F2008-04-023

F2008-04-023DEVELOPMENT OF ULTRA-HIGH-STRENGTH GALVANNEALED STEEL SHEETS WITH EXCELLENT BENDING FORMABILITY AND HOLE-EXPANSIBILITY1Toshiki Nonaka*, 1Nobuhiro Fujita, 1Toshimasa Tomokiyo, 1Kenichiroh Matsumura,1Takashi Miyagi1Nippon Steel Corporation, JapanKEYWORDS – Ultra-high-strength steel, Galvannealed steel, Absorbed energy, Bend radius, CrashworthinessABSTRACT - Increasing the strength of a steel sheet is essential for both reducing the weight of an automobile and improving its crashworthiness. Various kinds of high-strength steel sheets have been applied to date to automobile parts. The majority of steel sheets used for the main structural members have had tensile strength of up to 590 MPa, while ultra-high-strength steel sheets with tensile strength ranging 980 and 1470 MPa have been applied to members, such as bumper reinforcements and door beams, where high strength is required. The applications of ultra-high-strength steel sheets have expanded in recent years due to the demand for reduced weight vehicles. There are varieties of cold-rolled steel (CR) sheets, while few galvannealed steel (GA) sheets have been developed. Bending formability and hole-expansibility of conventional high-strength galvannealed steel sheets is poor.In addition to good bending formability and hole-expansibility, wettability is important for ultra-high-strength galvannealed steel sheets. Surface quality is required for galvannealed steel sheets. Although ultra-high-strength steel sheets normally contain silicon for enhanced elongation, bending formability and hole-expansibility, silicon leads to poor wettability with molten zinc [1]. It is therefore difficult to manufacture ultra-high-strength galvannealed steel sheets with good formability and wettability.In this study, ultra-high-strength galvannealed steel sheets with excellent bending formability, hole-expansibility and good wettability were developed by controlling the microstructures of steel sheets and reducing the silicon content. The minimum bend radius, which affects the higher absorbed energy, of the developed ultra-high-strength galvannealed steel sheets, is smaller than that of conventional ultra-high-strength galvannealed steel sheets, while the adhesion of the coating is equivalent to that of mild steels. It is possible that the ultra-high-strength galvannealed steel sheets developed will contribute to weight reduction of autobody and improved crashworthiness.INTRODUCTIONGlobal warming is becoming an importantissue. To protect the environment, automobilemanufacturers are making great efforts toimprove fuel economy. Figure 1 shows theofficial plan announced by the JapaneseGovernment [2]. According to this, the CO2maritime, air) in Japan should be reduced by12 million tons by fiscal year 2010. One of the most efficient methods of improving fuel economy is reducing the weight of vehicles. On the other hand, regulations such as EURO-N-CAP [3], have recently proved so restrictive for crashworthiness that additional reinforcement may be required, resulting in additional weight.To achieve both light-weight properties and crash safety, the application of high strength steel is essential, hence various types of high-strength steel sheets have been applied to date for automobile parts. The majority of the steel sheets used for the main structural members have had tensile strength ranging between 440 and 590 MPa, while ultra-high-strength steel sheets, with tensile strength ranging between 980 and 1470MP, have been applied to members, such as bumper reinforcements and door beams, where high strength is required. Given the poor bending formability and hole-expansibility of conventional high-strength galvannealed steel sheets, few galvannealed steel (GA) sheets have been developed, while a variety of cold-rolled steel (CR) sheets.In addition to good bending formability and hole-expansibility, wettability is important for ultra-high-strength galvannealed steel sheets. Surface quality is required for galvannealed steel sheets. Although Ultra-high-strength steel sheets normally contain silicon for enhanced elongation, bending formability and hole-expansibility, silicon leads to poor wettability with molten zinc. It is therefore difficult to manufacture ultra-high-strength galvannealed steel sheets with good formability and wettability.In this study, ultra-high-strength galvannealed steel sheets with excellent bending formability and hole-expansibility have been developed and various characteristics of developed steel for automobiles are introduced.CONCEPT OF MATERIAL DESIGN FOR ULTRA HIGH STRENGTH STEEL SHEETS Normally, the formabilities, such as the bending and hole-expansibility properties of steel sheets, deteriorate as its strength increases [4,5]. The alloying element and heat pattern in the annealing process should be relatively limited during the galvannealing process. To suppress such degradations, it is effective to control the microstructure of steels. Microstructures of ultra-high-strength steel incorporate a mixture of soft microstructure (ferrite) and hard one (martensite), while the bending formability and hole-expansibility can be improved by decreasing the variation in hardness of each microstructure. This is because the boundaries between soft and hard microstructures must be so weak that micro-cracks or voids can easily form [6,7]. To obtain good wettability, a method of reducing silicon content is developed for steel sheets, since silicon leads to poor wettability with molten zinc.EXPERIMENTAL PROCEDUREManufacturing materialsGalvannealed steel sheets in this study were produced with a thickness of 1.4 mm using commercial lines, on a continuous galvannealed line. The main compositions of galvannealed steel sheets are carbon and manganese, with wettability in mind. The coating weight of galvannealed steel sheets used in this work was controlled to approximately 40 g/m2.Certain properties was measured and analysed as follows:1) Mechanical properties; tensile tests, hole expanding tests and V-bend tests2) Analysis of the microstructure hardness; hardness tests using a Hysitron TriboIndenter. 3) Adhesive resistance of coatings; V-bend tests4) Spot weldability; joint strength testsFigure 2 Schematic illustration of holeexpansion test5) Crashworthiness; weight drop testsEvaluation of the mechanical propertiesTensile tests were performed under JIS (Japanese IndustryStandard, Z 2201). Hole expanding tests were performed under JFS (Japan iron and steel Federation Standard T 1001).Hole-expansibility was measured as the hole expansion ratio (λ) via the test shown in Figure 2, whereby a punch having atop angle of 60 degrees was forced into a punched hole 10 mm in diameter. The hole expansion ratio (λ) is given by:100}/){((%)00×−=d d d λwhere d was the diameter of the hole at the time when a crack propagated across the thickness of a specimen sheet and do was its original diameter.Figure 3 V-bend test Bending formability was measured as the minimum bend radius(R) through the V-bend test shown in Figure 3. Specimens were bent by a V-shaped punch with a tip of various radii at an angle of90 degrees to inspect the minimum bend radius without cracking. When the punch was forced, the smallest radius without crack of the specimens was defined as the minimum bend radius (R).Evaluation of microstructure hardnessA hardness test was performed to analyze microstructures, upon which bending formability and hole-expansibility have an effect. They can be enhanced by reducing the hardness difference within each microstructure. The test was performed using a Hysitron TriboIndenter, which had a cube corner diamond indenter probe, and a load function, while peak load of 500 μN was used for all indentation tests. The hardness of microstructures was estimated relative to the depth of indentation and peak load. This test analyzed the hardness of cross-sectional microstructures constructed of steel sheets. The size of the indentation is smaller than that of the Vickers or Brinell hardness tests, which are general hardness tests, and the microstructure hardness could be analyzed in detail. During this study, the distance of the measured points was 2 μm and the measurement area was 40 μm x 30 μm.Evaluation of adhesive resistance of coatingsV-bend tests were shown in Figure 4, whereby the specimens were bent using a V-shaped punch at an angle of 90 degree. When the punch was forced, metal powders were peeled from the surface of the specimen. After applying cellophane tape to the compressive surface of the specimen, the metal powders on the cellophane tape weredissolved in 5% HCl solution. The amounts of metal powders (Zn andFe) were quantified by Inductively Coupled Plasma spectroscopy and Figure 4 Schematic illustration of the V-bend testtheir sum was regarded as the amount of powdering. X-ray diffraction of each of the coatings was also performed for quantitative analysis for the Γ phase, which would influence the adhesive resistance of coatings. The Adhesive resistance of newly developed steels was compared with conventional mild steel with a tensile strength of 270 MPa.Evaluation of the spot weldabilityWeldability was evaluated in terms of joint strength and the available welding current ranges were decided as follows. The lower limit of the welding current value was defined as a current for which the weld diameter was equal to four times the square root of the sheet thickness. On the other hand, the upper limit of the welding current value was defined as the maximum current in the current range with no expulsion. Joint strengths were measured using both tensile shear and cross tension tests. The developed steel sheets were compared with other conventional galvannealed steel sheets with tensile strengths of 440 MPa and 590 MPa. Evaluation of crashworthinessA drop test was shown in Figure 5, where the specimens had asquare cross section of 70 x 50 mm with flanges for spot welding.Crash tests were performed under conditions where weights werebetween 145 and 160 kgf and heights were between 5 and 9 m.The absorbed energies were measured in both cases of axial andbending crash tests, while crash displacements and loads weremeasured using laser distance measurements and load cells. Theabsorbed energies in the axial and bending cases were calculatedat crash displacement of 100 and 30 mm, respectively.Simulation of crashworthiness Array A simulation of the absorbed energy was performed for thepurpose of quantifying the effect of the bend radius, with theabsorbed energies of specimens with various thickness andtensile strengths calculated by Computer Aided Engineering(CAE). The shapes of the specimens were shown in Figure 6.The specimens had square cross sections of 70x50 mm withflanges and thickness of between 1.4 and 2.0 mm withtensile strengths of 590 and 980 MPa. The absorbed energieswere calculated in the case of bending crash test. The strokelength was 15 mm and the inside bend radius was between0.5 and 10 mm.RESULTS AND DISCUSSIONMechanical propertiesTable 1 and Figure 7 show the mechanical properties of developed high strength galvannealed steel. Bending formability and hole-expansion of developed steel are superior to those of conventional steel. Regarding the formability of ultra-high-strength steel sheets, the higher the elongation, the lower the hole-expansibility [8]. However, the elongation of developed steel was almost equivalent to that of conventional steel this time.Hardness testThe Results of hardness tests are shown in Figures 8 and 9. Figure 8 shows the hardness distribution of cross-sectional microstructures, with the hard area colored green and yellow in Figure 8. Hard areas of development steel sheets with a tensile strength of 980 MPa are smaller than those of conventional steel sheets with a tensile strength of 980 MPa. Figure 9Figure 8 Hardness distributions of galvannealed steel sheets with tensile strength of 980 MPaPa（μm ）（μm ）hardnessPa（μm ）（μm ）hardnessshows their hardness histograms. More than 10 GPa areas comprise 22% of conventional microstructures, whereas more than 10 GPa areas comprise 10% of developed microstructures. The hardness of the soft areas of both conventional steel sheets and developed steel sheets are equivalent, whereas super hard areas of developed steel sheets are smaller than those of conventional steel sheets. This suggests that the hardness difference of each microstructure of developed steel sheets is smaller than that of conventional steel sheets.It is presumed that developed steel sheets have a smaller bend radius and higher hole-expansion ratio than conventional steel sheets because bending formability and hole-expansibility is enhanced by decreasing the hardness difference of each microstructure [5, 6].Adhesive resistance of coatingsFigure 10 shows the relationship between Fe content of coatings and amount of powdering. Adhesive resistance of developed steels tends to show the same behaviour as conventional mild steel with a tensile strength of 270 MPa. The higher Fe content, the greater amount of powdering in all cases.Figure 11 shows the relationship between Fe content and X-ray diffraction intensity of Γphase (Fe 3Zn 10). The higher Fe content, the higher diffraction intensity of Γ phase. This implies that the growth of hard and fragile Γ phase causes adhesive resistance of coatings to deteriorate. This is a general phenomenon affecting galvannealed steel sheets [9], and shows that developed galvannealed steel sheets have adhesive resistance of coating behaviour equivalent to that of conventional mild steel sheets.024*********51015Fe content (mass%)A m o u n t o f p o w d e r i n g (×10m g )Figure 11 Relationship between Fe content and X-ray diffraction intensity of Γ050100150051015Fe content (mass%)200250300350d i f f r a c t i o n i n t e n s i t y o f g a n m a p h e s e (c p s )Figure 10 Influence of Fe content on adhesiveSpot weldabilityFigure 12 shows the relation between the tensile strength and spot weld joint strength of high strength galvannealed steel sheets with tensile strengths ranging from 440 to 980 MPa. As the sheet strength increases, the tensile shear strength of the spot weld joint does likewise, while the cross tension strength remains unchanged. This is a general phenomenon in steel sheetsCrashworthinessFigure 13 shows changes in the absorbed energies with tensile strength ranging between 440 and 980 MPa. In both axial and bending cases, the absorbed energies increase in proportion to the sheets strength. Furthermore, during the axial crash test, the specimens buckled due to the formation of a series of continuous folds. According to this increase in absorbed energy, the application of ultra high strength steel sheets can be very useful in reducing the thickness ofSimulation of crashworthinessthe minimum bend radius oncalculated absorbed energy. Thevertical axis is absorbed energy ofspecimens constructed of steelsheets. The decrease in bendcorner radius small, the absorbedenergies of thin steel sheets canbe almost equivalent to those ofthick steel sheets. For example, incomparison of steel sheets with980 MPa tensile strength, theabsorbed energies of 1.4 mm thickness sample at a bend radius of 1.0 mm is almost equivalent to those of 1.6 mm thickness sample at a bend radius 8.0 mm.In addition, the absorbed energies of 980 MPa steel sheets with 1.4 mm thickness at bend radius of 1.0 mm is almost equivalent to those of 590 MPa steel sheets with 2.0 mm thickness at a bend radius of 4.0 mm. It is possible that the applications of the developed ultra-high-strength galvannealed steel sheets contribute to weight reduction and improved crashworthiness.SUMMARYIn this study, ultra-high-strength galvannealed steel sheets with excellent bending formability and hole-expansibility were developed. In addition, adhesion of coatings was found to be equivalent to that of mild steels. It is possible for developed ultra-high-strength galvannealed steel sheets to be formed with a small bend radius. The smaller the corner radius of the parts, the higher the absorbed energy. Therefore, it is possible that applications of the developed ultra-high-strength galvannealed steel sheets will contribute to weight reduction and improved crashworthiness.REFERENCES(1) Hirose,Y., Togawa, H., Sumiya, J. : Tetsu-to-Hagané. 68, 6, 121 (1982)(2)Home page of the Japan Automobile Manufacturers’ Association: http://www.jama.or.jp/(3)Home page of Euro N Cap: /(4)Abe, H.: Tetsu-to-Hagané. 68, 1208 (1982)(5)Sugizawa, S., Hamamatsu, S., Kikuchi K., Kunishige, K.: Tetsu-to-Hagané. 68, 1256(1982)(6)Mizui, M., Takechi, H., Sekine, T.: Tetsu-to-Hagané. 76, 414 (1990)(7)Yamazaki, K. : Journal of the J.S.T.P. 36 (416), 973 (1995)(8) Nonaka,T., Fujita,N., Tomokiyo,T., Goto,K. : Materia Japan,46,108(2007)(9) Suzuki,Y. Sugimoto, Y., Fujita, S.: J.Sur.Finish.Soc.Jpn,58,183(2007)(10) Oikawa,H.et. Kasuya, T., Tomokiyo, T.: Welding Technology, 56, 157 (2008)。