Mn-Depleted Zone Formation in Rapidly Cooled High-Strength Low-Alloy Steel Welds

The mechanism of formation of an Mn-depleted zone (MDZ) near the inclusion in a steel weld was elucidated based on quantification of MDZ depth and thermodynamic calculations. The effective inclusion phase for intragranular nucleation, which increased as a function of increasing chemical driving force, satisfied the requirements for presence of a considerable quantity of Mn in the phase, and a lower precipitation temperature compared with the solidus temperature of the matrix.     

Delamination Fracture Related to Tempering in a High-Strength Low-Alloy Steel

The delamination or splitting of mechanical test specimens of rolled steel plate is a phenomenon that has been studied for many years. In the present study, splitting during fracture of tensile and Charpy V-notch (CVN) test specimens is examined in a high-strength low-alloy plate steel. It is shown that delamination did not occur in test specimens from plate in the as-rolled condition, but was severe in material tempered in the temperature range 500 °C to 650 °C. Minor splitting was seen after heating to 200 °C, 400 °C, and 700 °C. Samples that had been triple quenched and tempered to produce a fine equiaxed grain size also did not exhibit splitting. Microstructural and preferred orientation studies are presented and are discussed as they relate to the splitting phenomenon. It is concluded that the elongated as-rolled grains and grain boundary embrittlement resulting from precipitates (carbides and nitrides) formed during reheating were responsible for the delamination.     

Origin and Propagation of Splits in High-Strength Low-Alloy Strip Steel

A high-strength strip steel (yield strength of ~700?MPa) with a ductile-brittle transition over a wide temperature range shows splits (fissures) on the fracture surfaces in the upper transition region. The steel, hot rolled to strip thicknesses of approximately 10.0?mm and 16.8?mm, had a predominantly fine-grained ferrite microstructure with some coarse grain patches (area percent of 11?pct and 42?pct, respectively). Low-blow Charpy tests were carried out at room temperature, corresponding to the upper transition region for these strips. The low-blow tests resulted in the formation of splits without main crack propagation from the notch; therefore, the energies at which the splits initiated could be determined. Acoustic emission (AE) sensors were used during low-blow Charpy testing of the strip steels and mild steel (where no splits occur); it was found that AE was able to detect signals from the hammer impact, split formation, and ductile deformation. Optical microscopy, scanning electron microscopy (SEM), and X-ray tomography were carried out, which verified the presence of splits and showed that they propagated by transgranular cleavage, preferentially following coarse-grained regions. No significant difference in the number or length of splits between the 10- and 16.8-mm strip was observed, but the 10-mm strip did produce deeper splits during the low-blow Charpy testing. The deeper splits contribute to a lower impact transition temperature for the 10-mm strip material.     

Micromechanical Testing of Fracture Initiation Sites in Welded High-Strength Low-Alloy Steel

Micropillar compression tests were performed on martensite (M) and retained austenite (A) containing constituents formed in high-strength low-alloy steel after simulated thermal weld heat treatment. Due to the complicated fine microstructure of the constituent, the resulted stress–strain curves were further analyzed by considering the post-mortem scanning electron micrographs and the morphology of the deformation of the pillars. It was possible to obtain relevant data for the uniaxial stress–strain behavior of the M–A constituent in the studied steel. Microvickers measurements, which were converted to macroscopic yield stress by empirical relations, were compared with the micropillar compression test results. Comparison showed that these empirical relations are overestimating the obtained mechanical properties of the M–A constituents.     

高强度低合金钢Q420N轧制工艺优化实践

试验高强度低合金钢Q420N(/％:0.16C,0.28Si,1.39Mn,0.015P,0.003S,0.11Cr,0.009N)的生产流程为120 t转炉-LF精炼-RH真空脱气-连铸300 mm×340 mm方坯-热连轧成Φ90 mm棒材.试验研究了普通轧制工艺(开轧1100～1150℃,终轧950～1000℃,...     

Ferrite Grain Size Distributions in Ultra-Fine-Grained High-Strength Low-Alloy Steel After Controlled Thermomechanical Deformation

Plane-strain compression testing was carried out above, around, and below the A _r3 temperature with the deformation temperature, T _def, varying between 1323 K and 973 K (1050 °C and 700 °C), using Gleeble 3500, to develop uniform distribution of ultra-fine ferrite (UFF) grains. Prior austenite (γ) grain structure, developed after soaking at 1473 K (1200 °C), was mixed in nature, comprising both coarse- and fine-γ-grain sizes. Applying heavy deformation in a single pass, just above the austenite-to-ferrite (α) transformation temperature (A _r3), and cooling to room temperature resulted in the formation of UFF grain sizes (average α-grain size ~2 to 3 μm), with the largest grain sizes extending up to ~10 to 12 μm. Water quenching just after deformation prevented the coarsening of UFF grains and restricted the largest grain sizes to under 6 μm. Although the ferrite grain structures appeared homogeneous in slowly cooled samples (cooling rate (CR) 1 K/s), careful observation revealed the presence of alternate bands of coarse- (5 to 10 μm) and fine-α grains (<1 to 3 μm). The final α-grain size distributions were explained in view of the starting γ-grain size variation, dynamic recrystallization (DRX) of γ, dynamic strain-induced γ-to-α transformation (DSIT), and DRX of α and grain growth during slow cooling. Electron backscattered diffraction analysis (EBSD) revealed the presence of a large fraction (70 to 80 pct) of high-angle boundaries, having misorientation ≥15 deg. Compared to the use of the single, heavy deformation pass, the application of a number of lighter passes between A _e3 and A _r3 temperatures is more suitable in industrial rolling conditions, and also has the potential of developing UFF grains with high-angle boundaries.     

Austenite Grain Structures in Ti- and Nb-Containing High-Strength Low-Alloy Steel During Slab Reheating

Austenite-grain growth was investigated in a couple of microalloyed steels, one containing Ti and the other containing Nb, Ti, and V, using different reheating temperatures between 1273 K and 1523 K (1000 °C and 1250 °C). Nature and distribution of microalloy precipitates were quantitatively analyzed before and after reheating. Interdendritic segregation (or microsegregation) during casting can result in an inhomogeneous distribution of microalloy precipitates in the as-cast slabs, which can create austenite grain size variation (even grain size bimodality) after reheating. Ti addition reduced the grain size variation; however, it could not eliminate the grain size bimodality in Nb-containing steel, due to the differential pinning effect of Nb precipitates. A model was proposed for the prediction of austenite grain size variation in reheated steel by combining different models on microsegregation during solidification, thermodynamic stability, and dissolution of microalloy precipitates and austenite grain growth during reheating.     

Prediction of Inhomogeneous Distribution of Microalloy Precipitates in Continuous-Cast High-Strength,Low-Alloy Steel Slab

Spatial distribution in size and frequency of microalloy precipitates have been characterized in two continuous-cast high-strength, low-alloy steel slabs, one containing Nb, Ti, and V and the other containing only Ti. Microsegregation during casting resulted in an inhomogeneous distribution of Nb and Ti precipitates in as-cast slabs. A model has been proposed in this study based on the detailed characterization of cast microalloy precipitates for predicting the spatial distribution in size and volume fraction of precipitates. The present model considers different models, which have been proposed earlier. Microsegregation during solidification has been predicted from the model proposed by Clyne and Kurz. Homogenization of alloying elements during cooling of the cast slab has been predicted following the approach suggested by Kurz and Fisher. Thermo-Calc software predicted the thermodynamic stability and volume fraction of microalloy precipitates at interdendritic and dendritic regions. Finally, classical nucleation and growth theory of precipitation have been used to predict the size distribution of microalloy precipitates at the aforementioned regions. The accurate prediction and control over the precipitate size and fractions may help in avoiding the hot-cracking problem during casting and selecting the processing parameters for reheating and rolling of the slabs.     

Latest Progress of Low-Alloy High-Strength Steels in Baosteel

After research and development for decades,low-alloy high-strength steels have been widely used and playing an important role in economy.This article introduces,from the perspective of environmental protection,the Baosteel’s latest progress of low-alloy high-strength steels continuously innovated with the focus of achieving high-strength,high-toughness,long service life and versatile functions,and with the aim of providing energy-saving and pollution-reduction solutions to down-stream sectors.     

低合金高强度调质钢控轧控冷工艺优化

研究了低合金高强度调质钢的轧制方式及轧后冷速对调质后微观组织及力学性能的影响。结果表明:在试验条件下,试验钢采用不同轧制方式及冷速获得不同淬火加热前的初始组织及力学性能,但调质后却具有相同的微观组织和力学性能。建议低合金高强度调质钢采用一阶段的轧制方式,终轧温度应高于奥氏体部分再结晶区的上限温度,轧后采用适宜的冷速(4~15℃/s)进行加速冷却。以此工艺调质后的试验钢具有较高的均匀性,与现行TMCP工艺相比,强塑性和强韧性更为优异。     

Atom-Probe Tomographic Investigation of Austenite Stability and Carbide Precipitation in a TRIP-Assisted 10 Wt Pct Ni Steel and Its Weld Heat-Affected Zones

Newly developed low-carbon 10 wt pct Ni-Mo-Cr-V martensitic steels rely on the Ni-enriched, thermally stable austenite [formed via multistep intercritical Quench-Lamellarization-Tempering (QLT)-treatment] for their superior mechanical properties, specifically ballistic resistance. Critical to the thermal stability of austenite is its composition, which can be severely affected in the weld heat-affected zones (HAZs) and thus needs investigations. This article represents the first study of the nanoscale redistributions of C, Ni, and Mn in single-pass HAZ microstructures of QLT-treated 10 wt pct Ni steels. Local compositions of Ni-rich regions (representative of austenite compositions) in the HAZs are determined using site-specific 3-D atom-probe tomography (APT). Martensite-start temperatures are then calculated for these compositions, employing the Ghosh-Olson thermodynamic and kinetics approach. These calculations predict that austenite (present at high temperatures) in the HAZs is susceptible to a martensitic transformation upon cooling to room temperature, unlike the austenite in the QLT-treated base-metal. While C in the QLT-treated base-metal is consumed primarily in MC and M₂C-type carbide precipitates (M is Mo, Cr, V), its higher concentration in the Ni-rich regions in the HAZs indicates the dissolution of carbide precipitates, particularly M₂C carbide precipitates. The role of M₂C carbide precipitates and austenite stability is discussed in relation to the increase in microhardness values observed in the HAZs, relative to the QLT-treated base-metal. Insights gained from this research on austenite stability and carbide precipitation in the single-pass HAZ microstructures will assist in designing multiple weld cycles for these novel 10 wt pct Ni steels.     

Correlation of Fractographic Features with Mechanical Properties in Systematically Varied Microstructures of Cu-Strengthened High-Strength Low-Alloy Steel

Fracture is often the culmination of continued deformation. Therefore, it is probable that a fracture surface may contain an imprint of the deformation processes that were operative. In this study, the deformation behavior of copper-strengthened high-strength low-alloy (HSLA) 100 steel has been investigated. Systematic variation of the microstructure has been introduced in the steel through various aging treatments. Due to aging, the coherency, size, shape, and distribution of the copper precipitates were changed, while those of inclusions, carbides, and carbonitrides were kept unaltered. Two-dimensional dimple morphologies, quantified from tensile fracture surfaces, have been correlated to the nature of the variation of the deformation parameters with aging treatment.     

Gas-Carburizing Kinetics of a Low-Alloy Steel

Gas-carburizing kinetics of a low-alloy steel (Pyrowear 53) was investigated by thermogravimetric experiments. Kinetic curves were modeled by adapting the approximate integral method, and the diffusion coefficient of carbon as well as the rate constant of the surface reaction were estimated. These parameters were evaluated after several carburizing procedures, which differ from each other in the surface treatments performed before the carburizing step. It is known that the carbon enrichment is low when this steel is carburized without any pretreatment, and this behavior was found to be related to a low value of carbon diffusivity. The interaction between the selective oxidation of alloying elements by the carburizing atmosphere and carbon diffusion is discussed. The pretreatment procedures investigated in this work consist of different combinations of oxidation, reduction, and grit-blasting processes. The most effective procedures involve oxidation in dry air or oxidation in wet air followed by grit blasting.     

Low-Temperature Toughening Mechanism in Thermomechanically Processed High-Strength Low-Alloy Steels

High-strength low-alloy (HSLA) steels were fabricated by varying thermomechanical processing conditions such as rolling and cooling conditions in the intercritical region, and the low-temperature toughening mechanism was investigated in terms of microstructure and the associated grain boundary characteristics. The steels acceleratedly cooled to relatively higher temperature had lower tensile strength than those acceleratedly cooled to room temperature due to the increased volume fraction of granular bainite or polygonal ferrite (PF) irrespective of rolling in the intercritical region, while the yield strength was dependent on intercritical rolling, and start and finish cooling temperatures, which affected the formation of PF and low-temperature transformation phases. The steel rolled in the intercritical region and cooled to 673 K (400 °C) provided the best combination of high yield strength and excellent low-temperature toughness because of the presence of fine PF and appropriate mixture of various low-temperature transformation phases such as granular bainite, degenerate upper bainite (DUB), lower bainite (LB), and lath martensite (LM). Despite the high yield strength, the improvement of low-temperature toughness could be explained by the reduction of overall effective grain size based on the electron backscattered diffraction (EBSD) analysis data, leading to the decrease in ductile-to-brittle transition temperature (DBTT).