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.