CN104152916A - Thermal treatment and plasma nitrocarburizing surface treatment process method for special wear-resistant die steel with ultrahigh heat conductivity for hot stamping - Google Patents

Abstract

A heat treatment and plasma nitrocarburizing surface treatment process for ultra-high thermal conductivity wear-resistant die steel (HDCM) for high-strength steel plate/tube hot stamping. The heat treatment process is quenching at 1050-1000°C, tempering twice at 560-600°C; the surface treatment process is voltage 600-850V, atmosphere ratio NH ₃ : CO ₂ =10:1-16:1, furnace pressure 450-700Pa, nitriding The temperature is 520°C-550°C, and the nitriding time is 8h-12h. In terms of comprehensive performance, the performance of the special hot stamping die steel is: the hardness is 51-53HRC, and the impact toughness value Ak is about 170-200J; The micro-Vickers hardness is 750-850HV ₃₀₀ , and the thickness of the infiltrated layer is 150-260μm. The hardness value of the mold steel of the invention is higher than that of general heat treatment H13, can improve the wear resistance of the mold, can effectively prevent the roughening of the mold surface, and can greatly prolong the service life of the mold after plasma surface nitriding.

Description

Process method of heat treatment and plasma nitrocarburizing surface treatment of ultra-high thermal conductivity wear-resistant die steel for hot stamping

technical field

The invention relates to a process method for heat treatment and plasma nitrocarburizing surface treatment of ultra-high thermal conductivity wear-resistant die steel special for high-strength steel plate/pipe hot stamping, which is applied in the technical field of alloy steel heat treatment and surface treatment.

Background technique

Die steel is usually divided into three categories according to the object of mold use: cold work die steel, hot work die steel and plastic mold steel. Hot stamping die steel belongs to hot work die steel. With the rapid development of the automobile industry and the continuous improvement of the demand for lightweight automobiles, the hot stamping forming process is mainly used to obtain ultra-high-strength stamping parts. When obtaining ultra-high-strength steel, due to the extremely high forming force required by the cold stamping forming process, it is easy to cause mold damage, equipment vibration, and problems such as cracking, wrinkling, rebound, and poor dimensional accuracy. Therefore, the original cold stamping forming process no longer meets the needs of technology and production development, while the hot stamping forming process utilizes metals at high temperatures, whose plasticity and ductility increase rapidly, and the yield strength decreases rapidly. Hot stamping die steel is one of the die steels whose usage has risen sharply and consumed the most in recent decades, and its working conditions are more complicated than that of general hot work die steel.

The steel plate hot stamping process is to heat a special high-strength steel plate to the austenitic temperature range, quickly move to the mold, quickly stamp, and quench the part through the mold (instead of air) with a cooling circuit in the state of the press holding pressure Cool (and ensure a certain cooling rate), and finally obtain ultra-high-strength stampings (the structure is martensite, and the strength is around 1500MPa or even higher). When working, because the mold is in direct contact with the heated blank, when the hot metal is put into the cavity of the hot stamping mold, the surface of the cavity heats up sharply, and the surface layer generates compressive stress and strain during stamping and holding pressure, which makes the mold need Good thermal strength and thermal stability; during the pressure holding process, the parts are quenched through the mold with cooling channels, in order to make the mold quickly take away the heat of the steel plate and ensure the precision of the mold during service , the mold material must have a large thermal conductivity and a small thermal expansion coefficient; when the metal part is taken out, the surface of the cavity is subjected to tensile stress and tensile strain due to the sharp drop in temperature. It is prone to thermal fatigue; and the hot stamping die steel is subject to a large impact load during service, so the die must have excellent toughness; in order to prevent the roughening of the die surface during service, the die must also have a high hardness. In addition, since the steel plate after austenitization is transferred from the furnace to the mold, a large amount of oxide scales will be produced in contact with oxygen and water, and the carbide particles that fall off during the use of the mold will be worn during the relative movement between the mold and the workpiece. The mold has good wear resistance. Therefore, complex working conditions require hot stamping die materials to have high thermal conductivity, thermal strength, hardness, impact toughness, wear resistance, hardenability, thermal stability, and thermal fatigue resistance.

At present, the longest used hot stamping die steel in my country is the national standard GB/T1299-2000 steel grade 4Cr5MoSiV1 (equivalent to North American standard H13 steel). The chemical composition of this hot stamping 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 0.20-0.5wt% %, P≤0.03wt%, S≤0.03wt%. At present, Japanese companies use SKD61 hot work die steel (equivalent to North American standard H13 steel), Swedish hot stamping die suppliers use ORVAR, and German companies use CR7V and 1.2379.

Heat treatment has a crucial influence on the performance of die steel. Heat treatment determines the internal microstructure of die steel, thereby determining the strength and toughness of die steel. The mold can obtain the required hardness through different heat treatments, but its structure and performance may be significantly different. In order to give full play to the toughness of the alloy elements of the mold steel, different mold steels for different purposes must have corresponding heat treatment processes. Plasma surface nitriding temperatures agree well with the tempering temperatures of hot work tool steels, and compared to other surface treatments, it is a well-established technology that is low cost and highly reliable. The phase composition and properties of the surface compound layer and diffusion layer can be well controlled by the atmosphere, nitriding temperature, and time. Through simple nitriding treatment, the service life of the mold can be greatly improved, and it has a very high cost performance.

Shanghai University invented a high-strength steel plate/tube hot stamping ultra-high thermal conductivity wear-resistant die steel HDCM, the weight percentage of chemical elements is: C: 0.33-0.40%; Si: <0.30%; Mn: <0.30%; W: 1.0-2.0%; Mo: 4.0-5.0%; Cr: <0.30%; V: 0.10-0.20%; the rest is Fe and unavoidable impurity elements, among which S: ≤0.01%; P: ≤0.02 %; O: ≤30ppm. . In addition, the invention also provides a manufacturing method of hot stamping die steel, which adopts electroslag remelting process, high temperature homogenization and ultra-fine heat treatment process to control the material structure, so as to strengthen the material and improve its performance index . Compared with existing materials, the thermal conductivity of the hot stamping die steel described in the invention is 80% higher than that of H13 steel in the range of 100-200°C, and has good wear resistance. the

Analysis of Chinese and foreign patent retrieval content for related technologies

Searching Chinese and foreign patents by entering keywords related to the content of the present invention found that the patent documents related to the heat treatment of hot work die steel and its surface treatment technology related to the present invention include:

1) Patent application number: CN201210146712.5, name: high silicon and high manganese type high thermal stability hot work die steel and its heat treatment process, the steel is characterized in that the ratio of high content of silicon and manganese in the chemical composition is kept at 1: 1 unchanged, the mass percentage of each main alloying element is: C0.25～0.45%, Si0.8～2.0%, Mn0.8～2.0%, Cr3.5～4.5%, Mo0.6～1.2%, V0. 4～0.8%, P<0.02%, S<0.02%, Fe balance. The preparation process of the hot work die steel of the present invention is as follows: batching, smelting, pouring, and then electroslag remelting; high-temperature homogenization heat treatment, and then multi-directional forging heat processing; then superfine heat treatment and isothermal annealing treatment; finally Quenching and tempering heat treatment, that is, heating it to 980-1100°C for austenitization, and after oil cooling or water mist cooling, tempering at 540°C-600°C for two to three times.

2) Patent application number: CN201210139562.5, name: Heat treatment process of automobile forging die steel H13, which is characterized in that it includes the following process steps: (1) Put the die steel into the heating furnace, the temperature of the heating furnace is <300°C, and the temperature is raised To 760~780°C, methanol is introduced as a protective atmosphere, and the amount of protective atmosphere is: 3~5×furnace volume/hour; (2) First, continue to heat the heating furnace to 820~850°C for the first stage heat treatment, The heating time is: the length of the mold steel × 1.2~1.8min; then the heating furnace is heated to 1020~1050°C for the second stage heat treatment, the heating time is: the length of the mold steel × 1.5~2.2min; (3) quenching and cooling: the mold The steel is cooled in a quenching nitrate furnace at 160~180°C, and the cooling time is 1.5~2.5 hours; then the mold steel is naturally cooled to 120~160°C in the air, and the cooling time is 1.5~2.5 hours; ( 4) Tempering treatment: Temper the quenched and cooled die steel at 530-600°C for a holding time of 5-9 hours. After tempering, naturally cool to room temperature in the air; (5) repeat steps (4) ) for 2 to 3 times to complete the heat treatment process of the mold steel; the unit of the length of the mold steel is mm.

3) Patent application number: CN200710039838.1 invention discloses a nitriding method for grain-oriented silicon steel, the main features include plasma nitriding treatment in the plasma nitriding chamber, by controlling the nitriding temperature in the plasma nitriding chamber to 500-700°C , The nitriding time is 10-30 seconds, the nitriding atmosphere is 0-10% of H2, 0-5% of NH3, and the rest is N2, the vacuum degree of the plasma nitriding chamber is controlled to be 1000-2000Pa, and the plate voltage is 700-1000V , to control the infiltration amount, distribution and nitride size of the N element infiltrated into the grain-oriented silicon steel. The plasma nitriding method used in the production of oriented silicon steel can make the nitriding effect uniform, enhance the inhibitory force of the inhibitor, and improve the process stability.

4) Patent application number: CN200410017949.9 A method for surface modification treatment of precision forging dies for valves, mainly using nitriding technology, the method is characterized by the following processes and steps: a. Using the original hot-worked die steel After quenching and tempering, the manufactured valve precision forging die is put into a special plasma heat treatment equipment, and a nitrogen-containing atmosphere mainly composed of ammonia gas is introduced, and plasma infiltration is carried out for 2-8 hours at a temperature range of 500-550°C. Nitrogen treatment; b. At the end of plasma nitriding, add ammonia and acetone mixed gas, and carry out plasma nitrocarburizing treatment at the above-mentioned temperature of 500-550°C for 1-3 hours. The volume ratio of ammonia gas and acetone gas 10:1~5:1; c. Then put it into a steam oxidation furnace, and carry out surface oxidation treatment on the mold under the steam pressure of 0.05~0.25MPa and the temperature range of 500-550℃. d. After the above composite process, the surface of the mold is uniform blue-gray, and the surface structure of the mold is formed into three layers, that is, the outermost layer is an oxide film with a thickness of 3-10 μm; the second layer is a nitrogen and carbon diffusion layer. , the thickness is 100 ~ 400μm, the third layer is the core of the matrix, below the subsurface, is the matrix structure of the hot work die steel.

Comparative analysis between the present invention and prior art

The comparative analysis is as follows: From the comparison of the components, it can be seen that the element content of the chemical composition of the present invention is obviously different from that of patent 1-2, the mechanism of the element's effect on the performance of the material is also different, and the application environment of the material is also different.

1) The similarity between the patent CN201210146712.5 and the present invention is that it adopts the process of quenching and tempering twice to obtain a stable tempered martensite structure. The invention obtains the compound-free nitriding layer by controlling parameters such as plasma nitriding atmosphere ratio, nitriding time, furnace pressure, voltage, nitriding temperature and the like. While improving the surface hardness of the mold, optimize the corrosion resistance, wear resistance and anti-adhesion performance of the mold. Compared with the process of ordinary plasma nitriding to obtain a compound layer and a diffusion layer sequentially from the surface to the core, the compound-free layer obtained in the present invention avoids the brittleness of the compound layer, improves the thermal fatigue performance of the mold, and avoids compound layer The hard abrasive grains produced by breaking and falling off are beneficial to the wear resistance of the mold.

2) Patent CN201210139562.5 is used for the heat treatment of H13 steel for automobile forging dies. The difference from the present invention lies in its quenching method. The die steel is cooled in a quenching nitrate furnace at 160-180 ° C, and the cooling time is 1.5 ~ 2.5 hours; then the mold steel is naturally cooled in the air to 120-160°C in the nitrate furnace, and the cooling time is 1.5-2.5 hours; the present invention is vacuum air quenching. Compared with patent CN201210139562.5, the present invention tempers at 560-600°C for different times according to the size of the mold. In this study, a vacuum high-pressure gas quenching furnace was used, and the cooling method was air cooling to room temperature. And after heat treatment, the service life of the mold is further improved by plasma nitriding process.

3) Patent CN201210099323.1 is a nitriding method for grain-oriented silicon steel. By adopting the plasma nitriding method in the production of grain-oriented silicon steel, the nitriding effect can be uniformed, the inhibitory force of the inhibitor can be enhanced, and the process stability can be improved. And the present invention is all different on processing material and nitrocarburizing process. This patent research improves the wear resistance of the mold by plasma nitrocarburizing on the mold material HDCM for hot stamping.

4) Patent CN200410017949.9 is a surface modification processing method of valve precision forging die, which adopts nitriding + oxidation process. The valve precision forging die made of original hot work die steel is put into special plasma heat treatment equipment for plasma nitriding treatment after quenching and tempering; when the plasma nitriding is finished, the mixed gas of ammonia and acetone is injected into the plasma nitrogen carbon Co-infiltration treatment, and then placed in a steam oxidation furnace for surface oxidation treatment of the mold. After the process treatment, the surface structure of the mold is formed into three layers, that is, the outermost layer is an oxide film with a thickness of 3-10 μm; the second layer is a nitrogen and carbon diffusion layer with a thickness of 100-400 μm, and the third layer is based on heat. As the matrix structure of die steel. And this patent designs a new type of hot stamping die material and its plasma nitrocarburizing process, wherein the nitrocarburizing process is different from the patent CN200410017949.9 in temperature, atmosphere and other nitriding process parameters, and the obtained surface structure is also Differently, the final structure obtained in the present invention is a surface compound white layer with a hardness of 6-17 μm and a hardness of 750-850HV ₃₀₀ ; the second layer is a diffusion layer, about 150-260 μm; the third layer is a mold material matrix.

Relevant mechanism of the present invention and the analytical explanation of selection process parameter

Below is the analysis explanation to each parameter of the present invention:

1. Quenching temperature 1050-1000°C: The special die steel for hot stamping is hypereutectoid steel. When the quenching temperature is low, the hardness of the mold after tempering is too low; when the quenching temperature is high, the austenite grain grows and the material Decreased strength and poor toughness. Finally, the quenching temperature is selected to be 1050-1100°C. After tempering, the mold steel strength and toughness ratio most suitable for hot stamping is obtained.

2. Tempering temperature 560-600°C: After quenching, the austenite carbon content of the mold steel has increased, and a certain amount of austenite remains after quenching, and this part of the residual austenite is decomposed into martensite after one tempering , resulting in poor impact toughness. Therefore, the present invention uses 560-580-600°C tempering three times to achieve the purpose of stabilizing the structure and adjusting the hardness. The final mold steel hardness is 51-53HRC, and the unnotched impact energy of 7mm×10mm×55mm is 170-200J.

3. Plasma nitrocarburizing voltage 600-850V: The voltage has a greater role in promoting the main nitriding process in plasma nitriding. Higher voltage can increase the bombardment energy, thereby increasing the thickness of the nitriding layer and high nitriding force layer hardness, and too high voltage will hinder the attachment and diffusion of ions on the surface of the substrate, so the voltage should be selected in an appropriate range.

4. Plasma nitrocarburizing atmosphere ratio NH3:CO2= 10:1-16:1: By changing the ratio of NH3:CO2, the thickness of the compound layer and the phase composition and thickness of the diffusion layer can be adjusted. Therefore, adjust the atmosphere ratio according to different needs. the

5. Plasma nitrocarburizing furnace pressure 450-750Pa: The higher the furnace pressure of the plasma nitriding furnace, the more concentrated the glow; when the furnace pressure is lower, the glow diverges. In actual operation, the furnace pressure can be adjusted within the range of 133-1066Pa. Under high pressure, the compound layer has a high content of α phase, and low furnace pressure is easy to obtain γ phase. Ion nitriding under the condition of lower than 40Pa or higher than 2660Pa, the compound layer is not easy to appear.

6. Plasma nitrocarburizing temperature 520-550°C: The temperature of the ion nitriding compound layer and diffusion layer increases significantly with the increase of temperature, but it does not have much influence on the structure of the compound layer, and even at a low temperature of 400°C , There is also an obvious nitriding effect, which is also a prominent feature of ion nitriding. The surface hardness has a maximum value within a certain range, the temperature is too low, the hardened layer is too shallow, and the strengthening effect is not good; the temperature is too high, the nitrides in the infiltrated layer are coarsened, resulting in a decrease in hardness.

7. Plasma nitrocarburizing time 8h-12h: The influence of nitriding on the thickness of α-phase and γ-phase compound layers has different rules. When it is less than 4h, the thickness of the γ-phase compound layer increases with time, and remains basically unchanged after 4h, while the thickness of the α-phase compound layer continues to increase with the prolongation of nitriding time. In general, prolonging the nitriding time always increases the thickness of the compound layer. The compound parabolic relationship between the depth of the diffusion layer and time is similar to that of gas nitriding. In addition, when the nitriding atmosphere in the furnace is constant, the nitrogen concentration of the compound layer is not affected by the nitriding time. As the nitriding time prolongs, the diffusion layer deepens, and the hardness gradient tends to be gentle; but the increase of the holding time causes the nitride structure to become rough, resulting in a decrease in hardness.

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

The invention relates to a heat treatment and surface treatment process of high-strength steel plates and die steels special for hot stamping of pipes. In the heat treatment process, the steel of the present invention is characterized in that the heat treatment temperature and time are strictly limited to obtain the mold state most suitable for hot stamping. In terms of surface treatment, through multi-factor composite control, a strong nitrided layer and diffusion layer with good wear resistance required by the hot stamping die are obtained. In terms of comprehensive performance, the performance of the special hot stamping die steel is: after quenching at 1080°C and tempering at 560-600°C for three times, the hardness is 51-53HRC, and the impact toughness value Ak is 170-200 J; after surface plasma nitriding , the thickness of the mold surface compound layer is 6-17μm, the micro-Vickers hardness is 750-850HV ₃₀₀ , and the thickness of the seepage layer is 150-260μm. The hardness value of the mold steel of the invention is higher than that of general heat treatment H13, can improve the wear resistance of the mold, can effectively prevent the roughening of the mold surface, and can greatly prolong the service life of the mold after plasma surface nitriding. The mold after wear can be put into use again through mold repair and plasma nitriding of the present invention, thereby greatly improving the total service life of the mold.

the

Contents of the invention

The purpose of the present invention is to provide a heat treatment and plasma nitrocarburizing surface treatment technology for high-strength steel plate/pipe hot stamping special ultra-high thermal conductivity wear-resistant mold material. Its purpose is to make the hot stamping die steel obtain the most suitable strength and toughness ratio for hot stamping production through heat treatment, and effectively improve the surface strength of the die through surface treatment, and enhance the wear resistance, corrosion resistance, adhesion resistance and resistance of the die. thermal fatigue performance. The worn mold can be put into use again after repair and plasma nitriding again, thus greatly improving the total service life of the mold. Its contents include:

a. The heat treatment process of ultra-high thermal conductivity wear-resistant die steel for high-strength steel plate/pipe hot stamping is as follows: use a vacuum high-pressure gas quenching furnace to quench at 1050-1000 ° C, temper three times at 560-600 ° C, after treatment, the special hot stamping die steel The performance is: the hardness of the matrix is 51-53HRC, and the impact toughness value Ak is 170-200J;

b. The surface plasma nitrocarburizing process of high-strength steel plate/pipe hot stamping special ultra-high thermal conductivity wear-resistant die steel is as follows: the treatment process voltage is 600-850V, the atmosphere ratio NH3:CO2= 10:1-16:1, the furnace pressure 450-700Pa, nitriding temperature 520°C-550°C, nitriding time 8h-12h. After surface plasma nitriding, a compound infiltrated layer is formed on the surface of the mold, the thickness of the white layer is 6-17μm, the micro-Vickers hardness is 750-850HV _0.3 , and the thickness of the infiltrated layer is 150-260μm.

the

Description of drawings

Fig. 1 is the microstructure optical and SEM photo of this heat treatment process die steel of embodiment 1;

Fig. 2 is the metallographic structure photo of the die steel section after heat treatment and plasma nitrocarburizing in embodiment 1;

Fig. 3 is the metallographic structure photo of the mold steel section after the heat treatment of embodiment 2 and plasma nitrocarburizing;

Fig. 4 is the metallographic structure photo of the mold steel section after the heat treatment of embodiment 3 and plasma nitrocarburizing;

Fig. 5 is the XRD phase analysis of the mold steel surface after heat treatment and plasma nitrocarburizing in embodiment 3;

Fig. 6 is the cross-sectional hardness gradient of the mold steel after heat treatment and plasma nitrocarburizing in Example 3.

Detailed ways

The specific embodiment of the present invention is described below in conjunction with accompanying drawing now.

Example 1

The invention relates to a heat treatment and surface treatment process of a high-strength steel plate and a special ultra-high thermal conductivity wear-resistant die steel for hot stamping of tubes. The specific process is as follows:

1. Chemical composition of mold material: C: 0.36%; Si: 0.07%; Mn: 0.14%; W: 1.70%; Mo: 4.51%;

Cr: 0.12%; V: 0.14%;

2. Heat treatment process: Quenching at 1080°C, tempering at 560°C, 580°C, and 600°C for 2 hours. Under the conditions of this heat treatment process, the hardness and impact toughness: the hardness value is 51-53HRC, and the impact toughness value is 170-200 J. The quenched and tempered microstructure is shown in Fig. 1.

3. Plasma nitriding process: voltage 600-650V, atmosphere ratio NH ₃ : CO2=10:1, furnace pressure 450-500Pa, nitriding temperature 520°C, nitriding time 8h. After nitriding, the surface hardness is 760-800HV _0.3 and the compound layer thickness is 6-9μm. The microstructure of the nitrocarburized sample section is shown in Fig. 2.

Example 2

The invention relates to a heat treatment and surface treatment process of a high-strength steel plate and a special ultra-high thermal conductivity wear-resistant die steel for hot stamping of tubes. The specific process is as follows:

1. Chemical composition of mold material: C: 0.38%; Si: 0.06%; Mn: 0.12%; W: 1.65%; Mo: 4.43%;

Cr: 0.12%; V: 0.13%;

2. Heat treatment process: Quenching at 1080°C, tempering at 560°C, 580°C, and 600°C for 2 hours. Under the conditions of this heat treatment process, the hardness and impact toughness: the hardness value is 51-53HRC, and the impact toughness value is 170-200 J.

3. Plasma nitriding process: voltage 800-850V, atmosphere ratio NH ₃ : CO2=16:1, furnace pressure 450-500Pa, nitriding temperature 550°C, nitriding time 8h. After nitriding, the surface hardness is 800-840HV ₃₀₀ , and the compound layer thickness is 11-15μm. The microstructure of the nitrocarburized sample section is shown in Fig. 3.

Example 3

In this example, a special ultra-high thermal conductivity wear-resistant die steel for hot stamping is carried out for heat treatment and surface treatment. The specific process is as follows:

1. Chemical composition of mold material: C: 0.37%; Si: 0.08%; Mn: 0.12%; W: 1.65%; Mo: 4.43%; Cr: 0.20%; V: 0.12%;

2. Heat treatment process: Quenching at 1080°C, tempering at 560°C, 580°C, and 600°C for 2 hours. Under the conditions of this heat treatment process, the hardness and impact toughness: the hardness value is 51-53HRC, and the impact toughness value is 170-200 J.

3. Plasma nitriding process: voltage 800-850V, atmosphere ratio NH ₃ : CO2=10:1, furnace pressure 650-750Pa, nitriding temperature 550°C, nitriding time 12h. After nitriding, the surface hardness is 820-850HV ₃₀₀ , and the compound layer thickness is 12-17μm. Figure 4 shows the cross-sectional structure picture of the nitrocarburizing sample; Figure 5 shows the X-ray diffraction analysis of the surface image of Example 3. Fig. 6 is the cross-sectional hardness gradient of the sample of Example 3.

Claims (1)

1. A heat treatment and plasma nitrocarburizing surface treatment process of high-strength steel plate/pipe hot stamping special ultra-high thermal conductivity wear-resistant die steel, characterized in that it has the following processes and steps: a. The heat treatment process of ultra-high thermal conductivity and wear-resistant die steel for high-strength steel plate/pipe hot stamping is as follows: use a vacuum high-pressure gas quenching furnace to quench at 1050-1000 ° C, temper three times at 560-600 ° C; after treatment, the special hot stamping die steel The performance is: the hardness of the matrix is 51-53HRC, and the impact toughness value Ak is 170-200J; b. The surface plasma nitrocarburizing process of high-strength steel plate/pipe hot stamping special ultra-high thermal conductivity wear-resistant die steel is as follows: the treatment process voltage is 600-850V, the atmosphere ratio NH3:CO2= 10:1-16:1, the furnace pressure 450-700Pa, nitriding temperature 520°C-550°C, nitriding time 8h-12h; after surface plasma nitriding, a compound infiltrated layer is formed on the surface of the mold, the thickness of the white layer is 6-17μm, and the micro Vickers hardness is 750-850HV _0.3 , The thickness of the seepage layer is 150-260μm.