-tempering heat treatment can promote the diffusion of carbon and nitrogen from martensite to austenite, enriching carbon and nitrogen atoms in the austenite and improving its stability. However, this process results in excessively high retained austenite content in the steel. Therefore, there is an urgent need to develop a novel heat treatment process for high-nitrogen stainless bearing steel to overcome the problems of high retained austenite content and poor microstructure stability, thereby further improving its performance.

[0004] Fang Ming's content

[0005] The high nitrogen content in third-generation high-nitrogen stainless bearing steel for aerospace applications easily leads to a large amount of blocky retained austenite in the steel after traditional quenching-deep cooling-tempering heat treatment. During service, this blocky retained austenite transforms into martensite under stress, causing dimensional changes and even crack initiation, resulting in premature bearing failure. To address these issues, this invention provides a heat treatment method that effectively improves the microstructural stability of high-nitrogen stainless bearing steel. After this heat treatment, the retained austenite content can be significantly reduced, its stability improved, and the strength and toughness of the steel enhanced. This provides guidance for the development of high-performance high-nitrogen stainless bearing steel in my country and accelerates its research and application.

[0006] To address the above problems, the present invention provides the following technical solution:

[0007] This invention provides a high-nitrogen stainless bearing steel, wherein the chemical element composition of the high-nitrogen stainless bearing steel, by weight percentage, comprises: 0.28%–0.34% C, 0.35%–0.44% N, 0.3%–0.8% Si, 0.3%–0.6% Mn, 14.5%–16.5% Cr, 0.95%–1.1% Mo, 0.5%–1.0% Ni, with the balance being Fe and other unavoidable impurities.

[0008] An ingot containing the aforementioned target elements was prepared, and then the ingot was forged and heat-treated. Before forging, a high-temperature diffusion annealing was performed to reduce element segregation.

[0009] The forged steel ingots are loaded into the furnace and subjected to subsequent heat treatments in sequence, specifically: normalizing, spheroidizing annealing, quenching, deep cooling, fractionation, and tempering.

[0010] This invention provides a heat treatment method for improving the microstructure stability of high-nitrogen stainless bearing steel, specifically including the following steps:

[0011] (1) High-temperature diffusion annealing

[0012] The surface of the high-nitrogen stainless bearing steel ingot is sprayed with stainless steel anti-oxidation coating and allowed to dry naturally. Then, it is loaded into the furnace at a temperature below 350°C and heated to 800°C to 900°C at a rate of 60°C / h to 80°C / h and held for 1h to 2h. After that, the temperature is increased to 1240°C to 1260°C at a rate of 100°C / h to 120°C / h and held for 10h to 12h. Then, the furnace is cooled to make the dendritic structure generated during the solidification process disappear.

[0013] The stainless steel anti-oxidation coating sprayed on the surface of the high-nitrogen stainless bearing steel ingot comprises, by weight percentage: 3%–5% ZrO2, 65%–70% SiO2, 4%–9% Na2O, 1%–5% B2O3, 10%–20% Al2O3, 2%–7% MgO, 2%–4% CaO, with the balance being water.

[0014] (2) Forging

[0015] The steel ingot obtained in step (1) is cooled in a furnace at a cooling rate of 40℃ / h to 60℃ / h to 1140℃ to 1160℃, and then held at that temperature for 1h to 2h. After that, it is forged using the "two upsetting and two drawing" method, with the forging ratio controlled at 3 to 5. That is, first, the steel ingot is pressed down along the longitudinal direction and upset to 1 / 2 of its original height, then it is pressed down along the transverse direction and drawn to twice its original length, then it is pressed down along the longitudinal direction and upset to 1 / 2 of its original height, and finally it is pressed down along the transverse direction and drawn to twice its original length. The final forging temperature of the steel ingot is controlled at 945℃ to 965℃. Since this temperature range is the region where austenite, carbides and carbonitrides coexist, this steel has good high-temperature deformability, which can avoid cracking during the forging process. After forging, the parts are first cooled by water spray at a rate of 200℃ / s to 300℃ / s until they reach 400℃ to 500℃, and then air-cooled to obtain the forging.

[0016] (3) Poisoning

[0017] The forgings obtained in step (2) are loaded into a furnace at a temperature below 300°C and heated to 980°C to 1000°C at a rate of 60°C / h to 80°C / h. After holding at this temperature for 1 to 2 hours, they are air-cooled to room temperature to refine the grains and reduce the tendency to crack.

[0018] (4) Spheroidizing annealing

[0019] The steel sample that has been normalized in step (3) is loaded into the furnace and heated to 820℃~840℃ at a rate of 60℃ / h~80℃ / h, held for 5h~6h, then cooled to 640℃~660℃ at a rate of 40℃ / h~60℃ / h, held for 2h~4h, and finally cooled to room temperature to obtain a steel sample, so as to obtain uniformly distributed granular carbides and nitrides.

[0020] According to the spheroidizing annealing process, a spheroidizing annealing process was formed as follows: Figure 1 The graph shows the temperature variation curve of spheroidizing annealing for high-nitrogen stainless bearing steel.

[0021] (5) Quenching

[0022] The steel sample spheroidized and annealed in step (4) is placed in a quenching furnace at a temperature below 300°C and heated to 1020°C to 1040°C at a rate of 60°C / h to 80°C / h. It is then held for 1 to 2 hours to austenitize. Subsequently, it is oil quenched and cooled to room temperature to form a martensite, carbonitride and retained austenite structure.

[0023] (6) Cryogenic

[0024] The quenched steel sample obtained in step (5) is cryogenically treated in liquid nitrogen for 1 to 2 hours to eliminate most of the blocky unstable residual austenite and obtain more martensite structure. After being restored to room temperature in air, the steel sample is obtained.

[0025] (7) Partitioning and tempering

[0026] The steel sample, which has been cryogenically cooled in step (6), is transferred into a salt bath furnace heated to 180℃~220℃ and held for 5min~90min for fractionation treatment to improve the stability of the residual austenite. Then it is transferred into a tempering furnace heated to 350℃~450℃ for two temperings, each held for 1h~2h, and air-cooled to room temperature to obtain the steel sample.

[0027] Based on the aforementioned quenching-deep cryogenic treatment-partitioning-tempering process, a process was formed as follows: Figure 2 The graph shows the temperature variation curves of high-nitrogen stainless bearing steel during quenching, cryogenic treatment, fractionation, and tempering.

[0028] Based on the above detailed description, the heat treatment method for high-nitrogen stainless bearing steel of the present invention has the following beneficial effects:

[0029] (1) The heat treatment method for high-nitrogen stainless bearing steel according to an exemplary embodiment of the present invention can refine the grains and obtain uniformly distributed granular carbides and carbonitrides;

[0030] (2) The heat treatment method for high-nitrogen stainless bearing steel according to an exemplary embodiment of the present invention can significantly improve the stability of residual austenite and improve the strength and toughness of steel. The unnotched impact absorption energy reaches 50J to 60J, the yield strength reaches 1950MPa to 2050MPa, and the tensile strength reaches 2150MPa to 2250MPa, which significantly improves the comprehensive performance of this type of steel.

[0031] (3) The heat treatment method for high-nitrogen stainless bearing steel according to the exemplary embodiment of the present invention has low equipment requirements, simple operation, low cost, and is easy to industrialize, which can meet the needs of the high-end stainless bearing market. Attached Figure Description

[0032] Figure 1 A graph showing the spheroidizing annealing temperature variation of high-nitrogen stainless bearing steel according to an exemplary embodiment of the present invention;

[0033] Figure 2 A graph showing the temperature variation of quenching-deep cryogenic treatment-partitioning-tempering for high-nitrogen stainless bearing steel according to an exemplary embodiment of the present invention;

[0034] Figure 3 and Figure 4 Microstructure diagram of high-nitrogen stainless bearing steel according to an exemplary embodiment of the present invention;

[0035] Figures 5 to 6 The image shows the microstructure of a high-nitrogen stainless bearing steel, which is an exemplary comparative example according to the present invention. Detailed Implementation

[0036] An exemplary embodiment of the present invention provides a heat treatment method for improving the microstructure stability of high-nitrogen stainless bearing steel. The method first involves high-temperature diffusion annealing to reduce elemental segregation; then, reheating and forging to obtain a deformed microstructure; followed by normalizing and spheroidizing annealing pretreatment to refine the grains and obtain uniformly distributed granular carbides and carbonitrides, thereby reducing hardness and facilitating machining; then, quenching to obtain a fine-grained martensite and retained austenite microstructure in the high-nitrogen stainless bearing steel, with carbides and carbonitrides uniformly and diffusely distributed in the matrix; next, cryogenic treatment to promote the transformation of unstable retained austenite into martensite, thereby reducing the content of large-sized retained austenite; then, a partitioning treatment to promote the diffusion of carbon and nitrogen from martensite into the retained austenite, thereby improving the stability of the retained austenite; finally, tempering to promote the secondary dispersion precipitation of carbides and carbonitrides in the steel. The high-nitrogen stainless bearing steel obtained by the heat treatment method has high strength, good toughness, and excellent dimensional stability, which can improve the product quality of bearings and extend their service life, thereby meeting the market demand for high-performance high-nitrogen stainless bearing steel.

[0037] The following description, with reference to the accompanying drawings and specific examples, illustrates a heat treatment method for improving the microstructural stability of high-nitrogen stainless bearing steel according to an exemplary embodiment of the present invention, wherein... Figure 3 and Figure 4 Microstructure of high-nitrogen stainless bearing steel according to an exemplary embodiment of the present invention. Figure 5 and Figure 6 The image shown is an exemplary comparative microstructure of high-nitrogen stainless bearing steel according to the present invention. However, the technical solution of the present invention is not limited to the following specific examples.

[0038] Example 1

[0039] The chemical composition of high-nitrogen stainless bearing steel, by weight percentage, includes: 0.29% C, 0.37% N, 0.40% Si, 0.44% Mn, 16.22% Cr, 0.82% Mo, 1% Ni, with the balance being Fe and other unavoidable impurities.

[0040] In this embodiment of the invention, a 25kg pressure induction furnace is used as the smelting equipment. The steel ingot is wire-cut to prepare a cast steel ingot sample with dimensions of Φ90mm×350mm, which is then prepared for heat treatment.

[0041] The steel ingot is heat-treated using the following steps:

[0042] (1) High-temperature diffusion annealing. The surface of the high-nitrogen stainless bearing steel ingot is sprayed with stainless steel anti-oxidation coating and naturally dried. Then it is loaded into the furnace at 300℃, heated to 830℃ at a rate of 65℃ / h and held for 1.5h. Then it is heated to 1250℃ at a rate of 100℃ / h and held for 10h. Then it is furnace cooled.

[0043] (2) Forging. The steel ingot obtained in step (1) is cooled to 1150°C in a furnace at 60°C / h and then forged. The forging adopts a two-upsetting and two-drawing method with a forging ratio of 4. The final forging temperature is 950°C. After forging, it is first cooled by water spray to 450°C and then air cooled to obtain the forging.

[0044] (3) Normalizing. The forging obtained in step (2) is loaded into the furnace at 280°C, heated to 1000°C at a rate of 60°C / h, held for 1h, and then air-cooled to room temperature;

[0045] (4) Spheroidizing annealing. The sample obtained in step (3) is loaded into the furnace, heated to 830℃ at 60℃ / h, held for 5h, then cooled to 660℃ at 50℃ / h and held for 3h. The furnace is cooled to room temperature, and 15 bar samples with dimensions of 10.4mm×10.4mm×56mm and Φ15mm×65mm are prepared from the obtained steel sample.

[0046] (5) Quenching. The two bar samples of different sizes that have undergone step (4) are placed in a furnace at 280°C and heated to 1035°C at a rate of 60°C / h. The temperature is held for 1 hour, and then quenched in oil and cooled to room temperature.

[0047] (6) Deep cryogenic treatment. The quenched steel sample obtained in step (5) is subjected to deep cryogenic treatment in liquid nitrogen for 1 hour, and then restored to room temperature in air to obtain the steel sample;

[0048] (7) Partitioning and tempering. The steel sample cleaned in step (6) is immediately transferred into a salt bath furnace heated to 180°C and held for 5 minutes. Then it is transferred into a tempering furnace heated to 350°C and tempered twice, holding for 1 hour each time. After air cooling to room temperature, the final product is obtained and is designated as 1#.

[0049] Twenty-eight untreated rod samples prepared in step (4) of Example 1 were selected and divided into two groups according to different sizes. These groups underwent different subsequent heat treatments, and each group was designated as Examples 2# to 9# and Comparative Examples 1# to 6#. Examples 2# to 9# underwent subsequent heat treatment, including cryogenic treatment, fractionation, and tempering. Cryogenic treatment used liquid nitrogen, the fractionation temperature was 180℃ to 220℃, and the tempering temperature was 350℃ to 450℃. The comparative examples underwent subsequent heat treatment. Comparative Examples 1# to 3# underwent cryogenic treatment and fractionation, while Comparative Examples 4# to 6# underwent cryogenic treatment and tempering. Cryogenic treatment used liquid nitrogen, the fractionation temperature was 180℃ to 220℃, and the tempering temperature was 350℃ to 450℃. The specific temperatures and holding times for cryogenic treatment, fractionation, and tempering in Examples 1# to 9# and Comparative Examples 1# to 6# are shown in Table 1.

[0050] Table 1 shows the subsequent heat treatment temperature and holding time of the high-nitrogen stainless bearing steels prepared in Examples 1# to 9# and Comparative Examples 1# to 6# of the present invention.

[0051]

[0052] The microstructure of Example 5# before deformation is as follows: Figure 3 As shown, the tissue deformed by the tensile test is as follows: Figure 4 As shown, by Figure 3 and Figure 4 It can be seen that the matrix is ​​martensitic with a small amount of retained austenite, and the proportion of austenite changes little before and after mechanical deformation.

[0053] The microstructure of Comparative Example 2# before deformation is as follows: Figure 5 As shown, the tissue deformed by the tensile test is as follows: Figure 6 As shown, by Figure 5 and Figure 6 It can be seen that the matrix is ​​martensitic with a small amount of retained austenite, and the proportion of austenite is significantly reduced after mechanical deformation.

[0054] Performance Experiment

[0055] Impact test: The steel used in the experiment was processed into impact specimens of 10mm×10mm×55mm, and a room temperature unnotched Charpy impact test was carried out using a ZBC2452 impact testing machine.

[0056] Tensile test: The steel used in the test was processed into a bar-shaped tensile specimen with a total length of 65 mm, a parallel length of 30 mm, an original diameter of 5 mm for the parallel length, and a transition circle radius of 5 mm. The tensile test was carried out on a Zwick Roell Z100 testing machine.

[0057] The prepared and heat-treated high-nitrogen stainless bearing steel samples were observed using a high-resolution ZEISS GeminiSEM 300 scanning electron microscope. The results are shown in Table 2.

[0058] Table 2 shows the changes in residual austenite content before and after tensile tests of the high-nitrogen stainless bearing steels prepared in Examples 1# to 9# and Comparative Examples 1# to 6# of the present invention.

[0059]

[0060] The prepared and heat-treated high-nitrogen stainless bearing steel samples were subjected to impact and tensile tests, and the results are shown in Table 3.

[0061] Table 3 shows the experimental results of the mechanical properties of the high-nitrogen stainless bearing steels prepared in Examples 1# to 9# and Comparative Examples 1# to 6# of the present invention.

[0062]

[0063] Testing, combined with Examples 1-9 and Comparative Examples 1-6, shows that the products obtained using the heat treatment process of this invention, specifically Examples 1-9, exhibit superior performance compared to the products obtained in Comparative Examples 1-6. Compared to traditional heat treatment processes, the high-nitrogen stainless bearing steel prepared by the heat treatment method of this invention exhibits a smaller residual austenite transformation and more stable residual austenite. Simultaneously, its yield strength, tensile strength, and impact toughness are significantly improved, achieving excellent results.

[0064] In summary, this process optimizes the traditional heat treatment process for high-nitrogen stainless bearing steel. It is a novel heat treatment process for high-nitrogen stainless bearing steel that is specifically designed for the microstructure and properties of high-nitrogen stainless bearing steel. This process significantly improves the strength and toughness of high-nitrogen stainless bearing steel and has broad market application prospects, making it easy to promote.

[0065] This description is written based on a specific preferred embodiment. The specific embodiments of the present invention should not be considered as limited to this description. For those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention. All products derived from this invention without departing from the spirit and scope of the technical solution of the present invention should be considered to fall within the patent protection scope defined by the claims of the present invention.

Claims

1. A heat treatment method for improving the microstructural stability of high-nitrogen stainless bearing steel, characterized in that: By weight percentage, high-nitrogen stainless bearing steel consists of the following components: 0.28%–0.34% C, 0.35%–0.44% N, 0.3%–0.8% Si, 0.3%–0.6% Mn, 14.5%–16.5% Cr, 0.8%–1.1% Mo, 0.5%–1% Ni, with the balance being Fe and other unavoidable impurities; The heat treatment method includes the following steps: (1) Spray stainless steel anti-oxidation coating on the surface of high nitrogen stainless bearing steel ingot and let it dry naturally. Then, load it into the furnace at a temperature below 350℃, raise the temperature to 800℃~900℃ at a rate of 60℃ / h~80℃ / h and hold it for 1h~2h. Then, raise the temperature to 1240℃~1260℃ at a rate of 100℃ / h~120℃ / h and hold it for 10h~12h. Then, cool the furnace to make the dendritic structure generated during the solidification process disappear. (2) The steel ingot obtained in step (1) is cooled in a furnace at a cooling rate of 40℃ / h to 60℃ / h to 1140℃ to 1160℃, and then held at that temperature for 1h to 2h. After that, it is forged by "two upsetting and two drawing" with a forging ratio controlled at 3 to 5 and a final forging temperature controlled at 945℃ to 965℃. After forging, it is first cooled by water spray at a cooling rate of 200℃ / s to 300℃ / s to 400℃ to 500℃, and then air cooled to obtain the forging. (3) The forging obtained in step (2) is loaded into the furnace at a temperature below 300°C and heated to 980°C to 1000°C at a rate of 60°C / h to 80°C / h. After holding at this temperature for 1h to 2h, it is air-cooled to room temperature to refine the grains and reduce the tendency to crack. Then, the temperature is raised to 820°C to 840°C at a rate of 60°C / h to 80°C / h and held for 5h to 6h. After that, the temperature is lowered to 640°C to 660°C at a rate of 40°C / h to 60°C / h and held for 2h to 4h. Then, it is furnace-cooled to room temperature to obtain uniformly distributed granular carbides and carbonitrides. (4) The steel sample obtained in step (3) is placed in a quenching furnace at a temperature below 300°C and heated to 1020°C to 1040°C at a rate of 60°C / h to 80°C / h. After holding at this temperature for 1h to 2h, it is oil quenched to room temperature. Then, it is cryogenically treated in liquid nitrogen for 1h to 2h. After being restored to room temperature in air, it is transferred to a salt bath furnace that has been heated to 180°C to 220°C and held for 5min to 90min for fractionation treatment. Then, it is transferred to a tempering furnace that has been heated to 350°C to 450°C for two temperings, each time holding for 1h to 2h. After that, it is air-cooled to room temperature to obtain the steel sample.

2. The heat treatment method for improving the microstructural stability of high-nitrogen stainless bearing steel according to claim 1, characterized in that: Cryogenic treatment uses liquid nitrogen, with holding time controlled between 1 and 2 hours; fractionation treatment is carried out in a salt bath furnace, with temperature controlled between 180°C and 220°C, and holding time controlled between 5 and 90 minutes; tempering temperature is controlled between 350°C and 450°C, and holding time controlled between 1 and 2 hours.

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