Effect of initial microstructure on mechanical properties in warm caliber rolling of high carbon steel

Materials Science and Engineering A 528 (2011) 5833–5839

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Effect of initial microstructure on mechanical properties in warm caliber rolling of high carbon steel Y.S. Oh a , I.H. Son b , K.H. Jung a , D.K. Kim a , D.L. Lee b , Y.T. Im a,∗ a

National Research Laboratory for Computer Aided Materials Processing, Department of Mechanical Engineering, KAIST, 373-1, Guseong-dong, Yuseong-gu, Daejeon 305-701, Republic of Korea Wire Rod Research Group, Technical Research Laboratories, POSCO, 1 Goedong-dong, Nam-gu, Pohang, Gyeongbuk 790-785, Republic of Korea

b

a r t i c l e

i n f o

Article history: Received 1 February 2011 Received in revised form 23 March 2011 Accepted 7 April 2011 Available online 13 April 2011 Keywords: Warm caliber rolling High carbon steel Initial microstructure Submicron grain structure Strength Toughness

a b s t r a c t In this study, the effect of initial microstructure on change of mechanical properties was investigated by warm caliber rolling (WCR) of high carbon steel. Experiments were carried out with two different kinds of initial microstructures of pearlite and tempered martensite at the temperature of 500 ◦ C. For comparison, the microstructure of austenite phase obtained from the conventional hot rolling at the temperature of 900 ◦ C up to about 83% of the accumulative reduction in area was assumed to be a reference case. It was found that the WCR provided better mechanical properties in terms of strength and toughness compared to the conventional hot rolling based on experimental results of micro-hardness, tension, and Charpy impact tests. The improvement of strength and toughness was attributed to smaller ferrite grain and dispersed cementite particles with smaller interspacing aligned to the rolling direction after the WCR owing to field emission scanning electron microscopy. The investigated WCR might be useful in obtaining the high strength material with better toughness without adding new alloying elements for industrial applications according to the present investigation. © 2011 Elsevier B.V. All rights reserved.

1. Introduction High carbon steel is used for tire cord and neptune wire because of its high strength. A critical problem of applying high carbon steel for engineering applications is the fact that the toughness deteriorates due to high carbon content. Grain refinement is one of the alternatives to improve toughness and strength of the material [1–3]. Severe plastic deformation (SPD) technique [4–6] is one of the typical methods of grain refinement that imposes large plastic deformation to produce ultrafine-grained (UFG) steel with an average grain size below micron level [7]. While these techniques are discontinuous processes and require relatively high accumulative strain to produce UFG steel, warm caliber rolling (WCR) is a continuous SPD technique which requires relatively low accumulative strain [8]. Ohmori et al. [9] conducted the WCR of low carbon steel at various temperatures at 500, 600, and 650 ◦ C to produce UFG ferrite structure. They studied a microstructural evolution depending on the accumulated strain and the relationship between grain size and Zener-Holloman parameter. According to their research, the WCR is one of the effective methods to obtain UFG ferrite structure.

∗ Corresponding author. Tel.: +82 42 350 3227; fax: +82 42 350 3210. E-mail address: [email protected] (Y.T. Im). 0921-5093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2011.04.016

Contrary to the conventional hot rolling, WCR is conducted in the non-recrystallization region (below A1 temperature). The initial microstructure transformed from austenite to ferrite/pearlite phases at this temperature can be obtained by cooling the billet. This structure evolves from lamellar structure to micro sized ferrite grains that consist of spheroidized and uniformly dispersed submicron cementite particles during the WCR. The ferrite grains and dispersed fine cementite particles contribute to improving strength and toughness of the material [10,11]. The benefit of the WCR might be avoiding adding additional alloying elements to increase the mechanical properties of the material. Ohmori et al. [10] studied the effect of dispersed cementite particles on the balance of yield strength and uniform elongation after the WCR of low carbon steels. They found that control of strain hardening was an effective method to achieve the balance of yield strength and uniform elongation for UFG steels. Torizuka et al. [12] produced UFG steels with different carbon contents (0.02–0.45 wt.% C) by the WCR and investigated their stress-strain behavior depending on the reduction in area. They found that UFG steels with fine cementite particles resulted in better mechanical properties than ferrite/pearlite, bainite, and tempered martensite steels. While these researches were focused on low and medium carbon steels and the effect of second phase particles on strength and ductility of the material, Kimura et al. [13] and Inoue et al. [14] investigated the influence of initial microstructures on the

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Y.S. Oh et al. / Materials Science and Engineering A 528 (2011) 5833–5839 Table 1 Chemical composition of the specimen (wt.%).

toughness of medium carbon steels after the WCR. Kimura et al. [13] conducted the WCR for medium carbon low-alloy steel of tempered martensite to enhance toughness of the material. They explained that the reason for having low toughness of UFG steels developed by Ohmori et al. [10] was induced by an equiaxed ferrite grain structure of low carbon contents. They also suggested a fibrous structure will enhance the toughness of UFG steels. Inoue et al. [14] studied the impact and tensile properties of UFG low carbon steel produced by the WCR. They applied ferrite/pearlite and martensite as an initial microstructure and showed enhanced impact properties affected by delamination. As mentioned above, most of studies were restricted to the low and medium carbon steels. Investigations for improvement of strength and toughness of high carbon steel were rarely conducted. In addition, there is no report on the comparison between the conventional hot rolling and WCR. In this study, the effect of initial microstructure of pearlite and tempered martensite on mechanical properties of high carbon steel produced by the WCR was investigated. In addition, comparisons between the conventional hot rolling and WCR were made by conducting micro-hardness, tension, and Charpy impact tests for the specimen prepared by each rolling condition. The microstructure evolution was investigated by field emission scanning electron microscopy to correlate mechanical behavior with the mode of fracture and to determine the microstructural changes depending on the processing conditions of the WCR.

To investigate the mechanical properties depending on initial microstructures, WCR experiments were carried out with two different kinds of initial microstructures of pearlite and tempered martensite. For comparison, the reference microstructure was obtained from the conventional hot rolling of austenite phase. Rolling experiments were conducted with an oval-round pass depending on the processing conditions without lubrication in a single stand reversible laboratory mill available at the POSCO technical research center with the rolling speed of 0.5 m/s.

Temp.( oC)

Mn

P

S

0.20

0.60

0.03

0.03

(b) Temp.( oC)

1100 oC, 1 h

A3

Si

0.80

To avoid the effect of alloying elements, plain high carbon steel with a larger volume fraction of cementite particles affecting strength improvement [15] was used in this study and its chemical composition is summarized in Table 1. Rods with a square cross section of 30 mm × 30 mm were deformed into round bars with a diameter of 14 mm by the WCR. The accumulative reduction in area ({Ainitial − Afinal }/Ainitial ) after 8 pass caliber rolling was about 83%. The accumulative reduction in area of 2 pass and 6 pass caliber rolling was about 36% and 74%, respectively. During rolling experiments, temperature at the surface of the specimen was measured by a thermal imaging camera to validate the rolling temperature. Fig. 1(a) shows the temperature history of the conventional hot rolling called by process A. The specimen was kept at 1100 ◦ C for 1 h in order to obtain fully recrystallized austenite phase and ensure homogeneous temperature distribution in the specimen. The specimen was air-cooled and subjected to 8 pass caliber rolling at 900 ◦ C. This process was carried out to get the reference microstructure starting from the austenite phase. Process B was conducted to apply pearlite as an initial microstructure for the WCR. Fig. 1(b) shows the temperature history of the process B. The specimen was heated up to 1100 ◦ C and kept for 1 h in order to obtain fully recrystallized austenite phase. The specimen was air-cooled to 900 ◦ C and 2 pass caliber rolling was applied to refine the austenite grain size. The specimen was cooled in air between 2 pass and 3 pass. Subsequently, 6 pass rolling was carried out at 500 ◦ C. Martensitic structure is considered to be beneficial as an initial microstructure because it has a relatively large amount of grain boundaries depending on dislocation densities [13]. To apply tempered martensite as an initial microstructure for the WCR in the process C, the specimen was heated up to 1100 ◦ C and kept for 1 h. The specimen was air-cooled to 900 ◦ C and 2 pass caliber rolling was

2. Experimental

(a)

C

1100 oC, 1 h

Rolling at 900 oC

AC

A3 A1

A1 500

Rolling at 900 oC

AC AC

WCR at 500 oC

500

AC

AC

Ms

Ms

Time

Time o (c) Temp.( C)

1100 oC, 1 h

A3

AC

Rolling at 900 oC

A1 500 Ms

WCR at 500 oC WQ 1h

AC

Time Fig. 1. Temperature histories of rolling experiments: (a) process A, (b) process B, and (c) process C.

Y.S. Oh et al. / Materials Science and Engineering A 528 (2011) 5833–5839

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Fig. 2. Measuring points in the cross section of the specimen after the warm caliber rolling for the micro-hardness test.

carried out and subjected to water quenching. Then, the specimen was reheated to 500 ◦ C, kept for 1 h for tempering, and subjected to 6 pass WCR as shown in Fig. 1(c). To examine local distribution of strength of the material depending on the initial microstructure investigated in this work, micro-hardness test in Vickers scale (Hv) was carried out with a micro-hardness tester (Mitutoyo HM-200, Japan). The specimens were polished by a SiC paper and diamond suspensions up to 1 ␮m for the cross section normal to the rolling direction (RD) of the specimens. The indentation load of 9.8 N was applied for 10 s. Hardness values were measured along the line at increments of 1 mm to avoid plastic deformation region as shown in Fig. 2. To investigate the effect of initial microstructure on strength of the rolled specimen, tensile tests were carried out using Instron 5583 (MA, USA). Tensile direction was in parallel to the rolling direction of the rods. Three tensile tests were carried out for each case. The dimension of gage length and the diameter was 25 and 6 mm, respectively. The crosshead speed was 0.0167 mm/s at room temperature and load-stroke curves were obtained as shown in Fig. 3. Then, nominal stress-strain curves were calculated accord-

Fig. 3. (a) The load–stroke curves and (b) tensile test specimens obtained depending on the initial microstructure.

ing to ASTM E8M-04 [16]. During the tensile tests, elongation was measured by a mechanical extensometer (Instron 2630-100 series clip-on type, MA, USA) with a gage length of 25 mm and a travel of −2.5 to +25 mm. To investigate the effect of initial microstructure on toughness of the rolled specimen, several Charpy V-notch impact tests were carried out. As shown in Fig. 4, specimens for the Charpy V-notch

Fig. 4. Subsized Charpy V-notch impact test specimen.

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Fig. 5. Microstructures of rolled specimens at the center of the cross section perpendicular to the rolling direction with different magnifications: (a and b) process A, (c and d) process B, and (e and f) process C.

400 350 300

Hardness (Hv)

impact test were machined off from the rolled bar prepared by the WCR to have a subsize cross section of 5 mm × 10 mm according to the ASTM E23-05 [17]. Impact test was conducted in a temperature range from −140 to 100 ◦ C with an impact speed of 5.4 mm/s. Liquid nitrogen and 98% isopentane were used as a coolant to obtain the temperature of −140 ◦ C for the preparation of the specimens and 99.9% ethyl alcohol was used to obtain the temperature range from −100 to −20 ◦ C, while heated silicone solution was used for the temperatures higher than 20 ◦ C. In order to confirm the test temperature of the heating or cooling chambers, a K-type thermocouple was used. All specimens were held for 600 s to obtain homogeneous temperature distribution in the specimen. All impact tests were carried out within 3 s after taking out the specimen from the heating or cooling chambers. Field emission scanning electron microscope (FE-SEM) was employed by using Philips XL30SFEG (Netherlands) to observe microstructural changes. The cross section perpendicular to the

250 200 150 Process A

100

Process B

50

Process C 0 -1

0

1

2

3

4

5

6

7

Distance from the center (mm) Fig. 6. Micro-hardness distribution depending on different initial microstructures.

Y.S. Oh et al. / Materials Science and Engineering A 528 (2011) 5833–5839 Table 2 Surface temperature of the process A measured at the inlet and outlet for different passes. Number of pass Temp. (◦ C)

Inlet Outlet

1

2

3

4

5

6

7

8

906 854

834 823

818 780

766 759

755 745

699 719

708 715

599 698

rolling and normal direction of the specimens was polished by a SiC paper and diamond suspensions up to 1 ␮m and etching with a 2% nital solution. SEM micrographs were obtained with the measurement condition of 15 kV accelerated voltage. 3. Results and discussion Fig. 5 shows microstructures of rolled specimens at the center of the cross section perpendicular to the rolling direction depending on the initial microstructure. The WCR of pearlite and tempered

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martensite as an initial microstructure resulted in the development of the submicron grain structure, where segmented and spheroidized cementite particles were uniformly dispersed in the ferrite matrix. The SEM micrograph of the process A showed complete pearlite colonies consisting of random lamellar structures because of phase transformation after deformation. On the other hand, for the case of the process B, austenite structures were transformed into pearlite colonies by air cooling. During the subsequent WCR, lamellar structures were broken down into submicron sized grains consisting of spheroidized and uniformly dispersed cementite particles. Especially, for the case of the process C, martensite structures obtained by water quenching were transformed to tempered martensite consisting of precipitated cementite particles at the tempering temperature. Such cementite particles obtained from the process B and C were considered as obstacles to the rearrangement of dislocation and the migration of grain boundaries because of grain boundary pinning effect [13]. As a result, the SEM micrographs of the process B and C showed submicron ferrite grains

Fig. 7. Microstructures of rolled specimens in the cross section perpendicular to the rolling direction below 1 and 0.1 mm from the surface: (a and b) process A, (c and d) process B, and (e and f) process C, respectively.

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Nominal Stress (MPa)

1200 1000 800 600 400

Process A Process B

200

Process C 0 0.00

0.04

0.08

0.12

0.16

Nominal Strain Fig. 8. The nominal stress–strain curves according to the initial microstructure.

Charpy Impact Energy (J)

60 Process A 50

Process B Process C

40 30 20 10 0 -140

-100

-60

-20

20

60

100

Temperature (ºC) Fig. 9. Charpy impact absorbed energy depending on the temperature for the different initial microstructures.

and spheroidized cementite particles in the ferrite matrix. In addition, cementite particle sizes obtained by the process B and C were in the range from 0.01 to 0.5 ␮m. While the cementite particles after the process B were distributed mostly at the grain boundaries [18], the ones of the process C were observed at the grain boundaries as well as inside of the grain [13]. Fig. 6 shows Vickers micro-hardness distribution from the center to the surface depending on initial microstructures prepared by the process A, B, and C. In this figure, the average micro-hardness in the specimen prepared by the process A was Hv 273. On the other hand, the average Vickers micro-hardness values for the specimens prepared by the process B and C were Hv 328 and 350, respectively. These results were attributed to the change of grain size and the interspacing of cementite particles. The movement of dislocation was interrupted by grain boundaries and the small interspacing of cementite particles. Because the ferrite grain size and the spacing between cementite particles processed by the process B and C were smaller than the one of the process A as shown in Fig. 5, the values of micro-hardness distribution for the process B and C were higher than the one of the process A. The micro-hardness distributions are uniform in the specimen obtained by the process B and C. However, in the case of process A, the micro-hardness values at the measured positions located at 5 and 6 mm from the center of the cross section were higher than the data for other measured positions. Since the surface of the specimen is cooled rapidly due to the contact with the roll as shown in Table 2, phase transformation from austenite to fine pearlite occurred in the vicinity of the surface of the specimen in the case of the process A. When austenite is transformed to pearlite

at low temperature, interlamellar spacing becomes shorter as the temperature of pearlite transformation decreases. The diffusion of carbon, as characterized by its diffusion coefficient, is temperature dependent. Based on the diffusion coefficient of carbon Dc = 0.12 e−32,000/RT (where R is the gas constant and T is the absolute temperature), the diffusion coefficient decreases exponentially with decreasing temperature. Thus, diffusion length of carbon becomes shorter due to less diffusion coefficient at lower temperatures [19]. Due to the small interspacing of cementite particles, micro-hardness value near the surface of the specimen was higher than for other measured positions. In addition, the micro-hardness values decreased at the surface (at the position below 0.5 mm from the surface) in the cases of the process A, B and C due to decarburization as shown in Fig. 7. However, there was relatively less decarburization for the process C compared to the other processes of A and B owing to the shorter exposure time to high temperature. Tensile test results are shown in Fig. 8. In this figure, low yield strength (LYS) and ultimate tensile strength (UTS) of the specimen prepared by the process A were 517 and 942 MPa, respectively. On the other hand, the specimens prepared by the process B and C showed LYS of 939 and 1058 MPa, respectively, and UTS of 1060 and 1063 MPa, respectively. The smaller ferrite grain size and interspacing of cementite particles increased the LYS and UTS for the cases of the process B and C. Based on the tensile test results, the increment of LYS was higher than the one of UTS depending on the initial microstructure. For a quantitative comparison, a yield ratio defined as LYS/UTS is introduced in the present investigation. These values for the specimen determined by process A, B and C were 0.549, 0.886 and 0.995, respectively. The yield ratio of approximately 1 means that necking occurs immediately after yielding without work hardening [20]. It is well known that increase of strength of the material trades off ductility. However, the elongations up to fracture were hardly decreased with the specimens obtained from the WCR. The elongations up to fracture were 17.1%, 14.6% and 14.4% for the cases of the process A, B and C, respectively, based on the tensile test results shown in Fig. 8. This is due to the spheroidized and uniformly dispersed cementite particles, which provide new source of dislocations, in the ferrite matrix after the WCR as shown in Fig. 5(c) and (e) [21,22]. Fig. 9 shows the results of Charpy impact test at seven different temperatures depending on the initial microstructure. In this figure, the specimen obtained from the process A showed about 10 J of absorbed energy at room temperature. On the other hand, the specimen obtained from the process B and C showed about 20 and 44 J of absorbed energy at room temperature, respectively. In the case of the process A, due to transformation after deformation, the microstructure consists of complete pearlite colonies. Because of such pearlite colonies, crack propagated normal to

Fig. 10. Fractured Charpy V-notch impact test specimens at 20 ◦ C: (a) process A, (b) process B, and (c) process C.

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4. Conclusions To investigate the effect of initial microstructure on change of mechanical properties of high carbon steel produced by the WCR, rolling experiments were carried out with three different kinds of initial microstructures. To evaluate improvement of the strength and toughness of the material, micro-hardness, tension, and impact tests were conducted. (1) WCR produced higher micro-hardness distribution and yield and tensile strength compared to the conventional hot rolling because of the smaller ferrite grain and interspacing between cementite particles induced by the WCR. (2) In the cases of the WCR, elongations up to fracture hardly decreased compared to the conventional hot rolling since the cementite particles were spheroidized and uniformly dispersed in the ferrite matrix. (3) Due to the dispersed cementite particles aligned to the rolling direction, the specimen processed by the WCR showed higher impact energy than the one of the conventional hot rolling. Especially, the specimen determined by the process C showed 4.4 times higher impact energy than the specimen determined by the process A and the highest toughness was obtained among the three cases. The mode of fracture varied depending on the initial microstructure as well. (4) The investigated WCR might be useful in obtaining the high strength material with better toughness without adding new alloying elements for industrial applications according to the present investigation. Acknowledgements This work was supported by the Ministry of Education, Science and Technology through the National Research Foundation (no. R0A-2006-000-10240-0) and POSCO without which this work was not possible. References [1] [2] [3] [4] [5] [6] [7] [8] [9] Fig. 11. Microstructures of rolled specimens in the plane perpendicular to the normal direction: (a) process A, (b) process B, and (c) process C.

the rolling direction in the impact test specimen as shown in Fig. 10(a). For the cases of the process B and C, the microstructure consists of spheroidized and dispersed cementite particles in the ferrite matrix due to deformation after transformation. Because of this microstructural change, straight crack propagations were not observed as shown in Fig. 10(b) and (c). Based on Fig. 11(b) and (c), the dispersed cementite particles aligned to the rolling direction were considered to obstruct the propagation of cracks in the normal direction, resulting in higher absorbed energy. Especially, for the case of the process C, lip shear was observed in Fig. 10(c) and the absorbed energy was the highest among the three cases.

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