Effect of microalloying on pearlite transformation of high carbon wire steels

Abstract

Microalloying has been shown to improve strength in eutectoid steels for wire applications, and previous work on vanadium-microalloyed eutectoid steels showed delayed pearlite transformation with additions of niobium and accelerated pearlite transformation with additional nitrogen. This study investigates the origin of the CCT shifts with microalloying additions and whether trends in hardness and microstructural feature sizes observed in continuous cooling tests persist through industrial hot rolling simulations. An industrially hot rolled 1080 wire rod with vanadium additions and three laboratory-prepared alloys were studied. The base alloy, denoted the V steel, had a composition of 0.80C-0.50Mn-0.24Si-0.20Cr-0.079V-0.0059N (wt pct). The V+N steel contained 0.0088 wt pct N, and the V+Nb steel contained an additional 0.010 wt pct Nb. All alloys were subjected to a Gleeble® 3500 torsion hot rolling simulation based on industrial wire rod hot rolling parameters. Microstructural constituents, Vickers hardness, pearlite colony size, and pearlite interlamellar spacing (ILS) were characterized for each alloy. All alloys exhibited pearlitic microstructures with some proeutectoid ferrite at prior austenite grain boundaries, with no evidence of shear transformation products. The V steel has the lowest overall hardness, while both nitrogen and niobium additions increase hardness by approximately 15 HV, correlating to a 43 MPa increase in yield strength. Niobium additions refined ILS, with an average ILS of 92 ± 3 nm for the V+Nb steel compared to 113 ± 5 nm for the V steel and 113 ± 3 nm for the V+N alloy. Vanadium additions produced precipitation strengthening for all alloys and heat treatments, and additional precipitation strengthening with nitrogen and niobium additions was not apparent based on a Taleff regression analysis. Atom probe tomography of an industrially processed wire rod with vanadium additions revealed vanadium enrichment of cementite, and vanadium supersaturation in ferrite. Enhanced nucleation of V(C,N) on prior austenite grain boundaries was hypothesized to accelerate pearlite transformation in the presence of additional nitrogen. Based on the CCT thermal profile, samples were heat treated by quenching from 1093 °C and 880 °C for assessment of the state of vanadium. Solubility predictions indicated full dissolution at 1093 °C, and precipitation at 880 °C. Electrochemical dissolution results showed almost no V(C,N) precipitation for either hold temperature. Atom probe tomography showed homogeneous vanadium distribution in austenite for all samples quenched from 880 °C. Therefore the theory of accelerated pearlite transformation by enhanced nucleation of V(C,N) was not confirmed. Results indicate that accelerated transformation effects with nitrogen additions are not due to austenite grain size effects, differences in volume fraction of precipitates in austenite, or inhomogeneity of vanadium distribution in austenite. Solute drag of niobium in austenite was identified as a possible mechanism for delay of pearlite transformation with niobium additions. Samples of the V and V+Nb alloys were heat treated by quenching from 1093 ° C for assessment of the state of Nb. Solubility modeling for mixed (Nb,V)(C,N) precipitates predicted full precipitation at 1093 °C, with no niobium dissolved in austenite. Austenite grain size (AGS) was determined to be refined by ~50 µm for the V+Nb steel compared to the V steel, but AGS refinement would produce the opposite transformation shift than experimentally observed. Electrochemical dissolution showed less than 0.0020 wt pct Nb dissolved in austenite. Atom probe tomography of a pearlite transformation interface indicated no niobium present in austenite adjacent to the boundary. Modeling predictions, dissolution results, and APT compositional analysis presented in the current study have not confirmed a solute drag mechanism, though it remains plausible given the resolution and sampling volume limitations of the experimental methods employed.