Cold rolling – Automation
Online roughness measurement of hot-dip
galvanized steel sheet for skinpass mill control
Salzgitter Flachstahl, Germany, is the first steel producer to use the EMG SORM 3plus
online roughness measurement system for process control at its hot-dip galvanizing line. The
system, installed directly downstream of the skinpass mill, measures the data needed for
automatic rolling force control.
The skinpass mill uses work rolls featuring a Pretex structure which generates a spherical
texture on the strip surface. This gives the steel strip improved adhesiveness for deepdrawing oil. The sturdy structure of the online roughness measurement system is highly
insensitive to the harsh operating conditions.
Salzgitter Flachstahl GmbH (1 SZFG) is the largest subsidiary of the German Salzgitter
group. With a staff of approx. 4,500 employees, SZFG produced about 4.1 million t of steel
and achieved sales in the range of EUR 1.9 billion in 2004. Products from the integrated iron
and steel works are hot-rolled wide strip, cold-rolled strip as well as surface finished and
coated flat products with thicknesses ranging from 0.4 to 25 mm and widths of up to 1,850
mm. The production range includes drawing, deep drawing and extra deep drawing grades,
structural steel, fine-grained steel as well as high-strength steel.
Major customers for their flat products are car manufacturers and their suppliers, cold-rolling
shops, pipe/large pipe manufacturers as well as the construction industry.
High strength isotropic steels for the automotive industry offer excellent forming and
processing characteristics. Their higher strength enables components to be produced with a
reduced sheet thickness yet complying with the required crash features. Innovations
such as the Pretex® method applied in the field of surface technology yield enhanced
processing features in terms of forming and painting.
For the production of high-grade steel products, the Salzgitter group banks on advanced
manufacturing technologies for steel production and processing. With major investments in
new equipment to enhance and modernize their process engineering, SZFG ensure that
quality requirements will be met now and in the future.
SZFG is the first company in the steel industry to use the EMG SORM 3plus online
measuring system in a closed control loop. The equipment provides process control
immediately downstream of the skinpass stand in the production line.
"We have quite a positive experience with the online measuring system", states Matthias
Geler, senior plant engineer, who is one of the persons in charge of hot-dip galvanizing line
No. 1 (FV1) where an average 45,000 t of steel is processed each month. "Cold-rolled steel
with a thickness ranging from 0.36 to 3 mm accounts for 95 percent of our production, the
remainder being hot-rolled steel up to a thickness of 4 mm. In terms of sheet thickness, this
is quite a large range for a hot-dip galvanizing plant which gives us a critical competitive
edge", underlines Matthias Geler who knows what surface finishing is all about. Both the
technological properties, i.e., tensile strength and yield point, and the surface finish are
crucial in determining the quality of hot-dip galvanized steel. "One of the most important
parameters of surface quality is the roughness centre line average height Ra. Lately however
the peaks value per centimetre is also becoming more and more important to our
customers."
Smooth finish using Pretex textured rolls
The FV1 skinpass stand caters for the roughness height and peak value stipulated by the
customer and thus produces a smooth surface finish. "We are the only hot-dip galvanizing
shop to use rolls with a Pretex texture. These rolls have a very hard surface featuring
hemispherical projections in the μm range. Besides their special texture, Pretex rolls have a
very long service life and little roughness wear. These projections cause spherical hollows to
be embossed into the galvanized steel strip. This gives a surface roughness in the usual
range of 1.2 to 1.8 μm which is needed e.g. for ensuring that the oil applied for corrosion
protection and for improving the deep-drawing properties of steel strip for the automotive
industry adheres to the strip", explains Matthias Geler.
The conventional method of roughness measurement uses a stylus instrument. "This is the
established approach which however has critical disadvantages", states Sven Schulz, FV1
plant manager at Salzgitter Flachstahl. "As the stylus instrument only provides for selective
measurement at the exit of the hot-dip galvanizing line, the average roughness, which is
determined at the leading end, in the middle and at the trailing end of the strip, does not give
information about the surface quality of the complete coil. Therefore we have been faced with
complaints in the past. On the other hand, however, this method is accepted by customers
for determining surface quality."
Striving for high degree of automation
This gave the experts of Salzgitter Flachstahl GmbH a dilemma which they did not want to
put up with. Therefore they looked for a system for continuous surface quality measurement
or more precisely surface roughness. They did not want any system though but it had to be
something which would offer a high potential for automation already from the start, when it
was integrated into the hot-dip galvanizing line.
"We tested various roughness measurement systems. In the end, we went for the EMG
SORM 3plus. There are decisive reasons in favour of this system", recalls Matthias Geler.
According to the practitioner, the system is absolutely suitable for steel mills, i.e., dependable
in use in the harsh steel processing environment. How does EMG SORM 3plus work? A
homogeneous
laser beam is focused on the steel surface. It hits the surface with a diameter of approx. 5 to
7 μm. The very fine surface unevenness reflects the beam. 20 receivers accommodated in a
semicircle detect the reflections. EMG SORM 3plus scans every 5 μm for 8 seconds some
million surface spots in the strip running direction. In a few seconds, it calculates the
roughness across this distance before it starts measurement again. The values determined
by way of reflection 1 are stored in an evaluation computer, which converts the data into a
curve which is used as a basis for computing the roughness and the number of peaks.
During measurement, a signal transmits the strip speed to the EMG SORM 3plus so that the
system knows the time intervals required for specific checking.
Trouble-free integration into production
"What convinced us in the end was the fact that EMG SORM 3plus provides data which can
be verified using a stylus instrument installed at the strip exit", explains Matthias Geler. "It is
a fact that our customers assess the quality of our coils amongst other factors by the
measured values determined using a stylus instrument.
The point is that our customers also have this method at hand to verify our measured results.
Therefore we use EMG SORM 3plus primarily as a process control system.
It enables us to use the roughness values determined and displayed online to modify the
surface quality during production by controlling the skinpass stand rolling force. Let me put it
this way: We know immediately after the skinpass process the result measured by the stylus
instrument at the strip exit."
EMG SORM 3plus enables non-contact online sensing of the surface roughness of coldrolled steel strip. As the strip needs not to be stopped for these measurements, the system
could be easily integrated into Salzgitter Flachstahl's hot-dip galvanizing line as a process
control
system. The FV1 engineers in Salzgitter went an extra mile however. The values measured
by EMG SORM 3plus not only provide data comparable to the stylus results but thanks to
their immediate availability they can also be used for direct control of the skinpass stand
using a PLC.
"For this reason we have had software developed which can immediately react to the
measured roughness parameters and correct the rolling force of the skinpass stand as
necessary using a PLC. The reaction is based on the values of critical parameters stored in
the PLC, e.g., strip dimensions, zinc layer, quality, strip speed. As the software is selflearning, we have achieved our goal of sustainably increasing the degree of automation of
FV1 in combination with the online measurement system provided by EMG", states Matthias
Geler.
Anno Jordan, Sales Director, Quality Assurance Systems;
Dr. Matthias Irle, General Manager, Quality Assurance Systems,
EMG Automation GmbH, Wenden, Germany
Contact: www.emg-automation.com
E-mail: info@emg-automation.com