High-strain deformation of dual-phase steel

Abstract

A commercial type dual-phase steel has been heat treated to develop a conventional dual-phase structure and, by a double-quench heat treatment, a dual-phase structure with a small martensite island size. These specially heat treated materials together with the normalized material have been plastically deformed by rolling to a reduction of 98% (ε_t = 4·0). The tensile properties have been determined after deformation and correlated with the microstructure. It has been found that within the strain range ε_t = 0·5–1·5 the work hardening modulus is similar to that of pure iron. Over a narrow strain range little work hardening occurs but within the range ε_t = 2·5–4·0 the work hardening modulus is greater than that of ferrite. The increase in modulus seems to be associated with the plastic deformation of the martensite islands which, at the highest strains, give a fibre reinforcing effect. The results are discussed in relation to the work hardening mechanisms involved. It is concluded that changes in the ferrite grain size, established during the development of deformation bands at lower strains and subsequently deformed at higher strains, greatly influence the flow stress through a Hall-Petch relationship.

MST/241     

热轧双相钢的发展现状及高强热轧双相钢的开发

介绍了热轧双相钢的发展现状及存在问题,指出低成本热轧双相钢、高延伸凸缘型铁素体+贝氏体热轧双相钢(F-B热轧双相钢)及高强度热轧双相钢的开发及应用,将促进我国热轧双相钢的发展,推动汽车工业的"以热代冷"进程。同时,探讨了纳米析出强化型热轧双相钢的强化机理及工艺控制原理,并在实验室进行了中试,开发出铁素体基体析出强化型的热轧双相钢,其抗拉强度达770~830 MPa,屈强比0.75,组织为铁素体+马氏体,且铁素体基体中存在大量细小的纳米级尺寸的TiC过饱和析出和相间析出。     

On the nucleation and growth of voids at high strain-rates

The nucleation and growth of voids at high strain-rate is studied in copper as a model face centered cubic (fcc) material using large scale molecular dynamics (MD) methods. After a brief introduction to dynamic fracture, results are presented for the homogeneous nucleation of voids in single crystal copper and the heterogeneous nucleation in nanoscale polycrystalline copper. The simulations suggest void growth occurs through anisotropic dislocation nucleation and emission in agreement with experiment and the observed anisotropy of the tensile flow stress in fcc crystals. A phenomenological model for the transition from intergranular to transgranular fracture at high strain-rate is presented. This revised version was published online in July 2006 with corrections to the Cover Date.     

Austenite formation in manganese-partitioning dual-phase steel

Abstract

Manganese-containing ferritic–pearlitic steels have been studied after intercritical annealing for various times at temperatures of 700 and 725°C, during which austenite formation occurred at ferrite grain boundaries. Light and electron optical microscopy, and scanning transmission electron microanalysis and microdiffraction were employed for the study, in which both microstructural development and manganese redistribution were examined in detail. It was observed that the formation of austenite is associated initially with a migration of ferrite grain boundaries. It is proposed that this boundary migration is induced by manganese diffusion along the boundaries, and that this in turn provides the mechanism for rapidly transporting manganese to the growing austenite. On this basis, we find that austenite formation in these steels is associated with manganese-rich migrated ferrite boundaries, and is not dependent on the presence of cementite particles for nucleation.

MST/467     

Formation of layer-like voids in irradiated polytrioxane

The formation of voids and defects is investigated by X-ray diffraction methods, by weight loss measurement and by electron microscopy, for an irradiated polytrioxane (PTOX) obtained by the-ray-induced polymerization of trioxane in the solid state. The main reasons for the formation of the layer-like voids are transverse decomposition at a selective point accessible to the radiation in the micro-fibrils, and the subsequent extensible depolymerization which may occur from the unstable chain end with increase of the radiation dose. The analysis of the SAXS curve for the irradiated PTOX, according to Tsvankin's method gives reasonable results for the void structure, where the reduction of the sub-crystal fraction and also the weight loss of the sample by irradiation are taken into account.     

Microstructure and fatigue crack growth behavior in tungsten inert gas welded DP780 dual-phase steel

The purpose of this study was to evaluate microstructural and mechanical change of DP780 steel after tungsten inert gas (TIG) welding and the influence of notch locations on the fatigue crack growth (FCG) behavior. The tempering of martensite in the sub-critical heat affected zone (HAZ) resulted in a lower hardness (~ 220 HV) compared to the base material (~ 270 HV), failure was found to originate in the soft HAZ during tensile test. The fusion zone (FZ) consisted of martensite and some acicular ferrite. The joint showed a superior tensile strength with a joint efficiency of 94.6%. The crack growth path of HAZ gradually deviated towards BM due to the asymmetrical plastic zone at the crack tip. The FCG rate of the crack transverse to the weld was fluctuant. The Paris model can describe the FCG rate of homogeneous material rather well, but it cannot precisely represent the FCG rate of heterogeneous material. The fatigue fracture surface showed that the stable expanding region was mainly characterized by typical fatigue striations in conjunction with secondary cracks; the rapid expanding region contained quasi-cleavage morphology and dimples. However, ductile fracture mechanism predominated with an increasing stress intensity factor range (ΔK). The final unstable failure fractograph was subtotal dimples.     

Hydrogen effects in a dual-phase microalloy steel

A dual-phase steel containing niobium, vanadium and titanium as microalloying elements was tested for hydrogen embrittlement (HE). The susceptibility to HE was observed to be closely related to the microstructural state. Hydrogenated specimens intercritically annealed at relatively low temperatures to develop martensite islands in a ferrite matrix basically exhibited quasi-cleavage fracture with some ductile dimpling. The mode of fracture in charged specimens quenched from higher intercritical annealing temperatures was predominantly intergranular fracture along prior austenite grain boundaries and cracking of martensite laths. The detrimental role of residual stresses, retained austenite and microalloying carbides in the process of HE is discussed.     

Simulation of ductile crack propagation in dual-phase steel

The modified Mohr–Coulomb and the extended Cockcroft–Latham fracture criteria are used in explicit finite-element (FE) simulations of ductile crack propagation in a dual-phase steel sheet. The sheet is discretized using tri-linear solid elements and the element erosion technique is used to model the crack propagation. The numerical results are compared to quasi-static experiments conducted with five types of specimens (uniaxial tension, plane-strain tension, in-plane shear, 45° and 90° modified Arcan) made from a 2 mm thick sheet of the dual-phase steel Docol 600DL. The rate-dependent J ₂ flow theory with isotropic hardening was used in the simulations. The predicted crack paths and the force–displacement curves were quite similar in the simulations with the different fracture criteria. Except for the 45° modified Arcan test, the predicted crack paths were in good agreement with the experimental findings. The effect of using a high-exponent yield function in the prediction of the crack path was also investigated, and it was found that this improved the crack path prediction for the 45° modified Arcan test. In simulations carried out on FE models with a denser spatial discretization, the prediction of localized necking and crack propagation was in better accordance with the experimental observations. In four out of five specimen geometries, a through-thickness shear fracture was observed in the experiments. By introducing strain softening in the material model and applying a dense spatial discretization, the slant fracture mode was captured in the numerical models. This did not give a significant change in the global behaviour as represented by the force–displacement curves.     

Manganese partitioning and dual-phase characteristics in a microalloyed steel

A two-stage intercritical annealing treatment has been attempted to develop dual-phase structure and properties in a commercial grade microalloyed steel containing Nb and V. Manganese concentration in austenite enhanced the hardenability which attributes to martensite formation on air cooling after annealing. The volume fraction of martensite and the manganese distribution were found to depend on both annealing time and temperature. Different morphologies of martensite were identified for various intercritical annealing conditions, e.g. dispersed type, ring type, acicular type. The tensile data and behaviour confirmed the dual-phase characteristics in the microalloyed steel.     

The extension of cracks at high temperature by growth and coalescence of voids

The problem of creep crack growth is studied analytically in this paper. The analysis assumes a sufficiently slow crack growth rate so that elastic effects can be neglected and hence the near tip stress field is well approximated by the C*(HRR) field. The crack is assumed to advance continuously due to the growth and coalescence of voids directly ahead of the crack tip. An exact solution for the entire growth historya(a), 0 a < , is obtained as a function of crack extensiona andC* for a wide variety of void growth mechanisms. The solution shows that, for a finite nucleation zone with sizeR, the steady state crack extension ratea _ss =a(a ) is attained exponentially fast with characteristic distanceR. Another consequence of the solution is that is proportional toC*. An important result is that, for crack extensiona less thanR, the transient crack growth ratea(a) can be approximated by a closed form asymptotic series.

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Résumé On étudie par voie analytique le problème de la croissance des fissures de fluage. On suppose dans l'analyse une vitesse de croissance de fissure lente assez pour que les effets élastiques puissent être négligés, et que le champ de contrainte au voisinage de l'extrémité de la fissure est rendue avec une approximation suffisante par un champC* (HRR). On suppose que la fissure avance continûment vu une croissance et une coalescence de lacunes immédiatement en avant de l'extrémité de la fissure. On obtient une solution pour l'entièreté de l'histoire de la croissancea(a) ou 0 a en fonction de l'étendue de la fissurea et deC*, pour une large variété de mécanismes de croissance. La solution montre que, dans une zone finie de nucléation de dimensionR, un taux de croissance de fissure stabiliséa _ss =a(a ) est atteint à une vitesse exponentielle lorsqueR atteint une valeur caractéristique.Une autre conséquence de la solution est quea _ss est proportionnel àC*. Un résultat important est que, lorsque la largeur de la fissure est inférieure àR, la vitesse de croissance transitoire de la fissurea(a) peut être obtenue avec une approximation suffisante par une série asymptotique de forme fermée.

     

High-efficiency fast-heating annealing of a cold-rolled dual-phase steel

In the present work, fast-heating annealing was performed on a cold-rolled Fe–0.07C–1.7Mn–0.429Si dual-phase steel. In contrast to commercial conventional continuous annealed steel, 6.6% higher ultimate tensile strength and 14.1% greater elongation were obtained. Transmission electron microscopy observations reveal the incomplete ferrite recrystallization, the formation of bainite and fine fiber-like martensite in fast-heating processed dual-phase steel. High-efficiency simplified fast-heating annealing process demonstrates great potential for large-scale production.     

Fatigue strength of high strength dual-phase steel sheet

Fatigue tests have been carried out on lean-alloyed dual-phase steels with tensile strengths ranging from 300–800 MPa. Smooth specimens and specimens with punched holes were tested. The fatigue strength of dual-phase steel was found to be similar to that of other types of steel (eg solution hardened or microalloyed steels) of equal tensile strength. The fatigue strength increases with increasing yield strength. For notched specimens it is also related to the yield ratio. Work and bake hardening increase the fatigue strength of smooth specimens in proportion to the increase in yield strength. For notched specimens this effect is less and is dependent on the yield ratio. Bake hardening of material which was not work hardened also increased the fatigue strength. The notch sensitivity of low yield ratio dual-phase steel is found to be low. The notch sensitivity seems to increase with increasing yield ratio.     

Simulation of hot-rolled dual-phase weathering steel 09CuPCrNi

The continuous cooling transformation diagram of deformed austenite for steel 09CuPCrNi was constructed by means of a combined method of dilatometry and metallography. The diagram exhibits an elongated polygonal ferrite C-curve with a delayed pearlite start and a metastable austenite gap between the polygonal ferrite and pearlite regions and between the ferrite and bainite regions. For this experimental steel, it is possible to obtain a dual-phase microstructure directly by hot rolling and appropriate cooling. Based on the diagram, the technical process of a hot-rolled dual-phase treatment was established and simulated using a thermal simulation testing machine. Dual-phase microstructures were obtained that show some bainite phase and are characterized by an irregular distribution of island-shaped martensite in a matrix of equiaxed ferrite grains. The morphology of the martensite phase is essentially of the lath type, with small areas of micro-twins appearing.