High Strength EDDQ Sheet
High Strength EDDQ Sheet
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Introduction. |
Steel is the most important material in the automotive industry, since it fulfills a variety of requirements at relatively low costs. Fuel economy and safety factors demand higher strength steels. Other considerations regarding styling and ease of fabrication require sheet steel of extra deep drawing quality (EDDQ). As shown in figure 1 (1,2), most of the traditional methods of increasing the strength of automotive sheet also reduce the deep drawing properties (demonstrated by the Lankford value r) and the stretch formability (demonstrated by the strain hardening exponent n). However, high strength sheet with excellent formability has been developed based on interstitial free (IF) steel and this material is experiencing a steadily increasing demand.
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The Market of IF steel |
The possibility of improving the cold formability of sheet steel by using interstitial
free steels has been well known for almost three decades (3). Starting in the early 1980's in Japan a remarkable
growth in the production of IF steels was observed and the tonnage has more than doubled every three years. Similar
trends have also been noted in other industrialized countries (1). This steady increase is continuing worldwide
and the annual production reached about 15 million t in 1995. This remarkable development is supported by the fact
that modern steel making technology via the RH degassing process, sometimes in combination with oxygen blowing,
allows the mass production of steel with very low interstitial contents, with levels of carbon and nitrogen below
30 ppm for each interstitial element. Such low interstitial levels need comparably small additions of stabilizing
elements, thus reducing the alloy costs in this steel type, but the breakthrough in the adoption of IF steels was
mainly influenced by other factors:
1) The continuous annealing of sheet, which allows for time and cost savings to be made in comparison to the batch
annealing process, has become more common. Furthermore, the more homogeneous conditions in this process reduce
the variation in mechanical properties over the length and width of the sheet and also from coil to coil. However,
the fast reheating velocity does not allow for the use of traditional AIN texture control technology resulting
in insufficient formability of the traditional EDDQ steel composition. When using interstitial free steels an excellent
Lankford value and also an improved stain hardening potential is obtained, independent, if the batch annealing
or continuous annealing process is applied (4).
2) Continuous annealing is also the standard processing route in hot dip metallizing lines for coating steel sheet
with zinc or other metals. The usage of such corrosion-resistant sheet products for the automotive industry is
growing very rapidly and many new production lines have been installed worldwide. These lines do not normally allow
for cementite precipitation after recrystallization (the so called overaging part in continuous annealing lines).
Since the interstitial free condition guarantees naturally no aging, IF steels are therefore also favorable for
galvanizing lines as well as for other strip processing routes.
3) Large integrated parts guarantee a remarkable increase in press shop productivity owing to reduced pressing
and welding operations and allow savings of equipment. One reported example is a door link (5) where by using just
one large sheet of IF steel with excellent deep drawing properties the conventional method of using six components
could be replaced, thus reducing the costs for punch and dies by 20% on top of the higher productivity. The required
formability of the sheet for such integrated parts can only be obtained by using IF steels.
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Physical Metallurgy of IF steel |
Figure 2 (6,7) exhibits that the Lankford value increases exponentially with lower carbon levels, a tendency which is enhanced by niobium-stabilization of the interstitials carbon and nitrogen. Thus the reduction in the level of the interstitials is of upmost relevance.
Up to the early 1980's standard steelmaking technology produced carbon levels of <80 ppm and nitrogen levels of <60 ppm, which then required large amounts of stabilizing elements. A typical IF steel at this time relied preferably on titanium stabilization and typical additions were as high as 0.12% (8). This decision was based on economical factors, since titanium has a more favorable atomic ratio Me/C than niobium and the titanium ferroalloy also costs less compared to niobium. With lower contents of interstitials these benefits for titanium decrease in significance.
Figure 3 demonstrates
the precipitation start temperature of the compounds observed in modern IF steel, when the carbon content is either
stabilized with titanium or with niobium. Titanium shows a high reactivity with all metalloids in the sequence
oxygen, nitrogen, sulfur, carbon and phosphorus. In a vacuum treated aluminum killed steel, the first compound
which will be formed at high temperatures, i.e. during solidification or in the delta ferrite region, is the TiN.
Then, dependent on the level of the total sulfur, titanium and manganese content often the formation of TiS might
occur. The titaniumsulfide will be transformed into a titaniumcarbosulfide by absorbing carbon in the austenite
region. This reaction has been studied in detail recently (9) and seems to have preference over the alternative
possibility for the formation of MnS and TiC. Therefore in order to fully stabilize the carbon content by titanium
at least the stoichiometric addition to fix nitrogen, sulfur and carbon is necessary. This can be calculated from
the atomic weight ratios:
%Ti = 48/14 x %N + 48/32 x %S + 48/12 x %C (Equ. 1 )
In case of excess free titanium an iron-titanium-phosphide might be formed in the ferrite region.
When stabilizing the carbon content by niobium, the sulfur content will be fixed by manganese and the nitrogen
by aluminum. Since both elements are already present in these steels, the required niobium level has to be just
stoichiometric with respect to carbon.
%Nb = 93/12 x %C (Equ. 2)
NbC will be formed only during or after the g/a transformation according to the solubility product. If additional
solute niobium is present, several benefits have been observed (9) and these studies are still continuing (10).
From calculations using typical contents of the relevant metalloids in modern IF steel, e.g. 30 ppm C, 35 ppm N
and 80 ppm S it can be shown that at least 0.036% Ti or 0.023% Nb is needed in the steel to obtain the IF status.
With this rather low amount of the necessary stabilizing elements, the alloy costs are low for both variants. Since
various surface defects (blisters, slivers, streaky defects) are more pronounced with titanium levels higher than
0.025%, dual stabilization. i.e. fixing only the nitrogen by titanium and using niobium to stabilize the carbon,
is gaining in importance.
Furthermore, the different behavior of titanium and niobium in forming compounds during hot strip rolling also
has an influence on the final properties. In niobium stabilized steels a delay in recrystallization during the
final deformation steps in the finishing stand of the hot strip mill (11) and a lower g/a transformation temperature
of approximately 20oC at a typical cooling rate (12) are observed. Both observations
can be related to a solute drag effect of the relatively large niobium atom. Therefore more nuclei for ferrite
formation are active resulting in a finer grain size. The finer grain size of the hot rolled band gives a higher
r value and a lower Dr value for a certain degree of cold
rolling reduction (13). The high planar isotropy by the use of niobium in IF steel is known already, for some time
(14) and it has the positive effect of reducing earing phenomena in EDDQ grades. Since the titanium stabilized
material shows the higher elongation data, the best combination of cold forming properties is achieved with dual
stabilization of low interstitial grades, as summarized in figure 4 (2).
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High Strength IF or ULC steel |
The tensile properties of interstitial free steels are rather low with a yield
strength of around 150 MPa and a tensile strength of around 300 MPa. Based on the IF steel alloy concept, new ultra low carbon (ULC) high strength steel sheet
with excellent deep drawing and stretch forming ability have been introduced into the market and actually steels
with even higher strength than indicated in the figure 1 are under development. These steels do not always need
to be interstitial free, therefore the term ULC is more correct.
The inherent finer grain size of a niobium stabilized hot band is passed on to a certain extent to the recrystallized
annealed sheet, thus resulting in somewhat higher strength. This is one of the reasons that high strength ULC steels
rely on niobium microalloying. Since good cold formability is possible only with fully recrystallized material
and the kinetics of this process depend on the coarsening of the carbonitrides, the possibility of applying precipitation
hardening by carbonitrides cannot be used to increase the strength in EDDQ steels. Therefore, solid solution hardening
is used as a strengthening mechanism. Relevant elements in this context are manganese and silicon,
which increase the tensile strength by 4 N/mm2 per 0.1% Mn or 10 N/mm2 per 0.1% Si. The effect of phosphorus is
an order of magnitude greater, the tensile strength being increased by 100 N/mm2 per 0.1% P in niobium stabilized
steels and slightly less in titanium steels, as shown in figure 5 (15).
The lower strengthening effect in the titanium steel can be explained by the formation of the FeTiP-phase, thus
lowering the solution hardening effect of phosphorus. This is an additional reason, besides the finer grain size,
for the use of niobium in preference to titanium for carbon stabilization.
One negative effect of phosphorus is its tendency to diffuse to the grain boundaries, causing embrittlement. This
is particularly effective in interstitial free steels, where no carbon exists in the form of solute atoms, which
could competitively occupy the grain boundaries and thus strengthen them. After batch annealing such embrittlement
may be quite significant in IF steel without any extra phosphorus additions. The typical phosphorus
content in the high strength EDDQ steels can cause grain boundary embrittlement even during the short continuous
annealing cycle time. In order to avoid this, the addition of boron to the high strength ULC steels has become
common practice (16). Figure 6
shows results achieved with a specific impact test. Boron, which is also an interstitial
element, appears to successfully prevent grain boundary segregation of phosphorus, and this effect is enhanced
by the finer grain size (larger grain boundary area) when microalloying with niobium. Thus niobium helps to reduce
the phosphorus embrittlement in addition to its strengthening effects.
When galvanizing or galvannealing is involved, the steel usually must not contain major additions of silicon. Table 1 (15) shows characteristic production data
of a hot dip galvanized high strength IF steel. indicating also the high planar isotropy of these steels.
Table 1: Chemical composition and mechanical properties of hot dip galvanized high strength IF steel
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Chemical Composition |
Mechanical Properties |
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| 30 ppm C, 30 ppm N, | Yield Strength | 220 N/mm2 | |
| 0.35% Mn, 0.05% P, | Tensile Strength | 390 N/mm2 | |
| 0.03% Al, 0.035% Nb, | Elongation A 80 | 37% | |
| 0.02% Ti, 10 ppm B | r-value | 1.9 | |
| D r-Value | 0.1 | ||
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n-value | 0.21 | |
The bake hardenability of sheet steel is very much appreciated by the automotive
industry, since it allows the pressing with rather low forces and guarantees a higher strength in the final product.
i.e. after press forming and paint baking. Bake hardening introduces the Cottrell effect of dislocation locking
by interstitials and therefore the steel needs a certain amount of solute carbon, which should not be below 5 ppm
(16). One possibility is to produce ultra low carbon steel with a slightly under-stoichiometric addition of the
carbon-stabilizing elements. This approach needs very precise melting shop technology, especially since the carbon
pick up between the vacuum treatment and the concast slab usually varies slightly. The process
has one disadvantage, in that even such low levels of solute carbon which are present at the start of the continuous
annealing process hamper the formation of a favorable texture for deep drawing and thus influence the r-value (17).
Figure 7 (18)
illustrates schematically the processing route to obtain the extra yield strength increase of around 50 N/mm2 due
to bake hardening of a high strength EDDQ sheet. The alloy design is based on complete carbon stabilization by
niobium and after cold rolling the sheet exhibits an IF status. As already discussed earlier, the NbC is formed
at fairly low temperatures, i.e. in the ferrite region. Thus it can be partially dissolved, if a rather high temperature
in the continuous annealing process is applied, higher than necessary for the complete recrystallization of the
cold rolled sheet. Fast cooling after this dissolution treatment guarantees, that a few ppm of carbon remain in
solution causing bake hardenability. In this condition the steel does not exhibit a total interstitial free status,
but the favorable texture for excellent cold formability is maintained.
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Conclusions |
Interstitial free steels have obtained a relevant market share in the recent years
especially for the automotive industry, i.e. mainly continuously annealed EDDQ grade as galvanized or galvannealed
sheet. Historically, titanium only was used for the stabilization of the interstitial elements due to economic
reasons. With even lower carbon levels in modern IF steels this argument diminishes and dual stabilization with
a combination of small additions of titanium stoichiometric to nitrogen and niobium stoichiometric to carbon now
provides the best compromise with regard to cost, surface quality and formability.
Another requirement of the automotive industry is weight reduction, which demands for higher strength sheet material.
Since most of the higher strength sheet steels exhibit a reduced formability, their application is restricted.
Based on the ultra low carbon concept high strength extra deep drawing quality sheet steels have been developed,
which also guarantee a high planar isotropy. Niobium stabilization has become standard in such high strength ULC
sheet, owing to better properties and the possibility to apply bake hardening.
The versatility and the potential of the IF and ULC steels allow steel to maintain its position as the most economic.
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References |
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