Roll classification – Semi-steel rolls

The carbon content of semi-steel rolls is usually 1.4~2.4%, and the mechanical properties are between cast steel and cast iron, with the characteristics of small hardness drop and high wear resistance, which are widely used in rough rolling and middle rolling stands of section steel rolling mills, rough rolling and finishing rolling of hot strip rolling mills, when the carbon content of semi-steel is greater than 1.9%, it is called high-carbon semi-steel, high-carbon semi-steel due to high carbon content, eutectic carbides and pseudo-eutectic carbides exist in the matrix structure, which are used for rolls for billet continuous rolling stands, Rolls for intermediate stands and finishing stands for section steel rolling mills. When semi-steel rolls are used as hot rolls, in order to improve crack resistance, silicon content is added to the composition to become graphite cast steel rolls, which concentrates the advantages of cast steel and cast iron, and is widely used in billet hot continuous rolling mill rolls and section steel rolling mill rolls.

Which rolls are suitable for monoblock casting production?

Primary mill, billet mill, large steel and rail beam mill, hot strip mill scale and edge mill, steel universal mill edge mill, and small steel, wire rod mill roughing mill stand and other mills using the roll, most of the whole casting method of production, this type of roll using thicker layers, deeper holes. In addition, the hot strip mill’s two roughing rolls are also suitable for the whole casting production. The whole casting roll process is relatively simple, low manufacturing cost.

Commonly used roll material selection

Selection of work roll material
Roughing front work rolls (R1): Roughing front work rolls must have strong toughness, wear resistance and resistance to thermal cracking. Hardness range of HS40-55 or so. Generally use 60CrNiMo cast steel and other materials roll.

Roughing after the section of the work roll (R2): roughing after the section of the work roll requires the use of good thermal cracking performance of the material. General use of semi-steel, high chrome steel, high-speed steel and other materials.
Finishing section work rolls (F1-4): finishing section work rolls have high temperature and large load. Generally use cast semi-steel and high chromium centrifugal composite cast iron and other materials. High chrome centrifugal composite cast iron has high wear resistance and resistance to thermal cracking of the roll surface, and can inhibit the roll surface spot band defects.
Finishing rolls (F5-7): finishing rolls are used in the final stage of rolling, product quality, surface condition has a very important impact. Roll performance needs are mainly high hardness, wear resistance, resistance to indentation, resistance to spalling and resistance to thermal cracking, generally choose unlimited cold hard cast iron (ordinary, improved) and other materials.
2. Support rolls, the choice of material rolls
Whether for roughing or finishing support rolls are required to have good resistance to thermal cracking, wear resistance, fatigue resistance and high strength. Materials generally choose composite cast steel, alloy forging steel (Cr3, Cr5) and other materials.

Rolls in the selection of what should be considered

Roll is to determine the efficiency of the mill and rolling quality of the important consumption of parts, it is an important part of the steel mill rolling mill, the use of rolling pressure generated by the rolling mill to roll the steel. So in the production of rolls we usually need to consider what factors in the selection of materials?

1, the roll must have a good bite, so the choice of slab thickness is required.

2, the choice of roll material must have a good resistance to thermal cracking and thermal fatigue;

3, the material for the roll diameter is large, long roll body length, the rolling force to withstand high

4, the material is required to have high fracture resistance and high strength.

5, due to roughing and finishing in the same stand to complete, so we must take into account the rough rolling when the thickness of the rolled parts, small width, and so on.

Rolls are the cornerstone of cold rolling quality.

The cornerstone of cold rolled steel strip quality is the roll, and cold rolled steel strip surface roughness depends mainly on the cold rolled steel grossing process. Furring rolls have the ability to improve slab shape and product surface quality, improve deep-drawn performance, improve coating performance, avoid sticking to the steel when rolling, reduce the rolling cross-hatch, eliminate the annealing of adhesion, have a longer service life and other characteristics.

In the face of the arduous production tasks, the roll center in the refinement of management and technology research in two aspects of the precise force. Roll center grinding two workshops with careful operation, careful inspection, dedicated research, comprehensively improve the surface quality of rolls. For every lifting of rolls, every drop of rolls, every adjustment of the center height of the tiles, all refine the standards and develop detailed operating procedures. Special emphasis on the lifting of the roll, pay attention to the air can not swing greatly, to prevent the roll surface and insulation shed, half door crane gently in contact; each drop roll, need to be strictly close to the top of the head box and the tail box, in the distance of 5 meters away from the location of the operation less than 1 cm of the space range of the drop roll. Whenever the rolls fall to the TOW, rolling spares workers will need to carefully wipe the roll surface, carefully check the roll surface of the tiny defects, if slight watermarks, all need to be polished with mirror sandpaper, which can start grossing operation.

Roll center aiming at peer advanced standards, find gaps, make up for shortcomings, by technical staff and various steel mills to establish the surface quality of the rolls between the group for communication, in order to improve management capabilities, tap the space for improvement. Roll center take the long and make up for their shortcomings, through exchanges and in-depth research to gradually improve the quality of roll grossing, to ensure the cornerstone of slab quality, to provide a strong guarantee for the cold rolling production line.

Roll defect repair and grinding countermeasures

From August 2015 to April 2016 unplanned roll replacement statistics can be seen accident rolls accounted for 69% of the total roll replacement. Therefore, after the occurrence of rolling accidents, timely and correct treatment of rolls can minimize the unnecessary damage to the rolls, extend the service life of the rolls and reduce roll consumption.

The following briefly introduces several common accidents roll treatment:

1 Thermal cracks

Thermal cracks, mostly for the card steel or pile of steel after the accident, the roll cooling and heating.

Treatment: R1 and F1-F4 rolls in the premise of ensuring smooth rolling, can be used with hot cracked rolls on the line, and cracks are closed (closed cracks do not extend, but with the use of time to extend), cracks will be gradually eliminated.

2 Roll sticking steel

Rolls stick to steel, dumping or pile of steel accident caused by the roll sticking with steel.

Processing method: roll roll body strip steel adhesion folding serious, large area, thickness of more than 3mm need to be adhered to the strip first turning off. Adhesion of the iron turning off the roll body after checking the surface, if there are cracks and flaking (meat) depending on the degree of damage to determine the depth of cutting, to ensure that the elimination of defects without increasing the excessive roller consumption.

3 Work roll body edge spalling

The edge of the work roll body spalling, roll shoulder stress concentration.

Treatment: Since the shoulder of the roll is not in direct contact with the steel plate, the cracks can be removed from the peeling part, grinding chamfering, and adjust the contact with the support roll chamfering and angle to improve the stress distribution of the roll body.

4 Support roller defects

Support roller defects, support rollers are not in direct contact with the strip and the hardness of the work roll relative to the lower, but the price is expensive, so the use of conventional repair and grinding methods is not appropriate.

Treatment: when the damage is in the roll edge, when grinding the appropriate increase in the length of the end of the chamfer. Defective residue is handled with an angle grinder and will be rounded around the excessive, to prevent flaking defects from spreading further. Damage in the middle of the roll, according to the conventional grinding and then the same with the angle grinder and the defects around the rounded smooth excess.

What is carbon steel rolls, what are its organizational characteristics?

BAOFENGMMC is specialized in making rolls, making rolls for many years, have a very good reputation in all areas, the quality is top quality, can be customized in various sizes and models, the warranty time is also longer than normal manufacturers.

Carbon steel roll is a lower alloy content (not more than 0.8%) of the eutectic steel composition roll, such as GB1503-89 “cast steel roll” listed in the Z-U70, ZU80, ZU70Mn, etc., for China’s early use of a cast steel roll. Its matrix organization is pearlite, but there are a large number of block or strip ferrite in the matrix organization, reducing the impact toughness of the matrix, while wear resistance is also low. At present, the domestic cross-type steel mill, compound two heavy wire rod mill and individual billet mill is still using carbon steel rolls, has tended to be eliminated.

Cold roll material selection guide

I. Cold rolls made of cast iron
Cast iron is currently a commonly used material for cold rolls, the advantages of which lie in the low cost, good machinability, good wear resistance and corrosion resistance, etc.. However, cast iron roll also has its shortcomings, namely, low hardness, insufficient strength, easy to crack, etc.. Therefore, excessive extrusion and high temperature should be avoided in use.

Second, alloy steel material cold rolls
Alloy steel is a high-quality materials, with high strength, high hardness, good wear resistance and so on, cold rollers also began to use alloy steel as a material. Compared with cast iron, alloy steel cold roll has better durability and stability and other advantages. However, the cost of cold rolls made of alloy steel is relatively high, and the manufacturing process is also more complicated.
Third, the cold roll of super-hard alloy material
Super hard alloy is currently one of the development direction of the roll material, it is made of tungsten and cobalt powder and tungsten carbide powder mixing press molding, and then after sintering, milling and other processes. Super-hard alloy cold rolls with high hardness, high strength, wear resistance, good corrosion resistance and other advantages, suitable for more demanding cold rolling processing conditions. However, due to the complexity of its production process, high cost, generally only for the manufacture of high-grade cold rolled products.
Comprehensive view, cast iron, alloy steel and super-hard alloys are currently commonly used cold roll materials, the choice of which material depends on the variety, quality and processing requirements. Therefore, the selection of suitable cold roll materials, not only to extend the service life, improve production efficiency, but also reduce costs and improve product quality.

Description and characteristics of alloy cast steel rolls

Alloy cast steel rolls is a composition in the range of eutectic steel and over-eutectic steel, alloy content greater than 0.8% of the cast steel rolls, its carbon content is generally between 0.4% – 1.3%, and contains some chromium, nickel, molybdenum, vanadium, and other alloying elements, and its matrix organization through the normalizing + tempering heat treatment process, generally controlled for the Soxhlet and tempered Soxhlet. Typical materials such as: 60CrMnMo.65CrNiMo, 70Mn2Mo, 75CrMo, 75CrNiMo and so on.

Alloy cast steel roll matrix organization to pearlite type is dominated by a small amount of carbide, hardness range in HS 35-50; due to the matrix contains a small amount of secondary carbide, so compared with the carbon steel roll wear resistance greatly improved, at the same time, due to the lifting of the matrix in the block striated ferrite, so the impact toughness of the roll has been improved. Alloy cast steel roll performance is characterized by high strength, good toughness, has a very good bite, resistance to thermal cracking and impact resistance.
Alloy cast steel roll material has reduced carbon content, improve the degree of alloying trend, and the use of a variety of micro-alloying technology, while further improving the quality of gold treatment, so that the alloy cast steel spokes can better meet the needs of different mills.
What are the main applications of alloy cast steel rolls?
Alloy cast steel rolls are used in square/slab primary mills, roughing mills of large section steel billet and girder mills, two-roll roughing mills of plate and strip mills and large vertical rolls of scale-breaking racks, as well as the roughing mills of section steel and bar and wire rod mills.
Integral casting and composite casting support rolls are strictly alloy cast steel rolls, according to the hardness requirements to control the carbon content and the addition of different alloying elements, the production of support rolls of alloy cast steel materials, containing a high degree of chromium, molybdenum, vanadium and other alloying elements, with the increase in the alloy content, the matrix organization by the transformation of the body of the soxhlet to bainite or martensitic, and at the same time contain a high degree of hardness of the secondary carbides, the range of hardness is HS55-70, with excellent resistance and resistance to spalling.

How to prevent roll hazards to improve operational efficiency?

In rolling mill assemblies, rolls operate under chaotic conditions. Residual and thermal stresses are generated during the preparation process prior to production and use of the roll. In use, it is also subjected to a variety of cyclic stresses, including torsion, variation, shear, contact stresses and thermal stresses. The distribution of these stresses along the roll body is uneven and constantly changing. The reason for this is not only the planning elements, but also the continuous changes in roll wear, temperature and roll shape during use. In addition, rolling conditions often exhibit abnormal conditions. If the rolls are not properly cooled after use, they are also harmed by thermal stresses. As a result, in addition to wear, rolls often exhibit a variety of localized and surface damage, such as cracks, crazing, peeling and indentations. A good roll should achieve a good match between strength, abrasion resistance and various other performance indicators. In this way, it is not only in the normal rolling conditions are good, but also in the presence of some abnormal rolling conditions will not cause too much damage. Therefore, when manufacturing rolls, the metallurgical quality of the rolls needs to be strictly controlled or supplemented by external measures to enhance the bearing capacity of the rolls. Reasonable roll shape, through, deformation guidelines and rolling conditions can also reduce the working load of the roll, to prevent some of the peak stress, and extend the service life of the roll. The reason is related to the residual stress of the roll itself, the mechanical stress during rolling and the thermal stress of the roll, especially when the temperature difference between the outer surface of the roll body and the core is large. Poor cooling of the rolls at the start of a new rolling cycle, interruption of cooling or overheating of the roll surfaces may cause this temperature difference. The temperature difference between the outer surface of the roll and the core results in large thermal stresses, and when the large thermal, mechanical and residual stresses of the roll exceed the strength of the core of the roll, the roll breaks.

Cold welding recovery method: Cold welder rolls use high frequency electric spark discharge principle for non-thermal overlay welding of metal surfaces, so there will be no deformation, annealing, nibbling and residual stresses, rolling holes and scratches during the recovery of defects, please do not change the state of metal arrangement. High accuracy of calibration. The thickness of the coating is from a few micrometers to several millimeters. It can correct the defects of metal workpieces, such as abrasion, scratches, pinholes, cracks, defects deformation, hardness reduction, trachoma, damage and other functions, only need to be polished and buffed, can also be used for turning, milling, planing, grinding and other machining, as well as plating and other post-treatment.