Study on dynamic recrystallization of ultra-high strength 22MnB5 steel during hot rolling

The effect of deformation temperature and strain rate on the recrystallization behavior of ultra-high strength hot formed 22MnB5 steel was systematically studied by isothermal compression experiments, and the microstructure was characterized and analyzed. The results show that the peak stress and peak strain of 22MnB5 steel decrease with increasing deformation temperature and increase with increasing strain rate. The dynamic recrystallization of 22MnB5 steel is more sensitive to temperature and less affected by strain rate. The recrystallization behavior is significant during isothermal deformation above 1323 K. Based on the hyperbolic sinusoidal constitutive equation, the accurate prediction model of dynamic recrystallization grain size and a dynamic recrystallization critical strain model for 22MnB5 steel were established. The relationship between recrystallization austenite grain size and deformation temperature and deformation amount was obtained as follows: d=4.1×103[ε·exp(350.38/RT)]. The critical strains of complete recrystallization and complete non-crystallization at each deformation temperatures were determined by the critical strain model, which can provide a basis for the optimization design of rolling process parameters.


Introduction
The development and application of ultra-high strength steel is an important means to realize automobile lightweight [1].As one of the most representative ultra-high strength steels, 22MnB5 steel has been widely used in the automotive field [2].In order to further improve the mechanical properties of 22MnB5 steel, some microalloying elements are often added to 22MnB5 steel, and combined with thermomechanical controlled process (TMCP) to improve its strength and toughness [3][4].In industrial production, the controlled rolling process usually includes rolling in recrystallization region and the non-recrystallization region.Rolling in the recrystallization region, austenite grains are refined through repeated dynamic recrystallization.Rolling at the temperatures within the nonrecrystallization region cause accumulation of stored strain energy due to no complete recrystallization, which results in the formation of elongated grains and deformation bands [5].If the rolling process parameters are not properly controlled, a large amount of deformation in the partial recrystallization zone will lead to serious mixed grain phenomenon.Therefore, clarifying the recrystallization behavior of titanium microalloyed 22MnB5 steel has important guidance for the optimization of rolling process parameters.In recent years, the recrystallization behaviors of different steel rolling processes have been extensively studied.However, there is still a lack of research on the recrystallization of titanium microalloyed 22MnB5 steel.In this study, a single-pass compression test was carried out on a Gleebe-3800D simulator for titanium microalloyed 22MnB5 steel to obtain stress-strain curves under different deformation conditions.Combined with the microstructure after deformation, the effects of deformation temperature and strain rate on austenite dynamic recrystallization of 22MnB5 steel were investigated, and the recrystallization growth model and dynamic recrystallization critical strain model of 22MnB5 steel were established.

Experimental materials and procedures
In this study, a 22MnB5 steel slab was chosen as the raw material, and its chemical composition is shown in table 1.The thickness of the slab was 230 mm and the width was 1720 mm.Experimental samples were taken from the slab, and the sampling position was 1/4 of both the width direction and the thickness direction of the slab, and the sample size was φ 8 mm × 12 mm.In this paper, the dynamic recrystallization process of 22MnB5 steel was studied by using a single pass compression experiment on Gleeble-3800D thermal simulation machine.Figure 1 shows the temperature change process and specific deformation parameters of single-pass compression experiment.The sample was heated to 1180 °C at 10 °C‧s -1 for 3 min to obtain a uniform initial austenite structure.The sample was then cooled to the deformation temperature at a cooling rate of 10 °C‧s -1 and held for 10 s to eliminate possible temperature gradient in the sample.The stress-strain curves under different deformation conditions were obtained by compressing the samples at different deformation temperatures and different deformation rates to make the compression amount of the samples reaching 50 % (true strain being 0.69).The compressed sample was rapidly quenched to room temperature and cut along the axis.After grinding, polishing and corrosion treatment, the microstructure was observed by metallographic microscope.IOP Publishing doi:10.1088/1742-6596/2635/1/0120223 2, when the deformation temperature is 880 °C and 950 °C , the true stress increases rapidly and then increases slowly with the increase of true strain, and there is no obvious peak stress.When the deformation temperature is 1150 °C , 1100 °C and 1050 °C , the true stress increases first, then decreases and then tends to be stable with the increase of true strain, and there is obvious peak stress.It shows that the obvious recrystallization behavior occurs at the deformation rate of 1 s -1 and the deformation temperature of 1150 °C , 1100 °C and 1050 °C .Figure 3 shows the statistical results of peak stress, peak strain and steady-state strain at different temperatures.The deformation rate is 1 s -1 and the true strain is 0.69.The results show that the deformation resistance of 22MnB5 increases with the decrease of deformation temperature, the corresponding peak stress increases significantly, and the peak strain increases significantly.This shows that the deformation temperature can significantly affect the high temperature deformation behavior of austenite.The higher the deformation temperature is, the smaller the critical strain is, and the more likely the austenite dynamic recrystallization occurs.The atomic activity of the material increases with the increase of deformation temperature, which promotes the dynamic recrystallization behavior [6]. Figure 4 shows the original austenite microstructure after deformation at different deformation temperatures, the strain rate is 1 s -1 , and the true strain is 0.69.When deformed at 880 °C and 950 °C , the austenite structure is mainly equiaxed fine grains and squashed coarse grains, and there is a clear mixed grain phenomenon.When deformed at 1050 °C , 1100 °C and 1150 °C , the recrystallized grains are mainly equiaxed grains with uniform size, and complete dynamic recrystallization behavior occurs.Moreover, the austenite grain size after recrystallization increases with the increase of deformation temperature, as shown in figure 4.Under high temperature conditions, small energy required to promote dislocation movement and atomic diffusion; and the weak interaction of internal defect stress fields in metals lead to easy migration of grain boundaries, which promotes the occurrence of dynamic recrystallization and increases the austenite grain size.

Effect of deformation rate on recrystallization behavior
Figure 5 is the true stress-true strain curve of 22MnB5 steel under single-pass compression at different deformation rates.The deformation temperature is 1150 °C and the true strain is 0.69.As shown in figure 5, when the deformation rates are 10, 5, 1 and 0.1 s -1 , the true stress increases first, then decreases and then tends to be stable with increasing true strain, and there is an obvious peak stress.Figure 6 shows the statistical results of peak stress, peak strain and steady-state strain at different strain rates.As the deformation rate decreases, the deformation resistance of the material decreases significantly, and the peak stress and peak strain also decrease.This shows that as the deformation rate increases, the softening effect of dynamic recrystallization is weakened, and a large amount of deformation is required to occur dynamic recrystallization.Although reducing the deformation rate reduces the dislocation density, the distortion energy released inside the material per unit time increases, and the process of dislocation movement and grain boundary migration is prolonged, which increases the softening degree of dynamic recovery and weakens the deformation resistance of the material.Figure 7 is the original austenite microstructure after deformation at different deformation rates.When the deformation rate is 0.1 s -1 and 1s -1 , the austenite structure in the sample is all equiaxed grains.When the deformation rate is 5 s -1 and 10 s -1 , the microstructure of the sample is composed of a large number of equiaxed grains and a small amount of flattened elongated grains, resulting in a mixed grain phenomenon.This indicates that dynamic recrystallization is more likely to occur at a lower deformation rate when the deformation temperature remains unchanged.

Recrystallization growth kinetics
Critical strain during dynamic recrystallization process εc, peak strain εp, steady-state strain εs, deformation temperature T, deformation rate ε, and initial grain size are closely related.Formula (1) is commonly used to represent the thermal deformation characteristics of materials: In the formula, ε is material strain; Qd is the activation energy, J• mol -1 ; T is the deformation temperature, K; R is the gas constant, R=8.314 J (mol‧K) −1 ; both A and α are constants, where α is often 0.012 [7]; σp is the peak stress, MPa; n is the stress index.Take natural logarithm on both sides of equation ( 1): The slope n of the straight line obtained by least squares regression fitting is 4.397.Combined with the peak stress σp at different temperatures, the values of Qd and A were determined to be Qd=350.38kJ• mol -1 , A=6.466×10 12 .The Z-parameter equation of thermal deformation of materials can be obtained: The dynamic recrystallization grain size d (μm) of austenite is mainly related to deformation temperature, deformation rate and peak strain.The calculation model of dynamic recrystallization grain size is [8]: According to the statistical results of austenite grain size after deformation at different deformation temperatures and deformation rates, the value of u is determined to be 0.152 and the value of H is 4.1×10 3 .Therefore, the model of dynamic recrystallization grain size for 22MnB5 steel is:

Recrystallization critical strain
The above studies show that the deformation temperature and deformation rate have an important influence on the recrystallization behavior of austenite.Determining the recrystallization critical condition is the key to adjust the rolling parameters reasonably.The critical strain model of austenite recrystallization is [9]: In the formula, P1, P2, and P3 are constants, and P2 is generally 0.5 [9] ; D0 is the initial grain size of the sample, μm.The initial grain size of the sample in this experiment is 106.4 μm.
The values of P1 and P3 can be determined by substituting the obtained peak strain εp.P1=1.085×10 -5 , P3=0.25. the peak strain model of austenite recrystallization of 22MnB steel is: Similarly, different deformation parameters and steady-state strain are substituted into Formula (6), and value of P1 is 6.44×10 -4 and the value of P3 is 0.132.The steady-state strain model of austenite recrystallization is: The relationship between the critical strain εc and the peak strain εp of austenite dynamic recrystallization is usually [10]: Therefore, the critical strain model of austenite recrystallization of 22MnB5 steel is: ) Figure 8 shows the dynamic recrystallization map of 22MnB5 steel.As shown in figure 8, the critical strain of complete non-recrystallization and complete recrystallization of 22MnB5 steel decreases with the increase of temperature.Moreover, as the temperature increases, the strain range of partial recrystallization gradually decreases.This also means that the higher the temperature, the easier it is to avoid partial recrystallization rolling, thereby reducing the mixed crystal phenomenon.

Conclusions
A systematic investigation was conducted on the recrystallization behavior of 2 steel during hot rolling, and the main conclusions obtained are as follows: 1) With the increase of deformation temperature and the decrease of deformation rate, the critical stress and peak stress decrease.
2) With the increase of deformation temperature, the critical strain for dynamics recrystallization of 22MnB5 steel decreases.

Figure 1 .
Figure 1.Schematic diagram of single pass deformation experiment.

Figure 2
is the true stress-true strain curve of 22MnB5 steel under single pass compression at different deformation temperatures.The deformation rate is 1 s -1 and the true strain is 0.69.As shown in figure 8th International Conference on Recrystallization and Grain Growth Journal of Physics: Conference Series 2635 (2023) 012022

Figure 2 .
Figure 2. The stress-strain curves at different deformation temperatures.

Figure 3 .
Figure 3.The effect of temperature on peak stress and strain: (a) peak stress; (b) strain.

Figure 5 .
Figure 5.The stress-strain curves at different deformation rates.

Figure 6 .
Figure 6.The effect of deformation rates on peak stress and strain: (a) peak stress; (b) strain.