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Metallurgical Processes and Defects in Steel Products

Defects in steel products are defined as deviations in appearance, shape, dimension, macro-structure / micro-structure, and/or chemical properties when compared with the specifications given in the technical standards or any other normative documents in force. Defects are detected either through visual inspection or with the help of instruments and equipments.

There are four main metallurgical processes for the manufacture of finished steel products where the steel products can pick up defects. The defects picked up during these processes are (i) casting defects, (ii) rolling defects, (iii) forging defects, and (iv) welding defects. (Fig 1).These defects are described below.

Fig 1 Metallurgical processes and steel product defects

Casting defects

Casting is a forming process which converts liquid steel into a solid product. In foundries liquid steel is cast into complex shapes by pouring of liquid steel into a mould in which it sets to the required shape. In steel plants, liquid steel is normally continuously cast in the form of slab (either thick or thin), bloom or billets. Casting defects are defined as those characteristics which create a deficiency or imperfection exceeding quality limits imposed by design and service requirements.

Defects in foundry cast steel products

There are in general three broad categories of defects in the foundry cast steel products. These are (i) the major or most severe defects which result in scraping or rejection of castings, (ii) intermediate defects which permit salvaging of castings through necessary repairs, and (iii) minor defects which can be easily repaired. Common defects which generally occur in castings are given below.

◦ Porosity – It consists of the spherical holes of varying size, with bright walls, usually evenly distributed and formed due to the gases in the liquid steel. The larger holes are tend to be found in the heavier section (i.e. last to solidify). If the gas content of liquid steel is low prior to casting then the pinhole type of porosity appears due to absorption of hydrogen (H2) from steam in the mould.

◦ Blowholes – Blowholes are mainly found in three forms namely (i) elongated cavities with smooth walls, found on or just below the surface of the top most part of castings and are caused by the entrapped air, (ii) round shaped cavities with smooth bright walls which are caused by mould or core gases, coupled with insufficient permeability or venting, and (iii) small cavities immediately below the ‘skin’ of the casting surface which are formed by the reaction of the liquid steel with the moisture in the moulding sand.

◦ Piping – This is the defect encountered in risers or within the casting proper.

◦ Inclusions – These are material discontinuities formed by the inclusion of oxides, dross, and/or slag in a casting. They are due to either careless skimming and pouring, or the use of a dirty ladle, or turbulence because of improper gating methods. Faulty closing of moulds can cause ‘crush’ and loose pieces of sand becoming incorporated in the casting.

◦ Sponginess – it is a defect which occurs during the early stages of solidification of a casting and has the appearance, as the name imply, of a sponge. It is usually local or general in extent. The major cause is failure to obtain directional solidification of the casting towards the desired heat centres, such as risers and ingates, insufficiently high pouring temperature and placing of ingates adjacent to heavy sections.

◦ Shrinkage – It is a casting defect which occurs during the middle and later stages of solidification of the cast steel. It has a branching formation. It is readily distinguishable from that of sponginess and is in form of void.

◦ Hot tears – These are discontinuities which result from stresses developed close to the solidification temperature while the steel is still weak. These are attributed to the resistance of the mould and the core, which hinder contraction of the casting, causing thermal stress. Hot tears resemble ragged cracks.

◦ Crack – Crack is well defined and normally straight. Cracks are formed after complete solidification of the liquid steel. Quite large stresses are needed to cause fracture, and the walls of such cracks are discoloured according to the temperature of the casting when the cracks are formed. Bad casting design coupled with restriction of contraction by the mould, core, or box bars contribute to the cracking.

◦ Cold shuts – These are discontinuities (a form of lack of fusion) caused by the failure of a stream of liquid steel to unite with another stream of liquid steel, or with a solid steel section such as a chaplet. They are linear in appearance, with perhaps a curling effect at the ends. A cold shut is caused by the fluidity of the liquid steel being too low (i.e. too cold surfaces) or perhaps unsatisfactory methods of feeding the liquid steel.

◦ Unfused chaplet – A chaplet is often used to support a section of a mould or a core within a mould and when the liquid steel is poured in the chaplets, they are to fuse into the casting. When unfused, the chaplet causes a discontinuity in the casting.

◦ Misplaced core – It is an irregularity of wall thickness, e.g. one wall thicker than the other, It is caused by core out-of- alignment, careless coring-up and closing of mould, or rough handling after the mould is closed.

◦ Segregation – Segregation is a condition resulting from the local concentration of any of the constituents of the steel. The segregation can be ‘general’ extending over a considerable part of a casting, ‘local’ when only the shrinkage voids or hot tears are wholly or partially filled with a constituent of low melting point or ‘banded’ which is mainly associated with centrifugal castings but can also occasionally occur in static castings.

Defects in continuous cast steel products

Defects of the continuous cast steel products are formed due to several factors which include material related factors, casting speeds and temperatures, mould oscillation, casting powder, segregation coefficient of solute elements, phase transformation, and mechanical and thermal stresses. Common defects in steel which occurs during the continuous casting are as follows.

◦ Longitudinal cracks – They are formed in the direction of extraction of the steel. The presence of these defects results into the rejection of the steel. Longitudinal cracks occurs mainly due to (i) uneven primary cooling in the mould, (ii) turbulent flow of liquid steel and a meniscus level variation in the mould, (iii) non uniform or very intensive secondary cooling, (iv) variance in thermal conductivity coefficient along the mould length causing unequal, advanced wear of the mould, (v) casting of liquid steel with high superheat, (vi) high speed of casting, and (vii) use of the casting powder with improper characteristics.

◦ Transverse cracks – These cracks usually appear due to the tensions in the longitudinal direction of the strand. The transverse cracks are usually ground within the permissible limits provided they are not deep. These cracks appear due to (i) the thermal stresses, (ii) variation in the meniscus level, (iii) presence of segregation at the bottom of oscillation mark, and (iv) friction of the strand in the mould.

◦ Corner cracks – These are cracks present in the edge of the cast steel product. They appear due to high temperature variations in the liquid steel, higher aluminum content in the steel, higher sulphur level in the steel, non-uniform edge temperature, excess friction in the edges during casting because of non-uniform distribution of casting powder, and lower superheat of the steel.

◦ Star cracks – These cracks are very fine and caused by fragile nature of the strand at high temperatures. They are visible only on scale free surface. The surface is usually ground locally to remove the defect. Intense local cooling and presence of copper at the austenitic grain boundary cause star cracks. To avoid the star cracks in the cast product it is necessary to have (i) correct correlation between the spray flow and the casting speed, (ii) a uniform layer of melted casting powder between the strand and the mould, and (iii) adequate secondary cooling of the strand for avoiding increase of the thermal stress.

◦ Depressions – These are local deformations in the cast surface. Depressions can be longitudinal or transverse. Longitudinal depressions appear like the shallow ditches oriented along the length of the cast product. They occur due to the uneven heat transfer in the mould. These depressions can be controlled by uniform cooling in the mould, by centering of the liquid steel jet in the mould, by controlling the fluctuations of the mould steel level, use of a casting powder with suitable viscosity and melting characteristics, and by regularly monitoring the degree and uniformity of the mould wear. Transverse depressions may occur cyclically along the strand length. The peritectic steels with low carbon and high manganese contents and the stainless steels are sensitive to this defect. The transverse depressions can be caused by the fluctuations in the mould level, large quantity of casting powder, and by the turbulence of steel the sub-meniscus level. These depressions are controlled by controlling the mould steel level, having proper mould taper, use of a casting powder with suitable viscosity and melting characteristics, and proper positioning of the input nozzle and its support.

◦ Blowholes – These are cavities in the outer surface of the cast product and are often associated with inclusions. They are caused by presence of gases in the steel, humidity and quality of the casting powder, variation in the mould level, presence of moisture in the tundish refractory lining. Blowholes are controlled by sufficient de-oxidation of steel, use of dry casting powder, use of casting powder compatible with the grade of steel grade, temperature and casting speed, control of mould level fluctuations, control of nozzle immersion depth, avoiding the high superheat and avoiding slag foaming around the nozzle.

◦ Interruptions in the physical continuity of the cast product – This defect occurs when there is a pause in the casting process. It often occurs when there is a change of heat during sequence mode of operation. This defect is caused by a short interruption of the casting process and occurs when there is sudden change in casting speed caused by the variations of steel temperature in the tundish, by the variations of steel level in the mould, cogging of the nozzle due to high alumina levels, or by the variations of casting mode. The corrective measures are maintenance of a constant casting speed, a narrow range of temperature variation in the tundish, and steel level in the tundish within the prescribed limits.

◦ Slag spots defects -This defect is caused by the penetration of tundish slag in the cast product. It is caused by high level of slag in the tundish, rise in the active oxygen percentage in the steel, lowering of steel level in the tundish resulting in slag to enter the mould, and high viscosity of casting powder.

Rolling defects

The common rolling defects of steels are given below.

During the hot rolling, if the temperature of the steel is not uniform then the flow of the material occurs more in the areas having higher temperature and less in the parts with lower temperature. High temperature difference results into cracking and tearing.

In flat steels, the flatness is a characteristic describing the extent of the geometric deviation from a reference plane. The deviation from complete flatness is the direct result of the steel relaxation after hot or cold rolling, due to the internal stress pattern caused by the non-uniform transversal compressive action of the rolls and the uneven geometrical properties of the steel being fed for the rolling. The transverse distribution of differential strain/elongation induced stress with respect to the material’s average applied stress is normally termed as shape. Due to the strict relationship between shape and flatness, these terms are generally used in an interchangeable manner. In the case of steel strips and sheets, the flatness reflects the differential elongation across the width of the steel.

Profile is made up of the measurements of crown and wedge. Crown is the thickness in the centre as compared to the average thickness at the edges of the steel strip or sheet. Wedge is a measure of the thickness at one edge as opposed to the other edge. Both are generally expressed as absolute dimensions or as relative dimensions. As an example, the steel piece can have a crown of 0.05 mm (the centre of the steel is 0.05 mm thicker than the edges), or the steel piece can have 2 % crown (the centre of the steel piece is 2 % thicker than the edges). It is typically desirable to have some crown in the steel piece since it causes the steel piece to tend to pull to the centre of the rolling mill, and thus the rolling takes place with higher stability.

Mill spring is a defect in which the rolled sheet is thicker than the required thickness because, the rolls have got deflected by high rolling forces. Elastic deformation of the mill takes place. If stiffer rolls are used, namely roll material of high stiffness or elastic constant, then mill spring can be avoided. Normally elastic constant for mills may range from 1 to 4 GN/m. Roll elastic deformation can result in uneven strip thickness across. Roll material is to have high elastic modulus for reducing the roll deformation.

Maintaining a uniform gap between the rolls is difficult since the rolls deflect under the load required to deform the steel piece. The deflection causes the steel piece to be thinner on the edges and thicker in the middle. This can be overcome by using a crowned rolls (parabolic crown), however the crowned rolls only compensate for one set of conditions, specifically the material, temperature, and amount of deformation.  Other methods of compensating for roll deformation include continual varying crown (CVC), pair cross rolling, and work roll bending. Another way to overcome deflection issues is by decreasing the load on the rolls, which can be done by applying a longitudinal force which is essentially drawing. Other method of decreasing roll deflection includes increasing the elastic modulus of the roll material and adding back-up supports to the rolls.

Flatness of rolled steel sheets depends on the roll deflection. Sheets become wavy as roll deflection occurs. If rolls are elastically deflected, the rolled sheets become thin along the edge, whereas at centre, the thickness is higher. Similarly, deflected rolls result in longer edges than the centre. Edges of the sheet elongate more than the centre. Due to continuity of the sheet, the centre is subjected to tension, while edges are subjected to compression. This leads to waviness along edges. Along the centre zipper cracks occur because of high tensile stress there. Cambering of rolls can prevent such defects. However, one camber works out only for a particular roll force. In order to correct roll deflection for a range of rolling conditions, hydraulic jacks are used, which control the elastic deformation of rolls according to the requirement.

Edge cracks occur if rolls have excess convexity. They occur when the centre of the steel sheet has more elongation than the edges. This leads to a defect called centre buckle. Edge defects are due to heavy reduction. Small thickness sheets are more sensitive to roll gap leading to greater defects. Thin strips are more likely to undergo waviness or buckling.

During rolling, the steel sheet has a tendency to deform in lateral direction. Friction is high at the centre. Therefore, spread is the least at the centre. This leads to rounding of ends of the sheet. The edges of the sheet are subjected to tensile deformation. This leads to edge cracks. If the centre of the sheet is severely restrained and subjected to excess tensile stress, centre split can happen. Non-homogeneous material deformation across the thickness leads to high secondary tensile stress along edge. This leads to edge cracks. Secondary tensile stresses are due to bulging of free surface.

Due to non-homogeneous flow of material across the thickness of the steel sheet, another defect called allegatoring occurs. This is due to the fact that the surface is subjected to tensile deformation and centre to compressive deformation. This is because greater spread of material occurs at the centre.

The different classifications for flatness defects are as below.

◦ Symmetrical edge wave – the edges on both sides of the steel piece are ‘wavy’ because of the steel material at the edges are longer than the material in the centre.

◦ Asymmetrical edge wave – one edge is ‘wavy’ because of the steel material at one side is longer than the other side.

◦ Centre buckle – The centre of the strip is ‘wavy’ because of the strip in the centre is longer than the strip at the edges.

◦ Quarter buckle – This is a rare defect where the steel grains are elongated in the quarter regions (the portion of the strip between the centre and the edge). This is normally due to the use of excessive roll bending force since the bending force may not compensate for the roll deflection across the entire length of the roll.

Surface defects are (i) lap which appears as seam across the surface of the metal and occurs when a corner or fin is folded over and rolled but not welded into the metal, (ii) mill shearing which occurs as a feather-like lap, (iii) rolled in scale which occurs when mill scale is rolled into the steel, (iv) scabs which are long patches of loose metal that have been rolled into the surface of the steel, (v) seams which are open, broken lines that run along the length of the rolled steel and caused by the presence of scale as well as due to pass roughness of roughing mill, and (vi) slivers which are prominent surface ruptures.

Seams are surface irregularities, such as cracks, on the semi-finished steel which are stretched out and lengthened during rolling. Seams can also be caused by folding of the steel due to improper rolling. Seams are surface discontinuities and on finished bars they appear as either continuous or broken straight lines. On round bars they appear as straight or slightly spiral lines, either continuous or broken.

Laminations are large porosity, pipe and non-metallic inclusions in the semi-finished steel which are flattened and spread out during the rolling process.

Stringers are non-metallic inclusions in semi-finished steel which thinned and lengthened in the direction of rolling by the rolling process.

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