Abstracts Book - IMRC 2018

• SE5-O005
EFFECT OF INTERRUPTED COOLING ON TRANSFORMATION OF
INTER-CRITICAL AUSTENITE IN AHSS-DP COLD ROLLED STEEL
STRIPS
Martin Fibela Esparza 1 , Armando Salinas Rodriguez 1 , Frank E Goodwin 2
1 Centro de Investigación y de Estudios Avanzados del IPN - CINVESTAV Unidad Saltillo,
Metalurgia, Mexico. 2 International Zinc Association, Tecnología y desarrollo del mercado,
United States.
Industrial production of dual phase hot-dip galvanized advanced high strength
steel strips (AHSS-DP) for automotive applications requires the design of fast
thermal treatments which include inter-critical austenization and interrupted
cooling at about 460 °C. Ms temperatures of AHSS-DP steel grades are generally
lower than 460 °C. Therefore, ferrite-martensite microstructures in these steels
are difficult to produce. In this article, the results of an experimental
investigation into the effects of cooling rate before and after interruption of
cooling and the interruption cooling time are presented. To this end an AHSS-
DP cold rolled steel strip is selected for the investigation. Using a quenching–
induction dilatometer, samples subjected to inter-critical austenization during
60 s at 800 °C are then rapidly cooled to 460 °C at rates varying from 1 to 100
°C/s. Interrupted cooling time is varied from 5 to 60 s and final cooling is
performed using cooling rates within the same range. The effects of cooling rate
and interrupted cooling time on the microstructure are investigated using
optical and electron microscopy techniques.
The results demonstrate that initial and final cooling rates faster that the critical
cooling rate (~50°C/s) required to form martensite on continuous cooling cause
that the amount of martensite and bainite in the final microstructure increase
and decrease linearly, respectively, as the time of interrupted cooling increases.
Any other combination of cooling rates within the ranges investigated cause that
the amount of martensite in the final microstructure decreases to values below
5% as the interruption time increases. This effect is attributed to the formation
of bainite during the interruption of cooling. The actual time needed to cause
this effect decreases as the magnitude of the initial cooling rate increases. Initial
cooling rates of 1 °C/s inhibit completely the formation of martensite during final
cooling from 460 °C resulting in ferrite-bainite microstructures. These
experimental observations are discussed and explained in terms of dynamic