Under the condition of high-temperature rolling, many factors affect the grain development and recrystallization process of low alloy high strength steel. In the high-temperature environment, the lattice diffusion rate of metal materials is accelerated, which promotes the growth and development of grain. Under high-temperature conditions, the level of dislocation activity increases, which promotes grain migration and fusion through grain boundaries, thus continuing to increase grain size.
In addition, under high temperature environment, the generation capacity of recrystalized core is enhanced, and the boundaries of recrystalized grains are clearly visible, showing a new crystal appearance.
The change of grain boundary energy is an important factor in the high-temperature rolling process, which directly restricts the process of grain boundary recrystallizing. At high temperatures, the grain boundary moves faster, which improves the clarity and stability of the grain boundary, thus reducing the surface energy value of the grain boundary.
At the same time, in the high temperature environment, the dislocation movement is active, which is conducive to reducing the local dislocation density near the grain boundary, thus reducing the grain boundary energy level. In the high temperature rolling environment, the synergy of these two factors makes the grain boundary energy decrease sharply, which provides a better condition for the grain boundary recrystaling.
Table 1 Kinetic data of grain boundary recrystalization under low temperature rolling
Temperature / ℃ | Grain Size / μm | Grain Boundary Energy / J/m2 |
400 | 5 | 0.02 |
500 | 4 | 0.025 |
600 | 3.5 | 0.03 |
From Table 1, the kinetic data of grain boundary recrystalization under low temperature rolling conditions, including grain size and grain boundary energy at temperature, can be obtained. As the temperature decreases, the grain size decreases and the grain boundary energy increases slightly.
This may indicate that the dynamics of grain boundary recrystalization is affected by grain size in low temperature environment, and the change of grain boundary energy may have a definite effect on grain boundary migration and recrystalization.
The level of grain boundary energy depends on the degree of high dislocation density, and the dislocation phenomenon appears as a defect form in the crystal structure, which leads to the aggravation of the local strain field near the grain boundary.
This optimized strain field will cause the atomic structure chaos near the grain boundary and stimulate the activity of the grain boundary. Experimental studies in the IBC Group show that an increase in dislocation density leads to a significant increase in grain boundary energy, which has a significant impact on grain boundary stability and migration.
The essence of the grain boundary migration phenomenon lies in the driving effect of the grain boundary energy, however, there is a direct connection between the grain boundary energy level and the dislocation density near the grain boundary.
The height of the grain boundary energy usually makes the grain boundary clarity relatively low, thus promoting the grain boundary migration process. In the process of grain boundary migration, the surrounding dislocations are driven to gradually converge and form recrystalized core.
The grain boundary activity rate in the high dislocation density region is relatively fast, which induces dislocation aggregation in the grain boundary region and forms recrystalized core.
At high temperature, the lattice diffusion activity is enhanced, which promotes the grain boundary migration speed and activates the recrystalization process. On the contrary, at low temperatures, the lattice diffusion rate decreases, which limits grain boundary migration and then hinders the recrystaling process of high strength steel.
Temperature fluctuation has an effect on dislocation activity and distribution density. At low temperature, the dislocation activity is relatively low and the dislocation density in the region adjacent to the grain boundary increases. This increase in dislocation density affects the clarity of grain boundary edges, enhances the strain field forces around grain boundaries, and improves the vigor of grain boundaries, as shown in Table 2.
Temperature / ℃ | Grain Boundary Migration Rate / mm/min3 | Dislocation Density
Number Of Dislocations / mm3 |
800 | 0.005 | 1000 |
1000 | 0.02 | 800 |
1200 | 0.03 | 600 |
The strain path has a significant effect on the recrystalization of grain boundaries. The strain distribution within the crystal and the variation of dislocation density under the strain path directly determine the possibility of recrystalization at the grain boundary.
In the process of metal processing, the strain distribution in the crystal is not uniform due to the restriction effect of various strain paths. The stability of grain boundary caused by the inhomogeneity is closely related to the formation of recrystallized core.
(1) Regulation of strength and plasticity
Under high temperature rolling conditions, the obvious recrystalization process improves grain boundary clarity, resulting in increased material strength. The recrystalization of grain boundaries helps the material to deform more uniformly under stress and improves the plastic properties. On the contrary, under the low temperature rolling condition, the restricted grain recrystaling process makes the grain boundary relatively less clear, which improves the strength of the material, but may lead to the reduction of plastic properties. By adjusting the rolling temperature, the balance between strength and plasticity can be realized, and the mechanical properties of high strength steel plate can be precisely regulated.
(2) The influence of toughness and fatigue resistance
Under the condition of high temperature rolling, the recrystalization of grain is intensified, the clarity of grain boundary is improved, and the toughness of the material is enhanced. The fine recrystaled grains formed by high temperature rolling have beneficial effects on energy absorption and elongation properties.
Recrystalization during low temperature rolling is limited and may result in relatively low toughness. In addition, the fatigue resistance of metal materials is also affected by the processing temperature. By optimizing the clarity of grain boundaries and adjusting grain size, it helps to optimize the fatigue performance and service life of materials.
The rolling temperature has a significant effect on the corrosion resistance of the material. Under the condition of high temperature rolling, the degree of recrystalization of grain is relatively high, which helps to form a more continuous and compact grain boundary structure. Such a structure can effectively prevent the penetration and diffusion of corrosive media, thus improving the corrosion resistance of the material. In addition, the small grains generated in high temperature rolling environment are tightly coupled with clear grain boundaries, which further enhances the corrosion resistance of the material. The smaller grain size and uniform grain boundary structure alleviate the accumulation of corrosive media at the grain boundary, thus improving the durability of the material in corrosive environments.
(1) Variation of grain size and distribution
Under the condition of high temperature rolling, the recrystaling phenomenon of grain is particularly obvious, and the diameter of crystal particles is generally large and the distribution is relatively uniform. This microstructural property helps to improve the plastic properties of the material because of grain boundaries. Recrystaling causes the material to deform more uniformly during loading. On the contrary, under low temperature rolling conditions, the grain size is smaller and more concentrated, which may lead to an increase in the strength of the material, but may trigger a decrease in the plastic properties.
(2) Regulation of grain boundary energy
During high temperature rolling, the increase of grain boundary migration speed leads to the improvement of grain boundary clarity and the relative decrease of grain boundary energy. This helps to enhance grain boundary stability and promote grain recrystalization. On the contrary, moderately increased grain boundary energy may limit grain recrystalization and raise the dislocation concentration in the grain boundary region.
By adjusting the rolling temperature, the grain boundary energy can be precisely controlled, which provides space for flexible adjustment of grain boundary characteristics. For example, under certain conditions, recrystalization can be promoted to form distinct grain boundaries and improve the plasticity of materials.
However, the rolling process at lower temperature may increase the grain boundary energy, which is beneficial to improve the strength of the material.
In summary, by studying the action mechanism of rolling temperature on the grain boundary recrystaling phenomenon of low-alloy high-strength steel, as well as the grain boundary recrystaling mechanism, the relationship between rolling temperature and grain boundary recrystaling is analyzed, and the importance of optimizing the microstructure and properties of materials is elaborated.
At the same time, the grain boundary energy, dislocation density, grain boundary migration, grain size distribution, and other indicators are deeply discussed, which provide a reliable basis for accurate determination of rolling temperature, precise regulation of material mechanical properties and microstructure, and provide crucial theoretical support for the research and development of low alloy high strength steel.