Strength/ductility relationships in ultra high strength sheet steels in the range 800 to 1 400 MPa

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EURO PEAN
COMMISSION
SCIENCE
RESEARCH
DEVELOPMENT
technical steel research
Properties and in-service performance
Strength/ductility
relationships in ultra
high strength sheet
steels in the range
800 to 1 400 M Pa
h
Report
w
EUR 18603 EN STEEL RESEARCH EUROPEAN COMMISSION
Edith CRESSON, Member of the Commission Ʊ
responsible for research, innovation, education, training and youth
ÍS
DG XII/C.2 — RTD actions: Industrial and materials technologies —
Materials and steel
> >
Contact: Mr H. J.-L. Martin
Address: European Commission, rue de la Loi 200 (MO 75 1/10),
B-1049 Brussels — Tel. (32-2) 29-53453; fax (32-2) 29-65987 à European Commission
technical steel research
Properties and in-service performance
Strength/ductility relationships in ultra
high strength sheet steels in the range
800 to 1 400 M Pa
P. J. Evans
British Steel pic
9 Albert Embankment
London SE1 7SN
United Kingdom
Contract No 7210-MB/815
1 July 1992 to 30 June 1995
Final report
Directorate-General
Science, Research and Development
1998 EUR 18603 EN LEGAL NOTICE
Neither the European Commission nor any person acting on behalf of the Commission
is responsible for the use which might be made of the following information.
A great deal of additional information on the European Union is available on the Internet.
It can be accessed through the Europa server (http://europa.eu.int).
Cataloguing data can be found at the end of this publication.
Luxembourg: Office for Official Publications of the European Communities, 1998
ISBN 92-828-4996-1
© European Communities, 1998
Reproduction is authorised provided the source is acknowledged.
Printed in Luxembourg
PRINTED ON WHITE CHLORINE-FREE PAPER SUMMARY
STRENGTH DUCTILITY RELATIONSHIPS IN ULTRA HIGH STRENGTH SHEET STEELS
IN THE RANGE 800 MPA TO 1400 MPA
British Steel pic
ECSC Agreement No. 7210.MB/815
Final Summary Report
The aim of the project is to design continuously annealed sheet steels processing tensile
strengths in the range 800 to 1400 MPa to meet future requirements of the automotive
industry, for applications aimed at increasing passenger safety or for weight reduction. To
obtain adequate formability at these high strength levels, the investigations have been
targeted at steels employing the Transformation Induced Plasticity (TRIP) mechanism of
metastable retained austenite. Steels with carbon contents in the range 0.17 to 0.37%,
manganese contents of 0.94 to 1.94% and silicons of 0.31 to 1.74% have been
subjected to simulated continuous annealing cycles to produce a wide range of structures
with varying proportions of ferrite, bainite, martensite and retained austenite. The simulated
cycles involve rapid heating, at typically 200°Cs'\ representative of the heating rate
achievable in practice with Transverse Flux (TFX) induction heating. A laboratory TFX
continuous annealing simulator and a direct resistance heating simulator were employed for
this purpose. Ac, and Ac3 temperatures were determined by dilatometry for both a slow
heating rate (near equilibrium conditions) and a fast heating rate (approaching that in TFX
continuous annealing).
For samples of a 0.17%C, 1.44%Mn, 1.41%Si steel processed on the TFX simulator with a
TRIP type annealing cycle, microstructures consisted of 70 to 80% ferrite plus varying
amounts of bainite, martensite and retained austenite depending on the combinations of
annealing temperature and the slow cooling range through the bainite/martensite region.
Lower annealing temperatures and higher slow cooling ranges favoured the development of
a bainitic second phase to give a discontinuously yielding product. Tensile strengths
increased with increasing annealing temperature, from 600 N/mm2 in material annealed just
above the Ac, to 765 N/mm2 for mean annealing temperatures around 800°C, due to an
increase in the amounts of bainite and retained austenite.
Signed by: P.J. Evans
Cover Pages: 1
Text Pages: 73 Approved by: Dr. T.J. Goodwin
Figure Pages: 59 Research Manager
Appendices Pages: -By introducing progressively more martensite into the structure, by means of higher
annealing temperatures or lower slow cooling ranges, the yield point disappears and tensile
strengths rise to around 950 N/mm2. Uniform elongations of 20% were achieved ate s of 765 N/mm2 (bainitic second phase) and also at 832 N/mm2 (bainite plus
martensite). Material with a predominantly martensitic second phase possessed a tensile
strength of 937 N/mm2 with 18% uniform elongation, to give a TRIP/Dual Phase hybrid steel.
Retained austenite volume fractions were typically 9 to 13%, increasing with mean
annealing temperature up to a maximum at around 790°C. Higher temperatures tended to
favour martensite formation during the subsequent slow cool, due to reduced carbon
enrichment of austenite and resulted in a decrease in the level of retained austenite. The
retained austenite content does not in itself determine the strength-ductility combination; of
more significance is the amount of austenite which transforms during straining, and the
strain-range over which it transforms (i.e. the stability of the retained austenite). There is
evidence to indicate that lower quench temperatures result in improved ductility for a given
amount of austenite transforming, suggesting increased stability and transformation over a
higher range of strain.
A good correlation was found to exist between the strength and chemical composition of
laboratory hot rolled material processed to contain ferrite/pearlite structures. Such
information may provide a first indication of the feasibility of cold reduction for new C-Mn-Si
steel compositions.
These laboratory hot rolled materials were cold reduced and annealed on a resistance
heating simulator, recommissioned due to the emergence of problems with the TFX
simulator.
For all the steels studied, the combination of intercritical annealing temperature and quench
temperature determines the volume fraction of ferrite in the final product. The morphology
and relative proportions of the other phases are also influenced by the intercritical
temperature, with high temperatures promoting martensite. Ferrite grain sizes ranged from
ASTM 12 to 13 in the lower carbon and manganese steels, to ASTM 13 to 14 with higher
carbon and manganese. Ferrite volume fractions ranged from zero to 40% (depending on
annealing temperature) in steels with high carbon or manganese, to around 70 to 80% in
steels with lower carbon and manganese, but high silicon.
The product of tensile strength and uniform elongation for the steels processed on the
resistance heating simulator was found to increase with increasing values of the slow cool
start and end temperatures. Optimum combinations of strength and ductility were generally
obtained with a slow cooling range of approximately 450 to 350°C, giving
ferrite/bainite/retained austenite structures. Depending on annealing temperatures, slow
cool end temperatures greater than 300°C were generally required for the avoidance of
martensite. Lower temperatures can be tolerated for the lower carbon steels. For a given
steel, high annealings tend to result in a reduction in the tensile
strength-uniform elongation product due to a coarser structure and a tendency for less
austenite to be retained due to reduced carbon enrichment. Tensile strength increases with
increasing annealing temperatures due to larger volume fractions of second phase and an
increased tendency for martensite formation. Tensile strength increases and yield stress
decreases with lower slow cooling temperature ranges due to the formation of harder forms
of bainite and ultimately martensite. For material processed with high slow cooling temperature ranges (to give
ferrite/bainite/retained austenite structures), tensile strengths from 675 N/mm2 in a 0.18%C,
0.94%Mn, 1.74%Si steel to 1070 N/mm2 in a 0.37%C, 1.44%Mn, 1.24%Si steel were
obtained. Uniform elongations slightly in excess of 22% can be maintained at tensile
strengths up to 900 N/mm2 for steels with greater than 1% Silicon, resulting in an increase in
the tensile strength-uniform elongation product from around 15,000 to 20,000 N/mm2.% as
carbon, manganese or silicon contents are increased. A uniform elongation of 21% was
obtained at 1070 N/mm2 in the high carbon steel, giving a tensile strength-uniform
elongation product of 22,600 N/mm2.%.
For constant carbon and manganese contents, an increase in silicon content results in a
simultaneous increase in tensile strength and uniform elongation due to an increase in the
amount of retained austenite and a more significant TRIP effect. For example, the tensile
strength increases from around 750 to 875 N/mm2 and the product of tensile strength and
uniform elongation increases from 9,000 to 20,000 N/mm2% with an increase in silicon from
0.3 to 1.5% in steels with approximately 0.25% carbon and 1.4% manganese. A
simultaneous reduction in the yield to ultimate ratio is also observed due to the presence of
more retained austenite at yielding and hence more strain induced martensite at fracture.
Typical retained austenite volume fractions ranged from around 3% in a 0.26%C, 1.46%Mn,
0.31 %Si steel, to 9-13% in a 0.17%C, 1.44%Mn, 1.41%Si steel, around 20% in 0.25%C
steels with Si>1.2% and 23% in a 0.37%C,, 1.24%Si steel.
It may be difficult in practice to produce high ductility ferrite/bainite/retained austenite TRIP
steels at tensile strengths much in excess of 1000 N/mm2 due to the high levels of carbon,
manganese and silicon which would be required. The use of lower slow cooling
temperature ranges, to introduce significant amounts of martensite into the structure, may
be a satisfactory means of producing ultra high strength steels without resorting to very high
C, Mn or Si contents if moderate ductility is required; eg. around 8% uniform elongation at a
tensile strength of 1365 N/mm2 and greater than 11% uniform elongation for tensile
strengths less than 12402 has been obtained in a 0.37%C, 1.44%Mn, 1.24%Si steel. CONTENTS
Page
1. INTRODUCTION 17
2. THE TRANSFORMATION INDUCED PLASTICITY MECHANISM 1
3. EXPERIMENTAL STEELS8
4. STEELS PROCESSED ON TFX INDUCTION HEATED
CONTINUOUS ANNEALING SIMULATOR 19
4.1 Annealing Of Steel 23 To Produce Ferrite/Bainite
Structures
4.2 Annealing of Steel 24 to Produce Ferrite/Martensite
Dual-Phase Structures 22
4.3 Annealing of Steel 24 to Produce Ferrite/Bainite/Retained
Austenite Structures3
4.4 Laboratory Ingot Steels 30
5. MATERIALS PROCESSED USING RESISTANCE HEATING CA
SIMULATOR
5.1 Annealing of Steel3 (0.26%C, 1.46%Mn, 0.31 %Si) 34
5.2g of Steel 17 (0.37%C, 1.44%Mn, 1.24%Si)5
5.3 Annealing of Steel 18 (0.33%C,1.86%Mn,1.27%Si)8
5.4g of Steel 14 (0.26%C, 1.94% Μ η, 1.21 %Si) 39
5.5 Annealing of 0.24%C, 1.4%Mn Steels 40
5.6g of Steels 10,11 and 12 (approximately 0.26%C,
0.9%Mn) 43
5.7 Comparison of 0.25%C Steels Processed with High Slow
Cooling Temperature Ranges6
5.8 Annealing of Steels 3,4 and 6 (0.17 to 0.18% Carbon) 48
6. GENERAL DISCUSSION FOR ALL STEELS PROCESSED ON
RESISTANCE HEATING SIMULATOR9
6.1 Material Processed with High Slow Cooling Temperature
Range 50
6.2 Amount and Stability of Retained Austenite 51
6.3 Material Processed with Lower Slow Cooling Ranges2
7. FURTHER WORK
8. CONCLUSIONS3
9. REFERENCES6
TABLES 59
FIGURES 91