Mill rolls. Rolls for rolling mills
As a joint owner of the South Korean machine building plant BUKANG (Incheon city) that was founded in 1986 and currently is a leading manufacturer in the South Korea specializing in production of interchanging equipment and spare parts for various metallurgical units, manufacturing and engineering company ENCE GmbH offers rolls of rolling mills (mill rolls).
- General description
- Roll components. Roll features and parameters
- Sheet and grooved rolls. Flat rolls
- Cogging section lines rolls
- Production of roll blanks
- Modern methods of rolls production. Centrifugal casting
- Forged rolls
- Material of construction for rolls
- Rolls application. Rolls durability and hardness
- Rolls for hоt sheet rolling mill.
- Roll for cold sheet rolling mill.
- Quality control
General description of rolling mills
Central operation during rolling process that performed by mill rolls is metal deformation. This is achieved by metal drafting creating specified dimensions and cross profile. As rolls rotate they transmit effort arising during rolling process to components of rolling mill stand.
See fig.1 with schematic drawing of a rolling mill
Fig. 1 – Schematic drawing of a rolling mill
1- rolls in a stand; 2- foundation; 3- Wobbler; 4- spindle; 5- mill stand; 6- gear box; 7- coupling; 8- reducer ; 9- electric motor
Fig. 2 (a and b)
Roll components. Roll features parameters
Three main components of rolling mill rolls are (see fig. 2a):
- roll body (Ø D and length L); it contacts directly deformed metal during rolling process; - support part or necks (Ø dn and length ln); they are located on one and another sides of the body supported by roll bearings;
- roll drive end.
Characterizing parameters of section mill rolls are work diameter (Db), caliber diameter and nominal diameter (Dn). In case roll to be rotated with a universal shaft, roll drive end shall have a form of flat blade or cylinder (to seat a bush with blades on it). In case roll shall be rotated with a wobbler, roll drive end shall have a form of crosspiece for roll connection with shaft (connection via intermediate wobbler coupling).
Sheet and grooved rolls. Flat rolls
Mill rolls can be sheet and grooved. Sheet rolls are meant for sheet, strip and band rolling. These rolls are known as flat rolls, their body is cylindrical. Grooved mill rolls are meant for section material with round or square section, rail and H-beam rolling.
At grinding of roll body in a roll-grinding workshop (for sheet hot rolling), they get a concave chamber. Metal warms up during rolling, so does central roll part. Warmed-up roll body turns cylindrical. For sheet cold rolling roll body is ground to have a little convex chamber.
During rolling process roll center part bends greatly in comparison with roll edges, roll generating line gets cylindrical.
Rolling material gets correct dimensions along the width.
Fig. 2a. Flat rolls
Fig. 2b. Grooved rolls
Grooved rolls of section rolling mills are for rolling of workpieces and section material.
Bodies of these rolls have so called grooves (passes). They track profile of section metal. These grooves have a definition; they are called “passes”. Passes of two rolls with distance between them are called a groove. Rolls themselves are defined as grooved roll (shaped). Please see grooved roll on fig. 2b.
Body length of sheet roll mounted on sheet rolling stand is determined by max. width of rolling material (sheet, strip).
Body length of grooved roll that is used for rolling on cogging and section lines depends on calibration conditions, on width of rolling material.
Important proportion of rolling mill rolls is a ratio of body length L to Ø D. Choosing a balanced ratio L/D one shall approach a high resistance at min. elastic yield.
For different rolling mills there are the following ratio of body length to diameter (L/D):
- Cogging mill 2,2—2,7;
- Section rolling mills 1,6—2,5;
- Plate mills 2,0—2,8;
- 4-hi mills а) 3—5 (for work rolls); б) 0,9—2,5 (for back up rolls).
Roll neck length usually equals to roll diameter. Diameter of roll necks with antifriction and friction (close type) bearings shall be in accordance with design. Diameter of rolls with antifriction bearings shall be depending on rolls dimensions.
Once dimensions are submitted and confirmed, final dimensions shall be confirmed by ENCE GmbH. It is necessary to hold additional checking calculations of roll durability and formability.
Rolls for cogging and section rolling mills
Rolls for cogging and section rolling mills can be distinguished by application, material, hardness and hardness change (drop) depending on roll cross section.
By application section rolls can be divided into
1) rolls for heavy cogging mills (blooming, slabbing and semifinishing mills).
2) rail and structural steel and heavy-section mills;
3) medium-section mills;
4) small-section mills;
5) wire mill;
6) skelp mill.
Main characteristic of grooved roll is nominal (average) Ø D and body length L. If a mill is multistand, with rolls of different diameters, nominal diameter of finishing stand roll has a determining meaning. In regard of material grooved rolls can be divided into steel (forged, cast) and cast-iron.
Production of blanks for rolls
At present time production of roll workpieces is proceeded by vertical casting, centrifugal casting or forging of original workpiece.
Vertical casting method is the least-costing but roll metal has a least-compact structure and shorter operation lifetime.
Modern rolls production methods. Centrifugal casting
One of modern methods to manufacture cast rolls is a centrifugal casting method. This method is more expensive but it allows to compact metal structure close to outer surface (work surface) and provides longer roll operation lifetime.
True centrifugal casting machines are used for this purpose. Machine includes a mold installed on support rolls. Two driven rolls with motors are located in one plane of front row. This provides synchronization of their rotation. Top safety roll is mounted with gap relative to mold thread. All rolls of the same row are mounted in one support frame what excludes any angular misalignment of rolls and mold rotation axes. Some pads for vibration damping are mounted between rolls thread and hub. Support frame is separated from a case and transmits vibration only to the support frame cast into concrete foundation. By means of vibration and mold wobbling reduction, casting defects are excluded, for example, defects of mill rolls made of steel and alloys.
Roll centrifugal casting is made of high-alloy cast-iron. Metal of work surface in volume of 0,95±0,05 of roll work surface value is poured into a casting mold rotating around vertical axis.
The most expensive manufacturing method is a forging method of cast blank. This allows to harden metal of the total volume of roll body and increase part durability and operating life.
Rolls made of steel, especially forged rolls, are high-duty. Forging is proceeded at automated high-tech forging plants 31,5МН, 60МН, 150МН.
These rolls are widely used, foremost on heavy cogging mills, such as blooming, slabbing, billet mills, rough stands of section mills. Steel rolls provide increased frictional factor during contact with rolled material. It contributes to better application in stands of high-reduction mills. Forging of solid back-up rolls has the same consequence as forging of work rolls and includes the following operations: roughing, prestressings, fullering and forging for blank size. Rolls go to complicated heat treatments process after forging, and at final stage – surface quenching with tempering.
Cast-iron rolls have a durability that is not so high, however, they have high wear-resistance. They are mounted, as a rule, into prefinishing and finishing stands, sometimes into intermediate stands of rolling lines.
Rolls for many section mills are cast in special metal molds, with rough passes that resemble design of future passes. It is reasonable to cast these rolls of cast-iron so the whole pass shall have set thickness of chill layer. In case of groove cutting into roll body, the most wear-resistance layer of very hard material shall be stripped.
The rolls of cogging and section rolling mills are made of different steel grades depending on their application:
|Steel grade||Rolling mills (application of rolls)|
|50, 55, 55Х, 50ХН and 60ХН||Bloomings, slabbings and semifinishing mills|
|50, 55, 55Х, 40ХН, 50ХН,||Rail and structural steel mills and heavy section mills|
|60ХН, 9Х and 9ХФ|
|50, 55, 5ГХ and 50ХН||Medium section mills|
|50, 55, 55Х and 60ХГ||Light section mills|
|50, 9Х, 9ХФ and 9Х2МФ||Wire mills|
|50, 60ХГ||strip mills|
|150ХГ1М||Continuous semifinishing mills, medium section mills, light section mills, skelp mills and wire mills|
The rolls are provided to the rolling-mill shops after the heat treatment bay. The mechanical characteristics and hardness of the rolls are regulated. For example the rolls
- made of 9ХФ steel grade shall have the HB241-285 hardness;
- made of 55—НВ215 carbon steel shall have the 255 hardness;
- made of 9Х2МФ steel shall have the HВ 352 -429 hardness.
Hot rolling section lines are usually equipped with rolls of different cast iron grades.
For medium and light section rolling mills the rolls are provided made of СПХН-60 cast iron (chemical composition with the percentage content of elements: С - 3,9; Si - 0,7; Мn-0,8; Р-0,5; Сr-0,16; Ni- 0,4—1,0; Мо- 0,6—1,2). Their Shore hardness is chosen between 60—70.
The material of the mill rolls is very important for getting of the high quality rolled products, given values of the material cobbing and high resistance of the rolls. The material for the rolls is determined specifically taking into account the actual operation conditions. Steel rolls (casting, forging) are chosen for cogging and roughing stands, rolls of half-hard cast iron are for intermediate trains. Finishing stands are equipped with the rolls of half-hard or hardened cast iron.
At new section rolling mills characterized by high rolling speed the high-strength mill rolls with the heightened wear resistance values are used for getting of precision rolled products with given tolerances and surface quality. They are made of the hypereutectoid steel and substitute the forging steel rolls and cast iron rolls.
Application of rolls. Wear resistance and hardness of rolls
It should be noted that the forging rolls are used a little at the metallurgical hot rolling section mills. The casting rolls are used mostly made of hypereutectoid steel grades (so called, adamite). Casing steel rolls are used sometimes even at heavy cogging mills, such as bloomings, slabbings.
Due to using of the newly developed technological lubricants during the rolling the wear resistance of the mill rolls is being increased. The wear resistance of the rolls is 1,5 – 2 times as high by using of such lubrications.
According to the quality of material the rolls can be made of
- soft grages,
- half-hard grades,
- hard grades,
- extra-hard grades.
The soft rolls have the hardness value lower than НВ 270. They are used at the cogging mills, cogging stands, sometimes at the roughing stands of the section rolling lines. They are made of the forging and casing steel. In order to increase the resistance the work surfaces of the soft rolls are usually machined by build-up welding with high-strength materials (3X2BS, 18ГСЛ and etc.).
For using at the roughing stands at the low load the soft rolls can be made of cast iron with a perlite-graphite working layer.
Half-hard rolls have the НВ 270-420 hardness. They are used at finishing, prefinishing and roughing stands of the section rolling lines. This is the main type of rolls for the rail and structural steel mills and heavy section mills.
Besides, half-hard rolls are widely used in medium section and roughing stands of fine rolling mills and wire rolling mills. They are made of mottled cast iron hypereutectoid steel.
Hard rolls (with hardness of НВ 420-600) are used in finishing and roughing stands of wire and small section mills. They are made of chilled cast iron and can be one- or two-layer.
Superhard rolls (with Shore hardness of over 100) are made of metal-ceramic alloys. Base is tungsten carbide. With such a base rolls have a long life in finishing stands of wire roll mills. Roll life with such a base is several tens longer than life of rolls made of chilled cast iron.
In rolling process passes of grooved rolls wear out. When roll passes are worn rolling drafting schedule is broken, and consequently quality of rolled product surface is worse. That is why worn-out rolls are subject to reconditioning with roll-grinding machines. Number of grooved roll reconditioning operations depends on mill type, roll quality, operation conditions and ranges from 3-4 to 8-9 times and more. As a result of multiple reconditioning of rolls their initial (max.) diameter Dmax gets smaller.
Rolls for hot rolling sheet mill
During the rolling of sheets high forces and significant thermal effects are produced on the rolls. Therefore the following quality requirements are higher for such rolling conditions:
- mechanical properties;
- wear resistance;
- surface quality and
The higher the wear resistance is, the less is the idle time while the roll changing. Therefore the productivity is higher. The roll consumption is less, it means that the technical and economic features of the production are better.
A wide sheet range (relative to thickness and width), a huge number of sheet rolling mills which differ in arrangement, quantity and performance of the working stands determine a large number of the roll sizes, strict requirements for their quality. In view of the characteristics mentioned above, the mill rolls for the sheet rolling mills are made of steel or cast iron.
Work rolls are manufactured as one-piece-forged or casting ones. Back-up rolls are manufactured both as one-piece-forged (casting) and composite ones (with forged or casting roll jacket, forged axle).
The standard defines the following types of heat treatment for each steel grade and rolls of each size. The cast iron rolls for the lines of hot-rolled strip and sheets are manufactured as casting one’s made of:
- unalloyed cast iron with flake graphite;
- alloy cast iron with spherical graphite and
- alloy cast iron with flake graphite.
The roll body surface hardness, the (working) layer depth of the chilled cast iron, the chemical composition are the main indicators of the cast iron roll quality which determine the service durability of the rolls in operation. These requirements by considering the conditions of the operating modes, product range and roll loads are specified in the special quality standards or technical terms of the company ENCE GmbH.
Rolls for flat product cold rolling mills
Rolls for sheet cold rolling mills are divided into the following applications: work and back-up rolls. See Fig. 3 and 4.
Roll diameter is selected based on the calculations done with due account for range of sizes (its thickness), operating conditions, mechanical properties of rolled products, max. forces, drafting, and line design.
Work roll body length depends on the width of strip, sheet and band.
Work rolls are usually used as drive rolls. In mill stands where ratio of body length to roll Ø = or > 5:1, and a very thin alloy steel strip is rolled, and in multi-roll machines back-up rolls are used as drive rolls. Rolls with antifriction bearings have stepped necks. Mills where journal bearings are used usually have flat roll necks. Some necks have max. Ø and rounded places of transition from necks to body to reduce bearing pressure and increase strength of roll necks in oil film bearings.
Fig. 3. Work roll
Fig. 4 Work and solid-forged back-up rolls of four-high mill stand 1700
Work rolls (at body Ø >160 mm) have axial through grooves, so-called “axial passages”. Such passages in big size rolls become wider in roll bodies. These wider passages have Ø which is much bigger than inlet hole Ø.
The axial passages facilitate roll center cooling during quenching. Such additional work roll cooling during line operation process creates stable thermal conditions and therefore increases roll life.
Back-up rolls can be solid-forged (as shown in Fig. 3 and 4), cast, and composite (see Fig. 5). Back-up roll quality requirements are especially strict. Back-up roll body runout with necks which can occur during operation can lead to non-uniform thickness of the rolled strip. Max. permissible runout of Ø1500 mm roll body will be 0,03 mm.
For cold roll mills rolls are to be of high-grade steels which contain little harmful components S and P. After heat treatment steel is evaluated not only by mechanical properties, but also by processing characteristics: hardening capacity, tendency to overheating and hardening strain, machinability, grindability and others.
The most important properties of steels destined for roll manufacturing are considered hardness and hardening capacity. Steel hardness of Grade 9Х in a hardened condition achieves 100 Shore numbers.
Work rolls for multiple roll mills are usually made of Steels 9Х and 9Х2, tool, medium alloy and high-speed steels. Work surface hardness as heat treated achieves HRC 61-66.
Recent technologies use work rolls made of sintered carbide alloys (tungsten carbide as a base) more and more often. Hard alloy roll manufacturing is based as a rule on hot pressing or sintering plasticized work pieces. Quantity of cobalt powder is taken to be 8-15 % (the other component is tungsten carbide).
Hard alloy rolls are more wear resisting in comparison with the rolls made of alloy steel grades. Their wear resisting rate is 30—50 times higher. They can achieve the best roughness values of the rolled material surface during rolling.
They are manufactured solid and composite. As work rolls for multiroll mills solid ceramic-metal rolls are used as a rule. When designing hard alloy rolls some certain ratios are to be considered: neck Ø to body Ø (≥ 0,6) and body Ø and length (≤ 4).
The main disadvantage of ceramic-metal rolls is high embrittlement which excludes their use with strokes, impact shocks or b deflections. When rolls are charged into the mill stand it is required to completely eliminate roll skews which affect the quality of the rolled material. Back-up rolls for cold rolling mills are usually made of the following steel grades: 9X2, 9XФ, 75ХМ, 65XНМ. Recently steel grade 75ХМ is widely used for solid-forged back-up rolls.
Steel grades 40ХНМА, 55Х, 50ХГ, and Steels 70 are used for composite (with sleeves) back-up roll spindle manufacturing (small and medium-size). Steel grades 45XHВ and 45XHМ are used for spindle manufacturing for big back-up rolls of heavy-duty mills.
Steels 9Х, 9ХФ, 75ХН, 9X2, 9Х2Ф, and 9Х2В are used to manufacture sleeves of composite back-up rolls. Sleeve surface hardness after final heat treatment is 60—85 of Shore.
It is efficient to use cast back-up rolls, they are cheaper than forging rolls and have a better wear resistance. Big cast back-up rolls are made of CrNiMo and CrMnMo steels. For example, back-up rolls are made of steel type of 65ХНМЛ. After heat treatment their hardness is 45—60 of Shore.
Multiroll mill back-up rolls are made of tool steel. It contains 1,5% С and 12 % Сг. Their hardness after heat treatment is HRC 56— 62.
Special attention is paid to quality control at material casting production stage and to finished product quality, which includes the following control stages:
- Raw material chemical composition
- Roll material structure analysis
- Geometrical dimensions and centricity analysis
- Work surface hardness control
- Work surface roughness control
- Surface layer metal structure
Quality package for rolls is not less important for customers to transport finished products from the manufacturer to the end user. If secure roll fixation and good protection of working and mounting surfaces are missing this will undermine all the manufacturer’s efforts aimed at high-quality roll manufacturing.
Our engineers are always ready to consult or provide additional technical information about the offered mill rolls.