CN116516242B - High-toughness low-yield ratio low-longitudinal-transverse-strength anisotropic 800 MPa-grade steel plate and manufacturing method thereof - Google Patents

Abstract

The invention discloses a 800MPa grade steel plate with high toughness, low yield ratio and low longitudinal and transverse strength anisotropy, and a manufacturing method thereof. The steel plate is based on low carbon C-low Si-medium Mn-free B (≤0.00040%) low alloy steel, and Pcm, ultra-low N content, (Cu+Ni+Mo) alloying, Ti+Nb microalloying, Ca treatment, Ca/S ratio of 1.0-3.0 and Ca×S ^0.28 ≤1.0× ^10-3 and other alloy optimization combination designs are controlled. The online quenching and tempering process is optimized to make the microstructure of the finished steel plate uniform and fine low carbon bainite + a small amount of dispersed granular bainite, and the average grain size of the microstructure is below 20μm. While obtaining the high strength, excellent low temperature 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 parent steel plate-40℃ impact toughness KV ₂ ≥120J, welding heat affected zone (HAZ)-40℃KV ₂ ≥60J.

Description

High toughness, low yield strength ratio and low longitudinal and transverse strength anisotropy 800MPa grade steel plate and its manufacturing method

Technical Field

The invention relates to a high-toughness, low-yield-to-strength ratio and low longitudinal and transverse strength anisotropy 800MPa grade steel plate and a manufacturing method thereof.

Background Art

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 the matching between them 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.

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 at low temperature (-40℃), while the elongation at fracture and elongation basically reach the level of 600MPa grade steel plate, and the longitudinal and transverse strength anisotropy is low, the steel plate has excellent weldability, the welding preheating temperature is low (≤50℃), and no heat treatment is required after welding; and under the conditions of low alloy content, especially the content of precious metals Cu, Ni and Mo, and low manufacturing cost, the comprehensive mechanical properties and performance of the steel plate are greatly improved. , to reduce the alloy content of steel and save costs, high strength and lightweight reduce the weight, stability and safety of steel components, and more importantly, to further improve the cold/hot workability of steel components and the safety and reliability during service; currently, a research boom on the development of a new generation of high-performance steel materials has been set off in Japan, South Korea, the European Union and North America, 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.

Traditionally, high-strength steel plates with a tensile strength of ≥780MPa are mainly produced by offline quenching and tempering (RQ+T), which requires that the steel plates must have sufficiently high hardenability and hardenability, that is, the hardenability index DI ≥ 2 × finished steel plate thickness [DI = 0.311C ^1/2 (1+0.64Si) × (1+4.10Mn) × (1+0.27Cu) × (1+0.52Ni) × (1+2.33Cr) × (1+3.14Mo) × 25.4 (mm)], to ensure that the steel plates have sufficiently high strength, excellent low-temperature toughness, and uniformity of microstructure and properties along the thickness direction of the plate. Therefore, it is inevitable to add a large amount of alloying elements such as Cr, Mo, Ni, and Cu to the steel, especially a large amount of Ni element (see "CAMP-ISIJ", Vol.4, 1991, 1949; CAMP-ISIJ, Vol.4,1991,1950; "CAMP-ISIJ",Vol.7,1994,836;"CAMP-ISIJ",Vol.7,1994,837; Japanese Patent Sho 59-129724; Hei 1-219121; "Nippon Steel Research"No.314-1984;"Japanese Steel Pipe Technical Bulletin"No.107-1985;"Nippon Steel Technical Bulletin"No.348-1993;"Kawasaki Steel Technical Bulletin"Vol.4(No.3)-1972;"Kawasaki Steel Technical Bulletin" Vol.7(No.2)-1975).

More importantly, the use of traditional quenched and tempered steel component system and manufacturing process not only has a long steel plate manufacturing cycle and high manufacturing cost, but also makes it difficult to handle scrap steel with high Cu and Ni content, limiting the efficiency of scrap steel recycling and reuse; and for the 80 kg quenched and tempered steel produced by the traditional quenched and tempered process, due to the high alloy content of the steel plate, the steel plate has low elongation, high yield strength ratio, poor weldability (high sensitivity to welding cold cracks, high embrittlement degree of welding heat affected zone, sensitivity to reheat cracks, etc.), and it is difficult to control the uniformity of performance in the plate thickness direction; and the low elongation is not only not conducive to the cold and hot processing performance of the steel plate, but also has a great influence on the fatigue resistance, stress concentration sensitivity and structural stability of the steel plate. When used in large-scale engineering construction and large-scale equipment such as penstocks and steel bifurcations in hydropower projects, thermal power steam turbine generators and offshore platform structures, ship floating cranes and giant excavators, there are great safety hazards; therefore, when large fatigue heavy-load steel structures use high-strength steel, it is generally hoped that the 80 kg high-strength steel has excellent toughness and plasticity matching, especially the tensile elongation δ ₅ is above 18%. A large number of existing patent documents only explain how to achieve the strength and low-temperature toughness of the parent steel plate, but there are few descriptions on improving the welding performance of the steel plate and obtaining excellent low-temperature toughness of the HAZ in the welding heat affected zone, and there is no description on how to improve the tensile strength of the steel plate while improving the tensile elongation and uniformity of mechanical properties in the thickness direction of the steel plate (see Japanese Patents Sho 63-93845, Sho 63-79921, Sho 60-258410, Special Opening 4-285119, Special Opening 4-308035, Hei 3-264614, Hei 2-250917, Hei 4-143246, U.S. Patent US Patent 5798004, European Patent EP 0288054A2 and "Xishan Memorial Technical Lecture" No. 159-160, P79~P80).

Baoshan Iron & Steel Co., Ltd. applied for a series of 80 kg-grade quenched and tempered steel patents between 2007 and 2018, such as "HT780 steel plate with excellent weldability and low yield ratio and its manufacturing method; steel plate with high toughness and high plasticity and its manufacturing method; 80 kg-grade ultra-thick quenched and tempered steel plate and its manufacturing method, low cost, high toughness and excellent weldability 800MPa-grade quenched and tempered steel plate and its manufacturing method".

Although the comprehensive mechanical properties of steel plates produced by these patented technologies have reached a very high level: tensile strength ≥780MPa, yield strength ≥690MPa, Charpy transverse impact energy (single value) ≥47J at -40℃ and below, and excellent weldability of steel plates; however, the steel plates are all produced by offline quenching and tempering process, and a certain amount of Cu and Ni alloy elements are inevitably added to the steel, especially a large amount of Ni element.

There is also the patent "Low-cost 80 kg extra-thick quenched and tempered steel plate and its manufacturing method" applied by Baoshan Iron & Steel Co., Ltd. in 2009. Although the chemical composition of the steel plate does not contain Cu and Ni elements, the impact toughness of the steel plate can only meet the temperature requirements of -20°C and above; in addition, the steel plate adopts controlled rolling + offline quenching + tempering process; this not only has many manufacturing processes, a long manufacturing cycle, and high manufacturing costs, but also the energy consumption of the manufacturing process is relatively high (the steel plate is rolled and naturally air-cooled to room temperature, then shot blasted, and then heated to the quenching temperature again), which is not conducive to energy saving and environmental protection, and it uses The offline quenching + tempering process cannot fully utilize the hardenability and hardenability potential of the alloy elements. The hardenability and hardenability of the elements cannot be maximized. Therefore, in order to obtain the same strength and toughness level, more alloy elements (especially Ni, Mo, Cr, etc.) must be added, which not only further increases the manufacturing cost, but also damages the weldability of the steel plate. Especially for high-strength quenched and tempered steel plates, the sensitivity of welding cold cracking is greatly increased, and welding preheating and post-heating (i.e. PWHT) at higher temperatures are required. The range of suitable welding heat input is narrower, and the corresponding processing and manufacturing costs are greatly increased.

In addition, Baoshan Iron & Steel Co., Ltd. applied for a patent in 2012 for "Nickel-free high-toughness 80 kg high-strength steel and its manufacturing method". Although it successfully developed an 800MPa-grade tempered steel plate with excellent performance by using the online tempering process DQ+T process without adding precious alloy elements such as Cu and Ni, its low-temperature toughness cannot meet the low-temperature requirement of -40°C, and the steel plate cannot be welded without preheating, and the welding heat input cannot be higher than 50kJ/cm; more importantly, all the above-mentioned developed steel plates do not involve the anisotropy and low yield ratio control of the longitudinal/transverse strength of the steel plates, and the actual yield ratio of the steel plates has reached more than 0.94, which cannot meet the design safety requirements of special large and heavy steel structures (such as large-span road and rail bridge structures, marine engineering structures and giant ship floating cranes, etc.).

Summary of the invention

The object of the present invention is to provide a 800MPa grade steel plate with high toughness, low yield ratio and low longitudinal and transverse strength anisotropy and a manufacturing method thereof. While obtaining high strength, excellent low-temperature toughness, low yield ratio and low longitudinal/transverse strength anisotropy of a base steel plate, the steel plate has excellent weldability and excellent low-temperature toughness of a welding heat affected zone (HAZ), that is, the base steel plate has an impact toughness KV ₂ ≥100J at -40°C and a welding heat affected zone (HAZ) KV ₂ ≥100J at -40°C. The steel plate 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 batch industrial production.

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

800MPa grade online quenched and tempered steel plate with 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, high strength, high toughness, low yield ratio and low longitudinal and transverse strength anisotropy, but also has excellent weldability - low preheating temperature before welding (≤50℃), 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 (≤0.09%), low Pcm and high strength and high toughness; B) high strength, high toughness and low yield ratio, low longitudinal and transverse strength anisotropy; C) high strength and excellent weldability (low welding preheating temperature, no need for post-weld heat treatment, and excellent low-temperature toughness of welded joints, especially welded heat-affected zones).

The above properties conflict with each other in the composition design and the online quenching and tempering process DQ design and are difficult to reconcile: when the C content is reduced and the Pcm is low, it is difficult to achieve high strength, high toughness and low yield ratio of the steel plate; while 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 high strength, the weldability of the steel plate is seriously deteriorated (high sensitivity to welding cold cracks, high preheating temperature before welding, heat treatment is required after welding, and the heat affected zone of welding is severely embrittled, etc.). How to balance high strength, high toughness, low yield ratio, low longitudinal and transverse strength anisotropy and excellent weldability is one of the biggest difficulties in the development of the product of the present invention and is also a key core technology.

Therefore, the present invention integrates the key factors affecting the high strength, high toughness, low yield ratio, low longitudinal and transverse strength anisotropy and excellent weldability of the steel plate in terms of key technical routes, components and process design. Starting from alloy design, low carbon C-low Si-medium Mn system-B-free (≤0.00040%) low alloy steel is used as the basis, Pcm, ultra-low N content, (Cu+Ni+Mo) alloying, Ti+Nb microalloying, Ca treatment and Ca/S ratio controlled between 1.0 and 3.0 and Ca×S ^0.28 ≤1.0× ^10-3 and other alloy optimization combination designs are controlled, and the online quenching and tempering process DQ is optimized: [ξ×(900-T _{start rolling} )×(％Nb)×(V _c )]/[H×ζ×(T _{start cooling} -T _{stop cooling} )]≤0.253, 0.22≤[H×(T _{start cooling} -T _{stop cooling} )×DI _OL ×ξ]/[(V _c )×( _{Tstop cooling} )×( _{tin furnace} )×( _Ttempering )]≤5.64, so that the microstructure of the finished steel plate is uniform and fine low-carbon bainite + a small amount of dispersed granular bainite, and the average grain size of the microstructure is below 20μm. While obtaining the high strength, excellent low-temperature 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 -40℃ _KV2 ≥120J, and the welding heat affected zone (HAZ) at -40℃ _KV2 ≥60J.

Specifically, the high toughness, low yield ratio and low longitudinal and transverse strength anisotropy 800MPa grade steel plate of the present invention has the following components in percentage by weight:

C: 0.06%～0.09%

Si: ≤0.20%

Mn: 1.15%～1.55%

P: ≤0.013%

S: ≤0.003%

Cu: 0.10%～0.30%

Ni: 0.40% to 0.70%

Cr: 0.25%～0.50%

Mo: 0.20%～0.45%

Nb: 0.020%～0.050%

Ti: 0.008%～0.016%

N: ≤0.0050%

B: ≤0.00040%

Ca: 0.0010%～0.0035%

Al: 0.020%～0.035%

The rest is Fe and other unavoidable inclusions; and the contents of the above elements must simultaneously satisfy the following relationship:

[ξ×(900-T _{start rolling} )×(%Nb)×(V _c )]/[H×ζ×(T _{start cooling} -T _{stop cooling} )]≤0.253, ensuring that the online quenched and tempered steel plate has a low yield ratio and low longitudinal and transverse strength anisotropy while obtaining high strength and high toughness, and successfully eliminates the contradiction between high strength and high toughness and low yield ratio and low longitudinal and transverse strength anisotropy, which is difficult to be compatible; among them,

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

_{Tstart rolling} is the starting rolling temperature of controlled rolling without recrystallization, 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;

_{Tstart cooling} is the temperature at which accelerated cooling starts, in °C;

_{Tstop cooling} is the temperature at which accelerated cooling stops, in °C;

0.22≤[H×( _{Tstart cooling} - _{Tstop cooling} )×DI _OL ×ξ]/[(V _c )×( _{Tstop cooling} )×(tin _furnace )×( _Ttempering )]≤5.64; while ensuring that the online quenched and tempered steel plate has high strength and high toughness, the steel plate has excellent weldability, and successfully eliminates the contradiction between high strength, high toughness and excellent weldability (low C, low Pcm) of the online quenched and tempered steel plate, which is mutually opposed and difficult to be compatible; among them,

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

_{Tstart cooling} is the temperature at which accelerated cooling starts, in °C;

_{Tstop cooling} is the temperature at which accelerated cooling stops, in °C;

DI _OL is the online hardenability index of the steel plate,

DI _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 is the accelerated cooling rate of the steel plate, in °C/s;

Ca treatment and Ca/S ratio controlled between 1.0 and 3.0 and Ca×S ^0.28 ≤1.5×10 ^－3 : Ensure sulfide spheroidization and minimize the influence of inclusions on low-temperature toughness and weldability. At the same time, 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 steel plate, and improving the online quenched and tempered steel plate with excellent weldability and weld joint toughness.

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.

The microstructure of the steel plate of the present invention is uniform and fine low-carbon bainite + a small amount of dispersed granular bainite, and the average grain size of the microstructure is less than 20 μm.

The steel plate of the present invention has a yield strength of ≥650MPa, a tensile strength of ≥780MPa, an anisotropy, i.e. a longitudinal/transverse strength difference of ≤30MPa, a base steel plate -40°C impact toughness KV ₂ ≥120J, a welding heat affected zone (HAZ) -40°C KV ₂ ≥60J, and a yield strength ratio ≤0.85.

In the composition system design of the steel plate of the present invention:

In order to obtain a steel plate with a yield strength of ≥650MPa, a tensile strength of ≥780MPa, a Charpy impact energy (single value) of ≥120J at -40°C, low anisotropy (i.e., longitudinal/transverse strength difference ≤30MPa) and excellent weldability, the chemical composition of the 800MPa grade steel plate subjected to online quenching and tempering of the present invention has the following characteristics:

C has a great influence on the strength, low-temperature toughness, elongation and weldability of online quenched and tempered steel plates, especially high heat input weldability. From the perspective of improving the low-temperature toughness and excellent weldability of steel plates, it is hoped that the C content in the steel is controlled relatively low; however, from the perspective of the strength, low-temperature toughness, low yield ratio and low longitudinal and transverse strength anisotropy control of steel plates and the microstructure control and manufacturing cost during the production process, the C content should not be controlled too low; too low C content is likely to lead to high yield ratio and longitudinal and transverse strength anisotropy, and too low C content leads to excessively high grain boundary mobility, coarse grains in the microstructure of the parent steel plate and the weld HAZ and easy generation of mixed crystals, 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 weld HAZ; therefore, the reasonable range of C content is 0.06% to 0.09%.

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 800MPa-grade high-strength steel with a high alloy content, when welding with a large heat input, Si not only promotes the formation of M-A islands, but also makes the formed M-A 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 ≤0.20%.

As the most important alloying element in steel, Mn not only improves the strength of the steel plate, but also has the functions of expanding the austenite phase and reducing Ar ₃ -point temperature, refine the TMCP steel plate grains to improve the steel plate strength (fine grain strengthening effect), improve the steel plate low temperature toughness (fine grain toughening), anti-fatigue characteristics, promote the formation of low temperature phase transformation organization (phase transformation strength effect) to improve the steel plate strength; however, Mn is easy to segregate during the solidification of molten steel, especially when the Mn content is high, it will not only cause casting operation difficulties, but also easily cause conjugate segregation with elements such as C, P, S, especially when the C content in the steel is high, aggravate the segregation and looseness of the center of the ingot, and severe segregation in the center of the ingot is easy to form abnormal organization during the subsequent online quenching and welding process, resulting in low low temperature toughness of the steel plate and cracks in the welded joints. In addition, for high-strength online quenched and tempered steel plates, when the Mn content is too high, it will not only cause the steel plate low temperature toughness, elongation and weldability to deteriorate sharply, but also cause the steel plate yield ratio and longitudinal and transverse strength anisotropy to increase sharply; therefore, the suitable Mn content is 1.15% to 1.55%.

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 800MPa grade high-strength online quenched and tempered steel plates that require high toughness, low yield ratio, low longitudinal and transverse strength anisotropy, and excellent weldability, the P content needs to be controlled at ≤0.013%.

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 800MPa grade high-strength online quenched and tempered steel plates that require high toughness, low yield ratio, low longitudinal and transverse strength anisotropy and excellent weldability, the S content needs to be controlled at ≤0.003%.

Cu is also an austenite stabilizing element. Adding Cu can also reduce the Ar ₁ and Ar ₃ 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 deterioration of the impact load fracture characteristics (i.e. plastic toughness) and welded joint performance of thick steel plates, 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 expensive 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%.

Adding Ni can not only reduce the friction force of the dislocation lattice of the BCC crystal structure (i.e., PN force), improve the low-temperature dislocation mobility of the ferrite phase, promote dislocation cross-slip, and improve the intrinsic plasticity and toughness of ferrite; in addition, Ni, as a strong austenite stabilizing element, greatly reduces the Ar ₁ and Ar ₃ point temperatures, increases the driving force for the phase transformation of austenite to ferrite, causes the phase transformation of austenite at a lower temperature, greatly refines the microstructure of the online quenched and tempered steel plate, increases the crack propagation resistance through the ferrite grains, and greatly improves 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); 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 of the steel plate and the alloy cost of the steel plate (Ni is a precious alloy element); therefore, the Ni content is controlled between 0.40% and 0.70%.

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 high-strength 800MPa 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 0.25% and 0.50%.

Adding Mo can greatly improve the hardenability of the steel plate, promote the formation of bainite/martensite low-temperature phase transformation structure, improve the tempering characteristics and tempering process window of the steel plate, and improve the strength and toughness, and strength-plasticity matching 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, 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, the Mo content is controlled between 0.20% and 0.45%.

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 800MPa online quenched and tempered steel plates, when the Nb addition is less than 0.020%, 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 0.050%, not only the yield strength ratio of the steel plate, the longitudinal/transverse strength anisotropy and the alloy composition of the steel plate are increased, but also the strength of the steel plate is increased. The Nb content (Nb is also a precious alloying element) remains high; and in the welding process, it 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 0.020% and 0.050%, obtaining the optimal controlled rolling effect, achieving the strength-toughness/strength-plasticity matching of the online quenched and tempered steel plate, low yield ratio, and low longitudinal and transverse strength anisotropy, while not damaging the high heat input weldability of the steel plate.

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 liquid molten steel but must precipitate in the solid phase; therefore, the precipitation temperature of TiN must be ensured to be lower than 1400°C; when the amount of Ti added to the steel is too small (<0.008%), the number of TiN particles formed is insufficient to inhibit the growth of austenite grains during TMCP and welding thermal cycles and improve the low-temperature toughness and weldability of the steel plate; when the Ti content is too much (>0.016%), the TiN precipitation temperature exceeds 1400°C, and some TiN particles precipitate large-sized TiN particles during the solidification of the molten steel. Such large-sized TiN particles not only cannot inhibit grain growth, but instead become the starting point for crack initiation; therefore, the preferred control range of the Ti content is 0.008% to 0.016%.

To ensure intrinsic fine-grained steel, the Al content in the steel must be ≥ 0.020%; to reduce the number of Al ₂ O ₃ inclusions in the steel and improve the fatigue resistance and weldability of the steel plate, the Al content in the steel must not be higher than 0.035%.

The control range of N corresponds to the control range of Ti. For controlling the grain of steel plate and improving the low temperature toughness and weldability of steel plate, 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 of steel plate and improving the low temperature toughness and weldability of steel plate, but will be harmful to the low temperature toughness and weldability of steel plate; however, when the N content is too high, the free [N] in the steel will increase, especially under high line energy welding conditions, the free [N] content in the heat affected zone (HAZ) will increase sharply, which will seriously damage the low temperature toughness and bending cold workability of HAZ, and deteriorate the processing and use characteristics of steel. Therefore, the N content is controlled at ≤0.0050%.

In order to ensure low yield ratio and low longitudinal and transverse strength anisotropy of the steel plate, the B content in the steel shall not exceed 0.00040%.

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 and weldability of the toughness. In addition, Ca treatment is used to improve the pouring of molten steel with a higher aluminum content. 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%.

The method for manufacturing the high toughness, low yield ratio and low longitudinal and transverse strength anisotropy 800MPa grade steel plate of the present invention comprises the following steps:

1) Smelting and casting

Smelting according to the above composition and casting by continuous casting;

2) Slab heating

The heating temperature is controlled at 1070℃～1150℃;

3) Rolling, rolling width ratio ≥ 1.3

The first stage is ordinary rolling;

The second stage adopts non-recrystallization controlled rolling, the controlled rolling start temperature is controlled at 750-820℃, the rolling pass reduction rate is ≥7%, the cumulative reduction rate is ≥50%, and the final rolling temperature is 730-790℃;

4) Cooling

After the controlled rolling, the steel plate is immediately transported to the accelerated cooling equipment for accelerated cooling. The starting cooling temperature of the steel plate is 720℃～760℃, the cooling rate is ≥6℃/s, and the stopping cooling temperature is 200℃～350℃; 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 is 320-370℃, 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. After tempering, the steel plate is naturally air-cooled to room temperature.

Preferably, in step 1), continuous casting is adopted, the tundish pouring superheat is controlled at 8-30° C., the pulling speed is controlled at 0.6-1.0 m/min, and the crystallizer liquid level fluctuation is controlled at ≤5 mm.

According to the composition system of the steel of the present invention and the requirements of strength, plasticity and low temperature toughness of the steel plate, in the manufacturing method of the present invention:

Continuous casting is adopted, the tundish pouring superheat is controlled at 8℃～30℃, the pulling speed is controlled at 0.6m/min～1.0m/min, and the crystallizer liquid level fluctuation is controlled at ≤5mm.

According to the above-mentioned C, Mn, Nb and Ti content range, the slab heating temperature is controlled between 1070℃ 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.

The rolling process adopts two-stage rolling.

The first stage is ordinary rolling, which uses the maximum rolling capacity of the rolling mill for continuous rolling, maximizing the rolling line capacity while ensuring the recrystallization of the deformed steel billet and refining the austenite grains. The second stage adopts non-recrystallization controlled rolling. According to the above-mentioned Nb element content range in the steel, in order to ensure the non-recrystallization controlled rolling effect, the controlled rolling start temperature is controlled at 750℃～820℃, the rolling pass reduction rate is ≥7%, the cumulative reduction rate is ≥50%, and the final rolling temperature is 730℃～790℃.

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 start cooling temperature is 720℃～760℃, the cooling rate is ≥6℃/s, and the stop cooling temperature is 200℃～350℃. 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.

The tempering temperature (plate temperature) of the steel plate is 320℃～370℃, 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.

Beneficial effects of the present invention:

The steel plate of the present invention reduces the content of precious alloy elements Ni and Mo, matches the main alloy elements, micro alloy elements and inclusion elements, and combines them with the online quenching and tempering process DQ to produce an online quenched and tempered 800MPa grade steel plate with excellent comprehensive performance at low cost.

The steel plate described in the present invention not only has high strength, 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. It greatly shortens the manufacturing cycle of the steel plate, 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.

The high performance and high added value of the steel plate of the present invention are mainly reflected in the perfect matching of the high strength, high toughness, low yield ratio, low longitudinal and transverse strength anisotropy and excellent weldability of the steel plate, which successfully solves the following contradictions that are difficult to reconcile in composition design and DQ process design: ① between low C, low Pcm and high strength, low yield ratio, ② between high strength, high toughness and low yield ratio, low anisotropy of longitudinal and transverse strength, and ③ between 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 of user enterprises, shortens the time of steel structure manufacturing of users, and creates huge value for users.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a photograph of the microstructure (1/4 thickness) of the steel of Example 3 of the present invention.

DETAILED DESCRIPTION

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

The embodiments of the present invention are shown in Table 1, and Tables 2, 3 and 4 are examples of the manufacturing process of the embodiments of the present invention. Table 5 shows the performance parameters of the steel plate of the present invention.

Referring to FIG. 1 , it can be seen from FIG. 1 that the microstructure of the finished steel plate is uniform and fine low-carbon bainite + a small amount of dispersed granular bainite (5-20%), and the average grain size of the microstructure is below 20 μm; this ensures that the steel plate obtains high strength (yield strength ≥ 650 MPa, tensile strength ≥ 780 MPa), excellent low-temperature toughness (-40°C Charpy impact energy single value KV ₂ ≥ 120 J), low yield ratio (≤ 0.85), low longitudinal and transverse strength anisotropy (i.e., longitudinal/transverse strength difference ≤ 30 MPa) and excellent weldability (no preheating before welding, no heat treatment after welding).

The 800MPa grade quenched and tempered steel plate with high toughness, low yield ratio and low longitudinal and transverse strength anisotropy described in the present invention can be widely used in infrastructure construction (such as large-scale road and railway bridge projects, municipal projects, tunnel projects, etc.), marine engineering structures (offshore platforms, offshore wind towers and pile legs, 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 and stable mass industrial production.

Claims (5)

1. High toughness, low yield strength ratio and low longitudinal and transverse strength anisotropy 800MPa grade steel plate, the weight percentage of its components is: C: 0.06%～0.09% Si: ≤0.20% Mn: 1.15%～1.55% P: ≤0.013% S: ≤0.003% Cu: 0.10%～0.30% Ni: 0.40% to 0.70% Cr: 0.25%～0.50% Mo: 0.20%～0.45% Nb: 0.020%～0.050% Ti: 0.008%～0.016% N: ≤0.0050% B: ≤0.00040% Ca: 0.0010%～0.0035% Al: 0.020%～0.035% The rest is Fe and other unavoidable inclusions; and the contents of the above elements must simultaneously satisfy the following relationship: [ξ×(900-T _{start rolling} )×(%Nb)×(V _c )]/[H×ζ×(T _{start cooling} -T _{stop cooling} )]≤0.253, where: ξ is the cumulative reduction rate of controlled rolling without recrystallization, unit: %; _{Tstart rolling} is the starting rolling temperature of controlled rolling without recrystallization, 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; _{Tstart cooling} is the temperature at which accelerated cooling starts, in °C; _{Tstop cooling} is the temperature at which accelerated cooling stops, in °C; 0.22≤[H×( _{Tstart cooling} - _{Tstop cooling} )×DI _OL ×ξ]/[(V _c )×( _{Tstop cooling} )×(tin _furnace )×( _Ttempering )]≤5.64; where, H is the thickness of the finished steel plate, in mm; _{Tstart cooling} is the temperature at which accelerated cooling starts, in °C; _{Tstop cooling} is the temperature at which accelerated cooling stops, in °C; t _{in furnace} is the tempering time in furnace, unit: min; _Ttempering is the tempering temperature, in °C; DI _OL is the online hardenability index of the steel plate, DI _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 is the accelerated cooling rate of the steel plate, in °C/s; In Ca treatment, the Ca/S ratio was controlled between 1.0 and 3.0 and Ca×S ^0.28 ≤1.5×10 ^－3 . 2. The high toughness, low yield ratio and low longitudinal and transverse strength anisotropy 800MPa grade steel plate as described in claim 1 is characterized in that the microstructure of the steel plate is uniform and fine low-carbon bainite + dispersed granular bainite, and the average grain size of the microstructure is less than 20μm. 3. The high toughness, low yield ratio and low longitudinal and transverse strength anisotropic 800MPa grade steel plate according to claim 1 or 2, characterized in that the yield strength of the steel plate is ≥650MPa, the tensile strength is ≥780MPa, the anisotropy, i.e. the longitudinal/transverse strength difference is ≤30MPa, the impact toughness of the base steel plate at -40°C KV ₂ is ≥120J, the KV ₂ of the welding heat affected zone HAZ-40°C is ≥60J, and the yield ratio is ≤0.85. 4. The method for manufacturing a steel plate with high toughness, low yield ratio and low longitudinal and transverse strength anisotropy of 800 MPa as claimed in any one of claims 1 to 3, characterized in that it comprises the following steps: 1) Smelting and casting The composition smelting according to claim 1 is performed by continuous casting; 2) Slab heating The heating temperature is controlled at 1070℃～1150℃; 3) Rolling, rolling width ratio ≥ 1.3 The first stage is ordinary rolling; The second stage adopts non-recrystallization controlled rolling, the controlled rolling start temperature is controlled at 750-820℃, the rolling pass reduction rate is ≥7%, the cumulative reduction rate is ≥50%, and the final rolling temperature is 730-790℃; 4) Cooling After the controlled rolling, the steel plate is immediately transported to the accelerated cooling equipment for accelerated cooling. The starting cooling temperature of the steel plate is 720℃～760℃, the cooling rate is ≥6℃/s, and the stopping cooling temperature is 200℃～350℃; 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 is 320-370℃, 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. After tempering, the steel plate is naturally air-cooled to room temperature. 5. The method for manufacturing a 800 MPa grade steel plate with high toughness, low yield ratio and low longitudinal and transverse strength anisotropy as described in claim 4 is characterized in that in step 1), continuous casting is adopted, the tundish pouring superheat is controlled at 8-30°C, the pulling speed is controlled at 0.6-1.0 m/min, and the crystallizer liquid level fluctuation is controlled at ≤5 mm.