From steel to stainless steel: types obtainable and differences
Steel is an iron alloy composed primarily of iron and carbon, produced in steel mills.
Steel can be reclassified and differentiated based on:
Carbon percentage
Extra soft: carbon between 0.05% and 0.15%;
Soft: carbon between 0.15% and 0.25%;
Semi-hard: carbon between 0.40% and 0.60%;
Hard: carbon between 0.60% and 0.70%;
Very hard: carbon between 0.70% and 0.80%;
Extra hard: carbon between 0.80% and 0.85%.
With a carbon percentage above 2.00%, cast iron will be produced.
B. Alloy Contents
Unalloyed steels
Alloyed steels
STEEL PRODUCTION
The steelmaking industrial process begins with the preparation of three raw materials: iron ores, coking coal, and coke, which are then crushed and ground. These will be washed of impurities before being melted in blast furnaces.
CAST IRON REFINING FOR STEEL PRODUCTION
From the melting of the three raw materials, cast iron will be obtained, which will have a high carbon content, typically over 4%.
This will then be sent to specialized facilities called converters, where it will undergo a decarburization process to reduce its carbon content.
As the final step, the cast iron will be transferred to another furnace where different metals will be added based on the type of steel and alloy desired (Nickel, Chromium, Manganese, Vanadium, Molybdenum, Cobalt).
STAINLESS STEEL
Stainless steel was discovered in 1872 by the Englishmen Woods and Clark, but its actual industrialization occurred in 1913 thanks to metallurgist Brearley, who was experimenting with carbon steels with various alloys to achieve better wear resistance for military rifle barrels.
Among his experiments, he noticed that a carbon steel alloyed with approximately 13% chromium did not perform well in terms of wear resistance but did not oxidize and showed good resistance to acids.
This was the birth of the first martensitic steel.
A few months later in Germany, other metallurgists, by adding nickel, produced the first austenitic stainless steels, while simultaneously in America, increasing the chromium content and reducing carbon content resulted in the first ferritic stainless steels.
WHY STAINLESS?
Stainless steels are defined as those steels characterized by greater resistance to oxidation and corrosion.
This capability is mainly due to the presence of chromium in the alloy, which, in contact with oxygen, forms a molecular layer of chromium oxide (passivation) that protects the underlying metal from corrosion. In case of scratches and abrasions, the passivation layer will quickly reform upon contact with air oxygen.
A steel is categorized as stainless when the minimum chromium percentage is 10.5%, although chromium values generally range between 12% and 17%. The concentration of chromium in the alloy, along with the possible presence of alloying elements such as nickel, molybdenum, and titanium, will determine the stainless steel's resistance.
The carbon percentage, however, must always be < 1.2%.
It can therefore be stated that stainless steel surfaces remain relatively unchanged over time, giving the product a distinct nobility.
- Series 2XX – austenitic steel with chromium-nickel-manganese
- Series 3XX – austenitic steel with chromium-nickel and chromium-nickel-molybdenum
- Series 4XX – ferritic or martensitic steels with chromium
- Series 5XX – medium chromium martensitic steel
- Series 6XX – precipitation hardening steel with chromium
- 304 – Cr (18%) Ni (10%) C (0.05%);
- 304 L – (Low Carbon): Cr (18%) Ni (10%) C (< 0.03%);
- 316 – Cr (16%) Ni (11.3/13%) Mo (2/3%);
- 316 L – (Low Carbon): Cr (16.5/18.5%) Ni (10.5/13.5%) Mo (2/2.25%) C (< 0.02%);
- 316 LN – (Low Carbon Nitrogen) (presence of nitrogen);
- 316 LN ESR (electro-slag remelting);
- 430: Cr (16/18 %) C (0,08%).
- 904 L – (Low Carbon): Cr (19/23%) Ni (23/28%) Mo (2/2,25%) C (< 0.03%) Cu (1-2%);
HOW IS STAINLESS STEEL PRODUCED?
Production generally starts with selected stainless steel scrap and ferroalloys, which are melted in an electric furnace at about 1600 degrees Celsius.
The molten steel is transferred to another container for refining, which occurs at 1680/1700 degrees Celsius. During this process, the necessary elements are added to achieve the desired chemical composition.
Through the injection of oxygen, argon, and nitrogen at pressure from the bottom of the container, carbon is removed via the decarburization process, and impurities and slags are refined from the molten steel.
The molten steel, at approximately 1500/1550 degrees Celsius, is poured into continuous casting molds where it is cooled with water jets and other substances to solidify.
From this process, a block of steel, called a SLABS, is obtained.
HOW IS IT ROLLED?
Starting from the bloom, which is brought to the appropriate temperature using a special furnace, the following products are obtained:
Hot-Rolled Steel Slabs Plates
These are produced by hot rolling on a reversible mill, yielding plates up to 3000 mm in width, 10,000/12,000 mm in length, and thicknesses of over 100 mm.
After rolling, the plates are sandblasted, annealed, pickled, and, if required, undergo ultrasonic testing.
- Hot-Rolled Coils
Also starting from blooms, after passing through the roughing mill, the hot strip is rolled on a multi-stand mill to achieve the desired final dimensions.
Subsequently, the hot strips undergo heat treatment to obtain better structure, stress relief, and optimal mechanical properties.
Hot coils are finally pickled before being used as:
- Raw material for cold rolling
- Materials for end-users in both coils and plates
- Cold-Rolled Coils
Hot coils are further rolled on cold rolling mills known as SENDZIMIR, which offer high productivity and precise control over product thickness.
After rolling, the coils undergo heat treatment and pickling to achieve a 2D finish, while a 2B finish requires an additional SKIN PASS operation.
To achieve a BA (bright annealed) finish, coils are conveyed immediately after cold rolling into a tower furnace conditioned with an inert atmosphere of nitrogen with a small percentage of hydrogen, followed by the skin pass procedure.
