Controlled rolling of seamless steel pipe

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

Controlled rolling is a rolling method that controls the state of austenite and the state of organization of phase transformation products by controlling process parameters such as heating temperature, rolling temperature, and deformation system on the basis of adjusting the chemical composition of steel, thereby achieving the control of the structural properties of steel. Controlled rolling can also be more broadly understood as the optimal control of the entire rolling process from pre-rolling heating to the end of the final rolling pass, so that the steel can obtain the expected good performance. 

The task of controlled rolling is to control the state of austenite by controlling and optimizing rolling parameters such as heating temperature, rolling temperature of each pass in the rolling process, and reduction, so as to accumulate conditions for obtaining fine phase transformation structures in the subsequent cooling process. The key point of controlled rolling is the control of austenite state, mainly including the size of austenite grains, the level of internal energy, the number of internal defects, etc.

Controlled rolling has outstanding advantages that conventional rolling methods do not have. In summary, there are roughly the following points:

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

2. Simplify the production process. Controlled rolling can replace normalizing isothermal treatment.

3. As the comprehensive properties of steel such as strength and toughness are improved, the scope of use of steel and the service life of products are naturally expanded and increased. From the overall perspective of the production process, due to the simplification of the production process and the improvement of product quality, the cost of steel will be reduced under suitable production conditions.

4. The equipment made of steel produced by controlled rolling is light in weight, which is conducive to lightweight equipment. 

However, controlled rolling also has some disadvantages. For some steel grades, a large amount of low-temperature deformation is required, and the mill load needs to be increased. The pressure per unit roll length of the medium and thick plate mill is increased from 0.01 MN/mm to 0.02 MN/mm. Since the deformation temperature and deformation amount and other parameters must be strictly controlled, a complete range of temperature, pressure and thickness measuring instruments is required. In order to effectively control the rolling temperature and shorten the cooling time, a strong cooling facility must be provided to accelerate the cooling speed. Controlled rolling cannot meet the performance requirements of all steel grades and specifications.

Controlled rolling is a method of improving the strength and toughness of steel by mainly refining the grains. The process of austenite recrystallization after controlled 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. Recrystallization type controlled rolling.

It is to heat the steel to the austenitizing temperature and then plastically deform it. Dynamic or static recrystallization occurs during each deformation process 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 formation of fine ferrite grains after phase transformation. In order to prevent the growth of austenite grains after recrystallization, the reduction, rolling temperature and rolling interval time close to the final rolling pass should be strictly controlled. The final rolling pass should be carried out at a temperature close to the phase 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 controlled rolling is suitable for low-carbon high-quality steel, ordinary carbon steel and low-alloy high-strength steel. 

2.Unrecrystallized controlled rolling.

It is that after the steel is heated to the austenitizing temperature, plastic deformation occurs below the austenite recrystallization temperature, and no recrystallization occurs after the austenite is deformed (that is, no dynamic or static recrystallization occurs). Therefore, the deformed austenite grains are elongated, there are a large number of deformation bands in the grains, there are many nucleation points during the phase transformation, and the ferrite grains are refined after the phase transformation, which plays an important role in improving the strength and toughness of the steel. 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 controlled rolling.

It is heated to the austenitizing temperature, deformed to a certain extent, and then cooled to the austenite plus ferrite two-phase region and continued plastic deformation. Experiments show that during the rolling process in the two-phase region, dynamic recrystallization of ferrite can occur; when the deformation is moderate, the ferrite has only moderate recovery and causes recrystallization; when the deformation is small (15%~30%), the recovery degree decreases. In the high temperature zone of the two-phase region, ferrite is prone to recrystallization; in the low temperature zone 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 are appropriately selected in the two-phase region, a fine mixture of ferrite and pearlite can be obtained, thereby improving the strength and toughness of the steel.

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

1.Reduce the heating temperature as much as possible to refine the austenite grains before rolling.

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

3. Increase the cumulative reduction in the unrecrystallized zone of austenite to increase the grain area and deformation zone area per unit volume of austenite.