Showing papers on "Microalloyed steel published in 2019"

Effect of hot deformation parameters on flow stress and microstructure in a low carbon microalloyed steel

Abstract: Hot deformation behavior of low carbon microalloyed steel was investigated by single-pass compression tests using a thermomechanical simulator. After the specimens were subjected to different hot deformation processes, their microstructures were obtained by constant cooling rate and then examined using optical microscopy and electron backscattering diffraction (EBSD) technique. The results showed that the absence of stress peak in the flow stress curves does not necessarily indicate the absence of dynamic recrystallization (DRX) because in some cases the partial DRX phenomenon does occur when the stress peaks are absent in the flow curves. The metallurgical events that occur during hot deformation directly influence the nature of phase transformation products. High dislocation density in the deformed austenite can promote the intragranular nucleation of acicular ferrite, while the DRX austenite grains prefer to form lath bainite/martensite. The EBSD results confirmed that the microstructures that were transformed from prior pancaked austenite grains always contain higher fraction of middle misorientation angle boundaries and larger average local strain as compared to those with prior DRX grain shape.

Effect of coiling conditions on the strengthening mechanisms of Nb microalloyed steels with high Ti addition levels

Abstract: Steels alloyed with high Ti addition levels present an interesting combination of high strength and formability, principally due to the high levels of precipitation hardening that can be attained. However, their mechanical properties can be highly sensitive to variations in the processing route. In this work, dilatometry tests were performed to study the effect of coiling conditions on the microstructures and hardening mechanisms of a reference Nb microalloyed steel (0.03%Nb) and two high Ti-Nb steels (0.05%Ti-0.03%Nb, 0.1%Ti-0.03%Nb). Coiling temperatures from 550 °C to 675 °C and cooling rates of 0.01 °C/s and 0.03 °C/s were considered. A significant increase in hardness was observed for the high Ti-Nb steel samples. While the grain size and dislocation hardening were similar for all steels, much higher precipitation strengthening values in the range of 69–163 MPa and 100–307 MPa were calculated for the 0.05%Ti and 0.1%Ti steels, respectively. As a consequence, high yield strength values (over 700 MPa) were estimated for coiling temperatures greater than 625 °C for the Ti10Nb3 steel. However, it was also observed that the mechanical behavior of this steel greatly depended on coiling conditions: maximum mechanical strength was achieved at 625–650 °C, while it decreased significantly for temperatures between 550 °C and 600 °C and at 675 °C. The small size and density of the precipitates detected with TEM support the large precipitation hardening effect calculated in these conditions.

Determining role of microstructure on crack arrest and propagation phenomenon in low-carbon microalloyed steel

Abstract: The relationship between microstructure and crack arrest characteristics of a microalloyed steel with a yield strength of 460 MPa was elucidated in this study with particular focus on the effect of the microstructure on the propagation of brittle cracks. The study emphasized that the acicular ferrite (AF) content could be increased to 41% by lowering the final rolling temperature and final cooling temperature, which reduces the average effective grain size (EGS) to as small as ~4.1 μm and increases the high-angle grain boundaries (HAGB) proportion to as high as ~48.8%. The presence of AF in large numbers could reduce the ductile-to-brittle transition temperature to approximately −87 °C through refinement of the microstructure. The presence of HAGB between adjacent grains could significantly arrest cracks by changing the propagation direction of brittle cracks and was a decisive factor in determining the characteristics of brittle fracture. Moreover, the fracture roughness varied directly with the proportion of the HAGB and inversely with the average EGS. Martensite/austenite islands with low content (0.92%) and small size (~1.5 μm) reduced the stress concentration significantly at the crack tip. The excellent crack arrestability was attributed to AF, which minimized the expansion of cleavage facets by the split nail effect ahead of the growth direction, so that the cleavage facets were divided into two or more branches, considerably reducing the stress concentration and retarding brittle crack growth during crack propagation at low ambient temperature.

Analysis of Hot- and Cold-Rolled Loads in Medium-Mn TRIP Steels

Abstract: The purpose of this work is to investigate the hot- and cold-rolling requirements to produce third-generation advanced high-strength steels (AHSS). Therefore, five medium-Mn (10 to 14 wt pct Mn) alloys that exhibit transformation-induced plasticity (TRIP) were compared to a commercially produced grade of AISI 1018 using hot- and cold-rolling experiments. Experimental data collected from a STANAT instrumented rolling mill was utilized to measure force and torque during hot- and cold-rolling. Experimental data were processed by a 1D analytical model, based on Orowan model, to determine rolling pressure. It was determined that pressures required to hot-roll TRIP alloys are 1.4 to 1.8 times greater than pressures for rolling AISI 1018 steel. Cold rolling of the medium-Mn TRIP steels was found to be 1.5 to 2.8 times greater than the AISI 1018 steel. Mechanical and microstructural characterization was also performed and the variation in rolling pressure was related to the starting microstructural constituents, and alloys containing greater starting quantities of e-martensite in the microstructure had higher flow stresses at equivalent rolling strains during cold rolling.

Complex phase quantification methodology using electron backscatter diffraction (EBSD) on low manganese high temperature processed steel (HTP) microalloyed steel

Abstract: Microalloyed steels are required to have good strength and toughness, properties that are mostly influenced by microstructure and phase distribution in the steel, obtained by controlling the thermomechanical processing. High temperature processing (HTP) Steels, with lower manganese content, has been recently developed in order to reduce segregation, but reduction of Mn decreases the yield strength. In order to understand the relationship between chemical composition, microstructure, mechanical properties and processing parameters, quantitative analysis of the final microstructure is required but identification and quantification of the ferrite microconstituents under optical microscopy or scanning electron microscopy is difficult due to their similarities. The present work presents a methodology for identification and quantification of complex microconstituents on a low manganese HTP steel, subjected to different hot deformation and cooling cycles, by means of electron backscatter diffraction (EBSD). The results showed that the EBSD methodology allowed to identify and quantify different microconstituents in a low manganese HTP steel with similar results as those obtained by point count methodology.

Delamination toughening in a low carbon microalloyed steel plate rolled in the dual-phase region

Abstract: It is still a big challenge to obtain excellent low-temperature toughness for bulk steel materials. Delamination is an effective method to improve low-temperature toughness. In the present study, delamination toughening in a low carbon microalloyed steel plate with elongated and ultrafine-grained microstructure rolled in the dual-phase region has been investigated in detail. When toughness was measured along normal direction, the steel plate had a high upper shelf energy and no delamination occurred in the upper shelf region. A large delaminated crack parallel to rolling plane started to appear and changed the propagation path of main crack when testing temperature was lower than −60 °C. We find this kind of delamination induces a second upper shelf in the Charpy transition–temperature curve. The second upper shelf, reaching up to 300 J in the temperature range of −60 °C to −140 °C, results in excellent low-temperature toughness for the steel plate, and the ductile-brittle transition temperature is lowered to −157 °C. The developed steel plate also has high low-temperature toughness measured along transverse direction due to delamination. The effect factors on upper shelf energy, delamination mechanism and delamination toughening are discussed.

Research of microalloy elements to induce intragranular acicular ferrite in shipbuilding steel

Abstract: Based on the minimum degree of disregistry mechanism in oxide metallurgy, laboratory and industrial research have been conducted on intragranular acicular ferrite (IAF) induced by microalloying elements in austenite. Based on the chemical compositions of DH36 steel and Mg, Al, Ti, V, Nb microalloyed steel, experimental results show that in ingots' organization, both V and Nb can induce IAF, but when the adding sequence was Al-Mg-Ti, smaller and dispersion inclusions were formed in austenite. When the Mg content was 0.005 wt%, the inclusion structure induced IAF in austenite is as follows: MgO and Al2O3 forms the core and TixOy adheres to the Al-Mg complex inclusions to produce smaller particle size and dispersions of Al, Mg, Ti complex inclusions. Finally, upon lowering the temperature, carbonitrides of Ti, V, and Nb were precipitated on the outermost layer of the inclusions. These carbonitrides with small disregistry contribute to induce intragranular acicular ferrite.

Development of Niobium Microalloyed Steel for Railway Wheel with Pearlitic Bainitic Microstructure

Abstract: Heavy haul transportation (load over 30 tons/axle), as well as the axle load, has been more and more used in Brazil and worldwide. The stress generated in the wheel-rail contact with loads up to 30 tons/axle is around 760 MPa, which causes premature wear and cracks of conventional wheels (AAR (Association of American Railroads) class C). Microalloyed wheels are fundamental on heavy haul transport, whose main function is to combine high hardness, ductility, and yield strength of the material in order to prevent shelling. The main purpose of this research is to develop a new microalloyed wheel steel with niobium addition that meets all the requirements of the AAR class D material with mixed microstructure composed of pearlite and bainite. The 0.71C/0.020Nb steel developed in this study (Nb material) for railroad achieved the standards required for AAR class D in all mechanical properties, with fracture toughness higher than the usual vanadium microalloyed steel used in comparison. The Continuous Cooling Transformation (CCT) diagram showed the presence of bainite even at very low cooling rate, in the range between 0.3 - 2 °C/s. These cooling rates to form bainite are much lower than those observed in other steels with similar composition.

Effects of cooling rate and strain rate on phase transformation, microstructure and mechanical behaviour of thermomechanically processed pearlitic steel

Abstract: In the present investigation, thermomechanical controlled processing of a high carbon steel and a Nb microalloyed high carbon steel have been conducted in a Gleeble 3800 simulator. Different microscopic techniques have been utilised for the characterisation of the microstructure and hardness data has been used for the evaluation of mechanical properties. In order to suppress the transformation enthalpy, experiments are performed under varying cooling rate and strain rate. The effect of niobium microalloying leads to the lowering of recalescence and suppresses austenite to pearlite transformation start and finish temperatures at every cooling rate which leads to the refinement of interlamellar spacing and thereby improve hardness and predicted yield strength values. It is evident that a higher strain rate accelerates the kinetics of pearlite transformation and elevates the pearlite start temperature. The increase of strain rate in the range of 1 s–1 to 100 s–1 followed by a constant cooling rate (free cooling) leads to the refinement of interlamellar spacing as well as improves mechanical properties. The true stress-true strain diagram at a lower strain rate indicates higher strain hardening with sharp yield point, whereas the same at a higher strain rate indicates the sudden occurrence of strain softening. The variation in recalescence due to the alternation of the cooling rate and strain rate has been correlated with the final microstructure and mechanical properties.

Evolution of Microstructure During Double-Sided Friction Stir Welding of Microalloyed Steel

Abstract: Microstructures observed by analytical scanning and transmission electron microscopy in the overlap region of a double-sided friction-stir-welded microalloyed steel (EH46) were recorded in detail. They are compared with microstructures in the thermomechanically affected region of the weld and with the base material. The differences suggest that the overlap region has been stirred in the single-phase ferrite, and consists mainly of small equiaxed ferrite grains with strain-induced precipitates, while the thermomechanically affected zone was processed in the austenite-ferrite-phase field, resulting in a mixture of bainite lath packets and ferrite grains. The almost complete absence of pearlite or cementite in the overlap region has led to the suggestion that it dissolved during friction stir welding, providing carbon for strain-induced precipitation. Also, in the complex microstructures of the overlap region, ferrite grains containing a high density of cell-like structures were observed, some of them having precipitates nucleated on their intersections. This implies that strain-induced continuous dynamic recrystallization has occurred.

Influence of plasma nitriding pressure on microabrasive wear resistance of a microalloyed steel

Abstract: Microalloyed steels have been studied not only by the advent of the pre-salt, but also by the continuous search for improvement of their tribological properties. In this work, the influence of the plasma nitriding (PN) pressure on the nitrided layer formed on top of an API 5L X70 microalloyed steel was studied. PN treatments were carried out using a pulsed plasma reactor at temperatures of 410 °C and 440 °C, in a gas mixture of N2 and H2, for 3 h and under different pressures. Results showed that the pressure has a significant effect on the uniformity and width of the plasma sheath and, consequently, on the structure and thickness of the resulting nitrided layer. It has been observed that higher pressures and temperatures have led to the formation of nitrided layers containing a thinner white layer and without nitride needles. The best wear results were offered by the thicker nitrided layers, besides thinner white layer. This type of structure can be produced when PN treatments are performed using low-width plasma sheaths.

Microstructural features and precipitation behavior of Ti, Nb and V microalloyed steel during isothermal processing

Abstract: Thermal simulations of Ti, Nb and V microalloyed steel were conducted using a thermomechanical simulator, and the microstructural evolution and the precipitation behavior during isothermal processing were analyzed. The results show that with increasing holding time, the microstructural constituents change from the martensite and bainitic ferrites to granular bainite and polygonal ferrite. The maximum hardness is obtained for the specimen after isothermal holding for 5 s due to the martensite strengthening effect. The hardness of the specimen decreases after isothermal holding for 10 s, because the strengthening contribution of fine dispersed precipitates becomes weaker. The hardness values of the specimens increase and then remain high after isothermal holding for 60 and 600 s. This is attributed to the contribution of the interphase precipitation hardening to the hardness of the studied steel. The precipitates in the specimen are coarsened after isothermal holding for 3600 s, even though the coarsening is not remarkable. These precipitates are fcc (Ti, Nb)(N, C) particles and belong to the MX-type precipitates. The beneficial effects of precipitation strengthening are lost. The hardness decreases to a minimum hardness value due to the presence of large amounts of polygonal ferrite after isothermal holding for 3600 s. Relatively coarse precipitates are the primary origin of the hardness decrease.

400 MPa-grade high-nitrogen composite microalloyed steel bar and production method thereof

Abstract: The invention relates to a 500 MPa level high-nitrogen composite microalloyed steel bar and a production method thereof. The 500 MPa level high-nitrogen composite microalloyed steel bar comprises thefollowing chemical components of, by mass, 0.18-0.25 parts of C, 0.40-0.80 parts of Si, 1.30-1.60 parts of Mn, less than or equal to 0.045 parts of P, less than or equal to 0.045 parts of S, 0.020-0.040 parts of V, 0.010-0.040 parts of Nb, 0.010-0.040 parts of Ti, 0.0090-0.0150 parts of N, and the balance iron and unavoidable impurity elements. In the production method, after the continuous casting process is completed, a continuous casting billet is heated to 1130-1250 DEG C and subjected to heat preservation for 90-150 minutes, and then cooled to 1020-1120 DEG C for rolling; after the rolling, water cooling is carried out with the water pressure being 0.1-0.5 MPa and the temperature of an upper cooling bed being controlled at 920-1010 DEG C; and the steel bar is naturally cooled after entering the cooling bed, so that the high-nitrogen composite microalloyed steel bar is obtained. The mechanical properties of the steel bar meet the requirements of the national standard, the production cost is low and a production process is simple and easy to implement.

Surface hardening characteristics of microalloyed steel during ex-situ and in-situ Al2O3 reinforcement under TIG arcing

Abstract: This work presents a comparative study of the influence of producing ex-situ and in-situ surface composite (by Tungsten Inert Gas (TIG) arcing process) on the hardenability of microalloyed steel, hereafter mentioned as steel. The ex-situ surface composite was prepared by adding hard Al2O3 particles into the fused surface matrix, while for in-situ surface composite, Al2O3 particles were made to grow in the fused surface matrix through the addition of Al. In the latter process, Al as deoxidizer reacts with the oxygen present in the steel to form the in-situ growth of Al2O3. The modified surface matrix exhibited the presence of martensite phase and the particle reinforcements when studied under optical microscopy, field emission scanning electron microscopy (FESEM) supported by energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). Vickers’ microhardness testing of the modified surface affirmed a noteworthy improvement in hardness (2.13 times) with respect to the base metal (BM) for both the cases (ex-situ and in-situ) of Al2O3 reinforcement. Although, the hardness improvement was found to be similar in both the cases of reinforcement, but the depth of peak hardening was observed to be greater in case of matrix reinforcement by in-situ grown Al2O3 (∼1.25 mm) than the ex-situ added Al2O3 (∼0.9 mm), which is more useful for tribological requirements of industries.

Tensile behavior of normalized low carbon Nb-microalloyed steel in the presence of rare earth elements

Abstract: In this study, attempts have been made to study the effects of RE on the tensile behavior of a low carbon Nb-microalloyed steel which confronts yield point phenomenon during its tensile test. The results of the tensile tests and microstructural examinations presented in this research showed that the upper and lower yield points increase by the RE addition which was mainly attributed to the indirect effects of RE on the Nb-precipitation manner and refinement of the microstructure. The results also demonstrated that the flow behavior (oscillation in stress level) during the Luders strain zone is considerably different for the base and RE-added steels. It was found that the RE-added steel undergoes a uniform propagation of the Luders bands while the base steel showed a distinct non-uninform Luders strain with rough fluctuations of stress level within this area. This could be due to the uniformity in distribution of nanoprecipitates and solute, e.g. C, atoms in the RE-added steels. Moreover, It was observed that the Luders strain increases in the presence of RE which could be probably attributed to the finer ferrite grains and the change in nanoprecipitation behavior caused by RE addition. A significant increase in total elongation of the RE-added steel was also observed.

The evolution of strain pattern induced by banded structure under uniaxial tension in low-carbon microalloyed steel

Abstract: The presence of banding inhomogeneities is detrimental to the mechanical behaviors of steels due to the strain anisotropy. In this paper, combined with the in-situ tensile stage, digital image correlation (DIC) method was used to investigate the influence of banding pattern including the size, quantity and morphology on the continuous localized deformation and fracture behavior of low-carbon microalloyed steel under the uniaxial tension load. A zero-deformation test was introduced at the beginning of the experiment, where the accuracy and stability of DIC for this application were validated. The results show that the effect of banded structure on local deformation and damage evolution is significant. Compared to the rolling direction (RD) tensile, the local deformation of banded structure exhibits a higher sensitivity to the transverse direction (TD), and the development of shear bands is clearly affected by the local high strains caused by the deformation of soft banding. The coarse banding (CB) in specimen could decrease the mechanical properties of material. Meanwhile, the mechanical anisotropy is explained by the evolution model of local deformation. This work is helpful to understand the effect of banded structure on the mechanical behavior of steel.

Interaction of Strain Refined Precipitates and Recrystallized Grains in Nb-Ti Microalloyed Steel during Continuous Casting, Hot-Core Heavy Reduction Rolling, and Reheating Process

Abstract: Hot-core heavy reduction rolling (HHR2) is an energy-saving process that directly rolls continuously cast slabs at the end of solidification that have a large temperature gradient. Here, we elucidated the interaction of strain refined precipitates and recrystallized grains in Nb-Ti microalloyed steel during continuous casting, hot-core heavy reduction rolling, and reheating process. Furthermore, theoretical models were established to calculate recrystallization driving force and pinning force at temperatures in the range of 850-1300 °C by a two-stage interrupted compression test using a Gleeble-3800 thermomechanical simulator. The study indicated that a random dispersion of strain refined precipitates with a size range of ~ 6-10 nm was obtained in the hot-core heavy reduction rolled slab, which was different from the coarse and uneven distribution of precipitates in a conventional slab. Upon reheating, the austenite grains in the hot-core heavy reduction rolled slab were significantly refined and evenly distributed along the thickness direction. This is attributed to the random dispersion of strain refined precipitates formed in the HHR2 process.

Effect of Strain-Induced Precipitation on the Austenite Non-recrystallization ( T nr ) Behavior of a High Niobium Microalloyed Steel

Abstract: The non-recrystallization temperature (Tnr) of high niobium microalloyed steel was determined from both multihit and double-hit compression tests obtained under plane strain condition. The Tnr was determined to be in the range of 985 °C to 1010 °C for multihit conditions. The double-hit tests carried out at an interpass time of 5 seconds gave very low Tnr in the range of 860 °C to 900 °C. In order to understand this, double-hit experiments were carried out for different interpass times (2, 15, 100 seconds) at three different temperatures (950 °C, 1050 °C, and 1150 °C). The negative softening behavior was observed at 950 °C for higher interpass times of 15 and 100 seconds. This implies that the Tnr of this steel is well above 950 °C. This was due to the high amount of strain-induced precipitation of niobium carbonitrides at higher interpass times as revealed from TEM investigation. The precipitate size evolution considering the Dutta and Sellars nucleation condition in TC-PRISMA agrees well with the experimentally observed precipitate sizes. However, the evolution of Zener pinning forces considering TC-PRISMA nucleation condition is similar to both reported and experimentally determined values. The evolution of tensile properties also correlates well with the observed austenite recrystallization softening behavior. Therefore, a minimum interpass time of 15 seconds is required during double-hit compression tests to effectively precipitate Nb(C, N) and delay the static recrystallization softening behavior of this steel. This lead to the determination of comparable Tnr values between double-hit and multihit methods.

Interphase Precipitation and Application to Practical Steels

Abstract: Interphase precipitation is a phenomenon in which precipitates are generated on interphase boundaries during phase transformation, creating carbides having a form like bands or fibers in micro-alloyed steels. In isothermal transformation, a lower transformation temperature reduces the carbide diameter and band spacing, and a higher cooling rate reduces them in the continuous cooling process. Various interphase precipitation mechanisms have been suggested, and some models for the ledge mechanism attempted to explain the regular band spacing in steels containing carbide-forming elements quantitatively. Recently, as the orientation relationship in the grains of the interphase boundary is not consistent with the ledge mechanism, three-dimensional interface structures have been suggested to explain the experimental results of observation by transmission electron microscopy in low carbon steel. The newly-proposed interphase structure model may explain both the ledge and quasi-ledge mechanisms. Steels with tensile strength of more than 590MPa, which are manufactured by using interphase precipitated carbides, have been developed and used practically not only in plate and sheet products but also in forged products to improve formability. In steels consisting of ferrite and a second hard phase, interphase precipitated carbides are used to realize high local ductility and to reduce the difference of hardness between the two phases. Ferritic steels strengthened by nanometer-sized carbides are developed to achieve excellent formability, realizing precipitationstrengthening of more than 300MPa. [doi:10.2320/matertrans.M2018197]

An improved method for the matrix dissolution extraction of nanoparticles from microalloyed steel

Abstract: The chemical extraction of niobium and titanium carbonitride precipitates from microalloyed steels was studied. Steel samples and chemically synthesized reference nanoparticles were subjected to commonly used extraction protocols, and conditions were systematically varied. High acid concentrations led to particle etching with losses above 10%; long extraction times and small etchant volumes led to the formation of dense SiOx networks that engulfed the extracted particles. The addition of surfactants was found to reduce agglomeration and limit etching. We developed an optimized extraction protocol that can extract and retain particles with diameters below 10 nm with reduced etching and negligible network formation. The resulting particle dispersions are suitable both for efficient electron microscopy of large particle numbers in a single run and colloidal analysis of large numbers of particles in dispersion.

Dynamic Phase Transformation Behavior of a Nb-microalloyed Steel during Roughing Passes at Temperatures above the Ae3

Abstract: A five-pass torsion simulation of the roughing passes applied during hot plate rolling was performed in the single-phase austenite region of a Nb-microalloyed steel under continuous cooling conditions. The deformation temperatures were approximately half-way between the Ae3 and the delta ferrite formation temperature (i.e., 250 °C above the Ae3) in which the free energy difference of austenite and ferrite is at maximum. The microstructures in-between passes were analyzed to characterize and quantify the occurrence of deformation-induced dynamic phase transformation. It was observed that about 7% of austenite transforms into ferrite right after the final pass. The results are consistent with the calculated critical strains and driving forces which indicate that dynamic transformation (DT) can take place at any temperature above the Ae3. This mechanism occurs even with the presence of high Nb in the material, which is known to retard and hinder the occurrence of DT by means of pinning and solute drag effects. The calculated cooling rate during quenching and the time–temperature–transformation curves of the present material further verified the existence of dynamically transformed ferrite.

The Effects of Cooling Mode on the Properties of Ti–Nb Microalloyed High-strength Hot-rolled Steels

Abstract: The industrial trials of two cooling modes, i e, water cooling in forepart + air cooling in later part (WAC) and air cooling in forepart + water cooling in later part (AWC), were carried out for a Ti-Nb microalloyed steel. The average cooling rates and coiling temperature were the same for two modes. The continuous cooling transformation (CCT) curve of the tested steel was drawn. The effects of the cooling mode on the microstructure, precipitates, and properties of the steels were investigated. Results show that the strength of the steel in the WAC mode is significantly larger than that in the AWC mode, mainly because the smaller the grain size, the more and finer the grain precipitates. Therefore, when the average cooling rate is constant, the fast cooling in the forepart is an effective method to increase the strength of steels. However, the increase in the strength is accompanied by the decrease in toughness, so that the toughness of the steel should be considered when changing the cooling mode.