Steel material for carbonitriding and carbonitrided steel material

A steel material for carbonitriding treatment, comprising:

• C at 0.1 mass % to 0.3 mass %;
• Si at 0.3 mass % or less;
• Mn at 0.4 mass % to 2.0 mass %;
• P at 0.03 mass % or less;
• S at 0.03 mass % or less;
• Cu at 0.3 mass % or les;
• Ni at 2.5 mass % or less;
• Cr at 0.5 mass % to 3.0 mass %;
• Mo at 0.001 mass % to 1.0 mass %;
• Al at 0.01 mass % to 0.08 mass %;
• and
• N at 0.005 mass % to 0.03 mass %;
• with a balance being Fe and inevitable impurities,
• satisfying the following Expression 1 of 0.6 to 1.4 and Expression 2 of >560 when subjected to carbonitriding,
• Surface C concentration(mass %)+12/14×Surface N concentration(mass %), Expression 1:
• 129.7805×[Cr(mass %)]−76.9797×[Cr(mass %)]2+339.3375×[Surface N concentration(mass %)]−539.345×[Surface N concentration (mass %)]2+181.4983×[Cr(mass %)]×[Surface N concentration(mass %)]+437.6799, and Expression 2:
• providing a carbonitrided steel material having a hardness of 560 HV or more at a portion from a surface thereof to a depth of 0.05 mm when subjected to a tempering treatment at 500° C. after the carbonitriding.