Bearing HSLA Steel Research Articles

Recrystallisation Characteristics of a Cu Bearing HSLA Steel Assessed Through High Temperature Compressive Deformation

Dynamic (DRX) operative duringdeformation and static recrystallization (SRX) operative after deformation are considered responsible for the changes in microstructure and texture of deformed materials. Especially in the case of advanced steels that are required in the form of plates of various thicknesses, hot rolling is the main manufacturing process during which the steel undergoes DRX under the rolls and SRX between rolling passes/strands. Knowledge on DRX and SRX characteristics of such steels is crucial for optimisation of hot rolling parameters,achieving the desired microstructure and consequently the targeted mechanical properties.In this study, certain key aspects of both dynamic (DRX) and static recrystallisation (SRX) behaviour of aCu-bearing HSLA steel, which was developed at DMRL, have been explored through high temperature deformation studies using Gleeble thermo-mechanical simulator. Through uniaxial compression testing in the austenitic regime, domains of continuous and discontinuous DRX prevalent in the steel were identified and critical parameters for initiation of dynamic recrystallisation viz., critical strain (ec), critical stress (sc), peak stress (sp) and peak strain (ep) were determined as a function of temperature and strain rate. SRX characteristics were assessed through uniaxial double hit compression tests with fixed strain rate and strain per hit but at different temperatures and with different imposed intermediate static recrystallisation (ISRT) times. From the fractional softening data, parameters such as time for 50% recrystallisation, t0.5, temperature for 50% recrystallisation, T0.5, and activation energy, QSRX have been estimated.Although the steel exhibited good plastic deformation characteristics, the results suggest that the role of copper in retarding recrystallisation is significant.

Effects of cooling rate and isothermal holding on the precipitation behavior during continuous casting of Nb–Ti bearing HSLA steels

The precipitates identified in as-cast slab were semi-dendritic, dendritic or rod-like Nb-rich (Nb,Ti)(C,N). The morphology and chemistry were different from these precipitates after reheating and/or hot rolling processes. Most Nb and Ti were mainly precipitated into carbonitrides during the isothermal holding at the temperature range from 900 to 1000 °C.

Influence of thermo-mechanical processing and different post-cooling techniques on structure and properties of an ultra low carbon Cu bearing HSLA forging

An ultra low carbon Cu bearing HSLA steel, micro alloyed with Nb and Ti was forged in two stages and subsequently cooled at different cooling rates. Variation in microstructures and the mechanical properties at different cooling rates have been studied. Volume fraction of bainitic phase has been found to vary with different cooling rates. Maximum strength was achieved in this steel at an intermediate cooling due to precipitation hardening of very fine microalloying carbides, carbonitrides and ε-Cu precipitates. Finer polygonal ferrite and granular bainite structure obtained at the slowest cooling rate resulted in high impact toughness values at ambient and sub ambient temperatures.

Effect of Thermomechanical Processing on the Microstructure and Properties of a Low Carbon Copper Bearing Steel.

A boron treated copper bearing HSLA steel containing austenite formers like manganese and nickel, somewhat lower in amount than that in HSLA 100 variety of steel is chosen for the study. The role of thermomechanical processing on the microstructure and mechanical properties of the above steel has been investigated. Differential scanning calorimetric study is carried out for understanding the precipitation behaviour of copper in HSLA steel under the influence of boron. The microstructure of the experimental steel is found to consist of laths of martensites and bainite. MA constituents of ribbon like morphology are observed at the lath boundaries. Higher strength properties of the steel are attributed to the presence of finely distributed precipitates of copper and microalloy carbides.

Zr-containing precipitates in a Ti [sbnd]Nb microalloyed HSLA steel containing 0.016 wt.% Zr addition

The effects of an 0.016 wt.% Zr addition on precipitation in a Ti Nb microalloyed HSLA steel were studied by analytical electron microscopy (AEM). Complex Zr-containing oxides and complex Ti Zr and Ti Nb Zr nitrides (carbonitrides) were found, but no single ZnN precipitates were observed. Microanalysis and lattice parameter measurements showed that in these complex Zr-containing nitrides, Zr and N were intermixed with either Ti or Ti and Nb rather than present as an isolated ZrN phase. Four groups of complex Zr-containing nitride were observed in terms of microchemistry, morphology and size. All the Ti Zr and Ti Nb Zr nitrides observed were greater than 100 nm in size. However, the majority of the Zr free Ti Nb precipitates were smaller than 100 nm. Therefore, the precipitation of these Ti Nb particles was not affected by an 0.016 wt.% Zr addition. It appears that the Zr addition increased the microsegregation in Ti Nb steel, which would have an adverse effect on toughness. As a comparison the lattice parameters of the Ti Nb precipitates in Ti Nb microalloyed steels were measured. Finally, the potential application of ZrN particles in Nb bearing HSLA steels is discussed.

Microstructures and Age Hardening Characteristics of Direct Quenched Cu Bearing HSLA Steel.

Microstructures and age hardening characteristics of 0.04C-3.5Ni-0.6Cr-1.8Cu-0.4Mo-0.03Nb Cu bearing HSLA steel manufactured by reheat quenching (RQ) and direct quenching (DQ) processes, were investigated. Especially, the effect of DQ treatment on quenched microstructures and niobium carbide and e-Cu precipitation were examined. A finer quenched microstructure was obtained in DQ processed steel compared with RQ processed steel, due to the heavily deformed austenite introduced by DQ. In DQ processed steel, lath and block sizes of martensite decreased and internal dislocation density increased with the increase of austenite deformation in the non-recystallization region. Hence, the strength of quenched DQ steel increased with the amount of deformation in non-recrystallization region. e-Cu precipitation characteristics in the aging stage changed very sensitively to the quenched microstructures. The size and growth rate of e-Cu precipitate decreased with the amount of deformation in the non-recrystallization region, because heavily dislocated structure supplied more preferred nucleation sites of e-Cu precipitates and retarded the precipitate growth. Also, DQ process affected the niobium carbide precipitation in this steel. In RQ steel, most of niobium precipitated as a coarse carbide in the reheating stage for austenitization. Whereas, in DQ steel, most of niobium precipitated very finely in the aging stage and thus highly contributed to the increase of the strength and tempering resistance. Therefore, it was councluded that DQ processed steel had higher strength and tempering resistance than RQ processed steel due to the finer e-Cu and niobium carbide precipitates.

Evolution of acicular ferritic microstructure in a titanium bearing HSLA steel

An attempt has been made to design and develop acicular ferritic steels by adopting three-stage controlled rolling, with titanium alone as microadditive. Process parameters such as finish rolling temperature (FRT), amount of deformation during the last pass and also the cooling rate after finishing have been altered to obtain different microstructural effects. Optical as well as transmission electron microscopy studies indicated that acicular ferritic microstructure can be engendered by applying a minimum deformation of 25 % at the finishing pass followed by water quenching. Decreasing the FRT from 850 °C, for water quenched steels deformed 33 %, resulted in the refinement of polygonal ferrite, whereas the reverse is the case with the aspect ratio of acicular ferrite. The volume fraction of acicular ferrite progressively increases as the FRT is lowered. Evaluation of tensile properties suggested that yield strength as high as ~ 600 MPa with an elongation of ~ 25 %, could be obtained by finishing at a lower temperature of 750 °C. An impact transition temperature (ITT) of -53 °C indicated that the steel possesses adequate toughness properties. As weldability is one of the criteria for structural applications, this aspect too has been investigated. Es wurde der Versuch unternommen, nadeliges Ferritgefüge in einem Ti-mikrolegierten Stahl durch dreistufiges, geregeltes Walzen zu erzeugen. Die Verfahrensparameter -wie Endwalztemperatur, Umformgrad im letzten Stich, Abkühlgeschwindigkeit nach dem Fertigwalzen- wurden variiert, um verschiedene mikrostrukturelle Einflüsse zu erhalten. Untersuchungen mit Lichtmikroskop und TEM ergaben, daß nadeliges Ferritgefüge schon bei minimaler Umformung von 25 % beim Fertigstich mit anschließendem Abschrecken in Wasser erzeugt werden kann. Senkt man die Endwalztemperatur von 850 °C, so führt dies zur Feinung des polygonalen Ferrits in Stählen nach 33 % Umformung und Abschrecken in Wasser. Der Schlankheitsgrad des nadeligen Ferrits verhält sich genau umgekehrt. Der Volumenanteil an nadeligem Ferrit nimmt in dem Maße zu, wie die Endwalztemperatur sinkt. Eine Betrachtung der Festigkeitseigenschaften führt zu der Annahme, daß Streckgrenzen von 600 MPa bei 25 % Dehnung durch Fertigwalzen bei nur 750 °C eingestellt werden können. Die Übergangstemperatur von -53 °C belegt die guten Zähigkeitseigenschaften dieses Stahls. Auch die Schweißbarkeit, als eine für die Weiterverarbeitung wichtige Eigenschaft, wurde untersucht.

Computer Model for Prediction of Carbonitride Precipitation during Hot Working in Nb-Ti Bearing HSLA Steels.

A computer model which predicts complex precipitation behavior quantitatively in Nb-Ti bearing steel has been developed on a theoretical basis. The solubility and composition of the complex precipitates, and the chemical driving force of the precipitates from supersaturated austenite are estimated by means of thermodynamic analysis of regular solution composed of four-binary compounds. The change in dislocation density which acts as a nucleation site during hot working is calculated by using dislocation theory. And the time dependence of volume fraction and the particle radius of strain induced precipitation are also predicted on the basis of classical nulceation theory. In order to estimate the effect of deformation on nucleation, the change in elastic energy of dislocation with nucleation is calculated.Experimental results showed that combination of Nb and Ti addition, decreased the solubility of carbonitrides and accelerated the precipitation rate from supersaturated austenite because of the formation of complex precipitates. Such experimental results are in good agreement with the prediction by the present model. And both the acceleration of precipitation rate and the refinement of precipitates particles due to hot deformation are also quantitatively explained.

Thermomechanical Treatment of a Titanium Bearing HSLA Steel

High-strength low-alloy steels have been studied extensively in recent years. Vanadium and/or niobium containing low carbon steels have demonstrated superior mechanical properties through controlled rolling. In the present investigation, the effect of thermomechanical treatment on the mechanical properties of a titanium bearing low carbon steel has been studied. The tensile and impact properties of this steel are related to the microstructures introduced by various rolling schedules.It has been found that it is possible to obtain a steel with 36% elongation to fracture, 401 MPa yield strength, 494 MPa tensile strength, and an impact transition temperature of 213K through controlled rolling.

Thermomechanical Treatment of a Titanium Bearing HSLA Steel

High-strength low-alloy steels have been studied extensively in recent years. Vanadium and/or niobium containing low carbon steels have demonstrated superior mechanical properties through controlled rolling. In the present investigation, the effect of thermomechanical treatment on the mechanical properties of a titanium bearing low carbon steel has been studied. The tensile and impact properties of this steel are related to the microstructures introduced by various rolling schedules.It has been found that it is possible to obtain a steel with 36% elongation to fracture, 401 MPa yield strength, 494 MPa tensile strength, and an impact transition temperature of 213K through controlled rolling.

Characteristics of precipitates and mechanical properties in Ti bearing HSLA steels.

The effect of reheating temperature on tensile strength of controlled rolled steels and on grain coarsening of austenite during solution treatment were investigated for three series of Ti bearing HSLA steels (Ti, V-Ti and Nb-Ti), by comparing with the stability and composition of precipitates in each steel. It was found that addition of 0.03wt% Nb decreases the tensile strength of Ti bearing steels reheated at a low temperature and retards austenite grain coarsening, whereas addition of 0.05wt% V increases the tensile strength and scarcely retards austenite grain growth. These differences arise from the different stability of precipitates in Nb-Ti and V-Ti bearing steels. The equilibrium thermodynamic model of quaternary compounds is proved to predict the solubility and composition of precipitates in Ti bearing steels. It is concluded that the difference in solubility of the precipitates in austenite corresponds to the different solubility between Nb and V in the precipitates.

Influence of cooling rate on the microstructure and retained austenite in an intercritically annealed vanadium containing HSLA steel

The influence of cooling rate from the intercritical αγ region on the microstructure of a vanadium bearing HSLA steel was investigated by transmission electron microscopy. Oil quenching produced an essentially ferrite-martensite dual phase structure with ∼4 vol pct of fine particle and thin film retained austenite. In contrast, the slower air cooling resulted in a larger amount (∼10 vol pct) of retained austenite in addition to the ferrite and martensite phases. A major portion of the retained austenite in the air cooled specimen was of the blocky morphology (1 to 6 μm , with the remainder eing the submicron variety, similar to that of the oil quenched specimen. In conformance with the terminology of earlier studies, “retained” ferrite and “transformed” ferrite were observed in the air cooled steel while oil quenching completely suppressed the transformed ferrite. Retained ferrite, the cleaner of the two in terms of precipitate content, is the high temperature ferrite that coexists with the austenite at the intercritical temperature and which is retained on cooling. The transformed ferrite, on the other hand, forms from the decomposition of the austenite and contains banded carbonitrides (row precipitation) much like the initial microstructure of the HSLA steel.

Fatigue crack initiation and propagation in a quenched and tempered niobium bearing HSLA steel

The fatigue crack initiation and propagation behavior of a niobium bearing HSLA steel heat treated to give two tempered martensitic microstructures presumably with and without fine niobium carbides has been studied by light microscopy, electron microscopy, and strain gage measurements of plastic zone deformation. The high cycle, stress controlled fatigue life of the steel in both heat treated conditions was quite similar with the steel presumably containing the fine niobium carbides having slightly better resistance at low stress amplitudes. This slightly better high cycle resistance is associated with better resistance to fatigue crack initiation for this heat treatment. The fatigue crack propagation behavior of the steel was the opposite. The steel presumably containing the fine niobium carbides exhibited a much faster fatigue crack growth rate than that without them. The difference in growth rates is explained in terms of the plastic work expended during the propagation of the fatigue crack.

Low cycle fatigue behavior of a quenched and tempered niobium bearing HSLA steel

The low cycle fatigue behavior of a quenched and tempered niobium (columbium) bearing high strength low alloy steel heat treated to give tempered martensitic microstructures presumably with and without fine niobium carbides was studied by transmission electron microscopy, stress relaxation, X-ray diffraction line broadening and strain-controlled fatigue testing. The steel without the niobium carbides cyclically softened rapidly at all strain amplitudes studied. This softening was attributed to the rearrangement of the dislocation substructure into a cell structure and to the accompanying decrease in internal stress. The steel presumably containing the fine niobium carbides cyclically softened to a lesser extent. This correlated with the observation that dislocations in this steel did not rearrange themselves into a cell structure and, hence, there was less change in the internal stress during cycling. The steel without the niobium carbides exhibited somewhat better strain-life behavior at large strain amplitudes. This was attributed to the cell structure being able to accommodate a greater amount of plastic strain in that steel.