High-strength steel sheet and heat treatment process for improving fatigue limit thereof

Abstract

This invention discloses a high-strength steel plate and a heat treatment process for improving its fatigue limit. First, a high-strength steel plate slab is smelted in a converter and continuously cast. Then, the slab is heated and hot-rolled to the desired thickness, followed by air cooling to room temperature to obtain a hot-rolled steel plate. Next, the steel plate is heated to the desired temperature and held for a certain time for annealing, followed by furnace cooling to room temperature. Then, the sample is heated from room temperature to 600±10℃ within 40±2 min, held for 30±5 min, and then heated from 600℃ to 860±10℃ within 30 min, held for 30±5 min, and then oil-quenched to 580±10℃, held at 580±10℃ for 2.5 h, and finally oil-quenched to room temperature to obtain a high-strength steel plate with a high fatigue limit. This invention has a simple process, can significantly improve the fatigue limit of high-strength steel plates, maintain good impact performance, and effectively improve the service life of alloy steel.

Description

Technical Field

[0001] This invention belongs to the field of inorganic materials and relates to a steel smelting technology, specifically to a high-strength steel plate and a heat treatment process to improve its fatigue limit. Background Technology

[0002] Metallic materials are among the most important materials for human development, playing a vital role in people's lives throughout history. Due to their high toughness, hardness, and strength, metallic materials are widely used in defense, industry, agriculture, and electronics.

[0003] Heat treatment plays a crucial role in improving the properties of metallic materials. It offers the advantage of adjusting strength, hardness, and toughness without altering the chemical composition of the metal, while also improving its machinability. Heat treatment processes have also diversified from the traditional "four-stage heat treatment" method.

[0004] A search revealed that Chinese Patent Publication No. CN114657472A discloses a marine ultra-high strength low-temperature steel with excellent fatigue properties and its manufacturing method, comprising: C: 0.080%–0.140%, Si: 0.20%–0.60%, Mn: 1.15%–1.60%, Nb: 0.020%–0.050%, and V: 0.040%–0.08%. The composition is as follows: Cu: 0.30%–0.50%, Ni: 0.50%–0.80%, N: 0.0140%–0.0170%, Cr: 0.10%–0.20%, P≤0.010%, S≤0.005%, Als: 0.015%–0.035%, with the balance being Fe and unavoidable impurities. The process involves smelting, continuous casting, heating, rolling, and cooling. The resulting low-temperature steel has a microstructure of ultrafine ferrite + bainite + a small amount of martensite, a room temperature high-cycle fatigue limit strength exceeding 320 MPa, a low-temperature steel yield strength exceeding 500 MPa, and a tensile strength exceeding 620 MPa. Although this invention relates to high-fatigue-performance steel, it does not involve heat treatment, and the performance levels are not comparable.

[0005] Patent document CN 114182174 A discloses a method for producing high-strength and tough bridge structural steel plates. The composition and weight percentages are as follows: C: 0.03%–0.08%, Si: 0.30%–0.50%, Mn: 1.50%–2.00%, P≤0.015%, S≤0.005%, Al: 0.02%–0.05%, Ti: 0.008%–0.030%, Nb: 0.03%–0.08%, Cr: 0.30%–0.50%, Cu≤0.20%, with the balance being Fe and unavoidable impurity elements. This invention produces steel plates with yield strength ≥500MPa, tensile strength ≥630MPa, and conditional fatigue limit ≥500MPa through processes such as thermomechanical controlled rolling, multi-stage controlled cooling, and tempering heat treatment. The invention successfully prepared bridge steel plates with a conditional fatigue limit ≥500MPa using tempering heat treatment. Summary of the Invention

[0006] The purpose of this invention is to provide a heat treatment process for improving the fatigue limit of high-strength steel plates, thereby enhancing both the tensile strength and fatigue strength of the steel, while also improving its impact toughness. The heat treatment temperature is 650–700℃, with a holding time of 5–10 hours, followed by furnace cooling to room temperature. Subsequently, a tempering treatment is performed, holding at 600±10℃ for 30 minutes, then at 860±10℃ for 30 minutes, followed by oil cooling to 580±10℃, holding at that temperature for 2.5 hours, and finally oil cooling to room temperature. After heat treatment, the tensile strength of the steel plate reaches 1200 MPa, the fatigue limit strength reaches 510 MPa, and the impact energy reaches 132 J, effectively improving the service life of the alloy steel.

[0007] To solve the above-mentioned technical problems, the present invention employs the following technical means:

[0008] On one hand, the present invention provides a heat treatment process for improving the fatigue limit of high-strength steel plates, comprising the following steps:

[0009] 1) Slabs smelted in a converter and continuously cast into high-strength steel plates;

[0010] 2) Hot rolling: The slab is heated and then hot rolled to the required thickness, and then air-cooled to room temperature to obtain a steel plate;

[0011] 3) Annealing heat treatment: Heat the steel plate to C1℃, hold it at that temperature for a period of time for annealing treatment, and then cool it to room temperature in the furnace;

[0012] 4) Quenching and tempering heat treatment: The annealed steel plate is heated from room temperature to B1℃ within A1 minutes, held at that temperature for A2 minutes, and then heated from B1℃ to B2℃ within A3 minutes, held at that temperature for A4 minutes, and then oil quenched to B3℃. After holding at B3℃ for A5 hours, it is oil quenched to room temperature to obtain a high-strength steel plate with high fatigue limit.

[0013] As a preferred technical solution, in step 1), the composition and weight percentage content of the slab are as follows:

[0014] C: 0.25-0.35%, Si: 0.10-0.40%, Mn: 0.5-0.8%, Ni: 1.8-2.2%, Mo: 0.3-0.5%, Cr: 1.8-2.2%, Ti: 0.010-0.020%, P≤0.020%, S≤0.0020%, with the remainder being iron and unavoidable impurities.

[0015] As a preferred technical solution, in step 1), the thickness of the slab is not less than 200mm.

[0016] As a preferred technical solution, in step 2), the slab heating temperature is 1150~1200℃.

[0017] As a preferred technical solution, in step 3), the annealing temperature C1 ranges from 650 to 700°C, and the holding time is 5 to 10 hours.

[0018] As a preferred technical solution, in step 4), the value ranges of A1, A2, A3, A4 and A5 are 35~45, 25~35, 26~33, 25~35 and 2~3 respectively; the value ranges of B1, B2 and B3 are 590~610, 850~870 and 570~590 respectively.

[0019] On the other hand, the present invention provides a high-strength steel plate with high fatigue limit strength, obtained by using any one of the heat treatment processes described above.

[0020] The functions of each component in the high-strength steel plate of this invention are as follows:

[0021] C: Carbon is a crucial element affecting the strength, hardness, toughness, hardenability, and wear resistance of cast steel. Excessive carbon content results in high-carbon martensite after heat treatment, which has high hardness but low toughness and is prone to cracking during heat treatment. Insufficient carbon content leads to low hardness and poor wear resistance.

[0022] Si: Silicon does not form carbides in steel, but exists as a solid solution in ferrite or austenite. It improves the strength of the solid solution in steel, while avoiding significant deterioration of the plasticity and toughness of the steel plate due to excessive addition.

[0023] Mn: Manganese can improve the strength and hardness of steel and significantly improve hardenability. However, high content will reduce the low-temperature toughness of steel.

[0024] Ti: Titanium is a strong nitrogen element. Fine TiN particles can effectively prevent the growth of austenite grains during slab reheating, and can also improve the impact toughness of the weld heat-affected zone.

[0025] Mo: Refines the grain size of steel, improves hardenability and hot strength, and maintains sufficient strength and creep resistance at high temperatures. In tool steels, it can improve red hardness and suppress temper brittleness.

[0026] Ni: Nickel can effectively improve the hardenability of steel, has a certain solid solution strengthening effect, and can also significantly improve the low-temperature toughness of steel. The addition of Ni can improve hot brittleness and is beneficial to toughness.

[0027] Cr: Chromium is a medium-strong carbide-forming element. It significantly improves strength, hardness, and wear resistance, but simultaneously reduces plasticity and toughness. It can improve the oxidation resistance and corrosion resistance of steel. It raises the A3 and A1 temperatures, shifting the GS line to the upper left.

[0028] P: Phosphorus is a harmful element in steel. When the phosphorus content in steel is too high, it will increase the temper brittleness and cold brittleness of the material, that is, reduce impact toughness and increase brittle transition temperature. In low alloy steel and austenitic steel with high carbon content, phosphorus can also promote hot cracking. Therefore, its content should be reduced as much as possible, and for cost considerations, it should be controlled at ≤0.020%.

[0029] S: Sulfur is a harmful element in steel. Sulfur increases the hot brittleness of steel. It exists in steel in the form of sulfide inclusions and has an adverse effect on the plasticity, toughness, weldability, thickness direction properties, fatigue properties and corrosion resistance of steel. Therefore, its content should be reduced as much as possible and controlled to ≤0.0020%.

[0030] This invention rationally configures the contents of Ni, Mo, and Cr elements. Mo increases the stability of supercooled austenite, inhibits the diffusion transformation of iron and carbon atoms, and delays γ-ray diffraction. Fe →α Fe Phase transformation; Cr element lowers the bainite transformation initiation temperature of steel, allowing the bainite transformation to occur at a lower temperature; Ni element prolongs the pearlite transformation incubation period, reduces its nucleation rate and growth rate, and delays or even inhibits the pearlite transformation. Under these combined effects, the critical cooling rate for the martensitic transformation is reduced, thereby effectively improving the hardenability of steel and reducing the high-temperature temper brittleness of steel.

[0031] The present invention controls the slab thickness to be no less than 200mm in order to ensure that the steel plate is subjected to high pressure at the high temperature of hot rolling, so that the grains are fully broken.

[0032] The present invention incorporates annealing after hot rolling and a prolonged holding time to eliminate structural defects, refine and homogenize the microstructure, reduce internal stress, and improve the plasticity and toughness of the steel, thus preparing for subsequent quenching and tempering. This technical point is also one of the main innovations of the present invention.

[0033] The reason for heating the specimen to about 600°C and holding it at that temperature for a certain period of time during the tempering stage is to reduce the temperature difference of the specimen and prepare it for phase transformation, so that the phase transformation can be carried out uniformly on the entire cross section of the specimen, rather than directly heating it to the quenching temperature. This can avoid generating large structural stress. This technical point is another innovation of the present invention.

[0034] The reason for controlling the temperature at 860±10℃ and holding it for 30±5 minutes is that austenite nucleation, growth, carbide dissolution, and composition homogenization will occur at this temperature. These processes are all diffusion migrations of iron, carbon, and alloying elements, so they require a certain amount of time. In the initial stage, austenite nuclei first form at the interface between ferrite and cementite phases and diffuse under the carbon concentration gradient, promoting austenite growth. The longer the time, the more complete this process is. However, if the holding time is too long, the austenite grains will grow, which will have an adverse effect on the properties of the steel. In this invention, by controlling the alloying elements and temperature, the grain size is controlled to prepare the microstructure for subsequent rapid cooling.

[0035] The reason for choosing oil quenching is that, compared to water quenching, oil quenching has a gentler cooling rate, which is less likely to cause cracking of the specimen and reduce deformation.

[0036] The reason for controlling the tempering process at 580±10℃ for 2.5 hours is to obtain a high-temperature tempered structure of tempered sorbite, which allows for adjustment of the strength and toughness of the steel plate, resulting in good overall mechanical properties.

[0037] This invention ensures the purity of the steel billet through a design with low S and P content, while simultaneously guaranteeing its basic properties with appropriate alloying elements. The designed long-term annealing process avoids the structural defects (segregation, cracks, coarse structure, etc.) that occur when directly quenching and tempering after rolling, leading to uneven strength, mismatched strength and toughness, and cracking. Heating to a specific temperature and holding it for a certain time after annealing successfully avoids the problem of inconsistent transformation products caused by excessively rapid heating, where the surface has already transformed into austenite while the core remains a pearlite + ferrite structure. After sufficient microstructural preparation, quenching and tempering occurs, resulting in carbon segregation in martensite, martensite decomposition, transformation of retained austenite, transformation of carbides, and aggregation and growth of cementite, as well as α-phase recovery and recrystallization. The residual internal stress of the experimental steel is also continuously eliminated. A tempered sorbite microstructure with good strength and toughness is obtained. This process effectively solves the problems of low or excessive strength, poor toughness, low fatigue strength, and component failure caused by uneven microstructure when high-alloy structural steel is used as a component. The present invention has a simple composition and uses a common hot rolling process, which effectively improves the overall performance of the steel plate. Attached Figure Description

[0038] Figure 1 This is a flow chart of the heat treatment process for improving the fatigue limit in an embodiment of the present invention. Detailed Implementation

[0039] The present invention will now be described in detail:

[0040] Table 1 is a list of values ​​for each embodiment and comparative example of the present invention;

[0041] Table 2 is a list of the main process parameters for each embodiment and comparative example of the present invention;

[0042] Table 3 is a list of performance test results for each embodiment and comparative example of the present invention.

[0043] Each embodiment of the present invention is produced according to the following steps:

[0044] The steps are as follows:

[0045] 1) Smelt in a converter and continuously cast into slabs with a thickness of not less than 200 mm;

[0046] 2) Hot rolling: The billet is heated to 1150-1200℃ and rolled to the required thickness; then air-cooled to room temperature.

[0047] 3) Annealing heat treatment: The temperature is controlled at 650-700℃, the holding time is 5-10h, and the furnace is cooled to room temperature;

[0048] 4) Quenching and tempering heat treatment: The sample is heated from room temperature to 600±10℃ within 40±2 min and held for 30±5 min. Then, the sample is heated from 600℃ to 860±10℃ within 30 min and held for 30±5 min. Then, it is oil quenched to 580±10℃ and held at 580±10℃ for 2.5 h. Finally, it is oil quenched to room temperature to obtain a high-strength steel plate with high fatigue limit strength.

[0049] Table 1. Chemical composition (wt%) of various embodiments and comparative examples of the present invention.

[0050] example mm 1 12.5 0.25 0.30 0.6 0.011 0.0010 2.2 0.010 0.30 1.9 2 14.0 0.28 0.22 0.68 0.010 0.0020 1.9 0.011 0.35 2.0 3 18.0 0.30 0.33 0.65 0.015 0.0010 2.0 0.010 0.45 2.0 4 26.0 0.32 0.40 0.56 0.020 0.0011 1.8 0.015 0.50 1.8 5 30.0 0.35 0.33 0.80 0.010 0.0015 2.0 0.020 0.40 2.2 Comparison 1 14.0 0.25 0.22 0.68 0.010 0.0020 2.5 0.002 0.35 2.5 Comparison 2 28.0 0.35 0.40 0.55 0.010 0.0011 2.0 0.011 0.45 1.8

[0051] Table 2. List of main process parameters for each embodiment and comparative example of the present invention.

[0052]

[0053]

[0054] Table 3. Performance results of various embodiments and comparative examples of the present invention.

[0055]

[0056] As can be seen from Table 3, the five steel plates produced in Examples 1 to 5, after long-term annealing and quenching segmented heating, have better fatigue limits and impact properties than the steel plates that have not undergone these two processes.

[0057] The above embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Although the invention has been described in detail with reference to the embodiments, those skilled in the art should understand that various combinations, modifications, or equivalent substitutions of the technical solutions of the invention do not depart from the spirit and scope of the invention and should be covered within the scope of the claims of the invention.

Claims

1. A heat treatment process for improving the fatigue limit of a high-strength steel sheet, characterized in that, Includes the following steps: 1) Slabs smelted in a converter and continuously cast into high-strength steel plates; 2) Hot rolling: The slab is heated and then hot rolled to the required thickness, and then air-cooled to room temperature to obtain a steel plate; 3) Annealing heat treatment: Heat the steel plate to C1 ℃, hold it at that temperature for a period of time for annealing treatment, and then cool it to room temperature in the furnace; 4) Quenching and tempering heat treatment: The annealed steel plate is heated from room temperature to E1 ℃ within D1 minutes, held at this temperature for D2 minutes, and then heated from E1 ℃ to E2 ℃ within D3 minutes, held at this temperature for D4 minutes, and then oil quenched to E3 ℃. After holding at E3 ℃ for D5 hours, it is oil quenched to room temperature to obtain a high-strength steel plate with high fatigue limit. In step 1), the components and weight percentage content of the slab are as follows: C: 0.25-0.35%, Si: 0.10-0.40%, Mn: 0.5-0.8%, Ni: 1.8-2.2%, Mo: 0.3-0.5%, Cr: 1.8-2.2%, Ti: 0.010-0.020%, P≤0.020%, S≤0.0020%, with the remainder being iron and unavoidable impurities; The thickness of the slab is not less than 200 mm; In step 4), the value ranges of D1, D2, D3, D4 and D5 are 35~45, 25~35, 26~33, 25~35 and 2~3 respectively; the value ranges of E1, E2 and E3 are 590~610, 850~870 and 570~590 respectively.

2. The heat treatment process for improving the fatigue limit of high-strength steel plates according to claim 1, characterized in that: In step 2), the slab heating temperature is 1150~1200℃.

3. The process for improving the fatigue limit of high-strength steel sheets according to claim 1, characterized in that: In step 3), the annealing holding temperature C1 ranges from 650 to 700 degrees Celsius, and the holding time is 5 to 10 hours.

4. A high-strength steel sheet having a high fatigue limit strength, characterized by, Obtained by using the heat treatment process described in any one of claims 1-3.

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