Advanced high strength steels Research Papers (original) (raw)

- by
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- Multidisciplinary, Advanced high strength steels, Sheet Metal, CO 2 laser

Ferritic–martensitic dual phase (DP) steels deform spatially in a highly heterogeneous manner, i.e. with strong strain and stress partitioning at the micro-scale. Such heterogeneity in local strain evolution leads in turn to a spatially heterogeneous damage distribution, and thus, plays an important role in the process of damage inheritance and fracture. To understand and improve DP steels, it is important to identify connections between the observed strain and damage heterogeneity and the underlying microstructural parameters, e.g. ferrite grain size, martensite distribution, martensite fraction, etc. In this work we pursue this aim by conducting in-situ deformation experiments on two different DP steel grades, employing two different microscopic-digital image correlation (lDIC) techniques to achieve microstructural strain maps of representative statistics and high-resolution. The resulting local strain maps are analyzed in connection to the observed damage incidents (identified by image post-processing) and to local stress maps (obtained from crystal plasticity (CP) simulations of the same microstructural area). The results reveal that plasticity is typically initiated within ‘‘hot zones’’ with larger ferritic grains and lower local martensite fraction. With increasing global deformation, damage incidents are most often observed in the boundary of such highly plastified zones. High-resolution lDIC and the corresponding CP simulations reveal the importance of martensite dispersion: zones with bulky martensite are more susceptible to macroscopic localization before the full strain hardening capacity of the material is consumed. Overall, the presented joint analysis establishes an integrated computational materials engineering (ICME) approach for designing advanced DP steels.

Joining steel plates and aluminum plates by means of using Fe/Al structural (Triclad) transition joints has been well-received in the shipbuilding industry, but is rarely applied in other industries. In this work, 12.7 mm thick plates of advanced high-strength steel and AA 6061 alloy were successfully joined by using hybrid laser-arc welding with the help of Triclad transition joints. The parameters were optimized for welding dissimilar steels (A516 to advanced high-strength steel) and welding dissimilar aluminum alloys (AA 5456 to AA 6061). The optimization controlled the total heat input and kept the Al/Fe interface
of the Triclad transition joint below the maximum allowable temperature of 315.56 C in order to minimize the growth of brittle intermetallic phases and retain the mechanical properties of the Triclad transition joint. A finite element model was developed to study the temperature evolution at the Triclad interface and the heat distribution along with the welded structure. A ‘‘4:1’’ ratio was used between the width
of the Triclad transition joint and the thickness of the webs. This ratio improved the load-bearing property of the Triclad transition joint and compensated for the microcracks and brittle Al-rich intermetallic
phases observed at the Triclad interface. Tensile strength of 220 MPa was achieved at the welded structure. The fracture occurred at the heat-affected zone of the AA 5456-to-AA 6061 weld.

El temple y partición, denominado internacionalmente como Q&P (Quenching & Partitioning) es un concepto de tratamiento térmico relativamente nuevo, para generar microestructuras que contienen austenita retenida, estabilizada por la... more

In recent years, the use of advanced high strength steels (AHSS) in the automotive industry has increased due to their potential in reducing weight, leading to lower fuel consumption and carbon dioxide emissions. The AHSS structures would be the optimum choice for many applications; however, there are many defects to overcome in their stamping. In this present study, different types of defects and remedies of AHSS stampings are presented.

- by Zeal Press
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- Failure Analysis, Advanced high strength steels, AHSS

Components press formed from layered or laminated metallic materials have wide engineering application in aerospace, automobile, electrical, electronic and process industries. High strength ferrous metals are widely used in automobile industries for utilization of its high strength to weight ratio. Such high strength metals offer poor formability. Formability of such metals can be improved by applying a layer of metal having higher formability. This paper deals with springback behavior of a laminated stainless steel and aluminum in
V bending. Investigation of bending characteristics like thickness ratio and bending angles of sheet before/after springback is carried out in this paper. First part of this paper is on numerical analysis for springback prediction of laminated sheet. In second part, experiments are performed for different cases on V bending machine. The experimental results show that springback of sheet metal laminate is greatly affected
by relative position of strong/weak layers and thickness ratio of each layer.

- by sundaramoorthy B
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- Advanced high strength steels
- by Hanlong Zhang
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- Advanced high strength steels

The phase transformation kinetics under continuous cooling conditions for intercritical austenite in a cold rolled low carbon steel were investigated over a wide range of cooling rates (0.1–200 °C/s). The start and finish temperatures of the intercritical austenite transformation were determined by quenching dilatometry and a continuous cooling transformation (CCT) diagram was constructed. The resulting experimental CCT diagram was compared with that calculated via JMatPro software, and verified using electron microscopy and hardness tests. In general, the results reveal that the experimental CCT diagram can be helpful in the design of thermal cycles for the production of different grades of dual-phase–advanced high-strengh steels (DP-AHSS) in continuous processing lines. The results suggest that C enrichment of intercritical austenite as a result of heating in the two phases (ferrite–austenite) region and C partitioning during the formation of pro-eutectoid ferrite on cooling significantly alters the character of subsequent austenite phase transformations.

We present a novel method to locally control the constitution, morphology, dispersion and transformation behavior of multiphase materials. The approach is based on the targeted, site-specific formation and confined dissolution of precipitated carbides or intermetallic phases. These dispersoids act as “vessels” or “containers” for specific alloying elements forming controlled chemical gradients within the microstructure upon precipitation and subsequent (partial) dissolution at elevated temperatures. The basic processing sequence consists of three subsequent steps, namely: (i) matrix homogenization (conditioning step); (ii) nucleation and growth of the vessel phases (accumulation step); and (iii) (partial) vessel dissolution (dissolution step). The vessel phase method offers multiple pathways to create dispersed microstructures by the variation of plain thermomechanical parameters such as time, temperature and deformation. This local microstructure design enables us to optimize the mechanical property profiles of advanced structural materials such as high strength steels at comparatively lean alloy compositions. The approach is demonstrated on a 11.6Cr–0.32C (wt.%) steel, where by using M23C6 carbides as a vessel phase, Cr and C can be locally enriched so that the thus-lowered martensite start temperature allows the formation of a significant quantity of retained austenite (up to 14 vol.%) of fine dispersion and controlled morphology. The effects of processing parameters on the obtained microstructures are investigated, with a focus on the dissolution kinetics of the vessel carbides. The approach is referred to as vessel microstructure design.

High strength multiphase steels have been
developed consisting of combination of pearlite, tempered
martensite and small amount of ferrite, by suitable heat
treatment of a high carbon low alloy rail steel (0.7 % C).
The desired microstructure has been obtained by holding
fully homogenized steel in pearlitic range for small dura-
tions followed by water quenching and subsequent tem-
pering at 773 K for 18 h. Variation in mechanical
properties has been studied with the change in volume
fraction of different phases. Yield strength, ultimate tensile
strength and elongation are observed to be in the range of
500–1,000 MPa, 900–1,185 MPa and up to 16.8 %,
respectively. Continuous and discontinuous yielding along
with substantial work hardening has been explained as a
function of tempered martensite content.

- by Abhinav Varshney
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- Advanced high strength steels

This paper gives an overview of recent progress in microstructure-specific hydrogen mapping techniques. The challenging nature of mapping hydrogen with high spatial resolution, i.e. at the scale of finest microstructural features, led to the development of various methodologies: thermal desorption spectrometry, silver decoration, the hydrogen microprint technique, secondary ion mass spectroscopy, atom probe tomography, neutron radiography, and the scanning Kelvin probe. These techniques have different characteristics regarding spatial and temporal resolution associated with microstructure-sensitive hydrogen detection. Employing these techniques in a site-specific manner together with other microstructure probing methods enables multi-scale, quantitative, three-dimensional, high spatial, and kinetic resolution hydrogen mapping, depending on the specific multi-probe approaches used. Here, we present a brief overview of the specific characteristics of each method and the progress resulting from their combined application to the field of hydrogen embrittlement.

- by Dierk Raabe
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- Microstructure, Hydrogen, Secondary Ion Mass Spectrometry, Hydrogen Storage

The roll forming process parameters play a major role in the quality of the final roll-formed product. Optimum configuration without any cost increase in the roll forming line could present accurate and flawless products. In this paper, a roll forming process experimental modelling of a symmetrical U-section profile from advanced high strength steel (AHSS) material (type DP600) is presented. The factors selected for this study are the roll forming line velocity, the inter-distance between roll stations, the roll gap, and the diameter of the rolls. An optimization procedure for the roll forming line, via statistical design of the experimental simulation runs, is also presented. The optimum values of process parameters are calculated for minimum elastic longitudinal strains and shear strains, at strip edge, for each roll station. A reduction of 20–35% in elastic longitudinal strains could occur for all roll stations, and 30–50% reduction in shear strains occurs for roll stations with a greater folding angle, as this leads to roll-formed products of a better quality. Finally, the contribution of each factor on the longitudinal and shear strains has been calculated, showing that the inter-distance between the roll stations plays a dominant role in the roll forming process.

- by John Paralikas
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- Engineering, Advanced high strength steels, Mathematical Sciences, FEA

We investigate the kinetics of the deformation structure evolution and its contribution to the strain hardening of a Fe–30.5Mn–2.1Al–1.2C (wt.%) steel during tensile deformation by means of transmission electron microscopy and electron channeling contrast imaging combined with electron backscatter diffraction. The alloy exhibits a superior combination of strength and ductility (ultimate tensile strength of 1.6 GPa and elongation to failure of 55%) due to the multiple-stage strain hardening. We explain this behavior in terms of dislocation substructure refinement and subsequent activation of deformation twinning. The early hardening stage is fully determined by the size of the dislocation substructure, namely, Taylor lattices, cell blocks and dislocation cells. The high carbon content in solid solution has a pronounced effect on the evolving dislocation substructure. We attribute this effect to the reduction of the dislocation cross-slip frequency by solute carbon. With increasing applied stress, the cross-slip frequency increases. This results in a gradual transition from planar (Taylor lattices) to wavy (cells, cell blocks) dislocation configurations. The size of such dislocation substructures scales
inversely with the applied resolved stress. We do not observe the so-called microband-induced plasticity effect. In the present case, due to texture effects, microbanding is not favored during tensile deformation and, hence, has no effect on strain hardening.

- by Dierk Raabe
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- Microstructure, Advanced high strength steels, Steel, Crystallographic Texture

Machining operations are very common for the production of auto parts, i.e., connecting rods, crankshafts, etc. In machining, the use of cutting oil is very necessary, but it leads to higher machining costs and environmental problems. About 17% of the cost of any product is associated with cutting fluid, and about 80% of skin diseases are due to mist and fumes generated by cutting oils. Environmental legislation and operators’ safety demand the minimal use of cutting fluid and proper disposal of used cutting oil. The disposal cost is huge, about two times higher than the machining cost. To improve occupational health and safety and the reduction of product costs, companies are moving towards sustainable manufacturing. Therefore, this review article emphasizes the sustainable machining aspects of steel by employing techniques that require the minimal use of cutting oils, i.e., minimum quantity lubrication, and other efficient techniques like cryogenic cooling, dry cutting, solid lubricants, air/vapor/gas cooling, and cryogenic treatment. Cryogenic treatment on tools and the use of vegetable oils or biodegradable oils instead of mineral oils are used as primary techniques to enhance the overall part quality, which leads to longer tool life with no negative impacts on the environment. To further help the manufacturing community in progressing towards industry 4.0 and obtaining net-zero emissions, in this paper, we present a comprehensive review of the recent, state of the art sustainable techniques used for machining steel materials/components by which the industry can massively improve their product quality and production.

Invited lecture, German-Chinese High-level Workshop on “Microstructure-driven Design and Performance of Advanced Metals” held 12-16. April 2013 in the Institute of Metals Research (IMR) of the Chinese Academy of Science (CAS), Shenyang,... more

We investigated the thermodynamics and kinetics of carbide precipitation in a cold-rolled Fe-7Mn-0.1C-0.5Si medium manganese steel during low temperature tempering. The material was annealed up to 24 h at 450 C in order to follow the kinetics of precipitation. Using thermodynamics and kinetics simulations , we predicted the growth of M 23 C 6 carbides according to the local-equilibrium negligible partition (LENP) mode where carbide growth is controlled by the diffusion of carbon, while maintaining local chemical equilibrium at the interface. Atom-probe tomography (APT) measurements performed on samples annealed for 1, 6 and 24 h at 450 C confirmed that LENP is indeed the mode of carbide growth and that Mn segregation is necessary for the nucleation. Additionally, we observed the heterogeneous nucleation of transition carbides with a carbon content between 6 and 8 at% at segregated dislocations and grain boundaries. We describe these carbides as a complex face-centered cubic transition carbide type (CFCC-TmC phase) obtained by the supersaturation of the FCC structure by carbon that will act as precursor to the more stable g-M 23 C 6 carbide that forms at the dislocations and grain boundaries. The results suggest that the addition of carbon does not directly favor the formation of austenite, since Mn is consumed by the formation of the carbides and the nucleation of austenite is thus retarded to later stages of tempering as every FCC nucleus in the initial stages of tempering is readily converted into a carbide nucleus. We propose the following sequence of transformation: (i) carbon and Mn co-segregation to dislocations and grain boundaries; (ii) formation of FCC transition carbides; (iii) growth controlled according to the LENP mode and (iv) austenite nucleation and growth.

- by Dierk Raabe and +1
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- Thermodynamics, Crystallography, Phase Transformations, Microstructure

Dual-phase (DP) steel sheets have high potential for utilization as automotive structures due to their good combination of strength and ductility. As sheet metal forming processes induce complicated stress-strain states, determination of forming limit is vital, particularly using numerical approaches. This current study aims to examine the fracture behavior of DP600 steel sheets through several ductile fracture criteria in a wide range of stress states. For a better and more accurate understanding of the experimental tests, parallel numerical simulations were performed. First, the models were calibrated using the results of Nakazima tests, and then the fracture loci in principal strains, and equivalent strain-stress triaxiality spaces were predicted by each model. The capability of the criteria was verified through cross-die and bulge tests. Also, errors were quantified for the calculated results using correlation coefficient and relative error methods. The results reveal that Maximum Shear Stress, Modified Mohr Coulomb, and Lou fracture models were able to predict the onset of fracture with acceptable accuracy. However, Maximum Shear Stress required only one experimental test to be calibrated.

We present a multiscale dislocation density-based constitutive model for the strain-hardening behavior in twinning-induced plasticity (TWIP) steels. The approach is a physics-based strain rate- and temperature-sensitive model which reflects microstructural investigations of twins and dislocation structures in TWIP steels. One distinct advantage of the approach is that the model parameters, some of which are derived by ab initio predictions, are physics-based and known within an order of magnitude. This allows more complex microstructural information to be included in the model without losing the ability to identify reasonable initial values and bounds for all parameters.
Dislocation cells, grain size and twin volume fraction evolution are included. Particular attention is placed on the mechanism by
which new deformation twins are nucleated, and a new formulation for the critical twinning stress is presented. Various temperatures were included in the parameter optimization process. Dissipative heating is also considered. The use of physically justified parameters enables the identification of a universal parameter set for the example of an Fe–22Mn–0.6C TWIP steel.

Share of high and advanced high strength steels in automobile is increasing, however, such steels generally have poor formability and high amount of springback. One of the focus areas of research in high strength automotive steel is to increase the normal anisotropy to get better formability. Effect of strength and process parameters on springback has been studied by many researchers but that of anisotropy has not been studied by many. In the present work the effect of anisotropy on springback is predicted using finite element analysis for the benchmark problem of Numisheet-2005 [2005 Numisheet Benchmark 2, Springback prediction of a cross member, Proceedings of the 6th International Conference and Workshop on Numerical Simulation of 3D Sheet Metal Forming Processes, Detroit, USA, August 15–19, 2005]. An analytical model is developed to cross check the trends predicted from the finite element analysis. The effective stress has not been treated as a constant and the radial stress is considered in the present model. Both the models (FE and analytical) predict that higher anisotropy, in general, gives higher springback. Finite element analysis of the problem shows that springback is minimum for an isotropic material.

- by L. Durrenberger and +1
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- Materials Engineering, Mechanical Engineering, Strain Rate, Numerical Simulation

Numerical simulations of straight tube hydroforming of a dual phase (DP600) advanced high strength steel were performed using a variant of the Gurson–Tvergaard–Needleman (GTN) constitutive model to account for the influence of void shape and shear on coalescence. The effect of axial-feed (end-feed) on damage development and formability is investigated for end-feed loads of zero and 133 kN. A parametric study was conducted to determine an appropriate void nucleation stress and strain and the numerical values compared with the experimental data. The calibrated GTN damage model gives good agreement with the experimentally determined burst pressure, formability and failure location with the best performance occurring for the high end-feed load.

- by Cliff Butcher
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- Materials Engineering, Mechanical Engineering, Civil Engineering, Modeling

S. Smith, N.J. den Uijl, T. van der Veldt, T. Okada, M. Uchihara, F. Fukui. The effect of ageing on the spot weld strength of AHSS and the consequences for testing procedures, IIW-1976-08, Welding in the World, p.p. R12 – R26, Vol. 54,... more

Uijl, NJ den, Smith, S, Nishibata, H, Okada, T, Uchihara, M & Fukui, K (2008). Prediction of post weld hardness of advanced high strength steels for automotive application using a dedicated carbon equivalent number. IIW-1873-07. Welding... more

- by Nick J den Uijl
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- Welding Technology, Welding Engineering, Automotive Industry, Welding and Joining

Uijl, NJ den (2006). Modelling the influence of resistance spot welding on material properties. In W. Lucas & V.I. Makhnenko (Eds.), Proc. Joint Int. Conf. "16th Int. Conf. Computer Technology in Welding and Manufacturing" and "3rd Int. Conf. Mathematical Modelling and Information Technologies in Welding and Related Processes" (pp. 306-313). Kiev, Ukraine: E.O. Paton Electric Welding Institute of the NAS.

The effect of martensite spatial distribution and its interface morphology on the bake hardening characteristics of a dual phase steel was investigated. In one case, typical industrial continuous annealing line parameters were employed to anneal a 67% cold rolled steel to obtain a dual phase microstructure. In the other case, a modified annealing process with changed initial heating rates and peak annealing temperature was employed. The processed specimens were further tensile pre-strained within 1–5% strain range followed by a bake hardening treatment at 170 °C for 20 min. It was observed that industrial continuous annealing line processed specimen showed a peak of about 70 MPa in bake-hardening index at 2% pre-strain level. At higher pre-strain values a gradual drop in bake-hardening index was observed. On the contrary, modified annealing process showed near uniform bake-hardening response at all pre-strain levels and a decrease could be noted only above 4% pre-strain. The evolving microstructure at each stage of annealing process and after bake-hardening treatment was studied using field emission scanning electron microscope. The microstructure analysis distinctly revealed differences in martensite spatial distribution and interface morphologies between each annealing processes employed. The modified process showed predominant formation of martensite within the ferrite grains with serrated lath martensite interfaces. This nature of the martensite was considered responsible for the observed improvement in the bake-hardening response. Furthermore, along with improved bake-hardening response negligible loss in tensile ductility was also noted. This behaviour was correlated with delayed micro-crack initiation at martensite interface due to serrated nature.

N. den Uijl, T. Okada, T. Moolevliet, A. Mennes, E. van der Aa, M. Uchihara, S. Smith, H. Nishibata, K. Fukui: IIW-2162-11 (III-1573-10) Performance of resistance spot welded joints in advanced high strength steel in static and dynamic... more

The effect of carburizing time and wear behavior of the carburized, hardened Cr and Ni-Cr steel were studied. For this purpose, Cr and Ni-Cr steel containing 0.15%C was taken. The specimen of 8mm diameter and 6.5mm thick from each steels were carburized at 945oc at three different holding times (1 hr, 2hr, 3hr respectively) and quenched at salt water. Surface hardness was measured by using Rockwell hardness testing machine. Wear test were carried out by using pin on disc type apparatus under dead loads of 1.5kg,2kg and 3kg at a linear speed of 2.64 ms-1 in ambient air. The sliding distance was 6336m.The structure then observed the worn surface by optical microscopy. It has been found that Ni increases the amount of retained austenite content in the case of the carburized low carbon Cr steel. Ni reduces the surface hardness, case hardness and core hardness of the low carbon Cr steel. The presence of retain austenite along with the martensite that the surface hardness found lower than the maximum hardness of the both the steels. Wear rate increases with the increases of the dead load. Wear rate of Ni-Cr steel found higher than that of Cr steel due to the presence of Retain austenite in Ni-Cr steel.

- by Muammer Koç
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- Materials Engineering, Mechanical Engineering, Powder Metallurgy, Lubrication
- by Ahmed Nasser and +1
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- Materials Engineering, Mechanical Engineering, Finite Element, Advanced high strength steels

""The plastic deformation behavior of a modified transformation-twinning induced plasticity steel is investigated
in a wide range of temperature from 25 to 1000°C through compression testing. The main emphasis is on different
plastic deformation mechanisms involved. The results show that the flow stress is rigorously dependent on
deformation temperature and this behavior is classified in three characteristic regions. The strain-induced martensite
is detected as the chief plastic deformation mechanism from 25°C to a comparably high temperature of 200°C. The
activation of deformation twinning as the second deformation mechanism starts with an ascending trend from 200 to
300°C. This is followed by a descending trend in the range of 300°C to 500°C. In addition, the remarkable presence
of deformation twins at temperatures above 200°C has led to the formation of a new fine grain structure. The
restoration processes as the third deformation mechanism is detected in the range of 700 to 1000°C. The dynamic
recrystallization is the most important softening mechanism for the experimental steel during hot compression from
900 to 1000°C.""

- by Alireza Asgari
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- Automotive Industry, Finite Element, Advanced high strength steels, Design process

A novel type of duplex microstructure is generated in a single-phase austenitic steel (AISI 316L; X2CrNiMo19-12), consisting of plastically compliant recrystallized austenitic grains as the matrix containing coarse non-recrystallized grains with a nanotwinned austenitic (nt-c) structure as strengthening inclusions. This novel type of single-phase yet duplex microstructured steel exhibits an excellent combination of strength and ductility. We study the plastic co-deformation mechanisms between the nanotwinned and the recrystallized grains under tension using electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). At tensile strains below 5%, the nt-c grains nearly deform homogeneously in conjunction with the surrounding statically recrystallized (SRX) grains without generating notable strain localization near their interfaces. The anisotropic plastic deformation of the nt-c grains with predominant shear parallel to the twin boundaries results in a higher dislocation density in the neighboring SRX grains. As the strain exceeds 12%, localized deformation occurs within the nt-c grains in the form of shear banding. A strain gradient is developed in the surrounding SRX grains as a function of distance from the nt-c/SRX interface. Deformation twinning is observed in the SRX grains near the nt-c grains, while away from nt-c grains dislocation slip dominates the deformation. The strengthening effect of the strong and ductile nt-c grains may offer a novel approach to strengthen austenitic steels and related alloys by generating a nanotwinned / recrystallized duplex microstructure.

The cold-roll-forming process is one of the main processes for mass production of the profiles with constant cross-section in many industrial sectors. The introduction of advanced high-strength steels (AHSS), such as DP-series and TRIP-series, into the production of roll-formed profiles has brought new challenges. The current paper exploits the finite elements (FE) method and investigates the effects of the main roll-forming process parameters, namely the roll-forming line velocity, rolls inter-distance, rolls gap, and rolls diameter, on quality characteristics. These characteristics are the distribution of longitudinal and transversal strains on final profile, total, and elastic longitudinal strains and longitudinal residual strains at strips edge along the forming direction and dimensional accuracy of the profile after the final roll station.

- by John Paralikas
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- Engineering, Finite Element, Advanced high strength steels, Mathematical Sciences

Fuel efficiency of vehicles and increase the safety of passengers lead the automotive industry to incorporate of new materials and technologies into today's vehicles. Advanced high strength steels offer excellent strength and formability properties. One the main important grade of advanced high strength steels is Dual Phase (DP) steel. Dynamic behavior of the material must be considered in crashworthiness studying of the vehicles. In this paper the dynamic tensile characteristics of Dual Phase (DP) steel sheets at low and moderate strain rates ranging from 0.0009 s-1 to 0.1 s-1 are investigated. All the tests were performed by the tensile test machine with the cross-head speed of 4 mm/min and 500 mm/min. Digital Image Correlation method was employed to study the strain rate sensitivity, stress-strain curves and the strain fracture of DP800. In the numerical analysis, an isotropic hardening model with rate dependence was used to predict the experimental behavior. The fracture behavior of the material was simulated by using of ductile criterion model. The local strains in fracture zone obtained by the experiments and finite element analysis are compared and good agreement is obtained.

Large strain warm deformation at different temperatures and subsequent intercritical annealing has been applied to obtain fine grained (2.4 m) and ultrafine grained (1.2 m) ferrite/martensite dual-phase (DP) steels. Their mechanical properties were tested under tensile and impact conditions and compared to a hot deformed coarse grained (12.4 m) reference material. Both yield strength and tensile strength follow a Hall-Petch type linear relationship, whereas uniform elongation and total elongation are hardly affected by grain refinement. The initial strain hardening rate as well as the post-uniform elongation increase with decreasing grain size. Ductile fracture mechanisms are considerably promoted due to grain refinement. Grain refinement further lowers the ductile-to-brittle transition temperature and leads to higher absorbed impact energies. Besides the common correlations with the ferrite grain size, these phenomena are explained in terms of the martensite particle size, shape and distribution and the more homogeneous dislocation distribution in ultrafine ferrite grains.

- by Dierk Raabe and +1
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- Microstructure, Electron Backscatter Diffraction (EBSD), Advanced high strength steels, Mechanical Properties of Materials