Strength and formability of ultra-low-carbon Ti-IF Steels
- PDF / 641,227 Bytes
- 3 Pages / 612 x 792 pts (letter) Page_size
- 11 Downloads / 207 Views
Table I. Chemical Compositions of Steels (Weight Percent) Steel A B
C
Si
Mn
P
S
Al
N
Ti
0.0009 0.25 1.23 0.086 0.0086 0.036 0.0015 0.044 0.0035 0.24 1.25 0.092 0.0073 0.027 0.0014 0.046
WOO CHANG JEONG Ultra-low-carbon interstitial free (IF) steel sheets bearing Ti and/or Nb have been extensively used for automotive panels because of superior formability and nonaging properties. It is well known that the interstitial elements such as C and N play important roles in the formability. The lower the contents of the C and N in steel, the better the formability of the steel. The demands for the steel with excellent formability from automotive industry will accelerate the progress in the steelmaking process, leading to the development of the ultra-low-carbon steel. With the advent and installation of improved vacuum degassing equipment in the steelmaking process, it is now possible to consistently produce ultra-lowcarbon content of 0.002 to 0.005 wt pct. It is expected that in the near future, the C and N contents can be lowered to as low as 0.001 pct or less. This study is focused on strength and formability in the extremely ultra-low-carbon IF steels containing about 0.001 pct carbon. Chemical compositions of the steels used in this study are shown in Table I. The steels are high strength Ti-IF steels with about 400 MPa tensile strength. There is only a difference in carbon content between two steels; 0.25 pct Si, 1.25 pct Mn, and 0.09 pct P are added for the strengthening. The ingots were hot-forged and hot-rolled to 4.0-mmthick hot bands. The hot-rolled sheets were cold-reduced 75 pct to the nominal thickness of 1.0 mm. Tensile specimens with a 50-mm gage length and a 12.5-mm width were machined. The cold-rolled sheets were heated to 800 8C to 890 8C at a rate of 8 8C/s, held at the temperature for 30 seconds, slowly cooled down to 680 8C at 5 8C/s, and then rapidly cooled to room temperature at a rate of 40 8C/s. Tensile tests were conducted for as-annealed steels at a crosshead moving speed of 10 mm/min. The plastic strain ratio (r value) was determined by measuring the width and thickness change after 15 pct tensile strain. Tensile strength, r value, and yield point elongation (YPEl) of steels A and B are plotted as a function of annealing temperature in Figure 1. Lowering carbon content from 0.0035 to 0.0009 pct resulted in a decrease in tensile strength and an increase in r value. The change in tensile strength with carbon content was constant regardless of the annealing temperature. The softening effect of the carbon was, on average, 12.6 MPa per 0.001 pct. The result seems to be surprising. As far as the ultra-low-carbon IF steels are concerned, most research has been on the formability improvement. The IF steels are strengthened by substitutional alloying elements such as P, Si, and Mn. Consequently, there
WOO CHANG JEONG, Assistant Professor, is with the School of Automotive Engineering, College of Engineering, Catholic University of TaeguHyosung, Kyongbuk 712-702, Korea. Manuscript submitted July
Data Loading...
