Steels for hot stamping - Usibor^® and Ductibor^®

Description

Usibor^® and  Ductibor^® are hot formed grades which are intended for use in automobile structural and safety components. Due to their high resistance, the 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 absorb energy during an impact. The graph below illustrates the typical levels of tensile strength and fracture strain of Usibor^® and Ductibor^® steels.

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

• To propose complex geometry in direct process but also in indirect process as the cold formability of the product is good. The very good hot formability allows to use steel solution which include several functions (suppress of reinforcement part or assemblies);
• Homogeneity of mechanical properties obtained on part;
• Excellent fatigue strenght and impact resistance allowing substantial weight reduction.

ArcelorMittal was the first steelmaker to offer the automotive industry a coated press hardened steel: Usibor^®-AS with an aluminum-silicon coating.

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

• Simplification of the process and economic earnings (elimination of the shot-blasting step (no formation of scale), no specific protective atmospheres in the ovens during heat treatment austénitisation);
• Excellent temporary corrosion resistance after stamping, not requiring oiling of parts before assembly;
• Absence of decarburization;
• Excellent resistance to perforating corrosion, current use in dry and wet areas (side sill for example) of the vehicle.

Usibor^® 1500-GI galvanized Zn (for process of indirect stamping only) and Usibor^® 1500-GA galvannealed ZnFe (for direct and indirect process) complete now 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 a resistance to anti-intrusion during crash.

Main current applications are:

• Front and rear bumper beams;
• Door reinforcements;
• Windscreen upright reinforcements;
• B-pillar reinforcements;
• Floor and roof reinforcements;
• Roof and dash panel cross members.

B-pillar (thickness: 1.85 mm)

Bumper beam (thickness: 2.3 mm)

Door reinforcement (thickness: 1 mm)

Windscreen upright (thickness: 1.2 mm)

Laser Welded Blanks Usibor® 2000 / Ductibor® 1000 stamping trials

Ductibor^® steel grades are proposed in association with Usibor^® for Laser Welded Blanks solutions allowing to obtain hot stamped parts presenting characteristics locally more ductile than Usibor^® steels. This solution meets the needs to control precisely deformations of particular zones of the vehicle during the crash (example of a B-pillar reinforcement), and to widen the spectre of use of hot stamping steels parts dedicated for energy absorption (example of front or rear rail).

Potential applications of Laser Welded Blanks Usibor^® - Ductibor^® are:

• Front rail (front extremity, elbow and extension underfloor);
• Rear rail;
• B-pillar reinforcement (high part in Usibor^®, low part in Ductibor^®).

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 stamping. To integrate these steels at the design stage, a team of experts is available to carry out specific studies based on modeling or laboratory tests.

Technical characteristics

Mechanical properties

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

	YS (MPa)		UTS (MPa)		A (%) (3)		Bending angle (4)
Ductibor® 450	≥ 350		≥ 460		≥ 16		≥ 120°
Ductibor® 500	≥ 400		≥ 550		≥ 16		≥ 120°
Ductibor® 1000	≥ 800		≥ 1000		≥ 6		≥ 80°
Usibor® 1500	1100		1500		≥ 3		≥ 50°
Usibor® 2000	≥ 1400		≥ 1800		≥ 3		≥ 45°
Hot rolled     Cold rolled

⁽¹⁾ 5 to 10 minutes 880°C to 930°C type heat treatment followed by quenching in perfectly cooled stamping tools (cooling speed > 30°C per second).
⁽²⁾ paint baking simulation: 170°C heat treatment during 20 minutes.
⁽³⁾ Elongations to rupture are only indicative. More relevant is the minimum bending angle for crash ductility.
⁽⁴⁾ Bending angle following to the VDA238-100, referring to 1.5 mm thickness test specimen.

Chemical Composition (%)

	C	Mn	Si	B
	Max	Max	Max	Max
Ductibor® 450	0.11	1.1	0.06	0.001
Ductibor® 500	0.1	1.3	0.5	0.001
Ductibor® 1000	0.12	2	0.75	0.005
Usibor® 1500	0.25	1.4	0.4	0.005
Usibor® 2000	0.37	1.4	0.7	0.005
Hot rolled     Cold rolled

Available coatings and worldwide availability

Uncoated

Extragal®

Galvannealed

Aluminised

EUR

NAM

SAM

RSA

CHI

EUR

NAM

SAM

RSA

CHI

EUR

NAM

SAM

RSA

CHI

EUR

NAM

SAM

RSA

CHI

Usibor® 1500

22MnB5

22MnB5

Ductibor® 450

Ductibor® 500

Ductibor® 1000

Usibor® 1500

Usibor® 2000

 Hot rolled     Cold rolled

Available in non-visible part quality Undergoing customer testing Under development Available in visible and non-visible part quality (Z)

EUR : Europe Region - NAM : North America Region - SAM : South America Region - RSA : South Africa Region - CHI : China

Before heat treatment, the microstructure of Usibor^® 1500 is composed of ferrite and perlite.

Usibor® 1500 microstructure before hot stamping (delivery state)

The coating at delivery state is split into one ternary layer of alloy at the steel-coating interface and an overlay of binary aluminum-silicon alloy.

Appearance of the coating of Usibor® 1500 prior to hot stamping

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

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

The Al-Si coating is transformed in the oven into a protective Al-Fe-Si alloy layer adhering perfectly to the substrate.

Appearance of the coating after hot stamping (optical microscopy)

Heat treatment

Usibor^®-AS and Usibor^®-GA were specially developed for a direct hot stamping process consisting of austenitization of blanks in the heat treatment oven, hot stamping of 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 austenitization of Usibor^®-AS and Ductibor^®-AS. Usibor^® 1500-GI is not compatible with the direct hot forming process. Hot-stamped Usibor^®-AS and Ductibor^®-AS parts have no micro-cracks inside the steel substrate.

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

Consult us for data and advices related to hot stamping of 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.

Indirect process of hot stamping for Usibor®-GI

Surface treatment

Después del estampado y temple, la superficie de las piezas en Usibor^®-AS y Ductibor^®-AS permite una aplicación directa de pintura, sin necesitar una limpieza previa de tipo granallado. La elevada rugosidad superficial permite una adherencia excelente de la cataforesis incluso sin el agarre de la fosfatación. La composición de los baños de fosfatación no se ve modificada ni se ha identificado ningún tipo de contaminación.

Las piezas en Usibor^® 1500-GI se deben limpiar (mediante granallado por ejemplo) para garantizar una buena adhesión de la pintura y una buena soldabilidad.

Las piezas en 22MnB5 se granallan sistemáticamente para eliminar la cascarilla superficial y garantizar un buen comportamiento posterior en pintura.

Welding

Usibor^® and Ductibor^® have good spot weldability for both matching and non-matching joints at both 50 and 1000 Hz.

The product has a wide welding range and the mechanical (tensile, shear) performance of the joints complies with automotive manufacturer requirements and with standards. Thanks to 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 characterization of its products for purposes of resistance spot welding and arc welding, ArcelorMittal can provide technical support for welding parameter adjustment.

Homogeneous spot welding Usibor® + Usibor®

3 sheet spot weld combination with multiphase steel

MAG weld

Fatigue strength

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

Usibor^® 1500-AS and Ductibor^® 500-AS offer excellent fatigue properties (superior to those obtained in uncoated steels for hot stamping with decarburized surface).

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.

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

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

Impact resistance strength

Impact strength is the area in which Usibor^® 1500 and 22MnB5 come into their own. We can supply detailed information about the exceptional impact strength and the anti-intrusion properties of these two products.

Due to their high resistance, the Usibor^® steels are particularly suitable for anti-intrusion parts for which an ultimate load before collapse is required. Usibor^® steels have been characterized in the 3-point bending test using top hat cross-section specimens impacted at 30 kph. The tests showed very good behavior of these steels. The following graph shows the mass reduction potential of these steels compared to that of an HSLA 380. We can supply detailed information about the exceptional impact strength and the anti-intrusion properties of these two products.

Mass reduction potential of  Usibor® steel compared to that of an HSLA 380 steel (reference)

Thanks to their excellent ductility, the Ductibor^® steels are particularly effective for the parts which need absorb energy during an impact.

Ductibor^® steels have been characterized in dynamic axial compression tests using an top hat structure with a spot-welded closure plate at an impact velocity of 56 kph. These tests have demonstrated the good impact behavior of these steels. The following graph shows the mass reduction potential of these steels compared to that of an HSLA 380.

Mass reduction potential of Ductibor® steels compared to that of an HSLA 380 steel (reference)