STRENGTH AND MICROSTRUCTURE OF COLD-ROLLED IF STEEL

Anh-Hoa Bui,Hoang Le

doi:10.12776/ams.v22i1.690

Abstract

With the emerge of vacuum technology, it is possible to produce ultra low carbon (ULC) steels with carbon content of less than 0.005 %mass which is called interstitial free (IF) steels. In this study, strength and microstructure of IF steel after cold-rolling have been determined. The initial steel plates were cold-rolled using two different cold reductions (CR) as 80 and 90% in total, thereafter the steel sheets were cut into specimens for tensile test and optical microscopy. Ultimate tensile strength (UTS) of the cold-rolled steel was high (650807 MPa), but the elongation (EL) was low (3.55.3%). Meanwhile, UTS of the annealed steels was decreased to 290 MPa when soaking temperature was 800oC because of stress relief and recrystallization. It was concluded that higher CR (more severe deformation) increased the strength but decreased the ductility of the IF steels. In consistence with micrograph of the steels, X-ray diffraction (XRD) results showed that microstructure of the cold-rolled and annealed IF steels was only ferrite. Textures, one of the most important factors affecting the recrystallization, were found in cold-rolled steels.

Highlights

The demands for the steel with excellence formability from automotive industry have accelerated the progress in the steelmaking process, leading to the development of the ultra low carbon (ULC) steels containing carbon less than 0.01 %mass [1, 2]
It is seen that the Ultimate tensile strength (UTS) and yield strength (YS) of the cold-rolled steels dramatically increased but attaching decrease in the ductility despite of that the steels contained a small amount of strengthening elements such as Si and Mn
After cold-rolling step, annealing treatment would be eliminated if the steels are not required high ductility but require high strength

Summary

Introduction

The demands for the steel with excellence formability from automotive industry have accelerated the progress in the steelmaking process, leading to the development of the ultra low carbon (ULC) steels containing carbon less than 0.01 %mass [1, 2]. Two important objectives being pursued by the automobile industry are a decrease in car weight and improvements in safety [7] To realize these requirements, several researches have been implemented to reduce thickness of the steel sheet, increase strength and improve press formability of the steel. Thereafter, hot-rolling is conducted before cold-rolling is used to produce IF steel sheets for press forming This brings some disadvantages for production, namely, low productivity, higher energy consumption, longer processing line, etc. It would be better if hot-rolling step could be eliminated but properties of IF steels still fulfill the requirement of application. T0 and tf are thickness of initial plates and final sheets, respectively

IF steel

Results and Discussion

Conclusions