Method for producing forged crankshaft

A production method of a forged crankshaft, the forged crankshaft including:

• four journals each defining a rotation center;
• three pins each decentered with respect to the journals, the pins being respectively disposed at a first position, a second position, and a third position at a phase angle of 120°;
• and a plurality of crank arms that connect the journals with the pins, respectively, the production method comprising:
• a first preforming process for obtaining a first preform from a billet;
• a second preforming process for obtaining a final preform from the first preform;
• and
• a finish forging process for forming the final preform into a finishing dimension of the forged crankshaft by at least a single die forging step, wherein
• in the first preforming process, by using a pair of first dies, a region to be the pin and a region to be the journal of the billet are pressed from a direction perpendicular to an axial direction of the billet, so that while a cross sectional area of each of the regions is decreased thereby forming a plurality of flat parts, a region to be a second pin and to be disposed at the second position of the flat parts is decentered such that the decentering amount of the region to be the second pin becomes equal to or less than the decentering amount of the finishing dimension;
• wherein
• the second preforming process includes:
• a process of pressing regions to be the plurality of journals in a pressing direction corresponding to a width direction of the flat part, by using a pair of second dies;
• and a process of, after starting pressing by the second dies, decentering a region to be disposed at the first position and to be a first pin, and a region to be disposed at the third position and to be a third pin from each other in opposite directions in a decentering direction corresponding to the width direction of the flat part by using third dies, such that decentering amounts of the regions to be the first pin and the third pin are equal to, or less than (√3)/2 of the decentering amount of the finishing dimension;
• and wherein
• in the final preform, a thickness of each region to be the plurality of crank arms is equal to a thickness of the finishing dimension.