Galvanisability of silicon free CMnAl TRIP steels

Abstract

Transformation induced plasticity (TRIP) steels are a promising solution for the production of cars with low body mass because of the combination of high strength and large uniform elongation that they offer. However, conventional CMnSi TRIP steels with more than 1 wt-%Si have the drawback of poor Zn coating quality after continuous galvanising. This problem is due to the presence of complex Si–Mn oxides on the strip surface. The present research work therefore focused on the full substitution of Si by Al in TRIP steels and the detailed analysis of the galvanising behaviours of these Si free CMnAl TRIP steels. If the hot dipping is done after a combination of intercritical annealing and isothermal bainitic transformation in a furnace atmosphere with a high dewpoint, the wetting of the strip by the liquid Zn is improved significantly. However, the improvement is limited and not enough to avoid bare spots and coating defects cannot be avoided on conventional CMnSi TRIP steel. In contrast, the Si free CMnAl TRIP steel has a much better wettability when annealed at a low dewpoint. The surface segregation of the elements, which have a high affinity for oxygen, i.e. Si, Al, and Mn, was studied in detail and this revealed that Si was much more readily enriched on the surface than Al during the annealing in the low dewpoint atmosphere. The difference in the surface segregation between Si and Al resulted in a clear difference in the galvanisability. The limited presence of Al on the strip surface is due to the fact that Al can be oxidised internally during hot rolling. As a result, an Al depleted surface region is formed owing to selective internal oxidation of Al before the continuous galvanising.     

Degradation behavior of a commercial 13Cr ferritic stainless steel (SS405) exposed to an ambient air atmosphere for IT-SOFC interconnect applications

The objective of this work was to verify the feasibility of using low Cr ferritic stainless steels to improve IT-SOFC interconnect properties in terms of electrical conductivity and Cr evaporation resistance by taking advantage that low Cr ferritic stainless steels may form conductive and moisture-stable oxide layers of chromite spinels. In this context, surface degradation of a commercial 13Cr ferritic stainless steel (SS405) was studied at 700 °C in ambient air (containing ca. 3% H₂O by volume) for up to nearly 5000 h. The results showed a slow, nearly linear oxidation rate indicating that the oxide scale could not act as a fully protective barrier during a prolonged exposure at 700 °C. Oxidation-induced degradation was mainly due to the effects of Cr depletion in the subscale matrix region and internal oxidation attack. However, analysis of the oxide scale after long-term exposure revealed the presence of an interesting example of all-spinel structure consisting of outer layer of iron-substituted Mn–Cr spinel particles on the top of an inner and continuous Fe–Cr spinel layer. Since SS405 steel contains also Al and Si as minor additions, the distribution of these elements in the spinel oxide scale was studied and found to be rather different. Whereas Si was present exclusively in the inner oxide regions, Al migrated also to the outermost part of the oxide scale. Possible explanations for this unexpected Al surface enrichment are discussed.     

Chromium interaction with TiO2 dispersoids in oxide dispersion strengthened ferritic steel

Abstract

Auger electron spectroscopy and energy filtering TEM were used to study alloying element distributions in the vicinity of matrix–TiO₂ dispersoid interphase boundaries in an Fe–Cr ferritic steel strengthened by dispersed TiO₂ particles. Pronounced interfacial Cr segregation was found to occur in the course of annealing at 700°C. The Cr segregation also leads to Cr diffusion into the dispersoids and to the formation of oxide phases with different Cr/Ti ratios. The observed Cr segregation is described in terms of diffusion into the field of elastic stresses.     

Chromium vaporization of the ferritic steel Crofer22APU and ODS Cr5Fe1Y<Subscript>2</Subscript>O<Subscript>3</Subscript> alloy

The chromium vaporization rate of the Crofer22APU steel as compared to Cr5Fe1Y₂O₃ alloy was studied under humidified air at 800 °C over a period of up to 1300 h by using the transpiration method. Under oxidizing atmosphere, the Crofer22APU steel forms a (Cr,Mn)₃O₄ spinel layer on the surface, which hinders chromium release at 800 °C by about a factor of 3 as compared to Cr5Fe1Y₂O₃ alloy forming a pure chromia scale. The impact of minor alloying additives in the Crofer22APU ferritic steel on the chromium vaporization was considered. The steel with the lowest amount of Al and Si in the matrix exhibits a low chromium vaporization rate over 1300 h. The microstructure and composition of the oxide scales and surfaces formed were investigated using high-resolution scanning electron microscopy, X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry. The results obtained give insight into the chromium vaporization mechanism.     

Healing layer formation in Fe–Cr–Si ferritic steels

Abstract

The addition of small amounts of Si can dramatically improve the oxidation resistance of Fe and Fe–Cr steels. It is found that steels with Si contents above a certain critical value oxidise at a much slower rate and also become virtually immune to breakaway oxidation in high pressure CO₂, The critical Si content for this behaviour is found to vary with the Cr content (wt-%) of the steel, from about 2·5% for mild steel to 0·7% for 9%Cr steel to 0·3% for 11%Cr steel in the temperature range 575–650°C. The lower Si content required for Cr steels than for mild steels is advantageous, because it is small enough not to degrade the other metallurgical properties such as creep strength. The beneficial effect of Si is thought to arise from the formation of a near continuous ‘healing’ layer of amorphous SiO₂ at the oxide/metal interface which acts as a diffusion barrier to further transport of metal ions to the scale. The conditions required for the development of such layers are analysed using standard models of selective oxidation. The synergistic effect of Cr and Si is ascribed to the action of Cr as a secondary getter, in which it reduces the oxygen solubility in the metal and so reduces the Si content required to form a healing layer. Chromium also discourages the SiO₂ from converting to fayalite (Fe₂SiO₄) which is a much poorer diffusion barrier. The conventional theory of selective oxidation and secondary gettering is found to describe reasonably well the compositional limits of healing layer behaviour in these ferritic steels. However, the silica also seems to encourage the formation of a chromia based layer at the base of the overlying oxide and the oxidation rate during healing seems to be limited eventually by this chromia layer rather than the silica layer, as would be expected in the conventional model.

MST/1074     

Effect of surface treatment on segregation of impurities in haematite

Surface segregation of various impurities such as Mg, Si, Ca, Al and Cr were determined for the haematite phase (Fe₂O₃) annealed in two different gas compositions involving (1) air at 1173 K, and (2) a gas mixture containing sulphur at 773 K. The objective of work was to establish the effect of the gas-phase composition on segregation of lattice defects. The near-surface segregation profiles of the impurities were determined by secondary ion mass spectrometry. The depth profile analysis was made by sputtering using an Ar⁺ primary beam of energy 30 keV. The surface charge was neutralized by an electron gun. It was found that annealing Fe₂O₃ under a gas phase containing sulphur resulted in the formation of an Fe(SO₄)₃ surface layer. It was observed that the two surface treatment procedures applied (both with and without sulphur) result in Mg enrichment in the near-surface region of Fe₂O₃. Si and Ca exhibit an enrichment and impoverishment after the surface treatments 1 and 2, respectively. Finally, the near-surface layer is impoverished in Cr and Al after both types of surface treatment. Experimental results are discussed in terms of segregation driving forces of the respective elements and the possible effect of sulphur on the gas-solid heterogeneous kinetics.     

连铸连轧10CrNiCu钢板厚度方向元素分布特征分析

采用激光原位分析技术,研究了连铸连轧10CrNiCu钢板中各元素在板厚方向的分布,结果表明:C在钢板下表面含量高,上表面含量低;P、S偏析微区弥散分布;Mn、Ni、Cr、Si基本沿板厚中心对称分布,为中心正偏析,Cu的分布最均匀。元素的偏析度基本与原子半径大小成反比关系,C的偏析度最大,P、S、Si偏析度居中,Mn、Cr、Ni、Cu的偏析度较小。     

Micro-hardness, microstructures and thermal stability of (Ti,Cr,Al,Si)N films deposited by cathodic arc method

(Ti,Cr,Al,Si)N films were deposited by cathodic arc method using TiCrAlSi alloy cathodes. It was found that the microstructures of (Ti,Cr,Al,Si)N were closely related to (Al+Si) content. The crystal structure of (Ti,Cr,Al,Si)N was NaCl-type structure up to the (Al+Si) content of 0.60, where it changed to a hexagonal structure. The maximum hardness of 33 GPa was obtained at the lowest (Al+Si) content of 0.56, still in the cubic structure. The micro-hardness decreased down to 28 GPa as the crystal structure changed from NaCl-type to wurtzite-type.To investigate the thermal stabilities of (Ti,Cr,Al,Si)N, the films were annealed in a vacuum furnace. In Ti_0.20Cr_0.20Al_0.55Si_0.05N with cubic structure, the phase segregation occurred by annealing at over 900 °C, while Ti_0.22Cr_0.22Al_0.44Si_0.12N remained in cubic phase up to 1000 °C. The micro-hardness of Ti_0.20Cr_0.20Al_0.55Si_0.05N increased and that of Ti_0.22Cr_0.22Al_0.44Si_0.12N decreased at 1000 °C. Ti_0.20Cr_0.11Al_0.58Si_0.11N with a cubic and hexagonal mixture phase held its (c,h)-mixture phase up to 1000 °C, while there was an indication of an increase both in micro-hardness and in cubic ratio after annealing.In this paper, the micro-hardness and microstructure of (Ti,Cr,Al,Si)N are discussed as a function of annealing temperature and investigated by X-ray diffraction and electron microscopy.     

Identification of the nitrides formed during the annealing of a low-carbon low-aluminium steel

The precipitation of a low carbon low aluminium steel with a sub-stoichiometric [Al]/[N] atomic ratio was investigated during an annealing at 600°C by Transmission Electron Microscopy (TEM) and chemical analyses by X-ray Energy Dispersive Spectroscopy (EDX) on thin foils and carbon extraction replicas. These studies showed that in this steel, the precipitates which form have a platelet-like morphology associated with a cubic structure (with a = 0.412 ± 0.005 nm) and the following orientation relationship with iron: (001)_p // (001)_Fe and [110]_p // [010]_Fe. The EDX microanalyses of these precipitates revealed that they contain, in addition to aluminium and nitrogen, chromium and/or manganese. It was suggested that in the early stages of annealing, the initial precipitates are of CrN or (Cr,Mn)N type. During a prolonged annealing, they serve as nucleation sites for the final precipitates of (Al,Cr)N and (Al,Cr,Mn)N type.     

Oxidation and abrasive wear of Fe–Si and Fe–Al intermetallic alloys

In the present work, intermetallic alloys Fe–Si and Fe–Al (Fe₃Si–C–Cr and Fe₃Al-C), produced by induction melting, were evaluated regarding their oxidation and abrasive resistance. The tests performed were quasi-isothermal oxidation, cyclic oxidation, and dry sand/rubber wheel abrasion. As reference, the ASTM A297-HH grade stainless steel was tested in the same conditions. In the oxidation tests, the Fe–Al based alloy presented the lowest oxidation rate, and the Fe–Si based alloy achieved the best results in the abrasion test, showing better performance than the HH type stainless steel.     

超低碳焊条钢的VOD冶炼

设计的初炼炉出钢成分(w,%)为:C0.70～0.90;Mn、Si0.20～0.30;Al0.010～0.020,出钢温度为1660～1680℃,为VOD精炼超低碳焊条钢提供了足够的热能。初炼炉热装50%铁水,采用氧化法冶炼能有效控制砷、铜、磷、铬、钒等微量元素的含量;VOD精炼过程中通过工艺手段获取"玻璃"渣是控制碳、氧、氮、铝、硅、锰等元素含量行之有效的方法。制定的EAF+VOD冶炼工艺保证了生产的焊条钢各项指标均满足技术要求,质量达到国际同类产品的水平。     

Effects of Cr,Mn, Si,Cu and Zr on microstructure and mechanical properties of high carbon Fe–16Al alloy

Abstract

Effects of alloying elements Cr, Mn, Si, Cu and Zr on the microstructure and mechanical properties of Fe₃Al (Fe–16Al) based alloy containing ～0·5 wt-%C have been investigated. Six alloys were prepared by a combination of air induction melting with flux cover and electroslag refining (ESR). ESR ingots were hot forged and hot rolled at 1373 K and were further characterised with respect to microstructure and mechanical properties. The base alloy and the alloys containing Cr, Mn, Si and Cu exhibit a two phase microstructure of Fe₃AlC_0·5 precipitates in Fe₃Al matrix whereas the alloy containing Zr exhibits a three phase microstructure, the additional phase being Zr rich carbide precipitates. Cr and Mn have high solubility in Fe₃AlC_0·5 precipitates as compared to Fe₃Al matrix whereas Cu and Si have very high solubility in Fe₃Al matrix compared to Fe₃AlC_0·5 precipitate and Zr has very low solubility in both Fe₃Al matrix and Fe₃AlC_0·5 precipitate. No significant improvement in room and high temperature (at 873 K) strengths was observed by addition of these alloying elements. Furthermore, it was observed that addition of these alloying elements has resulted in poor room and high temperature ductility. Addition of Cr, Mn, Si and Cu has resulted in marginal improvement in creep life, whereas Zr improved the creep life significantly from 22·3 to 117 h.     

Continuous cooling transformation behaviour of Si–Mn and Al–Mn transformation induced plasticity steels

Abstract

The continuous cooling transformation (CCT) behaviour of two transformation induced plasticity (TRIP) steels was investigated using quench dilatometry. One was an established steel grade with a composition (wt-%) of Fe–0·2C–2Si–1·5Mn while the other steel was a novel composition where 2 wt-% Al replaced the silicon in the former grade. Characteristics of the α→γ transformation during reheating and the subsequent decomposition of austenite during continuous cooling were studied by dilatometry, and CCT diagrams were constructed for both steels. The effects of accelerated cooling and steel composition on γ transformation start temperature Ar ₃, phase transformation kinetics, and microhardness were investigated. The results showed that the Al–Mn steel had a much wider α→γ transformation range during reheating, compared with the Si–Mn steel. Furthermore, the Al–Mn steel exhibited no significant change in the rate of expansion during α→γ transformation. On the other hand, during continuous cooling, the Al–Mn steel exhibited higher Ar ₃, faster transformation kinetics, a higher volume fraction of polygonal ferrite in the microstructure, and lower hardness, compared with the Si–Mn steel. The addition of aluminium was found to have a significant effect on the products of phase transformation, kinetics, and form of the CCT diagram. For both steels, an increase in cooling rate lowered the Ar ₃ temperature, decreased the time of transformation, and increased the hardness.     

Precipitation–segregation mechanism for high temperature temper embrittlement of steels revealed by Auger electron spectroscopy and internal friction measurements

Abstract

A precipitation–segregation mechanism of high temperature (reversible) temper embrittlement has been suggested on the basis of Auger electron spectroscopy, internal friction measurements, and scanning and transmission electron microscopy (SEM, TEM) of 0·3C–Si–Mn–Cr steel after various heat treatments. It was found that P segregation takes place during heating for quenching and tempering. The intensity of P segregation does not change when the steel transforms from a ductile (600°C tempered for 2 h with water cooling) to a brittle (600°C tempered with furnace cooling) state. Isothermal aging at 500°C for 200 h leads to further segregation of P, but the intensity remains unchanged after a reductilising treatment (600°C reheating for 2 h with water cooling). (The term reductilising is used to describe the treatment employed to attempt to return the material to its original ductile state.) The internal friction measurements and TEM examinations indicated that the temper embrittlement of the steel is always accompanied by precipitation of very fine particles of Fe₃C(N) causing dead pinning of dislocations within the grains while the reductilised state corresponds to the re-solution of these fine particles back into the a solid solution which explains the reversibility of the phenomenon. Therefore, the P segregation is a necessary factor for grain boundary fracture of the steel in the temper embrittled state, but is still not sufficient for the actual appearance of this mode of fracture. The leading factor is the precipitation of Fe₃C(N) particles on dislocations and a precipitation–segregation mechanism is suggested which can be used to explain all the specific features of high temperature temper embrittlement.

MST/707     

Oxidation behaviour of some modern heat-resistant cast steels

The kinetics and morphological development of the oxidation of a selection of modern heat-resistant cast steels have been examined and compared with those of the traditional material, HK40. The materials examined had Cr contents of 24 to 29 weight percent (wt %), Ni contents of 30 to 46 wt %and in several cases minority additions of Nb, W, or both. One steel contained 3.3 wt % Al. Kinetics were measured gravimetrically over periods of 6 to 100 h and found to be parabolic in all cases except for the Al-containing steel, which oxidized in an irregular and irreproducible fashion. All steels formed an external scale of Cr₂O₃ with a Mn-rich spinel layer at the outer surface. Beneath this scale was a layer of alloy depleted in both Cr and Mn. Within the depleted layer inter-dendritic carbides had been destroyed, leaving either oxide near the external alloy surface or voids deeper within the alloy.     

Thermal oxidation-resistant surface alloys processed by laser alloying on 35 NCD 16 ferritic steel

The hardness and the high-temperature oxidation resistance of the low-alloyed ferritic steel 35 NCD 16 (Fe-0.38 C-1.8 Cr-4 Ni, wt%) were increased by laser surface alloying of Cr₃C₂ or Cr₃C₂ and SiC. The obtained surface alloys always exhibit primary dendrites ( Fe solid solution with Cr and Si + martensite) and an interdendritic eutectic containing Fe + martensite + M₇C₃ carbides (M=Fe, Cr). In the first case, the formation of iron chromite, FeCr₂0₄, in contact with the coating accounts for the good oxidation resistance. In the second case, the beneficial influence of silicon lies in the formation in the presence of oxygen of a thin chromia scale only, with local silicon enrichment, showing excellent barrier properties. Kinetic and structural observations are discussed in the light of thermodynamics and diffusion processes.     

Effect of microalloying with silicon on high temperature oxidation resistance of novel refractory high-entropy alloy Ta-Mo-Cr-Ti-Al

Abstract

The effect of 1 at.% Si addition to the refractory high-entropy alloy (HEA) Ta–Mo–Cr–Ti–Al on the high temperature oxidation resistance in air between 900 °C and 1100 °C was studied. Due to the formation of protective chromia-rich and alumina scales, the thermogravimetric curves for Ta–Mo–Cr–Ti–Al and Ta–Mo–Cr–Ti–Al–1Si showed small mass changes and low oxidation rates which are on the level of chromia-forming alloys. The oxide scales formed on both alloys at all temperatures are complex and consist of outermost TiO₂, intermediate Al₂O₃, and (Cr, Ta, Ti)-rich oxide at the interface oxide/substrate. The Si addition had a slightly detrimental effect on the oxidation resistance at all temperatures primarily as a result of increased internal corrosion attack observed in the Si-containing HEA. Large Laves phase particles distinctly found in the Si-containing alloy were identified to be responsible for the more rapid internal corrosion.     

Oxygen potential distribution during disappearance of the internal oxidation zone formed in the steam oxidation of Fe–9Cr–0.26Si ferritic steel at 973 K

Abstract

For discussing quantitatively the disappearance of the internal oxidation zone (IOZ), which was observed in the high temperature steam oxidation of the commercial Fe–9Cr–0.26Si ferritic steel (ASME T91), the oxygen potential distribution in the IOZ was calculated as a function of the normalized thickness of the IOZ. The distribution of the internal oxides was predicted with the calculated oxygen potential distribution and thermodynamic data. The predicted distribution of the internal oxides was in good agreement with the TEM observation at the internal oxidation front. When a sheet of amorphous SiO₂ formed at the internal oxidation front, the oxygen potential at IOZ/SiO₂ interface was calculated to be almost as same as that at the inner scale/IOZ interface because of the low oxygen permeability in the sheet of the SiO₂. The increase in the oxygen potential oxidizes the matrix of iron in the IOZ and converts the IOZ to the inner scale.     

Structural evolution of Ti-Al-Si-N nanocomposite coatings

Ti-Si-Al-N films were prepared by rf reactive magnetron sputtering, in static and rotation modes, using a wide range of different deposition conditions, which created conditions to obtain Ti-Al-Si-N coatings with different structural arrangements.Films prepared below a critical nitrogen flow, under conditions out of thermodynamic equilibrium, revealed a preferential growth of an fcc (Ti,Al,Si)N_x compound with a small N deficiency. With nitrogen flow above that critical value, the reduction of the lattice parameter was no longer detected. However, a thermal annealing showed that a complete thermodynamically driven segregation of the TiN and Si₃N₄ phases was not yet obtained. The segregation upon annealing induced a self-hardening and showed a multiphase system, where the crystalline TiN, (Ti,Al)N and (Ti,Al,Si)N_x phases were identified by X-ray diffraction. This behavior is due to the de-mixing of the solid solution associated to a small N deficiency.     

HP40Nb钢热浸镀Al-Si高温氧化行为及组织研究

为提高高温抗氧化性能,对HP40Nb钢进行了热浸镀Al-10%(质量分数)Si,并进行不同温度扩散处理,研究了不同扩散处理试样在1000℃条件下的高温氧化行为,通过SEM,EDS和XRD分析了经不同扩散处理后的渗层在高温氧化过程中的组织结构变化.结果表明:经800℃/4h扩散处理,渗层由内层(NiAl+ Cr3 Si)...