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The microstructure of as-quenched martensite in four Fe–C–Cr alloys (0.15C-1Cr, 0.15C-4Cr, 1C-1Cr, 1C4Cr, mass%) has been investigated. Moreover, the microstructures served as input for setting up modeling of carbide precipitation during tempering of martensite. The modelling was conducted using the Langer-Schwartz approach and the software TC-PRISMA, which retrieves thermodynamic data from the Thermo-Calc databank. It was found that the martensite in the low carbon steels is predominantly lath martensite with units arranged parallel to each other. On the other hand, the plate martensite dominates the microstructure in the high carbon steels. The ratio of high-angle to low-angle grain boundaries was found to increase with increasing Cr in the low carbon steels, which indicates that Cr has a similar effect as C on the lath martensite microstructure, however, the micro-hardness remained unaffected by the addition of Cr. Finally, the precipitation modeling clearly demonstrates the importance of proper definition of the initial microstructure for predictive modelling. Parameters such as dislocation density and frequency of high-angle grain boundaries have a drastic effect on e.g. the mean size of carbides.

The effect of uniform distribution of fine cementite on resistance of ultra-high strength steels to hydrogen embrittlement was studied. The materials used were directly-quenched and tempered 1000–1300 MPa class low carbon steel plates for welded structures with lath martensite structure. Cementite morphology was different at different heating rates to tempering temperatures. Finer cementite was distributed in rapidly-heated steels (20°C/s) than in slowly-heated steels (0.3°C/s). The rapidly-heated steels showed higher resistance to hydrogen embrittlement than the slowly-heated steels for a slow strain rate test (SSRT), whereas they showed almost the same resistance to hydrogen embrittlement for a constant load test (CLT). The specimens fractured in a plastic region for the SSRT, on the other hand, the CLT was conducted in an elastic region. The difference in hydrogen embrittlement resistance between plastic and elastic loading methods was concluded to result from a change in the hydrogen trap state at cementite in association with plasticity. Hydrogen is more strongly trapped at and/or around the strained interfaces between the matrix and cementite after plastic deformation. A close observation of fracture surfaces, hydrogen thermal desorption analysis and hydrogen microprint technique revealed that the high resistance of the rapidly-heated and tempered steels to hydrogen embrittlement for the SSRT is due to a shift of the fracture mode from quasi-cleavage fracture to ductile fracture. This shift was caused by the suppression of the quasi-cleavage fracture due to less hydrogen at lath boundaries accompanied by the uniform distribution of fine cementite.

In this study, the development of a lath martensite structure in low carbon high alloy steel was observed in situ using high-temperature laser scanning confocal microscopy. The crystallography of the martensite structure was analyzed using electron backscatter diffraction patterns. It was observed that martensite transformation starts from the prior austenite grain boundary. Then, the variant (Σ1) of another packet belonging to the same bain correspondence was observed in the early stage of martensite transformation. Another block in the same packet was observed in the next stage of martensite transformation. Finally, transformation occurred among the neighbors of the transformed martensite block.

The microstructural evolution of Fe–0.2C–5Mn steel during intercritical annealing with holding time for up to 144 hours was examined by TEM and STEM. It was demonstrated by TEM that the martensite lath structure gradually transformed into a lamellar ferrite and austenite duplex structure. The partitioning of manganese from ferrite to austenite was found by STEM. Typical Kurdjumov-Sachs orientation relationship between austenite lath and ferrite lath was observed by electron back scattered diffraction (EBSD). Based on the analysis of the austenite lath thckening behavior, it was proposed that the Mn-partitioning in austenite dominated the microstructure evolution of the ultrafine lamellar ferrite and austenite duplex structure during annealing process.

Bainite lath nucleated WithIN Grain (BWING) has been clarified to be greatly effective for the increase in both strength and toughness. The nucleation of BWING has been enhanced by the introduction of inclusion into austenite, which would act as nucleation site for BWING. However, the inclusion itself would be brittle and at the interface between inclusion and matrix, high density of dislocations are apt to be accumulated under deformation, inducing cleavage at the interface. As density of inclusion increases, steel would be strengthened, but brittle. Therefore, the introduction of dislocation network, that is, Small-Angle Dislocation Network (SADN) into austenite is tried in the present work. SADN would be stable in the high temperature region such as in austenite, because pinned dislocation existed in SADN would prevent the bow-out of dislocations through absorbing point defects. The formation of SADN in austenite, which act as nucleation site for BWING, was clarified experimentally. The bainite lath neighbor to the nucleated one might be formed under relaxing the stress field around the nucleated one and/or inclusion. It was analyzed that the average size of Aggregates of bainite Laths having nearly Parallel Slip systems between neighboring bainite laths (ALPS) corresponds to that of dimples on fracture surface. The process for ductile fracture is offered based on the crystallographic analysis using transmission electron microscopy.

The inherent brittle mode in dislocated lath martensitic steel is cleavage on {100} planes in the microstructure. The transition to {100} cleavage fracture on cooling determines the minimum value of the ductiule-brittle transition temperature. A half-century of research on the microstructure and toughness of lath martensitic steels has produced a semi-quantitative understanding of the brittle transition to cleavage. The results identify the crystallographic “block” of lath martensite as the effective grain size that controls cleavage, and clarify why the internal structure of a block has the microstructure it adopts. The ductile-brittle transition temperature is strongly affected by the block size. Several effective metallurgical processes are now available to refine the block size without excessive strengthening, leading to martensitic structural steels that combine high strength with good low-temperature toughness.

The temperature dependence of austenite nucleation behavior was investigated in an ultralow carbon 13%Cr–6%Ni martensitic stainless steel. The martensitic structure was partially reversed to austenite by heat treatment at different temperature in (austenite+ferrite) two-phase region. With increasing the reversion temperature, the shape of austenite grains tend to be changed from acicular to granular, and their nucleation site is changed from lath boundaries to prior austenite grain boundaries. The transition of nucleation site was discussed in terms of energetics by considering the increases in interfacial energy and elastic strain energy by formation of an austenite nucleus. The calculation results suggested that lath boundary is more preferential nucleation site rather than prior austenite grain boundary because the increment of elastic strain energy is reduced with lowing the reversion temperature.

The effect of heat treatment on the microstructure and the mechanical properties of a 10%Cr steel with 0.008% boron was examined. The microstructure and the mechanical properties of this steel subjected to the normalizing were studied after tempering under different conditions. The layers of retained austenite are located along the lath boundaries. The formation of M23(B·C)6 phase having film-like shape takes place on interface boundaries of retained austenite/martensite during tempering at 525°C. As a result, the steel exhibits brittle fracture with a low value of Charpy V-notch impact toughness of 6 J/cm2. Particles of the M23(B·C)6 phase are highly resistant against the spheroidizing. The tempering at 770°C only leads to the coagulation of these particles; when the fraction of M23(B·C)6 phase significantly decreases while the fraction of M23C6 carbides increases. The tempered at 770°C steel exhibits a high value of Charpy V-notch impact toughness of 260 J/cm2. The effect of boron additives on the phase composition and the brittleness of high-chromium steels is discussed.

Designed Fe-0.25C-1.5Mn-1.2Si-1.5Ni-0.05Nb (mass%) hot-rolled plates with different thicknesses are subjected to a novel quenching-partitioning-tempering (Q-P-T) process suitable for engineering with water quenching followed by salt bath tempering or water quenching followed by air cooling. A passing underwater quenching equipment (PUQE) is specially designed for realization of the Q-P-T process suitable for engineering. The tensile results show that the Q-P-T samples exhibit both high yield strength of over 900 MPa and elongation of over 15%. Microstructural characterization indicates that the high strength is attributed to the dislocation-type lath martensite or bainite and the dispersively distributed NbC-carbides in the martensite matrix, and the adequate elongation results from the flake-like retained austenite between martensite laths. In addition, effects of the retained austenite in Q-P-T steels on mechanical properties and the feasibility of Q-P-T processes for engineering industry applications are discussed.

The microstructure evolution at interfaces of a layer-integrated steel sheet artificially constructed by ductile austenitic stainless (SUS304) and high-strength martensitic (SCM415) steel layers, which were bonded through a cold-rolling and a subsequent annealing at 1000°C, has been investigated using scanning transmission electron microscopy (STEM) combined with energy dispersive X-ray spectroscopy (EDS). We find that a significant microstructural reconstruction around the SUS304/SCM415 interface has been accomplished during a short-time annealing followed by water-quenching; the resultant microstructures are found to consist of recrystallized austenite and lath martensite grains for the SUS304 and SCM415 layers, respectively. Interestingly, the original SUS304/SCM415 interface appears to migrate and extend into the SUS304 side, an occurrence of which can be reasonably explained by the martensitic transformation across the composition-gradient interface during quenching. These microstructural evolutions fairly account for a microscopic mechanism on how hetero-interface bonding can be achieved via simple cold-rolling/annealing procedures.

To improve the delayed fracture strength of ultra high-strength low alloy TRIP-aided steels with bainitic ferrite matrix (TBF steels), the effects of aluminum content on hydrogen absorption behavior and delayed fracture properties of 0.2%C–0.2–1.5%Si–1.5%Mn TBF steel were investigated. When aluminum was added to the TBF steel, the diffusible hydrogen increased. It was expected that the hydrogen was charged not only in retained austenite films but also on lath boundary. Delayed fracture strength of TBF steels containing aluminum were significantly increased, compared with conventional TBF steel. This was mainly caused by (1) suppression of the stress-assisted martensite transformation resulting from the stabilized or carbon-enriched retained austenite, (2) hydrogen trapping to refined interlath retained austenite films and lath boundary, and (3) relaxation of localized stress concentration by TRIP effect of the retained austenite.

Formable 980–1470 MPa grade ultra high-strength TRIP-aided sheet steels with bainitic ferrite matrix (TBF steels) have been developed for cold forming of automotive applications. Complex additions of 1.0%Al–0.05%Nb–0.2%Mo to 0.2%C–1.5%Si–1.5%Mn base steel increased both the volume fraction and carbon concentration of retained austenite films of the TBF steel, with refining of microstructure. Extremely much retained austenite was realized in the TBF steel austempered at 450–500°C, corresponding to hot-dip galvanization just after annealing. Good combinations of strength and elongation and strength and stretch-flangeability were achieved when the TBF steel was austempered at 450–500°C and 425–475°C, respectively. These good combinations were caused by fine lath structure and a large amount of metastable retained austenite film. It is noteworthy that the TBF steel exhibited an excellent combination of total elongation and stretch-flangeability when austempered at 325–350°C lower than MS.

Metallurgical observation of 9%Cr–1%Mo–0.2%V steel tempered at 750°C for a maximum of 100 h has been made. Accompanying the recovery of martensitic structure, discontinuous changes in hardness, intensity of an X-ray diffraction peak of the matrix, the lattice strain calculated from the integral width of an X-ray peak, the amount of extracted residue and the size of M23C6 were observed when tempering time is around 10 h. These changes are caused by the annihilation of dislocations and the coalescence of martensite lath followed by the formation of subgrains in the later stage of tempering, which is supported by electron back scattered pattern (EBSP) measurements. The ultra fine grains of the order of 0.1 μm were confirmed around martensite lath and block by EBSP, which is obviously correlated with the discontinuous changes in the hardness and the lattice strain. Precipitation of M23C6 on excess dislocations of martensite induces a larger amount of precipitates as compared with the thermal equilibrium, which causes the dissolution of M23C6 during consequent tempering. The dissolution of M23C6 results in slower growth rate as compared with the Ostwald ripening. The observed over-all time exponent is 1/16.

The effect of microstructural factors on hardness was investigated in normalized, tempered, aged and crept materials for Mod. 9Cr-1Mo steel, using nanoindentation and microhardness tests. Nanohardness and microhardness decreased during tempering, aging and creep exposure. Dislocation spacing, lath width, high angle boundary (block and packet boundary) spacing and inter-particle spacing increased during tempering, aging and creep exposure. A converted Vickers hardness was introduced to compare directly nanohardness and microhardness to Vickers hardness. The converted Vickers hardness increased with indent size in all the materials tested. Hardness at an indent size less than 1μm mainly consists of dislocations inside lath grains. Hardness at an indent size larger than 1μm originates from not only dislocation but precipitates and high angle boundaries such as block and packet boundaries. Comparing the converted Vickers hardness with lath width and high angle boundary spacing in normalized material with no precipitates, it was found that the lath boundary does not contribute to hardness. The difference in converted Vickers hardness between tempered and aged material was obviously large at the indent size, greater than inter-particle spacing. The decrease in hardness during aging is caused by increase in inter-particle spacing due to coarsening and coalescence of precipitates. On the other hand, not only changes in precipitates but also increase in high angle boundary spacing and dislocation spacing contribute to decrease in hardness during creep exposure.

Ridging phenomenon was successfully suppressed in a ferritic stainless steel by controlling microstructure through recrystallization of lath martensitic structure. Fe-12Cr-1Ni alloy was quenched after the solution treatment in an austenite single phase region to obtain lath martensitic structure. Cold rolling was performed to the quenched materials up to 80 % reduction in thickness before the annealing for recrystallization. With increasing the reduction by cold rolling, the recrystallization was promoted and ferrite grain size was decreased to 20 μm after recrystallization in the 80 % pre-cold-rolled material. A weak ‹111›//ND recrystallization texture was formed by the cold rolling, but no grain colonies existed in the microstructure. As a result, the materials produced through the recrystallization of lath martensite did not cause ridging during tensile deformation, although an orange peel appeared when the grain size was not refined enough.

Lath martensite structures, tempered at various temperatures (723-923 K) were studied by changing heating rates (2 K/s to 1 000 K/s) to the tempering temperature in an alloy steel for machine structural use (SCM435; Fe-0.35C-0.24Si-0.77Mn-1.05Cr-0.17Mo). Hardness of the rapidly heated (at 100 K/s or 1 000 K/s) specimen is larger than that of the slowly heated (at 2 K/s) specimen when tempering temperature and time are the same. Cementite precipitates are formed on high-angle boundaries (prior austenite grain boundary, block and packet boundaries) as well as within laths and at low-angle boundaries (lath boundaries) by tempering. TEM observation has revealed that finer cementite is dispersed more uniformly in the rapidly heated specimen than in the slowly heated specimen. It is considered that the temperature where cementite precipitation starts is raised by increasing the heating rate to tempering temperature, resulting in a higher nucleation rate and a finer dispersion of cementite.

In Oct. 1998 Chinese government opened national program: Fundamental Research on New Generation of Iron and Steel Materials in China (N.G. Steels). The progress of on-going project has been briefly reported in this paper. In order to get microstructure refinement in structural steels, the following 5 ways have been studied: For Ferrite+Pearlite microstructure in plain carbon steels and low-alloy steels, (1) Purified steel-making→Fully equiaxed continuous casting→Rough-mill with higher Zener-Hollman parameter (y-DRX refinement)→Finishing-mill rolling by Deformation Induce Ferrite Transformation (DIFT) technology. (2) For thin-slab continuous casting and rolling (CSP) process, ultra-fine sulfides (20-60 nm) and oxides (5∼20nm) controlling technology. (3) For Low Carbon Bainite (and Ultra Low Carbon Bainite) microstructure (LCB/ULCB) in microalloyed steels, combination of Deformation Induced Precipitation (DIP) and subsequent middle temperature phase transformation controlling. For quench-temperated martensite microstructure in alloy structural steels: (4) Ultra-fine y grains by innovated heat treatment and alloy design; (5) Carbide-free bainite/martensite microstructure with retained y films along lath/sub-lath boundaries.

Retained austenite characteristics and stretch-flangeability in low alloy TRIP type bainitic sheet steels with different silicon and manganese contents were investigated for automotive applications. As increasing silicon and manganese contents, an initial volume fraction of retained austenite film along bainitic ferrite lath boundary was increased in accompany with a decrease in the carbon concentration. An excellent stretch-flangeability was completed in the steels containing a small amount of stable retained austenites (i.e., volume fraction of 2-4 vol% and carbon concentration of more than 1.0 mass%). This was caused by small surface damage on hole-punching and effective strain-induced transformation plasticity of untransformed retained austenite on hole-expanding. When austempered at temperatures less than M s of the steel after intercritical annealing, further superior stretch-flangeability was achieved due to absence of initial blocky martensite, resulting from developments of long shear section and severe plastic flow and difficult void-initiation on hole-punching.

It is very imperative to improve delayed fracture properties of high strength steel, which may enlarge its usage. The published literature shows that the susceptibility to hydrogen embrittlement of a novel 1 500 MPa bainite/martensite dual-phase high strength steel is inferior to that of conventional quench-tempered high strength steel. The stress corrosion cracking (SCC) in a 3.5% NaCl solution for novel 1 500 MPa bainite/ martensite dual-phase high strength steel was investigated in this paper by using modified wedge-opening-loading (WOL) specimens. The experimental results show that KISCC for novel 1 500 MPa bainite/martensite dual-phase high strength steel is larger than 50 MPa·m1/2, exceeding conventional high strength steel. Its crack growth rate (da/dt)II is about 1×10−5 mm/s, which is less than that of conventional high strength steel. Hydrogen trapping phenomena in the steel were investigated by electrochemical permeation technique. The lath boundaries and stable retained austenite are beneficial hydrogen trap, slowing down the segregation of hydrogen on the crack tip, hence KISCC increases and crack growth rate decreases.

Creep deformation and the corresponding microstructural evolution in high-Cr ferritic steels has been studied using model steels with the initial microstructures consisting of the various combination of a, MX and M 23 C 6 to clarify the role of each precipitate on creep deformation and the resultant creep strength of the steels. A carbon free steel strengthened by the a phase has exhibited high creep resistance at the initial stage of transient creep region but a quick transition to the acceleration creep at a small strain, due to less precipitation along boundaries such as lath, block, packet and the prior austenite grain boundary. Precipitation of M 23 C 6 along these boundaries has been found to delay the transition to the acceleration creep, which gives a larger offset creep strain. Fine dispersion of MX has reduced much the creep rate in the transient creep region but enhances the heterogeneous creep deformation in the acceleration creep region. It is thus concluded that an optimum microstructure consists of fine dispersion of the a phase and MX carbonitride inside lath grain decorated with M 23 C 6 along lath, block, packet and the prior austenite grain boundary.

The recrystallization behavior of lath martensite during tempering was investigated in high-chromium martensitic steels by means of hardness testing, optical and transmission electron microscopy. The role of carbide particles on the recrystallization was also discussed in terms of the grain boundary pinning effect. The hardness of tempered specimens was plotted as a function of the tempering parameter, T(logt+20), for a low-carbon steel (Fe-9Cr-0.1C mass%) and an ultra-low carbon steel (Fe-9Cr-1Ni-0.006C mass%). The low-carbon steel exhibited gradual softening with recovery but did not undergo recrystallization. However, the ultra-low carbon steel suffered abrupt softening owing to the discontinuous recrystallization of lath martensite. Microstructural observations in the ultra-low carbon steel indicated that the recrystallization of lath martensite occurs with the 'bulge nucleation and growth (BNG) mechanism'. The possibility of recrystallization via this mechanism depends upon both the spacing of carbide particles on grain boundaries and the dislocation density of martensite. An energetic analysis on the formation of a recrystallized grain revealed the critical carbide spacing minimum required for the occurrence of recrystallization as a function of dislocation density. In the case of the low-carbon steel, carbide precipitates on grain boundaries with spacing smaller than the critical value, thus suppressing recrystallization.

Effects of thermal cycling on the microstructures and mechanical properties of Fe-15%Ni and Fe-31 %Ni alloys having lath and lenticular martensite, respectively, have been studied. The average width of laths in lath martensite of Fe-15%Ni alloy decreased with the increase in number of thermal cycles, and the width of internal twins in lenticular martensite of Fe-31 %Ni alloy also decreased with thermal cycling. The hardness of martensite increased up to 3 thermal cycles in Fe-31 %Ni alloy, over which it remained constant. On the other hand, the hardness of martensite in Fe-15%Ni alloy increased up to 1 cycle, and remained nearly constant on further cycles. The tensile strength of martensite in Fe-31 %Ni alloy increased up to 3 cycles, showing a similar tendency of the hardness variation. However, the tensile strength of martensite in Fe-15%Ni alloy decreased with increasing the number of thermal cycles in spite of the increase in hardness with thermal cycling. The reason is that no strain hardening occurred in the thermal-cycled specimens owing to the brittle fracture due to the segregation of sulfur at prior austenite grain boundaries.

The susceptibility to hydrogen embrittlement of a bainite/martensite dual-phase high strength steel with different morphologies obtained by the conventional or thermomechanical heat-treatment has been investigated by means of electrolytic hydrogen charging in this paper. The results show that the finer the microstructure, the lower the sensitivity of steel to hydrogen embrittlement is. The fractographic analysis suggests that the fracture mode of the hydrogen-charged specimens is a mixture of quasicleavage and dimple for both treating processes. The quasicleavage facet of the thermomechanical treated specimens is smaller than that of the conventional heated ones. The observation of the fracture profile specimen shows that the crack propagates preferentially along bainite/martensite laths boundary, suggesting that the fracture mode is predominantly lath boundary separation. In addition, the fractographic analysis indicates that the rhombic Ti(N, C) inclusion is the predominant type of inclusions on the fracture surface of the hydrogen-charged specimens of thermomechanical heat-treatment. The separation between Ti(N, C) inclusions and matrixes is a brittle fracture with small facets, which indicates that inclusions such as Ti(N, C) are harmful to hydrogen embrittlement.

Effects of Mo and W on creep strength of low Cr heat resistant steels have been investigated from the stand points of the phase stability of precipitates during long-term exposure at the elevated temperatures. The study on 2.25%Cr-Mo-V-Nb (Mo-steel) and 2.25%Cr-W-V-Nb (W-steel) with a same amount of Mo-equivalent has shown that the substituting W for Mo retards the evolution of microstructure, and thus remarkably improves the creep rupture strength. TEM observations have suggested that the most important precipitates strongly affecting the stability of microstructure are M 6 C type carbide and MC type carbide in both Mo-steel and W-steel. It is found that the M 6 C carbide precipitates with concentration of Mo and W during long-term aging. Therefore, the amounts of solute Mo and W supersaturated in matrix have reduced to the thermal equilibrium levels with changing the substructure from bainite lath structure to sub-grain liked structure. The kinetics of Mo- and W-partitioning between matrix and M 6 C carbide has been successfully expressed by a Johnson-Mehl-Avrami type equation and estimated the activation energy for diffusion of solutes; 125 kJ/mol in Mo-steel and 219 kJ/mol in W-steel. Thus the diffusion rate of W is suggested to be definitely lower than that of Mo. Furthermore, Mo- and W-partitioning to MC carbides have also been confirmed to affect the coherency and the growth rate of MC carbides. TEM observations on the strain image have shown that the MC carbide in Mo-steel has already lost the coherency with matrix. On the other hand, the finer MC carbide in W-steel has kept the coherent relationship with matrix even after long-term aging. It is concluded that W-steel is superior to Mo-steel in the stabilization of bainite lath structure and precipitates at elevated temperatures, resulting in the higher creep strength.

Low-carbon steel weld with a high density of oxide inclusions prepared using a experimental metal-cored wire has been examined to study the effect of inclusion size on the formation of acicular ferrite, and to understand the role of inclusion in the nucleation of ferrite lath. Depending on the ferrite morphology associated with inclusions, a total of 282 inclusions observed under TEM could be classified into two groups, i.e. the non-nucleant and the nucleant. Experimental results showed that the group of inclusions acted as nucleant were appreciably larger in size compared with those of non-nucleant resulting in the increased probability of nucleation with the increase of inclusion size, even though the chemical and structural natures appeared to be the same. The group of nucleant-inclusion was further divided into two types depending on the degree of nucleation, which was evaluated by the number of ferrite lath nucleated. Statistical analysis performed on inclusion size indicated that the larger the inclusion size is the more ferrite laths could be nucleated. Those laths nucleated from a large single inclusion have grown in many different radial directions and mostly had a different crystallographic orientation from those of adjacent ferrite laths. As a result of this study, it is demonstrated that larger inclusions are indeed more potent nucleation sites when compared with those of smaller size. Thus it could be concluded that the provision of the inclusion surface as for the inert surface for the heterogeneous nucleation of acicular ferrite lath would be the principal role of inclusions playing in the weld metal of low alloy steels. Other possible mechanisms were also considered, but they were unlikely to be operated in the present weld metal system.

The stretch-flangeability of a newly developed high-strength bainitic sheet steel which is associated with the transformation-induced plasticity (TRIP) of retained austenite, or "TRIP type bainitic steel" was investigated for the automotive applications. An excellent stretch-flangeability was completed in the steel composing of bainitic ferrite matrix and interlath retained austenite films without initial blocky martensite. In this case, the stable or carbon-enriched retained austenite films enhanced the stretch-flangeability due to the reducion of the surface damage on hole-punching and the promotion of the TRIP effect on hole-expanding. Also, uniform fine bainitic ferrite lath structure contributed to improving the stretch-flangeability due to the increased localized ductility and development of severe plastic flow.

Retained austenite characteristics and tensile properties in a 0.2C–1.5Si–1.5Mn, mass%, high-strength cold-rolled "TRIP type bainitic steel" which was associated with the transformation induced plasticity (TRIP) of the retained austenite were investigated. The steel mainly consisted of bainitic ferrite lath matrix, blocky martensites and stable retained austenite films of 5–12 vol%. When austempered at temperatures above MS temperature, the steel possessed high tensile strength of 900 MPa, large total elongation of 15–20 % and large reduction of area of 40–60 %. The good ductility was mainly owing to uniform fine lath structure, initial martensite and the TRIP effect of retained austenite, as well as a small contribution of long range internal stress resulting from untransformed retained austenite films.

The effect of trace elements such as Mn and B on creep properties of 0.06C-2.25Cr-1.6W-0.1Mo-0.25V-0.05Nb has been investigated from the standpoints of a long-term microstructural stability. The chemical analysis of extracted residues and TEM observation show that the M23C6 carbide and/or M7C3 carbides, precipitated by tempering are replaced by M6C with a concentration of W; i.e., the amount of W in solution reduces during creep. On the other hand, MX type carbides such as VC and NbC are very stable during long-term aging and contribute to the creep strength by obstructing the dislocation annihilation. One of the most significant results is that a reduction in Mn-content lowers the minimum creep rate, resulting in an increase in the creep rupture time. The nucleation and/or growth of M6C are retarded with reducing Mn-content, thereby the increase in dissolved W seems to enhance the resistance to creep deformation. Another significant result is that an increase in B-content has delayed the transition from the primary creep to a tertiary creep stage. The principal roles of B are stabilizing M23C6 or M23(C, B)6 on former austenite grain boundaries and retarding the dynamic recrystallization during creep. In addition, in the specimens with higher amount of B, the bainite lath interface is covered by MX and some filmy precipitates with high density, thereby the softening resistance is enhanced.

This work was done to clarify the metallurgical mechanisms that are used in thermal treatments to refine the effective grain size of lath martensitic steels. The alloy chosen for this work was Fe–12Ni–0.25Ti, which provides a well-defined lath martensite structure in the as-quenched condition. The alloy was given four prototypic heat treatments: (1) an anneal-and-quench treatment, (2) an intercritical anneal, (3) a single or double anneal in the austenite range, and (4) a single or double reversion to austenite at a rapid heating rate. Two effective grain sizes were identified for each heat treatment: the packet size, or dimension over which adjacent martensite laths have a common crystallographic orientation, and the prior austenite grain size. The former controls the ductile–brittle transition temperature (DBTT), the latter controls intergranular embrittlement. The different heat treatments suppress the DBTT and increase resistance to intergranular embrittlement to the extent that they decrease the relevant grain size. Rapid reversion is the most successful of the heat treatments since it destroys the alignment of laths within a packet. Rapid reversion readily yields an effective grain size near 1 μm, leading to a very low ductile–brittle transition temperature and a substantial resistance to temper or hydrogen embrittlement.

The processes of upper bainite formation, the crystallographic aspects and the internal structure in a 1.83% silicon steel have been investigated. The T-T-T diagram is separated into two C-curves as in the case of steels containing strong carbide forming elements. In the early stage of upper bainitic transformation, very fine needlelike ferrite subunits with the parallelogram cross sections elongated in a γ // α direction form on a{111} γ sheet in a side-by-side fashion. In the later stage of transformation, these subunits are coalesced on a{111} γ plane and produces upper bainite laths. This bainite is always related to the parent austenite with the Kurdjumov-Sachs relationship and accompanies the surface reliefs. e carbide particles precipitate mainly after the ferrite subunits formation or after the coalescence of them. Therefore, the carbide precipitation is not a fundamental characteristic of bainite transformation but is a secondary effect. e carbide needles precipitating within a bainite lath by long time holding are aligned almost parallel to the α with almost constant intervals and are related to the ferrite by the Jack relationship. These aspects are exactly similar to those in tempered martensite. Such an e carbide configuration is thought to arise from the precipitation on the dislocations introduced during transformation.

A time-temperature-transformation (TTT) diagram has been determined for a copper-bearing steel, whicn is a low alloy TMCP variant of the ASTM A710 type of structural steel. Quantitative mesurements have been supplemented by optical microscopy and transmission electron microscopy to investigate the isothermal transformation behaviour as well as the associated precipitate morphologies. It is shown that the kinetics and product phases of the polymorphic transformation and precipitation reactions are sensitive to both temperature and time. The TTT-diagram shows a prominent transformation region for bainitic structures, at temperatures intermediate between those of polygonal ferrite and martensite. In the intermediate region, the microstructures were characterised by a ferritic matrix with a lath and/or plate shaped grains containing a high dislocation density, together with a minor dispersed "island" phase. For a short holding time (5 sec) at intermediate temperatures (580-430°C), the island phase was identified as untempered twinned and lath martensite, autotempered twinned and lath martensite, and martensite/austenite constituent, depending on the level of carbon partitioning in the remaining austenite before quenching in water. For a longer holding time, the carbon enriched austenite regions decomposed to carbide and ferrite by coupled growth. Polygonal and quasi-polygonal ferrite were formed at relatively high transformation temperatures and these microstructures contained a low dislocation density and were associated with interphase e-copper precipitates.

Fe-0.002%C, Fe-0.01%C, Fe-1.5%Mn-0.001%C and Fe-1.5%Mn-0.01%C steels were vacuum-melted and examined. Fe-0.05% and Fe-1.5%Mn-0.05%C steels were also melted and used for comparison. The effects of cooling rate and chemical composition on microstructures were examined by optical and electron microscopy. Autoradiography of boron was also performed in order to know relation between γ and α grain boundaries and effects of microstuctures on discontinuous yielding were also examined. The fraction of quasi-polygonal ferrite (αq) increases with the cooling rate and the contents of C and Mn. Dislocation density in αq is higher than that in polygonal ferrite (αp) and dislocations tend to form cell structures in αq matrix, whereas dislocations which do not form cell structures were also observed. Normal cooling rate dependence of ferrite grain size was not observed in Fe-0.002%C and Fe-1.5%Mn-0.001%C steels. By autoradiography of boron it was known that not a small fraction of grain boundaries of ferrite might be coherent. Even in the ultra-low carbon steels cooled at 360°C/s, cementite was observed on ferrite grain boundaries. Discontinuous yielding was suppressed by bainite transformed during continuous cooling, while Mn enhanced discontinuous yielding. Bainite and martensite could be observed in he specimens of Fe-0.002%C and Fe-1.5%Mn-0.001%C steels cooled at the rate higher than 360°C/s. In such specimens, the continuous change from αq to bainite was observed. Martensite can be distinguished from bainite by its higher microhardness and thinner lath width.

Effects of deformation in unrecrystallized austentie region on bainite microstructure were investigated in low carbon Nb-B bearing steels. Particular emphases were placed on the variation of the morphology of bainitic ferrite in isothermally transformed specimen.SEM observation shows that, in the case of non-deformation, a number of straightly elongated bainitic ferrite laths form parallel from austenite grain boundaries, showing an aspect of typical upper bainite structure. In the case of 30%-deformation, although bainitic ferrite laths are shaped like curves of bows, the deformation has a llittle influence on their length. In the case of 50%-deformation, on the contrary, the length is significantly decreased in the growth direction of the lath. Furthermore, the number of bainitic ferrite lath in a "bainite packet" (an aggregate of bainitic ferrite laths with same crystallographic orientation) is decreased by the enhancement of the nucleation within austenite grains. The decrease of both the length and the number of bainitic ferrite lath leads to complicate the appearance of a microstructure.TEM observation, however, confirmed that those bainitic ferrite laths were also surrounded by two sets of parallel planes close to {451}α in the case of 50%-deformation, as previously reported for typical bainitic ferrite laths in the case of non-deformation. Therefore it suggests that bainitic ferrite laths transformed from heavily deformed austenite has also the same crystallographic characteristics with typical one.

Widmanstatten austenite laths forming from δ-phase in a δ/γ duplex phase stainless steel have been investigated metallographically. The austenite laths form in the temperature range between 1100 and 750°C with a C-curve in a T-T-T diagram.The formation of Widmanstatten austenite laths accompanies a sharp surface relief similar in appearance to that of martensite. Widmanstatten laths formed at 1000°C where relatively large alloy partition occurs do not change the morphology by the further isothermal holding. Those formed at lower temperatures, 900 and 800°C, where the partition is relatively small, however, decompose into the rows of small austenite fragments, and the alloying elements are partitioned quite largely between the γ fragments and the δ-matrix. The driving force for this interphase boundary migration producing the γ-fragments is thought to arise from the partial supersaturation of alloying elements in the initially formed laths. The interphase boundary migration with alloy partition does not produce additional surface reliefs, suggesting that a pure diffusional transformation does not induce surface reliefs. Such an austenite lath formation can be explained consistently in terms of a shear-assisted diffusional transformation model where the lattice change occurs via a diffusional individual atomic jumps and the resulting elastic strain is relaxed by lattice invariant shear.

The microstructure of two HY-130 steel weldments prepared by submerged-arc welding (SAW) and gas-metal-arc welding (GMAW) processes using identical filler wires were investigated. Light and electron microscopy along with microhardness traverses were used to characterize and compare the microconstituents present in the base metal, and weld metal, and heat affected zone (HAZ) of both weldments. The base metal consisted of a bainite-martensite microstructure and was uniform through the plate thickness. The weld metal and HAZ structures consisted of a mixture of lath and twinned martensite, bainite, autotempered martensite, and retained austenite. The GMAW weld metal exhibited a finer lath martensite with more interlath retained austenite and transformation microtwins. The SAW weld metal structure was more bainitic and contained higher inclusion densities as a result of the flux used to make the joint. Microhardness traverses exhibited higher hardness values in the GMAW weld metal compared to the SAW. The HAZ microhardness followed a similar trend for both weldments and showed a maximum value in the fine-grained region near the base metal.

The effect of austenitizing temperature were investigated on the microstructure and mechanical properties of 12%Cr steel. Low-temperture austenitizing below 1000°C induced the carbide coarsening during subsequent tempering at 750°C for 1 hr due to the nucleation effect of undissolved M23C6. The large and spheroidized carbides enhanced the subgrain growth. On the other hand, the complete dissolution of M23C6 above 1000°C caused the fine carbide formation on lath boundaries, which retarded the subgrain growth during tempering. Furthermore, the dissolution of Nb(C, N) above 1100°C enhanced the tempering resistance through increasing the stability of lath morphology and reducing the growth rate of M23C6. The increase in strength with increasing austenitizing temperature was attributed to the fine carbide distribution and the high dislocation density. Further, as the austenitizing temperature increased, the impact energy markedly reduced, due to the large proir austenite grain size and the high strength.

The influence of cooling rate on transformation temperatures in low carbon irons is reviewed. Plateaux are obtained in plots of transformation temperature vs.cooling rate. Comparison with later published TTT diagrams show that these plateaux correspond with the nose of 'c' curves on TTT diagrams. This leads to a proposed TTT diagram for iron. Transformations in Fe-Ni, Fe-Cr and Fe-C alloys, and low carbon irons are reviewed. The morphology and kinetics of the individual transformations, namely equi-axed ferrite, massive ferrite, bainitic ferrite, lath (massive) martensite and twinned martensite are then discussed. A recent theoretical treatment of the equi-axed ferrite transformation is summarised. This theory indicates that the kinetics of the equi-axed ferrite transformation is controlled by growth. Correlation is made between microstructures obtained in binary alloys and those obtained in Fe-Nb-C alloys and commercial HSLA steels.

The relationship between second phase morphology and retained austenite morphology and the influences of these two kinds of morphology on tensile properties of a 0.17C-1.41Si-2.00Mn (mass%) TRIP-aided dual-phase steel have been investigated in a temperature range between 20 and 400°C.A large amount of fine retained austenite was obtained when the second phase morphology was "a network structure" or "an isolated fine and acicular one." The retained austenite particles were nearly isolated in the ferrite matrix away from bainite islands and were moderately stable. On the other hand, "an isolated coarse structure" of second phase resulted in a small amount of more stable retained austenite film along bainite lath boundary.The influence of second phase morphology on the flow curve significantly differed from that of a conventional ferrite-martensite dual-phase steel. Isolated retained austenite particles lowered the flow stress, and resultantly reduced the effects of second phase morphology (i.e., network effect or fine grain size effect) on flow stress. However, the isolated retained austenite particles enhanced effectively the ductility, particularly at 50-100°C, due to the moderate strain induced transformation. On the other hand, retained austenite films along bainite lath boundary scarcely influenced on tensile properties of the steel. These results were discussed on the basis of a continum theory.

Reversion mechanism from deformation induced martensite (α') to austenite (γ) has been investigated in two metastable austenitic stainless steels, 15.6%Cr-9.8%Ni (the 16Cr-10Ni) and 17.6%Cr08.8%Ni (the 18Cr-9Ni) steels, by means of magnetic analysis and transmission electron microscopy. Metastable γ almost completely transforms to lath α' by 90% cold rolling, and the α' again reverts to γ during annealing at temperatures above 700 K. Deformation induced α' in the 16Cr-10Ni steel undergoes a martensitic shear reversion during heating to 923 K annealing, while that in the 18Cr-9Ni steel does a diffusional nucleation-growth reversion on 923 K annealing. Grain refining processes are greatly influenced depending on the reversion mechanism. Martensitically reversed γ has a high density of dislocations immediately after the reversion and the γ grains are refined through recovery and recrystallization process just like that taking place in a deformed γ. On the other hand, diffusionally reversed γ is characterized by the nucleation of equiaxed γ grains within the α' matrix and the γ grains gradually grow during annealing.The reversion mechanism significantly depends on the chemical compositions of steels and annealing temperature. An increase in the Ni/Cr ratio causes an increase in the Gibbs free energy change between fcc and bcc structure, leading to a fall-down of austenitizing temperature for the martensitic shear reversion. The critical driving force required for the complete martensitic shear reversion is about -500 J/mol. To obtain the critical driving force in the 18Cr-9Ni steel, it should be heated to a high temperature above 1 023 K. However, the diffusional reversion can easily occur because the martensitic shear reversion temperature is too high in the 18Cr-9Ni steel. The 16Cr-10Ni steel also undergoes the diffusional reversion when it was annealed at low temperatures below the martensitic shear reversion, 923 K.