Method for eliminating mixed crystal structure of medium-carbon alloy steel

Abstract

The invention discloses a method for eliminating a mixed crystal structure of medium-carbon alloy steel, and belongs to the technical field of heat treatment of steel. The mixed crystal structure eliminating method comprises the following steps: S1, heating to-be-treated medium-carbon alloy steel to Ac1-Ac3, and then preserving heat; s2, the steel obtained after heat preservation is completed is rapidly cooled to the room temperature; s3, repeating the operation for multiple times according to the steps S1 to S2; and S4, the steel treated in the step S3 is heated to be subjected to high-temperature tempering treatment and then air-cooled to the room temperature, and the medium-carbon alloy steel with the uniform grain structure is obtained. According to the method, the defects in production can be overcome, the problem of crystal mixing easily caused by large size, deformation and uneven cooling in the large die steel production process is particularly solved, and a mixed crystal structure with the original grain size level of 4-8 can be adjusted into a uniform structure with the grain size level of about 8.

Description

Technical Field

[0001] The invention belongs to the technical field of heat treatment of steel, and in particular relates to a method for eliminating mixed crystal structure of medium carbon alloy steel. Background Art

[0002] Medium carbon alloy steel, especially chromium-molybdenum-vanadium steel, is widely used in the field of mold steel because of its excellent strength, toughness and wear resistance. In recent years, with the increasing large-scale mold forming, in order to improve production efficiency and product yield, one mold with multiple cavities, the demand for large-scale mold steel has gradually increased. However, as the size of mold steel increases, it is inevitable that the requirements for the organization and performance of steel will also be higher. The larger the size, the worse the uniformity of the organization. At present, the industry generally controls the grain size of large mold steel at level 7 to 8, but mixed crystals often occur in the actual production process. The generation of mixed crystals directly leads to a decrease in the strength and toughness of steel. In the high temperature and high pressure environment during the service process of the mold, mixed crystals will seriously affect the safety and reliability of the mold, reduce the service life of the mold, and cause early failure of the mold.

[0003] CN113088825B discloses a method for eliminating medium-carbon microalloyed steel and its mixed crystal structure. Through the heat treatment method of high-temperature solid solution + low-temperature aging precipitation and then normalizing, the mixed crystal structure of the forging is eliminated, and refined grains are obtained, so that the treated medium-carbon microalloyed steel has high strength and good toughness, meeting the various requirements of medium-carbon microalloyed steel for rail transit axles. This method is mainly aimed at steel for microalloyed axles, and the alloy component content in the steel is below 2.5%; although the high-temperature solid solution method can refine the grains, it has high energy consumption, and high-temperature treatment is prone to cause thermal stress inside large workpieces, causing deformation and even cracking risks.

[0004] CN107739788B discloses a heat treatment method for eliminating mixed crystal defects of 30CrNi3MoV alloy steel. The forging process is carried out by using the existing forging technology, and then the forged forging is heat treated. By controlling and improving the normalizing and quenching temperature and the holding time of the 30CrNi3MoV alloy steel, and removing the tissue inheritance through process treatment, a good effect of removing mixed crystal defects is obtained. The method adopts multi-stage temperature control (normalizing + quenching + tempering), and its process is complicated and is not suitable for the cooling hysteresis characteristics of large-sized workpieces. Summary of the invention

[0005] The technical problem to be solved by the present invention is to solve the mixed crystal problem that occurs in the existing large-scale hot working die steel production process, and propose a method for eliminating the mixed crystal structure of medium carbon alloy steel, which is mainly used for eliminating the mixed crystal structure of chromium-molybdenum-vanadium hot working die steel.

[0006] In order to achieve the above application purpose, the technical solution adopted in this application is as follows.

[0007] A method for eliminating mixed crystal structure of medium-carbon alloy steel, comprising the following steps:

[0008] S1 Heat the medium-carbon alloy steel to be processed to Ac1 - Ac3, and then hold for heat preservation;

[0009] S2 Rapidly cool the steel after heat preservation to room temperature;

[0010] S3 Repeat the operations in steps S1 - S2 for multiple times;

[0011] S4 Heat the steel processed in step S3 for high-temperature tempering treatment, and then air-cool to room temperature to obtain medium-carbon alloy steel with uniform grain structure.

[0012] In the above step S1, the grain size grade of the medium-carbon alloy steel is 4 - 8 levels, and the difference of the mixed crystal structure is higher than 3 levels.

[0013] In the above step S1, the chemical composition of the medium-carbon alloy steel is, by mass percentage, C 0.35% - 0.45%, Si ≤ 0.60%, Mn ≤ 0.80%, Cr 5.0% - 5.5%, Mo 2.5% - 3.5%, V 0.5% - 0.8%, and the balance is Fe and inevitable impurities.

[0014] Furthermore, in the chemical composition of the above medium-carbon alloy steel, the inevitable impurities include P ≤ 0.025% and S ≤ 0.020%.

[0015] In the above step S1, the medium-carbon alloy steel is steel for preparing large die-casting molds, with the steel size of length 1000 - 2500 mm, width 1000 - 2000 mm, height 500 - 1500 mm, and the steel weight of 6 - 17 t.

[0016] In the above step S1, the heating rate is ≤ 200 °C / h.

[0017] Furthermore, the above heating rate is 50 - 150 °C / h.

[0018] In the above step S1, the Ac1 - Ac3 is specifically 810 - 840 °C.

[0019] In the above step S1, the heat preservation time is 10 - 20 min.

[0020] In the above step S2, the method of rapid cooling is specifically water cooling or oil cooling, and the cooling rate of rapid cooling is ≥ 100 °C / s.

[0021] Furthermore, the above cooling rate is 100 - 300 °C / s.

[0022] In the above step S3, the operations in steps S1 to S2 are repeated 3 to 5 times.

[0023] In the above step S4, the heating rate of temperature rise is ≤ 200 °C / h.

[0024] Furthermore, the heating rate of temperature rise is 50 to 150 °C / h.

[0025] In the above step S4, the temperature of the high-temperature tempering treatment is 560 to 600 °C.

[0026] In the above step S4, the time of the high-temperature tempering treatment is 2 to 3 h.

[0027] Advantages of the present invention: The method of the present invention can make up for the deficiencies in production, especially solve the problem of mixed crystal caused by large size, uneven deformation and cooling during the production of large die steel. It can adjust the mixed crystal structure with the original grain size level of 4 to 8 grades (average grain size of 40 to 55 μm) into a uniform structure with a grain size level of about 8 grades (average grain size of 20 to 30 μm). The operation method of the present invention is simple and highly feasible. Finally, it can increase the strength and toughness of the steel, enhance the safety and reliability under the service conditions of the die, improve the service life of the die, and avoid the early failure of the die. Description of the Drawings

[0028] Figure 1 Metallographic structures of the medium-carbon alloy steel product before (left) and after (right) treatment in Example 1;

[0029] Figure 2 Metallographic structures of the medium-carbon alloy steel product before (left) and after (right) treatment in Example 5;

[0030] Figure 3 Metallographic structures of the medium-carbon alloy steel products after treatment in Comparative Examples 1 to 3;

[0031] Among them: (a) is Comparative Example 1, (b) is Comparative Example 2, and (c) is Comparative Example 3. Detailed Embodiments

[0032] In order to make the technical problems, technical solutions and beneficial effects to be solved by the present application clearer, the present application will be further described in detail below in conjunction with the embodiments. Unless otherwise defined, all technical terms used herein have the same meaning as commonly understood by those of ordinary skill in the art.

[0033] A method for eliminating the mixed crystal structure of medium-carbon alloy steel includes two-phase zone heating, rapid cooling and high-temperature tempering.

[0034] The chemical composition of the medium-carbon alloy steel is as follows by mass percentage: C 0.35% - 0.45%, Si ≤ 0.60%, Mn ≤ 0.80%, Cr 5.0% - 5.5%, Mo 2.5% - 3.5%, V 0.5% - 0.8%, and the balance is Fe and unavoidable impurities.

[0035] In an embodiment of the present invention, the chemical composition of the medium-carbon alloy steel is as follows by mass percentage: C 0.35% - 0.45%, Si ≤ 0.60%, Mn ≤ 0.80%, Cr 5.0% - 5.5%, Mo 2.5% - 3.5%, V 0.5% - 0.8%, P ≤ 0.025%, S ≤ 0.020%, and the balance is Fe.

[0036] In the method for eliminating the mixed crystal structure of the above medium-carbon alloy steel, the two-phase region heating and rapid cooling are as follows: Put the steel into a heating furnace and heat it to a temperature between 810 - 840°C at a heating rate of ≤ 200°C / h, and keep it warm for 10 - 20 minutes; then put the steel after heating and insulation into oil or water and rapidly cool it to room temperature; repeat the two-phase region heating and rapid cooling steps 3 - 5 times.

[0037] The purpose of doing this is to obtain a partially austenitized structure after heating the steel between the Ac1 - Ac3 temperatures, preparing for the next tissue transformation. Reheating after cooling can rapidly transform the structure heated and partially austenitized in the previous step into martensite, and the coarse grains are refined through repeated austenitization and martensite phase transformation. Another function of strictly controlling the number of quenching times is to consider the usability of the mold. An appropriate number of quenching times can improve the hardness, toughness, tissue uniformity, and heat-resistant fatigue performance of the steel. When the number of quenching times is less than 3 or more than 5, it may lead to insufficient hardness and toughness of the steel, increased brittleness, accumulation of residual stress, and size instability.

[0038] In the method for eliminating the mixed crystal structure of the above medium-carbon alloy steel, the high-temperature tempering is as follows: Put the steel after repeated heating and cooling into a tempering furnace and heat it to 560 - 600°C at a rate of ≤ 200°C / h, keep it warm for 2 - 3 hours, and then take it out of the furnace and air-cool it to room temperature. The purpose of tempering is to transform the martensite structure obtained in the previous step into stable tempered sorbite.

[0039] Specific embodiments will be listed below to explain the solution of the present invention. Those skilled in the art will understand that the following embodiments are only used to illustrate the present invention and should not be regarded as limiting the scope of the present invention. For those not specified in the embodiments regarding specific technologies or conditions, they shall be carried out according to the technologies or conditions described in the literature in this field or according to the product specifications. For the reagents or instruments not specified for the manufacturers, they are all conventional products that can be obtained through commercial purchase.

[0040] Embodiment

[0041] The present invention provides five sets of examples and comparative examples for solving the mixed crystal structure in existing large-sized medium-carbon alloy steel by using the mixed crystal structure elimination method of the present invention. Among them, the chemical compositions of the medium-carbon alloy steel designed in the examples and comparative examples are shown in Table 1, and the balance is Fe.

[0042] Table 1 Chemical Compositions of Each Example and Comparative Example (in mass percentage, %)

[0043]

[0044] The elimination of the mixed crystal structure of medium-carbon alloy steel includes the following steps:

[0045] (1) Prepare the medium-carbon alloy steel to be processed. The chemical composition of the medium-carbon alloy steel is shown in Table 1, and the size, mechanical properties and mixed crystal condition of the medium-carbon alloy steel are shown in Table 2.

[0046] Table 2 Mixed Crystal Conditions of Each Example and Comparative Example

[0047]

[0048] (2) Perform two-phase zone heating, rapid cooling and high-temperature tempering on the medium-carbon alloy steel to be processed. The specific implementation steps are as follows: Put the steel into a heating furnace and heat it to between 810 and 840 °C at a heating rate of 100 °C / h, and hold for 10 to 20 minutes; Put the steel after heating and holding into oil or water and rapidly cool it to room temperature at a cooling rate of 100 to 300 °C / s; Repeat the heating and cooling steps 3 to 5 times; Put the steel after repeated heating and cooling into a tempering furnace and heat it to 560 to 600 °C at a rate of ≤200 °C / h, hold for 2 to 3 hours, and then take it out of the furnace and air-cool it to room temperature. The specific process parameters are shown in Table 3.

[0049] Table 3 Process Parameters of Each Example and Comparative Example

[0050]

[0051] (3) Detect the processed medium-carbon alloy steel product, and its grain structure and mechanical properties are shown in Table 4.

[0052] Table 4 Properties of Medium-Carbon Alloy Steel Products

[0053]

[0054] Among them, the metallographic structures of the medium-carbon alloy steel products before and after treatment in Example 1 are as Figure 1 shown, and the metallographic structures of the medium-carbon alloy steel products before and after treatment in Example 5 are as Figure 2As shown, the metallographic structures of the carbon alloy steel products after treatment in Comparative Examples 1 to 3 are as Figure 3 shown, where (a) is Comparative Example 1, (b) is Comparative Example 2, and (c) is Comparative Example 3. From Figures 1 to 3 this, it can be seen that by using the method of the present invention, the mixed crystal structure with an original grain size level of 4 to 8 can be adjusted to a uniform structure with a grain size level of about 8. Compared with the comparative examples, the grain uniformity of the carbon alloy steel products treated by the method of the present invention is significantly improved.

Claims

1. A method for eliminating mixed crystal structure of medium carbon alloy steel, characterized in that: The steps include: S1 heats the medium carbon alloy steel to be treated to Ac1-Ac3, and then keeps the temperature; S2 quickly cools the steel to room temperature after the insulation is completed; S3 repeats steps S1 to S2 for multiple times; S4: The steel material treated in step S3 is heated for high temperature tempering treatment, and then air-cooled to room temperature to obtain a medium carbon alloy steel with uniform grain structure.

2. The method for eliminating mixed crystal structure of medium carbon alloy steel according to claim 1, characterized in that: In step S1, the grain size level of the medium carbon alloy steel is 4 to 8, and the mixed crystal structure range is higher than 3.

3. The method for eliminating mixed crystal structure of medium carbon alloy steel according to claim 2, characterized in that: In step S1, the chemical composition of the medium carbon alloy steel is C 0.35%-0.45%, Si≤0.60%, Mn≤0.80%, Cr5.0%-5.5%, Mo 2.5%-3.5%, V 0.5%-0.8%, and the balance is Fe and unavoidable impurities. Wherein, the unavoidable impurities include P≤0.025% and S≤0.020%.

4. The method for eliminating mixed crystal structure of medium carbon alloy steel according to claim 3, characterized in that: In step S1, the medium carbon alloy steel is used for preparing large die-casting molds, the size of the steel is 1000-2500 mm in length, 1000-2000 mm in width, 500-1500 mm in height, and the weight of the steel is 6-17 t.

5. The method for eliminating mixed crystal structure of medium carbon alloy steel according to claim 1, characterized in that: In step S1, the heating rate is ≤200°C / h, Ac1-Ac3 are specifically 810-840°C, and the insulation time is 10-20min.

6. The method for eliminating mixed crystal structure of medium carbon alloy steel according to claim 1, characterized in that: In step S2, the rapid cooling method is specifically water cooling or oil cooling, and the cooling rate of the rapid cooling is ≥100°C / s.

7. The method for eliminating mixed crystal structure of medium carbon alloy steel according to claim 6, characterized in that: The cooling rate of rapid cooling is 100-300°C / s.

8. The method for eliminating mixed crystal structure of medium carbon alloy steel according to claim 1, characterized in that: In step S3, the operations of steps S1 to S2 are repeated 3 to 5 times.

9. The method for eliminating mixed crystal structure of medium carbon alloy steel according to claim 1, characterized in that: In step S4, the heating rate is ≤200°C / h, the temperature of high temperature tempering treatment is 560-600°C, and the time of high temperature tempering treatment is 2-3h.

10. The method for eliminating mixed crystal structure of medium carbon alloy steel according to claim 5 or 9, characterized in that: The heating rate is 50-150°C / h.

Images