A Bainite-Ferrite Multi-Phase Steel Strengthened by Ti-Microalloying

Jianfeng Wang, Guangqiang Li* and Aida Xiao

Key Laboratory for Ferrous Metallurgy and Resources Utilization of Ministry of Education, Wuhan University of Scienceand Technology, No. 947 Heping Avenue, Wuhan 430081, P. R. China

Ti-microalloyed bainite multi-phase steels were prepared in laboratory scale. The tensile strength of examined steels are all higher than775 MPa and the maximum value is 875 MPa, with at least 27 J impact energy at 253 K which shows a good balance of high strength and hightoughness. The examined steels have uniform and fine multi-phase structure with bainite as main phase and ferrite as minor phase, the ratio offerrite phase gradually decreases with the cooling speed of post-rolling increasing. Most of the precipitates are nano scaled particles of TiN andTiC, which disperse at grain boundaries, dislocations and other places, having effect of grain refinment strengthening and precipitationstrengthening. [doi:10.2320/matertrans.M2011009]

(Received January 6, 2011; Accepted August 18, 2011; Published September 28, 2011)

Keywords: titanium, microalloying, bainite, multi-phase steel

1. Introduction

Low carbon bainite steel has high strength and hightoughness. It emerged as required by society and the progressof metallurgical technologies over these 30 years and canfulfill multi-purpose application.1–6) At present, most ofpretty fine, high dislocation density bainite matrix structureis obtained by adding less carbon (carbon content� 0:06 mass%) and alloying elements such as Ti, V, Nb,Cu, Ni, Mo, etc., and using secondary refining, TMCP,tempering technologies in manufacturing processes. Expertson steel materials developed low alloy steels of 600–800 MPagrades by means of different strengthening methods, how-ever, relatively high amount of different alloy elements wereused, which make the production cost high and requires morestrict production conditions.7–14)

This article gives the experimental results for the develop-ment of Ti-microalloyed bainite-ferrite multi-phase steelwith perfect comprehensive properties by Ti-microalloyingwhich has relative lower cost comparing with niobium andvanadium alloying. The developed steels make use of theinteraction among alloy elements like Mn and Ti, rollingcontrol, rapid cooling after rolling and the effect ofprecipitation strengthening and grain refinement strengthen-ing of Ti. The structures and properties of developed steelwere tested, providing a basis for wider range of applicationof Ti-microalloyed steel.

2. Experiment Procedures

The rule of alloying design is to ensure effective delayof polygon ferrite formation in cooling control and obtainbainite. In order to reduce the cost of alloys addition,expensive elements V, Mo, Cu, and Ni are not used. Thechemical composition is listed in Table 1.

The experimental steel was firstly melted in a vacuuminduction furnace and casted into a 50 kg billet, then heatedin reheating furnace, rolled to a 5.5 mm-thick plate by

�500 mm mill with rolling and cooling control. Consideringthe load capacity of the mill and reduction of each rollingpass, the exit thickness of the third pass of breakdownbar was 60 mm in widness, exit temperature was 1273 Kand overall reduction rate was 64.7%; the overall reductionrate at the fifth pass of finishing rolling was 90.8% andthe finishing temperature range was 1103 to 1143 K. Afterhot rolling, the steel plate was cooled by laminar watercooling, with cooling speed of 13 to 40 K/s. The rollingprocesses and the parameters are shown in Fig. 1 andTable 2, respectively. The Samples of 300 mm in length �5.5 mm in thickness � width were taken after differentrolling and cooling conditions for properties examination.The samples of as casted billets, billets after reheating andplates at different rolling passes were also taken for theanalysis of precipitates, microstructures and mechanicalproperties.

The continuous cooling transformation (CCT) curve of theexperimental steel was obtained from as casted billet by hotsimulation experiments using Gleeble1500 simulator. Themechanical properties of the experimental steel were testedaccording to China National Standard GB2975-82 and GB/T228-2002. The v-shaped impact sample size is 5� 10�55 (mm), and tested using impact testing machine ZWICKRKP450.

Fig. 1 Hot deformation and cooling process.

*Corresponding author, E-mail: ligq-wust@wust.edu.cn

Materials Transactions, Vol. 52, No. 11 (2011) pp. 2027 to 2031#2011 The Japan Institute of Metals

3. Results and Discussion

3.1 CCT curve of the experimental steelThe continuous cooling transformation (CCT) curve of the

experimental steel is shown in Fig. 2. It can be seen fromFig. 2 that the microstructures of experimental steel con-sisted of ferrite (F) and pearlite (P) or ferrite (F) and bainite(B), respectively, depending on different cooling rates, whichranged from 0.1 K�s�1 to 100 K�s�1. The volume fraction ofdifferent phases also depends on cooling conditions. Thefaster cooling rate results in the lower transformation startingtemperature of ferrite and also the finer ferrite grains.The microstructure of mixed F and B can be obtainedif the cooling rate varies between 5.0 K�s�1 to 100 K�s�1.The volume fraction of B increases with the cooling rateincreasing. The microstructure of steel is ferrite plus a certainamount of bainite, which show a good combination ofstrength and toughness. Therefore, the cooling rate afterrolling in the production should be controlled to be largeenough to obtain the mixed microstructure of F + B forbetter comprehensive mechanical properties.15)

3.2 MicrostructureThe microstructures of final plates are shown in Fig. 3,

where (a), (b), (c), (d) are steel specimens of No. 1#, 2#, 3#,and 4#, respectively, obtained in the conditions listed inTable 2. The experimental steel is composed of mainly lathbainite and a few ferrites. Matrix of experimental steel isrefined bainite laths, which show as 3–6 mm sheaves withinterrupted or film type retained austenite between the laths.The sheaves have different orientation and cross-segmented.There are irregular granular bainite or acicular ferritebetween the sheaves. This refined matrix structures havehigh strength and toughness.16) In addition, there are differentvolume fractions of polygonal ferrite distributed at originalaustenitic grain boundary or in the grain interior. With theincrease of cooling rate after rolling in the order of (a) to (d),the volume fraction of ferrite reduces gradually and theferrite morphology turns from approximately polygonal toacicular.

In this experiment, heavy deformation was imposed onaustenite, which led to increase of energy storage inaustenite, thus enhanced the driving force of ferrite nucle-ation and the nucleation rate of ferrite at grain boundaries as

well. The nucleation rate for proeutectoid ferrite increaseswith the increasing of deformation zone and dislocationstructures possible in austenite. The control rolling andcooling at proper rate after rolling can increases � ! �transformation driving force and ferrite nucleation rate withthe increasing of condensate depression. The time intervalbetween deformation and � ! � transformation is shorter,the recovery of austenite grain boundaries or transgranulardislocations and deformation band is harder, so the numberof bainite and intragranular ferrite increases with the decreaseof their sizes, and ferrite grain is significantly refined.

The details of microstructure of the experimental steel areshown in Fig. 4. Figure 4(a) is a typical microstructure ofgranular bainite composed of generally in parallel distributedplatelets shaped ferrite and bar-like islands. Ultrafine bainiteplates with the length about 0.5 mm could be observedin Fig. 4(b). Bainitic ferrite plates grow along a certaindirections, no rigorous orientation relationship betweenaustenite and the ferrite of granular structure is observed.The growth of some bainite plates is stifled by intragranularferrite grains and granular structure. As a result, the lengthof granular bainite plates is largely reduced. Additionally,the ferrite of granular structure may grow up striding acrossgrain boundary of the parent phase, which may help toexplain why the prior austenite grain boundary in Fig. 4(b)could not easily be distinguished. According to the formationmechanism of M/A islands in granular structure,17) thedistricts surrounded by ferrite grains are carbon-enrichedduring the formation of granular structure, which willtransform to M/A islands afterwards. Such kind of M/Aislands could be seen in Fig. 4(c).

3.3 PrecipitationThere exists a large amount of precipitates of TiN, TiC and

Ti4C2S2 in Ti microalloyed steels.18) A few nano-sized cubicand approximate spherical precipitates exist in the exper-imental steel. The EDS spectrum shows that cubic particle inFig. 5(a) is TiN. These particles play the role of grain

Table 2 Process parameters of simulated rolling.

No.

Finishing rolling

temperature

T/K

Finish cooling

temperature

T/K

Cooling rate

K/s

1# 1153 903 13

2# 1123 853 22

3# 1133 883 31

4# 1103 863 40

100 50 30 20 10 5 2 1 0.5 0.1

Cooling rate, K/s

PF

B

A

300

400

500

600

700

800

900

1000

1100

1200

Tem

pera

ture

/K

1 10 100 1000 10000

Time/s

Fig. 2 Continuous cooling transformation curve of the developed steel.

Table 1 Chemical composition range of experimental steels. (mass%)

C Si Mn Al Ti N Fe

0.04–0.06 0.25–0.45 1.40–1.60 0.02–0.04 0.06–0.12 0.0030–0.0050 Balance

2028 J. Wang, G. Li and A. Xiao

refinement strengthening through pinning austenite grainboundary during soaking and acting as ferrite core of non-spontaneous nucleation during tandem rolling, because of thehigh precipitation temperature of TiN. Figure 5(b) shows

approximate spherical precipitates, TiC, precipitated atdislocations and grain boundaries during rolling and laminarcooling. Meanwhile, because the crystal defects provide afavorable heterogeneous nucleation site for second-phase

(a)

M/A

(c)(b)

M/A

Fig. 4 The SEM image (a) and TEM images (b), (c) of the granular bainite structures of experimental steels.

(d)(c)

(b)

B

QFB

QF

AF

GB

(a)

Fig. 3 SEM images of Microstructure of the experimental steel with the process parameters of simulated rolling listed in Table 2 in order

of (a)-1#, (b)-2#, (c)-3# and (d)-4#.

TiC

(b)

TiN1 µm

(a)

Fig. 5 The SEM image of a TiN precipitate (a) and a TEM image of a TiC precipitate (b) in experimental steel.

A Bainite-Ferrite Multi-Phase Steel Strengthened by Ti-Microalloying 2029

particles and decreases nucleation energy, moreover, soluteatoms diffuse quickly along the dislocation pipes, thus,titanium carbide particles nucleate and precipitate alongdislocations.19) The small TiC particles in the ferrite grainand dislocations can prevent the dislocation movement andplay a distinct role in precipitation hardening by Orowanmechanism. This is the main strengthening mechanism of thetitanium microalloyed high strength steel.

3.4 Mechanical propertiesFigure 6 is the typical tensile curve of the experimental

steel. It is obvious that there is no yield point but presents acontinuous yield on the curve with small hardening rate. Thisyield characteristic has close relation with bainite content andmobile dislocation density of the microstructure.20)

Table 3 shows the mechanical properties and charpyV-Notch impact energy values of the experimental steels.The experimental steel has good mechanical properties withyield strength in the range of 686�796 MPa and tensilestrength of 780�875 MPa and elongation higher than 16%.The experimental steel has a good impact toughness at roomtemperature and low temperature, the impact energy values at253 K is more than 27 J. The precipitation of TiN particle athigh-temperature heating process can inhibit austenite graingrowth, and the ferrite to bainite transformation can bedelayed effectively by alloying elements dissolved inaustenite. And, titanium carbide precipitation obviouslyhinders the slipping of dislocations.21)

In the process of rolling, austenite grains get small anduniform by repeated recrystallization. During finishing roll-ing, austenite is elongated and deformation band is formed bylarge deformation in austenite non-recrystallization area.After laminar cooling, the steel is rapidly cooled to coilingtemperature, fine low-carbon bainite microstructure with

high-density sub-structures are formed. The increase ofcooling speed can prevent or delay the premature precip-itation of titanium carbide during the cooling, which is readyfor formation of more dispersed precipitates, further improv-ing strength and toughness of the steel. The adverse effectsof carbon on bainite toughness are minimized due to thelower carbon content, so, the experimental steel can havehigh strength and good toughness.

4. Summary

(1) The experimental steel mainly has refined bainitematrix and certain amount of polygonal ferrite. With theincrease of the cooling speed, the volume fraction of ferriteamount decreases and approximate polygonal ferrite changesto acicular ferrite.

(2) Bainitic ferrite plates grow along a certain directionsand their length is largely reduced. According to theformation mechanism of M/A islands in granular structure,the districts surrounded by ferrite grains are carbon-enriched,which transform to M/A islands afterwards.

(3) Nano-sized cubic shape TiN and approximate spher-ical TiC precipitates exist in the experimental steel, whichcan provide the strengthening effect by grain refinementstrengthening and precipitation hardening so as to increasethe strength and toughness.

(4) Through rational chemical composition design, controlover rolling and rapid cooling, making full use of titaniumprecipitation hardening and grain refinement strengthening,Ti microalloyed bainite-ferrite muli-phase steel was success-fully developed with good comprehensive properties andrelatively lower cost. Experimental steel has tensile strengthup to 875 MPa, impact energy of 27 J at 253 K, showing agood balance of strength and toughness.

Acknowledgements

This work was financially supported by the NaturalScience Foundation of Hubei province, China, with GrantNo. 2008CDA010.

REFERENCES

1) E. J. Czyryca and M. G. Vassilaros: Key. Eng. Mater. 84–85 (1993)

491–520.

2) C. Garcı́a-Mateo and G. C. Francisca: Mater. Trans. 46 (2005) 1839–

1846.

3) C. Y. Huang, J. R. Yang and S. C. Wang: Mater. Trans. 34 (1993) 658–

668.

4) T. Araki, M. Enomoto and K. Shibata: Mater. Trans. JIM 32 (1991)

Fig. 6 The tensile curves of the experimental steel.

Table 3 Results of tensile test and charpy V-Notch impact energy values of experimental steels.

Yield TensileElongation, V-Notch impact energy, Akv/J

No. Strength, Strength,A/%

RelMPa Rm/MPa 233 K 253 K 273 K 298 K

1# 686 780 18.2 69.37 87.64 96.72 95.67

2# 690 775 17.5 64.21 75.67 76.33 75.46

3# 711 790 17.2 43.67 43.33 66.53 68.33

4# 796 875 16.5 17.82 27.33 46.61 54.76

2030 J. Wang, G. Li and A. Xiao

729–736.

5) M. Katsumata, O. Ishiyama, T. Inoue and T. Tanaka: Mater. Trans. JIM

32 (1991) 715–728.

6) S. W. Ooi and G. Fourlaris: Mater. Charact. 56 (2006) 214–226.

7) O. Chiaki: ISIJ Int. 6 (2001) 542–553.

8) F. Yoshimasa, S. Tsuyoshi and T. Kunikazu: ISIJ Int. 11 (2004) 1945–

1951.

9) Y. T. Meng, W. H. Wang and Yung-Fu Hsu: Mater. Trans. 49 (2008)

559–564.

10) H. Ohtsuka: Mater. Trans. 48 (2007) 2851–2854.

11) Y. Ohmori, H. Ohtani and T. Kunitake: J. Metal. Sci. 11 (1974) 357–

363.

12) R. D. K. Misra, H. Nathani and J. E. Hartmann: Mater. Sci. Eng. A. 394

(2005) 339–352.

13) C. J. Herman, B. Donnay and V. Leroy: ISIJ Int. 7 (1992) 779–785.

14) N. Naoki and M. Matthias: ISIJ Int. 1 (2005) 82–90.

15) F. Y. Xu, Y. W. Wang, B. Z. Bai and H. S. Fang: J. Iron. Ste. Res. Int. 3

(2010) 46–50.

16) J. P. Wang, Z. G. Yang, B. Z. Bai and H. S. Fang: Mater. Sci. Eng. A

369 (2004) 112–118.

17) H. S. Fang, C. Feng, Y. K. Zheng, Z. G. Yang and B. Z. Bai: J. Iron. Ste.

Res. Int. 6 (2008) 1–9.

18) R. Z. Wang, C. I. Garcia and M. Hua: ISIJ Int. 9 (2006) 1345–1353.

19) A. Barbacki and M. Kawalec: J. Mater. Proce. Techn. 64 (1997)

33–39.

20) J. Krawczyk and J. Pacyna: J. Achie. Mater. Manuf. Eng. 1–2 (2006)

93–98.

21) H. L. Yi, L. X. Du, G. D. Wang and X. H. Liu: J. Iron Steel Res. Int. 4

(2009) 72–77.

A Bainite-Ferrite Multi-Phase Steel Strengthened by Ti-Microalloying 2031