800MPa-grade steel plate with ultralow cost and excellent weldability and manufacturing method of 800MPa-grade steel plate

Abstract

The invention discloses a 800MPa-grade steel plate with ultralow cost and excellent weldability and a manufacturing method thereof. The 800MPa-grade steel plate comprises the following components in percentage by weight: 0.050-0.090% of C, less than or equal to 0.10% of Si, 1.60-2.00% of Mn, less than or equal to 0.013% of P, less than or equal to 0.0030% of S, 0.25-0.60% of Cr, 0.030-0.060% of Nb, 0.008-0.016% of Ti, 0.0008-0.0018% of B, 0.030-0.070% of Als, less than or equal to 0.0055% of N, 0.001-0.0040% of Ca and the balance of Fe and inevitable impurities. (% Als) / [(% N)-0.292 (% Ti)] is greater than or equal to 33; (% C) * [0.85 (% Mn) + 1.19 (% Als) + 9.33 (% P) + 12.05 (% S) + (% Si)] < = 0.16; and in Ca treatment, the Ca / S ratio is 1-3, and (% Ca) * (% S) is less than or equal to 1.5 * 10 <-3 >. On the basis that precious alloy elements such as Cu, Ni and Mo are not added, the high strength of 800 MPa is obtained at ultra-low cost, meanwhile, the toughness and weldability of the steel plate are also excellent, preheating is not needed before welding, stress relief heat treatment is not needed after welding, and the steel plate is particularly suitable for large steel structures and equipment such as engineering machinery and coal mine machinery.

Description

Technical Field

[0001] The present invention relates to the technical field of high-strength steel, and in particular to an ultra-low-cost, excellent weldability 800MPa grade steel plate and a manufacturing method thereof. Background Art

[0002] As we all know, 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, boiler containers, building structures, automobile 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 which strength, toughness, plasticity and weldability are the most important properties of low-carbon (high-strength) low-alloy steel, which is ultimately determined by the microstructural state of the finished steel; with the continuous development of metallurgical technology, people have higher requirements for the toughness, plasticity and weldability of high-strength steel, that is, the steel plate has high strength, high elongation, high strength and high weldability at low temperature. While resisting brittle fracture and plastic instability fracture, the steel plate has excellent welding performance; and under the condition of low manufacturing cost, the comprehensive mechanical properties and performance of the steel plate are greatly improved to reduce the amount of steel used to save costs, reduce the weight, stability and safety of the steel components themselves, and more importantly, to further improve the cold and hot workability of high-quality steel and the safety and reliability during service; currently, Japan, South Korea and the European Union have set off a research climax on the development of a new generation of high-performance steel materials, trying to obtain better microstructure matching, ultra-fine organization and structure through alloy combination optimization design and innovative manufacturing process technology, so that high-strength steel can obtain better strength, plasticity and toughness matching.

[0003] Traditional high-strength steel plates with tensile strength ≥780MPa are mainly produced by offline quenching and tempering process (RQ+T); this requires that the steel plates must have sufficiently high hardenability and hardenability, that is, the 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 performance along the plate thickness direction. Therefore, it is inevitable to add a large amount of alloying elements such as Cr, Mo, Ni, Cu, etc. to the steel, especially adding a large amount of Ni element (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 Sho 59-129724; Hei 1-219121; Nippon Steel Research No. 314-1984; Japanese Steel Pipe Technical Bulletin No.107-1985; Nippon Steel Technical Bulletin No.348-1993; Kawasaki Steel Technical Bulletin Vol.4 (No.3)-1972; Kawasaki Steel Technical Bulletin Vol.7 (No.2)-1975).

[0004] More importantly, the use of traditional quenched and tempered steel component system and manufacturing process not only results in a long steel plate manufacturing cycle and high manufacturing cost, but also makes it difficult to handle scrap steel containing high Cu and Ni content, limiting the efficiency of scrap steel recycling and reuse; and for 80 kg quenched and tempered steel produced by traditional quenched and tempered process, due to the high alloy content of the steel plate, the steel plate has low elongation, high yield strength ratio, poor weldability (high sensitivity to welding cold cracks, high embrittlement degree of welding heat affected zone, sensitivity to reheat cracks, etc.), and it is difficult to control the uniformity of performance in the plate thickness direction; and the low elongation is not only not conducive to the cold and hot processing performance of the steel plate, but also has a great influence on the fatigue resistance, stress concentration sensitivity and structural stability of the steel plate. When used in large-scale engineering construction and large-scale equipment such as penstocks and steel bifurcations in hydropower projects, thermal power steam turbine generators and offshore platform structures, ship floating cranes and giant excavators, there are great safety hazards; therefore, when large fatigue heavy-load steel structures use high-strength steel, it is generally hoped that the 80 kg high-strength steel has excellent toughness and plasticity matching, especially the tensile elongation δ5 is above 18%.

[0005] A large number of existing patent documents only explain how to achieve the strength and low-temperature toughness of the parent steel plate, but there are few descriptions on improving the welding performance of the steel plate and obtaining excellent low-temperature toughness of the HAZ of the welding heat affected zone, let alone how to improve the tensile strength of the steel plate while improving the tensile elongation and uniformity of mechanical properties in the thickness direction of the steel plate (Japanese Patents Sho 63-93845, Sho 63-79921, Sho 60-258410, Special Opening 4-285119, Special Opening 4-308035, Hei 3-264614, Hei 2-250917, Hei 4-143246, US Patent US Patent 5798004, European Patent EP 0288054A2, Nishiyama Memorial Technical Lecture No. 159-160, P79~P80).

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

[0007] In the "Low-cost 80 kg-grade 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 requirement of -20°C and above.

[0008] In addition, a controlled rolling + offline quenching + tempering process is adopted; this not only has many manufacturing processes, a long manufacturing cycle, and high manufacturing costs, but also relatively high energy consumption in the manufacturing process (the steel plate is rolled and naturally air-cooled to room temperature, and then shot blasted and heated to the quenching temperature again), which is not conducive to energy saving and environmental protection; and the offline quenching + tempering process cannot give full play to the hardenability and hardenability potential of the alloy elements, and the hardenability and hardenability of the elements cannot be maximized; therefore, in order to obtain the same strength and toughness level, more alloy elements (especially Ni, Mo, Cr, etc.) must be added, which not only further increases the manufacturing cost, but also damages the weldability of the steel plate, especially for ultra-high strength steel plates, the sensitivity of welding cold cracking is greatly increased, and welding preheating and post-heating (i.e. PWHT) at higher temperatures are required, and the range of suitable welding heat input is narrower, and the corresponding processing and manufacturing costs are also greatly increased.

[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 DQ process without adding precious alloy 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 the steel plate of Chinese Patent ZL202010468912.7 can meet the requirements of high toughness, low yield strength ratio and excellent weldability, the content of precious alloy elements Cu, Ni and Mo in the steel plate is too high, the manufacturing cost of the steel plate remains high, and the market competitiveness of the steel plate is poor, which cannot meet the large-scale and wide-ranging needs of engineering machinery and coal mining machinery. Summary of the invention

[0011] The object of the present invention is to provide an ultra-low-cost, excellent weldability 800MPa grade steel plate and a manufacturing method thereof, which adopts the TMCP process and omits offline tempering. On the basis of not adding precious alloy elements such as Cu, Ni and Mo, 800MPa high strength is obtained at an ultra-low cost, and the toughness and weldability of the steel plate are also excellent (no preheating is required before welding and no stress relief heat treatment is required after welding). The contradictory problem between the strength, toughness, weldability and ultra-low cost of the 800MPa grade steel plate is successfully solved, and the steel plate is particularly suitable for large steel structures and equipment such as engineering machinery and coal mining machinery.

[0012] To achieve the above object, the technical solution of the present invention is:

[0013] An ultra-low-cost, excellent weldability 800MPa grade steel plate, the weight percentage of its components is:

[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 at the same time 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 is 1-3, and (%Ca)×(%S)≤1.5×10 -3 .

[0030] Furthermore, the balance is Fe and inevitable inclusions.

[0031] The microstructure of the steel plate of the present invention is low-carbon bainite + a small amount of low-carbon martensite, and the average crystal cluster size is less than 20 μm.

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

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

[0034] C has a great influence on the strength, low temperature toughness, elongation and weldability of high-strength steel. From the perspective of improving the intrinsic plasticity and toughness and weldability of high-strength tempered steel, it is hoped that the C content in the steel can be controlled at a lower level; however, from the perspective of hardenability of high-strength steel, matching of strength and plasticity with strength and toughness, microstructure control during the TMCP process and manufacturing cost, the C content should not be controlled too low, especially for 80 kg TMCP steel plates; therefore, the reasonable range of C content is 0.050% to 0.090%.

[0035] As the most important alloying element, Mn not only improves the strength of the steel plate, but also has the functions of expanding the austenite phase area, significantly reducing the Ar3 point temperature, refining the bainite / martensite packet structure size, and the orientation difference of the bainite lath / martensite lath grain boundary to improve the low-temperature toughness of the steel plate, and promote the formation of low-temperature phase transformation bainite / martensite to make the steel plate have excellent toughness / plasticity matching; however, Mn is prone to segregation during the solidification of molten steel, especially when the Mn content is high, which will not only cause casting operation difficulties, but also It is easy to cause conjugate segregation with elements such as C, P, S and Si. Especially when the C content in the steel is high, the segregation and looseness in the center of the ingot are aggravated. Severe segregation in the center of the ingot is easy to form abnormal structure in the subsequent TMCP and welding process, resulting in low toughness of the high-strength steel plate and cracks in the welded joint. Therefore, it is extremely necessary to select an appropriate Mn content range for 800MPa grade steel plates according to the strength level and the C content range in the steel. The suitable Mn content of the quenched and tempered steel plate of the present invention is 1.60% to 2.00%.

[0036] Si promotes the deoxidation of molten steel and can improve the strength of 800MPa steel plates. However, for molten steel deoxidized by Al, the deoxidation effect of Si is not significant. Although Si can improve the strength of the steel plate, Si reduces the critical cooling rate of martensite formation, inhibits the formation of lower bainite, and seriously damages the toughness, elongation and weldability of high-strength steel plates. Especially under large heat input welding conditions, Si not only promotes the formation of MA islands, but also the formed MA islands are relatively large in size and unevenly distributed, which seriously damages the toughness of the welding heat affected zone (HAZ) and the SR performance of the welded joint. Therefore, the Si content in the steel should be controlled as low as possible. Therefore, the Si content in the steel is controlled below 0.10%.

[0037] As a harmful inclusion in steel, P has a great detrimental effect on the mechanical properties of steel plates, especially toughness, elongation, weldability and SR performance of welded joints. In theory, the lower the better; but considering the operability and cost of steelmaking, for steel plates requiring excellent weldability and excellent toughness / plasticity matching, the P content needs to be controlled at ≤0.013%.

[0038] As a harmful inclusion in steel, S has a great damaging effect on the toughness of the steel plate. More importantly, S combines with Mn in steel (especially under high Mn content conditions) to form MnS inclusions. During the hot rolling process, the plastic deformation characteristics of MnS cause MnS to extend along the rolling direction, forming a MnS inclusion belt along the rolling direction, which seriously damages the toughness, elongation, Z-direction performance, weldability and SR performance of the steel plate. At the same time, S is also the main element that produces hot brittleness during hot rolling. In theory, the lower the better; but considering the operability of steelmaking, steelmaking cost and the principle of smooth logistics, for steel plates requiring excellent weldability and excellent strength and toughness / strength and plasticity matching, the S content needs to be controlled at ≤0.0030%.

[0039] Cr is a weak carbide-forming element. Adding Cr not only improves the hardenability of the steel plate and promotes the formation of martensite / bainite, but also increases the orientation difference of the martensite / bainite lath grain boundary, increases the resistance of cracks passing through the martensite / bainite grain boundary, and improves the strength of the steel plate while improving the toughness of the steel plate to a certain extent. However, when the amount of Cr added is too much, during the tempering and welding thermal cycle, chromium-containing carbides precipitate, aggregate and grow (precipitate in the shape of a necklace) at the original austenite grain boundary, which seriously damages the low-temperature toughness and weldability of the steel plate. However, for 800MPa grade TMCP steel plates, a certain Cr content must be present 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 a trace amount of Nb element to steel is to carry out non-recrystallization controlled rolling, increase the hardenability of rolled steel plates during the TMCP process, refine the microstructure of the base steel plate, and improve the strength, toughness and temper softening resistance of the steel plate; there is an optimal matching range between the Nb content range and the C content range. Within the low carbon range, the Nb content that plays the best role in non-recrystallization controlled rolling and strengthening and toughening can be appropriately increased to ensure that the strength, toughness and strength-toughness / strength-ductility matching of the steel plate are excellent, while Nb has little damage to the weldability of the steel plate. Therefore, when the Nb addition is less than 0.030%, the above-mentioned role of Nb in the low-carbon steel plate cannot be effectively exerted and the strength and toughness of the steel plate cannot be improved (especially under the condition of not containing toughening elements such as Cu and Ni); when the Nb addition exceeds 0.060%, the formation of upper bainite (Bu) and the secondary precipitation embrittlement of Nb (C, N) are induced under welding conditions, which seriously damages the low-temperature toughness of the welding heat affected zone (HAZ); Nb, as a strong carbide-forming element, reduces the phase difference of the bainite / martensite lath grain boundary, greatly damaging the toughness of the steel plate and the welding HAZ; therefore, the appropriate Nb content is controlled between 0.030% and 0.060%.

[0041] The Ti content is between 0.008% and 0.016%, which inhibits the excessive growth of austenite grains during slab heating and TMCP, improves the toughness of the steel plate, and more importantly, inhibits the growth of HAZ grains during welding, improving the toughness of the HAZ. In addition, Ti has the function of fixing nitrogen, eliminating free nitrogen in the steel, and ensuring that the B element exists in the form of solid solution B. However, when the Ti content exceeds 0.015%, under the conditions of high acid-soluble aluminum and low nitrogen content, excess Ti precipitates in the form of TiC coherently on the martensite / bainite laths and on the grain boundaries, seriously embrittles the toughness of the high-strength steel plate.

[0042] The B content is controlled between 0.0008% and 0.0018%, which ensures the hardenability of the steel plate without damaging the weldability, HAZ toughness and surface quality of the slab.

[0043] Als in steel can fix the free [N] in the steel. In addition to reducing the free [N] in the welding heat affected zone (HAZ) and improving the low-temperature toughness of the welding HAZ, it is more important to ensure that the steel has a certain solid solution [B] and improve the hardenability of the steel plate; therefore, the lower limit of Als is controlled at 0.030%; however, adding excessive Als to the steel will not only cause casting difficulties, but also form a large number of dispersed needle-shaped Al2O3 inclusions in the steel, which will damage the internal soundness, low-temperature toughness and weldability of the steel plate, so the upper limit of Als is controlled at 0.070%.

[0044] In order to ensure the existence of solid solution [B] in the steel plate and prevent a large amount of coarse AlN from precipitating in a necklace-like manner along the original austenite grain boundaries, thereby damaging the toughness and plasticity of the steel plate, the N content in the steel shall not exceed 0.0055%.

[0045] Ca treatment of steel can further purify the molten steel on the one hand, and modify the sulfides in the steel to make them non-deformable, stable and fine spherical sulfides, inhibit the hot brittleness of S, improve the low-temperature toughness, elongation and Z-direction properties of the steel plate, improve the anisotropy and weldability of the steel plate toughness, and improve the pouring of high acid-soluble aluminum molten steel by Ca treatment. The amount of Ca added depends on the S content in the steel. If the Ca addition is too low, the treatment effect is not great. If the Ca addition is too high, the size of Ca (O, S) is too large, the brittleness is also increased, and it can become the starting point of the fracture crack, reducing the low-temperature toughness, elongation and weldability of the steel plate, and at the same time reducing the purity of the steel and polluting 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, so the appropriate range of Ca content is 0.0010 ~ 0.0040%.

[0046] In particular, the contents of the above elements in 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 AlN is precipitated in a fine dispersed state, to avoid the coarse AlN from precipitating in a necklace shape on the original austenite grain boundary, to improve the hardenability of the steel plate, to refine the grain size of the steel plate, and to improve 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] completely forms fine dispersed AlN, eliminating the strong embrittlement effect of the solid solution [N] on the welding heat affected zone, and improving the toughness and crack resistance and crack arrest properties of the high-strength steel plate welding heat affected zone.

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

[0051] ① Inhibit the conjugate segregation of C, Mn, P and S elements during the solidification of molten steel, improve the three properties of the slab (soundness, homogeneity and purity), and improve the uniformity, toughness and weldability of the steel plate;

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

[0053] ③ Promote the precipitation of carbides, inhibit the precipitation of M / A islands in HAZ, reduce the number and size of M / A islands, improve the morphology of M / A islands, and improve the toughness and crack resistance and crack arrest properties of steel plate welding HAZ; this is one of the key core technologies of the present invention.

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

[0055] Ensure that the amount of sulfides in the steel is appropriate and completely spherical Ca(O,S), and is distributed in the steel in a fine and dispersed state; to improve the toughness, toughness and plasticity matching, weldability and resistance to lamellar tearing of the steel plate.

[0056] The composition data in the above relationship is calculated as a percentage. For example, if the carbon content is 0.10%, just substitute 0.10 when calculating the relationship.

[0057] The method for manufacturing the ultra-low-cost, excellent weldability 800MPa grade steel plate of the present invention comprises the following steps:

[0058] 1) Smelting and casting

[0059] Smelting and casting into slabs according to the above ingredients;

[0060] 2) Controlled rolling, the total compression ratio of the steel plate, i.e. the thickness of the slab / the thickness of the finished steel plate ≥ 3.5

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

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

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

[0064] Among them, T 终轧 is the final rolling temperature without recrystallization, the temperature is ℃;

[0065] ξ is the cumulative reduction rate without recrystallization, in %;

[0066] χ is the high-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) Controlled 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 steel plate cooling temperature is 730-780℃, the cooling rate is ≥5℃ / s, and the cooling stop temperature is controlled at 320-470℃; and the following requirements need to be met:

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

[0070] Among them, DI OL is the 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 is the cumulative reduction rate without recrystallization, in %;

[0075] V c is the accelerated cooling rate of the steel plate, in °C / s;

[0076] T 开冷 is the starting temperature of accelerated cooling, in °C;

[0077] T 停冷 is the stop temperature of accelerated cooling, in °C;

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

[0079] Preferably, step 1) is performed by continuous casting, the tundish pouring superheat is controlled at 8-0°C, the pulling speed is controlled at 0.6-1.0 m / min, the crystallizer liquid level fluctuation is controlled at ≤5 mm, and the soft pressure reduction process at the end of solidification is 2-5%.

[0080] Preferably, after the accelerated cooling in step 3) is completed, when the thickness of the steel plate is ≥40 mm, the steel plate is slowly cooled, and the slow cooling process is to keep the temperature at not less than 300° C. for more than 24 hours, and then the steel plate is naturally air-cooled to room temperature.

[0081] In the manufacturing method of the present invention:

[0082] According to the requirements of the composition system, mechanical properties, weldability, internal soundness (i.e. UT flaw detection) and the like of the steel plate of the present invention, the casting of the present invention preferably adopts continuous casting, the tundish pouring superheat is controlled at 8°C to 30°C, the pulling speed is controlled at 0.6m / min to 1.0m / min, the crystallizer liquid level fluctuation is controlled at ≤5mm, and the light pressure reduction process at the end of solidification is 2% to 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. In order to ensure that [Al]+BN→AlN+[B] occurs during heating and rolling, and to ensure that there is sufficient solid solution B in the steel, the slab heating temperature is controlled between 1080℃ and 1180℃. Large reduction rolling is adopted, and the reduction rate of each rolling pass is ≥8% to ensure that the microstructure of the intermediate billet is uniform and fine and has sufficient solid solution B, laying the foundation for the subsequent TMCP process.

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

[0087] After the non-recrystallization controlled rolling is completed, the steel plate is immediately transported to the DQ+ACC equipment, and then the steel plate is accelerated to cool; the steel plate start cooling temperature and the final rolling temperature are 730℃~780℃, the cooling rate is ≥5℃ / s, and the stop cooling temperature is controlled at 320℃~470℃. When the thickness of the steel plate is ≥40mm, the steel plate is slowly cooled, and the slow cooling process is to keep the temperature at not less than 300℃ for more than 24 hours, and then the steel plate is naturally air-cooled to room temperature to ensure that the steel plate is fully dehydrogenated and prevent hydrogen-induced cracks.

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

[0089] (T 终轧 ) / (ξ×χ)≤6.7;

[0090] Among them, T 终轧 is the final rolling temperature without recrystallization, the temperature is ℃; ξ is the cumulative reduction rate without recrystallization, the unit is %; χ is the high-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), the unit is %.

[0091] It can be seen from the formula that as the final rolling temperature T 终轧 The cumulative reduction rate ξ before recrystallization is reduced, the controlled rolling effect is enhanced, the degree of austenite recrystallization is suppressed to a large extent, the austenite flattening effect is enhanced, and the dislocation density and deformation band density inside the austenite grains are increased;

[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, resulting in an increase in the position of the ferrite phase deformation nucleus, an increase in the driving force for the austenite to ferrite phase transformation, a decrease in the size of the bainite / martensite crystal packet formed after the phase transformation, and an increase in the frequency of large-angle grain boundaries between variants in the block;

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

[0094] The above effects are superimposed on each other, resulting in the small and uniform size of bainite / martensite crystal clusters, a significant increase in the frequency of high-angle grain boundaries between variants in the block, resulting in higher stress and strain required for crack initiation (the length of the dislocation pileup group becomes smaller) and more energy consumed by repeated bending of the crack during the expansion process (the packet interface and the high-angle grain boundaries between variants are the blocking points for crack expansion), and the steel plate obtains higher strength, toughness and plasticity (HP relationship) and better strength-toughness / strength-plasticity matching. This is one of the key core technologies of the present invention.

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

[0096] Among them, DI OL is the 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(mm), unit: mm; ξ is the cumulative reduction rate before recrystallization, unit: %; V c T is the accelerated cooling rate of the steel plate, in °C / s; 开冷 T is the starting temperature of accelerated cooling, in °C; 停冷 is the stop temperature of accelerated cooling, in °C; H is the thickness of the finished steel plate, in mm.

[0097] With the online hardenability index DI OL Increase, the cumulative reduction rate without recrystallization increases, and the cooling rate V cThe temperature range from the start of cooling to the stop of cooling increases, the martensite content in the low-temperature phase transformation structure of the steel plate increases relatively, the dislocation density in the martensite increases (the genetic effect of the controlled rolling effect), the effective fracture element size of bainite / martensite decreases, and the strength of the steel plate increases; and as the stop cooling temperature increases and the thickness of the steel plate increases (the effect is a lower cooling rate, especially the cooling rate in the center of the steel plate is lower), the martensite content in the low-temperature phase transformation structure of the steel plate is relatively lower, and the strength of the steel plate is lower; in summary, when all indicators meet the above formula, it ensures that the steel plate has high strength, high toughness, excellent toughness / strong plasticity, and the steel plate has excellent weldability, and successfully eliminates the mutually opposing and difficultly compatible contradictions between low C, ultra-low cost and high toughness and excellent weldability of 800MPa steel plates. This is one of the key core technologies of the present invention.

[0098] Beneficial effects of the present invention:

[0099] The present invention adopts the composition system of low C-ultra-low Si-high Mn-Cr alloying-(Ti+high Nb+B) microalloying as the basis, appropriately increases the acid-soluble Als 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 1.00~3.00 and other metallurgical technology control means to optimize TMCP process, that is, (T 终轧 ) / (ξ×χ)≤6.7, {(DI OL )×ξ×(T 开轧 )×[(T 开冷 -T 停冷 )]} / [H×(T 停冷 )]≥405, the microstructure of the finished steel plate is low-carbon bainite + a small amount of low-carbon martensite, the average cluster size is below 20μm, and an 80 kg high-strength steel plate with excellent strong plasticity and toughness is obtained.

[0100] The present invention not only obtains excellent 800MPa grade high-strength quenched and tempered steel plates, but also has excellent strength / ductility and welding processability of the steel plates, and successfully eliminates the mutually contradictory and difficultly compatible contradictions between low C, ultra-low cost and high toughness and excellent weldability of 800MPa steel plates; not only the safety and reliability of large heavy steel structures are improved, but also the online TMCP process reduces the manufacturing process, shortens the manufacturing cycle, and reduces the manufacturing cost of the steel plates. More importantly, the steel plates of the present invention adopt a new generation of TMCP process to give full play to the potential of the hardenability of alloy elements, and the hardenability / hardenability of alloy elements is maximized, and high strength, excellent strength and plasticity and plastic toughness matching and excellent weldability can be obtained without the need for precious alloy content (such as Cu, Ni, Mo), low carbon and low carbon equivalent. This not only further reduces the manufacturing cost, but also improves the weldability of the steel plate. Especially for high-strength steel plates, the sensitivity to welding cold cracking is greatly reduced, the welding preheating temperature is greatly reduced, no heat treatment is required after welding (i.e. SR), and the range of suitable welding heat input is wider, which correspondingly reduces the user's processing and manufacturing costs and processes, shortens the user's steel structure manufacturing time, and creates huge value for the user. Therefore, this type of steel plate is not only a high-value-added, green and environmentally friendly product. BRIEF DESCRIPTION OF THE DRAWINGS

[0101] Figure 1 This is a photo of the microstructure (1 / 4 thickness) of the steel in Example 5 of the present invention. DETAILED DESCRIPTION

[0102] The present invention will be further described below in conjunction with the embodiments and drawings.

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

[0104] Depend on Figure 1 It can be seen 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%), and the average cluster size is less than 20 μm.

[0105] The present invention combines the combination design of steel plate alloy elements with the TMCP process. While obtaining an excellent 800MPa grade high-strength quenched and tempered steel plate, the steel plate also has excellent toughness / plastic toughness and welding processability, and successfully eliminates the mutually opposing and difficultly compatible contradictions between low C, ultra-low cost and high toughness and excellent weldability of 800MPa steel plates; not only does it improve the safety and reliability of large heavy steel structures, but the online TMCP process also reduces the manufacturing process, shortens the manufacturing cycle, and reduces the manufacturing cost of the steel plate.

[0106] More importantly, the steel plate of the present invention uses a new generation of TMCP technology to give full play to the potential of alloy element hardenability, and the hardenability / hardenability of alloy elements is maximized. High strength, excellent strength-plasticity and plastic-toughness matching and excellent weldability can be obtained without using precious alloy content (such as Cu, Ni, Mo), low carbon and low carbon equivalent and short process manufacturing process (omitting tempering heat treatment). This not only further reduces the manufacturing cost, but also improves the weldability of the steel plate. Especially for high-strength steel plates, the welding cold crack sensitivity is greatly reduced, the welding preheating temperature is greatly reduced, no heat treatment (ie SR) is required after welding, and the range of suitable welding heat input is wider, which correspondingly reduces the cost and process of user processing and manufacturing, shortens the time of user steel component manufacturing, and creates huge value for users. Therefore, this type of steel plate is not only a high value-added, green and environmentally friendly product.

[0107] The 800MPa grade high strength, high toughness and excellent weldability quenched and tempered steel plate of the present invention is mainly used to manufacture pressure water pipes, volutes and steel bifurcated pipes of hydropower projects, large 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 quenched and tempered steel plate has a broad market prospect 。

[0108]

[0109]

[0110]

Claims

1. An ultra-low-cost, excellent weldability 800MPa grade steel plate, the 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 includes Fe and unavoidable inclusions; and at the same time 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 is 1-3, and (%Ca)×(%S)≤1.5×10 -3 .

2. The ultra-low-cost, excellent weldability 800MPa grade steel plate according to claim 1, characterized in that: The balance is Fe and unavoidable inclusions.

3. The ultra-low-cost, excellent weldability 800MPa grade steel plate according to claim 1 or 2, characterized in that: The microstructure of the steel plate is low-carbon bainite + a small amount of low-carbon martensite, and the average crystal cluster size is less than 20 μm.

4. The ultra-low-cost, excellent weldability 800MPa grade steel plate according to claim 1, 2 or 3, characterized in that: The tensile strength of the steel plate is ≥770MPa, and the yield strength is ≥690MPa. -40℃ Charpy transverse impact energy (single value) ≥100J, elongation at break δ5 ≥15%; low preheating temperature, less than 50℃; no thermal effect heat treatment is required after welding, and the heat input range is wide, 15~35kJ / cm; The transverse impact energy of the heat affected zone at -40°C (single value) is ≥100J.

5. The method for manufacturing the ultra-low-cost, excellent weldability 800 MPa grade steel plate according to claim 1, 2, 3 or 4, characterized in that: The steps include: 1) Smelting and casting Smelting and casting into slabs according to the composition of claim 1 or 2; 2) Controlled rolling, the total compression ratio of the steel plate, i.e. the thickness of the slab / the thickness of the finished steel plate ≥ 3.5 The first stage is ordinary rolling, the slab heating temperature is controlled at 1080~1180℃; high reduction rolling is adopted, and the reduction rate of each rolling pass is ≥8%; The second stage adopts non-recrystallization controlled rolling, with the start rolling temperature of 780-860℃, the rolling pass reduction rate ≥7%, the cumulative reduction rate in the non-recrystallization zone ≥50%, and the final rolling temperature of 760-800℃; And, the following requirements need to be met: (T 终轧 ) / (ξ×χ)≤6.7, Among them, T 终轧 is the final rolling temperature without recrystallization, the temperature is ℃; ξ is the cumulative reduction rate without recrystallization, in %; χ is the high-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) Controlled cooling After the non-recrystallization controlled rolling is completed, the steel plate is immediately transported to the DQ+ACC equipment for accelerated cooling. The steel plate cooling temperature is 730-780℃, the cooling rate is ≥5℃ / s, and the cooling stop temperature is controlled at 320-470℃; and the following requirements need to be met: {(DI OL )×ξ×(V c )×[(T 开冷 -T 停冷 )]} / [H×(T 停冷 )]≥405; Among them, DI OL is the online hardenability index, OF 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 without recrystallization, in %; V c is the accelerated cooling rate of the steel plate, in °C / s; T 开冷 is the starting temperature of accelerated cooling, in °C; T 停冷 is the stop temperature of accelerated cooling, in °C; H is the thickness of the finished steel plate, in mm.

6. The method for manufacturing the ultra-low-cost, excellent weldability 800 MPa grade steel plate according to claim 5, characterized in that: Step 1) casting is performed by continuous casting, the tundish pouring superheat is controlled at 8-0°C, the pulling speed is controlled at 0.6-1.0 m / min, the crystallizer liquid level fluctuation is controlled at ≤5 mm, and the soft pressure reduction process at the end of solidification is 2-5%.

7. The method for manufacturing an ultra-low-cost, excellent weldability 800 MPa grade steel plate according to claim 5 or 6, characterized in that: Step 3) After the accelerated cooling is completed, when the thickness of the steel plate is ≥40 mm, the steel plate is slowly cooled. The slow cooling process is to keep the temperature at not less than 300° C. for more than 24 hours, and then the steel plate is naturally air-cooled to room temperature.

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