Residual Elements in Steel

Sommario:

Residual elements (Cu, Ni, As, Pb, Sn, Sb, Mo, Cr, etc.) are defined as elements which are not added on purpose to steel and which cannot be removed by simple metallurgical processes. The presence of residual elements in steel can have strong effects on mechanical properties. There is therefore clearly the need to identify and to quantify the effects of residual elements in order to keep these effects within acceptable limits.
Residual elements, or at least some of them, have an influence on processing conditions and regimes, from casting to the final annealing, and possibly on all mechanical properties.

Residual elements (Cu, Ni, As, Pb, Sn, Sb, Mo, Cr, etc.) are defined as elements which are not added on purpose to steel and which cannot be removed by simple metallurgical processes. The presence of residual elements in steel can have strong effects on mechanical properties. There is therefore clearly a need to identify and to quantify the effects of residual elements in order to keep these effects within acceptable limits.

Residual elements, or at least some of them, have an influence on processing conditions and regimes, from casting to final annealing, and possibly on all mechanical properties. A clear distinction has to be made between those residual elements which have an effect due to their presence in solid solution, such as Mo, Cr, Ni, and Cu, and those which have an effect due to their segregation at interfaces (surface and grain boundaries), such as Sn, As, and Sb.

The following non exhaustive list gives some possible metallurgical effects of residual elements on processing conditions and properties of steel products. Residuals may influence:

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%.

Table 1: Mean Residual Element Levels in EAF Produced Steels

  Cu Ni Cr Mo Sn
Flat products 0.050-0.2000 0.050-0.2000 0.025-0.1000 0.010-0.0300 0.010-0.0300
Reinforcing bar max 0.48 max 0.08 max 0.24 max 0.06 max 0.08

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.

Table 2: Effects of increase of residual elements content on various steel properties

Property Cu Ni Cr Mo Sn Sb
Strength and hardness + + +,– + + +
Ductility +,– +,–  
Strain hardening, n 0,–  
Strain ratio, r +,– 0 0,–   0  
Impact resistance + + 0 0,–  
Hardenability + + +   +,0 +,0
Weldability    
Corrosion resistance + + +   +  
Temper embrittlement         + +
(+) Indicates an increase
(-) Indicates a decrease

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.

Table 3: Effect of residuals on yield and tensile strength; strength increment per wt%

Base Material and Heat Treatment Yield strength, MPa/ksi Tensile strength, MPa/ksi
  Cu Ni Cr Mo Sn Cu Ni Cr Mo Sn
Low Carbon Steel normalized or annealed 41/6 0/0 -27/4 13/2 124/18 13/2 13/2 -34/5 -55/-8  
Low Carbon Steel normalized 76/11 41/6 55/8     55/8 34/5 69/10    
0.3 Carbon Steels normalized 82/12 55/8 62/9 13/2   62/9 34/34 89/89 69/10  
0.2 Carbon Steel as-rolled   55/8 89/13       69/10 131/19    

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