Four-roller type sizing mill apparatus for producing round steel rods

A four-roller type sizing mill apparatus for forming round steel rods, comprising two four-roller stands each having two pair of facing rollers, one pair of facing rollers is arranged in a rolling direction perpendicular to the rolling direction of the other pair of facing rollers, the two four-roller stands being arranged in line, with the rolling direction of one of the two four-roller stands being shifted by 45.degree. from the rolling direction of the other four-roller stand, wherein the two pair of facing rollers of a first four-roller stand of the two four-roller stands are positioned closer to a rolling material inlet and are separated from each other by a distance which is greater than zero and not greater than five times a projected contact length of one of the pair of facing rollers positioned closer to the rolling material inlet, the distance being measured between a first standard straight line which passes through centers of one of the pair of facing rollers and lies on a plane parallel to side surfaces of the same pair of facing rollers and a second standard straight line obtained by projecting axes of the other pair of facing rollers onto the above-mentioned plane, wherein, in the first four-roller stand, the pair of facing rollers closer to the rolling material inlet are connected to roller driving means for transmitting to the same pair of facing rollers a torque large enough for the pair of facing rollers to roll a rolling material, and the other pair of facing rollers are not connected to roller driving means, and wherein, in the second four-roller stand, one of the two pair of facing rollers is connected to roller driving means for transmitting to the same pair of facing rollers a torque large enough for the pair of facing rollers to roll a rolling material, and the other pair of facing rollers are not connected to roller driving means, the pair of facing rollers in both the first and second four-roller stands which are not connected to roller driving means are provided with pre-rotating means for rotating the same pair of facing rollers by a torque smaller than the torque of the pair of facing rollers driven by the roller driving means but at substantially the same speed as the speed of the pair of facing rollers driven by the roller driving means at least when a front end portion of a rolling material is bitten by the pair of facing rollers which are not connected to roller driving means.