Effect of Continuous Galvanizing Heat Treatments on the Microstructure and Mechanical Properties of High Al-Low Si Transformation Induced Plasticity Steels

Abstract

Heat treatments were performed using an isothermal bainitic transformation (IBT) temperature compatible with continuous hot-dip galvanizing on two high Al-low Si transformation induced plasticity (TRIP)-assisted steels. Both steels had 0.2 wt pct C and 1.5 wt pct Mn; one had 1.5 wt pct Al and the other had 1 wt pct Al and 0.5 wt pct Si. Two different intercritical annealing (IA) temperatures were used, resulting in intercritical microstructures of 50 pct ferrite (a)-50 pct austenite (c) and 65 pct a-35 pct c. Using the IBT temperature of 465°C, five IBT times were tested: 4, 30, 60, 90, and 120 seconds. Increasing the IBT time resulted in a decrease in the ultimate tensile strength (UTS) and an increase in the uniform elongation, yield strength, and yield point elongation. The uniform elongation was higher when using the 50 pct a-50 pct c IA temperature when compared to the 65 pct a-35 pct c IA temperature. The best combinations of strength and ductility and their corresponding heat treatments were as follows: a tensile strength of 895 MPa and uniform elongation of 0.26 for the 1.5 pct Al TRIP steel at the 50 pct c IA temperature and 90-second IBT time; a tensile strength of 880 MPa and uniform elongation of 0.27 for the 1.5 pct Al TRIP steel at the 50 pct c IA temperature and 120-second IBT time; and a tensile strength of 1009 MPa and uniform elongation of 0.22 for the 1 pct Al-0.5 pct Si TRIP steel at the 50 pct c IA temperature and 120-second IBT time.

Selective Oxidation and Reactive Wetting of 1.0 Pct Si-0.5 Pct Al and 1.5 Pct Si TRIP-Assisted Steels

“…[16] However, it has been shown that the required microstructures and mechanical properties can be obtained when using IBT temperatures compatible with continuous galvanizing thermal cycles. [17][18][19] The selective oxidation of the alloying elements, Mn, Si, and Al, at the steel surface can result in poor reactive wetting during galvanizing. [20][21][22][23][24] These oxides can prevent Al in the Zn bath from reacting with metallic Fe on the steel substrate to form the Fe 2 Al 5 Zn x interfacial layer, resulting in bare spot defects in the Zn coating and overall poor coating quality.…”

The Effect of Two-Step Heat Treatment Parameters on Microstructure and Mechanical Properties of 42SiMn Steel

“…The integrated intensities of (200) ferrite peak and (111), (002) and (022) austenite peaks were used for evaluation. Carbon content in retained austenite was calculated when considering the effect of alloying elements, according to [1,27]: a = 3.572 + 0.0012 Mn − 0.00157 Si + 0.0056 Al + 0.033 C…”

Stability of Retained Austenite in High-Al, Low-Si TRIP-Assisted Steels Processed via Continuous Galvanizing Heat Treatments

“…In particular, it was determined that an IA microstructure of 50 pct c combined with a 120 second IBT produced optimal mechanical properties for the 1.0Al-0.5Si steel and a 50 pct c IA microstructure combined with either a 90 or 120 second IBT time produced optimal properties for the 1.5Al steel. [17] However, Bellhouse and McDermid [17] did not investigate the effect of heat treatment parameters for the preceding alloys on the RA to martensite transformation kinetics during deformation and this is the focus of the present research. In this case, the term ''kinetics'' is used to describe the transformation of RA to martensite as a function of the applied deformation.…”

“…[5][6][7][8][9][10][11][12][13][14][15][16] In particular, the present authors have successfully coated two high Al, low Si (0.20C-1.5Mn-1.0Al-0.5Si and 0.20C-1.5Mn-1.5Al (wt pct)) steels [14][15][16] for which very good mechanical properties were also obtained. [17] A further challenge of processing TRIP-assisted steels within the CGL concerns the thermal cycles commonly used. For example, the IBT for TRIP-assisted steels is commonly carried out at temperatures between 643 K and 703 K (370°C to 430°C), [6] which is incompatible with the temperature of the molten zinc bath, which is typically 733 K (460°C).…”

“…A limited number of investigations have been conducted using IBT temperatures in excess of 723 K (450°C), [17][18][19][20][21][22] which have shown both negative [18][19][20] and positive [17,21,22] effects on the mechanical properties of TRIP-assisted steels when using higher IBT temperatures. In particular, the recent study of Bellhouse and McDermid [17] addressed the use of CGL heat treatments on the microstructural development and mechanical properties of the two previously mentioned high Al, low Si TRIP-assisted steels and determined that sufficient quantities of stable RA and good mechanical properties could be obtained when using an IBT temperature of 738 K (465°C). In particular, it was determined that an IA microstructure of 50 pct c combined with a 120 second IBT produced optimal mechanical properties for the 1.0Al-0.5Si steel and a 50 pct c IA microstructure combined with either a 90 or 120 second IBT time produced optimal properties for the 1.5Al steel.…”