Development of a hot-rolled 980 MPa advanced high-strength steel with excellent flangeability and bendability

A new 980 MPa advanced high-strength steel(AHSS) with excellent bendability and flangeability has been studied and industrially produced, typical of tensile strength, fractured elongation, and hole expansion ratio(HER) exceeding 980 MPa, 10%,and 30%,respectively.The 90° V-type bending perpendicular to the rolling direction can satisfy the R/t=1.0 requirement, indicating excellent bendability.Systematic evaluations of industrial trial-produced 980 MPa hot-rolled AHSS have been conducted, including microstructure, tensile properties in three directions, HER,bendability, fatigue limit strength, and forming limit.The microstructure of the newly developed 980 MPa AHSS primarily consists of fine bainite and a small amount of martensite-austenite constituent.The practical yield and tensile strength are higher than 800 and 980 MPa, respectively, with typical elongation of 13% and HER of around 40%.The good combination of the newly developed 980 MPa AHSS is primarily attributed to the fine bainitic microstructure, resulting in excellent flangeability and bendability.In addition, the newly developed 980 MPa AHSS has good fatigue and forming properties, making it suitable for the production of chassis and suspension components.

Improvement of the microstructural features and mechanical properties of advanced high-strength steel DP590 welds

The effects of the welding current mode in resistance spot welding on the microstructure and mechanical properties of advanced high-strength steel dual-phase 590(DP590)sheets were investigated.Results showed that a rough martensitic structure was formed in the weld zone of the sample welded via the single-pulsed mode,whereas the microstructure in the heat-affected zone consisted of a very rough martensitic microstructure and rough ferrite.However,using the secondary pulse mode led to the formation of tempered martensite in the weld zone.The maximum load and the energy absorption to failure of the samples with the secondary pulsed cycle were higher than those of the samples with the single-pulsed mode.Tensile shear results indicated that the secondary pulsed mode could significantly change the mode of failure upon shear tension testing.Therefore,the obtained results suggest that the use of secondary pulsed mode can improve the microstructural feature and mechanical properties of advanced high-strength steel DP590 welds.

Current Challenges and Opportunities Toward Understanding Hydrogen Embrittlement Mechanisms in Advanced High-Strength Steels: A Review

Hydrogen embrittlement(HE)is one of the most dangerous yet most elusive embrittlement problems in metallic materials.Advanced high-strength steels(AHSS)are particularly prone to HE,as evidenced by the serious degradation of their load-bearing capacity with the presence of typically only a few parts-per-million H.This strongly impedes their further development and application and could set an abrupt halt for the weight reduction strategies pursued globally in the automotive industry.It is thus important to understand the HE mechanisms in this material class,in order to develop effective H-resistant strategies.Here,we review the related research in this field,with the purpose to highlight the recent progress,and more importantly,the current challenges toward understanding the fundamental HE mechanisms in modern AHSS.The review starts with a brief introduction of current HE models,followed by an overview of the state-of-the-art micromechanical testing techniques dedicated for HE study.Finally,the reported HE phenomena in different types of AHSS are critically reviewed.Focuses are particularly placed on two representative multiphase steels,i.e.,ferrite–martensite dual-phase steels and ferrite–austenite medium-Mn steels,with the aim to highlight the multiple dimensions of complexity of HE mechanisms in complex AHSS.Based on this,open scientific questions and the critical challenges in this field are discussed to guide future research efforts.

A comparison study on interfacial properties of fluorine-bearing and fluorine-free mold flux for casting advanced high-strength steels

A comparison study on interfacial properties of a traditional fluorine-bearing(F-bearing)mold flux and a newly designed fluorine-free(F-free)mold flux to produce advanced high-strength steels(AHSSs)by compact strip production technology was conducted.The results showed that these two kinds of mold fluxes gradually spread out on the typical AHSS substrate when slags began to melt with the increase in heating temperature,and they had a good interfacial ability between the two mold fluxes and the AHSS substrate,and there was no other interfacial reaction except the oxidization of steel substrate by the mold fluxes.In comparison,the wettability of the designed F-free mold flux with the AHSS substrate was better than that of the F-bearing mold flux.The reason could be explained as the addition of B_(2)O_(3) would increase the complexity and polymerization degree of the melt structure and weaken the attractive force between the ions and ionic groups,then leading to a better wettability.Besides,B_(2)O_(3) is an effective flux,which can reduce the melting temperature obviously,and the surface tension of the liquid F-free mold flux would get reduced with the addition of B_(2)O_(3).

Delayed fracture of tensile strength 980 MPa grade steels for automotive applications

Three different types of tensile strength （TS） 980 MPa grade advanced high-strength steels used in automotive applications, namely, 980MS （ martensite steel ）, 980DP （ dual phase ） and 980QP （ quenching and partitioning） steels were examined. The delayed fracture resistance of the steels was evaluated using a U-bend test, slow strain rate test（SSRT） and a constant load tensile test. The results indicated that all the steels could pass the 300 h HC1 solution immersion test and none of the U-bend specimens was fractured in the test. However,the steels exhibited different susceptibilities to delayed fracture under SSRT and the constant load tensile tests. 980DP exhibited the highest resistance to delayed fracture among all the samples, while 980MS was found to be the most susceptible to delayed fracture.

Formability Investigations of High-Strength Dual-Phase Steels

Car manufacturing is always regarded as the key industry behind sheet metal forming, and thus, the requirements of and developments in car manufacturing play a decisive role in the development of sheet metal forming. The automotive industry is faced with contradictory demands and requirements： better performance with lower consumption and less harmful emissions, more safety and comfort; these are extremely difficult to supply simultaneously with conventional materials and conventional manufacturing processes. The fulfillment of these often contradictory requirements is one of the main driving forces in the automotive industry and thus in the material and process developments in sheet metal forming, as well. In recent years, significant developments can be observed in the application of high-strength steels. In this respect, the application of various dual-phase steels is one of the best examples. However, the application of these highstrength steels often leads to formability and manufacturing problems. One formability problem is the springback occurring after sheet metal forming. In the current research, we have dealt mainly with advanced high-strength steels, primarily with dual-phase steels. When applying them, the springback phenomenon is one of the most critical issues. To reduce the tremendous amount of experimental work needed, we also applied numerical simulation using isotropic–kinematic hardening rules. The isotropic–kinematic hardening behavior of a given material in the applied Auto Form numerical package may be characterized with three independent material parameters c, v and K（a detailed explanation of their meaning will be given in the main part of this paper）. However, we found that the material data included in simulation packages for these new high-strength steels are not fully adequate. For the determination of more reliable material parameters and to achieve better simulation results, a new testing device was developed. Numerical simulations were performed using the material parameters determined by the new device to show the sensitivity of springback behavior to these material parameters.

Influence of lanthanum on inclusions and as-cast microstructures in a low-alloy high-strength steel

Advanced high-strength steel ingots with total lanthanum(TLa)contents of 0,15×10^(–6),86×10^(–6)and 360×10^(–6)were prepared through laboratory experiments.The modification of inclusions and the variation of the as-cast microstructure with the content of lanthanum in the high-strength steel were analyzed.The result showed that with the increase in the TLa content in the steel from 0 to 360×10^(–6),the modification path of inclusions in the as-cast steel was Al2O_(3)and calcium aluminate→LaAlO_(3)→La_(2)O_(2)S→La_(2)O_(2)S–La2O_(3).The addition of La in the high-strength steel significantly refined the solidification structures.With the increase in the TLa content in the steel from 0 to 360×10^(–6),the ratio of the equiaxed crystal region in the macrostructure increased from 30.1%to 50.7%,the proportion of the high-angle grain boundary in the microstructure increased from 36.9%to 69.8%,and the area fraction of the acicular ferrite and the bainite increased from 0 to 93.3%.Inclusions of LaAlO_(3),La_(2)O_(2)S and La2O_(3)in the La-containing steel could act as heterogeneous nucleation cores ofα-Fe during the solidification.With the increase in the TLa content in the steel,the number density of inclusions that could act as effective heterogeneous nucleation cores in the steel gradually increased,which enlarged the ratio of the equiaxed crystal region and the proportion of intragranular acicular ferrite,and refined the as-cast microstructure of the high-strength steel.

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.