Macro-etching of Continuous Cast Steel

Investigations on service failures in line pipe steels have underscored the importance of control of macro-segregation during slab casting. In some investigations, crack initiation in line pipe service failures has been traced to martensitic or hard segregation bands outcropping on the pipe surface. The formation of a brittle martensitic band is due to a hardenability increase from Mn and carbon enrichment resulting from macro-segregation in the casting operation. Oxide and sulfide inclusions occur in the macro-segregated regions. Hydrogen diffusion to these inclusion sites in sour-gas line pipe is known to cause catastrophic in-service failures. Segregation related problems are also known to affect the quality of the slab during the bending and unbending operation in the caster, resulting in problems with the seam welding of plates and hot rolled strip used to make pipes. Macro-segregation also affects subsequent field operations involving hot bending and field welding of pipes. Hence a method of preventing macro-segregation by fluid flow control in the caster, and having a method to monitor segregation have become an important quality assurance i ssue in the manufacture of line pipe and heavy plates. Since the base chemistry of the modern line pipe steel consists of ultra-low sulfur ( < 0.005 wt % S) and low carbon ( <0.06wt %C), the traditional Baumann sulfur printing method does not provide any useful information. Extensive research was carried out at Dofasco at the time of installation of their new caster to develop enhanced techniques for etching and documenting the macro-structure of low residual, as-cast slabs. Details of the techniques involving sample surface preparation, deep etching (using three different techniques) followed by ink highlighting will be outlined. The relative merits of documenting the resulting macrostructures using four different recording methods will be examined (i.e., taping, paper printing, Intaglio printing and digitized imaging).