Edge Fracture in Mixed Microstructure Steels

Presentation Outline

• Edge Fracture – Background

• Past Case Studies

• Material Evaluations on Production & Trial Material

− Edge quality effects

− Hole expansion testing on current and new grades of AHSS

− Nano-indentation and microstructure

• Concluding Remarks

Edge Fracture – Background

• Fracture initiating at a trimmed edge, typically not predicted by an FLC

− A local formability phenomena: aninter-relationship between steel microstructure, damage during trimming,  and subsequent edge-stretch during forming.

− AHSS more susceptible than single phase materials (Low Carbon, HSLAs)

Edge Fracture – Background

• Edge fractures typically occur in the highest edge strain area in the presence of a rough trimmed edge.

−Typically during drawing or from a stretch flange after a rough die trim.

Edge Fracture Case Studies

• Edge fracture instances are not isolated to particular steel suppliers, grades of AHSS, gauge, or coating.

• Case study subset selected to show common root causes and resolutions.

Front Compartment Rail – CR780T / 420Y (DP).

• This design is frequently used for double attached left / right compartment rails (material utilization)

• Significant edge stretch condition during the draw in the ‘horse collar’ area

• Also, thin bypass condition between common trim lines (difficult to support) affected trim quality.

Rear Rail CR590T / 340YDP

• Intermittent edge fracture predominantly on one hand of part.

−Size and exact location of fractures were variable run-to-run.

Rear Rail CR590T / 340YDP

• LH vs. RH trim conditions made more robust.

• Trim steel insert maintenance required (sharpening).

Potential Sources of Poor Trim Quality

• Poor Nesting (Trimming in Air)

• Die Breathing and Flexing

• Die Guidance

• Improper Clearance

−Typically too tight

−13-15% recommended for most AHSS

• Sharpness of Trim Steels

Part with Flanged Hole – DP980T / 550Y

• Part hole expansion is 13%

• Material capability is 12-15%

− Other material properties within specification

− Hole Expansion (recently added to qualification approval process in May 2014)

• Design not robust for material capability

• Changed material to high yield ratio CR980T / 700Y-MP-LCE

− (HER ~ 30%)

• No issues-to-date

Try-out vs. Production Blanks

• Laser cut blanks in try-out material are not a good indicator of potential edge fractures in production with die struck blanks.

• Stamping plants are concerned about receiving dies for secondary try-out when the dies have not stamped die struck blanks in primary try-out.

• Timing of production-intended blanks needs to ensure that delivery occurs before dies are shipped to home line.

Hole Expansion Testing

• Variability of hole expansion testing exists due to variation in microstructure within a material, the quality of the sheared hole, and specific testing equipment site-to-site.

− A tool for qualification of material and general understanding of edge stretch performance with an adequate sample size; however, challenges exist for use for lot acceptance testing.

Hole Expansion Test

• Mechanical properties of select grades for hole expansion testing.

• Current production DP780 and DP980 vs. newer RA-bearing 1180 MPa grades.

• Mechanical properties of select grades for hole expansion testing.

• Current production DP780 and DP980 vs. newer RA-bearing 1180 MPa grades.

• Newer high strength 1180 MPa grades showing >= HER vs. more conventional DP steels and less sensitivity to edge condition.

• New grades need balance of global and local formability for most applications.

Nano-Indentation Evaluation – DP780

• Nano-indentation testing was performed to determine constituent hardness distributions in mixed microstructure DP780 steel.

• Two production samples were acquired representing two steel sources.

− Under similar blanking and stamping conditions, one steel exhibited edge fracture while the other did not.

• Samples were ground/polished using standard metallurgical techniques and finish-polished with colloidal silica.

• MTS Nanoindenter XP was used.

− Testing was performed at room emperature with a Berkovich tip

− Displacement control was used to indent to 100 nm maximum depth

− 12 x 12 array of indents was placed on each specimen, spaced

2 µm apart − Resulting hardness was averaged over a 60-90 nm depth to remove any surface abnormalities

Summary

• Edge fracture susceptibility is influenced by a variety of related factors: blank edge condition, material, part design / forming (strain distribution) / trimming process.

• A balance of global and local formability performance is required for most cold-stamped parts and shall be considered in steel development.

− Uniform, fine-grained structure required

− Reduced hardness differential between constituents

• An industry-wide test is needed for material lot acceptance to predict edge fracture susceptibility

− Performance not predicted by standard tensile testing.