Effect of Microstructure on Fracture Mechanisms in Galvannealed Coatings.

Abstract

Fracture mechanisms in galvannealed coatings have been studied by performing draw bead tests on galvannealed Ti stabilized interstitial free and Aluminum killed low carbon steel sheets and by investigating coating microstructures by scanning electron microscopy. Galvannealing treatments, on samples galvanized using an industrial hot-dip galvanizing process, were conducted at 450, 500 and 550°C for several time periods between 1 and 360 s in a laboratory induction furnace.In the coatings with low Fe content (up to 5 g/m 2 ), the amount of powdering during the draw bead test was minimal. Growth of cracks nucleated within the d 1 phase was arrested at the steel-coating interfaces where only a limited amount of decohesion occurred. A steep increase in the amount of powdering was observed in coatings with Fe contents between 6-9 g/m 2 . In these coatings, cracks originating from the d 1 phase reached G-G 1 -d 1 phase boundaries, which provided preferential crack growth paths and thus facilitated fracture within the coating. A fracture mechanics model was proposed to account for the powdering resistance of galvannealed coatings.

Crack Spacing Distribution in Coating Layer of Galvannealed Steel under Applied Tensile Strain

“…The coating layer exhibits multiple cracking under thermally induced stress during cooling from the heat treatment-to room-temperature due to the difference in thermal expansion between the coating layer and substrate steel. 1,[4][5][6][7][8][9][10] When tensile stress is applied, the multiple cracking progresses furthermore, 1,[4][5][6][7][8][9][10][11] as typically shown in Fig. 1, and then, as shown later, the coating layer is spalled.…”

“…1, and then, as shown later, the coating layer is spalled. 1,6,7,[12][13][14][15][16][17][18][19][20][21] Concerning the thermal and applied stress-induced multiple cracking of the coating layer, the authors [8][9][10][11] have recently measured the change of crack spacing with increasing applied strain e for two samples supplied as the common samples for the research group on structure and property of the coating on GA (galvannealed) steels, organized in The Iron and Steel Institute of Japan. One was the IF (Interstitial Free) steel substrate with 10 mm thickness-coating layer (hereafter noted as IF/10).…”

“…[1][2][3] They are composed of the brittle Fe-Zn intermetallic compound-coating layer and ductile substrate steel. The coating layer exhibits multiple cracking under thermally induced stress during cooling from the heat treatment-to room-temperature due to the difference in thermal expansion between the coating layer and substrate steel.…”

Plastic Deformation and Fracture Behavior of Galvannealed Coating.

“…Hertveldt et al 9) demonstrated in single lap shear test that as the shear strength of galvannealed coatings increased, the amount of fracture at the GFe interface decreased, and also the fraction of the fracture in the a-Fe grain boundaries and within the coating increased. Alpas et al 10) observed in draw bead test that the decohesion occurred along the G-Fe interfaces or along the a-Fe grain boundaries of substrates when Fe contents in coatings are low, i.e., Ͻ5 g/m 2 , while the decohesion occurred along the G-G 1 and the G-d 1 phase boundaries when Fe contents in coatings are between 6 and 9 g/m 2 . The deformation and fracture of the coating is assumed to be thus dependent on the microstructure of the coating and also deformation condition (i.e.…”

Influence of the Crack Spacing in the Coating Layer on the Progress of Interfacial Debonding in Galvannealed Steel Pulled in Tension

“…On the other hand, the cracks induced parallel to the tensile direction (namely the cracks in the sample width direction) were caused by the thermally induced residual stress. [13][14][15][16][19][20][21] However, the influences of the crack spacing in the tensile direction and that in the sample width direction have not been clarified until now, despite the importance to clarify the spalling behavior. In our recent study, concerning the fracture of GA coating layer with high tensile strength steel substrates, it was newly suggested that the interfacial debonding could be reduced through the enhancement of multiple cracking.…”