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The abilities of non-destructive magnetic control of mechanical properties of roll forming billetsLyudmila I. Dolgova, Andrey A. Morozov, Vladimir L. Kornilov,
Vadim G. Antipanov, Vladimir N. Urtsev
Joint Stock Company "Magnitogorsk Iron and Steel Works",
High reliability of the devices has been proved in practice.
KIFM-1 are ferritic probe coercimeters with the attached electromagnets.
Their output parameter is proportional to coercive force of metal as well as to other factors related to parameters of the investigated material and the device. The devices are used at MMK for monitoring of hot steel more than 2,0 mm thick.
The coercimeter KRM-Z is a new device, recommended by research-and-production corporation SNR.
Unlike KIFM-1, the measurement in KRM-Z is carried out by the Hall converter and has the greater universality, simple design etc.
The efficiency of the NMM application is stipulated by its basic advantages as compared to visual survey and metal destroying trials, being arbitrary, and is based on probability relation between the controlled quality index and magnetic properties of steel (OST 14-1-184-86, GOST 30415-96). The procedure of development, introduction and commercial application of the NMM method for rolled products is represented in the standard STP 101-2-11.03.
Certification of finished products should be no less than 95 % reliable, i.e. probability of an error of the second kind, the risk of the consumer should not exceed 5 %.
Hot-rolled and cold-rolled strip in coils is used as billet for roll-forming in the shop of roll-formed shapes ("ProkatGP", subsidiary company of the joint stock company "MMR"), the strip is slit in the slitting mill and is wound in the continuous hot strip mills "2000" and "2500" and cold strip mill "2500".
Width of hot rolled strips for shaping is to be 50-600 mm, thickness is to be 1,0 - 8,0 mm; range of products: commercial steel grades make about 45 %, constructional steel grades- 40 %, low-alloy steel grades- 15 %.
One of the requirements to supplied roll-formed shapes is the requirement to mechanical properties, namely to a yield stress s
T, ultimate strength s
B, specific elongation d
It is known that the mechanical properties of flat steel can vary over a wide range, lengthwise of coiled billet. For example, plasticity of the front and rear ends of such a billet is lower than that of the middle part of a strip (lengthwise), that is stipulated by features of hot strip mill.
Therefore it is possible, that conditions of roll-forming (bend angles and bend radii), suitable for the middle part of a strip are unacceptable for its relatively longer ends (total length of strip sections with unacceptable properties, according to the data of "ProkatGP" and "MMR" Central Laboratory, amounts to 15... 20 M).
At stability of technological process for mass production with rather high level of mechanical properties for the given range of products, the application of a nondestructive magnetic method of quality monitoring (NMM) of quality is possible, aiming at:
Investigations of three steel grades from the hot strip mill "2000" (the main supplier of hot rolled strips) St 3ps, St 3Gps and 08 ps have been carried out. Histograms for allocation of frequencies of mechanical properties are obtained. Level of rejects for the examined steel grades is not exceeding 1,0 %.
The correlation-regressive analysis of interrelations of mechanical properties and readings of the devices NMM KIFM-1 and KRM-Z for the examined grades St 3ps, (3,0-6,0 mm), St 3Gps (4,5 and 6,5 mm) and 08ps (4,0 - 5,8 mm) has revealed significant correlation factors.
Inspection of the defects, resulted from the roll-forming process is of great importance. Flaws (cracks) occur most frequently; the reason for them was earlier understood as the result of the surface marks and dents during the hot rolling process or during transportation, or as the result of the steelmaking process imperfection, such as small breaks in the non-metallic inclusions zone and also slivers.
Long-lived practice of manufacture of the roll-formed shapes on the mills of ProkatGP has shown, that propensity to cracking during roll-forming is determined, among others, by chemical composition and mechanical properties of billet (especially value of a ratio s
B), and also microstructure of metal.
The cracking tendency is connected to the value s T/s B (is correlative interdependent with value of coercive force of material), this value exceeding some concrete value detected in practice.
At rather large particular bending angles on edges, another defect - waviness - can be observed at low values s T/s B owing to significant tensile stresses. Hence, the value s T/s B of every particular steel grade should be in a definite range of values to avoid cracking and waviness.
The investigation of correlations s T/sB , d 5, s T/s B and observations of readings of the device KIFM-1 has revealed an opportunity of transition to NMM for the roll-formed shapes in industrial conditions. In case the properties of driven billet are defined by installation NMM of a non-contact type efficiently, production of high quality shapes practically from the whole length of a strip is real.
About 400 roll-formed shapes are mastered at "MMR"s. A set of rolls (from 5 to 30 t each) is to be prepared for every particular shape, forcing a manufacturer to maintain significant production of rolls. Average cost of 1 t of rolls amounts to 26000 RUR. After roll-forming of 300 t of billet the rolls are sent for remachining and are reused 1-2 times, after that they become unfit for use (for roll-forming).
On the basis of the above practice, the roll-forming mill layout can be offered, allowing the use factor of metal rise without deterioration of roll stock quality.
The distinctive feature of the mill is the presence of one or two backup stands with hydraulic screwdown mechanism and with an opportunity of free transition of a strip between rolls. Thus between a push stand and vertical frameworks, installed behind it, the installation of electromagnetic nondestructive monitoring (for example IMPOK), with electric connection to hydraulic screwdown mechanisms of backup stands, is additionally placed; and for profiling of channels the backup stand is installed only after the second rougher.
The reason for location of nondestructive monitoring device behind the first push mill is that the billet within the operation limits of the device should be, whenever possible, flat. Besides, smooth rolls of the first stand supporting the strips, fed to the roughers, can partially deform metal, changing its mechanical properties. The upright rolls, close to the installation, ensure stability of a driven strip in a vertical plane within the necessary limits, leaving a stable clearance between the examined metal and the device.
The location of a backup stand depends on a sort of shape and on total value of bend angles (provoking cracks), because in case of critical properties of metal, resulting in such a defect, it is necessary to make two passes instead of one, with smaller particular bending angles. Therefore one backup stand is enough for profiling of channels, but it should be installed after the second rougher, as cracking of the bent channels is most frequently observed with total bend angles, corresponding to the third pass.
Having received a signal, showing unacceptable mechanical properties of a billet, a fast-acting (for example hydraulic) screwdown mechanism of backup stands (stand) are actuated; the upper rolls are immediately drawn down to a required position in order to start bending a strip at a preset angle, thus preventing the above defects of roll-formed products (before a signal is detected, the rolls are moved apart, ensuring unobstructed transition of a billet through a stand).
Hence, the installation and reasonable use of backup stands, possible only with NMM, located behind the first push mill of the roll-forming mill, will considerably raise a capacity factor of the equipment, increasing life of rolls.
As a result metal rejects of strip ends are diminished, application of more sparing conditions of roll-forming is not necessary, thus decrease of metal consumption and labor expenditures can be noted.
The application for the present engineering solution has been justified.
Benefit is reached by production costs reduction by 15 % minimum by means of:
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