Heredity in the Microstructure and Mechanical Properties of Hot-Rolled Spring Steel Wire 60Si2mna During Heat Treatment Process

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J. Mater. Sci. Technol., 2013, 29(1), 82e88

Heredity in the Microstructure and Mechanical Properties of Hot-rolled Spring Steel Wire 60Si2MnA during Heat Treatment Process

Chaolei Zhang, Leyu Zhou, Yazheng Liu* School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China [Manuscript received February 25, 2012, in revised form May 21, 2012, Available online 27 December 2012]

Heredity in the microstructure and mechanical properties of hot-rolled spring steel wire 60Si2MnA during heat treatment process was investigated comprehensively. The steel was isothermally transformed to obtain various hot-rolled microstructure (pearlite fineness within the range of 140e510 nm) and mechanical properties, and followed by some quenchingetempering treatment. Afterwards, microstructure was characterized by optical microscopy, scanning electron microscopy and quantitative metallography, and mechanical properties were determined by tensile test. The results indicated that the hot-rolled microstructure with a coarsen pearlite structure had been changed after reheating, to a austenite microstructure with bigger and more uneven grain size, and finally to a coarsen tempered microstructure. And the average austenite grain size and standard deviation of its distribution in quenched microstructure were observed to depend linearly on the interlamellar spacing in hot-rolled microstructure. Besides, to obtain a good combination of the final strength and plasticity, an optimum value range (190e280 nm) of the interlamellar spacing had been determined for the interlamellar spacing in hot-rolled microstructure.

KEY WORDS: Spring steel; Hot-rolling; Heat treatment; Microstructure; Mechanical properties; Heredity

1. Introduction no orientation relationship between the hot-rolled microstructure and mechanical properties of the spring steel was established. Spring steel wire 60Si2MnA, the required microstructure of The fundamental understanding of this scientific question is which is sorbite with a few fine-grained free ferrite, is often essential to optimize hot-rolling parameters and improve the final produced by high-speed wire rolling mill and retarded Stelmor properties of the springs. cooling technology. Then in manufacturing process of springs, In general, heredity in microstructure during steel production the steel is heated to austenitizing temperature, coiled and oil is understood as the recovery of the shape and size of original quenched, followed by tempering to the required hardness austenite grains after g / a / g transformation, and is most level. Consequently, the final microstructure and properties of often related to the chemical composition, structural state, and e springs are decided by the hot-rolling and heat treatment heating rate[8 10]. In this work, various hot-rolled microstruc- process. ture and mechanical properties were obtained by isothermal Up to the present, the effect of controlled rolling and cooling transformation, and then the trail of them during quenchinge process parameters on the hot-rolled microstructure and tempering process was demonstrated to investigate heredity in mechanical properties were carefully studied, and an artificial the microstructure and mechanical properties of the spring neural network model was developed for prediction of that in the steel. steel by Ai et al.[1,2]. Besides, the heat treatment parameters, such as austenitizing temperature[3], austenitizing time[4], tempering 2. Experimental temperature[5] and induction heating[6,7], have been extensively investigated to improve the properties of the springs. However, 2.1. Material condition and processing

The hot-rolled spring steel wire 60Si2MnA used in this work * Corresponding author. Prof.; Tel.: þ86 10 62333174; E-mail address: had a chemical composition of 0.60%Ce0.75%Mne1.69%Sie [email protected] (Y. Liu). e e 1005-0302/$ e see front matter Copyright Ó 2013, The editorial ofﬁce of 0.14%Cr 0.011%P 0.008S (in wt%). The steel wire was Journal of Materials Science & Technology. Published by Elsevier machined into cylindrical specimens with a size of Limited. All rights reserved. F12 mm 75 mm. Isothermal transformation was carried out to http://dx.doi.org/10.1016/j.jmst.2012.12.012 produce a systematic variation in hot-rolled microstructure. The C. Zhang et al.: J. Mater. Sci. Technol., 2013, 29(1), 82e88 83

Fig. 1 Optical micrographs showing the hot-rolled microstructure of the samples transformed at different temperatures: (a) 570 C, (b) 600 C, (c) 630 C, (d) 660 C. specimens were austenitized at 950 C for 30 min to permit full quantitative metallography of the hot-rolled microstructure are austenitization, and then were quenched into a salt pot for summarized in Table 1. As expected, the prior-austenite grain isothermal transformation at 570, 600, 630 and 660 C, respec- size remained constant due to the fixed austenitization tively. Transformation time was chosen sufficient to ensure temperature 950 C. However, with increasing the trans- complete transformations, without appreciable spheroidization. formation temperature from 570 to 660 C, not only the After the transformation, the specimens were austenitized at interlamellar spacing increases from 140 to 510 nm, but also 880 C for 30 min, and oil quenched, followed by tempering at the pearlite colony size rises from 18 to 54 mm. The relationship 420 C for 60 min. between transformation conditions (i.e., transformation temperature, prior-austenite grain size) and microstructure 2.2. Microstructural analysis and mechanical properties characteristics of the steel has been investigated comprehen- measurements sively and reported in literature[11]. The optical micrographs of the quenched microstructure, The specimens after every step heat treatment were metallo- primarily characterized by acicular martensite, are given in graphically polished and then etched with 4% nital for micro- Fig. 2. In a qualitative comparison of the four micrographs, it structural observation by optical microscopy and scanning shows that the size of acicular martensite has little change electron microscopy (SEM, JSM-6510A, JEOL). Interlamellar between Fig. 2(a) and (b), but is observably coarsening from spacing was measured by random intercept methods, and Fig. 2(b) to (d). average grain size was determined by linear intercept method. The initial austenitic condition, particularly the grain size, will [12] The mechanical properties of the specimens at room temperature naturally have an effect on martensite lath or plate size .To after the isothermal transformation and the tempering were confirm this, it was decided to investigate the austenite grain size determined by the tensile tests carried out by using standard five- fold specimens (5 mm in diameter) following the international standard ISO 6892-1:2009. Table 1 Quantitative metallography of the hot-rolled microstructure of the samples after isothermal transformation 3. Results and Discussion Transformation Interlamellar Pearlitic Prior-austenization temperature (C) spacing (nm) colony grain size (mm) 3.1. Heredity in microstructure size (mm)

Fig. 1 shows the hot-rolled microstructure of the steel after 570 140 18 60 various isothermal transformation treatments. Groups of 600 190 22 60 630 280 31 60 pearlite colonies and some free ferrite grains between them 660 510 54 60 could be clearly seen for all the samples. The results of the 84 C. Zhang et al.: J. Mater. Sci. Technol., 2013, 29(1), 82e88

Fig. 2 Optical micrographs showing the quenched microstructure of the samples transformed at different temperatures then quenched: (a) 570 C, (b) 600 C, (c) 630 C, (d) 660 C. of the quenched microstructure. The optical micrographs of the a uniform equiaxed structure. And as seen in Fig. 4, the austenite austenite grain and its size distribution of the samples trans- grain size distribution is close to normal distribution as Eq. (1), formed at different temperatures then quenched are given in being consistent with the results presented by Han et al.[13], Figs. 3 and 4, respectively. Fig. 3 shows the austenite grains with Kurtz and Carpay[14] and Zhang et al.[15]

Fig. 3 Optical micrographs showing the austenite grain of the samples transformed at different temperatures then quenched: (a) 570 C, (b) 600 C, (c) 630 C, (d) 660 C. C. Zhang et al.: J. Mater. Sci. Technol., 2013, 29(1), 82e88 85

Fig. 4 Grain size distribution of the austenite grain of the samples transformed at different temperatures then quenched: (a) 570 C, (b) 600 C, (c) 630 C, (d) 660 C.

2 fl A ðddÞ is a noticeable in uence of the interlamellar spacing on the ð Þ¼pffiffiffiffiffiffi s2 þ f d se 2 B (1) average grain size and the standard deviation. In a word, the 2p microstructure of lamellar pearlite and austenite grains indicates heredity in size. As the coarser pearlite was heated, the bigger where d is the grain size; d is the average grain size; s is the and more uneven austenite was formed. Specifically, it may be standard deviation; A and B are the material constants. noted in Fig. 5 that the average grain size and the standard Besides, the grain size is within the range of 10e120 mm, and deviation s follow a linear relationship with the interlamellar the average grain size of the samples transformed at 570, 600, spacing S, which can be described by linear curve fit as Eqs. (2) 630 and 660 C then quenched is 44.8, 45.6, 46.3 and and (3), respectively. 49.1 mm, respectively. The relation of the average grain size (d) and the standard d ¼ 1:14 10 2S þ 43:26 (2) deviation (s) to the interlamellar spacing (S) of the hot-rolled microstructure is plotted in Fig. 5. The data suggest that there s ¼ 2:47 103S þ 2:44 (3)

The results of the hot-rolled and quenched microstructural analysis are well indicative of the existence of heredity in microstructures during the quenching process. The transformation of lamellar pearlite to austenite is a complex reaction. The equilibrium microstructure is composed of ferrite and pearlite, the latter being a composite of ferrite and cementite. Ferrite has a very low solubility of carbon and hence, on its own, only begins to transform to austenite at high temperature. The initiation of austenitization in a hypoeutectoid steel is in pearlite, where the diffusion distances for carbon are small. New grains of austenite nucleate at interface between ferrite lamellae and Fig. 5 Relations of the average grain size and the standard deviation to cementite lamellae as well as pearlite colony boundaries. Work the interlamellar spacing. by Nemoto[10] of a commercial containing-manganese steel has 86 C. Zhang et al.: J. Mater. Sci. Technol., 2013, 29(1), 82e88 reported that the cementite lamellae provides carbon for relationship of the steel has been reported in literature[11] that the the growing austenite grain but also acts as barriers, and strength and percentage reduction of area excluding the the migrating direction of the austenite grain is parallel to the percentage elongation, follow a HallePetch type of relationship cementite lamellae. Hence, the coarser quenched microstructure with the inverse of the square root of the interlamellar spacing is closely associated with the coarser lamellar pearlite structure. (see Eqs. (4)e(6)). For the spring steel, percentage elongation is In the temperature range of 300e500 C, the martensite governed mainly by the uniform deformation, but percentage decomposes into ferrite and the precipitation of fine particles of reduction of area is governed mainly by the necking. And the carbide occurs. The fine granular structure formed is known as stress condition in the uniform deformation region and the neck secondary troostite[12]. Following the tempering at 420 C for region is different during tensile test. In consequence, the stress 60 min, visible changes in the microstructure were observed as condition should be taken into account during analyzing the seen in Fig. 6. The microstructure seen here is secondary effect of interlamellar spacing on mechanical properties. troostite, a dispersion of fine carbides in a ferritic matrix. In 1=2 a qualitative comparison of four micrographs in Fig. 6, it shows Rm ¼ 4268:42S þ 661:37 (4) that the microstructure is gradually coarsening from Fig. 6(a) to (d). The results of the quenched to tempered microstructural analysis are well indicative of the existence of a heredity in 1=2 Rp0:2 ¼ 3773:26S þ 274:45 (5) microstructures during tempering process.

3.2. Heredity in mechanical properties Z ¼ 3493:45S1=2 þ 16:46 (6)

It is known that the microstructure governs the mechanical properties of steel. There is an implied correlation between the The tempered mechanical properties, including ultimate tensile hot-rolled and the tempered mechanical properties of the spring strength, yield strength, percentage elongation and percentage steel because of the heredity in microstructure. reduction of area, have been plotted in Fig. 8 against the inter- Earlier study of the microstructureemechanical properties lamellar spacing in hot-rolled microstructure. As can be seen in relationship has shown that the deformation behavior of ferritee Fig. 8(a), the ultimate tensile strength and yield strength show pearlite steels is mainly governed by the pearlite, and the inter- little change when the interlamellar spacing increases from 140 to lamellar spacing substantially affects the mechanical proper- 280 nm. However, up to a certain value of the interlamellar ties[16,17].InFig. 7, the hot-rolled mechanical properties, spacing 510 nm, the ultimate tensile strength and yield strength including ultimate tensile strength (Rm), yield strength (RP0.2), rapidly decrease. And the variations of the percentage elongation percentage elongation (A) and percentage reduction of area (Z), and percentage reduction of area with increasing the interlamellar have been plotted against the inverse of the square root of the spacing are given in Fig. 8(b). With increasing the interlamellar interlamellar spacing. The microstructureemechanical properties spacing from 140 to 280 nm, not only the percentage reduction of

Fig. 6 SEM micrographs showing the tempered microstructure of the samples transformed at different temperatures then quenched and tempered: (a) 570 C, (b) 600 C, (c) 630 C, (d) 660 C. C. Zhang et al.: J. Mater. Sci. Technol., 2013, 29(1), 82e88 87

spacing is increased further up to 540 nm mainly because of the large-scale reduction in the strength. The above results show that the interlamellar spacing in hot- rolled microstructure in the range of 140e280 nm weakly affects the tempered mechanical properties. Only when the interlamellar spacing is increased further up to 540 nm, the tempered mechanical properties will be sharply decreased. As a rule, preservation of coarse-grained structure leads to lowering of the mechanical properties. Hence, heredity in the mechanical properties is in line with that in the microstructure. Towards the principle of the best combination of strength and plasticity, the interlamellar spacing in hot-rolled microstructure is not the thinner the better for the final mechanical properties of the spring steel. It has an optimum value range. As seen from the foregoing results, when the interlamellar spacing is 190e280 nm characterized of sorbite, the final mechanical properties have a good combination of strength and plasticity. In general, the controlling on the hot-rolled microstructure and mechanical properties of the steel is mainly on account of the subsequent deformation process, such as drawing and coiling. However, the results explained clearly in this paper indicate that the hot-rolled microstructure and mechanical properties influence the final ones. Consequently, the controlling on the hot-rolled microstructure and mechanical properties of the steel plays an important role in the final properties of the springs. Fig. 7 Relations of the inverse of the square root of interlamellar spacing (S1/2) in hot-rolled microstructure to the hot-rolled 4. Conclusions mechanical properties. From the results above, one can see that the inherited features area increase from 31.4% to 34.3%, but also the percentage affect considerably the properties of the spring steel and this elongation rises from 6.3% to 7.7%. The maximum value of the should be taken into account in its hot-rolling and heat treatment percentage reduction of area and the percentage elongation (41.9% processes. and 9.3%, respectively) are obtained when the interlamellar Heredity in the microstructure is proposed for the spring steel with a ferriteepearlite structure, and heredity in the mechanical properties has been clearly interpreted. The hot-rolled microstructure with a coarsen pearlite structure leads to the austenite grains with lager average size and more uneven micro- morphology after reheating. The average grain size and standard deviation of its distribution of quenched microstructure are observed to depend linearly on the interlamellar spacing in hot- rolled microstructure. And the quenched microstructure with the coarsen martensite structure further leads to the coarse tempered microstructure. The existence of heredity in the microstructure results in heredity in the mechanical properties. The final mechanical properties had little change with the interlamellar spacing in hot- rolled microstructure, as it increases from 140 to 280 nm. However, when the interlamellar spacing is increased further up to 540 nm, the tempered mechanical properties will be sharply decreased. And for the giving quenching and tempering conditions, when the interlamellar spacing is 190e280 nm, the final mechanical properties have a good combination of strength and plasticity.

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