CN103938096A - High-strength high-toughness hot work die steel and preparation method thereof - Google Patents

Abstract

A high-strength, high-toughness hot work die steel and a preparation method thereof belong to the technical field of die steel. The chemical composition weight percent of the steel is: C:0.30～0.40%, Si:0.20～0.40%, S≤0.006%, P≤0.01%, Mn:0.40～0.6%, Mo:1.70～2.20%, Cr:5.00～ 5.40%, V: 0.50-0.60%, Co: 0.50-0.60%, and the rest are Fe and unavoidable impurities. Compared with the existing hot work die steel H13, the steel of the invention has the advantages of high strength, high toughness and good comprehensive performance. The steel of the present invention is quenched at 1030°C, tempered at a temperature above 540°C, has higher tempered hardness and strength, and has higher impact toughness than H13 on the basis of maintaining high hardness and strength, so that the present invention Steel can be used in workplaces that require higher hardness without losing the toughness of the material. It is especially suitable for the manufacture of die-casting molds of copper, aluminum and their alloys that require high strength, high toughness, and high fatigue resistance. service life.

Description

A kind of high-strength high-toughness hot work die steel and its preparation method

technical field

The invention belongs to the field of die steel, in particular to a high-strength, high-toughness hot-work die steel and a preparation method thereof. Suitable for hot extrusion, mandrel, forging die, die insert for precision forging machine, die-casting and other hot-working dies, especially for die-casting of copper, aluminum and their alloys that require high strength, high toughness and high fatigue resistance mold. the

Background technique

The mold is an important process equipment in the manufacturing industry. The high precision, high complexity, high consistency, high efficiency and low consumption of the parts produced by the mold are unmatched by other processing and manufacturing methods. With the rapid development of industrial technology In order to reduce production costs, improve production efficiency and product quality, and improve material utilization, domestic and foreign manufacturing industries widely adopt non-cutting and less-cutting processing techniques. During the service of the mold, the mold will fail due to the following three reasons, first, impact fracture and thermal fatigue cracks caused by impact load and repeated heating and cooling; second, mold control heat due to high temperature metal flow Erosion wear; third, the cavity size wear caused by the interaction of mechanical stress and thermal stress. In use, every hot work die will have thermal fatigue cracks, which is the fatigue of the hollow surface of the die caused by repeated deformation caused by the periodic change of temperature at each impact, and its limitation on the life of the die is considered normal , but after a certain number of impacts, the cracks become thicker, so that the surface finish of the casting cannot meet the requirements, and the mold is scrapped at this time. High hot hardness, strength, temper softening resistance, thermal conductivity and low thermal expansion coefficient have a great effect on improving the thermal cracking resistance of hot work die steel. Therefore, the use of high hardness and strength mold materials can delay the appearance of thermal fatigue cracks, but if the hardness and strength are too high, the mold is prone to large cracks and overall fracture. At this time, the mold is completely scrapped, and even industrial accidents may occur. High toughness can prevent thermal fatigue cracks from extending into the mold, so in order to reduce the tendency of hot cracking and enhance the resistance of coarse cracks, high and isotropic plasticity and toughness are very important. The traditional H13 steel is a hot work die steel with a wide range of applications. This steel is strong and tough, but its service temperature range is limited. It is not necessarily suitable for all hot work die applications. The toughness can be improved at the same time, so the use range of the mold material can be expanded, and the service life of the mold can be improved at the same time. the

Contents of the invention

The object of the present invention is to provide a high-strength and high-toughness hot work die steel and its preparation method, which can replace the traditional H13 steel, and the heat strength, toughness and tempering stability of the hot work die steel exceed the H13 steel. the

According to the above purpose, the steel of the present invention refers to steel grades such as S7 in the United States, appropriately reduces the content of alloy elements Si and V on the basis of H13 (4Cr5MoSiV1), and adds a small amount of Mo and Co, so that the comprehensive performance of the steel grade exceeds H13 steel. The technical scheme adopted in the present invention is: (1) reduce the content of Si and V, reduce the amount of primary carbide in the material, so that the material has higher toughness; (2) appropriately increase the content of carbide forming elements Mo, W, Nb, To make up for the lack of high-temperature strength caused by the reduction of V content, it also improves the grain level in the quenching process, improves the secondary hardening effect, precipitates nano-scale Mo ₂ C during tempering, and improves the high-temperature strength of the material; (3) Appropriately increase Co content, increase the solid solution content of Co element to make up for the lack of strengthening effect of low Si content, and further improve the high temperature strength of the material. Although the alloy elements added in the steel of the present invention are alloy elements often added in hot work die steel, the range of variation of each element is obtained through a large amount of experimental data, and the final composition of the steel of the present invention is hot work die steel with the same alloy content. The composition ratio with the best comprehensive performance in die steel.

According to above-mentioned purpose and overall technical scheme, the concrete technical scheme of the present invention is:

The specific chemical composition (weight %) of the steel of the present invention is: carbon C: 0.30-0.40%, silicon Si: 0.20-0.40%, sulfur S≤0.006%, phosphorus P≤0.01%, manganese Mn: 0.40-0.6%, molybdenum Mo: 1.50-2.20%, chromium Cr: 5.00-5.40%, vanadium V: 0.50-0.60%, cobalt Co: 0.50-0.60%, and the rest is Fe and unavoidable impurities. the

Preferably, the mold steel includes: carbon C: 0.34-0.39%, silicon Si: 0.20-0.40%, sulfur S≤0.006%, phosphorus P≤0.01%, manganese Mn: 0.40-0.6%, molybdenum Mo : 1.70-2.20%, chromium Cr: 5.00-5.40%, vanadium V: 0.50-0.60%, cobalt Co: 0.50-0.60%, and the rest are Fe and unavoidable impurities. the

Preferably, the die steel further includes: less than 0.2% niobium. the

Preferably, the mold steel also includes 0-1% tungsten W, and the chemical composition content should also satisfy: 1.5%≤W+Mo≤2.5%. the

The functions and ratios of the above-mentioned elements are as follows, and "%" in the following descriptions means "mass percentage":

C: The carbon content in the steel determines the hardness of the matrix of the quenched steel. For hot work die steel, part of the carbon in the steel enters the matrix of the steel to cause solid solution strengthening. Another part of the carbon will combine with the carbide forming elements in the alloying elements to form alloy carbides. For hot work die steel, except for a small amount of residual alloy carbide, it is also required to disperse and precipitate on the quenched martensite matrix during tempering to produce secondary hardening, so that the uniformly distributed residual alloy carbide and Tempered martensite structure to determine the performance of hot work die steel. A large number of studies on low-carbon martensite at home and abroad show that: in order to obtain better comprehensive mechanical properties, the carbon content in steel should be controlled at 0.34-0.39%. the

Si: As an alloying element in steel, silicon exists in ferrite or austenite in the form of solid solution, does not form carbides, increases annealing, normalizing and quenching temperatures, and improves hardenability. Because silicon has a promoting effect on segregation, it is easy to form a band structure in the steel, so that the transverse performance is lower than the longitudinal performance. Therefore, on the basis of H13 steel, the silicon content is appropriately reduced, and the silicon content in the steel of the present invention is controlled at 0.20-0.40%. . the

S: Sulfur is easy to combine with manganese in steel to form non-metallic inclusions MnS, which are usually elongated into strips along the processing direction during hot working, which has a great impact on the transverse toughness of steel and reduces the isotropy of steel Performance. Sulfur is often considered as a harmful element in hot work die steel. Therefore, it should be reduced as much as possible when metallurgical conditions permit. The sulfur content in the steel of the present invention should be controlled below 0.006%. the

P: Phosphorus forms microscopic segregation when the molten steel solidifies, and then segregates at the grain boundary when heated at the austenitizing temperature, which significantly increases the brittleness of the steel. Control the phosphorus content below 0.01%, and the lower the content, the better. the

Mn: In addition to improving hardenability, it can also eliminate the harmful effects of sulfur. In the present invention, the content of Mn is controlled at 0.4-0.6%. the

Mo, W: Both molybdenum and tungsten are strong carbide forming elements, which can improve the hardenability of steel in steel, and at the same time form special carbides in steel to improve the secondary hardening ability and tempering stability of steel. In the invention steel, in order to control the amount of VC primary carbides, the content of vanadium is reduced, in order not to affect the secondary hardening ability of the steel, the content of molybdenum element (1.70-2.20%) is appropriately increased, and adding part of tungsten can improve the thermal strength role. Experiments have proved that the added molybdenum is more combined with carbon, and more fine and short rod-shaped Mo ₂ C carbides are precipitated during tempering, which plays a great role in improving the tempering stability of the steel of the present invention.

Nb: Niobium is a strong carbide-forming element, and its solid solubility product with carbon is very small, so it is easy to form very stable MC-type carbides. Niobium and vanadium have many similar properties, therefore, the recombination of Nb and V increases the effect of V. In addition, Nb also has the effect of refining grains and refining the as-cast structure. However, in the hot work die steel, Nb is easy to form large liquefied carbides during the solidification process, which is unfavorable to the toughness. Therefore, the Nb in the steel of the present invention is controlled below 0.2%. the

Cr: Chromium forms carbides, which can improve the hardenability and high temperature wear resistance of steel in hot work die steel. Chromium dissolves in austenite when quenching and heating, and solid dissolves in martensite after quenching, which can improve the temper softening resistance of the steel. It is precipitated from the matrix during tempering and generally forms Cr ₂₃ C ₆ alloy carbides. The increase of the tempering temperature and the prolongation of the tempering time have a tendency to coarsen. The steel of the present invention uses a chromium content of 5.00-5.40% which is equivalent to that of the H13 steel.

V: Vanadium can reduce the overheating sensitivity tendency of steel. A small amount of vanadium can refine the steel grains, and when the carbides are dispersed and precipitated after proper heat treatment, vanadium can improve the high-temperature durable strength and creep resistance of the steel. Adding 0.1-0.3% vanadium to low-alloy steel will significantly Effect. In martensitic steel, a vanadium content of 0.5% can produce sufficient secondary hardening effect. Excessive vanadium content will increase the formation probability of primary carbide VC in steel, and the existence of a large amount of primary carbide will significantly affect the toughness of steel and reduce the ability of hot work die steel to resist large cracks. The vanadium content in the H13 steel is 0.80-1.2%, and the vanadium content in the steel of the present invention is controlled at 0.50-0.60%

Co: Cobalt is mainly dissolved in the matrix, almost no carbide is formed in the steel, and only a very small amount of cobalt atoms can enter the precipitated phase. Therefore, cobalt mainly plays a solid solution strengthening role at high temperatures. Cobalt prevents and delays the aggregation of special carbides of other elements during tempering or use. In the steel of the present invention, the addition of Co has a certain effect on delaying the aggregation and coarsening of Cr carbides. Therefore, the tempering of hot work die steel can be improved. stability. Cobalt is a particularly important element in the steel of the present invention, and its content is controlled at 0.50-0.60%. the

Preparation method of the present invention is that the technical parameters controlled in the forging and annealing process are as follows:

Forging: heating and holding at 1140-1180°C for 1 hour (please give the parameter range of holding time), start forging at 1100-1150°C, the final forging temperature shall not be lower than 900°C, and slowly cool to room temperature at ≤30°C/h;

Annealing process: 400 ℃ heat into the furnace, 880 ℃ heat preservation for 5 hours, the furnace is cooled to 500 ℃ with a cooling rate of less than 30 ℃, and the furnace is air-cooled. the

Compared with the existing hot work die steel H13, the steel of the invention has the advantages of high strength, high toughness and good comprehensive performance. The steel of the present invention is quenched at 1030°C, tempered at a temperature above 540°C, has higher tempered hardness and strength, and has higher impact toughness than H13 on the basis of maintaining high hardness and strength, so that the present invention Steel can be used in workplaces that require higher hardness without losing the toughness of the material. It is especially suitable for the manufacture of die-casting molds of copper, aluminum and their alloys that require high strength, high toughness, and high fatigue resistance. service life. the

Detailed ways

According to the chemical composition range designed above, 4 furnaces of the steel of the present invention and 1 furnace of comparative steel (H13) were smelted on a 25Kg vacuum induction furnace, and their specific chemical compositions are shown in Table 1. Molten steel is cast into ingots, heated and kept at 1140-1180°C, forged at 1100-1150°C, final forged at ≥900°C, and slowly cooled to room temperature. □15mm bar. The steel of the present invention and the comparison steel are hot-loaded into a furnace at 400°C at the same time, kept at 880°C for 5 hours, and cooled to 500°C at a cooling rate of less than 30°C. After annealing, it is processed into a sample, which is quenched and tempered (quenched at 1030°C and tempered at 510-650°C). The mechanical properties at room temperature are shown in Tables 2-5.

Steel of the present invention

1. After quenching at 1030°C and high-temperature tempering at 570°C and 600°C, the hardness of the steel of the present invention is higher than that of the comparison steel, and the hardness of the tempering at other temperatures is equivalent to that of the comparison steel (see Table 2).

2. After quenching at 1030°C and high-temperature tempering at various temperatures, the tensile strength of the steel of the present invention is equivalent to that of the comparison steel (see Table 3)

3. After quenching and tempering, the impact toughness of the steel of the present invention is higher than that of the comparison steel (see Table 4)

4. After quenching and tempering treatment (quenching and tempering process 1030°C×30min oil cooling quenching+600°C×2h air cooling and tempering twice), the steel of the present invention has stronger red hardness and temper softening resistance ( See Table 5). the

The chemical composition of table 1 steel embodiment of the present invention and contrast steel, weight %

Table 2 Hardness values of steel examples of the present invention and comparative steels quenched at 1030°C and tempered at different temperatures

Table 3 steel embodiment of the present invention and the tensile strength table of comparison steel

U-shaped notch impact toughness table of table 4 steel embodiment of the present invention and contrast steel

Table 5 steel embodiment of the present invention and the anti-temper softening hardness data table (HRC) of contrast steel

(Note: Quenching and tempering process 1030℃×3Omin oil cooling quenching + 600℃×2h air cooling tempering twice).

Claims (5)

1. A high-strength and high-toughness hot work die steel, characterized in that the chemical composition weight percent is: carbon C: 0.30～0.40%, silicon Si: 0.20～0.40%, sulfur S≤0.006%, phosphorus P≤0.01% , manganese Mn: 0.40-0.6%, molybdenum Mo: 1.50-2.20%, chromium Cr: 5.00-5.40%, vanadium V: 0.50-0.60%, cobalt Co: 0.50-0.60%, the rest is Fe and unavoidable impurities thing. the 2. The hot work die steel according to claim 1, characterized in that carbon C: 0.34-0.39%, molybdenum Mo: 1.70-2.20%. the 3. The hot work die steel according to claim 1 or 2, characterized in that the chemical composition further includes: 0.2% or less niobium Nb. the 4. The hot work die steel according to claim 1 or 2, characterized in that it further includes 0-1% tungsten W, and the chemical composition content should also satisfy: 1.5%≤W+Mo≤2.5%. the 5. A method for preparing the hot work die steel according to claims 1 to 4, characterized in that the technical parameters controlled in the forging and annealing process are as follows: Forging: heating and holding at 1140-1180°C for 1 hour, starting forging at 1100-1150°C, final forging temperature not lower than 900°C, slow cooling to room temperature at ≤30°C/h; annealing process: hot charging at 400°C, 880°C Keep warm for 5 hours, cool to 500°C at a cooling rate less than 30°C, and take out the furnace to air cool. the