High Tensile Strength Steel Sheets Research Articles

Forming of Tailor-Welded Blanks Through Centerline and Offset Laser Welding

The forming of low and high strength tailor welded steel sheets into preferred three-dimensional shapes by a homogeneous distribution of plastic strain is highly difficult. In this work, the formability of tailor-welded blanks prepared using laser welding of low (IS 513 CR2, IS 513 CR3) and high tensile strength (AISI 304) steel sheets were analyzed and the formation of the edge profile and weld line movement was also examined. The results showed a decrease in formability characteristics owing to the increase in Strength Ratio. Scanning Electron Microscope analysis of the fracture locations within unsuccessfully formed tailor welded blanks showed the presence of tempered martensite in the soft zone was found to be the most predominant factor affecting its failure. The formability characteristics of investigated tailored blanks were predicted using finite element simulation and compared with experimented results for validation.

Rapid Evaluation of Hydrogen Embrittlement Resistance for Spot-Welds of High Tensile Strength Steel Sheet by Slow Rate Tensile Shear Test under Hydrogen Charging Conditions

Automobile manufacturers are accelerating adoption of spot welding of Advanced High-Strength-Steels (AHSS) sheets to reduce weight of automobile bodies. Rapid evaluation of the hydrogen embrittlement (HE) resistance for the spot-welds of AHSS sheets is required, since it is worried that the HE resistance of the nugget will deteriorate compared to the base metal due to the difference in microstructure caused by rapid cooling and solidification during spot welding. However, evaluation of the HE resistance for the spot-welds has not been established. In this study, we prepared spot-welded specimens using AHSS sheets and performed tensile shear tests with varying tensile rates under hydrogen charging to evaluate the relationship between diffusible hydrogen content and tensile shear strength. As a result, the tensile shear strength of spot welds decreased as the amount of diffusible hydrogen increased. The quasi-cleavage fracture surface and intergranular fracture surface were observed at the nugget and inside the crack generated at the nugget-heat affected zone interface. Furthermore, as the results of crack growth behavior and hydrogen thermal desorption spectroscopy analysis, hydrogen embrittlement in spot welds can be attributed to the stress-induced diffusion of hydrogen and the hydrogen trapped in dislocation and vacancy clusters at crack tips. This Paper was Originally Published in Japanese in J. Japan Inst. Met. Mater. 85 (2021) 75–83.

Rapid Evaluation of Hydrogen Embrittlement Resistance for Spot-Welds of High Tensile Strength Steel Sheet by Slow Rate Tensile Shear Test under Hydrogen Charging Conditions

Rapid Evaluation of Hydrogen Embrittlement Resistance for Spot-Welds of High Tensile Strength Steel Sheet by Slow Rate Tensile Shear Test under Hydrogen Charging Conditions

Effect of weld line shape on laser welded joint strength of automotive high-strength steel sheets

ABSTRACT The optimum weld line shape for the remote laser welding was investigated to improve shear strength and peel strength of the lap joints of 980 and 1180 MPa grade automotive high-tensile strength steel sheets. Lap joints were prepared with two kinds of weld line shapes that were straight line and circle. These lap joints were exposed to three different tensile tests of tensile shear test, cross tension test and L-form tension test. In tensile shear test, weld line shapes did not affect on tensile shear strength (TSS), and TSS was proportional to weld line length. In cross tension test, weld line shapes much affected on cross tension strength (CTS). CTS of the circle weld line shape joints was much higher than that of the straight weld line shape joints. In L-form tension test, the weld line shapes also much affected on L-form tension strength (LTS). LTS of continuous straight line weld joints was much higher than that of circle shape weld joints. The factors which controlled TSS, CTS and LTS were discussed by conducting FEA of the tensile tests from a viewpoint of local stress concentration considering the loading types of shear stress and tensile stress.

Experimental observations of cyclic plastic deformation behaviors of 590MPa class high tensile strength steel sheet and its modeling

Experimental observations of cyclic plastic deformation behaviors of 590MPa class high tensile strength steel sheet and its modeling

Effect of Gap between Steel Sheet and Electrode on Resistance Spot Welding

If resistance spot welding (RSW) is conducted when there is a gap between a steel sheet and an unmovable electrode, the steel sheets are bended by a movable electrode, and the quality of RSW is influenced. Therefore, it is necessary to understand the effect of the gap on RSW. In this study, two high-tensile strength steel sheets were welded with the gap. In addition, cross-section observations and cross tension tests were conducted to verify the effect of the gap on weldability and joint strength. Consequently, two notable results were obtained. First, the observation indicated that deformation around the corona-bond was varied depending on the gap. Second, the cross tension tests showed that the gap decreased the joint strength even though the nugget was large enough. These result indicated that controlling the gap is important to ensure the quality of RSW.

Reduction of Cracks During Punching Process by Cementite in High Tensile Strength Steel Sheets

An experimental study on high tensile strength steel sheets with ferritic and ferrite–cementite microstructures was performed with the objective of determining the crack formation and propagation mechanism in the punching process. In addition, the effect of dispersed cementites was also investigated. SEM, EPMA, and EBSD analyses confirmed that voids initiate at the ferrite-inclusion/precipitate (TiN, TiS, and cementite) interface, and the voids then propagate into the ferrite matrix by cleavage. EPMA, EBSD, and TEM/EDS demonstrated that cracks which formed within and near a center segregation area propagate along the Mn center segregation by intergranular fracture due to Mn segregation at the grain boundary. In addition to TiN and TiS, observation of the number of cracks, the length of cracks in the punched surface, and the number density of voids in tensile fracture parts indicated that dispersed cementites are also void formation factors, and cementites decrease the number of cracks and average crack length in the punched surface of ferrite–cementite steel sheets by reducing the stress applied to the voids.

高張力鋼板の打抜き加工時における割れ発生・進展メカニズム

高張力鋼板の打抜き加工時における割れ発生・進展メカニズム

高張力鋼板のスプリングバック抑制に及ぼすクッション圧の影響

高張力鋼板のスプリングバック抑制に及ぼすクッション圧の影響

Theoretical predictions of fracture and springback for high tensile strength steel sheets under stretch bending

Abstract For the last two decades, strong demands for the light weighted structures of automobiles have been increasing in all over the world. In order to satisfy the demands, the high strength steel sheets (hereafter HSSs) have been widely utilized in the various automobile related companies. However, for the companies, it is still one of the hard tasks to apply HSSs for the automobile parts with complex shapes, due to low ductility and large springback of the HSSs. In the present research, we assumed fracture limits in stretch bending are theoretically obtained with the assumption that fracture occurs when stretching force reaches its maximum value. The authors proposed an explicitly theoretical analysis method based on the maximum load criterion that can easily and rapidly allow us to predict the fracture and springback of HSSs by using two no dimensional parameters (limited wall stretch L/L0 (L: limit wall length of a sheet, L0: initial wall length) and bending radius t/R (t: sheet thickness, R: bending radius)). For the evaluations of this method, the comparisons between stretching bending experiments of HSSs and the corresponding theoretical calculations were conducted. From the comparisons, the calculated results by our proposed method have good agreements with the experimental observations.

Crack Formation and Propagation Mechanism in the Punching Process of High Tensile Strength Steel Sheet

The microstructure (crystal grain and the center segregation of Mn) around the voids and cracks in punching process was investigated using Hot-rolled 780 MPa-grade high tensile strength steel sheets. Steel sheets without center segregation were prepared by grinding only one side, while ones with center segregation were prepared by grinding both side. Punching tests were conducted with these two kind of steel sheets. Crack length and number of cracks on the punched surface were measured and counted by an optical microscope. Steel sheets with center segregation had more cracks in total and longer average crack length than that without center segregation. The Mn mapping and the crystal orientation mapping around the crack obtained through EPMA and EBSD showed that the range larger than 70 µm in crack length tend to cause intergranular fracture along Mn segregated area. Also in the range between 10 µm and 70 µm in crack length, cracks forming both along and away from Mn segregated area were observed. Concentration of Mn around the grain boundary of the center segregation area measured by TEM/EDS revealed that grain boundary contains high Mn concentration. The calculation on distribution of equivalent stress just before the onset of crack for steel sheets with and without center segregation using finite element model showed that equivalent stress concentrates at both edges of the punch and die and the center segregation part. Next, interrupted punching tests were conducted with two kind of steel sheets. The observation around voids through SEM, EPMA, and EBSD showed that voids initiate at the ferrite-Ti precipitate interface. From these results, following tendencies were found, within and near Mn center segregated area, voids initiate at the ferrite-Ti precipitate interface, and crack propagates along Mn center segregation. Moreover, Mn segregates at the grain boundary, and Mn weakens grain boundary cohesion which leads to an intergranular fracture. However, without Mn center segregated area, voids initiate at the ferrite-Ti precipitate interface, and crack propagates easily into the ferrite matrix by cleavage which leads to cause a transgranular fracture.

ステレオ画像相関法を用いた高張力鋼板の応力3軸度および破断ひずみの評価方法の確立

ステレオ画像相関法を用いた高張力鋼板の応力3軸度および破断ひずみの評価方法の確立

高張力鋼板における打抜き加工時の割れに及ぼす非金属介在物と析出物の影響

高張力鋼板における打抜き加工時の割れに及ぼす非金属介在物と析出物の影響

Strengthening spot weld joint by autotempering acceleration at heat affected zone

High tensile strength steel sheets in automotive body are composed of high weight fraction carbon and other additional elements. Therefore, securing the joint strength is difficult in spot welding. This study proposes a new technique for improving the spot weld joint strength. Typically, high joint strength can be obtained by prolonging the post-heating duration. This post-heating is expected to prevent propagation of crack into the nugget and improve the toughness of the heat affected zone. Compared to the conventional spot welding and other methods, the heat input is higher, thereby increasing the highest reaching temperature of heat affected zone. Hence, the austenite grain size becomes larger and martensite start temperature increases. Consequently, autotempering is promoted, resulting in stronger joint strength.

FEM Analysis of Friction for Pressing Dies Controlled by Shot Treatment

AbstractThe finite element method (FEM) analysis of elastic-plastic deformation was applied to examine the effect of surface texture controlling of dies by means of hybrid-shot (double shot by two different kinds of particles) treatment on the galling property applied to high tensile strength steel sheets. The FEM analysis results showed that the apparent friction coefficient, which was calculated from pressing load and the drawing force, increased with each increase in the pressing load. The experimental results from the drawing tests with lubricant oil showed that the friction coefficient of the die after hybrid-shot treatment decreased slightly with the increase of the pressing load. On the other hand, the friction coefficient of the die without shot treatment, (i.e. nearly flat surface), slightly increased with the increase of the pressing load due to the digging effect. These findings reveal that the effect of surface texture of dies was caused not only by the elastic-plastic deformation but also by the oil lubrication condition. The FEM analysis was therefore performed by taking into account the digging effect as well as the micro-oil pool effect. The experimental result in the case of hybrid-shot dies corresponds well to the FEM result by adopting the virtual oil model where the oil segment was defined as an elastic solid body.

高張力鋼板の中心偏析が及ぼす打抜き特性への影響とその改善

高張力鋼板の中心偏析が及ぼす打抜き特性への影響とその改善

Improvement in Punching Properties of High Tensile Strength Steel Sheet by Heat-Treatments for Reduction of Center Segregation

We investigated punching properties (crack in punched surface and hole expansion ratio) of high tensile strength steel sheets with and without center segregation. High strength steel sheets were heat-treated to reduce center segregation. Tensile strength, shear surface ratio, depth of rollover and burr height were measured on heat-treated steel sheets to confirm the effect of heat-treatment on strength. The EPMA analysis showed that the center segregation of Mn was reduced by the diffusion during heat-treatment. Crack-formation frequency and hole expansion ratio were also measured. As a result, the center segregation of Mn in high tensile strength steel sheets decreased by the heat-treatment (600 oC for 100 h) with maintaining the tensile strength, the depth of rollover and the burr height. The crack-formation frequency of the steel sheets decreased through heat-treatments.

Manufacturing Process Evolution Method of Neutron Ellipsoidal Mirror Simulation Using Measured Point-Set

Small angle neutron scattering (SANS) analysis using ellipsoidal neutron mirrors are highly demanded for basic researches of high tensile strength steel sheets, proteins and polymer materials. Although the mirror was conventionally assembled from multilayer neutron super mirrors deposited on ground and polished glass segments, it was difficult to align segmented mirrors with high accuracy and it took several months for the grinding and polishing process. An optimized manufacturing process is proposed utilizing optical function evaluation based on measured mirror surface profile. Optical aberration caused by surface form error and misalignment is evaluated by ray-tracing using mirror surface profile measured by non-contact high precision profilometer.

Finite Element Analysis of High Tensile Strength Steel Sheet by Using Complex Step Derivative Approximations

The framework for the complex step derivative approximations (hereafter CDSA) to calculate the consistent tangent moduli is studied. The present methods is one of the most effective methods to implement any material constitutive equations to the commercial finite element codes and does not suffer from calculation conditions and errors. In order to confirm the efficiency of CDSA, we developed the user subroutine code based on the CDSA using associative J2 flow rules with general nonlinear isotropic hardening rules that is commonly and widely utilized in commercial finite element codes. In this study, the user material subroutine ‘Hypela2’ of MSC.Marc (ver.2013.0.0) was utilized. The finite element calculation result by the proposal method shows a good agreement with the corresponding result by the MSC.Marc default setting. Also we apply the Yoshida-Uemori back stress model to the CDSA and evaluate this new technique to predict the deformation behavior of high tensile strength steel sheet.

Mechanical Behavior of 980MPa NANOHITEN<sup>TM</sup> at Elevated Temperatures and its Effect on Springback in Warm Forming

High tensile strength steel sheets have large springback after being formend at room temperature. Warm forming can be a solution to reduce springback of high tensile strength steel parts. NANOHITENTM is a high strength ferritic steel precipitation-strengthened by nanometer-sized carbides developed by JFE Steel Corporation. Tensile strength of the steel at room temperature does not change before and after deformation at elevated temperatures up to 873K since the carbides in the steel are stable at high temperatures less than 973K. Therefore, the steel is suitable for warm forming. Springback of 980MPa NANOHITENTM parts warm formed at 873K is the same level of that of cold formed conventional 590MPa steel parts. In this study, two kinds of material testing at room temperature and at elevated temperatures between 573K and 937K were performed to understand the mechanical behavior of 980MPa NANOHITENTM: uniaxial tensile tests and bending tests. The steels flow stress depends on not only material temperature but also strain rate in uniaxial tensile tests. After a bending test, the specimen shows springback measured by the change of an angle between the two sides. Stress relaxation happens while a test specimen is held at the bottom dead point after bending. And the stress relaxation could be used to reduce springback of warm formed parts.