Influence of Cold Rolling Reduction on Microstructure and Mechanical Properties of Twip Steel

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Acta Metall. Sin. (Engl. Lett.) Vol. 20 No.6 pp441-447 Dec. 2007

ACTA METALLURGICA SINICA (ENGLISH LETTERS)

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INFLUENCE OF COLD ROLLING REDUCTION ON MICROSTRUCTURE AND MECHANICAL PROPERTIES OF TWIP STEEL Z.L. Mi*,D. Tang, Y.J. Dai,H.Q. Wang, and S . S . Li National Engineering Research Center for Advanced Rolling Technology, University of Science and Technology Beijing, Beijing 100083, China hlanuscript received 8 October 2007

The influence of cold rolling reduction on microstmcture and mechanical properties of the TWIP (nwinning induced plasticirj) steel was investigated. The results indicated that the steel hud better comprehensive mechanical properties when cold rolling reduction was about 65.0% and the annealing temperature was lOOO?Z. The tensile strength of the steel is about h4OMPa and the yield strength is higher than 255MPa, while the elongation is above 82%. The microstructure is composed of austenitic matrix and annealing twim at room temperature, at the same time, u signiJcant amount of annealing iwim and stacking faults are observed by transmission electron microscopy (TEM). Mechanical twins pluy u dominant role during deformution. and result in excellent mechanical properties. KEY WORDS cold rolling reduction; annealing temperature; TWIP (twinning induced plasticiv) steel; twinning

1. Introduction The research of reducing consume in automobiles was developed in the recent years. The development of high-strength and super high-strength steel is one of the main ideas"]. TWIP (twinning induced plasticity) steel is a kind of high-strength and high-plasticity steel, which has been researched for several years. TWIP steel is entirely austenite at room temperature. When steel is deformed, the deformation twins occur inside the annealing twins[*].Because of the twin transformation, high-plasticity is produced with no neck-shrink. TWIP steel has excellent mechanical properties, such as high-hardness, high-plasticity and high-~trengthl~"]. Now the research of TWIP steel is still in its initial stages. In order to acknowledge the mechanical properties and its transformation of microstructure in TWIP steel, the tensile test of TWIP steel with different cold rolling reduction was done. Through the analysis of microstructure and mechanical properties, the rolling process of TWIP steel was discussed. This study could give the reference to develop better 'Corresponding author. Tel.: +86 10 62332598 ext. 6311. E-mail address: [email protected],edu.cn ( Z . L . Mi)

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performance steel.

2. Experimental The electromagnetism induction furnace had been used in TWIP steel experiment. The test steel was melted in vacuum and protected by argon gas, and then casted to plate. The chemical composition (wt%) of TWIP steel is shown as follows: C 0.015, Si 2.89, Mn 25.00, P 0.061, S 0.0043, A1 3.02, balance is Fe. The plate was rolled through hot rolling mill, and then was rolled through cold rolling mill. The test schemes are shown as follows: four groups of specimens with different strains of 60.7%, 65.0%, 68.0%, and 73.5% (all are given at different annealing temperatures of 800,900,950, and 1000°C. According to the standard of GB 3076-82, the sample was cut along the direction of rolling. The tensile test of plate was made at the machine of MTS-8 10 at room temperature. The sample's tensile speed was 3mdmin. To observe the microstructure of the TWIP steel, the sample was cut from the plate fore-andaft tensile process along the vertical direction of rolling. Optics microscope, transmission electron microscopy (TEM), and diffraction of X-ray were used to observe the structure of fore-and-aft in tensile test.

3. Results and Discussion 3.1 Mechanical property The mechanical property of plate, which was rolled in different cold rolling reduction and in different annealing temperature are shown in Fig. 1. Because there was no typical yield floor on the curve of strain-stress, the yield strength was uf12. From Fig. 1, the annealing temperature increased from 800 to 1000°C, the tensile strength of TWIP steel decreased from 840 to 640MPa, while its yield strength is also decreased from 575 to 255MPa. Although the strength decreased, the TWIP steel is a kind of high-strength steel. However, the TWIP steel's plasticity increased as the annealing temperature increased. The changes are shown in Fig.2. When the annealing temperature is 800°C,the steel's elongation is 60%. And when the annealing temperature increases to 1000°C, the steel's elongation is above 80%. From the experiment, it can be concluded that increasing the annealing temperature can improve the steel's plasticity. At the same time, Fig.2 shows the influence to the steel's strength and plasticity of different reduction in cold rolling. It showed that the steel has better mechanical properties when the cold rolling is about 65% than others. The motivation of recrystallization is the aberrance energy of the grain deformed. As the deformation increased, the aberrance energy will continue accretion. And this will drive the recrystallization. So, the 600

850 - (a),

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Fig. 1 Tensile strength (a) and yeild strength (b) in different reduction ration and different annealing temperature.

443 recrystallization temperature will debase[’]. Therefore the more deformation, the lower the annealing temperature will be. The change of microstructure according with deformation prove that the grain size will increase after annealing if the deformation accretion. Fig.3 is the same as Fig.4 in the annealing temperature and 300 85 - ( a ) 80

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Fig.2 Elongation percentage (a) and hardness (b) of steel with different reduction ration and different annealing temperature.

Fig.3 Microstructures of steel with different annealing temperature when reduction is 60.7%: (a) 800’C ; (b) 900°C; (c) 950°C;(d) 1000‘C .

Fig.4 Microstructures of steel with different annealing temperature when reduction is 73.5%: (a) 800°C; (b) 900'C; (c) 950°C; (d) 1000°C.

preserving time. Deformation when increased will make the TWIP effect to occur easily. It is as same as decreasing the annealing temperature.

3.2 Analysis of optic microstructure The photos of steel microstructure through different cold rolling reduction and different annealing temperature are givenin Figs. 3 and 4. It is obvious that the steel was not been deformed. There are several twins in steel after annealing, whose boundaries are straight and smooth. Twins manifold with the annealing temperature hoist. The twins are about 2-3km when the annealing temperature is about 800°C. And when the annealing temperature is lOOO'C, the twins are 20-40km. There are massive annealing twins in steel at this time. For different cold rolling reduction as in Figs. 3 and 4, the grain with the big transmutation is greater than the grain with the small transmutationl61. The microstructures of steel that is deformed with different annealing temperatures are given in Fig.5. The cold rolling reduction of the steel is 73.5%. The more and more fine mechanical twins come into being at the base of annealing twins after the steel has been deformed. According to FigSa, there are less mechanical twins than in Fig.Sd, because of less annealing twins in Fig.4a than in Fig.4d. Because the steel is in the recrystallization zone when the annealing temperature is 8OO0C,the size of annealing twins is small and less. And then the mechanical twins product is also less. When the annealing temperature is

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Fig3 Deformed microstructures of steel with different annealing temperature when reduction is 73.5%: (a) 800'C; (b) 900°C; (c) 950°C; (d) 1000°C.

1OOO"C, almost all annealing twins occur and there is accretion. So the size of the mechanical twins with smooth boundary increases after being deformed. It is believed that the more mechanical twins are produced, the TWIP effect will be more obvious.

3.3 Analysis of TEM structure and X-ray diffraction Fig.6 shows the bright field images of steel with different annealing temperatures. The microstructure is observed by TEM. The organization in Fig.6a is composed with polygon ferrite and austenitic matrix that have a small quantity twins and stacking-fault. Because the steel is not done with recrystallizing at the annealing temperature of 800°C, this microstructure is unbalanced. If the steel is kept longer at 800'C, the microstructure will balance soon. However there are all the single-phase austenite in steel when the annealing temperature is 1000°C(Fig.6b). There are a lot of annealing twins (part A in Fig.6b) and stacking-fault (part B in Fig.6b) in austenitic matrixIT. Fig.7a shows deformation twins of the steel annealed at 1000°C.Fig.7b shows the X-ray diffraction of

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Fig.6 Bright field images of steel with different annealing temperature: (a) 800°C; (b) 1000°C.

Fig.7 Deformation twins of steel by 1000°C annealed: (a) mechanical twins; (b) diffraction pattern

what is twin character spot. The parallelism and intervein strip mechanical twins have been formed in steel after it is deformed[']. These mechanical twins come into being on base of annealing twins. The mechanical twins and the annealing twins work together in the process of deforming. These two systems play an important role in tensile experiment. So the twin-induced plasticity, named TWIP effect, occurs obviously.

4. Conclusions ( I ) The steel can get tensile strength of 640MPa and yield strength of 255MPa when the cold rolling reduction is 65% and the annealing temperature is 1000°C.At this time, the steel also has excellent elongation at 82%. This mechanical performance is the best one in the experiment. It can be concluded that the good cold rolling reduction is about 65% and the fine annealing temperature is about l OOO'C. (2) The annealing twin accretion with the deformation increased at the same annealing temperature. The cold rolling reduction changed the recrystallization temperature. The increase of the cold rolling reduction can decrease the annealing temperature. (3) The microstructure of the steel is organized by ferrite and austenite when the annealing temperature is 8OO0C,because it is in the part recrystallization zone. When the annealing temperature is lOOO"C, the microstructure of the steel is composed of single austenite where there is an amount of twin and stacking-fault.

(4) More and more fine mechanical twins come into being at the base of annealing twins after the steel has been deformed. These bring about the TWIP effect and increase the elongation of the steel. Acknowledgements-This

work was supported by the National Natural Science Foundation of China (No. -50.575022)

and Specialized Research Foundation for the Doctoral Program of Higher Education 20040008024).

(No.

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