Spring round steel with high strength, high toughness and long fatigue life and production method thereof

Abstract

The invention discloses spring round steel with high strength, high toughness and long fatigue life and a production method of the spring round steel, and belongs to the technical field of spring round steel production, the key of the chemical components of the spring round steel is that the spring round steel comprises 0.90-1.10% of Ni and 0.13-0.17% of V, and the content of harmful elements such as O, N, S and P is strictly controlled. Ultra-low oxygen content and extremely high purity are ensured by adopting VD vacuum refining, surface decarburization is effectively prevented by applying argon and vacuum atmosphere protection heating, non-recrystallization region low-temperature rolling is implemented to refine grains, and an optimized quenching and medium-temperature tempering heat treatment system is matched. The technical bottlenecks that the strength and toughness of traditional spring round steel are difficult to consider, and the fatigue life is easily influenced by inclusions and decarburized layers are successfully solved, the final product still has excellent impact toughness and extremely long fatigue life when the tensile strength reaches the ultra-high strength level of 1900 MPa or above, the comprehensive performance is remarkably superior to that of an existing product, and the production cost is reduced. And the harsh requirements of automobile light weight on high-performance spring materials are met.

Description

Technical Field

[0001] This invention relates to the field of spring round steel production technology, and in particular to a spring round steel with high strength, high toughness, and long fatigue life, and its production method. Background Technology

[0002] Spring round steel is widely used in automobiles, mainly for manufacturing various types of springs, including suspension springs, shock absorber springs, and transmission system springs. The harsh working environment and high elasticity of springs dictate that spring round steel must possess high yield strength and tensile strength. In the context of automotive lightweighting, further improving the strength of spring round steel is an effective means to achieve this goal, leading to extensive research. For example, patent CN107653417A, entitled "Low-Temperature Rolled High-Strength Spring Round Steel and Its Preparation Method," describes a process that obtains high-strength spring round steel by adding 0.4-0.6% C, 0.2-0.4% Si, 0.7-1.0% Mn, 0.9-1.1% Cr, and 0.10-0.15% V by mass, combined with an initial rolling temperature of 1100-1150℃, a final rolling temperature of 700-850℃, and post-rolling air cooling. The patent with publication number CN101928892A, entitled "A Spring Steel Round Bar and Its Production Process", describes how a high-strength spring steel round bar is obtained by adding 0.56~0.64% C, 1.60~2.00% Si, 0.70~1.00% Mn, and 0.15~0.35% Cr by mass fraction and then through processes such as smelting, continuous casting, and rolling.

[0003] Many spring round steel production technologies improve yield strength and tensile strength by adding alloying elements and combining them with conventional production processes. However, this strength increase inevitably leads to a decrease in impact toughness. Fatigue life is one of the most important technical indicators for evaluating spring steel. High fatigue life requires spring round steel to possess not only high strength but also high toughness. Furthermore, large inclusions and decarburized layer depths exceeding 0.2 mm significantly reduce the fatigue life of spring round steel. Therefore, high fatigue life also places high demands on the inclusion level and decarburized layer depth of spring round steel. However, current spring round steel composition design and production technologies cannot guarantee both high strength and high toughness simultaneously, nor do they strictly control the inclusion level and decarburized layer depth; therefore, they cannot guarantee a high fatigue life for spring round steel. Summary of the Invention

[0004] The main objective of this invention is to provide a spring round steel with high strength, high toughness, and long fatigue life, and a method for producing the same, which can effectively solve the problems mentioned in the background art.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A spring round steel bar possessing high strength, high toughness, and long fatigue life has the following chemical composition by weight percentage: C: 0.43~0.47%, Si: 0.80~1.20%, Mn: 1.30~1.50%, S≤0.012%, P≤0.012%, Cr: 0.80~1.20%, V: 0.13~0.17%, Ni: 0.90~1.10%, O≤0.0010%, N≤0.0030%, The remainder consists of Fe and unavoidable impurities.

[0006] A method for producing spring round steel with high strength, high toughness, and long fatigue life includes the following steps: S1. During smelting, hot metal pretreatment, converter refining, LF furnace refining and VD furnace refining are adopted; S2. Continuous casting: During continuous casting, the pouring temperature is controlled at 15~30 ℃ above the liquidus temperature, and electromagnetic stirring is used. The billet pulling speed during continuous casting is 0.6~0.8 m / min. S3, Hot rolling: A process route using low heating temperature and low final rolling temperature during hot rolling; S4. Post-rolling cooling: After hot rolling, the spring round steel is placed in a slow cooling pit for cooling, with a cooling rate of less than or equal to 0.05℃ / s; S5. Heat treatment: Quenching and medium-temperature tempering are performed on the hot-rolled and cooled spring round steel.

[0007] Preferably, after pretreatment of the molten iron in S1, the S and P contents in the molten iron should be less than or equal to 0.012% to ensure that the S and P contents in the final spring round steel meet the composition requirements. The converter steelmaking adopts a top-and-bottom blowing process, with a converter endpoint C ≥ 0.12% and a tapping temperature ≥ 1650℃. Refining is carried out using an LF furnace and a VD furnace, with aluminum deoxidation. After refining, the mass fraction of all elements should be adjusted to the target range. After refining, all types of inclusions should meet the following requirements: Class A inclusions should be ≤ 1, Class B inclusions should be ≤ 0.5, Class C inclusions should be ≤ 1, and Class D inclusions should be ≤ 0.5.

[0008] Preferably, in step S2, the billet is slowly cooled after continuous casting, with a cooling time of ≥24 h.

[0009] Preferably, the temperature of the heating furnace in S3 is 980~1020 ℃, the furnace is in an argon protective atmosphere, the argon pressure is slightly greater than atmospheric pressure, and after holding at the temperature for 2 hours, it is immediately taken out of the furnace for rolling, and the final rolling temperature is controlled at 770~790 ℃.

[0010] Preferably, the heating temperature during quenching in S5 is 850~870 ℃, and the heating furnace adopts a vacuum atmosphere; after holding at the temperature for 1 hour, it is oil-cooled to room temperature; the temperature during medium-temperature tempering is 360~380 ℃, and after holding at the temperature for 2 hours, it is water-cooled to room temperature.

[0011] Compared with the prior art, the present invention has the following beneficial effects: This invention innovatively employs an alloy composition system containing 0.90~1.10% nickel (Ni) and 0.13~0.17% vanadium (V), and closely integrates a complete production process centered on VD vacuum refining to control ultra-low oxygen content, argon / vacuum atmosphere protection to prevent decarburization, low-temperature rolling in the non-recrystallization zone to refine grains, and optimized quenching and tempering heat treatment. This successfully solves the industry problem of insufficient toughness and fatigue life of high-strength spring round steel, ultimately enabling the product to have a tensile strength of over 1900MPa while still possessing excellent impact toughness and ultra-high fatigue life. Attached Figure Description

[0012] Figure 1 This is a schematic diagram of the production method of the present invention; Figure 2 The image shows the microstructure of the experimental steel used in this invention. Figure 3 This is a micrograph of the experimental steel of this invention. Detailed Implementation

[0013] To make the technical means, creative features, objectives, and effects of this invention easily understood, the invention is further described below in conjunction with specific embodiments. This invention aims to provide a spring round steel with high strength, high toughness, and long fatigue life, as well as a method for producing the same. Its core lies in using an optimized chemical composition system, combined with a precisely controlled process of pure steel smelting, low-temperature rolling, and special heat treatment, to achieve ultra-high strength while effectively ensuring the material's toughness and fatigue performance.

[0014] I. Chemical Composition Design of Spring Round Steel The chemical composition of the spring round steel of this invention, by weight percentage, is as follows: C: 0.43~0.47%, Si: 0.80~1.20%, Mn: 1.30~1.50%, S≤0.012%, P≤0.012%, Cr: 0.80~1.20%, V: 0.13~0.17%, Ni: 0.90~1.10%, O≤0.0010%, N≤0.0030%, with the remainder being Fe and unavoidable impurities.

[0015] The design principles and functions of each element are as follows: C (carbon): 0.43~0.47% Carbon (C) is the most critical element for ensuring the strength and hardness of steel. This invention controls the carbon content within a relatively high medium-carbon range, aiming to achieve an ultra-high tensile strength of over 1900 MPa through solid solution strengthening and the formation of sufficient carbides. A carbon content below 0.43% will result in insufficient strength; while a content above 0.47% will severely impair toughness and plasticity, and exacerbate the tendency for surface decarburization during rolling, having a fatal impact on subsequent fatigue life.

[0016] Si (silicon): 0.80~1.20% Si is an important solid solution strengthening element that can significantly improve the elastic limit and yield strength ratio of steel. More importantly, Si can inhibit the precipitation and growth of cementite during tempering, improve tempering stability, and thus significantly enhance the steel's resistance to elastic decay (elastic decay resistance). However, excessively high Si content (>1.20%) will promote surface decarburization and reduce toughness, so its upper limit needs to be controlled at 1.20%.

[0017] Mn (manganese): 1.30~1.50% Mn can improve the hardenability of steel, ensuring uniform mechanical properties across the entire cross-section. It can also combine with sulfur in steel to form MnS, eliminating the hot brittleness caused by sulfur. Appropriate amounts of Mn help reduce decarburization, but excessive amounts increase the steel's overheat sensitivity and temper brittleness; therefore, its upper limit is set at 1.50%.

[0018] Cr (chromium): 0.80~1.20% Cr significantly improves the hardenability, strength, and tempering stability of steel. It forms fine (Fe,Cr)3C carbides with C, resulting in excellent precipitation strengthening. Furthermore, Cr oxidizes internally before Fe, forming a dense Cr2O3 film on the surface, effectively slowing down the oxidation and decarburization process. Excessive Cr content, however, leads to the formation of coarse carbides, deteriorating toughness.

[0019] Vanadium (V): 0.13~0.17% V is a strong carbide-forming element. During post-rolling cooling and heat treatment, V precipitates fine, dispersed, and stable VC or V(C,N) particles. These nanoscale precipitates produce a strong precipitation strengthening effect, which is one of the keys to improving strength. At the same time, they can pin grain boundaries, inhibit austenite grain growth, and refine grains, thereby simultaneously improving strength and toughness.

[0020] Ni (nickel): 0.90~1.10% Ni is the key element for achieving high toughness in this invention. Ni, dissolved in ferrite, significantly improves the low-temperature impact toughness of the material and inhibits the initiation and propagation of microcracks. This is crucial for springs operating under high stress and forms the basis for ensuring high fatigue life. Simultaneously, Ni also improves the corrosion resistance of steel.

[0021] P, S, O, N (impurity elements): Extremely low control P and S: As harmful elements, P tends to agglomerate at grain boundaries, leading to intergranular embrittlement; S forms low-melting-point FeS, causing hot embrittlement. This invention strictly controls these elements to ≤0.012% through molten iron pretreatment and ladle refining.

[0022] O and N: O forms oxide inclusions, which are the core origin of fatigue cracks; dissolved N increases the brittleness of steel. This invention uses vacuum degassing (VD) and aluminum deoxidation to reduce the O content to below 10 ppm and the N content to below 30 ppm, greatly improving the purity of the steel.

[0023] II. Production Method of Spring Round Steel The production method of spring round steel of the present invention mainly includes the following steps: smelting, continuous casting, hot rolling, post-rolling cooling, and heat treatment.

[0024] (1) Smelting This step is crucial for controlling the purity of the components. The process flow is "molten iron pretreatment → converter primary refining → LF furnace refining → VD vacuum degassing".

[0025] Example: First, the blast furnace molten iron is pretreated for desulfurization and dephosphorization to ensure that the [S] and [P] content in the molten iron entering the converter is ≤0.012%.

[0026] Converter steelmaking: Top and bottom blowing process is adopted to enhance smelting efficiency. The final control is [C] ≥ 0.12% (to prevent over-oxidation), and the tapping temperature is ≥ 1650℃ to leave sufficient temperature space for subsequent refining. Aluminum deoxidation is used during tapping.

[0027] LF furnace refining: In the LF furnace, the molten steel is subjected to deep desulfurization and deoxidation through the production of white slag and argon stirring, and alloying fine-tuning is carried out to precisely adjust all elemental composition to the target range.

[0028] Vacuum Degassing (VD): Molten steel is suspended in a VD furnace and maintained at a vacuum of ≤67 Pa for 15-20 minutes to thoroughly remove [H], [O], and [N] gases from the molten steel. This step is crucial for obtaining ultra-low oxygen content (O ≤10 ppm).

[0029] Purity control: After refining, inclusions must be strictly graded, requiring the following: Class A (sulfides) ≤ Grade 1, Class B (aluminates) ≤ Grade 0.5, Class C (silicates) ≤ Grade 1, and Class D (spherical oxides) ≤ Grade 0.5. This is a prerequisite for ensuring high fatigue life.

[0030] (2) Continuous casting Fully protective casting is used to prevent secondary oxidation of the molten steel.

[0031] Control the superheat of continuous casting within a lower range of 15-30°C above the liquidus temperature to reduce center segregation.

[0032] Electromagnetic stirring technology is used to break up dendrites, increase the equiaxed crystal ratio, improve the solidification structure of the billet, and reduce compositional inhomogeneity.

[0033] Control the casting speed to 0.6~0.8 m / min to ensure solidification quality.

[0034] After continuous casting, the billet must be slowly cooled (≥24 hours) to eliminate internal stress and prevent cracks.

[0035] (3) Hot rolling The process of "low-temperature heating and low-temperature rolling" is adopted.

[0036] The continuously cast billet is heated at a low temperature of 980~1020℃ and held for 2 hours. Low-temperature heating can effectively prevent excessive growth of austenite grains, laying the foundation for obtaining a fine-grained structure. Argon gas (pressure slightly higher than atmospheric pressure) is introduced into the heating furnace to fundamentally prevent decarburization and iron oxide scale from forming on the billet during heating.

[0037] After holding at the heat, the material is quickly removed from the furnace and rolled, with the final rolling temperature strictly controlled between 770 and 790°C. This temperature is located in the non-recrystallization region of austenite. The rolling deformation will form a large number of high-density deformation bands and dislocations inside the austenite grains. These defects will become ferrite nucleation points during subsequent cooling phase transformation, thereby greatly refining the grains and significantly improving the impact toughness of the material.

[0038] (4) Cooling after rolling After hot rolling, instead of conventional air cooling or rapid cooling, the round steel is sent into a slow cooling pit for extremely slow cooling, with the cooling rate controlled at ≤0.05 ℃ / s.

[0039] Objective: To transform austenite into a pearlite + ferrite structure under near-equilibrium conditions, avoiding the formation of hard and brittle martensite or bainite, thereby significantly reducing hardness and facilitating subsequent machining (such as coiling springs).

[0040] (5) Heat treatment Heat treatment is the decisive step in obtaining the final performance, and the tempering process of "quenching + medium temperature tempering" is adopted.

[0041] Quenching: Heat and hold at 850~870℃ in a vacuum atmosphere furnace for 1 hour. The vacuum environment ensures zero decarburization of the round steel during austenitization. This heating temperature ensures partial dissolution of carbides such as V without coarsening the austenite grains, thus obtaining a uniform and fine austenitic structure. After holding, rapidly oil cool to obtain a high-strength martensitic structure.

[0042] Tempering: Temper at a relatively low temperature of 360~380℃ for 2 hours. The purpose of medium-temperature tempering is twofold: firstly, to allow fine ε-carbides to precipitate from the quenched martensite, eliminating internal stress and significantly improving toughness; secondly, to avoid excessive strength reduction caused by tempering at excessively high temperatures. Water cooling is used after tempering to prevent temper brittleness during slow cooling.

[0043] III. Implementation Examples and Performance To verify the effectiveness of the present invention, the following four embodiments are provided, and their specific components (Table 1), process parameters (Table 2), and final performance (Tables 3 and 4) are shown below.

[0044] Table 1 Chemical composition (mass fraction, %) of Examples 1-4

[0045] Table 2 Production process parameters for Examples 1-4

[0046] Table 3. Inclusion levels and decarburization layer depths of Examples 1-4

[0047] The results show that the purity (inclusion level) and surface quality (decarburized layer) of all embodiments are far superior to conventional technologies (typically decarburized layer >0.15mm), which provides a guarantee for high fatigue life.

[0048] Table 4 Mechanical properties of Examples 1-4

[0049] Performance Analysis: As shown in Table 4, the spring round steel prepared using this invention exhibits extremely excellent comprehensive performance: Ultra-high strength: Tensile strength is higher than 1900 MPa, and can reach up to 2000 MPa.

[0050] Excellent plasticity: Elongation (A) is above 10% and reduction of area (Z) is above 39%.

[0051] High toughness: The impact energy of the 2mm U-notch is much higher than 35 J, which reflects the great contribution of Ni and fine grain structure to toughness.

[0052] Ultra-high fatigue life: Under alternating stress of 1000±200 MPa, the fatigue life of all embodiments exceeded 10 million cycles (>10). 7 (and not broken.)

[0053] In summary, this invention, through a two-pronged approach of "composition design" and "process control," particularly the addition of Ni to enhance toughness, VD vacuum treatment to control purity, protective atmosphere heating to prevent decarburization, rolling in the non-recrystallization zone to refine grains, and optimized quenching and tempering heat treatment, successfully solves the industry problem of achieving both high strength and high toughness, and obtains an extremely long fatigue life, perfectly achieving the invention's objective.

[0054] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of this invention is defined by the appended claims and their equivalents.

Claims

1. A spring round steel bar possessing high strength, high toughness, and long fatigue life, characterized in that: Its chemical composition by weight percentage is as follows: C：0.43～0.47%， Si: 0.80~1.20%, Mn: 1.30~1.50%, S≤0.012%， P≤0.012%， Cr：0.80～1.20%， V：0.13～0.17%， Ni: 0.90~1.10%, O≤0.0010%， N≤0.0030%， The remainder consists of Fe and unavoidable impurities.

2. A method for producing spring round steel as described in claim 1, characterized in that: Includes the following steps: S1. During smelting, hot metal pretreatment, converter refining, LF furnace refining and VD furnace refining are adopted; S2. Continuous casting: During continuous casting, the pouring temperature is controlled at 15~30 ℃ above the liquidus temperature, and electromagnetic stirring is used. The billet pulling speed during continuous casting is 0.6~0.8 m / min. S3, Hot rolling: A process route using low heating temperature and low final rolling temperature during hot rolling; S4. Post-rolling cooling: After hot rolling, the spring round steel is placed in a slow cooling pit for cooling, with a cooling rate of less than or equal to 0.05 ℃ / s; S5. Heat treatment: Quenching and medium-temperature tempering are performed on the hot-rolled and cooled spring round steel.

3. The method for producing spring round steel with high strength, high toughness, and high fatigue life according to claim 2, characterized in that: After pretreatment of the molten iron in S1, the S and P contents in the molten iron should be less than or equal to 0.012% to ensure that the S and P contents in the final spring round steel meet the composition requirements. The converter steelmaking process uses a top-and-bottom blowing method, with a converter endpoint C ≥ 0.12% and a tapping temperature ≥ 1650℃. Refining is carried out using an LF furnace and a VD furnace, with aluminum deoxidation. After refining, all types of inclusions should meet the following requirements: Class A inclusions should be ≤ 1, Class B inclusions should be ≤ 0.5, Class C inclusions should be ≤ 1, and Class D inclusions should be ≤ 0.

5.

4. The method for producing spring round steel with high strength, high toughness, and high fatigue life according to claim 2, characterized in that: In step S2, the billet is slowly cooled after continuous casting, with a cooling time of ≥24 h.

5. The method for producing spring round steel with high strength, high toughness, and high fatigue life according to claim 2, characterized in that: The temperature of the heating furnace in S3 is 980~1020 ℃, the furnace is in an argon protective atmosphere, the argon pressure is slightly greater than atmospheric pressure, and after holding at the temperature for 2 hours, it is immediately taken out of the furnace for rolling, and the final rolling temperature is controlled at 770~790 ℃.

6. The method for producing spring round steel with high strength, high toughness, and high fatigue life according to claim 2, characterized in that: The quenching temperature in S5 is 850~870 ℃, and the heating furnace adopts a vacuum atmosphere; after holding at this temperature for 1 h, it is oil-cooled to room temperature; the tempering temperature is 360~380 ℃, and after holding at this temperature for 2 h, it is water-cooled to room temperature.

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