**1. Introduction**

The demand for vehicle weight reduction and improved crash performance has led to the application of hot stamping of boron steel [1,2]. However, the components with high strength and low levels of ductility did not show a good performance in crash energy absorption [3,4]. To improve the energy absorption, tailored hot stamping (THS) components with tailored mechanical properties have been developed [5–8]. From continuous cooling transformation (CCT) curve, it can be seen that ferrite and bainite can be obtained at cooling rates lower than 30 ◦C/s [9,10]. Since the temperatures of two contact bodies will affect their cooling rates, THS with segmented heating and cooling tools can be used to obtain a multiphase quenched microstructure and achieve the components with tailored properties. There are some studies that analyze how the tool temperature affects the mechanical properties [11–13]. However, the temperature difference between the cooling tool and the heating tool is much larger when the tool is partitioned for cooling and heating, which makes the springback of the formed components more serious than conventional hot stamping. Some scholars have studied the influence of hot stamping process parameters on the springback of formed components [14–16]. They concluded that the springback of the formed component gradually decreased with the increase of blank temperature and holding time. However, for THS components with various quenching phases, the microstructure distribution has an important impact on the springback.

The springback measurement of components is a complicated process. In order to understand the effect of the quenching phase content on the component springback, a THS experiment was carried out in this paper, and the quenching phase was quantitatively measured by the color metallography and image recognition method. The springback angles of di fferent phase contents were obtained by non-contact 3D measurement. The relationship between phase content and the springback angle was established, which can be used for the determination of the springback of THS components.

## **2. Experimental Study**

## *2.1. THS Experiments*

Usibor1500P boron steel with a thickness of 1 mm was used in the experiments, and the chemical composition is shown in Table 1. Figure 1 shows the THS experimental apparatus. The tool material used for this study was H13 steel and did not have any surface or hardening treatments. H13 steel has excellent comprehensive mechanical properties and high anti-tempering stability and is a common material for hot stamping tools. Cooling channels were machined inside the cooling tool to take away the heat of the blank by the water in the channels. Through installing the electric cartridge heaters controlled by the PID system, the hot tool temperatures could be varied from 25 ◦C and 600 ◦C. An air gap of 2 mm was left between the heating tool and the cooling tool to prevent heat transfer between them.


 of Usibor 1500P (wt.%).

composition

**Table 1.** Chemical

**Figure 1.** Schematic of the experimental apparatus.

A furnace was used to heat the blank to the temperature of 930 ◦C for austenizing, and the heating time was about 5 min. After the blank was austenized, it was moved to the tool to be formed and quenched. It took about 10 s to move the hot blank from the furnace to the tools. The quenching time was 10 s with the press force of 400 kN.
