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Modern Bainitic Steels
Published in H. K. D. H. Bhadeshia, Bainite in Steels, 2015
Ordinary steels are troublesome in forming operations because they exhibit discontinuous yielding behaviour, which causes the formation of unsightly stretcher strains on the finished products. Dual-phase steels are similar in composition to conventional alloys, but are heat treated to generate a mixed microstructure of ferrite and martensite. Their mechanical properties are characterised by continuous yielding, a small proof to tensile strength ratio and a large uniform elongation (Fig. 13.14). The free dislocations in martensite, the dislocations in the ferrite due to the strain induced by martensitic transformation, Fig. 13.15, and the large difference in hardness between the phases all contribute to yielding behaviour (Furnémont et al., 2004). Although the 0.2% proof stress can be small, the dual phase steels strain harden rapidly so the ultimate tensile strength can be quite large at 500-650 MPa, without loss of formability. More details on dual phase steels can be found in a review by Owen (1980). A typical dual-phase steel would have a chemical composition Fe-0.09C-0.6Si-1.0Mn wt% and would be heat treated to give a mixed microstructure containing about 20% martensite and 80% ferrite. The high silicon concentration is known to enhance formability; alloying elements such as chromium are sometimes added in small concentrations (≈ 0.5 wt%) for hardenability and solid solution strengthening. The carbon concentration is normally less than 0.2 wt% to avoid brittle spot-welds.
Automotive Architecture
Published in Patrick Hossay, Automotive Innovation, 2019
So, HSLA steel might be used to reinforce a door structure for side impact and moderate energy absorption. Or dual-phase steel might be used since it can be tuned to different yield strengths and ductility, making it useful for crumple zones, beams and cross-members, as well as fasteners, doors, and other body components. Extremely strong and hard martensitic steel might be used for door beams or roof sections to provide intrusion protection or rollover protection when energy absorption is not viable. With high strength and ductility, TRIP steel may be used for frame rails, roof rails, seat frames, B pillars, and other components.
Effect of solution treatment on microstructure and properties of 00Cr32Ni7Mo3.5N
Published in Powder Metallurgy, 2022
Yang Zhao, Jianhao Chen, Meihuan Qi, Shubin Ren, Xuanhui Qu
Figure 7 shows the tensile results of HIP samples and forgings after solution treatment. The slope of tensile curve in the elastic stage is referred as tensile elastic modulus. Clearly, the HIP samples and forgings show large differences in their elastic stage (Figure 7(b)). However, the samples under the same fabrication conditions show similar elastic behaviour (Figure 7(b)). Based on the microstructural analysis, this is because phase distribution can be the same after solution treatments under the same fabrication conditions. To some extent, the dual-phase steel can be regarded as a type of composite which is composed of ferrite and austenite phases. The elastic behaviour of composites is affected by the distribution of matrix and the second phase. In this regard, the equiaxed grains of HIP samples can help to obtain comprehensive superior elastic properties, which helps to delay the yielding stage. Figure 7(b) shows that this dual-phase configuration contributes about 6% elongation in the elastic stage for HIP samples. In comparison, the stress rapidly rises to the yielding point under slight strain for forged samples, i.e. the deformation may be governed by one phase for banded dual-phase structures in forged samples. When the yielding stage reached, all the curves of HIP samples show obvious fluctuation before entering the deformation hardening stage. HIP samples possess more dispersed Cr2N particles in δ matrix than forged samples. This is probably because of the pinning effect of small Cr2N particles on dislocation movement in the initial plastic stage. In the following hardening stage, the HIP samples can maintain a much higher strain to about 40%, while the forged samples suffer the following necking onset at a low strain (below 20%).