Fusion-bonded product having high-strength part and manufacturing method thereof

A fusion-bonded product having a high-strength part, comprising:

• a first preform (2a) constituted by (a) integrally, frictionally pressure-welding a first low-carbon steel part (L1) made of low-carbon steel containing less than 0.45% of C and a high-carbon steel part (H) made of high-carbon steel containing not less than 0.45% C, and (b) deforming the first preform by forging such that a border between the first low-carbon steel part (L1) and the high-carbon steel part (H) comprises a V-shaped peak in cross section;
• and
• a second preform (2b) having a second low-carbon steel part (L2) made of second low-carbon steel containing less than 0.45% of C;
• wherein the high-carbon steel part (H) of the first preform (2a) is provided with a high-strength part (9) that has been previously formed into a desired shape and quenched,
• the second low-carbon steel part (L2) of the second preform (2b) has been previously formed into a predetermined shape,
• the first low-carbon steel part (L1) of the first preform (2a) and the second low-carbon steel part (L2) of the second preform (2b) are bonded to each other by fusion welding, and
• the high-carbon steel part (H) of the first preform is spaced from the second low-carbon steel part (L2) of the second preform with the first low-carbon steel part (L1) of the first preform disposed therebetween.