Over the past 25 years, microalloyed steels (MA) have dominated the field of high-quality steels. Developments in recent years have shown that microalloyed steel is the best choice when high performance and low cost are pursued. In the early days of the development of microalloyed steel (MA), around the 1970s, microalloyed steel was almost the same as today’s plain carbon steel. Early microalloyed steels were mostly used in pipeline steel, plate and strip, and forging. With a better understanding of the characteristics of the microalloying elements that have been studied, especially their optimal cooperation with the hot deformation process, later microalloyed steels began to show the same characteristics as ordinary carbon steels and early Microalloyed steel has completely different properties. Incremental improvements in microalloyed steels will undoubtedly maintain momentum well into the 21st century, and understanding how microalloying elements function in the steels of that era will aid in this improvement. The purpose of this paper is to discuss some recent advances in modern microalloyed strip steels as well as sheet steels. Hopefully, a better understanding of different metallurgical phenomena will not only allow us to produce good steel grades, but will also facilitate the creation of more realistic models of these materials.
1. Background As we all know, there are three main reasons why microalloying elements are added to structural stainless steel:
(1) The austenite grains are refined during the rolling process, thereby refining the transformed ferrite grains;
(2) Reduce the transformation temperature, refine the ferrite grains, and increase the dislocation density;
(3) Produce precipitation strengthening or solid solution strengthening [1-5]. Because the alloy design and thermomechanical processing of these steels have been discussed in numerous, in-depth papers [18, 31-33, 36-38], only some aspects of these topics will be discussed here. The following focuses on strip rolling of two grades of steel:
(1) High-strength microalloyed strip steel for annealing after hot rolling or cold rolling (350MPa≤σs≤420MPa);
(2) Ultra-low carbon (ULC) strip steel for cold rolling. The hot-rolled strip rolling process can be described as follows: the slab is reheated to 1250°C, followed by rough rolling from 250mm to 25mm, then finish rolling from 25mm to 2mm, water cooling to a predetermined temperature, and then coiling and cooling to room temperature. The interval between rough rolling passes is long, while the interval between finishing passes is short, which means that the austenite grains will recrystallize multiple times during the rough rolling process. When the carbon steel bar enters the finishing rolling unit When, it is very likely that fine grains have been produced. Although a large amount of precipitation will occur in austenite depending on the processing conditions, the very short pass intervals and the presence of at least some solid solution microalloying elements will cause the last few passes of the finishing rolling process to be It is carried out below T5 temperature, which is the recrystallization termination temperature (Figure 1). Therefore, when austenite begins to transform during cooling, the austenite grains will become “pancake” shaped. Under this metallurgical condition, the crystal defect structure has a high Sv value , providing a large number of cores for ferrite nucleation and at least some benefit to the final ferrite grain refinement [31-33, 36- 38]. As will be discussed below, ferrite grain refinement plays a significant role as it contributes approximately 80-90% to the final yield strength of 350MPa grade hot rolled strip.
Trace elements such as niobium, vanadium and titanium were added to early pearlitic steels in order to take advantage of this austenitization process [29-32] and reduce the transformation temperature, and may produce ferrite phase precipitation strengthening . In-depth research work has shown that the austenite adjustment effect can be confirmed by the total interface area Sv of planar crystal defects such as grain boundaries, deformation bands and disconnected twin boundaries. Because these defects provide ferrite nucleation sites for the phase transformation process, it has been found that uniform and fine ferrite grains are related to larger Sv values, as shown in Figure 2. Obviously, fine ferrite is the combined result of high Sv value and low Ar3 point.
The characteristics of different types of hot rolling are closely related to the actual process and perhaps to the accuracy of the rolling mill. Table 1 briefly summarizes the typical and general rolling parameters used in simulations for different types of hot rolling processes.
To simulate an actual production procedure discussed above, the austenite is isothermally deformed and then quenched to room temperature to observe the microstructure. Its behavioral characteristics are briefly represented in Figure 1. When the deformation temperature is higher than T95, recrystallized austenite grains are obtained. When the deformation temperature of the pass is lower than T5, an elongated pancake-like structure is obtained. When the pass deformation is between these two temperatures, a harmful mixed crystal structure is obtained.