Non-Metallic Inclusions in steels-Methods to detect inclusions

Methods to detect inclusions

The amount, size distribution, shape and composition of inclusions should be measured at all stages in steel production. Measurement techniques range from direct methods, which are accurate but costly, to indirect methods, which are fast and inexpensive, but are only reliable as relative indicators. Dawson et al reviewed 9 methods in 1988 by dividing them into two categories of “off-line” methods and “online” methods. Zhang and Thomas reviewed around 30 methods to detect inclusions in steel.

1. Direct methods

1.1. Inclusion evaluation of solid steel sections
Several traditional methods directly evaluate inclusions in a two-dimensional section through solidified product samples. The last five of these methods add the ability to measure the composition of the inclusions.

Metallographic microscope observation (MMO)
This method is can only reveal the 2-dimensional section of an inclusion, however, inclusions are 3 dimensional in nature.

Image analysis (IA)
This enhancement to MMO improves on eye evaluation by using highspeed computer evaluation of video-scanned microscope images to distinguish dark and light regions based on a gray-scale cutoff.

Sulfur print
This popular and inexpensive macrographic method distinguishes macroinclusions and cracks by etching sulfur-rich areas. It is subject to the same problems as other 2-D methods.

Scanning electron microscopy (SEM)
This method clearly reveals the three-dimensional morphology and the composition of each inclusion. Composition can also be measured with Electron Probe Micro Analyzer (EPMA). Extensive sample preparation is required, however, to find and expose the inclusion(s).

Optical emission spectrometry with pulse discrimination analysis (OES-PDA)
The OES method analyzes elements dissolved in liquid steel. Inclusions cause high-intensity spark peaks (relative to the background signal from the dissolved elements), which are counted to give the PDA.

Laser microprobe mass spectrometry (LAMMS)
Individual particles are irradiated by a pulsed laser beam, and the lowest laser intensity above a threshold value of ionization is selected for its characteristic spectrum patterns due to their chemical states. Peaks in LAMMS spectra are associated with elements, based on comparison with reference sample results.

X-ray photoelectron spectroscopy (XPS)
This method use x-rays to map the chemical state of individual inclusions larger than 10µm.

Auger electron spectroscopy (AES)
This method use electron beams to map the composition of small areas near the surface of flat samples.

Cathodoluminescence microscope
Under microscope, the steel or lining sample section is stimulated by a cathode-ray (energetic electron-beam), to induce cathodoluminescence (CL). The color of CL depends on the metal ions type, electric field, and stress, allowing inclusions to be detected.

1.2. Inclusion evaluation of solid steel volumes
Several methods directly measure inclusions in the three-dimensional steel matrix. The first four of these scan through the sample with ultrasound or x-rays. The last four of these volumetric methods first separate the inclusions from the steel.

Conventional ultrasonic scanning (CUS)
The transducer (typically a piezoelectric) emits a sound pressure wave that is transferred into the sample with the aid of a coupling gel. The sound waves propagate through the sample, reflect off the back wall and return to the transducer. The magnitude of the initial input pulse and the reflected signals are compared on an oscilloscope to indicate the internal quality of the sample. Obstructing objects in the path of the sound will scatter the wave energy. This nondestructive method detects and counts inclusions larger than 20µm in solidified steel samples.

Mannesmann inclusion detection by analysis surfboards (MIDAS)
Steel samples are first rolled to remove porosity and then ultrasonically scanned to detect both solid inclusions and compound solid inclusions / gas pores. This method was recently renamed as the Liquid Sampling Hot Rolling (LSHP).

Scanning acoustic microscope (SAM)
In this method, a cone-shaped volume of continuouscast product is scanned with a spiraling detector, such as a solid ultrasonic system, which automatically detects inclusions at every location in the area of the sample surface, including from surface to centerline of the product.

X-ray detection
Inclusions images are detected by their causing variation in the attenuation of x-rays transmitted through the solid steel. An inclusion distribution can be constructed by dividing a sample into several wafers and subjecting each to conventional x-rays to print penetrameter radiograghs for image analysis.

Chemical dissolution (CD)
Acid is used to dissolve the steel and partially extract inclusions. The inclusion morphology and composition can be detected by another method like SEM, or be fully extracted by dissolving all the steel sample. The three dimensional nature of inclusions can be revealed by this method. The disadvantage is that the acid will dissolve away FeO, MnO, CaO, MgO in the inclusions. Thus this method is good to detect Al2O3 and SiO2 inclusions.

Slime (electrolysis)
This method is also called Potentiostatic Dissolution Techniques. A relatively large (200g – 2kg) steel sample is dissovled by applying electric current through the steel sample immersed in a FeCl2 or FeSO4 solution. This method was used to reveal the individual, intact inclusions. One disadvantage of this method is the cluster inclusions possibly break into separate particles after extraction from steel.

Electron beam melting (EB)
A sample of Al-killed steel is melted by an electron beam under vacuum. Inclusions float to the upper surface and form a raft on top of the molten sample. The usual EB index is the specific area of the inclusion raft. An enhanced method (EB-EV - Extreme Value) has been developed to estimate the inclusion size distribution.

Cold crucible (CC) melting
Inclusions are first concentrated at the surface of the melted sample as in EB melting. After cooling, the sample surface is then dissolved, and the inclusions are filtered out of the solute. This method improves on EB melting by melting a larger sample and being able to detect SiO2.

Fractional thermal decomposition (FTD)
When temperature of a steel sample exceeds its melting point, inclusions can be revealed on the surface of the melt and decomposed. Inclusions of different oxides are selectively reduced at different temperatures, such as alumina-based oxides at 1400 or 1600 C, or refractory inclusions at 1900 C. The total oxygen content is the sum of the oxygen contents measured at each heating step.

Magnetic particle inspection (MPI)
This method also called magnetic leakage field inspection can locate inclusions larger than 30µm in sheet steel products. The test procedure consists of generating a homogeneous field within the steel sheet that is parallel to the sheet surface. If an inhomogeneity (such as an inclusion or a pore) is present, the difference in magnetic susceptibility will force the magnetic flux field to bend and extend beyond the surface of the sheet. The major disadvantage is poor resolution of inclusions that are close together.

1.3 Inclusion size distribution after inclusion extraction
Several methods can find 3-dimensional inclusion size distributions after the inclusions are extracted from the steel using a method from 2.1.2 5)-7). 1) Coulter Counter Analysis174): This method, by which particles flowing into this sensor through its tiny hole are detected because they change the electric conductivity across a gap, measures the size distribution of inclusions extracted by Slime and suspended in water.174) 2) Photo Scattering Method175, 176): Photo-scattering signals of inclusions (that have been extracted from a steel sample using another method such as slime) are analyzed to evaluate the size distribution. 3) Laser-Diffraction Particle Size Analyzer (LDPSA)2): This laser technique can evaluate the size distribution of inclusions that have been extracted from a steel sample using another method such as Slime.

1.4. Inclusion evaluation of liquid
There are several approaches can be used to detect the inclusion amount and size distribution in the molten melts.

Ultrasonic techniques for liquid system
This method captures the reflections from ultrasound pulses to detect on-line inclusions in the liquid metal.

Liquid metal cleanliness analyzer (LIMCA)
This on-line sensor uses the principle of the Coulter Counter to detect inclusions directly in the liquid metal. Commonly this method is used for aluminum and other metals, and it is still under development for steel.

Confocal scanning laser microscope
This new in-situ method can observe the behavior of individual inclusions moving on the surface of the molten steel, including their nucleation, collision, agglomeration, and pushing by interfaces.

Electromagnetic visualization (EV)
This Lorentz-force-based detection system is used to accelerate inclusions to the top free surface of the melted sample of metals and highly conductive opaque fluids. The technique has better resolution than other on-line methods.

2. Indirect methods

Owing to the cost, time requirements, and sampling difficulties of direct inclusion measurements, steel cleanliness is generally measured in the steel industry using total oxygen, nitrogen pick-up, and other indirect methods.

Total oxygen measurement
The total oxygen (T.O.) in the steel is the sum of the free oxygen (dissolved oxygen) and the oxygen combined as non-metallic inclusions. Free oxygen, or “active” oxygen can be measured relatively easily using oxygen sensors. It is controlled mainly by equilibrium thermodynamics with deoxidation elements, such as aluminum. Because the free oxygen does not vary much (3-5 ppm at 1600 C for Al-killed steel), the total oxygen is a reasonable indirect measure of the total amount of oxide inclusions in the steel. Due to the small population of large inclusions in the steel sample.

Nitrogen pickup
The difference in nitrogen content between steelmaking vessels is an indicator of the air entrained during transfer operations. Nitrogen pickup thus serves as a crude indirect measure of total oxygen, steel cleanliness, and quality problems from reoxidation inclusions. For example, Weirton restricts nitrogen pickup from ladle to tundish to less than 10 ppm for critical clean steel applications. Note that oxygen pickup is always many times greater than the measured nitrogen pickup, due to its faster absorption kinetics at the air steel interface.

Concentration measurement
For LCAK steels, the dissolved aluminum loss also indicates that reoxidation has occurred. However, this indicator is a less accurate measure than nitrogen pickup because Al can also be reoxidized by slag. The silicon pickup, manganese pickup can be also used to evaluate the reoxidation process.

Lining refractory observation
Analysis of the lining refractory composition evolution before and after operations can be used to estimate inclusion absorption to the lining and the lining erosion. Also, the origin of a complex oxide inclusion can be traced to lining refractory erosion by matching the mineral and element fractions in the slag with the inclusion composition.

Slag composition measurement
Firstly, analysis of the slag composition evolution before and after operations can be interpreted to estimate inclusion absorption to the slag. Secondly, the origin of a complex oxide inclusion can be traced to slag entrainment by matching the mineral and element fractions in the slag with the inclusion composition. These methods are not easy, however, due to sampling difficulties and because changes in the thermodynamic equilibrium must be taken into account.

Tracer studies for determining exogenous inclusions from slag and lining erosion
Tracer oxides can be added into slags and linings in ladle, tundish, mold, or ingot trumpet, and top compound. Typical inclusions in the steel are then analyzed by SEM and other methods. If the tracer oxides are found in these inclusions, then the source of these inclusions can be decided.

Submerged entry nozzle (sen) clogging
Short SEN life due to clogging is sometimes an indicator of poor steel cleanliness. The composition of a typical clog during LCAK steel continuous casting is: 51.7% Al2O3, 44% Fe, 2.3% 23 MnO, 1.4% SiO2, 0.6% CaO, which shows that nozzle clogs are often caused by a simultaneous buildup of small alumina inclusions and frozen steel. Thus, SEN clogging frequency is another crude method to evaluate steel cleanliness.

3. Final product tests The ultimate measure of cleanliness is to use destructive mechanical tests to measure formability, deep-drawing, and / or bending properties of the final sheet product, or fatigue life of test specimens or product samples. Other sheet tests include the Hydrogen Induced Crack test and magnetoscopy. Another example is the inclusion inspection method in ultrasonic fatigue test. These tests are needed to reveal facts such as the potential benefit of very small inclusions ( 1µm), which should not count against cleanliness.

Reference: Lifeng Zhang and Brian G. Thomas, INCLUSIONS IN CONTINUOUS CASTING OF STEEL, XXIV National Steelmaking Symposium, Morelia, Mich, Mexico, 26-28, Nov.2003, pp. 138-183.