High strength if steels

HIGH STRENGTH IF STEELS

MOL-22216 - Phase Transformations and Heat

Treatments of Metals

MADAN PATNAMSETTY

244978

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

13-04-2015MADAN PATNAMSETTY 2

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.


automotive.arcelormittal.com/saturnus/sheets/second.htm.

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


S. P. Bhat and A. P. Applications, “Advances in High Strength Steels,” East.

n-value and r-value


• 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 + 2r450+ 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.

HS IF grades and Normal IF grades


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

GradesPS0,2

(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

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.


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.

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)


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.


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

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


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.


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.

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


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


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.

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


Car hood

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.


http://automotive.arcelormittal.com/saturnus/sheets/second.html

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High strength if steels

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