Development of ultrahigh strength steel by electroslag refining: Effect of inoculation of titanium on the microstructures and mechanical properties

Abstract

A low alloy steel with the nominal chemical composition of C 0.3%, Mn 1%, Cr 4%, Mo 1%, V 0.4% with lower amount of P and S is prepared by electroslag refining (ESR). The result of the mechanical properties of this steel after tempering confirms that this steel may be placed in the family of ultrahigh strength steel. The tensile, yield and impact strength of this steel are 1 650 MPa, 1 450 MPa and 300kJ/m2 respectively, coupled with good ductility and hardness. Three more steels are prepared by inoculation of titanium during the ESR process similar to above composition. The analysis values of titanium in these steels are 0.07%, 0.20% and 0.4%. The addition of titanium resulted a sharp change in both microstructures and mechanical properties. It is noticed that 0.07% titanium results a sharp increase of tensile properties compared to un-inoculated steel. The tensile and yield strength of the 0.07% titanium steel are 1 730 MPa and 1 512 MPa, respectively. Further increase of titanium to 0.2% and 0.4% result a significant drop of mechanical properties compared to steel with no titanium. The optical, SEM, TEM and TEM-carbon replica studies confirms that at lower concentration of titanium (0.07%), finer Ti(C, N) particles are precipitated which are able to restrict the austenite grain sizes. The refinement of austenite grains and consequent finer lath martensite may be the possible reason for strengthening. Higher titanium (≥0.2%) led to larger carbonitride particles which has no role on grain refinement. © 2006 ISIJ.