KR101312822B1 - Die steel and manufacturing method using the same - Google Patents

Abstract

The present invention imparts rust resistance to the mold steel, and simultaneously secures hardness of HRC 38 or more and V notch Charpy impact energy of ¹¹ J / cm ² or more, thereby greatly increasing the service life and productivity of the mold. It is related with the mold steel which can be improved, and its manufacturing method. Mold steel according to the present invention by weight%, C: 0.03-0.09%, Si: 0.2-0.9%, Mn: 1.3-1.7%, Cr: 4.5-6.0%, Mo: 0.5-2.5, W: 0.5-2.0 %, Ni: 2.5 ~ 3.5%, Cu: 0.7 ~ 1.5%, Al: 0.7 ~ 1.5%, Nb: 0.01 ~ 0.1%, S: over 0 ~ 0.006%, balance Fe and other unavoidable impurities It consists of a mixed structure of knight and martensite, and satisfies the following formula.
7.5≤ [Cr] +2 {[[Mo] +0.5 [W]}
By such a configuration, the service life of the mold can be greatly extended, and the toughness can be reduced, so that the cracks at the corners which can be generated during handling of the mold can be reduced.

Description

Die steel and manufacturing method using the same

The present invention relates to a mold steel and a manufacturing method thereof, and more particularly to a mold steel excellent in rust resistance and a manufacturing method thereof.

In general, mold steel used for molding plastics is roughly divided into steel grades which are heat treated after being processed into molds and steel grades which are processed into molds after heat treatment of the mold steels. In the past, a number of carbon steels or low alloy steels corresponding to the former have been used, but due to problems of short lifespan due to poor physical properties such as hardness and toughness, precipitation hardening mold steels corresponding to the latter are mainly used.

As such, the precipitation hardening mold steel mainly used in the mold for plastic molding has to be excellent in hardness and toughness as well as machinability in order to shorten the manufacturing time and improve the service life of the mold. Because of its properties, it is not easy to ensure excellent hardness and toughness at the same time.

In order to solve this problem, many studies have been conducted as disclosed in Japanese Patent Laid-Open Publication Nos. 193-255732, 1995-278743 and 1997-021351, and Korean Patent Publication No. 2000-0057043.

Thus, even if the hardness and toughness is sufficient, the disadvantages caused by the use environment of the plastic mold steel, that is, rust generated on the surface of the mold due to the humid environment by a large amount of water vapor by the cooling water used to cool the mold, In addition, a decrease in the cooling capacity of the mold is often caused by the occurrence of thick rust in the cooling holes through which the cooling water flows. Product defects are caused by deterioration of cooling ability due to rust generation, and especially the surface aesthetics of the molten mold generated on the surface. In addition, the surface rust of the plastic material caused by the rust generated, product defects and the like. In particular, the necessity of the redundant work by manual work for removing the rust generated on the surface is a major obstacle to the increase in productivity, causing a rise in manufacturing costs.

The present invention controls the process conditions such as adjustment of the alloying components, solution treatment, and aging treatment to impart rust resistance to the mold steel, and also have hardness of HRC 38 or higher and V notch Charpy impact of ¹¹ J / cm ² or higher. It is an object of the present invention to provide a mold steel and a method for manufacturing the same, which are capable of greatly improving the service life and productivity of the mold by simultaneously securing the value (CVN impact energy).

Mold steel according to the present invention by weight%, C: 0.03 ~ 0.09%, Si: 0.2 ~ 0.9%, Mn: 1.2 ~ 1.7%, Cr: 4.5 ~ 6.0%, Ni: 2.5 ~ 3.5%, Cu: 0.7 ~ 1.5%, Al: 0.5 ~ 1.5%, Nb: 0.01 ~ 0.1%, S: over 0 ~ 0.006%, and Mo: 0.5 ~ 2.5% and W: 0.5 ~ 2.0% of at least one, balance Fe and other unavoidable It contains impurities and consists of a mixed structure of bainite and martensite, and satisfies the following equation.

7.5≤ [Cr] +3.3 {[[Mo] +0.5 [W]}

In addition, the mold steel may satisfy a [Mo] + [W] value of 1.0 to 4.5.

In addition, the mold steel may further include B: 0.001% to 0.01% by weight.

Moreover, the mold steel may further include Sn: 0.001 to 0.06% by weight.

In addition, the mold steel may further include Co: greater than 0 to 0.01% by weight.

In addition, the mold steel may further include at least one selected from a weight% of Ca: more than 0 to 0.01% or less and Mg: more than 0 to 0.005% or less.

Manufacturing method of the mold steel according to the present invention by weight%, C: 0.03 ~ 0.09%, Si: 0.2 ~ 0.9%, Mn: 1.2 ~ 1.7%, Cr: 4.5 ~ 6.0%, Ni: 2.5 ~ 3.5%, Cu: 0.7 to 1.5%, Al: 0.5 to 1.5%, Nb: 0.01 to 0.1%, S: over 0 to 0.006% and Mo: 0.5 to 2.5% and W: 0.5 to 2.0% of at least one, balance Fe and Re-dissolving a slab containing other unavoidable impurities, consisting of a mixed structure of bainite and martensite, and satisfying the following formula through electric slag dissolution (ESR); Hot working the slab; Slow cooling the slabs and heating them through a solution treatment; And air-cooling after the solution treatment.

7.5≤ [Cr] +3.3 {[[Mo] +0.5 [W]}

At this time, in the heating step through the solution treatment, it can be heated to the austenite region, and heated and maintained to a temperature higher than the austenite transformation complete point (Ac3) when reheating.

According to the present invention, by controlling the process conditions, such as the adjustment of the alloying components, solution treatment, aging treatment and the like to secure the hardness of HRC 38 or more and V notch Charpy impact value of 11J / cm ² or more at the same time, the service life of the mold Can be greatly extended, and the toughness can be reduced, so that the crack in the corner portion which can be generated during handling of the mold can be reduced.

In particular, the plastic surface can be formed at a high speed due to the improvement of the cleanliness of the mold surface and the cooling ability by improving the rust resistance by adjusting the alloy composition, and also the cleanliness due to electro slag remelting (ESR). And a homogeneous structure can be obtained together to obtain a mold steel having excellent mirror properties and also having various properties.

1 is a view showing a heat treatment pattern of the manufacturing method of the present invention.
2 is a graph showing changes in hardness and toughness of the inventive steels and the comparative steels according to [Cr] +3.3 {[[Mo] +0.5 [W]}.
Figure 3 is a graph showing the change in rust generation fraction during the tap water immersion test of the invention steel and the comparative steel according to [Cr] + 3.3 {[[Mo] + 0.5 [W]}.
Figure 4 is a photograph showing the rust generated during the tap water immersion test of the invention and comparative steel.
Figure 5 is a photograph showing the microstructure after aging heat treatment of the inventive steel and the comparative steel.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Reference will now be made in detail to embodiments of the present invention and other details necessary for those skilled in the art to understand the present invention with reference to the accompanying drawings. However, the present invention may be embodied in various different forms within the scope of the claims, and thus the embodiments described below are merely exemplary, regardless of expression.

In the following description of the present invention, a detailed description of known functions and configurations incorporated herein will be omitted when it may make the subject matter of the present invention rather unclear.

The present invention relates to a mold steel, which is a material used for manufacturing a mold, and a method for manufacturing the same, and more particularly, to a mold steel for producing a mold for molding a plastic product, and a method for manufacturing the same. The present invention relates to a precipitation hardening mold steel having excellent rust resistance, which gives rust resistance to prevent rust from being formed, secures sufficient hardness and toughness, and can greatly extend the life of the mold steel.

Hereinafter, the mold steel of this invention is demonstrated in detail.

The inventors of the present invention have excellent properties of the mold steel, particularly rust resistance in tap water used as cooling water, and at the same time to secure the hardness of HRC 38 or more and the V notch Charpy impact value of ¹¹ J / cm ² or more. The research and experiment were repeated to complete the present invention.

As a result, the present invention contains low carbon, contains a certain amount of Cr to secure rust resistance, and includes Mo and W excellent in rust resistance as a role of assisting the improvement of rust resistance of Cr. In order to secure the hardness, carbides are dispersed and precipitated with Ni-Al intermetallic compounds and a part of the added Cr and Mo, and the grain growth is suppressed and refined by Nb carbonitride precipitation. In addition, the precipitation hardening type steel having excellent rust resistance can be produced by optimizing process conditions such as solution treatment and aging treatment to produce a mixed structure of bainite and martensite.

Thereby, by greatly suppressing the occurrence of rust on the surface during use or storage of the mold, it is possible to greatly reduce the surface quality problems of the plastic injection molded product due to the surface rust, and to significantly reduce the mold management cost. Accordingly, the service life can be greatly improved by reducing the manufacturing cost and securing the hardness and impact toughness, and molding of plastic can be performed at high speed.

First, the reasons for limiting components of the present invention will be described.

Carbon (C) is an element that enhances hardenability, increases strength and hardness by forced cooling after heating, and contributes to aging hardening by precipitation. In the present invention, carbon (C) is used in the reaction for forming carbides with Cr, Mo, Nb, etc. Mainly consumed by When the C content exceeds 0.09, coarse carbides form on cooling to reduce toughness, or these coarse carbides increase tool wear during cutting, especially in combination with Cr, which is most important for securing rust resistance, Decreases the high capacity to reduce rust resistance. If the content of C is less than 0.03%, since hardness is low after forced cooling, even if precipitation hardening by aging is performed, there is a limitation that a hardness of HRC 38 or higher can not be obtained. Therefore, the content of C is limited to 0.03 to 0.09%.

Silicon (Si) is an element that is added as a deoxidizer during steelmaking, and is partially dissolved in steel to improve hardenability and hardness.It suppresses the formation of coarse carbides during aging treatment, thereby rapidly decreasing impact toughness due to precipitation of grain boundary carbides. prevent. When the mass of the material is large, Mn alone is difficult to control the hardness during the solution treatment, and therefore 0.2% or more should be added to the steel. However, when the content of Si exceeds 0.9%, segregation may occur or the graphitization of carbides may be greatly reduced during aging treatment for a long time, leading to a significant decrease in toughness. In particular, the content of Si may cause deterioration of machinability. It is limited to 0.2-0.9%.

Manganese (Mn), together with C, improves hardenability and improves hardness and wear resistance after aging treatment. It suppresses the formation of ferrite and promotes the formation of martensite. In order to achieve the target hardness and wear resistance in the present invention, at least 1.2% should be added. However, when the Mn content exceeds 1.7%, the machinability and hot workability are reduced due to excessive refining of bainite, so the Mn content is 1.2 to Limited to 1.7%.

Chromium (Cr) is an element in which the improvement of corrosion resistance, in particular, the improvement of rust resistance is indispensable, is an element that improves hardenability when added, and improves hardness and corrosion resistance. In general, it is effective to inhibit the formation of ferrite and pearlite during cooling, to promote the formation of martensite and bainite, which is a low temperature structure, and to improve core hardness. In particular, it is preferable to add at least 4.5% or more to secure the rust resistance of the steel. However, if the Cr content exceeds 6.0%, the ferrite phase is formed at high temperature, and the solution heat treatment is performed in the two-phase region of austenite and ferrite during the solution heat treatment, and the ferrite phase is formed at room temperature even when cooled to room temperature. The content of Cr is limited to 4.5 to 6.0% because it causes a decrease in the uniformity of hardness, which is one of the essential elements of steel, and in particular, causes a decrease in workability due to uneven hardness in mechanical processing.

Molybdenum (Mo) is an element that improves the corrosion resistance of steel, and is an element added to obtain the purpose of hardening by carbide precipitation during the hardness and aging treatment of the matrix by known solid solution and to obtain high toughness. It is preferable to add 0.5% or more with Cr and W to improve rust resistance, hardness due to solid solution strengthening, and toughness. However, when the content of Mo exceeds 2.5%, ferrite is formed at high temperature as Cr. As a result, the hardness nonuniformity and workability decrease of the steel, and excessive addition increases the manufacturing cost, so the content of Mo is limited to 0.5 to 2.5.

Tungsten (W), along with Cr, Mo, etc., is a typical corrosion resistance improving element, and in order to show the effect, it is preferable to add 0.5% or more.However, when the content of W exceeds 2.0%, it is combined with carbon to form tungsten carbide. Impairs workability. In addition, the ferrite phase is formed during the high temperature solution treatment, resulting in non-uniformity of the tissue at room temperature. Therefore, the content of W is limited to 0.5 to 2.0%.

Nickel (Ni) is an element that improves hardness, toughness, and photoetching, and forms a homogeneous solid solution with Cu in part to prevent red-brittle brittleness during hot working. Ni ₃ Al, etc.) are precipitated, and the ε-phase which acts as a nucleus when the Ni-Al intermetallic compound is precipitated together with Cu is preferably added at 2.5% or more. However, when the content of Ni exceeds 3.5%, the machinability is lowered, the thermal conductivity is lowered and the injection molding cycle is increased, so the content of Ni is limited to 2.5 to 3.5%.

Copper (Cu) is an element effective in improving corrosion resistance and machinability while causing precipitation hardening as an ε-Cu intermetallic compound during aging treatment. It is preferable to add more than 0.7% of Cu. However, if the content of Cu exceeds 1.5%, the hot workability is lowered, which causes grain cracking, and causes surface defects. Therefore, the Cu content is limited to 0.7 to 1.5%. .

Aluminum (Al) combines with Ni during aging to form Ni-Al intermetallic compounds to increase precipitation hardening to increase hardness, and to form Al nitrides to inhibit coarsening of austenite grains during solution treatment. It is preferable to add 0.5% or more since it promotes the formation of lower bainite to increase toughness. However, when the content of Al exceeds 1.5%, surface defects occur due to the formation of oxidation inclusions, and the balance between carbides of other elements is broken, which decreases toughness, machinability and specularity. It is limited to 0.5-1.5%.

Niobium (Nb) combines with carbon or nitrogen to form carbonitrides to improve hardness and wear resistance, as well as to refine the austenite grains by dispersing such Nb carbonitrides to improve toughness and form hard coarse AlN inclusions. It is preferable to add 0.01% or more in order to suppress and improve mirroring property. However, when the content of Nb exceeds 0.1%, machinability is degraded due to excessive Nb carbonitride precipitation during aging treatment, and segregation at the center forms a cluster of Nb nitrides, thereby reducing the specularity. It is limited to 0.01 to 0.1%.

Sulfur (S) is an element that improves machinability by generating MnS inclusions, but causes surface defects by dropping of MnS inclusions during polishing, or acts as a starting point of surface corrosion in the case of chlorine-based plastics. In addition, in the present invention, the toughness and the specularity are reduced due to the formation of coarse emulsion-based inclusions.

Boron (B) is an quenchability improving element, and it is preferable to add 0.001% or more, since it minimizes the microstructure of the steel upon cooling, improves the hardness, and improves the lack of machinability due to the low S content of the present invention. . However, when the content of B exceeds 0.01%, hot workability and repair weldability may be reduced due to the large precipitation of B nitride or intermetallic compound at the grain boundary during hot working, and the content of B may be delayed due to bainite transformation. It is limited to 0.001 to 0.01%.

Tin (Sn) is an essential element in order to secure the rust resistance to which the present invention aims, and requires 0.001% or more. However, when addition amount exceeds 0.06%, corrosion resistance will fall. Moreover, since surface oxidation becomes nonuniform at the time of quenching heating, and worsen abrasive property, it is made into 0.06% or less.

Cobalt (Co) is an element that improves toughness and corrosion resistance, and combines with carbon or nitrogen to form carbonitrides to improve hardness and abrasion resistance, as well as to refine toughness of austenite grains by dispersing Co carbonitride. Let's do it. However, excessively high cost is required, and since the residual austenite may be increased in the final structure to cause variation in hardness, the present invention is preferably managed at more than 0 to 0.01% or less.

Calcium (Ca) is an element that improves machinability by preventing stretching of MnS inclusions in the processing direction by hot working. MnS inclusions have high viscosity at high temperatures, so MnS inclusions Stretching in the processing direction causes deterioration of machinability and anisotropy of mechanical properties. When Ca is added, resistance to shape deformation of MnS inclusions is increased to maintain spherical shape, and MnS inclusions are uniform and fine. Cracking due to stress concentration during polishing is reduced to improve machinability and extend the life of the cutting tool. However, when the Ca content exceeds 0.01%, a large amount of CaS inclusions may be generated to reduce toughness, and the mirror property may decrease due to the formation of oxides. Thus, in the present invention, the shape of the MnS-based inclusions may be optimally controlled. In order to improve machinability and toughness, it is preferable to manage to more than 0 to 0.01% or less.

Magnesium (Mg) has a greater oxygen affinity than Al, thus suppressing the formation of oxides such as Al ₂ O _{3 in} steel and forming MgO-based oxides. At this time, MgO-based oxides are less likely to aggregate and coarsen even when remaining in steel. Compared to Al oxide, which is easy to be used, it is possible to make the inclusion finer and improve the specularity, and to form a protective film on the surface of the tool during cutting to improve the machinability. When the Mg content exceeds 0.005%, hot workability and Since the specularity can be reduced, it is preferable to manage it to more than 0 to 0.005% or less in the present invention.

At this time, the mold steel according to the present invention preferably satisfies the relational expression of 7.5≤ [Cr] +3.3 {[[Mo] +0.5 [W]}. If the relationship is less than 7.5, the corrosion resistance is insufficient, so that rust is easily generated in tap water. However, as the above value increases, the corrosion resistance increases, but the machinability may decrease due to the high alloy and the toughness may decrease.

And the mold steel according to the present invention preferably satisfies the relationship 1.0≤ [Mo] + [W] ≤4.5, which means that when the value of the relation is 1.0 or less, if the content of Cr is 4.5-6%, Cr alone The effect of improving the corrosion resistance is insufficient, and rust may easily occur. And, as the value of the relation increases, there is an effect of increasing the corrosion resistance, but if the value exceeds 4.5, the ferrite phase is formed at a high temperature so that the solution treatment is heat-treated in the two-phase region, that is, austenite and ferrite region Microstructure unevenness of the mold steel occurs. In other words, residual ferrite is present after the aging treatment, resulting in a variation in hardness, in particular a variation in machinability depending on the position during machining.

The mold steel of the present invention comprises the above components and consists of the balance Fe and other unavoidable impurities. And alloy elements may be further added to improve the properties of the mold steel of the present invention, if necessary, it is not interpreted to be excluded from the scope of the present invention that the alloy element is not disclosed in the embodiment of the present invention. .

On the other hand, before aging hardening heat treatment, martensitic ferrite formed during transformation in the case of martensitic structure employs carbon and alloying elements so that the hardness is high but very brittle. However, the bainite structure is a structure in which the same ferrite as the martensite is known as the matrix structure, but in the form of fine carbides of carbon or alloying elements dissolved therein, unlike martensite in the formation of bainite, it is similar to the tempered martensite. Organization. Although a decrease in hardness occurs than martensite, the ferrite formed in bainite can be more easily deformed in martensite and thus can easily absorb external impacts. Accordingly, the toughness of the basic microstructure can be improved without loss of hardness even after age hardening by the mixed structure of martensite and bainite. In addition, during cooling after the solution treatment, cracks due to cooling can be prevented regardless of the size of the product.

Hereinafter, the manufacturing method of the high hardness and high toughness precipitation hardening mold steel of this invention is demonstrated in detail.

In the manufacturing method of the present invention, the steel ingot formed as in the above-described component system is specially re-dissolved by electroslag remelting (ESR, Electro Slag Remelting) to secure cleanliness, to perform clean rolling, or to perform flat rolling after hot forging. Etc. Hot processing. Subsequently, when reheating, the solution is heated to a high temperature equal to or higher than the austenite transformation complete point (Ac3), subjected to a solution treatment, cooled to room temperature, and then subjected to an appropriate time aging heat treatment at a temperature at which precipitates are formed again. Precipitation hardening mold steel can be manufactured.

First, using the electric furnace or vacuum induction or atmospheric induction to cast a steel ingot formed as in the above-described component system, and performs the electric slag melting (ESR) process to re-dissolve the cast ingot to ensure cleanliness do. Then, hot rolling is performed by performing rolling or by flat rolling after hot forging. After manufacturing a certain size of product by hot working, slow cooling is performed to prevent cracking due to thermal stress and transformation stress during cooling.

And, in order to impart the hardness and toughness of the product produced by slow cooling, in the case of precipitation hardening mold steel is typically heated to the austenite region through the solution treatment as shown in Figure 1, where shown in Figure 1 Likewise, the reheating is maintained by heating to a temperature higher than the austenite transformation point Ac3. After the solution treatment, the solution is cooled by air to room temperature. This is because, in the case of the mold steel, martensite and bainite mixed structure are formed without formation of ferrite even when air-cooled because of excellent hardenability. After the cooling, as shown in FIG. 1, aging treatment is performed at an appropriate temperature at which Ni and Al, which are alloy elements of the precipitation hardening mold steel, precipitate out of the intermetallic compound, and 38 or more required in the precipitation hardening mold steel have hardness and 11 J / cm. Secure ^two or more toughness at the same time.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings.

After preparing the precipitation hardening mold steel composition as shown in Table 1, after heating the solution at a temperature of 870 ℃ or more at the transformation completion point (Ac3) of the formation of austenite is completed for 30 minutes or more, air-cooled and water-cooled The specimens were prepared by cooling the specimens, and the specimens were processed into shapes of standard Charpy V-Notch specimens having a thickness of 10 mm. In order to minimize the material variation of the specimen during aging treatment, the temperature of precipitation hardening by aging, which can be precisely controlled at 300 to 650 ° C., is used in a salt bath (temperature control range: ± 3 ° C.) After 30 hours of aging, the V notched Charpy specimens were finally reworked and subjected to an impact test at room temperature. Rockwell hardness (HRC) was measured. Here, the impact test was performed three times at the same temperature, the hardness measurement was measured five times, the average value of the values were obtained.

Figure 2 shows the hardness (HRC), V notch Charpy impact value measured after aging heat treatment of the invention steel and comparative steel in the method of FIG. The hardness and V notch Charpy impact values of the comparative steel and the inventive steel are expressed in PREN index. In the case of the comparative steel and the invention steel, the hardness can be obtained in the value of 38HRC or more, and the V notch Charpy impact value also indicates the value of ¹¹ J / cm ² or more (the arrow shown in FIG. 2 indicates the comparative steel). This means that the basic conditions required by the precipitation hardening mold steel can be used, and the mold steel can be used.

The rust resistance highlighted in the present invention was measured by the following method. The inventive steel and the comparative steel were subjected to heat treatment-aging hardening heat treatment according to the mold steel manufacturing process, and then the specimens were cut to a size of 50 mm x 50 mm having an appropriate thickness, and then precisely ground to 1 m. After grinding for 24 hours, the area of rust generated on the surface after immersion in tap water at room temperature was measured.

3 shows the rust generation area according to Cr + 3.3 {[Mo] +0.5 [W]} (unit weight%), which is an index indicating rust resistance. The rust generation fraction decreases significantly as the PREN index value increases. In the case of the invention steel having a value of 7.5 or more, the rust generation fraction is 50% lower than that of the comparative steel 2, which shows excellent rust resistance.

Figure 4 is a representative photograph (arrow rust) showing the surface where rust occurred after immersion in tap water for 24 hours. Some comparative steels show excellent V notch Charpy impact values at similar hardness levels, but the rust resistance is very inferior.

In addition, Figure 5 is a microstructure of the comparative steel and the invention steel has a very high PREN index value rust resistance is very excellent, but in the case of the comparative steel, the ferrite phase remains after aging heat treatment by expansion of the ferrite and austenite two-phase region at high temperature (Arrow sign).

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. It will be apparent to those skilled in the art that various modifications may be made without departing from the scope of the present invention.

The scope of the present invention is defined by the following claims. The scope of the present invention is not limited to the description of the specification, and all variations and modifications falling within the scope of the claims are included in the scope of the present invention.

Claims (8)

By weight%, C: 0.03-0.09%, Si: 0.2-0.9%, Mn: 1.2-1.7%, Cr: 4.5-6.0%, Ni: 2.5-3.5%, Cu: 0.7-1.5%, Al: 0.5- 1.5%, Nb: 0.01-0.1%, S: greater than 0 to 0.006% or less, Mo: 0.5-2.5% and W: 0.5-2.0% of at least one, balance Fe and other unavoidable impurities, bainite A mold steel composed of a mixed structure of martensite and satisfying the following formula, but having a value of [Mo] + [W] of 1.0 to 4.5.
7.5≤ [Cr] +3.3 {[[Mo] +0.5 [W]} delete The method of claim 1,
The mold steel is in weight percent, B: mold steel further comprising 0.001 to 0.01%. delete The method of claim 1,
The mold steel is by weight%, Co: more than 0 to 0.01% of the mold steel further comprises. The method of claim 1,
The mold steel is a mold steel further comprising at least one selected from a weight% of Ca: greater than 0 to 0.01% and less than Mg: greater than 0 to 0.005%. By weight%, C: 0.03-0.09%, Si: 0.2-0.9%, Mn: 1.2-1.7%, Cr: 4.5-6.0%, Ni: 2.5-3.5%, Cu: 0.7-1.5%, Al: 0.5- 1.5%, Nb: 0.01-0.1%, S: greater than 0 to 0.006% or less, Mo: 0.5-2.5% and W: 0.5-2.0% of at least one, balance Fe and other unavoidable impurities, bainite Re-dissolving a slab made of a mixed structure of and martensite and satisfying the following formula, but having an [Mo] + [W] value ranging from 1.0 to 4.5 through electric slag dissolution (ESR);
Hot working the slab;
Slow cooling the slabs and heating them through a solution treatment; And
And cooling the air after the solution treatment.
7.5≤ [Cr] +3.3 {[[Mo] +0.5 [W]} The method of claim 7, wherein
In the step of heating through the solution treatment, heating to the austenite region, the method of manufacturing a mold steel to be heated and maintained to a temperature higher than the austenite transformation complete point (Ac3) when reheating.

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