The microalloying additions like titanium, niobium and vanadium are being frequently used in modern steelmaking. These microalloying elements have a major influence on the final mechanical properties of steel. Increase in strength and toughness is achieved with grain refinement, while precipitation hardening increases strength but decreases toughness.
S1100QL is ultrahigh-strength microalloyed structural steel (UHSLA) used in heavy–lifting industry. Microalloying additions like titanium, boron, niobium and vanadium are being used to achieve the required mechanical properties. Titanium addition is used for boron protection as well as austenite grain refinement. However, many researchers argued that coarse TiN particles trigger cleavage fracture and thus drastically reduce the impact toughness of steels.
To decide, whether the titanium addition is really needed, we fabricated experimental steels with three different microalloying additions (Nb-B, V-Nb-B and Ti-Nb-B). All three steels were vacuum melted in induction vacuum furnace, cast into ingots, hot forget, hot rolled, water quenched and tempered at 200 °C, 570 °C and 640 °C for 1 hour.
The series of experiments were conducted to fully characterize and decide which microalloying addition can be used to achieve the required mechanical properties. Experimental tests included chemical analysis, metallographic analysis on light microscope, grain size measurement, automated SEM-EDS non–metallic inclusions analysis, differential thermal analysis, thermodynamic analysis using Thermo-Calc, construction of CCT and tempering diagrams, tensile tests and Charpy impact tests at -20 °C and -40 °C.
V-Nb-B microalloying addition showed best response in terms of tensile properties, which can be related to precipitation hardening of ferrite with VN particles. Highest Charpy impact toughness values were achieved using Ti-Nb-B microalloyed steel. Titanium can refine the grain size of steel during reheating and hot deformation (rolling, forging…) and thus increase impact toughness, which is closely related to grain size. Mechanical properties of all three steels were achieved during low-temperature tempering of quenched samples at 200 °C.