Seamless steel pipe for oil cylinder and manufacturing method thereof

Abstract

The present invention belongs to the technical field of low alloy steel manufacturing, and specifically relates to a seamless steel pipe for oil cylinder and a manufacturing method thereof. The chemical composition of the seamless pipe includes, by mass percentage: C: 0.32% to 0.37%, Si: 0.66% to 0.75%, Mn: 1.62% to 1.79%, RE: 0.21% to 0.35%, Sb: 0.01% to 0.05%, P≤0.010%, S≤0.005%, O≤0.0014%, N≤0.0030%, H≤0.00015%, As+Pb+Bi+Sn≤0.0030%, and the balance is Fe and unavoidable impurities; the RE is lanthanum and cerium. The present invention adopts the composition design concept of intensive deoxidation and alloying of C, Si and Mn and quantitatively strictly controlling harmful elements, and compositely adds Si, Mn, RE and Sb elements to manufacture and design seamless steel pipes, which provides a basic guarantee for the product of the present invention to have excellent tensile and compressive fatigue performance, strength and impact performance, and improves the service life and safety of fatigue-resistant cylinder pipes.

Description

Technical Field

[0001] The invention belongs to the technical field of low alloy steel manufacturing, and specifically relates to a seamless steel pipe for an oil cylinder and a manufacturing method thereof. Background Art

[0002] The working environment and changing working conditions of engineering equipment such as road rollers, excavators, bulldozers, cranes, loaders and tunnel engineering machinery affect various indicators of the machinery, such as strength, stiffness and dynamic characteristics, which can easily cause damage to parts and even fatigue fracture of parts, which has a serious impact on the service life of the working device of the mechanical equipment. In particular, the fatigue reliability life of the oil cylinder has an important impact on the construction safety. Therefore, the service life of the oil cylinder steel tube and piston rod has become the top priority of today's engineering machinery design.

[0003] In recent years, with the rapid development of my country's engineering machinery industry, it is required to reduce the weight and consumption of engineering equipment such as road rollers, excavators, bulldozers, cranes, loaders and tunnel engineering machinery, and the strength, toughness and fatigue performance of the cylinder materials are required to be high. In particular, the requirements for the fatigue resistance of the cylinder are becoming more and more important. The long-life design concept of high safety and reliability has gradually become the mainstream in large domestic machinery and equipment manufacturing companies. Due to the working characteristics of high-load reciprocating motion of this type of cylinder tube, almost 70% to 80% of the failures of engineering machinery equipment and parts failures are caused by fatigue. Therefore, the development of a seamless steel pipe material for cylinders suitable for high-load reciprocating motion has obvious practical significance and economic benefits.

[0004] Patent CN111304529A discloses a seamless steel pipe for oil cylinder, which is mainly composed of C, Si and Mn, and a small amount of Cr, Mo, Cu, Ni and Al alloy elements are added. The chemical element formula is C: 0.24-0.30%, Si: 1.20-1.40%, Mn: 1.20-1.40%, P: ≤0.015%, S: ≤0.010%, Al: 0.025-0.055%, Mo: ≤0.15%, Cr: ≤0.15%, Ni: ≤0.15%, Cu: ≤0.20%. It is manufactured by hot rolling + quenching + tempering. The shortcomings of this invention are as follows: 1. The fatigue resistance of the steel pipe product is low, and it cannot be used for the manufacture of high fatigue resistance steel pipes for oil cylinders. The steel cleanliness and grain fineness of the product are not high, and the ability to suppress the fatigue source area when stress concentration occurs is limited; 2. This invention mainly refines the grain structure by adding Al elements, which will inevitably produce Al2O3 inclusions with sharp-angle strings, and the particles are coarse and generally difficult to remove, which is very easy to form a fatigue crack source area, reducing the fatigue resistance of the product and increasing the risk factor during the use of the product. Patent CN112095049A discloses a seamless steel pipe material for oil cylinders, which limits the carbon equivalent, and the product alloying design formula is C: 0.15-0.19%, Si: 0.30-0.45%, Mn: 1.35-1.60%, P: ≤0.025%, S: ≤0.015%, Cr: ≤0.25%, Ni: ≤0.25%, Mo: ≤0.1%, Ti: ≤0.1%, V: ≤0.1%, Cu: ≤0.20%, TAl: ≤0.025%, Nb: ≤0.1%. The manufacturing process is: 1. Select high-quality scrap iron blocks and re-restore them into molten iron casting billets to make pipes, and then perform quenching and tempering heat treatment on the steel pipes. The shortcomings of this invention are as follows: using scrap iron as smelting raw materials, the harmful elements contained in the steel will inevitably be brought into the finished oil cylinder pipe, resulting in an increase in the instability of the product stress concentration crack initiation, reducing the product fatigue resistance, and affecting the product safety. Cannot meet the use requirements of high fatigue resistance.

[0005] Patent CN107746916A discloses a method for manufacturing a seamless steel pipe for oil cylinders, by adding V and Al alloy design C: 0.15-0.25%, Si: 0.10-0.40%, Mn: 1.0-1.5%, P: ≤0.015%, S: ≤0.020%, V: 0.02-0.08%, Al: 0.010-0.040%, Cr: ≤0.30%, Ni: ≤0.10%, Mo: ≤0.10%, Ti: ≤0.10%, Cu: ≤0.20%. The manufacturing method includes: tube billet rolling + normalizing heat treatment. The shortcomings of this invention are as follows: 1. The alloy element V is intentionally added, and the content is in the range of 0.02-0.08%. V has a strong affinity with N, O, and C, and is easy to form compounds in steel. It mainly produces a strong precipitation strengthening effect during hot rolling of steel. Normalizing heat treatment will cause the V compound phase to grow and aggregate, and the precipitation strength effect will disappear. It also reduces the ability of the oil cylinder pipe to suppress stress concentration crack sources and reduce the fatigue resistance of the product. 2. This invention mainly refines the grain structure by adding Al elements, which will inevitably produce Al2O3 inclusions with sharp corners, and the particles are coarse and generally difficult to remove. It is very easy to produce stress concentration, reduce the fatigue resistance of the product, and increase the risk factor during the use of the product. Therefore, this invention is not enough to meet the current and future requirements for high fatigue resistance of steel pipes for oil cylinders.

[0006] Patent CN104190740A discloses a method for manufacturing hot-rolled seamless steel tube blanks, which mainly improves the surface quality of the cylinder tube by selecting refined slag from the smelting process and performing plastic transformation treatment on brittle inclusions. The shortcomings of this invention are as follows: 1. The technical measures taken can only change the shape of the inclusions, but not the size of the inclusions, and do not fundamentally improve the ability of the cylinder tube to suppress stress concentration crack sources. The product fatigue resistance is not high, and the product safety risk is still very high. 2. The product strength is too low, the strength only reaches 266MPa, which leads to the fatigue resistance strength is inevitably too low, which cannot meet the current and future use requirements of hydraulic cylinders.

[0007] Patent CN113528954A discloses a seamless steel tube for cold-drawn hydraulic cylinder and a manufacturing method thereof. The unique composition design and manufacturing process are mainly used to improve the uniform elongation of the product, improve the cold drawing processing performance, and improve the surface quality of the product. It has certain advancements, but the product does not have high fatigue resistance and cannot meet the use requirements of high fatigue resistance cylinder tubes.

[0008] In summary, the existing technologies cannot meet the use requirements of steel pipes for oil cylinders on fatigue resistance, and seamless steel pipes for oil cylinders with high fatigue resistance have not been reported. Summary of the invention

[0009] The purpose of the present invention is to provide a seamless steel pipe for oil cylinders with excellent fatigue resistance and mechanical properties and a manufacturing process thereof, wherein the 10 million times tensile and compressive fatigue strength limit can reach more than 300MPa, the yield strength is ≥690MPa at room temperature, the tensile strength is ≥770MPa, and the V-notch impact energy is ≥27J at -20°C. At the same time, the surface quality and internal quality of the steel pipe are excellent. The use needs of domestic hydraulic cylinder manufacturing enterprises in the present and future development can be met.

[0010] The present invention adopts the composition design idea of ​​intensive deoxidation and alloying of C, Si, and Mn and strictly controlling the content of harmful elements, combined with a special manufacturing process, to provide a guarantee for the steel pipe to have sufficiently high fatigue resistance and comprehensive mechanical properties. The high-temperature section of plastic deformation of traditional seamless steel pipes is too large, and the low-temperature section is too small, resulting in the original metallographic structure of the product after hot rolling. The grain size is relatively large, the mechanical properties are not high, and the coarse grain boundaries are easily the source of fatigue cracks. According to a large number of experimental results of the inventor, the grain size and uniformity of the steel pipe, and the proportion of the hard phase, are the key factors that determine the quality of the product's fatigue resistance. Therefore, the material design and process technology route selection should focus on how to obtain the desired organizational properties while taking into account the compliance of other mechanical properties. To achieve the above purpose, the current method that is easier to think of is to use ultra-low carbon steel combined with ultra-fast cooling technology to produce intracrystalline ferrite structure, so that the steel has the characteristics of refined grain structure, high impact toughness, etc., but due to the particularity of the cross-section of seamless steel pipes, the ultra-fast cooling technology suitable for plate products is difficult to apply to the production of seamless steel pipes. Although some domestic steel enterprises and scientific research institutes have made attempts in this regard in the past two years, they still cannot meet the technical requirements of the products of the present invention. In addition, the implementation of metallurgical engineering is extremely difficult and costly. The uniformity and stability of the product's organizational performance are not very good, and it is not suitable for the manufacture of fatigue-resistant seamless cylinder pipes. Therefore, based on the characteristics of its own tooling equipment, the present invention, through the optimal design of chemical elements and the effective combination of innovative process means and methods, has obtained a seamless steel pipe material that meets the requirements of comprehensive mechanical properties, especially excellent fatigue resistance, which is very suitable for the preparation of fatigue-resistant cylinders. Moreover, the ingredient formula and manufacturing process provided by this method also determine the excellent quality of the internal and external quality of the product, especially it can meet the higher requirements of the cylinder processing technology for the inner surface quality of the steel pipe, is not prone to stress concentration, and enhances the safety of use. Based on the above considerations, a seamless steel pipe for fatigue-resistant cylinders with the following components as the core components is determined.

[0011] In order to achieve the above object, the technical solution of the present invention is as follows:

[0012] On one hand, the present invention provides a seamless pipe for an oil cylinder, wherein the chemical composition of the seamless pipe comprises, by mass percentage, C: 0.32% to 0.37%, Si: 0.66% to 0.75%, Mn: 1.62% to 1.79%, RE: 0.21% to 0.35%, Sb: 0.01% to 0.05%, P≤0.010%, S≤0.005%, O≤0.0014%, N≤0.0030%, H≤0.00015%, As+Pb+Bi+Sn≤0.0030%, and the remainder is Fe and unavoidable impurities; the RE is lanthanum and cerium.

[0013] Explanation of the selection range of each alloy element:

[0014] C: Carbon can greatly improve the hardenability of steel and has the best economic efficiency. It is an indispensable chemical element of the present invention. When the carbon content is less than 0.32%, its effect of improving hardenability and strengthening is insufficient, and it is difficult to achieve the purpose of the present invention. Too high carbon content will increase the content of hard phases such as cementite, reduce the uniformity of the refined grain structure and fatigue resistance of the steel pipe, and it is also difficult to achieve the purpose of the present invention. Therefore, the upper limit of the carbon content is set at 0.37%.

[0015] Si: Silicon plays a deoxidizing role on the one hand, and a strengthening role on the other hand. When the content is lower than 0.66%, the effect cannot be fully exerted. However, too high a silicon content will greatly damage the impact toughness of the steel, thus failing to meet the performance requirements of the product. Therefore, its upper limit is set at 0.75%.

[0016] Mn: Manganese can expand the austenite phase area, improve the stability of supercooled austenite during cooling, and greatly improve hardenability. Manganese can also combine with the S element in steel to form MnS, which can improve the properties and state of inclusions, and help refine the grains and improve the plasticity and toughness of steel. When the content is lower than 1.62%, the effect cannot be fully exerted, but when the content exceeds 1.79%, it will reduce the uniformity of the product structure grains and reduce fatigue resistance, which is not conducive to achieving the purpose of the present invention.

[0017] RE: Rare earth can effectively purify harmful elements such as oxygen and sulfur in steel, and can also refine the size of oxide and sulfide inclusions, making them into small and dispersed spheres, improving the toughness and plasticity of steel, and has a significant effect on reducing the stress concentration sensitivity of steel and inhibiting the initiation of fatigue cracks, and can improve the fatigue resistance of steel pipes. When the rare earth content is less than 0.21%, the effect is insufficient, and when the content exceeds 0.35%, the beneficial effects of the present invention are saturated, and negative effects may also be produced. Therefore, the upper limit of the rare earth content adopted in the technical solution of the present invention is 0.35%.

[0018] Sb: Antimony can precipitate on MnS inclusions and austenite grain boundaries at austenite temperature, thereby refining the grain size of the steel and effectively refining the grain size during multiple recrystallization processes. However, antimony has a strong affinity with oxygen and sulfur, which may produce impurities and embrittlement. It needs to be used in combination with other elements in steel, and its content itself should not be too high. Therefore, the antimony content adopted in the technical solution of the present invention is 0.01% to 0.05%.

[0019] P: Phosphorus is easily segregated at grain boundaries and can easily become a place for inducing stress concentration. In addition, due to its own cold brittleness tendency, a too high content is not conducive to achieving the purpose of the present invention, so it is limited to less than 0.010%.

[0020] S: Sulfur easily forms inclusions with manganese, etc. The increase of its content will lead to an increase in the number of inclusions, destroying the continuity of the material matrix during processing and increasing the risk of stress concentration. Therefore, it is limited to less than 0.005%.

[0021] O: Oxygen easily forms oxide inclusions with most chemical elements in steel, which damages the surface quality of steel and reduces the fatigue performance of products. In order to achieve the technical effect of the present invention, oxygen is limited to less than 0.0014%.

[0022] N: Nitrogen easily forms compounds with most alloying elements, with obvious sharp corners. When the content is high, it will aggregate and grow, which can easily aggravate the stress concentration tendency of the product and damage the fatigue resistance of the product. Therefore, it is limited to less than 0.0030%.

[0023] H: Hydrogen dissolved in steel can easily cause defects such as hydrogen embrittlement and white spots, reducing the fatigue resistance of the steel. Therefore, it is limited to less than 0.00015%.

[0024] As+Pb+Bi+Sn: Harmful impurity elements in steel, which make steel brittle, crack, segregate grain size, and reduce the fatigue resistance of the product. Therefore, it is limited to less than 0.0030%.

[0025] In the above technical solution, further, the seamless steel pipe has a fatigue limit strength of ≥300MPa after 10 million times of tension and compression, a yield strength of ≥690MPa at room temperature, a tensile strength of ≥770MPa at room temperature, and an impact energy CVN of ≥27J at -20°C.

[0026] Another aspect of the present invention provides a method for manufacturing the seamless steel pipe, the method comprising the following steps:

[0027] (1) After the molten steel is smelted in a converter, subjected to RH vacuum degassing, and refined in an LF furnace, it is cast into a steel billet. During the manufacture of the steel billet, RE and Sb alloys are added to the molten steel, and the addition station is in the middle and late stages of the LF treatment;

[0028] (2) After casting, the solidified ingot is sent to the continuous rolling heating furnace for blanking and rolling into round tube billets;

[0029] (3) After hot rolling, the steel pipe is pretreated as follows: heating to 880-890°C at a heating rate of not less than 30°C / s, and then cooling to below 100°C at a cooling rate of not less than 1°C / s, for 1-3 cycles;

[0030] (4) The pretreated steel pipe is heated at 840-860°C for 50-80 min for the first time, then cooled to room temperature at a rate of not less than 50°C / s, and then heated for a second time at 550-620°C for 70-120 min, and air-cooled to room temperature.

[0031] In the above technical solution, further, in step (1), RE and Sb alloys are added 10 to 19 minutes before the LF furnace is refined out of the tank.

[0032] In the above technical solution, further, in step (2), the steel pipe is formed into a seamless steel pipe.

[0033] The beneficial effects of the present invention are:

[0034] (1) The present invention adopts the composition design concept of intensive deoxidation and alloying of C, Si and Mn and quantitatively strictly controlling harmful elements, and compositely adds Si, Mn, RE and Sb elements to manufacture and design seamless steel pipes, which provides a basic guarantee for the product of the present invention to have excellent tensile and compressive fatigue performance, strength and impact performance, and improves the service life and safety of fatigue-resistant cylinder pipes.

[0035] (2) When manufacturing the steel billet of the present invention, RE and Sb are added to the molten steel in the form of an alloy, and the adding station is in the middle and late stages of the LF treatment, which ensures smooth production and a good yield, effectively reduces the particle size of inclusions, and at the same time avoids the formation of harmful impurities by the alloy, thereby greatly improving the fatigue resistance of the product.

[0036] (3) According to the present invention, the solidified ingot after casting is directly sent to the continuous rolling heating furnace, thereby avoiding energy waste, ensuring production rhythm and improving efficiency, and enhancing product competitiveness.

[0037] (4) The steel pipe of the present invention is manufactured in the form of a seamless steel pipe, thereby avoiding the risk of cracking of the welded pipe due to the presence of the weld, and providing a fundamental guarantee for the safety of the cylinder pipe under high fatigue working conditions.

[0038] (5) The steel pipe manufactured by the method of the present invention has a fatigue limit strength of more than 300 MPa after 10 million times, a tensile strength of more than 770 MPa, a yield strength of more than 690 MPa, and an impact energy of more than 27 J at -20°C, which meets the fatigue resistance performance requirements of oil cylinder manufacturing companies.

[0039] (6) In the steel pipe manufactured by the method of the present invention, the inclusions of type A are ≤0.5, the inclusions of type B are ≤1.0, the inclusions of type C are ≤1.0, the inclusions of type D are ≤0.5, the inclusions of type Ds are ≤0.5, and the total amount of inclusions is ≤3.0; the surface quality of the steel pipe meets the requirements of ultrasonic L2 technical level flaw detection; the grain size grade of the steel pipe organization is ≥10. The steel pipe manufactured by the present invention has excellent internal and external quality, low sensitivity to stress concentration of the steel pipe, especially the size grade of Ds inclusions is controlled below 0.5, which is more competitive in the market.

[0040] (7) The seamless steel pipe manufactured by the method of the present invention can be expanded and applied to other fields such as machinery, structure, bridge, etc., and has broad market prospects. DETAILED DESCRIPTION

[0041] The seamless steel pipe for oil cylinder of the present invention will be further described below based on specific embodiments, but the specific embodiments and related descriptions do not constitute improper limitations on the technical solution of the present invention.

[0042] Examples 1-8

[0043] The chemical composition of the seamless pipe for oil cylinder provided in Examples 1-8 is shown in Table 1.

[0044] Table 1 Chemical composition of the product of the present invention (wt%)

[0045]

[0046]

[0047] The manufacturing method of the seamless steel pipe for oil cylinder comprises the following steps:

[0048] (1) The molten steel having the chemical composition in Table 1 is subjected to RH degassing + LF furnace refining treatment, and the O content, N content, P content and S content are controlled to be less than 0.0014%, less than 0.0030%, less than 0.010% and less than 0.005% respectively. RE and Sb alloy are added 10 to 19 minutes before the LF refining tank is discharged. The molten steel is solidified by electromagnetic stirring using a continuous casting machine to cast the steel billet. The soft reduction technology is implemented during the straightening process of the cast billet.

[0049] (2) After the cast steel billet is sized, it is directly sent to the continuous rolling heating furnace with warm water to be opened and rolled into round tube billets. The production rhythm is compact and the energy loss rate is small. The steel pipe is formed by hot rolling seamless pipe making;

[0050] (3) After hot rolling, the steel pipe is pretreated as follows: the steel pipe is induction heated to 880-890°C, and then cooled to below 100°C at a cooling rate of not less than 1°C / s, and this cycle is repeated 1-3 times;

[0051] (4) The pretreated steel pipe is heated at 840-860°C for 50-80 min for the first time, then cooled to room temperature at a rate of not less than 50°C / s, and then heated for a second time at 550-620°C for 70-120 min, and air-cooled to room temperature;

[0052] (5) After the steel pipes have been inspected visually and by flaw detection, they are packaged and put into storage.

[0053] The manufacturing process parameters are shown in Table 2.

[0054] Table 2 Manufacturing process parameters of the products of the present invention

[0055]

[0056]

[0057] The mechanical properties of the product are shown in Table 3.

[0058] Table 3 Mechanical properties of the products of the present invention

[0059]

[0060] The above embodiments are only preferred embodiments of the present invention and are not intended to limit the implementation methods. The protection scope of the present invention shall be subject to the scope defined in the claims. Other different forms of changes or modifications may be made based on the above description. Obvious changes or modifications derived therefrom are still within the protection scope of the present invention.

Claims

1. A seamless pipe for oil cylinder, characterized in that: The chemical composition of the seamless pipe includes, by mass percentage, C: 0.32%-0.37%, Si: 0.66%-0.75%, Mn: 1.62%-1.79%, RE: 0.21%-0.35%, Sb: 0.01%-0.05%, P≤0.010%, S≤0.005%, O≤0.0014%, N≤0.0030%, H≤0.00015%, As+Pb+Bi+Sn≤0.0030%, and the balance is Fe and unavoidable impurities; the RE is lanthanum and cerium; The method for manufacturing seamless pipe comprises the following steps: (1) After the molten steel is smelted in a converter, subjected to RH vacuum degassing, and refined in an LF furnace, it is cast into a steel billet. During the manufacture of the steel billet, RE and Sb alloys are added to the molten steel, and the addition station is in the middle and late stages of the LF treatment; (2) After casting, the solidified ingot is sent to the continuous rolling heating furnace for blanking and rolling of round tube billets; (3) After hot rolling, the steel pipe is pretreated as follows: heating to 880~890℃ at a heating rate of not less than 30℃ / s, and then cooling to below 100℃ at a cooling rate of not less than 1℃ / s, and the cycle is repeated 1~3 times; (4) The pretreated steel pipe is heated at 840-860°C for 50-80 min for the first time, then cooled to room temperature at a rate of not less than 50°C / s, and then heated for a second time at 550-620°C for 70-120 min, and air-cooled to room temperature.

2. The seamless pipe for oil cylinder according to claim 1, characterized in that: The seamless pipe has a fatigue limit strength of ≥300MPa after 10 million times of tension and compression, a yield strength of ≥690MPa at room temperature, a tensile strength of ≥770MPa at room temperature, and an impact energy CVN of ≥27J at -20°C.

3. The seamless pipe for oil cylinder according to claim 1, characterized in that: In step (1), RE and Sb alloy are added 10 to 19 minutes before the LF furnace is refined and discharged from the tank.

4. The seamless pipe for oil cylinder according to claim 1, characterized in that: In step (2), the steel pipe is formed into a seamless steel pipe.

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