trip08
TRANSCRIPT
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Metallurgy of continuously annealed high strength TRIP steel sheetHiroshi Matsuda, Fusato Kitano, Kohei Hasegawa, Toshiaki Urabe and Yoshihiro Hosoya
The effects of heat-treatment conditions on mechanical properties are comprehensively investigated to optimise the industrial process of
the 590 MPa grade TRIP steel sheet with the metallurgical understanding. The substantial effect of the thermal conditions are first clari-fied by laboratory investigation, which includes the effects of annealing conditions, cooling conditions from intercritical temperature to
austempering temperature and austempering conditions. The results indicate that the optimum annealing temperature is between 800 and
850 C and the mechanical properties are hardly influenced by the annealing time between 30 and 120 s at an annealing temperature of
825 C.It is also suggested that the optimum quenching rate is 45 C/s to obtain the stable properties of the products and the optimum
austempering conditions are 425 C with over 300 s in case of a constant temperature austempering. Based on the laboratory investiga-
tion, mill trial is performed using the NKK No.4-CAL in Fukuyama works.The heat treatment conditions are intentionally varied to examine
minutely the stability of the production.The mechanical properties are sensitive to the austempering start temperature, when the austem-
pering temperature is gradually decreased during austempering in the industrial conditions for the stable operation without meanders.Ex-
cellent mechanical properties can be obtained by controlling the austempering start temperature between 445 and 460 C. On the con-
trary, the properties deteriorate in case of the austempering start temperature over 470 C although the amount of retained austenite is
the same or slightly larger than the material which exhibits excellent properties. This is because the retained austenite is less stable in the
high-temperature austempered material caused by less bainite transformation.
The weight reduction of automobile, maintaining suffi-
cient safety by using the high strength steel sheet, is of
great concern to the carmakers who have to fulfil envi-
ronmental standards. In particular for the structural and
chassis parts, there has been the demand for replacing the
conventional 390 - 440 MPa grades steel sheets by the
higher strength grades with sufficient formability. TRIP
steel is one of the candidate materials for the 590 - 780
MPa grades high strength steel sheet with superior form-
ability.
Numerous research works were conducted concerning
the influence of the chemical composition and the heat-treatment conditions on the microstructure and the result-
ing mechanical properties of TRIP steel for automotive use
[15]. Furthermore, remarkable efforts were made to
demonstrate the transformation behaviour of retained aus-
tenite into martensite during deformation and its effect on
the mechanical properties [69]. These efforts have been
contributed to the development of TRIP steels with high
performance. However, from the industrial point of view,
not only the high performance but also the stability of me-
chanical properties is of major importance. To obtain the
stable production, detailed research for the influence of
operating conditions on the mechanical properties is neces-
sary with the metallurgical understanding of its mechanism.
The purpose of this study is to optimise the industrial
process for the 590 MPa grade TRIP steel sheet, which ex-
hibits excellent ductility and good weldability, with the
metallurgical understanding. The substantial effects of the
thermal conditions on the mechanical properties were first
clarified by laboratory investigation, and then the mill trial
was performed using the industrial line. The effect of the
thermal conditions in the industrial process on the charac-
teristics of retained austenite was investigated to reveal its
effect on the mechanical properties.
Experimental procedure
Materials. Chemical compositions of steels used in this
study are listed in table 1. Steel A was used for the labo-
ratory investigation. Steel B is the steel produced on trial
by the industrial line.
Steel A was smelted in a 50-t electric furnace and cast
into a mould. After slabbing the ingot, they were soaked at
1200 C and hot-rolled in 7 passes to 3.2 mm thickness
with a finishing temperature of 870 C and a coiling tem-
perature of 600 C. Materials for the laboratory study were
cut from the hot-coil. After pickling, they were cold-rolledto 1.2 mm thickness by a laboratory mill, and then sub-
jected to the heat treatment using a laboratory annealing
simulator.
Steel B was smelted and then slabs were produced using
a continuous caster. Hot-rolling conditions were the same
as those of steel A. The hot-coils were pickled and cold-
rolled to 1.4 mm thickness by the industrial line. These
were then heat-treated by the NKK-No.4 CAL (continuous
annealing line) in Fukuyama Works, which has the roll-
quenching process for quenching from intercritical tem-
perature to austempering temperature and the over-aging
furnace for austempering.
Annealing conditions. The heat-treatment conditions
applied to steel A are shown in figure 1. The effects of an-
nealing temperature TA, annealing time tA, cooling rate of
gas-jet VG, quenching temperature TQ, cooling rate of
quenching VQ, austempering temperature TH and austem-
pering time tH were investigated to extract the substantial
Hiroshi Matsuda; Fusato Kitano; Kohei Hasegawa; Toshiaki Urabe; Dr.
Yoshihiro Hosoya, Materials and Processing Research Center, NKK
Corporation, Fukuyama, Japan.
Table 1. Chemical compositions of steels used in this study (mass
contents in %)
Steel C Si Mn P S Sol.Al N
A 0.097 1.08 1.66 0.009 0.001 0.040 0.0040
B 0.098 1.10 1.64 0.007 0.002 0.042 0.0035
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factors to produce the TRIP steel sheet by the CAL. No
temper-rolling was applied in the laboratory study.
Based on the laboratory study, 590 MPa grade TRIP
steel sheets were produced on trial by the CAL. The cold-
rolled coils were heat-treated with the conditions shown in
figure 2, followed by 0.5 % temper-rolling.
Testing and analysing. Mechanical properties were de-
termined by tensile tests with the JIS-13B specimen (GL:
50 mm, GW: 12.5 mm) on the laboratory study and the
JIS-5 specimen (GL: 50 mm, GW: 25.0 mm) for the mill-
trial materials. Yield strength was defined as a lower yield
point or as the strength at 0.2 % offset strain in case of the
absence of a yield point. The work-hardening behaviour
was described using the change in the instantaneous work-
hardening exponent ninst defined as the following equation
evaluated from the true stress-strain curve.
instd ln
nd ln
= (1)
where is the true stress and is the true strain.
Microstructures were investigated by scanning electron
microscopy. Samples were first annealed for 7200 s at 200
C and then etched with 2 % nital to distinguish retained
austenite from martensite [10]. By this technique, marten-
site appears as finely etched grains, whereas austenite ap-
pears perfectly smooth without any other microstructural
change. The volume fraction of ferrite, bainite, martensite
and retained austenite were quantitatively determined by
the observation coupled with image analysis.
The amount of retained austenite was also quantitatively
measured by X-ray diffraction (XRD) from the integrated
intensities of diffraction peaks (200), (220), (311),
(200) and (211) with CoK radiation using a rotating
and tilting specimen stage. The carbon content of retained
austenite was estimated from the lattice parametera0 (nm)
measured from (200), (220) and (311) peak with mono-
chromated CuK using the following equation:
[ ]0 0 3580 0 0033 C. .= + (2)
where [C] is the carbon mass content in % of retained
austenite. The equation is derived from the empirical rela-
tionship proposed by Dyson et al. [11] in order to take into
account the effect of other elements.
Results and discussion
Laboratory investigation of the effect of heat treat-
ment on mechanical properties. Effect of annealing con-
ditions. Figure 3 shows the effect of annealing tempera-
ture TA on the strength and ductility; the other conditions
were fixed as tA: 60 s, VG: 10 C/s, TQ: 700 C, VR: 45
C/s, TH: 400 C and tH: 180 s. Tensile strength slightly
decreases with elevating the TA up to 850 C and increases
with the higher TA. On the contrary, yield strength in-
Figure 1. Parameters of heat treatment on the laboratory investi-
gation
Figure 2. Continuous annealing cycle conducted in NKK No. 4
CAL
Figure 3. Effect of annealing temperature on the mechanical
properties
Figure 4. Effect of annealing temperature on the change in the
work-hardening exponent
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creases with elevating the TA from 775 to 800 C, and de-
crease with the TA over 850 C.High uniform elongation
is obtained between 800 and 850 C of TA. Effect of the
TA on the change in the instantaneous work-hardening ex-
ponent ninst is shown in figure 4. The ninst are maintained
stably up to the high strain level by annealing between 800
and 850 C, whereas it increases steeply at the beginning
of straining and then decrease continuously in the other
temperature ranges.The result indicates that the optimum
annealing temperature is between 800 and 850 C.
Figure 5 shows the effect of annealing time tA on the
mechanical properties; the other conditions were fixed as
TA: 825 C, VG: 10 C/s, TQ: 700 C, VR: 45 C/s, TH: 400
C and tH: 180 s. Significant difference in the mechanical
properties is hardly observed under the annealing time
between 30 and 120 s.
Effect of cooling conditions. In this temperature range,
ferrite grows and carbon is enriched into residual austenite,
whereas there is the possibility of pearlite precipitation
with slow cooling rate.
Figure 6 shows the effect of quenching temperature TQ
on the mechanical properties; the other conditions were
TA: 825 C, tA: 60 s, VG: 10 C/s, VR: 45 C/s, TH: 400 C
and tH: 180 s. Dependency of the mechanical properties on
the TQ is very small. The gas-jet cooling rate VG is also
unaffected on the mechanical properties between 5 and 10
C/s, although the figure is not illustrated here.
Figure 7 shows the effect of cooling rate of quenching
VQ on the mechanical properties. The effect of the VQ on
the change in the instantaneous work-hardening exponent
ninst is shown in figure 8. The ninst are maintained stably
under the wide range of strain in the samples quenchedbetween 30 and 60 C/s.
These results suggest that the pearlite formation is suffi-
ciently retarded in this range of cooling conditions for the
steel used. It is also indicated that the optimum VQ is 45
C/s to obtain the stable properties of the products, which
is convenient for applying the roll-quenching process.
Figure 5. Effect of annealing time on the mechanical properties Figure 6. Effect of quenching temperature on the mechanical
properties
Figure 7. Effect of cooling rate during quenching on the mechani-
cal properties
Figure 8. Effect of cooling rate during quenching on the change in
the work-hardening exponent
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Effect of austempering conditions. Figure 9 shows the
effect of austempering temperature TH on the mechanical
properties; the other conditions were TA: 825 C, tA: 60 s,
VG: 10 C/s, TQ: 700 C, VR: 45 C/s and tH: 180 s.Tensile
strength increases together with decrease of yield strength
and uniform elongation by quenching down to 350 C,
which would be caused by the transformation of austenite
into martensite. Excellent properties are obtained at 425 C
ofTH, but the increase ofTH over 425 C deteriorates duc-
tility. The effect of TH on the change in the work-
hardening exponent ninst is shown in figure 10.Stable ninst
is maintained up to the high strain region for the TH be-
tween 400 and 425 C. On the contrary, the samples cool-
ing down to 350 C and over 425 C show a peak at low
strain and then decreased continuously.
Figure 11 shows the effect of tH on the mechanical
properties; the other conditions were TA: 825 C, tA: 60 s,
VG: 10 C/s, TQ: 700 C, VR: 45 C/s and TH: 400 C.Ten-
sile strength decreases but yield strength and uniform
elongation increase with prolonging the tH.The effect oftH
on the change in the instantaneous work-hardening expo-
nent ninst is shown in figure 12. The sample which briefly
soaked for 30 s shows a sharp peak at the beginning and
then decreases continuously during straining. With pro-
longing tH, the sharp peaks ofninst in the small strain re-
gion are gradually flattened and the level of ninst in the
large strain region conversely increases. These results indi-
cate that the optimum austempering conditions are the TH
of 425 C with the tH over 300 s.
Trial production of the continuously annealed 590
MPa grade TRIP steel sheet. Based on the laboratory in-vestigation, 590 MPa grade TRIP steel sheets were pro-
duced on trial by the NKK No.4-CAL in Fukuyama works.
The austempering temperature is gradually decreased in
the industrial conditions because of the stable operation
without meanders. The heat treatment conditions were in-
tentionally varied to examine minutely the stability of the
production.
Figure 9. Effect of austempering temperature on the mechanical
properties
Figure 10. Effect of austempering temperature on the change in
the work-hardening exponent
Figure 11. Effect of austempering time on the mechanical proper-
ties
Figure 12. Effect of austempering time on the change in the work-
hardening exponent
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Figure 13 shows the relationship between austempering
start temperature TH1 and the mechanical properties. De-
spite the other conditions variation, the mechanical prop-
erties are strongly correlated with the TH1. The tensile
strength increases and total elongation decreases with
higherTH1. As a result, the product of tensile strength and
total elongation decreases with higher TH1. This is com-
patible with the laboratory result for the effect of the TH
from 425 to 475 C shown in figure 9, although the
austempering temperature is gradually decreased during
austempering in the industrial line. The stable production
can be obtained by paying much attention to control the
TH1 between 445 and 460 C in the industrial line.
From the following section, the effect of the TH1 on the
mechanical properties is revealed by investigating the two
specimens B1 and B2 indicated in figure 13.
Relationship between the stability of austenite and
mechanical properties. Tables 2 and 3 show the heat-
treatment conditions and the mechanical properties of the
samples B1 and B2
respectively. Fig-
ure 14 shows the
true stress-strain
curves and the
change in the in-
stantaneous work-
hardening expo-
nent ninst during
straining. The ninst increases gradually to a maximum for
the sample B1, whereas it increased steeply to a maximum
at low strain and then gradually reduced for the sample B2
during straining. It is conceivable that the sharp increase in
the ninst at low strain for the sample B2 is due to the fast
transformation of retained austenite into martensite, as re-
ported by Evans et al. [12].
Figure 13. Effect of austempering start temperature on mechani-
cal properties
Table 2. Industrial heat-treatment conditions of samples B1 and
B2
Steel line speed
m/s
TA
C
TQ
C
TH1
C
TH2
C
TH3
C
B1 2.0 812 651 456 389 351
B2 2.0 803 661 488 396 358
Table 3. Mechanical properties of sam-
ples B1 and B2
Steel YS TS
MPa
total uniform
elongation, %
B1 440 632 38.8 23.1
B2 421 690 34.2 21.2
Table 4. Volume fractions of the phases constituting the micro-
structures of samples B1 and B2 (volume fraction in %)
Steel image analysis XRD
ferrite bainite martensite retained austenite
B1 77.4 12.4 0.7 9.5 8.7
B2 76.6 4.1 9.6 9.7 10.9
Figure 14. Effect of austempering start temperature on mechani-
cal properties
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Figure 15 shows microstructures of these samples, and
table 4 presents the volume fractions of the phases consti-
tuting the microstructures.Pearlite and cementite were not
observed. The amount of the phases which formed during
or after austempering is significantly different, although
that of ferrite is almost identical between two samples. The
amount of bainite is larger but that of martensite is smaller
for the sample B1 than for the sample B2.The amount of
retained austenite is almost the same between these two
from the image analysis or slightly larger for the sample
B2 than for the sample B1 from the XRD result.This result
indicates that the sample B2 exhibits low ductility com-
pared to the sample B1 despite containing the same or a
large amount of retained austenite, and therefore the dete-
rioration of ductility for the sample B2 is not explained by
only the amount of retained austenite.
Table 5 shows the carbon content of the retained aus-
tenite estimated by equation (2). Figure 16 shows T0, T0
and Ae3 curves with the relationship between the carbon
content of the retained austenite and the austemperingtemperature range of the samples in the industrial line. The
T0 curve is the temperature at which austenite and ferrite
of same composition have identical Gibbs free energy, and
the T0 curve is the same as the T0 but the strain energy of
bainite formation (400 J/mol) is taken into account [13].
TheAe3 curve is the ( + )/ paraequilibrium temperature,
which means no partitioning of substitutional alloying
elements. Each curve is calculated using the Materials Al-
gorithms Project
(MAP) Programs
and Data Library,
http://www.msm.c
am.ac.uk/map/, Department of Materials Science and Met-
allurgy, University of Cambridge [14].
The carbon content of retained austenite in the sample
B2 is less than that in the sample B1. The relationship be-
tween carbon content in retained austenite and the austem-
pering temperature range corresponds well with the T0
curve, far from theAe3 curves, for both of them.This result
consists with the previous studies by Jacques et al. [15]
and Girault et al. [16], and suggests that the bainitic ferrite
growth is a diffusionless transformation such as martensite,
but the supersaturated carbon in the bainitic ferrite is parti-
tioned into residual austenite soon afterwards. The result
also indicates that austempering at low temperature but
above the martensite transformation start temperature leads
to a large amount of bainite if the austempering time is suf-
ficient to transform.
Table 6 shows the calculated martensite transformation
start temperatureMS and the calculated volume fraction of
martensite and retained austenite with the experimental re-
sults.TheMS was calculated using the MAP Programs and
Data Library [17; 18]. The volume fraction of martensite
and retained austenite was calculated using the empirical
equation proposed by Koistinen et al. [19] as:
( ){ }
( ){ }( )
res
res
0 011
1 0 011
s a
M s a
V exp . M T V
V exp . M T V
=
=
(3)
where Ta is the ambient temperature, VM and Vg is the vol-
ume fraction of martensite and retained austenite in % at
the ambient temperature respectively. Vres is the volume
fraction of residual austenite in % before cooling. The cal-
culated volume fractions are congruous to the experimen-
tal fractions. This suggests that less carbon content in the
sample B2 leads to high MS and results in a large amount
of martensite compared to the sample B1. The deficient
carbon content also deteriorates the stability of the retained
austenite for the sample B2. According to Sachdev [6], de-
creasing the stability of the retained austenite give rise to
the shift in the austenite to martensite strain transformationto lower strain and led to the deterioration of ductility. Al-
though the decreasing test-temperature caused the instabil-
ity of the retained austenite in his research, the same effect
Figure 15. SEM micrographs illustrating the microstructures of samples B1 and B2 after
200 C 7200 s tempering (: ferrite, B: bainite, : martensite and R: retained austenite)
Table 5. Carbon mass contents in % of
retained austenite of the samples
Steel carbon mass content in %
(200) (220) (311) average
B1 0.95 0.98 0.94 0.96
B2 0.76 0.79 0.75 0.77
Table 6. Martensite start temperature MSin C and volume fraction of martensite VMand retained austenite V in %
Steel MS calculated image analysis
VM V VM V
B1 -58 0 10.2 0.7 9.5
B2 95 10.4 8.9 9.6 9.7
Figure 16.T0, T0 and Ae3 curves with the relationship between the
carbon content of retained austenite and austempering tempera-
ture of the samples
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occurs in the sample B2 by the deficient carbon content.
Therefore, the sample B2 would exhibit high tensile
strength but low ductility compared to the sample B1 be-
cause of the large amount of martensite and the unstable
austenite, although the amount of retained austenite is the
same or slightly larger for the sample B2 than for the sam-
ple B1.
From these results, the effect of austempering conditions
on the microstructure and resulting mechanical properties
can be explained as follows. When cooling to the bainite
transformation temperature range, a certain amount of
austenite, 22-24 % in case of the sample B1 and B2, re-
mains metastable. During austempering, the austenite
transforms partially into bainite, and the carbon is enriched
into the residual austenite. The amount of bainite transfor-
mation depends on the austempering temperature, and the
higher austempering start temperature of the sample B2
leads to a less amount of bainite transformation compared
to the sample B1. This results in the larger amount but less
carbon content of the residual austenite in the sample B2 atthe end of austempering. When cooling to ambient tem-
perature, the residual austenite transforms partially into
martensite. Less carbon content of the residual austenite in
the sample B2 lead to the higherMS, and result in a larger
amount of martensite transformation. The austenite is fi-
nally retained almost the same or slightly larger in the
sample B2 than in the sample B1. However, the retained
austenite in the sample B2 is less stable than that in the
sample B1 because of its less carbon content, and therefore
it transforms into martensite with low strain during strain-
ing. Consequently, the sample B2 exhibits low ductility
compared to the sample B1.
Conclusions
Detailed investigation for the effects of heat-treatment
conditions on the mechanical properties is conducted to
optimise the industrial process of the 590 MPa grade TRIP
steel sheet with the metallurgical understanding. The me-
chanical properties are sensitive to the austempering start
temperature in the industrial process. Excellent mechanical
properties can be obtained by controlling the austempering
temperature between 445 and 460 C. On the contrary, the
properties deteriorates in case of a austempering start tem-
perature over 470 C although the amount of retained aus-
tenite is the same or slightly larger than the material which
exhibits excellent properties. This is because the retained
austenite is less stable in the high-temperature austempered
material caused by less bainite transformation.
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