SPHEROIDAL GRAPHITE CAST IRON, CAST ARTICLE AND AUTOMOBILE STRUCTURE PART MADE THEREOF, AND METHOD FOR PRODUCING SPHEROIDAL GRAPHITE CAST IRON ARTICLE

1 . A spheroidal graphite cast iron, wherein graphite particles observed in an arbitrary cross section of at least 1 mm 2 meet

N (5-) ≥250,

N (5-20) /N (5-) ≥0.6, and

N (30-) /N (5-) ≤0.2,

wherein N (5-) represents the number (/mm 2 ) of graphite particles having equivalent-circle diameters of 5 μm or more, N (5-20) represents the number (/mm 2 ) of graphite particles having equivalent-circle diameters of 5 μm or more and less than 20 μm, and N (30-) represents the number (/mm 2 ) of graphite particles having equivalent-circle diameters of 30 μm or more.

2 . The spheroidal graphite cast iron according to claim 1 , wherein said graphite particles meet N (2-5) ≥100, wherein N (2-5) represents the number (/mm 2 ) of graphite particles having equivalent-circle diameters of 2 μm or more and less than 5 μm.

3 . The spheroidal graphite cast iron according to claim 1 , wherein said graphite particles meet N (5-20) /N (5-) ≥0.65.

4 . The spheroidal graphite cast iron according to claim 1 , wherein said graphite particles meet D max ≥50.4 μm, wherein D max represents the maximum equivalent-circle diameter of graphite particles.

5 . The spheroidal graphite cast iron according to claim 1 , wherein said graphite particles meet −0.15≤[N (5-10) −N (15-20) ]/N (5-10) ≤0.25, wherein N (5-10) represents the number (/mm 2 ) of graphite particles having equivalent-circle diameters of 5 m or more and less than 10 μm, and N (15-20) represents the number (/mm 2 ) of graphite particles having equivalent-circle diameters of 15 μm or more and less than 20 μm.

6 . A cast article formed by the spheroidal graphite cast iron recited in claim 1 .

7 . The cast article according to claim 6 , wherein said cast article is a structure part for automobiles.

8 . A method for producing a cast article of spheroidal graphite cast iron meeting the following conditions:

N (5-) ≥250,

N (5-20) /N (5-) ≥0.6, and

N (30-) /N (5-) ≤0.2,

wherein N (5-) , N (5-20) , and N (30-) are the number (/mm 2 ) of graphite particles having equivalent-circle diameters of 5 μm or more, the number (/mm 2 ) of graphite particles having equivalent-circle diameters of 5 μm or more and less than 20 μm, and the number (/mm 2 ) of graphite particles having equivalent-circle diameters of 30 μm or more, respectively, among graphite particles observed in an arbitrary cross section of at least 1 mm 2 ; comprising

pressing a surface of a melt poured into a gas-permeable casting mold by a gas at pressure of 1-100 kPa, before said melt starts eutectic solidification; and

solidifying said melt while supplying said gas into said casting mold.

9 . The method for producing a cast article according to claim 8 , wherein said pressure is 10-50 kPa.

10 . The method for producing a cast article according to claim 8 ,

wherein said method meets

0≤ dt pE /dt E ≤1,

wherein dt E represents a time period from the start of eutectic solidification of said melt to the completion of the eutectic solidification, and dt pE represents a time period from the start of eutectic solidification of said melt to the completion of said pressing.