ETD

• Adria
• Danieli Universal Endless (DUE)
• Dual Phase (DP)
• Electric Arc Furnace (EAF)
• Interstitial Free (IF)
• Metallurgy
• Press Hardened Steel (PHS)

The decarbonization of the steel industry is pushing automotive flat production from the integrated blast furnace route toward the electric arc furnace (EAF). The electric route has long been considered unable to meet the cleanliness and formability demanded by exposed automotive panels. Interstitial free steel in particular has never been produced at automotive quality from an EAF, and only recently have certain advanced high-strength steel grades begun to be cast via this route. This thesis examines the plant-engineering and metallurgical conditions under which that gap can be closed, taking the Adria endless plant in Piombino as its reference case.
The work first explains the transition between blast furnace and EAF and why the electric route is structurally attractive in a fragmented market as the Italian one. The plant configuration adopted at Adria is a Danieli Universal Endless line fed by two 190-tonne electric arc furnaces, designed to produce hot-rolled coils at 2.7 million tonnes per year. Only three steel grades are examined: press hardened steel (PHS), dual phase (DP) and interstitial free (IF). The first one enhances strength because the sheet is quenched while formed inside the die and it is the automotive manufacturer’s responsibility. The DP requires high strength standard while IF is used for deep drawing. Each grade is characterized in terms of its alloy design, target microstructure and the formability parameters, including the strain hardening exponent, the plastic anisotropy ratio, the forming limit curve and the hole expansion ratio, against which the process must be validated. The core of the thesis indeed is focused on analysing the possible critical defects that can occur and it shows targeted plant level solutions to avoid or restrict it to an acceptable quality standard.
Two examples illustrate this approach. Centerline segregation of carbon, manganese, phosphorus and sulphur is one of the most persistent internal defects in continuous castings. The macro segregation leads to a solute enrichment of the core that cannot be corrected once solidification is complete. It is mitigated by dynamic soft reduction, which compresses the strand at the final stage of solidification. It basically applies controlled compressive force to the loose strand side where the core is supposed to be almost solid to understand if the core inside is still liquid. The mould has a wave receiver which detect differences with Fourier Frequency Transformation (FFT) analysis and then in feedforward it changes the pressure applied on the strand. The second example is a solution to partially solve castability barrier of interstitial free grades. Under no circumstances should calcium be used to improve castability of IF because then spherical calcium aluminates would harm formability. Here it is the argon injection gambit to clean SEN and avoid nozzle clogging.
A dedicated part of the work concerns the two classes of unintended contamination that enter the circuit without being intended, the tramp elements carried by the scrap charge and the nitrogen absorbed through the arc.
Copper and tin, which cannot be removed once dissolved, are governed through charge mix design and residual limits, since their surface enrichment during reheating triggers hot shortness within a well-defined temperature window. The copper equivalent content can lead to liquid metal embrittlement (hot shortness). It extends liquid state down to lower temperatures and it penetrates between austenitic grain boundaries. The copper melting temperature is well known, however many factors change it unpredictably. Different elements such as Sn ,Sb ,As ,Ni ,Mo change the Cu behavior , the rolling stress and the local surface enrichment. Predicting critical ranges should be traversed rapidly. This is more achievable if the range is over 1050°C because milling processes typically begin below that threshold.
Nitrogen is structurally higher in the electric furnace than in the blast furnace due to the pickup from the atmosphere. Surface segregation of sulfur and oxygen on the surface reduces nitrogen absorption by blocking the active sites. The vacuum degassing is mandatory for reach low level and titanium and niobium are carefully added to tie up all the remaining free atoms that alone would lead to a dangerous strain ageing undesirable especially for IF. In particular AlN precipitation must be controlled on the cooling profile in the caster to avoid prolonged exposure of the strand surface in the 700-900°C band.
The contribution of the thesis is to connect plant configuration and metallurgical practice into a single argument, showing that automotive grade steels, including the interstitial free grade, are a credible objective for the electric route when the limiting factors are addressed in a coordinated and preventive way. Data from the Q-Melt, which monitors the chemical composition, could reduce the restriction in chemistry of critical elements only for the hot rolling process purpose if the program can shorten the time exposition to certain range of temperature; clearly maintaining desired output performances.