Patent ID: 8361249

Claim:
A steel plate having a composition consisting of: by mass in %, a C content of from 0.05 to 0.18%, a Si content of from 0.10 to 0.50%, a Mn content of from 1.2 to 2.0%, an Al content of from 0.01 to 0.10%, a Cr content of from 0.10 to 0.22%, a P content of 0.01% or below, a S content of 0.01% or below, a V content of from 0.02 to 0.03%, optionally Cu, optionally Ni, optionally Mo, optionally Ti, optionally B, and optionally Ca, wherein the remaining balance of the composition is Fe and inevitable impurities, wherein the composition satisfies a condition expressed by: 6.7[Cr]+4.5[Mn]+3.5[V]≧7.2% (1) where [Cr], [Mn] and [V] represent the Cr content, the Mn content and the V content in percent by mass, respectively, and wherein cementite grains in the steel plate have a mean grain size equal to a circle-equivalent diameter of from 0.147 to 0.165 μm, and wherein the steel is made by a process comprising heating a steel slab having the composition of the steel plate at a temperature of from 1000 to 1200° C.; hot rolling and cooling the steel slab at a cooling rate of 4° C./s or above to form the steel plate; and tempering the steel plate at a temperature of from 600 to 700° C., wherein ferrite formation in the steel plate is suppressed during the process.