coated ahss issues · 2013. 8. 28. · gary dallin ahss - coating issues and concerns 3 trends in...
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Gary Dallin
AHSS - Coating Issues and Concerns 1
AHSSCoating Issues and
Concerns
Gary DallinGalvInfo Center
Trends in New Sheet Steels andGlobal Standards for Sheet Steels 2
Introduction Compared to HSS, AHSS grades use higher amounts
of alloys such as Mn, Si, Mo, and C - many of whichhave a higher affinity for O than Fe
Achieving good zinc adherence using traditionalcontinuous annealing furnace practices is an ongoingissue with higher alloy steel due to the difficulty inreducing their more stable oxides
Considerable research is underway that focuses onmodified furnace conditions from those used for lowcarbon and HSLA steels
Some producers are looking at other methods ofavoiding zinc adherence problems
Gary Dallin
AHSS - Coating Issues and Concerns 2
Trends in New Sheet Steels andGlobal Standards for Sheet Steels 3
Topics of Discussion Review of AHSS steel grades and
metallurgy Selective Oxidation during annealing Internal oxidation Control of oxidation/reduction Other methods to avoid selective
oxidation
Trends in New Sheet Steels andGlobal Standards for Sheet Steels 4
The Family of Coated Sheet Steels
Gary Dallin
AHSS - Coating Issues and Concerns 3
Trends in New Sheet Steels andGlobal Standards for Sheet Steels 5
AHSS Mechanical Properties
Trends in New Sheet Steels andGlobal Standards for Sheet Steels 6
AHSS Chemistries
For both DP and TRIP C level of 0.1 to 0.4%, but as low a possible for good weldability Mn level of 1.0 - 2.5% Cr & Mo up to 1.2% (together) Si level of 1.0 - 2.5%, and/or Al level of 1.0 - 2.5% Often micro-alloyed with Nb, V, or Ti for grain refining and precipitation
hardening
Gary Dallin
AHSS - Coating Issues and Concerns 4
Trends in New Sheet Steels andGlobal Standards for Sheet Steels 7
Stress-strain Behavior The high work
hardening rate andgood elongation ofDP steels give themmuch higher UTSthan conventionalHSS of similar YS
TRIP steels have alower initial workhardening rate thanDP steels, but thehardening ratepersists at higherstrains. This givesan advantage overDP in the mostsevere stretchforming applications
Trends in New Sheet Steels andGlobal Standards for Sheet Steels 8
Dual Phase Steels Ferritic matrix containing
a hard martensitic secondphase in the form ofislands
Higher volume fraction ofsecond phase generallyincreases strength
For coated products DP isproduced by controlledcooling from the two-phase ferrite plusaustenite phase totransform some austeniteto ferrite before controlledcooling to transform theremaining austenite tomartensite
Gary Dallin
AHSS - Coating Issues and Concerns 5
Trends in New Sheet Steels andGlobal Standards for Sheet Steels 9
TRIP Steels(TRansformation Induced Plasticity)
Retained austenite embeddedin a primary matrix of ferrite,with varying amounts ofmartensite and bainite
Requires the use of anisothermal hold at anintermediate temperaturewhich produces the bainite
Higher C and Si contentresults in the significantvolume fractions of retainedaustenite
During deformation retainedaustenite progressivelytransforms to martensite withincreasing strain therebyincreasing the work hardeningrate at higher strain levels
Trends in New Sheet Steels andGlobal Standards for Sheet Steels 10
Oxide Formation During Annealing Despite protective atmospheres oxides of elements with a higher
affinity for O than Fe form on the steel surface and cannot bereduced under normal conditions
These oxides cause zinc adhesion failure or interfere with thegalvannneal reaction
In CA furnaces very low dew points have been used to controloxides.
Research on highly alloyed AHSS steels has shown that ultra-thinoxide layers can form over large areas of the surface at very lowdew points - often referred to a Selective Oxidation
At higher dew points there tends to be selective accumulation ofalloying elements along the grain boundaries, resulting in increasedFe content near the surface
Gary Dallin
AHSS - Coating Issues and Concerns 6
Trends in New Sheet Steels andGlobal Standards for Sheet Steels 11
Selective Oxidation
Annealed AHHS surfaces after heat treatment at 800 °C at various dew points
OxideOxide
Oxide
Fe
Fe
Fe
DP -17 °C DP -30 °C DP -45 °C
3 µm 3 µm 3 µm
Source: Galvatech 07 Proceedings - B. Schuhmacher paper
Trends in New Sheet Steels andGlobal Standards for Sheet Steels 12
Selective Oxidation
At the AHSS production level, it has been foundthat mere addition of moisture to the incominggas will not ensure effective stabilization of thedew point in the furnace, sufficient to avoidselective oxidation
Gary Dallin
AHSS - Coating Issues and Concerns 7
Trends in New Sheet Steels andGlobal Standards for Sheet Steels 13
Oxidation/Reduction ThyssenKrupp have found that a better way to
achieve good zinc wetting on AHSS is to use anoxidation/reduction technique
Iron-based alloy surfaces exposed to anoxidizing atmosphere will primarily form a Feoxide layer that completely covers the surface
Unlike the oxides of Mn, Si, Cr, and Al, Fe oxidecan easily be reduced by hydrogen downstreamin the furnace
Trends in New Sheet Steels andGlobal Standards for Sheet Steels 14
Internal Oxidation
The water vapor formed during the oxidation/reduction processprevents formation of harmful oxides on the surface andembeds the stable alloy oxides, keeping them from interferingwith the Fe-Zn reaction
The name used to describe this process is “Internal Oxidation” The best method of accomplishing it is to directly preheat the
strip using a direct-fired furnace (DFF) Many new lines built to produce AHSS have only radiant tube
(RT) furnaces, so accomplishing internal oxidation is a hugechallenge
Gary Dallin
AHSS - Coating Issues and Concerns 8
Trends in New Sheet Steels andGlobal Standards for Sheet Steels 15
Internal Oxidation
SEM scans of the various stages of the oxidation/reduction technique
Source: Galvatech 07 Proceedings - B. Schuhmacher paper
Trends in New Sheet Steels andGlobal Standards for Sheet Steels 16
Control of Oxidation/Reduction Using oxidation/reduction requires that it be
closely controlled by being able to measure thethickness of the oxide layer formed Furnace sensors based on reflectance work well for
homogeneous oxide layers At temperatures above 700°C, use infrared
pyrometry just downstream from the oxygen feed-inpoint
The above techniques are being usedsuccessfully in Europe to produce coated AHSS
Gary Dallin
AHSS - Coating Issues and Concerns 9
Trends in New Sheet Steels andGlobal Standards for Sheet Steels 17
Control of Oxidation/Reduction Recent research had shown that reducing Si
and increasing Al improves the reactive wettingof TRIP steels at -30°C
While Mn does selectively oxidize, it has beenfound that at levels as high as 1.5% it does notadversely affect reactive wetting - the reasonbeing that MnO is reduced by the Al in the zincbath. This suggests that routine zinc potanalysis should include Mn - at least whenrunning high Mn steels
Trends in New Sheet Steels andGlobal Standards for Sheet Steels 18
Other Methods to AvoidSelective Oxidation Heat and soak sheet at 850 - 1000°C to cause
reduction of SiO2 by C from the steel. Still a labprocedure and is energy intensive
Use Mo as it does not selectively oxidize - veryexpensive
Treat hot band at 720°C prior to pickling tosegregate Si and P oxides at grain boundariesabout 10 microns below surface. May be aviable added process to avoid retrofittinggalvanize lines with a DFF
Gary Dallin
AHSS - Coating Issues and Concerns 10
Trends in New Sheet Steels andGlobal Standards for Sheet Steels 19
ASTM Specifications for AHSS Currently no ASTM standard(s) for AHSS Europe has prEN 10336 (Dec 2006)
describing the technical deliveryconditions for HD and EG sheet metal ofmulti-phase steels with respect toformability
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