15 05, 2018

What Is Steel Pickling?

2019-11-19T14:49:22+00:00May 15th, 2018|News Blog, NMC Media|

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: https://www.nationalmaterial.com or call (U.S.) 847-806-7200

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3 05, 2018

What are TRIP Steels?

2018-05-11T18:23:52+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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16 04, 2018

Complex Phase Steel: An Introduction

2018-04-16T23:30:52+00:00April 16th, 2018|News Blog, NMC Media|


WHAT IS COMPLEX PHASE STEEL?

Complex Phase steel (CP steels) is a part of the Advanced High-Strength Steel (AHSS) family.

Complex Phase steels gain their strength through extremely fine grain size and micro structure containing small amounts of martensite, pearlite and retained austenite embedded in a ferrite-bainite matrix. High grain refinement is achieved by precipitation of micro alloying elements such as Nb, Ti or V or retarded recrystallization.

Complex Phase steel is being produced as both hot-rolled and cold-rolled which can be hot dipped galvanized for corrosion protection. Hot-rolled products are available in the higher thickness ranges needed to produce structural type parts.

The chemistry and microstructure of CP steels is similar to Trip Steels, except for the addition of some quantities of the Nb, Ti and or V to cause a precipitation strengthening effect.

The bainite complex phase microstructure exhibits improved strain hardening and strain capacity over the bainite structure alone.

Properties of CP steel

CP steels have a higher minimum yield strength in comparison with dual phase steels of identical tensile strengths of 800 Mpa and greater. When compared to DP steels, CP steels have a much higher yield strength to tensile strength ratio.

CP steels have high work hardening capability at low strain, high fatigue strength, high energy absorption, wear resistance and bake hardening potential.

Heat treatment of hot-rolled CP steels at 500-700 C can further increase the yield point of the material by up to 100 MPa.

CP steels are readily welded to itself or other common grades of steel, spot welders for other lower strength grades can be used with the appropriate adjustments.

Current and developmental strength grades range from 780 to 1470 Mpa minimum tensile strength with 5-30% total elongation.

Formability

With high uniform elongation and continuous yielding CP steels have excellent formability and are suitable for stretch forming, roll-forming, bending and hole expansions.

Applications of CP steel

Due to CP steels high capability to absorb energy during a collision they are particularly well suited for weight saving manufacturing of cold formed crash relevant parts in automobiles. There are several automotive applications in body structure, suspension and chassis components.

Current production grades of CP steels and examples of automotive applications:

  • CP 600/900 Frame rails, B pillar reinforcements, tunnel stiffener
  • CP 680/780 Frame rails, chassis components, cross members
  • CP 800/1000 Suspension brackets, fender beams
  • CP 1000/1200 Frame rail reinforcements, rocker panel supports
  • CP 1050/1470 Bumper beams, side sills

To summarize complex phase steels’ properties:

  • Tensile strengths that meet and exceed 800 MPa
  • High ratio of yield to tensile strength
  • Great for cold forming, bending, and hole expansion
  • Strong bake-hardening qualities
  • High durability and wear resistance
  • High crash energy absorption
  • Good weldability

About National Material L.P. – With more than 3,000 employees from a multinational portfolio of companies, NMLP provides engineered metal products, which include steel processing, aluminum extrusion and stainless steel rolled product companies, to automotive, aerospace, construction, defense, electrical, and industrial markets.

Visit National Material: https://www.nationalmaterial.com or call (U.S.) 847-806-7200, Diana Pulido

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4 04, 2018

The Differences Between Hot and Cold Rolled Steel

2018-04-04T18:33:39+00:00April 4th, 2018|News Blog, NMC Media|

When approaching your project, it is crucial to know the type of steel that is best suited for your needs. Different types of steel manufacturing can produce material that performs better for the specific applications. The hot and cold rolled steel manufacturing methods specifically have a great effect on the overall performance of the steel.

Prior knowledge of distinctions between the two methods can help your company save on time, raw material cost, and additional processing. This article aims to explain the differences between hot and cold rolled steel and discuss the advantages (and limitations) of each.

NOTE: Hot-rolling and cold-rolling should not be mistaken for different grades of steel. Steels of different grades can be produced as hot-rolled or cold-rolled.

HOT-ROLLED STEEL

Hot-rolling refers to a mill process in which you roll the steel at a temperature above its recrystallization temperature; a heat that typically exceeds 1000° F.

When steel is heated past its recrystallization point, it becomes more malleable and can be properly formed and shaped. It also allows for the ability to produce larger quantities of steel. The steel is then cooled at room temperature, which “normalizes” it, eliminating the worry for stresses in the material arising when quenching or work-hardening.

When the steel cools off, it will shrink non-uniformly, which gives slightly less control on the overall size and shape of a finished hot-rolled product.

Hot-rolled steel typically has a scaly surface finish. For situations in which the appearance of the material is a concern, the scales can be removed by several techniques: pickling, grinding, or sand-blasting.

These properties make hot-rolled steel most suitable for structural components and other applications where incredibly precise shapes and tolerances are of less importance, such as:

  • Railroad tracks
  • I-beams
  • Agricultural equipment
  • Sheet metal
  • Automotive frames

COLD-ROLLED STEEL

As you might suspect, the manufacturing process behind cold-rolled steel is a bit different. Despite the name, this process refers to steel that is pressed with the pressure of a roller at room temperature.

Compared to hot-rolled steel, cold-rolled steel has a nearly 20% increase in strength through the use of strain hardening. It’s through a series of breakdown, semi-finishing, sizing, semi-roughing, roughing, and finishing that cold-rolled steel shapes can be created.

Cold-rolling steel allows for the creation of very precise shapes. Since the process is performed at room temperature, the steel will not shrink as it cools, as it does in the hot-rolled process.

The exterior finish of cold-rolled steel is very desirable when aesthetics and visual appeal are a priority in your project.

However, the applications of cold-rolled steel are somewhat limited to a couple of shapes – square, round, flat, and variations thereof.

Typical uses for cold-rolled steel:

  • Strips
  • Bars
  • Rods
  • Home appliances
  • Roof and wall systems
  • Metal furniture
  • Aerospace structural members

HOT AND COLD ROLLED STEEL. WHICH DO I NEED?

If you require large structural components, you will most likely need the hot-rolled steel process to create the parts. For smaller parts that require more precise and durable qualities, then the cold-rolled steel process is the way to go. If you have questions about […]

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23 03, 2018

Dual-Phase Steels: An Introduction

2018-04-04T18:04:25+00:00March 23rd, 2018|News Blog, NMC Media|


What are dual-phase steels?

Dual-Phase steels (DP steels) are a part of the Advanced High Strength Steel (AHSS) family.

Ferrite-Martensite dual-phase steel is a low-to-medium carbon material with between 5-50% volume fractional martensite islands that are dispersed in a soft ferrite matrix. In addition to martensite, bainite and retained austenite components can also exist; these are normally produced when improved edge stretch formability is desired.

These variations in microstructure give dual-phase steels a wide spectrum of strength and ductility. DP steels are known to be capable of absorbing large amounts of energy. Combined with a low cost of production, these properties make DP steels highly desirable for automotive applications.

Dual-phase steels can be produced as both hot rolled and cold rolled based material. When Hot Rolled, the rolling temperature and cooling process on the Hot Strip Mill is carefully controlled to produce the ferrite-martensite structure from austenite. When Cold Rolled, the properties are developed on Continuous Annealing lines where there is even greater control over thermal treatment.

DP Steels can be also produced as HD Galvanized, HD Galvannealed and Electro Galvanized.

Properties of DP steels

Both Hot and Cold-rolled DP steels offer an incredibly advantageous combination of low yield, high-tensile strength, easy cold working, and weldability due to their ferrite-martensite imbued lattice microstructure.

The carbon content of dual-phase steels enables the formation of martensite at practical cooling rates, which increases the hardenability of the steel. Generally, higher carbon will promote a stronger steel and a higher fractional percentage of martensite.

In DP steels the soft Ferrite phase is generally continuous, giving these steels excellent formability. When DP deforms the strain is concentrated in the lower strength Ferrite phase surrounding the hard islands of Martensite, which creates the very high initial work hardening rate exhibited by these steels.

Due to high-strain hardenability, dual-phase steels also have a high-strain redistribution capacity. This means improved drawability as well as finished part mechanical properties (yield strengths) that are higher than the initial blank.

DP steels also have a bake hardening effect that is an important benefit over conventional HSLA type materials. The bake hardening effect is the increase in yield strength resulting from elevated temperature aging created by the curing temperature of the paint bake cycle.

DP grades are currently being produced from 500 to 1200 Mpa minimum tensile strength with 5-35% total elongation.

dual-phase steels

Formability

DP steels offer an excellent combination of strength and drawability as a result of their strain hardening capacity from the beginning of deformation. This capability ensures homogeneous strain redistribution and reduces local thinning.

Dual-phase steels can be drawn on conventional tools, provided the settings are properly adjusted. For example, drawing pressure may be increased by approximately 20% for a Dual-Phase 600, compared to a micro-alloyed (HSLA) type steel of the same thickness.

Applications in Automobiles

As one could expect from a material with a high-tensile strength, dual-phase steels are well suited for automobile parts that are meant to absorb a lot of energy during an impact.

Dual-phase steels are often used in the following automobile applications:

  • DP300/500 Roof Outer, Door Outer, […]
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