| Low alloy high strength steels were widely used in shipbuilding, bridges, offshore platforms, high-rise buildings, pipelines, pressure vessels, large oil storage tanks and other large structures. In order to improve efficiency and reduce costs, high heat input welding method was more and more used. At present, only the two kinds of steels, large oil storage tanks and shipbuilding plate steels, can achieve high heat input welding level of100kJ/cm in china, the level of heat input was far behind Japan and other developed countries, and production of high heat input welding steel was still rare in other industrial applications. Development of high heat input welding steel was one of the key objectives in China’s steel industry science and technology. Because of tight blockade of this technology abroad, Selected as researched materials, shipbuilding plate and oil storage tanks steels production process as high heat input welding steel was studied in details Influencing factors of high heat input welding steel performance were studied and analyzed, including chemical composition, smelting process, rolling process, heat treatment process and welding process. Consequently, a relatively integrated process technology for producing high heat input welding steel was formed. The main study work and results were as follows:(1)The effect mechanism of chemical elements on mechanical properties and high heat input welding performance was analyzed. The proportion of each element was reasonably determined. The chemical composition of high heat input welding steel was designed conforming to the national standard about oil storage tanks and shipbuilding plate. The process route was determined considering smelting, rolling, heat treatment, high heat input welding and other aspects.(2)In the case of the same chemical composition, the smelting process, opportunity and order of alloys addition were adjusted. The number of small size inclusions in developed steel was more than several times or ten times in traditional steel and large size inclusions in developed steel were less than in traditional steel. The growth of prior austenite grain was inhibited during high heat input welding thermal cycle in developed steel, and a number of intragranular acicular ferrite (IAF) were formed; therefore the impact toughness in HAZ of developed steel was much higher than traditional steel’s.(3)After control rolling (CR) in the range of nonrecrystallization temperatures, three kinds of quenching-tempering processes were investigated for oil storage tanks steel. They are removed-line quenching and removed-line tempering (CR-RQ-T), direct quenching and removed-line tempering (CR-DQ-T) and direct quenching following heat-treatment on-line process (HOP)(CR-DQ-HOP).The results showed that:The order of second-phase particles number is CR-RQ-T<CR-DQ-T<CR-DQ-HOP, and the order of second-phase particles size is CR-RQ-T>CR-DQ-T>CR-DQ-HOP. After the same high heat input welding thermal cycle, the order of prior austenite grain size is CR-RQ-T>CR-DQ-T>CR-DQ-HOP, and toughness in HAZ is CR-RQ-T<CR-DQ-T<CR-DQ-HOP.(4)Effect of inclusions in test steel on the prior austenite grain or at the austenite grain boundaries was studied. The results showed that:after the welding thermal cycle with a peak temperature of1400"C, the pinning effect of TiN on austenite grain boundary decreased; however, grain coarsening of grain boundary ferrite (GBF) with a polygonal massive morphology was inhibited by Ti-oxide and film membrane GBF was not observed in the steel. The effective inclusions for the forming of IAF were compound inclusions containing Ti and MnS, and the size of the inclusion was0.6-5um;.the inclusions of5～8μm size can also be the intragranular ferrite nucleus. In the case of welding heat input of100-800kJ/cm(t8/5=138-730s) and peak temperature of1400℃(holding time was3-30s), the microstructure was mainly composed of GBF, intragranular polygonal ferrite (IPF) and IAF, and the area fraction of IAF was more than50%; however, lath bainite and granular bainite were not observed. The impact toughness of above test steels was enough high, namely, the impact value was higher than150J at-20℃.(5)In the case of welding heat input of400kJ/cm and peak temperature of1400℃, the toughness of trial steel of oil storage tanks was higher than150J at-20℃. the industrial applications effect exceeded the highest level of reported by exiting literature. After electro-gas welding (EGW) at a heat input of200kJ/cm, the impact energy in fusion line was higher than90J at-20℃, and met the requirements of building oil storage tanks of more than0.15million m3volume replacing of imported steel. As a result of this study, an integrated industrialization technology for producing high heat input welding steel was formed, and technical basis of high heat input welding steel was established. |
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