Ultrahigh-strength twip steel sheet and manufacturing method thereof

Abstract

The present invention provides a TWIP steel sheet that has an austenite matrix structure and includes 15-25 wt % of manganese. The TWIP steel sheet has a recrystallization texture in which brass orientation is suppressed and {3 5 2} <2 2 1> is developed as its main orientation during heat treatment subsequent to cold rolling, and has an average plastic strain ratio of 1.2 or more, preferably 1.5 or more. Since this TWIP steel sheet has high workability and strength, complicated vehicle body components can be easily manufactured through a press molding process without cracking or rupturing.

1 . An ultrahigh-strength TWIP steel sheet having an austenite matrix structure comprising 15˜25 wt % of manganese, wherein the TWIP steel sheet has a recrystallization texture in which brass orientation is suppressed and {3 5 2} <2 2 1> is developed as a main orientation during heat treatment subsequent to cold rolling, and has an average plastic strain ratio of 1.2 or more. 
   
   
       2 . The ultrahigh-strength TWIP steel sheet according to  claim 1 , wherein the TWIP steel sheet comprises 0.15˜0.30 wt % of carbon, 0.01˜0.03 wt % of silicon, 15˜25 wt % of manganese, 1.2˜3.0 wt % of aluminum, 0.020 wt % or less of phosphorus, 0.001˜0.002 wt % of sulfur, and residual iron. 
   
   
       3 . The ultrahigh-strength TWIP steel sheet according to  claim 1 , wherein a fraction of Σ3 grain boundaries having a misorientation angle of 60° is 10% or less, and a fraction of Σ3 grain boundaries having a misorientation angle of 5° or less is 50% or more. 
   
   
       4 . The ultrahigh-strength TWIP steel sheet according to  claim 1 , wherein a fraction of Σ3 grain boundaries having a misorientation angle of 15° or more is less than 40%. 
   
   
       5 . A method of manufacturing an ultrahigh-strength TWIP steel sheet having an improved plastic strain ratio, comprising:
 annealing a cold-rolled TWIP steel sheet that has an austenite matrix structure and comprises 15-25 wt % of manganese such that the cold-rolled TWIP steel sheet has a recrystallization texture in which brass orientation is suppressed and {3 5 2}<2 2 1> is developed as its main orientation,   wherein the annealing of the cold-rolled TWIP steel sheet comprises:   recovery heat treatment that is conducted at a first temperature range such that a fraction of a coincide site lattice (CSL) grain boundary is decreased; and   recrystallization heat treatment that is conducted at a second temperature range such that the recrystallization texture is developed.   
   
   
       6 . The method of manufacturing an ultrahigh-strength TWIP steel sheet according to  claim 5 , wherein the cold-rolled TWIP steel sheet comprises 0.15˜0.30 wt % of carbon, 0.01˜0.03 wt % of silicon, 15˜25 wt % of manganese, 1.2˜3.0 wt % of aluminum, 0.020 wt % or less of phosphorus, 0.001˜0.002 wt % of sulfur, and residual iron. 
   
   
       7 . The method of manufacturing an ultrahigh-strength TWIP steel sheet according to  claim 5 , wherein the recovery heat treatment is conducted at a temperature of 230˜280° C. for 20˜30 minutes. 
   
   
       8 . The method of manufacturing an ultrahigh-strength TWIP steel sheet according to  claim 5 , wherein the recrystallization heat treatment is conducted at a temperature of 700˜820° C. for 6˜8 hours.