A fine-grained, high-strength and toughness mirror plastic mold steel and heat treatment method thereof

Abstract

The invention discloses a fine-grained high-strength and toughness mirror plastic mold steel and a heat treatment method thereof. The components of the mold steel are as follows by mass percentage: C: 0.35-0.45%, Si: 0.40-0.60%, Mn: 0.40-0.70%, Cr: 6.50-8.00%, Mo: 0.50-0.70%, V: 0.50-0.70%, Al: 0.020-0.060%, P≤0.015%, S≤0.010%, O≤0.001 5%, N: 0.020-0.040%, the heat treatment method includes two quenching processes and two tempering processes, the quenching temperature of the first quenching process is 1020-1050°C, the holding time is 1-2h, the quenching temperature of the second quenching process is 980-1020°C, the holding time is 1-3h, the tempering temperature of the first tempering process is 530-550°C, the holding time is 2-6h, and the tempering temperature of the second tempering process is 500-520°C, the holding time is 2-6h. The present invention adopts an alloying design mainly composed of C, Cr, Mo, and V, and utilizes V, Mo, Al, and N multi-alloys to synergistically refine austenite grains. After a heat treatment process of two quenchings + two temperings, a martensitic structure with fine grains is obtained, achieving an excellent match of high strength and high toughness.

Description

Technical Field

[0001] The invention belongs to the technical field of plastic mould steel, and in particular relates to a fine-grained high-strength and toughness mirror plastic mould steel and a heat treatment method thereof. Background Art

[0002] Plastic molds are important process equipment in the plastic molding processing industry. With the rapid development of my country's plastics industry, the demand for plastic product molding is increasing, and higher requirements are also put forward for the purity, polishing performance, fine grain structure, homogenization and corrosion resistance of plastic mold steel.

[0003] At present, the advanced mirror plastic mold steels in the world mainly include Japan's CENA1 age-hardening steel, martensitic mirror stainless plastic mold steel HPM38, pre-hardened plastic mold steel NAK80 (10Ni3MnCuAl), SUS420J2 improved S-STAR and S-STAR-A, etc.; Sweden's ASSAB's POLMAX steel and CORRAX mold steel; Germany's Gritz's XPM VICTORY ESR and CRMHP VICTORY ESR, etc.

[0004] In order to meet the high requirements of transparent plastic and rubber products on mold quality, technicians in this field have conducted a lot of research and improvement on plastic mold steel. Although the prior art has made some explorations in the design of mirror plastic mold steel components, how to optimize the component design and use effective heat treatment processes to refine the material and improve its toughness at the same time under the existing equipment smelting technology level is a technical problem that technicians in this field urgently need to solve. Summary of the invention

[0005] In order to solve the above problems, the present invention provides a fine-grained high-strength and toughness mirror plastic mold steel and a heat treatment method thereof.

[0006] In one aspect of the present invention, the present invention provides a fine-grained high-strength and toughness mirror plastic mold steel, the chemical composition of the mirror plastic mold steel is controlled by mass percentage as follows: C: 0.35-0.45%, Si: 0.40-0.60%, Mn: 0.40-0.70%, Cr: 6.50-8.00%, Mo: 0.50-0.70%, V: 0.50-0.70%, Al: 0.020-0.060%, P≤0.015%, S≤0.010%, O≤0.0015%, N: 0.020-0.040%, and the rest is Fe and unavoidable impurity residue.

[0007] Preferably, in the above-mentioned fine-grained high-strength and toughness mirror plastic mold steel, the chemical composition of the mirror plastic mold steel is controlled by mass percentage as follows: C: 0.35-0.39%, Si: 0.40-0.50%, Mn: 0.50-0.60%, Cr: 6.50-7.50%, Mo: 0.50-0.65%, V: 0.52-0.60%, Al: 0.035-0.044%, P≤0.010%, S≤0.005%, O≤0.0015%, N: 0.020-0.035%, and the rest is Fe and unavoidable impurity residue.

[0008] In another aspect of the present invention, the present invention provides a heat treatment method for fine-grained high-strength and toughness mirror plastic mold steel, wherein the components of the mirror mold steel are as follows by mass percentage: C: 0.35-0.45%, Si: 0.40-0.60%, Mn: 0.40-0.70%, Cr: 6.50-8.00%, Mo: 0.50-0.70%, V: 0.50-0.70%, Al: 0.020-0.060%, P≤0.015%, S≤0.010%, O≤0.0015%, N: 0.020-0.040%, and the rest is Fe and unavoidable impurity residue, wherein the heat treatment method includes two quenching processes and two tempering processes:

[0009] (1) First quenching process: The mold steel is subjected to the first quenching treatment in the quenching medium. The quenching temperature of the first quenching process is controlled at 1020-1050°C, the heating and holding time is controlled at 1-2h, and the mold steel is rapidly cooled to room temperature.

[0010] (2) Second quenching process: After the first quenching treatment, the mold steel is subjected to a second quenching treatment in a quenching medium. The quenching temperature of the second quenching process is controlled at 980-1020°C, the heating and holding time is controlled at 1-3h, and the mold steel is rapidly cooled to room temperature.

[0011] (3) First tempering process: After two quenching treatments, the mold steel is subjected to the first tempering treatment. The tempering temperature of the first tempering process is controlled at 530-550°C, the holding time is controlled at 2-6h, and air-cooled to room temperature after tempering;

[0012] (4) Second tempering process: The mold steel after the first tempering treatment is subjected to the second tempering treatment. The tempering temperature of the second tempering process is controlled at 500-520°C, the holding time is controlled at 2-6h, and air-cooled to room temperature after tempering.

[0013] Preferably, in the above-mentioned fine-grained high-strength and toughness mirror plastic mold steel heat treatment method, in the mold steel heat treatment step, the quenching temperature of the first quenching process is controlled to 1025°C, and the heating and holding time is controlled to 1.5h; the quenching temperature of the second quenching process is controlled to 990°C, and the heating and holding time is controlled to 2h; the tempering temperature of the first tempering process is controlled to 540°C, and the holding time is controlled to 3h; the tempering temperature of the second tempering process is controlled to 510°C, and the holding time is controlled to 2h.

[0014] Preferably, in the above-mentioned fine-grained high-strength and toughness mirror plastic mold steel heat treatment method, in the mold steel heat treatment step, the quenching medium is water or water mist.

[0015] Furthermore, in the above-mentioned fine-grained high-strength and toughness mirror plastic mold steel heat treatment method, the mold steel is made by forging and rolling, including: proportioning the raw materials according to the component content requirements of the mirror mold steel, forging and rolling into mold steel after smelting and casting, the heating temperature of forging and rolling is controlled at 1200-1260°C, and the final forging or final rolling temperature is controlled at ≥900°C.

[0016] Furthermore, in the above-mentioned fine-grained high-strength and toughness mirror plastic mold steel heat treatment method, a converter, an electric furnace or a vacuum induction furnace is used for smelting, casting includes continuous casting or die casting, and the mold steel after forging and rolling is air-cooled and then heat-treated.

[0017] The fine-grained high-strength and toughness mirror plastic mold steel and the heat treatment method thereof of the present invention have the following advantages and beneficial effects: the present invention adopts an alloying design mainly composed of C, Cr, Mo and V, and utilizes V, Mo, Al and N multi-alloys to synergistically refine austenite grains, and obtains a martensitic structure with fine grains through a heat treatment process of two quenchings and two temperings, thereby achieving an excellent match of high strength and high toughness, and fully meeting the high requirements of the field for the purity, polishing performance, fine grain structure, homogenization and corrosion resistance of plastic mold steel. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0019] Figure 1 The present invention provides a 500-fold metallographic structure diagram of the mirror plastic mold steel after heat treatment using the heat treatment method for the fine-grained high-strength and toughness mirror plastic mold steel of the present invention. DETAILED DESCRIPTION

[0020] In order to make the purpose, technical solution and advantages of the present invention clearer, the technical solution of the present invention will be clearly and completely described below in conjunction with the specific embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0021] The chemical composition of the fine-grained high-strength and toughness mirror plastic mold steel of the present invention is controlled by mass percentage as follows: C: 0.35-0.45%, Si: 0.40-0.60%, Mn: 0.40-0.70%, Cr: 6.50-8.00%, Mo: 0.50-0.70%, V: 0.50-0.70%, Al: 0.020-0.060%, P≤0.015%, S≤0.010%, O≤0.0015%, N: 0.020-0.040%, and the rest is Fe and inevitable impurity remainder.

[0022] Preferably, the chemical composition of the fine-grained high-strength and toughness mirror plastic mold steel of the present invention is controlled by mass percentage as follows: C: 0.35-0.39%, Si: 0.40-0.50%, Mn: 0.50-0.60%, Cr: 6.50-7.50%, Mo: 0.50-0.65%, V: 0.52-0.60%, Al: 0.035-0.044%, P≤0.010%, S≤0.005%, O≤0.0015%, N: 0.020-0.035%, and the rest is Fe and unavoidable impurity residue.

[0023] The heat treatment method of fine-grained high-strength and toughness mirror plastic mold steel of the present invention comprises the following steps:

[0024] (I) Forging and rolling of die steel: raw materials are proportioned according to the component content requirements of the target product of mirror die steel, and forged and rolled into die steel after smelting and casting, wherein the component content of mirror die steel is as follows by mass percentage: C: 0.35-0.45%, Si: 0.40-0.60%, Mn: 0.40-0.70%, Cr: 6.50-8.00%, Mo: 0.50-0.70%, V: 0.50-0.70%, Al: 0.020-0.060%, P≤0.015%, S≤0.010%, O≤0.0015%, N: 0.020-0.040%, and the rest is Fe and unavoidable impurity residue, and in the forging and rolling of die steel, the heating temperature of forging and rolling is controlled to be 1200-1260°C, and the final forging or final rolling temperature is controlled to be ≥900°C;

[0025] (II) Heat treatment of mold steel: including two quenching processes and two tempering processes, specifically including:

[0026] (1) First quenching process

[0027] The die steel after forging and rolling is subjected to the first quenching treatment in the quenching medium. The quenching temperature of the first quenching process is controlled at 1020-1050°C, the heating and holding time is controlled at 1-2h, and it is quickly cooled to room temperature.

[0028] (2) Second quenching process

[0029] After the first quenching treatment, the mold steel is subjected to a second quenching treatment in a quenching medium. The quenching temperature of the second quenching process is controlled at 980-1020°C, the heating and holding time is controlled at 1-3h, and the mold steel is rapidly cooled to room temperature.

[0030] (3) First tempering process

[0031] After two quenching treatments, the mold steel is subjected to the first tempering treatment. The tempering temperature of the first tempering process is controlled at 530-550°C, the holding time is controlled at 2-6h, and air-cooled to room temperature after tempering;

[0032] (4) Second tempering process

[0033] The mold steel after the first tempering treatment is subjected to the second tempering treatment. The tempering temperature of the second tempering process is controlled at 500-520°C, the insulation time is controlled at 2-6h, and air-cooled to room temperature after tempering.

[0034] Preferably, in the heat treatment method of fine-grained high-strength and toughness mirror plastic mold steel of the present invention, in the mold steel heat treatment step, the quenching temperature of the first quenching process is controlled to 1025°C, and the heating and holding time is controlled to 1.5h; the quenching temperature of the second quenching process is controlled to 990°C, and the heating and holding time is controlled to 2h; the tempering temperature of the first tempering process is controlled to 540°C, and the holding time is controlled to 3h; the tempering temperature of the second tempering process is controlled to 510°C, and the holding time is controlled to 2h.

[0035] Furthermore, in the heat treatment method of fine-grained high-strength and toughness mirror-finished plastic mold steel of the present invention, in the mold steel heat treatment step, the quenching medium is water or water mist.

[0036] Furthermore, in the heat treatment method of fine-grained high-strength and toughness mirror plastic mold steel of the present invention, in the mold steel forging step, a converter, an electric furnace or a vacuum induction furnace is used for smelting, and the casting can be continuous casting or die casting. The mold steel after forging is air-cooled and then heat-treated.

[0037] In the fine-grained high-strength and toughness mirror plastic mold steel of the present invention, the chemical composition of the mirror plastic mold steel is controlled according to the above mass percentage because:

[0038] C: One of the most important strengthening elements and elements that improve hardenability. Carbon can combine with chromium, molybdenum or vanadium to form carbides, and can also form carbonitrides with chromium, molybdenum or vanadium under the synergistic effect of nitrogen. During the quenching process, carbon is dissolved in the matrix to form a martensitic structure to ensure the hardness of the material. If the carbon content is too high, it will significantly reduce the plasticity and toughness of the steel, and combine with chromium and molybdenum to form carbides, reducing the chromium and molybdenum in the matrix, thereby reducing corrosion resistance. Therefore, the C content should be appropriately reduced, and the lost strength is mainly compensated by the Cr, Mo, V alloy nano second phase. In the present invention, the C content is controlled at 0.35-0.45%.

[0039] Si: The main deoxidizing element in steel. It also has a significant solid solution strengthening effect, which can significantly improve the strength and hardness of steel, increase the quenching temperature, and improve hardenability. Si at 500-550°C can increase the dispersion of alloy carbides precipitated from die steel during tempering, thereby increasing the secondary hardening peak. However, excessive Si content will bring the risk of cracks in the ingot and steel plate, thereby damaging the toughness of the steel. Therefore, the Si content in the present invention is controlled to be 0.40%-0.60%.

[0040] Mn: An effective element for deoxidation and desulfurization, it dissolves in ferrite and austenite at the same time, expands the austenite region, reduces the phase transition point during the cooling process of steel, improves the hardenability of steel, and significantly improves the strength and hardness of steel. However, if the Mn content is too high, the toughness of the steel will be significantly reduced, and the hot working performance will decrease. Therefore, the Mn content in the present invention is controlled to be 0.40% to 0.70%.

[0041] Cr: It can not only significantly improve the corrosion resistance of steel, but also improve the uniformity of material structure. A part of it is dissolved in the matrix to play a role of solid solution strengthening, and the other part combines with carbon to form carbides. Cr dissolves in austenite during quenching and heating, and dissolves in martensite after quenching, which can improve the steel's resistance to tempering softening. During tempering, it precipitates from the matrix to form (Fe, Cr)3C alloy cementite, which can refine the grains and cementite, thereby obtaining uniform and fine quenched grains and tempered structure, and improving the toughness of the material. However, excessive Cr and C are prone to form carbides such as Cr7C3, which damage the toughness of the steel. Therefore, the Cr content in the present invention is controlled at 6.50-8.00%.

[0042] Mo: a strong carbide element. The present invention obtains fine austenite grains, significant nano-second phase precipitation strengthening effect and high tempering resistance through Mo and V alloying, thereby obtaining a high-temperature tempering structure with fine grains and high toughness. When the Mo content is lower than 0.50%, the above effect is limited. When the content is too high, the above effect is saturated or even leads to an excessive effect of improving tempering resistance, which easily leads to a decrease in impact toughness. Therefore, the Mo content in the present invention is controlled at 0.50-0.70%.

[0043] V: The die steel of the present invention is a medium carbon alloy steel, which is alloyed with a relatively high V. Firstly, (V, Mo) (C, N) can be precipitated during forging and rolling to refine the austenite grains. Secondly, a large amount of M (C, N) (M: Cr, Mo, V), M 23 C6 (M: Cr, Mo, V), VN nanoparticles refine the grain size of reheated austenite. In addition, V partially dissolved in the heat-treated reheated austenite improves hardenability, and at the same time precipitates in large quantities during high-temperature tempering, occupies a portion of C in the steel, and forms a large number of nano second phases mainly composed of V, C and Mo, thereby obtaining uniform and fine austenite grains and their phase transformation structures. However, too high a V content will lead to too high a VC precipitation temperature, too much precipitation, and easy coarse particle size, which is not conducive to refining austenite grains, and is not conducive to the strength and toughness of mold steel. Therefore, the V content in the present invention is controlled at 0.50-0.70%.

[0044] Al: a strong deoxidizing element, which can combine with N to form AlN, and can play a role in grain refinement. When the content is lower than 0.020%, the above effects are not obvious. Therefore, in the present invention, the Al content is controlled to be 0.020-0.060%.

[0045] N: As an interstitial element, it can increase the hardness of martensite. In addition, when V, Al, etc. are present in the steel, N will form carbonitrides with them, which can effectively inhibit the growth of austenite grains. Therefore, the N content in the present invention is controlled to be 0.020-0.040%.

[0046] O, P, S: are impurity elements in die steel, which are detrimental to toughness and plasticity and seriously reduce fatigue performance. Therefore, in the present invention, the O content, P content, and S content are controlled to be O≤0.0015%, P≤0.015%, and S≤0.010%, respectively.

[0047] Therefore, by controlling the chemical composition of mirror plastic mold steel, adopting medium and low C, Cr, Mo, V-based alloying design, and adopting Al, N synergistic multi-component composite microalloying ideas, the focus is on the reasonable matching of C, N, V, Mo, Cr, and Al elements, giving full play to their role in mold steel, obtaining a microstructure with fine grains and mainly martensite, so that the mold steel has comprehensive mechanical properties such as ultra-high strength and toughness, and thus has good service performance.

[0048] Moreover, in the heat treatment method of fine-grained high-strength and toughness mirror plastic mold steel of the present invention, the mold steel after forging and rolling is subjected to two quenching treatments and two tempering treatments: the first quenching is performed at a relatively high temperature (1020-1050°C) for austenitization, the purpose of which is to eliminate the uneven structure of forging or rolling, and to dissolve and homogenize V and other parts; the first quenching is performed at a relatively short holding time (1-2h) in order to inhibit the austenite grains and the precipitated M (C, N) (M: Cr, Mo, V), M 23 The purpose of coarsening C6 (M: Cr, Mo, V) and VN particles and rapid cooling is to obtain martensite / bainite structure; on the basis of obtaining a relatively uniform and fine martensite structure by the first quenching, a lower temperature (980-1020°C) is selected for the second quenching to perform secondary austenitization, mainly to obtain more nanoparticles and reduce the coarsening tendency of austenite grains, and finally obtain uniform and fine austenite grains and their phase transformation structure, thereby obtaining fine-grained high-strength and toughness mirror plastic mold steel. According to the test, the average grain size of the austenite grains in the mold steel structure of the present invention is 8-12μm, and the room temperature yield strength R p0.2 ≥1450MPa, room temperature tensile strength R m ≥1700MPa, room temperature elongation A≥10%, hardness above 50HRC, room temperature impact absorption energy KU2≥35J.

[0049] Hereinafter, conventional mirror plastic mold steel 4Cr13 is used as comparative steel, and three mold steels with different chemical composition ratios are selected from the fine-grained high-strength and toughness mirror plastic mold steel of the present invention as Example 1, Example 2 and Example 3 of the present invention, and the fine-grained high-strength and toughness mirror plastic mold steel and its heat treatment method of the present invention are described in detail. The chemical composition of the mold steel of Example 1, Example 2, Example 3 of the present invention and the comparative example, excluding Fe and the inevitable impurity remainder, is shown in Table 1 below.

[0050] Table 1 Chemical composition (mass percentage, %)

[0051] C Si Mn P S Cr Mo V Al N O Example 1 0.35 0.42 0.51 0.008 0.002 7.12 0.52 0.55 0.043 0.024 0.0012 Example 2 0.37 0.46 0.53 0.007 0.003 6.90 0.54 0.56 0.036 0.028 0.0014 Example 3 0.38 0.43 0.54 0.005 0.004 6.98 0.61 0.55 0.051 0.031 0.0009 Comparative Example 0.37 0.47 0.52 0.004 0.003 12.30 - 0.30 0.045 - 0.0016

[0052] The mold steels of Example 1, Example 2, and Example 3 of the present invention were subjected to the heat treatment of the present invention as described above, and mold steel samples were obtained after the heat treatment. As a control, the mold steel of the comparative example was subjected to conventional heat treatment, and mold steel samples were obtained after the heat treatment. Mechanical property tests were performed according to national standards, and the austenite grain size in the mold steel structure was detected by the intercept method. The mechanical property indicators and austenite grain size of the mold steel samples of Example 1, Example 2, Example 3, and the comparative example are shown in Table 2 below. The metallographic structure of the mirror plastic mold steel of Example 1 is shown as an example. Figure 1 shown.

[0053] Table 2 Mechanical properties and grain size of die steel specimens

[0054] <![CDATA[R m / MPa]]> <![CDATA[R p0.2 / MPa]]> A / ％ <![CDATA[KU2(20℃) / J]]> HRC Grain size / μm Example 1 1734 1486 12.4 47.8 50.3 8 Example 2 1786 1532 11.3 44.2 50.8 12 Example 3 1813 1498 10.7 37.6 51.4 11 Comparative Example 1871 1603 6.8 32.5 36.2 20

[0055] It can be seen that the mold steel of the present invention has a room temperature tensile strength R m The mold steel of the present invention has good comprehensive mechanical properties of more than 1700MPa, more than 10% elongation A at room temperature, more than 50HRC Rockwell hardness, and more than 35J impact absorption energy at room temperature KU2. The mold steel of the present invention has uniform and fine austenite grains, with an average grain size of 8 to 12μm, and has better fine grains and high strength and toughness matching than the comparative steel.

[0056] In summary, compared with the prior art, the fine-grained high-strength and toughness mirror plastic mold steel and the heat treatment method thereof of the present invention have the following advantages and beneficial effects:

[0057] The present invention adopts an alloying design mainly composed of C, Cr, Mo and V, and utilizes V, Mo, Al and N multi-alloys to synergistically refine austenite grains. A martensitic structure with fine grains is obtained through a heat treatment process of two quenchings and two temperings, thereby achieving an excellent match of high strength and high toughness, and fully meeting the high requirements of the field for the purity, polishing performance, fine grain structure, homogenization and corrosion resistance of plastic mold steel.

[0058] It should be noted that, in this article, the term "comprise" or any other variation thereof is intended to cover non-exclusive inclusion, so that an article or device including a series of elements includes not only those elements, but also includes other elements not explicitly listed, or also includes elements inherent to such article or device.

[0059] It should also be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the present invention.

Claims

1. A fine-grained, high-strength and toughness mirror plastic mold steel, characterized in that: By controlling the chemical composition of mirror plastic mold steel, adopting the alloying design of medium and low C, Cr, Mo, and V as the main elements, adopting Al and N synergistic multi-component microalloying, and reasonably matching C, N, V, Mo, Cr, and Al elements, a microstructure with fine grains and mainly martensite is obtained through the heat treatment process of two quenching + two tempering. The average grain size of austenite grains in the mirror plastic mold steel organization is 8-12μm, and the room temperature yield strength R p0.2 ≥1450MPa, room temperature tensile strength R m ≥1700MPa, room temperature elongation A≥10%, hardness greater than 50HRC, room temperature impact absorption energy KU2≥35J, so that the mold steel has ultra-high strength and toughness comprehensive mechanical properties, so as to have good service performance. The chemical composition of the mirror plastic mold steel is controlled by mass percentage as follows: C: 0.35-0.39%, Si: 0.40-0.50%, Mn: 0.50-0.60%, Cr: 6.50-7.50%, Mo: 0.50-0.65%, V: 0.52-0.60%, Al: 0.035-0.044%, P≤0.010%, S≤0.005%, O≤0.0015%, N: 0.020-0.035%, and the rest is Fe and unavoidable impurity residue.

2. A heat treatment method for fine-grained high-strength and toughness mirror plastic mold steel, wherein the components of the mirror mold steel are as follows by mass percentage: C: 0.35-0.45%, Si: 0.40-0.60%, Mn: 0.40-0.70%, Cr: 6.50-8.00%, Mo: 0.50-0.70%, V: 0.50-0.70%, Al: 0.020-0.060%, P≤0.015%, S≤0.010%, O≤0.0015%, N: 0.020-0.040%, and the rest is Fe and unavoidable impurity residue, characterized in that: The heat treatment method includes two quenching processes and two tempering processes: (1) First quenching process: The mold steel is subjected to the first quenching treatment in a quenching medium of water or water mist. The quenching temperature of the first quenching process is controlled at 1020-1050°C, the heating and holding time is controlled at 1-2h, and the mold steel is rapidly cooled to room temperature. (2) Second quenching process: After the first quenching treatment, the mold steel is subjected to a second quenching treatment in a quenching medium of water or water mist. The quenching temperature of the second quenching process is controlled at 980-1020°C, the heating and holding time is controlled at 1-3h, and the mold steel is rapidly cooled to room temperature. (3) First tempering process: After two quenching treatments, the mold steel is subjected to the first tempering treatment. The tempering temperature of the first tempering process is controlled at 530-550°C, the holding time is controlled at 2-6h, and air-cooled to room temperature after tempering; (4) Second tempering process: After the first tempering treatment, the mold steel is subjected to a second tempering treatment. The tempering temperature of the second tempering process is controlled at 500-520°C, the holding time is controlled at 2-6h, and the mold steel is air-cooled to room temperature after tempering. The die steel is made by forging and rolling, including: mixing raw materials according to the component content requirements of mirror die steel, forging and rolling into die steel after smelting and casting, the heating temperature of forging and rolling is controlled at 1200-1260°C, and the final forging or final rolling temperature is controlled at ≥900°C.

3. The heat treatment method of fine-grained high-strength and toughness mirror plastic mold steel according to claim 2 is characterized in that: In the mold steel heat treatment steps, the quenching temperature of the first quenching process is controlled at 1025°C, and the heating and holding time is controlled at 1.5h; the quenching temperature of the second quenching process is controlled at 990°C, and the heating and holding time is controlled at 2h; the tempering temperature of the first tempering process is controlled at 540°C, and the holding time is controlled at 3h; the tempering temperature of the second tempering process is controlled at 510°C, and the holding time is controlled at 2h.

4. The heat treatment method of fine-grained high-strength and toughness mirror plastic mold steel according to claim 3 is characterized in that: The smelting is carried out by using a converter, an electric furnace or a vacuum induction furnace, the casting includes continuous casting or die casting, and the die steel after forging and rolling is air-cooled and then heat-treated.

Images