Steel making

3. Stainless steels are iron based alloys containing a minimum of 10
wt-%Cr and varying amounts of other alloying elements.
Grade Classification C Mn Cr Ni Mo
304 Austenitic 0.05 1.5 18 .5 9 …
310 Austenitic 0.08 1.5 25 20 …
316 Austenitic 0.05 1 17 11 2
410 Martensitic 0.1 0.5 12 … …
430 Ferritic 0.03 0.4 16 .5 … …

4. Very low carbon contents are necessary to ensure that grain
boundary corrosion will not take place. Carbon is removed from
steels by oxidation, carbon being much more readily oxidised
than iron, as can be seen on the Ellingham diagram given as Fig.
1. The vertical axis will be ∆G ͦ, the standard Gibbs free energy
change. Chromium is clearly much more readily oxidised than
iron. At ~ 1200 ͦC the oxidation of carbon and chromium at unit
activity are equally likely, but as the temperature is increased
carbon becomes increasingly favoured.

5. As carbon is oxidised to very low levels its activity drops rapidly
and the C/O₂/CO line on the Ellingham diagram rotates
upwards, which therefore makes chromium oxidation
increasingly favourable. The main issue with producing stainless
steel is the restriction imposed by the carbon– chromium
equilibrium, i.e. the need to oxidise carbon without oxidising
much chromium. Failure to develop a strategy to achieve this
would lead to excessive loss of chromium and necessitate the
addition of expensive low carbon ferrochromium to achieve the
required grade of stainless steel, without simultaneously
increasing the carbon content beyond specifications.

7. Stainless steelmaking technology
In the early days of stainless steel production carbon steel scrap,
iron ore and lime (CaO) were charged into an electric arc furnace
(EAF). When the charge was molten, ferrosilicon, lime and
fluorspar (CaF₂) were added and the temperature of the bath was
increased so that a large amount of low carbon ferrochromium
could be added.
With the advent of tonnage oxygen, oxygen injection into the
molten steel using a lance greatly improved the rate of
decarburisation. However, the more oxidising conditions had the
expected adverse effect of extensive oxidation of chromium to
the slag. A reduction period using ferrosilicon to reduce the
oxidised chromium from the slag became necessary before low
carbon ferrochromium could be added.

8. Modern stainless steelmaking practice does not attempt to make
stainless steel in an EAF alone.
The EAF is used to provide a molten steel charge for converters,
without any significant decarburisation of the melt and with
minimum chromium oxidation to the slag. The melt is then
decarburised in a variety of converters, with argon oxygen
decarburisation (AOD) and vacuum oxygen decarburisation (VOD)
being the most widely used. As a result, these practices are
classified as ‘duplex’ in that there are two stages in the
production of stainless steel. Sometimes further refining takes
place under vacuum to produce stainless steels which have very
low carbon and nitrogen requirements and this is known as
‘triplex’ practice. The duty of the EAF is to efficiently and
economically deliver molten metal of the required composition
and temperature to a converter.

9. A typical heat commences with the charging of a total of 10 t
carbon steel scrap, 90 t stainless steel scrap and 10 t high carbon
ferrochromium (50 Cr, 7 C, 4 Si, balance Fe, wt-%) through the
open roof of the furnace. The roof is then swung into place, the
electrodes are lowered and current flow is initiated so that the
electrodes bore through the scrap to form a pool of liquid metal.
About 5 t fluxes containing mostly lime (CaO) and some magnesia
(MgO) are added soon after.
When the scrap is fully melted, carbon in the form of coke (~ 300
kg) is added to the steel bath together with ~ 250 Nm3 of oxygen
which is injected through a lance over a period of ~ 10 min.

10. 3Si + 4CrO 1:5 = 4Cr + 3SiO₂
∆G ͦ at 1600 ͦC = -- 422 kJ
∆H ͦ at 1600 ͦC = -- 588 kJ
The silica content of the slag increases during reduction and the
slag becomes more viscous, so before tapping ~ 100 kg fluorspar
(CaF₂) and 150 kg borax (Na₂O.2B₂O₃) is usually added to reduce
the viscosity. The tap-to-tap time is ~ 1 h and produces ~100 t
molten metal and 15 t slag.
After the oxygen blow, ~ 900 kg of 75 wt-%Si ferrosilicon is added
to reduce chromium oxides from the slag according to the
reaction :

11. An understanding of the competitive oxidation equilibrium of
carbon and chromium is the key to stainless steelmaking. While
carbon simply oxidises to carbon monoxide gas, the form of
chromium oxide in the slag may be CrO ( above 1700 ͦC) or
Cr₂O₃( < 1400 ͦC) . By taking a simple equation
The carbon–chromium equilibrium
Equilibrium constant :

12. During the EAF cycle there are two periods during which chemical
changes take place in the melt. The first period is when oxygen is
blown into the molten steel, and the second is when ferrosilicon is
added to return chromium to the molten steel.

14. The isotherms in Fig show that as the carbon content decreases,
so does the chromium content. At a temperature of 1600 ͦC, an
alloy containing 18 wt-% chromium has an equilibrium carbon
content of ~ 0. 42 wt-%. Decarburising below this carbon content
leads to a loss of chromium to the slag. If the melt were
decarburised to , 0.08 wt-% carbon as required by most stainless
steel grades, the chromium content would be reduced to < 5 wt-
% which is well below that required. It can also be seen that
increasing the temperature allows lower carbon contents at any
given chromium content.

15. For the 18 wt-% chromium alloy, the equilibrium carbon content
can be decreased to 0.1 wt-% if the temperature is raised to
1800 ͦC. While higher temperatures permit greater extents of
decarburisation there are a number disadvantages, including
higher energy costs and higher refractory consumption.
In practice EAF slags are not necessarily saturated with
chromium oxides so the values of the activity coefficients of CrO
and CrO1.5 in slag are significant, and depend on the
composition of the slag. Maximising the values of these activity
coefficients is desired because this minimises the concentration
of chromium oxides in slag and therefore minimises chromium
losses. Stainless steelmaking slags contain mostly CaO, SiO₂,
MgO and Al₂O₃.

16. Liquid region at 1600 ͦC in
CaO–SiO₂–MgO–5 wt-%Al2O3
system

18. The processing conditions favourable for minimising chromium
losses during decarburisation are:
(i) operating at as high a temperature as possible
(ii) choosing a slag with the maximum possible basicity
(iii) minimising the slag volume
(iv) reducing the input partial pressure of oxygen to reduce pCO
(v) lowering the oxygen partial pressure in the slag and steel
with ferrosilicon additions.

19. The activity coefficients of CrO and CrO 1.5 in the slag increase
with increasing basicity. Thus the most important chemical
property of the slag is its basicity.
Higher temperatures involve more severe attack on furnace
refractories, while operating with a reduced oxygen partial
pressure increases the time required to decarburise the steel
to the necessary low carbon contents. If replacement of
oxidised chromium is required, expensive low carbon
ferrochrome must be employed. The evolution of stainless
steelmaking technology has been guided by the need to
overcome or minimise these process constraints.