Ultra-low Carbon Bake Hardening Research Articles

Effect of grain size on the static strain aging of a ULC-bake hardening steel

Abstract Bake hardenability in ultra low carbon bake hardening (ULC-BH) steels is primarily influenced by the (i) solute carbon content in the matrix, (ii) dislocation density and (iii) grain boundaries. Grain boundaries in fine-grained steels significantly change the dislocation distribution and carbon-concentration profiles from grain interior to the grain boundary and correspondingly, the bake hardening response may change. The present study examines the effect of different grain sizes (66–15 µm) on the bake hardening behavior of a ULC-BH steel (total carbon content 24 wt.ppm) as a function of tensile strain (2–5%) and aging temperatures (50–170°C). The carbon concentration profiles across the grains of different sizes have been evaluated both through modeling calculations and experimental measurements. Results showed that the bake hardening decreases with decreasing grain size in general, but there is a critical fine grain size corresponding to a specific amount of tensile strain, at which the bake hardening increases significantly due to the contribution of grain boundary carbon and increased dislocation density adjacent to the grain boundaries.

Probable Maximum Sizes of Inclusions Predicted by SEV and PSD for BH Steels of Automobile Exposed Panel with Different Sulfur Contents

This study attempted to estimate the maximum size of inclusions in the ultra-low carbon Bake Hardening (BH) steels of automobile exposed panel. The Probable Maximum Sizes (PMS) of inclusions at the different steelmaking stages for BH steel with different sulfur contents were predicted by two methods of Statistics of Extreme Values (SEV) and Particle Size Distribution (PSD). The S content does not show a relationship with the PMS prediction of inclusions in the molten steel in which Al2O3 is the main inclusion, while the higher content of S leads to a larger PMS value in the slab, due to more number of large-sized Al2O3-MnS inclusions formed during solidification. The PMS value in the slab is greater than that in the molten steel for BH steel. Thus, the PMS of inclusions in the slab cannot be estimated from the molten steel samples. The SEV can be used to predict well the PMS values at different steelmaking stages for BH steels. However, the PSD of exponential function cannot predict well the PMS value in the slab for BH steel when considering all kinds of inclusions due to the large influence of small-sized MnS with high number density on the PSD of exponential function. When only considering Al2O3-MnS inclusions, the PSD of exponential function can make a reasonable PMS prediction in the slab, because the size distribution of Al2O3-MnS with large size can follow the exponential function.

Effect of annealing time and phosphorus addition on bake hardening behavior of ultra-low carbon bake hardening steel

The relationship of the P and C grain boundary segregation and its effect on bake hardening behavior were investigated in ultra-low carbon bake hardening (ULC-BH) steel with and without P addition annealed at 810 °C for various time using electron probe micro-analyzer, electron backscattered diffraction, and three-dimensional atomic probe techniques. Results revealed that P addition and annealing duration considerably affected the bake hardening behavior of experimental steel. The BH value of ULC-BH steel without P addition is lower than that with P addition within a short annealing time, and the difference in the BH value gradually decreases as the annealing duration is prolonged. P segregation is dominant in terms of a high P bulk content in steels with P addition at the expense of C segregation during annealing. By contrast, opposite effects are observed in low carbon bake hardening steel. The high residual solute C content in steel with P addition is due to P segregation at the grain boundary. Site competition is mainly responsible for the lower BH value in ULC-BH steel without P addition than that with P addition. As the annealing time is further extended, C segregation begins at grain boundary despite the delayed P segregation, leading to a gradual decrease in the solute concentration in the matrix of steels with P addition. C and P segregations reach the equilibrium as the annealing time increases to 60 min at 810 °C in the two steel samples. Theoretical calculations reveal that the residual solute C concentration in the matrix decreases to zero, and this finding is consistent with the change trend of the bake hardening value. Hence, the C segregation at grain boundary adversely influences the bake hardening property of ULC-BH steel.

Effects of prestrain and grain boundary segregation of impurity atoms on bake hardening behaviors of Ti+V-bearing ultra-low carbon bake hardening steel

The distribution pattern of solute atoms, interaction between C atoms and dislocations, and bake hardening behaviors of the P-added and P-free ultra-low carbon bake hardening (ULC-BH) steels were investigated, which were annealed at 810 °C for 2 min and subsequently treated with prestrains and baking. The three-dimensional atomic probe (3DAP) and internal friction results revealed that C atoms were homogeneously distributed in the matrix of the P-added ULC-BH steel, but they were strongly segregated in the P-free ULC-BH steel after annealing at 810 °C for 2 min. The solute C concentration of the P-added ULC-BH steel was higher than that of the P-free ULC-BH steel. The addition of P greatly strengthened the BH behavior of the ULC-BH steel. The variation in the BH behavior with the prestrain was apparently different between the P-added and P-free ULC-BH steels. The BH value of the P-added ULC-BH steel gradually increased with an increase in the prestrain ranging from 1% to 10%. However, the value of the P-free ULC-BH steel increased by increasing the prestrain up to 8% and decreased subsequently. The BH behavior showed higher sensitivity to the solute C concentration for the ULC-BH steel than for the low carbon (LC) steel. The BH value either increased or decreased with an increase in the prestrain, which mainly depended on the solute C concentration, i.e., whether the solute C concentration was sufficient or not to saturate the dislocation generated by the prestrain during the baking process.

Effect of P addition on texture evolution of Ti+V-bearing ultra-low carbon bake hardening steel

The grain boundary segregation of phosphorous, precipitate behavior, and texture evolution were investigated in a P-added ultra-low carbon bake hardening (ULC-BH) steel and in an ordinary ultra-low carbon bake hardening steel annealed at various times at 750°C and 810°C. No observable difference was found in terms of precipitate behavior of the P-added ULC-BH steel and the ordinary ULC-BH steel. The grain boundary segregation of phosphorous has a profound effect on the recrystallization texture of the ULC-BH steel during the annealing process. Phosphorous addition could delay recrystallization and could refine the grain of the P-added ULC-BH steels after annealing. Furthermore, the {111}//ND texture of the P-added ULC-BH steel was stronger compared with ordinary ULC-BH steel annealed at 750°C. However, the opposite results were observed after annealing at 810°C, thereby indicating that the grain boundary segregation of phosphorous can be both beneficial and deleterious for {111}//ND texture development of the ULC-BH steel, mainly depending on the annealing temperature. The complexity of the various effects could have been primarily caused by the relative distribution of phosphorous at the differently oriented grain boundary in the P-added ULC-BH steels annealing at certain temperatures.

Role of Mn and P in Texture of Bake Hardening Steel during Heat Treatment

Samples of three ultra-low carbon bake hardening (ULC-BH) steels (BH-0, BH-Mn, and BH-P) were prepared by two annealing processes followed by water quenching and overaging. The texture evolution in the as-treated steels was explored via EBSD and ODE, and the roles of Mn and P elements as well as heat treatment processes in the texture orientation and intensity were investigated. It is found that overaging significantly increases texture intensity of BH-0 and BH-Mn steel, while markedly hinders the texture development in BH-P steel. Based on the microstructure analysis, it can be deduced that the texture variations in BH-0 and BH-Mn steels mainly contribute to carbide (in BH-0 and BH-Mn steels) and manganese containing compound (in BH-Mn steel) or partially to C segregation to dislocation, while the texture variation in BH-P steel is mainly caused by the P and C co-segregation formed during quenching.

Interstitial Solution Carbon Concentration and Defects of Ti + Nb ULC-BH Steel by Internal Friction and Positron Annihilation Methods

The interstitial solution carbon concentration and defects in continuously annealed Ti+Nb bearing ULC-BH (ultra-low carbon bake hardening) steel samples are investigated by multi-functional internal friction apparatus, positron annihilation lifetime spectroscopy (PALS) and coincidence Doppler broadening spectroscopy (CDBS). The relationship of internal friction peaks, interstitial solution carbon concentrations and movable dislocations in the samples under different conditions is analyzed. A correlation of lifetime component τ1 values with interstitial solution carbon concentrations in the samples for different continuous annealing processes is established, while a correlation of lifetime component τ2 values with multi-vacancies, vacancy clusters, microvoids and other types of defects for various continuous annealing processes is also demonstrated. Furthermore, the average lifetime results illustrate the overall defect densities for various continuous annealing processes. The CDBS analysis reflects the chemical surroundings of the defects at the annihilation sites and reveals that the peak heights of the ratio curves relate to the total number of defects such as interstitial carbon atoms, dislocations, vacancies and other types of defects. The results show that internal friction, PALS and CDBS are effective techniques to identify and characterize the interstitial solution carbon concentration, multi-vacancies, vacancy clusters, microvoids and other types of microscopic defects in annealed Ti+Nb bearing ULC-BH steel.

Effects of Rolling Parameters on Texture and Formability of High Strength Ultra-Low Carbon BH Steel

The effects of hot rolling and cold rolling parameters on texture and r (plastic strain ratio) value of high strength ultra low carbon bake hardening (ULC-BH) steels are studied with orientation distribution function (ODF) structural analysis method. After hot rolling, the high strength ULC-BH steel sheet has weak γ-fiber with uniform orientation distributions, and weak γ-fiber, of which {445}>110< component forms a high intensity peak at coiling temperature of 750 °C. After cold rolling, both {111}>110<−{111}>112< intensity on the γ-fiber and {111}−{112} >110< intensity on the α-fiber enhanced. As a result of substitutional solute elements Mn and P being added to the steel, strong {112}>110< deformation texture component is observed on γ-fiber, especially at 80% cold rolling reduction, and this leads to the strong {111}>112< recrystallization texture after annealing. The increase of cold rolling reduction shifts the maximum intensity on the γ-fiber from {111}>112< to {111}>113<. After annealing, a very strong γ-fiber is obtained, with intensity peak at {111}>112< component when cold rolling reduction reaches 80%. Increasing coiling temperature and cold rolling reduction improve γ-fiber intensity and r value, resulting in good deep drawability.

Effect of Mn and P on precipitation behavior and solute distribution in ultra-low carbon bake hardening steels

The effect of Mn and P on types of precipitates as well as solute distributions in ultra-low carbon bake hardening (ULC-BH) steels was originally investigated in this paper. Three samples of hot-rolled ULC-BH steels (Ti–V, Ti–V–Mn, and Ti–V–P) were prepared. The types of precipitates in the three samples were characterized with scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and further calculated by Thermo-Calc. The different types of nanometer sized vanadium carbides are clearly characterized by SEM and TEM, and results of calculation and experiments are generally in agreement. Besides, the solute distributions in Ti–V–Mn and Ti–V–P steels were detected by three-dimensional atom probe (3DAP). It is found that no Ti, N, or S elements are present in both of the two steels, and the distribution patterns of the solute elements in the two steels are completely determined. Results show that SEM, TEM combining with 3DAP can well detect the existence of elements added into the bake hardening steels, and are very useful techniques for the base of the further investigation of mechanical properties and bake hardening phenomenon in bake hardening steels.

Effect of Baking on Spot Welds of a Prestrained Ultra-Low Carbon Bake Hardening Steel Sheet

The effect of bake hardening (BH) treatment on the strength of spot-welded joints made on prestrained (PS) sheets of an ultra-low carbon (ULC) BH grade steel has been investigated using tensile-shear specimens. A series of these experiments has been carried out on spot-welds made on as-received and PS sheets; a part of the latter being subjected to BH treatment. Determination of hardness profile along the weldments, characterization of the microstructures, and substructures by optical microscope, scanning electron microscope (SEM), and transmission electron microscope (TEM), as well as fractographic examinations are some necessary supplementary examinations. The results infer that the strength of the PS sheets increases by BH treatment, but this treatment does not exhibit any significant influence on the strength of the spot-welds made on PS sheets. The hardness profile along HAZ and fusion zone of the spot-welds made on as-received sheets remains almost unaltered for PS specimens or that subjected to BH treatment. The locations of failure around the spot-welds of as-received (AR), PR, and bake hardened specimens are similar in nature, and these failures usually originate from the HAZ-base metal interfacial region.

Nature of strain aging stages in bake hardening steel for automotive application

Strain aging behavior of industrially produced ultra low carbon bake hardening (BH) steel for automotive application was investigated. The aging process was studied by the dynamic Young's modulus and amplitude-independent dislocation internal friction measurements with acoustic methods. Analysis of the revealed strain aging stages was carried out and has resulted in the conclusion that formation of interstitial atoms atmospheres on the dislocations is accompanied and substantially affected by the dissolution of their grain boundary segregations.

Effect of grain size on the static strain aging of a ULC-bake hardening steel

Bake hardenability in ultra low carbon bake hardening (ULC-BH) steels is primarily influenced by the (i) solute carbon content in the matrix, (ii) dislocation density and (iii) grain boundaries. Grain boundaries in fine-grained steels significantly change the dislocation distribution and carbon-concentration profiles from grain interior to the grain boundary and correspondingly, the bake hardening response may change. The present study examines the effect of different grain sizes (66 – 15 lm) on the bake hardening behavior of a ULC-BH steel (total carbon content 24 wt.ppm) as a function of tensile strain (2 – 5 %) and aging temperatures (50 – 170 °C). The carbon concentration profiles across the grains of different sizes have been evaluated both through modeling calculations and experimental measurements. Results showed that the bake hardening decreases with decreasing grain size in general, but there is a critical fine grain size corresponding to a specific amount of tensile strain, at which the bake hardening increases significantly due to the contribution of grain boundary carbon and increased dislocation density adjacent to the grain boundaries.

A Model for the Cottrell Atmosphere Formation During Aging of Ultra Low Carbon Bake Hardening Steels.

A model describing the formation of the carbon atmosphere around a dislocation during aging of vacuum degassed ultra low carbon bake hardening steels has been developed by extending the original Cottrell theory taking into account the effects of the variation of the free carbon concentration, the saturation of dislocations and the segregation of carbon atoms to grain boundaries. Strain aging experiments have been carried out with an ultra low carbon steel and the experimental results been compared with the theoretical predictions. It is demonstrated that the model developed can describe the formation of Cottrell atmosphere in ultra low carbon bake hardening steels in industrial circumstances.