Nucleation of intragranular ferrite at Ti2O3 particle in low carbon steel
Introduction
Grain refinement of steel is a method which can enhance strength without deteriorating toughness among various strengthening methods of steels1, 2. Since the boundaries of prior austenite grains in steels act as preferential nucleation sites of ferrite grains rather than the interiors of prior austenite grains during the austenite–ferrite transformation, obviously the size of the austenite grains should be reduced through the hot-deformation and recrystallization process in order to refine the grain size of steels. A new approach has been recently attempted to obtain refined ferrite grains by dispersing fine non-metallic inclusions which could act as heterogeneous nucleation sites within austenite grains3, 4, 5, 6, 7, 8, 9, 10. This intragranular ferrite nucleates at a point, whereas intergranular ferrite nucleates at a surface such as an austenite grain boundary. This grain refinement method has long been used to refine the microstructure of the heat-affected zones (HAZs) of welded structures11, 12, 13, 14, 15, 16, 17, 18, 19and recently of medium carbon forging steels5, 6, 7for the improvement of toughness. It is also anticipated to be an effective strengthening or toughening method in a variety of steels8, 9, 10.
Among many non-metallic inclusions and precipitates which are known to provide heterogeneous nucleation sites in steels, considerable attention has been given to Ti-oxides which produce very interesting microstructural features in low carbon steels. In some of the Ti-containing low carbon steels, chaotic microstructures consisting of fine interlocking ferrite plates called “acicular ferrite” have been frequently observed in the HAZ following welding13, 14, 15, 16, 17, 18, 19. This acicular ferrite structure has been known to provide a desirable combination of high strength and good toughness because of its small grain size and the interlocking microstructure20, 21. This interlocking structure of acicular ferrite can deflect the propagation of cleavage cracks, thus enhancing the toughness[22].
The acicular ferrite has been known to intragranularly nucleate at Ti-oxides dispersed in austenite grains[23]. Although Ti2O3, among a number of other titanium oxides, is believed to be the most effective nucleant of intragranular ferrite in Ti-bearing low carbon steels8, 14, 17, 18, some researchers have reported that TiO particles are the most effective nucleation sites responsible for the formation of acicular ferrite13, 24, 25. Moreover, it has been a very difficult task to identify which non-metallic inclusions are effective in the formation of acicular ferrite[23]as commercial steels normally include many different non-metallic inclusions such as oxides of Al, Ca, Mg, Zr, Ti, Si and Mn, nitrides of Ti, Nb and V and sulfides of Mn, Ti, Cu and Zr. The development of local Mn-depleted zones (MDZs) around titanium oxide particles within austenite grains has been the most influential hypothesis for the intragranular nucleation of ferrite at titanium oxide particles8, 16, 17, 18, 23, 26, even though a number of different mechanisms have been proposed for nucleation at non-metallic inclusions4, 5, 6, 7, 13, 24, 26, 27.
The nucleation of acicular ferrite can be stimulated by the depletion of austenite stabilizing elements such as Mn around titanium oxide particles. However, there have been two different views on the explanation for the formation of MDZ around titanium oxide. One is that the MDZ is formed by the precipitation of MnS at a titanium oxide particle within austenite grains at high temperatures8, 16, 17, 18, 26, where MnS acts as a direct nucleation site. Another is that titanium oxide itself absorbs neighboring Mn atoms thus the MDZ being formed around it[23].
The aim of the present study is to investigate systematically the intragranular nucleation of ferrite in Ti-bearing low carbon steels with carefully controlled experiments in order to verify the role of Mn in the nucleation of intragranular ferrite.
Section snippets
Experimental procedure
Three 1 kg ingots of low carbon steels were made by vacuum induction melting and casting and then hot rolling to plates of 13 mm in thickness. The chemical compositions of the heats are given in Table 1. High-purity raw materials were used to exclude such elements as Al, Ca, P and S in non-metallic inclusions, which are commonly included in commercial steels. Titanium was added into Steels 1 and 3, but not Steel 2. The Mn concentration of Steel 1 was kept relatively high to maintain high enough
Thermodynamic calculation
The CALPHAD (calculation of phase diagrams) approach28, 29, 30, 31was employed to interpret thermodynamic stabilities of a variety of non-metallic inclusions in Ti-bearing low carbon steels. The data set of the parameters for the thermodynamic models describing the Fe–Mn–Si–Ti–C–O–N system was constructed by compiling both the Scientific Group Thermodata Europe (SGTE) solution database incorporated into the computer software, Thermo-Calc[32], and the recently published data. The details of this
Evolution of acicular ferrite
Fig. 2(a) and (b) show the microstructures of Steels 1 and 2, respectively, cooled at a rate of 5 K/s after austenitization at 1623 K for 20 min. In Steel 1, the interwoven microstructure of intragranularly nucleated acicular ferrites is dominant [Fig. 2(a)], though a minor fraction of ferrite sideplates is also found along the austenite grain boundary indicated by the arrow in the figure. The basket weave structure of the acicular ferrite contrasts with the well-aligned ferrite sideplates. Coarse
Conclusions
Systematic studies of heat treatment and diffusion bonding experiments as well as thermodynamic calculation have been carried out to elucidate the intragranular nucleation of acicular ferrite in Ti-containing low carbon steels. The fine acicular ferrite structure observed in Ti-bearing low carbon steels used in the present study originates from the heterogeneous nucleation of ferrite plates at non-metallic inclusions within austenite grains. The heterogeneous nucleation site has been identified
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