Acta Polytechnica Acta Polytechnica 53(4):384–387, 2013 © Czech Technical University in Prague, 2013 available online at http://ctn.cvut.cz/ap/ THE EFFECT OF STRAIN RATE ON THE MECHANICAL PROPERTIES OF AUTOMOTIVE STEEL SHEETS Miroslav Német∗, Mária Mihaliková Technical University of Košice, Faculty of Metallurgy, Department of Materials Science, Slovakia ∗ corresponding author: miroslav.nemet@tuke.sk Abstract. The automotive industry is currently seeking detailed information about various types of materials and their behavior under dynamic loading. Dynamic tensile testing of sheet steels is growing in importance. The experimental dynamic tensile technique depends on the strain rate. Each type of test serves for a specific range of strain rates, and provides specific types of information. This work deals with the influence of the strain rate on the mechanical properties of automotive steel sheets. Three different types of steel: IF steel, DP steel, and micro-alloyed steel (S 460) were used to compare static and dynamic properties. Keywords: dynamic tensile test, automotive steel sheets, rotation wheel. 1. Introduction An improved understanding of the behaviour of au- tomotive materials at high velocity is driven by the challenges of various types of crash legislation and by competition among car manufacturers. The strength of a sheet steel product is dependent on the speed at which it is deformed. It is well known that the yield strength and the ultimate tensile strength of materials are determined by the behavior of dislocations, and these depend on the pre-history of the loading and on the strain rate. For FCC metals, at low strain rates, the true stress increases linearly with the logarithm of the strain rate. At high strain rates exceeding 103 s−1, the true stress increases approximately linearly with the strain rate [1, 2]. The mechanical behavior of ma- terials under dynamic or impact loading is different from that under static loading. When a structure deforms in the dynamic state, the inertia effect and the propagation of stress waves are so great that the material properties are influenced by the strain rate [3]. Tensile testing of metallic sheet materials at high strain rates is important for making a reliable analysis of vehicle crashworthiness. During a crash event, the maximum strain rate often reaches 103 s−1, at which the strength of the material can be significantly higher than under quasi-static loading conditions. Thus, the reliability of crash simulation depends on the accuracy of the input data specifying the strain – rate sensitiv- ity of the materials [4]. On the basis of experimental and numerical calculations, the strain – rate range be- tween 10−3 and 103 s−11 is considered to be the most relevant to vehicle crash events. In order to evaluate the crashworthiness of a vehicle with accuracy, reliable stress-strain characterization of metallic materials at strain rates higher than 10−3 s−1 is essential [5]. 2. Automotive steel sheets An important and challenging issue in the automotive industry is lightweight, safe design and enhancement of the crash response of auto-body structures. The most widely-used automotive steels are IF steel, DP steel and microalloyed steel [6, 7]. IF steels have ultra- low carbon levels designed for low yield strengths and high work hardening exponents. These steels are de- signed to have more stretchability than mild steels. Some grades of IF steels are strengthened by a combi- nation of elements for solid solution, precipitation of carbides and/or nitrides, and by grain refinement. An- other common element added to increase the strength is phosphorus. The higher strength grades of the IF steel type are widely used for structural applications and also for closure applications [8]. Dual-phase steels have a microstructure which contains predominately martensite (there can be small amounts of retained austenite, bainite or pearlite) in a ferrite matrix, and these steels exhibit characteristic mechanical proper- ties, i.e. continuous yielding, a high tensile strength to yield strength ratio, and very high initial work hard- ening rates. The combination of high strength and high ductility has made DP steels very attractive to industry, particularly to the automobile sector. The mechanical properties of DP steels depend on a num- ber of parameters, including the strength, morphology and volume fraction of the constituent phases. The strength of ferrite is controlled by the steel chemistry and by its grain size, while the properties of martensite depend on its carbon concentration and on its scale. These groups of steels are strengthened primarily by micro-alloying elements contributing to fine carbide precipitation and grain-size refinement. High-strength low-alloy (HSLA) steels, or micro-alloyed steels, are de- 384 http://ctn.cvut.cz/ap/ vol. 53 no. 4/2013 The Effect of Strain Rate on Automotive Steel Sheets Material C S N Mn P Si Al Nb V Ti IF 0.0013 0.0105 0.0017 0.082 0.011 0.006 0.055 0.001 0.002 0.04 S 460 0.040 0.002 – 0.44 0.009 0.05 0.046 0.035 0.2 0.016 DP 600 0.072 0.006 0.005 1.18 0.017 0.01 0.057 0.002 0.003 0.001 Table 1. Chemical composition of the steels. Figure 1. The RSO Rotation wheel. Figure 2. Microstructure of IF steel. signed to provide better mechanical properties and/or greater resistance to atmospheric corrosion than con- ventional carbon steels. They are not considered to be alloy steels in the normal sense, because they are designed to meet specific mechanical properties rather than a chemical composition. The chemical compo- sition of a specific HSLA steel may vary for different product thicknesses to meet mechanical property re- quirements. [9]. 3. Experimental method Three automotive steel sheets were used for a static and dynamic tensile test: IF steel 1.5mm in thick- ness, DP steel 1.6mm in thickness, and micro-alloyed steel 1.5mm in thickness. A static test was performed Figure 3. Microstructure of DP steel. Figure 4. Microstructure of HSLA steel. according to the EN ISO 6892-1 standard. The dy- namic tensile test according the ČSN EN ISO 26203-1 standard used test equipment that can reach speeds greater than 10−3 s−1 of strain. A flywheel machine was used for the dynamic tensile tests. A scheme of the rotating flywheel machine and the stand set is shown in Fig. 1. The basic element, a wheel disc 600mm in diameter and 100mm in width, is equipped with a self-aligning forked hammer. The hammer is normally kept in a wheel pocket, and is blocked in this position by a slidable pin. The wheel is accelerated by an electric motor to the selected speed, which is measured by a rotary encoder. When the selected speed is achieved, the slidable pin is moved by an electromagnet, unlocking the hammer that rotates to working positions, striking an anvil connected to the sample. The velocity of the hammer ranges from 5 to 50m s−1, yielding available impact energy from 1.4 to 140 kJ. Because the work needed to sample defor- 385 M. Német, M. Mihaliková Acta Polytechnica M at er ia l M ec ha ni ca l pr op er ti es Lo ad in g ra te [m s− 1 ] 1. 6 ·1 0− 4 1. 6 ·1 0− 3 6. 6 ·1 0− 3 IF F [N ] 10 19 5 10 46 5 10 75 0 R e [M Pa ] 16 9 18 0 19 7 R m [M Pa ] 28 2 29 1 30 0 A 80 [% ] 34 .5 35 39 .5 D P 60 0 F [N ] 18 24 2 18 51 2 18 62 2 R e [M Pa ] 34 6 36 3 36 5 R m [M Pa ] 56 1 57 4 57 0 A 80 [% ] 23 .4 21 24 .2 S 46 0 F [N ] 21 61 1 21 72 2 22 02 2 R e [M Pa ] 42 7 45 7 46 4 R m [M Pa ] 50 9 53 8 53 6 A 80 [% ] 18 .8 20 .9 21 .3 T ab le 2. M ec ha ni ca lp ro pe rt ie s of st ee ls un de r a st at ic lo ad in g ra te . M at er ia l M ec ha ni ca l pr op er ti es Lo ad in g ra te [m s− 1 ] 1. 6 ·1 0− 4 1. 6 ·1 0− 3 6. 6 ·1 0− 3 IF F [N ] 71 58 75 87 79 61 R m [M Pa ] 50 2 53 2 55 9 D P 60 0 F [N ] 10 72 3 12 51 7 13 58 1 R m [M Pa ] 70 5 82 4 89 4 S 46 0 F [N ] 99 44 11 32 1 11 60 2 R m [M Pa ] 69 8 79 4 81 4 T ab le 3. M ec ha ni ca lp ro pe rt ie s un de r a dy na m ic lo ad in g ra te . Figure 5. Static tensile curves of IF steel. mation up to fracture is less than 60 J, the impact velocity is almost constant during the test [10]. 4. Result and discussion The chemical composition of the steels investigated here in mass [%] is presented in Tab. 1. The mi- crostructures of the steels are presented in Figs. 2–4. Static tensile tests were performed for a three-speed load. The tensile curves of samples deformed with strain rate 1.6 · 10−5 m s−1, 1.6 · 10−4 m s−1 and 6.6 · 10−3 m s−1 for IF steel are shown in Fig. 5, for DP steel in Fig. 6, and for microalloyed steel in Fig 7. Tab. 2 shows the results of the static tests. For the dynamic tensile tests, an RSO rotating flywheel machine was used at three-speed load 6m s−1, 12m s−1, 20m s−1. The tensile curves of samples deformed with strain rate 6, 12 and 20m s−1 for IF steel is shown in Fig. 8, for DP steel in Fig. 9, and for micro-alloyed steel in Fig. 10. Tab. 3 shows the results of the dynamic tests. Tab. 4 compares the Figure 6. Static tensile curves of DP steel. Figure 7. Static tensile curves of S 460 steel. static and dynamic yield strength of the tested steels. Three different types of steel were used: IF steel, DP steel, and microalloyed steel (S 460) to compare the static and dynamic properties. The samples for tensile testing were prepared according to the EN ISO 6892-1 norm. An RSO-type rotary hammer was used for the dynamic tensile test. The yield strength of the IF steel increased from 282MPa (1.6 · 10−4 m s−1) to 558MPa (20m s−1). The yield strength of the DP 600 steel 386 vol. 53 no. 4/2013 The Effect of Strain Rate on Automotive Steel Sheets Figure 8. Dynamic tensile curves of IF steel. Material Static Rm Dynamic Rm ∆Rm IF 282 559 276 DP600 561 894 332 S 460 509 814 305 Table 4. Comparison between the static and dynamic yield strength of the tested steels [MPa]. increased from 561MPa (1.6·10−4 m s−1) to 894MPa (20m s−1). The yield strength of the micro-alloyed S 460 steel increased from 509MPa (1.6·10−4 m s−1) to 814MPa (20m s−1). The dislocation density increased under dynamic conditions, and this influenced the increased yield strength in all the investigated steels. We conclude that with increasing strain rate in all three steels, there is an increase in the strength properties and a change in the plastic properties. 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Archives of Civil and Mechanical Engineering 8, 2008, pp. 129–137. 387 Acta Polytechnica 53(4):384–387, 2013 1 Introduction 2 Automotive steel sheets 3 Experimental method 4 Result and discussion Acknowledgements References