Influence of sub-temperature quenching temperature on the tensile strength, hardness and toughness of 60Si2Mn steel was studied, and the ferrite content, austenite grain size and martensite morphology of the microstructure of this steel after sub-temperature quenching were analyzed. The results show that duplex microstructure of martensite and ferrite is obtained by sub-temperature quenching of 60Si2Mn steel. The ferrite content decreases with the quenching temperature increasing. The strength and hardness of 60Si2Mn steel increase, however, its toughness decreases in the range of 770~810℃. The maximum strength and hardness can be obtained by quenching at 800℃, but the strength and hardness decease above 800℃. The small amount of strip ferrite is distributed among martensite lamellar to improve the toughness of the steel. All martensite structure can be obtained by quenching at 810℃

Anming Li

Mengjuan Hu

English

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Engineering Review Vol. 32, Issue 3, 181-185, 2012. 181______________________________________________________________________________________________________________________THE INFLUENCE OF SUB-TEMPERATURE QUENCHING TEMPERATURE ON THE MICROSTRUCTURE AND PROPERTIES OF 60Si2Mn STEELA.M. Li* – M.J. HuSchool of Materials Science & Engineering, Henan Polytechnic University, Jiaozuo 454000, ChinaARTICLE INFO Abstract:Article history:Received 29.05.2012.Received in revised form 27.07.2012Accepted 03.09.2012.The influence of sub-temperature quenching temperature on the tensile strength, hardness and toughness of 60Si2Mn steel has been studied, and the ferrite content, austenite grain size and martensite morphology of the microstructure of this steel after sub-temperature quenching analyzed. The results show that duplex microstructure of martensite and ferrite has been obtained by sub-temperature quenching of 60Si2Mn steel. The ferrite content decreases when the quenching temperature increases. The strength and hardness of 60Si2Mn steel increase, however, its toughness decreases within the range of 770~800°C. The maximum strength and hardness can be obtained by quenching at 800°C, but the strength and hardness decrease above 800°C. The small amount of strip ferrite is distributed among martensite lamellar to improve steel toughness.The entire martensite structure can be obtained by quenching at 810°C.Keywords:60Si2Mn steelSub-temperature quenchingTensile strengthHardnessMicrostructure* Corresponding author. Tel.: 0391 3980631E-mail address: anmingli2001@sina.com.cn1 Introduction60Si2Mn steel is traditionally used for makingsprings of automobiles, tractors, railway vehicles and other high elastic stress components. In recent years, it has been used instead of Cr12 steels [1] formanufacturing small cold moulds. With the expansion of 60Si2Mn steel application field,problems with conventionally completed quenching process arose, referring to the difficulty of obtainingdesirable high tenacity [2]. Consequently, it would cause mould fracture, structural failure and eventually collapse of blades. Improving the toughness of 60Si2Mn steel through the quenching process is an effective way of assuring a longer/better service life of cold mould [3].Sub-temperature quenching is an incomplete quenched process, in which materials are quenched in two-phase region. Compared with the traditional quenching process, the microstructure of lath martensite and ferrite banding can be obtained when the steel undergoes sub-temperature quenching, which is beneficial to steel toughness but only under the premise of not reducing or slightly lowering its strength and hardness so as to be widely applied in the production [4]. Certainly, research into the influence of sub-temperature quenching temperature on the microstructure and mechanical properties of 182 A. Li, M. Hu: Influence of sub-temperature quenching on microstructure and properties of 60Si2Mn steel______________________________________________________________________________________________________________________60Si2Mn steel is of practical significance for the quality of cold mould made from this steel. Hence this paper deals with microstructure and mechanical properties of 60Si2Mn steel quenched under different sub-hardening temperatures.2 Experimental procedure2.1 Experimental materialThe composition of experimental 60Si2Mn steel is (mass fraction, %): 0.59% C, 1.75% Si, 0.75% Mn, 0.32% Cr, 0.30% Ni, 0.0035% S, 0.032% P, and the balance is Fe, according to the standard of America ASTM 9260.2.2 Sample preparation The tensile sample is a short sample with thedimension of d0=10 mm, L0=5d0, where d0 and L0indicate the diameter and effective length of the tensile sample, respectively. The dimension of hardness samples is Ø16 mm × 20 mm. The dimension of an impact V-shaped specimen gap is 10 mm × 10 mm × 55 mm.2.3 Heat treatment processAll samples were heated in the same box-type electric furnace. Holding quenching time of 20 minwas followed by oil cooling. Holding time for tempering was 40 min. The sample number and heat treatment process are shown in Table 1.3 Experimental resultsThree tensile samples and three hardness samples intercepted from Ø16 mm × 2000 mm bar have beenprocessed respectively at each temperature point. The average property value is regarded as the experimental result. The average sample values of tensile strength and hardness are shown in Table 1.Table 1. Heat treatment parameters and mechanical properties of 60Si2Mn steelCodeQuenchingtemperature (°C)Temperingtemperature (°C)HardnessHRCUltimateTensile strength(MPa)Elongation(%)12345678910111213141577077077078078078079079079080080080081081081040043046040043046040043046040043046040043046031.030.127.838.036.933.143.341.640.246.544.441.843.540.236.41034101397112061173112013891312124515261436133513971320120716.116.316.515.515.816.014.815.115.313.314.114.212.312.913.4Engineering Review Vol. 32, Issue 3, 181-185, 2012. 183______________________________________________________________________________________________________________________4 Results and discussion4.1 Quenching temperature and the ferrite contentWithin the temperature range of 770~810°C, with increasing quenching temperature, the content of ferrite in the quenching structure decreases gradually, but the content of martensite increases, and consequently the distribution of ferrite morphology also undergoes transformations. A large amount of ferrite exists in the sample quenched at the lower temperature, and a part of ithas an independent existence when the block has shaped (Fig. 1a). Martensite and ferrite dual-phase structure can be obtained by quenching the sample at 800°C, in which/where ferrite distributes between the martensite plates (Fig. 1b, Fig. 3). The whole martensite structure can be obtained by quenching the sample at 810°C (Fig. 1c).(a) (b)(c)Figure 1. The quenched microstructure of 60Si2Mn steel (a) 780°C quenched (b) 800°C (c) 810°C4.2 Grain sizeMetallographic analysis shows that grain size of 60Si2Mn steel samples with sub-temperature quenched at 780°C is fine (see Fig. 2). The austenitestructure has grown in size mainly through the grain boundary migration, and has been split by ferrite that exists in the sub-temperature quenched microstructure and that impedes inhibiting austenite grain boundary migration.At the same time, as the lower quenching temperature is not beneficial to atomic diffusion, grain boundary migration is slow and the austenite grain growth is consequently hindered [5].4.3 Martensite morphologyFig. 2 shows the microstructure of 60Si2Mn steel, and Fig. 3 shows TEM image of the sample quenched at 800°C. The sub-temperature quenchedmicrostructure is mainly fine martensite, with a small amount of ferrite distributed between the lath martensite lamellar.780°C quenched 800°C quenched810°C quenched2020 20184 A. Li, M. Hu: Influence of sub-temperature quenching on microstructure and properties of 60Si2Mn steel______________________________________________________________________________________________________________________Figure 2. The grain size of 60Si2Mn steel quenched at 780°CFigure 3. The SEM image of 60Si2Mn steel quenched at 800°C4.4 Quenching temperature and the mechanical propertiesWith an increase of sub-temperature quenching temperature within the range of 770~810°C, which is shown in Fig. 4, the values of strength and hardness first increase and then decrease. The specimen quenched at 800°C has the best strength and hardness. The elongation of the sample decreases monotonically with an increase ofquenching temperature.Metallographic analysis shows that a small number of ferrite blocks exist in the sample when quenching temperature is reduced. Obviously, a ferrite block experienced a significant reduction in hardness and strength. Therefore, samples are of lower hardness and strength. A very small amount of lath martensite and ferrite duplex structure is distributed by ferrite between lath martensite when quenched at 800°C. The mechanical property of duplex structure is superior to a single martensite [6]. On the one hand, a small amount of ferrite decreases the strength and hardness of the steel. But compared with complete quenching, high carbon content martensite can be obtained by sub-temperature quenching while the hardness of martensite, on the other hand, can be increased by raising the carbon content, which is partly offset by decreasing strength and hardness caused by the presence of ferrite [7]. Moreover, both the grain size of austenite and martensite structure after sub-temperature quenching are fine. This is why specimen quenched at 800°C has higher strength and hardness.With increasing quenching temperature, strength and hardness first increase and then decreasewhereas elongation increases. The experimental test shows that the impact toughness value of 60Si2Mn steel completely quenched at 870°C and then tempered at 460°C is about 24.4 J/cm2, but the value is 29.3 J/cm2 when the samples are tempered at 450°C after sub-temperature quenching at 800°C.The duplex structure obtained by sub-temperature quenching improves the toughness of 60Si2Mn steel.5 Conclusion(1)  Within the temperature range of 770-810°C,when the quenching temperature increaseas, the ferrite content in 60Si2Mn steel graduallydecreases, and the distribution form is correspondingly changed. (2)  The structure of 60Si2Mn steel sub-temperature consists of lath martensite and ferrite, whichimproves steel toughness.(3)  When the quenching temperature is below 800°C, the strength and hardness of  60Si2Mn steel increase. When the temperature is higher than 800°C, the strength and hardness decrease. Withinthe experimental temperature range, specimens quenched at 800°C have the best strength and hardness. F MEngineering Review Vol. 32, Issue 3, 181-185, 2012. 185______________________________________________________________________________________________________________________(a ) the curves of hardness (b) the curves of tensile strength(c) the curves of elongation percentageFigure 4. Mechanical properties of 60Si2Mn steel’s quenched at 770°C -810°CReferences:[1] Zhang, X.Y., Xu, H.X., Li, Z.C.: Comparison of Strengthening and Toughing Craft of 60Si2Mn Cold-punched Mould Steel, Hot Working Technology, 3 (2005), 40-41.[2] Zhang, X.M.: Provement of Heat Treatment Process of 60Si2Mn Cold Forging Die,Fasteners Technology, 3 (2005), 8-9.[3] Zhao, X.P., et al.: Application of Sub-Temperature Quenching for Mould Parts, Heat Treatment of Metals Abroad, 2 (2002), 20-21.[4] Li, A.M., Li, X.F., Chen, H.: The Austenite Inverse Phase Transformation Sub-Temperature Quenching of 27SiMn Steel,Journal of China Coal Society, 9 (2008), 1063-1066.[5] Han, J.M., Cui, S.H., Li, W.J., Ma, X.Y.: Dual Phase Heat Treatment of Low-Alloy Steel,Journal of Iron and Steel Research, International, 12 (2005), 47-51.[6] Dong, J., Vetter, H., Zoch, H.W.: Shortening the Duration of Heat Treatment in the Lower Bainitic Range. Transactions of Materials and Heat Treatment, 25 (2004), 5, 555-560.[7] Huang, T.J.: Mechanical Properties of 40Cr Steel After Strengthening and Toughness Treatment, Hot Working Technology, 35(2006), 14, 56-57.

The influence of sub-temperature quenching temperature on microstructure and properties of 60Si2Mn steel

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