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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 CMnSiTRIP 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: Assoc.Prof.Dr Dinh Ba Tru 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 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 CMnSiTRIP 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 CMnSiTRIP 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 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: Features of AHSS-TRIP steels and their manufacturing processes Theory of microstructure and mechanical properties of TRIP steels Experimental Methods Influence of processing parameters on microstructure and mechanical properties of the sponge iron melted CMnSiTRIP 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 Chapter 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 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 TRIP steels have a balanced combination of strength and ductility TRIP steels have high strength and tensile-yield ratio TRIP steels have good ductility 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 AHSS steels melted from sponge iron AHSS steels refined in second refining furnaces 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 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 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 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 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 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.+fb.b+fd.d+f’.’ = f.+fb.b+fd.d+f’.’ f+fb+fd+f’ = 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 thépTRIP - 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 tăng dẻo chuyển biến pha thépTRIP 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 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: + b,supersaturated b,saturated + enriched b,unsaturated + enriched + ( + ) where: - austenite; b- bainitic ferrite; - ferrite; - 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 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: 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 780 -1 -0,237 388,2 0,64 13,2 473,5 TN 780 -1 -0,2 390 0,5 12,5 NTU -0,74 757,3 -1 400 10 469,2 TN -1 750 -1 400 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 T+ MH TN 780 780 -0,222 773,5 -0,333 770 + MH 0,667 0,6 1 B TB TN 13,3 13 15 15 MH -1 -1 0 TN 350 350 400 400 MH -1 -1 0 TN 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 T+ MH TN 780 780 -0,11 776,7 -0,333 770 + MH -1 -1 -0,83 -1 Elongation, A, % TB TN 5 5,8 MH -0,223 -0,2 0 TN MH 388,8 0,504 390 0,5 400 400 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 Mơ Thực hình nghiệm TN MH TN MH TN 5,3 -0,274 386,3 0,044 10,2 28590 4,1 0,3 385 10 27411 8,2 400 10 28790 2,9 400 10 27980 Variable Level PA NTU TN NTU TN T+ + MH TN MH 780 -0,943 780 -1 -0,02 779,4 -0,359 780 -0,4 3.7 Method for processing data on STATISTICA software 3.8 Conclusion of chapter The studied TRIP steel is enough for experiment Critical temperatures, Ac1, Ac3, Bs, Ms were detemined 12 Infering the preparing process mode (soak, hot forging and rolling) guarantees the refined microstructure with ferrite grain size being near 20 m 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 to 15 minutes; IBT from 350 to 4500C, holding from to 15 minutes Establishing functional relations of Rm, Rp, A and PSE with four processing variables which can be utilized to predict target values Chapter INFLUENCE OF PROCESSING PARAMETERS ON MICROSTRUCTURE AND MECHANICAL PROPERTIES OF THE SPONGE IRON MELTED CMnSiTRIP 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) 13 a, b, c, d, Fig 4.9 Relation between volume fraction of retained austenite and TMPPs 14 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 Range 2: T+: 770-7900C, +: 11-15 min; TB: 440-4500C; B: 5-7 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 Range 2: T+: 770-7900C, +: 5-9 min; TB: 350-4050C; B: 10-13 Range 3: T+: 800-8100C, +: 7.5-15 min; TB: 350-4050C; B: 7.5-15 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 Range 2: T+: 765-7900C, +: 11-15 min; TB: 350-3600C; B: 5-7 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 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 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 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 Range Range Volume fraction of phases (%) f fb fd 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 +, 5-8 370-400 5-7 350-370 5-10 350-370 11-15 440-450 11-15 350-370 B, 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 CMnSiTRIP 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 The studied TRIP steel has a high quality chemical composition enough for generating special properties of high strength and good ductility The studied TRIP steel microstructure has three phases including ferrite, bainite, retained austenite that be suitable to the criteria of TRIP steels Mechanical properties of the studied TRIP steel is sutable to several national criterias of TRIP steels Effects of TMPPs on microstructure of the TRIP steel were analysized 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 TMPP ranges for various balnaces of strength and ductility were detemined according to user requirements GENERAL CONCLUSION I Main results of the dissertation 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” 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 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 24 with volume fraction of ferrite from 38 to 74 %, bainite from 19 to 58 % and retained austenite from to 16 %, grain size of ferrite of to m, retained austenite of to m, thus, mechanical properties including yield strength of 410 to 480 MPa, ultimate tensile strength of 740 to 900 MPa and elongation at fracture of 24 to 36 % were obtained Initial condition: hot deformation with deformation ratio of 8; then, cold rolling in thickness reduction of 80 %; heat treatment with intercritical annealing at 750 to 8100C, hold at to 15 minutes and cooling in melted salt at 350 to 4500C, hold to 15 minutes It established the influences of four TMPPs on microstructure and mechanical properties of the studied TRIP steel and optimal determined TMPP areas for highest strength or medium strength and ductility or highest ductility from those These were confirmed truly and can apply in reality according to user’s requirement II New contributions of the dissertation This is a first research on an AHSS-TRIP steel in Vietnam, it opens an orientation for research on materials for economics and defense Researched on producing a TRIP steel from Mirex sponge iron - Melting and refining in an industrial vacuum furnace – Thermomechanical process to create a threephase microstructure of ferrite, bainite and retained austenite, mechanical properties of the studied steel are more exceedingly than of HSLA The dissertation used a combination of modern experimental methods, examined effects of four TMPPs on volume fraction, grain size of phases and mechanical properties of the studied steel Studied results can be used to establish the process schedule for producing steel billets utilized in stamping and spinning shell structures of missiles and pressure vessels… III Problems that need to be investigated in the future Because the first dissertation was done in Vietnam for a limited period of time, it only explored the initial stages of some theoretical foundations and the effect of four TMPPs on microstructure and mechanical properties The effects of chemical composition and deformation haven’t been mentioned in depth Therefore, these problems will need for further research and development List of published scientific works Hien Dinh Van, Chuc Nguyen Van, Thuc Tran Cong, Long Le Van and Tru Dinh Ba, “Experimental study of producing steel 09Mn2Si made from the sponge iron”, Journal of Science and Technology - 8th science and technology conference on dynamics and mechanical engineering, No 27, pp 162-165, Hanoi, April 2015 Hien Dinh Van, Thuc Tran Cong, Long Le Van and Tru Dinh Ba, “A study of producing a CMnSiAl transformation induced plasticity steel from the sponge iron raw material”, Journal of Military Science and Technology - Academy of Military Science and Technology, pp 160-167, Hanoi, Oct 2015 Hien Dinh Van, Tru Dinh Ba and Chuc Nguyen Van, “Investigation of the mechanical behaviour of thermo-mechanical treatmented MnSsi transformation induced plasticity steel”, Journal of Military Science and Technology - Academy of Military Science and Technology, pp 276-282, Hanoi, Sep 2016 Hien Dinh Van, Tru Dinh Ba, Chuc Nguyen Van and Long Le Van, “Effects of cold rolling on microstructure and mechanical properties of the thermo-mechanical treated 0,22C-1,4Mn-1,6Si steel”, Proceedings of 4th National Science and Technology Conference on dynamics and mechanical Engineering 2016, pp 468-473, Hanoi, Oct 2016 Hien Dinh Van, “Effect of intercritical annealing on the microstructure and mechanical properties of CMnSiTRIP steel”, Vietnam mechanical Engineering Journal, No 6, Hanoi, June 2017 HIEN Dinh Van et al., “Influence of heat treatment on microstructure and mechanical properties of a CMnSiTRIP steel using design of experiment”, Journal of materials today proceedings (Scopus Index) in IConAMMA 2017, Banglagore, India, August 2017 ... 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+ Rm1 = 3190,553 + 10,155.+ - 11,281.TB – 28,869.B... Mechanical properties of AHSS and TRIP steels TRIP steels have a balanced combination of strength and ductility TRIP steels have high strength and tensile-yield ratio TRIP steels have good ductility... 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