4 Nanostructured steel for automotive body structures Y.OKITSU,Honda R&D Co.Ltd.,Japan and N.TSUJI,Kyoto University,Japan Abstract:In this chapter.the results of studies on the application of nanostructured steel sheets to automotive body structures are introduced. A new route to fabricating nanostructured/ultrafine ferritic microstructures without severe plastic deformation is presented.The preparation and evaluation of two kinds of steel sheets,ultrafine-grained(UFG)ferrite- cementite(FC)steel and UFG multi-phase(MP)steel are also presented and discussed.The UFG-FC steel showed large strain rate sensitivity in flow stress,while the UFG-MP steel showed a good combination of high strength and large work hardening rate,which improved the dynamic collapse properties of hat columns.It is shown that UFG steels would help further weight reduction of automotive body structures. Key words:ultrafine grain,steel sheets,multi-phase,strain rate sensitivity, dynamic collapse. Note:This chapter was first published as Chapter 22 'Applying nanostructured steel sheets to automotive body structures'by Y.Okitsu and N.Tsuji in Nanostructured metals and alloys,ed.Sung H.Whang. Woodhead Publishing Limited,2011,ISBN:978 1 84569 670 2.It is reproduced without revision. 4.1 Introduction In recent times,reducing the body weight of automobiles has been considered one of the important criteria in auto manufacturing for reducing CO,emissions as well as improving the fuel efficiency of automobiles.Steels have been the most popular and indispensable material for automotive body structures in the past,and recently in particular,various kinds of high-strength steels (HSSs)!have been developed and applied to the body structures,which has contributed to a weight reduction.In order to achieve further weight reduction, new HSSs are required.As described in the next section,nanostructured or ultrafine-grained(UFG)steels are expected to become one of the new HSSs.2 However,it is still difficult to apply UFG steel sheets to automobile body parts.This is owing to the limited dimensions of UFG steels fabricated by severe plastic deformation(SPD)processes such as equal-channel angular extrusion (ECAE),3 high-pressure torsion (HPT).and accumulative roll bonding(ARB).5-7 which require very high plastic strain. 57 Woodhead Publishing Limited,2012
© Woodhead Publishing Limited, 2012 57 4 Nanostructured steel for automotive body structures Y. OKITSU, Honda R&D Co. Ltd., Japan and N. TSUJI, Kyoto University, Japan Abstract: In this chapter, the results of studies on the application of nanostructured steel sheets to automotive body structures are introduced. A new route to fabricating nanostructured/ultrafine ferritic microstructures without severe plastic deformation is presented. The preparation and evaluation of two kinds of steel sheets, ultrafine-grained (UFG) ferritecementite (FC) steel and UFG multi-phase (MP) steel are also presented and discussed. The UFG-FC steel showed large strain rate sensitivity in flow stress, while the UFG-MP steel showed a good combination of high strength and large work hardening rate, which improved the dynamic collapse properties of hat columns. It is shown that UFG steels would help further weight reduction of automotive body structures. Key words: ultrafine grain, steel sheets, multi-phase, strain rate sensitivity, dynamic collapse. Note: This chapter was first published as Chapter 22 ‘Applying nanostructured steel sheets to automotive body structures’ by Y. Okitsu and N. Tsuji in Nanostructured metals and alloys, ed. Sung H. Whang, Woodhead Publishing Limited, 2011, ISBN: 978 1 84569 670 2. It is reproduced without revision. 4.1 Introduction In recent times, reducing the body weight of automobiles has been considered one of the important criteria in auto manufacturing for reducing CO2 emissions as well as improving the fuel efficiency of automobiles. Steels have been the most popular and indispensable material for automotive body structures in the past, and recently in particular, various kinds of high-strength steels (HSSs)1 have been developed and applied to the body structures, which has contributed to a weight reduction. In order to achieve further weight reduction, new HSSs are required. As described in the next section, nanostructured or ultrafine-grained (UFG) steels are expected to become one of the new HSSs.2 However, it is still difficult to apply UFG steel sheets to automobile body parts. This is owing to the limited dimensions of UFG steels fabricated by severe plastic deformation (SPD) processes such as equal-channel angular extrusion (ECAE),3 high-pressure torsion (HPT),4 and accumulative roll bonding (ARB),5–7 which require very high plastic strain
58 Advanced materials in automotive engineering Recently the authors showed a new route to fabricate UFG steel sheets through conventional rolling and annealing procedures,8 which made it possible to overcome the dimensional problem,and evaluate the properties and performances required for automotive body structures systematically. Based on this processing route,UFG steel sheets with various microstructures such as ferrite-cementite (FC)89 and multi-phase (MP)structuresl0 were fabricated. In this chapter,future trends of automotive body engineering and demands for UFG steels are described first.Secondly,the new route to fabricate UFG steel sheets without SPD and the properties of fabricated UFG-FC steel sheets are shown.Next,UFG-MP steel sheets developed in order to improve the work hardening rate of UFG microstructures are described.Finally,there is an illustration of the crash-worthiness of UFG steel sheets,which is one of their important properties as body materials. 4.2 Potential demand for nanostructured steels for automotive body structures Nowadays,improving the fuel efficiency of automobiles is a very important subject,which comes from global demands for reducing CO2 emissions. For this purpose,it is necessary not only to improve the fuel efficiency of engines,but also to reduce body weight,rolling drag force,friction,etc. However,the actual body weight of automobiles has increased mainly due to the demands for improving the crash-worthiness of body structures during a collision.For this reason,considerable efforts have been made to suppress any weight increase.The applications of HSSs in automobiles have increased significantly since the 1990s.As a result,more than 50%of body parts are made of HSSs2 in recent cars.Although low-density metals such as aluminium and magnesium are also applied,low-carbon steels are still widely used for manufacturing various kinds of cars due to their high cost performance.The ULSAB (UltraLight Steel Auto Body)project showed that a significant weight reduction could be achieved by applying AHSSs (advanced high strength steels)and advanced manufacturing methods.A review of such projects is presented in the appendix to this chapter. In order to achieve further weight reductions by extending the application of HSSs,new steels with new microstructures and superior properties have been widely studied.3 An example is high-Mn TWIP(twinning-induced plasticity)steels,415 which have austenite microstructures and a combination of superior tensile strength and elongation.This steel is classified as'second- generation AHSS'.2A recent target,so-called'third-generation AHSSs',2 is steels with intermediate elongation between conventional HSS and TWIP steels but fewer alloy elements than TWIP steels.Grain refinement down to Woodhead Publishing Limited,2012
58 Advanced materials in automotive engineering © Woodhead Publishing Limited, 2012 Recently the authors showed a new route to fabricate UFG steel sheets through conventional rolling and annealing procedures,8 which made it possible to overcome the dimensional problem, and evaluate the properties and performances required for automotive body structures systematically. Based on this processing route, UFG steel sheets with various microstructures such as ferrite-cementite (FC)8,9 and multi-phase (MP) structures10 were fabricated. In this chapter, future trends of automotive body engineering and demands for UFG steels are described first. Secondly, the new route to fabricate UFG steel sheets without SPD and the properties of fabricated UFG-FC steel sheets are shown. Next, UFG-MP steel sheets developed in order to improve the work hardening rate of UFG microstructures are described. Finally, there is an illustration of the crash-worthiness of UFG steel sheets, which is one of their important properties as body materials. 4.2 Potential demand for nanostructured steels for automotive body structures Nowadays, improving the fuel efficiency of automobiles is a very important subject, which comes from global demands for reducing CO2 emissions. For this purpose, it is necessary not only to improve the fuel efficiency of engines, but also to reduce body weight, rolling drag force, friction, etc. However, the actual body weight of automobiles has increased mainly due to the demands for improving the crash-worthiness of body structures during a collision. For this reason, considerable efforts have been made to suppress any weight increase. The applications of HSSs in automobiles have increased significantly since the 1990s.11 As a result, more than 50% of body parts are made of HSSs12 in recent cars. Although low-density metals such as aluminium and magnesium are also applied, low-carbon steels are still widely used for manufacturing various kinds of cars due to their high cost performance. The ULSAB (UltraLight Steel Auto Body) project showed that a significant weight reduction could be achieved by applying AHSSs (advanced high strength steels) and advanced manufacturing methods. A review of such projects is presented in the appendix to this chapter. In order to achieve further weight reductions by extending the application of HSSs, new steels with new microstructures and superior properties have been widely studied.13 An example is high-Mn TWIP (twinning-induced plasticity) steels,14,15 which have austenite microstructures and a combination of superior tensile strength and elongation. This steel is classified as ‘secondgeneration AHSS’.2 A recent target, so-called ‘third-generation AHSSs’,2 is steels with intermediate elongation between conventional HSS and TWIP steels but fewer alloy elements than TWIP steels. Grain refinement down to
Nanostructured steel for automotive body structures 59 submicrometer grain sizes is thought to be one of the important microstructural controlling methods that will contribute to third-generation AHSS.2 Up to now,the improvement of automotive steels has been discussed in terms of formability.However,in order to achieve further weight reduction, other points of view are being considered.Generally the parts in an automotive body structure are classified roughly into two groups having different functions. One is the parts in crushable zones,i.e.the front and rear frame sections, which deform heavily in car collision and should absorb impact energy.The other is the parts making up the passengers'cabin,which deform little and prevent the passengers from injury during a collision.Ultra high strength steels (UHSSs)with tensile strength of over 1000 MPa are applied to the parts for making up the passengers'cabin.6 However,if UHSSs are applied to the parts in the crushable zones aiming for further weight reduction,other mechanical properties must be preferentially considered.For example,when UHSSs are applied to the crushable parts,their deformation mode tends to become unstable,which reduces the efficiency of energy absorption.10This is caused by the decreased work hardening rate of the material already highly strengthened.17The authors10 have shown using lab-fabricated UFG-MPsteel that a large work hardening rate(n-value)was effective in achieving a stable deformation in axial collapse.In the following sections,the practical results of evaluating microstructures,mechanical properties and crash-worthiness using fabricated UFG steel sheets are shown and discussed. 4.3 Fabricating nanostructured low-C steel sheets 4.3.1 A new route to fabricate nanostructured steel sheets without severe plastic deformation It has been already reported that UFG ferritic steels have superior properties such as high strength,8 high fracture toughness at around-190Cand high strain rate sensitivity in flow stress.20.21 Severe plastic deformation processes are generally applied in order to fabricate nanostructured or UFG steels. However,they do not seem to adapt to conventional mass production routes for steels.In this section,firstly,the concept of the new route for fabricating UFG steel sheets without SPD is described.The distinguishing feature of the process is conventional rolling of a duplex microstructure composed of soft ferrite and hard martensite.The key factor in the process is strain distribution between soft and hard phases.When such a duplex microstructure is deformed, a larger strain is introduced in the soft phase (ferrite),while a strain that is not so heavy but sufficient for final microstructural refinement is introduced in the hard phase (martensite).As a result,a number of recrystallization nuclei form uniformly from both ferrite and martensite,and uniform UFG microstructures are obtained after subsequent annealing. Woodhead Publishing Limited,2012
Nanostructured steel for automotive body structures 59 © Woodhead Publishing Limited, 2012 submicrometer grain sizes is thought to be one of the important microstructural controlling methods that will contribute to third-generation AHSS.2 Up to now, the improvement of automotive steels has been discussed in terms of formability. However, in order to achieve further weight reduction, other points of view are being considered. Generally the parts in an automotive body structure are classified roughly into two groups having different functions. One is the parts in crushable zones, i.e. the front and rear frame sections, which deform heavily in car collision and should absorb impact energy. The other is the parts making up the passengers’ cabin, which deform little and prevent the passengers from injury during a collision. Ultra high strength steels (UHSSs) with tensile strength of over 1000 MPa are applied to the parts for making up the passengers’ cabin.16 However, if UHSSs are applied to the parts in the crushable zones aiming for further weight reduction, other mechanical properties must be preferentially considered. For example, when UHSSs are applied to the crushable parts, their deformation mode tends to become unstable, which reduces the efficiency of energy absorption.10 This is caused by the decreased work hardening rate of the material already highly strengthened.17 The authors10 have shown using lab-fabricated UFG-MP steel that a large work hardening rate (n-value) was effective in achieving a stable deformation in axial collapse. In the following sections, the practical results of evaluating microstructures, mechanical properties and crash-worthiness using fabricated UFG steel sheets are shown and discussed. 4.3 Fabricating nanostructured low-C steel sheets 4.3.1 A new route to fabricate nanostructured steel sheets without severe plastic deformation It has been already reported that UFG ferritic steels have superior properties such as high strength,18 high fracture toughness at around –190 °C19 and high strain rate sensitivity in flow stress.20,21 Severe plastic deformation processes are generally applied in order to fabricate nanostructured or UFG steels. However, they do not seem to adapt to conventional mass production routes for steels. In this section, firstly, the concept of the new route for fabricating UFG steel sheets without SPD is described. The distinguishing feature of the process is conventional rolling of a duplex microstructure composed of soft ferrite and hard martensite. The key factor in the process is strain distribution between soft and hard phases. When such a duplex microstructure is deformed, a larger strain is introduced in the soft phase (ferrite), while a strain that is not so heavy but sufficient for final microstructural refinement is introduced in the hard phase (martensite). As a result, a number of recrystallization nuclei form uniformly from both ferrite and martensite, and uniform UFG microstructures are obtained after subsequent annealing
60 Advanced materials in automotive engineering Table 4.1 shows the chemical composition of the UFG-FC steel studied. This steel is made by adding Nb and B to a low C steel.Figure 4.1 shows a schematic illustration describing the fabricating process and conditions. The fabricating process consists of conventional hot-rolling,cold-rolling and annealing as indicated in Fig.4.1. An ingot was hot-rolled to a thickness of 6.8 mm at austenite region,air- cooled to 540C,which corresponded to the intercritical region of ferrite and austenite,and water-cooled to room temperature for obtaining a duplex microstructure of ferrite and martensite.The hot-rolled sheets were cold- rolled at room temperature with lubricant.Specimens having various total cold-rolling reductions from 85%to 94%were prepared.The cold-rolled sheets were annealed at various temperatures ranging from 525C to 700C for 120 seconds in a salt bath.Annealed sheets that were 150 mm wide and 0.4 to 1.0 mm thick were obtained.Microstructural observations were carried out by optical microscopy and scanning electron microscopy (SEM). An optical micrograph(OM)of the hot-rolled steel sheet observed from the transverse direction (TD)of the sheet is shown in Fig.4.2.The hot-rolled sheet showed a duplex microstructure composed of a ferrite matrix(bright region)and martensite islands(dark region).The mean intersect lengths along the normal direction (ND)of both regions,measured using the OM,were 5.55 um in the ferrite and 2.46 um in the martensite.The area of martensite was 42%of the whole.Transmission electron microscopy (TEM)observation has confirmed that the martensite present was 'lath-martensite'including a high density of dislocations.Cementite particles were not observed in the as-quenched martensite. Table 4.1 Chemical composition (mass%)of UFG-FC steel C Si Mn P S Al Nb B N 0.10 0.01 2.00 0.002 0.00130.035 0.0220.00150.0007 1200C Hot-rolling Cold-rolling Annealing AC 540°C 525-700C,120s 770°C RT 85-94% 入v Wo 4.1 Schematic illustration describing the fabricating conditions of UFG-FC steel. Woodhead Publishing Limited,2012
60 Advanced materials in automotive engineering © Woodhead Publishing Limited, 2012 Table 4.1 shows the chemical composition of the UFG-FC steel studied. This steel is made by adding Nb and B to a low C steel. Figure 4.1 shows a schematic illustration describing the fabricating process and conditions. The fabricating process consists of conventional hot-rolling, cold-rolling and annealing as indicated in Fig. 4.1. An ingot was hot-rolled to a thickness of 6.8 mm at austenite region, aircooled to 540 °C, which corresponded to the intercritical region of ferrite and austenite, and water-cooled to room temperature for obtaining a duplex microstructure of ferrite and martensite. The hot-rolled sheets were coldrolled at room temperature with lubricant. Specimens having various total cold-rolling reductions from 85% to 94% were prepared. The cold-rolled sheets were annealed at various temperatures ranging from 525 °C to 700 °C for 120 seconds in a salt bath. Annealed sheets that were 150 mm wide and 0.4 to 1.0 mm thick were obtained. Microstructural observations were carried out by optical microscopy and scanning electron microscopy (SEM). An optical micrograph (OM) of the hot-rolled steel sheet observed from the transverse direction (TD) of the sheet is shown in Fig. 4.2. The hot-rolled sheet showed a duplex microstructure composed of a ferrite matrix (bright region) and martensite islands (dark region). The mean intersect lengths along the normal direction (ND) of both regions, measured using the OM, were 5.55 mm in the ferrite and 2.46 mm in the martensite. The area of martensite was 42% of the whole. Transmission electron microscopy (TEM) observation has confirmed that the martensite present was ‘lath-martensite’ including a high density of dislocations. Cementite particles were not observed in the as-quenched martensite. Table 4.1 Chemical composition (mass%) of UFG-FC steel C Si Mn P S Al Nb B N 0.10 0.01 2.00 0.002 0.0013 0.035 0.022 0.0015 0.0007 1200 °C Hot-rolling Cold-rolling Annealing 770 °C AC WQ WQ 540 °C RT 85–94% 525–700 °C, 120 s 4.1 Schematic illustration describing the fabricating conditions of UFG-FC steel
Nanostructured steel for automotive body structures 61 ND 10μm RD 4.2 Optical micrograph of the hot-rolled sheet of the UFG-FC steel. Observed from TD. Figure 4.3 (a),(d)and(g)shows SEM microstructures of the specimens cold-rolled to reductions of 85%,91%and 94%in thickness,respectively.The ferrite matrix(dark gray region)exhibited a wavy microstructure elongated roughly to the rolling direction(RD)and bent along the martensite islands (light gray region). It was indicated that complex plastic flow occurred and higher strain was introduced in the softer ferrite matrix owing to the existence of hard martensite phase.TEM observation"of the 91%cold-rolled specimen confirmed a fine lamellar structure with a mean spacing of 0.14 um in the ferrite region.It was also confirmed by selected area diffraction that this region contained various crystal orientations.It is worth noting that the fine structure including large misorientations,which had been found in SPD-processed materials, already existed at the cold-rolled state in spite of the relatively low plastic strain of 2.8.It is well known that the hard second particles increase the rate of formation of high angle grain boundaries (HAGB)in the matrix through inhomogeneous deformation in two-phase metals and alloys.22 The large local misorientation is caused by local lattice rotations in the vicinity of the hard particles.In addition,the shear strain should be closely related to the significant grain refinement of ferrite matrix.Kamikawa et al.3 have reported that redundant shear strain introduced at sheet subsurface regions owing to high friction in rolling accelerates the microstructure refinement in the ARB of IF(interstitial-free)steel.The wavy-shaped ferrite and diamond- Woodhead Publishing Limited,2012
Nanostructured steel for automotive body structures 61 © Woodhead Publishing Limited, 2012 Figure 4.3 (a), (d) and (g) shows SEM microstructures of the specimens cold-rolled to reductions of 85%, 91% and 94% in thickness, respectively. The ferrite matrix (dark gray region) exhibited a wavy microstructure elongated roughly to the rolling direction (RD) and bent along the martensite islands (light gray region). It was indicated that complex plastic flow occurred and higher strain was introduced in the softer ferrite matrix owing to the existence of hard martensite phase. TEM observation8 of the 91% cold-rolled specimen confirmed a fine lamellar structure with a mean spacing of 0.14 mm in the ferrite region. It was also confirmed by selected area diffraction that this region contained various crystal orientations.8 It is worth noting that the fine structure including large misorientations, which had been found in SPD-processed materials, already existed at the cold-rolled state in spite of the relatively low plastic strain of 2.8. It is well known that the hard second particles increase the rate of formation of high angle grain boundaries (HAGB) in the matrix through inhomogeneous deformation in two-phase metals and alloys.22 The large local misorientation is caused by local lattice rotations in the vicinity of the hard particles. In addition, the shear strain should be closely related to the significant grain refinement of ferrite matrix. Kamikawa et al. 23 have reported that redundant shear strain introduced at sheet subsurface regions owing to high friction in rolling accelerates the microstructure refinement in the ARB of IF (interstitial-free) steel. The wavy-shaped ferrite and diamondND RD 10 µm 4.2 Optical micrograph of the hot-rolled sheet of the UFG-FC steel. Observed from TD
62 Advanced materials in automotive engineering (b) c (d) (e) (f) (g (h) () ND RD 2μm 4.3 SEM microstructures of (a),(d),(g),cold-rolled specimen,(b), (e),(h),specimens annealed at 600C after cold-rolling and (c),(f), (i),specimens annealed at 650C after cold-rolling.The cold-rolling reductions are (a),(b),(c),85%,(d),(e),(f),91%and (g),(h),(i),94%. Observed from TD. shaped martensite in Fig.4.3 suggested that large plastic strains including shear strain components were introduced to the ferrite grains. On the other hand,the martensite islands were also deformed to some extent in the cold-rolling and showed diamond shapes.Table 4.2 summarises the cold-rolling reduction and the mean thickness ratio,t/to,of the martensite islands in the microstructures measured using the OM of the hot-rolled sheet and the SEM micrograph of the cold-rolled specimens. Here,t and to are the mean intersect lengths of the martensite islands along ND after and before cold-rolling,respectively.The reduction of the martensite islands was much smaller than the reduction of the specimen, which indicated that a larger strain was introduced to the ferrite grains. By TEM analysis in the previous study,s large local misorientations in the deformed martensite regions were also confirmed in spite of the smaller strain in martensite.Ueji et al.24 have reported that 50%cold-rolled low-carbon martensite exhibited fine lamellar structure involving large misorientations. which was equivalent to the microstructure in SPD processed steels.This is thought to be attributed to the complex and fine microstructure of the Woodhead Publishing Limited,2012
62 Advanced materials in automotive engineering © Woodhead Publishing Limited, 2012 shaped martensite in Fig. 4.3 suggested that large plastic strains including shear strain components were introduced to the ferrite grains. On the other hand, the martensite islands were also deformed to some extent in the cold-rolling and showed diamond shapes. Table 4.2 summarises the cold-rolling reduction and the mean thickness ratio, t/t0, of the martensite islands in the microstructures measured using the OM of the hot-rolled sheet and the SEM micrograph of the cold-rolled specimens. Here, t and t0 are the mean intersect lengths of the martensite islands along ND after and before cold-rolling, respectively. The reduction of the martensite islands was much smaller than the reduction of the specimen, which indicated that a larger strain was introduced to the ferrite grains. By TEM analysis in the previous study,8 large local misorientations in the deformed martensite regions were also confirmed in spite of the smaller strain in martensite. Ueji et al. 24 have reported that 50% cold-rolled low-carbon martensite exhibited fine lamellar structure involving large misorientations, which was equivalent to the microstructure in SPD processed steels. This is thought to be attributed to the complex and fine microstructure of the (a) (d) (g) (b) (e) (h) (c) (f) (i) ND RD 2 µm 4.3 SEM microstructures of (a), (d), (g), cold-rolled specimen, (b), (e), (h), specimens annealed at 600°C after cold-rolling and (c), (f), (i), specimens annealed at 650°C after cold-rolling. The cold-rolling reductions are (a), (b), (c), 85%, (d), (e), (f), 91% and (g), (h), (i), 94%. Observed from TD
Nanostructured steel for automotive body structures 63 Table 4.2 Cold-rolling reduction and mean thickness ratio of the specimens and the martensite islands of the cold-rolled UFG-FC specimens Cold-rolling reduction t/to of the specimen Mean t/to of the of the specimen(%) martensite islands 85 0.15 0.47 91 0.09 0.32 94 0.06 0.27 as-transformed martensite,also involving a high density of dislocations. They also showed equiaxed UFG microstructure after annealing of the 50% cold-rolled specimen with a single phase of martensite.It can be concluded therefore that the strain applied to martensite in the present study was not very large but probably enough to introduce large local misorientations and to form UFG microstructure through subsequent annealing. Figure 4.3 (b),(e)and (h)show the microstructures of the cold-rolled sheets after annealing at 600C.In the 85%cold-rolled and annealed specimen (Fig.4.3 (b)),both equiaxed fine ferrite grains and elongated ferrite grains located in an arc-like row (in the lower part of Fig.4.3(b)) were observed.The fine equiaxed ferrite grains seemed to be formed by continuous coarsening of the finely subdivided regions in the cold-rolled microstructure together with recovery.8 It was difficult to distinguish clearly which area in Fig.4.3 (b)was originally ferrite or martensite,but according to the supposed mechanism discussed before,the ultrafine ferrite grains could originate from both ferrite and martensite.On the other hand, some coarse grains in an arc-like row seen were probably formed mainly by recovery of the ferrite regions such as the elongated and bent ferrite seen in the upper and left part of Fig.4.3(a).Those ferrite regions in the cold-rolled microstructure seem to be deformed to a smaller plastic strain because of the relatively low density of surrounding martensite islands. Also the fact that the coarse ferrite grains in the annealed microstructure contained few cementite particles suggests that they were originally ferrite When the cold-rolling reduction was higher than 91%,the specimens were filled mostly with equiaxed ultrafine ferrite grains(Fig.4.3 (e)and (h)). This seemed to be because the spacing of the martensite islands as seen in Fig.4.3(d)and (g)decreased due to larger cold-rolling reductions.When the specimens were annealed at 650C.significant grain growth occurred in all the specimens.Fine cementite particles dispersed in ferrite grains were also observed in all annealed specimens in Fig.4.3.Figure 4.4 shows the relationship between the annealing temperature and mean ferrite grain size measured on the SEM micrographs by the mean intersection method along the RD and ND.The larger the cold-rolling reduction was,the smaller the obtained ferrite grain size became. Woodhead Publishing Limited,2012
Nanostructured steel for automotive body structures 63 © Woodhead Publishing Limited, 2012 as-transformed martensite, also involving a high density of dislocations. They also showed equiaxed UFG microstructure after annealing of the 50% cold-rolled specimen with a single phase of martensite. It can be concluded therefore that the strain applied to martensite in the present study was not very large but probably enough to introduce large local misorientations and to form UFG microstructure through subsequent annealing. Figure 4.3 (b), (e) and (h) show the microstructures of the cold-rolled sheets after annealing at 600 °C. In the 85% cold-rolled and annealed specimen (Fig. 4.3 (b)), both equiaxed fine ferrite grains and elongated ferrite grains located in an arc-like row (in the lower part of Fig. 4.3 (b)) were observed. The fine equiaxed ferrite grains seemed to be formed by continuous coarsening of the finely subdivided regions in the cold-rolled microstructure together with recovery.8 It was difficult to distinguish clearly which area in Fig. 4.3 (b) was originally ferrite or martensite, but according to the supposed mechanism discussed before, the ultrafine ferrite grains could originate from both ferrite and martensite. On the other hand, some coarse grains in an arc-like row seen were probably formed mainly by recovery of the ferrite regions such as the elongated and bent ferrite seen in the upper and left part of Fig. 4.3 (a). Those ferrite regions in the cold-rolled microstructure seem to be deformed to a smaller plastic strain because of the relatively low density of surrounding martensite islands. Also the fact that the coarse ferrite grains in the annealed microstructure contained few cementite particles suggests that they were originally ferrite. When the cold-rolling reduction was higher than 91%, the specimens were filled mostly with equiaxed ultrafine ferrite grains (Fig. 4.3 (e) and (h)). This seemed to be because the spacing of the martensite islands as seen in Fig. 4.3 (d) and (g) decreased due to larger cold-rolling reductions. When the specimens were annealed at 650 °C, significant grain growth occurred in all the specimens. Fine cementite particles dispersed in ferrite grains were also observed in all annealed specimens in Fig. 4.3. Figure 4.4 shows the relationship between the annealing temperature and mean ferrite grain size measured on the SEM micrographs by the mean intersection method along the RD and ND. The larger the cold-rolling reduction was, the smaller the obtained ferrite grain size became. Table 4.2 Cold-rolling reduction and mean thickness ratio of the specimens and the martensite islands of the cold-rolled UFG-FC specimens Cold-rolling reduction t/t0 of the specimen Mean t/t0 of the of the specimen (%) martensite islands 85 0.15 0.47 91 0.09 0.32 94 0.06 0.27
64 Advanced materials in automotive engineering 0-85% 昌e △91% 0-94% 5 550 600 650 700 Annealing temperature,TC 4.4 The relationship of annealing temperatures and mean ferrite grain sizes measured by the mean intersect method of UFG-FC steels. The sizes of the equiaxed ultrafine ferrite grains shown in Fig.4.3(b),(e) and(h)were not so different.However,due to the existence of the elongated and arc-like ferrite grains the average ferrite grain size in the specimen 85% cold-rolled and annealed at 600C(Fig.4.3(b))was slightly larger than that in other specimens having larger cold-rolling reductions(Fig.4.3 (e)and (h)).On the other hand,the effect of the annealing temperature on the ferrite grain size was more significant as shown in Fig.4.4.The 85%cold-rolled specimens annealed at 600C and 625C contained some recovered ferrite grains as shown in Fig.4.3(b).The minimum grain size of the homogeneous grain structures obtained was 0.43 um in the specimen 94%cold-rolled and then annealed at 600C(Fig.4.3 (h)). The UFG ferrite formed throughout the specimens by large cold-rolling reductions followed by low annealing temperatures.In order to clarify the process of fine-grain formation,Fig.4.5 shows the microstructure of a specimen that was 94%cold-rolled and subsequently annealed at 525C for 120 seconds. The dotted lines roughly delineate the former martensite islands that had already changed to fine-grained ferrite and cementite.In the ferrite matrix, UFG ferrite also formed along the wavy microstructure in the vicinity of the former martensite.Finer equiaxed grains of ferrite were seen around the former martensite islands.The result suggests that a larger strain was introduced into the ferrite/martensite interface regions through cold-rolling. Therefore,in order to reduce the cold-rolling reductions required,it seems to be effective to decrease the spacing of martensite islands in the starting microstructure before cold-rolling. Woodhead Publishing Limited,2012
64 Advanced materials in automotive engineering © Woodhead Publishing Limited, 2012 The sizes of the equiaxed ultrafine ferrite grains shown in Fig. 4.3 (b), (e) and (h) were not so different. However, due to the existence of the elongated and arc-like ferrite grains the average ferrite grain size in the specimen 85% cold-rolled and annealed at 600 °C (Fig. 4.3 (b)) was slightly larger than that in other specimens having larger cold-rolling reductions (Fig. 4.3 (e) and (h)). On the other hand, the effect of the annealing temperature on the ferrite grain size was more significant as shown in Fig. 4.4. The 85% cold-rolled specimens annealed at 600 °C and 625 °C contained some recovered ferrite grains as shown in Fig. 4.3 (b). The minimum grain size of the homogeneous grain structures obtained was 0.43 mm in the specimen 94% cold-rolled and then annealed at 600 °C (Fig. 4.3 (h)). The UFG ferrite formed throughout the specimens by large cold-rolling reductions followed by low annealing temperatures. In order to clarify the process of fine-grain formation, Fig. 4.5 shows the microstructure of a specimen that was 94% cold-rolled and subsequently annealed at 525 °C for 120 seconds. The dotted lines roughly delineate the former martensite islands that had already changed to fine-grained ferrite and cementite. In the ferrite matrix, UFG ferrite also formed along the wavy microstructure in the vicinity of the former martensite. Finer equiaxed grains of ferrite were seen around the former martensite islands. The result suggests that a larger strain was introduced into the ferrite/martensite interface regions through cold-rolling. Therefore, in order to reduce the cold-rolling reductions required, it seems to be effective to decrease the spacing of martensite islands in the starting microstructure before cold-rolling. 85% 91% 94% 550 600 650 700 Annealing temperature, T/°C Mean ferrite grain size, d/mm 2 1.5 1 0.5 0 4.4 The relationship of annealing temperatures and mean ferrite grain sizes measured by the mean intersect method of UFG-FC steels
Nanostructured steel for automotive body structures 65 ND RD 1μm 4.5 SEM microstructure of UFG-FC steel that was 94%cold-rolled and annealed at 525C. 4.3.2 Mechanical properties of nanostructured ferrite- cementite steel sheets The tensile properties of nanostructured ferrite-cementite steels were investigated using a high-speed servo-hydraulic material test system produced by Saginomiya Inc.equipped with a special load-sensing block.25.26 With this machine,stress-strain (s-s)curves at a wide range of strain rates from quasi-static to dynamic deformations can be obtained.Figure 4.6 shows the appearance of the prepared tensile specimen having a gauge length of 6mm and a width of 2mm. The tensile direction was parallel to RD.Tensile tests were operated at various strain rates ranging from 10-s to 10s-at room temperature.Total elongation of the specimens was measured from the difference in the gauge length before and after testing.Fabricating conditions,microstructures and quasi-static mechanical properties measured at a strain rate of 102s-are summarised in Table 4.3. UFG-FCA,B,C and FCM specimens were prepared by 91%cold-rolling of the same hot-rolled sheet as shown in Fig.4.2 and subsequent annealing at 620C,635C,670C and 700C for 120 seconds,respectively.The mean ferrite grain sizes indicated in Table 4.3 were calculated by the EBSD (electron backscatter diffraction)data measured on the TD sections.UFG- FC steels showed microstructures of ultrafine ferrite and cementite,while the FCM specimen showed a microstructure composed of ferrite,cementite and martensite.FCM specimen contained about 14%of martensite in the Woodhead Publishing Limited,2012
Nanostructured steel for automotive body structures 65 © Woodhead Publishing Limited, 2012 4.3.2 Mechanical properties of nanostructured ferritecementite steel sheets The tensile properties of nanostructured ferrite-cementite steels were investigated using a high-speed servo-hydraulic material test system produced by Saginomiya Inc. equipped with a special load-sensing block.25,26 With this machine, stress–strain (s–s) curves at a wide range of strain rates from quasi-static to dynamic deformations can be obtained. Figure 4.6 shows the appearance of the prepared tensile specimen having a gauge length of 6mm and a width of 2mm. The tensile direction was parallel to RD. Tensile tests were operated at various strain rates ranging from 10–2s –1 to 103 s –1 at room temperature. Total elongation of the specimens was measured from the difference in the gauge length before and after testing. Fabricating conditions, microstructures and quasi-static mechanical properties measured at a strain rate of 10–2s –1 are summarised in Table 4.3. UFG-FC A, B, C and FCM specimens were prepared by 91% cold-rolling of the same hot-rolled sheet as shown in Fig. 4.2 and subsequent annealing at 620 °C, 635 °C, 670 °C and 700 °C for 120 seconds, respectively. The mean ferrite grain sizes indicated in Table 4.3 were calculated by the EBSD (electron backscatter diffraction) data measured on the TD sections. UFGFC steels showed microstructures of ultrafine ferrite and cementite, while the FCM specimen showed a microstructure composed of ferrite, cementite and martensite. FCM specimen contained about 14% of martensite in the ND RD 1 µm 4.5 SEM microstructure of UFG-FC steel that was 94% cold-rolled and annealed at 525 °C
66 Advanced materials in automotive engineering 0 27 4.6 Schematic drawing of the test piece for dynamic and quasi-static tensile tests. microstructure.The Acl transformation temperatures of UFG-FC steel measured by a dilatometer was approximately 700C,so that the FCM specimen contained both ferrite and austenite during the annealing at 700C and the austenite transformed to martensite during the subsequent water-cooling. Figure 4.7 shows s-s curves at strain rates of 10-2,102 and 103 s-1.Figure 4.7 (a).(b),(c)and (d)correspond to the test results of UFG-FC steels A. B,C and the FCM steel listed in Table 4.3. Both UFG-FC and FCM steels showed the yield drop phenomenon.The flow stress increased whereas the uniform elongation significantly decreased as the ferrite grain size in the UFG-FC specimens decreased(Fig.4.7(a). (b)and (c)).The same behavior has been reported for UFG IF steel sheets prepared by ARB and the annealing process.18 On the other hand,as shown in Fig.4.7(d),the work hardening increased when martensite was introduced in the microstructure. The flow stress significantly increased when the strain rate increased in UFG-FC steels.In order to investigate the strain rate dependence of the flow stress,the difference in tensile flow stress at 5%nominal strain between the strain rates of 103 s-and 10-2 s-,Ao,was evaluated.Figure 4.8 shows the Woodhead Publishing Limited,2012
66 Advanced materials in automotive engineering © Woodhead Publishing Limited, 2012 microstructure. The Ac1 transformation temperatures of UFG-FC steel measured by a dilatometer was approximately 700 °C, so that the FCM specimen contained both ferrite and austenite during the annealing at 700 °C and the austenite transformed to martensite during the subsequent water-cooling. Figure 4.7 shows s–s curves at strain rates of 10–2, 102 and 103 s–1. Figure 4.7 (a), (b), (c) and (d) correspond to the test results of UFG-FC steels A, B, C and the FCM steel listed in Table 4.3. Both UFG-FC and FCM steels showed the yield drop phenomenon. The flow stress increased whereas the uniform elongation significantly decreased as the ferrite grain size in the UFG-FC specimens decreased (Fig. 4.7 (a), (b) and (c)). The same behavior has been reported for UFG IF steel sheets prepared by ARB and the annealing process.18 On the other hand, as shown in Fig. 4.7 (d), the work hardening increased when martensite was introduced in the microstructure. The flow stress significantly increased when the strain rate increased in UFG-FC steels. In order to investigate the strain rate dependence of the flow stress, the difference in tensile flow stress at 5% nominal strain between the strain rates of 103 s–1 and 10–2 s–1, Ds, was evaluated. Figure 4.8 shows the 10 6 2 27 4.6 Schematic drawing of the test piece for dynamic and quasi-static tensile tests
