Super high-strength and high-toughness hot work die steel with superfine tissue structure and preparation method thereof

Abstract

The invention relates to hot work die steel with a super high-strength high-toughness superfine structure and a preparation method of the hot work die steel. The hot work die steel comprises the following components in percentage by mass: 0.30-0.45% of C, 0.80-1.20% of Si, 0.80-1.20% of Mn, less than or equal to 0.03% of P, less than or equal to 0.03% of S, 3.0-4.5% of Cr, 0.8-1.5% of Mo, 0.80-1.20% of V, 0.025-0.100% of Nb, 0.010-0.015% of RE and the balance of Fe and other inevitable impurities. The preparation method mainly comprises the following steps of smelting, homogenization treatment and cogging, intermediate annealing treatment, large-deformation medium-temperature rolling and tempering treatment after rolling. The structure is regulated and controlled through conventional equipment and a production process, martensite structure ultra-fining can be achieved, and the strength of the hot work die steel is remarkably improved. The method is simple in production process, low in cost, low in production equipment requirement, high in production efficiency and convenient for industrial production and application.

Description

Technical Field

[0001] The invention relates to the field of metal materials and processing technology, and in particular to hot working die steel with super high strength, high toughness and ultra-fine structure and a preparation method thereof. Background Art

[0002] With the rapid development of industry today, mold forming has gradually replaced traditional cutting and other forming methods due to its high efficiency, high precision and low consumption. The mold industry has become one of the hot industries in the world today, and mold steel is the most important material and technical foundation of the mold industry, and it is also an important standard for measuring a country's industrial level. Hot working die steel is a type of mold steel, mainly used to manufacture molds for pressing and forming solid metals and high-temperature liquid metals (Cu, Al alloys, etc.) heated to above the recrystallization temperature. Hot working die steel is generally a medium-carbon alloy steel containing strong carbide alloy elements such as Cr, Mo, V, and W. Its strength mainly comes from the tempered martensitic matrix and carbides with secondary hardening. This type of steel often has good hardenability. The matrix produced after quenching is a martensitic structure, and then a large number of dispersed and fine alloy carbides are precipitated through high-temperature tempering, thereby improving the strength, hardness, wear resistance and thermal stability of the mold. The main failure cause of hot working die steel is thermal fatigue failure caused by cracks, and high strength is not only conducive to delaying crack initiation, but also enables die steel to serve better in extreme environments; coupled with the rapid development of industry in recent years, the requirements for the material type, structural characteristics and mechanical properties of die steel are becoming more and more stringent. Therefore, the development of high-performance and ultra-high-strength hot working die steel has become an important task for my country's steel industry.

[0003] In metal alloy materials, strength and plasticity are generally not achieved at the same time, and the improvement of strength is generally accompanied by the decrease of plasticity. Grain refinement is the only method that can simultaneously improve the strength and plasticity of materials, and at the same time improve the processing and forming properties of materials. Therefore, how to obtain fine-grained structure has become an important direction of material research. Severe Plastic deformation (SPD) is a commonly used plastic deformation method. It can introduce a large strain during the deformation process, thereby effectively refining the grains and obtaining a complete large-size block specimen. By controlling the microstructure during the deformation process, a block with both strength and plasticity can be obtained at the same time. It has been widely used in the preparation of ultra-high strength martensitic steel. Representative severe plastic deformation processes include equal channel angular pressing (ECAP), high pressure and torsion (HPT), and cumulative roll bonding (ARB). However, these processes are relatively complex and have high requirements for equipment and molds. Traditional steel mills are difficult to meet such high requirements. In contrast, large deformation rolling is a simple method for preparing ultra-high strength and high toughness steel. It has a large deformation amount under multiple rolling passes, can process large-sized workpieces, has low requirements for workers and equipment, and is easy to achieve industrial production. Large deformation warm rolling is one of the large deformation rolling methods. It is carried out in the supercooled austenite region (temperature below A 1 , higher than M S ) is deformed and rolled during the rolling process. The dynamic recovery of austenite is inhibited, resulting in the formation of a large number of defects such as dislocations, dislocation cells and deformation bands inside the grains. These defects provide abundant nucleation sites for martensitic transformation, thereby promoting the refinement of the martensitic structure after the phase transformation, showing great potential in the production of materials with excellent mechanical properties.

[0004] Chinese patent CN202211278224.X discloses a method for producing high-strength hot working die steel. This patent produces a hot working die steel with a tensile strength ≥1900MPa through the method of high-temperature quenching + isothermal treatment + high-temperature tempering.

[0005] Chinese patent CN201911342992.5 discloses a production method for high-performance hot working die steel. This patent prepares a hot working die steel with a tensile strength ≥1450MPa and a hardness ≥46HRC by combining forging and spheroidizing annealing.

[0006] It is difficult to obtain ultra-high strength of hot working die steel through conventional heat treatment processes. It is difficult to increase the yield strength to more than 1500MPa, and the tensile strength is difficult to reach more than 2000MPa. In addition, the alloy elements required to be added to traditional hot working die steel are of high mass and cost. Therefore, optimizing the alloy composition of hot working die steel and developing new process methods are of great engineering significance for the production and application of hot working die steel. Summary of the invention

[0007] The purpose of the present invention is to provide a hot working die steel with super high strength, high toughness and ultra-fine microstructure and a preparation method thereof. By refining the sub-temperature austenite structure and then obtaining an ultra-fine martensite structure, the hot working die steel can be improved to an ultra-high strength level while maintaining a high toughness. The tensile strength is ≥2200MPa, the yield strength is ≥2000MPa, and the elongation at break is not less than 5.0%.

[0008] To achieve the above object, the technical solution of the present invention is:

[0009] Super high-strength, high-toughness and ultra-fine microstructure hot working die steel, whose components by mass percentage are: C: 0.30-0.45%, Si: 0.80-1.20%, Mn: 0.80-1.20%, P≤0.03%, S≤0.03%, Cr: 3.0-4.5%, Mo: 0.8-1.5%, V: 0.80-1.20%, Nb: 0.025-0.100%, RE: 0.010-0.015%, and the balance is Fe and other inevitable impurities.

[0010] Preferably, the composition mass percentage of the hot working die steel is, C: 0.33-0.42%, Si: 0.80-1.20%, Mn: 0.80-1.20%, P≤0.005%, S≤0.010%, Cr: 3.5-4.0%, Mo: 1.0-1.2%, V: 0.80-1.20%, Nb: 0.04-0.08%, RE: 0.010-0.015%, the remainder is Fe and other inevitable impurities, and the total mass percentage of the added alloying elements is less than 10%.

[0011] Further preferably, the mass percentage of each component in the alloy steel is: C: 0.35-0.40%, Si: 1.0%, Mn: 0.8%, P≤0.005%, S≤0.010%, Cr: 3.7-4.0%, Mo: 1.0-1.2%, V: 0.9-1.1%, Nb: 0.04-0.08%, RE: 0.010-0.015%, and the total mass percentage of the added alloy elements is less than 8.0%.

[0012] The present invention also provides a method for preparing a hot working die steel with super high strength, high toughness and ultra-fine structure, comprising the following steps:

[0013] Step 1, smelting: loading the alloy steel into a furnace after batching according to the mass percentage of each component in the alloy steel, uniformly stirring and smelting in a vacuum induction furnace, and casting into an ingot;

[0014] Step 2: Homogenization + blanking: Heat the ingot to 1100-1280℃, keep it at this temperature for no less than 4.0h, and cool it in air or furnace to obtain a blank with homogeneous structure and properties. Then reheat it to 1100-1200℃, and perform hot rolling or hot forging to blank. The blanking temperature is no less than 1050℃, and the final rolling or final forging temperature is no less than 900℃, finally forming an intermediate blank with a thickness of no more than 60mm.

[0015] Step 3: Intermediate annealing: The intermediate billet is subjected to isothermal spheroidizing annealing. The billet is heated to 830-900°C and kept at this temperature for 2.0-5.0h, then furnace cooled to 740-780°C and kept at this temperature for 2.0-5.0h, then furnace cooled to below 500°C and air cooled out of the furnace.

[0016] Step 4: austenitization + large deformation medium temperature rolling, heating the intermediate billet to 1100-1200°C, holding time for 2.0-4.0h, taking it out of the furnace and spray cooling, performing large deformation medium temperature rolling, and water cooling to room temperature after rolling;

[0017] Step 5: Tempering after rolling: Temper the steel plate after large deformation and medium temperature rolling, and air cool to room temperature after heat preservation;

[0018] Furthermore, the smelting method used in step 1 is any one of vacuum arc furnace smelting, vacuum induction plus electroslag remelting, vacuum smelting plus vacuum consumables, or a combination of two thereof;

[0019] Furthermore, the homogenization treatment temperature in step 2 is 1200° C., the holding time is 6.0 h, the starting rolling temperature during blanking is 1150° C., the final rolling temperature is 950° C., and the final rolled plate thickness is 20 mm;

[0020] Furthermore, the intermediate annealing in step 3 is isothermal spheroidizing annealing, wherein the billet is heated to 890°C and kept at this temperature for 2.0 hours, then furnace-cooled to 760°C and kept at this temperature for 4.0 hours, and then furnace-cooled to below 460°C and air-cooled out of the furnace;

[0021] Furthermore, the austenitizing temperature in the medium temperature rolling in step 4 is 1100° C. and the holding time is 2 h.

[0022] Furthermore, the starting rolling temperature of the large deformation medium-temperature rolling in step 4 is between 700 and 800° C., and the final rolling temperature is between 500 and 600° C.;

[0023] Furthermore, the medium temperature rolling process in step 4 adopts 2 to 4 rolling passes, the reduction of a single pass is not less than 25%, and the cumulative reduction is not less than 80%;

[0024] Furthermore, in step 5, the heating temperature of the post-rolling tempering treatment is 200-560° C., the heat preservation is 1.0 h, and the steel is air-cooled to room temperature.

[0025] The present invention realizes grain refinement and precipitation strengthening through large deformation rolling and post-rolling tempering treatment; utilizes the mechanical external stress in large deformation rolling to break and flatten the equiaxed grains in the original structure, while introducing a large number of defects such as dislocations and large-angle grain boundaries; after completing the large deformation rolling, tempering treatment is performed to precipitate carbon atoms in the martensite matrix to form carbides, thereby enhancing the effect of precipitation strengthening and reducing the carbon content in the matrix, thereby obtaining a hot working die steel with excellent comprehensive mechanical properties.

[0026] Compared with the prior art, the present invention has the following advantages:

[0027] 1. The hot working die steel prepared by the method of the present invention has a fine grain size, and the structure is mainly composed of martensite transformed from a metastable ultrafine lamellar austenite structure. The fine-grained martensite structure makes the alloy steel have ultra-high strength and high toughness. The prepared hot working die steel has a tensile strength of not less than 2200MPa, a yield strength of not less than 2000MPa, and an elongation at break of not less than 5%.

[0028] 2. The production process is simple. Compared with other SPD technologies for preparing ultrafine-grained materials, the present invention adopts conventional industrial rolling equipment, which overcomes the limitations of SPD technology on mold and product size. There is no need to design special molds, and the requirements for material size are low, which is conducive to industrial production.

[0029] 3. Hot working die steel is generally medium carbon alloy steel, which is generally rolled or forged above the austenitizing temperature, and the grains will inevitably grow during the rolling process. The preparation method of the present invention adopts medium temperature strong deformation heat treatment, and the deformation is carried out in the supercooled austenite region. Compared with high temperature deformation rolling or forging, when the austenite grains are rolled and deformed in the supercooled austenite region, it is difficult for the dynamic recovery and dynamic recrystallization to occur, thereby avoiding the grain growth caused by recovery and recrystallization; secondly, medium temperature rolling can significantly reduce energy consumption, improve production efficiency, and save production costs.

[0030] 4. The hot working die steel prepared by the method of the present invention achieves a significant reduction in alloy content compared with traditional hot working die steel by optimizing the ratio of alloy elements without sacrificing material properties. This not only helps to simplify the smelting and processing process of the material, but also plays a key role in reducing the cost of raw materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 A schematic diagram of the process for preparing a hot working die steel with super high strength, high toughness and ultra-fine structure according to the present invention;

[0032] Figure 2 This is a typical scanning electron microscope microstructure picture of the hot working die steel with super high strength, high toughness and ultra-fine microstructure prepared by the present invention before large deformation and medium temperature rolling;

[0033] Figure 3 This is a typical scanning electron microscope microstructure picture of the hot working die steel with super high strength, high toughness and ultra-fine microstructure prepared by the present invention after medium-large deformation and medium-temperature rolling;

[0034] Figure 4 This is a picture of a typical scanning electron microscope structure of the hot working die steel with super high strength, high toughness and ultra-fine structure prepared by the present invention after medium-large deformation and medium-temperature rolling and +500℃ tempering heat treatment;

[0035] Figure 5 Partial tensile mechanical property curve of the hot working die steel with super high strength, high toughness and ultra-fine structure prepared by the present invention. DETAILED DESCRIPTION

[0036] The present invention is further described below in conjunction with the accompanying drawings and specific embodiments. It should be noted that the composition of the alloy steel designed by the present invention can fluctuate within a certain range. For those of ordinary skill in the art, several improvements and adjustments can be made without departing from the concept of the present invention, which all belong to the protection scope of the present invention.

[0037] Example 1

[0038] The ultra-high strength hot working die steel provided in this embodiment has the following components in alloy steel: C: 0.35-0.40%, Si: 1.0%, Mn: 0.8%, P≤0.005%, S≤0.010%, Cr: 3.7-4.0%, Mo: 1.0-1.2%, V: 0.9-1.1%, Nb: 0.04-0.08%, RE: 0.010-0.015%, the remainder is Fe and other unavoidable impurities, and the total mass percentage of alloying elements added is less than 8.0%. The preparation method of ultra-high strength hot working die steel with the above composition, the preparation process of which includes smelting, homogenization treatment + blanking, austenitization + large deformation medium temperature rolling. The specific steps are as follows:

[0039] (1) Melting: Weigh alloy elements such as Cr, V, Mo, Mn and carbon steel with a purity greater than 99% according to the alloy steel composition ratio, place the raw materials in a vacuum arc furnace, and evacuate to 1×10 -3 MPa, each ingot block is repeatedly melted 5 to 7 times, combined with electromagnetic stirring in the cavity to ensure that the components in the ingot block are evenly distributed. After melting, it is cast into a 40mm thick rectangular ingot.

[0040] (2) Homogenization treatment + blanking: The ingot is heated to 1200°C in a high-temperature box-type resistance furnace and kept warm for 6.0 h. The ingot is taken out of the furnace and rolled. After two passes of rolling, the final rolling temperature is 950°C and the final thickness is 20 mm. It is then air-cooled to room temperature.

[0041] (3) Intermediate annealing: Heat the billet to 890℃ and keep it at this temperature for 2.0h, then cool it in the furnace to 760℃ and keep it at this temperature for 4.0h, then cool it in the furnace to below 460℃ and air cool it out of the furnace.

[0042] (4) Austenitization + large deformation medium-temperature rolling: The annealed billet is heated to 1100°C in a high-temperature box-type resistance furnace and kept warm for 2.0 h. The ingot is taken out of the furnace and spray-cooled to 750°C before rolling. After 4 rolling passes, the final rolling temperature is 550°C, the cumulative deformation is more than 80%, and the final thickness is 2.1 mm.

[0043] The typical tissue photos of the samples obtained by scanning electron microscopy are attached. Figure 3 The structure of the prepared mold steel is mainly composed of martensite transformed from metastable ultrafine lamellar austenite structure, with a lamellar width of ≤2.0 microns, martensite laths inside the lamellar, and the martensite domain is ultrafine equiaxed high-angle grains with a size of ≤1.0 microns. The tensile strength of the prepared hot working die steel is 2691MPa and the yield strength is 2416MPa.

[0044] In the present invention, each element component has the following characteristics and functions:

[0045] 1) C content: 0.30~0.45wt%

[0046] Generally speaking, the higher the carbon content, the higher the strength, the higher the hardness, and the better the wear resistance, but it will also be accompanied by a decrease in plasticity. Carbon in steel generally exists in two forms. One part of the carbon dissolves into the lattice gap of the matrix to play a solid solution strengthening role; the other part of the carbon forms carbides with alloy elements. The fine dispersed alloy carbides can greatly improve the effect of precipitation strengthening.

[0047] 2) Cr content: 3.0-4.5wt%

[0048] Chromium is a relatively low-priced alloying element that can enhance the hardenability of steel and easily form carbides with carbon, thereby increasing the hardness, strength and wear resistance of steel during high-temperature service. The addition of chromium can also improve the corrosion resistance of steel. 23 C 6 Carbides can easily spheroidize, aggregate and grow during tempering, resulting in a decrease in hardness.

[0049] 3) Mo content: 0.8-1.5wt%

[0050] Molybdenum is a strong carbide element and can easily form secondary hardening carbides with carbon during the tempering process, thereby improving the material's high temperature performance and tempering resistance stability.

[0051] 4) V content: 0.80~1.20wt%

[0052] Vanadium easily forms VC with carbon. VC has a high melting point and hardness, which can improve the strength, hardness and thermal stability of mold steel. VC that is not completely dissolved during quenching can effectively pin the original austenite grain boundary, thereby hindering grain growth and refining the grain.

[0053] 5) Nb content: 0.025~0.100wt%

[0054] The addition of trace alloying element niobium can significantly refine the grains. Niobium easily forms carbides and nitrides with carbon and nitrogen. The precipitation of niobium carbon or nitride can pin the grain boundaries and the solute dragging effect of solid-solution niobium atoms to inhibit grain growth.

[0055] 6) RE content: 0.010~0.015wt%

[0056] Rare earth elements are easy to combine with oxygen and sulfur in steel during smelting, which has the functions of deoxidation, desulfurization and purification, can reduce the segregation of alloy elements, refine the structure, and improve the fatigue resistance and softening resistance of steel. However, if the addition process is improper, it is easy to cause problems such as uneven distribution of components and oxidation and burning of rare earth.

[0057] 7) Si content: 0.80~1.20wt%

[0058] Silicon is a non-carbide-forming element that can inhibit the precipitation of cementite during tempering and delay the decomposition of martensite.

[0059] 8) The content of P and S is: P≤0.03%wt, S≤0.03%wt

[0060] Phosphorus has a large solubility in steel and has a strong solid solution strengthening effect. It significantly improves the strength and hardness of steel, but it will drastically reduce the low-temperature toughness of steel. Sulfur can only exist in solid steel in the form of FeS inclusions, which can easily lead to hot brittleness during hot processing of steel; however, appropriate sulfur and phosphorus can help improve the cutting performance of steel.

[0061] Example 2

[0062] Preparation method of hot working die steel with super high strength, high toughness and ultra-fine structure, such as Figure 2 The steps are as follows:

[0063] (1) Alloy smelting treatment is the same as step (1) of Example 1;

[0064] (2) Homogenization treatment + blanking treatment is the same as step (2) of Example 1;

[0065] (3) Intermediate annealing treatment is the same as step (3) of Example 1

[0066] (4) Large deformation and medium temperature rolling treatment is the same as step (4) of Example 1;

[0067] (5) Tempering after rolling: After the large deformation and medium temperature rolling treatment in step (4), the plate is tempered at 200°C for 1.0 h and air-cooled to room temperature. The tempering heat treatment causes the internal structure of the material to partially recover and reduce the dislocation density; at the same time, a large number of nano-precipitates are generated and evenly dispersed in the martensite matrix during the tempering process, which improves the plasticity and toughness of the material while slightly reducing the strength.

[0068] The tensile strength of the prepared hot working die steel is 2469MPa and the yield strength is 2253MPa.

[0069] Example 3

[0070] Preparation method of hot working die steel with super high strength, high toughness and ultra-fine structure, such as Figure 2 The steps are as follows:

[0071] (1) Alloy smelting treatment is the same as step (1) of Example 1;

[0072] (2) Homogenization treatment + blanking treatment is the same as step (2) of Example 1;

[0073] (3) Intermediate annealing treatment is the same as step (3) of Example 1

[0074] (4) Large deformation and medium temperature rolling treatment is the same as step (4) of Example 1;

[0075] (5) Tempering after rolling: After the large deformation and medium temperature rolling treatment in step (4), the plate is tempered at a temperature of 250° C. for 1.0 h and then air-cooled to room temperature.

[0076] The tensile strength of the prepared hot working die steel is 2501MPa and the yield strength is 2309MPa.

[0077] Example 4

[0078] Preparation method of hot working die steel with super high strength, high toughness and ultra-fine structure, such as Figure 2 The steps are as follows:

[0079] (1) Alloy smelting treatment is the same as step (1) of Example 1;

[0080] (2) Homogenization treatment + blanking treatment is the same as step (2) of Example 1;

[0081] (3) Intermediate annealing treatment is the same as step (3) of Example 1

[0082] (4) Large deformation and medium temperature rolling treatment is the same as step (4) of Example 1;

[0083] (5) Tempering after rolling: After the large deformation and medium temperature rolling treatment in step (4), the plate is tempered at 300°C for 1.0 h and then air-cooled to room temperature.

[0084] The tensile strength of the prepared hot working die steel is 2575MPa and the yield strength is 2318MPa.

[0085] Example 5

[0086] Preparation method of hot working die steel with super high strength, high toughness and ultra-fine structure, such as Figure 2 The steps are as follows:

[0087] (1) Alloy smelting treatment is the same as step (1) of Example 1;

[0088] (2) Homogenization treatment + blanking treatment is the same as step (2) of Example 1;

[0089] (3) Intermediate annealing treatment is the same as step (3) of Example 1

[0090] (4) Large deformation and medium temperature rolling treatment is the same as step (4) of Example 1;

[0091] (5) Tempering after rolling: After the large deformation and medium temperature rolling treatment in step (4), the plate is tempered at 350°C for 1.0 h and then air-cooled to room temperature.

[0092] The tensile strength of the prepared hot working die steel is 2499MPa and the yield strength is 2301MPa.

[0093] Example 6

[0094] Preparation method of hot working die steel with super high strength, high toughness and ultra-fine structure, such as Figure 2 The steps are as follows:

[0095] (1) Alloy smelting treatment is the same as step (1) of Example 1;

[0096] (2) Homogenization treatment + blanking treatment is the same as step (2) of Example 1;

[0097] (3) Intermediate annealing treatment is the same as step (3) of Example 1

[0098] (4) Large deformation and medium temperature rolling treatment is the same as step (4) of Example 1;

[0099] (5) Tempering after rolling: After the large deformation and medium temperature rolling treatment in step (4), the plate is tempered at 400°C for 1.0 h and then air-cooled to room temperature.

[0100] The tensile strength of the prepared hot working die steel is 2493MPa and the yield strength is 2266MPa.

[0101] Example 7

[0102] Preparation method of hot working die steel with super high strength, high toughness and ultra-fine structure, such as Figure 2 The steps are as follows:

[0103] (1) Alloy smelting treatment is the same as step (1) of Example 1;

[0104] (2) Homogenization treatment + blanking treatment is the same as step (2) of Example 1;

[0105] (3) Intermediate annealing treatment is the same as step (3) of Example 1

[0106] (4) Large deformation and medium temperature rolling treatment is the same as step (4) of Example 1;

[0107] (5) Tempering after rolling: After the large deformation and medium temperature rolling treatment in step (4), the plate is tempered at 450°C for 1.0 h and then air-cooled to room temperature.

[0108] The tensile strength of the prepared hot working die steel is 2391MPa and the yield strength is 2295MPa.

[0109] Example 8

[0110] Preparation method of hot working die steel with super high strength, high toughness and ultra-fine structure, such as Figure 2 The steps are as follows:

[0111] (1) Alloy smelting treatment is the same as step (1) of Example 1;

[0112] (2) Homogenization treatment + blanking treatment is the same as step (2) of Example 1;

[0113] (3) Intermediate annealing treatment is the same as step (3) of Example 1

[0114] (4) Large deformation and medium temperature rolling treatment is the same as step (4) of Example 1;

[0115] (5) Tempering after rolling: After the large deformation and medium temperature rolling treatment in step (4), the plate is tempered at a temperature of 500° C. for a holding time of 1.0 h; and air-cooled to room temperature.

[0116] The tensile strength of the prepared hot working die steel is 2423MPa and the yield strength is 2346MPa.

[0117] Example 9

[0118] Preparation method of hot working die steel with super high strength, high toughness and ultra-fine structure, such as Figure 2 The steps are as follows:

[0119] (1) Alloy smelting treatment is the same as step (1) of Example 1;

[0120] (2) Homogenization treatment + blanking treatment is the same as step (2) of Example 1;

[0121] (3) Intermediate annealing treatment is the same as step (3) of Example 1

[0122] (4) Large deformation and medium temperature rolling treatment is the same as step (4) of Example 1;

[0123] (5) Tempering after rolling: After the large deformation and medium temperature rolling treatment in step (4), the plate is tempered at a temperature of 560° C. for a holding time of 1.0 h; and air-cooled to room temperature.

[0124] The tensile strength of the prepared hot working die steel is 2217MPa and the yield strength is 2028MPa.

[0125] The present invention provides a detailed description of the production of ultra-high-strength, high-toughness, ultra-fine microstructure hot working die steel and its preparation method, with the aim of facilitating the technical personnel in this research field to quickly understand and grasp the content of the invention. At the same time, researchers familiar with the field can also make certain modifications based on the examples of the present invention to facilitate their own use. Therefore, the examples of the present invention are not limited to the contents mentioned above, and the modifications made by technical personnel in this field based on the basic ideas and content framework of the present invention should be within the scope of protection of the present invention.

Claims

1. Super high strength, high toughness and ultra-fine structure hot working die steel, characterized by: The chemical composition mass percentage of the alloy is: C: 0.30-0.45%, Si: 0.80-1.20%, Mn: 0.80-1.20%, P≤0.03%, S≤0.03%, Cr: 3.0-4.5%, Mo: 0.8-1.5%, V: 0.80-1.20%, Nb: 0.025-0.100%, RE: 0.010-0.015%, and the balance is Fe and other inevitable impurities.

2. The super high strength, high toughness and ultra-fine structure hot working die steel according to claim 1, characterized in that: The mass percentage of each component in the alloy steel is: C: 0.33-0.42%, Si: 0.80-1.20%, Mn: 0.80-1.20%, P≤0.005%, S≤0.010%, Cr: 3.5-4.0%, Mo: 0.90-1.30%, V: 0.80-1.20%, Nb: 0.04-0.08%, RE: 0.010-0.015%, the remainder is Fe and other inevitable impurities, and the total mass percentage of the added alloy elements is less than 10%.

3. The super high strength, high toughness and ultra-fine structure hot working die steel according to claim 2, characterized in that: The mass percentage of each component in the alloy steel is: C: 0.35-0.40%, Si: 1.0%, Mn: 0.8%, P≤0.005%, S≤0.010%, Cr: 3.7-4.0%, Mo: 1.0-1.2%, V: 0.9-1.1%, Nb: 0.04-0.08%, RE: 0.010-0.015%, the remainder is Fe and other inevitable impurities, and the total mass percentage of the added alloy elements is less than 8.0%.

4. The method for preparing super high strength, high toughness and ultra-fine structure hot working die steel according to claims 1 to 3, characterized in that: The following steps are involved: Step 1, smelting: the chemical composition of the alloy is as follows: C: 0.35-0.40%, Si: 1.0%, Mn: 0.8%, P≤0.005%, S≤0.010%, Cr: 3.7-4.0%, Mo: 1.0-1.2%, V: 0.9-1.1%, Nb: 0.04-0.08%, RE: 0.010-0.015%, the balance is Fe and other inevitable impurities, and the total mass percentage of the added alloying elements is less than 8.0%; after the above ingredients are prepared, the furnace is charged, and the alloy is uniformly stirred and smelted in a vacuum induction furnace, and cast into an ingot; Step 2, homogenization treatment + blanking: the ingot obtained in step 1 is heated to 1100-1280°C, the holding time is not less than 4.0h, and the cooling method is air cooling or furnace cooling to obtain a blank with homogeneous structure and performance; then it is reheated to 1100-1200°C, hot rolled or hot forged blanking is performed, the blanking temperature is not less than 1050°C, and the final rolling or final forging temperature is not less than 900°C, and finally an intermediate blank with a thickness of not more than 60mm is formed; Step 3, intermediate annealing treatment: intermediate annealing treatment is performed on the intermediate billet obtained in step 2, the billet is heated to 830-900°C and kept at this temperature for 2.0-5.0h, then furnace cooled to 740-780°C and kept at this temperature for 2.0-5.0h, then furnace cooled to below 500°C and air cooled out of the furnace; Step 4, austenitization + large deformation medium temperature rolling, heating the billet treated in step 3 to 1100-1200°C, holding time is 2.0-4.0h, taking it out of the furnace and spray cooling, performing large deformation medium temperature rolling, and water cooling to room temperature after rolling; Step 5: Tempering after rolling: The steel plate obtained after the four-step deformation warm rolling is tempered at a temperature of 180-600°C, and then air-cooled to room temperature after heat preservation.

5. The method for preparing super high-strength, high-toughness and ultra-fine structure hot working die steel according to claim 4, characterized in that: The smelting method in step 1 is any one of vacuum arc furnace smelting, vacuum induction plus electroslag remelting, vacuum smelting plus vacuum consumables, or a combination of two.

6. The method for preparing super high strength, high toughness and ultra-fine structure hot working die steel according to claim 4, characterized in that: The homogenization treatment temperature in step 2 is 1200°C, and the holding time is 6.0h. The starting rolling temperature during billet opening is 1150°C, and the final rolling temperature is 950°C, and finally an intermediate billet with a thickness of 20mm is formed; The intermediate annealing treatment in step 3 is an isothermal spheroidizing annealing treatment, wherein the intermediate billet is heated to 890°C and kept at this temperature for 2.0 hours, then furnace-cooled to 760°C and kept at this temperature for 4.0 hours, and then furnace-cooled to below 460°C and air-cooled out of the furnace; The austenitizing temperature in step 4 is 1100°C, and after keeping the temperature for 2.0h, the steel is spray-cooled to 750°C for large deformation and medium-temperature rolling, and the final rolling temperature is 550°C; the rolling process adopts 4 passes, the reduction of each pass is not less than 25%, and the cumulative reduction is not less than 80%; The post-rolling tempering treatment in step 5 is to heat the rolled steel plate to 200-560° C., keep the temperature for 1.0 h, and air-cool to room temperature.

7. The method for preparing super high strength, high toughness and ultra-fine structure hot working die steel according to claim 5, characterized in that: The microstructure of the prepared mold steel is mainly composed of martensite transformed from metastable ultrafine lamellar austenite structure, wherein the lamellar width is ≤2.0 microns, the interior of the lamellar is martensite laths, and the martensite domain is ultrafine equiaxed large-angle grains with a size of ≤1 micron.

8. The method for preparing super high strength, high toughness and ultra-fine structure hot working die steel according to claim 5, characterized in that: There are nano-scale carbides of Cr, Mo and V dispersedly distributed at high density in the martensitic matrix of the prepared die steel.

9. The method for preparing super high strength, high toughness and ultra-fine structure hot working die steel according to claim 5, characterized in that: The prepared die steel can also be used directly after warm rolling without tempering.

10. The method for preparing super high strength, high toughness and ultra-fine structure hot working die steel according to claim 5, characterized in that: The yield strength of the prepared mold steel is ≥2000MPa, the tensile strength is ≥2200MPa, and the elongation at break is not less than 5.0%.

11. The method for preparing super high strength, high toughness and ultra-fine structure hot working die steel according to claim 9, characterized in that: The yield strength of the prepared mold steel is ≥2200MPa, and the tensile strength is ≥2600MPa.

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