V-bearing Steel Research Articles

Effect of NbC and VC carbides on microstructure and strength of high-strength low-alloyed steels for oil country tubular goods

High-strength low-alloyed (HSLA) steels for oil country tubular goods (OCTGs) are usually microalloyed with niobium (Nb) and vanadium (V). In this work, the microstructure and strength of HSLA steels with different Nb and V contents were studied with the purpose of investigating the role of NbC and VC carbides. The results showed that NbC carbides were responsible for restraining the growth of prior austenite grains, and when the austenitizing temperature was below 910 °C, it has been found that the prior austenite grain size was similar in steels with 0.02 and 0.006 wt% Nb addition. VC carbides could impede dislocation annihilation and refine Cr-rich carbides and martensitic blocks during tempering. Moreover, VC carbides mainly contributed to the tempering softening resistance so that the V-bearing steels exhibited a higher strength than the V-free steel after tempering at 690 °C–730 °C. To illustrate the high tempering softening resistance of V-bearing steels, the changes in various strengthening contributions induced by V addition were quantitatively evaluated. The calculation results indicated that the main reason for the high tempering softening resistance of V-bearing steels was precipitation strengthening by nanosized VC carbides, in contrast to dislocation strengthening, substructure strengthening, and precipitation strengthening due to the refinement of Cr-rich carbides.

The Effect of Al2O3, CaO and SiO2 on the Phase Relationship in FeO\u2013SiO2 Based Slag with 20\xa0Mass% Vanadium

An innovative and sustainable way for production of vanadium (V) is to co-process V titanomagnetite ores with V-bearing steel slag, via reduction. V-slag produced from highly enriched Fe–V melt contains as high as 20 mass% V. The phase relationship in FeO–SiO2 based slag with V fixed at 20.4 mass% was investigated experimentally at 1673 K. The compositions covered the normal production range, viz. Al2O3 from 0 to 6 mass%, CaO from 1 to 5 mass% and SiO2 from 10 to 17 mass%. The content of MnO was fixed at 5.5 mass%. Samples quenched at 1673 K were found to be solid–liquid mixtures, either in a 2-phase region consisting of liquid and a solid spinel phase, or in 3-phase region consisting of liquid, spinel phase and silica phase. Free silica was only found in samples with a composition of 0 mass% Al2O3, 1 mass% CaO and 17 mass% SiO2. Formation of free silica was prohibited in samples with 1 mass% CaO and up to 17 mass% SiO2 by adding 5 mass% Al2O3. Addition of 4 mass% Al2O3 was also sufficient to prevent the presence of free silica in samples with 3 mass% CaO and 13.5 mass% SiO2. Most of the V was kept in the spinel phase, while the solubility of V in the liquid was generally less than 1 mass% as V2O3. Fe, Mn and Al coexisted in both liquid phase and spinel phase. On the other hand, Si and Ca only distributed in the liquid phase. The fraction of solid phase, viz. spinel was found to decrease with increasing SiO2 content.

HAZ softening in Nb-, Ti- and Ti + V-bearing quenched and tempered steel welds

Three high-strength Nb-, Ti- and Ti + V-microalloyed S690QL steels were welded to investigate the formation of softened HAZ and its impact on tensile properties. The welding was performed with three levels of heat input to produce softened zones with different characteristics (softening width, minimum hardness and softening ratio), and then, further tensile tests were done to study their influence on weld performance. The results showed that Ti bearing steel exhibited the lowest resistance to softening with the presence of largest soften width and lowest hardness value, causing final tensile failure occurred at softened HAZ. The metallurgical reason for the lower hardness is the high fraction of coarse ferrite. Nb- and Ti + V-bearing steels suffered moderate softening, due to high hardenability with addition of Mo, Nb and V, but the softening effect did not remarkably influence the tensile properties of these two steels.

Tearing Resistance Properties of Cr-Mo Steels with Internal Hydrogen Determined by the Potential Drop Method

The tearing resistance, dJ/da, of conventional 2.25Cr-1Mo steels and a V-bearing steel (2.25Cr-1Mo-0.3V steel) with internal hydrogen was measured using the effective offset potential drop method. Internal hydrogen refers to test specimens that are precharged (thermally charged) prior to testing. In general, Cr-Mo steels, used widely in the refining and petrochemical industries, are susceptible to temper embrittlement. However, very few studies have dealt with the effects of hydrogen and temper embrittlement on the tearing resistance. Test specimens were prepared by subjecting them to normalizing, tempering, and post-weld heat treatments that simulated actual conditions. Some specimens were embrittled by step cooling. Hydrogen substantially reduced dJ/da for all samples except for that for the V-bearing steel, and temper embrittlement caused additional adverse effects on dJ/da for samples with internal hydrogen for which the temper embrittlement parameter, i.e., the J-factor, was large.

Comparison of the Annealing Behavior of Three Cold Rolled (Ti-bearing, Nb-bearing and V-bearing) HSLA Steels

ABSTRACTThe annealing behavior of three HSLA steels is studied using the combined techniques of EBSD-KAM and Sub-grain Method. These techniques have been successfully used to assess the annealing behavior of AK, IF and other high strength steels. Stored energy maps in the hot band, cold rolled and after annealing are constructed and analyzed. The combined usage of the Sub-grain Method and EBSD-KAM techniques are employed to calculate and compare the evolution of the stored energy and recrystallization behaviour during the annealing of Ti-bearing, Nb-bearing, and V-bearing HSLA steels. Orientation dependent stored energy distribution maps at different annealing stages are constructed and analysed. The results show that the stored energy distribution through the thickness of the samples is not uniform and is independent of the steel composition. Similarly the recrystallization behaviour is strongly related to the initial microstructural condition and particularly to the grain boundary character distribution of the steels.

Delayed Fracture Using CSRT and Hydrogen Trapping Characteristics of V-bearing High-strength Steel

The delayed fracture characteristics of V-bearing steel were evaluated using conventional strain rate test (CSRT) and the hydrogen absorption and desorption behaviors were studied with the specimens hydrogen-charged and then exposed to air of 30°C for up to 2.5 months. CSRT was carried out at two test sites, and nearly the same delayed fracture resistance was obtained for the V-bearing steel. The fracture appearance changed from quasicleavage to intergranular with increasing hydrogen content. The hydrogen content of the boundary between fracture appearances was approximately 4 mass ppm. The hydrogen introduced into the V-bearing steel was composed of a diffusible one which decreased in concentration in 24 h when exposed to air of 30°C, and two types (weakly and strongly) of trapped ones. The strongly trapped hydrogen remained in the specimen after 2.5 months of exposure in air. By analyzing the thermal desorption profiles with Gaussian function, the peak temperatures of these hydrogen types were 100°C, 167°C and 198°C, corresponding to diffusible, weakly and strongly trapped hydrogen, respectively. The hydrogen-charged specimens of more than 4 mass ppm were fractured in the intergranular mode. After exposure in air and the hydrogen content became less than 4 mass ppm, the fracture mode changed to quasicleavage. After recharging the hydrogen to more than 4 mass ppm, the fracture mode became intergranular again.

Delayed Fracture Using CSRT Method and Hydrogen Trapping Characteristics of V-bearing High Strength Steel

The delayed fracture characteristics of V-bearing steel were evaluated using the CSRT method, and the hydrogen trapping and de-trapping behavior was studied with the specimens hydrogen-charged and then held in air for up to 2.5 months.The CSRT tests were carried out at two test sites and nearly the same delayed fracture characteristics were obtained using the V-bearing steel. The fracture appearance changed from quasi-cleavage to inter-granular fracture with an increase of hydrogen content. The boundary hydrogen content of the fracture appearance change is around 4 mass ppm.The hydrogen charged in V-bearing steel was composed of diffusible one, which comes out in 24 h when held in air of 30°C and two kinds of (weakly and strongly) trapped ones. The strongly trapped hydrogen remained in specimen after 2.5 months held in air. By analyzing the thermal desorption profiles with Gaussian function, the peak temperatures of these hydrogen were 100°C, 167°C and 198°C, which corresponds to diffusible, weakly and strongly trapped hydrogen.The specimens hydrogen-charged more than 4 mass ppm were fractured in inter-granular mode. After held in air and the hydrogen content became less than 4 mass ppm the fracture mode changed to quasi-cleavage. Re-charging the hydrogen more than 4 mass ppm, the fracture mode became inter-granular again.

自動車用熱延高張力鋼板の機械的性質に及ぼすＮｂおよびＶの影響

Nb-bearing and V-bearing steels have been well known as typical high-strength low-alloy (HSLA) steels. However, little comparison has been made so far between the formability of Nb-bearing steels and that of V-bearing steels for automotive use. The hot direct rolling (HDR) process has been noticed because it is a very energy saving process. This paper aims to clarify the differences between the formability of Nb-bearing hot-rolled steel sheets and that of V-bearing hot-rolled steel sheets produced by both HDR process and cold charge rolling (CCR) process. A low temperature (450°C) simulated coiling characterized by good formability of steels was applied in a laboratory to 0.025% Nb (25 Nb), 0.049% Nb (50 Nb) and 0.078% V (80 V) steels with the ultimate tensile strengths (TS) ranging from 490 to 590 MPa. Hole expansion test was carried out in addition to the tensile test because stretch flangeability is very important for automobile structural uses. As a result, TS-flangeability balance of Nb-bearing steels was found to be superior to that of V-bearing steel. This is because their microstructures were distinctly different: Nb-bearing steels had a ferrite+bainite microstructure and V-bearing steel had a ferrite+fine pearlite microstructure. For the linearly precipitated carbides, V-bearing steel had a low TS-flangeability balance. It was concluded that Nb-bearing steels is strengthened by ferrite grain refining and bainite transformation hardening rather than precipitation hardening, which is the main strengthening mechanism for V-bearing steels. Change from CCR to HDR increased the strengths of all the steels studied, resulting in nearly the same TS-elongation and TS-uniform elongation balances, but the TS-flangeability balance was still maintained through this change.

Relationship between microstructure and hydrogen absorption behavior in a V-bearing tempered martensitic steel

TEM observations and hydrogen thermal desorption analysis were performed to understand the relationship between microstructure and hydrogen absorption behavior in a V-bearing steel. The hydrogen absorption property was changed by controlling VC precipitation with changing tempering temperature from 573K to 973K. Hydrogen was absorbed into the specimen by cathode charging at a constant condition, and its content was examined by thermal desorption analysis. Energy-filtering and high-resolution TEM methods were employed to observe the size and distribution of VC precipitates. When the specimen was tempered at around 873K, the hydrogen absorption content was markedly increased; 4 times higher than that of the as-quenched specimen. TEM observations showed that this significant hydrogen absorption is attributed to nano-scale VC precipitates.

Computer Simulation of VC Precipitation at Moving γ/α Interfaces

A computer simulation technique was developed on the basis of the interaction between carbide precipitation and moving γ/α interfaces for predicting both the interphase precipitation kinetics and the microstructural evolution during austenite-to-ferrite transformation. Theoretical models for the calculation of the driving and pinning forces exerted on a moving interface boundary are described. The variations of the two forces lead to a phenomenon of periodic pinning and unpinning of the interface and, in turn, the characteristic microstructure of parallel sheet particles which is often associated with the interphase precipitation. The experimental data reported for a series of V-bearing steels were analyzed using the computer simulation technique. Three un- known physical parameters,i.e., the thickness of an incoherent interface or the height of the ledge of a coherent interface, the diffusion coefficient of V at the γ/α interface, and the co- herence loss parameter of a VC nucleus, were determined. The calculated intersheet spacing, precipitate size, and precipitation start time all show a good correlation with the experimental observations.

Heat treatment of some temperature-resistant carburizing steels

The heat treatment of some Mo-Cr-V secondary hardening carburizing steels was investigated. The compositions selected for study were chosen because 1) they can be readily softened by annealing, and 2) they do not require excessively high temperatures for carburizing. Both of these characteristics are desired for ease of manufacturing.After carburizing these steels at 955° C and oil quenching, the as-quenched core hardness is retained with little change for tempering temperatures up to 600° C, but the case hardness decreases slowly with increasing tempering temperature, reaching values of 52–54 Rc after tempering at 550° C. The case depths, case carbon content, retained austenite content and residual stress resulting from carburizing the experimental alloys were compared with the same properties measured on SA E 8620 steel. It was found that the surface oxides which form on V-bearing steels during heat treatment in endothermic gas-base carburizing atmospheres interfere with carburizing. For this reason, steels with 0.15 wt. pct. V or less are preferred for carburizing.Because of their enhanced resistance to softening at elevated temperatures compared to conventional carburizing steels, carburized secondary hardening steels should provide improved resistance to surface damage under conditions of marginal lubrication.

Creep Rupture Strength and Microstructure of Low C-10Cr-2Mo Heat-Resisting Steels with V and Nb

The new ferritic heat-resisting steels of 0.05C-10Cr-2Mo-0.10V-0.05Nb (Cb) composition with high creep rupture strength and good ductility have already been reported. The optimum amounts of V and Nb that can be added to the 0.05C-10Cr-2Mo steels and their effects on the creep rupture strength and microstructure of the steels have been studied in this experiment. The optimum amounts of V and Nb are about 0.10 pct V and 0.05 pct Nb at 600 °C for 10,000 h, but shift to 0.18 pct V and 0.05 pct Nb at 650 °C. Nb-bearing steels are preferred to other grades on the short-time side, because NbC precipitation during initial tempering stages delays recovery of martensite. On the long-time side, however, V-bearing steels have higher creep rupture strength. By adding V to the steels, electron microscopic examination reveals a stable microstructure, retardation during creep of the softening of tempered martensite, fine and uniform distribution of precipitates, and promotion of the precipitation of Fe2Mo.