The Concept And Process Of Rolling Mill

The Concept And Process Of Rolling Mill

Steel is widely used in our daily lives and holds a critical position in the national economy and most of the steel products we need are produced by rolling mills. The rolling process plays an important role as the final link of the steel-making process in modern steel enterprises.

Steel rolling, as its name, is a process of shaping the steel billet with rollers. And the rolling mill is what you need to get the job done. The complete rolling production process contains a series of different rolling techniques and it can be complicated. However, despite all those various requirements, the basic production process always contains the following procedures: reheating the steel billet and further treatments, rolling process, and the final finishing rolling process.

When the billets arrive, they need to be treated with chemicals like acids to remove the oxidation film and the obvious defects on the surface must be removed too. Then the billets will be reheated to a certain temperature to be ready for the coming rolling process. The rolling process is the core of this whole production, the billets will be rolled into different shapes and the rolling technique is crucial for the quality of the final product. It must meet the requirements from three different aspects: the product’s geometry dimension accuracy, internal organization, and performance, and clean and smooth surface. 

After all these processes, the product still has a complicated finishing rolling process. As a guarantee of the product, the finishing process can be different for the various requirements and standards. After the cooling, straightening, heat treatment, and other certain process procedures, the final product will meet all those customized requirements.

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What is Hot Rolling?

What is Hot Rolling?

Rolling mill is a metalworking process which divided into two types: hot rolling and cold rolling. Here we will analyze the hot rolling cause we had discussed cold rolling before.

In many theory books, which called hot rolling is a metalworking process that occurs above the recrystallization temperature of the material. On the contrary, it means that below the recrystallization temperature of the metalworking process is called cold rolling.

But what is hot rolling besides of this word. Here we have the detail explanation about it. After the grains deform during processing, they recrystallize, which maintains an equiaxed microstructure and prevents the metal from work hardening. The starting material is usually large pieces of metal, like semi-finished casting products, such as slabs, blooms, and billets. If these products came from a continuous casting operation the products are usually fed directly into the rolling mills at the proper temperature. In smaller operations, the material starts at room temperature and must be heated. This is done in a gas- or oil-fired soaking pit for larger workpieces and for smaller workpieces induction heating is used. As the material is worked the temperature must be monitored to make sure it remains above the recrystallization temperature. To maintain a safety factor, finishing temperature is defined above the recrystallization temperature; this is usually 50 to 100 °C (90 to 180 °F) above the recrystallization temperature. If the temperature does drop below this temperature the material must be re-heated before more hot rolling. All of this above, just part of the hot rolling actually.

Hot rolled metals generally have little directionality in their mechanical properties and deformation induced residual stresses. However, in certain instances, non-metallic inclusions will impart some directionality and workpieces less than 20 mm (0.79 in) thick often have some directional properties. Also, non-uniformed cooling will induce a lot of residual stresses, which usually occurs in shapes that have a non-uniform cross-section, such as I-beams. While the finished product is of good quality, the surface is covered in mill scale, which is the forms at high temperatures. It is usually removed via pickling or the smooth clean surface; process, which reveals a smooth surface.  Dimensional tolerances are usually 2 to 5% of the overall dimension.

Hot rolled mill steel seems to have a wider tolerance for amount of included carbon than cold rolled, making it a bit more problematic to use as a blacksmith. Also for similar metals, hot rolled seems to typically be less costly.

Hot rolling is used mainly to produce sheet metal, rebar, wire rod or other products. In a words, hot rolling is very popular used in the steel industry.

Since 1989 Hani Tech has been always involved in design, engineering, manufacturing and installation of complete hot rolling mill plant for long products (re-bars, SBQ bars, wire rod, small and medium sections), as well as revamping and upgrading of existing plants. 

Hani Tech is the one-stop manufacturer & supplier able to design, manufacture, install and commission your hot rolling mill plant from A to Z.

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And free send inquiry to us.
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Hot Rolling Process

Hot Rolling Process

The semi-finished billet from the steel plant is rolled to make it a qualified product.

Introduction of hot polling process

Process Flow of Hot Rolling: Process Flow of Main Rolling Mill Line:

The slab is directly sent to the slab warehouse of the hot rolling workshop from the slab discharging roller table of the continuous caster in the steelmaking and continuous casting workshop, and the directly hot charged slab is sent to the furnace loading roller table of the reheating furnace for heating. 

The slab that cannot be directly hot charged is lifted into the insulation pit by the crane, and then lifted to the upper material bench by the crane after insulation, and then heated by the furnace loading roller table of the heating furnace, leaving the possibility of direct rolling. 

The continuous casting slab is transported into the slab warehouse by the continuous casting workshop through the slab loading roller table or slab unloading roller table. 

Before the slab reaches the entry point, the technical data about the slab has been sent to the computer system of the hot rolling plant by the computer system of the continuous casting workshop, and the relevant data of the slab is displayed on the monitor, so that the staff can check and receive the defect free and qualified slab. 

In addition, the cleaned slab also needs to be checked, accepted and input into the computer. The flow direction of slabs entering the slab warehouse shall be determined by the slab warehouse computer management system according to the rolling plan.

Conventional slab charging and rolling: after the slab enters the slab warehouse, the slab is stacked to the designated stacking position in the slab warehouse by the slab clamp crane according to the unified instruction of the slab warehouse control system. During rolling, according to the rolling plan, the slab is lifted out of the stack one by one by the slab clamp crane, and lifted to the slab loading bench for loading. 

The slab is weighed and checked by the weighing roller table, and then sent to the furnace loading roller table of the heating furnace. After the slab is measured and positioned, it is loaded into the heating furnace by the steel loader for heating. 

Hot charging and rolling of carbon steel insulation pit: after the slab enters the slab warehouse, the slab is stacked to the designated stacking position in the insulation pit by the slab clamp crane according to the unified instruction of the slab warehouse control system. 

During rolling, according to the rolling plan, the slab is taken out of the insulation pit one by one by the slab clamp crane, lifted to the slab loading bench for loading. The slab is weighed and checked by the weighing roller table, and then sent to the furnace loading roller table of the heating furnace. 

After the slab is measured and positioned, it is loaded into the reheating furnace by the steel loader for heating.

Direct hot charging and rolling: when the production plans of continuous casting and hot rolling match, the qualified high-temperature continuous casting slab is transported to the weighing roller table through the charging roller table of the heating furnace. 

After weighing and checking, it enters the charging roller table of the reheating furnace. 

After the slab is measured and positioned in front of the designated reheating furnace, it is loaded into the reheating furnace by the charging machine for heating. 

Some of the directly hot loaded slabs transported through the unloading roller table need to be lifted by a crane once, placed on the loading roller table and directly sent to the reheating furnace area. 

If the steel-making plant can realize the direct hot charging and the slab is transported by the feeding roller table, some crane lifting operations can be reduced. 

After the slab is sent to the heating furnace through the charging roller table of the reheating furnace, it is loaded into the heating furnace by the supporting machine. 

After heating to the set temperature, it is supported by the tapping machine according to the rolling rhythm and placed on the discharging roller table of the reheating furnace.

After the heated slab is discharged from the furnace, it is transported through the conveying roller table. After descaling by the high-pressure water descaling device, the slab is sent to the fixed width press for side pressing and fixed width as required. The maximum one-time width reduction of the fixed width press is 350mm. 

Then it is transported by the roller table into the first two high reversible roughing mill and the second four high reversible roughing mill. The slab is rolled into an intermediate slab with a thickness of about 30-60mm according to the process requirements. 

The vertical roll mill in front of each roughing mill can control the width of the intermediate billet. 

An intermediate waste billet pushing device is arranged between R2 and the flying shear to push the intermediate waste billet onto the operation side bench of the intermediate roller table. 

The intermediate billet is transported by the intermediate roller table with thermal insulation cover to the cutting head and tail of the flying shear. The thermal insulation cover is conducive to reducing the heat loss of the intermediate billet and the temperature difference between the head and tail of the belt billet.

An edge heater is set in front of the flying shear, which can reduce the temperature difference between the edge and the middle part of the intermediate billet, improve the uniformity of strip steel performance and improve the quality of rolled piece shape. 

The cutting head flying shear is equipped with an intermediate blank head and tail shape detector and a cutting optimization control system to realize optimal cutting and reduce the loss of cutting head and tail. 

The strip blank after head cutting is removed from the secondary iron oxide scale by the high-pressure water descaling device before finishing rolling, and is guided into the finishing rolling unit by the vertical roll before finishing rolling. 

The intermediate billet is rolled into 1.2 ~ 25.4mm finished strip by F1 ~ F7 four roll finishing mill.

The threading speed, acceleration, maximum rolling speed, reduction of each stand, roll shifting stroke of work roll and roll bending force of each stand of the finishing mill are calculated and set by the computer control system according to the variety and specification of rolled strip to realize the closed-loop control of strip shape. 

In order to effectively control the quality of strip steel, the rolling line detection instruments such as crown, flatness, thickness, width and temperature are set at the outlet of F7 finishing mill, and the rolling line detection instruments such as strip surface quality, width and temperature are set at the inlet of coiler. 

The strip rolled by the finishing mill is cooled from the final rolling temperature to the specified coiling temperature by the strip laminar cooling system on the output roller table. 

The cooling water volume is set and calculated by the computer according to the steel strip cooling mode and final coiling temperature.

When the coiler passes through the strip before biting into the strip, the speed of the output roller table, pinch roll, auxiliary roll and drum is ahead of the rolling speed of the final frame; 

When the strip steel is bitten by the coiler, the output roller table, pinch roll and coiler will conduct speed-up rolling synchronously with the finishing mill; 

When the tail of the strip steel leaves the end frame, the output roller table and pinch roller shall slow down, that is, lag behind the coiling speed of the coiler until the tail of the hot rolled coil.

After coiling, the steel coil is lifted out by the coil unloading trolley to the baler for bundling. Then the steel coil is transported backward by the steel coil transportation system. 

After baling, weighing and marking, it is transported to the hot-rolled steel coil finished product warehouse, cold-rolled raw material warehouse and finishing raw material warehouse respectively. 

After the steel is returned to the hot rolling and coiling warehouse for inspection, the raw materials that need to be transported to the hot rolling and coiling warehouse for inspection are sent to the hot rolling and coiling warehouse for inspection, and the finished products are sent to the hot rolling and coiling warehouse for inspection respectively.

The steel coils cooled in the steel coil warehouse shall be sent to the leveling and winding unit, steel plate cross cutting unit, cold rolling workshop or delivered according to the sales plan according to the next processing process requirements. The steel coil is transported and stacked in the way of horizontal coil. 

The transportation system for the steel coil to the steel coil warehouse or cold rolling raw material warehouse adopts the pallet transportation system, and forms a transportation network together with the transportation system of 1780 hot rolling plant, which is uniformly controlled by the computer.

From the slab entering the slab warehouse to the delivery of finished products, the whole process tracks the slab, rolled piece and coil through the rolling line material tracking system and two warehouse management system, so as to realize the automatic production control of computer.

Please contact stella@hanrm.com for a quotation.
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Metallurgical Slitting Rolling Technology

Metallurgical Slitting Rolling Technology

Whether the rolled piece can be cut is not only related to the thickness of the connecting part and the relative reduction but also closely related to the pass shape and the size of the slit rolling. In the pass design of the slitting process, the design of pre-cutting and slitting holes is particularly important, followed by the vertical box pass. The pass design of these passes is related to the success of slitting and rolling.

The pre-cut split hole is a transitional pass to ensure the smooth slitting of the split hole. Its purpose is to reduce the unevenness of the split pass, so that the splitting wedge can complete the pressing and positioning of the opposite rolling piece, and accurately distribute the rolling. The cross-sectional area of ​​the piece reduces the burden of cutting the hole as much as possible, thereby improving the stability and uniformity of the cutting. The design should have sufficient reduction, the elongation coefficient should be 1.3 to 1.4, the expansion coefficient should be about 0.6, and the height of the connecting belt should be 0.45-0.48 times the height of the hole.

The slitting hole is basically formed by the connection of a double round pass and a slitting wedge. The function of the slitting pass is that the slitting wedge continues to press down the middle of the pre-slit rolling piece, rolling out the same shape as the pass Rolled pieces, so that the thickness of the connecting strip meets the needs of tearing apart multiple rolling pieces in parallel. In the process of slitting and rolling, due to the effect of the slitting wedge, the limitation of the non-porous side wall on both sides of the rolling piece in the slitting pass, and the rolling piece has a more complex cross-sectional shape, the changes of the rolling piece are as follows: Different from conventional rolling. Its main features are as follows. Severe uneven deformation. The compression deformation at the cutting wedge is often greater than that of other parts.

Splitting deformation is no extension or small extension deformation. When slitting the rolled piece in the slitting hole, the reduction at the bottom of the groove is small, while the reduction at the slitting wedge is larger, and the metal is subjected to the action of the slitting wedge, and the horizontal component force pointing to the widthwise direction is larger. It is a forced widening deformation, so the overall extension is small and the widening is larger.

When multiple roots are divided, the cross-sections of the rolling pieces in the middle and the sides are different, and the width of the rolling pieces on both sides is large and the extension is small; the head and middle dimensions of the rolling piece are also different, and the width of the head and tail is widened. The wedge angle of the slitting hole should be reasonable. If it is too small, it will not cut cleanly or cut. If it is too small, it will cause too much clamping force on the slitting wheel and increase the load of the slitting wheel, generally 60°-65°; The rounded corner of the tip is preferably 1～1.5mm. Too sharp will accelerate the wear of the roll; the thickness of the connecting belt should be close to the gap of the roll, 1-2mm is best; the elongation coefficient is 1.08～1.15, and a certain amount is reserved. Broad room for expansion.

Please contact stella@hanrm.com for a quotation.
And free send inquiry to us.
Email: stella@hanrm.com   stellarollingmill@gmail.com
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Hot Rolling VS Cold Rolling

 

Hot Rolling VS Cold Rolling

Both hot rolling and cold rolling are the forming processes of section steel or steel plate. They have a great influence on the structure and properties of steel. The rolling of steel is mainly hot rolling, and cold rolling is only used to produce small section steel and thin plate. 

Hot Rolling

Advantages: 

It can destroy the casting structure of ingot, refine the grain size of steel and eliminate the defects of microstructure so that the structure of steel is compact and the mechanical properties are improved. This improvement is mainly reflected in the rolling direction so that the steel is not isotropic to a certain extent. The bubbles, cracks, and porosity formed during pouring can also be welded under the action of high temperature and pressure. 

Disadvantages: 

1. After hot rolling, non-metallic inclusions (mainly sulfides and oxides, as well as silicates) in steel are pressed into thin sheets, resulting in stratification (interlayer). Layering deteriorates the tensile properties of steel along the thickness direction, and interlaminar tearing may occur when the weld shrinks. Local strain induced by weld shrinkage often reaches several times of yield point strain, which is much larger than that caused by the load.

2. Residual stress caused by uneven cooling. Residual stress is the internal self-balanced stress without external force. Hot rolled section steel of various sections has such residual stress. The larger the section size of general section steel, the larger the residual stress. Although the residual stress is self-balanced, it has some influence on the performance of steel members under external forces. For example, it may have adverse effects on deformation, stability, and fatigue resistance. 

Cold Rolling

Cold rolling refers to the process of cold drawing, cold bending, and cold drawing of steel sheets or strips into various types of steel at room temperature.

Advantage:

With fast forming speed, high yield, and no damage to the coating, it can be made into various cross-section forms to meet the needs of service conditions. Cold rolling can make steel produce large plastic deformation, thus improving the yield point of steel. 

Disadvantage:

1. Although there is no hot plastic compression in the forming process, there are still residual stresses in the section, which will inevitably affect the overall and local buckling characteristics of steel;

2. Cold-rolled section steel generally has an open section, which makes the free torsional stiffness of the section lower. Torsion is easy to occur in bending, flexural-torsional buckling is easy to occur in compression, and torsional resistance is poor;

3. Cold-rolled formed steel has a smaller wall thickness and no thickening at the corner of the plate connection, so it has a weak ability to withstand local concentrated loads.

The main differences between hot rolling and cold rolling as following:

Local buckling of the cold-rolled section is allowed to take full advantage of the post-buckling bearing capacity of members, while local buckling of the hot-rolled section is not allowed.

The causes of residual stresses in hot rolled and cold rolled sections are different, so the distribution of residual stresses in sections is also very different. The distribution of residual stress on the section of cold-formed thin-walled steel is bending, while that on the section of hot-bonded steel or welded steel is a thin film.

The free torsional stiffness of hot-rolled section steel is higher than that of cold-rolled section steel, so the torsional resistance of hot-rolled section steel is better than that of cold-rolled section steel.

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