Residual Elements in Steel

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Introduction to Residual Elements in Steel

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.

Metallurgical Effects of Residual Elements

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:
   • Recrystallization and rolling forces in the hot strip mill: Mo, Cr, Sn
   • Austenite to ferrite transformation, hardenability: All residual elements
   • Hot ductility during hot deformation: Zn, Sn
   • Recrystallization 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 and 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 residual elements
   • 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

Sources and Control of Residual Elements

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 residual elements in order to ensure the steel cleanliness required by the product performance. Most 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.

Typical Residual Element Levels in Steel Production

As far as flat products and reinforcing bars are concerned, Table 1 shows typical values of main residual elements for the EAF route, in weight percentage.

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

Impact of Residual Elements on Steel Properties

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

Strengthening Mechanisms and Quantitative Effects

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 these residuals are present, the yield and tensile strength increment due to solid solution may be 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