TRIP08

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TRIP-Aided Ferrous Alloys

Int. Conf. on TRIP-Aided High Strength Ferrous Alloys 113

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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Int. Conf. on TRIP-Aided High Strength Ferrous Alloys114

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.

References

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