A high life tool steel and a carburizing method for a tool

Abstract

The application belongs to the technical field of metal materials, and particularly relates to a high-life cutter steel and a carburizing method of a cutter. The chemical composition of the steel is as follows: C: 0.18%-0.22%, Si: 0.25%-0.30%, Mn: 0.50%-0.60%, P<=0.010%, S<=0.015%, Cr: 1.9%-2.0%, Ni: 1.80%-2.20%, Cu: 0.20%-0.50%, and the rest is Fe and inevitable impurities. The steel can effectively meet the performance requirements of fatigue resistance and high hardness of the cutter steel through component design and heat treatment methods, and a manufacturing method with significantly improved comprehensive mechanical properties and fatigue resistance is obtained. Through inhibition of carbide precipitation, optimization of carbon distribution and reduction of residual austenite, the surface hardness and fatigue performance are simultaneously improved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of metal materials, and particularly relates to a high-life cutter steel and a carburizing method of a cutter. BACKGROUND

[0002] Traditional cutters are manufactured from the same material, and when the hardness requirement is higher than 55HRC, the machining cost is significantly increased, and the service life of the cutter is generally short.

[0003] Currently, common cutter steels on the market, such as H13, SKH9 and the like high-hardness alloy steel, have high hardness, but the material cost is high, and there is a deficiency in fatigue performance, so that the durability and economy of the cutter under high-strength cutting operation cannot be well balanced.

[0004] In the prior art, patent application No. CN202211221368.1 discloses a high-hardness high-nitrogen martensitic stainless steel cutter material and a preparation method thereof, the chemical composition and austenite structure of the stainless steel material are adjusted, and the high-nitrogen austenitic stainless steel is prepared into a high-nitrogen supersaturated solid solution martensitic stainless steel through solid phase transformation. The heat treatment combination of solid solution-aging-quenching is adopted to adjust the composition of the austenite matrix phase, and the composition adjustment is suitable for obtaining a dispersion strengthened high-strength martensite according to the Schaeffler phase diagram, so as to improve the toughness of the high-hardness metal material. The overall hardness is higher than 55HRC, but the fatigue performance cannot meet the requirements of the cutting tool.

[0005] The hardness and toughness of the cutter are a double-edged sword, low toughness will affect the service life of the cutting edge, and when the cutter cuts hard objects at a high frequency, the same hardness of the cutting edge and the matrix will cause the cutting edge to collapse due to high hardness, thereby reducing the service life. If the service life is to be improved, a transition zone with a hardness gradient needs to be established between the cutting edge and the matrix, and the toughness gradually increases as the hardness of the matrix decreases, thereby relieving the high-strength load.

[0006] Therefore, it is particularly urgent to develop a high-life cutter steel, which aims to balance the material cost, machining cost and durability of the cutter through optimization of the chemical composition and heat treatment process, and the high life also reflects in corrosion resistance and high impact work. SUMMARY

[0007] To overcome the deficiencies of the prior art, the purpose of the present application is to provide a high-life cutter steel and a carburizing method of a cutter, which is designed according to the high strength, high wear resistance and economic performance of the cutting tool, and realizes different mechanical properties of the core and surface of the same material through chemical composition design and carburizing heat treatment. The matrix of the cutter steel shows good strength and toughness and supportability during use, and the cutting edge surface has good load-bearing strength and hardness.

[0008] To achieve the above object, the present application realizes the following technical scheme:

[0009] A high-life cutter steel, the chemical composition of the steel is as follows:

[0010] C: 0.18%~0.22%, Si: 0.25%~0.30%, Mn: 0.50%~0.60%, P≤0.010%, S≤0.015%, Cr: 1.9%~2.0%, Ni: 1.80%~2.20%, Cu: 0.20%~0.50%, the rest is Fe and inevitable impurities.

[0011] The present application selects the above alloy element types and contents because of the role of each element in the high-life cutter steel:

[0012] C: 0.18%~0.22%; Carbon is an important element for precipitation strengthening and martensite formation based on carbide. The carbon in this range will form the matrix hardness of the cutter after carburizing quenching, provide the support and toughness of the cutting edge. In order to obtain the above-mentioned effect, the core hardness is low (20~30HRC) before carburizing, the plasticity is good, the impact load can be borne, and the carbon below 0.22% is needed to avoid brittle fracture. On the other hand, if the matrix carbon content is too high, the surface carbon after carburizing may exceed 1.0%, which leads to coarse carbide or high brittle martensite easy to crack, and affects the inward diffusion of carbon, so the upper limit of carbon is set to 0.22%.

[0013] Si: 0.25%~0.30%; Si can be added as a deoxidizing element during manufacturing. Si produces ferrite solid solution strengthening effect, and 0.25% or more content is needed to obtain the above-mentioned effect. If Si exceeds 0.30%, the temper resistance strength of the alloy increases, and the low-temperature embrittlement phase becomes easy to precipitate, so the upper limit of Si is set to 0.30%.

[0014] Mn: 0.50%~0.60%; Mn has the same deoxidizing effect as Si, and can be added during manufacturing. MnS inclusions serve as pitting sources, and low Mn≤0.60% is designed to improve corrosion resistance in combination with 2%Cr / 2%Ni / 0.5%Cu. Excellent impact toughness and Ni+Cu combination are brought by 260℃ salt bath tempering+deep cooling process.

[0015] Cr: 1.9%~2.0%; Appropriate addition of Cr can increase strength and wear resistance, and reduce decarburization of steel during heat treatment, but Cr has the same effect as Ni to reduce the martensite phase transition temperature. Excessive addition of Cr will cause the strength to decrease due to the increase of residual austenite, so the upper limit is set to 2.0%. The Cr content of 1.9% is the threshold of the continuity of the protective rust layer, and when the Cr content is less than 1.7%, the rust layer resistivity drops by an order of magnitude, and when the Cr content is greater than or equal to 1.9%, the Cl- diffusion coefficient decreases by 45% (weight loss from 0.18 to 0.10 mm / a after 72h immersion); Cooperate with Cu and Ni, when Cr / Cu≥4 and Cr / Ni≈1, the pitting potential is +80mV, which increases the ability of the protective rust layer. When in salt bath, Cr forms <5nm M2C / M7C3 clusters with C, and the hardness increases by 1-1.5HRC, and if Cr<1.8%, almost no such dispersion strengthening peak is observed, and the hardness will decrease after cryogenic treatment.

[0016] Ni: 1.80%~2.20%; Ni is solid-solved in the parent phase, which helps to solid-solution strengthen the material, improve the hardenability, increase the strength of the steel without lowering the toughness, and also helps to improve the low-temperature toughness and fatigue resistance, so it must be added. Under the condition of low Mn, the Cl- diffusion coefficient decreases by 45% when the Ni content is close to 2%, which improves the corrosion resistance.

[0017] Cu: 0.20%~0.50%; In the present application, Cu can stabilize the microstructure during surface quenching, increase the strength of the alloy during quenching, and have a certain precipitation strengthening effect. On the other hand, if Cu is added in excess, it will increase the intergranular weakening of the metal, which is not conducive to plasticity and hot workability, so the upper limit of Cu is set to 0.50%. Under the condition of low Mn, the rust layer resistivity is increased by 1.8 times when the copper content is about 0.4%, which improves the corrosion resistance.

[0018] The balance is Fe and impurity elements inevitably mixed during manufacturing. As representative impurity elements, S, P, N, etc. can be considered. Ideally, the amount of these elements is less, and the amount that can be reduced according to the manufacturing equipment commonly used, the less each element is the better.

[0019] The work hardening of the steel is 30~35HRC. Its state is hot-rolled or stress-relief annealed state, as a raw material for carburizing heat treatment, and the microstructure state is bainite + ferrite.

[0020] It should be noted that the steel for carburizing heat treatment as a raw material, the steel billet smelting method, forging, rolling process, and rough machining shape before final machining are not particularly limited.

[0021] The steel with the aforementioned one carburizing high-life tool is subjected to carburizing heat treatment, the carburizing heat treatment is mainly used for surface carbon control, and after carburizing, the wear resistance and high-life requirement of the tool edge surface carbon mass fraction is controlled at 0.8% to 1.1%, which is specifically determined according to the gradient design, the carbon content of the carburized layer gradually decreases from the surface to the center, and the carburized layer depth of the tool edge subjected to the process heat treatment is 0.9 to 1.2 mm.

[0022] A carburizing method of a high-life tool steel tool, comprising the following steps:

[0023] (1) Carbon potential grading control:

[0024] The strong carburizing stage adopts dynamic control of low carbon potential 0.9% to 1.0%, inhibits the explosive nucleation of Cr carbide, and the temperature is controlled at 930 to 940℃, and the holding time is 3 to 4 hours.

[0025] The diffusion stage is divided into two stages, the high-temperature diffusion temperature is 930 to 950℃, the time is 2 to 3 hours, the carbon potential is 0.7% to 0.75%, the high-temperature promotes the partial dissolution of the formed small carbide, and releases carbon atoms to migrate to the inside. The low-temperature diffusion temperature is 880 to 900℃, the time is 3 to 4 hours, the carbon potential is 0.55% to 0.60%, the medium-temperature balances the carbon gradient, and avoids the steep drop of the surface carbon concentration.

[0026] (2) Austenitizing adjustment before quenching:

[0027] Two-stage austenitizing, the first stage, high-temperature stage temperature is 980 to 990℃, holding time is 30 to 40 minutes, dissolving large block Cr carbide, releasing carbon to austenite. The second stage, rapidly cooled to 880 to 890℃, holding time is 20 to 30 minutes, inhibiting carbide reprecipitation, and refining grains.

[0028] (3) Isothermal treatment:

[0029] The quenching medium adopts high-speed quenching oil, and the expected cooling speed is ≥110℃ / s, which compensates for the insufficient hardenability caused by low Mn, reduces the generation of bainite, and generates lower bainite+martensite mixed structure in the matrix in the 250 to 300℃ salt bath for 5 to 10 minutes, so as to improve the toughness. The tool edge surface layer has been completely quenched into high-carbon flaky martensite and 8% to 15% residual austenite, the internal stress is extremely high, the quenching stress dislocation climb+microzone yield surface pressure is reduced by 30% to 40%, the grinding crack rate is reduced by >90%, the brittleness is reduced, the carbide is precipitated by 2 to 4 nm, the impact value is increased by 20% to 30%, the tensile stress is removed, and microcracks are avoided during deep cooling.

[0030] (4) Deep cooling treatment:

[0031] The deep cryogenic treatment forces the transformation of residual austenite, and after isothermal treatment, the tool piece is immediately placed in a -75 to -85 DEG C liquid nitrogen-alcohol mixture for deep cryogenic holding, and the holding time is 4 to 5 hours, so that the residual austenite is reduced to less than 5%, and the phase transformation stress is reduced. The residual austenite of the high-Ni steel is reduced to reduce the negative influence on the hardness, and the dimensional stability is improved. After the holding is completed, the tool piece is placed in air to return to room temperature naturally.

[0032] (5) low-temperature tempering:

[0033] The low-temperature tempering temperature is 180 to 190 DEG C, and the holding time is 2 to 3 hours, so that more than 80% of the precipitation strengthening effect is retained, and the quenching stress is eliminated.

[0034] After the tool piece formed by the steel is heat treated, the hardness of the outer layer of the tool piece is 58 to 60 HRC; the cutting tool is tested by using 0.6 to 0.8 mm thick bluing packaging steel strips, the cutting frequency is 44 to 53 thousand times, the matrix impact energy KV2 is greater than or equal to 125 J, the blade impact energy KV2 is greater than or equal to 12 J, and the seawater immersion corrosion rate is less than or equal to 0.6 mm / a.

[0035] After the tool piece formed by the steel is heat treated, the hardness of the outer layer of the tool piece is 58 to 60 HRC; the cutting tool is tested by using 0.6 to 0.8 mm thick bluing packaging steel strips, the cutting frequency is 44 to 53 thousand times, the matrix impact energy KV2 is greater than or equal to 125 J, the blade impact energy KV2 is greater than or equal to 12 J, and the seawater immersion corrosion rate is less than or equal to 0.6 mm / a.

[0036] Compared with the prior art, the beneficial effects of the present application are:

[0037] 1. The present application can effectively meet the performance requirements of fatigue resistance and high hardness of the tool steel through component design and heat treatment method, and a manufacturing method with significantly improved comprehensive mechanical properties and fatigue resistance is obtained. By inhibiting carbide precipitation, optimizing carbon distribution and reducing residual austenite, the surface hardness and fatigue performance are simultaneously improved. The steel component cooperates with the heat treatment process, the surface carbide size is 1 to 3 microns, the effective hardening layer depth is 0.9 to 1.2 mm, the surface hardness is 58 to 60 HRC, the core impact energy is greater than or equal to 50, the cutting steel strip frequency is more than 40,000 times, and the high life characteristics are realized.

[0038] 2. Low alloying amount, controllable cost: under the basic heat treatment condition, the hardness is only 30 to 35 HRC, which effectively reduces the material cost and processing cost.

[0039] 3. Excellent performance after heat treatment: after the tool piece is formed and subjected to process heat treatment, the surface hardness can reach 58 to 60 HRC, while ensuring high wear resistance, the matrix still has high strength and toughness, and the degree of deformation is extremely small.

[0040] 4. The obvious advantages compared with traditional materials: compared with H13, SKH9 and other high hardness alloy steel, in the case of similar hardness, the material not only has lower cost, but also overcomes the problem of insufficient fatigue resistance of traditional high hardness materials. BRIEF DESCRIPTION OF DRAWINGS

[0041] Figure 1 is a metallographic diagram of the present application. DETAILED DESCRIPTION

[0042] The present application will be described in detail below, but it should be pointed out that the implementation of the present application is not limited to the following embodiments.

[0043] Example 1:

[0044] A 13-ton ingot manufactured by continuous casting + electroslag remelting was rolled into a steel plate with a thickness of 30 mm by hot forging as a tool blank. The composition of the ingot is shown in Table 1.

[0045] Table 1 Blank composition (mass%)

[0046]

[0047] The steel plate was cut into a tool blank of 30x70x250mm, and after finishing, carburizing heat treatment was performed. After cleaning the surface of the tool blank with isopropyl alcohol, it was loaded into the furnace, and a dynamic control of low carbon potential 0.9% was used in the strong penetration stage at a temperature of 930℃ for 3 hours. In the diffusion stage, two stages were performed, the high temperature diffusion temperature was 935℃, the carbon potential was 0.7%, and the time was 3 hours. The low temperature diffusion temperature was 890℃, the carbon potential was 0.6%, and the time was 3.5 hours.

[0048] Two-stage austenitizing, high temperature austenitizing 980℃ for 30 minutes, then rapid cooling to 880℃ for 20 minutes. The quenching medium is high-speed quenching oil cooling, isothermal for 10 minutes in a 260℃ salt bath. Immediately after quenching, deep cooling was carried out in a -80℃ liquid nitrogen-alcohol mixture (liquid nitrogen is added to alcohol and stirred until -80℃), the holding time was 4.5 hours, and the residual austenite content was reduced to 4%. After the tool was placed at room temperature, low temperature tempering was carried out at a temperature of 180℃ for 2 hours. The cutting tool was tested for cutting times using a steel strip with a thickness of 0.6mm, and the performance is shown in Table 2.

[0049] Table 2 Performance

[0050]

[0051] Example 2:

[0052] A 13-ton ingot manufactured by continuous casting + electroslag remelting was rolled into a steel plate with a thickness of 30 mm by hot forging as a tool blank. The composition of the ingot is shown in Table 3.

[0053] Table 3: Composition of the blank (in mass %)

[0054]

[0055] The steel plate was cut into a cutter blank of 30 x 70 x 250 mm, and after finish machining, carburizing heat treatment was performed. After cleaning the surface of the cutter blank with isopropyl alcohol, the blank was loaded into the furnace, and in the strong carburizing stage, dynamic control was used with a low carbon potential of 0.95% at a temperature of 930°C for 3 hours. In the diffusion stage, two stages were used, with a high temperature diffusion temperature of 950°C, a carbon potential of 0.75% for 2 hours. A low temperature diffusion temperature of 900°C was used for 4 hours, and the carbon potential was 0.65%.

[0056] Two-stage austenitizing was performed at a high temperature of 980°C for 30 minutes, followed by rapid cooling to 880°C for 20 minutes. The quenching medium was high-speed quenching oil, and the quenching medium was a salt bath at 300°C for 10 minutes. After quenching, the tool was immediately placed in a -85°C liquid nitrogen-alcohol mixture for cryogenic treatment for 4.5 hours, and the residual austenite content was reduced to 4%. After the tool was brought to room temperature, low-temperature tempering was performed at a temperature of 180°C for 2 hours. The cutting tool was tested for cutting times using a steel strip with a thickness of 0.7 mm, and the performance is shown in Table 4.

[0057] Table 4: Performance

[0058]

[0059] Example 3:

[0060] A 13-ton steel ingot manufactured by continuous casting + electroslag remelting was hot-forged into a steel plate with a thickness of 30 mm as a cutter blank. The composition of the steel ingot is shown in Table 5.

[0061] Table 5: Composition of the blank (in mass %)

[0062]

[0063] The steel plate was cut into a cutter blank of 30 x 70 x 250 mm, and after finish machining, carburizing heat treatment was performed. After cleaning the surface of the cutter blank with isopropyl alcohol, the blank was loaded into the furnace, and in the strong carburizing stage, dynamic control was used with a low carbon potential of 0.95% at a temperature of 930°C for 3 hours. In the diffusion stage, two stages were used, with a high temperature diffusion temperature of 935°C, a carbon potential of 0.7% for 3 hours. A low temperature diffusion temperature of 890°C was used for 3.5 hours, and the carbon potential was 0.65%.

[0064] Two-stage austenitizing, high temperature austenitizing 980℃ for 30 minutes, then rapid cooling to 880℃ for 20 minutes. Quenching medium adopts high speed quenching oil cooling, 280℃ salt bath isothermal for 10 minutes. After quenching, immediately into -75℃ liquid nitrogen alcohol mixture for deep cooling, the holding time is 4.5 hours, the residual austenite content is reduced to 4%. After the tool is placed at room temperature, low temperature tempering temperature is 180℃, holding for 2 hours. The cutting tool cutting frequency is tested by using steel strip with thickness of 0.6mm, the performance is shown in table 6.

[0065] Table 6 Performance

[0066]

[0067] Comparative example:

[0068] In order to compare the corrosion performance of the tool steel and the ordinary carbon steel tool with the same carbon content, the seawater immersion test is carried out. The test uses the comparative plain carbon steel T10, and the surface C0.97% group of the carburized tool is selected for testing, and the results are as follows:

[0069] Table 7 Results of seawater immersion test for 72h

[0070]

[0071] As shown in the above results, it can be known that only by using the method of the present application through component design and heat treatment can the performance requirements of high life and high hardness of the tool steel be effectively realized, and the index requirements of obtaining comprehensive mechanical properties and significantly improving the life are realized.

Claims

1. A carburizing method for a cutting tool made of high-life cutting tool steel, characterized in that, Includes the following steps: (1) Carbon potential hierarchical control: Strong infiltration stage: adopt dynamic control of low carbon potential (0.9%~1.0%), temperature (930~940℃), and heat preservation for 3~4 hours; Diffusion stage: This is carried out in two phases. The high-temperature diffusion temperature is controlled at 930~950℃ for 2~3 hours, with a carbon potential of 0.7%~0.75%. The low-temperature diffusion temperature is controlled at 880~900℃ for 3~4 hours, with a carbon potential of 0.55%~0.6%. (2) Austenitization adjustment before quenching: Two-stage austenitization: First stage, high-temperature austenitization to dissolve carbides at 980~990℃ for 30~40 minutes; Second stage, rapid cooling to 880~890℃ and holding for 20~30 minutes. (3) Isothermal treatment: The quenching medium is high-speed quenching oil, and the quenching isotherm ... (4) Cryogenic treatment: After isothermal treatment, the tool is immediately placed in a liquid nitrogen-alcohol mixture at -75~-85℃ for cryogenic insulation for 4~5 hours; after the insulation is completed, the tool is placed in the air to return to room temperature naturally. (5) Low-temperature tempering: The low-temperature tempering temperature is 180~190℃, and the holding time is 2~3 hours; The chemical composition of the steel, by weight percentage, is as follows: C: 0.18%~0.22%, Si: 0.25%~0.30%, Mn: 0.50%~0.60%, P≤0.010%, S≤0.015%, Cr: 1.9%~2.0%, Ni: 1.80%~2.20%, Cu: 0.20%~0.50%, with the remainder being Fe and unavoidable impurities.

2. The carburizing method for a cutting tool made of high-life cutting tool steel according to claim 1, characterized in that, The thickness of the carburized layer on the blade treated by the aforementioned carburizing method is 0.9~1.2mm.

3. The carburizing method for a cutting tool made of high-life cutting tool steel according to claim 1, characterized in that, The carbon mass fraction on the blade surface treated by the aforementioned carburizing method is controlled at 0.8%~1.1%.

4. The carburizing method for a cutting tool made of high-life cutting tool steel according to claim 1, characterized in that, The hardness of the outer layer of the cutting tool formed from the steel is 58~60HRC; the cutting tool can withstand 44,000 to 53,000 cutting cycles when tested with 0.6~0.8mm thick bluing steel strip, the matrix impact energy KV2≥125J, the cutting edge impact energy KV2≥12J, and the seawater immersion corrosion rate ≤0.6mm / a.

5. The carburizing method for a cutting tool made of high-life cutting tool steel according to claim 1, characterized in that, The outer layer of the cutting tool formed from the steel, after heat treatment, has a microstructure of tempered martensite + a small amount of retained austenite + precipitated copper and carbon nano-reinforcing phases, with the retained austenite content ≤3%; the inner layer of the cutting tool has a microstructure of more than 97% tempered martensite, with the remainder being a very small amount of retained austenite, bainite and precipitated copper and carbon nano-reinforcing phases.

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