Bake Hardening Research Articles

Bake Hardening Behavior and Plastic-strain-dependent Elastic Modulus of Steel Sheets for Automotive Outer Panels

Bake Hardening Behavior and Plastic-strain-dependent Elastic Modulus of Steel Sheets for Automotive Outer Panels

Investigation on deformation and strengthening mechanisms of Al-Zn-Mg-Cu alloy under a hybrid process of cryogenic incremental sheet forming and bake-hardening treatment

Incremental sheet forming process is featured with high flexibility and short manufacturing cycle, which has great potential in customized forming of complex components. However, the formability of high-strength aluminum alloy is still needs to be improved. In this paper, an efficient cryogenic incremental sheet forming (CISF) process combined with bake-hardening (BH) treatment is proposed to achieve excellent formability and strength for Al-Zn-Mg-Cu alloy. First, the effect of different cryogenic temperatures on the formability of Al-Zn-Mg-Cu alloy is investigated and the mechanism of plasticity enhancement during cryogenic incremental sheet forming was revealed. The fracture forming limit lines constructed from different components show a significant increase in the formability of as-quenched Al-Zn-Mg-Cu at −170°C. Specially, when forming cone component with variable wall angle at −170℃, the ultimate forming height is increased by 70.4 % compared with 25℃. The microstructure shows that dislocation cross-slip is suppressed at cryogenic temperatures, and the large number of subgrain structure and uniformly distributed dislocations reflect the improved deformation uniformity at cryogenic temperatures. Second, the effect of cryogenic incremental sheet forming on the strength of Al-Zn-Mg-Cu is explored. The vertical forming force can be increased by 37.4 % at −170°C compared to 25°C, and the components have higher post-forming strength. Moreover, the strengthening response of the components after the combination of cryogenic incremental sheet forming and bake-hardening treatments is investigated. The cryogenic specimens exhibited a rapid bake-hardening response compared to the room temperature specimens. The yield strength increased by 15.4 % to 387.2 MPa after 40 mins of bake-hardening treatment at 180°C. It has been shown that precipitation strengthening and dislocation strengthening are the main strengthening mechanisms during cryogenic incremental sheet forming and bake-hardening treatments. This study reveals the dual enhancement effect during the cryogenic incremental sheet forming process, and proposes an efficient approach which combines cryogenic incremental sheet forming with bake-hardening treatment to manufacture high-performance components.

Fracture toughness and fracture mechanism of Al-Mg-Si alloy sheet subjected to pre-cryorolling and subsequent bake hardening

The fracture toughness of AA6061 sheets subjected to pre-cryorolling and room-temperature pre-rolling with varying reductions (5 %, 10 %, 15 %, and 20 %) followed by bake hardening was investigated. In the direction parallel to the rolling direction (R-T), the unit crack initiation energy (UIE) of the pre-cryorolled samples after bake hardening exhibited an increase of 0.8–6 % compared to samples without pre-rolling. In contrast, the UIE of the room-temperature pre-rolled samples displayed a varying degree of decrease. This difference can be attributed to the smaller mean grain size in the pre-cryorolled samples compared to the room-temperature pre-rolled samples. Due to work hardening, the crack propagation resistance of the pre-rolled samples was diminished, as evidenced by a decrease in crack propagation energy (UPE) with increasing reduction. The variation trends of UIE and UPE in the direction perpendicular to the rolling direction (T-R) were similar to those observed in the R-T direction. The fracture mechanism of the samples during the tearing process involves a combination of intergranular and transgranular fractures. However, samples in the R-T direction exhibit higher resistance to crack propagation compared to those in the T-R direction, which is reflected in the larger UIE and UPE. The greater resistance in the R-T direction can be attributed to the significant elongation of grains prior to fracture, resulting in a higher degree of plastic deformation in the region near the crack. Additionally, the proportion of transgranular fracture along the crack propagation path is greater in the R-T direction.

The combined effect of pre-aging and Mg/Si ratio on the natural aging and bake hardening in Al-Mg-Si-0.7Cu alloys

The combined influences of pre-aging treatment (PA) and changing Mg/Si ratio on the natural aging (NA) effect and bake hardening response for a series of 6000 series alloys with excess Mg and Cu content was studied. The results showed that PA provides a clear improvement in alloy hardness after paint baking whereas a rising excess Mg (from 1.5 to 3.0) decreases this effect. The hardness in T4P temper however was generally relatively low, due to the combined effect of PA and a high Mg/Si ratio. The strong interaction energy between Cu and Mg was responsible for facilitating the clustering and precipitation processes inducing this behavior. During NA, Mg-Cu and Mg-Mg clusters were found in addition to the Mg-Si(-Cu) clusters, with the total cluster number density decreasing in excess-Mg alloys, resulting in reduced T4P temper hardness. After bake hardening, the disordered β″ precipitates, containing substructures of “low density cylinders” (LDC) and Cu sub-unit clusters, and the lath-like QP2 phases are responsible for the excellent hardening response regardless of Mg/Si ratio effects. The alloy with a Mg/Si ratio of 1.5 exhibits a low strength in T4P temper (134 MP) and very high strength (274 MPa), coupled with a good ductility (14%) after paint baking. This alloy has great potential for automotive application.

On Local Formability/Ductility of New Advanced High-Strength Steels: Temperature, Bake Hardening and Strain Rate Effects

On Local Formability/Ductility of New Advanced High-Strength Steels: Temperature, Bake Hardening and Strain Rate Effects

Revisit of bake hardening mechanism: Influence of baking on tensile properties of press hardening steels

Paint baking is a process in automotive industries to accelerate the curing of paint and increase yield strength of steel panel for improving the dent resistance due to the formation of well-known Cottrell's atmosphere. However, our careful study on bake hardening mechanism of three press hardening steels (PHS) indicates a different explanation. We developed two novel Nb/Si alloyed steels with superior tensile properties when compared with the commercial grade of 22MnB5. It was found that the paint baking at 170 °C led to the significant increase of yield strength but the decreases of both uniform and total elongation for all the three PHSs. According to the neutron diffraction results, both the baking and the tensile straining until the elastic-plastic transition strain (εep) led to the decrease of full width at half maximum (FWHM), indicating that both should release the residual stress (RS) that was generated during martensite transformation; whilst the internal friction examinations reveal no presence of Snoke peak and no influence of the baking on Snoek-Kê-Köster peak. Based on these results, the kinetics simulation on forming Cottrell's atmosphere and the relevant microstructural characterization results, we can safely conclude that the strengthening caused by the baking is not ascribed to the change of Cottrell atmosphere, carbide precipitates or retained austenite (RA) grains during the baking although commonly believed, but just due to the release of RS. Moreover, the relaxation of compressive RS in martensite and the transformation of RA grains to martensite are the main contributor to the strain hardening of invented PHSs before εep. Therefore, the baking treatment decreases both strain hardening capability and ductility of studied PHSs due to the release of the RS.

Effects of Pre-Strain on Bake Hardenability and Precipitation Behavior of Al-Mg-Si Automotive Body Sheets

Effects of Pre-Strain on Bake Hardenability and Precipitation Behavior of Al-Mg-Si Automotive Body Sheets

Integrating reversion ageing and forming of high-strength Al alloys: Principles and theoretical basis

Simultaneously improving the formability and post-formed strength of high-strength aluminum (Al) alloys, such as Al–Zn–Mg–Cu alloys, is essential in manufacturing complex-shaped panel components. The strict requirements on heat-treatment condition and high tooling costs limit the applications of current forming methods. A novel process called integrated reversion ageing and forming (IRAF) is proposed to form naturally aged (NA or T4 tempered) Al alloys. A principle-based concept analysis and systematic thermo-mechanical-metallurgical study of the IRAF process were performed. Additionally, tensile tests were conducted to evaluate the effects of parameters including heating rate, holding time, and forming temperature on formability and baked strength. The deformability of the AA7075-T4 alloy can be significantly enhanced through rapid heating to the reversion ageing temperature (150–300 °C), followed by short-term holding, as evidenced by the reduced yield strength of 200 MPa and increased uniform ductility. An instant strength increase to a value close to that of the T6 state was obtained after a short bake hardening (BH) treatment. Further, temperature-time-property (TTP) diagrams were established based on the correlation between the measured mechanical properties and through-process microstructure evolution to explain the mechanism underlying the optimised processing window of IRAF. The results indicate that fast-heating rate (>300 °C/min) promotes the reversion of NA clusters and inhibits re-precipitation of solutes, thereby improving the warm formability. Reversion ageing above 240 °C could induce the formation of coarse η'/η phases, leading to a considerably declined BH response. To accurately predict the strength evolution and deformation behavior during IRAF, a physical-based unified constitutive model was constructed by considering the reversion of NA clusters and solute re-precipitation. The bending and drawing tests on the AA7075-T4 alloy sheets verified that IRAF in the most-reverted state enabled optimum formability. The findings inspire promoting the reversion of pre-existing metastable particles to improve warm formability and post-formed age hardening.

Research on strain aging behavior of ultra-high strength dual-phase steel

The bake hardening value is one of the vital strength indexes of dual-phase steel, representing the strengthening ability of materials after pre-strain and baking, playing an important role in vehicle safety and lightweight design. Studying and improving the strain aging mechanism of dual-phase steel helps one to understand the material characteristics and enhances its utilization value. However, the ultra-high strength dual-phase steel is often prone to fracture outside the gauge length of a tensile specimen of the bake hardening value test. No suitable theory explains the fundamental law of dislocation pinning during the saturation stage at present. This paper used FEA, DIC, SEM, TEM, internal friction, and metallographic methods to study the strain aging behavior of dual-phase steels under different pre-strain, bake time, and bake temperature conditions. The results show that the fracture outside the gauge length is related to factors such as the uneven distribution of pre-strain and the ultra-high upper yield strength. The rolling pin shape tensile specimen testing has successfully solved this testing problem. The measured results at the saturation stage of dislocation pinning are in good agreement with the fitting results of the dislocation pinning strengthen mechanism based on the probability event quantization assumption.

Unprecedented bake hardening responses of interstitial high-entropy alloy by synergistic effect with lattice distortion

High-entropy alloys (HEAs) composed of multiple principal elements have attracted considerable interest in recent decades as a new class of metallic materials. Among them, the equi-atomic CoCrFeMnNi HEA system has great potential for application as a structural material due to its excellent properties in various fields, but it has a drawback of relatively low yield strength (YS) at room temperature. Here, for the first time, we propose the bake hardening (BH) as a breakthrough to overcome the relatively inferior YS of the CoCrFeMnNi HEA system. We found that the carbon-added CoCrFeMnNi HEAs are not only bake-hardenable, but also exhibit unprecedented BH responses than conventional bake-hardenable materials, with BH responses up to 120.0 MPa depending on the carbon concentration in the matrix. It is the outcome of the mutual synergistic effect between the massive solid solution strengthening characteristic from lattice distortion of the HEAs and the Cottrell atmosphere generated during the BH process. This study demonstrates that the carbon-added CoCrFeMnNi HEAs are more advantageous to apply the BH effect than conventional bake-hardenable materials, which suggests the possibility of industrialization of various interstitial HEA systems as well as carbon-added CoCrFeMnNi HEAs.

Effect of Room-Temperature Pre-rolling and Pre-cryorolling on Natural Aging and Bake Hardening Response of an Al–Mg–Si Alloy

Effect of Room-Temperature Pre-rolling and Pre-cryorolling on Natural Aging and Bake Hardening Response of an Al–Mg–Si Alloy

Resistance spot welding of high-strength low-alloyed (HSLA) 420 steel and bake-hardening (BH) 220 steel

Resistance spot welding of high-strength low-alloyed (HSLA) 420 steel and bake-hardening (BH) 220 steel

Influence of pre‐strain and bake hardening on the static and fatigue strength of a DP600 steel sheet

AbstractThe influence of pre‐strain on the tensile and fatigue properties of a dual phase DP600 steel was studied. The material was pre‐strained by uni‐axial tension in rolling and transverse direction. Thereafter, specimens were cut from the deformed plates in parallel or orthogonal to pre‐strain direction. It was found that pre‐strain increases yield and tensile strength. Results suggested that strain path change primarily affects the elastic‐plastic transition during early stage of reloading. Pre‐strained specimens showed an increase in high cycle regimes as a consequence of yield strength increment, irrespective of imposed pre‐straining direction. A modified stress life equation that accounts for pre‐strain was proposed and showed good agreement with experimental data. Bake hardening enhanced both tensile and high cycle fatigue resistance. Walker equation was successfully fitted to account for tensile mean stress. In low cycle fatigue, negligible influence of pre‐strain was observed due to cyclic softening and residual stress relaxation.

Effects of Prestrain and Baking on the Tensile and Fatigue Properties of Fe–0.1C–5Mn Transformation‐Induced Plasticity Steel

This study aims to investigate the effects of uniaxial tensile prestrain (PS) and bake hardening (BH) on tensile and force‐controlled (R = 0.1) high‐cycle fatigue (HCF) properties of Fe–0.1C–5Mn medium Mn transformation‐induced plasticity (TRIP) steel. Stress‐life data of the as‐received (AR: 0% prestrain) and 10%, 15%, and 20% prestrained (PS10, PS15, PS20) samples have been generated under both baked and unbaked conditions. It is manifested that prestraining increases the HCF life of the steel. The addition of BH further increases the HCF life. The fatigue limit is greatly improved from 433 MPa (AR) to 532 MPa (BH20), and the maximum incremental value is as great as 99 MPa. It is attributed to the austenite to martensite transformation (progressive TRIP effect) that occurs during the prestraining and cyclic loading processes.

Achieving unprecedented yield strength of 2.2 GPa with high ductility in formed parts using strain-aging

Ultra-high yield strength is important in extreme service conditions, but it severely deteriorates the ductility and also brings significant challenges for material forming. Here, we successfully developed a strategy involving the compositionally lean alloy design and strain-aging processing route to endow medium Mn steels with unprecedented ‘part performance’ of 2.0–2.3 GPa yield strengths and 8–14% uniform elongations. The significant bake hardening response with the maximum of 465 MPa higher than the most of existing researches is reported, which leads to the ultra-high yield strength above 2 GPa. By applying pre-strains, the morphology and volume fraction of retained austenite can be tailored, which affects the Lüders bands and the transformation-induced-plasticity (TRIP) effect. In the suitable pre-strain range, the film-like retained austenite with high stability can provide sustained TRIP effect, thus ensuring high uniform elongations in the strain-aged samples.

Novel bake hardening mechanism for bainite-strengthened complex phase steel

• Yield strength of materials can be greatly improved after pre-strain and baking. • Dislocation as fast channels for carbon diffusion promotes cementite formation. • Carbon clusters gathered at the dislocations act as precursors of cementite. • Cementite precipitation is main reason for increase in strength of bainite steel. In the automobile industry, stamping and paint baking processes are used to strengthen components, and this not only saves costs, but also further reduces the bodyweight of automobiles. In this work, the bake hardening mechanism of the complex phase (CP) steel CP980 was explored by comparing it with that of DP1180, which has a clear bake hardening mechanism and a carbon content similar to that of CP980. By analyzing the bake hardening response and the microstructural changes of the two steels, we found that the bake hardening process of CP980 was divided into three stages. In the first two stages, the carbon atoms diffused into dislocations to form Cottrell atmosphere pinning dislocations, and excess carbon atoms formed carbon clusters or low-temperature carbide pinning dislocations that were similar to DP1180. In the third stage, the dislocation acted as rapid channels for carbon diffusion, and fine cementite gradually formed when the carbon clusters gathered at the dislocations as precursors, resulting in precipitation hardening. This novel bake-hardening (BH) mechanism is crucial for obtaining a comprehensive understanding of the strain-baking behavior of advanced high-strength steals (AHSS).

Phase Field Simulation of the effect of Aging Precipitates on Grain Growth in Aluminum Alloys

Aluminum alloy skin for automobile is the focus of development, application and research of aluminum alloy for automobile. At present, the problems such as aging stability, formability, concavity resistance and bake hardening of anti-baking paint need to be solved urgently. The aging precipitates of baking varnish have the effect of pinning grain boundaries and refining grains. In this study, the topological transformation law of polycrystalline evolution during normal grain growth was analyzed by continuous phase field model, and the influence of different size, morphology and volume fraction of precipitated phase particles on polycrystalline growth were simulated. By verifying the Zener relationship between the particle size of the precipitate and the limit radius of the grain, an exponential function relationship different from the growth law of the ideal spherical particle pinned polycrystals was obtained. This research has an important scientific guiding role in the aging modification design of automobile skin and in improving the service life of aluminum alloy sheets.

The combined effect of pre-aging and Sn addition on age hardening response and precipitation behavior of Al-1.0Mg-0.6Si (−0.3Cu) alloy

The combined effect of Sn and pre-aging on hardening and precipitation behavior of Al-1.0Mg-0.6Si(-0.3Cu) alloy was investigated by using hardness measurement, tensile test, transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). The results indicated that the natural aging (NA) hardening of Al-1.0Mg-0.6Si (−0.3Cu) alloy was suppressed by pre-aging treatment, and the suppression of NA can be enhanced by the addition of Cu. However, the suppression of NA time of Al-1.0Mg-0.6Si and Al-1.0Mg-0.6Si-0.3Cu alloy, i.e., without obvious hardness increase after pre-aging treatment, is reduced to 5 and 7 days by the addition of Sn, which is not conducive to the suppression of NA. For subsequent artificial aging (AA) of the Al-1.0Mg-0.6Si (−0.3Cu) alloys, the AA precipitation of Al-1.0Mg-0.6Si alloy with pre-aging treatment is accelerated by the addition of Sn or Cu, particularly by adding both Sn and Cu, resulting in the increase in the bake hardening response. The effect of the evolution of clusters after pre-aging on underling mechanisms of NA and its AA were also discussed.

The Effect of Adding V and Nb Microalloy Elements on the Bake Hardening Properties of ULC Steel before and after Annealing

Bake hardening (BH) is a vital part of special steel production. Studies in this field have focused on steels under homogeneous yielding, but until now, none have been conducted on the phenomena that occur for steels under heterogeneous yielding. In the current study, the effect of adding Nb and V alloying elements on the strength of ultra-low carbon (ULC) steel after bake hardening was investigated. The effects of pre-strain, grain size, and recrystallization annealing temperature were analyzed, as well as the effect of Nb and V on the yield stress caused by the bake hardening process. For this purpose, five types of alloys with different V and Nb contents were melted, cast in an induction furnace, and subjected to hot hammering and hot rolling. Then, cold rolling was applied to the samples by ~80%. To eliminate the effects of cold working, tensile samples were subjected to recrystallization annealing at 750 and 800 °C for 30 min, and the samples were quickly quenched in a mixture of a NaCl solution and ice. The annealed samples were subjected to a pre-tensile strain in the range of 2-12% and then aged in a silicone oil bath at 180 °C for 30 min. Then they were subjected to a tensile test. The obtained results showed that with the increase of the pre-strain and the annealing temperature, the values of baking hardness increased. The presence of V in the composition of steel reduced the annealing temperature.

Analysis of stamping crack in bake hardening steel

The bake hardening steel can better solve the contradiction between stamping formability and dent resistance of steel plates due to the lower strength and higher formability during stamping, as well as the advantages of increased strength after coating and baking. However, bake hardened steel can suffer from cracking problems in the stamping process. In this paper, the mechanical properties and stamping properties of the bake hardening steel were tested, and the production process was also analyzed. The results showed that the material has aged due to the long storage time, decreasing elongation. The main reason for stamping cracks was the reduction of the amount of oil applied to the material anti-rust oil. When the material was stored for a long time, the anti-rust oil on the surface of the steel plate volatilized, resulting in cracks in the stamping. Based on the above research, the material delivery time was strictly controlled. The cracks disappeared by supplementing and oiling the materials that had been stored for more than five months. The stamping performance of the bake hardening steel is improved.