What is the general temperature of hot rolling rolls and what impact does it have on the quality of hot rolled strip steel?

Mastering and controlling the temperature of hot rolling rolls is very critical in hot-rolled strip manufacturing, which can effectively improve product quality and technical level, thereby promoting the development of the entire industry.

Keyword: Hot Rolling temperature

The temperature of hot rolling rolls is generally around 1000°C. If the hot rolling roll temperature is too high or too low, it will have an adverse effect on the quality of the hot rolled strip.

What is the general temperature of hot rolling rolls

Common value ranges of hot rolling roll temperatures

Hot rolling roll temperature is one of the indispensable parameters in the hot rolling process. Normally, the hot rolling roll temperature is adjusted according to actual production needs. Generally speaking, the temperature of hot rolling rolls is around 1000°C, and in some cases it can be as high as 1200°C. There will be certain differences in the hot rolling roll temperatures of different rolling mills and different materials, which need to be adjusted and monitored according to actual conditions.

Effect of hot rolling roll temperature on hot rolled strip quality

The quality of hot rolled strip is affected by many factors, one of the most important factors being the hot rolling roll temperature. Generally speaking, if the temperature of the hot rolling roll is too high or too low, it will have an adverse effect on the quality of the hot rolled strip, which is specifically reflected in the following aspects:

1). If the temperature of the hot rolling roll is too high, the surface roughness of the hot-rolled strip will increase, and defects such as oxide scale and local pitting will easily occur, thus affecting the surface quality of the hot-rolled strip.

2). If the hot rolling roll temperature is too low, the surface quality of the hot-rolled strip will be poor, and the mechanical properties of the hot-rolled strip will also decrease. Indicators such as strength and toughness will be significantly reduced.

Therefore, it is very important to understand and control the hot rolling roll temperature to help us produce higher quality hot rolled strip. At the same time, the temperature of the hot rolling roll not only affects the surface quality of the hot-rolled strip, but also has a great impact on its internal quality, such as grain size and structure. Therefore, in the actual production process, we need to comprehensively consider the impact of different factors on hot-rolled strip steel, and ultimately produce hot-rolled strip steel products that meet standard requirements and have stable quality.

Hot rolled strip production technology and common quality problems

  • Hot rolled strip technology

The billet required for hot-rolled strip steel is generally a rough-rolled slab or a continuous-cast slab. Its appearance, curvature, dimensional tolerance and chemical composition are all technical requirements that need to be focused on. The quality of the slab is the basis for normal hot rolling processing. It is required that the internal and external surfaces of the slab should be free of defects such as looseness, pores, inclusions, and segregation. Hot rolled strips mainly have the following technical requirements:

1)High dimensional accuracy requirements.

Dimensional accuracy mainly includes length, width and thickness accuracy. Dimensional accuracy is the most difficult technical indicator to control in the production of hot-rolled strips, and has a great impact on the subsequent reprocessing and performance of the strips. Among them, the accuracy of thickness is more prominent. If the error is too large, it will have a negative impact on the subsequent stamping process. Moreover, if the dimensional accuracy is poorly controlled, it is easy to cause metal waste in subsequent processing steps. Some small changes in thickness and size will affect the performance of the plate and metal consumption. When the billet is heated in the heating furnace, the heating temperature, heating time and steel passing speed are all process indicators that need to be controlled. The heating system for the preheating section, heating section, soaking section and other links should be reasonably formulated according to the process requirements of each steel type. In order to prevent uneven heating of the billet, as well as over-burning, oxidation, steel sticking, decarburization and other phenomena, hot-rolled products are relatively difficult to control thickness and size due to their processing characteristics. Therefore, hot rolling is often called the precursor process of cold rolling, and its products are mostly used as blanks for cold rolling. In order to improve the rolling accuracy as much as possible, it is generally produced according to the negative tolerance rolling process.

2) The shape of the plate should be good.

Hot-rolled products have relatively high requirements for plate shape. Not only is the plate shape required to have no obvious appearance defects, but the thickness, curvature, etc. must also strictly meet the requirements. The curvature of ordinary thin plates should be controlled within 20mm; the curvature of ordinary medium-thick plates per meter should not be greater than 15mm. For wide and thin plates, they are very sensitive to uneven deformation and are prone to uneven deformation. . Uneven deformation of the plate shape during strip rolling is the main cause of poor plate shape and uneven thickness, and is closely related to the accuracy of strip thickness control.

3) The surface quality should be good.

Hot-rolled strip steel is often used as structural steel. Therefore, defects such as cracks, scabs, folds, scratches, bubbles, rust, and intrusion of oxide scale are not allowed on its surface. Since these defective parts can withstand less stress than other normal parts, these defective parts can easily become weak links where stress is concentrated in the strip, which is the source of corrosion and cracking of the strip. For example, when producing silicon steel sheets, the iron oxide scale on the surface will also be quickly punched to the surface of the strip during the deep drawing process, which will lead to cracks, roughness on the surface of the strip, and rapid wear of the stamping tools. Therefore, the surface quality of hot-rolled strips must be ensured, and processes such as heating, rolling, and high-pressure water descaling must be strictly controlled to eliminate quality hazards.

4) Requirements for mechanical properties.

The requirements for the mechanical properties of plate and strip steel mainly include: process performance, mechanical properties, chemical properties and physical properties of special plates and strips, etc. The mechanical characteristics of hot-rolled products are mainly manifested in the uneven structure and properties, and their strength and plasticity indicators are between those of cold work hardened products and fully annealed products. General structural steel plates have higher requirements for process performance such as welding and cold bending. The requirements for mechanical properties are not strict. For steel plate products with special purposes, such as silicon steel sheets, stainless steel plates, high-temperature alloy plates, etc., there are relatively high requirements for their high-temperature performance, acid resistance, alkali resistance, corrosion resistance, etc. For structural steel plates with important uses, their chemical composition, strength, shaping and other process properties must be strictly controlled.

Analysis of hot rolling production process

The process flow of hot rolling production mainly includes: heating, high-pressure water descaling, rough rolling, decoiling, shearing, finish rolling, cooling, and curling. The specific process flow is as follows: First, take out the slab to be rolled from the slab warehouse, put it into the furnace side roller table, and load the slab into the furnace by the steel loading machine. In the heating furnace, the slab will be heated according to the established heating schedule. After the slab reaches a certain temperature, it is lifted out by the tapping machine and placed on the tapping side roller table. Then the slab is sent to the rough rolling process under the transport of rollers, and is sequentially subjected to high-pressure water descaling, width setting, rough rolling, shearing, finishing rolling, descaling and other processing processes. Finally, the slab is processed and rolled into strip products of certain specifications and then sent to the output roller table. After laminar cooling, the pinch rollers are sent to the curling section. The coilers and coil-assistant rollers complete the curling of the strips. The strips are packed and numbered by the baling machine and then transported to the warehouse.

1) Rough rolling.

After the slab comes out of the heating furnace, a layer of iron oxide scale with a thickness of about 2~5mm will be formed on the surface. It must be cleaned, otherwise it will be pressed into the slab during the subsequent rolling process, causing quality defects that affect the surface quality and mechanical properties of the slab. High-pressure water descaling equipment is generally used to remove oxide scale on the surface of the slab, and the scouring force of water pressure is used to clean the iron scale attached to the surface of the slab. Hot-rolled strips use continuous casting billets. In order to meet the rolling requirements of different varieties and specifications of slabs, the width of the slab needs to be adjusted through the width-fixing process.

2) Finish rolling.

Finish rolling is mainly high-precision rolling of strip thickness, plate shape and other appearance dimensions. Its precision control is achieved by the currently advanced AGC roll gap adjustment and compensation system. Various interference factors in rolling are eliminated through roll gap adjustment, thereby meeting the high-precision appearance and size requirements of high-quality strip steel products.

3) Cooling and curling.

After the strip is finished rolled, the final rolling temperature is about 800℃~900℃, and it needs to be cooled down to reach the temperature of 550℃~650℃ required for the coiling process. Commonly used strip cooling processes in hot rolling production include laminar flow cooling, high-pressure water spray cooling, water curtain cooling, etc. After the strip is cooled down, it immediately enters the coiling process. The curling machine uses the remaining temperature after the strip is cooled to quickly curl the strip under the action of the coiling roller. The main function of the curling machine is to curl the finished rolled products into a fixed roll shape and into steel coils for easy bundling and transportation.

4) Finish rolling process.

The hot-rolled strip finishing equipment unit includes smoothing, coiling, shearing and other processes. The shearing of strip steel is mainly accomplished by flying shear equipment. The flying shear equipment uses the rapid cutting effect of the shear blade to cut the strip steel into plates of certain specifications to facilitate subsequent plate processing.

Analysis of common quality problems and causes in hot-rolled strip production

1) Crack problem.

Cracks are a common surface quality problem of strip steel.

Surface cracks on hot-rolled strip steel mostly appear in the form of longitudinal cracks, with different depths and lengths. Various crack defects existing in the slab itself are the main reasons for cracks in strip processing. Moreover, the cracks contained in this raw material blank cannot be eliminated in subsequent processes. In some cases, due to unscientific control of the crystallizer and heat flow, defects on the surface of the copper plate of the crystallizer, etc., the slab shell grows unevenly. Stress imbalance causes slab cracks.

2) The problem of scars.

The mark problem mainly affects the flatness of the surface of strip steel products, and can easily become a weak link in stress, causing the strip to break in subsequent rolling processes. Marks will be distributed on the surface of the strip in the form of irregular strips, spots, etc. In severe cases, rough pitting will appear on the surface of the strip. The scar problem is mainly caused by the intrusion of iron oxide scale. Find the reasons from the production process: First, the slab heating time is too long. There are many and thick iron oxide scales formed on the surface, and it is difficult to remove these iron oxide scales during the descaling process. As a result, it is pressed into the surface of the strip during the subsequent rolling process. Second, the pressure of the descaling water is not enough or the descaling process is unscientific, resulting in poor descaling effect. Third, the roll tonnage is too high or the rolling force is set unreasonably, causing the surface of the strip to be worn and pitted.

3) Indentation problem.

The indentation defects on the strip surface are generally distributed periodically, and the length and width are relatively fixed. The occurrence of indentation defects is mainly caused by damage to the roller surface, slag adhering to the roller surface or pieces falling off the rolled piece. Indentation problems seriously affect the flatness of the surface of hot-rolled strip steel.

4) Surface bulge.

The problem of strip surface bulging generally occurs at the edge of the strip, which is distributed along the rolling direction of the strip. The cause of the edge bulging defect of hot-rolled strip is mainly caused by local wear of the work rolls.

Common quality problems and solutions for hot-rolled strip steel

1) Solutions to crack problems.

First of all, we must pay attention to the maintenance of the continuous casting process equipment to prevent the coating from peeling off during the use of the equipment. Attention should also be paid to improving the protective slag system, and by summarizing the production experience of different steel types, formulating mold flow field areas and taper tables suitable for different steel types and specifications, so as to improve the production quality of hot-rolled strip steel.

2) Solutions to the marking problem.

The main focus should be on preventing iron oxide scale from being pressed in. First of all, reasonable heating measures must be formulated to avoid excessive formation of iron oxide scale due to too long heating time and too high heating temperature. The next step is to ensure the descaling effect, by adjusting the descaling water pressure, cleaning the nozzle blockage and other measures to ensure the descaling water pressure; at the same time, the rolling force and tonnage of the rollers must be reasonably adjusted to avoid strip surface wear.

3) Solutions to the indentation problem.

To solve the indentation problem of hot-rolled strip steel, you must first check whether the roll surface is worn or stuck to steel. If the wear is light, the roll surface can be repaired. If the indentation is serious, the roll must be replaced immediately.

4) Solutions to the bulge problem.

Optimize the crown of the roll and improve the grinding accuracy of the roll. We should also avoid the problems of poor water cooling effect and uneven roll wear caused by nozzle clogging and other problems. Rolls with serious wear should be repaired and replaced in time.

 

MM GROUP is one of the professional roll manufacturing base in China, which supply all kinds of large-size rolls for iron and steel enterprises with production capacity of 100,000 tons of all kinds of hot strip mill rolls, section mil rolls, rod mil rolls, cold rolling m rolls, casting and forging backup rolls.

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