High-strength high-corrosion-resistant spring steel and method for producing the same

Abstract

The application discloses a kind of high-strength high corrosion resistance spring steel and preparation method thereof, including, C: 0.4~0.5%, Si: 1.8~2.0%, Mn: 0.7~0.8%, Cr: 2.0~2.5%, Ni: 0.9~1.2%, Cu: 0.2~0.4%, V: 0.1~0.2%, W: 0.15~0.40%, La: 0.01~0.03%, Nb: 0.02~0.04%, N≤0.006%, S≤0.005%, P≤0.005%, the balance is Fe and inevitable impurity element by weight percentage.The application constructs basic corrosion resistance system by Cr, Ni, Cu, and uses W to improve surface passivation film stability, uses La to purify steel liquid, modifies inclusion and improves film uniformity, to synergistically improve the corrosion resistance of material in chlorine-containing environment;The prepared spring steel mechanical property meets tensile strength ≥1800 MPa, yield strength ≥1500 MPa, reduction of area ≥40%, elongation ≥10%, impact energy KU2≥20 J, neutral salt spray test 240 h corrosion rate ≤0.8 g / m2·h;The preparation process of the application is simple, and the requirement to equipment is low.

Description

Technical Field

[0001] This invention belongs to the field of metallurgical technology, and particularly relates to a high-strength, high-corrosion-resistant spring steel and its preparation method. Background Technology

[0002] Spring steel is an alloy steel specifically used to manufacture springs or elastic elements. Spring steel operates in extremely harsh environments and must withstand various forces such as tension, compression, torsion, impact, fatigue, and corrosion during operation. Sometimes it also needs to withstand extremely high instantaneous sudden loads. Therefore, the comprehensive performance requirements for spring steel are higher.

[0003] The automotive industry is one of the most important application areas for spring steel. Because spring steel operates in environments exposed to the elements, especially in humid conditions with corrosive media containing chloride ions, it is highly susceptible to corrosion and fracture. Rust not only significantly weakens the mechanical properties of components but also threatens the service life of elastic parts, thus posing a significant threat to the safe operation of vehicles.

[0004] Chinese patent CN104674133A discloses a weather-resistant spring flat steel. By adding trace amounts of boron (B), the effect of chromium (Cr) in the steel is increased more than tenfold, thereby improving its corrosion resistance. However, this spring flat steel has poor tempering elasticity and low strength, making it unsuitable for the production of high-strength spring steel in the automotive industry. Chinese patent CN106048424A discloses a high-strength corrosion-resistant spring steel and its processing technology. Although it uses the addition of nickel (Ni) to inhibit corrosion pit formation and the addition of copper (Cu) to enhance atmospheric corrosion resistance, the relatively high carbon (C) content in this material leads to increased hydrogen embrittlement sensitivity. Chinese patent CN114107841A discloses a high-strength corrosion-resistant spring steel and its preparation method. Its preparation process employs multiple alloying methods, such as deoxidation alloying with ferrosilicon, some rare earth alloys, and ferrochrome alloys during the converter tapping process; the addition of ferronickel (NiFe), ferroniobium (NiO), and rare earth alloys during the LF refining process; and the addition of the remaining rare earth alloys after RH refining to break up air pockets. This complex alloying process increases the operational difficulty and cost of the production process. Chinese Patent CN118581304A discloses a heat treatment method for spring steel used in heavy-duty truck leaf springs, which replaces tempering by performing nitrocarburizing treatment in a vacuum heating furnace. However, this equipment has high investment costs, as well as high operating and maintenance costs.

[0005] Therefore, the technical problem to be solved by this invention is how to design a spring steel with high strength, good corrosion resistance, simple process and low equipment requirements. Summary of the Invention

[0006] The purpose of this invention is to provide a high-strength, high-corrosion-resistant spring steel and its preparation method, so as to achieve the preparation of spring steel with high strength, good corrosion resistance, simple process and low equipment requirements.

[0007] To solve the above-mentioned technical problems, the present invention is mainly achieved through the following technical solutions:

[0008] In a first aspect, the present invention provides a high-strength, high-corrosion-resistant spring steel comprising, by weight percentage: C: 0.4%~0.5%, Si: 1.8%~2.0%, Mn: 0.7%~0.8%, Cr: 2.0%~2.5%, Ni: 0.9%~1.2%, Cu: 0.2%~0.4%, V: 0.1%~0.2%, W: 0.15%~0.40%, La: 0.01%~0.03%, Nb: 0.02%~0.04%, N≤0.006%, S≤0.005%, P≤0.005%, with the balance being Fe and unavoidable impurity elements.

[0009] In this invention, the carbon content is controlled at 0.4% to 0.5%. As a core strengthening element, the carbon content is positively correlated with tensile strength. However, when the carbon content exceeds 0.5%, it is easy to cause lattice embrittlement, resulting in a decrease in fatigue limit of more than 30%. If the carbon content is too low, it will not meet the basic strength requirements of spring steel. By controlling the carbon content at 0.4% to 0.5%, it is beneficial to achieve a balance between the strength and toughness of spring steel.

[0010] With the Si content controlled between 1.8% and 2.0%, Si primarily enhances the matrix strength through substitutional solid solution strengthening, while the dense SiO2 layer generated by high-temperature oxidation improves corrosion resistance. By controlling the Si content between 1.8% and 2.0%, corrosion resistance can be maintained while reducing ferrite brittleness caused by excessive Si (>2.0%).

[0011] The Mn content is controlled between 0.7% and 0.8%. Mn improves hardenability by expanding the austenite region, ensuring that the spring steel obtains a uniform martensitic structure across the entire cross-section. When the Mn content is higher than 0.8%, it easily causes grain coarsening and temper brittleness. When the Mn content is too low, it does not have sufficient hardenability.

[0012] The Cr content should be controlled between 2.0% and 2.5%. Cr can promote the formation of a relatively stable Cr-rich passivation film on the steel surface, thereby improving corrosion resistance. When the Cr content exceeds 2.0%, a continuous oxide film is formed, effectively blocking the penetration of corrosive ions such as Cl⁻. When the Cr content exceeds 2.5%, it can easily lead to excessive Cr-rich carbides, thereby reducing toughness.

[0013] With the Ni content controlled between 0.9% and 1.2%, Ni can form an austenite-ferrite dual-phase structure with Cr, thus blocking the propagation of corrosion cracks. Ni can also stabilize the austenite phase, improve low-temperature toughness, and synergistically inhibit the propagation of corrosion pits with Cu. By controlling the Ni content between 0.9% and 1.2%, cost increases can be controlled while maintaining corrosion resistance.

[0014] The Cu content is controlled between 0.2% and 0.4%. Cu improves corrosion resistance by refining the interlamellar spacing of pearlite, especially enhancing resistance to seasonal corrosion. However, excessive Cu content can easily cause Cu embrittlement, so it needs to be added to the steel along with Ni.

[0015] With the V content controlled at 0.1%~0.2%, V forms V(C,N) nano-precipitates with C and N during rolling. The V(C,N) particles pin the grain boundaries and inhibit recrystallization, resulting in a grain refinement rate of 50%. By controlling V at 0.1%~0.2%, high-temperature creep strength can be improved while coarsening of the precipitates can be avoided.

[0016] With a W content controlled between 0.15% and 0.40%, W can improve the stability, density, and repassivation ability of the passivation film on the steel surface, enhance the film's resistance to chloride ion corrosion, and thus improve the material's resistance to pitting and crevice corrosion. When the W content is below 0.15%, the improvement in corrosion resistance is not significant; when it is above 0.40%, it easily leads to segregation and carbide coarsening, increasing the steel's embrittlement tendency and hindering the improvement of overall performance.

[0017] The La content is controlled between 0.01% and 0.03%. La can purify molten steel, modify inclusions, reduce harmful sulfides, and improve the uniformity of the microstructure. It can also reduce the preferential nucleation sites for local corrosion and help form a more uniform and dense surface passivation film. When the La content is below 0.01%, the purification and modification effects are limited. When it is above 0.03%, coarse rare earth inclusions are easily formed, which is not conducive to fatigue and corrosion resistance stability.

[0018] The Nb content is controlled at 0.02%~0.04%. Nb refines the original austenite grains by forming NbC and NbN, thereby improving strength and resistance to hydrogen-induced cracking. The Nb content is controlled at 0.02%~0.04% to avoid the formation of harmful Laves phase with high chromium content.

[0019] The spring steel of this invention is based on elements such as C, Si, Mn, V, and Nb, and is compounded with elements such as Cr, Ni, Cu, W, and La to achieve passivation and refinement of the microstructure, thereby synergistically improving the corrosion resistance of the spring steel, while also taking into account the strength, elastic limit, hardenability, and tempering stability of the spring steel.

[0020] As a preferred embodiment, C is 0.42%~0.45%.

[0021] As a preferred embodiment, W is 0.20%~0.30%.

[0022] As a preferred embodiment, the La content is 0.012%~0.020%.

[0023] As a preferred embodiment, the mechanical properties of the high-strength and high-corrosion-resistant spring steel are as follows: tensile strength ≥1800 MPa, yield strength ≥1500 MPa, reduction of area ≥40%, elongation after fracture ≥10%, impact energy KU2 ≥20 J, and corrosion rate ≤0.8 g / m²·h in a neutral salt spray test for 240 h.

[0024] In a second aspect, the present invention provides a method for preparing high-strength, high-corrosion-resistant spring steel as described in any one of the embodiments of the first aspect above, comprising the following steps: molten iron pretreatment, molten iron ladle heating, converter smelting, LF refining, RH vacuum degassing, continuous casting, slow cooling, rolling, and heat treatment;

[0025] Hot metal pretreatment: KR desulfurization process is adopted, and S is controlled to be ≤0.005%;

[0026] Heating the molten iron ladle: Cover the molten iron ladle and heat it to ensure that the temperature of the molten iron is ≥1490℃;

[0027] Converter smelting: control the final C value ≥ 0.08%, and the tapping temperature ≥ 1650℃;

[0028] LF refining: Deoxidation and alloying are carried out by adding rare earth alloys and deoxidizers;

[0029] RH vacuum degassing: control the vacuum degree ≤67Pa, and maintain the vacuum cycle for ≥20min;

[0030] Continuous casting: Use low superheat pouring, superheat 15~20℃; use crystallizer electromagnetic stirring, end electromagnetic stirring and dynamic light pressure of billet;

[0031] Slow cooling: After slow cooling, the billet is heated, and the temperature of the soaking zone is controlled at 1210~1250℃, with a furnace time of 150~180min;

[0032] Rolling: The initial rolling temperature of roughing is 1000~1050℃, and the reduction rate is 40%~50%; the initial rolling temperature of finishing is 880~900℃, and the final rolling temperature is 760~800℃.

[0033] Heat treatment: Quenching and tempering process is adopted.

[0034] As a preferred embodiment, RH vacuum degassing controls the O content in the molten steel to be ≤0.0015% and the H content to be ≤0.0001%.

[0035] This invention utilizes vacuum circulation to dehydrogenate and reduce total oxygen. By controlling the hydrogen content, white spot defects can be eliminated, hydrogen embrittlement and delayed cracking can be prevented, thereby improving the internal quality of the steel. Oxygen easily forms oxide inclusions in steel; the higher the oxygen content, the more inclusions there are. By controlling the oxygen content, non-metallic inclusions can be reduced, purity can be improved, and the impact toughness, elongation, and reduction of area of ​​the steel can be enhanced, fatigue life can be increased, and the processing and use performance of the steel can be improved. This, in turn, increases the alloy yield, reduces the cost of subsequent heat treatment, flaw detection and repair, and lowers the scrap rate.

[0036] As a preferred embodiment, the billet pulling speed is controlled at 1.2~1.5m / min during continuous casting, the secondary cooling water ratio is 0.25~0.45L / kg, and the billet surface temperature is ≥900℃.

[0037] This invention controls the billet casting speed to 1.2~1.5m / min during continuous casting, with a matching secondary cooling water ratio of 0.25~0.45L / kg. This helps ensure the thickness of the billet shell within the crystallizer. Excessive casting speed results in a thinner shell, easily leading to steel leakage; insufficient casting speed causes low production efficiency and surface cracks in the billet shell. Excessive water ratio causes rapid cooling of the billet surface, leading to a sudden drop in surface temperature and potential cracking; insufficient water ratio results in inadequate cooling and insufficient shell strength. By controlling the secondary cooling water ratio, surface cracks in the billet can be prevented, solidification rate can be controlled, internal quality can be improved, and water consumption can be saved. Controlling the billet surface temperature reduces corner cracks and longitudinal surface cracks, ensuring uniform and stable billet quality.

[0038] As a preferred embodiment, the quenching temperature is 880~920℃, the holding time is 30~35min, and the oil cooling is used; the tempering temperature is 380~400℃, the holding time is 120~130min, and the water cooling is used to room temperature.

[0039] Compared with the prior art, the advantages and positive effects of the present invention are:

[0040] 1. This invention constructs a basic corrosion-resistant system based on Cr, Ni, and Cu. Cr helps form a relatively stable Cr-rich passivation film on the steel surface, Ni helps improve the electrochemical stability of the material and reduce the tendency for localized corrosion propagation, and Cu helps improve the material's corrosion resistance in humid and chloride-containing environments. Based on this, the addition of W further enhances the stability, density, and repassivation ability of the surface passivation film, strengthens the passivation film's resistance to corrosive ions such as Cl ions, thereby effectively inhibiting the initiation and propagation of localized corrosion such as pitting and crevice corrosion, and improving the material's resistance to localized corrosion in salt spray and chloride-containing media environments.

[0041] 2. By adding trace amounts of La, this invention can purify molten steel, reduce impurities and improve the uniformity of the microstructure during the smelting process, reduce the adverse effects of harmful inclusions and micro-area segregation on corrosion resistance and fatigue performance, and promote the formation of a more uniform, continuous and dense surface passivation film, thereby improving the material's pitting corrosion resistance, passivation film stability and overall corrosion resistance.

[0042] 3. In this invention, W and La have a synergistic effect. La mainly reduces the tendency of local corrosion initiation by improving the cleanliness of molten steel, improving the morphology of inclusions and the uniformity of the structure. W mainly inhibits corrosion propagation by improving the stability of the passivation film and the ability to repassivate. After the two work together, the corrosion rate in the neutral salt spray environment can be further reduced while maintaining the high strength and good ductility of the material, thereby improving the overall service reliability and service life of the material. Detailed Implementation

[0043] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0044] It should be noted that in the description of this invention, terms such as "upper," "lower," "left," "right," "vertical," "horizontal," "inner," and "outer," which indicate directional or positional relationships, are based on directional or positional relationships and are used merely for ease of description. They do not indicate or imply that the device or element must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as limiting the invention. Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0045] In this invention, unless otherwise explicitly specified and limited, the terms "installation," "connection," "linking," and "fixing," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0046] In this invention, unless otherwise explicitly specified and limited, "above" or "below" the second feature can include direct contact between the first and second features, or contact between the first and second features through another feature between them. Furthermore, "above," "over," and "on top" of the second feature includes the first feature directly above or diagonally above the second feature, or simply indicates that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature includes the first feature directly below or diagonally below the second feature, or simply indicates that the first feature is at a lower horizontal level than the second feature.

[0047] The following disclosure provides many different embodiments or examples for implementing various structures of the invention. To simplify the disclosure, specific examples of components and arrangements are described below. These are merely examples and are not intended to limit the invention. Furthermore, reference numerals and / or letters may be repeated in different examples; such repetition is for simplification and clarity and does not in itself indicate a relationship between the various embodiments and / or arrangements discussed. In addition, examples of various specific processes and materials are provided in this invention, but those skilled in the art will recognize the application of other processes and / or the use of other materials.

[0048] In a first aspect, embodiments of this disclosure provide a high-strength, high-corrosion-resistant spring steel comprising, by weight percentage: C: 0.4%~0.5%, Si: 1.8%~2.0%, Mn: 0.7%~0.8%, Cr: 2.0%~2.5%, Ni: 0.9%~1.2%, Cu: 0.2%~0.4%, V: 0.1%~0.2%, W: 0.15%~0.40%, La: 0.01%~0.03%, Nb: 0.02%~0.04%, N≤0.006%, S≤0.005%, P≤0.005%, with the balance being Fe and unavoidable impurity elements.

[0049] Preferably, C: 0.42%~0.45%.

[0050] Preferably, W: 0.20%~0.30%.

[0051] Preferably, La: 0.012%~0.020%.

[0052] Preferably, the mechanical properties of high-strength and high-corrosion-resistant spring steel are as follows: tensile strength ≥1800 MPa, yield strength ≥1500 MPa, reduction of area ≥40%, elongation after fracture ≥10%, impact energy KU2 ≥20 J, and corrosion rate ≤0.8 g / m²·h in neutral salt spray test for 240 h.

[0053] In a second aspect, the present disclosure provides a method for preparing high-strength, high-corrosion-resistant spring steel as described in any of the embodiments of the first aspect above, comprising the following steps: hot metal pretreatment, hot metal ladle heating, converter smelting, LF refining, RH vacuum degassing, continuous casting, slow cooling, rolling, and heat treatment;

[0054] Hot metal pretreatment: KR desulfurization process is adopted, and S is controlled to be ≤0.005%;

[0055] Heating the molten iron ladle: Cover the molten iron ladle and heat it to ensure that the temperature of the molten iron is ≥1490℃;

[0056] Converter smelting: control the final C value ≥ 0.08%, and the tapping temperature ≥ 1650℃;

[0057] LF refining: Deoxidation and alloying are carried out by adding rare earth alloys and deoxidizers;

[0058] RH vacuum degassing: control the vacuum degree ≤67Pa, and maintain the vacuum cycle for ≥20min;

[0059] Continuous casting: Use low superheat pouring, superheat 15~20℃; use crystallizer electromagnetic stirring, end electromagnetic stirring and dynamic light pressure of billet;

[0060] Slow cooling: After slow cooling, the billet is heated, and the temperature of the soaking zone is controlled at 1210~1250℃, with a furnace time of 150~180min;

[0061] Rolling: The initial rolling temperature of roughing is 1000~1050℃, and the reduction rate is 40%~50%; the initial rolling temperature of finishing is 880~900℃, and the final rolling temperature is 760~800℃.

[0062] Heat treatment: Quenching and tempering process is adopted.

[0063] Preferably, the RH vacuum degassing controls the O content in the molten steel to be ≤0.0015% and the H content to be ≤0.0001%.

[0064] Preferably, during continuous casting, the billet pulling speed is controlled at 1.2~1.5m / min, the secondary cooling water ratio is 0.25~0.45L / kg, and the billet surface temperature is ≥900℃.

[0065] Preferably, the quenching temperature is 880~920℃, the holding time is 30~35min, and the oil cooling is used; the tempering temperature is 380~400℃, the holding time is 120~130min, and the water cooling is used to room temperature.

[0066] Example 1:

[0067] A high-strength, high-corrosion-resistant spring steel comprises, by weight percentage: C: 0.40%, Si: 1.8%, Mn: 0.77%, Cr: 2.03%, Ni: 0.9%, Cu: 0.31%, V: 0.12%, W: 0.2%, La: 0.012%, Nb: 0.02%, with the balance being Fe and unavoidable impurity elements.

[0068] The method for preparing high-strength, high-corrosion-resistant spring steel according to the embodiments of this disclosure includes the following steps:

[0069] (1) Hot metal pretreatment: KR desulfurization process is adopted. The hot metal is stirred in the hot metal ladle to form a vortex. The desulfurizing agent, active lime and magnesium powder, is added twice to desulfurize the hot metal and control the S of the hot metal leaving the station to be ≤0.005%;

[0070] (2) Heating of molten iron ladle: The molten iron ladle is covered and electromagnetic induction heating is used to ensure that the temperature of molten iron is ≥1490℃ during the process of transporting molten iron from the ironmaking plant to the steelmaking plant;

[0071] (3) Converter smelting: control the endpoint C≥0.08%, and the tapping temperature≥1650℃;

[0072] (4) LF refining: After melting the slag, LF refining is carried out, and rare earth alloys and deoxidizers are added for deoxidation and alloying.

[0073] (5) RH vacuum degassing: control the vacuum degree to 50Pa, the vacuum cycle holding time to 20min, and control the O in the molten steel to ≤0.0015% and H to ≤0.0001%; ​​after the RH process is completed, the chemical composition of the molten steel should meet the requirements;

[0074] (6) Continuous casting: Low superheat casting is used in the continuous casting process, with a superheat of 15~20℃; an integral tundish is used to reduce secondary oxidation; electromagnetic stirring of the crystallizer and electromagnetic stirring at the end are used in conjunction with dynamic light pressure of the billet to ensure the quality of the billet; the billet pulling speed is controlled at 1.5m / min, the secondary cooling water ratio is 0.25L / kg, and the billet surface temperature is ≥900℃;

[0075] (7) Rolling: The roughing rolling temperature is 1050℃ and the reduction rate is 40%~50%; the finishing rolling temperature is 890℃ and the finishing rolling temperature is 780℃.

[0076] (8) Heat treatment: Quenching + tempering process is adopted: Quenching temperature is 880℃, holding time is 30~35min, oil cooling; Tempering temperature is 380℃, holding time is 120~130min, water cooling to room temperature.

[0077] Example 2:

[0078] A high-strength, high-corrosion-resistant spring steel comprises, by weight percentage: C: 0.42%, Si: 1.83%, Mn: 0.74%, Cr: 2.1%, Ni: 1.1%, Cu: 0.25%, V: 0.1%, W: 0.25%, La: 0.015%, Nb: 0.03%, with the balance being Fe and unavoidable impurity elements.

[0079] The method for preparing high-strength, high-corrosion-resistant spring steel according to the embodiments of this disclosure includes the following steps:

[0080] (1) Hot metal pretreatment: KR desulfurization process is adopted. The hot metal is stirred in the hot metal ladle to form a vortex. The desulfurizing agent, active lime and magnesium powder, is added twice to desulfurize the hot metal and control the S of the hot metal leaving the station to be ≤0.005%;

[0081] (2) Heating of molten iron ladle: The molten iron ladle is covered and electromagnetic induction heating is used to ensure that the temperature of molten iron is ≥1490℃ during the process of transporting molten iron from the ironmaking plant to the steelmaking plant;

[0082] (3) Converter smelting: control the endpoint C≥0.08%, and the tapping temperature≥1650℃;

[0083] (4) LF refining: After melting the slag, LF refining is carried out, and rare earth alloys and deoxidizers are added for deoxidation and alloying.

[0084] (5) RH vacuum degassing: control the vacuum degree to 50Pa, the vacuum cycle holding time to 25min, and control the O in the molten steel to ≤0.0015% and H to ≤0.0001%; ​​after the RH process is completed, the chemical composition of the molten steel should meet the requirements;

[0085] (6) Continuous casting: Low superheat casting is used in the continuous casting process, with a superheat of 15~20℃; an integral tundish is used to reduce secondary oxidation; electromagnetic stirring of the crystallizer and electromagnetic stirring at the end are used in conjunction with dynamic light pressure of the billet to ensure the quality of the billet; the billet pulling speed is controlled at 1.2m / min, the secondary cooling water ratio is 0.45L / kg, and the billet surface temperature is ≥900℃;

[0086] (7) Rolling: The roughing rolling temperature is 1050℃ and the reduction rate is 40%~50%; the finishing rolling temperature is 880℃ and the finishing rolling temperature is 760℃.

[0087] (8) Heat treatment: Quenching + tempering process is adopted: Quenching temperature is 880℃, holding time is 35min, oil cooling; tempering temperature is 380℃, holding time is 130min, water cooling to room temperature.

[0088] Example 3:

[0089] A high-strength, high-corrosion-resistant spring steel comprises, by weight percentage: C: 0.45%, Si: 1.91%, Mn: 0.71%, Cr: 2.4%, Ni: 0.9%, Cu: 0.39%, V: 0.2%, W: 0.30%, La: 0.02%, Nb: 0.03%, with the balance being Fe and unavoidable impurity elements.

[0090] The method for preparing high-strength, high-corrosion-resistant spring steel according to the embodiments of this disclosure includes the following steps:

[0091] (1) Hot metal pretreatment: KR desulfurization process is adopted. The hot metal is stirred in the hot metal ladle to form a vortex. The desulfurizing agent, active lime and magnesium powder, is added twice to desulfurize the hot metal and control the S of the hot metal leaving the station to be ≤0.005%;

[0092] (2) Heating of molten iron ladle: The molten iron ladle is covered and electromagnetic induction heating is used to ensure that the temperature of molten iron is ≥1490℃ during the process of transporting molten iron from the ironmaking plant to the steelmaking plant;

[0093] (3) Converter smelting: control the steel output C ≥ 0.08% and the steel output temperature ≥ 1650℃;

[0094] (4) LF refining: After melting the slag, LF refining is carried out, and rare earth alloys and deoxidizers are added for deoxidation and alloying.

[0095] (5) RH vacuum degassing: control the vacuum degree to 50Pa, the pure degassing time to 20min, and control the O in the molten steel to ≤0.0015% and H to ≤0.0001%; ​​after the RH process is completed, the chemical composition of the molten steel should meet the requirements;

[0096] (6) Continuous casting: Low superheat casting is used in the continuous casting process, with a superheat of 15~20℃; an integral tundish is used to reduce secondary oxidation; electromagnetic stirring of the crystallizer and electromagnetic stirring at the end are used in conjunction with dynamic light pressure of the billet to ensure the quality of the billet; the billet pulling speed is controlled at 1.2m / min, the secondary cooling water ratio is 0.45L / kg, and the billet surface temperature is ≥900℃;

[0097] (7) Rolling: The roughing rolling temperature is 1050℃ and the reduction rate is 40%~50%; the finishing rolling temperature is 900℃ and the finishing rolling temperature is 800℃.

[0098] (8) Heat treatment: Quenching + tempering process is adopted: Quenching temperature is 920℃, holding time is 30min, oil cooling; tempering temperature is 400℃, holding time is 120min, water cooling to room temperature.

[0099] Example 4:

[0100] A high-strength, high-corrosion-resistant spring steel comprises, by weight percentage: C: 0.44%, Si: 1.88%, Mn: 0.73%, Cr: 2.2%, Ni: 1.2%, Cu: 0.31%, V: 0.13%, W: 0.22%, La: 0.018%, Nb: 0.02%, with the balance being Fe and unavoidable impurity elements.

[0101] The method for preparing high-strength, high-corrosion-resistant spring steel according to the embodiments of this disclosure includes the following steps:

[0102] (1) Hot metal pretreatment: KR desulfurization process is adopted. The hot metal is stirred in the hot metal ladle to form a vortex. The desulfurizing agent, active lime and magnesium powder, is added twice to desulfurize the hot metal and control the S of the hot metal leaving the station to be ≤0.005%;

[0103] (2) Heating of molten iron ladle: The molten iron ladle is covered and electromagnetic induction heating is used to ensure that the temperature of molten iron is ≥1490℃ during the process of transporting molten iron from the ironmaking plant to the steelmaking plant;

[0104] (3) Converter smelting: control the steel output C ≥ 0.08% and the steel output temperature ≥ 1650℃;

[0105] (4) LF refining: After melting the slag, LF refining is carried out, and rare earth alloys and deoxidizers are added for deoxidation and alloying.

[0106] (5) RH vacuum degassing: control the vacuum degree to 50Pa, the pure degassing time to 20min, and control the O in the molten steel to ≤0.0015% and H to ≤0.0001%; ​​after the RH process is completed, the chemical composition of the molten steel should meet the requirements;

[0107] (6) Continuous casting: Low superheat casting is used in the continuous casting process, with a superheat of 15~20℃; an integral tundish is used to reduce secondary oxidation; electromagnetic stirring of the crystallizer and electromagnetic stirring at the end are used in conjunction with dynamic light pressure of the billet to ensure the quality of the billet; the billet pulling speed is controlled at 1.5m / min, the secondary cooling water ratio is 0.25L / kg, and the billet surface temperature is ≥900℃;

[0108] (7) Rolling: The roughing rolling temperature is 1050℃ and the reduction rate is 40%~50%; the finishing rolling temperature is 900℃ and the finishing rolling temperature is 800℃.

[0109] (8) Heat treatment: Quenching + tempering process is adopted: Quenching temperature is 900℃, holding time is 30min, oil cooling; tempering temperature is 400℃, holding time is 120min, water cooling to room temperature.

[0110] Comparative Example 1:

[0111] The spring steel comprises, by weight percentage: C: 0.40%, Si: 1.82%, Mn: 0.71%, Cr: 2.3%, Ni: 1.1%, Cu: 0.37%, V: 0.2%, Nb: 0.03%, with the balance being Fe and unavoidable impurity elements. Its preparation method is the same as in Example 1.

[0112] Comparative Example 2:

[0113] The spring steel, by weight percentage, comprises: C: 0.42%, Si: 1.84%, Mn: 0.78%, Cr: 2.3%, Ni: 1.1%, Cu: 0.25%, V: 0.12%, W: 0.25%, Nb: 0.03%, with the balance being Fe and unavoidable impurity elements. Its preparation method is the same as in Example 2.

[0114] Comparative Example 3:

[0115] The spring steel, by weight percentage, comprises: C: 0.41%, Si: 1.8%, Mn: 0.73%, Cr: 2.2%, Ni: 0.9%, Cu: 0.26%, V: 0.16%, La: 0.015%, Nb: 0.04%, with the balance being Fe and unavoidable impurity elements. Its preparation method is the same as in Example 3.

[0116] Comparative Example 4:

[0117] The spring steel, by weight percentage, comprises: C: 0.45%, Si: 1.83%, Mn: 0.71%, Cr: 2.3%, Ni: 1.0%, Cu: 0.29%, V: 0.1%, W: 0.1%, La: 0.008%, Nb: 0.03%, with the balance being Fe and unavoidable impurity elements. Its preparation method is the same as in Example 4.

[0118] Comparative Example 5:

[0119] The spring steel, by weight percentage, comprises: C: 0.43%, Si: 1.85%, Mn: 0.77%, Cr: 2.5%, Ni: 1.2%, Cu: 0.31%, V: 0.19%, W: 0.25%, La: 0.040%, Nb: 0.02%, with the balance being Fe and unavoidable impurity elements. Its preparation method is the same as in Example 4.

[0120] The mechanical properties of the spring steels from Examples 1-4 and Comparative Examples 1-5 were tested, and the results are shown in Table 1 below:

[0121] Table 1: Mechanical properties of spring steel in Examples 1-4 and Comparative Examples 1-5

[0122]

[0123] As shown in Table 1, in Examples 1-4, under the Cr-Ni-Cu basic corrosion-resistant system, the addition of W and La resulted in spring steels with tensile strengths of 2110-2170 MPa, yield strengths of 1840-1878 MPa, and impact energy of 21-24 J, all higher than the tensile strengths (2028-2105 MPa), yield strengths (1805-1832 MPa), and impact energy (18-21 J) of Comparative Examples 1-5. This indicates that the high-strength, high-corrosion-resistant spring steel prepared using the method of this application can maintain high strength and good ductility and toughness. Comparative Example 1, without the addition of W and La, had a corrosion rate of 1.05 g / m²·h, while Examples 1-4 had corrosion rates of 0.68-0.75 g / m²·h, indicating that the addition of W and La can significantly improve corrosion resistance. To improve the corrosion resistance of spring steel, the corrosion rate of Comparative Example 2 was 0.88 g / m²·h. Comparative Example 2 added W but not La, indicating that adding W alone can reduce the corrosion rate by improving the stability and repassivation ability of the passivation film. The corrosion rate of Comparative Example 3 was 0.92 g / m²·h. La was added but no W was added, indicating that adding La alone can improve corrosion resistance stability by purifying the molten steel, removing impurities, and improving the uniformity of the microstructure. The amounts of W and La added in Comparative Example 4 were lower than those in Examples 1-4, while the amounts of W and La added in Comparative Example 5 were higher than those in Examples 1-4. However, the performance of the spring steel in Comparative Examples 4 and 5 was worse than that in Examples 1-4, indicating that controlling the content of W and La within a reasonable range helps maintain the overall performance of the spring steel.

[0124] Therefore, compared with the prior art, the beneficial effects of the present invention are as follows: The present invention constructs a basic corrosion-resistant system by increasing the content of Cr, Ni, and Cu elements. Cr helps to form a relatively stable Cr-rich passivation film on the steel surface, Ni helps to improve the electrochemical stability of the material and reduce the tendency for localized corrosion propagation, and Cu helps to improve the corrosion resistance of the material in humid and chloride-containing environments. Based on this, the addition of W further improves the stability, density, and repassivation ability of the surface passivation film, enhances the resistance of the passivation film to corrosive ions such as Cl ions, thereby effectively inhibiting the initiation and propagation of localized corrosion such as pitting and crevice corrosion, and improving the material's resistance to localized corrosion in salt spray and chloride-containing media environments.

[0125] This invention, by adding trace amounts of La, can purify molten steel, reduce impurities and improve the uniformity of the microstructure during the smelting process. It reduces the adverse effects of harmful inclusions and micro-segregation on corrosion resistance and fatigue performance, and promotes the formation of a more uniform, continuous and dense surface passivation film, thereby improving the material's pitting corrosion resistance, passivation film stability and overall corrosion resistance.

[0126] In this invention, W and La have a synergistic effect. La mainly reduces the tendency of local corrosion initiation by improving the cleanliness of molten steel, improving the morphology of inclusions and the uniformity of the microstructure, while W mainly inhibits corrosion propagation by improving the stability of the passivation film and the ability to repassivate. After the two work together, the corrosion rate in the neutral salt spray environment can be further reduced while maintaining the high strength and good ductility of the material, thereby improving the overall service reliability and service life of the material.

[0127] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A high-strength, high-corrosion-resistant spring steel, characterized in that, The composition by weight percentage is as follows: C: 0.4%~0.5%, Si: 1.8%~2.0%, Mn: 0.7%~0.8%, Cr: 2.0%~2.5%, Ni: 0.9%~1.2%, Cu: 0.2%~0.4%, V: 0.1%~0.2%, W: 0.15%~0.40%, La: 0.01%~0.03%, Nb: 0.02%~0.04%, N≤0.006%, S≤0.005%, P≤0.005%, with the balance being Fe and unavoidable impurity elements.

2. The high-strength, high-corrosion-resistant spring steel according to claim 1, characterized in that, The C content is 0.42%~0.45%.

3. The high-strength, high-corrosion-resistant spring steel according to claim 1, characterized in that, The W content is 0.20%~0.30%.

4. The high-strength, high-corrosion-resistant spring steel according to claim 1, characterized in that, The La content is 0.012%~0.020%.

5. The high-strength, high-corrosion-resistant spring steel according to claim 1, characterized in that, The mechanical properties of the high-strength and high-corrosion-resistant spring steel are as follows: tensile strength ≥1800 MPa, yield strength ≥1500 MPa, reduction of area ≥40%, elongation after fracture ≥10%, impact energy KU2 ≥20 J, and corrosion rate ≤0.8 g / m²·h in neutral salt spray test for 240 h.

6. A method for preparing high-strength, high-corrosion-resistant spring steel as described in any one of claims 1 to 5, characterized in that, The process includes the following steps: molten iron pretreatment, molten iron ladle heating, converter smelting, LF refining, RH vacuum degassing, continuous casting, slow cooling, rolling, and heat treatment. Hot metal pretreatment: KR desulfurization process is adopted, and S is controlled to be ≤0.005%; Heating the molten iron ladle: Cover the molten iron ladle and heat it to ensure that the temperature of the molten iron is ≥1490℃; Converter smelting: control the final C value ≥ 0.08%, and the tapping temperature ≥ 1650℃; LF refining: Deoxidation and alloying are carried out by adding rare earth alloys and deoxidizers; RH vacuum degassing: control the vacuum degree ≤67Pa, and maintain the vacuum cycle for ≥20min; Continuous casting: Use low superheat pouring, superheat 15~20℃; use crystallizer electromagnetic stirring, end electromagnetic stirring and dynamic light pressure of billet; Slow cooling: After slow cooling, the billet is heated, and the temperature of the soaking zone is controlled at 1210~1250℃, with a furnace time of 150~180min; Rolling: The initial rolling temperature of roughing is 1000~1050℃, and the reduction rate is 40%~50%; the initial rolling temperature of finishing is 880~900℃, and the final rolling temperature is 760~800℃. Heat treatment: Quenching and tempering process is adopted.

7. The method for preparing high-strength, high-corrosion-resistant spring steel according to claim 6, characterized in that, RH vacuum degassing controls the O content in molten steel to be ≤0.0015% and H content to be ≤0.0001%.

8. The method for preparing high-strength, high-corrosion-resistant spring steel according to claim 6, characterized in that, During continuous casting, the billet pulling speed is controlled at 1.2~1.5m / min, the secondary cooling water ratio is 0.25~0.45L / kg, and the billet surface temperature is ≥900℃.

9. The method for preparing high-strength, high-corrosion-resistant spring steel according to claim 6, characterized in that, The quenching temperature is 880~920℃, the holding time is 30~35min, and the oil is used for cooling; the tempering temperature is 380~400℃, the holding time is 120~130min, and the water is used for cooling to room temperature.

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