| It is well know that the additon of Ti can result in fine TiN precipitates,which can inhibit austenite grain growth while welding course.With the development of smelting technology,the content of C and N decrese.In addtion of microalloying elements,such as Nb,V,optimal adding amount of Ti and the appropriate Ti/N value also needs further study.The microstructures and toughness of HAZ in the high Nb X80 pipeline steel with different content of Ti and the ratio Ti/N were investigated systemically by useing welding thermal simulation technique,scanning electronic microscope (SEM),optical microscope (OM),transmission electronic microscope (TEM),impact strength and thermodynamics analysis software Thermo-cacl.By investigating the HAZ of industrial pipeline steels,the result are as follow, the shape decrease of toughness mainly results form coarse grain size besides martensite-austenite constituent(MA). As a result of increased Ti content and Ti/N ratio,the prior austenite grain size is decrease.Precipitations mainly are Ti-rich and square after simulated weld thermal cycle.With the increasing of Ti content in an appropriate range, the volume percent of precipitation is higher, which can get specific number from the thermodynamics analysis, while the prior austenite grain size is smaller.When the Ti/N ratio is lower than 2.3, austenite grain size is coarse.Afer Ti/N ratio reaches 3, the austenite grain size reduce slightly with Ti/N ratio increasing. It can get the result best rang of Ti/N ratio is 2～3 and the content of Ti is 0.01%-0.015%.The result also indicate,cooling time t8/5 has great effects on microstructure and properties of samples.When t8/5=30℃/S,the microstructures are lath-shape bainite and granular bainite,which have good impact toughness.Appropriate Ti/N ratio was proved to be effective in control austenite grain size of HAZ(the ratio is 2.6 in the experiments), especially the regions near the fusion line,make it had better strength. |
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