Sheet steel

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Thin steel sheet is a flat product made from hot or cold rolled strip and sheet metal in thicknesses between 0.3 and 3.0 mm. Other flat products made of steel are thin sheet (thicknesses less than 0.3 mm) and heavy sheet (thicknesses greater than 3.0 mm). The flat products are semi-finished products; H. Input material for further processing into ready-to-use products. Other semi-finished products are long products ( e.g. billets and steel bars) and wire .

Steel grades

Overview of mechanical properties: range of hot-rolled steel grades

The steel grades include unalloyed carbon steels with carbon mass fractions of a few hundredths to 1% with and without an alloy, e.g. T. micro- alloy and alloyed case- hardening and heat-treatable steels. This results in strengths of 400 to 1500 MPa.

For the performance properties, the increase in strength due to the forming process (" work hardening ") and, depending on the type of steel, due to the baking of the paint (" bake hardening ") must be taken into account.

The demand from consumers, especially from the automotive industry, for steels on the one hand for lightweight construction , i. H. The possibility of reducing the component thickness of steels with higher strength without loss of rigidity and, on the other hand, of building ever safer vehicles, has led to the development of higher and high-strength steels that also have good cold formability in the last 10 years . The potential in the development of easily formable steels with high strength has not yet been fully exploited.

The development trends also focus on improving processing behavior:

  • Improvement of formability (elongation), especially in the strength range above 800 MPa
  • Reduction in the variation in mechanical properties
  • Increased hole expansion behavior and improved bending behavior
  • Improvement of the welding behavior of the high-strength multi-phase steels.
Overview of the steel types
designation Surname Standard EN…
uncoated
[electrol. dam.]
{fire- coated }
Soft steels DD ... 10111
Micro-alloyed steels S ... MC 10149
HX… LAD + Z {10292}
Dual phase steels HDT ... X pr 10338
[10152]
{10346}
Ferrite-bainite phase steels HDT… F pr 10338
[10152]
{10346}
Complex phase steels HDT ... C pr 10338
[10152]
{10346}
Martensitic steels HDT ... C pr 10338
[10152]
{10346}
Manganese-boron steel for hot forming 22MnB5 type
(see manufacturer) 		s. Manufacturer
Alloyed and unalloyed engineering steels various various

Cold rolled strip and sheet

Range of services for cold-rolled steel grades

The carbon contents range from 30 ppm to about 0.35% for broadband and up to 1% (for narrowband). The alloy proportions range from 0 to micro-proportions up to the usual proportions of alloyed structural steels (mainly in the case of narrow strip). The demand from consumers, especially from the automotive industry, for steels on the one hand for lightweight construction, i. H. The possibility of reducing the component thickness of steels with higher strength without loss of rigidity and, on the other hand, of building ever safer vehicles, has led to the development of higher and high-strength steels that also have good cold formability in the last 10 years.

The main goals of the development trends are:

  • Increasing the strength range to over 1200 MPa
  • Expansion of the hot-dip galvanized product range with improved forming behavior
  • TWIP steels ( English twinning induced plasticity ) with elongation values of over 40%
  • Steels with a reduced specific weight
Overview of the steel types
designation Surname Standard EN…
uncoated
[electrol. dam.]
{fire- coated }
Soft steels DC…
DX… 		10130
[10152]
{10346}
High-strength IF steels H ... Y 10268
[10152]
{10346}
Bake hardening steels H ... B 10268
[10152]
{10346}
High-strength stretch-drawing steels HI 10268
[10152]
{10346}
Micro-alloyed steels H ... LA ... 10268
[10152]
{10346}
Work hardening steels s. Manufacturer s. Manufacturer
Dual phase steels HCT ... X pr 10338
[10152]
{10346}
TRIP steels HCT… T pr 10338
[10152]
{10346}
Complex phase steels HCT ... C pr 10338
[10152]
{10346}
Manganese-boron steel for hot forming 22MnB5 type
(see manufacturer) 		s. Manufacturer

"Sandwich sheet"

A sandwich sheet is understood to be the combination of two cold-rolled sheets (0.4 to 1.2 mm each) with an organic intermediate layer (25 to 50 µm). Sound waves cause internal friction in the viscoelastic intermediate layer, so that the vibration energy is “converted” into mechanical energy and thus ultimately into heat. This results in structure-borne noise attenuation of up to 20 decibels . Accordingly, this product is used, for example, in vehicle construction ( e.g. oil pans , end walls) or in other areas of noise protection ( e.g. containers for glass recycling ). For the cover sheets (also in combination) the o. A. Steel grades are used. Surface coatings are also possible.

Surface coatings and strip treatment processes

Due to its affinity for oxygen , steel must be protected against oxidation ( rust ) for functional and aesthetic reasons. This protection takes place by applying metallic and / or organic coatings, which act as barriers to prevent the entry of oxygen and corrosive media. In addition, the metallic coatings with less noble metals than iron also provide cathodic protection against damage or imperfections in the coating.

The coatings are applied on an industrial scale in continuous coating systems at the steel manufacturer, depending on the requirements, in different thicknesses from a few to 50 µm. They either consist of:

  • a metal that is less noble than steel, for example zinc , tin (cathodic protection and barrier effect) or nobler than steel, for example chromium , nickel (barrier effect and aesthetics). The application is carried out by electrolytic metal deposition (electrolytic coating) or by dipping the steel into the molten application metal or alloy (hot dip coating).
  • Varnish that is applied with application rollers (roll coaters).
  • the combination of metal and paint.

The metal coatings are suitable for welding, e.g. Some of the thin layers of lacquer are also formable, both when the parameters are adapted to the surface.

The active (cathodic) and passive (barrier) corrosion protection of steel is the primary goal of surface finishing from the point of view of component function and economy. In addition, advanced, modern coating technologies also enable surfaces with a decorative, functional and aesthetic effect to be produced. This means designing the visual appearance of the surface through surface structures, roughness or gloss and, above all, color. The latter is achieved by so-called coil painting of non-galvanized or mostly zinc or zinc alloy coated steel strips in coil coating systems. The surface refinement also achieves other functional properties. For example, such as self-cleaning surfaces, antibacterial properties or a low coefficient of friction to support the forming process by the processor, through the thinnest inorganic (e.g. zinc phosphate layers) or organic films. Both coatings are suitable as adhesive layers for a further coat of paint.

The most important sales markets for surface-refined sheet metal are the construction, household appliance and automotive industries, the latter being the main driving force behind technological progress over the past 25 years. The high levels of corrosion on the vehicle due to the increased use of road salt have meant that the corrosion protection of the steel sheets had to be continuously improved by coating them with zinc or zinc alloy coatings and organic corrosion protection primers.

As a result, the corrosion protection guarantees of the automobile manufacturers could be continuously increased to the corrosion protection guarantees of well over 10 years currently granted by most manufacturers, despite forced lightweight steel construction (reduced sheet metal thicknesses).

Accordingly, the proportion of surface-refined steel strips for use in car body construction has risen to an average of over 80% - for some manufacturers even to 100%.

Electrolytic coatings

This coating is carried out in a fully continuous process with layer thicknesses of typically 5; 7.5 and 10 µm zinc applied, whereby it can be applied either on one or both sides - with the same or different layer thickness (for example ZE 75/75: galvanized on both sides with nominal layers of 7.5 µm on each side). The coating adheres firmly, is easy to form, is suitable for welding and meets the highest surface requirements of the automotive and household appliance industries. The thin sheet is usually oiled to avoid the formation of white rust, on request with special oils to increase its suitability for forming. Pre-phosphating is also possible, which also reduces the formation of white rust and increases the suitability for forming.

Hot dip coatings

The hot-dip coating (such as galvanizing , hot-dip galvanizing or Sendzimirverzinkung is also fully continuous). In contrast to the electrolytic process, this is a combination of several manufacturing steps (recrystallizing annealing, coating and skin-passaging) in one system. The coating takes place in that the rolled strip is annealed to recrystallize in a continuous furnace and then passed through the molten coating bath. The individually adjustable layer thicknesses are typically between 7 and 20 µm (for example Z100, total coating weight 100 g / m² corresponding to approx. 7 µm per side), a one-sided coating as with electrolytic galvanizing is not possible, but different thicknesses per side are within limits.

In addition to the most common pure zinc coatings (Z), with about 0.5% Al in the layer, there are also the following zinc alloy coatings that are commonly used for special applications:

  • Zinc - 5% aluminum (ZA), Galfan ® (English galvanized fantastique )
  • Zinc 92% - magnesium, aluminum (ZM)
  • Zinc - 55% aluminum (AZ), Galvalume ® ( galvanized and aluminized )
  • Zinc - 10% iron (ZF), galvannealed (English galvanized and annealed )

In addition, aluminum coatings (AS) with typically 10% silicon and zinc-iron coatings (FZ) with over 50% iron are also used on a large scale for press-hardened steel.

These coatings each have different combinations of properties in terms of forming and welding behavior. A wide range of coatable materials means that the processor can choose the optimum material for the specific application.

With regard to the corrosion properties, corrosion rates for pure zinc coatings can be found in DIN EN ISO 14713. Alloy coatings with an aluminum and / or magnesium content show better corrosion resistance in short-term tests such as the salt spray test compared to pure zinc coatings. This increased corrosion resistance has not yet been proven under natural outdoor weather conditions. For example, outdoor weathering tests carried out by the Federal Highway Research Institute (BASt) (see BASt report "Strip-galvanized crash barrier rails BASt - B - 0059") showed no different corrosion rates between crash barriers with pure zinc coatings (Z) and those with zinc-aluminum coatings (ZA). An exposure test carried out by the Swerea KIMAP research institute in Swedish tunnels also showed no different corrosion rates between pure zinc coatings and zinc-aluminum-magnesium coatings (ZM).

The coated sheet is usually oiled to avoid the formation of white rust, if desired with special oils to increase its suitability for forming. Pre-phosphating is also possible, which also reduces the formation of white rust and increases the suitability for forming. The areas of application are diverse and range from the construction - to the automobile - to the household appliance industry.

Organic coatings

Organic coating is also a fully continuous process: in a system, after chemical cleaning, a conversion layer (<1 µm) is applied as an adhesion promoter for the paint layer (spray, dip or roll coater process ). The thin sheet to be coated can be with or without a metallic coating. Then the first coat of paint, the primer (about 5 µm) and then the top coat (about 20 µm) are applied. Both are applied using the roller application or roll coater method, with drying at 200–250 ° C after each process step. The organic coating is mostly carried out on both sides with a two-layer lacquered decorative side and a single-layer (primer) lacquered back. The coil-coated material is ready for use and does not require any further painting by the end user. This means that the surface properties such as color, gloss or structure are delivered in the ordered final state for the component, but processing into the component itself only takes place after the coating ( finish first - fabricate later ). The coated sheet metal is deformable but not suitable for welding if the deformation parameters are adjusted. It is mainly used in the construction, household appliance (white goods) and phono / computer industries (brown goods).

Furthermore, galvanized sheet metal coated with a weldable, a few micrometers thin corrosion protection primer is used in the automotive industry to increase the corrosion protection in flanges and cavities.

Development tendencies

Different priorities are set when developing the coatings. This ensures that high-strength and high-strength steels can be galvanized in the hot-dip process. Active and passive corrosion protection is also increased through both new types of metallic coatings (new alloys, new and further developed processes) and new active corrosion protection systems (for example, incorporation of nanocapsules with corrosion protection inhibitors).

Another focus is on improving the processing properties, in particular the weldability for laser beam and resistance welding. In addition, coatings for hot-formable steels are being developed to protect against oxidation during the hot-forming process and against corrosion on the finished component. Organically coated thin sheets receive self-cleaning, antibacterial coatings and coatings with special optical effects ( iridescent topcoats). Other aspects are coatings that prevent fingerprints , graffiti and scratches. In addition, the further development of the corrosion protection primer with regard to the optimal, component-specific corrosion protection with simultaneously improved processing properties, especially with a view to welding .

In cooperation with the automotive and automotive supplier companies, the use of coil-coated - pre-primed and / or prefilled - thin sheets in bodywork is being further developed.

Material selection for special applications

The choice of materials is based on the processing requirements as well as the usage properties of the component in its intended applications. Both, individually or in combination, differ from case to case and require careful analysis. This means that it does not make sense to set up common requirement profiles for the selection of materials for different applications.

The criteria for material selection from the point of view of the essential requirements of the automotive industry for body use are primarily:

  • Lightweight construction option
  • Crash construction
  • Freedom of design and shape
  • Pedestrian protection
  • longevity
  • Surface finish

The processing requirements and usage properties of the material are derived from this. Component as follows:

  • Formability
  • Compliance
    • Thermal processes (resistance and laser welding)
    • Mechanical processes (clinching)
    • Chemical processes (gluing)
  • strength
    • Stiffness
    • Fatigue strength (cyclical)
    • Crash resistance
    • Ductility
    • Buckling stiffness / strength
  • Paintability
  • Corrosion resistance

In addition, the material or component costs and processing costs are another criterion for material selection.

Further processing

The steel ordered by the processor is usually delivered by the steel manufacturer as a coiled strip ( coil ), if required also as cut sheet metal via the steel service center (dimensions: desired width times desired length). The coil is packed on delivery for reasons of transport protection (mechanical damage, corrosion). The processing of the steel takes place individually and depends on the use as well as on the processing units available, whereby the sequence of processing steps is basically the same for the different processors. In the automotive industry it is:

  • First work step : unwinding and straightening the strip as well as sheet metal or molded blank cutting. The strip, wound into a coil, is unwound, leveled and, depending on the requirements, the sheet metal or the blank (shaped blank) is cut.
  • Second work step : component production (forming, cutting). The forming (stretch drawing, deep drawing) takes place in a press (mostly multi-stage press). The forming parameters (holding-down force, punch radii, forming speed) depend on the type of steel used, the surface coating and the component geometry. Modern analysis methods allow the forming to be simulated on the computer.
  • Third work step : joining several formed parts to form an assembly.
  • Fourth work step : joining the assemblies to the body shell. Here, components are assembled into component groups (for example the components inside and outside of the door panel and door receptacle) and the various component groups into the body shell. The assembly is carried out using joining processes such as welding (predominantly spot welding, increasingly also laser welding), soldering (to a lesser extent), clinching and gluing.
  • Fifth work step : painting the body shell and secondary corrosion protection measures. For reasons of corrosion protection, but also for aesthetic and marketing reasons, the body shell is painted in the paint shop. The surfaces must be cleaned of oils, fats, dust and metal abrasion. The cleaning takes place in a process sequence with the phosphating, i.e. the application of a phosphate layer as an adhesion promoter for the paint in a multi-stage continuous spray / immersion system. After this pretreatment, the first coat of paint is applied wet-on-wet, using the KTL process (cataphoretic dip painting or electrodeposition painting) in a dip tank, also in a continuous process. The electrophoretic deposition of the paint on the body ensures that thin and even paint layers of 15 to 20 µm are formed even in angled places and even in cavities. The paint is then baked (dried) at 180 ° C. The application of the other layers of paint (filler, top coat, with metallic paint also clear coat) takes place in spray booths using spray robots.
  • Sixth work step : assembly (assembly of the automobile). The assembly of the finished, painted body shell with the add-on and built-in parts to form the ready-to-use automobile is what is known as assembly.

literature

  • Sheet steel for automobile construction, Library of Technology, Volume 202, Verlag Moderne Industrie, Landsberg / Lech, 2000, ISBN 3-478-93163-0 .

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