The process of strain compression test was employed to simulate the Thin Slab Direct Rolling (TSDR) and Conventional Controlled Rolling (CCR) for Nb and Nb-Timicroalloyed steel. In order to simulate the TSDR initial condition, reheating at elevated temperatures of 1400℃ was used and in the process of CCR, a lower temperature of 1200℃ was used. Ferrite and pearlite structures were found after the deformation and air cooling in both process, but the TSDR simulation showed more heterogeneous microstructure, as well as ferrite grain size, were found to be coarser in the CCR simulation for both specimen. The tests to do with the fracture toughness was also covered. It was found that the behavior was completely ductile for the two processing routes whereas there was a notable difference between the processing route of the Nb-Ti steel, where the behavior that was observed after the process of the CCR simulation was ductile compared to that of TSDR simulation that was ductile-brittle. According to the analysis that was contacted, it showed that the differences in the behavior were attributed to the heterogeneous grain size distribution that was present in the microstructure that was in the TSDR simulation when compared to that of CCR one as well as the presence of the coarse TiN particles. In the process of the TSDR, the big fraction of the coarse grains was the microstructural parameter that was controlling the cleavage fracture in the process of breaking the TiN particles.
From the mechanical behavior perception, the process of the grain refinement is regarded as one of the vital procedure to simultaneously improve strength and toughness and this is the main objective of thermomechanical treatment that is subjected to Nb-Ti and Nb steels. But the development of the new technology such as Thin Slab Casting and then Direct Rolling TSDR which gives Nb-Ti and Nb new microstructural phenomena. According to the studies, microalloyed processed steels may have a final microstructure that is not well refined giving rise to mixed ferrite grain that has a range of grains which can impair their toughness behavior. This paper studies the processing history, evolution and the development of the microalloyed steel that is deformed by plane strain compression after it was exposed to the heat of 1200 or 1400℃ so as to replicate the CCR as well as TSDR in that order have been examined.
Material and Procedure
The chemical configuration of the steel that was used is found in table 1. To simulate plate rolling, plane strain compression tests were used. The specimen was soaked for 15 minutes at different temperatures of 1200 ℃ and 1400℃ for CCR and TSDR simulations, then three deformation passes were applied at a rate of 10s-1 that was constant in the process of continuously cooling the specimen. The values figures of pass temperature that were used as shown in the second table. After the specimen going through the last deformation, it was air cooled.
Typical Processing History
Thesimulation process that precedes preheating of CCR is achieved at 1200oC. In the TSDR process that leads to coarser initial grain size previous to deformation, a preheating temperature of 1400℃ was chosen. A higher amount of microalloying elements that were in solid solution, was chosen for the simulation of the TSDR process. Table 3 has the austenite grain size that was obtained. The constituent volume of each microstructure, as well as the mean ferrite grain size, was determined. The results that were obtained are in table 3. The main constituent in both sheets of steel is polygonal ferrite.
Controlled rolling and controlled cooling
When a small size grain is developed, it increases both the strength and the toughness according to the studies that were contacted by Burello, E., and Worth (2011). The traditional route to a fine grain size in structural steel has been to incorporate grain refining elements that include aluminum. In case normalizing is done on the niobium steel so as to boost the impact properties, its strength advantage was lost in the process. In order to obtain a fine grain size in a niobium steel, then the low finishing temperature has to be applied.
Stages in Controlled Evolution of the material
In the process of the controlled rolling, the first step is to control the austenite grain size in the soaking stage. This is determined by the temperature that is necessary to take into the solution the microalloying particles, that was formed during the process of cooling that follows solidification during the casting. The grain size was found to relate to the size of the soaking temperature. Thus there should be a balance between the temperatures that is necessary to dissolve the particles of the resultant austenite grain size as well as the economic high soaking temperatures.
The importance of the correct soaking temperature for a particular steel was well explained by Burello and Worth (2011). The rolling process was divided into three stages that were as a result of the changes in the austenite and ferrite grain structures. The first one was the deformation of the austenite recrystallization temperature range. Deformation of the temperatures that are above 1000℃ lead to the development of the coarse recrystallized austenite that later turns to coarse ferrite and upper bainite. With the increase in the strain, the size of the austenite size decreases in the process of the recrystallization that is introduced in the rolling reduction. In the second place, it is the deformation of the recrystallized range.
Deformation that results from the temperatures that are above 1000℃ leads to the development of the recrystallized austenite grains size that is found by recrystallization reduces as the strain increase that is introduced by the rolling reduction. In the intermediate temperature that ranges from 1000℃ to 900℃ leads to the deformation that refines austenite through repeated recrystallization, therefore, causing fine-grained ferrite. Austenite grains were found to be elongated in nature, leading to the formation of the deformation bonds that increase the number of the potential sites for ferrite nucleation which is considered as one of the most important steps in the aspect of controlled rolling (Callister & Rethwisch, 2011).
The third stage is the deformation of the two-phase region. Deformation that was below recrystallization temperatures were found to produce warm worked austenite that results in the finer ferrite grained microstructure. Compared to the first two stages, the third deformation has more influence when it comes to the mechanical properties. Rolling that were above the Ar3 temperatures resulted in equiaxed ferrite grains as well as the substructure that was produced by the recently formed grains. In the process of the total reduction, the transition temperature was observed to decrease. It was suggested not to consider rolling below Ar1 or the changes in the microstructure in relation to the role of the dislocations. Non-recrystallisation temperature Tnr was found to be an important concept that represented the starting of the inhibition of the complete static recrystallization in the cooling between rolling passes.
According to the study, controlled rolling process was proposed to be extended to four regions (Saad, 2011). Data that was obtained in the microalloyed steel that was containing Ti and N, basing on the simulation of the 23 rolling pass schedule that had a fixed interpass time of the 20s and cooled at a rate of 21℃. Austenite was found to recrystallize between the passes with lack of accumulation of the dislocations. The increase that was observed in the flow stress was due to the decrease in the temperature. The flow stress was found to increase more rapidly due to the inhibition of the recrystallization between the passes. In the third region, there is a great decrease in the MFS and cRa starts in this phase. It is at this stage where the intercritical two-phase rolling occurs. The last phase corresponds to the rolling of the ferrite. Hot rolling takes place when the temperature decreases which influences the recrystallization of austenite on the transformation behavior as well as the continuous cooling transformation. When large strains were introduced in a temperature range that allowed only a fraction of the austenite to recrystallised, mixed ferrite grain sizes were found (Halda, Suwas & Bhattacharjee, 2009). The region where the unrecrystallisation grains were formed led to ferrite of the different grain sizes that were originating from the recrystallised austenite in the surrounding regions.
The state of the austenite before the process of transformation was found to be a major factor that determines the ferrite grain size. Additionally to the grain size of austenite, potential ferrite nucleation has to be considered concerning any relationship with the ferrite grain size.
During the various stages of annealing, when microalloying was slightly added, it postponed the ferrite to austenite transformation. The austenite that is formed during the process of heating was in TRIP980 decelerated as a result of the addition of the microalloying. Consequently, proeutectoid ferrite that was formed in the process of cooling to the overaging stage was postponed in TRIP980 as compared to the original material. Also, isothermal bainitic transformation in the overaging was slowed down in TRIP980 (Wang, 2012).
Despite the higher amount of the intercritical ferrite that was used in the TRIP980, the amount of ferrite after cooling to the overaging phase and also after the isothermal bainitic transformation was same for the two sheets of steel that were under investigation. Isothermal bainitic transformation in TRIP980, showed a lower kinetics with the also lower amount of bainitic during the end of the overaging as opposed to TRIP780. Generally, the amount of the retained austenite that was present in the microstructure that was obtained last, was slightly higher in TRIP980 when compared to TRIP780 contrary to their original contents.
Compared to TRIP780, the strain induced martensitic transformation was much faster in TRIP980, suggesting the lower stability of the retained austenite. Austenite that was retained in the TRIP980, underwent the complete transformation to strain induced martensite as a result of the engineering strain of 0 while approximately 5% volume of austenite that was retained was still present in the microstructure of the TRIP780 when the strain level was the same (Funk, 2014). Additionally, austenite that was retained continued to go through the transformation to form strain-induced martensite in TRIP780 even at higher strain though with a lower transformation rate compared to lower stains.
From the results, it was shown that after deformation of the large grains of austenite grains by the use of the TSDR process, the microstructure that was obtained were more coarse and heterogeneous as compared to those that were recorded in the lower austenite grain size. A high heterogeneity was in the microstructure was also found in other studies, but contrary to this study, some of them had fine mean ferrite sizes of grain in the TSDR simulation as compared to CCR.
According to the analysis of the static recrystallization kinetics of the current steel, it was found that materials were able to recrystallize after the first pass, something that did not happen in the subsequent passes. For all the cases, the temperatures for pass 2 and 3 were below the non-recrystallization temperatures that were determined for the steel. This suggests that strain is accumulated in the austenite as it deforms at low temperatures. What differentiates the two process routes is that the strain accumulates in the austenite on a different recrystallized size more coarse and heterogeneous in TSDR simulation. It implies that for the initial grain size, deformation schedules that have been applied were insufficient to reach to attain the homogeneous austenite microstructure before the formation. But contrary to this, the ferrite grain refinement in CCR simulation is caused often by the lower initial grain size.
The lower ferrite size of grains that were found after CCR simulations possibly will suggest higher tensile properties when compared to the process of TSDR. It was observed that the tensile properties that were obtained for the Nb-Ti steel in both processing routes are the same despite the differences in the ferrite grain size. Nb steel showed the same results. In order to explain this results, the softening that is caused by the coarse ferrite grain has to be compensated by the larger contribution to the strengthening as a result of the precipitation of the TSDR products. The high probability to precipitate during the process of transformation following this precipitate route was an expectation due to the high quantity of microalloying elements in solid solution when compared to CCR route. This leads to increase in the driving force for the precipitation. For the case of the Nb steel, the differences in the tensile properties can be explained due to higher pearlite and acicular volume fraction.
Regarding the toughness, the results that were obtained showed that Nb steel for both processing routes has a ductile behavior. The behavior of Nb-Ti was found to be ductile for the process of CCR and ductile-fragile for that of TSDR. The toughness in the behavior in the Nb-Ti can be attributed to the more heterogeneous ferrite grain size in the process of the TSDR simulation as compared to the CCR process. But this reason was not enough to explain the toughness difference that existed, basing on the knowledge that same heterogamous microstructure in the Nb steel does not result in the lower toughness. So as to analyze this change, the micromechanisms intervening the in the brittle process have to be considered.
In both simulations for the Nb-ti steel, broken TIN coarse particles were possibly observed on the cleavage surface. The particles were precipitated in the process of the solidification of the steel and the followed soaking at 1200℃ that was not able to dissolve them. According to the study, microcracks nucleated in the broken TIN coarse particle can encourage cleavage fracture initiation in the brittle. However, if these particles are in equal quantity and distribution of the particles in the matrix, microstructural parameters were found to control the behavior of the steel. For low carbon steels that have more composition of ferrite microstructure, the ferrite grain size was found to correspond to the cleavage facet size. In the CCR simulation, the broken TiN coarse particles were observed in the voids. In the TSDR simulation, the origin of the cleavage fracture as well as the secondary cleavages was associated with these particles.
The paper covered the different aspects of the microalloyed steel that included the processing and the microstructural control that is involved, how the microstructure evolves as well as control. The process of microstructural development was also highlighted. In the process of the TSDR simulation of Nb-ti as well as Nb microalloyed steels, in all cases, coarse and more heterogeneous microstructures as opposed to simulation of the conventional CCR processes that may results to an insufficient refinement of the coarse initial austenite grain size as a result of hot working. A combination of the heterogeneous microstructure due to the process of TSDR with the presence of coarse TiN particles in the Nb-Ti microalloyed steel was found to cause the brittle-ductile behavior.
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