Blooming Mill

a high-capacity rolling mill for the reduction, into blooms, of steel ingots of large cross section, with masses up to 12 tons and larger. In some cases a blooming mill is used for the rolling of slabs and also shaped pieces (for heavy I beams, channel iron, and the like). In metallurgical plants the blooming mill is an intermediate link between the steel-casting shops and the rolling shops, which turn out the finished product. In modern plants, blooming mills work together with continuous billet mills, which turn out billets for the shape mills. Blooming mills are classified by the diameter of the rolls and occur as (1) single-stand, including double-roll reversing two-high mills (large, 1,150–1,300 mm; medium, 900–950 mm; and small, 750–800 mm; see Table 1), and triple-roll nonreversing three-high mills (750–800 mm); (2) tandem: two two-high stands placed in tandem, with 1,150 mm rolls in the first stand and 900–1,000 mm rolls in the second; (3) continuous: several nonreversing two-high stands, placed in tandem, with 800–1,000 mm rolls; and (4) specialized: single-stand two-high reversing mills, 1,350 - 1,400 mm, which turn out billets for wide-flange beams.

In the USSR in metallurgical plants, 1,150-mm blooming mills have become widespread in large-capacity plants, and 900–1,000-mm mills in medium-capacity plants. Ingots of 1 to 3 tons are reduced with small blooming mills (in the production of high-quality alloyed steels and other steels). Three-high mills were first used for the rolling of Bessemer ingots in the USA by A. Holley (1871). In subsequent years John and George Fritz and also A. Holley built mechanized three-high blooming mills for the rolling of small ingots. In 1880 in England, Ramsbottom designed a two-high reversing mill with reversible rotational direction of the rolls for the rolling of ingots up to 5 tons and larger. The two-high reversing mill acquired wide distribution as a result of the electrical reversible drive mechanism proposed by K. Ilgner (1902). In the USSR blooming mills have been manufactured since 1931; the first modern blooming mill was put into production in the Makeevka Metallurgical Plant (1933).

The composition of the blooming mill itself includes the housing and the main electric motors and machinery which turn the rolls. A blooming mill shop includes accessory equipment (overhead cranes, ingot buggy, manipulators, roller tables) and shears for cutting to the required dimensions the strips coming out of the blooming mill. The housing consists of two cast-steel frames weighing 60–105 tons which are mounted on foundation slabs, steel rolls and their bearings, the mechanism for the adjustment (raising and lowering) of the upper roll, and the mechanism for changing the rolls. The total height of the stand reaches 7–9 m. The rotation of the rolls is accomplished by means of DC electric motors. In the case of a blooming mill with one electric motor, the mechanism transmitting power to the rolls consists of two universal spindles, a gear housing with a two-high set of pinions, situated one over the other, and a trunk coupling which connects the pinion drive shaft with the electric motor shaft. In the most recently constructed blooming mills, each roll is provided with a separate motor. In this case the rotational motion is transmitted by means of drive shafts and universal spindles.

The rolling process in the blooming mill shop includes delivery of hot ingots on railroad flatcars from the foundry to the soaking pits, the preheating of the ingots in a vertical position in the soaking pits to 1100–1300° C (depending on the type of steel), delivery of each ingot in an ingot buggy to the receiving roller tables of the blooming mill, weighing of the ingot and its conveyance along a roller table to the rolls of the blooming mill, and rolling of the ingot in 11–19 passes with a reduction of 40–120 mm during a pass and with intermediate 90-degree tiltings. (The turning and moving the piece along the rolls is accomplished by the manipulator.) Arriving at the shears, the strip has its front and back ends cut off, after which it is transferred to the billet mills. Frequently the piece is cut into separate blooms or slabs, which are transferred by roller tables to the cooler and then to storage. The yield of blooms and slabs comprises 85–90 percent of the mass of the ingot. The utilization of the blooming mill permits the casting of steel into large ingots and increases the quality of the rolled product.

The design of the majority of plants producing section-shaped rolled products corresponds to the traditional layout: the blooming mill—continuous billet mills—shaping mills. The leading unit of the shaping cycle is a two-high reversing blooming mill; hence, the capacity of the entire cycle is determined by the productivity of the blooming mill.

Several plants (for example, the Kuznetsk) have attained increased productivity for a two-high reversing mill with a relatively low rolling speed (approximately 3–4 m/sec) as a result of the maximum possible increase of the reduction in each pass (as much as 100–150 mm). At other plants (for example, Magnitogorsk and also at plants in the USA, the Federal Republic of Germany, and elsewhere) the capacity is increased by means of increasing the roll speed to 6–7 m/sec without essential increase in reduction, which is 60–90 mm per pass.

In the USSR powerful, completely automated 1,300 mm blooming mills have been built and installed with an annual production of up to 6 million tons of ingots at the Krivorozh’e (1965) and Cheliabinsk metallurgical plants. According to level of productivity attained, Soviet blooming mills occupy first place in the world.

The automated control system of a blooming mill consists of separate autonomous systems, each of which controls one mechanism or a group of mechanisms and is linked with a

computer. The computer receives information on the work of the machines during the process of advancing the ingots along the line of the blooming mill and adjusts the process variables and the operating schedules of the equipment. This system makes it possible to increase the productivity of the blooming mill as a result of a more efficient management of the rolling, and it ensures better metal quality. The blooming mill equipment is more fully utilized.

The bottleneck of the blooming process was the supplying of heated ingots from the soaking pits to the receiving roller table. In the USSR a circular ingot feeding method has been developed, which completely satisfies any level of blooming mill production.

In the USSR the pouring of slabs of billets in continuous steel-casting installations has become prevalent. In many cases (particularly for large production volume) this process is more expedient and economical than using a blooming mill. This method has several advantages (in comparison with the blooming mill), which permit the reducing of the prime cost of the rolled product by 7–10 percent. The operation of several industrial continuous casting installations has shown that this method is more economical than pouring the metal into molds with subsequent rolling of the ingots in a blooming mill. The prospects appear good for combining of continuous casting with subsequent rolling of the cast semifinished products (blooms).