High-toughness, fatigue-resistant, and ultra-low-haz hardness 700 mpa grade steel plate and method of making same

Abstract

High toughness, fatigue resistance and ultra-low HAZ hardness 700MPa grade steel plate and its manufacturing method, the component weight percentage 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 contains Fe and inevitable inclusions, and Ni / Cu≥1.1, control ε-Cu precipitation strengthening and toughening factor 0.32≤λ≤1.45, Ti+Nb microalloying, Ca treatment, and Ca / S ratio control in 1.0-3.0 and (%Ca)×(%S) 0.28 ≤1.5×10 ‑3 Alloy combination design; optimize TMCP+T process, obtain high strength, high toughness and excellent weldability of base metal steel plate, under the condition of wide heat input range, the welded HAZ has excellent low temperature toughness, ultra-low hardness and fatigue resistance.

Description

TECHNICAL FIELD

[0001] The present application relates to high-strength steel, in particular to a 700MPa grade steel plate with high toughness, fatigue resistance and ultra-low hardness of welding heat affected zone and a manufacturing method thereof. BACKGROUND

[0002] It is well known that low-carbon (high-strength) low-alloy steel is one of the most important engineering structural materials, which is widely used in oil and gas pipelines, offshore platforms, shipbuilding, hydropower engineering, bridge structures, boiler vessels, 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 the strength, toughness, plasticity, weldability and their matching are the most important properties of low-carbon (high-strength) low-alloy steel, which are 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 for the plasticity and toughness of high-strength steel, especially the weldability and low hardness of welding HAZ, that is, the steel plate has the ability to resist brittle fracture and plastic instability fracture at ultra-low temperature conditions (under the condition of-60℃), while the steel plate has excellent weldability and low hardness of welding HAZ, can withstand wide heat input welding (welding heat input 10-40kJ / cm); and under the condition of relatively low manufacturing cost, the comprehensive mechanical properties and use performance of the steel plate are greatly improved to reduce the alloy content of the steel and save the cost, the high-strength lightweight reduces the self-weight, stability and safety of the steel structure, and more importantly, further improves the cold / heat workability of the steel structure and the safety and reliability during service.

[0004] At present, a research climax of developing a new generation of high-performance steel materials has been launched in Japan, Korea, the European Union and North America, trying to obtain better microstructure matching through alloy composition optimization, control of sub-microstructure 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 the high-strength steel obtains better strength-plasticity and plasticity-toughness matching, weldability and low anisotropy.

[0005] The existing high-strength quenched and tempered steel plate with tensile strength ≥700MPa is mainly produced by offline quenching and tempering process (RQ+T); but for steel plate thickness ≤40mm, online quenching and tempering process can also be used for production (DQ / TMCP+T); in order to obtain high strength, the steel plate must have high enough hardenability, that is, the hardenability index DI of the steel plate ≥2.0×the thickness of the finished steel plate 〖DI=0.311(%C) 1 / 2[(1 + 0.64 (%Si)] x [(1 + 4.10 (%Mn)] x [(1 + 0.27 (%Cu)] x [(1 + 0.52 (%Ni)] x [(1 + 2.33 (%Cr)] x [(1 + 3.14 (%Mo)] x 25.4 (mm) to ensure that the steel plate has sufficiently high strength and excellent low-temperature toughness, thus inevitably adding a certain amount of alloying elements such as Cr, Mo, Ni, Cu, V, etc. to the steel, even adding Cr + Mo content to more than 0.70%, resulting in a high carbon equivalent and cold crack sensitivity index of the steel plate, which seriously affects the weldability of the steel plate (see Chinese patents ZL201210077114.7, ZL201010113835.X, ZL200810042088.8, ZL200810042124.0).

[0006] In addition, the surface (subsurface) layer of the steel plate with high alloy content is prone to over-quenching, forming coarse martensite structure, which seriously deteriorates the low-temperature toughness, elongation and fatigue expansion resistance of the surface (near-surface) layer of the steel plate (see “Electric Power Civil Engineering” (Japanese), 1986, Vol. 201, P33; “Iron and Steel”, 1986, Vol. 72, S612; “Iron and Steel”, 1986, Vol. 72, S614; “Iron and Steel”, 1985, Vol. 71, S1523; “Iron and Steel”, 1986, Vol. 72, S615; “Iron and Steel”, 1986, Vol. 73, S1398; “Kawasaki Iron and Steel Technical Report” (Japanese), 1988, Vol. 20, P233; “Iron and Steel Research” (Japanese), 1986, Vol. 322, P99; “CAMP-ISIJ” (Japanese), 1989, Vol. 3, P207; “NKK Technical Report” (Japanese), 1990, Vol. 133, P37; “Electric Power Civil Engineering” (Japanese), 1994, Vol. 249, P1; “Sumitomo Metal” (Japanese), 1995, Vol. 47, P1; “Nishiyama Memorial Technical Lecture” 191-192, 2008, P162).

[0007] There are a large number of existing patent literatures only explain how to realize the strength and low temperature toughness of the base steel plate, and there are few explanations on how to improve the weldability of the steel plate to obtain excellent low temperature toughness of the heat affected zone (HAZ), and there is no mention of how to improve the tensile elongation of the steel plate, the uniformity of the mechanical properties in the thickness direction, the excellent mechanical processing performance and the anti-fatigue crack propagation characteristics while improving the tensile strength of the steel plate (see Japanese Patent Nos. 63-93845, 63-79921, 60-258410, 4-285119, 4-308035, 3-264614, 2-250917, 4-143246, US Patent No. 5798004, European Patent No. EP 0288054A2, and Xiyama Memorial Technology Lecture, pp. 159-160, P79-P80).

[0008] China Baosteel has developed a series of high toughness and excellent weldability 600MPa quenched and tempered steel plates (see Chinese Patent Nos. ZL201210077114.7, ZL201110181293.4, ZL201010113835.X, ZL200910196233.2, ZL200810042088.8, ZL200810042124.0). The research and development of these steel grades mainly solves the problems of (super) high toughness, excellent weldability, low cost manufacturing, specification expansion of thick steel plates, and uniformity of properties in the thickness direction of thick steel plates, etc. of 600MPa quenched and tempered steel plates, and significant breakthroughs and good results have been achieved. The steel plates have achieved batch industrial production, and have been successfully applied to major engineering construction and major equipment manufacturing at home and abroad, achieving good supply performance, replacing imported products for some grades, filling the domestic gap, especially the successful development of thick 600MPa quenched and tempered steel plates resistant to strain aging embrittlement and eliminating residual stress annealing embrittlement (see Chinese Patent No. ZL201610463494.6). Not only excellent low temperature toughness and anti-SR embrittlement softening characteristics are obtained, but also the weakness of poor intrinsic fatigue resistance of high strength quenched and tempered steel plates is solved.

[0009] China Baosteel applied for a 700MPa low temperature quenched and tempered steel plate with excellent weldability and fatigue resistance and its manufacturing method in 2019 (see Chinese Patent No. ZL201910150314.2), which successfully solved the problems of poor fatigue resistance, high sensitivity to cold cracking of the quenched and tempered steel plate, and easy embrittlement of the welded joint. It has been successfully applied to 30-75M 3 giant excavators, 600-1500 ton giant cranes, and port machinery, etc., achieving import substitution (replacing imported products from SSAB), and strongly supporting major equipment manufacturing in China; however, the steel plate cannot withstand high heat input welding, and the hardness of the welded joint is relatively high (HV10≥300), which leads to poor low temperature fatigue resistance of the welded joint and the risk of fracture failure during service. SUMMARY

[0010] The present application aims to provide a high-toughness, fatigue-resistant and ultra-low HAZ hardness 700MPa grade steel plate and a manufacturing method thereof, which has high strength, high toughness and excellent weldability of the base steel plate, and has excellent low-temperature toughness, ultra-low hardness and fatigue resistance of the HAZ under a wide heat input range condition, with a yield strength ≥630MPa, a tensile strength ≥690MPa, a Charpy impact energy KV2 (single value) ≥100J at -60℃, excellent weldability (no preheating before welding and no heat treatment after welding) and ultra-low HAZ hardness (HV10 ≤260) in a wide heat input range (10-40kJ / cm); and is particularly suitable for ice sea area icebreaker hulls, offshore platforms, sea-crossing bridges, offshore wind pile legs and wind tower structures, port machinery, etc., and can realize low-cost stable batch industrial production.

[0011] The high-toughness, ultra-low HAZ hardness 700MPa grade steel plate is one of the most difficult products in thick plate products, because such steel plates not only require low C, low carbon equivalent CEV, high strength, excellent ultra-low temperature toughness and fatigue resistance, but also have to withstand a wide welding heat input range, have excellent ultra-low temperature toughness of the HAZ, and achieve ultra-low hardness of the HAZ under a wide welding heat input condition; these performance requirements are not only difficult to meet simultaneously, but also some of the performance exist in a mutually contradictory situation: 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 HAZ; C) the contradiction between high strength and excellent weldability; D) the contradiction between a wide welding heat input and excellent ultra-low temperature toughness and fatigue resistance of the HAZ.

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

[0013] The present application comprehensively considers the key factors affecting high strength, high toughness, high toughness and ultra-low hardness of HAZ in wide heat input welding of steel plate, such as key factors, from the design of alloy combination, uses the ultra-low carbon C-low Si-low Mn low alloy steel composition system as the basis, low carbon and low carbon equivalent, high Cu+high Ni alloying and Ni / Cu≥1.1, control εCu precipitation strengthening and toughening factor 0.32≤λ≤1.45, Ti+Nb micro-alloying, Ca treatment and Ca / S ratio control between 1.0-3.0 and (%Ca)×(%S) 0.28 ≤1.5×10 -3 Etc. Alloy combination design; optimization of TMCP+T process: [ξ×(%Nb)] / {H×[(T 开轧 )+(T 停冷 )] / 2}≥1.9×10 -5 , [(D 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 blocky ferrite, the average grain size of the microstructure is below 20μm, while obtaining high strength, high toughness and excellent weldability of the base material steel plate, the HAZ has excellent low temperature toughness, ultra-low hardness and fatigue resistance under the condition of wide heat input range.

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

[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 comprising Fe and other inevitable impurities; and the above element contents must simultaneously satisfy the following relationships:

[0028] Ni / Cu≥1.1;

[0029] controlling the ε-Cu precipitation strengthening factor 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] Further, the balance is Fe and other inevitable impurities.

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

[0034] The yield strength of the finished steel plate of the present application is ≥630MPa, the tensile strength is ≥690MPa, the Charpy impact energy at -60℃ is ≥100J (single value), the weldability is excellent, no preheating before welding and no heat treatment after welding are required, and the HAZ hardness HV10 in the input range of 10-40kJ / cm is ≤260.

[0035] In the component design of the steel plate described in the present application:

[0036] In order to obtain the steel plate with yield strength ≥630MPa, tensile strength ≥690MPa, and Charpy impact energy at -60℃ ≥100J (single value), and to achieve equal strength and equal toughness of the HAZ and the base steel plate while the HAZ hardness HV10 is ≤260, the TMCP+T steel plate chemical composition of the present application has the following characteristics:

[0037] C has a great influence on the strength, low temperature toughness, elongation and weldability, especially the hardness of the welded HAZ of the 700 MPa grade TMCP + T steel plate. In order to improve the low temperature toughness, weldability and the welded HAZ, the C content in the steel should be controlled to be relatively low. However, the C content should not be controlled to be too low from the perspective of the strength, fatigue resistance and the microstructure control and manufacturing cost. Too low C content not only leads to high grain boundary migration rate, coarse and mixed grain of the base steel plate and the welded HAZ and softening of the welded HAZ, but also causes grain boundary weakening and serious deterioration of the low temperature toughness of the base steel plate and the welded HAZ. Therefore, the reasonable range of the C content is 0.04-0.08%.

[0038] Si promotes the deoxidation of the molten steel and can improve the strength of the steel plate. However, the deoxidation effect of Si is not great when Al is used for deoxidation. Although Si can improve the strength of the steel plate, it seriously damages the low temperature toughness, elongation and weldability of the steel plate, especially for the 700 MPa grade high strength steel with high alloy content. Si not only promotes the formation of M-A islands in the welded HAZ, but also causes the M-A islands to be coarse and unevenly distributed, which seriously damages the low temperature toughness of the welded HAZ and hardens the HAZ to deteriorate the fatigue resistance. Therefore, the Si content in the steel should be controlled to be as low as possible. Considering the economy and operability of the steelmaking process, the Si content is controlled to be below 0.30%.

[0039] Mn is the most important alloying element in steel. In addition to improving the strength of the steel plate, Mn also has the effects of expanding the austenite phase region, reducing the Ar3 point temperature, refining the grains of the TMCP + T steel plate to improve the strength of the steel plate (fine grain strengthening effect), improving the low temperature toughness and fatigue resistance of the steel plate, promoting the formation of low temperature phase change structure (phase change strength effect) to improve the strength of the steel plate. However, Mn is prone to segregation during the solidification of the molten steel. When the Mn content is high, not only is the casting operation difficult, but also conjugate segregation phenomenon is prone to occur with C, P, S and other elements, especially when the C content in the steel is high, the segregation and porosity in the center of the casting blank are aggravated. Severe center area segregation of the casting blank is prone to form abnormal structure in the subsequent TMCP and welding process, which leads to low low temperature toughness and elongation of the steel plate. In addition, for the 700 MPa grade steel plate with high alloy content, too high Mn content not only causes the low temperature toughness, elongation and weldability of the steel plate to deteriorate sharply, but also leads to a sharp increase in the hardness of the welded HAZ of the steel plate, which causes the cold crack sensitivity of the welded joint to increase sharply and the fatigue resistance to deteriorate seriously. Therefore, the suitable Mn content is 0.45-0.85%.

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

[0041] S is a harmful inclusion in steel, which 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 relatively high, S and Cu easily form low melting point compounds at the grain boundary, which seriously deteriorate the toughness and elongation of the steel plate, and more importantly, seriously deteriorate the plasticity and toughness of the welded joint (fusion line, HAZ). At the same time, S is also the main element that causes hot shortness during hot rolling. In theory, the lower the better; but considering the steelmaking operability, steelmaking cost and smooth logistics principle, for steel plates requiring-60°C toughness, high strength and low hardness of the welded HAZ, the S content needs to be controlled at ≤0.003%.

[0042] Cu is also an austenite stabilizing element, and adding Cu can also lower 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, through ε-Cu precipitation, the steel plate is strengthened, 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 700 MPa, the lower limit of Cu content in steel cannot be lower than 0.95%. However, too much Cu addition not only easily causes copper brittleness, surface cracking and internal cracking of the casting blank, and especially the low temperature toughness, fatigue resistance and welded joint performance of thick steel plates are seriously deteriorated, and the upper limit of Cu is controlled at 1.55%. Therefore, the Cu content is controlled between 0.95-1.55%.

[0043] The addition of Ni can not only reduce the lattice friction of BCC crystal structure (i.e. P-N force), improve the low-temperature dislocation mobility of ferrite phase, promote dislocation cross-slip, and improve the intrinsic plasticity and toughness of ferrite; in addition, as a strong austenite stabilizing element, Ni can greatly reduce the Ar1 and Ar3 point temperatures, improve the driving force of austenite to ferrite phase transformation, cause the austenite to transform at a lower temperature, greatly refine the microstructure of TMCP+T steel plate, increase the expansion resistance of cracks through ferrite grains, and greatly improve the low-temperature toughness of TMCP+T steel plate, so Ni has the effect of simultaneously improving the strength and low-temperature toughness of TMCP+T steel plate without reducing the elongation (i.e. plasticity and toughness); the addition of Ni in 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 within a certain range, the better; however, too high a Ni content not only hardens the welding heat-affected zone, which is unfavorable for the weldability and toughness of the steel plate, but also greatly increases the alloy cost of the steel plate (Ni is a valuable alloying element). Considering the high Cu content of the precipitation-strengthening steel plate, the Ni / Cu must be ≥1.1; therefore, the Ni content is controlled between 1.10-1.80%.

[0044] As 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 misorientation between bainite laths (i.e. phase transformation variant) and increasing the resistance of cracks to pass through the bainite block structure, thereby improving the plasticity and toughness, and anti-cracking and crack-stopping properties of the steel plate while improving the strength of the steel plate; however, when the Cr addition is too much, it seriously damages the weldability of the steel plate, hardens and embrittles the HAZ, and causes the low-temperature toughness and anti-fatigue properties of the HAZ to deteriorate seriously; however, for 700MPa grade high-strength steel plate, a certain amount of Cr must be added to ensure that the steel plate has sufficient hardenability; therefore, the Cr content is controlled between 0.60-1.00%.

[0045] The addition of Mo can greatly improve the hardenability of the steel plate, promote the formation of low-temperature phase transformation structures such as bainite, improve the tempering properties and tempering process window of the steel plate, and improve the strength and plasticity match of the steel plate 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 of the steel plate, the elongation and weldability of the steel plate, worsens the low-temperature toughness of the HAZ, increases the hardness of the HAZ, and deteriorates the anti-fatigue properties of the HAZ, but also greatly increases the production cost; therefore, considering the phase transformation strengthening effect of Mo, the influence on the low-temperature toughness, elongation, weldability of the base steel plate, the low-temperature toughness and anti-fatigue properties (low HAZ hardness) of the HAZ, the Mo content is controlled between 0.05-0.35%.

[0046] The purpose of adding trace Nb element in steel is to control rolling without recrystallization, refine grain size of steel plate, improve strength and toughness of TMCP+T steel plate. When the content of Nb is less than 0.010%, the effect of controlling rolling without recrystallization is not obvious, and the ability of improving strength and toughness of TMCP+T steel plate is not enough. When the content of Nb is more than 0.035%, the alloy cost of steel plate is high, and more importantly, the brittleness and hardening caused by upper bainite (Bu) and Nb(C,N) secondary precipitation during welding process seriously damage the low-temperature toughness and anti-fatigue properties of HAZ. Therefore, the content of Nb is controlled between 0.010% and 0.035%.

[0047] The purpose of adding trace Ti element in steel is to combine with N in steel to generate TiN particles with high stability, and to inhibit the grain growth of steel plate and HAZ. The content of Ti added in steel should match the content of N in steel. The matching principle is that TiN should not precipitate in liquid steel but should precipitate in solid phase. Therefore, the precipitation temperature of TiN should be ensured to be lower than 1400℃. When the content of Ti added in steel is too low (<0.008%), the number of TiN particles formed is insufficient to inhibit the austenite grain growth during TMCP and welding thermal cycle and to improve the low-temperature toughness and weldability of steel plate. When the content of Ti added is too high (>0.016%), the precipitation temperature of TiN exceeds 1400℃, and some TiN particles precipitate in large size during the solidification of steel liquid. The large-size TiN particles not only cannot inhibit the grain growth, but also become the starting point of crack initiation. Therefore, the optimal control range of Ti content is 0.008% to 0.016%.

[0048] Ca treatment is performed on the steel. On the one hand, the steel liquid can be further purified. On the other hand, the sulfides in the steel are modified to become stable and small spherical sulfides that cannot be deformed, the hot brittleness of S is inhibited, the low-temperature toughness, elongation and Z-direction performance of the steel plate are improved, the anisotropy of strength and toughness and weldability of the steel plate are improved, and the pouring of high-acid-soluble aluminum steel liquid is improved. The amount of Ca added depends on the content of S in the steel. If the amount of Ca added is too low, the treatment effect is not good. If the amount of Ca added is too high, the size of Ca(O,S) is too large, the brittleness is increased, and the low-temperature toughness, elongation and weldability of the steel plate are reduced. At the same time, the purity of the steel is reduced and the steel liquid is contaminated. Generally, the content of Ca is controlled according to ESSP=(wt%Ca)[1-1.24(wt%O)] / 1.25(wt%S), wherein ESSP is the shape control index of sulfide inclusions, 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 component design of the present application also particularly requires:

[0050] Ni / Cu≥1.15, adding a certain proportion of Ni element in high Cu content steel plate, not only can refine, homogenize the microstructure of steel plate, improve the intrinsic plasticity and toughness of steel plate, more importantly, inhibit the Cu segregation at the original austenite grain boundary and cause the weakening, brittle mother steel plate and the low temperature toughness and fatigue resistance of welded HAZ.

[0051] Control the ε-Cu precipitation strengthening and toughening factor 0.32≤λ≤1.45, after TMCP process, the ε-Cu in the steel is precipitated in a fine and dispersed state by high temperature tempering, which greatly improves the strength of the steel plate and realizes the precipitation strengthening of the steel plate; but the ε-Cu precipitation will cause the serious deterioration of the plasticity and toughness of the steel plate, so it is necessary to improve the intrinsic of the steel plate, by controlling the ε-Cu precipitation strengthening and toughening factor in the appropriate range, not only can the steel plate be precipitation strengthened to meet the strength level of 700MPa, but also can improve the intrinsic plasticity and toughness of the steel plate, inhibit the Cu segregation at the grain boundary and eliminate the grain boundary embrittlement of high Cu steel, realize the matching of strength and toughness, and strength and plasticity. Wherein λ=[3.72(%Cu)-2.16(%Cu) 2 )] / (%Ni); this is one of the key core technologies of the present application.

[0052] Ca treatment and Ca / S ratio control in 1.0-3.0 and (%Ca)×(%S) 0.28 ≤1.5×10 －3 : ensure sulfidation spheroidization and minimize the effect of inclusions on low temperature toughness and weldability, and Ca(O, S) particles are uniformly and finely distributed in the steel, which inhibits the growth of austenite grains in the heat affected zone during welding process, and improves the low temperature toughness and fatigue resistance of the steel plate in the welding heat affected zone.

[0053] The component data in the above relationship is calculated in percentage, for example, if the carbon content is 0.10%, the relationship is calculated by using 0.10 in the calculation.

[0054] The manufacturing method of the high toughness, fatigue resistant and ultra-low welding heat affected zone hardness 700MPa grade steel plate disclosed by the present application comprises the following steps:

[0055] 1) smelting and casting

[0056] Smelting and casting into a slab according to the above components;

[0057] 2) slab heating

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

[0059] 3) rolling

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

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

[0062] 4) Cooling

[0063] After controlled rolling, the steel plate is immediately transported to the accelerated cooling area for accelerated cooling. The initial cooling temperature of the steel plate is 740-820℃, the cooling rate is ≥5℃ / s, and the cooling stop temperature is 250-500℃. When the thickness of the steel plate is ≥50mm, the steel plate is subjected to slow cooling. The slow cooling process involves holding the plate at a temperature of not less than 250℃ for at least 24 hours, and then allowing the steel plate to air cool naturally 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 that starts when the target temperature is reached in the furnace. After tempering, the steel plate is naturally air-cooled to room temperature.

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

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

[0068] Wherein, ξ is the cumulative reduction rate without recrystallization, in percentage (%).

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

[0070] T 开轧 This is the pre-recrystallization rolling temperature, expressed in °C.

[0071] T 终轧 This refers to the final rolling temperature before recrystallization, expressed 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 For online hardenability index,

[0074] DI OL = 0.51 x (%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)] x 25.4, unit mm;

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

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

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

[0078] T 停冷 is the stopping temperature of the accelerated cooling, unit is °C;

[0079] t 保温 is the holding time of the tempering, unit is min.

[0080] In the manufacturing method described in the present application:

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

[0082] The first stage of rolling is normal rolling, which uses the maximum rolling capacity of the rolling mill to continuously roll, maximizes the rolling line capacity, and ensures that the deformed slab recrystallizes and the austenite grains are refined. The second stage uses non-recrystallization controlled rolling, and according to the content range of Nb in the above steel, in order to ensure the effect of non-recrystallization controlled rolling, the controlled rolling starting temperature is controlled at 790-930°C, the rolling pass reduction rate is ≥7%, the cumulative reduction rate is ≥50%, and the finish 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 immediately subjected to accelerated cooling; the steel plate open cooling temperature is the finish rolling temperature of 740-820°C, the cooling rate is ≥5°C / s, and the stop cooling temperature is 250-500°C. When the thickness of the steel plate is ≥50mm, the steel plate is subjected to slow cooling, and the slow cooling process is to keep the temperature at not less than 250°C for more than 24 hours, and then the steel plate is naturally air cooled to room temperature.

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

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

[0086] 1.9×10 开轧 . 终轧 . -5 .

[0087] With the increase of the unrecrystallized cumulative reduction rate ξ and the Nb content, the degree of austenite flattening is greater, the inhibition of the recovery and recrystallization in the rolling gap and the cooling process is stronger, the deformation storage energy in the austenite is higher (i.e. the dislocation density, deformation band density, deformation twin boundary density and grain boundary deformation step density and other crystal defect densities in the austenite are higher), the austenite stability is lower, the nucleation sites in the austenite are more and the nucleation rate is greater, after the accelerated cooling phase change, the microstructure is more fine and uniform; on the contrary, with the increase of the thickness H of the finished steel plate and the unrecrystallized rolling average temperature {(T 开轧 )+(T 终轧 ) / 2}, the degree of austenite flattening is lower, the recovery and recrystallization degree in the rolling gap and the cooling process is smaller, the deformation storage energy in the austenite is lower (i.e. the dislocation density, deformation band density, deformation twin boundary density and grain boundary deformation step density and other crystal defect densities in the austenite are lower), the austenite stability is higher, the nucleation sites in the austenite are less and the nucleation rate is smaller, after the accelerated cooling phase change, the microstructure is more coarse and non-uniform.

[0088] It is found through the comprehensive analysis and research that when the above formula is met, within the thickness range (16-100 mm) of the steel plate of the present application, 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 application.

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

[0090] DI OL the greater, the accelerated cooling speed V c the greater, the accelerated cooling start temperature T 开冷 the higher, the accelerated cooling stop temperature T 停冷 the lower, the low-temperature phase transition structure bainite content, especially the higher the lath bainite content, the higher the strength of the steel plate. With the higher Cu content of the steel grade, the more the ε-Cu precipitates during the tempering process after the TMCP process, the higher the strength of the steel plate; with the increase of the tempering temperature and the extension of the tempering time, that is, the greater the value of [(T 回火 )-550] and lgt (note: 550℃ is the ε-Cu precipitation peak, at this temperature, the steel plate has the highest strength and the worst plasticity and toughness, so this temperature should be avoided for tempering, and overaging temperature tempering should be used), 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 is reduced, and the low-temperature toughness and plasticity are increased.

[0091] It is found through comprehensive analysis and research that when the above formula is satisfied, within the thickness range of the steel plate of the present application (16-100mm), not only does the steel plate ensure high strength, high toughness, and 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 application.

[0092] The beneficial effects of the present application are:

[0093] The steel plate of the present application is designed by matching and combining the main alloying elements, micro-alloying elements and inclusion elements, and combined with the TMCP+T manufacturing process, to efficiently produce TMCP+T steel plates with excellent comprehensive performance. The steel plate of the present application not only has high strength, high toughness and excellent fatigue resistance, but also has excellent weldability, without preheating before welding and without heat treatment after welding, and the welded heat-affected zone (HAZ) has low hardness, high plasticity and toughness, and fatigue resistance, ensuring the safety and reliability of the steel plate during processing and service. The use of TMCP instead of offline quenching process realizes green environmental protection during steel plate manufacturing and use.

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

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

[0096] B) high strength with ultra-low hardness in the HAZ of welding, high strength with excellent weldability;

[0097] C) the problem of conflicting between wide welding heat input and excellent ultra-low temperature toughness and fatigue resistance in the HAZ of welding in the composition design and TMCP+T process design, which is difficult to reconcile, greatly improves the safety, stability and durability of large heavy steel structures; good weldability (no preheating before welding and no heat treatment after welding) saves the cost and shortens the time of steel structure manufacturing for users, creating great value for users, so such steel plates are not only high value-added, low-carbon and green products in the whole life cycle. BRIEF DESCRIPTION OF DRAWINGS

[0098] Figure 1 The photograph of the microstructure (1 / 4 thickness) of the steel of Example 3 of the present application. DETAILED DESCRIPTION

[0099] The present application will be further described below in combination with examples and drawings.

[0100] The composition of the steel plate embodiment of the present application is shown in Table 1, and the remaining amount comprises Fe and inevitable impurities. Tables 2-4 are the manufacturing process parameters of the steel of the present application, and Table 5 is the performance parameters of the steel of the present application.

[0101] From the above, it can be seen that the microstructure of the finished steel plate is uniform and fine ultra-low carbon blocky ferrite, and the average grain size of the microstructure is less than 20 μm. Figure 1 As can be seen, the microstructure of the finished steel plate is uniform and fine ultra-low carbon blocky ferrite, and the average grain size of the microstructure is less than 20 μm.

[0102] In summary, the steel plate of the present application not only has excellent strength and toughness and strength-plasticity matching, but more importantly, the steel plate has excellent weldability (no preheating before welding and no heat treatment after welding, excellent ultra-low temperature toughness in the HAZ of welding joint, and excellent anti-cracking and crack-stopping properties), and the hardening degree of the welding joint is low (HV10≤260), the welding joint has excellent fatigue resistance, eliminates the risk of fatigue fracture during the service of large engineering machinery, improves the safety and stability during the service, prolongs the service cycle life, and realizes the green and low-carbon of the whole life cycle of the large engineering machinery. The steel plate of the present application is mainly used for ship structures, ocean engineering structures, bridge engineering structures, energy development engineering, offshore wind power engineering and engineering machinery, mine machinery and heavy vehicle manufacturing, etc., and can realize low-cost and stable batch industrial production.

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Claims

1. High-toughness, fatigue-resistant, and ultra-low weld heat-affected zone hardness grade 700MPa steel plate, with the following composition by weight percentage: 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 consists of Fe and other unavoidable inclusions; and the contents of the above elements must simultaneously satisfy the following relationship: Ni / Cu ≥ 1.1; The toughening factor for ε-Cu precipitation was controlled to be 0.32 ≤ λ ≤ 1.

45. λ=[3.72(%Cu)-2.16(%Cu) 2 )] / (%Ni); Ca treatment, with the Ca / S ratio controlled between 1.0 and 3.0, (%Ca) × (%S) 0.28 ≤1.5×10 －3 ; The microstructure of the finished steel plate is uniform and fine ultra-low carbon blocky ferrite, with an average grain size of less than 20 μm. The finished steel plate has a yield strength ≥630MPa, a tensile strength ≥690MPa, a Charpy impact energy KV2 value ≥100J at -60℃, excellent weldability, requires no preheating before welding and no heat treatment after welding, and has a HAZ hardness HV10 ≤260 within the input range of 10-40kJ / cm.

2. The manufacturing method of the high-toughness, fatigue-resistant, and ultra-low weld heat-affected zone hardness 700MPa grade steel plate as described in claim 1, characterized in that, Includes the following steps: 1) Smelting and casting The components described in claim 1 are smelted and cast into slabs; 2) Slab heating The heating temperature is controlled between 1030 and 1130℃; 3) Rolling The first stage is ordinary rolling, which is continuous rolling using the maximum rolling capacity of the rolling mill; The second stage adopts non-recrystallization controlled rolling, with the initial rolling temperature controlled at 790-930℃, the rolling pass reduction rate ≥7%, the cumulative reduction rate ≥50%, and the final rolling temperature 780-850℃. 4) Cooling After controlled rolling, the steel plate is immediately transported to the accelerated cooling area for accelerated cooling. The initial cooling temperature of the steel plate is 740-820℃, the cooling rate is ≥5℃ / s, and the cooling stop temperature is 250-500℃. When the thickness of the steel plate is ≥50mm, the steel plate is slowly cooled. The slow cooling process involves holding the plate at a temperature of not less than 250℃ for at least 24 hours, and then allowing the steel plate to air cool naturally 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 when the target temperature is reached in the furnace, and the steel plate is naturally air-cooled to room temperature after tempering. In addition to meeting the above requirements, the following relationship must also be satisfied: [ξ×(%Nb)] / {H×[(T 开轧 )+(T 终轧 )] / 2}≥1.9×10 -5 , in, ξ represents the cumulative reduction rate without recrystallization, in percentage (%). H represents the thickness of the finished steel plate, in mm; T 开轧 This is the pre-recrystallization rolling temperature, expressed in °C. T 终轧 This is the final rolling temperature before 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 For online hardenability index, 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 The unit for accelerating the cooling rate of the steel plate is ℃ / s; T 开冷 The starting temperature for accelerated cooling, measured in °C; T 回火 The holding temperature for tempering heat treatment, in °C; T 停冷 The stopping temperature for accelerated cooling, measured in °C; t 保温 This is the tempering holding time, in minutes.

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