CHEMICAL AND STRUCTURAL HETEROGENEITY OF CONTINUOUSLY CAST SLAB AND THICK-ROLLING SHEETS OF STRENGTH CATEGORIES K60 AND K65

Abstract

An increase in the operating pressure in the main pipelines for pumping gas is accompanied by an increase in the diameter and wall thickness of the pipe and, accordingly, an increase in the requirements for properties that are determined by the structure of the steel. The formation of the structure proceeds along the entire technological path of the production of hot rolled pipes from a continuously cast billet to a hot-rolled sheet intended for large-diameter welded pipes. Slab defects such as various types of liquation, which contribute to a decrease in the mechanical properties of pipes and premature destruction, pass into the sheet. The paper analyzes the structure of rolled products made of low-alloy tubular steel of strength categories K60 and K65, and its relationship to the structure of continuously cast blanks. In the final structure of rolled steel of the K60 strength category, a banded structure of two types is observed: along the cross-section of the rolled sheet, due to the movement of the temperature front during cooling after rolling, and an axial banded structural heterogeneity caused by the axial chemical heterogeneity of the continuously cast billet. In steel of strength category K65, the axial structural heterogeneity consists of plate bainite. The microhardness depends on the structural state and increases from the basic ferritic structure to the structure with pearlite banding and has a maximum value in the region of martensitic banding. In the axial zone of the rolled product, large carbonitrides are observed, inherited from a continuously cast billet, and many of them are located on sulfide inclusions, which were obviously the centers of nucleation.