Defects in Hot Strip Mill
The problem of surface quality of hot rolled strips belongs to the basic priority of the production process. Each imperfection of the material can cause defect or decreasing quality of final product. Possible source of surface defects for hot rolled strips can be at different step in the manufacturing process such as (i) production and casting of liquid steel, (ii) reheating of slabs, and (iii) rolling of hot slabs. Between the most problematic surface defects, the initiators of surface defects are created during process of production and casting of liquid steel and include (i) inclusions, (ii) blowholes, (iii) scabs, (iv) lines of oxides aluminum, (v) holes, and (vi) longitudinal, transverse and the edge cracks. The condition of reheating slabs in reheating furnace and hot rolling process represents further creation and evolution of defects. The technological operation of heating and rolling can also create the specific surface defects such as shell surface, thermal cracks, hangnails, slivers, scales, squeeze of rolls, scratches and other defects. The accumulation of defects in material during the technological operation causes decrease in mechanical properties and the fractographic analysis reveals another degradation mechanism in the material.
Defects from cast slab before rolling are as follows:-
- Some defects like porosity, cavity, blow hole occurred in the cast slab gets closed up during the rolling process.
- Longitudinal stringers of non-metallic inclusions or pearlite banding are related to melting and solidification practices. In severe cases, these defects can lead to lamination which drastically reduces the strength in the thickness direction.
Defects arising during rolling are described below:-
There are two aspects to the problem of the shape of a strip. These are (i) uniform thickness over the width and thickness which can be precisely controlled with modern gauge control system, and (ii) flatness which is difficult to measure accurately.
Shape problems are greatest when rolling in thin strip because fractional errors in the roll gap profile increase with decrease in thickness, producing larger internal stress. Thin strip is also less resistant to buckling. Mild shape problems can be corrected by stretch leveling the strip in tension or by bend flexing the strip in a roller-leveller.
Mill spring is a defect in which the rolled strip is thicker than the required thickness since the rolls get deflected by high rolling forces. Elastic deformation of the mill takes place during rolling. Mill spring can be avoided, if stiffer rolls are used which means that the roll material is having high stiffness or elastic constant. Normally elastic constant for mills may range from 1 to 4 GNm-1.
Roll elastic deformation can result into uneven strip thickness across. Roll material is required to have high elastic modulus for reducing the roll deformation. For producing very thin gauge strips, small diameter rolls are used. They are supported with larger rolls. Generally, the minimum thickness of rolled strips achieved is directly proportional to roll radius, friction, flow stress.
Roll flattening increases the roll pressure and eventually causes the rolls to deform more easily than the material being rolled.
Flatness of rolled strips depends on the roll deflection. Strips become wavy as roll deflection occurs.
The roll gap must be perfectly parallel to produce strips with equal thickness at both ends. The rolling speed is very sensitive to flatness. A difference in elongation of one part in 10,000 between different locations in the strip can cause waviness.
If rolls are elastically deflected, the rolled strips 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 strip elongate more than the centre. Due to continuity of the strip it can be said, that the centre is subjected to tension, while edges are subjected to compression. This leads to waviness along edges. Along the centre, zipper cracks occur due to 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 requirement.
If rolls have excess convexity then the centre of the strip material has more elongation than the edges. This leads to a defect called centre buckle.
Possible effects when rolling with insufficient camber include thicker centre means the edges are plastically elongated more than the centre, normally called long edges. This induces the residual stress pattern of compression at the edges and tension along the centre line. This can cause centre line cracking, warping or edge wrinkling or crepe-paper effect or wavy edge.
Possible effects when rolls are over-cambered include thicker edges than the centre means the centre is plastically elongated more than the edges, resulting in lateral spread. The residual stress pattern is now under compression in the centre line and tension at the edges. This can cause edge cracking, centre splitting, and centre line wrinkling.
Small thickness strips are more sensitive to roll gap defects leading to greater defects. Thin strips are more likely to undergo waviness or buckling. These defects are corrected by doing roller leveling or stretch leveling under tension. Stretch leveling is carried out between roller leveler rolls.
During rolling the strip 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 strip. The edges of the strip are subjected to tensile deformation. This leads to edge cracks. If the centre of the strip 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 the bulging of free surface. Edge cracks can be avoided by using edge rolls.
Edging defect is caused by inhomogeneous deformation in the thickness direction. If only the surface of the material being rolled is deformed (as in a light reduction on a thick slab), the edges are concaved. The overhanging material is not compressed in the subsequent step of rolling, causing this area under tensile stress and leading to edge cracking. This has been observed in initial breakdown of hot-rolling when h/Lp>2. With heavy reduction, the centre tends to expand more laterally than the surface to produce barreled edges. This causes secondary tensile stresses by barreling, which are susceptible to edge cracking.
Due to non-homogeneous flow of material across the thickness of the strip, another defect occurs. This defect is called allegatoring. 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 centre. Alligatoring occurs when lateral spread is greater in the centre than the surface (surface in tension, centre in compression) and with the presence of metallurgical weakness along the centre line.
Surface defects are more easily in rolling due to high surface to volume ratio. Grinding, chipping or descaling of defects on the surface of cast slabs are normally required to be done before being rolled. Laps due to misplace of rolls can cause undesired shapes. Flakes or cooling cracks along edges result in decreased ductility in hot rolling of extra coarse grained slabs. Scratches can be due to tooling and handling. A variation in thickness is due to deflection of rolls or due to the rolling speed.
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