TRIP: Transformation Induced Plasticity; HSLA: High Strength Low Alloy; DP: Dual-Phase; BH: BakeHardened

The automotive industry is currently under considerable pressure to adapt to environmental laws or standards to reduce emissions and increase the efficiency of their vehicles. Engineers are finding ways to reduce the amount of steel used in the vehicle to reduce the weight, while still maintaining the engineering and safety requirements. The main problem in the automotive industry is the effect of elastic recovery or the springback. Elastic recovery or springback is when the metal slightly reverts to its original shape after being formed. This becomes a problem when a part must be formed to a specific geometry that is essential to the safety and engineering standards requirements. Springback is a common phenomenon within metals such as steels, aluminum, etc. The springback depends on the initial Young’s modulus, and the amount of prestrain applied to the material in plastic deformation. Normally in the automotive industry mild steel is used for the bodies of vehicles, due to its high formability. The problem with this steel is that it exhibits low strength, which makes it limited in other applications. Most researchers conduct experiments on AHSS steels such as DP and TRIP for their high strength and high ductility compared to conventional steels. However, AHSS steels are limited in the automotive industry due to the challenges in forming, tool life, sheet metal coupling, and springback behavior. The springback is the main problem with AHSS steels, which compromises the mass-production of automotive structural components, Nikhare et al. [1-2]. Springback is affected by the microstructure of retained austenite and retained martensite, which affects the young’s modulus of steels Nikhare et al. [1]. Understanding the effects of young’s modulus can alter the springback, and other mechanical properties in steels.

Young’s modulus as stated in the introduction, is the measure of stiffness of the material when subjected to a stress. This is shown from subsequent loading/unloading tests in plastic region which characterizes the Young’s modulus behavior. As the AHSS steel becomes strained, the Young’s modulus degrades, until it reaches the saturation point, where it remains constant throughout further straining [1-10]. The orientation of the steel has a significant role in the degradation of the elastic modulus. The three common rolling directions of the steel are 0°, 45°, and 90° to the rolling direction Figure 1. These orientations show different non-linear paths of elastic modulus degradation, which can be fitted by an average elastic modulus (Eav) by using equation 1, Nikhare et al. [1].

Where E0 is the initial young’s modulus; εp is the prestrain value; ESAT is the saturated young’s modulus; and ξ is the material parameters indicating the modulus decay rate Figure 2. Various steels show change in Young’s modulus upon loading/ unloading as shown in Table 1. TRIP (Transformation induced plasticity), HSLA (High Strength Low Alloy), DP (dual-phase), and BH (bake-hardened). The difference in hat treatments of the steel, changes the behavior of the mechanical properties [1, 3-6].

Comparing the two metals DP780 and DP1000, it is observed that the DP780 steel has a smaller elastic recovery than the DP1000, Julsri et al. [5]. The elastic modulus decreases in a nonlinear fashion, when the steel is subjected to strain, Halilovič et al. [7]. Pre-straining the steel increases the rate of degradation of young’s modulus when the steel is subjected to strain [4,8]. Prestraining the steel perpendicular to the rolling direction, causes the young’s modulus to degrade at a greater rate [7,8]. It is also important to note that the degradation is also greater when the steel is not oriented in the rolling direction, meaning it has the most significant amount of degradation when the rolling direction is 90°, Panin et al. [9]. It has been reported by Cobo et al. [10], that the reduction in young’s modulus decreases with respect to decreasing the ferrite volume fraction. This is due to the dislocation movement within body dentered cubic ferrite structures is easier than in body centered tetragonal martensite structures. Consequently, the local micro-plastic strain is in smaller quantities when the steel has a greater ferrite volume fraction, but the higher martensite volume in the microstructure will generate a higher plastic micro-strain Nikhare et al. [1].

In this mini review, several effects are covered that change the young’s modulus degradation in steels. A table has been constructed to show the comparison of the Young’s modulus degradation in various types of steels. In short, Young’s modulus degrades more significantly with respect to pre-strain. The rolling direction orientation affects the young’s modulus degradation. It also decreases as ferrite volume fraction decreases or when the austenite volume fraction increases. It is recommended to characterize the AHSS material and model the material behavior accordingly to predict the springback in the desired part.

Authors would like to thank Penn State Erie, The Behrend College for support provided to perform this research.

1. Lajarin SF, Nikhare CP, Marcondes PVP (2018) Dependence of plastic strain and microstructure on elastic modulus reduction in advanced high-strength steels. Journal of the Brazilian Society of Mechanical Sciences and Engineering 40(2): 87.
2. Lajarin SF, Chemin Filho RA, Rebeyka CJ, Nikhare CP, Marcondes PVP (2020) Numerical study on variation of chord modulus on the springback of high-strength steels. The International Journal of Advanced Manufacturing Technology 106(11): 4707-4713.
3. Kim H, Kim C, Barlat F, Pavlina E, Lee MG (2013) Nonlinear elastic behaviors of low and high strength steels in unloading and reloading. Materials Science and Engineering: A 562: 161-171.
4. Dai HL, Jiang HJ, Dai T, Xu WL, Luo AH (2017) Investigation on the influence of damage to springback of U-shape HSLA steel plates. Journal of Alloys and Compounds 708: 575-586.
5. Julsri W, Suranuntchai S, Uthaisangsuk V (2018) Study of springback effect of AHS steels using a microstructure based modeling. International Journal of Mechanical Sciences 135: 499-516.
6. Mendiguren J, Cortés F, Galdos L, Berveiller S (2013) Strain path′s influence on the elastic behaviour of the TRIP 700 steel. Materials Science and Engineering: A 560: 433-438.
7. Vrh M, Halilovič M, Štok B (2011) The evolution of effective elastic properties of a cold formed stainless steel sheet. Experimental Mechanics 51(5): 677-695.
8. Luo L, Ghosh AK (2003) Elastic and inelastic recovery after plastic deformation of DQSK steel sheet. J Eng Mater Technol 125(3): 237-246.
9. Panin VE, Derevyagina LS, Panin SV, Shugurov AR, Gordienko AI (2019) The role of nanoscale strain-induced defects in the sharp increase of low-temperature toughness in low-carbon and low-alloy steels. Materials Science and Engineering: A 768: 138491.
10. Cobo Molina R, Pla M, Hernández Rossi R, Benito Páramo JA (2009) Analysis of the decrease of the apparent young's modulus of advanced high strength steels and its effect in bending simulations. In IDDRG 2009 international conference. pp. 109-117.