WO1994012673A1

WO1994012673A1 - A method of manufacturing stainless steel

Info

Publication number: WO1994012673A1
Application number: PCT/SE1993/001010
Authority: WO
Inventors: Mikael Brunner
Original assignee: Aga Aktiebolag
Priority date: 1992-11-26
Filing date: 1993-11-24
Publication date: 1994-06-09
Also published as: SE470020B; SE9203556L; SE9203556D0

Abstract

The present invention relates to a method of manufacturing stainless steel by treating a high carbon crude steel in a first stage with a gaseous mixture containing oxygen and inert gas, so as to lower the carbon content of the steel to a value of 0.2-0.1 %, and then treating the steel with oxygen and inert gas in a second stage. The invention is characterized by treating the steel in the second stage alternately with an oxygen-rich gas and inert gas until reaching a final carbon content corresponding to the carbon content of the finished steel.

Description

A METHOD OF MANUFACTURING STAINLESS STEEL

The present invention relates to a method of manufac¬ turing stainless steel by treating crude steel of high carbon content in a first stage with a gaseous mixture containing oxygen and an inert gas, wherein the carbon content of the steel is lowered to a value of 0.4 or lower, and then treat¬ ing the crude steel with oxygen and inert gas in a second stage. By stainless steel is meant generally a steel of high chromium content, for instance a chromium content of 18%, and a particularly low carbon content, for instance a carbon con¬ tent of 0.020%. The low carbon content is decisive to the non-corrosive properties of the steel. Thus, stainless steel which has a chromium content of 18% and a carbon content of 0.040% is inferior to stainless steel which has a carbon content of 0.020%, given by way of example above. The delete¬ rious effect of the carbon present is due to the formation of chromium carbide, Cr₂₃C. As evident from the formula for carbide, one carbon atom binds twenty-three chromium atoms, therewith depleting the steel of chromium and lowering the corrosion-resistant properties of the steel.

Consequently, stainless steel manufacturing processes are mainly directed to the removal of carbon from the crude steel smelt. The crude steel smelt usually contains initially about 1% carbon, due to the fact that the majority of crude materials, such as steel scrap and ferroalloys, etc., contain carbon. Although it is possible to produce stainless steel from carbon-free crude materials, such starting materials are particularly expensive and therefore uneconomical in use.

A chromium-containing steel smelt is decarburized by adding oxygen gas to the smelt, this oxygen reacting with the carbon dissolved in the steel to form carbon monoxide. The carbon monoxide departs from the smelt in a gaseous form as gas is generated. Decarburization with the aid of oxygen gaε is highly effective down to carbon contents of about 0.1-0.2% after which the generation of gas decreases and the oxygen gas delivered to the smelt reacts essentially with chromium to form chromium oxide.

As a result of equilibrium reactions, the truly low carbon contents can be achieved either by heating the smelt to temperatures about 2,000°C, which is very difficult and highly expensive to achieve, or by using inert gases, such as argon. The use of argon gas results in an equilibrium shift, because the formation of carbon monoxide is facilitated in the argon bubbles blown or injected into the smelt. Modern manufacture of stainless steel is today dominated by methods in which argon is used. These methods are designated AOD processes, where the acronym AOD stands for Argon Oxygen Decarburization.

The AOD process is characterized by introducing to the steel smelt a process gas which is comprised of a mixture of oxygen and argon. There is first introduced to the smelt a gaseous mixture of high oxygen content, for instance three parts oxygen and one part argon, until the carbon content or carbon concentration of the smelt has fallen to about 0.4%, whereafter the proportions of oxygen and argon in the process gas is lowered to one part oxygen and one part argon until the carbon content has fallen to 0.2%, whereafter there is introduced into the smelt a third gaseous mixture having the proportions of one part oxygen and three parts argon.

A characteristic feature of the AOD process is thus that the composition of the process-gas mixture is changed as a function of carbon content, in other words the proportion of oxygen in the process gas is lowered with decreasing carbon contents and the argon proportion increases correspondingly. In the AOD process, the composition of the process-gas mix- ture can be changed either stepwise or continuously, although always in a manner such that the argon content will increase with decreasing carbon contents. The reason for this method of procedure is probably because it is thought that an exces¬ sively high content of oxygen, i.e. too much oxygen, will lead to a more complex oxidation of chromium.

The present invention relates to a method of decarbur- izing a stainless steel smelt by treating crude steel of high carbon content in a first stage with a gaseous mixture which contains oxygen and inert gas, such as to lower the carbon content of the steel to a value of at most 0.4% by weight, and then treating the smelt with oxygen and inert gas in a second stage. The inventive method is characterized by treat- ing the steel in said second stage alternately with a gaseous mixture of high oxygen content and an inert gas until the carbon content of the steel smelt has fallen to a value which corresponds to the content of carbon in the finished steel. In contradistinction to the conventional AOD process, in which the composition of the gaseous mixture is changed so as to lower the oxygen proportion of the gaseous mixture with decreasing carbon contents, when practicing the inventive method oxygen is delivered to the smelt with a gaseous mix¬ ture that has a high oxygen content. Many comparison tests and theoretical considerations have led to the conclusion that the use of gaseous mixtures of low oxygen contents in the low carbon content range is less effective, i.e. results in a lower decarburization rate than the use of gaseous mixtures which have a high oxygen content in the low carbon content range. The formation of carbon monoxide is namely a function of the oxygen potential in the smelt, among other things, i.e. a function of the amount of oxygen dissolved in the smelt. When a mixture of oxygen and argon, or some other inert gas of low oxygen content, is used, the amount of oxygen dissolved in the smelt is minimized because the inert gas flushes or rinses the carbon dioxide from the smelt. Thus, when practicing conventional methods, decarburization is achieved in the smelt with a low content of dissolved oxygen. When practicing the present invention, the content of dissolved oxygen in the steel smelt is optimized so as to optimize the ability of the argon to flush carbon monoxide from the smelt. According to the invention, the content of dissolved oxygen in the smelt is optimized by using gaseous mixtures of oxygen and inert gas of high oxygen content. This high content of dissolved oxygen is maintained throughout the whole decarburization period in the range of low carbon content, by blowing or injecting a gaseous mixture of high oxygen content into the smelt over relatively short periods. The high content of dissolved oxygen is thus maintained during the whole of the decarburization period, by periodi¬ cally injecting an oxygen-rich gaseous mixture into the smelt during the decarburization period. The inert gas preferred is argo .

The invention will now be described in more detail with reference to the accompanying drawing, in which Figure la illustrates schematically the carbon content (on a loga- rithmic scale) as a function of the gas injected in accor¬ dance with the standpoint of techniques; and Figure lb illus¬ trates schematically the carbon content (on a logarithmic scale) as a function of gas injection in accordance with the present invention. Figure la illustrates schematically the process of gas injection according to the conventional AOD process. The composition of the gas is changed stepwise: At the high carbon contents, the ratio of 0₂/Ar is equal to 4/1, i.e. four parts by volume oxygen and one part by volume argon. At a carbon content of about 0.4%, the ratio is changed to 1/1, i.e. one part oxygen and one part argon. At a carbon content of about 0.2%, the composition of the gas is again changed, this time to a ratio of 1/4, i.e. to one part oxygen and four parts argon. It is evident that the oxygen content in the gaseous mixture falls with a decreasing carbon content in the smelt at the same time as the argon content increases. The high argon content optimizes decarburization, so that the content of dissolved oxygen in the smelt becomes low and remains low during the whole of the decarburization process. The low content of oxygen injected simultaneously into the smelt is unable to raise the content of dissolved oxygen in the smelt, since the simultaneous injection of a relatively large quantity of argon acts in an opposite direction, i.e. in a decarburizing direction in which the oxygen content is lowered at the same time as the carbon content is lowered to form carbon monoxide.

Figure lb illustrates schematically a gas injection pro¬ cess according to the present invention. The two procedures are identical up to a carbon content of about 0.2-0.1%, i.e. within the high carbon content range, since the present invention relates to the low carbon content range, i.e. a range in which the carbon content is lower than about 0.2%. It is evident from the diagram that the oxygen required is injected with an oxygen-containing gaseous mixture of high oxygen content, namely an 0₂/Ar ratio of 4/1, i.e. with a gaseous mixture of the same composition as the gaseous mix¬ ture used at the beginning of the decarburization process (the first stage or the first phase) . The high concentration of dissolved oxygen in the smelt is maintained by injecting the oxygen-rich mixture into the smelt at successive and intermittent intervals.

Example 1. Conventional AOD process. A crude steel smelt had the following composition:

The liquid crude steel was treated in a 5-tonne (metric tonne) converter, first with a gaseous mixture in which oxygen and argon were present in a ratio of 3/1 down to a carbon content of 0.3%, and then with a gaseous mixture containing oxygen and argon in a ratio of 1/1 down to a carbon content of 0.141%, thereby terminating decarburizing in the high carbon content range. Decarburization in the low carbon content range was commenced with a gaseous mixture containing oxygen and argon in a ratio of 1/4, of which 16 Nm^J 0₂ and 65.3 Nm³ Ar were injected into the smelt, whereafter a sample was taken. The sample was found to have a carbon content of 0.053%.

Thus, 3.2 Nm³ oxygen gas and 13.1 Nm³ argon were uses with each tonne (metric tonne) of steel when decarburizing in the low carbon content range. Example 2. Decarburization in accordance with the pres¬ ent invention.

A crude steel smelt had the following analysis:

The liquid crude steel was treated in a 5-tonne con¬ verter, first with a gaseous mixture containing oxygen and argon in a ratio of 3/1 down to a carbon content of 0.150%, whereafter decarburization in the low carbon content range was commenced in accordance with the invention, first by in- jecting into the smelt 10 Nm³ argon and then 5 Nm³ of a gas¬ eous mixture comprised of three parts oxygen gas and one part argon, i.e. the same mixture proportions as those used when decarburizing in the high carbon content range: 0₂/Ar = 3/1. The decarburization process was continued by repeating the two stages four times: 10 Nm³ argon followed by 5 Nm³ gaseous mixture in which oxygen and argon were present in proportions equal to 3/1. Thus, there was injected into the smelt a total of four times 10 Nm³ argon and four times 5 Nm³ 0₂/Ar = 3/1 mixture. The decarburizing process was terminated by inject- ing 10 Nm³ argon into the smelt. A total of 15 Nm³ oxygen gas and 52 Nm³ argon were injected into the smelt. 3.0 Nm³ oxygen gas and 10.4 Nm³ argon were injected into the smelt for each tonne of steel produced.

The values obtained are summarized in the following table:

It is evident from the table that gas consumption was lower when practicing the present invention while decarburization was greater.

Although the method has been described with reference to the use of argon in the decarburization process, it will be understood that other gases from the group of inert gases can also be used with similar results. Argon does not react chemically when injected into the liquid steel, but acts as a rinsing or flushing gas to remove carbon monoxide from the molten steel. Such gases as nitrogen gas, hydrogen gas, helium, etc. , could have been used equally as well to this end. The use of argon is given only by way of example and has no limiting significance on the inventive method.

Claims

1. A method of manufacturing stainless steel by treating high carbon crude steel in a first stage with a gaseous mixture containing oxygen and inert gas, so as to lower the carbon content of the steel to a value of 0.2-0.1%, and then treating the steel with oxygen and inert gas in a second stage, characterized by treating the steel in said second stage alternately with an oxygen-rich gaseous mixture and inert gas until the carbon content of the steel has fallen to the final value that corresponds to the carbon content of the finished steel.

2. A method according to Claim 1, characterized in that the oxygen-rich gaseous mixture is comprised of at least substan- tially oxygen together with an inert gas.

3. A method according to Claim 1 or 2, characterized in that the inert gas is argon.

4. A method according to one or more of Claims 1-3, charac¬ terized in that the oxygen-rich gaseous mixture contains 90- 70 percent oxygen.

5. A method according to Claim 4, characterized in that gaseous mixture contains 85-75 percent oxygen.