An ultra-low cost, excellent weldability 800mpa grade steel plate and a manufacturing method thereof

Abstract

An ultra-low cost, excellent weldability 800MPa grade steel plate and a manufacturing method thereof, the composition in weight percentage is: C 0.050-0.090%, Si≤0.10%, Mn 1.60-2.00%, P≤0.013%, S≤0.0030%, Cr 0.25-0.60%, Nb 0.030-0.060%, Ti 0.008-0.016%, B 0.0008-0.0018%, Als 0.030-0.070%, N≤0.0055%, Ca 0.001-0.0040%, the balance comprising Fe and inevitable inclusions; and satisfies: (%Als) / [(%N)-0.292(%Ti)]≥33; (%C)×[0.85(%Mn)+1.19(%Als)+9.33(%P)+12.05(%S)+(%Si)]≤0.16; Ca treatment, Ca / S ratio is 1-3, (%Ca)×(%S)≤1.5×10 ‑3 The application obtains 800MPa high strength at ultra-low cost without adding Cu, Ni and Mo precious alloy elements, and the toughness and weldability of the steel plate are also excellent, and preheating before welding and post-welding heat treatment are not needed, and the application is particularly suitable for large steel structures and equipment such as engineering machinery and coal mine machinery.

Description

Technical Field

[0001] This invention relates to the field of high-strength steel technology, specifically to an ultra-low cost, excellent weldability 800MPa grade 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, 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, ultimately determined by the microstructure of the finished steel. With the continuous advancement of metallurgical technology, higher requirements are being placed on the toughness, plasticity, and weldability of high-strength steel, meaning that the steel plate must maintain high strength, high elongation, and high performance even at low temperatures. While possessing resistance to brittle fracture and ductile instability fracture, the steel plate also exhibits excellent weldability. Furthermore, under conditions of lower manufacturing costs, it significantly improves the comprehensive mechanical and performance properties of the steel plate, thereby reducing steel consumption and saving costs, lightening the weight of steel components, and enhancing stability and safety. More importantly, it further improves the cold and hot workability of high-strength steel and its safety and reliability during service. Currently, a research boom in the development of next-generation high-performance steel materials has emerged in Japan, South Korea, and the European Union, striving to achieve better microstructure matching, ultra-refined microstructure and structure through alloy combination optimization design and innovative manufacturing processes, so that high-strength steel can achieve a better balance of strength, plasticity, and toughness.

[0003] 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., a 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)) is used to ensure that the steel plate has sufficiently high strength, excellent low-temperature toughness, and uniformity of microstructure and properties along the thickness direction. Therefore, 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-I SIJ, 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; Japanese 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.

[0004] 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, high embrittlement of the heat-affected zone, 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%.

[0005] Existing patent literature mainly describes how to achieve the strength and low-temperature toughness of the base steel plate, with little explanation on improving the weldability of the steel plate and obtaining excellent low-temperature toughness of the weld heat-affected zone (HAZ). Furthermore, it does not address how to improve the tensile strength of the steel plate while simultaneously improving its tensile elongation and the uniformity of its mechanical properties in the thickness direction (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, Nishiyama Memorial Technical Lecture Nos. 159-160, pp. 79-80).

[0006] Between 2007 and 2015, Baosteel applied for a series of patents for 80-kilogram-class quenched and tempered steel, such as Chinese patent CN200710042357.6, "HT780 steel plate with excellent weldability and low yield strength ratio and its manufacturing method"; Chinese patent CN200810036416.3, "steel plate with high toughness and high plasticity and its manufacturing method"; and Chinese patent CN201210078314.4, "80-kilogram-class ultra-high toughness, ultra-thick steel plate and its manufacturing method". Although the steel plates produced using these patented technologies have achieved a high level of comprehensive mechanical properties: tensile strength ≥780MPa, yield strength ≥690MPa, Charpy transverse impact energy (single value) at -40℃ and below ≥47J, and excellent weldability, a certain amount of Cu and Ni alloying elements are inevitably added to the steel plates, especially a relatively large amount of Ni.

[0007] In the "Low-cost 80 kg-class extra-thick quenched and tempered steel plate and its manufacturing method" disclosed in Chinese patent CN200910048287.4, the chemical composition of the steel plate does not contain Cu and Ni elements, but the impact toughness can only meet the temperature requirements of -20℃ and above.

[0008] Furthermore, the controlled rolling + offline quenching + tempering process not only involves more manufacturing steps, a longer manufacturing cycle, and higher manufacturing costs, but also consumes relatively more energy (the steel plate is rolled and naturally air-cooled to room temperature, then shot-blasted, and then reheated to the quenching temperature), which is not conducive to energy conservation and environmental protection. Moreover, the offline quenching + tempering process cannot fully utilize the hardening and hardening potential of alloying elements, and the hardening and hardening properties of the elements cannot be maximized. Therefore, in order to obtain the same strength and toughness level, more alloying elements (especially Ni, Mo, Cr, etc.) must be added, which not only further increases the manufacturing cost, but also impairs the weldability of the steel plate. Especially for ultra-high strength steel plates, the sensitivity to cold cracking during welding is greatly increased, requiring welding preheating and post-heating (i.e., PWHT) at higher temperatures, and the appropriate range of welding heat input is narrower, which correspondingly increases the processing and manufacturing costs significantly.

[0009] The technical solution disclosed in Chinese patent ZL201210209649.5 is to successfully develop an 800MPa grade quenched and tempered steel plate with excellent performance by using the DQ process without adding precious alloying elements such as Cu and Ni. However, the low temperature toughness cannot meet the low temperature requirement of -50℃, and the steel plate cannot be welded without preheating, and the welding heat input cannot be higher than 50kJ / cm.

[0010] Although Chinese patent ZL202010468912.7 can meet the requirements of high toughness, low yield strength ratio and excellent weldability, the steel plate has an excessively high content of precious alloying elements Cu, Ni and Mo, resulting in high manufacturing costs and poor market competitiveness, which cannot meet the needs of large-scale engineering machinery and coal mining machinery. Summary of the Invention

[0011] The purpose of this invention is to provide an ultra-low cost, high-weldability 800MPa grade steel plate and its manufacturing method. It adopts the TMCP process and omits offline tempering. Without adding expensive alloying elements such as Cu, Ni and Mo, it achieves 800MPa high strength at an ultra-low cost, while the steel plate also has excellent toughness and weldability (no preheating required before welding and no stress relief heat treatment required after welding). It successfully solves the contradiction between the strength, toughness, weldability and ultra-low cost of 800MPa grade steel plate, and is particularly suitable for large steel structures and equipment such as engineering machinery and coal mining machinery.

[0012] To achieve the above objectives, the technical solution of the present invention is as follows:

[0013] A low-cost, high-weldability 800MPa grade steel plate, with the following composition by weight percentage:

[0014] C: 0.050%~0.090%

[0015] Si: ≤0.10%

[0016] Mn: 1.60%~2.00%

[0017] P: ≤0.013%

[0018] S: ≤0.0030%

[0019] Cr: 0.25%–0.60%

[0020] Nb: 0.030%~0.060%

[0021] Ti: 0.008%~0.016%

[0022] B: 0.0008%~0.0018%

[0023] Als: 0.030%~0.070%

[0024] N: ≤0.0055%

[0025] Ca: 0.001%~0.0040%

[0026] The balance includes Fe and other unavoidable inclusions; and simultaneously satisfies the following relationship:

[0027] (%Als) / [(%N)-0.292(%Ti)]≥33;

[0028] (%C)×[0.85(%Mn)+1.19(%Als)+9.33(%P)+12.05(%S)+(%Si)]≤0.16;

[0029] Ca treatment, Ca / S ratio of 1–3, and (%Ca) × (%S) ≤ 1.5 × 10⁻⁶ -3 .

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

[0031] The microstructure of the steel plate described in this invention is low-carbon bainite plus a small amount of low-carbon martensite, with an average cluster size of less than 20 μm.

[0032] The steel plate of this invention has a tensile strength ≥770MPa, a yield strength ≥690MPa, a Charpy transverse impact energy (single value) ≥100J at -40℃, and a fracture elongation δ5 ≥15%; a low preheating temperature, less than 50℃; no heat effect heat treatment is required after welding; a wide heat input range, 15~35kJ / cm; and a transverse impact energy (single value) ≥100J at -40℃ in the heat-affected zone.

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

[0034] Carbon (C) has a significant impact on the strength, low-temperature toughness, elongation, and weldability of high-strength steel. From the perspective of improving the intrinsic ductility, toughness, and weldability of high-strength steel, it is desirable to control the C content in the steel to a low level. However, from the perspective of hardenability, the balance between strength and toughness, microstructure control during the TMCP process, and manufacturing costs, the C content should not be controlled too low, especially for 80 kg TMCP steel plates. Therefore, the reasonable range for C content is 0.050% to 0.090%.

[0035] Mn, as the most important alloying element in steel, not only improves the strength of steel plates, but also expands the austenite phase region, significantly lowers the Ar3 point temperature, refines the bainite / martensite packet structure size, improves the low-temperature toughness of steel plates by reducing the orientation difference between bainite laths and martensite laths, and promotes the formation of bainite / martensite in low-temperature phase transformation structures, thus giving steel plates an excellent balance of strength and toughness / strength and plasticity. However, Mn is prone to segregation during the solidification process of molten steel, especially when the Mn content is high, which not only causes difficulties in casting operations, but also... Furthermore, it is prone to conjugate segregation with elements such as C, P, S, and Si. Especially when the C content in the steel is high, it 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 toughness of high-strength steel plates and cracks in welded joints. Therefore, it is extremely necessary to select an appropriate Mn content range based on the strength grade and the C content range in the steel for 800MPa grade steel plates. The suitable Mn content for the quenched and tempered steel plate of this invention is 1.60% to 2.00%.

[0036] Si promotes deoxidation of molten steel and can increase the strength of 800MPa steel plates. However, when using Al for deoxidation, the deoxidation effect of Si is not significant. Although Si can increase the strength of steel plates, it reduces the critical cooling rate for martensite formation and inhibits the formation of lower bainite, severely impairing the toughness, elongation, and weldability of high-strength steel plates. Especially under welding conditions with high heat input, Si not only promotes the formation of martensite islands, but also forms large and unevenly distributed martensite islands, severely impairing the toughness of the heat-affected zone (HAZ) and the SR performance of the welded joint. Therefore, the Si content in steel should be controlled as low as possible, and the Si content in steel should be controlled below 0.10%.

[0037] P, as a harmful inclusion in steel, has a significant detrimental effect on the mechanical properties of steel plates, especially toughness, elongation, weldability, and the SR performance of welded joints. Theoretically, the lower the content, the better. However, considering the operability and cost of steelmaking, for steel plates that require excellent weldability and a good balance of strength and toughness / ductility, the P content needs to be controlled at ≤0.013%.

[0038] Sulfur (S), as a harmful inclusion in steel, significantly impairs the toughness of steel plates. More importantly, S combines with manganese (Mn) in steel (especially under high Mn content conditions) to form MnS inclusions. During hot rolling, the plastic deformation characteristics of MnS cause MnS to extend along the rolling direction, forming MnS inclusion bands along the rolling direction. This severely damages the toughness, elongation, Z-axis properties, weldability, and SR properties of welded joints of the steel plate. At the same time, S is also the main element that causes hot brittleness during hot rolling. Theoretically, the lower the content, the better. However, considering the principles of steelmaking operability, steelmaking cost, and smooth logistics, for steel plates that require excellent weldability and excellent strength-toughness / strength-plasticity matching, the S content needs to be controlled at ≤0.0030%.

[0039] As a weak carbide-forming element, Cr not only improves the hardenability of steel plates and promotes the formation of martensite / bainite, but also increases the orientation difference of martensite / bainite lath grain boundaries, increasing the resistance of cracks to passing through the martensite / bainite grain boundaries. While improving the strength of the steel plate, it also has a certain effect on improving the toughness of the steel plate. However, when the amount of Cr added is too large, during tempering and welding thermal cycles, chromium-containing carbides precipitate and aggregate at the original austenite grain boundaries (precipitating in a necklace-like pattern), which seriously impairs the low-temperature toughness and weldability of the steel plate. However, for 800MPa grade TMCP type steel plates, a certain Cr content is necessary to ensure that the steel plate has sufficient hardenability. Therefore, the appropriate Cr content is controlled between 0.25% and 0.60%.

[0040] The purpose of adding trace amounts of Nb to steel is to achieve non-recrystallization controlled rolling, increase the hardenability of rolled steel sheets during the TMCP process, refine the microstructure of the base steel sheet, and improve the strength, toughness, and resistance to tempering softening of the steel sheet. There is an optimal matching range between the Nb content range and the C content range. Within the low carbon range, the content of Nb that exerts the best non-recrystallization controlled rolling and toughening effect can be appropriately increased to ensure that the strength, toughness, and strength-toughness / strength-plasticity of the steel sheet are excellent, while Nb has minimal impact on the weldability of the steel sheet. Therefore, when the Nb content is below 0.030%, the aforementioned effects of Nb in low-carbon steel plates cannot be effectively utilized, thus failing to improve the strength and toughness of the steel plates (especially under conditions where toughening elements such as Cu and Ni are not present). When the Nb content exceeds 0.060%, it induces the formation of upper bainite (Bu) and secondary precipitation embrittlement of Nb (C,N) under welding conditions, severely impairing the low-temperature toughness of the weld heat-affected zone (HAZ). As a strong carbide-forming element, Nb reduces the phase difference between bainite / martensite lath grain boundaries, greatly impairing the toughness of the steel plate and the weld HAZ. Therefore, the appropriate Nb content should be controlled between 0.030% and 0.060%.

[0041] When the Ti content is between 0.008% and 0.016%, it inhibits excessive austenite grain growth during slab heating and TMCP, improving the toughness of the steel plate. More importantly, it inhibits HAZ grain growth during welding, improving HAZ toughness. In addition, Ti has a nitrogen-fixing effect, eliminating free nitrogen in the steel and ensuring that boron exists in solid solution form. However, when the Ti content exceeds 0.015%, under conditions of high acid-soluble aluminum and low nitrogen content, excess Ti coherently precipitates as TiC on martensitic / bainitic laths and grain boundaries, severely embrittles the toughness of high-strength steel plates.

[0042] The boron content is controlled between 0.0008% and 0.0018% to ensure the hardenability of the steel plate without compromising its weldability, HAZ toughness, and slab surface quality.

[0043] Al in steel can fix free nitrogen in the steel. In addition to reducing free nitrogen in the weld heat-affected zone (HAZ) and improving the low-temperature toughness of the weld HAZ, more importantly, it can ensure that there is a certain amount of solid solution [B] in the steel and improve the hardenability of the steel plate. Therefore, the lower limit of Al is controlled at 0.030%. However, adding too much Al to steel will not only cause casting difficulties, but also form a large number of dispersed needle-like Al2O3 inclusions in the steel, which will damage the integrity of the steel plate, low-temperature toughness and weldability. Therefore, the upper limit of Al is controlled at 0.070%.

[0044] To ensure the presence of dissolved [B] in the steel plate and to prevent the precipitation of large amounts of coarse AlN along the original austenite grain boundaries in a necklace-like pattern, which would impair the toughness and plasticity of the steel plate, the N content in the steel shall not exceed 0.0055%.

[0045] 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-grained spherical sulfides; thirdly, it suppresses the hot brittleness of sulfur; fourthly, it improves the low-temperature toughness, elongation, and Z-axis properties of the steel plate; and fifthly, it improves the anisotropy of the steel plate's toughness and weldability. Furthermore, Ca treatment improves the casting of high-acid-soluble aluminum steel. The amount of Ca added depends on the sulfur content in the steel. Too little Ca has little effect; too much Ca results in excessively large Ca(O,S) inclusions, increasing brittleness and potentially becoming crack initiation points, reducing the steel's low-temperature toughness, elongation, and weldability, while also lowering the steel's purity 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. Therefore, the suitable range for Ca content is 0.0010–0.0040%.

[0046] In particular, the element content of the present invention must simultaneously satisfy the following relationship:

[0047] (%Als) / [(%N)-0.292(%Ti)]≥33;

[0048] ① To ensure that there is a sufficient amount of solid solution [B] in the steel and that AlN precipitates in a fine and dispersed state, to avoid coarse AlN precipitating in a necklace-like manner on the original austenite grain boundaries, thereby improving the hardenability of the steel plate, refining the grain size of the steel plate, and improving the low-temperature toughness of the steel plate.

[0049] ② Ensure that during the multi-layer and multi-pass welding process of the steel plate, the solid solution [N] is completely formed into fine and dispersed AlN, eliminating the strong embrittlement effect of solid solution [N] on the weld heat-affected zone, and improving the toughness and crack resistance and crack arrest characteristics of the weld heat-affected zone of high-strength steel plate.

[0050] (%C)×[0.85(%Mn)+1.19(%Als)+9.33(%P)+12.05(%S)+(%Si)]≤0.16,

[0051] ① It inhibits the conjugate segregation of C, Mn, P and S elements during the solidification process of molten steel, improves the three internal properties of slab (soundness, homogeneity and purity), and enhances the uniformity, toughness and weldability of steel plate performance;

[0052] ②Increase the critical cooling rate of the martensitic phase transformation to promote the formation of lower bainite (B). L Generate, form B L +M multiphase structure effectively divides the original austenite grains, refines the grain size of 800MPa grade steel plate, and improves the toughness of the base steel plate;

[0053] ③Promoting carbide precipitation, inhibiting M / A island precipitation in HAZ, reducing the number and size of M / A islands, improving M / A island morphology, and improving the toughness and crack resistance and crack arrest characteristics of welded steel plate HAZ; this is one of the key core technologies of this invention.

[0054] Ca treatment, Ca / S ratio of 1–3, and (%Ca) × (%S) ≤ 1.5 × 10⁻⁶ -3 ;

[0055] Ensure that the amount of sulfides in the steel is appropriate and that the sulfides are completely and completely spherically formed into Ca(O,S) and distributed in the steel in a fine and dispersed state; so as to improve the toughness, toughness and strength-plasticity matching, weldability and resistance to lamellar tearing of steel plates.

[0056] 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.

[0057] The method for manufacturing ultra-low cost, high weldability 800MPa grade steel plate according to the present invention includes the following steps:

[0058] 1) Smelting and casting

[0059] The above-mentioned components are smelted and cast into slabs;

[0060] 2) Control the rolling process; the total compression ratio of the steel plate (i.e., slab thickness / finished steel plate thickness) should be ≥ 3.5.

[0061] The first stage is ordinary rolling, with the slab heating temperature controlled at 1080-1180℃; high reduction rolling is adopted, with a reduction rate of ≥8% per rolling pass;

[0062] The second stage adopts non-recrystallization controlled rolling, with a rolling start temperature of 780-860℃, a rolling pass reduction rate of ≥7%, a cumulative reduction rate of ≥50% in the non-recrystallization zone, and a final rolling temperature of 760-800℃.

[0063] Furthermore, the following requirements must be met: (T) 终轧 ) / (ξ×χ)≤6.7,

[0064] Among them, T 终轧 The final rolling temperature before recrystallization is ℃;

[0065] ξ represents the cumulative reduction rate without recrystallization, in percentage (%).

[0066] χ is the large-angle bainite lath / martensite lath grain boundary formation index, χ = 1.33 (%Mn) + (%Ni) + 0.55 (%Cu) + 0.21 (%Cr) - 0.93 (%Mo) - 2.55 (%Nb) - 1.91 (%Ti) - 1.42 (%V) - 4.63 (%Si), in %;

[0067] 3) Control cooling

[0068] After the non-recrystallization controlled rolling is completed, the steel plate is immediately transported to the DQ+ACC equipment for accelerated cooling. The initial cooling temperature is 730–780℃, the cooling rate is ≥5℃ / s, and the final cooling temperature is controlled at 320–470℃. Furthermore, the following requirements must be met:

[0069] {(DI OL )×ξ×(V c )×[(T 开冷 -T 停冷 )]} / [H×(T 停冷 )]≥405;

[0070] Among them, DI OL For online hardenability index,

[0071] DI OL =0.51C 0.5 [(1+0.7(%Si)][(1+3.33(%Mn)][(1+

[0072] 0.35(%Cu)][(1+0.36(%Ni)][(1+2.16(%Cr)][(1+3(%Mo)][(1+

[0073] 1.75(%V)][(1+1.77(%Al)][(1+200(%B)]×25.4(mm), unit mm;

[0074] ξ represents the cumulative reduction rate without recrystallization, in percentage (%).

[0075] V c The unit for accelerating the cooling rate of the steel plate is ℃ / s;

[0076] T 开冷 The starting temperature for accelerated cooling, measured in °C;

[0077] T 停冷 The stopping temperature for accelerated cooling, in °C;

[0078] H represents the thickness of the finished steel plate, in mm.

[0079] Preferably, in step 1), continuous casting is used, the superheat of the tundish is controlled at 8-0℃, the casting speed is controlled at 0.6-1.0m / min, the liquid level fluctuation in the crystallizer is controlled at ≤5mm, and the light reduction process at the end of solidification is 2-5%.

[0080] Preferably, after step 3) accelerated cooling ends, when the steel plate thickness is ≥40mm, the steel plate is subjected to slow cooling. The slow cooling process involves holding the steel plate at a temperature of not less than 300℃ for at least 24 hours, and then the steel plate is naturally air-cooled to room temperature.

[0081] In the manufacturing method described in this invention:

[0082] According to the requirements of the steel plate composition system, mechanical properties, weldability, and internal quality (i.e., UT testing) of the present invention, the casting of the present invention preferably adopts continuous casting, the superheat of the tundish is controlled at 8℃~30℃, the casting speed is controlled at 0.6m / min~1.0m / min, the liquid level fluctuation in the crystallizer is controlled at ≤5mm, and the light reduction process at the end of solidification is 2%~5%.

[0083] Controlled rolling

[0084] To ensure that the microstructure of the ultra-high strength steel plate is uniform and fine, the total compression ratio of the steel plate (slab thickness / finished steel plate thickness) is ≥3.5.

[0085] The first stage is ordinary rolling. To ensure that [Al] + BN → AlN + [B] occurs during heating and rolling, and to ensure that there is sufficient dissolved B in the steel, the slab heating temperature is controlled between 1080℃ and 1180℃. High reduction rolling is adopted, with a reduction rate of ≥8% per rolling pass, to ensure that the microstructure of the intermediate slab is uniform and fine and that there is sufficient dissolved B, laying the foundation for the subsequent TMCP process.

[0086] The second stage adopts non-recrystallization controlled rolling, with a rolling start temperature of 780℃~860℃, a rolling pass reduction rate of ≥7%, a cumulative reduction rate of ≥50% in the non-recrystallization zone, and a final rolling temperature of 760℃~800℃.

[0087] After the non-recrystallization controlled rolling is completed, the steel plate is immediately transported to the DQ+ACC equipment, where it undergoes accelerated cooling. The initial cooling temperature is 730℃~780℃ (final rolling temperature), the cooling rate is ≥5℃ / s, and the final cooling temperature is controlled between 320℃~470℃. When the steel plate thickness is ≥40mm, it undergoes slow cooling, which involves holding at a temperature of at least 300℃ for at least 24 hours. Subsequently, the steel plate is allowed to air-cool naturally to room temperature to ensure sufficient dehydrogenation and prevent hydrogen-induced cracking.

[0088] In addition to meeting the above requirements, the following relationship must also be satisfied:

[0089] (T 终轧 ) / (ξ×χ)≤6.7;

[0090] Among them, T 终轧 ξ is the final rolling temperature before recrystallization, in °C; ξ is the cumulative reduction rate before recrystallization, in %; χ is the large-angle bainite lath / martensite lath grain boundary formation index, χ=1.33(%Mn)+(%Ni)+0.55(%Cu)+0.21(%Cr)-0.93(%Mo)-2.55(%Nb)-1.91(%Ti)-1.42(%V)-4.63(%Si), in %.

[0091] The formula shows that as the final rolling temperature T increases... 终轧 The reduction and increase of the cumulative reduction rate ξ of non-recrystallization result in a greater controlled rolling effect, a greater degree of suppression of austenite recrystallization, enhanced austenite flattening effect, and increased dislocation density and deformation band density within austenite grains.

[0092] Secondly, as the cumulative reduction rate ξ of non-recrystallization increases, the dislocation density and deformation band density inside the flat austenite grains also increase significantly; this leads to an increase in the number of ferrite phase deformation nuclei, an increase in the driving force for the austenite-to-ferrite phase transformation, a decrease in the size of the bainite / martensite clusters after the phase transformation, and an increase in the frequency of large-angle grain boundaries between variants in the block.

[0093] Finally, as the large-angle bainite lath / martensite lath grain boundary formation index χ increases, the probability of large-angle grain boundaries between variants in the block increases.

[0094] The combined effects of these factors result in finer and more uniform bainite / martensite clusters, a significant increase in the frequency of large-angle grain boundaries between variants within the block, leading to higher stress-strain requirements for crack initiation (due to smaller dislocation pile-up lengths) and increased energy consumption during crack propagation due to repeated bending (packet interfaces and large-angle grain boundaries between variants act as points preventing crack propagation). This results in higher strength, toughness, and plasticity (HP relationship) and a better strength-toughness / strength-plasticity balance in the steel plate. This is one of the key technologies of this invention.

[0095] {(DI OL )×ξ×(V c )×[(T 开冷 -T 停冷 )]} / [H×(T 停冷 )]≥405;

[0096] Among them, DI OL DI is the online hardenability index. OL =0.51C 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(mm), unit mm; ξ is the cumulative reduction rate of non-recrystallization, unit %; V c T represents the accelerated cooling rate of the steel plate, expressed in °C / s. 开冷 The initial temperature for accelerated cooling, measured in °C (T). 停冷 The stopping temperature for accelerated cooling, in °C; H is the thickness of the finished steel plate, in mm.

[0097] With the online hardenability index DI OL Increase, increase in cumulative non-recrystallization reduction ξ, and increase in cooling rate V cIncreasing the temperature range from start to stop cooling leads to a relative increase in martensite content and martensite density in the low-temperature phase transformation microstructure of the steel plate (due to the inherited effect of controlled rolling), and a decrease in the effective fracture element size of bainite / martensite, resulting in increased steel plate strength. Conversely, as the stop cooling temperature increases and the steel plate thickness increases (resulting in a lower cooling rate, especially in the center of the steel plate), the martensite content in the low-temperature phase transformation microstructure decreases, leading to lower steel plate strength. In summary, when all indicators satisfy the above formula, the steel plate achieves high strength, high toughness, and excellent strength / toughness / plasticity, while also exhibiting excellent weldability. This successfully eliminates the inherent contradiction between low carbon content, ultra-low cost, and high toughness and excellent weldability in 800MPa steel plates. This is one of the key core technologies of this invention.

[0098] The beneficial effects of this invention are:

[0099] This invention uses a low-C-ultra-low-Si-high-Mn-Cr alloying-(Ti+high-Nb+B) microalloying composition system as the basis, and appropriately increases the acid-soluble Al content in the steel, (%Als) / [(%N) t [0.292%Ti]≥33, control (%C)×[0.85%Mn+1.19%Als+9.33%P+12.05%S+(%Si)]≤0.16, Ca treatment, Ca / S ratio between 1.00 and 3.00 and other metallurgical control measures to optimize the TMCP process, i.e. (T 终轧 ) / (ξ×χ)≤6.7、{(DI OL )×ξ×(T 开轧 )×[(T 开冷 -T 停冷 )]} / [H×(T 停冷 The concentration of 405 ensures that the microstructure of the finished steel plate is low-carbon bainite + a small amount of low-carbon martensite, with an average grain size of less than 20μm, resulting in a high-strength steel plate with excellent strength, plasticity, and toughness in the 80kg class.

[0100] This invention achieves excellent 800MPa grade high-strength quenched and tempered steel plates while also exhibiting superior strength / toughness / ductility and weldability. It successfully eliminates the inherent contradiction between low carbon content, ultra-low cost, and high toughness and excellent weldability in 800MPa steel plates. This not only improves the safety and reliability of large heavy steel structures but also reduces manufacturing steps, shortens the manufacturing cycle, and lowers steel plate manufacturing costs through the online TMCP process. More importantly, the new generation TMCP process fully utilizes the hardenability potential of alloying elements, maximizing their hardenability / hardening properties. This allows for the achievement of high strength, excellent strength-ductility and ductility-toughness balance, and excellent weldability without requiring high-value alloys (such as Cu, Ni, Mo) or low carbon content and carbon equivalent. This not only further reduces manufacturing costs but also improves the weldability of steel plates. Especially for high-strength steel plates, the sensitivity to cold cracking during welding is greatly reduced, the welding preheating temperature is significantly lowered, post-weld heat treatment (i.e., SR) is not required, and the suitable range of welding heat input is wider. Correspondingly, this reduces the cost and procedures for users to process and manufacture, shortens the manufacturing time of steel components for users, and creates huge value for users. Therefore, this type of steel plate is not only a high-value-added and environmentally friendly product. Attached Figure Description

[0101] Figure 1 This is a photograph of the microstructure (1 / 4 thickness) of the steel in Example 5 of the present invention. Detailed Implementation

[0102] The present invention will be further described below with reference to the embodiments and accompanying drawings.

[0103] The composition of the embodiments of the present invention is shown in Table 1, with the balance being Fe and unavoidable impurities. Tables 2 and 3 show the manufacturing process parameters of the embodiments of the present invention. Table 4 shows the performance parameters of the embodiments of the present invention.

[0104] Depend on Figure 1 It is known that, through the combination of composition, TMCP and subsequent tempering process, the microstructure of the steel plate is fine low-carbon bainite + a small amount of low-carbon martensite (less than 30%), with an average cluster size of less than 20μm.

[0105] This invention combines the design of alloying elements in steel plates with the TMCP process to obtain high-strength tempered steel plates of 800MPa grade. At the same time, the steel plates also exhibit excellent strength, toughness / ductility, and weldability. It successfully eliminates the contradiction between low carbon content, ultra-low cost, high toughness, and excellent weldability in 800MPa steel plates. This not only improves the safety and reliability of large heavy steel structures, but also reduces manufacturing steps, shortens the manufacturing cycle, and lowers the manufacturing cost of steel plates through the online TMCP process.

[0106] More importantly, the steel plate of this invention utilizes a new generation of TMCP process to fully leverage the hardenability potential of alloying elements. The hardenability / hardening properties of these elements are maximized, achieving high strength, excellent strength-to-ductility and toughness balance, and superior weldability without the use of expensive alloys (such as Cu, Ni, Mo), low carbon content and equivalents, and a short manufacturing process (omitting tempering heat treatment). This not only further reduces manufacturing costs but also improves the weldability of the steel plate. Especially for high-strength steel plates, the sensitivity to cold cracking during welding is significantly reduced, the welding preheating temperature is drastically lowered, post-weld heat treatment (SR) is unnecessary, and the suitable range of welding heat input is wider. Correspondingly, this reduces the user's processing costs and procedures, shortens the manufacturing time of steel components, and creates significant value for the user. Therefore, this type of steel plate is not only a high-value-added and environmentally friendly product.

[0107] The 800MPa grade high-strength, high-toughness, and excellent weldability tempered steel plate described in this invention is mainly used for manufacturing pressure water pipes, volutes and steel branch pipes for hydropower projects, large-scale engineering machinery structures, and offshore oil platforms, and is a key material for national economic construction. The 800MPa grade high-strength, high-toughness, and excellent weldability tempered steel plate has broad market prospects. 。

[0108]

[0109]

[0110]

Claims

1. A low-cost, high-weldability 800MPa grade steel plate, the composition by weight percentage of which is: C：0.050％～0.090％ Si: ≤0.10% Mn: 1.60%~2.00% P：≤0.013％ S：≤0.0030％ Cr：0.25％～0.60％ Nb: 0.030%~0.060% Ti: 0.008%~0.016% B：0.0008％～0.0018％ Als: 0.030%~0.070% N：≤0.0055％ Ca: 0.001%~0.0040% The balance consists of Fe and unavoidable inclusions; and simultaneously satisfies the following relationship: (%Als) / [(%N)-0.292(%Ti)]≥33; (%C)×[0.85(%Mn)+1.19(%Als)+9.33(%P)+12.05(%S)+(%Si)]≤0.16; Ca treatment, Ca / S ratio of 1–3, and (%Ca) × (%S) ≤ 1.5 × 10⁻⁶ -3 ; The microstructure of the steel plate is low-carbon bainite + a small amount of low-carbon martensite, with an average grain size of less than 20 μm. The steel plate has a tensile strength ≥770MPa, a yield strength ≥690MPa, a Charpy transverse impact energy of ≥100J at -40℃, and a fracture elongation δ5 ≥15%; a low preheating temperature, less than 50℃; no heat treatment is required after welding; the heat input range is wide, 15~35kJ / cm; and the transverse impact energy of the heat-affected zone at -40℃ is ≥100J per unit.

2. The method for manufacturing ultra-low cost, high weldability 800MPa grade steel plate as described in claim 1, characterized in that, Includes the following steps: 1) Smelting and casting Smelting and casting into slabs according to the composition described in claim 1; 2) Control the rolling process; the total compression ratio of the steel plate (i.e., slab thickness / finished steel plate thickness) should be ≥ 3.

5. The first stage is ordinary rolling, with the slab heating temperature controlled at 1080-1180℃; high reduction rolling is adopted, with a reduction rate of ≥8% per rolling pass; The second stage adopts non-recrystallization controlled rolling, with a rolling start temperature of 780-860℃, a rolling pass reduction rate of ≥7%, a cumulative reduction rate of ≥50% in the non-recrystallization zone, and a final rolling temperature of 760-800℃. Furthermore, the following requirements must be met: (T) 终轧 ) / (ξ×χ)≤6.7, Among them, T 终轧 The final rolling temperature before recrystallization is ℃; ξ represents the cumulative reduction rate without recrystallization, in percentage (%). χ is the large-angle bainite lath / martensite lath grain boundary formation index, χ = 1.33 (%Mn) + (%Ni) + 0.55 (%Cu) + 0.21 (%Cr) - 0.93 (%Mo) - 2.55 (%Nb) - 1.91 (%Ti) - 1.42 (%V) - 4.63 (%Si), in %; 3) Control cooling After the non-recrystallization controlled rolling is completed, the steel plate is immediately transported to the DQ+ACC equipment for accelerated cooling. The initial cooling temperature is 730–780℃, the cooling rate is ≥5℃ / s, and the final cooling temperature is controlled at 320–470℃. Furthermore, the following requirements must be met: {(DI OL )×ξ×(V c )×[(T 开冷 -T 停冷 )]} / [H×(T 停冷 )]≥405; Among them, DI OL For online hardenability index, DI OL =0.51C 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; ξ represents the cumulative reduction rate without recrystallization, in percentage (%). V c The unit for accelerating the cooling rate of the steel plate is ℃ / s; T 开冷 The starting temperature for accelerated cooling, measured in °C; T 停冷 The stopping temperature for accelerated cooling, in °C; H represents the thickness of the finished steel plate, in mm.

3. The method for manufacturing ultra-low cost, high weldability 800MPa grade steel plate as described in claim 2, characterized in that, Step 1) Casting adopts continuous casting. The superheat of the tundish is controlled at 8-0℃, the casting speed is controlled at 0.6-1.0m / min, the liquid level fluctuation in the crystallizer is controlled at ≤5mm, and the light pressure reduction process at the end of solidification is 2-5%.

4. The method for manufacturing ultra-low cost, excellent weldability 800MPa grade steel plate as described in claim 2 or 3, characterized in that, Step 3) After the accelerated cooling is completed, when the steel plate thickness is ≥40mm, the steel plate is slowly cooled. The slow cooling process is to keep it at a temperature of not less than 300℃ for more than 24 hours, and then the steel plate is naturally air-cooled to room temperature.

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