Study On Thermodynamic Model Of Precipitation Behaviour Of Inclusions In09CuPCrNi Weathering Steel During Solidification

China is abundant in rare earths (RE) resources and also the largest producer of RE in the world. The functions of rare earth elements in steel are cleaning molten steel, modifying inclusions and micro-alloying. Research shows that the occurrence and action of RE in steel will be more obvious as the improvement of steel cleanliness. Thus, the investigation and exploitation of high added value RE steel have great economic benefits and strategic meaning.09CuPCrNi steel is one kind of typical weathering steel in China. However, the comprehensive mechanical properties and corrosion resistance of09CuPCrNi steel should be promoted to meet the challenge of complex climate and bad working conditions. In view of the successful application of RE in weathering steel, such as09CuPTiRE steel and B450NbRE steel and so on, RE is chosen to add into09CuPCrNi steel. In the present study, the occurrence of RE and its effects on the types and contents of inclusions in09CuPCrNi steel were systematically investigated by establishing thermodynamic model. The results will provide the theoretical basis for the new type weathering steel development.In this paper, a coupling thermodynamic model was established to quantificationally describe elements segregation and inclusions precipitation in09CuPCrNi steel during solidification. The precipitation behavior of inclusions during solidification was investigated by the model. In addition, the effects of steel cleanliness which involved the content of oxygen, sulfur, phosphorus and nitrogen, and the varieties and addition of RE on inclusions types and contents in steel were discussed thoroughly. The results are obtained as follows:(1) Al2O3and MnS successively form in weathering steel without RE. When0.02%Ce is added, Al2O3, Ce2O2S and MnS precipitate in sequence in weathering steel with RE. When 0.02%La is added into the steel, LaAlO3, LaS and MnS precipitate in turns.(2) The types of inclusions in weathering steel with RE transform from Al2O3, Ce2O2S and MnS to CeN, Ce3S4and Ce2O2S with increasing the addition of Ce. And with the increase of La addition, the types of inclusions alter from Al2O3, MnS and LaAlO3to LaN, LaS and LaAlO3. When the additions of Ce and La respectively arrive to0.025%and0.01%, Al2O3can not precipitate. MnS can not precipitate when the additions of Ce and La are all arrive to0.045%.(3) With the increase of oxygen content, the types of inclusions transform from MnS, Ce3S4and Ce2O2S to MnS, Ce2O3,Al2O3and3Al2O3·2SiO2in weathering steel containing Ce, and the types of inclusions transform from MnS, LaAlO3, LaS to MnS, LaAlO3, Al2O3and3Al2O3·2SiO2in weathering steel containing La.(4) With the sulfur content increasing, the content of Ce2O3declines while that of Ce2O2S and Al2O3increases in the weathering steel containing Ce. MnS starts to precipitate when the sulfur content is25×10-6. While the precipitation amount of LaS increases with the sulfur ontent increasing in weathering steel containing La. When the sulfur content is40×10-6, and LaS reaches precipitation equalization and MnS begins to precipitate.(5) When Ce or La addition is0.05%, the critical nitrogen content of CeN or LaN precipitation is0.004%and that of AlN is0.028%.