Scavenging additions of boron in low C – low Al steels

Abstract

Scavenging of boron on nitrogen is almost complete on the hot strip for B/N ≥ 0.7–0.8. Nitrogen ageing does not appear and low coiling can be used (650°C). BN precipitates as polycrystalline aggregates (spherulites) and in association with other precipitates or inclusions. Solute carbon is somewhat reduced in B steels coiled at 650°C but a more important reduction is achieved when coiling at 750°C due to an Fe 23 (C, B) 6 precipitation in the optimum range 1.0 ≤ B/N ≤ 1.5. Yield stresses (YS) lower than 190 MPa are obtained for optimized compositions of low Al‐B steels coiled at 650°C and continuously annealed (825°C – 60 s – cooling rate: 50°C/s). Further softening is achieved when coiling at 750°C (YS < 160 MPa). Lower ageing (≤ 20 MPa) of B steels coiled at 650°C is also for c.a. steels when compared to low Al grades; bake hardening remains at a high level (> 50 MPa). No ageing is observed in B steels coiled at 750°C of which the bake hardening is reduced (25 MPa). Deep drawability of B steels is improved at reduced C, Al, Mn contents and at higher coiling temperatures; a detrimental effect of a B excess is observed. Low nitrogen contents and reduced Al additions are needed to reach best B steel properties.