Titanium-vanadium micro-alloyed steel for diesel engine piston and preparation method of titanium-vanadium micro-alloyed steel

Abstract

The invention provides titanium-vanadium microalloyed steel for a diesel engine piston and a preparation method, and relates to the technical field of steel for the diesel engine piston. The steel comprises the following chemical components in percentage by weight: 0.30 to 0.40 percent of C, 0.80 to 1.20 percent of Si, 0.60 to 1.00 percent of Mn, 1.20 to 1.60 percent of Cr, 0.15 to 0.25 percent of Mo, 0.05 to 0.10 percent of Ti, 0.05 to 0.10 percent of V, less than or equal to 0.025 percent of P, 0.015 to 0.025 percent of S, 0.020 to 0.040 percent of Al and the balance of Fe. And the balance of Fe and inevitable impurities. The method comprises the steps of raw material weighing and smelting, forging forming, hot rolling and thermal refining. The titanium-vanadium microalloyed steel for the diesel engine piston is prepared through simple vacuum melting, hot forging, preheating before hot rolling, hot rolling and thermal refining. The method is simple and easy to operate, green and environment-friendly, low in cost, short in process, high in efficiency and beneficial to industrial large-scale production and popularization.

Description

Technical Field

[0001] The invention relates to the technical field of steel for diesel engine pistons, in particular to titanium-vanadium microalloyed steel for diesel engine pistons and a preparation method thereof. Background Art

[0002] Diesel engines have been widely used in heavy-duty commercial vehicles, engineering machinery, ships, military industry and other fields due to their high thermal efficiency, good power and emission performance, and have played a huge role in promoting industrial production and social and economic development. In recent years, reducing carbon emissions and even achieving carbon neutrality has become a common pursuit of countries around the world. The emission regulations in Europe, the United States and China continue to tighten, which requires diesel engines to continue to develop in the direction of low carbonization, cleanliness and light weight.

[0003] In order to meet the above development requirements, improving the power density and combustion pressure of diesel engines has become a technical problem that needs to be solved urgently. If it is not solved, it will lead to a significant increase in the heat released per unit time in the cylinder, and then a significant increase in temperature and pressure. The relatively harsh working environment in the cylinder significantly increases the thermal load of each component, which in turn affects the reliability and service life of the entire diesel engine. Therefore, the thermal load and thermal strength of key components in the diesel engine cylinder have become one of the main factors limiting the further enhancement of the engine.

[0004] The piston is the "heart" of the diesel engine. It bears alternating mechanical and thermal loads, and has high requirements for mechanical and physical properties. It must have sufficient strength and rigidity, good thermal conductivity, and high temperature resistance. It is one of the key components in the engine with the worst working conditions. The earliest engine pistons were cast iron. Later, aluminum alloy materials were favored by engine companies for their light weight, good thermal conductivity, and low expansion coefficient, and began industrial manufacturing. With the rapid development of high-horsepower automobile engines, especially the application of heavy-duty diesel engine turbocharging and intercooling technology, as well as the continuous improvement of large-bore high compression ratio and low emission requirements. Traditional aluminum alloy piston materials cannot meet its use requirements, so many piston material research institutions and manufacturers at home and abroad have introduced many new piston materials, such as forged steel, ceramics and composite materials. At present, the cost of ceramic and composite pistons is too high and the technology is not yet fully mature, so steel die-forged pistons will show a blowout demand in the future and become one of the hot spots for design and development of domestic forging manufacturers.

[0005] With the implementation of the National V and National VI emission standards, new technical requirements such as high boost, low fuel consumption and low emissions have been put forward for diesel engines. It is imperative to increase the combustion temperature (500℃→600℃) and explosion pressure (20MPa→30MPa) in the cylinder. The existing piston steel is mainly 42CrMo and 38MnVS6 bars, which are typical quenched and tempered steel and non-quenched and tempered steel, with a market share of more than 95%. It faces the problem of insufficient comprehensive performance (including strength, toughness, thermal conductivity and heat resistance, etc.) in high-temperature service environments above 600℃. The composition and processing technology of piston steel must be optimized to improve the high temperature and stress limit that the piston can withstand, while meeting the lower thermal conductivity, in order to meet the reliability and durability requirements of high-strength diesel engines.

[0006] However, there are many problems in the prior art regarding the preparation of engine piston materials and their performance.

[0007] For example, Chinese patent CN105838965A discloses an alloy steel material for engine pistons, which has high carbon and silicon contents in the composition selection. However, the mechanical properties of the prepared material are low in strength and low in elongation. In addition, the prepared material is an ingot, which has not been machined or heat treated. The alloy liquid composition is relatively complex, and the raw materials need to be specially prepared with modified niobium carbide and friction reducer, which is costly. The same is true for Chinese patent CN117488210A, which has a high carbon content and a low silicon content. It is also necessary to spray a layer of carbon powder on the surface of the continuous casting billet, and then perform multiple hot rolling and rapid cooling and then slowly heat preservation, and finally slowly cool to obtain the material. The control is difficult, the cost is high, the process is long, and the efficiency is low.

[0008] Although Chinese patent CN107868904A does not disclose steel for engine pistons, it discloses a steel for hydraulic breaker pistons and its manufacturing process, in which the carbon content and silicon content are selected to be lower, and the nickel content and molybdenum content are selected to be higher. The preparation method focuses on the smelting process and forging and post-forging normalizing + high-temperature tempering + quenching + low-temperature tempering. Not only does the addition of high-cost alloy elements increase the production cost, but the multiple smelting and refining processes are complicated, the process is long and the efficiency is low, and the post-forging heat treatment process is even more so, which increases the difficulty of preparation. Summary of the invention

[0009] In order to solve the problem of insufficient comprehensive performance (including strength, toughness, thermal conductivity and heat resistance, etc.) of engine piston steel in high-temperature service environment above 600℃ in the prior art, although there are ways to improve performance such as adding alloy elements and increasing the content of existing elements, improving preparation process and adding heat treatment, there are more or less technical defects such as high cost, long process, difficult operation, low efficiency, and inability to improve performance synergistically; the present invention proposes a titanium-vanadium microalloyed diesel engine piston steel and preparation method that can solve the above technical defects. The technical scheme is as follows:

[0010] A titanium-vanadium microalloyed steel for a diesel engine piston. The cross-sectional diameter of the rod of the titanium-vanadium microalloyed steel for a diesel engine piston is φ60-φ120 mm, and the chemical composition is as follows by mass percentage: C 0.30-0.40%, Si 0.80-1.20%, Mn 0.60-1.00%, Cr 1.20-1.60%, Mo 0.15-0.25%, Ti 0.05-0.10%, V 0.05-0.10%, P≤0.025%, S 0.015-0.025%, Al 0.020-0.040%; the rest is Fe and unavoidable impurities.

[0011] Optionally, the hot-rolled microstructure of the titanium-vanadium microalloyed diesel engine piston steel is ferrite with a volume fraction of 30-60% and pearlite with a volume fraction of 40-70%, and the microstructure of the titanium-vanadium microalloyed diesel engine piston steel after quenching and tempering treatment is tempered bainite; wherein: the ferrite is equiaxed polygonal in shape, and the average grain size is 10-30μm; the pearlite is lamellar in shape, and the average grain size is 20-40μm; the tempered bainite is equiaxed and fine-grained in shape, and the average grain size is 10-15μm.

[0012] Optionally, the room temperature properties of the titanium-vanadium microalloyed diesel engine piston steel after quenching and tempering treatment: tensile strength R m ≥1000MPa, yield strength R eL or R p0.2 ≥900MPa, elongation after fracture A≥14%, Brinell hardness in the range of 290-340HBW, room temperature impact energy KV2≥18J, room temperature thermal conductivity λ is 20-25W / (m·K), 20-200℃ average linear expansion coefficient α m At (12-14)×10 -6 / ℃ range.

[0013] A method for preparing the titanium-vanadium microalloyed diesel engine piston steel is provided. The method for preparing the titanium-vanadium microalloyed diesel engine piston steel comprises the following steps:

[0014] S1, weighing and smelting raw materials: weighing raw materials according to the chemical composition of the titanium-vanadium microalloyed diesel engine piston steel, then adding the weighed raw materials into a vacuum induction furnace for smelting, and casting the smelted molten steel to obtain a steel ingot;

[0015] S2, forging: heating the S1 steel ingot and forging it to obtain a steel billet;

[0016] S3, hot rolling: the S2 steel billet is heated and then hot rolled, and then slowly cooled to room temperature to obtain a hot rolled bar;

[0017] S4, quenching and tempering treatment: the hot-rolled bars in S3 are subjected to quenching and tempering treatment to finally obtain titanium-vanadium micro-alloyed steel bars for diesel engine pistons.

[0018] Optionally, in S1, the smelting temperature is 1630-1680°C, the smelting time is 20-40 min; and the casting temperature is 1550-1560°C.

[0019] Optionally, the heating rate of heating in S2 is 5-10°C / min, the initial forging temperature is 1130-1180°C, the final forging temperature is 850-900°C, and the forging upsetting ratio is 1.5-2.5.

[0020] Optionally, the heating rate in S3 is 5-10°C / min, the steel is heated to 1130-1180°C and kept warm for 30-90min before hot rolling, the starting temperature of hot rolling is 1030-1080°C, the final rolling temperature is 850-950°C, the hot rolling adopts one-pass large reduction rolling or multiple-pass small reduction rolling, the total reduction is 50-87%, and the slow cooling cooling rate is 0.1-0.3°C / min.

[0021] Optionally, the tempering treatment in S4 is first quenching at 850-890°C for 30-60min, with oil cooling as the cooling method; and then tempering at 620-660°C for 60-120min, with air cooling as the cooling method.

[0022] Optionally, the size of the steel ingot in S1 is 200×200 mm, the size of the steel billet in S2 is 150×150 mm, and the size of the bar in S3 and S4 is φ60-φ120 mm.

[0023] The technical principle of the present invention for selecting the chemical components and their contents:

[0024] C: C is a key element that affects the strength and hardness of steel. Adding an appropriate amount of C can ensure that the steel has good hardenability and appropriate strength. At the same time, C can also form carbides with Mo, Ti, and V to refine the grains. When the C content is too low, it cannot ensure that the steel has sufficient tensile strength, resulting in reduced deformation resistance and shortened fatigue life of the piston. When the C content is too high, it will have an adverse effect on the subsequent processing of the piston. Taking all factors into consideration, the mass percentage of C is controlled between 0.30 and 0.40%.

[0025] Si: Si is a ferrite-forming element with a strong solid solution strengthening effect, which can effectively improve the strength of steel. However, when the Si content is too high, it will reduce the plasticity of the steel and have an adverse effect on fatigue performance. In addition, Si is a commonly used deoxidizer, and adding an appropriate amount of Si is beneficial to reducing oxygen in steel. Therefore, the mass percentage of Si is controlled between 0.80 and 1.20%.

[0026] Mn: Mn is an austenite-forming element with a strong solid solution strengthening effect. It can reduce the phase transition temperature of steel, improve hardenability and refine grains, and can effectively improve the strength and toughness of steel. However, when the Mn content is too high, it will aggravate the segregation of steel and deteriorate the toughness of steel. In addition, Mn is a good desulfurizer and deoxidizer, which can eliminate or weaken the hot brittleness caused by the reaction of S and Fe. Therefore, the mass percentage of Mn is controlled between 0.60~1.00%.

[0027] Cr: Cr is a ferrite-forming element that can reduce the austenite phase area, improve the hardenability of steel, improve the high-temperature oxidation resistance of steel, and increase the corrosion resistance of steel in complex atmospheres. Cr can form Cr2O3 with oxidation resistance at high temperatures, which slows down the overall diffusion rate in the oxide film. As the Cr content increases, the oxide film becomes denser, more complete, and more continuous. However, when the Cr content is too high, it will promote the segregation of impurity atoms, increase the temper brittleness tendency of steel, and form coarse carbides, which will deteriorate the cold deformation performance of steel. Therefore, the mass percentage of Cr is controlled between 1.20 and 1.60%.

[0028] Mo: Mo is a ferrite-forming element. Its ability to expand the ferrite phase is higher than that of Cr. It can effectively improve the hardenability and solid solution strengthening effect of steel, improve the strength and hardness of steel to a certain extent, and improve the intergranular corrosion resistance. In addition, Mo can improve the tempering stability of steel, improve impact toughness, and increase the thermal fatigue resistance of steel, which is very beneficial for pistons under high temperature working conditions. However, it should be noted that the alloy cost of Mo is relatively high, and its content should not be too high. Based on this, the mass percentage of Mo is controlled between 0.15~0.25%.

[0029] Ti, V: Ti and V can form precipitates with C and N in steel, effectively pinning grain boundaries and refining grains through the second phase strengthening mechanism. By controlling the precipitation of the second phase, as well as the number, size and distribution of these precipitates, the mechanical properties of microalloyed steel can be significantly improved, and the steel can still maintain a fine-grained structure at high temperatures, reduce the overheating sensitivity of steel, and significantly improve the high-temperature endurance strength of steel. At the same time, V can also promote the precipitation of Mn and Si, increase the Mn and Si content at the grain boundaries, increase the grain boundary strength, and improve the mechanical properties of steel. However, when the Ti and V content is too high, coarse inclusions will be formed, which will have an adverse effect on the mechanical properties of steel.

[0030] In addition, the addition of Ti and V in the present invention can form Ti-Mo-V composite microalloying with Mo, making full use of the synergistic effect of various microalloying elements, increasing the volume fraction of the precipitated phase, refining the structure and reducing the size of the precipitated phase, and the complex precipitated phase such as (Ti, Mo, V) C has higher thermal stability, and finally further improves the performance. Taking all factors into consideration, the mass percentage of Ti and V is controlled between 0.05-0.10%.

[0031] P: P is a harmful element that increases the cold brittleness of steel and reduces plasticity. At the same time, P is the main element that forms inclusions. When the P content is too high, multi-element eutectic structure is formed between dendrites, resulting in a serious decrease in the toughness of the steel. Therefore, the mass percentage of P is controlled within 0.025%.

[0032] S: Adding an appropriate amount of S can combine with Mn to form long strips of MnS. The presence of MnS can produce a stress concentration source, making the chips easy to break, thereby improving the cutting performance of the steel. At the same time, MnS also has a lubricating effect and can reduce the wear on the tool. However, when the S content is too high, it will increase the hot brittleness of the steel and reduce the plasticity and toughness of the steel. Therefore, the mass percentage of S is controlled between 0.015-0.025%.

[0033] Al: Al is an excellent deoxidizer, and Al can form fine AlN precipitates with N, which can effectively hinder the migration of grain boundaries, thereby refining grains and improving the toughness of steel. However, when the Al content is too high, it will increase the difficulty of molten steel casting and form coarse Al2O3 inclusions, which will have an adverse effect on the performance of steel. Therefore, the mass percentage of Al is controlled between 0.020-0.040%.

[0034] Compared with the prior art, the above technical solution has at least the following beneficial effects:

[0035] The above scheme, the present invention proposes a titanium-vanadium microalloyed diesel engine piston steel and a preparation method, which can solve the problem of insufficient comprehensive performance (including strength, toughness, thermal conductivity and heat resistance, etc.) of engine piston steel in high-temperature service environment above 600°C in the prior art. Although there are ways to improve the performance by adding alloy elements and increasing the content of existing elements, improving the preparation process and adding heat treatment, there are more or less technical defects such as high cost, long process, great operation difficulty, low efficiency, and inability to improve the performance synergistically.

[0036] The present invention optimizes the chemical composition of Ti and V microalloyed diesel engine piston steel: by increasing the content of Si and Cr elements, the increasingly enhanced heat resistance requirements of piston steel are met; Ti and V elements are added to form Ti-Mo-V composite microalloying, so that the piston steel has better mechanical and physical properties; the mass percentage of S element is controlled in the range of 0.015-0.025% to meet the cutting performance requirements of piston steel.

[0037] The present invention obtains steel ingots through simple vacuum smelting, which can reduce the content of gases such as hydrogen and oxygen during the smelting process, reduce the formation of pores and inclusions, and make the alloy elements more evenly distributed to avoid segregation, thereby improving the purity and structural uniformity of the steel ingot, effectively enhancing the mechanical properties, and improving the heat resistance, wear resistance and fatigue life of the piston under high temperature and high pressure conditions. Since the steel ingot has fewer internal defects, the probability of problems occurring in the subsequent processing process is reduced, which can effectively improve the yield rate, making the comprehensive performance of piston steel significantly better than that of traditional smelting processes, and achieving the best cost-performance balance.

[0038] The present invention can compact and close casting defects such as pores and shrinkage, break up coarse cast structures, promote dynamic recrystallization, significantly refine grains, and improve the density of the billet through hot forging with an upsetting ratio of 1.5-2.5. At the same time, hot forging can alleviate the segregation of alloy elements, ensure the uniform composition of the billet, and avoid performance fluctuations. The internal stress distribution of the billet after hot forging is more uniform, the deformation resistance during rolling is reduced, the rolling mill load and energy consumption are reduced, the crack risk during rolling is reduced, and the yield rate is improved. The organization optimization, defect control and performance improvement brought by hot forging are crucial, and are particularly suitable for the manufacture of diesel engine pistons with strict requirements on strength, toughness and fatigue life.

[0039] The present invention can maintain an appropriate temperature gradient during the rolling process of the steel by preheating before hot rolling and selecting the hot rolling method, avoid uneven structure caused by too fast heating, promote dynamic recrystallization and grain refinement, reduce residual stress after rolling, improve strength and toughness, and reduce the risk of subsequent heat treatment or machining deformation. Through multiple deformations of the rolling mill, large deformation processing can be completed quickly, which is suitable for mass production, and different diameters and lengths can be switched to meet diversified orders and meet the performance requirements of diesel engine pistons.

[0040] The present invention can make the piston steel have a uniform tempered bainite structure through the quenching and tempering treatment after hot rolling, significantly improve the strength, hardness and impact resistance, and refine the grains, improve the uniformity of the structure and fatigue resistance. The quenching and tempering treatment can also eliminate the residual stress of the piston steel during the thermal deformation process, reduce the risk of deformation and cracking, thereby extending the service life of the piston and enhancing reliability. In addition, by controlling the tempering temperature, the strength, hardness and toughness of the piston steel can be adjusted to meet the needs of different application scenarios of diesel engine pistons.

[0041] The room temperature properties of the titanium-vanadium microalloyed diesel engine piston steel after quenching and tempering treatment of the present invention are as follows: tensile strength R m ≥1000MPa, yield strength R eL or R p0.2 ≥900MPa, elongation after fracture A≥14%, Brinell hardness in the range of 290-340HBW, room temperature impact energy KV2≥18J, which can make the piston withstand huge pressure and impact force during service.

[0042] The room temperature thermal conductivity λ of the titanium-vanadium microalloyed diesel engine piston steel after quenching and tempering treatment is 20-25W / (m·K), so that the piston can transfer the absorbed heat to the cylinder wall in time to maintain the normal working temperature of the engine; the average linear expansion coefficient α of 20-200℃ m At (12-14)×10 -6 / ℃ range, so that there is good sealing between the piston and the cylinder wall, and the reciprocating motion of the piston will not be hindered due to excessively high thermal expansion coefficient.

[0043] The titanium-vanadium microalloyed diesel engine piston steel of the present invention has reasonable chemical composition and process design, a wide process window, and can be commercially produced in batches on a bar production line, thus having good promotion prospects and application value.

[0044] In summary, compared with other traditional methods, the method of the present invention prepares titanium-vanadium microalloyed diesel engine piston steel through simple vacuum melting, hot forging, preheating before hot rolling and hot rolling, and tempering treatment; the method is simple and easy to operate, green and environmentally friendly, low in cost, short in process, high in efficiency, and is conducive to large-scale industrial production and promotion. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0046] Figure 1is a metallographic structure diagram of titanium-vanadium microalloyed diesel engine piston steel according to Example 1 of the present invention;

[0047] Figure 2 It is the metallographic structure diagram of the Cr-Mo diesel engine piston steel of comparative example 1 of the present invention. DETAILED DESCRIPTION

[0048] The technical solution of the present invention is described below in conjunction with the accompanying drawings.

[0049] In the embodiments of the present invention, words such as "exemplarily" and "for example" are used to indicate examples, illustrations or explanations. Any embodiment or design described as "example" in the present invention should not be interpreted as being more preferred or more advantageous than other embodiments or designs. Specifically, the use of the word "example" is intended to present the concept in a specific way. In addition, in the embodiments of the present invention, the meaning expressed by "and / or" can be both, or it can be either of the two.

[0050] In the embodiments of the present invention, "image" and "picture" may sometimes be used interchangeably. It should be noted that when the distinction between them is not emphasized, the meanings they intend to express are consistent.

[0051] In the embodiments of the present invention, sometimes a subscript such as W1 may be written as a non-subscript such as W1. When the difference is not emphasized, the meanings to be expressed are the same.

[0052] In order to make the technical problems, technical solutions and advantages to be solved by the present invention more clear, a detailed description will be given below with reference to the accompanying drawings and specific embodiments.

[0053] A titanium-vanadium microalloyed steel for a diesel engine piston. The cross-sectional diameter of the rod of the titanium-vanadium microalloyed steel for a diesel engine piston is φ60-φ120 mm, and the chemical composition is as follows by mass percentage: C 0.30-0.40%, Si 0.80-1.20%, Mn 0.60-1.00%, Cr 1.20-1.60%, Mo 0.15-0.25%, Ti 0.05-0.10%, V 0.05-0.10%, P≤0.025%, S 0.015-0.025%, Al 0.020-0.040%; the rest is Fe and unavoidable impurities.

[0054] In particular, the hot-rolled microstructure of the titanium-vanadium microalloyed diesel engine piston steel is ferrite with a volume fraction of 30-60% and pearlite with a volume fraction of 40-70%, and the microstructure of the titanium-vanadium microalloyed diesel engine piston steel after quenching and tempering is tempered bainite; wherein: the ferrite is equiaxed polygonal in shape, and the average grain size is 10-30 μm; the pearlite is lamellar in shape, and the average grain size is 20-40 μm; the tempered bainite is equiaxed and fine-grained in shape, and the average grain size is 10-15 μm.

[0055] In particular, the room temperature properties of the titanium-vanadium microalloyed diesel engine piston steel after quenching and tempering are as follows: tensile strength R m ≥1000MPa, yield strength R eL or R p0.2 ≥900MPa, elongation after fracture A≥14%, Brinell hardness in the range of 290-340HBW, room temperature impact energy KV2≥18J, room temperature thermal conductivity λ is 20-25W / (m·K), 20-200℃ average linear expansion coefficient α m At (12-14)×10 -6 / ℃ range.

[0056] A method for preparing the titanium-vanadium microalloyed diesel engine piston steel is provided. The method for preparing the titanium-vanadium microalloyed diesel engine piston steel comprises the following steps:

[0057] S1, weighing and smelting raw materials: weighing raw materials according to the chemical composition of the titanium-vanadium microalloyed diesel engine piston steel, then adding the weighed raw materials into a vacuum induction furnace for smelting, and casting the smelted molten steel to obtain a steel ingot;

[0058] S2, forging: heating the S1 steel ingot and forging it to obtain a steel billet;

[0059] S3, hot rolling: the S2 steel billet is heated and then hot rolled, and then slowly cooled to room temperature to obtain a hot rolled bar;

[0060] S4, quenching and tempering treatment: the hot-rolled bars in S3 are subjected to quenching and tempering treatment to finally obtain titanium-vanadium micro-alloyed steel bars for diesel engine pistons.

[0061] In particular, in S1, the smelting temperature is 1630-1680°C, the smelting time is 20-40 min; and the casting temperature is 1550-1560°C.

[0062] In particular, the heating rate of heating in S2 is 5-10°C / min, the initial forging temperature is 1130-1180°C, the final forging temperature is 850-900°C, and the forging upsetting ratio is 1.5-2.5.

[0063] In particular, the heating rate in S3 is 5-10°C / min, the steel is heated to 1130-1180°C and kept warm for 30-90min before hot rolling, the starting temperature of hot rolling is 1030-1080°C, the final rolling temperature is 850-950°C, the hot rolling adopts a single large reduction rolling or multiple small reduction rolling, the total reduction is 50-87%, and the slow cooling cooling rate is 0.1-0.3°C / min.

[0064] In particular, the tempering treatment in S4 is first quenching at 850-890°C for 30-60min, and the cooling method is oil cooling; then tempering at 620-660°C for 60-120min, and the cooling method is air cooling.

[0065] Specifically, the size of the ingot in S1 is 200×200 mm, the size of the billet in S2 is 150×150 mm, and the size of the bar in S3 and S4 is φ60-φ120 mm.

[0066] The performance test of the titanium-vanadium microalloyed diesel engine piston steels of the following Examples 1-5 and Comparative Examples 1-2 is as follows: samples are taken respectively to conduct mechanical and physical property tests.

[0067] The specific detection method is as follows:

[0068] Tensile test: According to the national standard GB / T 228.1-2021 "Tensile test of metallic materials - Part 1: Room temperature test method", the test was carried out, and the tensile specimens were processed into a gauge section of Φ5×25mm and a clamping end with M10 thread. The test environment was controlled at room temperature of 10-35°C, and the tensile strength R of the piston steel of Examples 1-5 and Comparative Examples 1-2 can be effectively measured. m , yield strength R eL or R p0.2 And elongation after fracture A.

[0069] Brinell hardness test: The test was conducted in accordance with the national standard GB / T 231.1-2018 "Brinell hardness test for metallic materials - Part 1: Test method". The surface of the sample was flat and smooth, and the thickness was at least 8 times the indentation depth. The test environment was controlled to be room temperature 10-35°C to measure the Brinell hardness HBW of the piston steels of Examples 1-5 and Comparative Examples 1-2.

[0070] Impact test: The test was carried out in accordance with the national standard GB / T 229-2020 "Charpy Pendulum Impact Test Method for Metallic Materials", the sample size was 10×10×55mm, the notch was V-shaped, and the test environment was controlled to room temperature 23±5°C to measure the room temperature impact energy KV2 of the piston steels of Examples 1-5 and Comparative Examples 1-2.

[0071] Thermal conductivity measurement: The test was conducted in accordance with the national standard GB / T 22588-2008 "Flash method for measuring thermal diffusion coefficient or thermal conductivity", using a Φ10×2.5 mm disc sample and setting the test temperature to 25°C to measure the room temperature thermal diffusion coefficient of the sample, and combining the density and specific heat capacity of the sample (determined by the DSC method) to measure the room temperature thermal conductivity λ of the piston steels of Examples 1-5 and Comparative Examples 1-2.

[0072] Thermal expansion coefficient determination: According to the national standard GB / T 4339-2008 "Determination of thermal expansion characteristic parameters of metal materials", the test was carried out, the sample size was Φ4×25mm, 20°C was used as the reference starting temperature, and the linear thermal expansion of the sample in the temperature range of 20-200°C was recorded to measure the average linear expansion coefficient α of the piston steel of Examples 1-5 and Comparative Examples 1-2 m .

[0073] Example 1

[0074] A titanium-vanadium microalloyed steel for a diesel engine piston. The cross-sectional diameter of the rod of the titanium-vanadium microalloyed steel for a diesel engine piston is φ60 mm, and the chemical composition is as follows by mass percentage: C 0.30%, Si 1.13%, Mn 0.61%, Cr 1.31%, Mo 0.20%, Ti 0.055%, V 0.051%, P 0.009%, S 0.020%, Al 0.037%; the rest is Fe and unavoidable impurities.

[0075] A method for preparing the titanium-vanadium microalloyed diesel engine piston steel is provided. The method for preparing the titanium-vanadium microalloyed diesel engine piston steel comprises the following steps:

[0076] S1. Raw material weighing and smelting: The raw materials are weighed according to the chemical composition of the titanium-vanadium microalloyed diesel engine piston steel, and then the weighed raw materials are added to a vacuum induction furnace for smelting, and the vacuum is evacuated to 2.5×10 -3 Pa, then fill with argon to 0.04MPa, the smelting temperature is 1650℃, the smelting time is 30min; the smelted molten steel is cast to obtain a steel ingot, the casting temperature is 1550℃; the size of the steel ingot is 200×200mm;

[0077] S2, forging: the S1 steel ingot is heated and then forged, the heating rate is 5°C / min, the initial forging temperature is 1150°C, the final forging temperature is 880°C, the forging upsetting ratio is 1.5, and a steel billet is obtained; the size of the steel billet is 150×150mm;

[0078] S3, hot rolling: the S2 steel billet is heated and then hot rolled, the heating rate is 5℃ / min, the temperature is heated to 1150℃ and kept for 60min before hot rolling, the starting temperature of hot rolling is 1050℃, the final rolling temperature is 900℃, the hot rolling adopts one-pass large reduction rolling or multiple passes of small reduction rolling, the total reduction is 87%, and then slowly cooled to room temperature, the cooling rate of slow cooling is 0.1℃ / min, and the hot rolled bar is obtained; the size of the bar is φ60mm;

[0079] S4, quenching and tempering treatment: the hot-rolled bars in S3 are quenched and tempered by quenching at 870°C for 45 minutes, and the cooling method is oil cooling; then tempering at 620°C for 90 minutes, and the cooling method is air cooling, and finally a titanium-vanadium micro-alloyed steel bar for diesel engine piston is obtained.

[0080] The hot-rolled microstructure of the titanium-vanadium microalloyed diesel engine piston steel prepared in this embodiment is ferrite with a volume fraction of 32% and pearlite with a volume fraction of 68%. The microstructure of the titanium-vanadium microalloyed diesel engine piston steel after quenching and tempering is tempered troostite. Figure 1 As shown; among them: the shape of ferrite is equiaxed polygonal, and the average grain size is 15μm; the shape of pearlite is lamellar, and the average grain size is 35μm; the shape of tempered troostite is equiaxed and fine-grained, and the average grain size is 11μm.

[0081] The room temperature properties of the titanium-vanadium microalloyed diesel engine piston steel prepared in this example after quenching and tempering treatment are as follows: tensile strength R m The yield strength is 1202MPa, and the yield strength R eL or R p0.2 The average linear expansion coefficient of α at 20-200℃ is α, the elongation after fracture is 14%, the Brinell hardness is 333HBW, the impact energy KV2 at room temperature is 18J, the thermal conductivity λ at room temperature is 25W / (m·K), and the average linear expansion coefficient α at 20-200℃ is α. m 12.7×10 -6 / ℃.

[0082] Example 2

[0083] A titanium-vanadium microalloyed steel for a diesel engine piston. The cross-sectional diameter of the rod of the titanium-vanadium microalloyed steel for a diesel engine piston is φ80 mm, and the chemical composition is as follows by mass percentage: C 0.32%, Si 1.20%, Mn 0.69%, Cr 1.26%, Mo 0.25%, Ti 0.071%, V 0.065%, P 0.012%, S 0.022%, Al 0.034%; the rest is Fe and unavoidable impurities.

[0084] A method for preparing the titanium-vanadium microalloyed diesel engine piston steel is provided. The method for preparing the titanium-vanadium microalloyed diesel engine piston steel comprises the following steps:

[0085] S1. Raw material weighing and smelting: The raw materials are weighed according to the chemical composition of the titanium-vanadium microalloyed diesel engine piston steel, and then the weighed raw materials are added to a vacuum induction furnace for smelting, and the vacuum is evacuated to 2.5×10 -3 Pa, then fill with argon to 0.04MPa, the smelting temperature is 1650℃, the smelting time is 30min; the smelted molten steel is cast to obtain a steel ingot, the casting temperature is 1550℃; the size of the steel ingot is 200×200mm;

[0086] S2, forging: the S1 steel ingot is heated and then forged, the heating rate is 5°C / min, the initial forging temperature is 1150°C, the final forging temperature is 880°C, the forging upsetting ratio is 2, and a steel billet is obtained; the size of the steel billet is 150×150mm;

[0087] S3, hot rolling: the S2 steel billet is heated and then hot rolled, the heating rate is 5℃ / min, the temperature is heated to 1150℃ and kept for 60min before hot rolling, the starting temperature of hot rolling is 1050℃, the final rolling temperature is 900℃, the hot rolling adopts one-pass large reduction rolling or multiple passes of small reduction rolling, the total reduction is 78%, and then slowly cooled to room temperature, the cooling rate of slow cooling is 0.2℃ / min, and hot rolled bars are obtained; the size of the bars is φ80mm;

[0088] S4, quenching and tempering treatment: the hot-rolled bars in S3 are quenched and tempered by quenching at 870°C for 45 minutes, and the cooling method is oil cooling; then tempering at 640°C for 90 minutes, and the cooling method is air cooling, and finally a titanium-vanadium micro-alloyed steel bar for diesel engine piston is obtained.

[0089] The hot-rolled microstructure of the titanium-vanadium microalloyed diesel engine piston steel prepared in this embodiment is ferrite with a volume fraction of 44% and pearlite with a volume fraction of 56%, and the microstructure of the titanium-vanadium microalloyed diesel engine piston steel after quenching and tempering treatment is tempered bainite; wherein: the ferrite is equiaxed polygonal in shape, and the average grain size is 22 μm; the pearlite is lamellar in shape, and the average grain size is 30 μm; the tempered bainite is equiaxed and fine-grained in shape, and the average grain size is 13 μm.

[0090] The room temperature properties of the titanium-vanadium microalloyed diesel engine piston steel prepared in this example after quenching and tempering treatment are as follows: tensile strength R m The yield strength is 1118MPa, and the yield strength R eL or R p0.2The average linear expansion coefficient of α at 20-200℃ is α, the elongation after fracture is 15%, the Brinell hardness is 317HBW, the impact energy KV2 at room temperature is 21J, the thermal conductivity λ at room temperature is 23W / (m·K), and the average linear expansion coefficient α at 20-200℃ is α. m 13.1×10 -6 / ℃.

[0091] Example 3

[0092] A titanium-vanadium microalloyed steel for a diesel engine piston. The cross-sectional diameter of the rod of the titanium-vanadium microalloyed steel for a diesel engine piston is φ90 mm, and the chemical composition is as follows by mass percentage: C 0.35%, Si 1.05%, Mn 0.82%, Cr 1.60%, Mo 0.15%, Ti 0.086%, V 0.084%, P 0.008%, S 0.025%, Al 0.030%; the rest is Fe and unavoidable impurities.

[0093] A method for preparing the titanium-vanadium microalloyed diesel engine piston steel is provided. The method for preparing the titanium-vanadium microalloyed diesel engine piston steel comprises the following steps:

[0094] S1. Raw material weighing and smelting: The raw materials are weighed according to the chemical composition of the titanium-vanadium microalloyed diesel engine piston steel, and then the weighed raw materials are added to a vacuum induction furnace for smelting, and the vacuum is evacuated to 2.5×10 -3 Pa, then fill with argon to 0.04MPa, the smelting temperature is 1650℃, the smelting time is 30min; the smelted molten steel is cast to obtain a steel ingot, the casting temperature is 1555℃; the size of the steel ingot is 200×200mm;

[0095] S2, forging: the S1 steel ingot is heated and then forged, the heating rate is 8°C / min, the initial forging temperature is 1150°C, the final forging temperature is 880°C, the forging upsetting ratio is 2, and a steel billet is obtained; the size of the steel billet is 150×150mm;

[0096] S3, hot rolling: the S2 steel billet is heated and then hot rolled, the heating rate is 8℃ / min, the temperature is heated to 1150℃ and kept for 60min before hot rolling, the starting temperature of hot rolling is 1050℃, the final rolling temperature is 900℃, the hot rolling adopts one-time large reduction rolling or multiple-pass small reduction rolling, the total reduction is 72%, and then slowly cooled to room temperature, the cooling rate of slow cooling is 0.2℃ / min, and the hot rolled bar is obtained; the size of the bar is φ90mm;

[0097] S4, quenching and tempering treatment: the hot-rolled bars in S3 are quenched and tempered by quenching at 870°C for 45 minutes, and the cooling method is oil cooling; then tempering at 660°C for 90 minutes, and the cooling method is air cooling, and finally a titanium-vanadium micro-alloyed steel bar for diesel engine piston is obtained.

[0098] The hot-rolled microstructure of the titanium-vanadium microalloyed diesel engine piston steel prepared in this embodiment is ferrite with a volume fraction of 47% and pearlite with a volume fraction of 53%, and the microstructure of the titanium-vanadium microalloyed diesel engine piston steel after quenching and tempering treatment is tempered bainite; wherein: the ferrite is equiaxed polygonal in shape, and the average grain size is 23 μm; the pearlite is lamellar in shape, and the average grain size is 29 μm; the tempered bainite is equiaxed and fine-grained in shape, and the average grain size is 15 μm.

[0099] The room temperature properties of the titanium-vanadium microalloyed diesel engine piston steel prepared in this example after quenching and tempering treatment are as follows: tensile strength R m The yield strength R is 1051MPa. eL or R p0.2 The average linear expansion coefficient of α at 20-200℃ is α, the elongation after fracture is 16%, the Brinell hardness is 301HBW, the impact energy KV2 at room temperature is 22J, the thermal conductivity λ at room temperature is 24W / (m·K), and the average linear expansion coefficient α at 20-200℃ is 938MPa, the elongation after fracture is 16%, the Brinell hardness is 301HBW, the impact energy KV2 at room temperature is 22J, the thermal conductivity λ at room temperature is 24W / (m·K), and the average linear expansion coefficient α at 20-200℃ is α. m 13.2×10 -6 / ℃.

[0100] Example 4

[0101] A titanium-vanadium microalloyed steel for a diesel engine piston. The cross-sectional diameter of the rod of the titanium-vanadium microalloyed steel for a diesel engine piston is φ100 mm, and the chemical composition is as follows by mass percentage: C 0.38%, Si 0.97%, Mn 0.87%, Cr 1.43%, Mo 0.18%, Ti 0.100%, V 0.089%, P 0.005%, S 0.018%, Al 0.025%; the rest is Fe and unavoidable impurities.

[0102] A method for preparing the titanium-vanadium microalloyed diesel engine piston steel is provided. The method for preparing the titanium-vanadium microalloyed diesel engine piston steel comprises the following steps:

[0103] S1. Raw material weighing and smelting: The raw materials are weighed according to the chemical composition of the titanium-vanadium microalloyed diesel engine piston steel, and then the weighed raw materials are added to a vacuum induction furnace for smelting, and the vacuum is evacuated to 2.5×10 -3Pa, then fill with argon to 0.04MPa, the smelting temperature is 1630℃, the smelting time is 20min; the smelted molten steel is cast to obtain a steel ingot, the casting temperature is 1560℃; the size of the steel ingot is 200×200mm;

[0104] S2, forging: the S1 steel ingot is heated and then forged, the heating rate is 10°C / min, the initial forging temperature is 1130°C, the final forging temperature is 850°C, the forging upsetting ratio is 2, and a steel billet is obtained; the size of the steel billet is 150×150mm;

[0105] S3, hot rolling: the S2 steel billet is heated and then hot rolled, the heating rate is 10℃ / min, the steel billet is heated to 1130℃ and kept warm for 30min before hot rolling, the starting temperature of hot rolling is 1030℃, the final rolling temperature is 850℃, the hot rolling adopts one-pass large reduction rolling or multiple passes of small reduction rolling, the total reduction is 65%, and then slowly cooled to room temperature, the slow cooling rate is 0.2℃ / min, and the hot rolled bar is obtained; the size of the bar is φ100mm;

[0106] S4, quenching and tempering treatment: the hot-rolled bars in S3 are quenched and tempered by quenching at 850°C for 30 minutes, and the cooling method is oil cooling; then tempering at 640°C for 60 minutes, and the cooling method is air cooling, and finally a titanium-vanadium micro-alloyed steel bar for diesel engine piston is obtained.

[0107] The hot-rolled microstructure of the titanium-vanadium microalloyed diesel engine piston steel prepared in this embodiment is ferrite with a volume fraction of 46% and pearlite with a volume fraction of 54%, and the microstructure of the titanium-vanadium microalloyed diesel engine piston steel after quenching and tempering treatment is tempered bainite; wherein: the ferrite is equiaxed polygonal in shape, and the average grain size is 26 μm; the pearlite is lamellar in shape, and the average grain size is 32 μm; the tempered bainite is equiaxed and fine-grained in shape, and the average grain size is 14 μm.

[0108] The room temperature properties of the titanium-vanadium microalloyed diesel engine piston steel prepared in this example after quenching and tempering treatment are as follows: tensile strength R m The yield strength R is 1152MPa. eL or R p0.2 The average linear expansion coefficient at 20-200℃ is α, the elongation after fracture is A is 15%, the Brinell hardness is 322HBW, the impact energy KV2 at room temperature is 20J, the thermal conductivity at room temperature is λ is 22W / (m·K), and the average linear expansion coefficient at 20-200℃ is α. m 12.8×10 -6 / ℃.

[0109] Example 5

[0110] A titanium-vanadium microalloyed steel for a diesel engine piston. The cross-sectional diameter of the rod of the titanium-vanadium microalloyed steel for a diesel engine piston is φ120 mm, and the chemical composition is as follows by mass percentage: C 0.40%, Si 0.84%, Mn 0.98%, Cr 1.51%, Mo 0.22%, Ti 0.092%, V 0.099%, P 0.011%, S 0.019%, Al 0.026%; the rest is Fe and unavoidable impurities.

[0111] A method for preparing the titanium-vanadium microalloyed diesel engine piston steel is provided. The method for preparing the titanium-vanadium microalloyed diesel engine piston steel comprises the following steps:

[0112] S1. Raw material weighing and smelting: The raw materials are weighed according to the chemical composition of the titanium-vanadium microalloyed diesel engine piston steel, and then the weighed raw materials are added to a vacuum induction furnace for smelting, and the vacuum is evacuated to 2.5×10 -3 Pa, then fill with argon to 0.04MPa, the smelting temperature is 1680℃, the smelting time is 40min; the smelted molten steel is cast to obtain a steel ingot, the casting temperature is 1560℃; the size of the steel ingot is 200×200mm;

[0113] S2, forging: the S1 steel ingot is heated and then forged, the heating rate is 10°C / min, the initial forging temperature is 1180°C, the final forging temperature is 900°C, the forging upsetting ratio is 2.5, and a steel billet is obtained; the size of the steel billet is 150×150mm;

[0114] S3, hot rolling: the S2 steel billet is heated and then hot rolled, the heating rate is 10℃ / min, the steel billet is heated to 1180℃ and kept warm for 90min before hot rolling, the starting temperature of hot rolling is 1080℃, the final rolling temperature is 950℃, the hot rolling adopts one-pass large reduction rolling or multiple passes of small reduction rolling, the total reduction is 50%, and then slowly cooled to room temperature, the slow cooling rate is 0.3℃ / min, and the hot rolled bar is obtained; the size of the bar is φ120mm;

[0115] S4, quenching and tempering treatment: the hot-rolled bars in S3 are quenched and tempered by quenching at 890°C for 60 minutes, and the cooling method is oil cooling; then tempering at 640°C for 120 minutes, and the cooling method is air cooling, and finally a titanium-vanadium micro-alloyed steel bar for diesel engine piston is obtained.

[0116] The hot-rolled microstructure of the titanium-vanadium microalloyed diesel engine piston steel prepared in this embodiment is ferrite with a volume fraction of 55% and pearlite with a volume fraction of 45%, and the microstructure of the titanium-vanadium microalloyed diesel engine piston steel after quenching and tempering treatment is tempered bainite; wherein: the ferrite is equiaxed polygonal in shape, and the average grain size is 30 μm; the pearlite is lamellar in shape, and the average grain size is 22 μm; the tempered bainite is equiaxed and fine-grained in shape, and the average grain size is 13 μm.

[0117] The room temperature properties of the titanium-vanadium microalloyed diesel engine piston steel prepared in this example after quenching and tempering treatment are as follows: tensile strength R m The yield strength R is 1164MPa. eL or R p0.2 The average linear expansion coefficient of α at 20-200℃ is α, the elongation after fracture is 14%, the Brinell hardness is 325HBW, the impact energy KV2 at room temperature is 19J, the thermal conductivity λ at room temperature is 23W / (m·K), and the average linear expansion coefficient α at 20-200℃ is α. m 12.5×10 -6 / ℃.

[0118] Comparative Example 1

[0119] This comparative example provides a Cr-Mo diesel engine piston steel, the chemical composition and mass percentage of the piston steel are: C 0.40%, Si 0.24%, Mn 0.77%, Cr 1.11%, Mo 0.20%, P 0.018%, S 0.017%, Al 0.022%; the rest are Fe and unavoidable impurities.

[0120] In this comparative example, when preparing the Cr-Mo diesel engine piston steel, except for the ratio of each raw material, other parameters and operations are the same as those in Example 1.

[0121] The metallographic structure diagram of the Cr-Mo diesel engine piston steel of this comparative example is as follows: Figure 2 As shown, the metallographic structure of the piston steel is tempered troostite.

[0122] The room temperature properties of the Cr-Mo diesel engine piston steel prepared in this comparative example after quenching and tempering: tensile strength R m The yield strength R is 1036MPa. eL or R p0.2 The average linear expansion coefficient of α at 20-200℃ is α, the elongation after fracture is 17%, the Brinell hardness is 296HBW, the impact energy KV2 at room temperature is 97J, the thermal conductivity λ at room temperature is 49W / (m·K), and the average linear expansion coefficient α at 20-200℃ is 936MPa. m 12.6×10 -6 / ℃.

[0123] Comparative Example 2

[0124] This comparative example provides a Cr-Mo diesel engine piston steel, the chemical composition and mass percentage of the piston steel are: C 0.44%, Si 0.24%, Mn 0.80%, Cr 1.13%, Mo 0.16%, P 0.012%, S 0.024%, Al 0.025%; the rest are Fe and unavoidable impurities.

[0125] In this comparative example, when preparing the Cr-Mo diesel engine piston steel, except for the ratio of each raw material, other parameters and operations are the same as those in Example 3.

[0126] The room temperature properties of the Cr-Mo diesel engine piston steel prepared in this comparative example after quenching and tempering: tensile strength R m The yield strength is 926MPa, and the yield strength R eL or R p0.2 The average linear expansion coefficient of α at 20-200℃ is α, the elongation after fracture is 18%, the Brinell hardness is 260HBW, the impact energy KV2 at room temperature is 129J, the thermal conductivity λ at room temperature is 48W / (m·K), and the average linear expansion coefficient α at 20-200℃ is α. m 12.5×10 -6 / ℃.

[0127] In addition, it should be noted that, in the present invention, the chemical composition design and related processes of the Ti, V micro-alloyed diesel engine piston steels of Examples 1 to 5 all meet the design specification requirements of the present invention.

[0128] Compared with Example 1 of the present application, Comparative Example 1 has different chemical compositions: the C content of Example 1 is 0.30%, which is lower than 0.40% of Comparative Example 1; the Si content of Example 1 is 1.13%, which is higher than 0.24% of Comparative Example 1; the Cr content of Example 1 is 1.31%, which is higher than 1.11% of Comparative Example 1; 0.055% Ti and 0.051% V are added to Example 1, while Comparative Example 1 does not add Ti and V. Under the same preparation method, Example 1 can meet the increasingly enhanced heat resistance requirements of piston steel by increasing the content of Si and Cr elements; by adding Ti and V, the mechanical properties and physical properties of piston steel can be improved; by reducing the C content, the carbon equivalent can be controlled to meet the welding performance requirements. The performance indicators are as follows (all after quenching at 870°C for 45 minutes and oil cooling + tempering at 620°C for 90 minutes and air cooling): the tensile strength, yield strength and Brinell hardness of Example 1 reached 1202MPa, 1079MPa and 333HBW, which are higher than 1036MPa, 936MPa and 296HBW of Comparative Example 1, showing stronger adaptability and higher durability; the room temperature thermal conductivity of Example 1 is 25W / (m·k), which is significantly lower than 49W / (m·k) of Comparative Example 1, which is beneficial to the thermal management of the engine.

[0129] Compared with Example 3 of the present application, Comparative Example 2 has different chemical compositions: the C content of Example 3 is 0.35%, which is lower than 0.44% of Comparative Example 2; the Si content of Example 3 is 1.05%, which is higher than 0.24% of Comparative Example 2; the Cr content of Example 3 is 1.60%, which is higher than 1.13% of Comparative Example 2; 0.086% Ti and 0.084% V are added to Example 3, while Comparative Example 2 does not add Ti and V. Under the same preparation method, Example 3 can meet the increasingly enhanced heat resistance requirements of piston steel by increasing the content of Si and Cr elements; by adding Ti and V, the mechanical properties and physical properties of piston steel can be improved; by reducing the C content, the carbon equivalent can be controlled to meet the welding performance requirements. The performance indicators are as follows (all after quenching at 870°C for 45 minutes and oil cooling + tempering at 660°C for 90 minutes and air cooling): the tensile strength, yield strength and Brinell hardness of Example 3 reached 1051 MPa, 938 MPa and 301 HBW, which are higher than 926 MPa, 818 MPa and 260 HBW of Comparative Example 2, showing stronger adaptability and higher durability; the room temperature thermal conductivity of Example 3 is 24 W / (m·k), which is significantly lower than 48 W / (m·k) of Comparative Example 2, which is beneficial to the thermal management of the engine.

[0130] Comparison of Examples 1-5 shows that the chemical composition fluctuates within a given range, and the process parameters of smelting, forging, rolling and quenching and tempering are slightly different, but the fluctuation of performance is very small. The tensile strength of Examples 1-5 is in the range of 1051-1202MPa, the yield strength is in the range of 938-1079MPa, the elongation after fracture is in the range of 14-16%, the Brinell hardness is in the range of 301-333HBW, the impact energy at room temperature is in the range of 18-22J, the thermal conductivity at room temperature is in the range of 22-25W / (m·k), and the average linear expansion coefficient at 20-200℃ is in the range of 12.5-13.2×10 -6 / ℃ range, with stable and excellent performance, which can meet the use requirements of high-performance diesel engine pistons. Among them, the mechanical properties are more sensitive to the tempering process. By controlling the tempering temperature, the strength, hardness and toughness of the piston steel can be adjusted to meet the needs of different application scenarios of diesel engine pistons.

[0131] The above scheme, the present invention proposes a titanium-vanadium microalloyed diesel engine piston steel and a preparation method, which can solve the problem of insufficient comprehensive performance (including strength, toughness, thermal conductivity and heat resistance, etc.) of engine piston steel in high-temperature service environment above 600°C in the prior art. Although there are ways to improve the performance by adding alloy elements and increasing the content of existing elements, improving the preparation process and adding heat treatment, there are more or less technical defects such as high cost, long process, great operation difficulty, low efficiency, and inability to improve the performance synergistically.

[0132] The present invention optimizes the chemical composition of Ti and V microalloyed diesel engine piston steel: by increasing the content of Si and Cr elements, the increasingly enhanced heat resistance requirements of piston steel are met; Ti and V elements are added to form Ti-Mo-V composite microalloying, so that the piston steel has better mechanical and physical properties; the mass percentage of S element is controlled in the range of 0.015-0.025% to meet the cutting performance requirements of piston steel.

[0133] The present invention obtains steel ingots through simple vacuum smelting, which can reduce the content of gases such as hydrogen and oxygen during the smelting process, reduce the formation of pores and inclusions, and make the alloy elements more evenly distributed to avoid segregation, thereby improving the purity and structural uniformity of the steel ingot, effectively enhancing the mechanical properties, and improving the heat resistance, wear resistance and fatigue life of the piston under high temperature and high pressure conditions. Since the steel ingot has fewer internal defects, the probability of problems occurring in the subsequent processing process is reduced, which can effectively improve the yield rate, making the comprehensive performance of piston steel significantly better than that of traditional smelting processes, and achieving the best cost-performance balance.

[0134] The present invention can compact and close casting defects such as pores and shrinkage, break up coarse cast structures, promote dynamic recrystallization, significantly refine grains, and improve the density of the billet through hot forging with an upsetting ratio of 1.5-2.5. At the same time, hot forging can alleviate the segregation of alloy elements, ensure the uniform composition of the billet, and avoid performance fluctuations. The internal stress distribution of the billet after hot forging is more uniform, the deformation resistance during rolling is reduced, the rolling mill load and energy consumption are reduced, the crack risk during rolling is reduced, and the yield rate is improved. The organization optimization, defect control and performance improvement brought by hot forging are crucial, and are particularly suitable for the manufacture of diesel engine pistons with strict requirements on strength, toughness and fatigue life.

[0135] The present invention can maintain an appropriate temperature gradient during the rolling process of the steel by preheating before hot rolling and selecting the hot rolling method, avoid uneven structure caused by too fast heating, promote dynamic recrystallization and grain refinement, reduce residual stress after rolling, improve strength and toughness, and reduce the risk of subsequent heat treatment or machining deformation. Through multiple deformations of the rolling mill, large deformation processing can be completed quickly, which is suitable for mass production, and different diameters and lengths can be switched to meet diversified orders and meet the performance requirements of diesel engine pistons.

[0136] The present invention can make the piston steel have a uniform tempered bainite structure through the quenching and tempering treatment after hot rolling, significantly improve the strength, hardness and impact resistance, and refine the grains, improve the uniformity of the structure and fatigue resistance. The quenching and tempering treatment can also eliminate the residual stress of the piston steel during the thermal deformation process, reduce the risk of deformation and cracking, thereby extending the service life of the piston and enhancing reliability. In addition, by controlling the tempering temperature, the strength, hardness and toughness of the piston steel can be adjusted to meet the needs of different application scenarios of diesel engine pistons.

[0137] The room temperature properties of the titanium-vanadium microalloyed diesel engine piston steel after quenching and tempering treatment of the present invention are as follows: tensile strength R m ≥1000MPa, yield strength R eL or R p0.2 ≥900MPa, elongation after fracture A≥14%, Brinell hardness in the range of 290-340HBW, room temperature impact energy KV2≥18J, which can make the piston withstand huge pressure and impact force during service.

[0138] The room temperature thermal conductivity λ of the titanium-vanadium microalloyed diesel engine piston steel after quenching and tempering treatment is 20-25W / (m·K), so that the piston can transfer the absorbed heat to the cylinder wall in time to maintain the normal working temperature of the engine; the average linear expansion coefficient α of 20-200℃ m At (12-14)×10 -6 / ℃ range, so that there is good sealing between the piston and the cylinder wall, and the reciprocating motion of the piston will not be hindered due to excessively high thermal expansion coefficient.

[0139] The titanium-vanadium microalloyed diesel engine piston steel of the present invention has reasonable chemical composition and process design, a wide process window, and can be commercially produced in batches on a bar production line, thus having good promotion prospects and application value.

[0140] In summary, compared with other traditional methods, the method of the present invention prepares titanium-vanadium microalloyed diesel engine piston steel through simple vacuum melting, hot forging, preheating before hot rolling and hot rolling, and tempering treatment; the method is simple and easy to operate, green and environmentally friendly, low in cost, short in process, high in efficiency, and is conducive to large-scale industrial production and promotion.

[0141] It should be understood that the term "and / or" in this article is only a description of the association relationship of associated objects, indicating that there can be three relationships. For example, A and / or B can represent: A exists alone, A and B exist at the same time, and B exists alone. A and B can be singular or plural. In addition, the character " / " in this article generally indicates that the associated objects before and after are in an "or" relationship, but it may also indicate an "and / or" relationship. Please refer to the context for specific understanding.

[0142] In the present invention, "at least one" means one or more, and "plurality" means two or more. "At least one of the following" or similar expressions refers to any combination of these items, including any combination of single items or plural items. For example, at least one of a, b, or c can mean: a, b, c, ab, ac, bc, or abc, where a, b, and c can be single or multiple.

[0143] It should be understood that in various embodiments of the present invention, the size of the serial numbers of the above-mentioned processes does not mean the order of execution. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of the present invention.

[0144] The above is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art can easily think of changes or substitutions within the technical scope disclosed by the present invention, which should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention should be based on the protection scope of the claims.

Claims

1. A titanium-vanadium microalloyed diesel engine piston steel, characterized in that: The cross-sectional diameter of the titanium-vanadium microalloyed diesel engine piston steel bar is φ60-φ120 mm, and the chemical composition is as follows by mass percentage: C 0.30-0.40%, Si 0.80-1.20%, Mn 0.60-1.00%, Cr 1.20-1.60%, Mo 0.15-0.25%, Ti 0.05-0.10%, V0.05-0.10%, P≤0.025%, S 0.015-0.025%, Al 0.020-0.040%; the rest is Fe and unavoidable impurities.

2. The titanium-vanadium microalloyed diesel engine piston steel according to claim 1, characterized in that: The hot-rolled microstructure of the titanium-vanadium microalloyed diesel engine piston steel is ferrite with a volume fraction of 30-60% and pearlite with a volume fraction of 40-70%, and the microstructure of the titanium-vanadium microalloyed diesel engine piston steel after quenching and tempering is tempered bainite; wherein: the ferrite is in an equiaxed polygonal shape, and the average grain size is 10-30 μm; the pearlite is in a lamellar shape, and the average grain size is 20-40 μm; the tempered bainite is in an equiaxed and fine-grained shape, and the average grain size is 10-15 μm.

3. The titanium-vanadium microalloyed diesel engine piston steel according to claim 1, characterized in that: The room temperature properties of the titanium-vanadium microalloyed diesel engine piston steel after quenching and tempering: tensile strength R m ≥1000MPa, yield strength R eL or R p0.2 ≥900MPa, elongation after fracture A≥14%, Brinell hardness in the range of 290-340HBW, room temperature impact energy KV2≥18J, room temperature thermal conductivity λ is 20-25W / (m·K), 20-200℃ average linear expansion coefficient α m At (12-14)×10 -6 / ℃ range.

4. A method for preparing titanium-vanadium microalloyed diesel engine piston steel according to any one of claims 1 to 3, characterized in that: The preparation method of the titanium-vanadium microalloyed diesel engine piston steel comprises the following steps: S1, weighing and smelting raw materials: weighing raw materials according to the chemical composition of the titanium-vanadium microalloyed diesel engine piston steel, then adding the weighed raw materials into a vacuum induction furnace for smelting, and casting the smelted molten steel to obtain a steel ingot; S2, forging: heating the S1 steel ingot and forging it to obtain a steel billet; S3, hot rolling: the S2 steel billet is heated and then hot rolled, and then slowly cooled to room temperature to obtain a hot rolled bar; S4, quenching and tempering treatment: the hot-rolled bars in S3 are subjected to quenching and tempering treatment to finally obtain titanium-vanadium micro-alloyed steel bars for diesel engine pistons.

5. The method for preparing titanium-vanadium microalloyed diesel engine piston steel according to claim 4, characterized in that: In S1, the smelting temperature is 1630-1680℃, the smelting time is 20-40min; the casting temperature is 1550-1560℃.

6. The method for preparing titanium-vanadium microalloyed diesel engine piston steel according to claim 4, characterized in that: The heating rate in S2 is 5-10°C / min, the initial forging temperature is 1130-1180°C, the final forging temperature is 850-900°C, and the forging upsetting ratio is 1.5-2.

5.

7. The method for preparing titanium-vanadium microalloyed diesel engine piston steel according to claim 4, characterized in that: The heating rate in S3 is 5-10℃ / min, and the temperature is heated to 1130-1180℃ and kept warm for 30-90min before hot rolling. The starting temperature of hot rolling is 1030-1080℃, and the final rolling temperature is 850-950℃. Hot rolling adopts one-time large reduction rolling or multiple passes of small reduction rolling, with a total reduction of 50-87%, and the cooling rate of slow cooling is 0.1-0.3℃ / min.

8. The method for preparing titanium-vanadium microalloyed diesel engine piston steel according to claim 4, characterized in that: The tempering treatment in S4 is to first quench at 850-890℃ for 30-60min, and the cooling method is oil cooling; then temper at 620-660℃ for 60-120min, and the cooling method is air cooling.

9. The method for preparing titanium-vanadium microalloyed diesel engine piston steel according to claim 4, characterized in that: The size of the ingot in S1 is 200×200mm, the size of the billet in S2 is 150×150mm, and the size of the bar in S3 and S4 is φ60-φ120mm.

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