INDUCTION FURNACE FOR HEATING AND TEMPERATURE HOMOGENIZATION IN HOT-ROLLING OF THIN STEEL STRIPS

Abstract: An induction furnace is described which is capable to heat a thin strip from a continuous casting apparatus throughout the furnace itself after a previous rolling, at a homogeneous temperature adapted to the subsequent rolling steps. The induction furnace of the present invention comprises, in a known manner, an array of toroidal coils being fed at M.F., with intermediate feeding rollers having reduced size along the strip feed direction with respect to the known coils so that the rollers are less spaced from each other. Flux concentrators are also provided, being distributed preferably in pairs on the inductor and mounted transversely movable with respect to the strip forward movement and possibly also in a plane perpendicular thereto.

Description: . - ''INDyCTIO _FyR ACE_FOR_HEATING_AND_TEH NIZATION IN HOT-ROLLING OF THIN STEEL STRIPS" The present invention relates to an induction furnace for heating and temperature homogeni zation in hot-rolling of thin steel strips produced by continuous casting and already subjected to a preliminary rolling step. It is generally known to use H.F. induction furnaces in steel industry. These furnaces are used, although not widely, as an alternative to the gas furnaces for heating of the slabs having a thickness of more than about 100 mm, whereas the use with strips having a thickness of less than about 30 mm is limited to a thermal treatment of the edges in order to oppose the natural cooling which, being greater than in the central zones, causes such a temperature decrease as to produce possible cracks at the strip edges. However this induction treatment of the edges is accomplished by means of C-shaped inductors which enclose, like sliding shoes, only the fringe areas of the strip during the forward movement and certainly cannot be considered true and actual induction furnaces. On the contrary the induction furnaces for slabs comprise a row of inducing coils having a substan¬ tially toroidal shape, within which the slab is caus¬ ed to move forward in sequence, between one coil and the other there being provided driven and/or idle rollers for feeding said slab. These coils have a size that in the direction of movement are in the order of about 900 mm. On the other hand plants are provided at present for the continuous production of steel strip by the continuous casting method in order to have as final product the so-called "coil" or steel plates cut at prefixed lengths, wherein it is required that strips in an intermediate step of hot-rolling are heated with contemporaneous temperature homogenization in induction furnaces for reaching the required tempe- rature in the final steps of rolling. In this respect reference should be made to the Italian Patent Applica¬ tion No.20752 A/88, the PCT Application DE-88-00628 and German Patent Application No.P-3840812.0. In this respect the adoption of known induct- ion furnaces, already used for the slabs, cannot how¬ ever give the expected results due to the fact that inconveniences would occur, being caused e.g. to the strip stumbling and possible hitting against the co ls owing to its lower rigidity with respect to the slabs having a greater thickness. Furthermore the strip requires for temperature homogenization that in certain zones, possibly not always the same but such as to be Localized at each time, the heating has to be stronger with a greater concentration of flux Lines. As a matter of fact this is not required by the slabs, since the temperature is more homogeneous due to the greater thi ckness. Therefore it is an object of the present invention to provide an induction furnace for steel strips from continuous casting having a thickness of Less than 30 mm, as they have been already partially hot-rolled so as to heat homogeneously the strip up to the required temperature for the subsequent steps of rolling completion without facing the above-mention¬ ed drawbacks. The induction furnace according to the inven¬ tion comprises an array of coi Ls, each of which is embedded in an inductor unit, separately fed by one or more frequency converter and successively crossed by the strip being supported and driven through pairs of rollers between each coi l and the subsequent one, the size of each coi l in the forward direction of the strip being less than 500 mm, there being also provided flux concentrating devices which are distributed in pairs on each inductor, at least at one upper or lower side with respect to the plane defined by the strip itself. According to a preferred embodiment of the invention the flux concentrating devices are mounted movable in a transverse direction to the strip and preferably also perpendicularly to the strip itself for a better distribution of power in the areas where a stronger heating is required. These and additional objects, advantages and characteristics of the induct¬ ion furnace according to the invention will be clear to the persons skilled in the art from the following detai led description of a preferred embodiment thereof, given by way of a non-limiting example, with reference to the annexed drawings in which: FIGURE 1 shows a partial, diagrammatic side view of a heating element of the furnace according to the present invention; and FIGURE 2 shows a cross-section view taken along line II-II of Fig. 1. With reference to the drawings. Fig. 1 schema¬ tically represents a portion of the furnace according to the invention along the forward direction of strip 1, as regards only one heating element of the furnace itself, comprised of a coiL 2 having a substantially rectangular cross-section with rounded edges (better seen in Fig. 2) which is embedded within an inductor 3, as it is surrounded by refractory material. As better shown in Fig. 1 the strip 1 passes through the space defined by each coi l 2 (the height of which wi ll be not less than the maximum strip thickness which may be expected), as it is driven by rollers 10 positioned between each heating element or coi l 2 and the subsequent one. The rollers 10 can be all motorized or some of them may be idle. Through the inductor 3 embedding the coil 2, this is fed on one side by means of suitable conductors 6 usually called "bus bars" by a source of given power and frequency, in particular one or more converters (not shown). Advantageously the frequency will be fixed once and for all, and possibly the feeding power will be varied according to the energy required for heating, as a function both of the strip temperature upstream of the furnace and of its thickness with a finer regulation being responsive to the temperature at the outlet of the coil. At the opposite side of the feeding connector 6 there may be provided manifolds for the delivery and discharge of the cooling water genericalLy designated 7 in Fig. 2 and connected to t h e i nd u c t o r 3 . According to the present invention there are also pro ided flux concentrators 4 formed of packs of magnetic sheet iron for directing the primary electro- magnetic field flux so as to concentrate it in a direct¬ ion substantially parallel to the forward movement of the strip. In the area of strip 1 where the concentrat¬ ed flux closes its circuit an induced current is produc¬ ed, having a higher intensity and thereby greater heating. The flux concentrators have been represented as forming two pairs, an upper one and a lower one. The concentrators 4 pertaining to the upper pair corre¬ spond to the concentrators 4* of the lower pair, where¬ by the concentrators are coupled two by two on either side of the strip 1, as they are co-axial with an axis passing throughout strip 1 and coi l 2. Flux concen¬ trators 4 and 4' will be normally positioned in the proximity of the edges of strip 1, just where a greater heating power is required. However preferably, as shown in Fig. 2, the flux concentrating devices are designed to be movable firstly in the transverse direction both to follow the dimensional variations of the strip in width and to be positioned also in correspondence of innerer zones which may be at a lower temperature (cold spots), and possibly also in a pependicular direction to the plane defined by strip 1 for a better positioning in the height direction in function not only of the strip thickness but also of the power to be concentrated. As shown in the drawings, in particular in Fig. 2, the two pairs of flux concentrators 4, 4 ' are mounted, with each element opposite to the associated one of the other pair, to an inner nut thread on screws 9 operable from the outside by means of control hand- wheels 5, 5' respectively. The screws 9, 9 1 will have one half of their length with a thread to a direction and the other half with a thread oppositely directed, so that the movement of the two concentrators of each pair wi ll be symmetric and self-centering at each opera¬ tion of the associate handwheel. Additionally the screws 9 and 9' will be preferably mounted at their central portion to a lifting and lowering device which is also controllable by means of outer handwheels 5a, 5a' . Said handwheeLs may be for example fixed respectively to screws 11, 11 * being perpendicular to the screws 9, 9', each of them passing through a hole at right angle to the Longitudinal axis of the Latter. Of course a completely automatic control of the concentrators 4, 4' positioning may be provided, both in a transverse and in height direction. Instead of the operating handwheels, step by step motors will be used, being interlocked with a regulation and control unit adapted to process directly in real time the strip temperature, speed and thickness signals as received each time at the furnace inlet. In this way at each moment an optimal adjustment of the flux concentrators can be obtained for a better efficiency of the induction furnace according to the present invention. EXAMPLE For experimental purposes an induction furnace plant according to the invention was installed, having suitable longitudinal size of the coils and being provid- ed with pairs of flux concentrators in association with each inductor. The main data of installed power number of inductors, frequency and size of the coi l port in height, as well as of the results obtained with reference to the consumptions according to the strip cross-section and its feedings speed, efficiency etc. are listed in the following table also reporting the corresponding data for an induction furnace accord¬ ing to the prior art, namely of the type used for heat¬ ing slabs, in which only the size of coils in the for¬ ward direction of the strip have been reduced without however the presence of the flux concentrators. As regards the consumptions it should be noted that they have been detected at the highest speed of the strip, for temperature increases (ΔT) of about 150°C and, at the lowest speed for ΔT of about 300°C. TABLE With concentra Without tors according concentrators to the inven¬ tion Installed power (HW) 16 21 Frequency (kHZ) 6 10 Number of inductors 20 26 Inductor port (height in mm) 65 55 Consumptions ( wh/t-ΔT=150°C-300°C) 15x1050mm v=0,19 m/sec 127 168 15x1050mm v=0,237 m/sec 79 127 25x1330mm v=0,086 m/sec 100 169 25x1330mm v=0,143 m/sec 42 96 20x1330mm v=0,11 m/sec 110 177 20x1330mm v=0,178 m/sec 50 106 Efficiency of inductors 25x1330 80% 79% 15x1330 79% 67% As it appears from the data given with the furnace plant according to the invention, not only a reduced installed power can be foreseen, but lower consumptions of energy are obtained with the same cross- section and speed of the strip, as well as better effi¬ ciencies of the inductors. Possible additions/or modifications can be made by those skilled in the art to the above described and illustrated embodiment of the induction furnace according to the present invention without departing from the scope of the invention itself. On the other hand no limitation will be expected as to the number of coils or heating elements in succession to each other which form the furnace.

Claims: C L A I M S 1. An induction furnace for heating steel strips having a thickness lower than 30 mm and for rendering homogeneous the temperature up to a value required for the subsequent steps of hot rolling, com- prising an array of coils (2) each of which is embedded in a refractory material at the inside of an inductor unit (3), separately fed by one or more frequency conver¬ ters being crossed in succession by the strip (1) which is supported and housed to move forward by pairs of rollers (10) between each co l (2) and the subsequent one, characterized in that the size of each co l (2) in the strip forward direction is less than 500 mm, there being also provided flux concentrators devices (4, 4') distributed in pairs on each inductor (3) at least on an upper or lower side, with respect to the plane defined by the strip (1). 2. A furnace according to claim 1, characteriz¬ ed in that each of said flux concentrators (4, 4') is mounted substantially co-axial with the corresponding concentrator of the other pair at the opposite side of strip (1), said flux concentrator (4, 4*) being formed of packs of magnetic sheet iron. 3. A furnace according to claim 2, characteriz¬ ed in that said flux concentrators (4, 4') are all positioned stationary in the proximity of the side edges of strip (1) . 4. A furnace according to claim 1 or 2, chara¬ cterized in that said flux concentrator (4, 4') are mounted movable in the transverse direction to the strip (1). - 40 - 5. A furnace according to claim 4, characteriz¬ ed in that said flux concentrators (4, 4 1 ) are movable also in the direction perpendicular to the plane of strip (1 ) . 6. A furnace according to claim 4 or 5, chara¬ cterized by comprising screw devices (9, 9"; 11, 11 *) for the adjustment of the said flux concentrators, there being provided outer control handwheels (5, 5'; 5a, 5a'). 7. A furnace according to claims 4 or 5, character zed by comprising driving means for the automa¬ tic movement of said flux concentrators 4, 4* interlock¬ ed with a means for processing data of strip (1) speed, temperature and thickness as sensed by detecting means upstream of the furnace, as well as of the temperature at the outlet.

Inventors: ARVEDI GIOVANNI, GOSIO GIOVANNI, ARVEDI, GIOVANNI, GOSIO, GIOVANNI
Applicants: ARVEDI GIOVANNI, ARVEDI, GIOVANNI
Application year: 1990
Main classification: H05B6
IPC codes: C21D9, H05B6