Spheroidal graphite cast iron, cast article and automobile structure part made thereof, and method for producing spheroidal graphite cast iron article

Abstract

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 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.