Effect of coiling temperature on the microstructure and mechanical properties of DP 980 steel

The effect of coiling temperature on the microstructure and mechanical properties of annealed dual phase steels was investigated. It was found that the rate of ferrite recrystallization with coiling temperature 550°C is faster than that with coiling temperature 650°C. That is low coiling temperature accelerates the recrystallization of ferrite during heating compared with high coiling temperature, which in turn promotes the formation of austenite through the nucleation process. Under the same annealing conditions, the martensite content of annealed steels obtained at low temperature coiling 550°C is higher than that of annealed steels obtained at high coiling temperature 650°C. Compared with high coiling temperature, the low coiling temperature favors the formation of the homogeneous and fine microstructure. Thus, the dual phase steel obtained at low coiling temperature has finer and uniform microstructure and high hole expansion ratio at annealing temperature 800°C in comparision with the dual phase steel obtained at high coiling temperature.


Introduction
The production of dual phase (DP) steel for automotive applications is ramping up rapidly [1][2].However, expanding DP steel production is accompanied by tightening quality standards including requirements [3].One of the serious problems in DP steel production is the fluctuations in mechanical properties of DP steel.It was reported that the the non-uniformity of mechanical properties is related to the non-uniformity of microstructures [4].Two major process occur during the inter-critical annealing of cold rolled DP steels.These include ferrite recrystallization and the formation of austenite [5][6].The final microstructure is strongly dependent on the recrystallization of ferrite and the austenite formation during annealing, the evaluation of the features of two major process is of fundamental importance [7].In addition, the non-uniformity of microstructure and mechanical properties of dual phase steel are also affected by the hot rolling parameters, such as coiling temperature and finishing temperature [8][9].Especially, the coiling temperature is believed to be an important parameter.The coiling temperature has larger impact the banding and microstructure of hot rolled steel, and further affect the final microstructure and mechanical properties of DP steel after annealing [10].Therefore, the paper investigates effect of coiling temperature on the microstructure and mechanical properties of dual-phase steels in conjunction with the evolution of recrystallization of ferrite and austenite formations during annealing to obtain optimum coiling temperature and annealing temperature for producing a desired strength-ductility combination of dual phase steel.

Experimental procedure
The chemical compositions of DP steel used for this study is provided in Table 1.Experimental ingots made about 50kg in laboratory by vacuum induction furnace.They were homogenized at 1200°C for 1hours and then hot rolled to 3mm hot band.Finishing temperatures were kept above 870°C and coiling temperatures were kept at 650°C and 550℃, respectively, then air cooled to ambient temperature.In order to investigate the effect of the coiling temperature, two coiling temperatures were chosen.The coiling temperature used for this study is provided in Table 2.Then, the hot rolled steel are cold rolled to 1.6mm thick.To study the behavior of austenite formation, annealing experiments were conducted at temperatures in the range of 740-800 ℃ (with steps of 30℃ ), with 60s at each temperature, followed by water quenching.For ferrite recrystallization behavior analysis, annealing experiments were conducted isothermally at various inter-critical temperatures in the range of 690-840℃ (with steps of 30℃) with 60s at each temperature, followed by air cooling.Muffle furnace was used for different annealing conditions.The microstructure of recrystallization and phase transformation was observed using electron back scatter diffraction (EBSD) and scanning electron microscope (SEM).The volume fraction of recrystallization and martensite was investigated using Image J software.Micro-hardness testing was done under load of 200g.Hole expansion ratio (λ) was evaluated by the hole expanding test standardized by ISO/TC164 SC2.The hole expanding tests were repeated three times for each sample.

Results and discussions
Figure 1 presents EBSD image quality (IQ) maps obtained at different annealing temperatures at coiling temperature 550℃.At annealing temperature 720°C, ferrite is still mostly non-recrystallized, a small portion of ferrite recrystallization is observed.During annealing between 720°C and 750°C, ferrite recrystallization progresses by nucleation and growth.At the end of annealing at 750°C, ferrite is almost fully recrystallized.Figure 2 presents EBSD IQ maps obtained at different annealing temperatures at coiling temperature 650℃.The results showed the similar recrystallization behavior trend.At annealing temperature 720°C, no ferrite recrystallization is observed.The volume fraction of ferrite recrystallization at coiling temperature 550℃ is about 93% at annealing temperature 750°C, while the volume fraction of ferrite recrystallization at coiling temperature 650℃ is about 42%. Figure 1 and Figure 2 confirms that both, the rate of ferrite recrystallization increased with increase in annealing temperature.But, the rate of ferrite recrystallization with different coiling temperatures is different.The ferrite recrystallization at coiling temperature 550℃ is mostly completed at annealing temperature 750°C, while the ferrite recrystallization at coiling temperature 650℃ is only partially completed at the same temperature.The rate of ferrite recrystallization with coiling temperature 550℃ is faster than that with coiling temperature 650℃.The accelerated recrystallization of ferrite at low coiling temperature can be related to the effect of the initial refined  Figure 3 presents the microhardness of the as-received cold rolled steels and annealed steels obtained at coiling temperature 550°C and 650°C at different annealing temperatures.As the annealing temperature increases, the hardness first decreases and then increases.The hardness of the cold rolled steels obtained at a coiling temperature of 550℃ is greater than that obtained at a coiling temperature of 650℃.The minimum hardness value of the test steel obtained at a coiling temperature of 550℃ appears at annealing temperature of 750℃, while the minimum hardness value of the test steel obtained at a coiling temperature of 650℃ appears at annealing temperature of 780 ℃.Based on the microstructure of steel, it can be concluded that the test steel obtained at a coiling temperature of 550 ℃ reaches almost complete recrystallization at annealing temperature of 750 ℃, while the test steel obtained at a coiling temperature of 650 ℃ reaches almost complete recrystallization at annealing of 780 ℃.The hardness of the test steel obtained from the annealing temperature range of 690 ℃ to 750 ℃ and the coiling temperature of 650 ℃ is higher than that obtained from the coiling temperature of 550 ℃.This is attributed to high volume of unrecrystallized ferrite in the test steel during high coiling temperature.The hardness of the test steel obtained at 550 ℃ during the annealing temperature range from 780 ℃ to 840 ℃ is higher than that obtained at 650 ℃.This is attributed to the high volume fraction of hard phase nucleation in the test steel during low coiling temperature.Based on the results of microstructure and hardness analysis, it was found that compared with high coiling temperature, low coiling temperature accelerates the recrystallization of ferrite in annealed steels.Figure 4 and Figure 5 present SEM micrographs obtained at coiling temperature 550°C and 650°C at different annealing temperatures, respectively.As shown, at annealing temperature of 740 ℃, the annealed steel obtained at high coiling temperature has not yet fully recrystallized, and austenite is formed on the unrecrystallized ferrite matrix.However, during low coiling temperature, the annealed sample has basically fully recrystallized, and austenite is formed on the fully recrystallized ferrite matrix.Compared with the annealed steels obtained at high coiling temperature, the annealed steels obtained at low coiling temperature can obtain more uniform and fine martensite at annealing temperature 800℃.Under the same annealing conditions, the martensite content of annealed steels obtained at low temperature coiling is higher than that of annealed steels obtained at high coiling temperature.The martensite content of annealed steels at coiling temperature 550℃ is about 47% at annealing temperature 740°C, while the martensite content of annealed steels at coiling temperature 650℃ is about 9%.The results of the hole expansion testing is shown in Table 3.The annealed steel obtained at low coiling temperature shows higher HER of 61.63% than the annealed steel obtained at high coiling temperature.The difference in hole expansion values are closely associated with the hardness difference between the hard and soft phases.The lesser the hardness difference, more the hole expansion ratios.As mentioned above, the annealed steel obtained at low coiling temperature has finer and uniform microstructure compared with the annealed steel obtained at high coiling temperature.Therefore, the annealed steel obtained at low coiling temperature shows higher HER than the annealed steel obtained at high coiling temperature.

Conclusion
The rate of ferrite recrystallization with coiling temperature 550℃ is faster than that with coiling temperature 650℃.The volume fraction of ferrite recrystallization at coiling temperature 550℃ is about 93% at annealing temperature 750°C, while the volume fraction of ferrite recrystallization at coiling temperature 650℃ is about 42%.Decrease in coiling temperature increases the recrystallization rate of ferrite.Further, decrease in coiling temperature promotes the formation of austenite through the nucleation process.Under the same annealing conditions, the martensite content of annealed steels obtained at low temperature coiling is higher than that of annealed steels obtained at high coiling temperature.The martensite content of annealed steels at coiling temperature 550℃ is about 47% at annealing temperature 740°C, while the martensite content of annealed steels at coiling temperature 650℃ is about 9%.The low coiling temperature favors the formation of the homogeneous and fine microstructure.Thus, the dual phase steel obtained at low coiling temperature has finer and uniform microstructure and higher hole expansion ratio (λ=61.63%)at annealing temperature 800℃ in comparision with the dual phase steel obtained at high coiling temperature.

Figure 3 .
Figure 3. Microhardness of the as-received cold rolled specimen and annealed specimen obtained at coiling temperature 550°C and 650°C at different annealing temperatures

Table 1 .
The chemical compositions of the steel used for this study (wt%)

Table 2 .
Coiling temperature used for this study

Table 3 .
Hole expansion ratio of each annealed steel obtained at different coiling temperatures