A steel with low yield ratio and high strength and toughness for LNG storage tank and its production method

Abstract

A low yield ratio and high strength and toughness steel for LNG storage tanks and a production method thereof, wherein the chemical composition mass percentage of the steel is: C=0.03-0.05, Si=0.15-0.30, Mn=0.50-0.80, P≤0.005, S≤0.003, Alt=0.020-0.040, Ni=8.50-9.50, Nb≤0.01, V≤0.01, Ti≤0.01, Mo≤0.08, Sn≤0.005; the rest are Fe and residual elements. The process route is: converter smelting → slag removal and P removal → LF refining → VD vacuum treatment → continuous casting → slab surface cleaning → slab anti-oxidation coating → slab heating → controlled rolling and controlled cooling → critical point quenching + tempering. The steel plate also meets the performance indicators of yield strength ratio ≤ 0.92, impact absorption energy at ‑196°C ≥ 100J, non-plastic transition temperature NDT < ‑196°C, etc. The structure of the steel of the present invention is martensite + reversed austenite, the product surface quality is excellent, and the ultrasonic detection meets the requirements of NB / T47013.3 standard TⅠ level.

Description

Technical Field

[0001] The invention belongs to the technical field of metallurgy and relates to a steel for LNG storage tanks with low yield ratio and high strength and toughness and a production method thereof. Technical Background

[0002] Liquefied natural gas (LNG) is recognized as the cleanest and low-carbon fossil energy on earth, and its main component is methane. The liquefaction temperature of natural gas under normal pressure is -162℃, and the volume after liquefaction is only 1 / 625 of that in gaseous state, which is very convenient for storage and transportation. LNG storage tanks are the main devices for storing and transporting liquefied natural gas. The materials used to manufacture LNG storage tanks are mainly Invar alloy, austenitic stainless steel, aluminum alloy, 9Ni steel and high manganese steel. With the large-scale development of LNG storage tanks, 9Ni steel has high strength, excellent low-temperature toughness and good welding performance, and is still the mainstream material for large storage tanks.

[0003] The main technical requirements of 9Ni steel, which is currently widely used in the manufacture of LNG storage tanks, are as follows: ① Tensile properties: ReH ≥ 585MPa, 680MPa ≤ Rm ≤ 820MPa, elongation A ≥ 18%; ② Impact properties: -196℃ impact absorption energy average value ≥ 100J, single value ≥ 70J; ③ Non-plastic transition (NDT) temperature is lower than -196℃. Considering the operational safety of LNG storage tanks, it is not only required that the steel used in the manufacture of LNG storage tanks has high strength and excellent low-temperature toughness, but also that the material should have a low yield strength ratio ≤ 0.92. The existing 9Ni steel is difficult to meet the above requirements, and it is urgent to develop new steel for LNG storage tanks. Summary of the invention

[0004] The purpose of the present invention is to provide a low yield ratio and high strength and toughness steel for LNG storage tanks and a production method thereof, and the properties of the steel for LNG storage tanks with a thickness of 5 to 50 mm are as follows: tensile properties, ReH ≥ 585MPa, 680MPa ≤ Rm ≤ 820MPa, elongation A ≥ 18%, yield ratio ≤ 0.92; impact properties, average value of -196°C impact absorption energy ≥ 100J, single value ≥ 70J; non-plastic transition temperature NDT ≤ -196°C.

[0005] The technical solution of the present invention:

[0006] A steel for LNG storage tanks with low yield ratio and high strength and toughness. The chemical composition of the steel is as follows: C=0.03-0.05, Si=0.15-0.30, Mn=0.50-0.80, P≤0.005, S≤0.003, Alt=0.020-0.040, Ni=8.50-9.50, Nb≤0.01, V≤0.01, Ti≤0.01, Mo≤0.08, Sn≤0.005; the rest are Fe and essential impurities; the structure of the steel is martensite+reversed austenite, the yield ratio is ≤0.92, the -196°C impact absorption energy is ≥100J, and the non-plastic transition temperature NDT is less than -196°C.

[0007] A production method for low yield ratio and high strength and toughness steel for LNG storage tanks, the process route of which is converter smelting → slag removal and P removal → LF refining → VD vacuum treatment → continuous casting → slab surface cleaning → slab anti-oxidation coating → slab heating → controlled rolling and controlled cooling → critical point quenching + tempering, and the key process steps include:

[0008] (1) Converter smelting: end point C ≤ 0.05%, steel tapping P ≤ 0.008%, fixed oxygen ≥ 600ppm;

[0009] (2) Slag removal: After the steel is tapped from the converter, it is sent to the LF furnace for further decarburization, dephosphorization and slag removal;

[0010] (3) LF refining: After slagging, the molten steel returns to the LF refining furnace to heat up for alloying, and then white slag is made and heated. The total time on the station is ≥90min, and argon is blown from the bottom throughout the process. When the LF furnace leaves the station, calcification treatment is carried out, and the calcium wire feeding amount is ≥200m;

[0011] (4) VD vacuum treatment: VD vacuum degree is below 0.5tor and vacuum holding time is ≥15min, and soft blowing time before leaving VD furnace is ≥15min;

[0012] (5) Continuous casting: The continuous casting billet thickness is 220mm, and the continuous casting is carried out with full protection casting. The overheating temperature of the tundish is: ≤35℃ for the opening furnace and ≤30℃ for the continuous casting furnace;

[0013] (6) Slab surface cleaning and anti-oxidation coating: Surface cleaning and anti-oxidation coating are performed before heating to ensure the surface quality of the steel plate;

[0014] (7) Slab heating: The slab is heated in a step-beam furnace, with the furnace temperature limited to ≤1220°C and the tapping temperature controlled at 1150-1200°C.

[0015] (8) Controlled rolling and controlled cooling: The first-stage rolling temperature is 1050-1150℃, the final rolling temperature is ≥950℃, and low-speed and high-reduction rolling is adopted to ensure that the reduction rate of at least one pass is above 20%.

[0016] (9) Critical point quenching + tempering treatment.

[0017] The above step (8) controlled rolling and controlled cooling: the steel plate with a thickness of 5 to 10 mm is directly rolled without temperature control in the second stage, the steel plate with a thickness of ≥ 10 mm has a second stage starting rolling temperature of 800 to 930°C and a final rolling temperature of 740 to 810°C.

[0018] The above step (8) controlled rolling and controlled cooling: the steel plate with a thickness of 5 to 20 mm is air-cooled after rolling, and the steel plate with a thickness of more than 20 mm is water-cooled after rolling, and the red-return temperature is ≤300°C;

[0019] The above step (9) critical point quenching + tempering treatment: 5-20 mm thick steel plate heat treatment process: quenching process, heating temperature 700±10℃, holding time 25-40min; tempering process, heating temperature 590±10℃, holding time 40-60min.

[0020] The above step (9) critical point quenching + tempering treatment: 20-50 mm thick steel plate heat treatment process: quenching process, heating temperature 700±10°C, insulation time 40-60min; tempering process, heating temperature 590±10°C, insulation time 50-75min.

[0021] The chemical composition design principle of the steel of the present invention is described below:

[0022] C: Carbon often exists in the form of solid solution and carbide in steel, which increases the strength and hardness of steel and reduces its plasticity and toughness. Therefore, the C content should be as low as possible while ensuring the strength. Taking into account the performance requirements of the steel, the C content of the steel of the present invention is controlled at 0.03-0.05%.

[0023] Si: Silicon element dissolves in ferrite in steel to increase the strength and hardness of steel, while reducing plasticity and toughness. At the same time, silicon improves the fluidity of molten steel, which is beneficial to casting performance. Taking comprehensive consideration, the Si content of the steel of the present invention is controlled at 0.15-0.30%.

[0024] Mn: Manganese is beneficial to improving the strength and toughness of steel plates, but Mn is an element that easily segregates, which is disadvantageous for controlling center segregation. The Mn content of the steel of the present invention is controlled at 0.50-0.80%.

[0025] P: Phosphorus is a cold brittle element, which strongly reduces the low-temperature toughness of steel. For LNG storage tank steel that needs to meet the ultra-low temperature impact requirement of -196°C, the phosphorus content in the steel must be strictly controlled. The P content in the steel of the present invention is controlled below 0.005%.

[0026] S: Sulfur is a hot brittle element and is easy to form sulfide inclusions, which reduces the plasticity and toughness of steel. Therefore, the S content in this steel should be reduced as much as possible. The S content in the steel of the present invention is controlled within 0.003%.

[0027] Nb: Niobium has a strong affinity with nitrogen and carbon in steel, and can form extremely stable Nb (C, N) compounds with them, which are dispersed along the austenite grain boundaries and are not conducive to ultra-low temperature toughness. The Nb content of the steel of the present invention is controlled below 0.010%.

[0028] V: Vanadium is easily combined with nitrogen and carbon in steel, and has a strong precipitation strengthening effect, which can improve the strength of steel but reduce low-temperature toughness. The V content of the steel of the present invention is controlled below 0.010%.

[0029] Ti: Titanium is very easy to combine with nitrogen in steel to form TiN inclusions, which is not conducive to low temperature toughness. The Ti content of the steel of the present invention is controlled below 0.010%.

[0030] Mo: Mo is a carbide-forming element, which easily forms composite cementite with iron and carbon in steel, and is not conducive to low-temperature impact toughness of -196°C. The steel of the present invention controls Mo to be below 0.08%.

[0031] Sn: The influence of selenium on steel performance is similar to that of phosphorus, which strongly reduces the low-temperature toughness of steel. Its content must be strictly controlled. The Sn content in the steel of the present invention is controlled to be below 0.005%.

[0032] The beneficial effects and advantages of the present invention are as follows: through reasonable component design, the contents of elements such as P, S, Sn, Nb, V, Ti, etc. are strictly controlled to ensure the stability of low-temperature toughness at -196°C, and the contents of C and Mn are controlled in a narrow range to ensure the matching of strength and toughness; the steel is discharged from the converter and then put into the LF furnace for decarburization, dephosphorization and slag removal, and LF refining + VD vacuum degassing treatment is adopted to ensure the cleanliness of the steel, and the P content is controlled below 0.005%; the surface quality of the finished steel plate is ensured to be excellent by cleaning the surface of the slab and applying an anti-oxidation coating; the furnace temperature is strictly limited during continuous casting billet heating, which can not only inhibit the growth of austenite grains, but also reduce the oxidation of the slab surface, thereby ensuring the internal and external quality of the steel plate; the rolling adopts a combination of a first-stage high-temperature, low-speed and large reduction + a second-stage controlled rolling + a controlled cooling after rolling, so as to fully refine the grains of the rolled structure and provide a fine and uniform original structure for subsequent heat treatment; the heat treatment adopts critical point quenching + tempering to reduce the yield ratio and the matching of strength and toughness, and finally obtains a product with excellent physical quality and various properties. The structure of the steel of the present invention is martensite and reversed austenite. The steel for LNG storage tanks with a thickness of 5 to 50 mm produced by the present invention has the following properties: tensile properties, ReH ≥ 585 MPa, 680 MPa ≤ Rm ≤ 820 MPa, elongation A ≥ 18%, yield strength ratio ≤ 0.92; impact properties, -196°C impact absorption energy average value ≥ 100 J, single value ≥ 70 J; non-plastic transition temperature NDT ≤ -196°C. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1The metallographic structure of Example 1;

[0034] Figure 2 The metallographic structure of Example 2;

[0035] Figure 3 The metallographic structure of Example 3;

[0036] Figure 4 This is the metallographic structure of Example 4. DETAILED DESCRIPTION

[0037] The present invention will be further described below in conjunction with the embodiments.

[0038] Embodiment 1:

[0039] Production of a low yield ratio and high strength and toughness steel for LNG storage tanks, with a finished product thickness of 5mm.

[0040] Steelmaking process: C = 0.042% at converter end, P = 0.006% at steel tapping, oxygen setting 696ppm; after steel tapping from converter, it enters LF furnace for further decarburization, dephosphorization and slag removal, C = 0.0246% and P = 0.0028% after slag removal. After slag removal, the molten steel returns to LF refining furnace for heating and alloying, and then white slag is made and heated. The total station time is 105min, and argon is blown at the bottom of the whole process. Calcification treatment is carried out when the LF furnace leaves the station, the calcium line feed amount is 250m, and the exit S = 0.0019%; the vacuum degree of VD vacuum treatment is 0.4tor, the vacuum time is 15min, the soft blowing time before leaving VD furnace is 15min, and the hydrogen setting is 1.1ppm. The continuous casting section thickness is 260mm, the continuous casting tundish superheat is 20-25℃, and the smelting composition is shown in Table 2.

[0041] Implementation process of steel rolling: adopt two-fire rolling, first the billet is opened from 260mm to 105mm thick, and then the 105mm thick batch is loaded into the furnace again for heating and rolling to 5mm thick, the two-fire billet heating limit furnace temperature is 1220℃, the actual steel tapping temperature is 1185℃, the direct hot rolling start temperature is 1041℃, the two rough rolling reduction rates are 19% and 23.5% respectively, the intermediate billet thickness is 65mm, the finishing rolling is 7 passes, and the final rolling temperature is 803℃.

[0042] Heat treatment process: quenching heating furnace temperature set at 700°C, actual tapping temperature at 701°C, holding time at 32 minutes, tempering heating furnace temperature set at 590°C, actual tapping temperature at 588°C, holding time at 47 minutes. Finally, the steel plate is obtained, and its properties are shown in Table 3. Its metallographic structure is shown in Table 3. Figure 1 shown.

[0043] Embodiment 2:

[0044] Production of a low yield ratio and high strength and toughness steel for LNG storage tanks, with a finished product thickness of 20mm.

[0045] Steelmaking process implementation process: the implementation method is the same as Example 1.

[0046] Implementation process of steel rolling process: continuous casting billet heating limit furnace temperature is 1220℃, actual steel tapping temperature is 1188℃, first stage rolling temperature is 1134℃, the last three passes reduction rates are 20.5%, 22.6%, 27.1% respectively, final rolling temperature is 1086℃, rolling intermediate billet thickness is 70mm, second stage rolling temperature is 889℃, final rolling temperature is 796℃.

[0047] Heat treatment process: quenching heating furnace temperature set at 700°C, actual tapping temperature at 699°C, holding time at 38 minutes, tempering heating furnace temperature set at 590°C, actual tapping temperature at 589°C, holding time at 52 minutes. Finally, the steel plate is obtained, and its properties are shown in Table 3. Its metallographic structure is shown in Table 3. Figure 2 shown.

[0048] Embodiment 3:

[0049] Production of a low yield ratio and high strength and toughness steel for LNG storage tanks, with a finished product thickness of 25mm.

[0050] Steelmaking process implementation process: the implementation method is the same as Example 1.

[0051] Implementation process of steel rolling process: continuous casting billet heating limit furnace temperature is 1220℃, actual steel tapping temperature is 1177℃, first stage rolling temperature is 1148℃, last three passes reduction rates are 20.1%, 22.2%, 23.8% respectively, final rolling temperature is 1114℃, rolling intermediate billet thickness is 80mm, second stage rolling temperature is 828℃, final rolling temperature is 775℃, ACC cooling is carried out directly after rolling, and the temperature returns to 289℃.

[0052] Heat treatment process: quenching heating furnace temperature set at 700°C, actual tapping temperature at 700°C, holding time 43 minutes, tempering heating furnace temperature set at 590°C, actual tapping temperature at 589°C, holding time 52 minutes. Finally, the steel plate was obtained. The properties of the steel plate are shown in Table 3. Its metallographic structure is shown in Table 3. Figure 3 shown.

[0053] Embodiment 4:

[0054] Production of a low yield ratio and high strength and toughness steel for LNG storage tanks, with a finished product thickness of 50mm.

[0055] Steelmaking process implementation process: the implementation method is the same as Example 1.

[0056] Implementation process of steel rolling process: continuous casting billet heating limit furnace temperature is 1220℃, actual steel tapping temperature is 1184℃, first stage rolling temperature is 1153℃, the last three passes reduction rates are 16.7%, 20.3%, 25.7% respectively, final rolling temperature is 1107℃, rolling intermediate billet thickness is 110mm, second stage rolling temperature is 814℃, final rolling temperature is 782℃, ACC cooling is carried out directly after rolling, and the temperature returns to 281℃.

[0057] Heat treatment process: quenching heating furnace temperature set at 700°C, actual tapping temperature at 699°C, holding time at 51 minutes, tempering heating furnace temperature set at 590°C, actual tapping temperature at 588°C, holding time at 69 minutes. Finally, the steel plate is obtained. The properties of the steel plate are shown in Table 3. Its metallographic structure is shown in Table 3. Figure 4 shown.

[0058] Table 2 Example 1 Smelting chemical composition (wt.%)

[0059]

[0060] Table 3 Performance test results of each embodiment

[0061]

[0062] It can be seen from Table 2 that the composition of the embodiment meets the design composition requirements. It can be seen from Table 3 above that the steel of the present invention has a low yield strength ratio, good strength-toughness matching, good bending process performance, and a non-plastic transition (NDT) temperature below -196°C. All properties fully meet the technical requirements and can be used to manufacture LNG cryogenic storage tanks. Figure 1 to Figure 4 It can be seen that its structure is martensite + reversed austenite, and the structure is fine and uniform.

Claims

1. A method for producing low yield ratio and high strength and toughness steel for LNG storage tanks, characterized in that: The chemical composition of the steel is as follows: C=0.03~0.05, Si=0.15~0.30, Mn=0.50~0.80, P≤0.005, S≤0.003, Alt=0.020~0.040, Ni=8.50~9.50, Nb≤0.01, V≤0.01, Ti≤0.01, Mo≤0.08, Sn≤0.005; the rest are Fe and essential impurities; the structure of the steel is martensite + reversed austenite, the yield strength ratio is ≤0.92, the impact absorption energy at -196℃ is ≥100J, and the non-plastic transition temperature NDT is <-196℃; The key process steps include: (1) Converter smelting: end point C ≤ 0.05%, steel tapping P ≤ 0.008%, fixed oxygen ≥ 600ppm; (2) Slag removal: After the steel is tapped from the converter, it is sent to the LF furnace for further decarburization, dephosphorization and slag removal operations; (3) LF refining: After slagging, the molten steel returns to the LF refining furnace to heat up for alloying, and then white slag is made and heated. The total time on the station is ≥90min, and argon is blown from the bottom throughout the process. Calcification treatment is carried out when the LF furnace leaves the station, and the calcium wire feeding amount is ≥200m; (4) VD vacuum treatment: VD vacuum degree is below 0.5tor and the vacuum holding time is ≥15min, and the soft blowing time before leaving the VD furnace is ≥15min; (5) Continuous casting: The thickness of continuous casting billet is 220mm, and continuous casting is carried out with full protection casting. The overheating temperature of the tundish is: ≤35℃ for the opening furnace and ≤30℃ for the continuous casting furnace; (6) Slab surface cleaning and anti-oxidation coating: Surface cleaning and anti-oxidation coating are performed before heating to ensure the surface quality of the steel plate; (7) Slab heating: The slab is heated in a stepping furnace, the furnace temperature is limited to ≤1220°C, and the tapping temperature is controlled at 1150~1200°C; (8) Controlled rolling and controlled cooling: The first-stage rolling temperature is 1050-1150°C, the final rolling temperature is ≥950°C, and low-speed and high-reduction rolling is adopted to ensure that the reduction rate of at least one pass is above 20%; (9) Critical point quenching + tempering treatment: The quenching process heating temperature of 5-20mm thick steel plates is 700±10℃, the holding time is 25-40min, and the tempering process heating temperature is 590±10℃, the holding time is 40-60min; the quenching process heating temperature of 20-50mm thick steel plates is 700±10℃, the holding time is 40-60min, and the tempering process heating temperature is 590±10℃, the holding time is 50-75min.

2. The method for producing a steel for LNG storage tanks with low yield ratio and high strength and toughness according to claim 1, characterized in that Step (8) controlled rolling and controlled cooling: Steel plates with a thickness of 5 to 10 mm are directly rolled without temperature control in the second stage; steel plates with a thickness of more than 10 mm have a second stage starting rolling temperature of 800 to 930°C and a final rolling temperature of 740 to 810°C.

3. The method for producing a steel for LNG storage tanks with low yield ratio and high toughness according to claim 1, characterized in that Step (8) Controlled rolling and controlled cooling: Steel plates with a thickness of 5 to 20 mm are air-cooled after rolling; steel plates with a thickness of more than 20 mm are water-cooled after rolling, and the return temperature is ≤300°C.

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