Control of the yielding and ageing behaviour in temper rolling

Rolls and roll systems which are integrated into the pass line of a temper rolling plant, introduce bending deformations into the strip, which extend the yield strength of the rolled material and thus lead to plastic deformations of the strip. When setting the optimum temper rolling degree and thus the resulting mechanical strip properties these plastic bending deformations have not been taken into account so far. The effect of plastic bending deformations on the mechanical strip properties, namely Stress-strain behaviour Yielding behaviour Strain hardening behaviour and Luders behaviour was investigated in a bending unit. The essential geometric proportions of the bending unit were equivalent to production plants. The investigations were carried out with batch annealed low carbon deep drawable strips (LC-DDQ steel). The investigated strip width was 300 mm, the strip thickness was 0.4 mm and 0.7 mm. The test results show that plastic bending deformations influence mechanical strip properties in a similar way as skin pass rolling itself. Regarding the course of yield stress, the minimum of yield strength, especially for thicker strips, is shifted to higher degrees of deformation than compared to temper rolling. The work hardening, which takes place after the minimum of yield stress, is accompanied by a significantly lower gradient of the yield stress curve than the compression strain caused by skin pass rolling. This effect is stronger, the thinner the strip thickness. The strain hardening exponent n shows depending on the degree of deformation for both the bending and rolling deformation the same course in the compared deformation range. Independent from a bending or a rolling deformation, the resulting gradients of the work hardening exponent lead to the same values. Concerning the decline or rather the complete suppression of the yield point elongation, the same effects are achieved for both the bending deformation and the deformation by temper rolling. From the investigation, it can be concluded that it is necessary to conduct the process of skin pass rolling in such a way that those deformations caused by plastic bending have to be taken into account when the optimum degree of temper rolling is specified. This makes it possible to achieve the most favourable mechanical strip properties. 3.2 Introduction and task definition The deformability of skin-passed strip is essentially favoured by a high exponent n of strain hardening, corresponding to a great uniformity of elongation, as well as a minimum yield stress. The two last mentioned nominal values would be set most favourably if the optimum skin-pass degree was adjusted on skin-passing. This will be the case if the strip is temper rolled with a deformation degree corresponding to the minimum of the yield stress curve. Pass reductions below the optimum skin-pass degree should absolutely be avoided (under-skin-passed) to eliminate the appearance of Luders' lines in subsequent processing stages, e.g. on materials with upper and lower yield point. So, light overskin-passing, connected with a small increase of the yield stress and a small reduction of uniformity of elongation, will be tolerated. Apart from rolling deformation, the strip can receive additional plastic bending deformations at several roller bodies which are part of the temper rolling process, such as coiler, tension roller, deflecting rolls and anti-sticking rolls.