1000MPa-grade steel plate with high toughness, strain aging embrittlement resistance and excellent weldability and manufacturing method thereof

Abstract

The invention discloses a 1000MPa-grade steel plate with high toughness, strain aging embrittlement resistance and excellent weldability and a manufacturing method thereof. The 1000MPa-grade steel plate comprises the following components in percentage by weight: 0.09 to 0.13 percent of C, less than or equal to 0.20 percent of Si, 0.70 to 1.00 percent of Mn, less than or equal to 0.013 percent of P, less than or equal to 0.0030 percent of S, 0.15 to 0.40 percent of Cu, 1.50 to 3.00 percent of Ni, 0.45 to 0.70 percent of Cr, 0.40 to 0.65 percent of Mo, 0.040 to 0.070 percent of V, 0.010 to 0.030 percent of Nb, 0.004 to 0.012 percent of Ti, 0.040 to 0.070 percent of Als, 0.0009 to 0.0017 percent of B, less than or equal to 0.0050 percent of N, less than or equal to 0.0030 percent The tensile strength of the steel plate is larger than or equal to 950 MPa, the yield strength is larger than or equal to 890 MPa, the Charpy longitudinal / transverse impact energy (single value) at the temperature of-60 DEG C is larger than or equal to 100 J, the strain aging transverse impact energy (single value) at the temperature of-40 DEG C is larger than or equal to 47 J, the elongation at break delta 5 is larger than or equal to 15%, and the weldability is good (the preheating temperature is smaller than or equal to 150 DEG C, and the steel plate can bear welding heat input of 30 kJ / cm-50 kJ / cm). And the method is particularly suitable for hydropower engineering, ocean platform structures and major equipment manufacturing.

Description

Technical Field

[0001] The present invention relates to the field of low alloy steel, and in particular to a 1000MPa grade steel plate with high toughness, resistance to strain aging embrittlement and excellent weldability, 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, hydropower engineering, 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, weldability and their matching are the most important properties of low carbon (high strength) low alloy steel, which is ultimately determined by the microstructure of the finished steel.

[0003] With the continuous development of metallurgical technology, people have put forward higher requirements on the toughness, crack arrest characteristics, plasticity and weldability of 1000MPa ultra-high strength steel, that is, the steel plate has the ability to resist brittle fracture, strain aging embrittlement and plastic instability fracture under ultra-low temperature conditions (-60℃), while the strength and toughness and plasticity of the steel plate reach the level of 800MPa tensile strength steel plate; and under the conditions of relatively low alloy content, especially precious metal content, and relatively low manufacturing cost, the comprehensive mechanical properties and performance of the steel plate are greatly improved to reduce the use of steel and save costs, reduce the weight, stability and safety of steel components, and more importantly, to further improve the cold and hot processability of steel components and their safety and reliability during service.

[0004] At present, there has been a research boom in Japan, South Korea and the European Union to develop a new generation of high-performance steel materials. Efforts are made to achieve better microstructure matching, ultra-fine metallographic microstructure and substructure, dislocation configuration, packet, block, variant fine structure and phase angle control through alloy combination optimization design, sub-microstructure fine structure control and innovative manufacturing process technology, so that ultra-high strength steel can obtain better crack arrest characteristics, strong plasticity and plastic-toughness matching and excellent weldability.

[0005] 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 sufficiently high hardenability and hardenability, that is, 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 properties along the thickness direction, it is inevitable to add a large amount of alloying elements such as C, Cr, Mo, Ni, Cu, V, etc. to the steel, especially a large amount of precious alloying elements such as Cu, Ni, Mo and V, which not only leads to high alloy cost of steel plate, but also causes high carbon equivalent and cold crack sensitivity index of steel plate, which seriously affects the weldability of steel plate.

[0006] In addition, the surface (sub) layer of steel plates with high carbon and 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.

[0007] Low elongation and low-temperature toughness are not only not conducive to the cold and hot processing performance of steel plates, but also have a great impact on the fatigue resistance, stress concentration sensitivity, crack resistance and structural stability of steel plates; when used on fatigue heavy-load structures such as pressure steel pipes, steel branch pipes and volutes in hydropower projects, thermal power steam turbine generators and offshore oil 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 1000MPa-grade high-strength steel not only has excellent crack arrest properties, strength-toughness / strength-plasticity matching, resistance to strain aging embrittlement and weldability, but also the elongation is ensured to be above 15% to ensure the processing performance and fatigue resistance of the steel plate.

[0008] A large number of existing patents and technical documents only explain how to achieve the strength and low-temperature toughness of the base steel plate, but there is little explanation on improving the welding performance of the steel plate and obtaining excellent low-temperature toughness and crack arrest characteristics 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 and uniformity of mechanical properties in the thickness direction of the steel plate, and there is no indication of how to prevent overquenching of the surface (sub) layer of the steel plate.

[0009] There is also the "960MPa grade quenched and tempered steel plate with excellent toughness and plasticity and its manufacturing method" disclosed in Chinese patent ZL201010227961.8. Although the comprehensive mechanical properties of the steel plate have reached a relatively high level: tensile strength ≥980MPa, yield strength ≥890MPa, -60℃ Charpy transverse impact energy (single value) ≥47J, the maximum production thickness of the steel plate produced by the steel plate manufacturing technology of this invention can only reach 80mm.

[0010] Chinese patent ZL201110071217.8 discloses "a kind of HT960 steel plate with excellent plasticity and toughness and its manufacturing method", which is produced by TMCP+offline tempering process, giving full play to the quenching potential of alloy elements, greatly reducing the alloy content, manufacturing cycle and manufacturing cost of the steel plate, and greatly improving the comprehensive mechanical properties and weldability of the steel plate. However, the production thickness of the steel plate is still limited, and the maximum thickness can only reach 70mm. It is still impossible to produce thicker steel plates.

[0011] The "1000MPa grade steel plate with ultra-thickness, ultra-high toughness and excellent weldability and its manufacturing method" disclosed in Chinese patent CN112143958A adopts low-carbon, high-nickel and high DI index design, and successfully develops 1000MPa grade ultra-high toughness steel plate with a maximum thickness of 180mm through special controlled rolling + DQ + QT process. The strength, toughness and plasticity of the steel plate match the level of 800MPa grade quenched and tempered steel plate, especially the elongation, crack resistance and crack arrest characteristics and weldability of the steel plate are basically equivalent to those of 800MPa quenched and tempered steel plate, which can be used in hydropower engineering pressure steel pipes, volutes, steel bifurcated pipes and steel bifurcated pipe crescent ribs. The actual product The quality has reached the international advanced level (equivalent to the actual quality of similar products of Nippon Steel); but the manufacturing cost of the steel plate is relatively expensive, especially when the precious alloy element Ni is added at 1.50% to 4.00%, and the surface cracks of the slabs with high Ni content are frequent, and the slabs need mechanical grinding; this not only leads to many manufacturing processes, long manufacturing cycle, low metal recovery rate, difficulty in recycling scrap steel (containing a large amount of Ni elements), and high steel plate manufacturing cost, but more importantly, the steel plate fails to solve the problem of excessive quenching of the surface layer causing surface embrittlement and unstable strain aging resistance, which poses a certain risk to the safety and reliability of the steel plate after it is processed and deformed into components. Summary of the invention

[0012] The purpose of the present invention is to provide a 1000MPa grade steel plate with high toughness, resistance to strain aging embrittlement and excellent weldability and a manufacturing method thereof, so that the steel plate can obtain ultra-high strength, excellent crack arrest characteristics and strong toughness / strong plasticity matching, and the steel plate not only has excellent strain aging embrittlement resistance characteristics, but also has good weldability; and successfully solves the contradictions and difficult to reconcile problems of strength and plastic toughness, strength and strain aging embrittlement resistance characteristics, strength and weldability, and strength and excessive quenching of the surface (sub) of the steel plate of ultra-high strength tempered steel plate; successfully eliminates the problem of overquenching of the surface (sub) layer of the steel plate under the condition of high alloy content, which causes low plastic toughness, crack arrest characteristics and low strain aging embrittlement resistance characteristics of the steel plate surface; the tensile strength of the steel plate is ≥950MPa, the yield strength is ≥890MPa, the -60℃ Charpy longitudinal / transverse impact energy (single value) is ≥100J, the -40℃ strain aging transverse impact energy (single value) is ≥47J, and the elongation at break δ 5≥15% and good weldability (preheating temperature ≤150℃, can withstand 30-50kJ / cm welding heat input); especially suitable for hydropower projects such as volutes, pressure steel pipes, steel bifurcated pipes and metal structural parts, offshore platform structures and major equipment manufacturing such as shield machines, giant excavators, giant cranes and ship floating cranes.

[0013] To achieve the above object, the technical solution of the present invention is:

[0014] The present invention is based on the composition system of low C-low Si-medium Mn-(Cu+Ni+Mo+Cr) alloying-(Ti+Nb+V+B) microalloying, appropriately increases the acid-soluble Als content in the steel and Als / [(%N)-0.292(%Ti)]≥21, controls (%C)×(%Mn)≤0.132, Ni equivalent≥0.26, [(%Si)×H 1 / 2 ] / (%C)≤12.67, Ca treatment and Ca / S ratio between 1.00 and 3.00 and (%Ca)×(%S) 0.18 ≤1.5×10 -3 Through metallurgical technology control means such as fine low-carbon lath martensite + a small amount of lower bainite (≤30%), the average cluster size is below 20μm, and excellent plasticity and toughness 1000MPa grade quenched and tempered steel plate is obtained to solve the problem of excessive quenching of the surface layer of ultra-high strength steel plate.

[0015] Specifically, the 1000MPa grade steel plate with high toughness, resistance to strain aging embrittlement and excellent weldability described in the present invention has the following components in percentage by weight:

[0016] C: 0.09~0.13%

[0017] Si: ≤0.20%

[0018] Mn: 0.70~1.00%

[0019] P: ≤0.013%

[0020] S: ≤0.0030%

[0021] Cu: 0.15~0.40%

[0022] Ni: 1.50~3.00%

[0023] Cr: 0.45~0.70%

[0024] Mo: 0.40~0.65%

[0025] V: 0.040~0.070%

[0026] Nb: 0.010~0.030%

[0027] Ti: 0.004~0.012%

[0028] Als: 0.040~0.070%

[0029] B: 0.0009~0.0017%

[0030] N: ≤0.0050%

[0031] O≤0.0030%

[0032] Ca: 0.001~0.0035%

[0033] The balance includes Fe and other unavoidable inclusions; and the following relationship must be satisfied at the same time:

[0034] Als / [(%N)-0.292(%Ti)]≥21;

[0035] (%C)×(%Mn)≤0.132;

[0036] Ni equivalent ≥ 0.26,

[0037] Ni equivalent=(%Ni)+0.97(%Mn)-(%Mn) 2 +1.23(%Cu)-0.58(%Cr)-

[0038] 1.15(%Mo)-4.47(%Si);

[0039] [(%Si) × H 1 / 2 ] / (%C)≤12.67; H is the thickness of the steel plate, in mm;

[0040] Ca treatment, and Ca / S ratio is 1.00~3.20, (%Ca)×(%S) 0.18 ≤2.5×10 -3 .

[0041] Further, the balance is Fe and other unavoidable inclusions;

[0042] The microstructure of the steel plate of the present invention is fine low-carbon lath martensite + a small amount of lower bainite, and the average crystal cluster size is less than 20 μm.

[0043] The steel plate of the present invention has a tensile strength of ≥950MPa, a yield strength of ≥890MPa, a -60°C Charpy longitudinal / transverse impact energy (single value) of ≥100J, a -40°C strain aging transverse impact energy (single value) of ≥47J, and a fracture elongation of δ 5 ≥15%, good weldability: preheating temperature ≤150℃, can withstand 30kJ / cm-50kJ / cm welding heat input.

[0044] In the composition design of the steel plate of the present invention:

[0045] C is the most effective hardenability element and can effectively improve the hardness of quenched steel plates. It is the most effective alloying element for improving quenched and tempered steel plates. Therefore, for ultra-high strength steel plates, it is very important to control the C content range. Properly increasing the C content in steel can greatly reduce the amount of other alloying elements. More importantly, when the C content in steel is lower than the critical value, adding more other alloying elements cannot effectively improve the strength of the steel plate. This is especially important for 1000MPa grade quenched and tempered steel plates. However, it is well known that C has a great influence on the strength, low temperature toughness, elongation, surface (sub) surface overquenching, strain aging embrittlement resistance and weldability of ultra-high strength quenched and tempered steel plates. From the perspective of improving the intrinsic plastic toughness, strain aging embrittlement resistance of ultra-high steel plates, inhibiting surface (sub) surface overquenching and improving weldability, it is hoped that the C content in the steel can be controlled appropriately low; however, from the perspective of hardenability, strength and toughness matching, weldability, microstructure control and alloy cost control of ultra-high steel plates, the C content should not be controlled too low, especially for 1000MPa grade high toughness quenched and tempered steel plates, the reasonable range of C content is 0.09% to 0.13%.

[0046] 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 reduces the critical cooling rate of martensitic transformation, promotes martensitic transformation, inhibits the segmentation effect of lower bainite transformation on original austenite grains, coarsens the size of packet crystal clusters and reduces the angle of block lath grain boundaries, seriously damaging the low-temperature toughness, crack arrest characteristics, elongation, strain aging embrittlement resistance 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 and crack arrest characteristics of the welding heat affected zone (HAZ). Therefore, the Si content in steel should be controlled as low as possible. Considering the economy and operability of the steelmaking process, the Si content is controlled at ≤0.20%.

[0047] As the most important alloying element in steel, Mn not only improves the strength of the steel plate, but also has the functions of expanding the austenite phase and reducing Ar 3point temperature, reduce the size of packet crystals to improve the plasticity and toughness of the steel plate, promote the formation of low-temperature phase transformation organizations martensite and bainite to improve the strength of the steel plate; however, Mn is prone to segregation during the solidification of molten steel, especially under conditions of high C content. When the Mn content is high, it will not only cause casting difficulties, but also easily co-segregate with elements such as C, P, S, Mo, Cr, etc., aggravating the segregation and looseness of the center of the ingot. Severe segregation in the center of the ingot is prone to form abnormal tissues during subsequent manufacturing and welding processes, resulting in low low-temperature toughness of ultra-high strength steel plates and cracks in welded joints; more importantly, when the Mn content is too high, the orientation difference between the block structures is reduced (i.e., the probability of V1 / V4 interface is increased and the probability of V1 / V2 interface is reduced), which seriously deteriorates the crack arrest characteristics and strain aging embrittlement resistance of ultra-high strength steel plates; in addition, when the alloy contains a high content, especially a high C content, Mn increases the hardenability of martensite, coarsens the size of martensite crystals (i.e., packet size), causing over-quenching of the steel plate surface (extremely low surface toughness and prone to environmental brittleness), promoting the formation of coarse martensite laths in the welding heat affected zone and deteriorating the toughness and crack resistance and arrest properties of the welded joint; therefore, according to the steel component system and C content range of the present invention, the suitable Mn content is 0.70% to 1.00%.

[0048] As a harmful inclusion in steel, P has a great detrimental effect on the mechanical properties of steel plates, especially low-temperature toughness, crack arrest characteristics, strain aging embrittlement resistance, elongation, weldability and post-SR performance of welded joints. In theory, the lower the better; but considering the operability and cost of steelmaking, for ultra-high strength quenched and tempered steel plates that require low cost, excellent weldability, -60℃ toughness and excellent strength and toughness matching, the P content needs to be controlled at ≤0.013%.

[0049] 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 steel (especially when the C and Mn contents are both high), and it is very easy 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 toughness, crack arrest properties, strain aging embrittlement resistance, elongation, Z-direction performance and weldability 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 quenched and tempered steel plates that require low cost, excellent weldability, -60℃ toughness and excellent strength and toughness matching, the S content needs to be controlled at ≤0.0030%.

[0050] Cu is also an austenite stabilizing element. Adding Cu can also reduce Ar 3point temperature, improve the hardenability and atmospheric corrosion resistance of the steel plate; however, excessive Cu addition, higher than 0.40% (for ultra-high strength steel plates with high C and high Mn), is likely to cause copper brittleness, surface cracking of the ingot, internal cracking problems and especially performance degradation of ultra-high strength steel plate welded joints after SR; for 1000MPa grade quenched and tempered steel plates, Cu addition is too little, less than 0.15%, and any effect is small; therefore, it is appropriate to control the Cu content between 0.15% and 0.40%; in addition, the composite addition of Cu and Ni not only reduces the copper brittleness of copper-containing steel and alleviates the intergranular cracking during the hot rolling process, but more importantly, Cu and Ni are austenite stabilizing elements, and the composite addition of Cu and Ni can significantly reduce Ar 3 , increase the driving force for the phase transformation from austenite to ferrite, refine the packet structure size, promote the growth of martensite / bainite laths in various directions (i.e. increase the probability of V1 / V2 interface and reduce the probability of V1 / V4 interface), cause the orientation difference between martensite / bainite blocks to become larger, increase the resistance of cracks passing through martensite / bainite laths and improve the low-temperature toughness of ultra-high strength steel plates.

[0051] Ni is the most important alloying element for the toughness, strength-plasticity matching and weldability improvement of ultra-high strength quenched and tempered steel plates. Adding Ni can not only improve the low-temperature mobility of dislocations in the ferrite phase, promote dislocation cross-slip, and improve the low-temperature intrinsic plasticity and toughness of the BCC (body-centered cubic) crystal structure; secondly, Ni increases the orientation difference between martensite / bainite block laths (i.e., increases the probability of V1 / V2 interface and reduces the probability of V1 / V4 interface), and increases the resistance of cracks passing through martensite / bainite packet grain boundaries and variant laths; finally, Ni, as an austenite stabilizing element, reduces Ar 3 point temperature, refine the martensite / bainite packet structure size, so Ni has the function of improving the strength, elongation and low-temperature toughness of the quenched and tempered steel plate at the same time; adding Ni to the steel can also reduce the copper brittleness of copper-containing steel, reduce intergranular cracking during hot rolling, and improve the atmospheric corrosion resistance of the steel plate. Therefore, theoretically, the higher the Ni content in the steel is within a certain range, the better; but for low-cost, 1000MPa grade quenched and tempered steel plates, there must be a certain Ni content to ensure that the steel plate has sufficient hardenability and uniform performance in the thickness direction, while ensuring the strength and toughness, strong plasticity matching, low-temperature toughness, and crack arrest characteristics of the steel plate, especially for ensuring the anti-strain aging embrittlement characteristics; therefore, the Ni content is controlled between 1.50% and 3.00% to ensure the hardenability and toughness level of the steel plate without damaging the weldability of the steel plate.

[0052] 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 reduces the orientation difference between martensite / bainite laths to a certain extent (i.e., increases the probability of V1 / V4 interface and reduces the probability of V1 / V2 interface), reduces the resistance of cracks passing through the martensite / bainite packet structure, and has a certain effect of deteriorating the toughness of the steel plate while improving the strength of the steel plate. More importantly, when the amount of Cr added is too much, the weldability of the steel plate is seriously damaged, causing serious embrittlement of the HAZ and deterioration of the crack arrest characteristics, especially the toughness of the welded joint after stress relief treatment (i.e., after SR). However, for high-toughness 1000MPa grade quenched and tempered steel plates, a certain Cr content must be present to ensure that the steel plate has sufficient hardenability; therefore, the Cr content is controlled between 0.45% and 0.70%.

[0053] Adding Mo can greatly improve the hardenability of steel plates, promote the formation of martensite / bainite, improve the tempering characteristics and tempering process window of steel plates, and improve the toughness and plasticity matching of steel plates after tempering; however, Mo, as a strong carbide-forming element, while promoting the formation of martensite / bainite, increases the size of martensite / bainite packets and promotes the formation of small-angle grain boundaries between martensite / bainite block laths (i.e., increases the probability of V1 / V4 interfaces and reduces the probability of V1 / V2 interfaces), and reduces the resistance of cracks passing through the martensite / bainite packet grain boundaries and between lath variants. ; In addition, Mo promotes the 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 and elongation of the ultra-high strength quenched and tempered steel plate, inducing overquenching of the surface (sub) layer of the steel plate; and when Mo is added too much (especially when the C and Mn contents are high), it not only seriously damages the elongation, weldability and post-SR performance of the welded joint of the steel plate, but also increases the SR brittleness and production cost of the steel plate; However, for high-toughness 1000MPa grade quenched and tempered steel plates, a certain Mo content must be present to ensure that the steel plate has sufficient hardenability and resistance to tempering softening and to expand the tempering process window. Therefore, considering the phase transformation strengthening effect of Mo and its influence on the low-temperature toughness, elongation and weldability of the parent steel plate, the Mo content is controlled between 0.40% and 0.65%.

[0054] The V content is between 0.040% and 0.070%, and as the thickness of the quenched and tempered steel plate increases, the V content can be appropriately taken to the upper limit. The purpose of adding V is to expand the tempering process window, improve the strength and thermal stability of the quenched and tempered steel plate, and achieve the matching of strength, toughness and plasticity of the quenched and tempered steel plate by dispersing and precipitating V (C, N) in the bainite / martensite laths; if V is added too little, the number of precipitated V (C, N) particles is too small, and the strength of the 1000MPa quenched and tempered steel plate cannot be effectively improved and stabilized; if V is added too much, higher than 0.070%, the low-temperature toughness, crack arrest characteristics, elongation, weldability of the steel plate, especially the ultra-high strength quenched and tempered steel plate with high alloy content, and the low-temperature toughness and crack arrest characteristics of the welded HAZ are damaged.

[0055] The purpose of adding trace amounts of Nb elements in steel is to carry out non-recrystallization controlled rolling, refine the microstructure of the steel plate, especially refine the surface (sub) surface grains of the steel plate, reduce the hardenability of the surface (sub) surface layer of the ultra-high strength steel plate, inhibit the excessive quenching of the surface (sub) surface layer of the ultra-high strength steel plate, and improve the low temperature toughness and toughness matching of the 1000MPa grade quenched and tempered steel plate; when the Nb addition is less than 0.010%, the effect of controlled rolling to refine the surface (sub) grains cannot be effectively exerted; when the Nb addition exceeds 0. When the Nb content is 0.03%, the formation of coarse upper bainite (Bu) and secondary precipitation embrittlement of Nb (C, N) are easily induced during welding, which seriously damages the low-temperature toughness and crack resistance and arrest properties of the welding heat affected zone (HAZ). Therefore, the Nb content is controlled between 0.010% and 0.030% to obtain the best controlled rolling effect, achieve the strength and toughness matching of 1000MPa grade quenched and tempered steel plates, and inhibit excessive quenching of the surface (sub) layer without damaging the toughness of welding and multi-pass welding HAZ.

[0056] The Ti content is between 0.004% and 0.012%, which inhibits the excessive growth of austenite grains during slab heating, rolling and tempering heat treatment, improves the low-temperature toughness of the steel plate, and more importantly, inhibits the growth of HAZ grains during welding and improves HAZ toughness. 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 0.012%, under the condition of high acid-soluble aluminum content, excess Ti precipitates in the form of TiC coherently on the martensite / bainite laths and on the grain boundaries, seriously embrittles the microstructure of the steel plate.

[0057] Als in steel can fix the free [N] in steel. In addition to reducing the free [N] in the weld heat affected zone (HAZ) and improving the low temperature toughness of the weld 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 0.040%. However, adding too much Als to steel will not only cause casting difficulties, but also form a large number of dispersed needle-shaped Al in the steel. 2 O 3Inclusions damage the internal soundness, low temperature toughness and weldability of the steel plate, so the upper limit of Als is controlled at 0.070%.

[0058] The B content is controlled between 0.0009% and 0.0017%, which ensures the hardenability of the steel plate without damaging the weldability, HAZ toughness and surface quality of the slab.

[0059] In order to ensure the presence of sufficient 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 transverse low-temperature toughness and plasticity of the steel plate, the N content in the steel shall not exceed 0.0050%.

[0060] In order to ensure the plastic toughness of 1000MPa grade quenched and tempered steel plate, it is necessary to reduce the inclusions in the steel, among which alumina inclusions are the most harmful, followed by SiO 2 , therefore the O content in the steel is ≤0.0030%.

[0061] 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, 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%.

[0062] In addition, the content of the ingredients of the present invention must simultaneously satisfy the following relationship:

[0063] Als / [(%N)-0.292(%Ti)]≥21, eliminate the free [N] in the steel and welding heat affected zone;

[0064] 1) Ensure that there is enough solid solution [B] in the steel, maintain the steel plate with sufficient hardenability and stable hardenability, and achieve the strength, low temperature toughness and toughness matching of 1000MPa grade steel plate;

[0065] 2) Reduce the free [N] content in the heat affected zone of welding and improve the microstructure and low temperature toughness of the heat affected zone of welding.

[0066] (%C)×(%Mn)≤0.132, the purpose is:

[0067] 1) Reduce the degree of conjugate segregation during the solidification of molten steel with carbon, manganese and high alloy content, improve the internal soundness and homogeneity of the steel plate, and improve the UT pass rate and low-temperature toughness of the steel plate;

[0068] 2) Inhibit excessive quenching of the surface (sub) of steel plates with carbon, manganese and high alloy content, improve the low-temperature toughness and environmental brittleness of the surface (sub) layer of the steel plate, and improve the anti-crack and anti-strain aging embrittlement properties of the steel plate;

[0069] 3) Balancing the hardenability of the steel plate, inhibiting the generation of coarse martensite packet size, and improving the strength and toughness, strength-plasticity matching and low-temperature toughness of the steel plate is one of the key technologies of the present invention.

[0070] The low-temperature dislocation mobility index of the iron-based BCC crystal structure---Ni equivalent ≥ 0.26. For the requirements of ultra-low temperature toughness, crack arrest characteristics and strain aging embrittlement resistance at -60℃, improving the low-temperature intrinsic plastic toughness of the body-centered cubic (BCC) crystal structure ferrite is the key to the successful development of such steels; with the increase of Ni equivalent, the directionality of the dd electron cloud orbit of the d orbit of the electrons outside the nucleus of the iron atom under low temperature conditions is reduced (the trend of transformation to covalent bonds), the lattice friction force (i.e. PN force) of the low-temperature sliding of the ferrite dislocation is greatly reduced, the low-temperature sliding and cross-slip of the ferrite dislocation are promoted, the mobility of the ferrite dislocation under low temperature conditions is improved, the number and length of the dislocation pile-up groups in the microstructure are reduced, and the local stress concentration inside the steel plate is reduced, ensuring that the steel plate has excellent intrinsic plastic toughness and strain aging embrittlement resistance under ultra-low temperature conditions (-60℃); wherein Ni equivalent = (%Ni) + 0.97 (%Mn) - (%Mn) 2 +1.23(%Cu)-0.58(%Cr)-1.15(%Mo)-4.47(%Si); This is one of the key technologies of the present invention.

[0071] [(%Si) × H 1 / 2 ] / (%C)≤12.67; improve the critical cooling rate of martensitic transformation, form a gradient phase transformation mode, the lower bainite generated by the first phase transformation divides the original austenite grains, and refines the size of the martensitic packet crystal group, which not only improves the strength-toughness / strength-plasticity matching and low-temperature toughness of the steel plate, but also reduces the surface (sub) surface overquenching, and improves the strain aging embrittlement resistance of the ultra-high strength quenched and tempered steel plate; H is the thickness of the steel plate, in mm. This is one of the key technologies of the present invention.

[0072] Ca treatment, and Ca / S ratio between 1.00 and 3.20 and (%Ca)×(%S) 0.18 ≤2.5×10 -3; To improve the low temperature toughness, strength-toughness matching, weldability and lamellar tearing resistance of ultra-high strength steel plates, while suppressing the environmental brittle sensitivity of ultra-high strength steel plates (especially delayed hydrogen-induced cracking in the center of the plate thickness).

[0073] 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.

[0074] The present invention also provides a method for manufacturing the 1000MPa grade steel plate with high toughness, resistance to strain aging embrittlement and excellent weldability, which comprises the following steps:

[0075] 1) Smelting and rolling process

[0076] Smelting and casting into slabs according to the above ingredients;

[0077] 2) Rolling

[0078] The first stage of rolling is recrystallization rolling, the slab heating temperature is controlled at 1100-1170℃, the pass reduction rate is ≥5%, and the final rolling temperature is ≥960℃;

[0079] The second stage adopts controlled rolling in the non-recrystallization zone, with the start rolling temperature ≤800℃, the average rolling reduction rate ≥7%, the cumulative reduction rate ≥30%, and the final rolling temperature ≤780℃;

[0080] After rolling, the steel plate is naturally air-cooled to room temperature;

[0081] 3) Quenching and tempering treatment

[0082] The quenching treatment adopts two offline gradient quenching processes QQT, each quenching temperature is 870-920℃, the second quenching temperature is lower than the first quenching temperature, each quenching holding time is ≥15min, and the quenching holding time refers to the holding time starting from when the center temperature of the steel plate reaches the quenching target temperature; the plate passing speed is 0.3-4.0m / min;

[0083] Tempering treatment, tempering temperature is 575 ~ 620 ℃, tempering holding time ≥ 30min, tempering holding time is the holding time starting from when the center temperature of the steel plate reaches the tempering target temperature; after tempering, the steel plate is naturally air-cooled to room temperature; and the following relationship must also be met:

[0084] 4.6≤{(%Si)×DI×[(T 淬火1 )-Ar 3 ]×[(T 淬火 2) -Ar 3 ]×(T 精终轧 )×(H 1 / 2 )} / [(V 通板1 )×(V通板2 )×(T 加热 )×(T 粗终轧 )×ξ]≤5489.9, where

[0085] DI=0.367(%C) 0.5 [1+0.7(%Si)][1+3.33(%Mn)][1+0.35(%Cu)][1+0.36(%Ni)][1+2.16(%Cr)][1+3(%Mo)][1+1.75(%V)][1+1.77(%Al)]×25.4;

[0086] Ar 3 =910-310(%C)-80(%Mn)-20(%Cu)-15(%Cr)-55(%Ni)-80(%Mo)-0.35(H-8);

[0087] T 淬火1 is the first quenching temperature, °C;

[0088] T 淬火2 is the second quenching temperature, °C;

[0089] T 精终轧 is the final rolling temperature, °C;

[0090] V 通板1 is the first board passing speed, m / min;

[0091] V 通板2 is the second board passing speed, m / min;

[0092] T 加热 is the heating temperature, °C;

[0093] T 粗终轧 is the rough rolling and final rolling temperature, ℃;

[0094] H is the thickness of the steel plate in mm.

[0095] Preferably, in step 1), continuous casting is adopted for casting, the tundish pouring superheat is controlled at 10-30° C., the pulling speed is controlled at 0.6-1.0 m / min, and the crystallizer liquid level fluctuation is controlled at ≤3 mm.

[0096] Preferably, after the rolling in step 2) is completed, the steel plate with a thickness of ≥50 mm is immediately subjected to slow cooling, the slow cooling process is above 300° C., and the temperature is kept for more than 36 hours; then the steel plate is naturally air-cooled to room temperature.

[0097] In the manufacturing method of the present invention:

[0098] 1. The rolling process adopts two-stage rolling.

[0099] The first stage of rolling is recrystallization rolling. To ensure that Nb is completely dissolved during heating and rolling, the slab heating temperature is controlled between 1100°C and 1170°C, the pass reduction rate is ≥5%, and the final rolling temperature is ≥960°C.

[0100] The second stage adopts controlled rolling in the non-recrystallization zone, with a controlled rolling start temperature of ≤800℃, an average rolling reduction rate of ≥7%, a cumulative reduction rate of ≥30%, and a final rolling temperature of ≤780℃. The steel plate grains before heat treatment are refined and adjusted to lay the foundation for finally obtaining a fine and uniform martensitic structure. After rolling, the steel plate with a thickness of ≥50mm is immediately slow-cooled at a temperature of more than 300℃ for more than 36 hours. The steel plate is then naturally air-cooled to room temperature.

[0101] 2. Tempering treatment

[0102] Quenching treatment, using two offline gradient quenching process (QQT) for production, according to the steel plate hardenability index, Ac 3 The quenching temperature of the steel plate is adjusted dynamically to achieve the matching between the hardenability of the steel plate, the quenching temperature and the quenching roller speed, to suppress the over-quenching of the steel plate surface, to obtain a fine and uniform low-carbon martensite + a small amount of low-carbon lower bainite structure, so as to obtain the performance requirements of the steel plate of the present invention. Each quenching temperature is 870-920°C, the second quenching temperature is lower than the first quenching temperature, each quenching holding time is ≥15min, the plate passing speed (quenching roller speed) is 0.3-4.0m / min, and the quenching holding time refers to the insulation time starting from when the center temperature of the steel plate reaches the quenching target temperature.

[0103] Tempering treatment: the tempering temperature of the steel plate is 575℃~620℃, and the tempering holding time is ≥30min. The tempering holding time is the insulation time starting from when the center temperature of the steel plate reaches the tempering target temperature; after the tempering is completed, the steel plate is naturally air-cooled to room temperature.

[0104] 3. In addition to meeting the above requirements, the following relationships must also be met:

[0105] 4.6≤{(%Si)×DI×[(T 淬火 1) -Ar 3 ]×[(T 淬火2 )-Ar 3 ]×(T 精终轧 )×(H 1 / 2 )} / [(V 通板1 )×(V 通板2 )×(T 加热 )×(T 粗终轧 )×ξ]≤5489.9, where:

[0106] DI=0.367(%C) 0.5[1+0.7(%Si)][1+3.33(%Mn)][1+0.35(%Cu)][1+0.36(%Ni)][1+2.16(%Cr)][1+3(%Mo)][1+1.75(%V)][1+1.77(%Al)]×25.4;

[0107] Ar 3 =910-310(%C)-80(%Mn)-20(%Cu)-15(%Cr)-55(%Ni)-80(%Mo)-0.35

[0108] (H-8).

[0109] Through composition optimization coupled with two offline gradient low temperature quenching processes:

[0110] 1) A double-cycle phase transformation between austenite and martensite (including a small amount of lower bainite) (Bu (heating) → γ (quenching 1) → very small amount of B L +α'(heating)→γ(quenching 2)→very small amount of B L +α'), which not only fully refines the martensite cluster size packet, increases the proportion of high-angle grain boundaries between blocks, promotes the formation of martensite + lower bainite multiphase structure, inhibits surface (sub) surface overquenching, and ensures that the steel plate has excellent low-temperature toughness, crack arrest properties and resistance to strain aging embrittlement degradation;

[0111] 2) Balance the hardenability of ultra-high strength steel plates of different thicknesses to ensure that the ultra-high strength quenched and tempered steel plates have sufficient hardenability and strength, and ensure uniform microstructure and properties in the thickness direction of the quenched and tempered steel plates, and matching toughness / strength plasticity;

[0112] 3) Through the low-temperature gradient double quenching process, the austenite grains on the surface (sub-surface) of the steel plate are refined, the excessive quenching of the surface (sub-surface) of the steel plate is inhibited, and the steel plate's resistance to strain aging embrittlement is improved.

[0113] Beneficial effects of the present invention:

[0114] The present invention adopts a composition system of low C-low Si-medium Mn-(Cu+Ni+Mo+Cr) alloying-(Ti+Nb+V+B) microalloying, matched with a controlled rolling + secondary gradient quenching process, which not only fully exerts the potential of alloy elements for hardening and hardenability, but also effectively refines the steel plate packet (i.e., crystal cluster), increases the density of large-angle grain boundaries between martensite / bainite laths (i.e., variants), and effectively refines the martensite / bainite sub-fine structure. With a relatively small amount of precious alloy content (compared with similar Japanese steel grades, the Ni alloy content is relatively low), the steel plate can obtain ultra-high strength, excellent ultra-low temperature toughness, crack arrest characteristics, resistance to strain aging embrittlement characteristics, and strong toughness / strong plasticity matching, which not only further reduces the manufacturing cost, but also improves the weldability of the steel plate. In particular, for ultra-high strength steel plates, the welding cold crack sensitivity 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 user's processing and manufacturing costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0115] Figure 1 This is a photograph of the microstructure (1 / 4 thickness) of Steel Example 4 of the present invention. DETAILED DESCRIPTION

[0116] The present invention will be further described below in conjunction with the embodiments and drawings.

[0117] The composition of the steel of the present invention is shown in Table 1, and the remainder of the composition includes Fe and other inevitable impurities. The manufacturing process parameters of the steel of the present invention are shown in Tables 2 to 4, and the performance parameters of the steel of the present invention are shown in Tables 5 and 6.

[0118] Depend on Figure 1 It can be seen that the microstructure of the steel plate of the present invention is fine low-carbon tempered martensite + a small amount of tempered lower bainite, the average cluster size reaches level 9.5, and the average cluster size is below 20 μm.

[0119] In summary, the present invention combines the alloy element combination design with the controlled rolling + low-temperature multiple quenching heat treatment process to give full play to the hardenability effect of C and the hardenability effect of B, forming an effective match between the alloy elements and the special tempering process; while achieving ultra-high strength, excellent crack arrest characteristics and strong toughness / strong plasticity matching of the steel plate, the steel plate not only has excellent resistance to strain aging embrittlement characteristics, but also has good weldability; successfully solves the contradictory and difficult-to-reconcile problems of strength and plasticity and toughness, strength and resistance to strain aging embrittlement characteristics, strength and weldability, and strength and excessive quenching of the surface (sub) of the steel plate of ultra-high strength tempered steel plates; successfully eliminates the problem of over-quenching of the surface (sub) layer of the steel plate under the condition of high alloy content, resulting in low plasticity and toughness, crack arrest characteristics and low resistance to strain aging embrittlement characteristics of the steel plate surface.

[0120]

[0121]

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[0125]

Claims

1. High toughness, resistance to strain aging embrittlement and excellent weldability 1000MPa grade steel plate, the weight percentage of its components is: C：0.09～0.13％ Si: ≤0.20% Mn: 0.70~1.00% P：≤0.013％ S：≤0.0030％ Cu: 0.15~0.40% Ni: 1.50~3.00% Cr：0.45～0.70％ Mo: 0.40~0.65% V：0.040～0.070％ Nb: 0.010~0.030% Ti: 0.004~0.012% Als: 0.040~0.070% B：0.0009～0.0017％ N：≤0.0050％ O≤0.0030％ Ca: 0.001~0.0035% The balance includes Fe and other unavoidable inclusions; and the following relationship must be satisfied at the same time: Als / [(%N)-0.292(%Ti)]≥21; (%C)×(%Mn)≤0.132; Ni equivalent ≥ 0.26, Ni equivalent=(%Ni)+0.97(%Mn)-(%Mn) 2 +1.23(%Cu)-0.58(%Cr)- 1.15(%Mo)-4.47(%Si); [(%Si) × H 1 / 2 ] / (%C)≤12.67; H is the thickness of the steel plate, in mm; Ca treatment, and Ca / S ratio is 1.00~3.20, (%Ca)×(%S) 0.18 ≤2.5×10 -3 .

2. The 1000MPa grade steel plate with high toughness, resistance to strain aging embrittlement and excellent weldability as claimed in claim 1, characterized in that: The balance is Fe and other unavoidable inclusions.

3. The 1000MPa grade steel plate with high toughness, resistance to strain aging embrittlement and excellent weldability as claimed in claim 1 or 2, characterized in that: The microstructure of the steel plate is fine low-carbon lath martensite + a small amount of lower bainite, and the average crystal cluster size is less than 20 μm.

4. The 1000MPa grade steel plate with high toughness, resistance to strain aging embrittlement and excellent weldability as claimed in claim 1, 2 or 3, characterized in that: The steel plate has a tensile strength of ≥950MPa, a yield strength of ≥890MPa, a -60°C Charpy longitudinal / transverse impact energy (single value) of ≥100J, a -40°C strain aging transverse impact energy (single value) of ≥47J, and a fracture elongation δ5 of ≥15%; good weldability: preheating temperature ≤150°C, and can withstand 30kJ / cm-50kJ / cm welding heat input.

5. The method for manufacturing a 1000MPa grade steel plate having high toughness, resistance to strain aging embrittlement and excellent weldability as claimed in any one of claims 1 to 4, characterized in that: The steps include: 1) Smelting and rolling process Smelting and casting into slabs according to the composition of claim 1 or 2; 2) Rolling The first stage of rolling is recrystallization rolling, the slab heating temperature is controlled at 1100-1170℃, the pass reduction rate is ≥5%, and the final rolling temperature is ≥960℃; The second stage adopts controlled rolling in the non-recrystallization zone, with the start rolling temperature ≤800℃, the average rolling reduction rate ≥7%, the cumulative reduction rate ≥30%, and the final rolling temperature ≤780℃; After rolling, the steel plate is naturally air-cooled to room temperature; 3) Quenching and tempering treatment For quenching treatment, two offline gradient quenching processes QQT are used, each quenching temperature is 870-920℃, the second quenching temperature is lower than the first quenching temperature, and each quenching holding time is ≥15min; the plate passing speed is 0.3-4.0m / min; Tempering treatment, the tempering temperature is 575 ~ 620 ℃, the tempering holding time is ≥ 30min; after the tempering, the steel plate is naturally air-cooled to room temperature; and the following relationship must also be met: 4.6 ≤ {(%Si) × DI × [(T 淬火1 ) - Ar3] × [(T 淬火2 ) - Ar3] × (T 精终轧 ) × (H 1 / 2 )} / [(V 通板1 ) × (V 通板2 ) × (T 加热 ) × (T 粗终轧 ) × ξ] ≤ 5489.9, where, DI=0.367(%C) 0.5 [1+0.7(%Si)][1+3.33(%Mn)][1+0.35(%Cu)][1 +0.36(%Ni)][1+2.16(%Cr)][1+3(%Mo)][1+1.75(%V)][1+1.77(%Al)]×25.4; Ar3=910-310(%C)-80(%Mn)-20(%Cu)-15(%Cr)-55(%Ni)-80(%M o)-0.35(H-8); T 淬火1 is the first quenching temperature, °C; T 淬火2 is the second quenching temperature, °C; T 精终轧 is the final rolling temperature, °C; V 通板1 is the first board passing speed, m / min; V 通板2 is the second board passing speed, m / min; T 加热 is the heating temperature, °C; T 粗终轧 is the rough rolling and final rolling temperature, ℃; H is the thickness of the steel plate in mm.

6. The method for manufacturing a 1000MPa grade steel plate having high toughness, resistance to strain aging embrittlement and excellent weldability as claimed in claim 5, characterized in that: Step 1), casting is performed by continuous casting, the tundish pouring superheat is controlled at 10-30°C, the pulling speed is controlled at 0.6-1.0 m / min, and the crystallizer liquid level fluctuation is controlled at ≤3 mm.

7. The method for manufacturing a 1000MPa grade steel plate having high toughness, resistance to strain aging embrittlement and excellent weldability as claimed in claim 5 or 6, characterized in that: Step 2) After rolling, the steel plate with a thickness of ≥50 mm is immediately subjected to slow cooling, the slow cooling process is above 300° C., and the temperature is kept for more than 36 hours; then the steel plate is naturally air-cooled to room temperature.

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