A grading distribution process for reinforcing and plasticizing 60Si2CrVA steel

Abstract

The application discloses a heat treatment process for realizing reinforcement and plasticization of 60Si2CrVA steel. The 60Si2CrVA steel plate is heated to 860-910 DEG C and kept for 30-40 min to complete austenitization, then is quenched in quenching oil at 100-150 DEG C and kept for 60-120 s, then is quickly transferred to a salt bath furnace at 270 DEG C-320 DEG C and kept for 10-20 min, then is transferred to another salt bath furnace at 180-230 DEG C and kept for 3 hours, and finally is water-cooled or air-cooled to room temperature. On the basis of the Q&P (quenching and partitioning) process, the application utilizes the hierarchical partitioning design to increase the carbide precipitation in the martensite matrix while ensuring the residual austenite content in the final microstructure of the material, so that the strength and plasticity are obviously improved compared with the traditional quenching and tempering or the traditional Q&P process.

Description

Technical Field

[0001] This invention belongs to the field of metal heat treatment technology, specifically relating to a graded distribution process for reinforcing and plasticizing 60Si2CrVA steel. Background Technology

[0002] 60Si2CrVA steel, as a mature commercial spring steel, traditionally undergoes the following heat treatment process: austenitization at 860℃ → oil quenching → 410℃. Tempered at 50℃, the main microstructure consists of tempered martensite and a small amount of retained austenite. After conventional heat treatment, the yield strength, tensile strength, and elongation are 1606 MPa, 1857 MPa, and 8.3%, respectively. Although the 60Si2CrVA steel treated with conventional quenching and tempering has high strength, it has poor plasticity.

[0003] Based on the alloy composition analysis of 60Si2CrVA steel, the alloy composition of 60Si2CrVA steel is shown in the table below.

[0004] This alloy contains approximately 1.5 wt.% Si, which suppresses cementite precipitation, and the C, Mn, and V contents meet the design requirements for Q&P steel. The Q&P hot-process concept involves rapidly cooling the austenitized steel to a temperature between Ms and Mf, retaining a portion of austenite without transformation. Then, the temperature is rapidly increased to near the Ms point for distribution, causing carbon to move from martensite to the untransformed austenite, stabilizing this portion of austenite and obtaining a room-temperature microstructure composed of martensite and retained austenite. Under external force, the retained austenite undergoes a martensitic transformation, exhibiting a TRIP (transformation-induced plasticity) effect, thus achieving a high strength and plasticity combination.

[0005] As a mature commercial steel, 60Si2CrVA steel is well-suited for heat treatment using the Q&P (Quality and Process) steel concept to further enhance its mechanical properties. The traditional Q&P process involves austenitization at 860℃ → oil quenching at 100-150℃ → fractionation at 270℃. The yield strength, tensile strength, and elongation obtained using this traditional process are 1840 MPa, 2141 MPa, and 9.8%, respectively. While the strength is significantly improved compared to traditional quenching and tempering, the increase in plasticity is not substantial.

[0006] Chinese invention patent CN101597680B discloses a quenching and heat treatment process for 60Si2CrVA steel springs used in trains. Specifically, the spring is rapidly placed in a temperature of 280℃~350℃ for isothermal treatment for 50 to 800 seconds, then quenched in water or oil for cooling; finally, it is tempered at 250℃~470℃ for 60 to 120 minutes. The 60Si2CrVA steel springs produced by this invention not only have high yield strength and fatigue strength, and good hardenability, but also high tensile strength, yield strength ratio, and high strength and toughness. This method employs a two-step quenching process, but further improvements in strength and plasticity are needed. Summary of the Invention

[0007] To address the aforementioned problems, this invention aims to provide a graded distribution process for reinforcing and plasticizing 60Si2CrVA steel.

[0008] To achieve the above objectives, the present invention adopts the following technical solution: a graded partitioning process for reinforcing and plasticizing 60Si2CrVA steel, wherein after heating and austenitizing 60Si2CrVA steel and quenching it, graded partitioning is performed twice near the Ms point temperature, and the specific graded partitioning is as follows: First, quickly transfer the sample to a salt bath furnace at 270℃~320℃ and keep it warm for 10~20 minutes, then transfer it to another salt bath furnace at 180~230℃ and keep it warm for 3 hours.

[0009] Preferably, the process steps are as follows: Step 1: First, heat the 60Si2CrVA steel plate to 860-910℃ and hold for 30-40 minutes to fully austenitize it; Step 2: Then quench in quenching oil at 100-150℃ for 60-120 seconds; Step 3: Perform hierarchical allocation processing; Step 4: Finally, cool to room temperature.

[0010] Preferably, the thickness of the steel plate in step 1 is 2mm to 10mm, and the quenching holding time of the steel plate in the quenching oil is determined according to the thickness of the steel plate.

[0011] Preferably, for steel plates with a thickness of 2-5 mm, the quenching holding time in quenching oil at 100-150℃ is 60-80 s; for steel plates with a thickness of 5-10 mm, the quenching holding time in quenching oil at 100-150℃ is 80-120 s.

[0012] Preferably, the molten salt bath in the salt bath furnace contains 50 wt% KNO3 and 50 wt% NaNO2.

[0013] Preferably, the cooling to room temperature in step 4 is performed using water cooling or air cooling. Compared with the prior art, the present invention has the following advantages: 1. Through a two-stage fractionation process, the first fractionation after quenching is performed for a short period near the Ms point temperature to ensure sufficient diffusion of carbon from martensite to austenite and uniform carbon distribution within the austenite. Then, a longer fractionation period is performed at a lower temperature to allow carbide precipitation while suppressing the decomposition of excessive austenite. This heat treatment process achieves a good balance between carbide and austenite content in 60Si2CrV steel, resulting in improved strength and ductility compared to materials treated using traditional methods.

[0014] 2. Based on the Q&P (Quenching and Partitioning) process, this invention utilizes a graded partitioning design to ensure the content of residual austenite in the final microstructure of the material while increasing the precipitation of carbides in the martensitic matrix, achieving a significant improvement in strength and plasticity compared with traditional quenching and tempering or traditional Q&P processes. Attached Figure Description

[0015] To more clearly illustrate the technical solutions in the specific embodiments of the present invention, the drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0016] Figure 1 This is the equilibrium phase diagram of 60Si2CrV steel in this invention; Figure 2 This is a TTT transformation curve of supercooled austenite in 60Si2CrV steel used in this invention; Figure 3 This is a comparative schematic diagram of various heat treatment processes in this invention; Figure 4 This is a comparison chart of the mechanical properties of various heat treatment processes used in this invention. Detailed Implementation

[0017] The present invention will be further described below with reference to the accompanying drawings and specific embodiments. However, it should not be construed that the scope of the subject matter of the present invention is limited to the following embodiments. All modifications, substitutions and alterations made based on ordinary technical knowledge and common practices in the art without departing from the above-described technical concept of the present invention are included within the scope of the present invention.

[0018] In Q&P steel, the TRIP effect of vanadium carbide (VC) during deformation can improve plasticity and tensile strength, but the yield strength is determined by the high-strength martensitic matrix. Therefore, an increase in VC content is often accompanied by a significant decrease in yield strength. Introducing precipitation strengthening through tempering can improve the material's strength, but prolonged tempering often leads to the decomposition of metastable austenite, reducing the final retained austenite content. The equilibrium phase diagram was calculated using Thermo-Calc software, as shown in Figure 1. When the temperature drops below 500℃, the precipitation of vanadium carbide (VC) during the equilibrium phase transformation increases with decreasing temperature, while the amount of cementite decreases. Numerous finely dispersed nano-VC particles in the matrix effectively pin grain boundaries and dislocations, and during deformation, the dislocation bypass mechanism changes to a dislocation shearing mechanism, simultaneously improving both strength and plasticity. Figure 2 The TTT curves of the supercooled austenite in the experimental steel, calculated using JMatPro software, are shown. This indicates that during holding at temperatures ranging from 180 to 420°C, metastable austenite decomposes into bainite. When the partitioning temperature is below 350°C, the decomposition rate of metastable austenite decreases due to the reduced temperature.

[0019] Therefore, this invention designs a staged partitioning process based on the carbide precipitation law and the decomposition kinetics of easily precipitated austenite: First, after a single quenching, a short partitioning process is performed near the Ms point temperature to ensure sufficient diffusion of carbon from martensite to austenite and to ensure uniform carbon distribution within the austenite. Then, a longer period of time is performed at a lower temperature to allow carbide precipitation to continue, while simultaneously suppressing the decomposition of excessive austenite. This heat treatment process achieves a good balance between the carbide and austenite content in 60Si2CrV.

[0020] This invention employs a graded distribution process, which achieves a good balance between the content of carbides and austenite in 60Si2CrV steel through two distribution treatments. This results in 60Si2CrV steel with improved strength and plasticity compared to materials heat-treated by traditional processes.

[0021] A graded distribution process for reinforcing and plasticizing 60Si2CrVA steel includes the following steps: Step 1: Heat 60Si2CrVA steel plates with a thickness of 2mm to 10mm to 860 to 910℃ and hold for 30 to 40 minutes to fully austenitize them; Step 2: Then quench the steel plate in quenching oil at 100-150℃ for 60-120 seconds. Specifically, after the steel plate has been fully austenitized, quench it in quenching oil at 100-150℃ for a period of time to ensure that the entire steel plate (surface and core) cools to the set temperature of the quenching oil. The holding time can be determined according to the thickness of the ring: for steel plates with a thickness of less than 5mm, the quenching holding time in quenching oil at 100-150℃ is 60-80 seconds; for steel plates with a thickness of 5-10mm, the quenching holding time in quenching oil at 100-150℃ is 80-120 seconds.

[0022] Step 3: Perform a graded distribution process. After the above quenching and holding at that temperature for the corresponding time, quickly transfer the sample to a molten salt bath at 270℃~320℃ and hold for 10~20 minutes for short-term distribution. The molten salt bath consists of 50wt% KNO3 + 50wt% NaNO2. This ensures sufficient diffusion of carbon from martensite to austenite and its uniform distribution within the austenite.

[0023] After a short isothermal period in a salt bath furnace at 270℃~320℃, the material is transferred to another salt bath furnace at 180~230℃ for 3 hours of holding to allow carbides to continue to precipitate while suppressing excessive decomposition of austenite. Step 4: Finally, cool to room temperature with water or air.

[0024] The above heat treatment technology achieved a good balance between carbide and austenite content in 60Si2CrV steel. It effectively utilized carbide precipitation strengthening and the TRIP effect of retained austenite during deformation, resulting in a simultaneous improvement in both the strength and plasticity of the final material. Specific processes are as follows... Figure 3 As shown, red represents the process of this invention, while black and blue represent the traditional quenching and tempering process and the traditional Q&P process, respectively.

[0025] Example: A 5mm thick 60Si2CrVA steel plate was heated to 880℃ and held for 30 minutes to achieve complete austenitization. Then, it was quenched in 120℃ quenching oil for 120 seconds, then quickly transferred to a 270℃ salt bath furnace for 10 minutes, and then transferred to another 220℃ salt bath furnace for 3 hours. Finally, it was water-cooled or air-cooled to room temperature.

[0026] A novel graded partitioning process was adopted, firstly partitioning the material briefly near the Ms point temperature, then isothermally heating it at a lower temperature for a longer period. This is to allow carbon to fully diffuse from martensite to austenite and ensure its uniform distribution within the austenite. The subsequent isothermal treatment increases the precipitation of carbides (VC) while suppressing the decomposition of excessive austenite. This heat treatment process achieved a good balance between carbide and austenite content in 60Si2CrV steel. It effectively utilized carbide precipitation strengthening and the TRIP effect of retained austenite during deformation, resulting in simultaneous improvements in both strength and ductility of the final material. The mechanical properties are as follows: Figure 4 As shown.

[0027] The above provides a detailed description of a graded distribution process for reinforcing and plasticizing 60Si2CrVA steel, as provided by this invention. Specific examples have been used to illustrate the structure and working principle of this invention. The descriptions of the embodiments are merely for the purpose of helping to understand the method and core ideas of this invention. It should be noted that those skilled in the art can make various improvements and modifications to this invention without departing from its principles, and these improvements and modifications also fall within the scope of protection of the claims of this invention.

Claims

1. A graded distribution process for reinforcing and plasticizing 60Si2CrVA steel, characterized in that: After heating and austenitizing 60Si2CrVA steel and quenching it, it undergoes two graded partitioning treatments at different temperatures near the Ms point temperature. The specific graded partitioning is as follows: After quenching, quickly transfer the sample to a salt bath furnace at 270–320℃ and hold for 10–20 minutes, then transfer it to another salt bath furnace at 180–230℃ and hold for 3 hours.

2. The graded distribution process for reinforcing and plasticizing 60Si2CrVA steel according to claim 1, characterized in that: The process steps are as follows: Step 1: First, heat the 60Si2CrVA steel plate to 860-910℃ and hold for 30-40 minutes to fully austenitize it; Step 2: Then quench in quenching oil at 100-150℃ for 60-120 seconds; Step 3: Perform hierarchical allocation processing; Step 4: Finally, cool to room temperature.

3. The graded distribution process for reinforcing and plasticizing 60Si2CrVA steel according to claim 1, characterized in that: The salt bath furnace contains a molten salt bath, which contains 50 wt% KNO3 and 50 wt% NaNO2.

4. The graded distribution process for reinforcing and plasticizing 60Si2CrVA steel according to claim 2, characterized in that: The cooling to room temperature in step 4 is achieved by water cooling or air cooling.

Images