Correlation of Material Phase Hardness and Macroscopic Properties of 1180DP Steel Zack Capo, Alvin Laimana, Tina Landon, Michelle McKinney, Jason Sawa Faculty Advisor: Dr. David F. Bahr Industrial Sponsor: Dongwei Fan Project Background The microhardness and single grain hardness of two heat treated dual-phase steel products were examined. Two methods of measuring individual phase properties were utilized; performing a grid of indentations and targeting the hardness of indentations made in isolated phases. Nanoindentation data indicated hardness in heat treatment one of 3.9 GPa for ferrite and 6.2 GPa for the hard phase. Material receiving heat treatment two contained ferrite measuring 3.9 GPa and the hard phase at 6.1 GPa. Differences in yield strength, tensile strength and hole expansion ratio were not found to be associated with differences in phase hardness. Experimental Procedure Results Recommendations Discussion MSE 430 - 440: Materials Processing and Design Samples from different locations on the sheet width were cut via a water jet. They were mounted and polished to a 3 micron diamond stage and etched with 0.5% and 5% Nital (nitric acid and ethanol). Advanced High Strength Steels (AHSS) combine high yield and tensile strength with good formability. ArcelorMittal’s 1180DP product is a dual-phase product and it is offered as galvanized and uncoated product with two different heat treatments. The compositions and properties are: Product composition, yield strength, tensile strength and hole expansion ratio (HER) of galvanized and uncoated 1180DP steel Forming operations risk the creation of cracks and early material failure problems. Deformation behavior of dual- phase steels can be attributed to the yielding of ferrite and fracture of harder phases. Phases include martensite and bainite, depending on the heat treatment. Achieving a high HER requires a limit in the variation of phase hardness. For these products, bulk and nanohardness data was collected and SEM analysis was performed to gain knowledge in the contribution of each phase to macroscopic behavior. One sample set was etched and examined via optical microscopy to analyze grain size and phase fraction for each product and location. The Heyn Intercept Method was used to measure grain size. 48% of the volume fraction (VF) of HT1 was ferrite and 52% of the VF of HT2 was ferrite. The grid method of sampling is recommended for nanoindentation, combined with SEM imaging for phase analysis. Grid spacing should exceed the average grain size. The grain volume sampled by the indenter should be less than 15% and indentation data should be excluded when the observed grain diameter is less than 50% of known grain size. Sample preparation is critical for the collection of high quality results. Samples acquired from three positions on a half sheet of rolled steel. A directional tab on removed samples documents the rolling direction. Edge Quarter Center C Mn Si Other YS (MPa) TS (MPa) HER HT1 0.13 2.5 0.7 Confidential 850 1240 23.8% HT2 0.14 2.1 0.7 Confidential 910 1250 29.5% In this SEM image, ferrite can be seen as the darker phase and the lighter phase represents the harder phase. Indents can be observed within grain boundaries. (a) (b) The microstructure of HT1 material is shown, based on a sample from the center section. This work is sponsored by ArcelorMittal East Chicago, IN HT1 (a) and HT2 (b) grain sizes from quarter section. Grains were not determined to be shallow compared to their observed surface area. 95% confidence intervals ranged from 0.76 μm to 1.16 μm for ferrite and 1.18 μm to 1.82 μm for harder phases. GPa X (mm) X (mm) Y (mm) Y (mm) Nanoindentation tests with multiple loads indicated that a size effect problem exists on initial loading of the material. The lack of a fully developed plastic zone results in abnormally high hardness for the first loadings. Hardness was best obtained on the third or fourth loading of the material, particularly when these loads occurred between 1500 and 2500 μm. Isolated phase targeting on etched samples yielded a small number of usable indentations. These were used to confirm ranges identified with the grid sampling method, which produced a higher yield of usable indentations per hour of machine time. Key factors affecting the quality of nanoindentation results included the size of the grains relative to the size of the indentation probe, the roughness of the sample surface after polishing, and the extent to which grains were level with the surface. Nanoindentation tests utilizing a 10 by 10 grid of test points yielded the following results. Rolling direction indicated with the arrow. Scale bars on each face represent 50 μm. Grains showed minor variability in shape and orientation on different faces. Isolated phase targeting was completed with HT1 using a load of 1000 μN. To maximize the amount of data obtained for one material, all isolated phase targeting experimentation was completed on HT1 samples. Image from the scanning probe on the Hysitron. The lower three samples, highlighted in green, are indicative of usable placements on ferrite. The indentation at the top was placed too close to a grain boundary. A sample load-depth curve is shown to the right. Displayed hardness values for hard and soft phases are based on average measurements from accepted indentations from the grid sampling method. No statistically significant difference was observed between the hardness of either hard phase in the two products, although measurements from hard phases from HT1 were slightly elevated. Phase hardness differed by 2.25 GPa in HT1 and 2.20 GPa in HT2. These variations are likely due to variations in the microstructure formation during the heat treatment process. HT2 has a HER of 29.5% compared to 23.8% for HT1. Cracks during hole expansion operations typically form at the interface between hard and soft phases. Individual peak hardness in grains affects HER more than average hardness. The presence of a low number of harder grains in HT1 may be increasing the likelihood of crack formation and growth. It may be possible to increase the HER for HT1 by adjusting the process chemistry, temperature, or times. A second set of samples were prepared for nanoindentation with a Berkovich-tipped probe. These were etched using 5% Nital for 1.5 seconds. Two methods were used to measure the hardness of grains: random grid sampling with post- indentation phase analysis and direct targeting of isolated phases. After nanoindentation, a diamond scribe was used to mark grid locations to aid in finding them for SEM imaging. The nanohardness of ferrite and the hard phase displayed a similar trend to the measured Vickers hardness of a ferritic steel sample. As the applied load of Vickers hardness increased, readings approached the measured hardness of ferrite obtained from nanoindentation. Vickers loads of 10g have high variation, suggesting the measurement of multiple phases. Data taken from uncoated edge sample. Alloying effects on bainite formation in low-carbon DP steel during hot dip galvanizing cycle. Microalloyed Low-Carbon Multiphase Steels, DOI: 10.1002/srin.201500352 PAPG GF H = 2.74 H = 4.97 H = 4.75 H = 3.42 Grain size (μm) IPT Ferrite (GPa) St. Dev. 95% CI Hard phase (GPa) St. Dev. 95% CI HT1 3.97 0.42 0.36 5.31 0.14 0.14 50 μm - 1.0 2.0 3.0 4.0 5.0 6.0 7.0 Side Transverse Face Ferrite Hard Phase - 1.0 2.0 3.0 4.0 5.0 6.0 7.0 Side Transverse Face Ferrite Hard Phase Grain size (μm) Grid Method Ferrite (GPa) St. Dev. 95% CI Hard phase (GPa) St. Dev. 95% CI HT1 3.95 0.29 0.07 6.37 0.79 0.13 HT2 4.05 0.43 0.10 6.47 0.72 0.14 The following hardness data was obtained for each method using loads ranging from 2.350 to 2.450 mN. Vickers Martensite Ferrite Hardness maps of HT2 (L) and HT1 (R) material, quarter section; data recorded with 3000 μm load and third loading segment. In both cases product results in islands of the hard phase separated by a sea of ferrite due to diffusivity time. Special thanks to Raheleh M. Rahimi for help in nanoindentation
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Correlation of Material Phase Hardness and Macroscopic Properties of 1180DP Steel Zack Capo, Alvin Laimana, Tina Landon, Michelle McKinney, Jason SawaFaculty Advisor: Dr. David F. BahrIndustrial Sponsor: Dongwei Fan
Project Background
The microhardness and single grain hardness of two heat treated dual-phase steel products were examined.Two methods of measuring individual phase properties were utilized; performing a grid of indentations andtargeting the hardness of indentations made in isolated phases. Nanoindentation data indicated hardness in heattreatment one of 3.9 GPa for ferrite and 6.2 GPa for the hard phase. Material receiving heat treatment twocontained ferrite measuring 3.9 GPa and the hard phase at 6.1 GPa. Differences in yield strength, tensilestrength and hole expansion ratio were not found to be associated with differences in phase hardness.
Experimental Procedure
Results
Recommendations
Discussion
MSE 430-440: Materials Processing and Design
Samples from different locations on the sheet width were cutvia a water jet. They were mounted and polished to a 3micron diamond stage and etched with 0.5% and 5% Nital(nitric acid and ethanol).
Advanced High Strength Steels (AHSS) combine high yieldand tensile strength with good formability. ArcelorMittal’s1180DP product is a dual-phase product and it is offered asgalvanized and uncoated product with two different heattreatments. The compositions and properties are:
Product composition, yield strength, tensile strength and hole expansionratio (HER) of galvanized and uncoated 1180DP steel
Forming operations risk the creation of cracks and earlymaterial failure problems. Deformation behavior of dual-phase steels can be attributed to the yielding of ferrite andfracture of harder phases. Phases include martensite andbainite, depending on the heat treatment. Achieving a highHER requires a limit in the variation of phase hardness. Forthese products, bulk and nanohardness data was collectedand SEM analysis was performed to gain knowledge in thecontribution of each phase to macroscopic behavior.
One sample set was etched and examined via opticalmicroscopy to analyze grain size and phase fraction foreach product and location. The Heyn Intercept Method wasused to measure grain size. 48% of the volume fraction (VF)of HT1 was ferrite and 52% of the VF of HT2 was ferrite.
The grid method of sampling is recommended fornanoindentation, combined with SEM imaging for phaseanalysis. Grid spacing should exceed the average grainsize. The grain volume sampled by the indenter should beless than 15% and indentation data should be excludedwhen the observed grain diameter is less than 50% ofknown grain size. Sample preparation is critical for thecollection of high quality results.
Samples acquired from three positions on a half sheet of rolled steel. A directional tab on removed samples documents the rolling direction.
EdgeQuarterCenter
C Mn Si Other YS (MPa) TS (MPa)
HER
HT1 0.13 2.5 0.7 Confidential 850 1240 23.8%HT2 0.14 2.1 0.7 Confidential 910 1250 29.5%
In this SEM image, ferrite can be seen as the darker phase and the lighterphase represents the harder phase. Indents can be observed within grainboundaries.
(a) (b)
The microstructure of HT1 material is shown, based on a sample from the center section.
This work is sponsored byArcelorMittalEast Chicago, IN
HT1 (a) and HT2 (b) grain sizes from quarter section. Grains were not determined to be shallow compared to their observed surface area. 95% confidence intervals ranged from 0.76 µm to 1.16 µm for ferrite and 1.18 µm to 1.82 µm for harder phases.
GPa
X (mm)X (mm)
Y (m
m)
Y (m
m)
Nanoindentation tests with multiple loads indicated that asize effect problem exists on initial loading of the material.The lack of a fully developed plastic zone results inabnormally high hardness for the first loadings. Hardnesswas best obtained on the third or fourth loading of thematerial, particularly when these loads occurred between1500 and 2500 µm.
Isolated phase targeting on etched samples yielded a smallnumber of usable indentations. These were used toconfirm ranges identified with the grid sampling method,which produced a higher yield of usable indentations perhour of machine time. Key factors affecting the quality ofnanoindentation results included the size of the grainsrelative to the size of the indentation probe, the roughnessof the sample surface after polishing, and the extent towhich grains were level with the surface.
Nanoindentation tests utilizing a 10 by 10 grid of test points yielded the following results.
Rolling direction indicated with the arrow. Scale bars on each face represent 50 µm. Grains showed minor variability in shape and orientation on different faces.
Isolated phase targeting was completed with HT1 using a loadof 1000 µN. To maximize the amount of data obtained for onematerial, all isolated phase targeting experimentation wascompleted on HT1 samples.
Image from the scanning probe on the Hysitron. The lower three samples,highlighted in green, are indicative of usable placements on ferrite. Theindentation at the top was placed too close to a grain boundary. A sampleload-depth curve is shown to the right.
Displayed hardness values for hard and soft phases are based on averagemeasurements from accepted indentations from the grid sampling method.
No statistically significant difference was observed betweenthe hardness of either hard phase in the two products,although measurements from hard phases from HT1 wereslightly elevated. Phase hardness differed by 2.25 GPa inHT1 and 2.20 GPa in HT2. These variations are likely due tovariations in the microstructure formation during the heattreatment process. HT2 has a HER of 29.5% compared to23.8% for HT1. Cracks during hole expansion operationstypically form at the interface between hard and soft phases.Individual peak hardness in grains affects HER more thanaverage hardness. The presence of a low number of hardergrains in HT1 may be increasing the likelihood of crackformation and growth. It may be possible to increase theHER for HT1 by adjusting the process chemistry,temperature, or times.
A second set of samples were prepared for nanoindentationwith a Berkovich-tipped probe. These were etched using 5%Nital for 1.5 seconds. Two methods were used to measurethe hardness of grains: random grid sampling with post-indentation phase analysis and direct targeting of isolatedphases. After nanoindentation, a diamond scribe was used tomark grid locations to aid in finding them for SEM imaging.
The nanohardness of ferrite and the hard phase displayed a similar trend to the measured Vickers hardness of a ferritic steel sample.
As the applied load of Vickers hardness increased, readings approached themeasured hardness of ferrite obtained from nanoindentation. Vickers loadsof 10g have high variation, suggesting the measurement of multiple phases.Data taken from uncoated edge sample.
Alloying effects on bainite formation in low-carbon DP steel during hot dip galvanizing cycle. Microalloyed Low-Carbon Multiphase Steels, DOI: 10.1002/srin.201500352
PAPG GF
H = 2.74
H = 4.97
H = 4.75H = 3.42
Gra
in s
ize
(µm
)
IPT Ferrite(GPa)
St. Dev.
95% CI
Hard phase(GPa)
St. Dev.
95% CI
HT1 3.97 0.42 0.36 5.31 0.14 0.14
50 µm
-
1.0
2.0
3.0
4.0
5.0
6.0
7.0
Side Transverse Face
FerriteHard Phase
-
1.0
2.0
3.0
4.0
5.0
6.0
7.0
Side Transverse Face
FerriteHard Phase
Gra
in s
ize
(µm
)
GridMethod
Ferrite(GPa)
St. Dev.
95% CI
Hard phase(GPa)
St. Dev.
95% CI
HT1 3.95 0.29 0.07 6.37 0.79 0.13HT2 4.05 0.43 0.10 6.47 0.72 0.14
The following hardness data was obtained for each methodusing loads ranging from 2.350 to 2.450 mN.
Vickers
Martensite
Ferrite
Hardness maps of HT2 (L) and HT1 (R) material, quarter section; datarecorded with 3000 µm load and third loading segment. In both casesproduct results in islands of the hard phase separated by a sea of ferritedue to diffusivity time.
Special thanks to Raheleh M. Rahimi for help in nanoindentation
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