Uso de los diagramas de transformación de fases TTT y CCT para el diseño de rutas de fabricación de aceros avanzados de alta resistencia

Abstract

 Advanced High Strength Steels (AHSS) have been developed
to meet the demands of the automotive industry, which
are focused on making the use of fuel more efficient, with
a special interest in reducing the weight of vehicles but
without sacrificing the safety of passengers. Microstructural
control is the key in the development of these steels,
making it possible to obtain different degrees of resistance
but with the same chemical composition. In the present
work, TTT and CCT diagrams were used as a potential tool
for designing an appropriate thermal treatment for the
manufacture of two AHSS steels. The CCT and TTT diagrams
were simulated using the JMat-Pro software version 8.0.
The critical temperatures of transformation A1 and A3, were
determined experimentally by dilatometry. The steel was heat
treated following two routes. In the first route, a two-phase
microstructure consisting on ferrite+martensite was generated,
which is typical of dual phase steels (DP). The second was
designed to obtain a multiphase microstructure consisting on
ferrite+retained austenite+bainite+martensite, which is typical
of transformation induced plasticity (TRIP) steels. The strength
of the two types of steels was evaluated by uniaxial tensile
tests, obtaining average values of ultimate tensile strength
and % elongation of 1067 MPa and 7.51 % for DP steel and
890 MPa and 30.75 % for TRIP steel, respectively. These values
are in accordance with those expected for this type of steels.
The results show that CCT and TTT diagrams can be used to
determine optimal processing conditions for the development
of AHSS steels, by which time and processing costs can be
reduced.