Ultra-high Strength Steel Sheets Research Articles

Nanotwinning grain refinement induced by micro-needle peening in arc-welded ultra-high strength steel sheet

Generally, the fatigue strength of ultra-high strength steel (UHSS) and high strength steel (HSS) arc-welded joints are comparable regardless of base metal's strength. Still, the micro-needle peening (MNP) method can improve the fatigue strength to the level of those of base metals. To understand the mechanism of this improvement, this paper investigates the microstructure of UHSS (tensile stress grade of 980 MPa) arc-welded joints treated with MNP and compares it to HSS (tensile stress grade of 440 MPa) joints. We focus on the presence of nanotwins, which exhibited a minimum thickness of 4.7 nm, observed in the UHSS joints following the MNP treatment. Importantly, these nanotwins demonstrated remarkable stability even under cyclic loading conditions (nominal stress σn = 600 MPa, N = 3 × 106 cycles). This indicates that the nanotwins contribute to the significant improvement in fatigue strength demonstrated by MNP. However, the nanotwins were not observed in the HSS joints, suggesting sufficient driving stress is necessary for their occurrence. The dislocation pileup stress at the grain boundary during twinning was estimated by the thickness of the twin, which was 8.1 GPa. This value is of the same order of magnitude as the 3.7 GPa estimated by the Hall-Petch coefficient for ferritic steel. The lower levels of C, Si, and Mn can contribute to the lower pileup stress, resulting in absence of the nanotwins in the 440 MPa joint. Overall, this study provides insights into the microstructural changes induced by MNP treatment and their impact on the fatigue strength.

Delayed Fracture Enhanced by Martensite Transformed from Retained Austenite in Ultra-high Strength Steel Sheet

Delayed Fracture Enhanced by Martensite Transformed from Retained Austenite in Ultra-high Strength Steel Sheet

Effect of plastic strain on corrosion-induced hydrogen infusion and embrittlement behaviors of Zn-coated ultra-high strength steel sheet

This study examined the effects of plastic strain on the corrosion-induced hydrogen evolution, infusion, and embrittlement behaviors of ultra-high strength steel sheets with galvanized (GI) and galvannealed (GA) coatings. The tensile pre-straining of the coated steels did not increase the hydrogen permeation flux in the absence of applied cathodic polarization, in contrast to the case under cathodic polarization. This was closely associated with the rapid kinetics of corrosion scale formation under pre-strains, providing an inhibitory effect on the surface. From this aspect, the difference in the resistance to corrosion-induced hydrogen embrittlement between GI and GA-coated steels was mechanistically interpreted.

Short time resistance spot welding of 1.2Gpa ultra-high strength steel using high frequency power supply

The task of 1.2 GPa ultra-high strength steel sheet in short time’s high current spot welding has been clarified. High current and short time spot welding system and high frequency power supply has been developed. In order to prevent spatter, new dual-pulse welding method in which primary energization, cooling and secondary energization has been proposed. By using this novel welding method, nugget diameter and tensile shear strength satisfied the target and spatter did not occur. New method of 1.2 GPa ultra-high strength steel sheet in short time and high current spot welding has been established.

Rapid evaluation of the critical condition for hydrogen-induced cracking in ultrahigh-strength automotive steel sheets: A semiquantitative investigation based on U-bend specimens

The production of automotive steel plates, which involves pickling, electroplating, and welding among other steps, introduces hydrogen. Additionally, during the service life of components fabricated using such plates, electrochemical corrosion occurs, and hydrogen atoms are introduced. The hydrogen atoms are adsorbed and diffused into the interior of the metal, accumulating at any defects. Automotive parts are commonly fabricated using cold forming techniques. The hydrogen embrittlement sensitivity of ultrahigh-strength automotive sheets (UHSSs) is influenced by the stress, concentration of diffusible hydrogen, and plastic strain resulting from the cold forming process. The standard methods for quantitative studies on the UHSS hydrogen embrittlement sensitivity, such as the constant-load testing and low-strain-rate testing, require a specialized equipment and extended experimental period, making them impractical for original automotive equipment manufacturers. This study focuses on the use of the quenching and partitioning (QP) steel QP1180, which contains residual austenite, and martensite steel MS1500, which has a low deformation ability but high strength, as research objects. The equivalent strain and stress were controlled by adjusting the bending radius and U-bend opening distance, while the diffusible hydrogen concentration was charged through immersion. The fracture time of the U-bend sample was recorded, and the critical diffusible hydrogen concentration for hydrogen-induced cracking was measured. Additionally, the stress distribution during the U-bending process was simulated using the finite-element method. To evaluate the hydrogen embrittlement resistances of QP1180 and MS1500, the area enclosed by the load stress and equivalent strain (bending radius) and fracture time were semiquantitatively determined. Finally, the hydrogen-induced delayed fracture space regions of QP1180 and MS1500 were determined in the three-dimensional space, with equivalent strain, load stress, and diffusible hydrogen concentration as coordinate axes. This provided a convenient and reliable means of determination of the critical conditions for a safe service and rapid evaluation of the hydrogen embrittlement sensitivity of ultrahigh-strength automotive sheets.

Microstructural Control and Alloy Design for Improving the Resistance to Delayed Fracture of Ultrahigh-Strength Automotive Steel Sheets

The demand for higher-strength automotive steel sheets has increased significantly for lightweight and safe body concepts. However, the increment of the steel strength is often limited by the potential occurrence of delayed fracture. This paper discusses proper microstructure control and alloy design to improve the resistance against the delayed fracture of ultrahigh-strength automotive steel sheets in order to increase the usable upper limit of their strength and provides basic data serving as a practical guide for solving the problem of delayed fracture in ultrahigh-strength automotive steel sheets. It is confirmed that grain refinement, the appropriate dual-phase structure of martensite with ferrite or retained austenite, and surface decarburization, increase the resistance to delayed fracture. In terms of alloy design, the effects of Nb, Mo, and B on the delayed fracture resistance of hot-stamped steels have been investigated. The results suggest that there are other reasons for Nb to improve delayed fracture resistance in addition to grain refinement and the ability to trap hydrogen by its precipitates, as has been conventionally believed. Regarding Mo, it was clearly demonstrated that the segregation of this element at the grain boundary plays a main role in improving the delayed fracture resistance.

Wear characteristics of the tool in the cold stamping process of ultra high strength steel sheets by establishing a novel wear test method based on the progressive die

Demands for lightweighting and crashworthiness of the vehicle body have increased. For this purpose, advanced high strength steel to the automobile body was regarded as a solution with respect to manufacturing cost and impact energy absorption. However, increasing the strength of the automotive steel sheet lead to a diversity of problems caused by the tool wear due to higher forming load than that of commonly used steel sheets. Hence, systematic wear experiment and evaluation methods are required to quantitatively and qualitatively evaluate the tool wear amount in the sheet metal forming process. In this study, a methodology is proposed to quantitatively evaluate the wear of sheet metal forming tool based on the experimental results. In order to carry out a systematic wear test and save the time and cost, the progressive die set was designed to be suitable for wear test. The designed testing machine can simultaneously test four types of punches made under various tooling conditions such as materials, shapes, and coatings. Through the measurement of the wear depth, roughness, and surface imaging of the punch and product roughness, which represent the wear characteristics, quantitative evaluation methods for tool wear in the sheet metal forming process are established. By referring to the wear test results, it is confirmed that it is appropriate to analyze the reasonable tool wear characteristics by using the proposed methodology for quantifying the tool wear in the sheet metal forming process.

Laser-Assisted Robotic Roller Forming of Ultrahigh-Strength Steel QP1180 with High Precision.

Laser-assisted forming provides a perfect solution that overcomes the formability of low-ductility materials. In this study, laser-assisted robotic roller forming (LRRF) was applied to bend ultrahigh-strength steel sheet (a quenching and partitioning steel with a strength grade of 1180 MPa), and the effects of laser power density on the bending forces, springback, and bending radius of the final parts were investigated. The results show that LRRF is capable of reducing bending forces by 43%, and a compact profile with high precision (i.e., a springback angle smaller than 1° and a radius-to-thickness ratio of ~1.2) was finally achieved at a laser power density of 10 J/mm2. A higher forming temperature, at which a significant decrease in strength is observed, is responsible for the decrease of forming forces with a laser power density of higher than 7.5 J/mm2; another reason could be the heating-to-austenitization temperature and subsequent forming at a temperature above martensitic-transformation temperature. Forming takes place at a higher temperature with lower stresses, and unloading occurs at a relatively lower temperature with the recovery of Young's modulus; both facilitate the reduction of springback angles. In addition, the sharp bending radius is considered to be attributed to localized deformation and large plastic strains at the heating area.

1470 MPa級超高強度鋼板のピアス加工における金型損傷に及ぼすPVD皮膜特性の影響

We investigated the punch damage in the piercing of a 1470-MPa-class ultrahigh-strength steel sheet and discussed the effects of PVD coating properties on the punch damage behavior. Sequential piercing experiments were conducted using SKD11 punches with various types of PVD coating. A maximum of 240 shots of piercing were performed with a small clearance (5% of the pierced material thickness). The friction coefficient μ between the coatings and 1470 MPa steel was evaluated using a plate drawing tester at a high contact load without lubrication. For 120 shots of piercing, the higher the μ of coatings, the larger the damage area. For 240 shots, the effects of the heat resistance and adhesion of coatings additionally appeared. For the coatings with lower heat resistance, such as TiN and TiCN, the damage was greater and the effect of μ was more significant than for the coatings with high heat resistance, such as CrN, TiAlN, and TiCrAlN. In addition, among the low-heat-resistance coatings, the damage was smaller for the high-adhesion type. It was concluded that the frictional property, heat resistance, and coating adhesion are the major factors affecting punch damage in the piercing of ultrahigh-strength steel sheets.

Development of a novel forming method to restrain wall curvature for stamping products using general purpose ultra-high tensile strength steel sheets

Development of a novel forming method to restrain wall curvature for stamping products using general purpose ultra-high tensile strength steel sheets

Effect of Inclined Angle in Trimming of Ultra-high Strength Steel Sheets Having Inclined and Curved Shapes

Effect of Inclined Angle in Trimming of Ultra-high Strength Steel Sheets Having Inclined and Curved Shapes

Continuousness of interactions between hydrogen and plastic deformation of ultra-high strength steel sheet consisting of ferrite and nanometer-sized precipitates

ABSTRACT The continuousness of the interactions between hydrogen and plastic deformation of an ultra-high strength steel sheet consisting of ferrite and nanometer-sized precipitates has been investigated by tensile tests after or during cathodic hydrogen charging. In the tensile test in the air after hydrogen pre-charging, a hydrogen thermal desorption analysis shows that the amount of hydrogen desorbed decreases with increasing applied tensile strain from room temperature to 50 °C, but increases in the high temperature region. Upon tensile straining to 0.06, no hydrogen is desorbed in the low temperature region, and the change in desorption behaviour when strain exceeds 0.06 is negligible. This suggests that substantial interactions between hydrogen and plastic deformation in the test with hydrogen pre-charging occur only in the early stages of deformation. In contrast, in the tensile test during hydrogen charging, the hydrogen desorption which begins from room temperature continues even upon tensile straining to 0.06, suggesting continuous interactions, and a unique dislocation structure resembling sub-grain boundaries is observed. Upon aging at room temperature after tensile straining to 0.06 during hydrogen charging, all hydrogen desorption lower than 100 °C shifts to the high temperature region, but the recovery of elongation is not necessarily complete. When tensile strain is applied during hydrogen charging, continuous interactions presumably induce anomalous damage, thereby enhancing the degradation of ductility. The results of the present study strongly support the conclusion that the continuousness of the dynamic interactions between hydrogen and plastic deformation plays essential roles in hydrogen embrittlement of ferritic steel.

Effect of cold spray deposition on fatigue strength of arc-welded ultra-high strength steel sheet

This study investigates the effect of cold spray deposition on the fatigue property of a lap fillet arc-welded joint of ultra-high strength steel with a tensile strength grade of 980 MPa. Fatigue strength at 3 × 106 cycles was improved from 275 MPa to 450 MPa by the Fe + Al2O3 deposition but only improved to 300 MPa with the Zn + Al2O3 deposition. Further investigations of the Fe + Al2O3 coated joints revealed that the effect of the Fe + Al2O3 deposition consisted of compressive residual stress, reduction of stress concentration and the fatigue strength of the deposition itself.

Suppression of Springback for Longitudinally Curved Part by Imparting In-Plane Compression Stress in Press Forming

Dimensional accuracy defects caused by springback behavior are one of the serious problems in press forming for automobile parts using ultrahigh-strength steel sheets. In particular, in hat-shaped parts curved in the longitudinal direction, camber back, which is a kind of springback behavior, occurs. In this study, in order to suppress camber back, a new press-forming method that can impart in-plane compression stress was investigated using parts with a simple hat-shaped section and a W-shaped section simulating automotive parts. As a result, it was found that the amount of camber back decreases on applying compression stress in the longitudinal direction, because the bending moment caused by the difference between stress in the punch bottom area and that in the flange area was reduced. It was also clarified that the sensitivity of the amount of camber back to tensile strength was lowered when this forming method was used. Furthermore, a forming method in which buckling in both ends under a high compression condition can be prevented was developed.

Formation of a dark streaky edge defect on galvannealed ultra-high strength steel

To analyze the formation mechanism of a dark streaky edge defect on a hot-dip galvannealed (GA) ultra-high strength steel (UHSS) sheet, multiple metallographic characterizations were performed on samples collected from different processes. It was found that this streaky defect was directly caused by differences in the coating morphology. The coating in the edge zone showed a crater morphology, while the coating in the center zone showed a flatter morphology and uniform coating thickness. Corresponding streaky edge defects were found on the annealed steel prior to hot-dipping and on the pickled steel prior to cold rolling. The root cause was finally traced back to the hot-rolled steel after coiling. A formation mechanism of this defect was proposed based on the evolution of surface microstructures in multiple processes. The difference in the amounts of internal oxides formed in the subsurface of different zones of the hot-rolled strip influenced the pickled steel surface, cold-rolled steel surface, annealed steel surface and the galvannealed coating surface.

V-Bendability of Ultrahigh-Strength Low Alloy TRIP-Aided Steel Sheets with Bainitic Ferrite Matrix

V-Bendability of Ultrahigh-Strength Low Alloy TRIP-Aided Steel Sheets with Bainitic Ferrite Matrix

Effect of Minor Alloying Elements (C, Ni, Cr, Mo) on the Long-Term Corrosion Behaviors of Ultrahigh-Strength Automotive Steel Sheet in Neutral Aqueous Environment

This study examined the effects of minor alloying elements (C, Ni, Cr, and Mo) on the long-term corrosion behaviors of ultrahigh-strength automotive steel sheets with a tensile strength of more than 1800 MPa. A range of experimental and analytical results showed that the addition of Ni, Cr, and Mo decreased the corrosion current density and weight loss in electrochemical and immersion tests, respectively, in a neutral aqueous condition. This suggests that the minor addition of elements to steel can result in improved corrosion resistance even for long-term immersion periods. This is closely associated with the formation of thin and stable corrosion scale on the surface, which was enriched with the alloying elements (Ni, Cr, and Mo). On the other hand, their beneficial effects did not persist during the prolonged immersion periods in steel with a higher C content, suggesting that the beneficial effects of the minor addition of Ni, Cr, and Mo were overridden by the detrimental effects of a higher C content as the immersion time was increased. Based on these results, lower C and the optimal use of Ni, Cr, and Mo are suggested as a desirable alloy design strategy for developing ultrahigh-strength steel sheets that can be exposed frequently to a neutral aqueous environment.

Finding the pad load to suppress wrinkling in free bending of ultra-high tensile strength steel sheets

The critical pad load to suppress wrinkling in free bending is derived by linking the pad lifting behaviour and the wrinkle disappearing condition that depends on the clearance between the punch and the blank generated by pad lifting at the early stage of bending. The clearance is estimated based on the simulated results that the contacting zone of the blank with the pad at the early stage of bending is confined to the vicinity of the punch shoulder. The usefulness of the critical pad load is confirmed by forming experiments of a T-shaped product from high strength steel sheets.

Effects of Press Motion Control and Lubrication on Shape-freezing Properties of Hat-shaped Products of Ultra-high Strength Steel Sheet

Effects of Press Motion Control and Lubrication on Shape-freezing Properties of Hat-shaped Products of Ultra-high Strength Steel Sheet

Fundamental Study on Forming Method of Ultrahigh-Strength Steel Sheet to T Model Shape —Development of Forming Method of Ultrahigh-Strength Steel Sheet to L and T Shapes—

Fundamental Study on Forming Method of Ultrahigh-Strength Steel Sheet to T Model Shape —Development of Forming Method of Ultrahigh-Strength Steel Sheet to L and T Shapes—