roll forming process of impact beam for automobile using ... forming process of impact beam for...

Roll forming process of impact beam for automobile using advanced high-strength steel (AHSS)

1Tae-hyeon Park, 2Jae-Ho Jang, and *3Key-Sun Kim

1, Mechanical Engineering Dept., Graduate School, Kongju National University, Choenan, Korea, [email protected]

2.Div. of Mechanical & Automotive Engineering, Kongju National University, Choenan, Korea, [email protected]

3* Corresponding Author, Div. of Mechanical & Automotive Engineering, Kongju National University, Choenan, Korea, [email protected]

Abstract

Roll forming is generally a plastic working process where sheet metal coiled around the roll passes through rolls and dies in dozens of successive steps to undergo bending deformation gradually and produce the final cross-sectional shape. The side impact bar of automobile door is conventionally formed from structural steel with the tensile strength below 600 MPa through the process of roll forming and then strengthened by hardening in heat treatment furnace. The drawback of this process is the prolonged process time because of the inoperability of successive process. This paper proposes the production technique that shapes the advanced high-strength steel (AHSS) with the grade of 1500 MPa directly during roll forming to circumvent the strengthening heat treatment process. In this process, the high strength and elasticity of AHSS adversely affect its formability. To address this problem, cross-section and design parameters of product are optimized analytically, which was designed and fabricated for experiment. Experiments were also conducted and compared with experimental and analysis results. The analysis results in this study can be applied as basic technical data for the production of AHSS-based impact beam.

Keywords: Impact beam, Plasticity, Roll forming, Advanced high-strength steel (AHSS)

1. Introduction

Car crashes increase in number, and ensuring vehicle safety at the time of collision is of vital importance. For the front of the vehicle, an acceptable safety level is assured, but most of the impact on side doors is absorbed by an impact beam [1].

Side impact bars for automobile door are conventionally shaped from structural carbon steel with a tensile strength of below 600 MPa by the roll forming process and then strengthened by hardening treatment in a heat treatment furnace. The drawback of this process is the prolonged process time because of the inoperability of successive processes [2].

Recent research on improving the product safety and reducing the weight has focused on using high-strength steel (HSS) in the roll forming process to circumvent the need for the strengthening heat treatment process [3]. However, plastic deformation of high-strength material is difficult because of the high strength of the material and the frequent occurrence of spring-back after bending during roll forming, which makes fabrication of the final product a highly challenging task [4-5]. To overcome this technical challenge, determine the optimum shape of an automobile side impact beam using 1500 MPa grade ultra light material through numerical analysis and bending plastic analysis to establish product design parameters. Then designed and fabricated a die mold and compared the analysis and experimental results. Compared impact beams produced by using the proposed and conventional methods in terms of the safety and weight reduction. 2. Structural analysis for cross-section Fig. 1 shows the structure of an impact beam mounted on a car door. We proposed three models (models 1–3) with different shapes but the same cross-sectional area and material thickness. A 2.0-mm-

Roll forming process of impact beam for automobile using advanced high-strength steel (AHSS) Tae-hyeon Park, Jae-Ho Jang, and Key-Sun Kim

International Journal of Digital Content Technology and its Applications(JDCTA) Volume7, Number13, Sep 2013

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thick conventional circular model (model 4) was also tested. The 4 models are outlined in Table 1. To determine the optimal cross-section of the proposed models, numerical bending analysis was performed using the commercial software ANSYS. Each proposed model was uniformly modeled with a thickness of 1.5 mm, cross-sectional area of 1.639e-4 m2, and beam length of 900 mm. Used 1500 MPa grade material; the mechanical properties are summarized in Table 2. The shape-dependent strengths of the impact beams were compared using three-point bending analysis. The analysis conditions shown in Fig. 2 are as follows. A product-shaped support jig was fixed at two points, each 200 mm from the center, and a forced displacement was induced by placing a punch of 150 mm at the center of the impact beam. To create the same conditions as in the experiment, the z- and x-axes on two central lines inside the beam were fixed; the y-axis was also fixed to prevent slipping, and two points at the beam center were fixed along the x- and y-axes. Under the same conditions, models 1–3 were then subjected to bending stress applied at the center point of the beam, where the maximum deformation occurred. The bending stress was continuously increased from 1 MPa to 1950 MPa, and the bending load and deformation were calculated. The test results were compared with those from nonlinear structural analysis. Fig. 3 shows the schematic of the deformation analysis results when the bending load was applied to the proposed models. As shown in the graphs, model 1 showed the maximum bending load at 17.3 kN, and the corresponding maximum deformation was 17 mm; for model 2, these values were 18.6 kN and 16 mm, respectively.

The force reactions that induced the maximum deformation in models 2 and 3 were 18.6 and 18.5 kN, respectively, and showed a similar tendency. In other words, the maximum force reaction inducing the maximum deformation was about 20 % higher in models 2 and 3 compared to model 1. Given that all three proposed models had the same cross-sectional area, this implies an improved bending strength. Thus, models 2 and 3 showed similar performances; however, owing to its greater ease of production, model 3 was chosen as the final study model.

Figure 1. Door assembly with impact beam Figure 2. Boundary conditions of bending analysis

Table 1. Cross-section of impact beam

Items Proposed models (AHSS)

Conventional model (STEEL)

Model 1 Model 2 Model 3 Model 4

Cross-sectional shape

Heat treatment No heat treatment Strengthening heat

treatment after production

Cross-sectional area [ ]

1.639 1.639 1.639 1.872


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Length [mm] 900 900 900 900

Thickness [mm] 1.5 1.5 1.5 2.0

Table 2. Mechanical properties of material

Type Models 1–3 Model 4

Yield strength [MPa] 1292 950

Ultimate tensile strength [MPa] 1524 1200

Elongation [%] 4 14

Hardness [Hv] 470 400

Figure 3. Force-reaction graph of bending analysis

3. Plastic analysis of beam On the basis of the analysis results mentioned above, models 3 and 4 were subjected to bending and collision analysis and compared. To reduce the weight, the proposed model 3 had a material thickness of 1.5 mm, while the conventional model 4 was 2 mm thick. Model 3 used a sheet made of raw steel, and model 4 used a material that was heat-treated after production. The commercial code ANSYS was used for bending analysis, and the bending analysis conditions are shown in Fig. 2. A support jig was fixed at two points on the 900-mm-long beam, each 200 mm from the center, and a forced displacement was induced by placing a punch of 150 mm at the center.

The graphs in Fig. 4 show the deformation of the beam in relation to the force reaction occurring at the center region. The force reaction inducing the maximum stress for model 4 was 16 kN, whereas the force reaction for model 3 was 18 kN; thus, the proposed model demonstrated an increase of 14.17%. A comparison of the maximum deformations showed 24 and 16 mm for models 4 and 3, respectively; thus, the proposed model demonstrated considerably smaller deformation. Model 3 was thus confirmed to have improved properties in terms of maximum stress and deformation.

Collision analysis was performed to predict the deformation result at the time of collision under the conditions shown in Fig. 5: support brackets were fixed at both ends of the impact beam, a 1000 kg heavy barrier was placed at the center, and a shock was applied at a velocity of 60 km/h. The analysis time was set to 0.001 s, and the separation of the welded brackets by the collision was considered [6]. ANSYS Explicit Dynamics was used for the collision analysis. Fig. 6 is a schematic of the deformation results from the collision analysis; the resulting values are listed in Table 3. The collision analysis results revealed that the AHSS-based proposed model incurred a 15.968 mm deformation, which is a decrease of 3.33% compared to the 16.5 mm deformation of the conventional model. A substantial


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difference was observed in the maximum stress: 374.23 vs. 1248.9 MPa for the conventional and proposed models, respectively.

Figure 4. Graph of force reaction in bending analysis

Figure 5. Analysis conditions

Figure 6. Deformation at collision

Table 3. Result of collision analysis

Items Model 3 Model 4

Deformation [mm] 15.968 16.5

Max stress [MPa] 1248.9 374.23


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4. Design and manufacturing by roll forming process The proposed model was fabricated from 1500 MPa grade AHSS. This material shows a strong spring-back tendency when being formed; this was taken into consideration for the design of the successive bending processes [7]. First, a high-tensile design was provided for cassettes and stations to resist the plastic load of the 1500 MPa grade material, and a side roller jig was added between the roller stands to avoid overstressing the machine when it is bending the product. With regard to the rigidity of the product, the bottom edge groove of the product was redesigned to induce 1.2-fold over-bending compared to the design size, and the number of stations was increased from the conventional 12 processes to 18 by segmenting the bending forming stations for accommodating the increased elastic modulus of the 1500 MPa grade material [8]. The production steps of the roll forming process shown in Fig. 7 can be described as follows. The raw material in a coiled state was placed into alignment equipment and passed through multiple stations to form the cross-sectional shape; it was subjected to side rolling and formed into the final shape. Fig. 8(a) shows the step-by-step simulation process based on these steps [9]. The completed impact beam shape was then successively welded using tungsten inert gas (TIG) welding. Beads were removed and cut followed by bending and cutting to form the product shape. Fig. 8(b) shows the final product.

Figure 7. roll forming process

(a) Flower diagram of roll forming (b) Workpiece for impact beam

Figure 8. Flow diagram of roll forming process and workpiece


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5. Experimental results and discussion The performance of the manufactured impact beam was tested with a three-point bending tester, as shown in Fig. 10. The experimental conditions were the same as the analysis conditions. Fig. 11 compares the analysis and experimental results. The comparison revealed similar tendencies in general; both models 3 and 4 tended to incur slight errors. In the case of model 3, this may be due to the friction errors at the point of support during slipping and bending of the test piece during the experiment when the initial load was applied. For model 4, the errors might have occurred because the changes material properties of the product after heat treatment were reflected in the analysis conditions. The deformations of the two models are compared in Fig. 12. Model 4 incurred a 29 mm deformation under a maximum load of 16.52 kN, and model 3 incurred a 22 mm deformation under a maximum load of 17.9 kN; thus, model 3 incurred less deformation under a stronger load. When the maximum bending load for model 4 of 16.7 kN was applied to model 3, the maximum deformation was 15 mm compared to the 29 mm incurred by model 4. Thus, despite model 3 having a reduced steel thickness of 1.5 mm and lack of heat treatment, the deformation was reduced by 48%. The spring-back, which occurs when the load is removed after inducing forced deformation, was measured to compare the elastic modulus during the process. The measurements yielded similar results for models 3 (12.7 mm) and 4 (11.9 mm), which suggests that model 3 possesses sufficient elasticity despite not being heat-treated.

To investigate the welding-associated changes in the hardness of the welding surface and material component, the welded part was subjected to a hardness test clockwise from its center and analyzed using energy dispersive X-ray spectroscopy (EDS). Fig. 13 shows the measurement results from the micro vickers hardness tester. The hardness test results revealed that both models tended to decrease in hardness at the heat-affected zone and then increase, although the size of the affected area was not large enough to exert any significant influence. Model 4 showed an average hardness of 400 Hv, and model 3 showed a hardness of 470 Hv. Fig. 14 shows the EDS results for the component changes in the welded parts. Neither the welded area nor base metal zone showed any differences in the major components of C, Fe, and Mn beyond the error range of the EDS equipment; this suggests that welding did not significantly influence the experiment.

Figure 10. Three-point bending test

Model 3 Model 4


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(a) Model 3 (b) Model 4

Figure 11. Analysis and experimental comparison

Figure 12. Comparison of impact beam deformation

Table 7. Results of three-point bending test

Items Model 3 Model 4

Note Analysis Analysis Analysis Experiment

Load [N] 17902.93 18517 16527.93 16988

Deformation [mm] 22.619 16 29.6522 24

Spring-back [mm] - 12.725 - 11.881

Model 4 Model 3


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Figure 13. Graph of hardness test on welding zone

Figure 14. Result of EDS

4. Conclusion In the present study, developed a method of shaping a car door impact beam from 1500 MPa grade ultrahigh tensile strength steel sheets using a direct roll forming process and tested the performance. The test results are as follows.

First, determined the optimal shape of the car door impact beam through structural analysis and fabricated a prototype. To evaluate its performance, performed a bending test, collision analysis, and strength measurement. The results confirmed that the proposed model (model 3) showed a 48% reduction in deformation despite a reduced steel thickness from 2.0 mm to 1.5 mm and lack of heat treatment. Moreover, the strength increased by approximately 14.17%, and sufficient elasticity was demonstrated; thus, the performance of model 3 was improved compared to that of the conventional model 4 despite not being heat-treated. These results can serve as basic technical data for manufacturing impact beams using ultrahigh tensile strength steel. 10. References [1] Kee Joo Kim, Chang Pyung Han, Jong Han Lim, Young-Suk Lee, Si-Tae Won, Jae-Woong Lee,

“Light-weight Design and Simulation of Automotive Rear bumper Impact Beam Using Boron

Model 3 Model 4


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Steels”, Journal of The Korean Society of Automotive Engineers, KSAE, vol. 20, no. 2, pp98-102, 2012

[2] Hyun-Woo Lee, Jung-Bok Hwang, Sun-Ung Kim, Won-Hyuck Kim, Seung-Jo Yoo, Hyun-Woo Lim, Young-Jin Yum, “Construction of Vehicle Door Impact Beam using Hot Stamping Technology”, Journal of The Korean Society of Mechanical Engineers, KSME, vol. 34, no. 6, pp.797-803, 2010

[3] Dong Won Jung, Dong Hong Kim, Bong Chun Kim, “A Study on Development of Automotive Panel of Bumper Reinforcement with High Strenth Steel Using Roll Forming Process”, Journal of The Korean Society for Precision Engineering, KSPE, vol.29, no. 8, pp.840-844, 2012

[4] Hee-jin Son, Sung-yuk Kim, Beom-seok Oh, Key-sun Kim “Development of vehicle Door Side Impact Beam with High tensile Steel using Roll Forming Process”, Journal of The Korean Society of Manufacturing Process Engineers, KSTP, vol. 11, no. 6, pp.82-87, 2012.

[5] Dutton, Trevor Edwards, Richard Blowey, Andrew, “Springback Prediction and Compensation for a High Strength Steel Side Impact Beam.”, AIP Conference Proceedings, AIP, Vol. 778, p340-344, 2005

[6] Seil Song, Ikrae Cha, Kwon hee Lee, Gyung jin Park “Optimization of The automotive side Door Impact Beam Considering static Requirement”, Journal of The Korean Society of Automotive Engineers, KSAE, vol. 10, no. 3, pp.176-184, 2002

[7] Cai, Z. Li, M., "Mechanical mechanism of continuous roll forming for three-dimensional surface parts and the calculation of bending deformation", Journal of Chinese Journal of Mechanical Engineering, CJME, vol.49, no.2, pp.35-41, 2013

[8] Dong-Won Jung, Sang-Hu Park, Ji-Hyun Jeong, “Forming analysis for Optimization of 18 stage Roll Forming Process”, Journal of the Korean Society for Power System Engineering, KSPSE, vol. 17, no. 3, pp. 65-71, 2013

[9] Yong Kuk Park, “Study on Drawing Analysis of an Automotive Front Door and Stamping Die Manufacturing Process”, Journal of the Korean Society for Technology of Plasticity, KSTP, vol. 7, no. 6, 1998


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