A super-low temperature high-toughness quenched and tempered container steel with a yield strength of ≥ 500 mpa and a production method thereof

Abstract

This invention discloses an ultra-low temperature high-toughness quenched and tempered container steel with a yield strength ≥500MPa and its production method. The steel plate has a thickness of 60mm to 100mm and adopts the following chemical composition (wt%): C: 0.08~0.11, Si: 0.15~0.35, Mn: 1.25~1.48, P≤0.008, S≤0.003, Ni: 0.4~0.5, Mo: 0.12~0.25, Cr: 0.2~0.4, Alt≥0.020, and the rest are Fe and residual elements; the steel is produced by controlled rolling and controlled cooling. The steel plate, obtained through two quenching and tempering heat treatments, exhibits a good balance of strength and toughness, low alloy cost, and a ferrite + granular bainite microstructure. Testing revealed that its transverse tensile strength at the 1 / 4 thickness position is 590–644 MPa, yield strength ≥500 MPa, and transverse impact energy at -80℃ is 280–328 J; while at the 1 / 2 thickness position, the transverse tensile strength is 585–630 MPa, yield strength ≥500 MPa, and transverse impact energy at -80℃ is 180–250 J. The performance difference between the core and the 1 / 4 thickness position is minimal, resulting in a more stable and reliable cryogenic pressure vessel during service.

Description

Technical Field

[0001] This invention is applicable to the field of medium and heavy plate production, specifically relating to an ultra-low temperature high-toughness quenched and tempered container steel with a yield strength ≥500MPa and its production method. Background Technology

[0002] With the development of the petrochemical industry, the demand for materials for transportation pipelines and storage containers required in the extraction, production, transportation, and storage of various petroleum gases is constantly increasing. Among these, the most widely studied material is steel for cryogenic pressure vessels. Under the given liquid medium and service temperature conditions in the petroleum gas liquefaction and storage process, the better the impact toughness of cryogenic pressure vessels made of cryogenic pressure vessel steel, the better their stability and safety during service.

[0003] CN110878400A discloses a high-strength low-temperature pressure vessel SA537CL2 steel plate and its production method. In its chemical composition, the content of C element is relatively high, and the total content of Nb+Ti+V alloying elements is as low as 0.085%. This scheme only shows the low-temperature impact performance of the steel plate at -60℃, and the low-temperature performance at 1 / 2 thickness is still uncertain.

[0004] Publication number CN115141982A discloses a low-temperature pressure vessel steel plate and its manufacturing method, and publication number CN114807766A discloses a low-temperature pressure vessel micro-alloy steel plate and its production method. The steel plates obtained by using these two methods can only guarantee the impact toughness at -50℃, and the toughness is relatively low, which cannot meet the safety requirements for use at lower operating temperatures.

[0005] This invention is proposed to provide a container steel that can adapt to lower operating temperatures and has stable core impact performance. Summary of the Invention

[0006] To meet the above technical requirements, the present invention aims to provide an ultra-low temperature high-toughness quenched and tempered container steel with a yield strength ≥500MPa. This steel plate can adapt to lower operating temperatures, and its core impact performance also meets the application requirements.

[0007] Another objective of this invention is to provide a method for producing ultra-low temperature high-toughness tempered container steel with a yield strength ≥500MPa.

[0008] To achieve the above objectives, the technical solution adopted in this invention is: an ultra-low temperature high-toughness quenched and tempered container steel with a yield strength ≥500MPa, the steel plate thickness being 60mm~100mm, and the steel plate employing the following chemical composition by mass percentage: C: 0.08~0.11, Si: 0.15~0.35, Mn: 1.25~1.48, P≤0.008, S≤0.003, Ni: 0.4~0.5, Mo: 0.12~0.25, Cr: 0.2~0.4, Alt≥0.020, with the remainder being Fe and residual elements. This composition design is beneficial for improving the strength-toughness balance, reducing the amount of alloying added, improving the stability of impact performance at the 1 / 4 and 1 / 2 positions of the steel plate thickness, improving the hardenability of the steel plate during heat treatment, and reducing production costs.

[0009] To ensure impact toughness at the half-thickness position, a low-C, low-P, and low-S composition is adopted to reduce C segregation and P and S inclusions. As the C content increases, tensile strength improves, but impact energy decreases. To achieve both a suitable strength margin and good overall impact energy, the C content needs to be appropriately reduced; therefore, the designed C content is 0.08–0.11 wt%. During the quenching process, Si improves the hardenability of the steel, but high Si content can cause cracks after rolling. Therefore, the Si content needs to be limited to 0.15%–0.35% wt%. Increasing the Mn content increases the number of Mn-substituted atoms in the matrix, improving the strength of the steel plate. However, excessively high Mn content makes the banded structure more pronounced, reducing the low-temperature toughness of the steel plate. To provide appropriate strength while improving impact toughness, the Mn content needs to be limited to 1.25–1.48 wt%. Ni is a crucial component for ensuring low-temperature impact toughness at the half-thickness position. Increasing the Ni content improves the stability of austenite and enhances its inhibitory effect on ferrite and pearlite phase transformations; therefore, the Ni content is limited to 0.4–0.5 wt%. Adding a small amount of Mo can promote low-temperature bainitic phase transformation, resulting in more lath bainite in the final microstructure. Mo also promotes the bainitic phase transformation kinetics at low temperatures, effectively shortening the transformation time; therefore, the Mo content is limited to 0.12–0.25 wt%.

[0010] When Ni, Cr, and Mo are added together, Cr and Mo increase the activation energy of the γ→α transformation, while Ni increases the nucleation energy of the α phase during the γ→α transformation. This makes the effect of delaying the pearlite transformation more significant, the Bs point decreases more greatly, and a finer bainite structure is obtained, further improving the toughness of the steel.

[0011] The production method for ultra-low temperature high-toughness quenched and tempered container steel with a yield strength ≥ 500 MPa includes continuous casting, heating, controlled rolling and cooling, and heat treatment, as detailed below:

[0012] ① Continuous casting: Use billets with a thickness of ≥350mm, and the low magnification of the billets reaches Class C 1.5 or above;

[0013] ② Heating: The billet temperature is 200-500℃, the first heating temperature of the heating furnace is 960-1000℃, the second heating temperature is 1200-1270℃, the soaking zone heating temperature is 1220-1270℃, and the total heating time is 14-18 min / cm.

[0014] ③ Controlled rolling and cooling: A two-stage rolling process is adopted. The roughing rolling temperature is 1000-1200℃, and the reduction per pass is 35-45mm. The deformation rate per pass is controlled to be >16%. After each rolling pass, the steel plate is allowed to fully redden for 0.5-1.5 minutes before rolling again to ensure that the rolling force penetrates to the core of the steel plate and reduce segregation. The cumulative reduction in roughing rolling is 140-180mm. After roughing rolling, the steel plate is air-cooled to 860-880℃ before finishing rolling. The reduction per finishing rolling pass is 15-25mm, 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 warping. After rolling, the steel plate enters ACC cooling, and the cooling rate is controlled at 0.8-1.5℃ / s. The reddening temperature is 660-700℃.

[0015] ④ Heat treatment: A two-stage quenching and tempering process is adopted. The first quenching temperature is 880-900℃, the holding time is 2.0-2.2 min / mm, and the temperature is quenched to room temperature at a rate of 1.5-2.5℃ / s. The lower cooling rate is used to obtain a small amount of lower bainite, improve the ability to obtain lath martensite, and increase hardenability. The second quenching temperature is 760-780℃, the holding time is 1.5-1.8 min / mm, the temperature is 5-8℃ / s for 60-80mm thick steel plates, and the temperature is 3-5℃ / s for 80-100mm thick steel plates. The tempering temperature is 600-620℃, the holding time is 3.5-4 min / mm, and the temperature is cooled to room temperature in the furnace after the holding time.

[0016] During the secondary quenching and heating process in the two-phase region, when the reversible austenite begins to nucleate above Ac1, atomic diffusion is not active due to the low temperature, and grain boundary migration is slow. The reversible austenite does not grow rapidly but exists as fine grains, forming a fine martensite structure during subsequent quenching. Tempering cannot further alter the grain size, thus achieving grain refinement. Part of the reversible austenite transforms into bainite, while another part remains as stable retained austenite, forming island-like MA components. These retained austenite components effectively prevent crack propagation and improve the impact resistance of the steel. Furthermore, the retained austenite has a certain "adsorption effect," adsorbing carbon and other impurities from the steel, thus improving the steel's basic toughness. During tempering, the MA islands within the quenched granular bainite continuously decompose, and cementite continuously precipitates, ultimately resulting in a stable ferrite + granular bainite structure.

[0017] The steel plates obtained using this method were tested and found to have a transverse tensile strength of 590–644 MPa, a yield strength ≥500 MPa, and a transverse impact energy of 280–328 J at -80℃ at 1 / 4 of the thickness. At 1 / 2 of the thickness, the transverse tensile strength was 585–630 MPa, the yield strength ≥500 MPa, and the transverse impact energy at -80℃ was 180–250 J. The performance difference between the core and the 1 / 4 thickness was not significant, and the resulting cryogenic pressure vessels were more stable and reliable during service. Attached Figure Description

[0018] The present invention will now be described in further detail with reference to the accompanying drawings.

[0019] Figure 1 This is a 500X metallographic image of the 90mm thick steel plate at 1 / 4 of its thickness.

[0020] Figure 2 This is a 500x metallographic image of the 90mm thick steel plate of the present invention at half its thickness. Detailed Implementation

[0021] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0022] The features and performance of the present invention will be further described in detail below with reference to embodiments.

[0023] Several types of ultra-low temperature high-toughness quenched and tempered container steels with yield strength ≥500MPa were produced using the chemical compositions (unit: wt%) shown in Table 1 below:

[0024] Table 1: Chemical composition (Wt, %) of 60mm~100mm thick ultra-low temperature high-toughness quenched and tempered container steel

[0025] Example Thickness (mm) C Si Mn P S Alt Ni Mo Cr 1 60 0.09 0.32 1.40 0.005 0.003 0.025 0.41 0.21 0.22 2 80 0.09 0.28 1.33 0.006 0.003 0.023 0.43 0.25 0.24 3 100 0.10 0.23 1.36 0.005 0.003 0.027 0.46 0.25 0.25

[0026] The key processes in the steel production include converter smelting, LF refining, VD vacuum refining, continuous casting, billet heating, controlled rolling and cooling, and secondary quenching + tempering heat treatment, as detailed below:

[0027] (1) Converter smelting: P ≤ 0.010% and C ≤ 0.10% of steel are tapped. Slag must be blocked during steel tapping in the converter, and the slag layer thickness must be ≤ 55mm. During the steel tapping process, deoxidizers and alloys must be added into the ladle with the steel flow after tapping, and must not be added directly to the bottom of the ladle. Argon must be blown throughout the tapping process.

[0028] (2) LF refining: The white residue must be kept for ≥18 min during the refining process to ensure full desulfurization and control the finished product S ≤0.003%; pay attention to component control and control the finished product C at the target value of 0.08~0.10wt%.

[0029] (3) VD vacuum refining: The vacuum degree should reach below 67 Pa, and the holding time should be controlled at ≥15 min; the hydrogen content should be below 1.5 ppm; the VD outlet temperature should ensure that the superheat of the molten steel in the ladle is 15-25℃.

[0030] (4) Continuous casting: Ensure that the billet thickness is at least 350mm, the superheat of the casting is controlled at 15-25℃, and the casting speed is 0.78-0.83m / min. Low magnification must be at or above Class C 1.5 level.

[0031] (5) Billet heating: The billet temperature in the furnace is 200-500℃, the first heating temperature in the heating furnace is 960-1000℃, the second heating temperature is 1200-1270℃, the soaking zone heating temperature is 1220-1270℃, and the heating time is 14-18 min / cm.

[0032] (6) Controlled rolling and cooling: After the billet is taken out of the furnace, it is descaled to ensure that the descaling pressure is ≥18Mpa. Before rolling, the high pressure pre-water of the rolling mill must be shut off and the cooling water of the roll body must be turned off to below 10%. A two-stage rolling process is adopted. The first stage uses high-penetration rolling, with the initial rolling temperature controlled at 1000-1200℃ and the single-pass reduction controlled at 40-45mm. After each rolling pass, the steel plate is fully heated for 0.5-1.5min, and the temperature is raised to above 900℃ before rolling. The deformation coefficient is controlled to be >0.65 and the pass deformation rate is controlled to be >16% for more than two passes. The cumulative reduction in rough rolling is 140-180mm. After rough rolling, the steel plate is air-cooled to 860-880℃ before finishing rolling. The reduction in each finishing rolling pass is 15-25mm, and the final rolling temperature is controlled at 780-800℃. The last pass is used to level the steel plate, and the deformation is controlled at 1-3mm to prevent the steel plate from warping. After rolling, the steel plate enters ACC cooling, with the cooling rate controlled at 0.8-1.5℃ / s and the heating temperature at 660-700℃. After straightening, the steel plate is placed in a stack cooling pit for stack cooling at a temperature ≥450℃.

[0033] (7) Secondary quenching + tempering heat treatment: The heat treatment process route is primary quenching (quenching furnace → bidirectional quenching machine), secondary quenching (the steel plate exits the quenching machine and enters the chamber furnace for heat preservation, exits the chamber furnace, and continues to be quenched in the bidirectional quenching machine), and after quenching, it continues to be tempered in the chamber furnace. The primary quenching holding temperature is 880~900℃, the holding time is 2.0~2.2min / mm, the quenching machine roller speed is controlled at 0.5~0.8m / min, and quenching is carried out at 1.5~2.5℃ / s to room temperature; the secondary quenching temperature is 760~780℃, the holding time is 1.5~1.8min / mm, the 60~80mm thick steel plate is quenched at 5~8℃ / s to room temperature, and the 80~100mm thick steel plate is quenched at 3~5℃ / s to room temperature; the tempering temperature is 600~620℃, the holding time is 3.5~4min / mm, and the plate is cooled to room temperature with the furnace after the heat preservation is completed.

[0034] The mechanical properties of the steel plates obtained in the example were tested, and the details are shown in Table 2:

[0035] Table 2: Mechanical properties of 60mm~100mm thick ultra-low temperature high-toughness quenched and tempered container steel plates

[0036]

[0037] This trial production involved 10 batches each of 60mm, 80mm, and 100mm thick steel plates. Through reasonable chemical composition design and production process control, the microstructure of the steel plates is ferrite + granular bainite. The transverse tensile strength at 1 / 4 of the thickness is 590-644MPa, the yield strength is ≥500MPa, and the transverse impact energy at -80℃ is 280-328J. The transverse tensile strength at 1 / 2 of the thickness is 585-630MPa, the yield strength is ≥500MPa, and the transverse impact energy at -80℃ is 180-250J, which fully meets the requirements for use in low-temperature container steel.

Claims

1. A method for producing ultra-low temperature high-toughness quenched and tempered container steel with a yield strength ≥ 500 MPa, characterized in that, The steel has a thickness of 60mm to 100mm and contains the following chemical composition by mass percentage: C: 0.08 to 0.11, Si: 0.15 to 0.35, Mn: 1.25 to 1.48, P≤0.008, S≤0.003, Ni: 0.4 to 0.5, Mo: 0.12 to 0.25, Cr: 0.2 to 0.4, Alt≥0.020, with the remainder being Fe and residual elements; The steel has a microstructure of ferrite + granular bainite, with a transverse tensile strength of 590-644 MPa, a yield strength ≥500 MPa, and a transverse impact energy of 280-328 J at -80℃ at 1 / 4 of its thickness, and a transverse tensile strength of 585-630 MPa, a yield strength ≥500 MPa, and a transverse impact energy of 180-250 J at -80℃ at 1 / 2 of its thickness. The aforementioned ultra-low temperature high-toughness quenched and tempered container steel with a yield strength ≥ 500 MPa includes continuous casting, heating, controlled rolling and controlled cooling, and heat treatment, as detailed below: ① Continuous casting: Use billets with a thickness of ≥350mm, and the low magnification of the billets reaches Class C 1.5 or above; ② Heating: The billet temperature is 200-500℃, the first heating temperature of the heating furnace is 960-1000℃, the second heating temperature is 1200-1270℃, the soaking zone heating temperature is 1220-1270℃, and the total heating time is 14-18 min / cm. ③ Controlled rolling and cooling: Two-stage rolling is adopted. The roughing rolling temperature is 1000-1200℃, the reduction per pass is 35-45mm, and the deformation rate per pass is controlled to be >16%. After each rolling pass, the steel plate is allowed to fully redden for 0.5-1.5min before rolling again. The cumulative reduction in roughing rolling is 140-180mm. After roughing rolling, the plate is air-cooled until the temperature reaches 860-880℃ before finishing rolling. The reduction per finishing rolling pass is 15-25mm, and the final rolling temperature is controlled at 780-800℃. The last pass is used to level the steel plate, and the deformation is controlled at 1-3mm to avoid warping. After rolling, the steel plate enters ACC cooling, and the cooling rate is controlled at 0.8-1.5℃ / s, with a reddening temperature of 660-700℃. ④ Heat treatment: A two-stage quenching and tempering process is adopted. The first quenching temperature is 880-900℃, the holding time is 2.0-2.2 min / mm, and the temperature is quenched to room temperature at 1.5-2.5℃ / s. The second quenching temperature is 760-780℃, the holding time is 1.5-1.8 min / mm, the temperature is 5-8℃ / s for 60-80mm thick steel plates, and the temperature is 3-5℃ / s for 80-100mm thick steel plates. The tempering temperature is 600-620℃, the holding time is 3.5-4 min / mm, and the temperature is cooled to room temperature in the furnace after the holding time.

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