1. The processing conditions in terms of:
• Recrystallisation and rolling forces in the hot strip mill: Mo, Cr, Sn, ...
• Austenite to ferrite transformation, hardenability: All
• Hot ductility during hot deformation: Zn, Sn, ...
• Recrystallisation during annealing: Mo, Cr, Sn, ...
2. The surface aspect of the hot rolled and pickled strip: Cu, Ni, As, Sn, ...
• Due to hot shortness
• Due to possible synergy of Cu an Sn in hot shortness
3. The embrittlement of grain boundaries: Sb, Sn, As
• During strip coiling
• During batch or continuous annealing of low C steels
4. The precipitate/matrix interface segregation phenomena: Sn
• Ostwald ripening, precipitate growth, texture control
• Sn on Fe4N, Sn on MnS, Sb on TiC, ...
5. The mechanical properties of the final products: All
• Hot strips and cold rolled sheets
• Plates
• Long products
6. The coating by hot dip or electrodeposition
7. The weldability of HSS grades : Mo, Cr, Cu, Ni
Residual elements enter steel from impurities in ore, coke, flux and scrap; from these, scrap is considered to be the main source of residuals. The most commonly found residuals are Cu, Ni, Cr, Mo, and Sn. The acceptance limits of these residuals depend mainly on product requirements.
A major problem of the recycling process is to control the level of undesirable elements or residuals elements in order to ensure the steel cleanness required by the product performance. The most of steels used today are low carbon/low alloy and extra deep drawing grades of steel. The properties of these steels are very sensitive to the residual elements content and to the thermomechanical processing.
As far as flat products and reinforcing bars are concerned, Table 1 shows typical values of main residual elements for the EAF route, in wt%.
Although the effect of residuals on properties may be quite small, sometimes even a small change in some property can significantly increase the rejection rate of products with specified requirements. The general consensus about effects of residuals such as Cu, Ni, Cr, Mo, Sn and Sb on various steel properties is given in Table 2.
The strengthening mechanisms in steel include: solid solution strengthening, fine grain size, precipitation, amount of pearlite, dislocations introduced by cold work, and bainitic and martensitic transformations.
The residuals affect the tensile properties through solid solution strengthening. At the low concentrations that the these residuals are present, the yield and tensile strength increment due to solid solution may taken as proportional to solute concentration. Some estimates of strength increments contributed by various residuals are given in Table 3.
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