Specialized metallurgical processes – compositions for use therei – Processes – Process control responsive to sensed condition
Reexamination Certificate
2003-12-30
2004-12-14
Andrews, Melvyn (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Processes
Process control responsive to sensed condition
C075S382000, C075S384000, C706S904000
Reexamination Certificate
active
06830606
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for reducing the oxidation loss of chromium and the wear of refractories, by performing refining in accordance with a molten steel temperature and a [C] concentration, in decarburization-refining of chromium-contained steel under a normal or reduced pressure.
BACKGROUND INFORMATION
Methods for refining molten steel in accordance with a [C] concentration in the decarburization-refining of chromium-contained molten steel containing chromium by 11 mass % or more like stainless steel are know. One such method is a dilution-decarburizing method using which a CO gas partial pressure (P
CO
) in an atmosphere is reduced by injecting a dilution gas together with oxygen gas (hereunder referred to simply as “oxygen”) on and after the middle period of decarburization (for example, when a [C] concentration is 0.7 mass % or less). Another such method is a vacuum-decarburizing method, wherein molten steel is charged into a ladle and treated with the interior of the ladle depressurized. The former method is generally called the AOD method or the top-and-bottom blown converter method and the latter method the VOD method.
Further, as described in Japanese Patent Publication Nos. H3-68713 and H4-254509, the vacuum AOD method is used with which depressurized refining is applied on the way of decarburization in the AOD method has been employed recently. In the vacuum AOD method, the amount of supplied oxygen and the degree of vacuum are adjusted in accordance with a [C] concentration in the same way as in the VOD method.
Any of these methods may be intended to efficiently promote decarburization while the oxidation loss of [Cr] in molten steel is suppressed. However, in any of such conventional methods, the increase of the oxidation of [Cr] in proportion to the decrease of a [C] concentration cannot be avoided and thus the amount of the oxidation of [Cr] increases.
To suppress the oxidation loss of [Cr] in molten steel, in the VOD method for example, the amount of supplied oxygen and the degree of vacuum (not more than 100 Torr) are hitherto adjusted in accordance with the progress of decarburization as described in Japanese Patent Publication Nos. S55-89417 and S55-152118. Further, in the AOD method, the ratio of dilution gas is increased in accordance with the decrease of a [C] concentration, or the vacuum refining is applied on the way of decarburization.
In one of the above methods and, for example, in the method using which the ratio of dilution gas is increased in accordance with the decrease of a [C] concentration, when the ratio of dilution gas is increased excessively in an attempt to prevent the oxidation of [Cr], this causes excessive consumption of an expensive dilution gas and therefore an increase in the refining cost. In contrast, when it is attempted to reduce the amount of dilution gas, the oxidation loss of [Cr] cannot be avoided sufficiently.
Further, when chromium-contained molten steel is subjected to decarburization-refining by the VOD method, the AOD method or the vacuum AOD method, it takes a long time for the measurement of a molten steel temperature and the sampling and chemical analysis of the molten steel. For that reason, in practice, the measurement of a molten steel temperature is performed intermittently or is not performed and the concentration of [C] in the molten steel is not analyzed continuously and, therefore, the refining operation conforming to a molten steel temperature and a [C] concentration may not be carried out sufficiently. As a consequence, in actual operation, P
CO
is lowered excessively, namely the degree of vacuum is increased excessively, or a dilution gas is injected abundantly, the oxidation of [Cr] and the erosion of refractories are not suppressed sufficiently, and thus the refining cost increases and the productivity lowers as a result of the decrease in an oxygen supply rate.
As a method for solving the problem of the molten steel temperature measurement among the aforementioned problems, a refining method has been described in Japanese Patent Publication No. H11-124618. Using this method, it is possible to measure the temperature of chromium-contained molten steel continuously, to control the ratio of oxygen gas to the injected gas and the addition amounts of alloys, coolants and auxiliary materials in accordance with the measured molten steel temperature, to reduce the oxidation loss of [Cr], and to mitigate the erosion of refractories of a refining furnace.
In this method, however, the transition of a decarburization oxygen efficiency (the rate of the blown oxygen gas consumed for decarburization, hereunder referred to as “decarburization oxygen efficiency &eegr;”) during blowing and a [C] concentration in molten steel are likely not obtained accurately, thus the control of refining in the operation under a reduced pressure is particularly insufficient, and, as a result, the oxidation loss of [Cr] is still large and the productivity is not improved sufficiently.
SUMMARY OF THE INVENTION
The present invention is provided to address the problems of the conventional technologies, such as insufficient suppression of the oxidation loss of [Cr] and the excessive erosion of refractories, in the decarburization-refining of chromium-contained molten steel under a normal or reduced pressure. One of the objects of the present invention is to reduce the oxidation loss of [Cr] and the erosion of refractories by determining the optimum refining conditions on the basis of a molten steel temperature, molten steel components and gas blowing conditions determined by actual measurement or assumption. Another object of the present invention is to reduce the oxidation loss of [Cr] and the erosion of refractories by using a means for measuring a molten steel temperature continuously, determining a decarburized amount continuously, estimating a [C] concentration in molten steel, and refining the molten steel in accordance with the molten steel temperature and the [C] concentration.
In a thermodynamic equilibrium, a Hilty's equilibrium temperature, a Chipman's equilibrium temperature and a Fuwa et al's equilibrium temperature are known as a molten steel temperature T that balances with [C] and [Cr] concentrations (mass %) in molten steel and a CO partial pressure (P
CO
, atm.) in an atmosphere. Among those, a Hilty's equilibrium temperature T
H
(K) is adopted in the present invention because it is widely used. A Hilty's equilibrium temperature T
H
(K) is described by the following formula (5):
T
H
={−13,800/(−8.76+log([C]
P
CO
/[Cr]))} (5).
It has been determined that, when refining was controlled so that the difference between a molten steel temperature T and a Hilty's equilibrium temperature is approximately equal to a prescribed value or higher than the prescribed value in the refining of chromium-contained molten steel, specifically when refining was controlled so that P
CO
in an atmosphere and a molten steel temperature is adjusted on the basis of the transitions of [C] and [Cr] concentrations in the molten steel, the oxidation loss of [Cr] could be suppressed to the minimum and the erosion of refractories could be prevented without an excessive use of inert gas and an excessive increase of the molten steel temperature.
Further, in the control of refining based on a Hilty's equilibrium temperature, the accuracy of the refining control improves as the accuracy of the estimation of [C] and [Cr] concentrations during the refining improves. A method according to the present invention is provided for estimating [C] and [Cr] concentrations with high accuracy.
According t
Igarashi Masao
Nakao Ryuji
Tanaka Tomoaki
Yoshino Koichiro
Andrews Melvyn
Baker & Botts L.L.P.
Nippon Steel Corporation
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