Control of defects during continuous casting of line pipe (API) quality steels

摘要

Internal and surface quality of line pipe (API) grade steels are very important in addition to the various property requirements such as YS and UTS, YS/UTS ratio, elongation, fatigue strength at zero and sub zero temperature and weldability. Surface cracking is predominant in these steel grades because of the presence of various micro-alloying (Nb, V, etc.) elements for their precipitation in various forms like nitrides and carbo-nitrides at the prior austenite grain boundaries during continuous casting and making the solidified steel brittle. Besides this, internal soundness in terms of ultrasonic nondestructive testing is another important requirement of line pipe plates. High rejection of plates of these grades was experienced in the initial phase of production in terms of surface defects (cracks and entrapment of non-metallics) and internal defects (ultrasonic testing) including low yield at slab stage. These led to low yield of the final product. In view of this, development work was undertaken at Bhilai Steel Plant to improve processing technology to achieve higher yield of API grade plates. This paper discusses the reasons of low yield of slabs and plates in the initial stages of production and measures taken in continuous casting parameters to improve the same including the acceptability of final plates.