102 CHAPTER 3 Materials for consideration and use
3.3.1 Steel reduction and finishing processes
Processing improvements, which have enabled the increased range of properties previously highlighted, are summarized in the following below and many are featured in the flow chart shown inFigure 3.2, typical of most plants worldwide manufacturing automotive strip.
Regarding steel production, it is probably sufficient to know at this stage that most steel used for autobody manufacture has been smelted from iron (produced in a blast furnace) and recycled from scrap. Basic oxygen steelmaking (BOS) is the normal method used; impurities are oxidized by the injection of oxygen through the bottom of the converter, to produce refined material of composition typical of forming grades specified in Euronorm EN 10130. Normally these steels are aluminum-killed (AK), which refers to the addition of aluminum to minimize ageing effects by combining with nitrogen, and forming the characteristic ‘pancake sha- ped’, i.e. elongated, grains evident in the microstructure. These form the basic grades used for less demanding panel forms within the body structure. Carbon levels are typically 0.03e0.05% but for ultra-deep-drawing coated or high-strength grades, where extra drawability is required, this is reduced to less than 0.0002%. This is achieved by vacuum degassing of the molten steel prior to casting, resulting in the now well-known IF steels used for more complex shaped parts.
3.3.1.1 Vacuum degassing
This process involves the removal of gaseous and particulate inclusions, ensuring the levels of impurities are kept to a minimum. Additions of titanium or niobium ensure that interstitial elements such as carbon and nitrogen are reduced to extremely low levels and other compositional changes are effected to optimize texture develop- ment, resulting in high ‘r’ values. This explains the term ‘interstitial free’ and the
FIGURE 3.1
Steel grades with World Auto Steel Source
associated high level of formability of these IF variants. This is an important treatment for high-strength steels such as grades H180-260YD included in EN 10292, which show higher ‘r’ and ‘n’ values than other grades classified with similar strength levels. Similarly, IF substrates can boost the formability of hot-dip galva- nized products where annealing cycles might not be fully optimized. A typical vacuum-degassing rig is shown inFigure 3.2.3
3.3.1.2 Continuous casting
Following the steelmaking process the molten steel is cast. The traditional ingot casting processing route that led to the differentiation between ‘rimming’ and ‘killed’
steels has now largely disappeared since the continuous casting of slab has been introduced. Due to gas evolution in the final stages of solidification, shown by rimming steel replacing the ‘V’ shaped ‘pipe’ associated with killed steels (later discarded), better utilization of the cast ingot than the killed steel could be made, but it was prone to room temperature ageing in storage. As a consequence, the yield strength increased and the surface was prone to the appearance of secondary
‘stretcher-strain’ markings, which could show through the painted finish. The yield of the killed steel was lower due to the necessity to crop the ‘pipe’ due to its asymmetry and because it contained a higher content of residual impurities. Now continuous slab production using the type of rig shown inFigure 3.3ensures that the maximum yield is obtained and at least the same quality of material can be produced, but with a much higher level of consistency regarding cleanliness and property values. As shown in Figure 3.3, the ladle of steel is poured directly into a water-cooled copper mold; the reciprocating motion of the mold sides and a suitable dressing prevents sticking. The resulting slab thickness is typically around 250 mm, but this has been reduced in some specialized ‘mini mill’ operations to 50 mm, thus shortening the rolling process. More recent advances aimed at direct strip manufacture (see Figure 3.4 make a less than 2 mm material a realistic prospect.
3.3.1.3 Hot- and cold-rolling processes
After casting the slabs are progressively rolled down to the sheet thicknesses supplied in coil form to the automotive press shop. In order to do this the slabs are reheated and then hot rolled to produce ‘hot band’ at 900e1200C, an intermediate form of now recrystallized material about 3 mm thick. This process causes the development of a thick layer of oxides, or scale, and although this is removed by
‘pickling’ in hydrochloric acid the rough surface only renders it fit for selected underframe, chassis parts or bracketry. Process modifications have improved the quality of the hot-rolled product and it is now possible to utilize this in thicknesses as low as 1.6 mm, which gives a slight cost reduction to the part producer. Cold reduction is now essential to optimize dimensional accuracy, surface and properties, producing automotive material commonly 0.5e2.0 mm thick. This is normally carried out on a four- or five-stand sequence of four-high roll stations. The important influences on the final properties concerning formability are the grain size (which controls ‘n’ value) and the crystallographic texture (developed on subsequent 104 CHAPTER 3
Materials for consideration and use
FIGURE 3.2
(a) Typical sheet metal steel manufacturing process; (b) alternative continuous annealing line
(Courtesy of Corus) 105
r o t a l u m u c c a t i x E
4 r
B Final
g n il o o c
t i x E
s e r u t a e f n i a M
•Length112m
•Height28m
•Striplength2500m
•Processspeed350m/min
•50%H2gasjetcooilng
A O
R CGJC
3 r
B Br2
y r t n E y
r a d n o c e S
g n il o o c
e g a r e v
O HGJC
k a o S
g n i t a e H
r o t a l u m u c c a y r t n n E
o i t c e s e c a n r u
(b) F
FIGURE 3.2 (Continued)
106CHAPTER3
Materials for consideration and use
annealing), which in turn controls ‘r’ value. Both parameters will be defined in detail in Chapter 5 and their importance in component manufacture explained. The hot- rolling coiling temperature, percentage final reduction and annealing temperature/
rate critically affect these parameters, a high CR figure (of the order of 80%) being
Additions
RHOB vacuum degassing process
Vacuum pumping station Sollac Dunkerque
240 t steel ladle H: 10 m Plunger ID: 0.6 m ID: 2 m Immersion: 0.5 m Circulation rate:
100 to 150 t/min
2 oxygen tuyeres Q = 280 m3/h→ 1600 m3/h
8 intergas injection tuyeres Q = 60 m3/h
FIGURE 3.3
Vacuum degassing in the steelmaking process3
Mold: formation of a solid shell
Continuous liquid steel feed
Initial solidification front
Final solidification front
Continuous withdrawal at constant speed Oxy-gas cut-off
Bottom of the liquid pool:
end of solidification Secondary cooling:
continued solidification
The principle of continuous casting Ladle liquid steel
Tundish
Mold
Secondary cooling
StraighteningWithdrawal
Oxy-gas cut-off Layout of a curved mould type continuous slab caster.
Product transfer
FIGURE 3.4
Continuous casting showing solidification and extraction processes3
commensurate with the optimum ‘r’ value. This is controlled by the power of the mill equipment, and the roll diameter/configuration and final thickness. The thick- ness tolerance determines yield, and variation is corrected by automatic gauge control (AGC) systems working on an elaborate feedback system, which should achieve an accuracy of within 1%.
Immediately after cold reduction the strip is then annealed to restore maximum ductility, and finally receives a skin pass (normally 0.8e1.2% reduction) to impart the final surface texture and to remove any tendency for ‘stretcher-strain’
formation. This stage of steel manufacture is extremely influential on the metal- lurgy and processing characteristics of automotive grades of steel sheet, and the mechanisms associated with skin passing are presented after a consideration of annealing.
3.3.1.4 Continuous annealing
Strand annealing has been used in the tinplate industry for the annealing of thin strip4and other similar research programs were evident in the UK in the 1960s.
However, continuous annealing process line (CAPL) technology (seeFigure 3.2(b)) was only introduced for the processing of automotive sheet, as an alternative to batch annealing, in the last 15 years. Developed extensively in Japan, it is widely now used in Europe, but the differences between this product and that of batch annealed coil need to be fully understood (seeTable 3.5).
As stated previously, with CAPL, advantages are gained in terms of consistency of composition and properties, but because the length of the cycle is less than 10 minutes recrystallization and grain growth is not as complete as for the batch annealed (BA) product. This has been observed in the press shop where for lower grades of forming steels (DCO-2/3) the ‘as received’ properties have not matched the BA ductility levels. However, by correct allocation of this material to the less demanding jobs and a reasonable degree of rework to tools, the CAPL products are now in regular use. The rapid annealing capability has also been used to tailor the properties of high-strength steels by control of composition and temperature/time cycle with the result that bake-hardening options and duplex/multiphase structured steels can be produced more readily.
Although requiring lower investment, the batch annealing processes commonly need two to three days to process the more formable grades of steel as opposed to 10 minutes for the CAPL treatment. Even with the hydrogen atmosphere utilized by the Ebner process, which provides a higher heat-transfer efficiency, uniformity of properties is still a problem (seeFigure 3.5).
While CAPL provides a vastly increased throughput, the lower processing time limits the development of favorable crystallographic textures and grain size. As a consequence ‘r’ values are lower and drawability is impaired. Yield stress tends to be higher as it is related to grain size by the Petch equation5:\
sLYS ¼ siþ2d1=2 (3.1)