| In recent years, as the development of metallurgical and automobile industry,interstitial free steel(IF steel) has been widely used in industrial manufacturing, whichhas replaced?rimmed steel?and?killed steel?as the third generation of deep drawingsteel. The IF steel, with a perfect deep drawing performance, achieves its highstrength by solid solution strengthening(the strengthening elements used to be P, Siand Mn). At the same time, however, the solute atoms tend to segregate to grainboundaries thereby causing a sharp increase of brittleness.Some researches have shown that P grain boundary segregation is one of themost important reasons for the embrittlement of ferritic materials, especially for IFsteel. This is mainly because there are no extra dissolved C and N atoms at grainboundaries in IF steel so that the grain boundary is quite smooth and its strength israther low. Hence the grain boundary segregation of P in IF steel may considerablyreduce the grain boundary cohesion, which would further lead to a sharp increase ofbrittleness. There are many factors that can cause P grain boundary segregation andthis paper focuses mainly on thermally-induced P grain boundary segregation and itseffect on the brittleness of a P-strenghtened IF steel.In order to study the effect of P grain boundary segregation on the brittleness ofthe IF steel, the samples were firstly heated at960?for austenitization and thenquenched to room temperature. Next heat the samples at880?and hold there for30min followed by ice water quenching to achieve a unified microstructure of ferrite.The samples were aged at520?,600?and680?, respectively. It is observed bymetallographic analysis that all the samples with different aging conditions have thesame grain size, which is about225?m, and all the microstructures are pure ferrite.The hardness of all the samples measured by Vickers Hardness Tester is about92HV.Therefore, the influence of hardness and microstructure on brittleness should benearly the same for all the samples.In this research, the grain boundary concentration of P was examined by meansof Auger electron spectroscopy(AES). Through analyzing the Auger spectrums of thesamples, it is shown that the equilibrium grain boundary segregation concentrations ofP are approximately23at.%at520?,19.2at.%at600?and16.5at.%at680?,respectively. According to the theory of equilibrium grain boundary segregation, theimportant thermodynamics parameters of P grain boundary segregation can be calculated. The segregation entropy(?S) and enthalpy(?H) of P grain boundarysegregation are about28.7J/mol K and16.3kJ/mol, respectively. In addition, thephenomenon of P non-equilibrium grain boundary segregation in the IF steel wasobserved during aging at680?. The critical time of P non-equilibrium grainboundary segregation at680?is estimated to be about30min. The variation trend ofP grain boundary concentration over holding time at680?is consistent with thedynamic curve obtained by the theoretical calculation. So the theory ofnon-equilibrium grain boundary segregation has been verified to be effective inpractice.The ductile-to-brittle transition temperatures(DBTTs) of all samples aged at680?were measured through fracture appearance transition measurements andfound that the trend of DBTT changing with ageing time is in accordance with that ofP grain boundary concentration, showing a linear relationship between P grainboundary concentration and DBTT. It is obtained by linear fitting thatDBTT(?)=4.43Cp64.76?where Cprepresents the phosphorus grain boundaryconcentration in at.%. At last, this thesis figured out the relationship between DBTTand ageing temperature through associating the theory of grain boundary segregationwith DBTT and P boundary concentration relationship, and then analyzed theinfluence of P concentration in the matrix and ageing temperature on the DBTT of thesteel. |
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