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tóm tắt tiếng anh ảnh hưởng của thông số công nghệ cơ – nhiệt đến tổ chức và cơ tính của thép TRIP CMnSi luyện từ sắt xốp

MINISTRY OF EDUCATION AND TRAINING MINISTRY OF DEFENSE
ACADEMY OF MILITARY SCIENCE AND TECHNOLOGY

DINH VAN HIEN

INFLUENCE OF THERMO-MECHANICAL PROCESSING
PARAMETERS ON MICROSTRUCTURE AND MECHANICAL
PROPERTIES OF A SPONGE IRON-MELTED CMnSi TRIP STEEL

Major: Dynamics and Mechanical Engineering
Code: 9520116

SUMMARY OF ENGINEERING DOCTORAL DISSERTATION

HANOI – 2018


The work was completed at:
Academy of Military Science and Technology

Science Consultant Cadres:

1. Assoc.Prof.Dr Dinh Ba Tru
2. Assoc.Prof.Dr Nguyen Van Chuc
Reviewer 1: Prof.Dr Do Minh Nghiep
Hanoi University of Science and Technology
Reviewer 2: Assoc.Prof.Dr Chu Thien Truong
Military Technical Academy
Reviewer 3: Assoc.Prof.Dr Trinh Hong Anh
Academy of Military Science and Technology
The dissertation will be defended in front of Doctor Examining
Committee held at Academy of Military Science and Technology
at … … …, … … … …, 2018

For further details:
- Vietnam National Library
- Library of Academy of Military Science and Technology


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INTRODUCTION
Necessity of this dissertation: Nowadays, there has been being a
metallurgy revolution about direct-reduced iron process (sponge iron)
instead of blast furnace. The steel would be purer because of using sponge
iron for melting and refining that make strength higher and ductility better,
thus, advanced high strength steels (AHSS) including DP steels, TRIP
steels … have created from CMnSi HSLA steels.
Vietnam has been performing the industrialization, thus, needs to pursue
developed nations and utilize “Sponge iron – melting and refining –
thermomechanical process” to produce high quality steels – AHSS for
producing members of cars and missiles …
Vietnam has been having good sponge iron which will support for
producing high quality steels. The national science and technology project
(2014-2017) has supported for researchers, thus, a team “Research on alloy
steels melted sponge iron” melted and refined several high quality steels in
a vacuum induction melting furnace VIM300 such as TRIP steels.
The author show that it needs to make studies on thermomechanical
process of TRIP steel to confirm a new technology for applying to realistic
conditions of Vietnam. Therefore, the project “Influence of processing
parameters on microstructure and mechanical properties of a sponge ironmelted CMnSi TRIP steel” is necessary to perform.
Objectives of this dissertation:
- Study on establishment of relations between some thermomechanical


processing parameters (TMPP) and mechanical properties with considering
their relations with microstructure of a CMnSi TRIP steel to control the
process for finding out processing parameters which having optimal
strength and ductility according to user’s requirements.
Objects of this investigation: This study chosen a sponge iron meltedCMnSi steel which has chemical composition in TRIP steel’s standards,
utilized thermomechanical process to make a TRIP effect assisted steel
with a three-phase microstructure of ferrite, bainite and retained austenite
and has found relations from that.
Scopes of this investigation: Detect TMPPs to make steel sheets which


2
have a three-phase microstructure of ferrite, bainite and retained austenite
and find relations between TMPPs and mechanical properties of a sponge
iron melted CMnSi steel from there.
This study focus on influence of four TMPPs including intercritical
annealling (IA) time and temperature, isothermal bainite treatment (IBT)
temperature and time. Other processing variables are held constant.
- Main target functions: strength and ductility characteristics.
- Second target functions: volume fraction and grain size of phases.
Contents of this investigation:
1. Features of AHSS-TRIP steels and their manufacturing processes.
2. Theory of microstructure and mechanical properties of TRIP steels.
3. Experimental Methods.
4. Influence of processing parameters on microstructure and mechanical
properties of the sponge iron melted CMnSi TRIP steel
Methods of this investigation: Study on the world’s science and
technology achievements about the relations among chemical composition,
microstructure and mechanical properties of TRIP steels, infer processing
solutions from these relations and perform measurements to find processing
parameter ranges as well as determine microstructure, mechanical
properties and the relations of them.
Meaningfulness in science: It confirmed that with a CMnSi steel
melted from sponge iron and refined to have low content of impurities,
processed to generate three phases of ferrite, bainite and retained austenite
in certain volume fraction and extra-fined grain size will definitely have a
combination of strength and ductility better than that of the HSLA steel in
similar chemical composition. It established the relations among
mechanical properties, processing parameters and microstructure, thus,
found out optimal TMPP areas of strength and ductility. These TMPP areas
were tested which confirmed discovered laws truly.
Meaningfulness in reality: Discovered-optimal TMPP areas can use for
manufacturing TRIP steel billets in industry or processing to increase
formability of semi-finished forming products and strength of finished
products. Studied Results demonstrate reality of the sponge iron-melted high
quality steel as AHSS-TRIP stees for using in civil and defense industry.


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Chapter 1. FEATURES OF AHSS-TRIP STEELS AND THEIR
MANUFACTURING PROCESSES
1.1. Related concepts
1.1.1. HSLA steel
1.1.2. AHSS
1.1.3. TRIP steel

Fig 1.1. Relation between tensile strength and elongation of several steels.
1.2. Composition, microstructure and mechanical properties of AHSS
and TRIP steels
1.2.1. Composition, microstructure of AHSS and TRIP steels
- Chemical composition of TRIP steels includes main elements of 0.1 to
0.4 %C, 1.0 to 2.5 %Mn and 1.0 to 2.2 %Si. These steels have the low content
of impurities ( 0,025%P and  0,015%S) and of gases (O2, H2, N2).
- TRIP steels use the multiphase strenggthening effect through creating
hard phases including bainite, retained austenite disperse in a soft-ductile
ferrite matrix. The volume fraction of phases is in a certain range, normally,
50 to 60 % ferrite, 25 to 40 % bainite and 5 to 20 % retained austenite. The
grain size of phase is fine, size of ferrite grain is less than 20 µm and of
retained austenite grain is ultrafine, less than 40 µm.
1.2.2. Mechanical properties of AHSS and TRIP steels
1. TRIP steels have a balanced combination of strength and ductility.
2. TRIP steels have high strength and tensile-yield ratio.
3. TRIP steels have good ductility.


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a- strain-stress curves
b- strain-hardening exponent curves
Fig 1.7. Mechanical properties of TRIP, DP and HSLA steels with similar
yield strength.
1.3. Manufacturing process and applicatio of AHSS and TRIP steels
1.3.1. Manufacturing process of AHSS steels
1. AHSS steels melted from sponge iron
2. AHSS steels refined in second refining furnaces
3. AHSS steels produced by thermomechanical process
The core of generating multiphase microstructure which would obtain special
properties of AHSS steels is that having to use thermomechanical process with a
combination of deformation and IA (between Ac1 and Ac3 temperature), then
cooling in various conditions according to each grade of steels.
1.3.2. Thermomechanical process for producing TRIP steels
1.3.2.1. Deformation process in producing TRIP steels

Fig 1.12. Two thermomechanical process schemas for producing TRIP steels.
The main task of the deformation stage is to control microstructure to obtain a
fined microstructure which would platform for a more refined microstructure
with heat treatments later.
1.3.4.2. Heat treatment of TRIP steels
The main task of the heat treatment stage is to control the volume fraction of
phases. To own the thermomechanical process of the TRIP steel needs
controlling six-processing parameters at least. Recently, several countries have


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been possessing the manufacturing process of TRIP steels in industry. However,
their processing parameters have been secreted.
1.3.3. Application of AHSS and TRIP steels
They can be used to fabricate members of cars, metal pressure vessels
intead of HSLA steels in military ...
1.4. Mirex sponge iron – important raw material for producing AHSS
1.5. Conclusion of chapter 1
1. Advanced high strength TRIP steels have main characteristics:
- Chemical composition: 0.1 to 0.4%C, 1.0 to 2.5%Mn, 1.0 to 2.2%Si,
low contents of P, S, impurities and gases;
- Microstructure: A three-phase microstructure of ferrite, bainite and 5
to 20 % retained austenite, refined grain size of phases created by a
thermomechanical process procedure.
- Mechanical properties: Having a good combination of strength and
ductility. Tensile strength (MPa): 600 to 1050 and elongation (%): up to 40.
- Applications: make steel structures, sheet mebers of cars, pressure
vessels … with light weight.
2. Orientation of the investigation:
- Study on some theories relating to special microstructure and
mechanical properties of TRIP steels to make platforms of experiments
- Perform experiments to create steel sheets with multiphase
microstructure and determine data of mechanical properties.
- Establish laws of effect of TMPPs and determine processing
parameters for optimized strength and ductility from there.
Chapter 2. THEORY OF MICROSTRUCTURE AND
MECHANICAL PROPERTIES OF TRIP STEELS
2.1. Strength and ductility of TRIP steels
2.1.1. Law of phase mixture apply for TRIP steels
Strength and ductility of TRIP steels obey the rule of phase mixture:
 = f.+fb.b+fd.d+f’.’
 = f.+fb.b+fd.d+f’.’
f+fb+fd+f’ = 1


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where: f, ,  are volume fraction, stress and strain of phases, respectively.
Strength and ductility of TRIP steels can be controlled by volume
fraction, strength and ductility of phases.
2.1.2. Các nguyên lý hãm lệch để tăng bền sử dụng trong thép TRIP
- Solid solution strengthening
of ferrite, bainite and retained
austenite by using C, Mn, Si;
- Precipitation strengthening;
- Strengthening by fined grain;
-

Phase

transformation

strengthening.
2.1.3.

Solid

precipitation

solution
strengthening

and
in

Fig 2.5. Contribution of strengthening
mechanism to strength of steels.

TRIP steels
2.1.4. Strengthening and plasticizing by fine grained in TRIP steels
TRIP steels use methods for making ferrite finer ((≤ 20 µm):
- Increasing strength by Hall-Petch rule: c = 0c + Kd.d-1/2

- Increasing ductility by transiting deform mechanism in a small amount
of grains to that in a series of grains.
2.1.5. Hai nguyên lý hóa bền và tăng dẻo bằng chuyển biến pha trong thép TRIP
Two phase transformation strengthening use for TRIP steels:
(1) Austenite  bainite and retained austenite” during heat treatment;
(2) Retained austenite  martensite during plastic deformation.
2.1.5.1. Strengthening and plasticizing by generating bainite and retained austenite

Fig 2.10. Description of dislocation pile-ups on phase boundaries.
2.1.5.2. Strengthening and plasticizing by generating mechanically forcedmartensite - TRIP effect.


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In a certain condition, retained austenite could transform to martensite
which support in increasing strength and ductility. Contribution of phases
and marteniste transformation on hardening of the TRIP steel.
d  d
d
d
d
df
d
 f .   f b .  b  f d
 f a ' .  '  a ' (  '    d )
d
d
d
d
d
d

(2.18)

Fig 2.17. Strain-stress curves and hardening rate curves of a TRIP steel.
2.1.5.3. Effect of volume fraction of phases on strength and duclity of
TRIP steels
The volume fraction of phases is
decisive factor on strength and ductility
of the TRIP steels:
- To obtain the optimal combination
of strength and ductility necessarily
optimizes volume fraction of retained
austenite;
- To obtain the optimal strength
necessarily experiences to find the
suitable processing parameter range.

Fig 2.19. Relation between
tensile strength and elongation of
TRIP steels.

2.1.5.4. Effect of grain size of bainite and retained austenite on strength and
duclity of TRIP steels
2.2. Thermodynamics of formation of TRIP steel microstructure
2.2.1. Basic thermodynamics of formation of TRIP steel microstructure
2.2.1.1. Formation of ferrite and austenite during intercritical annealing
2.2.1.2. Bainitic transformation and formation of retained austenite
Bainitic transformation of the TRIP steel could be summarized as follow:


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   + b,supersaturated  b,saturated + enriched  b,unsaturated + enriched + ( + )
where: - austenite; b- bainitic ferrite; - ferrite; và - carbide or cementite.
2.2.2. Role of C, Mn and Si on formation of TRIP steel microstructure
2.2.3. Effect of processing parameters on thermodynamics of formation of
TRIP steel microstructure
2.2.3.1. Effect of plastic deformation
2.2.3.2. Effect of intercritical annealling parameters
IA temperature and time control volume fraction of ferrite and austenite,
carbon concertration in these two phase, simultanously, control indirectly
volume fraction of retained austenite. According to several sudies, exist a
range of intermidiate temperature and time that have maximal volume
fraction of retained austenite.
2.2.3.3. Effect of cooling rate
To obtain the TRIP steel microstructure necessarily choose cooling
environments which have cooling rate higher than crictical cooling rate.
2.2.3.4. Effect of bainitic isothermal treatment parameters
IBT temperature and time control volume fraction of retained austenite and
its carbon concerntration. According to several sudies, exist the range of
certain temprature (350 to 4500C) and time (2 to 20 minutes) which have
maximal volume fraction of retained austenite.
2.3. Relations between chemical compostion C, Mn and Si with,
microstructure and mechanical properties of TRIP steels
Ranges of CMnSi compostion for optimal strength and elongation:
- Optimal strength {0.2-0.24%C, 2.0-2.2%Mn, 1.8-2.2%Si}. Tensile strength
is over 900 MPa, elongation is 20 to 30%.
- Optimal elongation:{0.12-0.14%C, 1.4-1.8%Mn, 1.8-2.2%Si} and
{0.2-0.24%C, 1.2-1.7%Mn, 1.4-1.6%Si}. Elongation is over 30%.
2.4. Conclusion of chapter 2
1. Strength and ductility of TRIP steels obey the general law being
plastic deformation by dislocation motion and strengthening by hindering
their motion. However, special mechanical properties of these steels are due to
several specific principles as follow:


9
- Use grain refinement strengthening of extra-fined ferrite grain.
- Use phase transformation strengthening with two transformations: (1)
Austenite  bainite and retained austenite” during heat treatment; (2)Retained austenite  martensite during plastic deformation.
2. To obtain the required TRIP steel microstructure needs controlling:
- Previous deformation condition to obtain uniform, refined microstructure;
- Themomechanical condition consisting of cold-rolling reduction, IA
temperature and time, IBT temperature and time to control grain size, alloy
concentration and volume fraction of phases.
In the scope of this investigation, determined specific works include:
- Chemical composition of the studied steel, previous deformation
condition, cold-rolling reduction and cooling environment will be fixed.
- Varying 4 processing variables: temperature and time of IA and IBT.
Chapter 3. EXPERIMENTAL METHODS
3.1. Experimental process schema
3.2. Methods for preparing steel billet and determining
thermodynamic parameters
3.2.1. Studed steel
The studied steel (0.22C-1.4Mn-1.6Si-0.021P-0.009S) melted from Mirex
sponge iron and refined. It is enough for experimental investigation.
3.2.2. Determination of thermodynamic parameters
Critical temperatures, Ac1 to Ac3: 740 to 8700C.
3.2.3. Preparation of steel billet
Homogeneous soak: 10000C/3 h.
Hot forging: Forging reduction, y = 4. Finished forging temperature, 9000C.
Hot rolling: Total thickness reduction of 50%, finished rolling at 8800C. A
refined microstructure was obtained with ferrite grain size being less than 20 µm.
3.3. Methods for determining microstructure and structure of phases
3.3.1. Identifying microstructure via optical microscope
Highlight phase constituents via color etching.


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3.3.2. Measuring grain size and volume fraction of phases
Measuring grain size and volume fraction of phases are performed by
image analysis techniques on ImageJ and Axiovision softwares.
3.3.3. Identifying retained austenite and deformed-martensite via X diffraction
3.4. Mothods for determing ranges of thermomechanical parameters

Fig 3.7. Thermomechanical process Schema of the study.
The goal was to find the range of TMPPs for obtaining three-phase
microstructure of the studied steel.
3.4.1. Determining cold-rolling reduction in thickness
The cold-rolling thickness reduction of 80% was chosen for
thermomechanical process by the previous experiment in order to obtain a
refined microstructure with ferrite grain size being less than 10 m.
3.4.2. Determining the range of heat treatment parameters
The ranges of heat treatment parameters were as follow: IA from 750 to
8100C, holding from 5 to 15 minutes; IBT from 350 to 4500C, holding from 5 to
15 minutes. The cooling environment: 45%NaOH + 30%NaNO3 + 25%NaNO2.
3.5. Methods for determining mechanical properties
3.6. Design of Experiment
Table 3.11. Regression functions of real varialbes.
TT
1
2
3
4
5
6
7
8

Regression functions
Rp1 = -240,404 + 5,592.+ + 3,666.TB + 0,635.B – 0,434.+.B + 0,026.TB.B– 0,177.+2 –
0,005.TB2 – 0,03.B2
Rp2 = -5810,425 + 16,691.T+ - 54,66.+ + 0,068.T+ .+ - 0,011.T+ 2
Rm1 = 3190,553 + 10,155.+ - 11,281.TB – 28,869.B + 0,013.+.TB + 0,103.+.B +
0,043.TB.B– 0,569.+2 + 0,013.TB2 + 0,191.B2
Rm2 = -21676,679 + 56,908.T+ + 63,823.+ - 0,036.T+ 2 – 0,069.T+ .+ - 0,351.+2
A1 = - 315,565 - 0,338.+ + 1,715.TB + 3,341.B - 0,005.TB.B - 0,002.TB2 - 0,056.B2
A2 = -1801,481 + 4,709.T+ + 0,494.+ – 0,003.T+ 2 - 0,043.+2
PSE1 = - 184460,49 – 127,91.+ + 1051,58.TB + 1924,49.B - 2,73.TB.B - 1,32.TB2 - 41,66.B2
PSE2 = - 2163940 + 5614,56.T+ + 758,37.+ – 3,6.T+ 2 - 46,33.+2


11
Four validated regression functions which express the relations between
TMPPs and mechanical properties of the studied steel were determined
(Table 3.11). The optimized values of investigated TMPPs and responsevalues were predicted and experimentally validated (Table 3.12 to 3.15).
Table 3.12. Optimized variable and experimental value of yield strength.
Yield strength
Rp, MPa

Variable Level
PA

TB
T+
+
B
Model
MH
TN
MH
TN
MH
TN
MH
TN
NTU
0
780
-1
5
-0,237 388,2 0,64 13,2
473,5
1
TN
0
780
-1
5
-0,2
390
0,5
12,5
NTU -0,74 757,3
-1
5
0
400
0
10
469,2
2
TN
-1
750
-1
5
0
400
0
10
PA- Experiment mode; NTU- predicted value; TN - Experiment; MH- coded value.

TN

Error
(%)
1,9

492,8

1,4

475,2

Bảng 3.13. Optimized variable and experimental value of tensile strength.
Tensile strength
Rm, MPa

Variable Level
PA
NTU
TN
NTU
TN

1
2

T+
MH
TN
0
780
0
780
-0,222 773,5
-0,333 770

+
MH
0,667
0,6
1
1

B

TB
TN
13,3
13
15
15

MH
-1
-1
0
0

TN
350
350
400
400

MH
-1
-1
0
0

TN
5
5
10
10

Model

TN

Error
(%)

899

1,7

913,9
808,7

1,4

820,5

Bảng 3.14. Optimized variable and experimental value of elongation.
Variable Level
PA
NTU
TN
NTU
TN

1
2

T+
MH
TN
0
780
0
780
-0,11 776,7
-0,333 770

+
MH
-1
-1
-0,83
-1

Elongation, A, %
TB

TN
5
5
5,8
5

MH
-0,223
-0,2
0
0

TN MH
388,8 0,504
390
0,5
400
0
400
0

B
TN
12,5
12,5
10
10

Model

TN

37,1
35,4
36,7
34,8

Error
(%)
4,6
5,2

Bảng 3.15. Optimized variable and experimental value of PSE.
Product of Rm and
A, PSE, MPa%
Error
TB
(%)
B

Thực
hình
nghiệm
TN MH
TN
MH TN
5,3 -0,274 386,3 0,044 10,2 28590
4,1
5
0,3
385
0
10
27411
8,2
0
400
0
10 28790
2,9
8
0
400
0
10
27980

Variable Level
PA

1
2

NTU
TN
NTU
TN

T+
+
MH
TN
MH
0
780 -0,943
0
780
-1
-0,02 779,4 -0,359
0
780
-0,4

3.7. Method for processing data on STATISTICA software
3.8. Conclusion of chapter 3
1. The studied TRIP steel is enough for experiment.
2. Critical temperatures, Ac1, Ac3, Bs, Ms were detemined.


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3. Infering the preparing process mode (soak, hot forging and rolling)
guarantees the refined microstructure with ferrite grain size being near 20 m.
4. Infering the range of TMPPs guarantees the true microstructure of the
TRIP steel: cold-rolling reduction of 80%; IA from 750 to 8100C, holding from
5 to 15 minutes; IBT from 350 to 4500C, holding from 5 to 15 minutes.
5. Establishing functional relations of Rm, Rp, A and PSE with four
processing variables which can be utilized to predict target values.
Chapter 4. INFLUENCE OF PROCESSING PARAMETERS ON
MICROSTRUCTURE AND MECHANICAL PROPERTIES OF THE
SPONGE IRON MELTED CMnSi TRIP STEEL
4.1.

Several

judgements

on

compostion,

microstructure

and

mechanical properties of the studied TRIP steel
Microstructure. Ferrite, bainite, retained austenite (Fig 4.1).

a- microstructure (x50)
b- map of retained austenite (x500)
Fig 4.1. Microstructure of thermomechanical processed 80B-4 sample.
Mechanical properties. Mechanical properties of the studied TRIP steel is
similar to the same TRIP steel in standard of several nations.
4.2. Effect of TMPPs on microstructure of the studied TRIP steel
4.2.1. Effect of cold rolling on ferrite grain size
4.2.2. Effect of intercritical annealling temperature and time on volume
fraction and grain size of ferrite
4.2.3. Effect of TMPPs on volume fraction of bainite
4.2.4. Effect of TMPPs on volume fraction and grain size of retained austenite
(see Fig 4.9)


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a,

b,

c,
d,
Fig 4.9. Relation between volume fraction of retained austenite and TMPPs.


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4.3. Effect of TMPPs on strength
4.3.1. Effect of TMPPs on ultimate tensile strength
The range of TMPPs for optimized tensile strength, Rm ≥ 830 MPa:
Range 1: T+: 770-7900C, +: 11-15 min; TB: 350-3700C; B: 5-7 min.
Range 2: T+: 770-7900C, +: 11-15 min; TB: 440-4500C; B: 5-7 min.
The optimally predicted value is in the optimized ranges.
4.3.2. Effect of TMPPs on yield strength
The range of TMPPs for optimized yield strength, Rp ≥ 460 MPa:
Range 1: T+: 750-7700C, +: 5-8 min; TB: 350-4050C; B: 9-14 min.
Range 2: T+: 770-7900C, +: 5-9 min; TB: 350-4050C; B: 10-13 min.
Range 3: T+: 800-8100C, +: 7.5-15 min; TB: 350-4050C; B: 7.5-15 min.
The optimally predicted value is in the optimized ranges.
4.3.3. Effect of TMPPs on tensile-yield ratio
The range of TMPPs for optimized tensile-yield ratio, Rm/Rp ≥ 1,8:
Range 1: T+: 765-7900C, +: 11-15 min; TB: 430-4500C; B: 5-7 min.
Range 2: T+: 765-7900C, +: 11-15 min; TB: 350-3600C; B: 5-7 min.
4.4. Effect of TMPPs on ductility
4.4.1. Effect of TMPPs on elongation
The range of TMPPs for optimized elongation, A ≥ 32%:
T+: 750-7900C, +: 5-10.5 min; TB: 370-4100C; B: 8-14 min.
The optimally predicted value is in the optimized ranges.
4.4.2. Effect of TMPP on strain hardening exponent
The range of TMPPs for optimized strain hardening exponent, n ≥ 2,2:
T+: 750-7900C, +: 5-10 min; TB: 350-4100C; B: 8-15 min.
4.4.3. Effect of TMPP on PSE
The range of TMPPs for optimized PSE, ≥ 25000 MPa%:
T+: 765-7900C, +: 5-11 min; TB: 350-4050C; B: 7-13.5 min.
The optimally predicted value is in the optimized range.
(see Fig 4.12, 4.16, 4.17, 4.18, 4.19, 4.21)


15

a,

b,

c,
d,
Fig 4.12. Relation between ultimate tensile strength and TMPPs.


16

a,

b,

c,
d,
Fig 4.16. Relation between yield strength and TMPPs.


17

a,

b,

c,
d,
Fig 4.17. Relation between tensile-yield ratio and TMPPs.


18

a,

b,

c,
d,
Fig 4.18. Relation between elongation and TMPPs.


19

a,

b,

c,
d,
Fig. 4.19. Relation between strain hardening exponent and TMPPs.


20

a,

b,

c,
d,
Fig 4.20. Relation between PSE and TMPPs.


21
4.5. TMPP ranges for optimal combination of strength and ductility
With the obtained results, The TMPP ranges for various balances of
strength and ductility were found out as Fig 4.21.

Ranges
Range 1
Range 2
Range 3

Volume fraction of phases (%)
f
fb
fd
53-58
30-40
≥ 10
67-74
20-26
5-8
53-62
32-40
6-10
49-53
40-47
≤6
49-58
40-47
6-10

T+, 0C
775-785
750-760
765-780
775-790
770-790

Processing parameters
TB, 0C
+, min
5-8
370-400
5-7
350-370
5-10
350-370
11-15
440-450
11-15
350-370

B, min
8-14
10-14
7-10
5-7
5-7

Fig 4.21. Microstructural and TMPP ranges for various balances of strength
and ductility.
Verification: To confirm generality
of

studied

results,

an

verified

experiment on the 0.18C-1.8Mn-2.0Si0.023P-0.014S steel was performed.
This steel melted from sponge iron and
refined in a vacuum induction furnace,
VIM300. The results show the inferred
experimental laws are general and can
apply

for

predicting

mechanical

Hình 4.22. Relation between
strength and elongation of the
verified CMnSi TRIP steel.

properties of other TRIP steels according to required strength and ductility.


22
4.6. Application of optimal TMPPs in forming semi-finished product

Dimention

Real image

Before forming

Dimention

Real image

After forming

Fig 4.23. A semi-finished product before and after forming.
4.7. Conclusion of chapter 4
1. The studied TRIP steel has a high quality chemical composition enough
for generating special properties of high strength and good ductility.
2. The studied TRIP steel microstructure has three phases including ferrite,
bainite, retained austenite that be suitable to the criteria of TRIP steels.
3. Mechanical properties of the studied TRIP steel is sutable to several
national criterias of TRIP steels.
4. Effects of TMPPs on microstructure of the TRIP steel were analysized.
5. Effects of TMPPs on mechanical properties of the TRIP steel were
analysized in the relation with microstructure, have confirmed the relations
among TMPPs, microstructure and mechanical properties are suitalbe. From
that, TMPP ranges for optimized strength and ductility were detemined.
6. TMPP ranges for various balnaces of strength and ductility were
detemined according to user requirements.
GENERAL CONCLUSION
I. Main results of the dissertation
1. It overviewed “AHSS-TRIP steels and manufacturing processes” to
lead to affirm that the TRIP steel only need the certain CMnSi content, but


23
have excellent combination of strength and ductility because of low content
of impurities, three-phase microstructure of ferrite, bainite and retained
austenite with a certain volume faction and fine grain size of each phase
and manufactured by advance technology “melting the steel by direct
reduction iron – steel refining – special thermomechanical process”.
2. It overviewed some basic theories relating microstructure and
mechanical properties of TRIP steels to lead to affirm that strength and
ductility of the TRIP steel obey the mixture law of phases, depend on
volume fraction and strength, ductility of each phase. These steels have
excellent balance of strength and ductility because of using an overall
combination of theories of strengthening, thus, two typical theories of
strengthening and plasticizing.
- Phase transformation strengthening with two transformations: (1)“austenite  bainite and retained austenite” during heat treatment to
generate hard phases of bainite and retained austenite which have good
strength and ductility through the mixture law; (2)- “retained austenite 
martensite” during plastic deformation to strengthen due to forming hard
martensite and plasticize due to effect of transformation induced plasticity.
- Strengthening by fined grain. Making ferrite fine as well as making
bainite and retained austenite ultrafine dispersing in a matrix of ferrite to
strengthen by hindering dislocation motion increasingly and plasticize by
transiting deform mechanism in grain with small amount of deformed
grains to that in a series of grains with larger amount of deformed grains.
3. It melted and refined a TRIP steel using MIREX sponge iron which has
composition truly in required limit with low content of impurities (0.22C1.4Mn-1.6Si-0.021P-0.009S) to support for applying thermomechanical
process. From base on studied scientific laws, this dissertation determined the
influence of four TMPPs (temperature and time of IA and IBT) on
microstructure and mechanical properties of the studied TRIP steel;
determined range of TMPPs truly and controlled impact of four TMPPs to
obtain a three-phase microstructure of ferrite, bainite and retained austenite


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