	US 7,488,396 B2
	Superior in machinability and method of production of same
Masayuki Hashimura, Muroran (Japan); Atsushi Mizuno, Muroran (Japan); Kenichiro Naito, Tokyo (Japan); Hiroshi Hagiwara, Tokyo (Japan); Kohichi Isobe, Muroran (Japan); and Hiroshi Hirata, Muroran (Japan)
Assigned to Nippon Steel Corporation, Tokyo (Japan)
Appl. No. 10/534,858 PCT Filed Nov. 14, 2003, PCT No. PCT/JP03/14547 § 371(c)(1), (2), (4) Date May 13, 2005, PCT Pub. No. WO2004/050932, PCT Pub. Date Jun. 17, 2004.
Claims priority of application No. 2002-332658 (JP), filed on Nov. 15, 2002; application No. 2002-332665 (JP), filed on Nov. 15, 2002; application No. 2002-332668 (JP), filed on Nov. 15, 2002; application No. 2002-332669 (JP), filed on Nov. 15, 2002; application No. 2002-332695 (JP), filed on Nov. 15, 2002; application No. 2002-332707 (JP), filed on Nov. 15, 2002; application No. 2003-374489 (JP), filed on Nov. 04, 2003; application No. 2003-374511 (JP), filed on Nov. 04, 2003; and application No. 2003-374517 (JP), filed on Nov. 04, 2003.
Prior Publication US 2006/0013720 A1, Jan. 19, 2006
Int. Cl. C22C 38/60 (2006.01); C22C 38/02 (2006.01); C22C 38/04 (2006.01)

U.S. Cl. 148—330 [148/320; 148/328; 420/87; 420/88; 420/121; 420/120; 420/128]

2 Claims

1. Steel superior in machinability comprised of, by wt %,

C: 0.005 to 0.2%,

Si: 0.001 to 0.5%,

Mn: 0.2 to 3.0%,

P: 0.001 to 0.2%,

T.N: 0.002 to 0.02%,

T.O: 0.0005 to 0.035%,

restricting the amount of S to 0.25 to 0.75 wt % and the amount of B to 0.002 to 0.014 wt %, by containing amounts of S and B in a region surrounded by A, B, C, and D shown in FIG. 4 where the contents of S and B satisfy the following equation (1), and by containing sulfides with BN precipitated in MnS:

(B−0.008)²/0.006²+(S−0.5)²/0.25²≤1 (1)

the balance of Fe and unavoidable impurities, said steel satisfying one or both of Mn/S in the steel being 1.2 to 2.8 or an area ratio of pearlite over a grain size of 1 μm in a microstructure of the steel being not more than 5%, and

having a density of MnS having a circle equivalent diameter of 0.1 to 0.5 μm at a cross-section parallel to a rolling direction of the steel material, taken from an extraction replica and observed by a transmission electron microscope, of at least 10,000/mm².