Studies have shown that the energy consumption of the automotive body is reduced by 6–8% for every 10% reduction in vehicle body quality [
1,
2,
3]. Therefore, reducing the weight of the vehicle without reducing the overall performance can effectively reduce fuel consumption and pollution emissions. Ultra-high strength steel met the requirements of automobile safety performance and lightweight development in recent years, which has been widely used in automobile parts manufacturing [
4,
5]. However, the deformation ability of ultra-high strength steel at room temperature is very poor, and it is easy to produce forming quality defects such as cracking and springback [
6,
7]. Meanwhile, new challenges have been encountered in the design of key collision structures for automobiles: different yield strength needs to be designed in different positions, such as the A-pillar, B-pillar and C-pillar, to withstand the pressure of the roof, high strength and rigidity in the upper side, while to protect the safety of the occupants when the car is in a side collision, high shaping and toughness are required in the lower side [
8]. This means the automotive industry urgently needs to design parts with performance gradients to meet the requirements of lightweight and partition performance at the same time [
9,
10].
Currently, the main forming technologies for designing stamped parts with partition performance include tailor rolling blank (TRB) technology, tailor welded Blank (TWB) technology, tailor patch blank (TPB) technology, tailor tempering process (TTP) technology, tailor heating process (THP) technology and tailor cooling process (TCP) technology [
11].
A great deal of research has been carried out around TCP in recent years. Mori et al. [
14] changed the contact condition between the local area of sheet metal and the die, and high strength occurred in the contacted part due to being quenched, while the non-contacted part was low strength because it was not quenched. Mori et al. [
15] also developed local heating of bypass resistance during hot stamping, where the sheet metal was partially heated by a bypass resistor, and then hot stamped. There was no martensitic transformation in the local heating area, allowing for the obtention of partially softened tissue. George et al. [
16] designed segmented dies with partial heating and cooling zones to control sheet metal cooling at different cooling rates after hot stamping to obtain a B-pillar with partition properties. The Vickers hardness was 488 HV and 234 HV in the cooling and heating areas after hot stamping, respectively. The tensile strength of the rapid cooling zone was reduced by 49%, and the tensile strength of the slow cooling zone was increased by 84%, which proved the purpose of partition performance. Ota et al. [
17] set up an air compression cooling device in the mold in order to solve the problem of cracking in the hot stamping process of hat-shaped parts. The local formability was improved by adjusting process parameters, and they produced trial parts with improved local performance. Wang et al. [
18] selected 22MnB5 of ultra-high-strength steel as their research object to study the influence of mold temperature and process parameters on the cooling rate and mechanical properties of sheet metal. Then, the samples were tested for performance and microstructure analysis. The results showed that the temperature difference between die and sheet metal was reduced by increasing the die temperature, resulting in a reduction in the sheet metal cooling rate. Furthermore, the martensite transformation was avoided, and more bainite structures were obtained, which make the extension performance better, while the tensile strength and hardness were reduced. Yang et al. [
19] analyzed the non-uniform temperature field and hot forming process by the simulation method. Based on the metallographic test and tensile test, it was concluded that the martensite structure was obtained in the rapid cooling area, in which tensile strength can reach 1500 MPa, and the elongation is about 7.0%. However, while the tensile strength of ferrite and pearlite is only 680 MPa, the elongation is 11.2%. It was proved that the method of selective cooling can realize the regional distribution of materials and the different microstructure of materials, and reflect different properties. Mu et al. [
20] studied the influence of process parameters on the tensile strength and elongation of the forming parts in hot stamping process. Combined with finite element simulation and the non-dominated sorting genetic algorithm (NSGA)-II optimization algorithm, the forming quality in the process of hot stamping was optimized, and the stamped parts with partition properties were produced. Wang et al. [
21] studied the influence of influencing factors on the crashworthiness of the front longitudinal beam and optimized it by NSGA-II. The results showed that the method significantly improved the formability and crashworthiness.
To sum up, the forming quality, such as microstructure, mechanical properties etc. of stamped parts with partition properties is controlled by many factors. However, the influence of influencing factors on specific forming parts is different, and the research-oriented to specific products on multi-objective optimization of partition cooling in hot stamping is just beginning. It is necessary to study the forming quality influencing factors and multi-objective optimization of partition cooling in the hot stamping process.
Therefore, in order to realize the partition performance of the automotive B-pillar, the partition cooling after hot stamping is adopted. Although the springback of the B-pillar is small after hot stamping, the thickness change has further optimization space. The method of “finite element simulation + optimal Latin hypercube design (OLHD) + the response surface methodology (RSM) + multi-objective optimization” is used to optimize the influencing factors of thickening and thinning, and the influence of influencing factors on forming quality is analyzed to verify the validity of the approximate model and the accuracy of multi-objective optimization. Afterwards, the simulation is performed according to the optimized parameter combinations to analyze the temperature field, microstructure, tensile strength, hardness, thickening rate and thinning rate, and its quality. Finally, the hot stamping test of the B-pillar and the mechanical properties and microstructure test are performed, which correspond well with the optimized simulation results and verify the feasibility of preparing the parts with performance gradient by regional hot stamping.