Nickel-free high-toughness 80kg-grade high-strength steel and manufacturing method thereof

Abstract

The invention relates to nickel-free high-toughness 80kg-grade high-strength steel and a manufacturing method thereof. A component system of ultralow C, ultralow Si, medium Mn and Ti, V and B microalloy steel is used as a basis; the acid-soluble Als content in steel is properly improved, the Als is not less than 10*[(%Ntotal)-0.292(%Ti)], Mn/C is not less than 12, (%C)*(%Si) is not more than 0.01, and [(%Cr)+1.3(%Mo)]*[(%V)+(%C)] is not less than 0.087; Ca treatment is carried out, the Ca/S ratio is 1.00-3.00, and (%Ca)*(%S)<0.18> is not more than 2.5*10<-3>; F*DI index number * zeta is not less than 2.0* finished steel plate thickness, wherein zeta is on-line DQ (direct quenching) hardenability contribution factor, and F is B element hardenability contribution factor; DQ (thermo mechanical control process: TMCP) and off-line tempering process (T) are optimized; the microscopic structure of the steel plate is fine low-carbon tempered martensite and tempered lower bainite; the steel has the average colony size below 20 mu m, has excellent strength and plasticity and strong toughness, and is particularly suitable for hydroelectric pressure water pipes, steel branch pipes, scrolls, ocean platforms, large-sized engineering machines and other large-sized steel structures and equipment.

Description

technical field [0001] The present invention relates to low-carbon (high-strength) low-alloy steel, in particular to nickel-free high-toughness 80-kilogram high-strength steel and a manufacturing method thereof. Impact energy (single value) ≥ 47J, elongation at break δ 5 ≥18%, excellent weldability. Background technique [0002] As we all know, low-carbon (high-strength) low-alloy steel is one of the most important engineering structural materials, widely used in oil and gas pipelines, offshore platforms, shipbuilding, 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 and weldability are the most important properties of low-carbon (high-strength) low-alloy steel, which are ultimately determined by the finished product The micr...

AI Technical Summary

Problems solved by technology

More importantly, the traditional quenched and tempered steel composition system and manufacturing process are adopted, not only the steel plate manufacturing cycle is long, the manufacturing cost is high, and the scrap steel containing high Cu and Ni is difficult to deal with, which limits the efficiency of scrap steel recycling and reuse; and for the traditional quenched and tempered process The 80 kg grade quenched and tempered steel produced, due to the high alloy content of the steel plate, results in low elongation of the steel plate, high yield strength ratio, and poor weldability (high sensitivity to welding cold cracks, high embrittlement in the welding heat-affected zone, and Sensitivity to hot cracks, etc.), the difficulty of controlling the uniformity of properties in the thickness direction, etc.; and the low elongation is not only not conducive to the cold and hot processing performance of the steel plate, but also has a negative effect on the fatigue resistance, stress concentration resistance and structural stability of the steel plate. When used in large-scale engineering construction and large-scale equipment such as pressure water pipes and steel branch pipes in hydropower projects, thermal power turbine generators and offshore platform structures, marine floating cranes and giant excavators, there are relatively large safety hazards. hidden dangers; therefore, when high-strength steel is used for large-scale fatigue and heavy-duty steel structures, it is generally hoped that the 80-kg high-strength steel has excellent strength, toughness, and strong plasticity matching, especially the tensile elongation δ 5 Above 18%

[0005] A large number of existing patent documents only explain how to realize the strength and low-temperature toughness of the base steel plate, improve the weldability of the steel plate, and obtain excellent low-temperature toughness of the welding heat-affected zone HAZ. , to improve the tensile elongation of the steel plate and the uniformity of the mechanical properties in the thickness direction, such as Japanese patents No. 63-93845, No. 63-79921, No. 60-258410, special flat opening 4-285119, etc.

[0006] Disclosed in Chinese patents "HT780 steel plate with excellent weldability and low yield strength ratio and its manufacturing method", "strong toughness, strong plasticity steel plate and its manufacturing method", "80 kg ultra-thick quenched and tempered steel plate and its manufacturing method" The comprehensive mechanical properties of the steel plate have also reached a very high level: tensile strength ≥ 780MPa, yield strength ≥ 690MPa, Charpy transverse impact energy (single value) ≥ 47J at temperatures below -40°C and below, the steel plate has excellent weldability, but none of the steel plates can Avoid adding a certain amount of Cu and Ni alloy elements, especially a large amount of Ni elements; there is also a Chinese patent "low-cost 80 kg extra-thick quenched and tempered steel plate and its manufacturing method", the chemical composition of the steel plate does not contain Cu , Ni elements, but the impact toughness can only meet the temperature requirements of -20 ℃ and above

In addition, the controlled rolling + off-line quenching + tempering process is adopted; this not only has many manufacturing processes, long manufacturing cycle and high manufacturing cost, but also relatively high energy consumption in the manufacturing process (steel plate rolling is completed and naturally air-cooled to room temperature, followed by After shot blasting, reheat to the quenching temperature), which is not conducive to energy saving and environmental protection; and the off-line quenching + tempering process cannot give full play to the hardenability and hardenability potential of alloy elements, and the hardenability and hardenability of elements cannot be maximized Therefore, in order to obtain the same level of strength and toughness, more alloying 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 ultra-high-strength steel plates, The sensitivity of welding cold cracking is greatly increased, and welding preheating and postheating (ie PWHT) are required at higher temperatures. The range of suitable welding heat input is narrower, and the processing and manufacturing costs are correspondingly greatly increased.

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