Study On Microstructure, Mechanical Properties And Heat Treatment Of High Strength Low-carbon Low-alloy Cast Steel

In harsh environments (e.g. overloads, low temperature, impact, etc), the traditional cast steels cannot meet operating requirements of excellent cryogenic impact toughness and weldability, due to the higher carbon content. In order to solve this problem, two kinds of low-carbon (C≤0.1%) nickel and molybdenum-containing high strength low-carbon low-alloy cast steel were designed. The effect of alloy design, heat treatment on microstructure and properties of high strength low-carbon low-alloy cast steel was investigated by differential scanning calorimetry, optical microscopy, scanning electron microscopy, transmission electron microscopy, impact testing machine and universal testing machine, etc.The results are summarized as follows:(1) As-cast microstructure of0.07C-1.4Mn-0.35Ni,0.10C-0.45Mn-1.4Ni cast steel was consisted of coarse ferrite and pearlite; Rm, Re of0.07C-1.4Mn-0.35Ni cast steel reached499,262Mpa, room temperature impact energy was89J; Rm, Re, impact energy of the latter were506,294Mpa,47J; optimization austenitizing temperatures of two kinds of cast steels were880℃.(2) Microstructure of0.07C-1.4Mn-0.35Ni cast steel was transformed into polygonal ferrite, granular bainite after air cooled at880℃, effective grain was obviously refined, the cast steel tempered at580℃exhibited Rm of588Mpa, Re of475MPa, percentage elongatin of25%,cryogenic(-40℃) impact energy of101J; microstructure of0.10C-0.45Mn-1.4Ni cast steel was polygonal ferrite, acicular ferrite, granular bainite after air cooled at880℃, subsequently tempered at580℃, it displayed Rm of593Mpa, Re of474MPa, percentage elongatin of27%, cryogenic(-40℃) impact energy of113J.(3) After oil cooled at880℃, microstructure of0.07C-1.4Mn-0.35Ni cast steel was turned into granular bainite, lath bainite, a few of ferrite, grain size was further refined, after tempered at600℃, it revealed Rm of774Mpa, Re of721MPa, percentage elongatin of25%, cryogenic(-40℃) impact energy of83J; oil cooled microstructure of0.10C-0.45Mn-1.4Ni cast steel was granular bainite, lath bainite, a few of ferrite, subsequently tempered at600℃, it showed Rm of740Mpa, Re of650MPa, percentage elongatin of23%, low temperature(-40℃) impact energy of70J.(4) Water cooled microstructure of0.07C-1.4Mn-0.35Ni cast steel was tiny lath bainite, effective grain was refined greatly; after tempered at600℃, it displayed Rm of800Mpa, Re of750MPa, percentage elongatin of23%, low temperatrue(-40℃) impact energy of90J; water cooled microstructure of0.10C-0.45Mn-1.4Ni cast steel was lath martensite, bainite, after tempered at600℃, the cast steel displayed Rm of875Mpa, Re of825MPa, percentage elongatin of19%, cryogenic (-40℃) impact energy of50J.(5) In this paper, test cast steels with low alloy content(≈2.5%), carbon equivalent (CE≤0.46%) by a simple heat treatment, exhibit excellent comprehensive properties at room temperature and cryogenic impact toughness;it can fulfil the operating requirements of cast steel under rigorous conditions.