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Microstructural and magnetic studies on thermomechanically treated HSLA steel
Journal of Magnetism and Magnetic Materials 88 (1990) 71-78 North-Holland
71
MICROSTRUCTURAL AND MAGNETIC STUDIES ON THERMOMECHANICALLY TREATED HSLA STEEL
S.P. N A R A Y A N a, V. R A O b, S. DAS b and O.N. M O H A N T Y b a Regional Research Laboratory, Bhopal-462 026, India b National Metallurgical Laboratory, Jamshedpur-831 007, India
Received 18 October 1989; in revised form 16 January 1990
Therrnomechanical treatment of commercial grade HSLA steel microalloyed with Nb, V and Ti has been carried out in order to obtain microstructures w;.th spheroidised carbides evenly distributed throughout the matrix and therefore with good magnetic properties at high strength levels. Alloys were tempei'ed after rolling-quenching, as well as forging-quenching.In both cases, rolled as well as forged, good magnetic properties were obtained at a strength of 600-620 MPa after spheroidisation of carbides. Thus, for a given level of carbon, the spheroidised carbides lead to a lower coercive force compared to the case in which they are lamellar. This is believed to be associated with lower locked-up microstressesin the former situation.
1. Introduction High strength materials with moderately soft magnetic properties are needed by the heavy electrical industries for hydrogenerator rotor poles, rim laminations, traction machines and rotors of portable generators. Due to the small size and the high rotational velocity of the rotor structure, these materials experience high stresses. Moreover, there is a possibility of temperatures building up to 2 0 0 - 2 5 0 ° C due to poor heat dissipation. Therefore, materials for such applications should possess high yield strength as well as moderately soft magnetic properties which are stable up to 200250 o C. Honeycutt and Steigerwald [1] evaluated the properties of high strength steels such as SAE 4320, 4340, modified H-11 A M 367 and maraging steel with ,+.o-~u.,+zo .... "~ Co wmcn ' " '- were prepared under different heat-treatment conditions for use in high speed rotors. So far, there seems to have been very little effort made to explore the possibility of using structural steels such as H S L A steels for soft magnetic applications. It has been shown by Swisher et al. [2] and Jiles [3,4] that in the case of . . . .
carbon steels, the detrimental effect of carbon on the soft magnetic properties would be considerably mitigated by spheroidisation of carbides. The present work was aimed at causing spheroidisation of carbides in H S L A steel containing 0.2% C and very small amounts of Nb, V and Ti through thermomechanical treatment (TMT) in order to achieve the fight combinatio,~ of strength and magnetic properties.
2. Experimental Commercial grade HSLA steel, with about 0.2 wt% C and bearing Nb, V and Ti as microalloying elements in the hot rolled condition, was taken for the study. The material was processed through the following two different routes:
,i
tempered at 675°C (RQT), - forged at 1150°C, quenched at 800°C and tempered at 6 7 5 ° C (FQT). Microstructural studies of the as-received and heat treated (after rolling/forging) samples were made using optical, scanning (Jeol-35) and trans-
0304-8853/90/$03.50 © 1990 - Elsevier Science Publishe." B.V. (North-Holland)
72
S.P. Narayan et al. / Thermomechanically treated HSLA steel
~..
'..~ -/%. ~ . ~ _
, 00
.
~,._
~
~,'~:~..~~~"~
~a
Fig. 1. Structural characteristics of as-received material: (a) optical micrograph shows ferrite and pearlite colonies in a banded manner; (b) SEM photograph displays the lamellar nature of pearlite.
la
Fig. 2. Structural characteristics of the samples rolled and water-quenched (RQ). (a) Optical micrograph shows needle-like features. (b, c) TEM shows acicular ferrite and lath martensite with some precipitates. (d) TEM at e higher magnification reveals the habit (and morphology) of the precipitates.
S.P. Narayan et al. / Thermomechanically treot~,~ltISLA steel
mission (Philips EM 400) electron microscopes. The magnetic properties were determined on toroid samples using a Walker's MH-3020 Hysteresisgraph. Magnetic tests at elevated temperature were carried out using toroid samples in which the primary and secondary windings of copper wire were insulated by means of ceramic fibre sleeves. Evaluations of the hardness and the tensile properties were also made.
73
3. Results 3.1. Microstructure
The as-received material (in the hot rolled condition) showed pearlite colonies with cementite lamellae (fig. 1). Specimens which were water quenched after rolling (RQ) showed the presence of acicular ferrite and lath martensite (figs. 2a-c).
Fig. 3. Structural characteristics of the specimens forged and water quenched (FQ). (a) Optical micrograph shows thick ferrite present along the prior-austenite grain-boundaries. (b) TEM photograph while showing slabs of ferrite precipitation along the grain boundaries (of prior austentite), also shows lath martensite and acicular ferrite. (c) TEM of another area shows the acicular nature and the presence of precipitates. (d) At high magnification TEM shows micro-twins in some of the martensite needles.
74
S.P. Narayan et al. / Thermomechanically treated HSLA steel
iii!!!iiill
m
?
Fig. 4. Microstructural changes accompanying the tempering of the RQ samples. SEM photographs (a), (b) and (c) correspond to 4.5, 30 and 52 h of tempering, respectively• One notices the spheroidisation of carbides and the increase in their size. (d) and (e) TEM-photographs corresponding to 52 h clearly reveal the formation of equia×ed grains of fertile as well as spheroidisation of carbides.
75
S.P. Narayan et al. / Thermomechanica!ly treated HSLA steel
l
d Fig. 5. Microstructural changes accompanying the tempering of FQ-samples. SEM photographs (a) and (b) correspond to 4.5 h and 52 h of tempering, respectively. Spheroidisation of carbides and their coalescence is noted. (c) and (d) TEM features correspond to 52 h. Spheroidisation of carbides, annihiliation of disloeat;,ons in ferrites ard the polygonization of the latter are noted.
Precipitation within the ferritic areas was clearly seen (fig. 2d). In the case of samples quenched after forging (FQ), the presence of a ferrite slab at the prior austenite grain boundaries was observed (figs. 3a and b). Also seen in these samples were the acicuiar ferrite and martensite. Microtwins were observed inside some martensites and needle-like precipitates were observed within the ferfite (figs. 3b-d). Tempering the RQ and FQ specimens initially resulted in the breaking up of the martensite into carbides and ferrite. With further increases in the tempering time, the carbides spheroidised and coalesced (figs. 4 and 5). Further, annhilation of
dislocations within the ferfite led to an equiaxed structure. 3.2. Mechanical properties
RQ and FQ samples showed high hardness and strength but low ductility (table 1). With tempering (RQT and FQT), hardness and strength dropped sharply during the initial stages, and thereafter, more gradually. 3.3. Magnetic properties at room lemperature
The permeability (t.t,~) and remanence (B~) were lowest in the quenched condition and rose
76
S.P. Naravan et aL / Thermomechanically treated HSLA steel
Table 1 Mechanical properties of TMT (rdled/forged) samples Prior treatment
Temper- Hardness Ultimate Elonhag (VPN) tensile gation time strength (%) (h) (MPa)
Rolled + Quenced RQ RQ RQ RQ RQ
0 0 4.5 4.5 30 52
510-520 510-520 240-245 240-245 198-206 186-196
1140 1670 770 1240 620 600
Forged + Quenched 0 FQ 4.5 FQ 30 FQ 52
410-450 270-285 203-210 176-185
1180 760 600 560
~) b)
a) b) a) a)
with tempering (table 2). High coercive force (He) values, which were obtained in the quenched samples, fell almost exponentially with tempering time. Close to a tempering time of 50 h, He registered a small increase in both the RQT and FQT specimens, decreased at longer tempering times (77 h).
15 a) 10 b) 2t2 a) 16 b) 23 a) 30 a)
3. 4. Elevated temperature magnetic properties The elevated temperature magnetic properties of the rolled + queu(.hed + tempered (RQT) and forged + quenched + tempered (FQT) samples are presented in table 3. The fall in the values of Bs and B r above room temperature and up to 300 °C
10 20 30 30
a) Longitudinal; b~ transverse.
Table 2 Magnetic properties of TMT (rolled and forged) samples Prior treatment
Tempering time (h)
Induction (Bs) at 7560 A / m (T)
Remanence ( B, )
RQ RQ RQ RQ RQ
0 4.5 30 52 77
FQ FQ FQ FQ
4.5 30 52 77
~'ma.~
B~//~s
(T)
Coercive force (A/m)
1.59 ;.57 1.74 1.70 1.64
1.07 1.08 1.44 1.27 1.25
1703 907 597 644 557
300 475 1075 750 1150
0.67 0.69 0.83 0°75 0.76
1.58 1.75 1.7,4 1.71
1.33 1.45 1.45 1.43
589 461 565 501
900 1700 1050 1175
0.86 0.83 0.83 0.84
Table 3 Elevated temperature magnetic properties Prior treatment
RQ + tempered 675 °C for 30 h
FQ + tempered 675 °C for 30 h
~ Room temperature.
Test temp. (°C)
(T)
RT ~) !00 200 300 360
1.74 ! .74 1.72 1 .f9 1.66
1.40 1.31 1.27
RT a) 100 20O 3OO
1.75 1.75 1.74 1.74
1.45 1,45 1.39 1.35
;nduction (B~) at 7560 A / m
Remanence (B,)
(13
(A/m)
1.44
597
A1.4 ."¢-T
#max
B,/Bs
1075
0.83
1 t't~c IIU f.)
U.O.~
477 342 302
1275 1900 2000
0.8i 0.78 0.77
461 434 366 366
1700 2000 1600 1700
0.83 0.83 0.80 078
S.P. Narayan et al. / Thermomechanicaily treated H S L A steel
was only marginal. But the decrease in H~ and the increase in /Xm~X with temperature were substantial.
4. Discussion The metaHographic studies of the RQ and FQ samples have shown the microconstituents to be acicular ferrite and martensite. The ferritic slabs observed in the FQ samples suggest that some ferrite precipitates out from the austenite even before quenching. A part of the ferrite has also taken up the Widmanstatten pattern. Although no pronounced alignment of constituent phase is observed along the rolling direction in the RQ samples, the strength properties in the transverse and longitudinal direction show a pronounced difference with the transverse section displaying a 50% higher value. This may be associated with crystallographic texture. Another feature of the microstructure is the presence of carbides. These are coarser than, but similar in habit to, those seen in the quenched samples (without forging or rolling) [5] or those fine carbides reported by Gau et al. [6]. The martensites show a mixture of a dislocated low-carbon variety and a micro-twinned high carbon variety (fig. 3d). This feature endorses the established view that there coold be a distribution of carbon in the martensite. Upon tempering, similar structural changes take place in both the RQ and FQ samples. Precipitation of fine carbides occurs during the early stages of tempering while growth and spheroidisation of carbides takes place at the later stages of tempering. The purpose of rolling or forging is to accelerate precipitation and spheroidisation of carbides [7,8] while the presence of niobium retards the n r ~ o o e e c~f e n h o r n i c l i ~ n t l n n
From a comparison of the time required for bringing about spheroidisation through TMT (preset study) with the time required by other methods, such as annealing or straightforward quenching and tempering [5], it appears that TMT followed by quenching and tempering is the more effective me~hod. This is not surprising when one takes into account the presence of more numerous
77
defects which aid the diffusion for microstructural changes. Thc lower strength in the FQ samples compared to the RQ samples (table 1) appears to be corroborated by the presence of chunky ferrites in the former. The fall in strength and the increase in ductility of the quenched specimens upon tempering are attributed to the release of high residual stresses, to an increase in the volume percent of ferrite and to the growth of ferrite. Upon tempering, the improvement in the magnetic properties of the RQ and the FQ specimens is about the same. However, the rolled samples have shown a higher coercive force than the forged ones. This is ostensibly due to the strong anisotropic effect of rolling. The influence of precipitates such as carbides on magnetic properties and on coercive force in particular, has been shown to vary with their morphology and size [3,4,9-12]. The small rise in the coercive force which occurs at around 50 h of tempering at 675°C is probably caused by fine precipitates of alloy carbides that effectively pin down the doman w~,lls; a subsequent increase in size beyond some 'ciitical value' [9] would lead to an eventual fall in ~:he coercive force ',:uch as the one which occurs at a tempering time of 70 h. The literature, while predicting some correlation between the size of the precipitates and interactions with domain walls, is silent on the underlying mechanism for the influence of the morphology or aspect-ratio of the carbide precipitates. There is experimental evidence in the literature [2-4,9,12] which shows that the spheroidal shape brings about a lowering of the coercive force. Identical results have also been reported by the authors earlier [5]. It may be hypothesized that anisotropy of stresses associated with a high aspect-ratio particle in the matrix tends to cau:;e a stronger interaction with the domain-walls. The !ocked-~_~p microstresses between the spherical carbides and the equiaxed ferrite matrix would be lower, hence a lowering of the coercive forces. Studies by Jiles [4] have shown dis~hit:t difi'~lc,ces in coercivities and hysteresis loss~ between normalised and spheroidised specimens of carbon steels at carbon levels greater than 0.4% C. However. they report negligible differences for steels ,:ontaining 0.24% C or
78
S.P. Narayan et al. / Thermomechanically treated HSLA steel
below, which is the carbon level in the present case. Thus, there appears to be some discrepancy. A clear explanation has yet to be found. The magnetic and tensile tests at elevated temperatures of both RQT and FQT samples have shown the properties to be reasonably stal~le up to 300 °C (table 3). In the case of RQT samples, the permeability rises gradually; whereas in the ease of the FQT samples, the permeability initially increases up to 100 °C is followed by a fall around 200°C and then an increase has been observed. Probably this kind of a small peak in /~ as a function of temperature is associated with the transformation of the carbides from a ferro- to a paramagnetic state. The fall in Bs and H c with temperature i~ in accordance with the existing theories.
(4) Spheroidised HSLA steels exhibit nearly stable magnetic properties up to 300 ° C.
Acknowledgements The authors would like to express their grateful thanks to the Director, National Metallurgical Laboratory, Jamshedpur, for his keen interest in this investigation. One of the authors (S.P.N.) would like to thank the authorities of RRL (Bhopal) for the kind permission to undertake the work as a part of the M. Tech progrmnme.
References [11 C.R. Honeycutt and E.A. Steigerwald, Trans. ASM 59
5 Conclusions
(1966) 113.
[2] J.H. Swisher, A.T. English and R.C. Stoffers, Trans. ASM (1) The morphology and size of the second phase particles have a significant influence on the magnetic properties of HSLA steels. The spheroidised structure induces attractive soft magnetic properties, probably due to lower locked-up micro stresses~ (2) Spheroidisation of carbides in HSLA steels can be accomplished more quickly by thermomechanical treatment than by straight quenching and tempering or simple annealing. (3) Microalloying of steel with Nb, V, Ti, etc. gives rise to high strength at lower carbon levels without impairing the soft magnetic properties.
62 (1969) 257.
[31 D.C. Jiles, J. Appl. Phys. 63 (1988) 2980. [4] D.C. Jiles, J. Phys. D21 (1988) 1186. [51 S.P. Narayan, V. Rao and O.N. Mohanty, J. Magn. Magn, Mat. (submitted).
[61 J.S. Gau, J.K. Koo and G. Thomas, Proc. Conf. on Fundamentals of Dual Phase Steels, Chicago (1981) p. 48.
[71 S. Chattopadhyay and C.M. Sellars, Acta Metall. 30 (1982) 157.
[81 A.H. Holtzman, J.C. Danko and R.D. Stout, Trans. Metall. Soc. AIME 212 (1958) 475.
[91 A.T. English, Acta Met. 15 (1967) 1573. [lOl L.J. Dijkstra and C.A. Wert, Phys. Rev. 79 (1950) 979. [111 W.C. Leslie mad D.W. Stevens, Trans. ASM 57 (1964) 261. [121 S.K. Ray and O.N. Mohanty, J. Magn. Magn. Mat. 78 (1989) 255.
71
MICROSTRUCTURAL AND MAGNETIC STUDIES ON THERMOMECHANICALLY TREATED HSLA STEEL
S.P. N A R A Y A N a, V. R A O b, S. DAS b and O.N. M O H A N T Y b a Regional Research Laboratory, Bhopal-462 026, India b National Metallurgical Laboratory, Jamshedpur-831 007, India
Received 18 October 1989; in revised form 16 January 1990
Therrnomechanical treatment of commercial grade HSLA steel microalloyed with Nb, V and Ti has been carried out in order to obtain microstructures w;.th spheroidised carbides evenly distributed throughout the matrix and therefore with good magnetic properties at high strength levels. Alloys were tempei'ed after rolling-quenching, as well as forging-quenching.In both cases, rolled as well as forged, good magnetic properties were obtained at a strength of 600-620 MPa after spheroidisation of carbides. Thus, for a given level of carbon, the spheroidised carbides lead to a lower coercive force compared to the case in which they are lamellar. This is believed to be associated with lower locked-up microstressesin the former situation.
1. Introduction High strength materials with moderately soft magnetic properties are needed by the heavy electrical industries for hydrogenerator rotor poles, rim laminations, traction machines and rotors of portable generators. Due to the small size and the high rotational velocity of the rotor structure, these materials experience high stresses. Moreover, there is a possibility of temperatures building up to 2 0 0 - 2 5 0 ° C due to poor heat dissipation. Therefore, materials for such applications should possess high yield strength as well as moderately soft magnetic properties which are stable up to 200250 o C. Honeycutt and Steigerwald [1] evaluated the properties of high strength steels such as SAE 4320, 4340, modified H-11 A M 367 and maraging steel with ,+.o-~u.,+zo .... "~ Co wmcn ' " '- were prepared under different heat-treatment conditions for use in high speed rotors. So far, there seems to have been very little effort made to explore the possibility of using structural steels such as H S L A steels for soft magnetic applications. It has been shown by Swisher et al. [2] and Jiles [3,4] that in the case of . . . .
carbon steels, the detrimental effect of carbon on the soft magnetic properties would be considerably mitigated by spheroidisation of carbides. The present work was aimed at causing spheroidisation of carbides in H S L A steel containing 0.2% C and very small amounts of Nb, V and Ti through thermomechanical treatment (TMT) in order to achieve the fight combinatio,~ of strength and magnetic properties.
2. Experimental Commercial grade HSLA steel, with about 0.2 wt% C and bearing Nb, V and Ti as microalloying elements in the hot rolled condition, was taken for the study. The material was processed through the following two different routes:
,i
tempered at 675°C (RQT), - forged at 1150°C, quenched at 800°C and tempered at 6 7 5 ° C (FQT). Microstructural studies of the as-received and heat treated (after rolling/forging) samples were made using optical, scanning (Jeol-35) and trans-
0304-8853/90/$03.50 © 1990 - Elsevier Science Publishe." B.V. (North-Holland)
72
S.P. Narayan et al. / Thermomechanically treated HSLA steel
~..
'..~ -/%. ~ . ~ _
, 00
.
~,._
~
~,'~:~..~~~"~
~a
Fig. 1. Structural characteristics of as-received material: (a) optical micrograph shows ferrite and pearlite colonies in a banded manner; (b) SEM photograph displays the lamellar nature of pearlite.
la
Fig. 2. Structural characteristics of the samples rolled and water-quenched (RQ). (a) Optical micrograph shows needle-like features. (b, c) TEM shows acicular ferrite and lath martensite with some precipitates. (d) TEM at e higher magnification reveals the habit (and morphology) of the precipitates.
S.P. Narayan et al. / Thermomechanically treot~,~ltISLA steel
mission (Philips EM 400) electron microscopes. The magnetic properties were determined on toroid samples using a Walker's MH-3020 Hysteresisgraph. Magnetic tests at elevated temperature were carried out using toroid samples in which the primary and secondary windings of copper wire were insulated by means of ceramic fibre sleeves. Evaluations of the hardness and the tensile properties were also made.
73
3. Results 3.1. Microstructure
The as-received material (in the hot rolled condition) showed pearlite colonies with cementite lamellae (fig. 1). Specimens which were water quenched after rolling (RQ) showed the presence of acicular ferrite and lath martensite (figs. 2a-c).
Fig. 3. Structural characteristics of the specimens forged and water quenched (FQ). (a) Optical micrograph shows thick ferrite present along the prior-austenite grain-boundaries. (b) TEM photograph while showing slabs of ferrite precipitation along the grain boundaries (of prior austentite), also shows lath martensite and acicular ferrite. (c) TEM of another area shows the acicular nature and the presence of precipitates. (d) At high magnification TEM shows micro-twins in some of the martensite needles.
74
S.P. Narayan et al. / Thermomechanically treated HSLA steel
iii!!!iiill
m
?
Fig. 4. Microstructural changes accompanying the tempering of the RQ samples. SEM photographs (a), (b) and (c) correspond to 4.5, 30 and 52 h of tempering, respectively• One notices the spheroidisation of carbides and the increase in their size. (d) and (e) TEM-photographs corresponding to 52 h clearly reveal the formation of equia×ed grains of fertile as well as spheroidisation of carbides.
75
S.P. Narayan et al. / Thermomechanica!ly treated HSLA steel
l
d Fig. 5. Microstructural changes accompanying the tempering of FQ-samples. SEM photographs (a) and (b) correspond to 4.5 h and 52 h of tempering, respectively. Spheroidisation of carbides and their coalescence is noted. (c) and (d) TEM features correspond to 52 h. Spheroidisation of carbides, annihiliation of disloeat;,ons in ferrites ard the polygonization of the latter are noted.
Precipitation within the ferritic areas was clearly seen (fig. 2d). In the case of samples quenched after forging (FQ), the presence of a ferrite slab at the prior austenite grain boundaries was observed (figs. 3a and b). Also seen in these samples were the acicuiar ferrite and martensite. Microtwins were observed inside some martensites and needle-like precipitates were observed within the ferfite (figs. 3b-d). Tempering the RQ and FQ specimens initially resulted in the breaking up of the martensite into carbides and ferrite. With further increases in the tempering time, the carbides spheroidised and coalesced (figs. 4 and 5). Further, annhilation of
dislocations within the ferfite led to an equiaxed structure. 3.2. Mechanical properties
RQ and FQ samples showed high hardness and strength but low ductility (table 1). With tempering (RQT and FQT), hardness and strength dropped sharply during the initial stages, and thereafter, more gradually. 3.3. Magnetic properties at room lemperature
The permeability (t.t,~) and remanence (B~) were lowest in the quenched condition and rose
76
S.P. Naravan et aL / Thermomechanically treated HSLA steel
Table 1 Mechanical properties of TMT (rdled/forged) samples Prior treatment
Temper- Hardness Ultimate Elonhag (VPN) tensile gation time strength (%) (h) (MPa)
Rolled + Quenced RQ RQ RQ RQ RQ
0 0 4.5 4.5 30 52
510-520 510-520 240-245 240-245 198-206 186-196
1140 1670 770 1240 620 600
Forged + Quenched 0 FQ 4.5 FQ 30 FQ 52
410-450 270-285 203-210 176-185
1180 760 600 560
~) b)
a) b) a) a)
with tempering (table 2). High coercive force (He) values, which were obtained in the quenched samples, fell almost exponentially with tempering time. Close to a tempering time of 50 h, He registered a small increase in both the RQT and FQT specimens, decreased at longer tempering times (77 h).
15 a) 10 b) 2t2 a) 16 b) 23 a) 30 a)
3. 4. Elevated temperature magnetic properties The elevated temperature magnetic properties of the rolled + queu(.hed + tempered (RQT) and forged + quenched + tempered (FQT) samples are presented in table 3. The fall in the values of Bs and B r above room temperature and up to 300 °C
10 20 30 30
a) Longitudinal; b~ transverse.
Table 2 Magnetic properties of TMT (rolled and forged) samples Prior treatment
Tempering time (h)
Induction (Bs) at 7560 A / m (T)
Remanence ( B, )
RQ RQ RQ RQ RQ
0 4.5 30 52 77
FQ FQ FQ FQ
4.5 30 52 77
~'ma.~
B~//~s
(T)
Coercive force (A/m)
1.59 ;.57 1.74 1.70 1.64
1.07 1.08 1.44 1.27 1.25
1703 907 597 644 557
300 475 1075 750 1150
0.67 0.69 0.83 0°75 0.76
1.58 1.75 1.7,4 1.71
1.33 1.45 1.45 1.43
589 461 565 501
900 1700 1050 1175
0.86 0.83 0.83 0.84
Table 3 Elevated temperature magnetic properties Prior treatment
RQ + tempered 675 °C for 30 h
FQ + tempered 675 °C for 30 h
~ Room temperature.
Test temp. (°C)
(T)
RT ~) !00 200 300 360
1.74 ! .74 1.72 1 .f9 1.66
1.40 1.31 1.27
RT a) 100 20O 3OO
1.75 1.75 1.74 1.74
1.45 1,45 1.39 1.35
;nduction (B~) at 7560 A / m
Remanence (B,)
(13
(A/m)
1.44
597
A1.4 ."¢-T
#max
B,/Bs
1075
0.83
1 t't~c IIU f.)
U.O.~
477 342 302
1275 1900 2000
0.8i 0.78 0.77
461 434 366 366
1700 2000 1600 1700
0.83 0.83 0.80 078
S.P. Narayan et al. / Thermomechanicaily treated H S L A steel
was only marginal. But the decrease in H~ and the increase in /Xm~X with temperature were substantial.
4. Discussion The metaHographic studies of the RQ and FQ samples have shown the microconstituents to be acicular ferrite and martensite. The ferritic slabs observed in the FQ samples suggest that some ferrite precipitates out from the austenite even before quenching. A part of the ferrite has also taken up the Widmanstatten pattern. Although no pronounced alignment of constituent phase is observed along the rolling direction in the RQ samples, the strength properties in the transverse and longitudinal direction show a pronounced difference with the transverse section displaying a 50% higher value. This may be associated with crystallographic texture. Another feature of the microstructure is the presence of carbides. These are coarser than, but similar in habit to, those seen in the quenched samples (without forging or rolling) [5] or those fine carbides reported by Gau et al. [6]. The martensites show a mixture of a dislocated low-carbon variety and a micro-twinned high carbon variety (fig. 3d). This feature endorses the established view that there coold be a distribution of carbon in the martensite. Upon tempering, similar structural changes take place in both the RQ and FQ samples. Precipitation of fine carbides occurs during the early stages of tempering while growth and spheroidisation of carbides takes place at the later stages of tempering. The purpose of rolling or forging is to accelerate precipitation and spheroidisation of carbides [7,8] while the presence of niobium retards the n r ~ o o e e c~f e n h o r n i c l i ~ n t l n n
From a comparison of the time required for bringing about spheroidisation through TMT (preset study) with the time required by other methods, such as annealing or straightforward quenching and tempering [5], it appears that TMT followed by quenching and tempering is the more effective me~hod. This is not surprising when one takes into account the presence of more numerous
77
defects which aid the diffusion for microstructural changes. Thc lower strength in the FQ samples compared to the RQ samples (table 1) appears to be corroborated by the presence of chunky ferrites in the former. The fall in strength and the increase in ductility of the quenched specimens upon tempering are attributed to the release of high residual stresses, to an increase in the volume percent of ferrite and to the growth of ferrite. Upon tempering, the improvement in the magnetic properties of the RQ and the FQ specimens is about the same. However, the rolled samples have shown a higher coercive force than the forged ones. This is ostensibly due to the strong anisotropic effect of rolling. The influence of precipitates such as carbides on magnetic properties and on coercive force in particular, has been shown to vary with their morphology and size [3,4,9-12]. The small rise in the coercive force which occurs at around 50 h of tempering at 675°C is probably caused by fine precipitates of alloy carbides that effectively pin down the doman w~,lls; a subsequent increase in size beyond some 'ciitical value' [9] would lead to an eventual fall in ~:he coercive force ',:uch as the one which occurs at a tempering time of 70 h. The literature, while predicting some correlation between the size of the precipitates and interactions with domain walls, is silent on the underlying mechanism for the influence of the morphology or aspect-ratio of the carbide precipitates. There is experimental evidence in the literature [2-4,9,12] which shows that the spheroidal shape brings about a lowering of the coercive force. Identical results have also been reported by the authors earlier [5]. It may be hypothesized that anisotropy of stresses associated with a high aspect-ratio particle in the matrix tends to cau:;e a stronger interaction with the domain-walls. The !ocked-~_~p microstresses between the spherical carbides and the equiaxed ferrite matrix would be lower, hence a lowering of the coercive forces. Studies by Jiles [4] have shown dis~hit:t difi'~lc,ces in coercivities and hysteresis loss~ between normalised and spheroidised specimens of carbon steels at carbon levels greater than 0.4% C. However. they report negligible differences for steels ,:ontaining 0.24% C or
78
S.P. Narayan et al. / Thermomechanically treated HSLA steel
below, which is the carbon level in the present case. Thus, there appears to be some discrepancy. A clear explanation has yet to be found. The magnetic and tensile tests at elevated temperatures of both RQT and FQT samples have shown the properties to be reasonably stal~le up to 300 °C (table 3). In the case of RQT samples, the permeability rises gradually; whereas in the ease of the FQT samples, the permeability initially increases up to 100 °C is followed by a fall around 200°C and then an increase has been observed. Probably this kind of a small peak in /~ as a function of temperature is associated with the transformation of the carbides from a ferro- to a paramagnetic state. The fall in Bs and H c with temperature i~ in accordance with the existing theories.
(4) Spheroidised HSLA steels exhibit nearly stable magnetic properties up to 300 ° C.
Acknowledgements The authors would like to express their grateful thanks to the Director, National Metallurgical Laboratory, Jamshedpur, for his keen interest in this investigation. One of the authors (S.P.N.) would like to thank the authorities of RRL (Bhopal) for the kind permission to undertake the work as a part of the M. Tech progrmnme.
References [11 C.R. Honeycutt and E.A. Steigerwald, Trans. ASM 59
5 Conclusions
(1966) 113.
[2] J.H. Swisher, A.T. English and R.C. Stoffers, Trans. ASM (1) The morphology and size of the second phase particles have a significant influence on the magnetic properties of HSLA steels. The spheroidised structure induces attractive soft magnetic properties, probably due to lower locked-up micro stresses~ (2) Spheroidisation of carbides in HSLA steels can be accomplished more quickly by thermomechanical treatment than by straight quenching and tempering or simple annealing. (3) Microalloying of steel with Nb, V, Ti, etc. gives rise to high strength at lower carbon levels without impairing the soft magnetic properties.
62 (1969) 257.
[31 D.C. Jiles, J. Appl. Phys. 63 (1988) 2980. [4] D.C. Jiles, J. Phys. D21 (1988) 1186. [51 S.P. Narayan, V. Rao and O.N. Mohanty, J. Magn. Magn, Mat. (submitted).
[61 J.S. Gau, J.K. Koo and G. Thomas, Proc. Conf. on Fundamentals of Dual Phase Steels, Chicago (1981) p. 48.
[71 S. Chattopadhyay and C.M. Sellars, Acta Metall. 30 (1982) 157.
[81 A.H. Holtzman, J.C. Danko and R.D. Stout, Trans. Metall. Soc. AIME 212 (1958) 475.
[91 A.T. English, Acta Met. 15 (1967) 1573. [lOl L.J. Dijkstra and C.A. Wert, Phys. Rev. 79 (1950) 979. [111 W.C. Leslie mad D.W. Stevens, Trans. ASM 57 (1964) 261. [121 S.K. Ray and O.N. Mohanty, J. Magn. Magn. Mat. 78 (1989) 255.