Abstract
Prior carburization of semi-finished steel sheets is a new process variant in hot stamping to manufacture parts with tailored properties. Compared to conventional hot stamping processes, a complex phase typed steel alloy is used instead of 22MnB5. Yet recent investigations focused on final mechanical properties rather than microstructural mechanisms cause an increase in strength. Thus, the influence of additional carburization on the microstructural evolution during hot stamping of a complex phase steel CP-W®800 is investigated within this work. The phase transformation behavior, as well as the grain growth during austenitization, is evaluated by in-situ measurements employing a laser-ultrasound sensor. The results are correlated with additional hardness measurements in as-quenched condition and supplementary micrographs. The experiments reveal that the carburization process significantly improves the hardenability of the CP-W®800. However, even at quenching rates of 70 K/s no fully martensitic microstructure was achievable. Still, the resulting hardness of the carburized samples might exceed the fully martensitic hardness of 22MnB5 derived from literature. Furthermore, the carburization process has no adverse effect on the fine grain stability of the complex phase steel. This makes it more robust in terms of grain size than the conventional hot stamping steel 22MnB5.
Highlights
Additional carburization enhances the hardenability of the CP-W®800 steel sheets
For a given quenching rate, the phase transformation is shifted to lower temperatures, which is accompanied by the formation of harder phases
A multiphase microstructure was present for all quenching rates and material conditions, the hardness of fully martensitic 22MnB5 could partially be exceeded, which is a consequence of the influence of carbon content on martensitic hardness
Summary
Recent investigations focused on a complex phase steel CP-W®800 as a base material since it meets the two requirements for this process It is press hardenable and its carbon content in as-delivered condition allows a further increase in strength by carburization. As concluded in a previous study [13], the process of combined carburization and hot stamping might be suitable especially for small batch size productions and prototyping, where high flexibility is crucial Since these types of manufacturing are often associated with a lower degree of automation, small fluctuations of individual process steps cannot be precluded. While different studies already indicate the phase transformation behavior of the conventional CP-W®800 during quenching, the influence of the various carbon content after carburization is unknown. The experimentTshoenetxhpeeprihmaesenttsraonnsftohrempahtaiosentbreahnasfvoiromr aartieolnimbietheadvtioorthaerecalirmbuitreidzetdo the carburized and non-carburainzeddncoonm-cpalrebxuprihzaesdecsotmeepl lseixncpehpahseasseteterlasnisnfcoermphataisoentdraantasfoofrm22aMtionnB5daista of 22MnB5 is broadly availabbleroinadthlyealivtaeirlaatbulreei.nRtehgealridteinragtuthree.gRraeginargdrionwgtthhde ugrrianing aguroswtetnhitdizuartiinogn,atuhsetenitization, the tests are done wteitshts2a2rMe ndBon5easwwithell2.2MnB5 as well
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