High-mn steel and method for manufacturing same

Abstract

Provided is a high-Mn steel which not only has high strength and excellent low-temperature toughness but also has excellent CTOD property at low temperatures. The high-Mn steel has a chemical composition containing, in mass %, C: 0.10-0.70%, Si: 0.05-0.50%, Mn: 20-30%, P: 0.030% or less, S: 0.0070% or less, Al: 0.01-0.07%, Cr: 0.5-7.0%, Ni: 0.01% or more and less than 0.1%, Ca: 0.0005-0.0050%, N: 0.0050-0.0500%, O: 0.0050% or less, Ti: less than 0.0050%, and Nb: less than 0.0050%, the balance consisting of Fe and inevitable impurities, and a microstructure having austenite as a base phase, where the austenite has a grain size of 1 μm or more and a standard deviation of 9 μm or less.

1 . A high-Mn steel comprising
 a chemical composition containing, in mass %,
 C: 0.10% or more and 0.70% or less, 
 Si: 0.05% or more and 0.50% or less, 
 Mn: 20% or more and 30% or less, 
 P: 0.030% or less, 
 S: 0.0070% or less, 
 Al: 0.01% or more and 0.07% or less, 
 Cr: 0.5% or more and 7.0% or less, 
 Ni: 0.01% or more and less than 0.1%, 
 Ca: 0.0005% or more and 0.0050% or less, 
 N: 0.0050% or more and 0.0500% or less, 
 O: 0.0050% or less, 
 Ti: less than 0.0050%, and 
 Nb: less than 0.0050%, 
 the balance consisting of Fe and inevitable impurities, and 
   a microstructure having austenite as a base phase, where the austenite has a grain size of 1 μm or more and a standard deviation of 9 μm or less.   
     
     
         2 . The high-Mn steel according to  claim 1 , wherein the chemical composition further contains, in mass %, at least one selected from the group consisting of
 Cu: 1.0% or less,   Mo: 2.0% or less,   V: 2.0% or less,   W: 2.0% or less,   Mg: 0.0005% or more and 0.0050% or less, and   REM: 0.0010% or more and 0.0200% or less.   
     
     
         3 . A method of manufacturing a high-Mn steel, comprising heating a steel material having the chemical composition according to  claim 1  to a temperature range of 1100° C. or higher and 1300° C. or lower, then subjecting the steel material to hot rolling where a rolling finish temperature is 750° C. or higher and lower than 950° C. and an average rolling reduction for one pass is 9% or more, and then subjecting the hot rolled material to cooling where an average cooling rate from a temperature of (rolling finish temperature −100° C.) or higher to a temperature range of 300° C. or higher and 650° C. or lower is 1.0° C./s or more. 
     
     
         4 . A method of manufacturing a high-Mn steel, comprising heating a steel material having the chemical composition according to  claim 2  to a temperature range of 1100° C. or higher and 1300° C. or lower, then subjecting the steel material to hot rolling where a rolling finish temperature is 750° C. or higher and lower than 950° C. and an average rolling reduction for one pass is 9% or more, and then subjecting the hot rolled material to cooling where an average cooling rate from a temperature of (rolling finish temperature −100° C.) or higher to a temperature range of 300° C. or higher and 650° C. or lower is 1.0° C./s or more.