CN111549298A

CN111549298A - Hot work die steel and preparation method thereof

Info

Publication number: CN111549298A
Application number: CN202010429396.7A
Authority: CN
Inventors: 黄进峰; 张�诚; 张津; 赵超; 连勇; 李建强; 张程
Original assignee: University of Science and Technology Beijing USTB
Current assignee: University of Science and Technology Beijing USTB
Priority date: 2020-05-20
Filing date: 2020-05-20
Publication date: 2020-08-18
Anticipated expiration: 2040-05-20
Also published as: CN111549298B

Abstract

The invention provides hot work die steel and a preparation method thereof, wherein the hot work die steel comprises the following chemical components in percentage by mass: c: 0.20 to 0.32 wt%, Si: less than or equal to 0.5 wt%, Mn: less than or equal to 0.5 wt%, Cr: 1.5 to 2.8 wt%, Mo: 1.5-2.5 wt%, W: 0.5 to 1.2 wt%, Ni: 0.5 to 1.6 wt%, V: 0.15 to 0.7 wt%, Nb: 0.01-0.1 wt%, the balance being iron, the degree of alloying being 5-7%; the tensile strength of the hot work die steel at 700 ℃ is 560-700 MPa; the room temperature hardness value of the hot work die steel after heat preservation for 3-5 hours at 700 ℃ is 32-38 HRC; the hot-work die steel has the advantages of 14-16% of elongation at room temperature, 48-65% of reduction of area and 52-63J of room-temperature impact toughness, and has excellent thermal stability and room-temperature plastic toughness.

Description

Hot work die steel and preparation method thereof

Technical Field

The invention relates to the technical field of hot work die steel, in particular to hot work die steel and a preparation method thereof.

Background

Hot work die steel is mainly used for dies for pressing solid or liquid metal above recrystallization temperature into workpieces, such as hot forging dies, hot extrusion dies, die casting dies, and the like. The hot work die steel has severe service conditions, a die cavity of the hot work die steel is in direct contact with a high-temperature workpiece, the local temperature can reach 600-700 ℃, and the hot work die steel also bears the effects of strong high-temperature load, high-temperature strain fatigue, cold and hot fatigue and the like. The insufficient high-temperature strength can cause the softening, deformation and collapse of the die, and the insufficient high-temperature strain fatigue resistance and cold and hot fatigue resistance can cause the cracking and peeling of the die, so that the comprehensive improvement of the high-temperature strength, high-temperature fatigue, cold and hot fatigue and other properties of the hot-work die steel is a core and key index for prolonging the service life of the hot-work die steel.

The existing widely-applied hot work die steel is medium alloy chromium system H13 steel (4Cr5MoSiV1), H13 steel has good obdurability matching and thermal fatigue resistance below 550 ℃, but the strength and the thermal stability are rapidly reduced when the temperature is over 600 ℃, the tensile strength at 700 ℃ is only 260-320 MPa, the reduction of the high-temperature strength causes the deterioration of the thermal fatigue resistance, the tendency of high-temperature hot cracking is increased, and the high-temperature service condition of the hot work die steel is difficult to meet.

In order to improve the service temperature and the high-temperature strength of hot-work die steel, the prior art generally adopts the method of preparing the hot-work die steel by increasing the content of carbon and alloy, for example, high-alloy tungsten-molybdenum hot-work die steel (3Cr2W8V) and the like, the alloy content is more than 10%, the high-temperature strength at 700 ℃ can be improved to 300-400 MPa, but the room-temperature toughness is only 11-13J, the cold and hot fatigue resistance is poor, the early failure of the die due to cracking is often caused, and the application range is very limited no matter the use safety, the cost processing and the like.

Therefore, a hot die steel with high strength at high temperature and good room temperature ductility and toughness and fatigue resistance is needed.

Disclosure of Invention

The invention aims to provide hot-work die steel and a preparation method thereof, so that the hot-work die steel has good plasticity and toughness and high-temperature application stability. The specific technical scheme is as follows:

the invention provides hot work die steel in a first aspect, which comprises the following chemical components in percentage by mass:

c: 0.20 to 0.32 wt%, Si: less than or equal to 0.5 wt%, Mn: less than or equal to 0.5 wt%, Cr: 1.5 to 2.8 wt%, Mo: 1.5-2.5 wt%, W: 0.5 to 1.2 wt%, Ni: 0.5 to 1.6 wt%, V: 0.15 to 0.7 wt%, Nb: 0.01-0.1 wt%, the balance being iron, the degree of alloying being 5-7%;

the tensile strength of the hot work die steel at 700 ℃ is 560-700 MPa;

the room temperature hardness value of the hot work die steel after heat preservation for 3-5 hours at 700 ℃ is 32-38 HRC;

the hot-work die steel has the elongation of 14-16% at room temperature, the reduction of area of 48-65% and the room-temperature impact toughness of 52-63J.

In one embodiment of the invention, the hot work die steel further comprises at least one of the following chemical components:

zr: 0.01-0.03 wt%, Co: 0.10 to 0.50 wt%, B: 0.001 to 0.005 wt%, Re: 0.01-0.10 wt%, Ti: 0.02 to 0.06 wt%, and Y: 0.01 to 0.1 wt%.

In one embodiment of the invention, the hot work die steel has an S content of less than 0.02 wt.% and a P content of less than 0.02 wt.%.

In one embodiment of the invention, the tempered sorbite structure of the hot work die steel retains lath characteristics after being drawn at 700 ℃.

In one embodiment of the invention, after the hot-work die steel is stretched at 700 ℃, the carbide in the hot-work die steel is nano-scale acicular MC type alloy carbide.

In one embodiment of the invention, the nano-scale acicular MC type alloy carbide is: v_0.5～0.8Mo_0.5～ _0.6Cr_0.15～0.3W_0.06～0.14Nb_0.01～0.02C。

In one embodiment of the present invention, the hot work die steel has a tensile strength of 600 to 700MPa at 700 ℃.

The second aspect of the present invention provides a method for preparing the hot-work die steel according to any one of the above aspects, comprising the steps of:

a smelting step: preparing the following raw materials in percentage by mass:

c: 0.20 to 0.32 wt%, Si: less than or equal to 0.5 wt%, Mn: less than or equal to 0.5 wt%, Cr: 1.5 to 2.8 wt%, Mo: 1.5-2.5 wt%, W: 0.5 to 1.2 wt%, Ni: 0.5 to 1.6 wt%, V: 0.15 to 0.7 wt%, Nb: 0.01 to 0.1 wt%, the balance being iron,

the raw materials are processed by electric arc melting, external refining, vacuum degassing and forging in a forging furnace to form electrode bars;

electroslag remelting step: removing oxide skin of the electrode bar, then placing the electrode bar into a vacuum electroslag remelting device for secondary refining, keeping the water temperature of a water cooling system of the electroslag remelting device not higher than 70 ℃, and electroslag remelting the electrode bar to obtain an electroslag steel ingot, wherein the melting speed is 7-12 kg/min, and the water temperature of cooling water of a crystallizer is kept at 40-50 ℃;

a homogenizing annealing step: heating the electroslag steel ingot to 1200-1250 ℃, and preserving heat for 15-23 h;

forging: cooling the electroslag steel ingot to a forging heating temperature of 1150-1200 ℃ for forging, wherein the initial forging temperature is 1130-1160 ℃, and the final forging temperature is more than or equal to 850 ℃ to obtain a steel ingot;

annealing after forging: putting the steel ingot into an annealing furnace when the temperature is lower than 500 ℃, heating to 830-890 ℃ at a heating rate of not more than 100 ℃/h, then carrying out heat preservation for [120min + r (mm) x 2min/mm ] or [120min + d (mm)/2 x 2min/mm ], then cooling with the furnace to below 500 ℃ at a rate of 20-40 ℃/h, taking out of the annealing furnace, and carrying out air cooling to obtain an annealed steel ingot;

fine grain heat treatment: heating the annealed steel ingot to 930-1150 ℃, carrying out first heat preservation for [ (15-40) min + r (mm) x 2min/mm ] or [ (15-40) min + d (mm)/2 x 2min/mm ], cooling the steel ingot to 400-500 ℃ within 1-2 min, then carrying out air cooling to 250-280 ℃ for second heat preservation for 5-10 h; then preserving the heat for 5-10 h at the temperature of 660-700 ℃;

quenching and tempering: heating the steel ingot after heat preservation to 980-1100 ℃, preserving heat for [ (15-40) min + r (mm) x 2min/mm ] or [ (15-40) min + d (mm)/2 x 2min/mm ], cooling to 50-150 ℃, tempering and preserving heat at 580-660 ℃, preserving heat for 6-16 h, and obtaining the hot work die steel;

wherein r is the material radius and d is the material thickness.

In one embodiment of the invention, the feedstock further comprises at least one of the following ingredients: zr: 0.01-0.03 wt%, Co: 0.10 to 0.50 wt%, B: 0.001 to 0.005 wt%, Re: 0.01-0.10 wt%, Ti: 0.02 to 0.06 wt%, and Y: 0.01 to 0.1 wt%.

In one embodiment of the present invention, the forging step specifically comprises:

forming and forging by using a precision forging machine, wherein the forging heating temperature is 900-1050 ℃, the initial forging temperature is 850-950 ℃, and the final forging temperature is more than or equal to 800 ℃;

or forming and forging by using a hydraulic hammer or an oil press, wherein the forging heating temperature is 1150-1200 ℃, the initial forging temperature is 1130-1160 ℃, and the final forging temperature is more than or equal to 850 ℃.

In one embodiment of the invention, the holding time of the annealing after forging is 6-8 h.

In the present invention, the term "degree of alloying" means: the total content of other elements except iron and carbon in the steel.

The invention has the beneficial effects that:

compared with the traditional hot-work die steel, the hot-work die steel provided by the invention has the advantages that the tensile strength at 700 ℃ is 560-700 MPa, is about 2 times of that of H13 steel and is about 1.5 times of that of 3Cr2W8V, the use temperature is increased to about 700 ℃ compared with the use temperature of the existing H13 steel, and the increase amplitude is up to 100 ℃, so that the application stability of the hot-work die steel at higher temperature is improved, and the hot-work die steel has good room-temperature plastic toughness and high-temperature fatigue resistance, so that the application range of the hot-work die steel is enlarged.

According to the heat treatment method of the hot work die steel, the tensile strength of the prepared hot work die steel at 700 ℃ can reach 560-700 MPa, and the room temperature hardness value after heat preservation for 3-5 hours at 700 ℃ is 32-38 HRC by controlling the addition proportion of the raw materials and reasonable forging and heat treatment processes.

Of course, not all of the advantages described above need to be achieved at the same time in the practice of any one product or method of the invention.

Drawings

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below, it is obvious that the drawings in the following description are only some embodiments of the present invention, and for those skilled in the art, other drawings can be obtained according to the drawings without creative efforts.

FIG. 1 is a flow chart of a heat treatment process of hot work die steel according to the present invention;

FIG. 2 is a schematic view showing the change of tensile strength with temperature of hot work die steel according to example 5 of the present invention;

FIG. 3a is an electron micrograph of the hot work die steel of example 5 of the present invention at room temperature;

FIG. 3b is an electron micrograph of the hot work die steel of example 5 of the present invention after being stretched at 700 ℃;

FIG. 3c is an enlarged view of a portion of FIG. 3 b;

FIG. 4a is an electron micrograph of H13 steel of comparative example 1 at room temperature;

FIG. 4b is an electron micrograph of the H13 steel of comparative example 1 after drawing at 700 ℃;

FIG. 4c is an enlarged view of a portion of FIG. 4 b;

FIG. 5a is a microstructure of carbide after drawing at 700 ℃ in the hot work die steel of example 5 of the present invention;

FIG. 5b is a selected area electron diffraction pattern of the hot work die steel of example 5 of the present invention after being stretched at 700 ℃;

FIG. 5c is a high resolution image of MC type alloy carbides after drawing at 700 ℃ in the hot work die steel of example 5 of the present invention;

FIG. 6 is a diagram showing the analysis of the carbide composition of the hot work die steel of example 5 of the present invention.

Detailed Description

The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

Although the prior art also changes H13 steelFurthermore, the room temperature low temperature toughness and the high temperature strength of the hot work die steel are improved, but the content of carbon and alloy is generally improved, so that the formation of high melting point carbide is promoted, the high temperature strength of the hot work die steel is improved in a solid solution strengthening and carbide dispersion strengthening mode, although the high temperature strength at 600 ℃ has a certain improving effect, the strengthening effect is limited at higher temperature, such as 700 ℃, mainly because when the temperature exceeds 600 ℃, M has a limited strengthening effect₂The coherent relationship between the C or MC carbide and the matrix is destroyed and converted into non-coherent M which is easy to grow₆C or M₂₃C₆Carbides, resulting in a large reduction in strengthening effect. Therefore, the existing design principle and method for improving the high-temperature strength by improving the carbon content and high alloying have the limit of improving the high-temperature strength of hot-work die steel, and can cause the rapid reduction of ductility, high-temperature fatigue, cold-hot fatigue and the like.

In view of the above, the present invention provides a hot work die steel and a preparation method thereof, and in general, the inventors determine the high temperature strength of a material based on the stability of the high temperature coherent relationship between a carbide and a substrate, select carbon and alloy elements, and determine heat treatment parameters for hot working, and realize the regulation and control of the mismatching degree of a carbide/substrate interface by the multi-alloying design of W, Mn, Mo, V, Cr, Ni and Nb and the optimization of a heat treatment process, so as to obtain a nano-scale low mismatching degree MC type alloy carbide distributed in a dispersion manner, so that the stability of the coherent relationship between the carbide and the substrate can be maintained to 700 ℃, and dislocation motion and recrystallization of a sorbite lath are hindered, thereby obtaining high temperature and high strength; meanwhile, the invention keeps the medium-low carbon design (the content of C is 0.20-0.32%), and obtains the quenching dislocation martensite fine-grained structure through the fine-grained heat treatment step so as to ensure the toughness and the fatigue resistance of the material after tempering, thereby ensuring the service life of the novel hot die steel from the structural aspect.

The invention provides hot work die steel which comprises the following chemical components in percentage by mass:

c: 0.20 to 0.32 wt%, Si: less than or equal to 0.5 wt%, Mn: less than or equal to 0.5 wt%, Cr: 1.5 to 2.8 wt%, Mo: 1.5-2.5 wt%, W: 0.5 to 1.2 wt%, Ni: 0.5 to 1.6 wt%, V: 0.15-0.7 wt%, Nb: 0.01-0.1 wt% of iron and 5-7% of alloying degree;

the tensile strength of the hot work die steel at 700 ℃ is 560-700 MPa, preferably 600-700 MPa, and more preferably 650-690 MPa;

the hardness value at room temperature of the hot-work die steel after heat preservation for 3-5 hours at 700 ℃ is 32-38 HRC, the heat preservation time is not particularly limited, and for example, the heat preservation time can be 3-5 hours, specifically, 3 hours, 4 hours or 5 hours, and preferably 4 hours.

The elongation of the hot die steel at room temperature is 14-16%; the reduction of area is 48% -65%; the room temperature impact toughness is 52-63J.

The inventors have found that carbon (C) is an important element in hot-work die steel, determines the hardness and strength of martensite formed during quenching, plays a key role in temper secondary hardening, and has an important influence on both the strength and toughness of hot-work die steel. Without being limited to any theory, the low-carbon steel quenching structure is usually dislocation martensite, not only has high toughness, but also has certain plastic deformation capacity, and can avoid and reduce the formation of quenching cracks; and the acicular martensite formed by the medium-high carbon steel is formed in an explosive manner, so that the stress is large, the twin crystal martensite has low toughness, plastic deformation cannot occur, and micro cracks are easily generated during quenching.

Based on the research, the carbon content design needs to keep the medium-low carbon level, the carbon content in the matrix is below 0.25 wt%, and the full lath martensite structure can be obtained after quenching, but in consideration of the reasons that in actual preparation, strong carbide forming elements such as Mo, W, V and the like form primary carbide to consume carbon, the carbon content in the hot work die steel is controlled to be 0.20-0.32 wt%, so that the toughness and fatigue performance of the material are improved, the actual production requirements are met, and the large-scale production of the hot work die steel is facilitated.

The inventors have also found that silicon (Si) and manganese (Mn) mainly act to equally deoxidize in steel, and have a certain solid-solution strengthening effect and an effect of improving hardenability. The solid solution strengthening effect of Si is good, a small amount of Si can obtain good solid solution strengthening effect, and the toughness of the material is rapidly reduced due to excessive Si; and Mn is an austenitizing forming element, excessive Mn can cause the structure of the quenched material to retain retained austenite, and excessive retained austenite is not beneficial to the high-temperature performance of the material, so that the contents of Si and Mn are controlled as follows: less than or equal to 0.5 weight percent of Si and less than or equal to 0.5 weight percent of Mn.

Chromium (Cr) mainly acts to improve strength, hardenability and oxidation resistance of steel, and Cr is a carbide-forming element which can form various carbides with carbon, such as Cr₇C₃，Cr₂₃C₆However, these carbides have a large mismatch with the substrate, and the coherent relationship is difficult to maintain at high temperature, so they tend to grow and coarsen at high temperature, and too high Cr content is not favorable for improving the high-temperature strength of hot-work die steel, so the Cr content is controlled to 1.5 to 2.8 wt% in the present invention.

Tungsten (W) and molybdenum (Mo) can not only improve the hardenability of the material, but also form a large amount of W with high melting point in the material₂C、Mo₂C carbide can be dissolved into the carbide VC to form alloy carbide, so that a secondary hardening effect is generated, and aggregation and growth of the carbide can be inhibited, thereby improving the high-temperature strength. However, too high W, Mo content results in too great a mismatch between carbide and matrix at high temperature and loss of compatibility, and also promotes M₆C and the like tend to grow up and coarsen carbide formation, resulting in disappearance of the high-temperature strengthening effect. In the invention, by adjusting the content of Mo to be 1.5-2.5 wt%, the content of W is as follows: 0.5-1.2 wt%, and the contents of Mo, W and V are matched with each other to form low-mismatching MC type alloy carbide which can keep a coherent relationship with a matrix at high temperature, so that the high-temperature strength of the hot-work die steel is improved.

Vanadium (V) is a strong carbide forming element, formed carbide particles are fine and dispersed, and can be completely dissolved in austenite only when the temperature is over 1200 ℃, so that the carbide has the function of refining austenite grains, formed MC type alloy carbide has good mismatching degree with a matrix, but coarse primary carbide is easily formed due to excessively high vanadium content, and the plasticity and the toughness of steel are obviously reduced. The inventor unexpectedly finds that the high-temperature coherent relationship between MC type alloy carbide and a matrix can be kept to 700 ℃ by controlling the content of V to be 0.15-0.7 wt% through the combined action of W, Mo and V elements, so that the high-temperature strength and the thermal stability of the hot-work die steel are remarkably improved, and the plasticity and the toughness of the hot-work die steel can also be improved.

Nickel (Ni) can effectively increase the hardenability of steel and improve the low-temperature toughness, but too much nickel (Ni) not only increases the cost, but also lowers the critical point Ac1 of hot-work die steel, which is unfavorable for red hardness, so the present invention controls the content of Ni as Ni: 0.5 to 1.6 wt%.

Niobium (Nb) can be preferentially combined with C to form a strong carbide, the grain growth is controlled during high-temperature austenitizing, and the effect of grain refinement is achieved, but if the content is too high, the primary carbide formed during material solidification is too much and large in size, and the improvement of impact toughness and fatigue performance of hot-work die steel is not facilitated, so that the Nb content is controlled to be 0.01-0.1 wt% so as to maximally exert the effect of grain refinement.

The inventors have also found, without being limited to any theory, that when at least one of the above Zr, Co, B, Re, Ti and Y is contained in a hot work die steel, the high temperature stability, the purity and the grain size of the hot work die steel can be further improved, which may be due to:

zirconium (Zr) has powerful deoxidizing and denitrifying element effect in steel smelting process, so that small amount of Zr is added to combine Zr with oxygen and nitrogen to form oxide and nitride with fine dispersion distribution in the matrix, and this is favorable to refining grain structure. In addition, Zr element can be combined with impurity element S to generate sulfide, so that the hot brittleness of the steel is avoided. Therefore, in order to obtain a steel having a finer and purer structure, the Zr content is controlled to 0.01 to 0.03 wt% in the present invention.

Cobalt (Co) can form a continuous solid solution with iron like Ni and Mn, and can hinder and delay the precipitation and aggregation of other alloy carbides in the tempering process, so that the heat strength of the material is remarkably improved, but the cobalt element can reduce the hardenability of martensitic steel, so that excessive addition is not suitable, and the content of cobalt is controlled to be 0.10-0.50 wt%.

Boron (B) has a very strong ability to improve hardenability within a certain content range, but does not greatly improve hardenability after exceeding 0.005 wt% in steel, and B plays a role in strengthening grain boundaries in steel, and can significantly improve the high-temperature strength of the material, so that the content of B is controlled to be 0.001-0.005 wt% in the present invention.

Rhenium (Re) is used as a rare earth element, can control the form of sulfide in steel, has the functions of deoxidation, desulfurization, transverse performance improvement and low-temperature toughness, and also has the function of dispersion hardening in low-sulfur steel, so that the Re content is controlled to be 0.01-0.10 wt% in the invention for deoxidizing, desulfurizing and purifying molten steel and improving the toughness of the steel.

Titanium (Ti) can be preferentially combined with C to form strong carbide, grain growth is controlled during high-temperature austenitizing, and the effect of grain refinement is achieved, but if the content is too high, primary carbide formed during material solidification is too much and large in size, and improvement of impact toughness and fatigue performance of hot-work die steel is not facilitated, so that the content of Ti is controlled to be 0.02-0.06 wt% to achieve the effect of grain refinement.

The content of trace yttrium (Y) in the steel can be segregated in the grain boundary, so that the grain boundary can be strengthened at high temperature, and the high-temperature strength is improved, therefore, the content of Y is controlled to be 0.01-0.1 wt%.

Sulfur (S) and phosphorus (P) are impurity elements which are unfavorable for the toughness of the material, probably because S forms sulfide inclusions to reduce the plasticity, and (Fe + FeS) eutectic is easy to form in a sulfur-containing atmosphere to cause a cracking phenomenon, so the content of S is reduced as much as possible; too high a P content leads to a decrease in low-temperature toughness and an increase in cold-brittle transition temperature, so that the P content should be minimized to avoid or reduce adverse effects on plasticity. However, when the contents of S and P in the steel are lower, the cost for removing these elements will be higher, and in order to make the hot-work die steel maintain excellent performance and reduce the production cost as much as possible for mass production, the present invention controls the S content to be less than 0.02 wt% and the P content to be less than 0.02 wt%.

In one embodiment of the invention, after the hot-work die steel is stretched at 700 ℃, the tempered sorbite structure of the hot-work die steel keeps the characteristics of laths, and high-density nano MC type alloy carbides are distributed in the laths, which indicates that the nano carbides in the hot-work die steel have higher thermal stability.

In one embodiment of the present invention, after the hot-work die steel is drawn at 700 ℃, the carbide in the hot-work die steel is a nano-scale acicular MC type alloy carbide which is a multi-element alloy carbide, and after the nano-scale acicular MC type alloy carbide is analyzed by an atom probe, the nano-scale acicular MC type alloy carbide is: v_0.5～0.8Mo_0.5～0.6Cr_0.15～0.3W_0.06～0.14Nb_0.01～0.02C, without being limited to any theory, the carbide can keep a coherent relationship with a higher temperature of a matrix, thereby realizing high-temperature and high-strength of the hot-work die steel at a low alloy degree.

Compared with the existing hot-work die steel, the hot-work die steel provided by the invention has the advantages that the tensile strength at 700 ℃ is 560-700 MPa, and the room temperature hardness value after heat preservation for 3-5 hours at 700 ℃ is 32-38 HRC, so that the service temperature of the hot-work die steel can be increased from 600 ℃ to about 700 ℃, the increase range is up to 100 ℃, the application stability of the hot-work die steel at higher temperature is improved, and the hot-work die steel has good room temperature ductility and toughness, so that the application range of the hot-work die steel is enlarged.

The invention also provides a preparation method of the hot-work die steel according to any one of the embodiments, which comprises the following steps:

a smelting step:

preparing the following raw materials in percentage by mass: c: 0.20 to 0.32 wt%, Si: less than or equal to 0.5 wt%, Mn: less than or equal to 0.5 wt%, Cr: 1.5 to 2.8 wt%, Mo: 1.5-2.5 wt%, W: 0.5 to 1.2 wt%, Ni: 0.5 to 1.6 wt%, V: 0.15 to 0.7 wt%, Nb: 0.01-0.1 wt% and the balance of iron, and then arc melting, external refining, vacuum degassing, forging and cogging the raw materials to obtain the electrode bar.

The preparation process of the electrode rod is well known to those skilled in the art, and the present invention is not particularly limited, for example, the electrode rod may be prepared by the following process: mixing the raw materials, and then sequentially carrying out electric arc melting (EAF), external refining (LF), Vacuum Degassing (VD) and forging in a forging furnace to obtain the electrode rod. The present invention is not particularly limited as long as the object of the present invention can be achieved by the above-mentioned arc melting, external refining, vacuum degassing, and forging, and for example, the temperature of the furnace for arc melting may be not lower than 1690 ℃, and the gas content and the impurity element content in the molten steel are controlled as follows: [ nitrogen (N) ] + [ hydrogen (H) ] + [ oxygen (O) ] < 150 ppm; the heating temperature of the external refining is 1600-1700 ℃, high-alkalinity reducing slag can be produced in the refining process, and the desulfurization is enhanced by controlling the temperature; the vacuum degassing time is 15-20 min, the heating temperature is 1560-1675 ℃, and the absolute vacuum degree is 50-100 Pa.

Electroslag remelting step:

removing oxide skin of the electrode bar, then putting the electrode bar into a vacuum electroslag remelting device for secondary refining, keeping the water temperature of a water cooling system of the electroslag remelting device not higher than 70 ℃, and carrying out electroslag remelting on the electrode bar to obtain an electroslag steel ingot. The electroslag remelting is not particularly limited, as long as the purpose of the invention can be achieved, for example, the melting speed can be 7-12 kg/min, the water temperature of the cooling water of the crystallizer is kept at 40-50 ℃, the deoxidizer can be at least one of aluminum particles or silico-calcium powder, and inert gas, such as argon, is filled in the whole electroslag remelting process.

The inventor researches and discovers that when the temperature of cooling water of a crystallizer of an electroslag remelting device is not higher than 70 ℃, the prepared electroslag steel ingot has more uniform and fine structure and higher purity.

A homogenizing annealing step:

heating the electroslag steel ingot to 1200-1250 ℃, and preserving heat for 15-23 h;

forging:

and cooling the electroslag steel ingot to a forging heating temperature of 1150-1200 ℃ for forging, wherein the initial forging temperature is 1130-1160 ℃, and the final forging temperature is more than or equal to 850 ℃, so as to obtain the steel ingot.

The forging heating temperature of the invention is increased by about 50 ℃ compared with the heating temperature of the existing die steel, so as to improve the high-temperature solid solubility of carbon and alloy elements, and ensure that the forged structure and crystal grains are fine.

Annealing after forging:

and (2) putting the steel ingot into an annealing furnace when the temperature is lower than 500 ℃, heating to 830-890 ℃ at a heating rate of not more than 100 ℃/h, and then carrying out heat preservation, wherein the heat preservation time is [120min + r (mm) multiplied by 2min/mm ] or [120min + d (mm)/2 multiplied by 2min/mm ], the specific heat preservation time can be determined according to the material size, preferably 6-8 hours, and then cooling with the furnace to below 500 ℃ at a rate of 20-40 ℃/h, taking out of the annealing furnace, and carrying out air cooling to obtain the annealed steel ingot.

Wherein r is the radius of the material, d is the thickness of the material, the heat preservation time can be calculated by adopting the r when the steel ingot is a cylinder, the heat preservation time can be calculated by adopting the d when the steel ingot is a cube, and the specific calculation mode is determined by the actual shape of the material; and the steel ingot is cooled to a lower temperature (such as lower than 500 ℃) and then annealed, so that the coarsening of crystal grains caused by overlong high-temperature heat preservation can be avoided.

Fine grain heat treatment:

with reference to fig. 1, fig. 1 is a flow chart of a heat treatment process of hot work die steel, the annealed steel ingot is heated to 930-1150 ℃ and then subjected to first heat preservation for [ (15-40) min + r (mm) x 2min/mm ] or [ (15-40) min + d (mm)/2 x 2min/mm ], the specific heat preservation time can be determined according to the material size, the process is a normalizing process, and then water cooling is performed within 1-2 min to 400-500 ℃, then air cooling is performed to 250-280 ℃ and then secondary heat preservation is performed, wherein the heat preservation time is 5-10 h; then preserving the heat for 5-10 h at the temperature of 660-700 ℃;

and when the steel ingot is a cube, calculating the heat preservation time by using the d, wherein the specific calculation mode is determined by the actual shape of the material.

In the invention, after normalizing, water cooling to 400-500 ℃ and air cooling to 250-280 ℃ and heat preservation for 5-10 h are adopted, B/M (bainite/martensite) complex phase structure refined grains are formed, and then heat preservation is carried out at 660-700 ℃ to form dispersed secondary carbide, so that austenite grains are prevented from growing large during subsequent quenching and tempering heating.

Quenching and tempering:

heating the steel ingot after heat preservation to 980-1100 ℃, preserving heat for [ (15-40) min + r (mm) x 2min/mm ] or [ (15-40) min + d (mm)/2 x 2min/mm ], and then cooling to 50-150 ℃; and tempering and preserving heat at the temperature of 580-660 ℃ for 6-16 hours to obtain the hot work die steel.

In the quenching and tempering treatment step, the heating temperature is increased by 30-50 ℃ compared with the quenching heating temperature of the existing hot work die steel, and the purpose is to improve the solid solubility of alloy elements. In addition, the cooling method of the quality control step of the present invention is not particularly limited, and may be, for example, air cooling, water cooling or oil cooling,

in the tempering and heat preservation step, the tempering is carried out at 580-660 ℃, so that the hot die steel can form the nano-scale MC type alloy carbide with low mismatching degree, and the thermal stability of the material is further improved.

In one embodiment of the present invention, the feedstock may further comprise at least one of the following components:

In one embodiment of the present invention, the forging step may specifically include:

forming and forging by using a precision forging machine, wherein the forging heating temperature is 900-1050 ℃, the initial forging temperature is 850-950 ℃, and the final forging temperature is more than or equal to 800 ℃; or forming and forging by using a hydraulic hammer or an oil press, wherein the forging heating temperature is 1150-1200 ℃, the initial forging temperature is 1130-1160 ℃, and the final forging temperature is more than or equal to 850 ℃, so that the forged piece with a proper shape and size is obtained.

The present invention is not particularly limited in the type of the finish forging machine, hydraulic hammer or oil press as long as the object of the present invention is achieved, and for example, a finish forging machine manufactured by GFM of austria may be used as the finish forging machine.

According to the heat treatment method of the hot-work die steel, the tensile strength of the prepared hot-work die steel at 700 ℃ can reach 560-700 MPa, the room temperature hardness value after heat preservation for 3-5 hours at 700 ℃ is 32-38 HRC, and the hot-work die steel has good room temperature ductility and toughness, so that the application range of the hot-work die steel is enlarged.

Hereinafter, embodiments of the present application will be described in more detail with reference to examples and comparative examples. Various tests and evaluations were carried out according to the following methods. Unless otherwise specified, "part" and "%" are based on weight.

Example 1

< melting >

Preparing the following raw materials in percentage by mass:

c: 0.19 wt%, Si: 0.20 wt%, Mn: 0.30 wt%, Cr: 2.22 wt%, Mo: 2.30 wt%, W: 0.50 wt%, Ni: 0.50 wt%, V: 0.22 wt%, Nb: 0.20 wt% and the balance iron, and the raw materials are processed by arc melting, refining, vacuum degassing and forging in a forging furnace to form the electrode bar.

< electroslag remelting >

Removing oxide skin of the electrode bar, then placing the electrode bar into a vacuum electroslag remelting device, keeping the water temperature of a water cooling system of the electroslag remelting device at 70 ℃, and carrying out electroslag remelting on the electrode bar to obtain an electroslag steel ingot.

< homogenizing annealing >

Heating the electroslag steel ingot to 1200 ℃, and preserving heat for 23 h.

< forging >

And (3) cooling the electroslag steel ingot to a forging heating temperature of 1150 ℃ for forging, wherein the initial forging temperature is 1130 ℃, and the final forging temperature is 850 ℃, so that the steel ingot is obtained, the radius of the steel ingot is 40mm, and the length of the steel ingot is 100 mm.

< annealing after forging >

And (3) putting the steel ingot into an annealing furnace when the temperature is lower than 500 ℃, heating to 830 ℃ at a heating rate of 80 ℃/h, then preserving the heat for 200min, then cooling to 450 ℃ along with the furnace at a rate of 20 ℃/h, taking out of the annealing furnace, and air-cooling to obtain the annealed steel ingot.

< Heat treatment for Fine Crystal >

Heating the annealed steel ingot to 930 ℃, performing primary heat preservation for 2 hours, performing water cooling to 400 ℃ within 1min, then performing air cooling to 250 ℃ for secondary heat preservation, wherein the heat preservation time is 10 hours; then keeping the temperature at 660 ℃ for 10 h.

< thermal refining >

Heating the steel ingot after heat preservation to 1000 ℃, preserving heat for 2h, and then cooling to 50 ℃; and tempering and preserving heat for 16 hours at the temperature of 600 ℃ to obtain the hot work die steel.

Example 2

< melting >

Preparing the following raw materials in percentage by mass:

c: 0.23 wt%, Si: 0.20 wt%, Mn: 0.30 wt%, Cr: 2.48 wt%, Mo: 2.15 wt%, W: 0.50 wt%, Ni: 0.50 wt%, V: 0.28 wt%, Nb: 0.10 wt% and the balance iron, and the electrode rod is made by the raw materials through arc melting, refining, vacuum degassing and forging in a forging furnace.

< electroslag remelting >

Removing oxide skin of the electrode bar, then placing the electrode bar into a vacuum electroslag remelting device, keeping the water temperature of a water cooling system of the electroslag remelting device at 65 ℃, and carrying out electroslag remelting on the electrode bar to obtain an electroslag steel ingot.

< homogenizing annealing >

Heating the electroslag steel ingot to 1230 ℃, and preserving the temperature for 20 h.

< forging >

And (3) cooling the electroslag steel ingot to the forging heating temperature of 1170 ℃ for forging, wherein the initial forging temperature is 1150 ℃, and the final forging temperature is 860 ℃, so that the steel ingot is obtained, the radius of the steel ingot is 40mm, and the length of the steel ingot is 100 mm.

< annealing after forging >

And (3) putting the steel ingot into an annealing furnace when the temperature is lower than 500 ℃, heating to 850 ℃ at a heating rate of 90 ℃/h, then preserving the heat for 200min, then cooling to 480 ℃ along with the furnace at a rate of 30 ℃/h, taking out of the annealing furnace, and air-cooling to obtain the annealed steel ingot.

< Heat treatment for Fine Crystal >

Heating the annealed steel ingot to 980 ℃, then carrying out primary heat preservation for 2h, cooling to 450 ℃ in 1.5min by water, then carrying out air cooling to 260 ℃ for secondary heat preservation, wherein the heat preservation time is 6 h; then keeping the temperature at 660 ℃ for 5 h.

< thermal refining >

Heating the steel ingot after heat preservation to 1020 ℃, preserving heat for 1.5h, and then cooling to 100 ℃; and tempering and preserving heat for 10 hours at the temperature of 620 ℃ to obtain the hot work die steel.

Example 3

< melting >

Preparing the following raw materials in percentage by mass:

c: 0.27 wt%, Si: 0.04 wt%, Mn: 0.07 wt%, Cr: 2.72 wt%, Mo: 1.90 wt%, W: 0.95 wt%, Ni: 1.22 wt%, V: 0.40 wt%, Nb: 0.10 wt%, Y: 0.02 wt% and the balance iron, and the raw materials are processed by arc melting, refining, vacuum degassing and forging in a forging furnace to form the electrode bar.

< electroslag remelting >

Removing oxide skin of the electrode bar, then placing the electrode bar into a vacuum electroslag remelting device, keeping the water temperature of a water cooling system of the electroslag remelting device at 68 ℃, and carrying out electroslag remelting on the electrode bar to obtain an electroslag steel ingot.

< homogenizing annealing >

Heating the electroslag steel ingot to 1250 ℃, and preserving heat for 15 h.

< forging >

And (3) cooling the electroslag steel ingot to the forging heating temperature of 1200 ℃ for forging, wherein the initial forging temperature is 1160 ℃, and the final forging temperature is 870 ℃, so that the steel ingot is obtained, the radius of the steel ingot is 40mm, and the length of the steel ingot is 100 mm.

< annealing after forging >

And (3) putting the steel ingot into an annealing furnace when the temperature is lower than 500 ℃, heating to 900 ℃ at the heating rate of 100 ℃/h, then preserving the heat for 200min, then cooling to 490 ℃ along with the furnace at the heating rate of 40 ℃/h, taking the steel ingot out of the annealing furnace, and air-cooling to obtain the annealed steel ingot.

< Heat treatment for Fine Crystal >

Heating the annealed steel ingot to 1000 ℃, performing primary heat preservation for 2 hours, cooling the steel ingot to 500 ℃ within 2min by water, then cooling the steel ingot to 280 ℃ by air, and performing secondary heat preservation for 6 hours; then the temperature is kept for 5h at 680 ℃.

< thermal refining >

Heating the steel ingot after heat preservation to 1020 ℃, preserving heat for 1.5h, and then cooling to 150 ℃; and tempering and heat preservation are carried out for 6 hours at the temperature of 635 ℃ to obtain the hot work die steel.

Example 4

< melting >

Preparing the following raw materials in percentage by mass:

c: 0.30 wt%, Si: 0.12 wt%, Mn: 0.02 wt%, Cr: 2.00 wt%, Mo: 1.65 wt%, W: 1.10 wt%, Ni: 1.42 wt%, V: 0.42 wt%, Nb: 0.02 wt%, Zr: 0.02 wt%, Co: 0.10 wt%, B: 0.003 wt%, Re: 0.012 wt%, Ti: 0.03 wt%, Y: 0.02 wt% and the balance iron, and the raw materials are processed by arc melting, refining, vacuum degassing and forging in a forging furnace to form the electrode bar.

< electroslag remelting >

Removing oxide skin of the electrode bar, then placing the electrode bar into a vacuum electroslag remelting device, keeping the water temperature of a water cooling system of the electroslag remelting device at 69 ℃, and carrying out electroslag remelting on the electrode bar to obtain an electroslag steel ingot.

< homogenizing annealing >

Heating the electroslag steel ingot to 1250 ℃, and preserving heat for 15 h.

< forging >

< annealing after forging >

< Heat treatment for Fine Crystal >

Heating the annealed steel ingot to 1100 ℃, carrying out primary heat preservation for 2h, cooling the steel ingot to 500 ℃ within 2min by water, then carrying out air cooling to 270 ℃ for secondary heat preservation for 6 h; then keeping the temperature at 700 ℃ for 5 h.

< thermal refining >

Heating the steel ingot after heat preservation to 1050 ℃, preserving heat for 1h, and then cooling to 100 ℃; and tempering and preserving heat for 6 hours at the temperature of 640 ℃ to obtain the hot work die steel.

Example 5

< melting >

Preparing the following raw materials in percentage by mass:

c: 0.32 wt%, Si: 0.30 wt%, Mn: 0.15 wt%, Cr: 2.75 wt%, Mo: 2.30 wt%, W: 0.65 wt%, Ni: 0.63 wt%, V: 0.70 wt%, Nb: 0.04 wt%, Y: 0.01 wt% and the balance iron, and the electrode rod is made by the raw materials through arc melting, refining, vacuum degassing and forging in a forging furnace.

< electroslag remelting >

Removing oxide skin of the electrode bar, then placing the electrode bar into a vacuum electroslag remelting device, keeping the water temperature of a water cooling system of the electroslag remelting device at 66 ℃, and carrying out electroslag remelting on the electrode bar to obtain an electroslag steel ingot.

< homogenizing annealing >

< forging >

And (3) cooling the electroslag steel ingot to a forging heating temperature 1180 ℃ for forging, wherein the initial forging temperature is 1140 ℃, and the final forging temperature is 870 ℃, so that the steel ingot is obtained, the radius of the steel ingot is 40mm, and the length of the steel ingot is 100 mm.

< annealing after forging >

And (3) putting the steel ingot into an annealing furnace when the temperature is lower than 500 ℃, heating to 850 ℃ at a heating rate of 95 ℃/h, then preserving the heat for 200min, then cooling to 485 ℃ along with the furnace at a rate of 35 ℃/h, taking out of the annealing furnace, and air-cooling to obtain the annealed steel ingot.

< Heat treatment for Fine Crystal >

Heating the annealed steel ingot to 1140 ℃, carrying out primary heat preservation for 2h, cooling the steel ingot to 430 ℃ within 1min by water, and then carrying out secondary heat preservation for 6h by air cooling to 270 ℃; then the temperature is kept for 5h at 680 ℃.

< thermal refining >

Heating the steel ingot after heat preservation to 1050 ℃, preserving heat for 1h, and then cooling to 70 ℃; the temperature is kept at 580 ℃ for 4h for tempering, and then the temperature is kept at 640 ℃ for 2h for tempering, so that the hot die steel is obtained.

Example 6

Except that W in the raw materials is 1.00 wt%, Ni is 1.22 wt%, V is 0.60 wt%, Nb: 0.02 wt%, and contains Zr: 0.01 wt%, Co: 0.20 wt%, B: 0.001 wt%, Re: 0.05 wt%, Ti: 0.04 wt%, Y: the same as in example 5 was repeated except that the amount of the catalyst was changed to 0.02 wt%.

Example 7

Except that Cr in the raw materials is 1.50 wt%, W is 1.00 wt%, Ni is 1.22 wt%, V is 0.60 wt%, Nb: 0.02 wt%, and contains Zr: 0.03 wt%, Co: 0.40 wt%, B: 0.005 wt%, Re: 0.10 wt%, Ti: 0.06 wt%, Y: the same procedure as in example 5 was repeated except that the amount of the catalyst was changed to 0.10% by weight.

Comparative example 1

The comparative example is H13 hot work die steel, which has the specifications: the radius is 40mm, the length is 100mm, and the heat treatment process comprises the following steps:

quenching: heating the forged and formed steel ingot to 1050 ℃, preserving heat for 1h, and cooling by water;

tempering: and heating the quenched steel ingot to 590 ℃, preserving heat for 2h, then heating to 620 ℃, and preserving heat for 2 h.

Comparative example 2

The comparative example is 3Cr2W8V hot work die steel, and the specification is as follows: the radius is 40mm, the length is 100mm, and the heat treatment process comprises the following steps:

quenching: heating the forged and formed steel ingot to 1130 ℃, preserving heat for 1h, and cooling by water;

tempering: and heating the quenched steel ingot to 610 ℃, preserving heat for 2h, then heating to 630 ℃, and preserving heat for 2 h.

< Performance test >

And (3) testing high-temperature strength:

the hot work die steel of examples 1 to 7 and comparative examples 1 and 2 was tested for high temperature tensile strength at 700 ℃ by using GB/T4338-2006 "Metal Material high temperature tensile test method", and the test results are shown in Table 2.

And (3) testing thermal stability:

the hot-work die steels of examples 1 and 5 and comparative examples 1 and 2 were tested for room temperature Rockwell Hardness (HRC) after being kept at different temperatures for 4 hours, and the test results are shown in Table 3.

And (3) testing room temperature performance:

the hot-work die steels of examples 1 and 5 and comparative examples 1 and 2 were tested for room-temperature tensile properties and impact toughness (U-notch) and the results include elongation (A), reduction of area (Z) and room-temperature impact toughness (A)_ku) As shown in table 4.

And (3) testing fracture toughness:

GB/T4161-2007 metallic material plane strain fracture toughness K is adopted_ICExperimental methods the compact tensile test specimens of examples 1 and 5 and comparative examples 1 and 2 were selected and carried out on a fatigue test platform (model MTS810), and the test results are shown in table 5.

Testing the high-temperature strain fatigue life:

the fatigue life test is carried out on an MTS NEW810 electronic hydraulic servo fatigue testing machine by adopting GB/T15248-2002 'method for testing axial equal-amplitude low-cycle fatigue of metal materials', selecting the embodiment 5 and the comparative example 1, and the results are shown in Table 6.

TABLE 1 composition of hot-work die steel for each example and comparative example of the present invention

TABLE 2 high-temperature Strength test results of Hot-work die steels of examples and comparative examples

Examples	R_m(MPa)	R_p0.2(MPa)
			Example 1	560	345
Example 2	621	405
			Example 3	634	410
Example 4	642	420
			Example 5	678	450
Example 6	687	466
			Example 7	694	483
Comparative example 1	292	255
			Comparative example 2	415	364

TABLE 3 thermal stability test results (unit HRC) for examples 1 and 5 and comparative examples 1 and 2

Steel grade	600℃	620℃	660℃	700℃
					Example 1	45	43.5	39	32
Example 5	47	45.1	41.3	37.2
					Comparative example 1	47	40.2	31	24
Comparative example 2	48	46	38.2	29.8

Table 4 results of room temperature performance test of examples 1 and 5 and comparative examples 1 and 2

Steel grade	R_m(Mpa)	R_p0.2(Mpa)	A(％)	Z(％)	A_ku(J)
						Example 1	1310	1020	16	62	63
Example 5	1350	1050	14	48.3	52
						Comparative example 1	1389	1189	11.2	43.7	21.0
Comparative example 2	1647	1449	10	30.8	13

TABLE 5 fracture toughness test results for examples 1 and 5 and comparative examples 1 and 2

Table 6 results of high temperature strain fatigue life test of example 5 and comparative example 1

As can be seen from table 2, the high temperature strength at 700 ℃ of examples 1 to 5 are higher than that of H13 steel and 3Cr2W8V steel of comparative example 1 and comparative example 2, specifically, compared to comparative example 1, example 1 is improved by nearly 2 times, and examples 2 to 5 are improved by about 2 times or more; compared with the comparative example 2, the improvement of the example 1 and the example 2 is nearly 1.5 times, and the improvement of the examples 3-5 is about 1.5 times or more, which shows that the hot-work die steel has excellent high-temperature strength.

As can be seen from Table 3, the decrease of the room temperature hardness of the steel of examples 1 and 5 after being kept at the temperature of 600-700 ℃ for 4 hours is smaller than that of the H13 steel of comparative example 1 and that of the 3Cr2W8V steel of comparative example 2, which shows that the hot work die steel of the invention has high thermal stability.

As is clear from Table 4, the elongation (A), the reduction of area (Z) and the room temperature impact toughness (A) of examples 1 and 5_ku) Both higher than the H13 steel of comparative example 1 and the 3Cr2W8V steel of comparative example 2, indicate that the hot work die steel of the present invention has good room temperature ductility and toughness.

As can be seen from Table 5, examples 1 and 5 had fracture toughness K at 41HRC and 46HRC_ICIs 107.8 to 144.2 MPa.m^0.5The temperature is increased to about 1.5 times of that of the H13 steel of the comparative example 1 and more than about 3 times of that of the 3Cr2W8V steel of the comparative example 2, which shows that the hot work die steel of the invention has good room temperature fatigue resistance.

As can be seen from Table 6, the fatigue life of each diameter sample of example 5 is higher than that of the H13 steel of the same diameter sample of comparative example 1 under the condition of 0.2-0.6% strain amplitude, which shows that the hot-work die steel of the invention has better high-temperature and low-cycle fatigue resistance than the H13 steel.

FIG. 2 is a schematic view showing the change of tensile strength with temperature of hot-work die steel according to example 5 of the present invention, in FIG. 2, the tensile strength of H13 steel rapidly decays after the temperature exceeds 600 ℃, and the tensile strength is only 292MPa at 700 ℃; although the 3Cr2W8V steel has high carbon content and alloy elements and high room temperature strength, the tensile strength is sharply reduced after the temperature exceeds 650 ℃, and the tensile strength at 700 ℃ is only 415 MPa; the tensile strength of the hot-work die steel is slowly reduced along with the temperature rise, the tensile strength at the temperature of more than 650 ℃ is higher than that of H13 steel and 3Cr2W8V steel, and even if the tensile strength reaches about 700MPa at 700 ℃, the tensile strength is improved to about 2 times of that of H13 steel and about 1.5 times of that of 3Cr2W8V steel.

FIG. 3a is an electron micrograph of a hot work die steel of example 5 of the present invention at room temperature (25 ℃ C.); FIG. 3b is an electron micrograph of the hot work die steel of example 5 of the present invention after being stretched at 700 ℃; fig. 3c is a partial enlarged view of fig. 3 b.

FIG. 4a is an electron micrograph of H13 steel of comparative example 1 at room temperature; FIG. 4b is an electron micrograph of the H13 steel of comparative example 1 after drawing at 700 ℃; fig. 4c is a partial enlarged view of fig. 4 b.

By comparing fig. 3a and 4a, the steel structure of the hot work die steel of the present invention and comparative example 1 is a tempered sorbite structure retaining lath characteristics at room temperature; by comparing fig. 3b with fig. 4b, and by comparing fig. 3c with fig. 4c, after drawing at 700 ℃, the hot-work die steel of the present invention still maintains the lath characteristics, in which high-density nano-sized MC type alloy carbides are distributed, while the H13 steel of comparative example 1 completely loses the lath characteristics, and the carbides are coarsened and spheroidized, which indicates that the nano-carbides in the hot-work die steel of the present invention have higher thermal stability and do not grow up at 700 ℃, and thus the hot-work die steel of the present invention has excellent thermal stability.

FIG. 5a is a microstructure of carbide of the hot-work die steel of example 5 of the present invention after being stretched at 700 ℃, specifically TEM bright field image, as shown in FIG. 5a, the carbide is nano-scale acicular MC type alloy carbide.

FIG. 5B is a selected area electron diffraction pattern of the hot work die steel of example 5 of the present invention after being stretched at 700 ℃ as shown in FIG. 5B, in which the (200) plane of the α matrix is parallel to the (200) plane of the MC carbide and the [001] direction of the α matrix is parallel to the [011] direction of the MC carbide, indicating that the MC carbide maintains a good B-N orientation relationship with the α matrix after being stretched at 700 ℃.

FIG. 5c is a high resolution image of MC type alloy carbide after drawing at 700 ℃ of the hot-work die steel of example 5 of the present invention, as shown in FIG. 5c, the carbide/matrix interface still remains highly coherent, indicating that the hot-work die steel of the present invention has good high temperature stability.

FIG. 6 is a graph showing the composition analysis of carbide of the hot work die steel of example 5 of the present invention, and the results of the atom probe analysis show that the dotted box shows that the composition analysis of carbide derived from this region, which is a multi-alloy carbide (V)_0.5～0.8Mo_0.5～0.6Cr_0.15～0.3W_0.06～0.14Nb_0.01～0.02C) The special carbide can keep a coherent relationship with a matrix at a higher temperature, thereby realizing high-temperature and high-strength at a low alloy degree.

In conclusion, without being limited to any theory, the inventor believes that the carbide and the matrix of the hot die steel can keep a high-temperature coherent relationship through the mutual matching of the components and the innovative heat treatment process, the adjustment and control of the mismatching degree of the carbide/matrix interface are realized, the stability of the coherent relationship between the carbide and the matrix can be kept to 700 ℃, and the high-temperature tensile strength of the hot die steel is improved.

The above description is only for the preferred embodiment of the present invention, and is not intended to limit the scope of the present invention. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention shall fall within the protection scope of the present invention.

Claims

1. The hot-work die steel comprises the following chemical components in percentage by mass:

the tensile strength of the hot work die steel at 700 ℃ is 560-700 MPa;

2. The hot work die steel according to claim 1, further comprising at least one of the following chemical components:

3. The hot work die steel according to claim 1 or 2, wherein the S content is less than 0.02 wt% and the P content is less than 0.02 wt%.

4. The hot work die steel according to claim 1 or 2, which has a tempered sorbite structure retaining lath characteristics after drawing at 700 ℃.

5. The hot work die steel according to claim 1 or 2, wherein the carbide in the hot work die steel is a nano-scale acicular MC type alloy carbide after the hot work die steel is drawn at 700 ℃.

6. The hot work die steel of claim 5, the nano-scale acicular MC-type alloy carbide being: v_0.5～ _0.8Mo_0.5～0.6Cr_0.15～0.3W_0.06～0.14Nb_0.01～0.02C。

7. The hot-work die steel according to claim 1 or 2, which has a tensile strength of 600 to 700MPa at 700 ℃.

8. A method of producing the hot work die steel according to any one of claims 1 to 7, comprising the steps of:

a smelting step: preparing the following raw materials in percentage by mass:

wherein r is the material radius and d is the material thickness.

9. The method of making a hot work die steel according to claim 8, the feedstock further comprising at least one of: zr: 0.01-0.03 wt%, Co: 0.10 to 0.50 wt%, B: 0.001 to 0.005 wt%, Re: 0.01-0.10 wt%, Ti: 0.02 to 0.06 wt%, and Y: 0.01 to 0.1 wt%.

10. The method for producing a hot work die steel according to claim 8, wherein the forging step specifically includes:

11. The method for producing hot work die steel according to claim 8, wherein the holding time of the post-forging annealing is 6 to 8 hours.