Steel sheet and method for manufacturing the same

A steel sheet consisting essentially of 0.10 to 0.37% C, 1% or less Si, 2.5% or less Mn, 0.1% or less P, 0.03% or less S, 0.01 to 0.1% sol.Al, 0.0005 to 0.0050% N, 0.005 to 0.05% Ti, 0.0003 to 0.0050% B, by mass, and balance of Fe, [B−(10.8/14)N*] being not less than 0.0005%, average particle size of precipitate TiN being 0.06 to 0.30μm, grain size of prior austenite after hardening treatment being 2 to 25 μm, N* being zero for negative N* values calculated in a definition formula of N*=[N−(14/48)Ti].

A method for manufacturing a steel sheet, comprising the steps of:

• hot rolling a steel slab consisting essentially of 0.10 to 0.37% C, 1% or less Si, 2.5% or less Mn, 0.1% or less P, 0.03% or less S, 0.01 to 0.1% sol.Al, 0.0005 to 0.0050% N, 0.0003 to 0.0050% B, by mass, and balance of Fe, [14B/(10.8N)] being 0.5 or more, at temperatures of Ar3 transformation point or above;
• and
• coiling the hot rolled steel sheet at temperatures of from 500 to 720° C.

A method for manufacturing a steel sheet, comprising the steps of:

• hot rolling a steel slab consisting essentially of 0.10 to 0.37% C, 1% or less Si, 2.5% or less Mn, 0.1% or less P, 0.03% or less S, 0.01 to 0.1% sol.Al, 0.0005 to 0.0050% N, 0.0003 to 0.0050% B, 0.05 to 0.30% Cr, by mass, and balance of Fe, [14B/(10.8N)] being 0.5 or more, at temperatures of Ar3 transformation point or above;
• and
• coiling the hot rolled steel sheet at temperatures of from 500 to 720° C.

A method for manufacturing a steel sheet, comprising the steps of:

• hot rolling a steel slab consisting essentially of 0.10 to 0.37% C, 1% or less Si, 2.5% or less Mn, 0.1% or less P, 0.03% or less S, 0.01 to 0.1% sol.Al, 0.0005 to 0.0050% N, 0.005 to 0.05% Ti, 0.0003 to 0.0050% B, by mass, and balance of Fe, [B−(10.8/14)N*] being not less than 0.0005%, at temperatures of Ar3 transformation point or above;
• and
• coiling the hot rolled steel sheet at temperatures of from 500 to 720° C., N* being zero for negative N* values calculated in a definition formula of N*=[N−(14/48)Ti].

A method for manufacturing a steel sheet, comprising the steps of:

• hot rolling a steel slab consisting essentially of 0.10 to 0.37% C, 1% or less Si, 2.5% or less Mn, 0.1% or less P, 0.03% or less S, 0.01 to 0.1% sol.Al, 0.0005 to 0.0050% N, 0.005 to 0.05% Ti, 0.0003 to 0.0050% B, 0.05 to 0.30% Cr, by mass, and balance of Fe, [B−(10.8/14)N*] being not less than 0.0005%, at temperatures of Ar3 transformation point or above;
• and
• coiling the hot rolled steel sheet at temperatures of from 500 to 720° C., N* being zero for negative N* values calculated in a definition formula of N*=[N−(14/48)Ti].