Metal treatment – Process of modifying or maintaining internal physical... – Magnetic materials
Reexamination Certificate
1997-09-29
2001-04-17
Ip, Sikyin (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Magnetic materials
C148S112000, C148S120000
Reexamination Certificate
active
06217673
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to the production of electrical steels, and more specifically to cold rolled, batch annealed and temper rolled or levelled motor lamination steels having good processing and magnetic properties, including low core loss and high permeability.
Desired electrical properties of steels used for making motor laminations are low core loss and high permeability. Those steels which are stress relief annealed after punching also should have properties which minimize distortion, warpage and delamination during the annealing of the lamination stacks.
Continuously annealed, silicon steels are conventionally used for motors, transformers, generators and similar electrical products. Continuously annealed silicon steels can be processed by techniques well known in the art to obtain low core loss and high permeability. Since these steels are substantially free of strain, they can be used in the as-punched condition (in which the steel as sold is commonly referred to as fully processed) or if better magnetic properties are desired the steel can be finally annealed by the electrical apparatus manufacturer after punching of the laminations (in which case the steel as sold is commonly referred to as semi-processed) with little danger of delamination, warpage, or distortion. A disadvantage of this practice is that the electrical steel sheet manufacturer is required to have a continuous annealing facility.
In order to avoid a continuous annealing operation, practices have been developed to produce cold rolled motor lamination steel by standard cold rolled sheet processing including batch annealing followed by temper rolling. In order to obtain the desired magnetic properties of high permeability and low core loss, it has been considered necessary to temper roll the steel with a heavy reduction in thickness on the order of 7%. Electrical steels processed by batch annealing and heavy temper rolling followed by a final stress relief anneal after the punching operations develop acceptable core loss and permeability through a complete recrystallization process. Unfortunately, the heavy temper rolling necessary for development of magnetic properties often results in delamination, warpage and distortion of the intermediate product when it is annealed, to the degree that it is unsuitable for service.
Fully-processed electrical steels are used by customers in the as-punched/stamped condition without a subsequent annealing operation being required. Standard cold-rolled electrical steels are unsuitable for most fully-processed applications due to strain remaining in the material. Fully processed materials are produced utilizing continuous anneal lines since no additional strain is required to provide acceptable flatness. Batch annealed materials, however, do not have acceptable flatness and require some strain simply to provide a flat product, which generally degrades the magnetic properties beyond a usable range. This strain is usually provided by conventional temper rolling.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a batch annealed and temper rolled motor lamination steel having magnetic and mechanical properties similar to silicon electrical steels produced by continuous annealing without temper rolling.
A more particular object of the invention is to provide a batch annealed and temper rolled motor lamination steel which can be given a final stress relief anneal to achieve low core loss and high permeability without delamination, warpage or distortion of the intermediate product produced by the electrical product manufacturer.
Another object of the invention is to provide a batch annealed and temper rolled motor lamination steel which displays acceptable core loss and permeability without a final stress relief anneal operation.
The present invention applies to the production of batch annealed motor lamination steels which are semi-processed, i.e. steels which are given a final stress relief anneal after punching, and fully processed steels, i.e. steels which are used in the as-punched condition without a final stress relief anneal. In both instances, the process of the invention is characterized by a composition having an ultra low carbon content less than 0.01%, preferably less than 0.005%, and either leveling or light temper rolling with a reduction in thickness not greater than 1.0%, and, preferably, not greater than 0.5%.
A preferred embodiment of the process provided by the invention for making both semi-processed and fully processed electrical steel comprises the steps of:
hot rolling a slab into a strip having a composition consisting essentially of (% by weight):
C:
up to 0.01
Si:
0.20-1.35
Al:
0.10-0.45
Mn:
0.10-1.0
S:
up to 0.015
N:
up to 0.006
Sb:
up to 0.07
Sn:
up to 0.12, and
the balance being
substantially iron,
followed by coiling, pickling, annealing the strip in coil form, cold rolling and batch annealing the strip in coil form, and then temper rolling the strip with a reduction in thickness ranging from greater than 0 to not greater than 1.0%.
In the case of semi-processed steel which is given a final stress relief anneal after punching, the steel can be hot rolled with a finishing temperature in either the austenite or ferrite region. Hot rolling with a finishing temperature in the austenite region results in optimum permeability after the stress relief anneal. Hot rolling with a finishing temperature in the ferrite region results in optimum core loss with lower permeability after the final stress relief anneal. In the case of fully processed steels which are not given a final stress relief anneal, optimum core loss and permeability are achieved when the steels are hot rolled with a finishing temperature in the austenite region.
In the case of both semi-processed and fully processed steels, the combination of ultra low carbon content, pickle band annealing, batch annealing and light temper rolling results in low core loss and high permeability. If the punched steel product is given a final stress relief anneal, the light temper roll of not greater than 1.0% and more particularly not greater than 0.5%, minimizes the residual stresses that are thought to be responsible for the occurrence of delamination, warpage and distortion.
Another embodiment of the invention relates to a method for the production of electrical steel strip characterized by low core loss and high permeability comprising the steps of:
hot rolling a slab into a strip having a composition consisting essentially of (% by weight):
C:
up to 0.01
Si:
0.20-1.35
Al:
0.10-0.45
Mn:
0.10-1.0
S:
up to 0.015
N:
up to 0.006
Sb:
up to 0.07
Sn:
up to 0.12, and
the balance being
substantially iron,
followed by coiling, pickling, cold rolling and batch annealing the strip in coil form, and then flattening the strip with a leveling process. Although it is not required, the strip may also be pickle band annealed in coil form.
The hot rolling step is conducted in either the ferrite region or the austenite region. The leveling process includes roller leveling with a reduction in thickness of the strip greater than 0 and preferably not greater than about 0.1%, or tension leveling. The tension leveled strip has a reduction in thickness not greater than 1.0% and, preferably, not greater than 0.5%. The leveling method is advantageous in that it does not require a continuous anneal facility or temper rolling apparatus, but rather only requires standard batch annealing and leveling facilities.
Other objects and a fuller understanding of the invention will be had from the following description of preferred embodiments and the accompanying drawings.
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patent: 2986485 (1961-05-01), Fitz et al.
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patent: 3188250 (1965-06-01), Holbein et al.
patent: 3212942 (1965-10-01), Takahashi
patent: 3297434 (1967-01-01), Littmann
patent: 3415696 (1
Beatty Gerald F.
Blotzer Richard J.
Butler John F.
Larson Ann M. R.
Lauer Barry A.
Ip Sikyin
LTV Steel Company, Inc.
Watts Hoffmann Fisher & Heinke
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