Production method of high-strength alloy structural steel for high-temperature carburization

Abstract

The invention relates to a production method of high-strength alloy structural steel for high-temperature carburization, and belongs to the technical field of steel and iron manufacturing. The steel comprises the following chemical components in percentage by weight: 0.18 to 0.21 percent of C, less than or equal to 0.30 percent of Si, 0.7 to 0.95 percent of Mn, less than or equal to 0.012 percent of P, 0.020 to 0.045 percent of S, 1.5 to 2.0 percent of Cr, less than or equal to 0.20 percent of Cu, 0.04 to 0.08 percent of Al, 0.045 to 0.085 percent of Nb, 0.012 to 0.02 percent of N and the balance of Fe and essential impurities. The method comprises the key process steps of smelting, ingot casting, forging, forming and carburizing, a structural part formed through cold extrusion is subjected to heat preservation for 12 hours at the temperature of 980 DEG C for high-temperature carburizing, and the grain size from a carburizing layer to a core area after carburizing is 9.0-10 grades.

Description

Technical Field

[0001] The present invention belongs to the technical field of steel manufacturing, and relates to a production method for various high-strength alloy structural steels used for high-temperature carburizing. Background Art

[0002] As the economic society enters the stage of high-quality development, energy conservation, emission reduction, improving production efficiency, and reducing production costs have become common requirements in all industries, and steel material production enterprises are no exception. To meet the development requirements of high performance and lightweight in the automotive industry, the future key development directions are gear steels with narrow hardenability bands, ultra-low oxygen carburized steels, carburized steels with low grain boundary oxidation layers, ultra-fine grain carburized steels, carburized steels with improved high-temperature hardness and high-temperature anti-softening properties, free-cutting gear steels, and steels for cold-forged gears. It can be seen from this that the future development directions involve various carburized steels.

[0003] Carburizing consists of three parts: the decomposition of the carburizing medium, absorption by the surface of the steel part, and diffusion into the part. When the required carburized layer depth is certain, the higher the carburizing temperature, the shorter the required carburizing time. Therefore, high-temperature carburizing can significantly increase the carburizing speed. Generally, it is considered that carburizing at a temperature exceeding 950°C belongs to high-temperature carburizing, and for every 50°C increase in the carburizing temperature, the carburizing speed can be increased by nearly 1 time. Therefore, for carburized parts such as large bearing rings, gears, and gear shafts with a relatively thick carburized layer, appropriately increasing the carburizing temperature can greatly shorten the carburizing time, improve the carburizing efficiency, and reduce the production cost. However, if the carburizing temperature is too high, it is easy to damage the carburizing furnace and carburizing tooling, damage the carburized parts, make the grains of the carburized parts coarse, and reduce the mechanical properties. The research results of a large number of literatures show that short-time high-temperature carburizing will not make the grains of the carburized parts coarse and the mechanical properties will not be reduced; long-time high-temperature carburizing will make the grains of the carburized parts grow, and even cause problems such as coarse grains and mixed grains; high-temperature carburizing can greatly increase the carburizing speed, save energy and reduce consumption, and shorten the production cycle. The United States has listed high-temperature carburizing technology as a key project for the development of heat treatment technology in the 21st century, and has vigorously studied carburizing equipment, carbon potential detection and control technologies that meet the carburizing requirements above 1000°C.

[0004] The carburizing temperature is the main factor affecting the carburizing efficiency. When the carburizing temperature of gears is increased from the conventional 930°C to 1000°C, the carburizing time can be shortened by more than 50%, thereby saving the carburizing cost and improving the production efficiency. However, after the conventional gear steel is kept warm at a higher temperature for a long time, the phenomenon of grain growth is inevitable. After the grains grow excessively, the fatigue performance and service life of the material will be greatly reduced. Therefore, in recent years, scholars have been committed to the development of steel grades for carburizing at higher temperatures. Summary of the Invention

[0005] The object of the present invention is to provide a production method for various high-strength alloy structural steels used for high-temperature carburizing, so as to solve the technical problems such as coarse grains and mixed grains often appearing in the carburized layer and the central position of structural parts after high-temperature carburizing of this steel type.

[0006] The technical solution of the present invention: A production method for high-strength alloy structural steel used for high-temperature carburizing, the chemical composition of the steel by weight percentage is C = 0.18 - 0.21, Si ≤ 0.30, Mn 0.7 - 0.95, P ≤ 0.012, S = 0.020 - 0.045, Cr = 1.5 - 2.0, Cu ≤ 0.20, Al = 0.04 - 0.08, Nb = 0.045 - 0.085, N = 0.012 - 0.02, and the rest are Fe and inevitable impurities; it includes the following technological steps: (1) Smelting: Add industrial pure iron into a vacuum induction furnace for melting. After the pure iron is completely melted, raise the temperature to 1590 - 1610 °C, and then add weighed ferrosilicon, ferromanganese, ferrochrome, ferroaluminum, ferroniobium, and ferrosulfur alloys into the induction furnace from the charging hole of the induction furnace. After the alloys are melted, fill nitrogen into the vacuum induction furnace to increase the nitrogen content of the molten steel; (2) Ingot casting: Pour the molten steel into an ingot mold with an inner diameter of 280 mm × 280 mm to cast an ingot with a cross-section of 280 mm × 280 mm, air-cool it to room temperature, then demold it, take out the ingot, cut off the ingot riser and sample for analysis of composition, gas, and inclusions; (3) Forging: Put the ingot with the riser cut off into a heating furnace for heating, the heating temperature is 1100 - 1200 °C, heat for 2 hours, and the holding time is 2 hours; then forge the ingot into a φ80 mm round bar, and perform surface cutting on the φ80 mm round bar to process it into a φ75 mm round bar, and then cut the φ75 mm round bar into 70 mm long steel bars; (4) Forming: Put the φ75 mm × 70 mm round bar into an annealing furnace for spheroidizing annealing, and then cold-extrude the round bar after spheroidizing annealing into shape; (5) Carburizing: Keep the cold-extruded formed structural parts at 980 °C for 12 hours for high-temperature carburizing, and after carburizing, the grain size from the carburized layer to the core area is 9.0 - 10 grades.

[0007] Advantages of the present invention: Aiming at the problems of grain coarsening and mixed grains that occur after high-temperature carburization of processed parts of high-strength alloy structural steel, through thermodynamic calculations and software simulation analysis, etc., the influence of each alloying element on grain coarsening and mixed grains in steel is studied, and a new composition is designed to stably obtain high-strength alloy structural steel with fine and uniform grains and fully meet the hardness requirements after high-temperature carburization. After this steel grade undergoes high-temperature carburization at 980°C for 12 hours, no mixed grains occur, and the grains are uniformly fine, solving the problems of grain coarsening and mixed grains that often occur during high-temperature carburization of high-strength alloy structural steel with traditional compositions. Description of the Drawings

[0008] Figure 1 It is a diagram of the carburized layer structure after high-temperature carburization of a structural part formed by cold extrusion of high-strength alloy structural steel with the original composition.

[0009] Figure 2 It is a diagram of the core structure after high-temperature carburization of a structural part formed by cold extrusion of high-strength alloy structural steel with the original composition.

[0010] Figure 3 It is a diagram of the carburized layer structure after high-temperature carburization of a structural part formed by cold extrusion of high-strength alloy structural steel with the test composition.

[0011] Figure 4 It is a diagram of the core structure after high-temperature carburization of a structural part formed by cold extrusion of high-strength alloy structural steel with the test composition. Detailed Embodiments

[0012] The following is further described in conjunction with embodiments.

[0013] Embodiment 1 A production method of high-strength alloy structural steel for high-temperature carburization. Before smelting with a 500 kg vacuum induction furnace, first prepare the raw materials for smelting according to the composition of the steel grade to be smelted: 500 kg of industrial pure iron, 2.27 kg of ferrosilicon (containing 75% silicon), 5.75 kg of ferromanganese (containing 78% manganese), 18.41 kg of ferrochrome (containing 56% chromium), 0.71 kg of ferroniobium (containing 60% niobium), 0.31 kg of aluminum block (containing 98.5% aluminum), 0.86 kg of carburizer (containing 97% carbon), and 30 m of ferrosulfur wire; the recovery rates of each alloy are as follows: the recovery rate of ferrosilicon is 88%, the recovery rate of ferromanganese is 78%, the recovery rate of ferrochrome is 97%, the recovery rate of ferroniobium is 95%, the recovery rate of aluminum block is 70%, the recovery rate of carburizer is 90%, and the recovery rate of ferrosilicon is 88%. The key steps of the production process include: (1)Smelting: Add 500 kg of industrial pure iron into a vacuum induction furnace for heating and melting. After the pure iron is completely melted, raise the temperature to about 1595 °C. Then, add weighed ferro-silicon, ferro-manganese, ferro-chrome, ferro-aluminum, ferro-niobium, ferrosulfur wire and other alloys into the induction furnace through the charging hole. After the alloys are completely melted and the temperature is stabilized at about 1595 °C, charge nitrogen into the vacuum induction furnace to increase the nitrogen content in the molten steel; (2)Ingot casting: Take a sample of the molten steel to detect the composition, and fine-tune the composition of the elements that do not meet the requirements of the steel grade composition range until the composition of each element meets the requirements. Then, pour the molten steel into an ingot mold with an inner diameter of 280 mm × 280 mm to cast an ingot with a cross-section of 280 mm × 280 mm. Air-cool it to room temperature, and then demold it to take out the ingot; (3)Forging: Remove the riser of the ingot, and then take a sample to detect the composition, as shown in Table 1; Send the ingot with the riser removed into a heating furnace for heating, with a heating temperature of 1120 °C, heating for 2 hours, and holding time of 2 hours; Then forge the ingot into a φ80 mm round bar, and perform surface cutting on the φ80 mm round bar to process it into a φ75 mm round bar, and then cut the φ75 mm round bar into 70 mm long steel bars; (4)Forming: Put the φ75 mm × 70 mm round bar into an annealing furnace for spheroidizing annealing, and then cold-extrude the spheroidized annealed round bar into shape; (5)Carburizing: Keep the cold-extruded formed structural parts at 980 °C for 12 hours for high-temperature carburizing. After carburizing, the grain size of the carburized layer is 9.0 grades, and the grain size of the core area is 9.5 grades; Through the above composition design and process control in this embodiment, a high-strength alloy structural steel with fine and uniform grains after high-temperature carburizing and fully meeting the hardness requirements can be stably obtained, solving the problems of coarse grains and mixed grains that often occur during high-temperature carburizing of traditional high-strength alloy structural steel.

[0014] Example 2 A production method of high-strength alloy structural steel for high-temperature carburizing. Before smelting with a 500 kg vacuum induction furnace, first prepare the raw materials for smelting according to the composition of the steel grade to be smelted: 500 kg of industrial pure iron, 2.27 kg of ferro-silicon (containing 75% silicon), 7.8 kg of ferro-manganese (containing 78% manganese), 13.81 kg of ferro-chrome (containing 56% chromium), 0.38 kg of ferro-niobium (containing 60% niobium), 0.62 kg of aluminum block (containing 98.5% aluminum), 1.08 kg of carburizer (containing 97% carbon), 30 m of ferrosulfur wire; The recovery rates of each alloy are as follows: the recovery rate of ferro-silicon is 88%, the recovery rate of ferro-manganese is 78%, the recovery rate of ferro-chrome is 97%, the recovery rate of ferro-niobium is 95%, the recovery rate of aluminum block is 70%, the recovery rate of carburizer is 90%, and the recovery rate of ferro-silicon is 88%. The key steps of the production process include: (1)Smelting: Add 500 kg of industrial pure iron into a vacuum induction furnace for heating and melting. After the pure iron is completely melted, raise the temperature to about 1605 °C. Then, add weighed ferro-silicon, ferro-manganese, ferro-chrome, ferro-aluminum, ferro-niobium, ferrosulphur wire and other alloys into the induction furnace through the charging hole. After the alloys are completely melted and the temperature is stabilized at about 1605 °C, charge nitrogen into the vacuum induction furnace to increase the nitrogen content in the molten steel; (2)Ingot casting: Take a sample of the molten steel to detect its composition. Fine-tune the composition of the elements that do not meet the requirements of the steel grade composition range until the composition of each element meets the requirements. Then, pour the molten steel into an ingot mold with an inner diameter of 280 mm × 280 mm to cast an ingot with a cross-section of 280 mm × 280 mm. Air-cool it to room temperature, and then demold to take out the ingot; (3)Forging: Remove the riser of the ingot, and then take a sample to detect its composition, as shown in Table 1; Send the ingot with the riser removed into a heating furnace for heating at a temperature of 1180 °C for 2 hours and a holding time of 2 hours; Then, forge the ingot into a φ80 mm round bar, and perform surface cutting on the φ80 mm round bar to process it into a φ75 mm round bar. Then, cut the φ75 mm round bar into 70 mm long steel bars; (4)Forming: Put the φ75 mm × 70 mm round bar into an annealing furnace for spheroidizing annealing, and then cold-extrude the round bar after spheroidizing annealing into shape; (5)Carburizing: Keep the cold-extruded structural parts at 980 °C for 12 hours for high-temperature carburizing. After carburizing, the grain size of the carburized layer is 9.5 grades, and the grain size of the core area is 10 grades; Through composition design and process control in this embodiment, a high-strength alloy structural steel with fine and uniform grains and fully meeting the hardness requirements after high-temperature carburizing can be stably obtained, solving the problems of coarse grains and mixed grains that often occur during high-temperature carburizing of traditional high-strength alloy structural steels with conventional compositions.

[0015] Table 1 Chemical composition of the steel in the embodiment (wt%) 。

Claims

1. A production method of a high-strength alloy structural steel for high-temperature carburizing, characterized in that: The chemical composition of the steel by weight percentage is C = 0.18 - 0.21, Si ≤ 0.30, Mn 0.7 - 0.95, P ≤ 0.012, S = 0.020 - 0.045, Cr = 1.5 - 2.0, Cu ≤ 0.20, Al = 0.04 - 0.08, Nb = 0.045 - 0.085, N = 0.012 - 0.02, and the rest is Fe and inevitable impurities; It includes the following technological steps: (1) Smelting: Add industrial pure iron into a vacuum induction furnace for melting. After the pure iron is completely melted, raise the temperature to 1590 - 1610 °C, and then add weighed ferrosilicon, ferromanganese, ferrochrome, ferroaluminum, ferroniobium, and ferrosulfur alloys into the induction furnace through the charging hole. After the alloys are melted, charge nitrogen into the vacuum induction furnace to increase the nitrogen content in the molten steel; (2) Ingot casting: Pour the molten steel into an ingot mold with an inner diameter of 280 mm × 280 mm to cast an ingot with a cross-section of 280 mm × 280 mm. Air-cool it to room temperature, then demold, take out the ingot, cut off the ingot riser and sample for analysis of composition, gas, and inclusions; (3) Forging: Put the ingot with the riser cut off into a heating furnace for heating at a temperature of 1100 - 1200 °C for 2 hours and keep it warm for 2 hours; then forge the ingot into a φ80 mm round bar, and perform surface cutting on the φ80 mm round bar to process it into a φ75 mm round bar, and then cut the φ75 mm round bar into 70 mm long steel bars; (4) Forming: Put the φ75 mm × 70 mm round bar into an annealing furnace for spheroidizing annealing, and then cold-extrude the spheroidized annealed round bar into shape; (5) Carburizing: Keep the cold-extruded formed structural part at 980 °C for 12 hours for high-temperature carburizing. After carburizing, the grain size from the carburized layer to the core area is 9.0 - 10 grades.

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