In the steel-work the rolled coils of low carbon steel plates must be unended before pickling on the continuous mill. This was solved with butt welded joint by laser beam welding at ISD DUNAFERR Dunai Vasmő ZRt. The 8 kW light power, near TEM00 mode laser beam’s final part of optical system is a divided parabolic mirror, which creates two focal spot in the welding region. In the interest of productivity rising, the rolled bands’ ends have to be welded at the highest speed as far as possible. The welded seams have to tolerate the repeated bends on the pickling mill without cracking. The optimum parameters of laser welding technology were determined by metallography and calculation of energetic efficiency.

Stresses born in the solid state may trigger acoustic emission (AE). Stresses are traditionally caused by mechanical forces and/or pressures implemented to the surface of the material. However, inner stresses in solid state may be born due to changing of magnetic field or due to temperature changes especially when cooling down the material, for example in structure changing. It is also well known that elongation of material structures due to rolling may lead to different AE bursts, which may be used for characterization of the structure of material due to cold forming. In the presentation we show in experiments that alternating magnetic field leads to AE burst, which have larger magnitude where the gradient of the changes of magnetic field is larger in absolute value. We also demonstrate correlation of the RMS of AE signals with the direction of the elongation of steel structure. This opens the way to investigate anisotropy in steel sheets. We also investigated the dependence of the measure of acoustic emission on frequency and on different sinus, triangle and rectangle forms in alternating magnetic field. EWGAE 35, Ljubljana, Slovenia, 13th – 16th Sep. www.ewg

A new equipment was developed to investigate the thermal fatigue behaviour of laser surface treated layers. The equipment is appropriate to study the thermal fatigue of rollers. The compared test materials were CMo3 grade hardened and tempered steels with laser quenched, TiC alloyed and Stellite 6 coated surfaces. The comparison of the surfaces based on the time of the first crack net appearance, the hardness profile of the surface and the morphological parameters of the cracks.

A fully automated laboratory-scale workstation has been constructed for simulating industrial acid pickling processes of steel sheet products. The device built comprises 4 liquid tanks that can be heated up to 80 °C and an arbitrary immersion program can be performed with the sheets to be tested by varying the immersion time and the motion velocity in each tank. The weight loss has been measured for both hot-rolled and cold-rolled specimens as a function of all variable parameters. The weight loss associated with the removal of the oxide layer could be established. The average oxide-related loss was 58 g⋅m-2 and 2 g⋅m-2 for hot-rolled and cold-rolled samples, respectively. It was found that the immersion time is the decisive parameter in the weigh loss measured, while the motion velocity and the temperature of the sample treatment are of secondary importance.

In order to perform a continuous cold rolling of steel sheets, hot rolled coils need to be attached to each other in order to get an endless sheet. This process can be achieved by laser key-hole welding as well. The structure and mechanical properties (e.g. hardness) of the welded joint without added metal, as well its heat affected zone differs significantly from the base metal. During our laboratory experiments the aptitude of cold rolling of joints and their environment, as well as the change in the features of materials was analyzed. During the rolling phases the hardness of the joint is higher than the ground structure, the strip thickness is modified depending on the difference of the hardness during the passes. According to our findings, the significant difference in measured hardness profile and texture structure prior to cold rolling and during cold rolling is minimized due to a recrystallisation annealing treatment following the cold rolling procedure. It is obvious that the normalization before cold rolling has advantageous effect to the properties of the welded joint. Elongation of the normalized samples is bigger than elongation of the non normalized samples.