Medium-carbon fatigue-resistant tool steel and heat treatment method

Abstract

The present invention belongs to the technical field of metal materials, and particularly relates to a medium-carbon fatigue-resistant tool steel and a heat treatment method. The chemical composition of the steel is as follows by weight: C: 0.45% to 0.50%, Si: 0.25% to 0.30%, Mn: 0.50% to 0.60%, P≤0.010%, S≤0.015%, Cr: 2.00% to 2.20%, Ni: 1.80% to 2.20%, Cu: 0.20% to 0.50%, N≤0.0045%, with the remainder being Fe and unavoidable impurities. Advantageously, the present invention, through chemical composition design and specific heat treatment, effectively enables the tool steel to achieve high hardness and fatigue resistance while also meeting the economic requirements of tool manufacturing.

Description

Technical Field

[0001] The invention belongs to the technical field of metal materials, and in particular relates to a medium-carbon fatigue-resistant tool steel and a heat treatment method. Background Art

[0002] Traditional cutting tools are typically manufactured from a single material, which presents numerous challenges. For one thing, when the hardness exceeds 55HRC, machining costs rise significantly, while tool life is generally short, making it difficult to meet the comprehensive performance and cost-effectiveness requirements of actual use.

[0003] Currently, common tool steels on the market, such as H13, SKH9 and other high-hardness alloy steels, have high hardness, but the material cost is high and there are deficiencies in fatigue performance. They cannot give a good balance between the durability and economy of the tool under high-intensity cutting operations.

[0004] Prior art patent application CN202211221368.1 discloses a high-hardness, high-nitrogen martensitic stainless steel tool material and its preparation method. The chemical composition and austenite structure of the stainless steel are adjusted, and the high-nitrogen austenitic stainless steel is transformed through a solid-state phase transformation to produce martensitic stainless steel with a high nitrogen supersaturation solid solubility. A combined solution-aging-quenching heat treatment is used to adjust the composition of the austenite matrix phase. Combined with the Scheffler phase diagram, the composition is adjusted to produce dispersion-strengthened, high-strength martensite, improving the toughness of the high-hardness metal material. The overall hardness exceeds 55HRC, but the surface wear resistance does not meet the requirements of cutting tools.

[0005] Therefore, it is particularly urgent to develop a new type of medium-carbon fatigue-resistant tool steel, aiming to achieve a good balance between material cost, processing cost and tool performance by optimizing the chemical composition and heat treatment process. Summary of the Invention

[0006] In order to overcome the shortcomings of the existing technology, the purpose of the present invention is to provide a medium-carbon fatigue-resistant tool steel and a heat treatment method. In view of the high strength, high wear resistance and economic needs of cutting tools, through the design of chemical composition and staged heat treatment, different mechanical properties can be achieved in the core and surface of the same material, ensuring that the tool steel exhibits good strength and toughness during use, and the surface can withstand load-bearing strength and hardness.

[0007] To achieve the above object, the present invention is implemented through the following technical solutions:

[0008] A medium-carbon fatigue-resistant steel for cutting tools, wherein the chemical composition by weight of the steel is:

[0009] C: 0.45%~0.50%, Si: 0.25%~0.30%, Mn: 0.50%~0.60%, P≤0.010%, S≤0.015%, Cr: 2.00%~2.20%, Ni: 1.80%~2.20%, Cu: 0.20%~0.50%, N≤0.0045%, the rest are Fe and unavoidable impurities.

[0010] The present invention selects the above alloying element types and contents because of the role of each element in medium carbon fatigue-resistant tool steel:

[0011] C is an important element for precipitation strengthening and martensite formation by carbides. Therefore, to achieve the aforementioned effects, 0.45% or more of C is required. On the other hand, excessive C formation of martensite can cause blade dimensional changes and easily cause cracking during quenching, so the upper limit of C is set to 0.50%.

[0012] Si can be added as a deoxidizing element during production. Si produces a solid solution strengthening effect on ferrite, and to achieve this effect, a content of 0.25% or more is required. If Si exceeds 0.30%, the alloy's tempering resistance increases, and low-temperature brittle phases become more likely to precipitate. Therefore, the upper limit of Si is set to 0.30%.

[0013] Mn, like Si, has a deoxidizing effect and can be added during manufacturing. Mn also increases hardenability and proper addition can improve hardening properties.

[0014] The appropriate addition of Cr increases strength and wear resistance and reduces decarburization during heat treatment. However, like Ni, Cr also lowers the martensitic transformation temperature. Excessive Cr addition increases the amount of retained austenite, leading to decreased strength. Therefore, the upper limit is set at 2.2%.

[0015] Ni is dissolved in the matrix, which contributes to the solid solution strengthening and hardenability of the material, improves the strength of the steel without reducing the toughness, and helps to improve the low-temperature toughness and fatigue resistance. Therefore, it must be added, and the addition amount is 1.80%~2.20%.

[0016] In the present invention, Cu can stabilize the microstructure during surface quenching and improve the strength of the alloy during quenching. On the other hand, if Cu is added excessively, it will increase the intergranular weakening of the metal and be detrimental to plasticity and hot workability. Therefore, the upper limit of Cu is set to 0.50%.

[0017] The balance is Fe and impurity elements that inevitably enter during manufacturing. Typical impurity elements include S, P, and N. Ideally, the amount of these elements should be as low as possible, as the amount can be reduced during manufacturing using standard equipment. The lower the amount of each element, the better.

[0018] The processing hardness of the steel is 45-47HRC, and the hardness of the outer layer of the tool formed from the steel is 55-58HRC; the cutting tool is tested with a 0.8mm thick blued steel strip and the cutting times are more than 30,000.

[0019] A heat treatment method for medium-carbon fatigue-resistant tool steel comprises the following steps:

[0020] 1) Basic heat treatment: Heat the tool steel to 850~870℃, keep it warm until it is completely austenitized, then quench it in quenching oil and cool it to below 50℃. Then, perform medium temperature tempering at 330~350℃, keep it warm for 2~4h, and then water cool or air cool it to room temperature.

[0021] 2) Process heat treatment: The machined tool is heated and quenched in a salt bath at a temperature of 900~920℃. After the salt is put into the bath and kept warm for 10~360s, it is quenched and rapidly cooled to below 50℃ using quenching oil. Then, it is tempered at 180~200℃ and kept warm for 2~4h before being air-cooled to room temperature for stress relief. The structure is tempered martensite with a gradual hardness change.

[0022] Step 1) obtaining a tool steel substrate having a yield strength of 1350-1550 MPa, a tensile strength of 1600-1650 MPa, and a hardness of 45-47 HRC, while maintaining a tough state with an impact energy KV2 of 20-30 J at room temperature and a tempered martensite structure.

[0023] Step 2) The surface strength of the tool reaches a yield strength of 1100-1250 MPa, a tensile strength of 2000-2200 MPa, a hardness of 54-58 HRC, and an impact energy KV2 of 7-15 J at room temperature, and the microstructure is tempered martensite.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] The present invention can effectively achieve higher hardness and fatigue resistance for tool steel through chemical composition design and specific heat treatment, while taking into account the economic requirements of tool manufacturing. The tool steel of the present invention meets the index requirements of cutting tools and can obtain more significant fatigue toughness and economy than ordinary quenching + tempering alloy steel. In terms of economic benefits, the smelting cost is reduced by adding a low amount of alloy. The cost of processing hard materials in the mechanical processing process is relatively high. This material is formed in a low hardness state and then subjected to surface treatment to reduce processing difficulty and economic costs; in addition, the toughness of the matrix and the transition section will make the tool as a whole less likely to break and increase the service life of the tool; in addition, since the surface hardness is very high and the wear resistance is high, the service life of the tool is also increased.

[0026] The steel of this invention uses a low alloy addition and is processed based on basic heat treatment, achieving a hardness of 45-47 HRC, resulting in low material and processing costs. After the tool is formed, heat treatment is performed to achieve an outer layer hardness of 55-58 HRC, ensuring wear resistance while minimizing deformation. Compared to high-hardness alloy steels such as H13 and SKH9, this tool steel offers lower material costs, comparable hardness, and far superior fatigue resistance. DETAILED DESCRIPTION

[0027] The present invention will be described in detail below, but it should be noted that the implementation of the present invention is not limited to the following embodiments.

[0028] The present invention can effectively realize medium carbon fatigue-resistant tool steel through chemical composition design and specific heat treatment, meet the requirements of cutting tool indicators, and obtain more significant fatigue toughness and economy than ordinary quenching + tempering alloy steel.

[0029] The chemical composition weight percentage of a medium carbon fatigue-resistant tool steel is:

[0030] C: 0.45%~0.50%, Si: 0.25%~0.30%, Mn: 0.50%~0.60%, P≤0.010%, S≤0.015%, Cr: 2.00%~2.20%, Ni: 1.80%~2.20%, Cu: 0.20%~0.50%, N≤0.0045%, the rest are Fe and unavoidable impurities.

[0031] The processing hardness of the tool steel is 45-47HRC, and the hardness of the outer layer of the tool formed from the steel is 55-58HRC.

[0032] The steel material for the basic heat treatment of medium-carbon fatigue-resistant tool steel is not particularly limited in terms of the smelting method, forging, rolling process, or rough processing shape before final processing. The quenching heating time for the process heat treatment should be determined according to the thickness of the blade.

[0033] Medium-carbon fatigue-resistant tool steel undergoes a phased heat treatment. The practical realization of low-alloy, high-wear-resistant properties for medium-carbon fatigue-resistant tool steel is accomplished through a combination of basic heat treatment and process treatment. Basic heat treatment is performed on unprocessed steel, with the microstructure and hardness achieved through quenching and tempering serving as the toughness matrix for the tool. Process treatment heat treatment is the surface heat treatment of the processed tool. Specifically, it includes the following steps:

[0034] 1) Basic heat treatment: Heat the tool steel to 850~870℃, keep it warm until it is completely austenitized (holding time 2.2~2.5min / mm), quench it with quenching oil and cool it to below 50℃, then perform medium temperature tempering at 330~350℃, keep it warm for 2~4h, then water cool or air cool it to room temperature, and the structure is tempered martensite;

[0035] Through basic heat treatment, the high strength matrix hardness of low alloy tool steel is achieved. The matrix strength of the tool steel reaches a yield strength of 1350~1550MPa, a tensile strength of 1600~1650MPa, and a hardness of 45~47HRC. During the use of the tool, it has the ability to support the tool body firmly and not easily deformed, and maintain a tough state with a room temperature impact energy KV2 of 20~30J. At this time, the tool steel has high hardness and fatigue resistance while taking into account the economic requirements of tool manufacturing.

[0036] 2) The machined cutting tool is quenched in a salt bath furnace at a temperature of 900-920°C. After holding the bath in salt for 10-360 seconds, it is rapidly cooled to below 50°C using quenching oil. Surface salt bath quenching austenitizes the surface and near-surface structures with molten salt, thermally affecting the matrix without fully austenitizing it. The resulting transitional structure gradually changes in hardness. The holding time is adjusted based on the blade thickness and the desired martensite layer thickness. This creates a hardness gradient between the surface and matrix, extending tool life. The quenched tool is then tempered at 180-200°C, held for 2-4 hours, and then air-cooled to room temperature, resulting in a tempered martensite structure.

[0037] Through process heat treatment, the processed low-alloy tool steel can obtain higher surface hardness, with the surface strength reaching 1100~1250MPa yield strength, 2000~2200MPa tensile strength, and 54~58HRC hardness. During the use of the tool, it has good cutting performance, small deformation, and maintains the room temperature impact energy KV2 at 7~15J. At this time, due to the rapid cooling of the tool steel surface to produce cryptocrystalline martensite, the hardness and toughness are better than the conventional acicular martensite performance, which improves the wear resistance and fatigue resistance of the tool and increases the service life.

[0038] Example 1

[0039] A 21-ton steel ingot manufactured by continuous casting and electroslag remelting was rolled into a 30-mm-thick steel plate by hot forging to serve as a tool blank. The composition of the steel ingot is shown in Table 1.

[0040] Table 1 Blank composition (mass %)

[0041]

[0042] The steel plates were cut into 30×70×250 mm tool blanks for basic heat treatment. The steel plates were quenched in a box-type resistance furnace, heated to 870°C for 1 hour, and then quenched with quenching oil. The quenched steel plates were then tempered at 350°C for 4 hours, then water-cooled to room temperature. The steel plates were then cut and tested for mechanical properties. The mechanical properties are shown in Table 2. The steel plates that completed basic heat treatment were then finished into tool blanks for process heat treatment. Quenching was performed in a 450×450×750 mm salt bath furnace at 910°C. The tool was immersed in molten salt for 40 seconds, then quenched and cooled to room temperature. The quenching medium was quenching oil. Low-temperature tempering was then performed, holding at 180°C for 2 hours, and then mechanical properties were tested. Test specimens were cut from the tool and tested. The results are shown in Table 3. When the cutting tool was tested with 0.8mm thick blued steel strip, the number of cutting times was 42,000, exceeding the life requirement of 30,000 times.

[0043] Table 2 Basic heat treatment mechanical properties

[0044]

[0045] Table 3 Mechanical properties of heat treatment process

[0046]

[0047] As shown in the above results, it can be seen that the application of the composition of Example 1 and the staged heat treatment effectively achieves the high hardness and fatigue resistance of the material, and meets the requirements of low cost and long life.

[0048] Example 2

[0049] 11 tons of die-cast steel ingots were hot-forged into bars of 50 × 170 × 260 mm to serve as tool blanks. The composition of the steel ingots is shown in Table 4.

[0050] Table 4 Billet composition (mass %)

[0051]

[0052] The steel plates were cut into 24×60×210 mm tool blanks for basic heat treatment. The steel plates were quenched in a box-type resistance furnace, heated to 860°C for 1 hour, and then quenched with quenching oil. The quenched steel plates were then tempered at 350°C for 2 hours, then water-cooled to room temperature. The steel blanks were then cut and tested for mechanical properties. The results are shown in Table 5. The steel plates, which had undergone basic heat treatment, were then finished into tool blanks for process heat treatment. Quenching was performed in a 450×450×750 mm salt bath furnace at 900°C. The tool was immersed in molten salt for 40 seconds, then quenched and cooled to room temperature. The quenching medium was quenching oil. Low-temperature tempering was then performed, held at 180°C for 2 hours, and then mechanical properties were tested. Test specimens were cut from the tool. The test results are shown in Table 6. When the cutting tool was tested with 0.8mm thick blued steel strip, the number of cutting times was 44,000, exceeding the life requirement of 30,000 times.

[0053] Table 5 Basic heat treatment mechanical properties

[0054]

[0055] Table 6 Mechanical properties of heat treatment process

[0056]

[0057] As shown in the above results, it can be seen that the application of the composition of Example 2 and the staged heat treatment effectively achieves the high hardness and fatigue resistance of the material, and achieves the index requirements of low cost and long life.

[0058] Example 3

[0059] 11 tons of die-cast steel ingots were hot-forged into bars measuring 50 mm x 170 mm x 260 mm to serve as tool blanks. The composition of the steel ingots is shown in Table 7.

[0060] Table 7 Blank composition (mass %)

[0061]

[0062] The steel plates were cut into 24×60×210 mm tool blanks for basic heat treatment. The steel plates were quenched in a box-type resistance furnace, heated to 860°C for 1 hour, then quenched with oil as the medium. The quenched steel plates were then tempered at 340°C for 2 hours, then water-cooled to room temperature. The steel blanks were then cut and tested for mechanical properties. The results are shown in Table 8. The steel plates, which had undergone basic heat treatment, were then finished into tool blanks for process heat treatment. Quenching was performed in a 450×450×750 mm salt bath furnace at 910°C. The tool was immersed in molten salt for 50 seconds, then quenched and cooled to room temperature. The quenching medium was quenching oil. Low-temperature tempering was then performed, held at 180°C for 2 hours, and then mechanical properties were tested. Test specimens were cut from the tool. The test results are shown in Table 9. The cutting tool was tested using 0.8mm thick blued steel strip and the number of cutting times was 45,000, exceeding the life requirement of 30,000 times.

[0063] Table 8 Basic heat treatment mechanical properties

[0064]

[0065] Table 9 Mechanical properties of heat treatment process

[0066]

[0067] As shown in the above results, it can be seen that the application of the composition of Example 3 and the staged heat treatment effectively achieves the high hardness and fatigue resistance of the material, and meets the requirements of low cost and long life.

[0068] Example 4

[0069] 11 tons of die-cast steel ingots were hot-forged into bars measuring 50 mm x 170 mm x 260 mm to serve as tool blanks. The composition of the steel ingots is shown in Table 10.

[0070] Table 10 Billet composition (mass %)

[0071]

[0072] The steel plates were cut into 38×60×210 mm tool blanks for basic heat treatment. The steel plates were quenched in a box-type resistance furnace, heated to 860°C for 1 hour, and then quenched with oil as the medium. The quenched steel plates were then tempered at 330°C for 2 hours, then water-cooled to room temperature. The steel blanks were then cut and tested for mechanical properties. The results are shown in Table 11. The steel plates, which had undergone basic heat treatment, were then finished into tool blanks for process heat treatment. Quenching was performed in a 450×450×750 mm salt bath furnace at 900°C. The tool was immersed in the molten salt for 60 seconds, then quenched and cooled to room temperature. The quenching medium was quenching oil. Low-temperature tempering was then performed at 180°C for 2 hours. Mechanical properties were then tested. Test specimens were cut from the tool. The test results are shown in Table 12. When the cutting tool was tested with 0.8mm thick blued steel strip, the number of cutting times was 43,000, exceeding the life requirement of 30,000 times.

[0073] Table 11 Basic heat treatment mechanical properties

[0074]

[0075] Table 12 Mechanical properties of heat treatment process

[0076]

[0077] As shown in the above results, it can be seen that the application of the composition of Example 4 and the staged heat treatment effectively achieves the high hardness and fatigue resistance of the material, and achieves the index requirements of low cost and long life.

[0078] Example 5

[0079] 11 tons of die-cast steel ingots were hot-forged into bars measuring 50 mm x 170 mm x 260 mm to serve as tool blanks. The composition of the steel ingots is shown in Table 13.

[0080] Table 13 Blank composition (mass %)

[0081]

[0082] The steel plates were cut into 38×60×210 mm tool blanks for basic heat treatment. The steel plates were quenched in a box-type resistance furnace, heated to 860°C for 1 hour, and then quenched with quenching oil. The quenched steel plates were then tempered at 360°C for 2 hours, then water-cooled to room temperature. The steel blanks were then cut and subjected to mechanical property testing. The results are shown in Table 14. The steel plates, which had undergone basic heat treatment, were then finished into tool blanks for process heat treatment. Quenching was performed in a 450×450×750 mm salt bath furnace at 910°C. The tool was immersed in molten salt for 55 seconds, then quenched and cooled to room temperature. The quenching medium was quenching oil. Low-temperature tempering was then performed, with a holding temperature of 200°C for 2 hours. Mechanical property testing was then performed. Test specimens were cut from the tool. The test results are shown in Table 15. When the cutting tool was tested with 0.8mm thick blued steel strip, the number of cutting times was 45,000, exceeding the life requirement of 30,000 times.

[0083] Table 14 Basic heat treatment mechanical properties

[0084]

[0085] Table 15 Mechanical properties of heat treatment process

[0086]

[0087] As shown in the above results, it can be seen that the application of the composition of Example 5 and the staged heat treatment effectively achieves the high hardness and fatigue resistance of the material, and meets the requirements of low cost and long life.

Claims

1. A medium carbon fatigue-resistant tool steel, characterized in that: The chemical composition weight percentage of the steel is: C: 0.45%~0.50%, Si: 0.25%~0.30%, Mn: 0.50%~0.60%, P≤0.010%, S≤0.015%, Cr: 2.00%~2.20%, Ni: 1.80%~2.20%, Cu: 0.20%~0.50%, N≤0.0045%, the rest are Fe and unavoidable impurities; The heat treatment method for medium carbon fatigue-resistant tool steel comprises the following steps: 1) Basic heat treatment: Heat the tool steel to 850~870℃, keep it warm until it is completely austenitized, then quench it in quenching oil and cool it to below 50℃. Then, perform medium temperature tempering at 330~350℃, keep it warm for 2~4h, and then water cool or air cool it to room temperature. 2) Process heat treatment: The machined tool is heated and quenched in a salt bath at a temperature of 900~920℃. After the salt is put into the bath and kept warm for 10~360s, it is quenched and rapidly cooled to below 50℃ using quenching oil. Then, it is tempered at 180~200℃ and kept warm for 2~4h before being air-cooled to room temperature for stress relief. The structure is tempered martensite with a gradual hardness change.

2. The medium carbon fatigue-resistant tool steel according to claim 1, characterized in that: The processing hardness of the steel is 45-47HRC, and the hardness of the outer layer of the tool formed from the steel is 55-58HRC; the cutting tool is tested with a 0.8mm thick blued steel strip and the cutting times are more than 30,000.

3. A heat treatment method for medium carbon fatigue-resistant tool steel according to claim 1 or 2, characterized in that: The following steps are involved: 1) Basic heat treatment: Heat the tool steel to 850~870℃, keep it warm until it is completely austenitized, then quench it in quenching oil and cool it to below 50℃. Then, perform medium temperature tempering at 330~350℃, keep it warm for 2~4h, and then water cool or air cool it to room temperature. 2) Process heat treatment: The machined tool is heated and quenched in a salt bath at a temperature of 900~920℃. After the salt is put into the bath and kept warm for 10~360s, it is quenched and rapidly cooled to below 50℃ using quenching oil. Then, it is tempered at 180~200℃ and kept warm for 2~4h before being air-cooled to room temperature for stress relief. The structure is tempered martensite with a gradual hardness change.

4. The heat treatment method for medium carbon fatigue-resistant tool steel according to claim 3, characterized in that: The step 1) obtains a tool steel substrate having a yield strength of 1350-1550 MPa, a tensile strength of 1600-1650 MPa, and a hardness of 45-47 HRC, and maintains a tough state with an impact energy KV2 of 20-30 J at room temperature, and a structure of tempered martensite.

5. The heat treatment method for medium carbon fatigue-resistant tool steel according to claim 3, characterized in that: In step 2), the surface strength of the tool reaches a yield strength of 1100-1250 MPa, a tensile strength of 2000-2200 MPa, a hardness of 54-58 HRC, and an impact energy KV2 of 7-15 J at room temperature, and the structure is tempered martensite.

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