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A proposed alternative method for processing dual phase (DP) steels is the utilization of a cold-rolled duplex ferrite-martensite structure, followed by intercritical annealing. The obtained results demonstrate the feasibility of this approach in producing a desirable microstructure. The microstructure consists of ferrite grains with an average size of less than 6 μm, along with a uniformly distributed fine martensitic phase. Treatment of the steels using this alternative route results in improved strength-ductility properties and a greater work-hardening exponent. These steels exhibit an excellent combination of mechanical properties with tensile strength of 866.69 MPa, total elongation of 24.40%, and the product of tensile strength and total elongation of exceed 20 GPa%.

The aim of this research was to develop a medium Ni–Mn dual phase steel with a combination of relatively high strength and good formability for industrial applications. Accordingly, this research was conducted to examine the effects of intercritical annealing and subsequent ageing treatments on the microstructure and mechanical properties of an Fe-9.5Ni-6.5Mn (wt.%) low carbon martensitic steel. Electron back scattering diffraction (EBSD) analysis revealed that intercritical annealing treatment at 610 °C in the ferrite + austenite (α+γ) dual phase region for various holding times led to formation of the reversed austenite phase in the martensite matrix, producing a medium Ni–Mn dual phase steel. It was resulted that the volume fraction of austenite and its grain size were significantly increased with increasing the holding time. Also, high resolution transmission electron microscope (HRTEM) images illustrated the formation of nano-sized θ-NiMn precipitates within the martensite phase after subsequent ageing at 450 °C for 7.2 ks. Intercritical annealing followed by ageing yielded a good combination of an ultimate tensile strength (∼1160 MPa) and fracture strain (∼11.5%) in the developed dual phase steel compared to the initial martensitic steel. The high strength and good ductility of the developed dual phase steel were respectively attributed to the presence of θ-NiMn precipitates with a hard character and reversely formed austenite with a ductile nature in the microstructure. Furthermore, the fully brittle fracture mode in the aged initial steel changed to a mixture of brittle and ductile behavior in the dual phase steel produced by intercritical annealing and subsequent ageing.

Dissimilarities in metal laser welding lead to brittleness in welded joints due to differences in the thermophysical and chemical properties between dissimilar base materials. To overcome such brittleness, the addition of a preset coating onto the base materials as an interlayer is a method for attaining reliable welded joints. Nd:YAG laser butt welding of DP590 dual-phase steel and 304 stainless, both with a thickness of 1 mm, was performed with a preset nickel coating as an interlayer using an electroplating process. The relationship between the microstructure and the mechanical properties of the welded joints was researched, the microstructure and composition of the weldment were analyzed, and the microhardness, tensile strength and corrosion resistance were tested. The results showed that the preset nickel coating increased the content of Ni element in the welded joints, which is beneficial to the formation of lath martensite. The average hardness of the welded joints increased by 12%, and the tensile strength was higher than 370 MPa. The corrosion rate of the welded joints can be slowed down, and the corrosion resistance can be improved by increasing the nickel coating. [ABSTRACT FROM AUTHOR]

The present study aims at to study the effect of chemical composition, intercritical annealing parameters (temperature and holding time) on microstructure and mechanical properties of dual phase steels. Three normalized steels (A, B and C) with different compositions were subjected to inter-critical annealing at 760°C and 810°C for different holding times (3, 6 and 9 min). The microstructural characterization was carried out using a scanning electron microscope (SEM) and mechanical properties were evaluated using micro-hardness measurements and tensile tests. The strength has been found to increase from 500 MPa (steel A) to >1200 MPa (steel C) at the cost of a decrease in elongation from 25% (steel A) to 7.5% (steel C). Results have been analysed and discussed. [ABSTRACT FROM AUTHOR]

• Galvanized DP980 steel and 2 GPa press hardened steel were joined. • Characteristics of laser beam welding and cold metal transfer welding joints were analyzed. • The soft weld metal of CMT joint with acicular ferrite improved the plasticity of the joint. Galvanized DP980 steel and Al-Si coated 2 GPa press-hardened steel (PHS) were joined via laser beam welding (LBW) and cold metal transfer (CMT) welding, respectively. The weld metal (WM) of LBW joints contains martensite, acicular ferrite, and bainite. The WM of the CMT joint consists entirely of acicular ferrite. The tensile strength of the LBW joint is 895 ± 5 MPa, slightly higher than that of the CMT joint by 50 MPa due to the presence of martensite. The total elongation is 4.1 ± 0.2 % and 5.0 ± 0.3 % for the LBW and CMT joints, respectively. The enhanced plasticity of the CMT joint was due to the soft WM plastic deformation, which increased the total deformation. [ABSTRACT FROM AUTHOR]

This study uses direct quenching (DQ) heat treatment at an intercritical annealing temperature (IAT) of 800 °C to form a martensite-ferrite dual phase microstructure of X70 steel. The effects of tempering temperatures ranging from 200 to 500 °C on tensile properties in a dual-phase X70 steel are investigated. Carbon diffusion and redistribution in the microstructure are influenced by tempering. It was discovered that the amount of carbides increases with the tempered temperature, resulting in depleted carbon in martensite. Conversely, increasing the tempered temperature causes a decrease in ultimate tensile strength and yield strength while increasing elongation. [ABSTRACT FROM AUTHOR]

Dual phase steels are materials whose microstructure is composed of a ferrite matrix with martensite islands. Ferrite provides excellent ductility, while martensite increases the strength of steel, this provides a special appeal in the automotive industry. The main objective of this research is to obtain dual phase steels from AISI 8620 steel with a high Mn content, performing heat treatments in the intercritical range to obtain martensite percentages of 27, 33, 41, and 48%, respectively. Microstructural characterization is performed using optical microscopy and scanning electron microscopy techniques, the mechanical characterization is carried out using hardness, tension and charpy impact tests. The highest mechanical resistance is achieved in steel with 41% martensite phase, while the highest ductility is given for the material with 27% martensite, a fractographic analysis of all materials allowed to determine that the type of fracture presented is ductile. When the martensite fraction increases, the impact energy exhibits a decreasing behavior, while the hardness behaves in an increasing way.

Dual phase steels are materials whose microstructure is composed of a ferrite matrix with martensite islands. Ferrite provides excellent ductility, while martensite increases the strength of steel, which provides a special appeal in the automotive industry. The main objective of this research is to obtain dual phase steels from AISI 8620 steel with a high Mn content, using heat treatments in the intercritical range to obtain approximate percentages of martensite of 27, 33, 41, and 48%. Microstructural characterization is performed using optical microscopy and scanning electron microscopy techniques. The mechanical characterization is carried out using hardness, tension and charpy impact tests. The highest mechanical resistance is achieved in steel with 41% martensite phase, while the highest ductility was obtained by the material with 27% martensite. A fractographic analysis of all materials allowed determining that the type of fracture presented is ductile. When the martensite fraction increases, the impact energy exhibits a decreasing behavior, while the hardness behaves in an increasing way. [ABSTRACT FROM AUTHOR]

The ferrite–bainite microstructures and mechanical properties in a hot-rolled dual phase steel with different transformation temperatures were investigated in this paper. The deformation behavior of bainite and ferrite was analyzed by using in situ scanning electron microscope. The results showed that the ferrite and bainite can be refined and the volume fraction of bainite can be slightly increased by 5% by decreasing the ferrite transformation temperature from 690 to 635 °C and increasing the bainite transformation temperature from 400 to 450 °C. The strengthening mechanism was dislocation difference in polygonal and acicular ferrite and then the grain size difference of ferrite. The hole expansion ratio increased from 52.9 to 83.7% with the ferrite transformation temperature decreasing. The cracks or voids were smaller in size and also the number in steel with the lower ferrite transformation temperature after expanding. The microstructure separation along the boundary between bainite and ferrite and the slip band in the ferrite appeared at the non-uniform deformation stage. However, fewer appeared in the microstructure of steel with lower ferrite transformation temperature. Cooperative deformation of ferrite and bainite and finer microstructure in steel with lower ferrite transformation temperature were the mechanisms why the cracks could not easily nucleated and propagated, respectively. Thus this was also why there was a better hole expansion ratio. The ferrite–bainite microstructures and mechanical properties in a hot-rolled dual phase steel with different transformation temperatures were investigated. The deformation behavior of bainite and ferrite was analyzed by using in situ scanning electron microscope. The strengthening mechanism was dislocation difference in polygonal and acicular ferrite and then the grain size difference of ferrite. The hole expansion ratio increased from 52.9 to 83.7%. Cooperative deformation of ferrite and bainite and finer microstructure in steel with lower ferrite transformation temperature were the mechanisms why the cracks could not easily nucleated and propagated, respectively. Thus this was also why there was a better hole expansion ratio. [ABSTRACT FROM AUTHOR]

In this study, DP980 steel sheets were laser welded with a laser power of 4.5 kW and a welding speed of 4.5 m min ^−1 . The microstructural evolution and mechanical properties of the welded joint and the effect of notch location on tear toughness were evaluated. Results show that the fusion zone (FZ) was composed of lath martensite, the hardness (276 HV) of the heat-affected zone (HAZ) was lower than that of the base metal (305 HV) resulting from martensite tempering. The welded specimens failed at the soft HAZ with a 98.4% joint efficiency during the tensile test, and the ultimate tensile strength of the as-recieved steel and welded joint was 1026 MPa and 1010 MPa, respectively. The tear energy of the FZ and HAZ was lower than DP980 base metal (BM). Thus, it is considered that the fracture toughness of the joint decreased after welding. The crack growth path of the FZ gradually deviated toward the HAZ during tearing due to the asymmetrical plastic zone at the crack tip. Compared with the ductile fracture of the base metal, the significant decrease in the fracture toughness of the welded joint is due to the weak deformation resistance of tempered martensite.

Herein, a new kind of overaging strategy: two steps of overaging for tailor rolled blank of dual phase steel (DP-TRB) was investigated. The results indicate that two steps of the overaging process is a useful way to control the mechanical properties of DP-TRB. In the premise of satisfying the requirement for the strength of DP590 grade, the total elongation can be significantly increased (3~7% in most cases). Due to the different degrees of ferrite recrystallization (differences of densities of dislocation) among the thicknesses, the obvious changes of mechanical properties among thicknesses are found. The thicknesses zones of 1.0~1.4 mm show lower strength, while the zones of 1.6~1.8 mm present higher strength. Otherwise, the high density of dislocations in samples of 1.6~1.8 mm provide more locations for Cottrell atmospheres, yield plateau occur easier. The zones with different thicknesses for one DP-TRB show two kinds of yield behaviors (continuous yield and non-continuous yield) simultaneously. The subtle C diffusion control by two step overaging leads to the quite different pinning effect of Cottrell atmospheres. Thus, the pinning effect occurs in a gradual way, and a transition state of yield behavior, which combines the characteristic of smooth curve in continuous yield and the plateau in non-continuous yield, is found.

Abstract In the presented investigation, cold-rolled sheets of a selected dual-phase (DP) steel were heat-treated according to varying thermal profiles, thus reproducing continuous annealing process. Initially the samples were soaked at 780 and 810 °C for 0–60 s followed by water cooling. Next, samples were preliminary treated by applying the same conditions, however after water cooling these compositions were subject to tempering at 230, 380 and 460 °C for both 60 and 240 s. The characterization of the effect of heat-treatment parameters on the mechanical properties and structure is the main objective of this investigation. Mechanical properties of the samples after applied thermal profiles were in line with those requirements imposed on the commercial sheets of DP steels. The obtained results of the investigation showed that tempering deteriorates the Yield Ratio, defined as R p0.2 /R m. This was caused by the martensite decomposition combined with carbide precipitation processes. Transmission electron microscopy observations revealed precipitated carbides, Fe 3 C in martensitic and M 7 C 3 in ferritic areas. The quantitative results of the structural investigation were then applied to predict the R p0.2 and R m using the Perlade model. The results indicate that discrepancies between the measured tensile tests and calculated R p0.2 and R m do not exceed 10%. [ABSTRACT FROM AUTHOR]

Abstract A 0.1C5Mn3Al steel with laminated ferrite/martensite and layered ferrite/martensite microstructures was processed by hot-rolling, cold forging and subsequent annealing. Tensile tests revealed that hot rolled sample (laminated) and cold forged + 650 °C annealed sample (layered) showed similar tensile strengths but different elongations. It is confirmed that laminated structure is good for retaining elongation. Furthermore, nanoindentation tests were performed at locations at or near interfaces and in the ferrite or martensite matrix, revealing that the nanohardness difference across the interface is important in affecting the elongation of the material. The underlying mechanisms were discussed with regarding to the dislocation-interface interactions. Graphical abstract fx1 [ABSTRACT FROM AUTHOR]

The Hopkinson experiment system was used to do the dynamic tensile experiment of 800 MPa grade cold rolled dual phase steel (DP800). The strain rate was determined as 500, 1000 and 2250 s-1 By comparing the experimental results, both the yield strength (Rp0.2) and the tensile strength (Rm) of the dual phase steel increase with strain rate in the exponential form. The plastic deformation at high strain rate leads to adiabatic temperature rise effect. The adiabatic temperature rise is 89℃ at the 2250 s-1 strain rate. Based on the J-C (Johnson-Cook) model and Z-A (Zerilli-Armstrong) model, the constitutive model of the dual phase steel was researched. The quadratic polynomial of strain rate effect of the J-C model was modified. The average coefficient of determination increases from 0. 9228 to 0. 9886 by modifying the J-C model.

Continuous cooling transformation curves were established and the influence of finish rolling temperature on the microstructure was investigated for a titanium-microalloyed dual phase steel through simulation experiment. A feasible process based on simulation experiment was obtained and confirmatory experiment was performed. It was found that pro-eutectoid ferrite could form in the steel when the cooling rate was less than 5℃·s-1 at a temperature of 625 to 725℃. The amount of ferrite and martensite increased firstly and then decreased slightly with the increasing of finishing rolling temperature. A higher hardness of ferrite was obtained at a higher finish rolling temperature because of precipitation strengthening. Better performance with a tensile strength of 672 MPa and a yield ratio of 0.61 was obtained for the steel when the finish rolling and slow cooling temperatures were 840℃ and 700℃,respectively. The amount of this precipitation strengthening was about 78.5 MPa based on calculations.

Abstract The mechanical properties in tailor rolled blank of dual phase steel (DP-TRB) at different thickness zones present significant differences after annealing. In this paper, the differences in mechanical property of the tailor rolled blank were investigated. The work hardening behavior is analyzed by C-J analysis method. The results indicate that the microstructure of DP-TRB is composed of ferrite and martensite at 800–820 °C. It is interesting that the martensite islands gradually chang to equiaxed morphology from rod-like shape at annealing temperature of 800 °C. In addition, the differences in mechanical properties between different thicknesses zones become less with the increased intercritical temperature from 760 °C to 820 °C. It indicates that the difference in yield strength decreases from 230 MPa to 60 MPa with the intercritical temperature changing from 760 °C to 820 °C. Ferrite recrystallization or not is the main reason for the differential properties. The critical thickness for recrystallization is same ~ 1.6 mm, and the 1.6 mm zones is experimentally proved to possess the largest strength at 760 °C, 800 °C, and 820 °C. The samples in thinner zones are softened because of the recrystallization, while the samples in the larger thickness zone are not strong enough as 1.6 mm with less work-hardening. After the yield point, there is an extra work hardening stage in 1.6 mm and 1.8 mm samples relative to 1.0–1.4 mm samples, which can eliminate nonuniform deformation and prepare for the followed uniform deformation. Consequently, turning of this feature of properties differentiation rationally would play an important role in application of DP-TRB. [ABSTRACT FROM AUTHOR]

The effect of dew point on the selective oxidation behavior, microstructure and mechanical properties of a high-Al low-Si dual phase steel is elucidated. The results showed that as dew point is increased from-30°C to +5°C, internal oxidation of alloying elements occurs, and consequently the mass of external oxides decreases, which improves the galvanisability of steel sheets. It was also observed that the dew point had an impact on the microstructure and mechanical properties of the experimental steel. With the increase of dew point, the decarburization layer on the surface was increased, leading to inferior mechanical properties. Compared with the industrial steel, the high-Al-low-Si DP steel exhibited excellent galvanisability under identical annealing conditions. [ABSTRACT FROM AUTHOR]

In the present study, an as-cast strip of dual phase steel (Fe-0.16C-0.3Si-0.81Mn-0.03Ti wt%) was successfully produced by twin-roll strip casting (TRSC) process. The as-cast strip was subjected to two different processing routes. The microstructure of the as-cast strip, cold-rolled sheet and annealed sheet was studied and related to mechanical properties of the annealed sheet. It was observed that the as-cast strip microstructure consisted of grain boundary ferrite, Widmanstatten ferrite, pearlite and intragranular ferrite, and the solidification structure was characterized by dendritic structure without the equiaxed center zone. The microstructure consisted of ferrite, martensite and non-recrystallization (N-R) region was obtained in the annealed sheet when the as-cast strip was directly cold rolled and annealed at 740–780 °C for 3 min. However, a typical dual phase microstructure of ferrite-martensite was obtained in the annealed sheet when the martensitic structure in the as-cast strip was subjected to cold rolling and annealing at 740–780 °C for 3 min. The dual phase microstructure in the annealed sheet having martensite fraction of ~ 51–77% was obtained, leading to tensile strength of ~ 851–914 MPa and corresponding total elongation of ~ 16–11%. Tempering treatment reduced martensite hardness, leading to lower tensile strength (~ 702–518 MPa) and corresponding higher total elongation (~ 19.8–30%) compared to non-tempered steel. [ABSTRACT FROM AUTHOR]

Nowadays, a wide range of materials is used for car body structures in order to improve both the passengers' safety and fuel consumption. These are joined by laser welding and solid state fiber lasers being used more and more in present. The article is focused on the research of laser welding influence on the mechanical and deformation properties, microstructure and microhardness of advanced high-strength steels: high-strength low-alloyed steel HC340LA, dual phase steel HCT600X and multi-phase residual austenite steel RAK40/70. The proper welding parameters have been found based on weld quality evaluation. The specimens for tensile test with longitudinal laser weld were used to measure mechanical and deformation properties. Microstructure and microhardness of laser welds were evaluated in the base metal, heat affected zone and fusion zone. The higher values of strength and lower ones for deformation properties of laser-welded materials have been found for dual and multi-phase steel. The microhardness strongly depends on the carbon equivalent of steel. Deformation properties are more sensitive than strength properties to the change of microstructure in the fusion zone and heat affected zone. [ABSTRACT FROM AUTHOR]

The development of dual phase steels to meet the current world demands, for the purpose of decreasing the fuel consumption with increasing the strength to weight ratio, requires certain microstructural modifications. In the present research, a new morphology of DP steel, known as Laminated–DP steel, as well as its unique production method has been introduced. The new process developed involved properly selecting low carbon steels, stacking them in a laminated manner and performing a roll bonding process followed by short austenitization treatment. The martensite volume fraction was designed and obtained to be 24%. Scanning electron microscopy (SEM) was employed for microstructural examination. Moreover, deformation and tensile behavior of the newly developed steel were studied and compared with those of some ordinary DP steel (ODP). Room temperature uniaxial tensile tests also revealed mechanical properties comparable with those of the commercial DP600 steel, a kind of structural automotive steel. [ABSTRACT FROM AUTHOR]

Similar and dissimilar combinations of a 1000 MPa galvanised dual phase (DP) steel and a 980 MPa twining-induced plasticity (TWIP) steel were resistance spot welded under different welding parameters. The microstructure, expulsion situation, nugget size and mechanical properties of spot welds were evaluated systematically. The results showed that the differences of microstructure and chemical compositions caused that the weld nugget hardness increases in the order of TWIP/TWIP, DP/TWIP and DP/DP. The lower melting point and heat conductivity of the TWIP steel and the lower electric resistance of the zinc coat on the DP steel caused that the expulsion occurring current increased in the order of TWIP/TWIP, DP/TWIP and DP/DP and under the same welding condition the nugget diameter increased in the order of DP/DP, DP/TWIP and TWIP/TWIP. Furthermore, the tensile shear failure mode and location depends on the nugget size, microstructure and hardness distribution of spot weld. [ABSTRACT FROM PUBLISHER]

For low-carbon V-Nb micro-alloyed steel, dual phase ferrite-martensitic microstructures were prepared with a variable martensite fraction, which ranged from Vm 20 to 88%. By measuring the temperature in the sample centre, the parameters of intercritical annealing and the relationship between Vm and the intercritical annealing temperature 720 °C

Multi-pulse tempering (MPT) on the resistance spot welding (RSW) is a considerable method to improve the quality of spot weld joints. For its uncertain tempering temperature and large temperature fluctuation, some stable sequential coupling finite element (FE) models for RSW and MPT of DP590 dual phase steel are developed to control the tempering temperature and further determine the experimental parameters. The nugget shape and size of simulation are verified by experimental observation, and the effects of MPT on microstructure, microhardness, and mechanical property during tensile-shear loading condition are experimentally investigated. The results show that MPT and its tempering temperature have no effect on the tensile-shear failure mode and have slight impact on nugget size and tensile-shear peak load. Besides, MPT leads to lower nugget hardness and better mechanical properties of spot weld joints than that of ordinary RSW due to the formation of a tempered martensite, and the higher tempering temperature expands these effects of MPT. However, the extent of the impact by MPT temperature on microstructure and mechanical property of spot weld joints is inferior to that of the impact by the MPT process itself. [ABSTRACT FROM AUTHOR]

The strip casting is a recently appeared technology with a potential to significantly reduce energy consumption in steel production, compared to hot rolling and cold rolling. However, the quantitative dependences of the steel microstructure and mechanical properties on strip casting parameters are unknown and require investigation. In the present work we studied the effects of strain and interrupted cooling temperature on microstructure and mechanical properties in conventional dual phase steel (0.08C–0.81Si–1.47Mn–0.03Al wt%). The strip casting process was simulated using a Gleeble 3500 thermo-mechanical simulator. The steel microstructures were studied using optical, scanning and transmission electron microscopy. Mechanical properties were measured using microhardness and tensile testing. Microstructures consisting of 40–80% polygonal ferrite with remaining martensite, bainite and very small amount of Widmanstätten ferrite were produced. Deformation to 0.17–0.46 strain at 1050 °C refined the prior austenite grain size via static recrystallisation, which led to the acceleration of ferrite formation and the ferrite grain refinement. The yield stress and ultimate tensile strength increased with a decrease in ferrite fraction, while the total elongation decreased. The improvement of mechanical properties via deformation was ascribed to dislocation strengthening and grain boundary strengthening. [ABSTRACT FROM AUTHOR]

In this paper was analyzed the influence of annealing treatment on the microstructure and mechanical properties of ferrite-martensite of dual phase of high strength steel DP580. The analyzed microstructural transformations were the result of applying the annealing process in a temperature range of 150 to 500 ° C. Metallographic and fractographic analysis were performed on the initial steel and heat-treated steel by using optical and electron microscopy. Modifications in mechanical properties were evaluated by uniaxial tensile tests and by microhardness tests. It was observed that the used range of annealing temperatures affects gradually the microstructure, allowing obtaining different combinations of steel strength and ductility. The effects of annealing heat treatment on the dual phase steel analyzed are the result of a synergistic effect between the transformations in the martensite phase and ferrite phase. [ABSTRACT FROM AUTHOR]

Conventional dual phase (DP) steel (0.08C-0.81Si-1.47Mn-0.03Al wt.%) was manufactured using simulated strip casting schedule in laboratory. The average grain size of prior austenite was 117 ± 44 µm. The continuous cooling transformation diagram was obtained. The microstructures having polygonal ferrite in the range of 40-90%, martensite with small amount of bainite and Widmanstätten ferrite were observed, leading to an ultimate tensile strength in the range of 461-623 MPa and a corresponding total elongation in the range of 0.31-0.10. All samples exhibited three strain hardening stages. The predominant fracture mode of the studied steel was ductile, with the presence of some isolated cleavage facets, the number of which increased with an increase in martensite fraction. Compared to those of hot rolled DP steels, yield strength and ultimate tensile strength are lower due to large ferrite grain size, coarse martensite area and Widmanstätten ferrite. [ABSTRACT FROM AUTHOR]

A high-Al low-Si dual phase (DP) steel processed through thermo-mechanical processing practice involving combination of hot rolling and water quenching was studied in terms of structure–mechanical property relationship. The structure of the steel was strongly influenced by the start rolling temperature. At start rolling temperature of 1150 °C, blocky-type martensite was obtained, but on decreasing the start rolling temperature to 1000 °C, long strip-type martensite formed along the rolling direction. The steel with long strip-type martensite was subsequently tempered at different temperatures. Steel tempered at 200 °C for 30 min had the lowest hardness and was characterized by best combination of mechanical properties with tensile strength of 950 MPa, total elongation of 25.94%, and the product of tensile strength and total elongation of 24.62 GPa%. The decrease in the hardness of martensite enhanced the strain hardening ability, while ferrite influenced the fracture characteristics. There was a high tendency to cleavage, when large number of ferrite grains with {100} orientation and hardness was high in the adjacent area between martensite and ferrite. However, the refinement of ferrite grain size and increase in ferrite–ferrite grain boundary misorientation led to increase in ductile behavior. [ABSTRACT FROM AUTHOR]

The influence of intercritical annealing temperature on the microstructure and mechanical properties of an 0.23%C low alloy steel was undertaken in this work. The as-received steel was normalised and afterwards annealed in the (a+?) region at 730°C, 750°C, 770°C and 790°C followed by quenching in hot water at about 50°C.Mechanical testing (tensile, impact and hardness) of the annealed samples were conducted at room temperature. The fracture surfaces of the impact test samples were examined using the scanning electron microscope (SEM). Micros structural evolution of the samples was also examined with an optical microscope. The results showed that all the evaluated mechanical properties were improved by intercritical annealing, with the samples treated at 790°C possessing the optimum combination of properties. The ultimate tensile strength (UTS), yield strength (YS), impact strength (IS), total elongation (TEL) and hardness of these samples improved by 2.7%, 13.7%, 19.7% and 31.05% respectively over the normalised sample. The microstructure photographs of the intercritically annealed samples revealed duplex structures of predominantly martensite in a ferrite matrix with little dispersions of either carbide or retained austenite, which is a typical characteristic of conventional dual phase steel. [ABSTRACT FROM AUTHOR]

Dual phase (DP) steels consisting different volume fractions of ferrite and martensite and different ferrite grain size were produced by a new route utilizing cold-rolling and subsequent intercritical annealing of ferrite/martensite duplex starting structure at 770 °C for different times. Scanning electron microscopy has been supplemented by nanoindentation and tensile test to follow microstructural changes and their correlations to the variation in phase's hardness and mechanical properties. The results showed that longer holding times resulted in coarser and softer ferrite grains in DP microstructures. Martensite nanohardness variation with holding time is related to change in its carbon content. Mechanical properties such as strength, elongation and toughness are well correlated with the martensite/ferrite hardness ratio. [ABSTRACT FROM AUTHOR]

The mechanical properties of DP780 dual phase steel were measured by quasi-static and high-speed tensile tests at strain rates between 0.001 and 1000 s at room temperature. The deformation and fracture mechanisms were analyzed by observation of the tensile fracture and microstructure near the fracture. Dynamic factor and feret ratio quantitative methods were applied to study the effect of strain rate on the microstructure and properties of DP780 steel. The constitutive relation was described by a modified Johnson-Cook and Zerilli-Armstrong model. The results showed that the strain rate sensitivity of yield strength is bigger than that of ultimate tensile strength; as strain rate increased, the formation of microcracks and voids at the ferrite/martensite interface can be alleviated; the strain rate effect is unevenly distributed in the plastic deformation region. Moreover, both models can effectively describe the experimental results, while the modified Zerilli-Armstrong model is more accurate because the strain-hardening rate of this model is independent of strain rate. [ABSTRACT FROM AUTHOR]

In this work, we have employed a unique solid-sate joining process, called friction bit joining (FBJ), to spot weld aluminum alloy (AA) 7075-T6 and dual phase (DP) 980 steel. Static joint strength was studied in the lap shear tension configuration. In addition, weld-bonding (adhesive + FBJ) joints were studied in order to evaluate the ability of adhesive to mitigate the impact of corrosion on joint properties. Accelerated laboratory cyclic corrosion tests were carried out for both FBJ only and weld-bonding joints. The FBJ only joints that emerged from corrosion testing had lap shear failure loads that were significantly lower than freshly prepared joints. However, weld-bonding specimens retained more than 80% of the lap shear failure load of the freshly prepared weld-bonding specimens. Examination of joint cross sections confirmed that the presence of adhesive in the weld-bonding joints mitigated the effect of the corrosion environment, compared to FBJ only joints. [ABSTRACT FROM AUTHOR]

Microstructure and properties of DP590 steel in overaging process of continuous annealing were investigated in the laboratory. Effects of the overaging temperature (OT) in conventional process (Process A) and reheated overaging temperature (ROT) of the proposed process (Process B) on microstructure and mechanical properties were analyzed. The results showed that, in process A, with the increase of the OT from 280 °C to 400 °C, the amount of granular retained austenite increases, the tensile strength decreases, the yield strength and elongation both increase. In process B, with the increase of the ROT from 310 °C to 400 °C, the tensile strength decreases, whereas the yield strength changes little. Compared with the conventional process, the tensile strength of the experimental steel increases obviously and the ratio of yield-to-tensile strength decreases in process B. The variation of instantaneous work hardening index ( n ⁎ ) with tensile strain can be divided into three stages. The increase in the OT or ROT leads to the continuous increase of n ⁎ in the third stage. [ABSTRACT FROM AUTHOR]

Phase transformations in dissimilar resistance spot welds of dual phase steel and ferritic stainless steel are analysed. In contrast to a full martensitic microstructure predicted by the Schaeffler and Balmforth diagrams, a ferrite-martensite microstructure was observed in the fusion zone. The formation of ferrite phase in the fusion zone can be attributed to the rapid cooling rate of resistance spot welding, which suppresses the post-solidification ferrite-austenite transformation. The grain growth and martensite formation were main metallurgical features of the heat affected zone of ferritic stainless steel side. Microstructure gradient of heat affected zone in dual phase steel side was dictated by martensitic transformation. The effect of weld thermal cycle on the mechanical performance of the joint is discussed. [ABSTRACT FROM AUTHOR]

Tempering has been used as a method to develop a range of dual phase steels with the same martensite morphology and volume fraction, but containing phases with different relative strengths. These steels were used to examine the strain partitioning between the two constituent phases experimentally through mechanical testing and numerically through finite element modelling. It was found that increasing the differential in strength between the two phases not only produces regions of high strain, but also regions of low strain. On average, a larger difference in strength between the phases increased the strain carried by the softer phase. There was no discernible preferential strain localisation to the ferrite/martensite interface, with the regions of strain localisation being determined by the morphology of the microstructure. A direct correlation between the average strain in the ferrite, and the measured ductility has been found. [ABSTRACT FROM AUTHOR]

In this study, the effects of ferrite morphology and of ferrite volume fraction on the mechanical properties and the fracture toughness values ( KIc) of dual phase steels were investigated. The fracture toughness values were determined using an approach that was different from the standard one. A homogenization heat treatment was applied to the steels. Two different starting microstructures were then obtained by either the full annealing of or the quenching of the steels from austenite before applying intercritical annealing to the steels in the austenite region. After this process, dual phase steels with two types of morphology, coarse and fine, were obtained with 40, 20, and 10% ferrite volume fractions, depending on the starting microstructure. Tempered specimens with 10% ferrite exhibited improved fracture toughness values compared with those of the other steels. [ABSTRACT FROM AUTHOR]

In this study, strain ageing behaviour of dual phase steel with different martensite morphology was investigated. Three different types (Intercritical annealing, intermediate quenching and step quenching) of heat treatment was applied in the critical temperature (775 °C). Ageing experiments after 4 % prestrain were carried out at 180 °C for different times ranging from 10 to 160 min. The microstructure after different heat treatments applied to the work pieces with the network, fiber, and the bulky martesite morphologies have been identified. It was found that the martensite morphology has no effect on the trend of ΔY (an increase in strength due to strain ageing), YS and UTS at different ageing times for 180 °C. However, at a given ageing time for 180 °C ΔY, YS, UTS and hardness values of fibrous martensite are higher than network or bulky martensite. [ABSTRACT FROM AUTHOR]

Abstract: To reduce cost, optimise mechanical properties and improve process tolerance, a series of 1000MPa grade ultrafine-grained dual phase (DP) steels with nanosized precipitates have been developed based on the C–Si–Mn–Ti alloy system. The grain size of ferrite in ultra-high strength DP steels ranges from 1.1 to 1.7μm. The amount of precipitations in the annealed sheet is much more than that in the hot-rolled plates with the highest distribution frequency being between 5 and 10nm. The grain refinement and precipitation strengthening interact significantly and have a considerable effect on yield strength. Therefore, the strengthening effects cannot be expressed as a simple linear relationship. A modified root-mean-square (RMS) relationship has been proposed to express the yield strength for dual phase steel with obvious grain refinement and precipitation strengthening. [Copyright &y& Elsevier]