A fire-resistant and weather-resistant steel for welded structures with a tensile strength of 800 MPa and its production method

Abstract

The present invention relates to a fire-resistant and weather-resistant steel for welded structures with a tensile strength of 800 MPa and a production method thereof. The steel contains chemical components with the following mass percentage contents: C: 0.012 - 0.042%, Si: 0.12 - 0.25%, Mn: 1.42 - 1.68%, P ≤ 0.001%, S ≤ 0.001%, Mo: 0.32 - 0.51%, Cr: 0.28 - 0.46%, Cu: 0.12 - 0.23%, Ni: 0.27 - 0.41%, Nb: 0.034 - 0.057%, Ti: 0.007 - 0.021%, Ca: 0.0008 - 0.0022%, Sn: 0.010 - 0.025%, [O]: 0.0015 - 0.0028%, [N] ≤ 0.0018%, As ≤ 0.0008%, and the balance is Fe and unavoidable impurities; the production steps include: converter smelting, secondary refining, slab heating, controlled rolling, and laminar flow cooling; the finished steel plate of the present invention has characteristics such as high strength and toughness, fire resistance and weather resistance, resistance to lamellar tearing, a low yield ratio, and excellent welding performance.

Description

Technical Field

[0001] The present invention relates to the technical field of low alloy steel manufacturing, in particular to a fire-resistant and weather-resistant steel for welded structures with a tensile strength of 800 MPa and a production method thereof. Background Art

[0002] In recent years, the safety of steel structure projects in fields such as high-rise buildings under fire has received increasing attention. In addition to the conventional properties of steel, special property requirements such as fire and disaster resistance, and corrosion resistance have been put forward to reduce casualties and property losses in the event of a fire. That is, it is required that the steel has excellent high-temperature creep performance and corrosion resistance, which can not only reduce the thickness of fireproof coatings and weather-resistant coatings, but also improve the resource and energy utilization efficiency. Therefore, in order to achieve sustainable development, alleviate the overcapacity of the steel industry, and practice the supply-side structural reform of the steel industry, it is necessary to develop high-performance structural steels with composite functions of earthquake resistance, corrosion resistance, and fire resistance.

[0003] Before the present invention, the invention products disclosed in Chinese patent applications 201310160484.1, 201310033300.52, 201110080774.6, 201110247615.0, 200910045146.7, 201110247615.0, 200910011963.0, 200910272414.9, 200910045146.7, 201110080774.6, 201110247615.0 all have good fire resistance, but do not have corrosion resistance; in addition, the invention products disclosed in Chinese patent applications 201010113848.7, 200910056602.8, 200910180490.7, 200910056602.8 all have good corrosion resistance, but do not have fire resistance.

[0004] Chinese patent CN1354273A discloses a high-performance fire-resistant and weather-resistant building steel and a production method thereof. The invention product contains C, Si, Mn, P, S, Cr, Mo, Ti, Als, N, O, Cr, Ni, Cu, Ca, B, and also contains one or more of Nb, V, RE. After smelting, rolling, and heat treatment, the product has excellent fire and weather resistance, but the invention product requires normalizing + tempering treatment and has a low strength level.

[0005] Chinese invention patents CN103695772A and CN103695773A respectively disclose 550MPa grade fire-resistant weather-resistant earthquake-resistant construction steel and its production method and 690MPa grade fire-resistant weather-resistant earthquake-resistant construction steel and its production method. The construction steels in the two invention patent applications contain C, Si, Mn, P, S, Nb, Ti, Mo, W, Mg, O, and also contain Sb or Zr or a mixture of the two in any proportion. After hot metal desulfurization, converter smelting, vacuum treatment, addition of Mg element, conventional continuous casting, heating of the ingot, segmented rolling, cooling after final rolling, etc., the steel has excellent fire resistance, weather resistance and earthquake resistance, and excellent comprehensive performance. However, the disadvantages of the two invention patent products are that the corrosion resistance is not very ideal, and only requires -20℃ impact.

[0006] Chinese invention patents 202010127534.6 and 202010013530.5 respectively disclose a 690MPa-grade fire-resistant and weather-resistant steel plate for building structures and its manufacturing method, and a 690MPa-grade seismic, corrosion-resistant and fire-resistant medium plate steel and its manufacturing method. The two invention patents contain C, Si, Mn, P, S, Nb, V, Ti, Mo, Cr, Cu, Ni, and Al. The steel plates have good fire resistance and weather resistance, but the steel plates need to be tempered in the two-phase zone after rolling, and only -40°C impact toughness is required.

[0007] Chinese invention patent 202010804777.9 discloses a refractory steel for welding structure with excellent creep rupture performance at 700℃ and its production method. The invented product has excellent fire resistance, high temperature creep performance and welding performance, but the invention does not have corrosion resistance. Another Chinese invention patent 202110945932.3 discloses a high-performance corrosion-resistant fire-resistant steel with excellent low temperature toughness at -80℃ and its production method. The invented product has high strength, excellent low temperature toughness and fire resistance and weather resistance, as well as good welding performance and cold working performance, but the invented product contains more precious alloys such as Mo, Cr, Cu, Ni, etc., which is difficult to implement on a large scale. Summary of the invention

[0008] The present invention provides a fire-resistant and weather-resistant steel for welding structure with a tensile strength of 800MPa and a production method thereof, aiming to overcome the current deficiencies in the technical field. The present invention has a simple manufacturing process and a short production cycle, and can be implemented on a large scale in various metallurgical enterprises. The finished steel plate has the characteristics of high toughness, fire resistance, weather resistance, lamellar tear resistance, low yield strength ratio and excellent welding performance.

[0009] A fire-resistant and weather-resistant steel for welded structures with a tensile strength of 800 MPa, the steel comprising chemical components with the following mass percentage contents: C: 0.012 - 0.042%, Si: 0.12 - 0.25%, Mn: 1.42 - 1.68%, P ≤ 0.001%, S ≤ 0.001%, Mo: 0.32 - 0.51%, Cr: 0.28 - 0.46%, Cu: 0.12 - 0.23%, Ni: 0.27 - 0.41%, Nb: 0.034 - 0.057%, Ti: 0.007 - 0.021%, Ca: 0.0008 - 0.0022%, Sn: 0.010 - 0.025%, [O]: 0.0015 - 0.0028%, [N] ≤ 0.0018%, As ≤ 0.0008%, with the balance being Fe and unavoidable impurities. At the same time, the above chemical components must also satisfy the following relationships: (1) 13Cu + 2Ni = 2.2 - 3.7%, (2) (2Mo + 3Cr + 5Cu) / 3Ni = 2.0 - 3.9, (3) (2Ca + Sn) / [O] = 9.0 - 10.7, and the values of the chemical elements in the formula are their mass percentage contents.

[0010] Preferably, among the above chemical components, Ti: 0.010 - 0.020%, Sn: 0.012 - 0.022%, As ≤ 0.0006%.

[0011] The microstructure of the finished steel plate prepared from the above chemical components of the present invention is lath bainite + a small amount of granular bainite + quasi-polygonal ferrite with a volume percentage of 10 - 12% + M / A island structure with a volume percentage of 7 - 10%. Among them, the width of more than 90% of the lath bainite is ≤ 0.92 μm, and the width of the M / A island is ≤ 0.35 μm.

[0012] For the finished steel plate prepared from the above chemical components of the present invention, ReL is 636.0 - 652.8 MPa, Rm is 814.2 - 835.3 MPa, ReL / Rm is 0.772 - 0.798, A is 27.83 - 31.17%, the cross-sectional shrinkage rate Z in the thickness direction is 56.4 - 62.5%, -80°C KV2 is 258.6 - 280.5 J, the high-temperature yield strength R P0.2 at 700°C is 451.1 - 472.5 MPa, R P0.2 / ReL = 0.703 - 0.727, the corrosion weight loss rate is 0.2145 - 0.2473 g / m 2 ·h, and the submerged arc welding HAZ - 80°C KV2 is 187.8 - 211.1 J.

[0013] A production method of a fire-resistant and weather-resistant steel for welded structures with a tensile strength of 800 MPa according to the present invention includes the following steps:

[0014] 1) Converter smelting: The temperature of the hot metal charged into the furnace is ≥1280 °C. The double-slag process for ultra-low phosphorus smelting is adopted, with combined top and bottom blowing throughout the process, lance position of 1.5 - 1.6 m, slag basicity of 2.0 - 2.1, and oxygen supply intensity of 3.5 - 3.6 m³ / t·min;

[0015] 2) Secondary refining: The operation mode of pre-adding aluminum ingots, lime, heating, and intermittent circulating desulfurization is adopted to reduce nitrogen pickup. The LHF treatment time is 30 - 50 min, CaO / CaF2 < 4, slag basicity is 4.1 - 4.2, and the end-point [S] ≤ 10 ppm; The RH vacuum degree ≤ 50 Pa, and the treatment time is 18 - 25 min;

[0016] 3) Slab heating: The heating temperature is 1280 - 1300 °C, the heating rate is 8 - 11 min / cm, the holding temperature is 1220 - 1230 °C, and the holding time is 45 - 55 min;

[0017] 4) Controlled rolling: In the first stage of rough rolling, the starting rolling temperature ≤ 1080 °C, the finishing rolling temperature is 1030 - 1050 °C, and the reduction per pass ≥ 30 mm; In the second stage of finish rolling, the starting rolling temperature ≤ 980 °C, and the finishing rolling temperature is 780 - 850 °C;

[0018] 5) Lamellar cooling: The starting cooling temperature is (755 + h / 2) ± 3 °C, and the recrystallization temperature is 350 - 420 °C, where h is the thickness value of the finished steel plate in mm.

[0019] The reasons for the limited quantitative chemical components in the present invention are described in detail below:

[0020] The C content of the present invention is selected to be 0.012 - 0.042%. C is an important element to ensure the strength of the steel plate, but it is not conducive to weldability and low-temperature toughness; at the same time, the fine carbonitride particles formed by C and Nb, Ti in the steel have a significant effect of refining grains and precipitation strengthening, which is beneficial to improving strength and low-temperature toughness. C can also form high-melting-point carbides with Mo, Cr, improving the room-temperature strength and high-temperature performance. When the C content is lower than 0.012%, the above effects are limited. When the C content is higher than 0.042%, it is not conducive to low-temperature toughness and welding performance, so the C content is limited to 0.012 - 0.042%.

[0021] The Si content of the present invention is selected to be 0.12 - 0.25%. Si has a solid-solution strengthening and deoxidation effect, but it is not conducive to low-temperature toughness and ductility. Therefore, considering the product performance of the present invention comprehensively, the Si content is limited to 0.12 - 0.25%.

[0022] The Mn content of the present invention is selected to be 1.42 - 1.68%. Appropriate Mn has the effect of refining grains and is an important element to ensure the strength and low-temperature toughness of steel. When the Mn content is lower than 1.42%, the above effects are not obvious. When the Mn content is higher than 1.68%, the welding performance deteriorates and the welding crack sensitivity increases. Therefore, the Mn content is limited to 1.42 - 1.68%.

[0023] For the present invention, P ≤ 0.001% and S ≤ 0.001%. In the steel of the present invention, P and S are harmful elements, both of which are prone to segregate at grain boundaries, increasing the brittleness of grain boundaries and being unfavorable for low-temperature toughness and high-temperature fracture performance. In addition, S is also prone to form MnS composite inclusions, damaging the steel performance. Therefore, the contents of P and S in the steel should be reduced as much as possible.

[0024] The Mo content of the present invention is selected to be 0.32 - 0.51%. Mo mainly improves the room-temperature strength and high-temperature performance by solid-solution strengthening. Mo can also prevent the aggregation and growth of Nb carbonitride particles at high temperatures. At the same time, the high-melting-point Mo carbide formed in the steel is beneficial to improving the high-temperature performance. In addition, Mo can also promote the formation of high-density dislocation bainite, which is beneficial to ensuring the volume percentage of bainite in the steel to meet the performance requirements of the steel of the present invention. When the Mo content is less than 0.32%, the above effects are limited; when the Mo content is greater than 0.51%, it will not only greatly increase the manufacturing cost, but also significantly deteriorate the welding performance and low-temperature toughness, being unfavorable for popularization and application. Therefore, the Mo content is limited to 0.32 - 0.51%.

[0025] The Cr of the present invention is selected to be 0.28 - 0.46%. Most of the Cr improves the room-temperature strength and high-temperature performance by solid-solution strengthening. Cr can form a dense oxide protective film, improving the corrosion resistance and heat resistance. However, too high Cr increases the ductile-brittle transition temperature, reducing the low-temperature toughness and welding performance. Therefore, the Cr content is limited to 0.28 - 0.46%.

[0026] The Cu of the present invention is selected to be 0.12 - 0.23%. Cu improves the strength and corrosion resistance, and ε-Cu precipitates during tempering to improve the strength. However, too high Cu reduces the toughness and welding performance. Therefore, the Cu content is limited to 0.12 - 0.23%.

[0027] The Ni of the present invention is selected to be 0.27 - 0.41%. Appropriate Ni improves the low-temperature toughness and corrosion resistance. When compounded with appropriate Cr and Cu, the improvement of low-temperature toughness and corrosion resistance is more significant. However, Ni belongs to precious alloys, and when in excess, a large amount of scale that is not easily removed is easily formed, deteriorating the surface quality. Therefore, the Ni content is limited to 0.27 - 0.41%.

[0028] The Nb content of the present invention is selected to be 0.034 - 0.057%. The combined addition of Nb and Ti can significantly increase the recrystallization temperature, broaden the rolling temperature range in the non-recrystallized zone, and is beneficial to hot rolling. The fine Nb or complex carbonitride particles formed by Nb, Ti, C, and N not only prevent the austenite grain growth during the billet heating process, but also have significant grain refinement strengthening and precipitation strengthening effects to improve strength and low-temperature toughness. When Nb and Mo are added simultaneously, Mo can also promote the uniform precipitation of Nb carbide particles and prevent their aggregation and growth at high temperatures, thereby improving the high-temperature performance. When the Nb content is lower than 0.034%, the above effects are limited. When the Nb content is higher than 0.057%, the Nb carbonitride is prone to aggregation and growth at high temperatures, which is not conducive to the product performance of the present invention. Therefore, the Nb content is limited to 0.034 - 0.057%.

[0029] The Ti of the present invention is selected to be 0.007 - 0.021%. The combined addition of Ti and Nb can significantly increase the recrystallization temperature, broaden the rolling temperature range in the non-recrystallized zone, and is beneficial to hot rolling. The fine carbonitride particles formed by Ti, Nb, C, and N not only prevent the austenite grain growth during the billet heating process, but also have significant grain refinement strengthening and precipitation strengthening effects to improve strength and low-temperature toughness. In addition, the fine TiN particles can prevent the austenite grain growth during the welding process and effectively improve the low-temperature toughness of the heat-affected zone of welding. However, when the Ti content is less than 0.007%, the above effects are limited. When the Ti content is greater than 0.021%, large-sized Ti-containing inclusions are easily formed, which instead damages the ductility, resistance to lamellar tearing, low-temperature toughness, and welding performance. Therefore, the Ti is limited to 0.007 - 0.021%, preferably 0.010 - 0.020%.

[0030] The Ca of the present invention is selected to be 0.0008 - 0.0022%. Ca is an important element to ensure the product performance of the present invention. It can not only spheroidize and modify sulfide inclusions to purify the steel quality, but also the oxide particles can serve as the nucleation core for AF formation, promote AF formation, refine grains, and improve the low-temperature toughness, resistance to lamellar tearing, and low-temperature toughness of the heat-affected zone of welding of the steel. When the Ca content is less than 0.0008%, the above effects are limited. When the Ca content is greater than 0.0022%, large-sized complex inclusions are easily formed, reducing the steel quality purity and being unfavorable to the low-temperature toughness, resistance to lamellar tearing, and low-temperature toughness of the heat-affected zone of welding of the steel. Therefore, the Ca is limited to 0.0008 - 0.0022%.

[0031] The Sn content of the present invention is selected to be 0.010 - 0.025%. An appropriate amount of Sn can form a dense SnO2 protective film with O, having good corrosion resistance. When the Sn content is less than 0.010%, its corrosion resistance effect is limited. However, too much Sn is likely to form complex large inclusions with MnS, reducing the steel purity, deteriorating ductility, low-temperature impact toughness and welding performance. In addition, Sn is prone to segregate at grain boundaries, increasing grain boundary brittleness, reducing grain boundary strength and low-temperature toughness, and being disadvantageous to the resistance to lamellar tearing performance. Therefore, Sn is limited to 0.010 - 0.025%, preferably 0.012 - 0.022%.

[0032] The O content of the present invention is selected to be 0.0015 - 0.0028%. Generally, O belongs to harmful gas. However, to obtain the product performance of the present invention, the functions of Ca and Sn in the steel must be fully exerted, especially to form sufficient amounts of CaO and SnO2 oxide particles, so as to obtain the product performance of the present invention. Therefore, O is limited to 0.0015 - 0.0028%.

[0033] The N content of the present invention is selected to be ≤0.0018%. To prevent the increase of dissolved N in the steel and the risk of increasing strain aging sensitivity, N in the present invention is limited to ≤0.0018%.

[0034] As in the present invention is ≤0.0008%. As is prone to segregate at grain boundaries, increasing grain boundary brittleness, reducing grain boundary strength and low-temperature toughness, and being disadvantageous to high-temperature performance. Therefore, the content of As must be strictly limited to ≤0.0008%, preferably As ≤0.0006%.

[0035] At the same time, the above chemical components must also satisfy the following relationships: (1) 13Cu + 2Ni = 2.2 - 3.7%, (2) (2Mo + 3Cr + 5Cu) / 3Ni = 2.0 - 3.9, (3) (2Ca + Sn) / [O] = 9.0 - 10.7. When 13Cu + 2Ni < 2.2%, the weather resistance cannot be guaranteed. When 13Cu + 2Ni > 3.7%, the crack tendency increases, the manufacturing cost is increased, and it is also disadvantageous to low-temperature toughness. When (2Mo + 3Cr + 5Cu) / 3Ni < 2.0, the room-temperature and high-temperature performance and weather resistance of the steel cannot be guaranteed. When (2Mo + 3Cr + 5Cu) / 3Ni > 3.9, the manufacturing cost is increased, and the low-temperature toughness and welding performance of the steel are deteriorated. When (2Ca + Sn) / [O] < 9.0, the number of formed oxide particles is insufficient, and the product performance of the present invention cannot be obtained. When (2Ca + Sn) / [O] > 10.7, it is easy to form large and complex inclusions containing Ca and Sn, reducing the steel purity, being disadvantageous to ductility and low-temperature toughness, and deteriorating the resistance to lamellar tearing and welding performance.

[0036] In addition to the above chemical components, the balance of the steel of the present invention is Fe and unavoidable inclusions.

[0037] In the production method of the present invention, the production process parameters need to be strictly and precisely controlled, and the reasons for controlling the production process parameters are as follows:

[0038] 1) Converter smelting: Ultra-low phosphorus double slag method is adopted for smelting. The lance position is controlled at 1.5 - 1.6 m, the oxygen supply intensity is 3.3 - 3.5 m 3 / t·min, and the slag basicity is 2.0 - 2.1, so as to quickly melt the slag, ensure the stable progress of the dephosphorization reaction and improve the dephosphorization rate, ultimately achieving the purpose of deep dephosphorization, preventing P from segregating at the grain boundaries as much as possible, improving the brittleness of the grain boundaries, and reducing the low-temperature toughness and high-temperature performance.

[0039] 2) Secondary refining: The LHF treatment time is 30 - 50 min, CaO / CaF2 < 4, and the slag basicity is 4.1 - 4.2; the RH vacuum degree ≤ 50 Pa, and the treatment time is 18 - 25 min. The purpose of controlling the above parameters is to ensure the activity of the slag and promote the rapid progress of the deep desulfurization reaction through operations such as pre-adding aluminum ingots and lime at the bottom of the ladle and the operation mode of heating and circulating desulfurization at intervals, ensuring the end point [S] ≤ 10 ppm, reducing the nitrogen increase in the molten steel, and realizing the stable production of ultra-pure molten steel.

[0040] 3) Slab heating: The heating temperature, holding temperature, and holding time are restricted within a certain range. The purpose is to fully austenitize the steel, reduce the degree of central segregation as much as possible, and prevent the growth of austenite grains during the heating process to ensure that the performance of the steel plate meets the requirements of the present invention.

[0041] 4) Rolling and cooling: After the slab is heated and discharged from the furnace, it needs to be descaled by high-pressure water. Then, when the slab temperature drops below 1080 °C, stage I rolling is carried out, and the single-pass reduction is controlled to be ≥ 30 mm. The purpose is to carry out repeated rolling with a large single-pass reduction in the complete austenite recrystallization zone to break the columnar dendrites after multiple passes of rolling, fully refine the austenite grains. Controlling the finishing temperature of stage I at 1030 - 1050 °C is to shorten the waiting interval time between the end of stage I finishing and the start of stage II finish rolling, prevent the further growth of austenite grains, and thus refine the structure of the final product; controlling the starting rolling temperature of stage II ≤ 980 °C ensures that stage II is rolled in the complete austenite non-recrystallization zone, avoiding the occurrence of mixed crystal tissue structure in the product and improving the product performance.

[0042] 5) Control the starting cooling temperature of laminar cooling at (755 + h / 2) ± 3 °C, and the recarburization temperature at 350 - 420 °C to ensure the obtained tissue structure and mechanical property level required by the present invention.

[0043] The microstructure of the finished steel plate prepared by the present invention is lath bainite + a small amount of granular bainite + quasi-polygonal ferrite with a volume percentage of 10-12% + M / A island structure with a volume percentage of 7-10%. Among them, the width of more than 90% of the lath bainite is ≤0.92 μm, and the width of the M / A island is ≤0.35 μm; ReL is 636.0-652.8 MPa, Rm is 814.2-835.3 MPa, ReL / Rm is 0.772-0.798, A is 27.83-31.17%, the cross-sectional shrinkage rate Z in the thickness direction is 56.4-62.5%, KV2 at -80 °C is 258.6-280.5 J, and the high-temperature yield strength R P0.2 at 700 °C is 451.1-472.5 MPa, R P0.2 / ReL = 0.703-0.727, the corrosion weight loss rate is 0.2145-0.2473 g / m 2 ·h, and the KV2 of the submerged arc welding HAZ at -80 °C is 187.8-211.1 J.

[0044] Compared with the prior art, the manufacturing process of the present invention is simple, the production cycle is short, and it can be implemented on a large scale in each metallurgical enterprise. The finished steel plate has the characteristics of high strength and toughness, fire and weather resistance, resistance to lamellar tearing, low yield ratio, and excellent welding performance. Brief Description of the Drawings

[0045] Figure 1 is the metallographic structure diagram of the steel plate produced in Example 5 of the present invention. Detailed Description of the Invention

[0046] In order to better explain the technical solution of the present invention, the technical solution of the present invention will be further described below in conjunction with specific embodiments. The following embodiments are only exemplary descriptions of the technical solution of the present invention and do not limit the present invention in any form. The serial numbers of the following embodiments are only for description and do not represent the advantages and disadvantages of the embodiments.

[0047] Table 1 below is the list of chemical composition values of each embodiment of the present invention;

[0048] Table 2 below is the list of values of the main process parameters of each embodiment of the present invention;

[0049] Table 3 below is the test results of the mechanical properties of the embodiments of the present invention.

[0050] Among them, the product thicknesses of Examples 1-8 are 15 mm, 20 mm, 30 mm, 40 mm, 50 mm, 60 mm, 70 mm, and 80 mm respectively

[0051] A production method of a fire- and weather-resistant steel for welded structures with a tensile strength of 800 MPa according to each embodiment of the present invention includes the following steps:

[0052] 1) Converter smelting: The temperature of the hot metal charged into the furnace is ≥1280 °C. The double slag method for ultra-low phosphorus smelting process is adopted, with top and bottom combined blowing throughout the process, the lance position is 1.5 - 1.6 m, the slag basicity is 2.0 - 2.1, and the oxygen supply intensity is 3.5 - 3.6 m³ / t·min;

[0053] 2) Secondary refining: The operation mode of pre-adding aluminum ingots, lime, heating, and intermittent circulation desulfurization is adopted to reduce nitrogen absorption. The LHF treatment time is 30 - 50 min, CaO / CaF₂ < 4, the slag basicity is 4.1 - 4.2, and the end point [S] ≤ 10 ppm; The RH vacuum degree ≤ 50 Pa, and the treatment time is 18 - 25 min;

[0054] 3) Slab heating: The heating temperature is 1280 - 1300 °C, the heating rate is 8 - 11 min / cm, the holding temperature is 1220 - 1230 °C, and the holding time is 45 - 55 min;

[0055] 4) Controlled rolling: In the first stage of rough rolling, the starting rolling temperature ≤ 1080 °C, the finishing rolling temperature is 1030 - 1050 °C, and the reduction per pass ≥ 30 mm; In the second stage of finish rolling, the starting rolling temperature ≤ 980 °C, and the finishing rolling temperature is 780 - 850 °C;

[0056] 5) Laminar flow cooling: The starting cooling temperature is (755 + h / 2) ± 3 °C, and the recrystallization temperature is 350 - 420 °C, where h is the thickness value of the finished steel plate in mm.

[0057] Table 1 List of chemical composition values of each embodiment of the present invention (wt, %)

[0058]

[0059] Table 2 List of main process parameter values of each embodiment of the present invention Table 3 Test results of mechanical properties of each embodiment of the present invention

[0060]

[0061] Table 3 Test results of mechanical properties of each embodiment of the present invention

[0062]

[0063] Samples were taken at 1 / 4 of the thickness of the steel plate in the embodiments of the present invention for room-temperature tensile property, -80°C longitudinal impact energy, and 700°C high-temperature tensile property tests. The test results are shown in Table 3: The ReL of the finished steel plate produced by using the composition and method of the present invention is 636.0 - 652.8 MPa, the Rm is 814.2 - 835.3 MPa, the ReL / Rm is 0.772 - 0.798, the A is 27.83 - 31.17%, the thickness-direction reduction of area Z is 56.4 - 62.5%, the -80°C KV2 = 258.6 - 280.5 J, and the 700°C high-temperature yield strength R P0.2 is 451.1 - 472.5 MPa, and the R P0.2 / ReL is 0.703 - 0.727, the corrosion weight loss rate is 0.2145 - 0.2473 g / m 2 ·h, the -80°C KV2 of the HAZ of submerged arc welding is 187.8 - 211.1 J, and it has the characteristics of high strength and toughness, fire and weather resistance, resistance to lamellar tearing, a lower yield ratio, and excellent welding performance, etc.

[0064] Figure 1 is the microstructural photograph of the finished steel plate obtained in Example 5 of the present invention. It can be seen from the figure that the microscopic structure characteristics of the steel of the present invention are lath bainite + a small amount of granular bainite + quasi-polygonal ferrite with a volume percentage of 10 - 12% + M / A island structure with a volume percentage of 7 - 10%. Among them, the width of more than 90% of the lath bainite is ≤ 0.92 μm, and the width of the M / A island is ≤ 0.35 μm. It is this uniform and fine multiphase structure that ensures the performance of the products of the present invention.

[0065] The above embodiments are only specific examples cited to explain the present invention and do not limit the present invention in any form. Any non-substantive changes made by anyone based on the above content and form without departing from the protection scope of the claims of the present invention shall be considered to fall within the protection scope of the claims of the present invention. The present invention is not limited to the above specific embodiments.

Claims

1. A fire-resistant and weather-resistant steel for welded structures with a tensile strength of 800 MPa, characterized in that The steel contains chemical components with the following mass percentage contents: C: 0.012 - 0.042%, Si: 0.12 - 0.25%, Mn: 1.42 - 1.68%, P ≤ 0.001%, S ≤ 0.001%, Mo: 0.32 - 0.51%, Cr: 0.28 - 0.46%, Cu: 0.12 - 0.23%, Ni: 0.27 - 0.41%, Nb: 0.034 - 0.057%, Ti: 0.007 - 0.021%, Ca: 0.0008 - 0.0022%, Sn: 0.010 - 0.025%, [O]: 0.0015 - 0.0028%, [N] ≤ 0.0018%, As ≤ 0.0008%, and the balance is Fe and unavoidable impurities. At the same time, the above chemical components must also satisfy the following relationships: (1) 13Cu + 2Ni = 2.2 - 3.7%, (2) (2Mo + 3Cr + 5Cu) / 3Ni = 2.0 - 3.9, (3) (2Ca + Sn) / [O] = 9.0 - 10.

7. The values of the chemical elements in the formulas are their mass percentage values; the microstructure of the obtained finished steel plate is lath bainite + a small amount of granular bainite + quasi-polygonal ferrite with a volume percentage of 10 - 12% + M / A island structure with a volume percentage of 7 - 10%. Among them, the width of more than 90% of the lath bainite is ≤ 0.92 μm, and the width of the M / A island is ≤ 0.35 μm.

2. A fire-resistant and weather-resistant steel for welded structures with a tensile strength of 800 MPa as described in claim 1, characterized in that: Ti: 0.010 - 0.020%, Sn: 0.012 - 0.022%, As ≤ 0.0006%.

3. A fire- and weather-resistant steel for welded structures with a tensile strength of 800 MPa, according to claim 1 or 2, characterized in that: The ReL of the obtained finished steel plate is 636.0 - 652.8 MPa, the Rm is 814.2 - 835.3 MPa, the ReL / Rm is 0.772 - 0.798, the A is 27.83 - 31.17%, the cross-sectional shrinkage rate Z in the thickness direction is 56.4 - 62.5%, the -80°C KV2 is 258.6 - 280.5 J, and the high-temperature yield strength R P0.2 at 700°C is 451.1 - 472.5 MPa, and R P0.2 / ReL = 0.703 - 0.727, the corrosion weight loss rate is 0.2145 - 0.2473 g / m 2 ·h, and the submerged arc welding HAZ -80°C KV2 is 187.8 - 211.1 J.

4. The production method of a fire-resistant and weather-resistant steel for welded structures with a tensile strength of 800 MPa as claimed in claim 1 or 2, characterized in that It includes the following steps: 1) Converter smelting: The temperature of the hot metal charged into the furnace is ≥1280°C. The double slag method for ultra-low phosphorus smelting process is adopted, with top and bottom combined blowing throughout the process. The lance position is 1.5 - 1.6 m, the basicity of the slag is 2.0 - 2.1, and the oxygen supply intensity is 3.5 - 3.6 m 3 / t·min; 2) Secondary refining: Adopt the operation mode of pre-adding aluminum ingots, lime, heating, and cyclic desulfurization at intervals to reduce nitrogen pickup. The LHF treatment time is 30 - 50 min, CaO / CaF2 < 4, the slag basicity is 4.1 - 4.2, and the end point [S] ≤ 10 ppm; the RH vacuum degree ≤ 50 Pa, and the treatment time is 18 - 25 min; 3) Slab heating: The heating temperature is 1280 - 1300 °C, the heating rate is 8 - 11 min / cm, the holding temperature is 1220 - 1230 °C, and the holding time is 45 - 55 min; 4) Controlled rolling: In the first stage, the rough rolling starting temperature ≤ 1080 °C, the finishing rolling temperature is 1030 - 1050 °C, and the single-pass reduction ≥ 30 mm; in the second stage, the finishing rolling starting temperature ≤ 980 °C, and the finishing rolling temperature is 780 - 850 °C; 5) Laminar flow cooling: The starting cooling temperature is (755 + h / 2) ± 3 °C, and the recrystallization temperature is 350 - 420 °C, where h is the thickness value of the finished steel plate in mm.

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