Refining process based on top-blowing with oxygen

A process for refining iron melts by blowing oxygen onto the surface of the iron melt, the quantity or concentration of the stream of oxygen being kept constant for most of the blowing time and being reduced in one or more stages towards the end of the blowing time without any significant increase in the total blowing time.

This invention relates to a refining process based on the top-blowing of 
oxygen. 
Most conventional processes for refining pig iron into steel are based on 
the top-blowing of oxygen, as is the case for example with the LD-process, 
the Kaldo process and the LDAC-process. In the top-blowing of oxygen, it 
is known that a certain quantity of oxygen per unit of time is blown onto 
the surface of the molten pig iron, blowing being continued under constant 
conditions until the carbon content has fallen to the required level. 
Unfortunately, these processes are attended by a whole number of 
disadvantages which are particularly noticeable where losses have to be 
made up. Apart from the fact that blowing with a constant quantity or 
rather concentration of oxygen or under a constant pressure per unit of 
time gives rise to high temperatures at which the lining can be affected, 
which can lead to repeated renewal of the furnace setting, metallic 
elements are increasingly oxidised during the constant blowing process and 
have to be replaced after refining. The metallic elements in question are 
primarily manganese which is generally blown down to very low levels, with 
the result that large quantities of ferromanganese or manganese metal have 
to be added. 
The object of the present invention is to provide a process which reduces 
or completely avoids these disadvantages of the oxygen top-blowing 
technique. In principle, the process according to the invention consists, 
and is also characterised, in that the constancy of the blowing technique 
is abandoned. This variation comprises blowing a predetermined quantity of 
oxygen per unit of time onto the surface for a certain period, reducing 
the concentration or the pressure or the quantity to a lower level at a 
certain moment and thus completing the blowing process. This reduction in 
the quantity or flow of oxygen may of course also be effected in stages. 
Where the invention is used in the Kaldo process, the reduction in the flow 
of oxygen is also accompanied with advantage by a change in the rotational 
speed of the converter. 
In the example referred to above, the application of the process according 
to the invention resulted in the complete saving of the previous loss of 
manganese. It also resulted in less wear of the furnace lining, in a 
tangible gain by virtue of the smaller quantity of oxygen and also in the 
fact that certain work, for example paving, was no longer necessary. 
In a variant of the process according to the invention, which is a logical 
extension of the basic concept of reducing the flow of oxygen, the 
reduction in the flow of oxygen is obtained by increasingly adding other 
gases, for example argon, to the oxygen so that the same effect as 
described above is automatically obtained. 
The process according to the invention is by no means confined to the 
example given above, instead it may be used anywhere where refining is 
carried out by the top-blowing of oxygen. 
The process according to the invention is simple and does not require any 
special equipment beyond the apparatus normally used. Attempts were made 
to follow the course of decarburization mathematically and to control 
refining through measurements and computer programs. It is characteristic 
of the value of these inventions and publications that they have never 
been nor could be adopted for use in practice because, on the one hand, 
they would add significantly to the cost of producing cheap iron and 
because, on the other hand, they necessitate a whole number of preliminary 
investigations and investigations during the actual refining process which 
are extremely time-consuming. Accordingly, it is only the technique of the 
top-blowing of oxygen which hitherto has been adopted in practice. Proof 
of this is the fact that, hitherto, it was not known how the loss of 
manganese for example could be reduced or prevented. Hitherto, this loss 
has been tacitly accepted. 
In a steelworks with an annual output from the Kaldo process of around 
730,000 tonnes, the loss of manganese in the conventional refining process 
amounted to around 0.4%, the manganese content of the starting material 
being of the order of 0.6% Mn at the beginning of the refining process and 
having fallen to 0.2% by the end of the refining process. By applying the 
process according to the invention, the loss of manganese could be reduced 
by 0.1% which corresponds to an annual saving of around 1,000,000 German 
Marks. 
The process according to the invention is of considerable industrial value 
and represents a significant technical advance. In addition, the process 
according to the invention has a high degree of invention.

EXAMPLE 
A number of batches were melted in a 150-tonne Kaldo converter, 120 tonnes 
of pig iron and 30 tonnes of steel scap being used in each batch. From the 
beginning of melting onwards, oxygen was blown on at a rate of 200 
Nm.sup.3 /minute (normal cubic meters per minute). At the moment when the 
intensity of the refining process abated, as reflected in the decrease in 
the temperature of the waste gas, the rotational speed of the converter 
was reduced, as was also the quantity of oxygen blown on per minute, 
namely to 150 Nm.sup.3 /minute. Thereafter, the oxygen was blown on at 
this rate up to the moment at which the slag began to sputter. The supply 
of oxygen was cut off and the slag removed. Thereafter, the rotational 
speed was increased again and after the melt had cooled to around 
1640.degree. C., as measured with a pyrometer for example, blowing was 
continued with a further reduced flow of oxygen, namely 70 Nm.sup.3 
/minute, up to completion. The total blowing time for each batch was 60 
minutes, for approximately the first 55 minutes of which blowing was 
carried out normally with 200 Nm.sup.3 /minute of oxygen, the two 
reductions in the flow of oxygen being effected during the last 5 minutes 
of the total blowing time, firstly to 150 Nm.sup.3 /minute of oxygen for 
about 3 minutes and then to 70 Nm.sup.3 /minute of oxygen for about 2 
minutes. 
In the batches, the average composition of the starting material was as 
follows: 
______________________________________ 
C Mn 
______________________________________ 
120 t of pig iron 
approx. 4.0% approx. 0.55% 
30 t of steel scrap 
approx. 0.10% 
approx. 1.0% 
______________________________________ 
with the usual accompanying elements. 
Some of the batches were blown with a constant stream of oxygen in the 
usual way up to the end of the refining process. The end product obtained 
contained on average approximately 0.10% of C and approximately 0.2% of 
Mn. 
The rest of the batches were blown in accordance with the invention, i.e. 
the flow of oxygen was kept constant for most of the blowing time, being 
reduced in one or more stages towards the end of the blowing time. On 
completion of the refining process carried out in this way, the C-content 
amounted on average to approximately 0.10% and the Mn-content on average 
to 0.3%, i.e. was approximately 0.1% higher than in the conventional 
process. In the case of the process according to the invention, it was 
surprisingly found that, on completion of the blowing process, the carbon 
content was the same as that obtained in the conventional blowing process 
using a constant flow of oxygen although there was no change in the total 
blowing time, whereas the Mn-content left on completion of the blowing 
process was considerably higher. 
The adjustment of the Mn-content to 1% after the blowing process produced a 
saving of 0.1% of Mn of the usual addition. The required analysis of the 
final steel was as follows: 
approx. 0.10% C, approx. 1.0% Mn. 
The moment at which the intensity of the refining process abates may also 
be determined otherwise, for example by measuring the changes in the 
composition of the waste gases. 
All the particulars and features disclosed in the documents, where they are 
new either individually or in combination in relation to the prior art, 
are claimed as essential to the invention.