Por: MARCOS ALEXANDRE STUART NOGUEIRA (CBMM), Danilo Di Napoli Guzela (Steelmaking and Continuous Casting Consultant), Paulo de Tarso Rossi Haddad (CBMM,)
Resumo:
The steels for sour service pipelines must have homogeneous macrostructure, free from center line segregation, once these regions are characterised by having discontinuities like inclusions, coarse carbides, heterogeneous microstructures that can accumulate ions H+, resulting in failures by Hydrogen Induced Cracking or Sulphide Stress Corrosion. This paper presents the main variables that control the intensity of center line segregation on continuous casting of slabs, striving to have homogeneous macrostructure free from defects and coarse precipitates. The casting speed and superheat are the main parameters to be controlled once they define the solidification macrostructure, being the equiaxed structure preferable to the columnar structure for the reduction of central segregation. Center line segregation intensity depends on chemical composition, being carbon, sulphur, phosphorous and manganese the main elements subject to segregation that must be reduced as much as possible.
The paper presents the main caster variables to be controlled to minimize the center line segregation. Anything that affects the movement of solute-enriched liquid between the dendrites during solidification must be considered carefully. It is essential to keep the roll gap and roll alignment constant and to prevent from bulging. Higher cooling rate brings to high thickness of solidified shell, resulting on less bulging. Soft reduction is very effective but just reaches 100% of its performance when the Solidification End Point (SEP) is located at the last roll of a its segment. The sprays efficiency must be daily inspected, preventing from clogging. The water quality and its temperature control are important parameters to minimize center line segregation. The paper presents two cases pointing the importance of water and soft reduction control for minimizing center line segregation
