Effect of second phase distribution and morphology on the bake hardening behavior of dual phase steels

Abstract

The present scenario of rapid boom in automotive industry has ushered tremendous improvement and growth in the steel processing technology. Advanced High Strength Steels (AHSS) have fulfilled most of the aspects concerning better utilization and fabrication of light weight steels for automobile applications. Various grades of AHSS have been processed over the past few decades, however, the search for even better mechanical properties and subsequently fuel efficient-light weight structures has deemed a deep research prospect in dual phase steels (DP steels), a category of AHSS. DP steels is the most commonly used AHSS grade for automotive industry. Dual Phase steels also known as DP steels consist of a hard martensite/ or bainite phase embedded within a softer ferrite phase. This peculiar combination of a hard phase (martensite) and a soft phase (ferrite) provides a perfect balance of strength and ductility in these steels. The typical production process of DP steels involves inter-critical annealing of low carbon steels which is followed by a rapid quenching or cooling techniques to obtain DP microstructure wherein, martensite is distributed along the grain boundaries of the ferrite grains. DP steels are mostly used for fabricating the exterior members of automobile bodies like the roof or floor panels and the cross member regions. The finished or heavily formed (simulated by pre-straining in this research work) auto-body is given a finishing paint curing treatment which helps in proper curing or adhesion of the paint coat over the entire exterior panels in the vehicles. This finishing operation is industrially referred to as the Bake Hardening treatment. The term hardening is associated with the improvement in final yield strength of the automobile body after this treatment. The increase in final yield strength is due to the presence of free or available interstitial solute carbon atoms in DP steels during its processing. These interstitial solute atoms upon receiving sufficient diffusion energy (during baking treatment) pin or lock the dislocations created during various forming operations (stamping, bending, extrusion etc.) thus, a rise in final yield strength is always obtained. Hence, in addition to curing of the paint coat, the dent resistance of the final component also improves at no extra production cost. In the present research work, bake hardening characteristics of a conventionally processed DP steel viz. Continuous Annealing Line (CAL) process was evaluated against a modified Continuous Annealing Line (mod-CAL) process. A typical industrial continuous annealing line (CAL) process was employed to anneal a 67% cold rolled steel to obtain the dual phase iv microstructure. Subsequent to this conventional annealing, the steel was now subjected to an improved process (mod-CAL) with modified initial heating rate and peak annealing temperature. The processed specimens (through CAL and mod-CAL respectively) were further pre-strained in the range 1–5 % followed by the bake hardening treatment at 170 for 20 minutes. It was observed that the CAL processed specimen showed a peak of about 70 MPa in bake-hardening index at 2 % pre-strain level. At higher pre-strain values (in excess of 2 %), a gradual drop in bake-hardening index was observed. On the contrary, the mod- CAL processed specimens showed near uniform bake-hardening response at all pre-strain levels and a decrease could be noted above 4% pre-strain. The evolving microstructure at each stage of annealing process and after bake-hardening treatment was studied using field emission scanning electron microscopy. The microstructure analysis distinctly revealed the differences in the martensite spatial distribution and interface morphologies developed by the two annealing processes. The modified process showed predominant formation of martensite within the ferrite grains with serrated lath martensite interfaces. This nature of the martensite was considered responsible for the observed improvement in the bake-hardening response. Furthermore, along with improved bake-hardening response, negligible loss in tensile ductility was also noted. This behavior was correlated with delayed micro-crack initiation at martensite interface due to the serrated nature of the lath martensite.