Processing, Microstructures and Properties of Ultra-High Strength, Low Carbon and V-Bearing Dual-Phase Steels Produced on Continuous Galvanizing Lines

Abstract

One of the most popular elements in weight reduction programs in the automotive industry is high strength zinc coated dual-phase steel produced on continuous hot dipped galvanizing lines. The high strength is needed for mass reduction, while the protective zinc coating is needed to prevent corrosion of the thin gage cold rolled steel. The present study was aimed to explore an optimized way to produce such dual-phase steels with ultra-high tensile strength (UTS > 1280MPa), good global ductility (TE > 18%), excellent local ductility (sheared-edge ductility, HER > 40%) and products of UTS × TE > 22000 MPa × %, conforming to data of AHSS Generation III steel. A steel of this kind is referred to as a third-generation advanced high strength steel. By altering chemical compositions (0.15wt.% carbon), pre-annealing conditions (different hot band coiling temperatures and cold reductions), annealing conditions (different intercritical annealing temperatures) and annealing paths (standard galvanizing or supercool processing), this study set out to investigate the effects of these factors on the microstructures and mechanical properties of dual-phase steels. Results showed that the stored energy of cold rolled steel with 0.15wt.% carbon was much higher than that of carbon containing 0.1wt.% carbon, generating numerous lattice defects during deformation and providing more driving force for formation of austenite and v recrystallization of ferrite during the intercritical anneal. In addition, it was found that the volume fraction of martensite increased with the combination of low coiling temperature, high cold reduction, and high annealing temperature, thereby increasing the tensile strength. Furthermore, the microstructural analysis and tensile testing results and showed that the tensile strength of dual-phase steel with 0.15Wt.% carbon, combined with the ultrafine microstructures (average ferrite grain sizes reached 1-2µm) could approach 1300MPa without loss of ductility and with hole expansion ratios, in some cases, reaching 35%. Since the relative hardness of the hard martensite and soft ferrite is important in controlling sheared-edge ductility, the nanohardness results of these phases were measured. It was revealed that the martensite hardness decreased with increasing volume fraction, at a given carbon content, indicating the tensile strength was independent of the martensite hardness. Other mechanical properties, such as yield strength, YS/UTS ratio, hardness and work hardening behavior, of dual-phase steels controlled by the factors mentioned above were correlated to the microstructural features. The hypothesis that the bulk carbon content would be a major factor in controlling the strength of these steels was vindicated by the results of this study. vi

Investigation of effects of processing on stretch-flangeability of the ultra-high strength, vanadium-bearing dual-phase steels

“…Stored energy, associated with the lattice defects, mainly dislocations, results from the cumulative effects of the transformation strains associated with the decomposition of austenite during the coiling of the hot band coils and later by the plastic deformation in cold rolling. From previous studies [42,47], the combination of low coiling temperatures and high cold reductions introduced more dislocations into the ferrite, providing a large driving force for ferrite recrystallization and austenite formation during intercritical annealing at a short holding time. Also, in this research, the low coiling temperature combined with a constant reduction (~60%) can give rise to a higher amount of intercritically formed austenite and a higher volume percentage of hard components (i.e., fresh martensite, bainite and/or tempered martensite) as can be observed after full CGL simulations.…”

“…Taylor et al [13] emphasized the importance of the nanohardness difference between hard phase martensite and soft phase ferrite on the stretchflangeability of DP 980 steels. Reducing the nanohardness difference between hard and soft phases can improve the hole expansion behaviors of DP steels [13,42,47]. For instance, the HER value of A1, SC (CT580℃, supercooling process) improves by ~ 62%, compared with the one of A1, GI (CT580℃, standard galvanizing), which is due to the nanohardness difference for undeformed microstructures of ferrite and martensite from 4.9 GPa (steel condition A1, GI) to 1.5 GPa (A1, SC) for the difference between the undeformed ferrite and tempered martensite.…”