A 9NI STEEL PLATE FOR SHIP LNG STORAGE TANK WITH HIGH STRENGTH AND LOW YIELD RATIO AND ITS PRODUCTION METHOD

Abstract

The invention relates to a 9Ni steel plate for ship LNG storage tank with high strength and low yield ratio. According to the mass percentage, the chemical constituents are C: 0.02-0.05%, Si: 0.10-0.30%, Mn: 0.50-0.80%, Ni: 8.90-9.50%, P: ≤0.0070%, s: ≤0.0020%, Cr: 0.10-0.25%, Alt: 0.010-0.035%, Nb: 0.010-0.020%, Ca: 0.0005-0.0030%, O: ≤0.0012%, N: ≤0.004%, H: ≤0.00015%, and the balance is Fe and unavoidable impurity elements. The production process flow is: smelting in a converter or electric furnace->RH vacuum degassing->LF refining->RH high vacuum degassing->Ca Treatment->continuous casting->slab slow cooling treatment->slab surface cleaning->heating->rolling ->quenching->tempering. For the 9Ni steel, especially the 9Ni thin steel plate, the invention adopts the constituents design of low C, 9% Ni, addition of Nb and Cr. The steel plate is subject to high-temperature hot rolling, and then QLT heat treatment process to obtain 9Ni steel with good strength, toughness and low yield ratio.

Claims

1. A 9Ni steel plate with high strength and low yield ratio for ship LNG storage tank, characterized in that chemical constituents of the steel plate by mass percentage are C: 0.02-0.05%, Si: 0.10-0.30%, Mn: 0.50-0.80%, Ni: 8.90-9.50%, P: ≤0.0070%, s: ≤0.0020%, Cr: 0.10-0.25%, Alt: 0.010-0.035%, Nb: 0.010-0.020%, Ca: 0.0005-0.0030%, O: ≤0.0012%, N: ≤0.004%, H: ≤0.00015%, and the balance is Fe and unavoidable impurity elements.

2. The 9Ni steel plate with high strength and low yield ratio for ship LNG storage tank according to claim 1, characterized in that the thickness of the steel plate is 12-50 mm.

3. The 9Ni steel plate for ship LNG storage tank with high strength and low yield ratio according to claim 1 or 2, characterized in that the yield strength of the steel plate is ≥590 MPa, the tensile strength is 680-820 Mpa, the yield ratio is ≤0.90, the elongation is ≥20%, and the impact toughness at −196° C. is ≥200 J; the steel plate has a microstructure of tempered sorbite, and a ferrite lamella reaches 2 um.

4. A production method of 9Ni steel plate for ship LNG storage tank with high strength and low yield ratio, which is characterized in that it comprises the following steps: (1) Molten steel smelting: the smelting raw materials are successively processed by converter smelting, RH refining, LF refining and RH refining; (2) Casting into slab: the casting overheat is controlled at 5-25° C., and the soft reduction meets 0.35≤fs≤0.95, where fs is the fraction of solid in the slab; the center segregation of the slab is not higher than grade C1.0; (3) Slow cooling and surface cleaning of the slab: the slab is put into a pit or covered for slow cooling treatment, wherein an initial temperature of slow cooling is not lower than 600° C., the slow cooling lasts for more than 48 hours; after slow cooling, the slab surface is cleaned and polished; (4) Heating a stepping type furnace is adopted, wherein an average heating rate is controlled at 10-14cm/min, heating to 1180-1250° C., and the heating time in the range of 600-900° C. is not less than 0.32 min/mm, heat holding is started when a core temperature is consistent with a surface temperature, and the holding time is not less than 1 hour; the alloy elements are fully dissolved in the steel, to ensure uniform constituents and properties in the final product; (5) Rolling after the slab is removed out of the furnace, it is subject to high-pressure water descaling treatment, and two-stage controlled rolling including rough rolling and finish rolling, to refine the grain: a start rolling temperature of rough rolling is 1080-1150° C., the reduction rate of the last three passes of rough rolling is ≥15%, and the thickness of an intermediate slab is ≥1.8h, where H is the thickness of a finished product; high temperature rolling is adopted for the finish rolling, and a start rolling temperature is between 900-980° C.; after rolling, air cooling is adopted; (6) Quenching after rolling, the steel plate is quenched off-line, a primary quenching temperature is 840±10° C., the holding time is 30-60 min after the furnace temperature reaches the desired temperature; a secondary quenching temperature is 625±10° C., and the holding time is 30-60 min after the furnace temperature reaches the desired temperature; the quenching medium is water; (7) Tempering the quenched steel plate is tempered, wherein the tempering temperature is 560±10° C. and the holding time is 120-180 min.

5. The production method of 9Ni steel plate for ship LNG storage tank with high strength and low yield ratio according to claim 4, characterized in that step (1) molten steel smelting is based on the chemical constituents of C: 0.02-0.05%, Si: 0.10-0.30%, Mn: 0.50-0.80%, Ni: 8.90-9.50%, P: ≤0.0070%, s: ≤0.0020%, Cr: 0.10-0.25%, Alt: 0.010-0.035%, Nb: 0.010-0.020%, Ca: 0.0005-0.0030%, O: ≤0.0012%, N: ≤0.004%, H: <0.00015%, and the balance is Fe and unavoidable impurity elements.

6. The production method of 9Ni steel plate for ship LNG storage tank with high strength and low yield ratio according to claim 4, characterized in that in step (3), upper and lower surfaces of the casting slabs are polished respectively, so that 1.5 mm thick is removed from the upper and lower surfaces respectively.

7. The production method of 9Ni steel plate for ship LNG storage tank with high strength and low yield ratio according to claim 4, characterized in that in step (6), after secondary quenching, large and small sizes of initial austenite grains coexist, wherein the large grain size is ≥40 um and the small size is less than 10 um; finally, the microstructure is tempered sorbite, and the ferrite lamella reaches a maximum of 2 um.

Description

DESCRIPTION OF THE ATTACHED DRAWINGS

[0037] FIG. 1 is a typical microstructure metallographic diagram of the test steel in embodiment 2 of the present invention;

[0038] FIG. 2 shows the microstructure of the test steel in embodiment 2 of the invention after secondary quenching and the initial austenite grain morphology corroded by picric acid alcohol solution.

DETAILED DESCRIPTION OF EMBODIMENTS

[0039] The invention is described in further detail below in combination with the embodiments with the attached drawings.

[0040] The production process of 9Ni steel plate for LNG ship with high strength and low yield ratio is as follows: smelting in a converter or electric furnace->RH vacuum degassing->LF refining->RH high vacuum degassing->Ca Treatment->continuous casting->slab slow cooling treatment->slab surface cleaning->heating->rolling->quenching->tempering.

[0041] The production method of 9Ni steel plate for LNG ship with high strength and low yield ratio in embodiments 1-4 of the invention comprises the following steps: [0042] (1) Smelting: Select high-quality raw materials, use 150 t converter for smelting, send them to LF furnace for refining after RH high vacuum degassing treatment, break the air for Ca treatment, and then go through RH vacuum degassing. See Table 1 for constituents control. [0043] (2) Continuous casting: Cast molten steel into 150 mm thick continuous casting slab. Control the casting temperature at 5-25° C. above the liquidus. Dynamic soft reduction is implemented during casting. See Table 2 for continuous casting process parameters. [0044] (3) slab slow cooling treatment: continuous casting slab enters the pit for slow cooling and hydrogen expansion. See Table 2 for pit temperature and slow cooling time. After slow cooling, the slab surface shall be polished and cleaned by machine, so that the upper and lower surfaces shall be cleaned up by removing 1.5 mm thick respectively. [0045] (4) Heating: Put the continuous casting slab obtained in step (3) into the stepping type furnace, with an average heating rate of 10-14 cm/min, and heat it to 1180-1250° C. Heat holding shall be started when the core temperature reaches the surface temperature, and the heat holding time shall not be less than 1 hour. The alloy elements in the steel are fully dissolved to ensure the uniformity of constituents and properties of the final product. The heating time in the temperature range of 600-900° C. shall be controlled to be ≥0.32 min/mm. [0046] (5) Rolling: after the slab is removed out of the furnace and descaled by high-pressure water, the two-stage controlled rolling of rough rolling and finish rolling is carried out. The start temperature of rough rolling is 1080-1150° C., and the reduction rate of the last three passes of rough rolling is ≥12%. The thickness of an intermediate slab ≥1.8h, where h is the thickness of finished product. High temperature rolling is adopted for finish rolling. The start temperature of finish rolling is 900-980° C., and the end temperature of rough rolling is ≥820° C. After rolling, the steel plate is cooled by air without accelerated cooling by ACC unit. See Table 3 for relevant process parameters. [0047] (6) Quenching: The quenching temperature of steel plate is 840±10° C., and the holding time after the furnace temperature reaches the temperature is 30-60 min; The secondary quenching temperature is 625±10° C., and the holding time after the furnace temperature reaches the temperature is 30-60 min; The quenching medium is water. [0048] (7) Tempering: the tempering temperature of steel plate is 560±10° C., and the holding time is 120-180 min. [0049] (8) After tempering, the steel plate shall be subjected to transverse tensile and transverse impact tests.

[0050] See table 1-3 for specific constituents and process parameters. See Table 4 for the corresponding properties of each embodiment.

[0051] FIGS. 1 and 2 show the microstructure photos of the test steel in embodiment 1 and 2. The microstructure of the finished steel plate is tempered sorbite structure. After secondary quenching, the large grain size of the original austenite is ≥40 um and the small grain size is less than 10 um. It can be seen that through two-stage controlled rolling &cooling and the selection of appropriate quenching process parameters, multi-size original austenite grains can be obtained, the yield ratio of steel plate can be reduced and the elongation of steel plate can be improved. In the tempering process, close to AC.sub.1 point, conduct heat preservation treatment for a long time to coarsen the ferrite layer. Further reduce the yield ratio and improve the elongation.

[0052] The invention adopts high-temperature controlled rolling and off-line quenching+tempering process, which is controlled from the aspects of chemical constituents design, base metal structure, inclusions, center segregation, quenching and tempering temperature and time, so as to ensure that the elongation and −196° C. low-temperature impact toughness of the steel are good while realizing ultra-high strength, and achieve the purpose of reducing the low yield strength ratio of thin 9Ni steel plate.

TABLE-US-00001 TABLE 1 chemical composition of super strong steel plate in embodiment (wt%) Embodiment C Si Mn P S Cr Ni Nb Al Ca O N H 1 0.03 0.13 0.70 0.0033 0.0003 0.15 9.35 0.0145 0.025 0.0009 0.0009 0.00189 0.0001 2 0.02 0.18 0.72 0.0039 0.0007 0.22 9.45 0.0160 0.025 0.0012 0.0007 0.00171 0.0001 3 0.03 0.28 0.69 0.0048 0.0007 0.10 9.34 0.0134 0.021 0.0016 0.0010 0.00185 0.0001 4 0.04 0.16 0.78 0.0047 0.0006 0.18 9.10 0.0134 0.021 0.0016 0.0006 0.00191 0.0001

TABLE-US-00002 TABLE 2 continuous casting process control Start Dynamic temperature slab soft of slow Hydrogen Embodi- thickness, Overheat, reduction, cooling, expansion ment mm ° C. fs ° C. time, hour 1 150 25 0.35-0.95 690 48 2 150 18 0.35-0.95 650 48 3 150 15 0.35-0.95 680 48 4 150 15 0.35-0.95 700 48

TABLE-US-00003 TABLE 3 rolling process control Product Reduction rate Thickness Start Ending thickness of three passes to be temperature temperature specification, slab tapping after rough heated, of finish of finish Embodiment mm temperature, ° C. rolling mm rolling, ° C. rolling, ° C. 1 12 1230 26% + 26% + 26% 26 823 829 2 16 1220 23% + 25% + 26% 32 920 826 3 30 1230 16% + 17% + 16% 60 984 40 4 50 1220 12% + 15% + 16% 90 860 625

TABLE-US-00004 TABLE 4 Transverse tensile and transverse impact properties of embodiments of the invention Tensile Yield Tensile Thickness, strength, strength, Yield Elongation, Embodiment mm MPa MPa ratio % Impact energy, Akv, J 1 12 672 755 0.89 22 −196° C. 240 235 233 2 16 671 746 0.90 23 −196° C. 211 229 242 3 32 684 778 0.88 21 −196° C. 213 208 225 4 50 669 752 0.89 23 −196° C. 205 237 215

[0053] Although the preferred embodiments of the invention have been described in detail above, it should be clearly understood that the invention may have various modifications and changes or those skilled in the art. Any modification, equivalent replacement, improvement, etc. made within the spirit and principles of the invention shall be included in the protection scope of the invention.