A non-adjusted steel material for a piston rod

Abstract

This invention proposes a non-heat-treated steel for piston rods, relating to the field of alloy structural steel technology. The steel composition includes: C 0.18-0.25%, Si 0.20-0.45%, Mn 1.20-1.60%, P ≤0.025%, S ≤0.015%, V 0.08-0.15%, Nb 0.02-0.06%, Cu 0.30-0.50%, Ni 0.20-0.35%, with the remainder being Fe. This invention provides a non-heat-treated steel for piston rods, simplifying the process and reducing energy consumption; improving mechanical properties; optimizing corrosion resistance; and extending fatigue life.

Description

Technical Field

[0001] This invention relates to the field of alloy structural steel technology, and in particular to a heat-free steel for piston rods. Background Technology

[0002] As the core transmission component of hydraulic cylinders and pneumatic cylinders, piston rods are widely used in engineering machinery (such as hydraulic cylinders for excavators and lifting cylinders for loaders), industrial automation equipment (such as pneumatic actuators for production lines), transportation (such as shock absorber cylinders for commercial vehicles and brake cylinders for rail transit) and special equipment (such as hydraulic drive systems for offshore platforms).

[0003] In actual working conditions, the piston rod reciprocates at high frequency (2-5 times / minute, more than 2000 times a day), and the stress state changes frequently: when it extends, it bears the load compressive stress (such as several tons to tens of tons in engineering machinery scenarios), and when it retracts, it is subjected to tensile stress. The moment of reversal, it generates an impact stress of 2-3 times the static load. Furthermore, stress concentration (coefficient 1.3-1.8) easily forms in the transition fillets and threaded sections of the rod body, which can easily lead to microcracks under long-term action. Insufficient material fatigue strength can cause fracture—such failures account for more than 60% in engineering machinery. Multi-media synergistic corrosion: hydraulic oil corrosion: high temperature (60-80℃) and moisture cause oil oxidation and deterioration, generating corrosive substances that damage the surface film. Mixed metal / rubber debris exacerbates "abrasive + corrosion" damage. Compressed air corrosion: incomplete drying (dew point > -40℃) forms a "wet film," causing rust (Fe2O3・nH2O), damaging the sealing fit accuracy, and the system efficiency drops by more than 10% when leakage exceeds 3mL / min. External environmental corrosion: in environments such as mines and oceans, media such as Cl⁻ penetrate the protective layer to form pitting corrosion (depth 0.1-0.3mm / year), further shortening fatigue life.

[0004] Continuous friction between the piston rod, guide sleeve, and seals (specific pressure 5-15 MPa) leads to: ① Surface roughness increasing from Ra=0.4-0.8 μm to over 2.0-3.2 μm, causing leakage and media contamination; ② Frictional heat causing local temperatures to reach 70-100℃ (over 120℃ at high speeds), reducing the material's yield strength by 15-20% and shortening the seal's lifespan by 30-50%. Therefore, the material must meet the requirements of "high strength, high toughness, long fatigue life, high corrosion resistance, and good wear resistance." The mainstream technical routes in the industry have shortcomings: Quenched and tempered structural steel (40Cr, 35CrMo, etc.): requires "quenching at 850-900℃ + tempering at 500-600℃", which is a complex process with high energy consumption (according to the "Energy Saving Technology Guidelines for the Iron and Steel Industry", energy consumption per ton is 800-1000kWh). Piston rods with a length-to-diameter ratio > 10 have a deformation of 0.5-1.0mm / m, requiring additional straightening. Conventional non-quenched steel (20MnV, 25MnV, etc.): can be used directly after rolling, but has poor corrosion resistance (neutral salt spray corrosion ≥ 0.05mm / a, rusting in humid environments within 6 months), and a short fatigue life (8×10). 6 -1.0×10 7 This only meets the requirement of a 3-4 year equipment lifespan, far below the industry's design requirement of 5-8 years. Summary of the Invention

[0005] Based on the technical problems existing in the background art, the present invention proposes a non-modulated steel for piston rods.

[0006] The present invention proposes a piston rod without tempering steel, the composition of which includes: C 0.18-0.25%, Si 0.20-0.45%, Mn 1.20-1.60%, P≤0.025%, S≤0.015%, V 0.08-0.15%, Nb 0.02-0.06%, Cu 0.30-0.50%, Ni 0.20-0.35%, with the remainder being Fe.

[0007] Preferably, the C content in the steel is 0.20-0.23% by mass percentage.

[0008] Preferably, the steel contains ≤0.40% Si and 1.30-1.50% Mn by mass percentage.

[0009] Preferably, the steel contains 0.10-0.13% V and 0.03-0.05% Nb by mass percentage.

[0010] Preferably, the steel contains 0.35-0.45% Cu and 0.25-0.32% Ni by mass percentage.

[0011] Preferably, the steel further contains 0.01-0.03% Ti by mass percentage, wherein the Ti is used to further refine the grain size of the steel so that the grain size of the steel is ≤8μm.

[0012] Preferably, the steel is obtained through the following preparation process:

[0013] Heat the steel billet with the corresponding composition to 1150-1250℃ and hold for 2-3 hours;

[0014] Then, two-stage rolling is carried out, with rough rolling temperature of 950-1050℃ and finishing rolling temperature of 820-880℃;

[0015] After rolling, laminar flow cooling is adopted at a cooling rate of 15-25℃ / s, and air cooling is performed after cooling to 350-450℃.

[0016] Preferably, in the steel preparation process, the finishing rolling temperature is 840-860℃, and the post-rolling cooling rate is 18-22℃ / s.

[0017] Preferably, the fatigue life of the steel is ≥1.3×107 cycles, and the corrosion rate tested by neutral salt spray test is ≤0.025mm / a.

[0018] Preferably, the steel is used for the processing and manufacturing of hydraulic piston rods or pneumatic piston rods, and the rolled steel can be directly cut or ground.

[0019] The piston rod steel without tempering proposed in this invention has the following beneficial effects: Simplified process and reduced energy consumption: Eliminating the quenching + tempering process reduces energy consumption per ton of steel by more than 35% (from 800-1000kWh to 500-550kWh), and the deformation rate after rolling is ≤0.2%, eliminating the need for straightening; Improved mechanical properties: Yield strength ≥600MPa, tensile strength ≥750MPa, elongation after fracture ≥18%, and impact absorption energy at -40℃ ≥50J. Compared to conventional untreated steel, its strength is increased by 15-20%, and its low-temperature toughness is increased by 25-30%. Corrosion resistance is optimized: the synergistic effect of Cu and Ni results in a corrosion rate ≤0.03mm / a in a neutral salt spray test (5% NaCl, 48h), which is more than 40% better than conventional untreated steel without Cu and Ni. Fatigue life is extended: the synergistic effect of V and Nb grain refinement and precipitation strengthening results in a fatigue life (stress ratio R=0.1, stress amplitude 400MPa) ≥1.2×10⁻⁶. 7 This improves upon conventional non-modulated steel by 20-50%, meeting the long-term alternating load requirements of the piston rod. Attached Figure Description

[0020] Figure 1 This is a flowchart illustrating the preparation process of a piston rod without tempering steel, as proposed in this invention. Detailed Implementation

[0021] Reference Figure 1 This invention proposes a non-heat-treated steel for piston rods, the composition of which includes:

[0022] 1. Steel composition design (mass percentage, wt%)

[0023] Carbon (C): 0.18-0.25%, used to ensure the basic strength of steel and avoid a decrease in toughness due to excessive content;

[0024] Silicon (Si): 0.20-0.45%, plays a role in deoxidation and solid solution strengthening, and its content is controlled below 0.45% to prevent toughness deterioration;

[0025] Manganese (Mn): 1.20-1.60%, strength is improved through solid solution strengthening, and microstructure is optimized in synergy with V and Nb;

[0026] Phosphorus (P): ≤0.025%, Sulfur (S): ≤0.015%, strictly control the content of harmful elements to reduce the brittleness of steel;

[0027] Vanadium (V): 0.08-0.15%, refines grains by forming V(C,N) precipitates, thereby improving strength and fatigue life;

[0028] Niobium (Nb): 0.02-0.06%, works synergistically with V to further refine austenite grains and inhibit grain growth;

[0029] Copper (Cu): 0.30-0.50%, forms a dense oxide film on the surface of steel, improving corrosion resistance, while also providing a small amount of solid solution strengthening;

[0030] Nickel (Ni): 0.20-0.35%, improves the low-temperature toughness of steel, and when combined with Cu, avoids the hot brittleness problem caused by Cu;

[0031] The remainder consists of iron (Fe) and unavoidable impurities.

[0032] 2. Preparation process

[0033] Smelting: A converter + LF refining + RH vacuum degassing process is adopted to control the purity of molten steel and ensure that the P and S content meets the standards; Converter smelting stage: A top-and-bottom blown converter is used, with molten iron (molten iron composition requirements: S≤0.020%, P≤0.030%) as raw material, and the oxygen flow rate is controlled at 250-300 Nm during the blowing process. 3 / h, the final carbon content is controlled at 0.08-0.12% (to avoid excessively low final carbon content leading to excessively strong oxidizing properties in the molten steel), and the final temperature is 1620-1650℃ ("final temperature" specifically refers to the actual temperature of the molten steel at the end of the converter blowing process (i.e., when oxygen supply is stopped). It is one of the core control parameters of converter steelmaking—its function is to ensure that the composition of the molten steel (such as carbon content) reaches the expected target, and at the same time to provide temperature adaptability for subsequent processes such as LF refining and RH vacuum degassing (if the final temperature is too low, it may lead to increased energy consumption for heating during LF refining; if it is too high, it may easily cause excessively strong oxidizing properties in the molten steel and an increase in alloy burn-off rate)). In the later stages of blowing, lime (30-40 kg / t steel) and fluorite (5-8 kg / t steel) are added to create alkaline slag, initially removing P (target P ≤ 0.028%) and S (target S ≤ 0.020%). In the LF refining stage: after tapping from the converter, the molten steel is transferred to the LF refining furnace. First, silicon-calcium alloy (1.5-2.0 kg / t steel) is added for deoxidation, controlling the oxygen content of the molten steel to ≤ 50 ppm. Subsequently, high-alkalinity refining slag (CaO / SiO2 = 3.5-4.0, 15-20 kg / t steel) is added, and argon gas is used for stirring (stirring intensity 0.3-0.5 Nm). 3 / (t・min)), through deep desulfurization of slag steel reaction (final S≤0.015%); at the same time, according to the composition analysis results, the content of C, Si, and Mn elements is finely adjusted (by adding carburizing agent, ferrosilicon, and ferromanganese) to ensure that the composition deviation is controlled within ±0.02%; RH vacuum degassing stage: after LF refining, the molten steel is sent to the RH vacuum treatment device, and the vacuum degree is evacuated to ≤67Pa, and the treatment time is 15-20min. H (final H≤2ppm) and N (final N≤80ppm) in the molten steel are removed through the vacuum environment, reducing gaseous defects (such as porosity and white spots); at the same time, the inclusions are further promoted to float under vacuum conditions (final A-class inclusions in the molten steel ≤1.0 grade, B / C / D-class inclusions ≤0.5 grade), ensuring that the purity of the molten steel meets the requirements of subsequent rolling for uniformity of structure, laying the foundation for the uniform precipitation of V and Nb elements, and achieving precise control of molten steel quality to achieve the goal of "low harmful elements and high purity".

[0034] Heating: Heat the steel billet to 1150-1250℃ and hold for 2-3 hours to allow V and Nb elements to fully dissolve in the austenite. Heating curve control: A three-stage heating system is adopted. After the steel billet (specification: Φ150-300mm round billet or 200×200-400×400mm square billet) is put into the furnace, it is first heated in the preheating section (temperature 600-800℃) for 1-1.5 hours to avoid thermal stress cracking caused by the cold billet directly entering the high-temperature zone. Then it enters the heating section (temperature 1000-1100℃) for 1-1.2 hours to gradually increase the core temperature. Finally, it enters the soaking section (temperature 1150-1250℃) and holds for 2-3 hours (the holding time is adjusted according to the thickness of the steel billet: for every 50mm increase in thickness, the holding time is extended by 0.3-0.5 hours). Element dissolution mechanism: 1150-1250℃ The homogenization temperature ensures that V and Nb elements are completely dissolved in the austenite matrix—V's dissolution temperature is approximately 1050℃, and Nb's dissolution temperature is approximately 1100℃. This temperature range not only achieves complete solid solution of both elements but also avoids excessively high temperatures (>1250℃) that lead to coarse austenite grains (if the temperature exceeds 1300℃, the austenite grain size easily exceeds 50μm, making it difficult to refine through subsequent rolling). Temperature uniformity control: Multi-point temperature measurement using thermocouples in the furnace (3-4 temperature measurement points are arranged on each billet, located at the head, middle, tail, and core respectively) ensures that the temperature difference between the inside and outside of the billet is ≤30℃, and the temperature difference of the billet in the same furnace is ≤20℃. This avoids uneven deformation due to uneven temperature during subsequent rolling, which can lead to microstructure segregation. This achieves complete dissolution of V and Nb elements and uniform billet temperature, avoiding localized component segregation or abnormal grain growth.

[0035] Rolling: A two-stage controlled rolling process is employed, with roughing temperature at 950-1050℃ and finishing temperature at 820-880℃ (austenite non-recrystallization zone). Grain refinement is achieved through cumulative deformation. Roughing stage (austenite recrystallization zone rolling): Roughing is carried out in the 950-1050℃ range, using 5-7 passes, with a total reduction rate controlled at 60-70% (single pass reduction rate 8-15%). Within this temperature range, austenite can rapidly undergo dynamic recrystallization after deformation, and the grain size can be reduced from the initial 5 mm to a smaller size after each pass. The 0-80μm thickness is refined to 20-30μm; simultaneously, the billet temperature is stabilized before each rolling pass by waiting between roughing passes (waiting temperature 950-1000℃, waiting time 10-15min), avoiding a sudden increase in rolling force due to temperature drop; in the finishing rolling stage (rolling in the non-recrystallized austenite region): after roughing, the billet enters the finishing mill and is rolled in 3-5 passes in the 820-880℃ range, with the total reduction rate controlled at 40-50% (single pass reduction rate 10-15%). 0℃ represents the non-recrystallization region of austenite (in the composition system of this invention, the recrystallization temperature of austenite is approximately 880-900℃). Within this range, deformation does not induce dynamic recrystallization, but rather generates a large number of dislocations at the austenite grain boundaries through "deformation energy accumulation," providing more nuclei for ferrite nucleation during subsequent cooling. Simultaneously, the finishing rolling temperature is strictly controlled between 820-880℃: if the temperature > 880℃, it enters the recrystallization region, making deformation energy accumulation impossible; if the temperature < 820℃, the billet's plasticity decreases, making it prone to rolling cracks. Rolling precision control: Through the hydraulic pressing system and AGC (automatic thickness control) system of the finishing mill, the dimensional tolerance of the finished steel (specifications: Φ20-100mm round steel or 10-50mm flat steel) is ensured to be ≤±0.1mm (diameter / thickness direction), and the surface roughness Ra≤6.3μm. This reduces the allowance for subsequent piston rod cutting. The synergistic method of "accumulated deformation in rough rolling + controlled rolling in the non-recrystallization zone of finishing rolling" is adopted. Through precise matching of deformation amount and temperature, the austenite grains are gradually refined.

[0036] Cooling: Laminar flow cooling is used after rolling at a rate of 15-25℃ / s. After cooling to 350-450℃, air cooling is performed to obtain a fine-grained ferrite + pearlite microstructure (grain size ≤10μm), eliminating the need for subsequent quenching and tempering. The laminar flow cooling system is configured with a dual-spray laminar flow cooling device. The upper nozzle is a fan-shaped nozzle (spray angle 60-90°), and the lower nozzle is a flat nozzle (spray width matched to the steel width). The cooling water zone is 15-20m long, with water volume controlled by zones (each zone's water volume can be independently adjusted, with an adjustment range of 0-100m). 3 / h) to achieve precise control of cooling rate; Cooling parameter control: When the rolled steel enters the cooling zone, it is first rapidly cooled at a cooling rate of 15-25℃ / s (the cooling rate is controlled by adjusting the nozzle water flow and the steel running speed (1-3m / s)), quickly passing through the nose area of ​​the C-curve (to avoid coarse pearlite), and then cooled to a final cooling temperature of 350-450℃—this final cooling temperature can ensure sufficient precipitation of ferrite (ferrite nucleation temperature is about 600-700℃, and the ferrite precipitation is >90% when cooled to 350-450℃), while avoiding cooling to below the martensitic transformation temperature (Ms≈300℃) to prevent the formation of hard and brittle structures such as martensite; Subsequent air cooling and structure formation: After final cooling to 350-450℃, the steel enters the air cooling zone for natural cooling ( Air cooling rate 5-10℃ / h), during which the remaining austenite gradually transforms into pearlite, ultimately forming a composite structure of "fine-grained ferrite (volume fraction 70-80%, grain size ≤8μm) + fine pearlite (volume fraction 20-30%, lamellar spacing ≤0.2μm)", with an overall grain size ≤10μm. This fine-grained structure not only enhances strength through "fine-grain strengthening" (yield strength can be increased by 30-50MPa when grain size is refined from 20μm to 10μm), but also improves toughness and fatigue life through structural uniformity, meeting performance requirements without subsequent quenching and tempering treatment. The core of post-rolling cooling is to achieve precise precipitation of the "fine-grained ferrite + pearlite" structure by controlling the cooling rate and final cooling temperature, avoiding tempering treatment.

[0037] Composition design (wt%) of Examples 1-3 and Comparative Examples 1-2.

[0038]

[0039] Preparation process: Examples 1-3: The process described in this invention (heating at 1200℃ for 2.5h, final rolling at 850℃, cooling rate at 20℃ / s) is adopted, without the need for tempering;

[0040] Comparative Example 1: Using conventional heat-free steelmaking process (heating at 1200℃ / 2h, final rolling at 900℃, cooling rate at 10℃ / s), no heat treatment is required;

[0041] Comparative Example 2: The tempering process of "860℃ quenching + 550℃ tempering" was adopted.

[0042] Performance test results

[0043]

[0044] The test results show that Embodiments 1-3 of the present invention are superior to Comparative Example 1 in terms of mechanical properties, corrosion resistance, and fatigue life, and the energy consumption is much lower than that of Comparative Example 2, which fully meets the requirements for piston rod use.

[0045] 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. A type of non-heat-treated steel for piston rods, characterized in that, The steel composition includes: C 0.18-0.25%, Si 0.20-0.45%, Mn 1.20-1.60%, P ≤0.025%, S ≤0.015%, V 0.08-0.15%, Nb 0.02-0.06%, Cu 0.30-0.50%, Ni 0.20-0.35%, with the remainder being Fe; The steel is obtained through the following preparation process: Heat the steel billet with the corresponding composition to 1150-1250℃ and hold for 2-3 hours; Then, two-stage rolling is carried out, with rough rolling temperature of 950-1050℃ and finishing rolling temperature of 820-880℃; After rolling, laminar flow cooling is adopted at a cooling rate of 15-25℃ / s, and air cooling is performed after cooling to 350-450℃.

2. The piston rod without tempering steel according to claim 1, characterized in that, The steel contains 0.20-0.23% carbon by mass percentage.

3. The piston rod without tempering steel according to claim 1, characterized in that, The steel contains ≤0.40% Si and 1.30-1.50% Mn by mass percentage.

4. The piston rod without tempering steel according to claim 1, characterized in that, The steel contains 0.10-0.13% V and 0.03-0.05% Nb by mass percentage.

5. The piston rod without tempering steel according to claim 1, characterized in that, The steel contains 0.35-0.45% Cu and 0.25-0.32% Ni by mass percentage.

6. The piston rod without tempering steel according to claim 1, characterized in that, The steel also contains 0.01-0.03% Ti by mass percentage, which is used to further refine the grain size of the steel so that the grain size of the steel is ≤8μm.

7. The piston rod without tempering steel according to claim 1, characterized in that, In the steel preparation process, the finishing rolling temperature is 840-860℃, and the post-rolling cooling rate is 18-22℃ / s.

8. The piston rod without tempering steel according to claim 1, characterized in that, The fatigue life of the steel is ≥1.3×10⁻⁶. 7 The corrosion rate, as tested by the neutral salt spray test, is ≤0.025 mm / a.

9. The piston rod without tempering steel according to claim 1, characterized in that, The steel is used for the processing and manufacturing of hydraulic piston rods or pneumatic piston rods. The steel in the rolled state can be directly cut or ground.

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