High-toughness fatigue-resistant 700 MPa-grade steel plate with ultralow welding heat affected zone hardness and manufacturing method of high-toughness fatigue-resistant 700 MPa-grade steel plate

Abstract

The invention discloses a steel plate with high toughness, fatigue resistance and ultralow welding heat affected zone hardness of 700MPa grade and a manufacturing method thereof, the steel plate comprises the following components by weight percent: 0.04-0.08% of C, less than or equal to 0.30% of Si, 0.45-0.85% of Mn, less than or equal to 0.013% of P, less than or equal to 0.003% of S, 0.95-1.55% of Cu, 1.10-1.80% of Ni, 0.60-1.00% of Cr, 0.05-0.35% of Mo, 0.010-0.035% of Nb, 0.008-0.016% of Ti, 0.0010-0.0040% of Ca and the balance of Fe and inevitable impurities, Ni / Cu is greater than or equal to 1.1, epsilon-Cu precipitation strengthening and toughening factors lambda are controlled to be greater than or equal to 0.32 and less than or equal the Ca / S ratio is controlled to be 1.0-3.0, and (% Ca) * (% S) 0.28 is smaller than or equal to 1.5 * 10 <-3 >. The TMCP + T process is optimized, high strength, high toughness and excellent weldability of a base metal steel plate are obtained, and meanwhile, the HAZ welded under the condition of a wide heat input range has excellent low-temperature toughness, ultralow hardness and fatigue resistance.

Description

Technical Field

[0001] The present invention relates to high-strength steel, and in particular to a steel plate with high toughness, fatigue resistance and ultra-low welding heat-affected zone hardness of 700 MPa 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 and is widely used in oil and gas pipelines, offshore platforms, shipbuilding, hydropower projects, 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 and dislocation substructure of the finished steel.

[0003] With the continuous development of metallurgical technology, people have put forward higher requirements on the plastic toughness of high-strength steel, especially weldability and low hardness of welding HAZ, that is, the steel plate has the ability to resist brittle fracture and plastic instability fracture under ultra-low temperature conditions (-60℃), while the steel plate has excellent weldability and low welding HAZ hardness, and can withstand a wide range of heat input welding (welding heat input 10~40kJ / cm); and under relatively low manufacturing cost conditions, the comprehensive mechanical properties and performance of the steel plate are greatly improved to reduce the alloy content of steel and save costs. High strength and lightweight reduce the weight, stability and safety of steel components themselves, and more importantly, further improve the cold / hot processability of steel components and their safety and reliability during service.

[0004] At present, there is a research boom in Japan, South Korea, the European Union and North America on the development of a new generation of high-performance steel materials. Efforts are made to achieve better microstructure matching through alloy combination optimization design, control of submicrostructure fine structure and innovative manufacturing process technology, ultra-fine metallographic microstructure and substructure (dislocation configuration, packet, variant and second phase type and precipitation state) fine structure, so that high-strength steel can obtain better strength-plasticity and plastic-toughness matching, weldability and low anisotropy.

[0005] Existing high-strength quenched and tempered steel plates with tensile strength ≥700MPa are mainly produced by offline quenching and tempering process (i.e. RQ+T); however, for steel plates with thickness ≤40mm, online quenching and tempering process (i.e. DQ / TMCP+T) can also be used for production; in order to obtain high strength, the steel plate must have a sufficiently high hardenability, i.e. the steel plate hardenability index DI ≥2.0×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)〗 to ensure that the steel plate has sufficiently high strength and excellent low-temperature toughness. Therefore, it is inevitable to add a certain amount of alloying elements such as Cr, Mo, Ni, Cu, V, etc. to the steel, and even the content of Cr + Mo is added to more than 0.70%, resulting in a relatively high carbon equivalent and cold crack sensitivity index of the steel plate, seriously affecting the weldability of the steel plate (see Chinese patents ZL201210077114.7, ZL201010113835.X, ZL200810042088.8, ZL200810042124.0).

[0006] In addition, over-quenching is likely to occur in the surface (subsurface) layer of the steel plate with high alloy content, forming a coarse martensite structure, which seriously deteriorates the low-temperature toughness, elongation rate and fatigue crack growth resistance of the surface (near-surface) layer of the steel plate (see "DENRYOKU DOBOKU" (Japanese), 1986, Vol. 201, P33; "TETSU TO HAGANE", 1986, Vol. 72, S612; "TETSU TO HAGANE", 1986, Vol. 72, S614; "TETSU TO HAGANE", 1985, Vol. 71, S1523; "TETSU TO HAGANE", 1986, Vol. 72, S615; "TETSU TO HAGANE", 1986, Vol. 73, S1398; "KAWASAKI SEITETSU GIHO" (Japanese), 1988, Vol. 20, P233; "SEITETSU KENKYU" (Japanese), 1986, Vol. 322, P99; "CAMP-ISIJ" (Japanese), 1989, Vol. 3, P207; "NKK GIHO" (Japanese), 1990, Vol. 133, P37; "DENRYOKU DOBOKU" (Japanese), 1994, Vol. 249, P1; "SUMITOMO METAL" (Japanese), 1995, Vol. 47, P1; "NISHIYAMA KINEN GIKEN KOSHO" 191-192, 2008, P162).

[0007] A large number of existing patent documents only explain how to achieve the strength and low-temperature toughness of the parent steel plate, but there are few descriptions on improving the welding performance of the steel plate and obtaining excellent low-temperature toughness of the HAZ of the welding heat affected zone, and there is no description on how to improve the tensile strength of the steel plate while improving the tensile elongation of the steel plate, the uniformity of mechanical properties in the thickness direction, the excellent mechanical processing performance and fatigue crack growth resistance (see Japanese Patents Sho 63-93845, Sho 63-79921, Sho 60-258410, Special Patents 4-285119, Special Patents 4-308035, Ping 3-264614, Ping 2-250917, Ping 4-143246, U.S. Patent USPatent 5798004, European Patent EP 0288054A2, "Xishan Memorial Technical Lecture" No. 159-160, P79~P80).

[0008] China Baosteel has successively developed a series of 600MPa quenched and tempered steel plates with high toughness and excellent weldability (see Chinese patents ZL201210077114.7, ZL201110181293.4, ZL201010113835.X, ZL200910196233.2, ZL200810042088.8, ZL200810042124.0). The research and development of these steel grades is mainly to solve the (super) high toughness, excellent weldability, low-cost manufacturing, expansion of extra-thick steel plate specifications and extra-thick steel plate of 600MPa grade quenched and tempered steel plates. Major breakthroughs and good results have been achieved in ensuring uniformity of performance in the thickness direction. Steel plates have been put into mass industrial production and successfully applied to major engineering construction and major equipment manufacturing at home and abroad, achieving good supply performance. Some varieties have replaced imports to fill the domestic gap, especially the successful development of extra-thick 600MPa grade quenched and tempered steel plates that resist strain aging embrittlement and eliminate residual stress annealing embrittlement (see Chinese patent ZL201610463494.6). It not only obtains excellent low-temperature toughness and resistance to SR embrittlement and softening, but also solves the disadvantage of weak intrinsic fatigue resistance of high-strength tempered steel plates.

[0009] In 2019, China Baosteel applied for 700MPa grade low temperature quenched and tempered steel plate with excellent weldability and fatigue resistance and its manufacturing method (see Chinese patent ZL201910150314.2), which successfully solved the problems of poor fatigue resistance of high-strength quenched and tempered steel plate, high sensitivity of welding cold cracks and easy embrittlement of welded joints, and was successfully applied to 30-75M mines in service around the world. 3 Giant excavators, 600-1500 ton giant cranes and port machinery, etc., have achieved import substitution (replacing imports from SSAB), which has strongly supported the manufacturing of major equipment in my country; but steel plates cannot withstand large heat input welding, and the hardness of the welded joints is relatively high (HV10≥300), resulting in poor low-temperature fatigue resistance of the welded joints, and there is a risk of fracture and failure during service. Summary of the invention

[0010] The object of the present invention is to provide a steel plate with high toughness, fatigue resistance and ultra-low welding heat affected zone hardness of 700MPa and a manufacturing method thereof. While obtaining high strength, high toughness and excellent weldability of the base steel plate, the welded HAZ has excellent low-temperature toughness, ultra-low hardness and fatigue resistance under a wide heat input range, and its yield strength is ≥630MPa, tensile strength is ≥690MPa, and Charpy impact energy KV2 (single value) at -60°C is ≥100J. It has excellent weldability (no preheating before welding and no heat treatment after welding) and ultra-low HAZ hardness (HV10≤260) within a wide heat input range (10-40kJ / cm); it is particularly suitable for icebreaker hulls, offshore platforms, cross-sea bridges, offshore wind power pile legs and wind tower structures, port machinery, etc. in ice sea areas, and can achieve low-cost stable batch industrial production.

[0011] High toughness, ultra-low welding heat affected zone (HAZ) hardness 700MPa grade steel plate is one of the most difficult varieties among thick plate products. The reason is that this type of steel plate not only requires low C, low carbon equivalent CEV, high strength, excellent ultra-low temperature toughness, and fatigue resistance; but also the steel plate must be able to withstand a wide range of welding heat input, excellent ultra-low temperature toughness of the welding heat affected zone, and ultra-low hardness of the welding HAZ under a wide range of welding heat input conditions; these performance requirements are not only difficult to meet at the same time, but some performances are contradictory to each other: A) The contradiction between low C, low carbon equivalent CEV and high strength; B) The contradiction between high strength and ultra-low hardness of the welding HAZ; C) The contradiction between high strength and excellent weldability; D) The contradiction between a wide welding heat input and the excellent ultra-low temperature toughness and fatigue resistance of the welding HAZ.

[0012] The above properties conflict with each other in the composition design and TMCP+T process design and are difficult to reconcile: when reducing the C content and carbon equivalent CEV, it is difficult to achieve high strength of the steel plate; when increasing the strength of the steel plate, it is difficult to achieve ultra-low hardness of the steel plate welding HAZ; when the steel plate is subjected to a wide range of heat input welding, it is difficult to ensure ultra-high toughness and fatigue resistance of the welding HAZ. How to balance high strength, ultra-high toughness, high toughness and ultra-low hardness of the HAZ during wide heat input welding is one of the biggest difficulties of this product and is also a key core technology.

[0013] The invention integrates the key factors affecting the high strength, high toughness, high toughness and ultra-low hardness of the HAZ during wide heat input welding of the steel plate in terms of key technical routes, components and process design. Starting from the alloy combination design, the ultra-low carbon C-low Si-low Mn low alloy steel component system is adopted as the basis, low carbon and low carbon equivalent, high Cu+high Ni alloying with Ni / Cu≥1.1, controlling the εCu precipitation strengthening and toughening factor of 0.32≤λ≤1.45, Ti+Nb microalloying, Ca treatment with Ca / S ratio controlled between 1.0 and 3.0 and (%Ca)×(%S) 0.28 ≤1.5×10 -3 Alloy combination design; Optimization of TMCP+T process: [ξ×(%Nb)] / {H×[(T 开轧 )+(T 停冷 )] / 2}≥1.9×10 -5 、[(DI OL )×(%Cu)×(V c )×(T 开冷 )] / {[(T 回火 )-550]×(T 停冷 )}≥17.4, so that the microstructure of the finished steel plate is uniform and fine ultra-low carbon massive ferrite, and the average grain size of the microstructure is below 20μm. While obtaining the high strength, high toughness and excellent weldability of the parent steel plate, the welded HAZ has excellent low-temperature toughness, ultra-low hardness and fatigue resistance under a wide heat input range.

[0014] Specifically, the steel plate with high toughness, fatigue resistance and ultra-low welding heat affected zone hardness of 700MPa described in the present invention has the following components in percentage by weight:

[0015] C: 0.04~0.08%

[0016] Si: ≤0.30%

[0017] Mn: 0.45~0.85%

[0018] P: ≤0.013%

[0019] S: ≤0.003%

[0020] Cu: 0.95~1.55%

[0021] Ni: 1.10~1.80%

[0022] Cr: 0.60~1.00%

[0023] Mo: 0.05~0.35%

[0024] Nb: 0.010~0.035%

[0025] Ti: 0.008~0.016%

[0026] Ca: 0.0010~0.0040%

[0027] The balance includes Fe and other unavoidable inclusions; and the contents of the above elements must simultaneously satisfy the following relationship:

[0028] Ni / Cu≥1.1;

[0029] Control the ε-Cu precipitation toughening factor to 0.32≤λ≤1.45,

[0030] λ=[3.72(%Cu)-2.16(%Cu) 2 )] / (%Ni);

[0031] Ca treatment, Ca / S ratio controlled at 1.0-3.0, (%Ca)×(%S) 0.28 ≤1.5×10 －3 .

[0032] Furthermore, the balance is Fe and other inevitable inclusions.

[0033] The microstructure of the finished steel plate of the present invention is uniform and fine ultra-low carbon blocky ferrite, and the average grain size of the microstructure is below 20 μm.

[0034] The finished steel plate of the present invention has a yield strength of ≥630MPa, a tensile strength of ≥690MPa, a Charpy impact energy KV2 (single value) of ≥100J at -60°C, excellent weldability, no need for preheating before welding, no heat treatment after welding, and a HAZ hardness HV10≤260 within an input range of 10-40kJ / cm.

[0035] In the composition design of the steel plate of the present invention:

[0036] In order to obtain a steel plate with a yield strength of ≥630MPa, a tensile strength of ≥690MPa, a Charpy impact energy (single value) of ≥100J at -60°C, and a HAZ hardness of HV10≤260 while the welded HAZ and the parent steel plate have the same strength and toughness, the chemical composition of the TMCP+T-shaped steel plate of the present invention has the following characteristics:

[0037] C has a great influence on the strength, low-temperature toughness, elongation and weldability of 700MPa grade TMCP+T steel plates, especially the hardness of welded HAZ. From the perspective of improving the low-temperature toughness, weldability and welded HAZ of steel plates, it is hoped that the C content in the steel can be controlled at a relatively low level; however, from the perspective of the strength and fatigue resistance of steel plates and the microstructure control and manufacturing cost during the production process, the C content should not be controlled too low; too low C content not only leads to excessively high grain boundary mobility, coarsening and mixed crystallization of the microstructure of the parent steel plate and the welded HAZ, and softening of the welded HAZ; and too low C content in the steel can cause grain boundary weakening, seriously deteriorating the low-temperature toughness of the parent steel plate and the welded HAZ; therefore, the reasonable range of C content is 0.04-0.08%.

[0038] 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 seriously damages the low-temperature toughness, elongation and weldability of steel plates. Especially for 700MPa grade high-strength steel with a high alloy content, Si not only promotes the formation of MA islands in the welding HAZ, but also makes the formed MA islands relatively large and unevenly distributed, which seriously damages the low-temperature toughness of the welding HAZ and hardens the HAZ, thereby deteriorating its fatigue properties. 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 0.30%.

[0039] As the most important alloying element in steel, Mn not only improves the strength of steel plates, but also has the functions of expanding austenite phase region, reducing Ar3 point temperature, refining TMCP+T steel plate grains to improve steel plate strength (fine grain strengthening effect), improving low temperature toughness of steel plates (fine grain toughening), fatigue resistance, and promoting low temperature phase transformation structure formation (phase transformation strength effect) to improve steel plate strength. However, Mn is prone to segregation during the solidification of molten steel, especially when the Mn content is high, which not only makes casting operation difficult, but also easily causes conjugate segregation with elements such as C, P, and S, especially When the C content in the steel is high, the segregation and looseness in the center of the ingot are aggravated. Severe segregation in the center of the ingot is prone to form abnormal structure in the subsequent TMCP and welding process, resulting in low low-temperature toughness and elongation of the steel plate. In addition, for 700MPa grade steel plates with high alloy content, when the Mn content is too high, not only the low-temperature toughness, elongation and weldability of the steel plate will deteriorate sharply, but also the HAZ hardness of the steel plate will increase sharply, resulting in a sharp increase in the cold crack sensitivity of the welded joint and serious deterioration of the fatigue resistance; therefore, the suitable Mn content is 0.45-0.85%.

[0040] As a harmful inclusion in steel, P has a great detrimental effect on the mechanical properties of steel, especially low-temperature impact toughness, elongation and weldability (especially causing weld HAZ hardening and embrittlement). In theory, the lower the better; but considering the operability and cost of steelmaking, for steel plates requiring -60℃ toughness, high strength and low weld HAZ hardness, the P content needs to be controlled at ≤0.013%.

[0041] As a harmful inclusion in steel, S has a great damaging effect on the low-temperature toughness, weldability and fatigue resistance of steel. More importantly, when the Cu content in steel is high, S and Cu are easy to form low-melting point compounds at the grain boundaries, which seriously deteriorate the toughness and elongation of the steel plate, and more importantly, the plastic toughness of the welded joint (fusion line, HAZ). At the same time, S is also the main element that produces hot brittleness during hot rolling. In theory, the lower the better. However, considering the operability of steelmaking, steelmaking cost and the principle of smooth logistics, for steel plates requiring -60℃ toughness, high strength and low welding HAZ hardness, the S content needs to be controlled at ≤0.003%.

[0042] Cu is also an austenite stabilizing element. Adding Cu can also reduce the Ar1 and Ar3 point temperatures, improve the atmospheric corrosion resistance of the steel plate, refine the microstructure of the TMCP+T steel plate, and improve the low-temperature toughness of the TMCP+T steel plate. More importantly, the steel plate is strengthened by ε-Cu precipitation, and the critical content of ε-Cu precipitation strengthening is 0.55%. In order to ensure that the steel plate has a strength of more than 700MPa, the lower limit of the Cu content in the steel cannot be less than 0.95%. However, excessive addition of Cu will not only easily cause copper brittleness, surface cracking and internal cracking of the ingot, but also seriously deteriorate the low-temperature toughness, fatigue resistance and welded joint performance of the thick steel plate. The upper limit of Cu is preferably controlled at 1.55%. Therefore, the Cu content is controlled between 0.95 and 1.55%.

[0043] Adding Ni can not only reduce the friction force of the dislocation lattice of the BCC crystal structure (i.e., PN force), improve the low-temperature dislocation mobility of the ferrite phase, promote dislocation cross-slip, and improve the intrinsic plasticity and toughness of ferrite; in addition, Ni, as a strong austenite stabilizing element, greatly reduces the Ar1 and Ar3 point temperatures, increases the driving force for the phase transformation of austenite to ferrite, causes the austenite to undergo phase transformation at a lower temperature, greatly refines the microstructure of the TMCP+T steel plate, increases the crack extension resistance through the ferrite grains, and greatly improves the low-temperature toughness of the TMCP+T steel plate. Therefore, Ni has the effect of simultaneously improving the strength and low-temperature toughness of the TMCP+T steel plate without reducing the elongation (i.e., plasticity and toughness); adding Ni to 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 steel is within a certain range, the better. However, too high Ni content will not only harden the welding heat affected zone, which is detrimental to the weldability of the steel plate and the toughness of the welded joint, but also greatly increase the alloy cost of the steel plate (Ni is a precious alloy element). Considering that for high Cu content precipitation strengthened steel plates, Ni / Cu must be ≥1.1; therefore, the Ni content is controlled between 1.10 and 1.80%.

[0044] Cr is a weak carbide-forming element. The addition of Cr not only improves the hardenability of the steel plate and promotes the formation of bainite, but also has a strong effect of increasing the orientation difference between bainite laths (i.e., phase transformation variants), increasing the resistance of cracks passing through the bainite block structure, and has a strong effect of improving the plastic toughness, crack resistance and crack arrest properties of the steel plate while improving the strength of the steel plate. However, when the amount of Cr added is too much, the weldability of the steel plate is seriously damaged, the HAZ is embrittled and hardened, and the low-temperature toughness and fatigue resistance of the HAZ are seriously deteriorated. However, for 700MPa high-strength 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 0.60 and 1.00%.

[0045] Adding Mo can greatly improve the hardenability of steel plates, promote the formation of low-temperature phase transformation structures such as bainite, improve the tempering characteristics and tempering process window of steel plates, and improve the strength and toughness, and strength-plasticity matching of steel plates after tempering; however, as a strong carbide-forming element, when Mo is added too much, it not only seriously damages the low-temperature impact toughness, elongation and weldability of the steel plate, but also deteriorates the low-temperature toughness of the HAZ, increases the HAZ hardness and deteriorates the fatigue resistance of the HAZ, and greatly increases the production cost; therefore, considering the phase transformation strengthening effect of Mo, the influence on the low-temperature toughness, elongation, weldability, low-temperature toughness of the HAZ and fatigue resistance (low HAZ hardness) of the parent steel plate, the Mo content is controlled between 0.05 and 0.35%.

[0046] The purpose of adding a trace amount of Nb element to steel is to carry out non-recrystallization controlled rolling, refine the grain size of the steel plate, and improve the strength and toughness of the TMCP+T steel plate. When the Nb addition is less than 0.010%, in addition to the ineffective control rolling effect, the strengthening and toughening ability of the TMCP+T steel plate is also insufficient; when the Nb addition exceeds 0.035%, not only does it cause the alloy cost of the steel plate (Nb is also a precious alloy element) to remain high; more importantly, the embrittlement and hardening caused by the formation of upper bainite (Bu) and the secondary precipitation of Nb (C, N) during welding seriously damage the low-temperature toughness and fatigue resistance of the welding heat affected zone (HAZ). Therefore, the Nb content is controlled between 0.010 and 0.035%.

[0047] The purpose of adding a trace amount of Ti to steel is to combine with N in steel to generate highly stable TiN particles to inhibit the growth of steel plate grains and grains in the welding HAZ area; the Ti content added to the steel must match the N content in the steel, and the matching principle is that TiN cannot precipitate in liquid molten steel but must precipitate in the solid phase; therefore, the precipitation temperature of TiN must be ensured to be lower than 1400°C; when the amount of Ti added to the steel is too small (<0.008%), the number of TiN particles formed is insufficient to inhibit the growth of austenite grains during TMCP and welding thermal cycles and improve the low-temperature toughness and weldability of the steel plate; when the Ti content is too much (>0.016%), the TiN precipitation temperature exceeds 1400°C, and some TiN particles precipitate large-sized TiN particles during the solidification of the molten steel. Such large-sized TiN particles not only cannot inhibit grain growth, but instead become the starting point for crack initiation; therefore, the optimal control range of Ti content is 0.008-0.016%.

[0048] 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 performance of the steel plate, improve the longitudinal and transverse strength of the steel plate toughness and the anisotropy of toughness and weldability. 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 of the steel and the 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 0.0040%.

[0049] The composition design of the present invention also specifically requires:

[0050] Ni / Cu≥1.15. Adding a certain proportion of Ni element to high Cu content steel plate can not only refine and homogenize the microstructure of the steel plate and improve the intrinsic plasticity and toughness of the steel plate, but more importantly, inhibit the segregation of Cu at the original austenite grain boundary, which leads to the weakening and embrittlement of the base steel plate and the low-temperature toughness and fatigue resistance of the welding HAZ.

[0051] Control the ε-Cu precipitation toughening factor to 0.32≤λ≤1.45. After the steel plate undergoes the TMCP process, high-temperature tempering causes ε-Cu to precipitate in the steel in a fine dispersed state, greatly improving the strength of the steel plate and achieving precipitation strengthening of the steel plate. However, ε-Cu precipitation will cause serious deterioration of the plastic toughness of the steel plate, so the intrinsic properties of the steel plate must be improved. By controlling the ε-Cu precipitation toughening factor within an appropriate range, not only can the steel plate be precipitated and strengthened to meet the steel plate strength level of 700MPa, but also the grain boundary embrittlement of high-Cu steel can be eliminated by improving the intrinsic plastic toughness of the steel plate and inhibiting the segregation of Cu at the grain boundary, thereby achieving the matching of strength and toughness and plasticity of the steel plate. Where λ=[3.72(%Cu)-2.16(%Cu) 2 )] / (%Ni); This is one of the key core technologies of the present invention.

[0052] Ca treatment and Ca / S ratio controlled between 1.0 and 3.0 and (%Ca)×(%S) 0.28 ≤1.5×10 －3 : While ensuring sulfide spheroidization and minimizing the influence of inclusions on low-temperature toughness and weldability, Ca(O, S) particles are evenly and finely distributed in the steel, inhibiting the growth of austenite grains in the heat-affected zone during welding, and improving the low-temperature toughness and fatigue resistance of the heat-affected zone of the steel plate welding.

[0053] 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.

[0054] The method for manufacturing a steel plate with high toughness, fatigue resistance and ultra-low welding heat affected zone hardness of 700 MPa according to the present invention comprises the following steps:

[0055] 1) Smelting and casting

[0056] Smelting and casting into slabs according to the above ingredients;

[0057] 2) Slab heating

[0058] The heating temperature is controlled at 1030~1130℃;

[0059] 3) Rolling

[0060] The first stage is ordinary rolling, which uses the maximum rolling capacity of the rolling mill for continuous rolling;

[0061] The second stage adopts non-recrystallization controlled rolling, the starting rolling temperature is controlled at 790-930℃, the rolling pass reduction rate is ≥7%, the cumulative reduction rate is ≥50%, and the final rolling temperature is 780-850℃;

[0062] 4) Cooling

[0063] After the controlled rolling is completed, the steel plate is immediately transported to the accelerated cooling area for accelerated cooling; the steel plate start cooling temperature is 740-820℃, the cooling rate is ≥5℃ / s, and the stop cooling temperature is 250-500℃; when the steel plate thickness is ≥50mm, the steel plate is slowly cooled, and the slow cooling process is to keep the temperature at not less than 250℃ for more than 24 hours, and then the steel plate is naturally air-cooled to room temperature;

[0064] 5) Tempering process

[0065] The tempering temperature of the steel plate is 600-670℃, and the tempering holding time is ≥3×H, where H is the thickness of the finished steel plate in mm. The tempering holding time is the holding time starting from when the target temperature is reached in the furnace. After the tempering is completed, the steel plate is naturally cooled to room temperature in air;

[0066] In addition to meeting the above requirements, the following relationships must also be met:

[0067] [ξ×(%Nb)] / {H×[(T 开轧 )+(T 终轧 )] / 2}≥1.9×10 -5 ,

[0068] Where, ξ is the cumulative reduction rate without recrystallization, in %;

[0069] H is the thickness of the finished steel plate, in mm;

[0070] T 开轧 is the rolling temperature before recrystallization, in °C;

[0071] T 终轧 is the final rolling temperature without recrystallization, in °C;

[0072] [(DI OL )×(%Cu)×(V c )×(T 开冷 )] / {[(T 回火 )-550]×(T 停冷 )×lg(t 保温 )}≥17.4,

[0073] Among them, DI OL is the online hardenability index,

[0074] DI OL =0.51×(%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, unit: mm;

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

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

[0077] T 回火 is the holding temperature of tempering heat treatment, unit: °C;

[0078] T 停冷 is the stop temperature of accelerated cooling, in °C;

[0079] t 保温 It is the tempering holding time, in min.

[0080] In the manufacturing method of the present invention:

[0081] According to the C, Mn, Nb and Ti content ranges, the slab heating temperature is controlled between 1030 and 1130° C., ensuring that all Nb in the steel is dissolved into austenite during the slab heating process and that the austenite grains of the slab do not grow abnormally.

[0082] The first stage of rolling is ordinary rolling, which uses the maximum rolling capacity of the rolling mill for continuous rolling, maximizing the production capacity of the rolling line while ensuring the recrystallization of the deformed steel billet and refining the austenite grains. The second stage adopts non-recrystallization controlled rolling. According to the above-mentioned Nb element content range in the steel, in order to ensure the non-recrystallization controlled rolling effect, the controlled rolling start temperature is controlled at 790-930°C, the rolling pass reduction rate is ≥7%, the cumulative reduction rate is ≥50%, and the final rolling temperature is 780-850°C.

[0083] After the controlled rolling is completed, the steel plate is immediately transported to the accelerated cooling equipment, and then the steel plate is accelerated cooled; the steel plate starting cooling temperature and the final rolling temperature are 740~820℃, the cooling rate is ≥5℃ / s, and the stopping cooling temperature is 250~500℃. When the steel plate thickness is ≥50mm, the steel plate is slowly cooled. The slow cooling process is to keep the temperature at not less than 250℃ for more than 24 hours, and then the steel plate is naturally air-cooled to room temperature.

[0084] According to the steel plate hardenability index DI, Cu content, TMCP process performance, especially the cooling stop temperature and steel plate thickness, the steel plate tempering temperature is adjusted to achieve a match between the steel plate hardenability, TMCP process performance and the tempering temperature, and to ensure that the steel plate obtains excellent strength-toughness / strength-plasticity matching; the steel plate tempering temperature (plate temperature) is 600-670°C, the tempering holding time is ≥3×H (H is the thickness of the finished steel plate), the unit is min, the tempering holding time is the holding time starting from when the furnace reaches the target temperature, and the steel plate is naturally air-cooled to room temperature after the tempering is completed.

[0085] In addition to meeting the above requirements, the following relationships must also be met:

[0086] ①[ξ×(%Nb)] / {H×[(T 开轧 )+(T 终轧 )] / 2}≥1.9×10 -5 .

[0087] As the cumulative reduction rate ξ of non-recrystallization increases and the Nb content increases, the flattening degree of austenite increases, the recovery recrystallization is more strongly inhibited during the rolling gap and cooling process, the deformation storage energy in austenite increases (that is, the higher the crystal defect density such as local dislocation density, deformation band density, deformation twin boundary density and grain boundary deformation step density in austenite), the lower the austenite stability, the more nucleation sites in austenite, the higher the nucleation rate, and the finer and more uniform the microstructure after accelerated cooling phase transformation; on the contrary, as the thickness H of the finished steel plate increases and the average non-recrystallization rolling temperature {(T 开轧 )+(T 终轧 )] / 2}, the lower the flattening degree of austenite, the smaller the degree of recovery recrystallization during rolling gap and cooling, the lower the deformation storage energy in austenite (that is, the lower the crystal defect density such as local dislocation density, deformation band density, deformation twin boundary density and grain boundary deformation step density in austenite), the higher the austenite stability, the fewer nucleation sites in austenite, the smaller the nucleation rate, and the coarser and more uneven the microstructure after accelerated cooling phase transformation.

[0088] The above analysis and research found that when the above formula is satisfied, within the thickness range of the steel plate of the present invention (16-100 mm), the microstructure of the steel plate can be ensured to be uniform and fine (≤20 μm), and the steel plate has excellent low-temperature toughness and fatigue resistance. This is one of the key core technologies of the present invention.

[0089] ②[(DI OL )×(%Cu)×(V c )×(T 开冷 )] / {[(T 回火 )-550]×(T 停冷 )×lg(t 保温 )}≥17.4.

[0090] With the online hardenability index DI OL The larger the value, the faster the cooling rate V c The larger the value, the faster the cooling start temperature T 开冷 The higher the temperature, the faster the cooling stop temperature T 停冷 The lower the bainite content of the low-temperature phase transformation structure, especially the lath bainite content, the higher the strength of the steel plate. As the Cu content of the steel grade increases, the more ε-Cu precipitates during the tempering of the steel plate after the TMCP process, and the higher the strength of the steel plate; as the tempering temperature increases and the tempering time increases, that is, [(T 回火 )-550] and the larger the lgt value (Note: 550℃ is the peak value of ε-Cu precipitation. When tempered at this temperature, the steel plate has the highest strength and the worst plasticity and toughness. Therefore, it is necessary to avoid tempering at this temperature and use over-aging temperature tempering), the more serious the Ostwald Ripening of the precipitated ε-Cu particles (the number of fine ε-Cu particles is greatly reduced, the number of coarse ε-Cu particles is greatly increased, and the number of precipitated ε-Cu particles is reduced), the strength of the steel plate decreases, while the low-temperature toughness and plasticity increase.

[0091] The above analysis and research found that when the above formula is satisfied, within the thickness range of the steel plate of the present invention (16-100mm), not only the steel plate has high strength, high toughness, excellent fatigue resistance and weldability, but also the welded HAZ has excellent low temperature toughness, ultra-low hardness and fatigue resistance under a wide range of heat input conditions. This is one of the key core technologies of the present invention.

[0092] Beneficial effects of the present invention:

[0093] The steel plate of the present invention is designed by matching the main alloy elements, microalloy elements and inclusion elements, and is combined with the TMCP+T manufacturing process to efficiently produce TMCP+T steel plates with excellent comprehensive performance. The steel plate of the present invention not only has high strength, high toughness and excellent fatigue resistance, but also has excellent weldability, does not require preheating before welding, and does not require heat treatment after welding. The welding heat affected zone (HAZ) has low hardness, high plastic toughness and fatigue resistance, ensuring the safety and reliability of the steel plate during processing, manufacturing and service. TMCP replaces the offline quenching process, realizing green and environmental protection in the process of steel plate manufacturing and use.

[0094] The high performance and high added value of the steel plate are mainly reflected in the perfect match of the high strength and high plasticity and toughness of the steel plate with excellent weldability and high performance of the welded joint (high plasticity and toughness, fatigue resistance), which successfully solves the following problems:

[0095] A) Low C / low carbon equivalent CEV and high strength;

[0096] B) High strength and ultra-low hardness of welded HAZ, high strength and excellent weldability;

[0097] C) The wide welding heat input and the excellent ultra-low temperature toughness and fatigue resistance of the welding HAZ conflict with each other in the composition design and TMCP+T process design, which are difficult to reconcile. This greatly improves the safety, stability and durability of large and heavy steel structures; good weldability (no preheating before welding and no heat treatment after welding) saves the user company's steel structure manufacturing cost and shortens the user's steel structure manufacturing time, creating huge value for the user. Therefore, this type of steel plate is not only a high value-added, low-carbon, green and environmentally friendly product throughout its life cycle. BRIEF DESCRIPTION OF THE DRAWINGS

[0098] Figure 1 This is a photo of the microstructure (1 / 4 thickness) of the steel in Example 3 of the present invention. DETAILED DESCRIPTION

[0099] The present invention will be further described below in conjunction with the embodiments and drawings.

[0100] The composition of the steel plate of the embodiment of the present invention is shown in Table 1, and the remainder includes Fe and unavoidable impurities. Tables 2 to 4 are the manufacturing process parameters of the steel of the embodiment of the present invention, and Table 5 is the performance parameters of the steel of the embodiment of the present invention.

[0101] Depend on Figure 1 It can be seen that the microstructure of the finished steel plate is uniform and fine ultra-low carbon massive ferrite, and the average grain size of the microstructure is below 20μm.

[0102] In summary, the steel plate of the present invention has excellent toughness and strength-plasticity matching, and more importantly, excellent weldability (no preheating before welding, no heat treatment after welding, excellent ultra-low temperature toughness of the HAZ of the welded joint, and excellent crack resistance and crack arrest properties), and the welded joint has a low degree of hardening (HV10≤260), and the welded joint has excellent fatigue resistance, eliminating the risk of fatigue fracture during the service of large-scale engineering machinery, improving safety and stability during service, and extending the service cycle life, thus achieving green and low-carbon throughout the life cycle of large-scale engineering machinery. The steel plate of the present invention is mainly used for ship structures, marine engineering structures, bridge engineering structures, energy development projects, offshore wind power projects, engineering machinery, mining machinery, and heavy vehicle manufacturing, and can achieve low-cost stable batch industrial production.

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Claims

1. High toughness, fatigue resistance and ultra-low welding heat affected zone hardness 700MPa grade steel plate, the weight percentage of its components is: C：0.04～0.08％ Si: ≤0.30% Mn: 0.45~0.85% P：≤0.013％ S：≤0.003％ Cu: 0.95~1.55% Ni: 1.10~1.80% Cr：0.60～1.00％ Mo: 0.05~0.35% Nb: 0.010~0.035% Ti: 0.008~0.016% Ca: 0.0010~0.0040% The balance includes Fe and other unavoidable inclusions; and the contents of the above elements must simultaneously satisfy the following relationship: Ni / Cu≥1.1; Control the ε-Cu precipitation toughening factor to 0.32≤λ≤1.45, λ=[3.72(%Cu)-2.16(%Cu) 2 )] / (%Ni); Ca treatment, Ca / S ratio controlled at 1.0-3.0, (%Ca)×(%S) 0.28 ≤1.5×10 －3 .

2. The high toughness, fatigue resistance and ultra-low weld heat affected zone hardness 700MPa steel plate according to claim 1, characterized in that: The balance is Fe and other inevitable inclusions.

3. The high toughness, fatigue resistance and ultra-low weld heat affected zone hardness 700MPa steel plate according to claim 1 or 2, characterized in that: The microstructure of the finished steel plate is uniform and fine ultra-low carbon massive ferrite, and the average grain size of the microstructure is less than 20μm.

4. The high toughness, fatigue resistance and ultra-low weld heat affected zone hardness 700MPa steel plate according to claim 1, 2 or 3, characterized in that: The yield strength of the finished steel plate is ≥630MPa, the tensile strength is ≥690MPa, the Charpy impact energy KV2 (single value) at -60℃ is ≥100J, the weldability is excellent, no preheating is required before welding, no heat treatment is required after welding, and the HAZ hardness HV10≤260 within the input range of 10-40kJ / cm.

5. The method for manufacturing a steel plate with high toughness, fatigue resistance and ultra-low weld heat affected zone hardness of 700 MPa as claimed in any one of claims 1 to 4, characterized in that: The steps include: 1) Smelting and casting Smelting and casting the composition according to claim 1 or 2 into a slab; 2) Slab heating The heating temperature is controlled at 1030~1130℃; 3) Rolling The first stage is ordinary rolling, which uses the maximum rolling capacity of the rolling mill for continuous rolling; The second stage adopts non-recrystallization controlled rolling, the starting rolling temperature is controlled at 790-930℃, the rolling pass reduction rate is ≥7%, the cumulative reduction rate is ≥50%, and the final rolling temperature is 780-850℃; 4) Cooling After the controlled rolling is completed, the steel plate is immediately transported to the accelerated cooling area for accelerated cooling; The steel plate start cooling temperature is 740~820℃, the cooling rate is ≥5℃ / s, and the stop cooling temperature is 250~500℃; when the steel plate thickness is ≥50mm, the steel plate is slowly cooled, and the slow cooling process is to keep the temperature at not less than 250℃ for more than 24 hours, and then the steel plate is naturally air-cooled to room temperature; 5) Tempering process The tempering temperature of the steel plate is 600-670℃, and the tempering holding time is ≥3×H, where H is the thickness of the finished steel plate in mm. The tempering holding time is the holding time starting from when the furnace reaches the target temperature. After the tempering is completed, the steel plate is naturally cooled to room temperature in air. In addition to meeting the above requirements, the following relationships must also be met: [ξ×(%Nb)] / {H×[(T 开轧 )+(T 终轧 )] / 2}≥1.9×10 -5 , Where, ξ is the cumulative reduction rate without recrystallization, in %; H is the thickness of the finished steel plate, in mm; T 开轧 is the rolling temperature before recrystallization, in °C; T 终轧 is the final rolling temperature without recrystallization, in °C; [(DI OL )×(%Cu)×(V c )×(T 开冷 )] / {[(T 回火 )-550]×(T 停冷 )×lg(t 保温 )}≥17.4, where DI OL is the online hardenability index, DI DI. OL 0.51×(%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, unit: mm; V c is the accelerated cooling rate of the steel plate, in °C / s; T 开冷 is the starting temperature of accelerated cooling, in °C; T 回火 is the holding temperature of tempering heat treatment, unit: °C; T 停冷 is the stop temperature of accelerated cooling, in °C; t 保温 It is the tempering holding time, in min.

6. The method for manufacturing a steel plate with high toughness, fatigue resistance and ultra-low weld heat affected zone hardness of 700 MPa as claimed in claim 5, characterized in that: The microstructure of the finished steel plate is uniform and fine ultra-low carbon massive ferrite, and the average grain size of the microstructure is less than 20μm.

7. The method for manufacturing a steel plate with high toughness, fatigue resistance and ultra-low weld heat affected zone hardness of 700 MPa as claimed in claim 5 or 6, characterized in that: The yield strength of the finished steel plate is ≥630MPa, the tensile strength is ≥690MPa, the Charpy impact energy KV2 (single value) at -60℃ is ≥100J, the weldability is excellent, no preheating is required before welding, no heat treatment is required after welding, and the HAZ hardness HV10≤260 within the input range of 10-40kJ / cm.

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