## 1 Introduction  
Quenching and partitioning (Q&P) steels are C-Mn-Si multiphase steels with an excellent balance between strength and ductility, which receive their properties from a special heat treatment consisting of a Q&P process. These steels are very interesting for lightweight and safe automotive structures [1] and have been intensively investigated in recent years. Quenching and partitioning steels close the gap between the first generation of advanced high-strength steels (AHSS), such as dual phase (DP) steels, complex phase (CP) steels, martensitic (MART) steels or transformation induced plasticity (TRIP) steels and the second generation of AHSS, such as twinning-induced plasticity (TWIP) steels or Al-added lightweight steels with induced plasticity (L-IP), see Figure 1. Significant advantages of Q&P steels include (i) a reduced amount of alloying elements and thus lower costs in comparison to second generation AHSS, and (ii) improved mechanical properties compared to first generation AHSS, see Figure 1 [2,3]. Therefore, Q&P steels represent a promising approach for the third generation of AHSS [3]. With the Q&P process, the amount of martensite and retained austenite can be adjusted. Figure 2 illustrates a typical T-t-regime of such a Q&P process. After austenitization, the first process step (quenching) is rapid cooling to a quenching temperature (Tq) between the martensite-start temperature (Ms) and the martensite-finish temperature (Mf), which results in a well-defined microstructure of martensite and austenite. In the second step (partitioning), an isothermal holding at Tq (one-step Q&P) or above Tq (two-step Q&P) is performed. This leads to carbon diffusion from the supersaturated martensite into austenite, resulting in a stabilization of austenite after subsequent cooling to room temperature and in a reduction of internal stresses in the martensite phase [2; 4; 5; 6; 7]. The formation of carbides during this heat treatment is inhibited due to alloying with Si or Al [4; 8]. With this heat treatment, which was first proposed by [PERSON] et al. [4], a microstructure of martensite with a well-defined fraction of retained austenite can be obtained. This results in a good balance between high strength (1000-1400 MPa [9]) and ductility (10-20% [9]), and therefore in a mostly ductile fracture behavior. Depending on the selected temperatures (one-step or two-step Q&P) and holding times, the properties of the steel can be specifically influenced [2; 5; 6; 10; 11; 12]. For an industrial application of Q&P steels, developments and investigations of joining methods is an important field. In previous studies, in particular, resistance spot welding, which is the primary method in automobile manufacturing [13], was investigated for joining [9; 13; 14; 15; 16; 17]. It was shown that joining of Q&P steels by resistance spot welding is usually possible but that the microstructure of the Q&P steels is often entirely changed, resulting locally in a loss of the desired mechanical properties [13; 14; 17]. For example, in Reference [17] it was found that due to the welding process, the martensite exhibited a reduced hardness and the austenite was converted into bainite. Moreover,  
Figure 1: Tensile elongations and strengths for the first generation of advanced high-strength steels (AHSS) (lower-right) and second generation (upper-right) with a gap in between, representing the opportunity for the third generation of AHSS, according to References [2; 3]. Figure 2: Schematic T-t-regime of a Quenching and partitioning (Q&P) process. alloying elements, for example P and B, in the steel can affect the weldability by forming inclusions in the fusion zone and undesirable phases in the heat-affected zone [14]. Since the multi-phase microstructure of the Q&P steel is essential for the balance in terms of strength and ductility, a modification of the microstructure during subsequent joining processes should be avoided.