20CrNiMo steel circulating quenching fine grain preparation process

Abstract

The invention discloses a 20CrNiMo steel circulating quenching fine grain preparation process, and belongs to the technical field of metal material heat treatment. The method comprises the following steps: (1) preheating treatment: heating a heating furnace to 910-960 DEG C, putting the 20CrNiMo steel into the furnace, preserving heat for 50-70 minutes, transferring the temperature to 630-670 DEG C, preserving heat, and cooling in air; and (2) circulating quenching is conducted, specifically, the preheated 20CrNiMo steel is heated to 840-880 DEG C, heat preservation is conducted, the step is repeated for 2-4 times, and oil cooling is conducted. According to the method, the grain level of the 20CrNiMo steel can be greatly refined through the preheating treatment and circulating quenching process, it is guaranteed that the material has good original austenite structure uniformity, the grain size level can be increased to 9.5 or above, and the performance requirement is met. The method is low in process cost and high in operability, and can be widely applied to the fields of machine manufacturing, automobile industry and the like.

Description

Technical Field

[0001] The invention belongs to the technical field of heat treatment of metal materials and relates to a 20CrNiMo steel cyclic quenching fine grain preparation process. Background Art

[0002] 20CrNiMo steel is an alloy structural steel with excellent mechanical properties, including high strength, toughness, and wear resistance. It is widely used in machinery manufacturing, the automotive industry, and other fields. Its excellent comprehensive mechanical properties and wear resistance have made it a key material in machinery manufacturing and other industrial fields. With technological advancements and industrial development, the application of 20CrNiMo steel will become even more extensive.

[0003] With the continuous progress of industry, the requirements for product performance are increasing, especially in terms of high strength, high toughness and structural uniformity. Precision components that require good toughness and fatigue resistance (such as gears) usually need to undergo carburizing heat treatment. However, the original austenite grains of gear steel tend to coarsen during carburizing, which is a challenge faced by the industry. With the improvement of carburizing efficiency and layer depth requirements, high-temperature carburizing has become more popular due to equipment improvements, which places higher requirements on the uniformity of material structure. Controlling the uniformity of the original austenite structure has become the key to limiting the application of high-temperature carburizing. However, the austenite grain size obtained by the existing preparation process, especially the final heat treatment process, is large, which limits the improvement of the comprehensive performance of 20CrNiMo steel. Therefore, research on grain refinement has always been a hot topic. Summary of the Invention

[0004] In order to solve the above technical problems, the present invention provides a 20CrNiMo steel cyclic quenching fine grain preparation process, which controls the austenite grain boundaries by fewer cyclic quenching times, effectively refines the austenite grains, and improves the comprehensive performance of 20CrNiMo steel. It has high operability, low cost and simple process flow.

[0005] In order to achieve the above-mentioned object of the invention, the technical solution adopted by the present invention is: A 20CrNiMo steel cyclic quenching fine grain preparation process comprises the following steps: (1) Preheating treatment: After heating the heating furnace to 910-960℃, put the 20CrNiMo steel into the furnace and keep it warm for 50-70min. Then turn the temperature to 630-670℃ and keep it warm, then air cool. (2) Cyclic quenching: Heat the preheated 20CrNiMo steel to 840-880°C, keep it warm, and then oil cool it. Repeat this step 2-4 times.

[0006] Furthermore, in a preheating treatment step, the temperature is kept at 630-670° C. for 50-70 minutes.

[0007] Further, a preheating treatment step is performed, and the mixture is air-cooled to room temperature.

[0008] Furthermore, in the cyclic quenching step, the holding time t and the diameter d of the 20CrNiMo steel satisfy the relationship t=(2~3)d, where the unit of t is min and the unit of d is mm.

[0009] Furthermore, in the cyclic quenching step, the cooling rate of the oil cooling is 30 to 50° C. / s.

[0010] Furthermore, the quenching step is repeated and the oil is cooled to room temperature.

[0011] The technical principle of the present invention's solution is as follows: The present invention first performs a preheat treatment, which homogenizes the steel's microstructure, eliminates internal stresses, and refines the grains, preparing the steel for subsequent quenching. During this process, the steel is first heated to the austenite region and then rapidly cooled to the pearlite transformation region for isothermal holding, transforming the austenite into a pearlite-like microstructure. During this process, the austenite grains undergo nucleation and growth. However, due to the rapid cooling rate and high degree of undercooling during isothermal normalizing, the growth of the austenite grains is somewhat inhibited, resulting in grain refinement.

[0012] After preheating, cyclic quenching is performed. During the quenching process at 840-880℃ for a certain period of time, the steel is heated to the austenite zone, and the austenite grains undergo nucleation and growth. Since the steel has undergone a temperature-transfer heat treatment first, its structure is uniform at this time. The formation and growth of austenite nuclei are affected by the refined pearlite and other structures. The driving force for grain growth is relatively small, thereby suppressing the excessive growth of austenite grains. In addition, during cyclic quenching, internal stresses are generated inside the steel. These internal stresses can cause the grains to distort and break, thereby refining the grains. In addition, during cyclic quenching, the steel stays at high temperature for a relatively short time, and the austenite grains cool rapidly before they have time to fully grow, which also helps to obtain fine austenite grains.

[0013] The beneficial effects produced by adopting the above scheme are as follows: the present invention can obtain uniform austenite grains through preheating treatment plus cyclic quenching, eliminate the problem of uneven austenite grains caused by reasons such as poor heating process, ensure that the steel has a uniform austenite grain structure and increase the austenite grain size level of the steel to above level 9.5, thereby improving the comprehensive performance of 20CrNiMo steel. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 This is the original austenite morphology of 20CrNiMo steel after cyclic quenching in Example 1; Figure 2 This is the original austenite morphology of 20CrNiMo steel after cyclic quenching in Example 2; Figure 3 This is the original austenite morphology of 20CrNiMo steel after cyclic quenching in Example 3; Figure 4 This is the original austenite morphology of 20CrNiMo steel after cyclic quenching in Example 4. DETAILED DESCRIPTION

[0015] Example 1: The diameter of 20CrNiMo steel in this example is 30 mm, and its cyclic quenching fine grain preparation process includes the following steps: (1) Preheating treatment: After heating the heating furnace to 910℃, put the 20CrNiMo steel into the furnace and keep it warm for 52 minutes. Then transfer it to the furnace at 630℃ and keep it warm for 53 minutes, then air cool it to room temperature.

[0016] (2) Cyclic quenching: After preheating, heat the 20CrNiMo steel to 840°C and keep it at that temperature for 90 min. Then, cool it to room temperature with oil at a cooling rate of 30°C / s. Repeat this step twice.

[0017] After preheating and cyclic quenching, the austenite grain size of the 20CrNiMo steel in this embodiment is 9.5, and the grains in the entire cross section are uniform and fine.

[0018] Example 2: The diameter of 20CrNiMo steel in this example is 35 mm, and its cyclic quenching fine grain preparation process includes the following steps: (1) Preheating treatment: After heating the heating furnace to 920℃, put the 20CrNiMo steel into the furnace and keep it warm for 55 minutes. Then transfer it to the furnace at 635℃ and keep it warm for 56 minutes, then air cool it to room temperature.

[0019] (2) Cyclic quenching: After preheat treatment, the 20CrNiMo steel was heated to 850°C and kept at this temperature for 98 min, then cooled to room temperature at a rate of 35°C / s. This step was repeated three times, and the steel was oil-cooled to room temperature.

[0020] After preheating and cyclic quenching, the austenite grain size of the 20CrNiMo steel in this embodiment is 10, and the grains in the entire cross section are uniform and fine.

[0021] Example 3: The diameter of 20CrNiMo steel in this example is 40 mm, and its cyclic quenching fine grain preparation process includes the following steps: (1) Preheating treatment: After heating the heating furnace to 930℃, put the 20CrNiMo steel into the furnace and keep it warm for 60 minutes. Then transfer it to a furnace with a temperature of 650℃ and keep it warm for 60 minutes, then air cool it to room temperature.

[0022] (2) Cyclic quenching: After preheating, the 20CrNiMo steel was heated to 860°C and kept at this temperature for 104 min. Then, the steel was cooled to room temperature at a cooling rate of 40°C / s. This step was repeated four times.

[0023] After preheating and cyclic quenching, the austenite grain size of the 20CrNiMo steel in this embodiment is 10.5, and the grains in the entire cross section are uniform and fine.

[0024] Example 4: The diameter of 20CrNiMo steel in this example is 45 mm, and its cyclic quenching fine grain preparation process includes the following steps: (1) Preheating treatment: After heating the heating furnace to 940℃, put the 20CrNiMo steel into the furnace and keep it warm for 62 minutes. Then transfer it to the furnace at 655℃ and keep it warm for 64 minutes, then air cool it to room temperature.

[0025] (2) Cyclic quenching: After preheating, the 20CrNiMo steel was heated to 865°C and kept at this temperature for 108 min. Then, the steel was cooled to room temperature at a cooling rate of 45°C / s through oil. This step was repeated three times.

[0026] After preheating and cyclic quenching, the austenite grain size of the 20CrNiMo steel in this embodiment is 10, and the grains in the entire cross section are uniform and fine.

[0027] Example 5: The diameter of 20CrNiMo steel in this example is 50 mm, and its cyclic quenching fine grain preparation process includes the following steps: (1) Preheating treatment: After heating the heating furnace to 950℃, put the 20CrNiMo steel into the furnace and keep it warm for 66 minutes. Then transfer it to the furnace at a temperature of 660℃ and keep it warm for 67 minutes, and then air cool it to room temperature.

[0028] (2) Cyclic quenching: After preheating, heat the 20CrNiMo steel to 870°C and keep it at that temperature for 110 min. Then, cool it to room temperature with oil at a cooling rate of 48°C / s. Repeat this step 4 times.

[0029] After preheating and cyclic quenching, the austenite grain size of the 20CrNiMo steel in this embodiment is 10.5, and the grains in the entire cross section are uniform and fine.

[0030] Example 6: The diameter of 20CrNiMo steel in this example is 55 mm, and its cyclic quenching fine grain preparation process includes the following steps: (1) Preheating treatment: After heating the heating furnace to 960℃, put the 20CrNiMo steel into the furnace and keep it warm for 70 minutes. Then transfer it to the furnace at 670℃ and keep it warm for 70 minutes, then air cool it to room temperature.

[0031] (2) Cyclic quenching: After preheating, heat the 20CrNiMo steel to 880°C and keep it at that temperature for 110 min. Then, cool it to room temperature with oil at a cooling rate of 50°C / s. Repeat this step twice.

[0032] After preheating and cyclic quenching, the austenite grain size of the 20CrNiMo steel in this embodiment is 10, and the grains in the entire cross section are uniform and fine.

Claims

1. A 20CrNiMo steel cyclic quenching fine grain preparation process, characterized in that: The following steps are involved: (1) Preheating treatment: After heating the heating furnace to 910-960℃, put the 20CrNiMo steel into the furnace and keep it warm for 50-70min. Then turn the temperature to 630-670℃ and keep it warm, then air cool. (2) Cyclic quenching: Heat the preheated 20CrNiMo steel to 840-880°C, keep it warm, and then oil cool it. Repeat this step 2-4 times.

2. The 20CrNiMo steel cyclic quenching fine grain preparation process according to claim 1, characterized in that: The preheating step is to keep the temperature at 630-670°C for 50-70 minutes.

3. The 20CrNiMo steel cyclic quenching fine grain preparation process according to claim 2, characterized in that: Preheating step, air cooling to room temperature.

4. The process for preparing fine grains of 20CrNiMo steel by cyclic quenching according to claim 3, characterized in that: In the cyclic quenching step, the holding time t and the diameter d of 20CrNiMo steel satisfy the relationship t=(2~3)d, where the unit of t is min and the unit of d is mm.

5. The process for preparing fine grains of 20CrNiMo steel by cyclic quenching according to claim 4, characterized in that: In the cyclic quenching step, the cooling rate of oil cooling is 30 to 50°C / s.

6. The process for preparing fine grains of 20CrNiMo steel by cyclic quenching according to any one of claims 1 to 5, characterized in that: The quenching step was repeated and the oil was cooled to room temperature.

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