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Ferrite grain size refinement through γ→α transformation

Ferrite grain size refinement through γ→α transformation. < from TMCP steel to UFG steel >. Tomoyuki Yokota N KK 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.

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Ferrite grain size refinement through γ→α transformation

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  1. Ferrite grain size refinementthrough γ→α transformation < from TMCP steel to UFG steel > Tomoyuki Yokota NKK Corp. (JFE group)

  2. Today’s talk (1) What is TMCP ? (2) Ultra refinement in ferrite grain size by ultimate utilization of TMCP (3) Current interest

  3. TMCPThermo Mechanical Controlled Processing Accelerated Cooling (AcC) Contorolled Rolling (CR)

  4. TMCP I.Kozasu:Materials Science and Technology,vol.7 Constitution and Properties of Steels Ed. By F.B.Pickering,VCH,(1993),184.

  5. 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.

  6. Microstructure of TMCP steel 20μm 5μm Conventional rolling TMCP

  7. Mechanical Properties I.Kozasu:Trans.ISIJ, 12,(1972),241. Ferrite grain is refined, and strength-toughness balance is improved by TMCP

  8. Application of TMCP steels

  9. Historical progress of TMCP C.Ouchi:ISIJ Int., 41,(2001),542.

  10. What would be happenedby ultra refinement in grain size? 20μm 5μm 1μm Conventional rolling UFG TMCP

  11. Advantages and technical problems involved in ultrafine grain steels C.Ouchi:CAMP-ISIJ vol ,(1998), .

  12. 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)

  13. 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 1997 - 2001 from Japanese government (4) Participants: Nippon Steel, NKK, Kawasaki Steel, Sumitomo Metal Industries, Kobe Steel

  14. Three types of large-strain deformation Ultimate utilization of TMCP

  15. Achievement in the Ferrous Super Metal Project M.Niikura et al: Jour. of Mat. Proc. Tech,117 (2001), 341. 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

  16. 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

  17. Existing ferrite grain size prediction models 1) Empirical models dα = f(dγ,C.R.,εret) 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 kineticsdα = f(dγ,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),. 1377 M.Umemoto et.al.: ISIJ Int.,32(1992), 306.

  18. 2.26 Sv = dα 2.26 × σαα dα＝ － ΔG γ→(α+γ) Estimation of theoretical limit of α grain size All chemical driving force is assumed to be stored as ferrite grain boundary energy ΔG γ→(α+γ) = －Sv × σαα ΔG γ→(α+γ) ; Calculated by MTDATA σαα = 0.6J/m2

  19. Result Ae3 Experimental data is in good accordance with theoretical limit at around Ae3 temperature Deviation between experimental data and theoretical limit becomes larger with lowering in transformation temperature.

  20. 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

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