Analysis of Causes for Mischcrystal of 15CrMoG High-pressure Boiler Steel Tube as Occurring during On-line Normalization Process and Relevant Process Improvement Measures

An analysis is made on the fact that mischcrystals and exceptional coarse bainite grain clusters stay with the Φ530 mm×18 mm 15CrMoG high-pressure boiler tube as on-line heat-treated with the normalization +temper process. The result reveals the main cause responsible for the said problem,i.e.,the metal is re-crystallized due to the low temperature for metal deformation during the precision rotary-rolling process,and in this re-crystallization process the small grains are refined,whereas it is almost impossible for the big grains to be changed in the same way. Accordingly,a solution is proposed. That is to get the tube cooled to a temperature under 500 ℃ before being on-line normalized,and then treated with a new on-line normalization process. Thanks to this new process,a fine and homogeneous ferritic+pearlitic structure of the tube is obtained,and it is no other than the structure as resulting from the off-line normalization treatment.