PURPOSE: To annihilate the center porosity in a steel material by the compressive stress that the steel material receives from all directions and to improve the quality of the steel material by rolling down the steel material provided with a specific temp. difference between the surface and the central part in the thickness direction and/or transverse direction at a specific shape ratio.

CONSTITUTION: The temp. on the surface of the steel material is made lower than in the central part to provide ≥400°C temp. difference between. Such steel material is rolled down in the thickness direction and/or transverse direction at the shape ratio of the steel material, i.e., the effective contact length in the thickness direction/ thickness adjusted at ≥0.5. The rolling down is executed by a press or malleably by rolls. The temp. difference of ≥400°C between the surface and the central part of the steel material is needed in order to form efficiently the compressive stress in the high temp. region in the central part of the steel material which is a material to be worked by the synergistic effect of the rolling force acted in the thickness direction of the steel material and the low-temp. condensing force of the steel material surface. The shape ratio is required to have ≥0.5 in order to form stably the compressive stress necessary for the press welding of the center porosity under the working pressure to the center of the steel material.