Increasing geometrical accuracy at open ends of the roll-formed part is difficult due to the release of residual stress after end cutting.In this work,a typical rail with a high requirement of geometry accuracy was selected to realize the behaviors of residual stress release.First,residual stress distribution after roll forming is discussed in detail by finite element analysis with ABAQUS.In addition,two different approaches are proposed to check their capabilities in reducing the residual stress level.The results indicate that both additional rolling passes and multiple bending processes are beneficial to reducing uniform residual stress.
With the improvement of safety performance,car parts have different requirements for material strength and energy absorption performance.The conventional 1 500-MPa hot stamping steel cannot well meet the requirements.Considering the new generation 600-MPa hot stamping steel,this study investigates the applicable car parts and hot stamping process,then designs a new body-in-white(BIW)crash test for obtaining the crash performance of the new material.Through the actual part development and crash test,it is verified that the application of the new generation hot stamping steel can improve the crash performance of BIW.
To improve the competitive relationship between strength and toughness,the effect of low undercooling in austenite(γ)on the microstructure and mechanical properties of commercial vanadium-containing wheel steels was studied using an optical microscope(OM),a scanning electron microscope(SEM),a transmission electron microscope(TEM),and mechanical property tests.The results show that when the wheel steel is slightly cooled to an appropriate temperature above Ac3 point for a short time after it has been austenitized at an elevated temperature,the solid-solved vanadium is pre-precipitated in the form of V(C,N)second phase semicoherent with the matrix in the original γ grain.This phase hardly participates in matrix strengthening.Due to the small mismatch between V(C,N)and ferrite(α),during the subsequent-cooling phase transformation stage,the pre-precipitated second phase becomes the α nucleation point,causing granular and ellipsoidal intragranular ferrite(IGF,with an average size of 4-6 μm)to nucleate in the original γ.The IGF production and strength loss increases with the increasing undercooling degree.Based on this,Masteel Co.,Ltd.has developed a new heat-treatment step-cooling process that can promote the formation of IGF,considerably improving the level and uniformity of fracture toughness on the premise that the strength and hardness of the wheel are almost unchanged.
The function,features,and architecture of a robot that performs automatic temperature measurement and sampling applied on a 150-t AC electric arc furnace(EAF)production line of Baosteel were presented,and the key points of design and revamping experience on the site layout,device protection,lance tool,probe container,measuring position control,and system safety were summarized.Furthermore,a valuable reference for the application of automatic temperature measuring and sampling robots in EAF steelmaking plants will be provided.
Three kinds of ultra-high-strength steels are subjected to uniaxial tensile,forming limit,and hole expansion tests to characterize their material forming properties.Results show that the elongation of S1500 reaches 12.9％and is higher than that of MS1500 with the same strength grade but is lower than that of QP980.The forming limit of S1500 steel is higher than that of MS1500 but lower than that of QP980.The instantaneous n-value of the material changes with the volume fraction of retained austenite.The hole expansion ratios of S1500,MS1500,and QP980 steels are 31.3％,32.2％,and 28.3％,respectively.The hole expansion ratio of QP steel increases slightly with the increase in strength grade.This behavior is contrary to the change trend of elongation and forming limit.Among the three kinds of materials,QP980 steel has the best global formability,and S1500 steel has better global formability than martensitic steel with a similar strength grade.The local formability of the materials improves slightly with the decrease in the amount of retained austenite.MS1500 may have the best local formability in accordance with engineering practice.
Among the bottlenecks that hinder the improvement of the production efficiency of hot stamping are high strength and difficulty in edge cutting and hole punching.Starting from the preparation of hot stamping multiphase microstructure materials,this paper developed a plate quenching die system with controllable surface temperature and prepared four types of hot stamping plates with different martensite volume fractions.Then,straight edge cold cutting experiments were performed to study the influence of cutting clearance and cutting force on fracture quality.The results show that the bright zone is the largest when the cutting clearance is 0.14 mm,and the cutting experience coefficient of the hot stamping sheet with each martensite volume fraction is obtained when the cutting clearance is 0.14 mm.The research results of this paper were applied to the production of hot stamping parts.
The loading capacity of ultralarge container ships has reached 24 000 TEUs so far,and to ensure the safe operation of these ships,the maxmium thickness of crack arrest steel used in the upper deck areas reaches 100 mm,and crack arrest toughness(Kca)needs to be＞8 000 N/mm3/2.The EH47 steel was employed to study the effect of Nb on the phase transformation of supercooled austenite in the continuous cooling process after rolling and the effect of Nb on microstructure and mechanical properties of the crack arrest steel plate.It was found that the addition of 0.02％Nb can inhibit the ferrite transformation,improve the steel plate strength,and reduce the strength inhomogeneity in the thickness direction.Industrial production of 100 mm-thick EH47 was carried out based on the function of Nb in EH47 steel,and the test results reveal that high-strength EH47 shipbuilding plates with high toughness,excellent fracture and crack arrest toughness,and good welding properties can be produced using Nb-microalloyed composition design and the thermal mechanical control process(TMCP);furthermore,the value of the crack arrest toughness reached 9 450.7 N/mm3/2 at-10 ℃.
Strip-shaped chromatic aberration defects on the surface of an electro-galvanized fingerprint-resistant sheet were systematically studied using Thermo-Calc thermodynamic calculations,scanning electron microscope and X-ray diffractometer analyses,and rapid heat treatment simulation technology.The formation mechanism of these defects was also analyzed.The results show a strong correlation between the defects and the uneven distribution of the Fe(001)component on the surface of the steel substrate.The relatively high proportion of Fe(001)components on the surface of the steel substrate affects the distribution density of the Zn crystal cells in different orientations during the electrodeposition process,which causes the reflection intensity of the light of the galvanized layer to differ from various visual perspectives,and macroscopic strip-shaped chromatic aberration defects are finally formed.The high proportion of the Fe(001)component on the surface of the steel substrate is mainly related to the following two factors.First,when the strip steel is hot-rolled,the finishing hot-rolling temperature is close to the γ→α phase transition temperature.When the local temperature on the upper surface of the strip steel is low,γ+αtwo-phase rolling easily occurs,and this results in an uneven stress distribution between the γ and α phases after the hot-rolling process.This uneven distribution of hot-rolling stress results in the formation of a coarse grain structure in the local area on the surface of the hot-rolled sheet,which strongly affects the subsequent cold rolling and annealing process,and the annealed steel sheet substrate ultimately contains a greater proportion of the Fe(001)component.And second,a fast cooling rate(＞10 K/s)during the slow cooling stage in the continuous annealing process inhibits the transformation of the Fe(001)to the Fe(111)component on the surface of the cold-rolled steel sheet,and it is then not possible to effectively eliminate the influence of the unevenly distributed Fe(001)component on the surface of the annealed steel sheet(originating from the uneven hot-rolled microstructure).This uneven distribution of the Fe(001)component on the surface of the annealed steel sheet following the continuous annealing heat treatment has a strong effect on the electrodeposition behavior of the Zn crystal cell in the subsequent electroplating process and the formation of macroscopic strip-shaped chromatic aberration defects on the surface of electro-galvanized fingerprint-resistant sheet.
The technology for spraying a sintering bed and thus improving sinter quality indicators while reducing the emission of flue gas pollutants has recently become an important research topic.The impacts on sinter quality and emissions when spraying the sintering surface with different amounts and flow rates of steam were investigated in this study.The sinter quality indicators were most effectively improved by spraying 180 g of steam flow continuously at a rate of 0.02 m3/min for 15 min after ignition for 8 min.The optimal effect on emission reduction was obtained by spraying 90 g of steam flow continuously at a rate of 0.01 m3/min for 15 min after ignition for 8 min.
The microstructure and mechanical properties of titanium(Ti)-bearing medium-carbon nonquenched and tempered steel with different nitrogen content before and after hot forging were investigated through smelting,forging,and laboratory tests.The results show that the grain size of nonquenched and tempered steel was gradually refined,and the ferrite content gradually increased with an increase in nitrogen content.The grain size of the material with low nitrogen content increased abnormally,and its impact properties significantly decreased after hot forging.The grain size of nonquenched and tempered steel with higher nitrogen content was slightly larger than that before forging,and the tensile and yield strength increased,but the impact toughness was not significantly reduced.The Ti-bearing nonquenched and tempered steel showed better strength and toughness after hot forging with the addition of 0.010％-0.015％nitrogen.
The wear mechanism and tool life of four types of oil country tubular goods chasers are studied herein via metallographic microscopy,scanning electron microscopy,and energy dispersive spectroscopy.Results show that when the tool has a fine and uniform microstructure,the main failure mode is wear failure.Furthermore,observed phenomena including large chip bonding,coating peeling,cracks demonstrate that the main wear mechanisms of the tool are adhesive wear,abrasive wear,coating flaking,and microchipping.
This paper addresses the microstructure evolution in the Zn-1.2Al-1.2Mg coating of press-hardened steel during heat treatment at temperatures ranging from 600℃to 900℃.The presence of aluminum in the coating leads to substantial variation in the chemical composition and microstructure within the heat-treated layer.As the heat treatment temperature increases,the original Fe2 Al5 barrier layer between the coating layer and steel substrate is progressively dissolved and replaced by the aggregates of a new(FeAl)phase.At higher heat treatment temperatures,the(FeAl)phase gradually migrates to the subsurface layer of the coating.Meanwhile,Zn/Fe interdiffusion induces Γ-Fe3 Zn10-phase formation in the original coating layer and α-Fe(Zn)solid solution formation at the zinc －substrate interface.These microstructure features suppress the high-temperature oxidation and evaporation of Zn.
For the purpose of developing a 1500 MPa grade steel sheet with excellent strength and ductility,a 0.15C －10Mn －1.5Al steel was employed to study austenite stability and microstructural evolution based on a novel double annealing processing.After a conventional intercritical annealing process,the sample was heated again to a temperature higher than Ac3 for a very short time to generate austenite grains with different manganese content;thus,the microstructure of martensite plus austenite can be obtained at room temperature.The experimental results show that with increasing annealing temperature,the tensile strength and yield strength increase.When the annealing temperature was higher than 820℃,the microstructure consisted of martensite plus austenite,and the tensile strength almost remained invariant with the annealing temperature.A tensile strength of 1537 MPa and an elong-ation of 25.1%were achieved for the 820℃condition.The volume fractions of austenite and martensite were identified by X-ray diffraction.It was found that with increasing annealing temperature,the volume fraction of the retained austenite decreased,and the ductility also had a gradual downward trend.The related austenite stability was discussed here as well.
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.
Cold-rolled martensitic steel is an important type of advanced high-strength steel for automobile production.With martensite as its primary microstructure constituent,martensitic steel possesses exceptional high strength despite its low alloy content.As the strength of cold-rolled martensitic steel increases,the martensite and carbon content also increases,leading to a decrease in bending properties and toughness.In this paper,the effect of various tempering parameters on the bending property and impact toughness of a quenched cold-rolled martensitic steel sheet was studied.It is found that after quenching,the ductility and impact toughness of the experimental steel are improved using low-temperature heat treatment.The optimal tempering conditions for ductility and toughness are analyzed.
Bainite,the main microstructure of ultrahigh-strength complex-phase(CP)sheet steel,usually exhibits various micro-morphologies when subjected to different austempering treatments.In the current study,conventional austempering treatment at the bainite nose temperature resulted in two bainite types with distinct micro-morphologies:polygonal blocky bainite and acicular bainite,which resulted in large fluctuations in the mechanical properties of CP steel,particularly yield strength and hole expansion ratio.Therefore,the precise control of bainite micro-morphology was studied and applied to separate the two bainite types through austempering optimization.The two bainite types of different micro-morphologies had different effects on the mechanical properties of CP steel:the acicular bainite favored hole expansion and flangeability but deteriorated ductility,while the polygonal blocky bainite favored ductility but deteriorated hole expansion and flangeability.Accordingly,two types of ultrahigh-strength CP steels of different mechanical properties can be stably manufactured through the precise control of bainitic micro-morphology to satisfy the specific demands of vehicle components in terms of the mechanical properties of CP steels.
In this study,two sandblasting textures,namely,quartz and high-chromium(Cr)stainless steel sands,were used for sandblasting of the internal surface L80-13Cr tubing and casing.The contrastive analysis on the properties of the pipes before and after abrasive blasting was conducted,including the metallographic phase detection of the internal surface,simulating accelerated corrosion tests,and residual stress tests for the pipe after abrasive blasting.Based on the analysis results,it is explained why the API standard requires that no scale appears on the internal surface of L80-13Cr and iron contamination when internal sandblasting media are applied.The results show that abrasive blasting can effectively remove scales on the internal surface of L80-13Cr tubing and casing and improve corrosion resistance.The abrasive blasting process does not produce obvious residual stress on the internal surface of the tubing and casing.Also,the de-rusting effect of stainless sand is better than that of quartz sand,and the former does not produce iron contamination.
A three-dimensional electromagnetic-thermal coupled model of a 45#steel strip during longitudinal magnetic flux induction heating was established in this study.The influence of different power levels,frequencies,and electromagnetic shielding on the magnetic induction intensity and the heating of parts of different materials around the inductor was analyzed by calculating the magnetic induction intensity around the longitudinal flux induction heater under different conditions.The results show that leakage flux can be reduced up to 49％by electromagnetic shielding.The higher the frequency,the greater the electromagnetic shielding effect,and the lower the magnetic leakage around the inductor.Conversely,the higher the power,the greater the magnetic induction around the inductor.By adding electromagnetic shielding to reduce magnetic leakage of the inductor and replacing the metal part material around the inductor with stainless steel,the heat generation of the parts around the inductor can be greatly reduced.
The electromagnetic and temperature fields in the multistage intermediate-frequency induction heating of coated steel tubes were calculated using a numerical simulation.The influence of the voltage and frequency on the parameters of power circuit and heating temperature of the steel pipe was also analyzed.The results indicate that:(1)the electromagnetic field of induction heating is concentrated between the induction coil and steel tube and dissipated more at both ends of the coil;(2)during the induction heating,the temperature difference between the outer and inner walls of the steel pipe is approximately 30-40 K,and the two temperatures rapidly tend to be the same after exiting the induction heater;(3)during the natural cooling,the temperature drop of the steel tube in the low-temperature section is approximately 10 K,and that in the high-temperature section is approximately 20 K;(4)the appropriate induction heating frequency and matching capacitance should be selected,so that the temperature of the steel pipe,especially at the outer wall,meets the coating process requirements,and as to achieve a synchronous improvement of the power efficiency and heating efficiency.This study is beneficial to building a more low-consumption and high-efficiency coating line of large-diameter steel pipes in Baosteel.
Ultra-thin hot-rolled strip is the developing result of the modern plate and strip rolling technology and can be used to replace cold-rolled products for energy saving and emission reduction.This work reviews recent progress and breakthroughs in this field by focusing on three recent technologies,namely the conventional strip with hot coil box and endless rolling,thin slab continuous casting and rolling/semi endless rolling,and endless compact rolling.In summary,ensuring high-quality production is an important bottleneck for ultra-thin strip and ferritic hot rolling by endless compact rolling.This production technology has strong feasibility and broad prospects for future development.
