Physico-Chemical Processes from the X70 Steel Making and Continuous Casting that Influence its Properties

Abstract

The paper presents the processes of elaboration and casting that favourably influence the properties of microalloyed steel.
High strength microalloyed steel used to manufacture main oil and gas pipelines must meet, in addition to special technical conditions, economic conditions, which contribute to the protection of the environment. Secondary treatment in LF and RH installations as well as automatically controlled continuous casting can also help improving the physical, mechanical and corrosion properties of the products obtained from these steels. The making of X70 steels at OLD1-(Liberty Steel Group), according to existing technology, is the peak of performance at the current stage.
Blowing oxygen and argon into the converter is done according to a Blowing Pattern that takes into account the gas flow and the distance from the head of the blowing lance to the surface of the metal bath. Deoxidation and microalloying of the X70 steel take place in the casting ladle and during the secondary treatment in LF and RH.
For deoxidation and microalloying, some ferro-alloys which have strictly limited content of harmful elements (P, S) are used. LF microalloying materials such as: Mn-99%, Al-99%, FeTi-70%, FeV-80%, FeNb-65%, Ca-99% or SiCa60/30% are introduced into the steel as tubular ferro-alloys and not chunks. In this way, a superior assimilation and homogeneous diffusion of the elements into the metal bath are achieved.
Secondary treatment of the X70 steel for chemical and thermal homogenization of the metal bath is achieved by advanced metal bath desulfurization using synthetic slag, lime and bauxite. Vacuum degassing with RH procedure is done to reduce hydrogen from 8-9 ppm to less than 2 ppm. At the continuous casting of these steel types, the bubbling is not used because it is intended that the floating of inclusions be easier on the surface of the metal bath.