Controlled rolling of seamless steel pipe

Controlled rolling refers to a rolling method that uses technological measures such as enhanced reduction and controlled cooling to improve the strength, toughness and other comprehensive properties of hot-rolled steel under conditions that are slightly lower than conventional rolling temperatures. Controlling the properties of rolled steel can match or exceed the properties of existing heat-treated steel.

Controlled rolling is based on adjusting the chemical composition of steel and controlling the heating temperature, rolling temperature, deformation system and other process parameters to control the austenite state and the organizational state of phase transformation products, thereby achieving the rolling control of the structural properties of the steel. system method. Controlled rolling can also be understood more broadly as optimal control of the entire rolling process from heating before rolling to the end of the final rolling pass, so that the steel can obtain the expected good properties.

The task of controlling rolling is to control the austenite state through the control and optimization of heating temperature, rolling temperature of each pass in the rolling process, reduction amount and other rolling parameters, so as to obtain fine particles in the subsequent cooling process. Phase change organization and other accumulation conditions. The key point of controlling rolling is the control of austenite state, which mainly includes the size of austenite grain size, the level of contained energy, the number of internal defects, etc.

Controlled rolling has outstanding advantages that conventional rolling methods do not have. It can be summed up as follows:

(1) Many test data show that the comprehensive mechanical properties such as strength and toughness of steel produced by controlled rolling methods are greatly improved. For example, controlled rolling can refine the ferrite grains, thereby increasing the strength and toughness of the steel.

(2) Simplify the production process. Controlled rolling can replace normalized isothermal treatment.

(3) Due to the improvement of the comprehensive properties such as strength and toughness of steel, the scope of use of steel and the service life of products have also been expanded and increased. From the overall production process, due to the simplification of the production process, the improvement of product quality, and under suitable production conditions, the cost of steel will be reduced.

(4) Equipment made of steel produced by controlled rolling is light in weight, which is conducive to lightening the equipment.

However, controlled rolling also has some disadvantages. For some steel types, large low-temperature deformation is required, and the rolling mill load needs to be increased. The pressure per unit roll length of the medium and thick plate rolling mill is increased from 0.01 MN/mm to 0.02 MN/mm. Since parameters such as deformation temperature and deformation amount must be strictly controlled, it is necessary to have complete temperature measurement, pressure measurement, thickness measurement and other instruments; in order to effectively control the rolling temperature and shorten the cooling time, there must be strong cooling facilities to accelerate Cooling rate. Controlled rolling cannot meet the performance requirements of all steel grades and specifications.

Controlled rolling is a method that mainly refines the grains to improve the strength and toughness of steel. Controlling the austenite recrystallization process after rolling plays a decisive role in obtaining a fine grain structure. According to the conditions for plastic deformation of austenite, controlled rolling can be divided into three types:

(1) Controlled rolling of recrystallized type.

It heats the steel to the austenitizing temperature and then undergoes plastic deformation. Dynamic or static recrystallization occurs during the deformation process of each pass or between two passes, and the recrystallization process is completed. After repeated rolling and recrystallization, the austenite grains are refined, which provides a prerequisite for the generation of fine ferrite grains after phase transformation. In order to prevent the growth of austenite grains after recrystallization, the reduction, rolling temperature and rolling gap time close to the final rolling passes must be strictly controlled. The final rolling pass is mainly carried out at a temperature close to the transformation point. In order to prevent the growth of austenite grains before phase transformation and ferrite grains after phase transformation, it is particularly necessary to control the cooling rate after rolling. This kind of controlled rolling is suitable for low-carbon high-quality steel and ordinary carbon steel and low-alloy high-strength steel.

(2) Non-recrystallized controlled rolling.

It means that after the steel is heated to the austenitizing temperature, plastic deformation occurs below the austenite recrystallization temperature, and no recrystallization occurs after austenite deformation (that is, no dynamic or static recrystallization occurs). Therefore, the deformed austenite grains are elongated, and there are a large number of deformation bands within the grains. There are many nucleation points during the phase transformation process. The ferrite grains are refined after the phase transformation, which is helpful for improving the strength and toughness of the steel. important role. This control process is suitable for low carbon steel containing trace alloy elements, such as low carbon steel containing niobium, titanium, and vanadium.

(3) Two-phase zone controlled rolling.

It is heated to the austenitizing temperature, undergoes certain deformation, and then cooled to the austenite plus ferrite two-phase region before continuing to undergo plastic deformation. Experiments show that during the rolling process in the two-phase zone, dynamic reforming of ferrite can occur; when the deformation is medium, the ferrite only recovers moderately and causes recrystallization; when the deformation is small (15%~30 %), the degree of recovery decreases. In the high-temperature region of the two-phase region, ferrite is prone to recrystallization; in the low-temperature region of the two-phase region, only recovery occurs. The rolled austenite phase transforms into fine ferrite and pearlite. Since carbon is enriched in the austenite in the two-phase region, carbon precipitates as fine carbides. Therefore, as long as the temperature and reduction amount are appropriately selected in the two-phase zone, a fine ferrite and pearlite mixture can be obtained, thereby improving the strength and toughness of the steel.

Controlled rolling is a technology that artificially forms as many lattice heterogeneous ferrite phase transformation nuclei as possible in austenite and effectively refines the ferrite grains. The technical key points of controlled rolling are summarized as follows:

(1) Reduce the heating temperature as much as possible to refine the austenite grains just before starting rolling.

(2) Optimize the rolling pass program (pass reduction) in the intermediate temperature zone (for example, 900°C or above) and refine the austenite grains through repeated recrystallization.

(3) Increase the cumulative reduction of the austenite non-recrystallized area and increase the austenite grain area and deformation zone area per unit volume.