Experimental Determination of Continuous Cooling Transformation (CCT) Diagrams for Dual-Phase Steels from the Intercritical Temperature Range

Armando Salinas,Krishna Bräutigam–Matus,Alfredo Flores,Frank Goodwin,Gerardo Altamirano

doi:10.3390/met8090674

Abstract

The phase transformation kinetics under continuous cooling conditions for intercritical austenite in a cold rolled low carbon steel were investigated over a wide range of cooling rates (0.1–200 ∘ C/s). The start and finish temperatures of the intercritical austenite transformation were determined by quenching dilatometry and a continuous cooling transformation (CCT) diagram was constructed. The resulting experimental CCT diagram was compared with that calculated via JMatPro software, and verified using electron microscopy and hardness tests. In general, the results reveal that the experimental CCT diagram can be helpful in the design of thermal cycles for the production of different grades of dual-phase–advanced high-strengh steels (DP-AHSS) in continuous processing lines. The results suggest that C enrichment of intercritical austenite as a result of heating in the two phases (ferrite–austenite) region and C partitioning during the formation of pro-eutectoid ferrite on cooling significantly alters the character of subsequent austenite phase transformations.

Highlights

Reducing the weight of vehicles to increase fuel efficiency, improving passenger safety and reducing CO2 emissions are priorities for the automotive industry
The phase transformation kinetics under continuous cooling conditions for intercritical austenite in a cold rolled low carbon steel were fully investigated via quenching dilatometry, electron microscopy and hardness test
The results are presented in the form of a continuous cooling transformation (CCT) diagram which can be helpful in the design of thermal cycles for the production of different grades of DP–advanced high-strength steels (AHSS) in continuous processing lines

Summary

Introduction

Reducing the weight of vehicles to increase fuel efficiency, improving passenger safety and reducing CO2 emissions are priorities for the automotive industry. In contribution to obtaining these goals, the steel industry offers different grades of advanced high-strength steels (AHSS) with multiphase microstructures (ferrite, bainite, retained austenite, stress/strain-induced martensite, athermal martensite, etc.) with an excellent balance of low cost, light weight and mechanical properties [1,2]. The microstructure may contain small amounts of other phases such as retained austenite, pro-eutectoid ferrite, pearlite and bainite, depending on cooling rate during the heat treatment [5]. These complex microstructures provide steel products manufactured with high strength, good ductility, low yield

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