ISSN 1392–1320 MATERIALS SCIENCE (MEDŽIAGOTYRA). Vol. 19, No. 2. 2013 Investigation of Usibor 1500 Formability in a Hot Forming Operation ∗ Hande GÜLER Mechanical Engineering Department, Faculty of Engineering and Architecture, Uludag University, TR-16059, Bursa, Turkey http://dx.doi.org/10.5755/j01.ms.19.2.1484 Received 05 April 2012; accepted 05 March 2013 The hot forming process included heating metals to a temperature in the austenite range, transferring the austenitized sheet from the furnace to a press, forming and simultaneously quenched. In this work, Usibor 1500 steel was hot stamped using water-cooled prototype mould. Micro structural analyses as well as tensile tests and hardness measurements of hot stamped samples were performed. The results showed that most of austenite microstructure was changed into martensite by the hot forming. The tensile strength and hardness value were up to 1485 MPa and 492 HV1, respectively. On the other side, hot stamped parts presented mechanical properties higher than cold stamped parts. Keywords: Usibor 1500; 22MnB5; hot forming; hot stamping; optical microscopy; mechanical properties. ∗ 1. INTRODUCTION a tribo simulator under dry conditions [5]. Merklein et al. [6] developed a heat able quenching tool for determining Manufacturing technologies of sheet metal forming is the thermal properties of 22MnB5. Abdulhay et al. [7] rapidly developed in recent years. Nowadays; deep drawing, examined the thermal conductance estimation to improve incremental sheet forming, and hot forming become very the sheet metal during hot stamping process. For this popular in sheet metal forming. Reducing the vehicles purpose, they designed and developed an experimental weight and increasing strength bring use of hot stamping device developed to estimate the thermal contact resistance processes. Hot stamping is one of the most productive at the part/tool interface. Hongtu et al.[8] focused on crash methods of forming processes. The process was developed behavior performance of hot forming steels experimentally and patented in 1974 by NJA, which later merged to SSAB and analytically. A 2D coupled thermo mechanical FEM Hardtech and is today known as Gestamp Hardtech [1]. Hot was developed to simulate sheet metal hot forming process stamping is a non-isothermal sheet metal forming technique for U channel part by Liu et al [9]. Liu et al. [10] where the blank is heated at about 900 °C, hold there long investigated the influence of cooling rate and blank holder enough and then placed between cooled dies, formed and force on phase transformations and spring-back during hot quenched simultaneously. Two different hot stamping sheet metal forming. Bai-liang et al. [11] studied numerical processes exist as direct and indirect hot method. simulation of hot stamping process for anti- collision part. While the direct process starts with a plain blank that Ying et al. [12] examined crack and spring-back is heated up to austenitization temperature, directly phenomena of hot stamping parts. Güler et al. [13] formed, and subsequently quenched in one process step. investigated effect of heat treatment parameters on the The indirect process uses a preformed component microstructure and mechanical properties of 22MnB5 steel. which is heated up to austenitization temperature and The heat treatment was conducted within the temperature quenched in a water cooled dies afterwards [2].The boron range from 700 °C to 950 °C, air and water cooling alloy steel 22MnB5 boron steel with an Al-Si layer named techniques were applied to the specimens. Usibor 1500 is commonly used in the hot stamping. This The main aim of this study is to achieve hot forming process and material are used for A and B pillars, bumper process using a prototype mould. The other aim is focused beams, side rails, door beams etc. on investigation into microstructure and mechanical The advantages of hot stamping process of the boron properties of the Usibor 1500 steel after being hot stamped. steel are i) very high formability ii) forming of complex In this context, micro structural evaluation, hardness geometries iii) high toughness iv) high elongation at brake measurements, and tensile tests were performed. The v) independence of material properties on the forming results showed that the mechanical properties and depth vi) good dimensional tolerances vii) good microstructure of hot formed Usibor 1500 steel conformed weldability viii) well suited for crash applications [3]. to the literature. Several investigations were conducted to investigate the hot forming process [4 – 12]. Ying et al. [4] 2. EXPERIMENTAL DETAILS investigated the hot forming process of 22MnB5 steel. In this paper, the blank was formed and water-cooling mould 2.1. Characteristics of Investigated Material was quenched simultaneously during the process of hot Mechanical properties of the material examined in the stamping. Yanagida and Azushima studied also the study, is called Usibor 1500 are shown in Table 1. coefficient of friction in hot stamping was measured using Usibor 1500, with a material thickness of 1.7 mm has an aluminum-silicon precoating differently than 22MnB5. ∗ Corresponding author: Tel: + 90 224 2941944, fax: +90 224 2941903. The chemical composition of the material was measured E-mail: handeguler@uludag.edu.tr (Hande Güler} 144 by Optical Emission Spectroscopy (OES) machine and the begins. The samples were rectangular blanks with length chemical composition is shown in Table 2. and width of (642×264) mm with 1.7 mm thickness. Press- Table 1. Mechanical properties of Usibor 1500 ing force was 6300 kN and pressing speed was 24 mm/s. The Usibor1500 steel was placed into a 930 °C oven Young’s Modulus (GPa) 222 and held for 5 minutes to obtain a homogeneous austenitic Tensile Stress (MPa) 543 microstructure. After the 5-min hold time, the blank was Yield Stress (MPa) 418 removed from the oven and put on prototype hot forming mould, then quenched, and formed respectively in the pres. Hardness (HV1) 191 The elapsed time from removing the sheet to Table 2. Chemical composition of Usibor®1500 (in mass %) manufacturing the final product was 25 s. The final product is seen in Fig. 2. Elements C Si Mn P S Cr Ti B Ni 3. RESULTS OF MECHANICAL PROPERTIES, HARDNESS PROFILE AND 0.19 0.649 1.13 0.0096 0.0021 0.192 0.0327 0.0030 0.0189 MICROSTRUCTURE In the as-received condition, Usibor 1500 steel shows After hot forming process, tensile test samples were cut a fine grain ferritic-pearlitic microstructure with the off the formed material. Cutting process was achieved from hardness value of 191 HV1 (Fig. 1). The microstructures of upper and lateral surfaces of final product by using laser the ferritic-pearlitic microstructure were observed under method as seen in Fig. 3. Two samples were cut off a final optical microscope and the hardness of the specimen was product for each surface and six specimens were examined measured with a 9.81 N (HV1) load. for each condition. All tensile tests were performed on a UTEST 25 ton universal tensile testing machine. Dimensions of the tensile specimen are shown in Fig. 4. Fig. 3. Tensile specimen surfaces cut from final product Fig. 1. Microstructure of Usibor®1500-as received Fig. 4. Dimensions of the tensile specimens (mm) On the other hand, these samples were used to evaluate microstructure. For evaluating microstructures, the speci- mens were grinded, polished and etched with Nital 3 % to observe, with an optical microscope Nikon Eclipse LV150. These specimens were finally used to measure micro hardness. Hardness measurements were carried out by Vickers hardness in the HV1 scale using MH-3 Micro- Fig. 2. Hot formed product vickers hardness tester. After hot forming process, a 2.5 until 3 times increase of Table 3 presents average tensile strength of materials. the Usibor 1500 material yield and tensile strength above Hot-formed parts showed higher tensile properties than as 1100 MPa and 1500 MPa can be achieved. This situation is received materials and this process nearly tripled tensile related to martensitic microstructure transformation during strength. On the other hand, there is a little difference quenching operation. For that reason, the sheet must be among the surfaces. Tensile strength value of upper surface austenitized and then cooled with a rapid cooling. was higher compared to that of lateral surface. Table 4 shows average hardness value of lateral and 2.2. Hot Forming Process upper surfaces. According to the values, the hardness of The experimental set-up of hot stamping press has the specimens remained almost unchanged. Otherwise, the water-cooled punch and dies. The die and the punch were tensile strength increases as the hardness decreases. In this made by tool steel. The cooling systems were in the punch case, the findings of Table 3 and Table 4 are compatible and die so that quenching was started as soon as forming with each other depending on this assumption. 145 Table 3. Comparison of tensile strength Hot stamped material presented higher tensile strength than as delivered material. The tensile strength and hardness Tensile strength (MPa) value reached 1485 MPa and 492 HV1 respectively. The As-received 543 effect of cooling medium on mechanical properties and Upper surface 1485 hardness of Usibor 1500 steels are important, during hot Lateral surface 1466 forming process. According to micrographs, the microstructure of the hot formed material showed martensitic Table 4. Micro-vickers hardness results phases. These findings showed that the hot forming process Hardness (HV1) using a prototype mould was verified through comparison with literature. Furthermore, additional research should be Upper surface 483 done to investigate effect of cooling rate and measuring this. Lateral surface 492 REFERENCES Fig. 5 shows example micrographs of hot-formed Usibor 1500 steel cut off the upper and lateral surfaces. 1. Bardelcik, A., Salisbury, C. P., Winkler, S., Wells, M. A., According to these pictures, martensite laths were very Worswick, M. J. Effect of Cooling Rate on the High Strain Rate Properties of Boron Steel International Journal of clear for upper surface through comparison with lateral Impact Engineering 37 (6) 2010: pp. 694 – 702. surface. This was because cooling rate of upper surface 2. 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