Improvements in the manufacture of stainless steels and low carbon ferro-alloys

摘要

In a method of manufacturing stainless steels and low carbon ferro alloys by direct ore reduction, a non-carbonaceous reducing-agent, a slag-making agent and the ore are preheated, preferably to such a temperature and for such a time as to substantially dehydrate them, introduced into a furnace on a bath consisting primarily of molten steel or iron and worked up to a thick viscous basic slag having a high content of oxide of the alloying metal or metals and adapted to retain the reducing agent in suspension. The slag is then further heated, a further quantity of reducing-agent and slag-making agent added, whereby the slag is worked down to a more fluid state, and the oxides are reduced to the alloying metal or metals. The reducing agent, which may comprise ferro-silicon, silicon, calcium silicide, ferro-chrome-silicon, ferro-manganese-silicon or aluminium is preferably used in amounts slightly in excess of the theoretical equivalent necessary. The raw materials should be crushed to three-quarter inch size or less to expedite the reaction. Charging of the furnace may be effected by successive alternate batches of oxides and reducers. Separate mixtures of ore and a part of the slagging-material and reducing-agent with a further part of the slagging-material may be preheated and charged alternately, and when the thick slag is formed on the bath, further preheated reducing-agent with lime or other slagging-material may be spread over the bath to maintain the basic nature thereof, e.g. to maintain the ratio of total bases to total acids on the slag substantially between 1.25 and 2 to 1. In the production of ferro-chrome alloys, hot dry chromite and lime mixture is charged into the furnace and fused. Hot ferro-silicon or silico-ferro-chrome, mixed with lime and heated, is added to build up a thick basic slag, having a high content of oxide of the alloying metal, which is superheated to about 1525 DEG C. to 1725 DEG C. and worked down to a more fluid slag by addition of further hot reducing-agent and lime. Hot or cold iron or steel of appropriate carbon content is added and the whole charge brought to the pouring temperature. The original bath may be prepared from substantially rust-free scrap iron or scrap alloys which may or may not contain a part of the required alloying metal or metals. In carrying out the process in a direct arc furnace, the electrodes should be raised clear of the slag bath to avoid carbon pick-up by the bath.