8-9th August, 2008 School of Materials Science & Nanotechnology,Jadavpur University
DEVELOPMENT OF ULTRAFINE GRAINS IN INTERSTITIAL FREE HIGH STRENGTH STEEL
Piyali Rakshit, Prof.P.K.Mitra, Prof.M.K.Mitra
Department of Metallurgical & Material Engineering, JU
ABSTRACT:
Specific modification of the ferrite structure is possible in IF steels by niobium and titanium alloying in such concentrations as are sufficient to completely bind Carbon and Nitrogen. The result is a “purified ferrite”, i.e. interstitial free high strength steel. Further grain refinement and consequent strengthening of IFHS steel was the objective of this work. Controlled recrystallization was sought to achieve this. A wide variation of ferrite grain size in the range 153 nm to 65799 nm could be obtained by varying the heat treatment parameters. Various phases obtained from heat treatment were characterized by XRD. Variation of hardness has been correlated with grain size.
KEYWORDS: Interstitial free high strength steel, grain refinement, heat treatment, static recrystallization, grain size measurement.
INTRODUCTION:
Strengthening of steels without losing ductility is the main thrust of research in the recent times. While grain refinement increase strength without losing much of ductility, cold working increases strength but decreases the ductility [1]. Further, the ductile-brittle transition temperature is lowered in steels by grain refinement, making the steels tough over a wider range of temperatures. The fatigue resistance also improves by grain refinement. Thus, grain refinement is perhaps the most desirable strengthening method. There are various ways to produce ultrafine grains, viz., deformation induced ferrite transformation [2], a controlled rolling accelerated cooling technique [3], static recryatalization route [4], thermomechanical process [5], high current electropulsing technique [6], equal channel angular pressing technique [7], repetative deformation using the side extrusion method [8], magnetic field processing [9], inoculation technique [10], reversion of eutectoid tempareture [11], rapid heating and repeated cycling [12], influence of vibration on grain size[13],accumalative roll bonding process[14], relaxation precipitation controlling phase transformation technique[15]. Recrystallization route was selected for further refining the as received samples.
EXPERIMENTAL WORK:
SAMPLE COMPOSTION:
Interstitial free high strength steel (IFHS) was obtained from Tata Steel in form of cold rolled , batch annealed ,skin passed condition .The composition of IFHS is given in Table1.
Table1:
C
(wt%)
Mn
(wt%)
P
(wt%)
Ti
(wt%)
S
(wt%)
Nb
(wt%)
N
(wt%)
B
(wt%)
Si
(wt%)
Al
(wt%)
0.0027
0.54
0.052
0.06
0.006
0.0179
0.0027
0.0009
0.007
0.028
2
. Differential thermal analysis (DTA) was done for obtaining recrystallization temperature of the as received sample. A range of temperature was selected from DTA graph for recrystallizing the samples. Temperature of recrystallization, holding time at the RT and subsequent quenching from RT were taken as the experimental parameters.
DTA is given in fig.1.The samples were quenched from different temperatures as per the schedule given in Table2.
Fig 1.Differential Thermal analysis of IFHS steel as received
Table 2: Heat Treatment Details
SAMPLE NO
HOMOGENIZING TEMPERATURE
HOLDING TIME
QUENCHING MEDIUM
NOMENCLATURE
1
6600C
30min in muffle furnace
Water
A660T30
2
6800C
30min in muffle furnace
Water
A680T30
3
7200C
30min in muffle furnace
Water
A720T30
4
7400C
30min in muffle furnace
Water
A740T30
5
7600C
30min in muffle furnace
Water
A760T30
6
7000C
30min in muffle furnace
Water
A700T30
7
7000C
5sec in muffle furnace
Water
A700T5
8
7000C
5sec in vertical furnace
Water
A700T5v
9
7000C
10sec in muffle furnace
Water
A700T10
3
10
7000C
50sec in muffle furnace
Water
A700T50
11
7000C
100sec in muffle furnace
Water
A700T100
12
7000C
500sec in muffle furnace
Water
A700T500
13
7000C
1000sec in muffle furnace
Water
A700T1000
14
7000C
5000sec in muffle furnace
Water
A700T5000
15
7000C
10000sec in muffle furnace
Water
A700T10000
16
7000C
15min+15min,2times quench, in muffle furnace
Water
A700T2q
17
7000C
15min+15min+15min,3times quench, in muffle furnace
Water
A700T3q
18
7000C
500sec, 3 times quench, in muffle furnace
Water
A700T3q-500
19
7000C
5sec in vertical furnace
Liquid nitrogen
A700T5sn
20
7000C
15sec in vertical furnace
Liquid nitrogen
A700T15sn
21
10000C
5sec in vertical furnace
Liquid nitrogen
A1000T5sn
One sample was annealed at 9000C.Its nomenclature is A900T2a and nomenclature of as received IFHS steel sample is AIfhs
RESULTS: Microstructures
Fig 2.a. AIfhs at 50X
Fig 2.b. A660T30 at 50X
Fig 2.c. A680T30 at 50X
Fig 2.d. A720T30 at 50X
4
Fig 2.e. A740T30 at 50X
Fig 2.f. A760T30at 50X
Fig 2.g. A700T30 at 50X
Fig 2.h. A700T5 at 50X
Fig 2.i. A700T10 at 50X
Fig 2.j. A700T50 at 50X
Fig 2.k. A700T100 at 50X
Fig 2.l. A700T500 at 50X
5
Fig 2.m. A700T1000 at 50X
Fig 2.n. A700T5000 at 50X
Fig 2.p. A700T2q at50X
Fig 2.q. A700T3q at50X
Fig 2.r. A700T3q-500 at 50X
Fig 2.s. A700T5v at 50X
Fig 2.t. A700T5sn at 50X
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SEM Photomicrographs
Fig 3.b. A700T15sn at 1300X(Back scatter mode)
Fig 2.v. A1000T5snat 50X
Fig 2.w. A900T2a at 50X
Fig 2.u. A700T15sn at 50X
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Fig 3.c. A700T5sn at 1300X(Back scatter mode)
Fig 3.d. A700T5sn at 1300X(Secondary electron mode)
Table 3: XRD Data
TREATMENT
PEAKS OF α IRON & (MAX. INTENSITY)
PEAKS OF ALUMINUM IRON BORIDE &MAX. INTENSITY
PEAK OF IRON NIOBIUM NITRIDE
&MAX. INTENSITY
AIfhs
3,4,6(100), 16
5,7(76), 8,15
12(46)
A700T30
6,7(92), 14,15
3,4(60), 16
8,10(50), 11,13
A700T5
13(100), 18,19
3,6(28)
5(30), 10,15,17
A700T10
8(100), 13,14,15
4,6(54)
5,9(37), 10
A700T50
9(100), 11,13,14
3,5,12(31)
4(29)
A700T500
7(100), 13,15
3,5(38), 14
4(38), 8,10
A700T2q
8(100), 15,17
4(39), 6,16
5(38), 9,11,13
A700T3q
8(100), 15,17,19
4,6(43), 16,18
5(37), 9,10,11,13
A700T3q-500
6(100), 9,11,12
2,4,10(28)
3(32), 5
A700T5v
7(100), 14,15
2(30), 5,11,16
4,8,10(33), 11,13
A700T5sn
11(100), 18,23
4,7,9(41), 19,21,24,25
8,12,13(31), 14,15,17,20,22
A700T15sn
9(100), 17,25
4,5(31), 7,16,18,19,21,
22,23,26,27,28,29
6(28), 10,11,12,15,20,24
A1000T5sn
9(100), 17,20
5(48), 7,18,19,21
6,11(36), 12,13,15
A900T2a
5(100), 6,7,8
3(23)
4(25)
8
Table 4: Macro hardness of the samples
SAMPLE
HARDNESS (IN VPN)
AIfhs
132.0
A660T30
115.3
A680T30
110.2
A720T30
113.7
A740T30
105.4
A760T30
101.6
A700T30
107.2
A700T5
122.6
A700T10
120.8
A700T50
112.5
A700T100
110.0
A700T500
126.2
A700T1000
109.8
A700T5000
107.6
A700T10000
114.9
A700T2q
119.5
A700T3q
114.4
A700T3q-500
116.2
A700T5v
136.1
A700T5sn
135.4
A700T15sn
136.6
A1000T5sn
124.8
A900T2a
79.5
Percentage Composition Of Elements In Different Samples
Table 5: In Weight %
ELEMENTS
AIfhs
A700T5v
A700T15sn
N
0.40
0.54
0.18
Al
0.44
0.54
0.42
Ti
0.17
0.03
-
Fe
98.83
98.89
99.18
Nb
0.16
-
0.21
Table 6: Measured Ferrite Grain size
SAMPLE
GRAIN SIZE (in μm)
GRAIN SIZE (in nm)
0.25%carbon steel
17.89
17890.0
AIfhs
1.1725
1172.5
A660T30
2.7147
2714.7
9
A680T30
2.7397
2739.7
A720T30
3.0322
3032.2
A740T30
3.163
3163.0
A760T30
3.172
3172.0
A700T30
2.6622
2662.2
A700T5
1.4358
1435.8
A700T10
1.5846
1584.6
A700T50
1.8199
1819.9
A700T100
2.0215
2021.5
A700T500
1.6846
1684.6
A700T1000
1.7522
1752.2
A700T5000
1.7305
1730.5
A700T10000
1.7202
1720.2
A700T2q
2.2936
2293.6
A700T3q
2.2588
2258.8
A700T3q-500
1.8871
1887.1
A700T5v
1.1017
1101.7
A700T5sn
1.1465
1146.5
A700T15sn
1.0223
1022.3
A1000T5sn
1.5484
1548.4
A900T2a
65.799
65799.0
SAXS RESULTS
Fig 4.a. SAXS result of as received IFHS Steel Fig 4.b. SAXS result of A700T15sn
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DISCUSSION:
The microstructure of IFHS steel as received sample has mainly ferrite matrix, with some lumpy aluminum iron boride. Some small particles are also seen, which are supposed to be iron titanium phosphide [16]. At lower magnification (50X) grain boundaries are not visible but at higher magnification (1300X) with better resolution grain boundaries and sub grain boundaries are clearly visible. So X ray diffraction is done to identify the various phases.
Phases are identified from X ray diffraction analysis. In all the cases the matrix is ferrite and its amount varied with heat treatment. The other minor phases observe in heat treated IFHS steel samples are aluminum iron boride and iron niobium nitride. Presumably they are present in IFHS steel in finely dispersed form. On recrystallization morphology of these constituents changed, enabling their detection by XRD as well as by metallography. Whereas, phosphides, which are present in as received IFHS steel dissolve in the solid solution while recrystallization. It is seen from XRD analysis that [211]α iron got maximum intensity in IFHS as received sample but maximum intensity shifts to [110] α iron on annealing (A900T2a), direct quenching (A700T5v), repeated quenching (A700T2q, A700T3q, A700T3q-500), and deferred quenching (A700T5, A700T10, A700T50, A700T500). An interesting feature is noticed in deferred quenching (A700T30) sample. As holding time in furnace is increased here intensity of [110] α iron decreases as its I/I0 decreases from 100 to 52. It is clearly seen from XRD that in as received sample all higher indices lines are found, whereas in heat-treated samples lower indices lines are observed. This indicates that due to the heat treatment not only recrystallization occurs, orientation and relative amount of phases also vary. These variations depend on heating temperature, cooling rate, holding time in furnace and number of repeated quenching cycles. The presence of iron, titanium, niobium, nitrogen, aluminum are also found by EDX analysis.
Categories of microstructures (shown in Fig 2.a-2.w) obtain by different heat treatment are given below-
-Tendency of forming grain boundaries are found increasing with increasing temperature -A660T30, A680T30, A720T30, A740T30, A760T30
- Grain sizes and morphology of second phase particles vary with holding time, and rate of quenching, viz., A700T30, A700T5, A700T10, A700T50, A700T100, A700T500, A700T1000, A700T5000, A700T10000, A700T2q, A700T3q, A700T3q-500.
-Grain refined structures are found in A700T5v, A700T5sn, A700T15sn , A1000T5sn.
-Coarse grain single phase structure is found in A900T2a .
Aluminum iron boride and iron niobium nitride are the minor phases confirm by XRD analysis. Aluminum iron boride has lumpy morphology and its morphology is found to be the same wherever it is obtained. Iron niobium nitride is circular in shape and small in size. This nitride is found at grain boundaries and its morphology is found to be different for different heat treatments.
Grain size of ferrite is found to be varying from 1022.3 nm (A700T15sn) to 65799nm (A900T2a ) as shown in the grain size table (Table 6). Cooling rate and holding time in furnace play a strong role in reducing grain growth. When rate of cooling is fast grain growth cannot occur. Also when holding time in furnace is less, grain growth
11
cannot take place so grain size is found to decrease. But in some cases due to poor Metallography this cannot be developed. In liquid nitrogen quenching and direct cooling the rate of cooling is very fast and holding time in furnace is also low so grains are of small size, viz., microstructures of -A700T5sn (1146.5nm), A700T15sn (1022.3nm), A700T5v (1101.7nm) which are less than as received sample (1172.5nm). Repeated quenching has no effect on grain size, as there is no formation of martensite. Grain size of A700T3q-500 is coarser than A700T500 .In case of annealing (A900T2a) cooling rate was very slow so grain growth takes place and grain size becomes larger. In case of A1000T5sn the temperature is very high yet the grain size is fine. Perhaps because of small holding time at 1000oC and very fast cooling grain growth cannot take place. It is found from SEM photomicrograph that grain size of ferrite is 1000nm and grain size of nitride is 250nm in A700T15sn ,grain size of ferrite is 1125nm and grain size of nitride is 500nm in IFHS as received sample. Grain sizes, which are measured from microstructure, are almost equal with grain sizes obtained from SEM photomicrograph.
It is seen that hardness is deceasing with increasing grain size. But in some cases this is not valid due to the presence of iron niobium nitride. Iron niobium nitride increased samples hardness. A700T500 (1684.6nm) has bigger grain size than A700T10 (1584.6nm) but A700T10 contained less amount of iron niobium nitride than A700T500 (126.2 in VPN) so hardness is less in case of A700T10 (120.8 in VPN). Grain size of A700T2q (2293.6nm) is bigger than A700T3q (2258.8nm) but A700T2q (119.5 in VPN) having higher hardness than A700T3q (114.4 in VPN) due to present of higher amount of iron niobium nitride in A700T2q. Grain size of A700T3q-500 (1887.1nm) is bigger than A700T50 (1819.9nm) but A700T3q-500 (116.2 in VPN) having higher hardness than A700T50 (112.5 in VPN) due to present of higher amount of iron niobium nitride in A700T3q-500.
The average grain size of as received obtains from SAXS analysis is 210 nm and the average grain size of A700T15sn is 153nm.
CONCLUSIONS:
1. 7000C is found to be the correct recrystallization temperature.
2. Suitable combination of temperature, holding time and cooling rate enables development of wide varity of microconstituents like nitride, boride, silicide and phosphides.
3. Amount of nitride and boride in the structure increases with higher holding time and faster cooling rate. Liquid nitrogen quenching further increased the amount of nitride and boride.
4. Phosphides dissolve during recrystallization.
5. Microconstituents like nitride, boride, silicide and phosphides dissolve in the matrix during annealing.
6. A wide variation of ferrite grain size in the range 153 nm to 65799nm could be obtained by varying the heat treatment parameters.
7. Further grain refinement and consequent strengthening of IFHS steel could be achieved by suitable selection of the heat treatment parameters.
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REFFERENCES:
[1] J. Schiotz, F.D. Di Tolla, K.W. Jacobsen. Softening of nanocrystalline metals at very smallgrains.Nature.391(1998),p.561./ http://en.wikipedia.org/wiki/Grain_boundary_strengthening
[2] [Longxiu Pan,Teknillinen tiedekunta, Oulun yliopisto Konetekniikan osasto, Oulun yliopisto Academic Dissertation to be presented with the assent of the Faculty of Technology, University of Oulu, for public discussion in Raahensali (Auditorium L10), Linnanmaa, on October 29th, 2004, at 12 noon.
[3] http://www.freepatentsonline.com/6464807.html
[4] www.aimnet.it/pdf_pubbi_06/salvatori.pdf /[Longxiu Pan,Teknillinen tiedekunta, Oulun yliopisto Konetekniikan osasto, Oulun yliopisto Academic Dissertation to be presented with the assent of the Faculty of Technology, University of Oulu, for public discussion in Raahensali (Auditorium L10), Linnanmaa, on October 29th, 2004, at 12 noon.
[5] Jain, Mohit Kumar (1994-11-28) Processing and mechanical behavior of ultrafine grain materials. http://resolver.caltech.edu/CaltechETD:etd-10112007-090033
[6] Yizhou Zhou, Wei Zhang, Baoquan Wang, Guanhu He, Jingdong Guo
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[10] www.wipo.int/ (WO-2003-052156) GRAIN REFINEMENT OF ALLOYS USING MAGNETIC FIELD PROCESSING.htm, Jayoung Koo, Shiun Ling , Michael John Luton Hans Thomann , Narasimha-Rao Venkata Bangaru[11] Accession Number : AD0634343, Corporate Author : CASE INST OF TECH CLEVELAND OHIO DEPT OF METALLURGY,Report Date : 30 SEP 1965
[12] John D. Verhoeven ,Emeritus Professor Iowa State University , Metallurgy of Steel for Bladesmiths & Others ,who Heat Treat and Forge Steel p.69
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[14] R. Yoda*, H. Haren*, N. Tsuji**, R. Ueji***, T. Toyoda*** and Y. Minamino*** Kakogawa Laboratories, Kobelco Research Institute Inc., 1 Kanazawa-cho, Kakogawa, Hyogo, 675-0137,Japan.** Department of Adaptive Machine Systems, Graduate School of Engineering, Osaka University, 2-1Yamadaoka, Suita, Osaka, 565-0871, Japan.*** Graduate student of Osaka University.
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