Production of Stainless Steels: Part Three

Secondary metallurgy

The blowing of oxygen under vacuum conditions offers a time and cost saving method for the production of stainless, corrosion and heat resistant steels, which are difficult or impossible to produce with conventional methods. The application of vacuum metallurgy is enhanced by the use of oxygen refining. A significant feature of this process is cost-beneficial production of high-chrome steels with very low carbon, nitrogen and hydrogen levels. Another application of this process is heating of low and medium alloy steels by the oxidation of aluminum, carbon and silicon.

Secondary refining equipment used mainly in the final refining step for stainless steel includes AOD furnace and VOD furnace. Stainless steel contains a large amount of chromium as a basic component. Since chromium is a strong oxide-forming element, during normal refining it is difficult to decarburize stainless steel to a sufficiently low carbon level while preventing loss of chromium through oxidation to the slag phase. Thus, low carbon levels are achieved by decreasing the partial pressure of carbon monoxide in the refining atmosphere to ensure preferential decarburization in the presence of chromium. In practice, this is done in the AOD furnace by dilution using argon and in the VOD furnace by reducing the pressure. Figure 1 shows a typical stainless steel production.

Figure 1: A typical stainless steel production facility.

Vacuum Decarburization Process

Most vacuum production (probably 98 percent) was made using the VOD process. The process was developed at Witten (Thyssen) in Germany between 1962 and 1967. Its major attributes include a minimal consumption of argon (e.g., about 1 cubic meter/ton of steel), and the elimination of nitrogen pick-up during tap associated with converter processes (since the VOD ladle is the casting ladle). Silicon consumption in the VOD is only 3 kg/ton; however, because of the 0.3 percent carbon and less than 0.1 percent silicon VOD charge requirement, an additional 3 kg/ton of silicon is used to minimize chromium losses in the arc furnace.

The major drawbacks relative to converter processes are higher refractory consumption, a lower productivity rate in both the EF and VOD (EF time was reduced by 25 percent versus 50 percent, and VOD had a charge-to-tap time of 50-70 minutes versus 40-60 in the AOD), less flexibility relative to the use of lower cost charge mixes (EF carbon ideally is about 0.3 percent versus 1.8 percent, silicon content is less than 0.1 percent versus 0.3 percent, and decreased desulfurization capability), and lower scrap melting capability. Refractory consumption in the VOD is higher than in modern converters. Maintenance and operating costs associated with steam production are also drawbacks.

Generally speaking the main benefits of VOD process are:

• Economic production of ELCN (extra-low carbon nitrogen) grades (C+N < 150 ppm)
• Operation is possible with varying initial carbon contents
• Low chromium oxidation losses resulting from low CO partial pressure
• High rate of chromium recovery through optimized slag metallurgy
• Low final dissolved gas contents
• Improved steel cleanliness
• Adjustment of exact compositional values

Refining treatment

After adjusting the required argon purging rate the tank is closed and the pressure reduced to about 100 mbar. The oxygen lance is driven into the working position and the oxygen blow is directed onto the slag-free spot on the steel surface. The relationship between the initial carbon and the bath level governs the rate blowing, which can vary between 0.25 and 0.7 Nm³/t, min. The start of the refining reaction is indicated by the rise in the tank pressure and by the exhaust gas analyzer. The process is monitored according to the data provided or can be controlled using the same by altering the lance distance, oxygen blow rate and argon purging.

The carbon monoxide reaction can also be monitored by a waste gas analyzer by the measurement of the CO/CO₂ ratio.

There are three stages:

• During stage 1 only silicon is oxidized;
• During stage 2 the CO/CO₂ ratio increases markedly and indicates a strong CO-reaction;
• During stage 3 the changing curve signals an increase in CO₂ in the waste gas which represents the end of the refining reaction. The carbon content is now around 0.08% C and 0.7-1.0% of Cr has been oxidized.

In order to avoid over refining and increased Cr-oxidation the oxygen supply is shut off. This is followed by the switching in of further vacuum pump stages in order to create the low vacuum required for further decarburization with oxygen dissolved in the bath/slag system for another 5-30 minutes.

Figure 2: Vacuum Oxygen Decarburization (VOD) unit.

The initial carbon content of the melt and the selected oxygen blow rate determine the duration of the refining treatment. The second decarburization phase under low vacuum is timed with the final nitrogen content in mind.

The temperature rise of the exothermic refining phase can be calculated quite accurately by taking into consideration the start temperature, the initial carbon and silicon contents and the endpoint of the oxygen blow. Under normal conditions the temperature rise is between 80°C and 150°C.

The tank is flooded after the second decarburization phase, and analysis and a temperature sample are taken and the predetermined reducing and alloying materials are added together with lime and fluorspar. The quantity of lime is calculated so as to guarantee a slag basicity of not less than 2.5. In order to ensure complete de-oxidation and good cleanness of the steel produced, the silicon addition including the amount required for de-oxidation and reduction purposes is calculated to meet the upper specification limit.

After the addition of the reducing agents the melt is treated again under reduced pressure. The reduction reaction is rapid as lime dissolves quickly under vacuum conditions. At the same time hydrogen pick-up from the reducing agents is avoided. Sulfur contents below 0.005%S are achieved without problem. The “white” slag generated is free from metallic oxides. The actual percentage of chromium oxidized is less as some reversion takes place during the vacuum refining process of the iron and manganese in the slag. There may be the need for a further analysis correction after the reduction phase. The teeming temperature is adjusted by the addition of the same grade of cooling scrape.

During the production of titanium stabilized grades the melt will be de-slagged prior to the titanium addition. Homogenization takes place after the addition, with argon stirring for a short period of time.

For a production of supper ferritic grades with chromium content of 29% Cr and with a summation of C+N of less than 150 ppm a special treatment procedure has been developed. As the nitrogen solubility decreases with increasing carbon content, the initial carbon for the refining phase is raised to around 2% C. In order to intensify the bath agitation the treatment ladle is equipped with three inert gas purging plugs. The oxygen blowing during the vacuum refining is not stopped until 0.02% C content has been reached. At that point the initial range of 0.02% to 0.04% N has been reduced to the range of 0.001% to 0.004% N.

During the second decarburizing phase under further reduced vacuum the carbon content of the over-refined melt is lowered below 0.005% C. As these melts are very prone to the carbon and nitrogen pick-up, the choice of alloying material is important and teeming must take place under an inert gas shroud. This process based on intensive argon purging is known as strong-stirring VOD (SS-VOD).