Heat treatment process for refining C250 maraging steel grains

Abstract

The invention relates to the technical field of steel heat treatment, and particularly discloses a heat treatment process for refining C250 maraging steel grains. The heat treatment technology for refining the C250 maraging steel grains comprises the following steps that firstly, heat preservation is conducted on C250 maraging steel at the temperature of 945-955 DEG C for 1-1.5 h, and water cooling is conducted to the room temperature; then preserving heat for 1-1.5 hours at the temperature of 925-935 DEG C, and performing water cooling to room temperature; then preserving heat for 1-1.5 hours at the temperature of 895-905 DEG C, and performing water cooling to room temperature; and finally, preserving heat for 1-1.5 hours at the temperature of 815-825 DEG C, and air-cooling to room temperature. According to the heat treatment technology for refining the C250 maraging steel grains, the C250 maraging steel grains can be fully refined and homogenized, the C250 maraging steel grains reach the grade 8-9, and the C250 maraging steel grains have the excellent mechanical property.

Description

Technical Field

[0001] The present application relates to the technical field of heat treatment of steel materials, and in particular to a heat treatment process for refining the grains of C250 maraging steel. Background Art

[0002] C250 maraging steel is an ultra-high strength steel based on carbon-free or low-carbon martensite, which is hardened by precipitation of intermetallic compounds through aging treatment. It has the advantages of high strength, high hardness, high toughness, low hardening index, good formability and excellent corrosion resistance. Therefore, it is widely used in aerospace, military industry, petrochemical, marine engineering, automobile manufacturing and other fields.

[0003] Research has found that the grain size of maraging steel has an important influence on its mechanical properties and service life, so grain refinement has become the main method to improve the strength, plasticity and toughness of maraging steel. Traditional methods of grain refinement mainly include solution treatment and deformation treatment; since the deformation treatment process is prone to produce texture, the performance of steel is directional, which affects its application in products; and the effect of solution treatment on grain refinement is very limited, and it is usually difficult to meet the requirements of modern equipment; in addition, the grain refinement effect of existing maraging steel grain refinement treatment methods is still not ideal.

[0004] Therefore, a heat treatment process for refining the grains of C250 maraging steel is proposed, which has very important theoretical significance for the wide application of maraging steel. Summary of the invention

[0005] In order to improve the grain refining effect of C250 maraging steel, the present application provides a heat treatment process for refining the grains of C250 maraging steel.

[0006] In the first aspect, the heat treatment process for refining the grains of C250 maraging steel provided in the present application adopts the following technical solution: A heat treatment process for refining the grains of C250 maraging steel comprises the following steps: heat treating the C250 maraging steel according to the following process: S1: Keep at 945-955℃ for 1-1.5h, and cool to room temperature with water; S2: Keep at 925-935℃ for 1-1.5h, and cool to room temperature with water; S3: Keep at 895-905℃ for 1-1.5h, and cool to room temperature with water; S4: Keep at 815-825℃ for 1-1.5h, and air cool to room temperature.

[0007] The present application utilizes C250 maraging steel to undergo repeated transformations from α to γ ​​during cyclic phase transformation heat treatment, so that a large number of microscopic defects are generated in the martensitic structure, including dislocations, twins, stacking faults, etc. The above microscopic defects will be inherited into the austenite during the transformation of martensite to austenite, thereby increasing the energy storage capacity of the material, recrystallization will occur at locations with higher energy of the material, the structure will continue to be refined, and the storage energy of the austenite will continue to increase, thereby achieving full refinement and homogenization of the austenite grains through repeated phase transformation and recrystallization.

[0008] In the present application, the presence of microscopic defects such as dislocations, twins, and stacking faults can increase the storage energy of the material, and recrystallization mainly nucleates and grows at positions with higher energy; therefore, the presence of such microscopic defects can increase the driving force for recrystallization of the material, causing the structure to continue to fragment and the dislocation density to continue to increase.

[0009] Optionally, the temperature of the S2 process is 930° C. and the holding time is 1 hour.

[0010] Optionally, the temperature of the S3 process is 900° C. and the holding time is 1 hour.

[0011] Optionally, the temperature of the S4 process is 820° C. and the holding time is 1 hour.

[0012] Optionally, the heat treatment process of the C250 maraging steel further comprises an aging treatment, and the specific steps are: subjecting the C250 maraging steel treated by the S4 process to an aging treatment at a temperature of 475-485° C. for 5-8 hours, and air cooling to room temperature.

[0013] Optionally, the heat treatment process of the C250 maraging steel further includes aging treatment, and the specific steps are: keeping the C250 maraging steel after S4 process at 475-485°C for 2-3h, and air cooling to room temperature; then continue to keep at 475-485°C for 2-3h, and air cool to room temperature.

[0014] In the present application, by performing the above two-stage aging treatment on the C250 maraging steel after the cyclic heat treatment, the microstructure of the C250 maraging steel can be further optimized and the fatigue life of the C250 maraging steel can be extended.

[0015] In a second aspect, the present application provides a C250 maraging steel obtained by using the heat treatment process for refining the C250 maraging steel grains.

[0016] Optionally, the grain size of the C250 maraging steel is level 8-9.

[0017] In summary, this application has the following beneficial effects: 1. The present application provides a heat treatment process for refining the grains of C250 maraging steel. The heat treatment process firstly performs a phase transformation crystallization of the maraging steel at 945-955°C, and a large number of micro defects will be generated in the martensitic structure. The micro defects can be inherited into the austenite, thereby significantly improving the storage energy of the material and significantly enhancing the driver for the recrystallization of the material. Then, the second, third and fourth phase transformation heat treatments are performed, and the material is continuously recrystallized and the structure is continuously fragmented, so that the grains of the C250 maraging steel are fully refined and homogenized. Therefore, the heat treatment process for refining the grains of C250 maraging steel provided in the present application can significantly improve the grain refining effect of C250 maraging steel, and has very important theoretical significance for the wide application of maraging steel.

[0018] 2. The heat treatment process for refining the grains of C250 maraging steel provided in this application can refine the grains of C250 maraging steel to level 8~9. The tensile strength of the C250 maraging steel obtained after aging treatment is greater than 1900MPa, and the fatigue life is greater than 200000 times.

[0019] 3. The present application further optimizes the microstructure of the C250 maraging steel after cyclic treatment by adopting a two-stage aging treatment process, thereby extending the fatigue life of the C250 maraging steel, making the fatigue life of the C250 maraging steel greater than 215,000 times. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is the metallographic structure diagram of C250 maraging steel without cyclic heat treatment; Figure 2 This is the metallographic structure diagram of C250 maraging steel after treatment in Example 1 of the present application. DETAILED DESCRIPTION

[0021] The present application provides a heat treatment process for refining the grains of C250 maraging steel, comprising the following steps: (1) The C250 maraging steel is heated to 945-955°C and kept at this temperature for 1-1.5 hours, and then water-cooled to room temperature to achieve the first cycle treatment, thereby obtaining the heat-treated C250 maraging steel; (2) The material after the first cycle treatment is heated to 925-935°C and kept at this temperature for 1-1.5 hours, and then water-cooled to room temperature to achieve the second cycle treatment; (3) The material after the second cycle treatment is heated to 895-905°C and kept at this temperature for 1-1.5 hours, and then water-cooled to room temperature to achieve the third cycle treatment; (4) The material after the third cycle treatment is heated to 815-825°C and kept at this temperature for 1-1.5 hours, and then air-cooled to room temperature to achieve the fourth cycle treatment; (5) The C250 maraging steel treated in step (4) is aged at 475-485°C for 5-8h and air-cooled to room temperature.

[0022] Furthermore, step (5) is to keep the C250 maraging steel treated in step (4) at 475-485°C for 2-3 hours, and air cool it to room temperature; then continue to keep it at 475-485°C for 2-3 hours, and air cool it to room temperature.

[0023] The specification of the C250 maraging steel used in the embodiment of the present application is Φ150×500mm; its chemical composition, in terms of mass percentage, includes: C content 0.0039%, Ni content 17.93%, Co content 7.87%, Mo content 4.89%, Ti content 0.40%, Al content 0.11%, P content 0.0023, S content 0.0004%, and the balance is Fe.

[0024] The raw materials, solvents, etc. of the present application can all be obtained commercially.

[0025] The present application is further described in detail below in combination with embodiments, performance testing and accompanying drawings. Example 1

[0026] Embodiment 1 provides a heat treatment process for refining the grains of C250 maraging steel, comprising the following steps: (1) C250 maraging steel was heated to 950°C and kept at this temperature for 1 hour, and then water-cooled to room temperature to achieve the first cycle treatment; (2) The material after the first cycle treatment is heated to 930°C and kept at this temperature for 1 hour, and then water-cooled to room temperature to achieve the second cycle treatment; (3) The material after the second cycle treatment is heated to 900°C and kept at this temperature for 1 hour, and then water-cooled to room temperature to achieve the third cycle treatment; (4) The material after the third cycle treatment is heated to 820°C and kept at this temperature for 1 hour, and then air-cooled to room temperature to achieve the fourth cycle treatment, thereby obtaining the heat-treated C250 maraging steel; (5) The C250 maraging steel treated in step (4) is kept at 480°C for 6 hours and air-cooled to room temperature.

[0027] Example 1 The metallographic structure of C250 maraging steel after heat treatment is shown in Figure 2 shown. Embodiment 2-4

[0028] Examples 2-4 respectively provide a heat treatment process for refining the grains of C250 maraging steel.

[0029] The difference between the above embodiment and embodiment 1 is that the temperature and time of the four-cycle treatment are specifically shown in Table 1 below. Comparative Examples 1-7

[0030] Comparative Examples 1-7 respectively provide a heat treatment process for refining the grains of C250 maraging steel.

[0031] The difference between the comparative example and Example 1 is that the temperature and time of the four-cycle treatment are specifically shown in Table 1 below.

[0032] Table 1 Temperature of four cycles of treatment in Examples 1-4 and Comparative Examples 1-7 Example 5

[0033] Example 5 provides a heat treatment process for refining the grains of C250 maraging steel.

[0034] The difference between the above embodiment and embodiment 1 is that the aging treatment process in step (5) is as follows: (5) The C250 maraging steel treated in step (4) is kept at 480°C for 3 hours and air-cooled to room temperature; then, the temperature is continued to be kept at 480°C for 3 hours and air-cooled to room temperature. Comparative Example 8

[0035] Comparative Example 8 provides a heat treatment process for refining the grains of C250 maraging steel, comprising the following steps: heating the C250 maraging steel to 750°C, keeping it warm for 5 minutes, and then water-quenching it to room temperature to achieve a first cycle treatment; heating the material after the first cycle treatment to 930°C, keeping it warm for 5 minutes, and then water-quenching it to room temperature to achieve a second cycle treatment, thereby obtaining the heat-treated C250 maraging steel. Comparative Example 9

[0036] Comparative Example 9 provides a heat treatment process for refining the grains of C250 maraging steel, comprising the following steps: heating the C250 maraging steel to 750°C, keeping it warm for 1 hour, and then water-quenching it to room temperature to achieve a first cycle treatment; heating the material after the first cycle treatment to 930°C, keeping it warm for 1 hour, and then water-quenching it to room temperature to achieve a second cycle treatment, thereby obtaining the heat-treated C250 maraging steel. Detection test

[0037] The microstructure and performance of the C250 maraging steel obtained in Examples 1-5 and Comparative Examples 1-9 were tested, and the results are shown in Table 2 below.

[0038] 1. Microstructure observation: The microstructure of the C250 maraging steel after heat treatment in Examples 1-5 and Comparative Examples 1-9 was observed by scanning electron microscopy to obtain the grain size, and the grain grade was obtained according to the Grain Size Grade Comparison Table 12; 2. Tensile strength test: refer to GB / T 228.1 to test the tensile strength of C250 maraging steel; 3. Fatigue performance test: Kt=1.0, strain ratio 0, maximum strain 0.006mm / mm, frequency 0.5HZ, triangular waveform.

[0039] Table 2 Performance test results of C250 maraging steel obtained in Examples 1-5 and Comparative Examples 1-9

[0040] According to the test results in Table 2, the grain grade of the C250 maraging steel obtained in Examples 1-5 of the present application is 8-9, the compressive strength is greater than 1900 MPa, and the fatigue life is greater than 200000 times. Therefore, it is shown that the four-stage step-by-step cooling heat treatment process provided in Examples 1-5 of the present application can fully refine the grains and homogenize the structure of the C250 maraging steel, thereby enabling the C250 maraging steel after aging treatment to obtain excellent mechanical properties.

[0041] The grain grade of C250 maraging steel obtained in Comparative Examples 1-4 is only 7-8, the compressive strength is only 1812-1883 MPa, and the fatigue life is only 98246-217787 times. Therefore, it is difficult to achieve excellent grain and microstructure of C250 maraging steel by the three-stage step-by-step cooling heat treatment process.

[0042] The grain grade of C250 maraging steel obtained in Comparative Examples 5-6 is only 6-7.5, the compressive strength is only 1776-1790 MPa, and the fatigue life is only 33492-54222 times. Therefore, it is difficult to achieve excellent grain and microstructure effects for C250 maraging steel by the four-stage step-by-step heating heat treatment process, especially the final high-temperature heating will cause the grains to grow.

[0043] The grain grade of the C250 maraging steel obtained in Comparative Example 7 is 4.0-7.5, the compressive strength is only 1294 MPa, and the fatigue life is only 9493 times. Therefore, it is difficult for the four-stage step-by-step cooling heat treatment process provided in Comparative Example 7 to achieve excellent grain and microstructure of the C250 maraging steel.

[0044] The grain grade of the C250 maraging steel obtained in Comparative Examples 8-9 is 5.5-6, the compressive strength is only 1799-1831 MPa, and the fatigue life is only 18971-19564 times. Therefore, it is explained that the grain refinement and microstructure uniformity of the C250 maraging steel after heat treatment using the two-stage temperature increase treatment process in the related art are not good, which leads to poor mechanical properties of the C250 maraging steel.

[0045] It can be seen from the test results of Example 1 and Example 5 that Example 5 can further optimize the comprehensive performance of C250 maraging steel and further improve the fatigue life by adopting a two-stage aging treatment process.

[0046] In summary, the present application provides a heat treatment process for refining the grains of C250 maraging steel. The heat treatment process adopts a four-stage gradient cooling treatment method. The present application controls the temperature and holding time of the four stages within the following ranges: S1: holding at 945-955°C for 1-1.5h, and water cooling to room temperature; S2: holding at 925-935°C for 1-1.5h, and water cooling to room temperature; S3: holding at 895-905°C for 1-1.5h, and water cooling to room temperature; S4: holding at 815-825°C for 1-1.5h, and air cooling to room temperature, so that the grains of C250 maraging steel are fully refined and the structure is homogenized, thereby enabling the C250 maraging steel to obtain excellent mechanical properties. Furthermore, by performing a two-stage aging treatment on the heat-treated C250 maraging steel, the comprehensive performance of the C250 maraging steel is further optimized, and the fatigue life of the C250 maraging steel is increased to more than 215,000 times.

[0047] Although the present invention has been described in detail above with general descriptions and specific embodiments, it is obvious to those skilled in the art that some modifications or improvements may be made thereto based on the present invention. Therefore, these modifications or improvements made without departing from the spirit of the present invention all fall within the scope of protection claimed by the present invention.

Claims

1. A heat treatment process for refining the grains of C250 maraging steel, characterized in that: The method comprises the following steps: heat treating C250 maraging steel according to the following process: S1: Keep at 945-955℃ for 1-1.5h, and cool to room temperature with water; S2: Keep at 925-935℃ for 1-1.5h, and cool to room temperature with water; S3: Keep at 895-905℃ for 1-1.5h, and cool to room temperature with water; S4: Keep at 815-825℃ for 1-1.5h, and air cool to room temperature.

2. The heat treatment process for refining C250 maraging steel grains according to claim 1, characterized in that: The temperature of the S1 process is 950° C. and the holding time is 1 hour.

3. The heat treatment process for refining C250 maraging steel grains according to claim 1, characterized in that: The temperature of the S2 process is 930° C. and the holding time is 1 hour.

4. The heat treatment process for refining C250 maraging steel grains according to claim 1, characterized in that: The temperature of the S3 process is 900° C. and the holding time is 1 hour.

5. The heat treatment process for refining C250 maraging steel grains according to claim 1, characterized in that: The temperature of the S4 process is 820° C. and the holding time is 1 hour.

6. The heat treatment process for refining C250 maraging steel grains according to any one of claims 1 to 5, characterized in that: The heat treatment process of the C250 maraging steel also includes aging treatment, and the specific steps are: keeping the C250 maraging steel after the S4 process at 475-485°C for 2-3 hours, and air cooling to room temperature; then continue to keep the temperature at 475-485°C for 2-3 hours, and air cooling to room temperature.

7. C250 maraging steel obtained by the heat treatment process for refining the grains of C250 maraging steel according to any one of claims 1 to 6.

8. The C250 maraging steel according to claim 7, characterized in that: The grain size of the C250 maraging steel is grade 8-9.

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