Metal founding – Process – Shaping liquid metal against a forming surface
Reexamination Certificate
2002-09-13
2004-05-25
Dunn, Tom (Department: 1725)
Metal founding
Process
Shaping liquid metal against a forming surface
C164S486000, C164S455000, C148S111000, C148S112000
Reexamination Certificate
active
06739384
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method for producing a grain oriented electrical steel strip with good magnetic properties from a continuously cast thin strip. The cast strip is cooled in a manner whereby a grain growth inhibitor needed to develop the grain orientation by the process of secondary grain growth is precipitated as a finely and uniformly dispersed phase. The cast strips produced by the present invention exhibit very good physical characteristics.
BACKGROUND OF THE INVENTION
Grain oriented electrical steels are characterized by the type of grain growth inhibitors used, the processing steps used and the level of magnetic properties developed. Typically, grain oriented electrical steels are separated into two classifications, conventional (or regular) grain oriented and high permeability grain oriented, based on the level of the magnetic permeability obtained in the finished steel sheet. The magnetic permeability of steel is typically measured at a magnetic field density of 796 A/m and provides a measurement of the quality of the (110)[001] grain orientation, as measured using Millers indices, in the finished grain oriented electrical steel.
Conventional grain oriented electrical steels typically have magnetic permeability measured at 796 A/m of greater than 1700 and below 1880. Regular grain oriented electrical steels typically contain manganese and sulfur (and/or selenium) which combine to form the principal grain growth inhibitor(s) and are processed using one or two cold reduction steps with an annealing step typically used between cold reduction steps. Aluminum is generally less than 0.005% and other elements, such as antimony, copper, boron and nitrogen, may be used to supplement the inhibitor system to provide grain growth inhibition. Conventional grain oriented electrical steels are well known in the art. U.S. Pat. Nos. 5,288,735 and 5,702,539 (both incorporated herein by reference) describe exemplary processes for the production of conventional grain oriented electrical steel whereby one or two steps of cold reduction, respectively, are used.
High permeability grain oriented electrical steels typically have magnetic permeability measured at 796 A/m of greater than 1880 and below 1980. High permeability grain oriented electrical steels typically contain aluminum and nitrogen which combine to form the principal grain growth inhibitor with one or two cold reduction steps with an annealing step typically used prior to the final cold reduction step. In many exemplary processes for the production of high permeability grain oriented electrical steels in the art, other additions are employed to supplement the grain growth inhibition of the aluminum nitride phase. Such exemplary additions include manganese, sulfur and/or selenium, tin, antimony, copper and boron. High permeability grain oriented electrical steels are well known in the art. U.S. Pat. Nos. 3,853,641 and 3,287,183 (both incorporated herein by reference) describe exemplary methods for the production of high permeability grain oriented electrical steel.
Grain oriented electrical steels are typically produced using ingots or continuously cast slabs as the starting material. Using present production methods, grain oriented electrical steels are processed wherein the starting cast slabs or ingots are heated to an elevated temperature, typically in the range of from about 1200° C. to about 1400° C., and hot rolled to a typical thickness of from about 1.5 mm to about 4.0 mm, which is suitable for further processing. The slab reheating in current methods for the production of grain oriented electrical steels serves to dissolve the grain growth inhibitors which are subsequently precipitated to form a fine dispersed grain growth inhibitor phase. The inhibitor precipitation can be accomplished during or after the step of hot rolling, annealing of the hot rolled strip, and/or annealing of the cold rolled strip. The additional step of breakdown rolling of the slab or ingot prior to heating of the slab or ingot in preparation for hot rolling may be employed to provide a hot rolled strip which has microstructural characteristics better suited to the development of a high quality grain oriented electrical steel after further processing is completed. U.S. Pat. Nos. 3,764,406 and 4,718,951 (both incorporated herein by reference) describe exemplary prior art methods for the breakdown rolling, slab reheating and hot strip rolling used for the production of grain oriented electrical steels.
Typical methods used to process grain oriented electrical steels may include hot band annealing, pickling of the hot rolled or hot rolled and annealed strip, one or more cold rolling steps, a normalizing annealing step between cold rolling steps and a decarburization annealing step between cold rolling steps or after cold rolling to final thickness. The decarburized strip is subsequently coated with an annealing separator coating and subjected to a high temperature final annealing step wherein the (110)[001] grain orientation is developed.
A strip casting process would be advantageous for the production of a grain oriented electrical steel since a number of the conventional processing steps used to produce a strip suitable for further processing can be eliminated. The processing steps which can be eliminated include, but are not limited to, slab or ingot casting, slab or ingot reheating, slab or ingot breakdown rolling, hot roughing and hot strip rolling. Strip casting is known in the art and is described, for example, in the following U.S. Pat. Nos. (all of which are incorporated herein by reference): 6,257,315; 6,237,673; 6,164,366; 6,152,210; 6,129,136; 6,032,722; 5,983,981; 5,924,476; 5,871,039; 5,816,311; 5,810,070; 5,720,335; 5,477,911; and 5,049,204. When employing a strip casting process, at least one casting roll and, preferably, a pair of counter rotating casting rolls is used to produce a strip that is less than about 10 mm in thickness, preferably less than about 5 mm in thickness and, more preferably, about 3 mm in thickness. The application of strip casting to the production of grain oriented electrical steels differs from processes established for the production of stainless steels and carbon steels due to the technically complex roles of the grain growth inhibitor system (such as MnS, MnSe, AIN and the like), grain structure and crystallographic texture which are essential to produce the desired (110)[001] texture by secondary grain growth.
SUMMARY OF THE INVENTION
The present invention relates to a process for producing grain oriented electrical steel from a cast strip wherein rapid secondary cooling of the cast strip is employed to control the precipitation of the grain growth inhibiting phases. The cooling process can be accomplished by the direct application of cooling sprays, directed cooling air/water mist, or impingement cooling of the cast strip onto solid media such as a metal belt or sheet. While the cast strip is typically produced using a twin roll strip caster, alternative methods using a single casting roll or a cooled casting belt may also be used to produce a cast strip having a thickness of about 10 mm or less.
Specifically, the present invention provides a method for producing grain oriented electrical steel strip comprising the steps of:
(a) forming a continuously cast electrical steel strip having a thickness of no greater than about 10 mm;
(b) cooling said strip to a temperature of from about 1150° C. to about 1250° C. such that it becomes solidified; and
(c) subsequently performing a rapid secondary cooling on said steel strip wherein the strip is cooled at a rate of from about 65° C./second to about 150° C./second to a temperature of no greater than about 950° C.
In one embodiment, the steel strip produced by the foregoing process is coiled at a temperature below about 850° C., preferably below about 800° C.
In another embodiment, the present invention provides a method for producing a grain oriented electrical steel strip comprising the steps of:
(a
Huppi Glenn S.
Schoen Jerry W.
Williams Robert S.
AK Properties, Inc.
Dunn Tom
Frost Brown Todd LLC
Lin I,-H.
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