Internal oxidation of hot-rolled ultra-high strength steel and its effect on the surface quality of cold-rolled sheets

In this study,the scale and internal oxidation of hot-rolled ultra-high strength steel sheets were characterized.It was found that both the formation of the scale and the internal oxidation of Si and Mn depended on the coiling temperature and position of the steel sample on the strip coil.At a relatively high coiling temperature,a large amount of internal oxidation was observed on the samples cut from the middle of the coil.The depth of the internal oxidation zone exceeded 10 μm and a thin iron layer covering the scale was observed in some cases.Pickling and cold-rolling experiments were conducted on selected samples.Scale pickling was found to be greatly delayed by the formation of an iron layer,which frequently resulted in under-pickled defects.In addition,pickling of the entire internal oxidation zone was difficult,except at the grain boundaries,where the degree of internal Si and Mn oxidation was enriched.The surface of the cold-rolled steel sheet was ruined by the remaining oxidation zone in the subsurface of the pickled steel.The internal oxidation of hot-rolled ultra-high strength steel must be precisely controlled to improve the subsequent surface quality of cold-rolled steel.

SCC evaluation of ultra-high strength steel in acidic chloride solution

The stress corrosion crack(SCC) susceptibility of ultra-high strength steel AerMet 100 was investigated by slow strain rate technique(SSRT),tensile with polarization and surface analysis technique.The curves of t fC l/tfW-strain rate are divided into three regions:stress-dominated region,SCC-dominated region,and corrosion-dominated region,so as the curves of tfCl/tfW-strain rate and tm/tf-strain rate.The results of tensile tests with polarization show that the main SCC mechanism of AerMet 100 is anodic dissolution,which controls the corrosion process.The three regions have been discussed according to the relationship between the rate of slip-step formation and the rate of dissolution.Fracture appearances in different environments were analyzed by scanning electron microscopy(SEM).SCC fracture appears as a mixture of intergranular and dimples,while it is totally dimples in the inert environment.The εf becomes the parameter to predict tf because the relationship between tfCl/tfW and tfC l/tfW is a straight line for AerMet 100.

Microstructure and Mechanical Properties of Aer Met 100 Ultra-high Strength Steel Joints by Laser Welding

Aer Met100 ultra-high strength steel plates with a thickness of 2 mm were welded using a CO2 laser welding system. The influences of the welding process parameters on the morphology and microstructure of the welding joints were investigated, and the mechanical property of the welding joints was analyzed. The experimental results showed that the fusion zone of welding joint mainly consisted of columnar grains and a fine dendrite substructure grew epitaxially from the matrix. With the other conditions remaining unchanged, a finer weld microstructure was along with the scanning speed increase. The solidification microstructure gradually transformed from cellular crystal into dendrite crystal and the spaces of dendrite secondary arms rose from the fusion line to the center of the fusion zone. In the fusion zone of the weld, the rapid cooling caused the formation of martensite, which led the microhardness of the fusion zone higher than that of the matrix and the heat affected zone. The tensile strength of the welding joints was tested as 1 700 MPa, which was about 87% of the matrix. However, the tensile strength of the welding joints without defects existed was tested as 1832 MPa, which was about 94% of the matrix.

Manifestations in Corrosion Prophase of Ultra-high Strength Steel 30CrMnSiNi2 A in Sodium Chloride Solutions

The corrosion behaviors of ultra-high strength steel 30CrMnSiNi2A in sodium chloride solution were studied by weight loss and electrochemical methods. The morphology of corrosion products was observed using scanning electron microscopy(SEM) and the composition was analyzed using an energy dispersive spectroscopy(EDS) and X-Ray diffraction(XRD). The experimental results showed that the corrosion came from pitting corrosion and the rust layer was composed of outer rust layer γ-FeOOH and inner rust layer Fe2O3 with a little β-FeOOH. The correlation between corrosion rate and test time accorded with exponential rule. The corrosion current measured by polarization methods was higher than that calculated by weight loss method after a long-time immersion, the main reason was that β-FeOOH and γ-Fe2O3 transformed by γ-FeOOH led to overestimating corrosion rate. The processes of corrosion prophase were obtained from XRD and EIS results. The corrosion product, Fe(OH)2 formed at the initial stage stayed at a non-steady state and then consequently transferred to γ-FeOOH, γ-Fe2O3 or β-FeOOH.

Analysis of the effect of geometrical parameters on fatigue performance of spot-weld joint for ultra-high strength steel

This paper studied the spot welding structure of ultra-high strength steel 22MnB5.ANSYS software was adopted to simulate its static strength;BS5400 algorithm was used to calculate the fatigue life;and the grouping method was used to test the fatigue performance of tensile shear spot weld specimens.The simulation results were in good agreement with the experimental values.Based on the validation of the simulation method,influences of different structural parameters on static strength and fatigue life were explored by adopting single factor.The results showed that within the selected structure parameter range,increase of the sheet thickness,nugget diameter,sheet width and overlapping length can lead to longer fatigue life.Besides,the fatigue life of spot weld took on a linear relationship with the overlapping length,a DoseResp relationship with the sheet thickness,and a single exponential decay relationship with the sheet width and the nugget diameter.Moreover,in order to estimate the impact from various parameters on the fatigue life of the specimens,the Taguchi orthogonal design method was applied in the simulation design.The simulating result indicated that influence of the sheet thickness on fatigue life was the most significant.In addition,the effects of nugget diameter,sheet width and overlapping length on fatigue life were reduced in turn.

Corrosion Behavior of Ultra-high Strength Steel 300M in Different Simulated Marine Environments

Corrosion behavior of 300M in neutralcorrosion environments containing Na Clsimulated by totalimmersion(TI),salt spraying(SS)and periodic immersion(PI),was investigated by surface analysis techniques,corrosion weight-loss method,and electrochemicalmeasurements.In totalimmersion environment,rust on the steelconsisted of a porous outer rust layer with main constituent of γ-Fe OOH,and an inner rust layer of dense Fe_3O_4 film with network broad cracks.In salt spraying environment,outer rust with main composition of γ-Fe OOH/α-Fe OOH/Fe_3O_4 was compact,and inner rust showed dense Fe_3O_4 film.Rust formed by periodic immersion exhibited a compact outer rust layer with constituent of α-Fe OOH/γ-Fe OOH/Fe_3O_4 and an inner rust layer with composition of α-Fe OOH/α-Fe_2O_3;inner rust showed a ultra-dense film adherent to the steel.The corrosion rate showed a rule of vss(salt spraying)>vti(totalimmersion)>>vpi(periodic immersion)in 0-240 h,and vss≈vti?vpiin 240-720 h.The rust formed by periodic immersion was dense and compact,with stable electrochemicalproperties,and had excellent protection on the steel.Humidity and oxygen concentration in allthe environments played major roles in rust formation.

Effect of anions on stress corrosion cracking behaviors of ultra-high strength steel 23Co14Ni12Cr3Mo

The effects of chloride,sulfate and carbonate anions on stress corrosion behaviors of ultra-high strength steel 23Co14Ni12Cr3Mo were studied by stress corrosion cracking(SCC)test method using double cantilever beam(DCB)specimens.The SCC morphology was observed by using scanning electron microscopy(SEM)and the composition of corrosion products was analyzed by using energy dispersive spectrometer(EDS).The results show that the crack propagates to bifurcation in NaCl and Na2SO4 solution,while the crack in Na2CO3 solution propagates along the load direction.The SCC rate in NaCl solution is the highest,while lower in Na2SO4 solution and little in Na2CO3 solution.From the SEM morphologies,quasi-cleavage fracture was observed in NaCl and Na2SO4 solutions,but intergranular features in Na2CO3 solution.The mechanism of anion effect on SCC of steel 23Co14Ni12Cr3Mo was studied by using full immersion test and electrochemical measurements.

Research and development of hot-rolled ultra-high strength steel at Baosteel

The effects of the composition and cooling process on the microstructures and properties of hot-rolled ultra-high strength low alloy(HSLA)steel,complex phase steel and martensite steel were studied in the laboratory.And S700MC and MP1200 ultra-high strength steels were trial produced at the 1 880 mm hot-rolling line of Baosteel.Compared with conventional hot-rolled high strength products,the idea that water is alloy was applied in the newly developed hot-rolled ultra-high strength steel.By the use of the economical composition design and controlled cooling after hot-rolling effectively,ultra-high strength steel of different steel grades can be obtained.

Prediction and Verification of Resistance Spot Welding Results of Ultra-High Strength Steels through FE Simulations

Resistance spot welding (RSW) is the most common welding method in automotive engineering due to its low cost and high ability of automation. However, physical weldability testing is costly, time consuming and dependent of supplies of material and equipment. Finite Element (FE) simulations have been utilized to understand, verify and optimize manufacturing processes more efficiently. The present work aims to verify the capability of FE models for the RSW process by comparing simulation results to physical experiments for materials used in automotive production, with yield strengths from approximately 280 MPa to more than 1500 MPa. Previous research has mainly focused on lower strength materials. The physical weld results were assessed using destructive testing and an analysis of expulsion limits was also carried out. Extensive new determination of material data was carried out. The material data analysis was based on physical testing of material specimens, material simulation and comparison to data from literature. The study showed good agreement between simulations and physical testing. The mean absolute error of weld nugget size was 0.68 mm and the mean absolute error of expulsion limit was 1.10 kA.

Microstructural Transformation and Precipitation of an Ultra-high Strength Steel under Continuous Cooling

We investigated phase transition and precipitation of ultra-high strength steel(UHSS)in a new "short process" with controlled rolling and controlled cooling.Thermalexpansion test combined with metallographic observation was used to research the continuous cooling transformation(CCT)curve.Moreover,the microstructuraltransformation and precipitation law was revealed by morphologicalobservation and alloying elements by electron probe micro-analyzer(EPMA).Transmission electron microscopy(TEM)was utilized to analyze the composition and grain orientation of microstructure.The study showed that the measured criticaltransformation temperatures of Ac1 and Ac3 were 746 and 868 ℃,respectively.The CCT curve indicated that the undercooled austenite was transformed into proeutectoid ferrite and bainite with HV 520 in a broad range of cooling rate 0.1^(-1) ℃·s^(-1).When subjected to a cooling rate of 1 ℃·s^(-1),the undercooled austenite was divided into small-sized blocks by formed martensite.With further increase of cooling rate,micro-hardness increased dramatically,the microstructure of specimen was mainly lathe bainite(LB),granular bainite(GB),lath martensite(LM)and residualaustenite.By diffraction test analysis,it was identified that there was K-S orientation relationship between martensite and austenite for {110}_α//{111}_γ,{111}_α//{101}_γ.EPMA clearly showed that carbon diffused adequately due to staying for a long time at high temperature with a lower cooling rate of 2 ℃·s-1.Phase transition drive force was lower and the residualaustenite existed in the block form of Martensite austenite island(M-A).With the increase of cooling rate to 10 ℃·s^(-1),the block residualaustenite reduced,the carbon content of residualaustenite increased and α phase around the residualaustenite formed into a low carbon bainite form.

Effects of the shape and size of rectangular inclusions on the fatigue cracking behavior of ultra-high strength steels

The fatigue cracking behavior of ultra-high strength steels containing rectangular inclusions of small sizes were investigated based on in situ observations by scanning electron microscopy (SEM). The size and shape of rectangular inclusions affect markedly the initiation site and propagation path of a fatigue crack. Especially, the initiation site of a fatigue crack depends strongly on the angle between the long-axis of a rectangle inclusion and the loading direction, and the length/width ratio of this rectangle inclusion because the residual stress distribution fields vary with these conditions. The results coincide very well with those of finite element analysis.

Simulation about hot stamping of ultra-high strength steel on the basis of lightweight technology

With the development of automobile lightweight,it is very necessary to apply the ultra-high strength steel parts manufactured by hot stamping,which offers the possibility to reduce the weight of automobiles and maintain the safety requirement.In order to complete hot stamping,it is important to design the structure of parts reasonably,which is related with reasonable matching of strength.The objective of this paper is to guide the design of parts manufactured by hot stamping and find the forming technical requirements of vehicle performance.Through experiments,the paper obtains the stress and strain curves at different deformation temperatures and strain rates.Based on experimental data, the constitutive relationship model is established which can reflect the deformation capacity of ultra-high strength steel during the process of hot stamping.Combined with finite element simulation results of hot stamping by commercial software AUTOFORM,transfer path of load and matching law of strength,the paper determines the design criteria and forming technical requirements of parts manufactured by hot stamping.At the same time,the impact performance of front cross member internal plate is taken into consideration.

Microstructural evolution of Al-Si coating and its influence on high temperature tribological behavior of ultra-high strength steel against H13 steel

Al-Si coated ultra-high strength steel(UHSS)has been commonly applied in hot stamping process.The influence of austenitizing temperature on microstructure of Al-Si coating of UHSS during hot stamping process and its tribological behavior against H13 steel under elevated temperature were simulatively investigated.The austenitizing temperature of Al-Si coated UHSS and its microstructual evolution were confirmed and analyzed by differential scanning calorimetry and scanning electron microscopy.A novel approach to tribological testing by replicating hot stamping process temperature history was presented.Results show that the hard and stable phases Fe_2Al_5+FeAl_2 formed on Al-Si coating surface after exposure to 930°C for 5 min,which was found to be correlated to the tribological behavior of coating.The friction coefficient of coated steel was more stable and higher than that of uncoated one.The main wear mechanism of Al-Si coated UHSS was adhesion wear,while abrasive wear was dominant for the uncoated UHSS.

Influence of Rust Layers on the Corrosion Behavior of Ultra-High Strength Steel 300M Subjected to Wet–Dry cyclic Environment with Chloride and Low Humidity

The influence of rust layers on the corrosion behavior of ultra-high strength steel 300 M subjected to a simulated coastal atmosphere was investigated by corrosion weight loss, surface analysis techniques, and electrochemical methods.The results exhibit the presence of a large proportion of c-Fe OOH and a-Fe OOH and a small amount of Fe3O4 in the outer rust layer. During the wet–dry cyclic process, the bonding performance and the density of outer rust layer deteriorate with the thickness of outer rust. The inner rust layer plays a main role on protectiveness, which can be attributed to the formation of an ultra-dense and adherent rust film with major constituent of a-Fe OOH and a-Fe2O3 on the steel.

Dynamic mechanical behavior of ultra-high strength steel 30CrMnSiNi2A at high strain rates and elevated temperatures

During high speed machining in the field of manufacture,chip formation is a severe plastic deformation process including large strain,high strain rate and high temperature.And the strain rate in high speed cutting process can be achieved to 105 s^(-1).30CrMnSiNi2Asteel is a kind of important high-strength low-alloy structural steel with wide application range.Obtaining the dynamic mechanical properties of30CrMnSiNi2Aunder the conditions of high strain rate and high temperature is necessary to construct the constitutive relation model for high speed machining.The dynamic compressive mechanical properties of30CrMnSiNi2Asteel were studied using split Hopkinson pressure bar(SHPB)tests at 30-700°C and3000-10000s^(-1).The stress-strain curves of 30CrMnSiNi2Asteel at different temperatures and strain rates were investigated,and the strain hardening effect and temperature effect were discussed.Experimental results show that 30CrMnSiNi2Ahas obvious temperature sensitivity at 300°C.Moreover,the flow stress decreased significantly with the increase of temperature.The strain hardening effect of the material at high strain rate is not significant with the increase of strain.The strain rate hardening effect is obvious with increasing the temperature.According to the experimental results,the established Johnson-Cook(J-C)constitutive model of 30CrMnSiNi2Asteel could be used at high strain rate and high temperature.

Effect of Martensite Fine Structure on Mechanical Properties of an 1100 MPa Grade Ultra-high Strength Steel

An 1100 MPa grade ultra-high strength steel with different martensite fine structures,characterized by prior austenite grain size,martensite packet size,block width and lath width,was studied by various heat treatment processes.The result shows that with decreasing prior austenite grain size,both the packet size and block width decrease,while the lath width has virtually no change.Accordingly,both strength and toughness increase,while total elongation decreases.The yield strength has a Hall-Petch type relationship with the prior austenite grain size,packet size and block width,and the block width may be regarded as a key factor influencing strength.On the other hand,the ductile to brittle transition temperature(DBTT)is found to be more related to the packet size,which may be considered as a dominant factor influencing toughness.

Newly Designed Cr-Mn Alloyed Ultra-high Strength Steel without Boron Addition for Hot-stamping Processing

A newly designed hot-stamping steel alloyed by chromium(Cr)and manganese(Mn)without boron(B)addition was developed for automobile mass reduction.The experimental results showed the Cr-Mn alloyed steel could be quenched to full martensite microstructure when the cooling rate was higher than 14 ℃/s.Yield strength,tensile strength and elongation of the experimental hot stamping part reached 1 180 MPa,1 645 MPa and 8.4%,respectively.The experimental hot stamping part possessed higher tensile strength and elongation,compared with conventional hot-stamping steel of 22MnB5.Furthermore,excellent processing flexibility would be obtained in this novel hot-stamping steel because of its lower critical cooling rate and phase transformation temperature.The design of the composition and investigations of microstructure,mechanical properties and hot-stamping processing were also studied.

Effect of Heat Treatment Process on Properties of 1000 MPa Ultra-High Strength Steel

Two types of steel,C-Mn-Cr-Mo-B microalloyed steel and C-Mn-Mo-Nb-Cu-B microalloyed steel,are designed to develop 1000 MPa ultra-high strength steel. Two kinds of processes,thermomechanical controlled process (TMCP) combined with traditional off-line quenching and tempering (QT) process versus controlled rolling process (CR) combined with direct quenching and tempering (DQ+T) process,are applied. The effect of heat treatment processing mode on the microstructure and mechanical properties is studied. The relationship between microstructure and mechanical properties is investigated by SEM and TEM. After tempering at 450 to 550 ℃ for 1 h,the steel produced by TMCP+QT process shows combination of excellent strength and low temperature toughness. The yield strength is above 1000 MPa,elongation above 15% and impact energy at-40 ℃ more than 30 J. After tempering at 450 ℃,a large number of ε-Cu particles precipitated in C-Mn-Mo-Nb-Cu-B steel produced by CR+DQ+T process lead to a significant increase in yield strength. And after tempering at 500 to 600 ℃,the yield strength of the steel is further improved to 1030 MPa because of precipitates,such as nitride or carbide of niobium,carbide of molybedenum and vanadium. When the tempering temperature is increased above 620 ℃,the yield strength is still higher than 1000 MPa and elongation is above 20% and impact energy at-40 ℃ is more than 35 J. After tempering at above 500 ℃,the toughness of the steel treated by TMCP+QT process is superior to that of steel by CR+DQ+T process.

Effect of Thermomechanical Treatment Temperature on Structure and Properties of CFB/M Ultra-High Strength Steel

Modified CCT diagram of carbide-free bainite-martensite(CFB/M)ultra-high strength steel was established by applying controlled cooling of small samples.In addition,the influence of thermomechanical treatment temperature on the structure and properties was discussed.The experimental results showed that when deformed at 860℃ and below,ferrite transformation occurred due to strain.With the decrease of ausforming temperature,the quantity of ferrite increased and strength and toughness were deteriorated.Therefore,certain information was provided for optimizing technical parameter of ausforming process:firstly,the thermomechanical treatment temperature should not be lower than 860℃ in order to avoid ferrite formation induced by deformation;secondly,rapid cooling rate is also significant after deformation in order to avoid ferrite precipitation during subsequent cooling stage.