KR102707116B1 - Method for heat treatment of high speed steel and high speed steel - Google Patents

Abstract

The present invention relates to a heat treatment method for high-speed tool steel and to high-speed tool steel, comprising a high-temperature homogenization step of heating high-speed tool steel to 1,240 to 1,290°C and then maintaining the temperature for a predetermined period of time, a first cooling step of cooling the homogenized high-speed tool steel to room temperature, an austenizing step of heating the high-speed tool steel at room temperature to a temperature higher than the A1 transformation temperature and then maintaining the temperature for a predetermined period of time, a second cooling step of cooling the austenized high-speed tool steel to room temperature, and a low-temperature tempering step of heating the high-speed tool steel at room temperature to a predetermined temperature and then maintaining the temperature for a predetermined period of time.

Description

METHOD FOR HEAT TREATMENT OF HIGH SPEED STEEL AND HIGH SPEED STEEL

The present invention relates to a method for heat treatment of high-speed tool steel, and preferably to a method for heat treatment of high-speed tool steel including high-temperature homogenization and air cooling.

High-speed tool steel is a material with excellent wear resistance, and is widely used for various purposes such as high-speed cutting, cold forging tools, and hot forging tools. The most fundamental reason why high-speed tool steel is used in a wide range of areas is that it contains many alloying elements such as W, Mo, V, and Cr that can precipitate carbides (MC/M ₂ C/M ₆ C) that can secure high hardness and wear resistance.

However, high-speed tool steels, although they have high durability, also have the disadvantage of weak toughness due to uneven distribution, such as segregation of carbides distributed within the structure. In particular, in the case of high-speed tool steels produced by casting and forging, coarse carbides such as MC and M ₆ C, which are primary carbides generated during the solidification process, coarse carbides exist as agglomerates, and these carbides continue to exist as coarse carbides even after quenching. In addition, since the degree of segregation is very large, there is inevitably a disadvantage of weak toughness.

To compensate for these shortcomings, Korean Patent No. 1994-038977 introduces a method for manufacturing high-speed tool steel with improved toughness by applying a spray molding method, and Korean Patent Publication No. 10-2013-0052103 introduces high-speed tool steel with improved productivity by controlling the matrix solid solution of carbides. Korean Patent No. 10-1319695 introduces high-speed tool steel with improved toughness by controlling the degree of fine precipitation and distribution of carbides. In addition, many documents, such as Korean Patent Publication No. 10-2014-0087279 and Korean Patent No. 10-2072606, describe methods for manufacturing ultra-high strength high-speed tool steel with excellent impact toughness. However, the above inventions have specificity in the heat treatment method during the composition and manufacturing process to improve the impact toughness of high-speed tool steel, and therefore, it is difficult to apply them to high-speed tool steel parts and products manufactured with a conventional composition range and method.

Accordingly, in the present invention, in order to improve the impact toughness of the manufactured high-speed tool steel, a heat treatment method capable of improving the segregation of carbides distributed within the structure of the raw material is developed for high-speed tool steel produced in a large size by a traditional method (casting and forging), thereby introducing a high-speed tool steel material with improved toughness.

Republic of Korea Patent No. 1994-038977 (May 13, 1998) Republic of Korea Publication Patent No. 10-2013-0052103 (May 23, 2012) Republic of Korea Patent No. 10-1319695 (October 11, 2013) Republic of Korea Patent Publication No. 10-2014-0087279 (July 9, 2014) Republic of Korea Patent Publication No. 10-2072606 (January 26, 2020)

Evaluation of Charpy Impact Test Performance for Advanced High-Strength Steel Sheets Based on a Damage Model, Ning Ma, Material behavior and formability: T. Kuwabara Published: 12 June 2010

In order to solve the above problems, the present invention aims to provide a heat treatment method for high-speed tool steel including a high-temperature homogenization step, a first cooling step, and a second cooling step.

In addition, the purpose is to provide a heat treatment method for high-speed tool steel, which improves the density of carbides and increases impact toughness by performing quenching while heating high-speed tool steel to a temperature higher than the A1 transformation temperature at which austenite is formed, and adding a low-temperature tempering step maintained for a certain period of time.

One embodiment of the present invention for achieving the above object relates to a heat treatment method for high-speed tool steel, comprising: a high-temperature homogenization step of heating high-speed tool steel to 1,240 to 1,290°C and then maintaining the temperature for a predetermined period of time; a first cooling step of cooling the homogenized high-speed tool steel; an austenizing step of heating the first-cooled high-speed tool steel to an A1 transformation temperature or higher and then maintaining the temperature for a predetermined period of time; a second cooling step of cooling the austenized high-speed tool steel; and a low-temperature tempering step of heating the second-cooled high-speed tool steel to a predetermined temperature and then maintaining the temperature for a predetermined period of time.

In the above embodiment, the first cooling step may include an air cooling step of air cooling the homogenized high-speed tool steel to 850 to 1,000°C; and an air cooling step of cooling the air-cooled high-speed tool steel to room temperature.

In the above embodiment, the blowing cooling step can be performed by injecting an inert gas (N ₂ , Ar, He) or air (Air) into the homogenized high-speed tool steel.

In the above embodiment, the blowing cooling step may cool the homogenized high-speed tool steel at a rate of 3°C/min or more by rotating one or more fans.

In the above embodiment, the second cooling step can be performed by spraying a coolant onto the high-speed tool steel at a constant pressure in a vacuum or inert gas atmosphere.

In the above embodiment, the refrigerant in the second cooling step may be provided as an inert gas.

In the above embodiment, the second cooling step can cool the high-speed tool steel by injecting the coolant into the high-speed tool steel at a pressure of 0.5 MPa or more.

In the above embodiment, the austenizing step can be performed at 1,100 to 1,200°C.

In the above embodiment, the austenizing step may include a first austenizing preheating step of heating high-speed tool steel at room temperature to 550 to 650°C and then maintaining the temperature for a predetermined period of time; a second austenizing preheating step of heating the high-speed tool steel to 800 to 900°C after the first austenizing preheating and then maintaining the temperature for a predetermined period of time; and a third austenizing preheating step of heating the high-speed tool steel to 1,000 to 1,080°C after the second austenizing preheating and then maintaining the temperature for a predetermined period of time.

In the above embodiment, the low-temperature tempering step may be performed by heating the high-speed tool steel at room temperature to 150 to 350°C and then maintaining the temperature for a certain period of time.

In the above embodiment, the heat treatment method of the high-speed tool steel may further include a preheating step of heating to 600 to 700° C. and then maintaining the temperature for a predetermined period of time prior to the high-temperature homogenization step.

In another embodiment of the present invention, the present invention relates to a high-speed tool steel containing, in wt%, C 0.7 to 1.5%, Mn 0.1 to 0.5%, Si 0.1 to 0.5%, Cr 3.5 to 5%, Mo 4.5 to 6.5, W 4.5 to 6.5, V 1.5 to 4.0%, and the remainder Fe and unavoidable impurities, and having an impact toughness value at room temperature of 14.0 J/cm2 or more.

According to the present invention, heat treatment was performed on high-speed tool steel produced by casting and forging methods to improve strength, hardness and impact toughness at room temperature. Specifically, it was confirmed that the distribution of directional and segregated carbides existing in the raw material structure was improved through high-temperature homogenization heat treatment, and that the toughness among the final properties of the steel was improved through low-temperature tempering.

Through this, the mechanical properties of high-speed tool steel manufactured using conventional techniques can be improved, thereby contributing to the improvement of the rigidity and service life of high-speed tool steel.

FIG. 1 is a drawing for explaining a heat treatment method for high-speed tool steel according to an embodiment of the present invention.
FIG. 2 is a photograph of the microstructure of Example 1 in which air cooling was performed according to an embodiment of the present invention.
Figure 3 is a photograph of the microstructure of Comparative Example 1 in which air cooling was not performed.
FIG. 4 is a photograph of the microstructure of high-speed tool steel that has undergone high-temperature homogenization according to an embodiment of the present invention.
FIG. 5 is a photograph of the microstructure of Example 1 according to an embodiment of the present invention.
Figure 6 is a photograph of the microstructure of a cross-section of a high-speed tool steel when low-temperature tempering was performed after the austenitizing step.
Figure 7 is a photograph of the microstructure of a cross-section of a high-speed tool steel when high-temperature tempering was performed after the austenitizing step.
Figure 8 shows the results of an impact toughness comparison experiment of high-speed tool steel subjected to heat treatment according to an embodiment of the present invention and high-speed tool steel subjected to conventional heat treatment.
Figure 9 is a photograph of the microstructure of high-speed tool steel that was heat treated according to Comparative Example 10 of the present invention.

Hereinafter, a heat treatment method for high-speed tool steel according to the present invention will be described in detail. The drawings introduced below are provided as examples so that those skilled in the art can sufficiently convey the idea of the present invention. Therefore, the present invention is not limited to the drawings presented below and may be embodied in other forms, and the drawings presented below may be illustrated in an exaggerated manner to clarify the idea of the present invention. At this time, if there is no other definition in the technical and scientific terms used, they have the meaning commonly understood by those skilled in the art to which this invention belongs, and in the following description and the attached drawings, a description of well-known functions and configurations that may unnecessarily obscure the gist of the present invention will be omitted.

According to one feature of the present invention, the present invention relates to a heat treatment method for high-speed tool steel. Specifically, the present invention can improve the distribution of segregated carbides in high-speed tool steel by performing a high-temperature homogenization step, a first cooling step, an austenitizing step, a second cooling step, and a low-temperature tempering step on the high-speed tool steel. Through this, the present invention can improve the impact toughness of the high-speed tool steel to 14 J/cm2 or more based on the Charpy impact value at room temperature.

In the present invention, high-speed tool steel means high-speed tool steel including various types of carbides such as MC, M ₂ C, M ₆ C, and M ₂₃ C ₆ , and more preferably means high-speed tool steel manufactured by a casting or forging method, but is not limited thereto. In this case, MC is a carbide whose main component is vanadium, M ₂₃ C ₆ is a carbide whose main components are chromium, and M ₆ C and M ₂ C are carbides whose main components are tungsten and molybdenum.

According to an embodiment, the high-speed tool steel may include a plate-shaped high-speed tool steel produced by a casting and forging method, and may include a product produced through an electric slag remelting process after casting, a product produced through a forging process after casting, and a billet, product, and part produced through an annealing process after a forging process.

According to an embodiment, the high-speed tool steel may contain, in wt%, C 0.7 to 1.5%, Mn 0.1 to 0.5%, Si 0.1 to 0.5%, Cr 3.5 to 5%, Mo 4.5 to 6.5, W 4.5 to 6.5, V 1.5 to 4%, and the remainder iron (Fe) and unavoidable impurities.

The above carbon (C) may be included in an amount of 0.7 to 1.5 wt%.

Typically, C is an element that determines mechanical properties, such as inducing precipitation of carbides such as MC, M ₂ C, M ₆ C, and M ₂₃ C ₆ in high-speed tool steels and forming martensite structures during the quenching process.

If the C is included in an amount of less than 0.7 wt%, a sufficient amount of carbide cannot be formed, making it impossible to obtain the target strength and hardness. On the other hand, if the C is included in an amount of more than 1.5 wt%, excessive carbon may remain in the high-speed tool steel to form a retained austenite phase, which may significantly reduce the hardness. For this reason, the C may be included in an amount of 0.7 to 1.5 wt%, more preferably 0.8 to 1.3 wt%.

The above manganese (Mn) may be included in an amount of 0.1 to 0.5 wt% or less.

The above Mn is an austenite stabilizing element, and when it is contained at 0.1 wt% or more, it can form an appropriate level of austenite phase in the high-speed tool steel, thereby improving the impact toughness of the high-speed tool steel. However, when the content of the above Mn is less than 0.1 wt%, the effect is minimal, and when the above Mn exceeds 0.5 wt%, the retained austenite may be excessively increased, which may significantly reduce the hardness.

The above silicon (Si) is contained in an amount of 0.1 to 0.5 wt%.

The above Si can play the role of a deoxidizer in high-speed tool steel. For this reason, if the above Si is included in an amount of 0.1 wt% or more, it can further increase carbide precipitation in the high-speed tool steel, thereby contributing to improving hardness. However, if the content of the above Si is less than 0.1 wt%, the effect is insignificant, and if the above Si exceeds 0.5 wt%, the toughness of the high-speed tool steel can be excessively reduced.

The above chromium (Cr) is contained in an amount of 3.5 to 5 wt%.

The above Cr is an alloying element required to improve the hardenability of high-speed tool steel, and can improve the hardenability by delaying carbide precipitation, especially in the low-temperature tempering and high-temperature tempering stages described later. In addition, it can react with the above C to form carbides in the form of M ₂₃ C ₆ . However, if the above Cr is less than 3.5 wt%, it cannot contribute to improving the hardenability, and if the above Cr exceeds 5 wt%, excessive austenite may be formed and excessively increased, which may significantly reduce the hardness. For this reason, the above Cr may be formed at 3.5 to 5 wt%, more preferably 4 to 4.5 wt%.

The above molybdenum (Mo) is contained in an amount of 4.5 to 6.5 wt%.

The above Mo is a representative carbide-forming element of high-speed tool steel and can react with C to form carbides in the form of M ₆ C. When the above Mo is included in an amount of 4.5 wt% or more, it can form carbides in an appropriate range and contribute to improving strength and hardness. However, when the above Mo is less than 4.5 wt%, a sufficient amount of carbides cannot be formed, making it difficult to obtain the target strength and hardness. On the other hand, when the Mo exceeds 6.5 wt%, the impact toughness of the high-speed tool steel can be significantly reduced. For this reason, the Mo is preferably included in an amount of 4.5 to 6.5 wt%, more preferably 4.7 to 4.8 wt%. 5.5 Weight % may be included.

The above tungsten (W) may be included in an amount of 4.5 to 6.5 wt%.

The above W, like the above Mo, can react with C to form carbides in the form of M ₆ C. However, if the W is less than 4.5 wt%, a sufficient amount of carbides cannot be formed, making it impossible to obtain the target strength and hardness. On the contrary, if the W exceeds 6.5 wt%, the impact toughness of the high-speed tool steel can be significantly reduced. For this reason, the W is preferably 4.5 to 6.5 wt%, more preferably 5.0 to 6.0 wt%. Weight % may be included.

The above vanadium (V) may be included in an amount of 1.5 to 4.0 wt%.

When the above V is included in an amount of 1.5 wt% or more, MC-type carbides can be formed in the high-speed tool steel. However, when the above V exceeds 4.0 wt%, excessive carbides may be generated, which may reduce impact toughness. For this reason, the above V may be included in an amount of 1.5 to 4.0 wt%, more preferably 1.8 to 2.2 wt%.

In addition, Ni, etc. may be added, but are not limited thereto, and the remainder may include iron (Fe) and unavoidable impurities.

The composition of the high-speed tool steel of the present invention has been described above. Below, a heat treatment method of the high-speed tool steel according to an embodiment of the present invention will be described.

FIG. 1 is a drawing for explaining a heat treatment method for high-speed tool steel according to an embodiment of the present invention.

Referring to FIG. 1, a heat treatment method (1000) of high-speed tool steel according to an embodiment of the present invention may include a preheating step (S110) of heating to 600 to 800°C and then maintaining the temperature for a predetermined period of time, a high-temperature homogenization step (S100) of heating high-speed tool steel to 1,240 to 1,290°C and then maintaining the temperature for a predetermined period of time, a first cooling step (S300) of cooling the homogenized high-speed tool steel, an austenizing step (S500) of heating the cooled high-speed tool steel to a temperature higher than the austenizing temperature and then maintaining the temperature for a predetermined period of time, a second cooling step (S700) of cooling the austenized high-speed tool steel to room temperature, and a low-temperature tempering step (S900) of heating high-speed tool steel at room temperature to a predetermined temperature and then maintaining the temperature for a predetermined period of time.

According to an embodiment, the austenizing step (S500), the secondary cooling step (S700), and the low-temperature tempering step (S900) are performed in a vacuum heat treatment furnace to prevent the high-speed tool steel from being oxidized or corroded by reacting with oxygen, moisture, and impurities in the air during the heat treatment process, but is not limited thereto.

According to an embodiment, a preliminary heating step (S110) of heating to 600 to 700°C and then maintaining the temperature for a certain period of time may be optionally performed prior to the high-temperature homogenization step (S100).

According to an embodiment, the preheating step (S110) is a step of maintaining the surface and the interior of the high-speed tool steel to the same temperature for a predetermined period of time before heating to the temperature of the high-temperature homogenization step. In addition, through the preheating step, thermal deformation occurring during heating up to the high-temperature homogenization step can be prevented.

According to an embodiment, the preheating step (S110) may heat the high-speed tool steel to 600 to 700°C and maintain the temperature for 30 to 150 minutes. After the preheating is completed, the high-speed tool steel may be heated at 1 to 10°C/min to the temperature of the high-temperature homogenization step (S100).

According to an embodiment, if the high-temperature homogenization step (S100) is performed below 1,240°C, the carbides are not sufficiently re-dissolved in the matrix, and the carbides are maintained in a state of being distributed in a directionally and segregated form. On the contrary, if the high-temperature homogenization step (S100) is performed above 1,290°C, a liquid phase may be generated at the grain boundary or the grains may become excessively coarse. As a result, the strength and impact toughness of the high-speed tool steel at room temperature may be reduced. For this reason, the high-temperature homogenization step (S100) may be performed at 1,240 to 1,290°C, more preferably 1,250 to 1,280°C.

According to an embodiment, the holding time in the high-temperature homogenization step (S100) may vary depending on the thickness of the high-speed tool steel, but if it exceeds 600 minutes, the grains may grow coarsely, which is not preferable. On the other hand, if it is performed for less than 360 minutes, sufficient time is not secured for the carbides to be re-dissolved into the matrix, so that some of the carbides may not be re-dissolved and may remain. For this reason, the high-temperature homogenization step may be performed for 360 to 600 minutes, more preferably 400 to 500 minutes.

The above first cooling step (S300) is a step of cooling the high-speed tool steel homogenized through the above high-temperature homogenization step to room temperature.

According to an embodiment, the first cooling step (S300) may include a blowing cooling step (S310) of cooling the homogenized high-speed tool steel to 850 to 1,000°C and then maintaining the temperature for a predetermined period of time; and an air cooling step (S330) of cooling the blowing-cooled high-speed tool steel to room temperature.

The above-mentioned blowing cooling step (S310) is a cooling method for forcibly cooling the homogenized high-speed tool steel to a temperature below a certain level to prevent carbides re-dissolved in the base from being re-segregated and precipitated at grain boundaries during the cooling process.

The above-described blowing cooling step (S310) can be performed by supplying an inert gas (N ₂ , Ar, He) or air (Air) to the homogenized high-speed tool steel after moving the homogenized high-speed tool steel out of the furnace. At this time, a fan (Pan), an air gun, etc. may be used as a method of injecting the inert gas (N ₂ , Ar, He) or air (Air), but the present invention is not limited thereto, and any previously disclosed method may be applied. In this specification, a method of blowing an inert gas (N ₂ , Ar, He) by rotating one or more fans (Pans) is described, but the present invention is not limited thereto.

Through the above-mentioned blowing cooling step (S310), the present invention can forcibly cool the homogenized high-speed tool steel to 850 to 1,000°C at a cooling rate of 3°C/min or more. If the temperature at which the above-mentioned blowing cooling step (S310) ends is less than 850°C, microstructures such as martensite or bainite in the high-speed tool steel may be excessively formed due to supercooling. As a result, brittleness may increase and impact toughness at room temperature may decrease.

On the other hand, if the temperature at which the above-mentioned air cooling step (S310) ends exceeds 1,000°C, proeutectoid carbide may precipitate during the air cooling step after the air cooling, which may increase brittleness. In addition, the impact toughness at room temperature may be significantly reduced due to the proeutectoid carbide. For this reason, the above-mentioned air cooling step (S310) is preferably performed by forced cooling at 850 to 1,000°C, more preferably at 870 to 970°C, and even more preferably at 890 to 950°C.

In addition, if the cooling rate is less than 3°C/min in the above-described blowing cooling step (S310), the cooling rate is too low, and proeutectoid carbide may precipitate while reaching 850 to 1,000°C, which may increase brittleness. However, if the cooling rate exceeds 5°C/min, it is difficult to expect a significant difference in effect. For this reason, the above-described blowing cooling step (S310) may be cooled at 3°C/min or more, more preferably 3 to 5°C/min.

According to an embodiment, after the above-mentioned blow cooling, the high-speed tool steel moved out of the furnace is air-cooled to remove residual stress in the high-speed tool steel, adjust the microstructure of the high-speed tool steel to an appropriate range, improve the distribution of carbides, and improve the properties of the steel. This is defined as an air-cooling step (S330).

After the first cooling described above, an austenizing step (S500) can be performed in which the high-speed tool steel at room temperature is heated to a temperature higher than the A1 transformation temperature and maintained for an appropriate period of time to form an austenite structure, and a second cooling step (S700) can be performed in which the austenized high-speed tool steel is cooled to room temperature.

According to an embodiment, the second cooling step (S700) may be performed in a vacuum heat treatment furnace, and more preferably, may be performed in a vacuum or inert gas (N ₂ , Ar, He) atmosphere. Through this, the present invention can prevent the high-speed tool steel from being oxidized by reacting with oxygen in the air, etc. in the austenizing step (S500) and the second cooling step (S700).

According to an embodiment, the austenizing step (S500) may be performed by heating to a temperature higher than the A1 transformation temperature at which the austenite phase is formed. At this time, the temperature at which the austenizing step (S500) is performed may vary depending on the composition of the high-speed tool steel, but is preferably performed at 1,100 to 1,200°C. If the austenizing step (S500) is less than 1,100°C, the solid solution diffusion of carbides into the high-speed tool steel matrix is minimal, so that the austenite phase is not properly formed or is formed in an excessively small amount, which may reduce the mechanical properties. On the other hand, if the austenizing step (S500) exceeds 1,200°C, the austenite grain size becomes coarser, so that the final mechanical properties of the steel are poor.

According to an embodiment, the austenizing step (S500) may be performed by maintaining the temperature range described above for 5 to 20 minutes. If the maintaining time in the austenizing step (S500) is less than 5 minutes, the austenite phase may not be sufficiently formed, and the temperatures inside and outside the high-speed tool steel may not be uniform, resulting in thermal deformation. In addition, it is difficult to re-dissolve solid elements inside the high-speed tool steel, making it difficult to secure sufficient strength.

In fact, when the thickness of the high-speed tool steel was 100 mm or more, it was confirmed that the strength and impact toughness at room temperature were reduced for the reasons described above, and that thermal deformation occurred in some test specimens.

On the other hand, if the holding time of the austenizing step (S500) exceeds 20 minutes, the grains of the high-speed tool steel may grow coarsely, thereby reducing the strength and impact toughness. For this reason, the austenizing step (S500) may be performed for 5 to 20 minutes, more preferably 10 to 15 minutes.

According to an embodiment, the austenizing step may be performed by heating the high-speed tool steel at room temperature to 1,100 to 1,200°C at once, or may be performed through one or more preheating steps. For example, the present invention may perform the austenizing step (S500) by dividing it into a first austenizing preheating step of heating to 550 to 650°C and then maintaining the temperature for a predetermined period of time, a second austenizing preheating step of heating to 800 to 900°C after the first austenizing preheating step and then maintaining the temperature for a predetermined period of time, a third austenizing preheating step of heating to 1,000 to 1,080°C after the second austenizing and then maintaining the temperature for a predetermined period of time, and an austenizing step of heating to 1,100 to 1,200°C and then maintaining the temperature for a predetermined period of time. In this specification, the austenitizing preheating step is explained as an example of dividing into three stages, but it is not limited thereto, and it is obvious that the process can be performed by subdividing it into less than or more than three stages.

In an embodiment, when the austenizing preheating step is included, the high-speed tool steel may be maintained for 30 to 150 minutes in each of the austenizing preheating steps. For example, when the austenizing step includes the first austenizing preheating step, the second austenizing preheating step, and the third austenizing preheating step as described above, the high-speed tool steel may be maintained at 550 to 650°C for 30 to 150 minutes, at 800 to 900°C for 30 to 150 minutes, and at 1,000 to 1,080°C for 30 to 150 minutes.

However, in the austenitizing step, if the holding time exceeds 20 minutes, the grains of the high-speed tool steel may grow coarsely, thereby reducing the strength and impact toughness. For this reason, the austenitizing preheating step may be performed for 30 to 150 minutes, and the austenitizing step may be performed for 5 to 20 minutes, more preferably 10 to 15 minutes.

Through this, the present invention can prevent thermal deformation in the austenitizing step and gradually increase the temperature to eliminate the internal and external temperature gradient of the high-speed tool steel, thereby preventing grains from coarsening.

The above second cooling step (S700) is a step of cooling the austenitized high-speed tool steel to room temperature to transform the austenite phase formed through the austenizing step into a bainite or martensite phase.

According to an embodiment, the second cooling step (S700) may be performed by spraying a coolant at a constant pressure onto the high-speed tool steel in a vacuum or inert gas (N ₂ , Ar, He) atmosphere. At this time, the coolant may be provided as an inert gas (N ₂ , Ar, He), but is not limited thereto, and any previously disclosed coolant may be used.

According to an embodiment, the second cooling step (S700) can be performed by spraying the refrigerant at a pressure of 0.5 MPa or more.

The above low-temperature tempering step (S900) is a step of heating high-speed tool steel at room temperature to a temperature relatively lower than the normal tempering temperature and then maintaining it to precipitate finely divided carbides.

Tempering is usually performed at 400 to 600°C. In this case, there is an advantage in that carbides can be precipitated quickly, but there is a problem in that the carbides become coarse and are unevenly distributed, such as segregated in some areas.

To improve this, the present invention performs tempering for a sufficiently long time at a temperature of 150 to 350°C, which is relatively lower than conventional tempering, thereby making it possible to atomize carbides and evenly disperse the carbides within the structure.

According to an embodiment, the low-temperature tempering may be performed at 150 to 350°C. If the temperature of the low-temperature tempering is less than 150°C, the residual austenite phase in the high-speed tool steel may not sufficiently transform into a martensite phase, so that hardness may not be secured. On the contrary, if the temperature of the low-temperature tempering exceeds 350°C, the bainite phase and the martensite phase may be excessively formed, so that the impact toughness at room temperature may rapidly decrease. For this reason, the low-temperature tempering may be performed at 150 to 350°C, more preferably 180 to 280°C, and even more preferably 200 to 250°C.

According to an embodiment, the low-temperature tempering may be performed by heating to 150 to 350°C and then maintaining the temperature for 150 to 200 minutes. If the low-temperature tempering time is less than 150 minutes, the effect realized by the low-temperature tempering is reduced, and a sufficient amount of bainite phase and martensite phase cannot be secured. On the contrary, if the low-temperature tempering exceeds 200 minutes, the impact toughness may be reduced due to excessive tempering, and the productivity may be reduced. For this reason, the low-temperature tempering may be performed for 150 to 200 minutes, more preferably 170 to 190 minutes, and even more preferably 175 to 185 minutes.

Although not disclosed in the drawings, the present invention may perform a high-temperature tempering step of heating the high-speed tool steel to 500 to 600°C and then maintaining the temperature for a predetermined period of time at least once after the low-temperature tempering step (S900). More preferably, after the low-temperature tempering, a second tempering step of heating to 540 to 560°C, maintaining the temperature for 175 to 185 minutes, and cooling to room temperature may be performed, and after the second tempering, a third tempering step of heating the high-speed tool steel at room temperature again to 540 to 560°C, maintaining the temperature for 175 to 185 minutes, and cooling to room temperature may be performed. Through this, the present invention can further refine the carbides, thereby further improving the strength and impact toughness at room temperature.

Hereinafter, the high-speed tool steel and the heat treatment method of the high-speed tool steel according to the present invention will be described in more detail through examples. However, the following examples are only a reference for explaining the present invention in detail, and the present invention is not limited thereto, and can be implemented in various forms.

Also, unless otherwise defined, all technical and scientific terms have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. The terminology used in the description herein is only for the purpose of effectively describing specific embodiments and is not intended to limit the invention. Also, the unit of additives not specifically described in the specification may be weight percent.

[Example 1]

High-speed tool steel having the composition shown in Table 1 below is processed into a 120x120x268 (mm) plate shape to prepare a high-speed tool steel test piece. The prepared high-speed tool steel test piece is placed in a heat treatment furnace, heated from room temperature to 650°C, and maintained for 60 minutes to perform a preheating step. The preheated high-speed tool steel is heated to 1,260°C and maintained for 480 minutes to perform a high-temperature homogenization step.

After this, the high-temperature homogenized high-speed tool steel specimen was moved out of the furnace and forcibly cooled at a cooling rate of 3°C/min by rotating a fan until the temperature reached 900°C (air cooling stage). When the temperature of the specimen reached 900°C, the air blowing was stopped and the specimen was air-cooled outside the furnace (air cooling stage).

After this, the high-speed tool steel test specimens at room temperature were subjected to a first austenitizing preheating step of maintaining them at 600°C for 60 minutes in a vacuum heat treatment furnace, a second austenitizing preheating step of maintaining them at 850°C for 60 minutes, and a third austenitizing preheating step of maintaining them at 1,050°C for 60 minutes, and then an austenitizing step of maintaining them at 1,170°C for 10 minutes was performed, and the austenitized test specimens were cooled to room temperature by injecting nitrogen gas (second cooling step).

After this, the high-speed tool steel test piece cooled to room temperature was heated to 200°C and maintained for 180 minutes to perform low-temperature tempering.

Alloy composition (weight%) C Mn Si Ni Cr W Mo V 0.8515 0.33 0.28 0.16 4.10 6.1 4.92 1.91

[Example 2]

All processes were performed in the same manner as in Example 1, except that a high-temperature homogenization step was performed at 1,260°C, and a blowing cooling step was performed until the high-speed tool steel reached 880°C after the high-temperature homogenization.

[Example 3]

All processes were performed in the same manner as in Example 1, except that a high-temperature homogenization step was performed at 1,240°C, and a blowing cooling step was performed until the high-speed tool steel reached 980°C after the high-temperature homogenization.

[Comparative Example 1]

After the high-temperature homogenization step, all processes were performed in the same manner as in Example 1, except that an air cooling step was performed, excluding the blowing cooling step.

[Comparative Example 2]

After the high-temperature homogenization step, all processes were performed in the same manner as in Example 1, except that the high-speed tool steel was cooled by blowing air until it reached 840°C.

[Comparative Example 3]

After the high-temperature homogenization step, all processes were performed in the same manner as in Example 1, except that the high-speed tool steel was cooled by blowing air until it reached 820°C.

[Comparative Example 4]

All processes were performed in the same manner as in Example 1, except that the blowing cooling step was performed at 1,080°C.

[Comparative Example 5]

All processes were performed in the same manner as in Example 1, except that a high-temperature homogenization step was performed at 1,300°C.

[Comparative Example 6]

All processes were performed in the same manner as in Example 1, except that the tempering step was performed at 550°C for 180 minutes.

The microstructures of the high-speed tool steels manufactured in Examples 1 to 3 and Comparative Examples 1 to 6 were photographed, and the strength, hardness, and impact toughness according to the heat treatment conditions were compared. The property values according to the heat treatment conditions are shown in Table 2 below.

At this time, the tensile test was performed by pulling both ends of the specimen at a rate of 1×10 ^-3 /s at room temperature, and the hardness was measured by the Rockwell C-scale. The impact toughness was evaluated by the Charpy impact test using an unnotch. For detailed description, refer to Metals Handbook, 10th ed., Materials Park OH 2000, vol.8, pp. 785-86.

Microstructure measurements were performed using an optical microscope (OM), a scanning electron microscope (SEM), and a transmission electron microscope (TEM).

High temperature homogenization
Temperature (℃) End of blower cooling
Temperature (℃) Tempering
Temperature (℃) hardness
(HRC) Shock personality
(J/cm2) Example 1 1,260 900 200 61.3 14.5 Example 2 1,260 880 200 60.8 11.4 Example 3 1,240 980 200 60.9 14.2 Comparative Example 1 1,260 - 200 59.5 7.8 Comparative Example 2 1,260 840 200 58.9 9.2 Comparative Example 3 1,260 820 200 61.1 10.1 Comparative Example 4 1,260 1080 200 60.4 11.2 Comparative Example 5 1,300 900 200 59.7 5.8 Comparative Example 6 1,260 900 550 60.7 7.7

FIG. 2 is a photograph of the microstructure of Example 1 in which air cooling was performed according to an embodiment of the present invention, and FIG. 3 is a photograph of the microstructure of Comparative Example 1 in which air cooling was not performed.

Referring to Fig. 2, it can be confirmed that the high-speed tool steel that performed blow-cooling according to an embodiment of the present invention has a uniform cross-section without the carbides re-emerged in the base being re-segregated and precipitated at the grain boundaries during the cooling process. As a result, it was confirmed that the above-mentioned Examples 1 to 3 had a hardness of 60 HRC or higher and an impact toughness of 14.0 J/cm2 or higher.

On the other hand, in Comparative Example 1, which did not perform the blowing cooling step in the first cooling step, it can be confirmed that dense carbides were formed on the cross section and the carbide area increased. As a result, the impact toughness value of Comparative Example 1 at room temperature was significantly reduced by 31.6 to 46.2% or more to 7.8 J/cm2. In addition, it was confirmed that in the case of some test pieces, the carbides were segregated in a specific direction, which further significantly reduced the impact toughness value.

Comparative Examples 2 and 3, which were air-cooled to temperatures below 850℃, showed a decrease in the size of carbides, unlike Comparative Example 1, but the impact toughness was confirmed to have decreased by 11.4 to 30.3% to 9.2 and 10.1 J/cm2, respectively. This is interpreted as being because the high-speed tool steel was forcibly cooled to an excessively low temperature, which increased the dislocation density due to supercooling and formed an excessive amount of retained austenite.

Comparative Example 4, in which air cooling was performed up to 1,080℃, showed that proeutectoid carbide was precipitated in the air cooling stage after air cooling, and the impact toughness decreased to 11.2 J/cm2.

For this reason, it can be confirmed that the above-mentioned blowing cooling step is preferably forced cooling at a temperature of 850 to 1,000°C.

In addition, through Examples 1 to 3 and Comparative Example 5, the impact toughness values at room temperature of high-speed tool steels according to high-temperature homogenization temperatures can be compared.

FIG. 4 is a photograph of the microstructure of high-speed tool steel that has undergone high-temperature homogenization according to an embodiment of the present invention, and FIG. 5 is a photograph of the microstructure of Example 1 according to an embodiment of the present invention.

Referring to FIGS. 4 and 5, Examples 1 to 3 can perform high-temperature homogenization to granulate and uniformly distribute coarse and non-uniform carbides distributed in high-speed tool steel. As a result, high-speed tool steel having a hardness of 60 HRC or higher and an impact toughness of 14.0 J/cm2 or higher can be manufactured.

On the other hand, it can be seen that Comparative Example 5, in which the temperature in the high-temperature homogenization step exceeded 1,290°C, had a reduced impact toughness at room temperature of 5.8 J/cm2 compared to Examples 1 to 3. This is interpreted as a result of the high-temperature homogenization step exceeding 1,290°C, resulting in the grain size of the high-speed tool steel becoming excessively coarse.

For this reason, it can be confirmed that the high-temperature homogenization step is preferably performed at 1,240 to 1,290°C.

Through Examples 1 to 3 and Comparative Example 6, the impact toughness values at room temperature of high-speed tool steels subjected to low-temperature tempering can be compared.

Figure 6 is a photograph of the microstructure of a cross-section of a high-speed tool steel when low-temperature tempering was performed after an austenizing step, and Figure 7 is a photograph of the microstructure of a cross-section of a high-speed tool steel when high-temperature tempering was performed after an austenizing step.

Referring to FIGS. 6 and 7, it can be confirmed that Example 1 (FIG. 6) in which low-temperature tempering was performed after the austenizing step has more fine carbides precipitated during the tempering heat treatment distributed within the matrix than Comparative Example 6 (FIG. 7) in which high-temperature tempering was performed after the austenizing step. In addition, it can be confirmed that the particle size of the formed carbide is smaller in Example 1 in which low-temperature tempering was performed than in Comparative Example 6 in which high-temperature tempering was performed. This is data proving that by performing low-temperature tempering after austenizing, the number of nuclei of precipitated carbides increases during the precipitation process of the carbides, and due to the increased nuclei, the carbides become finer and more uniformly distributed within the matrix structure. Accordingly, it was confirmed that the distribution of carbides according to the present invention was improved, and as a result, while the impact toughness at room temperature of Example 1 was 14.5 J/cm2, the impact toughness value of Comparative Example 7 was 7.7 J/cm2, a sharp decrease of 46.9%.

Based on this, it can be confirmed that performing low-temperature tempering at 150 to 300°C after the austenitizing process plays a critical role in improving the impact toughness of high-speed tool steel at room temperature.

Finally, in order to compare the impact toughness of high-speed tool steel heat-treated according to an embodiment of the present invention and high-speed tool steel heat-treated by a conventional heat treatment process, a number of test pieces heat-treated under the conditions of Example 1 and high-speed tool steel test pieces heat-treated under the conditions of Comparative Example 10 below were manufactured, and impact toughness tests were repeatedly performed on the manufactured test pieces at least five times. The results are summarized in Fig. 8 and Table 3.

[Comparative Example 10]

Except for the high-temperature homogenization step, and the tempering step was performed by maintaining it at 550℃ for 180 minutes according to the normal high-speed tool steel tempering conditions, all processes were performed in the same manner as in Example 1. Thereafter, the results of the impact toughness test at room temperature of the high-speed tool steel manufactured according to Example 1 and the high-speed tool steel manufactured according to Comparative Example 10 are disclosed in Table 3 and FIG. 8 below. In addition, the microstructure photograph of Comparative Example 10, which was subjected to austenitizing and tempering without high-temperature homogenization treatment, is disclosed in FIG. 9.

High temperature homogenization
Temperature (℃) Tempering
Temperature (℃) Shock personality
(J/cm2) #1 #2 #3 #4 #5 average Example 1 1,260 200 15.3 12.4 16.7 16.7 11.4 14.5 Comparative Example 10 - 550 7.3 7.7 6.5 6.1 6.2 6.8

Referring to FIGS. 8 and 9 and Table 3, the high-speed tool steel subjected to heat treatment according to an embodiment of the present invention was compared with the high-speed tool steel not subjected to high-temperature homogenization and low-temperature tempering, and as a result, it was experimentally confirmed that the impact toughness value increased by 3.7 to 12.5 J/cm2 compared to the comparative invention due to the improvement in carbide distribution, and that it increased by 7.7 J/cm2 on an average value basis. Through this, it was confirmed that the present invention has a superior effect of improving impact toughness compared to conventional heat treatment.

Although the present invention has been described through specific matters and limited examples as above, these have been provided only to help a more general understanding of the present invention, and the present invention is not limited to the above examples, and those skilled in the art to which the present invention pertains can make various modifications and variations from this description.

Therefore, the spirit of the present invention should not be limited to the described embodiments, and all things that are equivalent or equivalent to the claims described below as well as the claims are included in the scope of the spirit of the present invention.

Claims (10)

In a heat treatment method of high-speed tool steel,
A high-temperature homogenization step of heating a high-speed tool steel containing C 0.7 to 1.5% by weight, Mn 0.1 to 0.5%, Si 0.1 to 0.5%, Cr 3.5 to 5%, Mo 4.5 to 6.5, W 4.5 to 6.5, V 1.5 to 4.0% by weight and the remainder Fe and unavoidable impurities to 1,240 to 1,290°C and then maintaining the temperature for a predetermined period of time;
Primary cooling stage for cooling homogenized high-speed tool steel;
An austenizing step in which the first-cooled high-speed tool steel is heated to a temperature higher than the A1 transformation temperature and then maintained for a certain period of time;
Secondary cooling step for cooling the austenitized high-speed tool steel; and
A low-temperature tempering step comprising heating the second-cooled high-speed tool steel to 150 to 350°C and then maintaining it for a certain period of time:
The above first cooling step is,
A blow-cooling step for blow-cooling the homogenized high-speed tool steel to 850 to 1,000°C; and
A method for heat treatment of high-speed tool steel, comprising an air-cooling step of cooling air-cooled high-speed tool steel to room temperature. delete In the first paragraph,
A method for heat treatment of high-speed tool steel, wherein the above-mentioned blowing cooling step is performed by injecting an inert gas (N ₂ , Ar, He) or air (Air). In the third paragraph,
A heat treatment method for high-speed tool steel, wherein the above-mentioned blowing cooling step cools the homogenized high-speed tool steel at a rate of 3°C/min or more. In the first paragraph,
A method for heat treatment of high-speed tool steel, wherein the second cooling step is performed by spraying a coolant onto the high-speed tool steel in a vacuum or inert gas atmosphere. In paragraph 5,
A heat treatment method for high-speed tool steel, wherein the second cooling step comprises spraying the coolant onto the high-speed tool steel at a pressure of 0.5 MPa or more to cool the high-speed tool steel. In the first paragraph,
A heat treatment method for high-speed tool steel, wherein the austenizing step is performed at 1,100 to 1,200°C. In Article 7,
The above austenitizing step is,
A first austenitizing preheating step in which high-speed tool steel at room temperature is heated to 550 to 650°C and then maintained for a certain period of time;
A second austenitizing preheating step in which the temperature is heated to 800 to 900°C after the first austenitizing preheating and maintained for a certain period of time; and
A heat treatment method for high-speed tool steel, comprising a third austenitizing preheating step of heating to 1,000 to 1,080°C after the second austenitizing preheating and then maintaining the temperature for a predetermined period of time. In the first paragraph,
The heat treatment method of the above high-speed tool steel is:
A heat treatment method for high-speed tool steel, further comprising a preheating step of heating to 600 to 700°C and then maintaining the temperature for a predetermined period of time prior to the high-temperature homogenization step. delete