What is the black skin of steel? What designers need to know and how to treat it correctly

Here.What is "Iron Black Skin? What Designers Need to Know and How to Properly Treat it."　I am making a note about the

When handling "iron" in machine design, you may see its surface covered with a black coating.　This is called "kurokawa" (black skin). Do you know what this black skin of iron is and how to use it properly?　While the unique texture of the surface can be used in design as it is, on the other hand, using it without understanding its characteristics can often lead to failures such as peeling paint and rusting.

This article provides a comprehensive overview of what designers and all those who work with iron need to know, from the true nature of iron's black skin to its advantages and disadvantages, as well as the proper removal methods.　Get the right knowledge and get the most out of the material iron is made of.

What is iron black skin?

What is black skin in the first place? What is mill scale?

Found in steel materialsBlack skin is an oxide film that is naturally formed when steel is rolled at high temperatures of approximately 900 to 1200°C, reacting with oxygen in the air on the surface.　It is called "kurokawa" (black skin) because of this appearance.　　It is called "kurokawa" (black skin) because of this appearance,It is also officially called "mill scale" or "oxidation scale.

The main component of this black skin is "black rust", chemically defined as tri-iron tetroxide (Fe₃O₄).It is.　Generally, when you hear the word rust, you might imagine "red rust," which corrodes steel into rags,Unlike red rust, black rust is a benign rust that forms a dense layer and to some extent protects the iron inside　It is believed that

However, since black skin occurs naturally, its hue is not uniform.　Some are bluish black, some are reddish black, some are almost gray, etc,Characterized by unique irregularities and shading depending on the productHowever, this uneven texture can be an attractive design feature in the architectural field (but not in mechanical design...). (It is not an attraction in mechanical design...)

Why is it sold with black skin?

There are several reasons why steel is sold with a black skin covering.　The most significant reason is,Black skin is a byproduct that inevitably occurs during the manufacturing process of hot rolling　That is why.

Hot rolling is a process in which iron softened at high temperatures is stretched under pressure, and is suitable for cost-effective mass production because it can be formed efficiently with relatively little force.A black skin is naturally formed on steel that has undergone this process during the cooling process.

In other words, the black-skinned steel does not require additional man-hours to remove, so to speak."In its raw, unprocessed state."It is a very good quality steel.　As a result, it is supplied at a lower cost than migaki material (steel from which the black skin has been removed).　　Also, as mentioned above,The black skin also has a temporary rust-preventive effect and thus plays a certain role in storage when not immediately processed.

Advantages of using black leather are cost and unique texture

Several advantages exist in the use of black-skinned ironwood.

The first and greatest advantage is its low cost.　Since black skin material does not undergo a process to remove black skin or a finishing process to smooth the surface, it is available at a lower cost than other steel materials such as Migaki material.　Especially for structural members that do not require high dimensional accuracy or a beautiful appearance, black skin material is often selected for its cost.

Second, its unique texture has design value. The unique color shading, unevenness, and industrial atmosphere of black hides have a depth of flavor that is difficult to express with paint.　For this reason, many architectural interior materials, furniture, and artwork incorporate the texture of black leather as it is as part of their design.

Further,Black skin materials produced by hot rolling also have the property of low internal distortion (residual stress) during processing.　By this,It also has the advantage of being less prone to warping and twisting during post-processing such as cutting and welding.

Disadvantages of using black skin are accuracy and post-processing issues

While there are many advantages, there are also some disadvantages that cannot be ignored with black skin materials, especially in mechanical design.

The most significant disadvantages are low dimensional accuracy and surface roughness.The thickness of the black coating is not uniform, and the surface is uneven and rough.　Therefore, it is not suitable for use in mechanical parts that require precise dimensions or parts that need to be precisely mated with other parts.

Another serious problem is incompatibility with post-processing.The black skin has low adhesion to the base steel and easily peels off due to impact or deformation.　This is the reason why the paint peels off with the black skin even if paint is applied on top of it.will be.

Likewise,Plating cannot be done directly on top of the black skin because it will also cause adhesion failure.

Further,In welding as well, black film can significantly degrade quality.　If the black skin becomes entrapped in the weld, defects such as blowholes (cavities) can occur and joint strength can be severely compromised.

The relationship between black skin and iron rust that you should know.

The true nature of the black skin is "black rust," and by covering the surface of the steel, it serves as a protective coating that to some extent inhibits the formation of the highly corrosive "red rust.　From this point of view alone, it would seem that black film has a rust-preventive effect.

However, its protective capacity is not perfect.Numerous microscopic holes (pinholes) exist on the surface of the black skin that are invisible to the naked eyeI am doing it.When moisture and oxygen enter through these pinholes, red rust forms from the underlying steel substrate and corrosion progresses from the inside.

In addition, the black skin does not adhere well to the base metal and may peel off partially at any time.　The areas where the black skin is peeled off to expose the steel substrate are naturally unprotected, and rust rapidly forms there.

For these reasons, the black skin is only a temporary protective coating and cannot be relied upon for full-scale corrosion protection over the long term.It should be understood that in humid environments or outdoor use, even black skin will rust easily.

Black skin of iron used in design｜Knowledge of treatment methods and processing

Fundamental difference between iron black dye and black skin

Although black skin and "black dyeing" are similar in appearance in that they both blacken the surface of iron, their origins and purposes are fundamentally different.

While black peeling is a byproduct that occurs naturally during the hot rolling process, black dyeing is a type of "surface treatment" that intentionally produces a coating of tri-iron tetroxide using a chemical reaction for the purpose of rust prevention and aesthetic improvement.It is.

The most significant difference between the two is in the quality of the coating.　The coating produced by black oxide is uniform, dense, and tightly bonded to the base material, so it does not peel off easily, whereas black oxide has a non-uniform thickness, many pinholes, and weak adhesion to the base material, making it easy to peel off.

In short, although the main component is the same, tritium tetroxide, they are completely different in that black oxide is a naturally occurring film of uncontrolled origin and quality, while black dye is a functional film produced through a controlled process.

The purpose of iron plating and black coating is completely different.

The plating process for steel is also very different from that of black skin.Plating is an intentional surface treatment technique that deposits a thin film of metal on the surface to impart specific functions such as corrosion resistance (rust prevention), wear resistance, decorative properties, and electrical properties to a material.

In contrast, black skin, as mentioned above, is merely an oxide film that is accidentally formed during hot rolling.

The most important point in the relationship between the two is that a good plating process is not possible in the presence of black skinThis means that the surface of the black skin is rough, uneven, and has low adhesion.　　Since the surface of the black skin is rough, uneven, and has low adhesion, plating over it will not produce a uniform and highly adherent film, causing premature peeling and swelling.

Therefore,When plating a steel product, it is always necessary to completely remove the black skin by shot blasting or pickling as a pre-process.　The relationship between black skin and plating is incompatible,It is understood that plating is a process that can only be applied to clean substrates that have had their black skins removed.　Please do so.

Why is it necessary to remove the black skin?

In some cases, black skins are used as they are, but for many industrial products, especially machine parts, it is essential to remove them. There are four main reasons for this.

The first is to ensure quality in subsequent processes such as painting and plating.　As mentioned above, painting or plating over a poorly adhering black skin will cause the entire black skin to peel off. In order to achieve a reliable surface treatment, it is necessary to expose the underlying steel substrate.

Second, to guarantee the quality of welding.　When iron oxide, the main component of black film, gets mixed in with the molten metal during welding, it causes welding defects such as blowholes (internal cavities) and poor penetration.　This can significantly reduce the strength of the weld zone and lead to serious problems related to product safety.

Third, because dimensional accuracy is required.　The uneven thickness and surface irregularities of black skin hinder the fabrication of precision parts.In order to produce accurate dimensions, the black skin must be removed to obtain a smooth metal surface as a reference.

Fourth, to prevent long-term corrosion.　The black skin has pinholes and partial peeling, which are the starting point for corrosion.　Increased product durability,For long-term protection against rust, it is wise to remove unstable black skin and apply a more reliable rust-preventive treatmentIt is.

What is the best way to remove black skin?

There are two main methods for removing black skin: physical and chemical.　The choice between the two is determined by taking into consideration the shape of the material, thickness, required finish, cost, and other factors.

physical method

This method uses physical force to remove or scrape off the black skin.

• Shot blasting / sand blasting: Small steel or cast iron balls (shot), sand, or other abrasive material are sprayed onto the steel surface at high speed, and the impact peels off the black surface.　The surface forms a fine irregularity called nashiji, which improves the paint's ability to bite, making it very effective as a base coat for painting.　However, care must be taken with thin materials, as they may be deformed by impact.
• Mechanical polishing (e.g., grinder)Black film removal : This method involves attaching an abrasive wheel or disc to a tool such as a disc grinder or sander and manually scraping off the black film.　This method is easy for small-scale work or partial removal, but it is difficult to finish a large area evenly and takes time and effort.　There is also a risk of "wall thickness reduction," which is a change in the thickness of the material if too much is removed.

chemical method

This method removes black skin by dissolving it with the power of chemicals.

• Acid washing (pickling): Steel is immersed in an acidic solution, such as hydrochloric acid or sulfuric acid, to chemically dissolve the black film.　The advantage of this method is that even parts with complex shapes that are difficult to reach with abrasives from shot blasting can be treated uniformly as long as the parts are in contact with the solution.　On the other hand, the acid used is highly hazardous and requires specialized equipment and attention to safety management of the work environment and disposal of waste solution after use.

Why do most flat bars come with black skins?

Flat bar, or flat steel, as the name suggests, is a flat bar-shaped steel material used in a wide range of applications, including architectural hardware, machine mounts, and reinforcement materials.　The main reason why most flat bars on the market have a black skin is because of the way they are manufactured.

Most flat bars are manufactured by "hot rolling," which is a cost-effective and mass-producible process.　Hot rolled steel is formed by rolling it with rollers, which naturally forms a black crust on the surface during the subsequent cooling process.

Also,Flat bars are used in many applications, such as structural members of buildings and machine frames, where high dimensional accuracy and aesthetics are not necessarily required.It is.　In cases like this,Flat bars with black skins, which offer significant cost advantages, are often used as is or with only a simple rust-proof coatingTherefore, there is a background of high demand for the black-skinned condition.

Of course, if accuracy is required, the black skin must be removed before usewill be.

Important precautions and correct knowledge of handling iron black crust

In handling steel black skin, a lack of proper understanding of its characteristics can lead to unexpected quality problems and increased costs. It is especially important to pay attention to the following points in machine design and machining.

Design Considerations

First, it should be kept in mind that black skin materials have large dimensional tolerances.　Although JIS standards (e.g., JIS G3193) also specify dimensional tolerances for hot-rolled steel,The tolerance is wider than that of Migaki material.　Where precision assembly is required, the use of black skin materials should be avoided, or the design should take into account the dimensions after the black skin has been removed.

Notes on processing and maintenance

At the processing site,It should be noted that black skin is very hard and easily wears out cutting tool edgesIt is.　Machining with a black skin will shorten the life of the tool, resulting in higher costs.

Rusting cannot be avoided if the texture of the black skin is to be used as a product.　Since black bark is not a perfect rust-proof film, regular maintenance is required, especially in humid environments. Regular application of rust-preventive oil can inhibit rust formation and preserve the texture of the black leather for a long time.

Given these factors, it is extremely important to have the correct knowledge to handle iron black crust by weighing its advantages and disadvantages, deciding whether to "remove it or make the most of it" depending on the application, and applying the appropriate treatment for each.

Related Information

Finally,Selected five pages that explain "iron and black skin" in a professional and detailed manner.　Please check the following information if necessary.

• Pro Materials Corporation (formerly Hitachi Metals, Ltd.)
  • URL:. https://www.proterial.com/blog/st/st024.html
  • Description: (in Japanese only) This is a technical blog from Proterial, a major specialty steel manufacturer. The blog provides expert explanations, with photos and diagrams, of what black scale (mill scale) on steel is, what it really is, how it affects coating quality, and how to physically remove it by shot blasting.  

• SK Blast Co.
  • URL:. https://sk-blast.com/column2/causes-blackscale-removal/
  • Description: (in Japanese only) This column is written by a company specializing in blast processing. Focusing particularly on the necessity of black film removal in welding operations, the column explains in detail the causes of black film formation and the strength loss and defects that can occur when welding without removal.  

• MISUMI
  • URL:. https://jp.meviy.misumi-ec.com/info/ja/howto/29893/
  • Description: (in Japanese only) This is a technical information site operated by MISUMI, a general manufacturer and trading company of machine parts. The site compares the differences between black skin (hot-rolled steel) and migaki material (cold-rolled steel) in terms of processing method, surface condition, dimensional accuracy, cost, and application, and is very helpful for designers in selecting materials.

• Okamoto Metal Co.
  • URL:. https://okakin-kk.co.jp/service/hot/
  • Description: (in Japanese only) This is a product introduction page for a company specializing in steel processing. The advantages and disadvantages of hot-rolled steel sheet (black skin material) and the differences from pickling steel sheet are explained in detail from the perspective of a company that handles actual products, providing practical knowledge .

• Aska Corporation
  • URL:. https://www.askk.co.jp/contents/blog/black-skin.html
  • Description: (in Japanese only) This is a technical blog of a company engaged in surface treatment and coating. The differences between black scale (mill scale) and other oxide coatings such as anodized aluminum are compared and explained in detail from various perspectives, including the purpose of formation, temperature, thickness, and adhesion. It also goes into physical and chemical properties, and is highly specialized.

That's it.