Steels_ Metallurgy and Applications, Third Edition

90 Steels: Metallurgy and Applications

to those employed in continuous annealing but generally without an over-ageing

stage. Therefore, the formability of hot dip galvanized steel is poorer than that

of uncoated, batch-annealed material. However, the formability of the hot dip

product can be improved through the use of modified annealing cycles but the

special IF (interstitial-free) steels can also be employed as a substrate for highly

formable, hot dip galvanized strip. The IF steels were referred to earlier on p. 39

and owe their good formability to the elimination of carbon and nitrogen from

solid solution and the development of a favourable crystallographic orientation

texture.

As described earlier, electro-galvanizing takes place at near-ambient tempera-

ture and therefore the coating operation has little effect on the forming behaviour.

Standard specifications
Hot dip galvanized products are covered by BS EN 10142:1991 Continuously
hot-dip zinc coated miM steel strip and sheet for coM forming and the available
grades together with the associated mechanical properties are given in Table 1.7.
These grades are arranged in increasing suitability for cold forming, ranging
from DX 51D (bending and profiling quality) to DX 54D (special deep-drawing
quality). Differentiation is made between coating type, namely zinc (Z) or zinc-
alloy (ZF) coatings. Further designations are also applied in order to denote
coating finish, surface quality and surface treatment. As indicated in Table 1.8,
the grades can be supplied with a range of coating mass, but it should be noted
that the figures for coating mass (g/m 2) include both surfaces.
A possible area of confusion in BS EN 10142:1991 is the change in designation
of the basic grades. When this standard was first introduced, the grades were

Table 1.7 BS EN 10142:1991 Steel grades and mechanical properties

Designation

Steel grade Symbol for the

Steel name Steel number type of hot-dip

coating

Yield strength Tensile strength Elongation

R~) Rm Aso

N/ram 2 N/ram 2 %

max. 2) max. 2) min. 3)

DX51D 1.0226 +Z

DX51D 1.0226 +ZF

DX52D 1.0350 +Z

DX52D 1.0350 +ZF

DX53D 1.0355 +Z

DX53D 1.0355 +ZF

DX54D 1.0306 +Z

DX54D 1.0306 +ZF

500 22

3004) 420 26

260 380 30

220 350 36

l) The yield strength values apply to the 0.2% proof stress if the yield point is not pronounced,

otherwise to the lower yield strength (ReL)

2) For all steel grades a minimum value of 140 N/mm 2 for the yield strength (Re) and of 270 N/mm 2

for tensile strength (Rm) may be expected.

3) For product thicknesses <0.7 mm (including zinc coating) the minimum elongation values (As0)

shall be reduced by 2 units.

4) This value applies to skin-passed products only (surface qualities B and C).