Journal of Minerals & Materials Characterization & Engineering, Vol. 8, No.4, pp 283-292, 2009
Printed in the USA. All rights reserved
Challenges of Producing Quality Construction Steel Bars in West Africa:
Case Study of Nigeria Steel Industry
Sanmbo Balogun, David Esezobor*, Samson Adeosun, Olatunde Sekunowo
Department of Metallurgical and Materials Engineering,
University of Lagos, Nigeria 23401
*Corresponding Author, contact:
The production of quality high-yield reinforcing steel bars continues to receive attention from
researchers across the globe due to its importance and contribution to a country GDP. In most
of the developing countries, particularly Nigeria, empirical studies have shown that bars
produced through conventional rolling requires appropriate modification of its chemical
composition in order to obtain the desired mechanical properties such as strength. However, the
high cost factor involved in composition adjustment makes such approach unattractive. Rather,
the application of the combination of systems of controlled rolling and controlled cooling proves
to be the best option. This system however, requires some variations in processing parameters to
suit individual plant production peculiarities. In this paper attempt is made to study the
production challenges and opportunities the steel millers are facing in Nigeria. Previous works
in this area are also reviewed with a view to charting the way forward. Experimental studies and
process monitoring were carried out at some designated rolling mills in Nigeria.
Key Words: Mild steel, bar, strength, controlled rolling/cooling, thermo-mechanical,
Despite stiff competition from other materials, steel remains one of the most important
engineering materials [1].
Indeed the quantum of its consumption per head forms the basis for
measuring the level of a nation’s technological advancement.
284 S. Balogun, D. Esezobor, S. Adeosun, O. Sekunowo Vol.8, No.4
Steel exhibits a wide range of mechanical characteristics of which the strength factor is the
dominant property. Engineering strength is however, evaluated in terms of yield strength σ
ultimate tensile strength (UTS), modulus of elasticity (E), percentage elongation and impact
strength. Thus, any increase in the strength characteristics of steel will enhance the reliability and
durability of the structure/machine in which it is used
[2]. Conversely, low strength
characteristics often result in short life span of structures, warpage, undesirable deflection and
even failure/collapse.
In common engineering applications mild steel, 0.1-0.3%C are used in preference to different
grades of plain carbon steels. The bars, mostly produced by hot rolling, constitute the bulk (90%
by weight) of all structural steel profiles commonly employed in construction and allied
engineering works
[3]. Other areas of mild steel application include structural (reinforcements
and trusses), automobile (car bodies, transmission shafts etc), plant construction, foundry,
agricultural machinery, etc.
A direct correlation exists between steel’s microstructures and its mechanical properties
Hence, the development of a relevant structure – property model in steel is therefore, one of the
effective methods of improving its mechanical properties
[5]. However, appropriate production
method needs to be developed to meet increasing global demand for steel bars of superior
strength characteristics appreciably at low cost. Empirical analyses of methods of producing hot
rolled steel bars indicate a radical departure from the conventional rolling practices. Against the
sole dependence on chemical composition adjustment, emphasis is currently placed on the
development of relevant structural property that guarantees enhanced strength characteristics.
Through skilful manipulation of metallurgical factors, higher strengths are induced in the bar on
the basis of better corresponding microstructures developed.
Regrettably, mild steel bars produced in Nigeria and some West African countries particularly
Ghana exhibit abysmally low strength characteristics. According to a report
[6], the world
average specification for high yield steel bar is 460-500MPa. Comparison of other relevant
strength parameters with locally produced bars is presented in Table 1.
Table 1. Strength Characteristics Specification Comparison
Strength Characteristics
International Standard (min.) Nigeria Bars
Yield Strength (MPa) 460 250 - 350
UTS (MPa) 600 410 - 500
Elongation (%) 10 - 25 9 - 14
Vol.8, No.4 Challenges of Producing Quality Construction Steel Bars 285
With particular reference to the locally produced bars, the reality of the consequences of the data
in Table 1 had always impacted negatively on local steel industry. Risk of failure of structure(s)
in which such bars are used is quite high, giving rise to lack of confidence in the quality of
locally made bars. This has made the massive importation of the better quality product an
imperative. The result is the neglect and underdevelopment of the local steel industry. These are
real engineering problems to which an effective solution must be found.
Several experimental studies and process monitoring were carried out at some designated rolling-
stock (billets, ingots) production plants and rolling mills in Nigeria.
Metallurgical factors investigated include chemical composition, rolling-stock internal soundness
(level of dissolved gases, inclusions, sulphur and phosphorus) and rolling process dysfunction
factors. The latter encompasses the soaking temperature regime, strain, strain-rate, static and
dynamic recrystallisation during rolling and cooling of the rolled product. Basic processing data
obtained from these exercises are used in the analysis.
From the analyses, it was established that the problem of low strength characteristics prevalent in
conventional hot rolled steel bars entails two factors namely: metallurgical and process
3.1. Metallurgical Factor
Table 2 is a compilation of the average chemical composition of various billets/ingots produced
in Nigeria.
Variations of the strain hardening factor parameters in terms of carbon, copper and the Ceq value
are presented in Figure 1. The standard deviation for carbon calculated from the data in Table 2
is 0.09 against the acceptable value of 0.05.
Although, mild steel mostly used for structural purposes contain 0.15-0.30 percent carbon
billet compositions in the range 0.15-0.25 percent are preferable. The choice of the low carbon
level is to prevent embrittlement of the material during strain hardening and the development of
undesirable microstructure in the heat-affected zone (HAZ) of such steel bar during welding.
The two metallurgical factors considered in the formulation of the carbon equivalent relation
are expressed in equation 1 as:
286 S. Balogun, D. Esezobor, S. Adeosun, O. Sekunowo Vol.8, No.4
Thus, for weldable steel, Ceq < 0.51 while Ceq > 0.51 is indicative of non-weldability [9]. From
Figure 1 it can be seen that five out of seven (71.4%) companies’ products tested had C
in the range 0.51 – 0.58 and a standard deviation (σ
) of 0.05 as against the normal 0.02. This
implies that most locally produced steel bars are non-weldable.
Table 2 Average Chemical Composition of Steel Billet Produced in Nigeria
Elements, %
C Si S P Mn Ni Cr Mo V Cu Fe C*
Federated Steel 0.266 0.164 0.018 0.018 0.637 0.026 0.025 0.502 0.001 0.220 Bal 0.40
Sankyo 0.209 0.203 0.048 0.036 0.876 0.096 0.118 0.019 0.003 0.266 Bal 0.41
Delta Steel 0.358 0.397 0.019 0.027 1.109 0.061 0.077 0.013 0.001 0.141 Bal 0.58
Major 0.354 0.365 0.037 0.033 0.801 0.107 0.118 0.017 0.003 0.291 Bal 0.54
Universal 0.345 0.239 0.032 0.028 0.699 0.080 0.128 0.019 0.002 0.232 Bal 0.51
African Steel 0.332 0.210 0.036 0.031 0.857 0.101 0.105 0.013 0.003 0.240 Bal 0.52
Nigerian Spanish 0.376 0.062 0.042 0.005 0.587 0.034 0.024 0.014 0.011 0.223 Bal 0.51
Figure 1: Variations in the concentrations of strain
hardening factor parameter (C, Cu, Ceq) in some Nigerian
Stee l
% C, Cu & Ceq (10
The quality of molten steel depends on the charge characteristics. These include levels of
deoxidation, inclusions, slag composition and cleanliness. Sulphur and phosphorus are the two
major deleterious elements that must be controlled.
It may be deduced from Figure 2, that all the rolling stocks (except a few) maintain the standard
level of concentration of these impurities (0.04 max for the grade of steel).
Vol.8, No.4 Challenges of Producing Quality Construction Steel Bars 287
Deoxidation may be either through oxygen lancing or via the modern practice of slag foaming
[10]. The usefulness of this method however, depends on the slag composition as steel
with high level of dissolved gases particularly oxygen and nitrogen can make the steel behave in
a brittle manner
[11] if not controlled.
Figure 2: Variations in the concentrations of
deleterious elements (S & P) in some Nigerian Steel
% Sulphur & Phosphorus (10
Steel purity entails minimizing the size and frequency of undesirable non-metallic inclusions. It
is well established
[12] that the presence of small-scale inclusions limits maximum stresses and
other desirable mechanical properties of steel. Hence, only a slim allowance is usually
considered for trace elements such as zinc, tin and lead. These elements affect negatively the
creep strength, ductility, susceptibility to corrosion and workability of mild steel
demonstrated that metallic inclusions largely account for the anisotropic behavior
of hot rolled steel bars. These inclusions spread along the rolling direction thus lowering the
ductility and toughness. Uniform mechanical properties at any orientation to the rolling direction
can be obtained if the inclusions are small and equiaxed or globular. Rapid sensor-based
technology or real time analysis of most of the processing parameters can greatly enhance the
properties of steel. Table 3 shows the average tramp elements analyzed for local steel making
charges in comparison with the allowable values in good quality hot rolled mild steel.
It is evident from the table that most steel plants in Nigeria have high scrap input compared with
the use of virgin charges such as direct reduced iron (DRI) and briquettes. This condition can be
improved through dilution with substantial amount of DRI or briquettes to give purer steel as in
the case of Delta steel. A process route for the production of low carbon, aluminum-killed steels
with purity index of 1.5mg of steel and total oxygen content of 27ppm have been developed [15].
With optimum control of the melting process, desirable requirements in terms of chemical
composition can be achieved through either EAF or BOF route. Based on his work, Breedijk, T.
288 S. Balogun, D. Esezobor, S. Adeosun, O. Sekunowo Vol.8, No.4
[16] has reiterated the fact that the silicon semi-killed BOF remains the best process route for
steel meant for concrete reinforcement.
Table 3. Average Tramp Elements in EAF Charges
Type of Charge
Tramp Element
Cu + Sn +
Zn (%)
Sankyo Ltd, Ikeja 100% Heterogeneous
steel scrap
0.50 0.46 Unsatisfactory
U-Steel, Ikeja 100% Heterogeneous
steel scrap
0.46 0.46 Satisfactory
Federated Ota 100% Heterogeneous
steel scrap
0.52 0.46 Unsatisfactory
African Steel
100% Heterogeneous
steel scrap
0.47 0.46 Unsatisfactory
Delta Steel Warri 20% scrap 80% DRI 0.28 0.46 Good
The cost effectiveness of the EAF or BOF route, however, depends on such factors as scale of
operation, cost and availability of raw materials (scraps, highly metalized pellets, etc) and
energy. Currently, gas based DRI is more commonly charged to the EAF
[17]. This could be
attributed to the fact that the former offers higher metallization than the coal based variety and a
higher carbon content that can provide chemical energy in the form of “carbon boil” during EAF
Thus, the control of composition within acceptable tolerance limits is a precursor to the
production of hot rolled bars that exhibit acceptable strength characteristics. Such specification
as published in the British Standard BS 4449 of 1988 is highlighted in Table 4. The data are
based on cast analysis of billets meant for the production of reinforcing bars. The values
specified have been harmonized with ISO 6935 parts I and II to ensure global application.
Table 4.
Chemical Composition of Steel Grades* (BS 4449)
Element Grade 250 Grade 460 Deviations
(% Max.) (% Max.) (% Max.)
Carbon 0.25 0.25 0.02
Sulphur 0.060 0.050 0.005
Phosphorus 0.060 0.050 0.005
Nitrogen 0.012 0.012 0.001
* Grades are given in term of the minimum yield strength: Grades 460 are for hot rolled high yield
deformed bars while grades 250 are for low yield plain bars
Vol.8, No.4 Challenges of Producing Quality Construction Steel Bars 289
3.2. Rolling Process Dysfunction
Hot rolling as a shaping method is the plastic deformation at about 0.6 melting temperature of an
engineering material. At this temperature range, recrystallisation is spontaneous and the structure
is substantially free of strain hardening. Generally, three stages namely, preheating, sequential
plastic deformation and finishing are involved.
Preheating of the rolling stock is part of the metallurgical requirements of the process, which is
regarded as solution treatment in the austenitic phase. This allows for dissolution of solute
precipitates. However, sufficient time is needed to ensure complete homogenization of the
structure by diffusion
[18]. The dependence of diffusion on temperature, T and time, t may be
expressed as
= (2)
)(DtX =
where, D is the diffusion coefficient, D
is proportionality constant, Q, is the activation energy,
R, gas constant, and X, the diffusion distance.
The temperature to which a rolling stock is preheated must be properly controlled in order to
avoid the debilitating effect of grain coarsening at high temperature
[19]. Most rolling mills in
Nigeria do not have temperature-monitoring devices such as thermostats and thermocouples that
are usually attached to preheating furnaces and rolling lines respectively. This deficiency impacts
negatively on the quality of the local rolled products. Emphasis is currently placed on
synchronization of the continuous caster with down-stream mill to avoid the need for reheating
before rolling [20]. This results in substantial reduction in the cost of energy and weight-loss due
to scaling. However, this does not preclude the imperative of inter-stand temperature monitoring
during rolling.
The various metallurgical phenomena namely strain, strain hardening, static and dynamic
recrystallisation occurring during the sequential plastic deformation of rolling stock are
influenced by the series of grooves on the rolls. Roll-pass design is thus a major factor in the
success or otherwise of any rolling process
[21]. For desirable goals to be achieved, careful
handling is required to ensure proper property changes of steel during deformation at varied
loads, temperature and composition. Roll-pass design is also known to have a significant effect
on the flow stress of the metal
[22]. The design that accomplishes rolling of bars within limited
number of passes is usually considered
Principal dynamic recovery in hot working is the softening mechanism for the work hardening of
rolling stock occurring through dislocation climb and cross-slip. There is a fundamental
between plastic strain rate and average dislocation velocity [24]. Thus, the extent of
290 S. Balogun, D. Esezobor, S. Adeosun, O. Sekunowo Vol.8, No.4
plastic deformation a material undergoes is proportional to its dislocation density [25].
Therefore, in hot working where dynamic recovery is not possible through dislocation climb and
cross-slip, dynamic recrystallisation occurs as the softening mechanism. These processes occur
continuously to varying extents depending on temperature and dwell time throughout the rolling
The finishing operation refers to activities aimed at conferring desired property characteristics on
rolled product via microstructural evolution. Both the structure and the corresponding
mechanical properties are determined by the rolling temperature and the
cooling pattern
Apart from the conventional air-cooling approach, a host of other innovative methods have been
developed to meet the ever-increasing demand for rolled products with superior strength
It may be deduced from the foregoing that, microstructure and mechanical properties of hot
rolled steel bars are determined by such factors as the chemistry of the rolling stock, rolling
temperature and the cooling pattern employed. In order to obtain bars of adequate strength levels,
all these parameters must be constantly monitored and controlled.
The reality, causes and consequences of low strength characteristics exhibited by mild steel bars
produced in Nigeria have been analyzed. It would appear that improved processing method is the
most effective and efficient panacea to the problem. Innovative processing methods that integrate
all critical factors namely chemically sound rolling stock, controlled thermal variation during
rolling and controlled cooling are imperative. These could be complemented by installing
relevant process-variables monitoring devices.
The authors acknowledge the technical assistance from Federated Steel Mills Ota and Universal
Steels Limited Ikeja Lagos, Nigeria.
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