Thermo-Mechanical Control Process_ Part One Key to Metals

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Thermo-Mechanical Control Process: Part One

Abstract:In the past, the roll of hot rolling was only to achieve the nominal dimensions likethickness, width and length.According to this fine and uniform Acicular Ferrite, TMCP steels have higher strength andbetter toughness.

TMCP (Thermo-Mechanical Control Process) technology was developed in Japan in the1980’s. With the use of the TMCP process, very fine and uniform acicular ferrite is attained asmicrostructure, and therefore TMCP steels have higher strength and better toughness. Alsoaccording to the TMCP process, lower hardenability, less susceptibility for cold cracking andthe availability for the extra high heat input welding have been improved. The overallconsequence is that the quality level of TMCP steels is quite high with the mechanicalproperties of TMCP steels remaining generally very stable.

What is TMCP?

In the past, the role of hot rolling was only to achieve the nominal dimensions like thickness,width and length. If the quality requirement was extremely high, an off-line heat treatment suchas normalizing or quench and tempering would be added. As the quality requirements increase,a new process for plate rolling had to be developed - TMCP.

With the TMCP process, the total control during reheating of slab, plate rolling and coolingafter plate rolling is critical. According to the exact definition of TMCP by IACS (Figure 1),TMCP includes TMR (Thermo-Mechanical Rolling) and AcC (Accelerated Cooling). Commonlyhowever, when people speak of TMCP, they usually think of AcC as TMCP.

Figure 1: The definition of TMCP

AIM of TMCP

The aim of TMCP is to achieve a fine and uniform acicular ferrite microstructure instead of aferrite/pearlite banded structure of conventional steels. According to this fine and uniformacicular ferrite, TMCP steels have a higher strength and superior toughness. Figure 2 showsthe relationship between tensile strength and Ceq (Carbon Equivalent). At the same Ceq level,the strength of TMCP steels is higher than those of conventional steels. As shown in Figure 3,toughness is improved with the decrease of grain size. Therefore it can be summarized thatTMCP steels have the better toughness than conventional materials.

Figure 2: Relationship between Tensile Strength and Ceq

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Figure 3: Relationship between Toughness and Grain Size

Manufacturing Technology for High-Performance Steel based on TMCP

The performance of steel plates was originally improved due to the controlled-rolling method.TMCP (Thermo-Mechanical Control Process) technology, which combines a controlled-rollingmethod and a controlled – cooling method, has contributed to further improvement of thehigh-strength and high-performance characteristics of steel plates.

Today’s steel plates are produced not by the ordinary rolling method, but by TMCP technology,which incorporates micro-structural control suitable for higher strength and thickness, togetherwith advanced micro-alloying technology.

The application of TMCP for bridge steel started around 1996 when the HPS (High-Performance Steel) began to be used in the construction of bridges. The production of HPS’sby TMCP is expected to steadily increase as the use of HPS for bridge applications alsoincreases.

The characteristics of TMCP from a metallurgical point of view are grain refining and formationof a fine bainitic microstructure, which are both induced by the controlled rolling and controlledcooling process. These micro-structural control methods raise the yield strength and thetoughness simultaneously so that alloys can be saved by TMCP and the Pcm value can belowered in comparison with the conventional rolling process.

As a result, TMCP steels have better weldability than conventional steels. Because it savesalloys and lowers Pcm value, TMCP offers various advantages to steels such as heavy gagingand acceptance of high heat-input welding. Some 780 MPa class steels of low Pcm, which areproduced by direct quenching and tempering or the precipitation hardening process, were usedin the Akashi-Kaikyo Bridge.

Figure 4: Types of rolling technologies in today’s steel production

Metallurgical Characteristics of TMCP

The concept of microstructure control of TMCP steels begins with the slab reheating stage. Inthis stage, prior Austenite grain size is controlled carefully. The next stage is the hot rollingstage. By the controlled rolling in the Non-recrystallization region, fine and worked Austenitegrains are formed. These fine Austenite grains are transformed into fine acicular ferrite orupper bainite in the following accelerated cooling stage after hot rolling. Figure 5 showsmicrostructures of TMCP steels compared with the conventional rolling process of plate. It isevident that the microstructure of TMCP steel is very fine and uniform.

Figure 5: Microstructure of TMCP plates compared with conventional plates

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