Formability of high-strength, low-alloy sheet steels

Abstract

High-strength, low-alloy steel sheet (HSLA) is now replacing some low-carbon steels in the manufacture of pressed components in both commercial and private vehicles; the need for higher payloads in the former, andfor higher strength to meet increasingly stringent safety regulations in the latter, require their utilization. In this paper, the authors describe the investigation of five high-strength steels and compare their tensile properties and microstructural features with laboratorydetermined forming limit diagrams (FLDs). The tension-compression region is relatively insensitive to microstructure and is n value (or uniform elongation) controlled, with the limit line at ∼45° to the ε2=0 axis in accordance with theory. The tension-tension region varies widely with microstructure, particularly with inclusion content and pearlite banding, but only significantly at strain states where ε2/ε1>0·5. It is concluded that high-strength, low-alloy steels, with their higher yield and tensile strengths and so lower elongations and n values, have limited formability; this is reflected in the lower levels of the FLDs of these steels. The mechanical property which has the most influence upon the levels of the FLDs is the strain-hardening behaviour, as represented by the n value; microstructure is only significant in the tension-tension region. The mechanical anisotropy of these steels means that in some cases it is essential to determine the limit strains at both 0° and 90° to the rolling direction, but as most industrial pressings fall into the band ε2=±0·15, for the niobium-containing steels investigated, a single FLD, an ‘average’ of those determined here, would be in order for press-shop use.