Abstract
The influence of the oxide inclusions on the microstucture, properties and the Heat-affected-zone (HAZ) toughness of low alloy steels has been investigated. The low alloying steels were smelted with special oxide introduction technique and the properties of HAZ have been studied with thermo-simulation. The type, size and distribution of the non-metal inclusions in steels has been studied by optical microscope and scanning electronic microscope (SEM). The microstructure and mechanical properties of base metal and samples undergoing welding thermo-simulation were also analyzed. The results show that the inclusions in experimental steel are mainly complex inclusions composed of oxide containing Ti and MnS, these complex inclusions are distributed homogenously and their size is between 1 and 3 micron. The microstructure of HAZ consists of intragranular acicular ferrite (IAF), intergranular ferrite and small amount of lath bainite while the cooling time during the phase transformation between 800°C and 500°C (t8/5) is short. After the thermo simulation with t8/5 increasing the toughness of HAZ decreased and the size of prior austenite grain increased. If the size of the austenite grain decreased the amount of IAF also decreases.
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