NIPPON STEEL TECHNICAL REPORT No. 99 JANUARY 2009 - 85 - T E C H N O L O G Y Fi
NIPPON STEEL TECHNICAL REPORT No. 99 JANUARY 2009 - 85 - T E C H N O L O G Y Fig. 1 Layout of Hikari Wire Rod & Bar Mill before installation of RSB 1. Introduction The Hikari Wire Rod & Bar Mill began commercial operation in 1955 as Japan’s first fully continuous wire rod mill (for carbon steel). The Mill started production of stainless steel wire rods in 1963. To- day it is dedicated to the production of stainless steel bars and wire rods. In the meantime, various improvements were introduced in the Mill. For example, the processes after the No. 2 intermediate train were modernized in 1981 and in-line solution treatment was intro- duced in 1982. In 1995, new technologies to reinforce the Mill were developed and introduced. They include the highly efficient billet heating system (a complex heating system that combines a walking beam-type reheating furnace and an induction heater) for improving product quality, and the helical rolling mill for implementing an in- line blooming process. Stainless steel bars and wire rods which undergo secondary and tertiary forming processes are widely used in the forming of screws, bolts, pins, shafts, springs and welding rod for automobiles, office automation equipment, etc. Because of their many uses, stainless steel bars and wire rods are manufactured in a wide variety of grades. Thus, they are characterized by small volume production in various sizes and grades. At the Mill, wire rods (5.5- to 34-mm φ ) and bars (22- to 62- mm φ ) are rolled by one strand. Wire rod is finish-rolled by a no- twist mill (NT mill) and wound by a laying head. A thicker wire rod (bar-in-coil) is wound on a pouring reel, and bars are pooled on a cooling bed installed on a line branched out from the intermediate train for rolling wire rods (see Fig. 1). In recent years, market demand for stainless steel products has become increasingly rigorous. Accordingly, the demand for higher dimensional accuracy, shorter delivery time and fewer surface de- fects in stainless steel bars and wire rods as materials for stainless steel products has become conspicuous. Under such conditions, to improve product quality and produc- tivity, we introduced a high-rigidity, 3-roll mill (reducing/sizing mill: RSB) made by Kocks and a new type of conveyor line in November 2002. Looking at the rolling equipment, the RSB was installed after the Outline of Free Size Rolling for Hikari Wire Rod and Bar Mill intermediate train as the finishing mill for thicker wire rods and bars to improve their dimensional accuracy and implement size-free roll- ing. In addition, the dimensional accuracy of thinner wire rods was improved by stabilizing the dimensions of steel material at the NT mill entrance after introduction of the RSB. Furthermore the time required for roll changing was shortened by adopting roll assemblies for the RSB rolling stands. As a measure to reduce surface defects, the repeater that had been used to guide the steel from the intermediate train to the pre-finish- ing mill was replaced with a new conveyor line using continuous roller guides. As a result of the above modifications, we were able to improve the dimensional accuracy, shorten the roll-changing time by imple- menting size-free rolling, and reduce surface defects. In this paper, we describe in detail the modification work, with the focus on development of the technology for size-free precision rolling that was industrialized for the first time in the world through introduction of an RSB. 2. Basic Concept To meet increasingly rigorous customer needs, at the Wire Rod & Bar Mill, we decided to reinforce the overall capabilities of the rolling lines from the following standpoints. 1) Improving the dimensional accuracy of all products—thin wire rods, thick wire rods, bars (the difference between maximum diameter and minimum diameter within 0.15 mm)1). 2) Implementing size-free rolling (i.e., rolling products of varying sizes without changing rolls) on a 3-roll mill while meeting this requirement: diameter difference ≦ 0.15 mm, within 9% of di- ameter for rolling sizes < 34-mm φ and within 1.5 mm for roll- ing sizes ≧34-mm φ . 3) Improving the productivity by speeding up the roll-changing work (shortening the roll-changing time). 4) Improving the product surface quality by preventing the occur- rence of surface defects during transportation in the rolling proc- ess. To meet all the above objectives, we decided to install an RSB after the intermediate train. The RSB permits using either a pinpoint caliber having grooved rolls for each different rolling size to allow for precision rolling or a size-free caliber. We adopted the size-free caliber to implement size-free rolling. Since thick wire rods (16- to 34-mm φ) and bars are finished by the RSB, size-free rolling can be applied to them. For thin wire rods finished by the NT mill, it is possible to improve the dimensional accuracy of the entry steel material. The dimensional accuracy of steel products finished by the NT mill depends on the dimensional accuracy of the entry steel material and the characteristics of the NT mill. During a preliminary study, we conducted a rolling test on an actual mill using entry steel mate- rials of varying dimensional accuracy. As a result, we confirmed that NIPPON STEEL TECHNICAL REPORT No. 99 JANUARY 2009 - 86 - Outline of Free Size Rolling for Hikari Wire Rod and Bar Mill Fig. 2 Layout of Hikari Wire Rod & Bar Mill after installation of RSB Fig. 3 Appearance of RSB Fig. 4 Spread ratio2) Table 1 Specifications of RSB Manufacturer Kocks GMBH & Co. (Germany) Stand 17 18 19 20 Roll (mmφ) 380×75 380×75 380×75 380×75 Motor (kW) DC450 DC750 DC450 DC200 Base/top (rpm) 650/1 750 500/1 000 650/1 750 650/1 750 Gear ratio 1/4.63, 1/2.43 1/2.16, 1/1.18 1/4.64, 1/1.92 1/4.64, 1/1.92 Interval (mm) 720 720 720 - by using entry steel material with a high dimensional accuracy, it was possible to improve the dimensional accuracy of NT mill-fin- ished steel products, that is, thin wire rods. Thus, by installing the RSB after the intermediate train, it was possible to improve the di- mensional accuracy of all wire rods (5.5- to 34-mm φ ). For steel products finished by the NT mill, it is necessary to trans- port them from the RSB exit to the NT mill. To prevent slide marks on them during transportation, we decided to adopt a new conveyor line using continuous roller guides. 3. Basic Specifications Fig. 2 shows the layout of the rolling line after the modification. The layout of the new facilities has the following characteristics. 1) The RSB was installed after the intermediate train to permit im- proving the dimensional accuracy of all sizes of products. 2) The steel product is transported from the RSB to the NT mill by a new conveyor line using roller guides to prevent it from sliding over the conveyor line. This new conveyor line was connected to the existing conveyor line. In addition, the conveyor line from the RSB to the pouring reel was renewed. 3) The No. 1 profile meter was newly installed after the RSB to permit feeding back dimensional data to the RSB rolling proc- ess. 4) The RSB installation work was conducted during operation of the existing rolling line to minimize the production stoppage pe- riod. 4. Contents of Modification of Rolling Equipment 4.1 Outline of modification The principal specifications of the RSB introduced are shown in Table 1, and its appearance is shown in Fig. 3. The salient features of the RSB are as follows. All these features address the purposes of the modification. 1) Since the RSB is a 3-roll type mill, the spread ratio of rolled steel is smaller than that on a 2-roll type mill. This is suitable for roll- ing with high dimensional accuracy (see Fig. 4)2). 2) The three-shaft drive system eliminates the need for the bevel gears inside the rolling stand, and the increased roll shaft diam- eter gives a high stiffness of 250 tons/mm to the roll. These are also suitable for rolling with high dimensional accuracy. 3) The stand-to-stand distance is comparatively short (720 mm). This is advantageous from the standpoint of preventing the steel material from buckling and falling. 4) The gap between the roll and the guide roller can be varied in increments of 0.01 mm by remote operation. 5) In order to implement size-free rolling, an independent drive system in which each of the four stands is driven by a motor was adopted. Therefore, there is no need to use rolls of the same di- ameter in the individual stands. 6) The rolling stands are of a unit type and can be changed all at once. The time required for changing stands on-line is about five minutes. 4.2 Development of size-free rolling technology 4.2.1 Outline of size-free rolling Size-free rolling means rolling steel materials of varying cross- sectional sizes using a single grooved roll. In the 3-roll method, the entire circumference of the steel material is rolled by three uploads/Voyage/ guide - 2023-05-30T063207.505.pdf
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