Effect of Plastic Hot Deformation on the Hardness and Continuous Cooling Transformations of 22MnB5 Microalloyed Boron St
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NTRODUCTION
INCREASING strength requirements for structural steel components in vehicles, aiming to reduce the weight of cars while preserving their safety characteristics, have brought increasing use of hot forming operations. The method of press quenching, using manganese boron steels, allows steels to achieve strengths greater than 1500 MPa. In these steels, high formability and high mechanical strength can be achieved in two different production process steps of a finished part. The required strengths in the components can be obtained by heating at least to the austenitizing temperature, where the material has an fcc structure, and then cooling sufficiently quickly. An aluminumsilicon surface coating protects against oxidation of the base material. The cooling rate during the hot forming operations is strongly dependent on the surface press level; therefore, there are different cooling conditions in the various areas of a press-quenched component. The physical properties and the performance of these components are greatly influenced by the microstructure, residual stresses, and geometrical defects imposed by the manufacturing process. In order to efficiently develop these products, it is necessary to quickly A. BARCELLONA, Associate Professor, and D. PALMERI, Assistant Researcher, are with the Dipartimento di Tecnologia Meccanica, Produzione ed Ingegneria Gestionale, University of Palermo, Viale delle Scienze, 90128 Palermo, Italy. Contact e-mail: [email protected] Manuscript submitted July 18, 2008. Article published online February 26, 2009 1160—VOLUME 40A, MAY 2009
simulate and analyze the manufacturing processes to predict the final microstructure. Proper simulation tools are computational numerical models or neuro-fuzzy models and require accurate material data. Unfortunately, for this innovative material, the continuous cooling transformations (CCTs) have been determined only on the thermally treated material, but little knowledge exists on the CCTs that reproduce the typical work conditions of the press quenching process. The purpose of this work is to describe the employed experimental methods to obtain the hardness and microstructural changes of prestrained and thermally treated microalloyed boron steel 22MnB5.[1–3]
II.
22MnB5 ALUMINUM PRECOATED STEEL PROPERTIES
Quenchable boron steels use this alloying element as a hardening agent during thermomechanical treatments and provide a material with excellent hardness and high strength. The quenching treatment, usually performed on these materials, determines the heterogeneous precipitation of boron carbide at the grain boundaries and also the boron segregation. The segregation phenomenon causes increasing hardenability of the material by suppressing the austenite to ferrite transformation.[4–6] Substitutional solid solution elements such as Mn are known to only slightly influence the strength after quenching; however, Mn is essential for securing hardenability. Hot stamping is a nonisothermal forming process for sheet metals, where forming and quenching take p
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