WO1991008089A1

WO1991008089A1 - A steel bar and billet heating system located upstream of shears for further processing

Info

Publication number: WO1991008089A1
Application number: PCT/IT1990/000098
Authority: WO
Inventors: Guido Beiletti; Giovanni Bernardi; Pier Angelo Dell'oca
Original assignee: Elind S.P.A.; Forn/O/Matic Engineering S.R.L.
Priority date: 1989-11-24
Filing date: 1990-11-23
Publication date: 1991-06-13
Also published as: ATE124311T1; DE69020587D1; EP0591151A1; IT8922506A1; US5415381A; IT8922506A0; IT1239118B; EP0591151B1

Abstract

A steel bar and billet heating system located upstream of shears (14) for further processing comprises a conventional fuel operated heating furnace (6) up to a material temperature of 780 - 1000 °C, followed by at least two subsequent individual induction heating stations (11, 11A) up to the shearing or hot-pressing temperature of 1150 - 1300 °C. The system further comprises induction furnace monitoring and thermal control (9, 10, 12) means, product forwarding means within the combustion furnace and from the latter to the shears through the induction furnaces, adapted as well to move the bars and billets backwards, in the opposite direction, there being provided a waiting area (5) inside the furnace, a loader for the slugs coming back from the shears, and means for traversing the latter so that they do not interfere with the path of the pieces.

Description

" A STEEL BAR AND BILLET HEATING SYSTEM LOCATED UPSTREAM

OF SHEARS FOR FURTHER PROCESSING " This invention concerns a steel bar and billet heating system located upstream of shears for further process-

ing, in particular hot-pressing.

It is known that a very large majority of pieces derived from steel bars and billets is obtained by press-forg¬ ing, after shearing, and preferably by hot-pressing at

temperatures of 1100-1300°.. In such a way it is possible to reach satisfactory results, not only cost-wise, but also concerning the quality of the finished product, compared to cold processing both on a machine tool and through shearing and pressing at temperatures lower than

1100°C. It is also known that, for heating steel bars and billets stocked at ambient temperature, to the above mentioned

temperatures, up to now use has been made of combustion furnaces or induction furnaces. In the former (also in

chronological terms) liquid or gaseous hydrocarbons (presently methane is preferred) are used to generate

heat. Heat transfer to the products forwarded therewith- in, for instance by means of pilger rolls or beams,

takes place by radiation from the ceiling and from the

walls of said furnace wherein the products are introduced

through the front or through the sides, and are retained for a predetermined time span, so that they reach, at the exit therefrom, the predetermined temperature. If the latter has to reach values of 1150-1300°C for hot-pressing, as it has been mentioned above, very thick and expensive refractory linings must be provided, which strongly increase the furnace thermal inertia whereby, when the furnace requires maintenance operations, rather frequent at said high temperature levels, very long waiting times are needed for cooling, in the order of several days, during which production has to be stopped. A further problem taking place when combustion furnaces are used to reach the hot-pressing temperature directly, besides the fact that the energy performance drops considerably above a certain temperature, is due to the strong oxidation and associated formation of surface scale taking place on the products in particular due to the long high temperature residence times. which cause as well the problem of a possible material decarburizing.

It should further be noted that these times are further increased when, for any reason like a shears failure,

forwarding of the pieces is interrupted.

On the other hand, also the usage of the induction

furnaces alone to reach hot-pressing temperatures start-

ing from ambient temperatures causes some important draw- backs, like the fact that furnaces having such performan¬ ce are necessarily expensive, although they provide some advantages like the fact that the furnace goes almost

5 immediately to rated conditions with an optimum thermal control capacity, reduction of scale owing to the short¬ ened high temperature residence time, as well as the reduced maintenance requirements with an associated shortening of the operation downtimes. 10 One of the reasons why use of the induction furnaces has not developed in proportion to what the advantages mentioned above would suggest, besides the costs recalled above, is the fact that steel has a Curie temperature level around 760°C, above which a

15 ferromagnetic material becomes amagnetic, whereby the relative magnetic permeability value ( μr) becomes equal

to 1. Therefore, there takes place a substantial heating

inductor performance reduction in that the power trans¬ ferred to the piece to be heated is given by the follow-

20 ing formula:

2 Pw = uo x P r x π x F x H x V x K

wherein :

Pw = power transferred to the piece located within the

heating inductor;

o .__ μo = absolute permeability of air = 4* x 10-7; μr = relative permeability = average value 20 below the Curie temperature; F = working frequency in Hz;

H = magnetic field intensity in Asp/m; V = volume of the piece to be heated; K = a function of the ratio between diameter of the piece and current penetration depth inside the piece.

The inductor performance may also be defined through the following formula: n

wherein:

n = inductor performance; pc = resistivity of the inductor material =

0,017 x 10^" . Ω x m; pw = resistivity of the piece being heated.In the case of a 0,23% carbon steel we have a value at 20°C

of 0,160 x 10^".0 x m; a value at 1200°C of: 1,22 x 10^" . Ω x m;

μr = relative permeability- average value 20 below the Curie temperature.

Therefore, it would seem to be preferable to use induct- ion furnaces only for heating to a level below the Curie

temperature. If it were necessary to reach higher temp¬ eratures a combustion furnace should be used. Therefore, it would seem to be advisable to use induction furnaces

until the Curie temperature is reached, passing then to a combustion furnace.

On the contrary, it has surprisingly been found that, in spite of that technical prejudice, the optimum solution to the above mentioned problem of heating steel bars and billets to the hot-pressing temperature is given, accord¬ ing to this invention, by a system providing an initial heating, from ambient temperature up to 780 - 1000°C, preferably 800 - 850°C in a combustion furnace, followed by heating up to 1150 - 1300°C in at least two separate induction furnaces.

With this type of system, according to this invention, a

large portion of the above mentioned drawbacks may be overcome, which affect in particular those systems where- in heating is performed in combustion furnaces only or

in induction furnaces only.

According to a particular feature of this invention, said system additionally includes means for forwarding

the pieces in the direction going from the combustion

furnace to the shears, operable in such a way as to make said pieces to move backwards in order to reach a dwell

area inside the combustion furnace. According to another feature of this invention, means are also provided to temporarily stock slugs possibly coming back from the shears, associated with means to move the latter apart

from the path of the pieces.

The above and other objects, advantages and features of the system according to this invention will become apparent to those skilled in this art from the following detailed description of a preferred embodiment thereof, made for exemplary and non limiting purposes, in refer¬ ence to the attached drawings, wherein: Figure 1 shows a general schematic plan view of the system according to this invention;

Figure 2 shows a more detailed elevational view of the part of the system including the shears; and Figure 3 is a top plan view of the same portion of the

system shown in Figure 2. Referring now to the drawings, the system according to

this invention includes, at the entrance to a combustion automatic , furnace 6, preferably a "gas" type furnace, an/ bar select¬ or 1 for front loading of the furnace, which might of

course also be provided in the side loading type with an

intake roller table on the left hand side of the Figure. Furnace 6 within which the bars are forwarded in the direction of arrow F, for instance, by means of a pilger

process bar forwarding assembly whose driving members have been schematically shown and indicated by reference number 3, provides progressive heating of the metal products therewithin until, at the exit, the latter

reach a temperature of 780-1000°C, ranging preferably between 800 and 850°C, for reasons of energy saving and of surface scale reduction. In the exit area the bars or

billets are withdrawn by an evacuating roller table 2 and they are forwarded longitudinally out of the furnace

in alignment with an outer roller table 8 connecting the gas furnace with the induction furnaces. The latter have been indicated with reference numbers 11, 11a and are connected, for instance through bus bars 16 with medium frequency generators 19 in order to produce heating, inside the metal products forwarded therethrough, up to a temperature of 1250 - 1300°C.

.Optical pyrometers 9, 10 and 12 are provided for temperat¬ ure monitoring respectively at the exit of furnace 6, at

the exit of first inductor 11 and of second inductor 11a, whereby pyrometer 9 determines a first control on

furnace 11 and pyrometer 10 determines in real time a final control of furnace 11a, in such a way as to make sure that the product comes out of the latter at the

desired temperature. Pyrometer 12 monitors the piece

final temperature, i.e. the process temperature. Of course more than two induction furnaces may be provided for a finer and more reliable control.

Shears 14, which are controlled by a conventional

hydraulic control center 17 and are possibly provided, in a way known, with an entrance bar-holder group 21,

with a bar shearing hydraulic cylinder 22, and with a square-cut cylinder 24, have an outlet motor driven front stopper 15 and a preferably three-way unloading chute 18 where the cut pieces come out from. According to a particular feature of this invention, shears 14 are mounted on slide rails 25, as it is best shown in Figure 3, in order to be able to traverse them and in any way to move them apart from the piece forwarding path where¬

by, in case some sheared slugs, for any reason, are not used downstream of the shears, they may be moved back-

wards, for being heated once more in view of further

processing, and may be recovered and stocked inside a so called "louvers type" elevator loader 7. At the furnace exit there is provided a fast roller unloader 13 adapted

to unload the slugs and avoid interference with the

shears. In fact, when the shears will have moved to the position shown by a chain line in Figures 1 and 3, the roller unloader 13 will be able to withdraw the sheared slugs in that it is axially aligned with the shearing

area (see Figure 3).

In the same way, in case of a failure or any machine

stoppage of shears 14, the bars or billets which have already been forwarded all the way to the induction furnaces 11 and 11a may be moved backwards, through a reverse motion of roller conveyor 8, until they come

back inside furnace 6, wherein they may be kept heated in view of a following manufacturing process resumption. Of course, unloading roller table 2 of furnace 6 is taken by the bars and billets already heated and ready to come out. In order to allow the bars already forward¬

ed out. to go back inside furnace 6 there is provided, within furnace 6, a waiting area 5 where the bars present on the unloading roller table 2 are transferred, for instance by means of overturning levers 4 adapted to free said roller table by moving the bars and billets

located thereon towards said waiting area 5 which may be located, as shown in Figure 1, opposite said roller table

relative to the product forwarding direction within furnace 6, in the direction of arrow F.

Additions and/or modifications may possibly be made by those skilled in this art to the embodimental form, described and shown herein above, of the system accord¬ ing to this invention, without exceeding the scope of said invention.

Claims

1. A steel bar and billet heating system located up¬

stream of shears for further processing, wherein shears

(14) work at temperatures of 1100 - 1300°C, comprising a heating combustion furnace (6) provided with means (3) for forwarding said bars and billets therewithin, in the direction of an arrow (F), towards a furnace unloading roller table (2), located upstream of said shears (14), characterized in that said furnace (6) performs an init¬ ial heating from ambient temperature up to a temperature of 780 - 1000°C, and said system further comprises at

least two induction furnaces (11, 11a) located in series to each other and downstream of said furnace (6), for

heating up to 1150 - 1300°C, immediatel before said shears (14), a roller path (8) being provided between the exit from said furnace (6) and said induction furnac¬ es (11, 11a) for forwarding the pieces towards shears (14).

2. The system of claim 1, characterized in that within

said furnace (6) there takes place a heating of the bars

and billets up to an exit temperature of 800-850°C.

3. The system of claims 1 or 2, characterized in that

it includes pyrometers (9, 10) immediately ahead of each induction furnace (11, 11a) for the purpose of detecting the inlet temperature to each of said furnaces and of controlling the operation thereof in order to reach a respective predetermined exit temperature from each of them, as well as a pyrometer (10a) at the exit from the last induction furnace (11a) in order to monitor the final processing temperature.

4. A system according to any of the preceding claims, characterized in that said roller path (8) may be operat- ed also in a direction opposite to the direction from furnace (6) to shears (14), in order to re-enter in said furnace some bars and billets located on said roller path between furnace (6) and shears (14) in case the latter are stopped.

5. The system of claim 4, characterized in that within said furnace (6) a waiting area (5) is provided for temporarily stocking the bars and billets located on roller table (2) close to the exit from the furnace, in order to enable other bars and billets already forwarded out to come back into furnace (6), following said back¬

wards operation of roller path (8) , there being provided means (4) for said movement.

6. The system of claim 5, characterized in that said

means (4) comprise overturning levers and said area (5)

is located inside furnace (6), opposite to roller table (2) in the direction of said arrow (F).

7. A system according to any of the preceding claims, characterized in that it includes means (13) for wi-th- drawing from shears (14) the slugs which have not been used in a working cycle and which are heated again for a further manufacturing process, and means (7) for stock¬ ing the same.

8. The system of claim 7, characterized in that said shears (14) are mounted on horizontal rails (25) in order to be moved apart from the path of the slugs in case they have to go back upstream.

9. The system of claim 7, characterized in that said temporary stocking means (7) for the slugs coming back from shears (14) comprise a "louvers type" elevator loader.

10. A steel bar and billet heating system located up¬ stream of shears for further processing, substantially as described and shown above.