## 1 Introduction  
The strength of steel utilized in the automobile body structures has been increasing rapidly, and the proportion of martensite-based ultra-high-strength steel (UHSS) has been expanding drastically in the last two decades [1; 2; 3; 4]. Among modern martensite-based UHSS, fully martensitic steel and hot-press-formed (HPF) steel have higher strength owing to the higher martensitic phase fraction than that in martensite-ferrite dual-phase (DP) steel and transformation-induced plasticity steel, which have a partial martensite fraction [4; 5; 6].  
HPF steel, a boron-alloy steel, has a ferrite-pearific microstructure with a tensile strength of approximately 600 MPa before the HPF process. During the HPF process, an HPF-steel sheet is heated for complete austenization and then quenched in a water-cooled die; this process enables the achievement of high dimensional accuracy owing to the high temperature forming process and a fully martensitic microstructure formation owing to the rapid cooling. Based on the process characteristics, HPF is also termed press hardening, hot forming, or die quenching. Since 2010, HPF steel has been widely commercialized in the automotive industry, and 22 MnB5 steel with a tensile strength of 1.5 GPa has been most commonly applied [7]. Many automotive companies have successfully applied HPF steel to passenger-safety-related components, such as the bumper beams and center pillars, and automobile manufacturers are focused on expanding their applications to higher-grade HPF steel with higher carbon content [8].  
Because HPF steel has a fully martensitic microstructure, relatively soft tempered martensite is formed at the heat-affected zone (HAZ) while welding: this is termed as the HAZ softening effect. The HAZ softening effect is unavoidable in welding the martensite-based UHSS; therefore, laser welding with a relatively low heat input is preferred to minimize the size and magnitude of HAZ softening. Most previous studies have investigated the laser weldability of 1.5 GPa grade HPF steel using a CO\({}_{2}\) laser [9] and high brightness lasers [10] and found substantial hardness degradation of the HAZ when the peak temperature was near the austenization (Ac\({}_{1}\)) temperature. Steel manufacturers such as SSAB AB and ThyssenKrupp Steel released higher grades of HPF steel. Furthermore, the laser overlap weldability has been evaluated on 1.8 GPa grade and 1.9 GPa grade HPF steel [11]. The detrimental influence of surface layers such as the AlSi coating layer and the surface oxide layer on the laser-weld strength was investigated on 1.5 GPa HPF steel [12] and 1.8 GPa HPF steel [13]. The commercial 2.0 GPa HPF steel has been launched, but studies on the welding of 2.0 GPa grade HPF steel have not been reported yet.  
In this study, the mechanical and metallurgical behavior of laser welds was investigated on 2.0 GPa HPF steel. A high brightness laser was chosen as the power source to minimize the thermal effect on the welds, and the microstructural transformation at different locations was observed by microscopy. The cause of HAZ softening and its effect on the tensile strength of the welds were studied through microstructural analysis.