Martensitic steel and preparation method thereof

Abstract

The present invention discloses a martensitic steel and a preparation method thereof, belonging to the technical field of metal materials. The present invention provides a martensitic steel comprising the following chemical elements by weight: C 0.3-0.6wt%, Mn 2-2.6wt%, Cr 0.3-0.7wt%, Ni 0.05-0.2wt%, Mo 0.4-0.8wt%, Si 1.3-1.7wt%, V 0.05-0.2wt%, Al 1-1.5wt%, with the remainder being iron and unavoidable impurities. The present invention also provides a preparation method of the martensitic steel, comprising smelting and forging, austenitizing, quenching, and aging treatment. By optimizing the alloy composition and preparation process of the martensitic steel, the martensitic steel has improved wear resistance and mechanical properties.

Description

Technical Field

[0001] The present invention belongs to the technical field of metal materials, and in particular relates to martensitic steel and a preparation method thereof. Background Art

[0002] Martensitic wear-resistant steel, due to its high strength and durability, has become the preferred wear-resistant steel. Furthermore, its simple manufacturing process, low material cost, and excellent and stable mechanical properties have led to its widespread application in coal mining, rail transit, and engineering machinery. Hardness, as a rating criterion for martensitic wear-resistant steel, significantly influences its wear rate. As the hardness of the matrix increases, the material's wear resistance also increases. However, the hardness and toughness of a material generally have an inverse relationship: increasing hardness leads to a significant decrease in toughness, which can easily lead to brittle fracture during service and significantly reduce safety. Furthermore, under the same hardness conditions, the greater the toughness of a wear-resistant material, the higher its wear resistance. By selecting specific elements, adjusting their content, and optimizing the heat treatment process, it is possible to improve both the overall mechanical properties and the wear resistance of martensitic wear-resistant steel. Therefore, exploring the mechanisms by which the composition and heat treatment process of martensitic steel affect its mechanical and wear properties, and developing high-wear-resistant martensitic steel with excellent combined mechanical properties, is of great research significance and industrial application value. Summary of the Invention

[0003] In response to the above-mentioned prior art, the present invention provides a martensitic steel and a preparation method thereof, which optimizes the alloy composition and heat treatment process of the martensitic steel, thereby improving the wear resistance of the material while enhancing its comprehensive mechanical properties.

[0004] In order to achieve the above object, the technical solution adopted by the present invention is: to provide a martensitic steel, wherein the mass percentages of chemical elements are as follows: C 0.3-0.6 wt%, Mn 2-2.6 wt%, Cr 0.3-0.7 wt%, Ni 0.05-0.2 wt%, Mo 0.4-0.8 wt%, Si 1.3-1.7 wt%, V 0.05-0.2 wt%, Al 1-1.5 wt%, and the balance is iron and inevitable impurities.

[0005] The beneficial effects of the present invention are as follows: in the high wear-resistant martensitic steel of the present invention, the carbon element directly affects the microstructure of the quenched martensite and the amount of carbide precipitation in the martensitic wear-resistant steel, thereby affecting the strength and plastic toughness. It is generally believed that the solid solution strengthening of carbon atoms is the main reason why carbon improves the strength in martensite. With the increase of carbon content, the more carbon dissolved in martensite, the higher the strength and the lower the toughness. In low-carbon martensitic steel, martensite mainly exists in the form of laths. Due to its uniform distribution of carbides and relatively weak solid solution strengthening caused by low carbon, it has better plasticity. In medium-carbon steel, martensite exists in the form of flakes and laths. Combined with the heat treatment process of low-temperature tempering, it can significantly improve the strength without sacrificing plasticity too much, thereby having relatively excellent comprehensive mechanical properties. Therefore, considering all factors, the C content is controlled to be 0.3~0.6wt%.

[0006] Manganese dissolves in ferrite or austenite in martensitic wear-resistant steel, increasing its strength and hardness. Manganese significantly lowers the pearlite transformation temperature, shifting the steel's C curve strongly to the right. This reduces the critical quenching rate, exhibiting good hardenability and facilitating the transformation to bainite and martensite. However, increasing manganese content can cause intragranular and centerline segregation, leading to a significant decrease in plasticity and toughness. Therefore, the Mn content is controlled within a range of 2–2.6 wt%.

[0007] Chromium, like manganese, provides solid solution strengthening, shifts the C curve to the right, lowers the martensitic transformation point (M), and increases hardenability. However, it also possesses many advantages over manganese. For example, chromium exhibits excellent tempering resistance, corrosion resistance, and oxidation resistance. Furthermore, chromium forms a continuous solid solution with iron and various complex carbides with carbon, such as M7C3 carbides, which enhance the strength and hardness of martensitic wear-resistant steels, thereby improving their wear resistance. Therefore, considering all factors, the Cr content is controlled between 0.3 and 0.7 wt%.

[0008] Nickel exists as a solid solution in the α and γ phases of steel, providing solid solution strengthening. Furthermore, it does not form carbides with carbon and is a key alloying element in the formation and stabilization of austenite, improving the plasticity and toughness of martensitic wear-resistant steel. However, nickel is relatively scarce, and adding more of it is cost-effective. Therefore, considering all factors, the Ni content is controlled between 0.05 and 0.2 wt%.

[0009] Molybdenum improves the hardenability of steel, promotes the formation of martensitic microstructures, and prevents temper brittleness. Furthermore, by refining grain size and inhibiting grain boundary embrittlement, it improves the plasticity, toughness, and impact resistance of martensitic wear-resistant steel. However, molybdenum is a precious metal and is expensive, so its synergistic effects with Cr and Ni must be considered in the design. Therefore, the Mo content is controlled within a range of 0.4–0.8 wt%.

[0010] Silicon readily dissolves in ferrite, providing solid solution strengthening. It also dissolves in cementite, destabilizing it and hindering its precipitation and segregation. Silicon also delays the transformation of carbides into cementite, increasing the temperature range for low-temperature temper brittleness, thereby improving temper resistance. However, excessive silicon content can easily cause quenching cracks and increase temper brittleness. Therefore, considering all factors, the Si content is controlled within a range of 1.3–1.7 wt%.

[0011] Vanadium improves the hardenability of martensitic wear-resistant steel and enhances the dispersion and precipitation of carbides. It also refines grains and inhibits grain growth, improving the material's strength, hardness, impact toughness, and wear resistance. However, excessive addition of vanadium significantly degrades the machinability of martensitic wear-resistant steel. Therefore, considering all factors, the vanadium content is controlled within a range of 0.05–0.2 wt%.

[0012] The addition of aluminum significantly reduces the material's density, making it lighter. It also refines the grain size, increasing its strength and hardness. Therefore, considering all factors, the aluminum content is controlled at 1-1.5 wt%.

[0013] Elements such as C, Mn, Cr, and Ni dissolve into the Fe matrix, causing lattice distortion and thus increasing the strength and hardness of the steel. Mn, Cr, Ni, and Mo improve the hardenability of the steel, affecting the transformation of austenite to martensite after quenching by expanding the austenite region and lowering the critical transition temperature. The combination of Ni and Si promotes uniform nucleation of precipitates, improving the plastic deformation capacity and toughness of the steel.

[0014] On the basis of the above technical solution, the present invention can also be improved as follows.

[0015] Furthermore, the mass percentages of chemical elements are: C 0.46 wt%, Mn 2.3 wt%, Cr 0.49 wt%, Ni0.09 wt%, Mo 0.68 wt%, Si 1.5 wt%, V 0.08 wt%, Al 1.12 wt%, and the balance is iron and inevitable impurities.

[0016] The beneficial effect of adopting the further technical solution is: optimizing the mass percentage of chemical components to obtain martensitic steel with excellent comprehensive properties.

[0017] Furthermore, the microstructure of the martensitic steel is composed of 95% to 97% tempered martensite and 3% to 5% M7C3 precipitation phase by volume percentage.

[0018] The beneficial effect of adopting a further technical solution is that the M7C3 precipitates (M7C3 carbides) are uniformly distributed within the martensite structure, effectively hindering dislocation motion and thereby enhancing the material's strength and hardness. Under appropriate heat treatment conditions, such as tempering after austenitization, the M7C3 carbides can undergo precipitation hardening and secondary hardening, increasing the material's hardness and tensile strength. This strengthening effect is primarily due to the carbides' strengthening effect on the matrix structure. However, excessive precipitation levels or improper heat treatment processes can also reduce the material's toughness. Therefore, while pursuing high strength and hardness, controlling the volume percentage of the martensitic steel to 3% to 5% maintains the good toughness of the martensitic steel.

[0019] The present invention also discloses a method for preparing martensitic steel, comprising the following steps:

[0020] S1: preparing raw materials according to the mass percentage of chemical elements, smelting and forging the raw materials to obtain billet 1;

[0021] S2: After hot rolling the billet 1 to a thin steel plate with a thickness of 2 mm, the thin steel plate is heated to Ac3 temperature at a rate of 15-25°C / min to Ac3 temperature + 50°C, and kept at this temperature for 5-25 minutes to obtain billet 2;

[0022] S3: After quenching the second blank, cool it to room temperature to obtain the third blank;

[0023] S4: Heat the billet 3 to 150-200°C at a rate of 15-25°C / min, keep it at that temperature for 3-7 hours, and then air-cool it to room temperature to obtain martensitic steel.

[0024] Further, the steps of smelting and forging are:

[0025] S1: Put the raw materials into the electric arc furnace and melt them into molten steel at 1630~1700℃;

[0026] S2: pouring the molten steel into a refining device and refining the molten steel at 1620°C under an inert gas atmosphere;

[0027] S3: Pour the refined molten steel into a vacuum device for degassing;

[0028] S4: Pour the degassed molten steel into the crystallizer of the continuous casting machine, and obtain billet 1 after cooling.

[0029] Furthermore, the Ac3 temperature is 844°C.

[0030] Furthermore, the quenching method is water quenching.

[0031] Furthermore, the air cooling rate was 20°C / min.

[0032] The beneficial effect of the above preparation method of the present invention is that the holding time of austenitization provided by the preparation method of the present invention is short, which can avoid the original austenite grains being too coarse due to the holding time being too long, affecting the martensite structure obtained after quenching, and then deteriorating the comprehensive mechanical properties of the martensitic steel. It is then water quenched to room temperature to obtain a full martensite structure; then it is heated to 185℃ again for low-temperature tempering to avoid the occurrence of M 23 C6 and other precipitates deteriorate the material's comprehensive mechanical properties. Different tempering times are used to obtain martensitic steels with varying hardness, strength, and ductility. Finally, the steel is air-cooled to room temperature at a rate of 15-25°C / s to obtain a tempered martensite structure with a small amount of M7C3 precipitates. This method optimizes the austenitization, quenching, and tempering steps in the existing martensitic steel production process. The resulting martensitic steel has a microstructure of tempered martensite with a small amount of M7C3 precipitates. The M7C3 precipitates significantly enhance the hardness and strength of the martensitic steel, thereby improving its wear resistance. Therefore, this method has excellent industrial prospects.

[0033] The beneficial effects of the present invention are as follows: the present invention optimizes the alloy composition and heat treatment preparation process of martensitic steel so that the microstructure of the obtained martensitic steel is a tempered martensitic structure + a small amount of M7C3 precipitation phase, which makes it have better wear performance, while improving the wear resistance and comprehensive mechanical properties, thereby obtaining a wear-resistant steel with excellent comprehensive performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 This is the preparation flow chart of martensitic steel;

[0035] Figure 2 This is the scanning electron microscope micrograph of the martensitic steel prepared in Example 1. DETAILED DESCRIPTION

[0036] The specific implementation methods of the present invention are described in detail below with reference to the embodiments.

[0037] Example 1

[0038] A martensitic steel comprises the following chemical elements in percentage by weight: C 0.46 wt%, Mn 2.3 wt%, Cr 0.49 wt%, Ni 0.09 wt%, Mo 0.68 wt%, Si 1.5 wt%, V 0.08 wt%, Al 1.12 wt%, and the balance being iron and unavoidable impurities.

[0039] The above martensitic steel is processed through the following steps (such as Figure 1 As shown) to produce:

[0040] S1: Raw materials are prepared according to the mass percentage of chemical elements, and the raw materials are placed in an electric arc furnace to be smelted into molten steel at 1650°C. The molten steel is then poured into an LF furnace for refining at 1620°C in an argon atmosphere. The refined molten steel is then poured into a VD vacuum furnace for degassing. Finally, the degassed molten steel is poured into a crystallizer of a continuous casting machine, and after cooling, billet 1 is obtained.

[0041] S2: After hot rolling the billet 1 to a thin steel plate with a thickness of 2 mm, the thin steel plate is heated to 885°C at a rate of 20°C / min and kept at this temperature for 15 minutes to obtain billet 2;

[0042] S3: After quenching the second blank, cool it to room temperature to obtain the third blank;

[0043] S4: Billet 3 was heated to 185°C at a rate of 20°C / min, kept at that temperature for 5 hours, and then air-cooled to room temperature at a rate of 20°C / min to obtain martensitic steel.

[0044] Example 2

[0045] A martensitic steel comprises the following chemical elements in percentage by weight: C 0.3 wt%, Mn 2.6 wt%, Cr 0.3 wt%, Ni 0.2 wt%, Mo 0.4 wt%, Si 1.7 wt%, V 0.05 wt%, Al 1.5 wt%, and the balance being iron and unavoidable impurities.

[0046] The above martensitic steel is processed through the following steps (such as Figure 1 As shown) to produce:

[0047] S1: Raw materials are prepared according to the mass percentage of chemical elements, and the raw materials are placed in an electric arc furnace to be melted into molten steel at 1630°C. The molten steel is then poured into an LF furnace for refining at 1620°C in an argon atmosphere. The refined molten steel is then poured into a VD vacuum furnace for degassing. Finally, the degassed molten steel is poured into a crystallizer of a continuous casting machine, and after cooling, billet 1 is obtained.

[0048] S2: After hot rolling the billet 1 to a thin steel plate with a thickness of 2 mm, the thin steel plate is heated to 894°C at a rate of 15°C / min and kept at this temperature for 5 minutes to obtain billet 2;

[0049] S3: After quenching the second blank, cool it to room temperature to obtain the third blank;

[0050] S4: Billet 3 was heated to 150°C at a rate of 15°C / min, kept at that temperature for 7 hours, and then air-cooled to room temperature at a rate of 15°C / min to obtain martensitic steel.

[0051] Example 3

[0052] A martensitic steel comprises the following chemical elements in percentage by weight: C 0.6 wt%, Mn 2 wt%, Cr 0.7 wt%, Ni 0.05 wt%, Mo 0.8 wt%, Si 1.3 wt%, V 0.2 wt%, Al 1. wt%, and the balance being iron and inevitable impurities.

[0053] The above martensitic steel is processed through the following steps (such as Figure 1 As shown) to produce:

[0054] S1: Raw materials are prepared according to the mass percentage of chemical elements, and the raw materials are placed in an electric arc furnace to be smelted into molten steel at 1700°C. The molten steel is then poured into an LF furnace for refining at 1620°C in an argon atmosphere. The refined molten steel is then poured into a VD vacuum furnace for degassing. Finally, the degassed molten steel is poured into a crystallizer of a continuous casting machine, and after cooling, billet 1 is obtained.

[0055] S2: After hot rolling the billet 1 to a thin steel plate with a thickness of 2 mm, the thin steel plate is heated to 885°C at a rate of 25°C / min and kept at this temperature for 25 minutes to obtain billet 2;

[0056] S3: After quenching the second blank, cool it to room temperature to obtain the third blank;

[0057] S4: Billet 3 was heated to 200°C at a rate of 25°C / min, kept at that temperature for 3 hours, and then air-cooled to room temperature at a rate of 25°C / min to obtain martensitic steel.

[0058] Comparative Example 1

[0059] The commercial martensitic wear-resistant steel NM400 in the prior art.

[0060] Comparative Example 2

[0061] The commercial martensitic wear-resistant steel NM500 in the prior art.

[0062] Comparative Example 3

[0063] A martensitic steel comprises the following chemical elements in percentage by weight: C 0.148 wt%, Mn 1.884 wt%, Cr 0.176 wt%, Ni 0.015 wt%, S 0.019 wt%, Si 0.204 wt%, V 0.0022 wt%, Nb 0.0005 wt%, and the balance being iron and unavoidable impurities.

[0064] The above martensitic steel is processed through the following steps (such as Figure 1 As shown) to produce:

[0065] S1: Raw materials are prepared according to the mass percentage of chemical elements, and the raw materials are placed in an electric arc furnace to be smelted into molten steel at 1650°C. The molten steel is then poured into an LF furnace for refining at 1620°C in an argon atmosphere. The refined molten steel is then poured into a VD vacuum furnace for degassing. Finally, the degassed molten steel is poured into a crystallizer of a continuous casting machine, and after cooling, billet 1 is obtained.

[0066] S2: After hot rolling the billet 1 to a thin steel plate with a thickness of 2 mm, the thin steel plate is heated to 885°C at a rate of 20°C / min and kept at this temperature for 15 minutes to obtain billet 2;

[0067] S3: After quenching the second blank, cool it to room temperature to obtain the third blank;

[0068] S4: Billet 3 was heated to 185°C at a rate of 20°C / min, kept at that temperature for 5 hours, and then air-cooled to room temperature at a rate of 20°C / min to obtain martensitic steel.

[0069] Comparative Example 4

[0070] A martensitic steel comprises the following chemical elements in percentage by weight: C 0.46 wt%, Mn 2.3 wt%, Cr 0.49 wt%, Ni 0.09 wt%, Mo 0.68 wt%, Si 1.5 wt%, V 0.08 wt%, Al 1.12 wt%, and the balance being iron and unavoidable impurities.

[0071] The above martensitic steel is processed through the following steps (such as Figure 1 As shown) to produce:

[0072] S1: Raw materials are prepared according to the mass percentage of chemical elements, and the raw materials are placed in an electric arc furnace to be smelted into molten steel at 1650°C. The molten steel is then poured into an LF furnace for refining at 1620°C in an argon atmosphere. The refined molten steel is then poured into a VD vacuum furnace for degassing. Finally, the degassed molten steel is poured into a crystallizer of a continuous casting machine, and after cooling, billet 1 is obtained.

[0073] S2: After hot rolling the billet 1 to a thin steel plate with a thickness of 2 mm, the thin steel plate is heated to 1245°C at a rate of 12°C / min and kept at this temperature for 15 minutes to obtain billet 2;

[0074] S3: After quenching the second blank, cool it to room temperature to obtain the third blank;

[0075] S4: Billet 3 was heated to 230°C at a rate of 30°C / min, kept at that temperature for 300s, and then air-cooled to room temperature at a rate of 25°C / min to obtain martensitic steel.

[0076] Experimental Example 1

[0077] Metallographic characterization experiment: The martensitic steel prepared in Example 1 was observed using a scanning electron microscope. Figure 2 As shown in FIG, the microstructure is statistically 96% tempered martensite and 4% M7C3 precipitates by volume; the martensitic steels prepared in Examples 2 and 3 were also observed, and the microstructure of Example 2 is 95% tempered martensite and 5% M7C3 precipitates by volume; the microstructure of Example 3 is 97% tempered martensite and 3% M7C3 precipitates by volume.

[0078] Experimental Example 2

[0079] Performance testing: The martensitic steels prepared in Examples 1 to 3, the commercial martensitic wear-resistant steels NM400 and NM500 in Comparative Examples 1 to 2, and the martensitic steels prepared in Comparative Examples 3 to 4 were tested for performance in the following manner:

[0080] Wear tests were conducted on a high-precision reciprocating friction and wear testing machine using a ball-plane contact method. Wear specimens were machined into 15 mm × 10 mm × 2 mm and ground and polished to a mirror finish (Ra 0.02 μm) before the test. Si3N4 ceramic balls with a hardness of 1500 HV and a diameter of 3 mm were used as the grinding pair material. All specimens and Si3N4 balls were ultrasonically cleaned in alcohol for 15 minutes and dried before the wear test to ensure that there were no contaminants on the specimen surface. The friction tests were conducted in atmospheric and room temperature environments with a relative humidity of 50% to 60%. The displacement amplitude was 2500 μm, the normal loads were 10 N and 30 N, the experimental frequency was 2 Hz, and the number of cycles was 1×10 4 After the wear test, a white light interferometer (Bruker GTK-16-0295) was used to quantitatively characterize the wear spot profile and wear volume to analyze the wear performance of the alloy steel. To avoid irregular shapes at both ends of the wear scar and ensure the reliability of the experimental data, each test was repeated three times, and the wear volume was only taken from the middle 4 mm area of ​​the wear scar, and its value was used as the wear resistance evaluation index.

[0081] The indentation load for the hardness test was set to 200 gN, and the dwell time was 15 s. Before the test, the specimens were ground and polished to obtain a smooth and uniform test surface. To ensure the accuracy of the test results, 10 points were measured for each sample and the average value was taken as the final hardness value. In the tensile test, the tensile rate was set to 1.5 mm / min, and the specimen tensile test was carried out in accordance with the ASTM (E-8) standard. The thickness was 2 mm, and the tensile direction of the specimen was kept parallel to the hot rolling direction. The total elongation was measured using an extensometer with a gauge length of 25 mm, and the maximum displacement of the extensometer was taken as the total elongation, which can maximize the accuracy of the tensile data curve. In order to reduce the random error of the experiment, the tensile test was repeated three times, and the average value was taken as the value of each mechanical property.

[0082] The test results are shown in Table 1. The martensitic steels provided by the present invention (Examples 1-3) have better wear resistance than the existing commercial martensitic wear-resistant steels NM400 and NM500 and the martensitic steels of Comparative Examples 3-4, and at the same time have more excellent comprehensive mechanical properties.

[0083] Table 1 Performance test results

[0084]

[0085] Although the specific embodiments of the present invention have been described in detail in conjunction with the embodiments, this should not be construed as limiting the scope of protection of this patent. Within the scope described by the claims, various modifications and variations that can be made by those skilled in the art without creative work still fall within the scope of protection of this patent.

Claims

1. A martensitic steel, characterized in that: The mass percentages of chemical elements are: C 0.3~0.6 wt%, Mn 2~2.6wt%, Cr 0.3~0.7 wt%, Ni 0.05~0.2 wt%, Mo 0.4~0.8 wt%, Si 1.3~1.7 wt%, V 0.05~0.2wt%, Al 1~1.5 wt%, and the balance is iron and unavoidable impurities; The microstructure of the martensitic steel is as follows: 95% to 97% tempered martensite and 3% to 5% M7C3 precipitation phase by volume percentage; The martensitic steel is prepared according to the following steps: S1: raw materials are prepared according to the mass percentage of chemical elements, and the raw materials are smelted and forged to obtain billet 1; S2: After hot rolling the billet 1 to a thin steel plate with a thickness of 2 mm, the thin steel plate is heated to Ac3 temperature at a rate of 15-25°C / min to Ac3 temperature + 50°C, and kept at this temperature for 5-25 minutes to obtain billet 2; S3: After quenching the second blank, cool it to room temperature to obtain the third blank; S4: Heat the billet 3 to 150-200°C at a rate of 15-25°C / min, keep it at that temperature for 3-7 hours, and then air-cool it to room temperature to obtain martensitic steel.

2. The martensitic steel according to claim 1, characterized in that The mass percentages of chemical elements are: C 0.46 wt%, Mn 2.3 wt%, Cr 0.49 wt%, Ni 0.09 wt%, Mo 0.68 wt%, Si 1.5 wt%, V 0.08 wt%, Al1.12 wt%, and the balance is iron and inevitable impurities.

3. The martensitic steel according to claim 1, wherein: The steps of smelting and forging are: S1: Put the raw materials into the electric arc furnace and melt them into molten steel at 1630~1700℃; S2: pouring the molten steel into a refining device and refining the molten steel at 1620°C under an inert gas atmosphere; S3: Pour the refined molten steel into vacuum equipment for degassing; S4: Pour the degassed molten steel into the crystallizer of the continuous casting machine, and obtain billet 1 after cooling.

4. The martensitic steel according to claim 1, wherein: The Ac3 temperature is 844°C.

5. The martensitic steel according to claim 1, wherein: The quenching method is water quenching.

6. The martensitic steel according to claim 1, characterized in that: The air cooling rate is 20°C / min.

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