Surface longitudinal crack defect and its control for vertical continuous casting round blooms of steel P91
Steel P91 is one of micro-alloyed enhanced 9Cr-1Mo martensitic heat-resistant steels,which shows ad-vanced service properties such as excellent high-temperature strength,high-temperature oxidation resistance and corrosion resistance.Steel P91 has been widely adopted now for the manufacture of important high-temperature pressure resistant steam pipe in power plants equipment.In order to develop big round bloom continuous casting in-stead of traditional ingot casting for efficient manufacture of the high alloy steel,the continuous casting round bloom surface longitudinal crack frequently occurred in P91 semi-products was studied.The crack appearance,macro-mor-phology and inclusions had been studied by the aid of OM,SEM and EDS analysis,and the steel phase transforma-tion characteristic at initial solidification was calculated in terms of commercial thermodynamic software.The re-search shows that steel P91 solidifies initially very similar to hypo-peritectic steels with inherent surface depression and/or longitudinal crack sensitive,whose main inner reason is strong shrinkage of δ→γ near the mold meniscus,but the uneven shell thickness and gap due to poor casting practices should be the important extra reasons to induce the cracks.And it is believed that the longitudinal crack in P91 continuous casting round bloom should initiate from the mold region.Then,it has been observed by casting tests that the crack frequency drops obviously through an improved operation like as changing the bifurcated SEN port direction to avoid the outlet liquid flow impinging di-rectly upon the shell,or adopting a new mold powder for thicker melt flux layer by lower melting point and larger viscosity.However,it is found additionally that the crack frequency can be remarkably reduced by the application of a new mold with denser internal cooling water channels,which shows a weaker mold cooling intensity but more even heat flux distribution at circumference direction.The research results can provide reference for the control of surface longitudinal cracks in the continuous casting production of high alloy steel round blooms such as steel P91.
steel P91round bloomcontinuous castingsurface longitudinal crackproduction and control
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