Abstract
Surface roughness caused by hot deformation in a 22.5Cr-6.25Ni steel with ferrite/austenite (δ/γ) duplex structure has been studied by using a Gleeble hot tensile machine. The results obtained are as follows :
( 1 ) Two types of surface roughness were observed to occur during hot tensile deformation; Type A : the isotropic severe surface roughness formed in the case of pre-heating in the δ-single phase region (_??_ 1 250°C), and Type B : ridging-like wrinkles along the rolling direction formed in the case of preheating in the δ-γ duplex phase region.
( 2 ) The surface roughness was found to depend on deformation temperature and prior thermal history especially in the case of Type A. The degree of surface roughness increased with either increasing pre-heating temperature or decreasing deformation temperature. Such observations can be explained in terms of the δ grain coarsening and the precipitation behavior of γ phase.
( 3 ) The surface roughness of the specimen pre-heated in the δ-single phase region increased by the deformation in δ-γ duplex phase region where γ grains precipitated continuously along δ grain boundaries in comparison with the deformation in the δ-single phase region.
( 4 ) The δ-matrix precipitation of γ grains during long time holding in the δ-γ duplex phase region reduced the surface roughness.
