Edge Fracture in Mixed Microstructure Steels
- 2018-12-05 13:26:00
- LUQIMENG Original
- 2425
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.
