STEEL PLATE AND METHOD OF PRODUCTION OF SAME

1 . A steel plate comprising, by mass %,

C: 0.10 to 0.40%,

Si: 0.01 to 0.30%,

Mn: 1.00 to 2.00%,

P: 0.020% or less,

S: 0.010% or less,

Al: 0.001 to 0.10%,

N: 0.010% or less,

O: 0.020% or less,

Cr: 0.50% or less,

Mo: 0.10% or less,

Nb: 0.10% or less,

V: 0.10% or less,

Cu: 0.10% or less,

W: 0.10% or less,

Ta: 0.10% or less,

Ni: 0.10% or less,

Sn: 0.050% or less,

Sb: 0.050% or less,

As: 0.050% or less,

Mg: 0.050% or less,

Ca: 0.050% or less,

Y: 0.050% or less,

Zr: 0.050% or less,

La: 0.050% or less,

Ce: 0.050% or less and

a balance of Fe and unavoidable impurities,

herein a metal structure of the steel plate

has a ratio of a number of carbides at ferrite grain boundaries with respect to a number of carbides in ferrite grains of over 1,

has a ferrite grain size of 5μm to 50 μm, and

has an area ratio of pearlite of 6% or less; and

a Vickers hardness of the steel plate is 100 HV to 170 HV.

2 . The steel plate according to claim 1 containing one or both of

Ti: 0.10% or less and

B: 0.010% or less

instead of part of the Fe.

3 . A method of production of a steel plate, comprising

hot rolling a steel slab of a chemical composition according to claim 1 during which completing finish hot rolling at a 750° C. to 850° C. temperature region to obtain hot rolled steel plate,

coiling the hot rolled steel plate at 400° C. to 550° C.,

pickling the coiled up hot rolled steel plate,

holding the pickled hot rolled steel plate at a 650° C. to 720° C. temperature region for 3 hours to 60 hours as first stage annealing, then

holding the hot rolled steel plate at a 725° C. to 790° C. temperature region for 3 hours to 50 hours as second stage annealing, and

cooling the annealed hot rolled steel plate to 650° C. by a cooling rate of 1° C./hour to 30° C./hour.

4 . A method of production of a steel plate, comprising

hot rolling a steel slab of a chemical composition according to claim 2 during which completing finish hot rolling at a 750° C. to 850° C. temperature region to obtain hot rolled steel plate,

coiling the hot rolled steel plate at 400° C. to 550° C.,

pickling the coiled up hot rolled steel plate,

holding the pickled hot rolled steel plate at a 650° C. to 720° C. temperature region for 3 hours to 60 hours as first stage annealing, then

holding the hot rolled steel plate at a 725° C. to 790° C. temperature region for 3 hours to 50 hours as second stage annealing, and

cooling the annealed hot rolled steel plate to 650° C. by a cooling rate of 1° C./hour to 30° C./hour.