Microstructural Characterization of Ultra-High Strength Martensitic Steels

Esterl, Raphael; Sonnleitner, Markus; Stadler, Manfred; Wölger, Günter; Schnitzer, Ronald

doi:10.3139/147.110491

Cited by 10 publications

(7 citation statements)

References 22 publications

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“…The specimens were hot embedded and then ground from 320 grit to 4000 grit SiC paper for at least 30 s. Subsequently, the samples were polished with 3 µm diamond paste for at least 3 min and with 1 µm for 30 s. For the metallographic investigation, a picric acid etchant was used to reveal the PAGs after direct quenching. A specially developed etching process was applied, which is described in detail elsewhere, based on the picric acid presented by Bechet‐Beaujard in 1955 . The etchant was modified by sodium dodecyl sulfate as wetting agent and performed at room temperature.…”

Section: Methodsmentioning

confidence: 99%

Microstructural Analysis of the Recrystallization Behavior of Low Alloyed Steels

Esterl

Sonnleitner

Schnitzer

2019

steel research int.

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The recrystallization behavior of five low alloyed steels is investigated using double hit deformation tests. It is shown, that Niobium has the biggest influence in retarding the recrystallization kinetics. Further, the microstructural evolution dependent on strain and temperature during deformation is studied with a picric acid etchant and light-optical analysis. It is shown how the microstructure of two differently alloyed ultra-high strength steels changes along with the peculiarities of the corresponding stress-strain curves including the evolution of grain size and aspect ratio of the prior austenite grain. The findings on the different recrystallization kinetics with the role of recrystallization retarding elements are further reinforced by investigations on the Zener-Hollomon parameter and the activation energy needed for dynamic recrystallization. A rolling scenario on a deformation dilatometer is simulated on a hardenable and a micro-alloyed steel to illustrate the microstructural evolution between the rolling steps. It is shown, how the two ultra-high strength steels perform different in their microstructural evolution, as the waiving of micro-alloying elements (MAE) provides finer austenite grains.

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Section: Methodsmentioning

confidence: 99%

Microstructural Analysis of the Recrystallization Behavior of Low Alloyed Steels

Esterl

Sonnleitner

Schnitzer

2019

steel research int.

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“…However, it is noteworthy that the PAG boundaries are not visible in sample I max . Generally, tempering is an effective way to improve the etching result for AHSS [32]. During this heat treatment, elements like phosphorus segregate to the PAG boundaries, resulting in an attack during the subsequent etching.…”

Section: Discussionmentioning

confidence: 99%

Microstructural characterization of a double pulse resistance spot welded 1200 MPa TBF steel

et al. 2019

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In the automotive industry resistance, spot welding is the dominant technology in sheet metal joining of advanced high strength steels (AHSS). In order to improve the mechanical performance of AHSS welds, in-process tempering via a second pulse is a possible approach. In this work, two different double pulse welding schemes were applied to a 1200 MPa transformation-induced plasticity (TRIP)-aided bainitic ferrite (TBF) steel. The different microstructures in the welds were characterized via light optical and scanning electron microscopy. Additionally, hardness mappings with several hundred indents were performed. It is shown that the second pulse, following a low first pulse which is high enough to produce a weld nugget that fulfills the quality criterion of a minimum spot weld diameter of 4*√t, leads to partial reaustenitization and consequently to a ferritic/martensitic microstructure after final quenching. Hardness mappings revealed that this inner FZ is harder than the surrounding FZ consisting of tempered martensite. In contrast, if the highest current without splashing is chosen for the first pulse, the same second pulse does not reaustenitize the FZ but only temper the martensite.

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“…The microstructure was investigated in the light optical microscope (LOM) and the scanning electron microscope (SEM) using a Versa 3D from Thermo Fisher Scientific equipped with an EDAX Hikari XP electron backscatter diffraction (EBSD) system. In addition to Nital etching to reveal the different phases, the samples were etched according to the procedure described in the previous study [28] to evaluate the average grain size and morphology. A deformation-free surface for EBSD measurements was obtained by flat ion milling for 10 min at 4 kV on a Hitachi IM5000.…”

Section: Methodsmentioning

confidence: 99%

Bending Behavior of Zinc‐Coated Hot Stamping Steels

Hofer

Stadler

Kurz

et al. 2021

steel research int.

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The contradictory requirements of increased passenger safety and a simultaneous mass reduction of the body-in-white drive the development of advanced high strength steels in the automotive industry. Especially, components for the safety cell, e.g., reinforcement of B-pillar, bumper, and roof rails, need to be rigid and impede intrusion in the case of a crash event to protect the occupants. [1] For these applications, ultrahigh strength steels with a tensile strength (TS) of up to 2000 MPa and a martensitic microstructure are used. [2] However, their exceptional high strength is accompanied by limited formability, which can be overcome by hot stamping. The combination of shaping by hot forming and the simultaneous microstructure adjustment by quenching in water-cooled dies enables the production of components with complex geometries. Besides the reduced press forces, this manufacturing process also offers the ability to produce tailored blanks and eliminate spring back, which increases with the sheet strength in cold-forming operations. [3,4] While hot stamping was initially used for built-in components, the expansion of their application to more exposed structures leads to the need for corrosion protection. [5] In contrast to AlSi coatings that only provide barrier protection, zinc coatings additionally offer cathodic corrosion protection and, therefore, maintain their protection effect even when the coating layer is breached, e.g., by stone chipping. The major drawback of zinc coatings is their susceptibility to liquid metal embrittlement (LME) during direct hot press forming due to the presence of liquid zinc during deformation. [6] Thus, zinc-coated press hardening steels are mainly produced via the "indirect" process, where the sheet is cold stamped and subsequently subjected to a quenching and calibration operation in the press after full austenitization. Another possibility to avoid LME is by elimination of liquid phases during hot forming. One idea is to increase austenitization time to fully transform the coating into a single-phase solid solution of α-Fe(Zn) ferrite. [7] The disadvantage of this approach, besides the longer process time, is the decreased corrosion protection due to the lower potential difference between matrix and α-Fe(Zn) ferrite compared with zinc-rich Γ-ZnFe. [8] A further possibility is to increase the transfer time [9] or add an additional precooling step [8,10] to solidify all zinc phases, i.e., Γ-ZnFe, prior to the stamping operation. Typical precooling temperatures range between 450 and 650 C. [11] To guarantee a stable process, the composition of the standard hot stamping alloy 22MnB5 has been slightly adapted to 20MnB8. An increased amount of

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Microstructural Characterization of Ultra-High Strength Martensitic Steels

Cited by 10 publications

References 22 publications

Microstructural Analysis of the Recrystallization Behavior of Low Alloyed Steels

Microstructural Analysis of the Recrystallization Behavior of Low Alloyed Steels

Microstructural characterization of a double pulse resistance spot welded 1200 MPa TBF steel

Bending Behavior of Zinc‐Coated Hot Stamping Steels

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