DEVELOPMENT OF HOT ROLLED COPPER/NICKEL ALLOYED TRIP STEELS WITH CARBIDE-FREE BAINITIC MATRIX

摘要

Recent works [1,2] have confirmed the interest for steel industry to develop a new generation of TRIP steels with a carbide-free bainitic matrix. Major improvements of the stretch flangeability are in particular expected by the substitution of polygonal ferrite by bainite. This type of microstructure composed of bainite, interlaths retained austenite films and Martensite-Austenite (MA) coarse islands are also known basically as carbide free bainite [3-5]. rnWe have shown in previous works that such microstructure could be obtained with highly alloyed 0.3%C steel containing 1.5%Si [4] along continuous cooling paths after hot-rolling. Many difficulties arose when considering steels containing lower carbon content and considering multi-stage cooling paths on the run-out table and coiling regime. rnConcepts based on a 0.2%C Si-TRIP steel + Mo + (Cu/Ni) have thus been tested. Mo is added as a well-known bainite promoter. Copper is an extra lever to increase the tensile strengths and is a strong gamma-stabilizer element. Ni is also added with copper to prevent the hot cracking phenomenon and participate to the stabilization of retained austenite. rnThe expected target microstructures have been obtained when coiling at temperature higher than 400°C. They are composed of carbide-free granular bainite, retained austenite and tempered martensite. The total fraction of second phases (MA) and stability of retained austenite are the main microstructural features suitable to explain the mechanical properties. Some examples of the best reached balances are reported below: rn• UTS = 1550 MPa / TEl = 8 % / UTSxTEl = 12 400 MPa.% (fully martensitic steels) rn• UTS = 1034 MPa / El = 15.9 % / UTSxTEl = 16 440 MPa.% (DP-like behavior) rn• UTS = 835 MPa / El = 21.1 % UTSxTEl = 17 620 MPa.% (TRIP-like behavior) rnSome evidences confirm that copper does not precipitate in bainite under these thermomechanical conditions. Nevertheless, the gamma-stabilizing elements have a noteworthy influence on phase transformation and mechanical properties.