The present invention belongs to the field of metallic materials and the manufacture thereof, and specifically relates to a X65 pipeline steel for service in a supercritical CO₂ environment and a manufacturing method therefor.

Against the backdrop of the dual carbon goals, CCUS (Carbon Capture, Utilization and Storage) technology has become a focal point, with CO₂ transportation and storage being a crucial component thereof. For long-distance and large-scale CO₂ transportation, pipeline transportation of supercritical CO₂ is the most cost-effective method. Compared with pipelines for conventional gaseous CO₂ or natural gas, pipelines for supercritical CO₂ faces a higher risk of fracture failure. Owing to the high saturation pressure of the supercritical state and the tendency of CO₂ to undergo phase transition during decompression, the decompression wave curve changes from a smooth one to one with a pressure plateau, and the decompression wave velocity is significantly reduced. This makes the rapid propagation of ductile fracture more likely and fracture arrest more difficult, thus imposing higher requirements on the fracture performance of pipe materials.

In addition, under low-temperature working conditions such as pipeline leakage, planned or unplanned pressure reduction, the temperature drops due to the Joule-Thomson effect, exposing the material to a low temperature of -40°C, or even to conditions below the triple point (-56°C). Pressure reduction in long pipeline sections may persist for an extended period, and the pipeline material is prone to embrittlement if the temperature is sufficiently low, which imposes higher requirements on the low-temperature performance of the material. Therefore, pipelines for carbon dioxide transportation must possess adequate low-temperature toughness.

In addition, supercritical CO₂ has a higher solubility in water and stronger corrosivity compared with conventional gaseous CO₂. This corrosivity is particularly exacerbated in the presence of trace gaseous impurities: for instance, hydrogen-containing impurities such as H₂S may react with O₂ to form water under certain conditions, and the presence of SO₂, CO, trace NO₂ and other impurities will all accelerate corrosion. Even under the in-service conditions of pipelines after raw gas treatment, the actual operating environment is relatively complex, and various raw gas impurities as well as unplanned operating conditions may all give rise to potential corrosion risks. Therefore, it is necessary to consider the material's resistance to service conditions with unqualified water content.

Chinese Patent Application CN202110097162.1 discloses "A steel plate resistant to CO₂ corrosion and its preparation method". The pipeline steel comprises the following components in percentage by weight: 0.03~0.07% of C, 4.0~6.0% of Cr, 0.15~2.50% of Ni, 0.01~0.06% of Nb, less than or equal to 0.005% of P, less than or equal to 0.005% of S, and a balance being Fe and unavoidable impurities. A relatively high content of Cr and Ni is added to ensure the material has good CO₂ corrosion resistance and low-temperature toughness, resulting in a high alloy cost. The steel plate has a yield strength of ≥ 460 MPa, a tensile strength of ≥ 510 MPa, a Charpy impact energy at -20°C of ≥ 80J, and a corrosion rate of ≤ 0.21 mm/a under the simulated service environment of oil production equipment for CO₂ flooding.

Chinese Patent Application CN201310217916.8 discloses "A CO₂ corrosion-resistant pipeline steel for surface gathering and transportation and a manufacturing method therefor". The steel plate comprises the following components in percentage by weight: 0.01~0.08% of C, 0.10~0.50% of Si, 0.50~1.50% of Mn, less than or equal to 0.02% of P, less than or equal to 0.006% of S, less than or equal to 0.1% of Nb+V+Ti, 1.0~3.0% of Cr, 0.10~0.30% of Mo, 0.10~0.50% of Cu, 0.10~0.50% of Ni, and a balance being Fe and unavoidable impurities. Alloying elements such as Cr, Nb, V, Ti and Mo are added in this patent. Both Cr and Mo are precious metal elements, and in addition, Cr is detrimental to weldability. The steel plate has a tensile strength of 600~700 MPa, a yield strength of 530~600 MPa, a total elongation of ≥ 23%, a yield ratio of ≤ 0.85 and an impact energy at -20°C of ≥ 220 J. Its corrosion resistance can meet the requirements for service in a CO₂-saturated environment under atmospheric pressure.

Chinese Patent Application CN201510650858.7 discloses "A pipeline steel with excellent CO₂ corrosion resistance and a manufacturing method therefor". The pipeline steel comprises the following chemical components in percentage by weight: 0.035~0.060% of C, 0.10~0.30% of Si, 1.00~1.50% of Mn, less than or equal to 0.012% of P, less than or equal to 0.0020% of S, 3.0~4.5% of Cr, 0~0.10% of Cu, 0~0.10% of Ni, 0~0.10% of Mo, 0.015~0.040% of Nb, 0.010~0.025% of Ti, 0.010~0.050% of Al, less than or equal to 0.008% of N, and a balance being Fe and unavoidable impurities. The alloy scheme of this patent also adds a relatively high content of Cr (3.0~4.5%), leading to increased cost and degraded weldability. The steel plate has a yield strength (Rt_0.5) of ≥ 450 MPa, a tensile strength (R_m) of ≥ 535 MPa, a KV₂ at -20°C of ≥ 120 J, and a DWTTSA (surface area of Drop Weight Tear Test) at -15°C of ≥ 85%. Under the test conditions of a temperature of 80°C, a CO₂ partial pressure of 2 MPa and a flow rate of 1.0 m/s, the corrosion rate is ≤ 5 mm/a.

Chinese Patent Application CN201610897282.9 discloses "An economical low-Cr X65 pipeline steel with CO₂ corrosion resistance and a manufacturing method therefor". The pipeline steel comprises the following chemical components in percentage by weight: 0.04~0.05% of C, 0.18~0.22% of Si, 0.5~0.6% of Mn, 0.1~0.2% of Cr, 0.1~0.15% of Mo, 0.035~0.050% of Nb, 0.020~0.030% of V, 0.010~0.020% of Ti, less than or equal to 0.01% of P, less than or equal to 0.003% of S, and a balance being Fe and unavoidable impurities. In addition, the following requirement is met: Nb+V+Ti ≤ 0.1%. The steel plate has a yield strength R_t0.5 of 460~500 MPa, a tensile strength R_m of 550~600 MPa, a yield ratio R_t0.5/R_m of ≤ 0.9, an elongation after fracture A₅₀mm of ≥ 18%, an impact energy KV₂ at -20°C of ≥ 120 J, and a shear area ratio of fracture surface DWTT SA at -15°C of ≥ 85%. Under the conditions of a temperature of 60°C, a CO₂ partial pressure of 1 MPa and a flow rate of 1.0 m/s, the corrosion rate is ≤ 0.5 mm/a. It is difficult to effectively ensure stable strength and low-temperature toughness. And the low-temperature toughness is relatively poor, and the corrosion resistance of the material is also reduced.

From the comparison with existing patent applications, it can be found that most existing CO₂ pipeline materials adopt a relatively high Cr content. This not only significantly increases the alloy cost, but also brings high temper embrittlement due to the increased Cr content. Cracks tend to occur during welding, resulting in poor weldability and great performance risks for long-distance pipelines. In addition, although some materials use a low Cr content, there are no effective corresponding measures to compensate for the performance loss, and their low-temperature toughness and corrosion resistance are relatively poor.

The purpose of the present invention is to provide a X65 pipeline steel for service in a supercritical CO₂ environment and a manufacturing method therefor. The obtained pipeline steel can achieve the X65 strength level and has good weldability, excellent low-temperature toughness and corrosion resistance. The pipeline steel has a R_t0.5 yield strength of 450~570 MPa, a tensile strength of 540~700 MPa, a yield ratio of ≤ 0.93, an A₅₀mm elongation of ≥ 35%, a Charpy impact energy KV₈ at -56°C of ≥ 300 J, a DWTT SA% at -46°C of ≥ 85%, and an average corrosion rate of ≤ 0.06 mm/a in the supercritical CO₂ phase under the conditions of 8 MPa, 50°C and saturated water.

To achieve the above objectives, the technical solution of the present invention is as follows:
A X65 pipeline steel for service in a supercritical CO₂ environment, comprising the following chemical components in percentage by weight: 0.020~0.070% of C, 0.10~0.30% of Si, 0.80~1.45% of Mn, less than or equal to 0.0080% of P, less than or equal to 0.0015% of S, 0.05~0.35% of Cu, 0.05~0.30% of Ni, 0.20~0.80% of Cr, 0~0.09% of Mo, 0.025~0.055% of Nb, 0~0.050% of V, 0.005~0.020% of Ti, 0.0010~0.0040% of Ca, 0.010~0.040% of Alt, less than or equal to 0.0004% of B, less than or equal to 0.0030% of O, less than or equal to 0.0050% of N, less than or equal to 0.0002% of H, 0.0005~0.0050% of Ce, and a balance comprising Fe and other unavoidable impurities, wherein the following requirements are met simultaneously: $$\mathrm{Ca}/\mathrm{S}\ge 1.5;$$ $$\mathrm{Nb}+\mathrm{V}+\mathrm{Ti}\le 0.120\%;$$ $$0.8\le \left(\mathrm{Nb}+\mathrm{V}+\mathrm{Ti}\right)/\left(\mathrm{C}+\mathrm{N}+5\times \mathrm{B}\right)\le 2.5;$$ $$\left(\mathrm{Cr}+\mathrm{Cu}/3+\mathrm{Ni}/3+\mathrm{Mo}+\mathrm{Ce}\times 25\right)/\left(\mathrm{S}+\mathrm{P}\right)\ge 50.$$

Further, the balance is Fe and other unavoidable impurities.

The microstructure of the pipeline steel according to the present invention is uniformly refined granular bainite + polygonal ferrite + pearlite + MA (martensite-austenite constituent), wherein the volume percentage of granular bainite is ≥ 85%. In some embodiments, the volume percentage of polygonal ferrite is ≤ 15%.

In some embodiments, the microstructure of the pipeline steel according to the present invention comprises 85~92% by volume of granular bainite, and the remainder being any one or more of polygonal ferrite, pearlite and MA.

The pipeline steel according to the present invention has a R_t0.5 yield strength of 450~570 MPa, a tensile strength of 540~700 MPa, a yield ratio of ≤ 0.93, an A₅₀mm elongation of ≥ 35%, a Charpy impact energy KV₈ at -56°C of ≥ 300 J, a DWTT SA% at -46°C of ≥ 85%, and an average corrosion rate of ≤ 0.06 mm/a in the supercritical CO₂ phase under the conditions of 8 MPa, 50°C and saturated water.

In some embodiments, the pipeline steel according to the present invention has a R_t0.5 yield strength of 500~570 MPa.

In some embodiments, the pipeline steel according to the present invention has a tensile strength of 580~700 MPa.

In some embodiments, the pipeline steel according to the present invention has a yield ratio ≤ 0.90.

In some embodiments, the yield ratio of the pipeline steel according to the present invention is in the range of 0.81~0.93 or 0.81~0.90.

In some embodiments, the pipeline steel according to the present invention has an A₅₀mm elongation of 36~47%.

In some embodiments, the Charpy impact energy KV₈ at -56°C of the pipeline steel according to the present invention is ≥ 340 J. In some embodiments, the Charpy impact energy KV₈ at -56°C of the pipeline steel according to the present invention is ≥ 370. In some embodiments, the Charpy impact energy KV₈ at -56°C of the pipeline steel according to the present invention is ≥ 400. In some embodiments, the Charpy impact energy KV₈ at -56°C of the pipeline steel according to the present invention is 300~470 J. In some embodiments, the Charpy impact energy KV₈ at -56°C of the pipeline steel according to the present invention is 340~470 J. In some embodiments, the Charpy impact energy KV₈ at -56°C of the pipeline steel according to the present invention is 400~470 J.

In some embodiments, the DWTT SA% at -46°C of the pipeline steel according to the present invention is ≥ 89%. In some embodiments, the DWTT SA% at -46°C of the pipeline steel according to the present invention is 85~97%.

In the chemical components design of the X65 pipeline steel for service in a supercritical CO₂ environment according to the present invention:
C: C is the most economical strengthening element in steel, which enhances the strength of steel through interstitial solid solution strengthening. Increasing the carbon content can significantly improve the hardenability of steel, reduce the addition of other precious alloys and thus lower production costs. However, an increase in C content is detrimental to the ductility, toughness, weldability and corrosion resistance of steel. For this reason, an ultra-low C design is adopted in the present invention, with the C content controlled at 0.020~0.070%.

Si: Si is a solid-solution strengthening element and also a deoxidizing element in steel. However, an excessively high content exerts an adverse effect on the surface quality and weldability of the steel. If the Si content exceeds 0.30%, the toughness may be impaired. For this reason, the Si content is controlled at 0.10~0.30% in the present invention.

Mn: Mn enhances the strength of steel through solid solution strengthening, and is the primary and most economical strengthening element in steel for compensating the strength loss caused by the reduction in C content. Mn contributes to the formation of fine phase transformation products and also facilitates the control of oxygen and sulfur during the steelmaking process. If the Mn content is too low, it is difficult to achieve the target strength level, but Mn may also exacerbate center segregation. For this reason, the Mn content is controlled at 0.80~1.45% in the present invention.

Cr: Cr exerts a certain solid solution strengthening effect and can effectively enhance the hardenability of steel. When the Cr content is 0.10% or more, it can effectively improve the corrosion resistance of steel, forming a relatively dense protective layer on the steel surface to protect the matrix. However, an excessively high Cr content in steel is detrimental to weld quality and tends to cause grey spot defects. For this reason, the Cr content is controlled at 0.20~0.80% in the present invention.

Nb: Nb is an essential element in low-carbon microalloyed steel. Solute Nb undergoes strain-induced precipitation during hot rolling to form niobium carbonitrides, which pin the grain boundaries and inhibit the growth of deformed austenite. Through controlled rolling and controlled cooling, the deformed austenite is transformed into fine products with a high dislocation density. After coiling of the steel strip, the solute niobium precipitates dispersively as secondary phase particles NbC within the matrix, exerting a precipitation strengthening effect. An excessive content of Nb tends to cause cracks in the slab, thereby impairing the surface quality, and furthermore, it deteriorates the weldability of the steel. For this reason, the Nb content is controlled at 0.025~0.055% in the present invention.

Ti: Ti is a good deoxidizing and degassing agent, and an effective element for fixing nitrogen and carbon. Undissolved Ti carbonitrides in steel can inhibit the growth of austenite grains during heating. TiN and TiC precipitated during rough rolling in the high-temperature austenite region can effectively suppress the coarsening of austenite grains thus refining the grain size, and can also increase the solubility of Nb and reduce the microcrack sensitivity of Nb-containing steel. Ti is generally added in combination with Nb. In addition, its precipitation during the welding process can also inhibit the growth of high-temperature grains thereby improving weldability. For this reason, the Ti content is controlled at 0.005~0.020% in the present invention.

V: Vanadium has a strong affinity for carbon, ammonia and oxygen, forming corresponding stable compounds with them. Vanadium exists mainly in the form of carbides in steel. Vanadium refines the structural grains of steel, raises the grain coarsening temperature, enhances the strength and low-temperature toughness of steel, and also improves the post-weld toughness and weldability of steel. However, an excessively high V content can cause an increase in the ductile-brittle transition temperature of steel. For this reason, the V content is controlled at 0~0.050% in the present invention.

Nb+V+Ti: A relatively high content of Nb, V and Ti can form dispersively distributed carbonitride-boron compounds with C, N and B. These carbonitride-boron compounds can exert a solid solution strengthening effect, and owing to their very high melting points, they act as exogenous nucleation sites during the subsequent preparation process, effectively refining the structural grains and improving the microstructural properties. However, an excessive addition of Nb, V and Ti tends to cause the agglomeration of carbonitride particles in the steel plate, in particularly, the formation of elongated segregations distributed along the rolling direction in the core region. Moreover, these particles are hard phases with a hardness higher than that of the matrix, which increases the local brittleness of the microstructure and deteriorates the low-temperature toughness of the material. For this reason, Nb+V+Ti is controlled to be ≤ 0.12% in the present invention.

Furthermore, the combination of Nb, V and Ti with C, N and B can effectively inhibit the formation of chromium carbonitride-boron compounds by Cr. On the one hand, this reduces the adverse effects of chromium carbonitride-boron compounds on toughness and strength. On the other hand, it increases the solute content of Cr, thus giving full play to the role of Cr in CO₂ corrosion resistance. For this reason, the ratio of Nb, V, Ti to C, N, B must be strictly controlled, and in the present invention, 0.8 ≤ (Nb+V+Ti)/(C+N+5×B) ≤ 2.5 shall be maintained. When (Nb+V+Ti)/(C+N+5×B) is >2.5, the secondary phase particles become coarse and unevenly distributed, which is prone to cause stress concentration and initiate brittle fracture, significantly reducing the low-temperature toughness of the base material and welds. When (Nb+V+Ti)/(C+N+5×B) is < 0.8, the number of precipitated phases is small and their size is excessively fine, making them prone to re-dissolution into the base material. This results in a weak grain boundary pinning effect, which exerts an adverse influence on low-temperature toughness. Meanwhile, the solute content of Cr decreases, and the CO₂ corrosion resistance is impaired.

Mo: Mo is a strong hardenability element that significantly retards ferrite transformation and inhibits the formation of ferrite and pearlite. It can effectively promote bainite transformation to exert a matrix strengthening effect, yielding a finer microstructure. A certain amount of Mo offers a marked improvement in the microstructural properties of the material, yet an excessive Mo content reduces the plasticity of the steel, and Mo is a high-cost element. For this reason, the Mo content is controlled at a low level of 0~0.09% in the present invention.

Cu and Ni: They can enhance the strength of steel through solid solution strengthening and improve its atmospheric corrosion resistance. However, a relatively high content of these elements tends to cause hot cracking in the steel plate. Ni can refine the grain size, mitigate the hot brittleness of steel induced by Cu, and exerts a marked effect in improving the low-temperature toughness of steel. For this reason, the Cu content is controlled at 0.05~0.35% and the Ni content is controlled at 0.05~0.30% in the present invention.

S and P: S and P are primary impurity elements in steel. Phosphorus tends to induce cold brittleness in steel, while sulfur is prone to cause hot brittleness, resulting in unstable mechanical properties of the steel. In particularly, as the S content increases, the amount of MnS inclusions rises, which significantly impairs the low-temperature toughness of the material. For this reason, the contents of phosphorus and sulfur in the steel should be minimized as much as possible. Accordingly, in the present invention, the S content is controlled to be ≤ 0.0015% and the P content is controlled to be ≤ 0.0080%.

Ca: Ca treatment enables the control of sulfide morphology, improves the anisotropy of the steel plate and enhances the low-temperature toughness. In addition, when the S content is relatively high, the Ca/S ratio shall be appropriately increased to reduce the amount of MnS inclusions in the steel. However, an excessively high Ca content tends to cause an increase and coarsening of inclusions such as CaO in the steel. For this reason, the Ca content is controlled at 0.0010~0.0040% in the present invention, and the Ca/S ratio is simultaneously controlled to be ≥ 1.5.

Alt: Al is used for deoxidation in steel, and an appropriate content of Al also contributes to grain refinement and improves the strength and toughness of the steel. However, if the Al content exceeds 0.05%, coarse precipitates may form, thereby impairing the low-temperature toughness of the steel. For this reason, the Alt content is controlled at 0.010~0.040% in the present invention.

N, O and H: With regard to low-temperature toughness, O, N and H may all induce brittle fracture, particularly at low temperatures. The presence of O and N leads to the formation of oxides and nitrides, which tend to segregate at grain boundaries and cause intergranular brittleness. Meanwhile, H may permeate into the grains, resulting in lattice distortion and void formation, thereby initiating brittle fracture. The low-temperature toughness of steel is thus related to the contents of O, N and H, and an excessively high or inappropriate content of these elements may give rise to brittle fracture. For this reason, in the present invention, the O content is controlled to be ≤ 0.0030%, the N content is controlled to be ≤ 0.0050%, and the H content is controlled to be ≤ 0.0002%.

B: The primary effect of boron in steel is to enhance the hardenability and strength of the steel, thereby saving the use of other relatively rare and precious metals. However, the addition of B element exerts a marked adverse effect on the low-temperature toughness of the material. For this reason, the B content is controlled to be ≤ 0.0004% in the present invention.

Ce: Ce functions to purify the molten steel, modify inclusions and refine the grain size. A trace amount of cerium can lower the passivation potential, widen the passivation interval, raise the self-corrosion potential and polarization resistance of the steel, and increase the compactness of the inner rust layer. The Ce content is controlled at 0.0005~0.0050% in the present invention.

In addition, Ce can also purify the molten steel, reduce the segregation of S and P at phase boundaries and render the distribution of alloying elements such as Cr, Ni and Mo more uniform in the two phases, thereby improving the CO₂ corrosion resistance and toughness of the material. For this reason, based on the research on the influence law of Ce on various elements in the present invention, an index control of (Cr+Cu/3+Ni/3+Mo+Ce×25)/(S+P) ≥ 50 is proposed. When (Cr+Cu/3+Ni/3+Mo+Ce×25)/(S+P) is < 50, the material exhibits a relatively high corrosion rate and poor toughness. In some embodiments, the value of (Cr+Cu/3+Ni/3+Mo+Ce×25)/(S+P) is in the range of 50~210.

Overall, the present invention adopts the following design philosophy for the aforementioned composition:

1. 1. Based on a design with a relatively low Cr content, a multi-component alloy of Mn-Cu-Ni-Mo is added for strengthening, and microalloying with Nb-Ti-V is employed to promote grain refinement. Thus, the material attains good strength and toughness, along with favorable weldability and economic efficiency.
2. 2. An ultra-pure steel is obtained by controlling the contents of impurity elements such as S, P, B, O and N at an extremely low level. Inclusion modification is conducted via a Ca treatment process, and a stable calcium feeding process is employed to achieve Ca/S ≥ 1.5, thereby reducing the content of non-metallic inclusions in the steel. In combination with technologies such as soft reduction, the center segregation of the cast slab is improved, and the core inclusions and compositional segregation are reduced, thus yielding good low-temperature toughness of the steel.
3. 3. The self-corrosion potential of the alloy is increased and the corrosion tendency is reduced by the addition of Cu, Ni and Mo elements. Meanwhile, a trace amount of Ce is added to purify the molten steel, and (Cr+Cu/3+Ni/3+Mo+Ce×25)/(S+P) ≥ 50 is controlled. This reduces the segregation of S and P elements at phase boundaries, renders the distribution of alloying elements such as Cr, Ni and Mo more uniform, lowers the passivation potential, widens the passivation interval, increases the polarization resistance and the compactness of the inner rust layer, and thus improves the CO₂ corrosion resistance of the material.
4. 4. The key alloying elements is controlled to satisfy 0.8 ≤ (Nb+V+Ti)/(C+N+5×B) ≤ 2.5, which ensures that the suitable secondary phase particles such as NbTi(CN) are fine, uniform and dispersively distributed, and also increases the solute content of Cr. Thereby, the low-temperature toughness and CO₂ corrosion resistance of the material are significantly improved.

The method for manufacturing X65 pipeline steel for service in a supercritical CO₂ environment according to the present invention comprises the following steps:

1. 1) Smelting and casting
   Smelt and refine in accordance with the aforementioned composition, and cast into a slab;
2. 2) Rolling
   Heating temperature: 1100~1250°C; rough rolling finishing temperature: 900~1000°C; finish rolling finishing temperature: 750~880°C; cumulative reduction ratio of finish rolling ≥ 75%;
3. 3) Cooling
   Cooling start temperature: 700~800°C; cooling stop temperature: 300~450°C; cooling rate: 10~30°C/s.

Preferably, in step 1), the refining process adopts LF refining and RH refining. During the LF refining process, the stirring time after the complete addition of all alloys is controlled to be ≥ 5 minutes; the calcium wire feeding amount in the RH refining is 300~500 meters, and the Ca/S ratio is controlled to be ≥ 1.5.

Preferably, in step 1), continuous casting is adopted for the casting process, the holding time of continuous casting is controlled to be ≥ 3 minutes, and the fluctuation of casting speed is controlled to be ≤ 0.1 m/min.

Preferably, in step 2), the pass reduction ratio is controlled to be ≥ 15% when the rough rolling finishing temperature is ≥ 900°C and < 920°C; the pass reduction ratio is controlled to be ≥ 14% when the rough rolling finishing temperature is ≥ 920°C and < 950°C; the pass reduction ratio is controlled to be ≥ 11% when the rough rolling finishing temperature is ≥ 950°C and ≤ 1000°C.

In the manufacturing method of the present invention:
Refining involves LF treatment for desulfurization to reduce the sulfur content in the steel, and an RH treatment process to modify inclusions such as MnS and Al₂O₃ in the steel, thereby lowering the content of impurity elements in the steel. By increasing the effective calcium content in the steel, high-melting-point CaS is formed in advance during the solidification of molten steel, which inhibits the total amount of MnS generated in this process and modifies all or part of the MnS into CaS. On the other hand, large-particle Al₂O₃ inclusions are converted into low-melting-point composite inclusions, which promotes their flotation and purifies the internal quality of the molten steel. In the present invention, the LF stirring time is controlled to be ≥ 5 minutes, the feeding amount of Ca-Si cored wire in RH treatment is 300~500 meters, and the Ca/S ratio is controlled to be ≥ 1.5. Through the combined LF and RH treatment, an ultra-pure steel with low inclusions is obtained, and the accurate target designed composition is achieved after fine composition adjustment. In some embodiments, the LF stirring time is controlled to be in the range of 5~12 minutes in the present invention.

The stable control of holding time and casting speed during the continuous casting process is of great importance for improving slab quality, enhancing steel quality and alleviating microstructural segregation. Since Ce is added in the present invention, it is necessary to control and reduce the content of inclusions such as cerium compounds, and ensure low contents of impurity elements including O ≤ 0.0030%, N ≤ 0.0050% and H ≤ 0.0002%. Therefore, the holding time is controlled to be ≥ 3 minutes in the present invention to enable the sufficient flotation and removal of relevant inclusions or gases in the steel. The fluctuation of casting speed is controlled to be ≤ 0.1 m/min to realize stable casting control, reduce center segregation of the slab and improve the uniformity of slab quality, thereby enhancing the low-temperature toughness and performance uniformity of the final finished steel plate. In some embodiments, the holding time is controlled to be in the range of 3~11 minutes in the present invention.

The slab heating temperature is controlled at 1100~1250°C. A sufficiently high rheating temperature ensures the complete solid solution of alloys and uniform heating, which is conducive to obtaining a homogeneous microstructure and good plate flatness. Dynamic recrystallization can eliminate the stress induced by deformation and fragmented grain boundaries, refining and homogenizing the material and thereby enhancing its toughness and plasticity. However, this process requires the achievement of a critical deformation amount and a relatively high deformation temperature; the lower the rolling temperature, the higher the reduction ratio required for dynamic recrystallization.

In the present invention, the rough rolling finishing temperature is controlled at 900~1000°C, and the pass reduction ratio is controlled in a temperature-matched manner: the pass reduction ratio is set to ≥ 15% when the rough rolling finishing temperature is ≥ 900°C and < 920°C; the pass reduction ratio is set to ≥ 14% when the rough rolling finishing temperature is ≥ 920°C and < 950°C; and the pass reduction ratio is set to ≥ 11% when the rough rolling finishing temperature is ≥ 950°C and ≤ 1000°C. This is because energy accumulates continuously with the movement of dislocations and the like, and dynamic recrystallization occurs when the strain reaches a critical value. Dynamic recrystallization is a thermally activated process. Based on the study on the stress-strain curve characteristics of the steel with the composition of the present invention, the hot deformation activation energy was determined, and the minimum critical deformation amount required for each deformation temperature range was further defined, thus ensuring the occurrence of dynamic recrystallization. Meanwhile, the occurrence of dynamic recrystallization causes grain fragmentation, which significantly refines precipitated phases such as NbTi(CN) and realizes their dispersive distribution, further refining the grain size and improving the toughness of the material. In some embodiments, the pass reduction ratio of rough rolling is controlled to be ≤ 18%, such as ≤ 16%.

The finish rolling finishing temperature is controlled at 750~880°C, with cumulative finish rolling reduction ratio ≥ 75%. The specific rolling temperature and high reduction ratio increase the nucleation sites of dislocations, promoting the precipitation of NbTi(CN) and refining the grain size. Since the steel of the present invention is designed to contain 0.025~0.055% of Nb in its composition, the γ→α phase transformation is inhibited during the cooling process after rolling, which refines grain boundary ferrite and promotes the transformation to granular bainite. The cooling start temperature after rolling is controlled at 700~800°C, combined with a relatively high cooling rate of 10~30°C/s, which increases the content of bainite. In addition, the cooling stop temperature is controlled at a relatively low range of 300~450°C to improve the cooling penetration and enhance the microstructural uniformity along the thickness direction. Ultimately, a composite microstructure of granular bainite + polygonal ferrite + pearlite + MA is obtained, with the volume fraction of granular bainite being ≥ 85%. The interlaced distribution of lath packets with high-angle grain boundaries of different orientations in granular bainite can effectively refine the grain size and improve the strength and toughness of the material. Furthermore, the presence of polygonal ferrite in the microstructure is beneficial to enhancing the deformation coordination ability of the overall microstructure and inhibiting crack propagation, thus improving the low-temperature toughness of the material. However, when its volume fraction exceeds 15%, it will aggravate the microstructural inhomogeneity, easily form microstructural segregation bands in the core region and reduce the overall strength and toughness of the material. In addition, compared with bainite, polygonal ferrite and pearlite are more susceptible to corrosion, which impairs the overall corrosion resistance of the material. In some embodiments, the cumulative reduction ratio of finish rolling is controlled in the range of 75~80%.

Compared with the prior art, the present invention has the following advantages:
In terms of composition design, the present invention adopts an ultra-low Cr content design, achieves strengthening by adding a Mn-Cu-Ni-Mo multi-component alloy and introducing a trace amount of Ce and other elements, and promotes grain refinement through Nb-Ti-V microalloying. Thereby, the material attains good strength and toughness, supercritical CO₂ corrosion resistance, as well as favorable weldability and economic efficiency. Most of the existing CO₂ pipeline materials employ a relatively high Cr content, which leads to high alloy cost, high temper brittleness and poor weldability, imposing considerable cost and failure risks on long-distance pipelines. In addition, although some materials adopt a low Cr content, they lack effective measures to compensate for the performance loss, resulting in relatively poor low-temperature toughness and corrosion resistance.

Based on the austenite continuous cooling transformation characteristics of the steel with the present Cr-Mn-Cu-Ni-Mo multi-component alloy and Nb-Ti-V microalloying composition, the present invention adopts precise control during the rolling process to promote the refinement and precipitation of precipitated phases such as NbTi(CN), further refining the grain size and improving the toughness of the material. Combined with subsequent cooling control, a relatively high cooling rate and a relatively low cooling stop temperature are adopted to facilitate the formation of bainite and improve the microstructural uniformity, ultimately obtaining a target microstructure with uniform refinement and no obvious segregation. The volume fraction of granular bainite in the microstructure is ≥ 85%, and the remainder is a small amount of polygonal ferrite, pearlite and MA. Thus, the steel exhibits good low-temperature toughness, CO₂ corrosion resistance, as well as favorable weldability and economic efficiency.

The pipeline steel according to the present invention has a R_t0.5 yield strength of 450~570 MPa, a tensile strength of 540~700 MPa, a yield ratio of ≤ 0.93, an A₅₀mm elongation of ≥ 35%, a Charpy impact energy KV₈ at -56°C of ≥ 300 J, and a DWTT SA% at -46°C of ≥ 85%, thus exhibiting significantly superior low-temperature toughness in comparison with the mainstream existing materials. Meanwhile, the material possesses good corrosion resistance, with an average corrosion rate of ≤ 0.06 mm/a in the supercritical CO₂ phase under the conditions of 8 MPa, 50°C and saturated water.

The present invention is further described below in conjunction with the examples.

The chemical components of the steels in the examples of the present invention are shown in Table 1 and Table 2, and the remainder comprising Fe and unavoidable impurities; the specific process parameters are listed in Table 3. The mechanical properties and microstructural results of the steel plates of the examples and comparative examples of the present invention are presented in Table 4.

As can be seen from Table 4, the pipeline steel obtained by the present invention has a R_t0.5 yield strength of 450~570 MPa, a tensile strength of 540~700 MPa, a yield ratio of ≤ 0.93, an A₅₀mm elongation of ≥ 35%, a Charpy impact energy KV₈ at -56°C of ≥ 300 J, a DWTT SA% at -46°C of ≥ 85%, and an average corrosion rate of ≤ 0.06 mm/a in the supercritical CO₂ phase under the conditions of 8 MPa, 50°C and saturated water.

The methods for the relevant performance tests are described as follows:

1. (1) Tensile and Charpy impact tests: Tensile and Charpy impact tests were conducted in accordance with ASTM A370 to determine the R_t0.5 yield strength, tensile strength, yield ratio, elongation A₅₀ at room temperature, and Charpy impact energy at -56°C of the pipeline steel of each example and the comparative pipeline steel of each comparative example, wherein the yield ratio = yield strength / tensile strength.
2. (2) Drop weight tear test (DWTT): The DWTT SA% at -46°C of the pipeline steel of each example and the comparative pipeline steel of each comparative example was measured in accordance with API RP 5L3.
3. (3) Microstructural observation: The microstructural morphology was observed by an optical microscope (manufacturer: ZEISS, model: Axio Imager. M2m), and the volume fraction of granular bainite was determined by the metallographic microscopy method.
4. (4) Corrosion rate measurement: The original weight and surface area of the test specimens were measured first. The specimens were placed in the supercritical CO₂ phase under the conditions of 8 MPa, 50°C and saturated water for a 168-hour corrosion test. After the test cycle, the specimens were taken out, the corrosion products were removed, and the specimens were weighed again. The average corrosion rate was calculated based on the weight loss.

For the pipeline steel of Comparative Example 1, (Nb+V+Ti)/(C+N+5×B) is < 0.8, the cumulative reduction ratio of finish rolling was relatively low, and the feeding amount of silicon-calcium wire was insufficient resulting in a low Ca/S ratio, with the Ce content failing to meet the control requirements of the present invention.

For the pipeline steel of Comparative Example 2, (Nb+V+Ti)/(C+N+5×B) is > 2.5, (Cr+Cu/3+Ni/3+Mo+Ce×25)/(S+P) is < 50, the pass reduction ratio of the final rough rolling pass was relatively low, and the Ce content also failed to meet the control requirements of the present invention.

Both Comparative Example 1 and Comparative Example 2 exhibited low individual values of Charpy impact energy at -56°C and DWTT SA at -46°C, and also showed a relatively high corrosion rate in supercritical CO₂. C Si Mn P S Cu Ni Cr Mo Nb V Ti Ca Alt B O N Example 1 0.062 0.27 1.30 0.0053 0.0007 0.15 0.10 0.20 0.04 0.054 0.050 0.015 0.0035 0.036 0.0003 0.0016 0.0048 Example 2 0.043 0.22 1.27 0.0059 0.0005 0.23 0.22 0.50 0.05 0.036 0.036 0.006 0.0040 0.034 0.0002 0.0014 0.0036 Example 3 0.052 0.25 1.43 0.0063 0.0002 0.25 0.19 0.29 0.07 0.043 0.042 0.019 0.0010 0.040 0.0001 0.0029 0.0049 Example 4 0.020 0.24 1.41 0.0065 0.0001 0.24 0.15 0.30 0.05 0.028 0.002 0.018 0.0014 0.039 0.0001 0.0021 0.0047 Example 5 0.070 0.18 0.80 0.0045 0.0003 0.33 0.30 0.41 0.02 0.026 0.032 0.017 0.0026 0.010 0.0002 0.0011 0.0013 Example 6 0.048 0.11 1.25 0.0079 0.0005 0.06 0.05 0.23 0.06 0.042 0.042 0.015 0.0025 0.027 0.0001 0.0024 0.0023 Example 7 0.062 0.19 0.98 0.0047 0.0003 0.34 0.21 0.80 - 0.027 0.019 0.007 0.0027 0.013 0.0003 0.0010 0.0022 Example 8 0.052 0.26 1.26 0.0052 0.0005 0.23 0.20 0.26 0.09 0.038 0.038 0.005 0.0033 0.033 0.0002 0.0013 0.0034 Example 9 0.033 0.12 1.45 0.0077 0.0015 0.07 0.06 0.65 0.08 0.041 0.010 0.014 0.0023 0.028 0.0004 0.0026 0.0024 Example 10 0.024 0.29 1.29 0.0051 0.0004 0.18 0.12 0.22 0.07 0.050 - 0.020 0.0022 0.026 0.0003 0.0018 0.0026 Comparative Example 1 0.069 0.19 1.23 0.0045 0.0009 0.13 0.08 0.18 0.05 0.026 0.012 0.006 0.0009 0.012 0.0003 0.0023 0.0039 Comparative Example 2 0.021 0.26 1.43 0.0061 0.0007 0.25 0.03 0.15 0.02 0.029 0.029 0.02 0.0025 0.011 0.0005 0.0024 0.0046 H Ce Nb+V+Ti (Nb+V+Ti)/(C+N+5×B) Ca/S (Cr+Cu/3+Ni/3+Mo+Ce×25)/(S+P) Example 1 0.0001 0.0005 0.119 1.74 3.3 56 Example 2 0.0001 0.0006 0.078 1.64 8.0 112 Example 3 0.0002 0.0031 0.104 1.81 5.0 90 Example 4 0.0002 0.0032 0.048 1.90 14.0 85 Example 5 0.0001 0.0022 0.075 1.04 8.7 145 Example 6 0.0002 0.0042 0.099 1.95 4.6 51 Example 7 0.0001 0.0021 0.053 0.81 9.0 207 Example 8 0.0001 0.0007 0.081 1.44 6.6 90 Example 9 0.0002 0.0049 0.065 1.74 1.5 97 Example 10 0.0002 0.0008 0.070 2.49 5.5 75 Comparative Example 1 0.0001 0.0003 0.044 0.59 1.0 57 Comparative Example 2 0.0002 0.0002 0.078 2.78 3.6 39 Stirring time (min) Ca wire feeding amount (m) Continuous casting holding time (min) Fluctuation of continuous casting speed (m/min) Slab heating temperature (°C) Rough rolling finishing temperature (°C) Final pass reduction ratio in rough rolling (%) Finish rolling finishing temperature (°C) Cumulative finish rolling reduction ratio (%) Cooling start temperature (°C) Cooling stop temperature (°C) Cooling rate (°C/s) Example 1 5 499 7 0.01 1114 948 15 804 75 766 443 11 Example 2 9 440 11 0.05 1166 930 14 752 77 710 328 25 Example 3 8 422 10 0.08 1122 956 13 830 76 790 325 30 Example 4 10 481 8 0.07 1191 975 13 851 76 796 442 12 Example 5 6 414 5 0.04 1169 902 16 790 78 743 370 25 Example 6 7 303 6 0.03 1196 989 12 820 77 785 367 29 Example 7 11 496 4 0.06 1250 925 15 827 75 752 408 12 Example 8 7 439 6 0.10 1100 976 14 831 79 765 398 23 Example 9 9 491 9 0.00 1146 998 11 879 80 800 449 12 Example 10 12 366 3 0.02 1205 958 13 753 78 702 302 27 Comparative Example 1 6 280 8 0.12 1134 991 12 806 70 754 392 18 Comparative Example 2 8 457 7 0.04 1116 905 12 856 76 792 412 25 R_t0.5 yield strength (MPa) Tensile strength (MPa) Yield ratio Elongation A₅₀ (%) Charpy impact energy at -56°C (J) DWTT SA at -46°C (%) Corrosion rate in supercritical CO₂ phase (mm/a) Volume fraction of granular bainite (%) Example 1 523 599 0.87 45 387 433 410 95 92 0.046 87 Example 2 452 540 0.84 41 469 499 429 92 86 0.045 85 Example 3 541 612 0.88 39 374 427 447 95 98 0.041 92 Example 4 561 605 0.93 46 363 421 423 96 90 0.058 92 Example 5 460 560 0.82 47 379 314 353 90 92 0.053 86 Example 6 536 603 0.89 37 329 361 365 93 96 0.042 90 Example 7 453 541 0.84 42 493 358 483 91 97 0.037 90 Example 8 516 581 0.89 45 412 384 332 93 97 0.038 92 Example 9 568 698 0.81 36 378 436 472 91 87 0.042 88 Example 10 451 552 0.82 38 446 443 403 96 96 0.046 87 Comparative Example 1 514 578 0.89 43 238 425 377 78 86 0.126 77 Comparative Example 2 530 599 0.88 40 322 250 208 87 77 0.112 93 Note: The three columns for the Charpy impact energy at -56°C represent the test results of three parallel specimens; the two columns for the DWTT SA% at -46°C represent the test results of two parallel specimens.