Steelworld on the Secret Doors
THE SECRET DOORS OF LINEBOW (*)
by Annibale Izzo (**)
Sounds like a new Indiana Jones adventure, but actually it's something
undocumented in the Linebow programs. These are software tools conceived to
provide effective solutions when designing roll passes for bar and rod mills.
The purpose of the secret doors is to give the programs a "positive" protection.
The message is: "Fully paid users only will be given the keys to an enhanced
level of performance". In this paper we deal with one important secret door.
Of course, we will only glance at what is behind it, without giving the key
to open it.
Continuous bar mills
As we all know, a continuous bar mill is structured to gradually reduce the
cross-sectional area (and correspondingly increase the length) of the rolled stock.
This is obtained by having the bar "bitten" between pairs of grooved rolls. The
direction of the "bite" rotates by 90 degrees at each stand for microstructural
reasons.
If not carefully designed, this process may create surface defects which would
seriously impair the product quality. Two critical parameters are i) the distribution
of the elongation coefficients; and ii) the shape of the grooves (the roll passes).
An optimal emulator of the rolling process is the Linebow program DESFILE, a
workhorse in roll pass design for continuous bar mills. Originally developed for
hot rolling of high quality steel products, DESFILE follows very strict
specifications about the type of passes to use in the various parts of the mill:
which type of passes must be used and where.
The secret door of DESFILE allows the roll pass designer to manually interfere with
these specifications. To see where exactly this door can be opened, we will first
illustrate the standard design process.
Preparing the design session
Let us assume we are working with an 18-stand mill. DESFILE aims to design the
complete rolling line, inclusive of: a roughing sequence (2 stands, with either
box-square or diamond-square passes); a stretching sequence (14 stands); and a
finishing sequence (2 stands, with leader oval and finishing round passes).
In this paper we will deal only with the stretching sequence. You will find a
number of useful illustrations in our Internet site1.
The stretching sequence comprises a number of pairs of passes (7 in our example).
In each pair the first pass is called an "intermediate pass" (see illustration
"Intermediate passes"1), and the second a "definite pass" (see illustration
"Definite passes"1). As a general rule, "rough operators" such as diamonds and
squares are used in the first part of the mill and "smooth operators" such as
ovals and rounds are used in the finishing zone. So the first design indication
is that squares come first, then false rounds, then rounds.
Basically, we start from a square billet and a finished round. The ratio between
their cross-sectional areas gives us the overall elongation coefficient ltot
(total lambda).
Elsewhere2 we discussed how to determine the elongation coefficients in the
roughing and finishing sequences. Here we say only that for billets with side
>100mm lr=1.36 and for rounds with diameter >14mm lf=1.32. This way we can
calculate, for the stretching sequence, ls=ltot/1.36/1.32.
The design process is a mix of geometry and spread calculations, but for clarity's
sake we will follow only the geometrical thread. Therefore, among the DESFILE
input data, what we need to know to determine the type of pass at each stand is:
· Billet side and radius
· Final round diameter
· Number of stands (suggested by the program)
· Roll barrel diameter at each stand
· zlk, the distribution factor of the lamk's
(lamk is the "square-to-square elongation coefficient", the ratio between cross-
sectional areas of square N and square N+2.)
The last piece of information, zlk, allows the human designer to influence the
automatic operations of DESFILE. In fact zlk applies to the average stretching
lamk (in our case the 7th root of ls), to increase elongation in the first part
of the mill and decrease it in the second part.
The value of zlk can be chosen in the range 1.03 to 1.07 (default is 1.05).
By increasing zlk the distribution curve rotates clockwise, this way increasing
loads on the first half of the mill.
Design of stretching sequence
The first thing to say is that as a first approximation we design as squares all
the 7 definite passes of our stretching sequence. (For the moment we won't be
considering the intermediate passes.)
To do this we use the 7 values of lamk we obtained from the preparation above.
The cross-sectional areas of the exit squares at all stands (4 to 16) are:
Q4=Qbillet/1.36/lamk1; Q6=Q4/lamk2; Q8=Q6/lamk3; Q10=Q8/lamk4; Q12=Q10/lamk5;
Q14=Q12/lamk6; Q16=Q14/lamk7.
Now we stated2 that the proper corner radius of a square should be 20% of its
side, unless Q<4000mm^2, in which case it should be increased to 25%. With this
we can immediately calculate 7 square sides in our stretching sequence (S1 to S7).
Square sides are crucial data to calculate eSI, a parameter allowing to determine
the type of intermediate pass to be placed between two squares. For example, let
us refer to stands 4, 5 and 6. Stands 4 and 6 have a square pass, stand 5 an as
yet unknown pass (diamond or oval?).
We know the value of lamk2, and can calculate2 eSI=Q4/(S4*D5)
with D5 = roll barrel diameter at stand 5. This way we obtain a pair of coordinates
identifying a point in a particular diagram (see illustration "Choosing between OV
and DI"1). It's easy to see that the position of this point determines the
intermediate pass to choose.
At this stage we have a rolling line with a single type of definite pass (square)
and two types of intermediate passes (diamond and oval). The program automatically
takes three actions:
1. All the squares with area<110mm^2 are changed into rounds, and if the preceding
pass is diamond this is changed into oval.
2. All the diamond-square pairs found after an oval-square pair are changed into
oval-false round pairs.
3. All the diamond-square pairs in square-to-square sequences with lamk<=1.5 are
changed into oval-false round pairs.
Behind the secret door
Stop! We don't want to open the complex chapter of dimensioning the intermediate
passes, the shape of which we have just determined. Remember where we started from?
Secret doors. Well, the secret door of DESFILE suddenly opens at a certain point
of the input procedure and you are asked whether you want to confirm the value 110
(action 1 above) or assign your own value; and, whether you want to confirm the
value 1.5 (action 3 above) or assign your own value.
If you think it's a minor detail for the human designer, think again. A squared
infinity of refining possibilities comes at the reach of roll pass designers.
After all, it's better not to leave too much power to machines!
References
1. "LINEBOW Roll Pass Design Software", website http://www.passdesign.com
2. U. Suppo, A. Izzo, P. Diana, "Anwendung eines elektronischen Rechners zur
Rundstahlkalibrierung", Archiv für das Eisenhüttenwesen, 46 (1975), Nr. 7 Juli
(*) Published by Steelworld (Mumbai, India), August 1999 Vol 5 No 8 p 7
(**) LINEBOW, Via Garda 2, I-10015 Ivrea (ITALY) - Phone +39 0125 616 768,
fax +39 02 700 500 073, e-mail: info@passdesign.com