Hole Expansion Ratio Research Articles

The development and characterization of ultra high GIGA-strength ferritic hot band steels

The correlations existing among the hot mill processing, as-coiled microstructure and mechanical properties of a Mo–Ti–V microalloyed hot rolled high strength steel were investigated in this current study. Discrete processing parameters, i.e., finish rolling temperatures (FRT) and coiling temperatures (CT), were applied and the corresponding microstructures and mechanical properties were analyzed. It was found that the FRT had only a very minor influence on either microstructures or mechanical properties. However, the CTs strongly affected both the microstructures and mechanical properties. The microstructure in the matrix was observed to change from mainly polygonal ferrite to quasi-polygonal ferrite to granular bainite and upper bainite with falling CTs, accompanied by the formation of martensite/austenite (M/A) constituents at the lower CTs. The strength appeared to be increased by the dislocations originating by the shear component of the displacive phase transformation, and by fine (Ti, Mo)C precipitates, both formed during the coiling process. Strengths reaching values as high as 1166 MPa in yield strength and 1225 MPa in tensile strength were observed in specimens after coiling at 610 °C, and the steels still had a reasonable total elongation of around 20%. However, the hole expansion ratios of these steel conditions were rather low in this study, especially for those with higher strength. Several factors appeared to contribute to the poor hole expansion ratios found in these steel conditions: the presence of coarse TiN inclusions, a large amount of strengthening precipitates and a high dislocation density as well as the presence of M/A constituents.

Fracture characteristics of advanced high strength steels during hole expansion test

This study was performed to estimate the stretch-flangeability of two important advanced high strength steel (AHSS) sheets, i.e. DP980 and TRIP1180, using hole-expansion test (HET). Moreover, the effect of hole edge-quality on stretch-flangeability of both steels was analyzed by adopting two different methods, (i) shearing and (ii) wire cutting, to create the central hole on test specimens. Formability of the sheets was measured in terms of fracture strain and hole expansion ratio, under the influence of two different edge-qualities. A detailed characterization at both micro- and macro-scale was carried out to analyze the fracture behavior of both steels. Experimental results showed that the role of localization and shear in addition to edge-quality is crucial to fracture behavior of both the steels. Apart from the macro- and micro-scale characterization, a finite element study was also carried out to model the HET numerically. Finally, a suitable fracture criterion was used to predict the onset of fracture during the test.

Design and Development of Complex Phase Steels with Improved Combination of Strength and Stretch-Flangeability

This study presents the design and development of a hot-rolled bainitic steel, presenting a good combination of strength and stretch-flangeability, for automotive applications. Ti, Nb, and Mo were added in the steel composition in order to control austenite grain sizes, enhance precipitation hardening, and promote the formation of bainite. This study focuses on the effect of process parameters on final microstructures and mechanical properties. These parameters are the finishing rolling temperature, which conditions the austenite microstructure before its decomposition, and the coiling temperature, which conditions the nature and morphology of the ferritic phases transformed. A preliminary study allowed to determine the austenite grain growth behavior during reheating, the recrystallization kinetics, and the continuous cooling transformation curves of the studied steel. Then, a first set of parameters was tested at a semi-industrial scale, which confirmed that the best elongation properties were obtained for homogeneous bainitic lath/granular microstructures, that can be produced by choosing a coiling temperature of 500 ∘ C . When choosing those parameters for the final industrial trial, the microstructure obtained consisted of a homogeneous lath/granular bainite mixture that presented a Ultimate Tensile Strength of 830 MPa and a Hole Expansion Ratio exceeding 70%.

Effect of Tempering on Stretch-Flangeability of 980 MPa Grade Dual-Phase Steel

In this study, the effect of tempering on the stretch-flangeability is investigated in 980 MPa grade dual-phase steel consisting of ferrite and martensite phases. During tempering at 300 oC, the strength of ferrite increases due to the pinning of dislocations by carbon atoms released from martensite, while martensite is softened as a consequence of a reduction in its carbon super-saturation. This strength variation results in a considerable increase in yield strength of the steel, without loss of tensile strength. The hole expansion test shows that steel tempered for 20 min (T20 steel) exhibits a higher hole expansion ratio than that of steel without tempering (T0 steel). In T0 steel, severe plastic localization in ferrite causes easy pore formation at the ferrite-martensite interface and subsequent brittle crack propagation through the highly deformed ferrite area during hole expansion testing; this propagation is mainly attributed to the large difference in hardness between ferrite and martensite. When the difference in hardness is not so large (T20 steel), on the other hand, tempered martensite can be considerably deformed together with ferrite, thereby delaying pore formation and hindering crack propagation by crack blunting. Eventually, these different deformation and fracture behaviors contribute to the superior stretch-flangeability of T20 steel.

Examining Failure Behaviour of Commercially Pure Titanium During Tensile Deformation and Hole Expansion Test

Hole expansion ratio is a material parameter which defines the extent to which sheet metals are formed. Research has shown that, the stress state observed at the hole edge after hole expansion test is similar to those observed during conventional uniaxial tensile test. However, conventional tensile test methods are not efficient in evaluating material edge formability. This work utilised optical non-contact measuring techniques to examine failure behaviour during tensile test and hole expansion test of commercially pure titanium sheet, fabricated with either abrasive water jet cutting or electric discharge machining. The work found that, the deformation mode in conventional tensile testing are governed by localised necking and subsequently diffused necking prior to failure. Deformation mode observed in hole expansion test is characterised by localised necking with no visible occurrence of diffused necking prior to failure. The highest strains are concentrated at the hole edge during hole expansion test due to their sensitivity to the hole preparation method with accompanying multiple localised necking sites resulting in non-uniform deformation. Strains become concentrated in the bulk material microstructure rather than the machined edge during tensile testing resulting in single localised deformation site and a more homogenous deformation.

Microstructure, Crystallographic Texture, and Stretch‐Flangeability of Hot‐Rolled Multiphase Steel

The microstructure, crystallographic texture, strain hardening in the hole edge‐punched and stretch‐flangeability of hot‐rolled multiphase sheet steel is investigated by scanning electron microscopy, electron backscatter diffraction, and microhardness measurements. The results show that the strain hardening in the hole edge‐punched is not the main factor that influences the hole expansion ratio. The bainite fraction, average grain area, and kernel average misorientation distribution mean are microstructure factors that improve the hole expandability and reduce the effect of strain hardening in the hole edge‐punched on the hole expansion ratio. The crystallographic texture affects the hole expandability. The high intensity of {332}<113>α and {111}<112>α components that have higher normal anisotropy () also reduces the effect of strain hardening at the hole edge‐punched. The cracks propagate through the interfaces following the banded microstructure direction at the hole edge‐punched during the punching process. The gamma distribution analysis of the kernel average misorientation distribution indicates that there are different energy transfer (β parameter) and dislocation densities due to different cooling rates.

Fracture evolution in ferrite/martensite dual-phase flange steel

ABSTRACT Ferrite/martensite dual-phase steel is indispensable for manufacturing different automobile parts. In the present study, uniaxial tension tests and metallographic analyses are performed to reveal the fracture evolution in ferrite/martensite dual-phase flange steel with a high hole-expansion ratio. The results indicate that in the ferrite/martensite dual-phase steel, cracks originate at the interface between ferrite grains and martensite islands, and at the Al2O3 and sulfide inclusions. Further, dislocations obviously aggregate at the interface between the ferrite grains and martensite islands, providing many preferential crack nucleation sites. Stress concentrations tend to occur at the edge of Al2O3 and sulfide particles. In addition, the fraction of low-angle grain boundaries increases with prolonged deformation, and the size of ferrite grains gradually decreases with deformation, weakening their effect of inhibition of crack propagation. Moreover, the existence of the γ fiber texture and the {001} texture in the tensile deformation area facilitates crack propagation.

Linking constituent phase properties to ductility and edge stretchability of two DP 980 steels

Two DP 980 steels were characterized with multiple methods to determine its microstructure characteristics, tensile properties and edge stretchability. The first (DP980-B) is a commercial grade DP980 steel produced by AK Steel, while the second (DP980-T) is a modification of the first one by a tempering process. Both contain two individual phases: ferrite and martensite with body centered cubic (BCC) and tetragonal (BCT) crystal structure respectively. ASTM standard tensile tests were performed for both materials, revealing that DP980-B has higher work hardening and larger total elongation than DP980-T. Standard hole piercing and subsequent hole expansion and extrusion tests were also performed. The critical hole expansion ratio (HER) of hole pierced sheet at different nominal clearances, on the other hand, show an opposite trend: the HER of DP980-T almost doubles that of DP980-B. This indicates the tensile ductility and edge stretchability are determined by different mechanisms due to different deformation modes. The ductility of sheet metal during uniaxial tension is determined by deformation instability, i.e. necking, due to less constraint at the region of deformation and fracture will occur quickly after necking. The contribution of post-necking deformation to the total elongation is insignificant for the high strength grades such as DP980 steels. The ductility during hole-expansion, however, is mainly determined by material fracture behavior that is dependent on phase property disparity and material intrinsic fracture parameters. With the individual phase properties for both DP steels determined by in situ tensile tests under high energy X-ray diffraction, the results show that DP980-T steel has lower property disparities between the two phases than DP980-B steel, due to the tempering process. This explains why the DP980-T steel has higher HER than DP980-B steel: lower phase disparity will lead to less local deformation during loading. The higher work hardening rate for DP980-B steel contributes to its higher uniform elongation compared with DP980-T steel based on the maximum load condition of deformation instability. An integrated finite element simulation framework for studying hole expansion is also presented here, based on the calculated individual phase properties from the combined high energy x-ray diffraction (HEXRD) and elastic plastic self-consistent modeling. The simulation results correlate well with the experimental results on the HER difference between the two materials.

Effect of austenite stability on the hole expansion behavior of δ-TRIP steels

This paper deals with the assessment of the stability of retained austenite in δ-TRIP steels at two different isothermal holding time durations. Longer holding time leads to an improvement in austenite stability by greater carbon enrichment. This caused a significant increase in the hole expansion ratio at a tensile strength of 800 MPa. The microstructure mediated approach was employed to characterize the difference arising from different holding durations. The deformation-induced transformation of austenite into martensite mapped on the specimens after hole expansion proved that the austenite stability plays a significant role in the hole expandability of δ-TRIP steels.

Microstructure–Property Correlation in High-Strength Formable Steel with Varying Nb-Si Content

High-strength formable quality (HSFQ) steel grades, steel A (0.038Nb, 0.031Si), steel B (0.034 Nb, 0.27Si), and steel C (0.044Nb, 0.26Si), with varying Nb and Si content has been investigated. The microstructure, texture, precipitation, mechanical properties and corrosion behavior of these steel grades have been explored. Very fine grain size in the range of 2.7-3.6 µm was obtained in all these steel grades through controlled hot rolling. However, a significant variation in the crystallographic texture and precipitation behavior in these steels was observed. A higher volume fraction of γ-fiber texture components and uniformly distributed fine Nb(C, N) precipitates were obtained in steel B as compared to steel A and C. This resulted in better properties in steel B with relatively high PSE (product of strength–elongation), high uniform elongation, and high hole expansion ratio. The corrosion properties of the steels were also evaluated, and steel B showed better corrosion resistance among the three grades. Considering the overall performance and properties of these steel grades, steel B with an optimum Nb and Si content is found to have better property and performance among the three grades.

Influence of processing factors on the sheared-edge formability of vanadium bearing dual-phase steels produced using continuous galvanizing line simulations

Dual-phase steels are commercially attractive since they offer good formability at a high strength and reasonable cost. In addition to high strength, these steels also exhibit high “n-values” and high uniform and total tensile elongations. However, one limitation in the application of high-strength dual-phase steels is the fairly limited ductility observed during sheared-edge stretching or punched hole expansion. Therefore, successful high-strength dual-phase steels must have controlled microstructures in order to achieve the desired performance. The failure mechanism observed during stretching of the sheared edge is closely related to the microstructures (i.e., amount, hardness, size and distribution of the different phases and microconstituents), and the cracks were found to initiate predominately at the interphase interfaces separating the ferrite and the martensite. In the present research, the influence of three main variables on the hole expansion ratio (HER) behavior was explored: the metallurgical condition going into the continuous galvanizing line (CGL), different post-anneal CG line thermal processing paths and the influence of vanadium. Depending on conditions, all three variables can influence the sheared-edge ductility as determined in the hole expansion test. Suggestions for improving the sheared-edge ductility while maintaining adequate strength are presented.

Development of Tool Paths for Multi-axis Single Stage Incremental Hole-flanging

In this paper, multi-axis single stage incremental hole-flanging is explored in place of three axis multi-stage tool paths in order to prevent flange conicity in hole-flanging by single point incremental forming (SPIF). The angle of inclination of the forming tool, the starting radius and step size of the tool paths are varied to investigate the feasibility of new tool paths. Using the proposed tool paths, an optimization is performed by Finite element (FE) analyses. The geometrical accuracy of the flange is maintained within tolerances relevant to industrial applications while minimizing the resultant forces acting on the tool and maintaining high formability (hole expansion ratio, HER). The FE analyses of the process reveal low formability and high forming forces for flanges formed at high angles of inclination of the forming tool. The tool paths with small diameters at the start point of the tool path result in high bulge heights and high resultant forces. Hence, the existing three axis tool paths are better than the proposed strategies. To prevent flange conicity, over bending of the flange edge to compensate for springback after forming is implemented on the three axis tool path strategy. This is achieved by adding a second tool path with an over bend angle at the end of the three axis tool path.

Artificial Neural Network for Predicting Edge Stretchability in Hole Expansion Test With Gpa-Grade Steel

This paper mainly proposes an artificial neural network (ANN) model for predicting edge stretchability of GPa-grade steels, which is substantially difficult to predict due to the complex nonlinear relation among the numerous sheared edge qualities. We newly suggest the physically characterized parameters, such as material properties, deformed shape, and work hardening of sheared edge, to predict the various materials and punching methods, simultaneously. The proposed parameters are trained with the pre-damage strain which is calculated by inherent fracture strain and experimental results in terms of hole expansion ratio. To prevent the overfitting issues, cross validation method with additional datasets from a different kind of edge stretchability test such as sheared edge tensioning test are utilized. Experimental validations have been conducted with various GPa-grade steels and sheared edge conditions, which are compared with the proposed ANN model and numerical simulation. The proposed ANN model exhibits remarkable performance in the prediction of hole expansion ratio having a mean absolute error of 1.5% when compared to the previous studies such as numerical simulation and ANN model with utilizing the maximum hardness measured at the sheared edge.

Development of a Device Compatible with Universal Testing Machine to Perform Hole Expansion and Erichsen Cupping Tests

The material characterization regarding sheet metal formability is usually assessed by the forming limit curve (FLC). The FLC requires specialized and expensive equipment, several samples with different geometries, and can be a very time-consuming procedure for data treatment. Alternatively, the hole expansion test (HET) and the Erichsen Cupping Test (ECT) can be used for routine evaluations of sheet metal mechanical behavior. These formability tests require fewer quantities of material and easy specimen preparation with a fast analysis of results. The HET and ECT procedures provide a proper evaluation of sheet material stretch-flangeability, formability, strength, and ductility. In this work, we developed a low-cost mechanical device capable of performing the HET and ECT tests using a universal testing machine. The equipment is designed to meet the test parameters set by ISO 16630 (HET) and ISO 20482 (ECT) standards. In order to verify its functionality, tests were carried out with the dual-phase steels DP600 and DP780. The corresponding values determined for the hole expansion ratio and the Erichsen index provided reliable results in terms of the accuracy and repeatability of the proposed testing device.

Role of grain size on deformation microstructures and stretch-flangeability of TWIP steel

The present study reports the effects of grain size on deformation microstructures and stretch-flangeability of the fully austenitic cold-rolled and annealed Fe–17Mn-0.6C-1.5Al (wt. %) twinning-induced plasticity (TWIP) steel. Mechanical properties of the materials annealed at various annealing temperatures were investigated by uniaxial tensile tests and hole expansion tests. Grain refinement led to an increase in both tensile strength and hole expansion ratio. Higher deformation twin fraction was observed with increasing grain size. The martensitic transformation was observed at the hole edge of the hole expansion tested specimen, where severe deformation was concentrated. Main crack initiation sites were identified as twin boundaries, grain boundaries and phase boundaries between martensite and austenite. Therefore, the coarse-grained TWIP steel could show a high probability of crack initiation due to its high density of crack initiation sites. We suggest that grain refinement is the unique way to develop high strength and damage tolerant TWIP steels which can be applied for complex structural parts.

A critical review on hole expansion ratio

Materials resistance to edge fracture in intricate shape forming is commonly quantified by hole expansion ratio (HER). Hole expansion test is normally used to evaluate HER. To date, the governing factors of HER have not well understood regardless of its importance for automotive part manufacturing with advanced high-strength steels. The present paper comprehensively discussed the recent progress on HER, including fundamental deformation aspects, the effect of punch geometries, correlation with tensile properties, and the influence of microstructure. This reviewed work explains why HER is currently an essential topic of engineering research.

Structure-property relationship in novel low carbon hot-rolled TRIP steels via thermo-mechanical controlled processing and coiling

A novel process involving thermo-mechanical controlled processing (TMCP) and coiling process was adopted to obtain multiple-phase mixed microstructure. The coiling temperature governed microstructural evolution, stability of retained austenite (RA) and microstructure-properties relationships. The results indicated that the optimum properties were obtained at a coiling temperature of 485 °C with combination of tensile strength of 1006 MPa, elongation of 13.3%, hole-expansion ratio of 28% and impact toughness of 14.7 J (1/4-Charpy size sample at −20 °C). A small amount of RA was present when the coiling temperature was 430 °C (close to the Ms point), resulting in low elongation. The size of blocky RA located at grain boundary was ~450 nm, whereas, the film-like RA embedded in bainite laths was ~40–110 nm. The film-like RA was more stable than blocky RA, which enabled superior formability. Both types of RA provided positive and continuous TRIP effect during deformation. But the presence of a large amount of high-carbon martensite adversely affected impact toughness and formability because it acted as a source for crack initiation as a hard phase in the matrix.

Modelling the surface roughness influence on the hole expansion ratio of multiphase steel

There exist both extrinsic manufacturing process-related and intrinsic material-immanent effects on the edge crack sensitivity of dualphase steels. Extrinsic influences can be further divided into effects of residual damage as well as roughness-induced effects. A new simulation framework is applied to quantitatively assess the surface roughness-induced influences on the edge crack sensitivity of cold rolled dualphase steel of grade DP1000. Hole expansion ratios are numerically predicted for four different edge manufacturing processes, including drilling, milling, waterjet cutting, and wire cutting. The simulation framework applies the idea to make use of a ductile damage mechanics model. It employs realistic material parameters for all elements situated in the bulk, whereas those elements situated at the edge to be expanded during the test apply artificial model parameters. These are identified with the help of sub-models with geometrical surface representation. For the sub-model construction, roughness profiles are experimentally characterized by white light confocal microscopy. With the new simulation framework, numerical predictions of hole expansion ratios can be significantly improved, even though a remarkable overestimation still remains. Among others, this general overestimation can be attributed to local strain hardening, residual stresses, and microstructural modifications.

Local ductility – key parameter for predicting formability of AHSS

In order to be able to make targeted use of the advantages of AHSS grades in forming processes it is necessary to determine local material properties. Hence, in addition to edge crack sensitivity measures, such as hole expansion ratio according to ISO 16630 (HERISO), it is currently intensively discussed whether measures, that assess the local ductility of the material, should also be specified in material specifications. One representative is given by the parameter true fracture strain (TFS) that is based on the measurement of the fracture area of a cracked flat tensile specimen. By means of 16 different cold- and hot-rolled steel batches, the relation between the parameters TFS and HERISO was investigated. The outcome is that these parameters do not correlate to a satisfying degree when considering all materials and consequently are regarded as representatives of different material properties. Thus, the issue of the reliability of the measures is considered separately. For the regarded steel grades, the overall scattering of TFS, i.e. neglecting the sample orientation, is found to be 13 – 44 % and the scattering of HERISO is within 5 – 35 %. The high scattering of TFS is mainly caused by the anisotropy of the materials. In case of parallel measurements of the same batch the relative standard deviations of TFS and HERISO are on the same level for cold-rolled strips, whereas in case of hot-rolled strips the relative standard deviation of HERISO is significantly higher. By investigating the influence of the tensile specimen shape for a bainitic hot-rolled strip numerically, it could be shown that the width-to-thickness ratio, that takes the hole fracture area into account, influences the TFS value directly.

Local formability of AHSS: Measurement technique, specimen types and robustness

The recently proposed classification scheme for AHSS with global and local formability enables a material specific process development, allows a designation based on mechanical properties, as well as an assessment of various properties: deep drawability, edge-crack sensitivity, bendability, damage tolerance and fracture toughness. Local formability is measured by local fracture strains, e.g. area reduction at fracture or true thickness strain at fracture. Based on investigations of AHSS with strengths between 600 and 1000 MPa from different steel suppliers, this contribution addresses the influence of the measurement technique, of different specimen types and different sheet thicknesses on the measure for local formability. Furthermore, the correlation between true thickness strain at fracture and hole expansion ratio and also the hardness difference of the microstructure, as well as no correlation between local formability and bending angle according to VDA 238-100 are confirmed. It is shown that the influence of the measurement technique for true thickness strain at fracture is neglectable regarding the differentiation between the investigated grades. Using “uniaxial” tensile tests with specimen types 2 and 3 according to ISO 6892-1 one can obtain similar local fracture strains. Same states for the notched tensile tests with radii of 4 mm and 5 mm. The values gained by the smaller A30 mm specimen show slightly lower values compared to the “uniaxial” tensile specimen types. With increasing sheet thickness, a slight decrease in true thickness strain at fracture resulting from the combination of a changing width-to-thickness ratio of the tensile specimen and different microstructures can be seen. The large range of the measure for local formability using materials from different suppliers and different sheet thicknesses motivates the definition of requirements for material specifications and source approvals.