Steel Composition, Properties, Manufacturing, and Classification

Steel

Iron in its pure state lacks the strength and toughness for common applications. However, combining it with small amounts of carbon creates steel, a metal with properties varying according to its carbon content and other alloying elements like manganese, chromium, silicon, and aluminum.

Steel: Chemical Composition

Steel primarily consists of iron and carbon. It also contains trace elements, including sulfur and phosphorus, which are difficult to remove but are reduced to harmless proportions (<0.05%). Easier-to-obtain elements like silicon and manganese (0.2-0.9%) prevent oxidation of the molten metal. The remaining 97.5-99.5% is iron. Steels with this composition are called carbon steels. Their carbon content determines their classification, as shown below:

Resistance vs. Carbon Percentage

Alloy and Special Steels

Besides carbon, alloy steels include elements like chromium, nickel, molybdenum, tungsten, and vanadium, which modify or improve steel's properties. The effects of each element are:

Sulfur: An impurity tolerated up to 0.05%. It causes hot brittleness, but manganese (0.1-0.3%, with a minimum of 0.6% manganese) can neutralize this, creating free-cutting steels with lower resistance but higher machinability.

Cobalt: Used in high-speed tool steel to increase hot hardness. It's also used in refractories and enhances steel's magnetic properties.

Chromium: Forms hard carbides, increasing hardness, strength, and toughness at any temperature. It provides stainless steel and refractory properties, alone or alloyed.

Manganese: Primarily used as a deoxidizer and desulfurizer.

Molybdenum: Along with carbon, it's most effective for hardening steel and preventing fragility.

Nickel: Increases strength, hardenability, and corrosion resistance.

Lead: Present as globules, not combined, it improves machinability (turning, planing, drilling) due to its lubricating properties (0.15-0.30%). Carbon content should be <0.5% due to tempering difficulties and reduced hot tenacity.

Silicon: Used as a rust remover and provides elasticity. High ratios (1-5%) create steel with good magnetic properties.

Tungsten: Forms stable carbides with iron, providing high-temperature resistance. At 14-18%, it creates high-speed steels, tripling cutting speeds compared to carbon tool steels.

Vanadium: A strong deoxidizer, forming complex carbides with iron, providing good fatigue resistance, tensile strength, and shear power in tool steels.

Steel Classification

Steel grades are grouped into five main classes: carbon steel, alloy steel, low alloy XDR steel, stainless steel, and tool steel.

Carbon Steels: Over 90% of steels are carbon steels. They contain varying carbon amounts and less than 1.65% manganese, 0.60% silicon, and 0.60% copper. Examples include machinery, car bodies, construction structures, ship hulls, beds, and hairpins.

Alloy Steels: These contain vanadium, molybdenum, and other elements, plus larger amounts of manganese, silicon, and copper than carbon steels. They're used for gears, engine shafts, and cutting blades.

Low Alloy XDR Steels: Cheaper than conventional alloy steels due to smaller amounts of expensive elements. Special treatment gives them greater resistance than carbon steel. Used for freight wagons and building structures, allowing for thinner, lighter, yet stronger materials.

Stainless Steels: Containing chromium, nickel, and other elements, they resist rust and oxidation despite humidity or corrosive substances. Some are very hard, others very resistant to extreme temperatures. Used for decorative purposes, pipes, tanks, aircraft fuselages, surgical instruments, and kitchen utensils.

Tool Steels: Used for tools, cutting heads, and machine modeling. Containing tungsten, molybdenum, and other alloying elements, they offer strength, hardness, and durability.

Basic Principles for Obtaining Steel

Obtaining steel involves eliminating impurities from pig iron and scrap and controlling the content of elements that influence its properties.

Chemical reactions during steelmaking require temperatures above 1000°C to eliminate harmful substances, either as gases or by transferring them to the slag bath.

Basic Oxygen Process

The Bessemer process, using a pear-shaped Bessemer converter, was the oldest method for mass steel production. Air was blown through molten metal to remove impurities. The basic oxygen process refines steel in a similar furnace, but uses a high-pressure stream of pure oxygen instead of air. Oxygen combines with carbon and other elements, rapidly removing impurities. This process takes 50 minutes or less, producing about 275 tons of steel per hour.

Electric Furnace Process

Following the process of industrial restructuring in the steel industry in Spain abandons the path of the blast furnace and decisively bet for obtaining steel through electric arc furnace.

In this process, the raw material is scrap, which he pays special attention in order to obtain a high degree of quality of it. For this, the scrap is subjected to some strict controls and inspections by the manufacturer of steel, both in their place of origin and in the time of receipt of material at the factory.

Scrap quality depends on three factors:

• your facility to be loaded into the oven
• of their melting behavior (density of the scrap, size, thickness, shape, etc.).

• its composition, being important the presence of residual elements that are difficult to remove in the process of the oven.

According to its origin, the scrap can be classified into three groups:

Recycled scrap: scrap is composed, rejections, etc. originated in the factory. This is an excellent scrap.

• Scrap transformation produced during the manufacture of steel parts and components (chips from machine tools, presses and guillotines clippings, etc.).
• Scrap Recovery: usually most of the scrap used in steel works and must be the dismantling of steel frame buildings, industrial plants, ships, automobiles, household appliances.

The oven

Electric furnace consists of a large cylindrical container of heavy plates (15 to 30 mm thick) lined with refractory material which forms the sill that holds the liquid steel bath and slag. The rest of the furnace is formed by water-cooled panels. The vault can be moved to allow the loading of scrap through a proper baskets.

The vault is equipped with a series of holes through which electrodes are inserted, usually three, which are thick graphite rods up to 700 mm in diameter. The electrodes are moved so that it can adjust its distance from the load as they are consumed.

The electrodes are connected to a processor that provides conditions suitable voltage and current to blow the bow, with varying intensity, depending on the phase of operation of the oven.

Another hole in the roof allows the capture of fumes, which are conveniently purified to prevent contamination of the atmosphere.

The furnace is mounted on a structure that allows you to swing pendulum to proceed to bleeding and discharge the slag bath.

Steel Manufacturing Process

The manufacturing process is basically divided into two phases: melting and refining stage.

Melting phase

Once you have entered the scrap into the furnace and reagents and slag (mainly lime) moves the dome to close the oven and the electrodes are lowered to the proper distance, making the bow jumping materials melt completely loaded. The complete process is repeated until the capacity of the oven, being this one casting steel.

Phase refinement

The refining is carried out in two stages. The first in the oven itself and the second in a ladle furnace.

In the first refinement we analyze the composition of the melt and proceed to the elimination of impurities and undesirable elements (silicon, manganese, phosphorus, etc..) And perform a first adjustment of the chemical composition through the addition of iron alloys containing the necessary elements (chromium, nickel, molybdenum, vanadium, titanium, etc.)..

The obtained steel is poured into a ladle, lined with refractory material, which makes the role of Cuba for a second refining furnace in which fit the ends of steel composition and given the right temperature for the next phase in the manufacturing process.

Continuous casting

Ended the ladle refining is carried to the trough receiving continuous casting which empties its contents into a receiving trough provided for this purpose.

The continuous casting steel is a process in which steel is poured directly into a movable bottom mold, whose cross section is the geometry of semi-finished product that you want to make, in our case, the billet.

The trough has a hole receiving the substance, or dip, which distributes the liquid steel casting multiple lines, each of which has its mold or mold, usually copper cavity walls for cooling water, that serves to shape the product. During the process the mold is moved alternately up and down to detach the solid crust that is formed during cooling.

Then applied a cooling system controlled by first cold showers, and air later in the semi-finished cutting the desired lengths by moving torch during cutting.