High-strength steel plate and manufacturing method thereof

Abstract

Disclosed is a high-strength sheet including: C: 0.15% to 0.35% by mass, total of Si and Al: 0.5% to 3.0% by mass, Mn: 1.0% to 4.0% by mass, P: 0.05% by mass or less, and S: 0.01% by mass or less, with the balance being Fe and inevitable impurities, wherein the steel structure satisfies that: a ferrite fraction is 5% or less, the total fraction of tempered martensite and tempered bainite is 60% or more, the amount of retained austenite is 10% or more, MA has an average size of 1.0 μm or less, retained austenite has an average size of 1.0 μm or less, retained austenite having a size of 1.5 μm or more accounts for 2% or more of the total amount of retained austenite, and a scattering intensity at the q value of 1 nm−1 in X-ray small angle scattering is 1.0 cm1 or less.

1 . A high-strength sheet, comprising:
 Fe,   C: 0.15% by mass to 0.35% by mass,   a total of Si and Al: 0.5% by mass to 3.0% by mass,   Mn: 1.0% by mass to 4.0% by mass,   P: 0.05% by mass or less, and   S: 0.01% by mass or less,   wherein the high-strength sheet comprises a steel structure wherein:   a ferrite fraction is 5% or less,   a total fraction of tempered martensite and tempered bainite is 60% or more,   an amount of retained austenite is 10% or more,   a martensite-austenite constituent has an average size of 1.0 μm or less,   the retained austenite has an average size of 1.0 μm or less,   retained austenite having a size of 1.5 μm or more accounts for 2% or more of a total amount of the retained austenite, and   a scattering intensity at a q value of 1 nm −1  in X-ray small angle scattering is 1.0 cm −1  or less.   
     
     
         2 . The high-strength sheet according to  claim 1 , comprising:
 0.30% by mass or less of C.   
     
     
         3 . The high-strength sheet according to  claim 1 , comprising:
 less than 0.10% by mass of Al.   
     
     
         4 . A method for manufacturing a high-strength sheet, the method comprising:
 preparing a rolled material comprising:
 Fe, 
 C: 0.15% by mass to 0.35% by mass, 
 a total of Si and Al: 0.5% by mass to 3.0% by mass, 
 Mn: 1.0% by mass to 4.0% by mass, 
 P: 0.05% by mass or less, and 
 S: 0.01% by mass or less, 
   heating the rolled material which has an A C3  point, to a temperature of the Ac 3  point or higher, thereby austenitizing the rolled material;   after the austenitizing, cooling the rolled material between 650° C. and 500° C. at an average cooling rate of 15° C./sec or more and less than 200° C./sec, followed by retention at a temperature in a range of 300° C. to 500° C. at a cooling rate of 10° C./sec or less for 10 seconds or more and less than 300 seconds;   after the retention, cooling the rolled material from a temperature of 300° C. or higher to a cooling stopping temperature between 100° C. or higher and lower than 300° C. at an average cooling rate of 10° C./sec or more;   heating the material from the cooling stopping temperature to a reheating temperature in a range of 300° C. to 500° C. at an average heating rate of 30° C./sec or more;   holding at the reheating temperature T for a holding time of 1 to 150 seconds so as to satisfy a tempering parameter P of 10,000 to 14,500; and   after the holding at the reheating temperature, cooling from the reheating temperature T to 200° C. at an average cooling rate of 10° C./sec or more, wherein
     P=T ×(20÷log( t/ 3600))  (1)
 
   where T: reheating temperature (K) and t: holding time (seconds).   
     
     
         5 . The method according to  claim 4 , wherein the retention comprises holding at a constant temperature in a range of 300° C. to 500° C. 
     
     
         6 . The method according to  claim 4 , wherein the tempering parameter P is from 11,000 to 14,000 and the holding time t is from 1 to 150 seconds.