Steel plate and method for producing the same

Abstract

Provided is a steel plate that has high strength and excellent low-temperature toughness as well as excellent manufacturability. A steel plate having a predetermined chemical composition and a microstructure in which a volume fraction of retained austenite at a ¼ thickness position is less than 3.0%, the maximum prior austenite grain size at a ½ thickness position is 100 μm or less, and a ratio b/a of an average value b in the top 5% of prior austenite grain sizes to an average prior austenite grain size a at the ½ thickness position is 4.5 or less, with a steel plate thickness of 40 mm or less, a yield stress of 585 MPa or more, and a tensile strength of 690 MPa or more.

1 . A steel plate comprising:
 a chemical composition containing, in mass %,   C: 0.01% or more and 0.15% or less,   Si: 0.01% or more and 1.00% or less,   Mn: 0.10% or more and 2.00% or less,   P: 0.010% or less,   S: 0.0050% or less,   Ni: 5.0% or more and 10.0% or less,   Al: 0.002% or more and 0.100% or less, and   N: 0.0080% or less   with the balance being Fe and inevitable impurities; and   a microstructure in which   a volume fraction of retained austenite at a ¼ thickness position is less than 3.0%,   the maximum prior austenite grain size at a ½ thickness position is 100 μm or less, and   a ratio b/a of an average value b in the top 5% of prior austenite grain sizes to an average prior austenite grain size a at the ½ thickness position is 4.5 or less, with   a thickness of 40 mm or less,   a yield stress of 585 MPa or more, and   a tensile strength of 690 MPa or more.   
     
     
         2 . The steel plate according to  claim 1 , wherein the chemical composition further contains, in mass %, at least one selected from the group consisting of
 Cu: 0.01% or more and 1.00% or less,   Cr: 0.01% or more and 1.50% or less,   Mo: 0.03% or more and 1.0% or less,   Nb: 0.001% or more and 0.030% or less,   V: 0.01% more and 0.10% or less,   Ti: 0.003% or more and 0.050% or less,   B: 0.0003% or more and 0.0050% or less,   Sn: 0.01% or more and 0.30% or less,   Sb: 0.01% or more and 0.30% or less,   W: more than 0% and 2.00% or less,   Co: more than 0% and 2.00% or less,   Ca: 0.0005% or more and 0.0050% or less,   Mg: 0.0005% or more and 0.0100% or less,   Zr: 0.0005% or more and 0.0050% or less,   Ta: 0.01% or more and 0.20% or less,   Y: 0.001% or more and 0.010% or less, and   REM: 0.0010% or more and 0.0200% or less.   
     
     
         3 . A method for producing a steel plate, comprising
 heating a steel material having the chemical composition according to  claim 1  to a heating temperature of 900° C. or higher and 1200° C. or lower,   hot rolling the heated steel material under the condition that a rolling reduction ratio is 5 or more and the number of passes where a rolling reduction per pass is 10% or more out of the final 5 passes is 2 or more to obtain a hot-rolled steel plate having a thickness of 40 mm or less,   cooling the hot-rolled steel plate,   subjecting the cooled hot-rolled steel plate to reheating quenching by which the cooled hot-rolled steel plate is reheated to a reheating temperature of Ac3 point or higher and 900° C. or lower and quenched, and   tempering the reheated and quenched hot-rolled steel plate at a tempering temperature of 500° C. or higher and 650° C. or lower.   
     
     
         4 . A method for producing a steel plate, comprising
 heating a steel material having the chemical composition according to claim  2  to a heating temperature of 900° C. or higher and 1200° C. or lower,   hot rolling the heated steel material under the condition that a rolling reduction ratio is 5 or more and the number of passes where a rolling reduction per pass is 10% or more out of the final 5 passes is 2 or more to obtain a hot-rolled steel plate having a thickness of 40 mm or less,   cooling the hot-rolled steel plate,   subjecting the cooled hot-rolled steel plate to reheating quenching by which the cooled hot-rolled steel plate is reheated to a reheating temperature of Ac3 point or higher and 900° C. or lower and quenched, and   tempering the reheated and quenched hot-rolled steel plate at a tempering temperature of 500° C. or higher and 650° C. or lower.