Application of Nb element as grain refinement material in cast steel

Nb has strong affinity with oxygen, nitrogen and carbon in steel, and its function is similar to that of Ti. Nb can combine with it to form stable compounds.Compounds of Nb in steel can be classified into two categories:

One type is oxides with NbO and NbO2.NbO has a high melting point of 1 935 C and is less incompatible with the delta-ferrite lattice. In steel it is possible to form heterogeneous nuclei precipitated from delta-ferrite.NbO2 does not match the delta-ferrite lattice to a great extent and can not become heterostructure nucleus.

The other is carbide (NbC) and nitride (NbN), which are easy to dissolve into carbon and nitride [Nb (C, N)] of Nb.

In the early stage, adding a small amount of Nb into the steel can improve the strength. It is thought that the mechanism of action is grain refinement, because the melting point of NbO formed in the liquid steel is high and it can be used as the heterogeneous nucleus of solidification and crystallization.It is also considered that the mechanism of Nb increasing strength is precipitation hardening.

Recently, in order to identify the mechanism by which Nb strengthens steel, Tuttle has carried out further experiments and studies on the role of Nb in steel .

The test steel is medium carbon steel (US No. 1030) with about 0.3% carbon content. It is melted in induction furnace and added NbO, NbO2 and ferroniobium alloys into the liquid flow during final deoxidization. Then, its influence on the structure and properties of the steel is evaluated.

Both NbO and NbO2 are made from fine powder with 99% purity and passed through 325 mesh screen, with the addition amounts of 0.05% and 0.10% respectively.The amount of ferroniobium alloy is calculated as 0.07% and 0.14% of Nb added.

When Nb or its oxides are added to the molten steel, the strength of the steel increases, but the elongation decreases.

When Nb or its oxides are added, there are fewer needle ferrites and more equiaxed grains in the microstructure, but the grain size is larger than that of steel without Nb or its oxides.This means that when Nb is added to the steel, no heterogeneous nuclei will be formed.

The observation and analysis of inclusions in steel give us a further understanding of the role of Nb in steel.

The results of SEM observation and energy scattering spectrometer analysis show that when NbO or NbO2 is added to the steel, there is no separate NbO or NbO2 in the structure, which acts with Al2O3 and FeO in the steel and forms a compound inclusion.Inclusions are the same in steels added with ferroniobium alloys.

It can be seen that although the melting points of NbO and NbO2 are very high, such as the melting point of NbO is 1 935 C, they are not stable in molten steel. Therefore, they can not be used as heterogeneous nuclei for solidification and crystallization of molten steel.

In order to further confirm this view, Tuttle carried out phase equilibrium tests, considering that there is little data on the thermodynamic stability of NbO and NbO2 in molten steel.

Experimental process: 3 small crucibles with caps, each filled with pure iron particles of 40g, then loaded with 2.5 g NbO, 2.5 g NbO2 and 3G ferroniobium alloy respectively, then heated to 1590 C at a rate of 5 C/min, and cooled for 8 h after holding for 2 h.

The results of study and analysis show that the inclusions in the samples with NbO and NbO2 are the same as those from the castings without separate NbO and NbO2.

No inclusions of Nb were found in the specimens added to the ferroniobium alloy because all Nb was dissolved in iron.

Therefore, it can be affirmed that NbO is unstable at temperatures far below its melting point (1 590 C), so it is impossible to be used as heterogeneous nucleus for solidification and crystallization of molten steel.

Adding a small amount of Nb to the steel can improve the strength, possibly due to precipitation hardening, which is easy to dissolve in the steel and may precipitate fine carbon and nitride in the grain during cooling after solidification, thus restraining grain dislocation.

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