| Weathering steels(WS)are low-alloy steels formed by adding a small amount of alloying elements(Cu,Cr,Ni,P,Re,etc.)to ordinary carbon steel,which is widely used in the manufacture of railway freight vehicles because of its good strength,toughness and corrosion resistance.In recent years,with the rapid development of railway construction in China,the Ministry of Railways has also put forward the requirements of high strength and high corrosion resistance for WS.The addition of alloying elements is essential to achieve the strengthening of the steel and improve corrosion resistance.In this thesis,three types of WS for railway vehicles were selected for study: steel without Sb,steel with Sb and steel with P.The structural and compositional characteristics of various surface films produced under different corrosive environments were investigated in detail using multi-scale characterisation methods such as optical microscope(OM),scanning electron microscope(SEM),electron backscatter diffraction(EBSD),transmission electron microscopy(TEM)and atom probe tomography(APT).In particular,the characteristics of element distribution at the interface between surface film and substrate have been systematically explored using APT technology.The distribution behaviour of alloying elements in the corrosion process has been revealed.The interaction of alloying elements and the mechanism of surface film formation has been explored,and the role of alloying elements on the corrosion resistance of steel has been thoroughly discussed.The following breakthroughs and innovations have been made:(1)Through systematically exploring the structural characteristics of WS,the characteristics and laws of alloying element distribution in steel are revealed,and the strengthening effect of various alloying element additions in WS is mastered.The addition of Nb and Ti elements in WS for micro-alloying can form fine diffuse carbon nitride inside the grains,which is not only conducive to grain refinement,pegging dislocations to improve the overall strength of steel,but also beneficial to reduce the formation of pearlite in steel,which improves the organisational uniformity of steel and contributes to the reduction of pitting corrosion tendency.Moreover,there is a strong interaction between Sb and Cu,which can lead to the formation of(Cu,Sb)-rich precipitates in steel with Sb and inhibit the segregation of Cu and Sb at grain boundaries.(2)The influence of high temperature oxidation on the distribution of alloying elements and on the substrate of near-surface steels is mastered by studying the distribution of alloying elements in high temperature oxide films.The important influence of the addition of Cr and Si elements in WS on the phase transformation of high temperature oxide films during continuous cooling is clarified.The Fe O phase in the inner scale on the steel with low Cr and Si contents is observed to incur a eutectoid reaction in a divorced mode during cooling.The reaction is responsible for a unique bimodal distribution of Ni in the steel substrate near the scale/substrate interface.In contrast,the inner scale on the steel with high Cr and Si contents develops a finegrained nanocomposite oxide band to separate Fe O from the ferrite substrate.Penetrative cracks in the oxide scale facilitate O2 to reach the Fe O phase and incur a combination reaction to form coarse-grained Fe3O4.(3)By studying surface films in atmospheric exposure and periodic infiltration environments,it is able to grasp the characteristics and laws of the distribution of surface film elements in WS of different compositions and to better understand the influence of alloying elements on corrosion resistance.The Cu,Ni elements of the three WS have obvious segregation at the interface between rust layer and matrix,and enrichment in the rust layer near the interface.The Cr element is affected by the phase transition of corrosion products in the rust layer and presents a strip-like distribution in the rust layer.Even if P element exists as impurity in the steel without Sb addition and the steel with Sb addition,it is obviously enriched in the rust layer near the interface formed after atmospheric corrosion.These indicate that Cu,P,Cr,Ni are the major elements to form a protective rust layer.Mn element is barren in the interior and interface of the rust layer,which proves that Mn element is not the element to form a protective rust layer.The content of Sb in the rust layer of the steel with Sb is 0.19 at%,which is much higher than 0.04 at% Sb detected in the matrix.This indicates that Sb is one of the major elements to form a protective rust layer.(4)The study on the initial corrosion of fresh metal surface of WS shows that Cr,Ni,Sb and P elements played a very important role in the rapid formation of surface film in the acid environment.The periodic infiltration test can accelerate the redistribution of Cr and Si elements in the rust layer,which lead to γ-Fe OOH transformation to stable α-Fe OOH.The periodic infiltration test can be used to simulate the rust layer formation after long-term exposure in industrial atmosphere,to a certain extent.However,it is necessary to consider the complexity of the climate change in the service environment of the WSs,and modify the periodic infiltration test conditions,in order to better predict the performance of WS. |
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