Strukova I.D. Sklyar V.A.
The difference in shape between the original continuously cast billet and the square one (the ‘rhomboidity’ effect) may result in: (a) great difficulties when rolling the billet in break-down stands; (b) the breakdown bar dumping in a pass; and (c) errors in the billet’s design parameters after it has passed through a set of roughing stands. This, in turn, causes unstable rolling and heavy wear-out of the finishing stand rolls. In addition, as the experience of the rolling of distinct rhomboidity continuously cast billets shows, the latter has the effect of fully opening the internal and external corner cracks formed during its casting . The above-mentioned problems take on special significance under the conditions of integrating a section mill and a CC [continuous-caster] into a single casting and rolling unit.
Analysis of the existing methods of rolling continuously cast billets, including those with original rhomboidity, allows the three basic methods to be highlighted:
Method A: Rolling by means of a system of break-down box passes;
Method B: Rolling by means of a plain roll-to-box pass system in which the scale-breaker or an individual roughing stand act as a plain roll cage ;
Method C: Pass-free rolling ;
Using the pass-free rolling method allows to achieve a substantial reduction in roll regrinding costs. However, its main drawback, which is becoming a significant problem while rolling rhomb-shaped cross-section billets.
In the light of what has been stated above, there is no doubt that the carrying out of research into the peculiarities of the process of deforming continuously cast billets with and without a distinct inequality in the diagonals, drawn into the box passes, is of vital importance from the point of view of assessing the stability of the billets when rolling, of obtaining the correct and stable shape of the rolling product as well as of forecasting the quality of the products produced.
When planning the experiment a second-order incomposite design has been used for three factors (see Table 1). The following values have been taken as the factors:
-Δh/Bo, where Δh is the absolute reduction, Bo is the original billet width;
-à=Âî/Âê, where a is a jamming ratio;
-Êð= D2–D1, where Êð is a ‘rhomboidity’ ratio; D1 and D2 are the greater and the smaller billet diagonals, respectively.
Table 1: Levels and Factor Ranges
|
Factor |
Factor Range |
Level |
||
|
Upper |
Major |
Lower |
||
|
Symmetric Pass |
||||
|
Δh/Bo |
0.15 |
0.34 |
0.19 |
0.04 |
|
à=Âî/Âê |
3.0 |
6.8 |
3.8 |
0.8 |
|
Êð |
0.03 |
1.06 |
1.03 |
1.0 |
|
Asymmetric Pass |
||||
|
Δh/Bo |
0.1 |
0.23 |
0.13 |
0.03 |
|
à=Âî/Âê |
2.0 |
4.6 |
2.6 |
0.6 |
|
Êð |
0.03 |
1.06 |
1.03 |
1.0 |
The pilot research has been done on lead sample billets simulating rolling a continuously cast billet in a break-down stand of the Rolling Mill 500/370, with the modeling scale being 1:5. After giving the sample billet the necessary shape and dimensions, a net with a 2mm-mesh was printed on its lateral and contact surfaces. Original appearance of got thus standard is presented on the picture 1.
To carry out the experiment, a special set of rolls has been produced for the Laboratory Rolling Mill 100.
Fig. 1 – Thesimulated sample
While being rolled, the sample billets stopped at the rolls, which resulted in the unfinished sections. On the sample billets, the change in the position of the net’s knots was captured with a digital camera. The measurement of the coordinate grid was carried out under the AutoCAD program.