CA1074976A

CA1074976A - Method of producing steel ingots from unkilled steel

Info

Publication number: CA1074976A
Application number: CA239,465A
Authority: CA
Inventors: Jan Terpstra; Andries Beumer
Original assignee: Hoogovens Ijmuiden BV
Current assignee: Hoogovens Ijmuiden BV
Priority date: 1974-11-20
Filing date: 1975-11-12
Publication date: 1980-04-08
Also published as: NL170817B; NL170817C; GB1473769A; ES442760A1; IT1050645B; NL7415103A; ATA876775A; SE7513019L; DE2551587A1; FR2291812A1; AT381726B; JPS5173928A; FR2291812B1

Abstract

ABSTRACT OF THE DISCLOSURE

In a known process, unkilled steel is cast into an ingot mould and after a time the remaining liquid contents are killed by a deoxidising agent and are homogenised by injection of a gas. The present invention, which makes it possible to avoid the use of a hot top in such a process and to improve yields when slab rolling the ingot, proposes force-cooling and solidifying of the remaining liquid contents from the upper surface downwards by means of cooling fluid applied to the upper surfaces after the homogenisation.

Description

~7~971~;

The invention relates to a method of producing steel ingots, especially ingots which are to be rolled, to ingots so produced and to rolled steel product made from the ingots.
In a known process for making ingots, unkilled steel is cast into an ingot mould in one single casting action, and after a certain period of time the remaining liquid contents of the ingot mould are fully killed by the addition of deoxidizing agent and are subsequently homogenized by the injection of a gas. In Dutch Patent Application No. 70.12060 (corresponding to British Patent Specification No. 1,304,826) the predecessors in law of the present applicants suggested a similar method, in which however, preference was given to a mode of performance in which the heat flow away from within the central part of the mould is reduced by applying an i'nsulating layer (a so-called "hot-top") locally against the inner side of the ingot mould and/or by sprinkling an insulating powdery substance on top of the ingot. By these measures it is intended to keep liquid the upper portion of the ingot as much as possible over the entire cross section, in order to counteract the formation of a shrin-kage cavity. The dirt which is present in the top of the ingot after solidification (also caused by and coming from the hot top) must be eliminated from the steel when rolled to slabs, which affects the yield of slabs rolled from the ingot unsatis-factorily. Also applying heat insulating material to the top end of the inner surface of the ingot mould, and the sprinkling of insulating and/or exothermic substance on top of the ingot both tend further to increase the cost of making the ingot.
It is an object of the invention at least partially to eliminate the abov ~entioned drawbacks, and particularly to in-crease the slab yield while reducing the cost.

According to the invention there is provided a methodof producing steel ingots in which unkilled steel is cast into an ingot mould in a single casting operation, and after a period

-2-~107497~i of time the remaining liquid contents of the mould are fully killed by the addition of deoxidizing agent and are subsequent-ly subjected to homogenization by injection of a gas in which method the ingot mould is an uncovered mould and the contents of the mould are, after the said homogenization, force-cooled and at least partially solidified from the upper surface of the steel downwards by means of cooling fluid. sy an "uncovered"
mould we mean a mould in which special measures to reduce heat loss at the top, for instance provision of a hot top, are not taken.
Preferably the cooling fluid is a liquid directly applied to the upper surface of the steel, e.g. by spraying.
With this new method, it has been found possible to increase the slab yield in a particular application in the ap-plicant's works from 81% to 86%.
It is remarked that British Specification No. 1,346,987 shows a method in which an ingot mould is filled by pouring in one single action without using a hot top, and in which also the top of the ingot is force-cooled. In that case, however, the amount of deoxidizing substance added is such that only the steel in the top of the ingot mould is killed, whereas inten-tionally the steel in the lower part of the ingot mould is kept unkilled. The deoxidizing substance supplied is not homogenized through the melt by the injection of a gas, since it is the spe-cial intention with this known method that the continuing forma-tion of gas in the lower part of the ingot ensures that the ingot solidifies with a convex upper surface because of trapped gas bubbles. A drawback of this known method is that the de-oxidizing substance is distributed unequally through the ingot, with the result that upon rolling materials of different quali-ties are produced from one and the same ingot.

Preferably in the method of the present invention the said homogenization is performed for a period of 20 to 30 seconds 107~976 by the injection of air from a lance which is introduced down-wardly into the liquid steel to near the bottom of the ingot mould, and water is thereafter sprayed onto the upper surface of the steel for a period of 15 to 30 minutes.
An important advantage obtainable with the method of the invention is that, compared to the known method first de-scribed above, the cast ingot needs substantially less time for solidifying in the ingot mould. Also it has been found that the ingot may be placed in the pit furnace while warmer, and also a shorter time is required in the pit furnace until it has suffi-ciently equalized in temperature in order that it may be slab rolled. These factors can result in substantial savings in investment in ingot moulds and in costs of pit furnace opera-tion.
The following comparative example illustrates the invention.
EXAMPLE
A number of ingots of 20 tons steel each were cast and subsequently slab rolled by the known method first described a-bove and by a method embcdying the invention respectively. The tests were performed with a standard low alloy low carbon un-killed steel.
According to the known method, this steel was cast in ingot mould provided with a hot top and was caused to rim against the mould for 4 minutes. Thereupon a sufficient quanti-ty of aluminium ribbon was injected into the remaining melt in order to kill it. By bubbling of air from a pipe which was moved downwardly to near the bottom of the mould the melt was homogenized for 24 seconds.
It was found that the ingots cast in this manner had to cool down for 300 minutes in the mould before the moulds could be stripped. A further cooling period of 30 minutes was then necessary before the ingots could be placed in the pit furnace. Equalization of temperature for 480 minutes in the pit furnace was found to be necessary in order that the ingots could be rolled to slabs. After the slab rolling the slab yield was found to be 81~.
Steel of the same quality was cast in single and un-interrupted casting operatings into similar ingot moulds, which however, were not provided with hot tops. The treatment was similar to that described above with the exception that after the injection of aluminium ribbon to fully kill the still-molten contents and the homogenization, water was sprayed onto the up-per surface of the steel for 20 minutes to force-cool and at least partially solidify the melt downwardly. This time it was found that after a "covered" cooling time of 95 minutes the ingots could be stripped. After a "naked" cooling time of 65 - minutes these ingots could be placed in pit furnaces in which after 300 minutes they had sufficiently equalized in temperature to be rolled into slabs. Twenty of such slab rolled ingots were found to have a slab yield of 86%, as compared with the 81%
yield of the slab rolled ingots produced according to the known method.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. Method of producing steel ingots in which unkilled steel is cast into an ingot mould in a single casting operation, and after a period of time the remaining liquid contents of the mould are fully killed by the addition of deoxidizing agent and are subsequently subjected to homogenization by injection of a gas characterized in that the ingot mould is an uncovered mould and the contents of the mould are, after the said homogenization, force-cooled and at least partially solidified from the upper surface of the steel downwards by means of cooling fluid.

2. Method according to Claim 1, characterized in that the cooling fluid is a liquid directly applied to the upper sur-face of the steel.

3. Method according to Claim 2, wherein the said homo-genization is performed for a period of 20 to 30 seconds by the injection of air from a lance which is introduced downwardly into the liquid steel to near the bottom of the ingot mould, and water is thereafter sprayed onto the upper surface of the steel for a period of 15 to 30 minutes.