Effect of trace Nb on phase transformation and precipitation dynamics of 1600 MPa high-carbon pearlite wire rod produced by online isothermal treatment

As a raw material for large building structural components, such as bridge cables and prestressed steel strands, high-carbon wire rods face increasingly prominent issues of insufficient plasticity and Ti(C, N) inclusions as their strength increases. The present study investigated the impact of trace Nb addition on the microstructure, properties, and the precipitate of Ti(C, N) in a 1600 MPa high-carbon wire rod (95Si, 0.95C-1.12Si-0.6Mn-0.36Cr) processed via an online salt bath. SEM, TEM, EBSD, 3D-APT, and in-situ nanoindentation were utilized to characterize the microstructure, precipitates, and mechanical properties of the wire rod. The addition of trace Nb reduced the strength of the 95Si steel by 2.9 %, while the elongation at break increased by 16.8 %. This was mainly due to the addition of trace amounts of Nb, which increased the interlayer spacing of pearlite from 64 nm to 102 nm, and reduced the pearlite colony size from 3.5 μm to 2.8 μm in the studied 95Si steel. In addition, trace Nb reduced the inclusion density by 67 %, achieved mainly by reducing the number of Ti(C, N) inclusions and promoting the precipitation of nanoscale (Nb, Ti)(C, N). Consequently, the influence of Nb on the strengthening mechanism of 95Si steel manifested as a reduction of 9.4 % in the contribution from grain refinement strengthening and an increase of 100 % in the contribution from precipitation strengthening. In summary, Nb microalloying technology offers a promising approach for enhancing plasticity and controlling Ti(C, N) inclusions in high-carbon wire rods.