Ferrite grain size refinement through γ→α transformation Tomoyuki Yokota NKK Corp. (JFE group)
Today’s talk (1) What is TMCP ? (2) Ultra refinement in ferrite grain size by ultimate utilization of TMCP (3) Current interest
TMCP Thermo Mechanical Controlled Processing Contorolled Rolling (CR) Accelerated Cooling (AcC)
TMCP I.Kozasu:Materials Science and Technology,vol.7 Constitution and Properties of Steels Ed. By F.B.Pickering,VCH,(1993),184.
Metallurgical aspects of TMCP I.Kozasu:Materials Science and Technology,vol.7 Constitution and Properties of Steels Ed. By F.B.Pickering,VCH,(1993),184.
Microstructure of TMCP steel 20μm5μm Conventional rollingTMCP
Mechanical Properties Ferrite grain is refined, and strength-toughness balance is improved by TMCP I.Kozasu:Trans.ISIJ, 12,(1972),241.
Application of TMCP steels
Historical progress of TMCP C.Ouchi:ISIJ Int., 41,(2001),542.
What would be happened by ultra refinement in grain size? 20μm5μm Conventional rolling TMCP 1μm UFG
Advantages and technical problems involved in ultrafine grain steels C.Ouchi:CAMP-ISIJ vol,(1998),. AdvantageTechnical problem Increase of yield strength Improvement of toughness Increase of fatigue strength Improvement of SPF property Improvement of corrosion resistance property Improvement of grain boundary failure resistance property Increase of yield ratio Decrease of uniform elongation Properties of weld and HAZ High temperature properties Creation of ultrafine grain in heavy section products
National projects involved in ultrafine grain steels ・ Japan 1) Ferrous Super Metal Project 2) STX 21 ・ China ・ Korea HIPERS 21 Univ. of Manchester (U.K.) Univ. of Deakin and BHP (Australia)
Ferrous Super Metal Project (1) Objective: Ultra refinement of grain size under 1μm, in carbon/low alloy steel (2) Key technology: Large-strain deformation higher than 50% per pass (3) Fund: ＄ 15M for from Japanese government (4) Participants: Nippon Steel, NKK, Kawasaki Steel, Sumitomo Metal Industries, Kobe Steel
Three types of large-strain deformation Ultimate utilization of TMCP
Achievement in the Ferrous Super Metal Project Some interesting metallurgical phenomena caused by heavy deformation has been found out 1) Strain assisted low temperature diffusional transformation 2) Spontaneous reverse transformation due to adiabatic deformation heating M.Niikura et al: Jour. of Mat. Proc. Tech,117 (2001), 341.
Current interest Can limit of ferrite grain refinement simply be predicted by thermodynamics ? Comparison between experimental data and thermodynamically estimated theoretical limit of ferrite grain size
Existing ferrite grain size prediction models 1) Empirical models d α = f (d γ,C.R.,ε ret ) Final ferrite grain size is related to initial austenite grain size, cooling rate and retained strain in the austenite prior to transformation. Y.Saito, M.kimura et.al.: Kawasaki Steel Tech. Rep., 9, (March 1984), 12. C.M.Sellars: Proc. Int. Conf. HSLA Steels, Beijing, ASM,(1985), 73. 2) Models based on transformation kinetics d α = f (d γ,I,G) These models have been developed based on Johnson-Mehl equation. The ferrite grain size depends on nucleation rate, parabolic rate constant and prior austenite grain size. The effect of work-hardening of austenite on transformation kinetics has been also developed. M.Umemoto, H.Ohtsuka and I.Tamura: Acta metal.,34(1986), M.Umemoto et.al.: ISIJ Int.,32(1992), 306.
Estimation of theoretical limit of α grain size ΔG γ→(α+γ) = － Sv × σ αα ΔG γ→(α+γ) ; Calculated by MTDATA σ αα = 0.6J/m 2 All chemical driving force is assumed to be stored as ferrite grain boundary energy Sv = dαdα 2.26 dα＝ －dα＝ － 2.26 × σ αα ΔG γ→(α+γ)
Result Experimental data is in good accordance with theoretical limit at around Ae 3 temperature Deviation between experimental data and theoretical limit becomes larger with lowering in transformation temperature. Ae 3
Further study To clarify the reason why deviation between experimental data and theoretical limit becomes larger with lowering in transformation temperature. To develop more accurate prediction model