JFE 1980 OLAC On-Line Accelerated Cooling

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1 TMCP JFE No p. 1 6 Recent Development in Microstructural Control Technologies through Thermo-Mechanical Control Process (TMCP) Applied for JFE Steel s High Performance Plates SHIKANAI Nobuo MITAO Shinji ENDO Shigeru JFE JFE JFE TMCP thermo-mechanical control process JFE 1980 On-Line Accelerated Cooling TMCP 1998 Super HOP (Heat-treatment On-line Process) Super- HOP TMCP TMCP Abstract: Thermo-mechanical control process (TMCP) is one of microstructural control techniques, combining controlled rolling and controlled cooling, to obtain excellent properties of steel plates, such as high strength, excellent toughness and weldability. JFE Steel has continued efforts to develop TMCP technologies, ever since JFE Steel started operation of the accelerated cooling facility, (On-Line Accelerated Cooling), in the plate mill at West Japan Works (Fukuyama) in 1980, which was the fi rst industrial accelerated cooling system in the world. In 2004, the epoch-making on-line induction heating facilities, HOP (Heat-treatment On-line Process), were installed also in the plate mill at West Japan Works (Fukuyama). Toughness of high strength steels, which are usually produced by the quenching and tempering (Q-T) process, has been improved with the refi ning carbides through the rapid tempering by HOP. In addition, since cooling stop-temperature before tempering can be accurately controlled by Super-, unique microstructural control techniques utilizing martensite-austenite constituent (M-A) as the hard phase, which are not be able to be achieved by the conventional Q-T process or TMCP, have been developed. These techniques have already been applied to a variety of new products. This paper describes fundamentals of microstructural control by TMCP, and the recent development in TMCP with some examples of new products in JFE Steel. 1. TMCP thermo-mechanical control process ) JFE 1980 : On-Line Accelerated Cooling 1) TMCP TMCP 1

2 HAZ: heat affected zone JFE EWEL 2,3) 2004 Super- 4) HOP Heattreatment On-line Process 5) quenching - tempering Q-T 600 MPa 6 8) Super- Q-T TMCP Super- HOP 2. TMCP MPa TMCP TMCP 900 normalizing 1(a) - TMCP 1(b) CR controlled rolling CR CR- CR as CR 1(b) 15,16) JFE Super- 4) TMCP - Ceq M-A martensite-austenite constituent Super- Ceq JFE EWEL 2,3) HAZ YS390, 460 MPa 2,3,9) TMCP TMCP Structure Non- Ferrite/Pearlite/ Temp.: Temperature Fig. 1 Temp. CR: Controlled rolling (a) Normalizing (b) TMCP Fs: Ferrite Trans. Start Temp. 1 (a) (b) TMCP Schematic diagrams of (a) Normalizing and (b) TMCP Fs As CR CR- 2

3 Structure Temp. (a) RQ-T (b) DQ-T Non- Ausforming Ferrite/Pearlite/ Temp.: Temperature RQ-T: Reheat-quenching and tempering DQ-T: Direct quenching and tempering 2 (a) - - RQ-T (b) - DQ-T Fig. 2 Schematic diagrams of (a) RQ-T and (b) DQ-T 500 DQ direct quenching tempering DQ-T 2(b) 600 MPa DQ-T - - RQ-T reheat-quenching and tempering 2(a) 10) ausforming TMCP 10,11) DQ-T 17) JFE HOP DQ-T DQ-T DQ-T Super--HOP TMCP 3 Super- HOP TMCP 7,12,13) 3. Super- HOP TMCP 3.1 HOP DQ-T 6 8) DQ-T 600 MPa HOP HOP /s 3(a) HOP DQ 2 20 /s 3(b) HOP 1 2 DQ-T DQ 0.15%C 780 MPa (T.P.) T.P. T (log t C) (1) T K t (h) C T.P. 4 T.P. T.P. 3

4 Structure Temp. (a) DQ-T (b) DQ-HOP Non- Ausforming Ausforming HOP Ferrita/Pearlite/ Heating rate: C/s Heating rate: 2 20 C/s Temp.: Temperature DQ-T: Direct quenching and tempering DQ-HOP: Direct quenching and HOP (Heat-treatment On-line Process) 3 (a) - DQ-T (b) -HOP Fig. 3 Schematic diagrams of (a) DQ-T and (b) DQ-HOP 4 Fig. 4 vt rs ( C) HV [ C/s] 0.3 ( C/s) ( C/s) [ C/s] parameter MPa HV10 vt rs Changes in Vickers hardness (HV10) and the transition temperatures by Charpy impact tests (vtrs) with the tempering parameter in the TS780 MPa grade steel T.P. 0.3 /s 5 /s MPa T.P HOP HOP 1(b) (a) Photo 1 1(a) HOP T.P. HOP JFE- HITEN 610U2, JFE-HITEN 780LE 600 MPa JFE HYD960LE, JFE HYD1100LE 960, MPa HOP 12) 3.2 M-A 13,14) 2 µm 2 µm MPa (a) (b) HOP YR yield ratio 100 (b) Cementite distributions in the TS610 MPa grade steels produced by DQ-T process; (a) Tempered with an atmospheric furnace, and (b) Tempered with HOP 4

5 Structure Non- Ferrite/Pearlite/ Temp. (a) RQ-Q -T (b) DQ-Q -T RQ DQ Q Q Temp.: Temperature RQ-Q -T: Reheat-quenching Q (Inter-critical heating and quenching), and DQ-Q -T: Direct-quenching Q (Inter-critical heating and quenching), and 5 (a) RQ-Q -T (b) DQ-Q -T Fig. 5 Schematic diagrams of (a): RQ-Q -T and (b): DQ-Q -T JIS G mm YR 80 TMCP 600 MPa YR YR RQ-Q -T 5(a) DQ-Q -T 5(b) JFE Super- HOP M-A YR M-A M-A C Si Al M-A M-A 6 Super- +HOP M-A YR Super- Bs (Bf) 6 Temp. R.T. Untransformed austenite (C-enriched) HOP Bs HOP Ac1 M-A M-A 2 TS780 MPa SEM 13) M-A 13 M-A MPa YR 80 0 ve0 216 J 13) Super- HOP M-A Bf Bainite Untransformed austenite Tempered bainite C C Temp.: Temperature R.T.: Room temperature Bs: Bainite transformation start temperature Bf: Bainite transformation finish temperature Tempered bainite M-A Super- HOP M-A Fig. 6 Schematic illustrations showing temperature profi le and microstructural changes in the new processing with HOP 5

6 1 Table 1 Typical mechanical properties of the TS780 MPa grade steel produced by the new processing with HOP no. 89 p JFE 2004 no. 5 p vol. 43 no. 3 p NKK 2002 no. 179 p JFE 2004 no. 5 p vol. 44 no. 2 p Nagao, A.; Hayashi, K.; Oi, K.; Mitao, S.; Shikanai, N. Materials Science Forum. 2007, vol , p JFE 2007 no. 18 p JFE 2007 no. 18 p p ASTM Standards. A841/A841M. ASTM International. PA. USA. 12 JFE 2007 no. 18 p JFE 2007 no. 18 p JFE 2005 no. 9 p p Kozasu, I. Accelerated Cooling of Steel. Southwich, P.D., ed. TMS- AIME, PA, USA, 1986, p Okamoto, K.; Yoshie, A.: Nakao, H. Physical Metallurgy of Direct Quenched Steels. Taylor, K.A., ed. TMS-AIME, PA, USA, 1993, p Thickness (mm) Super- HOP M-A YR 780 MPa YS (MPa) TS (MPa) El (%) YR (%) ve 0 C (J) YS: Yield strength TS: Tensile strength El: Elongation YR: Yield ratio ve 0 C : Charpy impact energy at 0 C JFE TMCP 2 10 µm Super- HOP M-A YR 780 MPa Photo 2 A scanning electron micrograph of the TS780 MPa grade steel with the low yield ratio produced by the new processing with HOP YR780 MPa 13) X80 JFE-HIPER 14) HOP 14) 4. Super- HOP TMCP 6