Iron rusts.
Rust occurs when iron bonds with oxygen, creating iron oxide.
When chromium is mixed into iron, the chromium oxidizes before the iron can, forming a chromium oxide film on the surface. (Chromium has a strong property that allows it to bond with oxygen more easily than iron does.)
This film is colorless, transparent, and extremely thin, making it impossible to identify with the naked eye. (An extremely thin layer of approximately 0.005 μm consisting mainly of chromium oxide.)
  * 1 μm (micrometer) = 1/1,000 mm    0.005 μm = 5/1,000,000 mm
Furthermore, this film is chemically stable and incredibly strong.
Because it blocks oxygen, it effectively prevents the formation of rust.
A surface protected by this chromium oxide layer is referred to as being in a "passive state." (An alloy surface forms what is known as a passive film.)
Leveraging these characteristics, an alloy created by adding chromium, or chromium and nickel, to iron is called stainless steel.
When an alloy is made by adding chromium to iron, this passive film covers the surface of the stainless steel, making it highly resistant to corrosion.
Even if this passive film is damaged by machining or cutting, as long as an adequate amount of chromium is present, it will instantly combine with oxygen in the air to regenerate itself.
However, because chromium from inside the stainless steel is consumed to regenerate this film, the chromium content near the surface will decrease over time.
Stainless steel can possess various distinct properties depending on the ratio of the metals it contains.

This is the most representative and widely used type of stainless steel. It is also frequently referred to as "18-8 stainless steel."
Classified as an austenitic stainless steel, it is composed of 72% iron (Fe), 18% or more chromium (Cr), 8% or more nickel (Ni), and 2% trace elements.
It offers outstanding corrosion resistance and excellent mechanical properties, making it widely utilized for everything from household goods to industrial equipment.
It is non-magnetic in a solution-treated state, but can develop weak magnetism when subjected to heavy cold working.
(Solution Heat Treatment: A process in which steel is heated sufficiently to dissolve alloying elements into a solid solution and then rapidly cooled. This treatment is performed to optimize structure and properties.)
(Reference)
The acronym "SUS" stands for:
    Steel
    Use
    Stainless
It takes the first letter of each word.

While SUS304 and SUS304L contain the same amounts of chromium (Cr) and nickel (Ni), which directly impact corrosion resistance, they differ significantly in their carbon (C) content.
SUS304 contains 0.03% to 0.08% carbon (C).
SUS304L restricts carbon (C) to 0.03% or less.
When stainless steel is shipped from raw material manufacturers, the carbon (C) concentration is adjusted to be uniform throughout the material. This represents the state where the stainless steel achieves its peak corrosion resistance.
However, if stainless steel undergoes heat during machining or operation stages, the carbon (C) within the steel bonds with chromium (Cr)—the element responsible for corrosion resistance.
The resulting "chromium carbides" precipitate continuously along the grain boundaries of the stainless steel, creating chromium-depleted zones nearby that suffer from severely diminished corrosion resistance.
This phenomenon depends heavily on the carbon (C) concentration within the steel.
SUS304L, which features a lower carbon (C) concentration than SUS304, is a material where these depleted zones are relatively difficult to form. For this reason, SUS304L is frequently selected to avoid intergranular corrosion in welded or high-heat environments.

SUS304 and SUS316 are the two most prominent grades of austenitic stainless steel.
The primary chemical difference between these two steel types is the presence or absence of molybdenum (Mo) in their composition.
SUS304 does not contain molybdenum (Mo).
In contrast, SUS316 contains approximately 2% molybdenum (Mo).
There is no major difference between them in terms of general mechanical properties such as tensile strength.
The primary distinction lies in their level of corrosion resistance.
The corrosion resistance of stainless steel relies on the extremely thin layer formed on the surface called a "passive film."
In the case of stainless steel, the main components that build and fortify this passive film are chromium (Cr) and molybdenum (Mo).
The higher the concentration of these elements, the more "dense" and "corrosion-resistant" the passive film becomes.
The environmental range in which it can withstand corrosion is broader for SUS316 compared to SUS304, making it a highly superior material for harsh conditions.
However, because molybdenum (Mo) can show inferior corrosion resistance in specific strongly oxidizing acid environments, the corrosion resistance rankings of SUS304 and SUS316 may occasionally reverse depending on the chemical solution.

These terms refer to the classification of metal crystalline microstructures.
Chromium-based stainless steels are broadly divided into those possessing a martensitic microstructure and those with a ferritic microstructure.
These are respectively referred to as martensitic stainless steel and ferritic stainless steel.
Chromium-nickel-based stainless steels generally possess an austenitic microstructure and are called austenitic stainless steels.
The magnetic properties of the steel change depending on differences in these crystal structures.
Ferritic, martensitic, and duplex stainless steels, which feature a body-centered cubic (BCC) lattice structure (or contain it in part), are ferromagnetic and strongly attracted to magnets.
The presence or absence of magnetism depends entirely on this underlying crystal structure.

• Martensitic Stainless Steel
　　Hardenable by quenching (heat treatment).
　　Exhibits magnetism under all conditions.
　　Representative Grade: SUS420J2

• Ferritic Stainless Steel
　　Not hardenable by quenching (heat treatment).
　　Exhibits magnetism under all conditions.
　　Representative Grade: SUS430

• Austenitic Stainless Steel
　　Not hardenable by quenching (heat treatment).
　　Exhibits significant work-hardening properties, making it suitable for use as springs or high-tensile steel.
　　Non-magnetic in a solution-treated state, but can develop weak magnetism when subjected to heavy cold working.
　　Representative Grades: SUS304, SUS301, SUS304L, SUS316

• Precipitation-Hardening Stainless Steel
　　A class of steel designed to allow heat-treatment hardening of an austenitic-based base structure that normally cannot be hardened by quenching.
　　While its corrosion resistance does not match pure austenitic grades, it remains highly superior to chromium-based grades.
　　Representative Grade: SUS631

　*****　JIS G4313 Hardness (HV) (HV: Vickers Hardness Symbol [JIS Z 2244])　*****
　Vickers Hardness: A value calculated from the test load applied to indent a test surface using a square-based diamond pyramid indenter with a face angle of 136 degrees, and the surface area derived from the diagonal length of the indentation.

　*****　Unit "N/mm2"　*****
　In the International System of Units (SI), the Newton (N) is used as the unit of force.
　The square meter (m2) is used as the unit of area.
　Therefore, the unit of pressure is the Newton per square meter (N/m2).
　This is referred to as the "Pascal (Pa)".

The role of chromium (Cr) is, quite simply, "to prevent rust."
Stainless steel literally means "steel that does not stain (rust)."
However, it can still rust under certain conditions, so it is more accurately described as "rust-resistant steel."
The "rusting" of iron occurs when iron bonds with oxygen to form "iron oxide."
Chromium (Cr) has a characteristic that makes it bond with oxygen more easily than iron does.
When iron contains 12% or more chromium (Cr), the chromium oxidizes before the iron can, forming a chromium oxide film over the entire surface.
This film is colorless, transparent, and extremely thin (an ultra-thin layer of about 0.005 μm), making it invisible to the naked eye, yet it is chemically stable and very strong.
　*1 μm (micrometer) = 1/1,000 mm
　　　0.005 μm = 5/1,000,000 mm
Furthermore, because it blocks oxygen, it prevents the formation of iron oxide (rust).
This state in which the surface is protected by the chromium oxide film is called "passivation."
Even if this chromium oxide film is damaged by machining or cutting, it will regenerate immediately by bonding with oxygen in the air, provided there is an adequate amount of chromium (Cr) (12% or more).
However, because chromium from inside the stainless steel is consumed to regenerate this film, the overall chromium content decreases.
Stainless steel can take on a wide variety of properties depending on the ratio of the metals it contains.

18-0 Stainless Steel (SUS430)
　　This stainless steel is made by adding 18% or more chromium to iron.
　　Since no nickel is added, it is less expensive than SUS304.
　　It is magnetic, so it is used as induction-compatible (IH-compatible) stainless steel.

18-8 Stainless Steel (SUS304)
　　This stainless steel is made by adding 18% or more chromium and 8% or more nickel to iron.
　　It is the most widely used type for general tableware and kitchenware, not just flatware.
　　The addition of nickel to chromium makes it more robust, improving both corrosion resistance and durability.

18-10 Stainless Steel (SUS304L)
　　This stainless steel is made by adding 18% or more chromium and 10% or more nickel to iron.
　　By increasing the nickel content and lowering the carbon concentration compared to SUS304, the risk of rust formation is extremely low, significantly boosting its corrosion resistance and durability.

As shown in the diagrams below, mixing various metals with iron creates different types of stainless steel, each with its own unique characteristics.

For example, let's say the weight per millimeter is 1 kg.
If the coil width is 30 mm, one coil will weigh 30 kg.
If the coil width is 60 mm, one coil will weigh 60 kg.
In short, the formula to find the weight per millimeter is: "Weight of 1 Coil ÷ Width = Weight per Millimeter".
Now, suppose we have a coil that weighs 180 kg.
If its width is 80 mm, the unit weight of this coil would be:
　180 ÷ 80 = 2.25
As a result, the weight per millimeter is 2.25 kg.

Then, what if we have a coil of SUS304 1/2H 0.2 × 50, and its unit weight is 0.6 kg to 0.7 kg?
　50 mm (Width) × 0.6 kg = 30 kg
　50 mm (Width) × 0.7 kg = 35 kg
This means the weight of one coil will be between 30 kg and 35 kg.

You can calculate the mass (weight: kg) from the weight per millimeter. (You can calculate the unit weight of a coil.)

　(1) Enter half-width numbers into the "Coil Mass (Weight/kg)" and "Coil Width (mm)" fields.
　(2) After entering the values, click [Calculate].
　(3) The result will be displayed in "Unit Weight (kg/1mm)".
　　　(The calculation result is rounded to two decimal places.)
　　　(kg/1mm: Represents the weight per 1 mm of width.)
　(4) Click [Reset] to clear the entered values.

Here, we will explain using the example of stainless steel sheets.
The specific gravity (density) of SUS301 and SUS304 is 7.93 kg/dm³.
The simple formula is:
　7.93 × Thickness × Width × Length　(Width and length must be converted to meters)
For example, for a sheet of SUS304 1/2H 0.5 × 320 × 1000:
　7.93 × 0.5 × (320 ÷ 1000) × (1000 ÷ 1000)
　＝ 7.93 × 0.5 × 0.32 × 1 ≒ 1.2688 kg (Weight per sheet)
For 100 sheets, it will be: 1.2688 kg × 100 sheets ≒ 126.88 kg.

You can calculate the weight for other steel grades as well by changing the specific gravity value.

You can calculate the mass (weight: kg) of flat stainless steel spring sheets.

CSP: A symbol representing the manufacturing method. It stands for cold-rolled steel strips for springs.
　　　Derived from the words Cold Strip Spring.

CP: A symbol representing the product shape. It stands for cold-rolled sheets/plates.
　　　Derived from the initials of Cold Plate.

Other Symbols (Representing product shapes):
　　W: Wire (Wire)
　　HS: Hot-rolled strip (Hot Strip)
　　HP: Hot-rolled plate (Hot Plate)
　　WR: Wire rod (Wire Rod)
　　TP: Tubes and pipes for piping (Tube Pipes)

Excerpted from "How to Read Steel Symbols" in the JIS Iron and Steel Handbook.

As shown in the figure below, it refers to the value of lateral curvature per 1 meter of length.
This value applies to products where the ratio of width (w) to thickness (t), "w/t", is 10 or greater.
However, depending on the agreement between the delivery parties, it can be specified up to 1/2 of the camber (mm) value.
***** Excerpted from JIS G4313 *****

Please also check out the helpful formulas: (13) "Formula to Find Camber Values"

There are 5 types of edge finishes for products. (The diagram below is for illustrative purposes.)

The table below shows the approximate values of HRC converted from HV, and the approximate values of HV converted from HRC.
Please use this as an approximate conversion chart.
Reference: The Rockwell C (HRC) hardness test method uses a diamond or ball indenter. First, a preliminary test force (*1) is applied, followed by a total test force (*2). When the force is returned to the preliminary test force, the hardness value (HR) is calculated using the definition formula (*3) based on the difference (h) between the two indentation depths.
　　*1 Preliminary test force: 98.07 N (10 kgf)
　　*2 Total test force: 1471.0 N (150 kgf)
　　*3 Definition formula: HR = 100 - (h ÷ 2)
　***** Excerpted from JIS Z 2245 *****

Reference: The Vickers (HV) hardness test method uses a square-based diamond pyramid indenter with an angle of 136 degrees between opposite faces. The hardness value is calculated based on the test force applied to form an indentation on the surface and the surface area determined from the diagonal lengths of the indentation.
　***** Excerpted from JIS Z 2244 *****

We will explain this by dividing it into two processing methods: Coil Slitting and Shearing.

First, let's look at the shape of stainless steel coils (spring steel) after slitting.
When a coil is slit, the shape (curl direction) varies for each slit strand.
This phenomenon is caused by the slitter knife setup (combination of cutters).
The diagrams below show a simple conceptual illustration of the main components of a slitting machine.
Figure 1 is the front view from where the product comes out, and Figure 2 is the side view from the side.
In the side view (Figure 2), the product moves from right to left.

As shown above, during the slitting process, the cutters are set up (Figure 1), and when the material passes through (Figure 2), it emerges facing either upward or downward.
・For upward curl, the burr is on the bottom side (Figure 3: Burr Down).
・For downward curl, the burr is on the top side (Figure 3: Burr Up).

Furthermore, as the material passes through the cutters, the direction of the burrs alternates between up and down.
・When the burr is on top, the curl faces downward (Figure 3: Burr Up).
・When the burr is on the bottom, the curl faces upward (Figure 3: Burr Down).

Examples of Terminology Usage:
・Burr Down: Directing the curl upward.
　　(Upward curling shape)
・Unified Burr Down: Keeping the curl consistently facing upward.
　　(Aligning the curling shape upward)
　　(This requires a rewinding process to convert Burr Up coils into Burr Down)
・Burr Up: Directing the curl downward.
　　(Downward curling shape)
・Unified Burr Up: Keeping the curl consistently facing downward.
　　(Aligning the curling shape downward)
　　(This requires a rewinding process to convert Burr Down coils into Burr Up)
・Unified Burr Direction: Keeping the burr surface consistent.
　　(Aligning them either upward or downward)
　　(This requires a rewinding process to make the burr direction the same)
・Specify Burr Direction: Clearly stating whether Burr Up or Burr Down is required.

Next, let's look at the shape after shearing (guillotine cutting).
When a coil is slit, the burr direction at both edges is the same (Figure 5).
In shearing, however, the burrs on the left/right or front/back edges can point in opposite directions (Figure 6).

Because the upper blade of a shearing machine is angled, it is structured so that the blade descends starting from one end.
This follows the same principle as scissors.
Please see Figure 7, which shows how the blades are installed.
Note that when thick material is sheared into narrow widths, twisting (twist warp) may occur.

Figures 8 to 11 explain the actual cutting action of the "Shearing Machine Operation (Figure 7)".
Please refer to the diagrams below.

Figure 8 illustrates the material (sheet metal) being held between the upper and lower blades. The clearance changes depending on the material thickness.
If the clearance is too small, the blade life shortens, requiring more frequent blade sharpening.
If the clearance is large, the blade life increases, but shear cracking can occur, which may cause defects in subsequent processes.

Figure 9 illustrates the upper blade descending and cutting (shearing) the material.

Figure 10 illustrates how cracks develop from both the upper and lower sides within the material during the cut.

Figure 11 illustrates the material being separated (fractured) by the connecting cracks.

Figure 12 illustrates the cross-section of the cut edge. From top-left to bottom: "Rollover", "Shear Band", "Fracture Zone", and "Burr". On the right side, from top to bottom: "Burr", "Fracture Zone", "Shear Band", and "Rollover".

A sheet with a width of 1 m and a length of 2 m is commonly referred to as a "Meter Sheet."
The standards are defined by JIS (Japanese Industrial Standards). (JIS G4305 is the standard code for cold-rolled stainless steel sheets.)
The chemical composition, mechanical properties, thickness tolerances, and weight per sheet for SUS304 2B meter sheets are as follows.

A sheet with a width of 1 m and a length of 2 m is commonly referred to as a "Meter Sheet."
The standards are defined by JIS (Japanese Industrial Standards). (JIS G4305 is the standard code for cold-rolled stainless steel sheets.)
The chemical composition, mechanical properties, thickness tolerances, and weight per sheet for SUS304 2B meter sheets are as follows.

A "Mill Sheet" is a document that certifies the quality and material grade of steel products.
It refers to the material test certificate issued by steel manufacturers when delivering products to buyers.
Please note that "Mill Sheet" is a Japanese-coined English term (Wasei-eiga), derived from a sheet issued by a manufacturing mill.
The document consists of details such as the mechanical properties of the steel (Hardness (HV), Tensile Strength (N/mm²), Elongation (%), Yield Strength (N/mm²)) and its chemical composition.
Depending on the order, specification limits and actual production values (such as measured thickness) may be recorded for each produced lot.
Globally, it is officially issued under names such as "Inspection Certificate" or "Mill Test Certificate (MTC)."
This certificate serves as the primary official document guaranteeing the quality of the steel.
Throughout the distribution process, copies of this certificate are passed down from trading companies and steel wholesalers to the end users as proof of material compliance.

Both SUS316 and SUS316L offer superior corrosion resistance compared to SUS304.
SUS316L is a variation of SUS316 that has been modified to improve workability and formability.
By reducing the Carbon (C) content and increasing the Nickel (Ni) content compared to standard SUS316,
SUS316L achieves better workability and weldability.
The "L" stands for Low Carbon.

Element Guide:
　　C: Carbon
　　Si: Silicon
　　Mn: Manganese
　　P: Phosphorus
　　S: Sulfur
　　Ni: Nickel
　　Cr: Chromium
　　Mo: Molybdenum