Ultralow-temperature high-toughness quenched and tempered container steel with yield strength larger than or equal to 500 MPa and production method thereof

Abstract

The invention discloses ultralow-temperature high-toughness quenched and tempered container steel with the yield strength larger than or equal to 500 MPa and a production method of the ultralow-temperature high-toughness quenched and tempered container steel. The thickness of a steel plate ranges from 60 mm to 100 mm, and the ultralow-temperature high-toughness quenched and tempered container steel is prepared from, by mass, 0.08 wt%-0.11 wt% of C, 0.15 wt%-0.35 wt% of Si, 1.25 wt%-1.48 wt% of Mn, smaller than or equal to 0.008 wt% of P, smaller than or equal to 0.003 wt% of S, 0.4 wt%-0.5 wt% of Ni, 0.12 wt%-0.25 wt% of Mo, 0.2 wt%-0.4 wt% of Cr, larger than or equal to 0.020 wt% of Alt and the balance Fe and residual elements. The steel plate obtained through controlled rolling and controlled cooling, two times of quenching and tempering heat treatment is matched in strength and toughness and low in alloy cost and has a ferrite and granular bainite structure, and through detection, the transverse tensile strength of the position with the thickness being 1 / 4 is 590-644 MPa, the yield strength is larger than or equal to 500 MPa, the transverse impact energy at the temperature of-80 DEG C is 280-328 J, the transverse tensile strength of the position with the thickness being 1 / 2 is 585-630 MPa, the yield strength is larger than or equal to 500 MPa, and the transverse impact energy at the temperature of-80 DEG C is 180-250 J. The performance difference between the core part and the 1 / 4 thickness part is small, and the prepared low-temperature pressure vessel is more stable and reliable in the service process.

Description

Technical Field

[0001] The invention is applicable to the field of medium and thick plate production, and specifically relates to an ultra-low temperature high-toughness quenched and tempered container steel with a yield strength of ≥500MPa and a production method thereof. Background Art

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

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

[0004] Publication No. CN115141982A discloses a low-temperature pressure vessel steel plate and a method for manufacturing the same, and publication No. CN114807766A discloses a microalloy steel plate for a low-temperature pressure vessel and a method for producing the same. The steel plates obtained by these two schemes can only guarantee the impact toughness of -50°C, which is relatively low and cannot meet the safety requirements for use at lower operating temperatures.

[0005] The present invention is proposed to provide a container steel which can adapt to lower working temperature and has stable core impact performance. Summary of the invention

[0006] In order to meet the above technical requirements, the purpose of the present invention is to provide an ultra-low temperature high-toughness quenched and tempered container steel with a yield strength ≥500MPa. The steel plate can adapt to lower operating temperatures, and the core impact performance also meets the use requirements.

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

[0008] To achieve the above object, the technical solution adopted by the present invention is: an ultra-low temperature high-toughness quenched and tempered container steel with a yield strength of ≥500MPa, the thickness of the steel plate is 60mm-100mm, and the steel plate adopts 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, and the rest are Fe and residual elements. This composition design is conducive to improving the strength and toughness matching, reducing the amount of alloy addition, improving the stability of the impact performance at the 1 / 4 position and the 1 / 2 position of the steel plate thickness, improving the hardenability of the steel plate heat treatment, and reducing the production cost.

[0009] In order to ensure the impact toughness at the 1 / 2 thickness position, a low C, low P, and low S design is adopted in the composition to reduce C segregation and P and S inclusions. As the C content increases, the tensile strength increases and the impact energy decreases. In order to have a suitable strength margin and to have a good impact energy for the thickness as a whole, the C content needs to be appropriately reduced. Therefore, the C content is designed to be: 0.08-0.11wt%. In the quenching process, Si is beneficial to improve the hardenability of steel, but high Si will cause cracks in the steel plate after rolling, so the Si content needs to be limited to: 0.15%-0.35%wt%. Increasing the Mn content can increase the number of Mn substitution atoms in the matrix and improve the strength of the steel plate, but too high a Mn content makes the banded structure more obvious and reduces the low-temperature toughness of the steel plate. In order to provide appropriate strength and improve impact toughness at the same time, the Mn content needs to be limited to 1.25-1.48wt%. Ni is an important component to ensure low-temperature impact toughness at 1 / 2 thickness. Increasing Ni content will improve the stability of austenite and enhance the inhibitory effect on ferrite and pearlite transformations. Therefore, the Ni content is limited to 0.4-0.5wt%. Adding a small amount of Mo element can promote low-temperature bainite transformation, and finally generate more lath bainite in the organization. Adding Mo can also promote the bainite transformation dynamics under low temperature conditions and effectively shorten the phase transformation time. Therefore, the Mo content is limited to 0.12-0.25wt%.

[0010] When Ni, Cr and Mo are added together, Cr and Mo increase the activation energy of γ→α transformation, and Ni increases the nucleation energy of α phase during γ→α transformation, making the effect of delaying pearlite transformation more significant, the Bs point is reduced to a greater extent, a finer bainite structure is obtained, and the toughness of the steel is further improved.

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

[0012] ①Continuous casting: Use slabs ≥350mm thick, and the slab low multiple reaches Class C 1.5 or above;

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

[0014] ③ Controlled rolling and controlled cooling: two-stage rolling is adopted, the rough rolling start temperature is 1000-1200℃, the pass reduction is 35-45mm, the pass deformation rate is controlled to be >16%, and the steel plate is fully reddened for 0.5-1.5min before rolling again after each rolling pass to ensure that the rolling force penetrates into the core of the steel plate and reduce segregation. The rough rolling cumulative reduction is 140-180mm. After the rough rolling is completed, air cooling is carried out to wait for the temperature to reach 860-880℃ before finishing rolling. The finishing rolling pass reduction 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 the steel plate from buckling; after rolling, the steel plate enters ACC cooling, and the cooling rate is controlled at 0.8-1.5℃ / s, and the reddening temperature is 660-700℃;

[0015] ④ Heat treatment: adopt two quenching + tempering process, among which, the first quenching holding temperature is 880~900℃, the holding time is 2.0~2.2min / mm, and the temperature is quenched to room temperature at 1.5~2.5℃ / s. A small amount of lower bainite is obtained at a lower cooling rate, the ability to obtain lath martensite is improved, and the hardenability is increased; the second quenching is 760~780℃, the holding time is 1.5~1.8min / mm, 60~80mm thick steel plates are quenched to room temperature at 5~8℃ / s, and 80~100mm thick steel plates are quenched to room temperature at 3~5℃ / s; the tempering temperature is 600~620℃, the holding time is 3.5~4min / mm, and the furnace is cooled to room temperature after the holding is completed.

[0016] During the secondary quenching and heating process in the two-phase region, when the reverse austenite is heated to above Ac1 and begins to nucleate, due to the low temperature, atomic diffusion is not active, and grain boundary migration is slow, the reverse austenite will not grow rapidly, but will exist as fine grains, and will form fine martensite structure in the subsequent quenching process. And the tempering treatment can no longer change the size of the grains, thereby achieving the effect of grain refinement. Part of the reverse austenite is transformed into bainite, and the other part is retained as stable residual austenite to form island-shaped MA components. These residual austenites can effectively prevent the expansion of cracks and improve the impact properties of steel. And the residual austenite also has a certain "adsorption effect", which absorbs C elements and other impurities in steel and improves the basic toughness of steel. During the tempering process, the MA islands in the quenched granular bainite are continuously decomposed, and cementite is continuously precipitated, and finally a stable ferrite + granular bainite structure is obtained.

[0017] The steel plates obtained by adopting this scheme were tested, and the transverse tensile strength at 1 / 4 thickness was 590-644MPa, the yield strength was ≥500MPa, and the transverse impact energy at -80℃ was 280-328J. The transverse tensile strength at 1 / 2 thickness was 585-630MPa, the yield strength was ≥500MPa, and the transverse impact energy at -80℃ was 180-250J. There was little difference in performance between the core and 1 / 4 thickness. The prepared cryogenic pressure vessel was more stable and reliable during service. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The present invention will be further described in detail below in conjunction with the accompanying drawings.

[0019] Figure 1 This is a 500X metallographic structure diagram of a 90 mm thick steel plate at 1 / 4 of the thickness of the present invention.

[0020] Figure 2 This is a 500x metallographic structure diagram of a 90 mm thick steel plate at 1 / 2 thickness of the present invention. DETAILED DESCRIPTION

[0021] In order to make the purpose, technical scheme and advantages of the embodiments of the present invention clearer, the technical scheme in the embodiments of the present invention will be described clearly and completely below. If the specific conditions are not specified in the embodiments, they are carried out according to conventional conditions or conditions recommended by the manufacturer. If the manufacturer of the reagents or instruments used is not specified, they are all conventional products that can be purchased commercially.

[0022] The features and performance of the present invention are further described in detail below in conjunction with the embodiments.

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

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

[0025] Examples 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 production process of the steel mainly includes converter smelting, LF refining, VD vacuum refining, continuous casting, billet heating, controlled rolling and controlled cooling, secondary quenching + tempering heat treatment, as follows:

[0027] (1) Converter smelting: steel tapping P ≤ 0.010%, steel tapping C ≤ 0.10%; slag must be blocked during converter tapping, and the slag layer thickness must be ≤ 55 mm; during converter tapping, deoxidizers and alloys are required to be added into the ladle along 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: During the refining process, the white slag retention time is required to be ≥18 min, and the sulfur is fully desulfurized, and the S content of the finished product is controlled to be ≤0.003%. Pay attention to the composition control, and the C content of the finished product is controlled according to the target value of 0.08-0.10 wt%.

[0029] (3) VD vacuum refining: The vacuum degree must be below 67Pa, and the pressure holding time must be controlled to be ≥15min; the hydrogen content must be below 1.5ppm; the VD leaving station temperature must ensure that the superheat of the molten steel in the tundish is 15-25℃.

[0030] (4) Continuous casting: Ensure that the thickness of the ingot is at least 350 mm, the pouring superheat is controlled at 15-25°C, and the casting speed is 0.78-0.83 m / min. The low multiple must be Class C 1.5 or above.

[0031] (5) Billet heating: billet charging temperature is 200-500°C, heating temperature of heating furnace 1 is 960-1000°C, heating temperature of heating furnace 2 is 1200-1270°C, heating temperature of soaking section is 1220-1270°C, heating time is 14-18 min / cm.

[0032] (6) Controlled rolling and controlled cooling: After the billet comes out of the furnace, dephosphorize it first to ensure that the descaling pressure is ≥18Mpa. Before rolling, the high-pressure pre-water of the rolling mill must be turned off and the roller cooling water must be turned off to less than 10%. Two-stage rolling is adopted. The first stage adopts high permeability rolling. The starting rolling temperature is controlled at 1000-1200℃, the single-pass reduction is controlled at 40-45mm, each rolling pass is fully reddened for 0.5-1.5min, the temperature returns to above 900℃ and then rolling is carried out, the deformation coefficient is controlled to be greater than 0.65 for more than 2 passes, and the pass deformation rate is greater than 16%. The cumulative reduction of rough rolling is 140-180mm. After the rough rolling is completed, air cooling is carried out until the temperature reaches 860-880℃ to start finishing rolling. The finishing rolling pass reduction 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 according to 1-3mm to avoid buckling of the steel plate; after rolling, the steel plate enters ACC cooling, the cooling rate is controlled to be 0.8-1.5℃ / S, and the reddening temperature is 660-700℃; after the steel plate is straightened, it is put into the pile cooling pit for pile cooling, and the pile cooling temperature is ≥450℃.

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

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

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

[0036]

[0037] This time, 10 batches of 60mm, 80mm and 100mm thick steel plates were trial-produced. Through reasonable chemical composition design and production process control, the structure of the steel plate 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 use requirements of low-temperature container steel.

Claims

1. An ultra-low temperature high toughness quenched and tempered container steel with a yield strength ≥ 500MPa, characterized in that: The steel plate has a thickness of 60 mm to 100 mm and contains the following chemical compositions 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, and the rest are Fe and residual elements; The structure of the steel is ferrite + granular bainite, with a transverse tensile strength of 590-644MPa, a yield strength ≥500MPa, and a transverse impact energy of 280-328J at a thickness of 1 / 4; a transverse tensile strength of 585-630MPa, a yield strength ≥500MPa, and a transverse impact energy of 180-250J at a thickness of 1 / 2.

2. The ultra-low temperature high toughness quenched and tempered container steel with a yield strength of ≥500MPa according to claim 1, characterized in that Including continuous casting, heating, controlled rolling, controlled cooling and heat treatment, as follows: ①Continuous casting: Use slabs ≥350mm thick, and the slab low multiple reaches Class C 1.5 or above; ② Heating: The temperature of the billet in the furnace is 200-500℃, the heating temperature of the first heating furnace is 960-1000℃, the heating temperature of the second heating furnace is 1200-1270℃, the heating temperature of the soaking section is 1220-1270℃, and the total heating time is 14-18min / cm; ③ Controlled rolling and controlled cooling: two-stage rolling is adopted, the rough rolling start temperature is 1000-1200℃, the pass reduction is 35-45mm, the pass deformation rate is controlled to be >16%, and the steel plate is fully reddened for 0.5-1.5min before rolling again after each rolling pass. The rough rolling cumulative reduction is 140-180mm. After the rough rolling is completed, air cooling is carried out until the temperature reaches 860-880℃ before finishing rolling. The finishing rolling pass reduction 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 the steel plate from buckling; after rolling, the steel plate enters ACC cooling, and the cooling rate is controlled at 0.8-1.5℃ / S, and the reddening temperature is 660-700℃; ④ Heat treatment: adopt two quenching + tempering process, among which, the first quenching holding temperature is 880-900℃, the holding time is 2.0-2.2min / mm, and the temperature is quenched to room temperature at 1.5-2.5℃ / s; the second quenching is 760-780℃, the holding time is 1.5-1.8min / mm, 60-80mm thick steel plate is quenched to room temperature at 5-8℃ / s, and 80-100mm thick steel plate is quenched to room temperature at 3-5℃ / s; the tempering temperature is 600-620℃, the holding time is 3.5-4min / mm, and the furnace is cooled to room temperature after the holding is completed.

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