Flow characteristics and hot workability of a typical low-alloy high-strength steel during multi-pass deformation

Heavy components of low-alloy high-strength(LAHS) steels are generally formed by multi-pass forging. It is necessary to explore the flow characteristics and hot workability of LAHS steels during the multi-pass forging process, which is beneficial to the formulation of actual processing parameters. In the study, the multi-pass hot compression experiments of a typical LAHS steel are carried out at a wide range of deformation temperatures and strain rates. It is found that the work hardening rate of the experimental material depends on deformation parameters and deformation passes, which is ascribed to the impacts of static and dynamic softening behaviors. A new model is established to describe the flow characteristics at various deformation passes. Compared to the classical Arrhenius model and modified Zerilli and Armstrong model, the newly proposed model shows higher prediction accuracy with a confidence level of 0.98565. Furthermore, the connection between power dissipation efficiency(PDE) and deformation parameters is revealed by analyzing the microstructures. The PDE cannot be utilized to reflect the efficiency of energy dissipation for microstructure evolution during the entire deformation process, but only to assess the efficiency of energy dissipation for microstructure evolution in a specific deformation parameter state.As a result, an integrated processing map is proposed to better study the hot workability of the LAHS steel, which considers the effects of instability factor(IF), PDE, and distribution and size of grains. The optimized processing parameters for the multi-pass deformation process are the deformation parameters of 1223–1318 K and 0.01–0.08 s^(-1). Complete dynamic recrystallization occurs within the optimized processing parameters with an average grain size of 18.36–42.3 μm. This study will guide the optimization of the forging process of heavy components.

Determining role of heterogeneous microstructure in lowering yield ratio and enhancing impact toughness in high-strength low-alloy steel

Here we present a novel approach of intercritical heat treatment for microstructure tailoring,in which intercritical annealing is introduced between conventional quenching and tempering.This induced a heterogeneous microstructure consisting of soft intercritical ferrite and hard tempered martensite,resulting in a low yield ratio(YR)and high impact toughness in a high-strength low-alloy steel.The initial yielding and subsequent work hardening behavior of the steel during tensile deformation were modified by the presence of soft intercritical ferrite after intercritical annealing,in comparison to the steel with full martensitic microstructure.The increase in YR was related to the reduction in hardness difference between the soft and hard phases due to the precipitation of nano-carbides and the recovery of dislocations during tempering.The excellent low-temperature toughness was ascribed not only to the decrease in probability of microcrack initiation for the reduction of hardness difference between two phases,but also to the increase in resistance of microcrack propagation caused by the high density of high angle grain boundaries.

Effect of nanosized NbC precipitates on hydrogen-induced cracking of high-strength low-alloy steel

We investigated the effect of nanosized NbC precipitates on hydrogen-induced cracking(HIC)of high-strength low-alloy steel by conducting slow-strain-rate tensile tests(SSRT)and performing continuous hydrogen charging and fracture analysis.The results reveal that the HIC resistance of Nb-bearing steel is obviously superior to that of Nb-free steel,with the fractured Nb-bearing steel in the SSRT exhibiting a smaller ratio of elongation reduction(Iδ).However,as the hydrogen traps induced by NbC precipitates approach hydrogen saturation,the effect of the precipitates on the HIC resistance attenuate.We speculate that the highly dispersed nanosized NbC precipitates act as irreversible hydrogen traps that hinder the accumulation of hydrogen at potential crack nucleation sites.In addition,much like Nb-free steel,the Nb-bearing steel exhibits both H-solution strengthening and the resistance to HIC.

Development in oxide metallurgy for improving the weldability of high -strength low-alloy steel-Combined deoxidizers and microalloying elements

The mechanisms of oxide metallurgy include inducing the formation of intragranular acicular ferrite(IAF)using micron-sized inclusions and restricting the growth of prior austenite grains(PAGs)by nanosized particles during welding.The chaotically oriented IAF and refined PAGs inhibit crack initiation and propagation in the steel,resulting in high impact toughness.This work summarizes the com-bined effect of deoxidizers and alloying elements,with the aim to provide a new perspective for the research and practice related to im-proving the impact toughness of the heat affected zone(HAZ)during the high heat input welding.Ti complex deoxidation with other strong deoxidants,such as Mg,Ca,Zr,and rare earth metals(REMs),can improve the toughness of the heat-affected zone(HAZ)by re-fining PAGs or increasing IAF contents.However,it is difficult to identify the specific phase responsible for IAF nucleation because ef-fective inclusions formed by complex deoxidation are usually multiphase.Increasing alloying elements,such as C,Si,Al,Nb,or Cr,con-tents can impair HAZ toughness.A high C content typically increases the number of coarse carbides and decreases the potency of IAF formation.Si,Cr,or Al addition leads to the formation of undesirable microstructures.Nb reduces the high-temperature stability of the precipitates.Mo,V,and B can enhance HAZ toughness.Mo-containing precipitates present good thermal stability.VN or V(C,N)is ef-fective in promoting IAF nucleation due to its good coherent crystallographic relationship with ferrite.The formation of the B-depleted zone around the inclusion promotes IAF formation.The interactions between alloying elements are complex,and the effect of adding dif-ferent alloying elements remains to be evaluated.In the future,the interactions between various alloying elements and their effects on ox-ide metallurgy,as well as the calculation of the nucleation effects of effective inclusions using first principles calculations will become the focus of oxide metallurgy.

Effect of microstructure variation on the corrosion behavior of high-strength low-alloy steel in 3.5wt% NaCl solution

The effect of microstructure variation on the corrosion behavior of high-strength low-alloy（HSLA） steel was investigated. The protective property of the corrosion product layer was also explored. Experimental results reveal that the type of microstructure has significant effect on the corrosion resistance of HSLA steel. The measurement results of weight loss, potentiodynamic polarization curves, and electrochemical impedance spectroscopy indicate that the steel with acicular ferrite microstructure exhibits the lowest corrosion rate. Martensite exhibits a reduced corrosion resistance compared with polygonal ferrite. It is found that the surface of the acicular ferrite specimen uniformly covered by corrosion products is seemingly denser and more compact than those of the other two microstructures, and can provide some amount of protection to the steel; thus, the charge transfer resistance and modulus values of the acicular ferrite specimen are the largest. However, corrosion products on martensite and polygonal ferrite are generally loose, porous, and defective, and can provide minor protectiveness; thus, the charge transfer resistance values for polygonal ferrite and martensite are lower.

Effect of oxide inclusions on MnS precipitates and tensile mechanical property of high-strength low-alloy steel

The key role of oxide inclusions on the microstructure and mechanical property of a high-strength low-alloy steel was investigated.The field emission scanning electron microscope equipped with energy-dispersive spectrometry was used to characterize MnS precipitates.Oxide inclusions play an important role in the shape control of MnS precipitates.More oxides fovored to decrease the size and the aspect ratio of MnS precipitates.With less oxide inclusions in the steel,approximately over 16.7%MnS precipitates were with aspect ratio a>5 and pure MnS precipitates accounted for 75.9%in number.However,with more oxide inclusions in the steel,only 7.4%MnS precipitates were with a>5 and pure MnS precipitates accounted for 60.1%in number.Refinement of MnS by oxide inclusions improved the strength and inhibited the anisotropy.More oxide inclusions in the steel increased the yield strength and tensile strength of the steel in both longitudinal and transverse directions,and lowered the anisotropy of the mechanical property.

Impact of Mo/Ni alloying on microstructural modulation and low-temperature toughness of high-strength low-alloy steel

The high-strength low-alloy steel plates with varying Ni/Mo contents were manufactured using the thermos-mechanical control process.The investigation was conducted to explore the effect of Ni/Mo microalloying on microstructure evolution and mechanical properties of the steel.The results revealed that the increase in Ni content from 1 to 2 wt.%reduced the transition temperature of ferrite and the growth range of ferritic grain was narrowed,which promoted grain refinement.The optimized combination of grain size,high-angle grain boundaries(HAGBs),and martensite-austenite(M-A)islands parameter contributed to the excellent impact toughness of S1 steel at-100℃(impact absorbed energy of 218.2 J at-100℃).As the Mo increases from 0 to 2 wt.%,the matrix structure changes from multiphase structure to granular bainite,which increases the average effective grain size to~4.62 pm and reduces HAGBs proportion to~36.22%.With these changes,the low-temperature impact toughness of S3 steel is weakened.In addition,based on the analysis of the characteristics of crack propagation path,it was found that M-A islands with low content(~2.21%)and small size(~1.76 pm)significantly retarded crack propagation,and the fracture model of M-A islands with different morphologies was further proposed.Furthermore,correlation between behaviour of delamination and toughness was further analysed by observing delamination size and impact energy parameters.

Multi-performance evaluation of high-throughput accelerated corrosion test for high-strength low-alloy 921A steel

The corrosion behavior of high-strength low-alloy 921A steel in a simulated marine atmospheric environment was studied using a high-throughput experimental method.The corrosion behavior,corrosion morphology,and corrosion products of 921A steels were analyzed using various techniques,including corrosion mass loss method,polarization curve,white-light interferometry,scanning electron microscopy,energy-dispersive spectrometry,microbeam X-ray fluorescence spectrometry,X-ray diffraction technique,and X-ray photoelectron spectroscopy.The test results indicated that 921A steel exhibits better corrosion resistance than Q450NQR1 steel in simulated harsh atmospheric environments,as evidenced by a lower corrosion mass loss rate throughout the corrosion tests.The corrosion products of both steels consisted ofα-FeOOH,Fe_(3)O_(4),andγ-FeOOH,withα-FeOOH being more prevalent in the rust layer of 921A steel than in Q450NQR1 steel.The inner rust layer of 921A steel also exhibited an appositional enrichment region of Cr,Ni,Mo,and V,leading to its superior corrosion resistance compared to that of Q450NQR1 steel.The efficacy of high-throughput accelerated corrosion experimental methods was highlighted for evaluating the corrosion resistance of steel materials in harsh environmental conditions.The findings suggest that 921A steel exhibits better corrosion resistance compared to Q450NQR1 steel and has the potential to be more suitable in harsh marine atmospheric environments.The characterization of the rust layer structures and composition reveals the parallel enrichment of certain elements in the inner rust layer of 921A steel,which enhances its corrosion resistance.

Changes in microstructure and properties of weld heat-affected zone of high-strength low-alloy steel

The evolution of the microstructure and toughness of APL5L X80 pipeline steel after thermal welding simulation was investigated by X-ray diffraction,electron backscatter diffraction,and transmission electron microscopy.The results indicated that primary heat-affected zones can be divided into weld,coarse-grained,fine-grained,intercritical,and sub-critical zones.The microstructure of the weld zone is mainly composed of bainitic ferrite and a small amount of granular bainite;however,the original austenite grains are distributed in the columnar grains.The structure of the coarse-grained zone is similar to that of the weld zone,but the original austenite grains are equiaxed.In contrast,the microstructure in the fine-grained zone is dominated by fine granular bainite,and the effective grain size is only 8.15μm,thus providing the highest toughness in the entire heat-affected zone.The intercritical and subcritical zones were brittle valley regions,and the microstructure was dominated by granular bainite.However,the martensite-austenite(M/A)constituents are present in island chains along the grain boundaries,and the coarse size of the M/A constituents seriously reduces the toughness.The results of the crack propagation analyzes revealed that high-angle grain boundaries can significantly slow down crack growth and change the crack direction,thereby increasing the material toughness.The impact toughness of the low-temperature tempering zone was equivalent to that of the columnar grain zone,and the impact toughness was between those of the critical and fine-grained zones.

Effect of cathodic potential on stress corrosion cracking behavior of 21Cr2NiMo steel in simulated seawater

This study aims at providing systematically insights to clarify the impact of cathodic polarization on the stress corrosion cracking(SCC)behavior of 21 Cr2 NiMo steel.Slow-strain-rate tensile tests demonstrated that 21 Cr2 NiMo steel is highly sensitive to hydrogen embrittlement at strong cathodic polarization.The lowest SCC susceptibility occurred at-775 mV vs.SCE,whereas the SCC susceptibility was remarkably higher at potentials below-950 mV vs.SCE.Scanning electron microscopy(SEM)and electron backscattered diffraction(EBSD)revealed that the cathodic potential decline caused a transition from transgranular to intergranular mode in the fracture path.The intergranular mode transformed from bainite boundaries separation to prior austenitic grain boundaries separation under stronger cathodic polarization.Furthermore,corrosion pits promoted the nucleation of SCC cracks.In conclusion,with the decrease in the applied potential,the SCC mechanism transformed from the combination of hydrogen embrittlement and anodic dissolution to typical hydrogen embrittlement.

High-resolution Transmission Electron Microscopy Characterization of the Structure of Cu Precipitate in a Thermal-aged Multicomponent Steel

High-dispersed nanoscale Cu precipitates often contribute to extremely high strength due to precipitation hardening,and whereas usually lead to degraded toughness for especially ferritic steels.Hence,it is important to understand the formation behaviors of the Cu precipitates.High-resolution transmission electron microscopy(TEM)is utilized to investigate the structure of Cu precipitates thermally formed in a high-strength low-alloy(HSLA)steel.The Cu precipitates were generally formed from solid solution and at the crystallographic defects such as martensite lath boundaries and dislocations.The Cu precipitates in the same aging condition have various structure of BCC,9 R and FCC,and the structural evolution does not greatly correlate with the actual sizes.The presence of different structures in an individual Cu precipitate is observed,which reflects the structural transformation occurring locally to relax the strain energy.The multiply additions in the steel possibly make the Cu precipitation more complex compared to the binary or the ternary Fe-Cu alloys with Ni or Mn additions.This research gives constructive suggestions on alloying design of Cu-bearing alloy steels.

Hot Deformation and Corrosion Resistance of High-Strength Low-Alloy Steel

The hot deformation characteristics and the corrosion behavior of a high-strength low-alloy(HSLA) steel were investigated at deformation temperatures ranging from 800 to 1100 ℃ and strain rates ranging from 0.1 to 10 s-1 using an MMS-200 thermal simulation testing machine. Based on the flow curves from the experiment, the effects of temperature and strain rate on the dynamic recrystallization behavior were analyzed. The flow stress decreased with increasing deformation temperature and decreasing strain rate. With the assistance of the process parameters, constitutive equations were used to obtain the activation energy and hot working equation. The hot deformation activation energy of HSLA steel in this work was 351.87 kJ/mol. The work hardening rate was used to determine the critical stress(strain) or the peak stress(strain). The dependence of these characteristic values on the Zener-Hollomon parameter was found. A dynamic recrystallization kinetics model of the tested HSLA steel was constructed, and the validity of the model was confirmed by the experimental results. Observation of the microstructures indicated that the grain size increased with increasing deformation temperature,which led to a lowered corrosion resistance of the specimens.

Strengthening and toughening mechanism of a Cu-bearing high-strength low-alloy steel with refined tempered martensite/bainite(M/B)matrix and minor inter-critical ferrite

The microstructure–mechanical property relationship of a Cu-bearing low-carbon high-strength low-alloy steel,subjected to a novel multistage heat treatment including quenching(Q),lamellarization(L)and tempering(T),is presented.Yield strength of 989.5 MPa and average toughness at-80℃of 41 J were obtained in this steel after quenching and tempering(QT)heat treatments.Specimen QLT gained a little lower yield strength(982.5 MPa),but greatly enhanced average toughness at-80℃(137 J).To further clarify the strengthening and toughening mechanisms in specimen QLT,parameters of microstructural characteristic and crack propagation process were compared and analyzed for specimens Q,QL,QT and QLT.The microstructure of tempered martensite/bainite(M/B)in specimen QT changed to refined tempered M/B matrix mixed with minor IF(inter-critical ferrite)in specimen QLT.Cu-rich precipitates existed in tempered M/B for both specimens QT and QLT,as well as in IF.Compared with QT,adding a lamellarization step before tempering made the effective grains of specimen QLT refined and also led to coarser Cu-rich precipitates in tempered M/B matrix.The weaker strengthening effect of coarser Cu-rich precipitates should be a key reason for the slightly lower yield strength in specimen QLT than in specimen QT.No austenite was found in all specimens Q,QL,QT and QLT.Specimen QLT showed purely ductile fracture mode at-80℃due to the refined effective grains.The greatly improved toughness is mainly attributed to the enhanced energy of crack propagation.The combination of refined microstructure,softened matrix and deformation of minor'soft'IF during crack propagation led to the most superior toughness of specimen QLT among all specimens.

Simultaneously Improving Mechanical Properties and Stress Corrosion Cracking Resistance of High-Strength Low-Alloy Steel via Finish Rolling within Non-recrystallization Temperature

The effect of hot rolling process on microstructure evolution,mechanical properties and stress corrosion cracking(SCC)resistance of high-strength low-alloy(HSLA)steels was investigated by varying the finish rolling temperature(FRT)and total rolling reduction.The results revealed granular bainite with large equiaxed grains was obtained by a total rolling reduction of60%with the FRT of 950℃(within recrystallization temperature T_(r)).The larger grain size and much less grain boundaries should account for the relatively lower strength and SCC resistance.A larger rolling reduction of 80% under the same FRT resulted in the formation of massive martensite-austenite(M/A)constituents and resultant low ductility and SCC resistance.In contrast,a good combination of strength,ductility and SCC resistance was obtained via 80% rolling reduction with the FRT of 860℃(within non-recrystallization temperature T_(nr)),probably because of the fine grain size and M/A constituents,as well as a high density of grain boundary network.

Effects of Zr addition on microstructure and toughness of simulated CGHAZ in high-strength low-alloy steels

The effect of Zr addition(0.005,0.013,and 0.054 wt.%)on the microstructure and toughness of simulated coarse-grained heat-affected zone in high-strength low-alloy steels was revealed using a Gleeble 2000 thermal simulator.It was observed that elongated MnS inclusions were formed in the lowest Zr-containing steel,while only pure equiaxed ZrO2 existed in the 0.054Zr steel(Zr content of 0.054 wt.%).Complex oxide-sulfide inclusions(ZrO2+MnS)with size of(1.40±0.25)μtm were formed in 0.013Zr steel(Zr content of 0.013 wt.%).The complex inclusions refined the prior austenite grain,and the nucleation of acicular ferrite was promoted compared to those of 0.005Zr steel(Zr content of 0.005 wt.%)and 0.054Zr steel.Consequently,the 0.013Zr steel possessed superior low-temperature impact toughness in relation to 0.005Zr and 0.054Zr steels.Thus,moderate Zr addition can be considered as an effective method to refine the structure and improve the mechanical properties of the coarse-grained heat-affected zone.

Optimization of Calcium Addition to High-strength Low-alloy Steels

Nozzle blockage is a common problem during continuous casting of Al-killed steel, and calcium treatment is widely used to resolve it. In consideration of the production costs, the technology of nonmetallic inclusion control was studied to optimize the Ca consumption. The proposed process of slag washing was employed, and the refining slag composition, deoxidation conditions and alloying systems were optimized. Using these measures, the steel cleanliness before Ca addition was improved significantly, and the corresponding Ca consumption was reduced. More- over, the continuous casting could be conducted smoothly.

Tailoring Variant Pairing to Enhance Impact Toughness in High-Strength Low-Alloy Steels via Trace Carbon Addition

Alloying can make conventional metals reach ultra-high strength,but this usually comes at dramatic loss of toughness.In this work,a desirable strength–toughness combination in high-strength low-alloy steel achieved via trace carbon addition.The significance of carbon in tailoring variant pairing and tuning impact toughness was elucidated from the perspective of crystallography and thermodynamics.As the carbon content increases,the packets and blocks are refined,and the-40 impact toughness improves.The enhancement of impact toughness results from the higher density of block boundaries,and the fracture mode shifts from brittle fracture to ductile–brittle combined fractures,then to ductile fracture due to the increased carbon.Increasing the carbon content would lower the martensite start temperature(M_S)temperature and driving force for martensitic transformation,and increase the strength of austenite matrix,which in turn contributes to producing more V1/V2 variant pairs to accommodate the transformation strain.

Visualization of microstructural factors resisting the crack propagation in mesosegregated high-strength low-alloy steel

While relationship between fracture mechanism and homogeneous microstructures has been fully understood,relationship between fracture mechanism and inhomogeneous microstructures such as the mesosegregation receives less attention as it deserves.Fracture mechanism of the high-strength low-alloy(HSLA)steel considering the mesosegregation was investigated and its corre s ponding micro structure was characterized in this paper.Mesosegregation re fers to the inhomogeneous distribution of alloy elements during casting solidification,and leads to the formation of positive segregation zones(PSZ)and negative segregation zones(NSZ)in ingots.The fracture surface of impact sample exhibits the quasi-cleavage fracture at-21℃,and is divided into ductile and brittle fracture zone.Meanwhile,the PSZ and NSZ spread across ductile and brittle fracture zone randomly.In ductile fracture zone,micro-voids fracture mechanism covers the PSZ and NSZ,and higher deformation degree is shown in the PSZ.In brittle fracture zone,secondary cleavage cracks are observed in both PSZ and NSZ,but present bigger size and higher quantity in the NSZ.However,some regions of the PSZ still present micro-voids fracture mechanism in brittle fracture zone.It reveals that the microstructures in the PSZ exhibit a higher resistance ability to crack propagation than that in the NSZ.All observations above provide a better visualization of the microstructural factors that resist the crack propagation.It is important to map all information regarding the fracture mechanism and mesosegregation to allow for further acceptance and industrial use.

Effect of heat input on microstructure and mechanical properties of dissimilar joints of AISI 316L steel and API X70 high-strength low-alloy steel

The microstructure and mechanical properties of dissimilar joints of AISI 316L austenitic stainless steel and API X70 high-strength low-alloy steel were investigated.For this purpose,gas tungsten arc welding（GTAW）was used in three different heat inputs,including 0.73,0.84,and 0.97 kJ/mm.The microstructural investigations of different zones including base metals,weld metal,heat-affected zones and interfaces were performed by optical microscopy and scanning electron microscopy.The mechanical properties were measured by microhardness,tensile and impact tests.It was found that with increasing heat input,the dendrite size and inter-dendritic spacing in the weld metal increased.Also,the amount of delta ferrite in the weld metal was reduced.Therefore,tensile strength and hardness were reduced and impact test energy was increased.The investigation of the interface between AISI 316L base metal and ER316L filler metal showed that increasing the heat input increases the size of austenite grains in the fusion boundary.A transition region was formed at the interface between API X70 steel and filler metals.

Effects of final rolling temperature and coiling temperature on precipitates and microstructure of high-strength low-alloy pipeline steel

The strength-to-weight ratio is an important property of high-strength low-alloy(HSLA)steel in pipeline,whose precipitation strengthening can be improved.The final rolling temperature(FRT)and coiling temperature(CT)are the key process parameters in the control of precipitates and microstructure.Continuous cooling rate was fixed at 10℃/s,and the effects of deformation and coiling temperatures on precipitates and microstructure of Ti-Nb microalloyed HSLA steel were investigated through thermo-mechanical controlled processing on Gleeble 3500.The microstructure is mainly acicular ferrite with high density dislocation and several microns scale.The size and volume fraction of the precipitates were studied under transmission electron microscopy.The results showed that the diameter of the precipitates was in the range between 4 and 240 nm.The optimized combination of parameters is FRT of 820℃and CT of 550℃,and the volume fraction of precipitates obtained under this process is 0.59%.