Abstract
Modern weldable high strength steel grades are typically based on low-carbon alloy concepts using microalloying for obtaining a good strength-toughness balance. Such steel grades having a yield strength in the range of 420 to 690 MPa are very commonly used in pipelines, heavy vehicles, shipbuilding and general structural applications. Thermomechanical processing during hot rolling combined with accelerated cooling is an established means of producing such steel grades. Considering the alloying concepts, the use of niobium and molybdenum, and in selected cases boron, is very efficient to achieve high strength and good toughness. However, all targeted applications of such high strength steels involve extensive welding. Thus, heat affected zone properties are of particular importance. The present paper investigates the effects of Nb, Mo and Ti on the heat affected zone properties. Variations of the Mn and Si contents are considered as well. Additionally, the influence of post-weld heat treatment in the coarse-grained heat-affected zone (HAZ) is considered. In this approach, HAZ subzones were generated using laboratory weld cycle simulations in combination with systematic variation of alloying elements to scrutinize and interpret their specific effects. The results indicate that Mo and Nb, when alloyed in the typical range, provide excellent HAZ toughness and guarantee sufficiently low ductile-to-brittle transition temperature. An alloy combination of Nb, Mo and Ti improves performance under hot deformation conditions and toughness after post-weld heat treatment.
Highlights
The development of steel for use in pipeline, structural and automotive applications has seen a remarkable evolution since the late 1960s [1]
The results indicate that Mo and Nb, when alloyed in the typical range, provide excellent heat-affected zone (HAZ) toughness and guarantee sufficiently low ductile-to-brittle transition temperature
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 the
Summary
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). Steel of 700 MPa yield strength (grade 100) is achievable with a maximum carbon content of 0.05 percent [2]. 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 the
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