Metal treatment – Process of modifying or maintaining internal physical... – Magnetic materials
Patent
1990-06-13
1992-11-17
Sheehan, John P.
Metal treatment
Process of modifying or maintaining internal physical...
Magnetic materials
148112, H01F 104
Patent
active
051640244
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to a method of making non-oriented electrical steel sheets having excellent magnetic properties.
BACKGROUND OF THE INVENTION
If a steel blankwork containing more than 1% Si is hot rolled, generally the hot rolled sheet is recrystallized at the surface layer only, and the middle layer is composed of a rolled and non-recrystallized structure. If such a hot rolled sheet is cold rolled and annealed as it is, magnetic properties can not be provided, since a texture beneficial to the magnetic properties develops insufficiently. For securing the magnetic properties after the cold rolling and annealing, the hot rolled structure should be perfectly recrystallized. For example, Japanese Patent Application Laid Open Specifications No.68717/79 or No.97426/80, aiming at such objects, disclose annealings on the hot rolled sheet by a batch annealing or a continuous annealing after hot rolling and coiling.
In the annealing of the hot rolled sheet as such, if the recrystallization treatment is carried out on the hot rolled sheet, and if the annealing is done in an insufficient non-oxidizing atmosphere, the scales remaining on the surface thereof develop and grow thick, and internal oxidized layers grow in the steel surface layer so that a pickling ability after the treatment is markedly deteriorated. On the other hand, in spite of the non-oxidizing atmosphere, if the annealing is done in the atmosphere containing nitrogen, a nitriding reaction is accelerated in the steel surface layer, and it combines with Al in the steel and brings about precipitations of AlN in the steel surface layer. Therefore, AlN particles considerably lower ferrite grain growth in a final annealing. As a result, the steel surface layer is formed with regions of fine ferrite grains of about 20 .mu.m in thickness of about 100 .mu.m, which remarkably deteriorate properties of iron losses and magnetic properties at low magnetic fields.
In view of these circumstances, Japanese Patent Application Laid Open Specification No.35627/82 discloses an art of performing the pickling after the coiling at high temperature and subsequently a batch annealing. However, at coiling temperatures of higher than 700.degree. C., not only does the scale on the surface grow thick, but also an oxidation is caused in the ferrite grains, if the steel sheet contains more than 1 wt % Si. The oxidized layer in the ferrite grain cannot be perfectly removed by the pickling before the annealing of the hot rolled sheet, and the magnetic properties are deteriorated as state above.
Further, in the annealing of the hot rolled sheet, it is necessary to perfectly precipitate AlN for satisfied ferrite grain growth at a final annealing, and coarsen the precipitated AlN, for which a sufficient soaking time should be carried out in the annealing. If the soaking time is short and the coarsening of AlN particles is insufficient, the grain growth at the final annealing is spoiled by the inhibiting effect of movements of the grain boundaries due to AlN particles.
DISCLOSURE OF THE INVENTION
Taking these problems into consideration, it is an object of the invention to provide a method of making non-oriented electrical steel sheets having excellent magnetic properties.
For accomplishing this object, the invention passes the steel of specific chemical composition through the following steps so as to cause the ferrite grains to grow satisfactorily in the final annealing for providing the non-oriented electrical steel sheets having excellent magnetic properties.
1) The steel material is heated at the low temperature for hot rolling, thereby to reduce as much as possible re-solution of AlN particles when a slab is cooled, so that the coarsening of AlN particles is facilitated during the annealing of the hot rolled sheet.
2) The coiling is carried out at the low temperature for checking the amount of scale generation, and a de-scaling is perfectly done after the hot rolling. The de-scaled hot rolled sheet is annealed in the non-oxidizing atmosphere, thereby to
Hosoya Yoshihiro
Jitsukawa Masaharu
Nishimoto Akihiko
Tomita Kunikazu
Urabe Toshiaki
NKK Corporation
Sheehan John P.
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