Chromium-containing metal and manufacturing method thereof

Abstract

The high-Cr-containing metal according to the present invention is a chromium-containing metal manufactured in an arc melting furnace, and contains at least 85% Cr, up to 0.005% Al, up to 0.1% Si, and up to 0.002% S. The manufacturing method of this high-Cr-containing metal comprises the steps of reducing chromium oxides heated and melted in the arc melting furnace with Si, obtaining a molten metal containing at least 85% Cr, then, discharging slag generated in this Si reduction from the arc melting furnace, adding a basic flux into the arc melting furnace after discharging slag, melting the basic flux by electric arc, refining the molten metal by contacting the slag generated through melting of the basic flux with the molten metal, and then, tapping the molten metal from the arc melting furnace and cast.

Description

TECHNICAL FIELD [0001] The present invention relates to high-purity metal chromium and ferrochromium used in electronic materials and corrosion-resistant and heat-resistant superalloys and so on, and a manufacturing method thereof. BACKGROUND ART [0002] High-purity metal chromium and ferrochromium low in impurities are used for electronic materials and corrosion-resistant and heat-resistant super alloys. In the present invention, these metal chromium and ferrochromium are generically called “chromium-containing metals”. An economically available ore serving as a chromium source of the chromium-containing metal is chromite (FeO.Cr2O3). Because it contains much iron, however, the upper limit of the Cr content in ferrochromium obtained from chromite is about 72 mass %. It is therefore the general practice to use chromium oxide (Cr2O3) available by refining chromite as a raw material for metal chromium. [0003] The known manufacturing methods of metal chromium include the aluminum-thermi...

AI Technical Summary

Benefits of technology

[0010] The present invention was developed in view of these circumstances as described above, and has an object to provide a method for economically and efficiently manufacturing a chromium-containing metal having slight contents of impurities such as aluminum, silicon and sulfur, and to provide a chromium-containing metal having small contents of impurity elements, manufactured by this manufacturing method.

[0011] The present inventor carried out studies to find a method for efficiently manufacturing a chromium-containing metal having small contents of impurities and applicable for electronic materials and corrosion-resistant and heat-resistant superalloys at a low cost. They paid attention to the silicon reduction process having the smallest cost variation from among the conventional arts, and examined a method for removing silicon remaining in molten metal in the silicon reduction process. As a result, the following findings were obtained; silicon can be easily removed by producing a molten metal by the silicon reduction process, and after discharging slag, conducting refining in the presence of an appropriate flux. The findings suggested the possibility to manufacture a chromium-containing metal having small contents of aluminum, silicon and sulfur.

[0014] Because CaO has a high desulfurization ability and is available at a low cost, the basic flux should preferably mainly comprise CaO. Application of a heating treatment in a vacuum atmosphere eliminates gas constituents such as nitrogen and oxygen in the chromium-containing metal and permits manufacture of a high purity chromium-containing metal. Therefore, it is desirable to crush the cast chromium-containing metal, and subject the crushed chromiumcontaining metal to a vacuum heating treatment. By this treatment, it is possible to achieve a nitrogen content of up to 0.005% in the chromium-containing metal. Furthermore, it is desirable to crush a chromium-containing metal, form the crushed metal into briquettes, and subject the resulting briquettes to a vacuum heating treatment.

[0016] More specifically, the chromium-containing metal of the invention solves, in terms of quality, the problem of the aluminum content in the aluminum-thermit process, the problem of the silicon content in the silicon reduction process, and the problem of the sulfur content in the electrolytic reduction process, and in terms of the manufacturing cost, permits continuous production in an arc melting furnace, and production at a lower cost as compared with the aluminum-thermit process and the electrolytic reduction process because of the use of low-cost silicon as a reducing agent.

[0018] The conventional silicon reduction process has a problem of a high silicon content. If the chromium-containing metal is manufactured in a state of insufficient reduction by reducing the amount of metal silicon as a reducing agent (known as a weak reduction state), there would be a decrease in reduction yield of chromium oxide, although the amount of silicon remaining in the metal chromium is reduced to some extent. A smaller amount of blended metal silicon serving as a reducing agent keeps a high concentration of chromium oxide in slag during operation. This results in an increase in viscosity of slag during operation, which in turn leads to a lower rate of the reducing reaction, and hence to a higher concentration of chromium oxide in slag after operation. Since there is a correlation between the amount of chromium oxide in slag and the oxygen content in metal chromium, there is encountered a problem in that a smaller amount of metal silicon as a reducing agent results in an increase in oxygen content in metal chromium.

[0020] In the two-stage refining method of the present invention, in contrast, produced slag is discharged after the first step. There is a correlation between the silicon oxide content in slag and the oxygen content in metal chromium, and between the chromium oxide content in slag and the oxygen content in metal chromium. Discharge of slag after the first step can prevent the oxygen content in metal chromium from increasing. By refining the molten metal by use of a basic flux newly charged, it is possible to reduce not only the silicon content in metal chromium, but also the oxygen content. The reduction yield of chromium oxide never decreases, and the oxygen content in metal chromium is also reduced.

Problems solved by technology

This is however based on a batch process, resulting in a small throughput per batch, a low production efficiency, and the process uses expensive metal aluminum as a reducing agent, thus leading to a high manufacturing cost.

Furthermore, the strong reducing atmosphere causes reduction of refractory constituents used for furnace lining, and the reduced constituents are entangled into the metal chromium, posing problems in purity.

It has however a serious quality problem in that silicon serving as a reducing agent remains in an amount of about 0.7 mass % in the manufactured metal chromium.

In the aluminum-thermit process and the silicon reduction process, if metal chromium is manufactured in a state of insufficient reduction by using a small amount of metal aluminum or metal silicon serving as a reducing agent, the amount of aluminum or silicon remaining in metal chromium decreases.

However, this leads to a deteriorated reduction yield of chromium oxide and an increase in the amount of oxygen in manufactured metal chromium, thus causing another problem in manufacturing cost and in quality.

Necessity of many treatments for refining the chromate solution results in economic problems such as complicated manufacturing steps, a high equipment cost, and a large power consumption.

However, the conventional deironing treatment, including this method, cannot be considered to be high in production efficiency because of complicated treatment process.

Regarding these requirements, the chromium-containing metals manufactured by the conventional methods mentioned above contain much impurity elements, and requires a higher manufacturing cost according as the purity is higher, and cannot be considered to satisfy these requirements.