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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.
Advanced high strength steels (AHSSs) for auto-making are primarily produced by rolling, plus heat treatment technologies if necessary. However, due to the metallurgical complexity of AHSSs, it is impossible to roll all of the AHSS grades in a rolling mill with the same rolling technology. Each of AHSSs has unique applications in vehicles, and specified rolling technologies are required to produce high quality AHSS products where they might be the best employed to meet performance demands of the automotive parts. Such background has prompted the publication of this scholarly book in the area of rolling of AHSSs with a purpose of providing readers with a valuable technical document that can be used in the research and development of AHSSs for automotive and other manufacturing industries. With contributors from USA, Germany, Poland, Italy, Spain, Austria, Australia, China, India and Iran, the book highlights the latest advances in rolling technologies of AHSSs. It focuses on the theory, simulation and practice of the rolling of AHSSs: The book introduces the history, types and advances of AHSSs and their processes; proposes new theory that is applicable to the rolling of AHSSs, presents mathematical and numerical modelling of AHSSs in rolling; covers thermomechanical processing technologies of AHSSs; provides case studies on the rolling practice of the most popular AHSSs and includes other rolling-related technologies of AHSSs. The book will be useful for both theoretical and applied research aimed at AHSSs rolling technologies, and will be a scientific and valuable literature for the metallurgists, engineers, materials scientists, academics and graduate students who are studying and working with AHSSs and their rolling technologies worldwide.
Examines the types, microstructures and attributes of AHSSAlso reviews the current and future applications, the benefits, trends and environmental and sustainability issues.
This volume comprises select proceedings of the AHSS 2017 conference. AHSS is an instrumental event in creating a platform for exchanging recent thoughts and results among a selective group of researchers working in the area of steel science and engineering. Twenty two selected papers have been included in this volume. This book will serve as a reference to many practitioners and researchers working in the areas of steel strength, characterization, and applications.
In recent years, significant developments have been made to increase the mechanical strength of steels in order to reduce the overall weight of structures, particularly in motor vehicles. Depending on the application, the increase in strength should not be at the expense of forming and in-use properties. The development of ultra-high strength steels requires a search for new trade-offs between these properties in order to optimize the final microstructure. New Advanced High Strength Steels analyzes the interactions between tensile mechanical properties and properties such as work hardening, anisotropy, resistance to rupture, fatigue life, corrosion resistance, crashworthiness, edge retention, hydrogen resistance and weldability. It also examines the links between the microstructural parameters of high-strength steels and the properties mentioned above. It highlights the metallurgical developments that have been necessary for the emergence of these new generations of steels. The book concludes with a look ahead to future developments in ultra-high strength steels
The main objective of this proposal is to develop AHSS both hot rolled and annealed cold rolled bainitic steels with an optimal ductility and work hardening comparable to drawing steels, and/or keeping a suitable bending and stretching behaviour. In addition, forming processes such as roll-forming and hardening, and press-hardening were optimised to achieve a carbide free bainitic microstructure in final products such as structural safety components in the car body i.e. A-beam, roof-beams, car bumpers and side impact beams. Hot rolling of carbide free bainitic steels with 1100 MPa of yield strength and high toughness (KV(-40oC)>30 J) was proved to be challenging. Apart of a high carbon content (̃0.3wt.%), chemical composition requires of a high manganese content (̃2wt.%), which leads to a high risk of banding. By contrast, annealed cold rolled bainitic steels designed for continuous annealing line achieved far higher uniform elongation, better stretching ability and formability than that in DP980 and Martensitic 1400 steels considering the same range of ultimate tensile strengths. Finally, roll-forming simulations of bainitic cold rolled steels obtained by interrupted quenching and salt bath at different temperatures showed an excellent formability without crack formation in comparison to martensitic 22MnB5 reference steel. Likewise, press-hardening trials of hat shaped profiles using a novel quench and partitioning post heat treatment of cold rolled designed steels showed promising tensile results in comparison to same commercial 22MnB5 profiles.
In the past 30+ years significant advancements have been made in the development of higher strength sheet steels with improved combinations of strength and ductility that have enabled important product improvements leading to safer, lighter weight, and more fuel efficient automobiles and in other applications. Properties of the primarily low carbon, low alloy steels are derived through careful control of time-temperature processing histories designed to produce multiphase ferritic based microstructures that include martensite and other constituents including retained austenite. The basis for these developments stems from the early work on dual-phase steels which was the subject of much interest. In response to industry needs, dual-phase steels have evolved as a unique class of advanced high strength sheet steels (AHSS) in which the thermal and mechanical processing histories have been specifically designed to produce constituent combinations for the purpose of simultaneously controlling strength and deformation behavior, i.e. stress-strain curve shapes. Improvements continue as enhanced dual-phase steels have recently been produced with finer microstructures, higher strengths, and better overall formability. Today, dual phase steels are the primary AHSS products used in vehicle manufacture, and several companies have indicated that the steels will remain as important design materials well into the future. In this presentation, fundamental results from the early work on dual-phase steels will be reviewed and assessed in light of recent steel developments. Specific contributions from industry/university cooperative research leading to product improvements will be highlighted. The historical perspective provided in the evolution of dual-phase steels represents a case-study that provides important framework and lessons to be incorporated in next generation AHSS products.