Metal treatment – Process of modifying or maintaining internal physical... – Magnetic materials
Reexamination Certificate
2002-06-06
2004-08-10
Sheehan, John P. (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Magnetic materials
C148S120000, C148S111000, C148S112000
Reexamination Certificate
active
06773514
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a method for producing non grain-oriented magnetic steel sheets in which hot strip is produced from an input stock made of steel, such as cast slabs, strip, roughed strip, or thin slabs, wherein the magnetic steel sheets have little hysteresis loss and high polarisation, as well as good mechanical properties. Such non grain-oriented magnetic steel sheets are predominantly used as core material in electrical machinery such as motors and generators with a rotating direction of magnetic flux.
In this document the term “non grain-oriented magnetic steel sheets” refers to magnetic steel sheets covered by DIN EN 10106 (“magnetic steel sheets subjected to final annealing”) and DIN EN 10165 (“magnetic steel sheets not subjected to final annealing”). Furthermore, more strongly anisotropic types are also included provided they are not deemed to fall into the category of grain-oriented magnetic sheets.
The processing industry demands non grain-oriented magnetic steel sheets whose magnetic properties are better than those of conventional sheets of this type. There is a demand for reduced hysteresis loss coupled with an increased polarisation in the particular induction range used. At the same time, the respective treatment and processing steps to which the magnetic steel sheets are subjected in the context of their use, place special demands on the mechanical/technological characteristics of said magnetic steel sheets. In this context, cuttability of the sheets, e.g. during stamping, assumes particular importance.
By increasing magnetic polarisation, the magnetisation requirement is reduced. At the same time, copper losses are reduced too, said copper losses forming a significant part of the losses which arise during the operation of electrical machinery. The economic value of non grain-oriented magnetic steel sheets with increased permeability is thus very considerable.
The demand for types of non grain-oriented magnetic steel sheets which have greater permeability, not only relates to non grain-oriented magnetic steel sheets with high losses (P1.5≧5−6 W/kg), but also sheets with medium losses (3.5 W/kg≦P1.5≦5.5 W/kg) and low losses (P1.2≦3.5). This is the reason for efforts to improve the entire spectrum of the magnetic polarisation values of lightly siliconised, medium-siliconised and highly siliconised electrotechnical steels.
One approach to producing magnetic steel sheets of increased permeability, said approach being based on medium-siliconised or lightly siliconised alloys, consists of subjecting the hot strip to hot strip annealing during production. Thus for example WO 96/00306 proposes that hot strip intended for the production of magnetic steel sheets, be finish-rolled in the austenitic region, and that coiling be undertaken at temperatures above the complete transformation to ferrite. In addition, annealing of the coil takes place directly from the rolling heat. In this way a final product with good magnetic characteristics is obtained. However, due to the high energy requirements for heating before and after hot-rolling as well as due to the required alloying additions, the associated increased costs have to be accepted.
According to EP 0 469 980 an increased coiling temperature in combination with an additional hot strip annealing should be aimed for, so as to obtain useful magnetic characteristics even with low alloying contents. This too can only be accomplished if the increased costs are accepted.
SUMMARY OF THE INVENTION
It is thus the object of the invention to provide an economical way of producing magnetic steel sheets with improved characteristics.
According to the invention, this object is met by a method for producing non grain-oriented magnetic steel sheets in which, starting with an input stock such as cast slabs, strip or thin slabs made from a steel comprising (in weight %) 0.001-0.05% C, ≦1.5% Si, ≦0.4% Al, with Si+2Al≦1.7%, 0.1-1.2% Mn, if necessary up to a total of 1.5% alloying additions such as P, Sn, Sb, Zr, V, Ti, N, Ni, Co, Nb and/or B, with the remainder being iron as well as the usual accompanying elements, a hot strip is produced in that the input stock is hot-rolled directly from the casting heat or after preceding reheating to a reheating temperature between min. 1000° C. and max. 1180° C. in several deformation passes, and subsequently coiled, wherein during hot-rolling at least the first deformation pass takes place in the austenitic region and at least one further deformation pass takes place in the two-phase mixing region austenite/ferrite, and wherein during rolling in the two-phase mixing region a total deformation &egr;
h
of at least 35% is achieved.
According to the invention, the magnetic characteristics of magnetic steel sheets are influenced in a targeted way by deformation during the individual deformation passes undertaken during hot-rolling, depending on the respective microstructural condition at the time. Rolling in the two-phase mixing region is to be a decisive component; by contrast, the component of deformation in the ferritic region should be kept as small as possible. Thus the method according to the invention is particularly suitable for processing those Fe—Si alloys that have a pronounced two-phase mixing region between the austenitic and the ferritic region.
Attuning the alloying additions of ferrite-forming and austenite-forming elements, taking into account the contents range according to the invention of the individual elements, is to be undertaken starting with a base composition of (Si+2Al)≦1.7, namely such that there is an adequate distinction of the two-phase mixing region.
If cast slabs are used as an input stock, they are reheated to a temperature ≧1000° C. so that the material is completely in the austenitic state. For the same reason, cast thin slabs or cast strip are/is used directly exploiting the casting heat and if necessary are heated up to an initial rolling temperature exceeding 1000° C. The required reheating temperature increases in line with an increase in the Si content, but an upper limit of 1180° C. is not to be exceeded.
As a rule, hot-rolling according to the invention is carried out in a finish-rolling line comprising several roll stands. The purpose of rolling in the austenitic region which takes place in a single pass or in several passes, consists of being able to carry out the transition from the austenitic region to the two-phase mixing region and from the two-phase mixing region to the ferritic region in a controlled way within the finish-rolling line. The deformation passes carried out in the austenitic region also serve the purpose of setting the thickness of the hot strip prior to the start of rolling in the two-phase mixing region so that the desired total deformation taking place during rolling in the two-phase mixing region (“mixing rolling”) is safely attained. Mixing rolling also involves at least one deformation pass. Preferably however, several deformation passes are carried out in the mixing region austenite/ferrite, so as to safely achieve the total deformation of at least 35% required during such mixing rolling, thus obtaining the desired setting of the microstructure of the hot strip.
The term “total deformation &egr;
h
” refers to the ratio of thickness reduction during rolling in the respective phase region to the thickness of the strip when it enters the respective phase region. According to this definition, the thickness of hot strip produced according to the invention, for example after rolling in the austenitic region, is h
0
. During subsequent rolling in the two-phase mixing region, the thickness of the hot strip is reduced to h
1
. According to the definition, this results for example, in a total deformation &egr;
h
attained during mixing rolling to (h
0
-h
1
)/h
0
with h
0
=thickness during entry into the first roll stand which is passed in the mixing state austenite/ferrite, and h
1
=thickness when leaving the last roll stand in the mixing state.
Accordi
Fischer Olaf
Friedrich Karl Ernst
Hammer Brigitte
Kawalla Rudolf
Pircher Hans
Proskauer Rose LLP
Sheehan John P.
Thyssen Krupp Stahl AG
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