US20240068078A1

US20240068078A1 - Die steel with a high thermal diffusion coefficient and its preparation methods

Info

Publication number: US20240068078A1
Application number: US18/015,946
Authority: US
Inventors: Shuang Li; Zhen Cao; Zhen Wang; Yanlin SHI; Long Liu; Lulu Zhao; Ziliang WANG; Yongliang Shi; Chenlong Wang; Yunchang Huo
Original assignee: Hebei Vocational University Of Industry And Technology
Current assignee: Hebei Vocational University Of Industry And Technology
Priority date: 2022-01-18
Filing date: 2022-03-09
Publication date: 2024-02-29
Also published as: CN114395738B; WO2023137842A1; CN114395738A; JP2024515143A

Abstract

A die steel with a high thermal diffusion coefficient includes 0.30-0.40 wt. % of C, 0.05-0.10 wt. % of Si, 2.50-3.40 wt. % of Mo, 0.01-0.05 wt. % of Nb, 0.30-0.50 wt. % of Co, 0.01-0.05 wt. % of RE, the rest is Fe and unavoidable impurities, wherein in the die steel, P≤0.15 wt. %, S≤0.025 wt. %. A preparation method of the die steel includes steps of melting, electroslag remelting, electroslag ingot annealing, forging, spheroidizing annealing, quenching and tempering.

Description

CROSS REFERENCE OF RELATED APPLICATION

The present invention claims priority under 35 U.S.C. 119(a-d) to CN 202210055527.9, filed Jan. 18, 2022, and titled as “A die steel with a high thermal diffusion coefficient and its preparation method”. The entire content of CN 202210055527.9 is incorporated into the present invention by reference.

BACKGROUND OF THE PRESENT INVENTION

Field of Invention

The present invention relates to the field of iron and steel technology, and more particularly to a die steel with a high thermal diffusion coefficient. Moreover, the present invention also relates to preparation methods of the die steel with the high thermal diffusion coefficient.

Description of Related Arts

As the cornerstone of manufacturing industry, steel industry plays a very important role in the development of modern industry.
As a kind of steel material, die steel is widely used in industry and manufacturing. According to the temperature at which the solid metal or liquid metal is processed, there are two kinds of die steel, namely, cold working die steel and hot-work die steel. The hot-work die steel is a material for manufacturing a die in which a solid metal or a liquid metal heated above the recrystallization temperature obtains a desired shape, such as hot forging die, die casting die and hot stamping die. The hot-work die steel and the cold working die steel often need to be quenched and tempered before the die is put into use, so as to obtain excellent comprehensive mechanical properties.
In the design and application of commonly used die steel, its mechanical properties, such as heat treatment hardness, impact toughness, impact toughness and abrasion resistance, are often considered, but the influence of physical properties of materials on the service quality and life of materials is often ignored. The physical properties of materials, such as thermal diffusion coefficient, are also important factors affecting the service quality of the die. In the die steel, especially in the hot-work die steel, the thermal diffusion coefficient of the die affects the quality of the product and the service life of the die. In die casting, the liquid metal needs to be solidified in the abrasives. In this process, a mold with a high thermal diffusion coefficient is able to provide a high cooling rate for the liquid metal, reduce the die casting time, and improve the castability. In the hot stamping die, the die with high thermal diffusion coefficient is able to provide high cooling rate for the steel plate, promote the martensitic transformation of the steel plate, improve the strength of the steel plate, reduce the holding time and improve the production efficiency. At the same time, the high thermal diffusion coefficient of the die is able to effectively reduce the heating temperature of the die surface during processing, improve the high-temperature abrasion resistance and resist the initiation and propagation of thermal fatigue cracks, and improve the service life of the die.
However, the physical properties of heat conduction of the die steel, which is prepared by the preparation method of the die steel provided by the present invention, have not been fully taken into account in the current preparation method of the die steel. Therefore, there is space for improvement in the thermal conductivity of the die made from the existing die steel. It is helpful to prolong the service life of the die by improving the heat conductivity of the die steel.

SUMMARY OF THE PRESENT INVENTION

In view of this, an object of the present invention is to provide a die steel with a high thermal diffusion coefficient and its preparation methods, so as to improve the thermal diffusion coefficient of the die steel, thereby improving the service life of the die made from the die steel.
To achieve the above object, the present invention provides technical solutions as follows.
A die steel with a high thermal diffusion coefficient comprises 0.30-0.40 wt. % of C, 0.05-0.10 wt. % of Si, 2.50-3.40 wt. % of Mo, 0.01-0.05 wt. % of Nb, 0.30-0.50 wt. % of Co, 0.01-0.05 wt. % of RE, the rest is Fe and unavoidable impurities, wherein in the die steel, P≤0.15 wt. %, S≤0.025 wt. %.
Preferably, a mass percentage content of RE and S in the die steel meets conditions of [RE]/[S]>2.0, [RE]×[S]<0.005 wt. %.
Compared with prior arts, the present invention has advantages as follows.
The die steel with the high thermal diffusion coefficient provided by the present invention is able to be obtained through the specific proportion of the above-mentioned chemical elements, especially the specific proportion of carbon and alloying elements, and the obtained die steel with the high thermal diffusion coefficient has excellent hardness, abrasion resistance, toughness, and comprehensive mechanical properties.
The combination of Mo, Co and Si elements is beneficial to obtain the die steel with the high thermal diffusion coefficient. The reaction combination of Mo, Co and C with specific contents makes the die steel provided by the present invention have outstanding high thermal diffusion coefficient performance, and excellent comprehensive mechanical properties.
A large number of molybdenum carbides are formed in the material structure to achieve the strengthening effect. The obtained carbides not only ensure the toughness of the die steel with the high thermal diffusion coefficient while increasing its hardness, but also make the die steel obtain excellent tempering stability, red hardness and heat strength. The specific content of cobalt is able to improve the melting point of the die steel with the high thermal diffusion coefficient, and then dissolve more molybdenum element, so that the secondary hardening ability, hardness, high temperature strength, the abrasion resistance and durability of the die steel with the high thermal diffusion coefficient are improved. The specific content of niobium is able to refine grains effectively.
Another object of the present invention is to provide a preparation method of a die steel with a high thermal diffusion coefficient, the method comprises steps of melting, electroslag remelting, electroslag ingot annealing, forging, spheroidizing annealing, quenching and tempering.
Preferably, melting is performed in a range of 1450-1600° C.
Preferably, the step of electroslag ingot annealing comprises performing heat preservation on electroslag ingots obtained by electroslag remelting in a range of 750-800° C. for 8-10 h and then cooling to room temperature with an electric furnace.
Preferably, the step of forging comprises heating annealed ingots to 1150-1180° C., performing heat preservation for 30 min, and multi-directionally forging above 950° C. with a forging ratio no less than 6.
Preferably, the step of spheroidizing annealing comprises performing heat preservation on forged ingots in a range of 650-750° C. for 12-16 h, and then cooling to room temperature with the electric furnace.
Preferably, the step of quenching comprises performing heat preservation on a die steel blank after spheroidizing annealing in a range of 1050-1150° C. for 1 h, and then performing oil-cooling to room temperature, and then tempering.
Preferably, the step of tempering comprises performing heat preservation on a die steel material after quenching in a range of 570-630° C. for above 2 h, and then performing oil-cooling to room temperature.
Preferably, tempering is performed for twice.
Compared with prior arts, the preparation method provided the present invention is able to prepare die steel materials with excellent heat conductivity and comprehensive mechanical properties based on the composition ratio of the die steel provided by the present invention, so that the reasonable and feasible process flow and related process parameters are provided for the production of the die steel with the high thermal diffusion coefficient, which guarantees the good reaction between the chemical elements in the die steel and the phase formation of Martensitic structure in the die steel, so as to further guarantee the realization of high thermal diffusion coefficient and comprehensive mechanical properties of the die steel.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a scanning electron microscope diagram of a die steel with a high thermal diffusion coefficient after tempering according to a first preferred embodiment of the present invention.

FIG. 2 is a transmission electron microscope diagram of the die steel with the high thermal diffusion coefficient according to the first preferred embodiment of the present invention.

FIG. 3 shows the comparison of thermal diffusion coefficient between the die steel with the high thermal diffusion coefficient according to the first preferred embodiment of the present invention and H13 steel.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

It should be noted that embodiments and characteristics in these embodiments provided by the present invention may be combined with each other without conflict.

First Embodiment

The present invention relates to a die steel with a high thermal diffusion coefficient, which comprises 0.30 wt. % of C, 0.07 wt. % of Si, 3.40 wt. % of Mo, 0.02 wt. % of Nb, 0.35 wt. % of Co, 0.01 wt. % of P, 0.01 wt. % of S, 0.02 wt. % of RE, the rest is Fe and unavoidable impurities.
A preparation method of the die steel with the above high thermal diffusion coefficient comprises steps of:

- (A) melting predetermined components of the die steel by an electric furnace at 1480° C., obtaining steel ingots by casting the molten predetermined components, performing electroslag remelting on the steel ingots, annealing which comprises performing heat preservation at 780° C. for 8 h and cooling electroslag ingots to 25° C. with the electric furnace, heating the annealed ingots to 1160° C. and performing heat preservation for 30 min, multi-directionally forging at 960° C. with a forging ratio of 7, performing heat preservation at 700° C. for 12 h, and cooling the electric furnace to 25° C. with the electric furnace, so as to obtain a die steel blank with the high thermal diffusion coefficient;
- (B) obtaining a Martensitic die steel material which comprises heating the die steel blank with the high thermal diffusion coefficient to 1050° C., performing heat preservation for 1 h, and performing oil-cooling to 25° C.;
- (C) tempering which comprises performing heat preservation on the Martensitic die steel material at 630° C. for 135 min; and
- (D) repeating the step of (C), thereby obtaining the die steel with the high thermal diffusion coefficient, wherein the die steel has a hardness of 50 HRC, an impact energy of 124 J and a thermal diffusion coefficient of 11.94 mm²/s at room temperature. The thermal diffusion coefficient of H13 steel with the same heat treatment hardness is only 6.23 mm²/s at room temperature.

The properties and comparison results of the die steel prepared by the above method are shown in the drawings. FIG. 1 is a scanning electron microscope diagram of the die steel with the high thermal diffusion coefficient after tempering. FIG. 2 is a transmission electron microscope diagram of the die steel with the high thermal diffusion coefficient. FIG. 3 shows the comparison of thermal diffusion coefficient between the die steel with the high thermal diffusion coefficient and H13 steel.
Names of elements involved in this embodiment are: C denotes carbon, Si denotes silicon, Mo denotes molybdenum, Nb denotes niobium, Co denotes cobalt, P denotes phosphorus, S denotes sulfur, RE denotes rare earth, and Fe denotes iron.
It should be noted that the reactive combination of molybdenum, cobalt and niobium elements according to the preferred embodiment of the present invention facilitates the formation of the die steel with the high thermal diffusion coefficient. The interaction of specific content of molybdenum and cobalt makes the die steel with the high thermal diffusion coefficient prepared in this embodiment have high thermal diffusion coefficient capacity while maintaining excellent comprehensive mechanical properties. The above specific ratio, process control parameters, and the number of tempering are the optimal specific values, which are able to be adjusted appropriately within the disclosed value range and principle requirements, so as to obtain the die steel with excellent thermal conductivity and mechanical properties.
For example, in the above electroslag ingot annealing, spheroidizing annealing and tempering, cooling with the electric furnace is performed to room temperature; in order to facilitate controlling the consistency with the standard, preferably, the temperature is lowered to below 25° with the electric furnace; oil-cooling in quenching is able to refer to the above condition. Tempering is performed once or repeated for twice, preferably, repeated for twice.
It should be noted that in order to ensure the comprehensive properties of the die steel, the content of impurities such as phosphorus and sulfur should be kept at P≤0.15 wt. % and S≤0.025 wt. % in the composition of the die steel. Preferably, the mass percentage content of RE and S in the die steel meets the conditions of [RE]/[S]≥2.0 and [RE]×[S]≤0.005%. Moreover, the die steel provided by the present invention does not contain chromium, manganese, nickel, vanadium and tungsten.
Among the above elements, the addition of molybdenum element is able to increase the thermal conductivity of the die steel with the high thermal diffusion coefficient. A large number of molybdenum carbides are formed in the material to achieve the strengthening effect. The obtained carbides not only ensure the toughness of the die steel with the high thermal diffusion coefficient while increasing its hardness, but also make the die steel obtain excellent tempering stability, red hardness and heat strength.
Cobalt is mostly in a-Fe in the annealed state and has a certain solubility in molybdenum carbides. According to the first preferred embodiment, the specific content of cobalt is able to increase the melting point of the die steel with the high thermal diffusion coefficient, for increasing the quenching temperature, so as to dissolve more molybdenum element and strengthen the matrix. At the same time, the specific content of cobalt in this embodiment is also able to delay the precipitation of alloy carbides during tempering, slow down the growth of carbides, refine carbides, for improving the secondary hardening ability, hardness and high temperature strength of steel, so as to improve the abrasion resistance and durability of the die steel with the high thermal diffusion coefficient. Cobalt has little influence on the thermal conductivity of steel, and at the same time, is able to make the steel obtain high temperature hardness and comprehensive mechanical properties. Therefore, according to the first preferred embodiment, a certain amount of cobalt is added to ensure the combination of comprehensive mechanical properties and thermal diffusion ability.
Specifically, the specific content of niobium element is able to effectively refine the microstructure grains. Niobium is able to delay the recrystallization of austenite in the process of forging, quenching, normalizing and other heat treatment of the die steel with the high thermal diffusion coefficient, which is able to strongly refine the grains. Niobium is able to form interstitial mesophase such as NbC or NbN in the steel. During the recrystallization process, the recrystallization time is greatly increased due to the effect of NbN and NbC on the pinning of dislocation and the inhibition of grain growth. In the forging and heat treatment processes of the die steel with the high thermal diffusion coefficient, niobium is able to effectively fine grains, provide activation energy for molybdenum carbide precipitation, promote carbide dispersion and fine precipitation, and ensure good comprehensive mechanical properties and thermal diffusion capacity.
The ratio of carbon content to composition element content (which is the sum of the mass percentage of molybdenum and niobium) of carbides used in the present invention is in a range of 0.09 to 0.16. According to the first embodiment, C is 0.3 wt. %, Mo is 3.40 wt. %, and Nb is 0.02 wt. %, so the ratio of carbon content to composition element content of carbides is about 0.09. The above specific content of carbon promotes that the composition elements of molybdenum strong carbides are precipitated in the form of fine dispersion characteristics during high temperature tempering, to produce secondary hardening. In this embodiment, due to the specific ratio of carbon and molybdenum, molybdenum element is precipitated in the form of fine secondary precipitated carbide, which ensures the comprehensive mechanical properties and enables the die steel to have ultra-high thermal diffusion coefficient.
According to the first preferred embodiment, the rare earth elements in the die steel with the high thermal diffusion coefficient are able to obviously optimize the quality of the casting billet, improve the plasticity and toughness of the die steel with the high thermal diffusion coefficient, and also improve the transverse performance and low temperature toughness of the steel.

Second Embodiment

The second embodiment of the present invention relates to a die steel with a high thermal diffusion coefficient, which comprises 0.33 wt. % of C, 0.06 wt. % of Si, 3.20 wt. % of Mo, 0.03 wt. % of Nb, 0.48 wt. % of Co, 0.05 wt. % of P, 0.01 wt. % of S, 0.04 wt. % of RE, the rest is Fe and unavoidable impurities.
A preparation method of the die steel with the high thermal diffusion coefficient comprises steps of:

- (A) melting predetermined components of the die steel with the high thermal diffusion coefficient by an electric furnace at 1530° C., obtaining steel ingots by casting the molten predetermined components, performing electroslag remelting on the steel ingots, annealing which comprises performing heat preservation at 790° C. for 9 h, heating the annealed ingots to 1170° C. and performing heat preservation for 30 min, multi-directionally forging at 980° C. with a forging ratio of 6.5, performing heat preservation at 720° C. for 12 h, and cooling to room temperature with the electric furnace, so as to obtain a die steel blank with the high thermal diffusion coefficient;
- (B) obtaining a Martensitic die steel material which comprises heating the die steel blank with the high thermal diffusion coefficient to 1090° C., performing heat preservation for 1 h, and performing oil-cooling to 25° C.; and
- (C) tempering which comprises performing heat preservation on the Martensitic die steel material at 600° C. for 135 min, thereby obtaining the die steel with the high thermal diffusion coefficient, wherein the die steel has a hardness of 52 HRC, an impact energy of 103 J and a thermal diffusion coefficient of 10.60 mm²/s at room temperature.

Third Embodiment

The third embodiment of the present invention relates to a die steel with a high thermal diffusion coefficient, which comprises 0.36 wt. % of C, 0.09 wt. % of Si, 2.80 wt. % of Mo, 0.05 wt. % of Nb, 0.50 wt. % of Co, 0.01 wt. % of P, 0.02 wt. % of S, 0.04 wt. % of RE, the rest is Fe and unavoidable impurities.
A preparation method of the die steel with the high thermal diffusion coefficient comprises steps of:

- (A) melting predetermined components of the die steel with the high thermal diffusion coefficient by an electric furnace at 1510° C., obtaining steel ingots by casting the molten predetermined components, performing electroslag remelting on the steel ingots, annealing which comprises performing heat preservation at 800° C. for 8 h, heating the annealed ingots to 1180° C. and performing heat preservation for 30 min, multi-directionally forging at 970° C. with a forging ratio of 8, performing heat preservation at 710° C. for 13 h, and cooling to room temperature with the electric furnace, so as to obtain a die steel blank with the high thermal diffusion coefficient;
- (B) obtaining a Martensitic die steel material which comprises heating the die steel blank with the high thermal diffusion coefficient to 1100° C., performing heat preservation for 1 h, and performing oil-cooling to 25° C.;
- (C) tempering which comprises performing heat preservation on the Martensitic die steel material at 600° C. for 135 min; and
- (D) repeating the step of (C), thereby obtaining the die steel with the high thermal diffusion coefficient, wherein the die steel has a hardness of 51 HRC, an impact energy of 117 J and a thermal diffusion coefficient of 11.32 mm²/s at room temperature.

Fourth Embodiment

The fourth embodiment of the present invention relates to a die steel with a high thermal diffusion coefficient, which comprises 0.40 wt. % of C, 0.05 wt. % of Si, 3.40 wt. % of Mo, 0.04 wt. % of Nb, 0.45 wt. % of Co, 0.02 wt. % of P, 0.01 wt. % of S, 0.03 wt. % of RE, the rest is Fe and unavoidable impurities.
A preparation method of the die steel with the high thermal diffusion coefficient comprises steps of:

- (A) melting predetermined components of the die steel with the high thermal diffusion coefficient by an electric furnace at 1580° C., obtaining steel ingots by casting the molten predetermined components, performing electroslag remelting on the steel ingots, annealing which comprises performing heat preservation at 760° C. for 10 h, heating the annealed ingots to 1160° C. and performing heat preservation for 30 min, multi-directionally forging at 980° C. with a forging ratio of 7, performing heat preservation at 750° C. for 12 h, and cooling the electric furnace to room temperature, so as to obtain a die steel blank with the high thermal diffusion coefficient;
- (B) obtaining a Martensitic die steel material which comprises heating the die steel blank with the high thermal diffusion coefficient to 1080° C., performing heat preservation for 1 h, and performing oil-cooling to 25° C.;
- (C) tempering which comprises performing heat preservation on the Martensitic die steel material at 590° C. for 135 min, thereby obtaining the die steel with the high thermal diffusion coefficient, wherein the die steel has a hardness of 51.5 HRC, an impact energy of 108 J and a thermal diffusion coefficient of 10.85 mm²/s at room temperature.

In summary, the die steel with the high thermal diffusion coefficient is able to be obtained through the specific proportion of chemical elements, especially the specific proportion of carbon and alloying elements, and the obtained die steel with the high thermal diffusion coefficient has excellent hardness, abrasion resistance, toughness, and comprehensive mechanical properties.
The above are only preferred embodiments of the present invention and are not intended to limit the present invention. Any modification, equivalent replacement or improvement made within the spirit and principles of the present invention shall fall within the protective scope of the present invention.

Claims

1. A die steel with a high thermal diffusion coefficient, the die steel comprising 0.30-0.40 wt. % of C, 0.05-0.10 wt. % of Si, 2.50-3.40 wt. % of Mo, 0.01-0.05 wt. % of Nb, 0.30-0.50 wt. % of Co, 0.01-0.05 wt. % of RE, the rest is Fe and unavoidable impurities, wherein in the die steel, P≤0.15 wt. %, S≤0.025 wt. %.

2. The die steel with the high thermal diffusion coefficient according to claim 1, wherein a mass percentage content of RE and S in the die steel meets conditions of [RE]/[S]>2.0, [RE]×[S]<0.005 wt. %.

3. A preparation method of the die steel with the high thermal diffusion coefficient according to claim 1, the method comprising steps of melting, electroslag remelting, electroslag ingot annealing, forging, spheroidizing annealing, quenching and tempering.

4. The preparation method according to claim 3, wherein melting is performed in a range of 1450-1600° C.

5. The preparation method according to claim 4, wherein melting is performed at 1530° C.

6. The preparation method according to claim 3, wherein the step of electroslag ingot annealing comprises performing heat preservation on electroslag ingots obtained by electroslag remelting in a range of 750-800° C. for 8-10 h and then cooling to room temperature with an electric furnace.

7. The preparation method according to claim 6, wherein the step of electroslag ingot annealing comprises performing heat preservation on electroslag ingots obtained by electroslag remelting at 780° C. for 9 h and then cooling to the room temperature with the electric furnace.

8. The preparation method according to claim 3, wherein the step of forging comprises heating annealed ingots to 1150-1180° C., performing heat preservation for 30 min, and multi-directionally forging above 950° C. with a forging ratio no less than 6.

9. The preparation method according to claim 3, wherein the step of spheroidizing annealing comprises performing heat preservation on forged ingots in a range of 650-750° C. for 12-16 h, and then cooling to room temperature with an electric furnace.

10. The preparation method according to claim 9, wherein the step of spheroidizing annealing comprises performing heat preservation on the forged ingots at 700° C. for 14 h, and then cooling to the room temperature with the electric furnace.

11. The preparation method according to claim 3, wherein the step of quenching comprises performing heat preservation on a die steel blank after spheroidizing annealing in a range of 1050-1150° C. for 1 h, and then performing oil-cooling to room temperature, and then tempering.

12. The preparation method according to claim 11, wherein the step of quenching comprises performing heat preservation on the die steel blank after spheroidizing annealing at 1100° C.

13. The preparation method according to claim 11, wherein the step of tempering comprises performing heat preservation on a die steel material after quenching in a range of 570-630° C. for above 2 h, and then performing oil-cooling to room temperature.

14. The preparation method according to claim 12, wherein tempering is performed for twice.

15. The preparation method according to claim 12, wherein the step of tempering comprises performing heat preservation on a die steel material after quenching at 600° C.