Non-quenched and tempered steel for automobile piston and production process of non-quenched and tempered steel

Abstract

The invention discloses non-quenched and tempered steel for an automobile piston and a production process of the non-quenched and tempered steel. The steel comprises the following chemical components in percentage by weight: 0.37%-0.40% of C, 0.60%-0.75% of Si, 1.40%-1.50% of Mn, less than or equal to 0.025% of P, 0.020%-0.040% of S, 0.10%-0.20% of Cr, 0.012%-0.025% of Alt, 0.08%-0.13% of V, 0.015%-0.030% of Ti, less than or equal to 0.05% of Mo, less than or equal to 0.10% of Ni, less than or equal to 0.10% of Cu, 0.015%-0.020% of N and the balance of Fe and inevitable impurities, and the ratio of Alt to N is 0.6-1.0. The RH-LF process is innovatively developed, and stable control over the nitrogen content, zero blockage of a water gap and reduction of bending deformation of steel are achieved in cooperation with the manganese nitride alloy, nitrogen control through bottom blowing of nitrogen, the Ca alloying process and the rolling heat preservation process.

Description

Technical Field

[0001] This invention belongs to the field of metallurgical technology, specifically relating to a non-quenched and tempered steel for automotive pistons and its production process. Background Technology

[0002] 38MnVS6, a medium-carbon vanadium-nitrogen microalloyed non-quenched and tempered steel, leverages the precipitation strengthening effect of vanadium (V) and nitrogen (N) elements to directly form a high-strength bainite / ferrite multiphase structure during controlled cooling after rolling, eliminating the need for traditional quenching and tempering heat treatment. It possesses excellent mechanical properties, including a tensile strength ≥850MPa and elongation after fracture ≥12%, as well as improved machinability due to the addition of sulfur (S). It is widely used in high-load moving parts such as pistons and connecting rods in automotive engines and is a key material for achieving lightweighting. However, its industrial production faces three major technical barriers: First, as a sulfur-added and nitrogen-controlled steel, the nitrogen content control range is extremely narrow (0.015-0.020%), with a nitrogen loss rate as high as 30% during vacuum degassing. Atmospheric nitrogen return after degassing leads to a rebound in nitrogen content, making precise control difficult. Second, after adding sulfur to improve machinability, a large amount of MnS inclusions in the molten steel easily adhere to the inner wall of the submerged entry nozzle during continuous casting, causing nozzle blockage and interrupting casting. Furthermore, uneven stress during rolling and cooling due to the phase transformation between the core and surface can easily lead to excessive bending deformation, significantly increasing scrap rate and straightening costs. Therefore, a production process that integrates precise nitrogen content control, nodule suppression mechanisms, and bending deformation control technologies is needed to overcome these bottlenecks.

[0003] Existing non-quenched and tempered steel production processes suffer from problems such as narrow nitrogen content control range leading to high nitrogen loss rate, sulfur addition easily causing nozzle blockage and interruption of casting, and uneven core-surface phase transformation stress during rolling and cooling easily causing bending deformation exceeding tolerance and increasing scrap rate. To address these issues, a non-quenched and tempered steel for automotive pistons and its production process are provided. Summary of the Invention

[0004] Addressing the three major challenges of nitrogen content fluctuation (nitrogen loss during vacuum degassing), MnS nodule formation at continuous casting nozzles, and rolling bending in 38MnVS6 non-quenched and tempered steel, this invention achieves precise and stable control of nitrogen content through a unique LF-RH-LF light degassing refining process combined with nitrogen alloying and calcium alloying technologies, completely eliminating the risk of nozzle nodule formation. Combined with a rolling heat preservation process to reduce internal stress in the material, this invention forms an efficient and stable production process for non-quenched and tempered steel used in automotive pistons, ensuring that the overall performance of the product meets the standards.

[0005] According to one aspect of the present invention, a non-quenched and tempered steel for automotive pistons is provided, wherein the chemical composition of the steel, by weight percentage, is as follows: C: 0.37%–0.40%, Si: 0.60%–0.75%, Mn: 1.40%–1.50%, P≤0.025%, S: 0.020%–0.040%, Cr: 0.10%–0.20%, Alt: 0.012%–0.025%, V: 0.08%–0.13%, Ti: 0.015%–0.030%, Mo≤0.05%, Ni≤0.10%, Cu≤0.10%, N: 0.015%–0.020%, with the balance being Fe and its unavoidable impurities, and Alt:N satisfying 0.6–1.0.

[0006] Based on the above technical solution, the tensile strength of the steel is ≥850MPa and the elongation after fracture is ≥12%.

[0007] According to another aspect of the present invention, a production process for non-quenched and tempered steel for automotive pistons is provided, the production process comprising: eco-electric furnace smelting - LF primary refining - RH refining - LF secondary refining - continuous casting - heating - rolling - cooling; LF primary refining: After the ladle arrives, slag formation and deoxidation are carried out, without adjusting the Mn composition. The LF inlet temperature is 1500-1525℃. Lime is added at 3.0-4.0 kg / t, and silicon carbide is added for diffusion deoxidation. Argon gas is adjusted, but large-scale argon gas stirring and heating are not allowed. After confirming the slag is white by dipping, the temperature is measured and a sample is taken. After the composition is reported back, the Si and Cr compositions of the molten steel are adjusted, but the Mn and Ti compositions are not adjusted. That is, 1.0-2.0 kg / t of alumina balls, 30-80 m of aluminum wire segments, 0.3-0.6 kg / t of coke particles, 1.7-2.7 kg / t of ferrosilicon alloy, 0.45-0.55 kg / t of high-carbon ferrochrome alloy, and 1.0-2.0 kg / t of ferrovanadium alloy are added. After the composition is adjusted, the molten steel temperature reaches 1640-1650℃, and the ladle is hoisted into the RH vacuum pump. RH refining: After the ladle is placed on the ladle car, adjust the argon flow rate to 300-450 NL / min. After entering the working position, start the lifting operation. Insert the impregnation tube to a depth greater than 400 mm, and then turn off the argon. Switch the RH lifting gas and the bottom blowing gas of the ladle to nitrogen. To reduce nitrogen content loss, when the vacuum degree is <100 Pa, maintain the duration for 5-8 minutes. When the vacuum degree is 4000 Pa, maintain it for 10-12 minutes. After breaking the vacuum, take a steel sample and determine the nitrogen content to be 45-90 ppm. Then, lift the ladle back to the LF position. LF Secondary Refining: After the ladle is hoisted back to the LF station, manganese nitride is used to increase nitrogen content. 0.1-0.5 kg / t of high-carbon ferrotitanium and 8.0-11.0 kg / t of manganese nitride are added to adjust the nitrogen content to 150-200 ppm. Then, 200 meters of silicon-calcium wire is fed in per furnace, and 300-400 meters of sulfur wire is fed in every 10 minutes. The bottom blowing gas is adjusted to nitrogen, and soft blowing is performed for 5-12 minutes. After soft blowing, the ladle is hoisted to the continuous casting station.

[0008] Based on the above technical solution, the specific process of the ecological electric furnace smelting is as follows: 45-55t of molten iron and 40-60t of scrap steel are added; at the end point, the C content is 0.05-0.10% and P≤0.010%; the tapping temperature is 1615-1620℃ and the tapping time is 5-6min. When the tapping amount is 15-20t, a baked alloy is added sequentially at a baking temperature of 600-800℃. The baked alloy includes ferrosilicon alloy, ferromanganese alloy, aluminum wire segments, and ladle lime, wherein the ferrosilicon alloy is 3.5-4.0kg / t, the ferromanganese alloy is 8.5-10.5kg / t, the aluminum wire segments are 0.5-1.0kg / t, and 500kg / furnace of ladle lime is added.

[0009] Based on the above technical solution, the continuous casting specifically involves: hoisting molten steel to the casting position; setting the superheat to ≤45℃ and the billet pulling speed to 0.43~0.47m / min; setting the superheat to >45℃ and the billet pulling speed to 0.35~0.37m / min; using a continuous slag detection device in the continuous casting ladle to prevent slag from falling into the ladle; using a current of 400A and a frequency of 2Hz for the crystallization electromagnetic stirring; using a current of 420A and a frequency of 7Hz for the end electromagnetic stirring; and using an alternating stirring method for stirring; controlling the hydrogen content in the tundish to ≤2.5ppm.

[0010] Based on the above technical solution, the heating process specifically includes: furnace inlet temperature of 200℃~400℃, preheating section temperature of 650℃~700℃; second heating section temperature of 850℃~870℃; first heating section temperature of 1210℃~1240℃; soaking section temperature of 1200℃~1230℃; tapping temperature of 1180℃~1210℃; and total heating time of 330min~780min.

[0011] Based on the above technical solution, the rolling process specifically involves: a primary rolling mill using a BD mill with 9 passes, followed by a turnaround on the extra pass, with a reduction of 34-90 mm, resulting in an intermediate billet height of 268 mm and a width of 232 mm. Depending on the specifications, the billet is then rolled through 11-18 stands at a rolling speed of 0.27-2 m / s, producing hot-rolled round steel with a diameter of 55-90 mm.

[0012] Based on the above technical solution, the cooling process specifically involves: placing round steel bars into a pit for heat preservation, with red steel padding at the bottom of the pit, a pit entry temperature ≥ 500℃, an exit temperature ≤ 100℃, and a heat preservation time ≥ 48h. Beneficial effects This invention employs an innovative production process, developing RH-LF refining technology and combining manganese nitride alloying with bottom-blown nitrogen to achieve precise nitrogen control. This, coupled with Ca alloying and optimized rolling and heat preservation processes, effectively solves the core challenges of industrial-scale production of 38MnVS6. Its beneficial effects are significant: firstly, it enables precise and stable control of nitrogen content in steel, greatly reducing the risk of substandard composition, thereby reducing scrap and improving cost-effectiveness; secondly, it ensures that each batch of steel possesses excellent comprehensive mechanical properties, with tensile strength consistently above 850MPa and elongation after fracture not less than 12%, while maintaining the good machinability imparted by sulfur, fully meeting the application requirements of high-quality materials. Detailed Implementation

[0013] The present invention will be further described below with reference to specific embodiments, but this does not limit the present invention in any way.

[0014] Unless otherwise specified, all raw materials used in the embodiments of the present invention are obtained through purchase.

[0015] The present invention provides a non-quenched and tempered steel for automotive pistons in the specific embodiments section. Its chemical composition, by weight percentage, is as follows: C: 0.37%–0.40%, Si: 0.60%–0.75%, Mn: 1.40%–1.50%, P≤0.025%, S: 0.020%–0.040%, Cr: 0.10%–0.20%, Alt: 0.012%–0.025%, V: 0.08%–0.13%, Ti: 0.015%–0.030%, Mo≤0.05%, Ni≤0.10%, Cu≤0.10%, N: 0.015%–0.020%, with the balance being Fe and its unavoidable impurities. The Alt:N content is 0.6–1.0.

[0016] Based on the above technical solution, the steel has a tensile strength ≥850MPa, elongation after fracture ≥12%, yield strength ≥570MPa, and reduction of area ≥25%.

[0017] The present invention provides a production process for non-quenched and tempered steel for automotive pistons in the specific embodiments section, which includes the following steps: smelting in an eco-electric furnace - primary refining with LF - refining with RH - secondary refining with LF - continuous casting of large billets - heating - rolling; Ecological electric furnace smelting: Iron-mixing smelting is adopted, with 45-55t of molten iron and 40-60t of scrap steel added. The final carbon content is 0.05-0.10% and P≤0.010%. When the temperature is 1615-1620℃, tapping begins, and the tapping time is controlled at 5-6 minutes. When 15-20t of steel has been tapped, baked alloys (including ferrosilicon alloy and ferrosilicon manganese alloy) are added in sequence at a baking temperature of 600-800℃. Among them, ferrosilicon alloy is 3.5-4.0kg / t, ferrosilicon manganese alloy is 8.5-10.5kg / t, aluminum wire is 0.5-1.0kg / t, and 500kg / furnace of ladle lime is added.

[0018] LF primary refining: After the ladle arrives, slag formation and deoxidation are carried out, without adjusting the Mn composition. The LF inlet temperature is 1500-1525℃. 3.0-4.0 kg / t of lime is added, and 1.5-2.0 kg / t of silicon carbide is added for diffusion deoxidation based on the slag condition. The argon gas flow rate is adjusted to 150-250 NL / min. Large-scale argon gas stirring and heating are not permitted. After confirming the slag is white by dipping, a temperature measurement and sample are taken. After the composition report is received, the Si and Cr compositions of the molten steel are adjusted, but the Mn and Ti compositions are not adjusted. 1.0-2.0 kg / t of alumina balls, 30-80 m of aluminum wire, 0.3-0.6 kg / t of coke particles, 1.7-2.7 kg / t of ferrosilicon, 0.45-0.55 kg / t of high-carbon ferrochrome, and 1.0-2.0 kg / t of ferrovanadium are added. After the composition adjustment is completed, the molten steel temperature reaches 1640-1650℃, and the ladle is hoisted into the RH vacuum system for vacuuming.

[0019] RH refining: After the ladle is placed on the ladle car, argon gas is turned on at a flow rate of 300-450 NL / min. After entering the working position, the lifting operation is started, and the impregnation tube is inserted to a depth greater than 400 mm. The argon gas is then turned off. The RH lifting gas is switched to nitrogen gas, and the bottom blowing gas of the ladle is switched to nitrogen gas. To reduce nitrogen content loss, the vacuum period is 5 minutes when the vacuum degree is <100 Pa, and about 10 minutes when the vacuum degree is 4000 Pa. After breaking the vacuum, a steel sample is taken to determine the chemical composition and nitrogen content. The ladle is then hoisted back to the LF station.

[0020] LF Secondary Refining: After the ladle is hoisted back to the LF station, the composition is fine-tuned based on the full analysis sample after the venting process. Manganese nitride is used to increase nitrogen content to 150–200 ppm. 0.1–0.5 kg / t of high-carbon ferrotitanium and 8.0–11.0 kg / t of manganese nitride are added, and other components and temperature are adjusted to the target range. Then, 200 meters of silicon-calcium wire is fed per furnace, followed by 300–400 meters of sulfur wire at 10-minute intervals. The bottom-blowing gas is changed to nitrogen, and soft blowing is performed for 5–12 minutes. After soft blowing, the ladle is hoisted to the continuous casting station.

[0021] Continuous casting: The molten steel is hoisted to the pouring position, and the billet pulling speed is controlled according to the superheat. When the superheat is ≤45℃, the billet pulling speed is 0.43~0.47m / min; when the superheat is >45℃, the billet pulling speed is 0.35~0.37m / min. Medium carbon steel mold mold mold mold slag is used. The electromagnetic stirring current of the mold is 400A and the frequency is 2. The electromagnetic stirring current at the end is 420A and the frequency is 7Hz. An alternating (20s-5s-20s) stirring method is adopted.

[0022] Heating: Preheating section temperature 650℃~700℃; second heating section temperature 850℃~870℃; first heating section temperature 1210℃~1240℃; soaking section temperature 1200℃~1230℃; tapping temperature 1180℃~1210℃; total heating time 330min~780min.

[0023] Rolling: The primary rolling mill uses a BD mill with 9 passes, and the billet is turned over in the extra passes. The reduction is 34~90mm, and the intermediate billet is 268mm high and 232mm wide. Depending on the specifications, it is then rolled through 11~18 stands at a rolling speed of 0.27~2m / s to produce hot-rolled round steel with a diameter of 55-90mm.

[0024] Cooling: The round steel bars are placed in the pit for insulation. Red steel bars must be placed at the bottom of the pit. The temperature when entering the pit is ≥500℃, the temperature when leaving the pit is ≤100℃, and the insulation time is ≥48h. Example The relevant parameters for the ecological electric furnace smelting described in Embodiments 1-3 of this invention are shown in Table 1, the relevant parameters for the LF primary refining are shown in Table 2, the relevant parameters for the RH refining are shown in Table 3, the relevant parameters for the LF secondary refining are shown in Table 4, the relevant parameters for continuous casting are shown in Table 5, the relevant parameters for heating are shown in Table 6, the relevant parameters for rolling are shown in Table 7, and the relevant parameters for cooling are shown in Table 8.

[0025] The chemical composition of the finished steel is shown in Table 9, the mechanical properties of the finished steel are shown in Table 10, and the non-metallic inclusions of the finished steel are shown in Table 11. The non-quenched and tempered steel for automobile pistons of this invention is a sulfur-containing steel, and the control of Class A inclusions is quite difficult. This invention can control the Class A inclusion level in the steel to below 3.5 for the coarse A series and 2.5 for the fine A series, which meets the user's requirements and is evaluated according to the GB / T10561-2023 standard.

[0026] Table 1 lists the relevant parameters for the eco-friendly electric furnace smelting described in Examples 1-3.

[0027] Table 2 shows the relevant parameters for the primary refining of LF in Examples 1-3.

[0028] Table 3 lists the relevant parameters for RH refining described in Examples 1-3.

[0029] Table 4 lists the relevant parameters for the secondary refining of LF in Examples 1-3.

[0030] Table 5 lists the relevant parameters for continuous casting described in Examples 1-3.

[0031] Table 6 lists the relevant heating parameters described in Examples 1-3.

[0032] Table 7 lists the relevant parameters for the rolling process described in Examples 1-3.

[0033] Table 8 lists the relevant parameters for the cooling described in Examples 1-3.

[0034] Table 9 shows the chemical composition of the finished steel products described in Examples 1-3.

[0035] Table 10 shows the mechanical properties of the finished steel products described in Examples 1-3.

[0036] Table 11 shows the non-metallic inclusions in the steels described in Examples 1-3.

[0037] Through the above technical solutions, 38MnVS6 non-quenched and tempered steel adopts a multi-element synergistic design with C, Si, Mn, V, Ti, and Al, combined with advanced processes such as RH-LF refining, bottom-blown nitrogen control, and Ca treatment, to obtain a fine-grained microstructure with high strength, good toughness, and excellent machinability. This steel, through V(C,N) precipitation strengthening, Mn and Si solid solution strengthening, and TiN / AlN grain refinement, can directly form a uniform and fine ferrite-pearlite microstructure after forging and air cooling, eliminating the need for quenching and tempering heat treatment and meeting user requirements.

[0038] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A non-quenched and tempered steel for automotive pistons, characterized in that, The chemical composition of the steel, by weight percentage, is as follows: C: 0.37%–0.40%, Si: 0.60%–0.75%, Mn: 1.40%–1.50%, P≤0.025%, S: 0.020%–0.040%, Cr: 0.10%–0.20%, Alt: 0.012%–0.025%, V: 0.08%–0.13%, Ti: 0.015%–0.030%, Mo≤0.05%, Ni≤0.10%, Cu≤0.10%, N: 0.015%–0.020%, with the balance being Fe and its unavoidable impurities, and Alt:N satisfying 0.6–1.

0.

2. The non-quenched and tempered steel for automotive pistons according to claim 1, characterized in that, The steel has a tensile strength ≥850MPa, elongation after fracture ≥12%, yield strength ≥570MPa, and reduction of area ≥25%.

3. The manufacturing process for non-quenched and tempered steel for automotive pistons according to any one of claims 1 to 2, characterized in that, The production process includes: ecological electric furnace smelting - LF primary refining - RH refining - LF secondary refining - continuous casting - heating - rolling - cooling; LF primary refining: After the ladle arrives, slag formation and deoxidation are carried out, without adjusting the Mn composition. The LF inlet temperature is 1500-1525℃. Lime is added at 3.0-4.0 kg / t, and silicon carbide is added for diffusion deoxidation. Argon gas is adjusted, but large-scale argon gas stirring and heating are not allowed. After confirming the slag is white by dipping, the temperature is measured and a sample is taken. After the composition is reported back, the Si and Cr compositions of the molten steel are adjusted, but the Mn and Ti compositions are not adjusted. That is, 1.0-2.0 kg / t of alumina balls, 30-80 m of aluminum wire segments, 0.3-0.6 kg / t of coke particles, 1.7-2.7 kg / t of ferrosilicon alloy, 0.45-0.55 kg / t of high-carbon ferrochrome alloy, and 1.0-2.0 kg / t of ferrovanadium alloy are added. After the composition is adjusted, the molten steel temperature reaches 1640-1650℃, and the ladle is hoisted into the RH vacuum pump. RH refining: After the ladle is placed on the ladle car, adjust the argon flow rate to 300-450 NL / min. After entering the working position, start the lifting operation. Insert the impregnation tube to a depth greater than 400 mm, and then turn off the argon. Switch the RH lifting gas and the bottom blowing gas of the ladle to nitrogen. To reduce nitrogen content loss, when the vacuum degree is <100 Pa, maintain the duration for 5-8 minutes. When the vacuum degree is 4000 Pa, maintain it for 10-12 minutes. After breaking the vacuum, take a steel sample and determine the nitrogen content to be 45-90 ppm. Then, lift the ladle back to the LF position. LF Secondary Refining: After the ladle is hoisted back to the LF station, manganese nitride is used to increase nitrogen content. 0.1-0.5 kg / t of high-carbon ferrotitanium and 8.0-11.0 kg / t of manganese nitride are added to adjust the nitrogen content to 150-200 ppm. Then, 200 meters of silicon-calcium wire is fed in per furnace, and 300-400 meters of sulfur wire is fed in every 10 minutes. The bottom blowing gas is adjusted to nitrogen, and soft blowing is performed for 5-12 minutes. After soft blowing, the ladle is hoisted to the continuous casting station.

4. The production process according to claim 3, characterized in that, The specific process of the ecological electric furnace smelting is as follows: 45-55t of molten iron and 40-60t of scrap steel are added; at the end point, the C content is 0.05-0.10% and P≤0.010%; the tapping temperature is 1615-1620℃ and the tapping time is 5-6min. When the tapping amount is 15-20t, a baked alloy is added sequentially at a baking temperature of 600-800℃. The baked alloy includes ferrosilicon alloy, ferromanganese alloy, aluminum wire segments, and ladle lime, wherein the ferrosilicon alloy is 3.5-4.0kg / t, the ferromanganese alloy is 8.5-10.5kg / t, the aluminum wire segments are 0.5-1.0kg / t, and 500kg / furnace of ladle lime is added.

5. The production process according to claim 3, characterized in that, The continuous casting process specifically involves: hoisting molten steel to the casting position; setting the superheat to ≤45℃ and the billet pulling speed to 0.43~0.47m / min; setting the superheat to >45℃ and the billet pulling speed to 0.35~0.37m / min; employing a continuous slag detection device in the continuous casting ladle to prevent slag from falling into the ladle; using a 400A current and a 2Hz frequency for the crystallization electromagnetic stirring and a 420A current and a 7Hz frequency for the end electromagnetic stirring; and using an alternating stirring method for stirring; and controlling the hydrogen concentration in the tundish to ≤2.5ppm.

6. The production process according to claim 3, characterized in that, The heating process is as follows: preheating section temperature 650℃~700℃; second heating section temperature 850℃~870℃; first heating section temperature 1210℃~1240℃; soaking section temperature 1200℃~1230℃; tapping temperature 1180℃~1210℃; total heating time 330min~780min.

7. The production process according to claim 3, characterized in that, The rolling process specifically involves: a primary rolling mill using a BD mill with 9 passes, odd-numbered passes for turning the steel, a reduction of 34-90 mm, and an intermediate billet height of 268 mm and a width of 232 mm. Depending on the specifications, the billet is then rolled through 11-18 stands at a rolling speed of 0.27-2 m / s, producing hot-rolled round steel with a diameter of 55-90 mm.

8. The production process according to claim 3, characterized in that, The cooling process specifically involves: placing round steel bars into a pit for insulation, with red steel padding at the bottom of the pit; the temperature upon entering the pit is ≥500℃, the temperature upon exiting the pit is ≤100℃, and the insulation time is ≥48h.

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