Metal treatment – Process of modifying or maintaining internal physical... – Magnetic materials
Reexamination Certificate
1999-12-09
2002-03-26
King, Roy (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Magnetic materials
C148S221000, C148S230000, C148S231000, C148S111000
Reexamination Certificate
active
06361620
ABSTRACT:
The present application is the national stage filing of and claims priority to International Application No. PCT/EP97/04088, filed Jul. 28, 1997 and Italian Application Serial No. RM97A000146, filed Mar. 14, 1997.
FIELD OF THE INVENTION
The present invention refers to a process for the inhibition control in the production of grain-oriented electrical sheets and, more precisely, it refers to a process by which, through control of copper, aluminium and carbon content, type and quantities of precipitated second phases are determined since the hot-rolled strip, to obtain optimum grain size during the decarburization annealing and some degree of inhibition, thus allowing to carry out a subsequent continuous high-temperature heat treatment in which aluminium as nitride is directly precipitated by diffusing nitrogen along the strip thickness, in order to obtain the second phases ratio necessary to control the grain orientation of the final product.
STATE OF THE ART
Grain-oriented silicon steels for magnetic uses are normally classified into two groups, essentially differentiated by the induction value induced by a magnetic field of 800 As/m and known as “B800”: the conventional grain-oriented group, where B800 is lower than 1890 mT, and the high-permeability grain-oriented group, where B800 is higher than 1900 mT. Further subdivisions are depending on the so-called “core-losses”, expressed as W/kg.
Conventional grain-oriented steel, used since the 1930's, and grain super-oriented steel, having a higher permeability and industrially used since the second half of the 1960's, are essentially used to realise cores for electric transformers, the advantages of the super-oriented steel rising from its higher permeability (which allows reductions of core sizes) and from its lower losses, which are energy-saving.
The permeability of the sheets depends on the orientation of the body-centred cubic-lattice iron crystals (or grains): one of the grain edges must be parallel to the rolling direction. By using some precipitates (inhibitors), also called “second phases”, of suitable sizes and distribution, which reduce grain boundary mobility, a selective growth of the sole grains having the wanted orientation is obtained; the higher the dissolution temperature of said precipitates into the steel, the higher the grain orientation and the better the magnetic characteristics of the final product. Manganese sulphide and/or selenide are the predominant inhibitors in an oriented grain steel, while precipitates including nitrogen linked to aluminium (referred to as “aluminium nitride” for simplicity purposes) are the predominant inhibitors in a super-oriented grain steel.
Nevertheless, when a grain-oriented sheet or a grain super-oriented sheet is produced, during the solidification of steel and the cooling of the solidified body, second phases allowing the above mentioned improving effect are precipitated in coarse form, useless for the wanted purposes; said second phases must be therefore dissolved, reprecipitated in the right form and maintained into said form until the grain having wanted sizes and orientation is obtained at the end of a complicated and expensive transformation process including a cold-rolling at the desired final thickness, a decarburization annealing and a final annealing.
It is evident that the production problems, linked essentially to the difficulties of obtaining high yields and constant quality, are mainly due to the precautions to be taken during the whole transformation process of the steel for maintaining the second phases and, in particular, the aluminium nitride in the wanted form and distribution. In order to relieve said problems, technics have been developed where, the aluminium nitride suitable to control the grain growth is obtained by nitriding the strip, preferably after cold-rolling, as it is disclosed in U.S. Pat. No. 4,225,366 and in European patent n. 0,339,474.
According to the last mentioned patent the aluminium nitride, coarsely precipitated during the slow steel solidification, is maintained into said state by using low slab-heating temperatures (lower than 1280° C., preferably lower than 1250° C.) before the hot-rolling; nitrogen is introduced after the decarburization annealing, which reacts immediately to produce, essentially near the strip surfaces, silicon and manganese/silicon nitrides, having comparatively low solution temperature, which are dissolved during the final annealing in box-annealing furnaces; the nitrogen so released diffuses into the sheet, reacts with the aluminium and precipitates again on the whole strip thickness in a thin and homogeneous form as mixed aluminium and silicon nitrides; said process involves that the material stays at 700-800° C. for at least four hours. The above patent states that nitrogen must be introduced at a temperature near the decarburization one (about 850° C.) and in any case no higher than 900° C. to avoid an uncontrolled grain growth due to the absence of suitable inhibitors. In fact, the optimum nitriding temperature should be of about 750° C., while 850° C. is the upper limit to avoid said uncontrolled growth.
Prima facie the above process has some advantages: relatively low slab-heating temperatures before hot-rolling, decarburation and nitriding, and the fact that no increase in production costs is due to the necessity to maintain the strip at 700-850° C. for at least four hours in the box-annealing furnace (to obtain the mix of aluminium and silicon nitrides required to control the grain growth), as the heating in the box-annealing furnaces in any case requires similar times.
However, together with the above mentioned advantages the above process has some disadvantages as: (i) due to the low slab-heating temperature the sheet includes practically no precipitates inhibiting the grain growth: all the heating steps of the strip, and in particular those belonging to the decarburization and to the nitriding steps, must be taken at comparatively low and critically controlled temperatures, in that at the above conditions grain boundaries are very mobile involving the risk of an uncontrolled grain growth; (ii) the nitrogen introduced is stopped near the strip surfaces as silicon and manganese/silicon nitrides, which must be dissolved to allow the nitrogen diffusion towards the core of the sheet and its reaction for creating the wanted aluminium nitride: as a consequence, no improvement speeding up the heating time can be introduced during the final annealing, for example by using another type of continuous furnace instead of box-annealing ones.
The Applicant, knowing the above difficulties, has developed an improved process which is new and involves a considerable inventive step over the prior art, from which it is distinguished with regard to both the theoretical bases and the process characteristics.
Such process is disclosed by Applicant's copending U.S. applications Ser. Nos. 09/243,000, filed Feb. 26, 1999; 09/242,992, filed Feb. 26, 1999; 09/331,273, filed Jun. 17, 1999; 09/331,506, filed Jun. 22, 1999; and 09/331,504, filed Jun. 22, 1999.
Said patent Applications clearly set forth that the whole process, and in particular the control of the heating temperatures, can be made less critical if some precipitation of inhibitors suitable to control the grain growth is allowed since the hot-rolling step, thus permitting a best control of the grain size during the primary recrystallisation (during the decarburization annealing) and then a deep nitriding of the sheet to directly create aluminium nitride.
In more detail, the process described in said patent applications provides for carrying out said primary recrystallization annealing continuously between 800 and 950° C., in a wet nitrogen-hydrogen atmosphere for a period of time of between 20 and 150 seconds to produce a primary recrystallized strip; continuously nitriding said recrystallized strip at a temperature between 850 and 1050° C. [for a time between 5 and 120 seconds], in a wet nitriding atmosphere comprising ammonia at a level
Abbruzzese Giuseppe
Cicale′ Stefano
Fortunati Stefano
Acciai Speciali Terni S.p.A.
Hedman & Costigan ,P.C.
King Roy
Loy Nicole
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