Investigation on Formability of 22MnB5 Steel at Elevated Temperatures using a Physical Simulator

Abstract

B-Mn steel 22MnB5 is known to have excellent mechanical properties after experiencing a proper hot stamping process Not surprisingly 22MnB5 has become an increasingly popular material in industry recently The need for formability information on 22MnB5 metal sheet is also increasing A forming limit diagram is one of most important types of formability information The 22MnB5 forming limit diagram should be made at elevated temperatures The limiting dome height test is currently the most common experiment used to establish a forming limit diagram but the specimens are rubbed and bent during the test both of which influence the experimental results On the other hand it is difficult to control the specimen temperature using a high temperature limit dome high test machine which is the main factor hindering establishing of the standardization of a limiting dome height test for elevated temperatures In order to obtain experimental data with more accurate temperature control and to exclude the bending and friction that occur in a limiting dome high test in this study a new method which includes an experimental analysis and theoretical predictions was developed to investigate the formability of 22MnB5 at elevated temperatures In this method two designs for tensile test specimens with laser engraving grids are developed and used to obtain a tension-compression limit strain and a limit strain close to the plane-strain state at elevated temperatures together with the Gleeble 3800 physical simulator On the other hand a modified Cockcroft criterion that takes strain path into consideration is employed to predict an entire forming limit curve A forming limit curve can be established as a result of using the modified Cockcroft criterion together with the limit strains obtained from the tensile tests based on the two novel specimen designs In this study forming limit curves at three strain rates and at temperatures ranging from 650oC to 850oC were established Today many different fracture criteria have been developed All studies on this topic have presented experimental data that can justify the criterion they presented However the experimental results and predictions in the first quadrant of the forming limit diagrams often diverge Therefore due to the fact that the forming limit curves established in this study were predicted using a fracture criterion an accuracy identification method was developed In this method all the experimental data are obtained through pure stretching which can avoid the problems that occur in a limiting dome height test In respect to the second quadrant of a forming limit diagram two different tensile test specimens are applied to obtain the fracture strains in the second quadrant of a forming limit diagram On the other hand for the first quadrant of the forming limit diagram a cruciform biaxial tensile specimen was developed while a biaxial tensile apparatus was adopted The proposed specimen has the feature of thickness reduction and a contour design to ensure the fracture position is in the central region of the specimen so that a biaxial tensile state can be obtained By combining the forming limit curve establishing method and the accuracy identification method there is an opportunity to obtain an entire forming limit curve at an elevated temperature using the Gleeble 3800 physical simulator which is a low-cost and more reliable alternative compared with a high temperature limiting dome height test

Date of Award	2014 Jul 30
Original language	English
Supervisor	Rong-Shean Lee (Supervisor)