KR20120072499A - Precipitation hardening typed die steel with excellent hardness and toughness, and manufacturing method thereof - Google Patents

Abstract

PURPOSE: Precipitation hardening type die steel and a manufacturing method thereof are provided to obtain high hardness and toughness at the same time by controlling processing conditions. CONSTITUTION: Precipitation hardening type die steel comprises C of 0.05-0.13weight%, Si of 0.2-1.2weight%, Mn of 1.3-1.7weight%, Cr of 0.2-1.0weight%, Mo of 0.2-1.0weight%, Ni of 2.5-3.5weight%, Cu of 0.7-1.5weight%, Al of 0.7-1.5weight%, Nb of 0.01-0.1weight%, S of 0.006weight% or less, and Fe and inevitable impurities of the remaining amount, satisfying -6.7Δ[Cr]+2[Mo]-2[Si]-3[Mn]-20[C]Δ-3.8.

Description

High Hardness and High Toughness Precipitation Hardening Mold Steel and Manufacturing Method Thereof {PRECIPITATION HARDENING TYPED DIE STEEL WITH EXCELLENT HARDNESS AND TOUGHNESS, AND MANUFACTURING METHOD THEREOF}

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 making a mold for molding a plastic product, and a method for manufacturing the same. It relates to a high hardness and toughness precipitation hardening mold steel and a method for manufacturing the same, which can greatly extend the service life.

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, which is mainly used in plastic molding molds, 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. For this reason, there is a problem that it is not easy to simultaneously secure excellent hardness and toughness.

In order to solve this problem, Japanese Patent Laid-Open Publication No. 199-255732 reduces the carbon content to improve the toughness of the entire structure, and the hardness deficiency caused by the reduction of the carbon content is due to the carbide and Ni- precipitated by high temperature heat treatment. Due to the Al intermetallic compound to increase the hardness, there is still a problem that the cracks may still occur due to the thermal stress during mold processing and use due to the low toughness after curing due to the high carbon content and high temperature heat treatment temperature.

In addition, Japanese Patent Laid-Open Publication Nos. 1995-278743 and 1997-021351 increase the amount of addition of chromium (Cr) and S and limit the relational expression to simultaneously improve hardness and toughness, but chromium (Cr) and When satisfying the relational formula for sulfur (S), there is a lack of explanation as to why the toughness is improved, and in fact, since sulfur (S) is contained in a large amount of 0.05% by weight or more, there is a problem that the toughness and the specularity may be reduced.

As such, even if the hardness is high, when the toughness is insufficient, burrs may be generated when the mold surface is flawed due to the engagement of the plastic material, and the burrs may be generated. In the case of handling, cracks may be generated at the corners, and in order to prevent such cracking, plastic molding cannot be performed at a high speed, which is a major obstacle to improving the life of the mold and increasing productivity.

The present invention has been made to solve the above-described problems, by controlling the process conditions such as alloying, solution treatment, aging treatment, etc., at the same time to the high hardness of HRC 38 or higher and VJ notch Charpy impact value of 20J or higher By securing, high hardness and high toughness precipitation hardening mold steel which can improve the service life of a metal mold | die and productivity are aimed at providing it.

In the present invention, C: 0.05 to 0.13%, Si: 0.2 to 1.2%, Mn: 1.3 to 1.7%, Cr: 0.2 to 1.0%, Mo: 0.2 to 1.0%, Ni: 2.5 to 3.5%, Cu: 0.7 to 1.5%, Al: 0.7 to 1.5%, Nb: 0.01 to 0.1%, S: 0.006% or less, the balance consists of residual Fe and other unavoidable impurities, and is characterized by being a mixed structure of bainite and martensite Provides high hardness and toughness precipitation hardening mold steel.

At this time, it is also characteristic to satisfy the relation of -6.7 ≦ [Cr] +2 [Mo] -2 [Si] -3 [Mn] -20 [C] ≦ −3.8.

In addition, the mold steel is B: 0.001 ~ 0.01% by weight, Co: 0.01% or less (not including 0%), Ca: 0.01% or less (not containing 0%) and Mg: 0.005% or less It is also characteristic that it contains at least 1 sort (s) chosen from 2 sort (s) (not containing 0%).

Furthermore, the die steel is characterized by having a hardness of HRC 38 or higher and a V notch Charpy impact value of 20 J or higher at room temperature.

In addition, the area fraction of the bainite is also characterized by 10 to 30%.

In addition, the present invention is in the weight% C: 0.05 ~ 0.13%, Si: 0.2 ~ 1.2%, Mn: 1.3 ~ 1.7%, Cr: 0.2 ~ 1.0%, Mo: 0.2 ~ 1.0%, Ni: 2.5 ~ 3.5%, Cu: 0.7 ~ 1.5%, Al: 0.7 ~ 1.5%, Nb: 0.01 ~ 0.1%, S: 0.006% or less, hot working steel made of residual Fe and other unavoidable impurities, and after reheating the austenite transformation completion point ( It is heated to a temperature of 10 ~ 30 ℃ higher than Ac3) and maintained for a predetermined time, cooled to room temperature at a cooling rate of 0.5 ℃ / min ~ 20 ℃ / second, and then aged at a temperature of 530 ~ 560 ℃ It provides a high hardness and high toughness precipitation hardening mold steel manufacturing method.

At this time, the mold steel is B: 0.001 ~ 0.01% by weight, Co: 0.01% or less (not containing 0%), Ca: 0.01% or less (not containing 0%) and Mg: 0.005% or less It is also characteristic that it contains at least 1 sort (s) chosen from 2 sort (s) (not containing 0%).

Furthermore, there is a feature in that the re-dissolution process of electric slag melting (ESR) is performed before the hot working.

In addition, the aging treatment is characterized in that it is carried out for 3 to 15 hours.

According to the present invention, the process life of the alloy components, the solution treatment, and the aging treatment are controlled to simultaneously secure high hardness of HRC 38 and high V notch Charpy impact value of 20J or higher, thereby greatly increasing the service life of the mold. It is able to extend, and it is excellent in toughness, so that the occurrence of flaws generated in the mating surface or the like due to the engagement of the plastic material, and the cracks in the corners which may occur during the handling of the mold, such as the attachment, detachment and storage of the mold to the molding machine, are reduced. As a result, not only the molding of the plastic can be performed at high speed, but also high hardness, cleanliness, and uniform 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 the change in hardness and toughness according to the solution treatment temperature.
3 is a graph showing the change in toughness according to the solution treatment temperature during aging treatment.
Figure 4 is a graph showing the change in microstructure according to the cooling rate during continuous cooling.
5 is a graph showing the change in hardness according to cooling conditions and aging treatment conditions.
6 is a graph showing a change in toughness according to aging treatment conditions.

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

The present inventors have completed the present invention by repeating the research and experiment to secure both the high hardness of HRC 38 or more and the VN notch Charpy impact value (CVN absorbed energy) of 20J or more while having excellent properties of the mold steel. As a result, the present invention contains low carbon, disperses and precipitates Ni-Al intermetallic compound, chromium (Cr) and molybdenum (Mo) carbide, and finely inhibits grain growth by precipitation of Nb carbonitride, and solution treatment By producing the mixed structure of bainite and martensite by optimizing the process conditions such as aging treatment and aging treatment, the precipitation hardening mold steel having high hardness and toughness can be manufactured to greatly improve the service life of the mold and speed up the molding of plastic. It can be done with

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 If the content of C exceeds 0.13%, the martensite structure is excessively formed after forced cooling, and the toughness is lowered. If the content of C is less than 0.05%, the hardness is low after forced cooling, so that even if precipitation hardening by aging occurs, Since the hardness of HRC 38 or more can not be obtained, the C content is limited to 0.05 to 0.13%.

Silicon (Si) is an element that is added as a deoxidizer during steelmaking and partially dissolved in steel to improve hardenability and hardness.It suppresses the formation of coarse carbides during aging treatment and prevents the rapid decrease in impact toughness due to precipitation of grain boundary carbides. In addition, when the mass of the material is large, Mn alone should not be less than 0.2% in steel because it is difficult to control the hardness during the solution treatment. However, if the content of Si exceeds 1.2%, segregation may occur or the fluidity of the steel may not be high, and a uniform structure may not be obtained, and the toughness may be greatly reduced by promoting graphitization of carbides during aging treatment for a long time. The content of Si is limited to 0.2 to 1.2%.

Manganese (Mn), together with C, improves hardenability and improves hardness and abrasion resistance after aging treatment. It inhibits 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 1.3% must 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 1.3 to Limited to 1.7%.

Chromium (Cr) is an element that improves hardenability, improves hardness, corrosion resistance, and corrosion resistance. It inhibits the formation of ferrite and pearlite upon cooling, promotes the formation of martensite and bainite, which are low-temperature structures, and improves core hardness. It is preferable to add 0.2% or more. However, if the Cr content exceeds 1.0%, the Cr content is limited to 0.2 ~ 1.0% because precipitation of coarse chromium carbide at the grain boundary during aging treatment results in a decrease in toughness and deteriorates the weldability. do.

Molybdenum (Mo) is an element added to improve the hardness and corrosion resistance of the structure by solid solution of the matrix and to obtain the purpose of hardening by carbide precipitation during aging treatment, and also to obtain high toughness, and promotes the formation of bainite. In order to improve hardness, corrosion resistance and toughness and to form bainite, it is preferable to add 0.2% or more.However, if the Mo content exceeds 1.0%, the manufacturing cost increases and the toughness decreases due to the increase in hardness due to excessive solidification. , Mo content is limited to 0.2 ~ 1.0%.

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%.

Since copper (Cu) is an element that precipitates as an ε-Cu intermetallic compound during aging treatment and leads to precipitation hardening and is effective for improving corrosion resistance and machinability, it is preferably added at least 0.7%. However, when the Cu content exceeds 1.5%, Since the hot workability is lowered, which causes particle cracks and causes surface defects, the Cu content is limited to 0.7 to 1.5%.

Aluminum (Al) binds with Ni to form Ni-Al intermetallic compound in aging treatment to increase precipitation hardening to increase hardness, and to form Al nitride to suppress coarsening of austenite grains in solution treatment. It is preferable to add 0.7% or more since it promotes 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 0.7-1.5%.

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 it causes surface defects by dropping MnS inclusions during polishing, or acts as a starting point for surface corrosion in the case of chlorine-based plastics. Since toughness and specularity are reduced due to inclusion formation, in the present invention, an upper limit is set and controlled at 0.006% or less.

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, if the B content exceeds 0.01%, the hot workability and repair weldability may be deteriorated due to the large precipitation of B nitride or intermetallic compound at the grain boundary during hot working, and the bainite transformation may be delayed. It is limited to 0.01%.

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, causing variation in hardness. In the present invention, it is preferable to set the upper limit and manage it 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 set an upper limit and to manage 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 specular property can be reduced, it is preferable to set the upper limit to manage at 0.005% or less.

At this time, the mold steel according to the present invention preferably satisfies the relationship of -6.7≤ [Cr] +2 [Mo] -2 [Si] -3 [Mn] -20 [C] ≤ -3.8, which is If the value of is less than -6.7, the formation of cementite and martensite structure may be excessively formed, thereby reducing the toughness. When the value of the relation exceeds -3.8, the toughness decreases due to hardness increase due to excessive solid solution strengthening. Because there is a problem that can be.

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 will not be interpreted to be excluded from the scope of the present invention that the alloy element is not added in the embodiment of the present invention.

On the other hand, steels having martensite as the basic microstructure can be obtained by relatively quenching to transform austenite into martensite without phases of ferrite, pearlite, and bainite, and such martensite structure has strength or hardness. On the other hand, due to the lack of toughness, particularly in the case of large products, there is a problem that is easily broken by the impact of the thermal stress due to the internal and external temperature deviation formed during cooling and the transformation stress due to the martensite transformation, making the manufacturing process difficult. Even though the 100% martensite structure is secured, coarse carbides are easily formed during the aging hardening heat treatment process, thereby rapidly reducing the impact toughness. When controlling and forming part of bainite structure during cooling, It was confirmed that it is possible to secure a uniform mixed tissue with improved toughness without reduction.

That is, the mixed structure of martensite and bainite defined in the present invention is largely dependent on the composition of the steel and the cooling rate after the solution treatment, optimizing the content of alloying components such as Cr, Mo, and B while including low carbon. In addition, the cooling rate after the solution treatment can be kept relatively slow to ensure a constant bainite fraction.

As such, the reason that the toughness is improved without loss of hardness even after age hardening by the mixed structure of martensite and bainite is that the martensitic ferrite formed during transformation in the case of martensite structure before the age hardening heat treatment employs carbon and alloy elements. Although the hardness is high but very brittle, the bainite structure has the same ferrite structure as the martensite, but when martenite is formed, carbon or alloying elements that are solid-solution precipitated in the form of fine carbides. As an advanced martensite-like structure, ferrites formed in bainite can be more easily deformed in martensite and thus absorb external impacts, although a decrease in hardness occurs than martensite. Therefore, it is possible to prevent cracking due to cooling, regardless of the size of the product during cooling after solution treatment.

In addition, in the age hardening process after the solution treatment, the bainite ferrite is composed of fine laths, and particularly contains a high dislocation density generated during the bainite transformation, and thus, the age of the bainite microstructure during the age hardening heat treatment. In this case, since carbides are already formed in the microstructure, they act as a function of forming intermetallic compounds rather than carbides, so that the hardness can be improved even after age hardening, thereby sufficiently compensating for the reduction in hardness due to bainite structure.

Due to the age-hardening heat treatment behavior of the bainite structure, the unbalance of plastic deformation caused by the formation of rapid coarse carbides generated during the aging hardening of martensite structure is alleviated to delay the pore generation due to the interfacial isolation or the crack of martensite. This is because the toughness is improved.

In this case, the martensite tissue may include some unaffected residual austenite tissue between the martensite crystals, but according to the present invention, since the residual austenite is extremely small, the effect of residual austenite will be hardly affected. .

At this time, it is preferable to form a bainite microstructure by securing 10-30% of the area fraction of bainite by optimally controlling the cooling rate after the solution treatment. When the area fraction of bainite is less than 10%, Difficulties in the manufacturing process due to thermal stress due to internal and external temperature variation of the material and transformation stress due to transformation stress due to martensite transformation, and impact toughness due to coarse carbides formed in martensite due to age hardening heat treatment, can easily be damaged. If the area fraction of the bainite exceeds 30%, there is a problem that the mechanical properties such as hardness may be degraded even after aging by bainite structure, and the area fraction of bainite is limited to 10-30%. do.

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. After hot processing, the solution is heated to a temperature of 10 to 30 ° C. higher than the austenite transformation completion point (Ac3) and maintained for 30 minutes or more at the time of reheating, and at room temperature at a cooling rate of 0.5 ° C./min to 20 ° C. seconds. After cooling to, aging treatment for 3 to 15 hours at a temperature of 530 ~ 560 ℃ can be produced a high hardness and high toughness precipitation hardening mold steel.

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 In addition, hot rolling is performed by performing rolling or by flat rolling after hot forging. After manufacturing a certain sized product by hot working, slow cooling is performed to prevent cracking by thermal stress and transformation stress during cooling.

And, in order to give the hardness and toughness of the product produced by slow cooling to heat to the austenite region through the solution treatment, as shown in Figure 1, when reheating 10 austenite transformation complete point (Ac3) It is heated to a temperature of ˜30 ° C. and maintained for at least 30 minutes. Since the transformation complete point Ac3 is 850 to 860 ° C, a temperature 10 to 20 ° C higher than the transformation complete point Ac3 corresponds to a temperature range of 860 to 890 ° C.

As the temperature of the solution treatment increases, the effect of inhibiting grain growth due to the re-use of Nb carbide decreases and the austenite grains grow, so that the solution treatment temperature is maintained at 30 ° C. or lower immediately above the austenite transformation completion point (Ac3). It is desirable to secure high toughness. However, when the solution treatment temperature is directly below the austenite transformation point (Ac3), the hardness of the steel increases due to abnormal coexistence of ferrite and austenite. It should be kept above 10 ℃ just above the completion point (Ac3).

After the solution treatment is cooled to room temperature at a cooling rate of 0.5 ℃ / min ~ 20 ℃ / second bar, which forms a mixed structure of bainite and martensite without the formation of ferrite in the interval of the cooling rate, the bainite This is because a relatively slow cooling rate must be maintained in order to secure an appropriate area fraction of. However, if the cooling rate is less than 0.5 ℃ / minute, the structure mainly consists of ferrite or pearlite, the hardness is very low even after aging hardening, and the anisotropy of the product properties due to the nonuniformity of the tissue occurs, so the cooling rate is 0.5 ℃ / minute It is necessary to maintain the above, and in order to avoid the structure mainly composed of ferrite or pearlite, both water cooling, oil cooling and air cooling are possible.

After cooling, the aging treatment is performed at a temperature of 530 to 560 ° C. to ensure high hardness and high toughness at the same time. In the present invention, the temperature of the aging treatment at which the maximum hardness is obtained is about 500 ° C., but at 20 ° C. at 23 ° C. In order to have the above-mentioned V notch Charpy impact value, the temperature of 530-560 degreeC which is some overaging conditions slightly higher than the temperature of the aging treatment from which maximum hardness is obtained is preferable. This is because when the temperature of the aging treatment is less than 530 ° C., the toughness at which the maximum hardness is obtained at around 500 ° C. is insufficient, and when the temperature of the aging treatment exceeds 560 ° C., the aging hardening by precipitation becomes excessively aged and causes a decrease in hardness. .

Moreover, although the time of the said aging treatment changes with thickness of a final product, 3 hours-15 hours are preferable. This is because the longer the aging treatment, the greater the effect of improving toughness, but the maximum hardness value is obtained when 3 hours or longer, and the effect of toughness improvement is saturated when 15 hours or longer.

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

After the precipitation hardening mold steel is prepared as shown in Table 1, and then heated to this solution solution solution for 30 minutes at a temperature of 850 ~ 860 ℃ or more transformation completion point (Ac3) to complete the formation of austenite, air-cooled And water cooled to prepare specimens, which were processed into the shape of standard Charpy-V Notch specimens of 10 mm thickness. In addition, in order to minimize the material deviation of the specimen during aging treatment, after the aging treatment for 1 to 30 hours using a salt bath (temperature control range: ± 3 ℃) capable of precise control at 300 to 600 ℃, finally Charpy V The notched specimens were reworked and subjected to an impact test at room temperature, and 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.

[Table 1]

Figure 2 is a graph showing the change in hardness and toughness according to the solution treatment temperature of the invention steel 2, the austenite grains grow as the solution treatment temperature increases, it can be seen that the toughness is sharply reduced before aging treatment, The change in hardness was relatively minor.

And, as shown in Figure 3, it can be seen that as the solution treatment temperature increases, the toughness of the age-treated invention steel 2 decreases, and solution solution having a V-notch Charpy impact value of 20J or more at room temperature (23 ° C). It can be seen that the treatment temperature is 860-890 ° C, which is 10-30 ° C higher than the austenite transformation complete point (Ac3).

4 is a "phase transformation graph measured after cooling at various cooling rates after maintaining heat at the solution treatment temperature of Comparative Steel 1 and Inventive Steel 2", in the case of Comparative Steel 1 when the cooling rate is 1 ° C / sec or less. It can be seen that a structure mainly composed of ferrite or pearlite is formed. Therefore, in the case of Comparative Steel 1, when the cooling rate is 1 ° C / sec or less, the hardness decreases even after the age hardening heat treatment by the ferrite and pearlite mixed structure. In particular, the nonuniform microstructure causes variation in hardness and toughness. On the other hand, in the case of the invention steel 2, the cooling rate is 0.5 ℃ / min ~ 20 ℃ / second in the section can be confirmed that all the tissue is formed of a mixed structure of bainite and martensite, therefore, the invention steel 2 is When the cooling rate is 0.5 ℃ / min ~ 20 ℃ / second by securing a uniform mixed structure it can be produced a precipitation hardening mold steel of high hardness and high toughness.

In addition, Figure 5 is a graph showing the change in hardness according to the cooling conditions and aging treatment conditions for Comparative Steel 1 and Inventive Steels 2 to 4, in the case of Comparative Steel 1 at the aging treatment temperature of 530 ℃ both water and air coolant It does not appear that the hardness decreases with the aging treatment time, but it can be seen that a softening phenomenon occurs in which the hardness decreases as the aging treatment time elapses at a temperature over 530 ° C. On the other hand, in the case of the invention steels 2 to 4, the softening phenomenon that the hardness decreases as the aging treatment time elapses at a temperature of more than 530 ℃ occurs, but the water coolant and air cooling up to 560 ℃ during the aging treatment time of 3 hours to 15 hours It can be seen that the hardness of all materials is maintained at HRC 38 or higher.

And, Figure 6 is a graph showing the change in toughness according to the aging treatment conditions, when the aging treatment temperature is 450 ℃ and 500 ℃, both the invention steels 2 and 4 V notch Charpy impact value of 20J or more at room temperature, but lower than about 10J, In the case of 530 ℃ and 550 ℃ it can be seen that both the toughness of the invention steels 2 and 4 increases by more than 20J due to the softening phenomenon due to the hardness decrease. Therefore, the optimum aging treatment temperature for producing a precipitation hardening mold steel that can satisfy both high hardness and high toughness at the same time can be confirmed that the aging treatment time is 3 hours to 15 hours.

On the other hand, Table 2 below is the hardness and impact value of the air-cooled material according to the age hardening conditions, Table 3 below shows the hardness and impact value of the water-cooled material according to the age hardening conditions. In Table 2 and Table 3, there were many comparative steels with hardness of HRC 38 or more, but V toughness Charpy impact value was about 10J at room temperature, so the toughness was poor, whereas in the case of the inventive steel, 15 hours at 550 ° C which is the scope of the present invention. When the aging hardening heat treatment was performed, it can be seen that VRC notched Charpy impact value of 20J or higher at a hardness of HRC 38 or higher and room temperature.

However, the values of [Cr] +2 [Mo] -2 [Si] -3 [Mn] -20 [C] are -8.75 and -3.28, respectively, as in inventive steels 5 and 6. In case of deviation, when the aging hardening heat treatment was performed at 550 ° C. for 15 hours, a relatively low V notch Charpy impact value was measured as compared with other inventive steels. This is because in the case of the inventive steel 5, the formation of cementite and martensite structure are excessively formed, and thus the toughness is slightly reduced. In the case of the inventive steel 6, the toughness is rather reduced due to the hardness increase due to excessive solid solution strengthening.

In the present invention, the above embodiment is an example, and the present invention is not limited thereto. Anything that has substantially the same configuration as the technical idea described in the claims of the present invention and achieves the same effect is included in the technical scope of the present invention.

TABLE 2

[Table 3]

Claims (9)

By weight%, C: 0.05 to 0.13%, Si: 0.2 to 1.2%, Mn: 1.3 to 1.7%, Cr: 0.2 to 1.0%, Mo: 0.2 to 1.0%, Ni: 2.5 to 3.5%, Cu: 0.7 to 1.5 %, Al: 0.7 to 1.5%, Nb: 0.01 to 0.1%, S: 0.006% or less, high hardness, characterized in that the composition consists of the balance Fe and other unavoidable impurities, and consists of a mixed structure of bainite and martensite High toughness precipitation hardening mold steel. The method of claim 1,
A high hardness and toughness precipitation hardening mold steel, characterized by satisfying a relation of -6.7≤ [Cr] +2 [Mo] -2 [Si] -3 [Mn] -20 [C] ≤−3.8. The method of claim 1,
The mold steel is B: 0.001 ~ 0.01% by weight, Co: 0.01% or less (not containing 0%),
High hardness and toughness precipitation hardening mold characterized by containing at least one selected from two kinds of Ca: 0.01% or less (not including 0%) and Mg: 0.005% or less (not including 0%) River. The method of claim 1,
The mold steel has a hardness of HRC 38 or more, and a high hardness and toughness precipitation hardening mold steel, characterized in that the V notch Charpy impact value of 20J or more at room temperature. The method of claim 1,
Hardness and toughness precipitation hardening mold steel, characterized in that the area fraction of the bainite is 10 ~ 30% .. By weight%, C: 0.05 to 0.13%, Si: 0.2 to 1.2%, Mn: 1.3 to 1.7%, Cr: 0.2 to 1.0%, Mo: 0.2 to 1.0%, Ni: 2.5 to 3.5%, Cu: 0.7 to 1.5 After hot working steel of%, Al: 0.7 ~ 1.5%, Nb: 0.01 ~ 0.1%, S: 0.006% or less, balance Fe and other unavoidable impurities,
When reheating, it is heated to a temperature higher than 10 ~ 30 ℃ higher than the austenite transformation complete point (Ac3) and maintained for a predetermined time, cooled to room temperature at a cooling rate of 0.5 ℃ / min ~ 20 ℃ / second, and then of 530 ~ 560 ℃ A method for producing a high hardness and toughness precipitation hardening mold steel, characterized by aging at temperature. The method of claim 6,
The mold steel is B: 0.001 ~ 0.01% by weight, Co: 0.01% or less (not containing 0%),
High hardness and toughness precipitation hardening mold characterized by containing at least one selected from two kinds of Ca: 0.01% or less (not including 0%) and Mg: 0.005% or less (not including 0%) Method of manufacturing steel. The method of claim 6,
Method for producing a high hardness and toughness precipitation hardening mold steel, characterized in that to perform the re-dissolution process of the electric slag melting (ESR) before the hot working. The method of claim 6,
The aging treatment is a method of manufacturing a high hardness and toughness precipitation hardening mold steel, characterized in that performed for 3 to 15 hours.

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