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Effects of microstructure and pipe forming strain on yield strength before and after spiral pipe forming of API X70 and X80 linepipe steel sheets
Materials Science & Engineering A 573 (2013) 18–26
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Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Effects of microstructure and pipe forming strain on yield strength before and after spiral pipe forming of API X70 and X80 linepipe steel sheets Seok Su Sohn a, Seung Youb Han a, Jin-ho Bae b, Hyoung Seop Kim a, Sunghak Lee a,n a b
Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang 790-784, Republic of Korea Sheet Products & Process Research Group, Technical Research Laboratories, POSCO, Pohang 790-785, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 October 2012 Received in revised form 20 February 2013 Accepted 25 February 2013 Available online 7 March 2013
API X70 and X80 linepipe steel sheets were shaped in the form of pipe with different strains (thickness/ diameter ratio) by spiral pipe forming. Tension specimens taken from steel sheets or pipes at an interval of 2.5 mm were tested, and their yielding behavior, yield strength, and yield ratio before and after forming were analyzed. In the pipes, the continuous yielding and low yield ratios were shown in the inner side, whereas the discontinuous yielding and high yield ratios were shown in the outer side. This was because the Bauschinger effect and the strain hardening effect were mainly dominant in the inner and outer sides, respectively. The overall yield strength after spiral piping was defined by the competing effect of the strain hardening and the Bauschinger effect. The competing effects depended on the microstructure and the pipe forming strains. The low-temperature transformation microstructures were preferred for achieving the larger increase of overall yield strength after pipe forming. For a specific microstructure an optimization of the yield strength can also be achieved by controlling the pipe forming strain in order to maximize the strain hardening effect and to minimize the Bauschinger effect. & 2013 Elsevier B.V. All rights reserved.
Keywords: Electron microscopy Bainite Martensite Thermomechanical processing Sheet forming Hardening
1. Introduction Linepipe steels used for long-range transportations of crude oil or natural gas generally require high strengths to endure high pressures [1,2]. Widely used API X70 or X80 grade linepipe steels are fabricated in a sheet form, and then shaped as pipe. Among properties required for the pipes, the minimum yield strength in the circumferential direction is important due to the yield strength change after forming. During the forming, outer and inner walls of pipes are subjected to different strains, i.e., tensile strains on the outer wall and compressive strains on the inner wall, which also varied with pipe forming strain of thickness/diameter (t/D) ratio and the thickness position of the sheet. According to this strain history, sheets flattened from pipes often show the lower yield strength than original sheets, which leads to uncertainties of supplying linepipe steels satisfying the yield strength standard required by the American Petroleum Institute (API). The reason why the yield strength decreases under repeated tension and compression strains is generally interpreted by the Bauschinger effect. The resistance to plastic deformation is lowered, when a material already deformed in one direction is strained again in an opposite direction [3,4]. It works
n
Corresponding author. Tel.: þ82 54 279 2140; fax: þ82 54 279 5887. E-mail address: [email protected] (S. Lee).
0921-5093/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2013.02.050
importantly in pipe forming processes. According to Orowan’s theory [5], back stresses increase as the density of mobile dislocations increases during forward plastic deformation. The Bauschinger effect is proportional to the density of mobile dislocations, whose generation, migration, and pile-up are affected by microstructural factors [6,7]. Park et al. [8] studied the Bauschinger effect of API X46 to X70 grade linepipe steels, and quantitatively analyzed effects of microstructures on variation of the yield strength after the pipe forming. The yield strength was varied with pipe forming methods and forming strains (t/D ratios), which should be carefully considered before the yield strength measurement. Choi et al. [9] investigated the variation in yield strength of spiral formed to be pipe X70 linepipe steels, and reported that the Bauschinger effect decreased by increasing the volume fraction of acicular ferrite (AF) and the occurrence of continuous yielding. In these linepipe steels, the deformation amounts subjected to steel sheets are varied with pipe forming methods, and the strain hardening effect and the Bauschinger effect work differently with the deformation amount of each microstructure. In X80 grade linepipe steels, their microstructures are rather complicated because they consist of low-temperature transformation microstructures such as acicular ferrite (AF) and granular bainite (GB), which leads to difficulties of precise estimation of yield strength before and after the pipe forming. In consideration of these microstructural factors and pipe forming strains (t/D ratios) varied with pipe forming methods
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and pipe diameters, the variation in yield strength is needed to be carefully analyzed. In general, the yield strength increases with increasing deformation amount and consequently the strain hardening effect, whereas it decreases with increasing the Bauschinger effect. Therefore, effects of deformation amount on the yield strength in consideration of microstructures and pipe forming strains should be systematically analyzed in order to identify the variation in the yield strength and to investigate the competing mechanism between strain hardening and Bauschinger effects, but very few studies have been conducted. In this study, API X70 and X80 steel sheets were shaped to be pipe with different pipe forming strains by the spiral pipe forming. Tension specimens taken from steel sheets or pipes at an interval of 2.5 mm were tested, and their yielding behavior, yield strength, and yield ratio before and after pipe forming were analyzed. From these results, correlations between yielding behavior, yield strength, yield ratio, and microstructural factors were investigated, and the methods for preventing or minimizing the reduction in yield strength after the spiral pipe forming were suggested.
2. Experimental 2.1. API X70 and X80 linepipe steel sheets Commercial API X70 and X80 grade linepipe steels having a minimum yield strength level of 483 MPa (70 ksi) and 552 MPa (80 ksi), respectively, were used in this study, and their chemical compositions are shown in Table 1. The carbon equivalent is higher in the X80 steel than in the X70 steel. An overall grain refinement effect was expected by rolling with a high rolling reduction ratio (over 80%) in the non-recrystallized region of austenite after austenitization at 1180–1200 1C [10–12]. The rolling was finished at 790–840 1C in the austenite region above Ar3. After finishing rolling, the steels were cooled from the finish rolling temperature to 500–525 1C at a cooling rate of 16 1C/s. The final sheet thickness was 19.0 mm and 18.4 mm for the X70 and X80 steels, respectively.
19
forming strains of the 7S and 8L steels were 1.9% and 1.53%, respectively. The forming strain is higher in the 7S steel by about 0.4% at most than in the 8L steel. 2.3. Microstructural analysis The steel sheets were polished and etched in a 2% nital solution, and microstructures of longitudinal–short transverse (L-S) planes were observed by an optical microscope and a scanning electron microscope (SEM, model: JSM-6330F, JEOL, Japan). Volume fractions of microstructures present in the steel sheets were measured by an image analyzer. 2.4. Tensile and hardness tests Schematic diagrams showing direction and location of tensile specimens obtained from a sheet or a pipe are illustrated in Fig. 2. Seven tensile specimens were obtained from the sheet or pipe at an interval of 2.5 mm, and the tensile specimen direction (bluecolored arrow) was deviated by 301 from the rolling direction (red-colored arrow). Plate-type specimens having a gage length of 30 mm, a gage width of 5 mm, and a gage thickness of 1 mm were prepared, and were tested at room temperature at a strain rate of 5 10 3 s 1 in accordance with the ASTM standard test method [13] by a universal testing machine (model; 8801, Instron, Canton,
2.2. Pipe forming processes
eðXÞ ¼ 2X=ðDtÞ
ð1Þ
where e(X), D, t, and X are the strain at the distance X, outer diameter of the pipe, thickness of the sheet, and distance from the center of the sheet thickness respectively [8]. A schematic diagram showing the pipe forming process and the pipe forming strain at the distance X from the center of the sheet thickness is illustrated in Fig. 1(a) and (b), respectively. The maximum
2.5 1.9%
2.0 1.5 Pipe Forming Strain (%)
Pipes of 1020 mm and 1220 mm in outer diameter were fabricated by the spiral pipe forming of the X70 and X80 steel sheets, respectively, along the 301 direction deviated from the rolling direction. In convenience, an X70 steel sheet subjected to the small-diameter spiral pipe forming is referred to as ‘7S’, and an X80 steel sheet subjected to the large-diameter spiral pipe forming is referred to as ‘8L’. Since the strain gradient subjected during the spiral pipe forming is varied with the sheet thickness, the strain gradient at the distance X from the center of the sheet thickness can be expressed by
1.0 1.53% 0.5 0.0 -0.5 -1.0 -1.5
7S Steel 8L Steel
-2.0 Table 1 Chemical compositions of the API X70 and X80 steels (wt%). Steel C X70 X80
Mn
Si
o 0.04 1.29 0.30 o 0.06 1.78
Ni þCu Cr o 0.6
Mo
Al
o 0.4 0.01 0.035 0.17
-2.5
Ti þNb þ V Ceq o0.15
o 0.35 o 0.5
-10 -8 Inner
-6
-4
-2
0
2
4
6
Distance from the Center, X (mm)
8
10 Outer
Fig. 1. (a) Schematic diagram showing the pipe forming process and (b) the pipe forming strain at the distance X from the center of a sheet thickness.
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S.S. Sohn et al. / Materials Science & Engineering A 573 (2013) 18–26
Pipe Sheet Welding Line
30°
Rolling Direction 30 °
Tensile Specimen
Rolling Direction
Outer Center Inner Rolling Direction 30° Outer Center Inner
Fig. 2. Schematic diagrams showing direction and location of tensile specimens obtained from a sheet or a pipe. Seven tension specimens are taken from the sheet or pipe at an interval of 2.5 mm, and the tensile specimen direction (blue-colored arrow) is deviated by 301 from the rolling direction (red-colored arrow). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
MA, USA) of 100 kN capacity. Strain was measured over a gauge length of 30 mm using a contact 30 mm extensometer. The 0.2% offset stress was determined to be the yield strength in the steel specimens showing continuous yielding behavior, whereas the lower yield point was determined to be the yield strength in the steel specimens showing discontinuous yielding behavior. Tensile tests were conducted three times at least for each test condition, and the data were averaged. Vickers Hardness was measured at the same position of tensile specimen under a load of 300 g.
3. Results 3.1. Microstructures of API X70 and X80 linepipe steel sheets SEM micrographs of the outer, center, and inner position (on the basis of the spiral pipe position) of the 7S and 8L steel specimens are shown in Fig. 3(a) through (f). The volume fractions of various microstructures and average grain size were measured, and the results are shown in Table 2. The 7S and 8L steel specimens are composed of quasi-polygonal ferrite (QPF), acicular ferrite (AF), granular bainite (GB), pearlite (P), and martensiteaustenite constituent (MA) [14–18]. According to shapes and characteristics of microstructures, QPF is transformed at lower temperatures and faster cooling rates than polygonal ferrite, and has irregular grain boundaries. Its strength is relatively low because secondary phases are hardly formed inside grains or along grain boundaries. AF is characterized by fine grain size, irregular-shape, and alignment in arbitrary directions, and can be grouped into packets, depending on orientations between neighboring laths. It is known as a microstructure with good
combination of strength and toughness because of fine size and high interior dislocation density. GB contains island-type MAs, and has low toughness because of large packet size. P is a stable microstructure composed of ferrite and cementite, and its strength is high. MA is secondary phases formed at fast cooling rates, and has high strength and low toughness. Each microstructure is classified by these morphological categories as marked in the micrographs of Fig. 3(a) through (f). Since the differentiation between MA constituents and other microstructures was difficult in SEM micrographs, the specimens were etched in a LePera solution [19]. MAs and other microstructures (QPF, AF, GB, and P) were colored in bright white and brown, respectively. Optical micrographs of the 7S and 8L specimens etched in a LePera solution are shown in Fig. 4(a) through (f). The volume fractions of MA of the 8L steel specimens ranged from 1.9% to 2.5%, and other microstructures occupy the rest of MA, whereas MAs are hardly observable in the 7S steel specimen. According to the volume fraction data of QPF, AF, GB, P, and MA in Table 2, the 7S steel specimens are mostly composed of QPF, together with AF and a small amount of P. The outer and inner positions have finer grain sizes than the center position because of faster cooling rates. The 8L steel specimens mostly consist of AF, together with GB, instead of QPF, as low-temperature transformation microstructures are formed during rapid cooling from the austenite region. They also contain a considerable amount of MA, while pearlites are hardly found, because they have high carbon equivalent. In the center position, the grain size is larger, and the volume fractions of GB and MA are higher than in the outer or inner position due to slower cooling rates. The Vickers hardness was measured with intervals of 2.5 mm outward from the inner surface of the sheet and pipe under a load
S.S. Sohn et al. / Materials Science & Engineering A 573 (2013) 18–26
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8L -Outer
7S -Outer AF P MA QPF AF
5 m
5 m
8L -Center
7S -Center AF P
GB
MA
AF
QPF
5 m
5 m
7S -Inner
8L -Inner GB
P
QPF AF
MA AF 5 m
5 m
Fig. 3. SEM micrographs of the inner, center, and outer position (on the basis of the spiral pipe position) of (a) through (c) the 7S and (d) through (f) the 8L steel specimens. Etched by a nital solution. Table 2 Volume fractions of quasi-polygonal ferrite, acicular ferrite, granular bainite, pearlite, and martensite-austenite constituent and average grain size in the X70 and X80 steels (unit: %). Steel
Position
Quasi polygonal ferrite
Acicular ferrite
Granular bainite
Pearlite
Martensite-austenite constituent
Average grain size (mm)
7S
Outer Center Inner Outer Center Inner
Bal. Bal. Bal. – – –
28 76.0 21 74.8 32 77.7 Bal. Bal. Bal.
– – – 237 6.4 447 3.3 177 6.3
0.9 70.1 1.1 70.2 0.7 70.2 o 0.2 0.6 70.2 o 0.2
o 0.2 o 0.2 o 0.2 1.97 0.5 2.57 0.2 2.07 0.4
11.07 1.2 13.2 7 2.6 10.57 1.7 9.7 7 1.3 13.2 7 4.3 9.1 7 2.2
8L
of 300 g, and the results are shown in Fig. 5. In the sheet state, the 8L steel is harder by about 30 VHN than the 7S steel. The hardness is lowest at the center region, and increases as the measurement location moves from the center to the surface. In the pipe state, the hardness of the two steels is higher by about 10 VHN than in the sheet state, and is the lowest at the center region as the pipe forming strain increases in the outer or inner surface region. 3.2. Tensile properties of sheets and pipes Room-temperature tensile stress–strain curves of tensile specimens collected from various positions of sheets and pipes of the 7S and 8L steels (Fig. 2) are shown in Fig. 6(a) through (d), and the
yielding behaviors are summarized in Table 3. In the sheet of the 7S steel, the discontinuous yielding occurs in the center region, and changes into quasi-continuous or continuous yielding mode as the specimen position moves to the outer or inner side. In the case of the 8L steel, the continuous yielding is shown in the center and inner side, while the quasi-continuous yielding is predominant in the outer side. The reason why the yielding behaviors of the inner and outer sides are different from those of the center region is related with the microstructural difference due to cooling rates. This difference in the yielding behavior is related with the formation of mobile dislocations. In general, the yielding curves become smooth when hard phases are uniformly distributed to more easily generate mobile dislocations [20,21]. In the
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S.S. Sohn et al. / Materials Science & Engineering A 573 (2013) 18–26
7S -Outer
8L -Outer
MA MA
20 m
20 m
7S -Center
8L -Center
MA MA
20 m
20 m
7S -Inner
8L -Inner
MA
MA
20 m
20 m
Fig. 4. Optical micrographs of the inner, center, and outer position (on the basis of the spiral pipe position) of (a) through (c) the 7S, and (d) through (f) the 8L steel specimens. Etched by a LePera solution [19].
270 Sheet Pipe
260
8L Steel
Hardness (VHN)
250 240 230 7S Steel 220 210 200 Inner
Center
Outer
190 -8
-6
0 4 -4 -2 2 Distance from the Center (mm)
6
8
Fig. 5. Vickers hardness as a function of distance from the center of the sheet and pipe.
surface region, the shear deformation amount and cooling rate during rolling and cooling are higher than in the center region. As a result, the volume fraction of AF is higher in the surface
region, as mentioned in Section 3.1 and Fig. 3(a) through (f). It is known that AF has fine grain size and contains a lot of hard secondary phases, which shows the smooth and continuous yielding behavior. Since the mode and amount of deformation applied to the sheet during the spiral pipe forming are varied with the sheet thickness, the yielding behaviors of the pipes are different from those of the sheets. It is noted that the yielding behaviors of the center of the sheet and pipe are the same because only a small amount of deformation is applied to the center. The yield strength and yield ratio measured from tensile stress–strain curves of Fig. 6(a) through (d) are summarized in Fig. 7(a) and (b). The yield strengths of the center of the sheet and pipe of the 7S steel are about 500 MPa, and those of the 8L steel are about 550 MPa (Fig. 7(a)), which satisfy the yield strength requirements of the API X70 and X80 steels. In both the sheets and pipes, the yield strength is the lowest at the center region. After the pipe forming, the yield strength increases as the specimen position moves from the center to the outer side, and decreases as it moves to the inner side. In general, the variation in yield strength is larger in the outer side than in the inner side. In the sheets, the yield ratios of the 7S steel are higher than those of the 8L steel (Fig. 7(b)). In the center region of the pipes, the yield ratios of the 7S steel are higher than those of the 8L steel, but are decreased down below those of the 8L steel as the specimen position moves to the outer or inner side.
800 700 600 500 400 300 200 100 0
Inner Center 0
Stress (MPa)
Stress (MPa)
7S-Sheet
5
10
15 20 Strain (%)
25
800 700 600 500 400 300 200 100 0
Outer 30 35
7S-Pipe
Inner Center 0
10
5
15 20 Strain (%)
25
Outer 30 35
800 700 600 500 400 300 200 100 0
23
8L-Sheet
Inner Center 0
Stress (MPa)
Stress (MPa)
S.S. Sohn et al. / Materials Science & Engineering A 573 (2013) 18–26
5
10
15 20 Strain (%)
800 700 600 500 400 300 200 100 0
25
Outer 30 35
8L-Pipe
Inner Center 0
5
10
15 20 Strain (%)
25
Outer 30 35
Fig. 6. Room-temperature tensile stress–strain curves of the tensile specimens collected from various positions of the sheets and pipes of (a) and (c) the 7S, and (b) and (d) the 8L steels.
Table 3 Yielding behavior observed from tensile stress–strain curves of tensile specimens collected from various positions of sheets and pipes of the X70 and X80 steels. Shape
Sheet Pipe
Steel
7S 8L 7S 8L
Tensile specimen position Inner
’
’
Center
-
-
Outer
Q C C C
Q C C C
D C C Q
D Q D Q
D Q D D
Q Q D D
C Q D D
‘C’, ‘Q’, and ‘D’ mean continuous, quasi-continuous, and discontinuous yielding behaviors, respectively.
4. Discussion 4.1. Variation in yielding behavior before and after spiral pipe forming Tensile specimens of the sheet of the 7S steel mainly composed of QPF show mostly the discontinuous yielding behavior, and have high yield ratios of about 88%. On the other hand, tensile specimens of the sheet of the 8L steel mainly composed of AF and GB show the quasi-continuous or continuous yielding behavior, and have low yield ratios of about 83%. This is because their microstructures and amounts of mobile dislocations formed inside them are different. Kim et al. [20] reported from researches on microstructures composed of AF and GB, together with some MA, that this MA formed many dislocations at interfaces of adjacent ductile phases. Since most of these dislocations are mobile ones which can cause easy yielding in the initial stage of deformation, they lead to the continuous yielding, and raise the tensile strength. Thus, the yield ratio is lower in the 8L steel. In the pipes of the 7S and 8L steels, the continuous yielding occurs in the inner side, whereas the discontinuous yielding
occurs in the outer side. This variation in yielding behavior is mainly related with the mode and amount of deformation deviated from the thickness position of the sheet. During the spiral pipe forming, the sheet is subjected to compressive deformation in the inner side and to tensile deformation in the outer side. These increased amounts of compressive and tensile deformation in the inner and outer sides, respectively, can be confirmed by the distributions of forming strain (Eq. (1)). In the inner side of the pipe, the compressive-tensile deformation is applied during the tensile test, while the Bauschinger effect is working [22,23]. During the forward compressive deformation, back stresses are formed at mobile dislocations and other dislocations, grain boundaries, and precipitates. The backward tensile deformation is helped by these back stress fields, and mobile dislocations are partially blocked by easily-sheared obstacles on slip planes or by asymmetrical micro-stresses between dislocation pile-ups in the initial stage of deformation, thereby leading to the transient portion in which the radical strain hardening occurs [24,25]. Thus, the continuous yielding occurring in the inner side of the pipes of the 7S and 8L steels is associated with the Bauschinger effect, which increases from the center to the inner side as the effective strains increase. In the outer side of the pipe, the tensile–tensile deformation is applied during the tensile test, while the strain hardening effect is largely working [22,23]. The strain hardening is determined by factors such as grain size, volume fraction of precipitates, and density of tangled immobile dislocations [20]. In the spiral pipe forming, the dislocation density is increased by the tensile deformation, and dislocation cell structures are formed [26], thereby resulting in the decreased strain hardening. Since the final tensile test data include a considerable amount of strain hardening caused by the spiral pipe forming, they show high yield ratios as well as discontinuous yielding. Along with these reasons, the discontinuous yielding can be caused by the strain aging. The linepipe steel pipes should be
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S.S. Sohn et al. / Materials Science & Engineering A 573 (2013) 18–26
100 Sheet Pipe
700
8L Steel
650 600 550
Sheet Pipe
95 Yield Ratio (%)
Yield Strength (MPa)
750
7S Steel
7S Steel 90 85
500
80
450
75
8L Steel -8 -6 -4 -2 0 2 4 6 8 Outer Inner Distance from the Center (mm)
-8 -6 -4 -2 0 2 4 6 8 Outer Inner Distance from the Center (mm)
Fig. 7. (a) Yield strength and (b) yield ratio measured from tensile stress–strain curves as a function of distance from the center of the sheet or pipe.
80
Table 4 Change of yield strength measured before and after the spiral pipe forming of the X70 and X80 steels (unit: MPa).
7S 8L
Tensile Specimen Position
Average
Inner
’
’
Center
-
-
Outer
41 20
26 7
20 2
1 þ2
þ 33 þ 44
þ 52 þ 65
þ63 þ68
þ9 þ21
subjected to the coating, e.g., epoxy coating, in the final manufacture process, and thus they are often strain-aged by the thermal effect [27]. Since they are often exposed to temperatures lower than 100 1C, they can be naturally aged. In order to prevent the natural aging during the storage, they should be kept in a refrigerator at 20 1C, but the 7S and 8L steels were stored for a long time in an open atmosphere. Thus, the discontinuous yielding occurring in their outer side might be affected by the strain aging. However, the continuous yielding occurs in the inner side. This might be because the strain aging does not completely eliminate the transient portion due to the Bauschinger effect [23]. Williams [25] conducted the intermediate-temperature thermal treatments at 150–315 1C for 1–24 h in order to induce the interstitial-solute strain aging, but the transient portion still remained because the permanent softening observed on the strain reversal was not completely eliminated. Therefore, discontinuous yielding in the outer side can be affected by aging, but its effect is not enough to suppress the Bauschinger effect in the inner side. 4.2. Effects of microstructure on yield strength The change of yield strengths measured before and after the spiral pipe forming is plotted in Fig. 8 as a function of distance from the center of the sheet or the pipe, and the detailed data are summarized in Table 4. In the center of the 7S and 8L steels, the change of yield strength is hardly shown. However, the change of yield strength decreases in the inner side, whereas it increases in the outer side. When the 7S and 8L steels are compared, the change of yield strength is larger in the 8L steel in the outer side than in the 7S steel, but it is smaller in the 8L steel in the inner side. The decreased yield strength in the inner side is caused by the Bauschinger effect. According to Singh and Ramaswamy [28] the strain hardening effect was overridden by the Bauschinger effect in materials having high yield ratios, while the strain hardening effect was predominant over the Bauschinger effect in materials having low yield ratios. Park et al. [26] reported from
Change of Yield Strength (MPa)
Steel
σy (Pipe) -σy (Sheet)
1.25%
0.8%
60
1.5%
0.4% 1%
40
0 -20 -40
Pipe Forming Strain
0.5%
20 -0.4% -0.8% -1.25% -1%
0%
-0.5% 7S Steel 8L Steel
-1.5%
-8 Inner
-6
-4
-2
0
2
4
6
8 Outer
Distance from the Center (mm) Fig. 8. Change of yield strength before and after the spiral pipe forming as a function of distance from the center of the sheet or the pipe.
the yielding behaviors of API X80 steels that the Bauschinger effect was large in steels showing the quasi-continuous or discontinuous yielding, whereas it was small in steels showing the continuous yielding. Thus, the 8L steel composed of AF and GB together with a small amount of MA shows the continuous yielding and consequently the smaller Bauschinger effect than the 7S steel mainly composed of QPF, and shows the smaller change of yield strength in the inner side (Fig. 8). Meanwhile, the compressive strains introduced into the steel in the inner side of the pipe are higher in the 7S steel, and thus may lead to a larger Bauschinger effect. For the direct comparison, as shown in Fig. 8, the change of yield strength at the 7.5 mm position of the 8L steel is similar to that at the 2.5 mm position of the 7S steel, although the forming strain at the 7.5 mm position is higher. The compressive strains at the 7.5 mm position of the 8L steel and that at the 2.5 mm position of the 7S steel are 1.25% and 0.5%, respectively, as shown in Fig. 1. In other words, the Bauschinger effect is lower in the 8L steel than the 7S steel. The increased yield strength in the outer side of the pipe is caused by the difference in strain hardening of the 7S and 8L steels. As mentioned above, the 8L steel has the larger strain hardening than the 7S steel, which can be expected by the lower yield ratio in the 8L steel. The largely increased amount of yield strength in the 8L steel is also confirmed by the variation in yield ratio of Fig. 7(b). Since strain hardening decreases in the tensile
S.S. Sohn et al. / Materials Science & Engineering A 573 (2013) 18–26
test as the amount of strain hardening during the spiral pipe forming increases, the yield ratio of the outer side of the 8L steel is higher than that of the 7S steel. In the inner side, the yield ratio of the 7S steel decreases as the Bauschinger effect becomes large, and the yield ratio of the 8L steel increases slightly as the Bauschinger effect is counterbalanced by the strain hardening effect of the spiral pipe forming. 4.3. Effects of pipe forming strain (thickness/diameter ratio) on yield strength The yield strength of the pipe is determined by competing mechanisms of strain hardening effect of the outer side and the Bauschinger effect of the inner side. The overall yield strength of the 7S steel increases by 9 MPa after spiral pipe forming, while that of the 8L steel increases by 21 MPa (Table 4 and Fig. 8). Here, in addition to the microstructural effects, the different pipe forming strains of the two steels should be considered. Fig. 8 shows that the hardening in the outer side is slightly higher in the 8L steel, while the Bauschinger effect of the 8L steel is lower than that of the 7S steel in the inner side. As shown in Fig. 1, the maximum forming strains are 1.9% and 1.53% for the 7S and 8L steels, respectively. Thus, the forming strain is higher in the 7S steel pipes by about 0.4% at most than in the 8L steel pipes. Since the Bauschinger effect is largely varied even with a small amount of deformation in the initial stage of deformation, it can sensitively affect the change of yield strength [29]. If the pipe forming strain of the 8L steel increases by controlling the t/D ratio, the strain hardening effect largely increases. As a result, the 8L steel can obtain the larger increase in yield strength after the pipe forming because the small Bauschinger effect acted at a certain forming strain. On the contrary, in the 7S steel, the increase in strain hardening effect is not high enough to suppress the Bauschinger effect. An increase in forming strain would lead to a significant increase in yield strength for the 8L steel, but the increase in yield strength would be lower in the 7S steel. Thus, the increased yield strength after the spiral pipe forming can be achieved by controlling thickness and diameter of pipes in order to maximize the strain hardening effect against the Bauschinger effect. These aforementioned results show that the variation in yield strength after the spiral pipe forming of the X70 and X80 steels can be explained by competing mechanisms between strain hardening effect and Bauschinger effect. In order to increase the yield strength, steels showing continuous yielding and low yield ratios should be selected because the strain hardening effect and the Bauschinger effect varied with the yielding behavior and yield ratio. For achieving these steels, low-temperature transformation microstructures, together with a small amount of MA, are more desirable than QPF. In addition to microstructural consideration, the pipe forming methods, whose strain hardening effect maximally acted while the Bauschinger effect is minimized, are positively utilized by understanding forming strains of each pipe forming process in advance.
5. Conclusions API X70 and X80 steel sheets were shaped to be pipe with different pipe forming strains (thickness/diameter ratio) by spiral pipe forming, and their yielding behavior, yield strength, and yield ratio before and after the pipe forming were investigated.
(1) There were variations in yielding behavior before and after the spiral pipe forming. In the pipes of the 7S and 8L steels,
25
continuous yielding as well as low yield ratio occurred in the inner side, whereas discontinuous yielding together with high yield ratio occurred in the outer side. The Bauschinger effect and strain hardening effect were mainly dominant in the inner and outer sides, respectively, and were increased from the center to the outer or inner side as the pipe forming strains increased. Along with these reasons, the re-aging could affect the yielding behavior, but the effect was not enough to suppress the Bauschinger effect in the inner side. (2) The microstructures of the 7S and 8L steels affected the change of yield strengths before and after the spiral pipe forming. In the inner side, the 8L steel composed of mainly low-temperature transformation microstructures such as acicular ferrite and granular bainite, together with a small amount of martensite-austenite constituent, showed the smaller decrease of yield strength than the 7S steel mainly composed of quasi-polygonal ferrite because of the smaller Bauschinger effect. In the outer side, the 8L steel had larger strain hardening than the 7S steel, and thus the 8L steel showed the larger increase of yield strength than the 7S steel after the pipe forming. (3) The pipe forming strain affected the change of yield strengths before and after the spiral pipe forming. The yield strength of the pipe was determined by competing mechanisms of strain hardening effect of the outer side and Bauschinger effect of the inner side. In order to achieve a larger increased yield strength after the spiral pipe forming, it was necessary to maximize the strain hardening effect and minimize the Bauschinger effect by controlling the thickness/diameter ratio.
Acknowledgments This work was supported by POSCO under a Contract no. 2011Y017 and by the Ministry of Knowledge Economy under a Grant no. 100400-25. The authors would like to thank Mr. Hyeokjae Jeong of POSTECH for their help with the spiral pipe forming process. References ¨ H.G. Hillenbrand, C.J. Heckmann, K.A. Niederhoff, in: Proceedings of [1] M.K. Graf, the 13th International Offshore and Polar Engineering Conference, International Society of Offshore and Polar Engineers, Honolulu, 2003. [2] J. Yoo, S. Ahn, D. Seo, W. Song, K. Kang, Mater. Manuf. Processes 26 (2011) 154–160. [3] A. Abel, H. Muir, Philos. Mag. 26 (1972) 489–504. [4] T. Taira, T. Osuka, Y. Ishida, in: Proceedings of 15th Mechanical Working and Steel Processing Conference, Pittsburgh, 1977. [5] G.E. Dieter, Mechanical Metallurgy, third ed., McGraw-Hill Book Co., New York, 1986, pp. 236–237. [6] L. Zhonghua, G. Haicheng, Metall. Trans. 21A (1990) 717–724. [7] N. Ibrahim, J.D. Embury, Mater. Sci. Eng. A19 (1975) 147–149. [8] K.J. Park, Y.R. Jeo, J. Korean Inst. Met. Mater. 36 (1998) 1777–1783. [9] C.W. Choi, H.J. Koh, S. Lee, Metall. Mater. Trans. 31A (2000) 2669–2674. [10] I. Tamura, H. Sekine, T. Tanaka, C. Ouchi, Thermomechanical Processing of High-strength Low-alloy Steels, Butterworth & Co., London, 1988. [11] J. Takamura, S. Mizoguchi, in: Proceedings of 6th International Iron and Steel Congress, ISIJ, Tokyo, 1990. [12] G. Huang, K.M. Wu, Met. Mater. Int. 17 (2011) 847–852. [13] ASTM Standard E8m-09, Standard Test Methods for Tensile Testing of Metallic Materials, Annual Book of ASTM Standards, ASTM, West Conshohocken, PA, 2009. [14] T. Araki, Atlas for Bainitic Microstructures, ISIJ, Tokyo, 1992. [15] G. Krauss, S.W. Thompson, ISIJ 35, 1995, pp. 937–945. [16] K.-T. Park, S.W. Hwang, J.H. Ji, C.H. Lee, Met. Mater. Int. 17 (2011) 349–356. [17] S.W. Thompson, D.J. Colvin, G. Krauss, Metall. Trans. 21A (1990) 1493–1507. [18] H.K.D.H. Bhadeshia, Mater. Sci. Eng. A378 (2004) 34–39. [19] F.S. LePera, Metallography 12 (1979) 263–268. [20] Y.M. Kim, S.K. Kim, Y.J. Lim, N.J. Kim, ISIJ Int. 42 (2002) 1571–1577. [21] S.K. Kim, Y.M. Kim, Y.J. Lim, N.J. Kim, in: Proceedings of 15th Conference on Mechanical Behaviors of Materials, Seoul, Korea, 2001, pp. 177–186.
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Contents lists available at SciVerse ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Effects of microstructure and pipe forming strain on yield strength before and after spiral pipe forming of API X70 and X80 linepipe steel sheets Seok Su Sohn a, Seung Youb Han a, Jin-ho Bae b, Hyoung Seop Kim a, Sunghak Lee a,n a b
Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang 790-784, Republic of Korea Sheet Products & Process Research Group, Technical Research Laboratories, POSCO, Pohang 790-785, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 October 2012 Received in revised form 20 February 2013 Accepted 25 February 2013 Available online 7 March 2013
API X70 and X80 linepipe steel sheets were shaped in the form of pipe with different strains (thickness/ diameter ratio) by spiral pipe forming. Tension specimens taken from steel sheets or pipes at an interval of 2.5 mm were tested, and their yielding behavior, yield strength, and yield ratio before and after forming were analyzed. In the pipes, the continuous yielding and low yield ratios were shown in the inner side, whereas the discontinuous yielding and high yield ratios were shown in the outer side. This was because the Bauschinger effect and the strain hardening effect were mainly dominant in the inner and outer sides, respectively. The overall yield strength after spiral piping was defined by the competing effect of the strain hardening and the Bauschinger effect. The competing effects depended on the microstructure and the pipe forming strains. The low-temperature transformation microstructures were preferred for achieving the larger increase of overall yield strength after pipe forming. For a specific microstructure an optimization of the yield strength can also be achieved by controlling the pipe forming strain in order to maximize the strain hardening effect and to minimize the Bauschinger effect. & 2013 Elsevier B.V. All rights reserved.
Keywords: Electron microscopy Bainite Martensite Thermomechanical processing Sheet forming Hardening
1. Introduction Linepipe steels used for long-range transportations of crude oil or natural gas generally require high strengths to endure high pressures [1,2]. Widely used API X70 or X80 grade linepipe steels are fabricated in a sheet form, and then shaped as pipe. Among properties required for the pipes, the minimum yield strength in the circumferential direction is important due to the yield strength change after forming. During the forming, outer and inner walls of pipes are subjected to different strains, i.e., tensile strains on the outer wall and compressive strains on the inner wall, which also varied with pipe forming strain of thickness/diameter (t/D) ratio and the thickness position of the sheet. According to this strain history, sheets flattened from pipes often show the lower yield strength than original sheets, which leads to uncertainties of supplying linepipe steels satisfying the yield strength standard required by the American Petroleum Institute (API). The reason why the yield strength decreases under repeated tension and compression strains is generally interpreted by the Bauschinger effect. The resistance to plastic deformation is lowered, when a material already deformed in one direction is strained again in an opposite direction [3,4]. It works
n
Corresponding author. Tel.: þ82 54 279 2140; fax: þ82 54 279 5887. E-mail address: [email protected] (S. Lee).
0921-5093/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2013.02.050
importantly in pipe forming processes. According to Orowan’s theory [5], back stresses increase as the density of mobile dislocations increases during forward plastic deformation. The Bauschinger effect is proportional to the density of mobile dislocations, whose generation, migration, and pile-up are affected by microstructural factors [6,7]. Park et al. [8] studied the Bauschinger effect of API X46 to X70 grade linepipe steels, and quantitatively analyzed effects of microstructures on variation of the yield strength after the pipe forming. The yield strength was varied with pipe forming methods and forming strains (t/D ratios), which should be carefully considered before the yield strength measurement. Choi et al. [9] investigated the variation in yield strength of spiral formed to be pipe X70 linepipe steels, and reported that the Bauschinger effect decreased by increasing the volume fraction of acicular ferrite (AF) and the occurrence of continuous yielding. In these linepipe steels, the deformation amounts subjected to steel sheets are varied with pipe forming methods, and the strain hardening effect and the Bauschinger effect work differently with the deformation amount of each microstructure. In X80 grade linepipe steels, their microstructures are rather complicated because they consist of low-temperature transformation microstructures such as acicular ferrite (AF) and granular bainite (GB), which leads to difficulties of precise estimation of yield strength before and after the pipe forming. In consideration of these microstructural factors and pipe forming strains (t/D ratios) varied with pipe forming methods
S.S. Sohn et al. / Materials Science & Engineering A 573 (2013) 18–26
and pipe diameters, the variation in yield strength is needed to be carefully analyzed. In general, the yield strength increases with increasing deformation amount and consequently the strain hardening effect, whereas it decreases with increasing the Bauschinger effect. Therefore, effects of deformation amount on the yield strength in consideration of microstructures and pipe forming strains should be systematically analyzed in order to identify the variation in the yield strength and to investigate the competing mechanism between strain hardening and Bauschinger effects, but very few studies have been conducted. In this study, API X70 and X80 steel sheets were shaped to be pipe with different pipe forming strains by the spiral pipe forming. Tension specimens taken from steel sheets or pipes at an interval of 2.5 mm were tested, and their yielding behavior, yield strength, and yield ratio before and after pipe forming were analyzed. From these results, correlations between yielding behavior, yield strength, yield ratio, and microstructural factors were investigated, and the methods for preventing or minimizing the reduction in yield strength after the spiral pipe forming were suggested.
2. Experimental 2.1. API X70 and X80 linepipe steel sheets Commercial API X70 and X80 grade linepipe steels having a minimum yield strength level of 483 MPa (70 ksi) and 552 MPa (80 ksi), respectively, were used in this study, and their chemical compositions are shown in Table 1. The carbon equivalent is higher in the X80 steel than in the X70 steel. An overall grain refinement effect was expected by rolling with a high rolling reduction ratio (over 80%) in the non-recrystallized region of austenite after austenitization at 1180–1200 1C [10–12]. The rolling was finished at 790–840 1C in the austenite region above Ar3. After finishing rolling, the steels were cooled from the finish rolling temperature to 500–525 1C at a cooling rate of 16 1C/s. The final sheet thickness was 19.0 mm and 18.4 mm for the X70 and X80 steels, respectively.
19
forming strains of the 7S and 8L steels were 1.9% and 1.53%, respectively. The forming strain is higher in the 7S steel by about 0.4% at most than in the 8L steel. 2.3. Microstructural analysis The steel sheets were polished and etched in a 2% nital solution, and microstructures of longitudinal–short transverse (L-S) planes were observed by an optical microscope and a scanning electron microscope (SEM, model: JSM-6330F, JEOL, Japan). Volume fractions of microstructures present in the steel sheets were measured by an image analyzer. 2.4. Tensile and hardness tests Schematic diagrams showing direction and location of tensile specimens obtained from a sheet or a pipe are illustrated in Fig. 2. Seven tensile specimens were obtained from the sheet or pipe at an interval of 2.5 mm, and the tensile specimen direction (bluecolored arrow) was deviated by 301 from the rolling direction (red-colored arrow). Plate-type specimens having a gage length of 30 mm, a gage width of 5 mm, and a gage thickness of 1 mm were prepared, and were tested at room temperature at a strain rate of 5 10 3 s 1 in accordance with the ASTM standard test method [13] by a universal testing machine (model; 8801, Instron, Canton,
2.2. Pipe forming processes
eðXÞ ¼ 2X=ðDtÞ
ð1Þ
where e(X), D, t, and X are the strain at the distance X, outer diameter of the pipe, thickness of the sheet, and distance from the center of the sheet thickness respectively [8]. A schematic diagram showing the pipe forming process and the pipe forming strain at the distance X from the center of the sheet thickness is illustrated in Fig. 1(a) and (b), respectively. The maximum
2.5 1.9%
2.0 1.5 Pipe Forming Strain (%)
Pipes of 1020 mm and 1220 mm in outer diameter were fabricated by the spiral pipe forming of the X70 and X80 steel sheets, respectively, along the 301 direction deviated from the rolling direction. In convenience, an X70 steel sheet subjected to the small-diameter spiral pipe forming is referred to as ‘7S’, and an X80 steel sheet subjected to the large-diameter spiral pipe forming is referred to as ‘8L’. Since the strain gradient subjected during the spiral pipe forming is varied with the sheet thickness, the strain gradient at the distance X from the center of the sheet thickness can be expressed by
1.0 1.53% 0.5 0.0 -0.5 -1.0 -1.5
7S Steel 8L Steel
-2.0 Table 1 Chemical compositions of the API X70 and X80 steels (wt%). Steel C X70 X80
Mn
Si
o 0.04 1.29 0.30 o 0.06 1.78
Ni þCu Cr o 0.6
Mo
Al
o 0.4 0.01 0.035 0.17
-2.5
Ti þNb þ V Ceq o0.15
o 0.35 o 0.5
-10 -8 Inner
-6
-4
-2
0
2
4
6
Distance from the Center, X (mm)
8
10 Outer
Fig. 1. (a) Schematic diagram showing the pipe forming process and (b) the pipe forming strain at the distance X from the center of a sheet thickness.
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S.S. Sohn et al. / Materials Science & Engineering A 573 (2013) 18–26
Pipe Sheet Welding Line
30°
Rolling Direction 30 °
Tensile Specimen
Rolling Direction
Outer Center Inner Rolling Direction 30° Outer Center Inner
Fig. 2. Schematic diagrams showing direction and location of tensile specimens obtained from a sheet or a pipe. Seven tension specimens are taken from the sheet or pipe at an interval of 2.5 mm, and the tensile specimen direction (blue-colored arrow) is deviated by 301 from the rolling direction (red-colored arrow). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
MA, USA) of 100 kN capacity. Strain was measured over a gauge length of 30 mm using a contact 30 mm extensometer. The 0.2% offset stress was determined to be the yield strength in the steel specimens showing continuous yielding behavior, whereas the lower yield point was determined to be the yield strength in the steel specimens showing discontinuous yielding behavior. Tensile tests were conducted three times at least for each test condition, and the data were averaged. Vickers Hardness was measured at the same position of tensile specimen under a load of 300 g.
3. Results 3.1. Microstructures of API X70 and X80 linepipe steel sheets SEM micrographs of the outer, center, and inner position (on the basis of the spiral pipe position) of the 7S and 8L steel specimens are shown in Fig. 3(a) through (f). The volume fractions of various microstructures and average grain size were measured, and the results are shown in Table 2. The 7S and 8L steel specimens are composed of quasi-polygonal ferrite (QPF), acicular ferrite (AF), granular bainite (GB), pearlite (P), and martensiteaustenite constituent (MA) [14–18]. According to shapes and characteristics of microstructures, QPF is transformed at lower temperatures and faster cooling rates than polygonal ferrite, and has irregular grain boundaries. Its strength is relatively low because secondary phases are hardly formed inside grains or along grain boundaries. AF is characterized by fine grain size, irregular-shape, and alignment in arbitrary directions, and can be grouped into packets, depending on orientations between neighboring laths. It is known as a microstructure with good
combination of strength and toughness because of fine size and high interior dislocation density. GB contains island-type MAs, and has low toughness because of large packet size. P is a stable microstructure composed of ferrite and cementite, and its strength is high. MA is secondary phases formed at fast cooling rates, and has high strength and low toughness. Each microstructure is classified by these morphological categories as marked in the micrographs of Fig. 3(a) through (f). Since the differentiation between MA constituents and other microstructures was difficult in SEM micrographs, the specimens were etched in a LePera solution [19]. MAs and other microstructures (QPF, AF, GB, and P) were colored in bright white and brown, respectively. Optical micrographs of the 7S and 8L specimens etched in a LePera solution are shown in Fig. 4(a) through (f). The volume fractions of MA of the 8L steel specimens ranged from 1.9% to 2.5%, and other microstructures occupy the rest of MA, whereas MAs are hardly observable in the 7S steel specimen. According to the volume fraction data of QPF, AF, GB, P, and MA in Table 2, the 7S steel specimens are mostly composed of QPF, together with AF and a small amount of P. The outer and inner positions have finer grain sizes than the center position because of faster cooling rates. The 8L steel specimens mostly consist of AF, together with GB, instead of QPF, as low-temperature transformation microstructures are formed during rapid cooling from the austenite region. They also contain a considerable amount of MA, while pearlites are hardly found, because they have high carbon equivalent. In the center position, the grain size is larger, and the volume fractions of GB and MA are higher than in the outer or inner position due to slower cooling rates. The Vickers hardness was measured with intervals of 2.5 mm outward from the inner surface of the sheet and pipe under a load
S.S. Sohn et al. / Materials Science & Engineering A 573 (2013) 18–26
21
8L -Outer
7S -Outer AF P MA QPF AF
5 m
5 m
8L -Center
7S -Center AF P
GB
MA
AF
QPF
5 m
5 m
7S -Inner
8L -Inner GB
P
QPF AF
MA AF 5 m
5 m
Fig. 3. SEM micrographs of the inner, center, and outer position (on the basis of the spiral pipe position) of (a) through (c) the 7S and (d) through (f) the 8L steel specimens. Etched by a nital solution. Table 2 Volume fractions of quasi-polygonal ferrite, acicular ferrite, granular bainite, pearlite, and martensite-austenite constituent and average grain size in the X70 and X80 steels (unit: %). Steel
Position
Quasi polygonal ferrite
Acicular ferrite
Granular bainite
Pearlite
Martensite-austenite constituent
Average grain size (mm)
7S
Outer Center Inner Outer Center Inner
Bal. Bal. Bal. – – –
28 76.0 21 74.8 32 77.7 Bal. Bal. Bal.
– – – 237 6.4 447 3.3 177 6.3
0.9 70.1 1.1 70.2 0.7 70.2 o 0.2 0.6 70.2 o 0.2
o 0.2 o 0.2 o 0.2 1.97 0.5 2.57 0.2 2.07 0.4
11.07 1.2 13.2 7 2.6 10.57 1.7 9.7 7 1.3 13.2 7 4.3 9.1 7 2.2
8L
of 300 g, and the results are shown in Fig. 5. In the sheet state, the 8L steel is harder by about 30 VHN than the 7S steel. The hardness is lowest at the center region, and increases as the measurement location moves from the center to the surface. In the pipe state, the hardness of the two steels is higher by about 10 VHN than in the sheet state, and is the lowest at the center region as the pipe forming strain increases in the outer or inner surface region. 3.2. Tensile properties of sheets and pipes Room-temperature tensile stress–strain curves of tensile specimens collected from various positions of sheets and pipes of the 7S and 8L steels (Fig. 2) are shown in Fig. 6(a) through (d), and the
yielding behaviors are summarized in Table 3. In the sheet of the 7S steel, the discontinuous yielding occurs in the center region, and changes into quasi-continuous or continuous yielding mode as the specimen position moves to the outer or inner side. In the case of the 8L steel, the continuous yielding is shown in the center and inner side, while the quasi-continuous yielding is predominant in the outer side. The reason why the yielding behaviors of the inner and outer sides are different from those of the center region is related with the microstructural difference due to cooling rates. This difference in the yielding behavior is related with the formation of mobile dislocations. In general, the yielding curves become smooth when hard phases are uniformly distributed to more easily generate mobile dislocations [20,21]. In the
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S.S. Sohn et al. / Materials Science & Engineering A 573 (2013) 18–26
7S -Outer
8L -Outer
MA MA
20 m
20 m
7S -Center
8L -Center
MA MA
20 m
20 m
7S -Inner
8L -Inner
MA
MA
20 m
20 m
Fig. 4. Optical micrographs of the inner, center, and outer position (on the basis of the spiral pipe position) of (a) through (c) the 7S, and (d) through (f) the 8L steel specimens. Etched by a LePera solution [19].
270 Sheet Pipe
260
8L Steel
Hardness (VHN)
250 240 230 7S Steel 220 210 200 Inner
Center
Outer
190 -8
-6
0 4 -4 -2 2 Distance from the Center (mm)
6
8
Fig. 5. Vickers hardness as a function of distance from the center of the sheet and pipe.
surface region, the shear deformation amount and cooling rate during rolling and cooling are higher than in the center region. As a result, the volume fraction of AF is higher in the surface
region, as mentioned in Section 3.1 and Fig. 3(a) through (f). It is known that AF has fine grain size and contains a lot of hard secondary phases, which shows the smooth and continuous yielding behavior. Since the mode and amount of deformation applied to the sheet during the spiral pipe forming are varied with the sheet thickness, the yielding behaviors of the pipes are different from those of the sheets. It is noted that the yielding behaviors of the center of the sheet and pipe are the same because only a small amount of deformation is applied to the center. The yield strength and yield ratio measured from tensile stress–strain curves of Fig. 6(a) through (d) are summarized in Fig. 7(a) and (b). The yield strengths of the center of the sheet and pipe of the 7S steel are about 500 MPa, and those of the 8L steel are about 550 MPa (Fig. 7(a)), which satisfy the yield strength requirements of the API X70 and X80 steels. In both the sheets and pipes, the yield strength is the lowest at the center region. After the pipe forming, the yield strength increases as the specimen position moves from the center to the outer side, and decreases as it moves to the inner side. In general, the variation in yield strength is larger in the outer side than in the inner side. In the sheets, the yield ratios of the 7S steel are higher than those of the 8L steel (Fig. 7(b)). In the center region of the pipes, the yield ratios of the 7S steel are higher than those of the 8L steel, but are decreased down below those of the 8L steel as the specimen position moves to the outer or inner side.
800 700 600 500 400 300 200 100 0
Inner Center 0
Stress (MPa)
Stress (MPa)
7S-Sheet
5
10
15 20 Strain (%)
25
800 700 600 500 400 300 200 100 0
Outer 30 35
7S-Pipe
Inner Center 0
10
5
15 20 Strain (%)
25
Outer 30 35
800 700 600 500 400 300 200 100 0
23
8L-Sheet
Inner Center 0
Stress (MPa)
Stress (MPa)
S.S. Sohn et al. / Materials Science & Engineering A 573 (2013) 18–26
5
10
15 20 Strain (%)
800 700 600 500 400 300 200 100 0
25
Outer 30 35
8L-Pipe
Inner Center 0
5
10
15 20 Strain (%)
25
Outer 30 35
Fig. 6. Room-temperature tensile stress–strain curves of the tensile specimens collected from various positions of the sheets and pipes of (a) and (c) the 7S, and (b) and (d) the 8L steels.
Table 3 Yielding behavior observed from tensile stress–strain curves of tensile specimens collected from various positions of sheets and pipes of the X70 and X80 steels. Shape
Sheet Pipe
Steel
7S 8L 7S 8L
Tensile specimen position Inner
’
’
Center
-
-
Outer
Q C C C
Q C C C
D C C Q
D Q D Q
D Q D D
Q Q D D
C Q D D
‘C’, ‘Q’, and ‘D’ mean continuous, quasi-continuous, and discontinuous yielding behaviors, respectively.
4. Discussion 4.1. Variation in yielding behavior before and after spiral pipe forming Tensile specimens of the sheet of the 7S steel mainly composed of QPF show mostly the discontinuous yielding behavior, and have high yield ratios of about 88%. On the other hand, tensile specimens of the sheet of the 8L steel mainly composed of AF and GB show the quasi-continuous or continuous yielding behavior, and have low yield ratios of about 83%. This is because their microstructures and amounts of mobile dislocations formed inside them are different. Kim et al. [20] reported from researches on microstructures composed of AF and GB, together with some MA, that this MA formed many dislocations at interfaces of adjacent ductile phases. Since most of these dislocations are mobile ones which can cause easy yielding in the initial stage of deformation, they lead to the continuous yielding, and raise the tensile strength. Thus, the yield ratio is lower in the 8L steel. In the pipes of the 7S and 8L steels, the continuous yielding occurs in the inner side, whereas the discontinuous yielding
occurs in the outer side. This variation in yielding behavior is mainly related with the mode and amount of deformation deviated from the thickness position of the sheet. During the spiral pipe forming, the sheet is subjected to compressive deformation in the inner side and to tensile deformation in the outer side. These increased amounts of compressive and tensile deformation in the inner and outer sides, respectively, can be confirmed by the distributions of forming strain (Eq. (1)). In the inner side of the pipe, the compressive-tensile deformation is applied during the tensile test, while the Bauschinger effect is working [22,23]. During the forward compressive deformation, back stresses are formed at mobile dislocations and other dislocations, grain boundaries, and precipitates. The backward tensile deformation is helped by these back stress fields, and mobile dislocations are partially blocked by easily-sheared obstacles on slip planes or by asymmetrical micro-stresses between dislocation pile-ups in the initial stage of deformation, thereby leading to the transient portion in which the radical strain hardening occurs [24,25]. Thus, the continuous yielding occurring in the inner side of the pipes of the 7S and 8L steels is associated with the Bauschinger effect, which increases from the center to the inner side as the effective strains increase. In the outer side of the pipe, the tensile–tensile deformation is applied during the tensile test, while the strain hardening effect is largely working [22,23]. The strain hardening is determined by factors such as grain size, volume fraction of precipitates, and density of tangled immobile dislocations [20]. In the spiral pipe forming, the dislocation density is increased by the tensile deformation, and dislocation cell structures are formed [26], thereby resulting in the decreased strain hardening. Since the final tensile test data include a considerable amount of strain hardening caused by the spiral pipe forming, they show high yield ratios as well as discontinuous yielding. Along with these reasons, the discontinuous yielding can be caused by the strain aging. The linepipe steel pipes should be
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S.S. Sohn et al. / Materials Science & Engineering A 573 (2013) 18–26
100 Sheet Pipe
700
8L Steel
650 600 550
Sheet Pipe
95 Yield Ratio (%)
Yield Strength (MPa)
750
7S Steel
7S Steel 90 85
500
80
450
75
8L Steel -8 -6 -4 -2 0 2 4 6 8 Outer Inner Distance from the Center (mm)
-8 -6 -4 -2 0 2 4 6 8 Outer Inner Distance from the Center (mm)
Fig. 7. (a) Yield strength and (b) yield ratio measured from tensile stress–strain curves as a function of distance from the center of the sheet or pipe.
80
Table 4 Change of yield strength measured before and after the spiral pipe forming of the X70 and X80 steels (unit: MPa).
7S 8L
Tensile Specimen Position
Average
Inner
’
’
Center
-
-
Outer
41 20
26 7
20 2
1 þ2
þ 33 þ 44
þ 52 þ 65
þ63 þ68
þ9 þ21
subjected to the coating, e.g., epoxy coating, in the final manufacture process, and thus they are often strain-aged by the thermal effect [27]. Since they are often exposed to temperatures lower than 100 1C, they can be naturally aged. In order to prevent the natural aging during the storage, they should be kept in a refrigerator at 20 1C, but the 7S and 8L steels were stored for a long time in an open atmosphere. Thus, the discontinuous yielding occurring in their outer side might be affected by the strain aging. However, the continuous yielding occurs in the inner side. This might be because the strain aging does not completely eliminate the transient portion due to the Bauschinger effect [23]. Williams [25] conducted the intermediate-temperature thermal treatments at 150–315 1C for 1–24 h in order to induce the interstitial-solute strain aging, but the transient portion still remained because the permanent softening observed on the strain reversal was not completely eliminated. Therefore, discontinuous yielding in the outer side can be affected by aging, but its effect is not enough to suppress the Bauschinger effect in the inner side. 4.2. Effects of microstructure on yield strength The change of yield strengths measured before and after the spiral pipe forming is plotted in Fig. 8 as a function of distance from the center of the sheet or the pipe, and the detailed data are summarized in Table 4. In the center of the 7S and 8L steels, the change of yield strength is hardly shown. However, the change of yield strength decreases in the inner side, whereas it increases in the outer side. When the 7S and 8L steels are compared, the change of yield strength is larger in the 8L steel in the outer side than in the 7S steel, but it is smaller in the 8L steel in the inner side. The decreased yield strength in the inner side is caused by the Bauschinger effect. According to Singh and Ramaswamy [28] the strain hardening effect was overridden by the Bauschinger effect in materials having high yield ratios, while the strain hardening effect was predominant over the Bauschinger effect in materials having low yield ratios. Park et al. [26] reported from
Change of Yield Strength (MPa)
Steel
σy (Pipe) -σy (Sheet)
1.25%
0.8%
60
1.5%
0.4% 1%
40
0 -20 -40
Pipe Forming Strain
0.5%
20 -0.4% -0.8% -1.25% -1%
0%
-0.5% 7S Steel 8L Steel
-1.5%
-8 Inner
-6
-4
-2
0
2
4
6
8 Outer
Distance from the Center (mm) Fig. 8. Change of yield strength before and after the spiral pipe forming as a function of distance from the center of the sheet or the pipe.
the yielding behaviors of API X80 steels that the Bauschinger effect was large in steels showing the quasi-continuous or discontinuous yielding, whereas it was small in steels showing the continuous yielding. Thus, the 8L steel composed of AF and GB together with a small amount of MA shows the continuous yielding and consequently the smaller Bauschinger effect than the 7S steel mainly composed of QPF, and shows the smaller change of yield strength in the inner side (Fig. 8). Meanwhile, the compressive strains introduced into the steel in the inner side of the pipe are higher in the 7S steel, and thus may lead to a larger Bauschinger effect. For the direct comparison, as shown in Fig. 8, the change of yield strength at the 7.5 mm position of the 8L steel is similar to that at the 2.5 mm position of the 7S steel, although the forming strain at the 7.5 mm position is higher. The compressive strains at the 7.5 mm position of the 8L steel and that at the 2.5 mm position of the 7S steel are 1.25% and 0.5%, respectively, as shown in Fig. 1. In other words, the Bauschinger effect is lower in the 8L steel than the 7S steel. The increased yield strength in the outer side of the pipe is caused by the difference in strain hardening of the 7S and 8L steels. As mentioned above, the 8L steel has the larger strain hardening than the 7S steel, which can be expected by the lower yield ratio in the 8L steel. The largely increased amount of yield strength in the 8L steel is also confirmed by the variation in yield ratio of Fig. 7(b). Since strain hardening decreases in the tensile
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test as the amount of strain hardening during the spiral pipe forming increases, the yield ratio of the outer side of the 8L steel is higher than that of the 7S steel. In the inner side, the yield ratio of the 7S steel decreases as the Bauschinger effect becomes large, and the yield ratio of the 8L steel increases slightly as the Bauschinger effect is counterbalanced by the strain hardening effect of the spiral pipe forming. 4.3. Effects of pipe forming strain (thickness/diameter ratio) on yield strength The yield strength of the pipe is determined by competing mechanisms of strain hardening effect of the outer side and the Bauschinger effect of the inner side. The overall yield strength of the 7S steel increases by 9 MPa after spiral pipe forming, while that of the 8L steel increases by 21 MPa (Table 4 and Fig. 8). Here, in addition to the microstructural effects, the different pipe forming strains of the two steels should be considered. Fig. 8 shows that the hardening in the outer side is slightly higher in the 8L steel, while the Bauschinger effect of the 8L steel is lower than that of the 7S steel in the inner side. As shown in Fig. 1, the maximum forming strains are 1.9% and 1.53% for the 7S and 8L steels, respectively. Thus, the forming strain is higher in the 7S steel pipes by about 0.4% at most than in the 8L steel pipes. Since the Bauschinger effect is largely varied even with a small amount of deformation in the initial stage of deformation, it can sensitively affect the change of yield strength [29]. If the pipe forming strain of the 8L steel increases by controlling the t/D ratio, the strain hardening effect largely increases. As a result, the 8L steel can obtain the larger increase in yield strength after the pipe forming because the small Bauschinger effect acted at a certain forming strain. On the contrary, in the 7S steel, the increase in strain hardening effect is not high enough to suppress the Bauschinger effect. An increase in forming strain would lead to a significant increase in yield strength for the 8L steel, but the increase in yield strength would be lower in the 7S steel. Thus, the increased yield strength after the spiral pipe forming can be achieved by controlling thickness and diameter of pipes in order to maximize the strain hardening effect against the Bauschinger effect. These aforementioned results show that the variation in yield strength after the spiral pipe forming of the X70 and X80 steels can be explained by competing mechanisms between strain hardening effect and Bauschinger effect. In order to increase the yield strength, steels showing continuous yielding and low yield ratios should be selected because the strain hardening effect and the Bauschinger effect varied with the yielding behavior and yield ratio. For achieving these steels, low-temperature transformation microstructures, together with a small amount of MA, are more desirable than QPF. In addition to microstructural consideration, the pipe forming methods, whose strain hardening effect maximally acted while the Bauschinger effect is minimized, are positively utilized by understanding forming strains of each pipe forming process in advance.
5. Conclusions API X70 and X80 steel sheets were shaped to be pipe with different pipe forming strains (thickness/diameter ratio) by spiral pipe forming, and their yielding behavior, yield strength, and yield ratio before and after the pipe forming were investigated.
(1) There were variations in yielding behavior before and after the spiral pipe forming. In the pipes of the 7S and 8L steels,
25
continuous yielding as well as low yield ratio occurred in the inner side, whereas discontinuous yielding together with high yield ratio occurred in the outer side. The Bauschinger effect and strain hardening effect were mainly dominant in the inner and outer sides, respectively, and were increased from the center to the outer or inner side as the pipe forming strains increased. Along with these reasons, the re-aging could affect the yielding behavior, but the effect was not enough to suppress the Bauschinger effect in the inner side. (2) The microstructures of the 7S and 8L steels affected the change of yield strengths before and after the spiral pipe forming. In the inner side, the 8L steel composed of mainly low-temperature transformation microstructures such as acicular ferrite and granular bainite, together with a small amount of martensite-austenite constituent, showed the smaller decrease of yield strength than the 7S steel mainly composed of quasi-polygonal ferrite because of the smaller Bauschinger effect. In the outer side, the 8L steel had larger strain hardening than the 7S steel, and thus the 8L steel showed the larger increase of yield strength than the 7S steel after the pipe forming. (3) The pipe forming strain affected the change of yield strengths before and after the spiral pipe forming. The yield strength of the pipe was determined by competing mechanisms of strain hardening effect of the outer side and Bauschinger effect of the inner side. In order to achieve a larger increased yield strength after the spiral pipe forming, it was necessary to maximize the strain hardening effect and minimize the Bauschinger effect by controlling the thickness/diameter ratio.
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