Behavior Of High Strength Steel Research Articles

Influence of directionality on strength and impact behaviour of high strength steels

The directionality of the strength and impact behaviour of control rolled 110 grade high strength steel has been examined. Tensile specimens tested along the rolling direction were found to give strengths of ∼60 MPa lower than when tested transversely in the rolling direction. Charpy V notch impact samples, in contrast, gave lower impact transition temperatures when taken in the rolling direction (35°C lower). The difference in strength between the two directions is believed to be mainly as a result of the strong texture produced on control rolling. The difference in impact behaviour is due to the more severe fissuring on the fracture surface of the longitudinally tested specimens. Fissuring is more marked on the longitudinally tested sample because of the greater grain elongation in that direction, the initial fracture path being along the grain boundaries.

Comparisons of Constitutive Models for Steel Over a Wide Range of Temperatures and Strain Rates

This study investigates and compares several available plasticity models used to describe the thermomechanical behavior of structural steel subjected to complex loadings. The main purpose of this comparison is to select a proper constitutive model that can later be implemented into a finite element code to capture localizations (e.g., shear bands and necking) in steel and steel structures subjected to low- and high-velocity impact. Four well-known constitutive models for viscoplastic deformation of metals, i.e., Johnson–Cook (JC), Zerilli–Armstrong (ZA), Rusinek–Klepaczko (RK), and Voyiadjis–Abed (VA), have been investigated and compared with reference to existing deformation data of HSLA-65 and DH-36 steel conducted at low and high strain rates and various initial temperatures. The JC, ZA, and RK models reasonably describe the flow stress and the strain hardening behavior only in the certain ranges of strain, strain rate, and temperature for which the models were developed. This was attributed to the inaccurate assumptions used in developing these models. In contrast, the VA model most effectively describes the flow stress and strain hardening in which very good predictions are observed for the constitutive behavior of high strength steel over a wide range of strains, strain rates, and temperatures.

Effects of inclusions on very high cycle fatigue properties of high strength steels

The effect of inclusion size on fatigue behaviour of high strength steels in the very high cycle fatigue (VHCF) regime (>107–109 cycles) is reviewed. Internal fatigue fractures of high strength steels in the VHCF regime initiate mostly at non-metallic inclusions. The critical inclusion size below which it is hard to initiate fatigue cracking of high strength steels in the VHCF regime is found to be about half the critical value characteristic of the high cycle fatigue (HCF) regime (about 105–107 cycles). A stepwise or duplex S–N curve is observed in the VHCF regime. The shape and form of the S–N curves are affected by inclusion size and other factors including surface condition, residual stress, environment and loading modes. Fatigue strength and fatigue life for high strength steels have been found to obey inverse power laws with respect to inclusion size D of the form σw∝D−n1 and Nf∝D−n2 respectively. For fatigue strength, the exponent n1 has been reported to be ∼0·33 in the literature for the HCF regime and, more recently, to fall in the range 0·17–0·19 for the VHCF regime. For fatigue life, the exponent n2 is reported to be ∼3 in the HCF regime, and in the range 4·29–8·42 in the VHCF regime. A special area was often observed inside a ‘fish eye’ mark in the vicinity of a non-metallic inclusion acting as the fracture origin for specimens having a long fatigue life. The major mechanisms of formation for this special area are discussed. To estimate the fatigue strength and fatigue life, it is necessary to know the size of the maximum inclusion in a tested specimen, and to be able to infer this value using data from a small volume of steel. The statistics of extreme value (SEV) method and the generalised Pareto distribution (GPD) method are introduced and compared. Finally, unresolved problems and future work required in studying the VHCF of high strength steels are briefly presented.

Biaxial Tensile Test of High Strength Steel Sheet for Large Plastic Strain Range

Deformation behavior of high strength steel with a tensile strength of 590 MPa under biaxial tension was investigated for a work equivalent plastic strain range of 0.002 0.16. The test material was bent and laser welded to fabricate a tubular specimen with an inner diameter of 44.6mm and wall thickness of 1.2 mm. Using a servo-controlled tension-internal pressure testing machine, many linear stress paths in the first quadrant of stress space were applied to the tubular specimens. Moreover, biaxial tensile tests using a cruciform specimen were performed to precisely measure the deformation behavior of the test material for a small strain range following initial yielding. True stress-true plastic strain curves, contours of plastic work in stress space and the directions of plastic strain rates were measured and compared with those calculated using selected yield functions. The plastic deformation behavior up to an equivalent plastic strain of 0.16 was successfully measured. The Yld2000-2d yield function most closely predicts the general work contour trends and the directions of plastic strain rates of the test material.

Identification of Process-Based Limit Stress States Applying a Stretch-Bending-Test

The evaluation of forming simulation by using the forming limit curve has only limited validity if it is applied on car body components with non-linear strain paths. If modern high strength materials are used, the forming limit criteria can also provide invalid predictions. Especially high strength multiphase steels show a specific behaviour in forming, necking and crack initiation. If bending loads are applied to these materials, the onset of cracking occurs partially not within the range of the forming limit curve (FLC). The stress limit indicates the failure beginning more accurate. It is independent of the forming history and should be less sensitive to the behaviour of high strength steels. In the post processing of a simulation it could be used similar to the forming limit. A limit curve applied on the in-plane-stress-diagram of an analysed component defines areas that are more vulnerable for cracking. The required stress limit curve will be obtained in this research by applying a stretch-bending-test. It is selected in order to reach loads, which are comparable to the forming process in the components’ production. The forming state that is affecting the specimen is a combination of bending and stretching load. Different load conditions can be applied at the test by altering the stamp-radius and the specimen geometry. Since stresses cannot be measured directly in the experiment, the test is modelled in the simulation where the stresses can be calculated for a given material model. Finally the stress limit criterion was applied on the test parts’ stress distribution diagram. Occurring stresses above the stress limit curve are displayed on the simulation. Thereby it is possible to show a good correlation in critical areas between the failure prediction in the simulation and occurring rupture on the test component.

Investigation on Tribological Behavior of Advanced High Strength Steels: Influence of Hot Stamping Process Parameters

In order to investigate the friction and wear behavior of high strength steel in hot stamping process, a hot strip drawing tribo-simulator is developed and the friction coefficient, which is an important parameter in the finite element modeling, is measured. The results have shown that the friction coefficient remains almost unchanged until temperature reaches 500 °C. It then increases sharply as temperature is increased from 500 to 600 °C. It has also been shown that the friction coefficient decreases as the drawing speed increases. The change in the dominate wear mechanism as the temperature and the drawing speed increases has been identified from SEM analyses of the worn surface. The dominate wear mechanism is the groove cutting at temperatures between room temperature and 500 °C, which changes to the adhesive wear at temperatures above 500 °C. The main wear mechanism is the adhesive wear at 25 mm/s, which changes to the slight groove cutting at 75 mm/s.

Measuring techniques for the very high cycle fatigue behaviour of high strength steel at ultrasonic frequencies

Abstract To analyse the fatigue behaviour of steels in the very high cycle fatigue regime, an ultrasonic testing facility was developed that allows describing the cyclic deformation behaviour at ultrasonic frequencies by measuring characteristic fatigue data at a sufficiently high frequency. In correlation with scanning electron micrographs, fatigue tests prove that the sensitive measuring techniques indicate fatigue induced microstructural changes by a significant rise in the process parameters more than 106 cycles before final failure occurs. By further analysis of the attenuation behaviour of the ultrasonic resonance system, the logarithmic decrement can be used as a reasonable physical value to indicate changes in the microstructure with a very high sensitivity. This allows an improved understanding of the fatigue mechanisms in the very high cycle fatigue regime.

Fatigue Strength and Internal Crack Growth Behavior of High Strength Steel under Variable Amplitude Stressing in Very High Cycle Regime

Ultrasonic fatigue tests were carried out for high carbon chromium bearing steel, SUJ2, under two-step variable amplitude stressing at a frequency of 20 kHz in order to investigate the growth of an interior crack. Since it is important to clarify the damage mechanisms under variable amplitude stressing for practical applications, fatigue tests were conducted under repeated two-step variable amplitude stressing. An inclusion was observed at the initiation site of fracture. Optically dark area (ODA) and fish-eye were observed around the inclusion. Sizes of ODA and fish-eye depended on the stress intensity factor range. Interior crack growth rate was slower than the surface one due to vacuum environment. Cumulative damage values calculated by Miner's rule were relevant to the size of inclusions under repeated two-step variable amplitude stressing. Characteristics of interior fatigue crack growth were determined by the fracture surface observations for beach marks and the ODA sizes. Based on the results, the formation mechanism of ODA was discussed in conjunction with the reports by the other researchers.

Design of a Tribo-Simulator for the High Strength Steel Friction and Wear Investigation in Hot Stamping

Hot stamping has become the main technology to produce complex-shaped high-strength steel components. However, hot sheet metal forming can cause high rates of die wear and the scratching of the productions. A hot drawing tribo-simulator was designed to investigate the friction and wear behavior of high strength steel at elevate temperature. The value of the friction coefficient was obtained to examine the function of the tribo-simulator. The results prove that the coefficient of friction in hot stamping can be measured using this tribo-simulator. Moreover, the coefficient of friction is able to be used in finite element simulation to improve the accuracy of the modeling results and it is also an important indicator to evaluate the tribological behavior between the die and sheet metal.

An Investigation on Effect of Residual Stress on Springback Behavior of High Strength Steel in U-Bending Process

Bending process is an important process in the metal sheet forming in many industries. The main problem of the bending process is the spring-back phenomenon after removing the punch. This research aims the investigation on effect of residual stress on springback behavior of sheet metal in U bending process. The corner setting technique and bottoming process were designed for experiments. The corner setting technique and bottoming has reduced the thickness in bending area to 5, 10, 15 and 20 percent of the original sheet thickness. Clearance between punch and die of both processes was equal to same the thickness. The residual stress value and springback phenomenal were investigated by commercial program code DEFORM 2D which was able to analyze the effect stress and force in bending area. Electrolytic zinc coated carbon steel grade JIS; SECC, JIS; 440 and JIS; 590 which having the thickness of 1 mm were employed as the workpiece material for all experiments. The result of three materials in conventional U bending die shows larger spring back than the corner setting technique and bottoming process. Moreover, the corner setting technique reduces spring back value in bending process but requires high bending force. Corner setting technique and bottoming process at 15% and 20% shows that the spring go of all parts. The FEM simulation results explained the effect of residual stress to springback phenomenal. Comparisons between experimental and finite element method results were also performed.

A simple model to predict the very high cycle fatigue resistance of steels

This work deals with the very high cycle fatigue behavior of high strength steels where fracture origins are mostly at non-metallic inclusions. From the analysis of several features involved in the crack initiation and propagation processes, and since the fatigue resistance usually observed for metals is mainly due to a microstructural threshold for pure fatigue crack propagation, the following expression is proposed to estimate the internal fatigue resistance for a given fatigue life for fractures produced by cracks initiated from an internal inclusion: σ e Int = 444 Δ K th n R i where σ e Int is given in MPa, the inclusion radio R i is in μ m, n is a dimensionless factor and the pure fatigue crack propagation threshold ΔK th is equal to the lower value given by the following expressions: Δ K th = 4 . 10 - 3 ( H V + 120 ) a 1 / 3 Δ K th - 1 = - 0.0038 σ u + 15.5 where the pure fatigue crack propagation thresholds Δ K th (function of crack length) and Δ K th −1 (a constant value for a given tensile strength or hardness) are in MPa m 1/2, the crack length a in μm, the Vicker harness H V in kgf/mm 2 and the ultimate tensile strength σ u in MPa. Such estimation becomes very important in the assessment of the fatigue resistance of components subjected to very high cycle fatigue, as this might become a very expensive and time consuming task.

Prediction of the endurance limit taking account of the microgeometry after finishing milling

The aim of this study is to investigate the influence on the high cycle fatigue behaviour of high strength steels of the surface micro-geometry induced by their machining. The machining process considered herein is high speed finishing milling with a ball-end tool. The micro-geometry of the machined surface is characterized by 2- and 3-D roughness parameters. The studied material is a 25MnCrSiVB6 resulphurized steel with a bainitic matrix. The high cycle fatigue behaviour of this material is characterised by its endurance limit evaluated from symmetric tension-compression (Rσ = −1) tests performed according to standard staircase method. This endurance limit was compared to the results of so-called self-heating tests as well as rupture tests performed according to Locati approach. It appears that the difference between the endurance limits measured on the machined or the polished specimens is not fully accounted for by the self-heating measurements. Furthermore, the Arola–Ramulu model also appears as being unable to predict the influence of the surface integrity induced by the milling process on the high cycle fatigue life of this material. Last, a sensitivity analysis of the lead angle of the tool on the anisotropy of the machined surface micro-geometry proves that the roughness of surfaces milled with ball-end tools must be characterized by surface—and not linear—parameters.

Hydrogen Influence on the Mechanical Behaviour of High Strength Steel

Abstract Though numerous studies have been devoted to hydrogen embrittlement in steels, up to date there is not a general agreement about the effect of hydrogen on the mechanical behaviour. The purpose of this paper is to analyze the influence of hydrogen on the mechanical response of high strength steels. Samples were cathodically charged with hydrogen, previous to low cyclic fatigue and tensile tests at room temperature. The presence of hydrogen produces softening effects on the cyclic behaviour and improvements in the fatigue life for low hydrogen contents. The stress-strain curves of tensile tests on pre-charged samples depend on the strain rate imposed. Both tensile and fatigue response could be explained by the hydrogen enhancement of dislocation mobility mechanism.

Very High Cycle Fatigue Behavior and Thermographic Analysis of High Strength Steel

Very high cycle fatigue behavior of high strength steel, were investigated using ultrasonic fatigue testing equipment at 20 kHz up to 109cycles. S-N curves at room temperature with different stress ratio (R=0.01 and R=0.1) was determined. The experimental results show that fatigue strength decrease with increasing number of cycles between 105 and 109. SEM examination of fracture surface reveals that fatigue damage was governed by the formation of cracks, and subsurface crack initiation was in the very long life range. The results shown that the portions of life attributed to subsurface crack initiation between 107 and 109 cycles are 99%.

Prediction of threshold for very high cycle fatigue ([formula omitted] cycles)

This work deals with the very high cycle fatigue behavior of high strength steels in which the fracture origins are mostly at nonmetallic inclusions. The estimation of the total fatigue life for these materials could only success if the fatigue crack initiation life could be properly quantified. This is a very hard task because the phenomenous involved in the fatigue crack nucleation and initiation processes are not well understood yet. This stage is not uniquely governed by fatigue, but also by other processes that are usually active helping the pure fatigue process. From an analysis of several features involved in the crack initiation and propagation processes and taking into account that the fatigue limit usually observed for metals is mainly given for a microstructural threshold for fatigue crack propagation, an expression was deduced to estimate the internal fatigue resistance for a given fatigue life for fracture produced by cracks initiated from an internal inclusion. Such estimation becomes very important when it is necessary to assess fatigue resistance of components subjected to very high cycle fatigue, for which the experimental measure of the fatigue limit could require very long time by using traditional fatigue testing machines.

Ultra-high cycle fatigue behavior of high strength steel with carbide-free bainite/martensite complex microstructure

The ultra-high cycle fatigue behavior of a novel high strength steel with carbide-free bainite/martensite (CFB/M) complex microstructure was studied. The ultra-high cycle fatigue properties were measured by ultrasonic fatigue testing equipment at a frequency of 20 kHz. It is found that there is no horizontal part in the S-N curve and fatigue fracture occurs when the life of specimens exceeds 10 7 cycles. In addition, the origination of fatigue cracks tends to transfer from the surface to interior of specimens as the fatigue cycle exceeds 10 7, and the fatigue crack originations of many specimens are not induced by inclusions, but by some kind of “soft structure”. It is shown that the studied high strength steel performs good ultra-high cycle fatigue properties. The ultra-high fatigue mechanism was discussed and it is suggested that specific CFB/M complex microstructure of the studied steel contributes to its superior properties.

Influence of Loading Frequency on Fatigue Behavior of High Strength Steel

In order to investigate the influence of loading frequency on the fatigue behaviors of the high strength steel, ultrasonic fatigue tests were carried out for a high-carbon-chromium steel and the results were compared with those of fatigue tests using conventional rotary bending fatigue test machine with a frequency of 52.5Hz. The different of fatigue strength at ultrasonic frequency level and conventional frequency level is very small and the S-N curve obtained from 20 kHz or 52.5 Hz shows the step-wise shape. The fatigue crack occurred from inclusions on the subsurface site in the long life regime and the typical surface fracture occurred in the short life one though the loading frequency level is different. It is indicated that ultrasonic fatigue method is an effective method to investigate the fatigue properties in super-long life region.

Effect of hydrogen on the fracture behavior of high strength steel during slow strain rate test

The effect of hydrogen on the fracture behavior of the quenched and tempered AISI 4135 steel at 1450 MPa has been investigated by means of slow strain rate tests on smooth and circumferentially-notched round-bar specimens. Hydrogen was introduced into specimens by electrochemical charging and its content was measured by thermal desorption spectrometry (TDS) analysis. Results showed that the steel had high hydrogen embrittlement susceptibility. For both smooth and notched specimens, the fracture mode was changed from microvoid coalescence (MVC) to brittle intergranular (IG) fracture after the introduction of a small amount of diffusible hydrogen. Fracture initiated in the vicinity of the notch root for notched specimens, while it started from around the center in smooth specimens. The fracture stress decreased with increasing diffusible hydrogen content, and the decreasing trend was more prominent for specimens with a higher stress concentration factor. Taking into account the stress-driven hydrogen diffusion and accumulation in the vicinity of the notch root, the local diffusible hydrogen concentration and local fracture stress in notched specimens have been calculated. According to numerical results, the relationship between the local fracture stress and local diffusible hydrogen concentration was independent of stress concentration factor, which could account for the effect of hydrogen on the fracture stress of the steel.

2314 高強度鋼の二段変動荷重下における超長寿命疲労特性と内部き裂進展挙動(S07-3 疲労強度,S07 構造材料の疲労強度とき裂進展問題)

This paper presents the fatigue behavior of high strength steel, SCM435, under constant amplitude loading and repeated two step loading in ultra high-cycle region. The axial loading fatigue tests of R=-1 was conducted at frequency of 1〜200Hz. The area that was called the Fish-eye and FGA was observed in the fracture surface. In addition, beach marks were observed in Fish-eye under repeated two-step loadings. However, signs of the Two-step loading like the beach mark were not observed in FGA. In addition, it was forecast that a low stress and a high stress did not mutually influence the growth of FGA since it became a value close to one by using the Minor low.

Plane bending fatigue behavior of interstitial-free steel at room temperature

Abstract Interstitial-free steels are widely used as thin sheet for automobile external panels. For this type of application, essential properties of the steel are deep-drawability and stretch-formability. However, low-strength interstitial-free steel is nearly pure iron and work on fatigue fracture behavior of this steel might be adopted as a basic study to understand the behavior of high-strength steels used for structural components. In this study, the fatigue fracture behavior of low-strength interstitial-free steel has been studied at room temperature in air and in an argon atmosphere. Fractographic observations on fatigue fracture surfaces reveal a mixed type of fracture mode (intergranular and transgranular cracking) for all specimens. The major portion of intergranular fracture has been found at the regions of low and medium crack lengths. Further increase in the crack length causes a decrease in the proportion of intergranular cracking and, finally, the fracture mode becomes completely transgranular. At the same test temperature, the proportion of intergranular fracture drastically decreased when the test was carried out in the argon atmosphere.