Showing papers on "Microalloyed steel published in 2000"

Mathematical modeling of the hot strip rolling of microalloyed Nb, multiply-alloyed Cr-Mo, and plain C-Mn steels

Abstract: Industrial mill logs from seven different hot strip mills (HSMs) were analyzed in order to calculate the mean flow stresses (MFSs) developed in each stand. The schedules were typical of the processing of microalloyed Nb, multiply-alloyed Cr-Mo, and plain C-Mn steels. The calculations, based on the Sims analysis, take into account work roll flattening, redundant strain, and the forward slip ratio. The measured stresses are then compared to the predictions of a model based on an improved Misaka MFS equation, in which solute effects, strain accumulation, and the kinetics of static recrystallization (SRX) and metadynamic recrystallization (MDRX) are fully accounted for. Good agreement between the measured and predicted MFSs is obtained over the whole range of rolling temperatures. The evolution of grain size and the fractional softening are also predicted by the model during all stages of strip rolling. Special attention was paid to the Nb steels, in which the occurrence of Nb(C, N) precipitation strongly influences the rolling behavior, preventing softening between passes. The present study leads to the conclusion that Mn addition retards the strain-induced precipitation of Nb; by contrast, Si addition has an accelerating effect. The critical strain for the onset of dynamic recrystallization (DRX) in Nb steels is derived, and it is shown that the critical strain/peak strain ratio decreases with increasing Nb content; furthermore, Mn and Si have marginal but opposite effects. It is demonstrated that DRX followed by MDRX occurs under most conditions of hot strip rolling; during the initial passes, it is due to high strains, low strain rates, and high temperatures, and, in the final passes, it is a consequence of strain accumulation.

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

Abstract: 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.

Static recrystallization behaviour of a wide range of austenite grain sizes in microalloyed steels

Abstract: The static recrystallization kinetics of four steels microalloyed with Nb, Ti and Nb-Ti have been investigated, covering conventional and hot direct rolling conditions. The influence of the grain size on the recrystallization time has been studied for a wide range of grain sizes (20-1000 μm). For the same deformation conditions it has been observed that the quadratic dependence on grain size works well in the range of finer grain sizes (< 160 μm). In contrast, the recrystallization times measured at coarse grain sizes are significantly lower than those predicted by this dependence. However, when the whole grain size range analysed is considered, a linear dependence on grain size gives the best fit to experimental data. The effect of strain on the recrystallization kinetics has been also analysed for different initial grain sizes. It has been observed that the time to reach a 50% recrystallized fraction (t 0.5 ) shows a dependence on strain of the type t 0.5 e -p . the exponent being influenced by the initial grain size. A unique equation has been proposed to describe the static recrystallization kinetics of Nb, Ti and Nb-Ti microalloyed steels. The predictions of the equations have been compared to previously published data on similar steels.

The Mechanism of Brittle Fracture in a Microalloyed Steel: Part I. Inclusion-Induced Cleavage

Abstract: The cleavage resistance of two microalloyed steels (steels A and B) was studied using several tests, including the instrumented precracked Charpy and Charpy V-notch (CVN) techniques. Ductile-to-brittle transition temperatures were measured for the base-metal and simulated heat-affected zone (HAZ) microstructures. Steel B showed inferior cleavage resistance to steel A, and this could not be explained by differences in gross microstructure. Scanning electron fractography revealed that TiN inclusions were responsible for cleavage initiation in steel B. These inclusions were well bonded to the ferritic matrix. It is believed that a strong inclusion-matrix bond is a key factor in why TiN inclusions are potent cleavage initiators in steel. Strong bonding allows high stresses in a crack/notch-tip plastic zone to act on the inclusions without debonding the interface. Once an inclusion cleaves, the strong bond allows for transfer of the TiN crack into the ferritic matrix. It was estimated that only 0.0016 wt pct Ti was tied up in the offending inclusions in steel B. This indicates that extended times at high temperatures during the casting of such steels could produce TiN-related toughness deterioration at even modest Ti contents.

Thermodynamic analysis of inclusions in Ti-deoxidised steels

Abstract: The precipitation of non-metallic inclusions in Ti-deoxidised steels is analysed with the multiphase equilibrium code CEQCSI which is based on the IRSID slag model. For that purpose, the slag model has been extended to systems containing titanium oxides, using data available in the literature, and validated with industrial results obtained on various steel grades. Applications concerning slag-metal reactions in ladle metallurgy are described. 2 applications concerning inclusions in Ti-deoxidised steels are also presented: (a) experimental study of the effect of steel contamination by magnesia refractory lining on the composition of oxide inclusions in a Ti-deoxidised steel: there is a good correlation between analysed oxide compositions and compositions predicted by the model; (b) evaluation of the optimised steel composition to obtain Al 2 O 3 -TiO 2 inclusions during solidification for steels pertaining to the so-called metallurgy of oxides in steels.

The mechanism of brittle fracture in a microalloyed steel: Part II. Mechanistic modeling

Abstract: In Part I of this study, cuboidally shaped inclusions were found to be responsible for cleavage initiation in a low-carbon, microalloyed steel. In Part II, electron microdiffraction was used to identify these inclusions as the fcc phase (NaCl prototype) in the titanium-nitrogen system. A model for cleavage as induced by these inclusions is proposed. A microcrack begins on one side of the TiN inclusion, propagates to the other side, and then transfers into the matrix. Initiation at a particular location in the particle is believed to be caused by dislocation pileup impingement and stress concentrations such as crystal defects and surface irregularities within the TiN. Dislocations in the TiN inclusions were imaged by transmission electron microscopy (TEM). After the TiN microcrack transfers into the matrix, propagation spreads radially. From the area of crack transfer, two simultaneous propagation paths reverse directions and rotate around the particle. The particle separates these cracks for a short distance, they travel on different cleavage planes, and upon rejoining, a ridge of torn matrix is created. The location of this ridge can be used to infer where cleavage began in the TiN and where the microcrack transferred into the matrix. Tessellated residual stresses arising from differential thermal contraction between the TiN and the matrix are suggested to increase the cleavage-initiating potency of TiN inclusions.

Alignment of Ferrite Grains during Austenite to Ferrite Transformation in a High Magnetic Field

Abstract: Effects of a high magnetic field on austenite to ferrite transformation behavior were investigated in Nb containing steel. It was found that the isothermal ferrite transformation was accelerated by applying a magnetic field, mainly due to an increase in the nucleation rate of ferrite and no fundamental change in the transformed structure was observed. Recently. Shimotomai et al. showed that an aligned, two-phase structure is obtained by a reverse-transformation from lath martensite in a magnetic field. Considering the conditions of this heat treatment, it is expected that the aligned structure can also be formed in austenite to ferrite transformation. In this paper, the alignment of ferrite grains during austenite to ferrite transformation in a high magnetic field is reported.

Correlation of the microstructure and fracture toughness of the heat-affected zones of an SA 508 steel

Abstract: In this study, microstructures of a heat-affected zone (HAZ) of an SA 508 steel were identified by Mossbauer spectroscopy in conjunction with microscopic observations, and were correlated with fracture toughness. Specimens with the peak temperature raised to 1350 °C showed mostly martensite. With the peak temperature raised to 900 °C, the martensite fraction was reduced, while bainite or martensite islands were formed because of the slow cooling from the lower austenite region and the increase in the prior austenite grain size. As the martensite fraction present inside the HAZ increased, hardness and strength tended to increase, whereas fracture toughness decreased. The microstructures were not changed much from the base metal because of the minor tempering effect when it was raised to 650 °C or 700 °C. However, fracture toughness of the subcritical HAZ with the peak temperature raised to 650 °C to 700 °C was seriously reduced after postweld heat treatment (PWHT) because carbide particles were of primary importance in initiating voids. Thus, the most important microstructural factors affecting fracture toughness were the martensite fraction before PWHT and the carbide fraction after PWHT.

Effect of Silicon on the Kinetics of Nb(C, N) Precipitation during the Hot Working of Nb-bearing Steels

Abstract: The effect of Si on the rate of Nb(C, N) precipitation was investigated by using fractional softening measurements. Compression specimens, with diameters of 7.6 mm and lengths of 11.4 mm, were prepared from four Nb microalloyed steels containing a range of Si concentrations from 0.01 mass% to 0.41 mass%. Double-hit compression tests, with a strain of 0.3 for each pass and a strain rate of 0.1/sec., were employed. A solution heat treatment was applied immediately prior to each test. It was found that the rate of Nb(C,N) precipitation in hot deformed austenite increases with Si concentration. This phenomenon is attributed to the increases in the activities of C and N that result from Si addition.

STUDY ON THE PRECIPITATION BEHAVIOR OF TiN IN THE MICROALLOYED STEELS

Abstract: The precipitated behavior of TiN in liquid steels and during solidification of the microalloyed steels containing Ti has been studied by experiment and thermodynamic calculation The results show that precipitation occasion and condition of TiN in the microalloyed steels can be controlled by adjusting the content of N and Ti, that may reduce its detriment to the steel properties The probability of utilizing the fine and disperse TiN in liquid steel as the nuclei for crystallization of the casting structure, which could obtain fine and equiaxial grains, was also discussed

Fatigue properties of a 4340 steel coated with a Colmonoy 88 deposit applied by high-velocity oxygen fuel

Abstract: The fatigue behavior of a quenched and tempered AISI 4340 steel has been evaluated in three different conditions: (a) uncoated; (b) grit-blasted with alumina; (c) grit-blasted and coated with a thermal-sprayed Colmonoy 88 deposit 220 μm thick, employing a high-velocity oxygen fuel (HVOF) gun. The results indicate that grit blasting the base steel can lead to a significant reduction in the fatigue properties of the material. The microscopic observation of the fracture surfaces of the blasted samples indicates that the fatigue processes are initiated at the alumina particles that were retained within the matrix near to the surface of the specimens, giving rise to the presence of stress concentrators that act as nucleation sites of the fatigue cracks. Coating the blasted substrate with this kind of deposit leads to a further reduction of the fatigue properties of the base steel. Such a reduction has been associated with the fracture and partial detachment of the coating from the substrate along the substrate-deposit interface and the reduction in the area of the load-carrying segments of the composite material during fatigue testing. SEM observations of the fracture surfaces of coated samples tested at low alternating stresses, that support this view, have been presented. Under some alternating stresses, the HVOF deposits are believed to contribute to endure the stress applied to the material and therefore the evaluation of the fatigue properties of this kind of coated materials must take into account the thickness of the deposit sprayed unto the substrate. The analysis of the fracture surfaces of the coated specimens revealed that, in this case, the crack nucleation sites are also associated with the presence of Al 2 O 3 particles at the substrate-deposit interface. The fatigue performance of the material under the different conditions analyzed has been quantified by determining the Basquin parameters from the fatigue life curves obtained.

Tensile Properties of Simulated Thin Slab Cast and Direct Rolled Low-carbon Steel Microalloyed with Nb, V and Ti

Abstract: Microalloying additions are made to low carbon steels for a number of purposes. In the conventional controlled rolling of reheated slabs, an important function of niobium is to control recrystallisation in the austenite phase field in order to refine the ferrite grain size of the controlled rolled product. A second benefit is that niobium makes a contribution to the strength of the product through precipitation in the ferrite. Vanadium carbides and nitrides are too soluble in austenite to have a significant effect on grain refinement, when added to a Nb-bearing steel, but may augment precipitation strengthening, particularly at raised nitrogen levels. Titanium is generally added to restrain grain growth in the austenite on reheating and in the heat affected zone of welds, but is also capable of restraining recrystallisation in the austenite during controlled rolling, and of contributing to tensile strength. In conventional processing of continuously cast thick slab, the steel is reheated to 1 200–1 250°C for up to 8 hr before rolling. This conditions the austenite by partial homogenisation and solution of the microalloying elements. The austenite grain size after soaking and at the start of the rolling process is then usually smaller than it was in the original casting, being determined by the dispersion of any undissolved microalloy nitrocarbides and undissolved inclusions. In recent years, however, the practice of hot direct rolling has been developed, in which the as-cast austenite is rolled directly, without the intermediate stages of cooling to a low temperature, reheating and soaking. The austenite then has the coarse-grained, dendritic and segregated microstructure of the original casting prior to the commencement of hot rolling. Although hot direct rolling has been practised using conventional thick slab, its most practical application is in the production of hot rolled strip from thinner slab in minimills. With this newer technology, slab in thicknesses down to 50 mm is continuously cast and directly hot rolled within, typically, 15–20 min after casting; the only “heat treatment” between casting and rolling is temperature equalisation at approximately 1 100–1 150°C during that time interval. Both niobium and titanium segregate to the liquid phase during freezing, to form eutectic carbonitrides or nitrides on solidification. In the absence of a prolonged soaking period at high temperature, it might be expected that the presence of such precipitates in the austenite at the commencement of direct hot rolling might reduce the effectiveness of the microalloying additions, compared to their effect in conventional processing. The purpose of this research, therefore, was to investigate the tensile properties of hot direct rolled low carbon steels microalloyed with niobium, the composition being varied to allow eutectic NbCN to form over part of the range. The effect of including V and Ti additions over part of the composition range was also investigated.

Grain refinement and stabilization in spray-formed AISI 1020 steel

Abstract: An AISI 1020-grade mild steel, with a small amount of aluminum, was spray-formed by nitrogen gas-atomization and deposition. The spray-formed 1020 steel contained 0.05 mass% of nitrogen and 0.06 mass% of aluminum. Rolling the spray deposit at 1123 K for a thickness reduction of 70% and subsequent normalizing at low austenitic temperatures produced a fully dense steel having a refined ferrite grain size as small as 3 μm. This grain refinement resulted from the pinning of prior austenite grain boundaries by fine AlN particles which precipitated during the thermomechanical treatments. Room-temperature tensile properties (YS: 550 MPa, UTS: 630 MPa, elongation: 23%), exceeding those of conventional microalloyed high-strength low alloy steels, were achieved in the normalized state. The AlN-pinned fine-grained microstructure survived subsequent re-austenizing at temperatures as high as 1273 K. When tensile-tested in the austenite region at 1123 K, the microalloyed 1020 steel showed a large elongation exceeding 200%.

Atmospheric Corrosion Performance of Quenched-and-Tempered, High-Strength Weathering Steel

Abstract: Eight-year atmospheric corrosion tests of A 588B (UNS K12040) weathering steel were conducted in industrial, marine, and rural environments. The material was tested following two types of heat treatment: quenched-and-tempered to produce a tempered martensite microstructure and normalized to produce a microstructure comprised of ferrite and pearlite. Results of these tests indicated that these heat treatments had no effect on the corrosion resistance and that the performance of weathering steels can be estimated solely on the basis of composition.

Effect of composition and thermomechanical processing on the ageing characteristic of copper-bearing HSLA steel

Abstract: 3 alloys of copper-bearing steels with or without boron have been studied in different thermomechanical controlled processing (TMCP) conditions. The influence of TMCP schedules onto the ageing behaviour in boron-treated and boron-free steels is examined. The structure property analysis of these steels is also carried out. It is found that when the finish rolling is carried out above Ar 3 , the steels direct cooled after TMCP are able to retain a higher amount of copper in solid, leading to higher ageing response. Boron is found to delay the precipitation of copper and the best ageing effect is recorded in B-Cr steel.

Influence of strain rate on recrystallisation– precipitation interaction in V, Nb, and V–Ti microalloyed steels

Abstract: A study has been made of the recrystallisation–precipitation interaction in three microalloyed steels, containing respectively V, Nb, and V–Ti, applying two different strain rates. Recrystallised fraction v.time curves were determined and used to draw recrystallisation–precipitation–time–temperature (RPTT) diagrams. The influence of strain rate has been shown to be similar in the three steels. On the basis of the results the value of }}SB0·19 has been found for the exponent of the strain rate, following Dutta and Sellars'model for the parameter t 0·05 , which differs from the value }}SB0·5 proposed by these authors. Simultaneously, the influence of strain rate on the static recrystallis ation critical temperature has been determined, it being observed that an increase in the former leads to a drop in the latter. Furthermore, strain rate is shown to have an influence on the recrystallisation–precipitation interaction, acting on those parameters that best contribute to defining RPTT diagrams In this...

Strength and formability of ultra-low-carbon Ti-IF Steels

Abstract: Ultra-low-carbon interstitial free (IF) steel sheets bearing Ti and/or Nb have been extensively used for automotive panels because of superior formability and nonaging properties. It is well known that the interstitial elements such as C and N play important roles in the formability. The lower the contents of the C and N in steel, the better the formability of the steel. The demands for the steel with excellent formability from automotive industry will accelerate the progress in the steelmaking process, leading to the development of the ultra-low-carbon steel. With the advent and installation of improved vacuum degassing equipment in the steelmaking process, it is now possible to consistently produce ultra-low-carbon content of 0.002 to 0.005 wt pct. It is expected that in the near future, the C and N contents can be lowered to as low as 0.001 pct or less. This study is focused on strength and formability in the extremely ultra-low-carbon IF steels containing about 0.001 pct carbon.

Development of an ultra high strength hot rolled steel

Abstract: The use of as-hot rolled steel has captured a lot of interest as an alternative to equivalent strength quenched and tempered products in industries such as automotive.

Performance of Pipeline Steels in Sour Service

Abstract: The demand for steel for the production of pipelines to transport gas and oil containing hydrogen sulphide prompted the development of steel that is resistant to hydrogen induced cracking (HIC).During the past two decades, combined research efforts in the areas of product and process metallurgy have made it possible to satisfy most of the main requirements for grades X-42 and X-60 microalloyed steel for mildly acidic (pH = 5) H2S environments. Building on the experience acquired in the area of microalloyed steel for a mildly acidic (pH ∼ 5) H2S environment, the industry launched a program to develop steel that would satisfy new requirements for H2S-resistant pipelines under NACE conditions (TM0177, pH∼3). In order to develop these steels, it was necessary to define qualitatively and quantitatively the specific effects on H2S resistance of the multiple intrinsic parameters of the product itself as well as those resulting from the process.In this paper, data will be presented that have made it possible to relate the HIC performance of steels to chemical content, inclusion levels and thermomechanical treatment parameters.Copyright © 2000 by ASME