A method for producing a hot work die steel

Abstract

The present application belongs to the technical field of steel metallurgy, and relates to a preparation method of hot work die steel. Nano mixed powder with a core-shell structure in which the outer layer is carbon-coated is added to hot work die steel liquid, including alloy nano powder and rare earth oxide nano powder. The nano level rare earth oxide is directly added, so that the addition of rare earth and the dispersion distribution of rare earth oxide are realized. The nano mixed powder with the core-shell structure can improve the wettability of nano particles and steel liquid at high temperature, the yield of the nano mixed powder is improved, the size distribution of the formed inclusions is uniform, the size is small, and the carbide is effectively refined, so that the industrialized popularization and application are facilitated.

Description

Technical Field

[0001] This invention relates to the field of iron and steel metallurgy technology, specifically to a method for preparing hot work die steel. Background Technology

[0002] The size, morphology, and volume fraction of carbides in mold steel play a crucial role in its performance. Due to their high C, Cr, Mo, and V content, large primary carbides tend to precipitate during solidification. On the one hand, the precipitation of primary carbides reduces the solid solution content of Cr, Mo, and V in the matrix, leading to a decrease in the precipitation of secondary carbides during tempering and affecting the uniformity of the microstructure. Adding rare earth elements can improve the shape and distribution of inclusions, causing harmful elements such as sulfur and oxygen to aggregate and form inclusions that float to the surface of the molten steel or remain in the steel, thus purifying the steel matrix and grain boundaries. However, this also increases the number and size of inclusions, adversely affecting the steel. Currently, the main methods for adding rare earth elements are direct addition or the use of rare earth slag systems. The method of directly adding rare earth elements may lead to problems such as large burn-off, low yield, refractory material erosion, and nozzle blockage due to the reaction between rare earth elements and air, steel slag, and refractory materials. The method of using rare earth slag system results in severe burn-off of rare earth elements in steel, with very low recovery rate and inability to accurately control the content of rare earth elements in steel. This fails to give full play to the good effects of rare earth elements on steel performance, and also leads to increased production costs and serious waste of rare earth resources. Summary of the Invention

[0003] To address the problems existing in the prior art, the main objective of this invention is to propose a method for preparing hot work die steel.

[0004] To address the aforementioned technical problems, according to one aspect of the present invention, the present invention provides the following technical solution:

[0005] A method for preparing hot work die steel includes the following steps:

[0006] S1. Control the temperature of the hot work die steel molten steel to 1600~1605℃ during the refining process;

[0007] S2. Add 1-3 kg / t to the molten hot work die steel. 钢 The nano-mixed powder includes alloy nano-powder and rare earth oxide nano-powder. The nano-mixed powder has a core-shell structure with an outer layer of carbon.

[0008] S3, casting.

[0009] As a method for preparing hot work die steel according to the present invention, in step S2, the mass ratio of alloy nanopowder to rare earth oxide nanopowder is 5 to 10:1.

[0010] The application discloses a preparation method of a hot work die steel.

[0011] The application discloses a preparation method of a hot work die steel.

[0012] The application discloses a preparation method of a hot work die steel.

[0013] The application discloses a preparation method of a hot work die steel.

[0014] B1, a ball milling device is used to pre-disperse raw materials of the nano mixed powder;

[0015] B2, the pre-dispersed nano mixed powder is sieved and then pressed into a nano mixed powder pressing body.

[0016] The application discloses a preparation method of a hot work die steel.

[0017] The application discloses a preparation method of a hot work die steel.

[0018] The application discloses a preparation method of a hot work die steel.

[0019] The application discloses a preparation method of a hot work die steel.

[0020] The application provides a preparation method of hot work die steel, wherein nano mixed powder with core-shell structure of carbon-coated outer layer of alloy nano powder and rare earth oxide nano powder is added into hot work die steel liquid, and nano rare earth oxide is directly added, so that the addition of rare earth and the dispersion distribution of rare earth oxide are realized, the nano mixed powder with core-shell structure can improve the wettability of nano particles and the steel liquid at high temperature, the yield of the nano mixed powder is improved, the size distribution of the formed inclusions is uniform and small, and the carbide is effectively refined, so that the industrialized popularization and application are facilitated. BRIEF DESCRIPTION OF DRAWINGS

[0021] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings needed to be used in the embodiments or the prior art description. Obviously, the drawings in the following description only show some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative effort on the basis of these drawings.

[0022] Figure 1 Inclusion distribution diagram of H13 steel prepared in Example 1 of the present application;

[0023] Figure 2 Carbide distribution diagram of H13 steel prepared in Example 1 of the present application;

[0024] Figure 3 Inclusion distribution diagram of H13 steel prepared in Comparative Example 1 of the present application;

[0025] Figure 4 Carbide distribution diagram of H13 steel prepared in Comparative Example 1 of the present application.

[0026] The implementation, functional features and advantages of the present application will be further described with reference to the embodiments and the drawings. DETAILED DESCRIPTION

[0027] The technical solutions in the embodiments will be described clearly and completely below. Obviously, the described embodiments are only some of the embodiments of the present application, but not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative effort belong to the protection scope of the present application.

[0028] The main objective of this invention is to propose a method for preparing hot work die steel. Traditionally, inclusions smaller than 1 μm are considered to have little effect on the surface defects and strength of steel, and inclusions of approximately 1 μm in size can induce intragranular ferrite (IGF), which can refine the microstructure and significantly improve strength and toughness. However, no one has yet added rare earth oxides (rare earth oxides, including rare earth oxysulfides, are used in hot work die steels such as H13 steel). Therefore, this invention directly adds nano-sized rare earth oxides to molten steel to modify inclusions and improve the properties of hot work die steel.

[0029] According to one aspect of the present invention, the present invention provides the following technical solution:

[0030] A method for preparing hot work die steel includes the following steps:

[0031] S1. Control the temperature of the hot work die steel molten steel to 1600~1605℃ during the refining process;

[0032] S2. Add 1-3 kg / t to the molten hot work die steel. 钢 The nano-mixed powder includes alloy nano-powder and rare earth oxide nano-powder. The nano-mixed powder has a core-shell structure with an outer layer of carbon.

[0033] S3, casting.

[0034] Preferably, in step S2, the mass ratio of alloy nanopowder to rare earth oxide nanopowder is 5–10:1. Specifically, the mass ratio of alloy nanopowder to rare earth oxide nanopowder can be, for example, but not limited to, any one or any two of 5:1, 5.5:1, 6:1, 6.5:1, 7:1, 7.5:1, 8:1, 8.5:1, 9:1, 9.5:1, and 10:1; the amount of nano-mixed powder added to the molten hot work die steel can be, for example, but not limited to, 1 kg / t. 钢 1.5kg / t 钢 2kg / t 钢 2.5kg / t 钢 3kg / t 钢 The range between any one or any two of them;

[0035] Preferably, in the step S2, the alloy nano-powder includes any one or more of AlSi alloy powder, SiCa alloy powder, SiBa alloy powder; the rare earth oxide nano-powder includes any one or more of CeO2 powder, Ce2O3 powder, La2O3 powder, Nd2O3 powder, Pr2O3 powder, Ce2O2S powder, La2O2S powder. The AlSi alloy powder, SiCa alloy powder, SiBa alloy powder and the rare earth oxide nano-powder are mixed, the AlSi, SiBa and SiCa not only play a role in pre-dispersing the rare earth oxide powder, but also can reduce the rare earth oxide in the molten steel.

[0036] Preferably, in the step S2, the nano-mixed powder further includes metal nano-powder, and the mass ratio of the metal nano-powder to the total mass of the alloy nano-powder and the rare earth oxide nano-powder is 10-20:1. Further preferably, in the step S2, the metal nano-powder includes Mo powder, Fe powder, Mn powder. After the alloy nano-powder and the rare earth oxide nano-powder are mixed with the metal nano-powder such as Mo powder, Fe powder or Mn powder, the problem of serious molten steel slagging and poor yield caused by the nano-powder floating up due to being too light is effectively prevented. Specifically, the mass ratio of the metal nano-powder to the total mass of the alloy nano-powder and the rare earth oxide nano-powder can be, for example but not limited to, any one or a range between any two of 10:1, 11:1, 12:1, 13:1, 14:1, 15:1, 16:1, 17:1, 18:1, 19:1, 20:1.

[0037] Preferably, the average particle size of the alloy nano-powder, the rare earth oxide nano-powder and the metal nano-powder is 50 nm-1 μm.

[0038] Preferably, in the step S2, the nano-mixed powder is added in the form of a nano-mixed powder compact, and the nano-mixed powder compact is prepared by the following process:

[0039] B1, using a cooling type ball milling device to pre-disperse raw materials of the nano-mixed powder;

[0040] B2, after the pre-dispersed nano-mixed powder is passed through a 200-mesh sieve, an oil press is used to press the nano-mixed powder into a nano-mixed powder compact.

[0041] Preferably, in the step B1, the ball milling time is 4-24 h, and inert gas or vacuum protection is used throughout the ball milling process to prevent reactions between the powders.

[0042] Preferably, in the step S2, a polydopamine method or a glucose hydrothermal method is used to prepare a core-shell structure with a carbon coating (and a core-shell structure of nano-mixed powder@C). For example, the following process can be used:

[0043] The core-shell structure of the nano mixed powder @C is obtained by adding the nano mixed powder and the polydopamine into an aqueous solution, ultrasonic stirring, magnetic stirring, then adding the tris-hydroxymethyl aminomethane, and then performing filtration, vacuum drying, and calcination; more specifically, 1 g of the nano mixed powder and 0.5 g of the polydopamine are added into 500 ml of the aqueous solution, ultrasonic stirring is performed for 30 min, magnetic stirring is performed for 8 h, 0.346 g of the tris-hydroxymethyl aminomethane is added, filtration is performed, vacuum drying is performed at 80℃ for 24 h, and calcination is performed at 550℃ for 3 h, so as to obtain the core-shell structure of the nano mixed powder @C.

[0044] Alternatively, the nano powder is added into the aqueous glucose solution for hydrothermal treatment, the product is washed with distilled water and centrifuged for several times, and then the core-shell structure of the nano mixed powder @C is obtained after drying; more specifically, 1 g of the nano powder is added into 20 ml of the 0.4M aqueous glucose solution, hydrothermal treatment is performed at 180℃ for 2 h, the product is washed with distilled water and centrifuged for several times, and then the core-shell structure of the nano mixed powder @C is obtained after drying at 80℃ for 24 h.

[0045] Preferably, in the step S2, the process of adding the nano mixed powder into the hot work die steel liquid is as follows: the nano mixed powder is wrapped with iron foil or aluminum foil, and is tied with iron wire on a molybdenum rod, and is inserted into the steel liquid to realize the addition of the nano mixed powder, and the insertion depth is 1 / 3-2 / 3 of the total depth of the steel liquid. The nano mixed powder pressed block wrapped with the iron foil or the aluminum foil can prevent the rapid oxidation of the nano mixed powder when the nano mixed powder is added into the steel liquid, can avoid the influence on the slagging of the steel liquid and the metal yield, and can further improve the technical effects of the present application.

[0046] Preferably, in the step S2, after the nano mixed powder is added into the steel liquid, the ultrasonic stirring device, the mechanical stirring device, or the electromagnetic stirring device can be used for stirring to promote the reaction.

[0047] The technical scheme of the present application is further described below in combination with specific embodiments.

[0048] The embodiments and the comparative examples of the present application are all directed to the H13 hot work die steel in the prior art.

[0049] Embodiment 1

[0050] The present embodiment provides a preparation method of a hot work die steel, which comprises the following steps:

[0051] S1, controlling the temperature of the hot work die steel liquid to be 1600℃ in a refining process;

[0052] S2, adding 3 kg / t of the nano mixed powder into the hot work die steel liquid; 钢

[0053] ​The nano mixed powder comprises AlSi nano powder, CeO2 nano powder and Fe powder, the mass ratio of the AlSi nano powder and the CeO2 nano powder is 10:1, and the mass ratio of the Fe powder to the total mass of the AlSi nano powder and the CeO2 nano powder is 20:1; the process of adding the nano mixed powder into the hot work die steel liquid is that the nano mixed powder is wrapped with iron foil and tied with iron wire on a molybdenum rod, and then inserted into the steel liquid to add the nano mixed powder, and the insertion depth is 1 / 2 of the total depth of the steel liquid;

[0054] The nano mixed powder compact is prepared by the following process:

[0055] B1, the raw material of the nano mixed powder is pre-dispersed by using a cooling type ball milling device, and the ball milling time is 24h; the nano mixed powder with a core-shell structure of which the outer layer is carbon-coated is prepared by using a polydopamine method;

[0056] B2, the pre-dispersed nano mixed powder with a core-shell structure of which the outer layer is carbon-coated is screened through a 200-mesh sieve, and then pressed into a nano mixed powder compact by using an oil press.

[0057] S3, casting, the inclusion distribution of the prepared H13 steel is as shown in Figure 1 , the carbide distribution is as shown in Figure 2 , and it can be known from Figures 1-2 that the inclusions are all modified into Ce-O-S, Ce-O-As and Ce-Al-O, the size is concentrated in the range of 0.5-1.5μm, and the size distribution is relatively uniform; the misfit degree of the rare earth inclusion with δ-Fe and γ-Fe is less than 12%, which can effectively promote the nucleation of δ-Fe and γ-Fe, and refine the structure; from the carbide distribution, the carbide is Mo-rich type and V-rich type, the carbide size is concentrated in the range of 1-3μm (as shown in (a) of Figure 2 ), the area is 0.5-1.5μm 2 (as shown in (b) of Figure 2 ), and the number of V-rich carbide is more than that of Mo-rich carbide.

[0058] Example 2

[0059] The embodiment provides a preparation method of a hot work die steel, comprising the following steps:

[0060] S1, controlling the temperature of the hot work die steel liquid to be 1605℃ in a refining process;

[0061] S2, adding 3kg / t 钢 of nano mixed powder into the hot work die steel liquid, and stirring by using an ultrasonic stirring device;

[0062] The nano mixed powder comprises SiCa nano powder, Ce2O2S nano powder and Mo powder, the mass ratio of the SiCa nano powder and the Ce2O2S nano powder is 5:1, and the mass ratio of the Mo powder to the total mass of the SiCa nano powder and the Ce2O2S nano powder is 10:1; the process of adding the nano mixed powder into the hot work die steel liquid is that: the nano mixed powder compression block is wrapped with aluminum foil, is tied on a molybdenum rod with an iron wire, and is inserted into the steel liquid to realize the addition of the nano mixed powder, and the insertion depth is 2 / 3 of the total depth of the steel liquid;

[0063] The nano mixed powder compression body is prepared by the following process:

[0064] B1, the raw material of the nano mixed powder is pre-dispersed by using a cooling type ball milling device, and the ball milling time is 12 hours; and the nano mixed powder with a core-shell structure with a carbon-coated outer layer is prepared by using a glucose hydrothermal method;

[0065] B2, the pre-dispersed nano mixed powder with a core-shell structure with a carbon-coated outer layer is pressed into a nano mixed powder compression body by using an oil press after being sieved through a 200-mesh sieve.

[0066] S3, casting, to obtain H13 steel.

[0067] Example 3

[0068] The embodiment provides a preparation method of hot work die steel, comprising the following steps:

[0069] S1, controlling the temperature of the hot work die steel liquid to be 1600 DEG C in the refining process;

[0070] S2, adding 1 kg / t of nano mixed powder into the hot work die steel liquid; 钢

[0071] The nano mixed powder comprises SiCa nano powder, SiBa nano powder, CeO2 nano powder and Mn powder, the total mass of the SiCa nano powder and the SiBa nano powder and the mass of the CeO2 nano powder have a mass ratio of 8:1, and the mass ratio of the Mn powder to the total mass of the SiCa nano powder, the SiBa nano powder and the CeO2 nano powder is 16:1; the process of adding the nano mixed powder into the hot work die steel liquid is that: the nano mixed powder compression block is wrapped with iron foil, is tied on a molybdenum rod with an iron wire, and is inserted into the steel liquid to realize the addition of the nano mixed powder, and the insertion depth is 1 / 3 of the total depth of the steel liquid;

[0072] The nano mixed powder compression body is prepared by the following process:

[0073] B1, the raw material of the nano mixed powder is pre-dispersed by using a cooling type ball milling device, and the ball milling time is 4 hours; and the nano mixed powder with a core-shell structure with a carbon-coated outer layer is prepared by using a glucose hydrothermal method;

[0074] ​B2, the pre-dispersed outer layer is carbon-coated core-shell structure of the nano mixed powder, after 200 mesh sieve, using oil press, pressed into nano mixed powder pressing body.

[0075] S3, casting, obtaining H13 steel.

[0076] Example 4

[0077] The embodiment provides a preparation method of hot work die steel, comprising the following steps:

[0078] S1, controlling the temperature of the hot work die steel liquid to be 1600 DEG C in the refining process;

[0079] S2, adding 2kg / t of nano mixed powder to the hot work die steel liquid; 钢

[0080] The nano mixed powder comprises SiCa nano powder, SiBa nano powder, CeO2 nano powder, La2O3 nano powder and Mn powder, the mass ratio of the total mass of SiCa nano powder and SiBa nano powder to the total mass of CeO2 nano powder and La2O3 nano powder is 6:1, the mass ratio of Mn powder to the total mass of SiCa nano powder, SiBa nano powder, CeO2 nano powder and La2O3 nano powder is 12:1, the process of adding nano mixed powder to the hot work die steel liquid is that the nano mixed powder pressing block is wrapped with iron foil, tied with iron wire on a molybdenum rod, and inserted into the steel liquid to realize the addition of nano mixed powder, and the insertion depth is 1 / 2 of the total depth of the steel liquid;

[0081] The nano mixed powder pressing body is prepared by the following process:

[0082] B1, pre-dispersing the raw material of the nano mixed powder by using a cooling type ball milling device, and the ball milling time is 12h; the nano mixed powder with outer layer of carbon-coated core-shell structure is prepared by using a glucose hydrothermal method;

[0083] B2, the pre-dispersed outer layer is carbon-coated core-shell structure of the nano mixed powder, after 200 mesh sieve, using oil press, pressed into nano mixed powder pressing body.

[0084] S3, casting, obtaining H13 steel.

[0085] Comparative example 1

[0086] The difference from example 1 is that,

[0087] In step S2, the nano mixed powder comprises AlSi nano powder and Fe powder;

[0088] The inclusion distribution of the prepared H13 steel is shown in Figure 3 , and the carbide distribution is shown in Figure 4 . From Figures 3-4 ​It can be seen that the inclusions in the steel are mainly Al2O3 and MnS, and the misfit degree of Al2O3 and MnS with carbide is higher than that of δ-Fe and γ-Fe, which promotes the nucleation of carbide. It can be seen from the distribution of carbide that the size of V-rich carbide and Mo-rich carbide is large, and the size is mostly between 1-5 μm, and the distribution range is wide (such as Figure 4 The area distribution of carbide is uneven, and the number of large-area carbide is much more than that of Example 1 (such as Figure 4 The number of carbide is about 1.5 times that of Example 1.

[0089] Comparative Example 2

[0090] The difference from Example 1 is that the nano mixed powder does not adopt the core-shell structure with carbon wrapping the outer layer.

[0091] The nano mixed powder compact is prepared by the following process:

[0092] B1, the raw materials of the nano mixed powder are pre-dispersed by a cooling type ball milling device, and the ball milling time is 24 h;

[0093] B2, the pre-dispersed nano mixed powder is pressed into a nano mixed powder compact by an oil press after passing through a 200 mesh sieve.

[0094] The nano mixed powder of this comparative example does not adopt the core-shell structure with carbon wrapping the outer layer, and its wettability with the molten steel is poor and easy to agglomerate, so that the nano powder is easy to float to the surface of the molten steel, resulting in a decrease in the yield of rare earth oxide nanoparticles. In addition, the nano mixed powder without carbon wrapping the outer layer is reduced to rare earth with a low probability, thereby reducing the effect of rare earth modification.

[0095] Comparative Example 3

[0096] The difference from Example 1 is that,

[0097] In step S2, the nano mixed powder includes AlSi nano powder and CeO2 nano powder; no metal nano powder is added to achieve counterweight;

[0098] The yield of nanoparticles of this comparative example is low, no metal nano powder is added to achieve counterweight, and the nano mixed powder floats to the surface of the molten steel in the initial stage, which cannot achieve effective addition.

[0099] Comparative Example 4

[0100] The difference from Example 1 is that,

[0101] In step S2, the mass ratio of AlSi nano powder and CeO2 nano powder is 20:1;

[0102] The prepared H13 steel has good dispersibility of CeO2 nano powder, but the AlSi nano powder increases, which increases the content of Al and Si elements in the steel liquid to a certain extent.

[0103] Comparative Example 5

[0104] Different from Example 1,

[0105] In step S2, the mass ratio of Fe powder to the total mass of AlSi nano powder and CeO2 nano powder is 5:1.

[0106] The yield of CeO2 nano powder of the prepared H13 steel is reduced, the ratio of Fe powder is reduced, the total mass of nano powder is reduced, the probability of mixed powder floating to the surface is increased, and the yield of CeO2 nano powder is reduced.

[0107] Comparative Example 6

[0108] Different from Example 1,

[0109] In step S2, the process of adding nano mixed powder to the hot die steel liquid is: directly adding nano mixed powder compacts;

[0110] This comparative example does not use iron foil to wrap the directly added nano mixed powder block. During the adding process, the nano mixed powder has different melting points, which causes the nano mixed powder block to be easy to scatter before being added to the steel liquid, resulting in part of the nano powder floating or being blown away. At the same time, the rare earth oxide nanoparticles are easy to spontaneously aggregate and cannot be uniformly distributed in the steel liquid.

[0111] The present application adds nano mixed powder including alloy nano powder and rare earth oxide nano powder to the hot die steel liquid, and the outer layer of the core-shell structure of the nano mixed powder is carbon-coated. Directly adding nano-sized rare earth oxides realizes the addition of rare earth and the dispersion of rare earth oxides. The core-shell structure of the nano mixed powder can improve the wettability of the nano particles and the steel liquid at high temperature, the yield of the nano mixed powder is improved, the size distribution of the inclusions formed is uniform, the size is small, and the carbides are effectively refined, which is convenient for industrialization and application.

[0112] The above only describes the preferred embodiments of the present application, and does not limit the patent scope of the present application. Any equivalent structural transformation made by using the content of the present application, or direct / indirect application in other related technical fields within the inventive concept of the present application is included in the patent protection scope of the present application.

Claims

1. A method of producing a hot work die steel, characterized by, It comprises the following steps: S1, controlling the temperature of the hot work die steel liquid to be 1600-1605℃ in the refining process; S2, adding 1-3 kg / t of nano mixed powder into the hot working die steel liquid 钢 The nano mixed powder has a core-shell structure with a carbon coating on the outside, and the core is composed of alloy nano powder, metal nano powder and rare earth oxide nano powder; the metal nano powder is Mo powder, Fe powder or Mn powder; the mass ratio of the mass of the metal nano powder to the total mass of the alloy nano powder and the rare earth oxide nano powder is 10-20:1; the mass ratio of the alloy nano powder to the rare earth oxide nano powder is 5-10:

1. S3, casting; In the step S2, the process of adding the nano mixed powder into the hot work die steel liquid is as follows: the nano mixed powder pressing block is wrapped with iron foil or aluminum foil, and is tied with iron wire on a molybdenum rod, and is inserted into the steel liquid to realize the addition of the nano mixed powder.

2. The method of producing a hot work die steel according to claim 1, characterized by, In the step S2, the alloy nano powder comprises any one or more of AlSi nano powder, SiCa nano powder and SiBa nano powder; and the rare earth oxide nano powder comprises any one or more of CeO2 nano powder, Ce2O3 nano powder, La2O3 nano powder, Nd2O3 nano powder, Pr2O3 nano powder, Ce2O2S nano powder and La2O2S nano powder.

3. The method of claim 1, wherein the hot work die steel is prepared by the steps of: In the step S2, the nano mixed powder is added in the form of a nano mixed powder pressing body, and the nano mixed powder pressing body is prepared by the following process: B1, using a ball milling device to pre-disperse raw materials of the nano mixed powder; B2, after pre-dispersing the nano mixed powder, sieving and pressing into a nano mixed powder pressing body.

4. The method of producing a hot work die steel according to claim 3, characterized by, In the step B1, the ball milling time is 4-24h, and inert gas or vacuum protection is used in the whole ball milling process to prevent the reaction between the powders.

5. The method of claim 1, wherein the hot work die steel is prepared by the steps of: In the step S2, the polydopamine method or the glucose hydrothermal method is used to prepare a core-shell structure with a carbon-coated outer layer.

6. The method of claim 1, wherein the hot work die steel is prepared by the steps of: The insertion depth is 1 / 3-2 / 3 of the total depth of the steel liquid.

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