SUS410 half martensitic stainless steel (0.08% ~0.15%C

12.5%~13.5%Cr) is easy to produce inner surface defects in production of hot rolled thick-walled seamless steel tubes. The localization analysis of the crack defects at inner surface of 139.7 mm x 38.1 mm tube

the results shows that the thermoplastic decline of steel is caused by the large precipitation of δ-ferrite in steel near crack. During the deformation process

the deformation of the two-phase region is not coordinated

and finally cracks appear at the boundary between austenite and δ-ferrite. According to tlie Schaeffler phase diagram

the chemical composition of steel is redesigned

the Cr equivalent /Ni equivalent ratio decreases from 4.51 of originel defect tube to ≤ 3.0

the heating temperature of tube bloom decreases from 1200~1300℃ to 1180~1280 ℃

the internal surface defects are effectively solved

and the qualified rate of ultrasonic flaw detection increases to 97.9%.