Setting Mechanical Properties of High Strength Steels for Rapid Hot Forming Processes

Abstract

Hot stamping of sheet metal is an established method for the manufacturing of light weight products with tailored properties. However, the generally-applied continuous roller furnace manifests two crucial disadvantages: the overall process time is long and a local setting of mechanical properties is only feasible through special cooling techniques. Hot forming with rapid heating directly before shaping is a new approach, which not only reduces the thermal intervention in the zones of critical formability and requested properties, but also allows the processing of an advantageous microstructure characterized by less grain growth, additional fractions (e.g., retained austenite), and undissolved carbides. Since the austenitization and homogenization process is strongly dependent on the microstructure constitution, the general applicability for the process relevant parameters is unknown. Thus, different austenitization parameters are analyzed for the conventional high strength steels 22MnB5, Docol 1400M, and DP1000 in respect of the mechanical properties. In order to characterize the resulting microstructure, the light optical and scanning electron microscopy, micro and macro hardness measurements, and the X-ray diffraction are conducted subsequent to tensile tests. The investigation proves not only the feasibility to adjust the strength and ductility flexibly, unique microstructures are also observed and the governing mechanisms are clarified.

The Effect of Rapid Heating and Fast Cooling on the Transformation Behavior and Mechanical Properties of an Advanced High Strength Steel (AHSS)

“…This behavior can be explained by the effect of a heating rate and holding time on the nucleation and growth of austenite and the dissolution of Fe 3 C carbides. Several studies [12][13][14] have indicated that during rapid reheating the classical view of phase transformations will deviate markedly from those observed during equilibrium conditions. That is, the kinetics of transformation will have a different behavior.…”

Flexibility in metal forming

“…The heat assisted sheet metal forming in progressive or transfer tools by local induction heating is another approach for the flexible setting of geometrical and mechanical properties in large-scale production. Compared to conventional hot stamping, only critical zones are heat treated so that required product properties are fulfilled either by the initial condition or by the rapid heat treatment [97,98]…”

Minimisation of Heating Time for Full Hardening in Hot Stamping Using Direct Resistance Heating

“…Mori et al [14] developed smart hot stamping of ultra-high strength steel products using rapid resistance heating and mechanical servo press. Löbbe et al [15] examined the influence of various austenitisation parameters such as rapid heating and cooling on the mechanical properties in hot stamping of ultra-high strength steel sheets.…”