Small steel section continuous rolling production process and equipment

Author: Chief Editor: Li Peilu
Publisher:
Publish Date: 1999-04-01
Features:
This book is compiled based on the collection and organization of production processes, equipment, and experience of small-section steel strip continuous rolling mills at home and abroad. It consists of 10 chapters, introducing the development, types, and scale of small-section steel strip continuous rolling mills; main types of rolling mills; billet selection and cleaning; billet heating and the heating furnaces used; the pass systems and their design methods and guide devices for rolling round bars, ribbed steel rebars, angle steel, and irregular-section steel in small continuous rolling mills; examples of various pass systems; selection of rolls, installation, and adjustment of rolling mills; application of control rolling technology in small continuous rolling mills and finishing processes of rolled materials; types, parameters, and structures of main and auxiliary equipment in small continuous rolling mills; and automatic control of small continuous rolling mills. The book includes 479 figures and 125 tables. It is mainly intended for engineering technicians engaged in steel production, design, and research, and can also be used as a reference for students and teachers of relevant majors in colleges and universities.
Excerpt:
2.4.2.2 Workshop Production Processes and Main Characteristics
A High-Alloy Steel Rod and Wire Products
This workshop is a rod and wire mill producing high-alloy steel, with an alloy ratio of 76.7%, and high-alloy ratio reaching 13.3%. The billets used are mainly rolled billets and forged billets, with a portion of continuous casting billets also used. The maximum size of wire products from high-speed wire rolling mills can reach φ20mm, and the maximum size of bar products can reach φ40mm. For high-alloy steel rolling mills, such a product range already covers a considerable scope, so the planned large-coil production line is temporarily postponed, and the market demand will be considered later.
B Full-Continuous Rolling of High-Alloy Steel
Full-continuous rolling can more effectively control the temperature of the rolled material, which is particularly beneficial for alloy steel, especially high-alloy steel. At lower rolling speeds (rough and intermediate rolling), the temperature of the rolled material slightly decreases (reduced significantly compared to lath rolling mills), and measures such as heat preservation rails can be used at this time. As the rolling speed increases, the deformation heat of the rolled material keeps the temperature relatively stable. At high speeds, the temperature of high-alloy steel rolled material rises rapidly, so a water cooling device is set up before the final rolling to control the temperature. The entire line adopts an alternating horizontal/vertical layout for untwisted rolling, avoiding twisting of the rolled material during rolling, which could lead to cracks at the corners. The elliptical-round pass system ensures more uniform deformation, preventing cracks at the corners due to temperature reduction and simplifying the adjustment for changing product specifications. The headstock and intermediate rolling units are decoupled, allowing the speed of the rough rolling units to be adjusted independently to meet the rolling speed requirements of different alloy steels.
C Alloy Steel Temperature Control System
The water cooling line is arranged along the rolling line, cooling and equalizing the temperature at the optimal distance between frames. Precise adjustment of the temperature curve between frames and after the rolling mill is possible based on the characteristics of different steel grades, ensuring the metallurgical and mechanical properties of various final rolling products in the product range. The cooling programs that can be utilized are:
(1) Thermomechanical Rolling and Tempcore Process, which can improve the mechanical properties of the product.
(2) Normalizing Rolling and Austenitic Quenching (austenitic stainless steel), which can save subsequent heat treatment.
The CCT (Control Cooling Technology) computer model developed by SMS for offline calculation and online control of rolling processes and operating temperatures has a major new feature: measuring the temperature of the rolled material at different points in the rolling mill and comparing it with reference values. Closed-loop loop control ensures that the rolling line and cooling section automatically adjust to reference values. The entire rolling process can be simulated offline, and multiple reference values can be stored in the computer. Such a computer model avoids the need for extensive trial materials and time to determine the best rolling and cooling conditions, which are essential for achieving optimal product quality.
a Rod Finishing Temperature Control
Controlling the final finishing temperature (near Ac3) with appropriate compression (about 40%) allows low- and medium-carbon steel, as well as medium-alloy steel such as alloy structural steel, spring steel, and bearing steel, to achieve ideal microstructure and optimal mechanical properties. To this end, cooling water tanks are installed before the last two stands of the rod finishing mill, and a temperature equalizing section is set up before the rod finishing mill to ensure uniform temperature inside and outside the rapidly cooled rolled material. A water cooling tank is also installed after the rod finishing mill to rapidly cool high-quality carbon steel and austenitic stainless steel (online quenching as low as 500℃) after rolling. For steel grades requiring slow cooling, such as alloy tool steel, high-speed steel, and martensitic stainless steel, the rolled material is first moved from the hot bed to the rapid transfer machine before the cooling bed, then cut to length and slowly cooled in a slow cooling tank.
b Wire Finishing Temperature Control
To control the finishing temperature of the wire, two pre-water cooling tanks are installed before the finishing mill, and the interstand cooling of the untwisted finishing mill is enhanced. After exiting the finishing mill, the wire enters a delay-type Stelmo control cooling line for cooling. High-temperature tank cooling equipment is installed after the delay section of the roller way for slowly cooled alloy tool steel, high-speed steel, and martensitic stainless steel wire. After the wire is collected by the high-temperature coiler, it is transported by a wire coil transport machine, and a robot loads the coils into the slow cooling tank. After control cooling, the wire is collected again at the coiling station, further cooled on a hook-type transport machine, then bundled and weighed before being stored.
D Large-Coil Production Line (Reserved)
The large-coil production line consists of guide rails, water cooling lines, double grip feeders, and two Garret coilers. Equipment is installed after the coilers to transfer coils from the coilers to a chain transport machine. The following cooling programs can be applied to coils on these two Garret coilers:
(1) Coils are manually removed from the drums and placed on a walking beam transport machine for standard cooling.
(2) Direct quenching on the drums during coiling is performed, primarily for austenitic stainless steel, with the aim of:
1) Reducing the risk of surface scratches during hot coiling;
2) Producing products suitable for medium-level drawing without the need for controlled grain size, thus eliminating the need for conventional solution quenching.
Online quenching can also be used for ferritic stainless steel to reduce the risk of surface scratches during hot coiling. The direct quenching on the drums of the Garret coiler is fully immersed in water, and coiling is also completed in water. This avoids temperature uniformity loss due to operations between coiling and quenching, as both operations are performed simultaneously.
2.4.3 Special (High-Precision Square and Flat Steel) Alloy Steel
Although some small alloy steel rolling mills produce square steel, flat steel, and hexagonal steel in addition to round bars, the majority of small rolling mills only produce round bars. The increasing application of non-cutting stamping and molding processes in machinery and electronics industries has driven the rapid development of mold manufacturing at a rate of 15%–20% per year, raising the requirements for both the quantity and quality of mold steel. This demands that the thickness accuracy of flat steel for mold manufacturing not exceed 0.1mm, the width dimension accuracy not exceed 0.2mm, and the corner radius not exceed 0.5mm. This ensures minimal machining of the mold, reducing manufacturing costs and shortening the product update cycle. To meet the demand for high-precision, multi-variety flat steel and square steel for molds and tools, a special reversible continuous rolling mill designed for high-precision flat and square steel has emerged. This is the reversible continuous rolling mill for flat and square steel designed and manufactured by Austria's GFM company (a manufacturer of forging production technology and equipment).
The FS Steel Plant, aiming at the growing demand for mold steel in domestic and Asian markets, was the first to build such a flat and square steel rolling mill. The main machine WF40－5 rolling mill was introduced from GFM, while the process and electrical control equipment of other rolling lines, as well as production software, were designed by the Beijing Iron and Steel Design and Research Institute and supplied domestically. The workshop was put into operation in October 1996, with the plant layout shown in Figure 2-13.