Oxidation phenomena in advanced high strength steels : Modelling and experiment

Triggered by the recent popularity of advanced high strength steels (AHSS) and aluminium alloys for weight-reduction in automotive components, industrial interest in deformation-induced ductile damage in sheet metal is increasing in the last decades. Severe deformation during forming or service triggers different damage micro-mechanisms in the multi-phase microstructures of these materials, leading often to unpredicted failures. These failures can be avoided by (a) the optimization of metal microstructures to be less susceptible to damage-induced failures, which requires experimental characterization of damage micro-mechanisms or (b) the incorporation of continuum damagemodels in forming simulations to design forming operations within safe deformation limits, which requires experimental quantification of damage accumulation. However, both strategies are hampered by the limitations of the currently available experimental diagnostics. Therefore, the aim of this work is to develop new experimental methodologies that allow for (i) characterization of damagemicro-mechanisms and (ii) accurate quantification of damage accumulation, with a focus on industry-relevant sheet metal. As a starting point, the influence of damage evolution on localization and fracture is investigated by deforming two steels of different microstructure in different strain paths. The results revealed that for microstructures with many damage mechanisms (e.g. AHSS), damage accumulation significantly affects both necking and fracture limits, verifying the strong need for thorough characterization of damage micromechanisms in different strain-paths. The analysis of these mechanisms requires the development of a miniaturized testing setup that could fit within a scanning electron microscope (SEM) to track deformation-induced microstructure evolution in real time. To this end, a miniaturized Marciniak test setup is designed, built and tested, which allows the real-time, multi-axial testing of industrial sheet metal to the point of fracture within a SEM. A major benefit of the in-situ analysis with miniaturized equipment is the possibility of obtaining evolution of local strain distribution at the microstructure level, as demonstrated in a case study that clarifies the mechanical influence of the morphology and properties of microstructural banding in steels. The effect of band continuity and hardness are elucidated, yielding a clear detrimental influence especially for hard bands with a continuous morphology. Finally, an improved experimental methodology is developed to analyse 3D features of ductile deformation, with minimum specimen preparation artifacts. For the damage quantification problem a wide variety of experimental methodologies have been proposed in the literature, without a thorough evaluation with respect to measurement accuracy, precision, practicality, etc. To determine the most suitable damage quantification strategy for continuum damage models, damage morphology-based damage quantification methodologies (the volume fraction methodology, area fraction methodology, or density measurement methodology) and material property-based damage quantification methodologies (the indentationbased methodology, modified indentation methodology and micropillar compression methodology) are comparatively analyzed. The obtained results clearly indicated that methodologies that quantify ductile damage through its morphology have limited accuracy and probe a narrow damage spectrum, revealing the need for accurate material-property based damage quantification techniques. The indentation based methodology is a widely used example of such methodologies, however, a numerical-experimental analysis revealed that it cannot be used for this purpose, as the damage-induced degradation of both hardness and modulus is masked by other deformation-induced microstructural mechanisms (e.g. grain shape change, texture development, etc.). To this end, two original mechanical property-based damage quantification methodologies are proposed in this work. A new indentation-based methodology is developed and evaluated, that eliminates the influence of the microstructural heterogeneity to properly capture the damageinduced degradation of indentation hardness and modulus. And finally, an elastic compression-based methodology is developed where the elastic damage parameter is obtained through the deformation-induced degradation of the compression modulus of electro-discharge machined micropillars. The results from these two methodologies clearly indicate that methodologies that quantify ductile damage through its influence on mechanical property (e.g. hardness, modulus) have significantly higher accuracy, and therefore more suitable for identifying damage parameters for continuum damage models.

Certain structural applications require materials to possess both high strength and high toughness, two properties which can often be inversely related with one increasing at the expense of the other. Currently, only two classes of steels exist that fulfill both of these application-critical criteria – maraging steels and some cobaltcontaining secondary hardening steels. Unfortunately, these alloys are expensive due to their high cobalt and nickel contents. It is thought that in these alloys nickel offsets the detrimental effects cobalt can have on the ductile-to-brittle transformation and that by removing cobalt, an inexpensive steel alternative with comparable mechanical properties to the currently available alloys could be developed. This research examines the effect of nickel additions to a base secondary hardening steel on mechanical properties and microstructure. The work was divided into separate studies in which five experimental alloys with differing nickel amounts were examined in order to determine the effect of nickel on the strength, ductile-to-brittle transition temperature (DBTT), and room temperature fracture toughness. In the first study, the effect of nickel on strength was investigated. Increasing nickel content generally increased strength, although the effect of nickel on yield strength and ultimate tensile strength was highly dependent on the quench rate from the austenitizing temperature. Results indicate that fine precipitates of VC and Mo2C contribute to the strength of these alloys. The second study looked at the effect of nickel on the DBTT. Test specimens were heat treated uniformly and then Charpy impact energies were determined for five test temperatures. The DBTT decreased approximately 200°C when nickel content was increased from 0 to 5 and 6 wt.%. Heat treatment did not have a significant effect on the DBTT. Results of testing for the nickel-free alloys suggest that rare-earth inclusions are slightly more effective in promoting quasi-cleavage than calcium or aluminum oxide inclusions. The DBTT was primarily controlled by nickel content, as the effects of prior austenite grain size and retained austenite content were negligible. In the third and last study, the effect of nickel on room temperature toughness was analyzed. Fracture toughness increased with increasing nickel content and tempering temperature. The experimental alloys with zero nickel additions had unexpectedly low toughness compared to previous work in heats with nearly identical composition. This was determined to be the result of the large inclusions in these steels which promote quasi-cleavage fracture in the absence of nickel. In addition, the difference in toughness between the two experimental base heats could be due to the low coefficient of thermal expansion of rare-earth inclusions, which could increase tensile circumferential stresses that promote quasi-cleavage. Examination of KIC fracture surfaces revealed no correlation between average area fraction of secondary voids and fracture toughness.

Surface defects formed on Si-containing steels in hot-rolling process depend largely on the character of the scale composed of FeO and Fe2SiO4 (FeO+Fe2SiO4 oxide scale) during de-scaling. The formation of FeO+Fe2SiO4 oxide scale is influence by internal oxide precipitates. In order to elucidate quantitatively the internal oxidation, it is required to analyze the distributions of number, radius and volume of internal oxide precipitates in internal oxidation zone (IOZ).Internal oxidation of Fe–1 mass%Si alloy was examined at 1473K in Ar–17.6%H2–12.2%H2O where an outer scale of FeO does not form. The growth rate of IOZ was obeyed the parabolic rate law, indicating that the rate-determining step was the diffusion of oxygen through alloy. SiO2 precipitated from the internal oxidation front to 0.23±0.02 of normalized thickness of IOZ, and Fe2SiO4 precipitated from the position to the gas/alloy interface, depending on the chemical potential distribution of oxygen. Fe2SiO4 is formed by the reaction between SiO2, Fe and dissolved oxygen in IOZ. The number of precipitates in unit volume is reciprocally proportional to depth in IOZ, suggesting that the grain growth of oxide obviously affected the distribution. The radius of the precipitates was proportional to the cubic root of depth so that the volume of the precipitate was proportional to depth. These results elucidated that the volume fraction of precipitates in IOZ was constant.

Advanced high strength steels have been widely used in the automobile structures to enhance passengers’ safety [1]. Hot stamping process is recently applied and the strength of press hardening steels is elevated via martensitic phase transformation [2; 3]. Nevertheless, the oxidation and decarburization generally occur during the heat treatment prior to stamping[4], which in turn deteriorate the properties of steels [5]. Hot-dip galvanizing has been applied in steel industries due to its effectiveness to produce a Zn coating with both barrier and cathodic protections again corrosion [6]. Recently, hot-dip galvanized coatings have been studied to evaluate the protection on steels during hot stamping and the corrosion resistance of the resultant coating after hot stamping. The critical issues of the galvanized steels for hot stamping are oxidation and liquid metal embrittlement [7; 8]. The former deteriorates the resistance spot weldability and paintability, whereas the later has a negative effect on formability [9]. Silane, a kind of Si-based coating on light metals, has been used in galvanized steels to offer barrier protection against corrosion [10]. In this study, silane coating was applied on galvanized (GI) and galvannealed (GA) steels to elevate the practicability for the hot stamping application. The GI and GA steel sheets were received from China Steel Co., Taiwan, with coating thickness of approximately 10µm. Silane coating film was formed by a laboratory rolling method. The treating solution was prepared by mixing tetraethyl orthosilicate (TEOS), acidified water (pH= 1.0, adjusted by HNO3), γ-glycidoxypropyltrimethoxysilane (GPTMS), followed by stirring for 2 h at room temperature. The GI and GA steel sheets with and without the silane coating were isothermally held at 900℃ under the atmosphere for 5 min to simulate austenitization prior to hot stamping. XRD, SEM, TEM, and XPS were employed to characterize the surface microstructure of the various steels. Corrosion behaviors were tested by electrochemical stripping and potentiodynamic polarization measurements. The GI steel displayed a gray color after austenitization. In contrast, the GI steel with the silane coating still exhibited a metallic color, suggesting the silane coating is effective in preventing the GI steel from oxidation at 900℃ (Fig(a)). This is consistent with the XRD pattern showing the absence of the peaks resulting from ZnO for the GI steel with the silane coating (Fig.(b)). However, the silane coating offered a less degree of protection on the GA steel. The cross section of the silane-coated GI and (b) GA steels was further characterized by the SEM. Zn oxides were readily observed on the GA steel; conversely, Zn oxide scale was hardly observed on the GI steel. After austenitization, the silane-coated GI coating transformed to a mixture of Γ phase and Zn saturated α-Fe, which retained the cathodic protection over the steel substrate. However, the alloy layer of the silane-coated GA steel was solely composed of Zn saturated α-Fe and the cathodic protection was largely reduced after austenitization. Reference [1] R. Kuziak, R. Kawalla, S. Waengler, Archives of civil and mechanical engineering 8 (2008) 103-117.[2] D.W. Fan, H.S. Kim, B.C. De Cooman, Steel Research International 80 (2009) 241-248.[3] H. Karbasian, A.E. Tekkaya, Journal of Materials Processing Technology 210 (2010) 2103-2118.[4] D.W. Fan, B.C. De Cooman, steel research international 83 (2012) 412-433.[5] L. Dosdat, J. Petitjean, T. Vietoris, O. Clauzeau, steel research international 82 (2011) 726-733.[6] A. Marder, Progress in materials science 45 (2000) 191-271.[7] C.W. Lee, D.W. Fan, I.R. Sohn, S.-J. Lee, B.C. De Cooman, Metallurgical and Materials Transactions A 43 (2012) 5122-5127.[8] R. Autengruber, G. Luckeneder, S. Kolnberger, J. Faderl, A.W. Hassel, steel research international 83 (2012) 1005-1011.[9] C.-W. Ji, I. Jo, H. Lee, I.-D. Choi, Y. Do Kim, Y.-D. Park, Journal of Mechanical Science and Technology 28 (2014) 4761-4769.[10] R. Figueira, C.J. Silva, E. Pereira, Journal of Coatings Technology and Research 12 (2015) 1-35. Figure 1

Role of the internal oxidation layer in the liquid metal embrittlement during the resistance spot welding of the Zn-coated advanced high strength steel

The concept of quality is strongly linked to the steel micro-inclusionary cleanness. The aims of the correct refining practices are: 1) to adequate the non-metallic inclusions amount to acceptable values; and 2) to guarantee that the remaining inclusions will not be detrimental to the mechanical properties of steels. To reach such aims, the present work will evaluate the non-metallic inclusions analysis during steel making via EAF process. The characterization of inclusions consisted in the sampling of steel during the SAE 8620 refining. This steel is used in the production of automotive components industry. The characterization of steel inclusions consisted in analyzing 1) area occupied by inclusions and their size and; 2) chemical composition. The results were analyzed via ternary and pseudo-ternary phase diagrams. Some correlations between inclusion composition and steel making process were also done. The inclusions appear mainly as Si/Mn oxides in the beginning of the ladle furnace processing, and as Ca-Al oxides in the beginning of the continuous casting. The inclusions medium size was determined in the range of 4 to 10Pm. It was also observed a gradual decreasing of the occupied area by inclusions during the process, which is an important aim of the refining practices. A thermodynamic study was carried out were the results of inclusion analysis was compared with FactSage simulations and the results are compatible. The results of this work shown that the application of different techniques becomes possible the study and development of high quality steels. Key words: SAE 8620 steel, characterization, inclusions, FactSage.

Interpreting the radiative properties of advanced high strength steel strip using a hybrid thin film/geometric optics model

High Strength low alloy steels containing various levels of C, Nb and Mn were used and for each of which, a simulated double thermal cycle was applied with the same first peak temperature and different second peak temperatures to produce HAZ microstructure corresponding to multi-pass weld. Effect of double cycle second temperature on the microstructure was observed and compared with single cycle results obtained from previous works, it was found that the percentage of martensite austenite constituent (MA) increases by Nb addition for all steels with the same Mn content and the increase in Mn content at the same Nb content shows an increase in MA area fraction as well. MA area fraction obtained for the double cycle is larger than that obtained for the single cycle for all steels used which imply that MA will have great role in the brittle fracture initiation for double cycle and the inter-pass temperature should be controlled for medium and high-carbon Mn steel to avoid large area fraction of MA. The beneficial effects of Niobium obtained in single pass weld were not observed for the double cycle or multi pass welds.

Interlamellar spacing is one of the most fundamental parameters to characterize the pearlite microstructure and its mechanical properties,the influence of some alloying elements on interlamellar spacing in high carbon steels has been extensively studied in the past.Unfortunately,few studies refer to the influence of vanadium.In this paper,the effects of different amounts of vanadium on phase transformation in high carbon steels during continuous cooling has been investigated on Gleeblel500 thermo-mechanical simulator,and the mechanical properties relevant to these effects have been measured.The results show that by the way of adding vanadium to the high carbon steel,the pearlite transformation temperature is depressed,the CCT curves of this transformation are postponed so that the interlamellar spacing is reduced,leading to the increase of the pearlite hardness and strength in the high carbon steel.There are two reasons why vanadium can depress the average temperature of pearlite transformation:firstly,vanadium depresses the start temperature of this transformation, secondly,it lowers the velocity of pearlite nodule growing.It is also found that ferrite appears on the austenite grain boundaries of the high carbon steel,introducing 0.1%vanadium into the high carbon steel can somewhat restrain the ferrite formation,nevertheless by the way adding 0.2%or 0.3% vanadium,the ferrite formation on the austenite grain boundaries is apparently promoted.Evidently, the ferrite formation on austenite grain boundaries is beneficial to the improvement of ductility of high carbon steels as confirmed in this paper.

Recently, increasing attention has been focused on the high strength low alloy (HSLA) steels mircoalloyed with multiple miroalloying elements, such as Nb-Ti, Nb-V and Ti-Mo, which can form synthetic carbide in steel, such as (Nb, Ti)C, (Nb, V)C and (Ti, Mo)C. Compared with the simplex carbide, such as NbC, TiC, those synthetic carbides with nanometer size exhibiting a superior thermal stability to exert their powerful influence mainly through their precipitation hardening in ferrite. It is reported that the precipitation hardening of approximate 300 MPa which can be obtained in Ti-Mo-bearing steel was developed by JFE steel, attributing to the synthetic (Ti, Mo)C particle precipitated in ferrite. However, as common microalloying elements, Nb and Mo are added synchronously in steel. The strengthening mechanism of Nb-Mo mircoalloyed as-rolled steel and the role of the carbide pre*国家重点基础研究发展计划项目2015CB654803和国家高技术研究发展计划项目2015AA034302资助 收到初稿日期: 2015-09-15,收到修改稿日期: 2016-01-05 作者简介:张正延,男, 1986年生,博士生 DOI: 10.11900/0412.1961.2015.00482

The researches on the behavior of very-high-cycle fatigue (VHCF) for high strength steels have become a new branch in the field of metal fatigue since 1980s. The characteristics of crack initiation and propagation for high strength steels in VHCF regime differ from those in low cycle and high cycle fatigue regimes. One of the most distinct phenomena for VHCF regime is the interior or subsurface crack initiation at inclusions or at other inhomogeneities. In fact, the period of crack initiation and early growth dominates the fatigue life, which is extremely evident for the fatigue process containing VHCF regime. This paper attempts to further investigate the crack initiation with the morphology of so-called fish-eye and fine granular area (FGA) or optical dark area (ODA) for high strength steels. Fatigue tests were carried out on a high carbon low alloy steel by using rotating bending (52.5Hz) and ultrasonic push-pull (20kHz) machines and the fatigue fracture surfaces were examined by scanning electron microscopy. The present results together with the previous ones showed the data of the stress intensity factor range (SIF) as a function of the dimension for fish-eye and FGA. The results also showed the data of FGA dimension as a function of related inclusion size. Based on the present results and the ones from literature, two models of fatigue life estimation were developed. One was taking into account the size of plastic zone at crack tip in relation with the value of SIF at FGA. The other was considering the fatigue damage cumulative process in relation with the microscopic parameters (inclusion size and FGA size) at the fracture region along with macroscopic quantities (tensile strength and stress level). The estimations are in good agreement with experimental results.

Fe-30.6Mn-6.8A1-0.9C alloy steel is employed in this study to coat the chromium carbide and related hard coatings on the surface by using pack chromization process. Electrochemical test in25℃,3.5wt% aqueous solution is used to evaluate the corrosion resistance of chromized specimen in room temperature. The immersion test in 10 wt% NaCl+90 wt% Na2SO4 mixed molten salt at877℃ for 48 hours is conducted to investigate the hot corrosion resistance of chromized specimen. The (Fe,Cr)2 N(subscript 1-x)and Cr23C6 phases are found on the chromized surface, while the internal part of chromized layer is Cr7C3 phase. In general, the corrosion resistance of the chromized specimen is apparently improved. The resistance to high temperature mixed molten salt of the chromized specimen is also better than that of 430 stainless steel. However, the corrosion resistance of the chromized Fe-Mn-Alalloy steel will be affected by denseness and oxide defects in the chromized layer.

Stationary solid oxide fuel cell (SOFC) applications typically have targeted lifetimes of more than 40 000 h with a tolerable power degradation of less than 1% per 1000 h. To achieve these requirements powerful methods and strategies to characterize and quantify SOFC stack degradation are needed. The objective of this work was the Investigation and prediction of SOFC stack degradation based on in-situ measurements and post-test analyses. These results were combined with the findings from dedicated laboratory experiments for the investigation of single degradation effects combined with computer models to develop strategies to predict and subsequently reduce SOFC stack degradation. This requires the development of a model for the reliable quantification and thorough Interpretation of the degradation behavior of SOFC stacks under real, non-ideal conditions. The model extracts the time-dependent internal stack resistances from current-voltage data. As major advantage over assessing stack degradation simply from the slopes of current-voltage curves, these internal resistances can be directly compared with the sum of the resistances of different stack components as obtained from dedicated laboratory degradation experiments. Due to recent progress in improving the oxidation re-reduction (redox) stability of electrolyte supported SOFCs, the main contribution to the power degradation of stacks operated of more than 10 000 h is induced by the ohmic losses caused by the time-dependent and continuously formation of oxide scales on the metallic interconnects (MICs). Moreover, the oxide scale growth rate and the extrinsic electrical conductivity of the formed semi conductive Cr2O3 is influenced by various parameters such as scale morphology, temperature, gas atmospheres, impurities, reactive elements and interaction with adjacent components. For this reason oxide scale growth on chromium (CFY) and ferritic (Crofer) based alloys were investigated by scanning electron microscopy (SEM) including samples from SOFC stacks manufactured by the Hexis AG1, which were operated for up to 40 000 h. Comparison of the measured increase in ohmic resistance with mean scale growth rates obtained from SEM cross section images revealed a non-trivial, non-linear relation-ship. To understand the correlation between scale evolution and resulting ohmic losses, 2D finite element (FE) simulations of electrical current distributions were performed for a large number of scale morphologies. Oxide scale morphologies favor non-homogeneous electrical current distributions, where the main current flows over rather few “bridges”, i. e. local spots with relatively thin oxide scales. Combining electrical conductivity and SEM measurements with FE simulations revealed two further advantages: it allows a more reliable extrapolation of MIC degradation and it provides a new method to assess the effective electrical conductivity of thermally grown Cr2O3 scales under stack operation. On the anode side the degradation behavior of oxide scales is even more complex. For example, Ni particles released during thermal redox cycles from adjacent Ni containing components might be interspersed into the oxide scale. To study the influence of this interaction Cr2O3 pellets admixed with different amounts of Ni (up to 20 vol.%) were produced. The electrical conductivity was investigated in-situ in reducing forming gas (95% N2; 5% H2) atmosphere and air at 850. Furthermore microstructure and crystal structure are studied at different time steps with SEM and X-ray diffraction (XRD), respectively. Based on the applied methods it can be confirmed that during oxidation in air Ni forms a NiCr2O4 spinel phase. Exposure again in a reducing environment leads to an instantaneous decomposition of this spinel phase, which in turn leads to a fine dispersion of reduced Ni particles. This rearrangement of Ni by spinel decomposition improves the electrical conductivity of the Cr2O3-pellets. In conclusion, this work established new methods and models to more reliably assess of SOFC stack-degradation based on current-voltage data, which allow extracting internal stack resistances for direct comparisons with dedicated laboratory experiments of single degradation phenomena. The focus of attention is on the quantification and understanding of the complex degradation behavior of metallic interconnects (as major contribution to the overall SOFC stack degradation) caused by oxide scale growth under reducing and oxidizing conditions.

Assessment of mechanical properties in unirradiated and irradiated zircaloys and steels with non-standard tests and finite element calculations

In different production steps steels are exposed for short times, from a few minutes to up to a few hours, to high temperatures, either under reducing conditions for iron (e.g. N2-H2 during recrystallization annealing) or in oxidizing environments (e.g. during hot rolling and cooling after hot rolling). These processes are accompanied by external and internal oxidation, which can affect further processing steps. For example, during cooling after hot rolling of high strength steels especially grain boundary oxidation can lead to significant problems during pickling and cold rolling. For some alloys, e.g in the Fe-Mn-Si system, a surprisingly pronounced and deep grain boundary oxidation is observed.In this paper a fundamental investigation of grain boundary oxidation during annealing under model atmospheres simulating the oxygen partial pressure beneath the scale and also under significant lower partial pressure will be addressed, based on model alloys which are relevant to industrial high strength steel grades. The focus will be on questions such as how the different alloying elements and different annealing conditions influence the internal selective oxidation.In order to develop a fundamental understanding, the internal oxidation of dedicated binary and ternary model alloys (Fe-Mn, Fe-Si, Fe-Mn-Si, of different composition) was investigated in depth. Besides a thorough analysis of the oxide morphology, one important focus was on monitoring the mass gain in-situ during isothermal oxidation for short term exposures of up to 2h. The partial pressure of oxygen during the experiments was kept just below the equilibrium pressure for FeO/Fe and for comparison also at much lower partial pressure. As the mass changes during internal oxidation are very low, errors from buoyancy effects in a standard set-up are in the same range as the mass change to be investigated. Hence, a novel under-pressure TG set-up was developed and applied here. Furthermore, in-operando photoelectron spectroscopy of the oxide evolution at the surface by Ambient Pressure Photoelectron Spectroscopy (APXPS) was carried out, which provided important additional information for achieving a fundamental understanding of the observed oxidation kinetics. Based on the results the enhancing effect of the combination of Si and Mn on internal oxidation will be discussed.