Method for refining molten iron containing chromium

Abstract

A refining method and refining apparatus, able to shorten the time required for refining and reduce the refining costs in decarburization refining of a chromium-contained molten steel, which refining method for chromium-contained molten steel etc. performing decarburization refining by blowing a gas containing oxygen gas into a chromium-contained molten steel under a vacuum or atmospheric pressure and vacuum, said refining method for a chromium-contained molten steel etc. characterized by having a first step for blowing oxygen gas while making the inside of the vessel a pressure of a range of 400 Torr (53 kPa) to atmospheric pressure, a second step for blowing oxygen gas while evacuating the inside of the vessel to 250 to 400 Torr (33 to 53 kPa), and third step for blowing gas while evacuating the inside of the vessel to not more than 250 Torr (33 kPa). Further, a refining method and refining apparatus for an ultra-low carbon chrome melt characterized by performing a first vacuum refining until the third step, then restoring the pressure in the vessel to at least 400 Torr (53 kPa), then performing second vacuum refining while making the bottom blowing gas blow rate at least 0.4 Nm<3>/min per ton steel.

Description

[0001] The present invention relates to a refining method and refining apparatus for chromium-contained molten steel which refine chromium-contained molten steel in a refining vessel while blowing a gas containing oxygen gas.[0002] When refining chromium steel, in particular stainless steel and other chromium steel including at least 9% of chrome, the method of decarburization refining by the AOD method of blowing oxygen gas or a mixed gas of oxygen gas and an inert gas into a melt contained in a refining vessel has been extensively used. In the AOD method, when the decarburization proceeds and the concentration of carbon in the melt drops, the chromium becomes oxidized more easily, so the method has been adopted of raising the ratio of the argon gas or other inert gas in the blown gas along with the drop in the concentration of carbon to suppress the oxidation of chromium. However, in the region of low concentration of carbon, the decarburization rate falls, so a long time is requi...

AI Technical Summary

Benefits of technology

0163] The sealing water height is designed so that at the time of a normal vacuum refining operation, due to the sealing water, the gas inside the hot well will not leak to the outside and the sealing water will not be cut off even with pressure fluctuations of positive pressure and negative pressure of the gas in the hot well. If however the water inside the hot well overflows and is filled to the inside of the water-sea

Problems solved by technology

However, in the region of low concentration of carbon, the decarburization rate falls, so a long time is required until reaching the desired concentration of carbon.

Further, to raise the ratio of the inert gas in the blown gas, the amount of consumption of the expensive inert gas greatly increases.

This is also not advantageous economically.

On the other hand, it is extremely difficult to refine ultra-low carbon chromium steel with a concentration of carbon of not more than 0.01% by the AOD method.

Therefore this cannot be said to be an efficient decarburization refining method.

Therefore, compared with usual refining of low carbon chromium steel, a drop in productivity of the decarburization refining is invited and an increase in the refining costs is caused.

The amount of this generated increases sharply when the degree of vacuum rises (when a high vacuum is reached) and deposits on the alloy addition port, furnace cover, ducts, etc. at the top of the refining vessel to block the same or cause trouble in various equipment and operations and obstruct productivity.

This will also become major trouble in the equipment and worsen the productivity.

Among these, the oxygen blow rate can be controlled to a certain extent by the flow adjustment valve of the oxygen gas, but no sufficient control method has been established for the degree of vacuum.

In the above prior art, when using ejectors, the method of successively starting and stopping a large number of ejectors does not allow extremely fine control of the degree of vacuum since the ranges of capacity of the ejectors themselves are broad.

Further, as seen in Japanese Unexamined Patent Publication (Kokai) No. 10-1716, the method of allowing gas to leak in from the outside while operating the exhaust unit (for example, using nitrogen) enables control of the degree of vacuum to a certain extent, but has the defect that the gas costs rise.

However, while control of the degree of vacuum itself is possible, the exhaust gas sucked in contains a high concentration of CO gas, so when mixing in air containing a combustion-assisting gas constituted by oxygen, there is the danger of combustion and explosion.

Employment for actual machinery is extremely dangerous.

Further, if allowing gas to leak in from the outside, the load on the exhaust unit increases.

Further, the method of controlling the amount of supply of steam to an ejector used in this patent relies on the fact that the optimum steam flow rate of an ejector is distinctive, so changing this remarkably reduces the exhaust performance of the ejector itself.

Further, at the same time, a slight fluctuation in the amount of steam is overly sensitively reflected in the ejector performance, so extremely fine control of the pressure inside the refining vessel becomes difficult.

On the other hand, the method of using a water-sealed type vacuum pump is currently employed for control of the degree of vacuum by pump units, but this is not used together with ejectors, the capa

Images