Showing papers on "Microalloyed steel published in 1991"

Interactions between recrystallization and precipitation in hot-deformed microalloyed steels

Abstract: The static recrystallization and precipitation characteristics of low alloy steels containing Nb were studied following high temperature compression in the austenite range. The recrystallization kinetics in the Nb-steels were slower than those in the plain carbon steels by approximately an order of magnitude when Nb was in solution. However, much greater retardation of recrystallization was observed in the Nb-steels when precipitates were formed. The initiation of both recrystallization and precipitation appeared to be localized in the early stage of their nucleation. Recrystallization initiated predominantely at the prior austenite grain boundaries. Boundaries were also the preferential nucleation sites for NbCN precipitation. Hence, the NbCN particles were distributed in a highly localized fashion, sometimes delineating what may have been the prior austenite grain or subgrain boundaries. The heterogeneous nature of the nucleation process for recrystallization and precipitation suggested that the critical factor which determined the retardation of recrystallization was the local pinning effect of precipitation. The local pinning force was estimated from the local distribution of precipitation and was found to be of magnitude comparable to that of the driving force for recrystallization. The precipitation pinning force increased in the beginning of precipitation, showed a peak in the intermediate stage, and finally decreased as particles coarsened and were distributed more uniformly.

Carbonitride precipitate growth in titanium/niobium microalloyed steels

Abstract: Transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) have been used to investigate the morphology, distribution, composition, particle size distributions, and growth kinetics of carbonitride precipitates in steels containing low levels of Ti, Nb, C, and N. During the aging, only the complex carbonitride precipitates of the form (TixNb1−x) (CyN1-y) were found in the newly nucleated and growing particles. The youngest of these particles which approach the size of critical nuclei tends to be Ti-rich. Almost all of these particles are nearly spherical. The initial growth of the precipitates, which is very rapid, lasts less than 30 seconds followed by slow ripening. A model predicting the growth kinetics of carbonitrides and composition variation within the precipitates for the initial stage (before coarsening) has been developed based on equilibrium thermodynamics with the inclusion of capillarity and multicomponent diffusional kinetics. Satisfactory agreement with the experimental results has been demonstrated.

Recrystallization controlled rolling and accelerated cooling for high strength and toughness in V-Ti-N steels

Abstract: Optimum thermomechanically controlled process parameters have been established for the production of Ti-V-N microalloyed high-strength low-alloy (HSLA) steels. On the basis of laboratory simulation and full-scale processing, it has been shown that nitrogen is an essential alloying element addition and full appreciation of its effects leads to the ability to utilize high nitrogen steel in connection with hot rolling in a high-temperature regime to produce HSLA products with very favorable combinations of yield strength and toughness. The effects of reheating temperature, rolling reduction, cooling rate, and finish-cooling temperature (FCT) on the ferrite grain size and mechanical properties have been examined. It has been shown that the potential for precipitation strengthening is dependent on vanadium, nitrogen, and cooling parameters. Accelerated cooling (ACC) prevents precipitation of vanadium nitrides in austenite and enhances both grain refinement and precipitation strengthening. By adjusting nitrogen content and processing parameters, a yield strength of 500 MPa and impact transition temperature (ITT) below -60 ‡C can be obtained in the as-hot-rolled condition in Ti-V-N steels, using high finish-temperature hot rolling and accelerated cooling.

Activation energy in the static recrystallization of austenite

Abstract: By means of torsion trials, the influence of various alloying elements on the parametert0.5 of Avrami's equation has been studied. It has been found that carbon accelerates the static recrystallization of the austenite and that the other elements, in solution or by forming precipitates, slow it down. It is verified that the activation energy is the parameter most sensitive to the chemical composition and its value has been defined in a collection of 11 steels of the C-Mn type and those, microalloyed with Nb and Ti. Finally an equation is proposed which allows the calculation of the activation energy as a function of the chemical composition of the steel with great accuracy.

Multiphase microalloyed steel and method thereof

Abstract: A steel of particular utility in forging applications has a composition, in weight percent, of from about 0.05 to about 0.35 percent carbon, from about 0.5 to about 2.0 percent manganese, from about 0.5 to about 1.75 percent molybdenum, from about 0.3 to about 1.0 percent chromium, from about 0.01 to about 0.1 percent niobium, from about 0.003 to about 0.06 percent sulfur, from about 0.003 to about 0.015 percent nitrogen, from about 0.2 to about 1.0 percent silicon, balance iron plus conventional impurities. The steel may be worked in the austenite region to produce a well-conditioned austenite structure, cooled to transform the microstructure to a mixture of ferrite and bainite, and then cold forged to a final form. The steel may also be hot forged without first producing the well conditioned austenite. Heat treating of the final product is not required.

Hot-rolled microalloyed steel and its use in variable-thickness sections

Abstract: A vanadium-nitrogen microalloyed steel is continuously hot rolled to C-shaped sections that meet property requirements for side rails of truck frames with no heat treatment. Sections with different thicknesses of the web and flange regions can also be prepared by the same processing. The composition of the steel is from about 0,16 to about 0.20 percent carbon, from about 1.2 to about 2.0 percent manganese, from about 0.45 to about 0.55 percent silicon, from about 0.10 to about 0.30 percent vanadium, from about 0.001 to about 0.030 percent aluminum, from about 0.010 to about 0.027 percent nitrogen, less than about 0.080 percent phosphorus, less than about 0.030 percent sulfur, balance iron totalling 100 percent, with all percentages by weight. Variations of this steel containing either from about 0.01 to about 0.02 percent titanium or 0.04 to about 0.07 percent aluminum are also permissible.

Fracture toughness characterization of a weldment in a microalloyed steel using resistance curves

Abstract: Various weldment regions, e.g. the weld zone, heat-affected zone (HAZ) and parent metal, are characterized in terms of strength and fracture toughness parameters, J integral and crack-tip-opening displacement (CTOD) δ after manual metal arc weld to a microalloyed steel plate. The critical J values are obtained by three methods, namely the ASTM Standard E 813-81, the stretched-zone width measurement with superposition on the R curve and the proposed method based on the experimental data. On the contrary the critical δ values are determined by four methods: (i) British Standard 5762; (ii) superposition of the blunting line on the R curve; (iii) stretched-zone width measurement and superposition on the R curve; (iv) the proposed method. A linear relationship between the CTOD and stretched-zone width with a slope of 0.43 is obtained in the present study. The crack growth toughness values are also obtained from the R curves. The crack initiation toughness values obtained by the above methods are compared. The study focuses on the J - δ relationship for the weld metal and the HAZ. Quantitative microstructural analysis and fractographic studies have also been conducted to rationalize the observed toughness values in the weld metal, HAZ and the parent metal.

Effect of interrupted accelerated cooling on mechanical properties and structure of vanadium and titanium microalloyed steel bars

Abstract: The effect of interrupted accelerated cooling, in the range from finish rolling temperature to 600°C at rates up to 17 K s−1, on the mechanical properties and structure of V and Ti microalloyed steel bars was investigated. The influence of reheating temperature, in the range 1000–1250°C, and finish rolling temperature (starting cooling temperature), in the range 800–1000°C, was also studied. Interrupted accelerated cooling caused a double enhancement of grain refinement and an increase of precipitation strengthening resulting in increases of yield strength up to 750 MN m−2 and elongations greater than 25%. Increasing the reheating temperature to 1100 and 1250°C for V and Ti steels, respectively, enhanced the increment of precipitation strengthening due to accelerated cooling, but had no effect on grain refinement. Interrupted accelerated cooling was found to be most effective for improving yield strength when it is started at higher temperatures in the γ region. Accelerated cooling effectively inh...

Manganese partitioning and dual-phase characteristics in a microalloyed steel

Abstract: A two-stage intercritical annealing treatment has been attempted to develop dual-phase structure and properties in a commercial grade microalloyed steel containing Nb and V. Manganese concentration in austenite enhanced the hardenability which attributes to martensite formation on air cooling after annealing. The volume fraction of martensite and the manganese distribution were found to depend on both annealing time and temperature. Different morphologies of martensite were identified for various intercritical annealing conditions, e.g. dispersed type, ring type, acicular type. The tensile data and behaviour confirmed the dual-phase characteristics in the microalloyed steel.

Austenite grain growth during hot forging of medium carbon engineering steels with or without V–Ti microalloying

Abstract: Medium carbon engineering steels which are used for automotive and machine components are usually forged at rather high temperatures and subsequently quenched and tempered or controlled cooled. The austenite grain structure at the beginning of the γ-α transformation greatly determines the transformation process. Two commercial steel grades (about 0·4%C) with and without V–Ti microalloying were investigated using hot compression tests to simulate forging. The development of austenite grain structure during reheating, forging, and subsequent holding or cooling to the transformation temperature was evaluated. On reheating of both steels, a narrow range of temperature close to l000°C was found in which formation of a mixed austenite grain structure was observed. After forging, the higher the hot strain the faster the grain growth after recrystallisation. In the microalloyed steel, onset of abnormal grain growth was observed after heavy deformation at 1200°C, presumably due to strain induced coalescenc...

Hydrogen effects in a dual-phase microalloy steel

Abstract: A dual-phase steel containing niobium, vanadium and titanium as microalloying elements was tested for hydrogen embrittlement (HE). The susceptibility to HE was observed to be closely related to the microstructural state. Hydrogenated specimens intercritically annealed at relatively low temperatures to develop martensite islands in a ferrite matrix basically exhibited quasi-cleavage fracture with some ductile dimpling. The mode of fracture in charged specimens quenched from higher intercritical annealing temperatures was predominantly intergranular fracture along prior austenite grain boundaries and cracking of martensite laths. The detrimental role of residual stresses, retained austenite and microalloying carbides in the process of HE is discussed.

Stack cracking by hydrogen embrittlement in a welded pipeline steel

Abstract: Arrays of cracks, parallel to the original plate rolling direction, were produced in a X65 microalloyed steel by hydrogen embrittlement of pipeline sections containing a weldment. A region of the heat-affected zone of the weldment was shown to have a lower yield strength (“soft” zone) than the surrounding material and cracking was concentrated in this throughthickness zone to produce the effect known as stack cracking. In situ cathodic hydrogen charging of tensile specimens under load led to failure by linking the rolling-plane cracks with transverse cleavage cracks, which were often initiated at inclusions. All cracking was predominantly by cleavage and failure occurred in tension in short times by hydrogen embrittlement when the applied tensile stress was above about half the uncharged yield stress. The influence of microstructure, hydrogen pressure and tensile loading conditions on the location of stack cracks and the mode of fracture is discussed.

Recrystallization behaviour of a CMn steel, a titanium-interstitial-free steel and a niobium-microalloyed steel during strip rolling

Abstract: In order to characterize the Lake Erie strip mill of Stelco, Canada, three grades of steel (CMn, interstitial free (IF) and niobium microalloyed) were subjected to torsional simulation. The flow stresses and microstructural behaviours have been compared. As anticipated, for long interpass times (typical of plate-rolling schedules) the temperature Tnr of no recrystallization can be clearly established only for the niobium steel, the other grades undergoing nearly full recrystallization at all temperatures. Short interpass times, typical of strip-rolling schedules, lead to little pass-to-pass strain accumulation in the three steels. In the temperature range 1000-900 °C, static recrystallization is largely responsible for the high degree of interpass softening in the CMn steel. In the case of the IF steel, during the latter stages of finishing, there is some strain accumulation, and both static and dynamic recrystallization occur to a significant degree. For the niobium steel, because of solute effects, the short interpass times (about 1 s) do not permit much static recrystallization. Instead, softening is brought about mostly by dynamic and post-dynamic recrystallization. This is possible because there is considerably less Nb(C, N) precipitation during the short interpass times than under reversing (plate) mill conditions. The decreased level of precipitation in the niobium steel followed by the initiation of dynamic recrystallization would lead to lower rolling loads in the strip mill for this grade than in the plate mill at the same temperature.

Copper brazed torque converter pump housing made from formable high strength microalloyed steel

Abstract: A method is provided for thermally treating a high strength, microalloyed steel so as to first induce sufficient ductility (and corresponding low strength) in the microalloyed steel to readily enable the room temperature forming and/or machining of the steel. This is accomplished by appropriately heating to a solutionizing temperature and then cooling from this temperature at an extraordinarily slow rate so as to induce coarse precipitation of any strengthening particles and to thereby minimize the strengthening contributions associated with precipitates and ferrite grain size. The formed low strength components are then thermally treated again (such as during a copper brazing cycle) so as to induce high strength and relatively lower ductility in the microalloyed steel.

Microstructure and toughness of low carbon steel weld metal

Abstract: As part of an investigation into the microstructure and properties of low carbon microalloyed steel weld metal, the influence of changes in the optically resolvable microstructure on Charpy impact transition temperatures were examined An analysis of the experimental data was carried out, based on the premise that minor phases were the primary sites for brittle crack initiation, and that successful propagation or otherwise of such cracks was a function of the surrounding gross microstructure The resulting correlation between microstructure and toughness provided a means of rationalizing the influence of a range of compositional and other variables on toughness, through their effects on the microstructure

Strengthening Nb–V microalloyed steel

Abstract: A process for producing a low pearlite steel containing small amounts of niobium and vanadium is described. Controlling rolling followed by a short annealing treatment results in grain refinement, precipitation strengthening, and matrix recovery, and produces plate with improved strength at increased toughness levels. The process, which has been successfully applied in practice, obviates the need for crystallisation control by continuous casting.MST/1329

Controlled rolling with intermediate recrystallization of type St3sp steel

Abstract: A promising variation of strengthening of type St3sp steel is thermomechanical treatment, controlled rolling with intermediate recrystallization. With it the strength of the steel increases by 95–200 N/mm2, the 50% ductile-to-brittle transition temperature drops by 60°C, and the work for crack development doubles in comparison with these properties of steel in the hot rolled condition.

Microalloyed Steel Preforms

Abstract: : An analysis was conducted on two types of microalloyed steel to study their metallurgical, mechanical, and processing characteristics, and also to evaluate the suitability of utilizing these alloys as forgings for small gun tube components. The primary benefit associated with these steels is that optimum properties can be attained by direct quenching from the forging temperature. Presently, typical hot forgings must be thermally treated after forging to obtain desired properties. A test matrix was constructed to facilitate examination of the previously stated characteristics. The criteria included alloy type, heat-treatment temperature, forging reduction, tempering temperature, and bar diameter. Test results were compiled in a mechanical property data base that included hardness, tensile strength and yield strength, impact toughness, and ductile-to-brittle transition temperature. Further evaluation of these data enabled the determination of a desired processing route to achieve optimum properties. Based on the limited drawing specification requirements of only hardness, it was determined that several different processing combinations could yield acceptable results. However, for optimum properties (hardness), it was found that section thickness should be limited to one inch or less.