4 Nanostructured steel for automotive body structures Y.OKITSU,Honda R&D Co.Ltd.,Japan and N.TSUJI,Kyoto University,Japan Abstract:In this chapter.the results of studies on the application of nanostructured steel sheets to automotive body structures are introduced. A new route to fabricating nanostructured/ultrafine ferritic microstructures without severe plastic deformation is presented.The preparation and evaluation of two kinds of steel sheets,ultrafine-grained(UFG)ferrite- cementite(FC)steel and UFG multi-phase(MP)steel are also presented and discussed.The UFG-FC steel showed large strain rate sensitivity in flow stress,while the UFG-MP steel showed a good combination of high strength and large work hardening rate,which improved the dynamic collapse properties of hat columns.It is shown that UFG steels would help further weight reduction of automotive body structures. Key words:ultrafine grain,steel sheets,multi-phase,strain rate sensitivity, dynamic collapse. Note:This chapter was first published as Chapter 22 'Applying nanostructured steel sheets to automotive body structures'by Y.Okitsu and N.Tsuji in Nanostructured metals and alloys,ed.Sung H.Whang. Woodhead Publishing Limited,2011,ISBN:978 1 84569 670 2.It is reproduced without revision. 4.1 Introduction In recent times,reducing the body weight of automobiles has been considered one of the important criteria in auto manufacturing for reducing CO,emissions as well as improving the fuel efficiency of automobiles.Steels have been the most popular and indispensable material for automotive body structures in the past,and recently in particular,various kinds of high-strength steels (HSSs)!have been developed and applied to the body structures,which has contributed to a weight reduction.In order to achieve further weight reduction, new HSSs are required.As described in the next section,nanostructured or ultrafine-grained(UFG)steels are expected to become one of the new HSSs.2 However,it is still difficult to apply UFG steel sheets to automobile body parts.This is owing to the limited dimensions of UFG steels fabricated by severe plastic deformation(SPD)processes such as equal-channel angular extrusion (ECAE),3 high-pressure torsion (HPT).and accumulative roll bonding(ARB).5-7 which require very high plastic strain. 57 Woodhead Publishing Limited,2012© Woodhead Publishing Limited, 2012 57 4 Nanostructured steel for automotive body structures Y. OKITSU, Honda R&D Co. Ltd., Japan and N. TSUJI, Kyoto University, Japan Abstract: In this chapter, the results of studies on the application of nanostructured steel sheets to automotive body structures are introduced. A new route to fabricating nanostructured/ultrafine ferritic microstructures without severe plastic deformation is presented. The preparation and evaluation of two kinds of steel sheets, ultrafine-grained (UFG) ferrite￾cementite (FC) steel and UFG multi-phase (MP) steel are also presented and discussed. The UFG-FC steel showed large strain rate sensitivity in flow stress, while the UFG-MP steel showed a good combination of high strength and large work hardening rate, which improved the dynamic collapse properties of hat columns. It is shown that UFG steels would help further weight reduction of automotive body structures. Key words: ultrafine grain, steel sheets, multi-phase, strain rate sensitivity, dynamic collapse. Note: This chapter was first published as Chapter 22 ‘Applying nanostructured steel sheets to automotive body structures’ by Y. Okitsu and N. Tsuji in Nanostructured metals and alloys, ed. Sung H. Whang, Woodhead Publishing Limited, 2011, ISBN: 978 1 84569 670 2. It is reproduced without revision. 4.1 Introduction In recent times, reducing the body weight of automobiles has been considered one of the important criteria in auto manufacturing for reducing CO2 emissions as well as improving the fuel efficiency of automobiles. Steels have been the most popular and indispensable material for automotive body structures in the past, and recently in particular, various kinds of high-strength steels (HSSs)1 have been developed and applied to the body structures, which has contributed to a weight reduction. In order to achieve further weight reduction, new HSSs are required. As described in the next section, nanostructured or ultrafine-grained (UFG) steels are expected to become one of the new HSSs.2 However, it is still difficult to apply UFG steel sheets to automobile body parts. This is owing to the limited dimensions of UFG steels fabricated by severe plastic deformation (SPD) processes such as equal-channel angular extrusion (ECAE),3 high-pressure torsion (HPT),4 and accumulative roll bonding (ARB),5–7 which require very high plastic strain