JP2002060827A - Method for melting stainless steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for melting a stainless steel by which stainless steel scale (SUS scale) produced in the step of rolling can be reused as steelmaking raw material and the more SUS scale can be treated without bringing about such danger as the over-flow of the slag frequently developed in the case of using Telstar (aluminum dust), in a method for producing the stainless steel with the melting of scrap in an electric furnace and a subsequent AOD treatment. SOLUTION: The melting of the stainless steel as the preceding step of the AOD treatment is performed according to the following processes: (1) an initial charging (scrap, CaO, ferro-silicon, recarburizer and SUS scale),-power-supply and melting, (2) a supplementary charging (scrap and SUS scale), (3) the addition of reducing agent (ferro-silicon and Telstar) and melt-down, (4) the power-supply and heat-up, (5) the addition of ferro-silicon and the blowing of oxygen and (6) the steel tapping-off.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an improvement in a method for producing stainless steel by dissolving a scrap-based raw material blend in an electric furnace, followed by AOD treatment.

[0002]

2. Description of the Related Art In the production of stainless steel, which involves melting a raw material mainly composed of scrap in an electric furnace and subjecting the obtained molten steel to AOD treatment, AOD dust (hereinafter simply referred to as "dust") is used as waste. Occurs. on the other hand,
A mill scale (hereinafter, referred to as “SUS scale”) is generated from a rolling process for manufacturing a rolled material that is a main intermediate product of stainless steel. The SUS scale, of course,
Oxides such as Fe, Ni, and Cr that constitute stainless steel are mainly used.

As described above, dust is composed of Fe and Ni,
Since the oxides of Cr are the main components, if these oxides are reduced and the metal components are recovered in the molten steel, effective utilization of resources is achieved. As a technique for realizing this, the "Telstar method" has come to be used. The `` Telstar method '' is a method of mixing aluminum ash, that is, a mixture of metallic aluminum and alumina, which is generated when molten aluminum is melted and formed into an ingot, into dust and scale, a so-called `` Telstar '', using an electric furnace. This technology is used as an additive for By making the dust a star, the scattering of the dust was prevented and the reduction efficiency was improved.

[0004] However, the Telstar method has a problem that, in addition to the Telstar containing alumina, the metallic aluminum therein is oxidized to further generate alumina, and as a result, the volume of the slag is significantly increased. Due to this problem, called "slag overflow" in electric furnace operations, the addition of SUS scale to electric furnaces cannot be used at very high rates. At present, it is at most about 1/4 of the generated amount.

[0005] The inventor has sought a means for realizing reduction of SUS scale without increasing the amount of slag, and has noticed that a large amount of carbon is contained in scrap melted in an electric furnace. The possibility of reduction of SUS scale by carbon was considered. Calculating the free energy, we concluded that reduction by carbon could theoretically occur. When we experimented, this expectation was correct and S
It was confirmed that the US scale was effectively reduced.

It would be of great advantage in the production of stainless steel if chromium oxide could be reduced by the carbon in the molten steel. Therefore, before, chromium ore was powdered and tried to be blown into an electric furnace through a hollow electrode. However, chromium ore could not be reduced, and silicon had to be added as a reducing agent. From such experience, it was thought that SUS scale, also an oxide, could not be reduced by the carbon in the molten steel. It is understood that this difference in reducibility is caused by a difference in stability as an oxide, but in any case, the present invention breaks this stereotype.

[0007]

SUMMARY OF THE INVENTION An object of the present invention is to make use of the above-mentioned new knowledge of the inventor and to provide a useful metal-containing S
It is an object of the present invention to provide a method for smelting stainless steel in which US scale can be used as a raw material for steelmaking, which is possible without the danger of slag overflow that is common in the Telstar method.

[0008]

SUMMARY OF THE INVENTION A method for melting stainless steel according to the present invention, which has been completed on the basis of the above-mentioned new facts, is obtained by melting a raw material mixture mainly composed of scrap in an electric furnace. In the production of stainless steel by subjecting molten steel to AOD treatment, SUS is used as a part of the raw material for melting.
It is characterized in that a scale is used and the scale is reduced by carbon in molten steel.

[0009]

DETAILED DESCRIPTION OF THE INVENTION The method for melting stainless steel of the present invention, which is characterized by the use of a SUS scale, is described in detail in FIG.
In general, the following steps are carried out as shown in the flow chart below: 1) Initial loading (scrap, CaO, ferrosilicon, carburizing agent and SUS scale) and electric melting, 2) Top-up (scrap and SUS scale) and electric current Dissolution, 3) addition of reducing agent (ferrosilicon and telstar) and burn-through, 4) energization and heating, 5) addition of ferrosilicon and oxygen blowing, and 6) tapping.

The above steps can be performed with minor modifications. For example, when using a relatively large-capacity electric furnace, it is possible to add scrap or scrap and SUS scale as a third equipment after reloading. However, in any case, it is important that the SUS scale is charged at the early or middle stage of melting and is reduced by carbon contained in the molten steel at the time of meltdown in a high amount of 1.0 to 2.0%. . The reason that the SUS scale should not be added at the latter stage of melting is that when the SUS scale is directly introduced into the electric furnace at the time when the burn-through is near, the amount of Cr ₂ O _{3 in} the slag is large. It has been experienced that the reduction proceeds at one time and sometimes causes boiling.

However, if a large amount of SUS scale is charged during initial loading, it may remain unreacted around the porous plug at the hearth, so it is wise to avoid charging all at once. It is. The charging of the other raw materials, CaO and ferrosilicon, is the same as in the case of melting scrap with a normal electric furnace.

[0012] After the reloading (after the third charging, after that), by the time the burnout occurs, the carbon in the steel is SUS
Ferrosilicon and Telstar are charged to complete the reduction as it is consumed to reduce the oxides of the scale and reduces its amount and becomes insufficient for the reduction. The purpose of charging ferrosilicon and blowing oxygen at a later stage of the heating is to increase the temperature of molten steel.

[0013]

EXAMPLE An electric furnace having a capacity of 70 tons was initially charged with the following composition. (First time) Scrap: 40 tons Ferrosilicon: 0.4 tons SUS scale: 2 tons Carburizing agent: 1.5 tons CaO: 1.75 tons

Twenty minutes after the power was supplied, the following additional charge was charged. (Applied) Scrap: 38 tons SUS scale: 1 ton

When electricity was further supplied for 40 minutes, it melted off. Immediately before, the following charges were made. (Charging before burn-through) Ferrosilicon: 0.5 ton Telstar: 0.5 ton

Ten minutes after melting down, the next amount of oxygen was blown, during which the next charge was made. (Oxygen blowing) 500 Nm ³ / charge (final charge) Ferrosilicon: 0.5 ton

After 30 minutes, 75 tons of molten steel was discharged at a hot water temperature of 1680 ° C. The composition of the steel was as follows. C: 1.5%, Si: 0.2%, Mn: 1.0%, Cr: 18.2
%, Ni: 8.0%, Fe: balance.

When the slag was analyzed, it was found that Ni and Cr were not substantially contained therein.
It was confirmed that the scale was completely reduced.
In the above example, the SUS scale could be charged at 3 tons / charge.
It turned out that it could be processed. In the conventional Telstar method, there was a possibility of the slag overflowing, and only about 1 ton / charge could be processed.
The use of S-scale has greatly advanced, and 100% can be recycled for recycling.

[0019]

According to the present invention, in the production of stainless steel by coordination of electric furnace melting and AOD processing, SUS scale generated from a rolling process is charged into a molten metal prepared in an electric furnace and reduced with carbon in the molten steel. It has been returned to metal and can be used effectively. In the practice of the Telstar method, the problem of slag overflow that often becomes a bottleneck can be avoided by the present invention. In this way, AOD dust and SUS scale generated in the production and processing of stainless steel are reused as resources.

[Brief description of the drawings]

FIG. 1 shows a method for melting stainless steel according to the present invention.
5 is a flowchart showing the process.

Claims (2)

[Claims] In the production of stainless steel, which comprises melting a raw material mixture containing scrap as a main raw material in an electric furnace and subjecting the obtained molten steel to AOD treatment, when stainless steel is rolled as a part of the molten raw material. A method for smelting stainless steel, characterized in that a scale generated in step (a) is used to reduce the oxides constituting the scale with carbon in the molten steel to recycle the raw material. 2. The method for producing stainless steel according to claim 1, which is carried out according to the following steps: 1) Initial loading (scrap, CaO, ferrosilicon, carburizing agent and stainless steel scale) and electric melting, 2) additional loading (Scrap and stainless steel scale) and dissolution by electric current; 3) Addition of reducing agent (ferrosilicon and Telstar) and burn-off; 4) Heating of electric current; 5) Addition of ferrosilicon and oxygen blowing; and 6) Tapping.