A high hot hardness hot extrusion die steel and its preparation method

Abstract

The present invention relates to a high hot hardness hot extrusion die steel and a preparation method thereof, belonging to the technical field of alloy steel manufacturing. Its component formula includes elements in the following weight percentages: C: 0.17% - 0.24%; Si: 0.10% - 0.45%; Mn: 0.20% - 0.45%; Cr: 7.00% - 9.50%; W: 0.80% - 1.25%; V: 0.20% - 0.40%; B: 0.004% - 0.009%, the balance is iron, as well as impurity elements S, P, N, O, H. The preparation method includes: electric furnace melting, LF furnace refining, VOD degassing in a vacuum furnace, electroslag remelting, high-temperature homogenization, upsetting, drawing, quenching, tempering and isothermal spheroidizing annealing. The spheroidized structure of the obtained high hot hardness hot extrusion die steel is above AS4 level, and the quenched and tempered hardness is 48 - 52 HRC, having excellent strength and toughness, wear resistance, hot strength and thermal fatigue performance.

Description

Technical Field

[0001] The present invention relates to a high red-hardness hot extrusion die steel and a preparation method thereof, belonging to the technical field of alloy steel manufacturing. Background Art

[0002] AISI H13 steel is one of the most widely used die steels in the current die industry. However, its high-temperature friction and wear performance, high-temperature hot strength, thermal stability, thermal fatigue performance, and thermal conductivity still cannot meet the harsh conditions of current high-strength steel hot stamping, and it gradually cannot meet the needs of the die industry. Related alternative products such as high-strength and tough 1.2367, 8418 / DIEVAR, and 3Cr2W8V are suitable for die-casting dies, QRO90, HTCS-130, and DHA-THERMO are used for hot stamping dies, and 5CrNiMo is used for die-casting and hot forging dies. However, each has its disadvantages. For example, QRO90, 1.2367, 5CrNiMo, and 3Cr2W8V have insufficient toughness, 8418 / DIEVAR has insufficient hot strength, and although HTCS-130 has a very high thermal conductivity, its hot strength is very low.

[0003] To comprehensively improve the die life, the general idea in the industry is to optimize the alloy composition and assist with relatively harsh production equipment and process conditions. However, two problems still have not been solved: (1) A suitable substitute for the expensive Mo element has not been found because Mo plays a role in improving the high-temperature hardness in H13 series steels; (2) The optimization of the composition generally requires more stringent production equipment conditions, making it impossible for special steel enterprises with medium and low production capacities to produce similar products.

[0004] The general alloying idea is to reduce the chromium content to improve the thermal conductivity and increase the manganese content to improve the wear resistance. However, chromium is the core element of hot-work die steel, and reducing chromium will make it difficult to ensure the oxidation resistance and corrosion resistance; currently, hot-work die steel still mainly uses free forging of large modules, assisted by super-refinement processes. Only by combining the two can a super-refined precipitation structure be obtained. However, the spheroidization time of the super-refinement process is too long, which increases the equipment load and seriously affects the product delivery schedule.

[0005] Existing solutions are as follows:

[0006] (1) The patent document with publication number CN114855071A discloses an H13 alloy die steel and a preparation method thereof. In this invention, the Mo content is significantly reduced to ~0.2 wt.%, the Cr content is reduced to 1.35 wt.%, and a small amount of Al element is added. Its measured impact energy is very high because the Cr carbides are significantly reduced, so the macrostructure is also good. However, this steel grade will inevitably have insufficient oxidation resistance at high temperatures, and its high strength and hardness will also rapidly decrease.

[0007] (2) The patent document with publication number CN109136765A proposed a composition of hot work die steel and also disclosed a preparation method of hot work die steel, including batching, smelting, casting, electroslag remelting; high-temperature diffusion annealing, multi-directional forging hot working; preliminary heat treatment; final heat treatment. The prepared steel has advantages such as high thermal stability, thermal strength and good toughness, meeting the high-temperature performance requirements of current die manufacturing for its materials. The special feature of this invention is to increase the Mo content to 3.2%, improving the thermal strength and thermal conductivity, but high Mo is very undesirable in the current market environment.

[0008] (3) The patent document with publication number CN109280849A proposed a composition and manufacturing process of high-performance hot work die steel, with low carbon C of 0.20 - 0.30%, reducing Cr to 3.10 - 4.00%, adding W of 0.50 - 1.00%, and reducing V to 0.10 - 0.30%. This invention adopts the alloying idea of reducing V and increasing W, which well improves the wear resistance and reduces the cost, but the Cr content is relatively low, resulting in not very good hot working performance of this steel grade.

[0009] (4) The patent document with publication number CN108265232A adopts three-pronged approaches of optimizing the raw material formula, optimizing the smelting process, and optimizing the heat treatment process, combining the high stability of H13, further improving the thermal fatigue resistance, tempering resistance, thermal strength and toughness, and significantly increasing the service life of the die. However, this invention did not disclose the alloy composition ratio, so it is difficult to judge the pros and cons of its performance.

[0010] (5) The patent document with publication number CN107974637A proposed an alloy composition formula of hot work die steel, using a high Mo content of 2.80% - 3.20% to improve the thermal strength and impact toughness of the die steel. This invention adopts the alloying idea of high carbon and high Mo, but still does not have a better V substitution plan, and the cost of the die steel is still relatively high.

[0011] (6) The patent document with publication number CN108220815A proposed a composition formula of hot work die steel with high thermal strength and high impact toughness for hot forging, using high C of 0.40 - 0.50%, low Cr of 3.00 - 3.80%, and adding rare earth elements of 0.002 - 0.008%. This invention purifies the grain boundaries by adding rare earth, and the impact toughness will be improved, but there is still no clear plan and steps for the addition method of rare earth.

[0012] (7) The patent document with the publication number CN107974632A proposes an austenitic hot work die steel composition formula and its preparation method. This steel makes full use of austenite-forming elements Mn and C to expand the austenite phase region to obtain a stable austenite structure; uses the directional solidification electroslag process to control the behavior of carbides and inclusions in the electroslag ingot; and uses an appropriate heat treatment process to control the grain size and the decomposition and precipitation behavior of carbides. Although the single austenite structure hot work die steel prepared by this invention can fully improve the heat resistance and hot strength of the die steel, it is not applicable to the heat treatment process of the commonly used H13 steel in the current market, which increases the difficulty of its popularization. Another problem is that the yield strength of the austenite structure is not as good as that of the martensite structure.

[0013] (8) The patent document with the publication number CN108070794A proposes a high wear-resistant hot work die steel composition formula and its preparation method. This steel adds 1.8 - 2.5% of nano tungsten carbide, reduces molybdenum to 0.8 - 1.0%, and adds 0.06 - 0.1% of cerium oxide, which fully improves the hot strength, grain size and tissue purity of the hot work die steel. Adding ceramic composite powder in this invention is beneficial to improving the wear resistance of the die steel, but the formula elements of this patent are too complex and diverse, increasing the production difficulty.

[0014] (9) The patent document with the publication number CN107557667A proposes a high-performance hot work die steel for large die casting molds and its manufacturing process. To realize the production of large-section die casting molds, it proposes a low C of 0.20% - 0.30%, adds W of 0.10% - 0.20%, and Nb of 0.02% - 0.04% to achieve the purpose of high toughness and high hot strength. By adding a small amount of W and Nb, the hot strength of the die steel is improved, but the content of W is relatively low, and the grain refinement effect of Nb is limited.

[0015] (10) The patent document with the publication number CN107699789A proposes a high-toughness, high-thermal-stability ZW866 hot work die steel for die casting. Its idea is to add a certain amount of Nb of 0.005% - 0.08% to improve the grain size and achieve the purpose of improving comprehensive performance. Adding a trace amount of Nb element increases the control difficulty of the high-temperature hot working process, otherwise it will be very difficult to achieve the grain refinement effect.

[0016] Therefore, it is necessary to improve the existing technology to obtain a high red hardness hot extrusion die steel, reduce costs and facilitate processing. Summary of the Invention

[0017] The purpose of the present invention is to provide a high red hardness hot extrusion die steel.

[0018] Another purpose of the present invention is to provide a preparation method for the above high red hardness hot extrusion die steel.

[0019] The present invention provides a high red hardness hot extrusion die steel, and its component formula includes elements in the following weight percentages: C: 0.17% - 0.24%; Si: 0.10% - 0.45%; Mn: 0.20% - 0.45%; Cr: 7.00% - 9.50%; W: 0.80% - 1.25%; V: 0.20% - 0.40%; B: 0.004% - 0.009%, the balance is iron, and trace residual elements S, P, N, O, H.

[0020] Among them, S, P, N, O, H are impurities with acceptable contents.

[0021] More preferably, the formula includes elements in the following weight percentages:

[0022] C: 0.18% - 0.21%; Si: 0.15% - 0.25%; Mn: 0.25% - 0.35%; Cr: 8.05% - 8.50%; W: 1.00% - 1.15%; V: 0.25% - 0.30%; B: 0.006% - 0.008%, the balance is iron, and trace residual elements S, P, N, O, H.

[0023] In a preferred embodiment of the present invention, the formula includes elements in the following weight percentages:

[0024] C: 0.20%; Si: 0.18%; Mn: 0.28%; Cr: 8.65%; W: 1.10%; V: 0.28%; B: 0.006%, the balance is iron, and trace residual elements S, P, N, O, H. Among them, S, P, N, O, H are impurities with acceptable contents, and the contents are 1 - 200 ppm respectively.

[0025] The preparation method of the above high red hardness hot extrusion die steel adopts the following technological processes and steps:

[0026] 1) Melting: Put the batching into an electric arc furnace for melting. After the alloy components in the melting reach the indexes, control the temperature of the molten steel to 1520 - 1540 °C and pour it into a mold to form an electrode steel bar. After demolding, remove the oxide scale and pit defects on the surface of the electrode bar;

[0027] 2) Electroslag remelting: The electrode bar is subjected to electroslag remelting, so that the molten steel passes through the slag system for filtration and then slowly crystallizes and solidifies into a round ingot;

[0028] 3) High - temperature homogenization: Heat the round ingot to 1200 - 1245 °C, and the holding time is (0.4 - 0.6) × D hours, where D is the diameter size of the ingot in cm, to make the components in the steel diffuse evenly, and then cool it to the forging temperature of 1150 ± 10 °C;

[0029] 4) Upsetting: Upset the steel ingot at 1150 ± 10 °C on the press along the height direction of the steel ingot to 30% - 40% of its height, then finish forging, and reheat in the furnace for 4 - 6 hours; then perform the second upsetting, finish forging, and keep the furnace temperature for 6 - 8 hours, and then perform the third upsetting, finish forging, always maintaining the final forging temperature above 866 °C;

[0030] 5) Drawing out: Draw out and forge the steel ingot after three repeated upsetting operations to the final size to obtain the module, maintaining the final forging temperature above 860 °C - 900 °C, and after drawing out, cool in a pit to 300 ± 10 °C;

[0031] 6) Quenching: Heat the module to 1030 ± 10 °C and keep it warm for 8 - 12 hours, then air-cool quench;

[0032] 7) Tempering: Heat the quenched module to 560 °C and keep it warm for (0.1 - 0.2) × d hours, where d is the effective size of the forging in cm, and air-cool to room temperature;

[0033] 8) Isothermal spheroidizing annealing: Heat the forged part after tempering to 740 ± 10 °C, isothermal for (0.6 - 0.9) × d hours, where d is the effective size of the forging in cm, and then cool to room temperature with the furnace.

[0034] Preferably, during the high-temperature homogenization in step 3), heat the round steel ingot to 1200 - 1225 °C, and the holding time is (0.5 - 0.6) × D hours, where D is the diameter size of the steel ingot (cm), to make the components in the steel diffuse evenly, and then cool to the forging temperature of 1150 ± 10 °C.

[0035] Preferably, during tempering in step 7), heat the module to 560 °C and keep it warm for (0.15 - 0.2) × d hours, where d is the effective size of the forging in cm, and air-cool to room temperature.

[0036] In step 2), through electroslag remelting, after removing most of the impure impurities in the electrode bar through the metallurgical slag system filtration, the molten steel slowly crystallizes and solidifies to form a round steel ingot.

[0037] Some of the terms involved in the present invention are explained as follows.

[0038] Electrode bar: The base material used for electroslag remelting, which is cast after being melted in an electric arc furnace.

[0039] Electroslag remelting: A melting method that uses the resistance heat generated when an electric current passes through the molten slag as the heat source. Its purpose is to improve the metal purity and improve the ingot crystallization.

[0040] Steel ingot: The molten steel is poured into the mold through the ladle and solidifies to form a steel ingot with a certain shape.

[0041] High-temperature homogenization: A heat treatment process that eliminates or reduces the in-grain compositional inhomogeneity and the non-equilibrium microstructure state under actual crystallization conditions at high temperatures, improving the processing performance and service performance of alloy materials.

[0042] Segregation: The phenomenon that the constituent elements in an alloy are unevenly distributed during crystallization.

[0043] Upsetting: A forging process that reduces the height of a billet while increasing its cross-sectional area. It improves the transverse mechanical properties of forgings and reduces anisotropy; repeated upsetting and drawing are used to break up the carbides in alloy tool steels and make their distribution uniform.

[0044] Drawing out: Refers to any forging process that reduces the cross-sectional area and increases the length.

[0045] Red hardness: The ability of hot work die steel to maintain a slow decrease in hardness in a high-temperature working environment.

[0046] W and V are effective elements for comprehensively improving wear resistance, high-temperature hot strength, and toughness. The addition of B can significantly improve hardenability and hardness. Based on the composition of 4Cr5MoSiV1 steel, this invention completely replaces the Mo element with W, reduces C, increases Cr, and adds B to form spherical CrxC wear-resistant precipitates. These types of precipitates densely pin around the grains, hindering grain growth, and removing the relatively expensive Mo element; therefore, this type of hot extrusion die steel will have a lower cost, better wear resistance, high-temperature hot strength and toughness than H13 steel.

[0047] The high red-hardness hot extrusion die steel of this invention can still maintain a hardness of 48 - 52 HRC after tempering at 600 °C, and has excellent toughness, high-temperature friction and wear performance, hot strength, thermal stability, and thermal fatigue performance.

[0048] The beneficial effects of this invention are as follows:

[0049] (1) The impact toughness of the steel grade of this invention exceeds the excellent level of H13, and has excellent high-temperature friction and wear performance, high-temperature hot strength, thermal stability, and thermal fatigue performance.

[0050] (2) Since the hardenability is improved through the B element from the compositional perspective, it will be beneficial to the stable trial production of the steel of this invention in conventional large-scale production.

[0051] (3) This invention will increase the hardness of the hot extrusion die steel in the range of 48 - 52 HRC, and the impact energy of a 7×10×55 non-notch specimen is greater than 280 J. Description of the Drawings

[0052] Figure 1 It is a 1000-fold metallographic structure diagram of the die steel in Example 1;

[0053] Figure 2Test comparison chart of the thermal stability of the die steel in Example 1 and H13 steel after tempering at 600°C;

[0054] Figure 3 Test comparison chart of the friction and wear coefficients of the die steel in Example 1 and H13 steel. Specific implementation manners

[0055] The technical solutions of the present invention will be described below in conjunction with specific embodiments.

[0056] Example 1

[0057] The chemical composition of the high hot hardness hot extrusion die steel contains the following elements by weight percentage:

[0058] C: 0.20%; Si: 0.18%; Mn: 0.28%; Cr: 8.65%; W: 1.10%; V: 0.28%, B: 0.006%, the balance is iron, and trace residual elements S, P, N, O, H.

[0059] In this example, the technological process and steps of the steel are as follows:

[0060] 1) Melting: Put the ingredients into an electric arc furnace for melting according to the ratio. After the metallurgical composition meets the requirements, control the temperature to about 1520 - 1540°C and cast it into an electrode bar with a diameter of φ290mm × 2050mm. After the electrode bar is demolded, remove defects such as oxide scale and pits.

[0061] 2) Electroslag remelting: Carry out electroslag remelting refining on the electrode bar, filter out most of the impure impurities in the electrode bar through the metallurgical slag system, and then the molten steel slowly crystallizes and solidifies into a 1-ton round ingot (with a diameter of about 320mm).

[0062] 3) High-temperature homogenization: Heat the 1-ton ingot to 1225 ± 10°C, hold for 11 hours, and then slowly cool to 1150 ± 10°C to prepare for forging.

[0063] 4) Upsetting: Upset the 1150°C ingot along the height direction of the ingot to 35% of its height, then finish (i.e., use a press to flatten the irregular edges), heat it in the furnace at 1150 ± 10°C for 5 hours, then carry out the second upsetting, finish, heat it in the furnace at 1150 ± 10°C and hold for 5 hours, then carry out the third upsetting, finish, and keep the final forging temperature above 860°C.

[0064] 5) Drawing out: Forge and draw out the ingot after three repeated upsetting operations to the final size of 165mm × 520mm × 1550mm (thickness × width × length), with an effective size of 165mm, keep the final forging temperature above 860°C, and cool it in a pit to about 350°C after drawing out.

[0065] 6) Quenching: Heat the module to 1030 ± 5 °C and hold for 3 hours, then quench (water cooling medium).

[0066] 7) Tempering: Heat the module to 560 ± 5 °C for tempering for 12 hours, then air cool.

[0067] 8) Isothermal spheroidizing: Heat the module to 740 ± 10 °C and hold for 20 hours, then cool with the furnace.

[0068] The obtained metallographic structure diagram (1000 times) of the die steel is as Figure 1 shown.

[0069] Performance test

[0070] Perform performance tests on the above steel, and the results are as follows:

[0071] (1) Hardness after quenching and tempering: 51 HRC;

[0072] (2) Impact energy of the specimen without notch (7×10×55) is greater than 280 J;

[0073] Impact energy of the specimen with U-notch: 32 J;

[0074] (3) Thermal stability: The expression of thermal stability here is as follows: When the steel is held at 600 °C for different times, the decreasing trend of its hardness is used to judge the thermal stability. At the same time, a comparative test of thermal stability is carried out with H13 steel. The composition comparison is shown in Table 1 (wt.%), and the thermal stability data comparison is shown in Table 2 (unit: HRC) and Figure 2 shown.

[0075] (4) Test the friction and wear coefficient of Example 1 and H13 steel, and the results are as Figure 3 shown.

[0076] Table 1 Composition comparison of H13 and the steel

[0077]

[0078] Table 2 Hardness comparison of H13 and the patented steel after tempering at 600 °C for different times

[0079]

Claims

1. A high red hardness hot extrusion die steel, characterized in that, Its composition formula includes elements in the following weight percentages: C: 0.17% - 0.24%; Si: 0.10% - 0.45%; Mn: 0.20% - 0.45%; Cr: 7.00% - 9.50%; W: 0.80% - 1.25%; V: 0.20% - 0.40%; B: 0.004% - 0.009%, with the balance being iron, as well as impurity elements S, P, N, O, H; The preparation method of the high red-hardness hot extrusion die steel includes the following steps: 1) Melting: Put the ingredients into an electric arc furnace for melting. After the alloying components in the melt reach the specified standards, control the temperature of the molten steel to 1520 - 1540 °C and pour it into a mold to form an electrode bar. After demolding, remove the oxide scale and pit defects on the surface of the electrode bar; 2) Electroslag remelting: Subject the electrode bar to electroslag remelting. After the molten steel passes through the slag system for filtration, slowly crystallize and solidify into a round ingot. After electroslag remelting, bury it in a sand pit and cool it to room temperature; 3) High-temperature homogenization: Heat the round ingot to 1200 - 1245 °C and keep it warm for (0.4 - 0.6) × D hours, where D is the diameter size of the ingot in cm; make the components in the steel diffuse evenly, and then cool it to the forging temperature of 1150 ± 10 °C; 4) Upsetting: Upset the round ingot at 1150 ± 10 °C on a press along the height direction of the ingot to 30% - 40% of its original height, then finish it, and heat it in the furnace for 4 - 6 hours; then perform the second upsetting, finish it, heat it in the furnace and keep it warm for 6 - 8 hours, and then perform the third upsetting, finish it, and always keep the final forging temperature above 866 °C; 5) Drawing out: Draw out and forge the round ingot after three repeated upsetting operations to the final size to obtain a forging, keep the final forging temperature above 860 °C, and cool it in a pit to 300 ± 10 °C after drawing out; 6) Quenching: Heat the forging to 1030 ± 10 °C and keep it warm for 8 - 12 hours, then air-cool quench it; 7) Tempering: Heat the quenched forging to 560 °C and keep it warm for (0.1 - 0.2) × d hours, where d is the effective size of the forging in cm, and air-cool it to room temperature; 8) Isothermal spheroidizing annealing: Heat the forging after tempering to 740 ± 10 °C, isotherm for (0.6 - 0.9) × d hours, where d is the effective size of the forging in cm, and then cool it in the furnace to room temperature.

2. The high red-hard hot extrusion die steel according to claim 1, wherein, The formula includes elements in the following weight percentages: C: 0.18% - 0.21%; Si: 0.15% - 0.25%; Mn: 0.25% - 0.35%; Cr: 8.05% - 8.50%; W: 1.00% - 1.15%; V: 0.25% - 0.30%; B: 0.006% - 0.008%, with the balance being iron, as well as impurity elements S, P, N, O, H. In this composition, W completely replaces the Mo element in traditional hot work die steel.

3. The high red-hard hot extrusion die steel according to claim 1, characterized in that, The formula includes elements in the following weight percentages: C: 0.20%; Si: 0.18%; Mn: 0.28%; Cr: 8.65%; W: 1.10%; V: 0.28%, B: 0.006%, with the balance being iron, as well as impurity elements S, P, N, O, H.

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