Effect of laser heating on the microstructure and hardness of TRIP590advanced high strength steel used for roll forming

TRIP590 advanced high strength steel sheets were heated by laser with different powers.Changes of the microstructure and the hardness of TRIP590 steel under laser heating with different powers were investigated by metallographic microscope,scanning electron microscope,and hardness tester.The purpose was to study the effect of laser power on microstructure and hardness of TRIP590 steel.It is shown that the power of laser plays an important role on the microstructure and hardness of heated steel sheets.The results are helpful to determine suitable power for the laser auxiliary forming of Trip590 steel in order to obtain uniform microstructure and high hardness.

Yield Loci of TRIP590 Advanced High Strength Steel Based on Biaxial Loading Test

The main purpose of the paper is to obtain the experimental yield loci of TRIP590 advanced high strength steel,and to compare it with the theoretical loci in order to obtain the best yield criterion for this material.First,the biaxial loading tests under different loading paths in the method of load control are carried out.Then,the experimental yield loci of different deformation stages are obtained.Finally,the experimental yield loci are compared with the theoretical loci of Mises criterion and Hill48 criterion,the parameters of which are calculated based on the r?value and the yield stress method,respectively.The results show that the accuracy of the theoretical yield loci of Hill48 based on the yield stress is higher than that of Mises criterion and Hill 48 criterion based on r?value method.

Tensile and Spring-Back Behavior of DP600 Advanced High Strength Steel at Warm Temperatures

In recent years, the use of advanced high strength steels in automotive industry has been increased remarkably. Among advanced high strength steels, dual phase （DP） steels have gained a great attention owing to a combination of high strength and good formability. However, high strength usually increases the spring-back behavior of the material, which creates problems for the parts during the assembly. Thus, the uniaxial tensile deformation and spring-back behaviors of DP600 advanced high strength steel were investigated in rolling （0°）, diagonal （45°） , and transverse （90°） directions in the temperature range from room temperature （RT） to 300 ℃. All tests were performed at a deformation speed of 25 mm/min. A V-shaped die （60°） was used for the spring-back measurements. The results indicated that the formability and spring-back of the material were decreased with increasing the temperatures. The material showed complex behaviors in different directions and at different temperatures.

Strain Hardening and Strain Rate Sensitivity Behaviors of Advanced High Strength Steels

The mechanical properties of commercial dual phase （DP）, transformation induced plasticity （TRIP）, and high strength low alloy （HSLA-340） steel sheets are investigated and compared at various strain rates ranging from 0.001 7to 0.17 s-1 at ambient temperature.TRIP steel outperforms the other two materials , having comparable ductility and twice as large strength relative to DP steel.TRIP has larger strength and much larger ductility than HSLA-340.The exceuent ductility of TRIP800is due to its high strain hardening capability , which promotes stable plastic deformation.It is observed that the strain hardening rate in TRIP800does not decrease to zero at failure , as common in most materials in which failure is preceded by necking.

Shear Fracture of Advanced High Strength Steels

Failure experiments were carried out through a stretch-bending test system for advanced high strength steels, i.e. dual-phase （DP） steels and martensitic steels （MS）. The die radius in this system was designed from 1 to 15 mm to investigate the failure mode under different geometries. Two failure modes were observed during the ex- periments. As a result, critical relative radii （the ratio of inner bending radius R to sheet thickness t） for DP590 and DP780 steels were obtained. The stretch-bending tests of DP980 display some trends unlike DP590 and DP780 steels, and curve of DP980 in different thicknesses does not coincide well. High blank holder force exhibits more possibility of shear fracture tendency than low blank holder force. The unique character of high strength martensitic steel （1500MS） is that no shear fracture is found especially over small bending radius （R =2 mm） under the same experi- mental conditions. Microstructure analysis indicates that there are obviously elongated grains on shear fracture sur- face. It shows smaller diameter and shallower depth of the dimples than the necking failure.

Recent progress in the development and application of the new Gen.AHSS at Baosteel

In recent years,despite the global economic recession and steel oversupply,the demand for high strength steel( HSS) and advanced high strength steel( AHSS) sheets has robustly increased. To supply the materials needed for automotive manufacturing,various AHSS products and application technologies have been developed by Baosteel,including the 1st,2nd,and 3rd Gen. AHSS,which makes Baosteel the world’s first steel maker to commercially provide all three generations of AHSS. Recently,the new Gen.( the 2nd and 3rd Gen. combined)AHSS products,which feature ultra-high strength and enhanced ductility,have drawn great attention from both steel makers and the automotive industry as these products can be used to produce auto parts with complicated shapes by cold forming and have great potential for manufacturing car bodies that are lighter in weight and have higher crash resistance. In this article,the concepts,properties,and applications of Baosteel’s new Gen. AHSS products,including quenching and partitioning( QP) steels,twinning-induced-plasticity( TWIP) steels,and Medium-Mn( Mn-TRIP)steels,are described and reviewed. Moreover,the progress of the Baosteel light-weight steel car body( BCB) was reported.

Feasibility of 0.02% Nb-Based Microalloyed Steel for the Application of One-Step Quenching and Partitioning Heat Treatment

To attain an enhanced combination of mechanical properties for low alloyed steel, the current study has been made to fulfill that growing need in the industry. Its results are introduced within this paper. One step Quenching and Partitioning (Q&P) heat treatment has been applied on Niobium-based microalloyed steel alloy with 0.2 %C, in the form of 2 mm thickness sheets. The target of this study is to investigate the viability of applying that significantly recommended, results-wise, heat treatment on the highly well-suited alloy steel samples, to achieve the main target of enhanced properties. A single temperature of 275°C was used as quenching and Partitioning temperature. Four Partitioning periods (30, 200, 500, and 1000 Seconds) were used for soaking at the same temperature. The results were analyzed in the light of microstructural investigation and mechanical testing. All applied cycles did not enhance the strength but moderately improved the ductility and toughness, mainly caused by the slightly high soaking temperature used. Niobium impact of grain refining was apparent through all cycles. The cycle of 500 Seconds Partitioning time obtained optimum values at that particular temperature. The 1000 Seconds Cycle obtained the worst combination of properties. A set of recommendations are set. More research is required at this point, where a lower Partitioning temperature is advised. In the light of the applied combination of parameters, the Partitioning period at such temperature is advised to be between 500 and 1000 Seconds. A high probability that periods closer to 500 than 1000 Seconds will produce better results. More research is needed between those two values of Partitioning time to precisely determine the optimum time at that temperature on that specific alloy.

Comparing Properties of Adhesive Bonding,Resistance Spot Welding,and Adhesive Weld Bonding of Coated and Uncoated DP 600 Steel

Zinc coated dual phase 600 steel （DP 600 grade） was investigated, utilisation of which has gradually increased with each passing day in the automotive industry. The adhesive bonding （AB）, resistance spot welding （RSW）, and adhesive weld bonding （AWB） ioints of the zinc coated DP 600 steel were investigated. Additionally, the zinc coating was removed using HCL acid in order to investigate the effect of the coating. The microstructure, tensile shear strengths, and fracture properties of adhesive bonding （AB）, resistance spot welding （RSW）, and adhesive weld bonding （AWB） joints of the coated and uncoated DP 600 steel were compared. In addition, a mechani cal-electrical-thermal coupled model in a finite element analysis environment was utilised. The thermal profile phe nomenon was calculated by simulating this process. The results of the tensile shear test indicated that the tensile load bearing capacity （TLBC） values of the coated specimens among the three welding methods were higher than those of the uncoated specimens. Additionally, the tensile strength of the AWB joints of the coated and uncoated specimens was higher than that of the AB and RSW joints. It was determined that the fracture behaviours and the deformation caused were different for the three welding methods.

Effect of Heat Treatment on Microstructures and Mechanical Properties of Hot-Dip Galvanized DP Steels

In order to simulate the hot-dipped galvanizing of dual-phase （DP） steel （wt%） 0.15C-0.1Si-1.7Mn, the DP steels were obtained by different annealing schedules. The effects of soaking temperature, time, and cooling rate on ferrite grain, volume fraction of martensite, and the fine structure of martensite were studied. Results showed that the yield strength （YS） of DP steel is sensitive to annealing schedule, while total elongation has no noticeable dependence on annealing schedule. Increasing soaking temperature from 790 to 850 ℃, the YS is the lowest at soaking temperature of 850 ℃. Changing CR1 from 6 to 24 ℃/s, the YS is the highest when CR1 is 12 ℃/s. Increasing soaking time from 30 to 100 s, the YS is the lowest at soaking time of 100 s. Besides, it was found that sufficient movable dislocations within ferrite grains and high martensite volume fraction can eliminate yield point elongation, decrease the YS, and increase ultimate tensile strength. Through TEM observations, it was also found that increasing annealing temperature promotes austenite transformation into twin martensite, and increases volume fraction of martensite at sufficient cooling rate. With increasing the martensite volume fraction, the deformation substructure in the ferrite is well developed.

Hot Stamping Parameters Optimization of Boron Steel Using a Response Surface Methodology Based on Central Composite Design

The effect of hot stamping parameters on the mechanical properties of 22MnB5 steel sheet with thickness of 1.1 mm is studied. The considered parameters are austenization temperature （800- 1 000 ℃ ）, austenitizing soa king time （60-540 s）, initial deformation temperature （560-800 C） and tool temperature （20-220 ℃）. In order to obtain hot stamped parts with optimal mechanical properties, response surface methodology based on the central composite design has been employed to design the experiment matrix. Tensile strength of hot stamped parts is deter- mined as the relation in the mathematical model. The optimal condition and objective effects of parameters are deter mined via this relation. The statistical analysis showed that all four factors significantly affect the tensile strength of the hot stamped parts. The optimum austenization temperature is found to be 918.89 ℃ with the austenitizing soa- king time, initial deformation temperature and tool temperature of 279.45 s, 684.69 C and 21.85 ℃, respectively. These optimal hot stamping parameters prove to have high tensile strength （1 631.84 MPa） where deviation between predicted and actual response falls within 2 %.

A review of friction stir welding of steels： Tool, material flow, microstructure, and properties

Considerable progress has been achieved in friction stir welding （FSW） of steels in every aspect of tool fab- rication, microstructure control and properties evaluation in the past two decades. With the development of reliable welding tools and precise control systems, FSW of steels has reached a new level of technical maturity. High-quality, long welds can be produced in many engineering steels. Compared to traditional fusion welding, FSW exhibits unique advantages producing joints with better properties. As a result of active control of the welding temperature and/or cooling rate, FSW has the capability of fabricating steel joints with excellent toughness and strength. For example, unfavorable phase transformations that usu- ally occur during traditional welding can be avoided and favorable phase fractions in advanced steels can be maintained in the weld zone thus avoiding the typical property degradations associated with fusion welding. If phase transformations do occur during FSW of thick steels, optimization of microstructure and properties can be attained by controlling the heat input and post-weld cooling rate.

Carbide precipitation behavior and mechanical properties of micro-alloyed medium Mn steel

The carbide precipitation behavior and mechanical properties of advanced high strength steel deformed at different temperatures are investigated by X-ray diffractometer(XRD),scanning electron microscope(SEM),transmission electron microscope(TEM) equipped with an energy dispersing spectroscopy(EDS),and tensile tests.The medium Mn steel was subjected to controlled deformation up to 70% at 750℃,850℃,950℃,and 1050℃,and then quenched with water to room temperature,followed by intercritical annealing at 630℃ for 10 min.In comparison with the undeformed and quenched specimen,it can be concluded that acicular cementite precipitates during the quenching and cooling process,while granular NbC is the deformation induced precipitate and grows during the following annealing process.As the deformation temperature increases from 750℃ to 1050℃,the product of strength and elongation increases at first and then decreases.The smallest average size of second phase particles(20 nm) and the best mechanical properties(32.5 GPa%) can be obtained at the deformation temperature of 950℃.