Dual-phase hot-press forming alloy

Abstract

Hot-press forming steels are formed in a fully austenitic state followed by die-quenching in order to generate martensite and achieve strong steel. The ductility however, tends to be limited. We explore in this work a novel steel design in which the forming operation is in the two-phase austenite and ferrite field, so that the quenching results in a dual-phase ferrite and martensite microstructure at ambient temperature. It is demonstrated that better properties are achieved. The interpretation of the mechanisms of deformation during tensile testing indicates that the ductility can be further enhanced without compromising strength. The new steel also can be heated to temperature which is lower than that used for conventional hot-press forming steels, before transfer into the forming press.

Introduction

One application of strong steels in the context of automobiles is to enhance passenger safety by resisting damage to the passenger compartment during collisions. There are many varieties of such steels, for example those which are TRIP-assisted [1], [2], [3], [4], [5], [6] or the dual-phase steels which are nicely reviewed in [7]. These steels have to be formable and that requirement is difficult to achieve when the strength exceeds about 1000 MPa. Strong steels also suffer from an exaggerated change in shape due to the relaxation of elastic stresses when the steel is removed from the forming press [8].

One solution to these difficulties was the invention in Sweden [9], [10] of the process known as hot-press forming, in which a hot sheet of steel in its austenitic state, is fed into a forming press with water-cooled dies which quench the material into a martensitic state. Following austenitisation at about 900 $°$ C [11], [12], the steel is transferred to the press and the deformation occurs at high temperatures approximately in the range 800–600 $°$ C [13] where the steel is soft and formability limitations insignificant. The quenching produces already-formed components with strength in the range 1200–1600 MPa. There is little or no change in shape when the component is removed from the press. The steels typically have a composition Fe–0.22C, 1–2 wt% Mn (depending on whether the steel also is alloyed with boron) and other trace elements to give a martensite-start temperature of about 400 $°$ C [14]. The steel in its final condition after hot-press forming is fully martensitic and has a ductility (total elongation) of approximately 6–8%. One variant of the process is warm forming [15], where the maximum temperature to which the steel is heated can be as low as 600 $°$ C in order to minimise springback and oxidation. However, the strength achieved in this case is much lower than in hot-press forming because the steel is not austenitic during the forming process.

The purpose of the present work was to explore another option with the hope of improving ductility in the press-formed condition whilst maintaining the strength. An alloy was designed so that it consists of a mixture of allotriomorphic ferrite and austenite at the forming temperature, so that subsequent quenching leads to a dual-phase steel. A potential advantage of this mixture of allotriomorphic ferrite and martensite could be that the latter phase occurs in a finer state than in fully martensitic steels; this in itself should improve the resistance of the martensite to cracking [16], [17], [18].