GB2622886A

GB2622886A - Low temperature impact resistant steel produced by continuous variable crown (CVC) steckel mill and preparation method and use thereof

Info

Publication number: GB2622886A
Application number: GB2216477.6A
Authority: GB
Inventors: Zhao Xianpeng; Liu Peng; Qiao Song; Qi Tiansheng; Han Qibiao
Original assignee: SD Steel Rizhao Co Ltd
Current assignee: SD Steel Rizhao Co Ltd
Priority date: 2022-09-27
Filing date: 2022-11-04
Publication date: 2024-04-03
Also published as: CN115739995A; GB202216477D0

Abstract

A method of making steel plate suitable for low temperature applications such as natural gas storage tanks by continuous casting a billet which comprises (by weight): 0.05-0.09 % carbon, 1.35-1.65 % manganese, 0.10-0.30 % silicon, 0.02-0.04 % niobium, 0.015-0.025 % titanium, 0.15-0.20 % chromium and 0.02-0.055 % aluminium, with the balance being iron and impurities. The billet is then sequentially heated, flat rolled, coiled and rolled in a CVCplus Steckel mill 4 with 5-7 rolling passes. The rolling reductions of the last three passes through the CVCplus Steckel mill 4 respectively are 19-21 %, 16-19 % and 11-13 %. The finished steel plate is then cooled at a rate of 5-10 0C/sec to form a ferrite-pearlite microstructure with a small amount of bainite. The steel is heated in coiling furnaces 3 & 5 to a temperature in the range 860-930 0C between rolling passes, with an initial rolling temperature in the range 990-1030 0C and a final rolling temperature in the range 780-862 0C.

Description

LOW TEMPERATURE IMPACT RESISTANT STEEL PRODUCED BY CONTINUOUS VARIABLE CROWN (CVC) STECKEL MILL AND PREPARATION METHOD AND

USE THEREOF

TECHNICAL FIELD

100011 The present disclosure belongs to the technical field of pressure vessels, and particularly relates to a low temperature impact resistant steel and a preparation method and use thereof

BACKGROUND

[0002] With the development of the green concept "carbon neutral", traditional coal and oil resources are gradually replaced by renewable natural gas. Along with the increase in natural gas demand, a large number of natural gas storage tanks are required. The natural gas storage tank is a low-temperature pressure vessel. The skin steel for its top structure needs to have a thickness of 4-6 mm and a width of 2,200-3,050 mm, and needs to be resistant to low-temperature (-40°C) impact. In order to meet the above technical requirements, temperature controlled rolling and controlled cooling are required. Rolling is extremely difficult, and the control difficulty of steel plate is unprecedented.

[0003] During the production of wide and thin steel plates by using the traditional medium and thick plate or wide and thick plate rolling line, limited by the high temperature drop rate of rolled pieces in the rolling process, steel plates become thinner and thinner in the rolling process, at the same time, the rolled pieces become longer, and the steel plates will see a rapid temperature drop. In the prior art, the skin steel for the top structure of the natural gas storage tank that meets the requirements can only be prepared by implementing two heating operations plus two rolling operations. However, the existing process has low production efficiency and poor production stability, making it difficult to achieve large-scale production.

SUMMARY

[0004] In view of this, the present disclosure provides a low temperature impact resistant steel and a preparation method and use thereof The present disclosure can efficiently produce the low temperature impact resistant steel by using the CVCplus steckel mill, and the preparation method provided by the present disclosure can enable large-scale production of the low temperature impact resistant vessel steel.

[0005] To solve the above technical problem, the present disclosure provides a preparation method for a low temperature impact resistant steel, including the following steps.

100061 conducting heating and flat rolling on a continuous casting billet in sequence, and coiling the continuous casting billet with a CVCplus steckel mill to obtain a steel plate, where the continuous casting billet includes the following chemical components with mass percentage: 0.05-0.09% of C, 1.351.65% of Mn, 0.10-0.30% of Si, 0.02-0.04% of Nb, 0.15-0.025% of Ti, 0.15%-0.20% of Cr, and the balance of Fe; and the coiling is conducted for 5-7 rolling passes, and the coiling has a reduction rate of 11-13% at a last rolling pass, a reduction rate of 16-19% at a second last rolling pass, and a reduction rate of 19-21% at a third last rolling pass; and 100071 cooling the steel plate at a rate of 5-10C/s to obtain the low temperature impact resistant steel.

[0008] Preferably, the CVCplus steckel mill includes a coiling furnace and a four-high reversing mill. The coiling furnace includes a front coiling furnace and a rear coiling furnace.

[0009] A strip steel between a drum of the coiling furnace and the four-high reversing mill has strip tension of 18-25 t.

[0010] Preferably, the coiling furnace has a temperature of 860-930°C.

[0011] Preferably, a working roll of the CVCplus steckel mill moves axially, and the CVC has an axial movement within 70 mm during last three rolling passes of coiling.

100121 Preferably, the coiling has a total reduction rate of 50-85%.

[0013] The coiling is conducted at an initial rolling temperature of 990-1,030°C and a final rolling temperature of 780-862'C.

100141 Preferably, the flat rolling is conducted for 6-8 rolling passes.

[0015] The flat rolling is conducted at an initial rolling temperature greater than or equal to 1,020°C.

100161 Preferably, the heating is conducted at 1,230-1,260t with a heating coefficient greater than or equal to 10.0 min/cm.

[0017] Heating equipment includes a holding section, and the holding section has temperature uniformity less than or equal to 10°C and conducts heat preservation for no less than 25 min. [0018] Preferably, the steel plate has a self-tempering temperature of 620-690°C after cooling. [0019] The present disclosure further provides a low temperature impact resistant steel prepared by the preparation method according to the above technical solutions, including the following chemical components with mass percentage: 100201 0.05-0.09% of C; [0021] 1.35-1.65% of Mn; [0022] 0.10-0.30% of Si; [0023] 0.02-0.04% of Nb; 100241 0.15-0.025% of Ti; 100251 0.02-0.055% of Al; 100261 0.15-0.20% of Cr; and [0027] the balance of Fe and impurities.

[0028] The low temperature impact resistant steel has a thickness of 4-6 mm and a width of 2,200-3,050 mm.

100291 The low temperature impact resistant steel has impact toughness greater than or equal to 50 J at -40t.

100301 The present disclosure further provides use of the low temperature impact resistant steel according to the above technical solutions as a skin steel for a top structure of a natural gas storage tank.

[0031] The present disclosure provides a preparation method for a low temperature impact resistant steel, including the following steps: conducting heating and flat rolling on a continuous casting billet in sequence, and coiling the continuous casting billet with a CVCplus steckel mill to obtain a steel plate, where the continuous casting billet includes the following chemical components: carbon, manganese, niobium, titanium, chromium, silicon, and iron, the coiling is conducted for 5-7 rolling passes, and the coiling has a reduction rate of 11-13% at a last rolling pass, a reduction rate of 16-19% at a second last rolling pass, and a reduction rate of 19-21% at a third last rolling pass; and cooling the steel plate at a rate of 5-10t/s to obtain the low temperature impact resistant steel. The low temperature impact resistant steel provided by the present disclosure can be used as the low temperature impact resistant vessel steel. The present disclosure uses the CVCplus steckel mill for coiling, and the coiling furnace is arranged in front and rear of the mill for heat preservation, which improves the preparation efficiency of the low temperature impact resistant steel and realizes the large-scale production of the low temperature impact resistant vessel steel. The present disclosure improves the low temperature impact toughness of the low temperature impact resistant vessel steel by adding niobium, and improves the mechanical strength of the low temperature impact resistant vessel steel by adding titanium and chromium. The low temperature impact resistant vessel steel with a thickness of 4-6 mm and a width of 2,200-3,050 mm can be efficiently prepared according to the preparation method provided by the present disclosure, and the steel has impact toughness greater than or equal to 50 Akv/J at -40C,

BRIEF DESCRIPTION OF THE DRAWINGS

[0032] FIG. 1 is a system diagram of preparing a low temperature impact resistant steel with a continuous casting billet as an treatment object, where 1 is a heating furnace, 2 is a descaling machine, 3 is a front coiling furnace, 4 is a four-high reversing mill, 5 is a rear coiling furnace, and 6 is a cooling system; 100331 FIG. 2 is a metallographic stnicture diagram of a low temperature impact resistant vessel steel prepared in Example 1; and [0034] FIG. 3 is a metallographic structure diagram of a low temperature impact resistant vessel steel prepared in Example 2.

DETAILED DESCRIPTION OF THE EMBODIMENTS

100351 The present disclosure provides a preparation method for a low temperature impact resistant steel, including the following steps.

[0036] Heating and flat rolling are conducted on a continuous casting billet in sequence, and the continuous casting billet is coiled with a CVCplus steckel mill to obtain a steel plate. The continuous casting billet includes the following chemical components: carbon, manganese, niobium, titanium, chromium, silicon, and iron. The coiling is conducted for 5-7 rolling passes, and the coiling has a reduction rate of 11-13% at a last rolling pass, a reduction rate of 16-19% at a second last rolling pass, and a reduction rate of 19-21% at a third last rolling pass.

100371 The steel plate is cooled at a rate of 5-10 'Cis to obtain the low temperature impact resistant steel.

[0038] The present disclosure conducts heating and flat rolling on the continuous casting billet in sequence, and coils the continuous casting billet with the CVCplus steckel mill to obtain the steel plate. In the present disclosure, the continuous casting billet includes the following chemical components: carbon, manganese, niobium, titanium, chromium, silicon, and iron. In the present disclosure, a preparation method for the continuous casting billet preferably includes the following steps.

[0039] The hot metal, carbon, manganese, niobium, titanium, chromium, and silicon are mixed for steelmaking to obtain molten steel.

[0040] The molten steel is cast into a billet to obtain the continuous casting billet.

[0041] The present disclosure mixes the hot metal, the carbon, the manganese, the niobium, the titanium, the chromium, and the silicon for steelmaking to obtain the molten steel. The present disclosure has no special requirements for the preparation method for the hot metal, and the conventional method in the art can be used. In the process of steelmaking, the present disclosure preferably conducts hot metal pretreatment desulfurization, converter deep desulfurization, ladle furnace (LF) deep desulfurization, and ultra-low residual element control. The residual elements include one or more selected from the group consisting of As, Sb, Sn, Pb, Bi, B, and Ca. The present disclosure has no special requirements for the hot metal pretreatment desulfurization, converter deep desulfurization, LF deep desulfurization, and ultra-low residual element control, and the conventional method in the art can be adopted. In the present disclosure, the molten steel has a temperature of preferably 1,500-1,600°C, more preferably 1,548°C.

[0042] In the present disclosure, phosphorus in the continuous casting billet has a mass percentage preferably less than or equal to 0.008%. Sulfur in the continuous casting billet has a mass percentage preferably less than or equal to 0.002%. B in the continuous casting billet has a mass percentage preferably less than or equal to 0.0005%. As in the continuous casting billet has a mass percentage preferably less than or equal to 0.03%. Sb in the continuous casting billet has a mass percentage preferably less than or equal to 0.010%. Sn in the continuous casting billet has a mass percentage preferably less than or equal to 0.020%. Pb in the continuous casting billet has a mass percentage preferably less than or equal to 0.010%. Bi in the continuous casting billet has a mass percentage preferably less than or equal to 0.010%.

[0043] The molten steel is cast into a billet to obtain the continuous casting billet. In the present disclosure, the continuous casting billet has a thickness of preferably 140-160 mm, more preferably 150 mm, and a width of preferably 2,200-3,250 mm, more preferably 3,100-3,200 mm, and further preferably 3,150 mm. In the present disclosure, the equipment for casting billet is preferably a continuous casting machine. In the present disclosure, the molten steel in the continuous casting tundish has superheat of preferably 12-16°C. In the present disclosure, the dynamic soft reduction technology is preferably used in the casting process, and the present disclosure improves the central segregation and looseness of the continuous casting billet by using the dynamic soft reduction technology. In the present disclosure, the central segregation of the continuous casting billet is preferably of grade C0.5 or grade C1.0.

[0044] In the present disclosure, the heating is conducted at preferably 1,230-1,260°C, more preferably 1,239-1,245°C. In the present disclosure, the heating is conducted with a heating coefficient preferably greater than or equal to 10.0 min/cm, more preferably 10.5-11 min/cm. In the present disclosure, heating equipment preferably includes a holding section, and the holding section has temperature uniformity preferably less than or equal to 10°C, more preferably 7-9 °C, and conducts heat preservation for preferably no less than 25 min, more preferably 32-35 min. 100451 The present disclosure can improve the rolling plasticity and the solid solution of the alloy after heating, thereby improving the impact toughness of the low temperature impact resistant steel.

100461 In the present disclosure, the methods preferably further includes: descaling the steel plate before the flat rolling. The present disclosure has no special requirements for the descaling, and the conventional method in the art can be adopted.

[0047] In the present disclosure, the flat rolling is conducted for preferably 6-8 rolling passes, more preferably 6-7 rolling passes. In the present disclosure, the flat rolling is conducted at an initial rolling temperature preferably greater than or equal to 1,020°C, more preferably 1,050-1,100°C. In the present disclosure, the flat rolling is preferably automatically distributed by a secondary model.

[0048] In the present disclosure, the CVCplus steckel mill includes a coiling furnace and a four-high reversing mill. The coiling furnace includes a front coiling furnace and a rear coiling furnace. A strip steel between a drum of the coiling furnace and the four-high reversing mill has strip tension of preferably 18-25 t, more preferably 22-25 t. In the present disclosure, the CVCplus steckel mill has a roll bending force of preferably 500-1,000 t, more preferably 600-900 t. The present disclosure preferably increases or decreases the roll bending force according to the rolled plate type of the last three rolling passes of coiling, specifically increasing the roll bending force when the rolled plate type is an edge wave plate type, and decreasing the roll bending force when the rolled plate type is a medium wave plate type. In the present disclosure, the coiling furnace has a temperature of preferably 860-950°C, more preferably 920-940°C, and further preferably 930°C.

[0049] In the present disclosure, the coiling is conducted for preferably 5-7 rolling passes, preferably 6-7 rolling passes. The coiling has a total reduction rate of preferably 50-85%, more preferably 60-75%. In the present disclosure, the coiling has a reduction rate of 11-13% at a last rolling pass, preferably 11.0342.01%, a reduction rate of 16-19% at a second last rolling pass, preferably 16.58-18.32%, and a reduction rate of 19-21% at a third last rolling pass, preferably 19.37-20.21%. In the present disclosure, the steel plate has a thickness preferably less than or equal to 25 mm at the beginning of coiling. The present disclosure automatically sets the total reduction rate of coiling through the secondary model to obtain the steel plate with the required thickness.

100501 In the present disclosure, a working roll of the CVCplus steckel mill moves axially, and the CVC has an axial movement within 70 mm, more preferably 20-50 mm, during last three rolling passes of coiling.

[0051] In the present disclosure, the coiling is conducted at an initial rolling temperature of preferably 990-1,030°C, more preferably 1,000-1,020°C, and a final rolling temperature of preferably 780-862t, more preferably 791-858t.

[0052] After the steel plate is obtained, the present disclosure cools the steel plate at a rate of preferably 5-10t/s, more preferably 6-8°C/s, to obtain the low temperature impact resistant steel. The present disclosure preferably adopts a high-density gap nozzle for cooling, which can improve the cooling efficiency and improve the uniformity of the structure in the low temperature impact resistant steel.

100531 By virtue of the advantages of the coiling process of the coiling furnace in front and rear of the mill, the present disclosure rationally distributes the coiling passes and the pass reduction rate to ensure the stability of the rolled plate type.

[0054] The present disclosure uses the CVC steckel mill coiling technology, and through the heat preservation function of the two coiling furnaces in front and rear of the mill, takes into account the three requirements of plate type, thickness accuracy and performance, which can realize stable and high-quality production of a low temperature vessel steel for the top of a liquefied natural gas (LNG) tank with a thickness of 4-6 mm and a width of 2,500-3,050 mm, has high production efficiency and stable quality, and has a qualified rate no less than 99.5%. [0055] In the present disclosure, the steel plate has a self-tempering temperature of preferably 620-690°C, more preferably 653-668°C, after cooling.

[0056] In the present disclosure, the methods preferably further includes shearing, cold straightening and flaw detection of the cooled plate in sequence after the cooling. The present disclosure has no special requirements for the shearing, cold straightening and flaw detection, and the conventional method in the art can be adopted.

[0057] The low temperature impact resistant steel prepared according to the preparation method of the present disclosure has excellent thickness uniformity, and a single steel plate has thickness tolerance of preferably 0-0.2 mm, more preferably 0.08-0.1 mm.

[0058] FIG. 1 is a system diagram of preparing a low temperature impact resistant steel with a continuous casting billet as an treatment object, where 1 is a heating furnace, 2 is a descaling machine, 3 is a front coiling furnace, 4 is a four-high reversing mill, 5 is a rear coiling furnace, and 6 is a cooling system. The present disclosure uses the four-high reversing mill for the flat rolling and the coiling.

[0059] The present disclosure further provides a low temperature impact resistant steel prepared by the preparation method according to the above technical solutions, including the following chemical components with mass percentage: [0060] 0.05-0.09% of C; [0061] 1.35-1.65% of Mn; [0062] 0.10-0.30% of Si; [0063] 0.02-0.04% of Nb; [0064] 0.15-0.025% of Ti; [0065] 0.02-0.055% ofAl; [0066] 0.15-0.20% of Cr; and [0067] the balance of Fe and impurities.

[0068] In the present disclosure, by mass percentage, the low temperature impact resistant steel includes 0.05-0.09%, preferably 0.071-0.077%, of C. [0069] In the present disclosure, by mass percentage, the low temperature impact resistant steel includes 1.35-1.65%, preferably 1.57-1.59%, of Mn.

[0070] In the present disclosure, by mass percentage, the low temperature impact resistant steel includes 0.10-0.30%, preferably 0.22-0.25%, of Si.

[0071] In the present disclosure, by mass percentage, the low temperature impact resistant steel includes 0.02-0.04%, preferably 0.033-0.036%, of Nb.

[0072] In the present disclosure, by mass percentage, the low temperature impact resistant steel includes 0.15-0.025%, preferably 0.017-0.018%, of Ti.

[0073] In the present disclosure, by mass percentage, the low temperature impact resistant steel includes 0.02-0.055%, preferably 0.03-0.04%, of Al.

[0074] In the present disclosure, by mass percentage, the low temperature impact resistant steel includes 0.15-0.20%, preferably 0.17-0.18%, of Cr.

[0075] The present disclosure improves the low temperature impact toughness of the low temperature impact resistant steel by decreasing the mass percentage of C and increasing the mass percentage of Nb in the low temperature impact resistant steel, and improves the mechanical strength of the low temperature impact resistant steel by adding Cr and Ti.

[0076] In the present disclosure, the low temperature impact resistant steel includes the balance of Fe and impurities by mass percentage. In the present disclosure, the impurities include residual elements, nitrogen, phosphorus and sulfur, and the residual elements include one or more selected from the group consisting of As, Sb, Sn, Pb, Bi, B, and Ca. In the present disclosure, phosphorus in the low temperature impact resistant steel has a mass percentage preferably less than or equal to 0.008%, more preferably 0.007%. Sulfur in the low temperature impact resistant steel has a mass percentage preferably less than or equal to 0.002%, more preferably 0.001%. Nitrogen in the low temperature impact resistant steel has a mass percentage content of preferably 32-33 ppm. B in the low temperature impact resistant steel has a mass percentage preferably less than or equal to 0.0005%, preferably 0.0003-0.0004%. As in the low temperature impact resistant steel has a mass percentage preferably less than or equal to 0.03%, preferably 0.003-0.004%. Sb in the low temperature impact resistant steel has a mass percentage preferably less than or equal to 0.010%, preferably 0.001%. Sn in the low temperature impact resistant steel has a mass percentage preferably less than or equal to 0.020%, preferably 0.001%. Pb in the low temperature impact resistant steel has a mass percentage preferably less than or equal to 0.010%, preferably 0.001%. Bi in the low temperature impact resistant steel has a mass percentage preferably less than or equal to 0.010%. Ca in the low temperature impact resistant steel has a mass percentage of preferably 0.0013-0.0014%.

100771 In the present disclosure, the low temperature impact resistant steel has a carbon equivalent value (CEV) preferably less than or equal to 0.4% and a welding cold crack sensitivity index preferably less than or equal to 0.22%. The low temperature impact resistant steel provided by the present disclosure has excellent welding performance.

100781 In the present disclosure, the low temperature impact resistant steel has a thickness of 4-6 mm, preferably 5-6 mm, and a width of 2,200-3,050 mm, preferably 2,500-3,050 mm. In the present disclosure, the low temperature impact resistant steel has impact toughness greater than or equal to 50 J, preferably 117-120 J, at -40C.

100791 In the present disclosure, the low temperature impact resistant steel has excellent mechanical strength, and the low temperature impact resistant steel has yield strength of preferably 381-390 MPa, tensile strength of preferably 567-572 N1Pa, and an elongation of preferably 27-27.5%.

[0080] The present disclosure further provides use of the low temperature impact resistant steel according to the above technical solutions as a skin steel for a top structure of a natural gas storage tank.

[0081] In order to further illustrate the present disclosure, the technical solutions provided by the present disclosure are described in detail below in connection with examples, but these examples should not be understood as limiting the claimed scope of the present disclosure.

[0082] Example 1

[0083] After pretreatment and desulfurization, hot metal was mixed with carbon, manganese, niobium, titanium, chromium, and silicon for steelmaking (converter deep desulfurization, LF deep desulfurization, and ultra-low residual element control) to obtain molten steel at 1,548C. The molten steel was transferred to a continuous casting tundish to control the overheating temperature at 12-16t, and a billet was cast (using a dynamic soft reduction technology) to obtain a continuous casting billet with a thickness of 150 mm and a width of 3,150 mm. The central segregation of the continuous casting billet was CO.5.

[0084] The continuous casting billet was heated at 1,239t with a heating coefficient of 10.5 min/cm, and a holding section in heating equipment had temperature uniformity of 8V. The continuous casting billet was subjected to heat preservation for 32 min in the holding section. The heated continuous casting billet was descaled.

[0085] After descaling, the continuous casting billet was subjected to flat rolling for 6 rolling passes at an initial rolling temperature of 1,020t.

[0086] The plate after flat rolling was subjected to coiling for 7 rolling passes using a CVCplus steckel mill to obtain a steel plate. In the CVCplus steckel mill, a strip steel between a drum of the coiling furnace and a four-high reversing mill had strip tension of 22 t. The CVCplus steckel mill had a roll bending force of 800 t, and the coiling furnace had a temperature of 920t. The coiling was conducted at an initial rolling temperature of 1,000V and a final rolling temperature of 780-858t. At the beginning of coiling, the steel plate had a thickness of 25 mm, the coiling had a total reduction rate of 73%, and the coiling had a reduction rate of 12.11% at a last rolling pass, a reduction rate of 18.32% at a second last rolling pass, and a reduction rate of 20.21% at a third last rolling pass. The CVC had an axial movement of 45 mm during last three rolling passes of coiling.

[0087] The steel plate was cooled with a high-density gap nozzle at a cooling rate of 6t/s and had a self-tempering temperature of 668t. The cooled plate was subjected to shearing, cold straightening and flaw detection in sequence to obtain a low temperature impact resistant vessel steel plate. The low temperature impact resistant vessel steel plate had a thickness of 5.5 mm and a width of 3,000 mm. The low temperature impact resistant vessel steel plate included 0.071% of C, 1.57% of Mn, 0.007% of P, 0.001% of S, 0.22% of Si, 0.03% of Al, 0.033% of Nb, 0.018% of Ti, 0.17% of Cr, 33 ppm of N, 0.001% of Pb, 0.001% of Sb, 0.001% of Sn, 0.003% of As, 0.0014% of Ca, 0.0003% of B, and the balance of Fe and impurities. The low temperature impact resistant vessel steel plate had a CEV of 0.37% and a welding cold crack sensitivity index of 0.17%.

100881 Example 2

[0089] After pretreatment and desulfurization, hot metal was mixed with carbon, manganese, niobium, titanium, chromium, and silicon for steelmaking (converter deep desulfurization, LF deep desulfurization, and ultra-low residual element control) to obtain molten steel at 1,548V C. The molten steel was transferred to a continuous casting tundish to control the overheating temperature at 12-16°C, and a billet was cast (using a dynamic soft reduction technology) to obtain a continuous casting billet with a thickness of 150 mm and a width of 3,100 mm. The central segregation of the continuous casting billet was C1.0.

[0090] The continuous casting billet was heated at 1,245°C with a heating coefficient of 10.5 min/cm, and a holding section in heating equipment had temperature uniformity of 8V. The continuous casting billet was subjected to heat preservation for 35 min in the holding section. The heated continuous casting billet was descaled.

[0091] After descaling, the continuous casting billet was subjected to flat rolling for 6 rolling passes at an initial rolling temperature of 1,020t.

100921 The plate after flat rolling was subjected to coiling for 7 rolling passes using a CVCplus steckel mill to obtain a steel plate. In the CVCplus steckel mill, a strip steel between a drum of the coiling furnace and a four-high reversing mill had strip tension of 25 t. The CVCplus steckel mill had a roll bending force of 700 t, and the coiling furnace had a temperature of 940t. The coiling was conducted at an initial rolling temperature of 1,000V and a final rolling temperature of 791-862'C. At the beginning of coiling, the steel plate had a thickness of 25 mm, the coiling had a total reduction rate of 72.7%, and the coiling had a reduction rate of 11.03% at a last rolling pass, a reduction rate of 16.58% at a second last rolling pass, and a reduction rate of 19.37% at a third last rolling pass. The CVC had an axial movement of 30 mm during last three rolling passes of coiling.

[0093] The steel plate was cooled with a high-density gap nozzle at a cooling rate of 6t/s and had a self-tempering temperature of 653°C. The cooled plate was subjected to shearing, cold straightening and flaw detection in sequence to obtain a low temperature impact resistant vessel steel plate. The low temperature impact resistant vessel steel plate had a thickness of 6 mm and a width of 3,050 mm. The low temperature impact resistant vessel steel plate included 0.077% of C, 1.59% of Mn, 0.007% of P, 0.001% of S, 0.25% of Si, 0.03% of Al, 0.036% of Nb, 0.017% of Ti, 0.18% of Cr, 32 ppm of N, 0.382% of CEV, 0.17% of Pcm, 0.001% of Pb, 0.001% of Sb, 0.001% of Sn, 0.004% of As, 0.0013% of Ca, 0.0004% of B, and the balance of Fe and impurities. The low temperature impact resistant vessel steel plate had a CEV of 0.382% and a welding cold crack sensitivity index of 0.17%.

[0094] The low temperature impact resistant vessel steel prepared in Examples 1 and 2 was cut at 1/4 of the thickness, and the metallographic structure of the cut surface was observed to obtain a metallographic structure diagram, as shown in FIG. 2 to FIG. 3. FIG. 2 is a metallographic structure diagram of the low temperature impact resistant vessel steel prepared in Example I. FIG. 3 is a metallographic structure diagram of the low temperature impact resistant vessel steel prepared in Example 2. It can be seen from FIG. 2 and FIG. 3 that the low temperature impact resistant vessel steel plate provided by the present disclosure had a uniform and fine structure, a grain size of 10.0, a structure type of ferrite + pearlite, a small amount of bainite, and a banded structure grade of 0.5.

[0095] Comparative Example 1 [0096] A steel plate was prepared according to the method of Example 1, with the difference that the coiling had a reduction rate of 8.1% at a last rolling pass, a reduction rate of 14.3% at a second last rolling pass, and a reduction rate of 17.1% at a third last rolling pass, and at the same time, the CVC function was not applied, and the working roll of the CVCplus steckel mill did not move axially.

[0097] Comparative Example 2 [0098] A steel plate was prepared according to the method of Example 1, with the difference that the CVC had an axial movement of 90 mm during last three rolling passes of coiling, and cooling was conducted at a rate of 3 t/s.

[0099] The mechanical properties of the low temperature impact resistant vessel steel prepared in Examples 1 and 2 and Comparative Examples 1 and 2 were tested according to GBT 713-2014 and GBT 3531-2014 The results are listed in Table 1.

[0100] Table 1 Mechanical properties of steel plates prepared in Examples 1 and 2 and Com arative Exam les 1 and 2 Example Yield Tensile Elongation Impact Axial Thickness tolerance strength strength (%) energy at movement of of corresponding (MPa) (MPa) -40'C (J) CVC (mm) single steel plate (mm) Example 1 381 567 27 120 45 0.1 Example 2 390 572 27.5 117 30 0.08 Comparative Example 1 325 497 36 47 Without 0.63 application Comparative Example 2 330 507 38 25 90 0.52 [0101] It can be seen from Table 1 that the low temperature impact resistant steel provided by the present disclosure has excellent mechanical properties and thickness uniformity, and can be used as the low temperature impact resistant vessel steel.

[0102] Although the above examples have described the present disclosure in detail, they are only a part of, not all of, the examples of the present disclosure. Other examples may also be obtained by persons based on the examples without creative efforts, and all of these examples shall fall within the protection scope of the present disclosure.

Claims

WHAT IS CLAIMED IS: 1. A preparation method for a low temperature impact resistant steel, comprising the following steps: conducting heating and flat rolling on a continuous casting billet in sequence, and coiling the continuous casting billet with a CVCplus steckel mill to obtain a steel plate, wherein the continuous casting billet comprises the following chemical components with mass percentage: 0.05-0.09% of C, 1.35-1.65% of Mn, 0.10-0.30% of Si, 0.02-0.04% of Nb, 0.15-0.025% of Ti, 0.15%-0.20% of Cr, and the balance of Fe; and the coiling is conducted for 5-7 rolling passes, and the coiling has a reduction rate of 11-13% at a last rolling pass, a reduction rate of 16-19% at a second last rolling pass, and a reduction rate of 19-21% at a third last rolling pass; and cooling the steel plate at a rate of 5-10°C/s to obtain the low temperature impact resistant steel.
2. The preparation method according to claim 1, wherein the CVCplus steckel mill comprises a coiling furnace and a four-high reversing mill; and the coiling furnace comprises a front coiling furnace and a rear coiling furnace; and a strip steel between a drum of the coiling furnace and the four-high reversing mill has strip tension of 18-25 t.
3. The preparation method according to claim 2, wherein the coiling furnace has a temperature of 860-930°C.
4. The preparation method according to claim 1, wherein a working roll of the CVCplus steckel mill moves axially, and the CVC has an axial movement within 70 mm during last three rolling passes of coiling.
The preparation method according to claim 1, wherein the coiling has a total reduction rate of 50-85%; and the coiling is conducted at an initial rolling temperature of 990-1,030t and a final rolling temperature of 780-862°C.
6. The preparation method according to claim 1, wherein the flat rolling is conducted for 6-8 rolling passes; and the flat rolling is conducted at an initial rolling temperature greater than or equal to 1,020C.
7. The preparation method according to claim 1, wherein the heating is conducted at 1,230-1,260 t with a heating coefficient greater than or equal to 10.0 min/cm; and heating equipment comprises a holding section, and the holding section has temperature uniformity less than or equal to lot and conducts heat preservation for no less than 25 min
8. The preparation method according to claim 1, wherein the steel plate has a self-tempering temperature of 620-690°C after cooling.
9. A low temperature impact resistant steel prepared by the preparation method according to any one of claims 1 to 8, comprising the following chemical components with mass percentage: 0.05-0.09% of C; 1.35-1.65% of Mn; 0.10-0.30% of Si; 0.02-0.04% of Nb; 0.15-0.025% of Ti; 0.02-0.055% of Al; 0.15-0.20% of Cr; and the balance of Fe and impurities, wherein the low temperature impact resistant steel has a thickness of 4-6 mm and a width of 2,200-3,050 mm; and the low temperature impact resistant steel has impact toughness greater than or equal to 50 J at -40t
10. Use of the low temperature impact resistant steel according to claim 9 as a skin steel for a top structure of a natural gas storage tank.