Transformation and Precipitation Behavior in Low-Carbon Microalloyed Steels

Toyohisa Shinmiya, Nobuyuki Ishikawa, Shigeru Endo

JFE Steel Corporation, 1 Kokan-cho, Fukuyama, Hiroshima-prf, 721-8510 Japan

Keywords: Low-carbon microalloyed steel, Second phase, Transformation, Precipitation

Abstract

Thermo-mechanical controlled processing (TMCP) is widely applied to manufacture high strength steel plates. In order to

fully utilize the benefit of alloying elements, such as Nb, V and Mo that are usually added to TMCP steels, precipitation

behavior of those elements in accelerated cooling (AcC) steel is investigated and compared with those of quenching and

tempering (Q-T) steels and isothermal heat treated steels after controlled rolling. Cementite morphology that affects

toughness of the materials is also investigated.

In the AcC steels, complex carbonitride was hardly precipitated. However, fine complex carbonitrides including mainly Nb

were precipitated randomly in the bainitic ferrite matrix during tempering treatment in the Q-T steel. Fine complex

carbonitrides were also found in the isothermal heat treated steel, same as the Q-T steel, but the morphology of precipitation

was different; row precipitation was observed. Cementite becomes finer in the Q-T steel when proper tempering treatment

was applied. These results give possibilities of getting materials with higher strength and superior toughness by controlling

the amounts and morphology of fine precipitates and cementite.

Introduction

Recent high strength structural steel plates are produced by applying thermo-mechanical controlled processing (TMCP),

which includes controlled rolling followed by accelerated cooling (AcC) or quenching and tempering (Q-T). Especially,

advance in the accelerated cooling process has enabled higher strength by transformation strengthening under higher cooling

rate [1]. Low carbon steels are usually used for the steels that are required of high strength and higher levels of base metal

toughness and HAZ (Heat Affected Zone) toughness, as well as good weldability. Micro-alloying elements, such as Nb and

Ti, are added for preventing grain coarsening during heating at austenization temperature by pinning effect of carbo-nitride

particles. Nb is also a quite important element in controlled rolling process because solute Nb can increase the re-

crystallization temperature [2] and insure the grain refining of transformed bainite microstructure after controlled rolling and

accelerated cooling. Therefore, many of recent high strength structural steels, such as linepipe steels, have fine bainitic

microstructure and relatively lower C content.

On the other hand, Ti and Nb are the alloying elements commonly used for high strength hot rolled thin steels in order to

improve strength and formability by precipitation hardened ferrite microstructure. Fine carbide precipitates, such as TiC or

(Ti, Mo)C, are formed at the austenite/ferrite interface during slow cooling in the coiling process after hot rolling of the steel

sheet [3,4]. Precipitation hardened technique has been also applied for the structural steel plates produced by quenching and

tempering process [5]. The steels with relatively higher C content, such as 0.1-0.15%C, are reheated and quenched or directly

quenched after hot rolling, then fine precipitate are formed in the martensite or bainite matrix, during tempering process. In

this case, dislocations induced by martensite or bainite transformation can be precipitation sites for fine carbides.

Strength and toughness of steel is strongly affected by its transformation and precipitation behavior, therefore, controlling

those phenomena based on complete understanding of transformation and precipitation mechanisms should be a key issue for

materials designing of high strength steels. As mentioned above, recent high strength linepipe steels has the chemistry with

lower C content and Nb and Ti addition, and is manufactured by applying controlled rolling and accelerated cooling process.

Microstructural characteristics and transformation behavior of recent high strength linepipe steel has been well investigated

[6, 7]. However, its capability for precipitation hardening and its possibility of improving toughness by controlling second

phase morphology has not been well explored. In this paper, change in second phase morphology of the steels with various

TMCP conditions, such as AcC, Q-T and isothermal heat treatment, was investigated. And, in order to fully utilize the benefit

of alloying elements, such as Nb, Mo and V, precipitation behavior of those elements during tempering and isothermal

holding subsequent to controlled rolling and accelerated cooling were examined.

MS&T 2004 Conference Proceedings 11

Experimental Procedure

Materials used for the test

Chemical compositions of the steels used in this study are shown in Table 1. Low-carbon Nb, Ti, Mo and V bearing steel,

which can be applied for high strength linepipe material [8], was used. Steel A and B were used to examine the morphology

of second phase, such as cementite and martensite-austenite constituent (M-A), in recent low-cabon AcC steels. C and Mo

contents, which enhance hardenability, were varied. Steel ingots were prepared by a laboratory vacuum furnace. Those ingots

were reheated at 1100oC or 1200oC and rolled into the plates with 20mm in thickness, then different cooling or heat treatment

were applied. Fig. 1 shows temperature profile in three different TMCP conditions; (a) accelerated cooling (AcC), (b)

quenching and tempering (Q-T), and (c) isothermal heat treatment. Hot rolling conditions were same for all TMCP conditions.

After the hot rolling, plates were cooled and heat treated in a different way. Fig. 1(a) shows the TMCP condition that is

commonly applied to linepipe steels. Accelerated cooling was applied from 820oC and cooling finished temperature was

varied from 300oC to 560oC to investigate morphological change of second phase using steel A and B, and then the steel plate

was cooled in the air. Fig. 1(b) and (c) are the TMCP conditions for obtaining precipitation hardening. In the Q-T process,

plate was water quenched from 820oC into the room temperature, then tempered at 650oC. Different heating rate and holding

time were applied in tempering process, in order to investigate the morphological change of cementite by tempering. Heating

rate was controlled by different furnace; heating rate of about 1oC/sec and 2

oC/sec were applied by gas furnace and salt bath,

respectively. Holding time in tempering was also changed using different furnace, holding time of 15minutes and 0 minute

were applied for gas furnace and salt bath, respectively. Fig. 1(c), isothermal heat treatment, is simulating the heat process of

cooling and coiling of steel sheet. After the hot rolling, plates was accelerated cooled down to 625oC, then plate was

immediately brought into the gas furnace with the same temperature and held for one hour.

Microstructural observation

Samples for microstructural investigations were taken from the steel plates produced by different TMCP conditions, and

etched by 3% nital acid after polishing the longitudinal section parallel to the rolling direction and normal to the plate surface.

Then microstructure of the steel was observed by optical microscope and scanning electron microscope (SEM). Close

observation was done on the morphology of second phase by SEM. Especially change in the cementite morphology of Q-T

steel during tempering process and comparison with AcC steel is of great interest. As quenched sample was also investigated

paying attention on martensite-austenite constituent (M-A), which can be observed by two-stage electrical etching.

Carbide precipitates were investigated by transmission electron microscope (TEM). Thin film samples were extracted from

the middle thickness portion of the steel plates. As well as the morphology of the precipitates, chemical compositions of the

C Mn Mo Nb V Ti

Steel A 0.048 1.55 0.07 0.031 0.032 0.01

Steel B 0.075 1.55 0.20 0.036 0.052 0.01

Steel C 0.050 1.25 0.10 0.040 0.050 0.01

Table 1: Chemical compositions of the steels used (mass %)

1200oC, 2hr

Ar3

820oC

650oC15min.

R.T.

650oC

WaterQuench

1200oC, 2hr

Ar3

820oC

650oC15min.

R.T.

650oC

WaterQuench

Air Cooling

1100-1200oC, 2hr

Ar3

820oC

300-560oC

AcC

Air Cooling

1100-1200oC, 2hr

Ar3

820oC

300-560oC

AcC

1200oC, 2hr

Ar3

820oC

1hr.

Air Cooling

625oC

AcC

1200oC, 2hr

Ar3

820oC

1hr.

Air Cooling

625oC

AcC

(a) (b) (c)

Fig.1: Schematic temperature profiles in hot rolling and heat treatment of

(a) AcC, (b) Q-T and (c) Isothermal heat treatment

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precipitates were investigated by EDX analysis on the TEM. Quantitative chemical analysis of the precipitates was also

conducted on the Q-T steel and the AcC steel by extract residue method.

Continuous cooling transformation diagram

In order to understand the transformation behavior during cooling process of the steel with different TMCP conditions,

continuous cooling transformation diagrams were investigated. Cylindrical samples with the diameter of 8mm and height of

12mm were taken from the steel ingot, and heating and hot working were applied same as Fig. 1, and then cooled at different

cooling rate, from 0.3 oC/s to 50 oC/s. Transformation starting and finishing temperature were detected by dilatometer.

Results and Discussion

Morphology of second phase in recent low-carbon AcC steels

To understand morphology of second phase in recent low-carbon AcC steel, effect of C content and accelerated cooling

finishing temperature on microstructure was examined using steel A and steel B. SEM microphotographs of the steels with

different cooling finishing temperature were shown in Fig.2 and Fig.3. Two different methods of etching were applied in

order to distinguish the type of the second phase, cementite and M-A. Nital etching was applied for the specimens in Fig.2,

and both cementite and M-A are observed as second phase. On the other hand, two stages electrical etching was conducted on

the specimens in Fig.3, showing only M-A as second phase.

Continuous cooling transformation diagrams of the both steels are shown in Fig.4. Cooling rate during accelerated cooling

was about 30oC/s, and bainite transformation starting temperature (Bs) is about 550

oC and 500

oC for steels A and B,

respectively. In both steels, microstructure is bainite in all cooling finishing temperature conditions, but morphology of grain

is different. Relatively polygonal grain is observed in the steels, which accelerated cooling was stopped above 500oC.

On the other hand, morphology of the bainite matrix turns into acicular shape as the cooling finishing temperature was

getting lower for both steels A and B, because shear type transformation was occurred in that condition. It is seen from Fig.3

that in steel A, which is lower C content, second phase is observed along grain boundary mainly. On the other hand, in steel

B, which is higher C content, second phase is observed along grain boundary and lath interface. Relation between volume

fraction of second phase and cooling finishing temperature is shown Fig.5. Volume fraction of cementite was evaluated by

subtracting M-A volume fraction, which can be determined in Fig.3, from total volume fraction of second phase in Fig.2. In

both steels, main second phase is cementite if the cooling finishing temperature is above 400oC, and M-A is produced mainly

under 400oC. It was found that volume fraction of second phase is larger for steel B that has higher C content.

Fig.2: SEM micrographs of the steels with various cooling finishing temperature, (a) 560oC, (b) 410

oC,

(c) 305oC, (d) 520oC, (e) 400oC and (f) 330oC, of steel A, (a)-(c), and steel B, (d)-(f), after nital etching

(b) (c)

(d) (e) (f)

10 m

(a)

MS&T 2004 Conference Proceedings 13

10 m

Fig.3: SEM micrographs of the steels with various cooling finishing temperature, (a) 560oC, (b) 410oC, (c) 305oC,

(d) 520oC, (e) 400oC and (f) 330oC, of steel A, (a)-(c), and steel B, (d)-(f) after two stage electrical etching;

Fig.4: Continuous cooling transformation diagram; (a) Steel A and (b) Steel B

300

400

500

600

700

800

900

1 10 100 1000

Tem

pera

ture

(oC

)

Time (sec)

0.31310

40

20Cooling rate (oC/s)

F

B

60

P

0.1

300

400

500

600

700

800

900

1 10 100 1000

Tem

pera

ture

(oC

)

Time (sec)

0.31310

40

20Cooling rate (oC/s)

F

B

60

P

0.1

300

400

500

600

700

800

900

1 10 100 1000

Te

mp

era

ture

(oC

)

Time (sec)

0.1

0.31310

40

20Cooling rate ( oC/s)

F

B

P

Accelerated cooling finishing temperature (oC)

Vo

lum

e f

ractio

n o

f se

co

nd

ph

ase

(%

)

cementite (steel A)

cementite (steel B)

MA (steel A)

MA (steel B)

Fig.5: Relation between volume fraction of second phase and cooling finishing temperature

(b) (c)

(d) (e) (f)

(a)

MS&T 2004 Conference Proceedings14

Microstructure and cementite morphology of the steel with various TMCP conditions

As mentioned above, second phase behavior in bainite transformation in AcC process, such as type of second phase

(cementite or M-A), volume fraction of second phase and its morphology, is quite sensitive to chemical composition and AcC

condition. It should be quite important for material designing to control the second phase and to obtain favorable morphology

or volume fraction by controlling manufacturing parameter in the AcC process. Another possibility of controlling the second

phase is heat treatment process in TMCP. Therefore, the second phase behavior in different TMCP process, such as

quenching and tempering (Q-T) and isothermal heat treatment, is investigated and compared with the AcC process in this

section.

Steel C was used and different TMCP process was applied as shown Fig.1. Microstructures observed by optical microscope

of the steels with different TMCP conditions, Q-T, AcC and isothermal heat treatment, are shown in Fig.6. Q-T steel and

AcC steel showed bainitic microstructure, and isothermal heat treated steel has ferrite-pearlite microstructure.

SEM microphotographs of the steels are shown in Fig.7. It was found that bainite microstructure of Q-T steel was consisted

of bainitic ferrite and cementite, which is observed along the grain boundary mainly, same as AcC steel. Cementite in Q-T

steel was finer than that of AcC steel. Continuous cooling transformation diagram of steel C is shown in Fig.8. Cooling rate

during quenching and accelerated cooling was about 50oC/s and 30oC/s, respectively, and bainite transformation occurs for

both quenching and AcC process, resulting in the similar microstructure for both processes. On the other hand, polygonal

ferrite formed in the isothermal heat treated steel, because cooling finishing and holding temperature was higher than bainite

transformation starting temperature.

Fig.6: Microstructure of the steels in various TMCP conditions by optical microscope

(a) AcC, (b) Q-T and (c) isothermal heat treatment

(b) (c)(a)

50 m

(b) (c)(a)

5 m

Fig.7: SEM micrographs of the steels in various TMCP conditions by optical microscope

(a) AcC, (b) Q-T and (c) isothermal heat treatment

MS&T 2004 Conference Proceedings 15

It is interesting that very fine cementite was obtained by quenching and tempering. So, in order to understand microstructure

change during tempering process, steels with as quenched condition and tempered in different conditions were prepared.

SEM microphotographs of as quenched steel and the steel tempered by salt bath without isothermal holding after quenching

are shown in Fig.9. In as quenched steel, M-A was dispersed in bainitic-ferrite matrix. Aggregation of cementite was found in

the salt bath tempered steel, which was dissolved by two-stage electrical etching confirming that second phase was cementite

not M-A. On the other hand, in the steel tempered by gas furnace and held isothermally in 15 minutes, the condensation of

cementite was not observed and fine cementite was observed along grain boundary, because solute C in M-A could be diffuse

along grain boundary sufficiently.

200

300

400

500

600

700

800

1 10 100 1000

Tem

pe

ratu

re (

oC

)

Time (sec)

0.05C-1.25Mn-0.1Mo-0.01Ti-0.04Nb-0.05V

FP

B

Cooling rate,oC/s

200

300

400

500

600

700

800

1 10 100 1000

Tem

pe

ratu

re (

oC

)

Time (sec)

0.05C-1.25Mn-0.1Mo-0.01Ti-0.04Nb-0.05V

FP

B

Cooling rate,oC/s

Quench

AcCIsothermal heat treatment

200

300

400

500

600

700

800

1 10 100 1000

Tem

pe

ratu

re (

oC

)

Time (sec)

0.05C-1.25Mn-0.1Mo-0.01Ti-0.04Nb-0.05V

FP

B

Cooling rate,oC/s

200

300

400

500

600

700

800

1 10 100 1000

Tem

pe

ratu

re (

oC

)

Time (sec)

0.05C-1.25Mn-0.1Mo-0.01Ti-0.04Nb-0.05V

FP

B

Cooling rate,oC/s

Quench

AcCIsothermal heat treatment

Fig.8: Continuous cooling transformation diagram of steel C

Fig.9: SEM micrographs of as Q and Q-T steels; (a) As quenched, (a’) As quenched (two stage electrical etching),

(b) Tempered by salt bath and (c) Tempered by gas furnace

5 m

(b) (c)

(a) (a’)

MS&T 2004 Conference Proceedings16

Carbon quantity as cementite in as quenched, Q-T and AcC steel by extract residue method was shown in Fig.10. It was

found that the amount of cementite that is formed during quenching is quite small and then cementite was formed during

tempering treatment. And the amount of cementite was almost the same for both Q-T steel and AcC steel.

As mentioned above, in AcC steel, accelerated cooling was terminated at the temperature higher than bainite transformation

finishing temperature, and bainite transformation progressed during air-cooling after accelerated cooling. Cementite was

produced in mainly grain boundary along with bainite transformation, because the transformation progress diffusively at high

temperature. On the other hand, microstructure of as quenched was bainitic-ferrite and M-A. It was reported that bainite

transformation is incomplete transformation [9]. So, untransformed austenite, which has higher carbon concentration, was

present at the time of finishing of bainite transformation, then it transformed into M-A, not cementite, during continuous

cooling to room temperature. During tempering process, M-A was dissolved and turned into cementite, resulting in large

increase in cementite, which is shown in Fig.10.

From the above results, it is possible to control cementite morphology by optimizing TMCP condition, for example, proper

tempering process.

Precipitation behavior of the steel with various TMCP conditions

TEM photographs in the middle thickness portion of the steels were shown in Fig.11. Large amount of fine precipitates,

which has the diameter of 5nm or less, was formed in the Q-T steel and isothermal heat treated steel. Fig.12 shows

distribution of precipitate size in Q-T and isothermal heat treated steel. Size of the precipitate was almost the same in both

steels, and average size was about 3nm. In the AcC steel, fine precipitates under 5nm in diameter were hardly found. Only the

large particles of (Nb,Ti)(C,N), which were not dissolved during slab re-heating, was observed occasionally. It was found

that the fine precipitates was complex carbonitride consisted of Nb, Ti, V, and Mo, by EDX analysis. Although size of the

precipitates was similar for Q-T steel and isothermal heat treated steel, precipitation morphology was different. Fine

precipitates were formed randomly in the Q-T steel. On the other hand, row precipitation was observed in the isothermal heat

treated steel.

As Q Q-T

(salt bath)

Q-T

650×15min.

Isothermal

heat treated

AcC

C a

s C

em

entite

(ppm

)

Fig.10: Carbon quantity as cementite in as quenched, Q-T, isothermal heat treated and AcC steel

by extract residue method

(b) (c)(a)

50nm

Fig.11: Microstructure of the steels with various TMCP condition by TEM;

(a) AcC, (b) Q-T and (c) Isothermal heat treatment

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Quantity of alloying element, Nb, V, Mo, and Te, as carbonitride in as quenched steel, Q-T, isothermal heat treated steel,

steel and AcC steel were investigated by extract residue method, and results are shown in Fig.13. Ratio of the alloying

element as precipitate to total addition was indicated in Fig.13. It was found that the most effective element for precipitation

hardening was Nb. On the other hand, V and Mo content in complex carbonitride were very small. Therefore, it is considered

that Mo and V contribute to transformation hardening mainly. The amount of carbonitride in AcC steel was very small, same

as as quenched steel. In general, precipitation of carbonitride, such as NbC or TiC, occurs in the temperature range from

550oC to 700 oC. However, cooling rate during those temperature ranges was very high for the AcC steel, preventing the

precipitation of carbonitride. In the Q-T steel, fine complex carbonitride including mainly Nb was precipitated randomly in

bainitic-ferrite matrix during tempering treatment. And the quantity of precipitates increased by holding for 15min at 650 oC,

as indicated in Fig.13. This precipitation behavior can be a so-called secondary hardened. That is to say, dissolved alloying

elements combine with supersaturated carbon in bainitic-ferrite, and fine carbonitride was formed randomly in the matrix. In

the isothermal heat treated steel, fine complex carbonitride was formed, same as the Q-T steel. But the morphology of

precipitation was different; row precipitation was observed in the isothermal heat treated steel. It was reported that fine

carbide precipitates, such as (Ti,Mo)C, are formed at the austenite/ferrite interface during slow cooling in the coiling process

after hot rolling of the steel sheet using IF steel, which is 0.05%C-0.2%Mo-0.08%Ti [4]. Although the content of carbonitride

forming elements were small comparing to the reported steels, precipitation can occur in the austenite/ferrite interface during

isothermal heat treatment. As a result of above investigation, recent low-carbon steel for high strength linepipe has sufficient

capability of precipitation hardening when quenching and tempering or isothermal heat treatment are applied.

0

20

40

60

80

100

1 2 3 4 5 6

Num

ber

Diameter of the precipitate (nm)

N=136Ave.=2.8nm

Isothermal

0

20

40

60

80

100

1 2 3 4 5 6

Num

ber

Diameter of the precipitate (nm)

N=88

Ave.=2.9nmQ+T(a) Q-T (b) Isothermal heat treatment

0

20

40

60

80

100

1 2 3 4 5 6

Num

ber

Diameter of the precipitate (nm)

N=136

Ave.=2.8nmIsothermal

0

20

40

60

80

100

1 2 3 4 5 6

Num

ber

Diameter of the precipitate (nm)

N=88

Ave.=2.9nmQ+T(a) Q-T (b) Isothermal heat treatment

Fig.12: Distribution of precipitate size in Q-T and isothermal holding steel; (a) Q-T and (b) Isothermal holding

Fig.13: Alloying element quantity as carbonitride in as quenched, Q-T steel and AcC steel by extract residue method;

(a) Nb, (b)V, (c) Mo and (d) Ti

As Q Q-T

(salt bath)

Q-T

650×15min.

Isothermal

heat treated

AcC

Nb a

s C

arb

ide (

ppm

)

As Q Q-T

(salt bath)

Q-T

650×15min.

Isothermal

heat treated

AcC

Mo a

s C

arb

ide (

ppm

)

As Q Q-T

(salt bath)

Q-T

650×15min.

Isothermal

heat treated

AcC

V a

s C

arb

ide (

ppm

)

As Q Q-T

(salt bath)

Q-T

650×15min.

Isothermal

heat treated

AcC

Ti as C

arb

ide (

ppm

)

MS&T 2004 Conference Proceedings18

Strengthening mechanisms for different TMCP conditions

As discussed above, the steels produced with different process, AcC, Q-T and isothermal heat treatment, have different type

of microstructure, and strength mechanisms should be different. Tensile properties of those steels were shown in Table 2. Q-

T steel has the highest tensile strength, and the tensile strength of isothermal heat treated steel was almost the same as that of

AcC steel. It is interesting to note that AcC steel and isothermal heat treated steel has totally different microstructure, but the

strength is almost the same. This means the strengthening mechanisms are different for those steels. Based on the present

investigation, the strengthening mechanism of each steel was schematically illustrated in Fig.14. Isothermal heat treated steel

is strengthened mainly by precipitation hardening, while transformation hardening is the main mechanism for the AcC steel.

Even though the transformation hardening is reduced by tempering to a some degree, Q-T steel is strengthened both

precipitation and transformation hardening, resulting in highest strength among three steels. Although the further

investigation, such as quantitative analysis of fine precipitation morphology or the effect of heat treatment temperature, is

necessary for the complete understanding of metallurgical and mechanical behavior, above-mentioned analysis can give the

important guideline for materials designing for high strength steels.

TMCP condition YS TS YR

(N/mm2) (N/mm

2) (%)

Q+T 596 654 91.1

AcC 540 613 88.1

Isothermal heat treatment 523 591 88.5

Table 2 Tensile properties of the steels with various TMCP conditions

AcC Q+T IT

Precipitation

Transformation

Matrix

TMCP condition

Microstructure B+ B + + MX P+ MX

Fig.14: Schematic illustration of the strengthening mechanism for the steel with various TMCP conditions.

( B:bainitic ferrite, :cementite, MX:carbonitride)

MS&T 2004 Conference Proceedings 19

Conclusions

(1) In the AcC steels, two kinds of second phases, such as cementite and M-A, were observed. Main second phase is

cementite if the steel was finished accelerated cooling above 400oC, and M-A is formed mainly when the cooling

finishing temperature wasunder 400oC. It was found that volume fraction of second phase is larger for steel B that has

higher C content.

(2) Q-T steel and AcC steel have similar microstructure, bainitic ferrite and cementite. The amount of cementite was almost

the same in both steel, but fine cementite was obtained by tempering treatment using a gas furnace in Q-T steel.

(3) Complex carbonitride with the size of 5nm or less including Nb mainly was precipitated in Q-T steel and isothermal heat

treated steel. Precipitation formation of complex carbonitride was random in Q-T steel, while row precipitation was

observed in isothermal heat treated steel.

(4) The most effective element for precipitation hardening was Nb, and Mo and V contribute to transformation hardening

mainly. Recent linepipe steel, which is low-carbon microalloyed steel, has sufficient capability of precipitation hardening.

References

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