High-toughness and low-heat-conductivity steel for commercial vehicle engine piston and preparation method of high-toughness and low-heat-conductivity steel

Abstract

The invention provides high-toughness and low-heat-conductivity steel for a commercial vehicle engine piston and a preparation method, and relates to the technical field of steel for a 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, 0.80 to 1.00 percent of Mn, 0.90 to 1.50 percent of Cr, 0.15 to 0.25 percent of Mo, 0.20 to 0.25 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, casting forming, casting blank heat treatment, controlled rolling and controlled cooling and quenching and tempering heat treatment. The high-toughness and low-heat-conductivity steel for the engine piston of the commercial vehicle is prepared through simple vacuum melting, selection of casting blank heat treatment, controlled rolling and controlled cooling 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 present invention relates to the technical field of steel for commercial vehicle engine pistons. The steel is mainly used for engine piston parts that withstand high temperature and high pressure environments, and has significant performance advantages in applications requiring high strength, high toughness and low thermal conductivity; in particular, it refers to a high-strength, high-toughness, low-thermal conductivity steel for commercial vehicle engine pistons and a preparation method thereof. Background Art

[0002] At present, most commercial vehicle engine pistons are made of cast iron or aluminum alloy materials. Although these materials have advantages in cost and processing performance, their mechanical properties are insufficient under high temperature and high pressure environments. Especially under high engine operating temperature conditions, these materials are prone to problems such as thermal expansion, thermal fatigue and oxidation, which affect the overall performance and service life of the engine. Therefore, the development of steel materials with high strength, good toughness and excellent oxidation resistance has become a hot topic in current research.

[0003] In commercial vehicle engines, the piston needs to withstand high temperatures from the combustion chamber, and the thermal conductivity of the piston material has an important impact on the thermal management of the engine. Existing steels have high thermal conductivity while providing sufficient strength, which leads to engine thermal management problems. Traditional materials fail to effectively balance strength and low thermal conductivity, resulting in uneven heat distribution in the engine, further exacerbating thermal stress and component wear. Therefore, there is an urgent need for a steel with low thermal conductivity that can work stably for a long time under high temperature conditions.

[0004] Although there are many high-strength materials used in the manufacture of engine pistons, their toughness and oxidation resistance are often insufficient, which makes the engine pistons prone to cracks and ruptures under service conditions.

[0005] For example, Chinese patent CN113236435A discloses a high-performance friction welded cast iron piston and its preparation method. However, the carbon content and silicon content in the cast iron piston are very high, the hardness is low, and the strength and elongation are also low. It also requires a manganese phosphating coating on the surface, and is not suitable for use as engine piston steel.

[0006] Chinese patent CN117488210A discloses a steel for automobile engine piston and a manufacturing method thereof, wherein the steel has a high carbon content and a low silicon content, and requires a layer of carbon powder to be sprayed on the surface of the continuous casting billet, followed by multiple hot rolling and rapid cooling followed by slow heat preservation, and finally slow cooling to obtain the material, which is difficult to control, costly, long process and inefficient.

[0007] Moreover, in the prior art, both magnesium-based composite materials and aluminum-based composite materials can be used to prepare engine pistons, but the preparation process is complicated and the operation is difficult, and the prepared composite materials have more or less technical problems that mechanical properties, thermal conductivity, toughness and oxidation resistance cannot be improved synergistically. For example, the magnesium-based composite engine piston and its preparation method disclosed in Chinese patent CN109881059A, and the aluminum-based composite material for piston and its preparation method disclosed in Chinese patent CN101463440A, the composite materials prepared by these methods prove this point. Summary of the invention

[0008] In order to solve the technical problems in the prior art that the steel for commercial vehicle engine pistons has high carbon content in the selected ingredients, the surface needs to be coated, the preparation method is difficult to control, the cost is high, the process is long, the efficiency is low, and the mechanical properties, thermal conductivity, toughness and oxidation resistance cannot be improved synergistically; the present invention proposes a high-strength, low-thermal conductivity steel for commercial vehicle engine pistons and a preparation method that can solve the above technical problems. The technical solution is as follows:

[0009] A high-strength, toughness and low-thermal conductivity steel for a commercial vehicle engine piston. The rod diameter of the high-strength, toughness and low-thermal conductivity steel for a commercial vehicle engine piston is Φ80-Φ120 mm, and the chemical composition is as follows by mass percentage: C 0.30-0.40%, Si 0.80-1.20%, Mn 0.80-1.00%, Cr 0.90-1.50%, Mo 0.15-0.25%, V 0.20-0.25%, P≤0.025%, S 0.015-0.025%, Al 0.020-0.040%; the rest is Fe and unavoidable impurities.

[0010] Optionally, the hot-rolled microstructure of the high-strength, toughness and low-thermal conductivity steel for commercial vehicle engine pistons is 50-60% ferrite and 40-50% pearlite, and the microstructure of the high-strength, toughness and low-thermal conductivity steel for commercial vehicle engine pistons after quenching and tempering is tempered bainite; wherein: the ferrite is curved or flaky in shape, with an average grain size of 15-25 μm; the pearlite is lamellar in shape, with an average grain size of 10-20 μm; the tempered bainite is curved or corrugated in shape, with an average grain size of 10-15 μm.

[0011] Optionally, the high-strength, toughness, and low-thermal conductivity commercial vehicle engine piston steel has an impact energy of more than 27J at -28°C after tempering treatment, and can effectively control production costs; room temperature performance: tensile strength ≥910MPa, yield strength ≥750MPa, elongation after fracture ≥16%, cross-sectional shrinkage ≥50%, room temperature V-neck impact toughness ≥26J, thermal conductivity ≤30W / (m·K); performance in high-temperature service environments above 550-600°C: tensile strength ≥700MPa, yield strength ≥600MPa, elongation after fracture ≥10%, cross-sectional shrinkage ≥40%.

[0012] A method for preparing the above-mentioned high-strength, low-thermal conductivity steel for commercial vehicle engine pistons, the method for preparing the high-strength, low-thermal conductivity steel for commercial vehicle engine pistons comprises the following steps:

[0013] S1. Raw material weighing and smelting: weighing the raw materials according to the chemical composition of the high-strength, toughness and low-thermal conductivity commercial vehicle engine piston steel, and then adding the weighed raw materials into a vacuum induction furnace for smelting;

[0014] S2, casting: casting the molten steel obtained by smelting in S1 into a casting billet, and performing continuous casting or ingot casting to obtain a casting billet;

[0015] S3, heat treatment of the ingot: preheating, heating and soaking the S2 ingot to obtain a heat-treated ingot;

[0016] S4, controlled rolling and controlled cooling: the S3 heat-treated ingot is subjected to rough rolling, intermediate rolling, finishing rolling, water cooling, cooling on a cooling bed, slow cooling in a pile and air cooling in sequence to obtain a rolled bar;

[0017] S5, quenching and tempering heat treatment: The S4 rolled bar is subjected to quenching and tempering heat treatment to finally obtain a steel bar for commercial vehicle engine piston with high strength, toughness and low thermal conductivity.

[0018] Optionally, the smelting temperature in S1 is 1600-1650° C., and the smelting time is 90-120 min.

[0019] Optionally, the preheating section temperature in S3 does not exceed 900°C, the heating time is 40-50min, the heating rate is 10-15°C / min, and the ingot is subjected to uniform heat treatment, the uniform heat section temperature is 1180-1250°C, and the uniform heat time is 60-90min.

[0020] Optionally, in S4, the starting rolling temperature of rough rolling is 1080-1120°C, the final rolling temperature is 830-860°C, and the rolling deformation rate of each pass does not exceed 10%; the starting rolling temperature of intermediate rolling is 1050-1080°C, the final rolling temperature is 900-930°C, and the rolling deformation rate of each pass does not exceed 8%; the starting rolling temperature of finish rolling is 950-980°C, the final rolling temperature is 850-880°C, and the rolling deformation rate of each pass does not exceed 6%; ensure that the surface crack depth of the steel after rolling is ≤0.20mm.

[0021] Optionally, in S4, the rough rolling units are 3-5, the intermediate rolling units are 4-6, and the finishing rolling units are 4-6; the inlet temperature of water cooling is 1000-1050℃, and the outlet temperature is 850-900℃; the upper cooling bed temperature of cooling bed cooling is 820-850℃, and the lower cooling bed temperature is 530-560℃; the cooling rate of stack cooling and slow cooling is 0.1-0.3℃ / min, the terminal temperature is 80-100℃, and then air cooling to room temperature.

[0022] Optionally, the tempering treatment in S5 is first quenching at 850-880°C for 20-30min, with oil cooling as the cooling method; and then tempering at 650-700°C for 30-60min, with air cooling as the cooling method.

[0023] Optionally, the size of the ingot in S2 is Φ150×150-Φ300mm×300mm, the size of the heat-treated ingot in S3 is Φ150×150-Φ300mm×300mm, and the size of the bar in S4 and S5 is Φ80×80-Φ120mm×120mm.

[0024] Optionally, the heat resistance of the S4 medium-high strength and low thermal conductivity commercial vehicle engine piston steel rod is that it can withstand high temperatures of 600-650°C, can be used for a long time of 1000-2000h under high temperature conditions of 500-550°C, and is not easily oxidized.

[0025] Technical principle of the present invention:

[0026] The elements in the steel composition of the present invention play the following roles: C is the most important element in determining the hardness and toughness of the material. The C content should be controlled to maintain the minimum value while satisfying the strength and hardness. At the same time, in order to maintain the welding performance of the steel without reducing, the addition amount of C is designed to be 0.30-0.40%, preferably 0.32-0.38%.

[0027] Cr is a ferrite-forming element. The addition of Cr can have a favorable effect on the high-temperature oxidation resistance and corrosion resistance of heat-resistant steel in complex atmospheres. This is mainly due to the fact that after the addition of Cr, Cr can form Cr2O3 with oxidation resistance at high temperatures, which slows down the overall diffusion rate in the oxide film. When the Cr content in the heat-resistant steel increases, the oxide film becomes denser, more complete and more continuous. Therefore, the Cr addition amount of the steel designed in the present invention is 0.90-1.50%.

[0028] Mo is a ferrite-forming element, and its ability to expand the ferrite phase region is higher than that of Cr. Mo can improve the solid solution effect of the alloy, and in steel, it can improve the high-temperature strength of the alloy to a certain extent, improve the intergranular corrosion resistance of the alloy, and improve the ability to resist chloride pitting and the ability to resist stress cracking caused by sulfides. 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. Therefore, the Mo addition amount of the steel designed by the present invention is 0.15-0.25%.

[0029] The role of Si element in oxidation resistance is consistent with that of Cr element, but taking into account the consideration of mechanical properties and other related aspects, excessive Si is usually not added to steel, and since the standard Gibbs free energy of these two elements is low, they will first form corresponding oxides during high-temperature oxidation, which has the effect of improving oxidation resistance. Therefore, the Si addition amount of the steel designed by the present invention is 0.80-1.20%.

[0030] Mn is a weaker austenite-forming element, which can expand the austenite phase region. It has a lower cost than Ni and is often used to replace Ni. Mn can also stabilize austenite structure. Mn can significantly improve the hardenability of steel. However, if the heating temperature is too high during quenching, it will cause coarse grains. If the Mn content is too high, it is easy to form a morphous structure, and a large amount of reticular ferrite will appear, which will increase the temper brittleness tendency of the steel. Therefore, the Mn addition amount of the steel designed by the present invention is 0.80-1.00%.

[0031] The precipitation temperature of V in steel is relatively low. It mainly precipitates in the phases during the transformation from austenite to ferrite and in ferrite, and improves the strength of steel through precipitation hardening. The efficient strengthening effect of V provides a basic guarantee for the high strengthening of quenched and tempered steel, but too high V will deteriorate the toughness. Therefore, the designed V addition amount is 0.20-0.25%.

[0032] The specific method for obtaining the ingot according to the mass fraction of the present invention comprises: performing converter smelting, LF ladle refining, VD vacuum refining and continuous casting according to the formula; wherein the V element is added in the converter process; and the Mo element is added in the VD vacuum refining process.

[0033] Compared with the prior art, the above technical solution has at least the following beneficial effects:

[0034] The above scheme, the present invention proposes a high-strength, toughness, low-thermal conductivity steel for commercial vehicle engine pistons and a preparation method, which can solve the technical problems in the prior art that commercial vehicle engine piston steel has a high carbon content in component selection, the surface needs to be coated, the preparation method is difficult to control, the cost is high, the process is long, the efficiency is low, and the mechanical properties, thermal conductivity, toughness and oxidation resistance cannot be improved synergistically.

[0035] The present invention optimizes the chemical composition and adopts appropriate amounts of micro-alloy elements such as V, Cr, and Mo to improve the strength of steel while reducing the thermal conductivity of steel and improving the toughness and oxidation resistance of steel. The V element helps to refine the grains and improve the toughness of steel; the addition of Cr and Mo elements improves the oxidation resistance of steel and ensures stability under high temperature conditions; at the same time, the low thermal conductivity characteristic can effectively reduce the thermal management pressure of the engine and improve the working performance of the commercial vehicle engine piston.

[0036] The present invention obtains steel ingots by simple vacuum melting and casting, which can make the chemical composition of the steel more uniform and avoid the pollution of inclusions and gases that may occur during the smelting process. The vacuum melting process can effectively reduce the oxygen content in the molten steel, thereby improving the purity and mechanical properties of the steel. At the same time, vacuum melting can reduce the evaporation loss of alloy elements, making the control of alloy elements more precise and ensuring that the performance of the steel meets the expected requirements. This method is simple and efficient, which not only reduces the production cost, but also improves the quality of the steel, especially improves its oxidation resistance, high temperature resistance and toughness, and is suitable for the manufacture of high-performance components such as commercial vehicle engine pistons.

[0037] The present invention can make the ingot obtain a more uniform internal structure in the subsequent rolling and tempering process, reduce the temperature difference and stress concentration in the ingot, and thus improve the overall mechanical properties of the steel through the selection of heat treatment of the ingot. The heat treatment process helps to eliminate defects that may occur in the casting process, such as segregation and grain coarsening, and improves the machinability and surface quality of the steel. By accurately controlling the preheating, heating and soaking stages of the ingot, it is ensured that the steel can achieve stable organization and excellent mechanical properties in subsequent processing, especially in high temperature and high load working environments, thereby enhancing the high temperature resistance, oxidation resistance and mechanical strength of the steel, and meeting the requirements of key components such as commercial vehicle engine pistons.

[0038] The present invention can maintain an appropriate temperature gradient during the rolling process of the steel by controlling rolling and cooling, avoiding uneven organization caused by excessive cooling or overheating, thereby significantly improving the microstructure of the steel. The controlled rolling and controlled cooling process ensures that the grains of the steel are refined and improves its mechanical properties, especially in terms of tensile strength, toughness and fatigue resistance. In addition, by precisely controlling the temperature and deformation rate at each stage, the occurrence of surface defects such as cracks is further reduced, ensuring the surface quality and internal density of the steel. This technology can improve the strength and toughness of steel, enhance its stability in high temperature and high pressure environments, and meet the use requirements of high-performance components such as commercial vehicle engine pistons.

[0039] The present invention can obtain a uniform tempered bainite structure of the steel after rolling through the tempering treatment after controlled rolling and controlled cooling, thereby significantly improving the comprehensive mechanical properties of the steel. The tempering treatment optimizes the tensile strength, yield strength and impact toughness of the steel, and enhances the stability and oxidation resistance of the material under high temperature conditions. At the same time, the microstructure of the tempered bainite helps to improve the thermal fatigue resistance and wear resistance of the steel, ensuring that it is not prone to fatigue damage during long-term use under high load and harsh environments. The tempering treatment process simplifies the manufacturing process and significantly improves the performance of the steel, meeting the strict requirements of key components such as commercial vehicle engine pistons for high strength, high toughness and low thermal conductivity.

[0040] The high-strength, toughness and low-thermal conductivity commercial vehicle engine piston steel of the present invention has an impact energy of more than 27J at -28°C after tempering treatment, and can effectively control the production cost; room temperature performance: tensile strength ≥910MPa, yield strength ≥750MPa, elongation after fracture ≥16%, cross-sectional shrinkage ≥50%, room temperature V-mouth impact toughness ≥26J, thermal conductivity ≤30W / (m·K); performance in a high-temperature service environment above 550-600°C: tensile strength ≥700MPa, yield strength ≥600MPa, elongation after fracture ≥10%, cross-sectional shrinkage ≥40%.

[0041] The present invention successfully obtains a high-strength and low-thermal conductivity steel for commercial vehicle engine pistons and its manufacturing method by precisely controlling the cooling method and cooling speed and combining the steel composition design. The prepared steel exhibits good strength and toughness, low thermal conductivity and excellent oxidation resistance in actual production applications, and is particularly suitable for key components with high requirements such as commercial vehicle engine pistons, and has high promotion value and broad application prospects.

[0042] In summary, compared with other traditional methods, the method of the present invention prepares high-strength, low-thermal-conductivity steel for commercial vehicle engine pistons through simple vacuum melting, selection of ingot heat treatment, controlled rolling and controlled cooling, and tempering treatment; the method is simple and easy to operate, green and environmentally friendly, low in cost, short in process, and high in efficiency, and is conducive to large-scale industrial production and promotion. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] 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.

[0044] Figure 1 This is an optical microscope micrograph of the high-strength, low-thermal-conductivity steel for commercial vehicle engine pistons according to Example 1 of the present invention. DETAILED DESCRIPTION

[0045] The technical solution of the present invention is described below in conjunction with the accompanying drawings.

[0046] 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.

[0047] 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.

[0048] 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.

[0049] 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.

[0050] A high-strength, toughness and low-thermal conductivity steel for a commercial vehicle engine piston. The rod diameter of the high-strength, toughness and low-thermal conductivity steel for a commercial vehicle engine piston is Φ80-Φ120 mm, and the chemical composition is as follows by mass percentage: C 0.30-0.40%, Si 0.80-1.20%, Mn 0.80-1.00%, Cr 0.90-1.50%, Mo 0.15-0.25%, V 0.20-0.25%, P≤0.025%, S 0.015-0.025%, Al 0.020-0.040%; the rest is Fe and unavoidable impurities.

[0051] In particular, the hot-rolled microstructure of the high-strength, toughness and low-thermal conductivity steel for commercial vehicle engine pistons is 50-60% ferrite and 40-50% pearlite, and the microstructure of the high-strength, toughness and low-thermal conductivity steel for commercial vehicle engine pistons after quenching and tempering is tempered bainite; wherein: the ferrite is curved or flaky in shape, with an average grain size of 15-25 μm; the pearlite is lamellar in shape, with an average grain size of 10-20 μm; the tempered bainite is curved or corrugated in shape, with an average grain size of 10-15 μm.

[0052] In particular, the high-strength, toughness and low-thermal conductivity commercial vehicle engine piston steel has an impact energy of more than 27J at -28°C after tempering treatment, and can effectively control the production cost; room temperature performance: tensile strength ≥910MPa, yield strength ≥750MPa, elongation after fracture ≥16%, cross-sectional shrinkage ≥50%, room temperature V-neck impact toughness ≥26J, thermal conductivity ≤30W / (m·K); performance in high-temperature service environment above 550-600°C: tensile strength ≥700MPa, yield strength ≥600MPa, elongation after fracture ≥10%, cross-sectional shrinkage ≥40%.

[0053] A method for preparing the above-mentioned high-strength, low-thermal conductivity steel for commercial vehicle engine pistons, the method for preparing the high-strength, low-thermal conductivity steel for commercial vehicle engine pistons comprises the following steps:

[0054] S1. Raw material weighing and smelting: weighing the raw materials according to the chemical composition of the high-strength, toughness and low-thermal conductivity commercial vehicle engine piston steel, and then adding the weighed raw materials into a vacuum induction furnace for smelting;

[0055] S2, casting: casting the molten steel obtained by smelting in S1 into a casting billet, and performing continuous casting or ingot casting to obtain a casting billet;

[0056] S3, heat treatment of the ingot: preheating, heating and soaking the S2 ingot to obtain a heat-treated ingot;

[0057] S4, controlled rolling and controlled cooling: the S3 heat-treated ingot is subjected to rough rolling, intermediate rolling, finishing rolling, water cooling, cooling on a cooling bed, slow cooling in a pile and air cooling in sequence to obtain a rolled bar;

[0058] S5, quenching and tempering heat treatment: The S4 rolled bar is subjected to quenching and tempering heat treatment to finally obtain a steel bar for commercial vehicle engine piston with high strength, toughness and low thermal conductivity.

[0059] In particular, the smelting temperature in S1 is 1600-1650°C, and the smelting time is 90-120 min.

[0060] In particular, the temperature of the preheating section in S3 does not exceed 900°C, the heating time is 40-50min, the heating rate is 10-15°C / min, and the ingot is subjected to uniform heat treatment, the uniform heat treatment section temperature is 1180-1250°C, and the uniform heat treatment time is 60-90min.

[0061] In particular, the starting rolling temperature of S4 rough rolling is 1080-1120℃, the final rolling temperature is 830-860℃, and the rolling deformation rate of each pass does not exceed 10%; the starting rolling temperature of intermediate rolling is 1050-1080℃, the final rolling temperature is 900-930℃, and the rolling deformation rate of each pass does not exceed 8%; the starting rolling temperature of fine rolling is 950-980℃, the final rolling temperature is 850-880℃, and the rolling deformation rate of each pass does not exceed 6%; ensure that the surface crack depth of steel after rolling is ≤0.20mm.

[0062] In particular, the rough rolling units in S4 are 3-5, the intermediate rolling units are 4-6, and the finishing rolling units are 4-6; the inlet temperature of water cooling is 1000-1050℃, and the outlet temperature is 850-900℃; the upper cooling bed temperature of cooling bed cooling is 820-850℃, and the lower cooling bed temperature is 530-560℃; the cooling rate of pile cooling and slow cooling is 0.1-0.3℃ / min, the terminal temperature is 80-100℃, and then air cooling to room temperature.

[0063] In particular, the tempering treatment in S5 is first quenching at 850-880°C for 20-30min, and the cooling method is oil cooling; then tempering at 650-700°C for 30-60min, and the cooling method is air cooling.

[0064] In particular, the size of the ingot in S2 is Φ150×150-Φ300mm×300mm, the size of the heat-treated ingot in S3 is Φ150×150-Φ300mm×300mm, and the size of the bars in S4 and S5 is Φ80×80-Φ120mm×120mm.

[0065] In particular, the heat resistance of S4 medium-high strength and low thermal conductivity steel rod for commercial vehicle engine pistons is 600-650°C, and it can be used for a long time of 1000-2000h under high temperature conditions of 500-550°C without being easily oxidized.

[0066] Example 1

[0067] A high-strength, toughness and low-thermal conductivity steel for a commercial vehicle engine piston. The bar size of the high-strength, toughness and low-thermal conductivity steel for a commercial vehicle engine piston is Φ80×80 mm, and the chemical composition is as follows by mass percentage: C 0.35%, Si 1.05%, Mn 0.90%, Cr 1.20%, Mo 0.20%, V 0.22%, P 0.018%, S 0.020%, Al 0.025%; the rest is Fe and unavoidable impurities, and P in the impurities is ≤100ppm.

[0068] A method for preparing the high-strength, toughness and low-thermal conductivity steel for commercial vehicle engine pistons is provided. The method for preparing the high-strength, toughness and low-thermal conductivity steel for commercial vehicle engine pistons comprises the following steps:

[0069] S1. Raw material weighing and smelting: weigh the raw materials according to the chemical composition of the high-strength, toughness and low-thermal conductivity commercial vehicle engine piston steel, and then add the weighed raw materials into a vacuum induction furnace for smelting at a smelting temperature of 1650° C. for 100 min; wherein, V element is added during converter smelting; Nb element is added during VD vacuum refining;

[0070] S2, casting: casting the molten steel obtained by smelting in S1 into a billet, and performing continuous casting or ingot casting to obtain a billet; the size of the billet is Φ150×150mm;

[0071] S3, billet heat treatment: the S2 billet is subjected to preheating, heating and soaking billet heat treatment, the preheating section temperature is 870°C, the preheating time is 45min, the heating section includes the first heating and the second heating, the first heating temperature is 1080°C, the heating time is 1h; the second heating temperature is 1180°C, the heating time is 1h; the soaking section temperature is 1180°C, the heating time is 1h, and the heat-treated billet is obtained; the size of the heat-treated billet is Φ150×150mm;

[0072] S4, controlled rolling and controlled cooling: the S3 heat-treated ingot is subjected to rough rolling, intermediate rolling, finishing rolling, water cooling, stack cooling and air cooling in sequence, wherein the rough rolling start temperature is 1090°C, and the inlet temperature is 1000°C and the outlet temperature is 920°C using a water tank for water cooling; the upper cooling bed temperature of the cooling bed is 830°C and the lower cooling bed temperature is 550°C; then the ingot is stacked and slowly cooled, the cooling rate is 0.2°C / min, the terminal temperature is 80°C, and then air-cooled to room temperature to obtain a rolled bar; the size of the bar is Φ80×80mm;

[0073] S5, quenching and tempering heat treatment: The S4 rolled bar is subjected to quenching and tempering heat treatment. The quenching and tempering treatment is first quenching at 850°C for 25 minutes, and the cooling method is oil cooling; then tempering at 650°C for 30 minutes, and the cooling method is air cooling, and finally a steel bar with high strength, toughness and low thermal conductivity for commercial vehicle engine pistons is obtained.

[0074] The high-strength, low-thermal-conductivity steel rod for commercial vehicle engine piston prepared in this embodiment has heat resistance of 600-650°C, can be used for a long time of 1000-2000h under high temperature conditions of 500-550°C, and is not easily oxidized.

[0075] The hot-rolled microstructure of the high-strength, low-thermal-conductivity commercial vehicle engine piston steel prepared in this embodiment is ferrite with a volume fraction of 55% and pearlite with a volume fraction of 45%. The microstructure of the high-strength, low-thermal-conductivity commercial vehicle engine piston steel after quenching and tempering is tempered troostite, such as Figure 1 As shown; wherein: the shape of ferrite is curved or flaky, and the average grain size is 20μm; the shape of pearlite is lamellar, and the average grain size is 15μm; the shape of tempered troostite is curved or corrugated, and the average grain size is 12μm.

[0076] The high-strength, toughness, and low-thermal conductivity commercial vehicle engine piston steel prepared in this embodiment has an impact energy of 28J at -28°C after tempering treatment, and can effectively control the production cost; room temperature performance: tensile strength of 920MPa, yield strength of 760MPa, elongation after fracture of 19%, cross-sectional shrinkage of 52%, room temperature V-neck impact toughness of 28J, and thermal conductivity of 29W / (m·K); performance in a high-temperature service environment above 550-600°C: tensile strength ≥800MPa, yield strength ≥700MPa, elongation after fracture ≥12%, and cross-sectional shrinkage ≥50%.

[0077] Example 2

[0078] A high-strength, toughness and low-thermal conductivity steel for a commercial vehicle engine piston. The bar size of the high-strength, toughness and low-thermal conductivity steel for a commercial vehicle engine piston is Φ90×90 mm, and the chemical composition is as follows by mass percentage: C 0.32%, Si 1.00%, Mn 0.98%, Cr 1.00%, Mo 0.18%, V 0.23%, P 0.019%, S 0.018%, Al 0.030%; the rest is Fe and unavoidable impurities, and P in the impurities is ≤100ppm.

[0079] A method for preparing the high-strength, toughness and low-thermal conductivity steel for commercial vehicle engine pistons is provided. The method for preparing the high-strength, toughness and low-thermal conductivity steel for commercial vehicle engine pistons comprises the following steps:

[0080] S1. Raw material weighing and smelting: weigh the raw materials according to the chemical composition of the high-strength, toughness and low-thermal conductivity commercial vehicle engine piston steel, and then add the weighed raw materials into a vacuum induction furnace for smelting at a smelting temperature of 1650° C. for 100 min; wherein, V element is added during converter smelting; Nb element is added during VD vacuum refining;

[0081] S2, casting: casting the molten steel obtained by smelting in S1 into a billet, and performing continuous casting or ingot casting to obtain a billet; the size of the billet is Φ200×200mm;

[0082] S3, billet heat treatment: the S2 billet is subjected to preheating, heating and soaking billet heat treatment, the preheating section temperature is 880°C, the preheating time is 50min, the heating section includes the first heating and the second heating, the first heating temperature is 1100°C, the heating time is 1h; the second heating temperature is 1200°C, the heating time is 1h; the soaking section temperature is 1210°C, the heating time is 1h, and the heat-treated billet is obtained; the size of the heat-treated billet is Φ200×200mm;

[0083] S4, controlled rolling and controlled cooling: the S3 heat-treated ingot is subjected to rough rolling, intermediate rolling, finishing rolling, water cooling, stack cooling and air cooling in sequence, wherein the rough rolling start temperature is 1080°C, and the inlet temperature is 1000°C and the outlet temperature is 920°C by water tank water cooling; the upper cooling bed temperature of the cooling bed is 840°C and the lower cooling bed temperature is 560°C; then the ingot is stacked and slowly cooled, the cooling rate is 0.2°C / min, the terminal temperature is 80°C, and then air-cooled to room temperature to obtain a rolled bar; the size of the bar is Φ90×90mm;

[0084] S5, quenching and tempering heat treatment: The S4 rolled bar is subjected to quenching and tempering heat treatment. The quenching and tempering treatment is first quenching at 850°C for 25 minutes, and the cooling method is oil cooling; then tempering at 650°C for 40 minutes, and the cooling method is air cooling, and finally a steel bar with high strength, toughness and low thermal conductivity for commercial vehicle engine pistons is obtained.

[0085] The high-strength, low-thermal-conductivity steel rod for commercial vehicle engine piston prepared in this embodiment has heat resistance of 600-650°C, can be used for a long time of 1000-2000h under high temperature conditions of 500-550°C, and is not easily oxidized.

[0086] The hot-rolled microstructure of the high-strength, toughness, and low-thermal conductivity steel for commercial vehicle engine pistons prepared in this embodiment is ferrite with a volume fraction of 52% and pearlite with a volume fraction of 48%. The microstructure of the high-strength, toughness, and low-thermal conductivity steel for commercial vehicle engine pistons after quenching and tempering is tempered troostite; wherein: the ferrite is curved or flaky in shape, and the average grain size is 18 μm; the pearlite is lamellar in shape, and the average grain size is 14 μm; the tempered troostite is curved or corrugated in shape, and the average grain size is 13 μm.

[0087] The high-strength, toughness, and low-thermal conductivity commercial vehicle engine piston steel prepared in this embodiment has an impact energy of 26J at -28°C after tempering treatment, and can effectively control the production cost; room temperature performance: tensile strength of 930MPa, yield strength of 770MPa, elongation after fracture of 18%, cross-sectional shrinkage of 50%, room temperature V-neck impact toughness of 26J, and thermal conductivity of 28W / (m·K); performance in a high-temperature service environment above 550-600°C: tensile strength ≥850MPa, yield strength ≥750MPa, elongation after fracture ≥15%, and cross-sectional shrinkage ≥50%.

[0088] Example 3

[0089] A high-strength, toughness and low-thermal conductivity steel for a commercial vehicle engine piston. The bar size of the high-strength, toughness and low-thermal conductivity steel for a commercial vehicle engine piston is Φ100×100 mm, and the chemical composition is as follows by mass percentage: C 0.38%, Si 1.10%, Mn 0.95%, Cr 1.30%, Mo 0.22%, V 0.24%, P 0.015%, S 0.023%, Al 0.030%; the rest is Fe and unavoidable impurities, and P in the impurities is ≤100ppm.

[0090] A method for preparing the high-strength, toughness and low-thermal conductivity steel for commercial vehicle engine pistons is provided. The method for preparing the high-strength, toughness and low-thermal conductivity steel for commercial vehicle engine pistons comprises the following steps:

[0091] S1. Raw material weighing and smelting: weigh the raw materials according to the chemical composition of the high-strength, toughness and low-thermal conductivity commercial vehicle engine piston steel, and then add the weighed raw materials into a vacuum induction furnace for smelting at a smelting temperature of 1650° C. for 100 min; wherein, V element is added during converter smelting; Nb element is added during VD vacuum refining;

[0092] S2, casting: casting the molten steel obtained by smelting in S1 into a billet, and performing continuous casting or ingot casting to obtain a billet; the size of the billet is Φ250×250mm;

[0093] S3, billet heat treatment: The S2 billet is subjected to preheating, heating and soaking billet heat treatment, the preheating section temperature is 880°C, the preheating time is 50min, the heating section includes the first heating and the second heating, the first heating temperature is 1100°C, the heating time is 1h; the second heating temperature is 1200°C, the heating time is 1h; the soaking section temperature is 1210°C, the heating time is 1h, and the heat-treated billet is obtained; the size of the heat-treated billet is Φ250×250mm;

[0094] S4, controlled rolling and controlled cooling: the S3 heat-treated ingot is subjected to rough rolling, intermediate rolling, finishing rolling, water cooling, stack cooling and air cooling in sequence, wherein the rough rolling start temperature is 1080°C, and the inlet temperature is 1000°C and the outlet temperature is 920°C using a water tank for water cooling; the upper cooling bed temperature of the cooling bed is 840°C and the lower cooling bed temperature is 560°C; then the stack cooling is performed at a cooling rate of 0.2°C / min and an end temperature of 80°C, followed by air cooling to room temperature to obtain a rolled bar; the bar size is Φ100×100mm;

[0095] S5, quenching and tempering heat treatment: The S4 rolled bar is subjected to quenching and tempering heat treatment. The quenching and tempering treatment is first quenching at 850°C for 25 minutes, and the cooling method is oil cooling; then tempering at 650°C for 40 minutes, and the cooling method is air cooling, and finally a steel bar with high strength, toughness and low thermal conductivity for commercial vehicle engine pistons is obtained.

[0096] The heat resistance of the high-strength, low-thermal-conductivity steel rod for commercial vehicle engine piston prepared in this embodiment is that it can withstand high temperatures of 600-650°C and can withstand high temperatures of 500-550°C for 1000-2000h.

[0097] The hot-rolled microstructure of the high-strength, toughness, and low-thermal conductivity steel for commercial vehicle engine pistons prepared in this embodiment is ferrite with a volume fraction of 51% and pearlite with a volume fraction of 49%. The microstructure of the high-strength, toughness, and low-thermal conductivity steel for commercial vehicle engine pistons after quenching and tempering is tempered bainite; wherein: the ferrite is curved or flaky in shape, and the average grain size is 19 μm; the pearlite is lamellar in shape, and the average grain size is 16 μm; the tempered bainite is curved or corrugated in shape, and the average grain size is 11 μm.

[0098] The high-strength, toughness, and low-thermal conductivity commercial vehicle engine piston steel prepared in this embodiment has an impact energy of 27J at -28°C after quenching and tempering treatment, and can effectively control the production cost; room temperature performance: tensile strength of 950MPa, yield strength of 780MPa, elongation after fracture of 16%, cross-sectional shrinkage of 512%, room temperature V-neck impact toughness of 27J, and thermal conductivity of 30W / (m·K); performance in a high-temperature service environment above 550-600°C: tensile strength ≥900MPa, yield strength ≥750MPa, elongation after fracture ≥12%, and cross-sectional shrinkage ≥50%.

[0099] Example 4

[0100] A high-strength, toughness and low-thermal conductivity steel for a commercial vehicle engine piston. The bar size of the high-strength, toughness and low-thermal conductivity steel for a commercial vehicle engine piston is Φ110×110 mm, and the chemical composition is as follows by mass percentage: C 0.30%, Si 0.85%, Mn 0.80%, Cr 0.95%, Mo 0.16%, V 0.252%, P 0.016%, S 0.015%, Al 0.022%; the rest is Fe and unavoidable impurities, and P in the impurities is ≤100ppm.

[0101] A method for preparing the high-strength, toughness and low-thermal conductivity steel for commercial vehicle engine pistons is provided. The method for preparing the high-strength, toughness and low-thermal conductivity steel for commercial vehicle engine pistons comprises the following steps:

[0102] S1. Raw material weighing and smelting: weigh the raw materials according to the chemical composition of the high-strength, toughness and low-thermal conductivity commercial vehicle engine piston steel, and then add the weighed raw materials into a vacuum induction furnace for smelting at a smelting temperature of 1650° C. for 100 min; wherein, V element is added during converter smelting; Nb element is added during VD vacuum refining;

[0103] S2, casting: casting the molten steel obtained by smelting in S1 into a billet, and performing continuous casting or ingot casting to obtain a billet; the size of the billet is Φ275×275mm;

[0104] S3, billet heat treatment: The S2 billet is subjected to preheating, heating and soaking billet heat treatment, the preheating section temperature is 880°C, the preheating time is 50min, the heating section includes the first heating and the second heating, the first heating temperature is 1100°C, the heating time is 1h; the second heating temperature is 1200°C, the heating time is 1h; the soaking section temperature is 1210°C, the heating time is 1h, and the heat-treated billet is obtained; the size of the heat-treated billet is Φ275×275mm;

[0105] S4, controlled rolling and controlled cooling: the S3 heat-treated ingot is subjected to rough rolling, intermediate rolling, finishing rolling, water cooling, stack cooling and air cooling in sequence, wherein the rough rolling start temperature is 1080°C, and the inlet temperature is 1000°C and the outlet temperature is 920°C using a water tank for water cooling; the upper cooling bed temperature of the cooling bed is 840°C and the lower cooling bed temperature is 560°C; then the ingot is stacked and slowly cooled, the cooling rate is 0.2°C / min, the terminal temperature is 80°C, and then air-cooled to room temperature to obtain a rolled bar; the size of the bar is Φ110×110mm;

[0106] S5, quenching and tempering heat treatment: The S4 rolled bar is subjected to quenching and tempering heat treatment. The quenching and tempering treatment is first quenching at 850°C for 25 minutes, and the cooling method is oil cooling; then tempering at 650°C for 40 minutes, and the cooling method is air cooling, and finally a steel bar with high strength, toughness and low thermal conductivity for commercial vehicle engine pistons is obtained.

[0107] The heat resistance of the high-strength, low-thermal-conductivity steel rod for commercial vehicle engine piston prepared in this embodiment is that it can withstand high temperatures of 600-650°C and can withstand high temperatures of 500-550°C for 1000-2000h.

[0108] The hot-rolled microstructure of the high-strength, toughness, and low-thermal-conductivity commercial vehicle engine piston steel prepared in this embodiment is ferrite with a volume fraction of 52% and pearlite with a volume fraction of 48%. The microstructure of the high-strength, toughness, and low-thermal-conductivity commercial vehicle engine piston steel after quenching and tempering is tempered bainite; wherein: the ferrite is curved or flaky in shape, and the average grain size is 21 μm; the pearlite is lamellar in shape, and the average grain size is 17 μm; the tempered bainite is curved or corrugated in shape, and the average grain size is 15 μm.

[0109] The high-strength, toughness, and low-thermal conductivity commercial vehicle engine piston steel prepared in this embodiment has an impact energy of 29J at -28°C after tempering treatment, and can effectively control the production cost; room temperature performance: tensile strength of 910MPa, yield strength of 750MPa, elongation after fracture of 21%, cross-sectional shrinkage of 50%, room temperature V-mouth impact toughness of 29J, and thermal conductivity of 30W / (m·K); performance in high-temperature service environment above 550-600°C: tensile strength ≥900MPa, yield strength ≥750MPa, elongation after fracture ≥15%, cross-sectional shrinkage ≥50%, high-temperature V-mouth impact toughness ≥25J, and thermal conductivity ≤30W / (m·K).

[0110] Example 5

[0111] A high-strength, toughness and low-thermal conductivity steel for a commercial vehicle engine piston. The bar size of the high-strength, toughness and low-thermal conductivity steel for a commercial vehicle engine piston is Φ120×120 mm, and the chemical composition is as follows by mass percentage: C 0.37%, Si 1.00%, Mn 0.95%, Cr 1.10%, Mo 0.25%, V 0.20%, P 0.017%, S 0.019%, Al 0.025%; the rest is Fe and unavoidable impurities, and P in the impurities is ≤100ppm.

[0112] A method for preparing the high-strength, toughness and low-thermal conductivity steel for commercial vehicle engine pistons is provided. The method for preparing the high-strength, toughness and low-thermal conductivity steel for commercial vehicle engine pistons comprises the following steps:

[0113] S1. Raw material weighing and smelting: weigh the raw materials according to the chemical composition of the high-strength, toughness and low-thermal conductivity commercial vehicle engine piston steel, and then add the weighed raw materials into a vacuum induction furnace for smelting at a smelting temperature of 1650° C. for 100 min; wherein, V element is added during converter smelting; Nb element is added during VD vacuum refining;

[0114] S2, casting: casting the molten steel obtained by smelting in S1 into a billet, and performing continuous casting or ingot casting to obtain a billet; the size of the billet is Φ300×300mm;

[0115] S3, billet heat treatment: the S2 billet is subjected to preheating, heating and soaking billet heat treatment, the preheating section temperature is 880°C, the preheating time is 50min, the heating section includes the first heating and the second heating, the first heating temperature is 1100°C, the heating time is 1h; the second heating temperature is 1200°C, the heating time is 1h; the soaking section temperature is 1210°C, the heating time is 1h, and the heat-treated billet is obtained; the size of the heat-treated billet is Φ300×300mm;

[0116] S4, controlled rolling and controlled cooling: the S3 heat-treated ingot is subjected to rough rolling, intermediate rolling, finishing rolling, water cooling, stack cooling and air cooling in sequence, wherein the rough rolling start temperature is 1080°C, and the inlet temperature is 1000°C and the outlet temperature is 920°C using a water tank for water cooling; the upper cooling bed temperature of the cooling bed is 840°C and the lower cooling bed temperature is 560°C; then the ingot is stacked and slowly cooled, the cooling rate is 0.2°C / min, the end temperature is 80°C, and then air-cooled to room temperature to obtain a rolled bar; the size of the bar is Φ120×120mm;

[0117] S5, quenching and tempering heat treatment: The S4 rolled bar is subjected to quenching and tempering heat treatment. The quenching and tempering treatment is first quenching at 850°C for 25 minutes, and the cooling method is oil cooling; then tempering at 650°C for 40 minutes, and the cooling method is air cooling, and finally a steel bar with high strength, toughness and low thermal conductivity for commercial vehicle engine pistons is obtained.

[0118] The heat resistance of the high-strength, low-thermal-conductivity steel rod for commercial vehicle engine piston prepared in this embodiment is that it can withstand high temperatures of 600-650°C and can withstand high temperatures of 500-550°C for 1000-2000h.

[0119] The hot-rolled microstructure of the high-strength, toughness, and low-thermal conductivity steel for commercial vehicle engine pistons prepared in this embodiment is ferrite with a volume fraction of 50% and pearlite with a volume fraction of 50%. The microstructure of the high-strength, toughness, and low-thermal conductivity steel for commercial vehicle engine pistons after quenching and tempering is tempered troostite; wherein: the ferrite is curved or flaky in shape, and the average grain size is 22 μm; the pearlite is lamellar in shape, and the average grain size is 18 μm; the tempered troostite is curved or corrugated in shape, and the average grain size is 14 μm.

[0120] The high-strength, toughness, and low-thermal conductivity commercial vehicle engine piston steel prepared in this embodiment has an impact energy of 29 J at -28°C after tempering treatment, and can effectively control the production cost; room temperature performance: tensile strength of 940 MPa, yield strength of 760 MPa, elongation after fracture of 18%, cross-sectional shrinkage of 53%, room temperature V-neck impact toughness of 30 J, and thermal conductivity of 27 W / (m·K); performance in a high-temperature service environment above 550-600°C: tensile strength ≥900 MPa, yield strength ≥750 MPa, elongation after fracture ≥15%, and cross-sectional shrinkage ≥50%.

[0121] Comparative Example 1

[0122] This comparative example provides a steel for a commercial vehicle engine piston, wherein the chemical composition and mass percentage of the steel for a commercial vehicle engine piston are: C 0.45%, Si 0.80%, Mn 1.10%, Cr 1.30%, Mo 0.12%, V 0.18%, P0.020%, S 0.022%, Al 0.020%; the rest are Fe and unavoidable impurities.

[0123] In this comparative example, when preparing the commercial vehicle engine piston steel, except for the ratio of each raw material, other parameters and operations are the same as those in Example 2.

[0124] The metallographic structure of the piston steel prepared in this comparative example is tempered troostite.

[0125] The impact energy of the commercial vehicle engine piston steel prepared in this comparative example after quenching and tempering at -28°C reaches 19J; room temperature performance: tensile strength is 880MPa, yield strength is 690MPa, elongation after fracture is 15%, cross-sectional shrinkage is 45%, room temperature V-neck impact toughness is 25J, thermal conductivity is 28W / (m·K); performance in a high temperature service environment above 550-600°C: tensile strength ≥900MPa, yield strength ≥750MPa, elongation after fracture ≥10%, cross-sectional shrinkage ≥45%.

[0126] Comparative Example 2

[0127] This comparative example provides a steel for a commercial vehicle engine piston, wherein the chemical composition and mass percentage of the steel for a commercial vehicle engine piston are: C 0.28%, Si 0.85%, Mn 1.00%, Cr 1.10%, Mo 0.15%, V 0.20%, P0.018%, S 0.020%, Al 0.022%; the rest are Fe and unavoidable impurities.

[0128] In this comparative example, when preparing the commercial vehicle engine piston steel, except for the ratio of each raw material, other parameters and operations are the same as those in Example 2.

[0129] The metallographic structure of the piston steel prepared in this comparative example is tempered troostite.

[0130] The impact energy of the commercial vehicle engine piston steel prepared in this comparative example after quenching and tempering treatment at -28°C reaches 27J; room temperature performance: tensile strength is 910MPa, yield strength is 710MPa, elongation after fracture is 16%, cross-sectional shrinkage is 48%, room temperature V-neck impact toughness is 24J, thermal conductivity is 26W / (m·K); performance in a high temperature service environment above 550-600°C: tensile strength ≥750MPa, yield strength ≥650MPa, elongation after fracture ≥15%, cross-sectional shrinkage ≥40%.

[0131] 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 1-5 all meet the design specification requirements of the present invention.

[0132] Compared with Examples 1-5 of the present application, Example 1 has obvious advantages over Comparative Examples 1 and Comparative Examples 2. Comparative Example 1 adopts a high alloying design, and the strength of the steel is higher by increasing the content of C and Cr. However, due to the excessive amount of alloying elements, the plasticity and toughness of the material are poor, and the impact toughness is only 22J, which makes it not as good as Example 1 under high load and impact conditions. In addition, the high alloy composition makes the cost of Comparative Example 1 high, which limits its large-scale application. In contrast, Example 1 optimizes the alloy composition, especially the content of C, Cr, and V, to ensure high strength and maintain good plasticity and toughness. The impact toughness is increased to 28J, and it has stronger impact resistance and is suitable for long-term use in more harsh environments. Comparative Example 2 selects lower C, Si and Mn contents in the alloy composition, resulting in a lower strength of the material, only 860MPa. Although it has good plasticity, its oxidation resistance is poor and it cannot adapt to long-term use at high temperatures. At the same time, due to the small amount of alloying elements, its thermal conductivity is high, resulting in poor thermal management capabilities in high temperature environments. Example 1 improves the stability and oxidation resistance of steel at high temperatures by appropriately adding alloying elements such as Cr, Mo, and V, while controlling the thermal conductivity at 29W / (m·K) to ensure the thermal management performance of the material. Overall, Example 1 is significantly superior to Comparative Examples 1 and 2 in terms of strength, plasticity, toughness, high temperature resistance at 550-600°C, and oxidation resistance, showing stronger adaptability and longer service life, and cost controllable, making it suitable for high-end applications such as high-performance commercial vehicle engine pistons.

[0133] The above scheme, the present invention proposes a high-strength, toughness, low-thermal conductivity steel for commercial vehicle engine pistons and a preparation method, which can solve the technical problems in the prior art that commercial vehicle engine piston steel has a high carbon content in component selection, the surface needs to be coated, the preparation method is difficult to control, the cost is high, the process is long, the efficiency is low, and the mechanical properties, thermal conductivity, toughness and oxidation resistance cannot be improved synergistically.

[0134] The present invention optimizes the chemical composition and adopts appropriate amounts of micro-alloy elements such as V, Cr, and Mo to improve the strength of steel while reducing the thermal conductivity of steel and improving the toughness and oxidation resistance of steel. The V element helps to refine the grains and improve the toughness of steel; the addition of Cr and Mo elements improves the oxidation resistance of steel and ensures stability under high temperature conditions; at the same time, the low thermal conductivity characteristic can effectively reduce the thermal management pressure of the engine and improve the working performance of the commercial vehicle engine piston.

[0135] The present invention obtains steel ingots by simple vacuum melting and casting, which can make the chemical composition of the steel more uniform and avoid the pollution of inclusions and gases that may occur during the smelting process. The vacuum melting process can effectively reduce the oxygen content in the molten steel, thereby improving the purity and mechanical properties of the steel. At the same time, vacuum melting can reduce the evaporation loss of alloy elements, making the control of alloy elements more precise and ensuring that the performance of the steel meets the expected requirements. This method is simple and efficient, which not only reduces the production cost, but also improves the quality of the steel, especially improves its oxidation resistance, high temperature resistance and toughness, and is suitable for the manufacture of high-performance components such as commercial vehicle engine pistons.

[0136] The present invention can make the ingot obtain a more uniform internal structure in the subsequent rolling and tempering process, reduce the temperature difference and stress concentration in the ingot, and thus improve the overall mechanical properties of the steel through the selection of heat treatment of the ingot. The heat treatment process helps to eliminate defects that may occur in the casting process, such as segregation and grain coarsening, and improves the machinability and surface quality of the steel. By accurately controlling the preheating, heating and soaking stages of the ingot, it is ensured that the steel can achieve stable organization and excellent mechanical properties in subsequent processing, especially in high temperature and high load working environments, thereby enhancing the high temperature resistance, oxidation resistance and mechanical strength of the steel, and meeting the requirements of key components such as commercial vehicle engine pistons.

[0137] The present invention can maintain an appropriate temperature gradient during the rolling process of the steel by controlling rolling and cooling, avoiding uneven organization caused by excessive cooling or overheating, thereby significantly improving the microstructure of the steel. The controlled rolling and controlled cooling process ensures that the grains of the steel are refined and improves its mechanical properties, especially in terms of tensile strength, toughness and fatigue resistance. In addition, by precisely controlling the temperature and deformation rate at each stage, the occurrence of surface defects such as cracks is further reduced, ensuring the surface quality and internal density of the steel. This technology can improve the strength and toughness of steel, enhance its stability in high temperature and high pressure environments, and meet the use requirements of high-performance components such as commercial vehicle engine pistons.

[0138] The present invention can obtain a uniform tempered bainite structure of the steel after rolling through the tempering treatment after controlled rolling and controlled cooling, thereby significantly improving the comprehensive mechanical properties of the steel. The tempering treatment optimizes the tensile strength, yield strength and impact toughness of the steel, and enhances the stability and oxidation resistance of the material under high temperature conditions. At the same time, the microstructure of the tempered bainite helps to improve the thermal fatigue resistance and wear resistance of the steel, ensuring that it is not prone to fatigue damage during long-term use under high load and harsh environments. The tempering treatment process simplifies the manufacturing process and significantly improves the performance of the steel, meeting the strict requirements of key components such as commercial vehicle engine pistons for high strength, high toughness and low thermal conductivity.

[0139] The high-strength, toughness and low-thermal conductivity commercial vehicle engine piston steel of the present invention has an impact energy of more than 27J at -28°C after tempering treatment, and can effectively control the production cost; room temperature performance: tensile strength ≥910MPa, yield strength ≥750MPa, elongation after fracture ≥16%, cross-sectional shrinkage ≥50%, room temperature V-mouth impact toughness ≥26J, thermal conductivity ≤30W / (m·K); performance in a high-temperature service environment above 550-600°C: tensile strength ≥700MPa, yield strength ≥600MPa, elongation after fracture ≥10%, cross-sectional shrinkage ≥40%.

[0140] The present invention successfully obtains a high-strength and low-thermal conductivity steel for commercial vehicle engine pistons and its manufacturing method by precisely controlling the cooling method and cooling speed and combining the steel composition design. The prepared steel exhibits good strength and toughness, low thermal conductivity and excellent oxidation resistance in actual production applications, and is particularly suitable for key components with high requirements such as commercial vehicle engine pistons, and has high promotion value and broad application prospects.

[0141] In summary, compared with other traditional methods, the method of the present invention prepares high-strength, low-thermal-conductivity steel for commercial vehicle engine pistons through simple vacuum melting, selection of ingot heat treatment, controlled rolling and controlled cooling, and tempering treatment; the method is simple and easy to operate, green and environmentally friendly, low in cost, short in process, and high in efficiency, and is conducive to large-scale industrial production and promotion.

[0142] 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.

[0143] 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.

[0144] 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.

[0145] 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 high-strength, low-thermal-conductivity steel for commercial vehicle engine pistons, characterized in that: The rod diameter of the high-strength, toughness and low-thermal conductivity commercial vehicle engine piston steel is Φ80-Φ120 mm, and the chemical composition is as follows by mass percentage: C 0.30-0.40%, Si 0.80-1.20%, Mn 0.80-1.00%, Cr 0.90-1.50%, Mo 0.15-0.25%, V 0.20-0.25%, P≤0.025%, S 0.015-0.025%, Al 0.020-0.040%; the rest is Fe and unavoidable impurities.

2. The high-strength, low-thermal-conductivity steel for commercial vehicle engine pistons according to claim 1, characterized in that: The hot-rolled microstructure of the high-strength, toughness and low-thermal conductivity steel for commercial vehicle engine pistons is 50-60% ferrite and 40-50% pearlite, and the microstructure of the high-strength, toughness and low-thermal conductivity steel for commercial vehicle engine pistons after quenching and tempering is tempered bainite; wherein: the ferrite is curved or flaky in shape, and the average grain size is 15-25 μm; the pearlite is lamellar in shape, and the average grain size is 10-20 μm; the tempered bainite is curved or corrugated in shape, and the average grain size is 10-15 μm.

3. The high-strength, low-thermal-conductivity steel for commercial vehicle engine piston according to claim 1, characterized in that: The high-strength, toughness and low-thermal conductivity commercial vehicle engine piston steel has an impact energy of more than 27J at -28°C after tempering treatment, and can effectively control production costs; room temperature performance: tensile strength ≥910MPa, yield strength ≥750MPa, elongation after fracture ≥16%, cross-sectional shrinkage ≥50%, room temperature V-neck impact toughness ≥26J, thermal conductivity ≤30W / (m·K); performance in high-temperature service environments above 550-600°C: tensile strength ≥700MPa, yield strength ≥600MPa, elongation after fracture ≥10%, cross-sectional shrinkage ≥40%.

4. A method for preparing a high-strength, low-thermal-conductivity steel for commercial vehicle engine pistons according to any one of claims 1 to 3, characterized in that: The preparation method of the high-strength, low-thermal-conductivity commercial vehicle engine piston steel comprises the following steps: S1. Raw material weighing and smelting: weighing the raw materials according to the chemical composition of the high-strength, toughness and low-thermal conductivity commercial vehicle engine piston steel, and then adding the weighed raw materials into a vacuum induction furnace for smelting; S2, casting: casting the molten steel obtained by smelting in S1 into a billet, and performing continuous casting or ingot casting to obtain a billet; S3, heat treatment of the ingot: preheating, heating and soaking the S2 ingot to obtain a heat-treated ingot; S4, controlled rolling and controlled cooling: the S3 heat-treated ingot is subjected to rough rolling, intermediate rolling, finishing rolling, water cooling, cooling bed cooling, pile cooling, slow cooling and air cooling in sequence to obtain a rolled bar; S5, quenching and tempering heat treatment: The S4 rolled bar is subjected to quenching and tempering heat treatment to finally obtain a steel bar for commercial vehicle engine piston with high strength, toughness and low thermal conductivity.

5. The method for preparing high-strength, low-thermal-conductivity steel for commercial vehicle engine pistons according to claim 4, characterized in that: The smelting temperature in S1 is 1600-1650°C and the smelting time is 90-120min.

6. The method for preparing high-strength, low-thermal-conductivity steel for commercial vehicle engine pistons according to claim 4, characterized in that: The temperature of the preheating section in S3 does not exceed 900°C, the heating time is 40-50min, the heating rate is 10-15°C / min, and the ingot is subjected to uniform heat treatment, the temperature of the uniform heat section is 1180-1250°C, and the uniform heat time is 60-90min.

7. The method for preparing high-strength, low-thermal-conductivity steel for commercial vehicle engine pistons according to claim 4, characterized in that: S4 has 3-5 rough rolling units, 4-6 medium rolling units, and 4-6 finishing rolling units; the inlet temperature of water cooling is 1000-1050℃, and the outlet temperature is 850-900℃; the upper cooling bed temperature of cooling bed cooling is 820-850℃, and the lower cooling bed temperature is 530-560℃; the cooling rate of pile cooling and slow cooling is 0.1-0.3℃ / min, the terminal temperature is 80-100℃, and then air cooling to room temperature.

8. The method for preparing high-strength, low-thermal-conductivity steel for commercial vehicle engine pistons according to claim 4, characterized in that: The tempering treatment in S5 is to first quench at 850-880℃ for 20-30min, and the cooling method is oil cooling; then temper at 650-700℃ for 30-60min, and the cooling method is air cooling.

9. The method for preparing high-strength, low-thermal-conductivity steel for commercial vehicle engine pistons according to claim 4, characterized in that: The size of the ingot in S2 is Φ150×150-Φ300 mm×300 mm, the size of the heat-treated ingot in S3 is Φ150×150-Φ300 mm×300 mm, and the size of the bar in S4 and S5 is Φ80×80-Φ120 mm×120 mm.

10. The method for preparing high-strength, low-thermal-conductivity steel for commercial vehicle engine pistons according to claim 4, characterized in that: The heat resistance of S4 medium-high strength and low thermal conductivity steel rod for commercial vehicle engine piston is 600-650℃, and it can be used for a long time of 1000-2000h under high temperature conditions of 500-550℃ without being easily oxidized.

Images