Effect of induction heating temperature on the microstructure and mechanical properties of HSLA square tubes

Abstract

Severe strain hardening, thickness reduction, and large outer corner radius are three typical characteristics in the corner section of cold roll-formed steel. This paper presents a new manufacturing technology called cold and hot composite roll forming to improve the shape and properties in the corner section. In order to study the effect of induction heating temperature, a multi-pass cold and hot composite roll forming experiment for high-strength low-alloy steel (HSLA) square tubes was carried out under three different heating temperature ranges namely: the non-austenitizing zone, partial austenitizing zone, and austenitizing homogenizing zone. The microstructure and mechanical properties in the corner section of square tubes were studied using the material testing machine, optical microscope (OM), scanning electron microscope (SEM), and transmission electron microscope (TEM). Results showed that with the increase of induction heating temperature, the geometrical morphology and mechanical properties in the corner section of the cold and hot composite roll-formed square tube were significantly improved. Thus, compared with cold roll-formed square tubes, the corner thickness of cold and hot composite roll-formed square tubes was increased by 59%, the product of strength and elongation was increased by 48%, the outer corner radius was decreased by 91%, and no cracks appeared during the flattening test. Moreover, the proportion of granular bainite in the outer corner section increased, and the grain distortion in the inner corner section was relieved. The experimental results further showed that the austenitizing homogenizing zone was preferable among the three heating temperature ranges to manufacture high-quality HSLA square tubes.

Keywords

• high-strength low-alloy steel
• induction heating
• cold and hot composite forming
• flattening test