Acicular ferrite nucleates intragranularly on nonmetallic inclusions, forming a microstructure with excellent fracture toughness. The formation of acicular ferrite is strongly affected by the size, content, and composition of nonmetallic inclusions, but also by the composition of the steel matrix. The potential of inclusions in medium carbon HSLA (high-strength low-alloyed) steels has been the main focus in the literature so far. The current study evaluates the acicular ferrite capability of various inclusions types in four different steel grades with carbon contents varying between 0.04 and 0.65 wt pct. The investigated steels are produced by melting experiments on a laboratory scale and subsequent heat treatment in a High-Temperature Laser Scanning Confocal Microscope. Inclusions are exclusively formed by deoxidation and desulfurization reactions. No synthetic particles are added to the melt. The inclusion landscape is analyzed by Scanning Electron Microscopy. Final ductility of the samples is evaluated based on performed tensile tests. Inclusion types in every steel grade are assessed regarding their nucleation potential always considering the interaction with the steel composition, especially focusing on the role of manganese. The effects of (Ti,Al)O x -, MnS-, and MgO-containing inclusions are discussed in detail.

No abstract

Up to now, the Fe content of nonmetallic particles has often been neglected in chemical evaluations due to the challenging analysis of matrix elements in nonmetallic inclusions (NMI) in steel by scanning electron microscope and energy dispersive spectroscopy analysis (SEM/EDS). Neglecting matrix elements as possible bonding partners of forming particles may lead to inaccurate results. In the present study, a referencing method for the iron content in nonmetallic inclusions in the submicrometer region is described focusing on the system Fe-Mn-O. Thermodynamic and kinetic calculations are applied to predict the inclusion population for different Fe/Mn ratios. Reference samples containing (Fe,Mn)-oxide inclusions with varying Fe ratios are produced by manganese deoxidation in a high-frequency induction furnace. Subsequent SEM/EDS measurements are performed on metallographic specimens and electrolytically extracted nonmetallic inclusions down to 0.3 µm. The limits of iron detection in these particles, especially for those in the submicrometric regime, as well as the possible influence of electrolytic extraction on Fe-containing oxide particles are examined. The measured inclusion compositions correlate well with the calculated results regarding segregation and kinetics. The examinations performed are reliable proof for the application of SEM/EDS measurements to evaluate the Fe content in nonmetallic inclusions, within the physical limits of polished cross-section samples. Only electrolytic extraction ensures the determination of accurate compositions of dissolved or bonded matrix elements at smallest particles enabling quantitative particle descriptions for submicrometric (particles ≤ 1 µm) steel cleanness evaluations.

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

No abstract