Macro Defects in Steel - Rolling defects

Rolling defects

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.

Reference: Website http://ispatguru.com/metallurgical-processes-and-defects-in-steel-products, 2017.