| High strength low alloy H-beam, as a typical low-carbon low-alloy structural steel, exhibits an outstanding combination of high strength, resistance to brittle fracture and good weldability. It has been extensively applied to high-rise buildings, bridges, construction of large ships and offshore oil drilling platforms. Recently, with exploration of high latitude and cold regions, significant attention has been focused on high-grade H-beam with good toughness at low temperature. However, ductile-brittle transition temperature (DBTT) of existing hot-rolled H-beam product is too high, products have a tendency to brittle fracture, which does not meet the request for utilization in cold areas. Therefore, improving low-temperature toughness and reducing DBTT of HSLA H-beams are not only of remarkable realistic significance but also of theoretical value.Based on chemical composition of boron-added HSLA H-beam, composition of the new H-beam was designed, and0.5%nickel was added in new H-beams together with trace of boron so as to reduce its DBTT further. Mechanical properties and microstructures of experimental steels were investigated by means of thermomechanical simulation, room temperature uniaxial tensile tests, instrumented Charpy impact test, Jominy test, micro-hardness test, optical microscopy (OM), scanning electronic microscopy (SEM), high resolution transmission electronic microscopy (HRTEM) and X-ray diffraction (XRD). The H-beam with good impact toughness at-50℃was producted. Optimal heat treatment process of this steel was studied. Ductile to brittle transition temperature (DBTT) of the steel reduced to-96℃by quenching and tempering. Its low temperature impact toughenss is excellent.Thermomechanical simulation results show that Ac1temperature and AC3temperature of experimental steel are770℃and906℃respectively, when heating rate is10℃/s. In phase equilibrium conditions, the Ar3temperature and Ar1temperature are775℃and560℃respectively. When cooling rate increases from0.1℃/s to50℃/s, microstructures of this new type of steel gradually transform from polygonal ferrite and pearlite, grain-boundary ferrite and lath bainite, lath bainite and martensite to single martensite. Accordingly, its hardness increases from50HRA to73HRA. When deformation increases from0%to40%, more ferrite is found in specimens and Ar3temperature increases by55℃, due to effect of deformation induced phase transformation.Mechanical properties and microstructures of boron-nickel added cryogenic H-beam, produced by Laiwu Iron and Steel Company, were investigated. The results show that strength and plasticity of all producets meet requirements, especially, the toughness tested at-50℃reaches80J, which is twice more than requirement of national standard. Deep hole in fracture surface means dimple fracture that increases impact toughness significantly. Addition of nickel has little influence on inclusions, microstructures and second-phase particles. Refining pearlite and transformation of lattice structures are main causes of increasing low temperature toughness by nickel.Results of Jominy tests show that hardenability of boron-nickel added HSLA steel does not increase linearly when quenching temperature increases from870℃to1000℃. Its hardenability peaks at950℃. After heated at1070℃, the hardenability does not increase but decrease and volume fraction of ferrite increase. Compared with boron added steel and no boron or nickel steel, decreasing of hardenability is due to disappearance of anxo-action of boron and refined carbonitride niobium. Maximum quenching thickness equivalent through Jominy test reaches19.34mm.Mechanical properties and microstructures of experimental steel through different heat treatments were investigatied. The results show that tensile strength of quenched specimens reaches1000MPa. After high temperature tempering, the tensile strength remains from500MPa to600MPa, which is100MPa higher than that of hot-rolled steel. Compared with hot-rolled steel, elongation of quenched and tempered specimens decreases by10%but its impact toughenss reaches as high as above200J. Ductile brittle transition temperature decreases to-96℃. The allowable temperature of steel contents0.5%nickel decreases by20℃. Increasing of low temperature toughness is due to recrystallization and spheroidization of carbides. Carbides precipitated along grain boundaries do not harm to toughness, on the contrary, they prevent grain coarsening during recrystallization.Effect of heat treatment parameters on toughness was studied. Results show that quenching temperature has great influence on ductile brittle transition temperature but has little influence on toughness. Tempering temperature has great influence on toughness but has little influence on ductile brittle transition temperature. Residual ferrite after quenching is major influence of ductile brittle transition temperature. Recrystallization is the main factor of toughness.
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