| The high content of Nb added in high Nb pipeline steel has a great influence on microstructure and mechanical peroperties,as well as the thermostability of the carbonitride in welding heat affected zone(HAZ).But there are few comparison analysis of strength and toughness in HAZ.At the same time,the effects of Nb added in steel on the thermostability of carbonitride and the grain growth of austenite are not clear.In this paper,high Nb microalloyed steels are taken as research objects,the effects of welding parameters,chemical component and microstructure during weld thermal cycle,as well as the strengthening and toughening mechanism,austenictic formation and its growth kinetics,the stability and precipitation kinetics of Ti and Nb carbonitrides in heat affected zone(HAZ)are investigated by Gleeble-3500 thermo-mechanical simulator and the modern methods of materials testing.The brittle zone of the HAZ are the the corse grain heat affected zone and the inter-critical zone.And the cooling rate of 20 ?/s during welding thermal cycle enables better toughness in HAZ.The soften zone in HAZ is the fine grain heat affected zone.The main ways of strengthen for high Nb pipeline steel are refinement strengthening,solution strengthening,dislocation strengthening and precipitation strengthening,in which the refinement strengthening plays a major role in strengthening mechanisms in HAZ.The major factors that affect the toughness in HAZ of high Nb steel is the carbon equivalent.At the same peak temperature,with the increase of carbon equivalent,the strength increases linearly but the toughness decreases.Within the range above 0.436 wt.%,reduce the value of carbon equivalent through alloy design is benificial to obtain good toughness and strength at the same time.However,when the difference of the carbon equivalent in different steels is small,the content of micro alloying element and the microstructure of the base metal have a great influence on the mechanical properties,as well.The Nb-rich precipitates in high Nb steel decreases the thermostability and full dissolution temperature of Ti-rich carbonitride,which rapidly dissolves during the heating process and almost fully dissolves when the temperature increases to 1200 ?.Howere some of the Ti-rich precipitates still remaine even at 1350 ?.Moreover,the undissolved Ti-rich particles coarsened when cooling temperature decreases to below 1200 ?,howere,the sizes of the coarsened carbonitride particles are still smaller than 100 nm.As a resut,the prior austenite grain of the high Nb pipeline steel grows both during heating and cooling processes.The growth rate of prior austenite grain is larger when temperature is higher than 1200 ?,and the growth rate is smaller when temperature is lower than 1200 ? during heating and cooling processes.The grain size of the final austenite is still smaller than 80 ?m.The Nb carbonitride precipitation occurs rapidly in austenite of high Nb steel after hot deformation at 925 ?C.With the increase of the Nb content in solid solution,the Nb carbonitride precipitation is accelerated,which increases the parameter n of the factor of nucleation rate.The amount of Nb carbonitride increases with the increase of Nb and C contents in the matrix.The Nb carbonitride precipitation decreases the microhardness of the final microstructures.The precipitation rate of Nb carbonitride in acicular ferrite at 600 ? is lower than that in austenite,and the amount of Nb carbonitrides is lower than the maximum amount of precipitation.The effect of dissolved Nb and C contents in the matrix on the parameter n of the factor of nucleation rate is considerable.The Nb carbonitride precipitation in acicular ferrite holding at 600 ? has relatively high potency of precipitation hardness.The amount of Nb carbonitrides and the microhardness increase with the increase of Nb and C contents and holding time.However,the microhardness decreases when holding time is over 300 s because of tempering of acicular ferrite. |
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