It has always been a constant request in improving the strength, toughness, ductility and weldability of steels. In order to achieve the required demands, grain refinement is one of the most effective methods without having an increase in the change in the transition temperature, as shown in Figure 1 below.
Figure 1
The most efficient industrial technology in maximizing grain refinement at reasonable costs are Thermomechanical Rolling Process (TM), i.e. deformation without recrystallization and Thermomechanical Controlled Process (TMCP), which combines thermomechanical rolling with accelerated cooling. Thermomechanical Controlled Processing is thus being utilized as a substitute for heat treatments that require additional material handling and furnace facilities.
The thermomechanical rolling process is characterized by deformation in the non-recrystallization region of austenite, usually in the range from 1300 °F to 1500 °F, which may be carried up to an 80% reduction in thickness so as to obtain the desired grain refinement. Usually this final deformation is concentrated totally in the finishing train.
In a continuous rolling process, the number of deformation steps is limited to the number of stands in the mill. As such, a high deformation per pass is required so as to obtain the thickness of the final product from a given original thickness, and the deformation per pass is directly proportional to the rolling force produced.
Due to the high reduction per pass and low processing temperature, (so as to remain within the non-recrystallization region of austenite), excessive high rolling forces are produced. In plate rolling, many deformation steps with a relatively low deformation per pass are used to overcome this problem. However in the case of hot strip rolling, the processing temperature is to be increased.
As the necessary total deformation is to be carried out without austenite recrystallization, the recrystallization stop temperature has to be increased, and this can be achieved most effectively by increasing the amount of niobium in solid solution. Niobium contributed largely in slowing down austenite recrystallization with its role as a solute atom and ability to form carbonitride precipitates.
The precipitation process consists of nucleation, which is predominating at dislocation, and precipitate growth. The combined effect of the higher finish rolling temperature of steel strip production and the incomplete strain induced precipitation when compared with typical plate production due to high deformation velocity and short interpass time in the finishing train, produces a larger amount of niobium in solid solution. This solute niobium content causes precipitation strengthening and can be precipitated in the ferrite during or after transformation. It also has a transformation retarding effect.
In thermomechanical rolling, the partially rolled slab/strip is often held or delayed before the final rolling in the finishing train. This is due tot the fact that for a thicker slab/strip, the temperature of the material is often too high for successful thermomechanical rolling. This can be overcome either by introducing a delay time before the material enters the finishing train as mentioned earlier or to reduce the rolling velocity during the roughing process, etc, so as to keep the temperature of the material below that of austenite recrystallization. The acceleration of the rolling velocity in the finishing train is another consideration in maintaining a constant finish rolling temperature over the whole length of the strip.
In the thermomechanical controlled process fro plate rolling, further grain refinement can be achieve with faster cooling rates as lower transformation start temperature provides more nuclei in the undercooled austenite. Figure 2 below illustrates the applied cooling regime.
Figure 2
Accelerated cooling of structural steels produces a ferrite-bainite microstructure compared to a ferrite-pearlite microstructure in air cooling. In practice accelerated cooling, it is typical interrupted at around 550°C, which is then followed by air cooling.
This has two effects on the grain size:
1. The grain size of the polygonal ferrite is being refined. The faster the rate of cooling, the finer the ferrite grain size will be. This can be seen in Figure 3.
2. The microstructure of the resultant steel consists around 50% bainite that exhibits a finer grain size than ferrite. Together with a higher dislocation density of bainite, the steel’s strength increases significantly and its toughness is improved as well.
Figure 3
Steels that were thermomechanically processed exhibit better toughness than normalization and the required strength can be obtained with a leaner composition resulting in lower costs and improved weldability.
The application range of thermomechanically rolled steels is within minimum yield strength of 350 to 500 N/mm2. The typical applications for such steels are in shipbuilding including icebreakers, offshore construction, vessels especially for low temperature environments, commercial vehicle construction, cranes and other general steel construction.
References
1. L.Meyer and H. de Boer, Welding of HSLA Structural Steel, ASM, Metals Park, Ohio, 1978, p. 42-62.
2. Niobium Information 7/94, http://www.cbmm.com.br
3. L.J. Cuddy, Accelerated Cooling of Steel, TMS of AIME, Warrendale (PA), 1986, p. 235-243.
4. W.M.Hof, M.K.Grdf, H.G.Hillenbrand, B.Hoh and P.A.Peters, ibid.Lit.3, p.467-474.
5. Niobium Information 8/94, http://www.cbmm.com.br