Alloy steel for coil spring and preparation method thereof

Abstract

The invention discloses alloy steel for a coil spring and a preparation method of the alloy steel. The alloy steel for the coil spring comprises the following chemical components in percentage by weight: 0.20%-0.32% of carbon, 0.20%-0.30% of silicon, 0.20%-0.60% of niobium, 0.02%-0.04% of scandium and the balance of iron and inevitable impurities. Through the innovative ultra-low carbon composite microalloying design, the technical bottleneck that strength and plasticity of traditional coil spring steel are difficult to consider at the same time is broken through, perfect unification of high strength, high toughness, excellent fatigue resistance and good cold forming performance is achieved, the service life of a coil spring product is remarkably prolonged, and the reliability of the coil spring product is remarkably improved; the urgent requirements of high-end equipment manufacturing fields such as automobiles and machinery on high-performance elastic elements can be met.

Description

Technical Field

[0001] This invention relates to the field of alloy steel technology, specifically to an alloy steel for coil springs and its preparation method. Background Technology

[0002] Coil springs, as an important elastic element, are widely used in many fields such as the automotive industry, machinery manufacturing, electronic equipment, and home appliances. With the rapid development of modern industrial technology, especially the trend of lightweighting and high performance in automobiles, increasingly higher technical requirements are being placed on the steel used for coil springs, including high strength, high toughness, excellent fatigue resistance, good cold forming properties, and stable dimensional accuracy.

[0003] Currently, the steels commonly used for coil springs on the market mainly include carbon structural steel, low-alloy high-strength steel, and some alloy spring steels. However, these traditional steels face many technical bottlenecks in practical applications: First, there are shortcomings in the design of material composition. Traditional coil spring steel often uses medium or high carbon steel, with a carbon content typically above 0.3%. While this achieves high strength, its plasticity and toughness are relatively poor, making it prone to cracking during cold forming and limiting the manufacture of complex-shaped coil springs. At the same time, the alloying degree of existing steels is relatively low, and the application of micro-alloying elements is insufficient, making it difficult to achieve the optimal balance of strength, plasticity, and toughness.

[0004] Secondly, there is still room for improvement in microstructure control technology. In existing technologies, grain refinement of steel mainly relies on traditional controlled rolling and cooling techniques, which have limited grain refinement effects and result in less than ideal microstructure uniformity. Particularly in obtaining a fine and uniform ferrite + pearlite microstructure, the current process parameters lack precision, easily leading to microstructure inhomogeneity and affecting the overall performance stability of the material.

[0005] Third, there are limitations in the manufacturing process technology. Traditional production processes do not allow for precise control over the degree of smelting and purification, resulting in higher inclusion content in the steel and affecting the fatigue performance of the material. In the hot rolling process, the temperature control range is relatively wide, lacking refined process parameter design, making it difficult to achieve precise control over the microstructure. In terms of cooling processes, a single cooling mode is mostly used, lacking composite cooling technologies tailored to specific microstructure requirements.

[0006] Fourth, existing technologies still have potential for performance optimization. Current coil spring steels do not fully meet the demands of high-end applications in key indicators such as fatigue resistance, cold forming performance, and dimensional stability. Particularly in the automotive industry, the increasing power density of engines and the harsher operating environments place higher demands on the overall performance of coil spring materials.

[0007] Furthermore, environmental protection and sustainable development requirements are becoming increasingly stringent. Traditional steel production processes consume a lot of energy and have a significant environmental impact, making it imperative to develop cleaner and more efficient production technologies.

[0008] In summary, the above analysis shows that developing a new type of alloy steel for coil springs with a reasonable composition design, optimized microstructure, and advanced manufacturing process, as well as its preparation method, is of great practical significance and has broad application prospects for meeting the needs of modern industrial development. Summary of the Invention

[0009] To address the shortcomings of existing technologies, the present invention aims to provide an alloy steel for coil springs and its preparation method. Through innovative ultra-low carbon composite micro-alloying design, the present invention breaks through the technical bottleneck of traditional coil spring steel, which struggles to balance strength and plasticity. It achieves a perfect balance of high strength, high toughness, excellent fatigue resistance, and good cold forming performance, significantly improving the service life and reliability of coil spring products. This can meet the urgent needs of high-performance elastic components in high-end equipment manufacturing fields such as automobiles and machinery.

[0010] To achieve the above objectives, the present invention adopts the following technical solution: An alloy steel for coil springs has the following chemical composition by weight percentage: 0.20%~0.32% carbon, 0.20%~0.30% silicon, 0.20%~0.60% niobium, 0.02%~0.04% scandium, with the balance being iron and unavoidable impurities.

[0011] Preferably, the chemical composition, by weight percentage, includes: 0.25% carbon, 0.25% silicon, 0.40% niobium, 0.03% scandium, with the balance being iron and unavoidable impurities.

[0012] Preferably, the alloy steel has a yield strength of not less than 1400 MPa and an elastic modulus of not less than 195 GPa.

[0013] This invention also claims a method for preparing alloy steel for coil springs, comprising the following steps: (1) Smelting and refining: The molten iron is pretreated and then smelted in a converter and refined in an LF furnace to obtain qualified molten steel; Through dual processing of converter smelting and LF furnace refining, especially the full-process argon blowing purification in the LF furnace, harmful gases such as oxygen, nitrogen, and hydrogen, as well as impurities such as sulfur and phosphorus, can be effectively removed from the molten steel, significantly improving the cleanliness of the steel. Precisely controlled smelting time and temperature ensure the full dissolution and uniform distribution of microalloying elements such as niobium and scandium, laying a high-quality raw material foundation for subsequent microstructure control.

[0014] Preferably, in step (1), the converter smelting time is 35~45min, the converter tapping temperature is 1570~1590℃; the LF furnace refining temperature is 1540~1560℃, the refining time is 40~50min, and the tapping temperature is 1560~1580℃.

[0015] Preferably, in step (1), argon is blown throughout the entire refining process in the LF furnace.

[0016] (2) Continuous casting: The refined molten steel is passed through a tundish and cast into a continuous casting billet by a continuous casting machine; By strictly controlling the tundish temperature, casting speed, and cooling water parameters, the continuous casting slabs were guaranteed to have good surface quality and uniform internal structure. In particular, the design of thin slabs of 55~65mm helped reduce segregation and shrinkage defects, while providing better deformation conditions for the subsequent hot rolling process and ensuring the uniform distribution of microalloying elements during solidification.

[0017] Preferably, in step (2), the temperature of the molten steel in the tundish is 1530~1540℃, the casting speed of the continuous casting machine is 3.4~3.8m / s, the cooling water flow rate of the continuous casting crystallizer is 5500~5700L / min, the water temperature is 35~45℃, a protective slag with an alkalinity of 1.30~1.35 is used, and the slab thickness is controlled to be 55~65mm.

[0018] (3) Hot continuous rolling: After heating and descaling the continuously cast billet, hot continuous rolling is performed to form strip steel; Precise control of the heating and rolling temperature ranges ensures effective control of the complete solid solution and re-precipitation process of microalloying elements. High-pressure water descaling guarantees excellent surface quality, while controlling the initial and final rolling temperatures allows the steel to fully deform in the austenitic region, creating favorable conditions for obtaining a fine and uniform grain structure.

[0019] Preferably, in step (3), the temperature of the continuously cast billet entering the heating furnace is 900~940℃, the heating temperature is 1230~1270℃, the residence time in the heating furnace is 230~250min, and the soaking time is 15~25min.

[0020] Preferably, in step (3), the inlet pressure of the high-pressure water descaling is 18~20MPa and the outlet pressure is 28~30MPa.

[0021] Preferably, in step (3), the initial rolling temperature is 1000~1040℃ and the final rolling temperature is 870~890℃.

[0022] (4) Cooling and winding: The strip is cooled by laminar flow and then wound to obtain the alloy steel for coil springs.

[0023] By employing a composite cooling mode of "air cooling-water cooling-re-air cooling" to precisely control the winding temperature, an ideal ferrite + pearlite microstructure was successfully obtained. This microstructure combines good strength and plasticity. The tension winding controlled by the secondary PSC model ensures the quality of the coil shape, while the subsequent air cooling process is equivalent to self-tempering treatment, effectively eliminating internal stress and stabilizing the microstructure and properties.

[0024] Preferably, in step (4), laminar cooling adopts an air cooling, water cooling, and then air cooling mode to cool the strip to a coiling temperature of 800~820℃ in order to obtain a ferrite plus pearlite microstructure.

[0025] Preferably, in step (4), the tension set by the secondary PSC model is used for winding, and the coil is air-cooled to room temperature between other offline coils.

[0026] Compared with the prior art, the present invention has the following beneficial effects: 1. This invention provides an alloy steel for coil springs, which controls the carbon (C) content within a low carbon range of 0.20% to 0.32% and the silicon (Si) content within a relatively low range of 0.20% to 0.30%. This improves the steel's plasticity, toughness, and fatigue resistance while reducing its hardness and brittleness, making it less prone to cracking during cold forming into coil springs and ensuring good formability. Furthermore, it incorporates 0.2% to 0.6% niobium (Nb) and 0.02% to 0.04% scandium (Sc). Niobium, as a strong carbide and nitride forming element, can form stable Nb(C,N) particles in the steel, effectively preventing austenite grain growth during hot rolling and significantly refining ferrite grains after cooling, playing a crucial role in microalloying strengthening and toughening. Scandium is a more potent grain-refining and purifying element. It not only further refines the grains but also reacts with impurities such as oxygen and sulfur in steel to form stable, high-melting-point compounds, thus purifying the molten steel and improving the morphology of inclusions. The synergistic effect of niobium and scandium enables the alloy steel of this invention to achieve an exceptionally fine and uniform grain structure on an ultra-low carbon basis, thereby simultaneously possessing high strength, high toughness, and excellent fatigue resistance, meeting the stringent requirements for long-term service of coil spring products.

[0027] 2. This invention provides a method for preparing alloy steel for coil springs. Through precise control of key processes such as smelting, continuous casting, hot rolling, and cooling, the superior properties of the alloy steel are consistently achieved. In the smelting and refining stages, by limiting the operating time and temperature of the converter and LF furnace, and supplementing this with argon blowing purification throughout the LF furnace process, the steel composition can be precisely controlled, effectively removing harmful gases and inclusions, laying the foundation for producing high-quality, high-cleanliness steel. In the continuous casting stage, strict control of parameters such as tundish temperature, casting speed, cooling water, and protective slag ensures that the continuously cast billet has good surface quality and a uniform internal structure. In particular, using thinner slab specifications is beneficial for uniform deformation and microstructure control in the subsequent rolling process. In the hot continuous rolling stage, by precisely controlling the temperature and residence time of the heating furnace, setting strict opening and closing rolling temperatures, and cooperating with high-pressure water descaling, the billet is ensured to be rolled in the optimal plasticity zone, effectively suppressing abnormal grain growth and obtaining fine recrystallized austenite grains. During the cooling and winding stages, a special laminar flow cooling mode of "air cooling-water cooling-re-air cooling" is employed, and the winding temperature is precisely controlled at 800~820℃, which is key to obtaining the ideal microstructure of "ferrite + pearlite". This microstructure combines good strength and plasticity. Simultaneously, winding is performed using tension set by a two-stage PSC model, ensuring the quality of the coil shape. The air cooling process between coils is equivalent to a slow self-tempering treatment, which helps eliminate internal stress, further stabilizes the microstructure and properties, and ultimately yields high-end alloy steel products for coil springs with uniform performance and stable quality. Detailed Implementation

[0028] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. Of course, the specific embodiments described herein are only for explaining the invention and are not intended to limit the invention.

[0029] Unless otherwise specified, all chemical reagents and materials in this invention are purchased from the market or synthesized from raw materials purchased from the market.

[0030] A method for preparing alloy steel for coil springs includes the following steps: (1) Smelting and refining: The molten iron is pretreated and then smelted in a converter and refined in an LF furnace to obtain qualified molten steel; In step (1), the carbon content is controlled at 0.20%~0.32%, the silicon content is controlled at 0.20%~0.30%, the niobium content is 0.20%~0.60%, and the scandium content is 0.02%~0.04%.

[0031] In step (1), the converter smelting time is 35~45min, the converter tapping temperature is 1570~1590℃; the LF furnace refining temperature is 1540~1560℃, the refining time is 40~50min, and the tapping temperature is 1560~1580℃.

[0032] In step (1), argon is blown throughout the entire refining process in the LF furnace.

[0033] (2) Continuous casting: The refined molten steel is passed through a tundish and cast into a continuous casting billet by a continuous casting machine; In step (2), the temperature of the molten steel in the tundish is 1530~1540℃, the casting speed of the continuous casting machine is 3.4~3.8m / s, the cooling water flow rate of the continuous casting crystallizer is 5500~5700L / min, the water temperature is 35~45℃, a protective slag with an alkalinity of 1.30~1.35 is used, and the slab thickness is controlled to be 55~65mm.

[0034] (3) Hot continuous rolling: After heating and descaling the continuously cast billet, hot continuous rolling is performed to form strip steel; In step (3), the temperature of the continuously cast billet entering the heating furnace is 900~940℃, the heating temperature is 1230~1270℃, the residence time in the heating furnace is 230~250min, and the soaking time is 15~25min.

[0035] In step (3), the inlet pressure of the high-pressure water descaling is 18~20MPa and the outlet pressure is 28~30MPa.

[0036] In step (3), the initial rolling temperature is 1000~1040℃ and the final rolling temperature is 870~890℃.

[0037] (4) Cooling and winding: The strip is cooled by laminar flow and then wound to obtain the alloy steel for coil springs.

[0038] In step (4), laminar flow cooling adopts the mode of air cooling, water cooling, and then air cooling to cool the strip to a coiling temperature of 800~820℃ in order to obtain a ferrite plus pearlite microstructure.

[0039] In step (4), the tension set by the secondary PSC model is used for winding, and the coil is air-cooled to room temperature between other offline coils.

[0040] The present invention will be further described below through specific embodiments.

[0041] Example 1 A method for preparing alloy steel for coil springs includes the following steps: First, the molten iron undergoes pretreatment, followed by converter smelting to control the carbon content at 0.20 wt% and the silicon content at 0.25%. The converter smelting time is 40 minutes, and the tapping temperature is 1580°C. After smelting, the iron is transferred to an LF furnace for refining, where niobium and scandium are added to achieve a niobium content of 0.40 wt% and a scandium content of 0.03 wt%. The refining temperature in the LF furnace is 1550°C, the refining time is 45 minutes, and the tapping temperature is 1570°C. Argon is blown throughout the entire process.

[0042] After refining, the molten steel enters the tundish through the sliding gate at the bottom of the ladle. The temperature of the molten steel in the tundish is 1535°C. Then, it is cast through a continuous casting machine with a casting speed of 3.6 m / s. The cooling water flow rate of the continuous casting crystallizer is controlled at 5600 L / min, and the crystallizer water temperature is 40°C. The protective slag used in the continuous casting crystallizer is a special protective slag for high basicity of medium carbon steel with a basicity of 1.32. The slab thickness is controlled at 60 mm.

[0043] After being cooled in the secondary cooling section, the cast continuous casting billet enters the soaking furnace. The billet's initial temperature in the furnace is 920°C, and the heating temperature is 1250°C. The billet's residence time in the furnace is 240 minutes, and the soaking time is 20 minutes. After being removed from the furnace, the heated billet undergoes descaling. The descaling inlet pressure is 19 MPa, and the outlet pressure is 29 MPa. After one high-pressure water descaling, the billet enters a 6-stand continuous rolling mill for rolling. The starting rolling temperature is 1020°C, and the finishing rolling temperature is 880°C. The secondary descaling water supply after stands F1 / F2 is shut off, as is the cooling water supply between stands F1-F3.

[0044] After the rolled strip exits the F6 last stand, it is inspected by a multi-function instrument and then cooled to the coiling temperature (810°C) through an 8-segment laminar flow cooling section. The cooling process employs an air-cooling + water-cooling + air-cooling mode, first performing short-time air cooling, then accelerated water cooling, and finally air cooling to obtain a ferrite + pearlite microstructure. After cooling, the coil is produced using the coiling tension set by the secondary PSC model and then placed among other coils from the production line for air cooling to room temperature.

[0045] The resulting coil has good corrosion resistance and high strength properties, with a yield strength of 1400MPa, an elastic modulus of 195GPa, an elongation after fracture of 15%, and an impact energy of 55J at room temperature. It can be used to manufacture coil spring buffer mechanisms.

[0046] Example 2 A method for preparing alloy steel for coil springs includes the following steps: First, the molten iron undergoes pretreatment, followed by converter smelting to control the carbon content at 0.25 wt% and the silicon content at 0.25%. The converter smelting time is 45 minutes, and the tapping temperature is 1590°C. After smelting, the iron is transferred to an LF furnace for refining, where niobium and scandium are added to achieve a niobium content of 0.45 wt% and a scandium content of 0.04 wt%. The refining temperature in the LF furnace is 1560°C, the refining time is 50 minutes, and the tapping temperature is 1580°C. Argon is blown throughout the entire process.

[0047] After refining, the molten steel enters the tundish through the sliding gate at the bottom of the ladle. The temperature of the molten steel in the tundish is 1540°C. Then, it is cast through a continuous casting machine with a casting speed of 3.8 m / s. The cooling water flow rate of the continuous casting crystallizer is controlled at 5700 L / min, and the crystallizer water temperature is 45°C. The protective slag used in the continuous casting crystallizer is a special protective slag with high basicity for medium carbon steel, with a basicity of 1.35. The slab thickness is controlled at 65 mm.

[0048] After being cooled in the secondary cooling section, the cast continuous casting billet enters the soaking furnace. The billet's initial temperature is 940°C, and the heating temperature is 1270°C. The billet's residence time in the furnace is 250 minutes, and the soaking time is 25 minutes. After being removed from the furnace, the heated billet undergoes descaling. The descaling inlet pressure is 20 MPa, and the outlet pressure is 30 MPa. After one high-pressure water descaling, the billet enters a 6-stand continuous rolling mill for rolling. The starting rolling temperature is 1040°C, and the finishing rolling temperature is 890°C. The secondary descaling water supply after stands F1 / F2 is shut off, as is the cooling water supply between stands F1-F3.

[0049] After the rolled strip exits the F6 last stand, it is inspected by a multi-function instrument and then cooled to the coiling temperature (820°C) through an 8-segment laminar flow cooling section. The cooling process employs an air-cooling + water-cooling + air-cooling mode, first performing short-time air cooling, then accelerated water cooling, and finally air cooling to obtain a ferrite + pearlite microstructure. After cooling, the coil is produced using the coiling tension set by the secondary PSC model and then placed among other coils from the production line for air cooling to room temperature.

[0050] The resulting coil has good corrosion resistance and high strength properties, with a yield strength of 1450MPa, an elastic modulus of 200GPa, an elongation after fracture of 13%, and an impact energy of 50J at room temperature. It can be used to manufacture coil spring buffer mechanisms.

[0051] Example 3 A method for preparing alloy steel for coil springs includes the following steps: First, the molten iron undergoes pretreatment, followed by converter smelting to control the carbon content at 0.30 wt% and the silicon content at 0.20%. The converter smelting time is 35 minutes, and the tapping temperature is 1570°C. After smelting, the iron is transferred to an LF furnace for refining, where niobium and scandium are added to achieve a niobium content of 0.40 wt% and a scandium content of 0.03 wt%. The refining temperature in the LF furnace is 1540°C, the refining time is 40 minutes, and the tapping temperature is 1560°C. Argon is blown throughout the entire process.

[0052] After refining, the molten steel enters the tundish through the sliding gate at the bottom of the ladle. The temperature of the molten steel in the tundish is 1530°C. Then, it is cast through a continuous casting machine with a casting speed of 3.4 m / s. The cooling water flow rate of the continuous casting crystallizer is controlled at 5500 L / min, and the crystallizer water temperature is 35°C. The protective slag used in the continuous casting crystallizer is a special protective slag for high basicity of medium carbon steel with a basicity of 1.30. The slab thickness is controlled at 55 mm.

[0053] After being cooled in the secondary cooling section, the cast continuous casting billet enters the soaking furnace. The billet's initial temperature in the furnace is 900°C, and the heating temperature is 1230°C. The billet's residence time in the furnace is 230 minutes, and the soaking time is 20 minutes. After being removed from the furnace, the heated billet undergoes descaling. The descaling inlet pressure is 18 MPa, and the outlet pressure is 28 MPa. After one high-pressure water descaling, the billet enters a 6-stand continuous rolling mill for rolling. The starting rolling temperature is 1000°C, and the finishing rolling temperature is 870°C. The secondary descaling water supply after stands F1 / F2 is shut off, as is the cooling water supply between stands F1-F3.

[0054] After the rolled strip exits the F6 last stand, it is inspected by a multi-function instrument and then cooled to the coiling temperature (800°C) through an eight-segment laminar flow cooling section. The cooling process employs an air-cooling + water-cooling + air-cooling mode, first performing short-time air cooling, then accelerated water cooling, and finally air cooling to obtain a ferrite + pearlite microstructure. After cooling, the coil is produced using the coiling tension set by the secondary PSC model and then placed among other coils from the production line for air cooling to room temperature.

[0055] The resulting coil has good corrosion resistance and high strength properties, with a yield strength of 1500MPa, an elastic modulus of 205GPa, an elongation after fracture of 11%, and an impact energy of 48J at room temperature. It can be used to manufacture coil spring buffer mechanisms.

[0056] Comparative Example 1 A method for preparing medium-carbon alloy steel includes the following steps: First, the molten iron undergoes pretreatment, followed by converter smelting to control the carbon content at 0.50 wt% and the silicon content at 0.25%. The converter smelting time is 40 minutes, and the tapping temperature is 1580°C. After smelting, the iron is transferred to an LF furnace for refining, where niobium and scandium are added to achieve a niobium content of 0.40 wt% and a scandium content of 0.03 wt%. The refining temperature in the LF furnace is 1550°C, the refining time is 45 minutes, and the tapping temperature is 1570°C. Argon is blown throughout the entire process.

[0057] After refining, the molten steel enters the tundish through the sliding gate at the bottom of the ladle. The temperature of the molten steel in the tundish is 1535°C. Then, it is cast through a continuous casting machine with a casting speed of 3.6 m / s. The cooling water flow rate of the continuous casting crystallizer is controlled at 5600 L / min, and the crystallizer water temperature is 40°C. The protective slag used in the continuous casting crystallizer is a special protective slag with high basicity for medium carbon steel, with a basicity of 1.32. The slab thickness is controlled at 60 mm.

[0058] After being cooled in the secondary cooling section, the cast continuous casting billet enters the soaking furnace. The billet's initial temperature in the furnace is 920°C, and the heating temperature is 1250°C. The billet's residence time in the furnace is 240 minutes, and the soaking time is 20 minutes. After being removed from the furnace, the heated billet undergoes descaling. The descaling inlet pressure is 19 MPa, and the outlet pressure is 29 MPa. After one high-pressure water descaling, the billet enters a 6-stand continuous rolling mill for rolling. The starting rolling temperature is 1020°C, and the finishing rolling temperature is 880°C. The secondary descaling water supply after stands F1 / F2 is shut off, as is the cooling water supply between stands F1-F3.

[0059] After the rolled strip exits the F6 last stand, it is inspected by a multi-function instrument and then cooled to the coiling temperature (810°C) through an 8-segment laminar flow cooling section. The cooling process employs an air-cooling + water-cooling + air-cooling mode, first performing short-time air cooling, then accelerated water cooling, and finally air cooling to obtain a ferrite + pearlite microstructure. After cooling, the coil is produced using the coiling tension set by the secondary PSC model and then placed among other coils from the production line for air cooling to room temperature.

[0060] The resulting coil has high strength characteristics, with a yield strength of 1700MPa, an elastic modulus of 203GPa, an elongation after fracture of 6%, and an impact energy of 25J at room temperature. It is not suitable for manufacturing coil spring buffer mechanisms.

[0061] Comparative Example 2 A method for preparing alloy steel for coil springs includes the following steps: First, the molten iron undergoes pretreatment, followed by converter smelting to control the carbon content at 0.20 wt% and the silicon content at 0.25%. The converter smelting time is 40 minutes, and the tapping temperature is 1580°C. After smelting, the iron is transferred to an LF furnace for refining, where niobium is added to achieve a niobium content of 0.40 wt%. The refining temperature in the LF furnace is 1550°C, the refining time is 45 minutes, and the tapping temperature is 1570°C. Argon is blown throughout the entire process.

[0062] After refining, the molten steel enters the tundish through the sliding gate at the bottom of the ladle. The temperature of the molten steel in the tundish is 1535°C. Then, it is cast through a continuous casting machine with a casting speed of 3.6 m / s. The cooling water flow rate of the continuous casting crystallizer is controlled at 5600 L / min, and the crystallizer water temperature is 40°C. The protective slag used in the continuous casting crystallizer is a special protective slag with high basicity for medium carbon steel, with a basicity of 1.32. The slab thickness is controlled at 60 mm.

[0063] After being cooled in the secondary cooling section, the cast continuous casting billet enters the soaking furnace. The billet's initial temperature in the furnace is 920°C, and the heating temperature is 1250°C. The billet's residence time in the furnace is 240 minutes, and the soaking time is 20 minutes. After being removed from the furnace, the heated billet undergoes descaling. The descaling inlet pressure is 19 MPa, and the outlet pressure is 29 MPa. After one high-pressure water descaling, the billet enters a 6-stand continuous rolling mill for rolling. The starting rolling temperature is 1020°C, and the finishing rolling temperature is 880°C. The secondary descaling water supply after stands F1 / F2 is shut off, as is the cooling water supply between stands F1-F3.

[0064] After the rolled strip exits the F6 last stand, it is inspected by a multi-function instrument and then cooled to the coiling temperature (810°C) through an 8-segment laminar flow cooling section. The cooling process employs an air-cooling + water-cooling + air-cooling mode, first performing short-time air cooling, then accelerated water cooling, and finally air cooling to obtain a ferrite + pearlite microstructure. After cooling, the coil is produced using the coiling tension set by the secondary PSC model and then placed among other coils from the production line for air cooling to room temperature.

[0065] The resulting coil has good corrosion resistance and high strength properties, with a yield strength of 1250MPa, an elastic modulus of 190GPa, an elongation after fracture of 13%, and an impact energy of 40J at room temperature. It is not suitable for manufacturing coil spring buffer mechanisms.

[0066] Comparative Example 3 A method for preparing alloy steel for coil springs includes the following steps: First, the molten iron undergoes pretreatment, followed by converter smelting to control the carbon content at 0.20 wt% and the silicon content at 0.25%. The converter smelting time is 40 minutes, and the tapping temperature is 1580°C. After smelting, the iron is transferred to an LF furnace for refining, where scandium is added to achieve a scandium content of 0.03 wt%. The refining temperature in the LF furnace is 1550°C, the refining time is 45 minutes, and the tapping temperature is 1570°C. Argon is blown throughout the entire process.

[0067] After refining, the molten steel enters the tundish through the sliding gate at the bottom of the ladle. The temperature of the molten steel in the tundish is 1535°C. Then, it is cast through a continuous casting machine with a casting speed of 3.6 m / s. The cooling water flow rate of the continuous casting crystallizer is controlled at 5600 L / min, and the crystallizer water temperature is 40°C. The protective slag used in the continuous casting crystallizer is a special protective slag with high basicity for medium carbon steel, with a basicity of 1.32. The slab thickness is controlled at 60 mm.

[0068] After being cooled in the secondary cooling section, the cast continuous casting billet enters the soaking furnace. The billet's initial temperature in the furnace is 920°C, and the heating temperature is 1250°C. The billet's residence time in the furnace is 240 minutes, and the soaking time is 20 minutes. After being removed from the furnace, the heated billet undergoes descaling. The descaling inlet pressure is 19 MPa, and the outlet pressure is 29 MPa. After one high-pressure water descaling, the billet enters a 6-stand continuous rolling mill for rolling. The starting rolling temperature is 1020°C, and the finishing rolling temperature is 880°C. The secondary descaling water supply after stands F1 / F2 is shut off, as is the cooling water supply between stands F1-F3.

[0069] After the rolled strip exits the F6 last stand, it is inspected by a multi-function instrument and then cooled to the coiling temperature (810°C) through an 8-segment laminar flow cooling section. The cooling process employs an air-cooling + water-cooling + air-cooling mode, first performing short-time air cooling, then accelerated water cooling, and finally air cooling to obtain a ferrite + pearlite microstructure. After cooling, the coil is produced using the coiling tension set by the secondary PSC model and then placed among other coils from the production line for air cooling to room temperature.

[0070] The resulting coil has good corrosion resistance and high strength properties, with a yield strength of 1150MPa, an elastic modulus of 180GPa, an elongation after fracture of 13%, and an impact energy of 45J at room temperature. It is not suitable for manufacturing coil spring buffer mechanisms.

[0071] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. An alloy steel for coil springs, characterized in that, The chemical composition includes, by weight percent: carbon 0.20%~0.32%, silicon 0.20%~0.30%, niobium 0.20%~0.60%, scandium 0.02%~0.04%, and the balance being iron and unavoidable impurities.

2. A method of making the alloy steel for clock springs of claim 1, characterized in that, The method comprises the following steps: (1) smelting and refining: pretreating the molten iron, then carrying out converter smelting and LF furnace refining to obtain qualified molten steel; (2) continuous casting: casting the refined molten steel through a tundish and a continuous casting machine to form a continuous casting billet; (3) hot continuous rolling: heating and descaling the continuous casting billet, then carrying out hot continuous rolling to form a strip steel; (4) cooling and coiling: cooling the strip steel by laminar flow and coiling to obtain the alloy steel for the spring.

3. The production method according to claim 2, characterized by, In step (1), the converter smelting time is 35~45 min, the converter tapping temperature is 1570~1590°C; the LF furnace refining inlet temperature is 1540~1560°C, the refining time is 40~50 min, and the outlet temperature is 1560~1580°C.

4. The production method according to claim 2, characterized by, In step (1), argon is blown during the whole LF furnace refining process.

5. The preparation method according to claim 2, characterized in that, In step (2), the tundish molten steel temperature is 1530~1540°C, the continuous casting machine pulling speed is 3.4~3.8 m / s, the continuous casting crystallizer cooling water volume is 5500~5700 L / min, the water temperature is 35~45°C, the protective slag with a basicity of 1.30~1.35 is used, and the slab thickness is controlled to be 55~65 mm.

6. The preparation method according to claim 2, characterized in that, In step (3), the continuous casting billet inlet temperature of the heating furnace is 900~940°C, the heating temperature is 1230~1270°C, the residence time in the heating furnace is 230~250 min, and the soaking time is 15~25 min.

7. The preparation method according to claim 2, characterized in that, In step (3), the inlet pressure of the high-pressure water descaling is 18~20 MPa, and the outlet pressure is 28~30 MPa.

8. The preparation method according to claim 2, characterized in that, In step (3), the opening rolling temperature is 1000~1040°C, and the final rolling temperature is 870~890°C.

9. The preparation method according to claim 2, characterized in that, In step (4), the strip steel is cooled to a coiling temperature of 800~820°C by adopting the mode of air cooling, water cooling and re-air cooling to obtain a ferrite plus pearlite structure.

10. The method of claim 2, wherein, In step (4), the coiling is carried out by adopting the tension set by a two-stage PSC model, and the coiled plate is air-cooled to room temperature between other offline coiled plates.