Showing papers on "Bainite published in 2009"

Effect of martensite distribution on damage behaviour in DP600 dual phase steels

Abstract: The effect of martensite morphology and distribution in a ferrite matrix on the mechanical properties and the damage accumulation in uniaxial tension was investigated in two different automotive-grade dual phase DP600 steels. The two sheet steels had roughly 20% volume fraction of martensite but dissimilar chemical composition. A detailed analysis of microstructure and damage accumulation has been conducted as a function of strain. SEM analysis revealed that voids nucleation occurs by martensite cracking, separation of adjacent martensite regions, or by decohesion at the ferrite/martensite interface. Martensite morphology and distribution had a significant influence in the accumulation of damage. The steel with a more uniform distribution of martensite showed a slower rate of damage growth and a continuous void nucleation during the deformation process, which resulted in a higher void density before fracture. On the other hand, the steel with a centre-line of martensite through the sheet thickness exhibited accelerated void growth and catastrophic coalescence in the transverse orientation to the applied load.

Void Nucleation and Growth in Dual-Phase Steel 600 during Uniaxial Tensile Testing

Abstract: Commercial dual-phase (DP) steel in sheet form and comprised of ferrite, martensite, and bainite was subjected to uniaxial tension up to fracture. The damage characteristics were studied through extensive quantitative metallography and scanning electron microscope (SEM) observations of polished sections and fracture surfaces of failed specimens. The observed void nucleation mechanisms include nucleation at the martensite/ferrite interface or triple junction (most predominant), nucleation due to the cracking of martensite particles, and nucleation at the inclusions. The void characteristics in terms of area fraction, void density, void size ranges, and void orientations were analyzed as a function of thickness strain from various regions of the different uniaxial tensile test specimens taken to fracture. The damage analysis suggests that the void nucleation occurs during the entire deformation process with an almost constant rate and this rate reduces before fracture. A nucleation strain of 0.15 has been estimated for this material.

High strength steel sheet and method for manufacturing the same

Abstract: Disclosed is a high-strength steel sheet having excellent processability and a tensile strength of 980 MPa or more. The high-strength steel sheet has the following composition (in mass%): C: 0.1 to 0.3% inclusive, Si: 2.0% or less, Mn: 0.5 to 3.0% inclusive, P: 0.1% or less, S: 0.07% or less, Al: 1.0% or less, and N: 0.008% or less, with the remainder being Fe and unavoidable impurities. The steel structure of the high-strength steel sheet comprises 50% or more of martensite, 50% or less (including 0%) of ferrite, 10% or less (including 0%) of bainite, and 10% or less (including 0%) of residual austenite. In the high-strength steel sheet, the half width in the degree distribution of the nano-hardness obtained by the measurement of the hardness distribution of the martensite is 2.0 GPa or more, and the steel sheet has a tensile strength of 980 MPa or more.

Microstructural Evolution of a Low-Carbon Steel during Application of Quenching and Partitioning Heat Treatments after Partial Austenitization

Abstract: The “quenching and partitioning” (Q&P) process has been studied in a low-carbon steel containing 1.1 wt pct aluminum by heat treatments consisting of partial austenitization at 900 °C and subsequent rapid cooling to a quenching temperature in the range between 125 °C and 175 °C, followed by an isothermal treatment (partitioning step) at 250 °C and 350 °C for different times. Characterization by means of optical and scanning electron microscopy, electron backscattered diffraction (EBSD), magnetization measurements, and X-ray diffraction (XRD) has shown a multiphase microstructure formed by intercritical ferrite, epitaxial ferrite, retained austenite, bainite, and martensite after different stages of tempering. A considerable amount of retained austenite has been obtained in the specimens partitioned at 350 °C for 100 seconds. Experimental results have been interpreted based on concepts of the martensite tempering, bainite transformation, and kinetics calculations of the carbon partitioning from martensite to austenite.

Effects of alloying elements and microstructure on the susceptibility of the welded HSLA steel to hydrogen-induced cracking and sulfide stress cracking

Abstract: Hydrogen-induced cracking (HIC) and sulfide stress cracking (SSC) susceptibility of the submerged arc welded API 5L-X70 pipeline steel with different amounts of titanium at two levels of manganese (1.4% and 2%) were studied. The centerline segregation region (CSR) observed in the X70 pipe steel played an important role in the HIC susceptibility. Increased acicular ferrite content in the microstructure improved HIC resistance and SSC resistance, while bainite and martensite/austenite constituents deteriorated the workability of the welded specimens in sour environments. The 2% Mn-series welds showed higher SSC susceptibility than the 1.4% Mn-series welds due to the higher hardness values of the welds. The precipitated titanium carbonitrides in the welds can act as beneficial hydrogen traps and delay cracking in hydrogen sulfide environments. By further addition of titanium, the appearance of bainite and martensite/austenite in the microstructure outweigh any beneficial effect of titanium carbonitrides. The weld metals contained high percentage of acicular ferrite and good distribution of titanium carbonitrides yielded the best performance in sour environments. In two series of the welds, the best sour service properties were obtained at two compositions, 1.40% Mn–0.08% Ti and 1.92% Mn–0.02% Ti.

High-strength steel sheet and process for production thereof

Abstract: Disclosed are an ultra-high-strength steel sheet having both a tensile strength of as high as 1400MPa or above and excellent formability and an advantageous process for manufacturing the same. A high-strength steel sheet having both a composition which contains by mass C: 0.12 to 0.50%, Si: 2.0% or less, Mn: 1.0 to 5.0%, P: 0.1% or less, S: 0.07% or less, Al: 1.0% or less, and N: 0.008% or less with the balance being Fe and unavoidable impurities, and a structure which comprises, in terms of area fraction, autotempered martensite: 80% or more, ferrite: less than 5%, bainite: 10% or less, and retained austenite: 5% or less and in which the average number of precipitated iron carbide particles of 5nm to 0.5[mu]m in the autotempered martensite is 5OE04 or above per mm2.

High-strength steel plate and manufacturing method thereof

Abstract: Disclosed is a high-strength steel plate having superior ductility and stretch flangeability and a tensile strength (TS) of 980 MPa or higher, and having 0.17-0.73% C, 3.0% or less Si, 0.5-3.0 or less Mn, 0.1% or less P, 0.07% S, 3.0% or less Al, 0.010% or less N, and 0.7% or more Si + Al, an area ratio of martensite of 10-90% with respect to the entire steel plate composition, a residual austenite amount of 5-50%, and an area ratio of bainitic ferrite in the upper bainite of 5% or less with respect to the entire steel plate composition. Twenty-five percent or more of the aforementioned martensite is tempered martensite, and the total of the area ratio of the aforementioned martensite with respect to the entire steel plate composition, the aforementioned residual austenite amount and the area ratio of the aforementioned bainitic ferrite in the upper bainite with respect to the entire steel plate composition is 65% or more. The area ratio of polygonal ferrite with respect to the entire steel plate composition is 10% or less (including 0%), and the average amount of C in the aforementioned residual austenite is 0.70% or more.

Effect of Plastic Hot Deformation on the Hardness and Continuous Cooling Transformations of 22MnB5 Microalloyed Boron Steel

Abstract: The strains, transformation temperatures, microstructure, and microhardness of a microalloyed boron and aluminum precoated steel, which has been isothermally deformed under uniaxial tensile tests, have been investigated at temperatures between 873 and 1223 K, using a fixed strain rate value of 0.08 s−1. The effect of each factor, such as temperature and strain value, has been later valued considering the shift generated on the continuous cooling transformation (CCT) diagram. The experimental results consist of the starting temperatures that occur for each transformation, the microhardness values, and the obtained microstructure at the end of each thermomechanical treatment. All the thermomechanical treatments were performed using the thermomechanical simulator Gleeble 1500. The results showed that increasing hot prestrain (HPS) values generate, at the same cooling rate, lower hardness values; this means that the increasing of HPS generates a shift of the CCT diagram toward a lower starting time for each transformation. Therefore, high values of hot deformations during the hot stamping process require a strict control of the cooling process in order to ensure cooling rate values that allow maintaining good mechanical component characteristics. This phenomenon is amplified when the prestrain occurs at lower temperatures, and thus, it is very sensitive to the temperature level.

Comparison of mechanical properties of martensite/ferrite and bainite/ferrite dual phase 4340 steels

Abstract: Different microstructures were produced by heat treatment of 4340 steel. These microstructures are bainite, martensite, ferrite–martensite and ferrite–bainite. Mechanical tests were carried out at room temperature. The results showed that steel with bainite–ferrite microstructure has better ductility and charpy impact energy than steels with martensite–ferrite and full bainite microstructures. But yield and tensile strengths of this steel are less than the yield and tensile strengths of the other two steels. Hardness measurements showed that their hardness is the same. Fracture surface observations of tensile specimens showed increase in toughness of bainite–ferrite in comparison to martensite–ferrite and full bainite microstructures.

Toughness deterioration in advanced high strength bainitic steels

Abstract: Carbide free bainitic steels alloyed with manganese have achieved the highest strength and toughness combinations to date for bainitic steels. Ultimate tensile strengths ranging from 1600 to 1800 MPa were achieved while keeping a total elongation higher than 10%. Their toughness at room temperature matches tempered martensitic steels, known to be the best-in-class regarding this property. This improvement in toughness is achieved suppressing the precipitation of cementite during bainite formation by alloying the steel with about 1.5 wt% of silicon. However, it has been observed that strongly orientated martensite bands, associated to inhomogeneous manganese redistribution during solidification, lead to a remarkable deterioration in toughness in these advanced bainitic steels. The stress concentration associated with highly heterogeneous hardness distribution in the microstructure contributes to the premature crack nucleation.

Martensite Formation in Partially and Fully Austenitic Plain Carbon Steels

Abstract: The progress of martensite formation in plain carbon steels Fe-0.46C, Fe-0.66C, and Fe-0.80C has been investigated by dilatometry. It is demonstrated that carbon enrichment of the remaining austenite due to intercritical annealing of Fe-0.46C and Fe-0.66C does not only depress the start temperature for martensite, but also slows the progress of the transformation with temperature compared to full austenitization. In contrast, such a change of kinetics is not observed when the remaining austenite of lean-Si steel Fe-0.80C is stabilized due to a partial transformation to bainite, which suggests that the stabilization is not of a chemical but of a mechanical nature. The growth of bainite and martensite is accompanied by a shape change at the microstructural scale, which leads to plastic deformation and thus strengthening of the surrounding austenite. Based on this stabilizing mechanism, the athermal transformation kinetics is rationalized by balancing the increase in driving force corresponding to a temperature decrease with the increase in strain energy required for the formation of martensite in the strengthened remaining austenite.

Effect of composition on kinetics of athermal martensite formation in plain carbon steels

Abstract: The kinetics of martensite formation in three Fe–C–Mn alloys has been determined using dilatometry, and is compared with the data for other steels reported in literature. Each curve can be well described by the Koistinen–Marburger equation using a composition dependent start temperature and rate parameter α m. The empirical relationship derived for α m as a function of the chemical composition can improve predictions with the Koistinen–Marburger model of the volume fraction martensite at a certain temperature.

Evaluation of the Crystallographic Orientation Relationships between FCC and BCC Phases in TRIP Steels

Abstract: The crystallographic orientation relationships that are active during the transformation of austenite to bainite are studied for two TRIP steels by means of Electron BackScatter Diffraction (EBSD). A detailed evaluation of about 360 retained austenite grains and their BCC neighbours was performed. Three relationships were considered, namely Kurdjumov-Sachs, Nishiyama-Wassermann and Pitsch. It was found that the majority of the austenite grains had at least one neighbour that could be related with one of the three orientation relationships. The Kurdjumov-Sachs relationship appeared to be dominant and no strong indication for variant selection could be retrieved from the studied data. It was, however, also demonstrated that some precautions need to be made since a clear distinction between the evaluation of a small region of the microstructure and conclusions made for the complete material is necessary.

Load Partitioning and Strain-Induced Martensite Formation during Tensile Loading of a Metastable Austenitic Stainless Steel

Abstract: In-situ high-energy X-ray diffraction and material modeling are used to investigate the strain-rate dependence of the strain-induced martensitic transformation and the stress partitioning between austenite and α′ martensite in a metastable austenitic stainless steel during tensile loading. Moderate changes of the strain rate alter the strain-induced martensitic transformation, with a significantly lower α′ martensite fraction observed at fracture for a strain rate of 10−2 s−1, as compared to 10−3 s−1. This strain-rate sensitivity is attributed to the adiabatic heating of the samples and is found to be well predicted by the combination of an extended Olson–Cohen strain-induced martensite model and finite-element simulations for the evolving temperature distribution in the samples. In addition, the strain-rate sensitivity affects the deformation behavior of the steel. The α′ martensite transformation at high strains provides local strengthening and extends the time to neck formation. This reinforcement is witnessed by a load transfer from austenite to α′ martensite during loading.

Mechanical stability of retained austenite during plastic deformation of super high strength carbide free bainitic steels

Abstract: New carbide free bainitic microstructures are gaining an increasing interest on behalf the scientific and industrial community. The excellent combination of mechanical properties achieved in those microstructures with no need of complex heat treatments or thermomechanical processes represents their main advantage. The strength is mainly achieved by means of the very fine bainitic ferrite plates, consequence of the transformation mechanism, but the parameters contributing to the ductility of those microstructures are still unclear in this type of microstructures, where a soft phase, retained austenite, is imbibed in a very strong matrix of bainitic ferrite. A priori is reasonable to assume that retained austenite will control the levels of ductility achieved. Further enhancement of ductility can be achieved by the transformation of retained austenite into martensite (strain or stress assisted), thus its mechanical stability plays an important role in the final ductility. In this study, by means of X-ray analysis of interrupted compression tests, it is studied the influence that different microstructural aspects of retained austenite may have on its mechanical stability.

The effect of bainite morphology on the mechanical properties of a high bainite dual phase (HBDP) steel

Abstract: 4340 steel bars were austenitized at 850 °C for 1 h followed by heating at 700 °C for 90 min and quenching into a salt bath at the temperature range of 300–450 °C for 1 h to obtain dual structures with 34 vol.% fraction ferrite and various bainite morphologies. SEM studies showed that by increasing the austempering temperature, bainite morphology varies from lower to upper bainite. Tensile, impact and hardness tests revealed that increasing the austempering temperature from 300 to 400 °C leads to a reduction in yield and ultimate tensile strength, hardness, uniform and total elongation and impact energy. But in dual phase steel produced by austempering at 450 °C, yield and tensile strength and hardness increased and severe reduction in total elongation and impact energy obtained. Fractography of tensile specimens showed brittle behavior for this austempering temperature. Fatigue test results showed that fatigue limit decreases with increasing austempering temperature from 300 to 400 °C. Finally, fractography studies showed cleavage fracture at the surface of fatigue specimens austempered at 400 °C, which confirms the tendency to brittle behavior.