Acicular Ferrite Laths Research Articles

Tuning heterogeneous microstructure and improving mechanical properties of twin-roll strip cast low carbon steels by on-line heat treatments

Coarse austenite grains are unavoidable in the low carbon steels processed by twin-roll strip casting. Unfortunately, the polygonal ferrite (PF) grains cannot be refined by conventional on-line cooling process, resulting in low strength. In this work, an on-line heat treatment including reaustenitizing and subsequent controlled cooling was performed in laboratory to form heterogeneous microstructures composed of PF and acicular ferrite (AF). It has been founded that increased prior austenite grain size and the cooling rate led to more AF laths, which inhibited the growth of PF grains. When the prior austenite grain size is less than 60 μm, the formation of AF is suppressed. The tensile tests indicate that the heterostructured steels exhibited higher yield strength than the samples without AF. The improvement of yield strength can be attributed to the grain boundary strengthening, dislocation strengthening and back stress strengthening. The higher AF volume fraction leads to higher dislocation density in PF grains, and more AF/PF interfaces may increase the amount of accumulated geometrically necessary dislocations. Therefore, the contributions of dislocation and back stress strengthening increase with the increase of AF volume fractions. High dislocation density and the generation of back stress contribute a high strain hardening rate to delay the necking and sustain good plasticity. Moreover, the heterostructured steels exhibit the two-stage work hardening behavior during tensile test, which indicates that plastic deformation starts in PF grains while the AF laths remain in elastic state, and then plastic deformation occurs in both strain hardened PF grains and AF laths.

Effect of cooling route on microstructure and mechanical properties of twin-roll casting low carbon steels with an application of oxide metallurgy technology

The oxide metallurgy was employed in the fabrication of twin-roll strip casting low carbon steels. The quantitative dependences of the microstructure and mechanical properties on cooling parameters were revealed and emphatically investigated. After twin-roll strip casting, a two-step cooling route closed to the actual process was designed and the cooling rates exhibited great influences on the proportion of polygonal ferrite (PF) and acicular ferrite (AF). With increasing cooling rates in both steps, the AF volume fraction raised from 0.04 to 0.75 because the rapid cooling restrained the formation of PF. The AF laths were induced by fine Al–Si–Ti–Mn composite oxides (0.2–2 μm), leading to a heterogeneous microstructure consisting of PF and AF clusters with high proportion of large-angle (≥15°) grain boundaries. It was found that the AF laths kept K–S relationship with austenite and AF laths with collinear growth direction belong to the same codirectional variants group in one AF cluster. With the increase of AF volume fraction, the yield strength (YS) and the ultimate tensile strength (UTS) were improved, while the strip retained excellent elongation. The improved YS was mainly ascribed to the grain boundary strengthening and dislocation strengthening.

Effect of microstructure and grain size on DWTT properties of 22 mm thick X80M hot rolled steel strip

Aiming at improving the performance of 22 mm thick X80M hot rolled steel strip DWTT performance, simulated tests and industrial trials were carried out on a 550 mm test mill and a 2250 mm hot tandem mill. The effects of different starting temperature and the end temperature of finish rolling, cooling rate, coiling temperature on the microstructure and DWTT properties of the test steel were studied. The effects of austenite grain size on DWTT properties were studied by comparing the rough rolling reduction rate and the accumulative deformation rate in the non-recrystallized zone. The results show that uniform and fine original austenite grains can obtain uniform, and fine acicular ferrite lath bundles. The microstructure of acicular ferrite, granular bainite and a large number of fine M/A islands dispersed on ferrite matrix with high density dislocations has better DWTT performance. Meanwhile, the average effective grain size of X80M pipeline steel can be refined to obtain more grain boundary area per unit volume. Large angle grain boundary is beneficial to prevent crack propagation. Improving grain size of X80M pipeline steel strip is an effective measure to improve fracture toughness. Under this chemical composition, the final pass reduction ratio in roughing mill of X80M pipeline steel is ≥26%, the thickness of intermediate slab is 68 mm,the starting temperature of finish rolling is 900 °C, the end temperature of finish rolling is 810 °C, the cooling rate is 25 °C/s, the coiling temperature is below 400 °C, and DWTT toughness fracture ratio can reach more than 85%, which can meet the application requirements of pipeline steel for the extreme conditions of −15 °C in the west-east gas pipeline.

In situ observation of growth behaviour of acicular ferrite in low-carbon steel containing 13 ppm magnesium

ABSTRACTSS400 steel with 13 ppm magnesium was prepared to study the effects of supercooling degree on the formation behaviour of acicular ferrite (AF) and microstructures of Mg-containing low-carbon steel. The inclusions characterised using an automated inclusion analyzer, ASPEX, were mostly MnS, MgAl2O4 and MgO with sizes of 1–2 μm. The growth behaviour of AF during the cooling process for austenite transformation to ferrite and continuous cooling temperature diagram was investigated using in situ observation with high-temperature confocal laser scanning microscopy. The initial formation temperature of AF decreased with increasing cooling rate. The temperature of AF transformation was in a narrow range of around 100–150 K. The probability of AF nucleation from inclusions and the average AF lath growth rate increased with increasing cooling rate due to the larger driving force and thermal strain energy around the inclusions during the cooling processes.

Toughness improvement by Cu addition in the simulated coarse-grained heat-affected zone of high-strength low-alloy steels

The effect of microstructure and Cu addition in a simulated coarse-grained heat-affected zone (CGHAZ) of a high-strength low-alloy (HSLA) steel subjected to 100 kJ cm−1 heat input welding was studied. It has been observed that the primary microstructure in Cu-free HSLA steels is dominated by bainite, whereas, in Cu-bearing HSLA steels, the predominant microstructure is acicular ferrite. The acicular ferrite nucleated at intragranular complex inclusions consisting of Al and Ti oxides, covered with layer of MnS and CuS. The presence of high intensity of acicular ferrite and hard impingements between acicular ferrite laths or plates has contributed to the fine-grained and interlocked microstructure. The enhanced toughness in CGHAZ of Cu-bearing HSLA steel is attributed to the fine-grained interlocked microstructure of acicular ferrite.

In situ observation of acicular ferrite formation and grain refinement in simulated heat affected zone of high strength low alloy steel

The phase transformation from austenite to acicular ferrite in the simulated coarse grained heat affected zone of a high strength low alloy steel was investigated by means of analytical characterisation techniques such as in situ microscopy, transmission electron microscopy and electron backscattered diffraction analysis. The acicular ferrite grains nucleated on inclusions (Zr–Ti oxides) in coarse austenite grain grew in different directions and effectively partitioned coarse austenite grain into several finer and separate regions. The crystallographic grain size became small for coarse austenite grains due to the effective partitioning by acicular ferrite laths or plates.

Effect of hot-wire on microstructure and mechanical property in weld metal formed with CO2 gas shielded arc welding method

The new welding method known as F-MAG, which has been developed based on the CO2 gas-shielded arc welding method (MAG), increases deposition rate of weld metal. The F-MAG welding method is combined with hot-wire, as has been used in TIG arc- and laser-welding methods. F-MAG is performed using hot-wire which is inserted into the rear part of the weld pool made with the leading electrode. Hot-wire melts by both electrical heating of itself and heat of the weld pool. Multi-layered weld metals were prepared using F-MAG and MAG. Microstructures and mechanical properties of both as-weld and reheated zones in the uppermost layer of the multi-layered weld metals formed with F-MAG and MAG were examined and the effects of hot-wire on microstructures and mechanical properties were analysed. Both the as-weld and reheated zones of the weld metal formed with F-MAG consisted of acicular ferrite (AF), equiaxed ferrite and so on. Both strength and elongation in as-weld and reheated zones formed with F-MAG were superior compared with those formed with MAG. It could be suggested that strength increased by refinement of AF due to increase in the concentration of alloying elements being contained in the hot-wire. Larger elongation of the weld metal in F-MAG compared with that in MAG could be analysed in terms of the Aggregate of AF Laths with nearly Parallel Slip systems between neighboring AF laths (ALPS). The number of AFs contained in an ALPS formed with FMAG is larger than that with MAG, in spite of the sizes of ALPSs formed with MAG and F-MAG being almost same. Deformation occurs over a lot of AFs in the case of finer AF formed with F-MAG through the rotation of the tensile test piece during deformation, resulting in the larger elongation.

In Situ Observations of the Formation of Fine‐Grained Mixed Microstructures of Acicular Ferrite and Bainite in the Simulated Coarse‐Grained Heated‐Affected Zone

The formation of fine‐grained mixed microstructure of acicular ferrite and bainite in the simulated coarse‐grained heated‐affected zone was investigated by means of in situ observation and EBSD analysis. Single, multiple, and sympathetic nucleation of acicular ferrite were observed. Acicular ferrite laths or plates were formed prior to bainite and effectively partitioned austenite grains into many smaller and separated regions. The later formed bainite at lower temperature was confined in those smaller regions, thus resulting in the fine‐grained mixed microstructures of acicular ferrite and fine bainite plate.

CO<sub>2</sub>ガスシールドアーク溶接金属部の微細組織と機械的性質に及ぼすホットワイヤの効果

The new welding method called as F-MAG, which has been developed based on CO2 gas-shielded arc welding method (MAG) increase deposition rate of weld metal. F-MAG welding method is combined with hot-wire, as having been used in TIG arc- and laser-welding methods. F-MAG is performed using hot-wire which is inserted into the rear part of weld pool made with leading electrode. Hot-wire melts by both electrical heating of itself and heat of weld pool.Multi-layered weld metals were prepared using F-MAG and MAG. Microstructures and mechanical properties of both as-weld and reheated zones in uppermost layer of the multi-layered weld metals formed with F-MAG and MAG were examined and the effects of hot-wire on microstructures and mechanical properties were analyzed.Both the as-weld and reheated zones of weld metal formed with F-MAG consisted of acicular ferrite (AF), equiaxed ferrite and so on. Both strength and elongation in as-weld and reheated zones formed with F-MAG were superior compared with those formed with MAG. It could be suggested that strength increased by refinement of AF due to increase in the concentration of alloying elements being contained in hot-wire. Larger elongation of weld metal in F-MAG compared with that in MAG could be analyzed in terms of the Aggregate of acicular ferrite Laths with nearly Parallel Slip systems between neighboring acicular ferrite laths (ALPS). The number of AFs contained in an ALPS formed with F-MAG is larger than that with MAG, in spite of the sizes of ALPSs formed with MAG and F-MAG being almost same. Deformation occurs over a lot of AFs in the case of finer AF formed with F-MAG through the rotation of tensile test piece during deformation, resulting in the larger elongation.

The formation mechanisms of interlocked microstructures in low-carbon high-strength steel weld metals

Microstructural features and the formation mechanisms of interlocked microstructures of acicular ferrite in a low-carbon high-strength steel weld metal were investigated by means of computer-aided three-dimensional reconstruction technique and electron backscattered diffraction analysis. Multiple nucleation on inclusions, sympathetic nucleation or repeated nucleation, hard impingement, mutual intersection, and fixed orientation relationships of acicular ferrite grains were observed. They were all responsible for the formation of interlocked microstructures in the weld metal. During the process of isothermal transformation, the pre-formed acicular ferrite laths or plates partitioned austenite grains into many small and separate regions, and the growth of later formed acicular ferrite grains was confined in these small regions. Thus, the crystallographic grain size became smaller with the increasing holding time. ► Acicular ferrite is formed by multiple nucleation and sympathetic nucleation. ► Hard impingement and intersection of ferrite grains occur at later stages. ► The pre-formed ferrite laths partition austenite grains into smaller regions. ► The growth of later formed ferrite grains is confined in the smaller regions.

Grain refinement in the coarse-grained region of the heat-affected zone in low-carbon high-strength microalloyed steels

The microstructural features and grain refinement in the coarse-grained region of the heat-affected zone in low-carbon high-strength microalloyed steels were investigated using optical microscopy, scanning electron microscopy, and electron backscattering diffraction. The coarse-grained region of the heat-affected zone consists of predominantly bainite and a small proportion of acicular ferrite. Bainite packets are separated by high angle boundaries. Acicular ferrite laths or plates in the coarse-grained region of the heat-affected zone formed prior to bainite packets partition austenite grains into many smaller and separate areas, resulting in fine-grained mixed microstructures. Electron backscattering diffraction analysis indicates that the average crystallographic grain size of the coarse-grained region of the heat-affected zone reaches 6–9 μm, much smaller than that of austenite grains.

Effect of acicular ferrite formation on grain refinement in the coarse-grained region of heat-affected zone

The microstructure of acicular ferrite and its formation for the grain refinement of coarse-grained region of heat-affected zone of high strength low-alloy bainite steels were studied using three-dimensional reconstruction technique. Crystallographic grain size was analyzed by means of electron backscatter diffraction. It was revealed that the microstructure in the coarse-grained region of the heat-affected zone consisted of predominantly bainite packets and a small proportion of acicular ferrite. Acicular ferrite was of lath or plate-like rather than needle or rod-like morphology. Tempering of the coarse-grained region of heat-affected zone showed that the acicular ferrite was more stable than the bainite, indicating that the acicular ferrite was formed prior to bainite. The acicular ferrite laths or plates divided the prior austenite grains into smaller and separate regions, and confining the bainite transformed at lower temperatures in the smaller regions and hence leading to the grain refinement in the coarse-grained region of the heat-affected zone.

The role of molybdenum additions and prior deformation on acicular ferrite formation in microalloyed Nb–Ti low-carbon line-pipe steels

Microstructures in Nb–Ti-microalloyed line-pipe steels with various molybdenum additions, consisted mostly of acicular ferrite plus polygonal ferrite after hot rolling and rapid cooling. Structure-sensitive surface relief after etching on shadowed extraction replicas, allowed quantification of the acicular and polygonal ferrite contents. Continuous cooling transformation diagrams of two alloys, one Mo-free and the other containing 0.22% Mo, were determined for cooling rates from 0.1 to 40 °C s − 1 without and with prior deformation of the austenite below the nil-recrystallisation temperature. Molybdenum additions slightly enhanced the acicular ferrite formation in the strain-free austenite whereas prior deformation had a much greater effect, and strongly promoted acicular ferrite formation in both alloys. Thin foil electron microscopy of acicular ferrite in these low-inclusion content alloys showed a preference for parallel acicular ferrite laths with less “chaotically” nucleated laths.

X80ラインパイプ用鋼管のGMAWおよびSMAW周溶接部の機械的性質および金属組織に及ぼす入熱量の影響

This study evaluated the weld joint performance of X80 line pipes welded using fully automatic Gas Metal Arc Welding and manual Shielded Metal Arc Welding in order to clarify welding conditions for yield stress overmatching with adequate fracture toughness and hardness taking into account the distribution of the yield stress of line pipes. In addition, we conducted metallurgical surveys of the weld metal to clarify factors dominating the yield stress of the weld metal.The results clarified that the welding conditions applicable to X80 line pipes were the heat input range of between 0.9 and 1.3 kJ/mm using solid wires classified as YGW24 and ER100S-G for GMAW and that of between 1.6 and 2.4 kJ/mm using electrode classified as E11016-G for SMAW. The metallurgical surveys revealed that the yield stress of the weld metal was dependent on the acicular ferrite lath width and percentage of the grain boundary ferrite, ferrite side plate and acicular ferrite. The heat input dependency of the yield stress of the weld metal could be explained from the viewpoint of the acicular ferrite lath width which was the main microstructure.

ＭＡＧ溶接によるパイプライン用鋼Ｘ８０溶接金属の強度特性および金属組織に関する基礎的検討

Tensile properties and microstructure of weld metal of X80 plates were evaluated. X80 plates were welded using full-automatic Gas Metal Arc Welding (GMAW) under several welding conditions. To clarify effects of welding conditions on weld joint performance in X80 pipeline, strength, toughness and hardness of weld joints were evaluated. Grain size and microstructural component of the weld metal were measured. Overmatching for the specified minimum yield stress (SMYS) of X80 was obtained for all the welding conditions. If distribution of the yield stress of X80 base metal was taken into consideration, however, welding conditions to obtain overmatching were limited. The strength and hardness of the weld metal decreased as the heat input increased. This was due to the change of acicular ferrite lath width and percentage of grain boundary ferrite and ferrite side plate. On the other hand, Charpy impact energy was high, irrespective of the welding conditions.

Acicular ferrite microstructure in titanium bearing low carbon steels

Different microstructures having acicular ferrite as the major phase but with various types and amounts of microphases were obtained by applying different cooling processes to C–Mn steels containing fine non-metallic inclusions. Optical and electron microscopy were carried out to identify the various microphases in the acicular ferrite microstructure, and their mechanical properties were measured and compared to study the effect of the microphases on the microstructure–properties relationship in C–Mn wrought steels. The existence and increase of the fraction of small isolated martensite between the acicular ferrite laths were found to play an important role in determining the tensile strength and low temperature impact toughness of the steels. However, the elongation and room temperature impact toughness were rather insensitive to the microphases. This may be attributed to the uniform distribution and isolation of relatively small martensite due to the fine interlocking character of the acicular ferrite microstructure.

Influence of heat input and travel speed on microstructure and mechanical properties of double tandem submerged arc high strength low alloy steel weldments

An evaluation is presented of the microstructural characteristics and mechanical properties of welds in 20 mm thickness high strength low alloy steel HSLA 80, of Australian manufacture. In total, nine butt joints were prepared using the double tandem (four wire) submerged arc welding process in which both heat input and travel speed were varied. The inclusion size distribution was determined for selected welds and showed that heat input had a major effect. Colour etching techniques were used to reveal the solidification structure, which in turn correlated with welding parameters. As the heat input increased, the cooling rate decreased resulting in a larger cellular dendritic cell spacing, decreased acicular ferrite content, and coarser acicular ferrite laths. The effect of travel speed on delta ferrite cell spacing and prior austenite grain size was found to be co-dependent on the heat input and the thermal profile resulting from multiple electrode welding. These results show that increased deposition rates can be achieved by increasing the travel speed and current density without sacrificing joint quality.

Influence of heat input and travel speed on microstructure and mechanical properties of double tandem submerged arc high strength low alloy steel weldments

An evaluation is presented of the microstructural characteristics and mechanical properties of welds in 20 mm thickness high strength low alloy steel HSLA 80, of Australian manufacture. In total, nine butt joints were prepared using the double tandem (four wire) submerged arc welding process in which both heat input and travel speed were varied. The inclusion size distribution was determined for selected welds and showed that heat input had a major effect. Colour etching techniques were used to reveal the solidification structure, which in turn correlated with welding parameters. As the heat input increased, the cooling rate decreased resulting in a larger cellular dendritic cell spacing, decreased acicular ferrite content, and coarser acicular ferrite laths. The effect of travel speed on delta ferrite cell spacing and prior austenite grain size was found to be co-dependent on the heat input and the thermal profile resulting from multiple electrode welding. These results show that increased deposition rates can be achieved by increasing the travel speed and current density without sacrificing joint quality.

Effect of Inclusion Size on the Nucleation of Acicular Ferrite in Welds.

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.

Role of non-metallic inclusions in formation of acicular ferrite in low alloy weld metals

A critical investigation into the role of non-metallic inclusions in the nucleation of acicular ferrite in low alloy C–Mn–Ni weld metals was carried out using rapid in situ quenching, transmission electron microscopy, energy dispersive X-ray spectroscopy analysis, and dilatometric techniques. The relationship between the transformed acicular ferrite, the inclusions, and the chemical nature of the inclusions which had actually nucleated ferrite laths was examined in detail. By employing weld metals with either intermediate (200–350 ppm) or low (15–30 ppm) oxygen levels, the inclusion chemistry and nucleation behaviour of acicular ferrite in welds with different oxygen contents were compared. In addition, the influence of the [Al]/[O] ratio in lower oxygen content weld metals was also examined. It was found that the first acicular ferrite laths nucleated from the inclusions at both oxygen levels, and the chemical composition of the active inclusions varied considerably. The inclusions nucleated acicular ferrite by acting simply as inert substrates with the ability to reduce the free energy barrier to nucleation. The number and size distribution of the inclusions appeared to be the dominant factors; the thermal stress-strain fields and lattice disregistry were less important. The most favourable type of inclusions were within the size range 0·3–0·9 μm for the higher oxygen content weld metals and 0·35–0·75 μm for the lower oxygen level weld metals. A high [Al]/[O] ratio depresses the formation of acicular ferrite by increasing the soluble matrix aluminium content, which encourages extensive ferrite sideplate formation; the concurrent variation in inclusion chemical composition did not appear to influence the nucleation of acicular ferrite.MST/3209