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.

Advanced high-strength steel (AHSS) is a highly competitive material for the automobile industry to resolve the challenge of weight reduction and passenger safety. With the increase in strength, defects such as microcrack, local thickness reduction, and large corner radius in the corner section of cold roll-formed steel products become more and more serious. Cold and hot composite roll forming (CHCRF) is a newly developed manufacturing technology that combines the advantages of cold and hot forming to overcome the challenge of forming AHSS. In this paper, the mathematical models that represent the effect of line velocity, heating power, and deformation amount on the yield strength, outer corner radius, and microcrack length of the corner section of cold and hot composite roll-formed AHSS square tube are investigated by the response surface methodology (RSM) with Box-Behnken design (BBD). From the analysis of variance (ANOVA), it is found that all the three process parameters play a significant role in the outer corner radius and microcrack length, while the line velocity and heating power influence the yield strength significantly. Furthermore, an optimization process is proposed, with the outer corner radius and microcrack length as objective functions and the yield strength as a constrained condition, to manufacture the AHSS square tube with a small outer corner radius and no microcrack defect.Experimental verification has been conducted using the optimal process parameters, and the average magnitude of error is about 5 %, which implies that the optimization design is accurate and reliable.