Steel plate and method of production of same

Abstract

Low carbon steel plate excellent impact resistance characteristics after carburizing and quenching and after tempering, characterized by having a predetermined chemical composition, an average grain size of carbides of 0.4 μm to 2.0 μm, an area ratio of pearlite of 6% or less, a ratio of a number of carbides at the ferrite grain boundaries to the number of carbides inside the ferrite grains of over 1, and a Vickers hardness of 100HV to 180HV.

1 . A steel plate being a low carbon steel plate having a chemical composition consisting of, by mass %,
 C: 0.10 to 0.40%,   Si: 0.01 to 0.30%,   Mn: 0.30 to 1.00%,   Al: 0.001 to 0.10%,   Cr: 0.50 to 2.00%,   Mo: 0.001 to 1.00%,   P: 0.020% or less,   S: 0.010% or less,   N: 0.020% or less,   O: 0.020% or less,   Ti: 0.010% or less,   B: 0.0005% or less,   Sn: 0.050% or less,   Sb: 0.050% or less,   As: 0.050% 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,   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 impurities,   wherein a metal structure of the steel plate has:
 a carbide grain size of 0.4 to 2.0 μm; 
 a pearlite area ratio of 6% or less; and 
 a ratio of the number of carbides at ferrite grain boundaries to the number of carbides inside ferrite grains of more than 1, and 
   the steel plate has a Vickers hardness of 100 HV to 180 HV.   
     
     
         2 . A method of production of the steel plate according to  claim 1 , the method of production comprising:
 hot-rolling a steel slab with a chemical composition according to  claim 1  to obtain a hot rolled steel plate, the hot-rolling in which finish hot-rolling is completed in a 650° C. to 950° C. temperature region;   coiling the hot rolled steel plate at 400° C. to 600° C.;   pickling the coiled hot rolled steel plate, and subjecting a first stage annealing to the pickled hot rolled steel plate, the first stage annealing in which the pickled hot rolled steel plate is heated, at a heating rate of 30° C./hour to 150° C./hour, up to an annealing temperature of 650° C. to 720° C. and the steel plate is held for 3 hours to 60 hours; then   subjecting a second stage annealing to the hot rolled steel plate, the second stage annealing in which the hot rolled steel plate is heated, at a heating rate of 1° C./hour to 80° C./hour, up to an annealing temperature of 725° C. to 790° C. and the steel plate is held for 3 hours to 50 hours as second stage annealing; and   cooling the annealed hot rolled steel plate to 650° C. at a cooling rate of 1° C./hour to 100° C./hour.