Iron and steel

Taking the rolling process of medium and thick plates as an example, this article introduces the rolling process.

1、 Medium thick steel plate is an indispensable steel variety in the modernization construction of the country. It is widely used in the fields of large-diameter conveying pipes, pressure vessels, boilers, bridges, ocean platforms, various types of ships, tank armor, vehicles, building components, machine structures, etc. The level of medium thick plate rolling mill in a country is also one of the symbols of the equipment level of a country's steel industry, And to some extent, it is also a reflection of a country's industrial level// With the development of China's industry, higher requirements have been put forward for medium and thick steel plate products, both in terms of quantity and variety quality// Therefore, it is necessary and promising to study the rolling process of medium and thick plates. What is a medium thick plate? According to convention, thick plates are divided into medium plates, thick plates, and extra thick plates according to their thickness. Traditionally, medium plates, thick plates, and extra thick plates are collectively referred to as medium thick plates. At present, most of the steel plates produced by medium and heavy plate mills in China have a thickness of 4-250mm. The width is 1200-3900mm, and the length generally does not exceed 12m.

2、 Introduction to Rolling Process

（1） Raw materials for medium and thick plate production

The raw materials for thick plate production have undergone the evolution of steel ingots, blooming billets, and continuous casting billets. Continuous casting billet: Using a slab continuous casting machine, molten steel is continuously injected into its mold, and a certain thickness and width of slab is continuously pulled out and cut into a certain length.

Advantages: 1. Simplify and shorten the production process, reduce equipment and funds, etc. 2. Save energy and improve metal yield. 3. Uniform physical and chemical properties. 

Therefore, using continuous casting billets is very meaningful for improving product quality and reducing production costs.

（2） Slab heating  

The slab heating adopts a continuous heating furnace, and the heating process is divided into preheating section, heating section, and soaking section. 

Preheating section: Firstly, the heating is slow to avoid excessive temperature difference inside the thick slab caused by too fast heating speed, resulting in thermal stress and internal cracks; The second is to use the waste heat from the heating section to preheat the slab, which can save energy. Heating section: Increase heating speed.

Homogenization section: to evenly heat the interior of the slab.

The continuous reheating furnace can be divided into pusher type reheating furnace and step-by-step heating furnace according to the different ways of slab moving in the furnace. Due to uneven heating of the pusher type heating furnace slab; There are scratches on the bottom surface of slab, so it is gradually replaced by step-by-step heating furnace.

Advantages of step-by-step heating furnace: 1. uniform heating; 2. no scratch on slab surface; 3. no arching of slab.

（3） Phosphorus removal

Before and during the rolling process of the slab, it is necessary to remove the furnace generated oxide scale during the heating process and the regenerated oxide scale generated during the rolling process, which is called phosphorus removal. Phosphorus removal is very important for ensuring the surface quality of the Hong Kong version. The phosphorus removal equipment is divided into: here we choose "high-pressure water phosphorus removal", which uses the impact force of high-pressure water to break and remove the oxide scale on the surface of the slab and has been widely used.

（4） Rough rolling

The phosphorus removed slab is fed into the roughing mill. The main task of the rough rolling stage is to widen the slab to the required width and conduct large compression and extension.

According to the different ways in which the slab enters the rolling mill, it can be divided into full transverse rolling method, full longitudinal rolling method, and transverse rolling longitudinal rolling method. When the length of the slab is greater than or equal to the width of the steel plate, the full transverse rolling method can be directly used. 

When the width of the slab is slightly greater than or equal to the width of the finished steel plate, the full longitudinal rolling method can be directly used.

Cross rolling - longitudinal rolling method: includes three processes: longitudinal rolling, transverse rolling widening, and longitudinal rolling. Use a rotating roller table to rotate the slab 90 ° during widening rolling.

Rotating roller table: The roller table rollers are in a stepped or conical shape, with adjacent rollers arranged alternately and turning in opposite directions, causing the slab to generate rotational torque and rotate. When the slab does not need to be rotated, all roller table rollers turn in the same direction and are used as general roller table rollers.

（5） Insulation device, head and tail cutting, and phosphorus removal

The insulation device located on the roller track between the roughing mill and the finishing mill is beneficial for improving product quality and saving energy. Head and tail cutting: Cut off the poorly shaped head and tail ends to facilitate the smooth entry of the slab into the finishing mill. In order to remove the secondary oxide scale generated during the rolling process, a phosphorus removal device is installed on the rolling mill.

（6） Precision rolling

The main task of the precision rolling stage is quality control, which is the core part of the rolling production process. At present, due to the increasingly high requirements for dimensional accuracy and quality of thick plates, the form of two four high rolling mills for precision rolling is increasingly valued.

（7） Straightening 

Straightening is to ensure that the flatness of the steel plate meets the standard requirements of the product. The straightening equipment for medium and thick plates can be roughly divided into roller straightening machines and pressure straightening machines. On plate 105, ordinary rolling requires the use of hot straightening, so we choose a roller type hot straightening machine. 

（8） Cooling

The cooling rate after rolling of steel plates has a significant impact on their metallographic structure and mechanical properties. Rapid cooling is adopted after rolling to restrain ferrite growth, further refine ferrite, make pearlite evenly distributed, produce fine bainite, and improve strength and impact toughness. After the steel plate passes through the hot straightening machine, a disc roller cooling bed is used to cool the steel plate. 3、 Flash process explanation uses continuous casting billets as rolling raw materials. The billets are placed on the receiving roller table and sent to the charging position. The billets are pushed into the furnace by the loading pusher. The billets are transported and heated by a stepper mechanism in the furnace. The heated billets are lifted from the furnace by the tapping machine and placed on the conveying roller table at the discharge end. They are then sent to the high-pressure water phosphorus removal box for primary phosphorus removal, and then enter the roughing mill for rough rolling. After secondary phosphorus removal, they enter the finishing mill for finishing rolling, The temperature of the finished rolled slab can reach up to 1000 ° C. The slab is quickly cooled to 700 ° C through a laminar cooling device for hot straightening, and the straightened steel plate enters the cooling bed for slow cooling.

Cold rolling section
 
       1. Steel and pig iron are both alloys of iron and carbon, and their main difference is their carbon content. The carbon content of steel is generally lower than 1.7%, while the carbon content of pig iron is generally between 1.7% and 4.3%.

2. The raw materials for ironmaking are iron ore, fuel, and flux, while the fuel used is coke

3. The methods of steelmaking include: 1. Oxygen converter steelmaking 2. Open hearth steelmaking 3. Electric furnace steelmaking, among which the most productive steelmaking method is converter steelmaking.

4. Baosteel's ladle powder spraying treatment is mainly used for desulfurization of molten steel during edge casting and controlling the morphology of inclusions in the steel.

5. A certain degree of deoxidation must be carried out at the end of steelmaking. The specific operation of deoxidation is to add deoxidizer with strong oxygen binding capacity into the furnace or ladle. The commonly used deoxidizers are Mn, Si and Al. Mn in the base is a weak deoxidizer, and Al is a strong deoxidizer. The deoxidization performance of Si is in the middle.

6. The deoxidation method of steel plays an important role in the quality of steel ingots. According to different deoxidation methods, steel ingots can be divided into boiling steel, gland steel, semi killed steel, and killed steel. 

7. Steel billets are very hard and have poor plasticity at room temperature, making them difficult to process. Generally, they need to be heated to 1150--1300 ℃ for rolling. This method is called hot rolling, and cold rolling is a method of first rolling the steel billets (ingots) to a certain size, and then rolling them in a cold state. 

8.At present, the most widely used method for removing iron oxide scale in cold rolling production is chemical treatment mainly based on acid washing.

9.After cold rolling, the process of removing rolling oil stains on the plate surface through cleaning is called "degreasing". The specific methods include electrolytic cleaning, spray cleaning, machine cleaning, and combustion degreasing.

10. Annealing treatment is the most important heat treatment process in the production of cold rolled steel strips. The steel strips are heated to a certain temperature (600-720 ℃) in the furnace according to the different types of steel, insulated, and then slowly cooled. After annealing, work hardening can be eliminated and good mechanical properties can be obtained. There are two methods of annealing: bell furnace annealing and continuous annealing. The working cycle of bell furnace annealing generally takes 3-5 days. There are two types of continuous annealing: horizontal and tower. The speed of strip steel passing through the unit can reach up to 600 meters/minute. Due to the high speed of strip steel operation, it only takes a few minutes in the furnace. After annealing, the finished steel strip should also be flattened (with a reduction rate of 0.50-4%) 

11. After cold rolling, the steel coil is directly sent to the hot dip galvanizing unit for treatment. After the surface of the steel strip is degreased, cleaned, and continuously annealed, it is immersed in a zinc pot (with molten zinc inside) for galvanizing. When the molten zinc comes out, the surface has already been coated with zinc, The method of blowing with an air knife on the two sides is used to control the thickness of the zinc layer and the size of the zinc flakes during the crystallization of the zinc layer is controlled by a zinc flake controller (the lead content in the zinc pot can also be used as a means to control the size of the zinc flakes until the product has zero zinc flakes).

12. The cold-rolled electrical steel strip enters a dedicated electrical steel annealing furnace for annealing and insulation coating. Medium and low grade electrical steel generally do not require intermediate annealing, and the purpose of finished annealing is to decarburize and control magnetism. Non oriented electrical steel can also be supplied in a state of incomplete annealing or without insulation layer, known as semi processed electrical steel.

1. Lubrication mechanism of hot rolling process

The lubrication of hot rolling process is a dynamic process, and the changes in lubricating oil under high temperature, high pressure, and high speed conditions in the deformation zone are difficult to simulate experimentally. One view is that during hot rolling, under harsh working conditions in the roll gap, the surface of the roll is prone to wear, resulting in the formation of pits and pits on the surface; And the pits and pits exacerbate wear, causing a vicious cycle, and the rollers are quickly scrapped due to wear. If lubricating substances can be sprayed on the surface of the rolled piece before it enters the roll gap, forming a lubricating film. Although a small part of this film may be burned off by the hot rolled piece before entering the roll gap, most of it is still bitten into the roll gap, forming a thin lubricating film on the contact surface between the work roll and the rolled piece. Although the contact time between the oil film and the roller is only a few seconds, the oil film can play a lubricating role before burning, reducing roller wear and avoiding pits and pits quickly.

Due to the characteristics of modern hot rolling process, such as high temperature, high pressure, and high speed, the instantaneous temperature of the hot rolling work roll surface can reach 600~700 ℃, and it is stable at 50~80 ℃ after cooling and heat dissipation. Therefore, to adapt to the characteristics of hot rolling process, hot rolling lubricating oil should have the following characteristics:

(1) Composed of highly polar molecules, it has excellent wettability on the surface of the roller and can quickly form a uniform thin lubricating film, reducing friction and wear.

(2) The lubricating film has strong adhesion and instantaneous high pressure resistance, providing stable and excellent lubrication in the roll gap, and preventing or reducing the formation of oxides on the work roll and support rod.

(3) The lubricating film in the roll gap can prevent direct contact between the roll and the rolled piece, avoiding adhesion between the roll and the rolled piece at high temperatures.

(4) The residual lubricating oil on the strip steel at the exit of the rolling mill should be burned out in the shortest possible time to prevent residual oil from remaining on the surface of the strip steel and forming new pollutants.

(5) Lubricating oil has high flash point and thermal decomposition stability.

2.The role of lubrication in hot rolling process has obvious advantages in applying lubrication technology in hot rolling:

(1) The friction coefficient between the roller and the rolled piece during hot rolling can be reduced. The friction coefficient without lubrication is generally 0.35. The friction coefficient can be reduced to 0.12 when using effective lubrication.

(2) It can reduce rolling force and easily roll thin gauge strip steel. Generally, the rolling force can be reduced by 10% to 25%, thereby reducing energy consumption.

(3) It can reduce roller consumption and reserve, improve operation efficiency. Under hot rolling conditions, the contact between the work roller and the strip steel and cooling water will generate oxides with high hardness such as Fe3O4 and Fe2O3, which stick to the surface of the roller, causing the roller to generate a dark surface, namely black skin. Black skin is the main cause of abnormal wear of the rolling rolls. After using hot rolling lubrication, a boundary lubrication film separates the roller from the workpiece, which can prevent the formation of black skin on the surface of the roller, reduce roller wear, and extend the service life of the working roller; It can reduce roll changing time, improve rolling operation rate, and reduce work roll reserve. At the same time, it also reduces the wear and reserve of the support roller.

(4) It can reduce the pressure of oxide skin and improve the surface condition of the rolling mill. After using hot rolling lubrication, a layer of lubricating film separates the rolling mill and the rolling piece, preventing the formation of black skin on the surface of the rolling mill. This improves the surface quality of the product and also improves the pickling efficiency.

(5) It can extend the rolling mileage, reduce the number of roll changes, and increase the operation time.