Specialized metallurgical processes – compositions for use therei – Processes – Process control responsive to sensed condition
Reexamination Certificate
1998-06-26
2001-02-20
Andrews, Melvyn (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Processes
Process control responsive to sensed condition
C266S080000, C266S099000
Reexamination Certificate
active
06190434
ABSTRACT:
FIELD OF INVENTION
The present invention refers to a method for determination and control of the amount of nitrogen dissolved in metallic liquid phases and to the device for its realization.
The invention particularly refers to a device permitting a high measurement rate.
STATE OF THE ART
It is known that the in-line control of production processes actually has a direct positive influence both on production economy and on the quality of the end product. It is therefore important to have data acquisition systems able to give quick and reliable answers, to permit a real-time monitoring of a process and relevant timely adjustments, if necessary.
In the iron and steel field, it is particularly needed the possibility of a continuous in-line monitoring of the concentration of such elements as nitrogen and hydrogen which are dissolved into the liquid steel from the gaseous environment and negatively influence the mechanical and physical characteristics of the end product.
Such on-line monitoring is described, for instance, in a Japanese patent application (Appl. No. JP800039365 of Mar. 26, 1980) in which flue gas leaving a converter is sampled and analyzed for CO, CO
2
and N
2
, in order to detect the occurrence of foaming in the converter.
Nitrogen leads to hardening and fragility of ferrous alloys, thus being highly dengerous for steel products such as plates, gas pipes, deep-drawing sheets utilized in car bodies or domestic appliances production, for instance.
The typical phase for nitrogen absorption from the atmosphere is during the steel transfer from the ladle to the mould, through the tundish.
The determination of nitrogen content in liquid steel, directly in the steel shop, and preferably within the mould, could determine in real-time any rise in nitrogen concentration, thus permitting to timely intervene, in well known ways, to lower nitrogen pick-up and eliminate its excess from the metal bath.
In the control of metallic materials production, particularly during the step in which such materials are in the liquid state, up to now the determination of dissolved chemical species, especially gases, was made through quickly solidified metal samples sent to the laboratory for analysis. Such a method, though accurate with errors within 5%, is not a satisfying one, due to excessive delay for the answer, tipically of 10-30 minutes, which does not allows for timely bath composition adjustments.
A device for nitrogen determination in the liquid steel is available from Hereus-ElectroNite International N.V. The measurement method requires the immission into the liquid bath of an inert carrier gas, specifically helium. The nitrogen dissolved into the liquid steel is then divided, according to known laws, between metal and gas, and in the latter tends to a concentration, according to the Henry-Sieverts law, in equilibrium with, and proportional to, the one within the liquid bath.
From the gas coming out from the bath, and containing all the chemical species extracted from the same, all the species interfering with the nitrogen analysis are eliminated through specific molecular sieves, and the purified gas is sent to the analysis made utilizing known apparatuses for thermal conductivity determination.
The probe for introducing into the liquid bath a specific amount of helium, and for sampling it and send it to the analysis, is a disposable one and can give an answer in about 90 seconds, with a declared accuracy of 10%.
It is possible to enhance this accuracy utilizing a well known artifice. After a first measure, a new extraction operation is started utilizing as carrier gas a mixture of helium and nitrogen in which the nitrogen content corresponds to the one previously determined, and the measure is repeated. If the new measure is identical to the previous one, the latter was correct; if a nitrogen content is determined higher than the previous one, the actual nitrogen content of the bath is higher than the one obtained in the previous measure, while if the newly determined content is lower, the bath has a lower nitrogen content. The above artifice permits to obtain an accuracy of about 5%. Such probes for the nitrogen determination, though representing an interesting progress with respect to the classic technique of laboratory analysis, still maintain even important drawbacks:
are of the disposable kind, allowing only a single determination at a time, thus not permitting a continuous monitoring;
it is practically impossible to introduce the probes into the mould with the necessary high cadence, first of all because it is forbidden to stay under the ladle during the casting and then because, even if it should be possible, the presence of casting powders and of semiliquid slag on the bath surface into the mould would make it highly difficult to correctly introduce the probe at the location and within the desired time;
the measure time of 90 seconds is still too long for some kind of intervention, and anyway it have to be strictly maintained: in fact, different residence times of the probe into the bath would make meaningless the measure;
in the nitrogen determination, it is necessary to utilize as carrier gas helium, which is very costly and usually not available in a steel shop.
It is still unsolved the problem of the quick and accurate determination of the nitrogen content in liquid metal baths, in a continuous way or at least with a very short time interval from measure to measure.
SUMMARY OF THE INVENTION
It was now realized, and is part of present invention, a device permitting, when placed in the steel producing plant and for instance in the tundish or in the continuous casting mould, the determination of nitrogen content directly from the liquid phase. Such a device can point out possible nitrogen pick-up in real-time, thus permitting to immediately intervene.
Another aspect of present invention is the method for the determination and control of the nitrogen content of metal baths through said device.
Other objects of present invention shall be evident from the following detailed desctiption of the invention.
REFERENCES:
patent: 5522915 (1996-06-01), Ono et al.
Patents Abstracts of Japan, Publication No. JP56136915, Oct. 1981.
Borgianni Carlo
Di Donato Antonello
Pistelli Maria Ilaria
Andrews Melvyn
Centro Sviluppo Materiali S.p.A.
Hedman, Gibson & Costigan, P.C.
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