- Email: [email protected]
Physical Simulation of Hot Deformation and Microstructural Evolution for 42CrMo4 Steel Prior to Direct Quenching
Availableonline at www.sciencedirect.com
ScienceDirect JOURNAL OF IRON AND STEEL RESEARCH, INTERNATIONAL. 2009, 16(6): 47-51
Physical Simulation of Hot Deformation and Microstructural Evolution for 42CrM04 Steel Prior to Direct Quenching A H Meysarni' ,
R Ghasemzadeh", R Ebrahirni" ,
S H Seyedein' , M ] avidani'
M R Aboutalebi' ,
(1. Department of Material Science and Engineering, Iran University of Science and Technology, Tehran 16844, Iran; 2. Department of Material Science and Engineering, Sabzevar Tarbiat-Moallem University, Sabzevar 96196, Iran)
Abstract: Direct quenching and tempering (DQ-T) of hot rolled steel section has been widely used in steel mill for the sake of improvement of mechanical properties and energy saving. Temperature history and microstructural evolution during hot rolling plays a major role in the properties of direct quenched and tempered products. The mathematical and physical modeling of hot forming processes is becoming a very important tool for design and development of required products as well as predicting the microstructure and the properties of the components. These models were mostly used to predict austenite grain size (AGS) , dynamic, meta-dynamic and static recrystallization in the rods immediately after hot rolling and prior to DQ process. The hot compression tests were carried out on 42CrM04 steel in the temperature range of 900 -1 100 'C and the strain rate range of o. 05 -1 s - I in order to study the high temperature softening behavior of the steel. For the exact prediction of flow stress, the effective stress-effective strain curves were obtained from experiments under various conditions. On the basis of experimental results, the dynamic recrystallization fraction (DRX), AGS, hot deformation and activation energy behavior were investigated. It was found that the calculated results were in good agreement with the experimental flow stress and microstructure of the steel for different conditions of hot deformation. Key words: 42CrM04 steel; hot compression test; dynamic recrystallization; hot deformation; direct quenching; physical simulation
Direct quenching and tempering (DQ-T) combined with controlled rolling has been widely used in the production of low and medium carbon steel for plates and rods. The steel produced by DQ process has an advantage of attaining better combination of strength, toughness and weldability in comparison with the steels produced by conventional reheating and quenching (RQ) processl!", In DQ process, the reheating temperature before rolling is much higher than austenitizing temperature used for the RQ process, so that it will be important to control austenite grain size (AGS). It is obvious that hot deformation of austenite refines ferrite[2J (in controlled rolled steels) and martensite (in ausformed martensite) [3J . The mathematical and physical model of forming and rolling processes is applied increasingly and Biography,A H Meysami
is becoming a very important tool for design and development of required products, and also for predicting the microstructure and the properties of the componentl "?". Accurate prediction of the grain size due to microstructural evolution will be helpful in improvement of the steel products after direct quenching. Therefore, many studies for modeling of hot deformation and microstructural evolution have been carried out so far[s.5J. Recently, studies have been concentrated on the prediction of microstructure in the hot rolling process[4.S.8.9J. However, a model which describes the dynamic recrystallization and AGS for carbon and alloyed steels has not been developed yet, but process simulation could be used to optimize the process parameters. In terms of physical simulation, hot torsion and/or compression tests are normally used to acquire the steel flow be-
E-mail: [email protected];
Revised Date: February 23, 2009
• 48 •
Table 1
havior , and these data were further used to model the hot rolling of strip. plate. bar and rodl,.l"'. Since the microstructural evolution model is the function of a large number of parameters. the complete models will take the form expressed as a function of tempera.ture. strain. strain rate. inter pass time. restoration process. pre-test thermal history and potential for precipitation, The rel~ti'on between t'He" austenite grain size and deformation conditions becomes complicated when the infl~ence of restoration process is considered. The contribution of dynamic recrystallization (DRX) to AGS modeling is very important; most researchers considered that 100 % of dynamic recrystallization occurred if the applied strain exceeded a critical strain (E,):1-6,1". In other words. it was assumed that dynamic recrystallization could be expressed as a function of strain rate and temperature distribution in material. However. the decrease of flow stress and AGS after peak strain (E p ) with increasing strain means that softening process by dynamic recrystallization IS still In progress. Therefore. for a more precise calculation of DRX as a function of strain rate. a combination of Zener- Hollomon parameter should be considered in the model!" - In, • In this work. a procedure was proposed and applied to investigate hot deformation of 42CrM04 steel by analyzing flow stress curves of the material prior to direct quenching. DRX. which affects the flow stress and AGS. is expressed by modified JMAK's equation. Modified JMAK' s equation includes the strain at maximum softening rate (E' ) as well as the critical strain (E,-) for DRX initiation. Based on the modified ]MAK' s equation. the relevant equations were derived for static recrystallization (SRX) , meta-dynamic recrystallization (MDRX). DRX and grain growth of 42CrM04 carbon steel. Predicted AGS was compared with the experimentally measured.
1
Vol. 16
Journal of Iron and Steel Research. International
Materials and Experimental Procedure
The chemical composition (mass percent) of 42CrM04 steel is given in Table 1. This grade of steel was produced by continuous casting process at Iran alloy steel Co. Hot compression tests were performed to investigate hot deformation behavior of the material. The high temperatures of material were acquired by enclosing the specimen in an electrical furnace. The
0.11
%
Chemical composition of 42CrM04 steel
Mn
s
p
O. 55
O. 035
0.035
Cr
Mo
j.Oj
O. 18
Si ~
24
specimens with a gauge length of 15 mm and diameter of 10 mm were carefully machined and pre-heated up to 1 150C in the furnace holding for 3 min to obtain a uniform temperature. It is noted that prior to hot compression test. the AGS of annealed samples at 1 100e for 5 min was measured as 120 /Lm according to ASTM standard £112 before deformation. The samples further placed into hot compression test chamber immediately for testing. To investigate the effects of deformation temperature and strain rate on the microstructure and flow stress. continuous compression tests were conducted in the temperat ure range of 900 - 1 100 C and the strain rate range of 0.05 -1 s - I . The hot compression instrument used for experiments was a 200 t Zouiek. In order to reduce friction effect between specimen and jaw. mica sheets were used. The water-quenched specimens. after compression tests. were polished in horizontal and vertical sections and then etched with solution included FeCI, - HCI solutions. The grain size was determined on immediately quenched specimens after deformation to evaluate the DRX model. For the SRX. MDRX and grain growth models, the grain size was obtained from the quenched specimens after a particular holding time following a deformation event.
2
Results and Discussion
Fig. 1 shows the stress-strain curves for the 42CrM04 steel obtained at various temperatures and strain rates. The flow curves exhibit the peak and softening to an extensive steady state. which indicates DRX behavior. As shown in Fig. 1, at lower strain rate. the flow stress and peak stress decrease. and steady state region is more extensive. Fig. 2 presents the varia tions of work hardening rate (da! ds ) versus flow stress (a) at different temperatures. From this figure. the critical stress (a,) for the onset of DRX at various temperatures is obtained. Each curve consists primarily of three distinct segments. First. the work hardening rate increases linearly with the flow stress over the stress-strain curve from where a= O. to the stress that the onset of sub-grain formation could not appear from curves (a,,). Second. the da/ de-a curve gradually changes into a lower slope lin-
• 49 •
Physical Simulation of Hot Deformation and Microstructural Evolution for 42CrMo4 Steel
Issue 6
(c)
35
I 00
0.5 S- I
0.8
0.4
1.2
(a)
1700 (a) 1300
500
... .
20
0.2
900 'C I
1.0
0.6 Strain
(c)
0.60
0.20
0
1.00
1 100 'C
40
:: i·· .'. ;.~~ . .
..-.. "'........., 60
• 0.5 S- I • 1 S- I
900 (bl··. 700 • . •
·0.5 S-I • 1 S-I • 0.05 S-I
500 I
0.05 S-
'.~
log I'
.\"
r' 0
0
Stress-strain curves for hot compression of 42CrM04 steel at various strain rates
• .... ..... .'
100
0.05 S- I
15
20
5
Fig. 1
b
25
20 0
'C
0.5 s-)
40
60
o
80 100
.,.. ",,~
10
20
30
40
~~;••••
".
...
• 1 S-,
..- .. "- ..
300 ••••••••
1~·
o
50 60
I
·0.5 S- I • 0.05 S- I
~~
5
10 15 20 25 30 35
alMPa (a)
Fig.2
900 'C I
(b) 1000 'C;
ear segment. Third. the curve drops towards dcr/
I 900
(a)
• 0.5 S- I • 1 S- I • 0.05 S- I
1000 •
1 100 'C
In Eqn. (1) and Eqn. (2). A. n' and a are material constants; Rand Q are the gas constant and activation energy. respectively. A and n' can be determined from the relationship between stress and strain rate. Therefore. n' in Eqn, (2) for 900. 1 000. and 1 100 'C are 5. 1. 7. 2. and 5. 8. respectively. Then. the activation energy of deformation Q can be calculated from the relationship between stress and reciprocal of absolute temperature (l/T). Fig. 5 illustrates values of n'ln[sinh(aap)] versus liT at various strain rates. Using the slope of the lines (Fig. 4). the activation energy of deformation can be calculated. Therefore. activation energy for strain rates of 1, 0.5. and O. 05 S-I will be 217. 14. 197.83. and 201. 61 kJ. respectively. Fig. 6 shows the calculated volume fraction of
(2)
.
(c)
do'/ele-a curve at various temperatures and strain rates
(b)
• 1 S-I .0.5 S- I • 0.05 S- I
700 500
:l2 600 b 200 0
100 0
0.2
0.4
0.6
0.8 1.0
• 1 S- I .0.5 S- I • 0.05 S- I
300
300
'C
500 (c)
100
..
o
0'------"_---'-_--'-_-'---' o 0.2 0.4 0.6 0.8 1.0
0
0.2
E
(a)
Fig. 3
900 'C I
(bJ 1000 'C;
(c)
1 100 'C
do'lde-s curve at various temperatures and strain rates
0.4
0.6
0.8 1.0
• 50 •
Vol. 16
Journal of Iron and Steel Research. International
Values of a,. a•.
Table 2
T ernperature.'
C
Strain rate/s-I
O. 5
and
8.
for hot compression tests 0
(71)'/
900 900
8,
MPa
£,
£"
-1
15.4
84.4
0.009
O. 18
17.62
66.23
0.019
O. 3D
-2
900
0.05
22. 7
58
0.039
0.22
I 000
I
6. 68
52.5
0.000 2
0.42
1 000
O. 5
9.10
47.7
0.01
O. 34
1 000
0.05
9.29
:15.4
0.015
0.21
':E -5 I::
4. 45
33.41
0.01
0.34
:s
1 100 1 100
O. 5
5.66
28. 71
0.21
O. 30
1 100
0.05
6.84
21. 9
0.012
O. 20
-3
""" -4
~
'(ij
'"
-6 -7 •
-8 0.5 r---------"':"":"="'---::c"":":::"""
A
-9
o
0.0007
0.0009
-0.5
=",
.s
IS-I
• 0.5s" A 0.05s· 1
-1.5
Fig. 5
n'ln[sinhCaa.) ] versus 1 IT at various strain rates
.900 't'
-2.5
• 1000 't'
-3.5 L -_ _~ -2.0 -1.5
Fig. 4
• 1100 ~
-1.0 In[sinh( au p)I
Plot of 108° versus In[sinhCaa.) ] at various temperatures 1.2
0.6
IS-I
0.5 S·I
0.4 0.2 0
the strain at maximum softening rate (e' ). critical (b)
1.2
S-I
0.2
0.4
0.6
0.8
1.0
0.8
0.8 0.6
0.4
0.4
0.2
0.2
0
0.55..1
1.0
0.6
(a) 900 C;
Fig. 6
0.05
1.0
0.8 ~
o
-0.5
1.0 (a)
...,,0
DRX using Eqn, (3) for different strain rates at 900 • 1000. and 1 100 C. respectively. X IJRX = 1 - e xp {- O. 8[ (e - e, ) / e' Y' } (3 ) This equation. which is modified version of JMAK's equation!'O]. means that X DRX depends on
r:
~_ _____'
0.2
0.6 0.4 Strain
(b) 1000 C;
0.8
1.0
0
0.2
0.4
0.6
0.8
1.0
(e) 1100 C
Variations of volume fraction for dynamic recrystallization with temperature and strain rate
strain (ee)' and applied strain (e). Since the DRX is a continuous process of deformation (with nucleation of grains and migration of grain boundaries). X nRx increases with increasing the strain. Dynamic recrystallization produces new austenite grains and these grains could grow during hot deformation dynamically. It is proposed that DRX austenite grain size (AGS) during deformation can be formulated as follows: DnRx=BZ' (4) where. B is 1. 8 X 10" and Z' is -0. 15. respectively' 12J. Fig. 7 shows the calculated AGS variations versus strain rate at different temperatures by Eqn. (4). Based on this plot. the increase of temperature
and decrease of strain rate cause that the austenite grains grow. By decreasing strain rate. there is more time for atom diffusion. and with the increase of temperature. the activation energy for atom diffusion increase. Both the decrease of strain rate and increase of temperature cause that dynamic growth increases in high temperatures and low strain rates. Fig. 8 illustrates microstructures of hot deformarion samples at 900. 1 000 and 1 100 'C at strain rate 1 s - I . The recrystallization grains and grain boundaries can be seen in this picture. Increasing temperature in a constant strain rate has increased the AGS; this phenomenon is related to increase of
Issue 6
Physical Simulation of Hot Deformation and Microstructural Evolution for 42CrMo4 Steel
the atom diffusion (as explained above). The sizes of austenite at 900. 1 000 and 1 100 'C are 8. 5. 7.5. and 6. 5. respectively. in ASTM scale.
200
§.150
3
~.... 100 0.8
0.2
Fig. 7
• 51 •
In this paper. a model was developed for predicting DRX. AGS. and activation energy of hot deformation for 42CrM04 medium carbon steel using hot compression test in the temperature range of 900-1 100 'C and the strain rate range of O. 05 -1 S-I. Based on the results obtained in the study. the following con-
1.0
Variations of AGS with strain rate at different temperatures
(a) 900
Fig.8
c,
Conclusions
(b) 1000
c,
(c) 1100'C
Optical metallograph of hot deformation samples at strain rate of 1
elusions can be obtained: (1) The n' or slope of lines In EO versus lnj sinh (at1 p ) ] in Fig. 5 at 900. 1 000 and 1 100 'C are 5. 1. 7.2. and 5.8. respectively. (2) Activation energy for hot compression test of this steel at strain rates of 1. O. 5 and O. 05 S-1 will be 217.14. 197.83. and 201. 61 kl , respectively. (3) The decrease of strain rate and the increase of temperature in hot compression process promote the dynamic grain growth. (4) The sizes of austenite at 900. 1 000 and 1 100 'C are 8.5. 7.5. and 6.5. respectively in ASTM scale.
[4J
[5J
[6J
[7J
[8J
The authors wish to thank Iran University of Science and Technology (lUST) and Iran Alloy Steel Co for their financial support.
S-1
and Fine Grain Production in Stainless Steel [JJ. Computational Materials Science. 2006. 40( 1): 90. Lee H-W. Kwon H-C, 1m Y-T. et at. Numerical Investigation of Austenite Grain Size Distribution in Square-Diamond Pass Hot Bar Rolling [JJ. Journal of Materials Processing Technology. 2007,191(3),114. Toloui M, Serajzadeh S. Modeling Recrystallization Kinetics During Hot Rolling of AA5083 [JJ. Journal of Materials Processing Technology. 2007. 184(2): 345. Kim Sung-L, Lee Youngseog , Lee Duk-lak , et at. Modeling of AGS and Recrystallized Fraction of Microalloyed Medium Carbon Steel During Hot Deformation [JJ. Materials Science and Engineering. 2003. 355A( 1): 384. Grass H. Krempaszky C. Reip T. et at. 3-D Simulation of Hot Forming and Microstructure Evolution [JJ. Computational Materials Science. 2003. 28(3): 469. Liu Z Y. Wang W-D. Gao W. Prediction of the Mechanical Properties of Hot-Rolled C-Mn Using Artificial Neural Networks [JJ. Journal of Materials Processing Technology. 1996. 57(4): 332.
[9J
References : [10J
[1]
Chang W S. Microstructure and Mechanical Properties of 780 MPa High Strength Steels Produced by Direct-Quenching and Tempering Process [JJ. Journal of Material Sscience , 2002. 37 (4): 1973.
[l1J
[2J
Wang F. Zhu Q. Lin J. et at. Prediction of Microstructural Evolution in Hot Rolling [JJ. Journal of Materials Processing Technology. 2006. 177(3): 530.
[12J
[3J
Bontcheva N. Petrov P. Petzov G. et at. Finite Element Simulation of Strain Induced Austenite-Martensite Transformation
Dyja H. Korczak P. The Thermal-Mechanical and Microstructural Model for the FEM Simulation of Hot Plate Rolling [JJ. Journal of Materials Processing Technology, 1999. 93(3): 463. Kim 5-1. Yoo y-c. Prediction of Dynamic Recrystallization Behavior of AISI Type 4130 Medium Carbon Steel [JJ. Materials Science and Technology. 2002. 18(3): 160. Hodgson P D. Mathematical Modeling of Recrystallization Process During Hot Rolling of Steels [DJ. Queensland: University of Queensland. 1993. Hodgson P D. Gibbs R K. A Mathematical Model to Predict the Mechanical Properties of Hot Rolled C-Mn and Microalloyed Steels [n lSI] Int. 1992. 32(6): 1329.
ScienceDirect JOURNAL OF IRON AND STEEL RESEARCH, INTERNATIONAL. 2009, 16(6): 47-51
Physical Simulation of Hot Deformation and Microstructural Evolution for 42CrM04 Steel Prior to Direct Quenching A H Meysarni' ,
R Ghasemzadeh", R Ebrahirni" ,
S H Seyedein' , M ] avidani'
M R Aboutalebi' ,
(1. Department of Material Science and Engineering, Iran University of Science and Technology, Tehran 16844, Iran; 2. Department of Material Science and Engineering, Sabzevar Tarbiat-Moallem University, Sabzevar 96196, Iran)
Abstract: Direct quenching and tempering (DQ-T) of hot rolled steel section has been widely used in steel mill for the sake of improvement of mechanical properties and energy saving. Temperature history and microstructural evolution during hot rolling plays a major role in the properties of direct quenched and tempered products. The mathematical and physical modeling of hot forming processes is becoming a very important tool for design and development of required products as well as predicting the microstructure and the properties of the components. These models were mostly used to predict austenite grain size (AGS) , dynamic, meta-dynamic and static recrystallization in the rods immediately after hot rolling and prior to DQ process. The hot compression tests were carried out on 42CrM04 steel in the temperature range of 900 -1 100 'C and the strain rate range of o. 05 -1 s - I in order to study the high temperature softening behavior of the steel. For the exact prediction of flow stress, the effective stress-effective strain curves were obtained from experiments under various conditions. On the basis of experimental results, the dynamic recrystallization fraction (DRX), AGS, hot deformation and activation energy behavior were investigated. It was found that the calculated results were in good agreement with the experimental flow stress and microstructure of the steel for different conditions of hot deformation. Key words: 42CrM04 steel; hot compression test; dynamic recrystallization; hot deformation; direct quenching; physical simulation
Direct quenching and tempering (DQ-T) combined with controlled rolling has been widely used in the production of low and medium carbon steel for plates and rods. The steel produced by DQ process has an advantage of attaining better combination of strength, toughness and weldability in comparison with the steels produced by conventional reheating and quenching (RQ) processl!", In DQ process, the reheating temperature before rolling is much higher than austenitizing temperature used for the RQ process, so that it will be important to control austenite grain size (AGS). It is obvious that hot deformation of austenite refines ferrite[2J (in controlled rolled steels) and martensite (in ausformed martensite) [3J . The mathematical and physical model of forming and rolling processes is applied increasingly and Biography,A H Meysami
is becoming a very important tool for design and development of required products, and also for predicting the microstructure and the properties of the componentl "?". Accurate prediction of the grain size due to microstructural evolution will be helpful in improvement of the steel products after direct quenching. Therefore, many studies for modeling of hot deformation and microstructural evolution have been carried out so far[s.5J. Recently, studies have been concentrated on the prediction of microstructure in the hot rolling process[4.S.8.9J. However, a model which describes the dynamic recrystallization and AGS for carbon and alloyed steels has not been developed yet, but process simulation could be used to optimize the process parameters. In terms of physical simulation, hot torsion and/or compression tests are normally used to acquire the steel flow be-
E-mail: [email protected];
Revised Date: February 23, 2009
• 48 •
Table 1
havior , and these data were further used to model the hot rolling of strip. plate. bar and rodl,.l"'. Since the microstructural evolution model is the function of a large number of parameters. the complete models will take the form expressed as a function of tempera.ture. strain. strain rate. inter pass time. restoration process. pre-test thermal history and potential for precipitation, The rel~ti'on between t'He" austenite grain size and deformation conditions becomes complicated when the infl~ence of restoration process is considered. The contribution of dynamic recrystallization (DRX) to AGS modeling is very important; most researchers considered that 100 % of dynamic recrystallization occurred if the applied strain exceeded a critical strain (E,):1-6,1". In other words. it was assumed that dynamic recrystallization could be expressed as a function of strain rate and temperature distribution in material. However. the decrease of flow stress and AGS after peak strain (E p ) with increasing strain means that softening process by dynamic recrystallization IS still In progress. Therefore. for a more precise calculation of DRX as a function of strain rate. a combination of Zener- Hollomon parameter should be considered in the model!" - In, • In this work. a procedure was proposed and applied to investigate hot deformation of 42CrM04 steel by analyzing flow stress curves of the material prior to direct quenching. DRX. which affects the flow stress and AGS. is expressed by modified JMAK's equation. Modified JMAK' s equation includes the strain at maximum softening rate (E' ) as well as the critical strain (E,-) for DRX initiation. Based on the modified ]MAK' s equation. the relevant equations were derived for static recrystallization (SRX) , meta-dynamic recrystallization (MDRX). DRX and grain growth of 42CrM04 carbon steel. Predicted AGS was compared with the experimentally measured.
1
Vol. 16
Journal of Iron and Steel Research. International
Materials and Experimental Procedure
The chemical composition (mass percent) of 42CrM04 steel is given in Table 1. This grade of steel was produced by continuous casting process at Iran alloy steel Co. Hot compression tests were performed to investigate hot deformation behavior of the material. The high temperatures of material were acquired by enclosing the specimen in an electrical furnace. The
0.11
%
Chemical composition of 42CrM04 steel
Mn
s
p
O. 55
O. 035
0.035
Cr
Mo
j.Oj
O. 18
Si ~
24
specimens with a gauge length of 15 mm and diameter of 10 mm were carefully machined and pre-heated up to 1 150C in the furnace holding for 3 min to obtain a uniform temperature. It is noted that prior to hot compression test. the AGS of annealed samples at 1 100e for 5 min was measured as 120 /Lm according to ASTM standard £112 before deformation. The samples further placed into hot compression test chamber immediately for testing. To investigate the effects of deformation temperature and strain rate on the microstructure and flow stress. continuous compression tests were conducted in the temperat ure range of 900 - 1 100 C and the strain rate range of 0.05 -1 s - I . The hot compression instrument used for experiments was a 200 t Zouiek. In order to reduce friction effect between specimen and jaw. mica sheets were used. The water-quenched specimens. after compression tests. were polished in horizontal and vertical sections and then etched with solution included FeCI, - HCI solutions. The grain size was determined on immediately quenched specimens after deformation to evaluate the DRX model. For the SRX. MDRX and grain growth models, the grain size was obtained from the quenched specimens after a particular holding time following a deformation event.
2
Results and Discussion
Fig. 1 shows the stress-strain curves for the 42CrM04 steel obtained at various temperatures and strain rates. The flow curves exhibit the peak and softening to an extensive steady state. which indicates DRX behavior. As shown in Fig. 1, at lower strain rate. the flow stress and peak stress decrease. and steady state region is more extensive. Fig. 2 presents the varia tions of work hardening rate (da! ds ) versus flow stress (a) at different temperatures. From this figure. the critical stress (a,) for the onset of DRX at various temperatures is obtained. Each curve consists primarily of three distinct segments. First. the work hardening rate increases linearly with the flow stress over the stress-strain curve from where a= O. to the stress that the onset of sub-grain formation could not appear from curves (a,,). Second. the da/ de-a curve gradually changes into a lower slope lin-
• 49 •
Physical Simulation of Hot Deformation and Microstructural Evolution for 42CrMo4 Steel
Issue 6
(c)
35
I 00
0.5 S- I
0.8
0.4
1.2
(a)
1700 (a) 1300
500
... .
20
0.2
900 'C I
1.0
0.6 Strain
(c)
0.60
0.20
0
1.00
1 100 'C
40
:: i·· .'. ;.~~ . .
..-.. "'........., 60
• 0.5 S- I • 1 S- I
900 (bl··. 700 • . •
·0.5 S-I • 1 S-I • 0.05 S-I
500 I
0.05 S-
'.~
log I'
.\"
r' 0
0
Stress-strain curves for hot compression of 42CrM04 steel at various strain rates
• .... ..... .'
100
0.05 S- I
15
20
5
Fig. 1
b
25
20 0
'C
0.5 s-)
40
60
o
80 100
.,.. ",,~
10
20
30
40
~~;••••
".
...
• 1 S-,
..- .. "- ..
300 ••••••••
1~·
o
50 60
I
·0.5 S- I • 0.05 S- I
~~
5
10 15 20 25 30 35
alMPa (a)
Fig.2
900 'C I
(b) 1000 'C;
ear segment. Third. the curve drops towards dcr/
I 900
(a)
• 0.5 S- I • 1 S- I • 0.05 S- I
1000 •
1 100 'C
In Eqn. (1) and Eqn. (2). A. n' and a are material constants; Rand Q are the gas constant and activation energy. respectively. A and n' can be determined from the relationship between stress and strain rate. Therefore. n' in Eqn, (2) for 900. 1 000. and 1 100 'C are 5. 1. 7. 2. and 5. 8. respectively. Then. the activation energy of deformation Q can be calculated from the relationship between stress and reciprocal of absolute temperature (l/T). Fig. 5 illustrates values of n'ln[sinh(aap)] versus liT at various strain rates. Using the slope of the lines (Fig. 4). the activation energy of deformation can be calculated. Therefore. activation energy for strain rates of 1, 0.5. and O. 05 S-I will be 217. 14. 197.83. and 201. 61 kJ. respectively. Fig. 6 shows the calculated volume fraction of
(2)
.
(c)
do'/ele-a curve at various temperatures and strain rates
(b)
• 1 S-I .0.5 S- I • 0.05 S- I
700 500
:l2 600 b 200 0
100 0
0.2
0.4
0.6
0.8 1.0
• 1 S- I .0.5 S- I • 0.05 S- I
300
300
'C
500 (c)
100
..
o
0'------"_---'-_--'-_-'---' o 0.2 0.4 0.6 0.8 1.0
0
0.2
E
(a)
Fig. 3
900 'C I
(bJ 1000 'C;
(c)
1 100 'C
do'lde-s curve at various temperatures and strain rates
0.4
0.6
0.8 1.0
• 50 •
Vol. 16
Journal of Iron and Steel Research. International
Values of a,. a•.
Table 2
T ernperature.'
C
Strain rate/s-I
O. 5
and
8.
for hot compression tests 0
(71)'/
900 900
8,
MPa
£,
£"
-1
15.4
84.4
0.009
O. 18
17.62
66.23
0.019
O. 3D
-2
900
0.05
22. 7
58
0.039
0.22
I 000
I
6. 68
52.5
0.000 2
0.42
1 000
O. 5
9.10
47.7
0.01
O. 34
1 000
0.05
9.29
:15.4
0.015
0.21
':E -5 I::
4. 45
33.41
0.01
0.34
:s
1 100 1 100
O. 5
5.66
28. 71
0.21
O. 30
1 100
0.05
6.84
21. 9
0.012
O. 20
-3
""" -4
~
'(ij
'"
-6 -7 •
-8 0.5 r---------"':"":"="'---::c"":":::"""
A
-9
o
0.0007
0.0009
-0.5
=",
.s
IS-I
• 0.5s" A 0.05s· 1
-1.5
Fig. 5
n'ln[sinhCaa.) ] versus 1 IT at various strain rates
.900 't'
-2.5
• 1000 't'
-3.5 L -_ _~ -2.0 -1.5
Fig. 4
• 1100 ~
-1.0 In[sinh( au p)I
Plot of 108° versus In[sinhCaa.) ] at various temperatures 1.2
0.6
IS-I
0.5 S·I
0.4 0.2 0
the strain at maximum softening rate (e' ). critical (b)
1.2
S-I
0.2
0.4
0.6
0.8
1.0
0.8
0.8 0.6
0.4
0.4
0.2
0.2
0
0.55..1
1.0
0.6
(a) 900 C;
Fig. 6
0.05
1.0
0.8 ~
o
-0.5
1.0 (a)
...,,0
DRX using Eqn, (3) for different strain rates at 900 • 1000. and 1 100 C. respectively. X IJRX = 1 - e xp {- O. 8[ (e - e, ) / e' Y' } (3 ) This equation. which is modified version of JMAK's equation!'O]. means that X DRX depends on
r:
~_ _____'
0.2
0.6 0.4 Strain
(b) 1000 C;
0.8
1.0
0
0.2
0.4
0.6
0.8
1.0
(e) 1100 C
Variations of volume fraction for dynamic recrystallization with temperature and strain rate
strain (ee)' and applied strain (e). Since the DRX is a continuous process of deformation (with nucleation of grains and migration of grain boundaries). X nRx increases with increasing the strain. Dynamic recrystallization produces new austenite grains and these grains could grow during hot deformation dynamically. It is proposed that DRX austenite grain size (AGS) during deformation can be formulated as follows: DnRx=BZ' (4) where. B is 1. 8 X 10" and Z' is -0. 15. respectively' 12J. Fig. 7 shows the calculated AGS variations versus strain rate at different temperatures by Eqn. (4). Based on this plot. the increase of temperature
and decrease of strain rate cause that the austenite grains grow. By decreasing strain rate. there is more time for atom diffusion. and with the increase of temperature. the activation energy for atom diffusion increase. Both the decrease of strain rate and increase of temperature cause that dynamic growth increases in high temperatures and low strain rates. Fig. 8 illustrates microstructures of hot deformarion samples at 900. 1 000 and 1 100 'C at strain rate 1 s - I . The recrystallization grains and grain boundaries can be seen in this picture. Increasing temperature in a constant strain rate has increased the AGS; this phenomenon is related to increase of
Issue 6
Physical Simulation of Hot Deformation and Microstructural Evolution for 42CrMo4 Steel
the atom diffusion (as explained above). The sizes of austenite at 900. 1 000 and 1 100 'C are 8. 5. 7.5. and 6. 5. respectively. in ASTM scale.
200
§.150
3
~.... 100 0.8
0.2
Fig. 7
• 51 •
In this paper. a model was developed for predicting DRX. AGS. and activation energy of hot deformation for 42CrM04 medium carbon steel using hot compression test in the temperature range of 900-1 100 'C and the strain rate range of O. 05 -1 S-I. Based on the results obtained in the study. the following con-
1.0
Variations of AGS with strain rate at different temperatures
(a) 900
Fig.8
c,
Conclusions
(b) 1000
c,
(c) 1100'C
Optical metallograph of hot deformation samples at strain rate of 1
elusions can be obtained: (1) The n' or slope of lines In EO versus lnj sinh (at1 p ) ] in Fig. 5 at 900. 1 000 and 1 100 'C are 5. 1. 7.2. and 5.8. respectively. (2) Activation energy for hot compression test of this steel at strain rates of 1. O. 5 and O. 05 S-1 will be 217.14. 197.83. and 201. 61 kl , respectively. (3) The decrease of strain rate and the increase of temperature in hot compression process promote the dynamic grain growth. (4) The sizes of austenite at 900. 1 000 and 1 100 'C are 8.5. 7.5. and 6.5. respectively in ASTM scale.
[4J
[5J
[6J
[7J
[8J
The authors wish to thank Iran University of Science and Technology (lUST) and Iran Alloy Steel Co for their financial support.
S-1
and Fine Grain Production in Stainless Steel [JJ. Computational Materials Science. 2006. 40( 1): 90. Lee H-W. Kwon H-C, 1m Y-T. et at. Numerical Investigation of Austenite Grain Size Distribution in Square-Diamond Pass Hot Bar Rolling [JJ. Journal of Materials Processing Technology. 2007,191(3),114. Toloui M, Serajzadeh S. Modeling Recrystallization Kinetics During Hot Rolling of AA5083 [JJ. Journal of Materials Processing Technology. 2007. 184(2): 345. Kim Sung-L, Lee Youngseog , Lee Duk-lak , et at. Modeling of AGS and Recrystallized Fraction of Microalloyed Medium Carbon Steel During Hot Deformation [JJ. Materials Science and Engineering. 2003. 355A( 1): 384. Grass H. Krempaszky C. Reip T. et at. 3-D Simulation of Hot Forming and Microstructure Evolution [JJ. Computational Materials Science. 2003. 28(3): 469. Liu Z Y. Wang W-D. Gao W. Prediction of the Mechanical Properties of Hot-Rolled C-Mn Using Artificial Neural Networks [JJ. Journal of Materials Processing Technology. 1996. 57(4): 332.
[9J
References : [10J
[1]
Chang W S. Microstructure and Mechanical Properties of 780 MPa High Strength Steels Produced by Direct-Quenching and Tempering Process [JJ. Journal of Material Sscience , 2002. 37 (4): 1973.
[l1J
[2J
Wang F. Zhu Q. Lin J. et at. Prediction of Microstructural Evolution in Hot Rolling [JJ. Journal of Materials Processing Technology. 2006. 177(3): 530.
[12J
[3J
Bontcheva N. Petrov P. Petzov G. et at. Finite Element Simulation of Strain Induced Austenite-Martensite Transformation
Dyja H. Korczak P. The Thermal-Mechanical and Microstructural Model for the FEM Simulation of Hot Plate Rolling [JJ. Journal of Materials Processing Technology, 1999. 93(3): 463. Kim 5-1. Yoo y-c. Prediction of Dynamic Recrystallization Behavior of AISI Type 4130 Medium Carbon Steel [JJ. Materials Science and Technology. 2002. 18(3): 160. Hodgson P D. Mathematical Modeling of Recrystallization Process During Hot Rolling of Steels [DJ. Queensland: University of Queensland. 1993. Hodgson P D. Gibbs R K. A Mathematical Model to Predict the Mechanical Properties of Hot Rolled C-Mn and Microalloyed Steels [n lSI] Int. 1992. 32(6): 1329.