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Final Report on Effect of Impurities in Steel
S. Bell, B. Davis, A. Javaid and E. Essadiqi
Report No. 2005-41(CF)
March 2006
Enhanced Recycling, Action Plan 2000 on Climate Change, Minerals and Metals Program —
The Government of Canada Action Plan 2000 on Climate Change Minerals and Metals
Program, managed by the Minerals and Metals Sector of Natural Resources Canada, is working
towards reducing Canada’s greenhouse gas (GHG) emissions from the minerals and metals
sector. By matching funds with other partners, this program supports initiatives that enhance
recycling practices and provide GHG emission reductions.
________________________________________________________
DISCLAIMER
Natural Resources Canada makes no representations or warranties
respecting the contents of this report, either expressed or implied,
arising by law or otherwise, including but not limited to implied
warranties or conditions of merchantability or fitness for a particular
purpose.
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i
FINAL REPORT ON EFFECT OF IMPURITIES IN STEEL
by
S. Bell*, B. Davis*, A. Javaid** and E. Essadiqi**
EXECUTIVE SUMMARY
Over the last decade, the use of secondary ferrous material as a feed material for steelmaking has
grown dramatically because of the advancement of electric-arc furnace (EAF) technology and its
acceptance from the North America (NA) steel industry. In 2002, the EAF surpassed the
integrated two-furnace system as the number-one method for manufacturing steel in NA, and this
trend is expected to continue over the next decade. The success of the EAF is due to its lower
capital investment and greater flexibility compared with the integrated-steel technique.
The increased use of scrap material as a source of iron has caused a sharp rise in impurities in
steel because the current scrap classification and sorting infrastructure in NA is insufficiently
advanced to produce a consistent and rich ferrous feed for steelmaking. In addition, de-coating
techniques have not been widely employed in ferrous scrap preparation, representing one of the
major avenues for metallic impurities to enter into the steel bath.
Most impurities in steel are metallic and can be classified in three categories: primary alloying,
secondary alloying, and tramp elements. The distinction between primary/secondary alloying
and tramp is that the former serve a useful purpose in the steel by enhancing at least one of its
material properties while the latter does not. Common primary and secondary alloying elements
are carbon, manganese, and silicon; and copper, nickel, chromium, and molybdenum,
respectively. The two main elements in the tramp category are sulphur and phosphorous,
followed closely by tin and zinc. It should be noted that phosphorous does offer some positive
benefits to the steel, but these are heavily outweighed by its negative effects. Therefore the
designation of phosphorous as a tramp element is still considered to be valid in the steel industry.
Because of the regular occurrence of impurities in both solidified and molten steel, a better
understanding of their effects on the quality of steel, either directly or indirectly, has been
achieved. If the impurity influences the properties of the steel, it is considered to have a direct
effect; direct effects are the main focus of this report. On the other hand, if the impurity causes
specific conditions to be used that are outside of the regular norm for steelmaking or produces an
unwanted by-product, it is considered to have an indirect effect.
Primary alloying elements increase the mechanical properties of the steel – both yield and
ultimate-tensile strength, as well as hardness. They also reduce the occurrence of hot shortness
*Kingston Process Metallurgy Inc., Kingston, Ontario, Canada
**CANMET Materials Technology Laboratory (CANMET-MTL), Ottawa, Ontario, Canada
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ii
during hot-working practices. However the increase in strength is offset by a decrease in
ductility caused by the formation of precipitates. Weldability is also adversely affected; in
particular, silicon tends to decrease the machinability of the steel.
The positive mechanical effects of secondary alloying elements are identical to those listed
above for primary with a further impact on toughness (except with copper additions). Fatigue
strength is also improved for nickel and molybdenum alloys. From a chemical standpoint, the
three common stainless steel additives and copper increase the corrosion resistance of the metal.
On the other hand, the negative effects, aside from a loss in ductility, are slightly different for
alloys containing copper. Copper decreases the surface quality of the steel and reduces its ability
to be acid pickled, and it increases the tendency for hot shortness to occur. The ability to acid
pickle molybdenum-containing steels is also reduced.
With the exception of phosphorous, tramp elements dramatically decrease the strength of steel;
phosphorous has the remarkable ability to increase its strength, hardness, and hardenability.
Sulphur and phosphorous adversely affect the material’s toughness, fatigue strength, and
weldability. Moreover sulphur, tin, lead, and zinc negatively affect the surface quality of the
steel. Hot shortness is more prevalent when tramp elements are present – especially tin. Tin
also reduces ductility and the ability of the steel to be acid pickled.
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iii
CONTENTS
Page
EXECUTIVE SUMMARY I
INTRODUCTION 1
TYPES OF IMPURITIES 1
SOURCES OF FERROUS SCRAP 2
ORIGIN OF COMMON STEEL IMPURITIES 3
Tramp Elements 3
Lead 3
Tin 3
Zinc 4
Secondary Alloying 5
Antimony 5
Arsenic 5
Bismuth 5
Copper 5
Chromium, Nickel, Molybdenum, and Vanadium 6
CATEGORIES OF STEEL 6
Chemical Composition of Each Category of Steel 6
INFLUENCE OF IMPURITIES ON THE QUALITY OF STEEL 7
PRIMARY ALLOYING AGENTS 7
Carbon 7
Manganese 8
Silicon 8
SECONDARY ALLOYING AGENTS 9
Copper 9
Nickel 9
Chromium 10
Molybdenum 10
Vanadium 10
Niobium 11
Aluminum 11
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Boron 11
Cobalt 11
Tungsten 11
TRAMP ELEMENTS 12
Sulphur 12
Phosphorous 12
Tin 12
Lead 13
Zinc 13
Antimony 14
Cadmium 14
Mercury 14
CONCLUSION 14
REFERENCES 15
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1
INTRODUCTION
Steel is, by weight, the world’s number-one material produced because it is relatively
inexpensive and highly versatile. However over the last two decades, the steel industry in North
America (NA) has changed dramatically with the decline in blast-furnace production and the
emergence of electric-arc furnace (EAF) technology. Current data show that more than half of
the steel produced in NA comes from EAFs, while the remainder is produced through an
integrated, two-stage system that employs both a blast furnace and a basic oxygen furnace
(BOF). This trend is expected to continue, and some analysts have predicted that the percentage
of U.S. steel produced by EAFs will increase at a rate of 4% per year over the next decade1.
One of the main differences between these two processes is in the feed material. In a typical
EAF charge, 25-100% of the feed comes from secondary material, depending on the quality of
the steel being produced. However in a typical BOF process, only 25-35% of the initial charge
is secondary material with the majority being pig iron. Thus, much more iron-bearing scrap is
being used in current steelmaking practices. In 2002, 69% of the U.S. crude steel was
manufactured from recycled material2.
Because of inadequate scrap classification systems and sorting technologies, ferrous scrap is
often contaminated with residual elements. This increases contaminant levels in the steel bath
and necessitates either a secondary treatment or dilution to reduce their concentration to meet
customer specifications. However, these struggles have led to a better understanding of how
certain elements affect the material properties of steel, both mechanically and chemically. Most
of this knowledge was developed as a result of difficulties experienced during fabrication or
through performance tests conducted on finished products.
The main goal of this report is to summarize the effect of various elements on the properties of
steel. However, it is also important to understand the different classes of impurity elements
commonly found in steel and the avenues through which they enter into the ferrous feed stream.
This will therefore be the focus of the first part of this report.
TYPES OF IMPURITIES
Impurities in steel can be classified under three different categories: primary alloying, secondary
alloying, and tramp elements.
Primary and secondary alloying elements are added to carbon steels to positively influence their
properties. Typically primary alloying elements include carbon, manganese, and silicon while
secondary alloying elements include copper, nickel, chromium, molybdenum, aluminum,
vanadium, niobium, boron, cobalt, and tungsten. Within the secondary alloying element class,
however, the term ‘residual elements’ is commonly used to describe copper, nickel, chromium,
and molybdenum and is sometimes extended to include cobalt and tungsten. This extension
usually occurs if specialty steels, with a high alloying content, are used in the initial feed
material. By definition, residual elements are classified as elements that are incorporated into
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2
the metal but have no minimum concentration specified for the grade of steel being
manufactured. In some instances, a maximum concentration for a specific residual element is
noted in the steel specification. However, the main distinction between secondary alloying and
residual elements lies solely in the fact that the latter can have both positive and, most
importantly, negative effects on the properties of the steel.
Tramp elements are summarized as elements that do not serve a useful purpose in the furnace
and are mostly found in the slag, furnace atmosphere, or bag-house systems. If they are present
in a large quantity, they can incorporate themselves into the metal and result in adverse metal
properties. Generally, the term ‘tramp elements’ refers to: sulphur, phosphorous, lead, tin,
antimony, zinc, cadmium, and mercury.
It should be noted that the terms ‘residual’ and ‘tramp’ are interchangeable within the steel
community with no distinction made between the two types of elements because some residual
elements negatively affect the steel and therefore fit within the definition of a tramp element.
For the purpose of this report, the term ‘residual elements’ will be omitted, and copper, nickel,
chromium, molybdenum, cobalt, and tungsten will be discussed under secondary alloying
elements.
SOURCES OF FERROUS SCRAP
Steel scrap can be subdivided into two categories: new and old scrap. New scrap encompasses
both home and industrial scrap and is the cleanest type of secondary material available for
steelmaking. Home scrap is reserved for metal that is manufactured during the initial production
of steel and usually occurs in the casting house. Typically the majority of the home scrap is
recycled in house by direct addition into either the BOF or the EAF process. Over the last
decade, the availability of home scrap has dropped dramatically due to advancements in casting
technology3.
Industrial scrap is scrap that is produced from processing operations such as cutting, drawing,
extruding, or machining, and it can represent a large fraction of the starting material. For
instance, a typical General Motors stamping plant loses approximately one fifth of its metal input
to industrial scrap4. Losses from secondary casting operations are also included in this scrap
class. In most cases, industrial scrap is dirtier than home scrap because of the use of lubricants
during the processing operations listed above.
Old scrap is most often referred to as ‘post-consumer scrap’ and is described as steel components
that have been discarded after serving a useful life. Because of this generic definition, old scrap
encompasses a variety of different sources the most common being old automobiles, representing
40% of the total amount of old scrap recycled in the U.S. in 2002. Other sources of post-
consumer scrap are home appliances, machinery, railroad cars and tracks, construction
structures, and cans.
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3
ORIGIN OF COMMON STEEL IMPURITIES
Old scrap represents a major portion of the feed material in steelmaking. However this scrap
class is relatively unclean because of the presence of secondary elements. These elements
generally exist in a variety of different forms but can be summarized under the following three
headings:
1. pure state,
2. coating material, and
3. alloying agents.
Impurity elements that exist in their pure state are generally a result of poor separation practices.
Recycled electric motors are one such example because steel and copper coexist in a pure state.
A variety of different coatings are used for different steel products to enhance corrosion
resistance. The most common coating material is zinc, which is used for galvanizing steel.
If specialty steel scrap in used in the production of normal carbon steel, its alloying elements can
introduce unwanted impurities. The most common type of specialty steel is stainless, and its
inclusion in normal feed streams results in high nickel, chromium, and molybdenum levels.
A more in-depth look at individual sources of contamination for common tramp and secondary
alloying elements will now be conducted.
Tramp Elements
Lead
Lead is an alloying agent used in some engineering alloys to improve specific properties of steel
(machinability, antifriction, and bearing). Melts become contaminated with this metal when
free-machining alloys, containing 0.15-0.35% lead, intertwine with other lead-free grades9. In
addition, lead is a major constituent of many solders and is also present in some brasses as an
alloying element. Lead brasses are often used for bearings, cable sheathing, and tubes. Another
source of lead is pigments for white and red lead paints. Red lead is a common protective
coating for iron and steel and can find its way into the melt via these products. Lead has also
found uses as a coating material for steel sheet (terne plate), which is a common material for
automotive gasoline tanks. However because of lead’s classification as a designated substance
(cancer) and its disastrous affect on the environment, it is slowly being replaced by more friendly
alternatives.
Tin
Tin is a major coating material frequently used in steel food and beverage cans as well as other
packaging materials. It has been estimated that approximately one third of the tin produced
worldwide was used for the consumer packaging industry8. In the late 1990s, the amount of tin-
plated products manufactured in the European Union (EU) alone was estimated to be around
4.1 million tons per year5. However, the quantity of tin being used for steel coatings has been
declining steadily since its first commercial application at the beginning of the 19th century8. In
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4
the 1930s, approximately 22 kg of tin per tonne of steel was produced, and by the late 1990s this
amount had dropped below 4 kg/tonne of steel8. Most of this decline can be attributed to the
decrease in thickness of a typical tin coating layer. However, this reduction has not eliminated
the problem of tin contamination in steel since only a very small amount of tin is required to
produce unacceptable levels in all grades of steel. If an entire bath made up of tin-plated steel
was melted and not pre-treated (de-tinning), a tin concentration around 0.3% would typically be
achieved8. This is well over the maximum concentration specified for commercial and drawing-
quality steels (Table 1). Some pre-treatment of tin-plated steel is currently being conducted
before melting (usually electrolytically), but its implementation is still being hampered by
economic and technical challenges.
A relatively new, but substantial, source of tin impurities is emerging from tin-based solders,
which are replacing the less environmentally friendly lead-based solders. Most tin-based solders
are used for electronics and are therefore finding there way into many post-consumer product
lines that incorporate electronic components.
The other main source of tin contamination is bronzes. Most bronzes are used as engineering
material for machinery parts, and, if they are not separated from the ferrous feed material, they
can be introduced into the melt. There has also been a recent push to incorporate bronzes,
mostly tin-zinc, into the automotive sector since they have experimentally exhibited a
remarkable corrosion resistance. Therefore, there remains the possibility that, in the future,
additional tin contamination could result through recycled automobiles.
Zinc
Most of the zinc impurities found in steel come directly from the recycling of zinc-coated scrap.
The process of coating steel with zinc is referred to as galvanizing, either through hot-dipped or
electrolytic operations, and it results in a zinc content between 1-4% of the product weight8.
Hot-dipped products have a much thicker layer of zinc than their electrolytic counterparts.
Roughly 50% of all galvanized steel used in the EU is sheet metal with a zinc concentration of
2.74%8. In 1994, almost a million tons of zinc was employed in the EU for galvanizing, which
was much higher than in the previous decade6. This was directly related to the increased number
of galvanized products produced in the EU, from 6.3 million tons in 1985 to
13 million tons in 19948. The main reason for the profound increase in galvanized products was
its incorporation into the automotive industry. Since the average life-span of a vehicle is
estimated at 10 years, a large amount of galvanized steel is now starting to be recycled. This has
had a significant influence on the amount of zinc entering into a typical steel bath and by-
products formed outside of the furnace atmosphere (EAF dust).
Zinc has also found new applications in other steel coatings, such as zinc-aluminium, zinc-iron,
and zinc-nickel, in order to improve the corrosion resistance of the products. The main purpose
behind their development and implementation was to reduce the cost of more widely used
coatings, like zinc.
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5
Secondary Alloying
Antimony
Antimony is generally found in scrap piles that contain brasses and lead-based alloys. In both of
these sources, antimony is used as an alloying agent. Antimony-containing alloys find
applications as machinery bearing materials and are commonly found in old vehicles.
Arsenic
Arsenic is used as a decolourizer (i.e., pigment) in glass. However, it has also found limited uses
in bearing alloys and in some grades of copper as an alloying agent.
Bismuth
Bismuth is used as an alloying element to improve the machinability of free-machining steels
and is often intermixed with lead. Bismuth is looked upon as a possible replacement for lead
since it is much less toxic than lead. Therefore, it is expected that the concentration of bismuth
in future scrap will rise dramatically.
Copper
The main source of copper impurities in steel is from end-of-life vehicles. Copper is commonly
used for electronic equipment such as wires for electric motors and cooling elements. It is
estimated that 40-50 electric motors are contained in a typical passenger car, and, with demands
for more sophisticated features, this figure is expected to rise7. In addition, the size of the
average vehicle has dropped dramatically in the last decade resulting in much smaller parts that
are nearly impossible to dismantle economically. This is the main reason why most copper-
bearing parts are not dismantled but are just compacted and shredded with the bulk of the
automobile. Since the wires are small and entangle easily into the steel parts, they typically by-
pass most magnet separators.
Copper is also a main constituent of bronze, which is a very popular class of alloys for many
small automotive parts such as gears, wheels, springs, fasteners, etc. Zinc and copper are
combined to produce brass, which is the material of choice for condenser tubing, machine parts,
screws, etc. Moreover, copper is commonly alloyed with nickel to produce a range of
components including heat exchanger tubing, condensers, and electrical-resistance parts.
The last main source of copper contamination is steel alloys that contain a high percentage of
copper. Typically, structural steels have a copper concentration up to a maximum of 0.5% in
order to increase its corrosion resistance8. Some steel sheet grades used for culverts, pipes, and
other manufacturing applications (washing machine boilers) may contain copper up to 0.25%9.
Lastly, some precipitation-hardening steel grades contain 0.4-1.25% copper10. Copper
contamination generally occurs when these steel grades are used for manufacturing non-copper-
bearing steel alloys.
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6
Chromium, Nickel, Molybdenum, and Vanadium
All four of these elements are regularly used as alloying elements in many different classes of
steel, therefore contamination only occurs when these scrap classes (i.e. stainless steel) are
introduced into steel melts that are intended for other classes of steel. As mentioned above,
nickel is also a minor part of zinc-nickel galvanized coatings, up to maximum of 15%.
Moreover nickel-copper alloys, employed for condenser and heat-exchanger tubing, are found in
steel scrap charges. Chromium and nickel, in some instances, are used as individual coating
materials, but applications are rare.
CATEGORIES OF STEEL
For the purpose of this report, carbon and alloy steel products have been categorized into four
groups structural, engineering, commercial quality, and drawing-quality steels. Structural and
engineering steels are closely related and pertain to most ‘long products’ for structural purposes,
including bars, hot-rolled plate and sheet, and tubular products. The steel products that fit in
either of these two categories must meet a number of quality standards set forth by the steel
industry. These standards include structural quality (SQ), merchant quality (MBQ), and special
bar quality (SBQ). Contained within these standards are high-strength low-alloy (HSLA),
degassed (DG), and American Petroleum Institute (API) quality steels. The most important
physical property of structural and engineering steels is their mechanical strength, which
includes high yield and ultimate-tensile strengths, impact toughness, high- and low-temperature
strength, and hardness. For structural steels, most of these properties are achieved through the
material being hot worked (hot rolling and forging) while engineering steels rely on the
material’s alloy composition and heat treating to achieve its desired properties.
On the other hand, commercial and drawing-quality steels involve mostly flat products, such as
sheet, but it also includes cold-drawn bar, rod, wire, and tubular pieces. The quality standards
these products must meet are commercial quality (CQ), drawing quality (DQ), and extra-deep
drawing quality (EDDQ). These three standards also encompass degassed and degassed-
interstitial free (DGIF) steels. The main properties shared by these steels are their ductility and
formability. However they must also possess adequate mechanical strength, achieved mostly by
‘cold forming’ techniques. The four most common types of cold forming are cold rolling, deep
drawing, stamping, and cold drawing, of which cold drawing is the only process that does not
involve the manufacture of sheet products and is specifically allocated for wire forming. In
addition to ductility, these steel grades must also have a high surface quality and ‘coatability’.
Chemical Composition of Each Category of Steel
Depending on the quality of steel being produced, the maximum concentration allowed for a
particular element will vary. The degree of variance can be seen in Table 1, indicating either the
range or the maximum concentration of a particular element for the four quality categories of
steel.
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7
INFLUENCE OF IMPURITIES ON THE QUALITY OF STEEL
Elements can affect the quality of steel either indirectly or directly, depending on the specific
element (alloying – either primary or secondary – or tramp). If the presence of the element
requires specific processing conditions for the steel – ranging from specific ladle treatments to
casting to annealing - then it is considered to affect the steel indirectly. However, if the element
affects the steel’s physical properties – either by being in solid solution, segregating to interfaces
(grain boundaries), or forming intermetallic compounds – then it is considered to influence the
quality of the steel directly. The majority of the upcoming discussion will focus on the direct
effects of primary and secondary alloying agents and tramp elements on the physical and
chemical properties of steel directly. However, some indirect effects will be mentioned if they
are deemed important. In addition, the majority of this section of the report will center on tramp
elements because of their detrimental or more pronounced effects on steel quality in relation to
alloying elements.
PRIMARY ALLOYING AGENTS
Carbon
Carbon is the most important element in the manufacture of steel because it is solely responsible
for providing its exceptional hardness through the formation of iron carbide (Fe3C) precipitates.
As steel is cooled through its transition temperature, gamma iron (austenite) converts to alpha
iron (ferrite) and Fe3C. Due to the precipitation of Fe3C particles, the tensile strength of the
material increases since the particles act as dislocation barriers. However an inverse relationship
between carbon concentration and ductility exists, which can be detrimental to commercial and
drawing-quality steels. In most circumstances, the carbon concentration for these two categories
of steel is much lower than for structural and engineering-quality steels (Table 1). It should be
noted that the ductility of the material could be increased with proper heat treatment. In addition
to tensile strength and ductility, the weldability of the steel is inversely affected by rising carbon
levels.
Because of the dual nature of carbon on the properties of steel, a ‘carbon equivalent’ factor was
established to meet specific properties for certain applications. One such example of a carbon
equivalent calculation is illustrated below based on the concentration of four different elements:
“Carbon Equivalent” = % Carbon + 0.25x % Manganese + % Vanadium + % Niobium
Carbon also has two main indirect influences on the processing of steel. In order to meet the
proper carbon-equivalent factor, oxygen must be used in excess to remove the carbon through
the formation of mostly carbon dioxide (CO2). This process is inconvenient since it requires
additional furnace time and reduces daily output. On a positive note, the addition of carbon to an
oxidizing slag can cause it to foam and produce an insulating layer over top of the steel melt.
This not only decreases heat loss in the furnace but can also prolong electrode life in EAF
system and decrease refractory wear.
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8
Manganese
Manganese is mostly known as an Fe3C stabilizer within ferrous metallurgy. Under certain
conditions, iron carbide can disassociate to metallic iron plus graphite, in a process known as
graphitization. However it is well known that a manganese addition, resulting in a minimal
concentration of 0.3 wt %, ensures that graphitization does not occur11. A high silicon level in
steel also promotes graphitization, but it can be counter balanced by adding at least 0.3 wt %
manganese into the melt11.
Manganese also prevents steel from tearing during either hot rolling or forging, which is known
as ‘hot shortness’. Hot shortness is a common phenomenon in steels that contain sulphur since it
combines with iron to form iron sulphide (FeS), a very low-melting point and brittle inclusion.
The occurrence of FeS impurities causes the metal to lose strength in localized areas, especially
during hot rolling, because of its brittleness. However, the addition of manganese promotes the
formation of manganese sulphide (MnS) rather than FeS. Manganese sulphide inclusions are
typically dispersed throughout the steel and are ductile enough to deform during normal hot
rolling or forging practices. A manganese to sulphur ratio of eight to one is typically sufficient
for inhibiting hot-shortness characteristics11. Manganese is also known to increase hardenability,
toughness, and tensile strength, but it hinders ductility through the formation of MnS
precipitates. Manganese also decreases weldability.
Silicon
Silicon indirectly benefits steel through a number of processes. Firstly, silicon oxidizes to form
silica (SiO2) during the oxygen blow to remove mainly carbon. The SiO2 formation is very
exothermic, providing a secondary source of heat to the metal. Silica also reduces the time
required to melt the lime by reducing its melting point.
Silicon also plays an important role in deoxidizing molten steel. Generally before continuous
casting is initiated, the melt must be deoxidized. Silicon, in a lump, powder, or rod form, is
added to the melt and is the primary choice of deoxidizer for manufacturing continuously cast
billets for bar and structural products. Other deoxidizers include aluminium, titanium, and
calcium.
However a specific ratio of lime to silica must not be exceeded since it will begin to affect the
refining capability of the slag to remove sulphur and phosphorous from the underlying metal.
This maximum ratio has been estimated at 2.5 to 111. Silica also decreases the life expectancy of
the refractory, and, as mentioned above in the manganese section, silicon increases the likeliness
of graphitization.
Directly, silicon has shown to increase tensile strength due to the formation of silicate inclusions,
but it reduces the material’s machinability. High-silicon alloys are also used for specialty
electrical steels because of their ability to increase permeability and electrical resistivity and
decrease hysteresis losses. This has made silicon steel an attractive choice for transformers,
electric motor laminations, generators, and relays.
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9
SECONDARY ALLOYING AGENTS
Copper
Copper is completely soluble in steel, and the only existing economically feasible technology for
reducing its concentration is through dilution, which makes it a very unattractive material in feed
streams. Moreover, copper has a very negative effect on the surface of the steel since it
promotes hot shortness during hot rolling, forging, and casting operations12. Typical hot-
working processes operate between 1050-1200ºC8. Since copper does not readily form
intermetallics with oxides, sulphides, or carbides in steel, it has a tendency to accumulate and
form copper-enriched zones. Surface scaling and the low solubility of copper in austenite
amplify these zones. When the temperature reaches the hot-working zone, a copper-rich liquid
phase is formed under the scale. This phase moves into the grain boundaries and leads to a loss
of ductility and eventually intergranular fracture. Laboratory hot-bending tests have been
conducted8, and the result demonstrated that the minimum temperature needed for hot shortness
to occur in copper-containing steels was 1050ºC and that copper concentrations around 0.1 wt %
resulted in complete fracture. These tests were later industrially confirmed by Niles13. Another
source estimated the concentration to be close to 0.2 wt %, but no indication was given on their
particular testing method11.
Other alloying elements can either enhance or reduce the degree of hot shortness caused by
copper, making the situation very complex. For instance, antimony, tin, and arsenic all amplify
the hot shortness caused by copper while nickel reduces it. Each one of these elements affects
the hot shortness caused by copper in different degrees as illustrated in the following equation,
which is loosely labelled as the copper-equivalent factor14:
“Copper-Equivalent Factor” = %Cu + 10 x %Sb + 5 x %Sn + 2 x % As - %Ni
Nickel
The effects of nickel on steel properties are mostly positive and are summarized below:
• Increases hardenability;
• Strengthens ferrite (‘stiffens’);
• Limits distortion in heat treating and allows the use of a milder quenching medium;
• Allows high strength and toughness to be achieved at lower carbon levels;
• Improves weldability, plasticity, and fatigue properties;
• Raises ability to case harden;
• Enhances corrosion resistance; and
• Stabilizes austenite in stainless steel.
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10
One drawback to nickel is that it reduces ductility if present in solid solution. In addition, like
copper, the concentration of nickel in steel can only be reduced by dilution, which can
dramatically increase processing costs if a high-nickel feed is used in either BOF or EAF.
Chromium
Chromium is mostly known for dramatically increasing the corrosion resistance of steel when
present in concentrations greater than 4%, which is linked to the vast number of current stainless
steel alloys. Chromium also enhances carbide formation, which can improve wear resistance
and, to a marginal degree, increase resistance to softening during normal tempering practices.
As with nickel, chromium in solid solution ‘stiffens’ ferrite but reduces ductility; it also
enhances hardenability depth.
Chromium does impose negative effects on steel properties. The most common is temper
embrittlement, which refers to a reduction in ductility when tempered or cooled in the range of
700 to 1100°F. This type of defect occurs even when phosphorous, tin, arsenic, and antimony
(the major players for causing temper embrittlement) are low.
Chromium also causes a few, but significant, indirect problems to steelmaking. Chromium oxide
is only stable at very high temperatures and therefore cannot be oxidized readily at normal
operating temperatures. This requires more heat, a longer heat time, and a larger slag volume to
remove this element through oxidation. In addition, hexavalent Cr can be found in baghouse
dust, causing the dust to be labelled as a hazardous waste.
Molybdenum
When molybdenum is present in solid solution with steel it ‘stiffens’ ferrite and decreases
ductility, which is a common trait of all three common stainless steel alloying elements.
Moreover, it is a strong carbide former and has a pronounced effect on the hardenability of steel.
Molybdenum also inhibits pearlite formation and other microstructural changes during heat
treating. Corrosion resistance, toughness and fatigue properties are also improved with
molybdenum additions while acid-solution pickling is inhibited.
The major drawback to molybdenum, from the indirect side, is that it does not oxidize easily and
is very hard to remove from molten steel if its concentration is too high.
Vanadium
Vanadium is a very aggressive grain refiner that works by restricting the growth of ausentite
grains. It also promotes carbide and nitride formation, which enhances wear (or abrasion)
resistance. The yield strength, toughness and hot hardness of the steel are all positively
influenced by vanadium. Softening during tempering is limited with the addition of vanadium,
and it is effective in eliminating or reducing strain aging by locking up free nitrogen.
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Niobium
Niobium is more commonly referred to as ‘columbium’ in the ferrous metallurgy world, and its
influence on steel is almost exactly that of vanadium. The only difference between the two
elements is that columbium is not a strong nitride former. Along with vanadium, columbium is
very expensive and is mostly found in exotic steels.
Aluminum
Aluminum’s main use in the steel industry is as a deoxidizer since its affinity for oxygen is
extremely high. It also reacts with nitrogen to form aluminum nitride, which reduces the
occurrence of strain aging. Only in small amounts is aluminum used for grain refining by
limiting austenite grain growth. Aluminum is also known to increase toughness, especially at
low temperatures. Because it is a strong nitride former, aluminum promotes high surface
hardness and wear resistance when added to nitride steels.
One of aluminum’s major disadvantages comes from the indirect side. Aluminum oxide can
precipitate during continuous billet casting and cause the tundish nozzles to clog.
Boron
Boron is only advantegeous in steel in very small amounts (under 0.003%) as it improves its
hardenability by inhibiting ferrite precipitation during transformation from austenite in common
heat-treating practices11.
However, boron has a high affinity for oxygen and nitrogen and is not generally used in EAF
processing because of the subsequent high levels of oxygen and nitrogen contained within the
melt.
Cobalt
Through solid-solution strengthening, cobalt is a ferrite ‘stiffener’, which is an effect that occurs
at relatively high temperatures and increases the steel’s hot hardness. However, cobalt has one
main negative indirect effect that limits its use within the steel industry. Cobalt isotopes are used
as a common radioactive source in many industries, and, if these isotopes become entrained in
the scrap feed and are subsequently melted, they can have dire consequences to the steelmaker.
It should be also noted that cobalt cannot be eliminated from the bath, and its concentration can
only be reduced through dilution.
Tungsten
Tungsten is mostly known for forming relatively hard and stable carbides within the steel’s
microstructure, and it is almost entirely used for high-speed tooling steels requiring exceptional
wear resistance and high hot hardness. However due to its high cost, it is rarely used.
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TRAMP ELEMENTS
Sulphur
There is only one benefit for incorporating sulphur in steel and that is to improve machinability
of the steel in applications where tensile strength and other mechanical properties are not
important. As discussed previously in the manganese section, sulphur tends to react with iron to
form FeS, and, because of its brittle nature, it acts as chip breakers during machining.
Otherwise, sulphur is considered to be an unwanted impurity. This is because sulphur is
responsible for reducing the impact strength, ductility in the transverse direction, and
weldability. Moreover it causes a dramatic decline in the surface finish of the steel product, and,
in low-manganese steels, it promotes hot shortness.
To remove sulphur from either an EAF or a BOF melt, a basic slag must be employed at very
high temperatures and with a carbon boil to improve slag-metal interaction. Common additives
used to reduce sulphur levels further are calcium carbide (CaC2), calcium silicide (CaSi), and/or
a magnesium oxide-lime mixture.
Phosphorous
Phosphorous does promote some positive benefits to the steel, such as improved hardenability,
corrosion resistance, and machinability (restricted to re-sulphurized steels only). However, these
positive effects are outweighed heavily by the large losses in plasticity, ductility, and impact
strength. Therefore phosphorous, in almost all cases, is considered an unwanted impurity.
Removing phosphorous from a steel bath is also very difficult. During initial melting of the iron
feed, phosphorous oxidizes and enters into the melt as phosphorous oxide (P2O5) and will
eventually migrate into the slag. In the slag, the P2O5 can cause a ‘carbon boil’ to occur by
reacting with carbon to form CO2 and elemental phosphorous. Since the CO2 formation is very
exothermic, the bath can overheat. To stop the carbon from oxidizing, blockers are generally
added to consume the oxygen from the P2O5. Even iron, at high temperatures, is known to react
with P2O5 and cause elemental phosphorous to re-enter the bath. The indirect negative effects of
having too much phosphorous in the bath are increased heat time, lime consumption, slag
volume, and energy consumption as well as a reduction in furnace yield.
Tin
Tin adversely affects many steel properties, and, because it is almost completely soluble in liquid
steel and does not oxidize, dilution is the only way of reducing its concentration. In addition, tin
has a very minute solubility in solid steel and, upon cooling, it tends to segregate out to various
interfaces, such as grain boundaries. This can lead to many surface defects, including hot
shortness15. This is why tin is also expressed in the ‘Carbon-Equivalent Factor’ equation.
Compared with copper, the presence of tin increases the likelihood for hot shortness to occur by
a magnitude of five times and is therefore a key player for this particular defect. Moreover, in
low-carbon steels, the segregation around grain boundaries can cause embrittlement during
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annealing. High tin levels have also been linked to deterioration in surface quality and
toughness properties, measured from standard Charpy tests16.
Lead
Lead has a very low solubility in steel, which can lead to a few direct effects on the properties of
steel. Because of the rejection during solidification, the lead mostly accumulates on the surface
of the steel and can reduce its surface quality. This accumulation can also cause hot shortness to
occur during hot working, resulting in either edge or surface tearing. Lead’s only positive role is
to increase the machinablility of the steel by forming inclusions that act as chip breakers.
However the applications for lead-containing steels are limited to situations where tensile
strength and other mechanical properties are less important.
Indirectly, lead has several negative effects on the steelmaking process because of its high
toxicity and permeability in refractory bricks. Lead generally exists in scrap metal as a coating
constituent and, when heated to regular steelmaking temperatures, it readily volatilizes in the
furnace exhaust and oxidizes. The lead oxide is then recovered in the baghouse system and is a
common part of BOF and EAF dust causing the dust to be classified as a hazardous material,
resulting in more stringent and costly disposal regulations.
If the lead does become encapsulated into the steel bath, it forms small liquid droplets of metal
that tend to collect at the bottom of the furnace because of their density. At the crucible bottom,
the lead accumulates and can penetrate the refractory lining of the furnace and reduce brick life.
If the brick is not changed in time, which requires the furnace to be taken out of service, the steel
can penetrate the refractory wall and spill out of the furnace. This not only represents a safety
risk but also requires expensive equipment repair and longer furnace downtime.
Zinc
Similar to lead, zinc is insoluble in steel and may lead to hot shortness during hot processing.
The tears generally appear on the surface of the metal because of its insolubility. This is zinc’s
only real direct affect on the properties of steel.
In most baths, zinc volatilizes at normal steelmaking temperatures and rarely becomes entrapped
in the metal. However if the zinc content in the bath is significantly high, a larger probability of
zinc being carried over the continuous-casting operation exists. If this occurs, the zinc can
segregate to the surface and completely separate from the steel by depositing on the inner surface
of the continuous-casting mould. This can reduce the heat transfer through the mould and, if
greatly affected, can cause the skin of the solidifying steel to break open. The end result is
molten steel pouring onto the continuous-caster equipment, which would require extensive
equipment repair and a long downtime. Another indirect effect of zinc occurs in the baghouse.
Most of the iron units in BOF and EAF dust are surrounded by a zinc shell, which does not
enable it to be recycled directly back into either process.
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Antimony
Antimony is not commonly found in molten steel and only results when lead batteries, solders,
and semiconductor materials enter into the feed stream. Antimony directly affects steel
properties by increasing the probability of hot shortness occurring and decreasing the mechanical
strength of the metal. However, in most cases, antimony vaporizes and is recovered in the
baghouse as dust contributing to the material being classified as a hazardous waste.
Cadmium
Cadmium can find its way into the steel scrap stream through a variety of avenues, such as steel
coatings for improved corrosion resistance (mostly electroplating), nickel-cadmium batteries,
and paint pigments. As these materials are melted in the furnace, cadmium mostly enters the
baghouse system and is a common constituent of the dust that is generated. Similar to lead and
antimony, cadmium contributes to the dust’s hazardous-waste classification.
No direct effects were found in the literature for cadmium, indicating that it is not commonly
found in solidified steel.
Mercury
The main sources of mercury in steel scrap are electrical switches in automotive, appliance, or
manufacturing equipment. Mercury is a major concern because of its ability to accumulate in
living tissue and not be expelled. Most of the mercury that does reach the molten bath vaporizes
immediately and incorporates into the baghouse by-product(s).
Because mercury volatilizes, it rarely enters into the steel. Therefore, no direct effects of
mercury on steel were found.
CONCLUSION
Table 2 provides a general overview of the direct effects of primary alloying, secondary alloying
and tramp elements on both direct and indirect properties of steel. The abbreviation “INC”
indicates that the specific property in question increases with rising element concentration and
vice versa with the “DEC” distinction. Blank cells indicate that, either there is ‘no effect’, or
that the effect(s) are very complex and are related to other components, such as (1) interaction
with other elements, (2) specific practices and conditions of hot working (reheat temperature,
furnace atmospheric conditions, etc), and/or (3) either inline or finishing heat-treatment
conditions (annealing, quench, etc). In addition, the distinctions “INC” or “DEC” do not
necessarily mean that the element in question causes the indicated effect for all circumstances
since it is also affected by the factors listed above: (1), (2), and (3). However, it is believed, that
Table 2 adequately simplifies the effects of elements on steel properties, which is a very complex
subject because of the amount of contradictory literature available.
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REFERENCES
1. Fenton, M.D., “Iron and Steel Recycling in the United States in 1998”, U.S. Geological
Survey Circular 1196-G, 1998, http://pubs.usgs.gov/circ/c1196g/c1196g.pdf.
2. Fenton, M.D., “Iron and Steel Scrap”, Minerals Commodity Study, 2004,
http://minerals.usgs.gov/minerals/pubs/commodity/iron_&_steel_scrap/festscmcs04.pdf.
3. Herman, J., and Leroy, V., “Influence of Residual Elements on Steel Processing and
Mechanical Properties”, Process Technology Conference Proceedings, Liege, Belgium,
Vol. 15, 1997, pp. 107-116.
4. Dudek, F.J., Hellickson, D.A., and Koros, P.J., “Issues in Recycling Galvanized Scrap”,
Galvatech Conference Proceedings: The Use and Manufacture of Zinc and Zinc Alloy
Coated Sheet Steel Products in the 21st Century, Chicago, U.S., September 17-21, 1995, pp.
535-42.
5. Marique, C., “Recycling of Tinplate”, Proceedings from the 3rd ECSC Workshop on
Steelmaking, Brussels, Belgium, December 4th, 1996, pp. 145-162.
6. Birat, J.P., “Zinc in the Steel Shop: Raising Actual and Virtual Issues”, Proceedings from the
3rd ECSC Workshop on Steelmaking, Brussels, Belgium, December 4th, 1996, pp. 101-113.
7. Iwase, M., Oshita, H., and Tokinori, K., “Separation of Copper from Solid Ferrous Scrap by
Using Molten Aluminum”, Iron Steelmaker, Vol. 6, No. 22, 1993, pp. 61-66.
8. Janke, D., and Savov, L., “Recycling of Scrap in Steelmaking in View of the Tramp Element
Problem”, Resources for Tomorrow – Materials Recycling, Freiberger Forschungshefte,
Erstes Freiberger Euorpa-Seminar, 1997, pp. 49-66.
9. “Materials Handbook”, Ed. by G.S. Brady et al., 14th Edition, McGraw Hill, New York,
1997.
10. Higgins, R.A., “Engineering Metallurgy-Part I: Metallurgical Process Technology”,
6th Edition, Edward Arnold, London, 1993.
11. Jaffre, D., “Effects of the Elements on Steel Properties: A Summary”, Chaparral Steel, 2003,
www.steelnet.org/new/20030210.a.htm.
12. Uchino, H., “Effects of C and P on Surface Hot Shortness of Steel Due to Cu Mixed from
Steel Scrap”, Journal of Advanced Science, Vol. 13, No. 3, 2001, pp. 260-264.
13. Niles, P.E., “Recycling and Virgin Materials in the Changing European Steel Industry”, Iron
and Steelmaker, Vol. 4, 24, 1997, pp. 33-40.
14. D’Haeyer, R., Leroy, V., et al., “Effects of Tramp Elements in Flat and Long Products”,
Commission of the European Communities, Final Report, Brussels, Belgium, 1995, pp. 160.
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15. Katayama, H., Matsuoka, S., Sano, N., and Sasabe, M., “Research Activities on Removal of
Residual Elements from Steel Scrap in Japan”, Scandinavian Journal of Metallurgy, Vol. 27,
1998, pp. 24-30.
16. Leadbeter, M., Morrison, W., and Roberts, I., “Recycling and Residual Element Build Up”,
Galvatech Conference Proceedings: The Use and Manufacture of Zinc and Zinc Alloy
Coated Sheet Steel Products in the 21st Century, Chicago, U.S., September 17-21, 1995, pp.
555-564.
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Table 1 – Elemental specifications for the four categories of steel products.
Impurity
type Element Structural quality
steels Engineering
quality steels Commercial
quality steels Drawing quality
steels
C 0.05-0.50% 0.06-0.90% <0.1% 0.005-0.08% max
Mn 0.50-1.5% 0.25-2.0% 0.25-1.0% 0.15-0.25% max
Primary
alloying
Si 0.10-0.40% Usually 0.20-
0.35% 0.030% max 0.01-0.03% max
Secondary
alloying Cu Usually 0.35% max 0.12–0.30% max Usually 0.06%
max Usually 0.01%
max
Ni Usually 0.25% max 0.12–3.0% max Usually 0.06%
max Usually 0.01%
max
Cr Usually 0.25% max 0.12-1.50% max Usually 0.06%
max Usually 0.01%
max
Mo Usually 0.02% max 0.03-0.3% max Usually 0.005%
max Usually 0.005%
max
Al Usually 0.01% max 0.005–0.01% max 0.005-0.01% max 0.005-0.01% max
Al (killed) N/A N/A 0.02-0.045% 0.02-0.045%
Al (fine grain) N/A 0.02-0.04% N/A N/A
V N/A –0.025%
addition N/A – 0.10%
addition Usually 0.005%
max Usually 0.005%
max
Nb N/A –0.015%
addition N/A-0.05%
addition Usually 0.005%
max Usually 0.005%
max
B Usually 0.0005%
max 0.0005-0.003%
max addition Usually 0.0005%
max Usually 0.0005%
max
Co Usually 0.01% max 0.005–0.01% max 0.005-0.01% max 0.005-0.01% max
W Usually 0.01% max 0.005– 0.01% max 0.005-0.01% max 0.005-0.01% max
Tramp
elements S Usually 0.04% max Usually 0.02%
max Usually 0.03%
max Usually 0.0015%
max
P Usually 0.02% max Usually 0.015%
max Usually 0.03%
max Usually 0.0015%
max
Sn Usually 0.01% max Usually 0.01%
max Usually 0.01%
max Usually 0.01%
max
Pb Usually 0.01% max 0.005-0.01% max 0.005-0.01% max 0.005-0.01% max
Zn Usually 0.01% max 0.00.5-0.01% max 0.005-0.01% max 0.005-0.01% max
Sb Usually 0.01% max 0.005-0.01% max 0.005-0.01% max 0.005-0.01% max
Cd Usually 0.01% max 0.005-0.01% max 0.005-0.01% max 0.005-0.01% max
Hg Not found Not found Not found Not found
18
Table 2 – Overall summary of the direct effects of various elements on the properties of steel11.
Property C Mn Si Cu Ni Cr Mo V Nb Al S P Sn Pb Zn Sb
Strength INC INC INC INC INC INC INC INC INC DEC INC DEC DEC DEC DEC
Hardness INC INC INC INC INC INC INC INC INC
Hardenability INC INC INC INC INC INC INC
Wear Resistance INC INC INC INC INC
Machinability INC DEC INC INC INC
Toughness INC INC INC INC INC INC INC DEC DEC
Fatigue INC INC DEC DEC
Weldability DEC DEC INC DEC DEC DEC
Electrical
Resistance INC
Corrosion
Resistance INC INC INC INC INC
Surface Quality DEC DEC DEC DEC DEC
Acid “Pickle
Ability” DEC DEC DEC
Hot Shortness DEC DEC INC DEC INC INC INC INC INC INC
Resistance to Hot
Deformation INC INC INC INC INC INC INC INC
Resistance to Cold
Deformation INC INC INC INC INC INC INC INC
Deviation from
Expected Heat
Treat Response
INC INC INC INC INC INC INC INC INC INC
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