| 摘要 |
At all points Pa included in zone “A” away from the central point of the core 4 at a distance of 1 mm or greater and less than 5 mm, the following mathematical expression (I) is satisfied, and at any point Pb included in zone “B” away from the central point of the core 4 at a distance of 5 mm or greater and 10 mm or less, the following mathematical expression (II) is satisfied. Ha2−Ha1<5  (I) Hb2−Hb1≧5  (II) Ha1 represents the hardness at a point Pa1 that is located inside the point Pa along the radial direction and away from the point Pa at a distance of 1 mm, and Ha2 represents the hardness at a point Pa2 that is located outside the point Pa along the radial direction and away from the point Pa at a distance of 1 mm. Hb1 represents the hardness at a point Pb1 that is located inside the point Pb along the radial direction and away from the point Pb at a distance of 1 mm, and Hb2 represents the hardness at a point Pb2 that is located outside the point Pb along the radial direction and away from the point Pb at a distance of 1 mm.
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