KR101379058B1 - Precipitation hardening type die steel with excellent hardness and toughness and the method of manufacturing the same - Google Patents

Abstract

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: High hardness and toughness precipitation hardening mold steel with its composition composed of 0.7 ~ 1.5%, Al: 0.7 ~ 1.5%, Nb: 0.01 ~ 0.1%, S: 0.006% or less, balance Fe and other unavoidable impurities It relates to a manufacturing method. The present invention is characterized in that the post-heat treatment at 530 ~ 550 ℃ temperature after welding in order to restore the hardness of the heat affected zone of the welded part and the base material, characterized in that the post-heat treatment holding time is specified as 3 to 7 hours.

Description

Precipitation hardening type die steel with excellent hardness and toughness and the method of manufacturing the same

The present invention relates to a mold steel which is a material used for manufacturing a mold, and a manufacturing method thereof, and more particularly, to a hardening and high toughness precipitation hardening mold steel for making a mold for molding a plastic product and a manufacturing method for manufacturing the same. will be.

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.

An object of the present invention is to provide a high hardness and high toughness precipitation hardening mold steel.

In addition, the present invention provides a method for manufacturing a mold steel that provides a post-weld treatment method after welding repair, which can greatly improve the service life and productivity of the mold by welding repair and repair of fine defects or cracks generated during long time use of the mold steel. It aims to do 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: It provides high hardness and toughness precipitation hardening mold steel containing 0.7 ~ 1.5%, Al: 0.7 ~ 1.5%, Nb: 0.01 ~ 0.1%, S: 0.006% or less, balance Fe and other unavoidable impurities.

In addition, in the present invention, the mold steel has a high hardness of HRC 38 or more and a V-notch Charpy impact value (CVN absorbed energy) of 20J or more.

Further, according to another embodiment of the present invention, by 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, steel that is made up of the remaining Fe and other unavoidable impurities after hot working It provides a high hardness and high toughness precipitation hardening mold steel manufacturing method to perform the aging treatment after performing.

Further, in the present invention, after the hot working, slow cooling is performed.

In addition, the solution treatment in the present invention is reheated to a range of 10 ~ 30 ℃ higher than the austenite transformation complete point (Ac3) during reheating and maintained for 30 minutes or more.

In the present invention, the solution treatment is cooled to room temperature to form a mixed structure of bainite and martensite.

In addition, the present invention further comprises the step of performing a post-heat treatment to the temperature of 530 ~ 550 ℃ after repairing the micro-defects or cracks generated during the use of the mold steel to restore the hardness of the welded portion and the base material heat affected zone for the mold steel. do.

In addition, in the present invention, the post-heat treatment time is 3 to 7 hours after welding repair of the micro-defects and cracks generated during the use of the mold steel.

According to the present invention, high hardness and high toughness die steel can be obtained through adjustment of the alloying component.

Further, according to the present invention, process conditions such as solution treatment, aging treatment, etc. are controlled to generate defects in the mating surface due to the engagement of the plastic material, or during the handling of the mold such as mounting, detaching and storing the mold into the molding machine. There is an effect that can greatly extend the service life of the mold by heat treatment after welding repair to the cracked portion of the corner.

1 is a schematic diagram showing a plastic mold steel manufacturing process.
Figure 2 is a graph showing the hardness distribution of the welded portion and the base material portion after repairing the welding of the plastic mold steel.
Figure 3 is a graph showing the hardness change of the base material and the weld portion according to the post-heat treatment conditions after welding repair simulation.
Figure 4 is a photograph showing the surface gloss after the weld post-heat treatment.

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.

Generally, the precipitation hardening mold steel mainly used in plastic molding molds should have excellent machinability, hardness and toughness. However, since hardness and toughness are mutually opposite properties, it is not easy to simultaneously secure excellent hardness and toughness. In order to solve such a problem, Japanese Laid-Open Patent Publication No. 199-255732 reduces the carbon content to improve the toughness of the entire structure, and the hardness deficiency due to the reduction of the carbon content is due to the carbide and Ni- precipitated by high temperature heat treatment. It was intended to increase the hardness due to the Al intermetallic compound. However, due to the high carbon content and high temperature heat treatment temperature, the toughness after curing is low, and there is a problem that cracking may still occur due to thermal stress during mold processing and use.

In addition, Japanese Patent Laid-Open Publication Nos. 1995-278743 and 1997-021351 attempt to improve the hardness and toughness simultaneously by increasing the amount of chromium (Cr) and sulfur (S) added and limiting the relationship. There exists a problem that toughness and specularity may fall by containing a large amount of (S) 0.05 weight% or more.

As described above, even when 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, such as mounting, detaching, and storing the mold in the molding machine. Cracks may occur in the corners when the mold is handled. In order to prevent such cracking, plastic molding cannot be performed at high speed, which is a major obstacle to improving the life of the mold and increasing productivity.

The present invention provides an alloy design of a mold steel which can improve both hardness and toughness as described above, and also proposes a post-heat treatment method of a welded part after welding repair of microcracks or defects during use of the mold steel, thereby simultaneously providing high hardness and high toughness. We will present the precipitation hardening mold steel and its manufacturing method.

The present invention provides a precipitation hardened mold steel having high hardness and toughness which is excellent in various properties and secures a high hardness of HRC 38 or higher and a CVN absorbed energy of 20J or more at the same time, and greatly improves the life of the mold. do. In addition, the present invention is a post hardening treatment after welding repair of the micro cracks or defects generated in the mold, the precipitation hardening mold to enable high-speed plastic molding of the high-hard surface by the post-heat treatment of the appropriate welding part and to increase the life of the mold by reworking Provides a method for manufacturing steel.

First, the reason for component limitation of the steel used by this invention is demonstrated in detail. Next, the manufacturing method of this mold steel, such as the post-heat-treatment conditions after welding repair of the metal mold | die which concerns on this invention, is demonstrated. (Hereinafter, weight% is only expressed in%.)

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 is achieved by aging, 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 solid solution in which a part thereof is homogeneous with Cu to prevent red brittleness during hot working, and Ni-Al intermetallic compounds during aging treatment ( 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.

1 is a schematic diagram showing a plastic mold steel manufacturing process, Figure 2 shows the hardness distribution of the welded portion and the base material after repairing the plastic mold steel welding. In the present invention, the post-heat treatment temperature after welding repair of the mold steel having the above composition is controlled to 530 to 550 ° C. As shown in FIG. 2, this is because when the post-heat treatment temperature is lower than 530 ° C., the hardness of the welded part and the heat-affected part is insufficient to restore hardness due to insufficient aging treatment, and thus the hardness variation with the base material occurs. In this case, not only the difference in the mirror surface of the welded portion and the base metal portion occurs during the re-mirror treatment of the mold after welding repair, but also the weak welded portion is a major cause of mold damage. However, if the post-heat treatment temperature is 550 ° C or higher, in the case of heat affected zone by excessive heat treatment, the alloy element re-dissolved during welding is re-precipitated and excessive hardening occurs, and in the case of the base material, the hardness is due to over-aging by re-aging heat treatment. Decreases. In this case, the hardness of the base material portion is lower than that of the welded portion, resulting in a difference in mirror properties of the base material portion and the welded portion at the time of mirror processing and a difference in the service life in use. Therefore, the post-heat treatment temperature after welding repair is preferably 530 ~ 550 ℃.

In addition, it is preferable to heat-treat after heat treatment for 3 to 7 hours after welding repair. When the heat treatment time is less than the time period, sufficient hardening of the welded part and the welded heat affected part does not occur, resulting in a variation in hardness and mirror surface with the base material. However, when the heat treatment time is 7 hours or more, sufficient hardening occurs in the welded part and the welded heat affected zone unlike the short time heat treatment, but the hardness decreases due to excessive aging of the base material, resulting in a difference in hardness and specularity between the base material and the welding repair part. do. Therefore, the post-heat treatment time of the weld is suitable for 3 to 7 hours.

Hereinafter, the production method of the high hardness and high toughness precipitation hardening mold steel of the present invention and post-heat treatment conditions after repairing the welded part will be described in more 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 working, etc., after reheating, a solution treatment may be performed, and after cooling to room temperature, the steel used in the present invention may be aged at an appropriate temperature and time to produce 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 ~ 30 ℃ high temperature and maintained for over 30 minutes. After the solution treatment, the solution is cooled to room temperature, whereby a mixed structure of bainite and martensite is formed without formation of ferrite in the cooling rate section. However, if the cooling rate is too slow, it becomes a structure mainly composed of ferrite or pearlite, and even after aging hardening, the hardness is very low and anisotropy of product properties due to non-uniformity of the tissue occurs. Therefore, the structure mainly composed of ferrite or pearlite is used. In order to avoid water cooling, oil cooling and air cooling are all possible.

After the cooling, in order to ensure high hardness and high toughness, it is composed of a final step of aging treatment at a temperature capable of aging precipitation of the alloying elements.

(Example)

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

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 0.108C-0.3Si-1.45Mn-0.8Mo-0.8Cr in steel composition consisting of%, Al: 0.7 ~ 1.5%, Nb: 0.01 ~ 0.1%, S: 0.006% or less, balance Fe and other unavoidable impurities After the precipitation hardened mold steel was produced in the form of a rolled flat steel having a thickness of 50 mm produced after the furnace-ESR-flat rolling of steel of -3.05Ni-1.0Cu-1.06Al, the heat treatment process as shown in FIG. That is, 100 mm (width) x 100 mm (length) x 50 mm (thickness) specimens for the welding repair simulation of the micro-defects and cracks generated during the use of the mold steel were prepared using the solution treatment-cooling-aging heat treatment. The raw material which is a normal mold steel welding repair condition was preheated at the temperature of 300-400 degreeC, and the welding repair test was done by the argon TIG welding which applied the electric current of 100A. FIG. 2 is a material before heat treatment in which welding defects of fine defects or cracks generated during use of a mold steel are advanced, and are formed of a base material part, a weld part, and a heat affected part. In particular, the minute indentations shown in FIG. 2 are traces of the change in hardness. 2, the hardness of the base material portion is higher than that of the welded portion in the welded state, which is a case where the post-heating treatment is not performed after welding. In particular, the hardness of the heat affected zone where deterioration due to heat generated during welding occurs is Vickers hardness value of about 150HV lower than that of the base material portion, and it can be seen that it is lower than the welded portion. The difference in the hardness of the above material causes the difference in the mirror property in mirror processing and the life in use. 3 is a view showing a change in the hardness of the base material, the welded portion, and the heat affected part according to the post-heat treatment conditions after repairing the micro-defects or cracks generated during the use of the mold steel.

In FIG. 3, the post-heat treatment temperature was performed under conditions of 475 ° C. × 5 hours, 500 ° C. × 3 hours, 530 ° C. × 6 hours, 545 ° C. × 3 hours, and 560 ° C. × 3 hours. In Figure 3 it can be seen that in the case of the post-heat treatment temperature is low 475 ℃, 500 ℃ the hardness of the heat affected zone (indicated by the dotted line), which is the boundary of the welded portion and the base material portion is weakly restored than the base material portion. However, when the temperature is 530 ° C and 545 ° C, the hardness of the base material portion, the heat-affected portion, and the weld portion is uniform due to aging hardening caused by the post-heat treatment, so that there is almost no difference in hardness between the base portion and the weld portion. However, when the post-heat treatment temperature is increased to 560 ° C., the hardness of both the base material portion and the welded portion is similar, but a decrease in hardness due to overheating occurs (indicated by arrow). This results in a reduction in the overall hardness of the mold, which causes a problem of shortening the life of the mold. Therefore, suitable post-heat treatment conditions after welding are suitable for a temperature of 530 to 550 占 폚, and for 3 to 7 hours in the case of time.

4 is a photograph of the surface of the polished surface after being treated to the mirror surface after the post-heat treatment of the weld portion after the matt (right) and the glossy (left) treatments. In Figure 4 it can be seen that both the welded part and the base material parts exhibit the same gloss so as not to make a difference with the naked eye.

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)

delete delete 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 ~ 1.5%, Nb: 0.01 ~ 0.1%, S: 0.006% or less, remainder Fe and other unavoidable impurities, and after hot working steel with high hardness of HRC 38 or higher after solution treatment Using the steel, and then aging treatment,
The solution treatment is reheated to a range of 10 ~ 30 ℃ higher than the austenite transformation complete point (Ac3) during reheating and maintained for 30 minutes or more,
The aging treatment further comprises the step of performing a post-heat treatment to the temperature of 530 ~ 550 ℃ after repairing the fine defects or cracks generated during the use of the mold steel in order to restore the hardness of the heat affected zone of the welded part and the base material part Degree and high toughness precipitation hardening mold steel manufacturing method. The method of claim 3,
High hardness and toughness precipitation hardening mold steel manufacturing method characterized in that the slow cooling is performed after the hot working. delete The method of claim 3,
The solution treatment is a high hardness and toughness precipitation hardening mold steel manufacturing method characterized in that the cooling to room temperature to form a mixed structure of bainite and martensite. delete The method of claim 3,
High hardness and toughness precipitation hardening mold steel manufacturing method characterized in that after the repair to weld the micro-defects or cracks generated during use of the mold steel, the post-heat treatment time is 3 to 7 hours.

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