**1. Introduction**

The automotive industry focuses on increasing the use of advanced high-strength steels (AHSS) in order to satisfy the current demand for decreasing the fuel consumption by reduced weight and increasing vehicle safety by using these steels in different energy-absorbing components [1]. Usually, these AHSS are produced by thermomechanical processes, which control the microstructure and grain size as well as the precipitation hardening of micro-alloyed steels.

During welding of AHSS, the weld area will be completely changed and the excellent properties (i.e., tensile strength, toughness) will be lost. This means that the welded area could be the best area for crack propagation [2]. Therefore, pre-and/or post-welding treatment is necessary to improve the properties of this zone. Laser welding is a popular method in the industry because it is fast, creates narrow and deep welds, and can be used for different materials and shapes. So, in this work, a quenching and partitioning [3] method has been applied for post-welding treatment in order to control the microstructure [4]. The final structure will contain tempered martensite (which increases the yield strength) with retained austenite (which improves the ductility), and, depending on the Q&P conditions and chemical composition of the steel, some bainite and fresh martensite can also be formed [3–5].

However, carbide/nitride precipitation during the partitioning step cannot be avoided, even in low-carbon [6] or in high-carbon, high-silicon steels [7]. Therefore, one of the critical aspects is to monitor the behavior of precipitates during the process. Although precipitates and/or particles are designed to strengthen the material by a precipitation hardening mechanism, they could act as cleavage initiation sites and deteriorate the toughness [8–10]. Fairchild et al. [11] showed that a strong inclusion-matrix bond is why TiN inclusions are potent cleavage initiators in steels even with modest Ti contents. Another study by Di Schino et al. [12] on the effect of Nb microalloying on the heat-affected zone (HAZ) showed that a small difference in Nb content is able to influence the size of the bainitic packet, which results in both toughness and hardness.

Several works have been done on the mechanism of Q&P up to now [4,13–17], while only a few papers have investigated the fracture mechanisms and toughness of Q&P steels [18,19]. Fracture causes could mainly be related to: (i) Kinetics of carbon partitioning and stabilization of retained austenite (RA) in the structure, because it has been shown that increasing volume fraction of RA due to the TRIP effect delays the crack propagation [20]; (ii) Kinetics, size, number density and shape of secondary precipitates; and (iii) Microstructural refinement can be very effective for improving the toughness. Wang et al. [21] found that, when the cleavage crack encounters another packet of martensite, it may be arrested and then largely changes its propagation direction.

Previous studies by the authors [4,22] on the effect of Q&P after laser welding of a low-carbon steel showed that samples partitioned at a higher temperature had the best tensile properties but very low Charpy V impact toughness results. In the present work, in order to investigate the origins of such contradictory results, the abovementioned main issues are studied. Carbon diffusion from α to retained austenite was modeled by a diffusion-controlled transformation tool (DICTRA) [23] during quenching and partitioning. The type, size, and distribution of the precipitates were evaluated using a scanning transmission electron microscope (STEM) in order to verify the precipitation prediction modeling by TC–PRISMA. Finally, the effect of the packet size of martensite/bainite laths on crack propagation has been investigated by electron backscatter diffraction (EBSD).
