Preparation method of pre-hardened high-toughness and high-polishability stainless die steel

Abstract

The present invention relates to a preparation method of a pre-hardened high-toughness and high-polishability stainless die steel, and the main steps are as follows: S1, smelting of electrode billets; S2, electroslag smelting; S3, high-temperature homogenization; S4, multi-directional forging; S5, heat treatment after forging. The first step: After multi-directional forging, the workpiece is water-cooled until the surface temperature of the forging is 450-500 °C, and then loaded into an annealing furnace with a furnace temperature of 400-450 °C, held for 2-8 h, heated to 640-660 °C and held for 5-10 h, heated to 980-1020 °C, and the holding time is 2D h. After the holding is completed, it is lifted off the trolley and air-cooled by wind and fog to 250-350 °C; The second step: The cooled workpiece is loaded into an annealing furnace at 280-320 °C again and held for 5-10 h, heated to 840-880 °C and then held for 2D-3D h, and then the workpiece is cooled in the furnace to 720-760 °C, and then cooled in the furnace to below 250-300 °C and taken out of the furnace for air cooling; S6, pre-hardening treatment. Based on the composition of 20Cr13 stainless steel, the present invention designs the composition by slightly adjusting the C, Si, and Mn elements, adding Ni, Mo, and V micro-alloying elements, and at the same time significantly reducing the harmful elements P and S and the residual gases [H] and [O].

Description

Technical Field

[0001] The present invention relates to a die steel and a preparation method thereof, in particular to a preparation method of a pre-hardened high-toughness and high-polishability stainless die steel. Background Art

[0002] The level of the die industry is an important symbol to measure the manufacturing level of a country, and is also one of the important guarantees for a country's industrial products to maintain international competitiveness.

[0003] The development of our plastic die steel shows that the price fluctuations of domestic and imported products have fallen into a polarized state. On the one hand, domestic low-end plastic die materials have fallen into a vicious price war. In the general environment of rising raw material prices, the price of this product has caused huge economic pressure on the sales of enterprises, and enterprises generally have operating difficulties. On the other hand, mid- to high-end products are severely dependent on imports. Taking imported products such as German Glitz 1.2738, Swedish ASSAB 718HH, and Japanese Daido Special Steel NAK80, P20, etc. as examples, and Swedish Uddeholm Corporation's 420Mod series stainless steels STAVAX, MIRRAX, RAMAX, POLMAX, etc., their prices are 2 to 7 times that of domestic similar products, with rich profits. On the one hand, due to the relatively low market access threshold for low-end products, the product price has fallen into vicious competition due to the increase in intervening manufacturers, resulting in a continuous decline in corporate profits. On the other hand, some high-end and high-profit die steel products have high technical thresholds and low corporate brand awareness, and cannot be mass-produced and sold in China, resulting in the phenomenon that imported products occupy a large share of the domestic high-end die market.

[0004] Based on the composition of 20Cr13 stainless steel (GB / T 1220-2007), the present invention finely tunes the C, Si, and Mn elements, increases the Ni, Mo, and V microalloying elements, and at the same time significantly reduces the harmful elements P and S and the residual gases [H] and [O]. The produced die steel has good polishing performance, good corrosion resistance, good hardenability, good toughness and ductility. It can be widely used in electronic parts, tableware, utensils, cameras, sunglasses, chemical instruments and other high-gloss plastic products. Summary of the Invention

[0005] The purpose of the present invention is to provide a preparation method of a pre-hardened high-toughness and high-polishability stainless die steel. Based on the composition of 20Cr13 stainless steel (GB / T 1220-2007), the C, Si, and Mn elements are finely tuned, the Ni, Mo, and V microalloying elements are increased, and at the same time, the harmful elements P and S and the residual gases [H] and [O] are significantly reduced for the design of the composition.

[0006] To achieve the above object of the invention, a preparation method of a pre-hardened high-toughness and high-polishability stainless die steel mainly comprises the following steps:

[0007] S1. Electrode blank smelting:

[0008] Charge scrap steel and alloy materials according to the component content of plastic mold steel. Among them, heavy materials ≥ 30%, medium and light materials ≤ 70%, carbon content ≥ 0.70%, and less rust and less oil stain, high-purity scrap steel. Then, smelt using an electric arc furnace + ladle refining furnace + vacuum refining furnace, and then cast using the ingot casting method to obtain an ingot-cast electrode blank;

[0009] Among them, the component content of plastic mold steel is by mass fraction, C 0.18 - 0.28%, Si 0.80 - 1.20%, Mn 0.40 - 0.80%, Cr 12.0 - 14.0%, Mo 0.20 - 0.60%, V 0.20 - 0.60%, Ni 0.30 - 0.80%, P ≤ 0.010%, S ≤ 0.002%, and the rest is Fe;

[0010] S2. Electroslag smelting:

[0011] Use a protective atmosphere electroslag furnace to perform electroslag remelting on the ingot-cast electrode blank prepared in step S1 under argon protection, and then obtain an electroslag ingot;

[0012] S3. High-temperature homogenization:

[0013] Perform high-temperature diffusion on the electroslag ingot at 1230 - 1270 °C, and the holding time is 20 - 40 h;

[0014] S4. Multi-directional forging:

[0015] Perform upsetting and drawing in the X, Y, and Z directions on the electroslag ingot after high-temperature homogenization treatment, and finally draw it along the Z direction to the finished product. Among them, X and Y are two mutually perpendicular diameter directions, Z is the axial direction of the original electroslag ingot, the forging start temperature is 1150 - 1200 °C, and the forging end temperature ≥ 800 °C;

[0016] S5. Post-forging heat treatment: The post-forging heat treatment is divided into two steps,

[0017] First step: After multi-directional forging, cool the workpiece in water until the surface temperature of the forging is 450 - 500 °C, then load it into an annealing furnace with a furnace temperature of 400 - 450 °C, hold for 2 - 8 h, raise the temperature to 640 - 660 °C and hold for 5 - 10 h, raise the temperature to 980 - 1020 °C, and the holding time is 2D h, where D is the maximum thickness of the workpiece in the furnace, and its unit is dm. After the holding is completed, lift it off the trolley and cool it with air mist to 250 - 350 °C;

[0018] Step 2: Re-load the cooled workpiece into an annealing furnace at 280 - 320 °C for heat preservation for 5 - 10 h, heat up to 840 - 880 °C and then keep it for 2D - 3D h, then cool the workpiece in the furnace to 720 - 760 °C, with the heat preservation time being 5D - 7D h, and then cool it in the furnace to below 250 - 300 °C and take it out of the furnace for air cooling. D is the maximum thickness of the workpiece in the furnace, and its unit is dm;

[0019] S6 Pre-hardening treatment;

[0020] Perform pre-hardening treatment on the workpiece processed in step S5.

[0021] Specifically, in step S1, the specific steps of electric arc furnace smelting + ladle furnace refining + vacuum refining furnace refining are as follows:

[0022] A1 Electric furnace smelting:

[0023] During the melting period, control the oxygen pressure at 0.4 - 0.5 MPa, T ≥ 1560 °C, take samples for analysis, and adjust Ni and Mo to the lower limit according to the requirements of the steel grade; during the oxidation period, control the oxygen pressure at 0.6 - 0.8 MPa, blow oxygen for oxidation to remove carbon and scale; slag removal conditions: T ≥ 1650 °C, P ≤ 0.001%; add 300 kg of CaO and 80 kg of CaF2 as thin slag materials to push the slag and make it uniform; open the furnace body and add FeCr, requiring the oxygen pressure to be ≥ 0.8 MPa; pre-reduction: add 3 - 5 kg / t of Fe-Si powder to the molten steel until the slag turns color and becomes thinner; measure the temperature of the molten steel ≥ 1630 °C, and the slag removal ends; add 0.5 kg / t of Al wire, supplement 500 kg of CaO and 100 kg of CaF2 as thin slag materials, push the slag evenly and add 3 - 5 kg / t of C-Si powder and Al powder each for reduction to make white slag; composition: adjust the contents of C, Mn, Cr, Mo, and Ni according to the process requirements; tapping: the molten steel ≥ 1620 °C, the slag condition is good, the alloy addition time is ≥ 8 min, add 1 kg / t of Al and 1 kg / t of CaSi blocks to the ladle;

[0024] Promote the steel slag reaction, promote dephosphorization, and promote the formation of foamed slag through blowing oxygen for decarburization to obtain molten steel with low P, residual gases H, N, and inclusions. As oxidizing inclusions enter the steel, the oxygen level in the steel continuously increases and the reducibility weakens. By removing the old slag and replacing it with new slag, the balance is broken again to further improve the ability to reduce the oxygen, sulfur, and inclusion contents in the steel. Achieve the purpose of clean molten steel.

[0025] A2 Ladle furnace refining:

[0026] After the molten steel is smelted in the electric furnace, it is transferred into a ladle, hoisted to the refining furnace seat, and 500 kg of CaO, 80 kg of CaF2, and 3 - 5 kg of C - Si powder are added as slag materials. The slag system is reduced and adjusted. The total slag amount is controlled at 5% of the tapping amount. Measure the molten steel temperature ≥ 1570°C. When the slag is white, take samples for analysis. Adjust the composition: according to the internal control requirements, the alloying time ≥ 8 min. When the molten steel temperature is 1600 - 1620°C and in the white slag state, hoist the ladle to the slag - draining platform, remove all the furnace slag and replace it with new slag, and conduct secondary refining in the ladle refining furnace. The molten steel temperature ≥ 1650°C, the white slag time ≥ 20 min, add 2 - 3 kg / t of fire bricks to adjust the fluidity of the furnace slag, add AL wire, and do not add Ca wire.

[0027] 1) Generally, the basic slag of the refining slag mostly selects the slag system at the low - melting - point position of the CaO - SiO2 - Al2O3 ternary phase diagram. The most important function of the basic slag is to control the basicity of the slag, and the basicity of the slag has a great influence on both deoxidation and desulfurization during the refining process. Increasing the basicity can reduce the equilibrium oxygen in the steel. The refining slag can be refined using white slag (without adding deoxidizer) to achieve the purpose of reducing the oxygen, sulfur, and inclusion content in the steel.

[0028] 2) When the content of (FeO + MnO) in the slag is low, the reducibility of the slag is stronger, and at this time, the color is white.

[0029] 3) The sulfur partition ratio is inversely proportional to (FeO + MnO) in the refining slag, that is, as the content of (FeO + MnO) increases, the sulfur partition ratio decreases, which is not conducive to desulfurization of the furnace slag.

[0030] 4) As oxidizing inclusions enter the steel, the oxygen level in the steel continuously increases and the reducibility weakens. By removing the old slag and replacing it with new slag, the balance is re - broken, and the ability to further reduce the oxygen, sulfur, and inclusion content in the steel is improved, achieving the purpose of clean molten steel.

[0031] A3 vacuum refining furnace refining:

[0032] Two - stage vacuum refining furnace refining is adopted. The vacuum pump pumps step by step, the vacuum degree reaches 67 Pa, and the vacuum - holding time ≥ 15 min to ensure that H ≤ 1.2 ppm, and the samples taken for analysis meet the composition requirements.

[0033] Further, in step S2, the starting melting rate value in the steady - state stage of electroslag smelting is 7 - 8 kg / min, the ending melting rate value is 6 - 7 kg / min, an electroslag ingot is obtained, and then the electric furnace is powered off and cooled for 100 - 150 min.

[0034] Specifically, in step S3, the specific heating process of the electroslag ingot at 1230 - 1270°C is as follows: First, heat the electroslag ingot to 400 - 450°C and keep it warm for 5 h; then heat it to 800 - 850°C and keep it warm for 8 h; then heat it to 1230 - 1270°C, and the holding time is 20 - 40 h.

[0035] Specifically, in the first step of step S5, the temperature is raised to 640 - 660 °C at a rate of 40 - 80 °C / h and held for 5 - 10 h, and then the temperature is raised to 980 - 1020 °C at a rate of 60 - 100 °C / h.

[0036] Specifically, in step S6, the specific steps for pre - hardening the workpiece are as follows:

[0037] B1 Quenching:

[0038] After the workpiece is subjected to flaw detection, the head and tail are sawed off, reheated to the quenching temperature of 980 - 1000 °C, held for 2D h, and then cooled by the method of water + quenching liquid cooling. Finally, after the core temperature of the workpiece is cooled to ≤250 °C, it is taken out of the quenching liquid and air - cooled.

[0039] B2 Tempering:

[0040] The workpiece is placed in a tempering furnace at 200 - 300 °C and held for 5 - 10 h, then the temperature is raised to the tempering temperature of 450 - 550 °C and held for 4D h, and then cooled in the furnace to below 250 °C and taken out of the furnace and air - cooled to room temperature.

[0041] B3 Secondary Tempering:

[0042] The workpiece is placed in a tempering furnace at 200 - 300 °C and held for 5 - 10 h, then the temperature is raised to the tempering temperature of 500 - 580 °C and held for 4D h, and then cooled in the furnace to below 250 °C and taken out of the furnace and air - cooled to room temperature.

[0043] Furthermore, in steps B2 and B3 of step S6, the workpiece is cooled in the furnace at a rate of 15 - 30 °C / h to below 250 °C and taken out of the furnace and air - cooled to room temperature.

[0044] The preparation method of a pre - hardened high - toughness and high - polishability stainless die steel of the present invention has the following advantages compared with the prior art:

[0045] (1) By carefully selecting scrap steel and alloy materials, foreign impurities and harmful elements are significantly reduced;

[0046] (2) At the end of the oxidation period of electric furnace smelting, in order to overcome the oxidation state in the furnace and prevent harmful impurities in the slag from being reduced and re - entering the molten steel, all the oxidation slag is removed.

[0047] (3) During the LF refining process, by removing the refining slag and remaking new slag, the balance state between the molten steel and the steel slag is broken to better remove impurities and harmful elements in the molten steel.

[0048] (4) Through two - stage VD vacuum pumping + argon soft stirring, hydrogen in the steel is removed to the greatest extent. At the same time, it is beneficial for non - metallic inclusions in the steel grade to float to the slag - metal interface for removal.

[0049] (5) Ultra-high temperature and long-time homogenization treatment can greatly dissolve the liquid segregation carbides in steel and reduce banded segregation at the same time;

[0050] (6) The forging-after water cooling scheme can inhibit the formation of network carbides after forging;

[0051] (7) Adopting the double-medium cooling scheme of water + quenching liquid can not only ensure rapid cooling in the high-temperature stage to inhibit the generation of network carbides, but also avoid cracking with a slightly slower cooling rate in the low-temperature stage. Description of the Drawings

[0052] Figure 1 The microscopic structure morphology of the die steel obtained in Example 1;

[0053] Figure 2 The microscopic structure morphology of the die steel obtained in Example 2;

[0054] Figure 3 The microscopic structure morphology of the die steel obtained in Example 3. Detailed Implementation Modes

[0055] The following further describes the present invention in conjunction with the drawings and specific embodiments.

[0056] Example 1

[0057] A pre-hardened high-polish plastic mold steel, by mass fraction, is basically composed of the elements in Table 1 as follows:

[0058] Table 1

[0059]

[0060] A preparation method of a pre-hardened high-toughness and high-polish stainless mold steel, characterized in that the main steps are as follows:

[0061] S1. Electrode blank smelting:

[0062] According to the component content of the plastic mold steel, scrap steel and alloy materials are proportioned, wherein the heavy materials ≥ 30%, the medium and light materials ≤ 70%, the carbon content ≥ 0.70%, and there is less rust and less oil stain, and high-purity scrap steel is then smelted by electric arc furnace smelting + ladle furnace refining + vacuum furnace refining, and then cast by die casting to obtain a die-cast electrode blank;

[0063] S2. Electroslag smelting:

[0064] Using a protective atmosphere electroslag furnace, the die-cast electrode blank prepared in step S1 is subjected to electroslag remelting under argon protection to obtain an electroslag ingot;

[0065] S3. High-temperature homogenization:

[0066] Subject the electroslag ingot to high-temperature diffusion at 1260 °C for 30 h of holding time;

[0067] S4. Multi-directional forging:

[0068] Upset and draw the electroslag ingot after high-temperature homogenization treatment in three directions of X, Y, and Z, and finally draw it along the Z direction to the finished product, where X and Y are two mutually perpendicular diameter directions, and Z is the axis direction of the original electroslag ingot. The forging start temperature is 1160 °C, and the forging end temperature is 820 °C;

[0069] S5. Post-forging heat treatment: The post-forging heat treatment is divided into two steps.

[0070] First step: After multi-directional forging, cool the workpiece by water until the surface temperature of the forging is 480 °C, then load it into an annealing furnace with a furnace temperature of 450 °C, hold for 6 h, raise the temperature to 650 °C and hold for 10 h, raise the temperature to 1020 °C, and the holding time is 10 h. After the holding is completed, lift it off the trolley and cool it by air mist to 320 °C;

[0071] Second step: Load the cooled workpiece into an annealing furnace at 300 °C again and hold for 6 h, raise the temperature to 860 °C and hold for 10 h, then cool the workpiece in the furnace to 740 °C, the holding time is 20 h, and then cool it in the furnace to below 300 °C and take it out for air cooling;

[0072] S6. Pre-hardening treatment;

[0073] Subject the workpiece treated in step S5 to pre-hardening treatment.

[0074] Among them, in step S1, the specific steps of electric arc furnace smelting + ladle furnace refining + vacuum furnace refining are as follows:

[0075] A1 Electric furnace smelting:

[0076] During the melting period, the oxygen pressure is controlled at 0.5 MPa, T is 1600 °C, sampling and analysis are carried out, and Ni and Mo are added to the lower limit according to the requirements of the steel grade; during the oxidation period, the oxygen pressure is controlled at 0.7 MPa, and oxygen blowing is used for decarburization and scale removal; slag removal conditions: T is 1680 °C, P is 0.001%; 300 kg of CaO and 80 kg of CaF2 are added as lean slag materials to push the slag and make it uniform; when the furnace body is taken out, FeCr is added, and the oxygen pressure is required to be ≥0.8 MPa; pre-reduction: 5 kg / t of Fe-Si powder is added to the molten steel until the slag turns color and becomes thinner; the temperature of the molten steel is measured at 1650 °C, and the slag removal is completed; 0.5 kg / t of Al wire is added, 500 kg of CaO and 100 kg of CaF2 are added as supplementary lean slag materials, and C-Si powder and Al powder are added at 4 kg / t each to push the slag evenly and reduce to make a white slag; Composition: Adjust the contents of C, Mn, Cr, Mo, and Ni according to the process requirements; Tapping: The molten steel is at 1650 °C, the slag condition is good, the alloy addition time is 10 min, 1 kg / t of Al and 1 kg / t of CaSi blocks are added to the ladle.

[0077] Refining in A2 ladle furnace:

[0078] After the molten steel is smelted in the electric furnace, it is transferred to the ladle, hoisted to the refining furnace seat, and 500 kg of CaO, 80 kg of CaF2, and 5 kg of C-Si powder are added as slag materials to reduce and adjust the slag system. The total slag amount is controlled at 5% of the tapping amount; the temperature of the molten steel is measured at 1600 °C, the slag is white, and sampling and analysis are carried out; Adjust the composition: According to the internal control requirements, the alloying time is 12 min; the molten steel is at 1620 °C, and the ladle is hoisted out to the slag removal platform in the white slag state. All the slag is removed and replaced with new slag for secondary refining in the ladle furnace. The temperature of the molten steel is 1680 °C, the white slag time is 25 min, 2.3 kg / t of fire bricks are added to adjust the fluidity of the slag, and Al wire is added, and Ca wire is not added.

[0079] Refining in A3 vacuum refining furnace:

[0080] Two-stage vacuum refining furnace refining is adopted. The vacuum pump is pumped step by step, the vacuum degree reaches 67 Pa, and the vacuum holding time is ≥15 min to ensure that H≤1.5 ppm and N≤120 ppm, and sampling and analysis meet the composition requirements.

[0081] Among them, in step S2, the starting melting rate value in the steady state stage of electroslag smelting is 7.5 kg / min, the ending melting rate value is 6.5 kg / min, an electroslag ingot is obtained, and then the electric furnace is shut down and cooled for 120 min.

[0082] Among them, in step S3, the specific heating process of the electroslag ingot at 1260 °C is as follows: First, the electroslag ingot is heated to 450 °C and held for 5 h; then it is heated to 850 °C and held for 8 h; then it is heated to 1260 °C, and the holding time is 30 h.

[0083] Among them, in the first step of step S5, it is heated to 650 °C at a rate of 60 °C / h and held for 10 h, and then heated to 1020 °C at a rate of 80 °C / h.

[0084] Among them, in step S6, the specific steps for pre-hardening the workpiece are as follows:

[0085] B1 Quenching:

[0086] After the workpiece is subjected to flaw detection, the head and tail are sawed off, reheated to the quenching temperature of 1000 °C, held for 10 h, and then cooled by water quenching + quenching liquid cooling. Finally, it is taken out of the quenching liquid and air-cooled after the surface temperature of the workpiece drops to 200 °C;

[0087] B2 Tempering:

[0088] The workpiece is placed in a tempering furnace at 250 °C and held for 10 h, then heated to the tempering temperature of 500 °C and held for 4D h, and then cooled in the furnace to below 250 °C and taken out for air-cooling to room temperature;

[0089] B3 Secondary Tempering:

[0090] The workpiece is placed in a tempering furnace at 250 °C again and held for 5 - 10 h, then heated to the tempering temperature of 540 °C and held for 4D h, and then cooled in the furnace to below 250 °C and taken out for air-cooling to room temperature.

[0091] Among them, in steps B2 and B3 of step S6, the workpiece is cooled in the furnace at a rate of 20 °C / h to below 250 °C and taken out for air-cooling to room temperature.

[0092] Example 2

[0093] A pre-hardened high-polish plastic mold steel, by mass fraction, is basically composed of the elements shown in Table 2:

[0094] Table 2

[0095]

[0096] A preparation method of a pre-hardened high-toughness and high-polish stainless mold steel, characterized in that the main steps are as follows:

[0097] S1. Electrode blank smelting:

[0098] According to the component content of the plastic mold steel, scrap steel and alloy materials are proportioned, where heavy materials ≥ 30%, medium and light materials ≤ 70%, carbon content ≥ 0.70%, and less rust and less oil and grease, high-purity scrap steel, and then smelted by electric arc furnace smelting + ladle furnace refining + vacuum refining furnace refining, and then cast by die casting to obtain a die-cast electrode blank;

[0099] S2. Electroslag smelting:

[0100] Using a protective atmosphere electroslag furnace, subject the ingot electrode blank prepared in step S1 to electroslag remelting under argon protection to obtain an electroslag ingot;

[0101] S3. High-temperature homogenization:

[0102] Perform high-temperature diffusion on the electroslag ingot at 1260 °C for a holding time of 30 h;

[0103] S4. Multi-directional forging:

[0104] Perform upsetting and drawing in the X, Y, and Z directions on the electroslag ingot after high-temperature homogenization treatment, and finally draw it to the finished product along the Z direction, where X and Y are two mutually perpendicular diameter directions, and Z is the axial direction of the original electroslag ingot. The forging start temperature is 1160 °C, and the forging end temperature is 820 °C;

[0105] S5. Post-forging heat treatment: The post-forging heat treatment is divided into two steps.

[0106] First step: After multi-directional forging is completed, cool the workpiece by water cooling until the surface temperature of the forging is 480 °C, then load it into an annealing furnace with a furnace temperature of 450 °C, hold for 6 h, raise the temperature to 650 °C and hold for 10 h, raise the temperature to 1020 °C, with a holding time of 10 h. After the holding is completed, lift it off the trolley and cool it by air mist to 320 °C;

[0107] Second step: Re-load the cooled workpiece into an annealing furnace at 300 °C and hold for 6 h, raise the temperature to 860 °C and hold for 10 h, then cool the workpiece in the furnace to 740 °C, with a holding time of 20 h, and then cool it in the furnace to below 300 °C and take it out for air cooling;

[0108] S6. Pre-hardening treatment;

[0109] Perform pre-hardening treatment on the workpiece processed in step S5.

[0110] Among them, in step S1, the specific steps of electric arc furnace smelting + ladle furnace refining + vacuum furnace refining are as follows:

[0111] A1 Electric furnace smelting:

[0112] During the melting period, the oxygen pressure is controlled at 0.5 MPa, the temperature T is 1600 °C, sampling and analysis are carried out, and Ni and Mo are added to the lower limit according to the requirements of the steel grade; during the oxidation period, the oxygen pressure is controlled at 0.7 MPa, and oxygen blowing is used for decarburization and scale removal; slag removal conditions: T 1680 °C, P 0.001%; add 300 kg of CaO and 80 kg of CaF2 as lean slag materials to push the slag and make it uniform; when the furnace body is taken out, add FeCr, and the required oxygen pressure is ≥0.8 MPa; pre-reduction: add 5 kg / t of Fe-Si powder to the molten steel until the slag turns color and becomes thinner; measure the temperature of the molten steel at 1650 °C, and the slag removal is completed; add 0.5 kg / t of Al wire, add 500 kg of CaO and 100 kg of CaF2 as additional lean slag materials, push the slag evenly, and add 4 kg / t of C-Si powder and Al powder each to reduce and make a white slag; composition: adjust the contents of C, Mn, Cr, Mo, and Ni according to the process requirements; tapping: the molten steel is at 1650 °C, the slag condition is good, the alloy addition time is 10 min, add 1 kg / t of Al and 1 kg / t of CaSi block to the ladle;

[0113] Refining in an A2 ladle furnace:

[0114] After the molten steel is smelted in the electric furnace, it is transferred to the ladle, hoisted to the refining furnace seat, and 500 kg of CaO, 80 kg of CaF2, and 5 kg of C-Si powder are added as slag materials to reduce and adjust the slag system. The total slag amount is controlled at 5% of the tapping amount; measure the temperature of the molten steel at 1600 °C, the slag is white, and sampling and analysis are carried out; adjust the composition: according to the internal control requirements, the alloying time is 12 min; when the temperature of the molten steel is 1620 °C and in the white slag state, the ladle is hoisted out to the slag removal platform, the furnace slag is removed and all new slag is replaced, and secondary refining in the ladle furnace is carried out. The temperature of the molten steel is 1680 °C, the white slag time is 25 min, 2.3 kg / t of fire bricks are added to adjust the fluidity of the furnace slag, and Al wire is added, and Ca wire is not added;

[0115] Refining in an A3 vacuum refining furnace:

[0116] Two-stage vacuum refining furnace refining is adopted. The vacuum pump is pumped step by step, the vacuum degree reaches 67 Pa, and the vacuum holding time is ≥15 min to ensure that H≤1.5 ppm and N≤120 ppm, and sampling and analysis meet the composition requirements.

[0117] Among them, in step S2, the starting melting rate value at the steady state stage of electroslag smelting is 7.5 kg / min, the ending melting rate value is 6.5 kg / min, an electroslag ingot is obtained, and then the electric furnace is turned off and cooled for 120 min.

[0118] Among them, in step S3, the specific heating process of the electroslag ingot at 1260 °C is as follows: first, the electroslag ingot is heated to 450 °C and held for 5 h; then it is heated to 850 °C and held for 8 h; then it is heated to 1260 °C and the holding time is 30 h.

[0119] Among them, in the first step of step S5, it is heated to 650°C at a rate of 60°C / h and held for 10 h, and then heated to 1020°C at a rate of 80°C / h.

[0120] Among them, in step S6, the specific steps for pre-hardening the workpiece are as follows:

[0121] B1 Quenching:

[0122] After the workpiece is subjected to flaw detection, the head and tail are sawed off, reheated to the quenching temperature of 1000°C, held for 10 h, and then cooled by water quenching + quenching liquid cooling, and finally cooled to an air-cooling state after the surface temperature of the workpiece reaches 200°C and taken out of the quenching liquid;

[0123] B2 Tempering:

[0124] The workpiece is placed in a tempering furnace at 250°C and held for 10 h, then heated to the tempering temperature of 500°C, held for 4D h, and then cooled in the furnace to below 250°C and taken out of the furnace for air-cooling to room temperature;

[0125] B3 Secondary Tempering:

[0126] The workpiece is placed in a tempering furnace at 250°C again and held for 5 - 10 h, then heated to the tempering temperature of 540°C and held for 4D h, and then cooled in the furnace to below 250°C and taken out of the furnace for air-cooling to room temperature.

[0127] Among them, in steps B2 and B3 of step S6, the workpiece is cooled in the furnace at a rate of 20°C / h to below 250°C and taken out of the furnace for air-cooling to room temperature.

[0128] Example 3

[0129] A pre-hardened high-polish plastic mold steel, by mass fraction, is basically composed of the elements shown in Table 3:

[0130] Table 3

[0131]

[0132] A preparation method of a pre-hardened high-toughness and high-polish stainless mold steel, characterized in that the main steps are as follows:

[0133] S1. Electrode blank smelting:

[0134] According to the component content of the plastic mold steel, scrap steel and alloy materials are proportioned, where heavy materials ≥ 30%, medium and light materials ≤ 70%, carbon addition ≥ 0.70%, and there is less rust and oil stain, and high-purity scrap steel is then smelted by electric arc furnace smelting + ladle furnace refining + vacuum furnace refining, and then cast by die casting to obtain a die-cast electrode blank;

[0135] S2. Electroslag smelting:

[0136] Using a protective atmosphere electroslag furnace, subject the ingot electrode blank prepared in step S1 to electroslag remelting under argon protection to obtain an electroslag ingot;

[0137] S3. High-temperature homogenization:

[0138] Subject the electroslag ingot to high-temperature diffusion at 1260 °C for a holding time of 30 h;

[0139] S4. Multi-directional forging:

[0140] Perform upsetting and drawing in the X, Y, and Z directions on the electroslag ingot after high-temperature homogenization treatment, and finally draw it in the Z direction to the finished product, where X and Y are two mutually perpendicular diameter directions, and Z is the axial direction of the original electroslag ingot. The forging start temperature is 1160 °C, and the forging end temperature is 820 °C;

[0141] S5. Post-forging heat treatment: The post-forging heat treatment is divided into two steps,

[0142] First step: After multi-directional forging, cool the workpiece in water until the surface temperature of the forging is 480 °C, then load it into an annealing furnace with a furnace temperature of 450 °C, hold for 6 h, raise the temperature to 650 °C and hold for 10 h, raise the temperature to 1020 °C, and the holding time is 10 h. After the holding is completed, lift it off the trolley and air-cool it with fog to 320 °C;

[0143] Second step: Load the cooled workpiece into an annealing furnace at 300 °C again and hold for 6 h, raise the temperature to 860 °C and hold for 10 h, then cool the workpiece in the furnace to 740 °C, the holding time is 20 h, and then cool it in the furnace to below 300 °C and take it out for air cooling;

[0144] S6. Pre-hardening treatment;

[0145] Perform pre-hardening treatment on the workpiece after step S5.

[0146] Among them, in step S1, the specific steps of electric arc furnace smelting + ladle furnace refining + vacuum refining furnace refining are as follows:

[0147] A1 Electric furnace smelting:

[0148] During the melting period, the oxygen pressure is controlled at 0.5 MPa, the temperature T is 1600 °C, sampling and analysis are carried out, and Ni and Mo are added to the lower limit according to the requirements of the steel grade; during the oxidation period, the oxygen pressure is controlled at 0.7 MPa, and oxygen blowing is used for decarburization and scale removal; slag removal conditions: T 1680 °C, P 0.001%; add 300 kg of lean slag material CaO and 80 kg of CaF2 to push the slag and make it uniform; when the furnace body is taken out, add FeCr, and the required oxygen pressure is ≥0.8 MPa; pre-reduction: add 5 kg / t of Fe-Si powder to the molten steel until the slag turns color and becomes thinner; measure the temperature of the molten steel at 1650 °C, and the slag removal is completed; add 0.5 kg / t of Al wire, add 500 kg of CaO and 100 kg of CaF2 as lean slag material, push the slag evenly, and add 4 kg / t of C-Si powder and Al powder each to reduce and make white slag; composition: adjust the contents of C, Mn, Cr, Mo, and Ni according to the process requirements; tapping: the molten steel is at 1650 °C, the slag condition is good, the alloy addition time is 10 min, add 1 kg / t of Al and 1 kg / t of CaSi block to the ladle;

[0149] Refining in A2 ladle furnace:

[0150] After the molten steel is smelted in the electric furnace, it is transferred to the ladle, hoisted to the refining furnace seat, and 500 kg of CaO, 80 kg of CaF2, and 5 kg of C-Si powder are added as slag materials to reduce and adjust the slag system. The total slag amount is controlled at 5% of the tapping amount; measure the temperature of the molten steel at 1600 °C, the slag is white, and sampling and analysis are carried out; adjust the composition: according to the internal control requirements, the alloying time is 12 min; when the molten steel temperature is 1620 °C and in the white slag state, the ladle is hoisted out to the slag removal platform, the furnace slag is removed and all new slag is replaced, and secondary refining in the ladle furnace is carried out. The molten steel temperature is 1680 °C, the white slag time is 25 min, 2.3 kg / t of fire bricks are added to adjust the fluidity of the furnace slag, and Al wire is added, and Ca wire is not added;

[0151] Refining in A3 vacuum refining furnace:

[0152] Two-stage vacuum refining furnace refining is adopted. The vacuum pump is pumped step by step, the vacuum degree reaches 67 Pa, and the vacuum holding time is ≥15 min to ensure that H≤1.5 ppm and N≤120 ppm, and sampling and analysis meet the composition requirements.

[0153] Among them, in step S2, the starting melting rate value in the steady state stage of electroslag smelting is 7.5 kg / min, the ending melting rate value is 6.5 kg / min, an electroslag ingot is obtained, and then the electric furnace is powered off and cooled for 120 min.

[0154] Among them, in step S3, the specific heating process of the electroslag ingot at 1260 °C is as follows: first, the electroslag ingot is heated to 450 °C and held for 5 h; then it is heated to 850 °C and held for 8 h; then it is heated to 1260 °C and the holding time is 30 h.

[0155] Among them, in the first step of step S5, the temperature is raised to 650°C at a rate of 60°C / h and held for 10 h, and then the temperature is raised to 1020°C at a rate of 80°C / h.

[0156] Among them, in step S6, the specific steps for pre-hardening the workpiece are as follows:

[0157] B1 Quenching:

[0158] After the workpiece is subjected to flaw detection, the head and tail are sawed off, reheated to the quenching temperature of 1000°C, held for 10 h, and then cooled by water quenching + quenching liquid cooling. Finally, it is taken out of the quenching liquid and air-cooled after the surface temperature of the workpiece drops to 200°C.

[0159] B2 Tempering:

[0160] The workpiece is placed in a tempering furnace at 250°C and held for 10 h, then the temperature is raised to the tempering temperature of 500°C and held for 4D h, and then cooled in the furnace to below 250°C and taken out and air-cooled to room temperature.

[0161] B3 Secondary Tempering:

[0162] The workpiece is again placed in a tempering furnace at 250°C and held for 5 - 10 h, then the temperature is raised to the tempering temperature of 540°C and held for 4D h, and then cooled in the furnace to below 250°C and taken out and air-cooled to room temperature.

[0163] Among them, in steps B2 and B3 of step S6, the workpiece is cooled in the furnace at a rate of 20°C / h to below 250°C and taken out and air-cooled to room temperature.

[0164] In order to verify the mechanical properties of the hot work die steel provided by the preparation method of the present invention, the inventor took 4 specimens of the die steel obtained in Examples 1 to 3 respectively, and carried out electron microscope scanning and performance testing. The test data are shown in Tables 4 and 5:

[0165] Table 4

[0166]

[0167] Table 5

[0168]

[0169] Thus, it can be seen that 1, as Figures 1 to 3 shown, the spheroidized annealing microstructure of the die material is uniform, and uniform granular pearlite is distributed on the ferrite matrix. Rated according to the North American Die Casting Association NADCA#229-2011 standard, it is at the 3-4 level;

[0170] 2, after the die material is pre-hardened and polished, the polishing scores are all above 90 at the 14000# polishing grade, belonging to the excellent level;

[0171] 3. From the hardness test data of Examples 1 - 3, it can be concluded that for die steel produced with the same process parameters under the same composition, the hardness is very uniform, and the hardness fluctuation range is about ±0.5 HRC.

[0172] 4. From the inclusion test data of Examples 1 - 3, it can be concluded that for die steel produced with the same process parameters under the same composition, the levels of various inclusions are all ≤ 0.5 grade.

[0173] Thus, it can be seen that the die steel prepared by the present preparation method has excellent tissue uniformity, hardness uniformity, high purity, and high polishing performance.

Claims

1. A preparation method of a pre-hardened high-toughness and high-polishability stainless die steel, characterized in that, The steps are as follows: S1. Electrode blank smelting: Charge scrap steel and alloy materials according to the component content of plastic mold steel, where heavy materials ≥ 30%, medium and light materials ≤ 70%, carbon addition ≥ 0.70%, and less rust and less oil and grease, high-purity scrap steel. Then, smelt using an electric arc furnace + ladle refining furnace + vacuum refining furnace, and then cast using the ingot casting method to obtain an ingot-cast electrode blank; Among them, the component content of plastic mold steel is by mass fraction, C 0.18 - 0.28%, Si 0.80 - 1.20%, Mn 0.40 - 0.80%, Cr 12.0 - 14.0%, Mo 0.20 - 0.60%, V 0.20 - 0.60%, Ni 0.30 - 0.80%, P ≤ 0.010%, S ≤ 0.002%, residual gas [H] ≤ 1.0 ppm, [O] ≤ 10 ppm, and the rest is Fe; In step S1, the specific steps of electric arc furnace smelting + ladle refining furnace refining + vacuum refining furnace refining are as follows: A1 Electric furnace smelting: During the melting period, control the oxygen pressure at 0.4 - 0.5 MPa, T ≥ 1560 °C, take samples for analysis, and add Ni and Mo to the lower limit according to the requirements of the steel grade; during the oxidation period, control the oxygen pressure at 0.6 - 0.8 MPa, blow oxygen for oxidation to remove carbon and scale; slag-dragging conditions: T ≥ 1650 °C, P ≤ 0.001%; add 300 kg of CaO and 80 kg of CaF2 as thin slag materials to push the slag and make it uniform; open the furnace body and add FeCr, requiring the oxygen pressure ≥ 0.8 MPa; pre-reduction: add 3 - 5 kg / t of Fe-Si powder to the molten steel until the slag turns color and becomes thinner; measure the molten steel temperature ≥ 1630 °C, and start full slag-dragging; after the slag-dragging is completed, add 0.5 kg / t of Al wire, supplement 500 kg of CaO and 100 kg of CaF2 as thin slag materials, push the slag evenly, and add 3 - 5 kg / t of C-Si powder and Al powder each to reduce and make a white slag; composition: adjust the contents of C, Mn, Cr, Mo, and Ni according to the process requirements; tapping: the molten steel ≥ 1620 °C, good slag condition, alloy addition time ≥ 8 min, add 1 kg / t of Al and 1 kg / t of CaSi block to the ladle; A2 Ladle refining furnace refining: After the electric furnace smelting, the molten steel is transferred to the ladle, lifted to the refining furnace seat, and add 500 kg of CaO, 80 kg of CaF2, and 3 - 5 kg of C-Si powder, reduce and adjust the slag system, and control the total slag amount at 5% of the tapping amount; measure the molten steel temperature ≥ 1570 °C, the slag is white, take samples for analysis; adjust the composition: according to the internal control requirements, the alloying time ≥ 8 min; when the molten steel temperature is 1600 - 1620 °C and in the white slag state, lift the ladle to the slag-dragging platform, remove all the furnace slag and replace it with new slag, and conduct secondary ladle refining furnace refining, the molten steel temperature ≥ 1650 °C, the white slag time ≥ 20 min, add 2 - 3 kg / t of fire bricks to adjust the fluidity of the furnace slag, add AL wire, and do not add Ca wire; A3 Vacuum refining furnace refining: Adopt two-stage vacuum refining furnace refining, the vacuum pump pumps step by step, the vacuum degree reaches 67 Pa, the vacuum holding time ≥ 15 min, ensure that H ≤ 1.2 ppm, and take samples for analysis to meet the composition requirements; S2. Electroslag Smelting: Using a protective atmosphere electroslag furnace, subject the ingot electrode blank prepared in step S1 to electroslag remelting under argon protection to obtain an electroslag ingot. S3. High-temperature Homogenization: Perform high-temperature diffusion on the electroslag ingot at 1230 - 1270 °C for a holding time of 20 - 40 h. S4. Multi-directional Forging: Perform upsetting and drawing in the X, Y, and Z directions on the electroslag ingot after high-temperature homogenization treatment, and finally draw it in the Z direction to the finished product, where X and Y are two mutually perpendicular diameter directions, and Z is the axial direction of the original electroslag ingot. The forging start temperature is 1150 - 1200 °C, and the final forging temperature is ≥800 °C. S5. Post-forging Heat Treatment: The post-forging heat treatment is divided into two steps. First step: After multi-directional forging, cool the workpiece in water until the surface temperature of the forging is 450 - 500 °C, then load it into an annealing furnace with a furnace temperature of 400 - 450 °C, hold for 2 - 8 h, raise the temperature to 640 - 660 °C and hold for 5 - 10 h, raise the temperature to 980 - 1020 °C, and the holding time is 2D h, where D is the maximum thickness of the workpiece in the furnace, with the unit of dm. After the holding is completed, lift it off the trolley and cool it with air mist to 250 - 350 °C. Second step: Re-load the cooled workpiece into an annealing furnace at 280 - 320 °C and hold for 5 - 10 h, raise the temperature to 840 - 880 °C and hold for 2D - 3D h, then cool the workpiece in the furnace to 720 - 760 °C, hold for 5D - 7D h, and then cool it in the furnace to below 250 - 300 °C and take it out of the furnace for air cooling. D is the maximum thickness of the workpiece in the furnace, with the unit of dm. S6 Pre-hardening Treatment; Subject the workpiece treated in step S5 to pre-hardening treatment. In step S6, the specific steps for pre-hardening treatment of the workpiece are as follows: B1 Quenching: After performing flaw detection on the workpiece, saw off the head and tail, reheat it to the quenching temperature of 980 - 1000 °C, hold for 2D h, and then cool it by the method of water quenching + quenching liquid cooling. Finally, cool it until the core temperature of the workpiece is ≤250 °C and then take it out of the quenching liquid for air cooling. B2 Tempering: Put the workpiece into a tempering furnace at 200 - 300 °C and hold for 5 - 10 h, then raise the temperature to the tempering temperature of 450 - 550 °C and hold for 4D h, and then cool it in the furnace to below 250 °C and take it out of the furnace for air cooling to room temperature. B3 Secondary Tempering: Put the workpiece into a tempering furnace at 200 - 300 °C and hold for 5 - 10 h, then raise the temperature to the tempering temperature of 500 - 580 °C and hold for 4D h, and then cool it in the furnace to below 250 °C and take it out of the furnace for air cooling to room temperature.

2. The preparation method of a pre-hardened high-toughness and high-polishability stainless die steel according to claim 1, characterized in that, In step S2, the starting melting rate value at the steady state stage of electroslag smelting is 7 - 8 kg / min, the ending melting rate value is 6 - 7 kg / min, obtain the electroslag ingot, and then cut off the power and cool it in the furnace for 100 - 150 min.

3. The preparation method of a pre-hardened high-toughness and high-polishability stainless die steel according to claim 1, characterized in that, In step S3, the specific heating process of the electroslag ingot at 1230 - 1270 °C is as follows: First, heat the electroslag ingot to 400 - 450 °C and hold for 5 h; then heat it to 800 - 850 °C and hold for 8 h; then heat it to 1230 - 1270 °C and the holding time is 20 - 30 h.

4. The preparation method of a pre-hardened high-toughness and high-polishability stainless die steel according to claim 1, characterized in that, In the first step of step S5, the temperature is raised to 640 - 660 °C at a rate of 40 - 80 °C / h and held for 5 - 10 h, and then the temperature is raised to 980 - 1020 °C at a rate of 60 - 100 °C / h.

5. The preparation method of a pre-hardened high-toughness and high-polishability stainless die steel according to claim 1, characterized in that, In steps B2 and B3 of step S6, the workpiece is cooled with the furnace at a rate of 15 - 30 °C / h to below 250 °C and then air-cooled to room temperature out of the furnace.

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