Development of Deep Drawing Processes Under Indirect Hot Stamping Method for an Automotive Internal Combustion Engine Oil Pan Made from Ultra-High-Strength Steel (UHSS) Sheets Using Finite Element Simulation with Experimental Validation

Abstract

This study presents the development of a deep drawing process under an indirect hot stamping method for manufacturing an automotive internal combustion engine oil pan from ultra-high-strength steel (UHSS) sheets, specifically 22MnB5. The forming process involves two stages—cold stamping followed by hot stamping—and is finalized with rapid quenching to achieve a martensitic microstructure. Finite element simulation using AutoForm R8 was conducted to determine optimal forming conditions. The simulation results guided the design of the forming tools and were validated through experimental trials. The final oil pan component exhibited no cracks or wrinkles, with maximum thinning below 18%, a hardness of 550.63 HV, and a fully martensitic phase. This research demonstrates a novel and effective solution for producing deep-drawn, high-strength components using indirect hot stamping, contributing to the advancement of automotive forming processes in Thailand.

1. Introduction

In modern automotive manufacturing, there is a growing demand for high-strength structural components to enhance vehicle safety and improve fuel efficiency [1]. Traditionally, internal combustion engine (ICE) oil pans in Thailand have been produced using low-strength sheet steel such as JAC270F. However, due to the limited mechanical properties of this material, an additional protective component—called a safety guard—made from polymer is typically required during vehicle assembly to ensure impact resistance and durability. To overcome these limitations, high-strength boron steel such as 22MnB5 has been considered for oil pan production. When heated to 920–950 °C and rapidly quenched, 22MnB5 transforms into a martensitic microstructure, achieving superior tensile strength, yield strength, and hardness [2,3,4,5]. Despite these benefits, its low formability at room temperature poses challenges for deep drawing applications, often resulting in tearing or wrinkling [6,7,8,9]. Figure 1 shows the target geometry of the ICE oil pan, which requires significant drawing depth. Conventional deep drawing or single-stage hot stamping methods are unsuitable for forming such a complex component from 22MnB5. Therefore, this study introduces a two-stage “indirect hot stamping method” combining cold pre-forming and subsequent hot stamping followed by rapid mist quenching. This hybrid process improves formability while achieving a fully martensitic structure in the final part [10,11,12,13,14,15].

This study presents the first successful application in Thailand of forming a deep-drawn oil pan from UHSS (22MnB5) using the indirect hot stamping method. The process was optimized through finite element simulation (AutoForm R8) and validated experimentally [16,17,18,19,20]. The proposed method eliminates the need for polymer safety guards and reduces tooling steps [21,22,23]. It demonstrates a novel approach that combines deep drawing with hot stamping to achieve a martensitic microstructure in complex automotive parts, addressing a key gap in UHSS-forming technology [24].

2. Materials for Generating the Oil Pan and Materials Model

2.1. Boron Steel Grade 22MnB5

This study employs cold-rolled boron steel grade 22MnB5, classified as an ultra-high-strength steel (UHSS), for the production of automotive oil pans. The material has a thickness of 1.20 mm and contains key alloying elements such as manganese and boron. Its chemical composition is presented in

Table 1. Chemical composition of boron steel grade 22MnB5 (wt.%).

Materials	Chemical Composition (wt.%)
	C	Mn	B	Cr	Si	Al	Ti	N
22MnB5	0.220	1.180	0.002	0.160	0.220	0.030	0.040	0.005

.

2.2. Uniaxial Tensile Test

Uniaxial tensile tests were conducted in three rolling directions, 0°, 45°, and 90°, for boron steel grade 22MnB5 under both non-quenched and quenched conditions. The sheet specimens used in the experiments had a thickness of 1.2 mm, and all tests were performed in accordance with ASTM E8 standards [25]. The tests were carried out using a Shimadzu universal testing machine, model AG-100KNI M2. Strain measurement during the tensile tests was performed using an extensometer to ensure high accuracy in the determination of mechanical properties. The specimen geometry is illustrated in Figure 2a,b. The mechanical properties obtained from each rolling direction are summarized in

Table 2. Mechanical properties of boron steel 22MnB5 in each direction.

Mechanical Properties	Boron Steel Grade 22MnB5
	Before Quenching			After Quenching
	Direction (Degree)			Direction (Degree)
	0	45	90	0	45	90
Young’s modulus (GPa)	180	192	180	195	219	204
Tensile strength (MPa)	521	545	548	1680	1683	1717
Yield strength (MPa)	388	394	404	1268	1190	1243
r-values	0.69	0.74	0.92	0.56	0.79	0.60
n (strain hardening exponent)	0.216	0.265	0.283	0.252	0.249	0.320

. Furthermore, the engineering stress–strain curves before and after quenching and the true stress–strain after quenching in different rolling directions are shown in Figure 3a,b, respectively.

As shown in Figure 3 and Table 2, the significantly higher tensile strength observed in the 22MnB5 samples after quenching can be attributed to the transformation of the microstructure into a fully martensitic phase. Martensite is known for its high dislocation density and hardness, which directly contributes to increased tensile and yield strength. This transformation occurs due to the rapid cooling from 920 °C to room temperature, as performed in the quenching step.

The mechanical properties presented in Table 2 demonstrate a significant improvement in tensile and yield strengths after quenching, with tensile strength increasing from an average of approximately 538 MPa before quenching to over 1693 MPa post-quenching. This improvement confirms the formation of a martensitic microstructure, which enhances the material’s load-bearing capacity and crashworthiness, essential for automotive structural parts. In contrast, the r-values, which indicate the material’s resistance to thinning during plastic deformation, show a notable decrease after quenching. Before quenching, higher r-values, especially 0.92 at 90°, suggest good formability and favorable anisotropy for sheet forming operations. After quenching, the r-values drop, for example, to 0.56 at 0°, due to the increase in hardness and brittleness of the quenched martensitic phase. Although reduced, the remaining r-values are still within a usable range for hot stamping applications, where high strength is prioritized over formability.

2.3. Materials Model

As this research introduces a novel approach for manufacturing automotive oil pans using the indirect hot stamping method, it is essential that the finite element simulation in AutoForm R8 accurately replicates the two-stage forming process. Accordingly, the material model is divided into two phases. For the pre-forming stage conducted at room temperature, a cold-stamping material model is applied. In the second stage, the preformed part is heated to 920 °C and formed under hot-stamping conditions, requiring a high-temperature material model. This approach ensures that the simulation closely reflects real-world industrial conditions. The following sections provide detailed background and governing equations for the three material models used in this study.

2.3.1. The Cold-Stamping Materials Model

Yield Criterion

This research employs the Barlat1989 yield criterion [19] to model the anisotropic behavior of boron steel grade 22MnB5 sheet material. The Barlat1989 formulation incorporates r-values measured in three rolling directions, 0°, 45°, and 90°, enabling a more accurate representation of the material’s directional properties. When implemented within the forming simulation software, this yield criterion has demonstrated high predictive accuracy [19]. The mathematical representation of the Barlat1989 yield function is presented in Equation (1).

$$\mathrm{f}={\mathrm{a}|{\mathrm{k}}_1+{\mathrm{k}}_2|}^{\mathrm{M}}+{\mathrm{b}|{\mathrm{k}}_1+{\mathrm{k}}_2|}^{\mathrm{M}}+{\mathrm{c}\left|{2\mathrm{k}}_2\right|}^{\mathrm{M}}=2{\overline{\mathsf{\sigma }}}^{\mathrm{M}},$$

(1)

The parameters ${\mathrm{k}}_1$ and ${\mathrm{k}}_2$ can be calculated using Equations (2) and (3), respectively.

$${\mathrm{k}}_1={\displaystyle \frac{{\mathsf{\sigma }}_{\mathrm{x}\mathrm{x}}+\mathrm{h}{\mathsf{\sigma }}_{\mathrm{y}\mathrm{y}}}{2}},$$

(2)

$${\mathrm{k}}_2={\left[\left({\displaystyle \frac{{\mathsf{\sigma }}_{\mathrm{x}\mathrm{x}}-\mathrm{h}{\mathsf{\sigma }}_{\mathrm{y}\mathrm{y}}}{2}}\right)+{\mathrm{p}}^2{\mathsf{\sigma }}_{\mathrm{x}\mathrm{y}}^2\right]}^{{\displaystyle \frac{1}{2}}},$$

(3)

Parameters a, c, h, and p are also calculated from Equations (4)–(7).

$$\mathrm{a}=2-2\sqrt{{\displaystyle \frac{{\mathrm{r}}_0}{1+{\mathrm{r}}_0}}·{\displaystyle \frac{{\mathrm{r}}_{90}}{1+{\mathrm{r}}_{90}}}},$$

(4)

$$\mathrm{c}=2\sqrt{{\displaystyle \frac{{\mathrm{r}}_0}{1+{\mathrm{r}}_0}}·{\displaystyle \frac{{\mathrm{r}}_{90}}{1+{\mathrm{r}}_{90}}}},$$

(5)

$$\mathrm{h}=\sqrt{{\displaystyle \frac{{\mathrm{r}}_0}{1+{\mathrm{r}}_0}}·{\displaystyle \frac{1+{\mathrm{r}}_{90}}{{\mathrm{r}}_{90}}}},$$

(6)

$$\mathrm{p}={{\displaystyle \frac{\overline{\mathsf{\sigma }}}{{\mathsf{\tau }}_{\mathrm{s}}}}\left({\displaystyle \frac{2}{2\mathrm{a}+2^{\mathrm{M}}\mathrm{c}}}\right)}^{{\displaystyle \frac{1}{\mathrm{M}}}},$$

(7)

Hardening Law

In this research, the Swift hardening law [20] is adopted, as presented in Equation (8). This equation is selected because it effectively captures the stress–strain behavior of sheet metal materials [20], making it particularly well-suited for forming simulations. Its implementation contributes to simulation results that closely align with actual forming processes, enhancing the accuracy and reliability of the numerical analysis.

$$\mathsf{\sigma }=\mathrm{k}{\left({{\mathsf{\epsilon }}_0+\mathsf{\epsilon }}^{\mathrm{p}}\right)}^{\mathrm{n}},$$

(8)

where K is strengthening coefficient and n is the strain hardening exponent.

The value of n characterizes the true strain behavior. A higher n value signifies a greater force necessity for subsequent forming procedures.

Forming Limit Curves

In this paper, the Forming Limit Curves (FLC) proposed by Keeler in 1977 [21] are utilized. These curves are widely recognized and employed as a criterion to predict whether sheet metal materials will undergo damage during the forming process. The Keeler equation considers the work-hardening exponent (n) and sheet thickness (t) to determine FLC₀, which represents the lowest point on the FLC. The equation for FLC₀ is provided in Equation (9).

$${\mathrm{F}\mathrm{L}\mathrm{C}}_0=\mathrm{l}\mathrm{n}[1+\left({\displaystyle \frac{23.3+14.13\mathrm{t}}{100}}\right){\displaystyle \frac{\mathrm{n}}{0.21}}],$$

(9)

The left and right side of FLC as shown in Equations (10) and (11), respectively.

$${{\mathsf{\epsilon }}_1=\mathrm{F}\mathrm{L}\mathrm{C}}_0-{\mathsf{\epsilon }}_2,$$

(10)

$${{\mathsf{\epsilon }}_1=(1+\mathrm{F}\mathrm{L}\mathrm{C}}_0){\left(1+{\mathsf{\epsilon }}_2\right)}^{0.5}-1,$$

(11)

2.3.2. Material Modeling and Advanced Options in AutoForm R8 for Indirect Hot Stamping Method

In this study, AutoForm R8 was utilized to simulate the forming process of an automotive oil pan using the indirect hot stamping method. AutoForm R8 was chosen because it allows the forming process to be divided into two main stages, as identified in the research. Within the software, the indirect hot stamping method is further broken down into four key steps: (1) cold forming at room temperature (25 °C), (2) heating the blank to 920 °C for 10 min, (3) high-temperature forming at 920 °C, and (4) a quenching process to achieve a martensitic microstructure in the final process, as illustrated in Figure 4.

AutoForm R8 is capable of accurately simulating forming processes involving hot forming, as it is primarily designed with a focus on this type of manufacturing. Although the software does not explicitly provide a materials model specifically for hot forming (as these models are embedded within the system and not openly accessible), it offers a special feature that allows users to freely define the forming temperature at each stage, as illustrated in Figure 4. AutoForm R8 also includes a comprehensive material database for alloys commonly used in hot stamping processes. For instance, it provides stress–strain curves at various elevated temperatures.

In this study, data stress–strain curves for the boron steel grade 22MnB5 supplied by AutoForm engineers was used to simulate high-temperature forming up to 920 °C, allowing for a more accurate simulation of the oil pan component, as shown in Figure 5a. Another advanced feature available in AutoForm R8 for hot forming simulations is the phase transformation data of 22MnB5. This enables prediction of the resulting microstructure after different stages of the hot stamping process, as shown in Figure 5b.

3. Methodology

This study proposes a novel approach for producing automotive oil pan components with improved mechanical properties using ultra-high-strength boron steel (22MnB5). A two-step indirect hot stamping method combining cold and hot forming is introduced to achieve a martensitic microstructure. The process steps of this study are illustrated in Figure 6. As this method has not been previously applied, the research aims to identify optimal process conditions for implementation in Thailand’s automotive industry. ChatGPT -4o-mini was used to assist with English editing, with all suggestions carefully reviewed by the authors for accuracy.

3.1. Indirect Hot Stamping Method

The indirect hot stamping method, as shown in Figure 7, combined with the deep drawing process, is a new approach proposed by the researcher to overcome the forming challenges of automotive oil pan components. This process involves a combination of hot and cold forming, followed by hardening, to produce parts with a martensite phase microstructure, ensuring high strength and durability. Consequently, the forming process is divided into two main steps: 1. First step: indirect hot stamping method by cold stamping. 2. Second step: indirect hot stamping method by hot stamping. After completing these two main forming steps, the part must be quenched according to the continuous cooling transformation (CCT) diagrams of 22MnB5 to obtain a fully martensitic microstructure. Further details of these two steps and the quenching process are as follows:

3.1.1. First Step of Indirect Hot Stamping Method by Cold Stamping at Room Temperature

In the first step of forming the automotive part, the oil pan, using the indirect hot stamping method, the part undergoes cold stamping at room temperature, serving as a preliminary forming process. The objective is to shape the 22MnB5 metal sheet into a form that roughly approximates the height and overall shape of the final product. However, the detailed forming of sections is not yet performed at this stage. In this process, Mill Oil was used as a lubricant with a friction coefficient of 0.11, and a constant Blank Holder Force of 26.7 kN was applied. The result from Step 1 is the workpiece as shown in Figure 8a. In Figure 8b, the stress distribution map is presented, showing the stresses developed in the workpiece after the cold stamping process in this step. This stress map provides valuable insight into the stress state of the part during the initial forming stage.

3.1.2. Second Step of Indirect Process for Generating the Oil Pan by Hot Stamping at 920 Celsius

In this step, the part formed in Step 1 (first step of indirect hot stamping method by cold stamping at room temperature) is heated to 920 °C for 10 min to transform the microstructure into the Austenite phase. Afterward, the part is transferred to Die Set 2. In this forming process, the friction coefficient was set to 0.45, and a constant Blank Holder Force of 373.7 kN was applied. In this step, the final product with its complete shape is formed, as shown in Figure 9a. Figure 9b shows the strain distribution map, which illustrates the strain developed in the workpiece after the hot stamping process. This provides valuable information about the strain evolution during the forming process and complements the stress distribution shown in Figure 8b from the previous step.

3.1.3. Quenching Process After Indirect Hot Stamping Method

At this stage, the workpiece, which has undergone the indirect hot stamping process, will be subjected to quenching to reduce its temperature from 920 °C to an ambient temperature (around 25 °C). This will be done by referencing the cooling rate of boron steel grade 22MnB5 according to the continuous cooling transformation (CCT) curve of 22MnB5 [26], as shown in Figure 10. In this process, water mist is sprayed directly onto the surface of the heated part to rapidly reduce its temperature. This quenching method is employed to minimize distortion or warping that may occur during the cooling phase. It can be observed that the automotive part, the oil pan, must cool from 920 °C to 25 °C within 8–27 s to achieve a martensite phase microstructure with optimal mechanical properties. In this study, a critical cooling rate of 30 °C/s within approximately 27 s is applied to ensure that the microstructure of the formed automotive part after the indirect hot stamping process transforms into martensite.

3.2. Determination of Optimal Conditions for Automotive Component Formation via Indirect Hot Stamping Method

The researcher focused on factors that ensure successful part formation: (1) the shape and size of the initial blank and (2) the die shape and design to optimize forming variables, prevent defects, and reduce die tooling production time. These optimizations effectively reduce production costs. To determine the best forming conditions, the researcher conducted simulations using the AutoForm program, which specializes in optimizing the indirect hot stamping method.

3.2.1. Find the Optimal Initial Blank Size for Generating the Oil Pan

In this research, the optimal shape of the blank sheet was determined to ensure the successful forming of the oil pan automotive component. This was achieved through forming simulations using the AutoForm software. The researcher referenced various forming parameters from the conventional oil pan production process. However, the primary focus was on the shape and size of the initial blank sheet. The study was conducted as follows:

• The Shape of Initial Blank Size

Traditionally, the oil pan part is formed from a rectangular sheet metal blank called a “shear blank” (Figure 11a). The researcher experimented by altering the blank shape using reverse calculation from the AutoForm program, resulting in the “rough blank” (Figure 11b). Simulations were then conducted under the same forming conditions with both blank shapes to compare their impact on the formability of the oil pan part.

2.

Dimension of the Initial Blank Size

Another important factor the researcher focused on was determining the optimal size of the initial blank for successfully forming the automotive oil pan part. After selecting the curved, special “rough blank” shape, the blank size was varied to find the best fit for the forming process. The researcher then simulated the forming of the oil pan part under identical conditions using AutoForm, with the only variable being the initial blank size, to highlight its impact on the formability of the oil pan part, as shown in Figure 12.

3.2.2. Find the Optimal Die Shape for Generating the Oil Pan

After determining the appropriate shape and size of the initial blank for forming the automotive oil pan using the deep drawing process with the indirect hot stamping process, the next step was to design the die. Proper die design, especially adjusting the concave and curved sections, is crucial to prevent part damage such as tearing, cracking, or wrinkling. The researcher used AutoForm to simulate the forming process and optimize the die design by editing the die’s shape curve for both steps: (1) cold stamping in the first step and (2) hot stamping in the second step.

• Determine Optimal Die Shape for Initial Indirect Hot Stamping Step

Once the appropriate initial blank was obtained, a study was conducted to refine the critical curved sections of die impacting material formability. The shape of the cold-forming die’s curved sections was adjusted to find the optimal die design for flawless forming of the oil pan part in the first step, without any issues, as shown in Figure 13.

2.

Determine Optimal Die Shape for Second Indirect Hot Stamping Step

A study was conducted to refine the critical curved sections affecting the formability of the sheet metal in areas h1 and h2 of the die in Step 2 of the indirect hot stamping process. These areas, which resemble the final, more detailed and complex workpiece, were optimized to ensure the oil pan part could be fully and flawlessly formed in the second step, without any issues. The areas of interest, h1 and h2, are shown in Figure 14.

3.3. Generate Oil Pan Die from AutoForm Simulation

The die was designed in the AutoForm program as a 3D die set (CAD file) to serve as the actual die tooling for forming the oil pan part using the deep drawing process. This die set aims to validate the feasibility of forming the oil pan from boron steel grade 22MnB5 sheet metal using the new indirect hot stamping process.

3.4. Generate Oil Pan Based on AutoForm Simulation Conditions Using Indirect Hot Stamping Method

Due to limited equipment and budget, a 1:2 scale die set (Half-Side Tool) was created. Two die sets were constructed using S45C cast steel (UPR and LWR Die) and surface hardened. The process to produce the oil pan part was carried out using boron steel grade 22MnB5 (1.2 mm thickness) and the indirect hot stamping method.

In the first step, cold stamping was performed at room temperature. The part was then heated to 920 °C for 10 min to convert its microstructure to the Austenite phase. In the second step, the oil pan was formed through hot stamping at 920 °C. Finally, the part was quenched to harden the structure and achieve a martensite microstructure. The designed die set was then installed for industrial-scale manufacturing.

3.5. Investigate Oil Pan Characterization and Percent Thinning After Indirect Hot Stamping Method

The investigation aimed to determine if the microstructure of the oil pan, formed using the indirect hot stamping method, was martensite. The microstructure, hardness, and percent thinning of the oil pan were examined after forming to ensure they met the standards for automotive parts in Thailand. The following steps and standards were applied.

3.5.1. Oil Pan Microstructure After Indirect Hot Stamping Method

A comparative microstructure analysis of the boron steel grade 22MnB5 sheet material before and after the indirect hot stamping method was conducted using cross-sectional specimens prepared according to ASTM A131 [27], as shown in Figure 15. This analysis aimed to verify whether the oil pan, produced from boron steel grade 22MnB5, has a martensite microstructure after forming through the deep drawing process with the indirect hot stamping method.

3.5.2. Oil Pan Hardness After Indirect Hot Stamping Method

The hardness of the automotive oil pan part after the indirect hot stamping method was checked to determine if the microstructure is in the martensite phase. The test was conducted according to ASTM E92 [28] “Standard Test Methods for Vickers Hardness of Metallic Materials.” A portion of the part was cut and prepared according to ASTM E92 standards to check if the hardness value exceeds 500 HV [29], as the martensite phase should have a hardness value greater than 500 HV.

3.5.3. Oil Pan Percent Thinning After Indirect Hot Stamping Method

The percent thinning of the oil pan, made from boron steel grade 22MnB5 and formed using the indirect hot stamping method, was investigated along three lines (A, B, and C), as shown in Figure 16. This ensured that the oil pan’s percent thinning did not exceed 20%, meeting the standards of the automotive parts industry in Thailand and customer specifications. Additionally, a comparison of the percent thinning was made between the AutoForm simulation results and the actual forming process to verify the accuracy and precision of the simulation and finite element method.

4. Results of Indirect Hot Stamping Process

4.1. Results of Optimal Conditions for Automotive Component Formation Using Indirect Hot Stamping Method

4.1.1. The Best Initial Blank Size for Generated the Oil Pan

From the simulation of the forming process of the oil pan automotive part in the AutoForm program, in order to determine the most suitable shape and size of the initial blank to be used, the following study results are presented.

• The Shape of Initial Blank Size

The simulation results of the forming process of the oil pan automotive part in the AutoForm program using the initial “shear blank” are shown in Figure 17a, and the “rough blank” is shown in Figure 17b.

Figure 17a,b show that using the “rough blank” as the initial blank resulted in better outcomes and a higher likelihood of successfully forming the oil pan compared to the “shear blank.” Although the rough blank presented some forming challenges, it showed a tendency toward successful formation. Further studies were conducted to investigate other factors to ensure successful formation in the next phase.

2.

Dimension of the Initial Blank Size

The forming simulation for the oil pan was conducted using an initial “rough blank.” The blank size was optimized to achieve the best forming outcome with minimal tearing. The simulations were performed under identical conditions using AutoForm, with the only difference being the blank size. The results indicated that the best outcome was achieved with an initial blank size of 373.50 mm × 449.4 mm, as shown in Figure 18.

4.1.2. Optimal Die Shape for Generating Oil Pan

After studying the initial blank shape and size for forming the oil pan, it was found that the AutoForm simulation still encountered issues like cracking and wrinkling. Adjustments were made to the die curvature in two sets of dies: (1) to optimize the die shape for the first step of the indirect hot stamping process and (2) to optimize the die shape for the second step. These adjustments aimed to resolve the issues and improve the forming process.

• Find Optimal Die Shape for First Step of Indirect Hot Stamping Process

The study and adjustments made to the die for forming the oil pan part in Step 1 of the indirect hot stamping process focused on optimizing the curvature of the concave sections. Based on AutoForm simulation results, the most effective modification was to increase the wall curvature by +3.0 mm, as shown in Figure 19a. The blue line represents the original die curvature before modification, while the green line indicates the modified geometry after optimization. These curves were used to evaluate how changes in die geometry influence material flow during forming. Figure 19b shows the final die geometry after the curve shape editing process.

2.

Find Optimal Die Shape for Second Step of Indirect Hot Stamping Process

The die modifications for forming the oil pan part at positions h1 and h2 in Step 2 of the indirect hot stamping process were aimed at optimizing the curvature to improve material flow and minimize forming defects. AutoForm simulation results identified the optimal adjustments, as illustrated in Figure 20a. The blue line denotes the original die curvature before modification, while the green line represents the revised die geometry. At position h1, the curvature of the wall in the deep-drawn section was increased to promote better material draw-in, whereas at position h2, the curvature was decreased by 3.0 mm to reduce stress concentration and material thinning. Figure 20b shows the final die geometry after these optimizations were applied. These modifications collectively contributed to better formability, improved thickness distribution, and fewer surface defects during the hot stamping stage.

4.2. Results Die of Oil Pan from Simulation in AutoForm Program

After designing the die set for forming the oil pan using the indirect hot stamping process, the main structure was made from cast steel grade S45C (UPR and LWR Die). The punch and die insert, replaceable during experiments, were made from tool steel grade DCMX after heat treatment. The deep drawing process was performed using a 600-ton hydraulic press machine with a cushion post for the blank holder. The die set was designed in CAD format (Figure 21). Due to equipment and budget limitations, a scaled-down die set (1:2 ratio) was fabricated (Figure 22).

4.3. Generate Oil Pan Based on AutoForm Simulation Conditions Using Indirect Hot Stamping Method

The oil pan component was produced using boron steel grade 22MnB5 sheet metal with a width of 373.50 mm, a length of 449.4 mm, and a thickness of 1.2 mm. The initial blank shape was a rough blank, and the indirect hot stamping method was applied. In Step 1, cold stamping was performed at room temperature. Mill Oil was used as a lubricant in this step, with a friction coefficient of 0.11, and a constant Blank Holder Force of 26.7 kN was applied. The part was then heated to 920 °C for 10 min to transform the microstructure to Austenite. In Step 2, hot stamping was carried out at 920 °C, followed by rapid quenching to achieve a martensite microstructure. In this step, the friction coefficient was set to 0.45, and a constant Blank Holder Force of 373.7 kN was applied. The temperature of the die used in the hot forming process was maintained at 600 °C to ensure proper forming and quenching conditions. The die set and final oil pan component are shown in Figure 23 and Figure 24, respectively.

4.4. Investigate Oil Pan Characterization and Percent Thinning After Indirect Hot Stamping Method

4.4.1. Microstructure of Oil Pan After Indirect Hot Stamping Method

The results of the microstructural analysis of the boron steel grade 22MnB5 sheet metal before and after undergoing the “indirect hot stamping method” in a cross-sectional view are shown in Figure 25a,b, respectively.

From Figure 25a,b, it can be observed that the boron steel grade 22MnB5 sheet metal is coated with a thin alumina–silica layer to prevent rust formation. This coating, applied at the micron level, does not affect the formability of the sheet metal. Before quenching, the microstructure of the 22MnB5 boron steel primarily consisted of ferrite (white zones) and pearlite (black zones), as shown in Figure 25a. After the forming process using the indirect hot stamping method, which involves rapid quenching from 920 °C to room temperature, the microstructure transformed predominantly into a martensitic phase. This martensite exhibits a fine needle-like morphology beneath the alumina–silica coating across the entire surface, as shown in Figure 25b.

The martensitic laths display an average width of approximately 0.8–1.2 μm and lengths ranging from 4 to 10 μm, indicating a relatively fine martensitic structure. This morphology is consistent with the measured Vickers hardness of approximately 550 HV, as reported in previous studies, which suggest that narrower martensitic laths (less than 1.5 μm in width) are associated with higher hardness due to increased dislocation density and internal stress [30,31].

4.4.2. Hardness of Oil Pan After Indirect Hot Stamping Method

The hardness of the automotive oil pan part after the indirect hot stamping process was tested in accordance with ASTM E92 using an INNOVATEST Vickers hardness testing machine. The test was performed by applying a square-based pyramid indenter onto the polished surface of the specimen, as shown in Figure 26a. The measured hardness value was 550.63 HV0.5, as illustrated in Figure 26b. Based on the hardness value of approximately 550 HV, and according to the reference data reported in [29], it can be confirmed that the microstructure of the formed part is predominantly martensitic.

The hardness value measured from the automotive oil pan part after the indirect hot stamping method was 550.63 HV0.5, which corresponds to the hardness of the martensite phase [26]. This indicates that, under the conditions studied by the researcher, forming the boron steel sheet (grade 22MnB5) results in a martensite microstructure after processing.

4.4.3. Oil Pan Percent Thinning After Indirect Hot Stamping Method

The results of the percent thinning inspection of the oil pan made from boron steel grade 22MnB5 after undergoing the deep drawing process with the indirect hot stamping method were evaluated along three longitudinal lines (A, B, and C) of the oil pan, as shown in Figure 27. The percent thinning values obtained were compared with the standard criteria for automotive component manufacturing set by the Thai automotive industry and the customer’s specifications. It is generally required that percent thinning should not exceed 20% to ensure the structural integrity and performance of automotive sheet metal parts [32]. The comparison of percent thinning between the simulated forming process in the AutoForm software, the actual industrial forming process, and the standard automotive criteria is shown in

Table 3. Comparison of percent thinning results along Cross-Sections A, B, and C between forming simulation, experimental results, and standard automotive criteria.

Section		Thinning
		1	2	3	4	5	6	7	8	%Maximum
A	FEM	0.182	−0.025	−0.052	−0.052	−0.057	−0.032	−0.034	−0.134	15.17%
	Experimental	0.190	−0.027	−0.054	−0.050	−0.050	−0.054	−0.029	−0.021	15.83%
	Customer	<20% (Maximum 0.240)								-
B	FEM	0.015	−0.094	−0.158	−0.096	−0.103	−0.095	−0.032	0.177	14.75%
	Experimental	0.020	−0.084	−0.150	−0.089	−0.154	−0.070	−0.041	0.190	15.83%
	Customer	<20% (Maximum 0.240)								-
C	FEM	−0.015	0.010	−0.120	−0.216	−0.131	−0.097	−0.035	0.192	18.00%
	Experimental	−0.029	0.018	−0.129	−0.190	−0.139	−0.210	−0.050	0.170	15.83%
	Customer	<20% (Maximum 0.240)								-

.

5. Conclusions

This research successfully developed a forming process for an automotive oil pan made of 22MnB5 boron steel using the indirect hot stamping method, which integrates cold stamping, hot forming at 920 °C, and rapid water spray quenching. The approach yielded a fully martensitic microstructure, enhancing the mechanical strength. The forming simulation using AutoForm R8, with temperature- and phase-dependent material properties, accurately predicted the thinning, final geometry, and microstructure. The use of a rough blank size of 373.50 mm × 449.40 mm ensured defect-free forming without tearing or wrinkling. The process simulation and actual experimental trials showed close agreement, with maximum thinning not exceeding 18%, confirming industrial suitability. Microstructure examination validated the presence of martensite in accordance with ASTM A131, while hardness measurements reached 550.63 HV0.5, in line with the martensitic phase per ASTM E92. Overall, this study demonstrated the feasibility of using indirect hot stamping for complex UHSS components in the automotive industry. The method not only ensures high mechanical performance but also confirms the accuracy of simulation tools in supporting advanced forming process design.