A 690MPa-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 particularly to a 690MPa grade H2S stress corrosion resistant oil casing coil and its manufacturing method. Its chemical composition by weight percentage is: C: 0.13%–0.17%, Si: 0.15%–0.30%, Mn: 1.40%–1.70%, P: ≤0.013%, S: ≤0.004%, Ti: 0.01%–0.03%, Nb: 0.06%–0.10%, V: 0.04%–0.08%, Mg: 0.005%–0.007%, Zr: 0.001%–0.004%, rare earth elements: 0.001%–0.003%, Al: 0.02%–0.06%, N: ≤0.008%, with the remainder being Fe and unavoidable elements. The material exhibits high strength, better low-temperature toughness, and excellent SSC resistance, preventing brittle fracture of the oil well tubing and providing higher safety. The final microstructure of the coil is acicular ferrite; Pcm is less than 0.25%, A / B / C / D type inclusions are below grade 1.0, the sum of each type is below grade 1.5, the banded structure is below grade 0.5, and the impact energy at -10℃ is greater than 180J.

Description

Technical Field

[0001] This invention relates to the field of oil casing manufacturing technology, and in particular to a 690MPa grade H2S stress corrosion resistant oil casing coil and its manufacturing method. Background Art

[0002] Oil casing is one of the most commonly used oilfield-specific pipe materials, used in large quantities and at high cost, and is 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, the growth in global energy demand is also becoming increasingly significant, and natural gas production has maintained a high growth rate. Corrosion and protection of oil casing and tubing have gradually become a major concern. H2S corrosion of oil casing and tubing has become a major type of corrosion faced by oil and gas fields, and developing oil casing and tubing with good corrosion resistance has significant social and economic implications.

[0004] The coiled steel has a tensile strength of 690 MPa and belongs to the N80 steel grade specified in API Spec 5CT. API Spec 5CT specifies that N80 grade steel has a yield strength between 552-758 MPa, a tensile strength ≥689 MPa, an elongation ≥19%, and a transverse Charpy impact energy ≥60 J at 0℃. For H2S stress corrosion resistance (SSC), samples were tested according to NACE TM0177 using method A solution with 72% of the nominal yield strength and did not crack after 720 hours. Since N80 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 H2S 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] In summary, ensuring that the steel coil with a tensile strength of 690MPa has high strength, good weldability, and excellent resistance to SSC (Special Stress Compressor) while simultaneously ensuring better low-temperature toughness of the steel pipe has become a pressing technical problem to be solved. Summary of the Invention

[0006] To overcome the shortcomings of existing technologies, this invention provides a 690MPa grade H2S stress corrosion resistant coiled steel plate for oil well casing and its manufacturing method. The material has high strength, better low-temperature toughness, and excellent SSC resistance, preventing brittle fracture of the oil well casing and providing higher safety.

[0007] To achieve the above objectives, the present invention employs the following technical solution:

[0008] A type of coiled steel sheet with 690MPa resistance to H2S stress corrosion in oil casing has the following chemical composition by weight percentage:

[0009] C: 0.13%–0.17%, Si: 0.15%–0.30%, Mn: 1.40%–1.70%, P: ≤0.013%, S: ≤0.004%, Ti: 0.01%–0.03%, Nb: 0.06%–0.10%, V: 0.04%–0.08%, Mg: 0.005%–0.007%, Zr: 0.001%–0.004%, rare earth elements: 0.001%–0.003%, Al: 0.02%–0.06%, N: ≤0.008%, the remainder being Fe and unavoidable elements.

[0010] The final microstructure of the coil is acicular ferrite; Pcm is less than 0.25%, A / B / C / D type inclusions are below grade 1.0, the sum of each type is below grade 1.5, the banded structure is below grade 0.5, and the impact energy at -10℃ is greater than 180J.

[0011] A method for manufacturing a coiled plate with 690MPa resistance to H2S stress corrosion in oil casing includes the following steps:

[0012] 1) Continuous casting in smelting:

[0013] Hot metal pretreatment.

[0014] Converter smelting is carried out through top blowing or a combination of top and bottom blowing.

[0015] Ladle refining, LF furnace desulfurization treatment, and calcium treatment.

[0016] Slabs are continuously cast into continuously cast billets, and the continuous casting process employs electromagnetic stirring or dynamic light reduction.

[0017] 2) Rolling:

[0018] The continuously cast slab is heated to 1210–1280°C in a heating furnace, and then subjected to thermomechanical rolling.

[0019] The roughing rolling temperature is 1020-1060℃, the finishing rolling temperature is 960-990℃, and the finishing rolling temperature is 830-880℃.

[0020] 3) Cooling:

[0021] Laminar flow cooling is used after rolling, with a cooling rate of 26–30 °C / s.

[0022] 4) Winding:

[0023] After cooling, the product is wound up at a temperature of 400–450°C.

[0024] Compared with the prior art, the present invention has at least the following technical effects or advantages:

[0025] 1. In this invention, C (0.13%–0.17%) ensures both the material's hardness and its weldability, plasticity, and impact toughness; Mg (0.005%–0.007%) effectively guarantees the steel's strength and resistance to HIC and SSC; Nb (0.06%–0.10%) ensures impact toughness; Ti (0.01%–0.03%) ensures the overall tube's strength after heat treatment meets the N80 high-strength performance requirements, and the alloy cost is low; V (0.04%–0.08%) improves the steel's hardenability. It has a solid solution strengthening effect, effectively improving strength. It has a grain-refining effect, thus benefiting low-temperature impact toughness. It forms carbides with carbon, improving resistance to hydrogen corrosion under high temperature and pressure.

[0026] This invention employs a low-carbon, magnesium-containing Nb-V-Ti composite design, resulting in a material with high strength, better low-temperature toughness, and excellent SSC resistance.

[0027] 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.

[0028] 3. The present invention combines the effects of Nb, V and Ti to effectively control Pcm below 0.25%, while effectively improving strength and toughness, and enhancing SSC resistance.

[0029] 4. The Pcm of this invention is below 0.25%, resulting in good weldability. Existing N80 weld metal generally uses C content above 0.20% and Mn content above 1.4%, with Pcm exceeding 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 0.25%, resulting in excellent weldability.

[0030] 5. The present invention controls the roughing and finishing rolling temperature to 1020-1060℃, the finishing rolling temperature to 960-990℃, and the finishing rolling temperature to 830-880℃. This temperature range is conducive to the precipitation of Nb, V, and Ti at different stages, refines the grain size, and improves strength and toughness.

[0031] 6. In this invention, the rolled steel strip is cooled at a rate of 26-30℃ / s. At this cooling rate, complete acicular ferrite is obtained. This microstructure significantly improves strength and toughness, and also enhances resistance to HIC. It is beneficial for refining the final microstructure, reducing the yield strength decrease caused by the Bauschinger effect during tube making, and can also reduce the banded microstructure to below grade 0.5, significantly improving strength, while simultaneously improving toughness and corrosion resistance.

[0032] 7. After cooling, the present invention is wound up at a temperature of 400-450℃. This temperature range is conducive to the uniformity and refinement of the acicular ferrite structure, especially to avoid the growth and aggregation of M / A components, and to avoid the adverse effect of uneven structure on SSC performance. Detailed Implementation

[0033] This invention discloses a 690MPa-grade H2S stress corrosion resistant coiled steel plate for oil casing and its manufacturing method. Those skilled in the art can refer to the content of 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, and 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.

[0034] A type of coiled steel sheet with 690MPa resistance to H2S stress corrosion in oil casing has the following chemical composition by weight percentage:

[0035] C: 0.13%–0.17%, Si: 0.15%–0.30%, Mn: 1.40%–1.70%, P: ≤0.013%, S: ≤0.004%, Ti: 0.01%–0.03%, Nb: 0.06%–0.10%, V: 0.04%–0.08%, Mg: 0.005%–0.007%, Zr: 0.001%–0.004%, rare earth elements: 0.001%–0.003%, Al: 0.02%–0.06%, N: ≤0.008%, the remainder being Fe and unavoidable elements.

[0036] Compared with existing technologies, this solution adopts a low-carbon magnesium-containing Nb-V-Ti 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. Nb and V effectively improve strength and enhance SSC resistance. Pcm is less than 0.25%, resulting in good weldability.

[0037] The following details the mechanism of action of each alloy component in this invention, where the percentage symbol % represents a weight percentage:

[0038] 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 0.13% to 0.17%.

[0039] 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%.

[0040] 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, shifting the C-curve to the right and promoting bainite transformation. However, excessive manganese content can increase the tendency of center segregation in continuously cast billets, leading to an increase in banded structures in steel plates. Ultimately, the steel plates will contain a certain amount of banded structures, increasing brittleness, reducing plasticity, and worsening corrosion resistance. Simultaneously, it readily forms MnS, which has a detrimental effect on HIC resistance; the optimal range is 1.40%-1.70%.

[0041] 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 ≤ 0.013%, S ≤ 0.004%, and N ≤ 0.006%.

[0042] 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. The precipitated TiC produces a strong precipitation strengthening effect, ensuring that the grains do not grow significantly during subsequent normalizing heat treatment of the steel pipe, thus guaranteeing that the overall strength of the pipe after heat treatment meets the N80 high-strength performance requirements. However, excessively high content leads to higher alloy costs. The optimal range is 0.01%-0.03%.

[0043] 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 0.06%-0.10%.

[0044] Vanadium (V): Vanadium shrinks the austenite phase region, is infinitely soluble in ferrite, and is a strong carbide and nitride forming element. Its solution in austenite improves the hardenability of steel; its solution in ferrite has a very strong solid solution strengthening effect, effectively increasing tensile strength and reducing the yield strength ratio. It has a grain-refining effect, thus benefiting low-temperature impact toughness. Vanadium forms carbides with carbon, which improves resistance to hydrogen corrosion under high temperature and pressure. The suitable range is 0.04%-0.08%.

[0045] 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 structure is finely dispersed in the steel, forming composite inclusions that improve steel properties and corrosion resistance. The suitable range is 0.005%-0.007%.

[0046] Zr: Trace amounts of zirconium have deoxidizing, purifying, and grain-refining effects, and significantly modify inclusions, improving the low-temperature toughness of steel. When added in combination with Cr, it can significantly inhibit recrystallization of the alloy and improve its resistance to SSC corrosion. The optimal range is 0.001%-0.004%.

[0047] Rare earth elements can refine sulfides in steel, improving its resistance to sulfur dioxide (SSC). They can also combine with phosphorus (P) to inhibit P segregation, thus suppressing the decrease in SSC resistance caused by P segregation. However, when the content is too high, the oxides become coarse, leading to a decrease in SSC resistance. The optimal range is 0.001%-0.003%.

[0048] Al: 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 in this invention is 0.02%-0.05%.

[0049] A method for manufacturing a 690MPa 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:

[0050] 1) Continuous casting process:

[0051] 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, slab continuous casting to produce continuous casting slabs – continuous casting adopts electromagnetic stirring or dynamic light reduction to improve the quality of continuous casting slabs.

[0052] 2) Rolling and Cooling Process: The continuously cast slab is heated to 1210-1280℃ in a furnace, and then thermomechanically rolled. The roughing rolling temperature is 1020-1060℃, the finishing rolling temperature is 960-990℃, and the final rolling temperature is 830-880℃. This rolling temperature range is conducive to the full precipitation of Nb, V, and Ti at different stages, refining the grain size and improving strength and toughness. After rolling, the steel strip is cooled at a rate of 26-29℃ / s. At this cooling rate, a complete acicular ferrite structure can be obtained, which has the best strength and toughness matching and can significantly improve strength and toughness. At the same time, this cooling rate is beneficial to refining the final structure, reducing the yield strength decrease caused by the Bauschinger effect during tube making, and can also reduce the banded structure to below grade 0.5, improving the resistance to SSC corrosion. After cooling, the material is wound up at a temperature of 400-450℃. This temperature range is conducive to the uniformity and refinement of the acicular ferrite structure, especially to avoid the growth and aggregation of M / A components, and to avoid the formation of uneven structure that would have an adverse effect on the SSC performance.

[0053] 3) The final microstructure of the coil is acicular ferrite; Pcm is less than 0.25%, A / B / C / D type inclusions are below grade 1.0, the sum of each type is below grade 1.5, and the banded structure is below grade 0.5.

[0054]

Example

[0055] A 690MPa-grade H2S stress corrosion resistant oil casing coil and its manufacturing method are described below:

[0056] 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.

[0057] Table 1 Chemical composition (wt%) of the examples

[0058]

[0059] Table 2 Heating, Rolling, and Cooling Process Parameters

[0060]

[0061]

[0062] Table 3 Mechanical and Corrosion Resistance Properties

[0063]

[0064] As can be seen from Tables 1-3, the 690MPa grade H2S stress corrosion resistant oil casing coil produced by adopting the composition design and rolling and coiling process of the present invention meets the requirements of API SPEC 5CT standard N80 grade mechanical properties and corrosion resistance.

[0065] 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 690MPa-grade H2S stress corrosion resistance for oil casing, characterized in that, The chemical composition of the coiled steel sheet by weight percentage is as follows: C: 0.13%–0.17%, Si: 0.15%–0.30%, Mn: 1.40%–1.70%, P: ≤0.013%, S: ≤0.004%, Ti: 0.01%–0.03%, Nb: 0.06%–0.10%, V: 0.06%–0.08%, Mg: 0.005%–0.007%, Zr: 0.001%–0.003%, rare earth elements: 0.001%–0.003%, Al: 0.02%–0.06%, N: ≤0.008%, Pcm less than 0.25%, with the remainder being Fe and unavoidable elements; The final microstructure of the coil is acicular ferrite; the inclusion level of A / B / C / D types is below 1.0, the sum of each type is below 1.5, the banded structure is below 0.5, and the impact energy at -10℃ is 210~260J; Specifically, the steps include the following: 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 1210–1280℃, and then subjected to thermomechanical rolling. The roughing rolling temperature is 1020-1060℃, the finishing rolling temperature is 960-990℃, and the finishing rolling temperature is 850-880℃. 3) Cooling: Laminar flow cooling is used after rolling, with a cooling rate of 26–30 °C / s. 4) Winding: After cooling, the product is wound up at a temperature of 400–450°C.

2. The method for manufacturing a 690MPa grade H2S stress corrosion resistant coiled plate for oil casing according to claim 1, characterized in that, The first step is that the converter smelting is carried out by top blowing or top-bottom combined blowing.

3. The method for manufacturing a 690MPa 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 1210-1280 in a heating furnace.

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