Metal treatment – Process of modifying or maintaining internal physical... – With casting or solidifying from melt
Reexamination Certificate
1999-07-09
2001-07-24
Yee, Deborah (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
With casting or solidifying from melt
C148S602000, C148S654000
Reexamination Certificate
active
06264767
ABSTRACT:
FIELD OF INVENTION
This invention relates to an apparatus combination in or for use in a steel-making mill, and a preferred method of operating same.
BACKGROUND OF THE INVENTION
In in-line rolling mills, high speeds of operation are essential if undue cooling of the steel below optimal rolling temperatures is to be avoided. Consequently, in such mills, control of both internal microstructure and of external surface properties is difficult to achieve to produce optimum metallurgical characteristics in the finished steel product.
In a conventional continuous casting steel mill using a reversing rolling mill such as a Steckel mill for rolling, steel is cast into a strand by a caster, is severed into slabs or other preferred portions downstream of the caster, and passes to a reheat furnace where it is heated to a uniform pre-rolling temperature. From the reheat furnace, the steel may be rolled by an initial roughing mill, but at least the final rolling steps are effected by a Steckel mill. If thicker plate product is produced by the Steckel mill, the Steckel mill rolling is confined to flat-pass rolling. For thinner, coilable plate and strip product the steel may be coiled within the coiler furnace during at least the later rolling passes. Following rolling, the steel is typically cooled, and if finished as a plate product, is conventionally passed through a hot leveller or cold leveller or both. Coilable products are typically up-coiled or down-coiled and offloaded for shipment; flat plate is cut to preferred length and may be subjected to final cooling on a cooling bed before stacking and shipment.
The use of a Steckel mill to roll steel is well established in industrial practice and in the technical literature. However, the optimization of steel quality from both a surface standpoint and an internal microstructure standpoint, especially for flat steel plate products, has not been achieved by others. In particular, it has not been understood prior to the development of the present invention by the present inventors that a unique combination of steel cooling, steel heating and steel reduction steps, taken in a controlled manner between the caster and the end of the downstream line, can lead to preferred surface and metallurgical characteristics in finished steel plate.
SUMMARY OF THE INVENTION
We have found that optimal metallurgical characteristics of the steel thus produced, and particularly steel plate products thus produced, may be optimized by a careful selection of the combination of apparatus that is provided between the caster and the more remote downstream apparatus, and if appropriate operational constraints are applied to this combination of equipment.
Generally speaking, preferred metallurgical results require an overall reduction in thickness of the steel of at least about 3:1. Consequently, if the objective is to make ½″ plate, the initial casting must be at least 1.5″ thick. Further, for various other reasons it may be preferred to use castings of greater thickness than three times the target end-product thickness. The present invention is preferably used with castings of at least about 3 inches in thickness.
Specifically, the apparatus combination protected by this invention comprises a quench facility located downstream of the caster and upstream of the reheat furnace (“upstream quench station”), and a controlled temperature reduction facility located closely downstream of a Steckel mill or other similar reversing rolling mill. Note that “closely” does not preclude the possible installation of devices between the Steckel mill and temperature reduction facility. A hot flying shear or other suitable severing device is also required; ideally two hot flying shears one upstream and one downstream of the temperature reduction facility would be used. However, depending upon the intended use of the rolling mill, it is possible to make do with one flying shear. Some preferred uses of the rolling mill favour upstream location of the flying shear; others favour downstream location. For example, some of the rolling optimization aspects of the invention and the objective of presenting a clean vertical leading edge of the steel to the downstream quench station favour an upstream location of the hot flying shear. On the other hand, the need to accelerate a leading severed portion of the steel relative to a trailing portion when the steel is cut to length favours a cut-to-length flying shear downstream of the temperature reduction facility. The applicable considerations will be reviewed further later in this specification. To accommodate all possible objectives, two shears may be provided—one upstream of the temperature reduction facility, and one downstream. If the mill is to produce steel of an appreciable range of thicknesses, one type of shear could be provided for thinner steel, another type for thicker steel. Further, some shears work well only for hot steel, others for cold. The mill designer will take these points into consideration in the overall equipment selection and design.
The upstream quench station may be located either upstream or downstream of the slab severing apparatus (typically a cutting torch). This upstream quench station is constrained to apply to the steel a controlled surface quench with a depth of penetration of at least ½ inch, and preferably no more than about ¾ inch (while quench penetration deeper than ¾ inch is metallurgically tolerable, it conveys no additional benefit so far as the steel surface quality is concerned, and the greater the depth of penetration of the quench, the greater the amount of heat required in the reheat furnace to heat the steel to uniform pre-rolling temperature, and thus the higher the cost of production). The quench imparted to the steel must be sufficient to alter the surface layer of the steel from austenite to some other microstructure such as ferrite or pearlite. Preferably the quench is initiated when the surface temperature is at or above the austenite-ferrite transformation start temperature Ar
3
, although start temperatures somewhat below the Ar
3
can be tolerated, even though such lower temperatures are not optimum. A reduction in temperature by the quench station in the order of about 250-300° C. is preferably effected. Assuming a preferred start temperature at or above the steel's transformation start temperature Ar
3
, a suitable completion temperature is at or below the steel's austenite-ferrite transformation completion temperature Ar
1
.
The steel transformation start and completion temperatures Ar
3
, Ar
1
depend on the type of steel that is cast and the cooling rate. Most types of steel cast in a conventional continuous casting mill are suitable for application of the invention; for example, typical plain carbon steels suitable for quenching in accordance with the invention include steels having 0.03-0.2% carbon content. The cooling rate of a steel product is not constant throughout its body; cooling rates differ at different depths beneath the product surface. Different cooling rates will transform austenite to different combinations of transformation products; as the steel's cooling rate varies with strand depth, it follows that the transformed microstructure will differ with strand depth.
For optimal results, the quench should be applied in a manner that responds to the surface temperature gradient of the casting, which typically is hotter at the inner surface portions near the longitudinal centre of the casting than at the outer side edges of the casting. To this end, a transversely differential spray is arranged within the quench unit. Since spray applied above the casting is typically more effective than spray applied underneath the casting, the ratio of bottom spray flow rates to top spray flow rates is preferably in about the range of 1.2 to 1.5. Since the side edges of the casting tend to cool more rapidly than the central portions, and since there is a tendency of any accumulation of surface water to flow from the central portion over
Boecker Robert Joseph
Collins Laurie E.
Dorricott Jonathan
Frank William R.
Russo Joseph Duane
Barrigar Robert H.
Barrigar & Moss
IPSCO Enterprises Inc.
Yee Deborah
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