Metal treatment – Stock – Magnetic
Reexamination Certificate
2001-12-10
2004-06-01
Sheehan, John P. (Department: 1742)
Metal treatment
Stock
Magnetic
C148S310000, C420S087000, C420S119000
Reexamination Certificate
active
06743304
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a non-oriented electrical steel sheet, which is used as an iron core material of an electrical apparatus, having unprecedentedly excellent magnetic properties such as exceedingly high magnetic flux density and low core loss, excellent formability such as excellent punching property, and excellent rust resistance to a product manufactured by using said non-oriented electrical steel sheet and to a production method thereof.
2. Description of the Related Art
In recent years, movements for improving efficiency are rapidly spreading in the field of electrical machinery and apparatuses, specifically rotating machinery and medium- and small-sized transformers, where non-oriented electrical steel sheets are used as iron core materials, amid the worldwide movement for the global environmental preservation, including the saving of electric power and energy and regulations against freon gas emission. For this reason, demands for improving the properties of non-oriented electrical steel sheets, namely, for higher magnetic flux density and lower core loss, are growing stronger.
The core loss reduction of a non-oriented electrical steel sheet has been carried out mainly by increasing the electrical resistivity through the addition of Si and Al and, by doing so, reducing the Joule heat loss caused by the loss of the eddy current that flows through each steel sheet constituting an iron core during its service.
However, among the energy losses of a rotating machine or an apparatus containing an iron core, the energy loss shared by copper loss, which is the Joule heat loss caused by the electric current flowing through a coiled wire wound round the core, cannot be neglected. In order to reduce the copper loss, it is effective to reduce the current density required to excite a core to a certain magnetic field strength, and therefore, the development of a material that exhibits a higher magnetic flux density with a same exciting current cannot be avoided. Namely, the development of a non-oriented electrical steel sheet having ultra-high magnetic flux density is essential.
By realizing a non-oriented electrical steel sheet having ultra-high magnetic flux density, it becomes possible to miniaturize both a rotating machine and an iron core and, for a movable body like an automobile or an electric car where a rotating machine and an iron core are mounted, it also becomes possible to reduce the energy loss during operation by the weight reduction of the whole body. Further, in case of a rotating machine, the torque is increased and a smaller-sized and higher-power rotating machine can be realized.
Thus, if a non-oriented electrical steel sheet having ultra-high magnetic flux density can be realized, not only the energy loss of an iron core and a rotating machine during their operation can be reduced, but also the pervasive effect inestimably extends to the entire equipment system incorporating them.
Conventional production methods of non-oriented electrical steel sheets having high magnetic flux density will be outlined. In Japanese Examined Patent Publication No. S62-61644, disclosed is a method of coarsening a crystal structure after hot-rolling by controlling the hot-rolling finishing temperature to 1000° C. or more, and also coarsening the crystal structure before cold-rolling while eliminating a finish-annealing process. However, in an actual finish hot rolling mill, there is a disadvantage of the difficulty in eliminating the uneven temperature distribution along the longitudinal direction of a steel coil and thus the magnetic properties varying along the longitudinal direction thereof, because the rolling speed at the time when rolls bite the tip of the steel coil is different from the one under a steady rolling state.
In the mean time, in Japanese Unexamined Patent Publication Nos. S54-76422 and S58-136718, disclosed is a method of self-annealing by coiling a hot-rolled steel sheet at a high temperature between 700° C. and 1000° C. and annealing the coil itself with the heat retained therein as a means to suppress a cost increase caused by the addition of a process for annealing the hot-rolled steel sheet and to coarsen the crystal structure before cold-rolling. In the embodiments of these patent publications, however, all of the self-annealing are carried out in the &agr;-phase region for an identical reason, and the coarsening of the crystal structure before cold-rolling is limited.
Further, in Japanese Examined Patent Publication No. H8-32927, disclosed is a technology of pickling a hot-rolled steel sheet consisting of a steel material containing less than 0.01% of C, 0.5% to 3.0% of Si, 0.1% to 1.5% of Mn, 0.1% to 1.0% of Al, 0.005% to 0.016% of P and less than 0.005% of S, thereafter cold-rolling the pickled sheet at a cold reduction ratio of 5% to 20%, annealing the cold-rolled sheet for 0.5 to 10 minutes at a temperature between 850° C. and 1000° C., or for 1 to 10 hours at a temperature between 750° C. and 850° C., and then applying finish-annealing. This method is insufficient in improving magnetic flux density as compared to the conventional hot-rolled steel sheet annealing method and cannot meet the customers' demands for improving the magnetic properties of a non-oriented electrical steel sheet.
In addition, as the methods of improving the magnetic properties of non-oriented electrical steel sheets by improving the primarily re-crystallized texture, disclosed are the methods of manufacturing non-oriented electrical steel sheets excellent in magnetic properties by improving the texture with the addition of Sn in Japanese Unexamined Patent Publication No. S55-158252, Sn and Cu in Japanese Unexamined Patent Publication No. S62-180014, or Sb in Japanese Unexamined Patent Publication No. S59-100217.
However, even the addition of these texture controlling elements, like Sn, Cu or Sb, cannot satisfy the customers demands for a non-oriented electrical steel sheet having ultra-high magnetic flux density and low core loss.
As another method, the improvement in the production process such as devising a finish-annealing heat cycle is implemented as disclosed in Japanese Unexamined Patent Publication S57-35626. However, the attempts reveal little effect on the magnetic flux density improvement, though core loss improvement is seen.
There are three known technologies for obtaining high magnetic flux density by adding Ni, as described below.
In Japanese Unexamined Patent Publication No. H6-271996, disclosed is a method of obtaining high magnetic flux density and low core loss by adding the elements of Sn, Sb, Cu and the like in addition to Ni. However, in actual production, there is a problem of increasing production cost since it is required to control the cooling rate in the two-phase region from the A
r3
point to the A
r1
point either after solidification by rapid cooling or by heating the material again to a temperature not less than the A
C3
transformation temperature after the rapid cooling. Further, in Japanese Unexamined Patent Publication No. H8-246108, disclosed is a material having high magnetic flux density and low anisotropy realized by the addition of Ni. However, in actual production, it is required to finish-anneal the material by heating it to a temperature not less than the A
C3
temperature, and therefore, there is a problem of easily deteriorating the core loss on account of the internal oxidation of the Ni-added steel. In addition, in Japanese Unexamined Patent Publication No. H8-109449, disclosed are a material claiming to have high magnetic flux density and low anisotropy by adding Ni and its production method. However, in the actual production method, the annealing of a hot-rolled steel sheet or the self annealing of the same is essential, and the problem of easily deteriorating the core loss on account of the occurrence of the internal oxidization of Ni during the annealing cannot be solved.
As described above, the conventional technologies can not produce a non-oriented electr
Kawamata Ryutaro
Kubota Takeshi
Kenyon & Kenyon
Nippon Steel Corporation
Sheehan John P.
LandOfFree
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