Carbon
At constant grain size C has very little effect
on yield strength (YS). Effect of C on YS and tensile strength (TS):
|
Steel |
Change in YS (MPa) |
Comments |
|
C-Mn (<1%) as rolled |
DYS = 350.D%C |
|
|
C-1,50Mn-0, 045Nb controlled
rolled |
DYS = 185.D%C |
0,05/0,13%C; FRT=800°C |
|
C-1,40Mn as rolled |
DTS = 850.D%C |
|
|
C-1,50Mn-Nb contr. rolled |
DTS = 750.D%C |
0,10/0,20%C; FRT=800/900°C |
As C content increases volume fraction of
pearlite increases.
Increase in YS due to 1% increase in pearlite
content is 1,48 MPa
Increase in TS due to 1% increase in pearlite
content is 2,8MPa (normalized steels).
As n increases with the lowering of the C
content stretch formability is improved.
Toughness of hot rolled steel as measured by
the Charpy impact energy or by the CTOD (Crack Tip Opening Displacement) value
decreases as C content increases.
Increasing C content decreases austenite to
ferrite transformation temperature and so causes grain refinement.
The Carbon Equivalent (CE) normally expresses
effect of C on weldability. One of commonly used empirical expressions for CE
is
CE (IIW) = C+Mn/6+(Ni+Cu)/15+(Cr+Mo+V)/5
In effect CE is a measure of the susceptibility
of the heat affected zone to cold cracking during welding. Cold cracking
susceptibility increases as CE rises. From the above formula it is evident that
C influences CE to a great extent.
Partition of solute atoms between solid and liquid
during solidification causes microsegregation in the cast slab.
Microsegregation of C in C-Mn and low alloy steels depends also on the
interaction between this element and P and Mn.
Dendrite arms are rich in C and low in P
Interdendritic spaces are rich in P and low in
C.
C
diffuses away from regions rich in P. This is because the activation energy for
C diffusion in austenite (32000cal/mol) is smaller than that of P. So, C
diffuses more readily.
When there is pronounced P segregation the
micrograph shows a banded structure in the rolled product. White bands
correspond to interdendritic regions rich in P and low in C.
C increases hardenability of steel and so
facilitates formation of martensite in thick sections during cooling from the
austenite phase..
Manganese
1% increase in Mn increases YS by 33MPa due to
solid solution hardening.
Increasing Mn lowers austenite to ferrite
transformation temperature and so results in grain refinement.
1% increase in Mn reduces ferrite grain size by
1d-1/2 (mm)-1/2. This is equivalent to an increase in YS
of 18MPa.
In C-Mn steels (0,40% to 1,25%C), 1% increase
in Mn raises pearlite content by 9%. This is equivalent to an increase in TS of
40 to 60MPa.
Increasing Mn raises the carbon equivalent (see
formula above under Carbon) and is therefore detrimental to weldability
although less effective than C.
Mn increases hardenability of steel and so
facilitates formation of martensite in thick sections.
In Mn segregated areas, for example in the
centerline region (mid thick position) of C-Mn-Nb plate produced from
continuous cast slabs, cooling rate required for martensite formation is
Log (°C/s) = 3,95-1,73. %Mn (B.Mintz, et al. Met. Trans. A, V 19
June,1988)
If Mncenter = %Mn at plate center
due to segregation, and
Mnheat = average %Mn in plate (in
heat analysis), then
Segregation Ratio of Mn is: SMn = Mncenter/Mnheat.
Similarly, segregation ratios SP, SSi,
SNb etc can be defined for P, Si, Nb etc. From the literature (S. E. Webster and P. H. Bateson…)
SP = (SMn)3,5
SSi = 0,69.SMn + 0,31
SNb = (SMn)3,6
General note: (D.
Dulieu and I. G. Davies, Meeting-Directionality of properties in wrought
products. The metals Soc; 27-28 Nov. London)
Intensity of solute segregation during
solidification depends on steel chemistry and cooling conditions.
Solidification occurs by growth of primary dendrites aligned roughly to
direction heat flow. Secondary dendrites grow normal to primary ones.
Dendrite arm spacing is influenced by rate of
cooling during solidification or by the “local solidification time, t”
t=constant/[1/(dT/dt)], T=temperature
So, Dendrite arm spacing=constant/[1/(dT/dt)n] n=1/3 to ˝
Microsegregation is more severe between primary
dendrites than between secondary dendrites arms.
Homogenization time=constant.Ö(Dend. arm spacing)
For 0,54C-1,10Mn steel (Ironmaking Steelmaking, V 17 No.6, 1990)
Dendrite arm spacing =109,2.(dT/dt)-0,44
Taking dendrite arm spacing=220mm, dT/dt=0,2K/s
Banding (Bastien…)
After solidification interdendritic arm spaces
are rich in C and P than near the axis.
P (and As, Sb) increases Ar3 (and Ac3)(S
has no effect on Ar3). P has little effect on Ac1 and no
effect on Ms.
O is expected to show high segregation because
if its low partition coefficient. O has no effect on Ar3 and its
solubility in Fe is negligible. O readily forms oxide inclusions and therefore
has little effect on segregation.
Sn, Sb and As are ferrite stabilizers and tend
to form solid ferrite phase when they exceed solubility in austenite at reheat
temp. 1200°C.