A heat treatment method for improving the yield ratio of a continuous extrusion die steel

Abstract

This invention discloses a heat treatment method for improving the yield strength ratio of steel used in continuous extrusion dies. Specifically, the steel is subjected to the following sequential heat treatments: high-temperature normalizing, low-temperature normalizing, spheroidizing annealing, high-temperature quenching, high-temperature tempering, and low-temperature tempering. The high-temperature normalizing temperature is 1070℃~1100℃; the low-temperature normalizing temperature is 710℃~730℃; the spheroidizing annealing temperature is 840℃~870℃; the high-temperature quenching temperature is 1070℃~1100℃; the high-temperature tempering temperature is 620℃~640℃; and the low-temperature tempering temperature is 530℃~550℃. The yield strength ratio of the die steel after heat treatment using this method is ≥0.91, and the hardness is 48~52 HRC.

Description

Technical Field

[0001] This invention belongs to the field of heat treatment of metallic materials, and specifically relates to a heat treatment method for improving the yield strength ratio of steel used in continuous extrusion dies. Background Technology

[0002] Continuous extrusion technology, with its significant advantages such as low energy consumption, high production efficiency, excellent dimensional accuracy, and continuous production capability, has been widely applied in many key fields such as construction, aerospace, transportation, and communications. During continuous extrusion, the die must withstand multiple harsh conditions including high temperature, high pressure, and strong friction over extended periods. This necessitates that the die material possess high strength and toughness, high wear resistance, excellent corrosion resistance, and resistance to thermal fatigue to ensure the die's structural stability and superior performance during long-term service.

[0003] Today, with the increasing strength of processed copper and aluminum alloy materials and the growing popularity of high-speed extrusion presses, the industry has placed more stringent demands on the heat resistance and quality stability of continuous extrusion die materials. Continuous extrusion die steels need to withstand high temperatures, high pressures, intense friction, and cyclic loads for extended periods. Improving the yield strength ratio of die materials can precisely match their core requirements and solve key issues during service.

[0004] Extrusion dies must withstand extrusion stress of 200-500 MPa during operation. A high yield strength ratio can prevent plastic collapse and dimensional distortion of the die cavity, thus extending the die's service life. During continuous extrusion, the die temperature can reach 300-600℃. A high yield strength ratio design can reduce creep deformation at high temperatures, ensuring greater dimensional accuracy of the extruded parts.

[0005] Therefore, developing a heat treatment method to improve the yield strength ratio to meet the high temperature, high pressure, and high wear requirements of continuous extrusion dies for copper alloys is of great practical significance. Summary of the Invention

[0006] The purpose of this invention is to provide a heat treatment method for improving the yield strength ratio of steel used in continuous extrusion dies.

[0007] To achieve the above-mentioned objectives, the technical solution adopted by this invention is as follows: A heat treatment method for improving the yield strength ratio of steel for continuous extrusion dies includes: sequentially performing high-temperature normalizing, low-temperature normalizing, spheroidizing annealing, high-temperature quenching, high-temperature tempering, and low-temperature tempering on the steel for the die.

[0008] Furthermore, the high-temperature normalizing described in this invention involves heating the forged steel billet for the mold to 1070℃~1100℃, holding it at that temperature for 15~25min + 0.6min / mm, and then water-cooling it to room temperature. This step thoroughly refines the microstructure of the forged billet, eliminates forging stress and microstructure inhomogeneity, lays a uniform microstructure foundation for subsequent heat treatment, and initially improves the density of the steel, avoiding performance fluctuations caused by microstructure differences in subsequent processes.

[0009] Furthermore, the low-temperature normalizing described in this invention involves heating the die-cast steel billet to 710℃~730℃, holding it at that temperature for 15~20min + 0.6min / mm, and then water-cooling it to room temperature. This temperature range is close to the Ac1 line of the steel, which can promote the homogenization of the pearlite structure, refine the pearlite lamellar spacing, and further release the internal stress remaining after high-temperature normalizing, reduce the brittleness of the steel, improve the stability of the structure, and create better microstructural conditions for subsequent spheroidizing annealing.

[0010] Furthermore, the spheroidizing annealing described in this invention involves heating the die-cast steel billet to 840℃~870℃, holding it at that temperature for 3~4 hours, then furnace cooling it to 660℃~700℃, holding it at that temperature for 3~5 hours, then furnace cooling it to <500℃, and finally air cooling it after removal from the furnace. The core of this step is to transform the lamellar cementite into uniform and fine spheroidal cementite, reducing the hardness of the steel, increasing its plasticity, and facilitating subsequent processing and forming. At the same time, the spheroidal cementite can reduce the tendency of austenite grain growth during subsequent quenching, ensuring the refinement and uniformity of the microstructure after quenching, thus laying the microstructure foundation for a high yield strength ratio.

[0011] Furthermore, the high-temperature quenching described in this invention involves heating the die steel forging billet to 1070℃~1100℃, holding it at that temperature for 15~25min + 0.6min / mm, and then oil quenching it to room temperature. High-temperature heating allows carbon and alloying elements to fully dissolve into the austenite, followed by rapid oil quenching for rapid cooling, resulting in a martensitic structure. The formation of martensite significantly improves the strength and hardness of the steel. Simultaneously, oil quenching provides a gentler cooling rate compared to water cooling, preventing defects such as cracks and deformation caused by excessively rapid cooling, thus balancing strength and processing safety.

[0012] Furthermore, the high-temperature tempering described in this invention involves heating the die steel forging billet to 620℃~640℃, holding it at that temperature for 55~65min + 1.5min / mm, and then air-cooling it to room temperature. This step is a crucial part of the tempering process, allowing the quenched martensite structure to fully transform into tempered sorbite, eliminating quenching internal stress, reducing the brittleness of the steel, and improving toughness while ensuring high strength. This results in the steel possessing excellent comprehensive mechanical properties, avoiding brittle fracture problems caused by high strength alone, and providing a guarantee for optimizing the yield strength ratio.

[0013] Furthermore, the low-temperature tempering described in this invention involves heating the forged steel billet for the mold to 530℃~550℃, holding it at that temperature for 60~65min + 1min / mm, and then air-cooling it to room temperature. This step further stabilizes the tempered sorbite structure, transforms a small amount of retained austenite into martensite, and precipitates fine carbide particles, dispersing and strengthening the matrix. Without significantly reducing strength, this further improves the yield strength of the steel, ultimately achieving an increased yield-to-tensile ratio, while ensuring the steel hardness remains within the suitable range of 48~52HRC, meeting the requirements for mold use.

[0014] Furthermore, the steel used for the continuous extrusion die described in this invention is a medium-carbon chromium-molybdenum steel.

[0015] Furthermore, the chemical composition of the continuous extrusion die steel of the present invention contains 0.47-0.51 wt% C, 3.6-4.1 wt% Cr, 1.8-2.2 wt% Mo, and 0.4-0.6 wt% V.

[0016] The mold steel heat-treated by the method described in this invention has a yield strength ratio ≥0.91 and a hardness of 48-52 HRC.

[0017] The inventive principle of this invention lies in: The heat treatment process described in this invention can refine grains, which increases the number of grain boundaries. Grain boundaries hinder dislocation slip, allowing the material to reach yield strength at lower strain while maintaining high tensile strength. On the other hand, it promotes the precipitates to pin dislocations, restricting their free movement and requiring greater stress at yield, thereby increasing the yield strength ratio.

[0018] The beneficial effects of adopting the above technical solution are as follows: The method of this invention can significantly improve the yield strength ratio of mold steel, with a yield strength ratio ≥ 0.91.

[0019] The heat treatment process described in this invention focuses on improving yield strength, while the increase in tensile strength is relatively moderate, ultimately achieving a higher yield-to-tensile strength ratio. This higher ratio optimizes material load-bearing efficiency and structural safety, while also considering lightweighting and resource utilization. Detailed Implementation

[0020] The present invention will be further described in detail below with reference to specific embodiments. Example 1

[0021] The chemical composition of the steel used for the continuous extrusion die in this embodiment is 0.47% C, 1.8% Mo, 3.6% Cr, and 0.4% V, with the remainder being iron and unavoidable impurities; its heat treatment process includes the following steps: (1) High temperature normalizing treatment: The steel forging billet for the mold is loaded into the heating furnace and heated to 1070℃. After holding for 15min + 0.6min / mm, it is cooled to room temperature by water.

[0022] (2) Low temperature normalizing treatment: The steel forging billet for the mold is loaded into the heating furnace, heated to 710℃, held for 15 min +0.6 min / mm, and then water cooled to room temperature.

[0023] (3) Spheroidizing annealing treatment: Heat to 840°C in the furnace, hold for 3 hours, cool to 660°C in the furnace, hold for 3 hours, cool to <500°C in the furnace, and then air cool out of the furnace.

[0024] (4) High temperature quenching treatment: The steel forging billet for the mold is placed into the heating furnace and heated to 1070℃. After holding for 15min + 0.6min / mm, it is oil quenched to room temperature.

[0025] (5) High temperature tempering treatment: heat to 620℃, hold for 55min + 1.5min / mm, and then air cool to room temperature.

[0026] (6) Low temperature tempering treatment: heat to 530℃, hold for 60min+1min / mm, and then air cool to room temperature to obtain the product. Example 2

[0027] The chemical composition of the steel used for the continuous extrusion die in this embodiment is C 0.47%, Mo 2.2%, Cr 3.8%, V 0.6%, with the remainder being iron and unavoidable impurities; its heat treatment process includes the following steps: (1) High temperature normalizing treatment: The steel forging billet for the mold is loaded into the heating furnace and heated to 1070℃. After holding for 25min + 0.6min / mm, it is cooled to room temperature by water.

[0028] (2) Low temperature normalizing treatment: The steel forging billet for the mold is loaded into the heating furnace and heated to 710°C. After holding at the temperature for 20 min + 0.6 min / mm, it is cooled to room temperature by water.

[0029] (3) Spheroidizing annealing treatment: Heat to 840°C in the furnace, hold for 4 hours, cool to 660°C in the furnace, hold for 5 hours, cool to <500°C in the furnace, and then air cool out of the furnace.

[0030] (4) High temperature quenching treatment: The steel forging billet for the mold is loaded into the heating furnace and heated to 1070℃. After holding for 25min + 0.6min / mm, it is oil quenched to room temperature.

[0031] (5) High temperature tempering treatment: heat to 620℃, hold for 65min + 1.5min / mm, and then air cool to room temperature.

[0032] (6) Low temperature tempering treatment: heat to 530℃, hold for 65min+1min / mm, and then air cool to room temperature to obtain the product. Example 3

[0033] The chemical composition of the steel used for the continuous extrusion die in this embodiment is 0.51% C, 2.2% Mo, 4.1% Cr, and 0.6% V, with the remainder being iron and unavoidable impurities; its heat treatment process includes the following steps: (1) High temperature normalizing treatment: The steel forging billet for the mold is loaded into the heating furnace and heated to 1100℃. After holding for 15min + 0.6min / mm, it is cooled to room temperature by water.

[0034] (2) Low temperature normalizing treatment: The steel forging billet for the mold is loaded into the heating furnace and heated to 730°C. After holding at this temperature for 15 min + 0.6 min / mm, it is cooled to room temperature by water.

[0035] (3) Spheroidizing annealing treatment: Heat to 870°C in the furnace, hold for 3 hours, cool to 660°C in the furnace, hold for 3 hours, cool to <500°C in the furnace, and then air cool out of the furnace.

[0036] (4) High temperature quenching treatment: The steel forging billet for the mold is placed into the heating furnace and heated to 1100℃. After holding for 15min + 0.6min / mm, it is oil quenched to room temperature.

[0037] (5) High temperature tempering treatment: heat to 620℃, hold for 55min + 1.5min / mm, and then air cool to room temperature.

[0038] (6) Low temperature tempering treatment: heat to 530℃, hold for 60min+1min / mm, and then air cool to room temperature to obtain the product. Example 4

[0039] The chemical composition of the steel used for the continuous extrusion die in this embodiment is 0.47% C, 2.2% Mo, 3.9% Cr, and 0.5% V, with the remainder being iron and unavoidable impurities; its heat treatment process includes the following steps: (1) High temperature normalizing treatment: The steel forging billet for the mold is loaded into the heating furnace and heated to 1100℃. After holding for 25min + 0.6min / mm, it is cooled to room temperature by water.

[0040] (2) Low temperature normalizing treatment: The steel forging billet for the mold is loaded into the heating furnace and heated to 730°C. After holding at this temperature for 20 min + 0.6 min / mm, it is cooled to room temperature by water.

[0041] (3) Spheroidizing annealing treatment: Heat to 870°C in the furnace, hold for 4 hours, cool to 700°C in the furnace, hold for 5 hours, cool to <500°C in the furnace, and then air cool out of the furnace.

[0042] (4) High temperature quenching treatment: The steel forging billet for the mold is placed into the heating furnace and heated to 1100℃. After holding for 25min + 0.6min / mm, it is oil quenched to room temperature.

[0043] (5) High temperature tempering treatment: heat to 640℃, hold for 65min + 1.5min / mm, and then air cool to room temperature.

[0044] (6) Low temperature tempering treatment: heat to 550℃, hold for 65min+1min / mm, and then air cool to room temperature to obtain the product. Example 5

[0045] The chemical composition of the steel used for the continuous extrusion die in this embodiment is 0.49% C, 2.2% Mo, 4.1% Cr, and 0.5% V, with the remainder being iron and unavoidable impurities; its heat treatment process includes the following steps: (1) High temperature normalizing treatment: The steel forging billet for the mold is loaded into the heating furnace and heated to 1085℃. After holding for 20min + 0.6min / mm, it is cooled to room temperature by water.

[0046] (2) Low temperature normalizing treatment: The steel forging billet for the mold is placed into the heating furnace and heated to 720°C. After holding at this temperature for 18 min + 0.6 min / mm, it is cooled to room temperature by water. (3) Spheroidizing annealing treatment: Heat to 850°C in the furnace, hold for 3.5 hours, cool to 680°C in the furnace, hold for 4 hours, cool to <500°C in the furnace, and then air cool out of the furnace.

[0047] (4) High temperature quenching treatment: The steel forging billet for the mold is placed into the heating furnace and heated to 1085℃. After holding at this temperature for 20 min + 0.6 min / mm, it is oil quenched to room temperature. (5) High temperature tempering treatment: heat to 630℃, hold for 60min + 1.5min / mm, and then air cool to room temperature.

[0048] (6) Low temperature tempering treatment: heat to 540℃, hold for 62min+1min / mm, and then air cool to room temperature to obtain the product. Example 6

[0049] The chemical composition of the steel used for the continuous extrusion die in this embodiment is C 0.47%, Mo 1.8%, Cr 4.1%, V 0.5%, with the remainder being iron and unavoidable impurities; its heat treatment process includes the following steps: (1) High temperature normalizing treatment: The steel forging billet for the mold is loaded into the heating furnace and heated to 1080℃. After holding at the temperature for 22min + 0.6min / mm, it is cooled to room temperature by water.

[0050] (2) Low temperature normalizing treatment: The steel forging billet for the mold is loaded into the heating furnace, heated to 720°C, held for 18 min + 0.6 min / mm, and then cooled to room temperature by water.

[0051] (3) Spheroidizing annealing treatment: Heat to 860°C in the furnace, hold for 3.5 hours, cool to 680°C in the furnace, hold for 4 hours, cool to <500°C in the furnace, and then air cool out of the furnace.

[0052] (4) High temperature quenching treatment: The steel forging billet for the mold is loaded into the heating furnace and heated to 1080℃. After holding for 22min + 0.6min / mm, it is oil quenched to room temperature.

[0053] (5) High temperature tempering treatment: heat to 630℃, hold for 60min + 1.5min / mm, and then air cool to room temperature.

[0054] (6) Low temperature tempering treatment: heat to 545℃, hold for 63min+1min / mm, and then air cool to room temperature to obtain the product. Comparative Example 1

[0055] The chemical composition of the steel used in this comparative continuous extrusion die is 0.47% C, 1.8% Mo, 4.1% Cr, and 0.4% V, with the remainder being iron and unavoidable impurities; its heat treatment process includes the following steps: (1) Normalizing treatment: heating temperature 1110℃, holding for 2.5 hours, water mist cooling to 150~200℃, and then air cooling.

[0056] (2) Spheroidizing annealing treatment: The annealing heating temperature is 840℃, the holding temperature is 4 hours, the furnace is cooled to 700℃, the holding temperature is 6 hours, the furnace is cooled to 460℃, and then the furnace is removed and air-cooled.

[0057] (3) Quenching treatment: heating temperature 1080℃, holding for 1 hour, oil quenching to room temperature.

[0058] (4) Tempering treatment: Tempering temperature 620℃, tempering time 2h, two temperings are performed, and the sample is cooled to room temperature after each tempering to obtain the final product.

[0059] Performance testing

[0060] Performance tests were conducted on samples of the mold steel obtained in Examples 1-6 and Comparative Example 1, and the results are shown in Table 1.

[0061] Table 1. Test results of hot work die steel in each embodiment and comparative example. .

Claims

1. A heat treatment method for improving the yield ratio of a steel for a continuous extrusion die, characterized by, The steel used for continuous extrusion dies is subjected to high-temperature normalizing, low-temperature normalizing, spheroidizing annealing, high-temperature quenching, high-temperature tempering and low-temperature tempering in sequence.

2. The heat treatment method for improving the yield ratio of a continuous extrusion die steel according to claim 1, characterized in that, The high-temperature normalizing process involves heating the steel forging billet for the mold to 1070℃~1100℃, holding it at that temperature for 15~25min+0.6min / mm, and then water-cooling it to room temperature.

3. The heat treatment method for improving the yield strength ratio of steel for continuous extrusion dies according to claim 1, characterized in that, The low-temperature normalizing process involves heating the steel forging billet for the mold to 710℃~730℃, holding it at that temperature for 15~20min + 0.6min / mm, and then water-cooling it to room temperature.

4. The heat treatment method for improving the yield strength ratio of steel for continuous extrusion dies according to claim 1, characterized in that, The spheroidizing annealing process involves heating the steel forging billet for the mold to 840℃~870℃, holding it at that temperature for 3~4 hours, then furnace cooling it to 660℃~700℃, holding it at that temperature for 3~5 hours, furnace cooling it to <500℃, and finally air cooling it after removing it from the furnace.

5. The heat treatment method for improving the yield strength ratio of steel for continuous extrusion dies according to claim 1, characterized in that, The high-temperature quenching process involves heating the steel forging billet for the mold to 1070℃~1100℃, holding it at that temperature for 15~25min+0.6min / mm, and then oil quenching it to room temperature.

6. The heat treatment method for improving the yield strength ratio of steel for continuous extrusion dies according to claim 1, characterized in that, The high-temperature tempering involves heating the steel forging billet for the mold to 620℃~640℃, holding it at that temperature for 55~65min+1.5min / mm, and then air-cooling it to room temperature.

7. The heat treatment method for improving the yield strength ratio of steel for continuous extrusion dies according to claim 1, characterized in that, The low-temperature tempering involves heating the steel forging billet for the mold to 530℃~550℃, holding it at that temperature for 60~65min+1min / mm, and then air-cooling it to room temperature.

8. A heat treatment method for improving the yield strength ratio of steel for continuous extrusion dies according to claim 1, characterized in that, The steel used for the continuous extrusion die is a medium-carbon chromium-molybdenum series steel.

9. A heat treatment method for improving the yield strength ratio of steel for continuous extrusion dies according to claim 1, characterized in that, The mold steel heat-treated by the method described above has a yield strength ratio ≥0.91 and a hardness of 48-52 HRC.