Shape Change of Sulphide Inclusions during Hot Bar Forging and Cold Wire Drawing and Its Influence on Mechanical Properties of High Carbon Steels

Abstract

This paper describes the deformation behaviour of sulphide inclusions (MnS) during hot working (forging and rolling) to bar and during cold drawing using high carbon steels.
During hot working to bar, MnS first elongates and then breaks into pieces and each MnS becomes smaller. On the other hand, sulphide inclusions in REM treated steel remain almost globular during hot working.
On cold drawing, MnS also becomes smaller due to both deformation and fragmentation. In REM treated steel, however, a large inclusion breaks into clustered pieces while a small inclusion stays globular and causes a crack in the vicinity of the inclusion in the drawing direction.
Mechanical properties of the forged bar are determined in both longitudinal and transvefse directions after being normalized at 780°C. Although the reduction of area of the transverse direction decreases with increasing S content, the effect of S content decreases with the increases of the hot forging ratio.
Up to 70% drawing reduction, an increase of S content decreases the ductility in the transverse direction of cold drawn steel, but over 70% the effect of S content disappears because the ductility is very poor.
It is found that there is a correlation between the fracture strain and the sulphide inclusion area fraction on the fracture surface of tensile test pieces. This relation is maintained regardless of the amount and the shape of the sulphide inclusion, or of the test direction.
However, the fracture strain of cold drawn steel, of over 70% drawing reduction, is determined mainly by the fiber structure of the matrix, caused by the severe drawing, and the above relationship disappears.