[en] The present paper focuses on the microstructure-based cleavage crack propagation in a Charpy impact tested fully pearlitic steel by correlating microstructure and crystallography with the overall fracture behavior. The importance of pearlite lamellae orientation in providing preferred fracture paths is discussed, encompassing the mechanism of interface decohesion and stepwise crack propagation through a mathematical model simulation. While the {100} cleavage cracking is well familiar in pearlitic steels, crack propagation along the {110} crystallographic planes can also prevail in some pearlite colonies or nodules. This is related to suppressing the crack tip dislocation emissions due to restricted slip transferability across the lamellae interfaces. Besides, the strain incompatibility due to large elastic modulus or Schmid factor mismatch across the pearlite nodule boundaries is responsible for triggering internodular cracking in the steel. Connecting the framework of fracture mechanics with the experimental observations, the mechanisms pertaining to different types of tear ridges formed within a pearlite colony are proposed. This certainly illuminates the role of lamellae orientation in the process of crystal bending and shearing at the tear ridges formed within the colonies or at the twist nodule boundaries.

[en] Effect of the initial as-cast structure on the microstructure–texture evolution during thermomechanical processing of 409L grade ferritic stainless steel was studied. Samples from the regions of cast slab having ‘columnar,’ ‘equiaxed,’ and a mixture of ‘columnar’ and ‘equiaxed’ grains were subjected to two different processing schedules: one with intermediate hot-band annealing before cold-rolling followed by final annealing, and another without any hot-band annealing. EBSD study reveals that large columnar crystals with cube orientation are very difficult to deform and recrystallize uniformly. Resultant variations in ferrite grain structure and retention of cube-textured band in cold-rolled and annealed sheet contribute to ridging behavior during stretch forming. Initial equiaxed grain structure is certainly beneficial to reduce or even eliminate ridging defect by producing uniform ferrite grain structure, free from any texture banding. Application of hot-band annealing treatment is also advantageous as it can maximize the evolution of beneficial gamma-fiber texture and eliminate the ridging defect in case of completely ‘equiaxed’ starting structure. Such treatment reduces the severity of ridging even if the initial structure contains typically mixed ‘columnar-equiaxed’ grains.

[en] Primarily ferrite-pearlite microstructure having coarse ferrite grain size (24 µm) and high pearlite fraction (42%) offered YS ∼ 575 MPa with poor impact properties such as, upper shelf energy (USE) of only 30 J and ductile brittle transition temperature (DBTT) as high as 27 °C in an industrially hot-rolled plate of 0.25 wt% C steel. In order to improve the strength along with the impact properties by developing ferrite-bainite microstructures, two different types of heat-treatments, namely step-cooling (SC) and intermediate cooling (IC) treatments, were carried out on the as-received material. Significant improvement in strength along with the impact toughness (with YS of 740 MPa, USE of 222 J and DBTT of − 57 °C) has been achieved by developing fibrous microstructure, with alternate thin-films (2–4 µm thick) of ferrite and bainite through intermediate cooling (IC) treatment. Fine film-like structure with large orientation difference across the ferrite-bainite interface boundaries not only increased the strength but also resulted in frequent deflection in cleavage crack propagation path which improved the low-temperature impact toughness and reduced the DBTT.

[en] Advanced high strength strip steel with bainitic-martensitic microstructure is widely used at present for the structural applications due to their high strength along with satisfactory ductility and toughness. Most of the research work has been carried out on improving the strength and toughness of such steels, however limited information is available about their bending performance. The present study is undertaken after failure was observed in industrial high strength strip steels while performing bend tests as per standard specification. Two different industrial steel grades having different compositions were used for this study. The hot rolled strip steels were rapidly cooled down (by water spray cooling) and coiled at different temperatures ranging from 450°C to 382°C. Microstructural and textural characterization were carried out by scanning electron microscopy attached with electron back scattered diffraction (EBSD) facility and transmission electron microscopy. Bend tests were conducted following ASTM E290 standard. Results showed that microstructures of the investigated steels were comprised of different constituents such as granular bainite, upper bainite, lower bainite and martensite. Factors such as superior microstructural homogeneity, high intensity and uniform distribution of gamma fiber texture (<111>//ND) along with {332}<113> texture component and high post uniform elongation played beneficial effect in terms of bending performance. On the other hand dominance of cube (C) and Goss {110} <001> texture components, higher martensite fraction, severe microstructural heterogeneity and high strain rate sensitivity (m) are the factors that deteriorated the bendability. An effort has been made to establish processing-microstructure-texture bendability correlation for the investigated steels. (author)

[en] Single-pass and multi-pass deformation schedules were applied over 973 K to 1323 K (700 °C to 1050 °C) using a Gleeble^® thermomechanical simulator to understand the effect of the number of deformation passes, strain per pass, and interpass interval on ferrite grain refinement, particularly for the formation of an ultrafine ferrite grain structure (ULFG) with favorable high-angle grain boundaries and texture components. The microstructure, texture, and grain boundaries were characterized using SEM and EBSD. In the single-pass schedule, a decrease in deformation temperature from 1323 K to 1073 K (1050 °C to 800 °C) refined the ferrite grain size from 9.5 to 2.0 μm and intensified the beneficial γ-fiber (〈111〉//ND) texture. The ferrite grain size remained unchanged, whereas Cube and Goss texture strengthened with a further decrease in deformation temperature to 973 K (700 °C). For the multi-pass schedule at 1073 K (800 °C), where the total deformation was applied in three successive passes with 10-second interpass time, an ULFG structure was also developed with 2.5 μm grain size. Grain growth during the interpass intervals weakened the texture in the multi-pass deformed samples compared to that of the single-pass deformed samples. The evolution of microstructure and texture has been explained considering that restoration mechanisms (recrystallization, grain growth, and phase transformation) acted during and after deformation. Finally, ferrite grain refinement during different processing schedules was predicted using a simple mathematical approach based on the experimental data, with suggestions for future improvements.

[en] The tensile and Charpy impact properties of four strip samples from two different B-added low-C ultra-high-strength steel strips (Al-treated and Ti-treated), coiled at two different temperature ranges (360–380 °C and 450–460 °C), have been evaluated and correlated to the microstructural parameters, dislocation density, and the intensity of high-angle boundaries. The effects of coiling temperatures on the microstructural evolution and mechanical properties have been discussed. The volume fraction of the individual phase constituents (namely, granular bainite, upper bainite, lower bainite and tempered martensite) and their hardness, local deformation response and the strain-hardening ability, as determined from nanoindentation testing, influenced the bulk properties such as hardness, tensile properties (strength and ductility), Charpy impact properties (upper shelf energy, USE, and ductile-to-brittle transition temperature, DBTT) and strain-hardening abilities under both quasi-static and dynamic loading conditions. The dominance of granular bainite and upper bainite (75–90 %) reduced the strength (670–722 MPa yield strength), improved ductility (16.7–19.5 % elongation to failure) and USE (35–42 J) in the samples coiled at the higher temperatures. In contrast, a higher fraction of tempered martensite and lower bainite (78–82 %) significantly increased the strength (808–814 MPa), reduced ductility (13.0–14.5 %) and USE (19–29 J) in the lower temperature coiled samples. The DBTT showed a complex trend with the microstructural parameters. It depended on the USE level, as well as on the ‘effective grain size’ of the matrix.

[en] Heavy deformation of metastable austenite at intercritical temperature is known to develop ‘ultrafine ferrite-grain’ (ULFG) structure and provide grain boundary strengthening. Systematic thermomechanical simulation was conducted in Gleeble®3500 by deforming the samples isothermally at two intercritical temperatures: 810 °C (~40 °C below Ae₃ and ~160 °C above Ar₃) and 710 °C (~140 °C below Ae₃ and ~60 °C above Ar₃) to identify the critical conditions for the formation of ULFG structure in a low carbon microalloyed steel. Both single-pass and multi-pass deformations with varying equivalent total strain level were considered in order to provide a solution towards the development of ULFG structure upon industrial rolling. Microstructure evolution suggested that multi-pass intercritical deformation can produce uniform distribution of ultrafine ferrite grains (grain size ≤ 2 µm) as a combined effect of static- and dynamic strain-induced transformations (SSIT and DSIT) and continuous dynamic recrystallization (CDRX). Based on the microstructural evidences and strain analysis following Militzer-Brechet model, a descriptive model has been proposed discussing the mechanism of grain refinement during isothermal intercritical deformation.

[en] Low-carbon microalloyed steel was subjected to warm rolling followed by rapid transformation annealing (RTA) at 800-850 °C and subcritical annealing (SCA) at 600 °C to develop ultrafine ferrite grain structures (UFFG) with grain size less than 3 μm. The present study investigated the influence of light (40%) and heavy (80%) warm rolling deformation (LWR and HWR) applied during the finishing pass of two-pass rolling schedules on the microstructural evolution after rolling and subsequent annealing treatments. RTA treatment of HWR sample at a lower intercritical temperature for an optimum duration (800 °C, 30 s) developed UFFG-martensite dual-phase structure that offered the best combination of strength (YS ~ 900 MPa and UTS ~ 1200 MPa) and ductility (25% elongation). The SCA treatment provided sufficient time to achieve a uniform distribution of carbide particles throughout the ferrite matrix. SCA treatment of HWR at 600 °C for 4 h developed UFFG-carbide structure achieving YS of 800 MPa with 20% ductility. The SCA of LWR resulted in coarser ferrite grain structures (grain size > 5 μm) having higher ductility (more than 30%) but lower strength (UTS of 400-550 MPa) as compared to RTA.

[en] Highlights: • The effect of B and N on the evolution of microstructure and precipitate stability in normalized and tempered specimens (for different austenitization temperatures, 1000−1100 °C) and the creep resistance of 9Cr-1Mo grade steels (5 steels varying B (0–100 ppm)and N(20–500 ppm) were discussed. • Auger electron spectroscopy revealed the enrichment of boron within the M23C6 precipitates at the vicinity of PAG boundaries in B added steels. • The 70 ppm B steel with 108 ppm N shows the best creep resistance (rupture time of 1536 h and minimum creep-rate of 1 × 10⁻⁵/h at 650 °C, 120 MPa) followed by 90 ppm B with 90 ppm N steel and 100ppm B with 20ppm N respectively. • Too high N concentration in B-free steel, on the other side, promotes the coarsening of MX precipitates that is detrimental to the creep resistance. Too low boron content (25 ppm B here) is also not a preferred combination. • It can be recommended that it is necessary to maintain optimum concentrations of B (70–100 ppm) and N (90–110 ppm) to improve the microstructural stability and creep resistance of modified 9Cr-1Mo steel. -- Abstract: The present study systematically varied the concentrations of B (0–100 ppm) and N (20–500 ppm) in modified 9Cr-1Mo steel to understand the combined effect of B and N addition on the microstructural stability and the creep resistance (650 °C, 120 MPa). The Auger Electron Spectroscopic analysis reveals the enrichment of B within the M₂₃C₆ precipitates at the vicinity of prior-austenite grain boundaries in B added steels both in normalized and tempered specimens and also in creep tested specimens. The 70 ppm B steel with 108 ppm N showed the best creep resistance (rupture time as high as 1536 h and minimum creep-rate as low as 1 × 10⁻⁵/h), followed by 90 ppm B, 90 ppm N steel. Too low B content (25 ppm or less) or too high N content (500 ppm) affected precipitate stability and consequently the creep resistance of the steel. Finally, it can be recommended that it is necessary to maintain optimum concentrations of B (70–100 ppm) and N (90–110 ppm) to improve the creep resistance of modified 9Cr-1Mo steel.