IF steels presentation and a case study example

1. HIGH STRENGTH IF STEELS
MOL-22216 - Phase Transformations and Heat
Treatments of Metals
MADAN PATNAMSETTY
244978

2. INTRODUCTION
• IF(INTERSTITIAL FREE) STEELS – Free from interstitial atoms
• Commercially developed in Japan in 1970’s
• Steels are produced with low interstitial elements (primarily carbon
and Nitrogen) and stabilized with Ti and Nb by forming carbides/
nitrides precipitates
• With out interstitial atoms - steels are soft and ductile
• High strength IF steels are produced by solid solution strengthening
viz. P, Si, Mn..
• Due to high ductility IF steels can be used for deep drawing and
stretch forming
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3. MICROSTRUCTURE
• Conventionally IF steels are produced with hot rolling, cold rolling and
annealing. And hot rolling is done Austenitic region.
• But Hot rolling in Ferritic region is also feasible with finished products having
pronounced annealing texture and also manifest good deep drawing
• Ti and Nb form carbides, nitrides and carbo-nitrides which segregates in
grain boundary or over the matrix.
• Addition of P in HSIF(High Strength Interstitial steels) also form precipitates
at grain boundaries which decrease formability and also low resistance to
CWE (Cold work embrittlement), and ductile to brittle transition temperature
increased.
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automotive.arcelormittal.com/saturnus/sheets/second.htm.

4. Mechanical properties
• The addition of Mn, Si and P hardens the IF steel and by the addition of Cu
the tensile strength can also reach up to ~566Mpa.
• And the graph below shows IF HS steels have Tensile strength in the range
of ~ 210 MPa to 400 Mpa
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S. P. Bhat and A. P. Applications, “Advances in High Strength Steels,” East.

5. n-value and r-value
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• Strain hardening Coefficient (n) is the measure for comparing stretching
capacity of various metals.
σT = K Єn
Where σT is true stress and Є is true strain
Higher the value of n more the metal is plastic.
• The Lankford co-efficient or r – value (Plastic strain ratio) is a measure of plastic
anisotropy of a rolled steel. This quantity is used as an indicator for deep drawing
r – Value = Єxy/Єz
Where Є is plastic strain through a direction in xy plane and z (thickness)
And r – values are varied according to test direction of anisotropic materials.
r - value = (rl + 2r45
0+ rt)/4
For the values approaching 1 the sheet is resisting thinning.
So Low r value and high n value ensures excellent deep drawability and good strain
distribution.

6. HS IF grades and Normal IF grades
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Grades
PS0,2
(MPa)
UTS
(MPa)
ef (%)
L0 = 80 mm
th < 3 mm r n
C Max
%
Mn
Max %
Si Max
%
IF 180 180 -230 340 -400 ≥ 35 ≥ 1.7 ≥ 0.19 0.01 1 0.25
IF 220 220-260 340-400 ≥ 33 ≥ 1.7 ≥ 0.19 0.01 0.7 0.5
IF 260 260-300 380-440 ≥ 30 ≥ 1.5 ≥ 0.18 0.01 1 0.5
IF 300 300-340 400-460 ≥ 28 ≥ 1.5 ≥ 0.17 0.01 1 0.5
Mechanical properties and Chemical compositions of HS-IF grades
Grades
PS0,2
(MPa)
UTS
(MPa)
ef (%),
L0 = 80
mm,
th < 3 mm
r n
C
Max
%
Mn Max
%
Si Max
%
P
man%
DX57 - GI Hyperform 120-160 260-310 ≥ 43 ≥ 2.3 ≥ 0.23 0.01 0.2 0.5 0.02
DX57(EN 10346) 120-170 260-350 ≥ 41 ≥ 2.1 ≥ 0.22 0.12 6 0.5 0.1
DX56 140 - 180 270 - 330 ≥ 40 ≥ 1.9 ≥ 0.21 0.01 0.25 0.03
DC06 120-160 270-330 ≥ 42 ≥ 2.2 ≥ 0.22 0.01 0.25 0.03
Mechanical properties and Chemical compositions of normal IF grades

7. Weldability
• For IF steels Resistance spot welding is widely
spread process used for automotive applications.
• For certain high r - value grades TIG and LASER
welding techniques are used due to appearance of
large grains in the vicinity of welded joints.
• HAZ undergoes micro structural modification, and
anisotropy is pronounced in grain morphology where
grain size in transverse direction/ perpendicular to
heat flow may reach around 30 μm and size in
longitudinal /parallel to heat flow is ~100 μm .
• Since the critical cooling temperatures for these
grades are so high as 105 0C/unit time to 2000 0C/unit
time of different gauge thicknesses, thus faster
cooling rate to produce brittle material is too hard, but
a little bit of Bainite forms at the centre.
• And mostly transformation only happens from
Austenite to ferrite.
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Source: E. Bayraktar, D. Kaplan, L. Devillers, and J. P. Chevalier, “Grain growth mechanism during the welding of
interstitial free (IF) steels,” J. Mater. Process. Technol., vol. 189, no. 1–3, pp. 114–125, Jul. 2007.

8. ALLOYING AND CHEMICAL
COMPOSITION
• Precipitation hardening is the impact of stabilizers Ti and Nb, Ti shows poor precipitation
strengthening due to coarser (200-1000 nm dia) precipitate as compared to Nb (5-30mm dia)
precipitate.
• The amount of Ti required to stabilize an IF steel is
Tistable = 4*C + 3.42*N + 1.5*S
and TiC, TiN kind of precipitates are formed
• From the ultra low carbon steels there is a possibility of formation of Ti4C2S2 and TiN, thus the
equation can be changed in this condition to
Tistable = 3.42N + 2*(1.5*S)
• Similarly the amount of Nb required is evaluated as
Nbstable = 7.75*C + 6.65*(N – Al*1.93)
Or
Nbstable = 7.75*C + 6.65*(N – Ti*3.42)
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9. ALLOYING AND CHEMICAL
COMPOSITION
• Solid solution strengthening is done by
alloying with P, Si, Mn.
• Since strength is increased the formability
and elongation decreases. P and Si have
less effect of r- value comparing to Mn
(which decreases formability)
• Addition of Boron around 0.0005% to
0.003% has a significant impact on
hardness. That is Boron forms phases with
iron which has high dislocation densities,
thus even at slow cooling the hardness rate
is large
• Copper addition from 1-1.3% significantly
increases hardness to ~550 MPa as copper
undergoes aging through copper nano
cubic clusters and thus causes mild solid
solution strengthening.
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E. Bayraktar, D. Kaplan, L. Devillers, and J. P. Chevalier, “Grain growth mechanism during the welding of
interstitial free (IF) steels,” J. Mater. Process. Technol., vol. 189, no. 1–3, pp. 114–125, Jul. 2007

10. BATCH AND CONTINUOUS ANNEALING
• Batch annealing The heating rate is approximately 30 0C/ Hour with a
soaking temperature of 700 0C and then the coil is slowly cooled at 100C/hour.
This process takes several days.
• Continuous annealing lines involves several processes including cleaning,
annealing, over aging or galvannealing, and some temper rolling in one
continuous operation. heating rates 10 -40 0C/sec with soaking temperature of
800-900 0C for 40-80 seconds are typical. And cooling rates at 20 – 200 0C
are used depending on the galvannealing stage
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11. BATCH ANNEALING CASE STUDY
• A Ferritic hot rolled Phosphorous added Ti-IF is analysed with a chemical composition
as following and the results at various finishing temperatures are as follows
• The variation of decrease and increase in r-value is cause of annealed texture
formation when finished rolling temperature is decreased.
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Table 1: Chemical composition of experimental steels (mass percent, %) [5]
Steel C Si Mn P S Al N Ti
HS -IF 0.002 0.005 0.16 0.085 0.0026 0.023 0.0021 0.065
Table 5: Mechanical properties and corresponding warm rolling parameters [5]
No
ST/
C
FT/
%
RD/
% R,/MPa (Variance) R,/MPa (Variance) A/% (Variance) n (Variance)
r
(Variance)
a 700 560 90 162(8.61) 325 (9.88) 40 (2. 36) 0.25 (0.017) 1.37 (0.15)
b 700 620 90 150(5.48) 320 (6.46) 36 (2.67) 0.26 (0.013) 1.14 (0.12)
c 700 640 90 158(6.76) 320 (6.84) 35 (3.33) 0. 27 (0.017) 1.18 (0.19)
ST-Start rolling temperature; FT-Finish rolling temperature; RD-Reduction.

12. BATCH ANNEALING CASE STUDY
• The significance of texture can be analysed with the knowledge of FeTiP particles,
these particles leaves Ti for TiC formation during recrystallization annealing.
• This results in the interstitial carbon in matrix which results in poorer texture and high
r - values and thus poor formability
• Increase in number of recrystallized grains with respect to increase in finishing
temperature
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13. HOT DIP GALVANIZING AND GALVANNEALING
• Hot dip Galvanizing - Steel is dipped into molten zinc bath of 450 0C and
exposed to atmosphere for Zn to react with O2 and CO2 to form carbonate
which acts as corrosion resistance
• Galvannealing is a part of continuous annealing process where steel after
hot galvanizing annealed over a critical temperature for which Iron forms
phases with Zn. Thus this process effectively provide additional strength,
scratch resistance, allows for easier coating and enhances adhesion
properties
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E. Bayraktar, D. Kaplan, L. Devillers, and J. P. Chevalier, “Grain growth mechanism during the welding of interstitial free (IF) steels,” J.
Mater. Process. Technol., vol. 189, no. 1–3, pp. 114–125, Jul. 2007.

14. Applications
• Complex parts such as wheel arches, toe boards, reinforcements
• Some grades are coated and used for door panels.
• IF300 with high UTS value can be used to design some complex structural
parts like long beams, cross members, suspensions and chassis
components
• One of the automobile application is Car hood from IF 220 grade which has
good indentation resistance and weight reduction potential increased by
deep drawing depth which is hot dip galvanized for the exposed part
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Car hood

15. REFERENCES
• [1] E. Bayraktar, D. Kaplan, L. Devillers, and J. P. Chevalier, “Grain growth mechanism during
the welding of interstitial free (IF) steels,” J. Mater. Process. Technol., vol. 189, no. 1–3, pp. 114–
125, Jul. 2007.
• [2] ArcelorMittal, “Extract from the product catalogue,” p. 131, 2012.
• [3] http://automotive.arcelormittal.com/saturnus/sheets/second.html
• [4] W. M. Guo, Z. C. Wang, S. Liu, and X. Bin Wang, “Effects of finish rolling temperature on
microstructure and mechanical properties of ferritic-rolled P-added high strength interstitial-free
steel sheets,” J. Iron Steel Res. Int., vol. 18, no. 5, pp. 42–46, 2011.
• [5] http://automotive.arcelormittal.com/europe/products/DS/DD/EN
• [6] P. Martin and J. T. Bowker, “Cold Work Embrittlement of Interstitial Free Steel by,” 2000.
• [7] S. I. Kim and Y. Lee, “Influence of cooling rate and boron content on the microstructure and
mechanical properties of hot-rolled high strength interstitial-free steels,” Met. Mater. Int., vol. 18,
no. 5, pp. 735–744, 2012.
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