HOT PRESS FORMING IN SHEET METAL FORMING

Transcription

1 MISKOLCI EGYETEM ANYAGSZERKEZETTANI ÉS ANYAGTECHNOLÓGIAI INTÉZET HOT PRESS FORMING IN SHEET METAL FORMING MicroCAD 2014, Miskolc konferencia cikk Kidolgozta: Kovács Péter Zoltán egyetemi adjunktus Készült: a TÁMOP A-11/1/KONV ajárműipari anyagfejlesztések projekt keretében A projekt szakmai vezetője: Dr. Tisza Miklós egyetemi tanár, tanszékvezető Miskolc 2014

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3 INNOVATIVE MECHANICAL DESIGN AND TECHNOLOGY SYMPOSIUM D1: MATERIAL PROCESSING TECHNOLOGIES 1. Ádám Dobosy, Gyula Nagy, János Lukács LCF Experiments on Different Yield Strength Steels and their Welded Joints 2. Gaszton Gál, Zsolt Lukács, Miklós Tisza New Equipment for Measuring Large Strain Cyclic Tension-compression Testing of Sheet Metal 3. Ákos Meilinger Finite Element Modeling of Friction Stir Welding 4. Nagy Nóra Humán implantátumok fejlődése 5. László Prém Spot Welding Experiments on Automotive Dual-phase Steel Sheets 6. Miklós Tisza, Péter Z. Kovács, Zsolt Lukács Hot Press Forming in Sheet Metal Forming 7. Gergely Dezső, Ferenc Szigeti, Attila Százvai Investigation on Force Demand of Steel Planing 8. Sándor Bodzás, Illés Dudás Production Technology of Spiroid Worm Surface Using Grinding Wheel Banking Angle Correction 9. Wojciech Zębala, Robert Kowalczyk Analysis of Cutting Data and Nose Radius Influence on WC-Co Turning Process with PCD Tools 10. Wojciech Zębala, Bogdan Słodki, Grzegorz Struzikiewicz Examples of Disturbances in Cutting Force Measurement in Inconel 625 Turning 11. Anatoly Grabchenko, Vladimir Fedorovich, Ivan Pyzhov Increase in Working Capacity of Diamond Dressers

4 HOT PRESS FORMING IN SHEET METAL FORMING Miklós Tisza 1, Péter Z. Kovács 2, Zsolt Lukács 3 1 DSc, Professor, Head of Department, 2 PhD, senior lecturer, 3 senior lecturer University of Miskolc, 3515 Miskolc-Egyetemváros 1 tisza.miklos@uni-miskolc.hu, 2 metkpz@uni-miskolc.hu, 3 lzsolt@kugli.met.uni-miskolc.hu Abstract. The automotive industry always had a determining role in the whole economy. In sheet metal forming both the material and the technological developments are strongly affected by the development tendencies in car manufacturing. One of the main trends in today s car manufacturing is to apply light weight construction principles to meet the increased customers demand and legal requirements. In this respect, application of high strength steels is regarded as one of the most promising possibilities. Applying high strength steels has a very positive response for many requirements: increasing strength results in the application of thinner sheets leading to significant mass reduction and simultaneously to lower consumption and less harmful emissions however it also leads to formability problems. Hot press forming is one of the most successful solutions to overcome these difficulties. In this paper, a general overview of hot press forming processes will be summarised. 1. INTRODUCTION Hot press forming also known as press is hardening is a relatively new forming process. It was recently developed particularly for the application of high strength steels in car body manufacturing in the automotive industry. Hot forming is a unique metal forming processing technique: in this process the material is heated up to a sufficiently high temperature to provide austenitic microstructure then it is cooled down rapidly in the forming tool. At certain cooling rate diffusion less martensitic phase transformation occurs leading to significant increase of the strength parameters. Since the martensitic transformation i.e. the hardening process occurs in the forming tool, this is the reason that this process is often termed as press hardening. Concerning the ever increasing requirements on the increase of crash resistance of vehicles high strength materials are more and more widely used. Among these high strength materials Manganese-boron steels are the most often used in hot press forming. Hot press formed parts offer very high resistance to deformation during crash due to their very high strength (R m = 1,500 to 2,000 MPa). These high strength steels besides the excellent crash resistance properties are also important from the point of view of mass reduction and thus reducing the fuel consumption and harmful CO 2 emissions. One of the main reasons that hot press forming is particularly used in car manufacturing for parts that must maintain their shape in the event of crash ensuring the safety of passenger compartment.

5 These parts include the A and B-pillars, bumpers, various side impact beams, cross member, rear seat and roof, subplate, rocker panel, etc. as shown in Figure 1. Roof rail inner B-pillar inner Subplate Tunnel Side impact beam Front bumper Rocker panel Figure 1. Typical hot press formed parts in car body 2. SHORT HISTORY OF HOT PRESS FORMING OF HIGH STRENGTH STEELS Though the application of hot press forming in the automotive industry looks back only for a relatively short period, however the process itself dates back even to 1977 when Erland Lundström a Swedish scientist patented a process to produce sheet steel products such as a reinforcement element in a larger structure [1]. The next important milestone in the application of hot press forming in the automotive industry was in The Swedish SAAB company introduced first this technology in serial production for manufacturing the side impact beam in SAAB 9000 series [2]. Since then the use of hot press forming was spreading in producing parts offering an extra high level of strength. Manganese-boron alloyed steel (22MnB5 as one of the most typical grade) was used as blank material. Until 1999 the material mentioned above was exclusively processed in uncoated condition. In 1999, the first time in France, Manganese-boron alloyed steel was hot press formed with aluminium-silicon coating offering much better protection against scale and corrosion [3]. Another trend-setting innovation should be mentioned in 2005 as the first published results on the application of Tailored Blanks for hot press forming: it was cited as HotForm Blanks and it is used in large series production since 2007 [4]. Galvanized Manganese-boron alloyed steel was first used in car-body manufacturing in 2008 [5]and the so-called Tailored tempering has been used in serial production since 2012 [6]. Tailored tempering means a process where locally different properties are set with high pinpoint precision on the hot press formed part utilising sophisticated tool design principles.

6 3. MATERIALS USED IN HOT PRESS FORMING There are intensive research works in progress on the application of various grades of high strength steels. Some examples for the so-called conventional Advanced High Strength Steels (AHSS steels) are shown in Figure 2. The most suitable group for hot press forming is shown in this Figure as HPF. This group includes the 22MnB5 Manganese-boron alloyed steel as the most typical one for this application, but some other types as 8MnCrB3, 20MnB5, 27MnCrB5 and 37MnB4 can also be used in hot press forming [7]. Figure 2. Ultimate tensile strength vs Total elongation for conventional AHSS steels As a general conclusion it can be sated that those steels are the most appropriate for hot press forming which produce a fully martensitic microstructure after hot forming if water cooled tool is used [8] Material properties of typical hot forming steels Some characteristic materials properties of typical Manganese-boron alloyed hot forming steels are summarised in Table 1. Steel grade Martensite start temperature, o C Table 1. Material properties of Manganese-boron alloyed steels Critical cooling rate, o C/s Yield strength, MPa Tensile strength, MPa as delivered hot formed as delivered 8MnCrB MnB MnB MnCrB MnB hot formed

7 The blank sheet has to be austenitized before forming at least for 5 minutes at about 950 o C. From Table 1., it can be seen well that for these hot forming steels a relatively low cooling rate is sufficient to achieve martensitic microstructure. This phase transformation occurring in the closed die is responsible for the high strength of the hot formed part. It is well known that the hardness increase is primarily due to the carbon content. The other alloying elements such as Mn, Cr have only a small influence on the increase of hardness but they are essential for increasing the socalled hardenability zone providing martensitic phase transformation. Boron alloying has the highest effect in this respect by slowing down the phase transformation i.e. the austenite transformation to softer microstructural elements, and also leading to martensitic phase transformation over the entire cross section of the formed part. It can be also observed from Table 1., that these materials have relatively low strength values and good formability properties in as delivery state due to the ferrite-pearlite microstructure, and after hot forming and quenching in the forming tool they can achieve very high strength increase up to 1,500 to 2,000 MPa. 4. THE HOT FORMING PROCESS As it was very shortly described in the Introduction section, hot press forming is a complex forming and tempering operation when the materials heated first up to the austenitic zone and holding until the carbon fully dissolve producing homogeneous austenite microstructure. The forming operation is performed in this state, and then the part is cooled down rapidly in the tool with the critical cooling rate to prevent the carbon diffusion, thus resulting in martensitic microstructure. Through the combination of heating, holding, forming and rapid cooling, complex parts can be produced with excellent strength properties. The usual process cycle of hot press forming can be seen in Figure 3. Figure 3. Temperature vs process time for hot press forming There are two main technological process variants applied in hot press forming: one of them is called direct hot forming, the other is the indirect hot forming as

8 shown in Figure 4. In direct (or often termed as single stage) hot press forming the blank sheet is directly austenitized, transferred to the stamping tool and cooled down rapidly with the forming tool, thus providing the excellent strength properties. In indirect (or two-stage) hot press forming, the initial blank first is formed in cold state. This first stage either can be just a pre-forming stage, i.e. an intermediate step in producing the final part, or may provide the final geometry of the part and just a calibration occurs in hot condition. The possible degrees of forming depend upon the specific boundary conditions such as the complexity of the part or the coating system used for preventing scaling and corrosion. Besides the second forming step which might be either a calibration or a second stage forming - the austenitization and the subsequent quenching is the inherent part of the process chain to provide the required high strength properties when the part produced is cooled down together with the tool under controlled cooling conditions. a) direct hot forming b) indirect hot forming Blank Austenitization Transfer Hot forming and quenching Part Blank Cold pre-forming Austenitization Transfer Calibration and quenching Part Figure 4. Direct (a) and indirect (b) process chain in hot press forming [7] In both cases that is either direct or indirect process is used it is absolutely essential to transfer the austenitized blank or part into the stamping tool as fast as possible. A further important requirement that forming must be completed before the martensitic phase transformation starts. Therefore a fast tool closing and forming operation are the precondition for a successful process control. 5. SUMMARY AND CONCLUSIONS In this paper a new, innovative process, the hot press forming was introduced and analysed. Hot press forming (also termed as press hardening) process recently is more and more widely used in the automotive industry to produce car body elements particularly for parts that must maintain their shape in the event of crash ensuring the safety of passenger compartment. The forming process is performed in the austenitic state thus the material has good formability, but during the subsequent rapid cooling within the tool a martensitic phase transformation occurs providing very high strength properties and excellent crash resistance.

9 The most appropriate materials for hot press forming are those ones which produce a fully martensitic microstructure after hot forming if water cooled tool is used. There are several types of Manganese-boron alloyed steels which exhibit these properties in hot press forming. Among them the 22MnB5 alloy is one of the most widely used grades in the automotive industry. 6. REFERENCES [1] Lundström, E.: Method of producing a sheet steel product such as a reinforcement element in a larger structure, Patent No , SSAB Hardtech AB, [2] Berglund, G.: The history of hardening of boron steel in northern Sweden, In: 1st International Conference on Hot Sheet Metal Forming of High- Performance Steel, Kassel, Germany, (2008) pp [3] Borsetto, F., Ghiotti, A., Bruschi, S.: Investigation of the high strength steel Al Si coating during hot stamping operations, Key Engineering Materials, vol , (2009) pp [4] Erhardt, R., Boke, J.: Industrial application of hot forming press simulation, In: 1st International Conference on Hot Sheet Metal Forming of High- Performance, Steel, Kassel, Germany, (2008) pp [5] Faderl, J., Manzenreiter, T., Radlmayr, M.: Press hardening of hot-dip galvanized 22MnB5: a stable and reproducible process, In: 1 st International Conference on Hot Sheet Metal Forming of High-Performance Steel, Kassel, Germany (2008) pp [6] George, R., Bardelcik, A., Worswick, M.: Localized die temperature control for tailored properties in hot forming of boron steels numerical study, IDDRG, Golden,Colorado, USA, (2009) pp [7] Karbasian, H., Tekkaya A. E.: A review on hot stamping, Journal of Materials Processing Technology, vol. 210 (2010) pp [8] Naderi, M.: Hot stamping of ultra high strength steels, PhD Theses, RWTH Aachen, [9] Paar, U., Becker, H.H., Alsmann, M.: Press-hardened components from Kassel-chances and challenges. In: 1st International Conference on Hot Sheet Metal Forming of High-Performance Steel, Kassel, Germany, (2008) pp ACKNOWLEDGEMENT This research work is based on the results achieved in the TÁMOP B- 10/2/KONV project and continued in the TÁMOP A-11/1/KONV project in the framework of the New Hungarian Development Plan. The realization of this project is supported by the European Union, co-financed by the European Social Fund. Both financial supports are gratefully acknowledged.