terms of alternating oscillation and a vibration rate of 1 200 rpm to repeatedly cause bending deformation of the test piece and the number of repeated vibrations deformations until reaching a fracture was measured the evaluation of fatigue characteristics was rated u201c a u201d when the number of repetitions was 10 7 or more and rated u201c b u201d when less than 10 7 incidentally the test specimen had a profile of 0 32 mmt u00d7 10 mmw u00d7 100 mml a plurality of 0 32 mmt test pieces of 15 mm u00d7 15 mm were sampled and attachments and the like on the surface layer were removed by pickling the test piece was subjected to chemical dissolution with 5 g in total of bromine and methanol and extraction of inclusions was performed by an extraction filter having a pore size u03c6 of 5 u03bcm the extracted residue was observed by sem and the shape and size of the inclusion were measured also the identification of the inclusion was performed by edx the long side a and short side b of nitride or carbonitride were measured and the size of the carbonitride based inclusion was evaluated by the maximum size of the long side a furthermore fig1 shows the microscopic observation results of example 1 as a representative of examples 1 to 24 fig2 a shows the results of chemical extraction test and in addition fig2 b shows the results of chemical extraction test results of observation by sem of comparative example 14 incidentally in table 2 the carbonitride based inclusion is a ti carbonitride based inclusion and in all planar views its shape is a square or substantially square angular shape in fig1 it is seen that a zr inclusion zro 2 is present in.