9Ni steel plate for high-quality Nb-containing ultralow-temperature pressure vessel and manufacturing method of 9Ni steel plate

Abstract

The invention relates to a 9Ni steel plate for a high-quality Nb-containing ultralow-temperature pressure vessel and a manufacturing method of the 9Ni steel plate. The steel plate comprises the following chemical components in percentage by mass: 0.02-0.05% of C; 0.15% to 0.30% of Si; mn: 0.65 to 0.75%; p: < = 0.005%; s: < = 0.002%; 0.020% to 0.030% of Al; 8.50%-10.0% of Ni (nickel); 0.008% to 0.013% of Nb; h < = 0.8 ppm; 0.10% or less of As + Sb + Bi + Sn + Pb; and the balance of Fe and inevitable impurities. The steel is subjected to high-temperature rolling and is matched with QT heat treatment, specifically, a high-temperature quenching and tempering process. The 9Ni steel has excellent tensile property and excellent low-temperature toughness, the transverse and longitudinal tensile strength Rm is 720-780 MPa, the yield strength Rel is larger than or equal to 650 MPa, and the ductility A is larger than or equal to 21%; the sample is bent at normal temperature under the condition that D is equal to 3a and b is equal to 2a, the bending angle is 180 degrees, and no crack exists; and at the temperature of-196 DEG C, the impact absorption energy KV2 of the impact sample is greater than or equal to 200J.

Description

Technical Field

[0001] The invention belongs to the technical field of special steel smelting, and specifically relates to a high-quality Nb-containing 9Ni steel plate for ultra-low temperature pressure vessels and a manufacturing method thereof. Background Art

[0002] At present, 9Ni steel plate is widely used in the inner tank of large LNG storage tanks that contact with LNG. It is a steel plate containing 9% Ni element. This steel plate has high strength and excellent low-temperature toughness. GB / T 3531-2014 stipulates that its yield strength Rel≥560MPa (5-30mm) and Rel≥550MPa (>30-50), tensile strength 680-820MPa, elongation after fracture ≥18%; under -196℃, 9Ni steel low-temperature impact absorption energy ≥100J. With the development of the times, the construction of LNG storage tanks is becoming larger and larger, and the corresponding material selection requirements are becoming more and more stringent, with the focus on higher and higher strength and toughness requirements. For example, under -196℃, the actual level of 9Ni steel plate impact absorption energy is ≥200J, the tensile strength of the steel plate is located at the midline of 680-820MPa, and the tensile strength of the steel plate is designed to be R m : 720-780MPa. Competition in the steel industry is becoming increasingly fierce. Improving the comprehensive performance of 9Ni steel plates without increasing or significantly increasing costs has become the focus of the industry.

[0003] Nb is known as the "MSG of steel". A very small amount of Nb can significantly improve the comprehensive performance of steel plates by rationally utilizing the manufacturing process. However, at present, there is no patent for 9Ni steel plates containing Nb. "A 9Ni steel and its production method" (patent publication number) CN114737114A mentions that the residual Nb in the steel is ≤0.0050%, and the steel is rolled using a two-stage rolling process, with a second-stage rolling temperature of 800-950℃, a final rolling temperature of 800-900℃, and a steel plate red-returning temperature of 400-600℃; the tensile strength of the resulting steel plate reaches about 1400MPa, far exceeding the requirements of GB / T 3531-2014.

[0004] As we all know, grain refinement can improve the low-temperature toughness and strength of steel plates at the same time. Nb carbides and nitrides are dissolved in austenite at 800-900℃ to a certain extent, so that more fine and dispersed Nb carbonitrides can be precipitated in the subsequent air cooling process, which has a strong inhibitory effect on grain growth, thereby refining grains and improving strength and toughness. Summary of the invention

[0005] The technical problem to be solved by the present invention is to provide a high-quality Nb-containing 9Ni steel plate for ultra-low temperature pressure vessels in response to the above-mentioned prior art. The beneficial Nb element is added to the steel, and an advanced high-temperature and high-pressure continuous rolling process is adopted. Warm rolling is no longer required, which improves production efficiency, further refines the grains, and improves and stabilizes the low-temperature toughness of the steel plate.

[0006] The technical solution adopted by the present invention to solve the above problems is: a high-quality Nb-containing 9Ni steel plate for ultra-low temperature pressure vessels, the chemical composition of the steel plate and its mass percentage are C: 0.02-0.05%, Si: 0.15-0.30%, Mn: 0.65-0.75%, P≤0.005%, S≤0.002%, Al: 0.020-0.030%, N≤0.0060%, Ni: 8.50-10.0%, Nb: 0.008-0.013%, and the balance is Fe and unavoidable impurities.

[0007] The performance of the normalized ultra-high temperature molten salt storage tank low alloy container steel plate of the present invention meets the following requirements: the yield strength R el : Transverse and longitudinal ≥ 650MPa, tensile strength R m : Transverse and longitudinal 720-780MPa, elongation after fracture A≥21%; -196℃ impact single value ≥200J.

[0008] The following is a detailed description of the effects and dosage of the components in the present invention:

[0009] C: is the most important strengthening element in steel and also the element that has the greatest impact on toughness, especially low temperature toughness. For the 9Ni steel plate of the present invention, the carbon content is controlled to be 0.02-0.05%.

[0010] Si: It is a weak deoxidizing element in steel and is cheap. When the Si content is less than 0.15%, the deoxidation effect is poor; when the Si content is higher, the toughness and welding performance will decrease. The Si content of the present invention is controlled to be 0.15-0.30%.

[0011] Mn: It has the effects of solid solution strengthening and deoxidation, but its deoxidation capacity is relatively weak. The Mn content in the present invention is controlled to be 0.65-0.75%.

[0012] Ni: It is a beneficial element that can significantly improve low-temperature toughness, especially has a significant impact on the impact toughness and tough-to-brittle transition process of the steel plate in an ultra-low temperature environment. The present invention controls its content to 8.50-10.0%.

[0013] Nb: Nb carbides and nitrides are dissolved in austenite at 800-900°C to a certain extent, so that more fine and dispersed Nb carbonitrides can be precipitated in the subsequent air cooling process, which has a strong inhibitory effect on grain growth, thereby refining the grains and improving strength and toughness; too high Nb content will greatly increase the strength of the steel plate. In order to reduce the strength within a reasonable level, a higher tempering temperature must be used. The tempering temperature easily enters the two-phase region, which sharply reduces the toughness of the steel plate, resulting in an imbalance in the strength and toughness of the steel plate, and the toughness is sharply reduced at -196°C. The present invention controls its content to 0.008-0.013%.

[0014] Al: It mainly plays a role in deoxidation and grain refinement. Because the product of carbon content and oxygen content in molten steel is basically constant at a certain temperature, the C content in 5.5Ni steel is extremely low, and the oxygen in the molten steel is at a relatively high level. [Al] combines with [O] in the molten steel to form (Al 2 O 3 ) into the slag to achieve the purpose of deoxidation; however, if the Al content is too high, it is easy to cause some deoxidation products Al 2 O 3 It remains in the steel to form inclusions. Therefore, the present invention controls its content [Al] to be 0.020-0.030.

[0015] H: It is one of the most harmful elements in steel. H atoms are small in size and can diffuse easily in steel. Two H atoms form H 2 Molecule, H 2 The molecules gather together to generate greater pressure, forming micro cracks in the weak links inside the steel plate, which appear as white spots in the macroscopic view, causing the steel plate to become brittle, leaving a safety hazard for the use of the steel plate. Therefore, the present invention controls the content of the molten state to be no higher than 0.8ppm.

[0016] P: is a harmful impurity element in steel. Phosphorus combines with iron to form iron phosphide, which is easy to form segregation at the grain boundary, weaken the grain boundary and produce cold brittleness. The present invention controls the [P] content to ≤ 0.005%.

[0017] S: is a harmful impurity element in steel, which is easy to form defects such as segregation and inclusions. During the solidification process of the slab, Mn is easy to segregate with S in the center of the slab to form lamellar MnS inclusions, reducing the formation of low-melting FeS at the austenite grain boundary, but MnS inclusions are easy to cause unstable impact toughness in the core of the steel plate. In the later stage of refining, the Ca treatment technology is used to make the inclusion morphology spheroidized and evenly distributed, reduce the viscosity of the molten steel, and promote the floating of the inclusions. The present invention controls S: ≤0.002%.

[0018] The present invention also provides a method for manufacturing a high-quality Nb-containing 9Ni steel plate for ultra-low temperature pressure vessels. The smelting process adopts a converter - LF refining outside the furnace - RH vacuum treatment - CC continuous casting into billets - billets entering the pit and slowly cooling. The rolling process adopts a hot rolling process without additionally controlling the rolling temperature. The heat treatment adopts a continuous furnace high-temperature quenching + high-temperature tempering heat treatment. The specific process steps are as follows:

[0019] ⑴ Smelting and continuous casting: The smelting raw materials are prepared according to the chemical composition, and are sequentially subjected to KR molten iron pretreatment, converter smelting, LF refining, RH refining and continuous casting. The molten iron is subjected to KR pretreatment in order to achieve deep desulfurization. Since the phosphorus content in 9Ni steel is required to be ≤0.005%, in order to maintain an oxidizing environment, no Al element is added during the converter steelmaking process, and the converter slag retention operation is performed to achieve P removal from the molten steel; since the carbon content in the steel is low, the oxygen content is high (at a certain temperature, the product of the concentrations of [C] and [O] in the molten steel is constant), Al deoxidation is used [Al] + [O] = (Al 2 O 3 ) produced after (Al 2 O 3 ) is significantly more than conventional steels. Therefore, after LF refining, 9Ni steel is soft-blown with argon for more than 20 minutes. During the rising process of argon gas flow, bubbles are generated, transformed, collided and broken to form small argon bubbles. The vortex in the argon bubble plume area is used to promote (Al 2 O 3 ) inclusions gather and float into the slag, improving the purity of molten steel. 2 O 3 ) inclusions to obtain high-purity molten steel, and Nb element is added to the high-purity molten steel for alloying to meet the chemical composition design requirements.

[0020] ⑵ Slow cooling of billets in the pit: The casting superheat is 20±5℃ above the liquidus temperature. The purpose is to ① promote the melting of the mold protection slag to obtain a liquid slag layer with reasonable viscosity; ② ensure the fluidity of the molten steel in the tundish and the crystallizer; ③ prevent the generation of coarse columnar crystals, and obtain high-purity molten steel and 150mm thick continuous casting slabs with C0.5 level center segregation and 0.5 level looseness. After the continuous casting is completed, the continuous casting billet is hoisted into a slow cooling pit with a heating function, and the pit entry temperature is not less than 850℃; after slow cooling to 300℃, the heating function is started to keep the billet cooled to 200℃ at a cooling rate of 2-4℃ / h. The purpose of this slow cooling step is to reduce the H content in the steel, thereby further improving the core quality of the continuous casting billet to ensure the uniform and stable performance of the steel plate. After the slow cooling is completed, the surface of the continuous casting billet is cleaned with temperature.

[0021] ⑶ Continuous casting billet heating: The above continuous casting billet is heated to 1180-1280℃, and kept warm for 2-3 hours. The high temperature section holding time is not less than 10 minutes, so that the alloy elements in the steel are fully dissolved to ensure the uniformity of the composition and performance of the final product. After the continuous casting billet is taken out of the furnace, it is descaled with high-pressure water.

[0022] ⑷Rolling process: The continuous casting billet is rolled at a high temperature of more than 900℃ and air-cooled after rolling. A small amount of Nb element is added to the 9Ni steel plate to form a very small amount of Nb (C, N) particles. The dispersed Nb (C, N) particles precipitate in the crystal during the air cooling process. Due to the pinning effect, the grain growth is hindered, achieving the purpose of grain refinement.

[0023] (4) Heat treatment: The rolled steel plate is subjected to high temperature quenching (Q) + high temperature tempering (T) heat treatment. All heat treatment processes are carried out in a continuous furnace. The high temperature quenching heating temperature is 800-900°C, the furnace time is 1.3-2.3min / mm (not less than 25min), and the steel is water-cooled after being taken out of the furnace; the high temperature tempering heating temperature is 550-630°C, and the furnace time is 3.0-5.0min / mm (not less than 50min); a grain structure with a grain size of 8 and a tempered bainite structure with a high density of dislocations + an inverted austenite structure are obtained. Elements such as Mn and Ni in steel are elements that strongly expand the austenite region in the Fe-C phase diagram. Through high-temperature quenching heat treatment, a lath-shaped martensite structure is obtained, but the chemical composition of this lath-shaped martensite structure is uneven, and there are concentration fluctuations; during the tempering process at 550-600℃, carbon atoms and nickel atoms have certain diffusion and migration capabilities, and some high-concentration, unstable native martensite phases decompose, breaking through the potential energy barrier under the action of carbon and nickel elements, and inverting to form austenite.

[0024] The high-quality Nb-containing 9Ni steel plate for ultra-low temperature pressure vessels of the present invention adopts a reasonable chemical composition design; combines a continuous casting process to produce a 150mm thick continuous casting slab with low center segregation and looseness; adopts multiple dehydrogenation processes to reduce the hydrogen content in the steel plate; utilizes high-temperature rolling; and optimizes the heat treatment process to manufacture a high-quality 9Ni steel plate with a thickness of 5-50mm and guaranteed mechanical properties and process properties.

[0025] Compared with the prior art, the advantages of the present invention are:

[0026] (1) A high-quality 9Ni steel plate for use in ultra-low temperature pressure storage tanks is proposed, and the corresponding properties are specified: transverse and longitudinal tensile strength of the steel plate, Rel ≥ 650MPa, tensile strength Rm 720-820MPa, elongation after fracture ≥ 21%; under -196℃ conditions, the low-temperature impact absorption energy of 9Ni steel is ≥ 200J.

[0027] (2) Chemical composition design: Additional Nb element is added to improve the strength and toughness of the steel plate.

[0028] (3) Desulfurization of molten iron by KR pretreatment; P removal in an oxidizing environment during steelmaking; and removal of (Al2O3) inclusions by using the argon bubble plume zone to obtain high-purity molten steel.

[0029] (4) A phased slow cooling process is used to further reduce the hydrogen content in the steel.

[0030] (5) The steel plate is rolled at a temperature above 900°C and air-cooled after rolling. DETAILED DESCRIPTION

[0031] The technical solution of the present invention is described in more detail in conjunction with the preferred embodiments of the present invention. However, these embodiments are only descriptions of the preferred implementation methods of the present invention and cannot impose any limitation on the scope of the present invention.

[0032] Example 1

[0033] The thickness of the steel plate in Example 1 is 12 mm.

[0034] The production process of 12mm thick steel plate is as follows:

[0035] The smelting raw materials are configured according to the chemical composition of the above steel plates and are sequentially subjected to KR molten iron pretreatment - converter smelting - LF refining - RH refining - 150mm continuous casting - continuous casting billet hooding and slow cooling - continuous casting billet cleaning - continuous casting billet heating - insulation treatment - high-pressure water descaling - hot rolling - air cooling - straightening - heat treatment.

[0036] Among them, after the molten steel is deoxidized, it needs to be left to stand and soft-blown with argon for 20 minutes; the high-temperature slab leaving the continuous casting machine is slowly cooled in stages into the pit, with an inlet temperature of 900°C. After slowly cooling to 300°C, the heating function is started to keep the slab cooled to 200°C at a cooling rate of 3°C / h. The purpose of this slow cooling step is to reduce the H content in the steel.

[0037] Furthermore, the specific process of heating, controlled rolling and cooling of slabs into steel plates is as follows: the slabs are heated to 1180-1280°C, kept warm for 140 minutes, kept warm for 13 minutes in the high temperature section, descaled by high-pressure water after being taken out of the furnace, and rolled at a high temperature of more than 900°C to form the slabs, with a final rolling temperature of 910°C and air cooling after rolling.

[0038] The completely cooled steel plate enters the continuous furnace for heat treatment. All heat treatment processes are carried out in the continuous furnace. The high-temperature quenching heating temperature is 850℃, the furnace time is 25min, and it is water-cooled after being taken out of the furnace; the high-temperature tempering heating temperature is 600℃, and the furnace time is 50min.

[0039] The finished steel plate formed by the above manufacturing process has excellent comprehensive performance, as shown in Table 1-Table 2 for details.

[0040] Example 2

[0041] The thickness of the steel plate involved in Example 2 is 25 mm.

[0042] The production process of 25mm thick steel plate is as follows:

[0043] The steel plate slab steelmaking and slab pile slow cooling are consistent with Example 1.

[0044] Furthermore, the specific process of heating, rolling and cooling of slabs into steel plates is as follows: the slabs are heated to 1180-1280°C, kept warm for 135 minutes, kept warm for 15 minutes in the high temperature section, descaled with high-pressure water after being taken out of the furnace, rolled at a high temperature of more than 900°C, the final rolling temperature is 923°C, and air-cooled after rolling.

[0045] The completely cooled steel plate enters the continuous furnace for heat treatment. All heat treatment processes are carried out in the continuous furnace. The high-temperature quenching heating temperature is 850℃, the furnace time is 40min, and it is water-cooled after leaving the furnace; the high-temperature tempering heating temperature is 600℃, and the furnace time is 80min.

[0046] The finished steel plate formed by the above manufacturing process has excellent comprehensive performance, as shown in Table 1-Table 2 for details.

[0047] Example 3

[0048] The thickness of the steel plate involved in Example 3 is 40 mm.

[0049] The production process of 40mm thick steel plate is as follows:

[0050] The steel plate slab steelmaking and slab pile slow cooling are consistent with Example 1.

[0051] Furthermore, the specific process of heating, rolling and cooling of slabs into steel plates is as follows: the slabs are heated to 1180-1280°C, kept warm for 135 minutes, kept warm for 12 minutes in the high temperature section, descaled with high-pressure water after being taken out of the furnace, rolled at a high temperature of more than 900°C, the final rolling temperature is 930°C, and air-cooled after rolling.

[0052] The completely cooled steel plate enters the continuous furnace for heat treatment. All heat treatment processes are carried out in the continuous furnace. The high-temperature quenching heating temperature is 850℃, the furnace time is 70min, and it is water-cooled after being taken out of the furnace; the high-temperature tempering heating temperature is 580℃, and the furnace time is 140min.

[0053] The finished steel plate formed by the above manufacturing process has excellent comprehensive performance, as shown in Table 1-Table 2 for details.

[0054] Table 1 Main chemical composition of finished product of embodiment 1 (wt%)

[0055] C Si Mn P S Ni Nb Al Example 1 0.04 0.16 0.68 0.003 0.001 8.92 0.012 0.018 Example 2 0.04 0.17 0.69 0.004 0.001 8.93 0.012 0.018 Example 3 0.05 0.17 0.70 0.004 0.001 8.93 0.012 0.018

[0056] Table 2 Mechanical properties of steel plates produced in Example (delivery state)

[0057]

[0058] Although the preferred embodiments of the present invention are described in detail above, it should be clearly understood that various modifications and variations are possible for those skilled in the art. Any modification, equivalent substitution, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A high-quality Nb-containing 9Ni steel plate for ultra-low temperature pressure vessels, characterized in that: The chemical composition and mass percentage of the steel plate are C: 0.02-0.05%; Si: 0.15-0.30%; Mn: 0.65-0.75%; P: ≤0.005%; S: ≤0.002%; Al: 0.020-0.030%; Ni: 8.50-10.0%; Nb: 0.008-0.013%; H≤0.8ppm; As+Sb+Bi+Sn+Pb≤0.10%; The balance is Fe and inevitable impurities.

2. The high-quality Nb-containing 9Ni steel plate for ultra-low temperature pressure vessels according to claim 1, characterized in that: The steel plate has a transverse and longitudinal tensile strength Rm of 720-780 MPa, a yield strength Rel ≥ 650 MPa, and an elongation A ≥ 21%; The specimen is bent at room temperature under the conditions of D=3a, b=2a, with a bending angle of 180° and no cracks; and at a temperature of -196°C, the impact absorption energy KV2 of the impact specimen is ≥200J.

3. A method for manufacturing a high-quality Nb-containing 9Ni steel plate for ultra-low temperature pressure vessels as claimed in claim 1, characterized in that: The method mainly comprises the following steps: ⑴ The smelting raw materials are processed by KR hot metal pretreatment, oxygen converter smelting, LF refining outside the furnace, RH vacuum refining and continuous casting process in sequence to obtain 150mm thick continuous casting billets; after the continuous casting is completed, the continuous casting billets are put into the pit for slow cooling; (2) Heat the continuous casting billet to 1180-1280℃, keep it warm for 2-3 hours, and keep it warm for no less than 10min in the high temperature section. Descale the continuous casting billet with high pressure water after it leaves the furnace, and then carry out high temperature rolling, with the rolling temperature ≥900℃, and air cool it after rolling. ⑶ The rolled steel plate is subjected to high temperature quenching + high temperature tempering heat treatment. All heat treatment processes are carried out in a continuous furnace and air-cooled after leaving the furnace.

4. The method for manufacturing a high-quality Nb-containing 9Ni steel plate for ultra-low temperature pressure vessels according to claim 1, characterized in that: In step (1), no Al element is added during the converter steel-making process, and a converter slag retention operation is performed to achieve P removal from the molten steel; and after LF refining, soft argon blowing is performed for more than 20 minutes.

5. The method for manufacturing a high-quality Nb-containing 9Ni steel plate for ultra-low temperature pressure vessels according to claim 1, characterized in that: In step (1), the casting superheat is 20±5°C above the liquidus temperature. After the continuous casting is completed, the continuous casting billet is hoisted into a slow cooling pit with a heating function, and the pit entry temperature is not lower than 850°C; after slow cooling to 300°C, the heating function is started to keep the billet cooled to 200°C at a cooling rate of 2-4°C / h.

6. The method for manufacturing a high-quality Nb-containing 9Ni steel plate for ultra-low temperature pressure vessels according to claim 1, characterized in that: In step (3), the high temperature quenching heating temperature is 800-900°C, the time in the furnace is 1.3-2.3 min / mm, the total time is not less than 25 min, and water cooling is performed after being taken out of the furnace; the high temperature tempering heating temperature is 550-630°C, the time in the furnace is 3.0-5.0 min / mm, and the total time is not less than 50 min.

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