【Major Equipment in the Chemical Engineering Sector】Medium to large diameter welded surface casing made of 135-N80 steel grade

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Oil casing is one of the most commonly used pipes specifically for oil in oil fields; it is used in large quantities and at high costs, making it an essential material for drilling and production operations in oil and gas fields. Compared with seamless pipes, HFff sleeves offer significant advantages such as uniform wall thickness, high dimensional accuracy, excellent perforation performance, strong resistance to crushing, and lower costs, and are gradually replacing traditional seamless steel pipes. The API Spec 5CT standard specifies that oil casing can be manufactured from seamless pipes and straight-seam welded pipes. For straight-seam welded casings, the traditional production process involves designing appropriate chemical compositions based on the requirements of the casing’s performance ； Smelted and rolled into hot-rolled sheets/coils ； The sheet coil is turned into a tube through forming and straight-seam resistance welding ； Heat treatment of steel pipe welds or the entire steel pipe ； Appropriate threading is also performed on the pipe ends, ultimately producing qualified oil casing. For N80 grade oil casing, the standards specify that the yield strength of the tube body should be between 552 and 758 MPa, the tensile strength should be at least 689 MPa, the elongation rate should be at least 19%, and the (TC impact energy Akv (transverse)) should be at least 24 J. As safety requirements for oil wells increase, users also demand higher impact toughness from steel strips; for example, CNOOC requires an impact energy of > 80 J and a shear area of SA ≥ 75%. Currently, the production process for N80 grade ERW sleeves involves a composition design with a high carbon content (>0.20%); in some cases, alloying elements (Mo, Ni, Nb, V, and Cu) are added, followed by smelting and rolling into hot-rolled steel strips ； Then, the steel strip is used to manufacture welded pipes via the ERW process; the welds or the entire pipe undergo heat treatment, followed by appropriate threading of the pipe ends, ultimately resulting in qualified N80 grade oil casing. Due to the high carbon content (>0.20%), it is not possible to obtain hot-rolled steel strips with excellent weldability, which not only increases the complexity of the HFW pipe manufacturing process but also reduces the production efficiency of pipe fabrication ； The higher carbon content also results in generally moderate impact toughness of the steel strip. Prior to the present invention, there were already multiple documents or patent applications related to steel for N80 grade ERW oil casing, such as I) Chinese Patent CN101096737A, a high-strength straight-seam welded oil casing steel and its manufacturing method. In this patented composition, the carbon content is 0.20%-0.30%; a high carbon level is detrimental to weldability and toughness ； Mo: 0.10%-0.30%, high alloy cost. 2) Japanese Patent JP58199818A, method for manufacturing high-strength steel pipes for oil wells. The composition consists of 0.28%–0.35% C, 1.40%–2.00% Mn, 0.1%–0.3% Mo, and 0.02%–0.10% V; after tube production, the entire tube undergoes normalizing heat treatment. This patent adds the precious element Mo, resulting in high alloy costs ； A high carbon content affects the impact toughness of the steel strip. 3) Japanese Patent JP58093821A, method for manufacturing resistance-welded steel pipes for oil wells. The composition consists of C: 0.23%-0.26%, Mn: 1.8%-2.0%, Cr: 0.30%-0.35%, and V: 0.03%-0.10%. After tube production, the entire tube undergoes normalizing or quenching and tempering heat treatment. The C content in this patent is relatively high, which affects the impact toughness of the steel strip ； At the same time, the Mn and Cr contents are also high, resulting in a high carbon equivalent (Ceq 0.60%-0.68%), which leads to poor weldability of the steel strip. 4) Japanese Patent JP58199819A, method for manufacturing high-strength steel pipes for oil wells. The composition consists of 0.30%-0.35% C, 1.60%-2.00% Mn, and 0.05%-0.09% V; the entire tube undergoes normalizing heat treatment after being manufactured into tubes. The patent specifies a high carbon content, which affects the impact toughness of the steel strip and results in high alloy costs ； At the same time, the Mn content is also high, resulting in a high carbon equivalent (Ceq of 0.58%-0.70%), which leads to poor weldability of the steel strip. 5) Japanese Patent JP01225722A, method for manufacturing high-strength resistance-welded pipes for oil wells. The composition consists of C: 0.14%-0.22%, Mn: 1.20%-2.00%, Ti: 0.04%-0.07%; Nb is added selectively in an amount of 0-0.045%, and V in an amount of 0-0.10%. The high carbon content in the composition affects the impact toughness of the steel strip ； The Ti content is high, resulting in high alloy costs. 6) Japanese Patent JP07102321A: Method for manufacturing high-strength, non-heat-treated steel pipes for oil wells with a tensile strength of 800 MPa. The composition includes 0.25%-0.40% C, 1.00%-2.00% Mn, 0.01%-0.10% Nb, and 0.01%-0.10% V. The C content in this patent is relatively high, which affects the impact toughness of the steel strip ； Adding both Nb and V at the same time increases the cost of the alloy. 7) Paper \"Research on the Trial Production Process of N80 HFW Casing\", Welded Pipe, 2010, 33 (7); composition: C: 0.09%, Mn: 1.68%, Si: 0.23%, P: 0.009%, S: 0.002%, Mo: 0.01%, Ni: 0.02%, Cr: 0.24%. The carbon content in this paper is low; to increase strength, expensive alloying elements such as Mo and Ni must be added, resulting in high alloy costs. In published patents and literature, the C content is relatively high, and the weldability of the steel strip is generally average ； Some incorporate valuable alloying elements such as Mo, Ni, Cu, etc., resulting in high alloy costs ； Some require whole-tube heat treatment. The steel for straight-seam welded oil casing disclosed above meets the requirements of N80 steel pipes as specified in API Spec 5CT by the American Petroleum Institute, but it has a relatively high carbon content and average weldability and toughness ； Adding alloying elements Mo, Ni, Cu increases the cost of the alloy ； Whole-tube heat treatment is a complex process. Summary of the Invention: To overcome the disadvantages of the existing technologies mentioned above, the objective of the present invention is to provide a straight-seam resistance-welded oil casing steel and a method for its production. In particular, it relates to an N80 grade ERW oil casing steel with high strength, featuring a low carbon content (<0.10%), excellent weldability, and good impact toughness ((TC, transverse Akv > 200 J, SA > 90%)), along with a method for manufacturing such steel. To achieve the above objectives, one of the technical solutions proposed in this invention is to develop a steel grade suitable for N80-class ERW oil casing. The chemical composition (by weight, %) is as follows: C 0.05%-0.09%, Si 0.15%-0.35%, Mn 1.70%-1.95%, P ≤ 0.020%, S ≤ 0.008%, Ti 0.010%-0.050%, Al 0.02%-0.06%, Nb 0.05%-0.08%, Cr 0.20%-0.30%, K 0.008%; the remainder consists of Fe and unavoidable elements. Compared with existing technologies, this solution has a low C content (〈0.10%), the steel strip exhibits excellent weldability and good impact toughness (0°C, Akv>200J, SA>90%); a small amount of Cr replaces expensive elements such as Mo, Ni, and Cu, thereby reducing the cost of the alloy ； After tube manufacturing, only the welds and heat-affected zones require heat treatment; whole-tube heat treatment is not necessary. C: Carbon is an element that contributes to the formation of carbides; it enhances the strength of steel through solid solution strengthening and precipitation strengthening. It is the most effective element for ensuring strength. However, too high a carbon content can affect the weldability and impact toughness of the product, so it is advisable to keep the carbon content between 0.05% and 0.09%. Si: Silicon can provide solid solution strengthening, but excessive amounts of it can reduce the plasticity and toughness of steel; the optimal range is 0.15–0.35%. Mn: Manganese provides solid solution strengthening, increases the stability of austenite, and is also beneficial for improving hardenability, thereby effectively ensuring the strength of the steel. However, an excessive manganese content can increase the tendency for central segregation in the continuous casting billet, affect the microstructural uniformity of the hot-rolled steel, and have a negative impact on weld quality; the optimal range is 1.70%-1.95%. P: Phosphorus is a harmful element in steel; it increases the cold brittleness of steel, deteriorates its weldability, reduces its plasticity, and worsens its cold bending properties. Its content should be controlled at < 0.020%. S: Sulfur is a harmful element in steel; it causes thermal brittleness, reduces the ductility and toughness of steel, and also has an adverse effect on its weldability. Its content should be controlled at less than 0.008%. Note: Niobium is an element that enhances strength through fine grain formation and precipitation, and it can compensate for the decrease in strength caused by reduced carbon levels as well as improve weldability; however, too high a content increases the cost of the alloy, with an appropriate range being 0.05%-0.08%. Ti: Titanium is a strong carbide-forming element that significantly refines austenite grains, thereby compensating for the decrease in strength caused by the reduction of carbon. If the content is too high, coarse TiN will form, reducing the material’s properties; the appropriate range is 0.010%–0.050%. Cr: Chromium can enhance strength through solid solution strengthening and fine grain strengthening, with an appropriate range of 0.20%-0.30%. Al: Aluminum is a commonly used deoxidizer; adding a small amount of aluminum to steel helps to refine the grain structure and improve impact toughness. The appropriate range is 0.02–0.06%. N: Soluble nitrogen has a strong effect of pinning dislocations, which negatively affects toughness; its content should be kept below 0.008%. The second aspect of the technical solution in this invention is to propose a manufacturing method for N80 grade ERW oil casing steel, which includes pre-treatment of molten iron, conversion furnace smelting, secondary refining outside the furnace, continuous casting of slabs, reheating of the cast slabs, rolling, cooling, and coiling. It is characterized by I) pre-treatment of molten iron in the smelting and continuous casting process; converter smelting via top blowing or combined top-bottom blowing; secondary refining in an LF furnace for mild desulfurization, as well as calcium treatment to control the morphology of inclusions and improve the ductility, toughness, and cold bending properties of the steel; and continuous casting of slabs into continuous cast slabs, with the use of electromagnetic stirring or dynamic reduced pressure to enhance the quality of these slabs. 2) The rolling process involves heating the cast slab in a heating furnace to 1200–1270°C, followed by two-stage controlled rolling in a hot continuous rolling mill. In the first stage, the starting temperature is 1100–1200°C, with the final rolling temperature exceeding 980°C; in the second stage, the starting temperature is below 960°C, and the final rolling temperature is 760–810°C. This approach effectively refines the grain structure, which helps to improve strength and toughness. 3) Cooling process: The rolled steel strip is rapidly cooled at a rate of 15–20°C/s, and the strip is coiled at a temperature of 470–520°C. When the steel strip is subjected to controlled cooling at a rapid cooling rate (15–20°C/s), combined with low-temperature coiling (470–520°C), the bainite transformation zone is avoided, resulting directly in a bainite structure along with a small amount of ferrite structure. Increasing the cooling rate simultaneously can minimize the volume fraction of the ferrite structure and relatively shorten its growth process; it also raises the phase transformation energy accumulated in the supercooled austenite, providing a large number of nuclei for ferrite transformation, thereby weakening grain refinement and banded structures. At the same time, the high volume fraction of bainite as a strengthening phase results in a significant increase in the strength of the steel strip, thereby yielding steel strips with high strength and high toughness. The special feature of the technical solution of this invention lies in 1) a low C content (〈0.10%), excellent weldability, and good impact toughness (0°C, Akv>200J, SA>90%) ； 2) Replace precious elements such as Mo, Ni, Cu, etc. with a small amount of Cr to reduce alloy costs ； 3) After the steel strip is used to form the pipe, heat treatment is only required for the welds and the heat-affected zone; there is no need for heat treatment of the entire pipe. The heat treatment process is simple, resulting in low production costs for the steel pipes. Specific embodiments: Further details of the present invention are provided below. The chemical composition of the test steel is shown in Table I, the heating, rolling, and cooling processes are listed in Table 2, and the mechanical properties are given in Table 3. Table I Chemical composition (wt, %) Claim 1. A steel for N80 grade resistance-welded oil casing, characterized in that its chemical composition, by weight percentage, is as follows: C 0.05%-0.09%, Si 0.15%-0.35%, Mn 1.70%-1.95%, P ≤ 0.020%, S ≤ 0.008%, Ti 0.010%-0.050%, Al 0.02%-0.06%, Nb 0.05%-0.08%, Cr 0.20%-0.30%, N ≤ 0.008%; the remainder being Fe and unavoidable elements. 2. A method for manufacturing N80-grade resistance-welded oil casing steel as described in Claim I, comprising hot metal pretreatment, converter smelting, secondary refining and slab continuous casting, reheating of the cast slab, rolling, cooling, and coiling. The characteristic of this method is that the rolling process involves heating the continuous cast slab in a heating furnace to 1200–1270°C, followed by two-stage controlled rolling in a hot strip rolling mill: in the first stage, the starting rolling temperature is 1100–1200°C with a final rolling temperature above 980°C; in the second stage, the starting rolling temperature is below 960°C with a final rolling temperature of 760–810°C. The cooling process involves rapidly cooling the rolled steel strip at a rate of 15–20°C/s, with the strip being coiled at a temperature of 470–520°C. 3. The manufacturing method for N80 grade resistance-welded oil casing steel as described in claim 2, characterized in that the molten iron is pre-treated and smelted in a converter through top blowing or combined top-bottom blowing ； Extracorporeal refining via LF furnace mild desulfurization and calcium treatment ； Continuous casting employs electromagnetic stirring or dynamic mild rolling. Abstract: The present invention discloses an N80-grade steel for resistance-welded oil casing and its manufacturing method. Its chemical composition, by weight percentage, is as follows: C 0.05%-0.09%, Si 0.15%-0.35%, Mn 1.70%-1.95%, P ≤ 0.020%, S ≤ 0.008%, Ti 0.010%-0.050%, Al 0.02%-0.06%, Nb 0.05%-0.08%, Cr 0.20%-0.30%, N ≤ 0.008%; the remainder being Fe and unavoidable elements. Excellent weldability and good impact toughness (0°C, Akv>200J, SA>90%) ； After steel strip is used to manufacture pipes, only the welds and heat-affected zones require heat treatment; there is no need for heat treatment of the entire pipe. The heat treatment process is simple, resulting in low production costs for steel pipes.