**1. Introduction**

In the automotive field, reducing the body weight is one of the most fundamental ways to save energy and reduce pollution. Therefore, more and more lightweight and high-strength materials have been developed for body manufacturing [1–3]. Galvanized dual phase steel has the advantages of low yield ratio, good formability, high tensile strength, good matching of strength and plasticity, corrosion resistance, and so on, which has been used in automobile body manufacturing [4–6]. The wide application potential of galvanized dual phase steel in the automotive field mainly depends on the welding methods and its welding quality.

In previous literature, several welding methods, such as laser welding [7–10], gas metal arc welding (GMAW) [11–13], and friction stir welding (FSW) [14–16], have been employed to weld the galvanized dual phase steel, and these welding methods have their own advantages and disadvantages, as well as being applicable to different base metal materials and joint shapes. Resistance spot welding has the advantages of high efficiency and low cost, which is an important welding method for manufacturing the automobile sheet structures [17,18]. Many scholars have studied the microstructure, softening zone characteristics, mechanical behavior, and welding spatter defect of resistance spot welding of dual phase steels [19–23]. The CR590T/340YDP galvanized dual phase steel sheet used

as an automobile manufacturing material not only has good corrosion resistance, but also high strength, which can effectively reduce the weight of the car body, and some research on resistance spot welding have been carried out [24–26]. Wang et al. [27] investigated the effect of base material chemical compositions on the properties of the resistance spot welding joint of DP590 steel, and the results indicated the tensile strength and toughness of welded joints were affected by the chemical compositions of the base material, especially the carbon content. Namely, for the same grade DP590 steel, the weld formation, microstructure, and mechanical properties of resistance spot welding will be different if the chemical compositions of the base material are not the same. During the resistance spot welding process, it is well known that the welding heat generation can be expressed as Q = *I* <sup>2</sup>*Rt* (*I* is the welding current; *R* is the resistance; and *t* is the welding time). In general, the welding time and welding current are 10−<sup>1</sup> *s* level and kA level, respectively. Therefore, the welding current is considered as the key factor to determine the welding heat input and influence of the welding quality [28]. Wang et al. [29] established a finite element model for resistance spot welding of DP590 steel, and the nugget formation process was investigated. The simulative result for nugget size was obviously bigger than that of the experiment under a large welding current, although they were well fitted under a suitable welding current. Therefore, the experimental investigation on the effect of the welding current on properties in resistance spot welding of DP590 steel has an important significance.

To explore the weldability, and provide the process with guidance for automobile body production, the CR590/340Y galvanized dual phase steel sheets employed for the automobile front longitudinal beam part were carried out by resistance spot welding, and the influence of the welding current on weld formation, microstructure, microhardness, and tensile strength was studied in detail.
