Controlling the rolling of the rolling mill can strengthen and toughen the steel. The essence is to control the metallurgical process (such as austenite in recrystallization, alloying elements and carbon and nitride solid solution and precipitation, phase transformation, work hardening, texture, etc.) of the steel during the whole rolling process by adjusting various rolling process parameters (such as heating temperature, deformation amount, finish rolling temperature, post-rolling cooling), and finally achieve the purpose of controlling steel structure and properties.

The following are the three main mechanisms for controlling the rolling of rolling mills to increase the strength and toughness of steel.

1. Grain refinement

For hypoeutectoid steel, the finer the ferrite grains, the higher the strength of the steel and the better the toughness. The smaller the austenite grains before the phase transformation, the smaller the ferrite grains after the phase transformation. Controlled rolling can be used to refine austenite grains by two methods; one is austenite processing and recrystallization alternately to refine grains; the other is rolling in austenite non-recrystallization zone.

Reducing the heating temperature of the billet to obtain smaller original austenite grains, increasing the deformation amount per pass, and lowering the finish rolling temperature, all of which are beneficial to the refinement of austenite recrystallized grains.

In order to achieve rolling in the austenite non-recrystallization zone, it is necessary to increase the recrystallization temperature of austenite, and when the steel contains trace elements such as barium, titanium, vanadium, etc., it has such an effect. Since the carbides and nitrides of these elements are precipitated from austenite, the austenite recrystallization can be remarkably suppressed, thereby effectively increasing the austenite recrystallization temperature and allowing the rolling process to proceed in the amorphous region.

2. Carbon and nitride strengthening

Vanadium, niobium and titanium are elements formed by relatively strong carbides or nitrides, and their carbides or nitrides strengthen the structure and properties of steel.

Carbides and nitrides dissolve in austenite at high temperatures, and austenite precipitates after ferrite transformation, which directly acts as a dispersion strengthening agent for steel.

3. Subgranular strengthening 

The change in the austenite grain is the same as that in the case of rolling in the austenite + ferrite region and rolling below the austenite recrystallization temperature. The transformed ferrite grains are strengthened by rolling (deformation) to produce subgrains, dislocations, and the like. The steel rolled in the two-phase region becomes a mixed structure of ferrite grains (deformation after deformation) and ferrite grains containing sub-products (deformation after transformation), thereby increasing the toughness and strength of the steel.