**1. Introduction**

In light of global environmental concerns, the automobile industry has become increasingly interested in fuel efficiency and weight reduction. With increasing demand for technological development to achieve high performance and high fuel efficiency, research on the application of advanced high-strength steel (AHSS) for automobile components has progressed [1–4].

The HPF process was introduced to enhance the stiffness and strength of automobile components. In the HPF process, a blank is heated to a high temperature to create an austenitic phase and then cooled rapidly in forming dies to form a martensite phase [5,6].

However, the use of hot stamped parts is restricted in automobile components and requiring collision absorption, owing to their low elongation. For example, in the case of a side crash, the B-pillar, also known as the center pillar, is among the most important automobile components with respect to passenger safety. During side crash impact, the center pillar must be ductile to absorb the collision energy and stiff to improve intrusion resistance. Much research has been conducted on center pillars made with alternative materials [7–9] or post tempering processes to obtain tailored properties [10,11].

Owing to productivity demands and manufacturing costs, the PW and PS techniques are well-known and widely used in the automobile industry to improve the collision characteristics of center pillars.

**Citation:** Lee, M.S.; Jin, C.K.; Suh, J.; Lee, T.; Lim, O.D. Investigation of Collision Toughness and Energy Distribution for Hot Press Forming Center Pillar Applied with Combination Techniques of Patchwork and Partial Softening Using Side Crash Simulation. *Metals* **2022**, *12*, 1941. https://doi.org/ 10.3390/met12111941

Academic Editors: Yao Shen and Ning Gao

Received: 14 October 2022 Accepted: 7 November 2022 Published: 12 November 2022

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To improve intrusion resistance and increase the stiffness of automobile components, a PW is attached to the center pillar to achieve reinforced structural stiffness. The formability of laser-welded PW blanks was investigated by Mori et al. and Shi et al. [12,13]. The spot weld conditions to attach PW to HPF parts was optimized by Ahmad et al. [14]. Chengxi et al. studied methods to improve the mechanical properties of B-pillars with PW and predict the temperature distribution using FE simulation [15].

PS can be applied to improve energy absorption and increase the ductility of automobile components. Gao et al. investigated the characteristics of temperature-dependent IHTC of 22MnB5 during the spray-quenching process [16]. Ota et al. [17] evaluated the damage value and tailored temperature using a forming limit diagram. Bok et al. performed a simulation to predict the microstructure and mechanical properties of a B-pillar in comparison with experimental results [18]. Kim et al. manufactured a B-pillar using the partial strengthening method with minimal shape change [19]. However, the above studies investigated only the manufacturing process and were limited in that they did not evaluate the impact toughness and energy distribution flow, which affect performance when applied to an actual vehicle. A few studies have been conducted involving impact tests to evaluate the effects of PW and PS on impact toughness and energy distribution. Recently, M. S. Lee et al. manufactured an HPF center pillar with PW and PS by controlling the cooling rate and conducted experimental and numerical drop weight tests [20]. Owing to the limitations of the experimental equipment, the results were limited in terms of evaluating the energy absorption characteristics of PW and PS, and test collisions could not be conducted at high speed or using real vehicles owing to high costs.

In this study, given the importance of the evaluation of safety in high-speed collisions, a vehicle and an impactor were modeled according to the IIHS guidelines to evaluate the collision absorption capacity at high speed based on a previously verified low-speed drop weight test, as well as energy distribution in a high-speed collision. To improve stiffness and ductility, PW and PS techniques were applied to the HPF center pillar, and experiments were performed for comparison with the simulations to verify the results.
