Mechanism Of Solid-state Reaction At The Interface Between Inclusion And Steel Matrix During Heat Treatment At 1473K

After heat treatment and rolling process, the type, quantity, size, morphology and physicochemical characters of non-metallic inclusions finally formed in steel influence the quality and properties of steel products directly. However, this kind of non-metallic inclusions often has great difference with the inclusions existing in molten steel before solidification. One of the major reasons for the difference is that the following heat treatment process which steel matrix goes through cannot only change the microstructure and properties of steel, but also effect the properties of internal non-metallic inclusions. During heat treatment process, solid-state reactions occur between solid steel matrix and inclusions which cause composition segregation of steel matrix, modification of original inclusions and precipitation of new inclusions.It is feasible to implement the control of physicochemical characters of non-metallic inclusions in steel matrix by appropriate heat treatment process. In this study, the equilibrium relationship of MnO-SiO2-FeO oxide and Fe-Mn-Si solid alloy. Al2O3-CaO-FeO oxide and Fe-Al-Ca solid alloy at 1873 K were constructed and verified by thermodynamic equilibrium calculation and high temperature equilibrium experiments. Then the influence of heat treatment at 1473 K, sulfur and oxygen on the interfacial solid-state reaction and the physicochemical characters of alloy matrix and oxide in the above two kinds of diffusion couple were also studied. Moreover, the mechanism of solid-state reaction and diffusion between steel matrix and inclusions were discussed and revealed. Finally effective thermodynamic model for controlling and predicting the two kinds of solid-state reaction and modification of inclusions was developed based on the experimental results.Results show that:1) thermodynamic equilibrium calculation and high equilibrium experiments showed, at 1873 K, equilibrium deoxidization products in Fe-Mn-Si alloy and Fe-Al-Ca solid alloy were ternary MnO-SiO2-FeO oxide and Al2O3-CaO-FeO oxide respectively; 2) equilibrium state of Fe-Mn-Si solid alloy and MnO-SiO2-FeO oxide, Fe-Al-Ca solid alloy and Al2O3-CaO-FeO oxide at 1873 K was broken during the heat treatment at 1473 K. occurring high temperature solid-state reactions and elements diffusion between the alloy and oxide; 3) With the prolongation of heat treatment time, the width of PPZ, MCZ and ACZ tended to increase gradually in the two kinds of diffusion couples. For the system of Fe- Mn-Si solid alloy and MnO-SiO2-FeO oxide, the width of PPZ and MCZ, from the 15 μm,15 μm before heat treatment, increased to 79 μm,120 μm and 138 μm,180 μm after heat treatment for 10 h and 50 h, respectively; For the system of Fe-Al-Ca solid alloy and Al2O3-CaO-FeO oxide, the width of PPZ and MCZ, from the 7 μm, 20 μm before heat treatment, increased to 31 μm,90 μm and 87 μm.170 μm after heat treatment for 10 h and 50 h, respectively:In addition, due to the separation of FeO, pure Fe particles formed in the two types oxide, and both the size and quantity of Fe particles tended to increase with the increase of heat treatment time; 4) For the two kinds of diffusion couple system, the increase of FeO content in oxide could improve the content of excess oxygen, then promote the interfacial solid-state reaction between alloy and oxide and increase the area of PPZ, MCZ and ACZ; 5) Sulfur could constrain the oxygen diffusion and interfacial solid-state reactions between MnO-SiO2-FeO oxide and Fe-Mn-Si solid alloy during heat treatment, however, there is no obvious influence for the system of Fe-Al-Ca solid alloy and Al2O3-CaO-FeO oxide.6) For the system of Fe-Mn-Si solid alloy and MnO-SiO2-FeO oxide, when the content of FeO or sulfur in oxide is relatively higher, in order to keep the whole electroneutrality of oxide, cations such as Mn2+ and Si4+ would diffuse from oxide to alloy matrix, causing the phenomenon that Mn and Si content in the alloy near the interface rose again after it decreased to a certain degree. However, this phenomenon was constrained when the FeO and sulfur content in oxide became lower;7) the quality and size of the dendritic Al2O3-CaO inclusions which precipitated near the interface in Fe-Al-Ca solid alloy showed negative correlation relationship with the FeO content in oxide, and positive correlation relationship with the heat treatment time.8) Based on the Wagner internal oxidation model, thermodynamic diffusion models for reflecting and predicting interface solid-state reaction between Fe-Mn-Si solid alloy and MnO-SiO2-FeO oxide, Fe-Al-Ca solid alloy and Al2O3-CaO-FeO oxide under different FeO content and heat treatment time were constructed. Model results could coincide well with experimental results in the two kinds of system.