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Calculation of Selective Oxidation of High Strength Steels in Grain Boundary and Grain

Author: LiuSai
Tutor: SuXuPing
School: Xiangtan University
Course: Materials Science and Engineering
Keywords: selective oxidation high strength steel grain boundary annealing FISHER model
CLC: TG156.2
Type: Master's thesis
Year: 2012
Downloads: 6
Quote: 0
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Abstract


High strength steels (transformation-induced plasticity(TRIP), interstitial free(IF), complex phase (CP), dual phase (DP), and martensitic steels) are of great interestfor the automotive industry following the demand of producing lighter and stiffer carbodies from the viewpoint of fuel consumption performance. On the other hand, howto prevent rust in automotive bodies has become another great important task indevelopment of the steels. Hot galvanized steels have been extensively used in theautomotive industry due to their excellent corrosion resistance, good formability,good weldability, and lower cost. However, the addition of alloying elements (Mn, Al,Si, Cr, etc.) which are satisfied the automotive industry’s need for steels with bothenhanced formability and higher strength causes significant problems in the hot-dipgalvanizing (HDG) process. It is because that selective oxidation on the steel surfacedeteriorate the wettability of zinc on steel to form so-called “bare-spot defects”.Silicon and manganese are the main alloying elements added to the steelsubstrate. High strength steel which contains silicon and manganese was taken as anexample in this paper and calculated the possible surface oxides to analyze theselective oxidation. And the experimental analysis of the influence of manganese inhot-dip galvanized. The study found that0.4wt.%Mn increases the total thickness ofdiffusion layer. But the total thickness of diffusion layer descended when themanganese content increased to more than1.2wt.%. Manganese in the steel canpromote the growth of δ phase at the initial stage of diffusion and advance thegrowth of ζ at later. The growth of δ phase became slower in the later.The quality of galvanized layers is related to surface condition greatly.Segregation and oxidation of minor alloying elements (Mn, Al, Si, Cr, etc.) occursduring recrystallization annealing before hot-dip galvanizing which worsen thesurface condition. This paper researches the selective oxidation process of alloyingelement in grain boundary and grain during recrystallization annealing. The workfirstly introduces the FISHER model, and gets the conditions from the literatures, thenanalysis the formed oxides at difference depths to study the selective oxidation ofhigh strength steel. The results show that the diffusion of oxygen in the grainboundary is faster than grain and the presence of grain boundary oxidation is less with the decrease of grain size. The diffusion of oxygen in γ-Fe is faster than α-Fe and thedifference of concentration between grain boundary and grain in γ-Fe is smaller thanα-Fe. The concentration of oxygen is enhanced with the increase of the dew point andthe concentration of oxygen in grain is smaller than grain boundary. The discrepancyof concentration between grain boundary and grain is decreased with the reduction ofdew point but increased when the dew point down to80℃. The concentration ofoxygen is decreased with the increase of PH2and the influence factor become smallerin grain boundary and grain but concentration of oxygen is smaller in grain also. Theminimum oxygen partial pressure to form the metal oxide is increased with the risingof annealing temperature and the diffusion rate of oxygen is enhanced. Oxygen cannotdiffuse into grain at993.15K and the difference of concentration between grainboundary and grain is lessened with the rise of annealing temperature. Theconcentration of oxygen in grain approaches grain boundary at the1373.15K.The calculations of CMnSi TRIP steel at ferritic annealing in a low dew pointN2-10%H2atmosphere of-30℃show that internal oxides within a depth of50nmbelow the surface are mainly SiO2and at the grain boundaries larger MnO oxideparticles are formed. The results are fit well with the experimental results. Theapplication of Fisher model for simultaneous diffusion and precipitation of chemicalelements in metallic matrices and grain boundary is feasible.

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CLC: > Industrial Technology > Metallurgy and Metal Craft > Metallurgy and Heat Treatment > Heat treatment > Heat treatment process > Annealing
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