u03c0 1 2 using l section micrographs next as for the toughness of a mother material a single phase microstructure is advantageous and the generation of pearlite upper and lower bainite and martensite in the microstructure must be suppressed to less than about 5 percent by volume the toughness of a weld portion is substantially determined by a chemical component it was confirmed that when a bainitic ferrite single phase a phase ratio of about 95 percent by volume or more has a pcm value of 0 17 or less superior toughness can be obtained bainitic ferrite has a ferrite microstructure transformed at low temperature in which a great number of dislocations are present in the grains and is apparently different from polygonal ferrite soft pro eutectoid ferrite transformed at a high temperature which is common ferrite in addition the area ratio of the bainitic ferrite obtained by image processing of sectional micrographs thereof is converted into the volume ratio of the bainitic ferrite precipitation hardening is performed by precipitation of alloy elements such as nb to ensure strength in response to a lower carbon content the precipitation of alloy elements is effective to increase the strength however the ratio in percent of the amount of precipitated nb to the total amount of nb hereinafter referred to as a u201c nb precipitation ratio u201d is advantageously controlled from about 5 to about 80 to obtain a preferably higher strength while the ctod toughness is retained since a large.