| Development of high-speed,large-scale and light-weight shipbuilding industry has put forward higher requirements for the properties of ship plate steel especially for the weldability.Oxide metallurgy technology,utilizing inclusions in micron scale to induce acicular ferrite(AF)nucleation during cooling(nucleating effect)and precipitates and/or inclusions in nano scale to pin austenite grain boundaries during heating(pinning effect),can effectively improve strength and toughness of heat affected zone(HAZ)and then the weldability of the steel can be enhanced.At present,the technology mostly focused on the nucleating or pinning effect of inclusions or precipitates,and obtaining micron inclusions and nano precipitates(or inclusions)in steels through complex deoxidation to achieve the nucleating and pinning effects has not been systematically reported yet.Therefore,based on previous studies,Ce,Ca,Ti and Mg were selected and added into EH36 ship plate steel to study variation of inclusion properties(composition,size,number density and distribution)in the steels with Mg-Ti-Ce/Ca treatment.The composition requirements(Ce/Ca contents)and process requirements(Ce/Ca killing time and heat treatment)were investigated to obtain a large amount of AF.Growth of prior austenite grain size(PAGS)in the Mg-Ti-Ce/Ca-treated steels with different austenitizing temperatures and holding time was also studied.Finally,strength and low temperature toughness of HAZ were tested and analyzed.The results were as follows.(1)Analysis of the characteristics of inclusions and microstructure in the MgTi-Ca-treated steels with different Ca contents showed that Ca had a good modification effect on Mg-Al-O inclusions in the steels.A large number of Mg-CaAl-O-(S)+MnS-CaS inclusions were obtained in the final quenched steel with 0.0017%Ca.Mean diameter of inclusions declined and quantity of inclusions raised with the addition of Ca,which effectively induced the formation of AF.While,as the Ca content further enlarged to 0.0051%,CaS in the shell of the inclusions enhanced obviously.Because CaS replaced MnS,which was not conducive to the formation of manganese depleted zone and the lattice disregisrtry between CaS andα-Fe was high(26.34%),AF nucleation rate on inclusions decreased,led to a decrease of the area fraction of AF in the steels.Moreover,a certain amount of TiNcontaining inclusions in the steels were obtained and the type of inclusions effectively induced AF formation.(2)Study on the fearures of inclusions and microstructure in the Mg-Ti-Cetreated steels showed that,the types of inclusions modified by Ce were very different with increasing Ce contents,and it was the same case with the ability of AF nucleation on the inclusions.With enhancing Ce contents,the complex inclusions gradually changed from Mg-Al-O+MnS(or TiN)to Ce-Mg-Al-O+MnS(or TiN)to Ce-Mg-O-S+MnS(or TiN).The lattice misfit between the Ce-containg inclusions and α-Fe was reduced and amount of pure MnS,non-nucleus for AF,in the steels declined obviously with raising the content of Ce,which enchanced the nucleation rate of AF on inclusions and ratio of AF.When the Ce content rised to 0.037%,the inclusions collided and aggregated in melts,lessened the nucleation rate of AF on inclusions.When the Ce content was 0.024%,the inclusions in the steel were refined and dispersed,the nucleation rate of AF on inclusions was the highest,and the same applied to the proportion of AF in the steels.(3)Fixed～0.03%Ce and～0.002%Ca,investigantion on types,mean diameter and quantity of inclusions and variation of micro structure with different Ce/Ca killing time showed that the types of inclusions were mainly affected by the contents of deoxidizers,and did not change significantly with the killing time(0-5 min),but the size and number density of inclusions increased with the increase of Ce/Ca killing time.For the Mg-Ti-Ce-treated steels,the change of inclusion size was mainly due to the collision and aggregation of inclusions in liquid steel,while for Mg-Ti-Ca-treated steels,the inclusion size was affected not only by the behavior of inclusions in liquid steel,but also by MnS precipitation.However,whether MnS precipitated on the existed inclusions or not,the nucleation rate of AF on inclusions first increased and then varied little with the increase of inclusions size.Because large inclusions tended to induce the formation of polygonal ferrite,their ability to induce AF was not further improved.By comprehensively considering the inclusions size,amount and the nucleation rate of AF on inclusions in different size ranges,the effective nucleation area of inclusions induced AF nucleation was proposed,which can evaluate the nucleation ability of AF on inclusions with different size and amount.(4)Study on the variation of PAGS of the steels under the condition of different austenitizing temperatures and holding time showed that Mg-Ti-Ce/Ca treatment increased the types of pinning particles and effectively inhibited the growth of primary austenite grains.Submicron Ce-O-S,Ce-S inclusions,CaS and CaO inclusions at the grain boundaries were found in addition to TiN in the steels with 0.015%Ce and 0.0028%Ca.Under the heat treatment conditions of holding at 1250-1400℃ for 20 min and holding at 1350℃ for 5-120 min,the growth trend of prior austenite grain in the Mg-Ti-Ce/Ca-treated steels was smaller than that in the Mg-Ti-treated steel,and the good pinning effect was obtained in the Mg-TiCe/Ca-treated steels.The heat treatment conditions affected the PAGS and the nucleation rate of AF on inclusions and finally affected the formation of AF in the steels.The austenitizing temperatures conducive to AF formation in Mg-Ti-Ce/Catreated steels was 1350℃ and the holding time was 20-40 min.Under this conditions,the PAGS was 119-143 μm and 161-202 μm for the steels with 0.015%Ce and 0.0028%Ca,respectively.(5)Results of mechanical properties of HAZ under heat input of 50 kJ/cm,100 kJ/cm and 150 kJ/cm showed that,Mg-Ti-Ce/Ca treatment effectively improved the yield and tensile strength of HAZ.When the heat input was 50 and 100 kJ/cm,MgTi-Ce/Ca treatment greatly improve the-20℃ impact toughness of HAZ.It is found that the inclusions in Mg-Ti-Ce/Ca-treated steels effectively induced the formation of AF and refined the grains of HAZ.Meanwhile,a lot of high-angle grain boundaries in the steels were found,which effectively prevented the crack propagation and improved the mechanical properties of HAZ.As the heat input reached 150 kJ/cm,the HAZ in the Mg-Ti-Ce-treated steels achieved the highest toughness among the steels,while TiN in the Mg-Ti-Ca-treated steel coarsened,the grain size increased,and the toughness of HAZ decreased accordingly.Mg-Ti-Ce treatment is better than Mg-Ti-Ca treatment in improving the strength-ductilitytoughness combination of HAZ. |
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