The CR660Y780T-CP-GI is a new member of the complex-phase steel family
28.10.2020 | Salzgitter Flachstahl GmbH
The CR660Y780T-CP-GI is a new member of the complex-phase steel family
The family of complex-phase steels is characterized by a high yield strength in relation to the tensile strength. Compared with dual-phase steels, the grades are slightly less deep-drawable and stretch-formable, but have significantly lower edge cracking sensitivity. This means that shear cutting results in only a slight reduction in the formability of the sheet edge. In addition, a complex-phase steel allows tighter component radii to be achieved compared with dual-phase steel.
Salzgitter Flachstahl has now developed a variant with a 90 MPa higher yield strength to complement the CR570Y780T-CP-GI. Despite the higher yield strength, the CR660Y780T-CP-GI still offers comparable formability.
In order to classify its properties, its mechanical characteristics from the tensile test and its formability are compared in a table. For comparison, two grades from the 780 MPa strength class are listed here: the dual-phase steel CR440Y780T-DP-GI and the complex-phase steel CR570Y780T-CP-GI.
In order to quantitatively evaluate the advantages of the CR660Y780T-CP-GI compared to the CR570Y780T-CP-GI without a concrete component, a simplified potential study is being conducted. For this purpose, a component made of CR570Y780T-CP-GI is assumed which is exclusively under tensile load. If the component is designed in such a way that it does not deform plastically during operation or in case of misuse, then sheet thickness and yield strength are the most important parameters. If the yield strength is 90 MPa higher when using CR660Y780T-CP-GI, the sheet thickness can be reduced by 16% accordingly. The lower material input in this way would reduce costs by approx. 14%. If less material is required for the same part performance, less CO2 is also produced in the manufacture of the part. As a result, the CO2 equivalent is reduced by 0.37 kg per kilogram of initial component weight (excluding processing by the supplier).
The positive result of the simplified potential study motivates to further pursue the approach of substituting CR570Y780T-CP-GI with CR660Y780T-CP-GI. In the next step, therefore, the formability of the grade will be considered in more detail. For this purpose, the forming properties of CR660Y780T-CP-GI are compared with those of the dual-phase steel and CR570Y780T-CP-GI on the basis of concrete, experimental results.
The stress-strain curves or flow curves show the typical hardening processes for the three grades. The high yield strength of CR660Y780T-CP-GI can be seen accordingly. The forming limit curve of CR660Y780T-CP-GI is at the same level as that of CR570Y780T-CP-GI for a comparable sheet thickness. This indicates that no problems are to be expected when changing from CR570Y780T-CP-GI to CR660Y780T-CP-GI in the forming process. One aspect of forming that is not covered by the forming limit curve is the forming of cut edges. For some steel grades, shear cutting can significantly reduce the formability of the edge. These grades are called edge crack sensitive. In the case of CR660Y780T-CP-GI, however, no deterioration is to be expected here, as the formability of the shear cut edges has been further increased. This is demonstrated by the hole expansion ratio of 84%, which is again significantly higher than that of CR570Y780T-CP-GI. The hole expansion ratio according to ISO 16630 describes how far a shear cut hole can be expanded with a conical punch.
From the point of view of forming, nothing would therefore stand in the way of a substitution of CR570Y780T-CP-GI by CR660Y780T-CP-GI.
Have we aroused your interest? Our Sales Department and our Technical Customer Service will be pleased to assist you with questions regarding the application and the provision of prototype material.