A High-Strength High-Ductility Ti- and Mo-Bearing Ferritic Steel
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I.
INTRODUCTION
IN an effort to achieve more vehicle weight reductions while still satisfying increasingly stringent safety regulations, the global automotive industry has been adopting various types of advanced high-strength steels (AHSSs) over the past decades.[1,2] Compared with conventional steels, AHSSs are cost effective due to their good combination of strength and ductility. The latest successful development is the Nanohiten sheet steel,[3] which is a Ti- and Mo-containing AHSS with a ferrite matrix strengthened by nanometer-sized carbides. The steel achieves an ultimate tensile strength (UTS) of up to 780 MPa, of which the contribution from the nanometer-sized carbides accounts for about 300 MPa. This value draws much attention, for it is two or three times higher than that of conventional Ti-containing hotrolled sheet steels. It is also important to note that the hole-expanding ratio of this AHSS can reach up to 120 pct incredibly. On the other hand, although the utilization of higher strength (UTS > 980 MPa) AHSSs such as dual-phase steels (DP steels) and transformation-induced plastic steels (TRIP steels) has increased significantly over past decades, the inadequate formability of these AHSSs has hindered their wider applications in the automotive industry.[4,5] YAO HUANG, Intermediate Engineer, is with the China Electric Power Research Institute, Beijing, China, and also with the Engineering Research Institute, University of Science and Technology Beijing, Beijing 100083, China Contact e-mail: [email protected] AIMIN ZHAO, Professor, is with the Engineering Research Institute, University of Science and Technology Beijing, and also with the Beijing Laboratory for Modern Transportation Advanced Metal Materials and Processing Technology, Beijing 100083, China. XIAOPEI WANG, Junior Engineer, is with the Engineering Research Institute, University of Science and Technology Beijing. JINGBO YANG, Professor of Engineering, and JUNKE HAN and FENGLI YANG, Vice-Senior Engineers, are with the China Electric Power Research Institute. Manuscript submitted October 28, 2014. Article published online November 16, 2015 450—VOLUME 47A, JANUARY 2016
In general, single-phase ferritic steels free of pearlite and large cementite particles have superior local elongation compared with multiphase steels, such as DP or TRIP steels.[6] Such steels are expected to offer largely improved stretch-flange formability which is an important property for AHSSs. To date, several studies have reported the development of UTS 980 MPa grade hot-rolled steels.[7,8] However, both the tensile elongation and stretch-flange formability are still inadequate. It is necessary to develop high-strength (UTS > 980 MPa) and high-elongation (minimum 18 pct) ferritic steels. This study reports the development of such a high-performance hot-rolled ferritic steel, which achieves a UTS of 995 MPa, a total elongation of 20 pct, and a hole-expanding ratio of ~44 pct. In addition, the precipitation behaviors of the nanometer-sized carbides under different conditi
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