Procedure for Slitting of Mild-Steel Strips

This is the process of cutting strips of different sizes from a hot rolled mild steel coil. The strips are used for making square rectangular and circular hollow sections or tubes. In this process, width of the strips range from 47/48 millimeters and thickness of 1.0 millimeters for making 12 millimeters by 12 millimeters square hollow sections to width of 670 mm and thickness of 5.9 millimeters to 6.0 millimeters for making 175 millimeters by 175 millimeters square hollow sections or 200 millimeters by 150 millimeters rectangular hollow sections. For this process, the hot rolled mild steel coil is bundled in a 6 to 10 tone-package with a width of 1200 millimeters to 1300 millimeters.

Materials needed for the process are: slitting line machine, overhead crane, weighing scale, micro meter screw gauge, 3 meter steel tape and the mild steel hot rolled coil.

Before the process begins the coil is visually checked for deformities, that is; burrs, dent edges, elongations, lamination and waviness. It should also be checked for supplier tolerance on mechanical properties; thickness and width.

If the coil is okay, the straps which seal the coil are welded off leaving one which would hold the coil preventing it from unfolding or uncoiling before loading it in the slitting line machine.

The coil is then placed in the slitting line machine with the help of an overhead crane and on to the slitting line car at the slitting line bay. At this point the machine is turned on, the coil is pushed forward using hydraulic force and raised up to be sandwiched by cone-shaped mandrel of the plate drums that straightens them. The remaining scrap is scaled off using a pinch roll-up, it is then used to support the coil to penetrate through the rollers. Prior to the material reaching the cutters, the coil width is measured using a 3 meter steel tape and the thickness measured using 0 to 25 millimeters micro meter screw gauge to ensure they are of the required dimensions. These steps should be done thrice; at the start of the process, when the material gets to the mid section and when it gets to the end.

If all is okay, the machine is turned back on and the material passed through the cutters. As the process continues, the strips are moved to the mandrel that straightens them, where the separator shaft is used to prevent the strips from intermingling. The sides - which are part of the additional width - are considered extra material (scrap), they are rolled and attached by the scrap winder unit which moves in a circular motion simultaneously with the machine as the material is being processed. Adjustment screws are fastened to lock the strips so that as the mandrel starts to recoil, the strips would not detach or slip off.

As the process proceeds, speed is gradually increased to allow easy flow of strips that are being cut into slits. The clutch at the main slitting drive is released to ensure the process is continuous. Strips are tightened by inserting short strip straps as the process is ongoing. At the end of the process, the strips are welded. At this point the separator shaft is moved upwards and the mandrel screws are unlocked to access and remove the strips. This point culminates the procedure for slitting of mild-steel strips.

Basic Procedure of Slitter Setting for Slitting Line Machine

 

The process of manufacturing mild steel hollow-section structures involves a step-by-step arrangement of cutters, plastic 'spacers' and industrial rings in the slitting-line machine to get good quality steel strips. The process results in hollow-section structures that are circular, rectangular and square in shape as well as 'multi-c' and flat bar strips.

Before the process begins, one should make sure the cutters, plastic 'spacers' and the industrial rings are clean. There are a couple of reasons for this; the first is because of durability of the equipment, the second is that, if left unclean, the cutters may increase in size by a few millimeters due to accumulation of dust particles resulting in incorrect strip sizes. In addition to these two main reasons, in the process of welding or using oxy-fuel to cut off sections of the strip that are damaged during the process, the sparks produced can attach themselves to the cutters if they were not properly cleaned and this can lower the quality of the hollow-section strips being manufactured. For quality results, it is always recommended to clean the cutters, plastic 'spacers' and the industrial rings before and after the slitting of strips in the slitting line machine.

Before arranging the cutters, you should have the slitting machine's setting plan and line up all materials needed. This will make the work easier and also avoid errors that sometimes cause a newly assembled setting not to work properly for the first time. With everything in place you should start by opening the machine. First step is to unscrew and loosen the stand-lock bolt then move the stand away from the shaft by unscrewing the movable stand-stud screw. You should then draw the slitting machine's top and bottom shafts apart, by turning its hand wheel in an anticlockwise direction till one has access to the overlapping cutters. After that is successfully done, remove the plastic 'spacers', cutters and rings then assemble them as per the size of strips to be produced.

The plastic 'spacers' with rubbers and those without should be arranged equally in an alternating order. The work of the rubber is to reinforce the strips in the slitting machine to prevent them from moving out of line thus causing damage to the resultant strips. With the plastic 'spacers' in place screw back the stand-lock bolt and lock it, then close the shaft-lock nut. Slowly turn the hand wheel to move the slitting machine's shafts to their original position ensuring the edges of each cutters fit over each other creating a clear overlap. Lastly, after the assembly process is complete, make sure you tighten the slitting machine's shaft-nut firmly so that the materials in the slitting-machine do not lose alignment once the slitting begins.

The bush, disk and rings must be arranged according to the slitting machine's setting to allow the strips to evenly recoil at the separator shaft during the slitting process. Also, instead of making a space of exact width between the spacer's disks, there should be an allowance; strips of thickness as small as 1 millimeter should have an allowance of 1.2 millimeter and those of 5 millimeter should have an allowance of 2 millimeters. This is mainly because strips of light thickness like 1 millimeter tend to get out of alignment easily, thus given that specified margin of allowance to stay in alignment from beginning to end of the process.

With everything set and working, let the process run till the number and shape of hollow-sections are all done. After the process is complete, confirm the accuracy of the setting by measuring the distance between each cutter. If there is a reduction in length between the cutters, insert a plastic shim with the exact required thickness between the 'spacers'. Normally, plastic shims' thicknesses range from 0.1 to 0.8 millimeters.

In storing the cutters after the process is completely done, do not mix light gauge cutters with those of heavy gauge, they should be kept in separate places. This is mainly due to the fact that light gauge cutters are used for cutting strips of 1.0 millimeter but can also be used in cutting strips of 5.0 millimeters, but once a light gauge cutter is used in cutting thicker material like 5.0 millimeter, they should never be used as light gauge cutters else this will result in low quality strips. Additionally, you should remember to clean all the materials as earlier mentioned.

The process of manufacturing mild steel hollow-section structures involves a step-by-step arrangement of cutters, plastic 'spacers' and industrial rings in the slitting-line machine to get good quality steel strips. The process results in hollow-section structures that are circular, rectangular and square in shape as well as 'multi-c' and flat bar strips.

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Inspecting the Quality and Weight of Strips From a Hot Rolled Mild Steel Coil

This is the checking of the quality of the raw material; before the process of slitting a hot rolled mild steel coil, during the process of slitting it into strips and after, so as to produce good quality strips and to prevent further damage - if any had occurred - to the strips. To make sure everything was done properly, measures are taken after the process is complete by calculating the weight of each strip produced.

After the coil has been inspected and placed into the slitting line machine and the process of slitting the coil into strips commences, the strips are inspected for un-slitted parts, burrs, dent edges, waviness, rubber marks, blisters, splits, laminations, notches and cracks.

Definition of Terms

Un-slitted part: This is a deformity caused by poor balancing of the extra material (scrap) at both ends of the coil. This is commonly found on the first strip and the last. It can also be caused by the reduction of the coil width at mid process thus resulting in reduction of the strip's width size. To prevent this, when the deformity is noticed, the slitting machine's cutters should be carefully lowered so that they create a uniform overlap.

Burr: This is a sharp edge protuberance on the strip facing upward or downward and is caused by poor slitting machine's setting or blunt cutters. Burrs may cause openings when the material is being processed in the making of tubes.

Dent edges: The mishandling of the strips, especially at the edges, during the process of slitting, causes this deformity. The mishandling mostly occurs when the strips are constantly moved during the slitting process.

It can also occur when the dents of the hot rolled coil are too deep for the extra material, known as scrap, to pass through at its sides. When this happens, the extra material is rolled to the scrap winder thus causing notches, cracks and dents.

Waviness: This is the deformity in which there is lack of flatness on a strip. It is caused when the coil itself is wavy or by poor slitting machine's setting at the beginning of the slitting process. To make sure the slitting machine settings are correct, so as to avoid a majority of the aforementioned deformities, before the process of slitting begins; pass a half a meter strip through the slitting machine's cutters. The strip's width is then measured to check if it is correct once it comes out of the cutters.

Once the process is completed, one can save time in knowing the weight of each strip without having to weigh them individually. To do this, one should start by measuring the weight of the waste material that includes the material at the sides, which is attached to the scrap winder. After getting the total weight of the material, subtract it from the coil's weight. The result is then divided by the total width of all the strips and multiplied by each strip to find individual weights.

Another method:
Use the length and width of the slitted strips to find the area then
multiply the results by the actual thickness to get the volume, after getting the volume multiply by the density of mild steel to get the weight.
Use the length, width and thickness of the strips to get the volume,
The weight can then be calculated by multiplying it by the density of mild steel.
The density of mild steel can be directly measured and used to calculate the weight of the slitted strips.

A defective strip should be marked or the defective parts cut off. Once a part of the strip is cut off, its weight should be taken and subtracted from the original strip's weight.

HOW TO MAKE MILD STEEL STRIPS INTO CIRCULAR, RECTANGULAR, AND SQUARE HOLLOW SECTIONS OR TUBES .

This process occurs after the mild strips have been slitted by the slitting line machine using a cold rolling mill. In this process, the largest strip would be 670mm with a thickness of 5.0 to 6.0mm for making a rectangular structural hollow section of 200 by 150mm. The smallest strip width is 47 to 48mm with a thickness of 1.0mm for making 12 by 12mm square hollow section.

Machines to be used may defer depending on the width of the strips. Larger strips like 670mm may have to use a bigger mill than that to be used on a 47mm strip width. Before beginning the process, ensure that the strips are free from any damages or defects such as burrs, lamination, dents, waviness, and unstilted parts. These damages and defects may cause poor welding resulting in openings or leakages in the circular hollow sections or pipes. In case any defects and damages occur, the deformed parts should be cut off and the good parts rejoined by welding. Once everything is in order, you lift the strips using an overhead crane and load them into the uncoiler, unsealed. The pinch roll-up should be positioned downwards to support the strip to be moved to the leveler rollers. Note that large width strips of 670mm with the thickness of 5.0 to 6.0 are supported by pinch roll up but for those of width of 47mm and thickness of 1.0, the pinch roll up may not be needed.

If the process is a continuation and the strip is not flat, it should be cut off at the beginning by the shearing machine and joined to the other strips. However, if it is the first time, you should pass it through rollers that will have to be adjusted to fit the strip’s width through the hoop cage. The hoop cage enables the loading of many strips as possible to enable the continuation of the process. From the hoop cage, the strip is moved to the forming rollers which is adjusted to the type of tube to be produced. It is then moved to the fin pass then to the weld and lastly to the scarfing post where excess material formed by the weld is trimmed off by the carbide tip tool which is rolled and attached to the scrap winder. From the scarfing post, the material goes through the main pass to the side rolls that are moulding the tubes into the desired final products.

You should now be able to move it through the turks-head to the cut-off carriage which is set to the length of the tube desired. Coolant water from the leveler should be used to help cool the welding so that it will not get damaged due to overheating as forming rolls moved to welding lastly to finalized at the side rolls. The coolant water also helps in preventing the rollers from slowly wearing off due to friction as well as in keeping the material straightened so that it does not bend due to the expansion and contraction of metal.

 

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