A Study on the Bake Hardenability of a Ti-Nb Stabilized Extra Low carbon Steel

Yakıt tüketimini düşürmek ve bu şekilde egzoz gazlarının çevreye verdiği zararı en aza indirmek amacıyla otomotiv gövde ağırlığının azaltılması son yıllarda otomotiv sanayindeki en önemli beklentilerden birisidir. Arzu edilen özellikler fırınlama sırasında sertleşen çeliklerle elde edilmiştir. Ancak fırınlama sırasında sertleşme özelliği ön deformasyon miktarı ve fırınlama sıcaklığına bağlı olarak değişmektedir. Bu çalışmada Ti-Nb ile stabilize edilmiş çok düşük karbonlu bir çelik sacın fırınlamayla sertleşme özelliği ön deformasyon miktarı ve fırınlama sıcaklığına bağlı olarak incelenmiştir.

Influence of Grain Size on Bake Hardenability for Low Carbon Steel by Internal Friction Method

We presented the comparative influence of two different grain size on bake hardenability for low carbon steel dependent internal friction (IF) spectra. Samples that had a ferrite structure consisting of two peaks and a linear background. Analyzed the grain size and grain boundary length of low carbon steel by EBSD technique. The results showed that: the grain refinement increased the number of initial solute carbon atoms and the effect of movable dislocations pinning by carbon. So grain refinement of low carbon steel could enhance its bake hardening properties.

Effects of Cu or Li Addition and Multi-Step Aging Conditions on the Bake-Hardenability of an Al-Mg-Si Alloy

In recent years, automobiles with lower fuel consumption are required because the exhaust fume is severely regulated. The weight-saving is quite effective to realize such low fuel consumption, and therefore aluminum alloy becomes attractive as an alternative material of steels due to its high specific strength. 6XXX series Al-Mg-Si alloys exhibit good bake-hardenability during paint-bake treatment in the automobile manufacturing process, but to reduce further environmental impact, the paint-baking temperature is supposed to be lowered than the present temperature of about 443K. In this study, it was aimed to investigate the attained hardness after paint-bake treatment at various temperatures of 408-443K for an Al-0.55wt%Mg-0.90wt%Si alloy with/without microalloying elements of Cu and Li. The effects of multi-step aging conditions; e.g. pre-aging, natural aging and paint-bake treatments, were also investigated through Vickers hardness test, TEM observation and DSC analysis. From the obtained experimental results, it was clarified that the addition of Cu or Li to the Al-Mg-Si alloy increases the attained hardness even at a paint-baking temperature of 408K due to the increased volume fraction of precipitates. Furthermore, pre-aging treatment at 373K for 18ks was also effective in suppressing the increase in hardness during natural aging, resulting in the highest attained hardness among the investigated multi-step aging conditions; i.e. HV100 in the Li-added alloy paint-baked at 408K.

Niobium Containing Advanced High Strength Steels for Automotive Applications – Processing, Microstructure, and Properties

Two major drivers for the use of advanced steels in the automotive industry are fuel efficiency and increased safety performance. Fuel efficiency is mainly a function of weight of steel parts, which in turn, is controlled by gauge and design. Safety is determined by the energy absorbing capacity of the steel used to make the part. All of these factors are incentives for the automobile manufacturers to use Advanced High Strength Steels (AHSS) to replace the conventional steels used to manufacture automotive parts in the past. AHSS is a general term used to describe various families of steels. The most common AHSS is the dual-phase steel that consists of a ferrite-martensite microstructure. These steels are characterized by high strength, good ductility, low tensile to yield strength ratio and high bake-hardenability. Another class of AHSS is the complex-phase or multi-phase steel which has a complex microstructure consisting of various phase constituents and a high yield to tensile strength ratio. Transformation Induced Plasticity (TRIP) steels is another class of AHSS steels finding interest among the U.S. automakers. These steels consist of a ferrite-bainite microstructure with significant amount of retained austenite phase and show the highest combination of strength and elongation, so far, among the AHSS in use. High level of energy absorbing capacity combined with a sustained level of high n value up to the limit of uniform elongation as well as high bake hardenability make these steels particularly attractive for safety critical parts and parts needing complex forming. A relatively new class of AHSS is the Quenching and Partitioning (Q&P) steels. These steels seem to offer higher ductility than the dual-phase steels of similar strengths or similar ductility as the TRIP steels at higher strengths. Finally, martensitic steels with very high strengths are also in use for certain parts. The most recent initiative in the area of AHSS is the so-called 3rd Generation AHSS. These steels are designed to fill the region between the dual-phase/TRIP and the Twin Induced Plasticity (TWIP) steels with very high ductility at strength levels comparable to the conventional AHSS. Enhanced Q&P steels may be one method to achieve this target. Other ideas include TRIP assisted dual phase steels, high manganese steels and higher carbon TRIP type steels. In this paper, some of the above families of advanced high strength steels for the automotive industry will be discussed with particular emphasis on the role of niobium.

COMPARING THE BAKE HARDENING OF NANO-GRAIN AA6056 WITH ITS COARSE-GRAIN STRUCTURE

In preset work, the bake hardening behavior of 6056 aluminum alloy with nanostructured layer, produced by surface severe plastic deformation (SSPD), was compared with that of AA6056 with conventional coarse grain structure. Wire brushing process was employed in order to produce surface layers with nanograins. After formation of nanostructured surface layer, the structure was characterized using X-ray Diffraction (XRD) and scanning electron microscopy (SEM) techniques. According to the microhardness measurements, the depth of nanostructured layer, with the grains of 50-110 nm, was about 40-50 μm on each side of specimens. The bake hardenability of the produced nanostructured AA6056 was then studied. The results indicate an increase of about 40% in the bake hardening of nanograin AA6056 comparing to the bake hardening of coarse-grain AA6956. The maximum microhardness of nanograin layer was about 250 HV, whereas it was 65 HV for coarse-grain AA6056.

Study of Bake Hardenability for Hot Rolling Low Carbon Steel

The TMCP and bake hardening experiments were carried out for the test steel，the microstructure analyses，mechanical properties and bake hardenability test were carried out by using SEM, TEM and tensile test machine. The results show that small volume fraction of bainite microstructure increased will improve BH value in the microstructure of F+P(little). compared with the microstructure that consists of ferrite mainly，which consists of bainite mainly has a higher BH value. The microstructure that consists of ferrite mainly shows the trendy of BH that increased first and then decreased, along with pre-strain increased. The microstructure that consists of bainite, under the condition of higher pre-strain, with the pre-strain increased, the BH value shows the trendy of increasing. While the BHT value is increasing monotonously.

Effect of Copper Sulfide on the Fatigue Properties of Bake Hardening Steels

Bake hardening steels have to resist strain aging to prevent the yield strength increment and stretcher strain during press process and to enhance the bake hardenability during baking process after painting. The bake hardening steels need to control the solute carbon and the solute nitrogen to improve the bake hardenability. Ti and/or Nb alloying for nitride and carbide precipitation and low carbon content below 0.003% are used to solve strain aging and formability problem for automotive materials. However, in the present study, the effect of micro-precipitation of copper sulfide on the bake hardenability and fatigue properties of extremely low carbon steel has been investigated. The bake hardenability of Cu-alloyed bake hardening (Cu-BH) steel was slightly higher (5MPa) than that of Nb-alloyed bake hardening (Nb-BH) steel, but the fatigue limit of Cu-BH steel was far higher (45MPa) than that of Nb-BH steel. All samples showed the ductile fracture behavior and some samples revealed distinct fatigue stages, such as crack initiation, stable crack growth and unstable crack growth.