Influence of nanoparticle reinforcements on the strengthening mechanisms of an ultrafine-grained dual phase steel containing titanium

In the present study, the simulated coarse-grained heat-affected zones (CGHAZs) of X90 pipeline steel were produced using a Gleeble 3500 simulator under different heat inputs (HIs) varying from 10 to 50 kJ/cm to study the effect of HI on grain boundary evolution and toughness properties in CGHAZ of X90 pipeline steel. The experimental results show that the microstructure and grain boundary is significantly influenced by different HIs. All the studied specimens have baintic dominant microstructure that composed of lath bainite (LB), granular bainite (GB) and M/A (martensite/austenite) constituents. The fraction of LB decreased with increasing of HIs, and the fraction of GB increased. The excellent combination of GB and LB obtained at an HI of 25 kJ/cm, resulting the highest impact absorbed energy, is 267 J. The impact toughness decreased sharply when HI increase from 25 kJ/cm to 30 kJ/cm, the impact toughness decreased to 18 J when the HI is larger than 30 kJ/cm. The number of HAGBs increased first and then decreased with increasing of HIs. However, for low energy coincidence lattice grain boundaries (low-ΣCSLs), the Σ3, Σ11, Σ25b boundaries decreased with increasing of HIs. Low fraction of M/A constituents are not the direct reason for the reduction of impact toughness in the simulated CGHAZs of the studied steel.

Effect of welding heat input on grain boundary evolution and toughness properties in CGHAZ of X90 pipeline steel

The contribution of interphase precipitation on yield strength was studied in Ti–Nb and Ti–Nb–Mo microalloyed steels. The high yield strength is partially attributed to strengthening induced by high degree of interphase precipitation of nanoscale carbides, which are nucleated during continuous cooling. The nanoscale carbides in Ti–Nb–Mo steel were identified as (Ti,Nb,Mo)C crystal structure similar to (Ti,Nb)C in Ti–Nb steel. The interphase precipitation of these carbides with planar or curved boundary, different row spacing and different orientation of the rows in the ferrite grain is related to the mobility of grain boundaries during γ/α transformation based on ledge mechanism. The contribution of interphase precipitation to yield strength was estimated to be ~320 MPa and ~170 MPa for Ti–Nb–Mo and Ti–Nb steels. The low yield ratio that is important from the perspective of structure application is attributed to the dual phase microstructural consisting of ferrite and bainite.

Contribution of interphase precipitation on yield strength in thermomechanically simulated Ti–Nb and Ti–Nb–Mo microalloyed steels

A Cu-bearing Nb-microalloyed bainitic steel with high yield strength (>700 MPa), good elongation (>35%) and low temperature toughness was developed for heavy plates. An intercritical annealing plus tempering (L–T) heat treatment was applied for combining reversed transformation with the precipitation during the tempering stage. The enrichment of alloying elements during the annealing process leads to the drop of A c1 temperature, which makes the reversed transformation and precipitation occur simultaneously in the subsequent tempering process. The secondary enrichment of Mn and Ni makes the reversed austenite stable enough to be retained at room temperature. About 29% retained austenite introduced by the L–T process contributes to excellent elongation more than 10% by the TRIP effect. The formation of Nb(C,N) and (Nb, Mo)C precipitates during the annealing and cooling stages inhibits the recovery of dislocations, while more Nb precipitates smaller than 10 nm and Cu precipitates with sizes of 20 nm play significant strengthening role on the matrix. The combination effects of retained austenite and nano-sized dual-precipitate increased yield strength, ductility and toughness remarkably.

High strength low-carbon alloyed steel with good ductility by combining the retained austenite and nano-sized precipitates

We describe here the precipitation behavior of copper and fine-scale carbides during thermo-mechanical processing and isothermal aging of copper-bearing niobium-microalloyed high strength steels. During thermo-mechanical processing, precipitation of ɛ-copper occurs in polygonal ferrite and at the austenite–ferrite interface. In contrast, during isothermal aging, nucleation of ɛ-copper precipitation occurs at dislocations. In the three different chemistries investigated, the increase in strength associated with copper during aging results only in a small decrease in impact toughness, implying that copper precipitates do not seriously impair toughness, and can be considered as a viable strengthening element in microalloyed steels. Precipitation of fine-scale niobium carbides occurs extensively at dislocations and within ferrite matrix together with vanadium carbides. In the presence of titanium, titanium carbides act as a nucleus for niobium carbide formation. Irrespective of the nature of carbides, copper precipitates and carbides are mutually exclusive.

Precipitation behavior during thin slab thermomechanical processing and isothermal aging of copper-bearing niobium-microalloyed high strength structural steels: The effect on mechanical properties

We describe here the precipitation behavior and mechanical properties of 560 MPa Ti–Nb and 770 MPa Ti–Nb–Mo–V steels. The precipitation characteristics were analyzed in terms of chemistry and size distribution of precipitates, with particular focus on the crystallography of precipitates through an analysis of electron diffraction patterns. In addition to pure carbides (NbC, TiC, Mo 2 C, and VC), Nb containing titanium-rich carbides were also observed. These precipitates were of a size range of 4–20 nm. The mechanism of formation of these Ti-rich niobium containing carbides is postulated to involve epitaxial nucleation of NbC on previously precipitated TiC. Interface precipitation of NbC was an interesting observation in compact strip processing which is characterized by an orientation relationship of [0 0 1] NbC //[0 0 1] α-Fe , implying that the precipitation of NbC occurred during austenite–ferrite transformation.

S. J. Jansto

Papers

Fine-scale precipitation and mechanical properties of thin slab processed titanium–niobium bearing high strength steels

Precipitation behavior during thin slab thermomechanical processing and isothermal aging of copper-bearing niobium-microalloyed high strength structural steels: The effect on mechanical properties