Study on welding process performance of HG70D high strength steel plate

In this paper, the performance of HG70D welded joint of ultra-high strength steel plates is presented, and the performance of HG70D and Q345B welded joints is studied. The high strength steel plate HG70D showed excellent weldability. Through the X-ray inspection of welded joints, tensile strength, impact test and bending performance test, the comprehensive performance of the joint was excellent. The macroscopic and microscopic metallographic analysises of the welded joints show that the welding seams included pearlite, sorbite, ferrite, etc. The influence of stress annealing temperature on HG70D of high strength steel plate was analyzed by heat treatment.

Ultra-high cycle fatigue behavior of high strength steel with carbide-free bainite/martensite complex microstructure

The ultra-high cycle fatigue behavior of a novel high strength steel with carbide-free bainite/martensite （CFB/M） complex microstructure was studied. The ultra-high cycle fatigue properties were measured by ultrasonic fatigue testing equipment at a frequency of 20 kHz. It is found that there is no horizontal part in the S-N curve and fatigue fracture occurs when the life of specimens exceeds 10^7 cycles. In addition, the origination of fatigue cracks tends to transfer from the surface to interior of specimens as the fatigue cycle exceeds 10^7, and the fatigue crack originations of many specimens are not induced by inclusions, but by some kind of ＂soft structure＂. It is shown that the studied high strength steel performs good ultra-high cycle fatigue properties. The ultra-high fatigue mechanism was discussed and it is suggested that specific CFB/M complex microstructure of the studied steel contributes to its superior properties.

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 tf^Cl/tf^W -strain rate are divided into three regions： stress-dominated region, SCC-dominated region, and corrosion-dominated region, so as the curves of εf^Cl/εf^W - 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 εf^Cl/εf^W and tf^Cl/tf^w is a straight line for AerMet 100.

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 Fe_2O_3 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-Fe_2O_3 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-Fe_2O_3 or β-FeOOH.

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

AerMet100 ultra-high strength steel plates with a thickness of 2 mm were welded using a COz 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.

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.

Formation mechanism of transverse crack in repair welding for thick plate of Q345 high strength steel

Transverse cracks occur usually in repair welding for thick plate of high strength steel. It needs multiple times of repair welding. The quality of production and deliver deadline will be influenced. Therefore, it is very significant to investigate the cause and control of transverse crack in repair welding. In this paper, both ends restraint crack experiment is developed to produce delay transverse crack for high strength steel. Metallographic results show that four types of cracks are found in repair welding metal zone and heat affected zone. Large chevron transverse cracks are found in repair welding zone. Lots of micro transverse cracks are found in inter-layer repair welding metal zone, root HAZ and two ends of repair welding individually. The distribution character and formation mechanism of the transverse crack are further analyzed through hardness testing and residual stress measurement.

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.

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.

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.

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 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.

Effect of short-time low-temperature austenitizing on microstructure and mechanical properties of DT300 ultra-high strength steel fabricated by laser powder bed fusion

To address the inhomogeneous microstructure and improve the mechanical properties of DT300 ultra-high strength steel specimens fabricated by laser powder bed fusion,different post-heat treatment schedules are performed.With the increase in austenitizing temperature and time,the migration rate of austenite grain boundaries continuously increases with the dissolution of nano-carbides,and the formation of nano-oxides and twin martensite is also inhibited accordingly.The rapid growth in the size of prior austenite grains and martensite laths,as well as the decrease in the content of nano-oxides and twin martensite,led to a rapid decrease in the strength(yield strength and ultimate tensile strength)from HT2 to HTF specimens.The HT1 specimens(austenitizing at 830℃for 30 min,then oil quenching and tempering at 300℃for 120 min and finally air cooling)display excellent mechanical properties of yield strength of 1572 MPa,ultimate tensile strength of 1847 MPa,elongation of 9.84%,and fracture toughness of 106 MPa m^(1/2),which are counterparts to those of conventional DT300 steel forgings after heat treatment.

Effect of plastic deformation on diffusion-rolling bonding of steel sandwich plates

Diffusion bonding is one of the most important techniques for composite materials, while bonding temperature, holding time,and rolling reduction are the key parameters that affect the bonding strength of sandwich plates. To study the effect of plastic deformation on the bonding strength, laboratory experiments were carried on a Gleeble Thermal Simulator to imitate the diffusion-rolling bonding under different reductions for steel sandwich plates. The bonding strength and interlayer film thickness were measured, and the element diffusion was analyzed using line scanning. The relationship between the bonding strength and “diffused interlayer” thickness was investigated. It has been found that the bonding strength increases with reduction, whereas the interlayer film thickness decreases gradually as the reduction increases. The diffusion under plastic deformation is obviously enhanced in comparison with that of nil reduction. The mechanism of plastic deformation effect on the diffusion bonding and related models have been discussed.

Recent development of UHSS and its hot-dip galvanizing processes in Baosteel

Baosteel has excelled in automotive steel sheets in the past three decades.It has made a significant contribution to the development of China’s automotive industry by producing a wide range of high-quality steel products.Some milestones achieved by Baosteel automotive steel sheet were briefly reviewed.The current challenges in producing ultra-high strength steel(UHSS),especially hot-dip galvanized UHSS,were summarized.The most current advancements in UHSS and the corresponding hot-dip galvanizing processes were discussed.The galvanizability of Si-Mn-added QP steel and DP steel, Mn-added TWIP steel, and Al-added low-density steel has been improved by different techniques in Baosteel.

The Influence of Preheat Temperature of Steel Plate on Steel-mushy Cu-graphite Bonding

The pressing bonding of steel plate to QTi3.5-3.5 graphite slurry was conducted. Under the conditions of 530 ℃ for the preheat temperature of dies, 45% for the solid fraction of QTi3.5- 3.5 graphite slurry, 50 MPa for the pressure and 2 min for the pressing time, the relationship between the preheat temperature of steel plate and interfacial mechanical property of bonding plate was studied. The results show that when the preheat temperature of steel plate is lower titan 618 ℃ , the interfacial shear strength of bonding plate increases with the increasing of the preheat temperature of steel plate. When the preheat temperature of steel plate is higher than 618 ℃ , the interfacial shear strength decreases with the increasing of the preheat temperature of steel plate. When the preheat temperature of steel plate is 618 ℃ , the highest interfacial shear strength of bonding plate of 127.8 MPa can be got.

Improving behavior of semi-supported steel plate shear walls

In spite of the good performance of the steel plate shear wall(SPSW)in recent earthquakes and experimental studies,the need for huge columns to surround the infill plate is a major shortcoming of the system.This shortcoming can be resolved by using semi-supported SPSW.The semi-supported SPSW has secondary columns that prevent the transfer of stress from the infill plate to the main columns.In spite of extensive experimental and numerical investigations on SPSWs,there are many ambiguities regarding the behavior of the semi-supported SPSW.Although stress in the columns is reduced,incomplete diagonal tension field action is formed in the infill plate that creates new problems.In this paper,a new type of semi-supported SPSW is presented in which the steel plate and the secondary columns are angled.The creation of the angle of the plate and the secondary column makes it possible to use the full capacity of the steel plate as well as the capacity of the secondary columns.Numerical results showed that the wall with a 60°angle has a favorable performance relative to the semi-supported wall.Moreover,with the 60°angle,stiffness,strength and energy absorption is increased.The angle of the secondary columns has little effect on the non-elastic stiffness.Nevertheless,using a wall with an angle of more than 90°can neutralize the wall’s behavior relative to conventional walls.Therefore,the wall with a 60°angle as an optimal angle is recommended.

IMPROVEMENT ON SUSCEPTIBILITY OF ULTAR-HIGH STRENGTH STEEL TO STRESS CORROSION CRACKING BY HIGH TEMPERATURE QUENCHING

The effect of quenching temperature on the stress corrosion cracking of 30Cr3SiNiMoV ultra-high strength steel in 3.5% NaCl aqueous solution has been studied.The threshold K_(ISCC) may continuously increase with the quenching temperature raised from 870 to 1200℃ . All of the fractures are intergranular.The analyses of the segregation along prior austenitic grain boundaries,grain size and other microstructural factors reveal that the inerease of K_(ISCC) is mainly due to the coarsening of prior austenitic grains.

Investigation on microstructure and properties of a combined precipitation hardening ultrahigh strength steel

The microstructure and properties of a combined precipitation hardening ultrahigh strength steel with nano-sized carbides and intermetallics were studied systematically.The results show that after tempering at 300℃lots ofε-carbides are precipitated in the martensite,the strength rises and the toughness falls slightly.After tempering at 430℃,much coarser cementite lamina are precipitated in martensitic laths,which causes the impact toughness falls to the minimum value.With temperature further increasing the cementites are dissolved and M_2C carbides,β-NiAl intermetallics and reverse austenite begin to precipitate.The tensile strength and yield strength achieve the peak value at 470℃,490℃respectively.The tested steel achieve a tensile strength of 2 120 MPa,a yield strength of 1 950 MPa and impact energy of 54 J/cm^2 after optimum tempering at 510℃.When tempering temperature is above 530℃the M_2C carbides and reverse austenite is coarsening.After tempering at 560℃the reverse austenite reaches the maximum volume fraction in present work.