(TMCP) with JFE Steel's High-Performance Plates
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JFE TECHNICAL REPORT No. 11 (June 2008) Recent Development in Microstructural Control Technologies through the Thermo-Mechanical Control Process (TMCP) with JFE Steel’s High-Performance Plates† SHIKANAI Nobuo* MITAO Shinji*2 ENDO Shigeru*3 Abstract: techniques have already been applied to various Thermo-mechanical control process (TMCP) is a advanced products. In this paper, the fundamentals of microstructural control technique combining controlled microstructural control by TMCP, and the recent devel- rolling and cooling. Thermo-mechanical control process opment of TMCP are described. Examples of the is used to obtain excellent properties for steel plates, advanced high-strength plates produced in JFE Steel are such as high strength, excellent toughness, and excellent also presented. weldability. JFE Steel has been developing TMCP tech- nologies ever since it started operating its accelerated 1. Introduction cooling facility, OLAC® (On-Line Accelerated Cooling), in its plate mill at West Japan Works (Fukuyama) in 1980 Thermo-mechanical control process (TMCP) is a (the world’s first industrial accelerated cooling system microstructural control technique combining controlled ever built). In 1998, JFE Steel developed Super-OLAC, rolling and cooling. TMCP is used to obtain excellent an advanced accelerated cooling system capable of properties for steel plates, such as high strength, excel- cooling plates homogeneously at high cooling rates lent toughness, and excellent weldability. In 980, JFE close to the theoretical limits. In 2004, the epoch- Steel started operating OLAC® (On-Line Accelerated making on-line induction heating facility, HOP® (Heat- Cooling), the world’s first industrial accelerated cool- treatment On-line Process), was also installed in the ing facility, in the plate mill at JFE Steel’s West Japan plate mill at West Japan Works (Fukuyama). High- Works (Fukuyama),2). In 998, JFE Steel developed strength steels, a grade of steel usually produced by the Super-OLAC5), an advanced accelerated cooling system quenching and tempering (Q-T) process, can be tough- capable of cooling plates homogeneously at high cooling ened by refining the component carbides through rapid rates close to the theoretical limits. tempering by HOP. Because Super-OLAC is capable of TMCP strengthens and toughens steel plates essen- accurately controlling the stop cooling temperature tially by refining the transformed microstructures. TMCP before tempering, JFE has managed to develop a new can also reduce alloying addition, and thus realizes other set of microstructural control techniques using M-A merits such as improved weldability. JFE Steel has also (martensite-austenite constituent) as the hard phase. established JFE EWEL®, a microstructural control tech- These are unique techniques unachievable with the con- nology for the heat-affected zone (HAZ) in high-heat- ventional Q-T process or conventional TMCP. These input welding, to ensure the excellent mechanical prop- † Originally published in JFE GIHO No. 18 (Nov. 2007), p. 1–6 *2 Dr. Eng., Senior Researcher Deputy General Manager, Plate & Shapes Res. Dept., Steel Res. Lab., JFE Steel * Dr. Eng., *3 Dr. Eng., General Manager, Plate and Shapes Res. Dept., Senior Researcher Deputy General Manager, Steel Res. Lab., Plate & Shapes Res. Dept., JFE Steel Steel Res. Lab., JFE Steel Recent Development in Microstructural Control Technologies through the Thermo-Mechanical Control Process (TMCP) with JFE Steel’s High-Performance Plates erties of welds performed by customers3,4). Needless to In TMCP, the transformed structure is refined by say, the TMCP technology with Super-OLAC is essential a suitable combination of controlled rolling (CR) and to JFE EWEL®. accelerated cooling (Fig. (b)). To increase the nucle- In 2004, JFE Steel started operation of an epoch- ation sites of ferrite during cooling, CR is used to refine making on-line induction heating facility, HOP®(Heat- the grains and strain the austenite. Next, the transformed treatment On-line Process), in the plate mill at West structure is further refined by accelerated cooling after Japan Works (Fukuyama)6). High-strength steel with a CR, that is, cooling to a reduced transformation temper- tensile strength of over 600 MPa, a type usually manu- ature at which the diffusion of the atoms is limited while factured by quenching and tempering (Q-T), can be the large driving force of the transformation is applied. produced by the Super-OLAC 1 HOP process online The fine microstructure thus obtained helps realize without any off-line heat treatment. Production capaci- excellent mechanical properties, such as high strength ties have been greatly increased and the lead-time from and toughness0,11). order to delivery has been shortened. And as an added JFE Steel has completed the installation of three advantage, the rapid tempering by HOP improves the Super-OLAC5) systems. The first was installed at the toughness of the high-strength steels by refining car- West Japan Works (Fukuyama) in 998; the second, at bides7–9). the West Japan Works (Kurashiki) in 2003; the third, at Because Super-OLAC 1 HOP is capable of accu- the East Japan Works (Keihin) in 2004. With our Super- rately controlling the stop cooling temperature before OLAC and HOP systems, we now have outstanding tempering, JFE Steel has been able to develop a capabilities for manufacturing high-performance steel new set of microstructural control techniques using plates. M-A (martensite-austenite constituent) as the hard Large amounts of alloying addition are usually phase. These are unique techniques unachievable with needed to obtain high strength in plates, especially conventional Q-T or conventional TMCP. These new in plates with high wall thicknesses. For this reason, microstructural control technologies have already been the carbon equivalent (Ceq) of the steels tends to be applied for various high-strength steel plates. increased. In steels of this type, an extremely hard This paper describes recent developments in TMCP, martensite-austenite constituent (M-A) that compro- including the Super-OLAC 1 HOP process, with some mises toughness tends to be formed in the upper-bainite examples of advanced steel plates produced in JFE Steel. structure at the HAZ of welds, especially in large-heat- input welding. To decrease the Ceq value via processing 3,4) 2. Fundamentals of Microstructural Control with Super-OLAC is an important technical essence of JFE EWEL®. JFE EWEL® is applied to high-strength by TMCP thick plates for container ships and building construc- Figure 1 shows schematic diagrams of rolling and tion, plates to be welded with large heat input3,4,2). heat pattern to improve the toughness of steel plates with In conventional TMCP, accelerated cooling is usually tensile strengths typically under 600 MPa. The tough- interrupted at the intermediate temperature range, for ness of the steels is improved by a off-line heat treat- example, at around 500°C, due to relief of internal resid- ment at about 900°C, followed by cooling to the ambient ual stress or other factors. In producing high-strength temperature (Normalizing (Fig. 1(a)). The refined trans- steel plates, plates with tensile strengths typically in formed structure from refined austenite contributes to excess of 600 MPa, steelmakers occasionally apply the toughening effect. direct quenching from the austenite region after hot roll- Structure Temp. (a) Normalizing (b) TMCP Recrystallized Austenite Ac3 Non-Recrystallized OLAC Austenite Fs Ar3 Ac1 As CR Ar1 Ferrite/Pearlite/ CR-OLAC Bainite/Martensite Fs: Ferrite Trans. Start Temp. Temp.: Temperature CR: Controlled rolling Fig. 1 Schematic diagrams of (a) Normalizing and (b) TMCP 2 JFE TECHNICAL REPORT No. 11 (June 2008) Recent Development in Microstructural Control Technologies through the Thermo-Mechanical Control Process (TMCP) with JFE Steel’s High-Performance Plates Structure Temp. (a) RQ-T (b) DQ-T Recrystallized Austenite Ac3 Non-Recrystallized OLAC Austenite Ar3 Ausforming Ac1 Tempering Tempering Ar1 Ferrite/Pearlite/ Bainite/Martensite Temp.: Temperature RQ-T: Reheat-quenching and tempering DQ-T: Direct quenching and tempering Fig. 2 Schematic diagrams of (a) RQ-T and (b) DQ-T ing down to around the ambient temperature, followed strength and toughness of steel plates produced by the by tempering (DQ-T process (Fig. 2(b)). The DQ-T pro- DQ-T process, our group assessed the hardness and cess is classified as a thermal refining process in some Charpy impact properties of lab scale DQ-T plates con- cases), because the process can be regarded as a ratio- taining 0.5% of carbon, tempered at various heating nalized combination of reheat quenching and tempering rates and temperatures. (RQ-T) (Fig. 2(a)). However, since mechanical proper- The mechanical properties of DQ-T plates, such as ties of the plates are usually improved by ausforming, hardness and toughness, are usually analyzed by the the DQ-T process is also classified as TMCP ,3). tempering parameter (T.P.), based on the following equa- In high-strength steels produced by the DQ-T pro- tion: cess, microstructural control of transformed structures at lower temperatures, such as bainite and martensite, is T.P. 5 T 3 (log t 1 C) .................................... () very important to obtain an excellent balance of strength and toughness. Grain refinement and straining of aus- where T is the tempering temperature (K), t is the tem- tenite by controlled rolling, followed by quenching to pering time (h), and C is a constant. around the ambient temperature (ausforming), bring The relationships between hardness, the ductile- about fine sub-structures in bainite or martensite, and brittle transition temperature in Charpy impact tests, thus help to improve both the strength and toughness4). and T.P. are shown in Fig. 4. The hardness of the DQ-T JFE Steel developed the on-line DQ-T process by plates was determined by the T.P. without showing introducing the Super-OLAC + HOP process. In the next apparent dependency on the heating rate in tempering. It section we explain some recent metallurgical innova- is interesting to note that the fracture appearance transi- tions achieved by JFE Steel using Super-OLAC + HOP.