How is a stainless steel sheet rolled

The production of stainless steel sheet is a multi-step precision process, with rolling being the core step in shaping the final sheet's shape and properties. The entire process can be broadly divided into the following main stages:

First, Smelting and Continuous Casting of Stainless Steel Sheet
1. Raw Material Melting: Scrap steel, ferroalloys (such as chromium, nickel, and molybdenum), and pig iron are melted in an electric arc furnace to form the initial molten steel.
2. Refining: The molten steel is transferred to an argon oxygen decarburization furnace or vacuum degassing unit for refining. This crucial step serves to:
(A) precisely adjust the chemical composition (such as Cr, Ni, C, and N)
(B) reduce carbon content and other impurities
(C) control oxygen content and gas inclusions.
3. Continuous Casting: The refined molten steel is poured into a continuous casting machine and continuously cast under a protective atmosphere into slabs of a specific width and thickness. This is the starting shape for stainless steel production.
4. Heating
(A) Hot slabs from continuous casting or cold slabs that have been cooled and then reheated are heated in a walking beam furnace or roller hearth furnace.
(B) The heating temperature is typically between 1100°C and 1250°C (the specific temperature depends on the steel type, with austenitic stainless steels requiring higher temperatures and ferritic and duplex stainless steels requiring lower temperatures).
(C) Purpose: To impart sufficient plasticity to the metal for subsequent rolling and deformation, while also ensuring uniform distribution of alloying elements within the steel (initiating solution treatment).
(D) Heating rate and time: Precise control is required to ensure uniform slab temperature and avoid overheating, overburning, and severe oxidation.

Second, Hot Rolling of Stainless Steel Sheet
1. Main Purpose: To roll thick slabs into coils or plates of the desired thickness, refine the grain size, and provide blanks for cold rolling or directly produce hot-rolled finished products. 2. Core Equipment: Hot rolling mill, typically consisting of:
(A) Roughing mill: Consists of one or two reversing mills (such as two-high or four-high reversing mills). The slab undergoes multiple reciprocating rolling cycles in the roughing mill, gradually reducing its thickness from approximately 200 mm to 30-50 mm, while increasing its length and width.
(B) Finishing mill: Consists of five to seven four-high or six-high tandem mills connected in series. The roughed strip passes continuously through the stands of the finishing mill at high temperatures, rapidly thinning it to the target thickness (usually above 1.5 mm).
3. Rolling Characteristics:
(A) Operated above the recrystallization temperature.
(B) Large deformation per pass.
(C) High temperature and high speed (finishing exit speed can reach over 10 m/s).
(D) Requires a strong rolling force.
(E) Requires cooling water to control roll and strip temperature (temperature drop). 
4. Key Process Control Points:
(A) Precise control of rolling force, roll speed, and reduction.
(B) Thickness control along the length and width of the strip.
(C) Control of strip shape (flatness).
(D) Rapid cooling of the rolled strip through laminar cooling or water curtain cooling to control microstructure and properties. Cooling rate is crucial to the microstructure of certain steel grades (such as duplex stainless steel).
5. Coiling: After hot rolling, the hot-rolled strip is coiled into coils on a coiler or cut into single hot-rolled sheets as required.

Third, Pickling of Stainless Steel Sheets
(A) After hot rolling, a dense oxide scale forms on the surface of stainless steel.
(B) This scale and any surface defects must be removed through a pickling process.
(C) Pickling Methods: Commonly used methods include nitric acid-hydrofluoric acid mixed acid pickling (for austenitic stainless steel), nitric acid pickling (for ferritic stainless steel), and neutral salt electrolytic pickling. Choose the method based on the steel grade. 
(D) Steps: Typically include: descaling (mechanically or chemically loosening the oxide scale), pickling (primarily a chemical dissolution process), rinsing, brushing, and drying.
(E) Purpose: To obtain a clean, bright, and passivated stainless steel surface in preparation for cold rolling or delivery.

Fourth, Cold Rolling of Stainless Steel Sheet
1. Main Purpose: For stainless steel plates that require a thinner thickness (usually ≤3mm, but can be reduced to approximately 0.1mm), higher surface quality requirements, stricter dimensional accuracy, and higher hardness and strength, cold rolling must be performed at room temperature.
2. Core Equipment:
(A) Single-Stand Reversing Cold Rolling Mill: The strip is rolled back and forth multiple times on the same rolling mill, gradually reducing its thickness. Suitable for small-batch, multi-variety, and thick-gauge production.
(B) Multi-Stand Tandem Cold Rolling Mill: Consists of 3-5 four-high or six-high rolling mills connected in series. The strip is continuously formed to the target thickness through each stand. It offers high production efficiency and precision, making it suitable for large-scale production. 
3. Rolling Characteristics:
(A) Conducted at room temperature (heat of deformation is actually generated).
(B) The strip undergoes work hardening, significantly increasing its strength and hardness while decreasing its plastic toughness.
(C) The amount of deformation per pass is smaller than that of hot rolling.
(D) Requires extremely high rolling forces.
(E) Requires extremely high surface finish and roll surface quality.
4. Key Process Control Points:
(A) Thickness Control: Use a precise automatic thickness control system.
(B) Shape Control: Use roll bending, tilting, and staged roll cooling to precisely control strip crown and flatness. Hydraulic roll bending and more advanced shape control systems are key.
(C) Tension Control: Controlling strip tension between the stands of a continuous rolling mill is crucial, affecting stability and thickness accuracy.
(D) Surface Quality Control: Rolls must be highly polished, and the rolling oil must be clean, lubricated, and effectively cooled. (E) Intermediate Annealing: Due to severe work hardening, after a certain number of cold rolling passes (or when continuous rolling reaches an intermediate thickness), the material becomes very hard and brittle, making further rolling impossible. Intermediate annealing is required to eliminate the hardening and restore plasticity. This is typically done using a continuous annealing line.
5. Annealing and Tempering
(A) After cold rolling to the target thickness, final annealing is required.
(B) Purpose: Eliminate the work hardening caused by cold rolling and restore the material's plasticity; homogenize the microstructure; and achieve the desired mechanical properties and corrosion resistance through a specific heating and cooling schedule (such as solution treatment and ductility restoration).
(C) Commonly used equipment: Continuous annealing and pickling lines.
(D) Process:
- Heating: The strip is rapidly heated in a protective atmosphere to the target temperature (approximately 1050-1150°C for austenitic steel and 800-900°C for ferritic steel).
- Holding: Homogenizes the microstructure and dissolves carbides (solid solution). - Rapid Cooling: Rapid cooling (usually by water spray or mist cooling) to maintain the solid solution state (especially for austenitic steels, to prevent carbide precipitation).
6. In-Line Pickling: After annealing, a new oxide scale forms on the surface. In-line pickling (by spray or immersion) is usually performed immediately after the continuous annealing and pickling line to achieve a bright surface and passivation.
7. Tempering/Quenching and Tempering Rolling
(A) Purpose: Eliminate the yield plateau and improve the material's formability (uniform deformation capability); improve surface finish; achieve specific mechanical properties (e.g., increase yield strength); and correct minor shape defects.
(B) Equipment: Single-stand two-roll or four-roll tempering mill.
(C) Features: Applies very small reductions (usually between 0.5% and 3%). The goal is not to reduce thickness, but rather to optimize the material's microstructure and properties and improve the surface.
8. Finishing
(A) Surface Treatment: Various levels of surface treatment are performed as required:
(B) Grinding/Polishing: Achieve mirror, brushed, frosted, and other effects. (C) Passivation: Strengthens the surface oxide film to further improve corrosion resistance.
9. Shearing/Slitting: Cuts large steel coils into customer-specified widths or cuts into cut-to-length plates.
10. Inspection: Performs rigorous final quality checks, including:
(A) Thickness accuracy
(B) Surface quality (smoothness, presence of scratches, indentations, etc.)
(C) Plate shape (straightness, waviness)
(D) Mechanical property testing (sampling)
(E) Chemical composition verification (sampling)
11. Packaging: Provides packaging and protection according to standards to prevent damage during transportation.

In summary, the stainless steel sheet production process has two main branches:
1. Hot rolling process: Slab -> Heating -> Hot rolling -> Cooling -> Coiling/Slitting -> Pickling -> (Possibly tempering/quenching and tempering) -> Finishing -> Shipment. This process produces relatively thick hot-rolled coils or single sheets of medium and heavy plate.
2. Cold Rolling Path: Hot-rolled coil (after pickling) -> Cold-rolled (possibly with intermediate annealing and pickling) -> Final annealing and pickling -> Temper/Quenched and Tempered Rolling -> Finishing -> Shipping. This produces thin plates and ultra-thin strips with excellent surface quality, high dimensional accuracy, and high strength.
Modern stainless steel plate production relies heavily on automated control, advanced equipment, and precision processes to ensure high quality and consistent performance of the final product.