Ultra-high toughness, low yield ratio and low longitudinal and transverse strength anisotropy 1000MPa grade steel plate and its manufacturing method

Abstract

Ultra-high toughness, low yield ratio and low longitudinal and transverse strength anisotropic 1000MPa grade steel plate and its manufacturing method, adopt low C-low Si-medium Mn low alloy steel design, appropriately increase the acid-soluble Als content in the steel, (%Als) / [(%N)-0.292(%Ti)]≥43, (%C)×[(6.53(%Si)+5.17(%Als)+0.71(%Mn)]≤0.215, low Pcm, ultra-low N content, (Cu+high Ni+Mo) alloying, (Ti+Nb+B) microalloying, Ca treatment, Ca / S ratio 1.0~3.0, Ca×S 0.28 ≤1.0×10 －3 The optimized combination design of alloys and the optimized online quenching and tempering process DQ are adopted. The microstructure of the finished steel plate is low-carbon bainite + a small amount of dispersed low-carbon lath martensite. The average grain size of the microstructure is below 20μm. While obtaining the ultra-high strength, ultra-high toughness, low yield ratio and low longitudinal / transverse strength anisotropy of the parent steel plate, the steel plate has excellent weldability and the low-temperature toughness of the welding heat affected zone HAZ is also excellent.

Description

Technical Field

[0001] The present invention relates to the field of low alloy steel, and in particular to a 1000MPa grade steel plate with ultra-high toughness, low yield ratio and low longitudinal and transverse strength anisotropy 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 for the toughness, plasticity, especially weldability and low anisotropy of high-strength steel, that is, the steel plate has the ability to resist brittle fracture and plastic instability fracture under ultra-low temperature conditions (-60℃ and below), while the elongation at fracture and uniform elongation reach the level of 800MPa grade steel plate, and the longitudinal and transverse strength anisotropy is low, the steel plate has excellent weldability, the welding preheating temperature is reduced (≤75℃), and no heat treatment is required after welding; and under the conditions of lower alloy content, especially the content of precious metals Cu, Ni and Mo (compared with offline quenching and tempering process), and relatively low manufacturing cost, the comprehensive performance of the steel plate is greatly improved. Mechanical properties and performance in use, to reduce the amount of alloy used in steel to save costs, high strength and lightweight to reduce the weight, stability and safety of steel components, and more importantly, to further improve the cold / hot workability of steel components and safety and reliability during service; currently, a research boom has been set off in Japan, South Korea, the European Union and North America to develop a new generation of high-performance steel materials, trying 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) fine structure, so that high-strength steel can obtain better strength-plasticity and plastic-toughness matching, weldability and low anisotropy.

[0004] Existing high-strength steel plates with tensile strength ≥950MPa are mainly produced by offline quenching and tempering process (i.e. RQ+T); however, for steel plates with thickness ≤60mm, online quenching and tempering process (i.e. DQ+T) can also be used for production; in order to obtain ultra-high strength, the steel plate must have sufficiently high hardenability and hardenability, that is, the steel plate hardenability index DI ≥3.50×the thickness of the finished steel plate〖DI=0.311(%C) 1 / 2[(1+0.64(%Si)]×[(1+4.10(%Mn)]×[(1+0.27(%Cu)]×[(1+0.52(%Ni)]×[(1+2.33(%Cr)]×[(1+3.14(%Mo)]×25.4(mm)〗, in order to ensure that the steel plate has sufficiently high strength, excellent low temperature toughness and uniform microstructure and properties along the thickness direction, it is inevitable to add a large amount of C and Cr to the steel. , Mo, Ni, Cu, V and other alloying elements, especially a large amount of precious alloying elements such as Cu, Ni, Mo and V, not only lead to high alloy costs of steel plates, but also cause high carbon equivalent and cold crack sensitivity index of steel plates, which seriously affect the weldability of steel plates; in addition, the surface (sub) layer of steel plates with high carbon and high alloy content is prone to over-quenching, forming a coarse martensitic structure, which seriously deteriorates the low-temperature toughness and elongation of the surface (near) layer of the steel plate. (See [Electric Power Civil Engineering (Japan) Article), 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 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 Bulletin (Japanese), 1990, Vol.133, P37; Electric Power and Civil Engineering (Japanese), 1994, Vol.249, P1; Sumitomo Metal (Japanese), 1995, Vol.47, P1; Nishiyama Memorial Technical Lecture 191-192, 2008, P162】).

[0005] Low elongation and low-temperature toughness are not only not conducive to the cold and hot processing performance of steel plates, but also have a great impact on the fatigue resistance, stress concentration sensitivity, crack resistance and structural stability of steel plates; when used on fatigue heavy-load structures such as pressure steel pipes, steel branch pipes and volutes in hydropower projects, thermal power steam turbine generators and offshore oil platform structures, there are great safety hazards; therefore, when large fatigue heavy-load steel structures use ultra-high-strength steel, it is generally hoped that 1000MPa-grade high-strength steel not only has excellent strength and toughness, strong-plastic matching and weldability, but also the elongation is ensured to be above 14% to ensure the processing performance and fatigue resistance of the steel plate.

[0006] A large number of existing patents and technical documents only explain how to achieve the strength and low-temperature toughness of the parent steel plate, but there are few explanations on improving the welding performance of the steel plate and obtaining excellent low-temperature toughness of the HAZ of the welded heat-affected zone. There is no mention of how to improve the tensile strength of the steel plate while improving the tensile elongation and uniformity of mechanical properties in the thickness direction of the steel plate, and there is no indication of how to prevent the surface (sub) layer of the steel plate from being overquenched. (See Japanese Patents Sho 63-93845, Sho 63-79921, Sho 60-258410, Te Ping Kai 4-285119, Te Ping Kai 4-308035, Ping 3-264614, Ping 2-250917, Ping 4-143246, US Patent USPatent 5798004, European Patent EP0288054A2 and Nishiyama Memorial Technical Lecture No. 159-160, P79-P80).

[0007] Chinese patent ZL201010227961.8 discloses "960MPa grade quenched and tempered steel plate with excellent toughness and plasticity and its manufacturing method". Although the comprehensive mechanical properties of the steel plate also reach a relatively high level: tensile strength ≥980MPa, yield strength ≥890MPa, -60℃ Charpy transverse impact energy (single value) ≥47J, the maximum thickness of the steel plate produced by the invention steel plate manufacturing technology can only reach 80mm.

[0008] Chinese patent ZL201110071217.8 discloses "a kind of HT960 steel plate with excellent plasticity and toughness and its manufacturing method", which is produced by TMCP+offline tempering process, giving full play to the quenching potential of alloy elements, greatly reducing the alloy content, manufacturing cycle and manufacturing cost of the steel plate, and greatly improving the comprehensive mechanical properties and weldability of the steel plate. However, the production thickness of the steel plate is still limited, and the maximum thickness can only reach 70mm. It is still impossible to produce thicker steel plates; more importantly, the steel plate has a high yield strength ratio (≥0.95), large anisotropy of longitudinal and transverse strength, and the difference in longitudinal and transverse strength reaches 70MPa~110MPa.

[0009] The "ultra-high toughness, ultra-thick 1000MPa grade steel plate and its manufacturing method" disclosed in Chinese patent 201910566603.0 adopts low-carbon, high-nickel and high DI index design, and successfully develops 1000MPa grade ultra-high toughness steel plate with a maximum thickness of 180mm through special controlled rolling + DQ + QT process. The strength, toughness and plasticity of the steel plate are matched to the level of 800MPa grade quenched and tempered steel plate, especially the elongation, crack resistance and crack arrest characteristics and weldability of the steel plate are basically equivalent to those of 800MPa quenched and tempered steel plate, which can be used in hydropower engineering pressure steel pipes, volutes, steel bifurcated pipes and steel bifurcated pipe crescent ribs. The actual quality of the product has reached the international advanced level (compared with The physical quality of similar products of Nippon Steel is comparable); however, not only is the manufacturing cost of the steel plate extremely expensive: the precious alloy element Ni is added up to 1.50% to 4.00%, the surface cracks of the high Ni content slabs are frequent, and the slabs need to be mechanically ground, resulting in many manufacturing processes, long manufacturing cycles, low metal recovery rates, and difficulty in recycling scrap steel (containing a large amount of Ni elements), the steel plate can only be used for special engineering components, such as pressure-bearing components of hydropower projects and polar marine engineering components and related equipment, which limits the widespread use of mass socialization; more importantly, the steel plate has a high yield strength ratio (≥0.92), a large anisotropy of longitudinal and transverse strength, and a longitudinal and transverse strength difference of more than 50MPa. Summary of the invention

[0010] The object of the present invention is to provide a 1000MPa grade steel plate with ultra-high toughness, low yield ratio and low longitudinal and transverse strength anisotropy and a manufacturing method thereof, so that the ultra-high strength, ultra-high toughness, low yield ratio and low longitudinal / transverse strength anisotropy of the base steel plate are obtained, and at the same time, the steel plate has excellent weldability and the low-temperature toughness of the welding heat affected zone HAZ is also excellent, that is, the base steel plate has an impact toughness of -60°C KV2≥100J, and the welding heat affected zone (HAZ) has a KV2≥100J at -40°C. It is particularly suitable for steel for hydropower engineering (pressure steel pipes, volutes, steel bifurcations, yoke steel plates and turbine seat ring plates, etc.), offshore platforms, large-span bridges, especially highway-railway bridges and cross-sea bridges and port machinery, etc., and can achieve low-cost stable mass industrial production.

[0011] To achieve the above object, the technical solution of the present invention is:

[0012] Ultra-high toughness, low yield strength ratio and low longitudinal and transverse strength anisotropy 1000MPa grade steel plate, the weight percentage of its components is:

[0013] C: 0.08~0.13%

[0014] Si: ≤0.15%

[0015] Mn: 0.95~1.35%

[0016] P: ≤0.013%

[0017] S: ≤0.003%

[0018] Cu: 0.10~0.30%

[0019] Ni: 1.10~1.60%

[0020] Cr: 0.40~0.70%

[0021] Mo: 0.35~0.65%

[0022] Nb: 0.010~0.030%

[0023] Ti: 0.007~0.015%

[0024] Als: 0.035~0.065%

[0025] B: 0.0009~0.0016%

[0026] N: ≤0.0045%

[0027] Ca: 0.0010-0.0035%, the rest is Fe and other unavoidable inclusions; and,

[0028] The above element contents must satisfy the following relationship at the same time:

[0029] (%Als) / [(%N)-0.292(%Ti)]≥43, eliminate free [N] in steel and welding heat affected zone; c. Ensure that there is enough solid solution [B] in the steel, maintain the steel plate with sufficient hardenability and stable hardenability, and achieve the strength, low temperature toughness and toughness matching of 1000MPa grade online steel plate; ② Reduce the free [N] content in welding heat affected zone, improve the microstructure and low temperature toughness of welding heat affected zone;

[0030] (%C)×[(6.53(%Si)+5.17(%Als)+0.71(%Mn)]≤0.215, c reduces the degree of conjugate segregation of C, Mn, P, S, etc. during the solidification of molten steel with high alloy content, improves the internal soundness and homogeneity of the steel plate, improves the UT qualified rate and low-temperature toughness of the steel plate; ② inhibits the excessive quenching of the surface (sub) of ultra-high strength steel plate with high alloy content, improves the low-temperature toughness and environmental brittleness of the surface (sub) layer of the steel plate, and improves the crack resistance of the steel plate Cracking characteristics; ③ Balance the hardenability in the thickness direction of the steel plate, increase the critical cooling rate of the martensitic phase transformation, promote the lower bainite phase transformation, realize the "lower bainite---martensite" time continuity, effectively divide the original austenite grains, inhibit the generation of coarse martensitic crystal group packet size, increase the appearance of phase transformation variant pairs between V1 / V2, refine the block size, improve the strength-toughness / strength-plasticity matching, elongation and low-temperature toughness of the ultra-high strength quenched and tempered steel plate, which is one of the key technologies of the present invention;

[0031] Ca treatment, Ca / S ratio controlled at 1.0-3.0, 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 welding heat-affected zone, reducing the anisotropy of the longitudinal and transverse properties (strength, toughness) of the online quenched and tempered ultra-high strength steel plate, and improving the online quenched and tempered ultra-high strength steel plate with excellent weldability and weld joint toughness.

[0032] The microstructure of the finished steel plate of the present invention is low-carbon bainite + a small amount of dispersed low-carbon lath martensite, and the average grain size of the microstructure, that is, the packet cluster size, is less than 20 μm.

[0033] The thickness of the steel plate of the present invention is ≤50mm, and its yield strength is ≥890MPa; the thickness of the steel plate is greater than 50mm, and its yield strength is ≥870MPa; the tensile strength of the steel plate is ≥950MPa, the single value of the Charpy impact energy KV2 at -60°C is ≥120J, the KV2 of the welding heat affected zone HAZ-40°C is ≥100J, the yield strength ratio is ≤0.90, the longitudinal / transverse strength difference is ≤30MPa, the weldability is excellent, and no preheating is required before welding and no heat treatment is required after welding.

[0034] 1000MPa grade online quenched and tempered steel plate with ultra-high toughness, low yield ratio and low longitudinal and transverse strength anisotropy 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 Pcm, ultra-high strength, ultra-high toughness, low yield ratio and low longitudinal and transverse strength anisotropy, but also has excellent weldability - low preheating temperature before welding (≤100℃), no need for heat treatment after welding, and excellent low-temperature impact toughness of welded joints (especially heat-affected zones). However, these performance requirements conflict with each other, are difficult to reconcile, and are difficult to meet at the same time: A) low C (≤0.11%), low Pcm and ultra-high strength and ultra-high toughness; B) ultra-high strength, ultra-high toughness and low yield ratio, low longitudinal and transverse strength anisotropy; C) ultra-high strength and excellent weldability (low welding preheating temperature, no need for post-weld heat treatment, welded joints, especially welded heat-affected zones, have excellent low-temperature toughness). The above-mentioned properties conflict with each other in composition design and online quenching and tempering process DQ design and are difficult to reconcile. When the C content and Pcm are reduced, it is difficult to achieve ultra-high strength, ultra-high toughness and low yield ratio of the steel plate; while greatly improving the strength and low-temperature toughness, it is difficult to achieve low yield ratio and low longitudinal and transverse strength anisotropy of the steel plate; when the steel plate obtains ultra-high strength, the weldability of the steel plate is seriously deteriorated (high sensitivity to welding cold cracks, high preheating temperature before welding, heat treatment required after welding, severe embrittlement of the welding heat affected zone, etc.).

[0035] How to balance ultra-high strength, ultra-high toughness, low yield ratio, low longitudinal and transverse strength anisotropy and excellent weldability is one of the biggest difficulties of this product and also the key core technology. Therefore, in terms of key technical routes, components and process design, the present invention integrates key factors affecting the ultra-high strength, ultra-high toughness, low yield ratio, low longitudinal and transverse strength anisotropy and excellent weldability of the steel plate, starting from alloy design, using low-carbon C-low Si-medium Mn low alloy steel as the basis, appropriately increasing the acid-soluble Als content in the steel and (%Als) / [(%N)-0.292(%Ti)]≥43, (%C)×[(6.53(%Si)+5.17(%Als)+0.71(%Mn)]≤0.215, low Pcm, ultra-low N content, (Cu+high Ni+Mo) alloying, (Ti+Nb+B) microalloying, Ca treatment and Ca / S ratio controlled between 1.0 and 3.0 and Ca×S 0.28 ≤1.0×10 －3 Optimized combination design of alloys and optimized online quenching and tempering process DQ: [13.67 (% B) 1 / 2 ]×{[ξ×(900-T 开轧 )×(%Nb)×(V c )] / [H×ζ×(T 开冷 -T 停冷 )]}≤0.117、0.62≤[H×(T 开冷 -T 停冷 )×DI OL ×ξ] / [(V c )×(T 停冷 )×(t 在炉 )×(T 回火 )]≤25.73, so that the microstructure of the finished steel plate is low carbon bainite + a small amount of dispersed low carbon lath martensite, the average grain size of the microstructure (i.e. packet size) is below 20μm, while obtaining the ultra-high strength, ultra-high toughness, low yield ratio and low longitudinal / transverse strength anisotropy of the parent steel plate, the steel plate has excellent weldability, and the low-temperature toughness of the welding heat affected zone (HAZ) is also excellent, that is, the impact toughness of the parent steel plate at -60℃ KV2≥100J, and the welding heat affected zone (HAZ) at -40℃ KV2≥100J.

[0036] 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.

[0037] In the steel plate component system design of the present invention:

[0038] C has a great influence on the strength, low-temperature toughness, elongation and weldability of online quenched and tempered steel plates. From the perspective of improving the low-temperature toughness and weldability of steel plates, it is hoped that the C content in the steel is controlled relatively low; however, from the perspective of strength, low-temperature toughness, low yield ratio and low longitudinal and transverse strength anisotropy control of steel plates and microstructure control and manufacturing cost during the production process, the C content should not be controlled too low; too low C content not only causes the steel plate to fail to reach 1000MPa grade ultra-high strength (unless a large amount of alloying elements are added, which will cause serious cracking of the steel plate weldability and extremely expensive manufacturing costs), the yield ratio and longitudinal and transverse strength anisotropy remain high, and too low C content leads to excessively high grain boundary mobility, coarse grains and easy generation of mixed crystals in the microstructure of the parent steel plate and 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 0.08% to 0.13%.

[0039] 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 1000MPa grade high-strength steel with high alloy content, when welding with large heat input, Si not only promotes the formation of MA islands, but also makes the formed MA islands coarse and unevenly distributed, which seriously damages the low-temperature toughness of the welding heat affected zone (HAZ). Therefore, the Si content in steel should be controlled as low as possible. Considering the economy and operability of the steelmaking process, the Si content is controlled at ≤0.15%.

[0040] As the most important alloying element, Mn not only improves the strength of steel plates, but also has the functions of expanding austenite phase region, reducing Ar3 point temperature, refining DQ 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 the 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, it aggravates the ingot. Segregation and looseness in the center of the ingot. Severe segregation in the center of the ingot is prone to form abnormal structure during the subsequent online tempering and welding process, resulting in low low-temperature toughness of the steel plate and cracks in the welded joints. In addition, for ultra-high strength online tempered steel plates with high alloy content, when the Mn content is too high, it not only causes a sharp deterioration in low-temperature toughness, elongation and weldability of the steel plate (a significant increase in welding cold crack sensitivity, a sharp increase in the M / A island content in the welding heat affected zone, and severe embrittlement of the welded joints), but also causes a sharp increase in the yield ratio and longitudinal and transverse strength anisotropy of the steel plate; therefore, the suitable Mn content is 0.95% to 1.35%.

[0041] 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. In theory, the lower the better; but considering the operability and cost of steelmaking, for 1000MPa grade high-strength online quenched and tempered steel plates that require ultra-high toughness, low yield ratio, low longitudinal and transverse strength anisotropy, and excellent weldability, the P content needs to be controlled at ≤0.013%.

[0042] 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, S combines with Mn in steel to form MnS inclusions. During the hot rolling process, the plasticity of MnS causes MnS to extend along the rolling direction to form a MnS inclusion belt along the rolling direction, which not only seriously damages the low-temperature impact toughness, elongation, Z-direction performance, fatigue resistance and weldability (especially large heat input weldability) of the steel plate, but also causes severe longitudinal and transverse anisotropy of longitudinal and transverse strength and toughness; at the same time, S is also the main element that produces hot brittleness during hot rolling, and theoretically the lower the better; but considering the operability of steelmaking, steelmaking cost and the principle of smooth logistics, for 1000MPa grade high-strength online quenched and tempered steel plates requiring ultra-high toughness, low yield ratio, low longitudinal and transverse strength anisotropy and excellent weldability, the S content needs to be controlled at ≤0.003%.

[0043] Cu is also an austenite stabilizing element. Adding Cu can also reduce the Ar1 and Ar3 point temperatures, improve the atmospheric corrosion resistance of steel plates, refine the microstructure of online quenched and tempered steel plates, and improve the low-temperature toughness of online quenched and tempered steel plates. However, if Cu is added too much, higher than 0.45%, it will not only easily cause copper brittleness, surface cracking of ingots, internal cracking, and especially the impact load fracture characteristics (i.e. plastic toughness) of thick steel plates, and the performance of welded joints, but also lead to high yield strength ratio and longitudinal and transverse strength anisotropy of steel plates. At the same time, considering that Cu is a relatively precious alloying element, from the perspective of cost-effectiveness, the upper limit of Cu should be controlled at 0.30%. If Cu is added too little, lower than 0.05%, it will basically have no effect. Therefore, the Cu content is controlled between 0.10% and 0.30%.

[0044] 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; secondly, as a strong austenite stabilizing element, Ni can greatly reduce the Ar1 and Ar3 point temperatures, increase the driving force of the austenite to ferrite phase transformation, cause the austenite to undergo phase transformation at a lower temperature, greatly refine the microstructure of the online quenched and tempered steel plate, improve the crack propagation resistance through the ferrite grains, and greatly improve the low-temperature toughness of the online quenched and tempered steel plate. Therefore, Ni has the effect of simultaneously improving the strength and low-temperature toughness of the online quenched and tempered steel plate without reducing the elongation (i.e., plasticity and toughness), especially for the ultra-high strength 1000MPa grade microstructure dominated by lath martensite, Ni is an indispensable alloy element for improving the plasticity and toughness of the lath martensite structure; thirdly, 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 yield ratio, longitudinal and transverse strength anisotropy and alloy cost of the steel plate (Ni is a precious alloy element); therefore, for 1000MPa grade high-strength online quenched and tempered steel plates that require ultra-high toughness, low yield ratio, low longitudinal and transverse strength anisotropy, and excellent weldability, the Ni content needs to be controlled at 1.10% to 1.60%.

[0045] Cr is a weak carbide-forming element. Adding Cr not only improves the hardenability of the steel plate and promotes the formation of martensite / bainite, but also increases the orientation difference between martensite / bainite laths, increases the resistance of cracks passing through the martensite / bainite packet structure, and improves the strength of the steel plate while improving the toughness of the steel plate to a certain extent. However, when the amount of Cr added is too much, the weldability of the steel plate is seriously damaged. However, for ultra-high strength 1000MPa grade online quenched and tempered steel plates, a certain Cr content must be present to ensure that the steel plate has sufficient hardenability; therefore, the Cr content is controlled between 0.40% and 0.70%.

[0046] Adding Mo can greatly improve the hardenability of steel plates, promote the formation of bainite / martensite low-temperature phase transformation structure, 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 greatly increases the yield ratio, longitudinal and transverse strength anisotropy, and production cost of the steel plate; therefore, considering the phase transformation strengthening effect of Mo, and its influence on the low-temperature toughness, elongation, weldability, yield ratio, and longitudinal and transverse strength anisotropy of the parent steel plate, for 1000MPa-grade high-strength online quenched and tempered steel plates requiring ultra-high toughness, low yield ratio, low longitudinal and transverse strength anisotropy, and excellent weldability, the Mo content needs to be controlled at 0.35% to 0.65%.

[0047] The purpose of adding trace amounts of Nb elements to steel is to carry out non-recrystallization controlled rolling, regulate the state of austenite grains, refine the grain size of steel plates, and improve the strength and toughness of online quenched and tempered steel plates; for 1000MPa online quenched and tempered steel plates, when the Nb addition is less than 0.010%, in addition to the inability to effectively exert the controlled rolling effect, the online quenched and tempered steel plate's strengthening and toughening ability is also insufficient; when the Nb addition exceeds 0.030%, it not only causes the steel plate's yield strength ratio, longitudinal / transverse strength anisotropy and steel plate's The alloy cost (Nb is also a precious alloying element) remains high; and the welding process induces the formation of upper bainite (Bu) and the secondary precipitation embrittlement of Nb (C, N), which seriously damages the low-temperature toughness of the heat-affected zone (HAZ) of high heat input welding. Therefore, the Nb content is controlled between 0.010% and 0.030% to obtain the best controlled rolling effect, realize the online quenched and tempered steel plate strength-toughness / strength-plasticity matching, low yield ratio, and low longitudinal and transverse strength of the agreement without damaging the weldability of the steel plate.

[0048] The purpose of adding a trace amount of Ti to the steel is to combine with N in the 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 the 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 (<0.007%), 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 (>0.015%), 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 preferred control range of the Ti content is 0.007% to 0.015%.

[0049] Als in steel can fix the free [N] in the steel. In addition to reducing the free [N] in the welding heat affected zone (HAZ) and improving the low-temperature toughness of the welding HAZ, it is more important to ensure that the steel has a certain solid solution [B] and improve the hardenability of the steel plate; therefore, the lower limit of Als is controlled at 0.035%; however, adding excessive Als to the steel will not only cause casting difficulties, but also form a large number of dispersed needle-shaped Al2O3 inclusions in the steel, which will damage the internal soundness, low-temperature toughness and weldability of the steel plate, so the upper limit of Als is controlled at 0.065%.

[0050] The B content is controlled between 0.0009% and 0.0016%, which ensures the online hardenability of the ultra-high strength steel plate without damaging the weldability, HAZ toughness and surface quality of the steel plate.

[0051] The control range of N corresponds to the control range of Ti. For controlling the grain size of steel plates and improving the low-temperature toughness and weldability of steel plates, if the N content is too low, the number of TiN particles generated will be small and the size will be large, which cannot play the role of controlling the grain size of steel plates and improving the low-temperature toughness and weldability of steel plates. Instead, it will be harmful to the low-temperature toughness and weldability of steel plates. However, when the N content is too high, the free [N] in the steel increases, especially the free [N] content in the heat-affected zone (HAZ) increases sharply under high-energy input welding conditions, which seriously damages the low-temperature toughness of the HAZ. In addition, excessively high N content causes coarse AlN to precipitate along the original austenite grain boundaries, which damages the transverse impact toughness, plasticity and bending cold workability of the steel plate, and deteriorates the processing and use characteristics of the steel. Therefore, the N content is controlled to be ≤0.0045%.

[0052] 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 and weldability of the steel plate, while also reducing the purity of the steel and polluting the molten steel. Generally, the Ca content is controlled according to ESSP=(wt%Ca)[1-1.24(wt%O)] / 1.25(wt%S), wherein ESSP is the sulfide inclusion shape control index, and the value range is preferably between 0.80 and 4.00. Therefore, the appropriate range of Ca content is 0.0010% to 0.0035%.

[0053] The method for manufacturing the ultra-high toughness, low yield ratio and low longitudinal and transverse strength anisotropic 1000 MPa grade online quenched and tempered steel plate of the present invention comprises the following steps:

[0054] 1) Smelting and casting

[0055] According to the above composition, the casting is carried out by continuous casting, the tundish pouring superheat is controlled at 8-30℃, the pulling speed is controlled at 0.6-1.0m / min, and the crystallizer liquid level fluctuation is controlled at ≤5mm;

[0056] 2) Slab heating

[0057] The slab heating temperature is controlled at 1060~1150℃;

[0058] 3) Rolling, rolling width ratio ≥1.3;

[0059] The first stage is ordinary rolling, which uses the maximum rolling capacity of the rolling mill for continuous rolling to ensure recrystallization of the deformed steel billet and refine the austenite grains;

[0060] The second stage adopts non-recrystallization controlled rolling, the controlled rolling start temperature is controlled at 720-800℃, the rolling pass reduction rate is ≥7%, the cumulative reduction rate is ≥50%, and the final rolling temperature is 710-760℃;

[0061] 4) Cooling

[0062] After the controlled rolling, the steel plate is immediately transported to the accelerated cooling equipment for accelerated cooling. The steel plate start cooling temperature is 700-740℃, the cooling rate is ≥5℃ / s, and the stop cooling temperature is 150-300℃. When the thickness of the steel plate is ≥50mm, the steel plate is slowly cooled. The slow cooling process is to keep the temperature at not less than 150℃ for more than 24 hours, and then the steel plate is naturally air-cooled to room temperature.

[0063] 5) Tempering

[0064] The tempering temperature of the steel plate is 530-650℃, and the tempering holding time is ≥15min. The tempering holding time is the holding time starting from when the center temperature of the steel plate reaches the tempering target temperature. The time in the furnace is 2.0-3.0 times the thickness of the finished steel plate. The unit of the furnace time is min, and the unit of the thickness of the finished steel plate is mm. After the tempering, the steel plate is naturally cooled to room temperature in air.

[0065] And the following relationship needs to be satisfied in the above process:

[0066] [13.67(%B) 1 / 2 ]×{[ξ×(900-T 开轧 )×(%Nb)×(V c )] / [H×ζ×(T 开冷 -T 停冷)]}≤0.117, ensuring that the online quenched and tempered steel plate has a low yield ratio and low longitudinal and transverse strength anisotropy while obtaining ultra-high strength and ultra-high toughness, and successfully eliminates the contradiction between ultra-high strength, ultra-high toughness and low yield ratio and low longitudinal and transverse strength anisotropy, which is difficult to be compatible. This is one of the key core technologies of the present invention; among them,

[0067] ξ is the cumulative reduction rate of controlled rolling without recrystallization, unit: %;

[0068] T 开轧 is the starting rolling temperature of non-recrystallization controlled rolling, unit: °C;

[0069] V c is the accelerated cooling rate of the steel plate, in °C / s;

[0070] H is the thickness of the finished steel plate, in mm;

[0071] ζ is the width-to-width ratio, i.e. the width of the finished steel plate / the width of the slab;

[0072] T 开冷 The cooling temperature at which accelerated cooling starts, in °C;

[0073] T 停冷 Stop cooling temperature for accelerated cooling, unit: °C;

[0074] 0.62≤[H×(T 开冷 -T 停冷 )×DI OL ×ξ] / [(V c )×(T 停冷 )×(t 在炉 )×(T 回火 )]≤25.73, ensuring that the online quenched and tempered ultra-high strength steel plate has ultra-high toughness while the steel plate has excellent weldability, and successfully eliminating the contradiction between ultra-high strength, ultra-high toughness and excellent weldability (low C, low Pcm) of the online quenched and tempered steel plate, which is difficult to be compatible; among them,

[0075] H is the thickness of the finished steel plate, in mm;

[0076] T 开冷 The cooling temperature at which accelerated cooling starts, in °C;

[0077] T 停冷 Stop cooling temperature for accelerated cooling, unit: °C;

[0078] Online hardenability index DI of steel plate OL =0.514(%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)][×(1+200(%B)]×25.4, unit is mm;

[0079] ξ is the cumulative reduction rate of controlled rolling without recrystallization, unit: %;

[0080] V c It is the accelerated cooling rate of the steel plate, in ℃ / s.

[0081] In the method for manufacturing the steel plate of the present invention:

[0082] According to the above-mentioned C, Mn, Nb and Ti content range, the slab heating temperature is controlled between 1060℃ and 1150℃ to ensure that all Nb in the steel is dissolved into austenite during the slab heating process, while the austenite grains of the slab do not grow abnormally. In order to ensure that the steel plate has low longitudinal and transverse strength anisotropy, the rolling width ratio is ≥1.3.

[0083] The rolling process is carried out in two stages. The first stage is ordinary rolling, which uses the maximum rolling capacity of the rolling mill for continuous rolling. While maximizing the production capacity of the rolling line, it ensures that the deformed steel billet undergoes recrystallization and refines 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 720℃~800℃, the rolling pass reduction rate is ≥7%, the cumulative reduction rate is ≥50%, and the final rolling temperature is 710℃~760℃.

[0084] 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 final rolling temperature are 700℃~740℃, the cooling rate is ≥5℃ / s, and the stopping cooling temperature is 150℃~300℃. 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 150℃ for more than 24 hours, and then the steel plate is naturally air-cooled to room temperature.

[0085] The tempering temperature (plate temperature) of the steel plate is 530℃~650℃, and the tempering holding time is ≥15min. The tempering holding time is the insulation time starting from when the center temperature of the steel plate reaches the tempering target temperature. The time in the furnace is 2.0~3.0 times the thickness of the finished steel plate. After tempering, the steel plate is naturally air-cooled to room temperature.

[0086] Beneficial effects of the present invention:

[0087] The steel plate of the present invention reduces the content of precious alloy elements Ni and Mo, matches the combination design between the main alloy elements, micro-alloy elements and inclusion elements, and combines it with the online quenching and tempering process DQ, so as to produce an online quenched and tempered 1000MPa grade steel plate with excellent comprehensive performance at low cost. The steel plate not only has ultra-high strength, ultra-high toughness, low yield ratio, and low longitudinal and transverse strength anisotropy, but also has excellent weldability, low welding preheating temperature, no need for heat treatment after welding, and excellent low-temperature toughness of the welded joint, especially the weld heat affected zone, which greatly shortens the manufacturing cycle of the steel plate and improves the safety and reliability of the steel structure throughout its life cycle, creates huge social and economic value for project owners and construction companies, and realizes green and environmental protection in the entire process of steel plate manufacturing and use.

[0088] The high performance and high added value of the steel plate are mainly reflected in the perfect match between the high strength and high toughness of the steel plate and the low yield ratio, low longitudinal and transverse strength anisotropy and excellent weldability. The present invention successfully solves the contradictions that are difficult to reconcile in composition design and DQ process design: low C, low Pcm and ultra-high strength, low yield ratio, ultra-high strength, ultra-high toughness and low yield ratio, low anisotropy of longitudinal and transverse strength, ultra-high strength and excellent weldability, which greatly improves the safety, stability and durability of large heavy steel structures; good weldability (especially low preheating temperature before welding and no need for heat treatment after welding) saves the cost of steel structure manufacturing for user enterprises and shortens the time of steel structure manufacturing for users, creating huge value for users. Therefore, this type of steel plate is not only a high value-added, full-life green and environmentally friendly product. BRIEF DESCRIPTION OF THE DRAWINGS

[0089] Figure 1 This is a photo of the microstructure (1 / 4 thickness) of Example 3 of the present invention. DETAILED DESCRIPTION

[0090] The present invention will be further described below in conjunction with the embodiments and drawings.

[0091] The composition of the embodiment of the present invention is shown in Table 1, Tables 2 to 4 are the manufacturing process of the embodiment of the present invention, and Table 5 shows the performance parameters of the steel plate of the embodiment of the present invention.

[0092] like Figure 1 As shown, the microstructure of the finished steel plate of the present invention is low-carbon bainite + a small amount of dispersed low-carbon lath martensite, and the average grain size of the microstructure (ie, packet size) is below 20 μm.

[0093] The steel plate of the present invention is mainly used for marine engineering structures (marine platform pile legs and rack steel plates, etc.), bridge engineering structures, energy development projects, large floating cranes and engineering machinery, mining machinery, port machinery and heavy vehicle manufacturing, etc., and can achieve low-cost stable batch industrial production. As a key material for infrastructure construction, marine engineering development, large-scale hydropower engineering construction and related equipment manufacturing industries, the 1000MPa grade quenched and tempered steel plate with ultra-high toughness, low yield ratio and low longitudinal and transverse strength anisotropy has broad market prospects.

[0094]

[0095]

[0096]

[0097]

Claims

1. Ultra-high toughness, low yield strength ratio and low longitudinal and transverse strength anisotropy 1000MPa grade steel plate, the weight percentage of its components is: C：0.08～0.13％ Si: ≤0.15% Mn: 0.95~1.35% P：≤0.013％ S：≤0.003％ Cu: 0.10~0.30% Ni: 1.10~1.60% Cr：0.40～0.70％ Mo: 0.35~0.65% Nb: 0.010~0.030% Ti: 0.007~0.015% Als: 0.035~0.065% B：0.0009～0.0016％ N：≤0.0045％ Ca: 0.0010~0.0035% The rest is Fe and other unavoidable inclusions; and the contents of the above elements must simultaneously satisfy the following relationship: (%Als) / [(%N)-0.292(%Ti)]≥43; (%C)×[6.53(%Si)+5.17(%Als)+0.71(%Mn)]≤0.215; Ca treatment, Ca / S ratio controlled at 1.0-3.0, Ca×S 0.28 ≤1.5×10 －3 ;and , obtained by the following manufacturing method, comprising: 1) Smelting and casting According to the above composition, the casting is carried out by continuous casting, the tundish pouring superheat is controlled at 8-30℃, the pulling speed is controlled at 0.6-1.0m / min, and the crystallizer liquid level fluctuation is controlled at ≤5mm; 2) Slab heating The slab heating temperature is controlled at 1060~1150℃; 3) Rolling, rolling width ratio ≥1.3; The first stage is ordinary rolling, which uses the maximum rolling capacity of the rolling mill for continuous rolling to ensure recrystallization of the deformed steel billet and refine the austenite grains; The second stage adopts non-recrystallization controlled rolling, the controlled rolling start temperature is controlled at 720-800℃, the rolling pass reduction rate is ≥7%, the cumulative reduction rate is ≥50%, and the final rolling temperature is 710-760℃; 4) Cooling After the controlled rolling, the steel plate is immediately transported to the accelerated cooling equipment for accelerated cooling. The steel plate start cooling temperature is 700-740℃, the cooling rate is ≥5℃ / s, and the stop cooling temperature is 150-300℃. When the thickness of the steel plate is ≥50mm, the steel plate is slowly cooled. The slow cooling process is to keep the temperature at not less than 150℃ for more than 24 hours, and then the steel plate is naturally air-cooled to room temperature. 5) Tempering The tempering temperature of the steel plate is 530-650℃, and the tempering holding time is ≥15min. The tempering holding time is the holding time starting from when the center temperature of the steel plate reaches the tempering target temperature. The time in the furnace is 2.0-3.0 times the thickness of the finished steel plate. The unit of the furnace time is min, and the unit of the thickness of the finished steel plate is mm. After the tempering, the steel plate is naturally cooled to room temperature in air. In the above process, the following relationship also needs to be satisfied: [13.67(%B) 1 / 2 ]×{[ξ×(900-T 开轧 )×(%Nb)×(V c )] / [H×ζ×(T 开冷 -T 停冷 )]}≤0.117, where ξ is the cumulative reduction rate of controlled rolling without recrystallization, unit: %; T 开轧 is the starting rolling temperature of non-recrystallization controlled rolling, unit: °C; V c is the accelerated cooling rate of the steel plate, in °C / s; H is the thickness of the finished steel plate, in mm; ζ is the width-to-width ratio, i.e. the width of the finished steel plate / the width of the slab; T 开冷 The cooling temperature at which accelerated cooling starts, in °C; T 停冷 Stop cooling temperature for accelerated cooling, unit: °C; 0.62≤[H×(T 开冷 -T 停冷 )×DI OL ×ξ] / [(V c )×(T 停冷 )×(t 在炉 )×(T 回火 )]≤25.73; in, H is the thickness of the finished steel plate, in mm; T 开冷 The cooling temperature at which accelerated cooling starts, in °C; T 停冷 Stop cooling temperature for accelerated cooling, unit: °C; Online hardenability index DI of steel plate OL =0.514(%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)][1+200(%B)]×25.4, unit is mm; ξ is the cumulative reduction rate of controlled rolling without recrystallization, unit: %; V c It is the accelerated cooling rate of the steel plate, in ℃ / s.

2. The ultra-high toughness, low yield ratio and low longitudinal and transverse strength anisotropy 1000 MPa grade steel plate according to claim 1, characterized in that: The microstructure of the finished steel plate is low-carbon bainite + dispersed low-carbon lath martensite, and the average grain size of the microstructure, i.e., the packet cluster size, is less than 20 μm.

3. The ultra-high toughness, low yield ratio and low longitudinal and transverse strength anisotropy 1000 MPa grade steel plate according to claim 1 or 2, characterized in that: The thickness of the steel plate is ≤50mm, and its yield strength is ≥890MPa; the thickness of the steel plate is >50mm, and its yield strength is ≥870MPa; the tensile strength of the steel plate is ≥950MPa, and the Charpy impact toughness K at -60℃ is V2 Single value ≥120J, welding heat affected zone HAZ-40℃K V2 ≥100J, yield strength ratio ≤0.90, longitudinal / transverse strength difference ≤30MPa, excellent weldability, no need for preheating before welding and no need for heat treatment after welding.

4. The method for manufacturing the 1000MPa grade steel plate with ultra-high toughness, low yield ratio and low longitudinal and transverse strength anisotropy as claimed in claim 1, 2 or 3, characterized in that: The steps include: 1) Smelting and casting According to the composition smelting of claim 1, the casting is carried out by continuous casting, the tundish pouring superheat is controlled at 8-30°C, the pulling speed is controlled at 0.6-1.0m / min, and the crystallizer liquid level fluctuation is controlled at ≤5mm; 2) Slab heating The slab heating temperature is controlled at 1060~1150℃; 3) Rolling, rolling width ratio ≥1.3; The first stage is ordinary rolling, which uses the maximum rolling capacity of the rolling mill for continuous rolling to ensure recrystallization of the deformed steel billet and refine the austenite grains; The second stage adopts non-recrystallization controlled rolling, the controlled rolling start temperature is controlled at 720-800℃, the rolling pass reduction rate is ≥7%, the cumulative reduction rate is ≥50%, and the final rolling temperature is 710-760℃; 4) Cooling After the controlled rolling, the steel plate is immediately transported to the accelerated cooling equipment for accelerated cooling. The steel plate start cooling temperature is 700-740℃, the cooling rate is ≥5℃ / s, and the stop cooling temperature is 150-300℃. When the thickness of the steel plate is ≥50mm, the steel plate is slowly cooled. The slow cooling process is to keep the temperature at not less than 150℃ for more than 24 hours, and then the steel plate is naturally air-cooled to room temperature. 5) Tempering The tempering temperature of the steel plate is 530-650℃, and the tempering holding time is ≥15min. The tempering holding time is the holding time starting from when the center temperature of the steel plate reaches the tempering target temperature. The time in the furnace is 2.0-3.0 times the thickness of the finished steel plate. The unit of the furnace time is min, and the unit of the thickness of the finished steel plate is mm. After the tempering, the steel plate is naturally cooled to room temperature in air. In the above process, the following relationship also needs to be satisfied: [13.67(%B) 1 / 2 ]×{[ξ×(900-T 开轧 )×(%Nb)×(V c )] / [H×ζ×(T 开冷 -T 停冷 )]}≤0.117, where ξ is the cumulative reduction rate of controlled rolling without recrystallization, unit: %; T 开轧 is the starting rolling temperature of non-recrystallization controlled rolling, unit: °C; V c is the accelerated cooling rate of the steel plate, in °C / s; H is the thickness of the finished steel plate, in mm; ζ is the width-to-width ratio, i.e. the width of the finished steel plate / the width of the slab; T 开冷 The cooling temperature at which accelerated cooling starts, in °C; T 停冷 Stop cooling temperature for accelerated cooling, unit: °C; 0.62≤[H×(T 开冷 -T 停冷 )×DI OL ×ξ] / [(V c )×(T 停冷 )×(t 在炉 )×(T 回火 )]≤25.73; in, H is the thickness of the finished steel plate, in mm; T 开冷 The cooling temperature at which accelerated cooling starts, in °C; T 停冷 Stop cooling temperature for accelerated cooling, unit: °C; Online hardenability index DI of steel plate OL =0.514(%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)][1+200(%B)]×25.4, unit is mm; ξ is the cumulative reduction rate of controlled rolling without recrystallization, unit: %; V c It is the accelerated cooling rate of the steel plate, in ℃ / s.

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