Effects of Cooling Conditions on Microstructure, Tensile Properties ...

Effects of Cooling Conditions on Microstructure, Tensile Properties, and Charpy Impact Toughness of Low-Carbon High-Strength Bainitic Steels HYO KYUNG SUNG, SANG YONG SHIN, BYOUNGCHUL HWANG, CHANG GIL LEE, and SUNGHAK LEE In this study, four low-carbon high-strength bainitic steel specimens were fabricated by varying finish cooling temperatures and cooling rates, and their tensile and Charpy impact properties were investigated. All the bainitic steel specimens consisted of acicular ferrite, granular bainite, bainitic ferrite, and martensite-austenite constituents. The specimens fabricated with higher finish cooling temperature had a lower volume fraction of martensite-austenite constituent than the specimens fabricated with lower finish cooling temperature. The fast-cooled specimens had twice the volume fraction of bainitic ferrite and consequently higher yield and tensile strengths than the slow-cooled specimens. The energy transition temperature tended to increase with increasing effective grain size or with increasing volume fraction of granular bainite. The fastcooled specimen fabricated with high finish cooling temperature and fast cooling rate showed the lowest energy transition temperature among the four specimens because of the lowest content of coarse granular bainite. These findings indicated that Charpy impact properties as well as strength could be improved by suppressing the formation of granular bainite, despite the presence of some hard microstructural constituents such as bainitic ferrite and martensiteaustenite. DOI: 10.1007/s11661-012-1372-5 Ó The Minerals, Metals & Materials Society and ASM International 2012

I.

INTRODUCTION

RECENTLY, demands for developing new advanced steels, which can meet with the requirements of environmental-friendliness and excellent mechanical properties simultaneously, have been emphasized. To achieve these requirements, studies on improving functionality by optimizing many properties obtained from various microstructural combinations have been actively conducted.[1–3] For example, thick plates of high-strength high-toughness steels having bainitic and martensitic microstructures are utilized in large-scale constructions of linepipes, buildings, bridges, industrial plants, and ships.[3–5] The addition of alloying elements into these steel plates is a simple method to produce high-strength high-toughness steels, but alloying poses problems of deteriorating ductility and toughness as strength is improved. Recently, efforts have been made to realize HYO KYUNG SUNG, Research Assistant, is with Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang 790-784, Korea. SANG YONG SHIN, Research Professor, is with Department of Materials Science and Engineering, Pohang University of Science and Technology, Pohang 790-784, Korea. BYOUNGCHUL HWANG and CHANG GIL LEE, Senior Researchers, are with Ferrous Alloys Department, Korea Institute of Materials Science, Changwon 641-831, Korea. SUNGHAK LEE, Professor, is with Center for Advanced Aerospace Materials, Pohang University of Science and Technology, and also with Department of Materials Science and Engineering, Pohang University of Science and Technology. Contact e-mail: [email protected] Manuscript submitted February 24, 2012. Article published online August 21, 2012 294—VOLUME 44A, JANUARY 2013

a good combination of high strength and toughness by promoting fine low-temperature transformation microstructures such as bainite at rapid cooling rates, while the addition of alloying elements is minimized.[6–9] Among alloying elements, carbon is the most effective and economical element for strengthening,[10] and chromium homogeneously distributes bainitic microstructures, while refining grains and preventing the formation of martensite.[11,12] Niobium plays a role in improving strength and toughness by refining austenite grains and forming carbonitrides.[12,13] In addition to alloying methods, various complex and mixed microstructures can be formed by varying cooling conditions, and bainite or martensite is readily formed with increasing cooling rates.[7,14] As the finish cooling temperature (FCT) during rolling of bainitic steels decreases, the yield strength tends to increase because of the grain refinement effect and the increased volume fraction of low-temperature transformation microstructures. The yield strength is rather reduced, while the tensile strength increases, however, when the FCT decreases further below a certain temperature. This is closely related to the volume fraction of secondary phases such as martensite-austenite constituents.[15,16] When the cooling rate increases, the grain size decreases overall, and the volume fraction of bainitic ferrite increases, which leads to higher strength and lower ductility and toughness.[15–17] As the optimization of various cooling conditions can successfully produce high-strength and high-toughness steel plates having bainitic microstructures, it is necessary to systematically investigate the METALLURGICAL AND MATERIALS TRANSACTIONS A

effects of cooling conditions on microstructures and mechanical properties. In the current study, four low-carbon high-strength bainitic steels were fabricated by varying FCTs and cooling rates, and their tensile and Charpy impact properties were investigated. Effective grain sizes of various microstructures were analyzed by electron backscatter diffraction (EBSD) analysis to identify factors affecting Charpy impact properties. Based on these results, the microstructural evolution due to varied cooling conditions was understood, and the correlation between microstructural factors and tensile and Charpy impact properties was examined. Appropriate microstructures of low-carbon high-strength bainitic steels were also suggested.

II.

EXPERIMENTAL

A low-carbon high-strength bainitic steel whose chemical composition was 0.045C-0.25Si-1.9Mn-0.5 Ni-0.2Cr-0.25Mo-0.04Nb-0.04V-0.015Ti-0.03Al-0.001B (wt pct) was fabricated by a vacuum induction melting method. After austenitizing at 1423 K (1150 °C) for 1 hour, rolling was started at about 1353 K (1080 °C) and finished in the austenite region above the Ar3 (1123 K (850 °C)). An overall grain refinement effect was expected by rolling with a high rolling reduction ratio (88 pct) in the non-recrystallized region of austenite,[6] and the plate thickness was reduced from 100 m to 12 m. The rolled plates were air-cooled to the start cooling temperature of 1043 K (770 °C), were watercooled to 673 K (400 °C) or 773 K (500 °C) at a cooling rate of 25 or 30 K/s, and then were air-cooled. For convenience, the steels with FCT of 673 K (400 °C) or 773 K (500 °C) and cooling rate of 25 K/s are identified to as ‘‘S4’’ and ‘‘S5’’, respectively, and the steels the steels having FCT of 673 K (400 °C) or 773 K (500 °C) and cooling rate of 30 K/s are identified to as ‘‘F4’’ and ‘‘F5’’, respectively. The longitudinal–transverse (L–T) plane of the steel specimens was polished and etched with 2 pct nital solution, and microstructures were observed by an optical microscope and a scanning electron microscope (SEM, model; S-4300E, Hitachi, Tokyo, Japan). Volume fractions of microstructures were measured on SEM micrographs by means of an image analyzer. Electron back-scatter diffraction (EBSD) analysis (resolution: 0.2 lm) was conducted by a field emission scanning electron microscope (FE-SEM, model: S-4300SE, Hitachi, Japan).[18] The data were then interpreted by orientation imaging microscopy analysis software provided by TexSEM Laboratories, Inc. Tensile and Charpy impact specimens were collected from the 1/2 thickness location of the rolled plate. Spherical tensile specimens having a gage diameter of 6 mm and a gage length of 30 mm were prepared in the transverse direction, and were tested at room temperature at a strain rate of 0.003 s1 by means of a universal testing machine (model: Instron 5582, Instron, Canton, MA, USA) with 100 kN capacity. Tensile tests were conducted three times at least for each test condition, and the results were averaged. Standard Charpy V-notch METALLURGICAL AND MATERIALS TRANSACTIONS A

specimens [size: 10 9 10 9 55 mm, orientation: transverse–longitudinal (T–L)] were tested in the temperature range from 77 K (196 °C) to 298 K (25 °C) by means of a Tinius Olsen impact tester (model: FAHC-J-500-01, JT Toshi, Tokyo, Japan) of 500 J capacity. Based on the regression analysis data for absorbed impact energy,[19,20] the energy transition temperature (ETT) which corresponds to the average value of upper shelf energy (USE) and lower shelf energy was determined.

III.

RESULTS

A. Microstructure Optical and SEM micrographs of the four bainitic steel specimens (S4, S5, F4, and F5 specimens) are shown in Figures 1(a) through (d) and 2(a) through (d), respectively. All the specimens are basically composed of (AF), granular bainite (GB), bainitic ferrite (BF), and martensite-austenite (MA) constituents.[12,21,22] These microstructural constituents are marked in the optical and SEM micrographs, and their approximate volume fractions were roughly measured, as provided in Table I. In terms of shapes and characteristics of microstructures, AF is generally characterized by irregular-shape, fine grain size, and arbitrary directional alignment.[21–23] GB is an equiaxed microstructure, and contains islandtype MA constituents.[2,12] BF is identified differently by researchers,[2,7,12] depending on the presence or distribution of secondary phases such as MA, retained austenite (RA), and cementite.[22] Each microstructure is classified by these morphological categories. As the differentiation between BF and MA was difficult to obtain in optical and SEM micrographs, the specimens were etched in a LePera solution.[24] Optical micrographs of the S5 and F5 specimens etched in a LePera solution are shown in Figures 3(a) and (b), respectively. Bainitic microstructures (BF, AF, and GB) and MA were colored in brown and white, respectively. The volume fractions of MA are about 2.3 pct and 1.8 pct in the S5 and F5 specimens, respectively, and the rest of MA shows bainitic microstructures of AF, BF, and GB. All the specimens are composed mostly of AF, together with small amounts of GB, BF, and MA, as shown in Table I. The S4 specimen contains about 70 vol pct of AF, together with 20 vol pct of GB and 10 vol pct of BF. The volume fractions of microstructures of the S5 specimen are similar to those of the S4 specimen, although the volume fraction of MA is lower in the S5 specimen. In the fast-cooled F4 and F5 specimens, considerable amounts of BF (about 20 pct), twice as large as the amounts in the slow-cooled S4 and S5 specimens, are formed. The volume fraction of MA tends to be lower in the F4 and F5 specimens than in the S4 and S5 specimens, respectively. The EBSD analysis data are shown in Figures 4(a) through (d). Inverse pole figure (IPF) maps can be represented in different colors, depending on the orientation of each point. The boundaries between grains having different orientations of 15 deg or higher are high-angle boundaries, and these VOLUME 44A, JANUARY 2013—295

(a)

S4

(b)

S5

10µ µm

(c)

F4

10µm

(d)

F5

10µm

10µm

Fig. 1—Optical micrographs (L–T plane) of the (a) S4, (b) S5, (c) F4, and (d) F5 specimens. Nital etched.

(a)

S4

(b)

S5

10 µm

(c)

F4

10 µm

(d)

F5

10 µm

10 µm

Fig. 2—SEM micrographs (L–T plane) of the (a) S4, (b) S5, (c) F4, and (d) F5 specimens. Nital etched. 296—VOLUME 44A, JANUARY 2013

METALLURGICAL AND MATERIALS TRANSACTIONS A

Table I. Volume Fractions of Acicular Ferrite, Granular Bainite, Bainitic Ferrite, and Martensite-Austenite Constituents and Effective Grain Size in the Low-Carbon High-Strength Steel Specimens Volume Fraction (Percent) Acicular Ferrite (AF)

Steel Specimen S4 S5 F4 F5

64.5 67.3 69.1 48.8

± ± ± ±

8.8 5.7 4.5 8.9

Granular Bainite (GB) 23.0 20.2 9.2 29.8

± ± ± ±

Bainitic Ferrite (BF)

6.1 3.3 1.9 5.0

9.6 10.2 19.3 19.6

± ± ± ±

2.8 2.6 3.5 3.6

Martensite-Austenite Constituents (MA) 2.9 2.3 2.4 1.8

± ± ± ±

Effective Grain Size* (lm)

1.1 0.8 0.8 0.9

15.7 16.8 13.2 19.7

± ± ± ±

2.6 2.9 2.2 2.3

*The effective grain size was measured from the EBSD analysis.

(a)

S4

(b)

10µ µm

(c)

F4

S5

10µm

(d)

10µm

F5

10µm

Fig. 3—Optical micrographs (L–T plane) of the (a) S5 and (b) F5 specimens. Etched in a LePera solution.[24]

grains are generally considered to be effective grains.[25,26] The effective grain size results are shown in Table I. The grain size is the smallest (13.2 lm) in the F4 specimen, and increases in the order of the S4, S5, and F5 specimens. B. Tensile and Charpy Impact Properties Figure 5 shows room-temperature stress–strain curves, and tensile properties obtained from them are listed in Table II. All the specimens show the continuous yielding behavior, and their yield strength, ultimate tensile strength, and elongation are present in the ranges of 700 through 800, 850 through 950 MPa, and 14 METALLURGICAL AND MATERIALS TRANSACTIONS A

through 18 pct, respectively. The strengths of the fastcooled F4 and F5 specimens are higher than those of the slow-cooled S4 and S5 specimens because the volume fraction of BF is higher in the F4 and F5 specimens. The S4 and F4 specimens having FCT of 673 K (400 °C) have the higher strengths than the S5 and F5 specimens having FCT of 773 K (500 °C). Figure 6 shows the Charpy absorbed energy data as a function of test temperature, from which the USE and ETT were obtained as listed in Table II. The USEs of all the specimens are higher than 240 J, but the USEs of the slow-cooled S4 and S5 specimens tend to be slightly higher than those of the fast-cooled F4 and F5 specimens. The VOLUME 44A, JANUARY 2013—297

(a)

S4

(b)

S5

F4

20

20

20

(c)

F5

(d)

20

Fig. 4—Inverse pole figure (IPF) color maps of the (a) S4, (b) S5, (c) F4, and (d) F5 specimens.

S4 S5 F4 F5

1000

Stress (MPa)

800

600

400

200

0

0

5

10

15

20

25

Strain (Pct) Fig. 5—Stress-strain curves obtained from the room-temperature tensile test of the (a) S4, (b) S5, (c) F4, and (d) F5 specimens.

ETT is the lowest in the fast-cooled F4 specimen, and increases in the order of the S5, S4, and F5 specimens.

IV.

DISCUSSION

In the low-carbon high-strength bainitic steel specimens considered for this study, microstructures having 298—VOLUME 44A, JANUARY 2013

different volume fractions of AF, GB, BF, and MA are formed with varying cooling conditions. With increasing cooling rate, grains are generally refined in conventional steels as the grain growth is constrained, and the volume fraction of MA increases as the martensite start temperature (Ms) is rapidly reached.[6] In the bainitic steels, the volume fractions of GB and BF formed at lower temperatures than AF increase under fast cooling rates, but the low-temperature toughness decreases as GB and BF have larger effective grain size than AF.[15] In the bainitic steel specimens of the current study, AF and GB are mostly formed during cooling, and the grain growth occurs without forming new phases after cooling. At high FCTs, the time for grain growth is extended, and the austenite region supersaturated with Mn and C atoms becomes smaller than in the case of low FCTs. When the austenite cools below the Ms, it transforms mostly to secondary phases, and thus the final structure contains secondary phases such as MA. Thus, the volume fraction of MA is lower in the case of high FCT than in the case of low FCT.[6,27] The F4 and F5 specimens fabricated under the fast cooling rate have the twice the volume fraction of BF than the S4 and S5 specimens (Table I). Between the F4 and F5 specimens rolled at the same cooling rate, the F5 specimen has the lower volume fraction of MA than the F4 specimen. This is because the formation of MA, which is transformed at lower temperatures than AF, is METALLURGICAL AND MATERIALS TRANSACTIONS A

Table II.

Steel Specimen

Tensile and Charpy Impact Properties of the Low-Carbon High-Strength Steel Specimens Ultimate Tensile Strength (MPa)

Yield Strength (MPa)

S4 S5 F4 F5

709 687 806 789

± ± ± ±

1 29 4 17

854 849 953 891

± ± ± ±

Total Elongation (Pct)

2 3 1 6

17.5 15.2 14.1 17.3

Charpy Absorbed Energy (J)


0.5 0.4 0.2 0.7

Energy Transition Temperature (K (°C)

253 266 242 241

172 (101) 165 (108) 162 (111) 175 (98)

300

300 250 200 150 100 50 0 50

± ± ± ±

Upper Shelf Energy (J)

100

150

200

250

300

250 200 150 100 50 0 50

100

Temperature (K)

150

(a)

300

250

300

300



250

(b)

300 250 200 150 100 50 0 50

200

Temperature (K)

100

150

200

250

300

250 200 150 100 50 0 50

100

150

200

Temperature (K)

Temperature (K)

(c)

(d)

Fig. 6—Charpy absorbed energy of the (a) S4, (b) S5, (c) F4, and (d) F5 specimens as a function of test temperature.

delayed by the higher FCT. The S5 specimen also has the lower volume fraction of MA than the S4 specimen under the same cooling condition. Consequently, the F4 specimen shows higher volume fractions of BF and MA because the AF formation is delayed by rapidly passing the start and finish temperatures of AF transformation during cooling. The strength generally increases when hard phases are populated or grains are refined.[13,28–30] As hard secondary phases such as MA are readily transformed at low temperatures, they show high strength. BF has higher


strength than AF as it has high dislocation density and well-developed lath boundaries because BF is formed at faster cooling rates and lower temperatures than AF.[2,12] The F4 and F5 specimens have higher volume fraction of BF than the S4 and S5 specimens which show the yield strength of about 800 MPa and the tensile strength of about 900 MPa. The F4 specimen containing higher volume fraction of MA as well as BF shows the highest yield and tensile strengths. In view of Charpy impact properties, the USE is mostly influenced by the volume fraction of various

VOLUME 44A, JANUARY 2013—299

boundaries,[2,12] and thus raise the effective grain size, which consequently leads to the increase in ETT.[33] The F4 specimen shows the lowest ETT among the four specimens because of the least content of coarse GBs. This indicates the close relation between the ETT and volume fraction of GB. The fractions of GB of the S4 and S5 specimens are similar at about 10 pct, but the MA fraction is higher in the S4 specimen than in the S5 specimen (Table I). The large content of hard MAs in the S4 specimen might be associated with the higher ETT than in the S5 specimen. As the volume fraction of AF increases, the ETT decreases, which shows the opposite trend of volume fraction of GB (Figure 7(c)). Thus, it is necessary to increase the volume fraction of fine AF and to prevent the formation of GB to have the low ETT. To examine the cleavage crack propagation path, the cross-sectional area beneath the fracture surface of the Charpy specimen fractured at 77 K (196 °C) was observed by an SEM after the fracture surface was coated by nickel. SEM micrographs of the fine-grained F4 specimen and the F5 specimen containing many

microstructural phases, whereas the ETT is mostly affected by the effective grain size,[3,31] and increases with increasing volume fraction of phases having good toughness.[32] For example, all the specimens show high USE above 240 J when their volume fraction of AF is higher than 50 pct, as shown in Tables I and II. Effects of microstructural factors such as grain size (d) on ETT are presented by the following equation:[33] ETT ¼ fðcompositionÞ þ gðstrengthÞ 11:5 d1=2 ½1

Energy Transition Temperature (K)

where f(composition) and g(strength) are the functions of chemical composition and strength, respectively. This equation indicates that the ETT increases with increasing hardenability elements, strength, and grain size. Figures 7(a) through (c) show the correlations between ETT and effective grain size, volume fraction of GB, and volume fraction of AF, respectively. The ETT tends to increase with increasing effective grain size or with increasing volume fraction of GB (Figures 7(a) and (b)). GBs in the F5 specimen have large effective grains due to the formation of many low-angle grain

S4 S5 F4 F5

175

170

165

160

12

13

14

15

16

17

18

19

20

21

Effective Grain Size (µm)



(a)

S4 S5 F4 F5

175

170

165

160

5

10

15

20

25

30

35

S4 S5 F4 F5

175

170

165

160

45

50

55

60

65

70

Volume Fraction of GB (Pct)

Volume Fraction of AF (Pct)

(b)

(c)

75

Fig. 7—Energy transition temperature (ETT) as a function of (a) effective grain size, (b) volume fraction of granular bainite, and (c) volume fraction of acicular ferrite. 300—VOLUME 44A, JANUARY 2013


(a)

F4

10 µm

(b)

F5

10 µm

Fig. 8—SEM micrographs of the cross-sectional area beneath the cleavage fracture surface of the Charpy impact specimens fractured at 77 K (196 °C) for the (a) F4 and (b) F5 specimens, showing the crack propagation path. The fractured surface was coated by Ni.

coarse GBs are shown in Figures 8(a) and (b), respectively. Points at which the cleavage crack propagation is bent are indicated by arrows. As the F4 specimen consists mainly of fine AF regions, the crack propagation is bent frequently, and thus the unit crack path is short below 2 to 5 lm (Figure 8(a)). The unit crack path is long (about 10 lm) in areas where coarse GBs are formed in the F5 specimen (Figure 8(b)). This result confirms the close relation between the unit cleavage crack path, effective grain size, and ETT. In the current study, the volume fractions of AF, GB, BF, and MA and their effective grain sizes are varied by controlling cooling rates and FCTs, and the effects of these microstructures on tensile and Charpy impact properties are investigated. The F4 specimen has the Charpy impact properties because the resistance to crack propagation is improved as it has the smallest effective grain size due to the least content of coarse GBs. These findings indicate that the low-temperature impact toughness as well as strength can be improved by suppressing the formation of GB, despite the presence of some hard microstructures such as BF and MA. To further develop economic and environmentally friendly bainitic steels having excellent mechanical properties, more systematic studies on alloying compositions and cooling conditions and on mechanisms are needed.

V.

CONCLUSIONS

In this study, four low-carbon high-strength bainitic steels were fabricated by varying FCTs and cooling rates, and their microstructures were analyzed to investigate effects of cooling conditions on tensile and Charpy impact properties. 1. All the bainitic steel specimens consisted mostly of acicular ferrite (AF), granular bainite (GB), bainitic ferrite (BF), and martensite-austenite (MA) constituents. The fast-cooled F4 and F5 specimens had twice the volume fraction of BF than the slow-cooled S4 and S5 specimens. The S5 and F5 specimens had lower volume fraction of MA than hat of the S4 and F4 specimens because the formation of MA was delayed by the higher FCT. METALLURGICAL AND MATERIALS TRANSACTIONS A

2. The fast-cooled F4 and F5 specimens having higher volume fraction of BF than that of the slow-cooled S4 and S5 specimens showed a yield strength of about 800 MPa and a tensile strength of about 900 MPa, which were higher than those of the S4 and S5 specimens. In particular, the F4 specimen containing a higher volume fraction of MA as well as BF showed the highest yield and tensile strengths. 3. The energy transition temperature (ETT) tended to increase with increasing effective grain size or with increasing volume fraction of GB. GB regions in the F5 specimen had large effective grain size because of the formation of many low-angle grain boundaries, and thus raised the effective grain size, which consequently led to the increase in ETT. The F4 specimen showed the lowest ETT among the four specimens because of the least content of coarse GBs. These findings indicated that the lowtemperature impact toughness as well as strength could be improved by suppressing the formation of GB, despite the presence of some hard microstructural constituents such as BF and MA. ACKNOWLEDGMENTS This study was supported by the Ministry of Knowledge Economy under a grant No. 100400-25.

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