Metal treatment – Process of modifying or maintaining internal physical... – Magnetic materials
Reexamination Certificate
2000-06-06
2003-02-25
Sheehan, John (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Magnetic materials
C148S113000
Reexamination Certificate
active
06524400
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a process for the production of grain-oriented electric quality sheet with low remagnetization loss and high polarization.
For the production of grain-oriented electric quality sheet, steels are smelted and cast which contain 2.5 to 4.0% Si, 0.010 to 0.100% C, up to 0.150% Mn, up to 0.065% Al and up to 0.0150% N, and optionally 0.010 to 0.3% Cu, up to 0.060% S, up to 0.100% P, up to 0.2% As, Sn Sb, Te and Bi respectively, residue iron and unavoidable impurities. Without being cooled below 700° C. the solidified strands with thicknesses of 25 to 100 mm are cut up into thin slabs and homogeneously thoroughly heated in an equalization furnace standing in line to a temperature up to 1170° C., the dwell time being 60 minutes at the most. These homogeneously thoroughly heated thin slabs are then rolled out continuously in a multi-stand hot rolling mill into hot strip having a thickness of 0.5 to 3.0 mm and coiled. Finally the hot strip is optionally annealed. The hot strip thus treated is cold rolled in one or more passes to a finished strip thickness of 0.15 to 0.50 mm. The cold strip is annealed with recrystallization and decarburization. Then after the application of a predominantly MgO-containing annealing separator, the strip is subjected to high temperature annealing with secondary recrystallization, so that a very sharp Goss texture is set up. In a final annealing the strip is then insulated and relieved of inner residual stresses.
Grain-oriented electric quality sheets are used more particularly in output transformers for guiding the magnetic flux. This requires as low remagnetization losses as possible and as high polarization values as possible. For this purpose, in grain-oriented electric quality sheets, a very sharply marked Goss texture is produced in a controlled manner, so that very satisfactory magnetic properties are obtained along the rolling direction as the preferred magnetic direction.
For more than 20 years, the production of flat steel products was begun by the continuous casting of melts by use of strand-casting technique. The melt is cast into a chill mold, from which it emerges as a solidifying strand approximately 100 to 300 mm in thickness. The strand is then normally guided in an arch from the perpendicular to the horizontal and cooled. After leaving the continuous casting installation the strand is divided up into individual slabs. Simple mild steel slabs are normally kept in a slab store, where they cool to ambient temperature. In contrast, slabs for grain-oriented electric quality sheet, which are alloyed with 2.5 to 4.0% Si, must be stored at higher temperatures since they are more liable to form cracks during re-heating prior to hot strip rolling, if they have previously been cooled to excessively low temperatures. In this instance the large slab thickness of the conventional continuous casting process proves to be particularly disadvantageous since it causes high and inhomogeneous temperature gradients during the reheating of the slabs, and these lead to high internal stresses. Slabs for grain-oriented sheet must therefore normally be stored in heated holding furnaces at temperatures of, for example, 100 to 500° C. The disadvantages of this method are the increased energy expenditure and the increased complication and cost of processing.
Slabs produced by the conventional continuous casting process are then introduced into a pusher furnace, a walking beam furnace or a unit of equivalent effect and heated to high enough temperatures, so that satisfactory shapeability is achieved. Hot rolling is then normally performed in two component steps: the slabs 100 to 300 mm thick are first rough rolled to a thickness of approximately 30 to 60 mm. The rough rolling is frequently performed in reversing stands. The roughed slabs are then continuously rolled out in a multi-stand finishing step to give hot strip in thicknesses of 2.0 to 6.0 mm.
Grain-oriented sheet produced by the conventional processes has the special feature that the slabs must absolutely be heated to temperatures up to 1400° C. to dissolve foreign phase particles in the slab, so that they can be separated finally dispersed during the subsequent hot rolling (U.S. Pat. No. 2,599,340). In one prior art process, for example, these particles are mainly Mn sulfides or Mn selenides (J. E. May and D. Turnbull: Trans. AIME, 212 (1958), 769). In another process additional Al nitrides are produced (U.S. Pat. Nos. 3,159,511; 3,287,183). In a further process MnSe and MnSb are produced (DE 23 51 141 A). Other nitrides are also known, such as VN, (Al, Si)N, . . . (DE 19 20 666 A, EP 0 219 611) and sulfides such as Cu
2
S, TiS, CrS, . . . (EP 0 619 376 A1, DE 23 48 249 A). In general the purpose of these particles is to block the movement of grain boundaries in the following production steps up to before the secondary recrystallization, thereby inhibiting normal grain growth during different annealing treatments. For this reason they are known as grain growth inhibitors. Only those particles from a distribution of particles of different size which are smaller than approximately 100 nm can adequately hinder the movement of the grain boundaries and function as inhibitors. Lastly, during the high temperature annealing the inhibitors control the process of secondary recrystallization which leads to the formation of the desired very sharp Goss texture.
In conventional continuous casting the precipitations following casting and solidification are usually so coarse that practically no particles exist below a size of 100 nm. For this reason the coarse particles must be dissolved during slab preheating. In the case of the Mn sulfides, for this purpose annealing temperatures up to approximately 1400° C. are required. The particles are again precipitated in the desired manner during subsequent hot rolling with suitably adjusted parameters (pass plans and shaping temperatures in the different stands, cooling).
The heating of the slabs to the high temperature required for dissolving the particles acting as grain growth inhibitors can be performed either directly in one furnace or in two furnaces successively. In the latter case the slabs are heated in the first furnace, for example, to 1250° C., then in the second furnace to temperatures up to 1400° C. Although it makes the process more complicated and expensive, it has proved advantageous for the magnetic properties of the finished product to perform between these two stages a first hot shaping of the slab (prerolling) in order to homogenize and refine its structure (EP 0 193 373 B1).
The high slab preheating temperature required for the formation of the inhibitor phase makes the manufacturing process for hot strip for grain-oriented sheet difficult and expensive, because investments are required for special costly high temperature furnaces, since the liquid scale occurring on silicone steel slabs above 1350° C. damages the furnace hearth, this being connected with damage to the underside of the slabs which causes production losses. The optionally interposed prerolling also makes production expensive and eliminates capacity which might be utilized for the production of other flat steel products.
MnS or MnSe particles have only a limited effect on grain growth inhibition. It must be offset by only a correspondingly adapted driving force for grain size enlargement to enable the desired Goss selection process to take place at the right moment during secondary recrystallization. This means that the degree of shaping during the final cold rolling stage to final thickness must not be excessive. The most advantageous reduction of thickness in this cold rolling stage of the Si steel inhibited with MnS or MnSe is 40 to 60%. Since the hot strip thickness can, only with difficulty, be reduced below 2.0 mm by the conventional hot rolling technology without losses of quality and production, the optimum degree of shaping during cold rolling to final thickness must be obtained by rolling in a number of stages, between which a recrys
Böttcher Andreas
Espenhahn Manfred
Günther Klaus
Huneus Hans
Kawalla Rudolf
Proskauer Rose LLP
Sheehan John
Thyssen Krupp Stahl AG
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