May 2018 | National Material Company - Steel Processing Facilities

WHAT IS STEEL PICKLING?

Steel pickling refers to a treatment that is used to remove impurities, rust, and scale from the surface of a material. During hot working processes, an oxide layer (referred to as “scale”, due to the scaly nature of its appearance) develops on the surface of the metal. Before most cold rolling processes, previously hot rolled steel goes through a pickling line to remove the scale from the surface and make it easier to work. To restore the best corrosion resistant performance, the damaged metal layer must be removed, exposing a fully alloyed stainless steel surface.

In order to remove this oxide layer, the material is dipped into a vat of what is called “pickle liquor”. Pickle liquor can come in many forms; carbon steels with an alloy content of less than 6% are often pickled in hydrochloric or sulfuric acid. For steels that have a higher carbon content, a two-step pickling process is required, with additional acids used (phosphoric, hydrofluoric, and nitric acid).

Steel processing companies that offer hot and cold rolling services typically encounter two different types of scale – high-temperature and low-temperature scale. High temperature milling processes create three layers of iron oxide, which forms on the material after at temperatures above 1070F. Low temperature mill scale develops in procedures that use temperatures below 1070.

TYPES OF ACID USED IN STEEL PICKLING:

Hydrochloric

Advantages:

Reduce heating costs since pickling solutions are used at room temperature
More extensive scale removal
Less penetration of hydrogen by diffusion
Less deposition of iron salts on the pickled surface

Disadvantages:

Fumes when heated above ambient temperatures
Acid recovery systems are expensive
More corrosive toward equipment
Magnesium Higher disposal costs than sulfuric acid

Sulfuric

Advantages:

Acid can be renewed more frequently
Raising temperature will allow lower acid concentrations to pickle effectively
Ease of recovering iron sulfate
The rate of pickling can be controlled by varying the temperature

Disadvantages:

Greater acid attack on base metal.
Greater hydrogen diffusion into the steel
Pickling residues are more adherent
Acid solutions must be heated

After the steel pickling process, sheet steel will typically oxidize after long exposure to atmospheric conditions that experience high humidity. To counteract this, a film of oil or other waterproof coatings are applied to create a shield of moisture in the air.

National Material’s Steel Pickling Capabilities:

About National Material L.P. – National Material Limited Partnership and its affiliates have a long history of quality and service dating back to 1964. Since its founding, National Material L.P. has grown to over 30 business units and is now one of the largest suppliers of steel in America. The National Material group of industrial businesses consists of the Steel Group, Stainless and Alloys Group, Raw Material Trading Group, Aluminum Group, and Related Operations.

Visit National Material: http://www.nationalmaterial.com or call (U.S.) 847-806-7200

What are TRIP Steels?

Kelly Smith2018-05-11T18:23:52+00:00May 3rd, 2018|News Blog, NMC Media|

WHAT ARE TRIP STEELS (Transformation Induced Plasticity Steels)?

TRIP Steels (Transformation Induced Plasticity Steel) are part of the Advanced High-Strength Steel (AHSS) family.

The microstructure of TRIP steels consists of at least five-volume percent of retained austenite, which is embedded in a primary ferrite matrix. The microstructure also contains hard phases like bainite and martensite in varying amounts.

TRIP steels are notable due to the higher carbon content than other members of the AHSS family, such as dual phase steels. They typically require the use of an isothermal hold at an intermediate temperature, which produces some bainite. Silicon and aluminum are added in order to both accelerate the ferrite and bainite formation process, as well as avoiding carbide buildup in the bainite region of the material.

Greater silicon, aluminum, and carbon content of TRIP steels result in large fractions of retained austenite in the material’s final microstructure. The increased carbon content also stabilizes the retained austenite phase below the usual ambient temperature.

Changing the carbon content helps to control the strain level at which the austenite begins to transform into martensite. At low carbon levels, the transformation of the retained austenite will begin almost immediately upon deformation, which will then improve the formability and work hardening rate during the stamping process.

At higher carbon content, the transformation will occur only at strain levels beyond those utilized during the forming processing. The retained austenite remains after the final stage of the forming process at these higher carbon levels – the transformation into martensite will occur only during subsequent deformation; in the case of automobiles, an example would be a crash event.

PROPERTIES OF TRIP STEELS

TRIP Steels can be produced as hot-rolled, cold-rolled, or hot dip galvanized, with a strength range from 500 MPa to 800 MPa.

TRIP Steels are highly sought after due to their high work hardening rate, which is created by the hard second phases that are dispersed in the soft ferrite during deformation. Despite the fact that initial work hardening rate of the material is lesser than that of, say, dual phase steels, TRIP steels sustain their hardening rate at much higher strain levels, where DP steel’s work hardening rate would deteriorate.

As a result of the high work hardening rates, TRIP steels also have substantial stretch forming properties.

The high strain hardening capacity and mechanical strength make these steels an excellent candidate for automotive parts that require a high energy absorption capacity. TRIP steels also have a strong bake hardening following deformation, which even further improves their crash performance.

To summarize TRIP steel’s properties:

Work hardening – When compared to other advanced high-strength steels, TRIP steels exhibit and retain a higher work hardening rate at higher levels of strain.
Formability – As a byproduct of the high work hardening rate, these steels have substantial stretch forming properties, and can be put through stamping processes in a relatively stable manner.
Bake hardening – TRIP steels have a very high bake hardening capacity, and can by doing so can increase their yield strength by close to 70 MPa.

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What Is Steel Pickling?

What are TRIP Steels?