Niobium-containing Steel Research Articles

Effect of Chromium and Nickel on Heat-Affected Zone Properties with Welding of X70 Strength Category Niobium-Containing Steels

Properties are evaluated for heat-affected zone (HAZ) metal with welding of X70 strength group steels microalloyed with niobium and additions of chromium and nickel. The evaluation criteria used were determination of parameters of the upper, medium, and lower threshold temperatures for HAZ metal cold brittleness with a coarse microstructure and austenite grains of significant magnitude.

The influence of Widmanstätten ferrite on yielding behavior of Nb-containing reinforcing steel bars

The engineering stress–strain curves for a series of civil-structured steel bars have been investigated. The yield plateau existed in the corresponding curves for niobium-free steel bars, but continuous plastic flow occurred for niobium-containing steel bars. The results from optical metallography and transmission electron microscopy provide evidence to suggest that the absence of a yield plateau in niobium-containing steel bars is associated with a substantial amount of Widmanstatten ferrite, which forms with a displacive mechanism.

Improvement of weldability and evaluation criteria for high-strength pipe steel heat-affected zone reliability

Criteria are formulated for evaluating weldability on the basis of methods simulating welding thermal cycles and analysis of austenite polymorphic transformation kinetics in the thermally affected zone. Results are provided for evaluating the joint heat-affected zone (HAZ) cold resistance under the influence of thermal action during welding of thick-walled pipes for considerable heat input and the tendency for cold cracks during welding with small heat input. The effect of alloying elements, i.e., nickel and chromium, and also vanadium and molybdenum in steels microalloyed with niobium on ductile-brittle failure parameters during impact testing is demonstrated. Microalloying niobium-containing steels with vanadium and molybdenum leads to HAZ metal embrittlement. Optimum cooling rates after welding are determined with which HAZ metal cold resistance at down to –30°C is provided are determined.

Analysis of the solubility of carbides, nitrides, and carbonitrides in steels using methods of computer thermodynamics: III. Solubility of carbides, nitrides, and carbonitrides in the Fe-Ti-C, Fe-Ti-N, and Fe-Ti-C-N systems

A critical analysis of available thermodynamic data for the Fe-Nb-C, Fe-Nb-N, and Fe-Nb-C-N systems has been performed. Based on this analysis, self-consistent thermodynamic descriptions of these systems have been selected and refined using the CALPHAD method. With the use of the thermodynamic descriptions constructed, phase equilibria in these systems have been calculated. On the basis of these calculations, the solubility of carbides, nitrides, and carbonitrides in the Fe-Nb-C, Fe-Nb-N, and Fe-Nb-C-N systems has been analyzed and some laws governing the effect of composition and temperature on the phase composition of niobium-containing steels have been considered.

Prediction of Rolling Load Recrystallization Kinetics and Microstructure During Hot Strip Rolling

The recrystallization kinetics and grain size models were developed for the C-Mn and niobium-containing steels to describe the metallurgical phenomenon such as softening, grain growth, and strain accumulation. Based on the recrystallization kinetics equations, the mean flow stress and the rolling load of each pass were predicted and the optimum rolling schedule was proposed for hot strip rolling. The austenite grain refinement is associated with the addition of niobium, the decrease of starting temperature of finish rolling, and the reduction of finished thickness. The mean flow stress curve with a continuous rising characteristic can be usually observed in the finish rolling of niobium-containing steel, which is formed as a result of the heavy incomplete softening and strain accumulation. The predicted rolling loads are in good agreement with the measured ones.

Bactericidal Activity of Copper and Niobium–Alloyed Austenitic Stainless Steel

Biofouling and microbiologically influenced corrosion are processes of material deterioration that originate from the attachment of microorganisms as quickly as the material is immersed in a nonsterile environment. Stainless steels, despite their wide use in different industries and as appliances and implant materials, do not possess inherent antimicrobial properties. Changes in hygiene legislation and increased public awareness of product quality makes it necessary to devise control methods that inhibit biofilm formation or to act at an early stage of the biofouling process and provide the release of antimicrobial compounds on a sustainable basis and at effective level. These antibacterial stainless steels may find a wide range of applications in fields, such as kitchen appliances, medical equipment, home electronics, and tools and hardware. The purpose of this study was to obtain antibacterial stainless steel and thus mitigate the microbial colonization and bacterial infection. Copper is known as an antibacterial agent; in contrast, niobium has been demonstrated to improve the antimicrobial effect of copper by stimulating the formation of precipitated copper particles and its distribution in the matrix of the stainless steel. Thus, we obtained slides of 3.8% copper and 0.1% niobium alloyed stainless steel; subjected them to three different heat treatment protocols (550 degrees C, 700 degrees C, and 800 degrees C for 100, 200, 300, and 400 hours); and determined their antimicrobial activities by using different initial bacterial cell densities and suspending solutions to apply the bacteria to the stainless steels. The bacterial strain used in these experiments was Escherichia coli CCM 4517. The best antimicrobial effects were observed in the slides of stainless steel treated at 700 degrees C and 800 degrees C using an initial cell density of approximately 10(5) cells ml(-1) and phosphate-buffered saline as the solution in which the bacteria came into contact with copper and niobium-containing steel.

Microstructures of Heat-Affected Zone in Niobium Containing Steels

In order to understand the effect of niobium additions on the microstructures of the HAZ (heat-affected zone) in mild steels, a plain C-Mn steel (without niobium content) and three niobium containing steels with 0.01, 0.02 and 0.04 mass%Nb, respectively were investigated through simulated HAZ experiments with the heat inputs of 2×106, 5×106 and 8×106 J/m. The microstructures of simulated coarse-grained HAZ have been examined by optical metallography and transmission electron microscopy. It was found that the addition of niobium had a significant effect on the transformation. For high energy heat inputs (8×106 and 5×106 J/m), the additions of niobium evidently retarded the pearlite formation even in the case of 0.01 mass%Nb containing steel; the microstructures of coarse-grained HAZ of niobiumcontaining steels consisted mainly of secondary Widmanstätten ferrite, but that of the niobium-free steel comprised a large amount of pearlite besides secondary Widmanstätten ferrite. In the condition of low energy heat input (2×106 J/m), the microstructures of coarse-grained HAZ of niobium-containing steels were all similar and consisted mainly of interlocking ferrite plates with small amounts of bainite and Widmanstätten ferrite; while that of the niobium-free steel comprised Widmanstätten ferrite and martensite with a small quantity of pearlite. The results from Charpy impact test indicated that the niobium-containing steels treated with the heat input 2×106 J/m could possess higher toughness than those treated with the heat inputs of 5×106 and 8×106 J/m. It is proposed that the interlocking ferrite structure, which forms in niobium-containing steels under 2×106 J/m heat input, improves the toughness property.

Influence of grain size and precipitation on hot ductility of microalloyed steels

The influence of grain size on the hot ducility of microalloyed steels (C–Mn–Al, C–Mn–V–Al, and C–Mn–Nb–Al) has been determined by heating them above their solution temperatures and cooling to the test temperature of 850°C. The C–Mn–Al steel showed excellent hot ductility which was independent of grain size. Dynamic recrystallization readily occurred and there was no evidence for AlN precipitation. Marked dynamic precipitation occurred during the tensile test for vanadium- and niobium-containing steels but this did not vary significantly with reheating temperature, provided complete dissolution of the precipitates had occurred. Isolating the influence of grain size from that of precipitation in these steels showed that a change in grain size from 150 to 300 μm reduced the reduction of area values by 15–20%. Precipitate distribution was also varied by heating to temperatures in the range 850–1330°C and tensile testing at 850°C. When present before testing at the γ grain boundaries in the form of a fine grain-refining precipitate, AlN reduced the hot ductility in the C–Mn–Al steel and delayed the onset of dynamic recrystallization. Coarser precipitates produced by raising the reheating temperature allowing dynamic recrystallization to occur gave improved ductility. For the niobium- and vanadium-containing steels, precipitate distributions which were in a coarse randomly precipitated form gave the best hot ductility. These occurred with the niobium-containing steel when heated to 1100°C and more generally in the vanadium-containing steel throughout a wide temperature range. The worst precipitate distribution occurred in the niobium containing steel when the NbCN was taken into solution before testing and reprecipitated in a fine form at the γ grain boundaries and within the matrix during the test.MST/490

Influence of grain size and precipitation on hot ductility of microalloyed steels

AbstractThe influence of grain size on the hot ducility of microalloyed steels (C–Mn–Al, C–Mn–V–Al, and C–Mn–Nb–Al) has been determined by heating them above their solution temperatures and cooling to the test temperature of 850°C. The C–Mn–Al steel showed excellent hot ductility which was independent of grain size. Dynamic recrystallization readily occurred and there was no evidence for AlN precipitation. Marked dynamic precipitation occurred during the tensile test for vanadium- and niobium-containing steels but this did not vary significantly with reheating temperature, provided complete dissolution of the precipitates had occurred. Isolating the influence of grain size from that of precipitation in these steels showed that a change in grain size from 150 to 300 μm reduced the reduction of area values by 15–20%. Precipitate distribution was also varied by heating to temperatures in the range 850–1330°C and tensile testing at 850°C. When present before testing at the γ grain boundaries in the form of a ...

Role of niobium in a dual-phase steel

AbstractTwo hot-rolled high-strength, low-alloy steels, one microalloyed with niobium, were intercritically annealed at 740°C for 10 min and cooled to produce a dual-phase microstructure. Although both steels showed tensile properties typical of dual-phase microstructures, only the niobium-microalloyed steel could match the mechanical characteristics of a typical commercial dual-phase steel. The role of the niobium would appear to be to provide a suitable level of strength in the initial, as-rolled state of the steel, since the properties in the dual-phase state were found to be a function of the initial state of the steel. A normalizing treatment resulted in grain refinement in both steels, but transmission electron microscopy observations showed that in the niobium-containing steel this was counterbalanced by an increase in Nb(C, N) precipitate size. The normalizing treatment thus equalized the mechanical properties of the initial state of the two steels, leading to similar dual-phase properties in both...

Formability of high-strength, low-alloy sheet steels

High-strength, low-alloy steel sheet (HSLA) is now replacing some low-carbon steels in the manufacture of pressed components in both commercial and private vehicles; the need for higher payloads in the former, andfor higher strength to meet increasingly stringent safety regulations in the latter, require their utilization. In this paper, the authors describe the investigation of five high-strength steels and compare their tensile properties and microstructural features with laboratorydetermined forming limit diagrams (FLDs). The tension-compression region is relatively insensitive to microstructure and is n value (or uniform elongation) controlled, with the limit line at ∼45° to the ε2=0 axis in accordance with theory. The tension-tension region varies widely with microstructure, particularly with inclusion content and pearlite banding, but only significantly at strain states where ε2/ε1>0·5. It is concluded that high-strength, low-alloy steels, with their higher yield and tensile strengths and so lower elongations and n values, have limited formability; this is reflected in the lower levels of the FLDs of these steels. The mechanical property which has the most influence upon the levels of the FLDs is the strain-hardening behaviour, as represented by the n value; microstructure is only significant in the tension-tension region. The mechanical anisotropy of these steels means that in some cases it is essential to determine the limit strains at both 0° and 90° to the rolling direction, but as most industrial pressings fall into the band ε2=±0·15, for the niobium-containing steels investigated, a single FLD, an ‘average’ of those determined here, would be in order for press-shop use.

Role op grain size in the strengthening of niobium-containing steel G13

Role op grain size in the strengthening of niobium-containing steel G13