Phase Steel Research Articles

Hole-flanging behaviour of dual-phase steel using conventional and two-stage incremental forming processes

ABSTRACT The present study aimed to analyse the hole-flanging behaviour of dual -phase steel using conventional and incremental forming processes. The sheet with different hole diameters (40, 30 and 20 mm) are prepared and tested. During conventional flanging operation, the effect of two different punch geometries (flat bottom and hemispherical) are used to study the hole-flanging ability. Whereas, in incremental forming operation, different tool paths are generated, i.e. single (90°) and two stage (45° + 90°). The results shows that the holes flanged using conventional process failed to flange the lower diameter hole which is attributed due to the higher tensile circumferential stresses acted along its circumference. Whereas, for hole flanged using incremental technique has shown similar observations except in case of two-stage strategy. The implementation two-stage strategy helps in successful formation of the flanges for lower diameter holes compared to the remaining one. Additionally, finite element simulations are performed to study various critical parameters such as strain path and thickness. The maximum value of strains and thickness reduction is found near the hole edge. The highest thinning (65%) is found in single-stage process and least (38.04%) in conventional flanging with hemispherical punch.

On the Measurability and Predictability of HAZ Softening in GMAW of Automotive DP980 Steel

Dual Phase (DP) steel, composed of a ferrite matrix with dispersed islands of martensite, has become popular in auto-body car construction due to its outstanding mechanical properties (i.e., high strength and good ductility). DP steel softens at the sub-critical heat-affected zone (SC-HAZ) when subjected to welding thermal cycles, owing to the tempering of the martensite phase. In this work, DP980 steel was subjected to varied thermal cycles: (a) furnace-tempering treatment, (b) gas metal arc welding (GMAW), and (c) resistance spot welding (RSW), in order to characterize the tempering of martensite below the Ac1 critical temperature and at the sub-critical heat-affected zone (SC-HAZ) in the case of the welded specimens. The coarsening stage of cementite phase was characterized through microstructure observations and hardness measurements. As expected, the comparative results indicated an advanced stage of the martensite tempering in the furnace heat-treated specimens, followed by the GMAW and the RSW specimens. Further, developed softening kinetic models have been suitably employed and adjusted in order to predict the extent of softening along the SC-HAZ of the GMAW specimen. Finally, as the advanced stage of cementite coarsening is due to the influence of the arc welded thermal cycle, a reasonable estimation of the hardness profile was obtained, particularly for tempering temperatures above 400 °C.

Experimental investigation on adhesive bonding of similar and dissimilar weld joint used for automotive applications

The automobile industry has started using adhesive bonding to join load bearing components which aerospace industry has been using for decades. Adhesive lap joints are used frequently in the manufacture of automobile. In present study, structural adhesives were used to join the aluminium alloy (AA5083 H111) with the HSS dual phase (DP780) steel. Adhesive bonding appears to be one of the appropriate methods of joining dissimilar materials. The aim of this work is to analyze the tensile strength of similar and dissimilar joints. The influence of various parameters was also investigated such as the overlap length and the bondline thickness of specimens. In DP steel, there is 22% increase in strength for similar lap joint when overlap length changes from 10 mm to 15 mm, while there is 45% increase in strength when it varies from 15 mm to 20 mm. Similarly in case of Al alloy, there is 26% increased strength for similar lap joints when length varies from 10 mm to 15 mm, while it increased to 42% when length changes from 15 mm to 25 mm and there is about 35% increase in strength for length varies from 20 mm to 25 mm. In case of dissimilar joints, firstly there is about 16% increase in strength then there is 5% decrease while after that there is 45% increase in strength. Adhesion failure, cohesion failure and mixed failure were obtained experimentally during failure mode analysis. As the strength of joint increases, failure mode shows a transition from adhesion failure to cohesion failure. From the literature survey it is evident that limited work has been carried out on analysis of shear-tensile strength of adhesively bonded steel and aluminium joint with variation in bonding parameters. Not much work on failure mode analysis of bonded joints during tensile testing has been reported. In present work a noval attempt has been made to analyze the shear-tensile strength and failure mode of adhesively bonded steel and aluminium joint with variation in bonding parameters.

Strain Hardening and Stretch Formability Behavior of Triple Phase (TP) Steel Strips

The current work explores the strain hardening and stretches formability behaviour of the developed Triple Phase (TP) steel. Double quenched TP steel strips posse three distinguished stages of strain hardening on tensile forming. 1st stage represents the highest n-value reflecting resistance to homogeneous deformation, where steel can be safely stretched. 2nd and 3rd stage reveals lower n-values, where localized thinning exist. On Erichsen testing, the relationship between punch forming force and punch stroke exhibits two forming regions. The 1st region is delineated by a straight line showing an ultra-high strain-hardening rate, which represents a reversible elastic stretch forming. The 2nd forming region continues to a higher Erichsen punch stroke than that of the 1st region and presents the permanent plastic stretch forming behaviour. It is found that bainite and martensite clusters created, by double quenching, in TP-steel exaggerated the elastic stretch forming limit 10 times higher than the as-hot rolled condition. 7 min. holding time of strips in the salt bath is considered the most effective for the creation of a useful volume fraction of the bainite phase. However, 21 min. holding time in salt bath grows martensite laths through the bainite aggregates, affecting negatively on stretch formability.

Monitoring of friction node surfaces in the context of their physico-chemical interactions with lubricating media of different surface activity

The article deals with tribochemical influence of the surface activity of biosynthetic oils on the operational efficiency and reliability of lubricated friction nodes under the critical conditions. It is proved that the oil surface activity is caused by the structure and properties of their molecules and interactions with steel surfaces. Evaluating criteria of oil tribochemical activity influence on the steel surface modification has been studied. The significant influence of biooils on the steel surface nanolayer phase transformations, friction and steel wearing is claimed. It has been established that the effect of the steel crystal structure deformation and strengthening its surface depends on activity of composition of lubricant. It is proved that biooils lead to improved friction and wear indicators of lubricated samples. X-structural analysis of the steel surfaces showed that during friction the austenite is destroyed and ferrite is formed. Penetration in the lubricated steel nanolayers shows increasing the ferrite and decreasing the austenite content, that cases reducing the deformation degree of metal crystals, which leads to the strengthening of its surface under the plasticized layer and to decreasing the level of friction and wear due to the formed intersurface servitotribopolymer film, which is stable under critical friction.

Micromechanical Simulation Approach of Dual Phase Steel Artificial Microstructure Using Random Field Model

In this study, a random field (RF) model with a Gaussian kernel was applied to generate an artificial microstructure of dual phase (DP) steels. Micrographs obtained from Scanning Electron Microscopy (SEM) were analyzed using image processing software to extract the grain size and the volume fraction of each phase. Based on watershed (Ws) segmentation and quantitative analysis, the real and artificial microstructures were compared by analyzing grain features related the solidity, grain size and aspect ratio (the proportional relationship between its width and its height). Consequently, this approach allows to simulate the overall stress-strain behavior of the analyzed microstructures. As a result, it was shown that the strain localization starts to develop at the ferrite/martensite interface and that the RF model could replicate the micromechanical behavior of DP steels.

Effect of post-weld heat treatment on microstructure and mechanical properties of DP800 and DP1200 high-strength steel butt-welded joints using diode laser beam welding

Among the available high-strength steels, there is growing demand for dual phase (DP) steels for wide application in the automotive industry owing to their good combination of high strength, ductility and formability. Also, the use of innovative welding technologies like laser beam welding (LBW) has growing importance in the field of high-strength steel because of its excellence in providing high-quality welds, high welding speed, high power density, low heat input, a narrow heat-affected zone and low heat distortions as compared to the conventional gas metal arc welding process. However, the hardening and softening in the heat-affected zone is a major issue when welding high-strength steel, i.e. DP steel grades, greatly affecting the strength, formability and plasticity of the whole-welded joint and thus affecting service performance and reliability. Based on preliminary experiments, the optimal welding condition was a nominal laser power of 1.0 kW and a welding speed of 8 mm/s. The aim of this work is to analyse and compare the weld and heat-affected zone characteristics, microstructure and mechanical properties of DP steels with 1-mm thick butt joints of DP800 and DP1200 high-strength steel (HSS) by diode laser beam welding. The effects of post-weld heat treatment (PWHT) on the strengthening of the laser-welded joints were evaluated by microstructural examinations under optical microscope and scanning electron microscope, and mechanical properties were examined by microhardness test, three-point bending tests and tensile tests.

Mechanical Properties Characterization of Welded Automotive Steels

Among the various welding technologies, resistance spot welding (RSW) and laser beam welding (LBW) play a significant role as joining methods for the automobile industry. The application of RSW and LBW for the automotive body alters the microstructure in the welded areas. It is necessary to identify the mechanical properties of the welded material to be able to make a reliable statement about the material behavior and the strength of welded components. This study develops a method by which to determine the mechanical properties for the weldment of RSW and LBW for two dual phase (DP) steels, DP600 and DP1000, which are commonly used for the automotive bodies. The mechanical properties of the resistance spot weldment were obtained by performing tensile tests on the notched tensile specimen to cause an elongation of the notched and welded area in order to investigate its properties. In order to determine the mechanical properties of the laser beam weldment, indentation tests were performed on the welded material to calculate its force-penetration depth-curve. Inverse numerical simulation was used to simulate the indentation tests to determine and verify the parameters of a nonlinear isotropic material model for the weldment of LBW. Furthermore, using this method, the parameters for the material model of RSW were verified. The material parameters and microstructure of the weldment of RSW and LBW are compared and discussed. The results show that the novel method introduced in this work is a valid approach to determine the mechanical properties of welded high-strength steel structures. In addition, it can be seen that LBW and RSW lead to a reduction in ductility and an increase in the amount of yield and tensile strength of both DP600 and DP1000.

The use of color etching to study the microstructure of laser welded steel used in the automotive industry

Abstract In the present article the results of a microstructure study of dual phase, multiphase (with transformation induced plasticity effect) and manganese-boron steel have been presented in the as-delivered state and after laser fusion by means of color metallography, which, up to now, has not been applied in those types of joints. In order to reveal the components of the microstructure of the steel types studied, five reagents were applied: nital, pikral, Klemm, LePera and sodium metabisulfite, in different combinations. Various phases were revealed in the material in the as-delivered state and after fusion, and this would not have been possible using standard procedures by means of a light microscope. In the as-delivered state all basic phases were observed. In the state after laser remelting, the observed microstructures were completely different than in the received state giving bainite and martensite an advantage. Measurements of the hardness of the as-received material and the welds of the tested steels were performed, showing a significant increase in weld hardness.

Application of the hybrid process ultrasound enhanced friction stir welding on dissimilar aluminum/dual‐phase steel and aluminum/magnesium joints

AbstractFriction stir welding as a solid‐state joining method with its comparatively low process temperatures is suitable for joining dissimilar materials like aluminum/magnesium or aluminum/steel. Such hybrid joints are of great interest regarding lightweight efforts in different industrial fields like the transportation area. The present work investigates the influence of additionally transmitted power ultrasound during the friction stir welding on the joint properties of EN AC‐48000/AZ91 and EN AW‐6061/DP600. Therefore, conventional friction stir welding was continuously compared to ultrasound enhanced friction stir welding. Light microscopic analysis and nondestructive testing of the joints using x‐ray and high frequency ultrasound show different morphologies of the nugget for the aluminum/magnesium joints as well as differences in the amount and size of steel particles in the nugget of aluminum/steel joints. Scanning electron microcopy proves differences in the thickness of continuous intermetallic layers for the aluminum/steel joints realized with and without power ultrasound. Regarding the tensile strength of the joints the power ultrasound leads to increased joint strengths for EN AC‐48000/AZ91 joints compared to a decrease for EN AW‐6061/DP600 joints. Corrosion investigations show an influence of the ultrasound power on the corrosion properties of EN AC‐48000/AZ91 joints which is attributed to a changed aluminum content in the nugget region. Because of the great potential difference between the magnesium and the nugget phase the transitional area exhibits strong galvanic corrosion. For EN AW‐6061/DP600 joints an increased corrosion caused by galvanic effects is not expected as the potentials of the EN AW‐6061 aluminum alloy and DP600 steel are very similar.

Real-time in-line steel microstructure control through magnetic properties using an EM sensor

Magnetic and electric properties (such as low field relative permeability and resistivity) are sensitive to changes in both steel microstructure and temperature. Recently an electromagnetic (EM) sensor system (EMspec™) has been installed to non-destructively on-line monitor the phase (microstructure) transformation in strip steels during the cooling process after hot rolling. To use an EM system to provide dynamic control via varying the cooling strategies or heat treatment using sensor feedback, which can give higher quality steel products with excellent mechanical properties at reduced cost, requires accurate interpretation of the EM sensor signals and predictive capability of the signals from desired microstructures at the relevant temperatures. A 3D FE model is reported here that allows the EMspec™ sensor output (Zero Crossing Frequency, ZCF) to be related to the steel microstructure (phase fraction) using the relationships between permeability and resistivity with microstructure and temperature. The model has been verified by room temperature measurements on various steel grades samples (varying microstructure and strip thickness). High temperature experimental tests have been carried out using a lab-based furnace and run-out table (ROT) with cooling system, mimicking the real-time monitoring of phase transformation of steel strip products. The experimental results have been compared to predicted sensor signals for the known transformation behaviour, determined independently using dilatometry. In this paper the process by which the model can be used to predict the ZCF values for different transformation behaviour, for example different ferrite fractions prior to bainite/martensite formation in a two phase steel, which in turn can be used to control the cooling strategy to achieve a desired microstructure and mechanical properties is discussed.

Back-stress-induced strengthening and strain hardening in dual-phase steel

Strain hardening for ductility remains challenging especially at high yield strength when dislocation plasticity is usually invalid. The hetero-deformation provides an effective route to induce extra back stress hardening specifically in hetero-structures inherently with large mismatch of mechanical responses, e.g. flow stress and strain hardening etc., upon applied loading. In this paper, both strengthening and strain hardening were investigated in a dual- phase steel, consisting of ductile γ-austenite and almost non-deformable B2 intermetallic phase as the second phase of volume fraction of 23%. The chemical composition was 0.86C, 16Mn, 10Al, 5Ni, balance Fe (wt.%). Of special note is two distinct hetero-deformation responses during both tensile and interrupted unload-reload testing. One is the yield-drop, while the other is hysteresis loop. Both unceasingly appear even from the elasto-plastic yield stage up to whole uniform deformation. The measured back stress and resultant back stress hardening account for a large proportion of global flow stress and strain hardening. Further, both Schmid factor and Kernel average misorientation (KAM) values were measured after tensile deformation. Un-expected, only γ-grains bordering on B2-phase show a significant decrease in the average Schmid factor, relative to almost unchanged in left γ-grains still next to γ-ones as well as B2-phase. This indicates that γ-grains adjacent to B2-phase bear the vast majority of plastic strains, not simple strain partitioning between γ and B2. Because of this, from the onset of yielding to end of tensile deformation, those γ-grains, in contrast to left γ-grains and B2 phase, exhibit a maximal increment in KAM values. This serves as a solid evidence of the generation of geometrically necessary dislocations to accommodate strain gradient near γ/B2 phase boundaries. It turns out that hetero-deformation due to plastic incompatibility induces the operation of back stresses, leading to both strengthening and strain hardening as well. Finally, a microstructure-based model was developed to calculate back stress which was well consistent with experimentally measured back stress.

Perspective directions of metallurgy and steel metal science development

The task of principal (multiple) increase of complex of service parameters (often being hard compatible) and steel quality under conditions of costs decrease becomes more and more actual. The solution is possible by creation and application of new principles and approaches based, first of all, on adequate physical and chemical predicting methods and effective technological skill of control non-metallic inclusions, forms of impurities, phase isolations, structure state, including homogeneity of the metal volume. A complex approach to prediction, analysis of phase composition, structure state and steel properties elaborated to implement this progressive direction. It has an important feature like elaborated in details scientific principles. The principles allow not only to make effective the development of new steel and technologies of their production at minimum costs, but also to reveal the content, directions and intensity of processes having place during metal processing. It was illustrated at the example of microalloyed (stabilized) by titanium super-high-carbon IF steel. Importance of accounting of regularity of phase isolations forming kinetics shown, that can result in considerable change of their characteristics and steel properties. This circumstance can be used for unification of chemical composition or development of cassette technologies production, for example, from IF-steels of the same chemical composition, of rolled products of different assortment or strength class. It was shown, that the complex phase isolations forming takes place with participation impurities and elements presented in the steel. Therefore there arose wide possibilities of considerable increase of complex of properties at decreasing of costs. Presence of deformation considerably accelerates the processes of nano-size phase isolations forming not only during rolling, but also during parts manufacturing by hot stamping, combined with steel quenching. This allows to increase their strength by 300–400 MPa additionally to martensitic structure. It is possible, apart from principal perfection of parameters of service characteristics stability, to reach considerable increase of composition homogeneity, structure and properties in the whole metal volume by non-metallic inclusions forming control. Forming of silicate glassy inclusions in low-alloyed steels improves principally the macro- and microstructure of CC billet and impact toughness of rolled products KCV–40 (420–445 J/сm2 ). Their presence in the steel stipulated by kinetic reasons. Additional substantial increase of composition homogeneity, structure and microalloyed steels properties can be reached during the rolling process by return redistribution of components from the axis zone to the surface of the rolled product. High efficiency of the above-mentioned approaches at present has been confirmed by development an array of pipe, structural, automobile steels and highly efficient technologies of their production.

Bare Spot Defect on a Hot Dip Galvanized DP Sheet Strip

Bare spots defect of galvanealed (GA) in High Strength Dual Phase (DP) steel strip with 1.5 % Mn contain was studied in detail. The surface morphologies of spot defects before and after partial and complete removal of the Zn layer, as well as the interface between the outermost coating layer and the sheet substrate were analyzed by optical microscope (OM), scanning electron microscope (SEM), energy dispersive spectrometer (EDS), X-ray Photoelectron spectroscopy (XPS), glow discharge optical emission spectrometry (GDOES) and laser scanning confocal microscope(LSCM). It was found that the bare spots defect was composed of a large number of pits with different sizes and depths. There were a lot of Fe - Zn alloy particles distributed in the pits, and many MnO formed on the surface, no effective Fe2Al5 inhibition layer formed. The results showed that the main reasons for the bare spots defect of the GA in High Strength DP steel strip are as follows: a silicon oxide film forms on the substrate during annealing prior to hot dipping because of Mn gathered on the surface of steel strip, thus strip surface wettability with liquid zinc is deteriorated and preventing the formation of the Fe2Al5 inhibition layer during hot dipping. In this paper, the dew point control process is introduced creatively, by increasing the dew point and the hydrogen content in the furnace area, and the bare spot defects on the surface of the high strength duplex steel galvanized sheet are solved.

Numerically predicted high cycle fatigue properties through representative volume elements of the microstructure

The investigation of high cycle fatigue (HCF) properties of materials is elaborate in experimental design. To reduce the costs, a multiscale numerical approach to predict the HCF-strength is proposed in this study. The intention is to correlate microstructural features and their related microdeformation mechanisms with macroscopic fatigue properties. The approach involves a series of numerical and mechanism-based analytical models. The microstructural features, including phase fraction, grain size and grain shape, are statistically characterized and represented in the generated representative volume element (RVE) model of the microstructure of a two phase steel. The underlying microdeformation mechanism is captured by an extended crystal plasticity (CP) model incorporating kinematic hardening on slip systems for cyclic loadings. The CP parameter set was calibrated on strain controlled low cycle fatigue tests. The numerical simulations of the cyclically loaded RVEs resulted in local plasticity fields. The highest plastically deformed grain of each RVE was identified and its grain size averaged plastic strain value extracted for further analysis. These scattered plasticity values fit extreme value distribution density functions. The parameters, which describe the shape of the functions, were used to calculate the HCF-strength. The obtained values showed a good agreement to experimental results.

Microstructure and Mechanical Properties of Laser Welded Dual Phase and Mild Steel Joints for Automotive Applications

Recently, the laser welding technology of carbon steel is being widely used compared with arc welding technology for its better welding characteristics. In the present study, the influence of welding conditions of both laser beam welding (LBW) and gas metal arc welding (GMAW) as a comparative study on the weld joint microstructures, hardness distribution and fatigue properties crosses the butt-welded joints of dual phase (DP) steel and mild steel are investigated. The results show that LBW produced narrow welds with complete penetration while GMAW produces wide fusion and heat affected zones. The microstructure of the fusion zone of laser welded DP steels contains mainly bainite, martensite, and a few amount of acicular ferrite phases. Hardness values of the heat-affected zone (HAZ) for dual-phase (DP) steels showing lower values for both LBW and GMAW processes due to the tempering action of the martensite phase. A narrow softening region was clearly observed in the HAZ welded for LBW compared with GMAW. In general, the fatigue life of the welded joints is improved by using laser welding technique.

Similar and dissimilar resistance spot weldability of galvanised DP1000 and TWIP980 steels

ABSTRACTSimilar 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.

Influences of Thickness and Geometrical Change from Draw Forming in Quasi-Static Axial Compression Simulation

In the current industrial practices in the automotive field, the forming effects are neglected in the automotive crash analysis simulation. However, in reality, the forming process alters the structural behaviour of these components significantly. The objective of this paper is to present the effect of draw forming in the quasi-static axial compression analysis. In the present work, a circular cup is draw formed using Dual Phase (DP600) steel followed by a compression analysis to quantify the effects of non-uniform thickness distribution and the geometrical change on the cup. Numerical compression simulation was carried out with RADIOSS to identify these effects which were modelled using, in case A, a CAD circular cup and, in case B, a formed circular cup from draw forming. The experimental work was done to validate the forming model before compression analysis. Quasi-static axial compression simulation results revealed that the peak load and the energy absorbed for case B are lower by 26.91% and 17.84% respectively compared to case A. The compression analysis results demonstrate that the effect of non-uniform thickness distribution from draw forming process gives significant effect in the subsequent analysis. Therefore, the forming effects should be considered when conducting a crash simulation to increase the crash prediction and safety.

Physical Simulation of Thermo-Mechanical Processing of Ferritic-Bainitic Dual Phase (FBDP) Steel

The present work is dealing with a physical simulation of thermo-mechanical processing of ferritic-bainitic dual phase (FBDP) steel alloy containing 0.1% C, 0.3% Si, 0.9% Mn and 0.7% Cr. The microstructure changes and allotropic transformations during thermo-mechanical simulation are investigated. A series of heating – cooling cycles to detect the critical and allotropic transformation temperature by dilatation were carried out on the thermo-mechanical simulator (Gleeble 3500). On the other hand, five – consecutive hits were used during the physical simulation of hot rolling process. Two hits were representing the roughening stage followed by three ones representing finish rolling. Holding at 500°C for 5, 7, 10, 12 and 15 min. after last hit has been applied and then followed by air cooling. Dilation curves appear that Ac1= 766 °C, while Ac3 was detected as 883 °C. Baintic allotropic transformation temperatures were clearly noticed as 618 °C for Bs and 542 °C for Bf. The recrystallization temperature was also detected as 1035 °C. Holding for 5-7 min. at 500 °C was concluded as the optimum for creation a bainite volume fraction. Rough hot deformation a higher temperature above the recrystallization temperature is essential, where no strain hardening and possibility for achieving high strains without excessive loads. Finishing deformation at temperature lower than Tr would create fine bainitic structure. The flow curve of the steel ensures continuous strain hardening. The strain hardening rate (σf/ε) was directly proportional to temperature difference from pass to pass.

An experimental study on fracture toughness of resistance spot welded galvanized and ungalvanized DP 450 steel sheets used in automotive body

The purpose of this study is to determine fracture toughness of Resistance Spot Welded (RSW) Dual Phase (DP) steels. RSW of galvanized and ungalvanized DP 450 steel sheets was carried out on spot welding machine. Fracture toughness of RSW joints of galvanized and ungalvanized DP 450 steel sheets was calculated from tensile-shear tests. New empirical equations were developed using Least Squares Method (LSM) between energy release rate, fracture toughness and critical crack size depending on the relationship between hardness and fracture toughness values. Results indicated that fracture toughness of joints welded by using RSW increased exponentially while the hardness decreased. In addition, fracture toughness and energy release rate of RSW galvanized DP 450 steel sheets were lower compared to RSW ungalvanized DP 450 steel sheets which had approximately the same hardness.