Abstract: An ultra-low carbon acicular ferrite steel heavy plate was obtained with an advanced thermo-mechanical control process-relaxed precipitation controlled transformation (TMCP-RPC) at Xiangtan Steel, Valin Group. The heavy plate has a tensile strength of approximately 600 MPa with a lower yield ratio. The impact toughness of the heavy plate achieves 280 J at −40°C. The fine-grained mixed microstructures of the heavy plate mainly consist of acicular ferrite, granular bainite, and polygonal ferrite. The high strength and excellent toughness of the heavy plate are attributed to the formation of acicular ferrite microstructure. The prevention of blocks of martensite/retained austenite (M/A) and the higher cleanness are also responsible for the superior toughness.
1. Introduction
Toughness is a very important factor for materials design and applications. High impact toughness in the base metal and heat-affected zone (HAZ) is required for high strength steels [1-3]. In recent years, extensive studies on the toughness improvement of weld metal and HAZ have been carried out [4-8]. Heavy plates are usually used to manufacture important structural parts such as bridges, buildings, and pressure vessels etc. [9-10]. Therefore, good toughness and low yield ratio are main considerations for heavy plate manufacturing and applications.
To improve the weldability and reduce welding costs for heavy plates, carbon content has been reduced gradually in the last decades. Many kinds of ultra-low carbon bainitic steels (ULCB) have been developed for structural applications with optimal mechanical properties in recent years [10-13]. Although the weldability is improved by carbon content reduction and the strength is raised by bainitic microstructure,the higher yield ratio and lower temperature toughness are still to be solved. In addition, the essential problems are associated with nonuniform microstructures in the production of heavy plates. The present work is to investigate the microstructural evolution in a steel heavy plate. It aims to obtain a microalloyed steel heavy plate with a good combination of high strength with lower yield ratio and good toughness at lower temperatures.
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