A Research On Steel Fining Process In Ancient China

Steel fining process in ancient China,known as Chao(炒)process,was actually a crucial steelmaking technology based on decarburization of cast iron,which could be dated back to the late Warring States period and was still in use in 1958.However,systematic research on such process is rare,the details of the Chao(炒)process in early time are unclear,the name of the process is still under debate,and distinction between the remains of Chao(炒)process and the remains of bloomery iron smelting technology is ambiguous.Here,the dissertation was conducted document research,13 steelmaking simulation experiments,analysis of slags,iron lumps,hammer scales,iron bars collected from simulation experiments and 12 iron artifacts collected from Wuku site in capital city of Western Han Dynasty(202BC-8AD),in order to address the previous problems and provide a comprehensive understanding of the Chao(炒)process,as well as a better approach to accurately identify the remains of the Chao(炒)process.The main points are as follows.Basically,the production process of the Chao(炒)technique can be divided into two basic steps:decarbonizing and forging.Ideally,the raw materials of cast iron will initially melt and then gradually solidify due to the increased melting point after decarburization.Indeed,the primary product should be aggregated into an iron lump with an average carbon content of less than 2.11%.Subsequently,the iron lump will be forged into intermediate or final product.The Chao(炒)process is varied in operations,production facilities,fluxes,fuels,chemical composition of cast iron and even the description in ancient document,demonstrating the diversity and a continuous history of the Chao(炒)process in ancient China.Regarding the facilities,the bowl-shaped hearth,the reverberatory and the rectangular pond connected to a blast furnace are three typical catogeries of decarburization furnaces used for the Chao(炒)processes in Chinese history.Especially,the bowl-shaped hearth is the most representative and probably the earliest type.During the Chao(炒)production of a "bowl-shaped hearth",pieces of cast iron are mixed with fuel,and air is blown through the blast facility to supply oxygen.Ideally,the iron would experience different phase states and undergo a series of oxidation and decarburization reactions step by step,such as the oxidation of silicon,manganese and other impurity elements in liquid cast iron,the decarburization in liquid state,the decarburization in solid-liquid two-phase state,the decarburization in solid state and the oxidation of iron.In order to decarburized successfully,the series of parameters of the blast facility,especially the air output,should be fit with the size and mophorgy of the hearth to make sure that the high-temperature zone around the tuyere is higher than the melting point of cast iron and allow it to melt completely,whilst the temperature at the bottom of hearth belows the melting point of primary product to ensure the successful solidification of the iron.Besides,oxygen supply should meet the need of the necessary decarburization and the volume of iron should fit the bottom of hearth to form an iron lump instead of pieces.On the other hand,fuel,cast iron and kinds of additives would be dropped into the same hearth.Then,these raw materilas and their high-temperature products become the main source of the slags and non-metallic inclusions.At least three different formating paths,named "SiO2 path" "CaO path" and "FeO path"respectively in this thesis,could be existed during the decarburization stage of the Chao(炒)process,which could generate slags with different features on bulk chemical composition.During the formation of slags,multiple kinds of endogenous and exogenous inclusions generated as well.The typical inclusion types of Chao(炒)process could be summarized as Si-Ca,Fe-Si-Ca,Fe-Si,Fe-Ca,CaO·nP2O5,MnS,MnS-FeS,SiO2,the agglomeration of multipe endogenous oxides(also known as Fe-Mn-Si-P system endogenous oxides),etc.The analysis results of the iron artifacts fromWuku site not only supplement the features of endogenous inclusions in the Chao(炒)products,but also reflect the skilled Chao(炒)technology of the bowl-shaped herath in the Western Han Dynasty.Through production control,the craftsman can obtain ideal primary products with different carbon content.The high carbon content steel were used to made high qulitiy spears and blades,while the wrought iron were used to made low quality nails.Finally,after concluding a series of technical characteristics of 6 kinds of typical iron and steel making technologies in ancient China,the thesis provides systematic identification methods of slags and iron artifacts of Chao(炒)process.