Critical Review Of Hot Stamping Technology For Automotive .
Proc. from the Materials Science & Technology Conference MS&T 2007, Sept.16-20, 2007, Detroit, MichiganCritical Review of Hot Stamping Technology for Automotive SteelsD. W. Fan, Han S. Kim*, S. Birosca and B. C. De Cooman*Graduate Institute of Ferrous Technology,Pohang University of Science and Technology, Pohang, South Korea*e-mail: decooman@postech.ac.kr; hansoo-kim@postech.ac.krKeywords: Hot stamping, Press hardening, Boron steel, Martensitic steelAbstractHot stamping is an alternative technology to produce ultra high strength steels forautomotive parts that has received much attention recently. In this paper the physical metallurgyprinciples of the hot stamping process are reviewed. The effect of composition on CCT curves ofhot stamping steels is discussed taking deformation during press forming into account. Moreoverthe thermo-mechanical behavior of press hardenable B-added CMn steel is reviewed in detail.The influence of temperature, cooling rate, strain and strain rate on the flow stress are discussed.The issues related to coatings on B-alloyed CMn hot stamping steel are also critically reviewed.Finally, the properties of both Zn-based and Al-based coating systems are compared, and thepossibility of the in situ formation of a diffusion barrier during press forming is discussed.IntroductionHot stamping, also often referred to press hardening, is a technology to produce ultra highstrength steels for automotive applications that has received much attention recently. The hotstamping process was originally developed by Carl-Erik Ridderstrale of Plannja Hard Tech in1973, which after having been a subsidiary of Swedish Steel AB now is part of the GESTAMPcompany [1-3]. The hot stamping steels are mainly used for automotive door beams, impactbeams, reinforcing beams, bumpers, pillars, roof rails, and tunnels [3, 4]. In 2004 the estimatedtotal consumption of hot stamping steels was considerable: about 60,000 to 80,000 tons inEurope. The 2008-2009 yearly consumption in Europe is expected to increase to about 300,000tons. Japanese and North American car makers are expected to follow this trend. Based on 2004predictions, more than 20 new hot stamping lines are expected to be built by 2009 throughout theworld [5]. The reason for the popularity of hot stamping steels with carmakers is due to the factthat the tensile strength of hot stamping steels can reach about 1500 MPa offering ultrahighstrength with lower weight. In addition, the hot stamping process achieves good repeatability inlong production runs without springback compared with cold press forming, and the material is alow carbon steel which has favorable welding characteristics.
Proc. from the Materials Science & Technology Conference MS&T 2007, Sept.16-20, 2007, Detroit, MichiganThermo-mechanical process of hot stampingHot stamping (Figure 1a) is a process during which the cold rolled steel sheets arecontinuously fed in a gas fired furnace, and the blanks are heated in the temperature range of900-950 C for 3-5 minutes to achieve full austenization. The red hot sheet is then fed into ahydraulic press. The press cycles down and remains in that position while the water-cooled diesquickly cool the formed blanks until the temperature reach below the martensitic transformationfinish temperature which is typically around 200 C. The required cooling rate is in the range of40-100 C/s [2, 5]. In order to avoid surface oxidation and the formation of oxide scale, hotstamping steel is often coated with aluminum-silicon or zinc [6, 7].Austenization800250A3Temperature (oC)Temperature 150200250300010Time (s)20Time (s)(a)3040(b)Figure 1 (a) Schematic thermal cycle of hot stamping steels. Figure 1(b) Thermal cycle of press diesIn hot stamping process, forming and hardening are combined in a single operation. Toachieve rapid and homogeneous cooling of steel sheet, the press dies have internal coolingcircuits. At the start of the deformation, the dies contact with the hot steel and their temperaturerise. The dies are then quenched and heat is removed from the steel sheet to the dies. Thetemperature cycle of press dies is schematically illustrated in Figure 1b.Before hot stamping process the microstructure of hot stamping steels consists of pearliteand ferrite, while after the process the microstructure becomes fully martensitic (Figure 2). Themechanical properties are also changed; the ultimate tensile strength rises from 600MPa to about1500MPa and the uniform elongation decreases from 20% to 6%.10 µm(a)10 µm(b)Figure 2 The schematics before hot stamping with ferrite and pearlite microstructures (a)and the full martensitic microstructure after hot stamping (b) [8].
Proc. from the Materials Science & Technology Conference MS&T 2007, Sept.16-20, 2007, Detroit, MichiganFigure 3 shows the two most common types of hot stamping production lines: the directhot stamping method (Figure 3a) and indirect method (Figure 3b). In indirect method the steelsheet is deformed about 90% to 95% of its final shape in a conventional die before heating. Theindirect method makes it easier to produce parts with larger dimensions and more complicatedshapes [5].(a)Direct hot stamping line:DecoilerBlankingHeatingPress formingFurnaceCooling systemShot blastingDie cooling(b)Indirect hot stamping line:PresspreformingFigure 3 Comparison between direct method (a) and indirect method (b) of hot stamping [4]Physical metallurgy of hot stamping steelsCompositions and CCT curves of hot stamping steelsThe compositions and calculated Ms temperatures of typical hot stamping steels are listedin Table 1. The Ms temperatures are typically about 400 C. This implies that press forming mustbe completed above 400 C otherwise the deformation will be more difficult when the martensitetransformation starts.Table 1 Compositions of hot stamping steels and their estimated Ms temperatures [6, 9-11]Ms 499-308 C-32.4 Mn-27.0 Cr-16.2 Ni-10.8 (Si Mo W) 10 CoSteelC [%]Si [%]Mn [%]Cr [%]P [%]S [%]B [%]Al [%]Ti [%]Mo [%]Ms [ 0.0350.10406It is believed that Manganese is extremely effective to increase the hardenability of steeland retards most transformation reactions. The relatively higher Mn content of steel 2 results in
Proc. from the Materials Science & Technology Conference MS&T 2007, Sept.16-20, 2007, Detroit, Michiganthe relatively lower Ms temperature estimation. The calculated Ms temperature of steel 3 is thehighest among the four steels due to the combination of relatively low C, and Mn contents.Boron is often added to press-hardenable steels. Its addition increases the hardenabilityby retarding the heterogeneous nucleation of ferrite at the austenite grain boundaries. This is dueto the reduction of interfacial energy when B segregates at the austenite grain boundaries, whichmeans that the presence of B increases the incubation time for the formation of ferrite and lowersthe nucleation rate after the onset of ferrite formation. The B content of steel 3 is relatively lowand this retards transformation less and shifts the ferrite transformation region in the CCTdiagram to the left (Figure 4c). Too much B addition leads to the formation of borides at theaustenite grain boundaries and this will enhance the nucleation of ferrite grains. B should be insolid solution because B oxide or nitride is not effective in retarding ferrite transformation. B hasa very high affinity for O and N, therefore addition of alloying elements such as Al and Ti areoften employed, in order to protect the solute B [12].A FA FA PMsA BA MA PA BMsA M(a)(b)A FA FMsA PA PA BA BMsA MA M(c)(d)Figure 4 CCT diagrams comparison of the steels in Table 1.(a) Steel 1 [12], (b) Steel 2 [11], (c) Steel 3 [15], ( d) Steel 4 [16]Chromium is another strong hardenability agent and it is especially important insuppressing the bainite transformation. Cr addition moves the bainite transformation region to a
Proc. from the Materials Science & Technology Conference MS&T 2007, Sept.16-20, 2007, Detroit, Michiganlower temperature. The Cr content of steel 1(Table 1) is relatively higher (around 0.2%),combining with the effect of B that makes its bainite transformation temperature lower thanothers (Figure 4a).The critical cooling rates for full martensitic transformation are shown in the CCT curves(Figure 4). Practically the maximum cooling rate is mostly decided by the design of dies, whichmeans the slower critical cooling rate will require a less complicated die design. This also haseconomical advantages. The critical cooling rate of steel 2 is the slowest among four steels. Thisimplies that the die design will be less costly.The plastic deformation of austenite has a profound effect on the austenite decompositionreactions, which is illustrated in Figure 5. The solid lines are the CCT curves without anydeformation while the dashed lines are the CCT curves after deformation. Straining austenitecauses two effects. First, austenite straining may increase the ferrite nucleation rate, and secondit increases the free energy of austenite, which consequently increases the transformationkinetics. Both effects lead to an increase in Ar3 temperature. Furthermore, the strain influencesthe bainite and marteniste transformations. It is believed that an increase in strain leads to shorterincubation time. In general, however, increased strains and lower deformation temperatures seemto stabilize the austenite phase and increase the retained austenite volume fraction in themicrostructure. In addition, strain lowers the Ms and Bs temperatures. These phenomenamentioned above are referred to as the mechanical stabilization of austenite [13]. Oneexplanation is that the increase in dislocation density by deformation raises the number ofpotential nucleation sites. However, too much deformation may introduce restrains to nucleigrowth [14]. Consequently, in practice, the critical cooling rate will be different from the staticcooling rate and this will require further study.900Deformation800A FA PTemperature (oC)600A BMs400A MMf20001s1min1hrTimeFigure 5 A schematic of deformation influence on CCT diagram.Solid line: no deformation. Dashed line: with austenite deformation.Thermomechanical properties of hot stamping steelsBefore hot stamping process the steel sheet is cold rolled. Merklein et al. investigated theinfluence of rolling direction and they concluded that rolling direction has no significant
Proc. from the Materials Science & Technology Conference MS&T 2007, Sept.16-20, 2007, Detroit, Michiganinfluence on the flow stress of 22MnB5 (steel 4 in Table 1) in the temperature range of500-800 C after austenization at 950 C [17].The thermomechanical properties at different temperatures and strain rates of 22MnB5steel were investigated by Merklein et al. [16] and Turetta et al. [18], respectively. In the work ofMerklein, the steel was austenized at 950 C for 3 minutes and the tests were conductedisothermally from 850 C to 500 C. The cooling rate was 80 C /s. In the case of Turetta’s workthe steel was austenized at 900 C for 5 minutes. Tests were conducted under continuous coolingconditions with a cooling rate of 40 C/s. The temperature range was 800 C-600 C.500oC650oCStrain rate 0.1 s-1Strain rate 1s-1Strain rate 0.1s-1Strain rate 0.01s-1T 650oC850oC(a)(b)600oC700oCStrain rate 0.5s-1T 600oC800oC(c)Strain rate 1s-1Strain rate 0.5s-1Strain rate 0.1s-1(d)Figure 6 Stress-strain curves of 22MnB5 at different temperatures and strain rates. (a) Different temperatures [16],(b) Different strain rates [16], (c) Different temperatures [18], (d) Different strain rates [18]The comparison of the stress-strain curves of 22MnB5 steel in references [16] and [18] isshown in Figure 6. In the original curves of Merklein, the stress was about 200MPa when thestrain was zero. The authors of this paper believe that 200MPa should be regarded as the offsetof stress, and the real stress data should be subtracted by 200MPa, which are redrawn in Figures6a and b.In Figure 6a, the curves show that the initial work hardening is balanced by dynamicrecovery. The higher the deformation temperature, the more dynamic recovery decreasesstrength. Figure 6c shows the same trend. A low strain rate has the same effect on strength as ahigh temperature. Figure 6b shows that a lower strain rate results in a lower strength.
Proc. from the Materials Science & Technology Conference MS&T 2007, Sept.16-20, 2007, Detroit, MichiganIn Figure 6b, the strain-stress curves with strain rate 0.01s-1 at 650 C shows a largedecrease in the stress value at strain 0.2 . The reason is that cooling from 950 C to 650 C with acooling rate 40 C/s takes 7.5s and the deformation with 0.4 true strain requires 40s, i.e. the totalcooling time is 47.5s, which reaches the ferritic transformation start curve (Figure 5 solid line)and the microstructure will change from austenite to soft ferrite during straining. And due to thedeformation enhanced ferritic transformation (Figure 5 dashed line), the stress drop starts evenearly. The microstructure of this specimen requires investigation.In Figure 6c steel is continuously cooled and deformed at the strain rate of 0.5s-1. Thetrue strain of 0.2 will take 0.4s and temperature will drop only 16 C, which means thattemperature change is small and its influence on stress can be ignored. The continuously cooledstrain-stress curves (Figure 6c, strain rate 0.5s-1) can be compared with the isothermal ones(Figure 6a, strain rate 0.1s-1) as follows. In the true strain range of 0-0.2, at high temperature thestress at 850 C (Figure 6a dashed line) is lower than the stress at 800 C (Figure 6c dashed line).This may be due to the higher temperature and lower strain rate. At medium temperature, thestress at 650 C (Figure 6a dot line) is lower than stress at 700 C (Figure 6c dot line). Though theformer one has relatively lower temperature, the relatively lower strain rate has a morepronounced influence on the decrease of stress. At low temperature, the stress with 500 C(Figure 6a solid line) is higher than stress at 600 C (Figure 6c solid line). This means thattemperature plays a more influential role in decreasing stress than the strain rate. This fact can beseen in Figure 6d; at 600 C the strain rate has much less effect on stress. Further investigation isrequired to clarify the influence of strain rate within specific temperature ranges.In Figures 6a and b, the maximum true strains are similar for different strain rates andtemperatures, but in Figure 6c and d the maximum true strains are quite different. In the originalpaper, it is not mentioned whether the test was halted when the specimen broke.Asai et al. [15] studied the deep drawability of steel 3 soaked at 800 C and 900 C. Hestudied the influence of microstructure, the forming start temperature and the oxide scale on theheated blanks. He reported that the deep drawability of samples soaked at 900 C was improvedwhen the forming start temperature was decreased to 700 C. The drawability was also improvedby increasing the oxide scale thickness on the blank.Coatings on hot stamping steelIf the hot stamping steel is bare, scale will form on the surface due to the thermaloxidation during the austenization heat treatment. In earlier hot stamping technologies, the scalewas removed by chromium shot blasting, which left a thin film of chromium and iron on thesurface, thus eliminated the need for re-oiling to prevent oxidation and corrosion [2].Nowadays coated steel sheets are used in the hot stamping process instead of bare steel.The coatings prevent surface oxidation and decarburization, and enhance corrosion resistanceafter painting. There are two types of coatings: Al-Si aluminized and Zn-Fe galvannealed [7].Al-Si hot-dip coatingIn this case, the hot stamping steel is hot dip coated with Al-10%Si (Figure 7a) [21]. Themorphologies before and after hot stamping process are compared in Figures 7a and b. After hot
Proc. from the Materials Science & Technology Conference MS&T 2007, Sept.16-20, 2007, Detroit, Michiganstamping the surface is much rougher due to the formation of Fe-Al intermetallic phases and thecontact with the press dies. The paintability is reported to be improved and good even withoutchemical pretreatment [20].Al-10%SiFe-Al-Si alloy layerFe(a)50% Al130% Al250% Al330% Al410% Al5Fe(b)FeAlSi(c)Figure 7 Cross sectional view before (a) and after (b) hot stamping.(c) Fe, Al and Si concentration profiles after heating at 950 C for 30s.During heat treatment the composition of surface layer is changed. From surface to insidethe layer consists of 5 sub-layers with Al concentration 50, 30, 50, 30 and 10% respectively(Figure 7c). The layers consist of 3 intermetallic phases, two ordered BCC phases (Fe2Al5, FeAl2)and one disordered BCC phase. As the soaking time increases, Al diffuses into the base steel andthe sub-layers disappear gradually [20].In Figure 7b there are some black spots inside the layer. These are defects formed duringthe formation of intermatallic phases: Kirkendall voids, which are due to the different diffusionrates of Al in Fe and Fe in the Al coating.AlN inhibition barrierThe Al-Si coated steels are also used in automotive exhausting systems. Research hasshown that if there is no free interstitial nitrogen in the steel substrate, the coating surfacedegrades quickly. This degradation results in a poorly reflecting grayish-to-black surface beyond450 C due to the growth of intermetallic phases. When interstitial N is present in the substrateAlN precipitates at the interface between steel and coating (Figure 8b), and the growth ofintermetallic phases will be limited. Figure 8c shows the clear N enrichment at the interfacebetween steel and coating [22].For hot stamping steel, the heating temperature is much higher than 450 C comparedwith exhaust parts, but it is held at this temperature for a very short time. Thus the intermetallicphases may not grow quickly enough to deteriorate the coating. And the solubility and diffusivityof interstitial N in the substrate will increase and an AlN diffusion barrier will be formed at theinterface. It will inhibit the growth of intermetallics. This could be an additional advantage ofAl-Si coating in hot stamping.
Proc. from the Materials Science & Technology Conference MS&T 2007, Sept.16-20, 2007, Detroit, MichiganAl-10%SiNSiFeIntermetallicphasesFe-Al-Si alloy layerAlNFeFe(a)(b)(c)Figure 8 Cross sectional view before (a) and after (b) heating at 450 C for 100 hours(c) Fe, Si and N concentration profiles attained by SIMSGalvannealingImai et al. [7] studied the behavior of galvannealed hot stamping steel. During thegalvannealing process the Zinc-Iron intermetallic phases are formed as shown in Figure 9a. Afterhot stamping the topmost layer is Zinc oxide and below is a Zinc-Iron solid solution layer. Thissolid solution layer is about 20μm and contains 20-30 wt% Zinc as shown in Figure 9c [7].Comparing the morphology in Figures 9a and b, after hot stamping the surface roughnessincreases and the Zinc-Iron layer becomes thicker.ZnOδZn-Fe intermetallicГZn-Fe intermetallicFe(a)Fe-Znsoild solutionFe(b)FeZnO(c)Figure 9 Cross sectional view of the Galvannealed coating before (a) and after (b) hot stamping(c) Fe, Zn and O concentration profiles after hot stamping.Suehiro et al. [20] tested and compared aluminized, galvannealed and bare presshardenable steels. An aluminized steel sheet with a coating weight of 160 g/m2 was heated to900 C for 2 min to form a Fe-Al alloy layer. A galvannealed steel sheet (coating weight 45 g/m2)and a cold rolled steel sheet without surface treatment were also selected. The three steel panelswere subjected to the same tests.
Proc. from the Materials Science & Technology Conference
Critical Review of Hot Stamping Technology for Automotive Steels . 250 0 10 20 30 40) (oC) M f 0 200 400 600 800 . In hot stamping process, fo
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