Studies on the effect of inclusions on material properties of the

The relationship between non-metallic inclusions and the mechanical properties of the steel product has been discussed in the literature. The effect of non-metallic inclusions on grains influences all processes of fracture, indirectly ‎[85]. Maropoulos and Ridley ‎[135] used electron microscopy and X-ray diffraction to study the microstructures and to relate the microstructural characteristics to the mechanical properties. They stated that the amount and size distribution of MnS inclusions directly affect Charpy upper shelf energy and ductility. Results from the literature show that inclusion distribution affects Charpy upper shelf energy (‎[136]‎[137] -‎[139]) directly by the following factors, ( ‎[140]-‎[144]),

 Sulfur level/ inclusion volume fraction

 Non-metallic inclusion number per unit area

 Inclusion shape

 Inclusion size and spacing

Thornton ‎[145] reviewed, extensively, the influence of inclusion parameters, shape, size, quantity, interspacing, distribution, orientation, interfacial strength, and physical properties relative to the matrix, on mechanical parameters such as tensile strength, impact strength, reduction of area, fatigue properties and fracture toughness. Some of these effects determine the function of specific steel products. For example, microstructure studies on rail steel gave some information about the effect of non-metallic inclusion characterization (volume fracture, yield

strength,…)‎on‎mechanical‎properties‎of‎material‎such‎as‎tensile,‎fatigue‎and‎fracture‎toughness.‎

These investigations revealed that tensile properties are independent of inclusion type and volume fraction level present in rail steel while toughness and high cycle fatigue properties are related to inclusion specifications ‎[146] However, toughness of a specific grade of steel can be improved economically by specifying the upper limits of the size, quantity and elongation of inclusions and required toughness, and machinability operating conditions ‎[85]. Studies on high strength steels show that hard and brittle oxide inclusions are the main factors accounting for initiation of fatigue damage ‎[147],‎[148]). Baker and Charles ‎[149] reported the effect of deformation of inclusions on the fracture properties of steel.

Therefore, scientists introduced different techniques to manipulate the inclusions in order to increase the quality of the steel products. Metallurgists suggest techniques like primarily, melting, oxidation, casting and re-melting procedures to control the quantity, nature, size, shape and distribution of inclusions ‎[85].

The influences of the non-metallic inclusions on mechanical properties are via their reaction to stress fields introduced to the matrix. Stress concentration around nonmetallic inclusions changes the mechanical properties of steels ‎[150]. Literature on stress concentration close to nonmetallic inclusions (‎[151]-‎[153]) and other experimental work (‎[154], ‎[155]) indicate the effect of the presence of inclusions on stress distribution in the steel matrix around the inclusions. Experimental work on major types of non-metallic inclusions in steel such as oxide inclusion, silicates, sulfides, carbides and Al2O3 shows the distribution of stress around the inclusions ‎[156].

These stresses change the shape of the inclusions by applying uneven forces on its contact surfaces with the matrix. Elongation of inclusions degrades the fracture properties ‎[149].

This can be observed especially if the direction of elongation is perpendicular to the principal stresses (‎[157]-‎[160]). Other papers in the literature (‎[161]-‎[163]), have investigated a number of factors which play roles in the response of inclusions to stresses such as, difference between strength in inclusion and matrix, composition of inclusion and matrix, contact surfaces between inclusion and matrix, temperature at imposed stresses, strain and strain rate of stress configuration, particle size, stress state and state of second phase particles. Based on these factors, different classifications for the inclusions have been considered. Hilty and Kay ‎[164]

have seen five different classes of inclusions found in the experiments on the basis of their behavior to the applied stresses on the matrix (Figure 13).

The hardness of the inclusion is temperature dependent. Waudby ‎[162] stated that deformation of steel and its mechanical behavior in both hot and cold working is influenced by the behavior of inclusions. In hot working, this behavior is influenced by several factors including temperature, mainly by plasticity of the inclusion, the relative plasticity of the various

phases within the inclusion, relative plasticity of the inclusion with matrix and friction force at the contact surface between the inclusion and matrix. At low temperature, the inclusion may become a potential source for defects in the final products. He showed that the initial shape of the silicate inclusions at low temperature has an effect on its fracture during hot working and small fragments are chipped off and spread out along the direction of flow of the material. He considered four factors, in addition to temperature, for governing the plasticity of silicates. These factors are;

a) Reduction during hot working b) Particle Size

c) Composition

d) Second phase particles

Behavior of the silicate during hot working was explained previously in many articles (‎[73], ‎[162], ‎[165]-‎[167]). Ekerot ‎[121] found the transition temperature for hard glassy silicate inclusions at which the deformation behavior of the glassy silicate changes and they become very soft and their index of deformability may even reach higher than . This transition temperature may be changed depending on the SiO2 content. He also stated that calcium stabilizes the glassy inclusion phase so the deformability of the inclusions can be improved. However, crystalline inclusions such as SiO2-MnO-Al2O3-CaO do not have this transition temperature and remained undeformed at all hot working temperatures.

Klevebring ‎[167] predicted the deformation behavior of an inclusion using the hot hardness data for manganese selenide and manganese telluride inclusions. He proposed that oxygen increases the hardness of MnS and consequently decreases the deformation index of

sulfides. This effect of oxygen content provides a deviation for the modeling of MnS deformation.

Researchers have attempted to predict the behavior of inclusions in the stress fields using continuum bifurcation analysis on a modified Gurson-type constitutive relation for porous media to predict changes of ductility behavior in low carbon steel containing stringered manganese sulfide inclusions ‎[168]. Ekerot ‎[121] approximated the stress-strain relationship of the glassy inclusions by Newtonian flow. Sundstrom ‎[169] used a power law stress-strain relationship in his plane strain model of elliptical inclusions in an infinite plate. He modeled a very soft inclusion behavior by assuming the same strain hardening exponents for the inclusions and the matrix.

Zeisloft and Hosford ‎[170] neglected inhomogeneity of the inclusion in the deformation of the specimen the for compressive plane strain of inclusions. Such an approach was found to result in too high a deformation index.