Low-temperature container steel with yield strength larger than or equal to 690 MPa and production method of low-temperature container steel

Abstract

The invention discloses low-temperature container steel with the yield strength larger than or equal to 690 MPa and a production method of the low-temperature container steel. Ni does not need to be added in component design, the production cost can be greatly reduced, grains are refined by adding a proper amount of Nb alloy, meanwhile, alloying elements such as Cr, Mo, Ti and B are added for alloying, two-stage controlled rolling and controlled cooling are conducted, a tempering or hardening and tempering process is adopted according to the thickness of a steel plate, and the yield strength of the low-temperature container steel is larger than or equal to 690 MPa. The obtained steel plate with the thickness of 80-120mm has good strength and toughness matching in a delivery state and a die welding state, specifically, the yield strength in a tempered state or a quenched and tempered state is 760MP or above, the tensile strength is greater than 880MPa, the elongation is greater than or equal to 22%, the transverse impact single value at the position of 1 / 4 at the temperature of 50 DEG C below zero is greater than 250J, and the transverse impact energy single value at the position of 1 / 2 thickness at the temperature of 50 DEG C below zero is greater than 180J; through simulated postweld heat treatment with the temperature being 630 DEG C and the heat preservation time being 10 h, the transverse impact single value of the 1 / 4 position at the temperature of-50 DEG C is larger than 160 J, the transverse impact single value of the 1 / 2 thickness position at the temperature of-50 DEG C is larger than 120 J, the yield strength is 720 MPa or above, and the tensile strength is larger than 830 MPa. And the requirements of low-cost, large-thickness, high-strength and high-toughness container steel can be met.

Description

Technical Field

[0001] The present invention relates to the technical field of steel plate production, and in particular to a low-temperature container steel with a yield strength of ≥690MPa and a production method thereof, wherein the container steel has good low-temperature impact performance. Background Art

[0002] At present, pressure vessel steels with high strength and good low-temperature impact toughness at home and abroad are generally thin in thickness, and in production they all adopt a method of low C and adding high contents of alloying elements such as Ni and Mo, which greatly increases the production cost. However, there has been no report on the production plan for low-cost, thick, high-strength and toughness container steels.

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

[0004] For example, Chinese patent CN202010127653.1 discloses a low-cost, high-strength and toughness pressure vessel steel plate and a production method thereof. The chemical composition of the steel plate is as follows by weight: 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 adds 0.30-0.40% Ni, which reduces the production cost and shortens the process flow in terms of chemical composition and production process. However, the steel plate thickness is only 29-46mm, and the maximum impact temperature is only -20℃, which does not have a clear advantage. Summary of the invention

[0005] In order to solve the above technical defects, the purpose of the present invention is to provide a low-temperature container steel with a yield strength ≥690Mpa. In terms of composition design, the steel plate does not need to add Ni elements, which can greatly reduce the production cost. At the same time, by adding Nb elements 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, and it has good low-temperature impact performance.

[0006] Another object of the present invention is to provide a method for producing low-temperature container steel with a yield strength ≥690Mpa.

[0007] To achieve the above-mentioned purpose, the technical solution of the present invention is: a low-temperature container steel with a yield strength of ≥690MPa, comprising the following chemical components in 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%, and the remaining content is Fe and unavoidable impurities; the steel thickness is 80-120mm, and its metallographic structure is tempered martensite + a small amount of tempered bainite.

[0008] In terms of ingredient design:

[0009] C: C is the main component element in steel. The strength of steel mainly depends on the content of C in steel. Too high C content will lead to poor toughness, plasticity and welding performance of steel; low C content will lead to low strength of steel. In order to ensure that the steel plate has high strength and good low-temperature impact toughness, the present invention requires that the C content be controlled within the range of 0.18% to 0.22%.

[0010] Si: Si is a common solid solution strengthening alloy element in steel, and is an essential element for the toughness and hardenability of steel and even for ensuring the deoxidation of steel. Therefore, the Si content in the present invention is required to be controlled within a range of 0.20% to 0.35%.

[0011] Mn: The Mn element can strengthen ferrite in steel by 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, the center segregation of the steel billet will be aggravated during the continuous casting process, which will affect the internal flaw detection quality of the steel plate and be detrimental to the stability of the impact performance at the 1 / 2 thickness position of the steel plate. Therefore, the present invention requires that the Mn content in the steel be controlled at 0.90% to 1.10%.

[0012] Nb: Nb is a strong carbide-forming element, which forms second-phase particles such as NbC and Nb(CN) in steel, hindering the growth of austenite grains, refining grains, and improving the strength and low-temperature toughness of steel plates. The action temperature of Nb is higher than that of Ti and V, and its contribution to the strength of steel plates is also greater than that of Ti and V. When its content is too high, intergranular cracks are likely to occur. Therefore, the Nb content of the present 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, and its effect in steel is second only to Nb and Ti. When V is added to steel, VC is formed, which improves the melting point, hardness and wear resistance of cementite. Therefore, the V content of the present invention is controlled at 0.03-0.08%.

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

[0015] Ti: Ti reacts with sulfur in the steel to form TiS2, making the sulfur 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, absorb hydrogen, and further optimize the mechanical properties of the steel. Therefore, the present invention requires that the Ti content in the steel be 0.030% to 0.050%.

[0016] Mo: Mo relies on solid solution strengthening and grain boundary strengthening to improve the strength of steel, so the present invention requires that the Mo content in the steel be controlled to be 0.15% to 0.25%.

[0017] B: B element can significantly improve the hardenability of steel. During the quenching process, the steel can be hardened more evenly, thereby obtaining higher strength and hardness. Adding B element can save precious alloys such as Cr, Ni, and Mo. Therefore, the present invention requires that the B content in the steel be controlled to be 0.0015% to 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 of the above-mentioned low-temperature container steel with a yield strength of ≥690MPa includes continuous casting, heating, rolling and heat treatment, as follows:

[0020] ①Continuous casting: The superheat of the tundish is controlled at 5-25℃, the pouring is protected throughout the process, the thickness of the ingot is 300-360mm, the low multiple of the ingot is controlled below Class C 1.0, and the non-metallic inclusions are A+B+C+D+Ds≤2.5 level;

[0021] ② Heating: The heating temperature of the ingot is 1150-1200℃, and the heating time is 360~450min. The alloy elements are fully dissolved and the temperature of the ingot is uniform. The furnace temperature is not higher than 1200℃. After the furnace is discharged, high-pressure water descaling is performed.

[0022] ③Rolling and controlled cooling: Two-stage rolling is adopted. The first stage adopts high permeability rolling. The single-pass reduction is controlled to be above 35mm, and the deformation coefficient of at least 2 passes is controlled to be greater than 0.55 and the pass deformation rate is greater than 16%. The cumulative reduction in one stage is 150-180mm. After the first stage, air cooling is performed to wait for the temperature to reach 850-880℃. The second stage rolling begins. The single-pass deformation is 10-25mm, and the final rolling temperature is controlled at 780-800℃. The steel plate is leveled in the last pass, and the deformation is controlled at 1-3mm to avoid buckling of the steel plate. After rolling, the steel plate enters ACC for rapid cooling, with the start cooling temperature at 740-760℃, and is quenched online with residual heat, and the cooling rate is controlled at 25-40℃ / s to cool to room temperature.

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

[0024] Preferably, in step ④, the tempering process requirements are that for steel plates with a thickness of 80mm≤t<100mm, a tempering temperature of 630-650°C and a holding time of not less than 300min are adopted, and the steel plate is naturally cooled after being taken out of the furnace; for a thickness of 100mm≤t≤120mm, a quenching + tempering process requirements are adopted, a quenching temperature of 880-900°C and a holding time of not less than 240min, and after quenching, the steel plate temperature is <80°C, a tempering temperature of 640-660°C and a holding time of not less than 480min, and the steel plate is naturally cooled after being taken out of the furnace.

[0025] The composition design of the present invention does not require the addition of Ni elements, which can greatly reduce the production cost. By adding an appropriate amount of Nb alloy to refine the grains, and adding Cr, Mo, Ti, B and other alloy elements for alloying, the rolling process is controlled by two stages of rolling and controlled cooling. According to the thickness of the steel plate, a tempering or quenching and tempering heat treatment process is used, so that the 80-120 mm thick steel plate has good strength and toughness matching in both the delivery state and the die welding state, which is specifically manifested as: the yield strength of the tempered state or the quenching and tempering state is 760MP Above, tensile strength>880MPa, elongation ≥22%, single value of transverse impact at 1 / 4 position -50℃ is>250J, single value of transverse impact energy at 1 / 2 position of thickness -50℃ is>180J; after simulated post-weld heat treatment at 630℃ and insulation for 10h, single value of transverse impact at 1 / 4 position -50℃ is>160J, single value of transverse impact at 1 / 2 position of thickness -50℃ is>120J, yield strength is above 720MPa, and tensile strength>830MPa. DETAILED DESCRIPTION

[0026] The present invention is further described below in conjunction with specific implementation methods. It should be noted that, under the premise of no conflict, the various embodiments or technical features described below can be arbitrarily combined to form a new embodiment.

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

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

[0029] ① Clean steel smelting: deep desulfurization treatment is adopted for molten iron, and the slag is clean after treatment. The S of molten iron before entering the converter is ≤0.005%; the LF furnace white slag holding time is ≥40min, the VD pressure holding time is not less than 18 minutes, and the hydrogen content after VD is ≤0.8ppm;

[0030] ②Continuous casting: The superheat of the tundish is controlled at 5-25℃, the pouring is protected throughout the whole process, and the thickness of the continuous casting billet is 300-360mm;

[0031] ③ Heating: The heating temperature of the steel billet is 1150-1200℃, and the heating time is 360-450min. This ensures that the alloy elements are fully dissolved and the temperature of the steel billet is uniform. The furnace temperature is not higher than 1200℃, and high-pressure water descaling is performed after the steel billet is discharged from the furnace.

[0032] ④ Rolling: Two-stage rolling is adopted. The first stage adopts high permeability rolling, and the single-pass reduction is controlled to be above 35mm, and the deformation coefficient is controlled to be greater than 0.55 and the pass deformation rate is greater than 16% for more than 2 passes. The cumulative reduction in one stage is 150-180mm. After the first stage, air cooling is carried out to wait for the temperature to reach 850-880℃; the second stage of rolling begins, the single-pass deformation is 10-25mm, and the rolling termination temperature is controlled at 780-800℃. The last pass is used to flatten the steel plate, and the deformation is 1-3mm to avoid buckling of the steel plate;

[0033] ⑤ACC cooling: ACC rapid cooling is carried out after rolling, the cooling temperature is 740-760℃, online residual heat quenching, cooling rate is 25-40℃ / S, and cooling to room temperature;

[0034] ⑥ Heat treatment: The thickness of the steel plate is t. When the thickness is 80mm≤t<100mm, tempering or quenching + tempering process is adopted. When the thickness is 100mm≤t≤120mm, quenching + tempering process is adopted. The tempering process requires that for the steel plate thickness between 80mm≤t<100mm, the tempering temperature is 630~650℃, the holding time is not less than 300min, and the temperature drops naturally after leaving the furnace; for the thickness of 100mm≤t≤120mm, the quenching + tempering process requirements are quenching temperature 880~900℃, holding time is not less than 240min, quenching is completed, the steel plate temperature is less than 80℃, the tempering temperature is 640~660℃, the holding time is not less than 480min, and the temperature drops naturally after leaving the furnace.

[0035] The following examples are implemented on the premise of the technical solution of the present invention, and provide detailed implementation methods and specific operation processes, but the protection scope of the present invention is not limited to the following examples. The methods used in the following examples are conventional methods unless otherwise specified.

[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 Table 3 and Table 4.

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

[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 Rolling process and heat treatment process of Examples 1-4

[0041]

[0042] Table 3 Mechanical properties of heat-treated steel plates of Examples 1-4

[0043]

[0044] Table 4 Mechanical properties of steel plates in heat treatment state + simulated post-weld heat treatment state of Example 1-4

[0045]

[0046] The 80, 90, 100, 120 mm steel plates produced in the above embodiments have good toughness and strength matching in the heat-treated state and the simulated post-weld heat-treated state. The yield strength in the tempered or quenched and tempered state is above 760 MP, the tensile strength is greater than 880 MPa, the elongation is ≥22%, the single value of the transverse impact at 1 / 4 position -50°C is greater than 250 J, and the single value of the transverse impact energy at 1 / 2 position -50°C in thickness is greater than 180 J. After 630°C, 10h die welding, the single value of the transverse impact at 1 / 4 position -50°C is greater than 160 J, and the single value of the transverse impact at 1 / 2 position -50°C in thickness is greater than 120 J, the yield strength is above 720 MPa, and the tensile strength is greater than 830 MPa. The steel plates obtained in Examples 1-4 all meet the requirements of NB / T 47013 flaw detection standard level I, and the sum of the grades of non-metallic inclusions of types A, B, C, D, and Ds in the steel is all less than level 2.5.

Claims

1. A low-temperature container steel with a yield strength ≥ 690 MPa, characterized in that: The chemical composition of the steel plate is as follows by weight: 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%, and the rest is Fe and unavoidable impurities; the thickness of the container steel is 80- 120mm, its metallographic structure is tempered martensite + a small amount of tempered bainite; the yield strength in the tempered or quenched and tempered state is above 760MP, the tensile strength is >880MPa, the elongation is ≥22%, the single value of the transverse impact at 1 / 4 position -50℃ is >250J, and the single value of the transverse impact energy at 1 / 2 position of thickness -50℃ is >180J; after 630℃, insulation for 10h simulated post-weld heat treatment, the single value of the transverse impact at 1 / 4 position -50℃ is >160J, the single value of the transverse impact at 1 / 2 position of thickness -50℃ is >120J, the yield strength is above 720MPa, and the tensile strength is >830MPa.

2. The method for producing low-temperature container steel with a yield strength of ≥690 MPa according to claim 1, characterized in that Including continuous casting, heating, rolling and heat treatment, as follows: ①Continuous casting: The superheat of the tundish is controlled at 5-25℃, the pouring is protected throughout the process, the thickness of the ingot is 300-360mm, the low multiple of the ingot is controlled below Class C 1.0, and the non-metallic inclusions are A+B+C+D+Ds≤2.5 level; ② Heating: The heating temperature of the ingot is 1150-1200℃, the heating time is 360-450min, the furnace temperature is not higher than 1200℃, and high-pressure water descaling is carried out after the furnace is taken out; ③Rolling and controlled cooling: Two-stage rolling is adopted. The first stage adopts high permeability rolling. The single-pass reduction is controlled to be above 35mm, and the deformation coefficient of at least 2 passes is controlled to be greater than 0.55 and the pass deformation rate is greater than 16%. The cumulative reduction in one stage is 150-180mm. After the first stage, air cooling is performed to wait for the temperature to reach 850-880℃. The second stage rolling begins. The single-pass deformation is 10-25mm, and the final rolling temperature is controlled at 780-800℃. The steel plate is leveled in the last pass, and the deformation is controlled at 1-3mm to avoid buckling of the steel plate. After rolling, the steel plate enters ACC for rapid cooling, with the start cooling temperature at 740-760℃, and is quenched online with residual heat, and the cooling rate is controlled at 25-40℃ / s to cool to room temperature. ④ Heat treatment: The thickness of the steel plate is t. When the thickness is 80mm≤t<100mm, tempering or quenching + tempering process is adopted. When the thickness is 100mm≤t≤120mm, quenching + tempering process is adopted.

3. The method for producing low-temperature container steel with a yield strength of ≥690 MPa according to claim 2, characterized in that: In step ④, the tempering process requires that for steel plates with a thickness of 80mm≤t<100mm, a tempering temperature of 630-650°C and a holding time of not less than 300min are adopted, and the temperature is naturally reduced after being taken out of the furnace; for a thickness of 100mm≤t≤120mm, a quenching + tempering process is adopted, with a quenching temperature of 880-900°C and a holding time of not less than 240min. After quenching, the steel plate temperature is <80°C, the tempering temperature is 640-660°C, the holding time is not less than 480min, and the temperature is naturally reduced after being taken out of the furnace.

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