WO2001034853A1

WO2001034853A1 - A process for the continuous direct resistance heating of metallic semifinished products

Info

Publication number: WO2001034853A1
Application number: PCT/IT2000/000446
Authority: WO
Inventors: Francesco Mancini Petrelli
Original assignee: DEVILLE MARKETING CORP
Current assignee: DEVILLE MARKETING CORP
Priority date: 1999-11-12
Filing date: 2000-11-07
Publication date: 2001-05-17
Anticipated expiration: 2002-05-12
Also published as: ITMC990091A0; ITMC990091A1; AU1884001A; IT1311691B1

Abstract

The present invention relates to a process for the continuous heating of metallic semifinished products, of the type used in fixed heating installations through which the semifinished products to be heated move continuously, characterised by the fact that each semifinished product is heated through the passage of electrical current generated by poles with different electrical potential in contact with different sections or points of the metallic semifinished product.

Description

A PROCESS FOR THE CONTINUOUS DIRECT RESISTANCE HEATING OF METALLIC SEMIFINISHED PRODUCTS

The present patent application for industrial invention relates to a process for the continuous heating of metallic semifinished products through the direct passage of electrical current inside the semifinished product to be heated. 5 Processes for the continuous heating of metallic semifinished products, such as flat/round strips or open/closed sections, are used in continuous manufacturing lines in order to give particular metallurgical characteristics and particular physical characteristics to the material.

The heating of metallic semifinished products (steels, particularly, but 10 also other metals, such as titanium and its alloys, copper and its alloys, aluminium and its alloys, etc.) may be required in temperature ranges included between the environmental temperature and hundreds or thousands of Celsius degrees, according to the properties of the metal before treatment and according to the properties to be obtained after treatment. 15 For instance, a typical annealing treatment for AISI 3XX stainless steels may require a temperature increase between 0/25°C and 1100/1200°C.

Thermal treatments may be required with controlled or non-controlled temperature gradients, in controlled or non-controlled atmosphere and can be followed by temperature holding for preset or non-preset time ranges and 20 cooling of the semifinished products with controlled or non-controlled temperature gradient; all of the above according to the metallurgical process to be performed on the processed metal.

However, it is always necessary to give energy to the metallic semifinished product so that energy is stored in the metallic material as 25 thermal energy, causing the desired temperature increase.

Typical processes used by metallurgical industries make use of fixed heating equipment (known as "heating furnaces or thermal treatment furnaces") for the continuous heating of metallic semifinished products. The fixed heating equipment generates heat, for example by means of gas combustion or through the passage of electrical current through resistive elements, which is in turn transferred to the metallic semifinished product by means of irradiation or convection.

In these processes the metallic semifinished product moves through the fixed heating equipment continuously.

In the currently used processes as illustrated above, the heat is generated outside the semifinished product and is then given to the semifinished product in order to increase its temperature: the quantity of heat generated in the time unit in the heating equipment compared with the quantity of heat stored by the semifinished product in the time unit determines the energy efficiency (output) of the heating system. A very popular heating system is the system used in many steelworks to heat stainless steel strips.

In this process an austenitic stainless steel strip with width between 1000 and 1500 mm and variable thickness from 0.4 to 6 mm moves through a thermal section made up of a heating annealing furnace and a cooling equipment.

The strip speed can vary according to the strip thickness and width in order to comply with the maximum temperature exiting the furnace (typically ranging between 1100X and 1200°C) and a preset temperature holding time (typically ranging between 0 and ³λ seconds). The furnace is made up of one or more modules and basically consists in a tunnel with metallic external structure coated with suitable refractory material and equipped with methane gas burners of suitable number and power in order to supply thermal power to be transferred to the strip to be heated. Obviously, the furnace dimensions for a given productivity K

(tons/hour) depend on many factors, among which burner characteristics, type of strip, refractory materials, and system geometry. In any case, even the most advanced combustion technologies and the most recent technological solutions, with power density up to over 2 tons/meter, require very long furnaces.

For instance, a furnace with productivity K approximately equal to 40 t/h dimensioned for power density of about 1.5 t/m can be 30 meters long, with output higher than 50% (energy transferred to the strip / the energy contained in the burnt combustible).

Similar processes provide for the production of heat by means of electrical resistance and the transfer of heat towards the surface of the metallic semifinished product moving through the heating equipment. This is the case, for example, of electric heating furnaces for silicon steel strips or heating furnaces for welded pipes of carbon stainless steel, titanium and its alloys, etc.

However, it must be noted that the known heating processes are undoubtedly penalised by some evident functional and design limits as briefly described below.

The current processes for the continuous heating of metallic semifinished products provide for heat transfer from an external source (burner flame, electric resistance or hot fumes) to the semifinished product to be thermally treated and are used at industrial level by installations (such as industrial furnaces) with technological limits, due to the law of thermal exchange between bodies at different temperatures, according to which the exchange is directly proportional to the difference in temperature between the bodies, to the surface of thermal exchange and also to the coefficient of global thermal exchange.

In practice, very large heating systems are often necessary, with all the resulting negative consequences, such as huge volumes, very expensive installations, very high power, sophisticated control systems to adapt the installed power to the needs of the product to be treated, in view of the non- linear correlation between supplied power and product temperature, large amount of fumes with environmental consequences, etc.

Moreover, these heating processes must often operate in controlled atmosphere: this condition is very difficult, if not impossible, to obtain with heating systems provided with gas burners, which are on the other hand very popular.

The use of thermal resistance is appropriate, but the costs of electrical energy often become prohibitive and unjustified.

Also the surface characteristics of the metallic semifinished products are often compromised by very large heating systems, since it is necessary to mechanically "support" the product at high temperature with rolls or other support means suitable to operate at high temperatures, which interfere with the metallic surface.

The use of vertical development systems does not represent always a solution to these problems and to the need of moving the semifinished products in a controlled way.

In order to overcome many of the problems related to the aforementioned systems for the continuous heating of metallic semifinished products, some designers have thought about using the generation of heat directly inside the semifinished product, by means of the Joule effect caused by the circulation of electrical currents inside the metal.

Metals, both in pure and alloy state, are usually good conductors and therefore electrical currents with suitable shape and intensity can be used to obtained a "controlled" heating.

In order to obtain heating through induction of electrically conductor materials, the most common method is represented by the realisation of a magnetic flow generated by a solenoid through the passage of electrical current.

For instance, in order to heat a flat metal sheet, the metal sheet can be surrounded with a coil of electrical conductors allowing to generate an alternatively variable magnetic flow in axial direction through the thickness of the metal sheet. The alternatively variable magnetic flow generates induced alternate currents in the metal sheet to be heated, according to the Faraday law on electromagnetic induction. The effects of the induced electrical current on the heating of the metal sheet are basically due to the Joule effect and allow to obtain the heating of the semifinished product.

However, in practical applications, this process has shown a first, significant inconvenience, due to the fact that the induced current does not distribute uniformly through the thickness of the metal sheet (known as "eddy- current" effect) and its intensity varies according to the thickness of the metal sheet along the thickness itself.

Moreover, the electrical, magnetic and thermal characteristics of the materials vary according to temperature and, in many practical applications, may limit the use of magnetic induction heating processes.

For instance, in carbon steels, the density of the magnetic flow is drastically reduced at a temperature ranging between 1350°F and 1400°F, making it difficult to obtain heating through induction above this temperature. The experimental relationships between magnetic saturation factors, the length of the inductor solenoid, the speed of the metal sheet through the fixed heating section, the metal sheet thickness and frequency, show that with lower thickness values and higher speed, the solenoid must be very long, with consequent problems of geometrical tolerances. In some cases, such as the heating of steel coils, this has lead to studying new shapes of inductors other than solenoid and capable of generating transversal magnetic flows.

Anyhow, the practical application of magnetic induction for the continuous controlled heating of metal sheets is still complex, and even more complex is the practical application in the case of other metallic semifinished products with non-axial-symmetric geometry. This principle is instead often applied for the continuos controlled heating of long products with cylindrical geometry, such as tubes.

In the event that technological innovation allows to overcome the problems related to the process as briefly illustrated above, induction heating could find an application in many areas for the treatment of metal sheets: from pre-heating, before traditional heating furnaces, to paint drying, from galvanic and galvanic annealing process to thermal treatment, etc.

Undoubtedly, the possibility of generating induced electrical currents in the metallic semifinished product allows for generating heating (due to the Joule effect) directly on the metallic mass to be heated, thus overcoming many of the problems related with the transfer of heat from an external environment (with higher temperature) to the metallic semifinished product (with lower temperature).

After carefully examining and evaluating the limitations of the known continuous heating processes, the new heating process according to the present patent application has been devised.

According to this new process, the continuous heating of metallic semifinished products - such as flat/round strips or hollow sections with open/closed section - is obtained through the passage of electrical current in the semifinished product due to the application of an electrical potential difference between different points or sections of the semifinished product.

For major clarity the description of the process according to the present invention continues with reference to the enclosed drawings, which are intended for purposes of illustration and not in a limiting sense, whereby:

- fig. 1 is a schematic drawing showing an embodiment of the process according to the present invention;

- fig. 2 is a schematic drawing showing a second embodiment of the process according to the present invention;

- fig. 3 is a schematic drawing showing a third embodiment of the process according to the present invention; - fig. 4 is a schematic drawing showing the fixed heating section of an installation using the process according to the present invention to heat steel strips;

- fig. 5 is a schematic drawing showing the fixed heating section of an installation using the process according to the present invention to heat metallic tubes or bars with circular section.

With reference to the preferred applications of the process shown in figs. 1 and 2, the heating section is fixed and the semifinished products move through it with speed (V).

In these applications, the passage of the electrical current inside the metallic semifinished product (S) is obtained through the direct (either sliding or rotating) contact of the semifinished product with poles having a different electrical potential.

In particular, as shown in fig. 1 , the semifinished product (S) comes into contact with three electrically connected poles (1, 2, 3) in transversal arrangement with respect to the feed direction of the semifinished product (S), in contact with its entire section. Generators of potential difference (4) are located between the external poles (1 , 3) and the central pole (2) in order to generate a current flow (i) with parallel direction with respect to the feed direction of the semifinished product

(S).

The external poles (1 , 3) are earthed so that there is no current circulation inside the semifinished product (S) before and after the heating station.

Fig. 2 shows a second embodiment of the process according to the present invention, in which the potential difference is applied from opposite sides to the borders of the semifinished product (S), in order to generate a current flow (i) with perpendicular direction with respect to the feed direction of the semifinished product (S).

The potential difference is applied by means of electrical contacts (50) -with a generator of potential difference (4) between them - adhering to the opposite borders of the semifinished product (S) and moving together with it for a short distance. Practically, each electrical contact (50) consists in a conductor element that rotates with closed circuit along a rectangular trajectory at a linear speed (V) synchronised with the feed speed (V) of the metallic semifinished product.

In particular, the contact between the conductor element (50) and the longitudinal border of the semifinished product (S) occurs in a longitudinal section (50a) of the rectangular trajectory with closed circuit and, more precisely, in the longitudinal section (50a) with speed having the same direction as the semifinished product (S).

Two poles (10, 30) are earthed to ensure that there is no current circulated inside the semifinished product (S) before and after the heating station. In the embodiment shown in fig. 3, the semifinished product (S) comes into contact each time with three pairs of poles (100, 200, 300) in transversal arrangement with respect to the feed direction of the semifinished product (S), in contact with its entire section.

To that end, two identical chains (500) with closed circuit - one above and one under the feed plane of the semifinished product (S) are provided, each of them supporting six poles, three poles for each longitudinal section.

During the rotation of the two chains (500), the poles come into contact with three pairs of cams (600) that are electrically connected and symmetrically opposed with respect to the feed plane of the semifinished product (S).

Generators of potential difference (4) are located between the two external cams and the central cam of each set of three cams, it also being provided that the external cams are earthed.

During the rotation of the chains (500), the poles assume the potential of the cam they come into contact with, so that the set of three poles (100,

200 300) which comes each time into contact with the set of three cams (600) generates a current flow (i) with parallel direction with respect to the feed direction of the semifinished product (S).

The semifinished product (S) is in contact with a set of three poles of the superjacent chain and the corresponding set of three poles of the underlying chain. The two sets of three poles in contact with the semifinished product move together with it for a certain distance.

This happens because the linear speed of the two chains (500) is exactly equal to the speed of the semifinished product (S). In view of the above, the main characteristics and advantages of the new continuous heating process according to the present invention are as follows: • The continuous heating process according to the present invention makes use of a section of the material to be heated as resistive conductor element of an electrical circuit. Therefore it can be stated that in this section of the material heat is generated by "Joule effect". • The temperature increase in time unit of the metallic mass affected by the passage of electrical current is proportional to the electrical power dissipated herein due to the "Joule effect"; being the electrical power directly transformed into heat inside the metallic mass to be heated, the process according to the present invention allows for obtaining a heating system with very high efficiency (efficiency is equal to the ratio between the heat stored in the time unit by the metallic semifinished product and the power dissipated in the entire system). The loss of heat in the time unit, that is the fraction of power which is not used, exclusively depends on the characteristics of thermal insulation of the thermal section in which the heating process is realised and on the power dissipation in the other elements of the electrical circuit. These aspects can obviously be managed and optimised in the designing stage.

• Once the flow of metallic material to be heated in the time unit is known, once the temperature range between the entrance and the exit of the thermal section has been defined and once the geometrical characteristics of the semifinished product (and therefore its electrical resistance according to temperature) are known, the electrical parameters for the heating process can be determined.

• The process according to the present invention and its preferred applications allow to obtain controlled temperature gradients and also extremely abrupt temperature gradients in the semifinished products, in absence of limiting thermal exchange phenomena.

• Unlike the processes based on the combustion of combustible materials, the process according to the present invention allows for simple operation with controlled atmosphere (such as in the presence of inert gas or gaseous hydrogen).

• The process according to the present invention can be used with any type of metallic material and geometrical shape of the semifinished product to be continuously heated.

• For a given thermal profile to be realised on the semifinished product

(heating curve), the process according to the present invention can be realised either individually or in series with other heating processes. An abrupt temperature increase on the metallic semifinished product can be for instance obtained by using the process according to the present invention in a pre-heating section and a section with methane burners to maintain the temperature for preset time intervals. • The process according to the present invention allows for overcoming many of the intrinsic limitations of heating processes characterised by the generation of heat outside the metallic semifinished product. In particular, it allows for obtaining rapid temperature increases in the metallic mass to be heated with a strong reduction of volumes in the thermal sections of continuous heating processes without thermal gradient limitations. It also allows for drastically reducing and completely eliminating polluting emissions of thermal sections of continuous heating processes associated with the generation of fumes in combustion processes. Moreover, in view of its high efficiency, it allows for reducing operating energy costs of thermal sections of continuous heating processes, also in presence of electrical energy costs that are usually higher that other combustibles.

Finally, in view of the minimum thermal inertia of the system, it allows for varying the treatment speed of the semifinished product from zero to the maximum value (and vice versa), also with very abrupt gradients, without the risk of breaking the semifinished product.

For additional clarity of the inventive idea on which the process according to the present invention is based, the description continuous with two practical applications of the said process.

The first case shown in fig. 4 and based on the scheme of fig. 1 relates to the continuous heating of flat stainless steel strips (such as AISI 3XX series) in controlled and non-controlled atmosphere. In this case the process according to the present invention can be realised with the passage of electrical current through two symmetrical, but not necessarily specular sections of strip (N1 , N2) with respect to a central roll (RC) to which a potential difference with respect to two external support rolls (RS) connected to the earth connection of the electrical circuit is applied. The two suitably dimensioned external rolls (RS) are made of good electrical conductor metal and support the strip, the electrical contact and the earth connection of the circuit at the same time, thus avoiding stray electrical currents along the strip outside of the thermal section.

The central deflector roll (RC) is coated of metallic material and electrically insulated with respect to the rest of the thermal section and allows for the contact of the strip surface (N) along the entire transversal section, thus ensuring good uniformity in the electrical current distribution through the entire strip section.

The solution with rotating contacts, with decreasing potential difference between the central roll (RC) and the lateral support rolls (RS) avoids electrical micro-discharges that could damage the strip surface (N).

The staggered arrangement of the rolls allows for obtaining good electrical contact and distribution of the electrical current through the entire strip section, reducing the phenomena of surface preferential paths (eddy current).

A temperature reading device, such as an optical pyrometer, at the end of the heating section allows to adjust the voltage or current of the electrical circuit, in order to maintain the temperature value constant in very reduced tolerance ranges. The second case shown in fig. 5 and based on the scheme of fig. 1 relates to the continuous heating of metallic tubes (T) with circular section in controlled and non-controlled atmosphere. In this case the process according to the present invention can be realised with the passage of electrical current through two symmetrical, but not necessarily specular sections of tube (T1 , T2) with respect to a pair of central rolls (RC) to which a potential difference with respect to two external support rolls (RS) connected to the earth connection of the electrical circuit is applied.

Claims

1) A process for the continuous heating of metallic semifinished products, of the type used in fixed heating installations through which the semifinished products to be heated move continuously, characterised by the fact that each semifinished product (S) is heated through the passage of electrical current generated by poles with different electrical potential in contact with different sections or points of the metallic semifinished product (S).

2) A process according to claim 1 , characterised by the fact that the contact between each metallic semifinished product (S) and the poles with different electrical potential is of sliding type. 3) A process according to claim 1 , characterised by the fact that the contact between each metallic semifinished product (S) and the poles with different electrical potential is of rotating type.

4) A process according to claim 1 , characterised by the fact that the contact between each metallic semifinished product (S) and the poles with different electrical potential is of adherent type.

5) A process according to claim 1 , characterised by the fact that the entire section of the metallic semifinished product (S) comes into contact with three electrically connected poles (1 , 2, 3) in transversal arrangement with respect to the feed direction of the semifinished product (S); it being provided that generators of potential difference (4) are located between the central pole (2) and the two external poles (1 , 3).

6) A process according to claim 1 , characterised by the fact that electrical contacts (50) are applied in opposite sections of the longitudinal borders of the semifinished product (S) between two earthed poles (10, 30) with a generator of potential difference (4) in intermediate position, capable of moving for a short distance in adherence with the semifinished product (S); it being provided that each electrical contact (50) is made up of a conductor element rotating with closed circuit along a rectangular trajectory at a linear speed (V) synchronised with the feed speed (V) of the metallic semifinished product (S), so that the contact between each conductor element and the border of the semifinished product (S) occurs in a longitudinal section (50a) of the rectangular trajectory with closed circuit with speed having the same direction as the semifinished product (S).

7) A process according to claim 1 , characterised by the fact that the semifinished product (S) is in contact in three transversal sections with two identical superimposed sets of three translating poles (100, 200, 300), with the central pole (200) having a higher electrical potential than the earthed external poles (100, 300); it being provided that each set of three poles (100, 200, 300) is supported and moved with closed circuit by means of a support chain (500) that supports a total of six poles, so that at least three poles (100, 200, 300) are simultaneously in contact with a side of the metallic semifinished product (S) during the rotation of the chain (500).

8) A process according to claims 1 and 7, characterised by the fact that during the rotation of each chain (500) three poles come into contact with three pairs of electrically connected cams (600) with the interposition of generators of potential difference (4), with the central cam (600) having a higher electrical potential than the earthed external cams.