The effect of microstructure on bendability of ultra-high strength steels
Liimatainen, Mia (2015)
Liimatainen, Mia
2015
Materiaalitekniikan koulutusohjelma
Teknisten tieteiden tiedekunta - Faculty of Engineering Sciences
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Hyväksymispäivämäärä
2015-06-03
Julkaisun pysyvä osoite on
https://urn.fi/URN:NBN:fi:tty-201505181286
https://urn.fi/URN:NBN:fi:tty-201505181286
Tiivistelmä
In recent years there has been an increasing interest to utilize ultra-high strength steels (UHSS) in various industries such as transportation, due to the possibility to produce structures with lighter weight without the need to make amends for performance level. Yet, a major challenge concerning the application of UHSS is their formability. Factors affecting bendability has been widely studied, yet there exists a lack of information concerning a fundamental comprehension of the effect of microstructure on bendability of UHSS. The aim of this work was to study the microstructural factors governing bendability of UHSS. In particularly the effects of surface hardness and homogeneity of surface microstructure, i.e. deviation of hardness, were studied.
The materials used in this work were direct quenched 8-12 mm thick UHSS microalloyed with niobium and comprising various carbon (C) and manganese (Mn) contents, rolled to different temperatures. In present work microstructural examination was performed by applying FESEM. The effects of finish rolling temperature (FRT), and C and Mn contents on the developing microstructure were studied. Bending tests were carried out for determining minimum bending radius and for comprehending failure in bending. Microhardness measurements were performed in order to comprehend the effects of hardness and homogeneity on bendability, and to relate microstructure with hardness.
The microstructures of the materials comprise principally a mixture of lath-like bainite and martensite, and a surface layer of granular bainite and/or ferrite reaching to various depths. It is found that microstructure is highly dependent on FRT, and C and Mn concentrations. The results suggest that strain localization, i.e. development of shear bands, is precursor for damage in bending in case of complex phase UHSS. Shear bands are observed to develop at the depth range of 0.1-0.5 mm from upper surface at maximum 45° shear stress directions. Hence it is suggested that the properties of the upper surface layers govern bendability. It is found that hardness of surface determines bendability due to its relation with several properties, which govern the onset of diffuse necking and strain localization. The soft layer is required to reach to 2.3-4.4 % relative to the total sheet thickness, and requirements concerning its hardness level are determined by the desired bending radius. The soft layer is achieved by low FRT and by adjusting the carbon and manganese concentrations. Furthermore it is found that a homogenous surface microstructure contributes to increased bendability. In an inhomogeneous microstructure resistance to dislocation movement varies locally. Therefore grains possessing softer orientations and phases possessing lower hardness levels are susceptible to strain localization.
The materials used in this work were direct quenched 8-12 mm thick UHSS microalloyed with niobium and comprising various carbon (C) and manganese (Mn) contents, rolled to different temperatures. In present work microstructural examination was performed by applying FESEM. The effects of finish rolling temperature (FRT), and C and Mn contents on the developing microstructure were studied. Bending tests were carried out for determining minimum bending radius and for comprehending failure in bending. Microhardness measurements were performed in order to comprehend the effects of hardness and homogeneity on bendability, and to relate microstructure with hardness.
The microstructures of the materials comprise principally a mixture of lath-like bainite and martensite, and a surface layer of granular bainite and/or ferrite reaching to various depths. It is found that microstructure is highly dependent on FRT, and C and Mn concentrations. The results suggest that strain localization, i.e. development of shear bands, is precursor for damage in bending in case of complex phase UHSS. Shear bands are observed to develop at the depth range of 0.1-0.5 mm from upper surface at maximum 45° shear stress directions. Hence it is suggested that the properties of the upper surface layers govern bendability. It is found that hardness of surface determines bendability due to its relation with several properties, which govern the onset of diffuse necking and strain localization. The soft layer is required to reach to 2.3-4.4 % relative to the total sheet thickness, and requirements concerning its hardness level are determined by the desired bending radius. The soft layer is achieved by low FRT and by adjusting the carbon and manganese concentrations. Furthermore it is found that a homogenous surface microstructure contributes to increased bendability. In an inhomogeneous microstructure resistance to dislocation movement varies locally. Therefore grains possessing softer orientations and phases possessing lower hardness levels are susceptible to strain localization.