Method of producing shaped steel changing in cross-sectional shape in longitudinal direction and roll forming apparatus for same

A roll forming apparatus for roll forming for producing a shaped steel which varys in cross-sectional shape in the longitudinal direction from a sheet, comprising:

• a first rolling die which has a rotation shaft and an annular ridge part which varies in cross-sectional shape in a circumferential direction which is centered about said rotation shaft, said first rolling die arranged so that the rotation shaft of said first rolling die becomes perpendicular to a sheet feed direction;
• a second rolling die which has a rotation shaft and an annular groove part which varies in cross-sectional shape in a circumferential direction which is centered about said rotation shaft, said second rolling die arranged so that said rotation shaft of said second rolling die becomes parallel to said rotation shaft of said first rolling die;
• and
• a drive device which synchronizes and rotationally drives said first rolling die and said second rolling die,
• wherein said first rolling die and second rolling die are arranged relatively so that a gap which is equal to a thickness of said sheet is formed between the two and the annular ridge part of said first rolling die and the annular groove part of said second rolling die engage,
• wherein the side surfaces of the annular ridge part of said first rolling die are provided with relief so that the gap with respect to side surfaces of the annular groove part of the second rolling die broadens over at least part of the circumferential direction and at an inner side in the radial direction of said first rolling die,
• wherein said annular ridge part of said first rolling die is configured so that the relative angle between the ridgeline and the rotation direction of said first rolling die varies at least partially in the circumferential direction, and
• wherein the relief amount at said relief is set to vary in accordance with the relative angle between the ridgeline of the annular ridge part of said first rolling die and the rotation direction of said first rolling die.