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Challenges Faced When Manually Rating Non-Metallic Inclusions (NMIs) to Determine Steel Quality


Rapid, accurate, and reliable rating of non-metallic inclusions (NMIs) is instrumental for the determination of steel quality. This article describes the challenges that arise from manual NMI rating, which is commonly used in steel production and component manufacturing, when striving for an efficient and cost-effective steel quality evaluation. Manual rating can produce inconsistent results which lead to a less confident determination of quality. The drawbacks of manual NMI rating performed by multiple users are: i) a large variability in results, e.g., NMI size and grouping; ii) long analysis times; and iii) difficulty to routinely apply various standards. In contrast, automated NMI rating is a more reliable and cost-effective alternative.


Due to an increasing demand over the last 20 years, steel has become one of the most widely used materials in the world [1]. As steel is used in large number of applications, e.g. manufacture of vehicles, machines, pipelines, and cable towers and construction of ships and buildings, determining steel quality is critical. Rating non-metallic inclusions (NMIs) is key to determining their influence on the quality of steel [2-5]. Fast, reliable, and flexible NMI rating solutions offer advantages for steel producers and component and part manufacturers. The challenges of using manual NMI rating solutions for efficient and cost-effective steel quality determination [3] are presented below.

Challenges with Manual Inclusion Rating

When attempting to rate NMIs to determine steel quality in an efficient and cost-effective way for steel production and industry manufacturing, the following challenges can arise with manual rating solutions.

Manual rating of NMIs can be subjective and inconsistent throughout all work shifts of a company producing steel or components made with steel. The variability is due to significant differences between how users perform the manual rating. As judgement based on eyesight and visual comparison can vary significantly between individuals, it is one reason why a consistent manual analysis of images showing NMIs by multiple users can be difficult.

NMI proximity parameters, such as area required for inclusion grouping [2], may not be consistently defined or applied by different users doing manual rating. Applying definitions of “grouped” inclusions in a consistent manner can be also very difficult.

Estimating the area, size, or inclination of NMIs can be also a challenge [3]. This difficulty arises because:

  • There are often a large variety of NMI shapes and
  • Measuring the true length of slightly inclined NMIs with only an overhead microscope view of NMIs visible on the steel surface can be troublesome

The analysis time for the manual NMI rating of a sample is often longer than what is considered acceptable for most steel producers and component manufacturers. Applying standard rating methods, like the ASTM E45 field-based evaluation [2,3,5,6], using a manual rating solution can be very time consuming.

Applying different international, regional, or organizational standards for steel quality when doing manual NMI rating can be quite challenging as well [5]. Similar to what has been mentioned above, changes to standard parameters, such as the definitions of inclusion roundness and grouping [2], can also make manual classification of inclusions confusing for users [3].

Overcoming the Challenges: Automated NMI Rating Solution

The Steel Quality Solution Suite from Leica Microsystems, utilizing the LAS X Steel Expert software, offers automated NMI rating solutions which overcomes the challenges of manual rating. The result is a rapid, flexible, and reliable way to determine steel quality [2].

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