Method of producing non-grain-oriented electrical sheet

Metal treatment – Process of modifying or maintaining internal physical... – Magnetic materials

Reexamination Certificate

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C148S111000

Reexamination Certificate

active

06582528

ABSTRACT:

BACKGROUND OF THE INVENTION
The invention relates to a procedure for manufacturing non-grain oriented electric sheet. In this connection, the term “non-grain oriented electric sheet” is understood as a steel sheet or steel strip that falls under the sheets mentioned in DIN EN 10106 regardless of its texture, whose loss anisotropy does not exceed that peak values set forth in European Standard DIN EN 10106. To this extent, the terms “electric sheet” and “electric strip” are here used synonymously.
In the following, “J2500” and “J5000” denote the magnetic polarization at a magnetic field strength of 2500 A/m and 5000 A/m. “P1.5” denotes the hysteresis loss at a polarization of 1.5 T and a frequency of 50 Hz.
The processing industry requires that non-grain oriented electric sheet be provided whose magnetic polarization values are increased relative to conventional sheets. This applies in particular to applications in which the induction of electric fields plays a special role. Increasing the magnetic polarization reduces the magnetization requirement. This is accompanied by a decrease in copper losses as well, which constitute a significant amount of the losses that arise during the operation of electrical equipment. Therefore, the economic value of non-grain oriented electric sheets with increased permeability is considerable.
The demand for higher-permeable non-grain oriented types of electric sheet relates not just to non-rain oriented electric sheets with high losses (P1.5≧5−6 W/kg), but also to sheets with average (3.5 W/kg≦P1.5≦5.5 W/kg) and low losses (P1.5≦3.5). Therefore, efforts are being made to improve the entire spectrum of slightly, moderately and highly silicated electrotechnical steels relative to their magnetic properties. In this case, the types of electric sheet with Si contents of up to 2.5 weight-% Si are especially important in terms of their market potential.
There are different known procedures for manufacturing highly permeable types of electric sheet, i.e., those with increased values of J2500 and J5000. For example, according to the procedure known from EP 0 431 502 A2, use is made of a non-grain oriented electric sheet by initially hot-rolling a steel input stock containing≦0.025% C, <0.1% Mn, 0.1 to 4.4% Si and 0.1 to 4.4% Al (figures in weight-%) to a thickness of at least 3.5 mm. The hot strip obtained in this way is subsequently cold-rolled without recrystallizing intermediate annealing at a deformation level of at least 86%, and subjected to annealing treatment.
The strip manufactured according to the known procedure exhibits a special cubic structure, a particularly high magnetic polarization of more than 1.7 T at a field strength J2500 of 2500 A/m and low hysteresis losses. However, this success is linked to the indicated special composition. This relates in particular to the Mn content, which was surprisingly found to be necessary to set the desired cubic texture. According to the known procedure, a specific ratio of Si and Al contents must also be maintained, which pivotally influences the properties of the respective electric sheet. Since these requirements are not satisfied for the entire range of products of interest here, the procedure described in EP 0 431 502 A2 only applies for the manufacture of sheets subject to particularly stringent requirements.
In addition to the procedures outlined above, technical literature also discloses other ways of improving the properties of electric sheets. For example, it has been proposed that the hot strip be subjected to intermediate annealing to produce highly permeable types of electric sheets (EP 0 469 980 B1, DE 40 05 807 C2).
Also known from EP 0434 641 A2 is a procedure for manufacturing a “semi-finished”, non-grain oriented steel sheet. According to the known procedure, steel containing 0.002-0.01% C, 0.2-2.0% Si, 0.001-0.1% S, 0.001-0.006% N, 0.2-0.5% Al, 0.2-0.8% Mn is used to cast a slab. This slab is subjected to heat treatment at 1100° C. to 1200° C., and then to final hot-rolling, wherein the final rolling temperature lies between 830° C. and 950° C. Subsequently, the hot strip undergoes an annealing treatment, during which it is subjected to a temperature lying between 880° C. and 1030° C. for 30 to 120 seconds. The annealed hot strip is then cold-rolled without intermediate annealing, during which a reduction in thickness of 70%to 85%is achieved during the course of cold-rolling. Finally, the cold-rolled strip is subjected to recrystallization annealing at temperatures of 620° C. to 700° C. for 30 to 120 seconds.
Such a “semi-finished” electric sheet fabricated according to the procedure known form EP 0 434 641 A2 is delivered to the user before annealing, is there deformed and undergoes final annealing only after deformation. The advantage to proceeding in this way is that the quality lost relative to the magnetic properties during deformation can be offset by conducting final annealing only after the deformation. However, the annealing step to be performed at the user leads to a considerable outlay during the manufacture of structural components out of electric sheet delivered in the “semi-finished” state. In addition, the electric sheets manufactured according to EP 0 434 641 A2 exhibit magnetic properties that do not exceed the usual level, despite the use of a steel with a special composition, and despite the fact that the sheets are delivered in the “semi-finished” state, processed by the user and only annealed in the processed state.
All known procedures described above share in common that they each require basic materials with special compositions or are tied to process parameters and steps that must be strictly adhered to. As a result, the known procedures are not suited to offer a wide range of high-quality electric sheets based on a uniform manufacturing process and manufactured cost-effectively.
Finally known from EP 0 263 413 A2 is a procedure for manufacturing finish-annealed, non-grain oriented electric sheets in which the slabs used to fabricate the sheets are not preheated in excess of 1150° C., and a steel alloy precisely adjusted in terms of its Al and Si content is used. Hot strip annealing is not described in EP 0 263 413 A2, so that it can be presumed that the costs usually encountered for this operation do not arise in this known procedure. However, both the limitation of preheating temperature and provision of exact stipulations for setting the steel composition greatly limits the range of electric sheet goods that can be subsequently manufactured according to EP 0 263 413 A2.
Proceeding from the prior art as summarized above, the object of the invention is to indicate a procedure with which a wide range of high-quality, non-grain oriented electric sheets with improved magnetic properties can be manufactured.
SUMMARY OF THE INVENTION
This object is achieved according to the invention by a procedure in which steel input stock, containing (in weight-%)≦0.06% C, 0.03-2.5% Si, ≦0.4% Al, 0.05-1.0% Mn, ≦0.02% S and, if desired, other alloying additives P, Sn, Sb, Zr, V, Ti, N and/or B with a content of up to 1.5 weight-% at most, with iron and other conventional companion elements as the residue, as a slab heated to a reheating temperature (T
BR
) which, with a maximal deviation of ±20° C., corresponds to a reheating target temperature (T
ZBR
)
T
ZBR
[° C.]=1195° C.+12,716*(
G
Si
+2
G
Al
)
wherein
T
ZBR
: Target temperature of reheated slab
G
Si
: Si content in weight-%
G
Al
: Al content in weight-%
and pre-rolled, or as a directly used cast strip or thin slab, is introduced into a group of finishing roll stands at an entry temperature of ≦1100° C., and hot-rolled into a hot strip with a thickness of <3.5 mm at a final rolling temperature (T
ET
)≧770° C., in which the hot strip is reeled up at a coiling temperature (T
HT
) determined as follows with a maximal deviation of ±10° C.:
T
HT
[° C.]=154−1.8
&agr;t+
0.577
T
ET
+111
d/d
0
wherein
d
0
: Reference

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