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Steels for hot stamping - Usibor® and Ductibor®

Description

Usibor® and  Ductibor® are hot-formed grades which are intended for use in automotive structural and safety components. Due to their high strength, Usibor® steels are particularly suitable for anti-intrusion parts for which an ultimate load before collapse is required. Thanks to their excellent ductility, the Ductibor® steels are particularly effective for the parts which need to absorb energy during an impact. The graph below illustrates the typical levels of tensile strength and breaking strain of Usibor® and Ductibor® steels.

The main advantages of Usibor® and Ductibor® steels are:

  • Ability to obtain complex geometry due to the fact that forming has taken place in the austenitic condition in the direct stamping process or that the material exhibits good formability before heat treatment in the indirect stamping process. Very good hot formability enables steel formulations to be used which incorporate several functions elimination of reinforcing parts or assemblies).
  • No springback
  • Uniformity of mechanical properties obtained in parts
  • Excellent fatigue strength and impact resistance enabling substantial weight reduction to be achieved

ArcelorMittal was the first steelmaker to offer a coated steel for hot forming: Usibor®-AS with an aluminium-silicon coating applied at quenching.

The additional benefits of Usibor®-AS and Ductibor-AS steels® are:

  • Simplification of the process and economic savings (elimination of the shot-blasting step after forming (no formation of scale), no specific protective atmospheres required in the austenitising furnaces
  • Excellent temporary corrosion resistance after stamping, parts not requiring oiling before assembly
  • Absence of decarburisation
  • Excellent resistance to perforation corrosion, current use in dry and wet areas (sills for example) of the vehicle

Usibor® 1500-GI Zn galvanised (for indirect stamping process only) and Usibor® 1500-GA ZnFe galvannealed (for direct and indirect processes) have now been added to the coating offer.

Two safety data sheets are available for each steel, one for the as-delivered product and one for the product after heat treatment. These steels require no special precautions.

Applications

Usibor® steels are particularly well suited for the entire range of structural parts which require anti-intrusion resistance in a crash.

The most common applications are:

  • Front and rear bumper beams;
  • Door reinforcements;
  • Windscreen pillar reinforcements;
  • B-pillar reinforcements;
  • Floor and roof reinforcements;
  • Roof and dash panel crossmembers.
  • B-pillar (thickness: 1.85 mm)

  • Bumper beam (thickness: 2.3 mm)

  • Door reinforcement (thickness: 1 mm)

  • Windscreen upright (thickness: 1.2 mm)

  • Usibor® 2000 / Ductibor® 1000 laser-welded blank stamping trials

Ductibor® steel grades may be offered with Usibor® in the form of laser-welded blanks enabling hot-stamped parts to be obtained with more ductile properties locally than Usibor® steels. This product meets the need to precisely control deformations of particular zones of the vehicle in a crash (B-pillar reinforcement for example), and to broaden the utilisation spectrum of hot-stamped steel parts for energy absorption (front or rear rail for example).

Potential applications for Usibor® - Ductibor® laser-welded blanks are:

  • Front rail (front end, corner joint and underfloor extension);
  • Rear rail;
  • B-pillar reinforcement (upper section in Usibor®, lower section in Ductibor®).
Example of potential applications of  Usibor® - Ductibor® steels

Example of potential applications of  Usibor® - Ductibor® steels

ArcelorMittal has a complete set of data relating to the forming and the service properties of steels for hot forming. In order to incorporate these steels at the design stage, a team of experts is available to carry out specific studies based on modelling or laboratory tests.

Mechanical properties

The table below lists the minimum characteristic values ​​after hot stamping (1) and paint baking simulation (2) for the part. These values ​​are indicative and depend on the hot-stamping process parameters.

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ArcelorMittal name Direction Yield stress Rp0.2 (MPa) (1)(2) Tensile strength Rm (MPa) (1)(2) Min. A(%) (3) Type 1 Min. A50mm (%) (3) Type 2 Min. A80mm (%) (3) Type 3 Min. A50mm (%) (3) Bending angle (4)
22MnB5 RD ≥ 1050 ≥ 1400 5 5 5
Ductibor® 450 RD ≥ 350 ≥ 460 15 15 15 ≥ 120
Ductibor® 500 RD ≥ 380 ≥ 550 15 15 15 ≥ 120
Ductibor® 1000 RD ≥ 800 ≥ 1000 6 6 6 ≥ 80
Usibor® 1500 RD ≥ 1050 ≥ 1400 5 5 5 ≥ 50
Usibor® 2000 RD ≥ 1400 ≥ 1800 5 5 5 ≥ 45
Show all
ArcelorMittal name Direction Yield stress Rp0.2 (MPa) (1)(2) Tensile strength Rm (MPa) (1)(2) Min. A(%) (3) Type 1 Min. A50mm (%) (3) Type 2 Min. A80mm (%) (3) Type 3 Min. A50mm (%) (3) Bending angle (4)

A80mm %: Percentage elongation after fracture using a specimen with gauge length L0 = 80 mm (ISO 6892-1 type 2 (EN20x80))
A50mm %: Percentage elongation after fracture using a specimen with gauge length L0 = 50 mm (ISO 6892-1 type 1 (ASTM12.5x50) or type 3 (JIS25x50)
A%: Percentage elongation after fracture using a proportional specimen with L0 = 5.65 (So)1/2
Ag %: Percentage plastic extension at maximum force
BH2: Increase in yield strength between a reference condition after a 2% plastic pre-strain and the condition obtained after heat treatment (170°C-20minutes)

(1) 5 to 10 minutes of 880°C to 930°C-type heat treatment followed by quenching in perfectly cooled stamping tools (cooling rate > 30°C per second).
(2) Paint baking simulation: 170°C heat treatment for 20 minutes.
(3) Breaking elongation A% measured on test pieces (ISO20x80) is given for information only, the minimum bending angle is more relevant to evaluating the ductility of the material in a crash.
(4) Bending angle in accordance with VDA238-100, on a 1.5 mm thick test piece.

For further information, we invite you to download the files below, containing typical tensile curves and the associated laws of physics.

Ductibor® 500

  

Ductibor® 1000

  

Usibor® 1500

  

Usibor® 2000

Chemical composition

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ArcelorMittal name Max. C (%) Max. Si (%) Max. Mn (%) Max. P (%) Max. S (%) Al (%) Max. B (%) Max. Ti + Nb (%) Max. Cr + Mo (%)
22MnB5 0.25 0.4 1.4 0.03 0.01 0.01 - 0.1 0.005 0.12 1
Ductibor® 450 0.1 0.5 1.9 0.03 0.03 0.015 - 0.2 0.001 0.24
Ductibor® 500 0.1 0.5 1.9 0.03 0.03 0.015 - 0.2 0.001 0.24
Ductibor® 1000 0.12 0.8 2 0.03 0.01 0.01 - 0.1 0.01 0.12 0.6
Usibor® 1500 0.25 0.4 1.4 0.03 0.01 0.01 - 0.1 0.005 0.12 1
Usibor® 2000 0.37 0.7 1.4 0.03 0.01 0.01 - 0.06 0.005 0.12 1.4
Show all
ArcelorMittal name Max. C (%) Max. Si (%) Max. Mn (%) Max. P (%) Max. S (%) Al (%) Max. B (%) Max. Ti + Nb (%) Max. Cr + Mo (%)
22MnB5 0.25 0.4 1.4 0.03 0.01 0.01 - 0.1 0.005 0.12 1
Usibor® 1500 0.25 0.4 1.4 0.03 0.01 0.01 - 0.1 0.005 0.12 1

Global availability


Under development     Customer trials     Commercial unexposed only     Commercial exposed and unexposed    

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ArcelorMittal name Generic name Uncoated (-UNC) Alusi® (-AS) Extragal® (-GI) Ultragal® (-GI) Galvannealed (-GA) Zagnelis® Protect (-ZMP) Galfan (-ZA) Electrogalvanized (-EG)
22MnB5
Ductibor® 450
Ductibor® 500
Ductibor® 1000
Usibor® 1500
Usibor® 2000
Show all
ArcelorMittal name Generic name Uncoated (-UNC) Alusi® (-AS) Extragal® (-GI) Ultragal® (-GI) Galvannealed (-GA) Galfan (-ZA)
22MnB5
Ductibor® 450
Ductibor® 500
Ductibor® 1000
Usibor® 1500
Usibor® 2000
Show all
ArcelorMittal name Generic name Uncoated (-UNC) Alusi® (-AS) Extragal® (-GI) Galvannealed (-GA) Galfan (-ZA)
22MnB5
Ductibor® 450
Ductibor® 500
Ductibor® 1000
Usibor® 1500
Usibor® 2000
Show all
ArcelorMittal name Generic name Alusi® (-AS) Galfan (-ZA)
22MnB5
Ductibor® 450
Ductibor® 500
Ductibor® 1000
Usibor® 1500
Usibor® 2000
Show all
ArcelorMittal name Generic name Uncoated (-UNC)
22MnB5
Ductibor® 450
Ductibor® 500
Ductibor® 1000
Usibor® 1500
Usibor® 2000
Show all
ArcelorMittal name Generic name Uncoated (-UNC) Alusi® (-AS) Galvannealed (-GA) Galfan (-ZA)
22MnB5
Ductibor® 450
Ductibor® 500
Ductibor® 1000
Usibor® 1500
Usibor® 2000

The microstructure of Usibor® 1500 before heat treatment exhibits a ferrito-pearlitic matrix.

Usibor® 1500 microstructure before hot-stamping heat treatment (delivery condition)

Usibor® 1500 microstructure before hot-stamping heat treatment (delivery condition)

The Usibor® 1500 Alusi® coating in the delivery condition is split into one ternary layer of alloy at the steel-coating interface and an overlay of binary aluminium-silicon alloy.

Section of Usibor® 1500 coating prior to hot stamping

Section of Usibor® 1500 coating prior to hot stamping

Following heat treatment and quenching, the microstructure is 100% martensitic.

Usibor® 1500 martensitic microstructure following hot-stamping heat treatment (example: 5-minute austenitisation at 900°C, followed by water quenching or die quenching). Scanning electron micrograph

Usibor® 1500 martensitic microstructure following hot-stamping heat treatment (example: 5-minute austenitisation at 900°C, followed by water quenching or die quenching). Scanning electron micrograph.

The Usibor® 1500 Al-Si coating is transformed in the furnace (interdiffusion and solidification reactions), forming different intermetal Al-Fe-Si alloy protective layers providing perfect adhesion.

Appearance of the coating after hot stamping (optical microscopy)

Appearance of the coating after hot stamping (optical microscopy)

Heat treatment

Usibor® Alusi® and Usibor®-GA were specially developed for a direct hot-stamping process consisting of austenitisation of blanks in the heat treatment furnace, hot stamping these hot blanks in a press and martensitic quenching in the water-cooled stamping tool. All strain during forming occurs at high temperature. We advise against cold pre-forming prior to austenitisation of Usibor® Alusi® and Ductibor® Alusi®. Usibor® 1500-GI is not compatible with the direct hot-forming process. Hot-stamped Usibor® Alusi® and Ductibor® Alusi® parts have no micro-cracks inside the steel substrate.

Indirect hot-stamping process for Usibor®-GI steels

 

Direct hot-stamping process for Usibor® Alusi® and Ductibor® Alusi®

 Please contact us for data and advice on hot-stamping Usibor® and Ductibor® steels.

Usibor® 1500-GI, Usibor® 1500-GA and 22MnB5 have a good cold formability, they can be used with an indirect process.

Direct hot-stamping process for Usibor®-AS and Ductibor®-AS

Indirect hot-stamping process for Usibor®-GI steels

Surface treatment

After hot stamping and quenching, Usibor® Alusi® and Ductibor® Alusi® components can be painted directly without a cleaning operation such as shot blasting. The high surface roughness of the part provides excellent adhesion with cataphoresis even without phosphate primer. The composition of phosphating baths need not be modified and no pollution is observed.

Parts in Usibor® 1500-GI must be cleaned (shot blasting for example) to ensure good paint adhesion and a good weldability.

Parts in 22MnB5 are systematically shot blasted in order to remove surface scale and thus ensure good paintability.

Welding

Usibor® and Ductibor® steels have good spot weldability for both similar and dissimilar welds at both 50 and 1000 Hz.

The product has a wide welding range and the mechanical (tensile, shear) strength of the joints complies with automotive manufacturers' requirements and standards. Thanks to the nature of the alloy layer obtained after hot stamping, welding electrode life is considered exceptional (several thousand spots without deterioration) compared to that of conventional metal coatings. MAG, MIG and conventional metal welding techniques, including brazing, can readily be applied.

Based on long shop-floor experience in the characterisation of its products for purposes of resistance spot welding and arc welding, ArcelorMittal can provide technical support for welding parameter adjustment.

  • Usibor® + Usibor® similar metal spot welding 

  • Triple thickness dissimilar metal spot weld with multiphase steel

  • MAG weld

Fatigue strength

Fatigue strength can be expressed as an endurance limit (maximum stress).

Usibor® 1500 Alusi® and Ductibor® 500 Alusi® offer excellent fatigue properties (superior to those obtained in uncoated steels for hot stamping with decarburised surfaces).

The table below shows 2 million cycle endurance limits, expressed in MPa, in a uniaxial tension-compression test for R = 0.1 and R = -1.

Brand name σD A 2.106 cycles (MPa)
R=-1
σD A 2.106 cycles (MPa)
R=0.1
Usibor® 1500 475 727
22MnB5* 305 617
Ductibor® 1000-AS 356 621

* Decarburized surface after hot stamping with a thickness of approximately 30 µm.

Impact resistance

Usibor® and Ductibor® steels are used for safety parts. A great deal of data is available to demonstrate their exceptional crash resistance.

Due to their high yield strength, Usibor® steels are particularly suitable for anti-intrusion parts for which an ultimate load before collapse is sought. The example below illustrates the potential for weight reduction afforded by Usibor® steels compared to other more conventional steels in a "dynamic three-point bend test" at 30 km/h for 10 kJ of energy.

Weight-saving potential of  Usibor® steel compared to that of an HSLA 380 steel (reference)

Weight-saving potential of  Usibor® steel compared to that of an CR340LA steel (reference)

Based on their high breaking load and excellent ductility, the Ductibor® steels are particularly effective for the parts required to absorb energy during an impact.

Ductibor® steels have been characterised in dynamic axial compression tests using a top-hat structure with a spot-welded closure plate at an impact velocity of 56 km/h. These tests have demonstrated very good impact behaviour for these steels. The following graph shows for information only, the minimum weight-saving potential of these steels compared to that of an CR340LA  steel. Ductibor® 1000 exhibits exemplary crash ductility for a steel of Rm>1000 MPa.

Weight-saving potential of Ductibor® steels compared to that of an HSLA 380 steel (reference) 

Weight-saving potential of Ductibor® steels compared to that of an CR340LA steel (reference) 

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