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Jafar Ghorbani, Abbas Zarei-Hanzaki, Hamid Azizi-Alizaminii, Seyyed MohammadHamed Hosseini:

An investigation into the influence of microstructure on thetoughness and tensile properties of V-microalloyed steels

In the last two decades, the significant market demands for microalloyed steels have led to enormous efforts as regards the op-timization of their properties. Following a national research program, the present work was scheduled to deal with a specialgrade of V-microalloyed steel. This grade was examined after a series of successive isothermal heat treatment to produce a va-riety of phase combinations (e.g., ferritic-martensitic, ferritic-martensitic-bainitic and ferritic-bainitic microstructures). Tensile andimpact tests were performed to gain knowledge about the mechanical properties. The resulting microstructures were evaluatedby means of SEM and optical microscopy. The results indicated that the corresponding tensile behaviour of the steels wasstrongly affected by microstructure and heat treatment parameters. Furthermore, the related ultimate tensile strength and impactvalues were broadly varied (750 to 1200 MPa and 5 J to 40 J, respectively) by the steels´ microstructure and chemical com-position. The corresponding fracture surfaces were found to vary with the steel´s microstructure.

Aims and scope. The high affinity of vanadium to com-bine with carbon and nitrogen along with the low solubilityof V(C/N) at medium temperatures of hot working are themain factors of how vanadium effects the final properties ofmicroalloyed steels. In low carbon steels, e.g., the growth

of austenite grains is retarded through the formation of VNdue to the presence of high nitrogen contents. However, incomparison with titanium and niobium, vanadium has alower capability of preventing the growth of austenitegrains at higher temperatures; therefore, its precipitation

strengthening effects are more considered [1].

Depending on the chemical composition and thesubsequent cooling after hot working, the final mi-crostructures of microalloyed steels may consist ofdifferent quantities and morphologies of pearlite,bainite, martensite/retained austenite and ferrite.Generally speaking, the ferrite in vanadium micro-alloyed steels may contain particles of a secondphase. The latter, i.e. V(C/N) precipitates, appearin the microstructure in two typical forms, i.e. asgeneral precipitates and interfacial ones. Generalprecipitates are distributed randomly and irregu-larly in the ferrite and follow the pattern of inter-face migration. These particles usually form dur-ing isothermal treatment in the �+� two-phase re-gion. Interfacial precipitates, however, form dur-ing continuous cooling and at temperatures higherthan the aforementioned ones. Obtaining a propervolume fraction, distribution and size of second-phase particles along with suitable dislocation sub-structure requires the selection of an appropriatechemical composition and heat treatment. Thismay affect the final strength of microalloyed steelsif the optimum austenite-to-ferrite transformationparameters are chosen [2].

Industrially applied microalloyed steels (in par-ticular those used in automotive industry) mostlycontain ferrite-pearlite microstructures. However,considering economical and environmental as-pects, other structures, such as dual-phase andmulti-phase grades, have also been discussed. Thevariation of microstructural components and theireffects on tensile properties have been mostlystudied in this area [2...3]. Nevertheless, as the im-pact properties are also important factors in appli-cation of these steels, the tensile and impact prop-erties of vanadium microalloyed forging steels

Fig. 1: Microstructural variation with bainite-formation time in mi-croalloyed steel Aa) before heat treatment; b) after 10 s; c) 30 s; d) 300 s heat treat-ment (F: ferrite, P: pearlite, B: bainite, M/A: martensite/retainedaustenite)

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have been investigated simultaneously in this paper afterapplying successive heat treatments.

Testing. Here, two types of medium-carbon steels withdifferent compositions were studied. The chemical compo-sitions of both grades are listed in table 1. The material wasreceived as radially forged to the shape of a billet. Both thetensile test specimens and impact specimens were ma-chined from the billet in longitudinal direction based onASTM-E8M and ASTM-E32 standards, respectively. Itshould be mentioned that the specimen final sizing was per-formed after the related heat treatment to prevent any mis-take in mechanical properties measurement from surfaceoxidation and/or decarburization due to subsequent heattreatment. The tensile tests were executed applying a fixedstroke displacement rate (i.e., 1 mm/min).

The experimental steels were subjected to a successiveheat treatment, i.e. austenitizing at 950 °C for 20 min in in-ert atmosphere, to produce the different microstructuralcomponents. The specimens were then quenched in a saltbath at 650 °C and held for 300 s at this temperature. Thisprocedure was followed by quenching the specimens in an-other salt bath at different temperatures (i.e., 360 °C, 410°C, and 460 °C) and isothermal holding for 10 s, 30 s, and300 s, respectively. Finally, quenching the specimens inwater ended the heat treatment.

The impact fracture surfaces were examined using ascanning electron microscope. In order to reveal the micro-structure, a colour etching method was applied (equal ratiosof 4%-pickral and 1%Na2S2O3 dissolved in distilled water).In this way, ferrite turns out as light grey, bainite as darkgrey, and martensite/retained austenite as white regions [4].

Evaluation and discussion of the results

Microstructure. Fig. 1 shows the initial and as-heattreated microstructures of steel A. Depending on thebainite-formation time and temperature, the microstructuremay consist of ferrite and martensite (fig. 1b), ferrite,

martensite and bainite (fig. 1c), and ferrite and bainite (fig.1d) microconstituents. As all the specimens were held iso-thermally at 650 °C for 300 s, allotriomorph ferrite formedin a continuous configuration at the prior austenite grain

boundaries. Therefore, all specimens con-tain a continuous ferrite microcomponentin a martensitic and/or bainite matrix. In-creasing the bainite formation time to 30 sresults in a three-phase (ferrite-martensite-

Fig. 5: Impact energy variations versus bainite start tempera-ture in ferritic-martensitic dual-phase microstructures (bainiteformation time: 10 s)

Fig. 4: Effect of bainite start temperature on ultimate tensileand yield strength of experimental steels (bainite formationtime: 300 s)

Fig. 3: Effect of bainite start temperature on ultimate tensileand yield strength of experimental steels (bainite formationtime: 30 s)

Fig. 2: Effect of bainite start temperature on ultimate tensileand yield strength of experimental steels (bainite formationtime: 10 s)

Table 1. Chemical composition of experimental steels (mass contents in %)Steel C Si Mn S P Cr Mo V N, ppmA 0.45 0.55 1.38 0.06 0.01 0.04 0.01 0.13 100 > NB 0.46 0.55 1.40 0.04 0.01 0.03 0.01 -- 100 > N

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bainite) microstructure. Furthermore, if the bainite forma-tion time increases to 300 s, the microstructure wouldmostly consist of ferrite in a bainitic matrix [5].

The effect of bainite forming parameters on tensileproperties. The variation of yield and tensile strength ofsteels A and B with bainite formation temperature and timeis illustrated in figs. 2 to 4. As can be seen in these figures,both the yield and ultimate tensile strength of steel A arehigher than for steel B under all conditions. This is attrib-uted to the presence of vanadium carbonitride precipitatesin steel A. Furthermore, this difference is higher in ferrite-martensite dual phase structures. It is well established thatVN, contrary to Ti(C/N), possesses a high heat expansioncoefficient and may, therefore, create tensile stress fieldsaround itself. These tensile areas may be partially compen-sated by the compressive stress state of the austenite tomartensite transformation. This phenomenon increases theaustenite to martensite transformation rate, thereby im-proving the ultimate tensile strength by generating a finermicrostructure [6].

As can be observed in fig. 2, the tensile properties werenot affected by bainite formation temperature for short-time(10 s) isothermal holding. The mechanical properties ofthese ferrite-martensite dual-phase microstructures aresomewhat higher than that of the conventional one due totheir higher carbon content. As is well illustrated in fig. 2,

the yield and ultimate tensile strength of these steels vary inthe range of 750-850 MPa and 1050-1200 MPa, respec-tively. The latter is attributed to the presence of ferrite andV(C/N) in a high-strength martensite matrix [7].

Increasing the bainite formation period from 10 to 30 sand from 30 to 300 s (figs. 1b, c and d) resulted in the for-mation of an appreciable amount of bainite. This led to adecrease in the final mechanical properties (figs. 2, 3, 4). Itis worth mentioning that the higher the bainite transformationtemperature in this case, the higher the strength drop. This isrelated to the effects of bainite morphology and to the fact thatlower bainite may affect the properties similar to martensite.

The effect of microstructure on the impact properties.The absorbed impact energies of steels A and B with fer-rite-martensite microstructures are shown in fig. 5. As canbe clearly seen, the absorbed impact energy in this case istoo low and varies from 6 J to 10 J. In addition, these dualphase structures exhibit a cleavage type fracture surface,fig. 6. The variation of the absorbed impact energies ofsteels A and B with the bainite start temperature is shown infig. 7. As is observed for a given bainite formation period,the higher the bainite start temperature, the lower the ab-sorbed impact energy [7]. Furthermore, decreasing thebainite start temperature from 460 to 410 ºC and 360 ºCchanges the fracture surface topography from cleavage tosemi-cleavage and ductile, respectively, figs. 8 to 10. It

Fig. 7: Impact energy variations versus bainite start tempera-ture in ferritic-martensitic dual-phase microstructures (bainiteformation time: 30 s)

Fig. 6: Impact specimen fracture surface with ferritic-martensitic dual phase microstructure

Fig. 8: Impact specimen fracture surface with ferrite-bainite-martensite microstructure (bainite formation temperature: 460 °C)

Fig. 9: Impact specimen fracture surface with ferrite-bainite-martensite microstructure (bainite formation temperature: 410 °C)

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should be noted that a similar behaviour was recognized byincreasing the bainite formation period up to 300 s, figs. 11and 12.

As is well established, several factors, such as the prioraustenite grain size, the state of microstructural componentsand their related aggregates, second phase characteristics,and so on, may dictate the impact properties of steels. Nev-ertheless, the microstructure of the steel should be refinedto enhance the impact toughness without sacrificing thestrength. A finer microstructure is obtained by refining theprior austenite grain size. This leads to finer microstruc-tures in any subsequent transformation.

As can be observed from figs. 5, 7 and 11, the absorbedimpact energy of steel A is higher than that of steel B inspite of the microstructure. In general, microalloying ele-ments affect the steels in two ways:⇒ reducing the prior austenite grain size thereby increas-

ing both the strength and toughness simultaneously;⇒ raising the strength and reducing the toughness by pro-

ducing carbonitride precipitates.Accordingly, the higher absorbed impact energy in mi-

croalloyed steel A is attributed to its finer prior austenitegrain size (40 �m) compared to that of steel B (75 �m) [8].

The variation of the impact energy with bainite start tem-perature in these steels may be rationalized as follows:Concerning upper bainite, it is well known that the carbidesprecipitated on the boundaries of bainitic ferrite do not in-

hibit crack growth. In addition, the low-angle boundaries ofadjacent bainitic ferrite laths do not act as potential barriersto crack growth. On the other hand, lower bainite showshigher strength and toughness. Reducing the bainite starttemperature results in an increase in dislocation densityalong with finer carbide precipitates within the bainitic fer-rite matrix. This finely dispersed carbide is not easily bro-ken and generates strong barriers to microcrack growth.The latter means that even if a cleavage microcrack nucle-ate may not easily propagate because of the presence ofstrong dislocation substructure [9].

Conclusions

The industrially applied microalloyed steels, particularlythose used in automotive industries, mostly contain ferrite-pearlite microstructures. However, considering the eco-nomical and environmental aspects, other structures ofthese steels such as dual-phase and multi-phase structureshave also been discussed.

In this study, the mechanical properties of medium car-bon steels with and without vanadium as a microalloyingelement after successive isothermal transformation were in-vestigated. The steels were examined after a series of suc-cessive isothermal heat treatment performed to produce avariety of phase combinations (e.g., ferritic-martensitic, fer-ritic-martensitic-bainitic and ferritic-bainitic microstruc-tures). The resulting microstructures were evaluated bymeans of SEM, optical microscopy, impact and tensile tests.

The results indicated that the corresponding tensile be-haviour of the steels was strongly affected by microstruc-ture and heat treatment parameters. Furthermore, the relatedultimate tensile strength and impact values were broadlyvaried (750 to 1200 MPa and 5 J to 40 J, respectively) bythe steels microstructure and chemical composition.

Applying a predetermined successive heat treatment toplain and microalloyed steels resulted in the followingpoints:⇒ short time isothermal holding (10 s) in the bainite for-

mation region did not lead to any bainite formation and,therefore, the final microstructure consists of continu-ous ferrite in a martensite matrix. The resulting ultimatetensile strength in this case was in the range of 1100-1200 MPa;

Fig. 12: Impact specimen fracture surface with ferrite-bainitemicrostructure (bainite formation temperature: 360 °C)

Fig.10: Impact specimen fracture surface with ferrite-bainite-martensite microstructure (bainite formation temperature: 360 °C)

Fig. 11: Impact energy variations versus bainite formationtemperature in ferrite-bainite dual phase microstructures(bainite formation time: 300 s)

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⇒ in spite of the state of microstructural components, theabsorbed impact energy was measured to be higher inmicroalloyed steel;

⇒ as the bainite start temperature was increased in thesteel containing bainite, the absorbed impact energy wasdecreased;

⇒ for the same bainite formation conditions, the ferrite-bainite microstructure showed the highest and the fer-rite-martensite one revealed the lowest impact energy.

References

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[2] S. Zajac: ISIJ 38 (1998) No. 10, p. 1130/39.[3] R. Kasper: stahl u. eisen 120 (2000) No. 10, p. 95/102.[4] E. Girault, P. Jacques, P. Harlet, K. Mols, J.V. Humbeeck, E. Aer-

naudt and F. Delannay: Mater. Character. 40 (1998), p. 111/14.

[5] H.K.D.H. Bhadeshia: Bainite in Steels, 1st edn., London, Great Brit-ain. The Institute of Metals, 1992.

[6] J. Park, S.Y. Ok, J.K. Park: Effect of Precipitation of V(C,N) on theFerrite Grain Refinement in a Tempered Martensite Structure of C-MN-V Steel", [in:] Intern. Symp. on ultra fine grained steels, 2001, p.172/76, The Iron and Steel Institute of Japan, ISUGS, 2001.

[7] M.J. Blant, C.L. Davis and M. Stangwood: Mater. Sci. Eng. A284(2001), p. 1/13.

[8] Z.G. Wang and S.H. Al: ISIJ Intern. 39 (1999) No. 8, p. 747/59.[9] F.B. Pickering: Physical Metallurgical and The Design of Steels, Ap-

plied Science Publishers LTD, London, 1978.

Jafar Ghorbani Abbas Zarei-Hanzaki CEO

Hamid Azizi-Alizaminii Seyyed Mohammad Hamed Hosseini

Dept. of Metallurgy and Material Eng., Tehran University, Iran