Abstract: The main quality indexes of hot rolled strip steel products include surface quality, shape and dimensional accuracy, mechanical properties and so on. With the wide application of automatic control technology in hot rolling production line, the strip shape quality control has been very precise, and the quality index of the product has been greatly guaranteed. The 2,250mm hot rolling line of Hangang West District has an annual designed production capacity of 4.6 million tons, and the whole line is controlled by automation. In the process of putting the equipment into production and use, due to the existing problems in the process and control system, product dimensions and other aspects of quality problems have occurred, causing some quality complaints and objections from customers. In this paper, the production process of hot rolling is discussed, and some quality problems existing in hot rolling production and solutions are analyzed.

 

Key words: hot rolled strip steel; Production technology; Quality control

 

 

 

1 Introduction

 

Hot rolling production line mainly produces high-strength, high-precision and high-value-added double-phase steel, multiple steel and other steel types. The products are mainly used in the production of hull steel, automobile steel, bridge steel, boiler, pressure vessel and other high-weatherability structural steel, and can also be used as cold-formed steel, welded pipe and cold rolling production of basic blank. The main quality indexes of hot rolled strip steel products include surface quality, shape and dimensional accuracy, mechanical properties and so on. With the wide application of automatic control technology in hot rolling production line, the control of strip shape quality has been very precise, and the quality index of each item of the product has obtained a great guarantee. The 2,250mm hot rolling line of Hangang West District has an annual designed production capacity of 4.6 million tons, and the whole line is controlled by automation. In the process of putting the equipment into production and use, due to the existing problems in the process and control system, product dimensions and other aspects of quality problems have occurred, causing some quality complaints and objections from customers. In this paper, the production process of hot rolling is discussed, and some quality problems existing in hot rolling production and solutions are analyzed.

 

2. Hot rolling production

 

Line main equipment introduction

 

The main equipment of the hot rolling production line includes: stepping beam heating furnace, high pressure water descaling, fixed width press, roughing mill, flying shear, finishing mill, coiler, flat and coiler, etc.

 

(1) heating furnace. Walking beam heating furnace is mainly used for production buffering and heating of slab before rolling. In order to meet the requirements of billet heating and realize the flexible heating of billet in different stages, the heating furnace is divided into eight heating stages. At the same time, the heating furnace adopts the automatic temperature control system to realize the accurate adjustment of the temperature of each heating section of the heating furnace, aiming at different steel types, different specifications of the slab along the length direction of continuous heating.

 

(2) High pressure water descaling machine. In the rolling process, oxide sheet and other scales will be produced and need to be removed by high-pressure water spraying.

 

(3) fixed width machine. The fixed-width press is used for the first width reduction before roughing, and is arranged between the dephosphorizer and the 2-high reversing roughing mill. The main function of the fixed-width press is to change the appearance of the material and obtain uniform material deformation through the extrusion action of the extrusion hammer.

 

(4) roughing mill. Roughing mill is mainly a kind of mechanical equipment for side rolling in order to obtain intermediate billet with uniform width and neat edge, so as to prevent crack edge and drum shape on the edge of rolling parts and reduce metal consumption coefficient.

 

(5) Flying scissors. The flying shear is mainly used to cut the billet according to a certain length size, so as to facilitate the subsequent rolling processing. The flying shear is powered by two DC motors and driven by tooth receiver, crank shaft, spur gear reducer, etc. Under the drive of the motor and crank, the flying shear quickly cuts up and down, so as to complete the cutting of the billet according to a certain length specification.

 

(6) Finishing mill. Finishing mill is a kind of high-precision rolling of slab by hydraulic system and roll, which uses hydraulic AGC and other automatic control and automatic compensation systems. It is mainly driven and driven by motor, coupling, reducer, etc., in order to obtain high-precision rolling parts with appearance size.

 

(7) coiler. The coiler mainly crimp the strip steel, making it into steel coil for baling, transportation and storage. The coiler is also driven by hydraulic pressure, automatic control by the roll seam hydraulic fine regulation, the coiler roll step control SC, crimping tension control system, etc. The equipment has the advantages of long service life, less scratches in the inner ring, wear and maintenance.

 

(8) Leveling unit. The flattening unit is mainly responsible for the carbon steel and other types of steel coil flattening, through the extrusion of the flattening roll to obtain the ideal plate plate type, so as to improve the quality of the plate, convenient for the later processing of the steel coil.

 

3. Hot rolling strip technology

 

The billet required for hot rolled strip steel is generally rolling slab or continuous casting slab, and its appearance form, bending degree, dimensional tolerance and chemical composition are the technical requirements that need to be focused on. The quality of slab is the basis of normal hot rolling. It should be required that there should be no defects such as porosity, porosity, inclusion and segregation on the inside and outside surface of slab. Hot rolled strip mainly has 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 index to control in hot-rolled strip production, which has a great influence on the subsequent reprocessing and performance of strip. The thickness accuracy is more prominent. If the error is too large, it will have a bad influence on the subsequent stamping process. And the dimensional precision control is poor, it is also easy to cause the waste of metal in the follow-up processing link, some small thickness size changes will affect the performance of the plate and metal consumption. When the billet is heated in the heating furnace, the heating temperature, the heating time and the steel passing speed are the technical indexes that need to be controlled. According to the process requirements of each steel, the heating system of preheating section, heating section and soaking section should be reasonably formulated to prevent uneven heating, overburning, oxidation, sticking steel, decarbonization and other phenomena of billet. Because of the processing characteristics of hot rolled products, it is difficult to control the thickness size. Therefore, hot rolling is also often referred to as the preceding process of cold rolling, and its products are mostly used as blank of cold rolling processing. In order to improve the rolling accuracy as much as possible, negative tolerance rolling process is generally produced.

 

(2) Good plate shape.

 

Hot rolled products have high requirements on the shape of the plate, not only requires that the shape of the plate has no obvious appearance defects, but also the thickness, curvature and so on should strictly meet the requirements. The buckling of ordinary thin plate should be controlled within 20mm; The curvature of ordinary medium and thick plates is no more than 15mm per meter length. For wide and thin plates and strips, they are very sensitive to uneven deformation and easy to produce uneven deformation. The uneven deformation of plate shape during strip rolling is the main cause of bad shape and uneven thickness, which is closely related to the precision of strip thickness control.

 

(3) The surface quality is good.

 

Hot rolled strip steel is often used as structural steel, therefore, its surface is not allowed to crack, scab, folding, scratching, bubbles, rust, oxide sheet pressing and other defects. Compared with other normal parts, these defective parts can withstand less stress, so these defective parts are easy to become the weak link of stress concentration in the strip, which is the root cause of corrosion and rupture of the strip. For example, in the production of silicon steel sheet, the surface of the oxide sheet will also be quickly stamped to the strip surface in the deep drawing link, which will lead to cracking, roughness and rapid wear of stamping tools on the strip surface. Therefore, the surface quality of hot rolled strip must be guaranteed, and the process of heating, rolling and high pressure water descaling must be strictly controlled to eliminate potential quality problems.

 

(4) Requirements for mechanical properties.

 

The requirements for mechanical properties of strip steel include: process properties, mechanical properties, chemical properties and physical properties of special strip steel. The mechanical characteristics of hot rolled products mainly show that the microstructure and properties are not uniform, and the strength and plasticity indexes are between cold work hardening products and fully annealed products. General structural steel plate for welding, cold bending and other technical performance requirements are higher, and the mechanical properties are not strict, for steel plate products with special uses, such as silicon steel sheet, stainless steel plate, high-temperature alloy plate, etc., for its high temperature performance and acid resistance, alkali resistance, corrosion resistance will have relatively high requirements, for structural steel plate with important uses, Must strictly control its chemical composition as well as strength, shaping and other technological properties.

 

4. Analysis of hot rolling production process

 

The process of hot rolling production mainly includes: heating, high pressure water descaling, rough rolling, uncoiling, shearing, finishing rolling, cooling and curling. The specific technological process is as follows: first, take out the slab to be rolled from the slab warehouse, put it into the roll table on the side of the furnace, and load the slab into the furnace by the steel loading machine. In the heating furnace, the slab will be heated in accordance with the established heating system. When the slab reaches a certain temperature, it will be pulled out by the steel drawing machine and placed on the roll table on the side of the oven. Then the slab will be sent to the rough rolling process under the conveyor roller, and the processing processes such as high-pressure water descaling, fixed width, rough rolling, shearing, finishing rolling and descaling will be carried out successively. Finally, the slab is processed and rolled into a certain specification of plate and strip products, which are sent to the output roller table. After laminar cooling, the clamping roller is sent to the crimping section. The crimping of plate and strip is completed by the coiler and coiler, etc., and the packing machine is sent to the warehouse after spraying.

 

(1) Rough rolling. After the slab comes out from the heating furnace, a layer of oxide sheet 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 in the subsequent rolling process, which will affect the surface quality and mechanical properties of the slab quality defects. High pressure water descaling equipment is generally used to remove the sheet iron attached to the surface of the slab, and the scouring force of the water pressure is used to clean the sheet iron. Continuous casting billet is used in hot rolled strip. In order to meet the rolling requirements of slab of different specifications, the width of slab should be adjusted by the fixed width process.

 

(2) Finishing rolling. Finishing rolling is mainly for strip thickness, plate shape and other appearance size of high precision rolling. The precision control is realized by the current relatively advanced AGC roll gap adjustment and compensation system, through the roll gap adjustment to eliminate various interference factors in rolling, so as to meet the high precision appearance size requirements of high quality strip steel products.

 

(3) cooling and curling. After finishing rolling, the final rolling temperature of the strip is about 800℃~900℃, and it needs to be cooled to reach the temperature of 550℃~650℃ required for curling process. The cooling processes of strip steel in hot rolling mainly include laminar flow cooling, high-pressure water jet cooling and water curtain cooling. After the strip is cooled, it immediately enters the crimping process. The crimping machine uses the remaining temperature after the strip is cooled to crimp the strip quickly under the action of the coiling roll. The main function of the crimp machine is to crimp the finished rolled product into a fixed coil type, which becomes a steel coil, so as to facilitate the baling and transportation.

 

(4) Finishing rolling process. Hot rolling strip finishing equipment unit includes leveling, winding, cutting and other procedures. The cutting of strip is mainly accomplished by flying shear equipment, which uses the rapid cutting function of cutting edge to cut strip into a certain specification sheet, so as to facilitate the subsequent sheet processing.

 

 

 

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

 

(1) Crack problem.

 

Crack is a common surface quality problem in strip steel.

 

The surface cracks of hot rolled strip are usually in the form of longitudinal cracks with different depths and lengths. All kinds of cracks and defects existing in slab itself are the main cause of cracks in strip processing, and the cracks included by this raw material billet can not be eliminated in the subsequent process, and some cases are due to the mold and heat flow control is not scientific, mold copper plate surface defects and other reasons, resulting in uneven growth of slab shell. The stress imbalance results in slab cracks.

 

(2) Spot problem.

 

The spot problem mainly affects the surface flatness of strip steel products, and is easy to become a weak stress link, which leads to strip fracture in the subsequent rolling process. The markings will be distributed on the surface of the strip steel in the form of irregular ribbons and spots, which will cause rough pitting on the surface of the strip steel in serious cases. The markings problem is mainly caused by the pressing of the oxide iron sheet. From the production process link to find the reasons: first, the slab heating time is too long, its surface formed more and thicker oxide sheet, descaling process is difficult to remove these oxide sheet clean, resulting in the subsequent rolling link is pressed into the strip surface; Second, the descaling water pressure is not enough or the descaling process is not scientific, resulting in poor descaling effect; Third, the roll tonnage is too high or the rolling force setting is unreasonable, resulting in strip surface wear, resulting in pockmarked surface.

 

(3) indentation problem.

 

The indentation defects on the strip surface are generally distributed periodically and the length and width are relatively fixed. Indentation defects are mainly caused by roll surface injury, roll surface sticky slag or rolling pieces falling off. Indentation problems seriously affect the surface flatness of hot rolled strip steel.

 

(4) Surface uplift.

 

Strip surface uplift usually occurs at the edge of the strip, which is distributed along the rolling direction of the strip. The main reason for the edge uplift of hot rolled strip is the local wear of the work roll.

 

 

 

6. Measures to solve common quality problems of hot rolled strip steel

 

(1) Solutions to crack problems.

 

First of all, attention should be paid to the maintenance of continuous casting process equipment to avoid coating peeling during the use of equipment; It is also necessary to improve the protection slag system. By summarizing the production experience of different steel types, the mold flow field area and taper table suitable for different steel types and specifications are worked out, so as to improve the production quality of hot rolled strip steel.

 

(2) Solutions to the markings problem.

 

We should prevent the pressing of the oxide sheet. Firstly, reasonable heating measures should be formulated to avoid excessive formation of oxide sheet caused by too long heating time and too high heating temperature. Thirdly, the descaling effect should be ensured by adjusting the descaling water pressure and cleaning up the blockage of the nozzle. At the same time, roll force and tonnage should be adjusted reasonably to avoid strip surface wear.

 

(3) Measures to solve the indentation problem.

 

To solve the indentation problem of hot rolled strip steel, first of all, we must check whether the roll surface is worn, sticking steel and other problems. If the wear is light, the roll surface can be milled. If the indentation is serious, the roll must be changed immediately.

 

(4) Measures to solve the uplift problem.

 

Optimize the convex of the roll, improve the grinding accuracy of the roll, but also avoid the problem of poor water cooling effect and uneven wear of the roll caused by the blockage of the nozzle, etc., for serious wear of the roll should be repaired and replaced in time.

 

 

 

Take part in the essay

[1] LONG Biao. Common Quality Problems and Process Optimization of Hot rolled Strip Steel Production [J]. Science and Technology Research, 2014, (4) : 218

[2] Feng Xudong. Discussion on Common Quality Problems in Hot Rolled Strip Production [J]. China Metal Bulletin, 2018, (12)

[3] WANG Yongxin. Research on Common Quality Problems in the Production Process of Hot rolled Strip Steel [J]. Urban Construction Theory Research, 2016, (15) : 56

[4] Dong Xinghui. Analysis of Common Quality Problems in Hot Rolled Narrow Strip Steel Production [J]. Welded Pipe, 2009, (3) 46

About the Author: Yang Fan, male, born in September 1985, Handan Branch of Hesteel Co., LTD., engaged in research and development of steel rolling technology