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Product definition

Major metallurgical families and characterization

ArcelorMittal's range of steels for the automotive sector comprises all the main metallurgical families:

  • Steels for drawing: aluminum killed and IF (Interstitial Free);
  • High-strength steels: high yield strength steels, rephosphorized steels, high strength IF steels, isotropic and bake hardening steels;
  • Very high strength multiphase steels: Dual Phase, TRIP, ferrite-bainite, Complex Phase steels.

The mechanical properties of these steels are the result of a combination of parameters that are defined throughout the steel manufacturing process. The two main parameters are:

  • Chemical composition;
  • Thermo-mechanical process.

Mechanical properties

To obtain the required mechanical properties, the steelmaker devises a range of strength/formability combinations suitable for the uses to which products are to be put in the automobile.A number of hardening processes are available. They can be employed alone or in combination:

Steel hardening mechanism

To activate and control these processes, the steelmaker varies:

a) Chemical composition

The composition of the alloy lends the steel its mechanical strength. Iron from the blast furnace, the first stage in the steel production process, is uniform for all products.

In the following process stage, alloying elements are added to or removed from the iron. This stage determines the main families of steel, from the strongest to the most formable. The proportion of carbon plays a crucial role in this determination, since it is the main hardening element added to iron. Other elements such as manganese, silicon and phosphorous are also used to adjust the strength of the steel. More selectively, further alloying elements such as titanium, niobium and vanadium can be added to lend specific hardness properties to the steel. These are called micro-alloyed steels, since these elements have an effect even when added in very small quantities compared to the other alloying elements.

In multiphase steels (Dual Phase, TRIP, Complex Phase...) it may be necessary to add chromium and molybdenum to obtain hard phases.

Nitrogen and carbon are chemical elements of small atomic size compared to iron. They are called interstitial elements because they are easily positioned within the iron crystal lattice (positions 2 and 3 in the figure below: positions 4 and 5 are occupied by substitution elements such as Mn, Si, etc., and position 1 is a vacancy). Placed in the interstices of the crystal lattice, they harden the crystal as a whole by preventing the atomic planes from sliding against each other. The quantity of interstitial elements in steel determines its mechanical properties. Carbon content is adjusted primarily by blowing oxygen through the molten metal and can be further lowered by vacuum treatment.There are two possible methods for removing carbides and nitrides, i.e. for inducing the precipitation of residual carbon and nitrogen atoms contained in compounds too voluminous to occupy interstitial positions. One-the method used for ordinary and high strength steels-consists in adding aluminum (in this case the steels are said to be "aluminum killed"). The other consists in adding titanium (these steels are then said to be "titanium killed"). The second method is the more efficient in reducing total interstitial nitrogen and carbon. This method is used to produce "Interstitial Free" (IF) type mild steels.

Various positions that alloying elements can occupy in the iron crystal lattice

b) Thermo-mechanical process

The grain structure of steel influences its mechanical behavior at two levels:

  • Microscopically, through alignment irregularities (dislocations) and interstitial or substitutional alloying elements within each grain, which is itself a single crystal of iron;
  • More macroscopically, through the shape (elongated or equi-axed) and size of the grains.

For a given chemical composition, these characteristics of a steel are related to the thermo-mechanical cycles it undergoes throughout the manufacturing process:

  • Solidification in slab form;
  • Hot rolling;
  • Cold rolling;
  • Annealing;
  • Skin-pass.

Rolling temperatures, cooling speeds, coiling temperatures, thickness reduction rates in the cold rolling mill, annealing cycles and skin-pass parameters are all varied in order to adjust the structure of the steel and hence the product's final properties.

Steel grain structure

Tensile test

Steel is characterized by the mechanical properties of products sold both in the cold rolled (thicknesses below 3.0 mm) and the hot rolled (currently, thicknesses higher than 1.8 mm) state. These properties reflect the product's propensity for secondary processing and for forming by means of drawing, bending, hydro-forming, etc. The method most commonly used to determine the mechanical properties of materials is the tensile test.

It has two advantages:

  • It is easy, rapid and standardized.
  • The resulting stress-strain curve provides a large amount of precise information.

The test consists in gradually elongating a specimen of the grade to be characterized. A uniaxial load is applied to the specimen in the rolling or transverse direction. The load needed to deform the specimen to failure and the elongation of the specimen are recorded simultaneously. These values are used to plot stress (load divided by the initial cross-section of the specimen) against strain (expressed as percent elongation of the initial gage L0).

This is the stress-strain curve, shown in the figure opposite. This uniaxial test is spelled out precisely in the EN 10002-1 standard and elsewhere. The importance of specimen preparation (machining), especially for high strength steels, should be borne in mind.

Shape of the stress-strain specimen

Configuration of the tensile test machine

Shape of the stress-strain curve

Remarks

Test specimen dimensions:

 

1. The dimension of tensile test specimens varies according to the thickness of the product tested:

a. thickness ≤ 3 mm: width 20 mm and length 80 mm;

b. thickness > 3 mm: width 30 mm and length 5.65 √S0. where S0 = width x thickness. Standard dimensions in Europe (EN standards).

2. Specimen dimensions also vary from one country to another:

a. Japan (JIS standard): width 25 mm and length 50 mm;

b. USA (ASTM standard): width 12.5 mm and length 50 mm.

Because of these variations in specimen size, the mechanical properties measured are not directly comparable. However, well-established conversions exist between the different standards.

JIS - EN - ISO elongation value correlations

These conversions are indicative. Our technical department can provide further information as required.

Tensile test direction
All parameters derived from the tensile test reflect the properties of the steel in a specific direction: that of the tensile test. These values depend on the direction in which the sample was taken with respect to the direction in which the thin sheet was rolled.

When indicating the mechanical properties of steel, the  sampling direction with respect to the rolling direction must always be specified:

  • Rolling direction RD (indicated by 0°);
  • Transverse direction TD (indicated by 90°);
  • Oblique direction (indicated by 45°).

Main mechanical properties

The tensile test measures the following parameters, which characterize the material:

a) Yield stress: YS

Point A on the stress-strain curve. It represents the load at which the elastic domain, in which deformation is reversible, ends and the plastic domain, in which deformation is irreversible, begins.

Typically, there are two types of transition:

  • The transition between the elastic and plastic domains shows a maximum followed by a sudden drop in yield stress and a plateau. A distinction is made between upper yield stress UYS, corresponding to the peak, and lower yield stress LYS, corresponding to the plateau. The length of the plateau is the yield point elongation es.
  • The transition is gradual. Yield stress is then defined conventionally. It is measured for 0.2% elongation and termed 0.2% proof stress (0.2% PS). The term YS will be used to cover both types in this document.

Definition of yield stress and plateau

b) Ultimate tensile stress (or tensile strength or mechanical strength): UTS

Point B on the stress-strain curve. This is the maximum load reached during the tensile test.

Beyond this point, deformation begins to concentrate locally in a phenomenon called "necking", which explains the drop in the load required for further deformation beyond Point B.

c) Fracture elongation: ef%

This is the residual elongation after failure of the specimen at point C on the stress-strain curve.

d) Strain hardening coefficient: n

In the tensile test, loads are measured with respect to the initial cross-section of the specimen. True stress σ and true strain ε are determined by calculating the load with respect to the instantaneous cross section, using the law of conservation of mass/matter.

The resulting plot of σ = f(ε) is called the true stress-strain curve. This curve can be described by the Holloman law: σ = k.εn, in which n is called the strain hardening coefficient. It describes the propensity of steel to harden during deformation in  the plastic domain (the higher the value of n, the more rapidly the steel hardens), to deform in the expansion mode and to redistribute strains.

e) Anisotropy coefficient: r

The anisotropy coefficient measures the tendency of the steel to resist thinning during the tensile test. It expresses the ratio between specimen width deformation and specimen thickness deformation and thus reflects the steel's ability to undergo severe deep drawing strains.

The values of r are usually on the order of 1 for hot-rolled sheet but can go up to nearly 3 for steels with the highest drawability.

f) Bake hardening

Bake hardening describes the steel's ability to harden during paint baking; this is used to increase the yield strength of the finished part.

These steels thus combine good drawability with good dent resistance after paint curing (YS value higher than in the initial blank) and good plastic deformation resistance in the finished part.

Bake hardening is determined by measuring the increase in YS following heat treatment at 170°C for 20 minutes, simulating paint curing conditions, after 2% uniaxial pre-strain (most representative value). This parameter is called BH2.

g) Work hardening

Work hardening describes the increase in yield stress compared to a reference level following plastic deformation. It is directly correlated with the steel's strain hardening coefficient n.

Low-carbon flat steel families

Low-carbon flat steels can be grouped into families according to their mechanical properties, their strength/ductility combination and the metallurgy (chemistry and thermo-mechanical processes) employed in their manufacturing. Within the metallurgical families, classification by YS and UTS range defines grades.

Metallurgical families

Range of ArcelorMittal steels for the automotive sector

Usibor® steels for hot forming are not shown. Their mechanical strength is on the order of 1500 MPa after hardening.

Coatings

In the automotive industry, autobody anti-corrosion protection, expressed as the anti-corrosion guarantee, has become a major selling point.

Several protection solutions have been developed and the most widely used can be divided into three groups:

  • Hot-dip metal coatings applied by immersion in a liquid metal bath (at temperatures up to 700°C);
  • Metal coatings applied by electrodeposition (at temperatures slightly above ambient);
  • Thin (0.5 to 6 µm) organic coatings applied on a substrate previously protected by electrodeposited or hot dip metal coating and pre-treated to increase corrosion resistance and adhesion of the organic coating.

Various families of coatings exist, depending on deposition process, chemical composition, thickness (or weight per unit area), application on one or both sides and surface appearance requirements.

 

Coating thickness is measured continuously on coating lines by means of x-ray gauges that scan the full width of the moving strip.

Other types of measurement can be carried out to obtain a point value:

  • Permascope, to determine the difference in product thickness with and without the coating;
  • Chemical measurement, to determine the difference in the weight of a sample with and without the coating (this is the most accurate measurement);
  • Optical microscopy, to determine highly local coating thickness values.

Surface condition

The surface condition of steels has a major impact on their service properties, particularly during the forming and painting processes.

Surface quality is characterized primarily by:

  • Surface topography;
  • Lubrication;
  • Chemical treatment.

Surface topography

Surface topography describes the surface micro-geometry of the steel sheet. This is essentially a two-dimensional parameter but it is usually characterized by a series of profiles (cross-sections). A profile is measured by means of a roughness tester, generally mechanical, and the profile is recorded by the vertical movements of a stylus placed on the surface. The signal can be broken down into several sinusoidal components with different wavelengths and amplitudes. The shortest wavelengths correspond to roughness and the longest to waviness.

Breakdown of a surface profile: the profile is a superimposed image of roughness, waviness and flatness defect, if any

Roughness:

Two parameters are primarily measured:

  • Roughness, Ra, i.e. the average depth of the surface profile. It generally ranges between 0.5 and 3 µm;
  • Peak count, RPc, i.e. the number of peaks that consecutively project beyond a band of given width (generally ±0.5 µm) centered around the mean profile depth, expressed in number per unit length (n.cm-1).

Increasing surface roughness while holding lubrication constant can help prevent seizing during drawing, especially in the case of uncoated products.

However, any increase in roughness must be assessed in terms of the entire process, with particular attention to surface appearance after painting.

Remark:

The calculation of roughness parameters is performed on the basis of a specific length for precise evaluation (at least five times the cut-off length). Depending on the measurement instrument, total length is generally 12.5 mm. The cut-off length is the long wavelength filtering threshold necessary for obtaining measurements representative of local micro-geometry.

Waviness:

Profile scanning also includes a measurement of waviness, which is the average value of the amplitudes within the wavelength limits set.

Waviness is major factor in appearance after painting (alongside, of course, painting process parameters). It is measured by, for example, the Wa 0.8 parameter.

For additional information, please contact our technical support department.

Surface texture control

Surface topography is imprinted on the strip by the roughness of the working rollers.

Roughness is transferred in the last stand of the cold rolling mill and during the skin pass operation after annealing or hot-dip galvanizing. Most of the roughness is transferred during the skin pass.

ArcelorMittal has developed special expertise in this area and can achieve the best possible combination of drawability and paint appearance.

Two main texturing processes are used:

  • EDT (Electrical Discharge Texturing) produces a stochastic surface texture;
  • EBT (Electron Beam Texturing) produces a fully controlled displacement of the electron beam, with impingement equally spaced along the axis and circumference of the work rolls.

Examples of roughness profiles (parallel scanning to obtain a 3D image)

Example of surface appearance after skin pass with EDT texture

Lubrication

Lubrication serves two purposes. It:

  • protects surfaces, both uncoated (red rust) and coated (white rust), against oxidation during storage and handling;
  • reduces friction and the tendency to seize during drawing.

Lubrication consists in depositing oils in a set quantity (ranging between 0.5 and 2.5 g/m2/side).

Lubricant suppliers offer a variety of products, from which ArcelorMittal has selected a range corresponding to the various requirements of its customers; certain oils called "prelubes", in particular, spectacularly improve the tribological performance of a given steel at constant texture.

ArcelorMittal also offers a range of dry films (drylubes) that can be applied to most coatings and to uncoated steels. These lubricants lend the steel very high friction performance and in most cases eliminate the need to re-oil, even in the most difficult situations. Because they are dry, they also have the advantage of helping to keep the shop floor clean. To develop an appropriate lubrication for an application, full-scale tests should be carried out to investigate forming behavior as well as possible impact on downstream processes (adhesive bonding, de-greasing and surface treatment in particular).

Chemical treatments

ArcelorMittal provides a wide range of chemical post-treatments designed to improve the drawing performance of coated steels:

  • Specific chemical treatments such as S250 improve the tribological behavior of electrogalvanized products;
  • Pre-phosphating electrogalvanized sheet improves tribological properties, limits particle formation during drawing, increases corrosion protection and facilitates paint adhesion;
  • NIT treatment achieves the tribological performance of pre-phosphating. It is available on electrogalvanized and pure zinc galvanized substrates. It is particularly useful in difficult drawing operations, ensuring uniform friction when the steel has been lightly oiled and limiting particle formation during drawing;
  • L-Treatment, which serves comparable purposes on Galvannealed substrate.

The friction behavior of NIT on galvanized sheet is similar to that of pre-phosphated electrogalvanized sheet

These post-treatments all strengthen the drawing process. They potentially reduce reject/rework rates.

They cannot be considered universal solutions. Their use must be examined on a case-by-case basis and they must be discussed with our technical support teams.

Surface appearance after painting

With ongoing improvements in steel substrates and painting techniques, it is now possible to obtain very good painting quality. Nevertheless, a film of paint is never completely flat and it never completely reflects light, as would a perfect mirror. The discrepancy between this ideal and the painted surface can be expressed in terms of distinctness of image (DOI) and gloss. DOI is the ability of the painted sheet to reflect an image distinctly.

It is measured, for example, by the DOI (Distinctness of Image) factor. Gloss is the capacity of the sheet to avoid distortions of the reflected object, often called the orange peel effect.

Painted appearance assessment: typical measurements

The painted appearance quality of a sheet to be used for skin parts is first dependent on painting process parameters-layer thicknesses and application and curing conditions.

Once the painting process has been optimized, the best results can be obtained by controlling the topographical parameters of the sheet. The waviness parameter (expressed as Wa 0.8), more than roughness, is crucial in this regard.

ArcelorMittal has developed its process for manufacturing coated steels for skin parts to control waviness in the initial blank and limit the recurrence of waviness after drawing.

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