Ichiro Kokubo

Width necking at the rougher in a hot-strip mill.

Abstract The accuracy of the width of a hot strip is very important for the improvement of yield, both for the steel maker and the steel user. One of the important technical problems concerning width accuracy is securing a final hot strip with uniform width along the whole length. In the top or tail portion of a hot strip, a local contraction of width unavoidably occurs, called “width necking” or simply “necking”. The amount of necking is determined mainly by the conditions of rougher rolling in the hot-strip mill. In this paper, the mechanism of necking, the influence of rolling conditions on the amount of necking and the optimum edger-rolling pass schedule are described. The main conclusions obtained are as follows: (i) Necking originates from the uneven distribution of dog-bone height along the length of a slab and the local steep width reduction in the top portion of the slab after edger rolling, but is lessened by the formation of flare during subsequent flat rolling; (ii) the amount of necking increases with the increase of slab width and edging draft and with decrease of slab thickness; and (iii) for the minimization of the amount of necking, heavy width-drafts in the early rougher-rolling stage are preferable to heavy width-drafts in the later rougher-rolling stage.

Newly Developed Zn-Fe/Zn-Ni Double-Layer Electroplated Steel Sheet

A new double-layer electroplated steel sheet has been developed. The steel sheet is composed of an upper Zn-Fe (80-90%) alloy layer which has good paint adhesion and a lower Zn-Ni (10-15%) alloy layer which has good corrosion resistance. Paintability, corrosion resistance and workability of the steel sheet were investigated. This coated steel sheet was revealed to be suitable for the automobile use.