PSI - Issue 19

Keiji Yanase et al. / Procedia Structural Integrity 19 (2019) 504–512 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

505

2

and the planned discard. However, the roll life can be significantly shortened by cracking during rolling, mostly by rolling incidents like cobble. Such cobbles tend to occur in the later hot strip mill stands more frequently than the earlier stands because of thinner gauge and higher rolling speeds. Fig. 1 shows the typical roll crack mechanism associated with the cobble incident. Strip steel may stick to the roll surface in such incidents. Due to the concentrated contact stress between the backup roll and the work roll at the adhered strip, the cracks are introduced during the cobble and are likely to propagate roll inside. Higher compressive residual stress promotes the shear-mode fatigue crack propagation when the initial plane of cracking is not at a right angle to the roll surface (Oda et al. (2013)). If the cracks penetrate deeply into the roll inside, the damage could be fatal for the roll, and in the worst case, they may lead to the roll spalling that causes the interruption of rolling operation and/or damages of other rolls like back up rolls as well. Although the crack propagation is associated with shear-mode fatigue crack growth (Figs. 2 and 3), there is no established method until now to estimate the resistivity of material against shear-mode crack growth. The investigation of such crack growth behavior will give a clue for the rational development of prospective material against spalling. Accordingly, aiming for such material development, this study conducts a series of fatigue tests for the roll materials to investigate the shear-mode fatigue crack growth behaviors, and thereby discusses their characteristics reporting the fundamental experimental data.

Later stand

Earlier stand

Later stand

Stuck strip

Occurrence of sticking and heat cracking of roll

Bent of strip

Rolling direction

Fig. 1. Hot-roll milling process and cobble incident.

Nomenclature a

half of total crack length

f test frequency ICDP indefinite chill double poured cast iron N number of cycles N f number of cycles to failure R stress ratio SEM scanning electron microscope  a amplitude of twist angle   a

increment of amplitude of twist angle from the onset of fatigue test

shear stress amplitude static compressive stress

 a

 static