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Improvement of toughness and strength balance in low-carbon steel bars with cube texture processed by warm bi-axial rolling
Materials Letters 240 (2019) 172–175
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Improvement of toughness and strength balance in low-carbon steel bars with cube texture processed by warm bi-axial rolling Tadanobu Inoue ⇑, Rintaro Ueji, Yuuji Kimura National Institute for Materials Science, 1-2-1, Sengen, Tsukuba 305-0047, Japan
a r t i c l e
i n f o
Article history: Received 26 November 2018 Received in revised form 19 December 2018 Accepted 3 January 2019 Available online 8 January 2019 Keywords: Fracture energy Bi-axial rolling Steels Nanocrystalline materials Toughening Texture
a b s t r a c t A novel rolling process characterized by multi-pass bi-axial reduction was proposed. An 800 MPa class low-carbon steel bar with ultrafine elongated-grain structures was fabricated using bi-axial rolling at a warm working temperature, and tensile and three-point bending tests were conducted at a temperature range from 77 K to 473 K. The texture in the developed steel bar was dominated by {0 0 1}h0 0 1i cube orientations. The toughness–strength balance of the developed steel was remarkably improved compared with that of conventional 0.15%C ferrite–pearlite steel and 0.29%C martensitic steel. The toughness reached its maximum when crack branching started to appear upon decreasing the test temperature, and the specimen did not separate into two pieces even at low temperature of 77 K. A microstructural design that improves toughness in steel was demonstrated. Ó 2019 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
The strengthening of materials is important to reduce weight in transportation. However, since toughness to ensure safety is lost by strengthening, the idea for improving strength–toughness balance is always sought. Although it was believed that the refinement of crystal grains is an effective method for developing this balance in materials without the addition of alloying elements, it has been reported that the balance is not always improved by grain refinement alone [1–3]. For ultrafine-grained (UFG) materials created through a severe plastic deformation process, improvement in toughness is achieved by controlling the texture rather than grain size [2,4–6]. In the present study, a novel rolling process characterized by multi-pass bi-axial reduction was proposed to create 800 MPa class low-carbon steel bars. The steel bar had an ultrafine elongated-grain (UFEG) structure characterized by a {0 0 1}h0 0 1i cube texture. The tensile and three-point bending tests were conducted at a temperature range from 77 K to 473 K. The strength– toughness balance in the UFEG steel bar with cube texture was compared with that in conventional ferrite–pearlite steel and martensitic steel.
A low-carbon steel with a chemical composition of 0.15 C, 0.3 Si, 1.5 Mn and the balance Fe (all in mass%) was used in this study. A 20 kg ingot was prepared by vacuum melting and casting, homogenized at 1473 K, and then hot rolled to a 40 mm square bar. A hotrolled bar was cut to 110 mm in length, heated to 1173 K, and held for 1 h followed by water quenching. The bar was soaked at a warm temperature of 823 K for 1 h and was then subjected to a rolling simulator with a roll diameter of 300 mm. A square bar was rolled without any lubricant and then rolled from another plane, as shown in Fig. 1a; that is, the reduction direction was changed in increments of 90° by rotating the bar. This process was repeated many times. The total reduction in area through multi-pass warm bi-axial rolling (WBR) was about 89% in 24 passes. Eventually, a 13 mm square bar (WBR sample) was produced. The bar was composed of a UFEG structure, and the average transverse grain size, dt, was 1.2 lm. For comparison, two conventional steels were prepared: a lowcarbon steel (SM sample) with a ferrite–pearlite structure and a quenched and tempered steel (QT sample) with a martensitic structure. The SM sample with a chemical composition of 0.15 C, 0.3 Si and 1.5 Mn was heated to 1173 K and held for 1 h, followed by air cooling [3]. The QT sample (0.29 C, 0.3 Si, 1.5 Mn) in the form of a 14 mm square bar was solution-treated at 1223 K for 0.5 h, fol-
⇑ Corresponding author. E-mail address: [email protected] (T. Inoue). https://doi.org/10.1016/j.matlet.2019.01.007 0167-577X/Ó 2019 Elsevier B.V. All rights reserved.
T. Inoue et al. / Materials Letters 240 (2019) 172–175
173
Fig. 1. Schematic drawing of (a) the proposed bi-axial rolling and (b) the mechanical test specimens taken from the rolled bar.
lowed by oil quenching. Next, the bar was tempered at 773 K for 1 h and then water-cooled. Samples in all mechanical tests were taken from the center of the rolled bars (Fig. 1b), and the tests were conducted at a temperature range from 77 K to 473 K. Tensile tests were conducted with a crosshead speed of 0.85 mm/min using specimens with a round cross section of 6 mm and a gage length of 30 mm as shown in Fig. 1b. A three-point bending test with a support distance of 40 mm at a crosshead speed of 0.5 mm min1 was conducted. A notch with a depth of 5 mm and a root radius of 0.13 lm was introduced using electro-discharge machining in rectangular bars of 10W 10B 55L mm shown in Fig. 1b. Nonlinear fracture mechanics methods based on the ASTM Standard E1820-01 were used to evaluate the fracture toughness. The apparent fracture energy, J, was calculated using the following formula:
J¼
2Apl 2 ¼ Bb Bb
Z
uF
P du
kJ=m2
ð1Þ
0
where Apl is the area under the bending load P–displacement u curve, b is the ligament length of 5 mm, B is the specimen width of 10 mm and uF denotes the displacement at which the test was terminated. Details of the mechanical tests and microstructural observations were given previously [3]. 3. Results and discussion Fig. 2a shows the EBSD maps along the ND for the WBR sample and the (0 0 1) pole figures. The texture was dominated by {0 0 1}h 0 0 1i cube orientations. The fraction of the cube texture was approximately 26% under a tolerance angle of 15°. The insets in Fig. 2b show that the {0 0 1}h0 0 1i grains have a cleavage plane parallel to the RD and the LD. In addition, the boundaries of UFEGs lying vertical to the initial crack orientation are existent. Generally,
the crack propagates parallel to the LD from the stress fields near the initial notch. However, in the WBR sample, the brittle fracture stresses, rF, which increase with a reduction in the grain size, have a relation of rF(RD) rF(LD) due to the UFEG structure, dt dL. Hence, it can be expected that a crack related to a brittle fracture preferentially propagates at these weak sites, {1 0 0} cleavage planes and grain boundaries, that are aligned in the RD. Fig. 3 shows the P–u curves at 77 K for the WBR, SM and QT samples and the appearance of the WBR and QT samples after the test. The SM and QT samples fractured immediately with peak loading, Pmax, and the cracks propagated directly across the center portion of the specimen. In other words, the conventional steels exhibited typical brittle fracture behavior at 77 K. On the other hand, in the WBR sample, the curve exhibited a unique change. The P sharply dropped after it attained Pm beyond two load drops, P1 and P2, became almost constant, and decreased again thereafter. Such a plateau region results from the delamination caused by crack branching on the aligned weak planes parallel to the RD. In fact, as shown in Fig. 3b, the crack propagated vertically to the LD and the specimen did not separate into two pieces. A similar plateau region resulting from the delamination is observed in the Charpy impact test for UFG low-carbon steel processed by warm caliber rolling [5] and a three-point bending test for the laminate composite [7]. Note that such behavior was not observed at 293 K due to ductile fracture (Fig. 3d), and it was observed below 173 K (Fig. 3c). Fig. 4 shows the correlation between yield stress and toughness. Here, the number in the figure denotes the test temperature and the curves denote J rys. The SM and QT samples exhibited a typical strength–toughness balance curve against temperature. The region, near 173 K, in which the J sharply drops with increasing rys in both samples, corresponds to the ductile-to-brittle transition temperature (DBTT); hence, the fracture shifts from a ductile to a brittle manner in that region. On the other hand, the WBR sample
174
T. Inoue et al. / Materials Letters 240 (2019) 172–175
Fig. 2. (a) Orientation map along the ND on the cross-sectional plane parallel to the RD for the WBR sample and the (0 0 1) pole figure. Here, grain boundaries with a misorientation above 15° are indicated by black lines. (b) Schematic illustration of weak interfaces, cleavage planes and grain boundaries for the elongated grains with a cube texture.
Fig. 3. (a) Bending load P–displacement u curves at 77 K. (b–e) Appearance of sample after the bending test.
showed an unusual strength–toughness balance, and the J rys became larger than that in the SM and QT samples at all temperatures. The J reaches its maximum at 173–123 K, which corresponds to the DBTT in the SM and QT samples, and decreases thereafter. However, even if the temperature was 77 K, the WBR sample did not exhibit low energy because of delamination toughening as
shown in Fig. 3b. When the temperature was over 233 K, the fracture was ductile and the crack propagated directly across the center portions of the specimen (Fig. 3d). When the temperature was low, i.e., as rys increased, a significant delaminating crack propagated along the longitudinal direction, as shown in Fig. 3b and c. Namely, the fracture shifted from ductile to delamination. Conse-
T. Inoue et al. / Materials Letters 240 (2019) 172–175
175
steel was remarkably improved at all temperature from 77 K to 473 K, compared with the conventionally quenched and tempered 0.29%C steel with a martensitic structure and the 0.15%C steel with a ferrite–pearlite structure. A strong texture is developed by plastic deformation at warm working temperatures and a finer grain microstructure provides a higher density of grain boundaries. For the toughness design in ultrafine elongated-grained steel, it is important to understand the spatial distribution of the weak sites such as the {1 0 0} cleavage planes and grain boundaries. Conflict of interest None. Acknowledgements
Fig. 4. Fracture energy J vs. yield stress rys at a temperature range from 77 K to 473 K. Here, rys denotes 0.2% proof stress.
We thank S. Kuroda, K. Iida, and T. Hibaru for material processing and E. Yasuda and Y. Kashihara for their assistance with the microstructural observations and illustrations. This study was partly supported by grant from JSPS KAKENHI, Japan, Grant Number 26249107. The grant is much appreciated. References
quently, the WBR sample exhibited an excellent strength–toughness balance. Incidentally, the J shows approximately 45% variation for two samples at 77 K in the WBR sample. This deviation may be caused by a difference in crack propagation behavior. We plan to examine this point by additional experiments. 4. Conclusions Multi-pass warm bi-axial rolling was proposed, and an 800 MPa class 0.15%C steel with an ultrafine elongated grain structure was fabricated. The steel was characterized by a {0 0 1}h0 0 1i cube texture. As a result, the toughness–strength balance of the developed
[1] A. Hohenwarter, C. Kammerhofer, R.J. Pippan, J. Mater. Sci. 45 (2010) 4805– 4812. [2] S.M. Toker, F. Rubitschek, T. Niendorf, C. Canadinc, H.J. Maier, Scripta Mater. 66 (2012) 435–438. [3] T. Inoue, Chapter 5: Toughening of low-alloy steel by ultrafine-grained structure (Development of fracture control from microstructure design), in: L.M. Alves (Ed.), Fracture Mechanics, InTech, Croatia, 2016, pp. 103–120, https://doi.org/ 10.5772/63797. [4] Y. Kimura, T. Inoue, F. Yin, K. Tsuzaki, Science 320 (2008) 1057–1060. [5] T. Inoue, F. Yin, Y. Kimura, K. Tsuzaki, S. Ochiai, Metall. Mater. Trans. 40A (2010) 341–355. [6] T. Inoue, Y. Kimura, J. Mater. Sci. 48 (2007) 4766–4772. [7] M. Pozuelo, F. Carreno, O.A. Ruano, Compos. Sci. Technol. 66 (2006) 2671–2676.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Improvement of toughness and strength balance in low-carbon steel bars with cube texture processed by warm bi-axial rolling Tadanobu Inoue ⇑, Rintaro Ueji, Yuuji Kimura National Institute for Materials Science, 1-2-1, Sengen, Tsukuba 305-0047, Japan
a r t i c l e
i n f o
Article history: Received 26 November 2018 Received in revised form 19 December 2018 Accepted 3 January 2019 Available online 8 January 2019 Keywords: Fracture energy Bi-axial rolling Steels Nanocrystalline materials Toughening Texture
a b s t r a c t A novel rolling process characterized by multi-pass bi-axial reduction was proposed. An 800 MPa class low-carbon steel bar with ultrafine elongated-grain structures was fabricated using bi-axial rolling at a warm working temperature, and tensile and three-point bending tests were conducted at a temperature range from 77 K to 473 K. The texture in the developed steel bar was dominated by {0 0 1}h0 0 1i cube orientations. The toughness–strength balance of the developed steel was remarkably improved compared with that of conventional 0.15%C ferrite–pearlite steel and 0.29%C martensitic steel. The toughness reached its maximum when crack branching started to appear upon decreasing the test temperature, and the specimen did not separate into two pieces even at low temperature of 77 K. A microstructural design that improves toughness in steel was demonstrated. Ó 2019 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
The strengthening of materials is important to reduce weight in transportation. However, since toughness to ensure safety is lost by strengthening, the idea for improving strength–toughness balance is always sought. Although it was believed that the refinement of crystal grains is an effective method for developing this balance in materials without the addition of alloying elements, it has been reported that the balance is not always improved by grain refinement alone [1–3]. For ultrafine-grained (UFG) materials created through a severe plastic deformation process, improvement in toughness is achieved by controlling the texture rather than grain size [2,4–6]. In the present study, a novel rolling process characterized by multi-pass bi-axial reduction was proposed to create 800 MPa class low-carbon steel bars. The steel bar had an ultrafine elongated-grain (UFEG) structure characterized by a {0 0 1}h0 0 1i cube texture. The tensile and three-point bending tests were conducted at a temperature range from 77 K to 473 K. The strength– toughness balance in the UFEG steel bar with cube texture was compared with that in conventional ferrite–pearlite steel and martensitic steel.
A low-carbon steel with a chemical composition of 0.15 C, 0.3 Si, 1.5 Mn and the balance Fe (all in mass%) was used in this study. A 20 kg ingot was prepared by vacuum melting and casting, homogenized at 1473 K, and then hot rolled to a 40 mm square bar. A hotrolled bar was cut to 110 mm in length, heated to 1173 K, and held for 1 h followed by water quenching. The bar was soaked at a warm temperature of 823 K for 1 h and was then subjected to a rolling simulator with a roll diameter of 300 mm. A square bar was rolled without any lubricant and then rolled from another plane, as shown in Fig. 1a; that is, the reduction direction was changed in increments of 90° by rotating the bar. This process was repeated many times. The total reduction in area through multi-pass warm bi-axial rolling (WBR) was about 89% in 24 passes. Eventually, a 13 mm square bar (WBR sample) was produced. The bar was composed of a UFEG structure, and the average transverse grain size, dt, was 1.2 lm. For comparison, two conventional steels were prepared: a lowcarbon steel (SM sample) with a ferrite–pearlite structure and a quenched and tempered steel (QT sample) with a martensitic structure. The SM sample with a chemical composition of 0.15 C, 0.3 Si and 1.5 Mn was heated to 1173 K and held for 1 h, followed by air cooling [3]. The QT sample (0.29 C, 0.3 Si, 1.5 Mn) in the form of a 14 mm square bar was solution-treated at 1223 K for 0.5 h, fol-
⇑ Corresponding author. E-mail address: [email protected] (T. Inoue). https://doi.org/10.1016/j.matlet.2019.01.007 0167-577X/Ó 2019 Elsevier B.V. All rights reserved.
T. Inoue et al. / Materials Letters 240 (2019) 172–175
173
Fig. 1. Schematic drawing of (a) the proposed bi-axial rolling and (b) the mechanical test specimens taken from the rolled bar.
lowed by oil quenching. Next, the bar was tempered at 773 K for 1 h and then water-cooled. Samples in all mechanical tests were taken from the center of the rolled bars (Fig. 1b), and the tests were conducted at a temperature range from 77 K to 473 K. Tensile tests were conducted with a crosshead speed of 0.85 mm/min using specimens with a round cross section of 6 mm and a gage length of 30 mm as shown in Fig. 1b. A three-point bending test with a support distance of 40 mm at a crosshead speed of 0.5 mm min1 was conducted. A notch with a depth of 5 mm and a root radius of 0.13 lm was introduced using electro-discharge machining in rectangular bars of 10W 10B 55L mm shown in Fig. 1b. Nonlinear fracture mechanics methods based on the ASTM Standard E1820-01 were used to evaluate the fracture toughness. The apparent fracture energy, J, was calculated using the following formula:
J¼
2Apl 2 ¼ Bb Bb
Z
uF
P du
kJ=m2
ð1Þ
0
where Apl is the area under the bending load P–displacement u curve, b is the ligament length of 5 mm, B is the specimen width of 10 mm and uF denotes the displacement at which the test was terminated. Details of the mechanical tests and microstructural observations were given previously [3]. 3. Results and discussion Fig. 2a shows the EBSD maps along the ND for the WBR sample and the (0 0 1) pole figures. The texture was dominated by {0 0 1}h 0 0 1i cube orientations. The fraction of the cube texture was approximately 26% under a tolerance angle of 15°. The insets in Fig. 2b show that the {0 0 1}h0 0 1i grains have a cleavage plane parallel to the RD and the LD. In addition, the boundaries of UFEGs lying vertical to the initial crack orientation are existent. Generally,
the crack propagates parallel to the LD from the stress fields near the initial notch. However, in the WBR sample, the brittle fracture stresses, rF, which increase with a reduction in the grain size, have a relation of rF(RD) rF(LD) due to the UFEG structure, dt dL. Hence, it can be expected that a crack related to a brittle fracture preferentially propagates at these weak sites, {1 0 0} cleavage planes and grain boundaries, that are aligned in the RD. Fig. 3 shows the P–u curves at 77 K for the WBR, SM and QT samples and the appearance of the WBR and QT samples after the test. The SM and QT samples fractured immediately with peak loading, Pmax, and the cracks propagated directly across the center portion of the specimen. In other words, the conventional steels exhibited typical brittle fracture behavior at 77 K. On the other hand, in the WBR sample, the curve exhibited a unique change. The P sharply dropped after it attained Pm beyond two load drops, P1 and P2, became almost constant, and decreased again thereafter. Such a plateau region results from the delamination caused by crack branching on the aligned weak planes parallel to the RD. In fact, as shown in Fig. 3b, the crack propagated vertically to the LD and the specimen did not separate into two pieces. A similar plateau region resulting from the delamination is observed in the Charpy impact test for UFG low-carbon steel processed by warm caliber rolling [5] and a three-point bending test for the laminate composite [7]. Note that such behavior was not observed at 293 K due to ductile fracture (Fig. 3d), and it was observed below 173 K (Fig. 3c). Fig. 4 shows the correlation between yield stress and toughness. Here, the number in the figure denotes the test temperature and the curves denote J rys. The SM and QT samples exhibited a typical strength–toughness balance curve against temperature. The region, near 173 K, in which the J sharply drops with increasing rys in both samples, corresponds to the ductile-to-brittle transition temperature (DBTT); hence, the fracture shifts from a ductile to a brittle manner in that region. On the other hand, the WBR sample
174
T. Inoue et al. / Materials Letters 240 (2019) 172–175
Fig. 2. (a) Orientation map along the ND on the cross-sectional plane parallel to the RD for the WBR sample and the (0 0 1) pole figure. Here, grain boundaries with a misorientation above 15° are indicated by black lines. (b) Schematic illustration of weak interfaces, cleavage planes and grain boundaries for the elongated grains with a cube texture.
Fig. 3. (a) Bending load P–displacement u curves at 77 K. (b–e) Appearance of sample after the bending test.
showed an unusual strength–toughness balance, and the J rys became larger than that in the SM and QT samples at all temperatures. The J reaches its maximum at 173–123 K, which corresponds to the DBTT in the SM and QT samples, and decreases thereafter. However, even if the temperature was 77 K, the WBR sample did not exhibit low energy because of delamination toughening as
shown in Fig. 3b. When the temperature was over 233 K, the fracture was ductile and the crack propagated directly across the center portions of the specimen (Fig. 3d). When the temperature was low, i.e., as rys increased, a significant delaminating crack propagated along the longitudinal direction, as shown in Fig. 3b and c. Namely, the fracture shifted from ductile to delamination. Conse-
T. Inoue et al. / Materials Letters 240 (2019) 172–175
175
steel was remarkably improved at all temperature from 77 K to 473 K, compared with the conventionally quenched and tempered 0.29%C steel with a martensitic structure and the 0.15%C steel with a ferrite–pearlite structure. A strong texture is developed by plastic deformation at warm working temperatures and a finer grain microstructure provides a higher density of grain boundaries. For the toughness design in ultrafine elongated-grained steel, it is important to understand the spatial distribution of the weak sites such as the {1 0 0} cleavage planes and grain boundaries. Conflict of interest None. Acknowledgements
Fig. 4. Fracture energy J vs. yield stress rys at a temperature range from 77 K to 473 K. Here, rys denotes 0.2% proof stress.
We thank S. Kuroda, K. Iida, and T. Hibaru for material processing and E. Yasuda and Y. Kashihara for their assistance with the microstructural observations and illustrations. This study was partly supported by grant from JSPS KAKENHI, Japan, Grant Number 26249107. The grant is much appreciated. References
quently, the WBR sample exhibited an excellent strength–toughness balance. Incidentally, the J shows approximately 45% variation for two samples at 77 K in the WBR sample. This deviation may be caused by a difference in crack propagation behavior. We plan to examine this point by additional experiments. 4. Conclusions Multi-pass warm bi-axial rolling was proposed, and an 800 MPa class 0.15%C steel with an ultrafine elongated grain structure was fabricated. The steel was characterized by a {0 0 1}h0 0 1i cube texture. As a result, the toughness–strength balance of the developed
[1] A. Hohenwarter, C. Kammerhofer, R.J. Pippan, J. Mater. Sci. 45 (2010) 4805– 4812. [2] S.M. Toker, F. Rubitschek, T. Niendorf, C. Canadinc, H.J. Maier, Scripta Mater. 66 (2012) 435–438. [3] T. Inoue, Chapter 5: Toughening of low-alloy steel by ultrafine-grained structure (Development of fracture control from microstructure design), in: L.M. Alves (Ed.), Fracture Mechanics, InTech, Croatia, 2016, pp. 103–120, https://doi.org/ 10.5772/63797. [4] Y. Kimura, T. Inoue, F. Yin, K. Tsuzaki, Science 320 (2008) 1057–1060. [5] T. Inoue, F. Yin, Y. Kimura, K. Tsuzaki, S. Ochiai, Metall. Mater. Trans. 40A (2010) 341–355. [6] T. Inoue, Y. Kimura, J. Mater. Sci. 48 (2007) 4766–4772. [7] M. Pozuelo, F. Carreno, O.A. Ruano, Compos. Sci. Technol. 66 (2006) 2671–2676.