Study On Toughening Mechanism And Production Process Optimization Of Wheel Steel For High Speed Train

Most recently, higher demand on safety level of railway wheels has been proposed as high speed railway operation speed break through350km per hour, so it is more important to develop high toughness wheels for high speed train transport. There are certainly researches on elements alloying mechanism and cleanliness improvement could be found in wheel steels, but investigations focus on toughness improvement in specific manufacturing process for high speed railway wheels is not adequate and need to be strengthened. The present study pays attention on toughness improvement of medium-high carbon pearlite wheel steels. A serial of analysis and testing methods, such as EBSD, Thermo-Calc thermodynamic calculation, Ansys finite element analysis and fracture toughness testing, et al, have been employed in present paper. Toughening mechanism of railway wheel steels has been researched through inclusion metallurgic, precision controlling of thermal process temperature parameters and special quenching process, so that toughening mechanism different from conventional way have been suggested and innovate research results have been attained.It is showed in this paper that there is makeable effect of sulphur content and its existing state on toughness of railway wheel steels. Traditionally, sulphur content was intently controlled at very low level with the purpose of toughening, but this attempting seldom worked. Distinct from traditional attempts, we find out that there is a best sulphur content scope. Fracture toughness increases about40%by increasing S from normal0.001to0.006～0.01%in this paper. And redundant safety assurance of wheels notablely increases as running at high speed. With sulphur adding in railway wheel steels, main inclusions change from naked oxides to complex inclusions which are specified as sulphides covering oxides. Stress concentration around complex inclusions essentially decreased with a cover of ductile MnS, so that microcracks original from oxides could be prevented, and that is the main reason of toughening. Adding with certain content of Ca, precipitation temperature increasing and proper decreasing in casting heating temperature of the steel, both could promote sulphides covered on oxides. Based on these work, thermodynamic model of MnS precipitation and dissolution from oxide has been proposed.It is found that another reason of wheel steels toughening is refining and uniformity of pearlite colony size. As key parameters in thermal process of wheels, deformation temperature, finish rolling temperature and tread quenching temperature must keep at lower levels, so prior austenite grain size get fined, what more, uniformity of pearlite colony size increase, which means probability of big colony size reduce, and toughness level of wheel steels increase. Further study in fracture surface reveals that single pearlite colony could form into a fracture facet and slip bands in a facet ended at pearlite colony boundaries. So it is reasonable to consider the pearlite colony as controlling submicrostructure of railway wheel steels.Adapted with interrupt tread quenching, which means a cooling way of a starting water quenching and then stop about100s and a further cooling, cooling rates and cooling fields better than that in continuous quenching could get, bainite phase transformation is depressed, hardness contribution gets uniform and pearlite colony size gets finer, toughness near tread get improved without changing in producing routine. It is indicated that interrupt quenching is more suitable for getting fine pearlite colony than continuous quenching.In summary, with introducing inclusions metallurgic technology firstly in railway wheel steels, brittle and hard, undeforming oxides could be covered by sulphides, which have similar ability of deformation with steel matrix. So that stress concentration around inclusions decrease and fracture toughness essentially improve and a new toughening mechanism is developed. At the same time, after precision controlling of thermal process temperature parameters, toughness of wheel steels improve for finer pearlite submicrostructure.