Plain carbon and low alloy steels
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17-1 Mild steels for deep drawing Interstitial-free (IF) or ultra low carbon (ULC) steels High purity iron is used essentially for its physical properties, particularly its fer ro magnetism. It has high ductility and excellent corrosion resistance. Very low carbon steels are close to pure iron and their high ductility enables them to be employed for components of complex geometry involving severe sheet forming processes. The good ductility is a consequence of a low yield stress. The grain size must therefore not be too fine. However, an excessive grain size must also be avoided in order to ensure a smooth surface finish after drawing, particularly when the component is to be coated. Extra deep drawing quality (EDDQ) steels must be able to withstand severe forming operations without tearing, and this is achieved by ensuring low concentrations of the interstitiai elements carbon and nitrogen, typically of the order of 20 to 30 ppm (Table 17-1-1). These low concentrations are achieved by vacuum degassing of the liquid metal, which also removes hydrogen. The residual carbon and nitrogen is tied up by adding small amounts of titanium and/or niobium, which form fine precipitates of TiN, Nb(C,N) and Ti 4C 2S2 (H phase). The latter compound forms in the presence of sulphur, by transformation of titanium sulphide TiS [Hua971.
M. Durand-Charre, Microstructure of Steels and Cast Irons © Springer-Verlag Berlin Heidelberg 2004
THE MICROSTRUCTURE OF STEELS AND
CAST IRONS
Figure 17-1-2: Optica! micrograph of an 5460 titanium and niobium micro-alloyed construction steel. Acicular ferrite is visible in the coarse grained rone of a weld. The small dark spots are titanium oxide inclusions, which sometimes act as nucleation sites for the ferrite. Courresy Arce/or Recherche
Oxide inclusions are also detrimental, ro an extent which depends on their nature and size. They can be kept very fine and sparse by the use of strong deoxidants such as calcium, cerium or zirconium, during refining. The first generation of EDDQ steels had yield strengths of the order of 150 MPa. This was later increased ro around 200-300 MPa, without loss of formabiliry, by the addition of phosphorus or the use of subsequent heat treatments [DeA98].
Table 17-1-1: Typica! composition of an IF stee/ Alloy
C
Si
S
P
N
Al
Typical
0.003
0.007
0.007
0.007
0.003
0.020
Maximum
0.08
0.D3
0.025
Mn
Ti 0.060
0.45
High strength low alloy (HSLA) or micro-alloyed steels . Many applications involve less severe forming but require higher strength levels. The steels used in this case belong to the family of high strength low alloy (HSLA) or micro-alloyed (MA) grades, which are similar ro the IF materials. However, slightly higher carbon levels are rolerated, being balanced by appropriate titanium and/or niobium levels. The hot rolling process is closely controlled and represents a veritable sequence of thermomechanical treatments below 1000°C, involving precipitation of carbides and/or cabonitrides and recrystallisation (cf Table 14-1-2). The strengthening
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