KR20190041729A - Hardfacing method of press die - Google Patents

Abstract

According to the present invention, a surface reinforcing method of a press die comprises: a preheating process of preheating a press die prepared as a base material structure; a laminating process of forming a surface reinforcing layer on the base material structure by providing high-speed tool steel powder while forming a molten pool using a laser on the preheated base material structure; and a post-heat treatment process in which a quenching step and a tempering step are sequentially performed in a state in which the surface reinforcing layer is formed on the base material structure.

Description

{HARDFACING METHOD OF PRESS DIE}

The present invention relates to a surface strengthening method of a press mold, and more particularly to a surface strengthening method of a press mold for press molding of a high strength material such as a high strength steel plate used in automobile, shipbuilding and aviation industries.

Parts used in automobile, shipbuilding and aerospace industries, and press die for manufacturing the same are required to have high mechanical abrasion resistance and strength. In order to process these high-strength parts, high-performance molds have been developed by adding alloying elements for high hardness to existing materials. However, very expensive alloying elements such as molybdenum (Mo), tungsten (W) and vanadium So it is costly to manufacture the mold material.

In order to reduce the cost, the mechanical properties are improved by modifying the surface of the mold or tool by nitriding, carburizing, etc. However, since such a process is performed for a long time in a high vacuum state, the productivity is deteriorated. In addition, although the surface modification method takes a long time, the thickness of the hardened layer on the surface is thin, and the bonding force between the hardened layer and the base material is low and peeled, thereby shortening the service life.

In particular, recently, a super high tensile strength steel sheet is applied to an automobile body in the domestic and overseas automobile market. The mold used for forming the super high tensile strength steel sheet with high strength is easily broken, and the wear rate of the mold is also high. In addition, shear forming such as piercing and trimming can not be performed in press molds with super high strength steel of 1.5 GPa or more, and there is no optimum alternative for strengthening or repairing high-load / high-impact press mold.

On the other hand, high speed tool steel has high hardness and high abrasion resistance, so it is suitable for hard facing. However, when the hard carbon layer is high and the hard layer is formed on the base material, There is a disadvantage that stacking is difficult. In addition, although high-speed tool steels have a high hardness and relatively low toughness, they often have a disadvantage that mold breakage such as chipping frequently occurs.

An object of the present invention is to provide a method of reinforcing a surface of a press mold capable of providing a press mold having improved toughness and hardness by forming a stable surface strengthening layer in a press mold by using high-speed tool steel.

A method of reinforcing a surface of a press mold for an object of the present invention includes a preheating step of preheating a press mold prepared as a base material structure; A laminating step of forming a surface strengthening layer on the base material structure by providing a high-speed tool steel powder while forming a molten pool using a laser on the preheated base material structure; And a post-annealing process for sequentially performing the annealing step and the tempering step in a state where the surface strengthening layer is formed on the base material structure.

In one embodiment, the hardness of the lamination region where the surface-strengthening layer having undergone the post-heat treatment is formed is at least 60 HRc and the total impact absorption energy by Charpy impact test is 2.0 J or more.

In one embodiment, the post-heat treatment process is configured to perform only one tempering step after performing the single etching step after the laminating process is completed.

In one embodiment, the step of post-annealing comprises raising the temperature of the surface-enhanced layer to between 1,000 and 1,100 ° C after lamination; Maintaining an isothermal state at a temperature of 1,000 to 1,100 DEG C; And cooling. At this time, the cooling in the quenching step may be performed by gas cooling at a cooling rate of 500 to 800 ° C / min using a cooling gas, or by air cooling at atmospheric conditions. In addition, the temperature raising rate may be 5 ° C / min in the step of raising the temperature to 1,000-1,100 ° C, and the time of maintaining the temperature at isothermal temperature may be one hour.

In one embodiment, the step of tempering the post-annealing process comprises heating to 500 to 600 ° C after cooling in a quenching step; Maintaining an isothermal state at a temperature of 500 to 600 DEG C; And furnace cooling.

In one embodiment, the parent material structure is formed of carbon steel for mechanical structure of AISI 1045 (JIS S45C) or AISI D2 (SKD11), and the high speed tool steel powder may be AISI M2 or AISI M4.

In one embodiment, the base material structure is AISI D2 (SKD11), the high speed tool steel powder is AISI M4, the isothermal state is maintained after the temperature is increased to 1,050 占 폚 in the quenching step, the temperature is raised to 550 占 폚 in the tempering step When the isothermal state is maintained, the hardness of the lamination region where the surface strengthening layer is formed is at least 60.3 HRc and the total impact absorption energy is 2.8 J. At this time, the preheating process may be performed at 300 ° C to 500 ° C.

In one embodiment, the preheating process may be performed at a temperature of at least 300 ° C but less than 500 ° C.

In one embodiment, when the preheating process is performed at a temperature of less than 300 ° C, cracks may be generated in the surface-strengthening layer formed by the laminating process, cracks may develop in the entire interface, and the preheating process may be performed at a temperature higher than 500 ° C Pores are formed in the surface strengthening layer formed by the laminating step due to excessive heat input. When the preheating process is performed at 300 to 500 ° C, generation of cracks, development of cracks and formation of pores in the surface strengthening layer are prevented do.

The press mold according to the present invention is characterized in that the surface is strengthened according to the method described above.

According to the surface strengthening method of the press mold of the present invention described above, it is possible to provide a press mold having improved toughness characteristics without lowering the hardness by forming a stable surface strengthening layer in the press mold using the high-speed tool steel. In particular, although hardness is generally reduced due to relaxation of compressive residual stress by a heat treatment process, in the present invention, a post-heat treatment process is performed through a quenching-tempering process to improve toughness, There is an advantage to be able to perform.

1 is a view for explaining a laminating step in a surface strengthening method of a press die according to an embodiment of the present invention.
FIG. 2 is a view for explaining a post-heat treatment process in a surface strengthening method of a press mold according to an embodiment of the present invention.
3 is a graph showing the results of analysis of the interfacial characteristics of Comparative Sample 1 and Samples 1 to 5, respectively.
4 is a graph showing the results of evaluating the hardness characteristics of Sample 1 and Comparative Samples 2 to 4.
Figs. 5 to 7 are diagrams showing the results of evaluating the wear resistance characteristics of Sample 1 and Comparative Samples 2 to 4. Fig.
Figs. 8 and 9 are views showing results of evaluation of toughness characteristics of Sample 1 and Comparative Samples 2 to 4. Fig.
FIG. 10 is a graph comparing the fracture profile after tempering immediately after the lamination process without the preheating process and the fracture profile when the preheating process, the lamination process and the post-heat treatment process are performed at 300 ° C for the ultra-high strength steel cold press die And FIG.

Hereinafter, the present invention will be described. The terminology used in describing the present invention in this application is used only to describe a specific embodiment and is not intended to limit the invention. The singular expressions include plural expressions unless the context clearly dictates otherwise. In the present application, the term " comprises " or " having ", etc. is intended to specify that there is a feature, step, operation, element, part or combination thereof described in the specification, , &Quot; an ", " an ", " an "

Unless defined otherwise, all terms used herein, including technical or scientific terms, have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Terms such as those defined in commonly used dictionaries are to be interpreted as having a meaning consistent with the contextual meaning of the related art and are to be interpreted as either ideal or overly formal in the sense of the present application Do not.

A surface strengthening method for strengthening the surface of a press mold according to the present invention includes a preheating process for preheating a prepared base material structure, a lamination process for forming a surface strengthening layer on the preheated base material structure, And a post-annealing process for performing annealing.

The base material structure used in the present invention is a press die to be subjected to surface strengthening, and the material forming the base material structure is not particularly limited, but the base material structure in the present invention is formed of ordinary carbon steel for mechanical structure used as a base material for mechanical structure . As the carbon steel for machine structure, AISI 1045 (JIS S45C), AISI D2 (SKD11) and the like can be used. In one embodiment, AISI D2 may be used as the base metal structure of a press mold capable of processing an ultra high strength steel sheet.

In the step of preparing the base material for the surface strengthening, when the base material structure is formed of ordinary carbon steel for mechanical structure, the base material structure can be processed so that the surface strengthening layer can be formed at the portion to be reinforced in the press die. For example, machining such as cutting can be performed on the edge portion of the base material structure.

Hereinafter, the surface strengthening method for strengthening the surface of the prepared press mold will be described step by step.

(1) Preheating process

First, a preheating process is performed as a preprocessing step for performing a lamination process on a base material, which is a prepared press mold. The preheating process is preferably carried out at a temperature of at least 300 캜, but not exceeding 500 캜.

In the laminating process performed after the preheating process, a process of applying energy locally using a direct laser to the base material structure is continuously or discontinuously performed several times. At this time, a laser is applied to form a melting pool in the deposition region of the base material structure, and the high speed tool steel powder is also melted and rapidly solidified together with a part of the base material structure. During the laser application, Immediately after rapid heating and laser passing, rapid cooling occurs. Among these processes, cracks are generated by the difference of the thermal expansion coefficient between the base material and the different materials of the high-speed tool steel and the occurrence of thermal stress due to the rapid solidification. In particular, a larger crack is generated at the interface of the lamination end, To delamination of the laminate portion as a whole advances to the interface, which may result in deterioration of mechanical strength and toughness. However, as in the present invention, the temperature of the base material structure is preheated to about 300 ° C to 500 ° C before the laminating process using the high-speed tool steel, so that the cooling rate in the cooling step is minimized even when the laser passes through the cooling step, It is possible to prevent defects from being generated.

In the case where the preheating process is not carried out, as described above, defects are generated in the entire stacking region, and the preheating process is performed at a temperature of less than 300 ° C, the occurrence of cracks is reduced However, if even a small number of cracks are present, it is a problem because the crack progresses to the entire interface. Therefore, the preheating process should be performed at least 300 ° C or more. On the other hand, when the preheating process is performed at a temperature higher than 500 ° C, the excessive heating of the base material causes excessive melting and cooling of the base material structure. At the same time, the inert gas generated during the process does not escape during cooling, . These pores may also cause crack initiation and propagation, lowering the bond strength and toughness at the lamination interface, thereby lowering the overall mechanical properties, so that it is preferable to perform the preheating process at 500 ° C or lower.

(2) Laminating process

The preheated base material structure is subjected to a laminating process in which a laser is directly irradiated and melted to form a surface strengthening layer using a high-speed tool steel. As the preheated base material structure is irradiated with a laser to form a melt pool, the high speed tool steel powder is supplied together with the laser powder to be supplied to the melt pool, so that the high speed tool steel melt is mixed with the molten pool and molten state of the base material structure, The surface hardened layer having a dense structure is formed as the portion is rapidly solidified. The laminating process will be described in more detail with reference to FIGS. 1 and 2. FIG.

1 is a view for explaining a laminating step in a surface strengthening method of a press die according to an embodiment of the present invention.

Referring to FIG. 1 (a), the apparatus used in the laminating process irradiates the laser beam directly onto the surface of the base material structure to form a molten pool in the base material structure, while supplying the high-speed tool steel powder to form a high- As shown in FIG. At this time, a powder gas, a coaxial gas and a shielding gas may be supplied together to prevent the transfer and oxidation of the high-speed tool steel powder. The powder gas and the coaxial gas may use argon (Ar), respectively. In this case, the laser power may be 700 to 900 W.

Referring to FIG. 1 (b), since the laser is a point energy source for locally supplying energy, the laser beam is scanned in order to supply energy to the entire surface having a predetermined area. The scanning direction of the laser beam may be a first direction. When scanning is completed in the first direction, the laser beam is moved in a second direction perpendicular to the first direction, and then the laser beam is moved in the first direction or the opposite direction to the first direction A second scanning can be performed. Such scanning may be repeated several times over the area where the surface enhancement layer should be formed in the parent material structure, and the next scanning may be performed to overlap the previous scanning trajectory. In addition, after the scanning in the first direction is completed, the scanning is performed in the second direction, the scanning is performed in the second direction or the reverse direction in the second direction by moving the laser beam in the first direction after the previous scanning, A layer-by-layer lamination process forms a surface strengthening layer on the surface of the base material structure. Therefore, the thickness of the press mold after the surface strengthening layer is formed in the lamination region becomes thicker than the thickness of the base material structure where no surface strengthening layer is formed. By way of example, seven layers can constitute a surface enhancement layer by layer by layer.

As the high-speed tool steel powder used in the present invention, a molybdenum-based high-speed tool steel, AISI M4 powder or M2 powder can be used. At this time, the average diameter of the particles constituting the high-speed tool steel powder may be from 100 mu m to 150 mu m.

The surface strengthening layer may be formed on the surface of the base material structure through the lamination process as described above, and the thickness of the surface enhancement layer may be 10% to 30% of the thickness of the base material structure. When the thickness of the surface strengthening layer is less than 10% of the thickness of the parent material structure, mechanical properties of the parent material structure hardly improve. When the thickness of the surface strengthening layer is more than 30%, the exposure time to the laser in the laminating process becomes long, There is a problem that the economic efficiency is low and the bonding strength with the base material structure is also deteriorated.

(3) Post-heat treatment process

After the surface strengthening layer is formed on the base material structure through the preheating process and the laminating process as described above, the post heat treatment process is performed. The post heat treatment process can prevent the decrease in hardness as a quenching-tempering process, so that the press mold can be surface-strengthened so as to secure both hardness and toughness. Typically, the toughness can be improved by heat treatment performed subsequent to the step of laminating the high-speed tool steel to the base material substrate. At the same time, the heat treatment causes a change in the component of the high-speed tool steel, It is inevitable. However, in the present invention, it is possible to prevent the hardness from being lowered by constituting the post-heat treatment step performed subsequent to the laminating step with a single etching step and a single tempering step, thereby ultimately improving not only toughness but also high hardness There is an advantage that it can be secured. The hardness of the laminated region where the surface strengthening layer is formed by one step and one tempering step may be at least 60 HRc and the total impact absorbing energy may be 2.0 J or more. Such a post-heat treatment process will be described later with reference to Fig.

FIG. 2 is a view for explaining a post-heat treatment process in a surface strengthening method of a press mold according to an embodiment of the present invention.

Referring to FIG. 2, the post-annealing process performed after the laminating process can be largely divided into a machining step and a tempering step, and one tempering step is performed after one machining step. The hardness is lowered due to a change in the composition of the high-speed tool steel due to the long heat treatment time even if the respective machining step and the tempering step are performed twice or more, or even if the work is performed twice or more in a machining-tempering process cycle. Therefore, the post-heat treatment process performed after the laminating process is limited to one cycle process in which one tempering step is performed after one crystallization step.

Specifically, the temperature of the base material structure itself in which the high-speed tool steel is laminated after the laminating process is raised from 1,000 ° C to 1,100 ° C, and the cooling is performed after being left at a high temperature for a predetermined time. At this time, the step of raising the temperature from 1,000 占 폚 to 1,100 占 폚 after the laminating step is gradually performed at a very low temperature raising rate of 5 占 폚 / min. It can be kept in an isothermal state for about 1 hour at a high temperature of 1,000 to 1,100 ° C. After the isothermal state is maintained, the cooling step can be terminated. Cooling of the quenching step can be performed by air cooling which causes the temperature to drop in the atmosphere and gas cooling using cooling gas. In the case of air cooling, the cooling rate may be 400 ° C / min and may be performed for 60 minutes or more. In gas cooling using a cooling gas such as nitrogen, the cooling rate may be 500 ° C / min to 800 ° C / min. Cooling of the quenching step can minimize the presence of retained austenite when quenching conditions such as gas cooling are used rather than air cooling, which is a desirable condition for ensuring toughness without lowering hardness.

After the heating step, the temperature is raised from 500 ° C to 600 ° C as a tempering step, the isothermal temperature is maintained for a predetermined time, and cooling is performed to finally complete the post-annealing process according to the present invention. The cooling of the tempering step is carried out by furnace cooling to cool slowly in the furnace. The cooling in the tempering step is allowed to cool slowly, unlike the cooling in the quenching step, so that the surface strengthening layer can be stably formed.

 Carrying out the quenching step leads to the formation of martensite with spherical carbide and can produce retained austenite. Through the tempering step, the toughness of the surface strengthening layer can be imparted as the martensite produced in the quenching step becomes tempered martensite.

In the case where only tempering is performed subsequent to the laminating step, the presence of tempered martensite through tempering, i.e., softening of martensite, increases the toughness and simultaneously removes carbon from the martensite to reduce the content of martensite Therefore, the hardness is inevitably reduced. However, according to the present invention, only one cycle of the casting step and the one of the tempering steps are sequentially performed one time, so that the martensite, carbide and retained austenite produced as a result of the quenching are subjected to the tempering step, Partially become martenite and some remain as retained austenite, resulting in a secondary hardening which results in a structure comprising tempered martenite, fine carbide precipitates and retained austenite. Thus, in the present invention, it is possible to produce a surface-hardened layer which does not deteriorate in hardness while securing toughness.

Hereinafter, the hardness and toughness securing effect of the sample prepared according to the present invention will be confirmed through production of an actual sample and evaluation of the characteristics of the prepared sample.

Conditions for sample preparation

AISI D2 (purchased from Carpenter) having a thickness of 10 mm (100 mm × 50 mm) was used as a base material substrate, and AISI M4 powder having an average particle diameter of 120 μm was used as a high-speed tool steel powder. AISI D2 and AISI M4 powder are shown in Table 1 below. In Table 1, the unit of each component is% by weight.

ingredient C Mn Si Cr Ni Mo W V Cu P S D2 1.56 0.25 0.24 11.31 0.18 0.83 - 0.25 - 0.03 0.001 M4 1.38 0.33 0.39 4.38 0.15 4.66 5.61 4.25 0.11 0.024 0.024

As shown in FIG. 1 (a), 0.5 mm, which is 50% of a single track width (1.0 mm), is superimposed for an area layer, and is line-by- Direction, and the single layer had a height of about 0.25 mm, and the seven layers were stacked in a layer-by-layer manner while crossing in the orthogonal direction. It was confirmed that the finally obtained laminate structure had a three-dimensional size of 10 mm × 20 mm × 1.5 mm on the base material substrate. Process conditions for irradiating the laser beam were as shown in Table 2, the laser power was fixed at 800 W, and argon gas was used as the powder gas and the coaxial gas, respectively.

Preparation of Sample 1

After the pre-heating temperature of the base material substrate is set to 300 ° C, a laminating process using the laser as described above is performed, and then a post-annealing process is performed through time and temperature conditions as shown in FIG. 2, Sample 1 was prepared. That is, the temperature was raised to 1,050 ° C at a heating rate of 5 ° C / min in a quenching step, the isothermal state was maintained at 1,050 ° C for 1 hour, and the cooling rate was 700 ° C / min. After the completion of the quenching step, the temperature was raised to 550 ° C at a heating rate of 5 ° C / min for the tempering step. After the isothermal state was maintained at 550 ° C for 1 hour, the quenching step was terminated Respectively.

Preparation of Samples 2 to 5

Samples 2 to 5 were prepared through substantially the same process as the preparation of Sample 1, except that the preheating temperature of the base substrate was 100 ° C, 200 ° C, 400 ° C and 500 ° C, respectively.

Preparation of Comparative Sample 1

A laminating process using a laser was performed immediately without preheating the base material substrate, followed by cooling in the furnace to obtain a comparative sample 1.

Confirmation of interfacial properties of surface strengthening layer according to preheating temperature

The prepared samples were cut and etched for 5 to 10 seconds using Nital (1% nitric acid in ethanol) as an etching solution in order to confirm the interfacial characteristics of each of Comparative Sample 1 and Sample 1 to 5 prepared as described above. FE-SEM (7100F, JEOL, Japan). The results are shown in Fig.

3 is a graph showing the results of analysis of the interfacial characteristics of Comparative Sample 1 and Samples 1 to 5, respectively.

Referring to FIG. 3, it can be seen that defects have occurred in Comparative Sample 1, which is a non-preheated sample, and Sample 2, which was prepared after preheating at 100 ° C. This can be attributed to the thermal stress caused by the difference between the thermal expansion coefficient (11.6 × 10 ^-6 / ° C) of the base substrate and the thermal expansion coefficient (9.5 × 10 ^-6 / ° C) of the laminated material. In other words, it can be seen that the shrinkage movement in the cooling process occurring in the lamination region is generated while being restrained by the base substrate, and in particular, induces cracking at the interface of the lamination end. Particularly, in the case of the comparative sample 1 in which the preheating is not performed, almost all the areas are corroded. In other words, it is not stably stacked, and thus the etchant penetrates into the cracks and is seriously damaged.

Sample 3 prepared by preheating at 200 占 폚 shows a remarkable reduction in cracking as compared with Sample 2, but since a slight crack may also have a large influence by crack propagation, it is preferable to preheat at least 300 占 폚 desirable.

Preparation of Comparative Samples 2 to 4

Comparative sample 2 (q-t D2) was prepared by performing substantially the same process as the post-heat treatment process in the production process of sample 1 without lamination of M4 on the base material substrate.

Comparative Example 3 (M4) was prepared by cooling to room temperature immediately after the laminating step without performing the post-heat treatment step.

Comparative sample 4 (t M4) was prepared by omitting the quenching step after the preheating process and the laminating process and performing only the tempering step by cooling in the furnace.

Characteristic evaluation-1 and result: hardness

For the evaluation of hardness characteristics, a depth-sensing indentation test was performed using a HR-521 digital Rockwell hardness tester (AKASHI, Japan). A load of 147.1 N was applied for 9 seconds according to ASTM Standard Test Method E18. The results are shown in Fig.

4 is a graph showing the results of evaluating the hardness characteristics of Sample 1 and Comparative Samples 2 to 4.

In FIG. 4, qt M4 represents Sample 1, and the hardness (unit HRc) of Sample 1 and Comparative Sample 2 to 4 is compared. As a result, when M4 is deposited regardless of whether post heat treatment is performed or not, 2 (qt D2). According to a conventional research report, it has been reported that the high hardness is mainly caused by the presence of martensite, and the secondary hardening is reported to occur by carbide precipitated in the martensite structure, Can be expected to be due to the rapid cooling rate and the martensite formed by the precipitated carbide.

Namely, carbide and tempered martensite are formed in the deposition region because repetitive reheating takes place by the post-annealing process of etching and tempering, and in the case of Sample 1 by the secondary curing mechanism of the precipitated carbide in the martensite structure It can be seen that it has high hardness. Carbides of high speed tool steels M4 are mainly present in the form of hard and stable MCs (e.g., vanadium carbide (VC)), M ₆ C and M ₂ C (e.g. molybdenum carbide (Mo ₂ C) Based on the characteristics of the carbide precipitated in the tool steel and the known studies, the carbide is vanadium-rich MC carbide, molybdenum-rich M ₆ C / M ₂ C carbide and chrome-rich M ₇ C ₃ / M ₂₃ C ₆ carbide. The carbide precipitated at low temperature becomes a fine precipitate. As a result, it can be considered that the softening of the martensite is reduced by the secondary hardening by brazing and tempering, and the hardness is prevented from being lowered.

In FIG. 4, both Sample 1 and Comparative Sample 4 (t M4) according to the present invention show higher hardness than Comparative Sample 2 (q-t D2), but lower than Comparative Sample 3 (M4).

In the conventional heat treatment process, the presence of tempered martensite and retained austenite causes a decrease in hardness, and even when most of the formed martensite is transformed into tempered martensite, the hardness decreases. During the normal tempering process, the carbon in the martensite can be trapped and precipitated as cementite or carbide during the tempering process. Although various types of carbides are precipitated, it is difficult to compensate for the decrease in hardness due to the decrease in the content of martensite due to the loss of carbon atoms in the presence of such carbides. To solve this problem, it can be considered to perform a long tempering process at a high temperature, but such a long annealing process rather reduces the residual stress caused during the deposition, thereby decreasing the hardness.

However, it can be seen that the hardness of the sample 1 according to the present invention shows a higher hardness than the comparative sample 2 (qt D2) and maintains a hardness similar to that of Comparative Sample 4 (t M4) and Comparative Sample 3 (M4) have. That is, the sample 1 obtained by carrying out the process according to the present invention has an advantage that toughness can be improved and hardness can be secured by preventing a decrease in hardness which is in a trade-off relationship with an increase in toughness can confirm.

Characteristic evaluation-2 and result: Abrasion resistance

A ball-on-disk wear tester (R & B Corp., Korea) was used to evaluate the wear characteristics. The balls were set to rotate at the top surface of the sample for 10 minutes under conditions of a load of 147.1 N (15 kgf) and a rotational speed of 10.49 rad / s (100 rpm). The width and depth of the wear track were measured using an Alpha Step Stylus Profiling System (Dektak XT Series; Bruker, USA) as an atomic force microscope (AFM). The results are shown in Figs. 6 to 9. Fig.

Figs. 5 to 7 are diagrams showing the results of evaluating the wear resistance characteristics of Sample 1 and Comparative Samples 2 to 4. Fig.

In FIG. 5, (a) shows the comparison sample 2 (qt D2), (b) shows the comparison sample 3 (M4) FIG. 6 is a photograph showing the wear profile after the disk wear test, FIG. 6 is a profile showing the wear profile, and FIG. 7 is the depth of wear after the ball-on-disk wear test.

5 to 7, it can be seen that the comparative sample 2 (qt D2) exhibits the greatest volume loss and that Sample 1 (qt M4) has less wear than the Comparative Sample 2 (qt D2) have.

Characteristic evaluation-3 and result: Toughness

In order to evaluate the toughness, which is resistant to breakage, a Charpy impact test was performed. The cracks in the high-pressure press die begin at the cavity surface and propagate inside the mold, so the V-notch was cut in the lamination region of the sample, taking into consideration the crack initiation and propagation characteristics in the high-pressure tool. Impact energy, impact velocity and collision angle were set at 50J, 3.8m / s and 150 각각, respectively, using Charpy Impactor at room temperature. The impact absorption energy was calculated after the impact test, and the shape of the fracture surface was confirmed using FE-SEM. The results are shown in Fig. 8 and Fig.

Figs. 8 and 9 are views showing results of evaluation of toughness characteristics of Sample 1 and Comparative Samples 2 to 4. Fig.

FIG. 8 shows the shape of the fracture surface. FIG. 9 shows the total absorbed energy as a result of the Charpy impact test. Referring to FIGS. 8 and 9, since there is no large plastic deformation in the deposition region, It can be seen that the total shock absorbed energy in case of quenching-tempered is the highest as 2.8 J. It can be confirmed that this shows a remarkably increased toughness characteristic as compared with the case of the other Comparative Samples 1 to 3, which does not exceed 2.0 J at the maximum of 1.79 J. In particular, it can be confirmed that the toughness is increased by 70% as compared with the case without post-heat treatment.

Preparation and Characterization of Comparative Sample 5

Although not shown in the results of FIGS. 5 and 9, the hardness and toughness evaluation was performed on the comparative sample 5 obtained by performing substantially the same process as the production of the sample 1, except that the tempering process was omitted. As a result, For Sample 5, the hardness (HRc) was found to be higher than that of Comparative Samples 2 to 4 and Sample 1 according to the present invention, and in particular, significantly higher than Comparative Sample 4, and the Charpy impact It can be confirmed that the total impact absorbed energy by the test is lower than that of the comparative sample 4. [ That is, in comparative sample 5 obtained only through the preheating process, the laminating process, and the weaving process, although it has a very high hardness, it is confirmed that it is easily broken even in a small impact due to a very low toughness characteristic. The preheating step, the laminating step and the post-annealing step are both performed, and both the annealing step and the tempering step are essential in the post-annealing step, so that the object of the present invention can be achieved.

Confirm the fracture plane

FIG. 10 is a graph comparing the fracture profile after tempering immediately after the lamination process without the preheating process and the fracture profile when the preheating process, the lamination process and the post-heat treatment process are performed at 300 ° C for the ultra-high strength steel cold press die And FIG.

10 (a) shows a case where tempering is performed immediately after a laminating process without a preheating process, and FIG. 10 (b) shows a case where a preheating process, a laminating process, and a post-heat treatment process are performed at 300 ° C .

Referring to FIG. 10, it can be seen that chipping and scratches occur and burrs are present in the case of (a), but in the case of (b) according to the present invention, the preheating process and the post- It can be confirmed that the surface strengthening is performed in a stable state in which burr occurrence is minimized.

Preparation of Sample 6 and Comparative Sample 6

Sample 6 according to the present invention was obtained through substantially the same process as that of Sample 1 except that the cooling in the quenching step was carried out for 60 minutes by air cooling.

Also, Comparative Sample 6 was obtained through substantially the same process as Sample 6 except that the post-heat treatment process after the preheating process lamination process was carried out only without the tempering step.

Evaluation of Hardness and Toughness Properties of Sample 6 and Comparative Sample 6

The same experiment as that for evaluating the hardness and toughness characteristics of Sample 1 was performed for each of Sample 6 and Comparative Sample 6.

As a result, the hardness of Sample 6 was 58.23 HRc, the impact absorption energy was 1.38 J, and in Comparative Sample 6, the hardness was 64.76 HRc and the impact absorption energy was 1.27 J.

Compared with the result of Sample 1, the shock absorption energy of Sample 6 shows a low value, so that it is more advantageous to exhibit high toughness characteristics than gas cooling in the quenching step.

Although it is confirmed that the toughness of Sample 6 is lower than that of Comparative Samples 2 and 3, but higher than that of Comparative Sample 6, the toughness characteristic is increased after the quenching step as a post heat treatment step It can be seen that either toughening or tempering can not optimize both toughness and hardness. That is, as in the present invention, after the preheating step and the laminating step, the etching step and the tempering step are sequentially performed one time, and then the post-annealing step is required to optimize both the toughness and the hardness.

It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the present invention as defined by the following claims. It can be understood that it is possible.

Claims (13)

A preheating step of preheating a press mold prepared as a base material structure;
A laminating step of forming a surface strengthening layer on the base material structure by providing a high-speed tool steel powder while forming a molten pool using a laser on the preheated base material structure; And
And a post-heat treatment step of sequentially performing a quenching step and a tempering step in a state that the surface strengthening layer is formed on the base material structure,
A method of strengthening the surface of a press die.
The method according to claim 1,
The hardness of the lamination region where the surface strengthening layer after the post-heat treatment process is formed is at least 60 HRc or more,
Characterized in that the total impact absorption energy by the Charpy impact test is 2.0 J or more.
A method of strengthening the surface of a press die.
The method according to claim 1,
The post-heat treatment process
Characterized in that after the laminating step is completed, only one tempering step is performed after performing the single quenching step.
A method of strengthening the surface of a press die.
The method according to claim 1,
The step of performing the post-heat treatment process includes
Raising the temperature of the surface strengthening layer to 1,000 to 1,100 ° C after lamination;
Maintaining an isothermal state at a temperature of 1,000 to 1,100 DEG C; And
Characterized in that it comprises a step of cooling
A method of strengthening the surface of a press die.
5. The method of claim 4,
The cooling of the step
Characterized in that it is cooled by gas cooling at a cooling rate of 500 to 800 DEG C / min using a cooling gas or by air cooling at atmospheric conditions.
A method of strengthening the surface of a press die.
5. The method of claim 4,
In the step of raising the temperature to 1,000 to 1,100 ° C, the temperature raising rate is 5 ° C /
Lt; RTI ID = 0.0 > 1 < / RTI > hour,
A method of strengthening the surface of a press die.
The method according to claim 1,
The tempering step of the post heat treatment step
Raising the temperature to 500 to 600 ° C after cooling in a quenching step;
Maintaining an isothermal state at a temperature of 500 to 600 DEG C; And
Characterized in that it comprises furnace cooling.
A method of strengthening the surface of a press die.
The method according to claim 1,
The base material structure is formed of carbon steel for mechanical structure of AISI 1045 (JIS S45C) or AISI D2 (SKD11)
Characterized in that the high-speed tool steel powder is AISI M2 or AISI M4.
A method of strengthening the surface of a press die.
The method according to claim 1,
AISI D2 (SKD11) was used as the base material, AISI M4 was used as the high-speed tool steel powder, the isothermal state was maintained after the temperature was increased to 1,050 ° C. in the quenching step, the temperature was increased to 550 ° C. in the tempering step, Occation,
Characterized in that the hardness of the lamination region in which the surface-strengthening layer is formed is at least 60.3 HRc and the total impact absorption energy is 2.8 J,
A method of strengthening the surface of a press die.
10. The method of claim 9,
Characterized in that the preheating step is carried out at 300 ° C to 500 ° C.
A method of strengthening the surface of a press die.
The method according to claim 1,
Characterized in that the preheating step is carried out at a temperature of at least 300 [deg.] C but less than 500 [
A method of strengthening the surface of a press die.
The method according to claim 1,
When the preheating process is performed at a temperature of less than 300 ° C, cracks may be generated in the surface strengthening layer formed by the laminating process, cracks may develop in the entire interface,
When the preheating process is performed at a temperature higher than 500 ° C., pores are formed in the surface strengthening layer formed by the laminating process due to excessive heat input,
Characterized in that when the preheating step is carried out at 300 ° C to 500 ° C, cracks are generated in the surface strengthening layer, crack progression and pore formation are prevented.
A method of strengthening the surface of a press die.
A press mold, characterized in that the surface is reinforced by the method according to any one of the claims 1 to 12.

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