Physical Metallurgy of Modern High Strength Steel Sheets

1. Introduction

One of the important tasks for the 21st century is themaintaining of sound ecology. The reduction of the burdenon the environment is an inevitable task assigned to indus-try. Recently, Japan internationally announced that it wouldreduce the generation of CO2 by 6% during the 5 yearsfrom 2008 to 2012 in comparison with the level in 1990. Tosatisfy this declaration, the Japanese automotive industrygenerating around 17% of total CO2 is now aiming at animprovement of fuel efficiency by 22.8% in 2010 comparedwith the level in 1995. A direct contribution of the steel in-dustry to the reduction of fuel consumption is the supply ofsteels enabling the lightening of automotive weight.

Figure 1 shows a relationship between the fuel mileageand automotive weight.1) The lightening of the weight di-rectly contributes to the improvement of the fuel consump-tion of cars. While light materials such as Al, Mg and plas-tics are applied to automotive parts to reduce weight, theapplication of high strength steel sheets is also promotedbecause of their advantage that in most cases, the conven-tional forming technology is applicable without great in-vestment.

Figure 2 shows the strength levels of high strength steelsheets already applied for automotive parts and expected tobe applied in the future. To pursue the ultimate lighteningof white body, the ULSAB-project was organized in 1994by 35 steel companies from 18 countries. This project suc-ceeded in a weight reduction of 25% using a great deal ofhigh strength steel sheets and sophisticated forming andwelding technologies such as hydroforming, tailored blank-ing, laser welding, etc.2) Such projects are being carried outfor closure parts and suspension parts by ULSUC- andULSAS-projects, respectively. It was reported that theseprojects achieved the same scale of weight reduction as theULSAB-project had achieved or succeeded in a significantdecrease in the production cost.3)

In these projects, the lightening of cars is mainly

achieved by optimizing the body design and by using newproduction technology, and the steels used are already com-mercially available. For further success in car lightening,development of more sophisticated high strength steelsheets is demanded.

In Fig. 3, the factors hindering the broad application ofhigh strength steel sheets to automotive parts are summa-rized. The first obstacle is an economical problem. The pro-duction cost of high strength steel sheets is relatively highbecause of the addition of valuable alloying elements forimproving their mechanical properties, and low yield andlow productivity. To reduce the amount of alloying ele-ments, the thermo-mechanical treatment (TMCP) has beenpositively applied. Recently, a new attempt has been madeto increase the strength of plain carbon steels by grain re-finement up to twice the original strength. The developmentof steel sheet with ultra-fine microstructure is mentioned inSec. 3.8.

Because the mechanical properties of high strength steelsheets are strongly influenced by production conditions inhot rolling and subsequent cooling, the fluctuation of theirmechanical properties is relatively large. To reduce the fluc-tuation, the line speed is often reduced and as a conse-quence, productivity decreases. In some cases, the yield of

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Review

Physical Metallurgy of Modern High Strength Steel Sheets

Takehide SENUMA

Yawata R&D Laboratory, Technical Development Bureau, Nippon Steel Corporation, Tobihata, Tobata-ku, Kitakyushu-shi, 804-8501 Japan. E-mail: [email protected]

(Received on September 8, 2000; accepted in final form on November 17, 2000 )

Lightening of automobile bodies is required from the viewpoint of saving energy which contributes toameliorating an ecological problem. A useful means of doing this is the application of high strength steelsheets to automobile bodies. The inferior formability of high strength steel sheets in comparison with thatof mild steel sheets, however, hinders their broad application. But in recent years, many high strength steelsheets with good formability have been developed using sophisticated physical metallurgy.

In this paper, the recent development of modern high strength steel sheets is reviewed paying special at-tention to their physical metallurgy which realized the improvement of their formability.

KEY WORDS: high strength steel; formability; BH; TRIP; DP; precipitation hardening; crash worthiness; fatigue resistance; delayed fracture.

Fig. 1. Relationship between the fuel mileage and automotiveweight.1)

product suffers remarkably. For example, overcooling ofTRIP steel sheets causes the martensitic transformation lo-cally and significantly deteriorates the homogeneity of me-chanical properties.

Another obstacle concerns their formability. In general,high strength steel sheets have poor formability and oftenhave a narrow or no formable range in press forming forcomplicated forms. Even though steel sheets are formedwithout fracture or wrinkles, this is not enough for practicaluse. Steel sheets are only applicable if the required profileis obtained within certain tolerance by press forming. Theprofile distortion caused by poor shape fixability such assurface deflection, spring back or distortion hinders the useof high strength steel sheets.

Weldability problems also hinder the wider use of highstrength steel sheets. The main problem is the softening ofthe heat affected zone of welded parts. The softened partbecomes a weak point of the material concerning fracturestrength and fatigue resistance. Another weldability prob-lem is the formation of blow holes in hot dip Zn coatedsteel sheets by arc welding often carried out for parts usinghigh strength steel sheets. The blow holes reduce thestrength and fatigue resistance.

Concerning fatigue resistance, it must be noted that withhigher strength, the fatigue resistance does not increaseproportionally to tensile strength and tends to saturate. Thismeans that it is no use applying high strength steel sheetsfor increasing fatigue resistance if the strength level ex-ceeds more than about 1 000 MPa.

To overcome these problems, much effort has been made.In this paper, the recent development of high strength steelsheets and their physical metallurgy are introduced and fu-ture research subjects are discussed.

2. Required Formability of High Strength Steel Sheets

For press forming of automotive bodies and parts, stretchformability, deep drawability, elongation-flangeability andbendability are most important. Because these propertiesare strongly influenced by microstructure and texture, theircontrol is of great importance.

For high strength steel sheets with a tensile strength levelof less than 450 MPa mainly used for outer and inner pan-

els, high stretch formability and good deep drawability aregenerally key requirements. On the other hand, good elon-gation-flangeability is often required for high strength steelsheets with a strength level between 450 and 1 000 MPamainly employed for members, frames, wheels, etc. Forultra high strength steel sheets with a strength level of morethan 1 000 MPa, the bendability is an important property.

Another important factor for formability is the planaranisotropy of mechanical properties such as elongation andr-value. Because fracture occurs in the weakest direction,the minimization of planar anisotropy is of practical inter-est.

As summarized, the development of highly formablehigh strength steel sheets aims at a steel sheet with requiredstrength and formability by controlling the microstructureand texture using sophisticated physical metallurgy.

3. Modern High Strength Steel Sheets

Figure 4 shows the strength–ductility balance and thestrength and hole-expansion ratio balance of various highstrength steel sheets.4) The formability of steel is remark-ably influenced by the strengthening mechanism and es-pecially by the resultant microstructure. Therefore, mi-crostructural control is important for obtaining the requiredformability. In the following, high strength steel sheetswhose microstructure is controlled in a sophisticated man-ner to obtain high formability are reviewed in detail.

3.1. Bake Hardenable (BH) Steel Sheets

BH steel sheets are ideal steel sheets for outer panels ofcars because they provide high formability, high surface de-flection resistance and high dent resistance simultaneously.The features of BH steel sheet are that it is as highlyformable in press forming as mild steel and is subsequentlyhardened by baking treatment. The baking treatment is usu-ally simulated by a heat treatment at 170°C for 20 min. ABH steel sheet is usually understood to be a steel sheetwhose yield strength is increased by a minimum value of30 MPa by the baking treatment. This increase in strength isbased on fixing the dislocations by interstitial atoms such ascarbon and nitrogen in solution.

Figure 5 shows the relationships between the BH and the

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Fig. 2. Strength levels of high strength steel sheets already applied forautomotive parts and expected to be applied in the future.

Fig. 3. Factors hindering the broad application of highstrength steel sheets to automotive parts.

amount of carbon in solution.5) With increasing amount ofcarbon in solution, the bake-hardenability increases. Theupper limit of BH is given by the occurrence of the stretch-er-strain phenomena due to yield elongation which deterio-rates the surface quality of the steel sheet. This limit isaround 60 MPa for conventional BH steel sheets.

Kinoshita and Nishimoto6) showed that BH does not de-pend only on carbon in solution but also on carbon segre-gated in the grain boundary. The bake hardening is a strainaging phenomenon which occurs at temperature higher thanthat at which the natural aging phenomenon occurs. The Csegregated at the grain boundary effectively resolves at ahigher temperature and contributes to the bake hardening.Figure 6 shows the relationship between aging deteriora-tion and BH for steel sheets with different grain sizes.6) Thegrain refinement and the decrease in the amount of ele-ments segregated at the grain boundary competitively withC such as P are reported to raise the limit of BH at whichthe stretcher-strain phenomena does not occur.

BH treatment at a lower temperature is demanded inorder to save energy in production. Tsukatani et al.7) inves-tigated the influence of Mn and Si content and Fe3C density

on BH using a 130°C and 170°C treatment. They recog-nized that BH at 130°C reveals two groups as seen in Fig.7. One group shows that BH is higher than AI while theother shows that BH is less than AI. The former is charac-terized by a high density of Fe3C of more than 53104/mm2

and Si content of less than 0.2%. Si influences the distribu-tion of Fe3C. The increase in the density of Fe3C increasesdislocation density formed by a pre-strain before the BHtreatment. The two phenomena realize a relatively high BHeven though the amount of C in solution is low.

In the present situation, steelmakers are compelled toguarantee that the stretcher–strain phenomena do not occurat a press forming carried out up to three months after pro-duction. Therefore, the maximum BH is limited in practice.The upper limit of BH can, however, be increased if agingis avoided by quick use of the steel sheet having a highamount of carbon or nitrogen in solution after skinpassrolling which introduces mobile dislocations and suppress-es yield elongation. For effective application of BH steelsheets, cooperation between steelmakers and automobilemanufacturers enabling a strict time schedule from produc-tion to press forming is necessary.

There are mainly three methods for producing BH steelsheets. ①Low carbon steel sheets with a limited amount ofcarbon content are annealed in a batch annealing furnace.②Low carbon steel sheets are annealed in a continuous an-nealing line. ③IF steel sheets are continuously annealed.

In batch annealing processes, steel sheets with a limitedamount of carbon fulfill the BH property. If the carbon con-tent is too low, the carbon remaining in solution is notenough to obtain the required minimum BH value. If thecarbon content is too high, the precipitation of carbide is

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Fig. 4. Strength–ductility balance and the strengthand hole-expansion ratio balance of varioushigh strength steel sheets.4)

Fig. 5. Relationships between the bake-harden-ability and the amount of carbon in solu-tion.5)

Fig. 6. Relationship between aging deteriora-tion and BH for steels with differentgrain size6) (coarse grain: 29 mm, finegrain: 9 mm).

Fig. 7. Relationship between aging indexand 170 and 130°C bake-hardenabil-ity.7)

accelerated and the carbon remaining in solution is also toolow to obtain the required minimum BH value. A properamount of carbon is from approximately 0.005% to approx-imately 0.02%, depending on annealing conditions.

In the case ②, a proper amount of carbon is from ap-proximately 0.015% to approximately 0.06% depending onannealing conditions. In a recent development, a continuousannealing consisting of high temperature annealing of ap-proximately 750–800°C, overcooling down to approximate-ly 250–300°C, reheating up to 350–450°C and temperaturedecreasing overaging has been performed using steel sheetswith approximately 0.015% carbon to provide both goodformability and bake hardenability.8)

Most BH steel sheets used at the present time are IF steelsheets because of their excellent formability. There are twotypes of IF-BH steel sheets; the excess C type and the ex-cess Ti and/or Nb type. In the former type, the amount of Tiand/or Nb to be added is reduced so that C in solution re-mains after the equilibrium formation of carbide with Tiand/or Nb. This method is advantageously characterized inthat bake hardenability can be obtained without high-tem-perature annealing, which is apt to cause heat buckling andrequires large energy consumption. However, as C in solu-tion exists during cold rolling, the formation of texture de-sirable for deep drawability is suppressed, resulting in alower r-value compared with conventional IF steel sheets.

On the other hand, excess Ti and/or Nb type BH steelsheets exhibit excellent deep drawability as C in solutiondoes not exist during cold rolling, but require high anneal-ing temperature. For this type of BH steel sheet, the soluteC required for bake hardenability must be secured by re-solving the carbide by high temperature annealing with sub-sequent rapid cooling. This phenomenon is quantitativelyshown in Fig. 8 in the case of an Nb-IF steel with Nb/C51.58. It will be noted that bake hardenability is enhanced asthe annealing temperature and cooling rate are increased.

As BH depends on the amount of the solute C, the fluctu-ation of the C content caused by the steelmaking processand by picking up in the succeeding processes affects thestability of BH. Figure 9 shows the solubility of NbC atvarious temperatures. To obtain the same amount of BH,the excess Nb type requires a higher annealing temperaturethan the excess C type. The fluctuation of solute C of theexcess Nb type at the annealing temperature is smaller thanthat of the excess C type even though the fluctuation of Ccontent in the steels is the same. Consequently, the fluctua-tion of BH of the excess Nb type is smaller than that of theexcess C type. This means that the excess Ti and/or Nb typeis preferable from the standpoint of BH stabilization.

Kawasaki et al.9) revealed that bake hardenability of Ti-IF steel is significantly influenced by S and Mn content. Ifthe amount of S is high, the main precipitate with Ti isTi4S2C2. On the other hand, if the S content is low, TiC isthe main precipitate with Ti. The latter dissolves at lowertemperature than the former and as a consequence, Ti-IFwith a lower S content has a higher amount of solute C atthe time of annealing and shows a higher BH. Even if the Scontent is high, bake hardenability can be maintained if thenecessary amount of Mn is added to form MnS, which hin-ders the precipitation of Ti4S2C2.

To obtain stable BH while maintaining relatively high r-

value, Yoshinaga et al.10) proposed continuous annealing inthe austenite region using Mn added Nb–Ti-IF steel sheets.Figure 10 shows the effect of the annealing temperature onBH and YP-El of Nb–Ti-IF and 1.5%Mn–Nb–Ti-IF steelsheets. The numerals in the figure mean the transformationratio. The specimens heated up to the austenite region havea high BH value because the solubility of C in austenite islarge. If transformation occurs, YP-El of 1.5%Mn–Nb–Ti-IF steel is suppressed while that of Nb–Ti-IF steel appears.The fact that YP-El of high-Mn–Nb–Ti-IF steel becomeszero suggests that the mobile dislocations which cannot befixed by solute C after aging remain because transformationfrom austenite to ferrite at a low temperature producedmany mobile dislocations.

The r-value at each annealing temperature is shown inFig. 11. In the case of Nb–Ti-IF steel sheets, r-values dras-tically decrease if the transformation from ferrite to austen-ite completely occurs. In the case of 1.5%Mn–Nb–Ti-IFsteel, r-values are hardly influenced by the occurrence oftransformation because the texture after annealing is similarregardless of the occurrence of transformation. The mecha-nism of the texture memory that the texture does notchange in high-Mn–Nb–Ti-IF steel sheets even though theyunderwent transformation may be due to the effect of resid-ual transformation stresses.10) Showing that the texturememory is hardly weakened by prolonged annealing wheregrain growth occurred and residual transformation stressesprobably were decreased, Hutchinson et al.11) suspected theresidual stress hypothesis.

Although this technology has the advantage of obtaininghigh BH without YP-El, the r-value of this steel is lowerthan that of conventional IF steel such as Nb–Ti-IF in Fig.11 because the high amount of Mn lowers the r-value.

Kimura et al.12) proposed a new technology by whichbake hardenability is imparted to Ti-IF steel keeping a highr-value by carbonizing at the time of continuous annealing.Figure 12 shows the influence of carbonizing time at850°C on the amount of C added and on the BH of IF steelsheets with a Ti content of 0.021% and 0.031%, respective-ly. It is clearly seen that the BH increases with the increas-ing amount of C added by carbonizing but the increase inthe BH is moderate if the amount of Ti content is high be-cause C then becomes carbide and cannot remain in solu-tion. If the carbonizing is carried out after completion of re-crystallization, the r-value is not lowered.

3.2. Surface Hardened Dent Resistant Steel Sheets

Another technology has been introduced to improve thedent resistance without deteriorating the r-value. The keytechnology is the nitriding or carbo-nitriding of Ti bearingIF steel at the time of continuous annealing in NH3 contain-ing atmosphere.13) The difference of this technology fromthat of carbonizing lies in the formation of a significantlyhardened surface layer. The hardening of the surface layeroccurs by the formation of clusters or fine precipitates con-sisting of titanium and nitrogen.

Figure 13 shows the change in hardness in the thicknessdirection with increasing nitriding time. The concentrationof ammonium is 4%, the nitriding temperature is 750°Cand the nitriding time is varied 0, 20, 40, 60 sec. It is clearlyseen that the hardened layer quite rapidly forms and extends

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in the thickness direction.This hybrid structure provides high dent resistance be-

cause the hardened surface layer increases the bending re-sistance and consequently the dent resistance. Figure 14shows the relationship between the dent resistance andyield strength of nitrided Ti bearing IF steel sheets com-pared with conventional IF steels. The nitriding temperatureis 750°C, the nitriding time is varied 20, 40, 60 sec, theconcentration of ammonium is varied 1, 2, 4, 10, 20%, andthe Ti content is 0.046%. The steel sheets marked 28 and38 K are conventional IF steel sheets with TS of 280 and380 MPa, respectively. Although the BH of the nitrided IFsteel sheets was only around 25 MPa, the nitrided IF steelsheets show significantly higher dent resistance than con-ventional IF steel sheets with the same YP. So, this technol-ogy offers high dent resistance without worsening the sur-face deflection which is mostly controlled by YP.

The diffusion and precipitation behaviors of nitridedsteel sheets were modeled and the mechanism of the precip-itation hardening of TiN is discussed elsewhere.14)

3.3. DP (Dual Phase) Steel Sheets

The microstructure of dual phase steel sheets consists ofa ferrite matrix with dispersed martensite. The DP steelsheets are made ductile by the ferrite and their strength iscontrolled by the amount of martensite therein. The proper-ties of the steel can be controlled in accordance with theamount of martensite as seen in Fig. 15.15) Besides a goodstrength and elongation balance, DP steel sheets are charac-

terized by a markedly low yield ratio and by the absence ofYP-El. Although YP-El does not appear, we obtain highBH up to 100 MPa. It means that a remarkably high BH isachievable without the occurrence of the stretcher strainphenomenon. These phenomena can be explained by theexistence of mobile dislocations around dispersed marten-

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Fig. 8. Influence of annealing temperature and rate ofsubsequent cooling on BH (annealing time is30 sec).

Fig. 9. Influence of C fluctuation of C excess type steeland Nb excess type steel on the BH stability.

Fig. 10. Influence of the annealing tempera-ture on BH and YP-El of Nb–Ti-IFand 1.5%Mn–Nb–Ti-IF steel sheets.Numbers of marks are volume frac-tion of austenite during annealing.10)

Fig. 11. Influence of annealing temperatureon r-value of Nb–Ti-IF and 1.5%Mn–Nb–Ti-IF steel sheets. Numbers ofmarks are volume fraction of austen-ite during annealing.10)

Fig. 13. Influence of nitriding time on the change in hardnessdistribution in the vicinity of surface in thickness direc-tion.

Fig. 12. Influence of carbonizing time on BHand amount of added carbon (SteelT1: 0.021%Ti, Steel T2: 0.031%Ti,carbonizing temperature: 850°C).12)

site formed by martensitic transformation.16)

DP steel sheets are produced either by hot rolling or coldrolling followed by continuous annealing. To obtain goodformability, the ferrite matrix should be soft, which is real-ized by g→a transformation of the steel sheets at a hightemperature. Afterwards, the steel is rapidly cooled belowMs temperature to suppress the formation of pearlite andbainite.

It has been recently reported17) that DP steel with finelydispersed martensite improved the crashworthiness of cars.Figure 16 shows the influence of a ferrite-martensiteperimeter on dynamic absorbed energy by a deformation ata strain rate of 23103 sec21. The ferrite–martensite perime-ter is defined as a length of the ferrite–martensite interfacein the unit area of 1 mm2. With the lengthening of the fer-rite–martensite perimeter, dynamic absorbed energy in-creases. This means that the dynamic absorbed energy in-creases if the martensite is finely dispersed. In deformationat a low strain rate of 231022 sec21, this dependence can-not be recognized, that is, the amount of the deformationenergy hardly depends on the dispersed state of martensite.

DP steel sheets contain a high amount of C and/or N insolution due to rapid cooling to a low temperature ofaround 150°C. BH treatment of DP steels, therefore, in-creases the strength of the steel significantly and thus sub-

stantially improves the crashworthiness of cars.Figure 17 shows a unique feature of DP steel sheets that

the BH increases upon increasing WH, which is uncommonfor other steels.18)

3.4. Residual Austenite Steel Sheets

Residual austenite steel sheets are also called TRIP(Transformation induced plasticity) steel sheets producedeither as hot bands or cold rolled products. TRIP steelsheets are most formable steel sheets in the strength levelhigher than 600 MPa because of TRIP phenomena. For theTRIP phenomena, quasi-stable austenite is needed at roomtemperature. The quasi-stable austenite transforms intomartensite by forming and the transformed part is strength-ened so that strain localization is avoided resulting in theimprovement of ductility.

Figure 18 shows the concept for obtaining quasi-stable

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Fig. 14. Influence of YP increase due to nitriding on the dent re-sistance (28K, 38K: unnitrided, parameter is nitridingtime).

Fig. 15. Influence of volume fraction of martensite on the me-chanical properties of a dual phase steel sheet.15)

Fig. 16. Effect of ferrite–martensite perimeter on dynamic ab-sorbed energy.17)

Fig. 17. Bake hardening behavior as a function of predeforma-tion.18)

Fig. 18. Concept for obtaining quasi-stable austenite at roomtemperature in the case of cold rolled product.19)

austenite at room temperature in the case of cold rolledproduct.19) In the inter-critical annealing, C in austenite isenriched according to the equilibrium condition and duringthe progress of ferrite transformation, C concentration inaustenite increases. If pearlite transformation is suppressedby rapid cooling and the cementite precipitation during bai-nite transformation is avoided by adding Si and Al, C inaustenite is enriched up to the concentration for To wherethe free energy of ferrite is equal to that of austenite in con-sideration of strain energy caused by transformation.Because To depends on the chemical composition, the satu-rated C concentration in austenite after completion of bai-nite transformation also depends on the chemical composi-tion. For example, Mn decreases the C concentration of re-tained austenite.19) If the concentration of C in austenitereaches around 1.2%, the austenite becomes quasi-stable atroom temperature.

The processing of TRIP steel sheets is similar to that ofDP steels. A significant difference between the two steelsheets is the coiling temperature for hot rolled products orover-aging temperature for cold rolled products. The tem-perature is around 150°C for DP steels and around 400°Cfor TRIP steels where the bainite transformation proceeds.

Figure 19 shows the change in the properties of coldrolled TRIP steel sheets during over-aging in a continuousannealing process.20) It is recognized that the strength–duc-tility balance is more closely related to the concentration ofC in austenite than to the amount of retained austenite. Thisindicates that the plastic stability of retained austenite playsan important role regarding the good strength–ductility bal-ance of TRIP steel sheets. It has been proved that the straininduced transformation of austenite is retarded if the C con-tent in it is high and as a consequence, the strain inducedtransformation lasts in a high strain region, which con-tributes to diffusing the strain concentration, resulting in amore uniform elongation.21,22) A mathematical model evalu-ating the stability of retained austenite and describing thekinetics of strain induced martensite transformation hasbeen proposed and the increase in the stability of retainedaustenite upon C content being increased has been quantita-

tively evaluated.21) The decrease in the amount of retainedaustenite during deformation depends on the formingmode.23) The amount of retained austenite of a specimenwhich undergoes shrink flanging is higher than that of auni-axially or equi-biaxially deformed specimen after de-formation at the same equivalent strain. Hiwatashi et al.23)

proposed that the difference can be explained by consider-ing the hydrostatic pressure acting in the forming and con-cluded that the stability of the retained austenite is con-trolled by the equivalent strain and the hydrostatic pressure.

The better deep drawability of TRIP steel sheets deter-mined by cylindrical drawing test than the other highstrength steel sheets with the same r-value was explainedby the difference in TRIP phenomena based on the differentforming mode.24) The wall of a cylindrical drawing speci-men undergoes uni-axial strain forming while the flange is formed by shrink flanging mode. The flange is lessstrengthened by TRIP phenomena than the wall, resultingin the less drawing resistance of the flange and better deepdrawability.

Figure 20 shows the formable range of various steelsheets for the formation of a door model as an example ofthe excellent formability of TRIP steel.23) Steel A is a TRIPsteel, Steel P is a precipitation hardened steel, Steel D is aDP steel and Steel S is a solution hardened steel. The TRIPsteel shows the widest formable range among tested steelsheets. In this case, Steels P and D having the same strengthlevel as that of Steel A possess no formable range. Thegood strength–ductility balance of TRIP steel enables appli-cation of a large blank holding force without rupture. Thelarge blank holding force contributes to the stabilization ofthe required shape of the product.

Regarding materials used for the structural parts of anautomobile, the fatigue resistance thereof is one of the keyfactors for determining the thickness of the steel sheet.Figure 21 shows the fatigue strength of steels with variousmicrostructures.25) The parameter in the figure means themicro-constituents, F: ferrite, B: bainite, gR: retainedaustenite, M: martensite, P: pearlite. The TRIP steelmarked with h shows a higher fatigue strength than theother steel sheets with the same TS level. The fact that thevolume fraction of retained austenite decreases during thestress controlled fatigue test indicates that the localizedtransformation of retained austenite into martensite at a tipof a fatigue crack improves the fatigue resistance.25) The

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Fig. 19. Influence of over-aging time on the properties of coldrolled TRIP steel sheets.20)

Fig. 20. Formable range of various steel sheets for the formationof a door model as an example of the excellent forma-bility of TRIP steel.23)

reason is that the localized transformation increases the re-sistance to the crack growth due to significant work harden-ing occurring in front of the crack and due to the formationof compressive stress associated with the volume increasecaused by the transformation.

Another important property of high strength steel sheetsis a capacity for collision impact energy absorption. Goodhigh strength steel sheets possess both good formabilityand high capacity for impact energy absorption. Figure 22shows the relationship between the quasi-static flow stressat 5% strain as a representative value of formability and thecalculated value of absorbed energy until 10 msec after acollision.26) The TRIP steel sheet and the DP steel sheetpossess both good formability and a high capacity for im-pact energy absorption and the application of these steelsheets contributes to improve the crashworthiness of auto-mobiles. The reason why these steel sheets improve crash-worthiness lies in their high strain rate sensitivity, highwork hardenability and high bake hardenability.

3.5. High Strength Steel Sheets with a High HoleExpanding Ratio

To obtain a high hole expanding ratio, the formation of ahomogeneous microstructure is important. The hole form-ing is usually carried out by punching or piercing. Thequality of the punched surface in the hole depends on thepunching condition and microstructure of the steel sheet.The punched surface consists of sheared and brittly frac-tured parts. Because the brittly fractured part contains more

micro-cracks than the sheared part, the smaller the ratio ofbrittly fractured part/sheared part is, the higher the hole expanding ratio is. Micro-cracks are often observed in asofter micro-constituent in the vicinity of harder micro-constituents because the strain concentrates there. The exis-tence of the micro-cracks reduces the hole expanding ratio.Steels showing inhomogeneous microstructure whose micro-constituents have a quite large difference in hardness areferrite–pearlite steels, DP steels and TRIP steels. On theother hand, steels showing relatively homogeneous mi-crostructure are single phase ferrite steels, bainitic ferritesteels, bainite steels, etc. Figure 4 shows that the steels with homogeneous microstructure have a markedly higherhole expanding ratio than those with inhomogeneous mi-crostructure.

Besides microstructure, it is known that the addition ofSi and P influences the hole expanding ratio. Adding aproper amount of Si improves the hole expanding ratio. Siincreases the strength of ferrite and decreases the differencein strength between ferrite and second phase particles. Theaddition of Si promotes the formation of polygonal ferritewhich also contributes to improve the hole expanding ratio.On the other hand, P segregated at grain boundaries de-creases the hole expanding ratio.

There are several new developments of high strengthsteel sheets with a high hole expanding ratio. One develop-ment is based on the precipitation hardening of single fer-rite phase steel sheets. To significantly increase the strengthof steel sheets by precipitation hardening, a large amount offine precipitates is necessary. Figure 23 shows a process forproducing hot rolled high strength steel sheets with a highhole expanding ratio.27) Adding a relatively high amount ofTi of around 0.1 to 0.2% to a low carbon steel with a car-bon content of around 0.05 to 0.1%, the ferrite matrix isstrengthened by TiC precipitated in a considerable amountduring and just after ferrite transformation during slowcooling. Because the precipitates are small, strain localiza-tion hardly occurs in their vicinity and the tendency to formmicro-cracks is reduced.

As an application of this technology, a steel sheet withbainitic ferrite microstructure is strengthened by TiC pre-cipitated in an off-line heat treatment to improve thestrength–hole expanding ratio balance.28)

Instead of TiC, Cu precipitates are used to produce ahigh strength steel sheet with a high hole expanding ratio

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Fig. 21. Relationship between cyclic yield stress and fatiguelimit in several types of steel sheets.25)

Fig. 22. Relationship between the quasi-static flow stress at 5%strain as a representative value of formability and thecalculated value of absorbed energy until 10 msec aftera collision.26)

Fig. 23. Process for producing hot rolled high strength steelsheets with a high hole expanding ratio.27)

according to the same principle mentioned above. In thiscase, the precipitation of Cu is carried out in the coilingprocess for hot bands and in the over-aging process for coldrolled products. Figure 24 shows the influence of agingtime and temperature on the tensile strength of 1.6% Cucontaining IF steel sheets. It is recognized that a strengthincrease of around 200 MPa is realized by a relatively shorttime heat treatment. The precipitates strengthening the steelare bcc-Cu with a size of several nanometers. If the precipi-tates become large and change into stable fcc-Cu, the pre-cipitation strength drastically decreases.29)

Another development for improving the hole expandingratio of steel sheets with inhomogeneous microstructure isbased on the concept that the difference in strength betweenferrite and hard second phase such as martensite and re-tained austenite by strengthening the ferrite matrix withfinely dispersed precipitates. Figure 25 shows a process forimproving the hole expanding ratio of DP steel sheets.30)

The principle is the same as shown in Fig. 23. Adding aproper amount of Ti of around 0.1% to a conventional DPsteel, the ferrite matrix is strengthened by TiC precipitatedjust after ferrite transformation during slow cooling. Thedecrease in the difference between the strength of ferritematrix and that of the martensite reduces the local strainconcentration in ferrite and improves local elongation andthe hole expanding ratio.

On the other hand, the improvement of the hole expand-ing ratio of TRIP steel sheets was carried out by changingthe microstructure consisting of ferrite, bainite and retainedaustenite into that of bainite matrix with film-like retainedaustenite.31) The concept is as follows: 1. The homogeneityof the microstructure is increased by forming bainitic orbainitic ferrite microstructure as the main microstructure. 2.The stability of retained austenite increases by reducing theamount of austenite. Most of the stabilized austenite is nottransformed into martensite during the punching processand first transformed during hole expanding. The TRIPphenomenon during punching enhances the difference inthe strength of micro-constituents and promotes the form-ing of micro-cracks and deteriorates hole expanding ratio.On the contrary, the TRIP phenomenon during hole ex-panding prevents strain localization and suppresses thegrowth of micro-cracks, resulting in good hole expandingratio. Although this kind of the TRIP steel sheet has a sig-nificantly lower elongation than conventional TRIP steel

sheet with the same strength level, it is reported that thestretch formability is hardly worsened. This suprising phe-nomenon is now being critically re-examined.

3.6. Deep Drawable High Strength Steel Sheets

In general, high strength steel sheets with a tensilestrength of more than 600 MPa possess poor deep drawabil-ity. Recently, a deep drawable high strength steel sheet wasdeveloped using two well-known facts: cold rolled IF steelsheets show excellent deep drawability, and Cu in steelshows high solubility in austenite and precipitates finely ata relatively low temperature and strengthens the steel sig-nificantly, as shown in Fig. 24. By preventing the precipita-tion of finely dispersed Cu in the coiling process and hard-ening by precipitation in the over-aging process in a contin-uous annealing line, a deep drawable high strength steelsheet was developed using a Cu bearing IF steel. Figure 26shows the influence of the coiling temperature on the r-value of cold rolled Cu bearing IF steel sheets. If Cu iscoarsely precipitated at high coiling temperature or the pre-cipitation of Cu is suppressed at low coiling temperature,cold rolled steel sheets with an r-value of around 1.5 areachievable.32) The existence of finely dispersed Cu in hotbands hinders the formation of texture desirable for gooddeep drawability of cold rolled products.

This steel sheet is characterized not only by good deepdrawability but also by a high hole expanding ratio as men-tioned in Sec. 3.5. Another good feature of Cu-bearing highstrength steel sheets is their high fatigue resistance. That is,Cu in solution retards the formation of cells in the disloca-tion structure, which is considered to retard the initiation offatigue cracks.33)

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Fig. 24. Influence of aging time and temperature on the tensilestrength of 1.6% Cu containing IF steel sheets.29)

Fig. 25. Process for improving the hole expanding ratio of DPsteel sheets.30)

Fig. 26. Influence of coiling temperature on the r-value of coldrolled Cu bearing IF steel sheets.32)

A drawback of the Cu-bearing steel is that expensive Nineeds to be added to prevent the occurrence of surfacecracks caused by the Cu-rich liquid phase in the grainboundary in the vicinity of the surface during hot rolling.34)

Besides, Cu is an element which is very difficult to removein the refining process and has a detrimental effect on steelrecycling.35)

3.7. High Strength Steel Sheets with High FatigueStrength at Welded Parts

One of the factors hindering the broad application ofhigh strength steel sheets is the apparent softening of theheat affected zone (HAZ) of welded parts. The softenedpart is a potential source of fracture by forming and by fa-tigue. The softening of the HAZ hinders the increase instrength and fatigue resistance expected by the use of highstrength steel sheets.

To prevent the softening, an Nb and Mo bearing highstrength steel sheet was developed.36) The feature of thissteel is that multiple component precipitates consisting ofNb, Mo and C rapidly precipitate in the HAZ during thewelding process and prevent the softening. The reason whyrapid precipitation is possible is that Mo suppresses the an-nihilation of the dislocations introduced by deformation inthe HAZ during the welding process and the precipitation ispromoted by the increase in nucleation sites and by pipediffusion through dislocations.

3.8. High Strength Steel Sheets with Ultra-fine FerriteMicrostructure

From the viewpoint of recycling, the use of alloying ele-ments should be avoided and the development of highstrength steel sheets with plain carbon composition shouldbe promoted. In the STX-21 project lead by the NationalResearch Institute in Japan,37) a high strength steel sheetwith a tensile strength of 800 MPa is being developed bygrain refinement of a conventional 400 MPa steel sheetfrom around 10 mm to around 1 mm. To realize this kind ofsignificant grain refinement, heavy reduction hot rollingprocesses in the unstable austenite region, inter-critical tem-perature region and ferrite region are proposed.38–40) Thedeformation induced transformation and dynamic recrystal-lization of ferrite greatly contribute to the remarkable grainrefinement of this TMCP. Even in a stable austenite regionabove Ae3, it was reported that a heavy reduction inducesthe massive transformation and produces an ultra-fine fer-rite microstructure.41)

To obtain a sub-micrometer ferrite microstructure, a min-imum reduction of 70% is required for one-pass deforma-tion and 50% for multi-pass deformation. These high re-

duction operations are not realistic in practice. The reasonsare: 1. Required accuracy of gauge is difficult to achieve. 2.The temperature increase due to heavy reduction at thecommercial high speed rolling can cause grain coarseninginstead of grain refinement. 3. Most of the conventionalmills lack the necessary power. Under a pass schedule real-ized in practice, the achievable minimum grain size isaround 2 mm even if the residual strain effect is fullyused.42)

The mechanical properties of the high strength steelsheets with ultra fine ferrite microstructure have been examined and improvement of the strength–toughness bal-ance and of fatigue resistance has been confirmed. Theirstrength–ductility balance is, however, inferior to that of DPand TRIP steel sheets. Besides, the high yield strength ofthis steel sheet based on the Hall–Petch relationship isdetrimental for profile fixability by press forming. Anotherdrawback of this steel sheet is the occurrence of softeningcaused by coarsening the ultra fine ferrite grains in the heataffected zone of welded parts. To suppress the grain coars-ening, proper precipitation treatment is needed.43)

Although a lot of effort has been made in the STX-21project to improve the drawbacks of these steel sheets, inthe present state, the application of high strength steelsheets with ultra fine ferrite microstructure for automotiveparts is quite restricted.

3.9. Super High Strength Steel Sheets

Super high strength steel sheets whose tensile strength ismore than 1 000 MPa are used for bumper reinforcements,door impact beams, etc. Yamazaki et al.44) showed that inthe case of a hut type press forming, the formability ofsuper high strength steel sheets can be evaluated by bend-ability and investigated the relationship between bendabili-ty and microstructure of these sheets. It was found thatbendability strongly depended on the homogeneity of themicrostructure, not on ductility and strength, as shown inFig. 27. Initial cracks were found mainly at the interfacebetween soft micro-constituents and hard micro-con-stituents.

To obtain a homogeneous microstructure of hot rolledsuper high strength steel sheets, it is important that the in-crease in C concentration in austenite during cooling is sup-pressed by means of rapid cooling. For cold rolled prod-ucts, intercritical annealing should be avoided so that theenrichment of C in austenite is suppressed. The inhomo-geneity of the microstructure means that there are manylocal sites in which micro-cracks can nucleate. If there areinitial cracks in the product caused by inclusions or scratchdamage, bendability is significantly deteriorated.

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Fig. 27. Relationship betweenhomogeneity index, elonga-tion, strength and bendabil-ity of super high strengthsteel sheets.44) (Homogeneityindex: average deviation ofHRC)

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On the other hand, super high strength steel sheets aresusceptible to delayed fracture. As shown in Fig. 28, de-layed fracture is likely to occur upon the amount of retainedaustenite being increased. It is thought that the reason forthis is that the volume increase caused by strain inducedtransformation from retained austenite into martensite canform voids or high intensity of dislocations in the vicinityof the transformed martensite, and hydrogen gathers inthese places, leading to fracture. To reduce the retainedaustenite, annealing in the austenite range is preferable tointer-critical annealing.45)

3.10. Zinc Dip Coated High Strength Steel Sheets withHigh Formability

A problem related to high strength steel sheets is that theelements added to strengthen the steel often cause dip-coat-ing defects and/or suppress the galvannealing reaction.Phosphor used as a solute hardening element, for example,enhances the formation of Fe–Al which impedes the gal-vannealing reaction.46) The addition of Si, which improvesthe strength–ductility balance, causes the formation of afirm silicon oxide film which decreases the wettability ofdip-coating and leads to coating defects.47) To improve thewettability and avoid coating defects, a pre-coating tech-nique is sometimes employed. Ni-, Cu- and Fe-pre-coatingsare reported to improve the wettability of Si bearing highstrength steel sheets.48)

Concerning the metallurgy in a continuous galvannealingline (CGL), the influence of the heating condition, the cool-

ing condition after heating and the galvannealing conditionon mechanical properties were mainly investigated. Thefirst two conditions are common subjects regarding the con-tinuous annealing process, but the last one is a unique prob-lem in the CGL process. The galvannealing process can de-teriorate the mechanical properties of steel sheets. A repre-sentative example is TRIP steel sheet. During the galvan-nealing process, Fe3C can be precipitated and the amount ofretained austenite and the cencentration of C in it are re-duced. Therefore, to produce TRIP steel, alloying elementswhich suppress the precipitation of cementite should be in-creased.

High strength steels used for member parts are often arc-welded. In this case, blow holes are formed in zinc dip coat-ed steel sheets, which deteriorate the strength and fatigueresistance of welded parts. To suppress blow hole forma-tion, welding conditions are optimized, but great successhas not been achieved yet.49) To enable the broad use ofzinc dip coated high strength steel sheets, the problem relat-ed to arc-welding must be solved.

3.11. Further Development of High Strength SteelSheets

High strength steel sheets to be developed further for ex-tending their use to automotive parts are given in Fig. 29.All the developments are very challenging but withoutthese new developments, wide application of high strengthsteel sheets to automotive parts cannot be expected.

4. Stable Manufacturing Method

Upon high strength steel sheets being formed, practicaluse thereof is not possible if the required shape is not ob-tained within the required gauge accuracy, even if fracturesor wrinkle formation does not occur. Deviation from the re-quired shape generally increases with increasing strengthdue to the spring back effect. Even for high strength steelsheets, a shape with the required accuracy can be obtainedif the spring back effect is properly considered in the tool-ing design. In this case, however, the fluctuation of thegauge and strength of the steel sheet must be controlled to acertain tolerance. Figure 30 shows that the fluctuation ofstrength generally increases with increasing strength if nocontrolling measure is met.

Recently, a technology to increase the gauge accuracyand to decrease the fluctuation of strength was developed.Figure 31 shows the principle of the technology for con-trolling the strength of hot rolled products.50) This technolo-

Fig. 28. Relationship between volume fraction of retainedaustenite and time to delayed fracture.45)

Fig. 29. High strength steel sheets expected to be developed fur-ther for extending their use to automotive parts.

Fig. 30. Relationship between strength and its fluctuation.

gy consists of software determining the strength from mi-crostructural factors predicted by a metallurgical model,51)

and hardware minimizing the fluctuation of strength bycontrolling the hot rolling and/or cooling condition. Thistechnology makes it possible to obtain the required strengthwithin a certain tolerance. Besides, this computer con-trolled system also predicts the resistance to hot deforma-tion accurately by considering the evolution of microstruc-ture and as a consequence contributes to increasing thegauge accuracy of products.52) Such a computer controlledsystem has been already installed in modern steelworks.53)

Another way to minimize the fluctuation of strength isrealized by the development of sophisticated new hot stripmills. Kawasaki Steel’s Chiba Works and Nippon Steel’sOhita Works realized endless hot rolling by welding the hotbar before finishing rolling.54,55) This technology makes itpossible to minimize unsteady operation in the top and bot-tom regions of hot strips, enables nearly isothermal hotrolling at a constant speed and realizes a product with mini-mum fluctuation of strength. Endless hot rolling is expectedto be applied in mini-mills as well.

Besides, endless rolling enables highly lubricated hotrolling because the roll biting problem of each hot bar canbe avoided by cutting lubricant only at the top of the firsthot bar. Lubricated hot rolling reduces the rolling load andtorque and enables low temperature rolling which often im-proves the mechanical properties of high strength steelsheets because of the grain refinement, but is usually diffi-cult to carry out for high strength steels because of theirhigh resistance to hot deformation. Another advantage oflubricated hot rolling is the decrease in the frequency ofsurface defects.

Recently, deep drawable hot bands were developed by lu-bricated hot rolling of IF steel sheets in the ferritic tempera-ture range.56,57) Lubricated hot rolling reduces the shearstrain in the surface region of hot bands and changes thesurface texture deteriorating the r-value to that similar tomidlayer texture desirable for deep drawability. An r-valueof around 2 was obtained for a hot band of 1mm in thick-ness hot rolled at around 750°C in the laboratory. Using a

deep drawable hot band, the deep drawability of cold rolledsteel sheets can be improved significamtly. For example,using a hot band with an r-value of around 1.5, a coldrolled steel sheet with an r-value of around 3 wasproduced.58,59) The deep drawability of Cu bearing highstrength steel sheets mentioned in 3.6. can also be improvedusing this technology.

5. Means Supporting the Increase of the Use of HighStrength Steel Sheets

If a steel sheet can be formed without fractures and wrin-kles and within tolerable gauge accuracy, it is due verymuch to forming technique. For example, as mentioned inthe Introduction, the ULSUB-project achieved around 25%weight reduction of a white body mainly by sophisticateduse of forming technology. The forming technology con-tributing to proper formation of high strength steel sheetsis, from the viewpoint of hardware, the hydroforming,50,61)

tailored blanking,62,63) warm press forming, etc., and from aviewpoint of software, forming simulation with FEM to op-timum tooling design and material selection. One of thelargest problems in the press forming of high strength steelsheets is poor shape fixability. To improve shape fixability,the following techniques are employed: 1. re-striking or re-pushing, meaning an additional light pressing after conven-tional press forming, 2. press forming with dynamicallycontrolled blank holding force, 3. press forming at hightemperature, 4. press forming1post heat treatment, mean-ing that a mild steel sheet is press-formed and strengthenedafterwards by heat treatment, etc.

Tribology is another means for extending the apparentformability of high strength steel sheets. The practicalformability of automotive steels is often affected as muchor more by superficial properties such as interfacial frictionwith the tooling as by the inherent formability of the steel.It is well known that the reduction of friction between theproduct and tooling due to proper lubrication improves theformability, especially deep drawability. In the use of liquidlubricants, a proper control of roughness improves forma-

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Fig. 31. Principle of the tech-nology for controllingthe strength of hotrolled products.50)

bility because the lubrication effectively works due to thelubricant existing in the properly roughened surface.

Besides liquid lubricants, the use of solid film lubricantsis increasing because of their extremely low coefficient offriction and of environment friendly property. There are two types of solid film lubricants. One is the solid film lu-bricant removed after press forming and the other is that re-mained and painted thereon. Solid film lubricants shouldprovide proper interfacial friction with tooling, good coat-ing property, anti-corrosion property, good weldability,good paintability, reasonable price, etc. For the solid filmlubricant removed after press forming, the removability ofthe film is also important.

6. Conclusion

In this paper, the recent developments of modern highstrength steel sheets were reviewed paying special attentionto their physical metallurgy. As shown here, a series ofhighly formable new high strength steels have been devel-oped using sophisticated physical metallurgy and have con-tributed to expanding the application of high strength steelsheets, especially to automobile bodies and parts.

Besides further developments of new types of highstrength steels with better mechanical properties, the devel-opment of proper forming and welding technology is re-quired to expand the use of high strength steel sheets.Therefore, a key to extending the use of high strength steelsheets for automobile parts is the cooperation of the auto-motive industry and steelmakers in developing materialsand forming and welding technologies simultaneously. Anew concept ‘early involvement and concurrent engineer-ing’ being carried out through the joint efforts of the auto-motive industry and steel industry leads us to expect apromising future.

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