## 1 Introduction  
Friction stir welding (FSW) is a well-known solid-state welding process with minimum thermal effect on weldments. FSW was initially applied to nonferrous metals and later to high-strength steels. [PERSON] et al. reported the feasibility of FSW for Cr and mild steels in 1999 [1]. Compared with the FSW of Al alloys, higher-strength tools are required to mitigate tool wear in the FSW of steel. Polycrystalline cubic boron nitride, W-based materials, and Si\({}_{3}\)N\({}_{4}\) have been proposed as tool materials instead of steel used for the FSW of Al alloys [2]. Another important feature of friction stir-welded steels is the welding system deflection due to the high applied load. The system deflection interferes with the exact plunging of the welding tool. Accurate depth control has been successfully achieved in the FSW of steels using high-stiffness welding systems [3, 4] and compensation methods such as offset control, constant force control [5, 6], and force-deflection models [7, 8]. Hybrid processes using an additional heat source can help reduce the yield strength of base materials and enhance the process capability [9, 10, 11, 12].  
As mild steel is being gradually replaced by high-strength steel in the automotive industry, studies have been conducted on the FSW of various high-strength steels. [PERSON] et al. introduced FSW for a high-carbon steel (S70C) under various welding conditions. When the peak temperature was under the austenization temperature (A1), refined microstructure containing fine ferrite was observed, and when the peak temperature was higher than A1 temperature, martensite was found [13]. [PERSON] et al. evaluated the friction stir spot welding of DP590 steel sheets. The weldment has a stronger microstructure than the base metal due to phase transformation and dynamic recrystallization, and the larger the heat input, the larger the weld nugget is formed [14]. [PERSON] et al. investigated the friction stir weldability of 780 MPa strength steels and boron steel. The 780 MPa steels, DP780 and TRIP780, showed hardening in the welds, while the boron steel showed softening in the heat affected zone (HAZ) [15]. [PERSON] et al. reported the metallurgical characteristics of friction-stir-welded DP980 joints and the local tensile deformation characteristics during tensile tests using the digital image correlation (DIC) method. Unlike the DP780 steel welds [15], DP980 steel welds has a softened white band on the HAZ [16]. [PERSON] et al. applied FSW to a 1.5 GPa-strength hot-stamped boron steel, one of the highest grades of automotive steel sheets, and reported that the welds had narrow ferrite band along the interface and prominent tempered martensite in the HAZ [17]. They devised a friction stir scribe welding scheme for Al/Fe lap joints using a special FSW tool [18]. [PERSON] et al. reported the behavior of a Fe-Al-Si intermetallic layer and material flow at the FSW interface of the Al alloy and Al-Si-coated boron steel. The Al-Si coating on the boron steel could influence the interface formation and the mechanical behavior of the friction stir welds [19].  
Despite the extensive studies on the FSW of various grades of high-strength steels, most studies have been conducted on a specific type of steel; it is necessary to comprehensively understand the FSW characteristics of steels of various grades. [PERSON] et al. evaluated the FSW weldments of high-strength steel sheets with strengths between 590 MPa and 1180 MPa, and provided the microstructure, tensile test results, and hardness profiles, but only under a few welding conditions [20]. In this study, FSW was carried out on an automotive high-strength steel with strengths ranging from 590 MPa to 1500 MPa under the same welding conditions, and the metallurgical and mechanical characteristics in terms of the location of weld zones were clarified for each grade of steel.