CN102212756A - Chromium-molybdenum-vanadium hotwork tool-die steel and heat treatment process thereof - Google Patents

Abstract

The invention relates to a low-cost chromium-molybdenum-vanadium-based hot work die steel and its heat treatment process, which belongs to the technical field of alloy steel manufacturing process. The composition and weight percentage of the steel are: C0.3～0.4%, Si0.6～1.2%, Mn0.6～1.2%, Cr3.5～4.5%, Mo0.8～1.2%, V0.4～0.8% , P<0.02%, S<0.02%, Fe balance. The preparation process of the hot working die steel of the present invention is as follows: batching, smelting, pouring, and then electroslag remelting; at 1200-1280 ° C, homogenization treatment for 8-10 hours, then forging, and then ultra-fine heat treatment and isothermal Annealing treatment, and finally quenching and tempering treatment, that is, heating it to 1000-1100°C for austenitization, and after oil quenching or high-pressure gas quenching, tempering at 560°C-620°C for two to three times. The hot work die steel of the invention has high thermal stability, high toughness and good thermal fatigue performance.

Description

Chromium molybdenum vanadium series hot working tool steel and its heat treatment process

technical field

The invention relates to a chromium-molybdenum-vanadium series hot working tool steel and its heat treatment process. The tool steel not only reduces the alloy cost (compared to the H13 series steel), but also has high thermal stability and good toughness and heat treatment. The invention relates to fatigue performance, which belongs to the technical field of alloy steel manufacturing technology.

Background technique

Tool steel can generally be divided into three major categories of hot work, cold work and plastic tool steel. Hot working tool steel is mainly used to manufacture aluminum alloy die-casting molds and copper-zinc die-casting molds. It is one of the most widely used and most consumed tool steels. Its working conditions are complicated and it takes a long time and heating during work. When the hot metal is put into the cavity of the hot work mold, the surface of the cavity heats up sharply, and the surface layer generates compressive stress and strain; when the metal part is taken out, the surface of the cavity is pulled due to the sharp drop in temperature. The effect of stress and tensile strain can easily cause thermal fatigue, etc., and the hot work die steel is also subjected to a large impact load during its service. Therefore, the tool and mold materials are required to have high thermal strength, high temperature hardness, impact toughness, hardenability, good thermal stability and thermal fatigue resistance. The hot working die steels widely used in my country include tungsten-based 3Cr2W8V, chromium-based H13, and 5CrNiMo and 5CrMnMo used in hot forging dies. Although the tungsten-based 3Cr2W8V has high tempering resistance and high thermal strength, its plasticity, toughness, thermal conductivity and thermal fatigue performance are poor; H13 steel is the most widely used hot work die steel, but its The high-temperature strength is not very high, and the general use temperature cannot exceed 540°C; the thermal strength of 5CrNiMo and 5CrMnMo is low, and it is easy to cause the collapse of the working part of the mold. The hot extrusion die steel currently used in my country is 4Cr5MoSiV1 in the national standard GB/T1299-2000. The chemical composition of this hot extrusion die steel adopts C 0.32-0.45wt%, Cr 4.75-5.50wt%, Mo 1.20-1.75wt%, V 0.80-1.20wt%, Si 0.80-1.2wt%, Mn 0.20-0.5 wt%, P≤0.03wt%, S≤0.03wt%. Since the chemical composition of this hot extrusion die steel contains relatively high molybdenum, chromium and vanadium elements and a certain amount of carbon elements, it belongs to hypereutectoid steel, so the segregation of its material electroslag ingot is serious, and there are A large number of large liquefied carbides make the material insufficient in toughness and prone to early crack failure. Because this material contains a large amount of secondary hardening elements, its tempered secondary carbides are easy to grow, coarsen and undergo type transformation under service conditions, and the alloying elements in the tempered martensite are also easy to precipitate and reduce the hardness of the steel. The strength, thereby reducing the high temperature performance of steel. The performance indicators of this steel are: after quenching at 1030°C + tempering at 590-610°C, the Rockwell hardness value is 44-46HRC, and the impact toughness value ("V" notch) Ak is ≥8J. Performance indicators such as hardness value and impact toughness value in the state are the key technical indicators of steel for hot extrusion dies, and are the main technical parameter indicators to measure the quality of steel for hot extrusion dies. In addition, the resistance to temper softening and thermal fatigue resistance are important performance indicators of hot work die steel.

The metallurgical manufacturing process of the above-mentioned hot work die steel is a process of electric furnace melting, electroslag remelting, and then forging. In its manufacturing process, after the electric furnace smelting and electroslag remelting process is completed, a 500Kg-3000Kg electroslag ingot is obtained, which is forged by a radial forging machine. This manufacturing process has the following problems: 1) The size of the electroslag ingot is small, and the small ingot size reduces the yield and manufacturing capacity of the product; 2) The original structure of the electroslag ingot has a large number of large particles or massive liquefied carbonization 3) The grain size of the material after forging is coarse, resulting in low impact toughness of the material after quenching and tempering, and the product grade is low, which cannot meet the market demand for high toughness hot work die steel need.

Under the current sustainable and economical development mode at home and abroad, the present invention breaks the traditional research direction of low-silicon and high-molybdenum-chromium from an economic point of view, adopts economical silicon and manganese as the main alloying elements, and makes full use of silicon, manganese and The solid solution strengthening of manganese alloy elements and the tempering stability of carbides, etc., and the content of noble alloy elements such as chromium, molybdenum and vanadium should be reduced as much as possible, so as to develop low-temperature alloys with high thermal stability, good impact toughness and thermal fatigue performance. Cost hot work tool steel.

Contents of the invention

The purpose of the present invention is to provide a low-cost chromium-molybdenum-vanadium series hot work tool steel and heat treatment process thereof.

Hot working tool steel of the present invention is characterized in that having following composition and weight percent:

C 0.3～0.4%, Si 0.6～1.2%,

Mn 0.6～1.2%, Cr 3.5～4.5%,

Mo 0.8～1.2%, V 0.4～0.8%,

P ＜0.02%, S ＜0.02%

Fe balance.

The heat treatment process of the above-mentioned low-cost chromium-molybdenum-vanadium series hot work tool steel has the following process steps:

A. Using induction melting or electric arc furnace melting: according to the chemical composition and weight percentage of low-cost chromium molybdenum vanadium series hot work die steel: C 0.3～0.4%, Si 0.6～1.2%, Mn 0.6～1.2%, Cr 3.5～ 4.5%, Mo 0.8~1.2%, V 0.4~0.8%, P<0.02%, S<0.02%, Fe balance, after batching, put it into induction melting or electric arc furnace for melting.

B. Electroslag remelting: Put the electroslag ingots poured out of melting into the electroslag remelting device for electroslag remelting.

C. High-temperature homogenization treatment: heat the steel ingot after electroslag remelting to 1200-1280°C for high-temperature homogenization treatment, keep it warm for 8-10 hours, uniform structure, and eliminate composition segregation and pseudo-eutectic carbide.

D. Forging: Adjust the temperature of the above steel ingots to a temperature range of 1050-1150°C for multi-directional forging; forging ratio ≥ 4, final forging temperature ≥ 850°C.

E. Ultra-fine treatment: the ultra-fine temperature is 1050-1150°C, and the ultra-fine time is 6-10h; then oil cooling (quenching medium) to below 200°C.

F. Isothermal annealing treatment: the first-stage isothermal annealing temperature is 830-850°C, and the annealing time is 6-10h; the second-stage isothermal annealing temperature is 730-750°C, and the annealing time is 6-10h;

G. Quenching and tempering heat treatment: heating to 1000-1100°C for austenitization, using oil quenching (quenching medium); followed by tempering at 560-620°C.

In the high-temperature homogenization treatment, the temperature of the steel ingot is raised in multiple stages to ensure that the temperature inside and outside the steel ingot is uniform, that is, isothermal at 800°C and 1100°C respectively; After the temperature is uniform, forging treatment is carried out.

In the quenching and tempering heat treatment, after heating to 1000-1100°C and oil quenching (quenching medium) to room temperature, tempering treatment is carried out immediately, and the tempering treatment is performed 2 to 3 times, each tempering temperature is 560-620°C, and the holding time for each tempering is 2～4h.

The theoretical basis of its compositional design of hot work tool steel of the present invention is as follows:

Compared with the traditional chromium-based hot work die steel, the hot work die steel reduces the content of Cr, Mo and V, and increases the content of Si and Mn. Recent studies have found that the addition of a certain amount of manganese can increase the matrix strengthening effect of steel and delay the transformation of martensitic structure, improving the temper softening resistance of steel. Although manganese is a weak carbide forming element and cannot form carbide strengthening, the addition of a certain amount of manganese can promote the decomposition of cementite and delay the precipitation and growth of carbides, which is beneficial to the thermal stability of steel . In addition, manganese can increase and stabilize the content of retained austenite in steel, which can improve the toughness and thermal fatigue resistance of steel. Silicon element is not a carbide forming element, but silicon element is an effective element to improve tempering resistance. Increasing the content of silicon element in steel can mainly slow down the decomposition of martensite in steel during tempering. The tempering process after the transformation from tensite to martensite effectively hinders the decomposition of martensite, which is mainly by inhibiting the growth of ε carbides and expanding the stable area of ε carbides, delaying the transformation of ε-carbides to θ- carbide transformation. Silicon delays the transformation of ε→θ, and can fully reduce the growth rate of cementite in steel during tempering. Silicon atoms precipitate from the θ phase and form a silicon atom-enriched region around the θ phase, inhibiting the growth of the θ phase. Growth and coarsening; in addition, silicon can effectively improve the temper softening resistance of steel. Due to the strong affinity between V and carbon, it is easy to form VC primary carbides during the smelting process. This carbide has a large particle size, which not only does not improve the performance of the steel, but also reduces the toughness and thermal fatigue performance of the steel. It is difficult to completely eliminate it in the subsequent heat treatment process. Therefore, appropriately reducing the V content in steel can effectively reduce the proportion of VC primary carbides and improve the performance of steel. However, during the tempering process, V can reduce the decomposition rate of martensite and delay the transformation of austenite, and V forms MC-type secondary carbides, which are fine and dispersed, and are not easy to aggregate and grow. During the tempering process, Enhanced secondary hardening effect, greatly improving the thermal stability and impact toughness of steel. Therefore, the content of V in the steel is controlled between 0.4% and 0.8%, so as to give full play to the alloying effect of V. Cr mainly forms Cr23C6 carbides in hot work die steel, which are easy to precipitate along the grain and grow and coarsen, reducing the thermal stability and thermal fatigue performance of the material. The chromium content of this hot work die steel is somewhat reduce this adverse effect.

The thermal stability, impact toughness and thermal fatigue performance of the hot working die steel of the present invention are better than those of the H13 steel after the above heat treatment.

Description of drawings

Fig. 1 is the annealed structure and tempered structure of the hot work die steel of the present invention after the above-mentioned heat treatment process.

Fig. 2 is the quenched structure of the hot work die steel of the present invention after the above heat treatment process.

Fig. 3 is the tempered structure of the hot work die steel of the present invention after the above heat treatment process.

Fig. 4 is a comparison of the thermal stability data of the hot working tool steel of the present invention at 620°C with that of H13 steel.

Fig. 5 is a photo of the thermal fatigue crack surface and cross-section of the hot work die steel of the present invention.

Figure 6 shows the thermal fatigue crack surface and section photos of H13 steel.

Fig. 7 is a comparison of cross-sectional hardness gradients of thermal fatigue samples of the hot working tool steel of the present invention and H13 steel.

Detailed ways

Specific embodiments of the present invention are now described below.

Embodiment one

In this example, the composition and weight percentage of the hot work die steel are as follows:

C 0.32%, Si 1.0%, Mn 1.0%, Cr 4.0%, Mo 1.0%,

V 0.6%, P 0.01%, S 0.01%, Fe balance.

In the present embodiment, the technical process and steps of hot working tool steel are as follows:

A Electric furnace smelting: smelting in an electric arc furnace according to the above alloy element ratio, the melting temperature is greater than 1500 ° C, casting into φ400mm-φ450mm electrode rods and air cooling;

B Electroslag remelting: The poured steel ingot is placed in the electroslag remelting device as a consumable electrode for electroslag remelting. The slag melting voltage is 56-62V, the current is 3000-5000A, and the electric system voltage is 57-59V. , current 11000-12000A, capping voltage 57-59V, current time 35-50Min, electroslag remelting into 500Kg-3000Kg electroslag ingot;

C High-temperature homogenization: heat the steel ingot after electroslag remelting to 1250°C for high-temperature homogenization treatment, keep it warm for 10 hours, uniform structure, and eliminate composition segregation and pseudo-eutectic carbide;

D Forging: Adjust the temperature of the above-mentioned high-temperature homogenized steel ingot to a temperature range of 1100°C for multi-directional forging; the forging ratio is ≥4, and the final forging temperature is ≥850°C.

E Superfine treatment: The superfine temperature is 1100°C, and the ultrafine time is 8h; then oil cooling (quenching medium) to below 200°C.

F Isothermal annealing treatment: the first stage isothermal annealing temperature is 830-850°C, annealing time is 8h; the second stage isothermal annealing temperature is 730-750°C, annealing time is 8h; then furnace cool to room temperature

G Quenching and tempering treatment: Quenching temperature is 1050°C, oil quenching is used, and tempering is carried out twice at 600°C, each tempering is 2 hours.

After the hot working tool steel of the present invention is through above-mentioned heat treatment, carry out performance test:

A hardness test:

Quenching hardness: 51HRC; Tempering hardness: 48HRC

B impact toughness test:

Take a transverse impact sample on the billet, and the sample size is 7mm×10mm×55mm (according to the standard of North American Die Casting Association).

Impact energy value at room temperature: ≥280J.

C thermal stability:

The hot working die steel of the present invention is subjected to a stability comparison test with H13 steel at 620°C. After quenching and tempering, the H13 steel has the same hardness value as the steel of the present invention, both of which are 49HRC. The test results are shown in Figure 4 . It can be seen from accompanying drawing 4 that although the hardness value of the hot work die steel of the present invention is consistent with that of H13 steel before the start of the experiment, at 620°C, the hot work die steel of the present invention has Steel is better than H13 steel.

D Thermal fatigue performance test:

Under the condition of 700 ℃, carry out cold and heat cycle, after 3000 times, compare the thermal fatigue surface morphology (as shown in Figure 5) and cross-sectional morphology (as shown in Figure 6) of the hot work die steel of the present invention and H13 steel ) and cross-sectional hardness gradient (as shown in Figure 7). It can be seen from the figure that after the thermal fatigue test of the hot work die steel of the present invention, the surface cracks are very uniform and small, and no relatively large main cracks are formed on the surface. However, the surface cracks of H13 steel are in the form of a network, and there are several main cracks with large widths, and the cracks are connected to each other in a cracked shape. In addition, it can be seen from the cross-sectional hardness gradient distribution that the hardness decrease of the H13 steel is significantly lower than that of the hot work die steel of the present invention. It can be seen from the comparison between the two that the thermal fatigue performance of the hot work die steel of the present invention is stronger than that of the H13 steel.

Claims (5)

1. A chromium-molybdenum-vanadium-based hot work tool steel, characterized in that the chemical composition and weight percentage of the steel are: C 0.3-0.4%, Si 0.6-1.2%, Mn 0.6-1.2%, Cr 3.5-4.5% , Mo 0.8～1.2%, V 0.4～0.8%, P<0.02%, S<0.02%, Fe balance. 2. The chromium-molybdenum-vanadium series hot work die steel according to claim 1, characterized in that the chemical composition and weight percentage of the steel are: C 0.32%, Si 1.0%, Mn 1.0%, Cr 4.0%, Mo 1.0 %, V 0.6%, P<0.02%, S<0.02%, Fe balance. 3. the heat treatment process of the chromium molybdenum vanadium series hot work tool steel according to claim 1 or 2, it is characterized in that the process has the following steps: A. According to the chemical composition and weight percentage of low-cost chromium-molybdenum-vanadium series hot work tool steel: C 0.3～0.4%, Si 0.6～1.2%, Mn 0.6～1.2%, Cr 3.5～4.5%, Mo 0.8～1.2% %, V 0.4～0.8%, P<0.02%, S<0.02%, Fe balance, batching, induction melting or electric arc furnace melting, and then electroslag remelting; B. High-temperature homogenization treatment: the homogenization temperature is 1200-1280 °C, and the homogenization time is 8-10 hours; then forging: heating to 1100-1150 °C for multi-directional forging processing, forging ratio ≥ 4, Final forging temperature ≥ 850°C; C. Ultra-fine treatment: the ultra-fine temperature is 1050-1150°C, and the ultra-fine time is 6-10h; then the oil is cooled to below 200°C; D. Isothermal annealing treatment: the first-stage isothermal annealing temperature is 830-850°C, and the annealing time is 6-10h; the second-stage isothermal annealing temperature is 730-750°C, and the annealing time is 6-10h; E. Quenching and tempering heat treatment: heating to 1000-1100°C, oil quenching; followed by tempering at 560-620°C. 4. the heat treatment process of chromium-molybdenum-vanadium series hot-working tool steel according to claim 3, is characterized in that described high-temperature homogenization treatment, divides multi-stage heating in the steel ingot heating process, guarantees that the temperature inside and outside the steel ingot is even, that is Isothermal at 800°C and 1100°C respectively; after high-temperature homogenization treatment, isothermal at 1100-1150°C, and forging when the temperature of the steel ingot is uniform. 5. The heat treatment process of chromium-molybdenum-vanadium-based hot work die steel according to claim 3, characterized in that the quenching and tempering heat treatment is heated to 1000-1100 ° C and oil-quenched to room temperature, and then tempered immediately. Fire treatment for 2 to 3 times, each tempering temperature is 560-620°C, and the holding time for each tempering is 2-4 hours.

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