Study On Microstructure And Properties Of Welding Heat Affected Zone Of Titanium Micro-alloyed Steel Q345B

In recent years,titanium micro-alloyed steel has been developed rapidly and widely used.The increase of titanium content in steel will cause the instability of the impact toughness of the plate in the actual production,and the increase of titanium content also affects the phase transition behavior and properties of the welding heat affected zone?HAZ?of the titanium micro-alloyed steel Q345B.In this paper,the influence of cooling rate on the microstructure and properties of the HAZ of titanium micro-alloyed steel Q345B is studied,the relationship between the welding heat input and the microstructure and properties of HAZ is clearly defined,and the data basis is provided for the determination of the welding process of the titanium micro-alloyed steel.The titanium micro-alloyed steel Q345B was studied as the research object.The microstructure and properties of welding heat affected zone of Ti micro-alloyed steel in different heat input were studied.The intragranular acicular ferrite?IAF?in the welding heat affected zone is studied based on this.The phase transformation was observed in situ by CLSM.The main contents and conclusions of this paper are as follows:?1?Based on the actual thermal cycle of the welding heat affected zone,the welding heat affected zone under different heat input was simulated by Gleeble3500,and the microstructure of titanium microalloy HAZ under different heat input?cooling rate?was determined by optical microscope and scanning electron microscope;Hardness in different heat input is obtained through hardness tester;the toughness in different heat input was tested:The results show that:when the cooling rate is less than 10?/s,the microstructure is mainly granular bainite?GB?and grain boundary ferrite?GF?.When the cooling rate is 2.5?/s,the intragranular acicular ferrite produced.When the cooling rate is more than 15?/s,the microstructure is mainly granular bainite?GB?,lower bainite?Bl?and martensite?M?;The law of hardness is as follows:the hardness of HAZ in Ti micro-alloyed steel has little change and the hardness value is about 230 HV when cooling rate is less than 10?/s.When the cooling rate is more than 10?/s,the hardness increases linearly with the cooling rate increasing,and the hardness is greater than 350 HV;The law of toughness is as follows:the impact energy is improved as a whole with cooling rate increasing.The peak occurs when the cooling rate is 2.5?/s and 40?/s.When the cooling rate is 2.5?/s,the formation of IAF significantly improves the impact toughness of HAZ.?2?The IAF,effects of inclusions and contents of different Ti on the precipitation of inclusions in Ti micro-alloyed steel Q345B were studied.It also provides relevant improvement measures for forming IAF.The results are as following:IAF takes micron scale inclusions as nucleation sites and generates more intragranular acicular ferrite in the way of induced nucleation.The ferrite produced is micron sized and interconnected to form“interlocking”.The most suitable and suitable austenite for producing IAF is about185?m.The inclusions is Al₂O₃?TiN?C₂S₂Ti₄?TiC in the Ti micro-alloyed steel calculated by FactSage.The precipitation priority of titanium inclusion:TiN>C₂S₂Ti₄>TiC.?3?The phase transition process of HAZ of titanium micro-alloy steel was observed in situ by CLSM simulation.The phase transformation type,phase transition temperature,morphology,phase transition position and so on were observed in situ.Research shows:Austenite and austenite are swallowed up and divided,and austenite grows obviously when temperature is 1245-1300?.During cooling process,austenite hardly grows up.The average size of austenite in the field is 50?m.The phase transition temperature range of the side plate ferrite is 639-634?,which is parallel to the grain boundary in a certain angle.BF is distributed parallel,the BF phase transition is shorter,and the speed of nucleation growth is faster.The temperature of acicular ferrite transformation is low,and the inclusion is the core of nucleation.