I. What is Iron and Steel

Iron and steel are alloys composed of iron (Fe), carbon (C), silicon (Si), manganese (Mn), phosphorus (P), sulfur (S), and small amounts of other elements. Among these, except for iron (Fe), the carbon content plays a major role in determining the mechanical properties of iron and steel, so they are collectively referred to as iron-carbon alloys. They are the most important and widely used metal materials in engineering technology.

2. Meaning of Steel

Steel is an iron-carbon alloy with a carbon content ranging from 0.03% to 2%. Carbon steel is the most commonly used ordinary steel, featuring easy smelting and processing. According to the difference in carbon content, carbon steel is further divided into low-carbon steel, medium-carbon steel, and high-carbon steel. Alloy steel, also called special steel, is made by adding one or more alloying elements to carbon steel to change the microstructure and properties of the steel. Common alloying elements added to steel include Si, Mn, Cr, Ni, Mo, V, Ti, etc.

3. Meaning of Pig Iron

Iron-carbon alloys with a carbon content between 2% and 4.3% are called pig iron. Pig iron is hard and brittle, and has good pressure resistance and wear resistance. It is divided into white cast iron, gray cast iron, and ductile iron. White cast iron has a silvery-white appearance, is hard and brittle, and cannot be machined; it is a raw material for steelmaking, so it is also called steelmaking pig iron. Gray cast iron has a silvery-gray fracture, is easy to cut and cast, and has good wear resistance. Ductile iron has properties close to those of steel. Special cast iron can be obtained by adding special alloying elements to cast iron.

II. Iron and Steel Production Process

The modern iron and steel smelting process involves smelting iron ore into pig iron in a blast furnace, pouring the molten iron into a converter or electric furnace to smelt into steel, then casting the molten steel into continuous casting billets or steel ingots, and processing them into steel products for various purposes through plastic deformation methods such as rolling.

2. Integrated Iron and Steel Works

An integrated iron and steel works generally includes production links such as raw material processing, ironmaking, steelmaking, steel rolling, energy supply, and transportation. It is a complex and large-scale production system. Most iron and steel enterprises in China are such full-process integrated enterprises.

III. Ironmaking Raw Materials and Process

The main raw materials for blast furnace smelting include iron ore (natural rich ore and artificial rich ore), fuel (coke and injected fuel), and flux (limestone and dolomite). To smelt one ton of pig iron, approximately 1.60-1.65 tons of iron ore with a grade of 63%, 0.3-0.6 tons of coke, and 0.2-0.4 tons of flux are required.

2. Ironmaking Process

Blast furnace ironmaking is a traditional ironmaking method based on coke as energy. Cooperated with converter ironmaking, it is currently the main method for iron production. It is expected that the leading position of blast furnace ironmaking will not change for a quite long period. The essence of blast furnace ironmaking is an iron reduction process, in which coke acts as fuel and reducing agent to reduce iron from iron ore or iron-bearing raw materials from the oxide or mineral state to liquid pig iron at high temperatures.

3. Ironmaking Process Flow

During the smelting process, the furnace charge (ore, flux, coke) is batch-loaded into the furnace from the top through charging equipment in a determined proportion. The high-temperature hot air blown in from the lower tuyeres reacts with coke to generate high-temperature reducing gas, which rises and heats, reduces, melts, and forms slag from the furnace charge, triggering a series of physical and chemical changes. Finally, liquid slag and iron accumulate in the hearth and are periodically discharged from the blast furnace. During the rising process, the temperature of the gas flow decreases continuously and its composition changes gradually, and finally, blast furnace gas is formed and discharged from the top of the furnace.

IV. Steelmaking Raw Materials and Process

Decarburization → dephosphorization → desulfurization → deoxidation → denitrification, dehydrogenation, etc. → removal of non-metallic inclusions → alloying → temperature rise → solidification and forming.

2. Steelmaking Principle and Raw Materials

The steelmaking process is an oxidation process. The main method to remove impurities is to blow oxygen into the molten pool and add slag-forming agents to form molten slag for removal. The decarburization reaction is the main means of the steelmaking process, and elements such as silicon, manganese, phosphorus, and sulfur are also removed through oxidation reactions. The raw materials for steelmaking include pig iron, scrap steel, flux (limestone), deoxidizers (ferrosilicon, ferromanganese, aluminum, etc.), and alloy materials.

3. Steelmaking Process

Hot metal pretreatment → converter or electric arc furnace steelmaking → secondary refining (ladle refining) → continuous casting.

V. Steelmaking Processes (Key Links)

Continuous casting of steel is a process that continuously casts molten steel into steel billets through a continuous caster. Compared with ingot casting, continuous casting has the following advantages: simplified process and energy saving; reduced billet head cutting rate, with metal yield 7-12% higher than that of ingot casting; efficient solidification; and optimized forming.

The continuous casting process flow is as follows: Molten steel is injected into the mold through a tundish and quickly cooled to form a solidified shell with a certain thickness, while the interior remains liquid (semi-solid billet). The lower part of the billet is connected to a dummy bar extending into the bottom of the mold. After casting starts, the billet withdrawal machine pulls the billet in the mold at a certain speed through the dummy bar. When the billet passes through the secondary cooling zone of continuous casting, it is further cooled by water spraying until it is completely solidified. After complete solidification, the billet is straightened by a straightener, cut into specified lengths, and transported out through a conveyor.

2. Steel Rolling

The rolling process is a process in which the friction between the rolled piece and the rolls pulls the rolled piece into the space between rolls rotating in different directions, causing it to undergo plastic deformation. A general steel rolling process can be divided into: heating furnace, rough rolling, intermediate rolling, finish rolling, and finishing.

VI. Mechanical Properties of Iron and Steel (Key Indicators)

Iron and steel have good physical, mechanical, and technological properties, with six key indicators as follows:

When a steel material or sample is stretched, if the stress exceeds the elastic limit, the steel material or sample will continue to undergo obvious plastic deformation even if the stress no longer increases. This phenomenon is called yielding, and the minimum stress value at which yielding occurs is the yield point.

2. Yield Strength

The yield point of some metal materials is not obvious, making measurement difficult. Therefore, to measure the yield characteristics of materials, the stress that causes permanent residual plastic deformation to reach a certain value (usually 0.2% of the original length) is defined as the conditional yield strength, or simply yield strength.

3. Tensile Strength

The maximum stress value reached from the start of stretching to fracture of the material. It represents the ability of steel to resist fracture. Corresponding to tensile strength, there are also compressive strength, flexural strength, etc.

4. Elongation

The percentage of the plastic elongation length of the steel after fracture to the original length of the sample is called elongation or percentage elongation.

5. Yield Ratio

The ratio of the yield point (yield strength) of steel to its tensile strength is called the yield ratio. The higher the yield ratio, the higher the reliability of structural parts. Generally, the yield ratio of carbon steel is 0.6-0.65, that of low-alloy structural steel is 0.65-0.75, and that of alloy structural steel is 0.84-0.86.

6. Hardness

Hardness refers to the ability of a material to resist the indentation of a hard object on its surface. It is one of the important performance indicators of metal materials. Generally, the higher the hardness, the better the wear resistance. Common hardness indicators include Brinell hardness, Rockwell hardness, and Vickers hardness.

VII. Factors Affecting the Properties of Iron and Steel

Influence of Chemical Composition on the Properties of Iron and Steel

Carbon is the most important element second only to iron. As the carbon content increases, the strength of steel increases, while the plasticity and toughness (especially low-temperature impact toughness) decrease. At the same time, the weldability, corrosion resistance, and cold bending performance are significantly reduced. Therefore, the carbon content of structural steel is generally not more than 0.22%, and that of welded structures should be less than 0.2%.

2. Manganese

Manganese is a weak deoxidizer. An appropriate manganese content can effectively improve the strength of steel and eliminate the hot brittleness effect of sulfur and oxygen on steel without significantly reducing the plasticity and toughness of steel. The manganese content in carbon structural steel is 0.3%-0.8%, and that in low-alloy steel is generally 1.0%-1.7%.

3. Silicon

Silicon is a strong deoxidizer. An appropriate silicon content can improve the strength of steel without obvious adverse effects on plasticity, toughness, cold bending performance, and weldability. However, when the silicon content is too high, it will reduce the plasticity, toughness, corrosion resistance, and weldability of steel.

4. Vanadium, Niobium, Titanium

Vanadium, niobium, and titanium can all refine the grain size of steel. Low-alloy steels in China all contain these three elements. As alloying elements other than manganese, they can not only improve the strength of steel but also maintain good plasticity and toughness.

5. Oxygen, Nitrogen

Oxygen and nitrogen are also harmful impurities, which can enter the metal from the air when the metal is in a molten state. Oxygen can cause hot brittleness of steel, and its effect is more intense than that of sulfur; nitrogen can cause cold brittleness of steel, similar to phosphorus.

VIII. Classification of Iron and Steel

Ordinary steel (P ≤ 0.045%, S ≤ 0.05%)

High-quality steel (both P and S ≤ 0.035%)

High-grade high-quality steel (P ≤ 0.035%, S ≤ 0.03%)

2. Classification by Chemical Composition

Carbon Steel

Low-carbon steel (C ≤ 0.25%)

Medium-carbon steel (C: 0.25%-0.6%)

High-carbon steel (C > 0.6%)

Alloy Steel

Low-alloy steel (total alloying element content ≤ 5%)

Medium-alloy steel (total alloying element content: 5%-10%)

High-alloy steel (total alloying element content > 10%)