Metal founding – Process – With measuring – testing – inspecting – or condition determination
Reexamination Certificate
2001-08-31
2004-03-30
Elve, M. Alexandra (Department: 1725)
Metal founding
Process
With measuring, testing, inspecting, or condition determination
C164S151500, C164S450300, C164S155600
Reexamination Certificate
active
06712122
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for continuous casting of steel, particularly to a method for estimating and controlling flow pattern of molten steel in continuous casting and apparatus therefor.
BACKGROUND OF THE INVENTION
Continuous casting of steel is carried out by injecting a molten steel at high speed into a mold via an immersion nozzle. The injected flow induces a molten steel flow in the mold, which molten steel flow gives significant influence on the surface and internal characteristics of produced slab. For example, when the surface flow speed of the melt surface in the mold, (hereinafter referred to simply as “meniscus”), is excessively high, or when vertical eddies are generated in the meniscus, mold powder is trapped into the molten steel. In addition, it is known that the floatation of deoxidized products such as Al
2
O
3
in the molten steel depends on the flow of molten steel. The mold powder and the deoxidized products which are trapped into the slab induce defects caused from the non-metallic inclusions on products.
Flow of molten steel in a mold varies during casting depending on the adhesion of Al
2
O
3
to inside surface of the immersion nozzle, the erosion of the immersion nozzle, the opening of sliding nozzle, and other variables, even under the same casting condition. The phenomenon is an important issue for improving the quality of slab. To this point, there are many proposed methods to detect the flow of molten steel, to control the intensity and direction of the magnetic field to be applied based on the detected state of the molten steel flow, thus to control the flow of molten steel in the mold.
For example, Japanese Unexamined Patent Publication No. 62-252650, (hereinafter referred to simply as “the Prior Art 1”), discloses a method for controlling flow of molten steel. According to the Prior Art 1, thermocouples are buried in a copper plate on shorter side of a mold to detect the difference in molten steel level between the right side and the left side to the immersion nozzle, and the direction of agitation and the thrust of agitation of the magnetic agitator are controlled to zero the level difference.
Japanese Unexamined Patent Publication No. 3-275256, (hereinafter referred to simply as “the Prior Art 2”), discloses a method for controlling deflected flow of molten steel. According to the Prior Art 2, thermocouples are buried in a copper plate on longer side of a mold to measure the temperature distribution on the copper plate on longer side of the mold, and the generation of deflected flow of molten steel is detected on the basis of the temperature distribution at the right half width and the left half width of the mold, thus controlling separately the current being applied to each of the two magnetic brakes of DC magnet type, located on the rear face of longer side of the mold, responding to the detected direction and magnitude of the generated deflected flow of molten steel.
Japanese Unexamined Patent Publication No. 4-284956, (hereinafter referred to simply as “the Prior Art 3”), discloses a method for controlling the speed of injection flow from an immersion nozzle in a magnetic agitator. According to the Prior Art 3, two non-contact distance meters are located above the meniscus between the immersion nozzle and the short side of the mold to measure the variations of melt level at the meniscus, and the propagation speed of the surface waves is derived from a mutual correlation function of these two measured values, thus controlling the injection flow speed from the immersion nozzle so as the propagation speed not to exceed a specified value.
The Prior Art 1 and the Prior Art 2 detect the flow of molten steel based on the temperature distribution on the mold copper plate, and control the flow on the basis of the detected molten steel flow. The variations in the temperature distribution on the mold copper plate are generated not solely caused from the variations of the flow state of molten steel, and they are generated also by the variations of the state of contact between the mold and the solidified shell, by the variations of inflow state of the mold powder, and other variables. Since there occur variations of temperature distribution on the mold copper plate owing to variables other than the flow of molten steel, the Prior Art 1 and the Prior Art 2 that detect the flow of molten steel from solely the temperature distribution on the mold copper plate cannot detect precisely the flow of molten steel.
Although no detail description is given here, investigations carried by the inventors of the present invention confirmed that, for reducing the amount of mold powder and of deoxidized products, solely the prevention of deflected flow in the mold to establish a flow symmetrical in right half width and left half width is not sufficient, and that an optimum flow pattern exists among several flows symmetrical in right half width and left half width.
The Prior Art 3 is an effective means of method for flow control. The Prior Art 3, however, controls only the flow speed of molten steel at meniscus, and is insufficient to detect the flow pattern of molten steel in the mold. In addition, both the Prior Art 1 and the Prior Art 2 cannot detect the flow pattern.
DISCLOSURE OF THE INVENTION
It is an object of the present invention to improve and stabilize the quality of slab manufactured by continuous casting, in particular to improve and stabilize the quality thereof through the prevention of dragging the mold powder, which is induced from a flow pattern of molten steel in the mold, thus assuring feed of good slab to succeeding stages.
In this regard, the present invention provides a method for controlling flow pattern of molten steel to maintain an optimum flow pattern in continuous casting, and further provides a temperature measurement device for mold copper plate to accurately estimate the flow state of molten steel, and a method for estimating the flow state of molten steel in the mold using the temperature measurement device.
To achieve the object, firstly, the present invention provides a method for estimating flow pattern of molten steel in continuous casting, which comprises the steps of:
continuously casting a molten steel injected into a mold through an immersion nozzle;
measuring temperatures of a copper plate in width direction thereof on longer side of the mold at plurality of points using a temperature measurement device; and
estimating a flow pattern of the molten steel in the mold based on the distribution of the copper plate temperatures at individual measurement points.
The method for estimating the flow pattern of molten steel preferably further comprises a step of applying a magnetic field to the molten steel that was injected into the mold so as the detected flow pattern to establish a specified pattern. The magnetic field applied is preferably a moving magnetic field that moves in the horizontal direction.
Furthermore, the method for estimating the flow pattern of molten steel preferably further comprises the steps of:
determining a heat flux being transferred from the molten steel in the mold to a cooling water for the mold copper plate using the mold copper plate temperatures measured by the temperature measurement device, thickness of the mold copper plate, distance between the surface of the mold copper plate on the molten steel side and the tip of a temperature measurement element, temperature of the cooling water for the mold copper plate, thickness of a solidified shell, thickness of a mold powder layer, and temperature of the molten steel in the mold;
deriving a convection heat transfer coefficient, corresponding to the heat flux, between the molten steel and the solidified shell; and
determining flow speed of the molten steel along the solidified shell based on thus derived convection heat transfer coefficient.
The method for estimating the flow pattern may further comprise the step of correcting the temperatures of copper plate on longer side of the mold.
The step of correcting the tempera
Isobe Yoshimitsu
Kubo Noriko
Kubota Jun
Monda Junichi
Nakada Masayuki
Elve M. Alexandra
Frishauf Holtz Goodman & Chick P.C.
McHenry Kevin
NKK Corporation
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