Starting the Mill After Major Roll Changing

General rules for mill adjustment after roll changing are as follows:

  • (a) Before the start of mill:
    • 1. Rolling scheme for the section should be as per roll pass Design and should be confirmed by in charge (operation)
    • 2. Passes of all stands should correspond to the roll pass design. Any deviation is only allowed with the permission with hi charge (operation).
    • 3. Fixing up of rolling tackles of entry and exit sides of rolls should correspond to the ones confirmed by In charge (operation). It is forbidden to use modified rolling tackles without consulting the In charge (operation).
    • 4. During the mill adjustments, twisting of the metal around its axis should be removed for all sections.
    • 5. Bending of pieces side wise, up and down, bending of front end and tail ends should be completely eliminated.
    • 6. Distortion of section in all stands should be removed during mill adjustments.
    • 7. Finished products should be free from all rolling defects.
    • 8. The final speed adjustments should be done only after all above operations are completed; after which the mill is considered to be adjusted.
  • (b) After the start of mill
  • 1. Mill adjustment is carried out by Foreman (operation).
  • 2. During mill adjustment, all changes in the reduction scheme and adjustment of rolling tackles can be made with the permission or by order of Foreman (operation). Slight changes adjustment in reduction in case of finishing and pre-finishing stands within the tolerance of finished product can be done by roller.
  • 3. Only the Shift Foreman (operation) will be allowed to start the mill or any individual stands after he has checked the same.
  • 4. Unauthorized persons should not be allowed in the Mill. Area mill, especially while Mill adjustment is being done. All Mill personnel should watch carefully the movement of bars in stands and outside the stands.
  • 5. Signal for pushing out the billets from the furnaces is to be given by Shift F/M (operation). He should see the condition of the billet, before the start of pushing of billet from furnace. Only one billet at a time is pushed out for Mill adjustment.

For the purpose of mill adjustment, return billets without heat numbers are to be charged into the furnaces. For this purpose, 3 to 4 billets depending upon the section to be rolled and are charged into the furnaces.

  • 6. For pass burning, sample pieces are to be kept in the furnaces for heating, which are taken out during pass burning. This piece should be selected by F/M (operation).
  • 7. Checking of dimension of pieces after the roughing, intermediate and finishing group should be carried out by Shift F/M (operation). The Foreman should also check loads on different stands for all sections. Corrections during Mill adjustment should be done in consecutive in order of rolling.
  • 8. When the bar is passing through pre-finishing and finishing stands, the roller should see the following:
    • (д) Proper gripping of metal by rolls.
    • (b) Direction of twist of the bar.
    • (c) The nature of bending of the bar.
    • (<7) Over filling and under filling of pass.

The roller should correct these inaccuracies and take a sample to examine the dimensions. Then again, necessary correction should be made in the mill to have correct dimension of the bar. The Foreman should see the profile and satisfy himself and give a permission to start the normal rolling.

9. Sample to be collected from sample saw with the permission from Foreman or roller.

Examination of the Finished Products

These are general rules for taking samples and examination of metal on the cooling beds.

  • 1. Final estimation of the section can only be given after double check of the finished product i.e., checking of technological sample at sample saw and examination and checking of dimensions of finished products on the cooling bed. It is forbidden to start normal rolling, if metal on the cooling bed is not checked and if there are no defects observed on the sample.
  • 2. Defective bars rolled during the mill setting should be separated from the rolled metal and afterwards cut separately. This separation should be carried out by recorder of the cooling beds on the instruction of the roller.
  • 3. The roller should examine the bar, out of which the sample is taken and check the dimensions at different distances from the back end.
  • 4. Samples from the saw are taken to check the dimensions as per the desired tolerances of given standard and also examine the surface of the product. In case of any rolling defect detected, the rolling should be stopped to take necessary action to rectify the defect.
  • 5. When examining the bar on the cooling bed, special attention should be paid to the defect. Periodicity of defects (roughness, scabs, sticking bar etc.) on any side of the section is the result of defect in the pass in one of the stand. In such case, rolling should be stopped to rectify the defect.
  • 6. In the process of steady rolling after every 100 to 200 pieces, sample should be taken at the hot saw. The last sample taken during the miming shift should remain on the control table for the next shift.
  • 7. The roller should periodically examine the pieces on the straightening machine approach roll table when rolling is going on.

Technical and Economical characteristics of Rolled Metal Production

Each ton of the annual output of rolled products, including that of the cogging division, requires capital investments 3-4 times greater than those for the blast furnace (excluding ore mining and coke production) and steel making processes. The unit capital investment in rolling increase parallel with the improvement of the rolled product quality.

Taking as an ordinary example, most popular structure of a hot rolling shop and comparing an approximate correlation of percent costs among the main stages of rolling, that the cost of the rolling mill, including shearing and strengthening facilities, will be around as high as 70%, that of the heating facilities to 16% and of the finishing installations and store rooms up to 14%.

It is customary to hold the rolling mill proper to be the main unit heading all other divisions of a rolling shop. Such an approach is also justified by the average upkeep expenditures of individual rolling divisions. These amounts to 20-30% for heating, 50-60% for rolling and 15-30% for finishing.

The following general formula may be used for determining the acmal hourly output of a rolling mill:

Where P = net yield of the mill, t/h

Q = mass of billet or ingot q = net yield factor

t = rolling rhythm к = rolling mill use factor

The rolling rhythm is determined in a number of ways, depending on the rolling mill arrangement and rolling scheme. For a single stand blooming mill this is the total time of rolling in the stand, including intervals between passing and successive ingots. For multi stand rolling mills this is the time of passing in the finished stand, including intervals between successive billets.

In general terms the rolling rhythm for a stand in which several passing are performed is determined from the formula :

Where t = rolling time for several passing or total productive time r. = time of intervals between passing.

If rolling speed v is constant (disregarding speed gain) and the length of the work piece after passing is L. then the productive time is determined from the formula

Where D = effective diameter of rolls

n = roll revolutions per minute

For reversing rolling mills, where the peripheral roll speed is variable, the productive time is expressed as

Where = speeding-up period

t2 = period of rolling at a constant speed t3 = slowing down period

The duration of the speeding up and slowing down periods is determined proceeding from the different between rotational speed nv n2 and ?/3 and from acceleration a or deceleration b rates (rev. sec/min.)

п7 - //,

Speeding up time ts = ---


w, —

Slowmg down tune tsl d = i-i


It should be borne in mind that in most cases the beginning and the end of a passing do not coincide with the zero speed values in the speed diagr am of a reversible motor and they are determined best with reference to the load diagrams of the motor or to oscillograms.

< Prev   CONTENTS   Source   Next >