Interstitial Free Steels

Interstitial Free Steels

 The term ‘Interstitial Free steel or IF steel’ refers to the fact, that there are no interstitial solute atoms to strain the solid iron lattice, resulting in very soft steel. IF steels have interstitial free body centered cubic (bcc) ferrite matrix. These steels normally have low yield strength, high plastic strain ratio (r-value), high strain rate sensitivity and good formability.

Conventional IF steels which were developed commercially in Japan during 1970s following the introduction of vacuum degassing technology contained carbon (C) in the range of 40 – 70 ppm and nitrogen (N) in the range of 30 -50 ppm. Later, niobium (Nb) and/or titanium (Ti) were added to these steels to stabilize the interstitial C and N atoms.

IF steel is termed as ‘clean steel’ as the total volume fraction of precipitates is very less. In spite of this, the precipitates appear to have a very significant effect on the properties of IF steels.

Liquid steel is processed to reduce C and N to levels low enough that the remainder can be ‘stabilized’ by small additions of Ti and Nb. Ti and Nb are strong carbide/nitride formers, taking the remaining C and N out of solution in liquid iron, after which these latter two elements are no longer available to reside in the interstices between solidified iron atoms.

IF steel has ultra low carbon content, achieved by removing carbon monoxide, hydrogen, nitrogen, and other gasses during steelmaking through a vacuum degassing process. Interstitial elements like nitrogen or carbon are also in the form of nitrides and carbides due to the alloying elements such as Nb and/or Ti used for the stabilization of the residual interstitials. Therefore, IF steels posses typically non aging properties. Because of their non ageing properties, IF steels are the standard base for hot dipped galvanized products.

In the past two decades, there has been a substantial growth in the production and demand for the IF steels. Nowadays, in these steels, normally the content of interstitial elements (C, N) is kept below 30 ppm by using modern day vacuum degassers followed by addition of Ti and/or Nb. These micro alloying additions form carbide and nitride precipitates and thus make the matrix almost ‘pure’, i.e. free from interstitial elements. Without free interstitial elements, these steels are very ductile and soft, donot age or bake harden, and do not form strain (Lüder’s) lines during forming due to the absence of yield point elongation (YPE).

In these steels, if titanium is used alone as the stabilizing agent then the aim Ti content is calculated from the formula: Ti = 4 × % C + 3.42 × % N + 1.5 × % S + 0.02. If niobium is also used then the aim Ti = 3.42 × % N + 1.5 × % S and Nb = 7.75 × % C. A typical IF steel composition is 0.002 % C, 0.01 % Si, 0.15 % Mn, 0.01 % P, 0.01 % S, 0.0025 % N, 0.04 % Al, 0.016 % Nb, and 0.025 % Ti.

The lack of interstitial atoms in the atomic structure enables IF steel to have extremely high ductility, ideal for deep-drawn products. In fact, IF steels are sometimes called extra deep drawing steels (EDDS). They have relatively low strength (although they are sometimes strengthened by the reintroduction of nitrogen or other elements), but high work hardening rates and excellent formability.

Stabilized, ultra low carbon (ULC), IF steel has the ability, during continuous annealing, to form crystal orientations favorable to deep drawing. This is not possible with low carbon, unstabilized steel on continuous annealing lines. The high r-values needed for good steel drawability require plentiful ‘cube-on-corner’ crystal orientations to form during annealing. This becomes increasingly possible when the carbon level is below 0.01 %, and is optimized at 0.001 % (10 ppm).

Typical micro structure of IF steel is shown in Fig 1.

Micro structure of IF steel

Fig 1 Micro structure of IF steel

 Texture and recrystallization of IF steel

 With modern steel making and continuous annealing, it is now possible to produce interstitial-free (IF) cold rolled steels. After cold rolling with high level of deformation and annealing, this matrix transform in structure with a strong recrystallization texture. This kind of texture is reason of high values of the average r- value, which is associated with a good formability of ultra low carbon.

The rolling texture of body centered cubic (bcc) low carbon and IF steel consists of two crystallographic fibers namely ? fiber and the ? fiber. Cells, sub grains, and micro bands, where these are not associated with significant lattice curvature, provide the driving forces for recrystallization. However, when these microstructures are associated with significant short range lattice curvature, as they are in deformation or shear bands, they can provide the nucleation sites. The process of recrystallization in IF steel occurs in two stages. The first stage is where the nuclei are contained in the original rolled out grains belonging to the ? fiber, and where the essential lattice curvature is derived from shear and deformation bands. The second stage involves the impingement/coalescence of several recrystallized grains in the as rolled envelope of an original hot band grain which provides a super-nucleation event, in which the as rolled boundaries bow out to give an equiaxed microstructure.

It has been seen that ? fiber texture components exhibit isotropic spring back behaviour in the plane of the sheet on account of their high in plane elastic isotropy, whereas, ? fiber component exhibit highly anisotropic spring back behaviour in plane strain bending operations. The presence of high volume fractions of ? fiber components in IF steel sheets, provides an excellent deep drawing properties as well as isotropic in plane spring back behaviour.

Properties of IF steel

Ultra low stabilized steel carbon (C) and nitrogen (N) in sheet steel results in higher mechanical properties, age hardening, and deterioration of the r-value (measure of resistance to thinning and drawability. Non-ageing IF steel has no yield point elongation, which means fluting and stretcher strains are never a problem.

IF steels made using only Ti are very common and are used to produce the best mechanical properties for deep drawing. It is very reactive in a zinc bath and is usually coated only as galvanized steel (GI).

Another popular type of IF steel is that is stabilized with both Ti and Nb. The synergy of these two elements allows complete stabilization to be achieved at lower levels of each element. Depending on the relative amounts of Ti and Nb, the steel needs to be annealed at a higher temperature during galvanizing and has slightly inferior mechanical properties to the Ti type. Ti-Nb type IF steel is also less reactive in a zinc bath and is usually employed when producing galvannealed steel (GA).

IF steels are ideal for directly producing deep drawing steels (DDS) and EDDS hot dip products by the continuous annealing process. During the zinc coating and galvannealing steps the strip is reheated above the over ageing temperature. If low carbon steels are being used, carbon would redissolve, and could cause strain ageing.  With IF steels, cooling and reheating is irrelevant, since carbon (and nitrogen if present) are not available to be re-dissolved and cause aging.

One type of EDDS made using stabilized steel is actually has a higher strength steel with minimum yield strength of 205 MPa. IF steel made using phosphorous additions of up to 0.06 %, combines good formability with high strength, producing good dent resistance on exterior panels

Most IF steels have manganese levels below 0.20 %, and the formability improves as the carbon level is lowered. Manganese becomes more damaging to r-values as the carbon level increases.

These steels are hardened by adding manganese, silicon and phosphorous in solid solution to the ferrite. The metallurgy of IF steels optimizes their drawability.

  • Their low YS/UTS ratio and high strain hardening coefficient n ensure excellent deep-drawability and good strain redistribution.
  • Their high strain ratio r ensures good deformation behavior, making them suitable for deep-drawing.

IF steels offer excellent drawability for their strength level as a result of their very good fracture elongation, normal strain ratios and strain hardening coefficients. These steels can be readily welded by all the welding processes.

Some advantages of IF steel include (i) superior stamping, forming, and drawing performance, (ii) the ability to make more complex parts, perhaps using a fewer numbers of dies, (iii) age hardening resistance (long shelf life for stored steel), and (iv) improved coating adhesion for galvanized products.

The main disadvantage of IF steel is that it can be very soft, resulting in shearing and punching difficulties, and its use may result in parts that are not as ‘strong’, i.e., dent resistant, compared to parts made from carbon steel.

Applications of IF steel

Applications for IF steel include elements of the body structure and closures. Nowadays, IF steel is widely known as the best affordable material for deep drawing operations. It has been utilized for broad applications ranging from automotive body to electronic components as well as from enamel wares to house hold appliances. IF steel has low yield strength but a poor dent resistance which are undesirable for certain automotive applications.

By 1975, the average vehicle contained 3.6 percent medium and high strength steels for a total vehicle steel content of 61 percent, mostly mild steel. In the 1980s, the use of IF and galvanized steels grew for complex parts, as styling, corrosion, and cost were key considerations.

IF steel was initially developed as a highly formable material, and used extensively for deep drawn applications requiring high ductility and resistance to thinning. It also became the standard base for hot-dipped galvanized steels, as the stabilizing alloy elements in IF prevent aging behavior. A third type of IF steel, with nitrogen or other elements re-introduced, could be used to meet higher dent resistance and strength requirements.

These steels have high strain hardening potential during forming, lending deep-drawn parts (trunks, tailgates, doors, linings, wheel arches, etc.) good dent resistance.

With their high mechanical strength guaranteeing good fatigue and impact resistance, these steels are intended for structural parts (longitudinal beams, cross members, B-pillars, etc. wheel arches, toe boards as well as for skin parts, in which they provide good indentation resistance. In contrast to that of conventional drawing quality steels, the weight reduction potential of these products increases with the depth of the drawing.