Powder metallurgy precipitation hardening high speed steel

Abstract

The application provides a powder metallurgy precipitation hardening high-speed steel, which comprises the following components in percentage by mass: Si: 0.5-1.2%; Co: 16.0-30.0%; W: 0-5.0%; Mo: 15.0-22.0%; (Mo+W / 2): 15.0-23.0%; Ce: 0.01-0.1%, and the rest is iron and impurities, and the steel is mainly composed of an iron-based cubic solid solution matrix and an intermetallic compound (IMC) mu phase in the matrix, wherein the mu phase is of the (Fe, Co)7(Mo+W / 2)6 type. The powder metallurgy precipitation hardening high-speed steel prepared by the application has small mu phase size and uniform distribution, excellent mechanical properties, a hardness of more than 65HRC after solid solution aging treatment, a notched impact toughness value of more than 11.0J, and outstanding tempering softening resistance.

Description

TECHNICAL FIELD

[0001] The present application relates to a new type of high speed steel, in particular to a powder metallurgy precipitation hardening high speed steel. BACKGROUND

[0002] High speed steel is the first choice for high speed machining of other materials, and has been widely used in the field of manufacturing. Precipitation hardening steel is a carbon-free iron-based martensitic precipitation hardening tool alloy. Because the content of C in the composition is low, basically no carbide is precipitated, and the structure is mainly iron-based matrix and Fe-Co-Mo-W intermetallic compound (i.e. IMC), and the hardening effect is due to the precipitation of IMC particles during aging process. This kind of precipitation hardening high speed steel has good grindability and resistance to temper softening, and good dimensional stability.

[0003] When prepared by traditional casting and forging process, the alloy composition is prone to segregation during solidification due to the characteristics of slow cooling of molten steel in the process, and this undesirable structure cannot be effectively solved by hot working method, which will have adverse effects on the performance of the alloy, resulting in low level of performance of high speed steel, including strength, toughness, grindability, etc. It is difficult to meet the requirements of high-end manufacturing on material performance and service life. SUMMARY

[0004] Therefore, the present application provides a powder metallurgy precipitation hardening high speed steel with good structure and excellent mechanical properties.

[0005] To achieve the above purpose, the technical scheme of the present application is as follows:

[0006] A powder metallurgy precipitation hardening high speed steel, characterized in that the steel comprises the following chemical components in mass percentage:

[0007] Si: 0.5%-1.2%;

[0008] Co: 16.0%-30.0%;

[0009] W: 0%-5.0%;

[0010] Mo: 15.0%-22.0%;

[0011] (Mo+W / 2): 15.0%-23.0%;

[0012] Ce: 0.01%-0.1%;

[0013] The balance is iron and impurities;

[0014] And the intermetallic compound (i.e. IMC phase) in the precipitation hardening steel is μ phase, and the type of μ phase is (Fe, Co)7(Mo+W / 2)6.

[0015] The present application can promote the precipitation of mu phase, improve the temper softening resistance and toughness of the steel by the reasonable design of alloy composition and ratio when the powder metallurgy process is used to prepare the precipitation hardening high speed steel.

[0016] The role of Co is to dissolve in the matrix, so that the alloy becomes martensitic steel, thereby increasing the hardness and strength of the alloy by one level compared with ferrite alloy. The increase of Co content will appropriately reduce the toughness of the steel. In the present application, the content of Co element is 16.0%-30.0%, preferably 18.0%-30.0%.

[0017] The melting point of W is high, which can increase the strength and temper stability of the steel, and the high temperature creep resistance, and also can increase the temper softening resistance of the steel, so that the surface temperature of the steel is less during processing and use, and the hardness decreases. In the present application, the content of W element is 0%-5.0%, preferably 0%-3.0%.

[0018] Mo can completely replace W and has lower price than W. On the other hand, the higher the Mo content, the higher the starting precipitation temperature of mu phase, and the larger the particle size of mu phase. In the present application, the content of Mo element is 15%-22.0%, preferably 15.0%-20.0%.

[0019] Si is not a carbide forming element, but is used as a deoxidizer and matrix strengthening element, which can improve the strength and hardness of the steel. However, too much Si will reduce the plasticity and toughness of the matrix. In the present application, the content of Si is controlled at 0.5%-1.2%, and preferably 0.5%-1%.

[0020] Ce is added in the refining stage, which can react with oxygen, sulfur and other elements in the steel liquid to form compounds and be discharged from the steel liquid as inclusions, thereby playing the role of deoxidation, desulfurization and modification.

[0021] As a limitation of the above-mentioned manner, the powder metallurgy precipitation hardening high speed steel comprises the following chemical components by mass percentage:

[0022] Si: 0.5%-1%;

[0023] Co: 18.0%-30.0%;

[0024] W: 0%-3.0%;

[0025] Mo: 15%-20.0%;

[0026] (Mo+W / 2): 15.0%-21.5%;

[0027] The balance is iron and impurities.

[0028] In order to achieve better comprehensive performance, the chemical components in the powder metallurgy precipitation hardening high speed steel should be controlled within the required range.

[0029] Further, the particle size of the mu phase is ≤1.5 μm at a volume fraction of at least 80%.

[0030] Further, the maximum particle size of the mu phase is not more than 5.5 μm.

[0031] Further, the volume fraction of the mu phase in the powder metallurgy precipitation hardening high speed steel is 12-20%.

[0032] Further, the hardness of the powder metallurgy precipitation hardening high speed steel is more than 65 HRC, and the impact toughness value of the unnotched sample is more than 11.0 J.

[0033] In the present application, the powder metallurgy process is used to prepare the precipitation hardening steel, which can solve the problem of element segregation and obtain uniform microstructure. The main steps of the powder metallurgy process for preparing the precipitation hardening high speed steel include: atomization and powdering, hot isostatic pressing forming, the steel liquid is rapidly cooled into powder, the alloying elements in the steel liquid cannot segregate and completely solidify, the powder is consolidated into a material with fine and uniform microstructure, and the performance of the precipitation hardening high speed steel produced by the traditional casting or electroslag process is greatly improved.

[0034] The present application also provides a preparation method for the powder metallurgy precipitation hardening high speed steel as described above, and the preparation method specifically comprises the following steps:

[0035] s1. preparing the precipitation hardening steel liquid according to the above chemical composition requirements and transferring it to a ladle;

[0036] s1.1. maintaining the superheat of the steel liquid by heating the protective slag covering the upper surface of the steel liquid in the ladle, and stirring the steel liquid by introducing inert gas at the bottom of the ladle;

[0037] s1.2. flowing the steel liquid into a preheated tundish through the flow guide pipe at the bottom of the ladle at a stable flow rate, and applying protective slag to the upper surface of the steel liquid when the steel liquid enters the tundish and covers the lower end surface of the flow guide pipe;

[0038] s1.3. continuously compensating and heating the tundish to maintain the superheat of the steel liquid;

[0039] s1.4. atomizing the steel liquid into powder after it enters the atomization chamber from the tundish using inert gas, and the obtained metal powder is settled at the bottom of the atomization chamber, then enters a powder storage tank body with a protective atmosphere, and is screened by a protective screening device before entering the powder storage tank body for storage;

[0040] s1.5 Under the protection of inert gas, the metal powder in the powder storage tank is transferred to the hot isostatic pressing jacket, the hot isostatic pressing jacket is subjected to vacuum degassing treatment after the metal powder is vibrated and packed tightly, the end of the hot isostatic pressing jacket is subjected to sealing welding treatment, then the hot isostatic pressing treatment is carried out to make the metal powder completely dense and solidified, and the powder metallurgy process is completed.

[0041] The powder metallurgy process described above comprises non-vacuum melting atomization powder preparation and hot isostatic pressing links, and the whole process is protected to control the oxygen content and the morphology of precipitated phases and optimize the performance of the precipitation hardening steel.

[0042] The protective slag of the ladle has the functions of air insulation and electric heating. Inert gas is introduced into the ladle through the air hole in the bottom of the ladle, so that the temperature of the molten steel at different positions in the ladle is balanced, and the removal of harmful inclusions is accelerated. The flow guide pipe in the bottom of the ladle plays a flow guiding role on the molten steel, reduces the turbulence generated in the molten steel flow process, avoids slag entrainment and prevents inclusions from entering the next link. On the other hand, the flow guide pipe prevents the molten steel from being exposed to air to prevent the oxygen content of the molten steel from rising. Before the molten steel enters the tundish, the tundish needs to be preheated to prevent local condensation or the second phase from precipitating too early when the molten steel enters the tundish.

[0043] The inside of the powder storage tank has the functions of atmosphere protection and forced cooling, the powder protection screening device plays a protective role in the powder screening process and prevents the powder from floating, the powder storage tank body is sealingly connected with the hot isostatic pressing jacket, and inert gas is introduced into the hot isostatic pressing jacket before powder loading to discharge air, so that the oxygen content in the powder can be prevented from rising.

[0044] The precipitation hardening high-speed steel prepared by the powder metallurgy process has reasonable component design, various effective protection means are adopted in the preparation process to prevent the molten steel and the powder from being polluted, and due to the specific chemical composition and the rapid condensation process of powder metallurgy, the intermetallic compound μ phase of the type (Fe, Co) 7 (Mo + W / 2) 6 is more fine and uniform, the hardness after heat treatment is above 65HRC, and the precipitation hardening high-speed steel has excellent hardness, tempering softening resistance and toughness, and can meet the application requirements of various types. BRIEF DESCRIPTION OF DRAWINGS

[0045] The accompanying drawings, which form a part of this application, are included to provide a further understanding of the application and are incorporated in and constitute a part of this specification. The illustrations are shown schematically in the drawings of the preferred embodiments of the application, and their description serves to explain the application without forming an improper limitation to the application. In the drawings:

[0046] Figure 1 The microstructure diagram of the precipitation hardening high-speed steel prepared in Example 1 of the application;

[0047] Figure 2 The microstructure diagram of the precipitation hardening high-speed steel prepared in Example 2 of the application;

[0048] Figure 3 Microstructure of the precipitation hardened high speed steel prepared in Example 4 of the present application;

[0049] Figure 4 Microstructure of the precipitation hardened high speed steel prepared in Example 5 of the present application;

[0050] Figure 5 Microstructure of the precipitation hardened high speed steel prepared in Example 6 of the present application;

[0051] Figure 6 Microstructure of the precipitation hardened high speed steel prepared in Example 7 of the present application;

[0052] Figure 7 Microstructure of the powder metallurgy high speed steel prepared in Comparative Example A of the present application;

[0053] Figure 8 Microstructure of the electroslag process high speed steel prepared in Comparative Example B of the present application;

[0054] Figure 9 Comparison chart of temper resistance of Examples 1 to 8 and Comparative Examples A and B of the present application. DETAILED DESCRIPTION

[0055] It should be noted that the embodiments in the present application and the features in the embodiments can be combined with each other without conflict.

[0056] The present application relates to a powder metallurgy precipitation hardened high speed steel, which comprises the following chemical components in percentage by mass: Si: 0.5%-1.2%; Co: 16.0%-30.0%; W: 0%-5.0%; Mo: 15.0%-22.0%; Cr: 3.0%-5.0%; V: 3.0%-5.0%; C: 0.5%-1.0%; N: 0.01%-0.05%; S: 0.01%-0.05%; P: 0.01%-0.05%; (Mo+W / 2): 15.0%-23.0%; Ce: 0.01%-0.1%; the balance being iron and impurities; and, the intermetallic compound (i.e. IMC phase) in the powder metallurgy high speed steel is μ phase, and the type of the μ phase is (Fe,Co)7(Mo+W / 2)6.

[0057] The present application relates to a powder metallurgy precipitation hardened high speed steel, which comprises the following chemical components in percentage by mass: Si: 0.5%-1.2%; Co: 16.0%-30.0%; W: 0%-5.0%; Mo: 15.0%-22.0%; Cr: 3.0%-5.0%; V: 3.0%-5.0%; C: 0.5%-1.0%; N: 0.01%-0.05%; S: 0.01%-0.05%; P: 0.01%-0.05%; (Mo+W / 2): 15.0%-23.0%; Ce: 0.01%-0.1%; the balance being iron and impurities; and, the intermetallic compound (i.e. IMC phase) in the powder metallurgy high speed steel is μ phase, and the type of the μ phase is (Fe,Co)7(Mo+W / 2)6.

[0058] The present application promotes the precipitation of μ phase when preparing the precipitation hardened high speed steel by powder metallurgy process through the reasonable design of alloy components and proportion, thereby improving the temper softening resistance and toughness of the steel.

[0059] The present application also relates to a method for preparing the above precipitation hardened high speed steel. When prepared by traditional ingot casting or electroslag process, the slow solidification speed is prone to cause segregation and thus performance decline. Therefore, in order to ensure that the prepared precipitation hardened high speed steel ingot is uniform in composition and organization, small in μ phase and high in purity, the present application adopts powder metallurgy process to prepare the steel ingot, and then obtains the required rod product through forging.

[0060] Specifically, the preparation method of the present application comprises the following steps:

[0061] s1. Pouring the molten steel of the present application into a melting ladle, and the molten steel loading weight is 1.5-8 tons;

[0062] s1.1. Heating the protective slag covering the upper surface of the molten steel in the ladle by electric heating with graphite electrodes, and stirring the molten steel by introducing argon or nitrogen gas into the bottom of the ladle, and opening the molten steel flow guide pipe when the superheat of the molten steel reaches 100-200℃;

[0063] s1.2. Flowing the molten steel into the tundish preheated to 800-1200℃ through the flow guide pipe at the bottom of the ladle at a flow rate of 10-50 Kg / min, and applying the protective slag when the molten steel enters the tundish and covers the lower end surface of the flow guide pipe;

[0064] s1.3. Continuously compensating the heating of the tundish during the atomization and powdering process, and maintaining the superheat of the molten steel at 100-200℃;

[0065] s1.4. The molten steel enters the atomization chamber through the bottom of the tundish, the atomization gas injection valve is opened, nitrogen gas is used as the gas medium for atomization and powdering, the purity of the nitrogen gas is ≥99.999%, the oxygen content is ≤2ppm, and the outlet pressure of the gas nozzle is 1.0-5.0MPa; the molten steel is broken into droplets under the action of nitrogen gas injection, and is rapidly cooled into metal powder, flies to the bottom of the atomization chamber, and then enters the powder storage tank body with a protective atmosphere; after the atomization and powdering is completed, the metal powder in the powder storage tank body is cooled to room temperature, and the metal powder is screened through a protective screening device; a positive pressure inert protective gas is introduced into the cavity of the protective screening device, and the powder storage tank is in a positive pressure inert gas protective atmosphere;

[0066] s1.5. Filling the metal powder in the powder storage tank body into a hot isostatic pressing jacket, first introducing inert gas into the hot isostatic pressing jacket to discharge air, then sealing the hot isostatic pressing jacket and the powder storage tank body, and performing vibration operation during the filling process to increase the filling density of the metal powder; after completion, the hot isostatic pressing jacket is subjected to vacuum degassing treatment, the hot isostatic pressing jacket is heated and kept at 200-600℃ during the vacuum process, degassing to 0.01Pa, and then continuing to heat and keep for more than 2h, then performing sealing welding treatment on the end part of the jacket, and finally performing hot isostatic pressing treatment on the jacket, the hot isostatic pressing temperature is 1120-1180℃, and the pressure is ≥100MPa, and the holding time is ≥1h, so that the metal powder is completely densified and solidified, and then the furnace is cooled down, and the powder metallurgy process is completed.

[0067] s2. Opening the blank by forging

[0068] The precipitation hardening high speed steel of the present application is further forged and deformed as needed to obtain a rod or a forged piece with a preset shape and size, and different properties are obtained by using different heat treatment systems, including annealing, solid solution and aging.

[0069] The annealing treatment involves heating the rod or the forged piece to 870-890℃, with a holding time ≥ 2h, and then cooling to below 550℃ at a rate of ≤ 15℃ / h, and then furnace cooling or static air cooling to below 50℃; the solid solution treatment involves preheating the annealed rod or forged piece at a temperature of 810℃-850℃, and after the temperature is uniform, placing it in a temperature of 1170-1200℃ for 15-40 minutes and then oil cooling; and then aging at a temperature in the range of 580-650℃, with a holding time of 3-4 hours, and then air cooling to 50℃.

[0070] The powder metallurgy process used in the present application uses various effective protection measures to prevent the molten steel and the powder from being contaminated during the preparation process, and the high-purity ingot provides a guarantee for finally obtaining high-performance alloy ingots.

[0071] The powder metallurgy precipitation hardening high speed steel of the present application and its preparation will be further described below with reference to the accompanying drawings, specific preparation examples and comparative examples, and corresponding performance tests.

[0072] Eight kinds of precipitation hardening high speed steels with different component compositions were obtained by the aforementioned preparation method, and were compared with powder metallurgy tool steel (alloy A) and cast-forged tool steel (alloy B), and the results are as follows:

[0073] Table 1: Chemical composition of each example of high speed steel

[0074]

[0075] In the table, "-" means that the element is not contained or the content of the element is very small and is not analyzed.

[0076] Among them, examples 1 to 8 are powder metallurgy precipitation hardening high speed steels of the present application, which are prepared by powder metallurgy process, first using gas atomization powder preparation process to prepare powder, then densifying the powder by hot isostatic pressing, and then making ingot blanks with a diameter of φ400mm, and further hot deformation processing to obtain rods with a diameter of φ60mm.

[0077] Comparative example A is prepared by powder metallurgy process and hot deformation processing to a diameter of φ57mm; comparative example B is prepared by electroslag remelting process and hot deformation processing to a diameter of φ61mm.

[0078] (I) Microstructure analysis

[0079] Based on the scanning electron microscope to obtain the microstructure image, Figures 1 to 6are microstructure schematic diagrams of the prepared precipitation hardened steel forgings of example 1, example 2, example 4, example 5, example 6 and example 7 respectively, Figure 7 is a microstructure schematic diagram of alloy A, Figure 8 is a microstructure schematic diagram of alloy B.

[0080] It is obvious that, Figures 1 to 6 The hardened phase dispersed in the matrix in the middle gray white can significantly improve the wear resistance, toughness and service life of the material. Figure 7 and Figure 8 The microstructure contains two types of precipitated phases, one type is bright white and large in size, and the other type is gray white and small in size.

[0081] The precipitated phase content, particle size and composition in examples 1 to 8 and alloys A and B after heat treatment are compared, as shown in table 2.

[0082] Table 2: content and chemical composition of precipitated phase

[0083]

[0084] In which "-" represents no or little content of the element, which is not analyzed.

[0085] As can be seen from table 2, in examples 1 to 8, the detected IMC is mainly μ phase, the type is (Fe, Co)7(Mo+W / 2)6, and the main components are Fe, Co, Mo, W and a small amount of Si and other alloying elements. In alloys A and B, the detected strengthening phase is mainly two types, one type is VC type carbide, and the composition is mainly C, V, Cr and Fe, and the other type is Cr-rich carbide, the type is (Cr, Fe)C type carbide, and the composition is mainly C, V, Cr and Fe, and contains a small amount of Mo.

[0086] The volume fraction of the μ phase of the precipitation hardened high speed steel of the application reaches 12%-20%, the particle size is small, most of the μ phase particle size is ≤1.5μm, and the maximum size is not more than 5.5μm. These precipitated phases are small in size and high in dispersion, and the high temperature aggregation resistance of the μ phase is higher than that of the carbide, so that the material has better service life.

[0087] In the alloy A prepared by the powder metallurgy process, the VC type carbide is the smallest, but there are a large number of (Cr, Fe)C type carbides with a size range of 5-9μm in the structure. In the B alloy prepared by the electroslag remelting process, although the size of the VC type carbide is similar to that of the μ phase in the precipitation hardened high speed steel example of the application, there are a large number of (Cr, Fe)C type carbides with a size range of 5-9μm in the structure.

[0088] (II) hardness and impact toughness analysis after heat treatment

[0089] To verify the effect of heat treatment regime on the properties of precipitation hardening high-speed steel prepared by the present invention, heat treatment processes with different solution temperature and aging temperature were set to heat treat the prepared bars. The verification parameters were set with reference to GB / T230.1 and GB / T 229.

[0090] The hardened steels and alloys A and B prepared in Examples 1 to 8 were subjected to heat treatment, and the results of the comparison of hardness and impact toughness are shown in Table 3.

[0091] Table 3: Comparison of Mechanical Properties

[0092]

[0093] As can be seen from Table 3, the precipitation-hardening high-speed steel of the present invention has relatively low impact toughness, but the measured values ​​meet the toughness requirements of the application field. The precipitation-hardening high-speed steel of the present invention is particularly suitable for occasions with less impact load.

[0094] (III) Analysis of resistance to tempering softening

[0095] The tempering softening resistance of the hardened steels and alloys A and B prepared in Examples 1 to 8 was compared. The heat treatment processes used are shown in Table 4, and the results of the tempering softening resistance are as follows: Figure 9 As shown, the verification parameter settings refer to GB / T 230.1 and GB / T 229.

[0096] Table 4: Tempering Softening Resistance Process

[0097]

[0098] Depend on Figure 8 It can be seen that the precipitation hardening steel of the present invention exhibits superior resistance to tempering softening.

[0099] Furthermore, due to limitations of the image analysis software for statistical analysis of precipitated phase particles, the precipitation-hardening steel prepared under the above conditions may contain individual μ-phase particles with sizes exceeding the maximum size. However, since their numbers are extremely small, they do not substantially affect the toughness and other mechanical properties of the precipitation-hardening steel and can therefore be disregarded. Many other even finer μ-phase particles cannot be identified by the analysis software; the statistical results of their volume fraction and particle size are only used for comparison.

[0100] Based on the above description, it can be found that, as preferred by mass percentage, the precipitation hardening high speed steel of the present application comprises the following chemical components: Si: 0%-0.8%; Co: 18.0%-30.0%; W: 0%-3.0%; Mo: 15%-20.0%; (Mo+W / 2): 15.0%-22.0%; the balance being iron and impurities. The precipitation hardening high speed steel formed by the above components can obtain ideal microstructure and excellent performance to meet the requirements.

[0101] In order to achieve better comprehensive performance, each chemical component in the powder metallurgy precipitation hardening high speed steel of the present application should be controlled within the required range. Specifically, it includes that the size of μ phase is ≤1.5 μm at a volume fraction of at least 80%, the maximum size of μ phase is not more than 5.5 μm, and the volume fraction of μ phase is 12-20%.

[0102] Due to the use of specific alloy composition design and the use of powder metallurgy process for preparation, the material has high intermetallic compound μ phase content, fine particle size, high dispersion, and high high-temperature aggregation resistance than carbide, so that the material obtains better toughness and better service life, can meet the application requirements of different types, can be used to make (1) cutters under high speed cutting, cutting difficult-to-machine materials; (2) high-precision gauges; (3) small and thin blade cutters.

[0103] In summary, the powder metallurgy precipitation hardening high speed steel of the present application has excellent mechanical properties, especially excellent temper softening resistance. Due to the characteristics of its alloy composition and the difference from the traditional high speed steel strengthening mechanism, its temper softening resistance is greatly superior to that of traditional high speed steel and other tool steels.

[0104] The above only describes the preferred embodiments of the present application and is not intended to limit the present application. Any modification, equivalent replacement, improvement, etc. made within the spirit and principles of the present application should be included in the protection scope of the present application.

Claims

1. A powder metallurgy precipitation-hardening high-speed steel, characterized in that, Its chemical components, by mass percentage, include: Si: 0.5%-1.2%; Co: 16.0%-30.0%; W：0％-5.0％； Mo: 15.0%-22.0%; (Mo+W / 2): 15.0%-23.0%; Ce: 0.01%-0.1%; The balance consists of iron and impurities; Furthermore, the intermetallic compound in the powder metallurgy precipitation-hardened high-speed steel is a μ phase, and the type of the μ phase is (Fe,Co)7(Mo+W / 2)6.

2. The powder metallurgy precipitation hardening high-speed steel according to claim 1, characterized in that, Its chemical components, by mass percentage, include: Si: 0.5%-1%; Co: 18.0%-30.0%; W：0％-3.0％； Mo: 15.0%-20.0%; (Mo+W / 2): 15.0%-21.5%; Ce: 0.01%-0.1%; The balance consists of iron and impurities.

3. The powder metallurgy precipitation hardening high-speed steel according to claim 1 or 2, characterized in that: The particle size of the μ phase, at least 80% by volume, is ≤1.5 μm.

4. The powder metallurgy precipitation hardening high-speed steel according to claim 1 or 2, characterized in that: The maximum particle size of the μ phase does not exceed 5.5 μm.

5. The powder metallurgy precipitation hardening high-speed steel according to claim 1 or 2, characterized in that: The volume fraction of the μ phase in the powder metallurgy precipitation hardening high-speed steel is 12-20%.

6. The powder metallurgy precipitation hardening high-speed steel according to claim 1 or 2, characterized in that: The powder metallurgy precipitation hardening high-speed steel has a hardness of 65 HRC or higher, and the impact toughness of the unnotched specimen is 11.0 J or higher.

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