Research On Fine Structure Control Mechanism Of Ultra-low Carbon Bainite Steel

Low carbon bainitic steel is a result of the development of modern metallurgical technology.Due to its high strength and toughness,low carbon bainitic steels have been widely used in cross-sea bridges,offshore platforms and various engineering machinery field etc.to meet the needs from national economic development.For thick/ultra-thickness structural steels,it is often difficult to obtain ideal through-thickness low-temperature bainitic structure due to the limitations of current rolling conditions and cooling facility.In this paper,from the perspective of bainite transformation crystallography,the relationship between alloy compositions,TMCP process and bainite transformation variant selection behavior has been established.The present work aims at building a theoretical model for the design of alloy composition and process to realize the refinement of the bainitic multi-level substructure in thick gauge structural steel which is composed of packet,blocks and laths.Based on this,the following work was carried out:(1)For the purpose of optimizing the variant selection,three ultra/extremely low-carbon bainitic steels with different chemical compositions were designed for comparison.Static continuous cooling transformation studies have been conducted.Results show that,when the alloy content is relatively low(1#experimental steel)and the hardenability is limited,pre-eutectoid ferrite was formed in the entire investigated cooling rate range,while low temperature transformed bainite was also obtained.With the increase of alloy content and hardenability(2#,3#experimental steels),bainitic structure was achieved over a wide cooling rate range of 1?50?/s,and the pre-eutectoid ferrite transformation was suppressed.The microstructure gradually changed from granular bainite to lath bainite.(2)Static isothermal transformation experiments have been carried out on experimental steels.In the case of insufficient hardenability(1#experimental steel),a partial transformation occurred before reaching the preset isothermal temperature,and a high-temperature bainitic microstructure was formed.When the hardenability is sufficient(2#,3#experimental steels),bainitic transformation took place during the isothermal transformation at 520?,and the substructure which has the characteristic block and packet in prior austenite grains was clearly visible.At 550?,the substructure features in prior austenite grains became blurred.At a higher temperature of 580?,the pro-eutectoid ferrite formed at prior austenite grain boundaries.Further decreasing the isothermal transformation temperature i.e.at 420?,450? and 480?,the complete lath bainitic micro structure was obtained and the characteristics of the multi-layered substructure were obvious.(3)EBSD characterization was used to analyze the relationship between alloy composition,isothermal transformation temperature,and variant selection behavior during bainitic transformation.Results showed that,for 2#experimental steel,at 500?,520?,and 550? where the transformation temperature is relatively high,the driving force for phase transformation increases with decreasing temperature.The fraction of high-angle grain boundaries in prior austenite grains gradually increases.Variant pairs changed from Bain group to CP group.For 3#expeirmental steel,when the bainite transformation is performed at 420? and 450?,variant pairs tend to be in the form of CP group,that is,the strength of prior austenite increases due to the lower transformation temperature which facilitate the self accommodation of transformation variant and promote the fraction of high-angle grain boundaries.(4)Hot rolling experiments of 2#and 3#experimental steels with relatively high alloy content and high hardenability were carried out.Results show that,for 2#experimental steels with relatively high bainitic transformation temperature,granular and lath bainite microstructure was obtained.The yield strength is 642 MPa,the tensile strength is 682 MPa,the yield ratio is relatively large(?0.93),and the fractured surface when impacted at-40? appeared to be brittle.For the 3#experimental steel with the highest hardenability,although a complete lath bainitic structure was achieved,the strength was significantly improved and the yield ratio was even larger,but the low temperature toughness was significantly reduced.EBSD analysis showed that large fraction of low-angle grain boundary is the root cause of poor low temperature toughness.