Low-temperature high-strength and high-toughness steel manufacturing method, high-strength and high-toughness track shoe and square head

Abstract

According to the low-temperature high-strength and high-toughness steel manufacturing method, the high-strength and high-toughness track shoe and the high-strength and high-toughness square head, fine grains and a nano-micron low-temperature tempered martensite structure are obtained by optimizing component design and adopting a circulating and multi-stage heat treatment process, and heat treatment stress minimization is achieved. And the fatigue limit is improved by improving the yield strength. Through the low-carbon high-silicon-manganese solid solution alloying design, high strength is achieved, meanwhile, excellent toughness is achieved, and therefore the use requirements under the low-temperature and high-impact-load conditions are met.

Description

Technical Field

[0001] The invention belongs to the technical field of low-temperature high-strength and high-toughness steel preparation, and more specifically relates to a method for manufacturing low-temperature high-strength and high-toughness steel, and a high-strength and high-toughness track shoe and square head. Background Art

[0002] Driven by the national energy conservation and emission reduction requirements, the use of high-strength steel has become an important technical means to achieve lightweight engineering equipment and vehicles. However, high-strength steel generally has the problem of insufficient impact toughness under high-strength conditions, especially in low-temperature environments, where its impact toughness is even weaker.

[0003] In recent years, research on ultra-high strength and toughness steel materials has made extensive progress worldwide. Metal strengthening and toughening technology has become one of the most dynamic frontier research directions in the field of metal structural materials. In practical applications, advanced high-performance steel materials not only need to have excellent strength, but also need to achieve excellent toughness, welding performance and stamping performance under low-cost conditions. Generally speaking, improving the plasticity and toughness of a material helps to improve its low-temperature toughness, but the trend of low-temperature toughness changing with temperature is closely related to the material's organizational structure.

[0004] In the prior art, the toughness of steel is often improved by increasing the content of Ni (nickel) and Mo (molybdenum). However, although this method can improve toughness, it will lead to an increase in the austenite content in the matrix, which will have a negative impact on the strength of the material. In addition, the increase in Mo content will also significantly increase the material cost, limiting its economic feasibility and promotion and application in actual production.

[0005] Track shoes are crucial load-bearing components in crawler machinery and are widely used in machinery such as tractors, bulldozers, cranes, tanks and mining equipment. During mechanical operation, track shoes not only need to support the weight of the machine, but also withstand complex alternating impact loads, strong friction and wear of ground sand and gravel, and severe vibrations, which will significantly accelerate the damage of track shoes. Fatigue fracture and rapid wear are common failure modes of track shoes. In order to ensure the service life of track shoes under harsh conditions, their materials are usually required to have high strength, high hardness, excellent plasticity and high impact toughness. At the same time, considering the operating environment in high-altitude cold areas, the material of the track shoes must be able to maintain sufficiently high impact absorption energy at a low temperature of -40°C.

[0006] The square head is a key component that connects the drill rod of a rotary drilling rig to the cylindrical drill bit. It has a fixed size and can be adapted to drill bits of different specifications. As the size of the drill bit increases, the square head needs to withstand greater torque and impact loads during the piling operation. The fatigue strength of traditional materials such as 35CrMo or 30CrMnSi quenched and tempered steel can no longer meet the high torque requirements, and fatigue cracks often occur, even leading to square head breakage.

[0007] Since fatigue-induced fracture is the main failure mode of the above two types of products, the development of a steel with high strength and toughness becomes the key to solving the problem. By improving the yield strength and impact toughness of the material, the generation of fatigue cracks can be effectively suppressed, thereby extending the service life of the track shoe and square head and avoiding the occurrence of fracture accidents.

[0008] The present invention aims to overcome the above technical bottlenecks and provide a method for preparing low-temperature high-strength and high-toughness steel that can take into account high strength, high toughness and economy. Summary of the invention

[0009] In order to solve the above technical problems, the present invention provides a method for manufacturing low-temperature, high-strength and high-toughness steel, as well as a track plate and square head. By optimizing the composition design and adopting a cyclic, multi-stage heat treatment process, fine grains and nano-micron-level low-temperature tempered martensite structure are obtained, and the heat treatment stress is minimized. By increasing the yield strength, the fatigue limit is increased. Through the low-carbon, high-silicon-manganese solid solution alloying design, high strength is achieved while having excellent toughness, thereby meeting the use requirements under low temperature and high impact load conditions.

[0010] As one aspect of the present invention, a method for manufacturing low-temperature high-strength and high-toughness steel is provided, wherein the raw material chemical composition mass percentage of the low-temperature high-strength and high-toughness steel is: Carbon: 0.18% to 0.20%, silicon: 1.40% to 1.48%, manganese: 0.8% to 1.0%, chromium: 0.7% to 1.0%, molybdenum: 0.20% to 0.40%, nickel: 0.9% to 1.2%, sulfur: 0% to 0.01%, phosphorus: 0% to 0.01%, and the rest are Fe and unavoidable impurities.

[0011] Furthermore, the method for manufacturing the low-temperature high-strength and high-toughness steel further includes a smelting process, and the smelting process includes the following steps: The raw materials are smelted by an electric arc furnace or a medium frequency induction furnace to obtain molten steel; The molten steel is subjected to LF refining and VD vacuum treatment in sequence; wherein the tapping temperature is 1530° C. to 1570° C.; The refined molten steel is poured into an ingot mold to obtain an ingot, or poured into a continuous casting crystallizer to obtain a continuous casting billet.

[0012] Furthermore, the method for manufacturing the low-temperature high-strength and high-toughness steel further includes a forging or hot rolling forming process, specifically comprising the following steps: The steel ingot or continuous casting billet is subjected to forging or hot rolling forming process, wherein: When the forging process is adopted: the initial forging temperature is 1150℃±50℃, the final forging temperature is 900℃±30℃; forged into square billet; When the hot rolling forming process is adopted: the starting rolling temperature is 1150℃±50℃, the final rolling temperature is 900℃±30℃; hot rolling is performed into bars.

[0013] It should be noted that the hot-rolled bar stock is used as the blank stock for the track shoe, and the forged square billet is used as the blank stock for the square head.

[0014] Furthermore, the method for manufacturing the low-temperature high-strength high-toughness steel further includes a heat treatment process, wherein: Before the heat treatment process, a cutting or free forging process is also included: cutting the bar material after the hot rolling forming process to obtain a track plate blank of a preset size; or free forging the forged square billet to obtain a square head billet of a preset size; Heat treatment process: including preliminary heat treatment; wherein, Preliminary heat treatment, the track plate blank of preset size or the square head blank of preset size is heated to between 800°C and 880°C, kept at the temperature for a preset time of 1.0-4.0 hours, and then cooled to room temperature in air to obtain a forging after preliminary heat treatment.

[0015] The heat treatment process also includes a first quenching, a first low-temperature tempering, a second quenching, and a second low-temperature tempering; wherein, A primary quenching process is to heat the forging after the preliminary heat treatment to 910°C ± 10°C and keep the temperature for a preset time; the forging after heating and keeping the temperature is placed in a cooling medium, the cooling medium is water or PAG coolant, and when the cooling medium is PAG coolant, the concentration of the PAG coolant is 4% to 12%; One low-temperature tempering, the forgings after the first quenching are subjected to low-temperature tempering treatment, the tempering temperature is 200℃~300℃, the tempering time is 1.5h~4h, and after tempering, they are cooled in water or PAG quenching liquid; Secondary quenching, the forgings after the first low temperature tempering are repeated once using the first quenching process; Secondary low temperature tempering, the forgings after secondary quenching are subjected to a repeated low temperature tempering process.

[0016] Furthermore, in the quenching process, the preset holding time after the forging is heated to 910°C±10°C is calculated based on the cross-sectional size of the forging, 1-2 min / mm.

[0017] As another aspect of the present invention, a high-strength and high-toughness steel track is also provided. The track plate heat-treated parts prepared by the above-mentioned low-temperature high-strength and high-toughness steel manufacturing method also need to perform medium-frequency induction quenching and low-temperature tempering on parts of the track plate. The induction quenching heating temperature is 900-950°C. After reaching the preset temperature, water or PAG coolant is sprayed for quenching. After quenching, it is placed in a 180-200°C furnace for low-temperature tempering; the hardness of the medium-frequency induction quenched part is HRC52-56.

[0018] Furthermore, the tensile strength of the high-strength and high-toughness steel track after local medium-frequency induction quenching and low-temperature tempering is: 1350~1520MPa, the yield strength is: 1250~1380MPa, the elongation is: 13~16%, the cross-sectional shrinkage is: 55~60%, the impact absorption energy at room temperature is 110~130J, the impact absorption energy at -50℃ is 80J~100J, and the hardness is: HRC38~45.

[0019] Furthermore, the microstructure of the high-strength and high-toughness steel track is a lath carbide-free bainite / lath martensite multiphase structure, and the lath substructure thickness or width is 20nm to 200nm.

[0020] The low-temperature high-strength and high-toughness steel manufactured by the manufacturing method of the present invention has high strength and high toughness. The high-strength and high-toughness steel track plate provided fundamentally solves the technical problems that the existing high-strength steel track plates have low yield strength, the yield strength is only 700-800MPa; the fatigue limit is low and fatigue cracks are prone to occur; the low-temperature impact toughness value at -50℃ impact absorption energy is only 13-18J, and it is easy to break when used in high-cold areas. At the same time, the high-strength performance is used to reduce the weight of parts and components, and the economic cost is reduced.

[0021] As another aspect of the present invention, a square head for a drill rod of a rotary drilling rig is provided, which adopts the manufacturing method of the above-mentioned low-temperature high-strength and high-toughness steel, undergoes preliminary heat treatment to eliminate the mixed crystal structure after forging, and performs rough processing, and then quenches and tempers the semi-finished square head after rough processing; the specific heat treatment process is: Quenching: heat the forging to 910℃±10℃ and keep it warm for the corresponding time (1-2min / mm) according to the cross-sectional dimensions of the forging. The heated and heat-insulated forging is placed in a cooling medium, the cooling medium is water or PAG coolant, when the cooling medium is PAG coolant, the concentration of the PAG coolant is 10% to 12%; The steel is tempered twice at a temperature of 260-280°C, and then cooled to room temperature in a PAG coolant. After heat treatment, the steel is finely processed to obtain the final square head.

[0022] The square head for the drill rod of the rotary drilling rig has a tensile strength of 1450-1520 MPa, a yield strength of 1350-1430 MPa, an elongation of 11-13%, a cross-sectional shrinkage of 50-55%, an impact absorption energy of 110-130 J at room temperature, an impact absorption energy of 100 J-110 J at -20°C, and a hardness of HRC43-45.

[0023] Furthermore, the microstructure of the square head for the rotary drilling rig drill rod is a lath carbide-free bainite / lath martensite multiphase structure, and the lath substructure thickness is 50nm to 500nm.

[0024] The low-temperature high-strength and high-toughness steel manufactured by the manufacturing method of the present invention has high strength and high toughness. The square head for the drill pipe of the rotary drilling rig provided fundamentally solves the problem that the existing drill pipe square head is made of 35CrMo steel with low fatigue limit and is prone to early torsional fatigue fracture, resulting in a major quality accident where the drill bit falls into a deep hole. After using the invented material, the fatigue limit is increased by more than 1 times, which can ensure that the service life of the square head increases by more than 2 times without fracture. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] By reading the detailed description below with reference to the accompanying drawings, the above and other purposes, features and advantages of the exemplary embodiments of the present disclosure will become readily understood. In the accompanying drawings, several embodiments of the present disclosure are shown in an exemplary and non-restrictive manner. In the accompanying drawings, the same or corresponding reference numerals represent the same or corresponding parts.

[0026] Figure 1 The grain structure of the track shoe after die forging without preliminary heat treatment; Figure 2 The track plate is heat treated by keeping at 880℃ for 2 hours after die forging. DETAILED DESCRIPTION

[0027] In order to make the purpose, features, and advantages of the present disclosure more obvious and easy to understand, the technical solutions in the embodiments of the present disclosure will be clearly and completely described below in conjunction with the drawings in the embodiments of the present disclosure. Obviously, the described embodiments are only part of the embodiments of the present disclosure, not all of the embodiments. Based on the embodiments in the present disclosure, all other embodiments obtained by those skilled in the art without creative work are within the scope of protection of the present disclosure.

[0028] Example 1: Manufacturing of low temperature resistant and high strength track shoe (1) Special steel mills smelt according to the composition. The chemical composition is: carbon: 0.18% to 0.20%, silicon: 1.40% to 1.48%, manganese: 0.8% to 1.0%, chromium: 0.7% to 1.0%, molybdenum: 0.20% to 0.40%, nickel: 0.9% to 1.2%, sulfur: 0% to 0.01%, phosphorus: 0% to 0.01%, and the rest is Fe and unavoidable impurities. The smelting process includes the following steps: The raw materials are smelted by an electric arc furnace or a medium frequency induction furnace to obtain molten steel; The molten steel is subjected to LF refining and VD vacuum treatment in sequence; wherein the tapping temperature is 1530° C. to 1570° C.; The refined molten steel is poured into a continuous casting crystallizer to obtain a continuous casting billet.

[0029] (2) Hot rolling of continuous casting billet. The continuous casting billet is heated to 1150℃ and hot rolled to form round steel bars of required size; the final rolling temperature is heated to 900℃.

[0030] (3) Stress relief and hardness reduction annealing. The hot-rolled round steel bar is reheated to 680℃ for annealing for 4 hours, and then cooled to 450℃ before being taken out of the furnace and cooled to room temperature.

[0031] (4) Cutting and die forging. Cutting is done by saw blade cutting machine, and the bar of designed length is sent into induction heating furnace, heated to 1200℃, kept at temperature for 5 minutes, and then sent into die forging machine for forging. The track plate forging is air cooled to room temperature.

[0032] (5) Preliminary heat treatment to eliminate mixed crystal structure: Place the track plate forging in a heating furnace, heat it to 880℃ and keep it at that temperature for 2 hours, then take it out of the furnace and air cool it.

[0033] (6) Quenching and tempering heat treatment. The quenching temperature is 920℃, the holding time is 1.5 hours, and then quenching in 8% PAG coolant. Tempering is carried out in time after quenching, the tempering temperature is 260℃, and the tempering time is 3 hours. Tempering is performed twice.

[0034] (7) Machining: Machining to the required accuracy and size.

[0035] (8) Local medium frequency induction quenching and low temperature tempering. In order to improve the wear resistance of the local part of the track shoe, the local medium frequency induction quenching is performed. The heating temperature is 950℃, kept warm for 5 seconds, and then cooled by spraying PAG coolant. After cooling, it is quickly placed in a 200℃ furnace for stress relief tempering, and the tempering time is 3 hours.

[0036] (9) Mechanical properties.

[0037]

[0038] Comparative Example 1: Manufacturing of high-strength and tough track shoes Compared with Example 1, Comparative Example 1 is only that the preliminary heat treatment process of step (5) is not performed, and the grain structure obtained is as follows: Figure 1 As shown, there is a serious mixed crystal structure, the length: width of the mixed crystal structure is 3:1, and the maximum grain size is about 150µm×100µm.

[0039] The low temperature resistant high strength and toughness track shoe manufactured by the method of Example 1 has a grain structure as follows: Figure 2 As shown, the grain size is uniform, the large grains disappear, all are equiaxed crystals, and the maximum grain diameter is about 50µm.

[0040] The mechanical parameters of the low temperature resistant and high strength track shoes prepared in Example 1 and Comparative Example 1 are shown in Table 2:

[0041] Conclusion: After quenching and tempering after the preliminary heat treatment in Example 1, the impact absorption energy of the high-strength and toughness steel is significantly improved, by about 46%.

[0042] Comparative Example 2: Changing the Nickel Content Compared with Example 1, Comparative Example 2 is only that the nickel content is changed multiple times, and the other components remain unchanged, and the rest of the process flow is the same. The experimental results obtained are shown in Table 3.

[0043]

[0044] Since an increase in nickel content will lead to an increase in austenite content in the matrix, which will have a negative impact on the strength of the material, the nickel content is preferably 0.9-1.2%, more preferably 1.0%, but due to the reason of error, 0.9-1.2% is generally applicable.

[0045] The data shown in Table 4 were obtained by testing the impact absorption energies of the high-strength and tough track shoe obtained in Example 1 and the 42CrMo track shoe in the prior art at different lower temperatures.

[0046]

[0047] Conclusion: The high-strength and tough track shoe obtained by Example 1 has better impact absorption ability.

[0048] Example 2: Manufacturing of Drill Pipe Square Head for Rotary Drilling Rig (1) Special steel mills smelt according to the composition. The chemical composition is: carbon: 0.18% to 0.20%, silicon: 1.40% to 1.48%, manganese: 0.8% to 1.0%, chromium: 0.7% to 1.0%, molybdenum: 0.20% to 0.40%, nickel: 0.9% to 1.2%, sulfur: 0% to 0.01%, phosphorus: 0% to 0.01%, and the rest is Fe and unavoidable impurities. The smelting process includes the following steps: The raw materials are smelted by an electric arc furnace or a medium frequency induction furnace to obtain molten steel; The molten steel is subjected to LF refining and VD vacuum treatment in sequence; wherein the tapping temperature is 1530° C. to 1570° C.; The refined molten steel is poured into a continuous casting crystallizer to obtain a continuous casting billet.

[0049] (2) Forging of continuous casting billets. The continuous casting billets are heated to 1150°C and forged to the required size. The final forging temperature is not less than 900°C and the billets are forged into square billets.

[0050] (3) Preliminary heat treatment to eliminate the mixed crystal structure after forging. Place the track plate forging in a heating furnace, heat it to 860℃ and keep it for 4 hours, then take it out of the furnace and air cool it.

[0051] (6) Quenching and tempering heat treatment. The quenching temperature is 920℃, the holding time is 3 hours, and then quenching in 12% PAG coolant. Tempering is carried out in time after quenching, the tempering temperature is 280℃, and the tempering time is 4 hours. Tempering is performed twice.

[0052] (7) Machining: Machining to the required accuracy and size.

[0053]

[0054] The data shown in Table 6 were obtained by testing the performance of the rotary drilling rig drill pipe square head obtained in Example 2 and comparing it with the 35CrMo square head in the prior art.

[0055]

[0056] Conclusion: The square head obtained by using Example 2 has better performance.

[0057] The above is only a specific embodiment of the present disclosure, but the protection scope of the present disclosure is not limited thereto. Any person skilled in the art who is familiar with the technical field can easily think of changes or substitutions within the technical scope disclosed in the present disclosure, which should be included in the protection scope of the present disclosure. Therefore, the protection scope of the present disclosure should be based on the protection scope of the claims.

Claims

1. A method for manufacturing low-temperature high-strength and high-toughness steel, characterized in that: The raw material chemical composition of the low-temperature high-strength and high-toughness steel is as follows: carbon: 0.18% to 0.20%, silicon: 1.40% to 1.48%, manganese: 0.8% to 1.0%, chromium: 0.7% to 1.0%, molybdenum: 0.20% to 0.40%, nickel: 0.9% to 1.2%, sulfur: 0% to 0.01%, phosphorus: 0% to 0.01%, and the rest is Fe and unavoidable impurities; The raw material is subjected to a smelting process, a forging or hot rolling forming process, a cutting or a free forging process to obtain a corresponding blank; Performing preliminary heat treatment on the blank, heating the blank of a preset size to between 800°C and 880°C, keeping the blank at the temperature for a preset time of 1.0-4.0 hours, and cooling the blank to room temperature in air to obtain a forged piece after preliminary heat treatment; The heat treatment process includes a first quenching, a first low-temperature tempering, a second quenching, and a second low-temperature tempering. A primary quenching process is to heat the forging after the preliminary heat treatment to 910°C ± 10°C and keep the temperature for a preset time; the forging after heating and keeping the temperature is placed in a cooling medium, the cooling medium is water or PAG coolant, and when the cooling medium is PAG coolant, the concentration of the PAG coolant is 4% to 12%; One low-temperature tempering, the forgings after the first quenching are subjected to low-temperature tempering treatment, the tempering temperature is 200℃~300℃, the tempering time is 1.5h~4h, and after tempering, they are cooled in water or PAG quenching liquid; Secondary quenching, the forgings after the first low temperature tempering are repeated once using the first quenching process; Secondary low temperature tempering, the forgings after secondary quenching are subjected to a repeated low temperature tempering process.

2. The manufacturing method according to claim 1, characterized in that: The smelting process of the raw materials specifically comprises the following steps: The raw materials are smelted by an electric arc furnace or a medium frequency induction furnace to obtain molten steel; The molten steel is subjected to LF refining and VD vacuum treatment in sequence; wherein the tapping temperature is 1530° C. to 1570° C.; The refined molten steel is poured into an ingot mold to obtain an ingot, or poured into a continuous casting crystallizer to obtain a continuous casting billet.

3. The manufacturing method according to claim 1, characterized in that: The forging or hot rolling forming process specifically includes: When the forging process is adopted: the initial forging temperature is 1150℃±50℃, the final forging temperature is 900℃±30℃; forged into square billet; When the hot rolling forming process is adopted: the starting rolling temperature is 1150℃±50℃, the final rolling temperature is 900℃±30℃; hot rolling is performed into bars.

4. The manufacturing method according to claim 1, characterized in that: The cutting or free forging process specifically includes: The bar material after the hot rolling forming process is cut to obtain a track plate blank of a preset size; or the forged square billet is free forged to obtain a square head billet of a preset size.

5. The manufacturing method according to claim 1, characterized in that: In the quenching process, the preset holding time after the forging is heated to 910℃±10℃ is calculated based on the cross-sectional size of the forging, 1-2min / mm.

6. A track shoe made of low-temperature high-strength and high-toughness steel obtained by the manufacturing method according to any one of claims 1 to 5, characterized in that: It is also necessary to perform medium frequency induction quenching and low temperature tempering on parts of the track plate. The induction quenching heating temperature is 900-950℃. After reaching the preset temperature, water or PAG coolant is sprayed for quenching. After quenching, it is placed in a 180-200℃ furnace for low temperature tempering; the hardness of the part after medium frequency induction quenching is HRC52~56.

7. The track shoe according to claim 6, characterized in that: After local medium frequency induction quenching and low temperature tempering, the track shoe has a tensile strength of 1350-1520MPa, a yield strength of 1250-1380MPa, an elongation of 13-16%, a cross-sectional shrinkage of 55-60%, an impact absorption energy of 110-130J at room temperature, an impact absorption energy of 80J-100J at -50℃, and a hardness of HRC38-45; The microstructure of the track shoe is a lath carbide-free bainite / lath martensite multiphase structure, and the lath substructure thickness or width is 20nm to 200nm.

8. A square head for a drill rod of a rotary drilling rig, characterized in that: The low-temperature, high-strength and high-toughness steel is prepared by the manufacturing method according to any one of claims 1 to 5 and then finely processed.

9. The square head for a drill rod of a rotary drilling rig according to claim 8, characterized in that: The square head for the rotary drilling rig drill pipe has a tensile strength of 1450-1520 MPa, a yield strength of 1350-1430 MPa, an elongation of 11-13%, a cross-sectional shrinkage of 50-55%, a room temperature impact absorption energy of 110-130 J, a -20°C impact absorption energy of 100 J-110 J, and a hardness of HRC43-45; The microstructure of the square head for the drill rod of the rotary drilling rig is a lath carbide-free bainite / lath martensite complex phase structure, and the lath substructure thickness is 50nm-500nm.

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