A METHOD OF MANUFACTURING STAINLESS STEEL

Abstract: The invention relates to a method of manufacturing stainless steel, by treating a raw high-carbon steel in a first stage with a gaseous mixture that contains oxygen and inert gas, therewith lowering the carbon content of the raw steel to a value of 0.2-0.1 %. The steel bath is then treated in a second stage with solely an inert gas, until the carbon content has fallen to the desired value. The invention is characterized in that the steel bath is treated in the first stage alternately with an oxygen-rich gas and with an inert gas, until the carbon content of the steel has fallen to the desired value.

Description: A Method of Manufacturing stainless Steel It is known to produce stainless steel by processing high-carbon raw steel with a mixture of oxygen gas and inert gas. During the decarburizing stage of the process, gaseous mixtures are blown into the melt and the ratio of oxygen gas to inert gas is changed progressively from an oxygen-rich mixture to an oxygen-lean mixture, as the carbon content of the raw steel diminishes. The method is known under the designation AOD, Argon Oxygen Decarburization. The process is begun with a gaseous mixture that con¬ tains four parts of oxygen gas and one part of inert gas, or a gaseous mixture of 4/1. The carbon content of the raw steel at the beginning of the process is between 1 and 1.5%. When the carbon content of the raw steel has fallen to about 0.8%, the oxygen gas/inert gas ratio is changed to 3/1, and then successively to 1/1 and finally to 1/3 at the end of the blow, when the carbon content of the steel has fallen to beneath 0.2%. This mixture ratio is then maintained until decarburization is complete, at which stage the carbon con- tent should have fallen to 0.04%. In addition to carbon, some chromium is also oxidized during the blowing process. This oxidation of chromium repre¬ sents a disadvantage, since chromium shall be recovered by reduction of chromium oxide with silicon. Silicon is an expensive alloy. The efficiency of the process can be mea¬ sured by the quantity of silicon consumed. The change in the mixing ratio between oxygen gas and inert gas is intended to avoid oxidation of chromium to the greatest possible extent. This can only be achieved partially under operating conditions, and consequently some chromium is always oxidized, resulting in the consumption of silicon for the reduction of chromium oxide. The differing mixture ratios of oxygen gas to inert gas cause difficulties and time losses during the decarburizing process. For example, it is necessary to interrupt the pro¬ cess and to check the carbon content of the molten bath in order, to ascertain the correct mixture ratio to be used. Conventional interruptions and sampling processes prolong the process time, increase sampling costs and analysis costs and also increase the heat losses that occur during the process. It has been proposed that endeavors are made to calculate the carbon content of the bath with the use of expensive analysis instruments, without interrupting the process for the purpose of taking samples and sample analysis. Even though these endeavors achieve the result desired, the instrumentation required in this regard complicates the process of manufac¬ ture and therewith also results in higher costs for instru- ment investment and for maintenance. An object of the present invention is to provide a decarburization process for the manufacture of stainless steel in which oxygen gas and inert gas are used without changing the mixture ratio as a function of the carbon con- tent. The extent to which chromium is oxidized is equally as low as the level of chromium oxidation that occurs in the conventional method, in which differing oxygen gas/inert gas mixture ratios are applied. According to the present invention, the decarburizing process is effected with solely one mixture ratio, for exam¬ ple a mixture ratio of four parts of oxygen gas and one part of inert gas, 4/1. This mixture ratio is maintained until the carbon content has fallen to low values, for example a value of 0.2%. Thus, the same mixture ratio is used while decarbur- izing from 1.5 to 0.2%. During this period, considerable quantities of inert gas are blown into the bath in short periods of time. These periodic injections of inert gas into the bath result generally in the same decarburizing process as that achieved with the changed mixture ratios applied in accordance with conventional techniques. The short periods of inert gas injection can take place at a predetermined periodicity, for example a two-minute inert-gas injection at five minute intervals. The inert-gas blowing or injection periods are predetermined and are inde- pendent of the carbon content of the bath, and consequently the process can be continued without needing to interrupt the process for the purpose of taking samples and analyzing said samples. When proceeding in accordance with the invention. the extent to which chromium is oxidized becomes smaller rather than greater, and the process time becomes shorter and productivity therefore higher. The difference between the present invention and the conventional method is considerable and also fundamental. According to the conventional method, the chromium is safe¬ guarded against oxidation by the changed mixture ratios. Oxidation of the chromium is restrained successively by mixtures which are progressively leaner in oxygen, while oxidation of the carbon is promoted at the same time. Thus, according to the conventional method, the purpose of the inert gas is to promote the oxidation of carbon as compared with the oxidation of the chromium. The present invention is based on the use of a totally different mechanism. The periodically repeated injections of inert gas into the bath result in a chemical reaction between the carbon that is present in the bath and the oxygen that is bound to chromium in the form of chromium oxide. The chromium oxide is thus reduced with carbon during the injection of inert gas into the melt. This would explain the surprising fact that the process can be carried out without changing the oxygen gas/nitrogen gas ratio in the gaseous mixture blown into the bath. It should be mentioned, however, that this explanation need not constitute a scientific truth. Another explanation may be that the conventional method which utilizes varying ratios of oxygen gas/inert gas is quite simply unnecessary and can therefore be omitted without serious detriment. In practice, however, the most significant difference between the inventive method and the conventional method is found in the greater efficiency that is achieved when prac¬ ticing the present invention, especially since the process of changing the mixture ratio between oxygen and inert gas at specific carbon contents, involving stopping the process, sampling, and prolonged waiting times, cannot always be carried out with satisfactory accuracy, despite the time taken and the investment made in expensive measuring instru¬ ments. Example 1 A raw steel had the following analysis (%) : C Si Mn Cr Ni 1.21 0.31 1.13 17.21 8.95 The molten raw steel was treated in a 5-tonne converter, by blowing oxygen gas and nitrogen gas into the bath. The treatment process was carried out in three stages: A first stage using four parts of oxygen and one part of nitrogen gas and then one part of oxygen and one part of nitrogen gas, and finally with a mixture containing one part of oxygen and three parts of argon. The switching points between the dif¬ ferent stages was based on carbon concentrations of 0.53% C and 0.18% C. The final carbon content was 0.041%. Time taken: Stage 1 32 min. Stage 2 9 min. Stage 3 12 min. Sample 1 5 min. Sample 2 4 min. Sample 3 5 min. Reduction, including slagging-off 17 min. Adjustment 11 min. Tapping 3 min. Total 98 min. Material consumption: Oxygen gas 29.04 Nm /t Nitrogen gas 18.10 " Argon 10.07 " Silicon 20.32 kg/t Lime 70.28 " Example 2 A raw steel was treated in a 5-tonne converter, The steel had the following analysis (%) : C Si Mn Cr Ni 1.24 0.37 1.32 18.3 9.01 The treatment process was carried out in accordance with the present invention. A gas mixture consisting of four parts of oxygen gas and one part of nitrogen gas was first blown (injected) into the molten bath. 7 minutes after commencing the blow, nitrogen gas was blown into the bath over a blow- period of 30 seconds, the flow of nitrogen gas reaching to 3 . . . . 0.81 Nm /min., tonne. The infection of four parts of oxygen gas and one part of nitrogen into the bath was then recom¬ menced. After a total blowing time of 12 minutes, nitrogen gas was injected or blown into the bath over a blow-period of 3 30 seconds, reaching a flow level of 0.81 Nm /min., tonne. The nitrogen-gas injection period was then repeated at 5- minute intervals. A total of six nitrogen blowing periods was applied. A carbon sample was taken after a blowing time of 38 minutes, and was found to contain 0.17% carbon. Thereafter, only argon was blown into the bath, over a blow-period of 7 minutes, the argon flow reaching to 0.73 Nm 3/min., tonne. The final carbon content was 0.039%. Time taken: Stage 1, including nitrogen blows 38 min. Stage 2 12 min. Sample 1 5 min. Reduction, including slagging-off 18 min. Adjustment 11 min. Tapping 4 min. Total 88 min. Material consumption: 3 Oxygen gas 27.22 Nm /t Nitrogen gas 17.10 " Argon 11.02 " Silicon 18.02 kg/t Lime 62.31 " It will be seen from the two examples that a time saving of 10 minutes was achieved with the inventive method, corre¬ sponding to a productivity gain of about 10%. The inventive method also results in a lower silicon consumption, whereas the remaining consumption values are generally the same. Thus, it will be seen from the examples that the inven¬ tive method results in an increase in productivity and also in a lower consumption of materials. When reference is made to an oxygen-rich gas in the description and Claims of this document, there is meant, in accordance with the present invention, a gaseous mixture which consists of at least 15 percent by volume oxygen, with the remainder consisting of an inert gas, for instance nitro- gen or argon. The gaseous mixture will preferably contain at least 70% oxygen. A preferred oxygen range is 75-90%. By inert gas is meant a gaseous mixture containing at least 70 percent by volume of an inert gas, e.g. nitrogen. According to the present definition, air can also be considered as an inert gas. The present inert gas contains at least 90%, particularly at least 99% inert gas, for instance argon or nitrogen.

Claims: CLAIMS 1. A method of producing stainless steel by treating a raw high-carbon steel in a first stage with a gaseous mixture containing oxygen and inert gas, therewith lowering the carbon content of the steel to a value of 0.2-0.1%, and thereafter introducing solely inert gas into the steel bath in a second stage until the carbon content has fallen to the desired value, characterized by treating the steel in the first stage alternately with an oxygen-rich gas and with an inert gas until the carbon content of the steel has fallen to the indicated value. 2. A method according to Claim 1, characterized by treating the steel bath with inert gas in the first treatment stage over a period of time which constitutes 50-5% of the time over which the steel bath is treated with the oxygen-rich gas. 3. A method according to Claim 1 or 2, characterized by repeating the treatment with inert gas in the first stage at constant time intervals. 4. A method according to one or more of Claims 1-3, characterized by effecting inert-gas treatment in the first stage over time periods of mutually equal durations. 5. A method according to one or more of Claims 1-4, characterized in that the oxygen content of the oxygen-rich gas is constant. 6. A method according to one or more of Claims 1-4, characterized in that the oxygen content is at least 70%, preferably 75-90%. 7. A method according to one or more of Claims 1-6, characterized in that the inert gas used in the first stage is nitrogen. 8. A method according to one or more of Claims 1-6, characterized in that the inert gas used in the first stage is argon. 9. A method according to one or more of Claims 1-6, characterized in that the inert gas used in the first stage is air.

Inventors: BRUNNER MIKAEL, BRUNNER, MIKAEL
Applicants: AGA AB, AGA AKTIEBOLAG
Application year: 1992
Main classification: C21C5
IPC codes: C21C5, C21C7