Heat treatment method for improving performance of die steel by introducing trace bainite and retained austenite

Abstract

The invention discloses a heat treatment method for improving the performance of die steel by introducing trace bainite and retained austenite. The heat treatment method comprises the following isothermal quenching steps: heating a test block to 1020-1060 DEG C, preserving heat for 40-80 minutes, performing oil cooling to 260-300 DEG C, putting the test block into fused salt with the same temperature, performing salt bath for 20-60 minutes, then taking out the test block, and performing oil cooling to room temperature. According to the method, the stability of the die in production and processing is improved while the performance is improved. The structure of the material is controlled in the heat treatment process through the heat treatment process and complete process parameters, a horse-bainite-residual austenite multi-phase structure with good mechanical properties is formed, and the determined heat treatment method can comprehensively improve the mechanical properties of the hot work die steel and prolong the service life of the hot work die steel.

Description

Technical Field

[0001] The present invention belongs to the technical field of martensite-bainite-retained austenite multi-phase hot working die steel. Specifically, it relates to a heat treatment method for introducing trace bainite and retained austenite to improve the properties of die steel. Background Art

[0002] The large-scale of dies is an inevitable trend in the development of modern high-end industries. For example, Tesla's vehicle body has adopted an integrated die-casting process to manufacture the body, which can greatly improve the manufacturing efficiency, significantly reduce the cost of the vehicle body, and greatly improve the strength of the body. Heat treatment is a very crucial process in the production of large-scale hot working dies. In order to meet the service performance requirements, hot working die materials need to be quenched and tempered to obtain excellent strength and good toughness. For large-scale hot working dies, there is a large difference between the surface temperature and the center temperature during the quenching process. During the cooling process, the core of the large die may be held at a temperature near the MS point, generating a multi-phase microstructure mixture of martensite, bainite, and retained austenite. Among them, martensite is the product of non-diffusive phase transformation, which contains a very high density of dislocations and has high strength and hardness. Bainite is the isothermal transformation product obtained by the decomposition of austenite. Carbides may or may not exist in bainite, usually depending on the composition of the steel and the isothermal holding time. Generally, it is considered that the strength of bainite is lower than that of martensite, but it has better toughness. Retained austenite can transform into martensite during the strain process, increasing the strain hardening rate and improving the toughness of the material.

[0003] The heat treatment method of traditional H13 steel easily causes carbides to dissolve into austenite, and it is difficult to transform into martensite during isothermal quenching. The retained austenite in the die structure increases, and the retained austenite reduces the hardness of the die, resulting in poor mechanical properties of the die steel and still not achieving the purpose of improving the service life of the die.

[0004] Therefore, there is an urgent need for a heat treatment method for introducing trace bainite and retained austenite to improve the properties of die steel. Summary of the Invention

[0005] The present invention aims to overcome the defects of the prior art and aims to provide a heat treatment method for introducing trace bainite and retained austenite to improve the properties of die steel. This method can make the hot working die steel obtain a uniform and refined martensite-bainite-retained austenite multi-phase structure, and can improve the mechanical properties and service life of the hot working die steel.

[0006] To achieve the above object, the steps of the technical solution adopted by the present invention are as follows:

[0007] A heat treatment method for introducing trace bainite and retained austenite to improve the properties of die steel, the heat treatment method includes the following isothermal quenching steps:

[0008] The test block is heated to 1020-1060℃ and kept at this temperature for 40-80min, cooled to 260-300℃ by oil cooling and placed in a molten salt bath at the same temperature for 20-60min, then taken out and cooled to room temperature by oil cooling.

[0009] The material of the hot working die steel is 4Cr5MoSiV1.

[0010] Preferably, the 4Cr5MoSiV1 die steel test block

[0011] Preferably, the test block is heated to 1040°C.

[0012] Preferably, the insulation time is 60 minutes.

[0013] Preferably, the salt bath time is 40 minutes.

[0014] Compared with the prior art, the present invention has the following beneficial effects:

[0015] 1. The present invention adopts special isothermal quenching conditions for hot working die steel, that is, after austenitization and heat preservation, oil cooling to the bainite transformation temperature range is carried out for isothermal salt bath. This process does not produce martensitic transformation. By controlling the salt bath temperature, trace bainite is produced during the salt bath process, and then oil cooling to room temperature occurs. At this time, martensitic transformation occurs and residual austenite exists in the structure. By controlling the temperature of the isothermal salt bath during isothermal quenching, the formation of trace bainite and residual austenite structure of hot working die steel is promoted during the heat treatment process. This structure can improve the mechanical properties of die steel such as toughness and thermal stability. The hot working die steel is provided with a uniform and refined martensitic-bainite-residual austenite multiphase structure, which can improve the mechanical properties and service life of the hot working die steel.

[0016] 2. The heat treatment method of the present invention for introducing trace bainite and residual austenite to improve the performance of mold steel has a complete process route. The microstructure of the obtained hot working die steel is analyzed by scanning electron microscopy, and thin-flaked and blocky residual austenite can be clearly observed, wherein the lamellar residual austenite is mainly distributed at the boundary between needle-shaped bainite and lath martensite. Various mechanical properties of the hot working die steel are greatly improved, the stability is significantly improved, and the service life of the hot working die steel is greatly extended.

[0017] Therefore, the present invention can obtain a uniform and refined Ma-B-O multiphase structure after heat treatment of the hot working die steel, and the determined heat treatment method can comprehensively improve the mechanical properties and service life of the hot working die steel. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 The tensile strength and yield strength curves of the hot working die steel in the isothermal quenching state and the tempering state at different temperatures of the embodiments 1 and 2 of the present invention;

[0019] Figure 2 Curves of elongation and impact energy absorption for the isothermal quenching state and tempering states at different temperatures of the hot work die steel in Embodiment 1 to Embodiment 2 of the present invention;

[0020] Figure 3 SEM micrographs of the microstructures of the materials after isothermal quenching at different temperatures.

[0021] In the figure, (a) Comparative Example 1 (isothermal quenching at 220°C), (b) Embodiment 1 (isothermal quenching at 260°C), (c) Embodiment 2 (isothermal quenching at 300°C). Specific Embodiments

[0022] The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments, which is not a limitation to its protection scope.

[0023] The test blocks used in the following examples are 4Cr5MoSiV1 die steel, and the element contents are shown in Table 1.

[0024] Table 1 Composition of 4Cr5MoSiV1 Die Steel

[0025]

[0026] The molten salt used in the following examples has a composition of 55% KNO3 + 45% NaNO2 and is purchased from East China Chemical and Glass Co., Ltd.

[0027] Embodiment 1

[0028] A heat treatment method for introducing trace bainite and retained austenite to improve the properties of die steel. The steps of the heat treatment method in this embodiment are:

[0029] Heat the 4Cr5MoSiV1 die steel test block to 1040°C and hold for 60 min, then cool it in oil to 260°C, place it in the molten salt at the same temperature for salt bath for 40 min, and then take it out and cool it in oil to room temperature.

[0030] In Comparative Example 1, the 4Cr5MoSiV1 die steel specimen was heated to 1040°C and held for 60 min, then cooled in oil and placed in the molten salt at 220°C for salt bath for 40 min, and then taken out and cooled in oil to room temperature.

[0031] The materials obtained in Embodiment 1 and Comparative Example 1 were respectively subjected to tempering treatments at 350°C, 500°C and 650°C (place the materials in a tempering furnace, heat up to the corresponding temperature, hold for 2 h, then cool naturally to room temperature, take out, and obtain the hot work die steel after tempering). Subsequently, their tensile strength, yield strength, elongation and impact energy absorption were compared, and the comparison results are shown in Figure 1 And Figure 2, the specific data is shown in Table 1.

[0032] Table 1 Comparison of Tensile Strength, Yield Strength, Elongation and Impact Absorption Energy between the Steel in Example 1 and the Steel in Comparative Example 1

[0033]

[0034] Figure 3 (a) is the SEM micrograph of the microstructure of the material after isothermal quenching at 220 °C (Comparative Example 1), and the microstructure is composed of martensite, self-tempered martensite and retained austenite;

[0035] Figure 3 (b) is the SEM micrograph of the microstructure of the material in Example 1 after isothermal quenching at 260 °C, and the microstructure is composed of martensite, acicular bainite and retained austenite.

[0036] It can be seen that the important mechanical properties of the material in Example 1 have been improved.

[0037] Example 2

[0038] A heat treatment method for improving the properties of die steel by introducing trace bainite and retained austenite. The steps of the heat treatment method in this example are as follows:

[0039] Step 1, isothermal quenching

[0040] Heat the 4Cr5MoSiV1 die steel specimen to 1040 °C and hold for 60 min, then cool it in oil to 300 °C, put it into a molten salt bath at the same temperature for 40 min, and then take it out and cool it in oil to room temperature.

[0041] The materials in Example 2 were respectively subjected to tempering treatment at 350 °C, 500 °C and 650 °C (put the materials into a tempering furnace, heat up to the corresponding temperature, hold for 2 h, then cool naturally to room temperature, take out, and obtain the hot work die steel after tempering). Then, compare its tensile strength, yield strength, elongation and impact absorption energy with those in Comparative Example 1. The comparison results are shown in Figure 1 and Figure 2 , the specific data is shown in Table 2.

[0042] Table 2 Comparison of Tensile Strength, Yield Strength, Elongation and Impact Absorption Energy between the Steel in Example 2 and the Steel in Comparative Example 1

[0043]

[0044] Figure 3 (c) is the SEM micrograph of the microstructure of the material in Example 2 after isothermal quenching at 300 °C, and the microstructure is composed of martensite, acicular bainite and retained austenite.

[0045] It can be seen that the mechanical properties of the material in Example 2 have been improved.

Claims

1. A heat treatment method for improving the properties of die steel by introducing micro-bainite and retained austenite, characterized in that The heat treatment method described above includes the following austempering steps: heating the test block to 1020 - 1060 °C and holding for 40 - 80 min, cooling it in oil to 260 - 300 °C and putting it into a molten salt bath at the same temperature for 20 - 60 min, and then taking it out and cooling it in oil to room temperature.

2. The heat treatment method for improving the properties of die steel by introducing micro bainite and retained austenite according to claim 1, characterized in that The material of the hot working die described above is 4Cr5MoSiV1.

3. The heat treatment method for improving the properties of die steel by introducing micro bainite and retained austenite according to claim 1, characterized in that The test block is heated to 1040 °C as described above.

4. The heat treatment method for improving the properties of die steel by introducing micro bainite and retained austenite according to claim 1, characterized in that The holding time is 60 min as described above.

5. The heat treatment method for improving the properties of die steel by introducing micro bainite and retained austenite according to claim 1, characterized in that The salt bath time is 40 min as described above.

Images