# Metallurgical Effects of Niobium and Molybdenum on Heat-Affected Zone Toughness in Low-Carbon Steel
## 1 Introduction  
The development of steel for use in pipeline, structural and automotive applications has seen a remarkable evolution since the late 1960s [1]. The increasing demands in terms of strength and toughness in general and weldability, in particular, have led to the design of alloys with reduced carbon content (Figure 1a). Nowadays, steel of 700 MPa yield strength (grade 100) is achievable with a maximum carbon content of 0.05 percent [2]. Reducing carbon increases upper shelf energy and lowers the ductile-to-brittle transition temperature (Figure 1b). The required strength is provided by either microstructural refinement, precipitation hardening or dislocation hardening. Typically a combination of these mechanisms is used. Since grain refinement is the only mechanism simultaneously improving strength and toughness (Figure 1b), it should be always applied in the first place before employing other strengthening mechanisms. Microstructural refinement is efficiently achieved by niobium micro-alloying in combination with thermo-mechanical controlled processing (TMCP). This approach of producing high strength low alloy (HSLA) steel containing below 0.1 percent carbon and niobium micro-alloying between 0.02 and 0.04 percent is covering a wide portfolio of applications in theyield strength range of 300 to 500 MPa. Typically, the carbon content is between 0.06 and 0.09 percent for many of these steel grades resulting in a ferritic-pearilite or pearlite-reduced ferritic microstructure. The trend of further reducing the carbon content is reasoned by improving upper shelf toughness and reducing ductile-to-brittle-transition temperature (DBTT). Besides, avoiding the peritectic range has clear benefits with regards to slab surface quality. In the 1980s, [PERSON] and [PERSON] [3] proposed an extraordinary alloy concept using particularly low carbon content in combination with a high niobium micro-alloy addition of 0.1 percent. The intrinsic benefits of modern low-carbon alloy concepts are a particularly high upper shelf toughness and very low DBTT. The microstructure of such alloys is nearly pearlite-free ferrite or by using increased cooling rate, acicular ferritic. The high recrystallization stop temperature of such high-niobium alloyed steel principally widens the processing window for TMCP rolling and enables higher production throughput, especially on hot strip mills. On the other hand, it allows producing ferritic-bainitic microstructure even by air-cooling after finish rolling. Fine-tuning of the original low-carbon high-niobium alloyed steel concept using chromium and molybdenum as co-alloying elements has opened a wide range of interesting applications for producing steel in the range of 460 to 700 MPa yield strength. These can be found in pipe as well as automotive applications. Considering structural applications, some traditional specifications do not allow the use of such innovative alloy concepts since the niobium content is often restricted although several studies indicated that concerns may not be justified [4; 5; 6]. Historically, the limitation of niobium addition relates to concerns about weldability and low heat affected zone (HAZ) toughness. Thereby, it is not accounted for that higher niobium additions are typically used in extra-low-carbon steels. Previous investigations applying industrial welding processes to such steels revealed that heat-affected zone toughness is not negatively affected by niobium additions in the range from 0.02 to 0.11% [7; 8]. The present study will focus on the metallurgical effects and microstructural changes occurring in the individual sub-zones of the heat-affected zone in extra-low-carbon steel (0.035% C) primarily considering the effects of niobium and molybdenum alloying. The effects of titanium and silicon alloying are additionally addressed.