1000MPa grade steel plate with matched toughness / plasticity and manufacturing method thereof

Abstract

The invention belongs to the technical field of steel materials, and specifically discloses a 1000MPa grade steel plate with matched toughness / high plasticity and a manufacturing method thereof, wherein the steel plate comprises Fe, C, Si, Mn, P, S, Cu, Ni, Cr, Mo, V, Nb, Ti, N, O, Ca and inevitable inclusions, and through the combination design of alloy elements combined with the TMCP+high temperature T process, while obtaining an excellent 950MPa grade ultra-high strength TMCP+T steel plate, the steel plate also has excellent toughness / high plasticity, strain aging embrittlement resistance and weldability, and successfully solves the contradictions among the strength, plasticity, low temperature toughness, aging embrittlement resistance and weldability of the ultra-high strength steel plate, thereby improving the safety stability and fatigue resistance of large heavy steel structures.

Description

Technical Field

[0001] The invention belongs to the technical field of alloy steel materials and relates to a 1000MPa grade steel plate with matched toughness and plasticity and a manufacturing method thereof. Background Art

[0002] Existing high-strength steel plates with tensile strength ≥950MPa are mainly produced by offline quenching and tempering process (i.e. RQ+T); however, for steel plates with thickness ≤60mm, online quenching and tempering process (i.e. DQ+T) can also be used for production; in order to obtain ultra-high strength, the steel plate must have a sufficiently high hardenability, i.e. the steel plate hardenability index DI ≥3.50×the thickness of the finished steel plate〖DI=0.311(%C) 1 / 2 [(1+0.64(%Si)]×[(1+4.10(%Mn)]×[(1+0.27(%Cu)]×[(1+0.52(%Ni)]×[(1+2.33(%Cr)]×[(1+3.14(%Mo)]×25.4(mm)〗, in order to ensure that the steel plate has sufficiently high strength, excellent low-temperature toughness and uniform microstructure and performance along the plate thickness direction, it is inevitable to add a large amount of alloying elements such as Cr, Mo, Ni, Cu, V, etc. to the steel, especially when the Ni content is added to more than 2.00%, resulting in a high carbon equivalent and cold crack sensitivity index of the steel plate, which seriously affects the weldability of the steel plate. In addition, the surface (sub) layer of the steel plate with a high alloy content is prone to over-quenching, forming a coarse martensitic structure, which seriously deteriorates the low-temperature toughness and elongation of the surface (near) layer of the steel plate; low elongation and low-temperature toughness are not only not conducive to the hot and cold workability of the steel plate It can not only have a great influence on the fatigue resistance, stress concentration sensitivity, crack resistance and structural stability of the steel plate. When used on fatigue heavy-load structures such as pressure water pipes and volutes in hydropower projects, thermal power steam turbine generators and offshore oil production platform structures, there are great safety hazards. Therefore, when large fatigue heavy-load steel structures use ultra-high-strength steel, it is generally hoped that 100 kg high-strength steel not only has excellent strength and toughness, strong plasticity matching and weldability, but also ensures that the elongation is above 14% to ensure the processing performance and fatigue resistance of the steel plate. A large number of existing patents and technical documents only explain how to achieve the strength and low-temperature toughness of the parent steel plate, but there are few explanations on improving the welding performance of the steel plate and obtaining excellent low-temperature toughness of the HAZ of the welding heat affected zone. There is no mention of how to improve the tensile strength of the steel plate while improving the tensile elongation of the steel plate and the uniformity of the mechanical properties in the thickness direction, and there is no indication of how to prevent the surface (sub) layer of the steel plate from being overquenched.

[0003] CN 102337478 A discloses a 100 kg-class quenched and tempered steel plate with excellent toughness and plasticity and a manufacturing method thereof. Although the comprehensive mechanical properties of the steel plate also reach a relatively high level: tensile strength ≥950MPa, yield strength ≥890MPa, -60°C Charpy transverse impact energy (single value) ≥47J, the steel plate manufacturing technology of the application adopts controlled rolling + offline quenching and tempering process (offline quenching + tempering); this not only has many manufacturing steps, a long manufacturing cycle, and high manufacturing costs, but also the energy consumption of the manufacturing process is relatively high (the steel plate is rolled and naturally air-cooled to room temperature, then shot blasted, and then heated to the quenching temperature again), which is not conducive to energy saving and environmental protection. Although CN116516243 A discloses the production of 1000MPa grade steel plate and its manufacturing method by online quenching and tempering + high temperature tempering process, the steel plate does not involve the requirements for the strain aging embrittlement resistance, and more importantly, the actual performance of the steel plate's strain aging resistance is also poor, which cannot meet the requirements of large hydropower stations and high-head pumped storage power stations. In addition, the offline quenching + tempering process cannot fully exert 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 Cu, 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 welding cold crack sensitivity is greatly increased, and welding preheating and post-heating (i.e. PWHT or SR) at higher temperatures are required, and the range of suitable welding heat input is narrower, and the processing and manufacturing costs are correspondingly greatly increased. Summary of the invention

[0004] The present invention provides a 1000MPa grade steel plate with matched toughness / high plasticity and a method for manufacturing the same. On the basis of less alloy addition, ultra-high strength is obtained while the steel plate has excellent toughness / high plasticity matching, excellent strain aging embrittlement resistance and weldability. The invention successfully solves the contradictions among the strength, plasticity, low temperature toughness, strain aging embrittlement resistance and weldability of the ultra-high strength steel plate and the problem of overquenching of the surface (sub) layer of the steel plate.

[0005] In order to achieve the above-mentioned object, the present invention adopts the following technical solution: a 1000MPa grade steel plate with matched toughness / plasticity, the steel plate contains the following components by mass percentage:

[0006] C: 0.08% to 0.13%,

[0007] Si: ≤0.20%,

[0008] Mn: 1.00% to 1.50%,

[0009] P: ≤0.013%,

[0010] S: ≤0.0030%,

[0011] Cu: 0.15% to 0.45%,

[0012] Ni: 1.30% to 2.00%,

[0013] Cr: 0.40% to 0.70%,

[0014] Mo: 0.30% to 0.60%,

[0015] V: 0.030%~0.060%,

[0016] Nb: 0.010% to 0.030%,

[0017] Ti: 0.008% to 0.016%,

[0018] B: ≤0.00050%,

[0019] N: ≤0.0040%,

[0020] O≤0.0030%、

[0021] and Ca: 0.0010% to 0.0035%, the rest is Fe and unavoidable impurities;

[0022] And the contents of the above ingredients satisfy the following relationship at the same time:

[0023] (Mn / C)≥10;

[0024] 2.15(%C)×[(%Si)+10(%S)+30(%P)+15(%O)]≤0.122;

[0025] Mo equivalent ≥ 0.66,

[0026] Wherein Mo equivalent = (%Mo) + 0.67 (%Cr) + 0.83 (%Si) + 2.67 (%Nb) + 1.62 (%V) - 0.11 [(%Cu) + (%Ni)], in %;

[0027] Ca / S is between 1.00 and 3.00 and (%Ca)×(%S) 0.18 ≤1.5×10 -3 .

[0028] Furthermore, the microstructure of the steel plate is mainly low-carbon tempered martensite + a small amount of low-carbon tempered lower bainite, the martensite and bainite laths are uniform and small in size, the average crystal cluster packet size is controlled below 20 μm, and lath clusters with different orientations appear in the packet structure.

[0029] Furthermore, the steel plate has a tensile strength of ≥950MPa, a yield strength of ≥890MPa, a -60°C transverse impact energy (single value) of ≥70J, a -40°C aging transverse impact energy (single value) of ≥70J, a fracture elongation δ5 of ≥15%, and excellent weldability.

[0030] The present invention also relates to a method for manufacturing the 1000MPa grade steel plate with matched toughness / plasticity, comprising the following steps:

[0031] S1. According to the composition of the steel plate, the steel plate is smelted and cast into billets by continuous casting. The tundish pouring superheat ΔT is controlled at 15℃~30℃, the casting speed is controlled at 0.5m / min~1.0m / min, and the crystallizer liquid level fluctuation is controlled at ≤3mm;

[0032] S2. The blank obtained in S1 is subjected to TMCP process, and the total compression ratio of the steel plate, i.e., the blank thickness / the finished steel plate thickness is ≥ 4.0;

[0033] S3. Temper the steel plate. The tempering temperature is controlled at 575-635°C. The tempering holding time is ≥ (0.60-1.0) × the thickness of the finished steel plate, in min. After the tempering, the steel plate is naturally cooled to room temperature.

[0034] Furthermore, S2 specifically includes the following processing:

[0035] The first stage is ordinary rolling. The slab heating temperature is controlled between 1100℃ and 1180℃, and the slab rolling speed is controlled at ≤1.0m / sec. The rolling shape factor (ΔH / R) 1 / 2 ≥0.18, where ΔH is the pass reduction and R is the working roll diameter;

[0036] The second stage is controlled rolling without recrystallization, the start rolling temperature of controlled rolling is ≤830℃, the rolling pass reduction rate is ≥7%, the cumulative reduction rate of the non-crystallized zone (≤830℃) is ≥50%, and the final rolling temperature is 750℃~800℃;

[0037] After the non-recrystallization controlled rolling is completed, the steel plate is accelerated cooled, the start cooling temperature and the final rolling temperature are 730℃~780℃, the cooling rate is ≥5℃ / s, the stop cooling temperature is controlled to be ≤400℃, and after the stop cooling is completed, it enters the heating slow cooling pit for insulation. During the insulation, the surface temperature of the steel plate is ≥250℃, and the insulation time is more than 24 hours.

[0038] Furthermore, S2 needs to satisfy the following relationship:

[0039] [(Ni equivalent) × (% Nb) × ξ] / [(T 终轧 )×H]≥7.0×10 -5 ;

[0040] Wherein Ni equivalent = (%Ni) + 0.33 (%Mn) + 0.65 (%Cu) - 0.47 (%Cr) - 0.63 (%Mo) - 0.81 (Mo + V), unit is %; ξ is the cumulative reduction rate in the non-crystallized zone (≤830℃), unit is %; T 终轧 is the final rolling temperature of controlled rolling without recrystallization, in °C; H is the thickness of the finished steel plate, in mm.

[0041] Furthermore, the interval time from the end of cooling of the steel plate to the entry into the heating slow cooling pit for insulation is ≤60min.

[0042] Furthermore, the following relationship needs to be satisfied in S2 and S3:

[0043] 8.9≤{DI OL ×Mo equivalent×[(T 开冷 )-(T 停冷 )]×(V c )} / [H×(T 回火 )×lgt]≤102;

[0044] Where 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 is mm; T 开冷 T is the starting temperature of accelerated cooling, in °C; 停冷 is the accelerated cooling stop temperature, in °C; V c is the cooling rate for accelerated cooling, in °C / s; H is the thickness of the finished steel plate, in mm; T 回火 is the tempering temperature, in °C; t is the tempering holding time, in min;

[0045] Mo equivalent = (%Mo) + 0.67 (%Cr) + 0.83 (%Si) + 2.67 (%Nb) + 1.62 (%V) - 0.11 [(%Cu) + (%Ni)], in %.

[0046] The invention also relates to the application of the steel plate in pressure water pipes, steel bifurcated pipes, volutes, marine structures and large-scale engineering mechanical structures or equipment in hydropower projects.

[0047] The present invention has the following beneficial effects:

[0048] The present invention adopts a component system of low C-ultra-low Si-medium-high Mn-(Ti+Nb+V) microalloyed steel as a basis, controls (Mn / C)≥10, (Cu+high Ni+Mo+Cr) alloying, 2.15(%C)×[(%Si)+10(%S)+30(%P)+15(%O)]≤0.122, Mo equivalent≥0.66, Ca treatment and Ca / S ratio between 1.00 and 3.00 and (%Ca)×(%S) 0.18 ≤1.5×10 －3 and other metallurgical technology control measures; optimize the TMCP+offline high-temperature tempering process (T), use the aus-forming process, that is, quickly cool the deformed austenite, inherit the deformation strengthening effect to the final microstructure, and promote the nucleation of martensite / bainite laths and growth along different directions, so that the newly formed martensite / bainite laths are small, effectively dividing the original austenite grains, resulting in small and uniform packets and a high probability of large-angle grain boundaries between blocks (that is, the frequency of large-angle grain boundaries between phase transformation variants V1 / V2 is much higher than the proportion of small-angle grain boundaries between phase transformation variants V1 / V4), so that the microstructure of the finished steel plate is fine low-carbon tempered martensite + a small amount of tempered bainite, the average cluster packet size is below 20μm and the block structure is extremely fine, so as to obtain a 1000MPa grade steel plate with strong toughness / strong plasticity matching, resistance to strain aging embrittlement and excellent weldability, and solve the problem of excessive quenching of the surface (sub) layer of ultra-high strength steel plate.

[0049] The present invention not only obtains excellent 950MPa grade ultra-high strength TMCP+T steel plates, but also has excellent toughness / plasticity, strain aging embrittlement resistance and weldability. It successfully solves the contradictions among the strength, plasticity, low temperature toughness, aging embrittlement resistance and weldability of ultra-high strength steel plates, and improves the safety, stability and fatigue resistance of large heavy steel structures. Good weldability saves the cost of steel component manufacturing for users, shortens the time of steel component manufacturing for users, and creates huge value for users. Therefore, such steel plates are not only high value-added and green environmentally friendly products. In addition, the invention adopts online TMCP+ The high temperature tempering process gives full play to the hardenability and hardenability potential of alloy elements, and the hardenability and hardenability of elements are maximized. With relatively less alloy content, especially lower content of Ni, Mo, Cr, etc. (compared with offline quenching and tempering), steel plates with ultra-high strength, high toughness / high plasticity, resistance to strain aging embrittlement, and excellent weldability can be obtained; this not only further reduces the manufacturing cost, but also improves the weldability of the steel plate, especially for ultra-high strength steel plates, the sensitivity of welding cold cracking is greatly reduced, the welding preheating and post-heating temperatures are reduced, and the range of suitable welding heat input is wider, which correspondingly reduces the processing and manufacturing costs of users. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Figure 1 1 and 2 are microstructures of the newly invented steel 4 of Example, wherein (a) is the surface, (b) is 1 / 4 thickness, and (c) is 1 / 2 thickness. DETAILED DESCRIPTION

[0051] The present invention provides a 1000MPa grade steel plate with matched toughness / plasticity and a manufacturing method thereof, specifically an ultra-high strength steel plate with tensile strength ≥950MPa, yield strength ≥890MPa, -60°C transverse impact energy (single value) ≥70J, -40°C aging transverse impact energy (single value) ≥70J, fracture elongation δ5 ≥15% and excellent weldability.

[0052] In order to meet the above indicators, the microstructure design of the steel plate is very important, and the microstructure design includes the organization type, size, microscopic substructure and second phase precipitates. As we all know, for high-strength steel, the strength, toughness and plasticity of the mixed organization of martensite and lower bainite are the best match. In order to meet all the above performance requirements at the same time, the microstructure of the 950MPa grade steel plate should be mainly low-carbon tempered martensite + a small amount of low-carbon tempered lower bainite to improve the intrinsic plasticity and toughness of martensite and bainite; the size of martensite and bainite laths is uniform and small to improve the deformation coordination ability of the lath itself and improve the plasticity and toughness of the lath itself; the crystal cluster size (i.e., packet structure) is controlled below 20μm and the lath clusters of different orientations appear in the packet structure (i.e., the V1 / V2 grain boundary ratio of the block structure is high) to further refine the block structure and improve the energy required for crack propagation and the deformation coordination ability of packet grain boundaries and blocks. Through the control of unrecrystallized controlled rolling and phase transformation process, A) the deformation dislocations and phase transformation dislocations generated during the unrecrystallized controlled rolling process are inherited to the formed martensite and bainite, forming huge dislocation strengthening; B) the entanglement nodes of the deformation dislocations are used as the nucleation points of martensite and bainite, further refining the packet and block structures, improving the steel plate's strength-toughness / strength-plasticity matching, low-temperature toughness and resistance to strain aging embrittlement; C) high-density dislocation cellular structures and dislocation subgrain boundaries are evenly distributed in the martensite and bainite laths, and nano-scale carbonitride particles are evenly dispersed and precipitated on the dislocation line network, which greatly improves the strength of the steel plate while minimizing the damage to plasticity and toughness. In addition, the use of aus-forming technology can not only greatly reduce the amount of alloying elements, especially the amount of precious alloying elements such as Ni, Mo, and Cr, but also obtain ultra-high strength while also having excellent plasticity, toughness, and weldability, reaching the level of 800MPa high-strength steel.The so-called AUS-forming process is that the deformed austenite undergoes a martensite / bainite phase transformation during the accelerated cooling process, and the martensite / bainite laths of different orientations nucleate and grow on the high-density deformation band and the dislocation steps of the original austenite grain boundary. When the lath clusters of different orientations (i.e., packet structures) meet during the growth process, the growth of the packet structure stops. Since the high-density packet structures of different orientations have a high probability of meeting during the growth process and the growth space is limited, a large number of packet structures of different orientations are formed, which divide the original austenite grains and form a packet structure of smaller size. Secondly, when the martensite / bainite laths in the same packet structure grow, they are blocked by the high-density dislocation network, and the martensite / bainite laths are not only small; but also And for the martensite / bainite of dislocation phase transformation, when growing along the direction of the screw dislocation line and the direction of the Burgers vector of the edge dislocation, the energy of lath growth is minimum, and a high-density dislocation network is generated during the deformation process, causing the martensite / bainite laths to grow in different variant forms, forming a block structure with a high proportion of V1 / V2 interface, further refining the substructure of the microstructure; again, the deformation dislocation structure is inherited through the martensite / bainite phase transformation, greatly improving the dislocation strengthening effect; finally, the high-density dislocation network provides a large number of places for carbonitride precipitation, resulting in a large number of nano-scale carbonitrides dispersedly precipitated on dislocations in a non-brittle form during the tempering of the steel plate; thereby, ultra-high strength can be obtained under low alloy content conditions, while plasticity, toughness and weldability are also excellent. The specific design principles of the present invention are as follows:.

[0053] C has a great influence on the strength, low temperature toughness, elongation, strain aging embrittlement resistance and weldability of ultra-high strength steel plates. From the perspective of improving the intrinsic plastic toughness, strain aging embrittlement resistance and weldability of ultra-high strength steel plates, it is hoped that the C content in the steel can be controlled at a lower level; however, from the perspective of hardenability, strength assurance, strength-toughness matching, microstructure control during TMCP process and manufacturing cost of ultra-high strength steel plates, the C content should not be controlled too low, especially for ultra-high strength 1000MPa grade steel plates; therefore, the reasonable range of C content is 0.08% to 0.13%.

[0054] Si promotes the deoxidation of molten steel and can improve the strength of steel plates. However, for molten steel deoxidized by Al, the deoxidation effect of Si is not significant. Although Si can improve the strength of steel plates, Si promotes the coarsening of packet size, which seriously damages the low-temperature toughness, strain aging embrittlement resistance, elongation and weldability of ultra-high strength steel plates. Especially under large heat input welding conditions, Si not only promotes the formation of MA islands, but also the size of the formed MA islands is relatively large 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 steel should be controlled as low as possible. Considering the economy and operability of the steelmaking process, the Si content is controlled below 0.20%.

[0055] As the most important alloying element, Mn not only improves the strength of steel plates, but also has the functions of expanding the austenite phase region, reducing the Ar3 point temperature, refining the packet structure scale of the microstructure, increasing the V1 / V2 grain boundary ratio of the block structure, improving the plasticity and toughness of the steel plate, and promoting the formation of low-temperature phase transformation structures such as martensite and bainite to improve the strength of the steel plate. However, Mn is prone to segregation during the solidification of molten steel, especially when the Mn content is high, which not only makes the casting operation difficult, but also easily causes conjugate segregation with elements such as C, P, S, Mo, Cr, etc., especially when the C content in the steel is high. When the steel is quenched and tempered, the segregation and looseness in the center of the ingot are aggravated. Severe segregation in the center of the ingot is prone to form abnormal tissue in the subsequent TMCP and welding process, resulting in low low-temperature toughness of the ultra-high strength steel plate and cracks in the welded joints. Therefore, it is extremely necessary to select an appropriate Mn content range for the ultra-high strength quenched and tempered steel plate according to the C content range. According to the steel component system of the present invention, the C content is 0.08% to 0.13% and the manufacturing process characteristics, the Mn content is suitable for 1.00% to 1.50%, and when the C content is high, the Mn content is appropriately reduced, and vice versa; and when the C content is low, the Mn content is appropriately increased.

[0056] As a harmful inclusion in steel, P has a great detrimental effect on the mechanical properties of steel plates, especially low-temperature impact toughness, elongation, resistance to strain aging embrittlement, weldability and SR performance of welded joints. In theory, the lower the better; but considering the operability and cost of steelmaking, for ultra-high strength TMCP steel plates that require excellent weldability, -60℃ low-temperature toughness, resistance to strain aging embrittlement and excellent strength-toughness / strength-ductility matching, the P content needs to be controlled at ≤0.013%.

[0057] As a harmful inclusion in steel, S has a great damaging effect on the low-temperature toughness of the steel plate. More importantly, S combines with Mn in the steel to form MnS inclusions. During the hot rolling process, the plasticity of MnS causes MnS to extend along the rolling direction, forming a MnS inclusion belt along the rolling direction, which seriously damages the low-temperature impact toughness, strain aging embrittlement resistance, 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 ultra-high strength TMCP steel plates requiring excellent weldability, -60℃ low-temperature toughness, strain aging embrittlement resistance and excellent strength-toughness / strength-plasticity matching, the S content needs to be controlled at ≤0.0030%.

[0058] Cu is also an austenite stabilizing element. Adding Cu can also reduce the Ar3 point temperature, improve the hardenability of the steel plate and the atmospheric corrosion resistance of the steel plate; however, if the Cu addition is too much, higher than 0.45%, it is easy to cause copper embrittlement, surface cracking of the ingot, internal cracking problems and especially deterioration of the SR performance of the welded joints of the ultra-high strength steel plate; for 1000MPa grade ultra-high strength TMCP steel plates, if the Cu addition is too little, lower than 0.15%, any effect is very small; therefore, the Cu content is controlled between 0.15% and 0.45%. The composite addition of Cu and Ni can not only reduce the copper embrittlement of copper-containing steel and alleviate the intergranular cracking during the hot rolling process, but more importantly, Cu and Ni are both austenite stabilizing elements. The composite addition of Cu and Ni can significantly reduce Ar3 and increase the driving force for the phase transformation from austenite to ferrite, resulting in the growth of martensite / bainite laths in all directions, leading to a larger orientation difference between martensite / bainite blocks (i.e., increasing the V1 / V2 grain boundary ratio), and increasing the resistance of cracks passing through martensite / bainite laths.

[0059] Adding Ni can not only improve the dislocation mobility in the ferrite phase and promote dislocation cross-slip, but also increase the orientation difference between the martensite / bainite block structures (i.e., increase the V1 / V2 grain boundary ratio); Ni, as an austenite stabilizing element, reduces the Ar3 point temperature and refines the martensite / bainite packet structure size. Therefore, Ni has the function of simultaneously improving the strength, elongation, low-temperature toughness and strain aging embrittlement resistance of TMCP steel plates; adding Ni to steel can also reduce the copper embrittlement phenomenon of copper-containing steel, alleviate intergranular cracking during hot rolling, and improve the atmospheric corrosion resistance of steel plates. Therefore, theoretically, the higher the Ni content in steel is within a certain range, the better. However, too high Ni content will harden the welding heat affected zone, which is not good for the weldability of the steel plate and the SR performance of the welded joint. However, for ultra-high strength 1000MPa grade TMCP steel plates, there must be enough Ni content to ensure that the steel plate has sufficient hardenability and uniform performance in the thickness direction, while ensuring the plastic toughness and strain aging embrittlement resistance of the steel plate. Therefore, the Ni content is controlled between 1.30% and 2.00% to ensure the hardenability and toughness of the steel plate, and the level of strain aging embrittlement resistance of the steel plate without damaging the weldability of the steel plate.

[0060] Cr is a weak carbide-forming element. Although the addition of Cr improves the hardenability of the steel plate and promotes the formation of martensite / bainite, it reduces the orientation difference between martensite / bainite laths (i.e., increases the V1 / V4 grain boundary ratio) and reduces the resistance of cracks passing through the martensite / bainite packet structure. While improving the strength of the steel plate, it has a certain effect of deteriorating the toughness of the steel plate and the resistance to strain aging embrittlement. In addition, when the amount of Cr added is too much, the weldability of the steel plate, especially the SR performance of the welded joint, is seriously damaged. However, for ultra-high strength 1000MPa grade TMCP steel plates, a certain Cr content must be present to ensure that the steel plate has sufficient hardenability and matches the strength and toughness of the steel plate. Therefore, the Cr content is controlled between 0.35% and 0.65%.

[0061] Adding Mo improves the hardenability and strength of the steel plate and promotes the formation of martensite / bainite. However, as a strong carbide-forming element, Mo increases the size of the martensite / bainite packet while promoting the formation of martensite / bainite, and the orientation difference between the formed martensite / bainite blocks is very small (i.e., increasing the V1 / V4 grain boundary ratio), reducing the resistance of cracks passing through the martensite / bainite packet structure. In addition, Mo promotes overquenching of the surface (sub) layer of ultra-high strength steel; therefore, while Mo greatly improves the strength of the steel plate, it reduces the low-temperature toughness, strain aging embrittlement resistance, and elongation of the ultra-high strength steel plate, and induces overquenching of the surface (sub) layer of the steel plate; and when Mo is added too much, it not only seriously damages the elongation, weldability and SR performance of the steel plate, but also increases the SR brittleness and production cost of the steel plate; however, for ultra-high strength 1000MPa grade ultra-high strength TMCP steel plates, a certain Mo content must be present to ensure that the steel plate has sufficient hardenability and resistance to temper softening. Therefore, considering the phase transformation strengthening effect of Mo and its influence on the low temperature toughness, elongation and weldability of the base steel plate, the Mo content is controlled between 0.30% and 0.60%.

[0062] The V content is between 0.030% and 0.060%, and as the thickness of the steel plate increases, the V content can be appropriately taken to the upper limit. The purpose of adding V is to disperse and precipitate V (C, N) in the bainite / martensite laths to improve the strength of the ultra-high strength TMCP steel plate. If V is added too little, less than 0.030%, too little V (C, N) is precipitated, and the strength of the ultra-high strength TMCP steel plate cannot be effectively improved; if V is added too much, higher than 0.060%, the steel plate's low-temperature toughness, strain aging embrittlement resistance, elongation, weldability and welding SR performance will be damaged.

[0063] The purpose of adding trace amounts of Nb elements to steel is to carry out non-recrystallization controlled rolling, refine the microstructure of the steel plate and increase dislocation strengthening, improve the quenching structure of the surface (sub) layer of the steel plate, prevent excessive quenching of the surface (sub) layer of the steel plate, improve the hardenability of the TMCP steel plate, and improve the strength, low-temperature toughness and strain aging embrittlement resistance of the ultra-high strength TMCP steel plate. When the Nb addition is less than 0.010%, the controlled rolling effect cannot be effectively exerted; when the Nb addition exceeds 0.030%, the high heat input welding Under the welding conditions, the formation of upper bainite (Bu) and the secondary precipitation embrittlement of Nb (C, N) are induced, which seriously damages the low-temperature toughness of the heat affected zone (HAZ) of high heat input welding. Therefore, the Nb content is controlled between 0.010% and 0.030% to obtain the best controlled rolling effect, realize the strength-toughness / strength-plasticity matching of ultra-high strength TMCP steel plates, resist strain aging embrittlement characteristics, and prevent excessive quenching of the surface (sub) layer, without damaging the toughness of the HAZ of high heat input welding and multi-pass welding.

[0064] 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 low-temperature toughness of the steel plate, and more importantly, inhibits the growth of HAZ grains during welding and improves HAZ toughness; however, when the Ti content exceeds 0.016%, when the Al content in the steel is high, excess Ti precipitates in the form of TiC on the martensite / bainite laths and on the grain boundaries, seriously deteriorating the low-temperature toughness, strain aging embrittlement resistance and weldability of the steel plate.

[0065] In order to ensure that the orientation difference between the martensite / bainite blocks becomes larger (i.e., increase the V1 / V2 grain boundary ratio), the B content shall not be higher than 0.00050%.

[0066] In order to ensure the surface quality of the invention steel plate slab and inhibit the occurrence of star cracks and edge cracks on the steel plate surface, the nitrogen content in the steel is ≤0.0040%.

[0067] In order to ensure the plastic toughness and resistance to strain aging embrittlement of ultra-high strength TMCP steel plates, the inclusions in the steel must be reduced, among which alumina inclusions are the most harmful, so the O content in the steel is ≤0.0030%.

[0068] Ca treatment of steel can, on the one hand, further purify the molten steel, and on the other hand, 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 of the toughness and weldability of the steel plate. In addition, Ca treatment is used to improve the pouring of high-acid-soluble aluminum molten steel; 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 the Ca(O,S) formed is too large, the brittleness is also increased, and it can become the starting point of the fracture crack, reducing the low-temperature toughness, strain aging embrittlement resistance, elongation and weldability of the steel plate, while also 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), wherein ESSP is the sulfide inclusion shape control index, and the value range is preferably between 0.80 and 4.00. Therefore, the appropriate range of Ca content is 0.0010% to 0.0035%.

[0069] Therefore, in the end, the composition design scheme of the present invention is to provide a 100 kg-class ultra-high strength TMCP steel plate with fine and uniform structure and excellent performance and a manufacturing method thereof, wherein the constituent elements include: Fe, C, Si, Mn, P, S, Cu, Ni, Cr, Mo, V, Nb, Ti, N, O, Ca and inevitable inclusions, and the weight percentages of the constituent elements are:

[0070] C: 0.08%~0.13%

[0071] Si: ≤0.20%

[0072] Mn: 1.00%~1.50%

[0073] P: ≤0.013%

[0074] S: ≤0.0030%

[0075] Cu: 0.15%~0.45%

[0076] Ni: 1.30%~2.00%

[0077] Cr: 0.40%~0.70%

[0078] Mo: 0.30%~0.60%

[0079] V: 0.030%~0.060%

[0080] Nb: 0.010%~0.030%

[0081] Ti: 0.008%~0.016%

[0082] B: ≤0.00050%

[0083] N: ≤0.0040%

[0084] O≤0.0030%

[0085] Ca: 0.0010%~0.0035%

[0086] And the contents of the above elements must satisfy the following relationship at the same time:

[0087] Controlling (Mn / C)≥10, refining the fine structure of the microstructure and substructure of the ultra-high strength steel plate (i.e., refining the packet crystal size and increasing the V1 / V2 ratio on the block boundary), improving the proportion, distribution and state of the second phase organization, improving the low-temperature plasticity and toughness, strong toughness / strong plasticity matching and resistance to strain aging embrittlement characteristics, while at the same time, the plasticity and toughness in the thickness direction of the steel plate (especially the center of the plate thickness) are uniform and excellent, and the ductile-brittle transition temperature of the fracture is lower than -60°C, ensuring that the steel plate has excellent crack resistance and crack arrest characteristics; this is one of the key technologies of the present invention.

[0088] 2.15(%C)×[(%Si)+10(%S)+30(%P)+15(%O)]≤0.122, A) greatly reduces the damage of P, S, O inclusion elements to the plastic toughness and strain aging embrittlement resistance of the steel plate; B) reduces the degree of co-segregation of C, Mn, P, S and O during the solidification of molten steel, and improves the low-temperature toughness, strain aging embrittlement resistance and weldability of the base steel plate; C) inhibits the precipitation of MA islands in the HAZ during welding, reduces the size of MA islands in the HAZ, reduces the number of MA islands in the HAZ and improves their morphology and distribution, greatly improving the low-temperature toughness and crack resistance and crack arrest properties of the welded HAZ; this is one of the key technologies of the present invention.

[0089] Mo equivalent ≥ 0.66, ensuring the high hardenability and tempering softening resistance of 1000MPa grade quenched and tempered steel plates, as well as a wide tempering strength platform and temperature range, and ensuring the toughness / plasticity matching and low-temperature toughness of 1000MPa grade quenched and tempered steel plates; wherein Mo equivalent = (%Mo) + 0.67 (%Cr) + 0.83 (%Si) + 2.67 (%Nb) + 1.62 (%V) - 0.11 [(%Cu) + (%Ni)], in %; this is one of the key technologies of the present invention.

[0090] Relationship between Ca and S: Ca / S is between 1.00 and 3.00 and (%Ca)×(%S) 0.18 ≤1.5×10 －3; Ensure sulfide spheroidization and minimize the influence of inclusions on low-temperature toughness, strain aging embrittlement resistance and weldability, while achieving high efficiency of inclusions: Ca (O, S) particles are evenly and finely distributed in the steel, inhibiting the growth of austenite grains in the heat-affected zone of the slab heating and high heat input welding, ensuring the uniform and fine microstructure of the steel plate, reducing the anisotropy of the longitudinal and transverse properties (strength, toughness) of the steel plate, improving the low-temperature toughness and strain aging embrittlement resistance of the steel plate, and improving the low-temperature toughness and crack resistance and crack arrest properties of the heat-affected zone of the steel plate in high heat input welding.

[0091] The specific manufacturing process includes

[0092] 1. Smelting and TMCP process

[0093] According to the composition system of the steel of the present invention and the requirements of strength, plasticity and low-temperature toughness of the steel plate, the manufacturing process design scheme is: continuous casting is adopted, the tundish pouring superheat ΔT is controlled at 15°C to 30°C, the drawing speed is controlled at 0.5m / min to 1.0m / min, and the crystallizer liquid level fluctuation is controlled at ≤3mm.

[0094] In order to ensure that the microstructure of the ultra-high strength quenched and tempered steel plate is uniform and fine, especially the surface (sub) layer of the steel plate has a certain strain storage rate, the total compression ratio of the steel plate (slab thickness / finished steel plate thickness) is ≥4.0.

[0095] The first stage is ordinary rolling, and the slab heating temperature is controlled between 1100℃ and 1180℃; A) Ensure that Nb in the steel is completely dissolved and the slab grains do not grow abnormally; B) At least 2 passes are rolled at low speed and high pressure, and the slab rolling speed is controlled at ≤1.0m / sec., and the rolling shape factor (ΔH / R) 1 / 2 ≥0.18, where ΔH is the pass reduction and R is the working roll diameter; this ensures that the microstructure of the rolled intermediate billet is uniform and fine, creating conditions for the second stage of controlled rolling to refine and homogenize the non-recrystallized grains.

[0096] The second stage is controlled rolling before recrystallization. The starting rolling temperature is ≤830℃, the reduction rate of each rolling pass is ≥7%, the cumulative reduction rate in the non-recrystallized zone (≤830℃) is ≥50%, and the final rolling temperature is 750℃~800℃, which lays the foundation for accumulating sufficient deformation in the non-recrystallized austenite, forming a high-density dislocation network structure, and realizing the aus-forming process.

[0097] After the non-recrystallization controlled rolling is completed, the steel plate is immediately transported to the ACC equipment at the maximum conveying speed of the roller, 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 to ≤400℃.

[0098] The interval time between the end of cooling and the entry of the heated slow cooling pit for insulation of the steel plate shall not exceed 60 minutes. The insulation process shall be at least 24 hours under the condition that the surface temperature of the steel plate is ≥250°C to ensure sufficient dehydrogenation of the ultra-thick steel plate to prevent the generation of hydrogen-induced delayed cracks.

[0099] 2. Tempering process

[0100] The tempering temperature of the steel plate (plate temperature) is 575-635°C. When the steel plate is relatively thin, the tempering temperature is on the upper limit, and when the steel plate is relatively thick, the tempering temperature is on the lower limit. The tempering holding time is ≥ (0.60-1.0) × the thickness of the finished steel plate. The tempering holding time is the holding time starting from when the center temperature of the steel plate reaches the tempering target temperature, and the time unit is min. After the tempering is completed, the steel plate is naturally air-cooled to room temperature.

[0101] In addition to meeting the above requirements, the following relationships must also be met:

[0102] [(Ni equivalent) × (% Nb) ×] / [(T 终轧 )×H]≥7.0×10 -5 , with the increase of Ni equivalent, under low temperature conditions, the depth of dislocation PN valley is reduced, the PN force of dislocation slip is reduced, dislocations are prone to cross slip, the length of dislocation plug cluster is reduced, the local stress concentration at the front end of the plug cluster is reduced, and the intrinsic plastic toughness of the steel plate is improved, ensuring the excellent low-temperature toughness and strain aging embrittlement resistance of the steel plate. With the increase of Nb content in steel, the increase of cumulative reduction rate ξ of non-recrystallization controlled rolling, the decrease of final rolling temperature of non-recrystallization controlled rolling, and the decrease of thickness of finished steel plate (rolled steel plate thickness effect), the flattening degree of austenite in steel plate is greatly increased, and the dislocation density inside the steel plate is sharply increased. Not only the position of martensite / bainite phase deformation nucleus is greatly increased, but also the probability of phase transformation variants growing in different directions is higher, the proportion of V1 / V2 interface of block is higher, the microstructure of steel plate after phase transformation is more uniform and fine, and the proportion of high-angle grain boundary in block fine structure is higher, and the strength-toughness / strength-plasticity matching, low-temperature toughness and strain aging embrittlement resistance of ultra-high strength steel plate are better; vice versa. Therefore, in order to ensure the toughness / ductility matching, low temperature toughness and strain aging embrittlement resistance of 1000MPa steel plate, the above relationship must be satisfied. Where Ni equivalent = (%Ni) + 0.33 (%Mn) + 0.65 (%Cu) - 0.47 (%Cr) - 0.63 (%Mo) - 0.81 (Mo + V), unit is %; ξ is the cumulative reduction rate without recrystallization, unit is %; T 终轧 is the final rolling temperature of the non-recrystallization controlled rolling, in °C; H is the thickness of the finished steel plate, in mm; this is one of the key technologies of the present invention.

[0103] 8.9≤{DI OL ×Mo equivalent×[(T 开冷 )-(T停冷 )]×(V c )} / [H×(T 回火 )×lgt]≤102, with the online hardenability index DI OL As the difference between the start-cooling temperature and the stop-cooling temperature increases, the cooling rate increases, and the strength of the steel plate increases; as the thickness of the steel plate increases, the tempering temperature increases, and the tempering holding time increases, the strength of the steel plate decreases; however, as the tempering temperature increases and the tempering time increases, the plastic toughness, low-temperature toughness, and resistance to strain aging embrittlement of the steel plate are greatly improved. Therefore, in order to ensure the strength, plastic toughness, low-temperature toughness, and resistance to strain aging embrittlement of 1000MPa steel plates, the above relationship must be satisfied. Among them, 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 is mm; T 开冷 T is the starting temperature of accelerated cooling, in °C; 停冷 is the accelerated cooling stop temperature, in °C; V c is the cooling rate for accelerated cooling, in °C / s; H is the thickness of the finished steel plate, in mm; T 回火 is the tempering temperature, in °C; t is the tempering holding time, in min; Mo equivalent = (%Mo) + 0.67 (%Cr) + 0.83 (%Si) + 2.67 (%Nb) + 1.62 (%V) - 0.11 [(%Cu) + (%Ni)], in %; this is one of the key technologies of the present invention.

[0104] Example

[0105] The technical solution of the present invention is further explained below in conjunction with specific implementation cases, with specific reference to Tables 1 to 5. Table 6 shows the properties of the prepared steel plates.

[0106] The microstructure of the newly invented steel 4 is shown in Figure 1 From the fine structure of the full-thickness microstructure of the steel plate, it can be seen that the microstructure of the finished steel plate is fine low-carbon tempered martensite + a small amount of tempered lower bainite, the average cluster packet size is below 202m and the block structure is extremely fine; it not only achieves the matching of ultra-high strength and toughness / plasticity, resistance to strain aging embrittlement, and weldability, but also solves the problem of excessive quenching of the surface (sub) layer of ultra-high strength steel plates. The steel plate is particularly suitable for large steel structures and equipment such as pressure water pipes, steel branch pipes, volutes, marine structures and large-scale engineering machinery in hydropower projects.

[0107]

[0108]

[0109]

Claims

1. A 1000MPa grade steel plate with matching toughness and plasticity, characterized in that: The steel plate is made of the following components by mass percentage: C: 0.08% to 0.13%, Si: ≤0.20%, Mn: 1.00% to 1.50%, P: ≤0.013%, S: ≤0.0030%, Cu: 0.15% to 0.45%, Ni: 1.30% to 2.00%, Cr: 0.40% to 0.70%, Mo: 0.30% to 0.60%, V: 0.030% to 0.060%, Nb: 0.010% to 0.030%, Ti: 0.008% to 0.016%, B: ≤0.00050%, N: ≤0.0040%, O≤0.0030% and Ca: 0.0010% to 0.0035%, the rest is Fe and unavoidable impurities; And the contents of the above ingredients satisfy the following relationship at the same time: The mass percentage ratio of Mn to C is ≥10; 2.15(%C)×[(%Si)+10(%S)+30(%P)+15(%O)]≤0.122; Mo equivalent ≥ 0.66, Wherein Mo equivalent = (%Mo) + 0.67 (%Cr) + 0.83 (%Si) + 2.67 (%Nb) + 1.62 (%V) - 0.11 [(%Cu) + (%Ni)], in %; The mass percentage ratio of Ca to S is between 1.00 and 3.00, and (%Ca)×(%S) 0.18 ≤1.5×10 -3 ; The specific preparation comprises the following steps: S1. According to the composition of the steel plate, the steel plate is smelted and cast into billets by continuous casting. The tundish pouring superheat ΔT is controlled at 15℃~30℃, the casting speed is controlled at 0.5m / min~1.0m / min, and the crystallizer liquid level fluctuation is controlled at ≤3mm; S2. The blank obtained in S1 is subjected to TMCP process, and the total compression ratio of the steel plate, i.e., the blank thickness / the finished steel plate thickness, is ≥4.0; The first stage is ordinary rolling, the slab heating temperature is controlled between 1100℃ and 1180℃, the slab rolling speed is controlled at ≤1.0m / s, and the rolling shape factor (ΔH / R) is 1 / 2 ≥0.18, where ΔH is the pass reduction and R is the working roll diameter; The second stage is controlled rolling without recrystallization, the start rolling temperature of controlled rolling is ≤830℃, the rolling pass reduction rate is ≥7%, the cumulative reduction rate of the non-crystallized zone is ≤830℃ is ≥50%, and the final rolling temperature is 750℃~800℃; After the non-recrystallization controlled rolling is completed, the steel plate is accelerated cooled, the start cooling temperature is 730℃~780℃, the cooling rate is ≥5℃ / s, the stop cooling temperature is controlled to be ≤400℃, and after the stop cooling is completed, it enters the heating slow cooling pit for insulation. During the insulation, the surface temperature of the steel plate is ≥250℃, and the insulation time is more than 24 hours; S3. Temper the steel plate. The tempering temperature is controlled at 575-635°C. The tempering holding time is ≥ (0.60-1.0) × the thickness of the finished steel plate, in minutes. After the tempering is completed, the steel plate is naturally cooled to room temperature. In step S2, it is also necessary to satisfy [(Ni equivalent) × (% Nb) × ξ] / [(T 终轧 )×H]≥7.0×10 -5 ; Wherein Ni equivalent = (%Ni) + 0.33 (%Mn) + 0.65 (%Cu) - 0.47 (%Cr) - 0.63 (%Mo) - 0.81 (Mo + V), unit is %; ξ is the cumulative reduction rate of the non-crystallized area, unit is %; T 终轧 is the final rolling temperature of the non-recrystallization controlled rolling, in °C; H is the thickness of the finished steel plate, in mm; In steps S2 and S3, it is also necessary to satisfy 8.9≤{DI OL ×Mo equivalent×[(T 开冷 )-(T 停冷 )]×(V c )} / [H×(T 回火 )×lgt]≤102; Where 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 is mm; T 开冷 T is the starting temperature of accelerated cooling, in °C; 停冷 is the accelerated cooling stop temperature, in °C; V c is the cooling rate for accelerated cooling, in °C / s; H is the thickness of the finished steel plate, in mm; T 回火 is the tempering temperature, in °C; t is the tempering holding time, in min; Mo equivalent = (%Mo) + 0.67 (%Cr) + 0.83 (%Si) + 2.67 (%Nb) + 1.62 (%V) - 0.11 [(%Cu) + (%Ni)], in %.

2. The 1000MPa grade steel plate with matched toughness and plasticity according to claim 1, characterized in that: The steel plate microstructure is mainly low-carbon tempered martensite + a small amount of low-carbon tempered lower bainite. The lath sizes of low-carbon tempered martensite and low-carbon tempered lower bainite are uniform and small, the average crystal cluster packet size is controlled below 20 μm, and lath clusters with different orientations appear in the packet structure.

3. The 1000MPa grade steel plate with matched toughness and plasticity according to claim 1, characterized in that: The steel plate has a tensile strength of ≥950MPa, a yield strength of ≥890MPa, a single value of -60°C transverse impact energy of ≥70J, a single value of -40°C aging transverse impact energy of ≥70J, and a fracture elongation δ5 of ≥15%.

4. The 1000MPa grade steel plate with matched toughness and plasticity according to claim 1, characterized in that: The interval time from the end of cooling of the steel plate to entering the heating slow cooling pit for insulation is ≤60min.

5. Application of the 1000MPa grade steel plate with matched toughness / ductility as described in any one of claims 1 to 4 in pressure water pipes, steel bifurcated pipes and volutes of hydropower projects.

6. Application of the 1000MPa grade steel plate with matched toughness / ductility properties as claimed in any one of claims 1 to 4 in marine structures.

7. Application of the 1000MPa grade steel plate with matched toughness / ductility as claimed in any one of claims 1 to 4 in large-scale engineering machinery structures.

Images