Effect of microstructure on bendability of ultra-high...
Transcript of Effect of microstructure on bendability of ultra-high...
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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