Effect of microstructure on

bendability of ultra-high strength

steelsCASR Seminar 8.6.2015

Mia Liimatainen

8.6.2015

8.6.2015

Contents

• Background

• Ultra-high strength steels (UHSS)– Introduction

– Applications

• Bending– Deformation during bending

– Failure in bending

– Factors affecting bendability of UHSS

• Experimental– Studied materials

– Effect of microstructure homogeneity on bendability

– Effect of surface microstructure on bendability

– Practice to attain desired microstructure

• Conclusions

• Questions

Background

• This presentation is based on Master’s Thesis

“The effect of microstructure on bendability of

UHSS”, which was done at the department of

Research and Development at SSAB

• Part of a project, which goal was to develop UHSS

comprising a combination of a yield strength of

960 MPa together with a minimum bending radius

of 2.5t

• Typically the issue is:

– High strength and formability are typically

incompatible, i.e. as strength increases

formability in turn decreases8.6.2015

Ultra-high strength steels (UHSS)

• Typically steels with yield strength greater than 700 MPa are graded as UHSS

• UHSS encompasses several steel families

• UHSS can be manufactured by various manners including direct quenching

• Depending on the steel type, the desired strength level is attained due to different

strengthening mechanisms, such as grain refinement, precipitation hardening and phase

hardening 8.6.2015

Applications

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Bending

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• Bending is a common metalworking operation in sheet metal forming, in

which a force is applied to the material resulting it to bend at an angle.

• Typically bendability is defined by the ratio of minimum bending radius (𝑅𝑚𝑖𝑛) achieved without damage to the sheet thickness (𝑡).

• During a bending process the specimen does not deform homogeneously at

any point. The outer fiber of the specimen is subjected to tensile stresses as

against the inner fiber is exposed to compression.

• The maximum strains occur at the surfaces of the specimen.

Deformation and surface appearance

during bending

1. Yield strength is exceeded at upper surface layers uniform plastic

deformation at upper layers.

2. Tensile strength is exceeded at upper surface layers diffuse necking

initiates at upper layers.

3. Eventually as strains increase strain localization, i.e. formation of shear

bands, occur.

4. Formation of shear bands finally leads to fracture.

Materiaalitekniikka, Mia Liimatainen, 8.6.2015

Failure in bending complex

phase steels

• Initiation of strain localization, i.e. formation of

shear bands is precursor for damage in bending

complex phase steels.

• Shear bands are observed to form at maximum

45° shear stress directions at the upper surface

(subjected to tensile stresses) in bending.

• Strain localization is a highly complex

phenomenon, in which an initially uniform

deformation localizes into a narrow region, which is

termed as shear band.

• Shear bands are considered as non-

crystallographic band-like regions, where

deformation is concentrated at.

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Shear

band

Material properties affecting formation

of shear bands and thus bendability

Defects TextureSurface

roughness

Segregation bands

Homogeneity

Physical and mechanical properties of

surface

8.6.2015 Studied in

present work

Experimental

• Studied materials

• The effect of microstructure homogeneity

on bendability

• The effect of surface hardness on

bendability

Materiaalitekniikka, Mia Liimatainen, 8.6.2015

Studied materials• Direct quenched ultra-high strength strip steels

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Wt.% C Wt.% Mn FRT Thickness YS

0.06-0.1 1.3-1.9 788-920C 8-12 mm 700-1200 MPa

• Complex phase microstructures consisting mainly of a mixture of

bainite and martensite, and a surface layer of granular bainite/ferrite,

which extent depends on chemical composition and FRTLath-like martensite

Effect of microstructure

homogeneity on bendability• The variation of hardness measurements was applied as an indicator of the microstructural

homogeneity

• HI= 100x (standard deviation of hardness/ average hardness)

Homogeneity of microstructure

• The significance of homogeneous microstructure on bendability of

UHSS has been widely verified. It has been proposed e.g. by Yamazaki

et al. (1995) that homogeneity of microstructure governs bendability of

UHSS.

• An inhomogeneous microstructure refers to a structure where are

several local sites, where strain localization can occur and thus

microcracks can nucleate

• Inhomogeneity and hence non-uniform plastic deformation can result

from:

– Various combinations of crystal orientation

– Phases with different hardness levels

– imperfections

• �

Effect of surface hardness on

bendability• In case of 8 mm strip thickness a soft surface layer reaching to the

depth of 0.2-0.35 mm from surface contributes to increased

bendability.

– That is 2.3-4.4 % relative to the total strip thickness.

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Grounds for the governing

influence of surface hardness

Hardness

Work-hardening capability (n)

Initial dislocation density (N)

Strain rate sensitivity (m)

Local Taylor factor (M)

Criterion for onset of diffuse necking:

• Hart:

• Considere:

Criterion for the onset of strain

localization:

• Dillamore:

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Desired microstructure in terms

of bendability and strength

Materiaalitekniikka, Mia Liimatainen, 8.6.2015

Surface microstructure to the depth of 0.2-

0.35 mm (≈300-380 HV) Microstructure at centre (>400

HV)

G

B

PFB + M

Practice to attain a combination

of YS>960 Mpa and Rmin>2.5

• The microstructures and hence hardness are highly dependent on FRT, and C

and Mn concentrations

250

300

350

400

450

500

0 100 200 300 400 500

Vic

ke

rs H

ard

ne

ss

[10

00

m

N]

depth [um]

0.06C-1.3Mn920C

0.08C-1.8Mn920C

CL

Regression Analysis:

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Conclusions

• Bendability of direct quenched ultra-high

strength steels is governed by subsurface

hardness and additionally a homogeneous

microstructure contributes to increased

bendability.

• In order to achieve a minimum bending radius of

2,5, the microhardness is required to be less

than 380 HV to the depth of 2.3-4.4 % relative to

the total strip thickness.

• Subsurface hardness can be controlled by C and

Mn contents and final rolling temperature.

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THANK YOU!

QUESTIONS?

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References

• C. D. Horvath, The Future Revolution in Automotive High Strength Steel Usage,

Materials and Appearance Center General Motors Corporation, ArcelorMittal, Great

Designs in Steel Seminar, p. 9.

• D. Porter, New Hot-Rolled, High-Strength Structural Steels, High Technology

Finland. Available (accessed on 01.02.2015): http://hightechfinland.com/di-

rect.aspx?area=htf&prm1=577&prm2=article

• D. Reche, T. Sturel, A.F. Gourgues-Lorenzon, J. Besson, Damage Mechanisms of

ultrahigh strength steels in bending application to a trip steel, European Conference

of Fracture 18, Dresden, 2010, p. 7.

• S. Zajac, J. Komenda, P. Morris, P. Dierickx, S. Matera, F. Penalba, Diaz, Technical

Steel Research, Report EUR 21245EN, Luxembourg, 2005, p. 10.

• M. Mohrbacher, Microstructural Optimization for Multiphase Steels with Improved

Formability and Damage Resistance, NiobelCon, pp. 1-4.

• K. Yamazaki, M. Oka, H. Yasuda, Y. Mizuyama, H. Tsuchiya, Recent Advances in

Ultrahigh-Strength Sheet Steels for Automotive Structural Use, Nippon Steel

Technical Report, No. 64, 1995, pp. 37-44.

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