Steel plate and method of producing same

A method of producing a steel plate, comprising:

• heating a slab to 1050° C. or higher and 1200° C. or lower, the slab having a chemical composition containing, in mass %
• C:
• 0.01% to 0.07%,
• Si:
• 0.5% or less,
• Mn:
• 1.0% to 2.0%,
• P:
• 0.01% or less,
• S:
• 0.0005% to 0.0050%,
• Al:
• 0.030% or less,
• Ni:
• 0.5% to 2.0%,
• Ti:
• 0.005% to 0.030%,
• N:
• 0.0015% to 0.0065%,
• O:
• 0.0010% to 0.0050%,
• Ca:
• 0.0005% to 0.0060%, and
• at least one element selected from the group consisting of
• Cu:
• 0.05% to 2.0%,
• Cr:
• 0.05% to 0.30%,
• Mo:
• 0.05% to 0.30%,
• Nb:
• 0.005% to 0.035%,
• V:
• 0.01% to 0.10%,
• W:
• 0.01% to 0.50%,
• B:
• 0.0005% to 0.0020%,
• REM:
• 0.0020% to 0.0200%, and
• Mg:
• 0.0002% to 0.0060%,
• with a balance being Fe and inevitable impurities;
• and
• having Ceq of 0.45% or less, where Ceq is defined by the following Formula (1):
• Ceq (%)=[C]+[Mn]/6+([Cu]+[Ni])/15+([Cr]+[Mo]+[V])/5 (1);
• and
• having Pcm of 0.22% or less, where Pcm is defined by the following Formula (2):
• Pcm (%)=[C]+[Si]/30+([Mn]+[Cu]+[Cr])/20+[Ni]/60+[Mo]/15+[V]/10+5[B] (2),
• where the brackets in Formula (1) and Formula (2) indicate a content by mass % of an element enclosed in the brackets and have a value of 0 if such an element is not contained;
• hot rolling the heated slab to obtain a hot-rolled steel plate;
• cooling the hot-rolled steel plate to a stop cooling temperature of 600° C. or lower under a condition of at an average cooling rate of 3° C./sec to 50° C./sec while a mid-thickness part of the hot-rolled steel plate has a temperature from 700° C. to 550° C.,
• wherein the hot rolling comprises:
• (1) rolling with an average rolling reduction of 10% or more per pass and a cumulative rolling reduction of 20% or more when the temperature at a mid-thickness part of the heated slab is 1050° C. or higher to obtain a heated plate;
• (2) rolling with a cumulative rolling reduction of 30% or more when the temperature at the mid-thickness part of the heated plate is lower than 1050° C. and 950° C. or higher;
• and
• (3) rolling with an average rolling reduction of 8% or more per pass and a cumulative rolling reduction of 60% or more when the temperature at the mid-thickness part of the heated plate is lower than 950° C.