JP2024515143A - Mold steel having high thermal diffusion coefficient and its manufacturing method - Google Patents

Abstract

The present disclosure relates to a die steel having a high thermal diffusion coefficient and a manufacturing method thereof, the die steel being, in mass percent, 0.30-0.40% C, 0.05-0.10% Si, 2.50-3.40% Mo, 0.01-0.05% Nb, 0.30-0.50% Co, 0.01-0.05% RE, the balance being Fe and unavoidable impurities, and in the die steel, P≦0.15% and S≦0.025%. The manufacturing method of the die steel includes process steps of smelting, electroslag remelting, electroslag ingot annealing, forging, spheroidizing annealing, quenching and tempering, which are arranged in sequence. The mold steel with high thermal diffusion coefficient disclosed in the present invention adopts a new compounding ratio of chemical elements and a reasonable process setting, thereby obtaining a new mold steel with ultra-high thermal conductivity, and the mold processed therewith has both excellent high-temperature performance and very high comprehensive mechanical properties.

Description

This disclosure claims priority to a patent application filed with the China Patent Office on January 18, 2022, bearing application number 202210055527.9 and entitled "Mold steel with high thermal diffusion coefficient and manufacturing method thereof," the entire contents of which are incorporated herein by reference.

The present disclosure relates to the technical field of steel, and in particular to a die steel having a high thermal diffusion coefficient. The present disclosure further relates to a method for manufacturing a die steel having a high thermal diffusion coefficient.

As the foundation of the manufacturing industry, the steel industry plays a very important role in the development of modern industry.

As one of the steel materials, die steel is widely used in industry and manufacturing. Die steel is divided into cold die steel and hot die steel according to the temperature at which metal or liquid metal is processed. Hot die steel is used to obtain die material of a desired shape by heating metal or liquid metal above the recrystallization temperature. Examples include hot forging press dies, pressure casting dies, and hot stamping dies. Hot die steel and cold die steel often need to be quenched and tempered to obtain good comprehensive mechanical properties before being put into the die for service.

In the design and application of commonly used die steels, mechanical properties such as heat treatment hardness, impact toughness, wear resistance, etc. are often considered, and the influence of the physical properties of the material on the quality of the material's service state and service life is ignored. The physical properties of the material, such as the thermal diffusion coefficient, are also important influencing factors that affect the service expression of the die. In die steels, especially hot die steels, the high thermal diffusion coefficient of the die affects the quality of the processed product and the service life of the die. In pressure casting dies, the liquid metal needs to be solidified and formed on the grinding tool. In this process, a die with a high thermal diffusion coefficient can increase the cooling rate of the liquid metal, reduce the pressure casting time, and improve the performance of the casting structure. In hot stamping dies, a die with a high thermal diffusion coefficient can increase the cooling rate of the steel plate, promote the martensitic transformation of the steel plate, increase the strength of the steel plate, reduce the pressure holding time, and improve production efficiency. In addition, the high thermal diffusion coefficient of the mold effectively reduces the heated temperature of the mold surface during processing, improving its high-temperature wear resistance and thermal fatigue crack resistance and expansion performance, and extending its service life.

However, currently, none of the types of die steels obtained by the die steel production process can fully take into account the physical property of thermal conductivity, so dies manufactured from conventional die steels have room for improvement in terms of thermal conductivity. Improving the thermal conductivity performance of die steels will contribute to extending the service life of dies.

In view of this, the present disclosure aims to provide a mold steel having a high thermal diffusion coefficient, which contributes to extending the service life of a mold manufactured from the mold steel by improving the thermal diffusion coefficient of the mold steel, and a method for manufacturing the same.

To achieve the above objective, the technical solution of this disclosure is realized as follows:

A mold steel having a high thermal diffusion coefficient, the mold steel comprising, in mass percent:
The die steel comprises C: 0.30-0.40%, Si: 0.05-0.10%, Mo: 2.50-3.40%, Nb: 0.01-0.05%, Co: 0.30-0.50%, RE: 0.01-0.05%, and the balance being Fe and unavoidable impurities, and in the die steel, P≦0.15% and S≦0.025%.

Furthermore, the mass contents of RE and S in the mold steel satisfy the conditions: [RE]/[S]>2.0 and [RE]×[S]<0.005%.

Compared to conventional technology, this disclosure has the following advantages:

In the die steel with a high thermal diffusion coefficient described in this disclosure, the above-mentioned compounding ratios of chemical element components, particularly the compounding ratios of specific carbon and alloy elements, result in a die steel with a high thermal diffusion coefficient, which has excellent hardness, wear resistance, and toughness, and has very high overall mechanical properties.

The combining action of Mo, Co and Si elements contributes to obtaining a die steel with a high thermal diffusion coefficient. The specific contents of Mo, Co and C are combined to produce a die steel manufactured according to the present disclosure that has outstanding high thermal diffusion coefficient performance and excellent overall mechanical properties.

Among these, the strengthening effect is achieved by forming a large amount of molybdenum carbide in the structure of the material, and the obtained carbide can increase the hardness of the die steel with a high thermal diffusion coefficient, ensure toughness, and provide the die steel with a high thermal diffusion coefficient with good tempering stability, red-heat hardness, and heat resistance. A certain content of cobalt can increase the melting point of the die steel with a high thermal diffusion coefficient, and more molybdenum element is dissolved, thereby improving the secondary hardening performance, hardness, and high-temperature strength of the die steel with a high thermal diffusion coefficient, and improving the wear resistance and durability of the die steel with a high thermal diffusion coefficient. A certain content of niobium element can effectively refine the crystal grains of the structure.

Another object of the present disclosure is to provide a method for producing a mold steel having a high thermal diffusion coefficient, the method comprising the steps of:

The manufacturing method includes the process steps of smelting, electroslag remelting, electroslag ingot annealing, forging, spheroidizing annealing, quenching and tempering, arranged in sequence.

Furthermore, in the smelting process step, the smelting temperature is 1450-1600°C.

Furthermore, in the electroslag ingot annealing process step, the electroslag ingot obtained after electroslag remelting is kept at 750-800°C for 8-10 hours and then cooled to room temperature in the furnace.

Furthermore, in the forging process step, the steel ingot obtained after annealing the electroslag ingot is heated to 1150-1180°C and kept at that temperature for 30 minutes, after which the steel ingot is multi-directionally forged, with a final forging temperature of 950°C or higher and a forging ratio of 6 or higher.

Furthermore, in the spheroidizing annealing process step, the forged steel material is kept at 650-750°C for 12-16 hours, and then cooled to room temperature in the furnace.

Furthermore, in the quenching process step, the mold steel blank after spheroidizing annealing is kept at 1050-1150°C for 1 hour, then oil-cooled to room temperature, and then tempered.

Furthermore, in the tempering process, the rapidly cooled mold steel material is kept at 570-630°C for at least 2 hours, and then cooled to room temperature in a furnace.

The tempering process is then repeated twice.

Compared to the prior art, the manufacturing method disclosed herein can manufacture die steel material with excellent thermal conductivity performance and comprehensive mechanical properties based on the component ratio of the die steel disclosed herein, and provides a reasonable and reliably executable process flow and related process parameters for the production of die steel with a high thermal diffusion coefficient, ensuring good reactions between the chemical elements in the die steel and the generation of compatibility of the martensitic structure in the steel material, contributing to further ensuring the realization of the high thermal diffusion coefficient and comprehensive mechanical properties of the die steel.

FIG. 2 is a scanning electron microscope image of the mold steel having a high thermal diffusivity according to Example 1 of the present disclosure after tempering treatment. FIG. 2 is a transmission electron microscope image of a mold steel having a high thermal diffusivity according to Example 1 of the present disclosure. FIG. 1 is a comparison diagram of the thermal diffusion coefficients of a die steel having a high thermal diffusion coefficient described in Example 1 of the present disclosure and H13 steel.

It is important to explain that, unless contradictory, the embodiments and features in the embodiments in this disclosure may be combined with each other.

Example 1
This example relates to a mold steel having a high thermal diffusion coefficient, and is composed of, in mass percent, 0.30% C, 0.07% Si, 3.40% Mo, 0.02% Nb, 0.35% Co, 0.01% P, 0.01% S, 0.02% RE, with the remainder being Fe and unavoidable impurities.

The manufacturing method of the above mold steel having a high thermal diffusion coefficient includes the following process steps:

a. The specified components of the die steel are smelted in an electric furnace at 1480°C, cast into a steel ingot, and then electroslag remelted. After that, the steel is annealed by keeping it at 780°C for 8 hours. The electroslag ingot is cooled in the furnace to room temperature of about 25°C, and the electroslag ingot annealing process is completed. After the annealed steel ingot is heated to 1160°C and kept at that temperature for 30 minutes, it is multi-directionally forged, with a final forging temperature of 960°C and a forging ratio of 7. The forged material is kept at 700°C for 12 hours, and cooled in the furnace to room temperature of about 25°C, to obtain a die steel blank with a high thermal diffusion coefficient.

b. The resulting die steel blank with a high thermal diffusion coefficient is heated to 1050°C and kept at that temperature for 1 hour, and then cooled in oil to 25°C to obtain a die steel material with a martensitic structure.

c. The resulting martensite structured mold steel material is kept at 630°C for 2 hours and 15 minutes (tempering treatment).

By repeating steps d and c, a die steel material with a hardness of 50 HRC, an impact energy of 124 J, and a high thermal diffusion coefficient of 11.94 mm ² /s at room temperature can be obtained. The thermal diffusion coefficient of H13 steel with the same heat treatment hardness at room temperature is only 6.23 mm ² /s.

The relevant performance and comparative situation of the die steel produced by the above process are shown in the figures. Figure 1 shows a scanning electron microscope photograph of the structure of the die steel with a high thermal diffusion coefficient obtained after tempering, and Figure 2 shows a transmission electron microscope photograph of the structure of the die steel with a high thermal diffusion coefficient. Figure 3 shows a comparison of the thermal diffusion coefficient of the die steel with a high thermal diffusion coefficient and H13 steel.

The names of the elements in this example are: C: carbon, Si: silicon, Mo: molybdenum, Nb: niobium, Co: cobalt, P: phosphorus, S: sulfur, RE: rare earth, Fe: iron.

It should be noted that the reaction combination of molybdenum, cobalt and niobium elements in this embodiment contributes to obtaining a mold steel with a high thermal diffusion coefficient. Due to the interaction of the specific contents of molybdenum and cobalt, the mold steel with a high thermal diffusion coefficient produced in this embodiment has a high thermal diffusion coefficient and excellent comprehensive mechanical properties. The specific compounding ratio values, process control parameters, and tempering treatment times, etc. given above are all suitable specific values, and appropriate adjustments can be made within the value ranges and principle requirements published in this disclosure, and mold steel products with excellent thermal conductivity performance and mechanical properties can be obtained.

For example, in the above electroslag ingot annealing, spheroidizing annealing and tempering processes, the furnace cooling process only requires lowering the temperature to room temperature, and for ease of control and standard consistency, it is preferable to cool the temperature in the furnace to 25° or less, and the oil cooling and temperature reduction in the quenching process may also be performed with reference to the above furnace cooling and temperature reduction. The tempering process can be performed once or twice in a repeated manner, and preferably, the tempering process is performed twice.

It should be particularly noted that in the composition ratio of the die steel of the present disclosure, in order to ensure the overall properties of the steel material, the impurity contents of phosphorus, sulfur, etc. should be maintained at P≦0.15% and S≦0.025%. It is preferable that the mass contents of RE and S in the die steel satisfy [RE]/[S]≧2.0 and [RE]×[S]≦0.005%. In addition, the die steel of the present disclosure does not contain elements such as chromium, manganese, nickel, vanadium, and tungsten.

Among the above elements, the addition of molybdenum can improve the thermal conductivity performance of die steel with a high thermal diffusion coefficient. In this embodiment, the enhancing effect is achieved mainly by forming a large amount of molybdenum carbide in the structure of the material, and the obtained carbide increases the hardness of die steel with a high thermal diffusion coefficient while ensuring toughness, and can provide die steel with a high thermal diffusion coefficient with good tempering stability, red-hot hardness, and heat resistance.

Cobalt is mostly in α-Fe in the annealed state, and also has a certain solubility in molybdenum carbide. In this embodiment, a certain content of cobalt can increase the melting point and quenching temperature of this mold steel with a high thermal diffusion coefficient, and more molybdenum elements are dissolved and strengthen the matrix. In addition, in this embodiment, a certain content of cobalt can further delay the precipitation of alloy carbides during tempering, delay the growth of carbides, refine carbides, improve the secondary hardening performance, hardness and high-temperature strength of the steel, and improve the wear resistance and durability of the mold steel with a high thermal diffusion coefficient. Cobalt element has little effect on the thermal conductivity performance of the steel, and at the same time, it can give the steel high high-temperature hardness and comprehensive mechanical properties, so in this embodiment, a certain amount of cobalt is added to ensure the combination of comprehensive mechanical properties and thermal diffusion performance.

Specifically, a certain content of niobium element can effectively refine the crystal grains of the structure. Niobium can delay austenite recrystallization during the forging and heat treatment processes such as quenching and normalizing of die steel with a high thermal diffusion coefficient, and has a very strong effect of refining crystal grains. Niobium can form gap intermediate phases such as NbC or NbN in the steel. In the recrystallization process, the recrystallization time is greatly increased due to the effects of dislocation pinning and grain growth inhibition by NbC and NbN. Niobium can effectively refine the crystal grains in the forging and heat treatment processes of die steel with a high thermal diffusion coefficient, provide activation energy for the precipitation of molybdenum carbide, promote the diffusion and refinement precipitation of carbide, and ensure good comprehensive mechanical properties and thermal diffusion performance.

The ratio of the carbon content to the carbide-forming element content used in this disclosure is 0.09-0.16 (the carbide-forming element is the sum of the mass percentages of molybdenum and niobium). In this embodiment, the carbon content is 0.3%, Mo is 3.40%, and Nb is 0.02%, and as can be seen from the calculation, the ratio of the carbon content to the carbide-forming element content is about 0.09. Due to the above specific carbon content, the strong carbide-forming element of molybdenum is precipitated with the characteristics of fine diffusion during the high-temperature tempering process, resulting in a secondary hardening phenomenon. In this embodiment, due to the specific carbon and molybdenum ratio, the molybdenum element is precipitated in the form of fine secondary precipitated carbides, which ensures the comprehensive mechanical properties and at the same time, the steel according to the embodiment has an ultra-high thermal diffusion coefficient.

In this embodiment, the trace rare earth elements in the die steel with high thermal diffusion coefficient can significantly optimize the quality of the cast piece, improve the plasticity and toughness of the die steel with high thermal diffusion coefficient, and further improve the transverse performance and low temperature toughness of the steel material.

Example 2
This example relates to a mold steel having a similarly high thermal diffusion coefficient, and is composed of, in mass percent, 0.33% C, 0.06% Si, 3.20% Mo, 0.03% Nb, 0.48% Co, 0.05% P, 0.01% S, 0.04% RE, with the remainder being Fe and unavoidable impurities.

The manufacturing method for the above mold steel includes the following process steps:

a. The specified components of die steel with a high thermal diffusion coefficient are smelted in an electric furnace at 1530°C and then cast into a steel ingot. After electroslag remelting, the steel is annealed by keeping it at 790°C for 9 hours. The annealed steel ingot is heated to 1170°C and kept at that temperature for 30 minutes, after which it is multi-directionally forged, with a final forging temperature of 980°C and a forging ratio of 6.5. The forged material is kept at 720°C for 12 hours, and then cooled to room temperature in the furnace to obtain a die steel blank with a high thermal diffusion coefficient.

b. The resulting die steel blank with a high thermal diffusion coefficient is heated to 1090°C and kept at that temperature for 1 hour, and then cooled in oil to 25°C to obtain a die steel material with a martensitic structure.

and c) maintaining the obtained martensite structure die steel material at 600° C. for 2 h 15 min (tempering treatment). Through the above steps, a die steel material having a hardness of 52 HRC, an impact energy of 103 J, and a thermal diffusion coefficient of 10.60 mm ² /s under room temperature conditions can be obtained.

Example 3
This example shows an embodiment for producing a die steel having a high thermal diffusion coefficient with different compounding ratios and process parameters. In this example, the die steel of the present disclosure is composed of, in mass percent, 0.36% C, 0.09% Si, 2.80% Mo, 0.05% Nb, 0.50% Co, 0.01% P, 0.02% S, 0.04% RE, and the remaining Fe and unavoidable impurities.

The manufacturing method for the die steel having the above-mentioned high thermal diffusion coefficient includes the following process steps:

a. The specified components of the die steel with a high thermal diffusion coefficient are smelted in an electric furnace at 1510°C and then cast into a steel ingot. After electroslag remelting, the steel is annealed by keeping it at 800°C for 8 hours. The annealed steel ingot is heated to 1180°C and kept at that temperature for 30 minutes, after which it is multi-directionally forged, with a final forging temperature of 970°C and a forging ratio of 8. The forged material is kept at 710°C for 13 hours, and then cooled to room temperature in the furnace to obtain a die steel blank with a high thermal diffusion coefficient.

b. The resulting die steel blank with a high thermal diffusion coefficient is heated to 1100°C and kept at that temperature for 1 hour, and then cooled in oil to 25°C to obtain a die steel material with a martensitic structure.

c. The resulting martensite structured mold steel material is kept at 600°C for 2 hours and 15 minutes (tempering treatment).

By repeating steps d and c, a die steel material can be obtained that has a hardness of 51 HRC, an impact energy of 117 J, and a high thermal diffusion coefficient of 11.32 mm ² /s under room temperature conditions.

Example 4
This embodiment provides another specific manufacturing form of a die steel having a high thermal diffusion coefficient, and in this embodiment, the die steel consists of, in mass percent, 0.40% C, 0.05% Si, 3.40% Mo, 0.04% Nb, 0.45% Co, 0.02% P, 0.01% S, 0.03% RE, and the remainder being Fe and unavoidable impurities.

The manufacturing method for the die steel having the above-mentioned high thermal diffusion coefficient includes the following process steps:

a. The specified components of the die steel with a high thermal diffusion coefficient are smelted in an electric furnace at 1580°C and then cast into a steel ingot. After electroslag remelting, the steel is annealed by keeping it at 760°C for 10 hours. The annealed steel ingot is heated to 1160°C and kept at that temperature for 30 minutes, after which it is multi-directionally forged, with a final forging temperature of 980°C and a forging ratio of 7. The forged material is kept at 750°C for 12 hours, and then cooled to room temperature in the furnace to obtain a die steel blank with a high thermal diffusion coefficient.

b. The resulting die steel blank with a high thermal diffusion coefficient is heated to 1080°C and kept at that temperature for 1 hour, and then cooled in oil to 25°C to obtain a die steel material with a full martensite structure.

and c) maintaining the temperature of the resulting die steel material having a martensite structure at 590° C. for 2 h 15 min (tempering treatment). By the above process, a die steel material having a high thermal diffusion coefficient of 10.85 mm ² /s at room temperature and a hardness of 51.5 HRC, an impact energy of 108 J, and a hardness of 51.5 HRC is obtained.

As described above, in the mold steel with a high thermal diffusion coefficient disclosed herein, the blending ratio of specific chemical element components, particularly the blending ratio of specific carbon and alloy elements, results in a mold steel with a high thermal diffusion coefficient, and the mold steel obtained with a high thermal diffusion coefficient has high hardness, wear resistance, and toughness all at the same time, and has very high overall mechanical properties.

The above are merely preferred embodiments of the present disclosure and are not intended to limit the present disclosure, and any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present disclosure are intended to be included within the scope of protection of the present disclosure.

Claims (15)

A die steel with a high thermal diffusion coefficient, characterized by the following mass percentages: C: 0.30-0.40%, Si: 0.05-0.10%, Mo: 2.50-3.40%, Nb: 0.01-0.05%, Co: 0.30-0.50%, RE: 0.01-0.05%, the balance being Fe and unavoidable impurities, and P≦0.15% and S≦0.025%. The die steel having a high thermal diffusion coefficient according to claim 1, characterized in that the mass contents of RE and S in the die steel satisfy [RE]/[S]≧2.0 and [RE]×[S]≦0.005%. A method for producing die steel with a high thermal diffusion coefficient according to claim 1 or 2, characterized in that it comprises the process steps of smelting, electroslag remelting, electroslag ingot annealing, forging, spheroidizing annealing, quenching and tempering, which are arranged in sequence. The manufacturing method described in claim 3, characterized in that in the smelting process step, the smelting temperature is 1450-1600°C. The manufacturing method described in claim 4, characterized in that in the smelting process step, the smelting temperature is 1530°C. The manufacturing method described in claim 3, characterized in that in the electroslag ingot annealing process step, the electroslag ingot obtained after electroslag remelting is kept at 750-800°C for 8-10 hours and then cooled to room temperature in the furnace. The manufacturing method described in claim 6, characterized in that in the electroslag ingot annealing process step, the electroslag ingot obtained after electroslag remelting is kept at 780°C for 9 hours and then cooled to room temperature in the furnace. The manufacturing method described in claim 3, characterized in that in the forging process step, the steel ingot obtained after annealing the electroslag ingot is heated to 1150-1180°C and kept at that temperature for 30 minutes, and then the steel ingot is multi-directionally forged, with a final forging temperature of 950°C or higher and a forging ratio of 6 or higher. The manufacturing method described in claim 3, characterized in that in the spheroidizing annealing process step, the forged steel material is kept at 650-750°C for 12-16 hours, and then cooled to room temperature in the furnace. The manufacturing method described in claim 9, characterized in that in the spheroidizing annealing process step, the forged steel material is kept at 700°C for 14 hours and then cooled to room temperature in the furnace. The manufacturing method described in any one of claims 3 to 10, characterized in that in the quenching process step, the die steel blank after spheroidizing annealing is kept at 1050-1150°C for 1 hour, then oil-cooled to room temperature, and then tempered. The manufacturing method described in claim 11, characterized in that in the quenching process step, the mold steel blank after spheroidizing annealing is kept at a temperature of 1100°C. The manufacturing method described in claim 11 is characterized in that in the tempering process, the rapidly cooled mold steel material is kept at 570-630°C for 2 hours or more, and then cooled to room temperature in a furnace. The manufacturing method described in claim 12, characterized in that the tempering treatment is repeated twice. The manufacturing method described in claim 12, characterized in that in the tempering treatment, the rapidly cooled mold steel material is kept at a temperature of 600°C.