where the larger atom is straining the lattice while smaller carbon, oxygen and nitrogen atoms occupy the interstices between the iron atoms.
Bake- hardening effect
Interstitial atom
Edge dislocation
Interstitial-free ‘IF’ steels are vacuum degassed to remove the carbon and oxygen atoms which impede the movement of dislocations and, therefore, increase the ease of deformation (positive effect on forming, negative effect on dent resistance). IF high-strength steels, therefore, combine the increased ductility associated with the ferritic matrix but gain enhanced strength from substitutional phosphorus, silicon and manganese additions.
eulavr
2.5
2.0
1.5
1.0
IF steel IF
HSS Future development Rephos-
phorized steel
Dual phase steel
0 10 20 30 40 50 60 70 80 90 100 Tensile strength (kgf/mm2)
Relationship between r value and tensile strength
Precipi- tation hardened steel
Bake-hardening steels derive their increase in strength from a strain ageing process that takes place on paint baking at circa 180C.
Sufficient carbon is retained in solution during either batch or continuous annealing to allow
(Continued)
3.3Steel119
Table 3.10 HSS Strengthening Mechanisms (Courtesy of Thyssen Krupp Stahl)dCont’d migration to dislocations following
cold deformation. These are effectively locked, requiring a higher subsequent stress to recommence deformation, thereby increasing dent resistance.
The strength of both rephosphorized and IF high strength steels can be enhanced by this mechanism but different modes of carbon retention are required to prevent premature diffusion of carbon at either room temperature in storage, or during the application of zinc by hot dipping. In continuous processing this can be achieved by
incorporating an over-ageing treatment and alloying, whereby just enough carbon is retained in solution to allow the mechanism to occur at elevated temperatures.
B
A
Bake hardening Strain hardening
ssertS
The degree of cold deformation will reduce theDBH response correspondingly (see the diagram below).
High-strength low-alloy steels gain their increased strength from the fine grain structure (smaller than ASTM No. 10) and fine dispersion of precipitates (e.g. niobium and titanium carbo-nitrides) both of
Grain boundaries Precipitations
Coarse- grained Coarsely dispersed
Fine-grained Finely dispersed
120CHAPTER3Materialsforconsiderationanduse
Multiphase steels derive their strength from thermo-mechanical processing, i.e. carefully balanced rolling, coiling and compositional control within the boundaries shown in the diagram opposite.
Types of steel included in this category are dual phase, TRIP/
TWIP, complex phase and martensitic phase as described below.
erutarepmeT
Time
Influence of alloying elements on transformation behaviour Ac1
Ac3 Al,P Si, P, Al Ferrite C, Mn, Cr, B
Si, P
bainite C, Mn, Cr, Mo pearllte C, Mn, Cr, Mo, Al Al,P
Al MsC, Mn
DP – steel TRIP – steel
Dual-phase steels normally contain a matrix of ductile ferrite plus a proportion of the hard martensite phase induced by alloying and heat treatment. The characteristic high work- hardening rate results from the generation and piling up of dislocations around the martensite fraction on straining. The combination of the high strength developed, associated with relatively high elongation values, enlarge the area under the stress/
strain curve resulting in improved energy absorption compared with other steels of similar strength.
These steels also exhibit bake hardenability but unlike normal BH steels theDBH increase is not
DP
Rm: 500–600
(Continued)
3.3Steel121
Table 3.10 HSS Strengthening Mechanisms (Courtesy of Thyssen Krupp Stahl)dCont’d limited by the amount of cold work
received (see below).
TRIP steels feature the transformation of metastable austenite to martensite during deformation thereby imparting a similar (but increased) strengthening compared to dual phase. The mechanism is similar to DP with dislocation pile-ups at the martensite/ferrite phase boundaries.
RA (TRIP)
600–800
(TWIP steels depend on the occurrence of mechanical twinning during deformation to achieve the necessary austenite phase change. These have a significantly different
composition, e.g. 18% Mn, 3% Si and 3% Al, and are under development for energy absorbing structural parts.)
Complex steels are typically hot rolled, fine grain steels featuring ferrite, bainite and martensitic phases with a fine grained microstructure and uniformly dispersed superfine precipitates.
CP PM MS-W (TMS)
> 800 > 800 > 1.000 MPa Ferrite
Bainite
Meta-stable austenite Martensite
Martensitic steels are hot rolled with extremely high strength levels imparted by the predominantly martensitic phase.
122CHAPTER3Materialsforconsiderationanduse
requirements and ensure compliance with warranty levels it is imperative that the protective coating withstands the surface friction associated with higher forming loads or the heat treatment if warm forming methods are used.
Reference to the latest extended ‘banana curves’ shows transformation induced plasticity (TRIP) and twinning induced plasticity (TWIP) grades, together with martensitic grades, exhibit the characteristics required. However, the ductility of the latter generally precludes it from all but simple channel shapes.
Tandem mill
(*) optional processing EBT
Sibertex©-technology: working principle
SB, EDT or EBT
Electrolytic galvanization (*) Temper mill
Hot-dip galvanization (*) Annealing
FIGURE 3.12
Tandem and temper mill treatments offered by the Sibertex process
(Courtesy of Sidstahl)
ZStE 180/220 BH (cold rolled) 500
400 300 200 100 0
)aPM(esaercnihtgnertsdleiY
0 2 4 6 8 10 12 14 16
500 400 300 200 100
00 2 4 6 8 10 12 14 16
RA-K 42/80 (cold rolled)
Prestrain (%)
0 2 4 6 8 10 12 14 16
0 2 4 6 8 10 12 14 16
MS-W, CP-W (hot rolled) MS-W CP-W
Work hardening Bake hardening DP-K 34/60
(cold rolled) 500
400 300 200 100 0 500 400 300 200 100 0
FIGURE 3.13
Contribution of work hardening and bake hardening in HSS
(Courtesy of TKS)
TRIP steels, already used in many production cars, derive their high-strength properties from transformation of their retained austenitic fraction to martensite during forming and also on impact. A high silicon and carbon content promotes the retention of austenite. Developments of this grade by ‘quenching and parti- tioning’ technology now being researched should ensure higher strength levels or improved formability. As evident from the ‘banana curve’ (see page 103) TWIP steels (austenitic stainless steel (ASS) zone) show more attractive ductility/high strength combinations, derived from the twinning behavior of the austenitic structure on deformation. They promise exceptional formability but currently suffer from delayed hydrogen cracking, associated with hydrogen diffusion to strained areas, and so require modified welding procedures. These steels are already under assessment by several motor manufacturers. They are generally high-manganese steels. They pose problems for manufacture by conventional processing methods, and the indications are that direct casting could be the most viable production route, resulting in a high-cost product.
To overcome cold-forming difficulties with AHSS (Advanced High Strength Steel) and UHSS (Ultra High Strength Steel) in order to produce the complex shapes demanded by door pillars, etc. and take advantage of strength levels up to 1500 MPa, most manufacturers are adopting hot-pressed boron-containing steels, e.g. Usibor 1500. VW manufacture such parts in-house. BMW have their own direct (one hot-pressing operation) and indirect (hot-pressing plus a cold-working stage) processes using a specially formulated zinc-based coating SOP 2009;11this reduces their outsourcing premiums. However, these grades are so effective that the additional costs of outsourcing (if required) can usually be justified. Hot pressings can be prone to defects such as elongated manganese sulphide inclu- sions, which affect collapse behavior on impact, and surface imperfections
700°C
Casting Scalping Preheating Hot rolling
10 mm
Continuous heat treatment Cold rolling
Cut to length
FIGURE 3.14
Aluminum production process