A low-temperature vessel steel with a yield strength of ≥ 690 mpa and a production method thereof

Abstract

This invention discloses a low-temperature container steel with a yield strength ≥690MPa and its production method. The composition design eliminates the need for Ni addition, significantly reducing production costs. Grain refinement is achieved by adding an appropriate amount of Nb alloy, while alloying elements such as Cr, Mo, Ti, and B are added for alloying. Through two-stage controlled rolling and cooling, and by employing either tempering or quenching and tempering processes depending on the steel plate thickness, the resulting 80-120mm thick steel plate exhibits excellent strength and toughness matching in both the delivered and die-welded states. Specifically, this is demonstrated in the tempered or quenched and tempered state. The yield strength is above 760 MPa, the tensile strength is >880 MPa, the elongation is ≥22%, and the single transverse impact energy at -50℃ at the 1 / 4 position is >250 J, while the single transverse impact energy at -50℃ at the 1 / 2 thickness position is >180 J. After simulated post-weld heat treatment at 630℃ for 10 hours, the single transverse impact energy at -50℃ at the 1 / 4 position is >160 J, and the single transverse impact energy at -50℃ at the 1 / 2 thickness position is >120 J. The yield strength is above 720 MPa, and the tensile strength is >830 MPa. This meets the requirements for low-cost, large-thickness, high-strength and high-toughness container steel.

Description

Technical Field

[0001] This invention relates to the field of steel plate production technology, specifically to a low-temperature container steel with a yield strength ≥690MPa and its production method, which has good low-temperature impact performance. Background Technology

[0002] Currently, pressure vessel steels with high strength and good low-temperature impact toughness, both domestically and internationally, are generally thinner, and are produced using low carbon content with high levels of alloying elements such as Ni and Mo, which significantly increases production costs. There are no reports on production solutions for low-cost, thick, high-strength and high-toughness pressure vessel steels.

[0003] For example, Chinese patent CN111850401A discloses a low-cost, high-strength, and high-toughness pressure vessel steel plate and its production method. The chemical composition of the steel plate, by mass percentage, is: C: 0.13%–0.18%, Si: 0.25%–0.35%, Mn: 1.10%–1.30%, P≤0.010%, S≤0.002%, Mo: 0.25%–0.35%, Al: 0.030%–0.040%, Ti: 0.070%–0.130%. Although this technical solution does not add Ni, it adds 0.25–0.35% Mo and 0.070%–0.130% Ti. While the addition of a large amount of Ti can improve the strength of the steel, it easily forms compounds with elements such as C, N, O, and S, especially with O, which has a strong affinity. This affects the purity of the molten steel and has an adverse effect on low-temperature impact performance and welding. Moreover, the thickness of the steel plate produced is only 6-36 mm.

[0004] For example, Chinese patent CN202010127653.1 describes a low-cost, high-strength, and high-toughness pressure vessel steel plate and its production method. The chemical composition of the steel plate, by weight percentage, is as follows: C: 0.13-0.16%, Si: 0.15-0.35%, Mn: 0.80-1.00%, P≤0.015%, S≤0.005%, Ni: 0.30-0.40%, Cr: 0.25-0.35%, Al: 0.020-0.035%, Ti: 0.010-0.020%, V: 0.06-0.15%, Cu: 0.20-0.30%, N: 0.010-0.020%. The solution incorporates 0.30-0.40% Ni, which reduces production costs and shortens the process flow in terms of chemical composition and production technology. However, the steel plate thickness is only 29-46mm, and the maximum impact temperature is only -20℃, which does not offer any significant advantages. Summary of the Invention

[0005] To address the aforementioned technical deficiencies, the present invention aims to provide a low-temperature container steel with a yield strength ≥690 MPa. In terms of composition design, this steel plate does not require the addition of Ni element, which can greatly reduce production costs. At the same time, by adding Nb element to refine the grains and adding appropriate amounts of Cr, Mo, Ti, B elements for alloying, the strength and toughness of the steel are improved, resulting in good low-temperature impact performance.

[0006] Another objective of this invention is to provide a method for producing cryogenic container steel with a yield strength ≥ 690 MPa.

[0007] To achieve the above objectives, the technical solution of the present invention is: a low-temperature container steel with a yield strength ≥690MPa, comprising the following chemical composition by weight percentage: C: 0.18~0.22%, Si: 0.20~0.35%, Mn: 0.90~1.10%, P≤0.010%, S≤0.002%, Cr: 0.40~0.60%, Mo: 0.15~0.25%, Nb: 0.015~0.030%, V: 0.03~0.08%, Al: 0.020~0.040%, Ti: 0.030~0.050%, B: 0.0015~0.0050%, with the remainder being Fe and unavoidable impurities; the steel thickness is 80~120mm, and its metallographic structure is tempered martensite + a small amount of tempered bainite.

[0008] Regarding ingredient design:

[0009] C: C is a major component of steel. The strength of steel mainly depends on the C content. Excessive C content leads to poor toughness, plasticity, and weldability; low C content leads to lower strength. To ensure that the steel plate has high strength and good low-temperature impact toughness, this invention requires the C content to be controlled within the range of 0.18% to 0.22%.

[0010] Si: Si is a common solid solution strengthening alloying element in steel. It is an essential element for the strength, toughness, hardenability, and even deoxidation of steel. Therefore, the Si content in this invention is required to be controlled between 0.20% and 0.35%.

[0011] Mn: Mn can strengthen ferrite in steel through solid solution strengthening. C-Mn strengthening is also the main way to improve the strength of low carbon steel. However, if the Mn content is too high, it will aggravate the segregation in the center of the billet during continuous casting. This will affect the quality of internal flaw detection of the steel plate and will also be detrimental to the stability of the impact performance at the 1 / 2 position of the steel plate thickness. Therefore, this invention requires that the Mn content in the steel be controlled between 0.90% and 1.10%.

[0012] Nb: Nb is a strong carbide-forming element that forms second-phase particles such as NbC and Nb(CN) in steel, hindering the growth of austenite grains, refining the grains, and improving the strength and low-temperature toughness of the steel plate. The effective temperature of Nb is higher than that of Ti and V, and its contribution to the strength of the steel plate is also greater than that of Ti and V. However, excessive Nb content can easily lead to intergranular cracks. Therefore, the Nb content in this invention is controlled at 0.015–0.030%.

[0013] V: V is one of the carbide-forming elements that effectively improves the strength of steel plates, second only to Nb and Ti in its effect on steel. Adding V to steel will form VC, which increases the melting point, hardness, and wear resistance of cementite. Therefore, the V content in this invention is controlled at 0.03–0.08%.

[0014] Al: Adding a small amount of Al to steel can effectively refine the austenite grains, thereby refining the ferrite grains and microstructure, and improving the impact toughness of the steel. Therefore, this invention requires the Al content in the steel to be 0.030% to 0.040%.

[0015] Ti: Ti reacts with sulfur in steel to form TiS2, which makes sulfur more evenly distributed in the steel, thereby improving the mechanical properties of the steel. Ti can also react with nitrogen in molten steel to form TiN, which absorbs hydrogen gas and further optimizes the mechanical properties of the steel. Therefore, this invention requires the Ti content in the steel to be 0.030% to 0.050%.

[0016] Mo: Mo strengthens steel by solid solution strengthening and grain boundary strengthening. Therefore, this invention requires that the Mo content in the steel be controlled at 0.15% to 0.25%.

[0017] Boolean (B): Boolean element significantly improves the hardenability of steel, allowing for more uniform hardening during quenching, resulting in higher strength and hardness. Adding boolean can also save on expensive alloys such as Cr, Ni, and Mo. Therefore, this invention requires the boolean content in the steel to be controlled at 0.0015%–0.0050%.

[0018] Furthermore, the thickness of the steel plate is 80-120 mm, and its metallographic structure is tempered martensite + a small amount of tempered bainite.

[0019] The production method for the aforementioned low-temperature container steel with a yield strength ≥ 690 MPa includes continuous casting, heating, rolling, and heat treatment, as detailed below:

[0020] ① Continuous casting: The superheat of the tundish is controlled at 5-25℃, and the casting is carried out under full protection. The billet thickness is 300-360mm, the low magnification of the billet is controlled below Class C 1.0, and the non-metallic inclusions A+B+C+D+Ds ≤ 2.5.

[0021] ② Heating: The billet is heated to 1150-1200℃ for 360-450 minutes to ensure that the alloying elements are fully dissolved and that the billet temperature is uniform. The billet temperature is not higher than 1200℃. After the billet is removed from the furnace, it is descaled by high pressure water.

[0022] ③ Rolling and Controlled Cooling: A two-stage rolling process is adopted. The first stage uses high-penetration rolling, with a single-pass reduction controlled at more than 35mm, and at least two passes with a deformation coefficient >0.55 and a pass deformation rate >16%. The cumulative reduction in the first stage is 150-180mm. After the first stage, the steel plate is air-cooled to 850-880℃. The second stage of rolling begins, with a single-pass deformation of 10-25mm. The final rolling temperature is controlled at 780-800℃. The last pass is used to level the steel plate, with the deformation controlled at 1-3mm to prevent the steel plate from warping. After rolling, the steel plate enters ACC rapid cooling, with an initial cooling temperature of 740-760℃, and online residual heat quenching is performed, controlling the cooling rate at 25-40℃ / s to cool to room temperature.

[0023] ④ Heat treatment: The thickness of the steel plate is t. When 80mm≤t<100mm, tempering or quenching+tempering process is adopted. When 100mm≤t≤120mm, quenching+tempering process is adopted.

[0024] Preferably, in step ④, the tempering process requires that for steel plates with a thickness between 80mm and t < 100mm, the tempering temperature is 630–650℃, the holding time is not less than 300min, and the plate is allowed to cool naturally after being removed from the furnace; for steel plates with a thickness between 100mm and t < 120mm, the quenching and tempering process requires that the quenching temperature be 880–900℃, the holding time be not less than 240min, the steel plate temperature be < 80℃ after quenching, the tempering temperature be 640–660℃, the holding time be not less than 480min, and the plate be allowed to cool naturally after being removed from the furnace.

[0025] This invention eliminates the need for Ni in its composition design, significantly reducing production costs. Grain refinement is achieved by adding an appropriate amount of Nb alloy, while alloying elements such as Cr, Mo, Ti, and B are added for alloying. The rolling process employs two-stage controlled rolling and cooling, and tempering or quenching-tempering heat treatment processes are used depending on the steel plate thickness. This ensures that 80–120 mm thick steel plates exhibit good strength and toughness matching in both the delivery and die-welded states. Specifically, the yield strength in the tempered or quenched-temper state is 760 MPa. The above parameters are as follows: tensile strength > 880 MPa, elongation ≥ 22%, single transverse impact energy at -50℃ at the 1 / 4 position > 250 J, and single transverse impact energy at -50℃ at the 1 / 2 position > 180 J; after simulated post-weld heat treatment at 630℃ for 10 hours, the single transverse impact energy at -50℃ at the 1 / 4 position > 160 J, and single transverse impact energy at -50℃ at the 1 / 2 position > 120 J; yield strength > 720 MPa; and tensile strength > 830 MPa. Detailed Implementation

[0026] The present invention will now be further described in conjunction with specific embodiments. It should be noted that, without conflict, the various embodiments or technical features described below can be arbitrarily combined to form new embodiments.

[0027] A container steel with a thickness of 80-120 mm, a yield strength ≥690 MPa, and good low-temperature impact performance, wherein the steel plate contains the following chemical composition by weight percentage: C: 0.18-0.22%, Si: 0.20-0.35%, Mn: 0.90-1.10%, P≤0.010%, S≤0.002%, Cr: 0.40-0.60%, Mo: 0.15-0.25%, Nb: 0.015-0.030%, V: 0.03-0.08%, Al: 0.020-0.040%, Ti: 0.030-0.050%, B: 0.0015-0.0050%, with the remainder being Fe and unavoidable impurities.

[0028] The steel production method includes the following steps:

[0029] ① Clean steel smelting: Deep desulfurization treatment of molten iron is adopted. After treatment, the slag is removed and the sulfur content of molten iron before entering the converter is ≤0.005%; the white slag holding time in the LF furnace is ≥40min, the VD holding time is not less than 18min, and the hydrogen content after VD is ≤0.8ppm.

[0030] ② Continuous casting: The superheat of the tundish is controlled at 5-25℃, and the casting is carried out under full protection. The thickness of the continuously cast billet is 300-360mm.

[0031] ③ Heating: The billet is heated to 1150~1200℃ for 360~450min to ensure that the alloying elements are fully dissolved and that the billet temperature is uniform. The billet temperature is not higher than 1200℃ when it is taken out of the furnace. After taking it out of the furnace, it is descaled by high pressure water.

[0032] ④ Rolling: Two-stage rolling is adopted. The first stage adopts high penetration rolling, with the single-pass reduction controlled at more than 35mm, and the deformation coefficient of more than 2 passes controlled at >0.55 and the deformation rate of the pass >16%. The cumulative reduction in the first stage is 150-180mm. After the first stage, the plate is air-cooled to 850-880℃. The second stage rolling begins, with a single-pass deformation of 10-25mm. The rolling termination temperature is controlled at 780-800℃. The last pass is used to level the steel plate, with a deformation of 1-3mm to avoid warping.

[0033] ⑤ACC cooling: After rolling, perform ACC rapid cooling with an initial cooling temperature of 740-760℃, followed by online residual heat quenching at a cooling rate of 25-40℃ / s, and cooling to room temperature.

[0034] ⑥ Heat Treatment: For steel plates with a thickness of t, when the thickness is 80mm ≤ t < 100mm, tempering or quenching + tempering is used; when the thickness is 100mm ≤ t ≤ 120mm, quenching + tempering is used. The tempering process requires the following: for steel plates with a thickness between 80mm ≤ t < 100mm, a tempering temperature of 630–650℃ and a holding time of not less than 300 minutes are used, followed by natural cooling after removal from the furnace; for steel plates with a thickness between 100mm ≤ t ≤ 120mm, a quenching + tempering process is used. The quenching temperature is 880–900℃ and the holding time is not less than 240 minutes. After quenching, the steel plate temperature is < 80℃. The tempering temperature is 640–660℃ and the holding time is not less than 480 minutes. After removal from the furnace, the steel plate temperature is allowed to cool naturally.

[0035] The following embodiments are implemented based on the technical solution of the present invention, providing detailed implementation methods and specific operation processes. However, the scope of protection of the present invention is not limited to the following embodiments. Unless otherwise specified, the methods used in the following embodiments are conventional methods.

[0036] Example

[0037] In this embodiment, the chemical composition (wt%) of each embodiment is shown in Table 1, the rolling and heat treatment processes are shown in Table 2, and the performance indicators are shown in Tables 3 and 4.

[0038] Table 1 Chemical composition (Wt, %) of steel plates from Examples 1-4

[0039] Thickness (mm) C Si Mn P S Cr Mo Nb V Als Ti B 80 0.18 0.25 0.93 0.009 0.001 0.45 0.2 0.025 0.06 0.025 0.035 0.0022 90 0.18 0.28 0.99 0.007 0.001 0.52 0.22 0.025 0.065 0.035 0.038 0.0025 100 0.20 0.33 1.02 0.009 0.001 0.55 0.23 0.028 0.07 0.028 0.032 0.0033 120 0.21 0.3 1.05 0.008 0.002 0.58 0.24 0.03 0.075 0.03 0.04 0.0035

[0040] Table 2 Examples 1-4 Rolling process and heat treatment process

[0041]

[0042] Table 3 Examples 1-4 Mechanical Properties of Steel Plates in Heat-Treated State

[0043]

[0044] Table 4 Mechanical properties of steel plates in heat-treated state + simulated post-weld heat-treated state (Examples 1-4)

[0045]

[0046] The 80, 90, 100, and 120 mm steel plates produced in the above examples, in both heat-treated and simulated post-weld heat-treated states, exhibit good toughness and strength matching. Their tempered or quenched-tempered state yield strength is above 760 MPa, tensile strength is >880 MPa, elongation is ≥22%, and the single transverse impact energy at -50°C at the 1 / 4 position is >250 J, while the single transverse impact energy at -50°C at the 1 / 2 thickness position is >180 J. After being molded at 630°C for 10 hours, the single transverse impact energy at -50°C at the 1 / 4 position is >160 J, and the single transverse impact energy at -50°C at the 1 / 2 thickness position is >120 J. The yield strength is above 720 MPa, and the tensile strength is >830 MPa. The steel plates obtained in Examples 1-4 all meet the requirements of NB / T 47013 flaw detection standard Class I, and the sum of the grades of non-metallic inclusions (A, B, C, D, and Ds) in the steel is all <2.5.

Claims

1. A method for producing cryogenic container steel with a yield strength ≥ 690 MPa, characterized in that, The chemical composition of the steel, by weight percentage, is as follows: C: 0.18–0.22%, Si: 0.20–0.35%, Mn: 0.90–1.10%, P≤0.010%, S≤0.002%, Cr: 0.40–0.60%, Mo: 0.15–0.25%, Nb: 0.015–0.030%, V: 0.03–0.08%, Al: 0.020–0.040%, Ti: 0.030–0.050%, B: 0.0015–0.0050%, with the remainder being Fe and unavoidable impurities; the thickness of the steel for this container is 80–100 mm. 20mm thick, its metallographic structure consists of tempered martensite + a small amount of tempered bainite; the yield strength in both tempered and quenched-tempered states is above 760MPa, tensile strength is >880MPa, elongation is ≥22%, and the single transverse impact energy at -50℃ at the 1 / 4 position is >250J, while the single transverse impact energy at -50℃ at the 1 / 2 position is >180J; after simulated post-weld heat treatment at 630℃ for 10h, the single transverse impact energy at -50℃ at the 1 / 4 position is >160J, and the single transverse impact energy at -50℃ at the 1 / 2 position is >120J, with a yield strength above 720MPa and tensile strength >830MPa; The production method of the aforementioned low-temperature container steel with a yield strength ≥ 690 MPa is characterized by including continuous casting, heating, rolling, and heat treatment, as detailed below: ① Continuous casting: The superheat of the tundish is controlled at 5-25℃, and the casting is carried out under full protection. The billet thickness is 300-360mm, the low magnification of the billet is controlled below Class C 1.0, and the non-metallic inclusions A+B+C+D+Ds ≤ 2.

5. ② Heating: The billet is heated to 1150~1200℃ for 360~450min. The temperature at which it is taken out of the furnace should not exceed 1200℃. After taking it out of the furnace, it is descaled by high pressure water. ③ Rolling and Controlled Cooling: A two-stage rolling process is adopted. The first stage uses high-penetration rolling, with a single-pass reduction controlled at more than 35mm, and at least two passes with a deformation coefficient >0.55 and a pass deformation rate >16%. The cumulative reduction in the first stage is 150-180mm. After the first stage, the steel plate is air-cooled to 850-880℃. The second stage of rolling begins, with a single-pass deformation of 10-25mm. The final rolling temperature is controlled at 780-800℃. The last pass is used to level the steel plate, with the deformation controlled at 1-3mm to prevent the steel plate from warping. After rolling, the steel plate enters ACC rapid cooling, with an initial cooling temperature of 740-760℃, and online residual heat quenching is performed, controlling the cooling rate at 25-40℃ / s to cool to room temperature. ④ Heat treatment: For steel plate thickness t, when 80mm≤t<100mm, tempering or quenching+tempering process is used; when 100mm≤t≤120mm, quenching+tempering process is used. The tempering process requires that for steel plate thicknesses between 80mm≤t<100mm, the tempering temperature is 630~650℃, the holding time is not less than 300min, and the plate is allowed to cool naturally after being taken out of the furnace. For steel plate thicknesses between 100mm≤t≤120mm, the quenching+tempering process requires that the quenching temperature is 880~900℃, the holding time is not less than 240min, and after quenching, the steel plate temperature is <80℃. The tempering temperature is 640~660℃, the holding time is not less than 480min, and the plate is allowed to cool naturally after being taken out of the furnace.

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