Non Metallic Inclusions in Steel: definition, risks, possible solutions
Non metallic inclusions are compound materials, with different chemical
origins than the steel, introduced into the steel during the manufacturing
It is impossible to manufacture steel that does not contain non metallic
inclusions to some extent. These materials also provide different mechanical
properties to steel like the corrosion resistance, toughness, formability, etc.
at the same time – non metallic inclusions are responsible for defects and
failures in steel, however the degree of these effects is yet to be defined. The
less are the non metallic inclusions in the steel the better the quality, thus
the ability to monitor these inclusions is an important part of the quality
control of steel, especially taking into account the array of various important
applications steel i (i.e the structural steel is the widest used material for
Already in 1918, at a May meeting of the British Iron and Steel Institute, A. McCance presented
a study proving that the more defective steel is bad solely due to the
concentration of non metallic inclusions it contains, and as high as 90% of the
faulty material under this notice, are faulty due to the amount of non metallic
inclusions. In 2019, the global structural steel market size reached as high as
290 billion U.S.
dollars , and the recent projections show that the size of the structural
steel market is expected to top 503.8 Billion
U.S. dollars by 2027.
In The US only, 110 million tons of steel is consumed every year,
40 million tons of which is used in the construction industry. (The
automotive and the equipment machinery industries are the one following in terms
of overall steel consumption). This surely gives the idea of the sheer volumes
this material is used in around the globe, and more importantly, the critical
applications it is used in.
Non metallic inclusions: basic definitions, causes and classification
The types of the non metallic inclusions in steel depend on various factors
such as the steel making process, the treatment and casting of steel,
the grade of steel, secondary metallurgy, etc, however there is no
particular measure the degree of the inclusions, dependent on these
Inclusions with large particles, even if in a small quantity, usually make
the steel more defective compared to the ones with larger quantity yet small
sizes of the inclusions. Here, another necessity for monitoring not only the
content but also the morphology and the distribution of the particles is of
utmost importance. ( The cracks on the steel material for example are likely to
be passing through groups of inclusions.)
Depending on the source of origin, non metallic inclusions are classified
into two groups
The endogenous inclusions are formed within the metal, due
to the chemical processes and within the precipitation of these materials in the
liquid phase. These cannot be fully controlled or eliminated, however, through
strict control in the forming processes it would be possible to noticeably
decrease the volume of these inclusions
The exogenous inclusions formed due to the entrapment of
external non metallic materials (particles of the mould, slag, dust, powders
used to protect the steel from sticking in the casting process, etc.). Process
control is hereby highlighted, as only through monitoring is it possible to
avoid these effects, especially taking into account the overall damage they
cause to the quality of the material.
The chemical composition is another method to classify the non metallic
inclusions. The most common types of those being oxides, sulfides , carbides, phosphides,
The Conventional Methods to detect and control non metallic inclusion
There are different internationally recognized standards that define the
acceptable quality of steels, thus also the acceptable amounts and types of
inclusions. One of those is the CEN EN 10247 – the Micrographic examination
of the non-metallic inclusion content of steels using standard pictures. Over
the time the industry has been evolving and changing opinions in terms of how
specific should the standards get, as there is not much, clear evidence how
“clean” should a steel material be to survive various workloads, this is one
of the reasons, for example, why some standards have permitted some percentage
of defects above the usually specified limits (i.e the EN 19277–1, standard
for bright bars in relation to surface cracks).
The main questions still remain
How to monitor the quality of steel (in regards to non
metallic inclusions especially)?
How to adapt this process to make it more accessible and trustworthy
for the user?
How to clearly classify the usability of a steel material for
applications with different demands?
Direct methods of detection of non metallic inclusions:
these methods are known as more accurate, yet costly and hardly accessible.
We will go through some of them below:
Evaluation of solid steel sections: these are two-dimensional section
evaluation methods like Image Analyses, Sulfur Print, Optical emission
spectrometry with pulse discrimination analysis (OES-PDA), X-ray photoelectron
Evaluation of solid steel volumes: These are more likely to provide also
three-dimensional information about the inclusions and include the types of
detecting like Ultrasonic scanning, Chemical Dissolution, Scanning Acoustic
Microscope (SAM), Fractional Thermal Decomposition, Magnetic Particle
Distribution of the inclusion size, after inclusion extraction: After the
inclusions are extracted from the steel, some of the above mentioned methods can
be used to identify the distribution of the inclusions in a 3D way. I.E. a
Coulter Counter Analysis method that uses a sensor with a miniature hole: when
the particles get inside the hole they can be detected as across the gap they
change the electric conductivity
Inclusion evaluation: liquid. An example is the Electromagnetic
Visualization, Liquid Metal Cleanliness Analyzer(LIMCA), etc.
Indirect methods of detection of non metallic inclusions
These are fast and relatively inexpensive methods, however it comes with the
cost, reliability, and the indications are usually relative.
Measurement using Total Oxygen: then oxygen in steel can be in a form of
dissolved oxygen and combined as non metallic inclusion. This method uses the
sum of both (the Total Oxygen). As the free/active oxygen has little variation,
it is easy to detect this quantity.
Nitrogen: Nitrogen pickup is an indirect measure of quality problems, total
oxygen and steel cleanliness
Is the Non metallic inclusion monitoring stuck?
Today, the modern steel production plants are getting increasingly automated,
each parameter is strictly controlled, the conventionally known methods of
monitoring the production and post production processes are consolidated by the
industry and the best available fit is found per manufacturer. A standard
process is to mark an anomalous steel material as “defective” and reject its
further use. The thresholds are given by international standards, which as we
found are evolving, and yet to fully explore the sector as well.
One thing is clear, neither the available standards, nor the control methods
are enough. Even if the controlled parameters are modified and put into the
automotion, there is no conventionally available system, simple and elegant
enough to showcase the potential defects introduced by the process of steel
production itself, or what is not less common, by previous processes (recycled
steel materials). The uncertainty of the measures is one side of the issue, the
more complicated side of the things is the fact that the available
methods cannot perform with the same effectiveness in the extremely variable
processes by different manufacturers. The aforementioned material has
proven the versatility and complexity of non metallic inclusions, their types,
classifications and other affecting factors.
Each of the mentioned methods to identify non metallic inclusions comes with
several limitations: the ultrasonic controls for example is not normally useful
to detect any defects just below the surface of the steel, Chemical dissolution
method is more fitting for a 3D detection, however it also dissolves away FeO,
MnO, CaO, MgO in the inclusions, thus is useful for mainly detect Al2O3 and SiO2
inclusions, and is a destructive detection method. Destructive detection
methods, in general, are known to be the better options for the final product
tests (formability, bending, deep-drawing properties measurement).
RVmagnetics, a company from Slovakia, EU presents an alternative method, that
is non-destructive, yet does not compromise the quality of necessary data to be
gathered about non metallic inclusions. The technology works on magnetic
principles, it is not however, the conventionally known Magnetic Principle
Inspection method (direct), which locates inclusions larger than 30µm, the
system has a different logic, is elegant, simple, and most importantly
Essentially the solution consists of MicroWire sensors and the sensing system . The MicroWire
sensors are able to detect all inclusions, including the inner ones (this opens
an opportunity to detect internal cracks or other anomalies that occur during
the operation of the steel material as well). The magnetic principle used to
receive the valuable data is rather unique and unlike other conventionally
Detection and tagging:
The solution consists of two checkpoint MicroWire lines installed on the
steel, excitation coil – which creates the magnetic field going through the
sensor, and another, sensing coil – which detects the response from
MicroWire. When the magnetic field is excited, once a non-magnetic inclusion(or
a scratch, crack, deformations both on surface and inner inclusion) occurs a
sharp signal can be obtained. A dedicated software receiving the signal at full
production speed, can quickly ensure the marking of the spots where the
MicroWire sensor detected the non metallic inclusion or any other anomalies. Our
focus within the industry is currently put on the Cold rolled steel sheets manufacturing), and even though there is a
substantial amount of research and development to be done in exploration of the
industry, signal detection and non metallic inclusions measurement in various
shapes and types of steels – our solution is essential and unique in
The RVmagnetics task is to customize the solution according to the demand
from the customer, the sensors are inexpensive to produce, the company has no
issues with the scalability and is ready for business . Especially now, when the
market observations clearly designate the need for a physical data production
unit that is simple, yet precise enough to be accessible for manufacturers of
all sizes, and to be used for the necessary R&D to increase the certainty of
international standards for different uses of steel materials.
With a B2B sales & marketing background in INGO & Foreign Investments in government sectors, Tigran is now responsible for extensive industry research in RVmagnetics focused on marketing the company both in R&D and Business spaces. Tigran is up to date with trends in deep tech, sensors, and innovative startups in need of niche growth. He shares the knowledge with RVmagnetics communities via blogs, publications, and news releases, while also using his experience to Manage RVmagnetics' Key Partners' accounts.