SPHEROIDAL GRAPHITE CAST IRON, CAST ARTICLE AND AUTOMOBILE STRUCTURE PART MADE THEREOF, AND METHOD FOR PRODUCING SPHEROIDAL GRAPHITE CAST IRON ARTICLE
A spheroidal graphite cast iron meeting 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, among graphite particles observed in an arbitrary cross section of at least 1 mm 2 .
1 . A cast article formed by a spheroidal graphite cast iron, wherein graphite particles observed in an arbitrary cross section of at least 1 mm 2 meet
N
(
5
-
)
≥
3
0
0
,
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,
wherein the cast article has a thickness of 40 mm or more in as-cast casting.
2 . The cast article 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 cast article according to claim 1 , wherein said graphite particles meet
N
(
5
-
20
)
/
N
(
5
-
)
≥
0
.
6
5.
4 . The cast article 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 cast article 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 μm 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 . The cast article according to claim 1 , wherein said cast article is a structure part for automobiles.
7 . A method for producing a cast article of spheroidal graphite cast iron of claim 1 , 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.
8 . The method for producing a cast article according to claim 7 , wherein said pressure is 10-50 kPa.
9 . The method for producing a cast article according to claim 7 , wherein said method meets
0
≤
d
t
p
E
/
d
t
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.