Metal founding – Process – With measuring – testing – inspecting – or condition determination
Reexamination Certificate
2002-02-19
2003-05-06
Dunn, Tom (Department: 1725)
Metal founding
Process
With measuring, testing, inspecting, or condition determination
C164S154700, C164S486000, C148S547000, C148S654000
Reexamination Certificate
active
06557622
ABSTRACT:
FIELD OF THE INVENTION
The present invention comprises methods of quenching a continuously cast steel product upstream of a reheat furnace that brings the steel to a uniform initial rolling temperature. One purpose served by the invention is to eliminate or reduce the incidence and severity of surface defects in the steel that occur during reduction rolling. There are a number of inventive aspects of the applicant's methods that collectively may comprise more than one invention, but for convenience, reference will be made to “the invention” on the understanding that the term covers the collectivity of inventions claimed herein.
BACKGROUND OF THE INVENTION
In conventional continuous casting mills with direct hot charging, steel in a caster assembly is cast into a continuous strand, and passes through a strand containment apparatus in which the steel surface is cooled and the strand changes direction from the vertical to the horizontal. The strand is then conveyed to a severing apparatus where it is severed into slabs, blooms, billets or other products. The slab or other product then enters a reheat furnace for heating to a uniform temperature suitable for downstream rolling and other processing.
Problems encountered with plate steel product produced by such continuous casting mills include the tendency for areas around one or more surfaces of the steel product to exhibit brittleness, cracking, sponging, and other surface defects (hereinafter collectively referred to as “surface defects” for convenience). Surface defects are especially prevalent after the interim steel product is subjected to downstream rolling or other stresses. Although the causes of such surface defects are not completely understood, it has been observed that surface defects tend to occur frequently in steel products having surfaces that are at or above the steel's austenite-to-ferrite transformation start temperature (Ar
3
) when the product exits the caster assembly, and which cool to a temperature above the steel's austenite-to-ferrite transformation completion temperature (Ar
1
) as the product enters the reheat furnace, then are reheated to a temperature above the transformation start temperature when the product is inside the reheat furnace. Steel products that tend to be particularly susceptible to surface defects include low- to high-carbon steels and low-alloy steels, all of which may contain aluminum (Al) and residual elements such as sulphur (S), phosphorus (P), nitrogen (N), and copper (Cu).
While an understanding of the causes of the surface defects is not per se necessary for the practice of the invention, some discussion of the applicant's understanding of the phenomenon may be helpful to the reader. Steel product exiting the caster assembly has a coarse austenite grain structure. As the steel product cools to a temperature above the transformation completion temperature Ar
1
of the metal, various elements including residual elements migrate to the austenite grain boundaries where they will reside as solute elements, or eventually combine to form precipitates. If the steel product has not cooled to below the transformation completion temperature Ar
1
before reheating in the reheat furnace, these elements, in either solute or precipitate form, remain at or near the partially transformed austenite grain boundaries. The presence of these elements on grain boundaries and/or the development of precipitate-free zones adjacent to grain boundaries can be detrimental to the ductility of the steel product and may also contribute to the manifestation of one or more types of surface defects. It appears that the principal culprit in many cases is the copper and/or aluminum nitride present.
If the interim steel product is taken off-line and left for several hours to cool slowly in still air, the entire product will have completely transformed from coarse-grained austenite to other microconstituents, such as ferrite or pearlite. Reheating this product in a reheat furnace to above the Ac
3
,(about 900° C. for most steels of interest) the critical temperature above which there is austenite, re-transforms the product into fine-grained austenite. It has been found that a product having such a fine-grained austenitic microstructure tends to be free from surface defects. However, such slow cooling requires the product to be taken off-line for an undesirably lengthy period of time, thereby slowing down steel production.
It has been found that instead of re-transforming the entire steel product into fine-grained austenite, it is necessary to re-transform only the surface layers to a suitable depth to achieve a product that is for the most part free of surface defects. However, off-line slow air cooling to achieve a re-transformed layer of sufficient depth requires an undesirably lengthy time.
Previously known methods have been devised in which a slab is taken offline, immersion-quenched in a quench tank, then returned on-line for transfer into the reheat furnace. In such methods, the temperature of the slab surfaces is often reduced below the Ar
1
, i.e. the steel's transformation completion temperature, before the slab is reheated in the reheat furnace. It has been found that an immersion-quenched slab tends to exhibit undesirably inconsistent metallurgical properties along its length. This inconsistency appears to be due to the formation of a lengthwise temperature gradient on the slab prior to its immersion; since the slab is cast from a continuous caster, its downstream portions have had more time to cool than its upstream portions.
In another known method, the casting is spray-quenched prior to severing into slabs and prior to entering the reheat furnace. An example of such a method is described in U.S. Pat. No. 5,634,512 (Bombardelli et al.). According to Bombardelli, quenching the strand is accomplished by a quench apparatus that sprays water under pressure through a plurality of sprayer nozzles onto the surfaces of the strand so that the surfaces are rapidly cooled.
A problem associated with Bombardelli's teaching is that the quench apparatus tends to create a transformed surface layer having an inconsistent depth and microstructure in steel products that, because of casting line speed variations, have developed irregular transverse and longitudinal temperature profiles along their surfaces prior to entering into the quench apparatus. Because the spray intensity in the Bombardelli apparatus cannot be varied amongst nozzles in a group of nozzles directed at a product surface, a product surface having a non-uniform pre-quench temperature profile will have a non-uniform post-quench temperature profile after being sprayed by the Bombardelli quench apparatus, thereby causing inconsistent surface layer properties, including inconsistent microstructures at any given depth of the surface layer.
SUMMARY OF THE INVENTION
The invention comprises a method for in-line quenching a steel product. Apparatus suitable for the practice of the method would include, in downstream progression: (1) a caster mould and a strand containment and straightening apparatus, all within a caster assembly; (2) a severing apparatus for severing the steel product from a strand into slabs or other products; and (3) a reheat furnace for reheating the steel product after it has been severed, as well as the apparatus more particularly described and claimed in U.S. application Ser. No. 09/350,319. The steel is normally conveyed from the caster to the reheat furnace on a plurality of spaced conveyor rolls (table rolls).
According to the invention, quenching is effected by applying a plurality of controlled pressurized sprays of cooling fluid (preferably air-mist) to selected portions of one or more surfaces of the steel product exiting the caster, so as to effect in a surface layer of the steel casting a metallurgical change from the initial austenite to desired microconstituents such as ferrite, pearlite, or other transformation products. The quench effects this change to a desired depth of penetration from the surfa
Boecker Robert J.
Collins Laurie E.
Dorricott Jonathan
Frank William R.
Russo Joseph D.
Barrigar Robert H.
Dunn Tom
IPSCO Enterprises Inc.
Lin I.-H.
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