Types Of High-strength Steels Research Articles

Simulation of ductile fracture in pipeline steels under varying constraint conditions using cohesive zone modeling

In this study, the effect of non-singular T-stress (a measure of crack tip constraint) on crack growth resistance curves (R-curves) and crack tip opening angle (CTOA) was investigated. The investigation proceeded in two parts; first using a modified boundary layer (MBL) model to systematically assess the effect of T-stress on the R-curves and CTOA, and second comparing the CTOA under different loading modes, namely, bending and tension. Three steels were examined in this work; the first was a typical high strength steel referred to as TH, the second and third were pipeline steels referred to as X70 and X100. Surface-based cohesive zone models with four sets of bilinear traction-separation (TS) laws were used for TH steel. The models of X70 and X100 were computed using element-based cohesive zone modeling with a trapezoidal TS law. All simulations were conducted using the finite element (FE) program ABAQUS. It was assumed in these simulations that there was no effect of T-stress on the TS laws per se. With this assumption, it was found that the T-stress and loading mode have only a small effect (1–2°) on the CTOA for the materials studied.

Determination of uniaxial large-strain workhardening of high-strength steel sheets from in-plane stretch-bending testing

This paper proposes a novel approach to determining the large-strain uniaxial stress-strain curves of sheet metals by using in-plane stretch-bending test data. In this approach, a stress-strain curve given by uniaxial tension is extrapolated by a constitutive equation of the combined Swift and Voce laws. The weighting coefficient in the combined Swift-Voce equation is determined by minimizing the difference in the stretch-bending mechanical responses, i.e., punch stroke (or load) vs. maximum bending strain, between the finite element (FE) simulation and the corresponding experimental result. This method is verified by performing experiments on four types of high-strength steel (HSS) sheets (the dual-phase type 590Y, 780Y, and 980Y and the precipitation type 590R). For all sheets, stress-strain curves are determined for strain levels of more than twice the achieved uniaxial-tension uniform elongations. The effects of sheet anisotropy and the Bauschinger effect in this method are discussed. The determined workhardening characteristic by this method for 590R at a large strain range is found to differ from that under an equi-biaxial stress state, as obtained from the bulge biaxial test.

The prediction for fatigue strength in very high cycle regime of high strength steel

BACKGROUND: Fatigue strength is an important parameter of evaluating the material fatigue property. There are many prediction models for fatigue strength in the VHCF regime. Murakami’s model is extensively applied in numerous prediction models. The initiation mode of VHCF cracks in high strength steel is very different from the assumption in Murakami’s model that the internal inclusion is considered as a disc crack. OBJECTIVE: We tried to analyze the effect of inclusion shape on the prediction by Murakami’s model and further examined the prediction effects of several generalized models. METHODS: We reviewed the prediction models for fatigue strength in the VHCF regime. On the assumption that the fatigue crack which initiated from the internal inclusion is a circumferential crack around a spherical cavity and crack propagation is the critical condition, we modified Murakami’s model. The predicted values of Murakami’s model, the modified model and Liu’s model were compared with the test results of 28 groups of high strength steel in the literature. RESULTS: The deviations between predictions by several generalized models and test results are mostly within 20%. The predicted values of four groups of high strength steel by the modified and Murakami’s models and six groups of high strength steel by Liu’s model have large deviations from the test results. The predicted fatigue strength values of Liu’s model are higher than that of the modified model, and the predicted values of the modified model are slightly higher than that of Murakami’s model. The mean value of fitting coefficient C of all types of high strength steel is closer to the coefficient of the modified model, and the sums of squares of coefficient deviations of the modified model is smaller than that of Murakami’s model. CONCLUSIONS: The effects of inclusion shape on the prediction of fatigue strength in the VHCF regime seem small. Murakami’s model, the modified model and Liu’s model coincide well with the test results in the literature. The prediction effect of the modified model is higher than that of Murakami’s model, and the prediction effect of Murakami’s model is higher than that of Liu’s model.

The influence of triggers geometry upon the stiffness of cylindrical thin walled tubes

AbstractToday’s automobile manufacturers are increasingly using lightweight materials to reduce weight; these include plastics, composites, aluminium, magnesium alloys, and also new types of high strength steels. Many of these materials have limited strength or ductility, therefore in many cases the rupture being serious consequences during crashes, underscore Picketta et al. in their studies.Automotive structures must deform plastically in a short period of time, a few milliseconds, to absorb the crash energy in a controllable manner. It must be light and enable economically mass-production [1].FE models rapidly gained acceptance among engineers. Many other factors facilitated the development of vehicle models by shell finite elements since most of the geometry of the structural surfaces was already on computer graphic files.Kee Poong Kim and Hoon Huh emphasize that the crashworthiness of each vehicle part needs to be evaluated at the initial stage of design for good performance of an assembled vehicle. As the dynamic behaviour of structural members is different from the static one, the crashworthiness of the vehicle structures has to be assessed by impact analysis.The paper analyzes the influence of trigger geometry upon the compression of thin-walled cylindrical tubes. Simulations performed on a simple model showed the dependence between triggers area and deformation times as well as the maximum deformations obtained for various speeds at which the simulations ware carried out. Likewise, the geometry of trigger leads to different results.

Springback Prediction of High-strength Steels in Large Radius Air Bending Using Finite Element Modeling Approach

The springback analysis of different types of high-strength steels (Weldox 1100 and Weldox 1300) has been studied using finite element modeling. The two standard types of finite elements have been used: shell and solid elements. All calculations have been implemented in a commercial finite element software, Abaqus. The model for precise springback prediction has been implemented and the accuracy in function of the number of elements through the thickness - has been analyzed. The results of the finite element analysis have been validated experimentally by monitoring the bending process using a camera system that is aligned with the bend line. Deflections of the sheet during and after bending have been measured according to the images recorded by the camera.

<i>In</i>-<i>Situ</i> TEM Study of Hydrogen-Induced Cracking in Carbide-Free Bainitic Steel

Carbide-free bainitic steel is a typical high-strength steel and it is very sensitive to hydrogen embrittlement. Studies have shown that the hydrogen embrittlement in bainitic steels was decreased by the addition of aluminium. However, hydrogen embrittlement still exists in these steels. The in-situ tension in transmission electron microscope (TEM) was used to observe and analyse the effect of hydrogen on the dynamic process of dislocation motion and crack nucleation in carbide-free bainitic steel. A special self-made constant deflection device was used to install the specimen in TEM. Before and after hydrogen charging, the movement of dislocations and microcracks can be clearly observed. The results showed that hydrogen can facilitate the emission, multiplication and motion of dislocations. It also demonstrated that hydrogen can enhance the microcrack nucleation, growth and connection of carbide-free bainitic steel. [doi:10.2320/matertrans.M2012263]

An experimental method for characterizing friction properties of sheet metal under high contact pressure

Contact friction is present on interacting surfaces of steel thin-walled structural members under axial folding during vehicle crash, and often occurs under relatively high contact pressure (∼100MPa) and sliding velocity (∼6m/s). The contact friction could affect structural deformation mode and kinetic energy absorption capacity, and yet has not received sufficient attention in the area of automotive crashworthiness. The lack of knowledge on contact friction behavior and friction data of sheet metal has long affected the reliability and predictability of vehicle crash simulations. The objective of this study is to develop an appropriate experimental method for friction measurement on self-paired sheet metal under high contact pressure conditions. A fixture is designed for applying clamping pressure of 1–100MPa on contact interfaces, under which sliding friction is generated by additional loading devices. Using a standard material test machine and a drop tower, two experimental configurations are set up for measuring the friction properties of steel sheet couples under the pressures of 1–100MPa and the sliding velocities of 0–6m/s. The test method is proved to be valid and viable in friction tests on a typical high-strength steel sheet. The results show distinct friction behaviors of the high steel under different loading conditions, which has also been discussed.

Metallographic study of non-homogenous armour impacted by armour-piercing incendiary ammunition

In this paper, the influence of impact and incendiary effect on the microstructure changes and crack development in non-homogenous add-on armour is studied. The main objective was to determine the parameters that may cause the yielding of the material, leading to a possible decrease in multi-hit resistance. For that purpose, patented wire fence and perforated plates made from two types of high strength steel, Hardox 450 and 50CrV4 tempered at 170 °C were tested. Various techniques were applied: high speed video camera was used, visual macroanalysis, light microscopy, scanning electron microscopy and energy dispersive X-ray analysis. It was found that incendiary effect, being of a brief nature, does not have any influence on microstructure of the add-on armour. However, impact greatly influences crack development and propagation, which occurs in 50CrV4 steel, the material with lower ductility, diminishing its multi-hit resistance. In accordance with obtained results, non-homogenous add-on armour behaviour models were devised, describing the influence of material ductility and geometry on fracture mode, stresses and deformability.

The Effect of Different Surface Pretreatment Methods on Nano-adhesive Application in High Strength Steel and Aluminum Bonding

Epoxy adhesives (single and two components) modified with SiO2 nano-particles were used in this investigation to glue aluminum alloy and also two types of high strength steel (dip-galvanized steel DP 600 and micro-alloyed steel ZStE340). To improve the adhesion between metal surfaces and adhesives, the metal surfaces were pretreated with: a self-indicating pretreatment (SIP*); corundum blasting; corundum blasting + a SIP coating; and a Pyrosil® treatment + SurALink® primer (PG 15 for epoxy adhesive). A single-lap shear tension test, done in accordance to DIN EN 1465, was used to determine the adhesive strength. Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray spectroscopy (EDX) analysis were used to analyze fractures that took place in the samples. The results showed that the adhesion strength of glued samples, containing the nano-particles modified adhesive, had significantly higher strength than unmodified ones. Pretreatment of the metal surfaces affected the adhesion, using nano-adhesives, only slightly. The adhesive strength values for single component epoxy resins were higher than those for two component epoxy resins. It was found that steel samples fractured adhesively at the steel surfaces. Aluminum treated samples indicated after pretreatment an increase in adhesive strength and the fracture occurred adhesively at the aluminum surfaces. Aluminum glued with two-component adhesives and pretreated with corundum blasting plus a SIP coating showed a mixed fracture mode; adhesively at the aluminum surface and cohesively in the adhesive layer.

スカシ張力制御式加工法による<br>高張力鋼板のスプリングバック低減

To enhance automobile weight reduction and to improve collision safety, the use of high-strength steel for press-formed parts has been expanding. However, dimensional accuracy defects occur because the use of a press-formed sheet metal induces springback behavior. In this paper, we propose a new forming method as a solution to this problem, here referred to as tension control (TC) forming. TC forming uses a technique that entails adding tension to a material after bending, it is effective for reducing springback. To implement this technique, the early stage of forming has some clearance between the die and the blankholder, then, the clearance is closed during the midst of forming, and then tension is added to the material during the latter half of the forming process. Experiments were carried out to investigate the effects of what on practical parts of high-strength steel. As a result, the proposed method is found effective for reducing springback in four types of high-strength steel. Furthermore, the effects of clearance and strokes with tension are clarified by elasto-plastic FEM, taking into account the Bauschinger effect.

고장력강의 구멍 확장 실험을 이용한 자동차부품 설계연구

Current need of weight reduction in automotive part increases the application for high strength steel (HSS). The various types of high strength steels have been used to produce chassis part, control arms and trailing arms for weight reduction and increasing of fatigue durability such as dual phase steel (DP) and ferrite bainite steel (FB). But, DP and FB steels have proven to show inferiority in durability as well as press formability. Edge cracking occurred often in flange forming and hole expansion processes is the major failure encountered. This paper discussed the behavior of edge stretchability of high strength steel of DP and FB steels. Experimental works have been conducted to study the effect of punch clearance and burr direction on hole expansion ratio (HER). Also finite element simulation (FEM) has been preformed to clarify the mechanism of flange crack and support the experimental results on HER of DP and FB steels. It was simulated the whole process of blanking process following by hole expansion process and ductile fracture criterion named the modified Cockcroft-Latham model which was used to capture the fracture initiation. From the hole expansion tests and FEM simulation studies it was concluded that ferrite bainite steel showed better stretch-flangeability than dual phase steel. It was attributed to the lower work hardening rate of ferrite bainite steel than dual phase steel at the sheared edge.

핫스탬핑에 의한 고강도 차체 부품 개발

Quenchable boron steel is a new type of high strength steel to reduce the weight of automobiles and maintain the safety conditions. Quenchable blanks can be hot-stamped and hardened in a water-cooled tool to achieve high strength. In this paper, new alloy for hot stamping is designed based on requirement of mechanical properties and two types of surface coating are investigated in viewpoints of oxidization and exfoliation. An automotive part of center pillar is manufactured by hot-stamping using Al-Si coated sheet. The performance of developed part is compared by static compression test and side impact crash test.

Modeling of reverse loading effects including workhardening stagnation and early re-plastification

This paper deals with the Bauschinger effect in sheet metal forming simulation. The concept of a new material model is presented able to accurately handle the phenomena of early re-plastification, transient softening and workhardening stagnation at load reversal. Besides the standard data used for simulations based on isotropic hardening, the model requires only four additional, physically comprehensible material parameters. The model is verified by means of tension-compression tests and springback simulations for hat profiles for three typical high-strength steel alloys.

７８０ＭＰａ級Ｄｕａｌ Ｐｈａｓｅ鋼の静的，動的強度特性に及ぼす炭素と珪素の影響

Various types of high strength steel have been developed to improve the impact safety and reduce the weight of cars. For the steels used in anti-crash equipments, mechanical properties, especially, at high strain rates such as 103/s is of importance. In this study, 0.110%C–1.44%Si–1.29Mn–0.65%Cr–0.29%Mo containing hot rolled dual phase steel with 780 MPa grade in tensile strength was employed as a base steel and the effects of carbon and silicon on static (strain rate: 10−3/s)/ dynamic (strain rate: 103/s) mechanical properties of the dual phase steel were investigated. Carbon and silicon contents were changed in a range of 0.076–0.190% and 1.44–2.39%, respectively. Grain size of the steels was varied by hot rolling reduction: 53% (named coarse grain process), 73% (middle grain process) and 88% (fine grain process). Dynamic absorbed energy up to 10% tensile strain had a linear relationship with tensile strength, regardless of microstructures, i.e., neglecting carbon and silicon contents, and hot rolling conditions. All absorbed energy to fracture had a close relationship with tensile strength–ductile balance parameter (tensile strength×total elongation), reflecting microstructural change through chemical and rolling conditions. All the processed 0.190% C steels, and the fine grain processed 1.93% Si and 2.39% Si steels showed the highest all absorbed energy of all the steels tested. The 0.190% C steel was characterized by almost 100% martensite with some content of retained austenite, and the 1.93% Si and 2.39% Si steels were fine grained ferrite+martensite. It was found that carbon improves all absorbed energy through increase in volume fraction of martensite and silicon raises it through solid solution hardening of ferrite matrix.

高張力鋼の低温割れを防止する新低変態温度溶接材料の開発

There has been growing interest in high strength steel with a tensile strength higher than 800 MPa, which would be ideal for weight reduction and better performance of steel structures. However, there are two major problems in the application of high strength steel to structures. One is that the fatigue strength of welded joints is far lower than that of the base metal, and the other is that the weld metal and the heat-affected zone (HAZ) are likely to have cold cracking. These problems must be solved before high strength steel can be widely used in steel structures. As a solution to the problem of cold cracking, a reduced preheating type of high strength steel has recently been developed in which cold cracking in the HAZ is reduced by reduction of alloy components (lower Ceq, Pcm) without loss of strength. However, since the resistance to cold cracking of the weld metal has not been improved, preheating of the weld metal is still necessary, and the problem remains toward its wider use. Recently, Ohta et al. developed a low transformation-temperature welding consumable, in which compressive residual stress is induced by transformation expansion of martensitic transformation of weld metal at a low temperature near room temperature, and demonstrated that this consumable improves the fatigue strength of welded joints. We considered that the reduction of tensile residual stress in the low transformation-temperature welding consumable might also be effective in decreasing the cold cracking and that this consumable would be useful in manufacturing high performance welded joints with improved fatigue strength and cold-cracking resistance. In the present study, we examined the effects of low temperature-transformation weld materials on the prevention of cold cracking in high strength steel. Firstly, the y-shaped weld cracking tests demonstrated that reduction of residual tensile stress induced by martensitic transformation expansion of weld metal was effective in reducing cold cracking. Secondly, the effects of the degree of joint constraint on cold cracking in low transformation-temperature weld materials were examined by the y-shaped and H-shaped weld cracking tests. The cracking ratio was high at low degrees of joint constraint and low at high degrees of joint constraint. Two causes were considered: one was that the reduction of tensile residual stress by transformation expansion is higher at a higher degree of joint constraint, and the other was that the weld metal of martensitic structure alone is sensitive to cold cracking. According to the above investigation, we attempted to develop a weld material with high resistance to cold cracking at different degrees of constraint. To maintain the effects of reduction of tensile residual stress by transformation expansion to reduce the amount of diffusible hydrogen and the sensitivity of cracking, which are other causes of cracking, we designed and developed a 2-microstructure phase weld metal (martensite + retained austenite) by modifying the low temperature-transformation weld material to obtain a lower Ms point. We examined the effects of the degree of joint constraint on cold cracking, and confirmed that the cracking rate of the modified rod was almost 0% at all degrees of constraint, and its resistance to cold cracking was high.

Impact of thin-walled high-strength steel structural sections

An experimental programme is reported which examines the progressive collapse behaviour of some thin-walled closed-section structural sections made from high-strength steels under quasi- static and impact axial loads. A comparison is made with theoretical formulae, which have been used successfully for predicting the behaviour of mild steel thin-walled structures. Ten quasi-static crush tests and 46 impact tests were conducted on spot-welded top-hat and laser- welded square sections. The thin-walled sections were made from two different types of high-strength steel and one mild steel, the mechanical properties of which were determined experimentally from quasi-static and dynamic tensile tests. Although no specific change in the collapse mode was observed, the limited weight-specific energy absorption e ciency of the high-strength steels under dynamic loadings hinders the weight-reduction potential of crashworthy designs. An unexpected difference in the structural effectiveness of spot-welded top-hat sections made from mild steels and high-strength steels was identified, but was not present for similar square sections. This difference further accentuated the loss in the advantage of high-strength steels over mild steels, especially for spot-welded top-hat sections, and led to differences in the agreement between the experimental results and the theoretical predictions.

Intergranular Cracking in Hydrogen-Charged High Strength Steel.

The fracture behavior of high strength steel under the influence of hydrogen was studied, with an emphasis on the conditions for the formation of intergranular (IG) cracks. Mechanical tests were carried out using two types of high strength steels (Series A and B) which had been heat-treated to obtain the same level of yield stress bus different grain boundary properties. The specimens were cathodically charged with hydrogen under constant stress conditions. Experimental analyses revealed that a high stress concentration at a fatigue pre-crack tip is required for generation of IG cracks. The morphologies of the fracture surfaces were intergranular (IG) and transgranular (TG) modes, respectively, in the two types of specimen, and the differences in morphology were found to result from differences in the amount of impurities segregated at the grain boundaries of the specimens.

Effect of Weld Parameters on Residual Stress Distribution in Flux Cored Arc Welding of High Strength Low Alloy Steel

Various types of high strength steels are increasingly used for producing welded structures with reduced weight and improved performance. Welding problems related to residual stress and distortion can arise, since yield stress of these steels are high, there is always a possibility of producing very high residual stresses in some locations near the weld. In general residual stresses become dangerous if they lead to local multi axiallity of tension stresses, which favour brittle fracture, knowledge of the residual- stress distribution in welds is necessary in order to assess the risk of crack growth in service The distribution of residual stresses after welding depends strongly on the weld parameters Attempts were made to study the influence of weld parameters (arc voltage, travel speed, welding current) on residual stresses pattern of high strength low alloy steel weldments This study was mainly performed by using X-ray stress analyzer.

Decomposition Process of Martensite-Austenite Phase in a Heat-Affected Zone of a High Strength Steel

Abstract A systematic study has been made of the decomposition of the martensite-austenite phase (M-A phase) formed in the heat-affected zone (HAZ) of a 800 MPa type of high strength steel. A heat-treatment to improve the toughness of HAZ was also proposed as a result of this study.Electron metallography indicated that the M-A phase decomposed by the precipitation of carbides even at temperatures of the order of 300°C. The activation energies for the decompositions of martensite and austenite have been determined to be 71.6 and 13.8 kJ/mol, respectively. The decomposition products of the M-A phase appear to be tempered martensite and hypereutectoid upper bainite.A drastic improvement in the ductility and toughness of the HAZ was obtained by annealing at temperatures ranging from 175 to 300°C, which can be attributed to the decomposition of martensite. A lower annealing temperature is preferable in order to retain the presence of the austenite constituent which improves the toughness.

Decomposition Process of Martensite-Austenite Phase in a Heat-Affected Zone of a High Strength Steel

A systematic study has been made of the decomposition of the martensite-austenite phase (M-A phase) formed in the heat-affected zone (HAZ) of a 800 MPa type of high strength steel. A heat-treatment to improve the toughness of HAZ was also proposed as a result of this study.Electron metallography indicated that the M-A phase decomposed by the precipitation of carbides even at temperatures of the order of 300°C. The activation energies for the decompositions of martensite and austenite have been determined to be 71.6 and 13.8 kJ/mol, respectively. The decomposition products of the M-A phase appear to be tempered martensite and hypereutectoid upper bainite.A drastic improvement in the ductility and toughness of the HAZ was obtained by annealing at temperatures ranging from 175 to 300°C, which can be attributed to the decomposition of martensite. A lower annealing temperature is preferable in order to retain the presence of the austenite constituent which improves the toughness.