High-End Inspection Technology To Ensure Best Quality

Surface roughness and coating adhesion are controlled and measured for each coil.
A special laboratory and inspection station equipped with state-of-the-art testing machines guarantee continuous quality control.
An inspection room in the line, which is equipped with a special worldwide most-developed lighting system, is used for surface control.
A surface inspection system from Parsytec is used as an online inspection system which consists of CCD-cameras scanning the surface, combined with highly sophisticated software processing. The system is able to inspect HDG coils at high speed and to detect defects of a size of 1 mm2. Details of the defects are displayed and stored digitally as pictures, thus supporting the quality inspector and resulting in high-quality accuracy.
Customized quality certificates provide elaborate information about the products, chemical analysis and mechanical properties.
Product data and quality-relevant process parameters are recorded for each metre of strip and saved in an internal database, providing the basis for all material reports required by customers.
All important order specifications, set point values and incoming material data can be visualized at all operating stations.

The internal information system, including quality management data, quantity and time reports can be viewed at any time at all relevant points in the facility.

Wide Quality Range Satisfies All Customer Requirements
Quality Range
All variants of hot-dip galvanized sheet meet the standards of the mild steels and structural steels listed in the product mix. Most variants can also be produced in high-strength grades. The mild steel product range extends from basic quality for simple forming through drawing quality to excellent stretch and deep-drawing qualities. Research and application technology have recently developed, or rather further developed, high-strength steels and built them into an extensive product range, specially for the automotive sector. The use of these steels allows an improvement in component behaviour and, due to the reduction in sheet thickness, the lowering of weight and so a saving in fuel while the strength level is maintained. Various strengthening methods are included in the product range. High-strength qualities suitable for deep-drawing accordance with DIN EN 10292 with graduated minimum yield strength and greater elongation :

Phosphorus alloyed steel (works designation PHZ...) has good drawability thanks to a comparatively high r-value. The strength is achieved through small additions of phosphorus and manganese (solid-solution hardening).
The high-strength special deep-drawing qualities (works designation HX??) are even more deep-drawable and stretch-formable. The basis is the IF character (Interstitial Free), i.e. the carbon and nitrogen released in the lattice are bound by titanium or titanium and niobium. Small additions of phosphorus and manganese produce a solid-solution hardening.
With bake-hardening steel (works designation BHZ??) the diffusion of free carbon atoms during heat treatment after. painting is used, e.g. bake-hardening for external sheets. This steel is excellently suited to outer auto body applications thanks to its good dent resistance.
With the micro alloyed steel (works designation MHZ??) a higher strength is achieved due to the fine grain and the carbonitride precipitation of Ti and Nb to the grain boundaries. The steel is well suited for structural components.
With the dual-phase steel (works designation DP-K) the development to multiplephase steels is achieved. Martensite islands have a strengthening effect in the ferritic microstructure. Besides good iso-tropic forming properties and a bake-hardening potential, it has a high strengthening capacity and thus a high energy absorption capacity. Its obvious use is in crash-relevant parts.

Corrosion Protection
A zinc coating applied to steel in an immersion process has for a long time been ideal in corrosion protection. The zinc actually protects the steel underneath in two ways:
First the zinc layer forms an adhering protective jacket, which protects the steel when subjected to weathering (barrier effect).

On the cut edges and in those places where this jacket sustains damage through to the steel base, the base zinc first dissolves in a conducting medium (dirt, drops of water), i.e. "is consumed", before the steel is attacked. This is termed the "cathode protective effect" of zinc.

The corrosion protection can be significantly enhanced by a paint coating.
If the hot-dip galvanized products are arranged according to the most important corrosion criteria and compared to cold-rolled steel, the results are :

Criterion	Hot-dip galvanized	Galvannealed	Cold-rolled sheet
Corrosion resistance unpainted	good	-	-
Corrosion resistance painted (KTL)	good	excellent	acceptable
Paintability (KTL)	good	satisfactory	excellent
Phosphateability	good	excellent	excellent
Fuel stability	satisfactory	-	inadequate

Formability And Welding Of Zinc Coated Sheets
Formability
All forming processes for the cold-rolled steel can be used for hot-dip galvanized sheets, if tool surface and geometry suit these materials. The last finishing operation of the tool surface must take place in the direction of the sheet metal flow. This also applies to the drawing besds and draw radii.

The tool surface must be absolutely scratchfree to avoid abrasion from the coating which commonly occurs at high forming unit cycle times. The abrasion can be minimized by using a suitable drawing aid or a phosphate-treated strip surface. If possible a thin metallic coating should always be chosen, as far as the corrosion protection requirements allow. The resulting friction forces are significantly lower than with cold-rolled sheet of comparable roughness because of the metallic coating. Thus the forming efficiency is increased, which often results in a higher limiting drawing ratio or in a greater depth of draw. Due to the lower friction, higher blank holder forces are required when deep-drawing than with cold-rolled sheet, with the result that, although the applicable working range shifts in terms of level, it is not restrictive.

Just as for cold-rolled sheet, a certain roughness of the hot-dip galvanized sheet surface permits optimal production conditions and safe transportation of parts through suction cups. This roughness is adjusted during temper-rolling.

Hot-dip galvannealed sheet is just as suitable for all deep-drawing processes as normal hot-dip galvanized sheet. Due to the iron content in the zinc layer, the frictional behaviour is more similar to that of cold-rolled sheet. Because of the higher frictional values, smaller blank dimensions can be chosen with stretch-forming parts. The fact that the zinc-iron layer is somewhat less ductile than the pure zinc layer has to be borne in mind when lubricating, maintaining tools and designing the radii.

Welding
Welding is the most commonly used joining technique. Hot-dip galvanized sheet can be easily resistance-spot welded. However, a higher welding current and greater electrode force have to be used compared with coldrolled sheet because the coating has less transitional resistance. As a result of this, the normal copper electrodes become alloyed and the electrode life, i.e. the number of spot welds is lower than with cold-rolled sheet.

This can be counteracted by using suitable electrode material, e.g. CuCr1Zr or CuC1 and adapted electrode geometry as well as sufficient cooling. A staggered control, for example raise the welding current in stages and thereby increases the electrode life significantly. Hot-dip galvannealed sheet can be welded more economically, as the copper electrode alloys up to a lesser degree because of the zinc-iron layer. The figure in the table compares the welding current range of hot-dip galvanized materials with cold-rolled sheet and its electrolytically coated variants. It shows that the welding-current range of the galvannealed variant is almost identical to that of cold-rolled sheet. Seam-welding can also be used. The electrodes have to be continuously cleaned using scrapers or brushes or with a knurled drive, which smoothes the contact area of the electrodes.

Projection-welding and stud-welding can also be used, even if, compared to cold-rolled sheet, the welding current and electrode force are increased or the stud is pressed with somewhat greater force into the melt.

The familiar, conventional methods, such as GMA and TIG welding are also suitable for joining hot-dip galvanized sheet. Other possibilities are soldering and brazing, whereby the inert gas technique is advantageous for the latter. Because the coating metal burns in the melt of the welding seam, a welding method should be chosen which introduces little heat into the welding area in order to reduce the corrosion protection by as little as possible. A welding speed, less than that used for coldrolled sheet allows the melt to degas and prevents pore formation as far as possible. The welding fumes must be drawn off.

Plasma-welding and laser-welding are recommended welding methods. The heat-affected area is comparatively small, the seam is almost free of pores and only slightly overfilled.

CERTIFICATE