Metal treatment – Process of modifying or maintaining internal physical... – Magnetic materials
Reexamination Certificate
1999-09-17
2001-09-11
Sheehan, John (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Magnetic materials
C148S113000
Reexamination Certificate
active
06287392
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a grain-oriented silicon steel sheet suitable for use as the iron core of transformers and other electric machines, and also to a process for producing the same. The silicon steel sheet possesses both good coating properties and good magnetic properties.
2. Description of the Related Art
Grain-oriented silicon steel sheets are used mainly as a material of the iron core of transformers and rotating machines. They are required to have such magnetic properties as high magnetic flux density, low iron loss, and small magnetostriction. Nowadays, there is an increasing demand for grain-oriented silicon steel sheets superior in magnetic properties from the standpoint of energy saving and material saving.
In the production of grain-oriented silicon steel sheets superior in magnetic properties, it is important that the resulting product has a structure such that the grains of secondary recrystallization are densely arranged along the (110)[001] orientation or so-called Goss orientation.
Grain-oriented steel sheets as mentioned above are produced by the following steps. First, grain-oriented silicon steel slabs are produced which contain MnS, MnSe, AlN, BN, or the like as an inhibitor necessary for secondary recrystallization. After heating, they undergo hot rolling. The resulting hot-rolled sheets undergo annealing, if necessary, and then undergo cold rolling (down to the final thickness) once or twice or more, with any intermediate annealing interposed. The cold-rolled sheets undergo decarburization annealing. With an annealing separator (composed mainly of MgO) coated, the steel sheets undergo final finishing annealing.
The grain-oriented silicon steel sheets obtained in this manner usually have their surfaces coated with an insulating film composed mainly of forsterite (Mg
2
SiO
4
) (which is simply referred to as “forsterite coating” hereinafter). This forsterite coating gives the steel sheets not only surface electrical insulation but also tensile stress resulting from low thermal expansion. Therefore, it improves iron loss as well as magnetostriction.
After final finishing annealing, grain-oriented silicon steel sheets are usually given a vitreous insulating coating (simply referred to as glass coating hereinafter) on the forsterite coating. This glass coating is very thin and transparent. Therefore, it is forsterite coating rather than glass coating that eventually determines the external appearance of the product. In other words, the appearance of forsterite coating greatly affects the product value. For example, any product would be regarded as inadequate if it had forsterite coating formed such that the base metal is partly exposed. Thus, the properties of forsterite coating seriously affect the product yields. That is, forsterite coating is required to have an uniform appearance without flaws, and with good adhesion to prevent peeling at the time of shearing, punching, and bending. Moreover, forsterite coating is required to have a smooth surface because the steel sheets laminated to form the iron core need to have a high space factor.
There have been disclosed various technologies to improve the magnetic properties of grain-oriented silicon steel sheets. One of them involves the use of an auxiliary inhibitor that makes up for the function of the main inhibitor such as MnS, MnSe, AlN, and BN. Among the known elements which function as auxiliary inhibitors are Sb, Cu, Sn, Ge, Ni, P, Nb, V, Mo, Cr, Bi, As, and Pb. Of these elements, Bi is known to give a much higher magnetic flux density than before (For example, Japanese Patent Publication Nos. 32412/1979 and 38652/1981, Japanese Patent Re-publication No. 814445/1990, Japanese Patent Laid-open Nos. 88173/1994 and 253816/1996). However, adding Bi to steel presents difficulties in producing good forsterite coating at the time of finishing annealing. Products with poor coating are usually rejected.
Forsterite coating is formed at the time of final finishing annealing. The formation of forsterite coating affects the decomposition of inhibitors (such as MnS, MsSe, and AlN) in steel. In other words, it also affects the secondary recrystallization which is an essential step to obtain good magnetic properties. In addition, forsterite coating absorbs the components of inhibitor which become unnecessary after the completion of secondary recrystallization, thereby purifying steel. This purification also contributes to improvement in the magnetic properties of steel sheets.
Consequently, forming a uniform forsterite coating by controlled steps is very important to obtain grain-oriented steel sheets with good magnetic properties.
Forsterite coating is usually formed by the following steps. First, a grain-oriented silicon steel sheet which has been cold-rolled to a desired final thickness is annealed in wet hydrogen atmosphere at 700-900° C. This annealing is called decarburization annealing. It has the following functions.
(1) To subject the texture (after cold rolling) to the primary recrystallization so that the secondary recrystallization takes place adequately in the final finishing annealing.
(2) To reduce the content of C in cold-rolled steel sheets from about 0.01-0.10 wt % to about 0.003 wt % or less so as to protect the magnetic properties of the product from aging deterioration.
(3) To cause subscale (containing SiO
2
) to form in the surface layers of steel sheets by oxidation of Si that is present in steel.
After decarburization annealing, the steel sheet is coated with an annealing separator (composed mainly of MgO) and then coiled. The coil undergoes final finishing annealing (which serves also for secondary recrystallization and purification) in a reducing or non-oxidizing atmosphere at about 1200° C. (maximum). Forsterite coating is formed on the surface of steel sheet according to the solid-phase reaction shown by the following formula.
2MgO +SiO
2
→Mg
2
SiO
4
Forsterite coating is a ceramic coating densely composed of fine crystalline particles about 1 &mgr;m in size. As the formula shows, one raw material of forsterite coating is subscale containing SiO
2
which has formed in the outer layer of the steel sheet at the time of decarburization annealing. Therefore, the kind, amount, and distribution of subscale are deeply associated with the nucleation and grain growth of forsterite coating. They also greatly affect the strength of grain boundary and grain of coating crystals and further affect the quality of coating after final finishing annealing.
The annealing separator (composed mainly of MgO as another raw material) is applied to the steel sheet in the form of an aqueous slurry. Therefore, steel sheets retain physically adsorbed water even after drying, and MgO partly hydrates to form Mg(OH)
2
. As the result, steel sheets continue to give off water (although small in quantity) until the temperature reaches about 800° C. during final finishing annealing. This water oxidizes the surface of the steel sheet during final finishing annealing. The oxidation by water also affects the formation of any forsterite coating and the behavior of inhibitors. Added oxidation by water is a factor tending to deteriorate magnetic properties. In addition, the ease with which oxidation by water takes place depends greatly on the physical properties of subscale formed by decarburization annealing.
Also, any additives other than MgO incorporated into the annealing separator, however small in quantity, greatly affect the film formation as a matter of course.
In the case of grain-oriented silicon steel sheets having a nitride inhibitor (such as AlN and BN), the physical properties of subscale greatly affect the behavior of denitrification during finishing annealing or the behavior of nitrification from the annealing atmosphere. Therefore, the physical properties of subscale greatly affect the magnetic properties of the sheet.
As mentioned above, controlling the physical properties of subscale formed in the outer layer of steel sheets dur
Honda Atsuhito
Kurosawa Mitsumasa
Senda Kunihiro
Toda Hiroaki
Watanabe Makoto
Kawasaki Steel Corporation
Schnader Harrison Segal & Lewis LLP
Sheehan John
LandOfFree
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