The key problem of the spot joining technology based on the use of aluminum and steel as components is to create an extended mechanically effective bonding area without formation of brittle Fe-Al reaction layers and undesirable geometry of hooks. The present study explores the process of friction stir spot welding of 2 ± 0.2 mm AA5754-H111 aluminum alloy to 3 mm Strenx 960 DP steel under different combinations of two ferrite-martensite steel structures obtained by intercritical heat treatment (IHT) at 725 ∘C and 775 ∘C followed by quenching, and three tool rotational velocities. Volume content of martensite after IHT and water quenching reached 0.38 and 0.61, correspondingly, while hardness values equaled 327 and 377 HV. Welds have been performed by means of a 4 mm cylindrical WC-Co pin, 12 mm flat shoulder, plunging rate of 12 mm/min, zero tilt of the tool axis to the surface of material being welded, 3 s dwell time and 0.7 mm penetration into the steel at rotational speeds of 800, 1200, and 2000 rpm. What are the roles of rotational speed and microstructure of initially used steel in governing process load, annular bond formation, hook shape evolution, structural transformations in steel, hardness variations, and brittle intermetallic compounds? An increment in rotational velocity resulted in a considerable decrease of the representative value of peak plunging force from about 16.5 kN at 800 rpm to about 10.4 kN at 2000 rpm proving that heating and plastic softening due to friction were the predominant factors here. Microstructure of initially applied steel did not significantly influence the peak plunging load for the selected range of speeds, but played an important role in formation of final structure. High-martensite IHT775 steel was responsible for the increase of hook heights at 1200 and 2000 rpm, while low-martensite IHT725 steel demonstrated the highest growth in width of bond at 2000 rpm. Martensitic stir zone has been revealed in the steel below exit hole, which afterwards became thermomechanically and thermally affected zones approaching the initial ferrite-martensite ratio. Reaction products of Fe-Al interfacial layers have been detected at tips of hooks, along the interface of the exit hole and at its corner. Hardness of intermetallic compounds varied in range 456 to 937 HV0.1, brittle nature was proved by local crack formation. The results show that rotation speed should be high for minimizing forces and forming bonds, although it works under the conditions of controlling hook formation and chemical reactions occurring at the interface.