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The effect of primary thermo-mechanical treatment on TRIP steel microstructure and mechanical properties
Author’s Accepted Manuscript The effect of primary thermo-mechanical treatment on TRIP steel microstructure and mechanical properties Ghavam Azizi, Hamed Mirzadeh, Mohammad Habibi Parsa www.elsevier.com/locate/msea
PII: DOI: Reference:
S0921-5093(15)00578-X http://dx.doi.org/10.1016/j.msea.2015.05.045 MSA32373
To appear in: Materials Science & Engineering A Received date: 21 April 2015 Revised date: 15 May 2015 Accepted date: 16 May 2015 Cite this article as: Ghavam Azizi, Hamed Mirzadeh and Mohammad Habibi Parsa, The effect of primary thermo-mechanical treatment on TRIP steel microstructure and mechanical properties, Materials Science & Engineering A, http://dx.doi.org/10.1016/j.msea.2015.05.045 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
The effect of primary thermo-mechanical treatment on TRIP steel microstructure and mechanical properties Ghavam Azizi a, Hamed Mirzadeh a,*, Mohammad Habibi Parsa a,b,c a
School of Metallurgy and Materials Engineering, College of Engineering, University of Tehran, P.O. Box 11155-4563, Tehran, Iran b
Center of Excellence for High Performance Materials, School of Metallurgy and Materials Engineering, University of Tehran, Tehran, Iran
c
Advanced Metalforming and Thermomechanical Processing Laboratory, School of Metallurgy and Materials Engineering, University of Tehran, Tehran, Iran
Abstract The initial microstructure of transformation-induced plasticity (TRIP) steel was found to be an important factor in determining the final microstructure and mechanical properties and their sensitivity to TRIP heat treatment. The presence of banded structure rendered inhomogeneous distribution of retained austenite and resulted in inferior and anisotropic mechanical properties.
Keywords: TRIP steels; Thermo-mechanical processing; Mechanical properties.
*
Corresponding author. Tel.: +982182084127; Fax: +982188006076. E-mail address: [email protected] (H. Mirzadeh). E-mail addresses: [email protected] (G. Azizi), [email protected] (H. Mirzadeh), [email protected] (M.H. Parsa).
1
1. Introduction The rejection of alloying elements (such as Mn) from the first formed δ-ferrite dendrites results in the enrichment of the interdendritic spaces from solutes and this makes the transformed austenite chemically inhomogeneous [1]. During hot rolling operations in the austenitic regime, the rich and lean regions elongate and provide basis for the appearance of alternative ferrite/pearlite bands [2]. The ferrite first forms in the regions with high Ar3 temperature and rejects the carbon atoms. Eventually, the carbon content in some regions reaches the eutectoid composition and when the temperature goes below Ar1, pearlite forms [3]. Inhibiting the formation of banded structure in high strength steels is an important technological boundary condition to allow utilizing the full potential of their mechanical properties [4]. The aim of this work is to systematically investigate the effect of thermo-mechanical history (chemical homogeneity, cooling rate and hot rolling) on the microstructure and mechanical properties of the starting and final sheets of a low-alloyed TRIP-assisted steel. The results of the present work might be important in industrial applications.
2. Experimental details A 0.2C-1.6Mn-0.7Si-0.9Al TRIP steel was prepared by vacuum induction melting (VIM). The homogenization treatment was performed by soaking at 1350 °C for 300 minutes. The hot rolling operations were performed on both the as-cast and as-homogenized slabs (with 20 mm thickness) at nominal temperature of 1150 °C by consecutive pass reductions of 10%, 20%, 20%, and 20% to achieve sheets with a thickness of 6 mm. The cold rolling operations were performed to achieve a thickness of 1 mm.
2
The intercritical annealing (IA) and bainitic isothermal transformation (BIT) treatments were performed at 790 °C for 6 min and 400 °C for 6 min to produce the TRIP microstructure. It should be noted that the Ar1 and Ar3 temperatures were determined by dilatometry as 713 and 890 °C, respectively. The Ms temperature (°C) of completely austenitic microstructure was calculated by Ms=539-423C-30.4Mn-7.5Si+30Al [5,6] to be about 430 °C. Based on the lever rule, by intercritical annealing at 790 °C, a microstructure consisting of 35% austenite and 65% ferrite is expected to be achieved and the level of carbon in the austenite phase becomes as high as 0.5. As a result, the Ms temperature of the austenite phase will be around 300 °C, which is below the BIT temperature of 400 °C. Optical microscopy and SEM/EDS analysis were used for microstructural investigations and elemental analysis, respectively. The TRIP microstructures were etched in the LePera’s reagent (1g Na2S2O5 in 100 ml H2O + 4g picric acid in 100 ml ethanol) while other microstructures were revealed by the 2% Nital solution (2ml HNO3 in 98 ml ethanol). The mechanical properties were investigated by tensile tests using a universal testing machine. The tensile specimens were prepared according to JIS-Z-2201 with the gage length of 8 mm for investigation of mechanical properties along the rolling direction, transverse direction, and 45° to rolling direction.
3. Result and discussion 3.1. Microstructural evolution during thermo-mechanical treatment The optical micrograph of the as-cast specimen is shown in Fig. 1a. The appearance of pearlitic microstructure in the interdendritic regions is a good sign of the microsegregation during solidification, which can be proved by severe fluctuations in the amount of alloying elements in different regions as sown in Fig. 1b. This inhomogeneity is the basis for the appearance of banded structure during hot rolling as shown in Figs. 1c-f. As can be seen in Fig. 1c, the 3
dendritic structure is being destroyed by 10% hot reduction. By further reduction to 30% (Fig. 1d), the initial microstructure has vanished completely. The signs of the banded structure can be seen in Fig. 1e and identifiable bands of ferrite and pearlite forms after 70% hot reduction. Therefore, it can be concluded that while the hot rolling operations can destroy the dendritic microstructure, it can result in the development of banded structure by pancaking the regions that are lean and rich from alloying elements. To investigate the effect of these bands on the properties of TRIP steel, it is necessary to compare with non-banded microstructures. Accordingly, two methods were used to inhibit the appearance of banded structure: (I) Homogenization: The hot rolled microstructures of the homogenized specimen at 1350 °C for 300 min and the manifestation of increased homogeneity by EDS analysis are shown in Figs. 1 g-i. Comparison of Fig. 1i with Fig. 1b reveals that the distribution of alloying elements is much more homogeneous. It can also be easily seen that by homogenization treatment, the dendritic structure has vanished and the banded structure does not appear even after severe hot reduction of 70%. (II) Normalizing: As shown in Fig. 1j, for the 70% hot rolled and air cooled specimen without prior homogenization, the banded structure has vanished and a complex and fine microstructure appeared inside prior austenite grains, which are themselves decorated by grain boundary allotriomorphs. It seems that rapid cooling will yield higher driving forces for the ferrite reaction, and differences in the Ar3 temperatures between different parts of the austenite phase, should have a lesser effect [7]. However, by re-austenitization and slow cooling, the banded structure returns as shown in Fig. 1k. This can be ascribed to the presence of chemical inhomogeneity as shown in Fig. 1l, which is very similar to that of the as-cast sample (Fig. 1b).
4
Fig. 1: (a) and (b) are related to the as-cast microstructure. The 10%, 30%, 50%, and 70% hot rolled and furnace cooled microstructures are respectively shown in (c), (d), (e), and (f). The 10% and 70% hot rolled and furnace cooled microstructures after homogenization and the corresponding EDS spectra are shown in (g), (h), and (i), respectively. (j) shows the 70% hot rolled and air cooled microstructure from the as-cast condition, (k) shows the subsequent re-austenitized and furnace cooled microstructure, and (l) exhibit the corresponding EDS spectra. Note that all the micrographs were taken at the same magnification.
5
3.2. Mechanical properties after thermo-mechanical treatment Fig. 2a shows the microstructures and engineering tensile stress-strain curves along the rolling direction of cold-rolled sheets to 1 mm, which correspond to the microstructures shown in Fig. 1f (denoted by NF0), Fig. 1h (denoted by HF0), and Fig. 1j (denoted by NA0).
Fig. 2: The room-temperature tensile curves and the corresponding microstructures of cold-rolled sheets with various thermo-mechanical histories that tested along different directions with respect to the rolling direction. NF and NA refer to hot rolled and respectively furnace/air cooled specimens without prior homogenization, while HF refers to hot rolled and furnace cooled specimen with prior homogenization.
It is seen that the non-banded microstructures (NA0 and HF0) have superior properties compared with the banded microstructure (NF0), which can be ascribed to the highly sandwiched ferritic phase between pearlite areas in HF0 and the very fine microstructure of NA0 due to its complex 6
and fine initial microstructure (Fig. 1j); whereas the microstructure of NF0 simply consists of ferrite/pearlite bands after cold rolling. It can be deduced that in the case of the nonhomogenized steel, the fast cooling-induced removal of the banded structure (NA) is helpful in achieving sheets with improved mechanical properties but the fully homogenized microstructure (HF) has slightly better mechanical properties. Fig. 2b shows the microstructures and engineering tensile stress-strain curves of cold-rolled sheets to 1 mm with initial microstructure of Fig. 1f along the rolling direction (NF0), 45° to the rolling direction (NF45), and transverse direction (NF90). It can be seen that the mechanical properties of the sheet with banded structure is anisotropic and depends on the direction of tensile specimen. This anisotropy is being healed considerably by homogenization as shown in Fig. 2c.
3.3. Mechanical properties after TRIP heat treatment Fig. 3 shows the microstructures and engineering tensile stress-strain curves along the rolling direction after TRIP-heat treatment (TIA = 790 °C and TBIT = 400 °C), which correspond to the initial microstructures of NF0, NA0, HF0. It can be seen that the distribution of the retained austenite (and bainite) completely depends on the microstructure of the starting sheet. In the aircooled specimen (NA0), the retained austenite is uniformly distributed due to the fine and complex initial microstructure, which effectively distributed the carbon atoms before intercritical annealing. In the case of HF0 (homogenized), the austenite has formed in place of pancaked pearlite during intercritical annealing, which resulted in a less homogenous distribution of retained austenite. However, in the case of NF0, the presence of distinct ferrite/pearlite bands in the starting sheet has resulted in large bands of ferrite/bainite-austenite in the final microstructure
7
as shown in Fig. 3. Therefore, there are very large bands of ferrite without retained austenite. These differences can exert impressive effects on mechanical properties and consideration of these issues will be very helpful in application of TRIP steels in the industry.
Fig. 3: The microstructures and room-temperature tensile curves along the rolling direction after TRIP-heat treatment (TIA = 790 °C and TBIT = 400 °C). Note that all the micrographs were taken at the same magnification.
It can be seen in Fig. 3 that the strength of the NF0 specimen is inferior compared with those of the other specimens, which can be ascribed to its inappropriate microstructure that consists of large bands of ferrite without retained austenite. The NA0 specimen shows unusual high yield 8
strength compared with other specimens. This can be ascribed to the formation of some martensite during BIT due to the presence of some prior austenite areas with high Ms temperature. The latter is a result of rapid cooling, which disturbs the distribution of carbon during pre-processing and subsequent formation of austenite during intercritical annealing in both lean and rich regions from alloying elements. This is not the case for NF0 specimen because furnace cooling during pre-processing partitions carbon atoms and subsequently pearlite areas toward high-Mn locations. The formation of martensite is also not a case for HF0 due to its high chemical homogeneity. Therefore, it can be deduced that the chemical homogeneity and cooling rate during pre-processing can exert important effects on redistribution of alloying elements, especially carbon, with the resultant sensitivity of the mechanical properties on the parameters of TRIP heat treatments. It should also be noted that the martensitic areas etch with the same color as the retained austenite and cannot be easily distinguished.
4. Conclusions In summary, the homogenization state and the initial microstructure of TRIP steel sheets were found to be important factors in determining the final microstructure and mechanical properties of TRIP steels. The presence of banded structure was shown to be responsible for inhomogeneous distribution of the retained austenite and inferior mechanical properties of TRIP steels. The chemical inhomogeneity of the austenite after intercritical annealing was found to be an important factor in determining the final microstructural constituents and the sensitivity of the final product on the parameters of TRIP heat treatment.
References [1] J.A. Eckert, P.R. Howell, S.W. Thompson, J. Mater. Sci. 28 (1993) 4412-4420.
9
[2] L.E. Samuels, Optical Microscopy of Carbon Steels, ASM International, Metals Park, OH, USA, 1980. [3] G. Krauss, Steels: Processing, Structure, and Performance, ASM International, Metals Park, OH, USA, 2005. [4] W. Xu, P.E. J. Rivera-Diaz-Del-Castillo and S. van der Zwaag, ISJI Int. 45 (2004) 380-387. [5] M. De Meyer, J. Mahieu, B.C. De Cooman, Mater. Sci. Tech. 18 (2002) 1121-1132. [6] M.V. Li, D.V. Niebuhr, L.L. Meekisho, D.G. Aatteridge, Metall. Mater. Trans. B 29 (1998) 661-672. [7] S.W. Thompson, P.R. Howell, Mater. Sci. Tech. 8 (1992) 77-86. Figure Captions Fig. 1: (a) and (b) are related to the as-cast microstructure. The 10%, 30%, 50%, and 70% hot rolled and furnace cooled microstructures are respectively shown in (c), (d), (e), and (f). The 10% and 70% hot rolled and furnace cooled microstructures after homogenization and the corresponding EDS spectra are shown in (g), (h), and (i), respectively. (j) shows the 70% hot rolled and air cooled microstructure from the as-cast condition, (k) shows the subsequent re-austenitized and furnace cooled microstructure, and (l) exhibit the corresponding EDS spectra. Note that all the micrographs were taken at the same magnification. Fig. 2: The room-temperature tensile curves and the corresponding microstructures of cold-rolled sheets with various thermo-mechanical histories that tested along different directions with respect to the rolling direction. NF and NA refer to hot rolled and respectively furnace/air cooled specimens without prior homogenization, while HF refers to hot rolled and furnace cooled specimen with prior homogenization. Fig. 3: The microstructures and room-temperature tensile curves along the rolling direction after TRIPheat treatment (TIA = 790 °C and TBIT = 400 °C). Note that all the micrographs were taken at the same magnification.
10
PII: DOI: Reference:
S0921-5093(15)00578-X http://dx.doi.org/10.1016/j.msea.2015.05.045 MSA32373
To appear in: Materials Science & Engineering A Received date: 21 April 2015 Revised date: 15 May 2015 Accepted date: 16 May 2015 Cite this article as: Ghavam Azizi, Hamed Mirzadeh and Mohammad Habibi Parsa, The effect of primary thermo-mechanical treatment on TRIP steel microstructure and mechanical properties, Materials Science & Engineering A, http://dx.doi.org/10.1016/j.msea.2015.05.045 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
The effect of primary thermo-mechanical treatment on TRIP steel microstructure and mechanical properties Ghavam Azizi a, Hamed Mirzadeh a,*, Mohammad Habibi Parsa a,b,c a
School of Metallurgy and Materials Engineering, College of Engineering, University of Tehran, P.O. Box 11155-4563, Tehran, Iran b
Center of Excellence for High Performance Materials, School of Metallurgy and Materials Engineering, University of Tehran, Tehran, Iran
c
Advanced Metalforming and Thermomechanical Processing Laboratory, School of Metallurgy and Materials Engineering, University of Tehran, Tehran, Iran
Abstract The initial microstructure of transformation-induced plasticity (TRIP) steel was found to be an important factor in determining the final microstructure and mechanical properties and their sensitivity to TRIP heat treatment. The presence of banded structure rendered inhomogeneous distribution of retained austenite and resulted in inferior and anisotropic mechanical properties.
Keywords: TRIP steels; Thermo-mechanical processing; Mechanical properties.
*
Corresponding author. Tel.: +982182084127; Fax: +982188006076. E-mail address: [email protected] (H. Mirzadeh). E-mail addresses: [email protected] (G. Azizi), [email protected] (H. Mirzadeh), [email protected] (M.H. Parsa).
1
1. Introduction The rejection of alloying elements (such as Mn) from the first formed δ-ferrite dendrites results in the enrichment of the interdendritic spaces from solutes and this makes the transformed austenite chemically inhomogeneous [1]. During hot rolling operations in the austenitic regime, the rich and lean regions elongate and provide basis for the appearance of alternative ferrite/pearlite bands [2]. The ferrite first forms in the regions with high Ar3 temperature and rejects the carbon atoms. Eventually, the carbon content in some regions reaches the eutectoid composition and when the temperature goes below Ar1, pearlite forms [3]. Inhibiting the formation of banded structure in high strength steels is an important technological boundary condition to allow utilizing the full potential of their mechanical properties [4]. The aim of this work is to systematically investigate the effect of thermo-mechanical history (chemical homogeneity, cooling rate and hot rolling) on the microstructure and mechanical properties of the starting and final sheets of a low-alloyed TRIP-assisted steel. The results of the present work might be important in industrial applications.
2. Experimental details A 0.2C-1.6Mn-0.7Si-0.9Al TRIP steel was prepared by vacuum induction melting (VIM). The homogenization treatment was performed by soaking at 1350 °C for 300 minutes. The hot rolling operations were performed on both the as-cast and as-homogenized slabs (with 20 mm thickness) at nominal temperature of 1150 °C by consecutive pass reductions of 10%, 20%, 20%, and 20% to achieve sheets with a thickness of 6 mm. The cold rolling operations were performed to achieve a thickness of 1 mm.
2
The intercritical annealing (IA) and bainitic isothermal transformation (BIT) treatments were performed at 790 °C for 6 min and 400 °C for 6 min to produce the TRIP microstructure. It should be noted that the Ar1 and Ar3 temperatures were determined by dilatometry as 713 and 890 °C, respectively. The Ms temperature (°C) of completely austenitic microstructure was calculated by Ms=539-423C-30.4Mn-7.5Si+30Al [5,6] to be about 430 °C. Based on the lever rule, by intercritical annealing at 790 °C, a microstructure consisting of 35% austenite and 65% ferrite is expected to be achieved and the level of carbon in the austenite phase becomes as high as 0.5. As a result, the Ms temperature of the austenite phase will be around 300 °C, which is below the BIT temperature of 400 °C. Optical microscopy and SEM/EDS analysis were used for microstructural investigations and elemental analysis, respectively. The TRIP microstructures were etched in the LePera’s reagent (1g Na2S2O5 in 100 ml H2O + 4g picric acid in 100 ml ethanol) while other microstructures were revealed by the 2% Nital solution (2ml HNO3 in 98 ml ethanol). The mechanical properties were investigated by tensile tests using a universal testing machine. The tensile specimens were prepared according to JIS-Z-2201 with the gage length of 8 mm for investigation of mechanical properties along the rolling direction, transverse direction, and 45° to rolling direction.
3. Result and discussion 3.1. Microstructural evolution during thermo-mechanical treatment The optical micrograph of the as-cast specimen is shown in Fig. 1a. The appearance of pearlitic microstructure in the interdendritic regions is a good sign of the microsegregation during solidification, which can be proved by severe fluctuations in the amount of alloying elements in different regions as sown in Fig. 1b. This inhomogeneity is the basis for the appearance of banded structure during hot rolling as shown in Figs. 1c-f. As can be seen in Fig. 1c, the 3
dendritic structure is being destroyed by 10% hot reduction. By further reduction to 30% (Fig. 1d), the initial microstructure has vanished completely. The signs of the banded structure can be seen in Fig. 1e and identifiable bands of ferrite and pearlite forms after 70% hot reduction. Therefore, it can be concluded that while the hot rolling operations can destroy the dendritic microstructure, it can result in the development of banded structure by pancaking the regions that are lean and rich from alloying elements. To investigate the effect of these bands on the properties of TRIP steel, it is necessary to compare with non-banded microstructures. Accordingly, two methods were used to inhibit the appearance of banded structure: (I) Homogenization: The hot rolled microstructures of the homogenized specimen at 1350 °C for 300 min and the manifestation of increased homogeneity by EDS analysis are shown in Figs. 1 g-i. Comparison of Fig. 1i with Fig. 1b reveals that the distribution of alloying elements is much more homogeneous. It can also be easily seen that by homogenization treatment, the dendritic structure has vanished and the banded structure does not appear even after severe hot reduction of 70%. (II) Normalizing: As shown in Fig. 1j, for the 70% hot rolled and air cooled specimen without prior homogenization, the banded structure has vanished and a complex and fine microstructure appeared inside prior austenite grains, which are themselves decorated by grain boundary allotriomorphs. It seems that rapid cooling will yield higher driving forces for the ferrite reaction, and differences in the Ar3 temperatures between different parts of the austenite phase, should have a lesser effect [7]. However, by re-austenitization and slow cooling, the banded structure returns as shown in Fig. 1k. This can be ascribed to the presence of chemical inhomogeneity as shown in Fig. 1l, which is very similar to that of the as-cast sample (Fig. 1b).
4
Fig. 1: (a) and (b) are related to the as-cast microstructure. The 10%, 30%, 50%, and 70% hot rolled and furnace cooled microstructures are respectively shown in (c), (d), (e), and (f). The 10% and 70% hot rolled and furnace cooled microstructures after homogenization and the corresponding EDS spectra are shown in (g), (h), and (i), respectively. (j) shows the 70% hot rolled and air cooled microstructure from the as-cast condition, (k) shows the subsequent re-austenitized and furnace cooled microstructure, and (l) exhibit the corresponding EDS spectra. Note that all the micrographs were taken at the same magnification.
5
3.2. Mechanical properties after thermo-mechanical treatment Fig. 2a shows the microstructures and engineering tensile stress-strain curves along the rolling direction of cold-rolled sheets to 1 mm, which correspond to the microstructures shown in Fig. 1f (denoted by NF0), Fig. 1h (denoted by HF0), and Fig. 1j (denoted by NA0).
Fig. 2: The room-temperature tensile curves and the corresponding microstructures of cold-rolled sheets with various thermo-mechanical histories that tested along different directions with respect to the rolling direction. NF and NA refer to hot rolled and respectively furnace/air cooled specimens without prior homogenization, while HF refers to hot rolled and furnace cooled specimen with prior homogenization.
It is seen that the non-banded microstructures (NA0 and HF0) have superior properties compared with the banded microstructure (NF0), which can be ascribed to the highly sandwiched ferritic phase between pearlite areas in HF0 and the very fine microstructure of NA0 due to its complex 6
and fine initial microstructure (Fig. 1j); whereas the microstructure of NF0 simply consists of ferrite/pearlite bands after cold rolling. It can be deduced that in the case of the nonhomogenized steel, the fast cooling-induced removal of the banded structure (NA) is helpful in achieving sheets with improved mechanical properties but the fully homogenized microstructure (HF) has slightly better mechanical properties. Fig. 2b shows the microstructures and engineering tensile stress-strain curves of cold-rolled sheets to 1 mm with initial microstructure of Fig. 1f along the rolling direction (NF0), 45° to the rolling direction (NF45), and transverse direction (NF90). It can be seen that the mechanical properties of the sheet with banded structure is anisotropic and depends on the direction of tensile specimen. This anisotropy is being healed considerably by homogenization as shown in Fig. 2c.
3.3. Mechanical properties after TRIP heat treatment Fig. 3 shows the microstructures and engineering tensile stress-strain curves along the rolling direction after TRIP-heat treatment (TIA = 790 °C and TBIT = 400 °C), which correspond to the initial microstructures of NF0, NA0, HF0. It can be seen that the distribution of the retained austenite (and bainite) completely depends on the microstructure of the starting sheet. In the aircooled specimen (NA0), the retained austenite is uniformly distributed due to the fine and complex initial microstructure, which effectively distributed the carbon atoms before intercritical annealing. In the case of HF0 (homogenized), the austenite has formed in place of pancaked pearlite during intercritical annealing, which resulted in a less homogenous distribution of retained austenite. However, in the case of NF0, the presence of distinct ferrite/pearlite bands in the starting sheet has resulted in large bands of ferrite/bainite-austenite in the final microstructure
7
as shown in Fig. 3. Therefore, there are very large bands of ferrite without retained austenite. These differences can exert impressive effects on mechanical properties and consideration of these issues will be very helpful in application of TRIP steels in the industry.
Fig. 3: The microstructures and room-temperature tensile curves along the rolling direction after TRIP-heat treatment (TIA = 790 °C and TBIT = 400 °C). Note that all the micrographs were taken at the same magnification.
It can be seen in Fig. 3 that the strength of the NF0 specimen is inferior compared with those of the other specimens, which can be ascribed to its inappropriate microstructure that consists of large bands of ferrite without retained austenite. The NA0 specimen shows unusual high yield 8
strength compared with other specimens. This can be ascribed to the formation of some martensite during BIT due to the presence of some prior austenite areas with high Ms temperature. The latter is a result of rapid cooling, which disturbs the distribution of carbon during pre-processing and subsequent formation of austenite during intercritical annealing in both lean and rich regions from alloying elements. This is not the case for NF0 specimen because furnace cooling during pre-processing partitions carbon atoms and subsequently pearlite areas toward high-Mn locations. The formation of martensite is also not a case for HF0 due to its high chemical homogeneity. Therefore, it can be deduced that the chemical homogeneity and cooling rate during pre-processing can exert important effects on redistribution of alloying elements, especially carbon, with the resultant sensitivity of the mechanical properties on the parameters of TRIP heat treatments. It should also be noted that the martensitic areas etch with the same color as the retained austenite and cannot be easily distinguished.
4. Conclusions In summary, the homogenization state and the initial microstructure of TRIP steel sheets were found to be important factors in determining the final microstructure and mechanical properties of TRIP steels. The presence of banded structure was shown to be responsible for inhomogeneous distribution of the retained austenite and inferior mechanical properties of TRIP steels. The chemical inhomogeneity of the austenite after intercritical annealing was found to be an important factor in determining the final microstructural constituents and the sensitivity of the final product on the parameters of TRIP heat treatment.
References [1] J.A. Eckert, P.R. Howell, S.W. Thompson, J. Mater. Sci. 28 (1993) 4412-4420.
9
[2] L.E. Samuels, Optical Microscopy of Carbon Steels, ASM International, Metals Park, OH, USA, 1980. [3] G. Krauss, Steels: Processing, Structure, and Performance, ASM International, Metals Park, OH, USA, 2005. [4] W. Xu, P.E. J. Rivera-Diaz-Del-Castillo and S. van der Zwaag, ISJI Int. 45 (2004) 380-387. [5] M. De Meyer, J. Mahieu, B.C. De Cooman, Mater. Sci. Tech. 18 (2002) 1121-1132. [6] M.V. Li, D.V. Niebuhr, L.L. Meekisho, D.G. Aatteridge, Metall. Mater. Trans. B 29 (1998) 661-672. [7] S.W. Thompson, P.R. Howell, Mater. Sci. Tech. 8 (1992) 77-86. Figure Captions Fig. 1: (a) and (b) are related to the as-cast microstructure. The 10%, 30%, 50%, and 70% hot rolled and furnace cooled microstructures are respectively shown in (c), (d), (e), and (f). The 10% and 70% hot rolled and furnace cooled microstructures after homogenization and the corresponding EDS spectra are shown in (g), (h), and (i), respectively. (j) shows the 70% hot rolled and air cooled microstructure from the as-cast condition, (k) shows the subsequent re-austenitized and furnace cooled microstructure, and (l) exhibit the corresponding EDS spectra. Note that all the micrographs were taken at the same magnification. Fig. 2: The room-temperature tensile curves and the corresponding microstructures of cold-rolled sheets with various thermo-mechanical histories that tested along different directions with respect to the rolling direction. NF and NA refer to hot rolled and respectively furnace/air cooled specimens without prior homogenization, while HF refers to hot rolled and furnace cooled specimen with prior homogenization. Fig. 3: The microstructures and room-temperature tensile curves along the rolling direction after TRIPheat treatment (TIA = 790 °C and TBIT = 400 °C). Note that all the micrographs were taken at the same magnification.
10