9Ni steel plate and its production method

Abstract

The present invention discloses a 9Ni steel plate and a production method thereof. The chemical composition of the steel plate: C 0.04 - 0.075%, Si 0.15 - 0.33%, Mn 0.5 - 0.8%, Ni 8.6 - 9.4%, Al 0.02 - 0.04%, with the balance being iron and impurities. The production method includes: heating, with the soaking temperature being 1150 - 1200°C; first rolling into an intermediate billet, with the starting rolling temperature being 1050 - 1090°C, then holding temperature, and then rolling into a hot-rolled plate, with the starting rolling temperature T fr ±10°C, and the finishing rolling temperature T fr ±18°C; water cooling, with the water inlet temperature T fc ±15°C, the final cooling temperature ≤200°C, and the average cooling rate throughout the process being 25 - 35°C / s; after discharging water, air cooling to room temperature; tempering treatment, with the tempering temperature being 540 - 580°C, and then air cooling to room temperature. The present invention can solve the problem of high yield ratio.

Description

Technical Field

[0001] The present invention belongs to the technical field of steel material preparation, and relates to a 9Ni steel plate and a production method thereof. Background Art

[0002] Compared with traditional energy sources, natural gas has the characteristics of high combustion calorific value and less pollution. In order to facilitate the transportation and storage of natural gas, natural gas is generally liquefied into liquefied natural gas (LNG) under the conditions of normal pressure and -162°C.

[0003] In view of the flammability and ultra-low temperature of LNG, the structural materials used for storage and transportation equipment need to have the characteristics of high strength and excellent low-temperature toughness. Among them, 9Ni steel has become the mainstream structural material in the field of LNG storage and transportation equipment due to its outstanding toughness in cryogenic environments.

[0004] However, 9Ni steel usually has the characteristic of a high yield ratio, and the yield ratio of thin-gauge 9Ni steel plates with the same composition is higher than that of thick-gauge ones. A high yield ratio means a greater potential safety hazard in cryogenic environments. Therefore, with the increasing emphasis on safety, in addition to improving strength and low-temperature toughness, how to reduce the yield ratio has also become an important research direction for 9Ni steel.

[0005] For example, Chinese Patent CN110541110A discloses a thin-gauge 9Ni steel plate and a production method thereof, aiming to reduce the yield ratio, but it can only control the yield ratio below 0.9, for example, within the range of 0.87 to 0.89, and the yield ratio is still relatively high. Summary of the Invention

[0006] In order to solve the problem of the high yield ratio existing in the existing 9Ni steel plates, the purpose of the present invention is to provide a 9Ni steel plate and a production method thereof.

[0007] To achieve the above object of the invention, an embodiment of the present invention provides a production method of a 9Ni steel plate. The chemical composition of the steel plate in mass percentage includes: C 0.04 - 0.075%, Si 0.15 - 0.33%, Mn 0.5 - 0.8%, Ni 8.6 - 9.4%, Al 0.02 - 0.04%, P≤0.005%, S≤0.002%, O≤0.002%, N≤0.0035%, H≤0.00015%, and the balance is iron and unavoidable impurities;

[0008] The production method includes the following processes,

[0009] Heating process: heating the steel billet, and the soaking temperature is 1150 - 1200°C;

[0010] Rolling process: First, the steel billet is made into an intermediate billet with a thickness of t1 through the first-stage rolling, then it is held at a certain temperature, and then the intermediate billet is made into a hot-rolled sheet with a thickness of t0 through the second-stage rolling; among them, the starting rolling temperature of the first-stage rolling is 1050 - 1090 °C, and the starting rolling temperature of the second-stage rolling is (T fr - 10) - (T fr + 10) °C, and the finishing rolling temperature is (T fr - 18) - (T fr + 18) °C, 1.3 ≤ t1 / t0 ≤ 12, and T fr is obtained from the following formula 1;

[0011] Cooling process: The obtained hot-rolled sheet is cooled by water cooling. The water inlet temperature of the hot-rolled sheet is (T fc - 15) - (T fc + 15) °C, and T fc is obtained from the following formula 2. The final cooling temperature is ≤ 200 °C, and the average cooling rate throughout the process is 25 - 35 °C / s; after water outlet, it is air-cooled to room temperature to obtain a cooled sheet;

[0012] Heat treatment process: The obtained cooled sheet is subjected to tempering treatment. The tempering temperature is 540 - 580 °C, and the holding time is more than 30 min. After the tempering is completed, it is air-cooled to room temperature;

[0013] T fr = 879 - 97C + 48Si - 9.5Mn - 5.1Ni - 200Al Formula 1;

[0014] T fc = 720 + 116C + 27Si - 10Mn + 6Ni - 177Al Formula 2;

[0015] Among them, the element symbols in Formula 1 and Formula 2 represent the mass percentages of each element in the steel billet.

[0016] As an optional implementation method, the hot-rolled sheet is a single-layer steel sheet with a thickness t0 > 8 mm;

[0017] The production method further includes a continuous casting billet process: A continuous casting billet with a thickness of 220 - 250 mm is prepared through steelmaking and continuous casting;

[0018] And, in the heating process: The obtained continuous casting billet is heated, and the soaking temperature is 1150 - 1180 °C; the total heating time is 160 - 220 min.

[0019] Optionally, in the cooling process: The recarburization temperature is ≤ 300 °C.

[0020] Optionally, in the cooling process:

[0021] The hot-rolled sheet is water-cooled on the post-rolling accelerated cooling control system;

[0022] The post-rolling accelerated cooling control system has 24 groups of cooling headers. Among them, the 1st to 6th groups, the 7th to 18th groups, and the 19th to 24th groups respectively form the first zone, the second zone, and the third zone;

[0023] The total cooling water flow rate of the post-rolling accelerated cooling control system is 1500 - 3600 L / s. The total cooling water flow rate of the first zone is 500 - 530 L / s. All the cooling headers in the first zone and the third zone are opened, and the cooling headers in the second zone are opened alternately.

[0024] Optionally, in the rolling process: the reduction rate of each pass in the first stage of rolling is 10 - 35%, and the reduction amount of at least three of the first 5 passes is not less than 8 mm.

[0025] Optionally, in the heat treatment process: when the thickness t0 is more than 10 mm, the heat preservation duration ranges from 2t0 to 14t0 min.

[0026] As another alternative implementation, the hot-rolled sheet is a composite sheet stacked by n single sheets, n takes the value of 2, 3, or 4, and the thickness t of at least one single sheet;

[0027] The production method further includes the following processes,

[0028] Continuous casting billet process: A continuous casting billet with a thickness of 220 - 250 mm is prepared through steelmaking and continuous casting;

[0029] Blooming process: The continuous casting billet is heated, the soaking temperature is 1200 - 1240 °C, and the total heating duration is 200 - 300 min; the heated continuous casting billet is rolled into a large slab with a thickness of 4.5t - 10t;

[0030] Composite process: The obtained large slab is cut into multiple small slabs, n small slabs are stacked and welded into a composite billet; this composite billet enters the heating process and is heated as the steel billet;

[0031] Sheet separation process: The tempered sheet obtained in the heat treatment process is separated into n single-layer 9Ni steel sheets by trimming the edges.

[0032] Optionally, in the heating process: the steel billet is heated, and the heating process includes preheating, heating, and soaking. The preheating temperature is 750 - 850 °C, and the heating rate is 20 - 30 °C / min; the heating temperature is 1100 - 1200 °C, and the heating rate is 30 - 50 °C / min; the soaking temperature is 1150 - 1180 °C; the total heating duration is 160 - 220 min.

[0033] Optionally, among the n single-layer 9Ni steel plates, the thickness t of at least one 9Ni steel plate is less than 8 mm.

[0034] Optionally, among the n single-layer 9Ni steel plates, the thickness t of at least one 9Ni steel plate is 3 to 8 mm.

[0035] Optionally, in the composite process: there is an antioxidant isolation agent between two adjacent small slab billets of the composite billet;

[0036] The antioxidant isolation agent is a coating composed of 55 to 65 parts by mass of hydrothermal leaching product of blast furnace slag, 7 to 15 parts by mass of corn starch, 12 to 33 parts by mass of polyvinyl alcohol, 5 to 8 parts by mass of sodium tripolyphosphate and water.

[0037] Optionally, the preparation method of the antioxidant isolation agent includes the following steps:

[0038] After the blast furnace slag is crushed, the slag material with the required particle size is screened out, and the slag material, hydrochloric acid and water are put into a reaction vessel, and then reacted at a temperature of 130 to 150 °C for 8 to 10 h;

[0039] The solid-phase slag is separated from the reacted system, and then the solid-phase slag is washed, ground and screened to obtain the hydrothermal leaching product of blast furnace slag;

[0040] Using the hydrothermal leaching product of blast furnace slag, corn starch, polyvinyl alcohol and sodium tripolyphosphate as powders, prepare materials according to the mass ratio of water: the powders of 1:2 to 3:4;

[0041] After heating the water to 90 to 95 °C, add polyvinyl alcohol and stir, let it stand until it cools to 60 to 70 °C, then add corn starch and stir for 0.8 to 1.5 h under constant temperature conditions, and then add the leaching product and sodium tripolyphosphate and stir to obtain the antioxidant isolation agent.

[0042] Optionally, in the step "After the blast furnace slag is crushed, the slag material with the required particle size is screened out, and the slag material, hydrochloric acid and water are put into a reaction vessel, and then reacted at a temperature of 130 to 150 °C for 8 to 10 h":

[0043] The particle size of the slag material is r mm, the concentration of hydrochloric acid is C mol / L, the volume ratio of the slag material to hydrochloric acid is 0.2 to 0.8, and the volume ratio of water to hydrochloric acid is 0.45 to 0.7, where C ranges from (ln(r * 60) - 0.7) to (ln(r * 60) + 0.7).

[0044] Optionally, in the cooling process: the reoxidation temperature ≤ 300 °C.

[0045] Optionally, in the cooling process:

[0046] The hot-rolled sheet is water-cooled on the post-rolling accelerated cooling control system;

[0047] The post-rolling accelerated cooling control system has 24 groups of cooling headers. Among them, the 1st to 6th groups, the 7th to 18th groups, and the 19th to 24th groups respectively form the first zone, the second zone, and the third zone;

[0048] The total cooling water flow rate of the post-rolling accelerated cooling control system is 1500 - 3600 L / s. The total cooling water flow rate of the first zone is 500 - 530 L / s. All the cooling headers in the first zone and the third zone are opened, and the cooling headers in the second zone are opened alternately.

[0049] Optionally, in the rolling process: the reduction rate of each pass in the first stage of rolling is 10 - 35%, and the reduction amount of at least three of the first 5 passes is not less than 8 mm.

[0050] Optionally, in the heat treatment process: when the thickness t0 is above 10 mm, the heat preservation duration ranges from 2t0 to 14t0 min.

[0051] To achieve the above-mentioned invention purpose, an embodiment of the present invention provides a 9Ni steel plate. The chemical composition of the steel plate includes, by mass percentage: C 0.04 - 0.075%, Si 0.15 - 0.33%, Mn 0.5 - 0.8%, Ni 8.6 - 9.4%, Al 0.02 - 0.04%, P ≤ 0.005%, S ≤ 0.002%, O ≤ 0.002%, N ≤ 0.0035%, H ≤ 0.00015%, and the balance is iron and inevitable impurities;

[0052] The thickness of the steel plate is below 8 mm, the tensile strength is 730 - 810 MPa, the yield strength is 590 - 680 MPa, the yield ratio is ≤ 0.87, the elongation is ≥ 23%, and the impact energy at -196 °C is ≥ 210 J.

[0053] Furthermore, the thickness of the steel plate is 3 - 8 mm.

[0054] Compared with the prior art, the beneficial effects of the present invention include: whether it is a thick specification or a thin specification, the specific technologies of the rolling process and the cooling process can lay a foundation for reducing the yield ratio. While ensuring excellent low-temperature toughness and high strength of the steel plate, it reduces the increase amplitude of the yield ratio caused by the heat treatment process, and finally reduces the yield ratio of the steel plate, thereby achieving all-round improvement of the steel plate in terms of strength, low-temperature toughness, and yield ratio. BRIEF DESCRIPTION OF THE DRAWINGS

[0055] Figure 1 It is the metallographic structure diagram at the 1 / 2 thickness of the steel plate in Example 2A of the second embodiment of the present invention;

[0056] Figure 2 It is the metallographic structure diagram at half of the thickness of the steel plate in Example 2B of the second embodiment of the present invention;

[0057] Figure 3 It is the metallographic structure diagram at half of the thickness of the steel plate in Example 2C of the second embodiment of the present invention;

[0058] Figure 4 It is the metallographic structure diagram at half of the thickness of the steel plate in Example 2D of the second embodiment of the present invention;

[0059] Figure 5 It is the metallographic structure diagram at half of the thickness of the steel plate in Example 2E of the second embodiment of the present invention;

[0060] Figure 6 It is the metallographic structure diagram at half of the thickness of the steel plate in Example 2F of the second embodiment of the present invention. Specific embodiments

[0061] The technical solution of the present invention will be further introduced below in conjunction with specific embodiments.

[0062] An embodiment of the present invention provides a production method of 9Ni steel plate. The produced 9Ni steel plate not only has the advantages of high strength, high and low temperature toughness, but also has a low yield ratio.

[0063] In terms of chemical composition, the chemical composition of the steel plate includes, by mass percentage: C 0.04 - 0.075%, Si 0.15 - 0.33%, Mn 0.5 - 0.8%, Ni 8.6 - 9.4%, Al 0.02 - 0.04%, P ≤ 0.005%, S ≤ 0.002%, O ≤ 0.002%, N ≤ 0.0035%, H ≤ 0.00015%, and the balance is iron and unavoidable impurities.

[0064] The functions of each element in the above chemical composition will be introduced below.

[0065] C is a strengthening element and also an austenite stabilizing element; if the C content is too low, it is difficult to ensure the strength of the steel plate; if the C content is too high, it will increase the ductile - brittle transition temperature and affect the low - temperature toughness of the steel plate. Therefore, on the premise of ensuring strength, the lower the C content, the better. In the present invention, the C content is controlled at 0.04 - 0.075%.

[0066] Si is a deoxidizing element and can also inhibit the grain - boundary segregation behavior of P; however, when the Si content is high, it affects the low - temperature toughness of the steel plate. In the present invention, the Si content is controlled at 0.15 - 0.33% to ensure the deoxidation effect and reduce the oxide inclusions in the steel without affecting the low - temperature toughness.

[0067] Mn is an element for solid solution strengthening and grain refinement, and is also an austenite stabilizing element. Adding Mn can significantly improve the hardenability of the material. At the same time, Mn is also an element prone to segregation and inclusion formation. Excessive addition will affect the fracture toughness and low-temperature impact toughness of the steel plate. In the present invention, the Mn content is controlled at 0.5 - 0.8%.

[0068] Ni is an austenite stabilizing element, which is particularly important for stabilizing reversed austenite. Reversed austenite is the main component providing the cryogenic toughness of the steel plate. It can improve the hardenability of the steel plate and increase the strength through solid solution strengthening. In the present invention, the Ni content is controlled at 8.6 - 9.4%.

[0069] Al is a deoxidizing element, which can effectively reduce the inclusion content in the steel and refine the grains. Excessive aluminum is likely to increase the Al2O3 inclusions in the steel and affect the low-temperature toughness of the steel. In the present invention, Al is controlled at 0.02 - 0.04%.

[0070] P, S, N, H, O are impurity elements and should be controlled in a relatively low range as much as possible. In the present invention, P ≤ 0.005%, S ≤ 0.002%, N ≤ 0.0035%, H ≤ 0.00015%, O ≤ 0.002%.

[0071] In terms of the production process, the production method uses a steel billet meeting the above chemical composition and obtains a steel plate through the following heating process, rolling process, cooling process, and heat treatment process.

[0072] Heating process: Heat the steel billet, and the soaking temperature is 1150 - 1200 °C;

[0073] Rolling process: First, roll the steel billet into an intermediate billet with a thickness of t1 through the first-stage rolling, then keep it at a certain temperature, and then roll the intermediate billet into a hot-rolled plate with a thickness of t0 through the second-stage rolling. Among them, the starting rolling temperature of the first-stage rolling is 1050 - 1090 °C, and the starting rolling temperature of the second-stage rolling is (T fr - 10) - (T fr + 10) °C, and the finishing rolling temperature is (T fr - 18) - (T fr + 18) °C, and 1.3 ≤ t1 / t0 ≤ 12;

[0074] Cooling process: Cool the obtained hot-rolled plate by water cooling. The water inlet temperature of the hot-rolled plate is (T fc - 15) - (T fc + 15) °C, the final cooling temperature is ≤ 200 °C, and the average cooling rate throughout the process is 25 - 35 °C / s; After coming out of the water, air-cool it to room temperature to obtain a cooled plate;

[0075] Heat treatment process: Temper the obtained cooling plate at a tempering temperature of 540 - 580 °C for a holding time of 30 minutes or more, and then air-cool to room temperature after the tempering is completed.

[0076] The above-mentioned T fr can be obtained from the following formula 1, T fc is obtained from the following formula 2:

[0077] T fr = 879 - 97C + 48Si - 9.5Mn - 5.1Ni - 200Al Formula 1;

[0078] T fc = 720 + 116C + 27Si - 10Mn + 6Ni - 177Al Formula 2;

[0079] Among them, the element symbols in Formula 1 and Formula 2 represent the mass percentages of each element in the steel billet. For example, if the C content in the steel billet is 0.04% by mass, then the element symbol C in Formula 1 and Formula 2 is substituted with the mass percentage of C, which is 0.04.

[0080] Therefore, the production method of the present invention can not only produce thick-specification 9Ni steel plates, but also produce thin-specification 9Ni steel plates. Whether it is thick-specification or thin-specification, through the specific techniques of the rolling process and the cooling process, the yield strength can be reduced while the tensile strength is stabilized, so as to not only ensure excellent low-temperature toughness and high strength of the steel plate, but also reduce the increase amplitude of the yield ratio caused by the heat treatment process, and finally reduce the yield ratio of the steel plate, thereby achieving all-round improvement of the strength, low-temperature toughness and yield ratio of the steel plate; in particular, for thin-specification 9Ni steel plates, the difficulty of reducing the yield ratio is much greater than that of thick-specification 9Ni steel plates. The production method of the present invention can solve the problem of high yield ratio of thin-specification 9Ni steel plates, which is of great significance for thin-specification 9Ni steel plates.

[0081] Trying to explain the technical solution of the present invention from the principle, for the production of 9Ni steel plates, on the one hand, fine grain strengthening needs to be carried out through temper heat treatment to improve strength and low-temperature toughness. However, the fine grain strengthening in temper heat treatment will cause a large increase in the yield strength, and thus in addition to the simultaneous increase in tensile strength, low-temperature toughness and yield ratio; in this application, through the specific operations in the rolling and cooling processes, the yield strength can be reduced while the tensile strength is stabilized, thereby eliminating the problem of excessive increase in the yield ratio during the heat treatment process, and finally achieving the effect of reducing the yield ratio without causing a decrease in tensile strength or low-temperature toughness.

[0082] Specifically, the tensile strength of the 9Ni steel plate of the present invention is 730-810 MPa, the yield strength is 590-680 MPa, and the yield ratio is ≤0.87, having excellent properties of high strength and low yield ratio.

[0083] Furthermore, in one embodiment of the present invention, the elongation of the steel plate is ≥23%, and the impact energy at -196 °C is ≥210 J, with excellent low-temperature toughness.

[0084] In addition, in one embodiment of the present invention, the metallographic structure of the cooling plate obtained in the cooling process is martensite + ferrite + reverse transformation austenite; and further, after tempering, the metallographic structure of the finally obtained steel plate is a multiphase structure of tempered martensite + ferrite + reverse transformation austenite, and among them: the volume ratio of tempered martensite is 75-85%, the volume ratio of reverse transformation austenite is 10-18%, and the remainder is ferrite.

[0085] Further, in one embodiment, the chemical composition of the steel plate further satisfies 68 ≤ Ni / (C + Al) ≤ 173 in terms of mass percentage. Thus, C, as a stable austenite element, can increase the content of reverse transformation austenite without affecting the composition standard, low-temperature toughness, and welding. For example, the content of reverse transformation austenite can be increased from 5%-10% of general 9Ni steel to 10-18%.

[0086] Furthermore, in another embodiment, the chemical composition of the steel plate further satisfies Mn = (1.8-2.5)Si + 0.17% in terms of mass percentage. Thus, Mn can improve the hardenability of the steel plate. By using the post-rolling accelerated cooling control system for cooling treatment, the offline quenching treatment in the conventional technology can be omitted, reducing the complexity of the subsequent processes after the cooling process, lowering the production cost, and improving the production efficiency.

[0087] As described above, the production method of the present invention can be used to prepare thick-specification 9Ni steel plates and thin-specification 9Ni steel plates, which will be introduced in detail below respectively.

[0088] First Embodiment

[0089] This embodiment provides the implementation details of the above-described production method for preparing thick-specification 9Ni steel plates.

[0090] It can be understood that the chemical composition of the 9Ni steel plate in this embodiment is the same as that above. For example, in terms of mass percentage, it includes: C 0.04-0.075%, Si 0.15-0.33%, Mn 0.5-0.8%, Ni 8.6-9.4%, Al 0.02-0.04%, P ≤ 0.005%, S ≤ 0.002%, O ≤ 0.002%, N ≤ 0.0035%, H ≤ 0.00015%, and the balance is iron and unavoidable impurities.

[0091] More preferably, the chemical composition of the steel plate further satisfies 68 ≤ Ni / (C + Al) ≤ 173, and / or further satisfies Mn = (1.8 - 2.5)Si + 0.17%.

[0092] Specifically, the production method includes a process flow of a continuous casting process, a heating process, a rolling process, a cooling process, and a heat treatment process in sequence, and a single-layer 9Ni steel plate with a thickness t0 > 8 mm is prepared.

[0093] Continuous casting process: A continuous casting slab with a thickness of 220 - 250 mm is prepared through steelmaking and continuous casting, and the chemical composition of the continuous casting slab is consistent with the chemical composition of the finally obtained 9Ni steel plate.

[0094] Heating process: The continuous casting slab obtained from the continuous casting process is heated, with a soaking temperature of 1150 - 1200°C; the total heating duration is 160 - 220 min.

[0095] Rolling process: First, the steel billet is rolled into an intermediate billet with a thickness of t1 through the first-stage rolling, then kept at a certain temperature, and then the intermediate billet is rolled into a hot-rolled sheet with a thickness of t0 through the second-stage rolling; among them, the starting rolling temperature of the first-stage rolling is 1050 - 1090°C, and the starting rolling temperature of the second-stage rolling is (T fr - 10) - (T fr + 10)°C, the finishing rolling temperature is (T fr - 18) - (T fr + 18)°C, 1.3 ≤ t1 / t0 ≤ 12, and T fr is obtained from the formula 1.

[0096] Cooling process: The obtained hot-rolled sheet is cooled by water cooling, and the water inlet temperature of the hot-rolled sheet is (T fc - 15) - (T fc + 15)°C, where T fc is obtained from the formula 2, the final cooling temperature ≤ 200°C, the average cooling rate throughout the process is 25 - 35°C / s; after discharging water, it is air-cooled to room temperature to obtain a cooled sheet.

[0097] Heat treatment process: The obtained cooled sheet is subjected to tempering treatment, with a tempering temperature of 540 - 580°C, a holding time of more than 30 min, and after the tempering is completed, it is air-cooled to room temperature to obtain the 9Ni steel plate.

[0098] In this embodiment, the thickness t0 of the obtained 9Ni steel plate is t0 > 8 mm, for example, it can be 8 - 12 mm, the tensile strength is 730 - 810 MPa, the yield strength is 590 - 680 MPa, and the yield ratio ≤ 0.87; the elongation ≥ 23%, and the impact energy at - 196°C ≥ 210 J.

[0099] The metallographic structure of the steel plate is a multiphase structure of tempered martensite + ferrite + reverse-transformed austenite, and among them: the volume proportion of tempered martensite is 75 - 85%, the volume proportion of reverse-transformed austenite is 10 - 18%, and the rest is ferrite.

[0100] Among them, in the continuous casting billet process, specifically in the steelmaking and continuous casting methods, any feasible technology in the technical field can be adopted for implementation. For example, the process route of hot metal KR desulfurization - converter smelting - LF refining - RH refining - continuous casting can be adopted for implementation. Since it is not the invention design point of this application, no more introduction will be given.

[0101] Preferably, in the heating process, the soaking temperature is controlled at 1150 - 1180 °C.

[0102] Preferably, in the rolling process: the reduction ratio of each pass in the first-stage rolling is 10 - 35%, and the reduction amount of at least three of the first 5 passes is not less than 8 mm.

[0103] Furthermore, in the rolling process, before the first-stage rolling, descaling can be carried out on the steel billet. Specifically, rough descaling can be carried out first after exiting the heating furnace, and fine descaling can be carried out again before entering the rolling mill.

[0104] In one embodiment, 6 ≤ t1 / t0 ≤ 12, and further controlled to 7 ≤ t1 / t0 ≤ 12.

[0105] In one example, after the first-stage rolling, the thickness t1 of the intermediate billet is 60 - 120 mm.

[0106] Preferably, in the cooling process: the average cooling rate throughout the process is preferably controlled at 28 - 33 °C / s, and more preferably controlled at 28 - 30 °C / s.

[0107] Or, the average cooling rate throughout the process can specifically be controlled at 28, 29, 30, 31, 32, 33 °C / s.

[0108] Preferably, in the cooling process: the recrystallization temperature ≤ 300 °C.

[0109] In another preferred embodiment, in the cooling process:

[0110] The hot-rolled sheet is water-cooled on the post-rolling accelerated cooling control system (full English name: Accelerated-cooling Control System, abbreviated as ACC); this post-rolling accelerated cooling control system has 24 groups of cooling headers. Among them, the 1st - 6th groups, the 7th - 18th groups, and the 19th - 24th groups respectively form the first zone, the second zone, and the third zone;

[0111] The total cooling water flow rate of the post-rolling accelerated cooling control system is 1500 - 3600 L / s, the total cooling water flow rate of the first zone is 500 - 530 L / s. All the cooling headers in the first zone and the third zone are opened, and the cooling headers in the second zone are opened alternately.

[0112] In this way, by controlling the cooling of the first zone, the second zone and the third zone, the shape of the steel plate can be improved, and the head and tail warping defects of the steel plate can be reduced.

[0113] In addition, preferably, in the heat treatment process: when the thickness t0 is more than 10 mm, the holding time can further take values in 2t0 - 14t0 min. For the thickness t0 less than 10 mm, the holding time can specifically take the value of 30 min, but not limited thereto, as long as it is more than 30 min.

[0114] Next, for the production method of this embodiment, some corresponding embodiments are provided here. The chemical compositions of the steel plates of these embodiments are shown in Table 1.

[0115] Table 1

[0116]

[0117] The specific preparation processes of the steel plates of these embodiments are as follows:

[0118] Heating process: The continuous casting billet prepared by steelmaking and continuous casting is sent into the heating furnace for heating; among them, the thickness, soaking temperature, and total heating time of the continuous casting billet are shown in Table 3 respectively;

[0119] Rolling process: After the continuous casting billet exits the heating furnace, it is first rolled in the first stage to form an intermediate billet with a thickness of t1, then kept warm, and then rolled in the second stage to form a hot-rolled plate with a thickness of t0; among them, the thickness t1, the thickness t0, the rolling start temperature of the first stage (i.e., 1 st rolling start temperature), the rolling start temperature of the second stage (i.e., 2 nd rolling start temperature), and the finishing rolling temperature (i.e., 2 nd finishing rolling temperature) are shown in Table 2 respectively;

[0120] Table 2

[0121]

[0122] Cooling process: On the ACC system, the obtained hot-rolled plate is cooled by water cooling; among them, the water inlet temperature, the final cooling temperature, the average cooling rate throughout the process, and the recarburization temperature of the hot-rolled plate are shown in Table 3; after the steel plate exits the water, it is air-cooled to room temperature to obtain a cooled plate;

[0123] Heat treatment process: Temper the obtained cooling plate; among them, the tempering temperature and holding time are shown in Table 3; after the tempering is completed, air-cool to room temperature to obtain 9Ni steel plate;

[0124] Table 3

[0125]

[0126] Perform performance testing on the obtained 9Ni steel plate, and the test results are shown in Table 4; in addition, perform metallographic structure testing on the obtained 9Ni steel plate. The metallographic structure of the steel plate in each example is a multiphase structure of tempered martensite + ferrite + reverse transformed austenite, as shown in Table 4 specifically;

[0127] Table 4

[0128]

[0129] Thus, it can be seen that the production method of the present embodiment, when producing 9Ni steel plates with a thickness of more than 8 mm, the obtained steel plates have the characteristics of high strength, high and low temperature toughness and low yield ratio, and the comprehensive performance is excellent.

[0130] Second Embodiment

[0131] This embodiment provides the implementation details of the aforementioned production method for preparing thin-gauge 9Ni steel plates.

[0132] It can be understood that the chemical composition of the 9Ni steel plate in this embodiment is the same as that in the previous text. For example, by mass percentage, it includes: C 0.04~0.075%, Si 0.15~0.33%, Mn 0.5~0.8%, Ni 8.6~9.4%, Al 0.02~0.04%, P≤0.005%, S≤0.002%, O≤0.002%, N≤0.0035%, H≤0.00015%, and the balance is iron and unavoidable impurities.

[0133] More preferably, the chemical composition of the steel plate further satisfies 68≤Ni / (C+Al)≤173, and / or further satisfies Mn=(1.8~2.5)Si+0.17%.

[0134] In this embodiment, the production method includes a process flow of a continuous casting blanking process, an ingot blooming process, a composite process, a heating process, a rolling process, a cooling process, a heat treatment process, and a plate splitting process in sequence to prepare a single-layer 9Ni steel plate with a thickness t≤8 mm.

[0135] Specifically, in the continuous casting blanking process: Prepare a continuous casting blank with a thickness of 220~250 mm through steelmaking and continuous casting. The chemical composition of this continuous casting blank is consistent with that of the finally obtained 9Ni steel plate.

[0136] Blooming process: The continuous casting billet is heated to a soaking temperature of 1200~1240°C for a total heating duration of 200~300 min; the heated continuous casting billet is rolled into a large slab with a thickness of 4.5t~10t.

[0137] Composite process: The obtained large slab is cut into multiple small slabs, and n small slabs are stacked and welded into a composite billet, where n = 2, 3 or 4.

[0138] Heating process: The obtained composite billet is heated to a soaking temperature of 1150~1200°C; the total heating duration is 160~220 min.

[0139] Rolling process: First, the billet is rolled into an intermediate billet with a thickness of t1 in the first-stage rolling, then it is temperature-held, and then the intermediate billet is rolled into a hot-rolled sheet with a thickness of t0 in the second-stage rolling; among them, the starting rolling temperature of the first-stage rolling is 1050~1090°C, and the starting rolling temperature of the second-stage rolling is (T fr -10)~(T fr +10)°C, and the finishing rolling temperature is (T fr -18)~(T fr +18)°C, 1.3≤t1 / t0≤12, and T fr is obtained from the formula 1.

[0140] Cooling process: The obtained hot-rolled sheet is cooled by water cooling. The water inlet temperature of the hot-rolled sheet is (T fc -15)~(T fc +15)°C, where T fc is obtained from the formula 2, the final cooling temperature is ≤200°C, and the average cooling rate throughout the process is 25~35°C / s; after water outlet, it is air-cooled to room temperature to obtain a cooled sheet.

[0141] Heat treatment process: The obtained cooled sheet is subjected to tempering treatment, the tempering temperature is 540~580°C, the holding time is more than 30 min, and after the tempering is completed, it is air-cooled to room temperature.

[0142] Sheet splitting process: The tempered sheet obtained in the heat treatment process is separated into n single-layer 9Ni steel sheets by trimming, that is, the required thin-specification 9Ni steel sheets are manufactured.

[0143] Preferably, for the production of single-layer thin-specification 9Ni steel sheets with a target thickness of t, t1 can satisfy 7t≤t1≤16t, and t0 can satisfy (0.8~1.2)*n*t. Preferably, t0 = n*t.

[0144] In one embodiment, 2≤t1 / t0≤8, which is further controlled to 2≤t1 / t0≤5, and further controlled to 2.5≤t1 / t0≤5, and further to 2.5≤t1 / t0≤4.

[0145] Optionally, the finally obtained n 9Ni steel plates may have substantially the same thickness of t, or may be implemented with different thicknesses.

[0146] In this embodiment, among the obtained n single-layer 9Ni steel plates, the thickness t of at least one 9Ni steel plate is below 8 mm, more preferably 3 - 8 mm, and may also be preferably 4 - 8 mm.

[0147] In one embodiment, the thickness t of a 9Ni steel plate is below 8 mm, the tensile strength is 730 - 810 MPa, the yield strength is 590 - 680 MPa, and the yield ratio ≤ 0.87; the elongation rate ≥ 23%, and the impact energy at -196 °C ≥ 210 J.

[0148] Among them, in the continuous casting billet process, specifically in the steelmaking and continuous casting methods, any feasible technology in the technical field can be adopted for implementation. For example, the process route of hot metal KR desulfurization - converter smelting - LF refining - RH refining - continuous casting can be adopted. Since it is not the invention design point of this application, no more introduction will be made.

[0149] Preferably, in the heating process: the steel billet is heated, and the heating process includes preheating, heating, and soaking. The preheating temperature is 750 - 850 °C, and the heating rate is 20 - 30 °C / min; the heating temperature is 1100 - 1200 °C, and the heating rate is 30 - 50 °C / min; the soaking temperature is 1150 - 1200 °C; the total heating duration is 160 - 220 min. Through research, it is found that after blooming, through the specific operation of this heating process, not only can the steel billet be heated to facilitate subsequent rolling, but also the yield ratio can be reduced. Specifically, in the rolling of thin-gauge steel plates, a large reduction ratio is inevitably involved, and the corresponding large reduction ratio will generate high-density dislocations and substructures, which will in turn cause a significant increase in the yield strength and lead to the problem of a high yield ratio. This is also an important reason why the defect problem of the high yield ratio of thin-gauge steel plates is more prominent than that of thick-gauge steel plates. However, through the operation in the heating process of this application, unexpectedly, the problem of excessive increase in the yield strength caused by high-density dislocations and substructures is greatly eliminated, and a low yield ratio of thin-gauge steel plates is achieved.

[0150] Preferably, in the heating process, the heating temperature can be controlled at 1100 - 1180 °C, and the soaking temperature can be controlled at 1150 - 1180 °C.

[0151] Of course, the heating process of this embodiment is not limited to this.

[0152] Preferably, in the rolling process: the reduction ratio of each pass in the first stage of rolling is 10 - 35%, and the reduction amount of at least three of the first 5 passes is not less than 8 mm.

[0153] Furthermore, in the rolling process, before the first-stage rolling, descaling can be performed on the billet. Specifically, rough descaling can be carried out after the billet exits the heating furnace, and fine descaling can be carried out before the billet enters the rolling mill.

[0154] Preferably, in the cooling process: the average cooling rate throughout the process is preferably controlled at 28 - 33 °C / s.

[0155] Alternatively, the average cooling rate throughout the process can specifically be controlled at 28, 29, 30, 31, 32, 33 °C / s.

[0156] Preferably, in the cooling process: the return red temperature ≤ 300 °C.

[0157] In another preferred embodiment, in the cooling process:

[0158] The hot-rolled plate is water-cooled on the post-rolling accelerated cooling control system (full English name: Accelerated-cooling Control System, abbreviated as ACC); this post-rolling accelerated cooling control system has 24 groups of cooling headers. Among them, the 1st - 6th groups, the 7th - 18th groups, and the 19th - 24th groups respectively form the first zone, the second zone, and the third zone;

[0159] The total cooling water flow rate of the post-rolling accelerated cooling control system is 1500 - 3600 L / s, the total cooling water flow rate of the first zone is 500 - 530 L / s. All the cooling headers in the first zone and the third zone are opened, and the cooling headers in the second zone are alternately opened.

[0160] In this way, by controlling the cooling of the first zone, the second zone, and the third zone, the shape of the steel plate can be improved, and the head and tail warping defects of the steel plate can be reduced. Especially for the preparation of the thin-gauge 9Ni steel plate in this embodiment, the shape problem is more difficult to control. Through this cooling method, the shape quality can be greatly improved.

[0161] Among them, for the "cooling headers in the second zone are alternately opened", specifically, among the 7th - 18th groups of cooling headers, the 7th, 9th, 11th... 17th groups of cooling headers are opened, while the 8th, 10th... 18th groups of cooling headers are closed.

[0162] In addition, preferably, in the heat treatment process: when the thickness t0 is more than 10 mm, the holding time can further be in the range of 2t0 - 14t0 min.

[0163] Furthermore, when the thickness t0 is more than 10 mm, the holding time can further be in the range of 2t0 - 5t0 min, and can preferably be 2t0 - 4t0 min.

[0164] When the thickness t0 is less than 10 mm, the heat preservation duration can specifically take a value of 30 min, but it is not limited thereto, as long as it is more than 30 min.

[0165] Furthermore, in this embodiment, in the composite process, n small slab blanks stacked can be welded into a composite blank by surrounding welding, spot welding or other welding methods, as long as they can form an integral whole fixed together. The present application does not limit the specific welding form and welding parameters.

[0166] Preferably, in this embodiment, in the composite process: there is an antioxidant isolation agent between two adjacent small slab blanks of the composite blank. In this way, on the one hand, the final surface quality of each steel plate can be improved, and on the other hand, it is convenient for the smooth separation between two adjacent steel plates in the subsequent plate splitting process.

[0167] The antioxidant isolation agent can specifically be implemented by using known antioxidant isolation agents in the technical field, and the present application also provides a preferred implementation manner of the antioxidant isolation agent, which is as follows.

[0168] In a preferred embodiment, the antioxidant isolation agent is a coating composed of 55 - 65 parts by mass of the hydrothermal leaching product of blast furnace slag, 7 - 15 parts by mass of corn starch, 12 - 33 parts by mass of polyvinyl alcohol, 5 - 8 parts by mass of sodium tripolyphosphate and water. By using the antioxidant isolation agent in this embodiment, on the one hand, the treatment and reuse of blast furnace slag can be realized, and on the other hand, compared with the commonly used antioxidant isolation agents in the prior art, the surface quality of the steel plate can be greatly improved and the difficulty of plate splitting can be reduced.

[0169] In one embodiment, the preparation method of the antioxidant isolation agent is as follows:

[0170] After the blast furnace slag is crushed, the slag material with the required particle size is screened out, and the slag material, hydrochloric acid and water are put into a reaction vessel, and then reacted at a temperature of 130 - 150 °C for 8 - 10 h;

[0171] The solid-phase slag is separated from the reacted system, and then the solid-phase slag is washed, ground and screened to obtain the hydrothermal leaching product of blast furnace slag;

[0172] Taking the hydrothermal leaching product of blast furnace slag, corn starch, polyvinyl alcohol and sodium tripolyphosphate as powders, the materials are prepared according to the mass ratio of water: the powders of 1:2 - 3:4;

[0173] After the water is heated to 90 - 95 °C, polyvinyl alcohol is added and stirred and left standing until it cools to 60 - 70 °C, then corn starch is added and stirred under constant temperature conditions for 0.8 - 1.5 h, and then the leaching product and sodium tripolyphosphate are added and stirred to obtain the antioxidant isolation agent.

[0174] Preferably, in the step of "after crushing the blast furnace slag, screening out the slag material with the required particle size, putting the slag material, hydrochloric acid, and water into a reaction vessel, and then reacting at a temperature of 130-150°C for 8-10 hours":

[0175] The particle size of the slag material is r mm, the concentration of hydrochloric acid is C mol / L, the volume ratio of the slag material to hydrochloric acid is 0.2-0.8, and the volume ratio of water to hydrochloric acid is 0.45-0.7, where C ranges from (ln(r*60)-0.7) to (ln(r*60)+0.7).

[0176] Here, water or hydrochloric acid can be measured by using a container with volume scales to determine the volume of water or hydrochloric acid.

[0177] The measuring method of the slag material can be: take a container with volume scales, first pour liquid (such as the measured hydrochloric acid and / or water) into the container, then put the slag material into the container and make the slag material completely submerged below the liquid level, and based on the change of the liquid level scale, determine the volume of the slag material in the container.

[0178] In this embodiment, a thin-gauge 9Ni steel plate is further provided, and its chemical composition in mass percentage includes: C 0.04-0.075%, Si 0.15-0.33%, Mn 0.5-0.8%, Ni 8.6-9.4%, Al 0.02-0.04%, P≤0.005%, S≤0.002%, O≤0.002%, N≤0.0035%, H≤0.00015%, and the balance is iron and inevitable impurities.

[0179] More preferably, the chemical composition of the thin-gauge 9Ni steel plate further satisfies 68≤Ni / (C+Al)≤173, and / or also satisfies Mn=(1.8-2.5)Si+0.17%.

[0180] Furthermore, the thickness t of the thin-gauge 9Ni steel plate is less than 8 mm, preferably 3-8 mm or even 4-8 mm, the tensile strength is 730-810 MPa, the yield strength is 590-680 MPa, and the yield ratio is ≤0.87; the elongation is ≥23%, and the impact energy at -196°C is ≥210 J.

[0181] In addition, the metallographic structure of the thin-gauge 9Ni steel plate is a multiphase structure of tempered martensite + ferrite + reverse transformation austenite, and among them: the volume fraction of tempered martensite is 75-85%, the volume fraction of reverse transformation austenite is 10-18%, and the rest is ferrite.

[0182] Next, for the production method and steel plate of this embodiment, some corresponding examples are provided here, and the chemical compositions of the steel plates in these examples are shown in Table 5.

[0183] Table 5

[0184]

[0185] The thickness t and properties of the steel plates in each embodiment are shown in Table 6; and in addition to the properties shown in Table 6, the steel plates in each embodiment also have excellent plate shape and surface quality, and the flatness tolerance of the steel plates is less than 0.8 mm / m; in addition, the metallographic structures of the steel plates are respectively as Figures 1 to 6 shown, all of which are multiphase structures of tempered martensite + ferrite + reverse transformed austenite, as specifically shown in Table 6.

[0186] Table 6

[0187]

[0188] The specific preparation processes of the steel plates in these embodiments are as follows:

[0189] Blooming process: The continuous casting billet prepared by steelmaking and continuous casting is sent into a heating furnace for heating, and the heated continuous casting billet is rolled into a large slab; among them, the soaking temperature during the heating process and the thickness of the large slab are shown in Table 7;

[0190] Composite process: The obtained large slab is cut into multiple small slabs, and n small slabs are stacked and welded into a composite billet; among them, the number n of small slabs contained in the composite billet in each embodiment is shown in Table 7;

[0191] Table 7

[0192]

[0193] Heating process: The obtained composite billet is sent into a heating furnace for heating; among them, the soaking temperature and the total heating duration are shown in Table 8 respectively;

[0194] Rolling process: After the composite billet exits the heating furnace, it is first rolled in the first stage to form an intermediate billet with a thickness of t1, then kept at a certain temperature, and then rolled in the second stage to form a hot-rolled plate with a thickness of t0; among them, the thickness t1, the thickness t0, the starting rolling temperature of the first stage rolling (i.e., 1 st starting rolling temperature), the starting rolling temperature of the second stage rolling (i.e., 2 nd starting rolling temperature), and the finishing rolling temperature (i.e., 2 nd finishing rolling temperature) are shown in Table 8 respectively;

[0195] Table 8

[0196]

[0197] Cooling process: On the ACC system, the obtained hot-rolled plate is cooled by water cooling; among them, the water inlet temperature of the hot-rolled plate, the average cooling rate throughout the process, and the recalescence temperature are shown in Table 9; after the steel plate is cooled below 200 °C, it is taken out of the water and then air-cooled to room temperature to obtain a cooled plate;

[0198] Heat treatment process: The obtained cooled plate is subjected to tempering treatment; among them, the tempering temperature and the holding time are shown in Table 9; after the tempering is completed, it is air-cooled to room temperature;

[0199] Table 9

[0200]

[0201] Plate splitting process: The tempered plate obtained in the heat treatment process is trimmed to separate the tempered plate into n single-layer steel plates, that is, the 9Ni steel plates of each embodiment shown in Tables 5 and 6 above are obtained.

[0202] Thus, it can be seen that the 9Ni steel plate of this embodiment not only has high strength and high and low temperature toughness compared with the existing thin-gauge low-temperature steel plates, but also has a low yield ratio that the existing technology does not have; furthermore, the production method of this embodiment can be used to prepare thin-gauge 9Ni steel plates, solving the technical problem of high yield ratio that could not be solved in the existing technology, and the obtained steel plates have the characteristics of high strength, high and low temperature toughness and low yield ratio at the same time, with excellent comprehensive performance.

[0203] Furthermore, in the composite process of each embodiment, an antioxidant isolation agent with the composition shown in Table 10 is also used between two adjacent small slab billets to separate the two adjacent small slab billets.

[0204] Table 10

[0205]

[0206] The preparation process of each antioxidant isolation agent is as follows:

[0207] After the blast furnace slag is crushed, slag materials with a particle size r shown in Table 10 are screened out, and the slag materials and hydrochloric acid with a concentration C shown in Table 10 are put into a reaction vessel according to a volume ratio of 1:5, and then 63% of the volume of hydrochloric acid of water is added, and then heated to 130-150 °C and held for 8-10 h;

[0208] The solid-phase slag is separated from the reaction system, and then the solid-phase slag is washed, ground and screened to obtain the hydrothermal leaching product of blast furnace slag; specifically, the ball milling process is used for grinding, the grinding balls are 5 mm SiC balls, and the ball-to-material ratio is 3:1; the screening can be carried out with a 400-mesh sieve;

[0209] Prepare a certain mass of hydrothermally leached blast furnace slag, corn starch, polyvinyl alcohol, sodium tripolyphosphate and water as shown in Table 10;

[0210] After heating the water to 90 - 95 °C, add polyvinyl alcohol and stir for 30 min. Let it stand until cooled to 60 - 70 °C, then add corn starch and stir for 0.8 - 1.5 h under constant temperature conditions. After that, add the leached material and sodium tripolyphosphate and stir to obtain the antioxidant isolation agent.

[0211] In addition, it should be noted here that in this application, the metallographic structure of the steel plate can be obtained by conducting tissue detection according to the standard of GB / T 15125 - 2009 "Metallic materials - Samples and test methods for metallographic examination"; the mechanical properties can be detected in accordance with GB / T 228.1 - 2021 "Metallic materials - Tensile testing - Part 1: Method of test at room temperature"; the low-temperature toughness can be detected in accordance with GB / T 229 - 2007 "Metallic materials - Charpy pendulum impact test method".

Claims

1. A production method of 9Ni steel plate, characterized in that, The chemical composition of the steel plate includes, by mass percentage: C 0.04 - 0.075%, Si 0.15 - 0.33%, Mn 0.5 - 0.8%, Ni 8.6 - 9.4%, Al 0.02 - 0.04%, P ≤ 0.005%, S ≤ 0.002%, O ≤ 0.002%, N ≤ 0.0035%, H ≤ 0.00015%, 68 ≤ Ni / (C + Al) ≤ 173, Mn = (1.8 - 2.5)Si + 0.17%, and the balance is iron and unavoidable impurities; The production method includes the following processes, Heating process: Heating the steel billet, with the soaking temperature being 1150 - 1200°C; Rolling process: First, the steel billet is rolled into an intermediate billet with a thickness of t1 through the first-stage rolling, then it is kept warm, and then the intermediate billet is rolled into a hot-rolled sheet with a thickness of t0 through the second-stage rolling; among them, the starting rolling temperature of the first-stage rolling is 1050~1090 °C, and the starting rolling temperature of the second-stage rolling is (T fr -10)~(T fr +10) °C, and the finishing rolling temperature is (T fr -18)~(T fr +18) °C, 1.3≤t1 / t0≤12, T fr is obtained by the following formula 1; Cooling process: The obtained hot-rolled sheet is cooled by water cooling. The water inlet temperature of the hot-rolled sheet is (T fc - 15)~(T fc + 15) °C, where T fc is obtained from the following formula 2. The final cooling temperature is ≤200 °C, and the average cooling rate throughout the process is 25~35 °C / s. After water outlet, it is air-cooled to room temperature to obtain a cooled sheet; Heat treatment process: Tempering the obtained cooling plate, with the tempering temperature being 540 - 580°C, the holding time being more than 30 min, and after the tempering ends, air cooling to room temperature; T fr = 879 - 97C + 48Si - 9.5Mn - 5.1Ni - 200Al Formula 1; T fc = 720 + 116C + 27Si - 10Mn + 6Ni - 177Al Formula 2; Among them, the element symbols in Formula 1 and Formula 2 represent the mass percentages of each element in the steel billet.

2. The production method of the 9Ni steel plate according to claim 1, characterized in that, The hot-rolled plate is a single-layer steel plate with a thickness t0 > 8 mm; The production method further includes a continuous casting billet process: Preparing a continuous casting billet with a thickness of 220 - 250 mm through steelmaking and continuous casting; And, in the heating process: Heating the obtained continuous casting billet, with the soaking temperature being 1150 - 1180°C; the total heating time being 160 - 220 min.

3. The production method of the 9Ni steel plate according to claim 1, characterized in that, The hot-rolled plate is a composite plate stacked by n single-layer plates, n takes the value of 2, 3, or 4, and the thickness of at least one single-layer plate is t; The production method further includes the following processes, Continuous casting billet process: Preparing a continuous casting billet with a thickness of 220 - 250 mm through steelmaking and continuous casting; Blooming process: Heating the continuous casting billet, with the soaking temperature being 1200 - 1240°C, the total heating time being 200 - 300 min; Rolling the heated continuous casting billet into a large slab with a thickness of 4.5t - 10t; Composite process: Cutting the obtained large slab into multiple small slabs, stacking n small slabs and welding them into a composite billet; This composite billet enters the heating process and is heated as the steel billet; Plate separation process: Separating the tempered plate obtained in the heat treatment process into n single-layer 9Ni steel plates through trimming the edges.

4. The production method of the 9Ni steel plate according to claim 3, characterized in that, In the heating process: Heating the steel billet, the heating process includes preheating, heating, and soaking, the preheating temperature is 750 - 850°C, the heating rate is 20 - 30°C / min; the heating temperature is 1100 - 1200°C, the heating rate is 30 - 50°C / min; the soaking temperature is 1150 - 1180°C; the total heating time is 160 - 220 min.

5. The production method of the 9Ni steel plate according to claim 3, characterized in that, Among the n single-layer 9Ni steel plates, the thickness t of at least one 9Ni steel plate is less than 8 mm.

6. The production method of the 9Ni steel plate according to claim 5, characterized in that, Among the n single-layer 9Ni steel plates, the thickness t of at least one 9Ni steel plate is 3 - 8 mm.

7. The production method of the 9Ni steel plate according to claim 3, characterized in that, In the composite process: There is an antioxidant isolation agent between adjacent two small slabs of the composite billet; The antioxidant isolation agent is a coating composed of 55 - 65 parts by mass of hydrothermally leached material of blast furnace slag, 7 - 15 parts by mass of corn starch, 12 - 33 parts by mass of polyvinyl alcohol, 5 - 8 parts by mass of sodium tripolyphosphate, and water.

8. The production method of the 9Ni steel plate according to claim 7, characterized in that, The preparation method of the antioxidant isolating agent comprises the following steps: After the blast furnace slag is crushed, the slag material with the required particle size is screened out. Then, the slag material, hydrochloric acid and water are put into a reaction vessel, and then reacted at a temperature of 130 - 150 °C for 8 - 10 h; The solid-phase slag is separated from the reaction system. Then, after the solid-phase slag is washed, ground and screened, the hydrothermally leached material of the blast furnace slag is obtained; Using the hydrothermally leached material of the blast furnace slag, corn starch, polyvinyl alcohol and sodium tripolyphosphate as powders, the material is prepared according to the mass ratio of water: the powders being 1:2 - 3:4; After the water is heated to 90 - 95 °C, polyvinyl alcohol is added and stirred, and left standing until cooled to 60 - 70 °C. Then, corn starch is added and stirred under constant temperature for 0.8 - 1.5 h. After that, the leached material and sodium tripolyphosphate are added and stirred to obtain the antioxidant isolating agent.

9. The production method of the 9Ni steel plate according to claim 8, characterized in that, In the step "After the blast furnace slag is crushed, the slag material with the required particle size is screened out. Then, the slag material, hydrochloric acid and water are put into a reaction vessel, and then reacted at a temperature of 130 - 150 °C for 8 - 10 h": The particle size of the slag material is r mm, the concentration of hydrochloric acid is C mol / L, the volume ratio of the slag material to hydrochloric acid is 0.2 - 0.8, and the volume ratio of water to hydrochloric acid is 0.45 - 0.7, where C ranges from (ln(r * 60) - 0.7) to (ln(r * 60) + 0.7).

10. The production method of the 9Ni steel plate according to any one of claims 1 to 9, characterized in that, In the cooling process: the return red temperature ≤ 300 °C.

11. The production method of the 9Ni steel plate according to any one of claims 1 to 9, characterized in that, In the cooling process: The hot-rolled sheet is water-cooled on the post-rolling accelerated cooling control system; The post-rolling accelerated cooling control system has 24 groups of cooling headers. Among them, the 1st - 6th groups, the 7th - 18th groups, and the 19th - 24th groups respectively form the first zone, the second zone and the third zone; The total cooling water flow rate of the post-rolling accelerated cooling control system is 1500 - 3600 L / s, the total cooling water flow rate of the first zone is 500 - 530 L / s. All the cooling headers in the first zone and the third zone are opened, and the cooling headers in the second zone are opened alternately.

12. The production method of the 9Ni steel plate according to any one of claims 1 to 8, characterized in that, In the rolling process: the reduction rate of each pass in the first stage of rolling is 10 - 35%, and the reduction amount of at least three of the first 5 passes is not less than 8 mm.

13. The production method of the 9Ni steel plate according to any one of claims 1 to 9, characterized in that, In the heat treatment process: when the thickness t0 is more than 10 mm, the heat preservation duration ranges from 2t0 to 14t0 min.

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