Microstructure Evolution and Work Hardening Behavior of Hot-Rolled DP780 Ferrite/Bainite Dual-Phase Steel

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Introduction
With the development of TMCP (thermo-mechanical control process) technology, hot rolling process can be used to produce thin strip produced by cold rolling process, and the production range can be broadened, shortened production cycle, and energy saving, and emission reduction can be achieved by "replacing cold rolling with hot rolling" [1,2].Replacing cold rolling with hot rolling has gradually become the development direction of major steel mills to improve production efciency and reduce production costs [3,4].Hot-rolleddual-phase steel has the characteristics of high strength, low yield ratio, continuous yield and high initial hardening rate, which can meet the combination of high strength and cold forming, excellent welding performance and fatigue performance [5], and it is considered an ideal steel for automobiles [6].Compared with traditional ferritic/martensitic dual-phase steel, ferrite/bainite dualphase steel (FB steel) has good strength and toughness matching and extended fange performance [7,8] and is widely used by the moving parts such as car chassis and spokes.
At present, NKK of Japan, POSCO of South Korea, TyssenKrupp of Germany, and SSAB of Sweden are in the forefront of FB steel research in the world and have successfully developed DP600, DP780, and higher-gradehotrolleddual-phase steels [9,10].Besides, Gao et al. developed a the strength-ductility synergy of dual-phase (DP) steels that was obtained by properly tailoring the structural heterogeneity, including the distribution and fraction of constituent phases [11][12][13][14].Due to the lower yield strength, good performance stability, and uniformity of DP780 steel, it has broad market application prospects.Terefore, diferent experimental conditions were studied in this article.
Hot-rolleddual-phase steels for automobiles produced by domestic steel mills are mainly below 600 MPa [15], and higher-strengthdual-phase steels require cold rolling combined with annealing processes, which have a long production cycle [16][17][18][19].However, research on the production of high-strengthDP780-gradehot-rolleddual-phase steel through bainite transformation interval coiling is relatively lacking.In this article, by adopting V-N-Cr microalloying design, using TMCP process combined with medium temperature crimping process, DP780 grade ferrite/ bainite dual-phase steel is obtained, and the microstructure evolution and mechanical properties are studied, and the work hardening behavior of the experimental steel is analyzed.Te research results provide reference and basis for the process determination and production of hot-rolled DP780 grade dual-phase steel.

Experimental Details
Te chemical composition of the experimental steel is shown in Table 1, and the balance is Fe.Te phase diagram of the experimental steel was calculated by Termo-Calc.Te specifc results are shown in Figure 1.Te measured A 1 � 678 °C, A 3 � 830 °C the cementite precipitation temperature is around 685 °C, and the V (C, N) is in the equilibrium state.Te precipitation begins at 1100 °C and reaches the maximum at 700 °C.A vacuum melting furnace was used to smelt the experimental steel into ingots, then the ingots were forged into long billets, and hot-rolled experimental billets of appropriate length were cut for rolling experiments.Te specifc controlled rolling and controlled cooling process is shown in Figure 2. A billet of 80 × 90 × 110 mm is heated in a box-type heating furnace to 1200 °C and held for 2 hours.When it is air-cooled to 1100 °C, the frst stage of austenite recrystallization zone rolling is carried out.Te billet is from 80 mm.After 2 passes, it is thinned to 45 mm (rough rolling), and then air-cooled to 900 °C in the second stage of rolling in the austenite unrecrystallized zone.Te steel plate is fnally rolled into 10 mm thick strip after 5 passes (Finish rolling), the pass reduction is controlled between 20% and 25%, and the fnal rolling temperature is controlled around 850 °C.Te strip is cooled to 570 °C and 540 °C by water quenching (30-40 °C/s), and then is coiled by a coiler and slowly cooled to room temperature.For ease of description, the experimental steel coiled at 570 °C is recorded as steel A, and the experimental steel coiled at 540 °C is recorded as steel B.
Metallographic specimens were cut from the side of the rolled plate along the rolling direction.After grinding and mechanical polishing, they were corroded by 4% nitric acid alcohol solution (volume percentage).Te microstructures of diferent coiling temperatures were taken by OLYMPUS optical microscope (OM) and Zeiss Ultra 55 Field emission scanning electron microscope (SEM) was used for observation, and the fne structure and precipitates of the sample were observed by FEI Tecnai G2 F20 transmission electron microscope (TEM).Electron backscatter technology (EBSD) was used to analyze the phase distribution and grain boundary angle distribution in the microstructure.Te sample was prepared by electrolytic polishing with a 12.5% vol perchlorate alcohol solution (volume percentage), and then measured on a Zeiss Ultra 55 feld emission scanning electron microscope with 0.06 μm Step size scans point by point.
According to GB/T 2975 national standard, take the fullthickness standard tensile plate sample along the rolling direction.At room temperature, assume that the strain rate is 3 mm/min on the CMT5105 microcomputer controlled universal testing machine according to GB/T 228-2002 "Metal Tensile Test Method" to conduct a tensile test.Charpy V-type impact test is taken along the width and extension direction.Te size of the sample is 5 × 10 × 55 mm (1/2 standard sample).It is carried out on the MTS ZBC2452-B pendulum test machine according to ASTME 23, and the test temperature is −20 °C, after the impact test is  Each experimental result is determined by more than three experimental data, so as to ensure the accuracy of the experimental results.Tere is little diference in the amount of ferrite introduced between the two, which is mainly related to the rolling process and is the same as the cooling process before coiling.For the steel in this experiment, the microstructures obtained at these two coiling temperatures are ferrite + bainite type dual-phase steels.As a soft phase structure, ferrite can ensure the plasticity of the steel to a certain extent, and at the same time, it still has good ductility.Te rate is better than the full bainite structure, and bainite as a hard phase structure ensures the strength of the steel.Te crystallographic characteristics of the experimental steel were analyzed by EBSD technology, and the experimental results are shown in Figure 4. Figures 4(a) and 4(d) are the quality maps of steel A and steel B, respectively.Te colored massive structure is retained austenite, which is distributed near the ferrite grain boundary, and diferent colors represent diferent orientations.Te retained austenite in steel A is small, while the size of retained austenite in steel B is diferent.Tis may contribute to the plasticity of the experimental steel, because the retained austenite is prone to normal strain and reduces the stress concentration at the crack tip.It causes crack propagation, dullness, and other normal deformation, which is manifested by phase change induced plasticity (TRIP) [16].It can be found that black massive structures are found in the quality maps of steel A and steel B, which are high-carbon martensite.Te high-carbon martensite lattice distortion is large and the band contrast value is low, so the color in the quality map is darker.Figures 4(b) and 4(e) are the orientation maps of steel A and steel B, respectively.Te color shades at diferent positions represent diferent orientation relationships, and the orientation diference of steel B is relatively large.Figures 4(c) and 4(f ) are the grain boundary distribution maps of steel A and steel B, respectively, and the corresponding azimuth angle and grain boundary ratio relationship diagrams are shown in Figures 4(g) and 4(h).Generally speaking, those with an orientation angle between 2 °and 15 °are small-angle grain boundaries, and those with an orientation angle above 15 °are high-angle grain boundaries.Te large-angle grain boundary can hinder the movement of dislocations during the deformation process, which helps to improve the strength of the steel [13], and a large proportion of the large-angle grain boundary determines the crack propagation work and can improve the impact toughness [17].By comparison, the ratio of high-angle grain boundaries of steel B is 62.2%, which is higher than that of steel A (49.3%).

Results and Discussion
In order to analyze the fne structure of the experimental steel more accurately, the morphology of the structure was observed under a transmission electron microscope.Te specifc morphology is shown in Figure 5. Figure 5(a) shows the TEM morphology of steel A. Te granular bainite is a fne lath structure under TEM, which is diferent from the spherical morphology under OM, and there is a twin martensite structure in steel A. Te formation of twinned martensite is mainly due to deformation and carbon desorption of ferrite.Some austenite with high-carbon content but unstable morphology undergoes martensite transformation during the quenching process, and its substructure is twinned [17].Large-sized square and spherical precipitates were found on the ferrite matrix, as shown in Figure 5(b).Te element distribution of the precipitates at the dotted line in the fgure is shown in Figure 5(c).Te precipitates are VC (vanadium carbide).In steel B, fner spherical VC precipitates with a size of about 40 nm were found, as shown in Figure 5(d), and the corresponding energy spectrum is shown in Figure 5(f ).It can be observed that there are fne and dispersed nano-scale precipitates on the ferrite matrix, and there are more entangled dislocations on the hard phase structure adjacent to the ferrite, as shown in Figure 5(e).Dislocation strengthening and precipitation strengthening are important strengthening methods for this experimental steel.More important, it can be clearly seen from Figures 4(a) and 4(b) that the quantity of retained austenite in steel B is more than that in steel A, due to the reduction of coiling temperature is conducive to the transformation of martensite into austenite and retained austenite is also an important aspect to improve ductility.Terefore, ductility of steel B is signifcantly improved.As shown in Figure 6(a).

Te Infuence of Coiling Temperature on Mechanical
Properties. Figure 6 is an analysis diagram of the mechanical properties and work hardening behavior of two experimental steels.Figure 6(a) is the engineering stressengineering strain curve.It can be found that the dualphase steel has the characteristic of continuous yielding, which is caused by the existence of internal stress, which allows the dislocation to remain mobile [18,19].Te yield strength, tensile strength, and elongation of steel A with higher coiling temperature are 620 MPa, 764 MPa, and 17% respectively.When the coiling temperature is lowered, the yield strength, tensile strength, and elongation of steel B are 663 MPa, 785 MPa, and 23%.Te strong plastic product of    steel B is as high as 18.1 GPa%, which is much higher than the strong plastic product of steel A (13.0 GPa%).As shown in Figure 3, the grain size of A and B steels is calculated by using the intercept method.It is found that the grain size of B steel is 2 times smaller than that of A steel.Terefore, reducing the grain size will enhance the comprehensive mechanical properties of the steel, especially the toughness.It can be found that when the coiling temperature drops to 540 °C, the mechanical properties of steel B have met the requirements of the national standard for DP780 dual-phase steel, which is mainly due to the improvement of structure strengthening and precipitation strengthening.Te relationship between work hardening index and true strain of experimental steel is shown in Figure 6(b).In the early stage of deformation (true strain <0.075), the work hardening index decreased signifcantly, and the work hardening index of the two experimental steels was almost the same.Dislocation slip in ferrite at this stage provides the main work hardening, and the interaction of high-density dislocations will lead to a decrease in the length of the active dislocation chain, which ultimately leads to an increase in fow stress [20].When the deformation continues (true strain <0.032), the work hardening index of steel A gradually decreases, but it is higher than that of steel B. At this stage, the TRIP efect may occur prematurely because of retained austenite in steel A. Te work hardening index of B steel gradually increases after the true strain is greater than 0.012.When the true strain is greater than 0.032, its value is higher than that of steel A, indicating that the forming performance of steel B is better than that of steel A. It is reported that the diference of work hardening index of steel A and B is related to TRIP efect austenite and high-density dislocations [21].Te retained austenite in steel B can still have the TRIP efect at relatively large strains, and the fne hard phase structure such as bainite or MA can provide high-density dislocations.Besides, wrong delivery and entanglement also improve the work hardening ability [21,22].Te work hardening index "n" is calculated as follows:

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where ε T and σ T are the strain and stress measured, respectively, and dε and dσ are ture strain and stress, respectively.
When n � ε during the stretching process, the experimental steel was necked, and the uniform elongation of steel B was 13.5%, which was higher than 5.7% of steel A. Te impact energy of steel A and steel B at −20 °C are 81.5 J and 93.1 J, respectively.Te low-temperature impact fracture morphology of the two experimental steels is shown in Figure 7, showing uniform and small dimples, which are typical ductile fractures.It can be seen from Figure 6(c) that the comprehensive mechanical properties of steel B are better than steel A, and the experimental steel structure type is ferrite/bainite dual-phase steel.Tus, the coiling temperature of 540 °C can be as a coiling process to product DP780 grade dual-phase steel.

Conclusions
Trough reasonable composition design, high-strengthhotrolled ferrite/bainite dual-phase steel can be produced through the TMCP process and the coiling process.When the coiling temperature is 540 °C, the yield strength of the experimental steel, the tensile strength, and elongation are 663 MPa, 785 MPa, and 23% respectively, and the strong plastic product is as high as 18.1 GPa%.Its mechanical properties meet the requirements of the DP780 level dualphase steel national standard.Te coiling temperature is too high, the microstructure is relatively coarse and it is not conducive to obtaining uniform and fne VC precipitates, resulting in a decrease in yield strength and tensile strength.Te experimental steel with a coiling temperature of 540 °C has a uniform elongation of 13.5%, and the work hardening efect is signifcant at higher strains.Te work hardening index is higher than that of the experimental steel with   Advances in Materials Science and Engineering a coiling temperature of 570 °C, and its forming properties are excellent.

Figure 1 :Figure 2 :
Figure 1: Phase diagram calculation for experimental steel based on thermo-calc.

3. 1 .
Te Infuence of Coiling Temperature on Microstructure.Te optical microstructure and secondary electron image of the experimental steel are shown in Figure3.It can be observed from Figure3(a) that when the coiling temperature is 570 °C, the microstructure in steel A is a typical granular bainite structure, which is a mixture of ferrite and islands surrounded by it.Te structure, in which the black spheres are retained austenite or M-A structure, under the electron microscope, it appears as a convex block structure, as shown in Figure3(b).When the coiling temperature is 540 °C, the microstructure of steel B contains granular bainite and a small amount of acicular ferrite, and the M-A structure is relatively small, as shown in Figure3(d).Te ferrite volume fraction of steel A is 63.3%, which is shown in Figure3(b), and the ferrite volume fraction of steel B is 64.1% as shown in Figure3(d).

Figure 4 :
Figure 4: Crystallographic characteristics of experimental steels analyzed by EBSD: (a-c) image quality map, orientation image map, and grain boundary misorientation map of steel A; (d-f ) image quality map, orientation image map, and grain boundary misorientation map of steel B; (g) plot of misorientation angle versus percentage of steel A; and (h) plot of misorientation angle versus percentage of steel B.

Figure 3 :
Figure 3: Microstructure images of experimental steels.(a, b) OM micrograph and SEM micrograph of steel A. (c, d) OM micrograph and SEM micrograph of steel B.

Figure 5 :
Figure 5: TEM micrographs of experimental steels: (a) morphology of granular bainite in steel A as coiling temperature is 570 °C; (b) morphology of precipitates in steel A as coiling temperature is 570 °C; (c) chemical composition of precipitates in steel A as coiling temperature is 570 °C; (d) fne VC precipitate in steel A as coiling temperature is 570 °C; (e) fne precipitates on ferrite in steel A as coiling temperature is 570 °C; and (f ) chemical composition of precipitates in steel B as coiling temperature is 540 °C.

Figure 6 :
Figure 6: Mechanical properties and work hardening behavior: (a) engineering stress-engineering strain curves; (b) instantaneous work hardening exponent as a function of true strain; and (c) comparison of mechanical properties between steel A and steel B.