1. Introduction

Hot-rolled high strength sheet steels have been applied tosuspension parts in automobiles1) for the weight reduction.A problem in press forming sheet steels having tensilestrength of 780 MPa grade was lack of stretch flange forma-bility.2) In general, stretch flange formability of highstrength sheet steel can be improved by the increase of localelongation.3) In the view point of microstructure, ferriticsteel without pearlite and large cementites has superiorlocal elongation since it has less stress-concentratingregion4) compared with multiphase microstructure such asdual-phase steel. Based on the knowledge, stretch flangeformability of 780 MPa grade sheet steel should be im-proved if microstructure of the steel can be controlled to beonly ferrite without pearlite and large cementites. However,the precipitation-strengthening of ferrite up to 780 MPagrade has not been achieved so far since very fine carbidesare required.

In this study, a new hot-rolled high strength sheet steelwith excellent stretch flange formability is successfully de-veloped not only by lowering precipitation temperature toobtain very fine carbides but also by preventing large ce-

mentites and pearlite from being generated in low carbonsteel.

2. Experimental Procedure

Carbides precipitating just after austenite–ferrite trans-formation such as TiC easily grow5) in hot-rolled low car-bon steel because of high austenite–ferrite transformationtemperature. Moreover, large cementites at grain bound-aries and pearlite, which deteriorate stretch flange formabil-ity,6,7) are generated in low carbon steel coiled at high tem-peratures for ferrite forming. Subjects to obtain ferriticsteel precipitation-strengthened by very fine carbides with-out pearlite and large cementites are not only to loweraustenite–ferrite transformation temperature but also tosuppress the deposition of cementite and pearlite underhigh temperature coiling.

Table 1 shows chemical composition of steels investi-gated. Chemical composition of the base steel was0.04%C–1.5%Mn–0.2%Mo. Manganese was added to pre-vent fine carbides from growing by lowering austenite–fer-rite transformation temperature (Ar3). The small amount ofmolybdenum was included to retard the deposition of both

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Development of High Strength Hot-rolled Sheet Steel Consistingof Ferrite and Nanometer-sized Carbides

Yoshimasa FUNAKAWA, Tsuyoshi SHIOZAKI, Kunikazu TOMITA,1) Tetsuo YAMAMOTO2) and Eiji MAEDA2)

Steel Research Laboratory, JFE Steel Corporation, Kawasaki-cho, Chuo-ku, Chiba 260-0835 Japan.1) Steel Research Laboratory, JFE Steel Corporation, Mizushima Kawasaki-dori, Kurashiki 712-8511 Japan.2) East Japan Works, JFE Steel Corporation, Ogishima, Kawasaki-ku, Kawasaki 210-0868 Japan.

(Received on May 24, 2004; accepted in final form on August 17, 2004 )

A ferritic steel precipitation-strengthened by nanometer-sized carbides was developed to obtain a highstrength hot-rolled sheet steel having tensile strength of 780 MPa grade with excellent stretch flange forma-bility.

Manganese in a content of 1.5% and molybdenum in a content of 0.2% were added to 0.04% carbon Ti-bearing steel in order to lower austenite–ferrite transformation temperature for fine carbides and to retardgenerating of pearlite and large cementites, respectively. Tensile strength of hot-rolled sheet steel increasedwith titanium content and it was achieved to 800 MPa in a 0.09% Ti steel. Microstructure of the 0.09%Tisteel was ferrite without pearlite and large cementites. Fine carbides of 3 nm in diameter were observed inrows in the ferrite matrix of the 0.09% Ti steel with transmission electron microscope. The characteristicarrangement of the nanometer-sized carbides indicates that the carbides were formed at austenite–ferriteinterfaces during transformation. By energy dispersive X-ray spectroscopy, the carbides were found to con-tain molybdenum in the same atomic concentration as titanium. Crystal structure of the nanometer-sizedcarbides was determined to be NaCl-type by X-ray diffractometry. The calculated amount of precipitation-strengthening by the carbides was approximately 300 MPa. This is two or three times higher than that ofconventional Ti-bearing high strength hot-rolled sheet steels.

Based on the results obtained in the laboratory investigation, mill trial was carried out. The developed hot-rolled high strength sheet steel exhibited excellent stretch flange formability.

KEY WORDS: precipitation-strengthening; ferritic steel; nanometer-sized carbide; hot-rolled high strengthsheet steel; stretch flange formability.

pearlite and large cementites at grain boundaries. Titaniumcontent was varied in steels from A to D to investigate theinfluence of titanium content on tensile strength.8) Nitrogencontent was reduced because nitrogen exhausts titaniumthrough forming large TiN.9) Furthermore, titanium andmolybdenum contents were varied in steels from E to I toinvestigate the influence of Ti/Mo atomic concentrationratio on tensile strength under a condition that the sum ofatomic concentrations of titanium and molybdenum wasequal to that of Steel C.

These steels were vacuum-smelted and cast to 50 kgingot. The ingots were rolled to 30 mm thick bars aftersoaking at 1 473 K. Laboratory slabs in 120 mm length and100 mm width were cut off from the rolled bars. The slabsreheated at 1 523 K for 3.6 ks were hot-rolled to 4.5 mmthick by 7 passes. Finishing temperature was controlled at1 173 K. After finishing rolling, they were air-cooled to893 K (approximately 10 K/s) and held at 893 K for 3.6 ksfollowed by furnace-cooling to room temperature for hotcoiling simulation. After reducing the thickness to 2.3 mmby grinding to remove scale defects, tensile test was carriedout in the longitudinal direction using JIS No.13B typespecimen (GW; 12.5 mm, GL; 50 mm). Cross head speed inthe tensile test was kept at 1.67�10�1 mm/s.

Cylindrical specimens of 12 mm height and 8 mm diame-ter were prepared from the rolled bar and dilatometric ex-periment was carried out. Figure 1 shows a schematic dia-gram of the dilatometric experiment. The specimens wereaustenized at 1 523 K for 300 s to resolve carbides and thendeformed three times with a strain of 0.3 and a strain rate of20/s. Cooling rate was fixed at 10 K/s in the measurementof austenite–ferrite transformation temperature.

Microstructure of the specimens in a cross section alongrolling direction was observed with a scanning electron mi-croscope (SEM). Thin foils were prepared from the speci-mens for the observation of fine carbides with a transmis-sion electron microscope (TEM). Atomic concentrations ofmetallic elements in carbides were determined by energy

dispersive X-ray spectrometry (EDS).All precipitates were extracted electrolytically using

10% acetyle aceton–tetramethyl ammonium chloride–methanol (10%AA) and filtered with a mesh of 0.2 mm f bychemical extraction method. The precipitates were dis-persed on a micro grid coated by organic film for TEM ob-servation. Crystal structure of the precipitates was deter-mined by X-ray diffractometry. Quantitative analysis of theprecipitates was also carried out by inductively coupledplasma spectrometry.

3. Results

3.1. Effect of Titanium Content on Mechanical Prop-erties

Figure 2 shows the influence of titanium content on me-chanical properties. The yield strength (YS; MPa) and ten-sile strength (TS; MPa) increased with titanium content sig-nificantly. The tensile strength successfully achieved to800 MPa with 0.09% titanium addition and saturated in ti-

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Table 1. Chemical composition of steels investigated (mass%).

Fig. 1. Schematic diagram of thermomechanical treatment indilatometric test.

Fig. 2. Influence of titanium content on mechanical properties ofhot-rolled sheet steel; (a) elongation, (b) tensile strengthand (c) yield strength.

tanium contents more than 0.09%. The elongation (EL; %)decreased with titanium content.

Figure 3 shows a scanning electron micrograph of across section of Steel C containing 0.09% titanium. Whilepearlite and large cementites were not observed, only ferritegrains can be seen. The SEM observation and saturation oftensile strength with titanium addition more than 0.09%suggest that almost all carbon in Steel C formed carbidescontaining titanium.

Figure 4 shows a transmission electron micrograph andan EDS spectrum of carbides in Steel C. Carbides of 3 nmin diameter can be seen in rows. Titanium, molybdenumand carbon were detected in the fine carbides except ironand manganese included in matrix. The Ti/Mo atomic con-centration ratio measured by quantitative analysis with theEDS spectrum was 1.03. Steel C is found to be strength-ened by the nanometer-sized carbides containing titanium

and molybdenum.

3.2. Effect of Titanium/Molybdenum Atomic Concen-tration Ratio on Mechanical Properties

To obtain 800 MPa in tensile strength effectively, opti-mum titanium and molybdenum contents were investigatedwith Steel C and Steels from E to I shown in Table 1. Steelsfrom E to I had various titanium and molybdenum contents.The sum of titanium and molybdenum atomic concentra-tions in these steels was the same as that of Steel C. Figure5 shows the influence of Ti/Mo atomic concentration ratioon mechanical properties. The yield strength and tensilestrength increased dramatically with Ti/Mo atomic concen-tration ratio and exhibited the maximum with the Ti/Moatomic concentration ratio of 1 at Steel C. Beyond the peak,tensile strength slightly decreased with the increase inTi/Mo atomic concentration ratio and then approached tothe tensile strength of Steel E containing only titaniumwithout molybdenum. The elongation decreased monoto-nously with the increase in Ti/Mo atomic concentrationratio and then approached to the elongation of Steel E.Consequently, the chemical composition of Steel C is thebest for high tensile strength.

4. Discussion

4.1. Nanometer-sized Carbides in Rows

Rows of carbides in hot-rolled high strength sheet steelare characteristic morphology. In 1963, Morrison10) report-ed that NbC in lows had been found in Nb-bearing hot-rolled high strength sheet steel. Furthermore, carbides inrows were observed in several steels; for example, in Ti-

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Fig. 3. Scanning electron micrograph showing microstructure ofSteel C (0.04%C–1.5%Mn–0.2%Mo–0.09%Ti).

Fig. 4. Transmission electron micrograph showing fine carbidesin Steel C; (a) bright field image and (b) EDS spectrumof fine carbides.

Fig. 5. Influence of Ti/Mo atomic concentration ratio on me-chanical properties of hot-rolled sheet steel; (a) elonga-tion, (b) tensile strength and (c) yield strength.

bearing steel,11) V-bearing steel12) and Mo-bearing steel.13)

Gray et al.14) and Davenport12) explained the rows of car-bides in terms of precipitation at austenite–ferrite interfacesduring transformation.

It has been demonstrated that distance among rows be-comes shorter with lowering austenite–ferrite transforma-tion temperature.14) A distance among rows in Steel C isconstant at approximately 15 nm in the transmission elec-tron micrograph shown in Fig. 4. The observation suggeststhat the austenite–ferrite transformation occurred at a con-stant temperature after rolling in the hot-rolling process.The Ar3 transformation temperature of Steel C was deter-mined by the dilatometric experiment to be 893 K and it isthe same as the coiling temperature. This indicates that theinterface precipitation of the nanometer-sized carbides inSteel C takes place at the coiling temperature.

4.2. Characterization of Nanometer-sized Carbides

Figure 6 shows transmission electron micrographs andEDS spectra of chemically extracted precipitates of Steel C.In Fig. 6(a), fine precipitates on the micro grid can be seenwith a large precipitate. The large precipitate will be TiNsince only titanium was detected as shown in Fig. 6(c).Figures 6(b) and 6(d) show a magnification of Fig. 6(a) and

EDS spectrum of the fine precipitates, respectively. The di-ameter of the fine precipitates was approximately 3 nm.Except carbon, oxygen and copper from the grid, titaniumand molybdenum was detected as shown in Fig. 6(d). Thefine precipitate was the nanometer-sized carbide in Fig. 4.

Figure 7 shows an X-ray diffraction spectrum of the pre-cipitates. Several peaks that should be from the nanometer-sized carbides can be seen with peaks of TiN. Each peak ofthe nanometer-sized carbides located at lower angle side ofthat of TiN. This result indicates that a crystal structure ofthe nanometer-sized carbides is NaCl-type. Moreover, a lat-tice parameter of the nanometer-sized carbides was0.433 nm which is similar to that of TiC (0.4327 nm).

Table 2 shows a result of quantitative analysis for thechemically extracted precipitates of Steel C. The amount ofcarbon as Fe3C was determined using the amount of Fe. Allthe nitrogen combined with titanium as TiN while no nitro-gen formed AlN. All the carbon in Steel C probably precip-itated as the nanometer-sized carbides containing titaniumand molybdenum since carbon as Fe3C was not detected.Moreover, a Ti/Mo atomic concentration ratio in thenanometer-sized carbides was 1.16. The ratio is in goodagreement with the ratio obtained with the EDS spectrumin Fig. 4(b). This result confirms that the nanometer-sizedcarbides in Steel C contain molybdenum as much as titani-um in atomic concentration.

Consequently, the nanometer-sized carbides is deter-mined to be (Ti,Mo)C with the lattice parameter of0.433 nm. The fact that (Ti,Mo)C precipitates directly inferritic steel have not been reported yet since (Ti,Mo)C hasbeen observed only in a quench-tempered 0.5%Mo–Ti–Bsteel.15,16)

4.3. Amount of Precipitation-strengthening

An amount of precipitation-strengthening by thenanometer-sized carbides in Steel C can be calculated bysubtracting strength of pure iron, solid-solution strength,grain-refinement strength from the yield strength (737 MPa)shown in Fig. 2. Yield strength of ferritic steel without pre-cipitation-strengthening can be given by the followingequation17):

YS (MPa)�53.9�32.34 [%Mn]�83.16 [%Si]

�354.2 [%N]�17.402 d�1/2................(1)

Here, [%Mn], [%Si] and [%N] mean mass% of man-ganese, silicon and nitrogen in solution, respectively, d(mm) is a ferrite grain diameter. In this study, [%N] is zerosince all the nitrogen made the large TiN. [%Si] and [%Mn]are 0.18 and 1.62, respectively. An ASTM nominal graindiameter d measured by image analysis with the scanningelectron micrograph shown in Fig. 3 was 3.1 mm.

Figure 8 shows the calculation result. The obtainedamount of precipitation-strengthening was approximately300 MPa. This is two or four times higher than that of hot-rolled sheet steels reported in the past.18)

Ashby–Orowan mechanism is well known as a relation-

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Table 2. Results of quantitative analysis for precipitates in Steel C.

Fig. 6. Transmission electron micrographs showing chemical ex-tracted precipitates of Steel C; (a) bright field image, (b)magnification of fine precipitates, (c) EDS spectrum of alarge precipitate, (d) EDS spectrum of fine precipitates.

Fig. 7. X-ray diffraction spectrum of chemically extracted pre-cipitates in Steel C.

ship between diameter of precipitates and amount of pre-cipitation-strengthening.8) But the amount of precipitation-strengthening of Steel C can not be determined correctly byAshby–Orowan mechanism since Ashby–Orowan mecha-nism assumes random dispersion of precipitates on a slipplane. However, calculation of the amount of precipitation-strengthening according to Ashby–Orowan mechanism willbe important. All the carbon in Steel C can be assumed toform the nanometer-sized carbides since no cementite wasgenerated in Steel C as shown in Table 2. Next, a volumefraction f (%) of the nanometer-sized carbides is determinedto be 4.35�10�3 with the lattice parameter of (Ti,Mo)C(0.433 nm), atomic weight of titanium (48), that of molyb-denum (96), that of carbon (12) and carbon content in SteelC (0.043%). An amount of precipitation-strengthening Ds(MPa) is given by the following equation6):

..........................(2)

Here, K is a constant (5.9 N/m), b is Burger’s vector(0.246 nm), and d is a diameter of precipitates. Figure 9shows a relationship between d and Ds . When the diameterof the carbides was 3 nm, the amount of precipitation-strengthening was determined to be approximately300 MPa. This is in good agreement with the amount ofprecipitation-strengthening in Fig. 8.

5. Mill Trial

Mill trial was carried out for Steel X in Table 3. A finish-ing temperature and coiling temperature were 1 173 K and923 K, respectively. A width and thickness of the hot-rolledsheet steel were 700 mm and 3.2 mm, respectively. Afteracid-pickling, tensile test was carried out in the transversedirection using JIS No. 5 specimen (GW; 25mm, GL;50mm) with a cross head speed of 1.67�10�1 mm/s. Hole-expanding test for evaluation of stretch flange formabilityobeyed JFST1001 of Japan Iron and Steel FederationStandard. Figure 10 shows the schematic diagram of hole-expanding test. A hole of 10 mm in diameter is punched outand then the hole is stretched by a corn punch with a top

angle of 60°. When a crack through the thickness is gener-ated, a diameter of expanded hole d (mm) is measured.Hole-expanding ratio (l ; %) as an index of stretch flangeformability is calculated by the following equation:

l�(d�10)/10�100...........................(3)

Figure 11 shows a scanning electron micrograph of Steel

∆σ �K

df

d

b1 2/ ln

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Table 3. Chemical composition of mill trialed steel (mass%).

Fig. 8. Amount of solid-solution strengthening, grain-refinementstrengthening and precipitation-strengthening in Steel C.

Fig. 9. Relationship between carbide diameter and amount ofprecipitation-strengthening.

Fig. 10. Schematic illustration of hole-expanding test.

Fig. 11. Scanning electron micrograph showing microstructureof Steel X.

X. Only fine-grained ferrite can be seen while large cemen-tites and the second phase such as martensite were not ob-served.

Figure 12 shows transmission electron micrographs ofcarbides in Steel X and a conventional 780 MPa grade pre-cipitation-strengthened hot-rolled sheet steel (0.12%C–0.4%Si–1.8%Mn–0.08%Ti–0.04%Nb) with EDS spectra ofcarbides. Very fine carbides in rows were observed in SteelX. The diameter of the fine carbides was 3 nm. A Ti/Moatomic concentration ratio of the fine carbides was approxi-mately 1. On the other hand, large carbides over 30 nm indiameter were observed in the conventional sheet steel. Thelarge carbides contained titanium and niobium.

Table 4 shows mechanical properties of Steel X.Forming limit strain19) means the sum of true strains alongthickness and width at a fracture point of JIS No.5 tensilespecimen. The tensile strength more than 800 MPa was at-tained with excellent hole-expanding ratio.

Figure 13 shows hole-expanding ratios and elongationsof several 780 MPa grade hot-rolled high strength sheetsteels; Steel X, ferritic–martensitic steel (F+M),20,21) ferrit-ic–bainitic–martensitic steel (F+B+M),20) ferritic–bainiticsteel (F+B) 22) and bainitic steel (B).22,23) The stretch flangeformability of Steel X is superior to those of the steels.

Nakagawa et al.24) have demonstrated that hole expand-ing ratio correlated closely with forming limit strain ex-pressing local elongation. Figure 14 shows the hole-ex-panding ratio and forming limit strain of Steel X plotted onthe correlation reported by Nakagawa et al. The plot of

Steel X on the line indicates that the excellent stretch flangeformability of Steel X is attributed to the excellent localelongation.

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Fig. 12. Transmission electron micrographs showing carbides in Steel X and conventional precipitation-strengthened sheet steel with EDS spectra;(a) bright field image of Steel X; (b) bright field image of conventional steel; (c) EDX spectrum of carbides in Steel X; (d) EDS spectrumof carbides in conventional steel.

Fig. 13. Comparison of hole-expanding ratio and elongation ofdeveloped steel with those of conventional hot-rolledhigh strength sheet steels.

Fig. 14. Correlation between forming limit strain and hole-ex-panding ratio of Steel X compared with that of conven-tional steels reported by Nakagawa et al.

Table 4. Mechanical properties of mill trialed steel.

6. Conclusion

A new hot-rolled high strength sheet steel consisting offerrite matrix and nanometer-sized carbides with excellentstretch flange formability was successfully developed. Thefollowing results are obtained.

(1) The newly developed hot-rolled high strength sheetsteel in tensile strength of 780 MPa grade can be producedwhen 0.04%C–1.5%Mn–0.09%Ti–0.2%Mo steel is coiledat 923 K in the hot-rolling process.

(2) The maximum tensile strength is attained when tita-nium is added as much as molybdenum in atomic concen-tration.

(3) Fine carbides of 3 nm in diameter precipitate in rowsin the developed sheet steel. The nanometer-sized carbideshas NaCl-type structure with a lattice parameter of0.433 nm.

(4) An amount of precipitation-strengthening by thenanometer-sized carbides is approximately 300 MPa. Thisis two or four times higher than that of conventional precip-itation-strengthened hot-rolled high strength sheet steels.

(5) Excellent stretch flange formability of the developedsteel is attributed to excellent local elongation.

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