Medium carbon steel sheet for cold working and method for manufacturing the same

1. A medium carbon steel sheet for cold working that has a hardness of 500 HV to 900 HV in a case of being subjected to high-frequency quenching in which a temperature is raised at an average heating rate of 100° C./second, the temperature is held at 1,000° C. for 10 seconds, and a quick cooling to a room temperature is carried out at an average cooling rate of 200° C./second, the medium carbon steel sheet comprises,

C: 0.30 to 0.60 mass %,

Si: 0.06 to 0.30 mass %,

Mn: 0.3 to 2.0 mass %,

P: 0.030 mass % or less,

S: 0.0075 mass % or less,

Al: 0.005 to 0.10 mass %,

N: 0.001 to 0.01 mass %, and

Cr: 0.001 to 0.10 mass %,

and a balance composed of Fe and inevitable impurities, wherein

an average diameter d of a carbide is 0.37 μm or less, a spheroidizing ratio p of the carbide is equal to or more than 70% and less than 90%, and the average diameter d in μm of the carbide and the spheroidizing ratio p in % of the carbide satisfy d≦0.04×p−2.6.

2. The medium carbon steel sheet for cold working according to claim 1 ,

further comprises one or more of,

Ni: 0.01 to 0.5 mass %,

Cu: 0.05 to 0.5 mass %,

Mo: 0.01 to 0.5 mass %,

Nb: 0.01 to 0.5 mass %,

Ti: 0.001 to 0.05 mass %,

V: 0.01 to 0.5 mass %,

Ta: 0.01 to 0.5 mass %,

B: 0.001 to 0.01 mass %,

W: 0.01 to 0.5 mass %,

Sn: 0.003 to 0.03 mass %,

Sb: 0.003 to 0.03 mass %, and

As: 0.003 to 0.03 mass %.

3. The medium carbon steel sheet for cold working according to claim 2 ,

wherein a Cr content [Cr mass %] and a Mo content [Mo mass %] satisfy [Cr mass %]+[Mo mass %]/10<0.10.

4. The medium carbon steel sheet for cold working according to claim 1 or 2 ,

wherein the hardness prior to cold working is greater than or equal to 120 HV and less than 170 HV.