Effect of Heat Treatment on Formability of Hot-dip Galvanized Low Carbon Steel Sheet

Abstract

Fig. 1. One series of galvannealed samples after stretching. Strain Distribution ProfilesThe major and minor diameters of the ellipses in the longitudinal direction and the corresponding distances of the ellipses from the pole were measured. From these data, strain distribution profiles were drawn by plotting the distance from the pole on the abscissa and the corresponding major and minor strains on the ordinate. FractographyThe fractured surfaces were studied using a scanning electron microscopy. The specimens were cut close to the region of the fracture. The fracture cross sections were cleaned with alcohol before study. The nature of fracture was analyzed from the features of the fracture to study the relationship between the features and the formability parameters. Results and Discussion Coating LayersThe experimental results of galvanized coating microstructure examined using SEM, EDS semi-quantity analysis, XRD patterns, and liner analysis of coating sections revealed that the coating microstructure consisted of z (Zn-4.2%Fe) and h (Zn-1.1%Fe) layers on steel surface (Fig 2(a)). The surface of the galvanized samples had shiny spangles but then changed to matt with no spangles. Figures 2(b), 2(c) show the section and the microstructure of galvanized samples and those annealed at 520°C and for varying durations. The apparent surface of coating at 120 s was silvery and bright without spangles but at 135 s, it turned gray and matt. Considering EDS semi-quantity analysis and XRD patterns, the galvannealed coating at 120 s consisted of G, d, z, and h layers while at 135 s, it only contained G and d layers. It can be seen in Fig. 2(c) that cracks initiated and propagated along the d phase. FLDs of the SteelsThe formability limit diagrams for the galvanized and galvannealed steels under investigation are presented in Fig. 3. The diagrams are similar in shape (V-shaped). The vertical and horizontal axes in these diagrams represent the percentages of maximum strain and minimum strain, respectively. The strains were measured for the necked ellipses and around the fracture area. That is why the forming limit diagrams were drawn under the obtained points.On the FLDs, the regions corresponding to the negative minor strain (tension-compression region) and the positive minor strain (tension-tension region) are not symmetrical. Thus, in the tension-compression region, the slope of the forming limit curve is higher and the safe zone is wider than those of the tension-tension region. It is clear from Fig. 3 that the level of forming limit diagrams and the major strains decrease with increasing heat treatment time at a constant temperature. The lowest limit strain of a FLD, which corresponds to 0 % minor strain, is known as the plain strain (FLD 0 ). Since failure appearing in cold-formed parts under pressed conditions often occurs in this state, the value for FLD 0 represents the most critical strain state on the FLD. High values of FLD 0 mean better formability. 18)FLD 0 values for galvanized and annealed samples are pre...

The Impact of Heat Treatment on the Behavior of a Hot-Dip Zinc Coating Applied to Steel During Dry Friction

“…The structure formed after the conducted experiment is similar to the result observed in hot-formed steel samples galvanized in a zinc bath with temperature stabilized at 460 °C; samples were cooled to room temperature, reheated to 500 °C, and then cooled again at a rate of 10°/s to room temperature [ 39 ]. Although the structure of the zinc coating after the HT described in [ 37 ] was composed of four layers, most of the data [ 4 , 38 , 39 ] agree that there are only three phases in a zinc coating after heat treatment. During HT, the δ and Γ phases grow at the expense of the ζ phase [ 39 ], and at higher temperatures, the δ phase grows towards the surface of the coating, consuming the ζ layer [ 5 ].…”

“…The time of treatment was 7 min for disc-shaped samples and 11 min for bolts. The HT parameters were selected on the basis of our own preliminary studies, a literature review [ 37 , 38 , 39 ], and measurements of the heating rates of the treated elements. After the heat treatment, samples were taken out of the furnace chamber and were air-cooled to ambient temperature.…”

“…In the publication [ 37 ], an increase of the zinc coating hardness of more than double was reported, but no information was given about the HT parameters. The coating microstructure also changed during the formability of hot-dip galvanized DC01 steel [ 38 ]. During the experiment, temperatures in the range of 500–540 °C were applied for 10–180 s. It has been found that the use of higher temperatures in combination with shorter processing times improves the coating formability behavior.…”

Comparison of the Tribological Properties of the Thermal Diffusion Zinc Coating to the Classic and Heat Treated Hot-Dip Zinc Coatings

“…Most of the data [ 22 , 55 ] confirm that there are only three phases in a HD zinc coating after heat treatment: Г (23.5–28.0 wt% Fe), δ (7.0–11.5 wt% Fe) and ζ (6.0–6.2 wt% Fe)— Figure 4 [ 46 , 56 ]. During the heat treatment, the δ and Γ phases grow at the expense of the ζ phase [ 57 ], and at higher temperatures, the ζ layer disappears and in its place the δ phase grows reaching to the surface of the coating [ 24 ].…”

Influence of Aluminum on the Formation Behavior of Zn-Al-Fe Intermetallic Particles in a Zinc Bath

“…[1] The growing importance of hot-dip galvanized coatings on automotive parts has led in recent years to studies on the mechanisms of their formation on interstitial-free (IF) steel substrates as well as high strength transformation-induced plasticity (TRIP) aided steels and twinning-induced plasticity (TWIP) aided steels of high strength and good formability. [2][3][4][5][6][7][8][9][10][11][12] Dross in the Zn pot can be classified by oxide type (Zn and/or Al) and intermetallic compound type (Zn-Fe and Fe-Al). The latter type tends to cause dross problems and forms in the Zn pot when Al and Fe are present in concentrations above the solubility limits.…”