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The current work aimed to develop cold rolled steel sheets with Rm> 1000 MPa optimised for roll forming to be produced in existing continuous annealing lines with either gas (CAL-GQ) or water quenching (CAL-WQ) and in hot dip galvanising (HDG) lines. The result of the project is a significant improvement in roll formability for materials produced in all three lines, compared to state of the art materials of the same strength level. The three steels developed within the project also reached the target values in terms of bendability (Ri/t 2) and hole expansion ratio (punched)25%. For the HDG line, the optimum microstructure consisted of bainite and small amounts of untempered martensite to reach the necessary strength level. A key to produce these steels is to avoid polygonal ferrite by using a relative high annealing temperature and well balanced amount of hardenable elements. For CAL-GQ with overaging section, the optimum microstructure consists of lower bainite, tempered martensite, and possibly a few percent ferrite. The process window in this line is largest, even Rm> 1200 MPa materials were produced in this line with good formability. For CAL-WQ with tempering section, the optimum microstructure is a highfraction martensite DP steel tempered at relative high temperature. This steel can be produced with very lean chemistries but controlling the amount and distribution of martensite is challenging.
The current work aimed to develop cold rolled steel sheets with Rm > 1000 MPa optimised for roll forming to be produced in existing continuous annealing lines with either gas (CAL-GQ) or water quenching (CAL-WQ) and in hot dip galvanising (HDG) lines. The result of the project is a significant improvement in roll formability for materials produced in all three lines, compared to state of the art materials of the same strength level. The three steels developed within the project also reached the target values in terms of bendability (Ri/t 2) and hole expansion ratio (punched) 25%. For the HDG line, the optimum microstructure consisted of bainite and small amounts of untempered martensite to reach the necessary strength level. A key to produce these steels is to avoid polygonal ferrite by using a relative high annealing temperature and well balanced amount of hardenable elements. For CAL-GQ with overaging section, the optimum microstructure consists of lower bainite, tempered martensite, and possibly a few percent ferrite. The process window in this line is largest, even Rm > 1200 MPa materials were produced in this line with good formability. For CAL-WQ with tempering section, the optimum microstructure is a highfraction martensite DP steel tempered at relative high temperature. This steel can be produced with very lean chemistries but controlling the amount and distribution of martensite is challenging.
The book covers all types of advanced high strength steels ranging from dual-phase, TRIP. Complex phase, martensitic, TWIP steels to third generation steels, including promising candidates as carbide free bainitic steels, med Mn and Quenching & Partitioning processed steels. The author presents fundamentals of physical metallurgy of key features of structure and relationship of structure constituents with mechanical properties as well as basics of processing AHSS starting from most important features of intercritical heat treatment, with focus on critical phase transformations and influence of alloying and microalloying. This book intends to summarize the existing knowledge to show how it can be utilized for optimization and adaption of steel composition, processing, and for additional improvement of steel properties that should be recommended to engineering personal of steel designers, producers and end users of AHSS as well as to students of colleges and Universities who deal with materials for auto industry.
Material properties -- Sheet deformation processes -- Deformation of sheet in plane stress -- Simplified stamping analysis -- Load instability and tearing -- Bending of sheet -- Simplified analysis of circular shells -- Cylindrical deep drawing -- Stretching circular shells -- Combined bending and tension of sheet -- Hydroforming.