Steel Deoxidation: Part One

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Introduction to Deoxidation Processes

Deoxidation, the final stage in steelmaking, involves removing excess oxygen from molten metal through materials with high oxygen affinity. These materials form either gaseous oxides or readily form slags. While Mn, Si, and Al serve as primary deoxidizers, elements like Cr, V, Ti, Zr, and B offer alternative solutions.

Basic Oxygen Furnace Operations

In BOF steelmaking, the steel bath contains 400 to 800 ppm oxygen activity at tapping. Deoxidation occurs during tapping through calculated additions in the tap-ladle. While recarburization may be necessary for below-specification carbon content, large ladle additions are avoided due to temperature considerations.

Ingot Structure Classifications

Figure 1: Typical ingot structures from fully killed to violently rimmed

The eight typical ingot conditions demonstrate varying degrees of gas evolution suppression, ranging from dead-killed (No. 1) to violently rimmed (No. 8).

Steel Categories by Deoxidation Level

Rimmed Steel

Minimal deoxidizer additions maintain sufficient oxygen for carbon monoxide evolution. Processing varies with carbon content: 0.12-0.15% for higher ranges, ≤0.10% for lower ranges. The resulting outer ingot skin shows relatively clean metal with low carbon content.

Capped Steel

A rimmed steel variation where initial rimming proceeds normally before termination by cast-iron cap sealing. Preferred for steels exceeding 0.15% carbon, particularly for sheet, strip, wire, and bar production.

Semi-killed Steel

Moderate deoxidation maintains sufficient oxygen for carbon monoxide formation to offset solidification shrinkage. Carbon content typically ranges from 0.15-0.30%, finding wide application in structural shapes.

Killed Steel

Complete deoxidation prevents gas evolution during solidification. Aluminum, combined with manganese and silicon ferro-alloys, serves as the primary deoxidizer. Essential for homogeneous structures in alloy steels, forging steels, and carburizing applications.

Deoxidation Equilibria and Mathematical Models

The fundamental deoxidation reaction follows: M_xO_y = xM + Y_O

With equilibrium constant: K = (h_M)^x(hO)^y

Table 1: Complete solubility data for deoxidation products in liquid iron

Equilibrium constant K*	Composition range	K at 1600°C	log K
[aAl]2[aO]4	< 1 ppm Al	1.1 x 10-15	-71600/T + 23.28
[aAl]2[aO]3	< 1 ppm Al	4.3 x 10-14	-62780/T + 20.17
[aB]2[aO]3		1.3 x 10-8
[aC] [aO]3	> 0.02% C	2.0 x 10-3	-1168/T - 2.07
[aCr]2[aO]3	> 3% Cr	1.1 x 10-4	-40740/T + 17.78
[aMn] [aO]	> 1% Mn	5.1 x 10-2	-14450/T + 6.43
[aSi] [aO]2	> 20 ppm Si	2.2 x 10-5	-30410/T + 11.59
[aTi] [aO]2	< 0.3% Ti	2.8 x 10-6
[aTi] [aO]	> 5% Ti	1.9 x 10-3
[aV]2[aO]4	< 0.10 V	8.9 x 10-8	-48060/T + 18.61
[aV]2[aO]3	> 0.3% V	2.9 x 10-6	-43200/T + 17.52

 

Activity relationships follow: H_i = f_i(wt.% i)

And interaction parameters: (d log f_i/d log wt%j) = e^j_i

Table 2: Interaction coefficients for carbon and stainless steels

Metal		Al	C	Mn	P	S	Si	Ti	H	N	O	Cr	Ni
Carbon steel 1600°C	%i		0.05	0.45	0.02	0.01	0.3	0.05
	fi	1.05	1.06	1.0	1.1	1.0	1.15	0.93	1.0	0.97	0.85
	ai		0.053	0.45	0.022	0.01	0.345	0.046
Stainless steel 1600°C	%i		0.05	0.45	0.02	0.01	0.3	0.05				18	8
	fi	3.6	0.49	1.0	0.32	0.66	1.24	9.4	0.93	0.17	0.21	0.97	1.0
	ai		0.025	0.45	0.006	0.007	0.372	0.47				17.5	8.0

For practical applications in low alloy steels: K_M = (%M)^x(%O)^y

Example calculation for 0.1% Si at 1600°C: K_Si = (%Si)(%O)² = 2.2 x 10⁻⁵

Therefore, (%O) ≈ 0.015 or 150 ppm

Contemporary Applications and Limitations

While aluminum deoxidation effectively suppresses carbon monoxide formation, certain applications require alternative approaches. Large ingot casting and continuous casting operations often prefer silico-manganese deoxidation or vacuum carbon deoxidation.