Case hardened steel having reduced thermal treatment distortion

1. A case hardened steel having a cross section having a macrostructure including an equiaxed zone and a columnar zone disposed around the equiaxed zone, the case hardened steel having a composition comprising, in mass %:

C: 0.05 to 0.45%;

Si: 0.01 to .0%;

Mn: more than 0 to 2.0%;

Al: 0.001 to 0.06%;

N: 0.002 to 0.03%;

S: more than 0 to 0.1%;

P: more than 0 to 0.05%; and

balance: Fe and inevitable impurities, wherein

Equation (1) described below and Equation (2) described below are satisfied in the equiaxed zone, or

Equation (3) described below is satisfied in the columnar zone,

Re =( Ae/Ao )×100≦30%  Equation (1)

( C min, 1/ Co )≧0.95  Equation (2)

( C min, 2/ Co )≧0.95  Equation (3)

where,

Re: area fraction (%) of the equiaxed zone,

Ae: area of the equiaxed zone,

Ao: area of the cross section,

Co: average concentration (mass %) of C in the cross section, or concentration (mass %) of C in molten steel in a ladle or continuous casting tundish,

Cmin, 1: minimum concentration (mass %) of C in the equiaxed zone, and

Cmin, 2: minimum concentration (mass %) of C in the columnar zone.

2. The case hardened steel according to claim 1 , wherein

Equation (1) and Equation (2) are satisfied in the equiaxed zone, and

Equation (3) is satisfied in the columnar zone.

3. The case hardened steel according to claim 1 , wherein

at least one of Equation (4) described below and Equation (5) described below is satisfied in the equiaxed zone,

( L/F )≧0.6  Equation (4)

( L/S )≧0.6  Equation (5)

where,

L: distance (mm) from a center of the cross section to a position closest to the center of the cross section and located on the periphery of the equiaxed zone,

F: distance (mm) from the center of the cross section to a position located on the periphery of the equiaxed zone and in a direction opposed, with respect to the center of the cross section, to the position closest to the center of the cross section and located on the periphery of the equiaxed zone, and

S: larger distance (mm) from among distances from the center of the cross section to positions at which the periphery of the equiaxed zone crosses a line passing through the center of the cross section of all lines perpendicular to a line connecting the center of the cross section and a position closest to the center of the cross section and located on the periphery of the equiaxed zone.

4. The case hardened steel according to claim 3 , wherein

Equation (4) and Equation (5) are satisfied in the equiaxed zone.

5. The case hardened steel according to 1 , wherein the composition of the steel further comprises at least one of, in mass %:

Mo: more than 0 to 1.5%;

V: more than 0 to 1.5%;

Nb: more than 0 to 1.5%;

Cu: more than 0 to 1.0%;

Ni: more than 0 to 2.5%;

Cr: more than 0 to 2.0%;

Sn: more than 0 to 1.0%;

Ca: more than 0 to 0.01%;

Zr: more than 0 to 0.08%;

Pb: more than 0 to 0.4%;

Bi: more than 0 to 0.3%;

Te: more than 0 to 0.3%;

Rem: more than 0 to 0.1%;

Sb: more than 0 to 0.1%;

Ti: more than 0 to 0.30%;

B: more than 0 to 0.005%; and

W: more than 0 to 2.0%.

6. The case hardened steel according to claim 2 , wherein

at least one of Equation (4) described below and Equation (5) described below is satisfied in the equiaxed zone,

( L/F )≧0.6  Equation (4)

( L/S )≧0.6  Equation (5)

where,

L: distance (mm) from a center of the cross section to a position closest to the center of the cross section and located on the periphery of the equiaxed zone,

F: distance (mm) from the center of the cross section to a position located on the periphery of the equiaxed zone and in a direction opposed, with respect to the center of the cross section, to the position closest to the center of the cross section and located on the periphery of the equiaxed zone, and

S: larger distance (mm) from among distances from the center of the cross section to positions at which the periphery of the equiaxed zone crosses a line passing through the center of the cross section of all lines perpendicular to a line connecting the center of the cross section and a position closest to the center of the cross section and located on the periphery of the equiaxed zone.

7. The case hardened steel according to claim 6 , wherein

Equation (4) and Equation (5) are satisfied in the equiaxed zone.

8. A mechanical component obtained by machining the case hardened steel according to claim 1 , and applying a thermal treatment to the machined case hardened steel.

9. A mechanical component obtained by machining the case hardened steel according to claim 2 , and applying a thermal treatment to the machined case hardened steel.

10. A mechanical component obtained by machining the case hardened steel according to claim 3 , and applying a thermal treatment to the machined case hardened steel.

11. A mechanical component obtained by machining the case hardened steel according to claim 4 , and applying a thermal treatment to the machined case hardened steel.

12. A mechanical component obtained by machining the case hardened steel according to claim 5 , and applying a thermal treatment to the machined case hardened steel.

13. A mechanical component obtained by machining the case hardened steel according to claim 6 , and applying a thermal treatment to the machined case hardened steel.

14. A mechanical component obtained by machining the case hardened steel according to claim 7 , and applying a thermal treatment to the machined case hardened steel.