Electric heating – Inductive heating – Specific heating application
Reexamination Certificate
2001-10-04
2003-08-19
Van, Quang T. (Department: 3742)
Electric heating
Inductive heating
Specific heating application
C219S653000
Reexamination Certificate
active
06608290
ABSTRACT:
TECHNICAL FIELD
The invention concerns a method of heating metal strips or other coilable strand metal object to an elevated temperature without oxidation of said metal object, which is oxidisable in air at said elevated temperature wherein the metal object is passed in a heating section through a gas tight heating chamber that is made at least partly of an insulating and electrically non-conductive material while being heated by action of at least two opposite walls of the heating chamber through transverse flux induction heating by means of transverse flux induction heating elements located outside of said chamber which contains a protective non-oxidising gas or gas mixture.
The invention also concerns an apparatus for heating a metal strip or other coilable metal object to an elevated temperature without oxidising said metal object which is oxidisable in air or other oxidising gas at said elevated temperature, wherein the apparatus comprises a gas tight heating chamber through which the metal object is provided to be passed, said chamber being made at least partly of an insulating and electrically non-conductive material and containing a protective, non oxidising gas, said heating chamber having entrance and exit ports for the metal object at the ends thereof, and wherein transverse flux induction heating elements are located opposite to each other outside the heating chamber for transverse flux induction heating of said metal object by action of flux induction heating elements through two opposite walls of the heating chamber as the metal object, is being conveyed through the chamber.
Further, the invention concerns a process line forming an integrated furnace for heat treating a metal strip or other coilable strand metal object, including an entry end and an exit end; a passage-way for the strand object extending along the process line from the entry end to the exit end, said passage-way being enclosed against the ambient environment; the process line further including an apparatus for heating said metal strip or other coilable strand metal object to an elevated temperature without oxidising said metal object which is oxidisable in air or other oxidising gas at said elevated temperature, wherein the apparatus comprises a gas tight heating chamber through which the metal object is provided to be passed, said heating chamber forming part of said enclosed passage way and being made at least partly of an insulating and electrically nonconductive material and containing a protective, non-oxidising gas, wherein transverse flux induction heating elements are located opposite to each other outside the heating chamber for transverse flux induction heating of said metal object by action of the flux induction heating elements through to opposite walls of the heating chamber as the metal object is being conveyed through the heating chamber, and a cooling section including a cooling chamber down-stream of said heating chamber, said cooling chamber also forming part of said enclosed passage-way.
BACKGROUND OF THE INVENTION
Strips of various types of metals and alloys strain harden when they are cold rolled, cold drawn, or cold stretched. Therefore they need to be heated and annealed in order to be re-crystallised. This particularly concerns stainless steel strips but is valid for metals in general. Conventionally, continuous annealing furnaces are used, which employ fuel or radiant electric heating in chambers through which the strip passes to be heated by conduction and/or radiation. The rate of heating is relatively slow, wherefore the overall furnace lengths need to be correspondingly long.
It is also known in the art to employ induction heating for heating metal strip and other strand metal objects. In principle, there exist two types of induction heating techniques; axial induction heating (AIH) and transverse flux induction heating (TFIH).
AIH is effected by passing an electric current through a wire, which is coiled around, but not touching, the metal to be heated. The electric current induces magnetic currents in the metal, whereby the metal is heated. To be heated in this way, the metal has to be essentially magnetic. Thus metals, such as copper, aluminium, and austenitic stainless steel, can not be easily heated by this technique.
TFIH employs electromagnets of opposite poles positioned on opposite sides of the metal to be heated. The action of passing a magnetic field through the metal heats the metal. The metal, in this case, needs to be electrically conductive but needs not to be magnetic. Thus also copper, aluminium, and stainless steel can be heated by this technique. The use of TFIH for heating metal strip is disclosed e.g. in GB 2 155 740 A, U.S. Pat. No. 4,585,916, EP 0 246 660 B1, EP 0 346 547 B1 and EP 0 667 732 A2.
A facility for producing cold rolled or finally annealed stainless steel strip normally includes at least two annealing sections; a preparatory annealing section and a bright annealing section. In the preparatory annealing section, hot rolled coil is heat-treated to make it easier to form during subsequent cold rolling. Because hot rolled austenitic steel strip, for example, will have a surface layer of scale remnant from the hot rolling process it is sufficient to anneal the strip at an appropriate temperature in a continuous annealing furnace which is open to the atmosphere (air). This process is followed by a descaling operation which removes the hot rolled scale and scale formed by the annealing process. After washing and drying the strip is in a condition suitable for cold rolling.
For surface critical intermediate annealing and final annealing, where a high degree of surface reflectivity is required, it is necessary to protect the surface of the strip from oxidation. This is effected in a continuous strand furnace which contains a protective, non-oxidising gas. Such furnaces can employ direct radiant heat aided by the conduction of the protective gas or by indirect radiant heat from a metal retort, which contains the protective gas and which, is externally heated. A main drawback with these prior art methods is that radiant heating, particularly radiant heating via a gas medium, is a slow process. Annealing furnaces of this type therefore are usually relatively long because of the time needed to raise the temperature of the strip throughout its thickness to the desired annealing temperature and in order to maintain an adequate throughput rate. Thus the capital cost of such furnaces is relatively high. In spite of these drawbacks, this type of annealing furnaces are regularly employed also for new installations, while use of the TFIH technique in practice basically has been restricted to the non-ferrous industry, typically for heating copper and aluminium strip materials to moderate temperatures.
DISCLOSURE OF THE INVENTION
It is the object of the invention to suggest a method and provide an apparatus and a process line which enable the above mentioned disadvantages to be overcome. Accordingly, the invention suggests a method as defined in the above preamble in which the metal object to be heated is a stainless steel object that has been cold rolled to a very high degree of surface reflectivity; the cold rolled stainless steel object is passed through said heating chamber and is heated in said chamber to a processing temperature between 700 and 1200° C. the cold rolled stainless steel object is maintained at said temperature between 700 and 1200° C. for long enough for the steel to recrystallize completely; and the heat treated metal object then is rapidly cooled directly from the processing temperature, in an airtight cooling section through which the non-oxidising gas is passed, to below a temperature of 600° C.
Within the above temperature range, for example, austenitic stainless steels may continuously be annealed at temperatures in the range 1050-1200° C., the exact choice of temperature for each grade depending on its specific chemistry. In contrast, cold rolled martensitic stainless steels may be softened in the heating chamber in the range
Avesta Sheffield Aktiebolag
Nixon & Vanderhye
Van Quang T.
LandOfFree
Method of heating metal strip and apparatus thereof does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method of heating metal strip and apparatus thereof, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method of heating metal strip and apparatus thereof will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3088795