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Effects of macro segregation and homogenizing heat treatment on creep strength of 9Cr–1Mo–V–Nb (Grade T91) steel have been investigated and it has been concluded that texture is useful feature to evaluate creep strength of the steel as a straightforward index of macro segregation.

Ultrasonic welding is a solid-state bonding technique available in bonding steels and other metals. To understand the effect of roughness on the evolution of bonding strength, Ni and steel were emp...

The hydrogen embrittlement behavior of multilayer steel consisting of 7 alternating layers of martensitic (MART) steel and twinning-induced plasticity (TWIP) steel was investigated through variation in the ratio of the hard layer to the soft layer, and the size of the grains according to the autenization temperature. Multilayer steels with a high fraction of MART was predicted to be vulnerable to hydrogen embrittlement, but was found to be less susceptible to hydrogen embrittlement due to the presence of a TWIP layer, which is supported in the center of the multilayer steels. In addition, when the grain size is large, twinning in the TWIP layer and subdivision in the MART layer occur, and so hydrogen embrittlement is promoted due to the trapping of hydrogen. The effect of electrochemically-charged hydrogen on the multilayer steel during deformation was analyzed using electron backscatter diffraction in terms of the microstructure-mechanical property relationships.

Cross-sectional observations of the deformation bands formed during in situ tensile observation at the free surface of both ultralow- and low-carbon lath martensitic steels were conducted. The cross-sectional observations clearly revealed the formation of steps along substructure boundaries of the ultralow-carbon lath martensite during deformation. In contrast, severe undulation indicating strain accumulation was observed in the vicinity of high-angle boundaries for the low-carbon lath martensite, and no clear steps were formed along substructure boundaries, even though small filmlike ferrites were found. The “greasy-film” mechanism proposed by Maresca [Maresca et al., Modelling and Simulation in Materials Science and Engineering 24(2016)025006, Maresca and Curtin, Acta Materialia 134(2017)302] as well as the plastic instability theory proposed by Dao and Asaro [Dao and Asaro: Mech. Mater., 23(1996)71] was examined considering the non-Schmid effects and the anisotropic activation of slip systems in lath martensite.

Multilayered steel composites consisting of alternating martensitic and austenite layers were successfully fabricated with a combination of high strength and ductility. In situ SEM tensile experiments were performed to investigate the fracture processes. Results indicated that the martensitic layer distributed between two ductile austenite layers has better deformation ability, and the dimple fracture mode dominated the fracture of the whole specimen. In situ synchrotron microbeam X-ray diffraction experiments were carried out to study the microscopic deformation in austenitic layer and martensitic layer ahead of notch root under incremental tensile loading. Results indicated that for austenite layer, the maximum strain is located in the middle of the layer at the lower applied load. When at the higher applied load, larger strain is concentrated on the notch root. For martensitic layer, uniform strain is distributed at the region at the lower applied load, with increasing the applied load, small strain appears in the middle of martensitic layer. However, the variation in the strain in the martensitic layer is relatively smaller, which is actually affected by both the notch and the ferrite content in the martensitic layer. The changing trends of the strain in the austenite layer and martensitic layer can still show the better combination of brittle martensitic phase and ductile austenite phase.

Ultrasonic welding (USW), a solid-state bonding technique, was applied to join ultra-low carbon steel and Al5052 aluminium alloy. The evolution of bonding strength and microstructure at the bonding interface during ultrasonic welding was investigated with an emphasis on the early stage before the formation of intermetallic compound. Initially, adhesive wear starts when steel and Al alloy have sliding contact, and thereafter, a thin layer of Al is attached onto the surface of the steel while the wear of Al base metal is continued owing to the sliding. As the welding time increases, bonding sections are gradually formed with the increase of interfacial temperature owing to the sliding friction, whose process is illustrated in this article.

Ultrasonic welding is a solid-state welding technique that can bond materials at a relatively low temperature and pressure. In this study, steel/steel and steel/Ni combinations were successfully bo...

The orientation of austenite (γ) reverted from lath martensite has been studied using electron backscatter diffraction (EBSD) in Fe-2%Mn-1.5%Si-0.3%C (mass%) alloy. Two morphologies of γ, acicular and globular, are formed during reversion. Nucleation sites for acicular γ are lath, block and sub-block boundaries, while those for globular γ are prior γ grain boundaries, as well as inside packets. Both acicular and globular γ hold a near Kurdjumov-Sachs (K-S) orientation relationship with at least one of its surrounding tempered martensite blocks. Most acicular γ have almost identical orientations with the prior γ, while only a part of acicular γ have a specific twin-related variant with the prior γ. Furthermore, grain boundary globular γ has almost identical orientations with the prior γ into which it does not grow. The formation of acicular and grain-boundary globular γ was attributed to strong variant selection during γ nucleation at lath, block and sub-block boundaries. On the other hand, orientation relationship analyses among intragranular globular γ, tempered martensite and cementite suggest that the nucleation of γ at cementite and tempered martensite interfaces causes weakening of the variant selection for the formation of intragranular globular γ.

Solid-state bonding between ultralow-carbon steel and pure nickel was conducted by hot pressing with various compressive strain ranging from 5 to 15% and subsequent isothermal holding at 923 K. It was found that the interfacial strength of contact area is accounted for by the evolution of the intrinsic strength of the interface and the amount of plastic energy dissipation at the crack tip during interface fracture. The compression induces severe deformation around the interface and consequently inhibits the plastic energy dissipation during interface fracture. In the first stage of isothermal holding, the residual strain around the interface on the steel side is reduced by recovery process, which concurrently decreases in the yield stress of the area adjacent to the interface. This promotes plastic energy dissipation of the area, leading to a significant increase in interfacial strength in the first stage.

In the present study, different combinations of multilayer sheets were prepared from 1 and 2 mm Mg AZ31 along with 0.25, 0.5, and 1 mm 304 L stainless steel. The texture and microstructure of the elongated samples (20 and 30 pct strain) were studied. It was found that the transversal stress plays an important role in both texture evolution and twinning in these composites. The obtained pole figures revealed an axial texture tilt with increasing steel layer volume fraction (V f). It was found that this is a direct effect of transverse stress, which becomes more significant upon reducing Mg V f. This extra stress component tilts the basal planes away from the original normal direction in monolithic samples. Moreover, our results indicate that with decreasing Mg V f, twinning activity was increased in the 20 pct deformed samples but reduced in the samples with 30 pct elongation. It is known that at high strains where sufficient transverse stress is generated, the activity of prismatic slip is significantly enhanced, which promotes the motion of dislocations and reduces the necessity of twinning. With decreasing Mg V f, stronger transversal stress is generated and Mg reaches the critical threshold of prismatic activity at lower strains.

The deformation modes of Mg in a steel-Mg-steel multilayer composite were studied by step-by-step deformation and EBSD for the same region at 0, 20 and 30% deformation. Texture evolution was characterized using pole figures. The results indicate that the basal texture intensity significantly increases up to 20% deformation. However, from 20% to 30% deformation, activity of prismatic slip increases and the intensity of the overall basal texture is reduced. Shear stress and Schmid factor analysis of the multilayer system showed the probability of prismatic slip increases up to 35% by increasing the stress ratio up to 0.4 and decreases afterwards. Keywords: Light alloys, Magnesium, Laminated metal composite, Texture evolution

Different multilayer Mg AZ31 and SS304L steel sheet combinations were prepared with different volume fractions of Mg. Isolated stress–strain curves of the Mg layers showed significant improvements in the strength and elongation of multilayer samples. Results indicated that in the most extreme situation with the lowest Mg volume fraction (V f = 0.39), the ultimate strength was increased by 25 pct to 370 MPa and the elongation was improved by 70 pct to 0.34. Investigation of the fracture surface showed that failure occurs by the coalescence of cracks close to the interface region. The improved strength of the multilayer samples was due to the combined effect of surface crack prevention by the steel layer and the higher work-hardening rate caused by the possible increased activity of non-basal systems. It is suggested that the stronger work-hardening behavior and the enhanced activity of non-basal systems in the multilayer samples were due to the formation of new stress components in the transverse direction. The larger the volume fraction of steel in the multilayer, the longer the distance remaining unstrained before the UTS.

A three-dimensional crystal plasticity finite element method (3D CPFEM) modelling of a real equal channel angular pressing (ECAP) process for investigating the mechanical properties and texture evolutions of single-crystal aluminium has been developed for the first time. The challenge of modelling such a severe plastic deformation via 3D CPFEM is how to accurately predict the deformation mechanism under the complicated contact conditions between a billet and a die. The validation by comparison with experimental observations demonstrates that the developed 3D CPFEM ECAP model is able to precisely capture the deformation characteristics at the microscale. Furthermore, this research clarified the previously remaining disputes such as the microstructural formation mechanism in the deformed area and the deformation nature in the plastic deformation zone. It is also the first time to extensively discuss the orientation-dependent deformation feature of the ECAP-processed billets, including morphology, la...

The solidification mechanism of austenitic stainless steels solidified with primary ferrite is clarified in terms of crystallography. The formation of austenite was expected to take place by the peritectic/eutectic reaction in the interdendritic region of the primary ferrite, but no unique orientation relationship was confirmed between the primary ferrite and the austenite. The austenite was found to keep growing along its direction, independently of the primary ferrite, even after the primary ferrite changed its growth direction. This was confirmed by solidification experiments with changing heat flow directions, and was supported by the observation that the austenite forms crystallographic domains different from those of the primary ferrite in the final microstructure. Thus, it is concluded that the austenite is not crystallographically restricted by the primary ferrite, and that the growth of the primary ferrite and that of the austenite are crystallographically independent of each other.

Changes in bonding strength and interfacial formation during the ultrasonic welding (USW) of steel and Ti were investigated. It was found that when the temperature of the joint was increased above 600 °C during USW, the bonding strength increased drastically and resulted in base-metal fracture. Interfacial, fracture, compositional, and crystallographical analyses revealed that phase transformation in Ti from α (hexagonal close-packed: HCP) to β (body-centered cubic: BCC) led to improved deformability at elevated temperatures, which promotes extensive bonding formation through the elimination of gaps near the bonding interface. Through this process, joints with good bonding and strength can be obtained.

To realize deformation mechanisms of twinning induced plasticity (TWIP) steels is always challenging. It is because that two controversial theory (deformation twins induced dynamic Hall and Petch effect vs Portevin-Le Ch a ˆ telier effect) have been long existed and both are thought of contributing to the resulting work-hardening behavior during straining of TWIP steels. In this study, a new class of multilayered steel (MLS) architected by Fe–22Mn-0.6C (wt%) TWIP steel and martensitic steel alternatively was designed to eliminate the Portevin-Le Ch a ˆ telier (PLC) effect, in order to systematically study the relationship of deformation twins and the work hardening behaviors of TWIP steels. After tensile deformation, it was found that the fraction of twins contribute to the higher toughness near the area of the interface in TWIP steel layers annealed at two different temperatures, suggesting that the deformation twins occur more significantly near the interface. It was also found that deformation twins are the main reason resulting in the improvement of the strength-ductility synergy of TWIP steels as well as MLS.

Multilayered composite steels consisting of alternating layers of martensitic phase and austenitic phase exhibit an excellent combination of strength and elongation compared with conventional advanced high-strength steels. The deformation processes underlying these properties are of considerable interest. In this article, microstructure, grain size, and phase are characterized by scanning electron microscopy (SEM) and electron backscattering diffraction. The hardness of each layer is analyzed by a microindentation hardness testing system. Finally, the deformation and failure processes in multilayered steel are investigated by in-situ SEM. The hardness results indicate that various hardening modes occur in the soft austenitic layer and the hard martensitic layer. In-situ SEM results combined with microstructure analysis and hardness results reveal that annealing temperature and annealing time have a significant impact on final microstructure, fracture behavior, strength, hardness, and ductility.

The solid-state bonding of ultralow-carbon steels was conducted by hot pressing and subsequent isothermal holding at low temperatures ranging from 873 to 923 K. The evolution of the interfacial strength was found to consist of two stages; the first stage, where the increase in interfacial strength is rapid and significant, and the second stage, where the increase is gradual. The evolution of strength in the first stage primarily takes place in contact regions produced by hot pressing. In the second stage, on the contrary, the evolution seems to result from the increase in the contact regions due to the shrinkage of voids. A molecular dynamics simulation was performed to clarify the atomic behaviour at the interface during the first stage. The results revealed that the disordered atomic arrangement caused by compression was rearranged with increasing isothermal holding time, leading to improved coherency between the contact regions and increased interfacial strength.

In order to improve the mechanical properties and to optimize grain refinement of Al-Mg-Si alloy, ECAP processing with an addition of hard particle TiB 2 is applied in this work. Mechanical property and microstructural evolution of Al-0.3Mg-7Si+1.5 wt.% TiB 2 specimen were investigated by using hardness-testing, optical micrograph observation and electron-backscattering diffraction (EBSD). ECAP processing was done through B A route for 4 passes at room temperature. Hardness test results show that the ECAP process doubled the hardness of the specimen compared to annealed specimen, and from EBSD/OIM analysis, the ECAP processing refined grains from an average grain size of 35 μm to 0.79 μm and led to producing grains having high misorientation angle (≥15).

This study investigated the effect of interlayer metals, particularly Al and Ni, on strength evolution during the ultrasonic welding (USW) of steel and Ti. It was found that the Al interlayer had an exceptional effect on accelerating strength evolution from 0 up to 1000 N in 0.1 s and up to 2000 N (26.7 MPa of lap shear strength) in 0.25 s. This causes base metal fracture with a higher bonding strength than the case without interlayer (700 N in 0.25 s). It was also found that when using Ni interlayer, the bonding strength was gradually increased to approximately 1000 N until 1.5 s of welding, slower than using Al interlayer. Microstructural analyses revealed that the Al interlayer was subjected to severe deformation and adhesion on base metals, leading to extensive contact to achieve bonding. In contrast, the less-deformable Ni interlayer exhibited a slow effect on strength evolution owing to less contact and the fracture within Ti during USW.

A sandwiched structure made of alternate layers of martensitic and austenitic steel has been quenched in both water and liquid nitrogen, and their deformation behaviors have been investigated via uniaxial tensile loading with an elongation of more than 40%. It demonstrates that quenching temperature, compared with the large-strain tensile deformation, has an even more significant influence on the improvement of the mechanical properties of martensitic steel.

Herein, the factors influencing the deformation behavior of lath martensite along with the crystallographic features, we used multilayered steels (MLS) containing martensitic steel and transformation-induced plasticity (TRIP) steel. Since martensite has an extremely strong and low ductility, ductile TRIP steels are coupled with the Martensitic steels to increase overall the plasticity of MLS. The influence factors were divided into three types such as the size of prior austenite grains (PAGs), the alignment of blocks with a tensile axis, and position of block (e.g., vicinity or center of PAG) around the blocks, and analyzed by matching microstructure and mechanical properties. The martensitic features up to an engineering strain of 20% were observed by means of an ex-situ electron backscatter diffraction (EBSD), compared with the operative slip systems of each influencing factor. The operative slip system was derived from an activation priority of the slip systems during deformation with the classification in two types such as in-lath-plane and out-of-lath plane systems. A fine block located near the PAG boundary with an initial shape parallel to the tensile axis is significantly deformed because of an activated multiple slip system, matching with the result by means of the crystal plasticity simulations.

In this work we present a novel in-situ approach to study the recrystallization behavior of age hardening alloys. We use confocal laser scanning microscopy (CLSM) at 400 °C to investigate the static recrystallization of an AlMg4Sc0.4Zr0.12 alloy in-situ. The results are combined with electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) analyses. It was found that CLSM is a powerful tool to visualize both the local initiation and temporal sequence of recrystallization. After fast nucleation and initial growth, the grain growth rate decreases and the grain boundary migration stops after some minutes due to Zener pinning from Al 3 (Sc,Zr) precipitates produced during the heat treatment. EBSD and TEM analyses confirm both the boundary movements and the particle-boundary interactions.

Significant crystallographic refinement was observed in the as-quenched lath martensitic steel during tensile deformation up to a strain of 20 pct. Crystallographic investigation by electron backscattering diffraction analysis coupled with the use of crystal plasticity model confirmed that the activation behavior of slip systems is inhomogeneous even within each martensite block. In particular, the activation behavior of slip systems is initially affected by the initial crystal orientation; however, the effect becomes negligible with increasing deformation. In addition, slip systems parallel to the lath plane are preferentially activated during elongation. Our results indicate that the nonuniform crystal rotation within individual blocks is induced not only by grain interaction phenomena but also by the anisotropic activity of slip systems. These are key factor that determines the grain refinement behavior in the as-quenched lath martensitic steel during deformation.

The microscopic strain distribution of lath martensitic steel during tensile deformation up to a strain of 10% has been measured in situ. Strain localization, which indicates grain interaction, is clearly observed in the vicinity of subblock boundaries, and it affects the inhomogeneous crystal rotation behavior within the block (or subblock), and hence the resultant rapid grain subdivision of the lath martensitic steel during elongation.

Multilayered steel composites consisting of alternating martensite and austenite layers and exhibiting a combination of high strength and ductility were successfully fabricated. To understand the microplasticity mechanisms responsible for such exceptional mechanical behavior, 3D X-ray microscopy with a submicron beam size was employed to probe the stress/strain distribution within the top two layers during incremental tensile loading. The 3D depth-dependent strain gradients were monitored in situ near the martensite/austenite interfaces as a function of the load level. It was observed that the strain gradients redistributed during loading. Specifically, large compressive strains developed in the top martensite layer transverse to the loading direction, while small tensile strains were found across the layer interface into the underneath austenite layer.

The development of a martensitic structure in a low-carbon and low-alloy steel was characterized using in situ confocal laser microscopy, high-speed photography and crystallographic analysis, including the nature of variant selection. The initial stage of transformation involves the partitioning of the austenite grain into packets, after which the rate of transformation is gradual. The crystallographic orientation of the plates that form is not random, but involves selection determined by the relationship between the shape deformation direction and the free surface. The vicinity of austenite grain and twin boundaries, and martensite/austenite interfaces also affect variant selection.

The fracture toughness of Fe­Al intermetallic compounds (IMCs), FeAl and Fe2Al5, that form as a thin layer on steel substrate was investigated. A model for evaluating the fracture toughness of a brittle thin layer on an elastoplastic substrate was applied, and the fracture toughness was evaluated from the thickness of the IMC layer and the crack interval in the IMC layer after uniaxial tensile testing. The phase and microstructure of the IMC layer were varied to investigate their effects on the fracture toughness of the IMC. The relationship between layer thickness and crack interval was in a good agreement with the theoretical model, and the fracture toughness was evaluated adequately using the model. It was clarified that FeAl has higher fracture toughness than Fe2Al5, and that fine-grained Fe2Al5 has higher fracture toughness than coarse-grained Fe2Al5. [doi:10.2320/matertrans.M2012425]

Multi-layered steel (MLS) consisting of alternating soft/ductile austenitic and hard/brittle martensitic stainless steel layers is a new class of hybrid material for structural application as it offers excellent combinations of strength and ductility. In this study, the contributions of each of the constituent layers to the overall strength and ductility of an MLS (having tensile strength > 1.4 GPa and ductility > 20%) were examined by recourse to nanoindentation experiments on each of them. By adapting two different indenter tip radii for the spherical nanoindentation experiments, constituent layers' stress-strain responses within the plastic regime were obtained and then compared with the macroscopic flow curve of the MLS that was obtained through tensile tests, to show that the strength contributions of the constituent steels to the global strength of MIS is as per the rule of mixtures. In order to examine the sources of tensile ductility of the MLS, sharp tip nanoindentation experiments were conducted on specimens extracted from tensile coupons that were subjected to predetermined plastic strains a priori. Results of these experiments show that the tensile failure occurs at a strain at which hardness of the austenitic layer, which is found to be dependent on the prior-plastic strain, is almost equal to the strain independent hardness of the martensitic layer. The results are discussed in terms of martensitic transformation within austenitic layer and the role of the mechanical environment change imposed by the neighboring martensite layers on it. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

This study investigates the mechanical properties and microstructure of friction stir butt welded high strength/ductility multilayered steel consisting of 15 alternating layers of SUS 301 austenitic stainless steel (eight layers) and SUS 420J2 martensitic stainless steel (seven layers) with a total thickness of 1·2 mm. With optimised welding parameters, defect free welds with an ultimate tensile strength (UTS) of 1240 MPa and a fracture elongation of 13% were accomplished. This corresponds to a joint efficiency of 90%. In this case, fracture occurred in the heat affected zone as a result of a very pronounced hardness drop in the martensitic layers resulting from the formation of a large amount of grain boundary precipitates, which were formed at temperatures ∼750°C slightly below Ac1. By applying post-weld heat treatment, the hardness drop in the martensitic layers was removed and the tensile properties were enhanced to UTS of 1310 MPa (95% joint efficiency) and a fracture elongation of 22%.

Liquid-phase bonding experiments were performed at 1073 K (800 °C) between ZIRCALOY-2 and type 316 austenitic stainless steel by inserting zinc as an interlayer. The evolution of the microstructure at the interface was studied and the formation of various phases was detected. On the zirconium side, the very rapid formation of Zn3Zr was detected, whereas on the steel side, an unexpectedly large amount of the base austenitic steel was observed to react with liquid Zn. The reacted iron solidified into a nickel-poor ferritic phase containing around 10 mol pct zinc, which grew into the austenite accompanied by a formation of a zinc-rich phase containing nickel. The reaction stopped when the zinc-rich phase reached saturation with a nickel content between 20 and 25 mol pct. Thermodynamic calculations showed that the addition of nickel to liquid zinc greatly decreases the free energy of the liquid phase, thus enabling a large stability range for the ferrite + liquid zone and reducing the stability range of the austenite. The primary equilibrium between the austenite and the liquid phase is thus metastable, and thus, the austenite transforms into ferrite and a high-nickel-content liquid. The transformation front then progresses until ternary equilibrium is reached between austenite, ferrite, and the zinc-rich phase.

Stress partitioning in multilayered steels consisting of martensitic and austenitic layers was measured during tensile deformation by in situ neutron diffraction measurements to investigate the mechanism of the improved strength–elongation balance. The deformation mode can be classified into three stages, and the results indicate that the applied stress is effectively transferred to the martensitic phase, because no stress concentration sites exist, owing to the multilayered structure. Hence, even as-quenched martensite deformed uniformly, resulting in improved strength–elongation balance in multilayered steels.

Liquid-phase bonding between a Mg alloy (AZ31) and low-carbon steel was attempted at 773 K (500 °C) using Ag as an interlayer that forms a eutectic melt with the Mg alloy at this temperature. On the AZ31 side, eutectic melting and subsequent isothermal solidification were observed, and it was confirmed that the solidification of the eutectic liquid was promoted by the diffusion of Ag into the AZ31 base metal. On the steel side, Al was transported from AZ31 during the eutectic melting and isothermal solidification. This transported Al was enriched at the steel surface and reacted with steel to form a uniform, thin Fe-Al intermetallic compound layer. After the isothermal solidification, strong bonding was achieved via the thin intermetallic compound layer between AZ31 and steel, and no Ag remained at the bonding interface. The strength of the joint was found to be higher than the yield strength of AZ31.

Mild steel with/without zinc coating and magnesium alloy AZ31B were lap joined by friction stir welding during the tool plunging through the magnesium into the steel. With increasing welding speed, the fracture strength increases. At the interface of zinc coated steel and magnesium, a liquid eutectic layer was detected for higher but not for lower welding speeds. At the interface between the steel without zinc coating and magnesium, no melting could be detected.

The effect of chemical composition of B1 compounds on the formation of intragranular ferrite during isothermal austenite-to-ferrite transformation was investigated by embedding single crystalline grains of chemically pure B1 compounds in low-alloy steel. Two kinds of B1 compounds having similar misfit against ferrite, TiO and TiN, were adopted in this study. Isothermal transformation experiments were carried out both above and below the bainitic transformation temperature. The orientation relationships between compound and ferrite and between ferrite and prior austenite were characterized by scanning electron microscope equipped with electron backscatter diffraction analyzer.It was found that the Baker-Nutting (B-N) orientation relationship played a dominant role in ferrite formation from B1 compounds at higher transformation temperature, but some low-index orientation relationships other than the Baker-Nutting orientation relationship were also observed as the transformation temperature decreased, which led to an increase in ferrite formation having the Kurdjumov-Sachs (K-S) orientation relationship with prior austenite. Ferrites from TiO had larger misorientation angles from the exact B-N orientation relationship and thus tended to have the K-S orientation relationship with prior austenite at lower transformation temperatures.

Microstructure and precipitates of creep ruptured specimens were investigated in order to understand the effects of carbon (C), nitrogen (N) and nickel (Ni) on long-term creep rupture strength of 10Cr heat-resistant steels containing boron (B). The low-N steels showed higher creep rupture strength than the high-N steels. In long-term creep rupture region such as over 10,000 h at 650°C, the deterioration of creep rupture strength was not observed in the low-N, high-C steel. On the other hand, the creep rupture strength of the low-N, low-C steel dropped to the strength level of the high-N steels. The addition of N to the B containing steels promoted the recovery of microstructure. The formation of coarse BN in the high-N steels led to the decrease of the amount of effective B that dissolved in M23(C,B)6 and suppressed its coarsening. From EDS analysis of precipitates, the fraction of M23C6 in the low-C steels was less than that in the high-C steels, while the fraction of coarse Laves phase in the low-C steels was more than that in the high-C steels. In the low-N, low-C steel, the coarsening of precipitates caused the deterioration of the creep rupture strength after 10,000 h exposure. Ni lowered Ac1 and Ac3 transition temperature, but it did not affect the fraction of precipitates according to the calculation of thermodynamic equilibrium using Thermo-Calc. It is concluded that C and N are more effective to the stability of microstructure than Ni.