Steel for 500mpa grade high toughness carbon dioxide corrosion resistance and method for manufacturing the same

Abstract

The present application relates to a kind of 500MPa grade high toughness carbon dioxide corrosion resistant steel and its manufacturing method, steel chemical composition includes C, Si, Mn, Ni, Cu, Nb, Co, Sn etc., the rest is Fe and impurity element.For the existing 500MPa grade pipeline steel coil plate does not have carbon dioxide corrosion resistance, cannot adapt to supercritical carbon dioxide delivery service environment, and impact energy is low, poor safety and other problems, with low-carbon low-manganese design, through Ni and Co synergistically promote steel strength, toughness and carbon dioxide corrosion resistance;Through Cu and Sn compound addition synergistic effect, solid solution strengthening improves the tensile strength of steel and carbon dioxide corrosion resistance.

Description

Technical Field

[0001] This invention relates to the field of pipeline steel production technology, and in particular to a 500MPa grade high-toughness steel resistant to carbon dioxide corrosion and its manufacturing method. Background Technology

[0002] Carbon capture, utilization and storage (CCUS) technology is a key technology for addressing global climate change, and carbon dioxide transportation is a crucial link in the CCUS industry chain that connects carbon dioxide capture and storage. The efficiency and cost of carbon dioxide transportation will directly affect the overall scale and economic benefits of CCUS.

[0003] The critical pressure of pure carbon dioxide is 7.38 MPa, and its critical temperature is 31.1℃. Supercritical transport refers to transport at pressures higher than the critical pressure, characterized by high density and low viscosity. When the entire pipeline transport process is in a supercritical state, transportation is most efficient, and wear is minimal. However, the free water and impurity gases present in supercritical carbon dioxide pipelines are highly corrosive. Existing API carbon steel pipeline systems used for transporting natural gas experience occasional corrosion, potentially leading to leaks and failures.

[0004] Pipes are fundamental to ensuring the safe transportation of goods through pipelines. They constitute a significant portion of the total investment in pipeline construction. Due to factors such as corrosion and damage from third parties, pipe walls may thin or fail, potentially leading to various safety accidents. Key technical requirements for supercritical pipes include: low-temperature brittleness when carbon dioxide leaks and the temperature drops rapidly to -50°C, and the corrosion rate in a supercritical carbon dioxide environment.

[0005] Currently, no 500MPa-class carbon dioxide transmission pipelines for large-capacity, long-distance transportation have been developed, either domestically or internationally. A search revealed the following relatively close patent documents: 1) Chinese patent application CN112941422A discloses "a CO2 corrosion-resistant steel plate and its preparation method." The steel composition includes C: 0.03%–0.07%, Cr: 4.0%–6.0%, Ni: 0.15%–2.50%, Nb: 0.01%–0.06%, P≤0.005%, and S≤0.0050%. It is produced using a medium-thick plate rolling mill, and the rolled steel plate requires quenching and tempering heat treatment. The high Cr content of this steel plate classifies it as stainless steel, making it unsuitable for conventional smelting and continuous casting production lines. Furthermore, the excessively high Cr content prevents straight seam welding, and the steel plate's impact toughness is insufficient to meet the crack-stopping requirement of -50℃ during carbon dioxide leakage.

[0006] 2) Chinese patent application CN106498279A discloses "a low-Cr economical X65 pipeline steel resistant to CO2 corrosion and its production method." The steel composition includes C: 0.04%–0.05%, Si: 0.18%–0.22%, Mn: 0.50%–0.60%, Cr: 0.1%–0.2%, Mo: 0.10%–0.15%, Nb: 0.035%–0.050%, V: 0.020%–0.030%, Ti: 0.010%–0.020%, P≤0.01%, and S≤0.0030%. The low Mn content in the steel plate leads to poor hardenability during subsequent straight seam welding, making it impossible to guarantee the low-temperature toughness of the weld and heat-affected zone at -50℃. Furthermore, the base material's low-temperature toughness at -20℃ is generally poor, failing to meet the crack-stopping requirement at -50℃ during carbon dioxide leakage. Summary of the Invention

[0007] This invention provides a 500MPa grade high-toughness steel resistant to carbon dioxide corrosion and its manufacturing method. Addressing the problems of existing 500MPa grade pipeline steel coils lacking carbon dioxide corrosion resistance, being unsuitable for supercritical carbon dioxide transport environments, and exhibiting low impact energy and poor safety, this invention employs a low-carbon, low-manganese design. It utilizes N and Co synergistically to enhance the steel's strength, toughness, and carbon dioxide corrosion resistance; and employs Cu and Sn composite addition for synergistic enhancement, solid solution strengthening to improve the steel's tensile strength and carbon dioxide corrosion resistance.

[0008] To achieve the above objectives, the present invention employs the following technical solution: This is a 500MPa grade high-toughness steel resistant to carbon dioxide corrosion. The chemical composition of the steel, by weight percentage, is: C: 0.02%–0.05%, Si: 0.40%–0.60%, Mn: 1.20%–1.50%, P≤0.020%, S≤0.002%, Ni: 0.40%–0.60%, Cu: 0.35%–0.55%, Nb: 0.09%–0.12%, Co: 0.10%–0.30%, Sn: 0.03%–0.05%, N≤0.006%. Among these, Ni / Co = 2.0–4.0, Nb / Co = 0.2–0.6, Cu / Sn = 8.0–15.0; the remainder is Fe and unavoidable impurity elements.

[0009] The finished steel plate has a single sub-bainitic structure.

[0010] The mechanical properties of the finished steel plate are as follows: transverse / longitudinal yield strength ≥500MPa, tensile strength ≥600MPa, yield strength ratio ≤0.85, elongation ≥30%, impact energy at -50℃ ≥300J, DWTT at -20℃ ≥95%, and average corrosion rate in a 14.5MPa supercritical carbon dioxide environment ≤0.15mm / a.

[0011] The manufacturing method of 500MPa grade high toughness and carbon dioxide corrosion resistant steel includes steelmaking, ladle refining, slab continuous casting, continuous casting billet heating, rolling, cooling and coiling. The following process is controlled: 1) Ladle refining: using RH+LF refining process; 2) Slab continuous casting: The continuous casting process adopts electromagnetic stirring or dynamic light reduction; 3) Heating of continuous casting slab: Heating the continuous casting slab to 1000-1100℃ and holding for 150-250 minutes; 4) Rolling: TMCP rolling process is adopted; the single-pass reduction rate of roughing is 20% to 40%, and the finishing temperature of roughing is 960 to 1000℃; the starting temperature of finishing rolling is 850 to 890℃, and the finishing temperature of finishing rolling is 700 to 750℃. 5) Cooling: The cooling rate is 31-45℃ / s, and the final cooling temperature is 200-270℃.

[0012] Compared with the prior art, the beneficial effects of the present invention are: 1) The steel used in this invention is designed to be low in carbon and manganese, with low carbon content, resulting in a small tendency for hardening of the heat-affected zone during welding and making it less prone to welding cracks. 2) The finished product has a uniform lower bainite single structure, which makes the material more tough; and the microcouple effect is weak, which makes it more resistant to carbon dioxide corrosion. 3) The synergistic addition of Ni and Co enhances both strength and toughness, and improves resistance to carbon dioxide corrosion. 4) Cu and Sn work synergistically to enhance the tensile strength and carbon dioxide corrosion resistance of steel through solid solution strengthening. Detailed Implementation

[0013] The 500MPa grade high-toughness carbon dioxide corrosion resistant steel of this invention has the following chemical composition by weight percentage: C: 0.02%–0.05%, Si: 0.40%–0.60%, Mn: 1.20%–1.50%, P≤0.020%, S≤0.002%, Ni: 0.40%–0.60%, Cu: 0.35%–0.55%, Nb: 0.09%–0.12%, Co: 0.10%–0.30%, Sn: 0.03%–0.05%, N≤0.006%; wherein, Ni / Co = 2.0–4.0, Nb / Co = 0.2–0.6, Cu / Sn = 8.0–15.0; the remainder is Fe and unavoidable impurity elements.

[0014] The rationale for the chemical composition design of the 500MPa grade high-toughness, carbon dioxide corrosion-resistant steel described in this invention is as follows: Carbon (C) is a carbide-forming element. It significantly improves strength through solid solution strengthening and phase transformation strengthening, making it the most effective element for ensuring strength. Carbon also enhances hardenability, ensuring both strength and hardness. C interacts with Nb to form stable carbonitrides, refining grains, achieving precipitation strengthening, and improving the strength and toughness of steel. In this invention, sufficient carbon is required to form lower bainite. Too low a carbon content cannot guarantee the tensile strength and hardness of the material; however, too high a content can easily cause center segregation in the steel plate, negatively impacting its corrosion resistance and crack arrest toughness, and affecting the weldability of the product. Therefore, this invention controls the carbon content to be within the range of 0.02% to 0.05%.

[0015] Si can dissolve in ferrite and austenite, playing a certain role in solid solution strengthening, significantly improving the hardness and tensile strength of steel, while promoting ferrite grain coarsening and reducing the anisotropy of the transverse and longitudinal properties of the steel plate. SiO2 and dense ferrosilicon inclusions can disperse on the steel surface like "ceramic particles," hindering the contact between CO2 molecules and the steel matrix, reducing the occurrence of carbon dioxide corrosion reaction, and improving the resistance to carbon dioxide corrosion. SiO2 particles can enhance the adhesion between the corrosion product film and the steel matrix, preventing "secondary corrosion" caused by film detachment, and extending the service life of the steel. However, an increase in silicon content will reduce the low-temperature toughness, plasticity, and weldability of the steel. Therefore, this invention controls the silicon content to be 0.40% to 0.60%.

[0016] Mn: Manganese has a solid solution strengthening effect. The solid solution formed by manganese and iron can improve the hardness and strength of ferrite and austenite in steel. Manganese is also a carbide-forming element, capable of entering cementite and replacing some iron atoms. In steel, manganese lowers the critical transformation temperature, increases austenite stability, strongly increases hardenability, promotes bainite formation, and effectively ensures the strength and toughness of steel. Manganese can compensate for the strength decrease caused by the reduction of carbon content, making it the most important and economical strengthening element. Excessive manganese content increases the tendency for center segregation in continuously cast billets, leading to an increase in banded structures in steel plates, thereby increasing brittleness and reducing plasticity. Therefore, this invention controls the manganese content to be 1.20%–1.50%.

[0017] P, S, and N are unavoidable impurity elements in steel. The lower their content, the better. However, if the content is too low, it will increase the production cost. Therefore, this invention controls P≤0.020%, S≤0.002%, and N≤0.006%.

[0018] Ni (Ni): Nickel can improve the strength of steel while maintaining good plasticity and toughness, and it also has high resistance to acid and alkali corrosion. Ni is insoluble in carbides and completely enters austenite, thus its effect of improving hardenability is fully realized. Adding Ni can inhibit pearlite formation. In this invention, Ni and Co are added simultaneously, ensuring a Ni / Co ratio of 2–4. These two elements are the core pairing for high-strength, high-toughness steel, balancing low-temperature toughness and resistance to carbon dioxide corrosion. The addition of Ni can also compensate for the reduced C content and its impact on strength, giving the steel pipe good mechanical properties and weld strength. The addition of Ni can increase the self-corrosion potential of the steel, improving its resistance to localized corrosion and effectively preventing the risk of pipeline corrosion perforation. Furthermore, the addition of Ni can improve the weldability of the steel, making it easier for cold and hot working and forming, and facilitating the preparation of steel pipes. Excessive Ni content will increase the alloy cost; therefore, this invention controls the nickel content to 0.40%–0.60%.

[0019] Cu (Copper): Copper improves hardenability and enhances the strength of steel during hot rolling and self-tempering. Copper also strengthens steel through precipitation hardening and improves corrosion resistance. In particular, copper forms a passivation film on the steel surface. When added in combination with Sn (ensuring Cu / Sn = 8-15), the two work synergistically to improve corrosion resistance, with copper primarily enhancing resistance to carbon dioxide corrosion and tin improving corrosion durability. Cu also promotes the formation of a more stable oxide film (CuO) on the steel surface, enhancing film adhesion and corrosion resistance, preventing localized corrosion caused by film detachment, and improving resistance to carbon dioxide corrosion. Adding an appropriate amount of copper can enhance the yield strength and yield ratio of steel without affecting weldability. However, high copper content can easily lead to copper embrittlement. To avoid this, this invention adds chromium in equal proportions. However, excessive copper content can cause difficulties in smelting and continuous casting and increase alloy costs. Therefore, this invention controls the copper content at 0.35%-0.55%.

[0020] Niobium (Nb) enhances strength primarily through two mechanisms: first, by forming NbC / Nb(CN) dispersed precipitates, achieving efficient precipitation strengthening; and second, by strongly inhibiting austenite grain growth, achieving grain refinement strengthening. Niobium is a synergistic element for strengthening and toughening; by refining grains and reducing grain boundary brittleness sources, it simultaneously inhibits pearlite transformation and refines precipitates, significantly improving the low-temperature impact toughness and fracture toughness of steel. In this invention, the combined addition of niobium and cobalt (controlling Nb / Co = 0.2–0.6) improves the steel's resistance to carbon dioxide corrosion; grain refinement ensures a uniform steel matrix structure, reducing micro-cell corrosion sites; and refined precipitates inhibit corrosion crack propagation, reducing the risk of stress corrosion cracking. In this invention, the niobium content is controlled at 0.09%–0.12%.

[0021] Cobalt (Co) exhibits significant solid solution strengthening effects, enhancing the strength and hardness of steel. At high temperatures, it suppresses dislocation movement, significantly improving the endurance strength of steel. Cobalt solution also improves toughness and hardenability. Since cobalt does not form carbides with carbon, it promotes carbon diffusion in steel, indirectly increasing the carbide precipitation efficiency of other carbide-forming elements, thus synergistically strengthening the steel. The combined addition of cobalt and niobium (Nb) accelerates the precipitation of Nb carbonitrides, refines grains, and achieves a triple effect of "grain refinement strengthening + solid solution strengthening + precipitation strengthening." Cobalt exists primarily in solid solution form in steel, forming small amounts of compounds such as Co3O4 and Co(OH)2 (dense films / dispersed particles), which improve CO2 corrosion resistance by strengthening corrosion product films and increasing cathodic polarization resistance. This invention controls the cobalt content to be 0.10%–0.30%.

[0022] Sn: Tin can enhance the tensile strength and hardness of steel through solid solution strengthening (an increase of approximately 50~120 MPa). The combined addition of Sn and Nb can ensure impact toughness through grain refinement strengthening. Simultaneously, tin can help improve the precipitation stability of Nb carbonitrides, achieving a dual benefit of "corrosion resistance + strength and toughness." SnO and SnO2 can form a dense oxide film on the steel surface, physically blocking CO2 molecules and H2 produced by hydrolysis. ＋ When in contact with the steel substrate, tin slows down the corrosion reaction. When tin participates in the corrosion process, it promotes the formation of an Fe3O4-SnO2 composite corrosion film on the steel surface. SnO2 fills the pores of the film, reducing its permeability, enhancing its stability, and minimizing secondary corrosion caused by the shedding of corrosion products. This invention controls the tin content to be 0.03%–0.05%.

[0023] The manufacturing method of the 500MPa grade high toughness and carbon dioxide corrosion resistant steel of the present invention includes steelmaking, ladle refining, slab continuous casting, continuous casting billet heating, rolling, cooling and coiling. The following process is controlled: 1) Ladle refining: The RH+LF refining process is adopted; RH refining controls the hydrogen and oxygen content, while LF refining carries out light desulfurization and calcium treatment to control the morphology of inclusions and improve the ductility, toughness and cold bending performance of steel.

[0024] 2) Slab continuous casting: The continuous casting process adopts electromagnetic stirring or dynamic light pressure.

[0025] 3) Heating of continuous casting slab: Heating the continuous casting slab to 1000-1100℃ and holding for 150-250 minutes; this temperature range and holding time can allow alloys such as Nb, Ni, and Cu to be fully dissolved, which is beneficial to improving yield strength and tensile strength.

[0026] 4) Rolling: The TMCP rolling process is adopted; the single-pass reduction rate of roughing is 20% to 40% to ensure the grain breakage effect and improve strength and toughness; the finishing rolling temperature of roughing is 960 to 1000℃, which is conducive to preventing austenite grain growth, making the grains uniform, and improving the uniformity of microstructure and properties; the starting rolling temperature of finishing rolling is 850 to 890℃, and the finishing rolling temperature is 700 to 750℃. This temperature range can refine the flattening degree of austenite grains, thereby refining the lower bainite grain size and ensuring high strength and low temperature impact toughness.

[0027] 5) Cooling: After rolling, ultra-fast cooling is adopted, with a cooling rate of 31-45℃ / s and a final cooling temperature of 200-270℃. This cooling rate can suppress pearlite transformation. The combination of cooling rate and final cooling temperature results in a bainitic structure with high strength and high toughness, thus ensuring that the impact energy of the product is greater than 300J.

[0028] The finished steel plate has a single sub-bainitic structure.

[0029] The mechanical properties of the finished steel plate are as follows: transverse / longitudinal yield strength ≥500MPa, tensile strength ≥600MPa, yield strength ratio ≤0.85, elongation ≥30%; impact energy at -50℃ ≥300J, DWTT at -20℃ ≥95%, and average corrosion rate in a 14.5MPa supercritical carbon dioxide environment ≤0.15mm / a.

[0030] To more intuitively illustrate the present invention, the embodiments of the present invention will be further described in conjunction with the examples. The following examples are merely preferred embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any technical solutions that can be obviously obtained by those skilled in the art within the scope of the technology disclosed in the present invention, including simple variations or equivalent substitutions, are all within the scope of protection of the present invention.

[0031]

Example

[0032] Table 1 Chemical composition of steel (wt%) Table 2 Main Process Parameters Table 3 Mechanical Properties of Finished Products Note: The corrosion rates in the table are the average corrosion rates in a supercritical carbon dioxide environment of 14.5 MPa. The corrosion test medium was a standard NACE A solution, the test temperature was 30℃, the CO2 pressure was 2.0 MPa, the stirring speed was 3 m / s, the test time was 72 h, and the corrosion rate was obtained by the weight loss method.

[0033] As can be seen from the above embodiments, by adopting the component design and production process of the present invention, it is possible to manufacture hot-rolled coils with high toughness and resistance to carbon dioxide corrosion for supercritical carbon dioxide transportation at the 500MPa level.

[0034] 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. 500MPa grade high-toughness steel resistant to carbon dioxide corrosion, characterized in that, The chemical composition of the steel, by weight percentage, is as follows: C: 0.02%–0.05%, Si: 0.40%–0.60%, Mn: 1.20%–1.50%, P≤0.020%, S≤0.002%, Ni: 0.40%–0.60%, Cu: 0.35%–0.55%, Nb: 0.09%–0.12%, Co: 0.10%–0.30%; Sn: 0.03%~0.05%, N≤0.006%; of which, Ni / Co=2.0~4.0, Nb / Co=0.2~0.6, Cu / Sn=8.0~15.0; the remainder is Fe and unavoidable impurity elements.

2. The 500MPa grade high-toughness carbon dioxide corrosion resistant steel according to claim 1, characterized in that, The finished steel plate has a single sub-bainitic structure.

3. The 500MPa grade high-toughness carbon dioxide corrosion resistant steel according to claim 1, characterized in that, The mechanical properties of the finished steel plate are as follows: transverse / longitudinal yield strength ≥500MPa, tensile strength ≥600MPa, yield-to-tensile ratio ≤0.85, elongation ≥30%, impact energy at -50℃ ≥300J, DWTT at -20℃ ≥95%, and average corrosion rate in a 14.5MPa supercritical carbon dioxide environment ≤0.15mm / a.

4. The method for manufacturing 500MPa grade high-toughness carbon dioxide corrosion-resistant steel as described in any one of claims 1 to 3, characterized in that, The process flow includes steelmaking, ladle refining, slab continuous casting, continuous casting billet heating, rolling, cooling and coiling; The following process is controlled: 1) Ladle refining: using RH+LF refining process; 2) Slab continuous casting: The continuous casting process adopts electromagnetic stirring or dynamic light reduction; 3) Heating of continuous casting slab: Heating the continuous casting slab to 1000-1100℃ and holding for 150-250 minutes; 4) Rolling: TMCP rolling process is adopted; the single-pass reduction rate of roughing is 20% to 40%, and the finishing temperature of roughing is 960 to 1000℃; the starting temperature of finishing rolling is 850 to 890℃, and the finishing temperature of finishing rolling is 700 to 750℃. 5) Cooling: The cooling rate is 31-45℃ / s, and the final cooling temperature is 200-270℃.

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