A steel for automobile engine pistons and its manufacturing method

Abstract

This invention relates to a steel for automobile engine pistons and its manufacturing method. The chemical composition of the steel, by weight percentage, is: C: 0.41–0.47%, Si: 0.15–0.35%, Mn: 0.70–1.00%, Cr: 1.10–1.40%, P: ≤0.020%, S: 0.015–0.035%, Mo: 0.17–0.27%, Al: 0.010–0.030%, N: 0.0040–0.0060%, with the balance being Fe and unavoidable impurities. The microstructure consists of ferrite and pearlite, with uniform distribution of ferrite and pearlite structures. The geometric center distance between each ferrite and pearlite structure is 15–30 μm. The hot-rolled hardness of the steel is ≤230 HBW, and the banded structure in the steel is ≤1.5 grade. The product features a narrow range of carbon deviation in cross-section, shallow decarburized layer and microcrack depth on the steel surface, low hot-rolled hardness, good banded structure, and high tensile strength after quenching and tempering.

Description

Technical Field

[0001] This invention belongs to the field of iron and steel metallurgy, and specifically relates to a steel for automobile engine pistons and its manufacturing method. Background Technology

[0002] The piston in an automotive engine is a key reciprocating component within the engine cylinder block. Pistons operate under conditions of high temperature, high pressure, high speed, and poor lubrication. During operation, the piston temperature is extremely high, reaching 600–700K at the piston crown, with very uneven temperature distribution. The piston crown also experiences significant gas pressure, reaching 3–5 MPa in gasoline engines and 6–9 MPa in diesel engines. The piston reciprocates at a very high speed (8–12 m / s) within the cylinder, and this speed is constantly changing, generating substantial inertial forces and subjecting the piston to significant additional loads. Under these harsh conditions, the piston deforms and wears faster, and also experiences additional loads, thermal stress, and surface corrosion. Therefore, the steel used for automotive engine pistons needs to possess high strength and uniform microstructure. Summary of the Invention

[0003] This invention aims to meet the requirements for automotive engine piston steel by developing an automotive engine piston steel and its manufacturing method. The steel has a cross-sectional carbon deviation range of ≤±5%; a surface decarburization layer depth of ≤0.20mm; a surface microcrack depth of ≤0.20mm; a hot-rolled hardness of ≤230HBW (for piston steel, lower hardness is better); and is graded according to GB / T34474.1-2017 "Evaluation of Banded Structure in Steel", with a banded structure grade of ≤1.5. After oil quenching at 850℃ and tempering at 600℃, the tensile strength of the steel is ≥950MPa.

[0004] The technical solution adopted by the present invention to solve the above problems is as follows: a steel for automobile engine pistons, wherein the chemical composition of the steel of the present invention is as follows (by weight percentage): C: 0.41-0.47%, Si: 0.15-0.35%, Mn: 0.70-1.00%, Cr: 1.10-1.40%, P: ≤0.020%, S: 0.015-0.035%, Mo: 0.17-0.27%, Al: 0.010-0.030%, N: 0.0040-0.0060%, with the balance being Fe and unavoidable impurities.

[0005] The main functions and design basis of each chemical element in the steel of this invention are as follows:

[0006] C: Carbon is a crucial element affecting the strength of steel, and also a key element influencing banded microstructure. Increasing the carbon content in steel can significantly improve its tensile strength, while also increasing the content of carbide microstructure and reducing ferrite banding. However, with increasing carbon content, carbon segregation in the steel becomes more severe; the hardness of the steel also increases, which is detrimental to sawing or shearing. The automotive engine piston steel of this invention adopts a medium-carbon design, with a carbon content selected in the range of 0.41% to 0.47%.

[0007] Si: Silicon does not form carbides in steel, but exists as a solid solution in ferrite or austenite, which can improve the strength of the solid solution in steel. The Si content in this invention is selected in the range of 0.15% to 0.35%.

[0008] Mn: Manganese forms a solid solution with iron, which can effectively improve the strength of ferrite and austenite in steel; at the same time, it is a carbide-forming element, entering cementite to replace some iron atoms, thus refining the pearlite structure and indirectly improving the strength of the pearlite structure. The Mn content in this invention is selected in the range of 0.70% to 1.00%.

[0009] The main function of chromium is to improve the hardenability of steel, enhance its oxidation resistance, and increase its corrosion resistance. The chromium content in this invention is selected within the range of 1.10% to 1.40%.

[0010] P: Phosphorus is a harmful element in steel, increasing its cold brittleness and reducing its plasticity. The P content in this invention is ≤0.020%.

[0011] S: The steel of this invention contains an appropriate amount of sulfur, which combines with manganese to form elongated manganese sulfide. This elongated manganese sulfide improves the machinability of the steel. The sulfur content in this invention ranges from 0.015% to 0.035%.

[0012] Mo (Mo): Molybdenum is a medium-strong carbide-forming element. In steel, it mainly exists in the form of carbides, which are dispersed in the matrix to strengthen the matrix and improve the strength and hardness of the steel. However, as the molybdenum content increases, the number of molybdenum carbides increases, and the carbides distributed at the grain boundaries have a stronger cutting effect on the matrix, which leads to a decrease in the toughness of the steel and makes it easier for deep surface cracks to occur during the steel rolling process. Therefore, the Mo content selected in this invention is in the range of 0.17-0.27%.

[0013] Al: The aluminum element in the steel of this invention plays a role in deoxidation and nitrogen fixation. The Al content in this invention is selected in the range of 0.010-0.030%.

[0014] N: In this invention, nitrogen in the steel combines with aluminum to form aluminum nitride particles. After the steel is forged into a semi-finished piston, it requires quenching and tempering. The semi-finished piston parts are quenched and tempered at a medium temperature, with a quenching temperature of 850℃ and a tempering temperature of 600℃. At this temperature, the austenite grains in the steel are not easily grown, and only a small amount of aluminum nitride is needed to pin the grain boundaries. However, adding too much nitrogen will lead to the formation of too many aluminum nitride particles in the steel. These aluminum nitride particles precipitate during continuous casting and accumulate at the grain boundaries on the steel surface, cutting the matrix and reducing the surface strength of the continuously cast billet. This makes the surface of the continuously cast billet more prone to microcracks. As the steel is heated and rolled, the microcracks on the steel surface will continue to expand and deepen. Therefore, the N content in this invention is selected in the range of 0.0040% to 0.0060%.

[0015] One method for manufacturing steel for automobile engine pistons includes the following process steps:

[0016] (1) The steelmaking process uses an electric furnace or converter for primary refining. During the primary refining process, the nitrogen content of the scrap steel added is ≤40ppm. During the refining process, alloys are added to adjust the composition to the internal control. After vacuum degassing of the molten steel, the nitrogen content in the molten steel is increased to 0.0040-0.0060% by bottom blowing nitrogen. Then it is hoisted to the continuous casting platform for casting. It is preferred to use protective casting to prevent the increase of nitrogen content. During the casting process from ladle to tundish, it is preferred to use long nozzle argon gas seal protection casting. For the protection from tundish to crystallizer, it is preferred to use submerged nozzle and select appropriate tundish covering agent and crystallizer protective slag, which can effectively prevent the molten steel from contacting air. The nitrogen content of the molten steel will not change during the continuous casting process.

[0017] As a preferred option: continuous casting adopts a stable medium-low superheat of 15-25℃; the secondary cooling zone adopts strong cooling with a specific water volume of 0.45L / kg and a water temperature of 15℃; the crystallizer, secondary cooling zone, and end electromagnetic stirring all adopt a low power frequency with stronger penetration and a larger current, with an electromagnetic stirring frequency of 1Hz and a current of 400A, so as to improve carbon segregation of the continuously cast billet through a reasonable continuous casting process.

[0018] (2) During the high-temperature removal process of the continuously cast billet, a layer of carbon powder with a thickness of more than 2 mm is sprayed on its surface. The high-temperature continuously cast billet (preferably ≥850℃) is then placed in a heating furnace for low-temperature heating at a temperature of 1050-1100℃, with a total heating time of ≤2 hours. Spraying carbon powder on the surface of the continuously cast billet during the high-temperature removal process effectively reduces decarburization of the billet surface during the furnace entry and heating process. The lower heating temperature and shorter heating time also contribute to reducing decarburization of the billet surface in the furnace. The heated billet is then rolled in multiple passes with an initial rolling temperature of 980-1030℃ and a final rolling temperature of 830-860℃. The lower initial and final rolling temperatures help reduce surface decarburization of the steel during the rolling process. The deformation rate of each rolling pass is ≤10%, and the total rolling reduction ratio is ≥10. The heated billet is rolled in multiple passes, with a deformation rate of ≤10% per pass. This prevents excessive deformation in a single pass from causing microcracks on the surface of the continuously cast billet to extend and deepen, ensuring that the surface crack depth of the rolled steel is ≤0.20mm. The total rolling compression ratio is ≥10. A higher total rolling compression ratio improves the uniformity of the steel's microstructure, resulting in a more uniform distribution of carbides. Therefore, a higher total rolling compression ratio further reduces carbon segregation in the steel cross-section, ensuring that the carbon deviation range of the steel cross-section is ≤±5%.

[0019] (3) The rolled high-temperature steel is rapidly immersed in a water tank for rapid cooling (isolation from air). The steel temperature must be 680℃ after exiting the water. Compared with water spraying on the steel surface, the overall cooling intensity of the steel immersed in the water tank is greater, and the rapid cooling time is shorter, which can be stably controlled within 30 seconds, resulting in higher production efficiency. In this rapid cooling process, the steel is completely isolated from the air, and the temperature drops very quickly, which helps to prevent decarburization on the steel surface. Rapid cooling facilitates the rapid transformation of the rolled microstructure from austenite to ferrite. After the steel exits the water, it is covered with an insulation hood for heat preservation. During this heat preservation process, the steel cools more slowly, allowing the remaining austenite to gradually transform into uniformly sized pearlite. This achieves a uniform distribution of ferrite and pearlite within the steel. The geometric center distance between each ferrite and pearlite structure is controlled at 15-30 μm. This uniform ferrite and pearlite structure helps reduce the banding grade of the steel, effectively controlling the banding level to ≤1.5. The uniform distribution of ferrite and pearlite prevents excessive concentration of pearlite, which could lead to excessively high local hardness, ensuring the hot-rolled hardness of the steel is ≤230 HBW. Nitrogen gas is used inside the insulation hood to effectively prevent surface decarburization during slow cooling. After slow cooling to 500℃, the steel is removed from the insulation hood and air-cooled, saving time to cool to room temperature and improving production efficiency.

[0020] This invention relates to a steel for automotive engine pistons, characterized by a medium-carbon composition with added molybdenum and trace amounts of aluminum and nitrogen. The primary refining furnace utilizes self-produced scrap steel with low nitrogen content. Vacuum degassing followed by bottom-blowing nitrogen precisely increases the nitrogen content in the steel. Continuous casting employs protective casting to prevent nitrogen accumulation in the molten steel. The continuous casting process utilizes medium-low superheat, strong cooling in the secondary cooling zone, and low-frequency electromagnetic stirring. The high-temperature continuously cast billet is sprayed with carbon powder and then hot-fed into a low-temperature heating furnace. The heated billet is then rolled in multiple passes. After rolling, the high-temperature steel is rapidly cooled in a water tank. After being removed from the water, the steel is covered with an insulation cover to maintain its temperature. The final product is steel for automobile engine pistons. The steel production specifications are Φ60-110mm, the cross-sectional carbon deviation of the steel is ≤±5%; the surface decarburization layer depth of the steel is ≤0.20mm, the surface microcrack depth of the steel is ≤0.20mm; the hot-rolled hardness of the steel is ≤230HBW; the banded structure of the steel is ≤1.5 grade; after oil quenching at 850℃ and tempering at 600℃, the tensile strength of the steel is ≥950MPa.

[0021] Compared with the prior art, the advantages of the present invention are as follows:

[0022] (1) The composition uses medium carbon, with an appropriate amount of molybdenum and trace amounts of aluminum and nitrogen. With reasonable smelting, continuous casting, rolling and cooling processes, compared with the existing technology, the smelting, continuous casting, rolling and cooling processes of this invention are more operable, the production process is simpler and the production cost is lower.

[0023] (2) The electric furnace or converter uses self-produced scrap steel with low nitrogen content. After vacuum degassing, bottom blowing nitrogen can accurately increase the nitrogen content in the steel. Continuous casting adopts protective casting to prevent nitrogen addition to the molten steel. The continuous casting process uses medium to low superheat, strong cooling in the secondary cooling zone, and low power frequency electromagnetic stirring to improve carbon segregation in the cross-section of the continuously cast billet.

[0024] (3) The surface of the high-temperature continuous casting billet is sprayed with carbon powder and then fed into the heating furnace for low-temperature heating, which can effectively prevent decarburization of the steel during the heating process. The heated billet is rolled in multiple passes, with a rolling deformation rate of ≤10% per pass. This prevents the microcracks on the surface of the continuous casting billet from expanding and deepening due to excessive deformation in a single pass during rolling, ensuring that the crack depth on the surface of the rolled steel is ≤0.20mm. The total rolling compression ratio is ≥10. A larger total rolling compression ratio can improve the uniformity of the steel structure and make the distribution of carbides in the rolled material more uniform. Therefore, a larger total rolling compression ratio further reduces the carbon segregation in the cross-section of the steel, thereby ensuring that the carbon deviation range of the steel cross-section is ≤±5%.

[0025] (4) The rolled high-temperature steel is rapidly immersed in a water tank for rapid cooling. The temperature of the steel after exiting the water is 680℃. Compared with water spraying on the surface of the steel, the overall cooling intensity of the steel immersed in the water tank is greater, and the rapid cooling time is shorter, which can be stably controlled within 30 seconds, resulting in higher production efficiency. During this rapid cooling process, the steel is completely isolated from the air, and the temperature drops very quickly, which helps to prevent decarburization on the surface of the steel. Rapid cooling is beneficial for the rolling structure to quickly transform from austenite to ferrite. After the steel exits the water, it is covered with an insulation cover for heat preservation. During the heat preservation process, the cooling rate of the steel is relatively slow, which is conducive to the slow transformation of the remaining austenite structure into a uniform pearlite structure. This achieves a uniform distribution of ferrite and pearlite structures in the steel, effectively controlling the banded structure of the steel to ≤1.5 grade and the hot-rolled hardness of the steel to ≤230HBW. Nitrogen protection is used inside the insulation cover to effectively prevent surface decarburization of the steel during the slow cooling process. After the steel is slowly cooled to 500°C, it is removed from the insulation cover and air-cooled, which saves the time for the steel to cool to room temperature and improves the production efficiency of steel.

[0026] (5) A steel for automobile engine pistons produced according to the present invention has the characteristics of narrow carbon deviation range of steel cross section, shallow decarburized layer and microcrack depth on steel surface, low hot rolling hardness of steel, good band structure and high tensile strength of steel after quenching and tempering. Detailed Implementation

[0027] The present invention will be further described in detail below with reference to the embodiments. The embodiments are exemplary and intended to explain the present invention, but should not be construed as limiting the present invention.

[0028] Example 1 and Example 2:

[0029] The two embodiments involve a method for manufacturing steel for automotive engine pistons: smelting all-in-one scrap steel in a 100t electric arc furnace → refining in a 100t refining furnace → vacuum degassing in a VD furnace → continuous casting with protective casting to produce 300mm*340mm square billets → high-temperature carbon powder spraying on the continuously cast billets → high-temperature continuous casting billets entering a heating furnace → low-temperature heating → multi-pass rolling → high-temperature steel cooled in a water bath after rolling → slow cooling with insulation after removal from the water → slow cooling to 500℃ followed by air cooling under an insulation cover. Two batches of automotive engine piston steel were manufactured.

[0030] The steelmaking process uses electric arc furnaces to smelt entirely self-produced scrap steel. The nitrogen content of the self-produced scrap steel is 28 ppm (Example 1) and 31 ppm (Example 2). During the refining process, alloys are added to adjust the composition to the internal control level. After vacuum degassing of the molten steel, the nitrogen content in the molten steel is increased to 0.0045% (Example 1) and 0.0058% (Example 2) by bottom blowing nitrogen. Then, it is hoisted to the continuous casting platform and protective casting is used to prevent the nitrogen content from increasing. The argon gas seal protection of the long nozzle can be used for the pouring process from ladle to tundish. The protection of the tundish to the crystallizer is achieved by using an immersion nozzle and selecting appropriate tundish covering agent and crystallizer protective slag, which can effectively prevent the molten steel from contacting air. The nitrogen content of the molten steel does not change during the continuous casting process. The continuous casting process uses a medium-low superheat, with superheat of 19℃ (Example 1) and 21℃ (Example 2). The secondary cooling zone uses strong cooling with a specific water volume of 0.45L / kg and a water temperature of 15℃. The crystallizer, secondary cooling zone, and end electromagnetic stirring all use low power supply frequencies, with an electromagnetic stirring frequency of 1Hz and a current of 400A. Through a reasonable continuous casting process, the carbon segregation of the continuously cast billet is improved.

[0031] During the high-temperature cooling process of the continuously cast billet, a 2mm thick layer of carbon powder was sprayed onto its surface. The high-temperature continuously cast billet was heated in a heating furnace at 891℃ (Example 1) and 878℃ ​​(Example 2) for low-temperature heating at 1058℃ (Example 1) and 1096℃ (Example 2), respectively, for a total heating time of 110min (Example 1) and 116min (Example 2). The heated billet was then rolled in multiple passes at initial rolling temperatures of 989℃ (Example 1) and 1021℃ (Example 2), and final rolling temperatures of 837℃ (Example 1) and 851℃ (Example 2). The deformation rate per pass was ≤10%, and the total rolling compression ratio was 36.1 (Example 1) and 10.7 (Example 2). The produced steel specifications were Φ60mm (Example 1) and Φ110mm (Example 2).

[0032] The rolled high-temperature steel is rapidly cooled in a water tank. The temperature of the steel after exiting the water is 680°C. The rapid cooling process takes 27 seconds (Example 1) and 28 seconds (Example 2). The steel after exiting the water is covered with an insulation cover to keep it warm. The atmosphere inside the insulation cover is nitrogen. The steel is slowly cooled to 500°C and then air-cooled after exiting the insulation cover.

[0033] The smelting composition of the steels obtained in Examples 1 and 2 is shown in Table 1.

[0034] Table 1 Smelting composition (wt%)

[0035] C Si Mn Cr P S Mo Al N Example 1 0.42 0.17 0.73 1.15 0.012 0.018 0.18 0.015 0.0045 Example 2 0.45 0.29 0.95 1.37 0.015 0.032 0.27 0.027 0.0058

[0036] The decarburization layer depth, microcrack depth, hot-rolled hardness, and banded structure of the steel obtained in Examples 1 and 2 are shown in Table 2.

[0037] Table 2

[0038] Surface decarburization layer Microcrack depth Hot-rolled hardness Band tissue Example 1 0.15mm 0.07mm 225HBW Level 1.0 Example 2 0.18mm 0.10mm 206HBW Level 1.5

[0039] The carbon deviation of the steel cross section is shown in Table 3.

[0040] Table 3

[0041]

[0042] The steels obtained in Examples 1 and 2 were oil quenched at 850°C and tempered at 600°C. The tensile strengths of the steels were 968 MPa (Example 1) and 1097 MPa (Example 2), respectively.

[0043] This application targets steel for automotive engine pistons, employing a medium-carbon composition with added molybdenum and trace amounts of aluminum and nitrogen. The primary refining furnace uses self-produced scrap steel with low nitrogen content. Vacuum degassing followed by bottom-blowing nitrogen precisely increases the nitrogen content in the steel. Continuous casting utilizes protective casting to prevent nitrogen accumulation in the molten steel. The continuous casting process employs medium-low superheat, strong cooling in the secondary cooling zone, and low-frequency electromagnetic stirring. The high-temperature continuously cast billet is sprayed with carbon powder and then hot-fed into a low-temperature heating furnace. The heated billet undergoes multiple rolling passes. After rolling, the high-temperature steel is rapidly cooled in a water bath. After removal from the water, the steel is covered with an insulation cover for heat preservation. This method of manufacturing steel for automotive engine pistons fills a domestic gap.

Claims

1. A method of manufacturing a steel for an automobile engine piston, characterized by: The steel has the following chemical composition by weight percentage: C: 0.41–0.47%, Si: 0.15–0.35%, Mn: 0.70–1.00%, Cr: 1.10–1.40%, P: ≤0.020%, S: 0.015–0.035%, Mo: 0.17–0.27%, Al: 0.010–0.030%, N: 0.0040–0.0060%, with the balance being Fe and unavoidable impurities. Manufacturing methods include: Step 1: Steel smelting and casting The steelmaking process uses an electric furnace or converter for primary refining. During the primary refining process, the nitrogen content of the scrap steel added is ≤40ppm. During the refining process, alloys are added to adjust the composition to meet the internal control requirements. After vacuum degassing of the molten steel, the nitrogen content in the molten steel is increased to 0.0040-0.0060% by bottom blowing nitrogen. Then it is hoisted to the continuous casting platform and continuous casting is carried out under protection. During the continuous casting process, the nitrogen content of the molten steel does not change. The molten steel is then cast into a continuous casting billet. Step 2: During the high-temperature removal process of the continuous casting billet, a layer of carbon powder with a thickness of more than 2mm is sprayed on its surface. The high-temperature continuous casting billet is then placed in a heating furnace for low-temperature heating at a temperature of 1050-1100℃. The total heating time is ≤2 hours. After heating, the billet is rolled into shape through multiple passes. The initial rolling temperature is 980-1030℃, the final rolling temperature is 830-860℃, the deformation rate of each rolling pass is ≤10%, and the total rolling reduction ratio is ≥10. Step 3: The rolled high-temperature steel is rapidly immersed in a water tank for rapid cooling. The rapid cooling time is controlled within 30 seconds. The timing of the steel exiting the water is controlled to ensure that the temperature of the steel after exiting the water is 680℃. Rapid cooling causes the rolled microstructure to quickly transform from austenite to ferrite. After the steel exits the water, it is covered with an insulation cover for slow heat preservation, which allows the remaining austenite microstructure to slowly transform into a pearlite microstructure of uniform size. Inert gas is used inside the insulation cover to prevent surface decarburization of the steel during the slow cooling process. After the steel is slowly cooled to below 500℃, it is removed from the insulation cover and air-cooled.

2. The method of manufacturing a steel for automotive engine piston according to claim 1, characterized in that: The microstructure of the steel consists of ferrite and pearlite, with ferrite and pearlite structures evenly distributed. The geometric center distance between each ferrite structure and the pearlite structure is 15-30 μm. According to GB / T34474.1-2017 "Evaluation of Banded Structures in Steel", the banded structure in the steel is ≤1.5 grade.

3. The method of manufacturing a steel for automotive engine piston according to claim 2, characterized in that: The hot-rolled hardness of the steel is ≤230HBW.

4. The method for manufacturing steel for automobile engine pistons according to claim 1, characterized in that: The cross-sectional carbon deviation of the steel is ≤±5%; the decarburized layer depth of the steel surface is ≤0.20mm; the microcrack depth of the steel surface is ≤0.20mm; and the tensile strength of the steel after oil quenching at 850℃ and tempering at 600℃ is ≥950MPa.

5. The method for manufacturing steel for automobile engine pistons according to claim 1, 2, 3, or 4, characterized in that: The steel is produced in sizes ranging from φ60 to 110mm.

6. The method for manufacturing steel for automobile engine pistons according to claim 1, characterized in that: In step one, the argon gas seal protection is used for the pouring process from ladle to tundish. The protection from tundish to crystallizer is achieved by using an immersion nozzle and selecting tundish covering agent and crystallizer protective slag to prevent the molten steel from contacting air and ensure that the N content in the molten steel does not fluctuate.

7. The method for manufacturing steel for automobile engine pistons according to claim 1, characterized in that: In step one, the continuous casting adopts a stable medium-low superheat of 15-25℃, the secondary cooling zone adopts strong cooling, the specific water volume is 0.45L / kg, and the water temperature is 15℃; the crystallizer, the secondary cooling zone, and the end electromagnetic stirring all adopt a low power frequency with stronger penetration and a larger current, the electromagnetic stirring frequency is 1Hz, and the current is 400A, which improves the carbon segregation of the continuous casting billet.

8. The method for manufacturing steel for automobile engine pistons according to claim 1, characterized in that: In step two, the temperature of the continuously cast billet entering the furnace is ≥850℃.

9. The method for manufacturing steel for automobile engine pistons according to claim 1, characterized in that: In step three, nitrogen gas is used for protection inside the insulation cover, and the cooling time of the steel billet in the water tank is ≤30s.

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