FI97626C - Procedure for manufacturing stainless steel - Google Patents

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

97626

Method for the production of stainless steel Förfarande för att tillverka rostfritt Stal

The present invention relates to a process for producing low carbon stainless steel, in which gaseous oxygen is blown into a steel melt in a boiler and in which the mixing function is achieved by blowing inert gas.

It is known from SE-A-435936 to refine chromium-containing steel by blowing gaseous oxygen onto or into a steel melt in a boiler in order to achieve a lower carbon content of 0.03%, whereby the mixing function is achieved by blowing inert gas. In this process, the first refining step is carried out using gaseous oxygen at atmospheric pressure until the carbon content has dropped between 0.2 and 0.4% and the second refining step is carried out in said boiler by continuing to stir the melt with inert gas but interrupting the gaseous oxygen supply. In this way, the pressure above the defrost bath is continuously reduced to 10 torr (> 1.33 KPa) or less.

From Canadian Metallurgical Quarterly, Vol. 10, No. 2, pp. 129-136 (Choulet, R J et al.) It is known to produce stainless steel by processing high carbon crude steel with a mixture of oxygen gas and an inert gas. During the Melloing step of the process, gaseous mixtures are blown into the melt and the ratio of oxygen gas to inert gas is progressively changed from an oxygen-rich mixture to an oxygen-lean mixture as the carbon content of the crude steel decreases. The method is known as AOD, Argon Oxygen Decarburization.

It is further known from the latter reference that continuously exchanged oxygen / argon proportions to minimize the use of argon can be approximated by two or more stepwise changes in gas ratios.

Such a process can be started with a gaseous mixture containing four parts of oxygen gas and one part of an inert gas, i.e. a 4/1 gaseous mixture. The carbon content of the crude steel at the beginning of the process is between 1 and 1.5%. When the carbon content of the crude steel has dropped 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, whereby the carbon content of the steel has dropped below 0, 2%. This mixture ratio is then maintained until decarbonization is complete, at which point the carbon content should have dropped to 0.04%.

In addition to carbon, chromium is also oxidized during the blowing process. This oxidation of chromium is detrimental because chromium has to be regenerated by reduction of chromium oxide with silicon. Silicon is an expensive alloying agent. The efficiency of the process can be measured by the amount of silicon consumed.

The goal of changing the mixture ratio between oxygen gas and inert gas is to avoid oxidation of chromium as much as possible. This can only be achieved partially during the operating conditions and thus part of the chromium is always oxidized, resulting in the consumption of silicon to reduce the chromium oxide.

Varying mixture ratios of oxygen gas and inert gas cause inconveniences and time losses during the riot process.

For example, it is necessary to interrupt the process and check the carbon content of the melt bath to ensure the correct mixture ratio to be used. Conventional interrupts and sampling processes extend process time, increase sampling and analysis costs, and further increase heat loss that occurs during the process. It has been proposed that experiments be performed to calculate the carbon content of the bath using expensive analytical instruments without interrupting the 3,97626 process for sampling and analyzing samples. Although these calcifications achieve the desired result, the instrumentation required in this context complicates the manufacturing process and, as a result, also results in higher costs for instrument investment and maintenance.

It is an object of the present invention to provide a annealing process for the production of stainless steel in which oxygen gas and inert gas are used without changing the mixture ratio as a function of carbon content. The amount of oxidizable chromium is as low as the level of chromium oxidation that occurs in a conventional method using varying oxygen gas / inert gas mixture ratios.

According to the present invention, the quenching process is carried out with a single mixture ratio, for example four parts oxygen gas and one part inert gas with a mixture ratio, i.e. 4/1.

This seasonal ratio is maintained until the carbon content has dropped to low values, for example to 0.2%. Thus, the same mixture ratio is used to remove carbon from 1.5% to 0.2%. During this period, substantial amounts of inert gas are blown into the bath for short periods of time. These periodic injections of inert gas into the bath generally result in the same decarbonization process as has been achieved with the modified mixing ratios applied in conventional techniques.

Short periods of inert gas injection may occur at predetermined intervals, for example, two minutes of inert gas injection at five minute intervals. The periods of blowing or injecting the inert gas are predetermined and independent of the carbon content of the bath, and as a result the process can be continued without the need to interrupt the process for taking samples and analyzing said samples. In the process according to the invention, the amount of oxidized chromium becomes smaller than 97,926 rather than higher and the processing time becomes shorter and the productivity thus higher.

The difference between the method according to the present invention and the conventional method is considerable and further essential. According to the conventional method, chromium is protected against oxidation by altered alloy ratios. Chromium oxidation is curbed by successive alloys that are progressively leaner in oxygen content while promoting carbon oxidation. Thus, according to the conventional method, the purpose of the inert gas is to promote the oxidation of carbon compared to the oxidation of chromium.

The present invention is based on the use of a completely different mechanism. Periodic repetitive injections of inert gas into the bath result in a chemical reaction between the carbon present in the bath and the oxygen bound to the chromium in the form of chromium oxide. Thus, carbon reduces the inert gas of chromium oxide during melt injection.

This would explain the surprising fact that the process can be performed without changing the oxygen gas / nitrogen gas ratio in the gaseous mixture blown into the bath. It should be noted, however, that this explanation should not be considered a scientific truth. Another explanation may be that a conventional method that utilizes varying oxygen gas / inert gas ratios is simply unnecessary and can thus be omitted without serious inconvenience.

In practice, however, the most significant difference between the method according to the invention and the conventional method can be found in the higher efficiency achieved in the practice of the present invention, especially since the process of changing the oxygen to inert gas mixture at certain carbon concentrations, including stopping, sampling and extended waiting times, with satisfactory accuracy, despite the 5 97626 time spent and the investment in expensive measuring instruments.

Example 1

The analysis of the crude steel was as follows (%): C Si Mn Cr Ni 1.21 0, 31 1.13 17, 21 8.95

The molten crude steel was treated in a 5 ton converter by blowing oxygen gas and nitrogen gas into the bath. The treatment process was performed in three steps: In the first step, four parts oxygen and one part nitrogen gas were used, followed by one part oxygen and one part nitrogen gas, and finally a mixture containing one part oxygen and three parts argon.

The exchange points between the different phases were based on carbon concentrations of 0.53% C and 0.18% C. The final carbon content was 0.041%.

Used time:

Step 1 32 min.

Step 2 9 min

Step 3 12 min.

Sample 1 5 min.

Sample 2 4 min.

; Sample 3 5 min.

Reduction, includes slag removal 17 min.

Setting 11 min.

Running out 3 min.

A total of 98 min.

Material consumption:

Oxygen gas 29.04 Nm3 / t

Nitrogen gas 18, 10 "

Argonia 10.07 "

Silicon 20.32 kg / t

Lime 70.28 6 97626

Example 2

The crude steel was processed in a 5 ton converter. The analysis of the steel was as follows (%): C Si Mn Cr Ni 1.24 0, 37 1.32 18, 3 9.01 The treatment process was performed according to the present invention. Initially, a gas mixture containing four parts of oxygen gas and one part of nitrogen gas was blown (injected) into the melt bath. 7 minutes after the start of the blowing, nitrogen gas was blown into the bath for a blowing period of 30 seconds, when the nitrogen gas flow rate reached 0.81 Nm 3 / min. , per tonne. The injection of four parts of oxygen gas and one part of nitrogen into the bath was then continued. After a total purge time of 12 minutes, nitrogen gas was injected or blown into the bath for a 30 second purge period, achieving a flow level of 0.81 Nm 3 / min. , per tonne. The nitrogen gas injection period was then repeated at 5 minute intervals. A total of six nitrogen purge periods were performed. The 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 for a 7 minute blowing period when the argon flow reached 0.73 Nm 3 / min. tonnes. The final carbon content was 0.039%.

Used time:

Step 1, includes nitrogen blows for 38 min.

Step 2 12 min.

Sample 1 5 min.

Reduction, includes slag removal 18 min.

Setting 11 min.

Running out 4 min.

A total of 88 min.

7 97626

Material consumption:

Oxygen gas 27.22 Nm3 / t

Nitrogen gas 17, 10 "

Argonia 11.02 "

Silicon 18.02 kg / t

Lime 62.31

It can be seen from the two examples that the method according to the invention achieved a time saving of 10 minutes, which corresponds to an increase in productivity of about 10%. In addition, the method according to the invention results in lower silicon consumption, while the other consumption values are generally the same.

Thus, it can be seen from the examples that the method according to the invention leads to an increase in productivity and, in addition, to a lower consumption of materials.

In the description and claims of this document, when referring to an oxygen-rich gas according to the present invention, there is a gaseous mixture containing at least 15% by volume of oxygen, the remainder containing an inert gas such as nitrogen or argon. The gaseous mixture preferably contains at least 70% oxygen. The preferred oxygen range is 75 to 90%. Inert gas means gaseous! a mixture containing at least 70% by volume of an inert gas, e.g. nitrogen. According to the previous definition, air can also be considered as an inert gas. The inert gas used contains at least 90%, in particular at least 99% of an inert gas, such as argon or nitrogen.

Claims (9)

A process for the manufacture of stainless steel by treating a high-carbon scaffold in a first stage with a gas mixture containing oxygen and inert gas, the carbon content of the scaffold being reduced to a value of 0.2-0.1%, and then in a second step by feeding only inert gas until the carbon content has dropped to the desired value, characterized in that in the first step the steel is treated alternately with an oxygen-rich gas and inert gas until the carbon content of the base has dropped to the specified value. Process according to claim 1, characterized in that the treatment time of the inert gas with inert gas in the first treatment stage is 50-5% of the treatment time with the oxygen-rich gas. Method according to claim 1 or 2, characterized in that the treatment with inert gas in the first step is repeated at constant time intervals. Method according to one or more of claims 1-3, characterized in that the treatment periods for the inert gas treatment in the first stage are the same length. Process according to one or more of claims 1-4, characterized in that the oxygen content of the oxygen-rich gas is constant. Process according to one or more of claims 1-4, characterized in that the oxygen content is at least 70%, preferably 75-90%. Process according to one or more of claims 1-6, characterized in that the inert gas in the first stage is nitrogen. 97626 Process according to one or more of claims 1-6, characterized in that the inert gas in the first stage is argon. Process according to one or more of claims 1-6, characterized in that the inert gas in the first stage is air.