Intragranular Ferrite Formed in a V–Ti–N Medium‐Carbon Steel Containing MnS Inclusions

Abstract

This paper studies the formation of intragranular ferrite in a V-Ti-N medium-carbon steel containing MnS inclusions. The (Ti, V)(C, N) particles precipitating on MnS inclusions are confirmed to be the main nucleation sites of intragranular ferrite. Because of the small lattice mismatch and low-energy ferrite/V(C, N) interphase boundary, the vanadium-rich edge of (Ti, V)(C, N) particles increases the nucleation potency of intragranular ferrite. The interphase of MnS inclusions and austenite provides suitable sites for (Ti, V)(C, N) particles to grow big enough to reach the critical size for intragranular ferrite nucleation. Furthermore, the efficiency of the formation of intragranular ferrite starts to increase rapidly when the length of MnS inclusions decreases to less than 10 mm. The shape of intragranular ferrite is also significantly influenced by the length of MnS inclusions. In addition, the intragranular ferrite is confirmed to have a new type of orientation relationship with the particles precipitating in liquid phase, which indicates that the orientation relationship between the intragranular ferrite and (Ti, V)(C, N) particles is not sole, but diversified.

Effect of Zr, Al Addition on Characteristics of MnS and Formation of Intragranular Ferrite in Non-Quenched and Tempered Steel

“…The sizes of most V (C, N) particles are about 0.2-0.3 μm, which are suitable for IPF nucleation. 20)…”

“…It has been proved that the V (C, N) particles precipitated on MnS inclusion have a B-N relationship with α-Fe, which is critical to the formation of IPF. 6,20) Recent researches 20) indicate that the size of MnS inclusions has a significant influence on the utilization efficiency of the nuclei for IPF formation and with the increase of MnS inclusions length, the utilization efficiency of the nuclei has been improved remarkably. As stated above, the sizes of pure MnS inclusions and compound inclusions in Steel 2 are both much smaller than those in Steel 1.…”

Effect of Mg on the Evolution of Inclusions and Formation of Acicular Ferrite in La–Ti‐Treated Steels

“…MDZ is the other mechanism for AF nucleation. The origin of MDZ is mainly in two aspects: 1) the Ti oxides absorb Mn atoms from the steel matrix and 2) MnS precipitates on inclusion during the cooling stage . In this work, Ti existed mainly in the form of TiN rather than Ti oxide and the content of Mg did not affect the formation of TiN.…”

“…The origin of MDZ is mainly in two aspects: 1) the Ti oxides absorb Mn atoms from the steel matrix and [17,18] 2) MnS precipitates on inclusion during the cooling stage. [12,19,38] In this work, Ti existed mainly in the form of TiN rather than Ti oxide and the content of Mg did not affect the formation of TiN. Figure 6 shows that MnS was usually distributed in the outer surface of inclusions in the samples.…”

The Coarse Microstructure of Medium‐Carbon Microalloyed Steel Crankshafts: Formation Mechanism and Finish Forging Control

“…On one hand, titanium has been added to the medium-carbon vanadium microalloyed steels, for the (Ti, V)(C, N) particles can strongly inhibit austenite grain growth at high temperatures. [1][2][3][4][5][6] On the other hand, the intragranular ferrite (IGF) nucleated on particles in austenite has been used to refine the coarse microstructure of forged steels. [7][8][9][10][11][12][13][14][15] The aforementioned two methods can increase the high angle boundaries (≥15 ) in steels, which can provide effective barriers to cleavage fracture and improve the toughness.…”