1. Introduction

The initial development of microalloyed steel for forgingsuch as crankshafts was in the spring of 1972. Since then,this group of steel is now being used for several automotivecomponents. The original idea for development of thesesteels was to obtain definite values of the yield and tensilestrength in crankshafts during the cooling phase after dropforging. Eliminating the cost of heat treating steel forgingsis the main target for the introduction of microalloyedsteels, which achieve the desired properties during con-trolled cooling after forging. In such steels, the strength lev-els and other properties achieved after cooling from hotworking temperature are comparable to those obtained fromconventional quenched and tempered steels. The elimina-tion of heat treating and often straightening operations fa-cilitates just-in-time delivery schedules. One company re-ports shipping components in the afternoon that wereforged in the morning.1–6)

Microalloying with V is an efficient method of strength-ening medium carbon ferrite–pearlite steels. From electronmicroscopy it has long been known that this occurs by fineN-rich V–carbonitrides in the polygonal ferrite as well as inthe ferrite–lamellae of pearlite. The investigation of thetemperature dependence of solubility of V–carbonitridesmakes dissolution possible to convenient temperatures andthereby gives potential for a considerable precipitationstrengthening. V-microalloying is therefore commonly pre-ferred to other microalloyed additions in these types of

steels.7)

The forging parameters, direct-cooling rate and composi-tions of microalloyed steels is highly influenced on the mi-crostructure and therefore play an important role on finalmechanical properties.8) In this study, the influence of re-heating temperature and direct-cooling rate after forging onthe microstructure and mechanical properties of V-microal-loyed steel are investigated.

2. Experimental Procedure

A medium carbon V-microalloyed steel 38MnSiVS5,composition of which is given in Table 1, was selected forexperimental procedure. Small samples, 8 mm in length and14 mm in diameter, were utilized to investigate the austenitegrain size evolution with temperature by heating specimensin the rang 800–1 100°C, and quenching in a brine solution.Polished sections of austenitized specimens were examinedoptically after etching in saturate aqueous picric acid solu-tion with some drops of surfactant at 60–80°C to determineaustenite grain size.

To investigate the influence of reheating temperature bil-lets with 45 mm in diameter and 250 mm in length wereaustenitized in an induction furnace for 5 min at 1 000,1 100 and 1 200°C.

Three cooling rate was employed in this investigation:still air-cooling, forced air-cooling and water spray-cooling.In order to determining the effect of direct-cooling rateafter forging on microstructure and mechanical properties

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The Influence of Reheating Temperature and Direct-cooling Rateafter Forging on Microstructure and Mechanical Properties of V-microalloyed Steel 38MnSiVS5

Ghasem DINI, Mahmood Monir VAGHEFI and Ali SHAFYEI

Department of Materials Science and Engineering, Isfahan University of Technology, 84154 Isfahan, Iran. E-mail: gh_dini@materials.iut.ac.ir, vagh-mah@cc.iut.ac.ir, shafyei@cc.iut.ac.ir

(Received on July 15, 2005; accepted on October 6, 2005 )

Wiht the aim of replacing quenched and tempered forging parts and reducing by this way costly and timeconsuming operations; an industrial forging procedure was developed to evaluate the influence of thermo-mechanical processing parameters on the microstructure and mechanical properties of vanadium microal-loyed steel. In order to study the influence of reheating temperature, after determining the dissolving tem-perature of vanadium carbonitrides precipitates, samples were heating in temperature rang 1 000 to1 200°C. After austenitization at 1 100°C, the microalloyed steel billets were forged in a hydraulic press andthen cooled with different cooling rates. The metallography and mechanical testing results indicated that byincreasing the reheating temperature, the strength and toughness of V-microalloyed steel have not changesignificantly and so the temperature of 1 200°C was selected for forging. By increasing cooling rate, bothstrength and toughness improve.

KEY WORDS: forging; microalloyed steel; strength; toughness.

of billets, after forging in a 1 200 t hydraulic press (reduc-tion of 50%) at a temperature range of 1 150–1 100°C, thesamples cooled in different rates.

Mechanical properties were determined through tensileand impact tests at room temperature. The ferrite–pearlitemicrostructure characteristics and pearlite colony size weredetermined using optical metallography.

3. Result and Discussion

3.1. Reheating Temperature Grain Size of Effect

The relation between the reheating temperature and prioraustenite grain size is shown in Fig. 1. The figure showsthat the amount of vanadium present in the alloy is effec-tive, as a grain size inhibitor, only below 950°C. Above thistemperature the grains coarsen rapidly.

3.2. Effect of Reheating Temperature on Microstruc-ture and Mechanical Properties

Figure 2 shows micrographs of samples reheated at1 000, 1 100 and 1 200°C, respectively. Figure 3 shows me-chanical properties of these samples as a function of soak-ing temperature. In general, there is an increase in tensilestrength with increasing reheating temperature in microal-loyed steels9) (Fig. 3(a)). This is associated with a coarserferrite networks in Fig. 2 on reheating, which is influencedby the final prior austenite grain size and result in a greaterproportion of pearlite and also with increased the amount ofdissolved vanadium in the austenite and accordingly the po-tential for precipitation hardening after cooling (Fig. 1). Buttoughness of vanadium-containing microalloyed steels de-creases with increasing reheating temperature due to coars-ening of the austenite grains. This effect is shown in Fig.3(b), which compare Charpy V-notch impact strength formicroalloyed steel 38MnSiVS5 reheated at three tempera-tures. As indicated in Figs. 3(a) and 3(b), the variation ofstrength and toughness with temperature increase are notsignificant and for reduce die wear, the 1 200 °C was select-ed for forging. At this higher temperature, resistance to diefilling is lower and therefore die life increases.

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Table 1. Chemical composition (wt%) of the steel investigated.

Fig. 1. The grain size as a function of asutenitizing temperature(soaking time of 120 s).

Fig. 2. Microstructures of samples reheated at: (a) 1 000, (b)1 100 and (c) 1 200°C.

Fig. 3. Mechanical properties of samples as a function of soak-ing temperature: (a) tensile strength and (b) toughness.

3.3. Effect of Direct-cooling Rate after Forging on Mi-crostructure and Mechanical Properties

Figure 4 illustrates the effect of cooling rate on the me-chanical properties of V-microalloyed steel and the corre-sponding microstractural change is shown in Fig. 5. Thetensile strength increase with increase in the cooling rate, but little or no reduction in toughness as shown in Fig. 4. As shown in Fig. 5, with increasing cooling rate,

the microstructure changes from coarse structure to fer-rite/pearlite mixed and fine structure.

The effect of cooling rate on pearlite colony size andhardness in V-microalloyed steel are represented in Figs. 6and 7, respectively. An increase in the cooling rate immedi-ately after hot working also restricts growth of the austenitegrains and increase driving force for reprecipitations of dis-solved carbonitrides. Therefore the nucleation sites for pre-cipitation increased and produce finer carbonitrides.10) Theresults indicated that, it is possible to improve the tensilestrength without impairing the impact toughness values inV-microalloyed steel.11–14)

4. Conclusions

The effect of reheating temperature and direct-coolingrate after forging on the microstructure and mechanicalproperties were evaluated and the following conclusionscan be drawn:

(1) The vanadium containing microalloyed steels is ef-fective as a grain size inhibitor only up to temperaturesbelow 950°C.

(2) In V-microalloyed steel 38MnSiVS5 variation oftemperature from 1 000 to 1 200°C have not important ef-fect on mechanical properties.

(3) Under low cooling rate conditions the final mi-crostructure is composed of ferrite-pearlite with somecoarse pearlite colony size, which in turn is not suitable forthe desired application.

(4) Increasing the cooling rate results in an increasingin the strength level, with little or no reduction in tough-ness.

Acknowledgments

Authors would like to acknowledge Haftom TirIndustries, Iran for the financial support and also wish to

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Fig. 4. Effect of cooling rate on: (a) strength and (b) toughness.

Fig. 5. Effect of cooling rate on microstructure of V-microal-loyed steel: (a) still air, (b) forced air and (c) water spraycooling.

Fig. 7. Effect of cooling rate on hardness of V-microalloyedsteel.

Fig. 6. Effect of cooling rate on pearlite colony size of V-mi-croalloyed steel.

thank Dr. M. R. Toroghinejad of Isfahan University ofTechnology for his technical discussions.

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