Low-cost thick gauge high toughness x70 pipeline steel and method of production thereof

Abstract

This invention discloses a low-cost, thick-gauge, high-strength and high-toughness X70 pipeline steel and its production method. Its chemical composition is: C: 0.05–0.07%, Si: 0.15–0.25%, Mn: 1.70–1.85%, Nb: 0.035–0.055%, Ti: 0.030–0.045%, Cr: 0.30–0.35%, Ce: 0.0015–0.0035%, Als: 0.015–0.035%, S: ≤0.003%, P: ≤0.010%, N: ≤0.0035%. The production method includes smelting and continuous casting, heating and rolling, cooling, and coiling processes. Compared with existing technologies, this invention, without adding Mo or Ni, achieves uniform and fine acicular ferrite with a small amount of Mao island (M / A) or granular bainite structure through optimized design of elements such as Nb, Ti, and Ce, combined with comprehensive control of continuous casting and controlled rolling and cooling processes. Niobium-titanium nanoprecipitates further strengthen the ferrite, enabling the pipeline steel of this invention to not only have excellent mechanical properties, weldability, and low-temperature tear resistance, but also significantly reduce production costs, achieving good service safety and economy.

Description

Technical Field

[0001] This invention relates to the field of pipeline steel production technology, specifically to a low-cost, thick-gauge, high-strength and high-toughness X70 pipeline steel and its production method. Background Technology

[0002] With the optimization and adjustment of the global energy structure, the demand for oil and natural gas has increased, promoting the rapid development of pipeline engineering. High-grade oil and gas pipeline steel has become the mainstream trend in the construction of long-distance pipelines. Improving the steel grade of pipeline steel and reducing production costs directly affects the competitiveness of pipeline steel in the industry. High-grade pipeline steel of X70 and above is mostly reinforced with heavy metal elements such as Mo and Ni to meet its performance requirements of high strength and high and low temperature crack arrest toughness. However, its production cost remains high, and there is currently a challenge in matching low manufacturing cost with excellent comprehensive performance of pipeline steel.

[0003] To address these challenges, steel companies have conducted research on pipeline steel. For example, patent CN121082686A discloses a hot continuous rolling production method for Mo- and Ni-free X70M pipeline steel, which is strengthened by Nb-Ti-Cr composite microalloying. However, this method can only produce thin-gauge pipeline steel with a thickness range of 8-18 mm. When the steel plate thickness increases to more than 18 mm, the material is prone to problems such as uneven microstructure and increased performance gradient in the thickness direction during the rolling process and subsequent cooling process, which adversely affects the toughness of the weld heat-affected zone and the service safety. Patent CN117467891A discloses a low-cost X70 grade high-strength pipeline steel plate coil and its production method. It is strengthened by Mn-Nb-Cr and trace amounts of Ti. However, the finishing rolling temperature needs to be controlled below 950℃ for low-temperature rolling. Therefore, the intermediate billet may need a long time to wait for the steel to warm up, which has a significant impact on the production rhythm. In addition, the yield strength of its product is between 500-524MPa, with a small strength margin. After pipe making, there is a high risk that the yield strength of the steel pipe will be less than the lower limit of 585MPa. Patent CN1746326A discloses a needle-shaped ferritic X70 pipeline steel with high crack-arresting toughness and its manufacturing method. The pipeline steel uses microalloying elements such as Nb: 0.050~0.080%, Ti: 0.005~0.025%, V: 0.010~0.060%, Mo: 0.10~0.30%, Cu≤0.30%, and Ni≤0.30%. However, molybdenum and nickel are precious metals and are expensive. Therefore, using this alloying method to produce pipeline steel is not only costly but also wasteful of resources.

[0004] Therefore, how to achieve stable control of the microstructure uniformity of X70 pipeline steel under thick gauge conditions without relying on high-cost alloying elements, while taking into account high strength and toughness and good weldability, remains a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0005] To address the aforementioned issues, this invention proposes a low-cost, thick-gauge, high-strength and high-toughness X70 pipeline steel and its production method. This method, without adding high-cost alloying elements such as Mo and Ni, achieves uniform and fine microstructure in thick-gauge X70 pipeline steel through synergistic control of composition design and production process. It also ensures high strength and toughness, good weldability, and crack arrest capability, thus meeting the comprehensive requirements of safety and economy for oil and gas pipelines.

[0006] To achieve the above-mentioned objectives, the present invention adopts the following technical solution: In a first aspect, the present invention provides a low-cost, thick-gauge, high-strength and tough X70 pipeline steel, the chemical composition of which is as follows by mass percentage: C: 0.05-0.07%, Si: 0.15-0.25%, Mn: 1.70-1.85%, Nb: 0.035-0.055%, Ti: 0.030-0.045%, Cr: 0.30-0.35%, Ce: 0.0015-0.0035%, Als: 0.015-0.035%, S: ≤0.003%, P: ≤0.010%, N: ≤0.0035%, with the balance being Fe and unavoidable impurities.

[0007] Furthermore, the microstructure of the low-cost, thick-gauge, high-strength and tough X70 pipeline steel is mainly composed of uniform and fine acicular ferrite with a small amount of Mao islands (M / A) or granular bainite. The average grain size of the acicular ferrite is 2.3~3.1μm, and the average size of the Mao islands (M / A) is 0.2~1.1μm. No banded structures are present in the microstructure.

[0008] Furthermore, the mechanical properties of low-cost, thick-gauge, high-strength and high-toughness X70 pipeline steel are as follows: tensile strength (Rm) 640~720MPa, yield strength (Rt0.5) 520~590MPa, yield ratio ≤0.82, and elongation A 50 ≥37%, impact toughness Akv≥280J at -30℃, drop shear area≥92% at -20℃.

[0009] Secondly, the present invention provides a method for producing low-cost, thick-gauge, high-strength and high-toughness X70 pipeline steel, including smelting and continuous casting, heating and rolling, cooling and coiling processes, the specific production process of which is as follows: (1) Smelting and continuous casting process: The converter is used for smelting. Molten steel is injected into the tundish through the sliding nozzle at the bottom of the ladle. The temperature of the molten steel in the tundish is controlled at 1525~1540 ℃. The billet casting speed is 1.4~2.0 m / min. The dynamic reduction during the continuous casting process is controlled at 4~6 mm to eliminate the segregation and porosity of the central component, and at the same time refine the as-cast structure to ensure the compactness and uniformity of the billet structure. (2) Heating and rolling process: The continuously cast slab is heated to 1220~1240 ℃ and held for 120~180 min; the rolling process includes roughing and finishing. The starting temperature of the roughing mill is controlled at 1100~1150 ℃, the inlet temperature of the first finishing mill is controlled at 982~990 ℃, the finishing mill termination temperature is controlled at 840~846 ℃, the total reduction rate is ≥90%, the reduction rate of the last pass of finishing mill is controlled at 14%~18%, and the thickness of the strip after rolling is 18~24 mm. No steel slab temperature control is required during the rolling process. (3) Cooling and coiling process: Laminar flow cooling is adopted after rolling, and the cooling rate is controlled at 26-32℃ / s to ensure that fine and uniform acicular ferrite with a small amount of Mao island (M / A) or granular bainite structure is obtained along the thickness direction. The coiling temperature is controlled at 340-400℃.

[0010] The main alloying elements and their mechanisms of action in the low-cost, thick-gauge, high-strength and tough X70 pipeline steel of this invention are as follows: Carbon (C) is the most economical and effective strengthening element. It not only provides solid solution strengthening but also combines with elements like Nb and Ti to form carbonitrides, thus providing precipitation strengthening. A certain amount of C can stabilize austenite and promote the formation of acicular ferrite and ferrite (M / A), thereby increasing the strength of X70 pipeline steel. However, excessive carbon content will reduce the toughness and weldability of the steel. This invention selects 0.05–0.07% C.

[0011] Si: Silicon is a strong deoxidizer and an effective solid solution strengthening element. Its content is controlled at a low level, primarily to ensure deoxidation. Excessive silicon content will impair the toughness, weldability, and surface quality of the steel. This invention selects 0.15–0.25% Si.

[0012] Mn: Manganese is a major solid solution strengthening element that can significantly improve the strength and hardness of steel. Simultaneously, manganese can lower the γ→α phase transformation temperature, promote the formation of acicular ferrite, and, within a suitable range and under appropriate processing conditions, improve the strength of steel while having a limited effect on toughness. This invention selects 1.70–1.85% Mn.

[0013] Nitrogen (Nb) can produce significant grain refinement and moderate precipitation strengthening. During controlled rolling, Nb(C,N) precipitates in austenite, strongly inhibiting austenite recrystallization and ensuring that the austenite is rolled in the non-recrystallized zone during the finishing rolling stage. This refines the ferrite grains after phase transformation, allowing the non-recrystallized austenite to form extremely fine ferrite grains during the phase transformation. The NbC nanoparticles precipitated in the ferrite provide a significant precipitation strengthening effect. However, the Nb content should not be too high; exceeding 0.055% Nb significantly weakens the strengthening effect. This invention selects 0.035–0.055% Nb. This content range is the economically effective range for achieving the above-mentioned effects. For thick steel plates, sufficient Nb content is one of the effective methods to ensure a uniform and fine microstructure throughout the thickness direction, thereby achieving stable microstructure and properties.

[0014] Ti: High-temperature precipitated TiN and Ti(N,C) particles can pin austenite grain boundaries and, in conjunction with Nb, prevent austenite recrystallization. By controlling the rolling and cooling processes, the amount of low-temperature (post-rolling cooling and coiling) nano-sized TiC precipitation can be increased, improving the precipitation strengthening effect. Simultaneously, adding Ti improves weldability. When the weld cools, Ti-containing oxides become nucleation sites, inducing the formation of intragranular acicular ferrite and preventing grain coarsening in the weld heat-affected zone. This microstructure significantly improves the toughness of the weld metal. However, the Ti content should not be too high, otherwise coarse liquid-precipitated TiN, solid-precipitated TiN, and Ti4C2S2 will easily form, adversely affecting the strength and toughness of the steel. This invention selects 0.030–0.045% Ti.

[0015] Cr (Cr): Improves the hardenability of steel, inhibits pearlite transformation, and promotes the formation of finer and harder acicular ferrite or bainite at low temperatures. Especially in thicker steel sections where cooling rates are slower, it helps to obtain a more uniform bainite / acicular ferrite microstructure, ensuring uniform cross-sectional properties. The addition of Cr promotes the precipitation of fine carbides, increasing the strength of the steel while reducing its yield strength ratio. Simultaneously, adding Cr to steel can improve the density of the surface oxide scale, enhancing the steel's resistance to uniform corrosion and pitting. This invention uses 0.30–0.35% Cr.

[0016] Ce: On the one hand, it can effectively improve the morphology and distribution of inclusions, making them spherical or nearly spherical, and more diffusely distributed. Compared with elongated or angular inclusions, spherical inclusions have less impact on the continuity of the steel matrix, thus reducing the probability of steel fracture caused by inclusions as crack initiation factors. On the other hand, after rare earth elements are added to steel, they react with other elements to achieve microalloying and refine grains, thereby also improving the strength and toughness of steel to a certain extent. This invention selects 0.0015~0.0035% Ce.

[0017] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. A chemical composition formula for low-cost, thick-gauge, high-strength and tough X70 pipeline steel and a matching production method are provided.

[0018] 2. Excellent weldability. Adding an appropriate amount of Ti can improve weldability. When the weld cools, the Ti-containing oxides become nucleation sites, inducing the formation of intragranular acicular ferrite and preventing grain coarsening in the heat-affected zone. This structure can significantly improve the toughness of the weld metal, making it suitable for large-diameter, thick-walled pipeline projects.

[0019] 3. Without adding high-cost alloying elements such as Mo and Ni, comprehensive performance that meets the requirements of strength, toughness and service safety of X70 pipeline steel can be obtained through the synergistic control of chemical composition and production process, which is conducive to reducing material costs and engineering application costs.

[0020] 4. Improved crack arrest capability and service safety. The synergistic effect of fine needle-like ferrite, M / A islands, and nanoscale precipitates effectively inhibits crack initiation and propagation while maintaining high strength and toughness, thereby improving the safety and reliability of pipeline steel under complex service conditions.

[0021] 5. The reason why the low-cost X70 pipeline steel produced by this invention has both high strength and high toughness is: (1) Dynamic light pressure during continuous casting can eliminate central component segregation (such as the segregation of elements such as Mn) and porosity, while refining the as-cast structure and ensuring the compactness and uniformity of the billet structure. (2) Without adding Mo and Ni, through the optimized design of elements such as Nb, Ti, Mn, Cr, Ce and N, combined with the comprehensive control of continuous casting and controlled rolling and cooling processes, uniform and fine acicular ferrite with a small amount of Mao island (M / A) or granular bainite structure is obtained. The acicular ferrite contains dispersed fine nanoscale precipitates of elements such as Nb and Ti to further strengthen the ferrite. The average grain size of acicular ferrite is 2.3~3.1μm, the average size of M / A is 0.2~1.1μm, and there is no banded structure in the structure. At the same time, through the comprehensive control of composition and process, a uniform and fine structure can be obtained in the entire thickness direction, thereby achieving uniform and stable performance. (3) Adding trace amounts of rare earth element Ce to steel can effectively improve the morphology and distribution of inclusions, making long strip-shaped inclusions such as MnS spherical or nearly spherical, and the distribution is also more diffuse, thereby reducing the probability of steel fracture caused by inclusions as crack initiation factors; on the other hand, after rare earth elements are added to steel, through the synergistic effect of Nb and Ti, the grains are further refined, thereby further improving the strength and toughness of steel. At the same time, the content of N is determined according to the Ti content in the steel of this invention, thereby avoiding the precipitation of coarse liquid-precipitated TiN, solid-precipitated TiN and Ti4C2S2, and combined with the control of rolling and cooling process, the Ti precipitates are precipitated as fine titanium carbide in the post-rolling cooling and coiling, thereby achieving the purpose of effectively strengthening ferrite without basically affecting the toughness.

[0022] 6. Excellent overall performance: While reducing alloy costs, the tensile strength (Rm) is 640~720MPa, the yield strength (Rt0.5) is 520~590MPa, the yield ratio is ≤0.82, and the elongation A is... 50 ≥37%, impact toughness Akv≥280J at -30℃, drop shear area≥92% at -20℃. It fully meets the comprehensive performance requirements of X70 pipeline steel for strength, toughness, and service safety, and is suitable for large-diameter, thick-walled pipeline projects. Attached Figure Description

[0023] Figure 1 The microstructure of the X70 pipeline steel produced in Embodiment 1 of the present invention.

[0024] Figure 2 The microstructure of the X70 pipeline steel produced in Embodiment 2 of the present invention.

[0025] Figure 3 The microstructure of the X70 pipeline steel produced in Embodiment 3 of the present invention.

[0026] Figure 4 The microstructure of the X70 pipeline steel produced in Example 4 of this invention.

[0027] Figure 5 The microstructure of the X70 pipeline steel produced in Embodiment 5 of the present invention.

[0028] Figure 6 The microstructure of the X70 pipeline steel produced in Comparative Example 1 of this invention is shown.

[0029] Figure 7 The microstructure of the X70 pipeline steel produced in Comparative Example 2 of this invention is shown. Detailed Implementation

[0030] To make the objectives, technical solutions, and advantages of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are some embodiments of the present invention, but not all embodiments.

[0031] Unless otherwise specified, the experimental methods used in the embodiments and comparative examples of this invention are conventional methods. Unless otherwise specified, the materials and reagents used are commercially available. Example 1

[0032] This embodiment provides a low-cost, thick-gauge, high-strength and tough X70 pipeline steel and its production method. Its chemical composition by weight percentage is: C: 0.05%, Si: 0.25%, Mn: 1.85%, Nb: 0.045%, Ti: 0.040%, Cr: 0.30%, Ce: 0.0035%, Als: 0.020%, S: 0.003%, P: 0.009%, N: 0.0028%, with the balance being Fe and unavoidable impurities.

[0033] The production process includes smelting, continuous casting, heating, rolling, cooling, and coiling. Smelting employs conventional pipeline steel smelting methods. Specific parameters for other processes are as follows: ① Continuous casting: Tundish temperature controlled at 1530 ℃, billet casting speed 1.5 m / min, dynamic reduction 5 mm; ② Heating: Furnace temperature 1220 ℃, holding time 170 min; ③ Rolling: Roughing stage initial rolling temperature 1100 ℃, finishing stage inlet temperature 982 ℃, termination temperature 840 ℃, total reduction 94%, final finishing stage reduction 14%, product thickness 18 mm; ④ Cooling and coiling: Laminar flow cooling rate 26 ℃ / s, coiling temperature 400 ℃.

[0034] The final room temperature microstructure is a uniform, fine acicular ferrite with a small amount of Mao islands (M / A) or granular bainite (see [reference]). Figure 1The average grain size of acicular ferrite is 2.3 μm, the average M / A size is 0.2 μm, and no banded structure is present in the microstructure.

[0035] The mechanical properties are as follows: tensile strength (Rm) 720MPa, yield strength (Rt0.5) 590MPa, yield ratio 0.82, elongation A 50 The value is 39%, the impact toughness Akv at -30℃ is 300J, and the drop shear area at -20℃ is 93%. Example 2

[0036] This embodiment provides a low-cost, thick-gauge, high-strength and tough X70 pipeline steel and its production method. Its chemical composition by weight percentage is: C: 0.07%, Si: 0.15%, Mn: 1.70%, Nb: 0.055%, Ti: 0.030%, Cr: 0.35%, Ce: 0.003%, Als: 0.035%, S: 0.0027%, P: 0.010%, N: 0.0035%, with the balance being Fe and unavoidable impurities.

[0037] The production process includes smelting, continuous casting, heating, rolling, cooling, and coiling. Smelting employs conventional pipeline steel smelting methods. Specific parameters for other processes are as follows: ① Continuous casting: Tundish temperature controlled at 1540 ℃, billet casting speed 2.0 m / min, dynamic reduction 4 mm; ② Heating: Furnace temperature 1240 ℃, holding time 120 min; ③ Rolling: Roughing stage initial rolling temperature 1150 ℃, finishing stage entry temperature 990 ℃, termination temperature 846 ℃, total reduction 90%, final finishing stage reduction 18%, product thickness 24 mm; ④ Cooling and coiling: Laminar flow cooling rate 28 ℃ / s, coiling temperature 390 ℃.

[0038] The final room temperature microstructure is a uniform, fine acicular ferrite with a small amount of Mao islands (M / A) or granular bainite (see [reference]). Figure 2 The average grain size of acicular ferrite is 3.1 μm, the average M / A size is 1.1 μm, and no banded structure is present in the microstructure.

[0039] The mechanical properties are: tensile strength (Rm) 640MPa, yield strength (Rt0.5) 520MPa, yield ratio 0.81, and elongation A. 50 The value is 37%, the impact toughness Akv at -30℃ is 280J, and the drop shear area at -20℃ is 92%. Example 3

[0040] This embodiment provides a low-cost, thick-gauge, high-strength and tough X70 pipeline steel and its production method. Its chemical composition by weight percentage is: C: 0.06%, Si: 0.20%, Mn: 1.75%, Nb: 0.035%, Ti: 0.045%, Cr: 0.32%, Ce: 0.0027%, Als: 0.015%, S: 0.003%, P: 0.0085%, N: 0.0021%, with the balance being Fe and unavoidable impurities.

[0041] The production process includes smelting, continuous casting, heating, rolling, cooling, and coiling. Smelting employs conventional pipeline steel smelting methods. Specific parameters for other processes are as follows: ① Continuous casting: Tundish temperature controlled at 1525 ℃, billet casting speed 1.4 m / min, dynamic reduction 6 mm; ② Heating: Furnace temperature 1230 ℃, holding time 180 min; ③ Rolling: Roughing stage initial rolling temperature 1110 ℃, finishing stage entry temperature 983 ℃, termination temperature 842 ℃, total reduction 92%, final finishing stage reduction 16%, product thickness 20 mm; ④ Cooling and coiling: Laminar flow cooling rate 32 ℃ / s, coiling temperature 340 ℃.

[0042] The final room temperature microstructure is a uniform, fine acicular ferrite with a small amount of Mao islands (M / A) or granular bainite (see [reference]). Figure 3 The average grain size of acicular ferrite is 2.5 μm, the average M / A size is 0.5 μm, and no banded structure is present in the microstructure.

[0043] The mechanical properties are as follows: tensile strength (Rm) 710MPa, yield strength (Rt0.5) 570MPa, yield ratio 0.80, elongation A 50 The value is 42%, the impact toughness Akv at -30℃ is 320J, and the drop shear area at -20℃ is 95%. Example 4

[0044] This embodiment provides a low-cost, thick-gauge, high-strength and tough X70 pipeline steel and its production method. Its chemical composition by weight percentage is: C: 0.065%, Si: 0.22%, Mn: 1.80%, Nb: 0.047%, Ti: 0.038%, Cr: 0.34%, Ce: 0.0015%, Als: 0.030%, S: 0.0018%, P: 0.0095%, N: 0.0028%, with the balance being Fe and unavoidable impurities.

[0045] The production process includes smelting, continuous casting, heating, rolling, cooling, and coiling. Smelting employs conventional pipeline steel smelting methods. Specific parameters for other processes are as follows: ① Continuous casting: Tundish temperature controlled at 1535 ℃, billet casting speed 1.8 m / min, dynamic reduction 5.5 mm; ② Heating: Furnace temperature 1225 ℃, holding time 135 min; ③ Rolling: Roughing stage initial rolling temperature 1120 ℃, finishing stage entry temperature 985 ℃, termination temperature 844 ℃, total reduction 93%, final finishing stage reduction 17%, product thickness 19 mm; ④ Cooling and coiling: Laminar flow cooling rate 30 ℃ / s, coiling temperature 360 ​​℃.

[0046] The final room temperature microstructure is a uniform, fine acicular ferrite with a small amount of Mao islands (M / A) or granular bainite (see [reference]). Figure 4 The average grain size of acicular ferrite is 2.6 μm, the average M / A size is 0.7 μm, and no banded structure is present in the microstructure.

[0047] The mechanical properties are: tensile strength (Rm) 695MPa, yield strength (Rt0.5) 545MPa, yield ratio 0.78, and elongation A. 50 The value is 43%, the impact toughness Akv at -30℃ is 314J, and the drop shear area at -20℃ is 93.5%. Example 5

[0048] This embodiment provides a low-cost, thick-gauge, high-strength and tough X70 pipeline steel and its production method. Its chemical composition by weight percentage is: C: 0.063%, Si: 0.24%, Mn: 1.78%, Nb: 0.051%, Ti: 0.034%, Cr: 0.31%, Ce: 0.0021%, Als: 0.023%, S: 0.0023%, P: 0.0089%, N: 0.0025%, with the balance being Fe and unavoidable impurities.

[0049] The production process includes smelting, continuous casting, heating, rolling, cooling, and coiling. Smelting employs conventional pipeline steel smelting methods. Specific parameters for other processes are as follows: ① Continuous casting: Tundish temperature controlled at 1532 ℃, billet casting speed 1.7 m / min, dynamic reduction 4.5 mm; ② Heating: Furnace temperature 1232 ℃, holding time 130 min; ③ Rolling: Roughing stage initial rolling temperature 1140 ℃, finishing stage entry temperature 987 ℃, termination temperature 845 ℃, total reduction 91%, final finishing stage reduction 15%, product thickness 22 mm; ④ Cooling and coiling: Laminar flow cooling rate 27 ℃ / s, coiling temperature 380 ℃.

[0050] The final room temperature microstructure is a uniform, fine acicular ferrite with a small amount of Mao islands (M / A) or granular bainite (see [reference]). Figure 5 The average grain size of acicular ferrite is 2.8 μm, the average M / A size is 0.9 μm, and no banded structure is present in the microstructure.

[0051] The mechanical properties are: tensile strength (Rm) 670MPa, yield strength (Rt0.5) 530MPa, yield ratio 0.79, and elongation A. 50 The value is 38%, the impact toughness Akv at -30℃ is 295J, and the drop shear area at -20℃ is 92.5%. Comparative Example 1

[0052] This comparative example provides a traditional Mo and Ni microalloyed 70 pipeline steel and its production method. Its chemical composition, by weight percentage, is as follows: C: 0.06%, Si: 0.20%, Mn: 1.70%, Mo: 0.10%, Ni: 0.14%, Nb: 0.040%, Ti: 0.017%, Als: 0.030%, S: 0.003%, P: 0.009%, N: 0.0030%, with the balance being Fe and unavoidable impurities.

[0053] The production process route is basically the same as in Example 3, namely smelting → continuous casting → heating → rolling → cooling and coiling. Specific parameters are: ① Continuous casting: tundish temperature controlled at 1525 ℃, billet casting speed 1.4 m / min, dynamic reduction 6 mm; ② Heating: furnace temperature 1230 ℃, holding time 180 min; ③ Rolling: roughing stage starting temperature 1110 ℃, finishing stage entry temperature 980 ℃, termination temperature 840 ℃, total reduction 92%, final finishing stage reduction 16%, product thickness 20 mm; ④ Cooling and coiling: laminar flow cooling rate 32 ℃ / s, coiling temperature 350 ℃.

[0054] The final room temperature microstructure is a uniform, fine acicular ferrite with a small amount of Mao islands (M / A) or granular bainite (see [reference]). Figure 6 The average grain size of acicular ferrite is 2.4 μm, the average M / A size is 0.5 μm, and no banded structure is present in the microstructure.

[0055] The mechanical properties are: tensile strength (Rm) 700MPa, yield strength (Rt0.5) 567MPa, yield ratio 0.81, and elongation A. 50 The value is 43%, the impact toughness Akv at -30℃ is 325J, and the drop shear area at -20℃ is 94%.

[0056] The results show that the microstructure and properties of the X70 pipeline steel without expensive Mo and Ni are basically the same as those with added Mo and Ni, but the cost per ton of steel is significantly reduced. The alloy cost of the system with added Mo and Ni is 1236 yuan / ton, while the alloy cost of the system with the composition of this invention is 850 yuan / ton, which reduces the cost per ton of steel by 386 yuan. Comparative Example 2

[0057] To further compare and verify, this comparative example provides a continuous casting process without dynamic soft pressure, with an elemental composition similar to that of Example 4 of this invention, in the following chemical composition (mass percentage): C: 0.065%, Si: 0.21%, Mn: 1.78%, Nb: 0.047%, Ti: 0.038%, Cr: 0.34%, Ce: 0.0016%, Als: 0.030%, S: 0.0020%, P: 0.0095%, N: 0.0028%, with the balance being Fe and unavoidable impurities.

[0058] The production process is basically the same as in Example 4, but the cooling rate is reduced, namely smelting, continuous casting, heating, rolling, cooling, and coiling. Specific parameters are: ① Continuous casting: Tundish temperature controlled at 1535 ℃, billet casting speed 1.8 m / min; ② Heating: Furnace temperature 1225 ℃, holding time 135 min; ③ Rolling: Roughing stage initial rolling temperature 1120 ℃, finishing stage entry temperature 984 ℃, termination temperature 843 ℃, total reduction rate 93%, final finishing stage reduction rate 17%, product thickness 19 mm; ④ Cooling and coiling: Laminar flow cooling rate 30 ℃ / s, coiling temperature 360 ​​℃.

[0059] The final room temperature microstructure is a heterogeneous acicular ferrite with a small amount of Mao islands (M / A) or granular bainite (see [reference]). Figure 7 The average grain size of acicular ferrite is 3.8 μm, the average M / A size is 1.7 μm, and banded structures are present in the microstructure.

[0060] The mechanical properties are: tensile strength (Rm) 675MPa, yield strength (Rt0.5) 520MPa, yield ratio 0.86, and elongation A. 50 The value is 30%, the impact toughness Akv at -30℃ is 205J, and the drop shear area at -20℃ is 84%.

[0061] The results show that although the performance of this comparative example can meet the standard requirements of X70 pipeline steel, the strength and toughness are reduced due to the large and uneven grain size and the presence of banded structure. In particular, the low-temperature impact toughness and drop hammer shear area are significantly reduced, which poses a risk of pipe cracking.

[0062] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; if these modifications and variations fall within the scope of the claims of the present invention and their equivalents, they should all be considered to be within the protection scope of the present invention.

Claims

1. A low-cost, thick-gauge, high-strength and high-toughness X70 pipeline steel, characterized in that, Its chemical composition by mass percentage is as follows: C: 0.05-0.07%, Si: 0.15-0.25%, Mn: 1.70-1.85%, Nb: 0.035-0.055%, Ti: 0.030-0.045%, Cr: 0.30-0.35%, Ce: 0.0015-0.0035%, Als: 0.015-0.035%, S: ≤0.003%, P: ≤0.010%, N: ≤0.0035%, with the balance being Fe and unavoidable impurities.

2. The low-cost, thick-gauge, high-strength and high-toughness X70 pipeline steel according to claim 1, characterized in that, The microstructure of X70 pipeline steel consists of uniform and fine acicular ferrite with a small amount of Mao islands or granular bainite. The average grain size of the acicular ferrite is 2.3~3.1μm, and the average size of the Mao islands is 0.2~1.1μm. No banded structures are present in the microstructure.

3. The low-cost, thick-gauge, high-strength and high-toughness X70 pipeline steel according to claim 1, characterized in that, The mechanical properties of X70 pipeline steel are as follows: tensile strength 640~720MPa, yield strength 520~590MPa, yield ratio ≤0.82, and elongation A. 50 ≥37%, impact toughness Akv≥280J at -30℃, drop shear area≥92% at -20℃.

4. A method for producing low-cost, thick-gauge, high-strength and high-toughness X70 pipeline steel as described in any one of claims 1-3, characterized in that, The production process is as follows: (1) Smelting and continuous casting process: The converter is used for smelting. Molten steel is injected into the tundish through the sliding nozzle at the bottom of the ladle. The temperature of the molten steel in the tundish is controlled at 1525~1540 ℃. The billet casting speed is 1.4~2.0 m / min. The dynamic reduction during the continuous casting process is controlled at 4~6 mm. (2) Heating and rolling process: The continuously cast slab is heated to 1220~1240 ℃ and held for 120~180 min; the rolling process includes roughing and finishing. The starting temperature of the roughing mill is controlled at 1100~1150 ℃, the inlet temperature of the first finishing mill is controlled at 982~990 ℃, the finishing mill termination temperature is controlled at 840~846 ℃, the total reduction rate is ≥90%, the reduction rate of the last pass of finishing mill is controlled at 14%~18%, and the thickness of the strip after rolling is 18~24 mm. No steel slab temperature control is required during the rolling process. (3) Cooling and coiling process: Laminar flow cooling is adopted after rolling, and the cooling rate is controlled at 26-32℃ / s to ensure that fine and uniform acicular ferrite with a small amount of Mao island or granular bainite structure is obtained along the thickness direction. The coiling temperature is controlled at 340-400℃.

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