Steel Plate Production Process

Steel plate is a widely used metallic material in construction, machinery manufacturing, transportation, and other fields. Its production process involves multiple stages, from raw material preparation to final forming, requiring a series of rigorous technological steps. Modern steel plate production mainly employs two process routes: long-process and short-process. The core objective is to obtain plates with the required strength, toughness, dimensional accuracy, and surface quality.

 

Raw Materials for Steel Plate Production

The basic raw material for steel plate production is iron. The form of raw materials varies depending on the process route.

 

1. Iron Ore

This is the starting point of the long-process. It mainly uses hematite and magnetite. After beneficiation to improve the iron grade, it is made into sinter or pellets for use in blast furnaces.

 

2. Scrap Steel

This is the main raw material for short-process electric arc furnace steelmaking. It comes from recycled scrap steel and industrial offcuts. Using scrap steel effectively achieves resource recycling.

 

3. Pig Iron

In the long-process, liquid pig iron is obtained by smelting iron ore in a blast furnace; in the short-process, a certain proportion of pig iron is sometimes added to adjust the composition.

 

4. Alloying Elements

To impart specific properties to steel plates, various alloying elements, such as manganese, silicon, chromium, nickel, and molybdenum, are added during the smelting process.

 

5. Auxiliary Materials

These include lime and fluorite for slag formation, and coke, pulverized coal, and natural gas as fuel and reducing agents.

Steel Plate Production Process Flow

Steel plate production is a complex system engineering project, mainly divided into four stages: ironmaking, steelmaking, continuous casting, and rolling.

 

1. Ironmaking Stage

The long-process steelmaking process begins with ironmaking. This process takes place in a blast furnace.

 

The blast furnace is a vertical cylindrical furnace. Iron ore, coke, and flux (limestone) are alternately charged from the top in a specific ratio.

 

High-temperature hot air is blown in through the tuyeres in the hearth, causing the coke to burn and produce high-temperature and carbon monoxide-reducing gases.

 

As the charge descends, the ore is gradually reduced and melted into molten iron, while the gangue combines with the flux to form slag.

 

Finally, the molten iron and slag, with their different densities, separate within the hearth and are discharged from the taphole and slag outlet, respectively. The resulting liquid pig iron is then transported to the steelmaking process.

 

2. Steelmaking Stage

The core task of steelmaking is to adjust the carbon content of the molten iron and remove harmful impurities such as phosphorus and sulfur, while precisely adding alloying elements to obtain steel with the required composition.

 

Converter Steelmaking (Mainly suitable for long processes)

Molten iron from the blast furnace is charged into the converter, a small amount of scrap steel is added, and then high-pressure oxygen is blown in. The oxygen reacts with elements such as carbon, silicon, manganese, and phosphorus in the molten iron, releasing heat.

 

By adding slag-forming agents and controlling the blowing process, slag is formed to remove impurities. At the end of smelting, samples are taken for analysis, and alloying is performed to adjust the composition, obtaining the target molten steel.

 

Electric Furnace Steelmaking (Mainly suitable for short processes)

Using scrap steel as the main raw material, it is melted using the arc heat generated by graphite electrodes. After melting, impurities are removed and the composition is adjusted through refining methods such as oxygen blowing and slag formation. Electric arc furnaces (EAFs) offer flexibility and are suitable for small-batch, multi-variety alloy steel production.

 

Ladle Refining

Whether from a converter or EAF, molten steel typically needs to be further purified and its composition and temperature precisely adjusted in a refining furnace (such as an LF furnace or RH vacuum treatment unit) to achieve higher quality standards and meet the production requirements of high-end steel plates.

 

3. Continuous Casting

This is the process of converting qualified molten steel into solid billets.

 

The molten steel is poured into a water-cooled crystallizer through a ladle and tundish.

 

Cold water circulates around the crystallizer, rapidly cooling and solidifying the molten steel inside to form a billet shell.

 

The billet with its liquid core is continuously pulled out by a leveling machine and enters a secondary cooling zone for further water cooling until complete solidification.

 

Finally, it is cut into slabs of specific lengths according to subsequent rolling requirements. Continuous casting replaces the traditional ingot casting-billing process, significantly improving production efficiency and metal yield.

 

4. Rolling

This is the crucial process that gives the steel plate its final shape, dimensions, and properties. The main processes include heating, rolling, and finishing.

 

Slab Heating

The continuously cast slab is fed into a walking beam furnace and uniformly heated to the temperature required for rolling (typically around 1200°C) to improve its plasticity and reduce its resistance to deformation.

 

Rough Rolling

The hot slab first passes through a roughing mill (usually a reversible mill) for multiple passes, rolling the thick slab into a strip of intermediate thickness.

 

Finish Rolling

The strip enters a finish mill (composed of multiple four-high mills connected in series) and is continuously rolled at high temperatures to further reduce its thickness, achieving the target specifications and forming a specific strip shape.

 

Cooling and Coiling

The thin strip exiting the finish mill is cooled to a predetermined temperature at a controlled rate by a laminar flow cooling system. This process has a decisive impact on the microstructure and final mechanical properties of the steel sheet. Subsequently, it is coiled into a hot-rolled coil by a coiler or directly proceeds to subsequent processes.

 

Cold Rolling

For steel sheets requiring thinner thickness, higher surface quality, or specific properties, hot-rolled coils must be pickled to remove iron oxide scale before cold rolling at room temperature. After cold rolling, the steel sheet hardens and requires annealing heat treatment to restore its plasticity. Finally, leveling rolling may be performed to achieve the desired surface and properties.

Subsequent Processing and Finishing

Rolled steel sheets may require various treatments depending on their application.

 

1. Heat Treatment

Through processes such as annealing, normalizing, quenching, and tempering, the internal microstructure of the steel sheet is adjusted to obtain different combinations of strength, hardness, and toughness.

 

2. Surface Treatment

To prevent corrosion or meet special requirements, the surface of the steel sheet is treated. Common treatments include galvanizing (hot-dip galvanizing or electroplating), tin plating, color coating, and film coating.

 

3. Mechanical Processing

This includes leveling (improving sheet shape), tension leveling (relieving stress and correcting shape), cross-cutting (cutting the steel coil into sheets of fixed length), and slitting (slitting).

 

Quality Control and Testing

Strict quality control is implemented throughout the entire steel plate production process.

 

1. Composition Analysis

Rapid and accurate chemical composition analysis is performed on molten steel, billets, and finished products using equipment such as spectrometers.

 

2. Performance Testing

Samples are taken from finished steel plates and tested in the laboratory for their mechanical properties (such as tensile strength, yield strength, elongation, impact toughness, etc.) and physical properties.

 

3. Non-destructive Testing

Using techniques such as ultrasonic waves, eddy currents, and X-rays, defects such as inclusions, delamination, and bubbles that may exist within the steel plate are detected without damaging the material.

 

4. Dimensional and Appearance Inspection

Thickness gauges, shape gauges, vision systems, etc., are used to inspect the thickness, width, straightness, surface finish, scratches, rust, etc., of the steel plates online or offline.

 

Main Applications of Steel Plate

Steel plates come in a wide variety of types. Based on thickness, they can be divided into thick plates, medium plates, and thin plates; based on rolling state, they can be divided into hot-rolled plates and cold-rolled plates; and based on material, they can be divided into carbon structural steel plates, low-alloy high-strength steel plates, stainless steel plates, electrical steel plates, etc.

 

1. Structural Steel Plates

Used in construction, bridges, shipbuilding, pressure vessels, engineering machinery, etc., emphasizing strength, toughness, and weldability.

 

2. Automotive Steel Plates

Require good stamping formability, surface quality, and certain strength, including deep-drawn plates and high-strength plates.

 

3. Pipeline Steel Plates

Used in the manufacture of oil and gas transmission pipelines, requiring high strength, high toughness, good weldability, and corrosion resistance.

 

4. Electrical Steel Plates

Mainly used in the manufacture of motor and transformer cores, requiring high magnetic permeability and low core loss.

 

5. Stainless Steel Plates

Possessing corrosion resistance, heat resistance, and aesthetic appeal, they are widely used in kitchenware, architectural decoration, chemical equipment, and other fields.

 

Summary

In conclusion, steel plate production is a crucial component of the metallurgical industry, and its technological development has consistently focused on improving efficiency, reducing costs, enhancing quality, developing new varieties, and achieving green manufacturing. Meticulous control at every stage, from raw materials to finished products, ensures that this fundamental material can meet the ever-growing and diversified needs of various sectors in modern society.