Hot- and Cold-Rolled Steel: Defects Affecting Laser Cutting

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Thanks to Ramazan Lee for the help!

Differences between cold-rolled (CR) and hot-rolled (HR) sheets, and which to choose

Rolled products (formed under pressure) differ significantly depending on the rolling process. The differences in the properties of hot-rolled and cold-rolled steel are determined by the temperature at which the rolling is performed.

Main differences:

The feedstock for hot rolling is a slab; for cold rolling, it is a hot-rolled sheet.
Unlike hot-rolled products, cold-rolled products are made exclusively from high-quality steel grades.
Cold-rolled sheet has a more uniform thickness distribution and does not warp during welding, which makes it preferred in instrument-making, automotive, and aerospace industries.
Cold-rolled sheet thickness does not exceed 5 mm; hot-rolled thickness can reach 200 mm.
Dimensional precision (angles, length, width) is higher for cold-rolled products.
Cold-rolled sheet does not crack when bent.
In terms of strength, durability, and reliability, hot-rolled and cold-rolled steel are equivalent.

Hot-rolled sheet

A hot-rolled steel sheet is a flat product whose thickness is many times smaller than its length and width. It is produced by hot rolling — the process that gives the product its name.

HR defects

1. Dimensional and shape inaccuracy. Deviations in thickness, length, width; longitudinal and transverse thickness variation; waviness.

Causes: incorrect mill setup, deviation from the rolling (deformation) schedule.
High longitudinal thickness variation — caused by poor roll profiling and roll wear.
Waviness — appears near the side edges of the strip due to excess reduction in those areas. Remedy: increase roll crown or reduce reduction.

2. Loss of metal continuity. Through cracks, surface cracks, torn edges, laminations, etc.

Causes: mostly metallurgical — violations of melting, deoxidation, and casting technology.
Rolling-related origin:
1. Through cracks form where the metal has a sharply reduced ductility.
2. Such areas are continuous non-metallic inclusions and oxidized internal blisters, which cause cracks and torn edges. They also appear when the metal is overheated or burnt before rolling.
Scabs (slivers) — tongue-shaped delaminations. Causes: ingot origin (formed during casting), non-metallic inclusions in the surface layer, opening of gas blisters, deep grooves on ingot and slab surfaces.
Lamination — heavy contamination of internal metal layers with non-metallic inclusions.

3. Surface defects. Rolled-in scale, scratches, imprints from the roll surface.

4. Unsatisfactory structure and physico-mechanical properties. If the chemical composition is correct, deviations are caused by violations of the deformation schedule (especially in the final passes), and by failure to meet the prescribed finishing-rolling and coiling temperatures.

Cold-rolled sheet

Cold-rolled sheet is a type of flat-rolled product made by cold rolling (abbreviated CR).

Defects are promoted by the small thickness of cold-rolled products, which is much less than that of hot-rolled.

Loss of homogeneity produces cracks, holes, torn edges, and laminations. These indicate poor raw-material quality or technological violations by the manufacturer.

Surface defects include dark streaks, bumps, dents, and under-pickling or over-pickling. They result from process violations — in particular, pickling. They can also be caused by incorrect oxidation, dents and protrusions on roll surfaces.

Rolled-in surface crumb — another defect that occurs when the strip and roll surfaces are not properly cleaned before rolling.

Cold-rolled sheet is produced from hot-rolled product by removing the scale through acid pickling. The pickled hot-rolled sheet is then passed through the rolling mill without preheating, achieving the required thickness. The final step is annealing, which produces the required properties. Cold-rolled sheet is supplied in coils or as cut sheets.

CR defects

1. Loss of metal integrity — roll wear.

Because cold-rolled sheets are mostly much thinner than hot-rolled, the dominant defects are transverse and longitudinal thickness variation, waviness, and buckling. They are prevented by optimal roll profiling, use of roll-bending counter-forces, and automatic rolling control.

The main cause of defects of this type (holes, cracks, torn edges, scabs, laminations) is poor quality of the original hot-rolled feedstock. Some defects may also arise from incorrect rolling. When buckled strips are fed into the rolls, with a tendency to form a longitudinal fold, one part of the strip shifts relative to another in the deformation zone. Light lines appear on the surface at an angle to the rolling direction — a defect known as a shear mark (or "herringbone" if the lines run symmetrically longitudinally). The cause is poor roll profiling and uneven distribution of reduction across the strip width.

2. Pickling defects.

During pickling of hot-rolled strip, under-pickling or over-pickling is possible. In the first case, dark stripes or spots of unstripped scale remain on the surface; in the second, the metal surface becomes rough and corroded by the acid solution. These defects require adjustment of the pickling regime.

During rolling, dents (depressions) or bumps (protrusions) sometimes form on the strip surface. Dents of various shapes and sizes usually appear due to metal particles welding onto the roll surface — in this case, the rolls must be cleaned, e.g., with emery cloth or an abrasive stone. Bumps appear when the roll surface has dents or pits (from spalling); rolls with severe surface defects must be replaced.

A common defect of cold-rolled sheets and strip is rolled-in metal crumb, occurring when metal particles fall onto the strip surface — often these come from the strip edges if they have cracks or burrs.

When the metal touches sharp edges of guide hardware, during transport, and in other operations, scratches and scores appear on strip surfaces. They can also occur from relative slippage of strip windings within a coil during winding, unwinding, and handling.

3. Annealing defects.

Some surface defects form during annealing of cold-rolled metal. If significant residues of process lubricant (emulsion) remain on the surface after rolling, dark spots and streaks may appear during annealing, mostly near the strip edges. This defect is often called emulsion burn-on. To prevent it, avoid overly concentrated emulsions and remove lubricant residue from the strip surface after rolling as completely as possible — for example, by air-blowing.

Summary

Deviations in structure and physico-mechanical properties depend mainly on compliance with prescribed heat-treatment regimes. At the same time, deformation regimes have a major influence and must be selected with the final properties of the metal in mind. A separate issue is internal stress in the sheet due to short thermal cycle time, shallow heat-affected zone, etc.

Cutting problems faced by operators

Internal stresses

Normally, internal stresses are fully balanced and have no visible effect on the sheet until this equilibrium is disturbed for some reason. When the equilibrium is broken (by external load, removal of a material layer as a machining allowance, or cutting), the sheet begins to deform until stress redistribution brings it to a new equilibrium. Such deformations are called residual deformations.

The direct cause of internal stresses is the non-uniformity of linear or volumetric changes in macro- and micro-volumes of the metal.

At the high temperatures of laser cutting, phase transformations occur: the metal changes structure, the crystal lattice and the unit-cell volume change, and internal stresses arise.

The system becomes non-uniform. In the area of laser action (the workpiece), zones form with properties different from the bulk material. If their concentration becomes significant and the stresses exceed a certain threshold, the system (sheet, part, sample) loses stability and deforms.

The claim of "deformation-free" laser processing should be understood as follows: the deformations can be much smaller than with traditional thermal processing methods (possibly by orders of magnitude), but they exist. The technologist's job is to understand their causes and minimize them — and the laser beam provides such opportunities.

In laser cutting, the heat input is greatest at the metal surface and smaller in the lower layers. The metal expands when heated, but because the surface temperature is higher, the upper fibers elongate more; due to the smaller elongation of the lower fibers, internal stresses develop. When internal stress reaches a critical value (upper-fiber elongation many times greater than lower-fiber elongation), the workpiece bows upward, and because of irreversible structural changes in the metal, these deformations are plastic (irreversible).

Methods to combat thermal deformation in laser cutting

Stress state of the sheet before cutting. For sheet metal that has not been previously heat-treated, annealing or tempering should be applied to relieve internal stresses.
Part overall dimensions (linear size and thickness). The greater the part thickness and the smaller the ratio of linear size to thickness, the smaller the deformation, because heating is more uniform.
Sheet clamping. Secure the sheet with clamps or other fixtures. Continuous cutting with the leftover micro-joints (tabs) cut after full sheet cooling is recommended.
Scrap after cutting. Minimum deformation is achieved when the area of the part being cut is comparable to the blank area — in that case, the scrap deforms more than the part. For precision part cutting, the scrap (offcut) should have greater freedom of movement than the part.
Cutting speed. Higher speed reduces the heat absorbed per unit length of cut, which reduces deformation.
Sheet position during cutting. The sheet must not sag under local heating. Tables with a high number of slats are preferred.
Uniform heating significantly reduces internal stresses. For straight cuts, cut from the middle outward to the edges. Use proper part sorting in nesting; cut complex parts in sections, ideally opposing each other; use the back-step (reverse step) method, etc.
Additional cooling by air blowing.
Pulsed cutting mode.
Piercing should be done with the pre-pierce-holes function enabled.
With visible buckling ("belly") of the sheet, place it defect-side up; use proper part-to-part spacing (≥10 mm); use micro-joints; you may flip the sheet if cutting with common cuts and properly placed micro-joints.

Remember: before cutting, the sheet must be cleaned of rust and degreased. If the sheet arrives oiled, wipe it down.