The bake hardening behaviour of electro‐galvanized cold rolled CMnSi and CMnAlSi TRIP steels

Cold rolled, intercritically annealed TRIP steels will very likely be used as electrogalvanized sheet steel for passenger safety‐related automotive body parts. This implies that their properties during and after manufacturing, i.e. in the deformed and paint‐baked state, must be known to evaluate their actual in‐service properties. The ageing behaviour of electrogalvanized TRIP steels and the effect of controlled ageing of the complex microstructure by paint baking on the mechanical properties was therefore investigated. Electroplating tests showed that the H uptake during pretreatment and electrogalvanization is limited and that the paintability of the electroplated sheet steel is unaffected even after deformation induced transformation of the austenite. In addition, a strong bake hardening effect was found in the case of the CMnAlSi TRIP steel. Internal friction measurements indicated that a damping peak for interstitial C could be obtained but that the free C content was less than 1 ppm prior to paint baking. A BH mechanism for TRIP steels is proposed.     

plane-strain work hardening and transient behavior of interstitial-free steel

Stress transients resulting from abrupt changes in strain path have been shown to be important to subsequent formability. In order to investigate whether these transients result from strain aging or related interstitial effects, two-stage experiments were performed on Armco interstitial-free steel After a prestrain in plane-strain tension, the material was strained in uniaxial tension in the direction of zero initial extension. The stress-strain curve in plane strain was found to deviate markedly from that predicted by usual plasticity theory (Hill’s theory withM = 2.0). Comparison of monotonie curves from uniaxial and plane-strain tension using a newly-developed, self-consistent calculation suggested that IF steel follows Hill’s new theory with constantM ≈ 2.9. After the change from plane strain to uniaxial tension, positive stress transients (flow stress exceeds the monotonie flow stress) were measured. This form of transient agrees with ones measured for other steels. It therefore appears that the origin of the transient phenomenon is independent of interstitial content, and that static strain aging is not the mechanism by which these stress transients occur. Formerly A. E. Browning, Graduate Research Associate, Department of Metallurgical Engineering, The Ohio State University     

Effect of free carbon and aging condition on bake hardening

Abstract

Though the basic science behind bake hardening has been known for some time, designing alloys is difficult in that the final bake hardening property depends mainly on the aging conditions and the amount of free C, which itself is a function of alloying additions, cooling rates, etc. In the present work, the amount of free C along with aging condition and deformation was measured before aging and taken into consideration to make a model which was used to resolve certain issues related to their effects on the final strength increment.     

Precipitation hardening of HSLA steels

A process model to describe the strength contribution from precipitation hardening during coiling after hot rolling has been developed for V and Nb HSLA steels. Experimental measurements of ageing behaviour on the V steel were conducted on coil material which was received in an underaged condition. The size and composition of precipitates was examined on replicas using scanning transmission electron microscopy. The precipitates were observed to be V rich and a substantial increase in precipitate size occurred as a function of ageing time. The modelling approach developed by Shercliff and Ashby for aluminium alloys was extended to microalloyed steels. The model assumes particle coarsening is the rate controlling process and that the precipitates are initially sheared by dislocations with a transition to non-shearable precipitates at peak strength. After calibration of the model, good agreement was observed between the model predictions and experimental data. A model for ageing in a Nb HSLA steel was also developed using literature values for the ageing behaviour.     

Optimizing continuous annealing of interstitial-free steels for improving deep drawability

The crystallographic textures and the resulting plastic anisotropy of five interstitial-free (IF) steels with different carbon equivalents and Nb and Ti microalloying content have been investigated. The steels were industrially hot rolled, cold rolled, annealed, and finally hot-dip galvanized. An alternative heat treatment of the cold rolled samples was conducted in laboratory scale using parameters close to those in industry-scale continuous annealing lines. The anisotropy parameters were both measured and predicted on the basis of the measured texture data. The calculated values were corrected by using functions that were fitted to the experimental data. It was found that for a given hot and cold rolling state even minor changes in the annealing conditions can improve the anisotropy parameter by up to 13 pct. Increase in recrystallization texture and improvement of the resulting anisotropy parameters are discussed in terms of partial transformation of ferrite to austenite in the intercritical regime.     

Development of {111} transformation texture in interstitial-free steels

The evolution of the transformation texture in two Ti₄C₂S₂-stabilized interstitial-free (IF) steels (Ti-and Ti/Nb-) was studied as a function of different thermomechanical processing (TMP) parameters using orientation distribution function (ODF) analysis. The ensuing transformation texture, largely independent of the steel composition and TMP path of the kind used in these experiments, is strongly related to the combination of initial austenite grain size and the amount of deformation applied in the early rolling passes. It is proposed that the formation of crystallographic heterogeneities in austenite induces the gamma-fiber texture development around the {111} components in the ferrite.     

Recrystallization of deep drawing columbium (Nb)-treated interstitial-free sheet steels

The softening response after isochronal annealing of cold-rolled, Cb-treated, vacuum decarburized-deoxidized, interstitial-free steels was investigated by hardness measurements, tension tests, and optical and transmission electron microscopy. Softening after annealing, following cold reduction, occurs by recovery-recrystallization and a reduction in precipitation hardening due to the coarsening of CbC precipitates. The recrystallization start temperature increases markedly with increasing amounts of Cb in solid solution in ferrite. Fine CbC precipitates retard recrystallization to a much lesser degree, and their effects can be eliminated by coarsening prior to cold reduction. Recrystallization initiates at free surfaces. Partially recrystallized structures consist of two layers of recrystallized matrix separated by an unrecrystallized zone. The recrystallized layers thicken with increasing temperature or time. This phenomenon is an inherent characteristic of these steels and not a result of external influences.     

Texture in deep drawing columbium (Nb)-treated interstitial-free steels

The development of the crystallographic texture in hot-rolled, cold-rolled, and recrystallized Cb-treated interstitial-free steels was investigated using the crystallite orientation distribution analysis as well as X-ray pole figures. The influence of chemical composition of the steel and processing variables on texture and on normal and planar anisotropy of the γ-value of cold-rolled and annealed sheet are discussed and compared with those of aluminum-killed deep drawing steels. While, in terms of ideal orientation components, the re-crystallization texture of aluminum-killed steels can be described as having significant amounts of {lll}〈110〉 and {lll}〈112〉 components, Cb-treated steels show these components and in addition even stronger {554}〈225〉 and {322}〈296〉 components. Distinctions in the hot-rolled texture, the cold-rolled texture, and the recrystallization texture are described. Cb-treated steels have an entirely different planar distribution of γ values or plastic strain ratios compared with aluminum-killed steels. The resulting average γ value, γ_m, is significantly higher for Cb-treated steels and results in superior deep drawing characteristics. A. ELIAS is deceased.     

Texture in deep drawing columbium (Nb)-treated interstitial-free steels

Metallurgical and Materials Transactions A - The development of the crystallographic texture in hot-rolled, cold-rolled, and recrystallized Cb-treated interstitial-free steels was investigated...     

A contribution to the increase in yield strength during the bake hardening process

Bake hardening denotes the controlled aging of steels during a thermal treatment as applied for example by paint baking outer panels of automobiles. Dependent on the duration of the thermal exposure and the temperature, the increase in yield strength can reach up to 80 MPa. The temporal progress is a function of the steel composition and the degree of prestrain and can appear in one or two stages. It seems that the interaction between precipitating ternary iron carbides and the formation of Cottrell atmospheres around mobile dislocations count responsible for the different shapes of the precipitation curves. In the present work an attempt is made to simulate the atmosphere forming and carbide precipitation with the use of classical models and take into account the degree of prestrain. The results are then compared with experimental data on the bake hardening process.     

Recrystallization of deep drawing columbium (Nb)-treated interstitial-free sheet steels

The softening response after isochronal annealing of cold-rolled, Cb-treated, vacuum decarburized-deoxidized, interstitial-free steels was investigated by hardness measurements, tension tests, and optical and transmission electron microscopy. Softening after annealing, following cold reduction, occurs by recovery-recrystallization and a reduction in precipitation hardening due to the coarsening of CbC precipitates. The recrystallization start temperature increases markedly with increasing amounts of Cb in solid solution in ferrite. Fine CbC precipitates retard recrystallization to a much lesser degree, and their effects can be eliminated by coarsening prior to cold reduction. Recrystallization initiates at free surfaces. Partially recrystallized structures consist of two layers of recrystallized matrix separated by an unrecrystallized zone. The recrystallized layers thicken with increasing temperature or time. This phenomenon is an inherent characteristic of these steels and not a result of external influences.     

不同成分体系对超低碳烘烤硬化钢性能的影响

研究了不同成分体系超低碳烘烤硬化钢的性能特征,生产结果表明:Ti-BH钢屈服强度和BH值较低,伸长率和"值富裕量较大,并且BH值波动较大.通过对不同成分体系钢板的微合金化原理进行分析,确定了不同成分体系烘烤硬化钢性能不同的原因,并据此将烘烤硬化钢的成分体系由Ti-ULC改为Nb+Ti复合体系.更改后的烘烤硬化钢性能指标...     

The role of coincident site lattice boundaries during selective growth in interstitial-free steels

The development of textures in interstitial-free (IF) steels as a result of annealing after cold rolling is described with the help of a combined nucleation and growth model. Nucleation is simulated by assuming that high stored energy nucleation occurs preferentially in high Taylor factor regions in the 75 to 85 pct cold reduced materials. Growth of the nuclei then takes place by means of Σ (110) type as well as by Σ 7 (111) type coincident site lattice (CSL) transformations. Of the six symmetrically equivalent (110) transformation axes, only the ones near the maximum shear stress poles are assumed to operate. The effects of the migration of individual Σ 9, Σ 11, Σ 17c, Σ 19a, Σ 33a, and Σ 33c (110) boundaries are analyzed. Their relative mobilities and contributions to the final texture are deduced by matching the simulated and experimental preferred orientations using a /ldleast-squares” method. On the basis of experimental results for two steels, the various boundary types are observed to have the following mobility ratios: Σ 33a: 12, Σ 19a:4, Σ 9:1, Σ 33c:l, and Σ 17c: 2.     

Effect of bake hardening treatment on the mechanical behaviors of aluminum alloy spot welding joints

In this paper, the mechanical behaviors of the spot welding aluminum alloy in both baked and unbaked conditions are investigated. The spot welded specimens with two different base aluminum alloy sheets are studied by tensile and fatigue tests. The test results of unbaked specimens are compared with the baked which are baked at 185°C for 20 min. The stress distribution of the specimens and the fatigue life under different stress is simulated by finite element method (FEM). The paint bake treatment can increase the ductility of both base materials and spot joints. The fatigue life is increased with high loading stress due to the pinning effect; while with low fatigue load, it has the opposite rule because the compressive residual stress is released during the bake hardening process, which can restrain the fatigue crack growth rate in low stress. The simulated results of FEM model can well verify the experiment data.     

Effect of prestrain on aging and bake hardening of cold-rolled, continuously annealed steel sheets

To understand the bake hardening (BH) behavior in an actual automotive part, 40 tensile specimens were machined from the actual press-formed outer-door panel of a compact car and both bake and work hardenability distribution data were determined. Strain applied by actual press forming was estimated from the work hardening data. Finally, the effects of prestraining mode and amount on ambient aging and BH response were also investigated. The BH widely ranged from 10 to 54 MPa and the work hardenability was between 17 and 82 MPa, depending on panel location. Bake hardening in the outer-door panel decreased as the work hardening increased, indicating that the BH steel must be applied to the shallow drawn parts in order to maximize the BH effect in dent resistance. In order to establish the effects of prestrain and ambient aging time on the age and subsequent BH, the specimens were prestrained and aged at ambient temperature for various time intervals, and then baked at 170 °C for 20 minutes. In the as-temper-rolled and press-formed condition, the steels were extremely resistant to ambient aging. However, it was found that a 0.3 pct tensile prestrain was sufficient to initiate ambient aging within 1 day, and the effect was accelerated with greater prestrain. With 8 days of ambient aging, all prestrained steels exhibited 20 to 25 MPa of age hardening. Irrespective of prestrain amount in the range of 0.5 to 5.0 pct, the BH decreased as the aging time increased.     

Aspects of Orientation-Dependent grain growth in extra-low carbon and interstitial-free steels during continuous annealing

The present work concentrates on the application of orientation imaging microscopy (OIM) based on the electron backscattered diffraction (EBSD) technique to the investigation of the microstructural evolution of an extra-low carbon (ELC) steel and a Ti-Nb-bearing interstitial-free (IF) steel, during continuous annealing. Aspects like the nucleation, the evolution of the recrystallized volume fraction and grain size of grains with different orientations, the interface area limiting recrystallized {111} regions, and the apparent growth rates have been considered. Different criteria have been applied in order to identify crystallites produced during annealing. During the first stages of annealing, a network of grain boundaries with misorientations higher than 10 deg is produced, mainly inside the deformed γ-fiber grains. The crystallites formed within this network, free from cells or subgrains at their interiors, can be considered as potential nuclei. However, among all, only some of them become effective due to an improtant selection. The {111} recrystallized grains have a significant size and number advantage as compared with other texture components, and a hard impingement between clusters of {111} grians is produced during grain growth. The effect of grain growth behind the recrystallization front seems to be negligible as compared with the grain coarsening produced by the migration of this front, driven by the cold-work stored energy. J.L. Bocos, formerly Researcher with CEIT     

Aspects of orientation-dependent grain growth in extra-low carbon and interstitial-free steels during continuous annealing

The present work concentrates on the application of orientation imaging microscopy (OIM) based on the electron backscattered diffraction (EBSD) technique to the investigation of the microstructural evolution of an extra-low carbon (ELC) steel and a Ti-Nb-bearing interstitial-free (IF) steel, during continuous annealing. Aspects like the nucleation, the evolution of the recrystallized volume fraction and grain size of grains with different orientations, the interface area limiting recrystallized {111} regions, and the apparent growth rates have been considered. Different criteria have been applied in order to identify crystallites produced during annealing. During the first stages of annealing, a network of grain boundaries with misorientations higher than 10 deg is produced, mainly inside the deformed γ-fiber grains. The crystallites formed within this network, free from cells or subgrains at their interiors, can be considered as potential nuclei. However, among all, only some of them become effective due to an important selection. The {111} recrystallized grains have a significant size and number advantage as compared with other texture components, and a hard impingement between clusters of {111} grains is produced during grain growth. The effect of grain growth behind the recrystallization front seems to be negligible as compared with the grain coarsening produced by the migration of this front, driven by the cold-work stored energy.     

A combined model of oriented nucleation and selective growth for the recrystallization of interstitial-free steels

A combined model of oriented nucleation and selective growth is proposed to describe primary recrystallization in interstitial-free (IF) steels. The model includes oriented nucleation at high Taylor-factor orientations together with orientation transformations (OTs) about selected 〈110〉 axes over a misorientation range that includes the Σ9, Σl 1, Σ19a, and Σ33a type coincidence site lattice (CSL) relationships. It is shown that only one or two of the six possible 〈110〉 rotation axes operate in a given grain and at a particular grain boundary. These are the ones closest to the maximum shear stress (MSS) poles for cold rolling. The present study suggests that IF steels display particularly sharp textures (compared to plain C steels), because the factors that favor the selection of the trans-formation axes and the more rapid growth of CSL boundaries operate more effectively in these materials.