Evaluation of the Form-Forming Mechanism for High-Tensile-Strength Steel Plate

Abstract

Steel plate with tensile strengths of from 440 MPa to 980 MPa is increasingly being adopted for the structural elements of automobile bodies to both reduce weight and increase collision safety. However, the use of such material brings defects in press forming and other problems. The research reported here focuses on the form-forming method, which is effective for forming high tensile strength steel plate. To clarify the features of form-forming, we used eight types of steel plate having tensile strengths of from 270 MPa to 980 MPa in three forming processes, draw-forming, one-step form-forming, and two-step form-forming, to evaluate the shape freezing property. The results showed that the shape freezing property depends on the mechanical properties of the material; speci cally, a correlation with tensile strength, 0.2% proof stress, and elongation was seen. The effectiveness of form-forming for materials of higher strength was con rmed. Improved shape freezing by dividing the form-forming process into two steps was also con rmed. For draw-forming, the entire shaped surface was affected by unbending, so form-forming effectively suppresses unbending. The spatial distribution of unbending was evaluated quantitatively by measuring the residual stress distribution, con rming that residual stress serves as a useful index for clarifying the forming mechanism.

Ductile Fracture Behavior of Mild and High-Tensile Strength Shipbuilding Steels

“…Ductile fracture resistance is an important structural performance requirement for the steel components of naval structures. As discussed by Matic et al [1], computational tools are capable of implementing a range of elastic-plastic constitutive relations and ductile fracture initiation models for analyzing the structural problems relevant to ship steels that involve considerable strain beyond moderate plasticity, such as an analysis of the plastic zone ahead of the crack tip and, on a larger scale, the plasticity generated due to forming [2,3] or extreme loads. Therefore, there is a demand for ductile fracture models that can be calibrated and implemented easily in finite element analysis but represent the micro-mechanisms of the failure process as accurately as possible.…”