Next Generation Vehicle: Stainless Steels for automotive lightweight applications

The objectives of the NGV (Next Generation Vehicle) Project were to demonstrate that stainless steel can be used to reduce weight and costs, and to improve safety and sustainability in structural automotive systems. The deliverables include enabling technologies, virtual technology for design and development, processing and testing. The compilation of the results as well as aspects such as new design criteria for the application of stainless steels in automotive components were arranged in an Engineering Guideline for car manufacturers and their suppliers. The program was approved by constructing several newly designed B-pillars where the deliverables were successfully applied. After performing some regular crash tests the cost efficiency was estimated by the NGV cost model. The NGV Project deliverables and data base establish a sound basis for the use of stainless steels in automotive series production. Introduction The automotive industry of today is characterized by faster cycles in materials invention, development and application, coupled with the ability to tailor materials for specific end-users requirements i.e. multi material solutions. It is therefore essential for materials development to be closely integrated with the final product and process concurrent engineering practice. This means being aware of the market and customers, industrial and environmental trends and forces, recycling, cost efficiency, and technology development. The aim of the project is to point out to the automotive industry that stainless steel can be used to reduce weight and cost in the manufacture of motor vehicles and to improve safety and sustainability in automotive body structures. The competiveness of stainless steels should be approved in the same process steps which a standard automotive development follows: the virtual development supported by FE-simulations, the analysis of forming, tooling, joining and determination of surface and corrosion properties. These different areas mark the structure of the NGV Project. The material choice for the project and the main results of the different research areas are explained in the following. Because of the complexity of the topic and the large experimental effort the project was organised by the three stainless steel producers ThyssenKrupp Nirosta GmbH , ArcelorMittal Stainless and Outokumpu Oyj. The European car manufacturers were represented by AUDI AG, BMW AG, Daimler AG, Saab Automobile AB, Volvo Cars and the Centro Ricerche Fiat. For the different working groups experts were integrated to ensure that the experiments conducted are according to the current state of the art. Material Traditionally, stainless steels are classified mainly by their microstructure. The major basic groups are martensitic, ferritic, austenitic and duplex (austenitic & ferritic) materials. The area of use for stainless steels is very vast and comprises mainly applications taking advantage of properties such as resistance against corrosion and/or very high or low temperatures as well as hygienic surfaces and aesthetic appearance. Increasingly, stainless steels are being used also for their mechanical properties such as the combination of very high strength and excellent formability together with high energy absorption capability in finished components. The stainless steels used in the NGV Project are all but one austenitic and that one is duplex, Table 1. The chemical composition of the different grades is given in Table 2. Table 1: Material selection for the NGV Project EN Type Finishing Supplier 1.4376 Austenitic 2B ThyssenKrupp Nirosta (TKN) ArcelorMittal Stainless Europe (AMSE) Outokumpu (OS) 1.4318 1.4318 C1000 Austenitic 2B Temper C1000 1.4310 1.4310 C1000 Austenitic 2B Temper C1000 1.4162 Duplex 2E 1 Cold rolled, annealed to retrieve material properties after cold rolling, pickled and skin passed 2 Reduced by cold rolling and achieved desired mechanical properties maintained, C1000 stating the tensile strength 3 Cold rolled, heat treated, mechanically descaled and pickled Table 2: Chemical composition of materials investigated Grade C N Cr Ni Mo Mn 1.4376 0.03 0.19 17.6 4.2 0.15 6.5 1.4318 0.025 0.11 17.5 6.6 0.20 <1.3 1.4310 0.10 0.03 17.0 7.0 <0.6 <2.0 1.4162 0.03 0.22 21.5 1.5 0.30 5.0 The austenitic materials referred to in these Guide Lines have mostly a more or less pronounced unique feature and that is deformationor strain-induced hardening through a forming of martensite in the material. Thus facilitating cold rolling to very high strength levels or creating strength during forming operations. Mechanical properties Tensile tests on stainless steels are done according to EN 10002 standard, i.e. specimen geometry and preparation, test conditions (position relatively to the rolling direction, temperature indicated, etc). Tests are typically performed at room temperature (296 K), on as-received material in the three directions (0°, 90° and 45°). The mechanical properties and r-values are summarized in Table 3. The n-value is generally determined according to the standard above mentioned; computed between 18-40% for annealed grades and between 5-17% for temper rolled C1000 grades. Even if n-values are available, they are not very meaningful when considering austenitic stainless steels prone to deformation martensite formation since they do not describe the complete curve. Figure 1 shows such an example in the case of 1.4318 where the Hollomon model cannot fit the tensile curve because of two slopes due to the TRIP effect. Table 3: Mechanical Properties (non-isothermal test conditions) Value (unit) 1.4376 1.4318 1.4318 C1000 1.4310 1.4310 C1000 1.4162 Figure 1: Comparison between the Hollomon model and the tensile curve for 1.4318 Rp0.2 (MPa) 410 420 800 300 950 600 Rm (MPa) 740 765 1050 80