Micro-alloyed Mg-Ca:Corrosion susceptibility to heating history and a plain problem-solving approach

The exceptionally low corrosion rate(∼0.1 mm y^(–1)in concentrated NaCl solution for 7 days)enables lean Mg-Ca alloys great potential for diverse applications,particularly if relevant properties(e.g.mechanical strength,electrochemical performance,etc.)can be enhanced by thermomechanical processing.However,herein it is demonstrated that the corrosion performance of lean Mg-Ca is susceptible to the heating process.The corrosion rate of Mg-0.15 wt%Ca alloy is remarkably accelerated after annealing even for a short time(4 h at 400℃)because Fe precipitation readily takes place.Fortunately,it is found that micro-alloying with dedicated additional elements is able to solve this problem.Nevertheless,the problem-solving capability is dependent on the element category,particularly the ability of the alloying element to constrain the Fe precipitation.Among the three studied elements(i.e.Sn,Ge and In),only In shows good competence of restricting the formation of Fe-containing precipitates,thereby contributing to retention of the superior corrosion resistance after annealing even at a rigorous condition(24 h at 450℃).The finding creates good foundation for follow-up work of developing lean Mg-Ca-based alloys combining high corrosion resistance,superior electrochemical performance with excellent mechanical properties for applications as biodegradable implants and anode materials for aqueous batteries.

Influence of Ti(C,N)precipitates on austenite growth of micro-alloyed steel during continuous casting

Austenite grain size is an important influence factor for ductility of steel at high temperatures during continuous casting. Thermodynamic and kinetics calculations were performed to analyze the characteristics of Ti(C,N) precipitates formed during the continuous casting of micro-alloyed steel. Based on Andersen-Grong equation, a coupling model of second phase precipitation and austenite grain growth has been established, and the influence of second precipitates on austenite grain growth under different cooling rates is discussed. Calculations show that the final sizes of austenite grains are 2.155, 1.244, 0.965, 0.847 and 0.686 mm, respectively, under the cooling rate of 1, 3, 5, 7, and 10 ℃·s^(-1), when ignoring the pinning effect of precipitation on austenite growth. Whereas, if taking the pinning effect into consideration, the grain growth remains stable from 1,350 ℃, the calculated final sizes of austenite grains are 1.46, 1.02, 0.80, 0.67 and 0.57 mm, respectively. The sizes of final Ti(C,N) precipitates are 137, 79, 61, 51 and 43 nm, respectively, with the increase of cooling rate from 1 to 10 ℃·s^(-1). Model validation shows that the austenite size under different cooling rates coincided with the calculation results. Finally, the corresponding measures to strengthen cooling intensity at elevated temperature are proposed to improve the ductility and transverse crack of slab.

Static Recrystallization Behavior of Hot Deformed Austenite for Micro-Alloyed Steel

Static recrystallization behavior of austenite for micro-alloyed steel during hot rolling was studied and the influence (τ-ε diagram) of holding time and deformation at different deformations and isothermal temperatures on microstructuralstate of austenite were discussed. Corresponding to parameter Z in the dynamic recrystallization diagram, parameterY was then introduced to simplify static recrystallization diagrams.

Effect of Chromium Micro-alloying on the Corrosion Behavior of a Low-carbon Steel Rebar in Simulated Concrete Pore Solutions

A new low-cost corrosion-resistant rebar（HRB400 R） was designed and fabricated by chromium micro-alloying. The effects of Cr on the passivation and corrosion behavior of this rebar in the simulated concrete pore solutions were studied systematically, and its improved corrosion resistance was revealed. In the Cl--free saturated Ca（OH）_2 solution, the HRB400 R rebar presented nearly the same passive film and similar passivation ability compared to the common carbon steel rebar. In the long-term immersion corrosion test in the Cl--contained Ca（OH）_2 solution, the HRB400 R rebar presented improved corrosion resistance and obvious longer passivation-maintaining period. Micro-alloying of Cr element in the rebar matrix enhanced its corrosion resistance against Cl--attack and retarded the corrosion initiation in the matrix. In the alkaline Na Cl salt spraying test, the HRB400 R rebar also presented obviously lower mass-loss rate. The enrichment of Cr element in the rust layer improved its retardant effect to the penetration of aggressive medium, and decreased the corrosion propagation rate of the rebar.

Effect of Nb content on strength of Nb micro-alloyed steels

Microstructures and properties of three Nb micro-alloyed steels were studied through hot rolling experiment. The result indicates that the ferrite grain size （dF ） decreases with increasing Nb content （Nb）, and the bainite fraction （fB） increases with increasing Nb content （Nb）. The effect of ferrite grain size （dF） on yield strength （δy） is related to Nb content （Nb）, and the effect of bainite fraction （fB） on yield strength （δy） is unrelated to Nb content （Nb）. Modelling of yield strength （δy） for Nb micro-alloyed steels with high accuracy has been built up with Nb content （Nb） and bainite fraction （fB） taken into account as new parameters, and formulas for ferrite grain size （dF ） and bainite fraction （fB） vs Nb content （Nb） have also been established under the experiment conditions. The research results could provide instructions for industrial productions.

Effect of Mo on the continuous cooling transformation behavior of Nb-Ti micro-alloyed low carbon steel

The effect of molybdenum on the continuous cooling transformation behavior of the micro-alloyed low carbon steel containing niobium and titanium was investigated by a Gleeble 3800 thermo-mechanical simulator. The phase transformation temperature of the steel at various cooling rates was detected. The microstmcture was observed by optical microscope （OM） and scanning electronic microscope （ SEM）, and its Vickers hardness was tested. Based on these, its dynamic continuous cooling transformation （CCT） diagrams were determined. The results show that the transformation temperature from deformed austenite to acicular ferrite （AF） is decreased when Mo is added, and the formation of pro- eutectoid ferrite （F） and pearlite （P） is either inhabited or postponed. Mo can also enlarge the range of the cooling rate in forming AF, and refine the microstructure effectively.

Hot Ductility of Vanadium Micro-alloying Steel Continuous Casting Slabs

The hot ductility of the V-containing micro-alloying steel CC (continuouscasting) slabs and precipitation of vanadium carbide in the tensile specimens were investigated. Theresults indicate that the precipitation ratio and precipitation rate of vanadium in the specimensreach maximum respectively at 900, -825 and 825 deg C. There is still l0 percent-l7 percent ofvanadium precipitated when the deformation temperature decreases to 800-700 deg C. Vanadium largelyaffects the ductility of the steel in the low ductility temperature Region III. Embrittlement ofsteel with higher V content is severer in the region and the embrittlement extends to lowertemperature.

Improving mechanical properties and high-temperature oxidation of press hardened steel by adding Cr and Si

This work investigated the effect of Cr and Si on the mechanical properties and oxidation resistance of press hardened steel.Results indicated that the microstructure of the Cr-Si micro-alloyed press hardened steel consisted of lath martensite,M_(23)C_(6)carbides,and retained austenite.The retained austenite and carbides are responsible for the increase in elongation of the micro-alloyed steel.In addition,after oxidation at 930℃for 5 min,the thickness of the oxide scales on the Cr-Si micro-alloyed press hardened steel is less than 5μm,much thinner than 45.50μm-thick oxide scales on 22MnB5.The oxide scales of the Cr-Si micro-alloyed steel are composed of Fe_(2)O_(3),Fe_(3)O_(4),mixed spinel oxide(FeCr_(2)O_(4)and Fe_(2)SiO_(4)),and amorphous SiO_(2).Adding Cr and Si significantly reduces the thickness of the oxide scales and prevents the generation of the FeO phase.Due to the increase of spinel FeCr_(2)O_(4)and Fe_(2)SiO_(4)phase in the inner oxide scale and the amorphous SiO_(2)close to the substrate,the oxidation resistance of the Cr-Si micro-alloyed press hardened steel is improved.

Effect mechanisms of micro-alloying element La on microstructure and mechanical properties of hypoeutectic Al-Si alloys

The rare earth influence on the as-cast micro structures and mechanical properties of aluminum alloys attracted great attentions in the last decades.But up to date no reports can be found on the effect of microalloying element La(La addition is below 0.1 wt.%) on the solidification of hypoeutectic Al-Si alloys.This study carried out solidification experiments with Al-6 Si alloys micro-alloyed by element La.The α-Al grain refinement,the eutectic Si modification and the tensile properties improvement caused by microalloying element La were investigated.The effect mechanisms of La were discussed.It is demonstrated that the addition of La as low as 100 ppm can deprave the more effective heterogeneous nucleation conditions for the eutectic Si caused by the impurity P.The addition of 0.06 wt.% La is sufficient to achieve an ideal α-Al grain refinement,eutectic Si modification and ductility improvement of the alloys.LaAlSi phase forms in the Al-Si alloy with the additive amount of La higher than 0.06 wt.%.It has a tetragonal structure.Micro-alloying element La refines the α-Al grains by working as a surfactant and modifies the eutectic Si by promoting the formation of the significant multiple Si twins.

Effect of Annealing Time on Microstructure and Mechanical Properties of Cold-Rolled Niobium and Titanium Bearing Micro-alloyed Steel Strips

Effects of annealing time on microstructure of cold-roiled niobium-titanium bearing micro-alloyed steel strips were investigated by optical microscopy, scanning electron microscopy, electron back-scatter diffraction （EBSD） and transmission electron microscopy. The complete recrystallization annealing temperature of 670 ℃ and complete annealing time of 9 min were determined using Vickers-hardness testing and EBSD analysis. The ferrite mi-crostructure with spheric cementite particles and nano-scale precipitates of Nb（C,N） in matrix was obtained. The ki-netics of the ferrite grain growth is lowered due to ferrite grain boundaries pinned by the cementite particles, so the ferrite grain size of 5.5 μm remains unchanged among the annealing time ranging from 9 to 30 min. In addition, the strength of tested steel also keeps unchanged with the increase of annealing time. The higher yield strength of ap-proximately 420 MPa can be obtained by grain refinement and precipitation hardening and the higher elongation of approximately 40% and work-hardening exponent of approximately 0.2 can be gained due to grain refinement and presence of cementite particles, indicating that the balance of strength, ductility and forming property is realized.

Effect of coarse TiN inclusions and microstructure on impact toughness fluctuation in Ti micro-alloyed steel

Toughness is an important property for steels used in engineering applications. However, recent toughness testing has shown the existence of a significant fluctuation in toughness in a single rolled plate of titanium micro-alloyed steel. The underlying causes of this fluctuation were investigated by fractography, analysis of microstructure and measurement of inclusions. Coarse and distributed TiN inclusions were responsible for the toughness variation, as they tended to act as the potential cleavage initiators to form micro-cracks. From a calculation of the local fracture stress, the critical size of coarse TiN inclusions for dominating micro-crack propagation was 4.93 μm, and similarly that of ferrite grains was 36.6μm. Under current casting and thermo-mechanically controlled processing schedules, the toughness fluctuation of rolled steel plates can be primarily attributed to the fraction of coarse TiN inclusions larger than 5μm. A corresponding relationship between impact energy and the proportion of coarse TiN inclusions was established. Finally, a normalizing treatment was applied to refine the ferrite grains of rolled steel plates. Despite the presence of coarse TiN inclusions, this refinement in ferrite grains minimized the toughness fluctuation and improved the uniformity of the impact properties of the steel plates.

Effects of micro-alloying and production process on precipitation behaviors and mechanical properties of HRB600

Effects of micro-alloying elements and production process on microstructure, mechanical properties and precipitates of 600 MPa grade rebars were studied by using pilot test, metallographic observa- tion, tensile test, thermodynamic calculation and transmission electron microscopy. The results show that the tested steels are composed of ferrite and pearlite, in which the conterlt range of pearlite is 33%-45%. For vanadium micro-alloyed steel, interphase preeipitation Strengthening effect of V can be promoted and the yield strength of tested steels can be increased with increasing V content and de- creasing finishing rolling temperature. The temperature of terminated cooling should be more than 700 ℃ when the water cooling is used. When niobium is added to the steel, more coarse （Nb,V）C,N precipitates are generated at high temperature, so that the solid solubility of precipitated phases of vanadium is reduced and the precipitation strengthening effect of vanadium is weakened.

Effect of micro-alloying La on precipitation behavior, mechanical properties and electrical conductivity of Al-Mg-Si alloys

The rapid industrial development calls for alloys that possess higher comprehensive properties. In this study, the effect of microalloying La addition on the precipitation behavior during artificial aging as well as the mechanical properties and electrical conductivity of Al-Mg-Si alloys were investigated by thermal analysis, microstructural characterizations and properties tests.The results demonstrated that micro-alloying La addition does not change the whole precipitation sequence during the artificial aging of Al-Mg-Si alloys as well as the atomic structure of the precipitates. However, the higher La-vacancy binding energy as well as the strong La-Si and La-Mg interactions can decrease the solubility of Si and Mg in the Al matrix and the β″ precipitation activation energy from 89.9 to 76.7 kJ/mol, leading to the improvement of the strength and electrical conductivity of Al-Mg-Si alloys simultaneously. The microstructural features affecting the strength and electrical conductivity were theoretically discussed in terms of the La addition.

Strain-induced Precipitation in Ti Micro-alloyed Interstitial-free Steel

Stress relaxation method was carried out on a Ti micro-alloyed interstitial-free （IF） steel at the temperature ranging from 800 to 1000℃. The results show that the softening kinetics curves of deformed austenite can be divided into three stages. At the first stage, the stress has a sharp drop due to the onset of recrystallization. At the second stage, a plateau appears on the relaxation curves indicating the start and finish of strain-induced precipitation. At the third stage, the stress curves begin to descend again because of coarsening of precipitates. Precipitation-time temperature （PTT） diagram exhibited a ＂C＂ shape, and the nose point of the PTT diagram is located at 900 ℃ and the start precipitation time of 10 s. The theoretical calculation shows that the strain-induced precipitates were confirmed as almost pure TiC particles. The TiC precipitates were heterogeneously distributed in either a chain-like or cell-like manner observed by transmission electron microscopy （TEM）, which indicates the precipitates nucleated on dislocations or dislocation substructures. In addition, a thermodynamic analytical model was presented to describe the precipitation in Ti micro-alloyed IF steel, which shows a good agreement between the experimental observation and the predictions of the model.

Corrosion behavior of low-carbon Cr micro-alloyed steel for grounding grids in simulated acidic soil

To improve the corrosion resistance of steels for grounding grids, a low-carbon Cr micro-alloyed steel was developed （C 1 steel）, and corrosion behavior of Q235 steel and newly developed C1 steel in simulated acidic soil was investigated. The corrosion rate was evaluated with the mass loss measurements, while the corrosion morphology of surface and cross section of rust layer was observed by scanning electron microscopy. The corrosion products were analyzed by energy- dispersive X-ray spectrometry, X-ray diffraction and X-ray photoelectron spectroscopy, and the polarization curve was measured using potentiodynamic polarization method. Results indicated that C 1 steel displayed good corrosion resistance in the simulated acidic soil, of which the corrosion rate was only 30% of that of Q235 steel after corrosion for 360 h. The analysis of rust layer showed that lower carbon content in steel could reduce the tendency of micro cell corrosion and appropriate amount of chromium could improve the corrosion potential of metal matrix. Moreover, the analysis of X-ray photoelectron spectroscopy revealed that the chromium enriched in inner rust layer of C1 steel existed mainly in the form of Fe2CrO4, which facilitated the formation of Cr-goethite and improved the protection of corrosion products.

Enhancement of glass-forming ability and thermal stability of a soft magnetic Co_(75)B_(25)metallic glass by micro-alloying Y and Nb

Soft magnetic Co-based Co-Y-Nb-B bulk metallic glasses(BMGs)without Fe have been developed by micro-alloying Y and Nb into a C075B25 alloy.First,addition of 3^at.%Y promotes the occurrence of glass transition and increases the supercooled liquid stability and magnetic softness.C0_(71.5)Y_(35)B_(25) metallic glass possesses a large supercooled liquid region(△T_(x))of 33 K and low coercivity(H_(c))of l.5 A/m.Subsequent alloying 2-4 at.% Nb into C0_(71.5)Y_(35)B_(25) alloy further enlarges △T_(x) to 50 K,lowers H_(c) to 0.9 A/m,and enables the formation of BMGs with a critical sample diameter up to 2.0 mm.The alloying Nb causes the formation of complex(Co,Nb,Y)_(23)B_(6) competing phase during crystallization and widens the melt undercooling during solidification,which improves the supercooled liquid stability and glass-forming ability,respectively.Co-Y-Nb-B BMGs also exhibit good soft magnetic and mechanical properties,i.e.,lowH_(c)of 0.9-1.2 A/m,relatively high saturation magnetic flux density of 0.36-0.57 T,high yielding strength of 3877-3930 MPa with plastic strain of 0.2%-0.3%,and high Vickers hardness of 1156-1201.The developed soft magnetic Co-based BMGs are promising for applications as structural and functional materials.

In-situ analysis of retained austenite transformation in high-performance micro-alloyed TRIP steel

Microstructures and mechanical properties of Ti-V micro-alloyed TRIP（ transformation-induced plasticity） steel with different compositions were investigated by tensile test,scanning electron microscopy（ SEM）,transmission electron microscopy（ TEM）,X-ray diffraction（ XRD） and thermodynamic calculation（ TC）. The results indicated that the steel exhibited high ultimate tensile strength（ 1 079MPa）,sufficient ductility（ 28%） and the highest product of strength and ductility（ 30 212 MPa·%） heat treated after intercritical annealing at 800℃ for 3 min and bainitic annealing at 430 ℃ for 5 min. In addition,the change of volume fraction of retained austenite（ VF-RA） versus tensile strain was measured using in-situ analysis by X-ray stress apparatus and micro-electronic universal testing machine. It was concluded that a-value could be used to evaluate the stability of retained austenite（ S-RA） in the investigated Ti-V micro-alloyed TRIP steel. The smaller a-value indicated the higher stability of retained austenite（ S-RA） and the higher mechanical properties of Ti-V micro-alloyed TRIP steel.

Effect of strain-induced precipitation on microstructure and properties of titanium micro-alloyed steels

The strain-induced precipitation behavior of titanium micro-alloyed steel was examined through the stress relaxation method.In addition,the relationship between strain-induced precipitation and isothermal precipitation was explored.The findings revealed that the strain-induced precipitation and recrystallization processes of titanium micro-alloyed steel coexist and compete at the same time.The results also showed that the recrystallization process was inhibited with straininduced precipitation.Moreover,a large amount of nano-sized TiC particles precipitated in Ti micro-alloyed steel.Notably,the strain-induced precipitated TiC had a size of 10 nm and isothermally precipitated TiC had a size of 3–6 nm.Additionally,there was a clear competitive relationship between strain-induced precipitation and isothermal precipitation.The findings also showed that strain-induced precipitation had an obvious effect on the refinement of austenite although the effect was not obvious on the increase in the yield strength.Furthermore,isothermal treatment was shown to be more advantageous than strain-induced precipitation.Finally,the major increase in the yield strength was mainly attributed to the precipitation strengthening of nano-sized TiC during isothermal precipitation.

Technology Development for Controlling Slab Transverse Corner Crack of Typical Micro-alloyed Steels

A mathematical model for simulating the fluid flow, heat transfer and solidification in the conventional mold and the chamfer mold, together with a finite element stress-strain model in the straightening process of both molds, were established for the typical niobium, vanadium, and titanium micro-alloyed steels. On the basis of both numerical analysis, the mold copper plate with an optimum chamfered shape was designed and applied in industrial tests. The predicted results from numerical simulation of fluid flow, heat transfer and solidification in the conven tional mold and the chamfer mold show that the increased chamfered angle leads to an approximately linear increase o[ the slab surface temperature, but it also causes strong flow near the slab corner. Very small chamfered length can lead to a significant increase of the temperature near the slab corner. However, with further increasing the chamfered length, the temperature of the slab corner increased slightly. The calculated results from the finite element analysis of stress-strain during the straightening process show that at the same slope width, the tangential strain on the slat） edges and corners is minimum when the chamfered angle is 30° and 45°, which is only 40° to 46° of rectangular slabs with the same cross-section area. At the same chamfered angle of 30°, when the chamfered length is controlled between 65-85 mm, the tangential strain on the part of the slab edges and corners is relatively smaller. Industrial test results show that the slab corner temperature at straightening segment increases about 100 ℃ by using chamfer mold compared to the conventional molds. The slab transverse corner cracks have been reduced more than 95° in comparison with those in the conventional mold.

Influence of Microstructures on Polarization Resistance and Hydrogen-Induced Cracking of a Micro-Alloyed Steel Submitted to Different Cooling Rates

The effect of different microstructures on the polarization resistance (Rp) and the hydrogen-induced cracking (HIC) of a micro-alloyed steel austenitized and submitted to different cooling rates was studied.Samples 19.1 x 6 x 2 mm,containing the whole thickness of the plate were extracted from a 20 mm plate and heat treated on a quenching dilatometer,were submitted to Rp and HIC corrosion tests.Both Rp and HIC tests followed as close as possible ASTM G59 and NACE standard TM0284-2003,in this case,modified only with regard to the size of the samples.Steel samples transformed from austenite by a slow cooling (cooling rate of 0.5℃.s-1) showed higher susceptibility to hydrogen-induced cracking,with large cracks in the middle of the sample propagating along segregation bands,corresponding to the centerline of the plate thickness.For cooling rates of 10℃.s-1,only small cracks were found in the matrix and micro cracks nucleated at non-metallic inclusions.For higher cooling rates (40℃.s-1) very few small cracks were detected,linked to non-metallic inclusions.This result suggests that structures formed by polygonal structures and segregation bands (were eutectoid microconstituents predominate) have higher susceptibility to HIC.Structures predominantly formed by acicular ferrite make it difficult to propagate the cracks among non-oriented and interlaced acicular ferrite crystals.Smaller segregation bands containing eutectoid products also help inhibit cracking and crack propagation;segregation bands can function as pipelines for hydrogen diffusion and offer a path of stress concentration for the propagation of cracks,frequently associated to non-metallic inclusions.Polarization resistance essays performed on the steel in theas received condition,prior to any heat treatment,showed larger differences between the regions of the plate,with a considerably lower Rp in the centerline.The austenitization heat treatments followed by cooling rates of 0.5 e 10℃.s-1 made more uniform the corrosion resistance along the thickness of the plate.The effects of heat treatments on the corrosion resistance are probably related to the microconstituent formed,allied to the chemical homogenization of the impurities concentrated on the centerline of the plate.