equal stress

Stress-strain power curve coefficient, K, numerically equal to the extrapolated value of true stress at a true strain of 1.00.

It is its yield stress which is equal to 235 N/mm2

The stress you induce on the beam is equal to weight hanging on the beam. The answer for the where part of the question is in the question. You said it yourself; you hang the weight from the center of the beam, therefore the stress of the weight will be in the center of the beam.

The forces are equal magnitude but opposite directions act tangent the surfaces of opposite ends of the object

the shear stress as force "f" acting tangent to the surface,dived by the "area"{a}

shear stress=f/a