Within the sector of precision cold-rolled steel, reaching the highest quality is always the most important added-value requested by industrial customers, especially in the highly demanding automotive sector. This affects not only the finished product, but to all the processes interacting along the value chain, from the design of a product to its manufacturing, including human resources management, environmental protection, industrial safety, etc.
ARANIA, a leading manufacturer in Southern Europe for cold rolled high and low carbon steel, has an important presence in the automotive sector, with more than 75% of its production going into this market. We develop high added-value products, tailored to highly demanding final customers’ requirements on thickness tolerances, surface quality and material characteristics, among other properties.
Considering the company’s target market and our tailored product manufacturing, we can greatly benefit from the technical support and new initiatives coming from research organisations and industries. This is the case of FormPlanet, an EU project, developing novel testing methodologies to characterise sheet metal properties, predict part performance and prevent production loses to the sheet metal forming industries.
The creation of a European Union’s Open Innovation Test Bed (OITB) for metal materials characterisation, aiming at increasing the productivity of the sheet metal forming industries through the development of new experimental and modelling methodologies to assure zero-defects production and the optimisation of sheet material development, production and performance, is a very useful source to go to when specific laboratory tests – out of the current validation tests performed at the manufacturing plants – are requested to take important decisions linked to the daily production, the prototype phase or the product design.
ARANIA’s contribution to the FormPlanet project, as industrial partner, is related to the tests and trials execution to determine the effect of skin-pass reduction in deep drawing operations in order to prove the effectiveness and reliability of novel testing methodologies and models to solve relevant industrial problems in sheet metal forming, employing demonstrators from different sectors and metallic alloys.
Thanks to tests developed within the FormPlanet project, which on the other hand are not available in the standard quality control of the manufacturing plants, it will be possible to control the effect of skin-pass reduction in deep drawing operations. The final operation in most of the cold rolled products is the skin-pass rolling, which is essential to remove the yield point elongation and to obtain the surface quality and flatness required by customers.
Due to these surface and flatness requirements, sometimes a higher skin pass percentage is applied, which can influence the mechanical and material properties. For example, it is noted that while a small increase of the skin-pass rate does not have any large effect on “r” value (normal anisotropy), it can change drastically the “n” value (work-hardening coefficient). These “r” and “n” values are representatives of the material formability. Thus, the need to understand and quantify the consequences that this decrease of “n” value has on material drawability emerges.
In consequence, any test able to determine the skin-pass limit to keep the material characteristics in terms of drawability leads to a great advantage to any cold rolled steel manufacturer. It allows setting process limits that ensure the material will fulfil the requested characteristics, avoiding unnecessary rejections or, in the worst case, delivering troublesome material to the final customer.
As an example of the work carried out is the study of the effect of skin-pass reduction in deep drawing operations for the DC05LC steel grade. The analysis of the normal anisotropy (R) and planar anisotropy (Δr) it has been performed by axial tensile test at 0º, 45º and 90º. Drawability is also analyzed by Limiting Drawing Ratio (LDR) & Earing Test. The results show that in the analyzed skin-pass range there are no significant differences on tensile properties depending on skin pass strain in the analyzed range (Figure 1).
Figure 1. Results of the tensile test for different skin pass strain.
Regarding anisotropy as it can see in Figure 2, r0 decreases very slightly with skin pass strain, r45 decreases slightly and r90 increases, also very lightly. If average values are analyzed: normal anisotropy (R) decreases very slightly, while planar anisotropy (∆r) increases. A similar trend is observed in the height of the EARS, related to planar anisotropy (∆r), but no differences are appreciated in LDR values, related to normal anisotropy (R).
Figure 2. Results of Anisotropy in terms of R, ∆r, LDR and EARING–
So, the differences on drawability depending on skin pass strain are not significant in the analyzed range.
In summary, the application of test methodologies to characterize the metal sheets properties within the FormPlanet project has allowed ARANIA to define the skin pass limits that achieves fulfils the drawability requirements for different steels. The studies performed permit to ARANIA minimize the risk of non-achievement the customer´s requirement.
ARANIA S.A., located in Amorebieta (Bizkaia) and founded in 1940, is a leading manufacturer in Southern Europe for cold rolled high and low carbon steel, as well as high-yield strength alloyed and microalloyed steel sectors. The company counts with the most advanced technology currently available on the market to provide a competitive product which is adapted to its customer’s needs, a working force of around 170 people and a maximum production capacity of 140,000 tons per year. The company has a high export rate, reaching 65% in recent years, and is present in more than 20 countries, mainly in Europe but also in America or Eastern Europe.