Study On The Microstructures And Mechanical Properties Of The Special Thickness Of The Plate Steel Produced By Ultra Fast Cooling

Based on the key project of NSFC(Control of nano-carbide precipitates in low carbon steels under ultrafast cooling and comprehensive strengthening mechanism),the mechanical properties and microstructures of plate in different position along thickness direction were emphatically studied by hot rolling and thermal simulation experiments.The effects of cooling process and component on microstructure and mechanical properties were investigated through a controlled experiment of single variable.Furthermore,the strengthening mechanism of low carbon microalloyed steel plate produced by UFC process was discussed,which provides a theoretical basis for the composition design and the cooling process determination of plate.According to the results of hot-rolling and thermal simulation experiments,the cooling rate of different position along thickness direction is different after ultra fast cooling of the low carbon microalloyed steel plate.The cooling rate is the slowest in the core and the fastest at the surface.As the position is close to the surface,the temperature equalization time is getting longer.In general,the microstructure of the plate center consists of polygonal ferrite and blocky pearlite.The granular bainite may occur at certain cooling process.From the corn to surface,pearlite quickly disappears and the polygonal ferrite gradually evolves into acicular or granular ferrite and even disappears completely,and the volume fraction of granular bainite rises rapidly and reaches a maximum at the surface.The lath bainite and martensite may also appear at certain cooling process,so the phase transformation strengthening effect continues to enhance as the position approaches to the surface.Furthermore,the closer to the plate surface,the smaller the grain size and the volume fraction and the size of the precipitate particles.The contribution of precipitate size to the precipitation strengthening is greater than the precipitation volume fraction,so the closer to the surface,the stronger the fine grain strengthening and the second phase strengthening effect.Under the synthetic effect of various strengthening mechanisms,the yield strength of the plate increases from 500MPa to 650MPa from its center position to the surface.The toughness at different thickness of the plate is determined by the grain size and the volume fraction of the different phases,and the impact energy at 0? can vary from 30J to 140J under the condition of different cooling process.When the final cooling temperature is high,the high cooling rate will cause the re-reddening time too long and the re-reddening temperature range very large and result in large grain size,which will greatly reduce the impact power from 138J to 80J of the 1/4h at-40?.When the final cold temperature is moderate or low,the high cooling rate leads to the decrease of the volume fraction of the polygonal ferrite,which affects the impact toughness of the steel,and the 1/4h-40? impact power is reduced from 149J to 130J and 111J to 17J,respectively.Therefore,in order to obtain more excellent comprehensive mechanical property,the cooling speed of the medium and heavy plate should not be too large after the control rolling.When the cooling speed is suitable,the high re-reddening temperature will lead to large grain size and decrease the mechanical properties of the plate,and the yield strength of 1/4h is only 535MPa.The lower re-reddening temperature will cause the microstructure of each thickness to be nearly all granular or lath bainite,and thus reduce the plate impact toughness.The impact power of 1/4h at-40? is only 111 J at this condition.Therefore,in order to obtain good impact toughness,the re-reddening temperature should be improved properly,which can ensure the yield strength reaching 550MPa and the impact power at-40? reaching 149J of 1/4h.Furthermore,600MPa yield strength and 750MPa tensile strength of 1/4h can be reached by reducing the re-reddening temperature,which is not below the limit value.The micro-alloying element can enhance the strength of the medium-thick plate by comprehensive strengthening mechanism including the precipitation enhancement and phase transformation enhancement,and the yield strength of the 1/4h increases from 459MPa to 629MPa,but the precipitation and the decrease of the ferrite volume fraction will result in the decrease of the impact power of 1/4h of the steel from 144J to 80J at-40?.The reduction of C content can enhance the ductility and reduce the strength of the steel.Ni is good for strength and has little harm to the toughness,so the rational use of Ni can improve the mechanical properties of the steel at the same time.