Technical Info Sheet #21
Steel producers/steel rolling mills are increasingly supplying steel with MC quality (also called micrograin steel). In the coding MC, the M stands for thermo-mechanically rolled and the C for cold forming.
The specific composition and rolling temperature create a very fine grain structure. This structure and associated mechanical properties make this material excellent for cold forming. In addition, this material is suitable for laser cutting because the steel surface is very smooth and tight and has a higher hardness. Due to its low carbon equivalent (CEV), the steel is also easy to weld.
In short: a type of steel with special properties that is particularly appreciated by machine and equipment manufacturers. The steel is supplied according to the European standard NEN-EN 10149-2.
MC-QUALITY HOT-DIP GALVANIZING
A characteristic feature of MC steel is its smooth surface and higher surface hardness. In addition, there is very little silicon present in MC steel and often a somewhat higher aluminum content. Because of these characteristics, in practice it appears that the formation of the zinc layer during hot-dip galvanizing is less, so the zinc layer thickness does not meet the layer thickness requirements listed in Table 3 of the international standard EN-ISO 1461. The achieved zinc coating thickness varies from a few micrometers to in some cases as much as 30 micrometers below the prescribed local or average coating thickness according to EN-ISO 1461 (Table 3). The most recent version of this standard therefore includes a comment in the text that in these cases the zinc coating thickness may not be linked to the steel section thickness.
Because the galvanizing plant cannot exert any influence on the zinc coating thickness to be achieved with these steels, there are therefore no requirements with regard to the acceptance criteria in terms of coating thicknesses.
Remark:
The thickness of the zinc coating depends on the following parameters: the temperature of the zinc bath, the shape of the object to be zinc-coated, the dipping time and curing speed and the chemical composition of the zinc melt as well as the chemical composition and surface condition of the steel.
Various trials have shown that adjustments to the hot-dip galvanizing process as described above have no or insufficient effect and therefore do not result in a zinc coating thickness that complies with Table 3 in standard EN-ISDO 1461. In addition to other aspects already mentioned, it should be borne in mind that at Manganese contents above 1.35%, deviations may occur in the form of a non-uniform and somewhat thicker galvanized coating. A pattern of grooves or channels may then develop. In the event of a higher Manganese content than 1.35%, the steel will have to be shot blasted beforehand in any case if the appearance of the coating is required.
EFFECTS THINNER ZINC COATING
Thus, despite the fact that the measured layer thicknesses may not meet the values in the table of EN-ISO 1461, there is often a nice smooth and regular surface of the zinc coating. So aesthetically a nice end result is achieved in most cases.
In various circumstances, the zinc coating thickness is essential for the life of the corrosion protection. In other situations, on the other hand, such as mechanical engineering for example, the service life is important but does not necessarily have to be half a century.
In the case of a zinc coating thickness of 85 microns, the steel will be protected for a period of at least 77 years and on average 113 years (according to ISO 9224, Corrosion Class C3).
REMEDY
If one prefers to maintain the zinc coating thickness according to Table 3 of EN-ISO 1461 when using MC steel, one can consider (or have the material blasted (SA 2.5) before delivery to the galvanizing plant.) As mentioned above, insufficient zinc coating thickness is partly a result of the high surface hardness and very smooth steel surface. Blasting roughens the smooth steel surface and will also reduce the surface hardness so that the values in the table can almost always be met. An additional advantage is that in this case the cutting edges are also slightly abraded and any oxidation residues that are difficult to remove are blasted off. The latter results in better cohesion of the zinc layer.
| Voorwerp en dikte voorwerp | Plaatselijke dikte van de verzinkte deklaag (minimum) μm | Plaatselijke massa (a) van de verzinkte deklaag (minimum) g/m2 | Gemiddelde dikte van de verzinkte deklaag (minimum) μm | Gemiddelde massaa van de verzinkte deklaag (minimum) g/m2 |
|---|---|---|---|---|
| Staal > 6 mm | 70 | 505 | 85 | 610 |
| Staal > 3 mm tot ≤ 6 mm | 55 | 395 | 70 | 505 |
| Staal ≥ 1,5 mm tot ≤ 3 mm | 45 | 325 | 55 | 395 |
| Staal < 1,5 mm | 35 | 250 | 45 | 325 |
| Gietstukken > 6 mm | 70 | 505 | 80 | 575 |
| Gietstukken ≤ 6 mm | 60 | 430 | 70 | 505 |
| OPMERKING: Deze tabel is voor algemeen gebruik: afzonderlijke productnormen kunnen andere eisen bevatten, waaronder afwijkende diktecategorieën. Eisen voor de plaatselijke en gemiddelde masse van de verzinkte deklaag in deze tabel dienen ter r eferentie bij geschillen die hieruit voort kunnen komen. |
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| (a) Gelijkwaardige massa van de verzinkte deklaag bij een nominale deklaagdichtheid van 7,2 g/cm3 (zie bijlage D). | ||||
DUPLEX SYSTEMS
If an organic topcoat (paint or powder coating) is applied after the thermal zinc coating, this is called a duplex system. If the smooth appearance of the zinc layer is the basis for the choice of applying this additional layer, it should be understood that cut edges must be rounded off (at least R=2 mm or a chamfer of 1 mm) and cut edges/surfaces must be reworked.
The cohesion of the zinc-iron alloy layers on the MC steel decreases, with the result that in the event of mechanical damage, the zinc layer may come off, and with it the organic coating from the substrate at the cut edge.
EN ISO 1461
Coatings applied by hot-dip galvanizing to iron and steel objects – Specifications and test methods.
ISO 9224
Corrosion of metals and alloys – Atmospheric corrosivity – Guideline values for corrosivity categories
NEN EN ISO 10149-2
Hot-rolled flat products made from high yield strength steels for cold forming – Part 2: Delivery conditions for thermomechanical rolled steels
