Thermodynamic Basis of Physical Metallurgy of Microalloyed Steel

The development and application of microalloyed steel is considered as the most outstanding achievement in the field of physical metallurgy in the steel industry in the 20th century. By adding trace amounts of vanadium, niobium, and titanium (usually less than 0.1%), the strength of ordinary carbon-manganese steel can be doubled, and the combination of strength and toughness of the steel  is significantly improved. In the past half a century, people have carried out in-depth and systematic research on the physical metallurgy of microalloyed steels, and accumulated rich experience and a large number of research results.

1. The physical metallurgy of microalloyed steels: reinforcement 

The reinforcement of microalloyed steel elements comes from the dispersion, precipitation and enhancement of fine carbon and nitrides, and the grain refinement of carbon and nitrides which prevent grain growth, or it is due to the combined effects of the above two. From the perspective of grain refinement, in order to keep the austenite grains small before the phase transformation, carbon and nitride particles are required to be partially insoluble in the austenite or partially analyzed during the hot rolling process. From the perspective of precipitation strengthening, the microalloying elements are required to be solid-dissolved in austenite, so that precipitation can occur during or after the phase transformation process of austenite/ferrite, and fine precipitates (that is, the particle diameter of 3nm to 5nm) can be obtained to achieve the dispersion strengthening effect.

2. The physical metallurgy of microalloyed steels: solubility

The solubility of carbides and nitrides of microalloying elements in austenite and ferrite is usually expressed by the solid solubility product of the mass fractions of microalloying elements and carbon and nitrogen.

(1) For each microalloying element, nitrides in austenite are more stable than carbides. The difference in solubility of microalloying nitrides and carbides is related to the types of microalloying elements. There is a big difference in the solid solubility product between vanadium nitride and vanadium carbide, titanium nitride and titanium carbide, while the difference in solid solubility between niobium nitride and niobium carbide is relatively small.

(2) Except for titanium nitride and vanadium carbide, the solubility of other microalloying elements nitrides and carbides in austenite is very similar. The stability of titanium nitride is much higher than that of nitrides and carbides of other microalloying elements, and the solubility difference is about 103 orders of magnitude. On the contrary, vanadium carbide is the easiest to dissolve among all microalloying elements, nitrides and carbides, and the difference in solubility is 103 orders of magnitude lower than the others.

(3) Only titanium nitride can be precipitated in liquid molten steel. The solid solubility of titanium nitride in liquid molten steel is 1-2 orders of magnitude higher than the solid solubility in austenite at the same temperature.

(4) The solid solubility of vanadium carbide, niobium carbide and niobium nitride in ferrite is about one order of magnitude lower than that of austenite at the same temperature.

3. Application of the physical metallurgy of microalloyed steels

The above-mentioned law of the change of the solid solubility product of microalloy carbides and nitrides points out the direction for the selection of microalloying elements. For example, in the application of vanadium in normalizing steel and high-carbon steel, vanadium is the most soluble compared to the two microalloying elements of niobium and titanium. It is not easy to precipitate in austenite, and will precipitate in the subsequent cooling process, thereby increasing the strength level of steel through precipitation strengthening. Titanium nitride has high stability and does not dissolve during the austenite process. It is used to control the coarsening of high-temperature austenitizing crystal grains, so as to achieve the purpose of grain refinement. These data provide a basis for the selection of microalloying elements in steel. Titanium nitride has high stability and is insoluble during the austenite process. It is used to control the grain coarsening of high-temperature austenitization, so as to achieve the purpose of grain refinement. These data provide a basis for the selection of microalloying elements in steel.

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