High-toughness, low yield strength ratio, and low longitudinal and transverse strength anisotropy of 800MPa grade weathering steel plate and its manufacturing method

Abstract

A high-toughness, low-yield-strength ratio, and low longitudinal and transverse strength anisotropy 800MPa grade weathering steel plate and its manufacturing method are disclosed. The steel plate has the following composition by weight percentage: C 0.060~0.010%, Si≤0.40%, Mn1.00%~1.40%, P≤0.015%, S≤0.003%, Cu 0.25~0.45%, Ni 0.15~0.55%, Cr 0.40~0.70%, Mo 0.05~0.35%, Nb 0.010%~0.040%, Ti 0.008~0.016%, Als 0.040~0.070%, N≤0.0050%, Ca 0.0010~0.0035%, balance includes Fe and unavoidable inclusions; the steel plate of this invention has a yield strength ≥650MPa, tensile strength ≥770MPa, low yield-to-tensile ratio (≤0.86), low longitudinal and transverse strength anisotropy (i.e., longitudinal / transverse strength difference ≤50MPa), excellent weldability, and can withstand a welding heat input of 50kJ / cm; impact toughness KV2 ≥120J at -40℃, and welding heat-affected zone (HAZ) Akv ≥70J at -40℃.

Description

Technical Field

[0001] This invention relates to the field of weathering steel plate technology, and in particular to a high-toughness, low yield strength ratio, and low longitudinal and transverse strength anisotropy 800MPa grade weathering steel plate and its manufacturing method. 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, hydropower projects, bridge structures, boiler containers, building structures, 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, weldability, and the matching between them are the most important properties of low-carbon (high-strength) low-alloy steel, which are ultimately determined by the microstructure and dislocation substructure of the finished steel.

[0003] With the continuous advancement of metallurgical technology, higher demands are being placed on the toughness, plasticity, especially weldability, and low anisotropy of high-strength steel. Specifically, the steel plate must possess resistance to brittle fracture and ductile instability fracture at low temperatures (-40℃), while achieving fracture elongation and uniform elongation at levels comparable to 600MPa grade steel plates. Furthermore, the steel plate must exhibit excellent weldability and low anisotropy, capable of withstanding high heat input welding (up to 50kJ / cm). Moreover, under conditions of relatively low alloy content, especially precious metal content, and lower manufacturing costs, the overall mechanical and performance properties of the steel plate must be significantly improved to reduce steel consumption. The reduction in alloy usage saves costs, and the high strength and lightweight design reduces the weight, stability, and safety of steel components. More importantly, it further improves the cold / hot workability and safety and reliability of steel components during service. Currently, Japan, South Korea, the European Union, and North America have launched a research boom in developing next-generation high-performance steel materials. They strive to achieve better microstructure matching through alloy combination optimization design, control of submicroscopic fine structure, and innovative manufacturing process technology. The goal is to achieve ultra-fine metallographic microstructure and substructure (dislocation substructure configuration, packet, block, etc.) fine structure, so that high-strength steel can obtain better strength-ductility and toughness matching, weldability, and low anisotropy.

[0004] Traditional high-strength steel plates with a tensile strength ≥780MPa are mainly produced through offline quenching and tempering (RQ+T); this requires the steel plate to have sufficiently high hardenability and hardenability, i.e., hardenability index DI ≥ 2 × finished steel plate thickness [DI = 0.311C]. 1 / 2[(1+0.64Si)×(1+4.10Mn)×(1+0.27Cu)×(1+0.52Ni)×(1+2.33Cr)×(1+3.14Mo)×25.4(mm)], to ensure that the steel plate has sufficiently high strength, excellent low-temperature toughness and uniformity of microstructure and properties along the thickness direction, it is inevitable to add a large amount of alloying elements such as Cr, Mo, Ni, and Cu to the steel, especially the addition of a large amount of Ni. (See CAMP-ISIJ, Vol.4, 1991, 1949; CAMP-ISIJ, Vol.4, 1991, 1950; CAMP-ISIJ, Vol.7, 1994, 836; CAMP-ISIJ, Vol.7, 1994, 837; Japanese Patent No. 59-129724; Hei 1-219121; Nippon Steel Research No. 314-1984; Japan Steel Pipe Technical Report No. 107-1985; Nippon Steel Technical Report No. 348-1993; Kawasaki Steel Technical Report Vol.4 (No.3)-1972; Kawasaki Steel Technical Report Vol.7 (No.2)-1975).

[0005] More importantly, the traditional quenched and tempered steel composition system and manufacturing process not only result in long steel plate manufacturing cycles and high manufacturing costs, but also make the processing of scrap steel with high Cu and Ni content difficult, limiting the efficiency of scrap steel recycling and reuse. Moreover, for 80 kg-class quenched and tempered steel produced by traditional quenching and tempering processes, the high alloy content of the steel plate leads to low elongation, high yield strength ratio, poor weldability (high sensitivity to cold cracking during welding, high embrittlement of the heat-affected zone during welding, sensitivity to reheat cracking, etc.), and difficulty in controlling the uniformity of properties in the thickness direction. The low elongation is not only detrimental to the cold and hot working performance of the steel plate, but also has a significant impact on the fatigue resistance, stress concentration sensitivity, and structural stability of the steel plate. This poses significant safety hazards when used in large-scale engineering construction and large equipment such as pressure water pipes and steel branch pipes in hydropower projects, steam turbine generators in thermal power plants and offshore platform structures, floating cranes on ships, and giant excavators. Therefore, when using high-strength steel for large fatigue heavy-load steel structures, it is generally desirable for 80 kg-class high-strength steel to have excellent strength-toughness and strength-plasticity matching, especially a tensile elongation δ5 of more than 18%.

[0006] Existing patent literature largely describes how to achieve strength and low-temperature toughness in the base steel plate, with limited explanation of improving weldability and obtaining excellent low-temperature toughness in the weld heat-affected zone (HAZ). Furthermore, it fails to address how to improve the tensile strength of the steel plate while simultaneously increasing its tensile elongation and the uniformity of its mechanical properties in the thickness direction. (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 Patent 5798004, European Patent EP 0288054A2, and the Nishiyama Memorial Technical Lecture, pp. 159-160, pp. 79-80.)

[0007] Other patents include Chinese patent CN200710042357 "HT780 steel plate with excellent weldability and low yield strength ratio and its manufacturing method", Chinese patent CN200810036416 "steel plate with high toughness and high plasticity and its manufacturing method", and Chinese patent CN201810163615 "low cost, high toughness and excellent weldability 800MPa grade quenched and tempered steel plate and its manufacturing method". The steel plates produced by these patented technologies also have a very high level of comprehensive mechanical properties: tensile strength ≥780MPa, yield strength ≥690MPa, Charpy transverse impact energy (single value) at -40℃ and below ≥47J, and the steel plates have excellent weldability. However, the steel plates are all produced by offline quenching and tempering process, and a certain amount of Cu and Ni alloying elements are inevitably added to the steel, especially a large amount of Ni element.

[0008] Chinese patent CN200910048287, "Low-Cost 80kg-Grade Extra-Thick Quenched and Tempered Steel Plate and Its Manufacturing Method," describes a steel plate whose chemical composition does not contain Cu or Ni elements. However, the impact toughness of the steel plate only meets the requirements for temperatures of -20℃ and above. Furthermore, the steel plate employs a controlled rolling + offline quenching + tempering process. This not only involves numerous manufacturing steps, a long manufacturing cycle, and high manufacturing costs, but also relatively high energy consumption (the steel plate is rolled and then naturally air-cooled to room temperature, followed by shot blasting, and then reheated to the quenching temperature), which is not conducive to energy conservation and environmental protection. Moreover, the use of off-grid... Linear quenching and tempering processes cannot fully utilize the hardening and quenching potential of alloying elements. The hardening and quenching properties of the elements cannot be maximized. Therefore, in order to obtain the same strength and toughness, more alloying elements (especially Ni, Mo, Cr, etc.) must be added. This not only further increases the manufacturing cost, but also impairs the weldability of the steel plate. In particular, for high-strength quenched and tempered steel plates, the sensitivity to welding cold cracking is greatly increased. Higher temperature welding preheating and post-heating (i.e., PWHT) are required, and the appropriate range of welding heat input is narrower, which in turn greatly increases the processing and manufacturing cost.

[0009] Chinese patent CN201210209649, "Nickel-free high-toughness 80 kg-grade high-strength steel and its manufacturing method," successfully developed an 800 MPa-grade quenched and tempered steel plate with excellent performance using an online quenching and tempering process (DQ+T) without adding precious alloying elements such as Cu and Ni. However, its low-temperature toughness cannot meet the -40℃ low-temperature requirement, and the steel plate cannot be welded without preheating, and the welding heat input cannot exceed 50 kJ / cm. More importantly, none of the steel plates developed in the above inventions involve corrosion resistance requirements, and therefore cannot meet the design safety requirements of special large-scale heavy steel structures (such as long-span railway and highway bridge structures, marine engineering structures, and giant ship floating cranes). Summary of the Invention

[0010] The purpose of this invention is to provide a high-toughness, low-yield-strength ratio, and low longitudinal / transverse strength anisotropy 800MPa grade weathering steel plate and its manufacturing method. This steel plate achieves the high strength, excellent low-temperature toughness, low yield-strength ratio, high weather resistance, and low longitudinal / transverse strength anisotropy of the base weathering steel plate. Simultaneously, it exhibits excellent low-temperature toughness in the HAZ during high heat input welding. The steel plate has a yield strength ≥650MPa, tensile strength ≥770MPa, low yield-strength ratio (≤0.86), low longitudinal / transverse strength anisotropy (i.e., longitudinal / transverse strength difference ≤50MPa), excellent weldability, a preheating temperature ≤50℃, and can withstand a welding heat input of 50kJ / cm. Its impact toughness at -40℃ (KV2) ≥120J, and its weld heat-affected zone (HAZ) at -40℃ (Akv) ≥70J, makes it particularly suitable for offshore platforms, cross-sea bridge steel structures, hydropower metal structural components, port machinery, and engineering machinery in cold regions. Furthermore, it enables low-cost, stable, and mass industrial production.

[0011] To achieve the above objectives, the technical solution of the present invention is as follows:

[0012] High-toughness, low yield strength ratio, and low anisotropy TMCP weathering steel plates are among the most challenging types of thick plates. This is because these steel plates not only require low carbon, low phosphorus content (Pcm), high strength, excellent low-temperature toughness, fatigue resistance, weather resistance, and low yield strength ratio and low anisotropy, but also the ability to withstand high heat input welding and excellent low-temperature impact toughness in the weld heat-affected zone. However, it is difficult to simultaneously meet these performance requirements: A) low carbon, low Pcm, and high strength and low yield strength ratio; B) high strength, high toughness, weather resistance, and low yield strength ratio, with low anisotropy in longitudinal and transverse strength; C) high strength, weather resistance, and excellent weldability, especially high heat input weldability.

[0013] The aforementioned properties conflict with each other in composition design and TMCP process design, and are difficult to reconcile: when reducing C content and Pcm, it is difficult to achieve high strength and low yield strength ratio of steel plate; while improving strength, low temperature toughness and weather resistance, it is difficult to achieve low yield strength ratio and low longitudinal and transverse strength anisotropy of steel plate; when steel plate achieves high strength and high weather resistance, weldability of steel plate is especially difficult to guarantee for steel plate with high heat input.

[0014] Balancing low Pcm, high strength, high toughness, high weather resistance, low yield strength ratio, low longitudinal and transverse strength anisotropy, and high heat input weldability is one of the biggest challenges of this invention and also a key core technology. Therefore, this invention integrates key factors that affect the high strength, high toughness, low yield strength ratio, low longitudinal and transverse strength anisotropy, and high heat input weldability of steel plates in its key technical routes, composition, and process design.

[0015] This invention starts with alloy design, using low-carbon C-low-Mn low-alloy steel as the base (a composition system of low C-medium Mn-low (Cu+Ni+Mo) alloying-Nb microalloying-ultra-micro Ti treatment). It aims to maximize the Al content in the steel, achieving Als / N ≥ 12, Pcm ≤ 0.26%, low N content, (Cu+Ni+Cr+Mo) alloying with a weather resistance index I ≥ 6.15, Ti+Nb microalloying, Ca treatment with a Ca / S ratio controlled between 1.0 and 3.0, and Ca×S... 0.28 ≤2.0×10 －3 Optimized alloy combination design and optimized TMCP process: [(%Nb)×ξ×10 5 ] / [ζ×H×(T 开轧 )]≤3.70、[ξ×(T 开冷 )×(DI OL )×(V c )] / [(T 停冷 [×H]≥1340 ensures that the microstructure of the finished weathering steel plate is uniform and fine low-carbon lower bainite, with an average grain size of less than 20μm. This results in high strength, excellent low-temperature toughness, low yield strength ratio, high weather resistance, and low longitudinal / transverse strength anisotropy of the base weathering steel plate. At the same time, the low-temperature toughness of the HAZ is also excellent during high heat input welding.

[0016] Specifically, the high-toughness, low-yield-strength ratio, and low longitudinal and transverse strength anisotropy 800MPa grade weathering steel plate described in this invention has the following composition by weight percentage:

[0017] C: 0.060%~0.010%

[0018] Si: ≤0.40%

[0019] Mn: 1.00%~1.40%

[0020] P: ≤0.015%

[0021] S: ≤0.003%

[0022] Cu: 0.25%–0.45%

[0023] Ni: 0.15%–0.55%

[0024] Cr: 0.40%–0.70%

[0025] Mo: 0.05%–0.35%

[0026] Nb: 0.010%~0.040%

[0027] Ti: 0.008%~0.016%

[0028] Als: 0.040%~0.070%

[0029] N: ≤0.0050%

[0030] Ca: 0.0010%~0.0035%

[0031] The balance includes Fe and other unavoidable inclusions; and the following relationship must be satisfied simultaneously:

[0032] Pcm≤0.26%, and Als / N≥12; where Pcm is the cold crack sensitivity index of steel plate welding, Pcm=C+Si / 30+(Mn+Cu+Cr) / 20+Ni / 60+Mo / 15+V / 10+5B, unit, %;

[0033] Weather resistance index I ≥ 6.15;

[0034] I=26.01Cu+3.88Ni+1.2Cr+1.49Si+17.28P-7.29Cu×Ni-9.10Ni×P-33.39Cu 2 ;

[0035] [(%Nb)×ξ×10 5 ] / [ζ×H×(T 开轧 )]≤3.70,

[0036] in,

[0037] H represents the thickness of the finished steel plate, in mm;

[0038] ξ represents the cumulative reduction rate of controlled rolling without recrystallization, in %;

[0039] T 开轧 The starting rolling temperature for non-recrystallized controlled rolling, in °C;

[0040] ζ is the width-to-width ratio, which is the width of the finished steel plate / the width of the slab.

[0041] [ξ×(T 开冷 )×(DI OL )×(V c )] / [(T 停冷 [)×H]≥1340;

[0042] in,

[0043] ξ represents the cumulative reduction rate of controlled rolling without recrystallization, in %;

[0044] T 开冷 The initial temperature for accelerated cooling, in °C;

[0045] T 停冷 The stopping temperature for accelerated cooling, in °C;

[0046] DI OL DI is the online hardenability index. OL =0.513C 0.5 (1+0.7Si)(1+3.33Mn)(1+0.35Cu)(1+0.36Ni)(1+2.16Cr)(1+3Mo)(1+1.75V)(1+1.77Al)×25.4, unit: mm;

[0047] V c The unit for accelerating the cooling rate of the steel plate is ℃ / s;

[0048] H represents the thickness of the finished steel plate, in mm;

[0049] The Ca / S ratio should be controlled between 1.0 and 3.0, and the Ca×S ratio should be within the range of 1.0 to 3.0. 0.28 ≤2.0×10 －3 .

[0050] Furthermore, the balance consists of Fe and other unavoidable inclusions;

[0051] The steel plate of this invention has a yield strength ≥650MPa, tensile strength ≥770MPa, Charpy impact energy KV2 (single value) ≥120J at -40℃, yield strength ratio ≤0.86, longitudinal and transverse strength anisotropy (i.e., longitudinal / transverse strength difference) ≤50MPa, preheating temperature before welding ≤50℃, and can withstand welding heat input of 50kJ / cm.

[0052] In the composition design of the steel plate described in this invention:

[0053] Carbon (C) has a significant impact on the strength, low-temperature toughness, elongation, and weldability, especially the weldability under high heat input, of TMCP weathering steel plates. From the perspective of improving the low-temperature toughness and weldability under high heat input of weathering steel plates, it is desirable to control the C content in the steel to be relatively low. However, from the perspective of controlling the strength, low-temperature toughness, low yield strength ratio, and low longitudinal and transverse strength anisotropy of weathering steel plates, as well as the control of microstructure and manufacturing costs during the production process, the C content should not be controlled too low. Too low a C content can easily lead to a persistently high yield strength ratio and excessively high grain boundary mobility, resulting in coarse grains in the microstructure of the base steel plate and the welded HAZ, which are prone to mixed crystal formation. Moreover, too low a C content in the steel can cause grain boundary weakening, which seriously deteriorates the low-temperature toughness of the base steel plate and the welded HAZ. Therefore, the reasonable range for C content is 0.06% to 0.10%.

[0054] Si promotes deoxidation of molten steel and can improve the strength of weathering steel plates. However, when using Al for deoxidation, the deoxidation effect of Si is not significant. Although Si can improve the strength of weathering steel plates, it severely impairs their low-temperature toughness, elongation, and weldability. Especially for high-strength weathering steels with high alloy content, during high heat input welding, Si not only promotes the formation of weathering islands, but also results in large and unevenly distributed weathering islands, severely damaging the low-temperature toughness of the weld heat-affected zone (HAZ). Therefore, the Si content in steel should be controlled as low as possible. Considering the weathering effect of Si and the economic efficiency and operability of the steelmaking process, the Si content is controlled below 0.40%.

[0055] As the most important alloying element, manganese (Mn) in steel, besides improving the strength of weathering steel plates, also plays a role in expanding the austenite phase region, lowering the Ar3 point temperature, refining the grains of TMCP weathering steel plates (grain refinement strengthening effect), improving the low-temperature toughness of weathering steel plates (grain refinement toughening), enhancing fatigue resistance, and promoting the formation of low-temperature phase transformation structures (phase transformation strength effect), thus improving the strength of weathering steel plates. However, Mn is prone to segregation during the solidification process of molten steel, especially when the Mn content is high. This not only causes difficulties in casting operations but also makes it easy to combine with elements such as C, P, and S. Conjugate segregation occurs, especially when the carbon content in the steel is high, which aggravates the segregation and porosity in the center of the billet. Severe segregation in the central region of the billet can easily form abnormal structures during subsequent TMCP and welding processes, resulting in low low-temperature toughness of the weathering steel plate and cracks in the welded joints. In addition, for high-strength TMCP weathering steel plates, excessively high Mn content not only causes a sharp deterioration in the low-temperature toughness, elongation and weldability of the steel plate, but also leads to a sharp increase in the yield strength ratio, longitudinal and transverse strength anisotropy and weather resistance. Therefore, the suitable Mn content is 1.00% to 1.40%.

[0056] Phosphorus (P) is a harmful inclusion in steel that significantly impairs the mechanical properties of weathering steel, especially its low-temperature impact toughness, elongation, and weldability. Theoretically, the lower the content, the better. However, considering P's excellent weather resistance, steelmaking operability, and steelmaking cost, for TMCP weathering steel plates that require high heat input welding, -40℃ toughness, high strength, and high heat input welding, the P content needs to be controlled at ≤0.015%.

[0057] Sulfur (S), as a harmful inclusion in steel, significantly impairs the low-temperature toughness, weldability, fatigue resistance, and weather resistance of weathering steel. 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 not only severely damages the low-temperature impact toughness, elongation, Z-axis properties, fatigue resistance, and weldability (especially high heat input weldability) of weathering steel plates, but also causes severe anisotropy in longitudinal and transverse strength and toughness. At the same time, S is also the main element that causes hot brittleness during hot rolling, and theoretically, the lower the better. However, considering the principles of steelmaking operability, steelmaking cost, and smooth logistics, for TMCP weathering steel plates that require high heat input welding, -40℃ toughness, high strength, low longitudinal and transverse strength anisotropy, and high heat input welding, the S content needs to be controlled at ≤0.003%.

[0058] Cu is a key weathering element and an austenite stabilizing element. Adding Cu can lower the Ar1 and Ar3 point temperatures, improve the atmospheric corrosion resistance of steel plates, refine the microstructure of TMCP weathering steel plates, and improve the low-temperature toughness of TMCP weathering steel plates. However, excessive Cu addition, exceeding 0.45%, can easily lead to copper embrittlement, surface cracking of billets, reduced weather resistance, internal cracking, and especially deterioration of the impact load fracture characteristics (i.e., ductility and toughness) and weld joint performance of thick weathering steel plates. It also results in persistently high yield strength ratio and longitudinal and transverse strength anisotropy in weathering steel plates. Considering that Cu is a relatively expensive alloying element, from a cost-effectiveness perspective, the upper limit of Cu should be controlled at 0.55%. Insufficient Cu addition, below 0.25%, has a relatively small weathering effect; therefore, the Cu content should be controlled between 0.25% and 0.45%.

[0059] Adding Ni not only reduces the lattice friction force (PN force) of dislocations in the BCC crystal structure, improves the low-temperature dislocation mobility of the ferrite phase, promotes dislocation cross-slip, and improves the intrinsic ductility and toughness of ferrite; in addition, as a strong austenite stabilizing element, Ni significantly lowers the Ar1 and Ar3 point temperatures, increases the driving force for the austenite-to-ferrite phase transformation, causing austenite to undergo phase transformation at lower temperatures, significantly refines the microstructure of TMCP weathering steel plates, increases the resistance to crack propagation through ferrite grains, and significantly improves the low-temperature toughness of TMCP weathering steel plates. Therefore, Ni has the effect of simultaneously improving the strength and low-temperature toughness of TMCP weathering steel plates without reducing elongation (i.e., ductility and toughness); adding Ni to steel can also reduce copper embrittlement in copper-containing steels, alleviate intergranular cracking during hot rolling, and improve the atmospheric corrosion resistance of steel plates. Therefore, theoretically speaking, the higher the Ni content in steel within a certain range, the better. However, excessively high Ni content will not only harden the heat-affected zone of the weld and negatively affect the weldability and toughness of the welded joint of the weathering steel plate, but also significantly increase the yield strength ratio, longitudinal and transverse strength anisotropy, and alloy cost of the weathering steel plate (Ni is a precious alloying element). Therefore, the Ni content should be controlled between 0.15% and 0.55%.

[0060] For weathering steel, Cr plays a significant role in promoting a dense amorphous oxide layer, making the addition of a certain amount of Cr essential. Adding a certain amount of Cr (≤0.70%) can improve the weathering resistance of the steel plate without compromising its low-temperature toughness and bending cold working properties. However, if the Cr content is too low (<0.40%), Cr contributes little to the weathering resistance of the steel plate, failing to meet high weathering resistance requirements. Adding too much (>0.70%) impairs the low-temperature toughness and bending cold working properties of the weathering steel plate, especially its weldability, making it unable to withstand high heat input during welding, and causing deterioration of the low-temperature toughness of the weld heat-affected zone (forming coarse, feathery upper bainite structures in the weld heat-affected zone). Therefore, the Cr content should be controlled between 0.40% and 0.70%.

[0061] Adding Mo can significantly improve the hardenability of weathering steel plates, promote the formation of bainite / martensite low-temperature phase transformation structures, improve the tempering characteristics and tempering process window of weathering steel plates, and improve the strength-toughness and strength-plasticity matching of weathering steel plates after tempering. However, as a strong carbide-forming element, excessive addition of Mo can not only seriously impair the low-temperature impact toughness, elongation, and weldability of weathering steel plates, but also significantly increase the yield strength ratio, longitudinal and transverse strength anisotropy, and production costs of weathering steel plates. Therefore, considering the phase transformation strengthening effect of Mo and its influence on the low-temperature toughness, elongation, weldability, yield strength ratio, and longitudinal and transverse strength anisotropy of the base steel plate, the Mo content is controlled at 0.05% to 0.35%.

[0062] The purpose of adding trace amounts of Nb to steel is to control rolling without recrystallization, refine the grain size of weathering steel plates, and improve the strength and toughness of TMCP weathering steel plates. When the Nb content is below 0.010%, in addition to failing to effectively exert the rolling control effect, the strengthening and toughening ability of TMCP weathering steel plates is also insufficient. When the Nb content exceeds 0.040%, it not only causes the yield strength ratio, longitudinal / transverse strength anisotropy, and high alloy cost of the steel plate (Nb is also a valuable alloying element), but also induces the formation of upper bainite (Bu) and the secondary precipitation embrittlement of Nb (C,N) under high heat input welding conditions, which seriously impairs the low-temperature toughness of the high heat input weld heat-affected zone (HAZ). Therefore, the Nb content is controlled between 0.010% and 0.040% to obtain the best rolling control effect, achieve a balance of strength and toughness / strength and plasticity, low yield strength ratio, and low longitudinal and transverse strength of TMCP weathering steel plates, while not compromising the high heat input weldability of the weathering steel plates.

[0063] The purpose of adding trace amounts of Ti to steel is to combine with N in the steel to generate highly stable TiN particles, which inhibit grain growth in weathering steel plates and the weld HAZ zone. The Ti content added to the steel must match the N content in the steel. The principle of matching is that TiN cannot precipitate in liquid steel but must precipitate in the solid phase. Therefore, the precipitation temperature of TiN must be ensured to be below 1400℃. When the amount of Ti added to the steel is too small (<0.008%), the number of TiN particles formed is insufficient to inhibit the growth of austenite grains during TMCP and welding thermal cycling, thus failing to improve the low-temperature toughness and weldability of weathering steel plates. When the Ti content is too large (>0.016%), the TiN precipitation temperature exceeds 1400℃, and some TiN particles precipitate as large-sized TiN particles during the solidification of molten steel. These large-sized TiN particles not only fail to inhibit grain growth but also become the starting point for crack initiation. Therefore, the optimal control range for Ti content is 0.008% to 0.016%.

[0064] Al in steel plates can fix free nitrogen in the steel, reduce free nitrogen in the weld heat-affected zone (HAZ), and improve the low-temperature impact toughness of welded HAZ with large heat input; however, adding too much Al to the steel will not only form a large number of dispersed needle-like Al2O3 inclusions in the steel, but also impair the low-temperature impact toughness and weldability of weathering steel plates. According to the analysis of the composition system of weathering steel plates, the optimal Al content is controlled between 0.040% and 0.070%.

[0065] The control range of nitrogen (N) corresponds to that of titanium (Ti). For controlling the grain size of weathering steel plates and improving their low-temperature toughness and weldability, N content is crucial. If the N content is too low, the number of TiN particles generated will be small and their size large, failing to control the grain size and thus not improving the low-temperature toughness and weldability; in fact, it may even be detrimental. However, if the N content is too high, the amount of free nitrogen ([N]) in the steel increases, especially under high heat input welding conditions, where the free nitrogen content in the heat-affected zone (HAZ) increases dramatically, severely damaging the low-temperature toughness and bending workability of the HAZ and worsening the steel's processing and service characteristics. Therefore, the N content should be controlled at ≤0.0050%.

[0066] 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; thirdly, it suppresses the hot brittleness of sulfur; fourthly, it improves the low-temperature toughness, elongation, and Z-axis properties of weathering steel plates; fifthly, it improves the longitudinal and transverse strength, anisotropy of toughness, and weldability of weathering steel plates; and sixthly, it improves the castability of high-acid-soluble aluminum steel. 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, increasing brittleness and potentially becoming the initiation point for fracture cracks, reducing the low-temperature toughness, elongation, and weldability of the steel plate, while also reducing the purity of the steel and contaminating the molten steel. Generally, the Ca content is controlled according to ESSP = (wt% Ca)[1-1.24(wt% O)] / 1.25(wt% S), where ESSP is the sulfide inclusion shape control index, and the value should be between 0.80 and 4.00. Therefore, the appropriate range of Ca content is 0.0010% to 0.0035%.

[0067] The composition design of this invention also specifically requires:

[0068] Pcm≤0.26%, and Als / N≥12; A) Improve the weldability of weathering steel plates to ensure that weathering steel plates have high heat input weldability; B) Eliminate the free [N] content in the heat-affected zone of high heat input welding, improve the microstructure of the heat-affected zone (forming high-density fine AlN particles, pinning austenite grain growth, refining the microstructure of the HAZ) and low-temperature toughness and crack arrest characteristics (eliminating free [N] in the HAZ, eliminating the embrittlement effect of free [N]). Wherein, Pcm is the cold crack sensitivity index of steel plate welding, Pcm=C+Si / 30+(Mn+Cu+Cr) / 20+Ni / 60+Mo / 15+V / 10+5B, in percentage.

[0069] Weather resistance index I ≥ 6.15; ensuring the steel plate has excellent weather resistance, where I = 26.01Cu + 3.88Ni + 1.2Cr + 1.49Si + 17.28P - 7.29Cu×Ni - 9.10Ni×P - 33.39Cu2 .

[0070] [(%Nb)×ξ×10 5 ] / [ζ×H×(T 开轧 )]≤3.70, ensuring that the weathering steel plate achieves high toughness while also possessing a low yield strength ratio and low longitudinal and transverse strength anisotropy; where H is the thickness of the finished steel plate, in mm; ξ is the cumulative reduction rate of non-recrystallized controlled rolling, in %; T 开轧 The rolling temperature is the initial rolling temperature before recrystallization, in °C; ζ is the width-to-width ratio, i.e., the width of the finished steel plate / the width of the slab; this is one of the key core technologies of this invention.

[0071] [ξ×(T 开冷 )×(DI OL )×(V c )] / [(T 停冷 [×H]≥1340; This ensures that under low C and low cold cracking sensitivity index Pcm conditions, the steel plate possesses high strength, high toughness, and low yield strength ratio, while also exhibiting excellent weldability and weather resistance with high heat input. It successfully eliminates the inherent contradiction between low C and low cold cracking sensitivity index Pcm in weathering steel plates and high strength, high toughness, low yield strength ratio, and low longitudinal and transverse strength anisotropy. Where ξ is the cumulative reduction rate of non-recrystallized controlled rolling, in %; T 开冷 The initial temperature for accelerated cooling, measured in °C (T). 停冷 The stopping temperature for accelerated cooling, in °C; DI OL DI is the online hardenability index. OL =0.513C 0.5 (1+0.7Si)(1+3.33Mn)(1+0.35Cu)(1+0.36Ni)(1+2.16Cr)(1+3Mo)(1+1.75V)(1+1.77Al)×25.4, unit mm; V c H represents the cooling rate of the steel plate, in °C / s; H represents the thickness of the finished steel plate, in mm; this is one of the key core technologies of this invention.

[0072] Ca treatment with the Ca / S ratio controlled between 1.0 and 3.0 and Ca×S 0.28 ≤2.0×10 －3 While ensuring spheroidization of sulfurization and minimizing the impact of inclusions on low-temperature toughness and weldability, Ca(O,S) particles are uniformly and finely distributed in the steel, inhibiting the growth of austenite grains in the high heat input weld heat-affected zone, reducing the anisotropy of longitudinal and transverse properties (strength, toughness) of the steel plate, and improving the low-temperature toughness of the high heat input weld heat-affected zone of the steel plate.

[0073] The component data in the above formulas are calculated as percentages. For example, if the carbon content is 0.10%, simply substitute 0.10 into the formula for calculation.

[0074] The manufacturing method of the high-toughness, low-yield-strength ratio, and low longitudinal and transverse strength anisotropy 800MPa grade weathering steel plate of the present invention includes the following steps:

[0075] 1) Smelting and casting

[0076] Smelting is carried out according to the above composition, and casting is carried out by continuous casting. The superheat of the tundish is controlled at 7-26℃, the casting speed is controlled at 0.6-1.0m / min, and the liquid level fluctuation in the crystallizer is controlled at ≤5mm.

[0077] 2) Slab heating

[0078] The slab heating temperature is controlled at 1080–1180℃;

[0079] 3) Rolling, with a rolling width ratio ≥ 1.3

[0080] The first stage is ordinary rolling, which is continuous rolling using the maximum rolling capacity of the rolling mill;

[0081] The second stage employs non-recrystallization controlled rolling, with an initial rolling temperature of T. 开轧 The rolling temperature is 750–850℃, the reduction rate per rolling pass is ≥7%, the cumulative reduction rate is ≥50%, and the final rolling temperature is 740–790℃; and simultaneously, it must satisfy: [(%Nb)×ξ×10 5 ] / [ζ×H×(T 开轧 )]≤3.70,

[0082] in,

[0083] H represents the thickness of the finished steel plate, in mm;

[0084] ξ represents the cumulative reduction rate of controlled rolling without recrystallization, in %;

[0085] T 开轧 The starting rolling temperature for non-recrystallized controlled rolling, in °C;

[0086] ζ is the width-to-width ratio, which is the width of the finished steel plate / the width of the slab.

[0087] 4) Cooling

[0088] After controlled rolling, the steel plate is immediately transported to the accelerated cooling equipment for rapid cooling, with the starting temperature T. 开冷 The temperature ranges from 720 to 760°C, and the cooling rate V is... c ≥5℃ / s, cooling stop temperature T 停冷 The temperature is set at 200–350℃, after which the steel plate is allowed to air cool naturally to room temperature; and simultaneously, the following conditions must be met:

[0089] [ξ×(T 开冷 )×(DI OL )×(V c )] / [(T 停冷 [)×H]≥1340;

[0090] in,

[0091] ξ represents the cumulative reduction rate of controlled rolling without recrystallization, in %;

[0092] T 开冷 The initial temperature for accelerated cooling, in °C;

[0093] T 停冷 The stopping temperature for accelerated cooling, in °C;

[0094] DI OL DI is the online hardenability index. OL =0.513C 0.5 (1+0.7Si)(1+3.33Mn)(1+0.35Cu)(1+0.36Ni)(1+2.16Cr)(1+3Mo)(1+1.75V)(1+1.77Al)×25.4, unit: mm;

[0095] V c The unit is the cooling rate of the steel plate, expressed in °C / s.

[0096] H represents the thickness of the finished steel plate, in mm;

[0097] 5) Tempering

[0098] The tempering temperature is 220–350℃, and the furnace time t is 1.0–3.0 times the thickness H of the finished steel plate.

[0099] After tempering, the steel plate was naturally air-cooled to room temperature.

[0100] Preferably, in step 4), when the steel plate thickness is ≥50mm, the steel plate is subjected to slow cooling. The slow cooling process involves holding the plate at a temperature of not less than 150℃ for at least 24 hours, followed by natural air cooling to room temperature.

[0101] In the method for manufacturing the steel plate described in this invention:

[0102] 1) The slab heating temperature is controlled between 1080℃ and 1180℃ to ensure that Nb in the steel is completely dissolved into austenite during the slab heating process, while the austenite grains in the slab do not grow abnormally.

[0103] 2) To ensure that the steel plate has low longitudinal and transverse strength anisotropy, the rolling width-to-width ratio is ≥1.3.

[0104] The first stage is ordinary rolling, which uses the maximum rolling capacity of the rolling mill for continuous rolling. This maximizes the production capacity of the rolling line while ensuring that the deformed steel billet recrystallizes and refines the austenite grains.

[0105] The second stage employs controlled rolling without recrystallization. Based on the aforementioned Nb content range in the steel, to ensure the effectiveness of controlled rolling without recrystallization, the initial rolling temperature is controlled between 750℃ and 850℃, the reduction rate per rolling pass is ≥7%, the cumulative reduction rate is ≥50%, and the final rolling temperature is 740℃ to 790℃. Furthermore, the following conditions must be met simultaneously: [(%Nb)×ξ×10 5 ] / [ζ×H×(T 开轧 )]≤3.70.

[0106] Studies have shown that the higher the Nb content, the greater the cumulative reduction rate of non-recrystallized controlled rolling, and the higher the initial rolling temperature T. 开轧 The lower the yield strength, the higher the yield strength ratio and the greater the anisotropy; however, the higher the Nb content, the better the toughness of the steel plate; conversely, the larger the ζ-width ratio and the larger the thickness H, the lower the yield strength ratio and the smaller the anisotropy.

[0107] Higher Nb content and lower unrecrystallized controlled rolling temperature, combined with a higher unrecrystallized region reduction rate ξ, result in a higher degree of austenite flattening, higher density of dislocations and deformation bands within the austenite body, more nucleation points for lower bainite, smaller lower bainite cluster size, and better toughness. Higher dislocation density leads to higher strain hardening of the steel plate and a higher yield strength ratio. Conversely, a larger width-to-width ratio ζ results in higher uniformity of the longitudinal and transverse microstructure within the steel plate and lower anisotropy. The thickness effect of the steel plate leads to a lower density of dislocations and deformation bands within the austenite body, fewer nucleation points for lower bainite, larger lower bainite cluster size, lower dislocation density, lower yield strength ratio, and poorer toughness in thicker steel plates.

[0108] The above formula ensures that weathering steel plates achieve high toughness while also having a low yield strength ratio and low longitudinal and transverse strength anisotropy.

[0109] 3) After controlled rolling, the steel plate is immediately transported to the accelerated cooling equipment at the maximum conveyor speed of the roller conveyor, and then accelerated cooling is carried out on the steel plate; the initial cooling temperature T of the steel plate is... 开冷 The temperature range is 720℃~760℃, and the cooling rate is V. c ≥5℃ / s, cooling stop temperature T 停 The temperature is set at 200℃~350℃, after which the steel plate is naturally air-cooled to room temperature; and simultaneously, the following condition must be met: [ξ×(T 开冷 )×(DI OL )×(V c )] / [(T 停冷 [)×H]≥1340;

[0110] With T 开冷As the temperature rises, the phase change undercooling ΔT increases (T 开冷 —The bainite initiation phase transformation temperature. The greater the phase transformation driving force, the greater the growth rate of lower bainite laths, the larger the size of lower bainite clusters, and the higher the lower bainite content. Conversely, the higher the cumulative reduction rate ξ of non-recrystallization controlled rolling, the higher the degree of austenite flattening, the higher the density of internal dislocations and deformation bands in austenite, and the more nucleation points of lower bainite. At the same time, the higher the internal dislocation density in austenite, the smaller the dislocation cell structure, the more stable the dislocation cell wall, the greater the resistance to bainite lath growth, and the smaller the size of lower bainite clusters. The higher the dislocation density and the more nucleation sites of lower bainite, the worse the stability of austenite, the lower the potential for forming lower bainite, and the lower the lower bainite content. During accelerated cooling, the two interact to form uniform and fine lower bainite.

[0111] Online hardenability index DI OL The higher the temperature, the higher the hardenability of the steel plate, and the greater the cooling rate Vc. 停冷 The lower the DI value, the higher the lower bainite content; while the thicker the steel plate, the higher the lower bainite content at the same cooling rate and the same online hardenability index DI. OL Under certain conditions, the lower the content of lower bainite in the steel plate, the better.

[0112] By using the coupling relationship of the above parameters, it is ensured that the steel plate has high strength, high toughness, and low yield strength ratio, while also having excellent weldability and weather resistance with high heat input. It also successfully eliminates the contradiction between the low carbon content and low cold cracking sensitivity index Pcm of weathering steel plate and the high strength, high toughness, low yield strength ratio, and low longitudinal and transverse strength anisotropy, which are difficult to reconcile.

[0113] The beneficial effects of this invention are:

[0114] This invention produces high-performance TMCP-type weathering steel plates at low cost by reducing the content of precious alloying elements, matching and combining the main alloying elements, microalloying elements, and inclusion elements, and integrating them with the TMCP manufacturing process. These weathering steel plates not only possess high strength, high toughness, low yield strength ratio, and low longitudinal and transverse strength anisotropy, but also exhibit excellent weather resistance and weldability, especially with high heat input welding capabilities. This significantly shortens the manufacturing cycle of steel structures, enables paint-free steel structures, creates enormous value for construction companies, and achieves green and environmentally friendly manufacturing and use of the steel plates.

[0115] The high performance and high added value of steel plates are mainly reflected in the perfect match between the high strength, high toughness, and excellent weather resistance of steel plates and the low yield strength ratio, low longitudinal and transverse strength anisotropy, and excellent weldability, which successfully solves the following problems:

[0116] ① Between low C, low Pcm and high strength, low yield strength ratio;

[0117] ② The relationship between high strength, high toughness and excellent weather resistance and low yield strength ratio and low anisotropy of longitudinal and transverse strength;

[0118] ③ The high strength, excellent weather resistance, and excellent weldability, especially the high heat input welding, which are conflicting and difficult to reconcile in composition design and TMCP process design, greatly improve the safety, stability, and durability of large heavy steel structures. The good weldability (especially high heat input welding) saves users the cost of steel structure manufacturing and shortens the manufacturing time. The excellent weather resistance eliminates the need for anti-rust coating on the steel structure surface, which not only creates great value for users, but more importantly, reduces carbon emissions (low preheating before welding, no heat treatment required after welding, high-efficiency welding) and reduces the pollution and toxicity of surface coating to the environment. Therefore, this type of steel plate is also a high value-added, green and environmentally friendly product with a full life cycle. Attached Figure Description

[0119] Figure 1 The microstructure (1 / 4 thickness) of the steel in Example 6 of this invention. Detailed Implementation

[0120] The present invention will be further described below with reference to the embodiments and accompanying drawings.

[0121] The composition of the steel in this embodiment of the invention is shown in Table 1. Tables 2 to 4 show the manufacturing process parameters of this embodiment of the invention. Table 5 shows the performance parameters of the steel in this embodiment of the invention.

[0122] Depend on Figure 1 It can be seen that the microstructure of the finished steel plate of the present invention is uniform and fine low-carbon lower bainite, with an average grain size of less than 20 μm; it achieves a low yield strength ratio (a continuous yielding phenomenon occurs during tensile testing, resulting in a lower Rp0.2) and low longitudinal and transverse strength anisotropy (the single low-carbon lower bainite structure greatly eliminates longitudinal and transverse strength anisotropy).

[0123] The steel plate of this invention can be used in ship structures, marine engineering structures, bridge engineering structures, energy development projects, wind power projects, and the manufacture of engineering machinery, mining machinery, and heavy vehicles, and can achieve low-cost, stable, and mass industrial production.

[0124]

[0125]

[0126]

[0127] Table 4

[0128] Example 1 300 48 Example 2 310 55 Example 3 330 100 Example 4 350 105 Example 5 280 80 Example 6 250 120 Example 7 220 150

[0129]

Claims

1. 800MPa grade weathering steel plate with high toughness, low yield strength ratio, and low longitudinal and transverse strength anisotropy, its composition by weight percentage is: C：0.060%～0.010% Si: ≤0.40% Mn: 1.00%~1.40% P：≤0.015% S：≤0.003% Cu: 0.25%~0.45% Ni: 0.15%~0.55% Cr：0.40%～0.70% Mo: 0.05%~0.35% Nb: 0.010%~0.040% Ti: 0.008%~0.016% Als: 0.040%~0.070% N：≤0.0050% Ca: 0.0010%~0.0035% The balance includes Fe and other unavoidable inclusions; and the following relationship must be satisfied simultaneously: Pcm≤0.26%, and Als / N≥12; where, Pcm is the cold cracking sensitivity index for steel plate welding, Pcm = C + Si / 30 + (Mn + Cu + Cr) / 20 + Ni / 60 + Mo / 15 + V / 10 + 5B, in % (%). Weather resistance index I ≥ 6.15; I=26.01Cu+3.88Ni+1.2Cr+1.49Si+17.28P-7.29Cu×Ni-9.10Ni×P-33.39Cu 2 ; [(%Nb)×x×10 5 ] / [ζ×H×(T 开轧 )]≤3.70, in, H represents the thickness of the finished steel plate, in mm; x represents the cumulative reduction rate of non-recrystallized controlled rolling, in % . T 开轧 The starting rolling temperature for non-recrystallized controlled rolling, in °C; ζ is the width-to-width ratio, which is the width of the finished steel plate / the width of the slab. [x×(T 开冷 )×(DI OL )×(V c )] / [(T 停冷 )×H]≥1340; in, x represents the cumulative reduction rate of non-recrystallized controlled rolling, in % . T 开冷 The initial temperature for accelerated cooling, in °C; T 停冷 The stopping temperature for accelerated cooling, in °C; DI OL DI is the online hardenability index. OL =0.513C 0.5 (1+0.7Si)(1+3.33Mn)(1+0.35Cu)(1+0.36Ni)(1+2.16Cr)(1+3Mo)(1+1.75V)(1+1.77Al)×25.4, unit: mm; V c The unit is the cooling rate of the steel plate, expressed in °C / s. H represents the thickness of the finished steel plate, in mm; The Ca / S ratio should be controlled between 1.0 and 3.0, and the Ca×S ratio should be within the range of 1.0 to 3.

0. 0.28 ≤2.0×10 －3 ; The steel plate has a yield strength ≥650MPa, tensile strength ≥770MPa, Charpy impact energy KV2 (single value) ≥120J at -40℃, yield strength ratio ≤0.86, longitudinal and transverse strength anisotropy (i.e., longitudinal / transverse strength difference) ≤50MPa, preheating temperature before welding ≤50℃, and can withstand welding heat input of 50kJ / cm.

2. The high-toughness, low-yield-strength ratio, and low longitudinal and transverse strength anisotropy 800MPa grade weathering steel plate as described in claim 1, characterized in that, The balance consists of Fe and other unavoidable inclusions.

3. The manufacturing method of the 800MPa grade weathering steel plate with high toughness, low yield strength ratio, and low longitudinal and transverse strength anisotropy as described in claim 1 or 2, characterized in that, Includes the following steps: 1) Smelting and casting Smelting according to the composition described in claim 1 or 2, casting adopts continuous casting, the superheat of the tundish is controlled at 7 to 26°C, the casting speed is controlled at 0.6 to 1.0 m / min, and the liquid level fluctuation in the crystallizer is controlled at ≤5 mm; 2) Slab heating The slab heating temperature is controlled at 1080–1180℃; 3) Rolling, with a rolling width ratio ≥ 1.3 The first stage is ordinary rolling, which is continuous rolling using the maximum rolling capacity of the rolling mill; The second stage employs non-recrystallization controlled rolling, with an initial rolling temperature of T. 开轧 The rolling temperature is 750–850℃, the reduction rate per rolling pass is ≥7%, the cumulative reduction rate is ≥50%, and the final rolling temperature is 740–790℃; and simultaneously, it must satisfy: [(%Nb)×x×10 5 ] / [ζ×H×(T 开轧 )]≤3.70, in, H represents the thickness of the finished steel plate, in mm; x represents the cumulative reduction rate of non-recrystallized controlled rolling, in % . T 开轧 The starting rolling temperature for non-recrystallized controlled rolling, in °C; ζ is the width-to-width ratio, which is the width of the finished steel plate / the width of the slab. 4) Cooling After controlled rolling, the steel plate is immediately transported to the accelerated cooling equipment for rapid cooling, with the starting temperature T. 开冷 The temperature ranges from 720 to 760°C, and the cooling rate is V. c ≥5℃ / s, cooling stop temperature T 停冷 The temperature is 200–350℃, after which the steel plate is naturally air-cooled to room temperature; and simultaneously, the following condition must be met: [x×(T 开冷 )×(DI OL )×(V c )] / [(T 停冷 [)×H]≥1340; in, x represents the cumulative reduction rate of non-recrystallized controlled rolling, in % . T 开冷 The initial temperature for accelerated cooling, in °C; T 停冷 The stopping temperature for accelerated cooling, in °C; DI OL DI is the online hardenability index. OL =0.513C 0.5 (1+0.7Si)(1+3.33Mn)(1+0.35Cu)(1+0.36Ni)(1+2.16Cr)(1+3Mo)(1+1.75V)(1+1.77Al)×25.4, unit: mm; V c The unit is the cooling rate of the steel plate, expressed in °C / s. H represents the thickness of the finished steel plate, in mm; 5) Tempering The tempering temperature is 220-350℃, and the furnace time t is 1.0-3.0 times the thickness H of the finished steel plate. After tempering, the steel plate is naturally air-cooled to room temperature.

4. The manufacturing method of the 800MPa grade weathering steel plate with high toughness, low yield strength ratio, and low longitudinal and transverse strength anisotropy as described in claim 3, characterized in that, In step 4), when the steel plate thickness is ≥50mm, the steel plate is slowly cooled. The slow cooling process involves holding the plate at a temperature of not less than 150℃ for at least 24 hours, and then the steel plate is naturally air-cooled to room temperature.

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