Influence of Holding Time After Deformation on Bainite Transformation in Niobium Microalloyed Steel

Available online at www.sciencedirect.com

ScienceDirect JOURNAL OF IRON AND STEEL RESEARCH, INTERNATIONAL. 2007, 14(5) : 62-65 ~~~~

Influence of Holding Time After Deformation on Bainite Transformation in Niobium Microalloyed Steel YI Hai-long,

DU Lin-xiu,

LIU Xiang-hua

WANG Guo-dong,

(State Key Laboratory of Rolling Technology and Automation, Northeastern University, Shenyang 110004, Liaoning , China)

Abstract: Using GIeeble1500 system, the influence of holding time on bainite transformation in deformed niobium microalloyed steel during continuous cooling was analyzed, and the carbides in upper bainite were also systematically researched. T h e results show that the occurrence of the static recrystallization decreases the amount of bainite with an increase in the holding time and the emergence of retained austenite ( R A ) with the longer holding time. Two types of carbides were observed in upper bainite with regard to their precipitation sites. They either existed between the bainite ferrite laths or co-existed with RA. T h e formation mechanism of two kinds of carbides was analyzed by combining T E M micrographs with the model. Key words: niobium microalloyed steel; bainite; carbide; static recrystallization; retained austenite

Although several research on bainite transformation mainly focused on alloy construction steels or bainitic steels, those on the plain carbon steels are rarely reported. Carbide is one of the constitutive of bainite, the carbide precipitated in a pleurites has major influence on the mechanical property of the bainite, therefore the research about the carbide has a significant effect on the study of material property. The continuous cooling transformation of deformed niobium microalloyed low carbon steel at different holding time was discussed in this study. T h e microstructure evolution at different holding time after deformation was analyzed. The carbides in upper bainite were also systematically researched. '

1 Experimental Method A niobium bearing steel was used, and the composition of the experimental steel is presented in Table 1. Cylindrical specimens ($8 mm X 15 mm) were prepared for the experiment on Gleeble-1500 system. The specimen was heated to 1 200 "C and held for 3 min. And then, it was cooled to 950 "C at 10 C / s and held for 30 s. T h e specimen was then de-

Si

C ~

0.14

%

Composition of experimental steel

Table 1

0. 27

Mn

P

S

Nb

1.25

0.025

0.010

0.026

~~~

formed compressively with strain of 0. 4 at a rate of 1 s-l. After t h a t , the specimen was held at different time (10 s , 50 s , 100 s , 500 s , and 1 000 s ) , and cooled to ambient temperature ( A T ) at 10 C / s . T h e softening curve was measured using double-hit test. T h e specimen was taken from experimental steel bar in axial direction, and was ground, polished, and etched in 4% nitric alcohol. T o show the prior austenite grain boundaries, the specimen was etched in saturated picric liquor with a small amount of the wetting agent.

2 2.1

Results Softening curve and prior austenite grain

Fig. 1 shows the softening curve of the deformation of the specimen at 950 "C. T h e whole curve can be divided into three parts: T h e first part indicates the stage when the holding time is less than 10 s ; static recrystallization fraction rapidly increases as the

Foundation Item: Item Sponsored by High Technology Development Program of China (863) (2001AA332020) ; National Natural Science Foundation of China (50271015) Biography:YI Hai-long(1979-), Male, Doctori E-mail: longhaiyi-2004@tom. comi Revised Dale: October 28, 2005

No. 5

Influence of Holding Time After Deformation on Bainite Transformation in Niobium Microalloyed Steel

*

63

-

1

10

100

1 OOL

Timels

Fig. 1

Softening curve deformed at 950 “c

(a)

Fig. 2

holding time increases, which is attributed to the fact that the static recrystallization is promoted by large deformation storage. T h e second part represents the stage when the holding time is between 10 s and 100 s ; static recrystallization is suppressed because of the precipitation of niobium carbides and nitridesc’O1, and deformation storage is also reduced because of the occurrence of static recrystallization, but the effect is less. The third part represents the stage when the holding time is longer than 100 s ; the effect on static recrystallization diminishes because of the increase in the precipitation, and therefore, the recrystallization fraction again increases rapidly. Fig. 2 shows the prior austenite grain at different holding time. Flattened grains were produced when the holding time was short. As the holding time was increased to 50 s , a small recrystallization grain appeared among big grains. T h e static recrystallization was complete, and the austenite grain became large with increase of holding time.

Influence of holding time on microstructure of bainite Fig. 3 shows the microstructures of specimen deformed at 950 ‘Ccooled to A T at 10 C / s with dif-

2.2

ferent holding time. Generally, the microstructure is bainite, and the influence of holding time on microstructure is less, but static recrystallization had taken

(a) 10 s ;

Fig. 3

(b) 50 s ;

10 s;

( b ) 50 s

Prior austenite grain at different holding time

place with increasing holding time. T h e amount of bainite decreased with increasing holding time. Meanwhile, ferrite was precipitated at the boundary, and the amount of ferrite increased with increasing holding time. The longer the holding time, the more is the retained austenite ( R A ) , as shown in TEM. Fig. 4 shows the morphology and diffraction pattern of typical RA. Because of the occurrence of the static recrystallization with increasing holding time, the deformation energy was consumed, the austenite grain had already grown up, and t h e stability of austenite increased. T h e phenomenon, which shows that the amount of bainite decreased with increasing holding time was also verified in the light microscope and is shown in Fig. 3.

2. 3

Pattern and distribution of upper bainite carbides

Bainite transformation occurs in a manner different from that of diffusion transformation in the pearlite range. T h e bainite transformation proceeds by a shear mechanism, which is similar to the martensite transformation””. But as distinct from the latter, the bainite reaction is accompanied by carbon diffusion and carbide formation. With regard to the precipitation sites, two types of carbides in upper bainite were observed and are shown in Fig. 5. Fig. 5 ( a >

( c ) 100 s ;

(d) 500 s ;

( e ) 1000

s

Microstructures of specimens deformed, held at 950 “c for different time and cooled to AT at 10 “c /s

Journal of Iron and Steel Research, International

64

Fig. 4

Morphology and diffraction pattern of RA

(a)

Fig. 5

Vol. 14

10 s;

( b ) 50 s ;

( c ) 500 s

TEM micrographs of three types of carbides in upper bainite

and ( b ) show the same carbides. In Fig. 5 ( a ) , carbides were relatively small and precipitated between the parallel platelets of bainite ferrite, and these carbides precipitated from the carbon-enriched austenite between platelets of upper bainite. In Fig. 5 ( b ) , besides the carbides that are parallel to the platelets of bainite ferrite, carbides that are approximately perpendicular to the platelets of bainite ferrite were also observed, and these carbides formed in the section of two platelets of bainite ferrite were close to each other. In Fig. 5 ( c >, the pattern and distribution of carbides were different from that observed previously ; carbides were surrounded by upper bainite ferrite and were confirmed by diffraction. This type of carbide is similar to RA and generally co-exists with RA in the T E M micrograph. T h e interpretation coupled with the model mentioned below can be taken into account for the better understanding of the formation principle of carbide. Transformation temperature of the upper bainite is relatively high; therefore, the diffusion veloci-

ty of carbon is high. During transformation from austenite to ferrite, large quantity of carbon atoms enter the periphery of ferrite or the austenite which is in between the platelets of upper bainite; Although there is a drop in supersaturation because of carbon diffusion, the concentration of the carbon adjacent t o the austenite continuously increases, leading to critical shear temperature that is lower than the isothermal temperature. Therefore, the phase transformation became slow. Another effect of the increasing carbon concentration in austenite is that the carbide will precipitate from carbon-enriched austenite region. T h e carbide that exists in austenite, which is related t o ferrite and austenite, is called the upper bainite carbide. Fig. 6 shows the two types of carbides precipitated using the During early stage of transformation, a first nucleated and grew in the boundary of Y, and a platelet was formed, as shown in Fig. 6 (a). Fig. 6 ( b ) shows the variation of carbon concentration at the a / r interface. By exciting nucleation, an-

No. 5

Influence of Holding Time After Deformation on Bainite Transformation in Niobium Microalloyed Steel

*

65

cause of the large deformation storage. In the second part, the rate of slope decreases because of the precipitation. In the third part, as the precipitation has grown larger, the rate of slope increases again. (2) By increasing the holding time, the amount of bainite decreased and more ferrite precipitated in the grain boundary. I Distance (arbitrary unit) ( 3 ) With regard to the distribution of carbide, two types of carbides in upper bainite were observed. They exist between laths of bainite ferrite or coexist with RA. References:

1%

Y interg

Fig. 6

i-

L

Y interg

Y interg

el,

Precipitation model for , BIZ, and 0, carbide in upper bainite

other a platelet was formed besides the a platelet that was formed, and obviously, 7-rich carbon region existed between platelets of upper bainite. If the kinetics and thermodynamics parameters are satisfied, carbide will precipitate from Y-enriched carbon region, as shown in Fig. 6 ( c ) , which is defined here as The platelets of upper bainite evolution with increasing holding time are shown in Fig. 6 ( d ) . Another carbide defined as 0, was precipitated adjacent to two a subunits. From Fig. 6 , it can be seen that B,, is parallel to a platelet, but the alignment of is determined by separation angle between terminal subunit of a and primary axis of a . But, BL1and belong to the same carbide, which is called intergranular carbide. In the experiment, carbide that was surrounded by upper bainite ferrite was observed, which is defined here a s 6,. This is attributed to the fact that the upper bainite ferrites grow after the precipitation of carbides, and the precipitated carbides are surrounded by the growing ferrite, and they can also be precipitated from austenite surrounded by ferrite ; therefore, these carbides coexist with RA. These carbides in upper bainite are analogous t o lower bainite carbides, which exist in ferrite platelet. This type of carbide was also observed by Hung et a1 in their research

ell.

3

Conclusions

(1) T h e softening curve can be obviously divided into three parts because of the addition of niobium. In the first part, the rate of slope is large be-

Hsu T Y , XU Zu-yao. On Bainite Formation [J]. Metallurgical Transactions A: Physical Metallurgy and Materials Science, 1990, 21A(4): 811-816. Reynolds W T. LIU S K , LI F 2 , e t al. Investigation of the Generality of Incomplete Transformation to Bainite in Fe-C-X Alloys [J]. Metallurgical Transactions A: Physical Metallurgy and Materials Science, 1990, 21A(6): 1479-1491. CHANG L C. Bainite Transformation Temperatures in HighSilicon Steels [J]. Metallurgical Transactions A : Physical Metallurgy and Materials Science, 1999, 30(4): 909-916. WANG Jia-jun, FANG Hong-sheng, YANG Zhi-gang, e t al. Fine Structure and Formation Mechanism of Bainite in Steels [J]. ISIJ International, 1995, 35(8) : 992-1000. Ohmori Y, Jung Y C, Ueno H , et al. Crystallographic Analysis of Upper Bainite in Fe-9%Ni-C Alloys [J]. Materials Transformations, JIM, 1996, 37(11): 1665.1671. Takezawa K , Maruyama S, Marukawa K, et al. Discussion on the Formation of Bainite and Other Precipitates in Cu-Zn and Ag-Zn Alloys [J]. Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science, 1994, 25(12) : 2621-2629. BO Xiang-zheng, FANG Hong-sheng, WANG Jia-jun. Three Dimensional Morphology and Microstructural Evolution of Bainite in Steels [J]. Journal of Materials Science and Technology, 1998, 1 4 ( 6 ) : 559-563. FANG Hang-sheng, WANG Jia-jun, YANG Zhi-gang, et al. Formation of Bainite in Ferrous and Nonferrous Alloys Through Sympathetic Nucleation and Ledgewise Growth Mechanism [J]. Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science, 1996, 27A(6): 15351545. Quidort D , Brechet Y. T h e Role of Carbon on the Kinetics of Bainite Transformation in Steels [J]. Scripta Materialia, 2002, 47(3) : 151-156. Medina S F. T h e Influence of Niobium on the Static Recrystallization of Hot Deformed Austenite and on Strain Induced Precipitation Kinetics [J]. Scripta Metallurgica et Materialia, 1995, 32(1): 43-48. Kurdomov G V , Utevsky L M , Entin R I. Transformations in Iron and Steel [MI. Moscow: Nauka, 1977. FANG Hong-sheng, WANG Jia-jun, YANG Zhi-gang, et al. Bainite Transformation [MI. Beijing: Scientific Press, 1999. H U A N G D H , Thorns G. Metallography of Bainitic Transformation in Silicon Containing Steels [J]. Metall Trans, 1977, 8A: 1661-1674.