**2. Experiment**

Boron bearing steel with an Al-Si coating was used in the present study (GMW14400M-ST-S-HS1300T/950Y-MS). Rectangular specimens for spot welding tests with a dimension of <sup>100</sup> × <sup>25</sup> × 1.5 mm3 were cut from the hot-stamped hardened A-pillars or B-pillars. The chemical composition of the base steel was measured, and the results are shown in Table 1. The yield strength *R*P0.2, tensile strength *R*m, and elongation of the investigated hot-stamped hardened steel were determined as about 1100 MPa, 1500 MPa, and 5.4~7.7%, respectively. The microstructure of the hot-stamped hardened steel consists entirely of fine martensite, including martensite laths and plates, as shown in Figure 1. Microhardness of the hot-stamped hardened steel was determined as HV432.

**Table 1.** Chemical compositions of hot-stamped steel sheets (wt %). **C Si Mn Cr Ni Mo V P S B Fe** 0.24 0.35 1.43 0.22 0.06 0.02 0.02 0.0317 0.0103 0.0025 base

**Figure 1.** Optical microstructure of the hot-stamped hardened steel.

In order to clarify the phase transformation of the experimental steel after experiencing reheating in spot welding, its continuous cooling transformation (CCT) diagram was obtained by dilatometry measurements. The steels were heated to 1000 ◦C at a heating rate of 100 ◦C/s for 2 min, and then cooled to room temperature with different cooling rates.

In resistance spot welding experiments, DC (direct-current) and variable frequency AC (alternating-current) power controllers (Sunke Co. LTD, Tianjin, China) were used in this study. Three types of current modes were applied: (1) DC, with current size of 8~9 kA and welding time of 300~350 ms; (2) general mid-frequency AC, with frequency of 110 Hz, current size of 10~11 kA, and welding time of 20 cycles; (3) mid-frequency AC with cooling intervals (about 6 ms) between any two neighboring current pulses, with frequency of 110 Hz, current size of 11~12.7 kA, and welding

time of 25~35 cycles. An electrode force of 550 kg was applied, and the diameter of the used electrode was 6 mm.

Specimens for microstructural examination were cut along a diameter of the spot welding nugget, as shown in Figure 2 by red dotted line of A-A. The specimen includes the whole cross section and it was embedded in epoxy resin with the cross section of the nugget at surface. One of the two cross sections was grounded and polished, and then etched with 3.5% nital reagent. Microstructural observations were conducted by optical microscopy (OM) (Leica DFC 450, Leica, Solms, Germany) and scanning electron microscopy (SEM) (Zeiss, Jena, Germany). Microhardness was determined to evaluate the property of different microstructural areas around the spot welding nuggets, by applying a load of 200 g. In order to examine the mechanical property of spot welding joints, tensile shear tests were performed using samples with dimensions as shown in Figure 2. Three samples were prepared for each welding condition and the average value of the three samples was used.

**Figure 2.** Specimen dimensions for tensile shear tests (unit: mm).

#### **3. Results and Discussion**

#### *3.1. Continuous Cooling Phase Transformation*

To elucidate the effect of the cooling rate on the phase transformation behavior of the experimental steel, the continuous cooling transformation diagram is established, as shown in Figure 3. Figure 4 shows the microstructures under different cooling rates. It can be seen that the critical cooling rate for martensite transformation was close to 17 ◦C/s, in which the microstructure consisted entirely of martensite, indicating the high hardenability of the steel. Under the slow cooling rate of 1.5 ◦C/s, as shown in Figure 4a, the microstructure consisted of polygonal ferrite and a small amount of pearlite, which was formed in the range 603~678 ◦C, as seen from Figure 3. With the cooling rate increasing to 3 ◦C/s, polygonal ferrite transformed to an acicular one, and a small amount of granular bainite could be found, as indicated by arrows in Figure 4b. The acicular ferrite nucleates at prior austenite grain boundaries and grows into the prior austenite grains. In the case of 3 ◦C/s, the phase transformation occurred in the temperature range 556~620 ◦C, as seen from Figure 3. As the cooling rate further increased to 8 ◦C/s, granular bainite tended to predominate in the produced microstructure, as shown in Figure 4c. As the cooling rate went higher than 17 ◦C/s, the product was completely martensite, as shown in Figure 4c. The martensite start and finish temperatures (Ms and Mf) were respectively determined as 438 and 339 ◦C.

**Figure 3.** Continuous cooling transformation diagram of the experimental hot stamping steel.

**Figure 4.** Optical microstructures under typical cooling rates: (**a**) 1.5 ◦C/s, (**b**) 3 ◦C/s, (**c**) 8 ◦C/s, (**d**) 17 ◦C/s.
