Somani et al. [96] examined the effects of plastic deformation on dilatation during martensitic transformation in a B-bearing steel. Their results show that
plastic deformation of austenite at high temperatures enhances ferrite formation significantly and consequently, the dilatation decreases markedly even at a cooling rate of 280°C/s.
It was found that, without plastic deformation, Ms and Mf were about 425°C and 280°C, respectively. The change in diameter was about 0.53%
corresponding to a relative volume change of 3.2%. They mentioned that the reason for the drastic decrease of dilatation and drop of the Ms value to 375°C due to an increase in the prior plastic strain could be justified as a result of the stabilization of austenite by means of plastic deformation and the presence of retained austenite in this regard. There were, however, distinct differences in the high temperature slopes of the dilatation curves. The slope in the deformed specimens is being smaller than that in non-deformed ones. This presumably indicates that some ferrite formed at higher temperatures as strain-induced, consequently, less martensite is present.
Microstructural examination also revealed that, at a cooling rate of 50°C/s, the ferrite content was about 20~40%. Hardness measurements also confirmed that the structure formed after severe plastic deformation was markedly softer, about 295~375 HV10, compared to the martensite hardness of 490~500HV10. However, martensite was still present in considerable amount, even though the dilatation became very small. Therefore, they suggested that some other factors, such as residual stresses due to prior plastic deformation may be an additional reason for the decrease of dilatation.
Finally they found that, the severe plastic straining (strain 0.8~1.0) during continuous cooling at 50°C/s results in a much lower final flow stress level (800-950 MPa at 300~200°C) than that obtained for martensitic structure in isothermal tests (1650~1900 MPa).
Another investigation by the mentioned authors [102] revealed that the Ms
temperature is lowered by about 25-70°C with increasing plastic strain from 0.16-0.39. As the reason for this, they proposed that, as a consequence of ferrite formation, carbon becomes enriched in the remaining austenite, which
therefore transforms into martensite at a somewhat lower temperature.
It was also observed that ferrite with an ultra-fine grain size can be formed as strain-induced by subjecting austenite to severe plastic straining at temperatures slightly above Ar3. Hardness measurements also confirmed that the microstructure formed after a high-temperature plastic deformation was remarkably softer, 302-440 HV10, while the martensite had a hardness of 490-510 HV10, which was justified due to the presence of ferrite in the microstructure as described before.
In case of hardness measurements, their image analysis data were in contrast with their hardness values and made it rather impossible to determine the martensite or bainite phases based on the optical microscopy images. They believed that the distinction between the bainite and martensite phases might require transmission electron microscopic examinations, which had, however not been performed, because this matter was not very important in their discussions.
To avoid the strain-induced phase transformation, it was suggested that the consequences of the prior plastic deformation should be small enough or disappear before the temperature reaches the ferritic regime level. This means that forming should take place at a high temperature, >800°C, where the driving force for the austenite decomposition is low, or the time should be long enough for static recrystallization to occur. Another, more realistic alternative might be forming at low temperatures, such as <600°C, i.e., below the ferrite regime. In that case, ferrite nucleation is not accelerated, although some enhancement of bainite formation may occur. This may not be so detrimental, however, due to the notably smaller strength difference between martensite and bainite. Furthermore, in order to avoid straining to continue at the martensitic stage, which would mean excessive forming loads, a major spring back and high residual stresses, forming should be finished above 420°C, which means that the proper temperature range is quite narrow.
Overall, it was proposed that, minimization of the plastic strain, maximization
of the cooling rate and/or forming at 450-600°C may be suitable ways to avoid excessive ferrite formation and to achieve the desired mechanical properties in formed and quenched components [102].
In the last reviewed work here, Jun and coworkers [97] studied the effects of thermo-mechanical processing on the microstructures and transformations of low carbon HSLA steels with and without boron. Microstructures observed in continuous cooled specimens were composed of pearlite, quasi-polygonal ferrite, granular bainite, acicular ferrite, bainitic ferrite, lower bainite, and martensite depending on cooling rate and transformation temperature. Fast cooling rate depressed the formation of pearlite and quasi-polygonal ferrite, which resulted in higher hardness. However, hot deformation slightly increased transformation start temperature, and promoted the formation of pearlite and quasi-polygonal ferrite. Hot deformation could also strongly promote the acicular ferrite formation which was not formed in non-deformation condition. Small boron addition effectively reduced the formation of pearlite and quasi-polygonal ferrite and broadened the cooling rate region from bainitic ferrite and martensite. Impurity boron segregates to grain boundaries and improves the grain boundary cohesive strength. This causes the mentioned effective suppression of pearlite and/or ferrite formation compared to other substitution elements. Microhardness of granular bainite varied from 220 to 250 HV, which resulted from high dislocation density and hard constituents. Transformation of these bainitic microstructures had both aspects of diffusional and shears mechanisms. It was suggested that granular bainite forms because carbon quickly diffuses away from the ferrite/austenite interface at relatively slow cooling rates, preventing the formation of cementite. The increased carbon content in the remaining austenite can stabilize austenite from further transformation, and this entrapment of residual austenite leads to granular bainite morphology. Shear mechanism for bainite-like transformation was proposed to be more dominated as increasing cooling rates.
It was also found that the deformation causes the formation of acicular ferrite, pearlite, and quasi-polygonal ferrite, otherwise prevents the martensite compared to that of non-deformed condition. The corresponding transformation curves of deformed CCT moves toward left side compared to those of non-deformed CCT [97].
Chapter Two
2 Experimental
Ultra high strength steels are increasingly used in the automotive industry, due to their significantly improved strength. However, these ultra high strengthening mechanisms of UHSS leads to unacceptable high stresses during forming and remarkable spring-back phenomena, thus making traditional sheet metal forming technologies unsuitable. The possibility to perform stamping operations at elevated temperatures, i.e., press hardening, represents a solution of these problems, allowing lower loads on tools and higher accuracy of formed components.
The aim of present research is to develop a general approach that will be able to offer accurate evaluations of the influence of process parameters on the properties of final ultra high strength steel sheets produced in press hardening operation. The stamping tests carried out in this research are run with ten grades of UHSS. In addition to direct hot stamping process, tests also carried out into cold stamping plus quench hardening processes to compare the
materials characteristics and formability.
In the following sections, materials and press hardening process as well as micro structural and mechanical investigation methods are represented.