High toughness, high ductility and super large heat input steel plate with high impact resistance and manufacturing method thereof

Abstract

A high-toughness, high-ductility steel plate with strong impact resistance and capable of ultra-large heat input, and its manufacturing method, wherein the weight percentage composition is as follows: C 0.09~0.15%, Si≤0.20%, Mn 1.30~1.60%, P≤0.013%, S≤0.0030%, Nb 0.008~0.022%, Ti 0.008~0.016%, Als 0.005~0.015%, N 0.0055~0.0085%, B 0.0012~0.0030%, Ca 0.0010~0.0035%, with the balance including Fe and unavoidable inclusions. This invention does not add precious elements such as Ni, Cu, and Mo. It uses a medium-C, ultra-low (Si+Al), high-Mn, high-N, and micro-(Ti+B) treated alloy system as the basis. By controlling the solidification co-segregation index ≤2.52, Ti / N ≤2.0, and Ca treatment, the Ca / S ratio is 1.0~3.0, and (%Ca)×(%S) 0.28 =1.0×10 －3 ~3.0×10 －3 Metallurgical techniques such as metallurgy are employed to optimize the TMCP process, resulting in high strength, high impact resistance, high toughness (especially high crack arrest properties), excellent weldability, and the ability to weld with ultra-high heat input.

Description

Technical Field

[0001] This invention relates to high-strength steel, and particularly to a high-toughness, high-ductility steel plate with strong impact resistance and ultra-large heat input, and a method for manufacturing the same. Background Technology

[0002] As is well known, low-carbon (high-strength) low-alloy steel is one of the most important engineering structural materials, widely used in oil and gas pipelines, offshore platforms, shipbuilding, bridge structures, boilers and vessels, building structures, the automotive industry, railway transportation, and machinery manufacturing. The performance of low-carbon (high-strength) low-alloy steel depends on its chemical composition and manufacturing process. Among these, strength, toughness, plasticity, and weldability are the most important properties of low-carbon (high-strength) low-alloy steel, which are ultimately determined by the microstructure of the finished steel.

[0003] With the continuous advancement of metallurgical technology and on-site control technology, people are placing higher demands on the toughness, plasticity, and weldability of high-strength steel. Specifically, steel plates should possess high strength, high elongation (especially high uniform elongation), and high crack arrest characteristics (i.e., resistance to brittle fracture and ductile instability fracture) at low temperatures, while also exhibiting excellent weldability and high-efficiency welding with high heat input. Furthermore, under conditions of lower manufacturing costs, the comprehensive mechanical and performance properties of steel plates should be significantly improved to reduce steel consumption, save costs, reduce the weight of steel components, and enhance stability and safety. More importantly, this will further improve the cold and hot workability of high-strength steel and its safety and reliability during service.

[0004] Currently, Japan, South Korea, the European Union, and North America are experiencing a surge in research into developing next-generation high-performance steel materials. The aim is to achieve better multiphase microstructure matching by combining alloy composition optimization with next-generation online deformation heat treatment technology, resulting in ultra-fine microstructures and uniformly distributed dislocation configurations within the cell structure. Without adding expensive alloying elements (such as Cu, Ni, Mo, etc.), by optimizing composition design and matching it with next-generation TMCP (Transformer Transformer Composition Processing) technology, better microstructure morphology, size, and substructure can be obtained, leading to higher strength / toughness / plasticity, higher elongation, and better weldability.

[0005] In the current technology for manufacturing thick steel plates with a yield strength ≥355MPa and a low-temperature impact toughness ≥34J at -60℃, a certain amount of Ni or (Cu+Ni) element (≥0.30%) is generally added to the steel. (The Firth (1986) International Symposium and Exhibit on Offshore Mechanics and Arctic Engineering, 1986, Tokyo, Japan, 354; “DEVELOPMENTS IN MATERIALS FOR ARCTIC OFFSHORE STRUCTURES”; “Structural Steel Plates for Arctic Use Produced by Multipurpose Accelerated Cooling System” (Japanese), Kawasaki Steel Technology News, 1985, No. 1 68~72; “Application of Accelerated Cooling For Producing 360MPa Yield Strength Steel plates of up to 150mmin Thickness with Low Carbon Equivalent”, Accelerated Cooling) RolledSteel, 1986, 209~219; "High Strength Steel Plates For Ice-Breaking VesselsProduced by Thermo-Mechanical Control Process", Accelerated Cooling RolledSteel, 1986, 249~260; "420MPa Yield Strength Steel Plate with Superior FractureToughness for Arctic Offshore Structures", Kawasaki steel technical report, 1999, No. 40, 56; "420MPa and 500MPa Yield Strength Steel Plate with High HAZtoughness Produced by TMCP for Offshore Structure", Kawasaki steel technicalreport, 1993, No.29, 54; "Toughness Improvement in Bainite Structure by Thermo-Mechanical Control Process" (Japanese), Sumitomo Metal, Vol. 50, No. 1 (1998), 26; "Steel Plates for Offshore Platform Structures Used in Icy Sea Regions" (Japanese), Iron and Steel Research, 1984, No. 314, 19-43], to ensure that the base steel plate has excellent low-temperature toughness, when welding with a heat input energy of <100KJ / cm, the toughness of the heat-affected zone (HAZ) can reach Akv≥34J at -60℃; however, when welding with a high heat input energy (≥200KJ / cm), the low-temperature toughness of the weld heat-affected zone (HAZ) is generally difficult to achieve, and the low-temperature toughness of the HAZ deteriorates severely.

[0006] Many patent documents only describe how to achieve low-temperature toughness in the base steel plate, with little explanation on how to obtain excellent low-temperature toughness in the heat-affected zone (HAZ) under welding conditions. This is especially true when using ultra-high heat input welding, where even less is provided on how to ensure low-temperature toughness in the HAZ. Furthermore, to ensure the low-temperature toughness of the steel plate, a certain amount of Ni or Cu+Ni elements are generally added to the steel. Even with high heat input welding, the low-temperature toughness of the HAZ in the steel plate rarely reaches -60℃. (See Japanese Patents Sho 63-93845, Sho 63-79921, Sho 60-258410, Japanese Patent Application Publication No. 4-285119, Japanese Patent Application Publication No. 4-308035, Japanese Patent Application Publication No. 3-264614, Japanese Patent Application No. 2-250917, Japanese Patent Application No. 4-143246; US Patents US Patent 4855106, US Patent 5183198, US Patent 4137104).

[0007] Currently, the main technologies for improving the low-temperature toughness of the heat-affected zone (HAZ) of high heat input welded steel plates include oxide metallurgy (see US Patent 4629505, WO 01 / 59167 A1), Ti-B treatment technology (see Japanese Patent Publications 59-2733, 59-3537, 56-127555, 56-209177) and ultra-low N-high Al-micro Ti treatment (Journal of the Japan Welding Society, 1982, Vol.51(2), 118).

[0008] China Baosteel has developed a series of low-temperature steel plates that can be welded with high heat input (such as Chinese patents ZL201410300713.X, ZL201310244712.3, ZL201310244706.8, ZL201310124065.2, ZL201310244713.8, ZL201210209637.2, ZL201410815614.5, ZL201710183350.X, and ZL201910149978.7). To ensure the low-temperature toughness of the heat-affected zone (HAZ) in high-heat-input welds, a certain amount of precious alloying elements such as Cu and Ni are added to steel plates (especially ultra-thick steel plates). While the weld performance of these steel plates under high heat input is relatively good, the toughness of the HAZ, particularly the low-temperature toughness, is not very stable; especially regarding crack arrest characteristics, high crack arrest characteristics at -40°C (Kca(-40°C) ≥ 8000 N / mm) are also problematic. 3 / 2 Not only are the base steel plate and the heat-affected zone (HAZ) not included, but also the impact resistance and the ability to weld with ultra-high heat input (welding heat input ≥300kJ / cm). Summary of the Invention

[0009] The purpose of this invention is to provide a high-toughness, high-ductility steel plate with strong impact resistance and capable of ultra-high heat input, and a method for manufacturing the same. While achieving excellent low-temperature toughness, especially high crack arrest properties, in the base steel plate, the steel plate also possesses excellent impact resistance (high uniform elongation, high fracture elongation) and weldability. Furthermore, under ultra-high heat input welding conditions, the low-temperature toughness and crack arrest properties of the welded HAZ are equally excellent. This invention successfully solves the problems of low alloy cost, low carbon equivalent, high strength, impact resistance, excellent low-temperature toughness (especially high crack arrest properties), and excellent weldability (especially weldability under ultra-high heat input). The contradiction lies in the fact that it integrates key factors affecting the high strength, impact resistance, excellent low-temperature toughness, high crack arrest properties, ultra-high heat input weldability, and low-cost manufacturing of steel plates. Its yield strength is ≥355MPa, tensile strength is ≥490MPa, uniform elongation is ≥22%, elongation after fracture is ≥40% (i.e., impact resistance), and transverse impact toughness KV2 ≥100J at -40℃. It is a low-temperature crack-arresting steel plate that can be welded with high heat input, and the KV2 (-40℃) of the welded HAZ is ≥70J. It is particularly suitable for offshore wind power, low-temperature pressure vessels, offshore platforms, and bridge steel.

[0010] To achieve the above objectives, the technical solution of the present invention is as follows:

[0011] This invention utilizes a simple alloy element combination design, without adding precious elements such as Ni, Cu, and Mo. It employs a medium-C, ultra-low (Si+Al), high-Mn, high-N, and micro-(Ti+B) treated alloy system as its foundation, and controls the solidification co-segregation index (%C). 0.5×[10.2(%P)+30.7(%S)+3.3(%Mn)]×[7.8(%Si)+5.6(%Als)]≤2.52, [1.27(%B)] / [(%N)-0.292(%Ti)-0.518(%Als)-0.147(%Nb)]≤61.5, Ti / N≤2.0, and Ca treatment, Ca / S ratio controlled between 1.0 and 3.0, and (%Ca)×(%S) 0.28 ≤1.5×10 -3 Metallurgical techniques; optimization of TMCP (Thermo-mechanical control process), i.e., [ξ×(T 开冷2 )×(DI OL )×(V c2 )] / {H×(T 开轧 )×[Ar3-(T 开冷2 )]×(T 停冷2 )}≥7.6×10 -3 This process results in a finished steel plate with a microstructure consisting of uniform and fine ferrite and a small amount of dispersed lower bainite, with an average grain size of about 20 μm. This produces high strength, high impact resistance, high toughness (especially high crack arrest properties), excellent weldability, and the ability to weld with ultra-high heat input.

[0012] Specifically, the high-toughness, high-ductility, and ultra-high heat input steel plate with strong impact resistance described in this invention has the following composition by weight percentage:

[0013] C: 0.09~0.15%

[0014] Si: ≤0.20%

[0015] Mn: 1.30%~1.60%

[0016] P: ≤0.013%

[0017] S: ≤0.0030%

[0018] Nb: 0.008%~0.022%

[0019] Ti: 0.008%~0.016%

[0020] Als: 0.005%~0.015%

[0021] N: 0.0055%~0.0085%

[0022] B: 0.0012%~0.0030%

[0023] Ca: 0.0010%~0.0035%

[0024] The balance includes Fe and other unavoidable inclusions; and the contents of the above elements must simultaneously satisfy the following relationship:

[0025] Cosegregation index:

[0026] (%C) 0.5 ×[10.2(%P)+30.7(%S)+3.3(%Mn)]×[7.8(%Si)+5.6(%Als)]≤2.52;

[0027] [1.27(%B)] / [(%N)-0.292(%Ti)-0.518(%Als)-0.147(%Nb)]≤61.5;

[0028] Ti / N≤2.0;

[0029] Ca treatment, Ca / S ratio = 1.0–3.0, (%Ca) × (%S) 0.28 =1.0×10 －3 ~3.0×10 －3 ;

[0030] Furthermore, the balance consists of Fe and other unavoidable inclusions.

[0031] The microstructure of the steel plate described in this invention consists of uniform and fine equiaxed ferrite with a small amount of dispersed bainite.

[0032] The steel plate of this invention has a yield strength ≥355MPa, tensile strength ≥490MPa, uniform elongation ≥22%, elongation after fracture (i.e., impact resistance) ≥40%, transverse impact toughness KV2 ≥100J at -40℃, and KV2 (-40℃) ≥70J for welded HAZ.

[0033] In the composition design of the steel plate described in this invention:

[0034] Carbon (C) has a significant impact on the strength, impact resistance (high uniform elongation, high fracture elongation), low-temperature toughness, and weldability of steel plates. From the perspective of improving the low-temperature toughness and weldability of steel, it is desirable to control the C content in the steel to a low level. However, from the perspective of steel strength and control of multiphase structure during production, the C content should not be too low. Especially for impact resistance steel plates, an excessively low C content (<0.090%) causes a serious imbalance in the phase ratio, morphological characteristics, phase distribution, and relative size of the grains of each component phase in the multiphase structure, which severely degrades the impact resistance and low-temperature toughness of the steel plate. In addition, it is also detrimental to the strength-toughness / strength-plasticity balance of the steel plate. While increasing the carbon content can refine the microstructure of the steel plate, it can also impair its weldability, especially under ultra-high heat input welding conditions. This is because the heat-affected zone (HAZ) grains become severely coarsened, and the cooling rate during the welding thermal cycle is very slow. This leads to the formation of coarse ferrite sideplates (FSP), Widmanstätten (WF), and upper bainite (Bu) in the HAZ, along with an increase in the number and size of macromolecular islands, severely damaging the toughness of the HAZ. Furthermore, excessively high carbon content not only deteriorates the plasticity (uniform elongation, fracture elongation) of the steel plate, negatively impacting its resistance to impact, but also worsens the phase ratio, morphology, phase distribution, and grain size matching of the multiphase structure, further deteriorating the steel plate's impact resistance and low-temperature toughness. Therefore, the carbon content should not exceed 0.15%.

[0035] Si promotes deoxidation of molten steel and can improve the strength of steel plates. However, the deoxidation effect of Si is not significant when using Al-deoxidized molten steel. Although Si can improve the strength of steel plates, it increases the degree of solidification segregation in molten steel, which seriously impairs the low-temperature toughness and weldability of steel plates. Especially under ultra-high heat input welding conditions, Si not only promotes the formation of blocky MA islands, but also forms large and unevenly distributed MA islands, which seriously impairs the toughness of the weld heat-affected zone (HAZ). Therefore, the Si content in steel should be controlled as low as possible. Considering the economy and operability of the steelmaking process, the Si content is controlled below 0.20%.

[0036] As the most important alloying element, manganese (Mn) in steel not only improves the strength of steel plates, but also expands the austenite phase region, lowers the Ar3 point temperature, refines the uniform grain size to improve the low-temperature toughness of steel plates, and promotes the formation of bainite during the TMCP process, thus increasing the strength of steel plates. The internally controlled Mn content in steel should not be lower than 1.30%. Mn is prone to segregation during the solidification process of molten steel, especially when the Mn content is too high (when the Mn content > 1.60%). This not only easily leads to co-segregation with elements such as C, P, and S, aggravating the segregation and porosity in the center of the continuously cast billet, but also, in severe cases, the segregation in the center of the continuously cast billet can easily form abnormal structures during subsequent controlled rolling and welding. Furthermore, excessively high Mn content can also form coarse MnS particles. These coarse MnS particles extend along the rolling direction during hot rolling, severely deteriorating the base steel plate (especially in the transverse direction) and the weld heat-affected zone (HAZ). Especially under ultra-high heat input welding conditions, Mn content significantly reduces impact toughness (i.e., promotes the formation of coarse upper bainite and increases the size and quantity of blocky MA), and results in poor resistance to lamellar tearing. Furthermore, excessively high Mn content also increases the hardenability of steel, raises the weld cold cracking susceptibility coefficient (Pcm), and affects the weldability of the steel (under lower heat input welding, brittle hard structures such as martensite are easily formed; under ultra-high heat input welding, abnormal structures such as coarse ferrite side strips (FSP), Widmanstätten structure (WF), and upper bainite (Bu) are easily formed). Therefore, the upper limit of Mn content in steel should not exceed 1.60%.

[0037] Phosphorus (P) is a harmful inclusion in steel that has a significant detrimental effect on the mechanical properties of steel, especially low-temperature impact toughness and weldability. Theoretically, the lower the content, the better. However, considering the operability and cost of steelmaking, for steel plates that require welding with ultra-high heat input and toughness at -40℃, the P content needs to be controlled at ≤0.013%.

[0038] Sulfur (S), as a harmful inclusion in steel, significantly impairs its low-temperature toughness. More importantly, S combines with manganese (Mn) in steel to form MnS inclusions. During hot rolling, the plasticity of MnS causes it to extend along the rolling direction, forming MnS inclusion bands along the rolling direction. This severely damages the impact toughness (especially transverse impact toughness), Z-axis properties, and weldability of the steel plate. Furthermore, S is a major element contributing to hot brittleness during hot rolling. Theoretically, the lower the S content, the better. However, considering the principles of steelmaking operability, cost, and smooth logistics, for steel plates requiring -40℃ toughness and capable of welding with extremely high heat input, the S content needs to be controlled at ≤0.0030%.

[0039] The purpose of adding trace amounts of Nb to steel is to effectively implement the TMCP process. When the Nb content is below 0.008%, the effect of non-recrystallization controlled rolling cannot be effectively utilized. When the Nb content exceeds 0.022%, it is easy to induce the formation of abnormal structures such as coarse ferrite side strips (FSP), Widmanstätten structure (WF), and upper bainite (Bu) under ultra-high heat input welding conditions, and promote the formation of blocky MA islands, which seriously impairs the low-temperature toughness of the heat-affected zone (HAZ) of ultra-high heat input welding. Therefore, the Nb content is controlled between 0.008% and 0.022% to obtain the best TMCP effect without compromising the low-temperature toughness of the HAZ of ultra-high heat input welding.

[0040] The purpose of adding trace amounts of Ti to steel is to combine with N in the steel to generate highly stable TiN particles. This controls austenite grain growth in the weld hard zone (HAZ), inhibits the formation of abnormal structures such as coarse ferrite side strips (FSP), Widmanstätten (WF), and upper bainite (Bu), and improves the weldability and low-temperature toughness of the HAZ. The Ti content added to the steel must match the N content. The principle of matching is that TiN must precipitate in the solid phase, not in the liquid steel. If the Ti content is too low (<0.008%), the number of TiN particles formed is insufficient to inhibit austenite grain growth in the HAZ, suppress the formation of coarse FSP, Bu, and other embrittlement structures, and thus improve the low-temperature toughness of the HAZ. If the Ti content is too high (>0.016%), large-sized TiN particles may precipitate. These large-sized TiN particles not only fail to inhibit austenite grain growth in the HAZ but also become the initiation point for crack initiation. Therefore, the optimal range for controlling the Ti content is 0.008% to 0.016%.

[0041] To ensure that Al and N combine to form AlN and B combine to form BN during the welding thermal cycle cooling process, thus improving the low-temperature toughness of steel plates in the high-heat-input welding HAZ, the lower the Al content in the steel, the better. However, when the Al content in the steel decreases below 0.005%, the deoxidation of the molten steel is insufficient, the inclusion content in the steel increases dramatically, and the low-temperature toughness, impact resistance, and weldability of the steel plate are severely deteriorated. On the other hand, when the Al content in the steel exceeds 0.015%, the precipitation of BN during the high-heat-input welding thermal cycle is inhibited, and a sufficient amount of BN cannot be formed. In summary, the appropriate Al content is controlled between 0.005% and 0.015%.

[0042] The control range of nitrogen (N) corresponds to that of Ti and boron (B). For welded steel plates with ultra-high heat input, the optimal Ti / N ratio is between 1.0 and 2.0. If the N content is too low, the number of TiN particles generated will be small and their size will be large, failing to improve the weldability of the steel and instead harming it. However, if the N content is too high, the free [N] content in the steel increases, especially under ultra-high heat input welding conditions, the free [N] content in the heat-affected zone (HAZ) increases sharply, severely impairing the low-temperature toughness of the HAZ and worsening the weldability of the steel. Therefore, the N content should be controlled between 0.0055% and 0.0085%.

[0043] Boron (B) is the most critical alloying element for improving the welding of steel plates with ultra-high heat input. B plays a dual role in this process: firstly, through solid solution, B segregates at the original austenite grain boundaries in the high heat input zone (HAZ), inhibiting the nucleation of ferrite at grain boundaries and forming abnormal structures such as coarse ferrite side strips (FSP), Widmanstätten structure (WF), and upper bainite (Bu); secondly, through intragranular BN particles acting as heterogeneous nucleation sites, it promotes the nucleation of equiaxed ferrite within the austenite grains of the HAZ, refining the microstructure of the HAZ in ultra-high heat input welding. To achieve these objectives, the B content in the steel is controlled between 0.0012% and 0.0030%.

[0044] Ca treatment of steel serves two purposes: firstly, it further purifies the molten steel; secondly, it modifies the sulfides in the steel, transforming them into non-deformable, stable, fine spherical sulfides, suppressing the hot brittleness of sulfur, improving the low-temperature toughness and Z-axis properties of the steel, and improving the anisotropy of the steel plate's toughness. The amount of Ca added depends on the sulfur content in the steel. If the amount of Ca added is too low, the treatment effect is minimal; if the amount of Ca added is too high, the Ca(O,S) ions will be too large, the number of inclusions in the steel will increase, and the brittleness will also increase, potentially becoming a fracture crack initiation point, reducing the low-temperature toughness and weldability of the steel, while also reducing the purity of the steel and contaminating the molten steel. Therefore, the appropriate range for the Ca content is 0.0010% to 0.0035%.

[0045] Cosegregation index (%C) 0.5 ×[10.2(%P)+30.7(%S)+3.3(%Mn)]×[7.8(%Si)+5.6(%Als)]≤2.52.

[0046] Intrinsic segregation in slabs significantly impacts the impact resistance of steel plates. Because the microstructure of the segregated zone differs from that of the non-segregated zone, the segregated zone not only contains abnormally brittle structures (such as high-carbon bainite, high-carbon martensite, and M / A islands), but also accumulates a large number of harmful inclusions (such as elongated MnS inclusions, Al2O3, and SiO2 inclusions), severely degrading the impact resistance of the steel plate. Suppressing the co-segregation of C, Mn, P, and S, and the promoting effect of Si and Als on the co-segregation of C, Mn, P, and S, reducing intrinsic segregation during steel solidification, and ensuring the impact resistance of the steel plate are extremely important. The fact that C, Mn, P, S, Si, and Als satisfy the above formula can suppress the co-segregation of C, Mn, P, and S, and the promoting effect of Si and Als on the co-segregation of C, Mn, P, and S; this is one of the key technologies of this invention.

[0047] [1.27(%B)] / [(%N)-0.292(%Ti)-0.518(%Als)-0.147(%Nb)]≤61.5;

[0048] 1) Ensure that B and N form BN particles during heating, rolling and accelerated cooling, eliminate the presence of solid solution [B] in the steel, eliminate the phase transformation strengthening effect of B, and achieve high toughness, excellent impact resistance and strong toughness / strong plasticity matching of the steel plate.

[0049] 2) Both Ti and B are strong nitride-forming elements, but their roles in welding thermal cycling are fundamentally different. TiN exhibits high stability at relatively high temperatures (≤1250℃), effectively suppressing HAZ grain growth far from the fusion line. However, near the fusion line (FL), the peak temperature of welding thermal cycling becomes very high (>1250℃), causing TiN particles to dissolve and failing to effectively suppress HAZ grain growth. Although BN particles are less stable than TiN particles at high temperatures and completely dissolve in the steel during welding thermal cycling heating and holding, due to the high diffusivity of B in steel, BN rapidly re-precipitates during the cooling process of welding thermal cycling (Ti and Al have very low diffusivity, so TiN and AlN cannot re-precipitate during welding thermal cycling cooling, i.e., the precipitation kinetics of TiN and AlN are very slow). Due to the special nature of its crystal structure, BN particles have a low-energy orientation relationship with ferrite (100). BN / / (110) α BN particles can serve as effective nucleation sites for ferrite, promoting the nucleation of equiaxed ferrite grains within austenite grains, refining the HAZ microstructure, and improving the low-temperature toughness and crack resistance and arrest properties of the HAZ in ultra-high heat input welding; this is one of the key technologies of this invention.

[0050] Ti / N≤2.0;

[0051] Ensuring excess [N] during the [Ti]+[N]→TiN reaction: 1) Inhibiting Oswald ripening of TiN particles and forming fine, dispersed TiN particles; 2) Ensuring sufficient residual dissolved [N] to combine with [B], eliminating the phase transformation strengthening effect of [B] during TMCP, and ensuring the steel plate has excellent strength-toughness / strength-plasticity matching and impact resistance; 3) Eliminating excess dissolved [N] in the weld heat-affected zone, i.e., all dissolved [N] is consumed by nitrogen-fixing elements Ti, Als, B, and Nb, eliminating dissolved [N] in the weld HAZ of the steel plate, eliminating the huge embrittlement effect of dissolved [N] on the ferrite structure, and improving the low-temperature toughness and crack resistance and crack arrest characteristics of the HAZ; This is one of the key technologies of this invention.

[0052] Ca treatment, with the Ca / S ratio controlled between 1 and 3; 1.0 × 10 －3 ≤(%Ca)×(%S) 0.28 ≤3.0×10 －3 .

[0053] To ensure spheroidization of sulfurization and minimize the impact of inclusions on low-temperature toughness and weldability, a certain number of Ca(O,S) particles are uniformly and finely distributed in the steel to suppress excessive growth of austenite grains in the heat-affected zone (HAZ) of high heat input welding. Secondly, Ca treatment is used to form a certain number of ultrafine Ca(O,S) inclusions, which promotes the precipitation and growth of BN particles in the Ca(O,S) inclusions. That is, BN particles often nucleate and grow non-uniformly on Ca(O,S) inclusions. Then, BN particles promote the nucleation of ferrite within the austenite grains in the HAZ, segmenting the austenite grains in the HAZ, refining the grain size in the HAZ, and improving the low-temperature toughness of the weld HAZ.

[0054] The steel plate of this invention has extremely excellent ultra-low temperature impact toughness (-50℃) and its microstructure consists of uniform and fine equiaxed ferrite plus a small amount of dispersed bainite.

[0055] The method for manufacturing high-toughness, high-ductility, and ultra-large heat input steel plates with strong impact resistance according to the present invention includes the following steps:

[0056] 1) Smelting and casting

[0057] The above-mentioned components are smelted and cast into slabs; among which, the casting adopts continuous casting, the superheat is controlled at 10-30℃, and the continuous casting light pressure is controlled at 2-4%.

[0058] 2) Slab heating

[0059] The slab heating temperature is controlled between 1080 and 1150℃;

[0060] 3) Controlled rolling

[0061] The non-recrystallization controlled rolling start temperature is 810~920℃, the rolling pass reduction rate is ≥7%, the cumulative reduction rate is ≥50%, and the final rolling temperature is 800~850℃;

[0062] 4) Control cooling

[0063] After controlled rolling, the steel plate immediately undergoes the first stage of accelerated cooling. The initial cooling temperature of the steel plate is 780-840℃, the final cooling temperature is 700-750℃, and the cooling rate is 1-4℃ / s.

[0064] The steel plate was then naturally air-cooled to 620-680℃ and then subjected to a second stage of accelerated cooling. The cooling temperature of the steel plate was 300-400℃ and the cooling rate was ≥5℃ / s. The steel plate was then naturally air-cooled to room temperature.

[0065] In addition to meeting the above requirements, the following relationship must also be satisfied:

[0066] [ξ×(T 开冷2 )×(DI OL )×(V c2 )] / {H×(T 开轧 )×[Ar3-(T 开冷2 )]×(T 停冷2 )}≥7.6×10 -3 ;in,

[0067] ξ represents the cumulative reduction rate without recrystallization, in percentage (%).

[0068] T 开轧 The rolling temperature before recrystallization, in °C;

[0069] T 开冷2 This is the starting temperature of the second stage of accelerated cooling, in °C.

[0070] V c2 The second stage accelerates the cooling rate, measured in °C / s;

[0071] T 停冷2 The temperature at which the second stage of accelerated cooling stops is measured in °C.

[0072] DI OL For online hardenability index,

[0073] DI OL =0.51 (%C) 0.5 [1+0.7(%Si)][1+3.33(%Mn)][1+0.35(%Cu)][1

[0074] +0.36(%Ni)][1+2.16(%Cr)][1+3(%Mo)][1+1.75(%V)][1+

[0075] 1.77(%Al)][1+200(%B)]×25.4, unit mm;

[0076] H represents the thickness of the finished steel plate, in mm;

[0077] Ar3 is the temperature at which the austenite-ferrite phase transformation begins under deformation, and the unit is °C.

[0078] Ar3=910-310(%C)-80(%Mn)-20(%Cu)-15(%Cr)-55(%Ni)

[0079] -80(%Mo)-0.35(t-8), where t is the thickness of the finished steel plate in mm.

[0080] In the method for manufacturing the steel plate described in this invention:

[0081] To ensure the excellent "three properties" (soundness, homogeneity, and purity) of the slab, the superheat of continuous casting is controlled between 10 and 30°C, and the light pressure of continuous casting is controlled between 2% and 4%.

[0082] To ensure uniform and fine initial austenite grains, a low-temperature slab heating process must be employed, while simultaneously ensuring complete solid solution of the microalloying element Nb. The slab heating temperature is controlled between 1080 and 1150°C to ensure that the initial slab has uniform and relatively fine austenite grains.

[0083] The initial rolling temperature for non-recrystallization controlled rolling is 810–920℃, the reduction rate per rolling pass is ≥7%, the cumulative reduction rate is ≥50%, and the final rolling temperature is 800–850℃.

[0084] After controlled rolling, the steel plate immediately undergoes the first stage of accelerated cooling. The initial cooling temperature is 780–840℃, and the final cooling temperature is 700–750℃, with a cooling rate of 1℃ / s ≤ 4℃ / s. Subsequently, the steel plate is naturally air-cooled to 620–680℃ and then undergoes the second stage of accelerated cooling. The final cooling temperature is 300–400℃, and the cooling rate is ≥ 5℃ / s. Afterward, the steel plate is naturally air-cooled to room temperature.

[0085] In addition to meeting the above requirements, the following relationship must also be satisfied:

[0086] [ξ×(T 开冷2 )×(DI OL )×(V c2 )] / {H×(T 开轧 )×[Ar3-(T 开冷2 )]×(T 停冷2 )}≥7.6×10 -3 .

[0087] As the cumulative reduction rate ξ of non-recrystallization increases, the initial rolling temperature T开轧 The reduction in austenite flattening and the increase in the density of crystal defects such as dislocations, deformation bands and grain boundary steps within the austenite body lead to an increase in the density of ferrite nucleation sites within the austenite body, thus refining the microstructure of TMCP steel plates and improving the low-temperature toughness of the steel plates.

[0088] With the second stage of accelerated cooling, the cooling temperature T 开冷2 Within a certain range (620~680℃), the amount of proeutectoid ferrite increases, the carbon content of untransformed austenite increases, and after accelerated cooling, the amount of ferrite in the microstructure of the steel plate increases and the amount of bainite decreases. Bainite is more easily embedded in the ferrite matrix and the hardness of bainite increases. During deformation, it is easier to realize the Oromann dislocation bypass mechanism, which can obtain a very good uniform elongation and fracture elongation (i.e. impact resistance).

[0089] With the second stage of accelerated cooling stopping at temperature T 停冷2 Reducing the thickness H of the finished steel plate increases its strength. This is due to the increase in alloy content (Pcm) and the accelerated cooling rate V in the second stage. c2 Increase the size of the steel plate, thus increasing its strength.

[0090] The steel plate's strength, toughness, impact resistance, weldability under ultra-high heat input, composition, thickness, cumulative reduction without recrystallization, initial rolling temperature, second-stage accelerated cooling start and stop temperatures, and second-stage accelerated cooling rate V are all considered. c2 The relationships between these factors are used to form the above-mentioned formula to ensure the steel plate's strength, low-temperature toughness, impact resistance, and weldability under ultra-high heat input. This is one of the key technologies of this invention.

[0091] The beneficial effects of this invention are:

[0092] This invention successfully mass-produces 500MPa grade steel plates with impact resistance and the ability to be welded with ultra-high heat input by designing a low-cost composition without adding expensive alloying elements Cu and Ni, and matching it with the corresponding TMCP process. The steel plates of this invention are manufactured using online controlled TMCP process technology, which not only significantly reduces the overall manufacturing cost and shortens the manufacturing cycle of the steel plates, creating huge value for enterprises, but also achieves green and environmentally friendly manufacturing process. The high performance and high added value of steel plates are mainly reflected in their high strength, high toughness, excellent impact resistance and crack resistance. At the same time, the steel plates also have excellent ultra-high heat input welding performance. They have successfully solved the key technical problem of the conflict and difficulty in reconciling the low carbon content, low carbon equivalent, and low cost (i.e., no precious alloying elements such as Cu and Ni) with high strength, high toughness, and excellent impact resistance in composition design and process design. This has greatly improved the safety and stability of large heavy steel structures. Ultra-high heat input welding saves users the cost of manufacturing steel components (ultra-high heat input high-efficiency welding) and significantly shortens the manufacturing time of users' steel components, creating huge value for users. Therefore, this type of steel plate is not only a high-value-added and environmentally friendly product.

[0093] The steel plate manufacturing technology of this invention not only reduces steel plate manufacturing costs and shortens the steel plate production cycle; secondly, the excellent weldability of the steel plate and its ability to be welded with ultra-high heat input greatly improve the on-site welding efficiency of users, save users' component manufacturing costs, shorten users' component manufacturing time, and create huge value for users; more importantly, the steel plate has excellent impact resistance characteristics, ensuring the safety and reliability of the steel structure throughout its service life. Therefore, this type of steel plate is a high-value-added and environmentally friendly product. Attached Figure Description

[0094] Figure 1 This is a micrograph of the steel from Example 5 of the present invention. Detailed Implementation

[0095] The present invention will be further described below with reference to the embodiments and accompanying drawings.

[0096] The composition of the steel plate in the embodiments of the present invention is shown in Table 1, with the remainder including Fe and unavoidable impurities. Tables 2 to 4 show the manufacturing process parameters of the steel in the embodiments of the present invention, and Table 5 shows the performance parameters of the steel in the embodiments of the present invention.

[0097] Depend on Figure 1 It can be seen that the microstructure of the steel plate consists of fine ferrite and a small amount of dispersed bainite, with an average grain size of less than 20 μm, and the cold deformation morphology of proeutectoid ferrite is clearly visible.

[0098] In summary, this invention achieves high-quality YP355MPa grade steel plate with excellent impact resistance by using a low-cost composition design without adding expensive alloying elements Cu and Ni, and matching it with a corresponding special TMCP process. This fully explores the potential for matching composition design with TMCP process.

[0099] This invention's steel plate manufacturing technology not only significantly reduces the overall manufacturing cost and shortens the steel plate manufacturing cycle, creating enormous value for enterprises, but also achieves a green and environmentally friendly manufacturing process. The high performance and high added value of the steel plate are concentrated in its high strength, excellent low-temperature toughness, and impact resistance, while its weldability (especially its weldability under ultra-high heat input) is equally excellent. It successfully solves the key technical problem of the conflicting and difficult-to-reconcile relationship between low C content, low carbon equivalent, low cost (i.e., no or trace amounts of precious alloying elements such as Cu and Ni) and high strength, impact resistance, and weldability under ultra-high heat input in composition and process design, greatly improving the safety and stability of large heavy steel structures.

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Claims

1. A high-toughness, high-ductility steel plate with strong impact resistance and capable of ultra-large heat input, its composition by weight percentage is: C：0.09～0.15％ Si: ≤0.20% Mn: 1.30%~1.60% P：≤0.013％ S：≤0.0030％ Nb: 0.008%~0.022% Ti: 0.008%~0.016% Als: 0.005%~0.015% N：0.0055％～0.0085％ B：0.0012％～0.0030％ Ca: 0.0010%~0.0035% The balance consists of Fe and other unavoidable inclusions; and the contents of the above elements must simultaneously satisfy the following relationship: Cosegregation index: (%C) 0.5 x [10.2 (%P) + 30.7 (%S) + 3.3 (%Mn)] x [7.8 (%Si) + 5.6 (%Als)] < 2.52; [1.27(%B)] / [(%N)-0.292(%Ti)-0.518(%Als)-0.147(%Nb)]≤61.5; Ti / N≤2.0; Ca treatment, Ca / S ratio = 1.0 to 3.0, 1.0 x 10 －3 ≤ (% Ca) x (% S) 0.28 ≤ 3.0 x 10 －3 ; The microstructure of the steel plate is uniform and fine equiaxed ferrite with a small amount of dispersed bainite. The steel plate has a yield strength ≥355MPa, tensile strength ≥490MPa, uniform elongation ≥22%, post-fracture elongation (i.e., impact resistance) ≥40%, transverse impact toughness KV2 ≥100J at -40℃, and KV2 ≥70J at -40℃ for welded HAZ.

2. The method for manufacturing a high-toughness, high-ductility steel plate with strong impact resistance and ultra-high heat input as described in claim 1, characterized in that, Includes the following steps: 1) Smelting and casting The slab is smelted and cast according to the composition of claim 1; wherein the casting adopts continuous casting, the superheat is controlled at 10-30℃, and the continuous casting light pressure is controlled at 2-4%. 2) Slab heating The slab heating temperature is controlled between 1080 and 1150℃; 3) Controlled rolling The non-recrystallization controlled rolling start temperature is 810~920℃, the rolling pass reduction rate is ≥7%, the cumulative reduction rate is ≥50%, and the final rolling temperature is 800~850℃; 4) Control cooling After controlled rolling, the steel plate immediately undergoes the first stage of accelerated cooling. The initial cooling temperature of the steel plate is 780-840℃, the final cooling temperature is 700-750℃, and the cooling rate is 1-4℃ / s. The steel plate was then naturally air-cooled to 620-680℃ and then subjected to a second stage of accelerated cooling. The cooling temperature of the steel plate was 300-400℃ and the cooling rate was ≥5℃ / s. The steel plate was then naturally air-cooled to room temperature. In addition to meeting the above requirements, the following relationship must also be satisfied: [ξ×(T 开冷2 )×(DI OL )×(V c2 )] / {H×(T 开轧 )×[Ar3-(T 开冷2 )]×(T 停冷2 )}≥7.6×10 -3 ; wherein, ξ represents the cumulative reduction rate without recrystallization, in percentage (%). T 开轧 recrystallization, in °C; T 开冷2 T2 is the second stage accelerated cooling start temperature in °C; V c2 acceleration cooling rate for the second stage, in °C / s; T 停冷2 T2 is the second stage accelerated cooling stop temperature in °C; DI OL For online hardenability index, DI DI. OL 0.51(%C) 0.5 [1+0.7(%Si)][1+3.33(%Mn)][1+0.35(%Cu)][1 +0.36 (% Ni)][1+2.16 (% Cr)][1+3 (% Mo)][1+1.75 (% V)][1+1.77 (% Al)][1+200 (% B)]×25.4, unit mm; H represents the thickness of the finished steel plate, in mm; Ar3 is the temperature at which the austenite-ferrite phase transformation begins under deformation, and the unit is °C. Ar3=910-310(%C)-80(%Mn)-20(%Cu)-15(%Cr)-55(%Ni)-80(%M o)-0.35(t-8), where t is the thickness of the finished steel plate in mm.

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