Titanium nitride precipitation behavior in thin-slab cast high-strength low-alloy steels

To assess the potential for obtaining and utilizing titanium nitride (TiN) refinement via the increased postsolidification cooling rates associated with thin-slab casting, TiN particle size distributions were evaluated by transmission electron microscope (TEM) examination of carbon extraction replicas. Eight commercially produced thin-slab cast TiN steels, nominally 0.05 pct C, 1.2 pct Mn, and one conventionally cast steel were received. Thin slab samples were taken from three locations in the production process: quenched after casting before the tunnel furnace, quenched after tunnel furnace soaking, and the as-rolled and air-cooled final product. Effects of cooling rate were evident in the results and agree with previously documented behavior, where precipitate size decreases with increased cooling rate. Statistical differences in particle size between specimens from steels with different chemistries were shown. These variations result from differences in the driving force for precipitation, rates of coarsening, and differences in volume fraction due to changes in steel composition. The interaction of composition and processing, such as soaking in the tunnel furnace and rolling, was found to be important. For example, the hyperstoichiometric steel (excess Ti) exhibited fine TiN after casting and soaking, but dramatic coarsening after hot rolling. This behavior was attributed to deformation enhanced particle coarsening, or incomplete precipitation after soaking, followed by continued growth during subsequent processing.     

Continuous-cooling-precipitation kinetics of Nb(CN) in high-strength low-alloy steels

Although isothermal precipitation has been frequently studied with respect to industrial hot deformation processing, the temperature decreases continuously under these conditions so that isothermal data cannot be applied directly to predict the precipitation kinetics. This study therefore was concerned with the continuous-cooling-precipitation (CCP) behavior of Nb carbonitride in austenite. In the present work, the Liu-Jonas (L-J) model was used to calculate the precipitation start (P _s) time at a given temperature from experimental data. A new calculation method for predicting the precipitation finish (P _f) time, based on reaction kinetics and classical nucleation and growth theory, was also developed. The additivity rule was then used to calculate the extent of precipitation during continuous cooling. Isothermal precipitation rates for 0.04 pct Nb steels were measured experimentally by the stress relaxation method. The CCP behavior was then calculated from the model, and the accuracy of the predictions was evaluated by carrying out continuous-cooling tests using a deformation dilatometer. The precipitate size distributions were determined by the transmission electron microscopy of specimens quenched after increasing intervals of cooling at various cooling rates. TheP _s andP _f times estimated from the particle size data show good agreement with the calculated CCP behavior.     

Self-propagating high-temperature synthesis of ferrovanadium nitride for use in smelting high-strength low-alloy steels

A new alloying material for smelting high-strength low-alloy steel is considered: FERVANIT fused ferrovanadium nitride. Self-propagating high-temperature synthesis in the ferrovanadium-nitrogen system permits the development of a new industrial production technology for alloys based on vanadium nitride. Self-propagating high-temperature synthesis requires no electrical energy, is environmentally benign, and results in a product with good operational properties.     

Strain-aging of vanadium,niobium or titanium-strengthened high-strength low-alloy steels

Four commercially available high-strength low-alloy (HSLA) steels were evaluated in this study. It was determined that all four steels were susceptible to strain-aging by interstitial solutes. The increase in strength due to strain-aging was similar to that observed in a low carbon steel studied for comparison. At high levels of prestrain, the percent loss in ductility in the HSLA steels was comparable to that observed in the low-carbon steel in specimens prestrained to the same fraction of the total elongation of the as-received metal. However, when considered on an absolute basis, the residual ductility in the HSLA steels was 25 to 50 pct of that observed in the low-carbon steel. The kinetics of strain-aging were briefly examined. Indications are that the kinetics are slower in the HSLA steels than they are in the low-carbon steel.     

Strain-aging of vanadium, niobium or titanium-strengthened high-strength low-alloy steels

Four commercially available high-strength low-alloy (HSLA) steels were evaluated in this study. It was determined that all four steels were susceptible to strain-aging by interstitial solutes. The increase in strength due to strain-aging was similar to that observed in a low carbon steel studied for comparison. At high levels of prestrain, the percent loss in ductility in the HSLA steels was comparable to that observed in the low-carbon steel in specimens prestrained to the same fraction of the total elongation of the as-received metal. However, when considered on an absolute basis, the residual ductility in the HSLA steels was 25 to 50 pct of that observed in the low-carbon steel. The kinetics of strain-aging were briefly examined. Indications are that the kinetics are slower in the HSLA steels than they are in the low-carbon steel.     

Coarsening kinetics of multicomponent MC-type carbides in high-strength low-alloy steels

Morphology and coarsening kinetics of MC-type carbide (MC-carbide) precipitating during the tempering process have been investigated in V- and Nb-bearing Cr-Mo martensitic steels. Detailed transmission electron microscopy (TEM) observations show that the addition of V and Nb stabilizes the B1-type MC-carbide instead of L’3-type M₂C-carbide. The morphology of the MC-carbide is characterized as disk-like with Baker and Nutting orientation relationships with the matrix. When the specimens are fully solution treated followed by quenching, the MC-carbide precipitates as a multicomponent system with continuous solid solution of VC, NbC, and MoC. The V-, Nb-, and Mo-partitioning control the lattice parameter of MC-carbide and consequently affect the coherency between MC-carbide and the matrix. The coherent MC-carbide grows into an incoherent one with the progress of tempering. The numerical analysis on TEM observations has shown that the coarsening kinetics of MC-carbide is equated to (time)^1/5 criteria, while the coarsening kinetics of the coexisting cementite is equated to (time)^1/3 criteria. It is thus suggested that the Ostwald ripening of MC-carbide is controlled by pipe diffusion of V, Nb, and Mo along dislocations. It has been confirmed that the coarsening rate of the multicomponent MC-carbide is affected by V, Nb, and Mo content. Applying the thermodynamic solution database, the rate equation for MC-carbide coarsening can be expressed as a function of V, Nb, and Mo content, and the activation energy for pipe diffusion can be estimated as ΔQ _v: ΔQ _Nb: ΔQ _Mo=1:3.9:0.6.     

Microstructural model for hot strip rolling of high-strength low-alloy steels

The microstructural evolution during hot-strip rolling has been investigated in four commercial high-strength low-alloy (HSLA) steels and compared to that of a plain, low-carbon steel. The recrystallization rates decrease as the Nb microalloying content increases, leading to an increased potential to accumulate retained strain during the final rolling passes. The final microstructure and properties of the hot band primarily depend on the austenite decomposition and precipitation during run-out table cooling and coiling. A combined transformation-ferrite-grain-size model, which was developed for plain, low-carbon steels, can be applied to HSLA steels with some minor modifications. The effect of rolling under no-recrystallization conditions (controlled rolling) on the transformation kinetics and ferrite grain refinement has been evaluated for the Nb-containing steels. Precipitation of carbides, nitrides, and/or carbonitrides takes place primarily during coiling, and particle coarsening controls the associated strengthening effect. The microstructural model has been verified by comparison to structures produced in industrial coil samples.     

Microstructures of hot-rolled high-strength steels with significant differences in edge formability

The relationship between microstructure and hole expansion was investigated for three industrial mill-processed steels with similar yield strength (about 525 MPa) and total elongation (about 25 pct). The nominal steel composition was (in mass pct) 0.1C, 1.4Mn, 0.1Si, 0.02Al, 0.04Nb, and 0.02Ti; any variations in composition or processing history were unintentional. The microstructures of all steels consisted of about 80 pct of proeutectoid ferrite and 20 pct of a carbon-enriched, high-hardness, low-temperature transformation product (LTTP). Despite these similarities, the hole-expansion values for the steels were 44, 74, and 115 pct. Detailed microstructural characterization revealed significant differences in the LTTPs of the three steels, as well as several important differences in the proeutectoid ferrite grains. Previously reported negative effects of large quantities of martensite, microstructural banding, and a high hardness ratio (LTTP/ferrite) were validated. Different hardness ratios correlated with differences in (1) dislocation substructures of proeutectoid ferrite grains, (2) grain-size distribution, and (3) the fine structure of bainitelike/pearlitelike regions. Superior hole-expansion performance (or edge formability) was associated with a microstructure consisting of 78 pct of uniformly fine-grained proeutectoid ferrite and 22 pct of a bainitelike microconstituent, a minimum amount of microstructural banding, and a low hardness ratio. Tensile-bar fracture surfaces of a material with this microstructure showed the largest amount of microplasticity. At the time the work was carried out R.D.K. Misra was at LTV Steel, Technology Center.     

Microstructures of hot-rolled high-strength steels with significant differences in edge formability

The relationship between microstructure and hole expansion was investigated for three industrial mill-processed steels with similar yield strength (about 525 MPa) and total elongation (about 25 pct). The nominal steel composition was (in mass pct) 0.1C, 1.4Mn, 0.1Si, 0.02Al, 0.04Nb, and 0.02Ti; any variations in composition or processing history were unintentional. The microstructures of all steels consisted of about 80 pct of proeutectoid ferrite and 20 pct of a carbon-enriched, high-hardness, low-temperature transformation product (LTTP). Despite these similarities, the hole-expansion values for the steels were 44, 74, and 115 pct. Detailed microstructural characterization revealed significant differences in the LTTPs of the three steels, as well as several important differences in the proeutectoid ferrite grains. Previously reported negative effects of large quantities of martensite, microstructural banding, and a high hardness ratio (LTTP/ferrite) were validated. Different hardness ratios correlated with differences in (1) dislocation substructures of proeutectoid ferrite grains (2) grain-size distribution, and (3) the fine structure of bainitelike/pearlitelike regions. Superior hole-expansion performance (or edge formability) was associated with a microstructure consisting of 78 pct of uniformly fine-grained proeutectoid ferrite and 22 pct of a bainitelike microconstituent, a minimum amount of microstructural banding, and a low hardness ratio. Tensile-bar fracture surfaces of a material with this microstructure showed the largest amount of microplasticity. At the time the work was carried out R.D.K. Misra was at LTV Steel, Technology Center.     

Niobium carbonitride precipitation and austenite recrystallization in hot-rolled microalloyed steels

The response of austenites to thermomechanical treatments is studied in a series of niobium (columbium) HSLA steels. Interactions between composition, plastic deformation, strain-induced precipitation, and austenite recrystallization are described and related to previous work in the field. Niobium in solution prior to deformation leads to significant retardation of subsequent austenite recrystallization if Nb(C,N) precipitation takes place prior to or during the early stages of recrystallization. Such straininduced precipitation proceeds in two stages: initially at austenitic grain boundaries and deformation bands, and later on substructural features in the unrecrystallized austenite. The latter precipitation is accelerated only if it occurs in the unrecrystallized austenite; if recrystallization precedes Nb(C,N) precipitation, then the precipitation reaction is much slower. Thus, the Nb(C,N) precipitation and austenite recrystallization reactions are coupled phenomena. The conditions necessary for such an interaction are analyzed, and it is proposed that the level of supersaturation of Nb(C,N) in the austenite at the deformation temperature is a critical factor in determining whether or not an effective interaction will operate at that temperature. This paper is based on a presentation made at a symposium on “Precipitation Processes in Structural Steels” held at the annual meeting of the AIME, Denver, Colorado, February 27 to 28, 1978, under the sponsorship of the Ferrous Metallurgy Committee of The Metallurgical Society of AIME.     

Strain aging kinetics of vanadium or titanium strengthened high-strength low-alloy steels

The strain aging kinetics of two commercially available high-strength low-alloy (HSLA) steels were investigated. Strain aging was found to be caused by interstitial solutes and is thought to occur in two stages: Snoek rearrangement and “Cottrell atmosphere” formation. The latter phenomenon can be satisfactorily described by an Arrhenius relationship with an average activation energy of 34.5 kcal/mole. This high activation energy is believed to be the result of interactions between interstitial solutes and strain fields of the coherent precipitates which strengthen HSLA steels. Consequently, strain aging in HSLA steels is considerably slower than in plain carbon steel. A simple relationship was developed for predicting equivalent strain aging times in these steels. It was shown that the relationship: logt ₁/t ₂= 7500[1/T ₁ - 1/T ₂], whereT ₁ <T ₂ < 478 K can be used to predict the timet ₁ necessary at temperatureT ₁ for producing strain aging identical to that observed in a shorter time at a higher temperature.     

Strain aging kinetics of vanadium or titanium strengthened high-strength low-alloy steels

The strain aging kinetics of two commercially available high-strength low-alloy (HSLA) steels were investigated. Strain aging was found to be caused by interstitial solutes and is thought to occur in two stages: Snoek rearrangement and “Cottrell atmosphere” formation. The latter phenomenon can be satisfactorily described by an Arrhenius relationship with an average activation energy of 34.5 kcal/mole. This high activation energy is believed to be the result of interactions between interstitial solutes and strain fields of the coherent precipitates which strengthen HSLA steels. Consequently, strain aging in HSLA steels is considerably slower than in plain carbon steel. A simple relationship was developed for predicting equivalent strain aging times in these steels. It was shown that the relationship: logt ₁/t ₂= 7500[1/T ₁ - 1/T ₂], whereT ₁ <T ₂ < 478 K can be used to predict the timet ₁ necessary at temperatureT ₁ for producing strain aging identical to that observed in a shorter time at a higher temperature.     

Thermomechanical controlled processing of hot-rolled multiphase steel containing Nb

Influence of thermo-mechanical controlled processing(TMCP),including two-stage rolling with laminar cooling,air cooling and ultra-fast cooling,on the microstructure and mechanical properties of three kinds of Nb-microalloeyed steels was investigated by hot-rolling experiment.Effect of chemistry compositions and microstructure on mechanical properties and the relationship between the multiphase microstructure' s formation with TMCP were analyzed.The results showed that the mixed microstructure containing ...     

Effect of Nb content on strength of Nb micro-alloyed steels

Microstructures and properties of three Nb micro-alloyed steels were studied through hot rolling experiment.The result indicates that the ferrite grain size(dF) decreases with increasing Nb content(Nb),and the bainite fraction(fB) increases with increasing Nb content(Nb).The effect of ferrite grain size(dF) on yield strength(σy) is related to Nb content(Nb),and the effect of bainite fraction(fB) on yield strength(σy) is unrelated to Nb content(Nb).Modelling of yield strength(σy) for Nb micro-alloyed steels with high accuracy has been built up with Nb content(Nb) and bainite fraction(fB) taken into account as new parameters,and formulas for ferrite grain size(dF) and bainite fraction(fB) vs Nb content(Nb) have also been established under the experiment conditions.The research results could provide instructions for industrial productions.