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Steels for hot stamping - Usibor®

Description

Usibor® 1500 and 22MnB5 are hot formed grades intended for use in automobile structural and safety components. The very high mechanical strength of the final part makes it possible to achieve weight savings of 30% to 50% compared to conventional cold forming grades.

The main advantages of Usibor® 1500 and 22MnB5 are:

  • Ability to achieve complex geometry in both the direct process (forming in austenitic state) and the indirect process (good cold formability). Their very good hot formability makes it possible to offer steel solutions that  integrate several functions (elimination of reinforcement parts and assemblies);
  • Total absence of springback;
  • Uniform mechanical properties obtained on the part;
  • Exceptional fatigue strength (Usibor® 1500) and impact resistance, allowing substantial weight reduction.

ArcelorMittal was the first steelmaker to deliver a coated press hardened steel, Usibor® 1500-AS with an aluminum-silicon coating.

 

The additional advantages of Usibor® 1500-AS, above and beyond those of 22MnB5, are:

  • Simplified process and cost savings: elimination of the shot-blasting step (no formation of scale), no need for protective atmosphere in austenitization ovens;
  • Excellent temporary corrosion resistance after stamping, no need to oil parts before assembly;
  • No decarburization;
  • Excellent resistance to pitting corrosion, currently used in dry and wet areas (e.g. side sill) of the vehicle.

Usibor® 1500-GI galvanized Zn (for indirect stamping process only) and Usibor® 1500-GA galvannealed ZnFe (for both direct and indirect processes) now round out the coating offer.

 

ArcelorMittal has now added the Ductibor® 500 grade to its range. This product is offered in association with Usibor® 1500 in Laser Welded Blank (LWB) solutions that locally provide more ductile properties than those of Usibor® 1500. This makes it possible to precisely control the crash deformation of specific vehicle parts (e.g. B-pillar reinforcement) and expands the use of hot stamping steels to energy absorption parts (e.g. front or rear rail).

 

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® 1500 and 22MnB5 are particularly well suited for the entire range of structural parts requiring good crash resistance.

Current applications include:

  • 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)

 

Potential applications of Usibor® 1500 - Ductibor® 500 Laser Welded Blanks include:

  • Front rail (front end, bend and underfloor extension);
  • Rear rail;
  • B-pillar reinforcement (upper part in Usibor® 1500, lower part in Ductibor® 500).

Examples of potential applications of  Usibor® 1500 - Ductibor® 500 Laser Welded Blanks

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

As delivered, before hot stamping (for information only)

 YS (MPa)UTS (MPa)ef (%)
L0 = 80 mm
th < 3 mm
 Usibor® 1500350 - 550500 - 700≥ 10
 22MnB5320 - 550500 - 700≥ 10
 
 Hot rolled     Cold rolled

After hot stamping according to best practices - typical values* (for information only)

 YS (MPa)UTS (MPa)ef (%)
L0 = 80 mm
th < 3 mm
 Usibor® 1500110015006
 22MnB5110015006
 
 Hot rolled     Cold rolled

* 5 to 10 minutes 900°C to 950°C type heat treatment followed by quenching in perfectly cooled stamping tools (cooling speed > 50°C per second).

Chemical Composition (%)

 CMnSi
 MaxMaxMax
Usibor® 15000.251.40.35
Usibor® 15000.251.40.35
22MnB50.251.40.35
22MnB50.251.40.35
 
 Hot rolled     Cold rolled

Available coating

 UncoatedExtragal®GalvannealedAluminised
 EURNAMSAMRSACHIEURNAMSAMRSACHIEURNAMSAMRSACHIEURNAMSAMRSACHI
Usibor® 1500           
Usibor® 1500                  
22MnB5                  
22MnB5                 

 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 and 22MnB5 is composed of ferrite and perlite.

Usibor® 1500 microstructure before hot stamping (delivery state)

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

Cross-section of the Usibor® 1500 coating 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 (interdiffusion and solidification) into protective Al-Fe-Si intermetallic layers that adhere perfectly to the substrate.

Appearance of the coating after hot stamping (optical microscopy)

Heat treatment

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

 

Consult us for data and advice relating to hot stamping of Usibor® 1500.

 
 
22MnB5
Usibor® 1500-AS
Usibor® 1500-GI
Usibor® 1500-GA

 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

Direct process

Indirect process

Shotblasting required

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® 1500-AlSi, Usibor® 1500-GA and 22MnB5

Surface treatment

After hot stamping and quenching, Usibor® 1500-AS components can be painted directly without a cleaning operation such as shotblasting. The high surface roughness of the part ensures good paintability (by cataphoresis or any other organic painting system). The composition of phosphating baths does not need to be modified and no pollution is observed.

 

The parts made with Usibor® 1500-GI must be cleaned (by shotblasting for example) to ensure good paint adhesion and a good weldability.

 

A cleaning step may be necessary on parts made with Usibor® 1500-GI, depending the end customer's  weldablity and paintability requirements.

 

Parts made with 22MnB5 are systematically shotblasted to remove surface scale and ensure good paintability.

Welding

Usibor® 1500 has excellent 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.

 The weldability of 22MnB5 is comparable to that of other carbon steels of similar composition.

Matching Usibor® + Usibor® joint

Triple thickness non-matching joint with multiphase steel

MAG weld

Fatigue strength

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

 

Usibor® 1500 offers 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® 1500727475
22MnB5*617305
 
 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.

When these steels are used, it is possible to reduce weight by 30% to 50% compared to conventional steel grades while achieving the same crash performance.

The example below shows the results of a three point bending flexural test at 29 kph with 10 kJ energy equivalent. The maximum loads recorded on specimens with an top-hat type cross-section and 1.5 mm thickness are shown. This example demonstrates the potential weight reduction offered by hot stamped steels compared to more conventional steels.

Specimen shape used in the three point bending flexural test

Ultimate load measured during a bending flexural test at 29 kph (10kJ)(thickness: 1.5 mm)

Three point bending flexural test specimen before and after crash test

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