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Repairing martensitic steel in automotive body structures

Saturday, September 1, 2018 - 07:00
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In today’s world, innovation is key for the automotive and steel industries. Steel’s role in automotive is constantly evolving as automakers face increased demands for performance, lightweighting, value and sustainability. To meet these challenges, the steel industry has been working with its automotive customers to develop and apply advanced high-strength steel (AHSS) in future vehicles, providing tailored solutions for each application. In fact, there are more than 200 automotive steel grades (Figure 1) featuring an array of properties, including formability up to 60 percent, and strengths from 200 MegaPascals (MPa) to 2000 MPa. Steel’s versatility offers automakers a high-value solution to meet fuel economy and performance requirements.

Figure 1 - Steel strength ductility diagram

As these new steels are introduced, it is crucial for the steel industry to work closely with the automotive repair industry to ensure proper technological knowledge is available. In order to make repairs, technicians must be able to recognize vehicle materials in order to create a plan of repair, understand the tools and techniques needed for the repair, and provide the consumer an accurate cost and time estimate. Thus, quality structural repairs are necessary to maintain the strength and properties of steel and ensure restoration of the vehicle to original performance, durability and level of crash protection.  

Looking back
In the December 2017 article, we discussed the repairability of AHSS, evaluating several steel grades’ sensitivity to thermal exposure taking place during heating to soften the material for straightening, typically by flame. AHSS are typically produced by non-traditional thermal cycles and contain microstructural constituents, such as martensite, whose mechanical properties can be altered by exposure to elevated temperatures.

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This is problematic, as this temperature sensitivity can alter the mechanical behavior during welding repair or flame straightening, seriously affecting the structural performance of the AHSS components after the repair. To assess the response of AHSS to repair, metal inert gas (MIG) welding was evaluated on selected samples (HSLA 340, DP 600, DP 780 and TRIP 600 steels). This process was deemed as an acceptable weld repair process for AHSS with no concerns over reducing the performance of the as-repair components.

Building on this insight, the Auto/Steel Partnership (A/SP), in collaboration with our members – AK Steel Corporation, ArcelorMittal, FCA US LLC, Ford Motor Company, General Motors Company, Nucor Corporation, and POSCO – has completed another phase of repair evaluation of AHSS body components. In particular martensitic (MS) steel grades were studied using various weld repair processes to create new options for weld repair. The results of this study will allow OEMs to update their repair process strategies as necessary.

In this work, roll-formed martensitic (MS) steels and press hardened steels (PHS) were selected for evaluation at a thickness of 1.0 mm. These steel grades are used in areas where exceptional strength and anti-intrusion are needed, including such applications as the A-pillars, B-pillars, rockers and rails. Specifically, the materials evaluated were: MS 1500, MS 1700 and PHS 1500.

Martensitic Steels
Martensitic steels have very high tensile strengths (800 MPa to 2000 MPa) and are produced by transforming the austenite from the hot-rolling or annealing process to martensite during quenching on the run-out table or in the cooling section of the continuous annealing line. MS steels are characterized by a martensitic matrix containing small amounts of ferrite and/or bainite as shown in Figure 2.

Figure 2 - Microstructure of martensitic steel
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