A 590MPa-grade H2S stress corrosion resistant coil for oil casing and its manufacturing method

Abstract

This invention relates to the field of oil casing manufacturing technology, and more particularly to a 590MPa grade H2S stress corrosion resistant oil casing coil and its manufacturing method. The composition is as follows: C: 0.07%–0.10%, Si: 0.15%–0.30%, Mn: 0.50%–1.00%, P: ≤0.015%, S: ≤0.004%, Ti: 0.04%–0.06%, Nb: 0.02%–0.04%, Mg: 0.004%–0.006%, B: 0.002%–0.004%, Mo: 0.10%–0.20%, Co: 0.2%–0.4%, Al: 0.02%–0.06%, N: ≤0.008%, with the remainder being Fe and unavoidable elements. The impact energy at -20℃ is greater than 150J, and the Pcm is less than 0.17%, indicating good weldability. The material has high strength, better low-temperature toughness, and excellent SSC resistance, which can prevent brittle fracture of oil well pipes and provide higher safety.

Description

Technical Field

[0001] This invention relates to the field of oil casing manufacturing technology, and in particular to a 590MPa grade H2S stress corrosion resistant oil casing coil and its manufacturing method. Background Technology

[0002] Oil casing is one of the most commonly used oilfield-specific pipe materials, with large usage and the highest cost, making it an indispensable construction material in oil and gas field drilling and production operations. Compared with seamless pipe, HFW (high frequency welded) casing has significant advantages such as uniform wall thickness, high dimensional accuracy, good perforation performance, strong resistance to crushing, and low cost, and is gradually replacing traditional seamless steel pipe.

[0003] With the continuous increase in population and the growing global demand for energy, oil and natural gas production is increasing year by year, making the corrosion and protection of oil casing a growing concern. The development of deep and ultra-deep wells is increasing, and oil extraction conditions are becoming increasingly harsh. In addition to low-molecular-weight alkanes and inert gases, oil and natural gas also contain CO2 and H2S. During natural gas extraction, the water content in the natural gas increases, and H2S dissolves in the water film on the metal surface, forming a corrosive medium. This causes different types of corrosion in different parts of the downhole casing steel, making the service conditions of oil casing increasingly demanding. This requires not only excellent mechanical properties but also good resistance to acid corrosion.

[0004] The coiled steel with a tensile strength of 590 MPa belongs to the M65 steel grade specified in API Spec 5CT. API Spec 5CT specifies that M65 grade steel has a yield strength between 448-586 MPa, a tensile strength ≥586 MPa, an elongation ≥22%, a transverse Charpy impact energy ≥20 J at 0℃, and H2S stress corrosion resistance (SSC) of samples subjected to 72% of the nominal yield strength load using method A solution according to NACE TM0177 without cracking after 720 hours. To ensure that the M65 straight seam welded oil casing requires resistance welding, good weldability and a low cold cracking sensitivity coefficient (Pcm) are required. To ensure impact and extrusion resistance, a certain level of impact toughness is necessary. To ensure HIC resistance, the fineness and uniformity of inclusions and microstructure need to be controlled. To ensure strength, certain alloys need to be added to refine the grain and provide hardenability.

[0005] Chinese patent document CN104357756A discloses "a straight-seam welded oil casing resistant to hydrogen sulfide stress corrosion and its manufacturing method". The composition includes C: 0.08%-0.15%, Si: 0.15%-0.30%, Mn: 0.50%-1.00%, P≤0.010%, S≤0.0050%, Cr: 0.40%-0.65%, Ni≤0.25%, Mo: 0.30%-0.50%, Nb≤0.02%, V: 0.05%-0.10%, Zr: 0.0005%-0.01%, Al: 0.005%-0.01%, Ca: 0.001%-0.003%, B: 0.0005%-0.001%, and RE: 0.001%-0.005%. The steel grade is 90ksi, which is C90 according to API standards. The production process involves ladle refining, RH furnace vacuum treatment, and continuous casting. The steel is then thermomechanically rolled into coils on a continuous rolling mill, processed into tubes via HFW (High-Temperature Winding), and finally tempered at high temperatures and then stress-relieved to obtain qualified steel pipes. However, due to the low C and Mn content and high strength requirements, large amounts of expensive alloying elements such as Mo, Ni, V, and Nb must be added, resulting in high alloy costs. Furthermore, the addition of Zr, Ca, B, and RE complicates the smelting process.

[0006] Chinese patent document CN102296233A discloses "Steel for High-Frequency Resistance Welding of Oil Casing and its Manufacturing Method". The composition includes C: 0.15%-0.35%, Si: 0.10%-0.60%, Mn: 0.30%-1.00%, P≤0.015%, S≤0.003%, Mo: 0.1%-0.5%, V: 0.03%-0.20%, Nb: 0.01%-0.05%, Ti: 0.01%-0.05%, Al: 0.01%-0.08%, Ca: 0.001%-0.005%, and B: 0.001%-0.003%. The steel grade is 80ksi, which is API standard N80. The product is produced through ladle refining, RH furnace vacuum treatment, and continuous casting. It is then thermomechanically rolled into plates and coils on a continuous rolling mill production line, and qualified steel pipes are obtained through HFW pipe making and online weld normalizing treatment. However, HFW has poor weldability and is not conducive to SSC performance; it also has a high content of precious elements such as Mo, V, Nb, and Ti, resulting in high alloy costs. Summary of the Invention

[0007] To overcome the shortcomings of existing technologies, this invention provides a 590MPa grade H2S stress corrosion resistant coiled steel plate for oil well casing and its manufacturing method. It exhibits an impact energy greater than 150J at -20℃, a Pcm less than 0.17%, and good weldability. The material boasts high strength, better low-temperature toughness, and excellent SSC resistance, preventing brittle fracture of the oil well casing and providing enhanced safety.

[0008] To achieve the above objectives, the present invention employs the following technical solution:

[0009] A type of coiled steel sheet for oil casing with H2S stress corrosion resistance of 590MPa grade, the chemical composition of which is as follows by weight percentage:

[0010] C: 0.07%–0.10%, Si: 0.15%–0.30%, Mn: 0.50%–1.00%, P: ≤0.015%, S: ≤0.004%, Ti: 0.04%–0.06%, Nb: 0.02%–0.04%, Mg: 0.004%–0.006%, B: 0.002%–0.004%, Mo: 0.10%–0.20%, Co: 0.2%–0.4%, Al: 0.02%–0.06%, N: ≤0.008%, with the remainder being Fe and unavoidable elements.

[0011] The steel grade in API SPEC 5CT for coiled steel is M65, which is suitable for normalized conditions and has an impact energy greater than 150J and a Pcm less than 0.17 at -20℃.

[0012] The microstructure of the coiled plate is ferrite-bainite, with a ferrite volume fraction of less than 5%. The Pcm is less than 0.17, indicating good weldability. The inclusion levels of A / B / C / D types are below 1.0, the sum of all inclusion levels is below 1.5, and the banded structure is below 0.5.

[0013] A method for manufacturing a coiled plate with 590MPa resistance to H2S stress corrosion in oil casing includes the following steps:

[0014] 1) Continuous casting in smelting:

[0015] Hot metal pretreatment.

[0016] Converter smelting is carried out through top blowing or a combination of top and bottom blowing.

[0017] Ladle refining, LF furnace desulfurization treatment, and calcium treatment.

[0018] Slabs are continuously cast into continuously cast billets, and the continuous casting process employs electromagnetic stirring or dynamic light reduction.

[0019] 2) Rolling:

[0020] The continuously cast slab is heated to 1200–1280°C in a heating furnace, and then subjected to thermomechanical rolling.

[0021] The roughing rolling temperature is 970-1010℃, the finishing rolling temperature is 900-950℃, and the finishing rolling temperature is 780-850℃.

[0022] 3) Cooling:

[0023] The rolled steel strip is cooled at a rate of 17–24 °C / s;

[0024] 4) Winding:

[0025] After cooling, the product is wound up at a temperature of 500–580°C.

[0026] Compared with the prior art, the present invention has at least the following technical effects or advantages:

[0027] 1. In this invention, C (0.07%–0.10%) ensures both the strength and hardness of the material, as well as its weldability, plasticity, and impact toughness. Mg (0.004%–0.006%) reduces the oxygen and sulfur content and the number of inclusions in the steel, purifying the molten steel and significantly modifying inclusions, thus improving the steel's properties. Nb (0.02%–0.04%) gives the steel higher strength and toughness, while maintaining low alloy cost. Mo (0.10%–0.20%) increases the stability of supercooled austenite after dissolving, shifting the C-curve to the right and improving the hardenability of the steel. Mo lowers the phase transformation temperature, inhibits the formation of polygonal ferrite, promotes the transformation of acicular ferrite, and enhances the precipitation strengthening effect of Nb(C,N), simultaneously increasing the steel's strength and lowering the ductile-brittle transition temperature, thus improving resistance to hydrogen sulfide corrosion. B (0.002%–0.004%) significantly improves hardenability and toughness.

[0028] This invention employs a low-carbon, magnesium-containing Nb-Mo-B composite design, resulting in a material with high strength, better low-temperature toughness, and excellent SSC resistance.

[0029] 2. The addition of Mg to refine inclusions in this invention can effectively change the morphology and size of inclusions. Containing magnesium oxide, this structure is finely dispersed in the steel to form composite inclusions, which significantly improves corrosion resistance and enhances the performance and impact resistance of SSC.

[0030] 3. In this invention, boron B replaces expensive alloying elements, reducing alloy costs. 0.002% boron is equivalent to 0.6% manganese, 0.7% chromium, 0.5% molybdenum and 1.5% nickel. Therefore, a small amount of boron can be added to replace expensive alloying elements.

[0031] 4. The Pcm of this invention is below 0.17%, resulting in good weldability. Existing M65 welds generally use C content above 0.20% and Mn content above 1.0%, with a Pcm close to 0.25%. When the Pcm value is greater than 0.25, the tendency for cold cracking increases significantly, and weldability decreases. The lower the Pcm, the better the weldability; this invention has a Pcm below 0.15%, resulting in excellent weldability.

[0032] 5. The roughing and finishing rolling temperature of this invention is 970-1010℃, the finishing rolling temperature is 900-950℃, and the finishing rolling temperature is 780-850℃. This temperature range is conducive to the precipitation of Nb and Ti, refines the grain size, and improves strength and toughness.

[0033] 6. The steel strip of this invention is cooled at a rate of 17-24℃ / s after rolling. This cooling rate promotes the formation of bainite, which is beneficial for refining ferrite and bainite, reduces the yield strength decrease caused by the Bauschinger effect during tube making, and can also reduce the banded structure to below grade 0.5, significantly improving strength, toughness and corrosion resistance.

[0034] 7. After cooling, the product is wound up at a temperature of 500-580℃. This temperature range is conducive to the uniformity and refinement of the bainite structure, and avoids the adverse effects of uneven structure on the SSC performance. Detailed Implementation

[0035] This invention discloses a 590MPa-grade H2S stress corrosion resistant coiled steel plate for oil casing and its manufacturing method. Those skilled in the art can refer to this document and appropriately modify the process parameters to achieve the desired result. It should be particularly noted that all similar substitutions and modifications are obvious to those skilled in the art and are considered to be included in this invention. The methods and applications of this invention have been described through preferred embodiments; those skilled in the art can clearly modify or appropriately change and combine the methods and applications described herein without departing from the content, spirit, and scope of this invention to realize and apply the technology of this invention.

[0036] A type of coiled steel sheet for oil casing with H2S stress corrosion resistance of 590MPa grade, the chemical composition of which is as follows by weight percentage:

[0037] C: 0.07%–0.10%, Si: 0.15%–0.30%, Mn: 0.50%–1.00%, P: ≤0.015%, S: ≤0.004%, Ti: 0.04%–0.06%, Nb: 0.02%–0.04%, Mg: 0.004%–0.006%, B: 0.002%–0.004%, Mo: 0.10%–0.20%, Co: 0.2%–0.4%, Al: 0.02%–0.06%, N: ≤0.008%, with the remainder being Fe and unavoidable elements.

[0038] Compared with existing technologies, this solution adopts a low-carbon magnesium-containing Nb-Mo-B composite design, which results in high material strength, better low-temperature toughness, and excellent SSC resistance. Mg refines inclusions, effectively changing their morphology and size, which is beneficial to SSC performance and impact resistance. Mo and B effectively improve strength and enhance SSC resistance. Pcm is less than 0.17%, resulting in good weldability.

[0039] The following details the mechanism of action of each alloy component in this invention, where the percentage symbol % represents a weight percentage:

[0040] Carbon (C) is a carbide-forming element and the most effective element for ensuring strength. It improves hardenability and guarantees the material's strength and hardness. Only with sufficient carbon can enough carbon-rich retained austenite be formed and stabilized at room temperature, ultimately forming a small amount of uniform and fine M / A microstructure. If the carbon content is too low, the material's hardness cannot be guaranteed, resulting in poor threading performance. If the content is too high, it will affect the product's weldability, plasticity, and impact toughness. The optimal range is 0.07%-0.10%.

[0041] Si can dissolve in ferrite and austenite, playing a certain role in solid solution strengthening. It can significantly improve the hardness and strength of steel, as well as increase fatigue strength and fatigue ratio. However, excessive content will significantly reduce the plasticity and toughness of steel. Its optimal range is 0.15%-0.30%.

[0042] Mn: Manganese has a solid solution strengthening effect and can increase the stability of austenite, which is also beneficial to improving hardenability and effectively ensuring the strength of steel. Manganese and iron form a solid solution, increasing the hardness and strength of ferrite and austenite in steel. At the same time, it is a carbide-forming element, entering cementite to replace some iron atoms. In steel, manganese lowers the critical transformation temperature, shifts the C-curve to the right, and promotes bainite transformation. However, excessive manganese content can increase the tendency of center segregation in continuously cast billets, resulting in more banded structures in the steel plate. Ultimately, the steel plate will contain a certain amount of banded structures, increasing brittleness, reducing plasticity, and worsening corrosion resistance. At the same time, it easily forms MnS, which has a detrimental effect on SSC performance. Its optimal range is 0.50%-1.00%.

[0043] P, S, and N are unavoidable impurity elements in steel, and it is desirable to keep them as low as possible. However, excessively low levels will increase production costs. In this invention, P ≤ 0.015%, S ≤ 0.004%, and N ≤ 0.006%.

[0044] Titanium (Ti): Titanium is a strong nitrogen-fixing element. Adding approximately 0.015% Ti can form high-temperature stable, fine TiN precipitates during slab continuous casting. These fine TiN precipitates effectively prevent austenite grain growth during heating and significantly improve the toughness of the heat-affected zone during welding. Higher Ti content yields more TiC particles, increasing steel strength through strain-induced precipitation and phase transformation precipitation. Simultaneously, the precipitated TiC provides strong precipitation strengthening, ensuring minimal grain growth during subsequent normalizing heat treatment, thus guaranteeing the overall tube's strength meets M65 performance requirements. However, excessively high Ti content increases alloy cost. The optimal range is 0.04%-0.06%.

[0045] Niobium (Nb) is a grain refiner and precipitation strengthening element that can compensate for the decrease in strength caused by the reduction in carbon content and improve impact performance. It also plays a role in inhibiting grain growth during heating and hot rolling, thereby refining the grains after quenching, ensuring impact toughness, and giving the steel higher strength and toughness. However, excessive amounts will increase the cost of the alloy. The appropriate range is 0.02%-0.04%.

[0046] Mg: Magnesium has strong chemical reactivity and a strong affinity for oxygen and sulfur, making it an effective refining agent. It can reduce the oxygen and sulfur content and the number of inclusions in steel, purifying the molten steel. It can also significantly modify inclusions in steel, improving its properties. Trace amounts of magnesium can alter the quantity, type, size, and distribution of sulfides, carbides, and carbonitrides. Magnesium-treated inclusions are mixed phases, with magnesium oxides at their core. This finely dispersed structure forms composite inclusions in the steel, improving its properties and corrosion resistance. The suitable range is 0.004%-0.006%.

[0047] B: Boron can significantly improve hardenability and toughness. Boron has a strong ability to improve hardenability; 0.002% boron is equivalent to 0.6% manganese, 0.7% chromium, 0.5% molybdenum, and 1.5% nickel. Therefore, adding a small amount of boron can replace expensive alloying elements. Too high or too low a boron content will affect hardenability. The suitable range is 0.002%-0.004%.

[0048] Mo (Mo): Molybdenum can dissolve into solid solutions, thus increasing strength. After dissolving into austenite, Mo increases the stability of supercooled austenite, shifting the C-curve to the right and improving the hardenability of steel. Mo lowers the phase transformation temperature, inhibits the formation of polygonal ferrite, promotes the transformation of acicular ferrite, and enhances the precipitation strengthening effect of Nb(C,N). Therefore, it can simultaneously increase the strength of steel and lower the ductile-brittle transition temperature, improving resistance to HIC (high brittle fracture). The suitable range is 0.10%-0.20%.

[0049] Co: Under acidic H2S conditions, a certain amount of cobalt will accumulate on the surface of steel, inhibiting hydrogen penetration into the steel and thus improving its resistance to saturation corrosion cracking (SSC). If the content is too low, the SSC resistance effect will not be achieved; if the content is too high, it will reduce the hardenability of the steel, which is detrimental to improving its strength. The suitable range is 0.2%-0.4%.

[0050] Als: Aluminum is a commonly used deoxidizer. Adding a small amount of aluminum to steel can refine the grains and improve impact toughness. The Al content of this invention is 0.02%-0.05%.

[0051] A method for manufacturing a 590MPa grade H2S stress corrosion resistant oil casing coil includes hot metal pretreatment, steelmaking, ladle refining and slab continuous casting, continuous casting billet reheating, rolling, cooling and coiling, specifically including the following steps:

[0052] 1) Continuous casting process:

[0053] Hot metal pretreatment, converter smelting – top blowing or top-bottom combined blowing, ladle refining, LF furnace light desulfurization treatment and calcium treatment to control inclusion morphology and improve the ductility, toughness and cold bending performance of steel.

[0054] Slabs are continuously cast into slabs – continuous casting employs electromagnetic stirring or dynamic light pressure to improve the quality of the slabs.

[0055] 2) Rolling and cooling processes:

[0056] The continuously cast slab is heated to 1200-1280℃ in a heating furnace and then subjected to thermomechanical rolling.

[0057] The roughing rolling temperature is 970-1010℃, the finishing rolling temperature is 900-950℃, and the finishing rolling temperature is 780-850℃. This temperature range is conducive to the precipitation of Nb and Ti, refines the grain size, and improves strength and toughness.

[0058] After rolling, the steel strip is cooled at a rate of 17-24℃ / s. This cooling rate promotes the formation of bainite, which is beneficial for refining ferrite and bainite, reduces the yield strength decrease caused by the Bauschinger effect during tube making, and can also reduce the banded structure to below grade 0.5, significantly improving strength, toughness and corrosion resistance.

[0059] After cooling, the material is wound up at a temperature of 500-580℃. This temperature range is conducive to the uniformity and refinement of the bainite structure, and avoids the adverse effects of uneven structure on the SSC performance.

[0060] The final microstructure of the coil is ferrite-bainite, with a ferrite volume fraction of less than 5%, Pcm of less than 0.17%, inclusions of type A / B / C / D below grade 1.0, inclusions of all types below grade 1.5, and banded microstructure below grade 0.5.

[0061]

Example

[0062] A 590MPa-grade H2S stress corrosion resistant oil casing coil and its manufacturing method are described in detail below:

[0063] Table 1 shows the chemical composition of the coils in the examples; Table 2 shows the heating, rolling, and cooling process parameters in the examples; and Table 3 shows the mechanical property test results of the examples.

[0064] Table 1 Chemical composition (wt%) of the examples

[0065]

[0066]

[0067] Table 2 Heating, Rolling, and Cooling Process Parameters

[0068]

[0069] Table 3 Mechanical and Corrosion Resistance Properties

[0070]

[0071]

[0072] As shown in Tables 1-3, the 590MPa grade H2S stress corrosion resistant oil casing coil produced by adopting the composition design, rolling and coiling process of the present invention meets the requirements of API SPEC 5CT standard M65 grade mechanical properties and corrosion resistance.

[0073] 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 method for manufacturing a coiled plate with 590MPa-grade H2S stress corrosion resistance for oil casing, characterized in that, Its chemical composition by weight percentage is as follows: C: 0.07%~0.10%, Si: 0.15%~0.30%, Mn: 0.50%~0.90%, P: ≤0.015%, S: ≤0.004%, Ti: 0.04%~0.06%, Nb: 0.02%~0.04%, Mg: 0.004%~0.006%, B: 0.003%~0.004%, Mo: 0.10%~0.20%, Co: 0.2%~0.4%, Al: 0.02%~0.06%, N: ≤0.008%, Pcm less than 0.17%, the remainder being Fe and unavoidable elements; The microstructure of the coiled plate is ferrite-bainite, with a ferrite volume fraction of less than 5%; the inclusion level of A / B / C / D types is below 1.0, the sum of each type is below 1.5, and the banded structure is below 0.5; the impact energy at -20℃ is ≥200J; The manufacturing method specifically includes the following steps: 1) Continuous casting in smelting: Ladle refining, LF furnace desulfurization treatment, and calcium treatment. Slabs are continuously cast into continuously cast billets, and the continuous casting process employs electromagnetic stirring or dynamic light reduction. 2) Rolling: The continuously cast slab is heated to 1200~1280℃, and then subjected to thermomechanical rolling. The roughing rolling temperature is 970~1010℃, the finishing rolling temperature is 900~950℃, and the finishing rolling temperature is 780~850℃. 3) Cooling: The rolled steel strip is cooled at a rate of 17~24℃ / s; 4) Winding: After cooling, the product is wound up at a temperature of 500~580℃.

2. The method for manufacturing a 590MPa grade H2S stress corrosion resistant coiled plate for oil casing according to claim 1, characterized in that, The first part refers to converter smelting via top blowing or a combination of top and bottom blowing.

3. The method for manufacturing a 590MPa grade H2S stress corrosion resistant coiled plate for oil casing according to claim 1, characterized in that, The 2) continuously cast slab is heated to 1200~1280℃ in a heating furnace.

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