TMCP stands for Thermo Mechanical Control Process and TMCP steels are those steels which are produced by this process. TMCP is a microstructural control technique combining controlled rolling (thermo mechanical rolling) and controlled cooling (accelerated cooling). These steels are sometimes microalloyed. Thermo mechanical control process is normally used to obtain excellent properties for steel plates such as high strength, excellent toughness along with excellent weldability through maximizing of grain refinement. These steels have almost same formability and weldability compared with mild steels. The superior mechanical properties introduced to the steel through this processing route are virtually equivalent to those obtained by heat treating conventionally rolled or forged steel and hence Thermo mechanical control process is used as a substitute for heat treatments that require additional material handling and furnace facilities.
TMCP technology was developed early in 1980s and major Japanese plate rolling mills have started to produce TMCP steels by middle of 1980s.
TMCP steels production
When TMCP is chosen as the process route, the input material (i.e. the slab) is heated to a temperature regularly used for hot rolling operations (around 1200 deg C). The roughing operation during rolling in the rolling mill is carried out in a normal way, but the finish rolling is carried out at a lower temperature (around 750 deg C to 800 deg C) than the temperatures used in a normal rolling process. Plastic deformation at this lower temperature promotes fine grain sizes and retards precipitation. The final hot working may continue down to temperatures below the critical temperature of transformation from austenite to ferrite. This requires heavy rolling equipment capable of deforming the steel at low hot working temperatures. The optimum precipitate size and dispersion is obtained when the finish rolling temperature is around 775 deg C
The cooling which follows brings the steel to the transformation temperature range, and the austenite to ferrite transformation results in fine ferrite grains and fine dispersed precipitates. For some TMCP steels, this last stage of cooling, during which transformation is completed, is accelerated by water cooling, to give a finer grain size. Accelerated cooling (AcC) can sometimes result in bainite formation as well as, or instead of, ferrite formation.
There are several methods for TMCP, some of which are illustrated against conventional processes in Fig.1.
Fig 1 Comparison of TMCP routes and conventional rolling. Zigzag part of the line show rolling
TMCP route broadly fall into the following three main categories:
- Controlled rolling (CR) from the normalizing temperature that is fully austenitic structure followed by a rapid cooling at a rate of approximately 10 deg C per sec. The aim of this process is to refine the grain size by controlled rolling and to increase the strength by suppressing the formation of ferrite and pearlite in favour of a strong tough bainite.
- Controlled rolling both above the Ar3 (temperature at which transformation of the austenite into ferrite starts during cooling) and below that temperature, in the austenite ferrite mixed region. In addition to austenite grain refinement, the recrystallized grains are flattened and nucleation of fine ferrite is encouraged by the deformation. At a temperature above the Ar1 (temperature at which transformation of the austenite into ferrite and cementite ends during cooling) the controlled rolling is interrupted, and followed by rapid cooling to room temperature or an intermediate temperature.
- Controlled rolling is performed as part of a preliminary processing, followed by cooling and reheating to just above the Ar3 temperature and then rapid cooling to well below the Ar1 temperature. The purpose of this technique is to develop the finest equiaxed austenite grain size before the controlled cooling begins.
By appropriate choice of deformation temperature and strain rate, the strength of steel can be increased. The strength of TMCP steel is higher than for normalized steel of the same composition. Thus TMCP steel has a leaner composition (lower alloy content) than conventional normalized steel of the same strength.
Metallurgical aspects associated with TMCP steels
The aim of TMCP is to achieve a fine and uniform acicular ferrite microstructure instead of a ferrite/pearlite banded structure of conventional steels. The thermomechanical rolling process is characterized by deformation in the non-recrystallization region of austenite which may be carried up to an 80 % reduction in thickness so as to obtain the desired grain refinement.
In the production of the TMCP steels, the control of microstructure starts at the reheating stage of the slabs itself when the grain size of the austenite is carefully controlled. In hot rolling stage, since the controlled rolling is carried out in the non recrystallization region, fine and worked austenite grains are formed. These fine austenite grains are transformed into fine acicular ferrite or upper bainite in the following stage of accelerated cooling. The final microstructure of TMCP steels is very fine and uniform.
Due to the fine and uniform acicular ferrite, TMCP steels have a higher strength and superior toughness. The relationship between carbon equivalent and tensile strength, yield strength and grain size and comparison of microstructures of conventional steel and TMCP steel is given in Fig.2.
Fig 2 Comparison of conventional steel with TMCP steel
Advantages of TMCP steels
- TMCP steels have higher strength and better toughness because of the fine and uniform acicular ferrite.
- TMCP steels have higher arrestability than normalized steels.
- As regards to formability, TMCP steels have the better strain ageing property and line heating property when compared with conventional steels.
- Generally, TMCP steels are good at resisting a reduction in toughness as a result of welding, because such steels usually have low carbon content. Some TMCP steels are excellent at resisting the toughness decrease, even at high heat inputs, as they have been processed to contain particles which control grain growth. These steels also have sufficiently high carbon content, or carbon equivalent levels to ensure that joint strength is not impaired at high heat inputs. Such steels are designated ‘high heat input resistant steels’.
Weldability of TMCP steels
As regards weldability of the TMCP steels due to its extra low equivalent carbon, TMCP steel virtually eliminates the need for preheating. This fact significantly improves welding efficiency, which has been the greatest problem in welding high strength steels. Hardenability is another important factor of weldability. Hardness level of TMCP steel is lower than conventional steel. TMCP steel is least susceptible to cold cracking or hydrogen cracking. Under adequate welding conditions, specific non low hydrogen type gravity electrodes can be used. The result is higher welding efficiency and weld joints with smooth beads that deliver remarkably improved performance.
Heat affected zone fracture toughness properties are generally better in TMCP steels than normalized steels and adequate fracture toughness can often be obtained up to higher heat inputs. Indeed, some ‘high heat input resistant’ grades of steel are made by a TMCP route.
There will be some degree of softening in the heat affected zones of TMCP steels after welding. Reduction in joint strength, however, is unusual, in those that have not had accelerated cooling as part of the manufacturing process. TMCP steels that have been manufactured using an accelerated cooling method to achieve the desired properties are not generally resistant to high heat inputs, and a degradation of properties can occur on welding. This is because the cooling rate in the weld region may be slower than that of production. In these steels it is important that the cooling rate is high, so that the grain size of the weld and HAZ can be maintained to give the desired properties. This is achieved by the use of moderately low heat input levels.
Application of TMCP steels
The application range of TMCP steels is within minimum yield strength of 350 to 500 N/Sq mm. The typical applications for such steels are in shipbuilding including icebreakers, offshore structures, tanks and vessels especially for low temperature environments, pipelines, building structures, commercial vehicle, cranes and other general steel construction activities.