Effects of the Conditions of Direct Hot Rolling after Continuous Casting upon the Mechanical Properties of Hot Rolled C-Mn-Nb Steel Sheets

Abstract

A study has been made of the effects of the conditions of direct hot rolling after continuous casting on the mechanical properties of hot rolled C-Mn-Nb steel sheets.
The following results are obtained.
(1) Austenite (γ) grain size after casting is very large compared with that after reheating at 1 250°C for 1 h. Ferrite grain sizes after hot rolling are almost equal in both processes. This is probably because y grain boundaries before hot rolling do not contribute to the ferrite refinement because of their very low densities.
(2) Tensile test results of steel sheets produced by both processes are almost the same if niobium is almost completely soluted before hot rolling.
(3). In case, total hot rolling reduction after casting is higher than 40%, total elongation becomes very large.
(4) Under the experimental condition of total hot-rolling reduction being 88%, the strengthening effect of niobium is saturated when niobium content is more than about.0.035%.
(5) The effects of the cooling rate in solidification (about 18 and 235°C/min at 1 500-1 300°C) and the heat history difference in the slab width on tensile properties are small in this direct hot rolling simulation test.