T. M. Maccagno

Effect of cooling rate after hot rolling and of multistage strain aging on the drawability of low-carbon-steel wire rod

Tensile testing was used to simulate the multistage strain aging occurring in low-C steel during the relatively short intervals between dies in a multiple-die wire-drawing machine. The effects were examined of three simulated post-hot-rolling cooling rates and three thermal treatments on the strain-aging susceptibility of a high- and a low-N steel. This was measured by applying a 6 pct tensile strain, followed by aging at either 65° or 100 °C for 20 seconds, and then pulling the specimen to failure at room temperature. Increases in flow stress and decreases in the elongation to fracture both indicated high susceptibility to strain aging. It was found that the nitrogen content, the cooling rate from the hot-rolling temperature to about 300 °C, as well as the cooling rate below 300 °C, all have dramatic effects on the strain-aging behavior. Moreover, multistage strain aging is more severe than single-stage strain aging. The implications of these observations on increasing the drawability of low-carbon-steel wire are discussed.

Behavior of steels near the incipient melting temperature

A new method of incipient melting temperature (IMT) detection has been developed in which a constant-strain-rate tensile deformation is applied to a specimen whose temperature is simultaneously increasing. The IMT is determined in a single test, and any phase transformations before the IMT will also be detected by the effects on the stress vs strain behavior in the same experiment. By means of such tests, the incipient melting behavior of a series of steels with carbon levels from 0.031 to 0.45 wt pct was examined. For the steels containing 0.08 to 0.097 pct C and about 1.5 pct Mn, it was found that incipient melting occurs in the two-phase (γ + δ) region in the temperature range from 1470 °C to 1480 °C and is significantly influenced by microalloying elements. In the ultralow-carbon steel (0.031 pct C), the IMT is in the single-phase δ region, and for the medium-carbon steel containing 0.42 pct C (hyperperitectic) it is in the γ single phase.