Bainite Transformation Under Continuous Cooling of Nb-Microalloyed Low Carbon Steel

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O C I R N C R DlRBCT. @ JOURNAL OF IRON AND STEEL RESEARCH, INTERNATIONAL. 2006, 13(3) : 36-39, 67

Bainite Transformation Under Continuous Cooling of Nb-Microalloyed Low Carbon Steel

YI Hai-long, DU Lin-xiu, WANG Guo-dong, LIU Xiang-hua (State Key Laboratory of Rolling and Automation, Northeastern University, Shenyang 110004, Iiaoning, China)

Abstract: Utilizing Gleeble-1500 thermomechanical simulator, the influences of hot deformation parameters on continuous cooling bainite transformation in Nb-microalloyed low carbon steel were investigated. The results indi- cate that bainite starting temperature decreases with raising cooling rate and increases with increasing deformation temperature. Deformation has an accelerative effect on the bainite transformation when the specimens are deformed at 950 %. When the deformation temperature increases, the effect of deformation on bainite starting temperature is weakened. The amount of bainite is influenced by strain, cooling rate, and deformation temperature. When the specimens are deformed below 900 *C, equiaxed ferrites are promoted and the bainite transformation is suppressed. Key words: Nb-microalloyed low carbon steel 8 bainite starting temperature; bainite; equiaxed ferrite; retained austenite

Grain ultra-fining is the main characteristic for new generation C-Mn steels. But the grain refine- ment will result in the increase of yield ratio of the steel and the steel having lower work-hardening abili- tyC1**'. In order to obtain high comprehensive prop- erties, the complex strengthening mechanism, inclu- ding microstructure strengthening and precipitation strengthening, is employed in the development of ultra-fine grained steels. It was reported that intro- ducing some bainite in the matrix of ultra-fine grained ferrite for plain C-Mn steel with micro-Nb is an effective approach to obtain good comprehensive proper tie^[^*^'. So bainite transformation during continuous cooling process after deformation is impor- tant for controlling microstructure of the ultra-fine steels.

Previous on the austenite/bainite transformation mainly focused on some bainitic steels or alloyed construction steels. Studies about the austenite/bainite transformation, especially the bainitic transformation during continuous cooling process after deformation, of low carbon steels are

seldom available. The aim of present study is mainly about the continuous cooling transformation of Nb- microalloyed low carbon steel with different hot deformation parameters. The bainite starting temperatures at different conditions are measured and the microstructure evolutions under different hot deformation parameters are analyzed.

1 Experimental

The tested material was a Nb-microalloyed low carbon steel with the composition ( mass percent, %) of C 0.14, Si 0.27, Mn 1.25, P 0.025, S 0.010 and Nb 0.026. Cylindrical specimens (#8 mmX 15 mm) were prepared for hot deformation experiment on Gleeble-1500 thermomechanical simulator.

In the tests, the specimens were heated to 1 200 'C and held for 3 min, and then cooled to deformation temperatures (850 'C , 900 "C , 950 'c , 1 000 "c 1 at 10 C/s . After 30 s of holding at deformation temperature, the specimens were compressed into a single hit to different true strains ( 0, 0. 4, 0. 6 ) at strain rate l / s and held for 10 s after deformation,

Foundation Item,Item Sponsored by High Technology Develob.nent Program of China (863) (2001AA332020) and National Natural Science

Biography:YI Hai-long(1979-), Male, Doctor; Foundation of China (50271015)

E-mail: longhaiyi- 2004@tom. comi Revised Date: June 8, 2005

No. 3 Bainite Transformation Under Continuous Cooling of Nb-Microalloyed Low Carbon Steel 37

and then cooled to room temperature at different cooling rates (0. 5 "C/s, 1 C / s , 5 C / s , 10 "C/s, 15 C / s ) . Bainite starting temperatures were measured through thermal dilation method and metallographic method.

The substructures of the bainite were observed by H-700 transmission electron microscopy. The microstructures were characterized using an optical microscope.

2 Results and Discussion

2.1 Influence of hot deformation parameters on

Bainite starting temperatures at different deformation temperatures and different cooling rates are listed in Table 1. From this Table, it is shown that at the same deformation temperature, the bainite starting temperature decreases with increasing cooling rate. At the same cooling rate, the bainite starting temperature decreases with decreasing deformation temperature, and the following two rea- sons result in this phenomenon: First, displacive transformation involves the cooperative movements of atoms, and such movements can be restricted by dense lattice defects. Formation of a higher density of defects in the austenite structure at lower temperature suppressed the bainite transformation proceed- ing by a shear mechanism; Second, as the deformation temperature decreases, more ferrite will be pre- cipitated; the austenite stability increases because of the concentration of carbon in austenite increases, so the deformation will have stabilized effects on bainite transformation.

Fig. 1 shows the relationship between cooling rate and bainite starting temperature for different strains at 950 "C and 1 000 "C. From this figure, the bainite starting temperature decreases as the cooling rate increases and increases as the strain increases. For the specimens deformed at 950 ' C , the bainite starting temperature of the specimens deformed with strain of 0. 6 is about 10 'C higher than that of unde- formed specimens. But for the specimens deformed at

bainite starting temperature

Table 1 Bainite starting temperature of samples deformed with strain of 0 .4 *C

Deformation Cooling rate/( C s-I )

5 10 15 temperature/% 1 ~_____

1000 575 570 559 550

950 570 565 555 544

900 565 560 550 540

850 560 555 545 535

E 575

565

555

I 0 4 8 12 16

Cooling rale/(C.s-')

Fig. 1 Relationship between cooling rate and bainite transformation temperature for different true strains at 950 "c and 1 000 'c

1 000 "C , the bainite starting temperature changes slightly when the specimens are deformed, that is to say, deformation at 1 000 'C has little influence on bainite starting temperature. Therefore, deformation has an accelerated effect on the bainite transformation when the specimens are deformed at 950 'C. When the deformation temperature increases, the effects of deformation on bainite starting temperature diminish.

At a fixed temperature, bainite starting temperature is increased with increasing strain; as shown in Fig. 1 ( a ) , this phenomenon contradicts the descrip- tion above. But Maki TC15' found that ausforming markedly increases the rate of transformation of upper bainite, and has a smaller effect on the lower bainite reaction. The bainite reaction is accompanied by carbon diffusion and formation of carbides during the transformation. The structural defects were pro- duced by deformation, and most of the dislocations introduced into austenite can also act as nucleation sites. The defects increased with the increase of strain. This promotes carbide formation during bainite transformation and also stimulates the nucle-

38 Journal of Iron and Steel Research, International Vol. 13

ation of bainite ferrite, which requires formation of areas with decreased carbon c~ncent ra t ion"~~. So bainite transformation is accelerated by deformed austenite, and the bainite starting temperature increases with increasing strain. The higher the deformation. temperature, the faster the defect density decreases and as a result, the degree of acceleration of austenite transformation to ferrite is reduced; so the influence of strain on bainite starting temperature diminishesC163. In order to explain this accurately, experimental studies should be carried out in depth.

2.2 Influence of hot deformation parameters on

Fig. 2 shows the microstructures of the steel obtained from the specimens deformed with strain of 0. 4 at 1 000 IC with different cooling rates. It can be observed that when the cooling rate is 0.5 C / s , the amount of bainite is little, almost no bainite transformation, and the amount of bainite increases with increasing cooling rate.

Fig. 3 is the optical micrographs showing the microstructures of the specimens deformed at 950 "C

bainite

with different amount of strains and cooled to the room temperature at 1 C / s . From Fig. 3 , the amount of bainite obviously reduces with the increase of strain. This is because the increase of strain increases va- cancy, dislocation and other crystal defects, the sites of heterogeneous nucleation increase, and the stability of austenite reduces, so the ferrite nucleation is facilitated, the amount of ferrite increases and the amount of bainite decreases. This is distinctively different from the reduction of bainite caused by austenite machinery stability.

Fig. 4 shows the microstructures of the steel obtained from the specimens deformed with strain of 0.4 at different temperatures. It can be found that the microstructures of the specimens deformed at 1 000 OC and 950 'C are ferrite and bainite, and the morphology of ferrite is acicular ferrite and small amount of equiaxed ferrite. When deformation temperature decreases to 900 'C and 850 OC, some martensites are found in the microstructures and the equiaxed ferrite increases and the bainite decreases. The lath of bainite is refined with decreasing temperature, as shown in Fig. 5. Therefore, it can be considered that

(a ) 0.5 C / s ; ( b ) 1 C / s ; (c ) 5 C / s

Fig. 2 Microstructures of specimens deformed at 1 000 'c with strain of 0 .4 for different cooling rates

(a ) 0; ( b ) 0.4; ( c ) 0.6

Fig. 3 Microstructures of specimens deformed at 950°C and cooled at 1 "C /s for different strains

No. 3 Hainite Transformation Under Continuous Cooling of Nb-Microalloyed Low Carbon Steel 39

(a ) 850 Y:; (b) 900 C ; (c) 950 C i (d) 1 000 C Fig. 4 Microstructures of specimens deformed with

strain of 0 .4 at different temperatures

when the specimens are deformed below 900 “C , the formation of equiaxed ferrite is facilitated, and the stability of untransformed austenite increases, so the bainite transformation is hindered.

Some retained austenite is also observed in the microstructure of specimen deformed at 1 000 ‘C through TEM, as shown in Fig. 6. This is primarily due to the occurrence of static recrystallization during short delay time after deformation at high temperature, which could increase the stability of austenite and make some austenite retained at the end of transformation.

(a ) 850 % ; (b) 900 %; (c ) 950 %; ( d ) 1000 C Fig. 5 TEM micrographs of bainite of specimens deformed

with strain of 0.4 at different temperatures

3 Conclusions

( 1 ) Bainite starting temperature decreases with raising cooling rate, and increases with the increase of deformation temperature because of the increase of density of defects and austenite stability. Deformation has an accelerative effect on the bainite transformation when the specimens are deformed at 950 ’C. When the deformation temperature increases, the effect of deformation on bainite starting temperature is weakened.

Fig. 6 Morphology and diffraction pattern of retained austenite in specimen (Continued on Page 67)

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(Continued from Page 39) ( 2 ) The amount of bainite is influenced by

strain, cooling rate, and deformation temperature because these factors influence the stability of austenite.

( 3 ) When the specimens are deformed below 900 'C , the formation of equiaxed ferrite is facilitated and the bainite transformation is hindered. Some retained austenite is observed in the microstructure of specimen deformed at 1 000 'C.

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Bainite Transformation Under Continuous Cooling of Nb-Microalloyed Low Carbon Steel