420MPa-grade ultra-wide-thick-specification low-yield-ratio structural steel for bridge and manufacturing method thereof

Abstract

The invention discloses 420 MPa-grade ultra-wide and thick specification low-yield-ratio structural steel for a bridge and a manufacturing method of the structural steel. The steel plate comprises the following chemical components in percentage by mass: less than or equal to 0.08% of C; 0.15 to 0.35 percent of Si; mn: 1.45 to 1.65%; the content of Als is 0.015 to 0.045 percent; 0.025% to 0.050% of Nb; ti: 0.008 to 0.02%; 0.10% to 0.30% of Cu; 0.10% to 0.30% of Ni; less than or equal to 0.015% of P; s: < = 0.005%; and the balance of Fe. A continuous casting slab with the thickness larger than 300 mm is adopted, and after heating, controlled rolling and DQ / ACC cooling, the ultra-wide and thick specification 420 MPa-level low-yield-ratio bridge steel is produced. The mechanical property can meet the requirements of Q420MPa-grade products, and the yield ratio is less than or equal to 0.84.

Description

Technical Field

[0001] The present invention belongs to the technical field of metal materials, and specifically relates to a 420MPa-grade ultra-wide and thick specification low-yield strength ratio structural steel for bridges and a manufacturing method thereof. The "controlled rolling + controlled cooling" process is adopted to manufacture ultra-wide and thick specifications with a thickness of 48-60mm and a width of 4200-4600mm, a yield strength ReL≥490MPa, a tensile strength Rm≥580MPa, an elongation after fracture A≥22%, a yield strength ratio≤0.84, an impact energy KV2≥280J at -40°C, and a Z-direction section shrinkage rate≥40%, which can meet the first-level flaw detection requirements of the GB / T 2970-2016 standard for bridge structural steel. Background Art

[0002] The ultra-wide 420MPa grade structural steel for bridges is specially developed for large and complex bridge projects. Its necessity lies not only in meeting the urgent demand for higher strength and larger span in bridge construction, but also in achieving the dual improvement of efficient material utilization and construction convenience through ultra-wide specification design. The yield strength of this steel is as high as 420MPa. Combined with its ultra-wide width, it makes it possible to process it into large components, thereby greatly improving the construction efficiency and the overall bearing capacity of the bridge. Its significant advantage is that it effectively reduces the amount of materials and construction costs while ensuring the stability and safety of the bridge structure, while significantly extending the service life of the bridge. Therefore, ultra-wide 420MPa grade structural steel for bridges has become an indispensable and important material in the field of modern bridge construction. With the increase in the width and thickness of steel plates, more stringent requirements are put forward for the equipment capacity and process control of the steel plate production process. Due to factors such as insufficient compression ratio and poor deformation penetration effect, its performance, especially low-temperature impact performance, and internal quality assurance are difficult, which greatly limits the application and promotion of this product.

[0003] Comparative document 1, patent "An ultra-wide non-anisotropic steel plate with a width ≥3500mm and its production method" (application number 202010804048.3), this method adopts a composition design of 0.10-0.16% C, 0.3-0.8% Mn, and 0.010-0.025% Nb, and adopts two-stage controlled rolling of rough rolling and finish rolling and post-rolling controlled cooling process to obtain an ultra-wide non-anisotropic steel plate with a width ≥3500mm and a thickness range of 10-100mm. From the actual width and performance of the application examples, the maximum width of the steel plate produced is 3720mm, and there is no example verification of steel plates with a width of ≥4200mm. The applicability of the method is questionable; the minimum yield strength is 425MPa, the minimum tensile strength is 548MPa, the minimum impact energy at -40℃ is 125J, and the maximum impact energy at -40℃ is 195J. Its performance can only barely meet the "GB / T714-2015 Structural Steel for Bridges" standard (yield strength ≥420MPa, tensile strength ≥540MPa, impact energy ≥120J), and the low-temperature impact toughness needs to be improved; Comparative Document 2, Patent "Ultra-wide Specification High-strength and High-toughness Marine Engineering Steel and Its Manufacturing Method" (Application No. 201811165261.3), this method adopts a low-carbon copper and nickel component design, and adopts a three-stage controlled rolling and post-rolling controlled cooling process to obtain a 75-90mm thick and 3980-4500mm wide ultra-wide specification high-strength and high-toughness marine engineering steel. From the actual performance of the application example, the steel plate manufactured by this method has good low-temperature toughness, but its yield strength is 370-431MPa and tensile strength is 502-556MPa, which cannot meet the performance requirements of 420MPa grade bridge structural steel; Comparative Document 3, patent "A steel plate with low yield ratio and ultra-wide bridge and its production method" (application number 201610075654.X), this method adopts 905-910℃*1.8t (t is the thickness of the steel plate) normalizing process to obtain a steel plate with low yield ratio and ultra-wide bridge. From the actual width and performance of the application example, the width of the production is 3760-4000mm and the thickness is 40-50mm. There is no example verification of wider and thicker steel plates, and the applicability of the method is questionable; the minimum yield strength is 375MPa, which cannot meet the performance requirements of 420MPa grade structural steel for bridges. In addition, this method is produced by normalizing treatment, and mass application will affect the manufacturing cost and production efficiency of the steel plate, which is not easy to apply and promote the method.

[0004] From the above patent comparison, it can be seen that the current 420MPa grade ultra-wide and thick structural steel for bridges has the following main deficiencies: The maximum width of steel plates that can be produced needs to be further improved; The yield strength and tensile strength of the steel plate are difficult to meet the requirements of 420MPa grade bridge structural steel; Low temperature impact toughness needs to be further improved. Summary of the invention

[0005] The purpose of the present invention is to provide a 420MPa-grade ultra-wide and thick low-yield strength ratio structural steel for bridges and a manufacturing method thereof. By using the alloy element ratio, using ultra-thick continuous casting slabs with a thickness of more than 300mm, and adopting a two-stage controlled rolling and post-rolling controlled cooling process, a low-yield strength ratio ultra-wide specification steel plate with performance that can stably meet the requirements of 420MPa-grade structural steel for bridges is obtained, the thickness of which is 48-60mm and the width specification can reach 4200-4600mm.

[0006] The present invention is realized by the following technical scheme: a 420MPa grade ultra-wide and thick low yield ratio structural steel for bridges and a manufacturing method thereof, comprising the following components in mass percentage: C: ≤0.08%; Si: 0.15-0.35%; Mn: 1.45-1.65%; Als: 0.015-0.040%; Nb: 0.025-0.050%; Ti: 0.008-0.020%; Cr: 0.20%-0.40%; Cu: 0.10%-0.30%; Ni: 0.10-0.30%; P: ≤0.015%; S: ≤0.005%; the rest are Fe and unavoidable impurities. Pcm≤0.21%; The calculation formula of the welding crack sensitivity index Pcm is: Pcm(%)=C+Si / 30+Mn / 20+Cu / 20+Ni / 60+Cr / 20+Mo / 15+V / 10+5B.

[0007] Reasons for designing the ingredients of the present invention: As an efficient strengthening element, C can form interstitial solid solution and interact with alloying elements such as Nb to precipitate fine carbides. These carbides precipitate especially during the rolling deformation process or before the austenite phase transformation, effectively hindering grain growth, refining the organizational structure, and hindering dislocation movement, thereby improving material strength. Therefore, the carbon content should not be set too low. However, the increase in carbon content will have an adverse effect on the plasticity and weldability of the material, especially low-temperature toughness, and increase the solid solution temperature of carbides when the billet is reheated. Therefore, the carbon content should not be too high. The present invention believes that it is more appropriate to control the upper limit of the carbon content to 0.08%.

[0008] Si in the present invention mainly plays the role of solid solution strengthening and can improve hardenability. However, too high Si content will lead to decreased plasticity and toughness of steel and increase the risk of cold brittleness. Therefore, the appropriate content range of Si in the present invention is 0.15% to 0.35%.

[0009] Mn also has a solid solution strengthening effect, which can effectively improve the strength and has a positive effect on hardenability. In addition, Mn can also increase the stability of austenite, reduce the austenite phase transition temperature, and inhibit the growth of phase transition grains of the steel plate before accelerated cooling, thereby achieving the effect of grain refinement. However, too high a Mn content can easily cause segregation, affect the uniformity of the structure and the toughness of the thickness center of the thick gauge steel plate, and is also not conducive to welding. Therefore, the present invention believes that it is more appropriate to control the Mn content between 1.45% and 1.65%.

[0010] Nb has significant solid solution and precipitation strengthening effects. By making full use of the precipitation pinning effect of Nb before rolling and accelerated cooling, the grains can be effectively refined and the strength and toughness can be improved. At the same time, Nb can also reduce the austenite phase transformation temperature, inhibit austenite recrystallization, and increase the recrystallization rolling temperature, thereby reducing the rolling force, increasing the rolling pass deformation rate, and achieving the expected fine grain effect. In addition, Nb can also reduce the mixed crystal problem caused by the temperature difference in the thickness direction of thick gauge steel. However, too high Nb content will lead to increased costs. Therefore, the present invention believes that it is more appropriate to control the Nb content between 0.025% and 0.050%.

[0011] Ti can form a precipitated phase mainly composed of TiN in the present invention, inhibiting the grain growth of austenite under high temperature conditions, which is beneficial to refining the microstructure of the ingot and the steel plate, and improving the toughness of the heat-affected zone after welding. At the same time, fine particles mainly composed of Ti oxides can promote grain refinement during the austenite phase transformation process, which is beneficial to improving the organizational properties near the thickness center of the thick gauge steel plate. Therefore, the present invention believes that it is more appropriate to control the Ti content between 0.008% and 0.020%.

[0012] Cr can improve the stability and hardenability of austenite, reduce the austenite phase transition temperature, and has a solid solution strengthening effect. In addition, the Cr element is highly economical and can partially replace precious alloy elements such as Mo and Ni to make up for the strength loss caused by the reduction of C content, and has a good effect on improving the uniformity of the structure in the thickness direction of thick gauge steel plates. However, too high a Cr content will have an adverse effect on weldability and plasticity. Therefore, the present invention believes that it is appropriate to control the Cr content between 0.20% and 0.40%.

[0013] Ni plays a role of solid solution strengthening in the present invention, which is beneficial to improve low temperature toughness, delay pearlite transformation, promote the formation of medium temperature transformation structure, and reduce the cooling rate limit of thick gauge steel plates. However, considering the high price of Ni, it is advisable to control its content to between 0.10% and 0.30%.

[0014] Cu also has a solid solution strengthening effect in the present invention and can improve corrosion resistance. However, a high Cu content is prone to induce cracks and impair weldability. Therefore, it is appropriate to control its content between 0.10% and 0.30%.

[0015] As a deoxidizing element, Al has a strong bonding ability with O. In order to promote the formation of fine particles mainly composed of Ti oxides, the present invention believes that the Al content should not be too high, and Als is preferably controlled between 0.015% and 0.040%.

[0016] P and S are regarded as harmful impurity elements in the present invention, and the lower their content, the better. P has a significant adverse effect on low-temperature toughness, so the present invention controls the P content to no more than 0.015%; and the increase in S content will promote the formation and growth of inclusions and deteriorate material properties, so the S content should be controlled to no more than 0.005%.

[0017] The present invention controls Pcm to ≤0.21%, which can ensure the toughness of the steel plate and enable the steel plate to have suitable weldability.

[0018] The present invention also discloses a 420MPa grade ultra-wide and thick low yield strength ratio structural steel for bridges and a manufacturing method thereof, which is characterized by comprising the following steps: molten steel smelting, slab continuous casting, slab heating, steel plate rolling, and DQ+ACC cooling process steps; wherein: (1) Molten steel smelting steps: The molten iron is desulfurized and slag-stripped, then enters the converter for smelting, and is refined and vacuum treated in the "LF furnace + RH furnace"; the vacuum treatment time is ≥12 minutes, calcium treatment is performed before steel tapping, and the static argon blowing time after wire feeding is ≥8 minutes; (2) Slab casting steps: Use continuous casting slabs with a thickness of more than 300 mm, the tundish superheat is ≤30°C, and the casting process maintains a constant pulling speed in the range of 0.40m-0.60m / min to ensure that the center segregation of the slab is C ≤1.0; (3) Slab heating step: The slab is heated in sections to a set soaking temperature of 1200-1250°C using a walking beam heating furnace. The slab is kept in the furnace for 1.0-1.4 t (t is the thickness of the continuous casting slab), ensuring that the slab is fully austenitized while avoiding overburning. (4) Steel plate rolling steps: During the rolling process, a two-stage controlled rolling and controlled cooling process is adopted. The first stage rolling starting temperature is 1050-1085℃, and the total reduction rate is 60-75%; the intermediate billet waiting temperature thickness is 1.8-2.5 times the finished steel plate thickness; the second stage rolling final rolling temperature is 790-820℃, and the total reduction rate is 50-60%; (5) ACC water cooling step: using the "DQ / ACC" cooling system, the initial cooling temperature is 790-800℃, the red-return temperature is 480-520℃, and the cooling rate is 9-11℃ / s; after the cooling is completed, the stacking is slowly cooled offline, the stacking slow cooling temperature is 450±50℃, and the stacking time is ≥72h; Further: it also includes a mechanical properties test step, that is, sampling and preparing samples from the steel plate specimens, and testing the tensile, impact and Z-direction tensile properties according to GB / T228, GB / T 229, and GB / T 5313; Furthermore, the performance of the above-mentioned 420MPa grade ultra-wide and thick specification low yield strength ratio bridge steel is as follows: yield strength ReL≥490MPa, tensile strength Rm≥580MPa, elongation after fracture A≥22%, -40℃ impact energy KV2≥280J, yield strength ratio ≤0.85, and Z-direction section shrinkage rate ≥40%.

[0019] The advantages of the present invention are: through a suitable and economical composition system and a relatively low carbon equivalent design, the addition of microalloying elements Nb, Ti, Al, and alloying elements Ni and Cu, and the use of a controlled rolling and controlled cooling process, it is possible to stably produce ultra-wide Q420MPa-grade structural steel for bridges with a thickness of 48-60mm and a maximum width of up to 4600mm. While having good low-temperature impact toughness, it also has good application safety and resistance to lamellar tearing. The internal quality of the steel plate is good, and the flaw detection can meet the first-level flaw detection requirements in the GB / T 2970-2016 standard. In summary, the alloy of the present invention has relatively low cost, simple process, and has the conditions for mass industrial production. It has broad application prospects in the field of large-scale and complex bridge engineering. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is the microstructure of the steel plate of Example 4. DETAILED DESCRIPTION

[0021] The invention discloses a 420MPa grade ultra-wide and thick structural steel for bridges with a low yield strength ratio and a manufacturing method thereof. The structural steel comprises the following components in percentage by mass: C: ≤0.08%; Si: 0.15-0.35%; Mn: 1.45-1.65%; Als: 0.015-0.040%; Nb: 0.025-0.050%; Ti: 0.008-0.02%; Cr: 0.20%-0.40%; Cu: 0.10%-0.30%; Ni: 0.10-0.30%; P: ≤0.015%; S: ≤0.005%; Pcm≤0.21%; and the rest are Fe and unavoidable impurities.

[0022] The present invention also discloses a 420MPa grade ultra-wide and thick low yield strength ratio structural steel for bridges and a manufacturing method thereof, which is characterized by comprising the following steps: molten steel smelting, slab continuous casting, slab heating, steel plate rolling, and DQ+ACC cooling process steps; wherein: (1) Molten steel smelting steps: The molten iron is desulfurized and slag-stripped, then enters the converter for smelting, and is refined and vacuum treated in the "LF furnace + RH furnace"; the vacuum treatment time is ≥12 minutes, calcium treatment is performed before steel tapping, and the static argon blowing time after wire feeding is ≥8 minutes; (2) Slab casting steps: Use continuous casting slabs with a thickness of more than 300 mm, the tundish superheat is ≤30°C, and the casting process maintains a constant pulling speed in the range of 0.40m-0.60m / min to ensure that the center segregation of the slab is C ≤1.0; (3) Slab heating step: The slab is heated in sections to a set soaking temperature of 1200-1250°C using a walking beam heating furnace. The slab is kept in the furnace for 1.0-1.4 t (t is the thickness of the continuous casting slab), ensuring that the slab is fully austenitized while avoiding overburning. (4) Steel plate rolling steps: During the rolling process, a two-stage controlled rolling and controlled cooling process is adopted. The first stage rolling starting temperature is 1050-1085℃, and the total reduction rate is 60-75%; the intermediate billet waiting temperature thickness is 1.8-2.5 times the finished steel plate thickness; the second stage rolling final rolling temperature is 790-820℃, and the total reduction rate is 50-60%; (5) ACC water cooling step: using the "DQ / ACC" cooling system, the initial cooling temperature is 790-800℃, the red-return temperature is 480-520℃, and the cooling rate is 9-11℃ / s; after the cooling is completed, the stacking is slowly cooled offline, the stacking slow cooling temperature is 450±50℃, and the stacking time is ≥72h; Preferably, the method further comprises a mechanical property testing step, i.e. sampling and preparing samples from the steel plate specimens, and testing the tensile, impact and Z-direction tensile properties according to GB / T 228, GB / T 229 and GB / T 5313; Preferably, the above-mentioned 420MPa grade ultra-wide and thick low yield strength ratio structural steel for bridges has the following properties: yield strength ReL ≥ 490MPa, tensile strength Rm ≥ 580MPa, elongation after fracture A ≥ 22%, -40℃ impact energy KV2 ≥ 280J, yield strength ratio ≤ 0.85, Z-direction section shrinkage ≥ 40%. The UT flaw detection of the steel plate can meet the first-level flaw detection requirements in the GB / T 2970-2016 standard; the microstructure of the steel plate is a composite structure of acicular ferrite / bainite + polygonal ferrite. Among them, the volume percentage of polygonal ferrite is 10% to 30%, and the grain size is ≥ 11 levels.

[0023] Example 1: A 420MPa grade ultra-wide and thick low yield strength ratio structural steel for bridges and its manufacturing method, with a thickness of 48mm and a width of 4600mm. The chemical composition and percentage content of the steel plate are: C: 0.068%, Si: 0.28%, Mn: 1.51%, Als: 0.034%, Ti: 0.012%, Cr+Ni+Cu+Nb: 0.60%, P: 0.015%, S: 0.0028%, N: 27ppm, H: 23ppm, and the rest are Fe and unavoidable impurities; wherein Pcm is: 0.18%.

[0024] The production steps and process parameters of the steel plate are: 1. Smelting: After desulfurization and slagging, the molten iron enters the converter for smelting, and is refined and vacuum treated in the "LF furnace + RH furnace". The vacuum treatment time is 13 minutes, and the static argon blowing time before steelmaking is 9 minutes. 2. Continuous casting: superheat 23°C, slab thickness 323mm, drawing speed 0.45m / min, center segregation C class 0.5; 3. Heating: The temperature of the heating and soaking section is 1247℃, and the total time of the slab in the furnace is 398min; 4. Rolling and cooling: A two-stage controlled rolling and controlled cooling process is adopted. The first-stage rolling temperature is 1044℃. After one pass of flattening, the widening rolling is carried out for 6 passes of rough rolling and cross rolling, with an average reduction rate of 12.13%. There are 2 passes of down-rolling, and the reduction rates are 17.68% and 18.87% respectively; the intermediate billet waiting temperature thickness is 126.54mm, and the total reduction rate of the first stage is 64.02%; the finishing rolling start temperature is 816℃, the final rolling temperature is 806℃, and the total reduction rate of the second stage is 58.70%; the "DQ+ACC" cooling system is used for water cooling, the starting cooling temperature is 790℃, and the steel plate red-returning temperature is 494℃; the stacking slow cooling temperature is 458℃, and the stacking time is 72.6h; The mechanical properties of the steel plate obtained in this embodiment are as follows: yield strength ReL 500MPa, tensile strength Rm 631MPa, elongation after fracture A27%, -40℃ impact energy KV2 average value 331J, Z-direction section shrinkage average value 46%. UT flaw detection of the steel plate can meet the first-level flaw detection requirements of GB / T2970-2016 standard.

[0025] Example 2: A 420MPa grade ultra-wide and thick low yield strength ratio structural steel for bridges and its manufacturing method, with a thickness of 60mm and a width of 4200mm. The chemical composition and percentage content of the steel plate are: C: 0.058%, Si: 0.26%, Mn: 1.53%, Als: 0.033%, Ti: 0.014%, Cr+Ni+Cu+Nb: 0.62%, P: 0.015%, S: 0.0022%, N: 33ppm, H: 20ppm, and the rest are Fe and unavoidable impurities; wherein Pcm is: 0.17%.

[0026] The production steps and process parameters of the steel plate are: 1. Smelting: After desulfurization and slagging, the molten iron enters the converter for smelting, and is refined and vacuum treated in the "LF furnace + RH furnace". The vacuum treatment time is 12 minutes, and the static argon blowing time before steelmaking is 11 minutes. 2. Continuous casting: superheat 21°C, slab thickness 450mm, casting speed 0.41m / min, center segregation C grade 1.0; 3. Heating: The temperature of the heating and soaking section is 1241°C, and the total time of the slab in the furnace is 512 minutes; 4. Rolling and cooling: A two-stage controlled rolling and controlled cooling process is adopted. The first-stage rolling temperature is 1063℃. After one pass of flattening, the widening rolling is performed for 9 passes of rough rolling and cross rolling, with an average reduction rate of 7.75%. There are 4 passes of down-rolling, and the reduction rates are 13.48%, 14.91%, 16.68% and 19.45% respectively; the intermediate billet waiting temperature thickness is 126.54mm, and the total reduction rate of the first stage is 71.88%; the finishing rolling start temperature is 784℃, the final rolling temperature is 811℃, and the total reduction rate of the second stage is 52.58%; the "DQ+ACC" cooling system is used for water cooling, the starting cooling temperature is 790℃, and the steel plate red-returning temperature is 499℃; the stacking slow cooling temperature is 471℃, and the stacking time is 73.2h; The mechanical properties of the steel plate obtained in this example are as follows: yield strength ReL 530MPa, tensile strength Rm 638MPa, elongation after fracture A22.5%, -40℃ impact energy KV2 average 324J, Z-direction section shrinkage average 47%. UT flaw detection of the steel plate can meet the first-level flaw detection requirements of GB / T2970-2016 standard.

[0027] Example 3: A 420MPa grade ultra-wide and thick low yield strength ratio structural steel for bridges and a manufacturing method thereof, with a thickness of 60mm and a width of 4420mm. The chemical composition and percentage content of the steel plate are: C: 0.061%, Si: 0.27%, Mn: 1.52%, Als: 0.033%, Ti: 0.012%, Cr+Ni+Cu+Nb: 0.63%, P: 0.014%, S: 0.0030%, N: 29ppm, H: 21ppm, and the rest are Fe and unavoidable impurities; wherein Pcm is: 0.18%.

[0028] The production steps and process parameters of the steel plate are: 1. Smelting: After desulfurization and slagging, the molten iron enters the converter for smelting, and is refined and vacuum treated in "LF furnace + RH furnace". The vacuum treatment time is 15 minutes, and the static argon blowing time before steelmaking is 9 minutes. 2. Continuous casting: superheat 24°C, slab thickness 450mm, pulling speed 0.41m / min, center segregation C class 0.5; 3. Heating: The temperature of the heating and soaking section is 1245℃, and the total time of the slab in the furnace is 533min; 4. Rolling and cooling: A two-stage controlled rolling and controlled cooling process is adopted. The first-stage rolling temperature is 1081℃. After one pass of flattening, the steel is widened and rolled for 8 passes, with an average reduction rate of 7.97%. The steel is rolled for 5 passes, with reduction rates of 11.95%, 12.80%, 14.82%, 16.50% and 14.83% respectively. The intermediate billet waiting temperature thickness is 126.59mm, and the total reduction rate in the first stage is 71.87%. The finishing rolling start temperature is 817℃, the final rolling temperature is 819℃, and the total reduction rate in the second stage is 52.60%. The "DQ+ACC" cooling system is used for water cooling, the starting cooling temperature is 800℃, and the steel plate red-returning temperature is 488℃. The stacking slow cooling temperature is 459℃, and the stacking time is 72.6h. The mechanical properties of the steel plate obtained in this example are as follows: yield strength ReL 533MPa, tensile strength Rm 646MPa, elongation after fracture A 22%, -40℃ impact energy KV2 average 328J, Z-direction section shrinkage average 51%. The UT flaw detection of the steel plate can meet the first-level flaw detection requirements in the GB / T2970-2016 standard.

[0029] Example 4: A 420MPa grade ultra-wide and thick low yield strength ratio structural steel for bridges and a manufacturing method thereof, with a thickness of 60mm and a width of 4600mm. The chemical composition and percentage content of the steel plate are: C: 0.074%, Si: 0.27%, Mn: 1.52%, Als: 0.026%, Ti: 0.012%, Cr+Ni+Cu+Nb: 0.62%, P: 0.011%, S: 0.0023%, N: 33ppm, H: 27ppm, and the rest are Fe and unavoidable impurities; wherein Pcm is: 0.19%.

[0030] The production steps and process parameters of the steel plate are: 1. Smelting: After desulfurization and slagging, the molten iron enters the converter for smelting, and is refined and vacuum treated in "LF furnace + RH furnace". The vacuum treatment time is 13 minutes, and the static argon blowing time before steelmaking is 10 minutes. 2. Continuous casting: superheat 24°C, slab thickness 450mm, pulling speed 0.41m / min, center segregation C class 0.5; 3. Heating: The temperature of the heating and soaking section is 1244°C, and the total time of the slab in the furnace is 546 minutes; 4. Rolling and cooling: A two-stage controlled rolling and controlled cooling process is adopted. The first-stage rolling temperature is 1079℃. After one pass of flattening, the widening rolling is performed for 9 passes of rough rolling and cross rolling, with an average reduction rate of 8.58%. There are 4 passes of down-rolling, and the reduction rates are 12.34%, 13.33%, 14.34% and 16.36% respectively; the intermediate billet waiting temperature thickness is 126.33mm, and the total reduction rate of the first stage is 71.93%; the finishing rolling start temperature is 810℃, the final rolling temperature is 807℃, and the total reduction rate of the second stage is 52.51%; the "DQ+ACC" cooling system is used for water cooling, the starting cooling temperature is 794℃, and the steel plate red-returning temperature is 519℃; the stacking slow cooling temperature is 497℃, and the stacking time is 73.1h; The mechanical properties of the steel plate obtained in this example are as follows: yield strength ReL 494MPa, tensile strength Rm 587MPa, elongation after fracture A 24%, -40℃ impact energy KV2 average 317J, Z-direction section shrinkage average 49%. The UT flaw detection of the steel plate can meet the first-level flaw detection requirements in the GB / T2970-2016 standard.

[0031] As shown in the attached drawings, the microstructure of the steel plate of Example 4 is observed, and the microstructure of the steel plate is a composite structure of acicular ferrite / bainite+polygonal ferrite, wherein the volume percentage of polygonal ferrite is 10% to 30%, and the grain size is ≥11.

[0032] In summary, the method of the present invention, through a suitable and economical composition system and a relatively low carbon equivalent design, adds microalloying elements Nb, Ti, Al, and alloying elements Ni and Cu, and adopts a controlled rolling and controlled cooling process, which can stably produce 420MPa-grade ultra-wide and thick specification low yield ratio bridge structural steel, with a thickness of 48-60mm and a width of up to 4200-4600mm. While having good low-temperature impact toughness, it also has good application safety and resistance to lamellar tearing. The internal quality of the steel plate is good, and the flaw detection can meet the first-level flaw detection requirements in the GB / T2970-2016 standard. In summary, the alloy of the present invention has relatively low cost, simple process, and has the conditions for mass industrial production, and has broad application prospects in the field of large and complex bridge engineering.

Claims

1. A 420MPa grade ultra-wide and thick low yield strength ratio structural steel for bridges, characterized by: It includes the following components in mass percentage: C: ≤0.08%; Si: 0.15-0.35%; Mn: 1.45-1.65%; Als: 0.015-0.040%; Nb: 0.025-0.050%; Ti: 0.008-0.02%; Cr: 0.20%-0.40%; Cu: 0.10-0.30%; Ni: 0.10-0.30%; P: ≤0.015%; S: ≤0.005%; Pcm≤0.21%; the rest are Fe and unavoidable impurities; among which Pcm (%) = C+Si / 30+Mn / 20+Cu / 20+Ni / 60+Cr / 20+Mo / 15+V / 10+5B.

2. The method for manufacturing a 420MPa grade ultra-wide and thick low yield strength ratio bridge structural steel as claimed in claim 1, characterized in that: The process includes the following steps: molten steel smelting, slab continuous casting, slab heating, steel plate rolling, ACC cooling, steel plate stacking slow cooling, and flaw detection inspection steps, wherein: (1) Molten steel smelting steps: Molten iron is desulfurized and slag-stripped, then enters the converter for smelting, and is refined and vacuum treated in the "LF furnace + RH furnace"; the vacuum treatment time is ≥15 minutes, and the static argon blowing time before steelmaking is ≥8 minutes; (2) Slab casting steps: Use continuous casting slabs with a thickness of more than 300 mm, the tundish superheat is ≤30°C, and the casting process maintains a constant pulling speed in the range of 0.40m-0.60m / min to ensure that the center segregation of the slab is C ≤1.0; (3) Slab heating step: The slab is heated in sections to a set soaking temperature of 1200-1250°C using a walking beam heating furnace. The slab is kept in the furnace for 1.0-1.4 t (t is the thickness of the continuous casting slab), ensuring that the slab is fully austenitized while avoiding overburning. (4) Steel plate rolling steps: During the rolling process, a two-stage controlled rolling and controlled cooling process is adopted. The first stage rolling starting temperature is 1050-1085℃, and the total reduction rate is 60-75%; the intermediate billet waiting temperature thickness is 1.8-2.5 times the finished steel plate thickness; the second stage rolling final rolling temperature is 790-820℃, and the total reduction rate is 50-60%; (5) ACC water cooling step: using the "DQ / ACC" cooling system, the initial cooling temperature is 790-800℃, the red-return temperature is 480-520℃, and the cooling rate is 9-11℃ / s; after cooling, the stack is slowly cooled offline, the stacking slow cooling temperature is 450±50℃, and the stacking time is ≥72h.

3. The method for manufacturing a 420MPa grade ultra-wide and thick low yield strength ratio bridge structural steel as claimed in claim 2, characterized in that: Using extra-thick continuous casting slabs with a thickness of more than 300mm, we produce ultra-wide structural steel for bridges with a thickness of 48-60mm and a width of 4200-4600mm.

4. The method for manufacturing a 420MPa grade ultra-wide and thick low yield strength ratio bridge structural steel as claimed in claim 2, characterized in that: In the first stage, the steel is leveled after the first pass and then widened by rolling. After reaching the target width, the steel is switched to forward rolling. In the forward rolling stage, a low-speed, high-reduction rolling strategy is adopted, with the instantaneous maximum torque reaching 8510-9712 ​​kNm. In the final stage, the reduction rate for at least 2 passes is ≥14%.

5. The method for manufacturing a 420MPa grade ultra-wide and thick low yield strength ratio bridge structural steel as claimed in claim 2, characterized in that: The DQ / ACC water cooling process adopts low-speed interval water cooling, with a roller speed of 0.4-0.5m / s and a cooling rate of 9.0-10.0℃ / S.

6. The method for manufacturing a 420MPa grade ultra-wide and thick low yield strength ratio bridge structural steel as claimed in claim 2, characterized in that: Mechanical properties of steel plate: yield strength ReL≥490MPa, tensile strength Rm≥580MPa, elongation after fracture A≥22%, -40℃ impact energy KV2≥280J, Z-direction section shrinkage ≥40%.

7. The method for manufacturing a 420MPa grade ultra-wide and thick low yield strength ratio bridge structural steel as claimed in claim 2, characterized in that: The yield strength ratio of the steel plate can be ≤0.

84.

8. The method for manufacturing a 420MPa grade ultra-wide and thick low yield strength ratio structural steel for bridges as claimed in claim 2, characterized in that: UT flaw detection of steel plates meets the first-level flaw detection requirements of GB / T 2970-2016 standard.

9. The method for manufacturing a 420MPa grade ultra-wide and thick low yield strength ratio structural steel for bridges as claimed in claim 2, characterized in that: The microstructure of the steel plate is a composite structure of acicular ferrite / bainite + polygonal ferrite; wherein the volume percentage of polygonal ferrite is 10% to 30%, and the grain size is ≥11 levels.

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