Abstract
Titanium additions are often used for boron factor and primary austenite grain size control in boron high- and ultra-high-strength alloys. Due to the risk of formation of coarse TiN during solidification the addition of titanium is limited in respect to nitrogen. The risk of coarse nitrides working as non-metallic inclusions formed in the last solidification front can degrade fatigue properties and weldability of the final product. In the presented study three microalloying systems with minor additions were tested, two without any titanium addition, to evaluate grain size evolution and mechanical properties with pre-defined as-cast, hot forging, hot rolling, and off-line heat-treatment strategy to meet demands for S1100QL steel. Microstructure evolution from hot-forged to final martensitic microstructure was observed, continuous cooling transformation diagrams of non-deformed austenite were constructed for off-line heat treatment, and the mechanical properties of Nb and V–Nb were compared to Ti–Nb microalloying system with a limited titanium addition. Using the parameters in the laboratory environment all three micro-alloying systems can provide needed mechanical properties, especially the Ti–Nb system can be successfully replaced with V–Nb having the highest response in tensile properties and still obtaining satisfying toughness of 27 J at –40 °C using Charpy V-notch samples.
Highlights
High yield strength levels above 550 MPa are required when talking about advanced high-strength steels (AHSS)
Tensile and toughness testing was conducted for Q and QT, to evaluate the effect of low-temperature tempering on the mechanical properties of S1100QL steel
The refined mixed microstructure is observed in the hot-rolled state (Figure 2b,e,h) as a part of forged-rolled technology used in this paper with higher final absolute deformation
Summary
High yield strength levels above 550 MPa are required when talking about advanced high-strength steels (AHSS). Non-standardized S1100QL steel belongs to the group of ultra-high-strength fine-grained martensitic steels It must meet the minimum yield strength of 1100 MPa and Charpy impact toughness of at least 27 J transverse and 30 J longitudinal to the rolling direction at −40 ◦ C. Ti · N product TiN forms non-metallic inclusions in the last solidification front These coarse non-metallic inclusions do not provide prior austenite grain (PAG) size control, they degrade fatigue properties, weldability, and impact toughness of the final product [8,9,10]. In the case of simultaneous Ti and Nb addition co-precipitation of niobium-rich particles on existing TiN is possible [23] Boron is another important microalloying element, used to increase hardenability. Each microalloying system was evaluated to determine the most promising system which will meet the demands for S1100QL steel
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