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![The Press Hardening Manufacture Line with Rapid-Cooling Process The inner metallographic structure transformation is the principle lead to the hardening and high tensile property of the steel. The continuous cooling transformation diagram CCT of 22MnB5 shown as FIGURE 2(a), describes this mechanism. The start temperature of ferrite to austenite transformation point Ac3 amounts to 830°C and the martensite start point Ms at 400°C. As illustrated in FIGURE 2(a), at the cooling rates higher than 27°C/s, bainite transformation can be avoided and fully martensitic microstructure can be formed resulting in higher hardness and strength levels. At 900 the steel is almost austenite (A) when it started to cooling. At different cooling rate austenite will change into different metallic crystal structures. When the cooling rate is higher than 27 /s, austenite transforms to martensite (M), which is a kind of crystal structure with high stiffness and stress. When the cooling rate is lower than 27 /s, austenite transforms to bainite (B), ferrite (F) and pearlite (P), which are structures with lower stiffness and stress[3].](profile/Ping-Hu-3/publication/258740600/figure/fig1/AS:392754468278277@1470651370444/The-Press-Hardening-Manufacture-Line-with-Rapid-Cooling-Process-The-inner-metallographic.png)
The Press Hardening Manufacture Line with Rapid-Cooling Process The inner metallographic structure transformation is the principle lead to the hardening and high tensile property of the steel. The continuous cooling transformation diagram CCT of 22MnB5 shown as FIGURE 2(a), describes this mechanism. The start temperature of ferrite to austenite transformation point Ac3 amounts to 830°C and the martensite start point Ms at 400°C. As illustrated in FIGURE 2(a), at the cooling rates higher than 27°C/s, bainite transformation can be avoided and fully martensitic microstructure can be formed resulting in higher hardness and strength levels. At 900 the steel is almost austenite (A) when it started to cooling. At different cooling rate austenite will change into different metallic crystal structures. When the cooling rate is higher than 27 /s, austenite transforms to martensite (M), which is a kind of crystal structure with high stiffness and stress. When the cooling rate is lower than 27 /s, austenite transforms to bainite (B), ferrite (F) and pearlite (P), which are structures with lower stiffness and stress[3].
Source publication
In this study, a new rapid-cooling process in press hardening based on
theoretical analysis, experimental test and optimal formability
simulation were investigated for improving formability and obdurability
of 22MnB5 boron steel. A series of non-isothermal flow behaviors in
different plastic strain rates from 0.001s-1 to 0.1s-1 was investigated
by...
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Citations
... During the hot stamping process, a PHS blank is heated at 850-950°C for 3-10 min to obtain a fully austenite structure first and is transferred to a forming die where the forming is conducted, followed by subsequent die quenching [4][5][6]. The strength of PHS is enhanced to approximately 1500 MPa via martensitic transformation upon die quenching [6][7][8][9]. During the austenitization heat treatment, the PHS is exposed to ambient atmosphere, where oxidation and decarburization take place. ...
The high temperature tensile test of hot dip 55 wt.% Al-Zn coated steel was performed to study the fracture behaviors of hot dip 55 wt.% Al-Zn coating as a protective coating for hot-stamping steel. During austenitization, the 55 wt.% Al-Zn coating had transformed to an alloy layer composed an Fe2Al5Znx (with few FeAl and Zn) outer layer and an FeAl inner layer. When the austenitization heat treatment was fixed at 5 min, the sample heated at a higher heating rate (20 K/s) was subjected to a longer soaking time at 900 °C. This resulted in a thicker FeAl inner layer at the steel / Fe2Al5Znx (FeAl, Zn) interface. During the initial stages of deformation, cracks formed in Fe2Al5Znx (FeAl, Zn) layer.
Subsequently, cracks developed in the FeAl inner layer. With continued deformation, cracks widened and debonding occurred at the FeAl / Fe2Al5Znx (FeAl, Zn) interface for the samples heated at lower heating rates (5 K/s or 10/s). Finally, crack widening accelerated the exposure and subsequent oxidation of the steel substrate. The absence of debonding can be beneficial for reducing the susceptibility to powdering during hot
stamping.
Boron-manganese steel 22MnB5 is extensively used in structural automotive components. Knowledge about its microstructural evolution during hot stamping and resistance spot welding (RSW) is extremely relevant to guarantee compliance with application requirements. Particularly, corrosion properties are critical to the application of uncoated sheet steels. However, microstructural studies are usually simplified to top-hat geometries, which might not be fully representative of the complex thermomechanical cycles locally faced by a real component. Therefore, the present work brings an extensive characterization of a hot-stamped 22MnB5 automotive B-pillar in terms of microstructure, hardness and corrosion resistance, which were correlated with a reverse engineering of the process using numerical simulation. Physical simulations of the subcritical heat affected zone (SCHAZ) of RSW were done to assess the influence of microstructure on martensite tempering. Results showed that the component undergoes a complex strain distribution along its body during hot stamping. Most heavily strained regions presented higher amounts of ferrite, leading to poorer corrosion resistance, since ferrite behaves as an anode. Physical simulations of the SCHAZ showed that the softening degree due to martensite tempering is solely affected by peak temperature, while other microstructural features appear to exert negligible or no influence.
In the hot stamping process of ultra-high-strength steel sheets, it is a significant issue to reveal the nonuniformity of cooling rate in space–time domain by finite element method and even uncover the inner cause, which contributes to the further adjustment of the phase transformation. In this work, a series of heat transfer experiments between dies and sheets were conducted on the self-developed experimental apparatus. The temperature evolution curves of die and BR1500HS ultra-high-strength steel sheets under different pressures and holding time were obtained. Moreover, the transient heat transfer coefficients (HTC) under different mean interface temperatures and pressures were calculated by the inverse heat transfer algorithm. Subsequently, based on the HTC curves, a thermal–mechanical-phase dynamic coupling finite element model was developed for modeling the hot stamping process, and a series of simulations for analyzing the non-uniform microstructures distribution in hot stamping parts were implemented. Finally, the simulation results were validated by actual hot stamping experiments. Two significant influence factors on the nonuniform distribution of microstructures were summarized as follows: the existence of incomplete contact between steel sheets and dies due to the sheet thickness reduction in sidewall and circular bead regions, and the temperature differences between dies and steel sheets.
With the current development of hot stamping technology, quality products have been striving for not only uniformity in hardness but also a minimum of springback. Considering the fact that the uniformity distribution of hardness and the randomness distribution of the springback are inevitable in hot stamping process due to many complex factors such as contact sequence, contact clearance, scale and initial stamping temperature, a new method named RCP method was proposed to improve the hardness and springback of hot stamping product in this paper. The cooling rate control before stamping is applied based on CCT curves through a rapid air cooling device which can provide cooling rate of 60°C/s. Three parameters included initial stamping temperature, contacting pressure and dwelling time have been investigated in this paper. According to the experiment results, the hardness deviation is markedly decreased and the springback can be controlled within the threshold value ±0.5mm.
