Materials

www.elsevier.com/locate/matdes

Materials and Design 28 (2007) 1760–1766

& Design

Development of hot-rolled dual-phase weathering steelCu–P–Cr–Ni–Mo

Chunling Zhang a,b,*, Bo Liao a,b, Dayong Cai a,b, Tianchen Zhao c, Yunchang Fan c

a Key laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, PR Chinab College of materials Science and Engineering, Yanshan University, Qinhuangdao 066004, PR Chinac Department of Materials Science, Shijiazhuang Railway Institute, Shijiazhuang 050043, PR China

Received 18 October 2005; accepted 10 May 2006Available online 10 July 2006

Abstract

The continuous cooling transformation (CCT) diagrams of weathering steels Cu–P–Cr–Ni–Mo were constructed by means of a com-bined method of dilatometry and metallography. The diagram of weathering steel with 0.41% Mo exhibits an elongated polygonal ferriteC-curve with a delayed pearlite-start, and a metastable austenite gap between the polygonal ferrite/pearlite and the bainitic ferriteregions. Weathering steel with 0.41% Mo is possible to obtain a dual-phase microstructure directly by hot-rolling and appropriate cool-ing. Dual-phase microstructures with some bainite and pearlite have been obtained by hot-rolled simulating on hot-rolled dual-phasetreatment procedures deduced by the CCT diagram. Hot-rolled dual-phase weathering steel with 0.41% Mo has more excellent compre-hensive mechanical properties and formability than commercial weathering steel 09CuPCrNi.� 2006 Elsevier Ltd. All rights reserved.

Keywords: Cu–P–Cr–Ni–Mo weathering steel; Continuous cooling transformation diagram; Hot-rolling; Metastable austenite; Dual-phase

1. Introduction

Weathering steel is one of the common candidates forsteel constructions in the field of transportation [1,2]. Nor-mally, the chemical composition of weathering steelincludes a small amount of Cu, Cr, Ni and P which are ableto promote, after exposure in the atmosphere, the forma-tion of a stable protective oxide layer, reducing remarkablythe corrosion rate [3,4]. Nowadays weathering steel 09CuP-CrNi is widely used in manufacturing rolling stock inChina. To increase the velocity and decrease the weightof the train, new weathering steel, which has not only highperformance in corrosion resistance under the condition ofexposure to sunshine, raining and snowing, and tempera-ture fluctuation, but also higher strength to reduce the

0261-3069/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.matdes.2006.05.011

* Corresponding author. Key laboratory of Metastable MaterialsScience and Technology, Yanshan University, Qinhuangdao 066004, PRChina. Tel.: +86 335 8074729; fax: +86 335 8060103.

E-mail address: zhangchl@ysu.edu.cn (C. Zhang).

weight of the train itself, must be developed. Dual-phaseweathering steel which has both the excellent atmosphericcorrosion resistance of the weathering steel and the favor-able comprehensive mechanical behavior and propertiesof the dual-phase steel, can satisfy these necessaries.

Dual-phase weathering steel produced by intercriticalanneal has not only high performance in corrosion resis-tance, but a low yielding stress, a high elongation valueand a smooth flow-stress curve with a high strain-harden-ing coefficient [5–7]. However, the intercritical annealmethod have some disadvantages: large investment ofequipment, high energy consumption and low productionefficiency and quality. Based on these conditions, develop-ing directly hot-rolled dual-phase weathering steel isnecessary.

For high strength low alloy steel, it is possible to obtaina dual-phase microstructure directly by hot-rolling andappropriate cooling, if its continuous cooling transforma-tion (CCT) diagram exhibits a gap between the pearliteand bainite regions where the austenite is stabilized [8,9].

1 10 100 1000 10000 1000000

200

400

600

800

1000

0.030.050.150.30.91.891850Vs( )

Ms=455

Ac1=725

Ac3=915

F P

B

Tem

pera

ture

,

Time, s

Fig. 1. The CCT diagram of weathering steel 09CuPCrNi.

C. Zhang et al. / Materials and Design 28 (2007) 1760–1766 1761

The main objective of this study is to design a weather-ing steel suited for hot-rolled dual-phase treatment, andthen establish the CCT diagram of the steel, analyze thefeasibility of hot rolled dual-phase treatment according tothe CCT diagram, put forward a scheme of hot-rollingand make hot-rolling simulation with Gleeble-3500, finallyhot roll dual-phase weathering steel with a experimentroller and test its tensile property.

2. Materials and experimental procedures

The alloys used in this research were vacuum-melted in a high fre-quency furnace; the chemical compositions of the alloys are shown inTable 1. The ingots were forged to diameter bars by 10 mm, rectangularbars by 17 mm thick by 22 mm wide by 150 mm long and rolling blanksby 60 mm thick by 80 mm wide by 80 mm long.

Dilatometry. Cylindrical dilatometer specimens used in the unde-formed conditions (B3 mm · 10 mm) and in the deformed conditions(B8 mm · 12 mm) were machined from the diameter bars. The static(without hot deformation) CCT diagrams were conducted on Forma-stor-Digital dilatometer, and the corresponding specimens were firstlyaustenitized for 5 min at 1050 �C, then cooled at different linear coolingrates. While the dynamic (with hot deformation) CCT diagrams were con-ducted on THERMECMASTOR-Z thermal simulator, and the corre-sponding specimens were firstly austenitized for 5 min at 1050 �C, thengiven 5 passes compression deformation with 50% total reduction, andfinally cooled at the linear cooling rates of 60, 30, 20, 10, 5, 2, 1, 0.5,0.2, 0.1, 0.05 �C/s, respectively.

Hot-rolling simulating. For the hot-rolled dual-phase treatment simulat-ing test, plane strain samples, 20 mm in length, 15 mm in width and 10 mmin height, were machined from the forged rectangular bars. The hot-rolleddual-phase treatment simulating was conducted on Gleeble-3500 hot sim-ulator, and the corresponding specimens were firstly austenized for 5 minat 1150 �C, and then given 3 passes compression deformation with 50%total reduction, finish-rolling temperature are 850 �C and 890 �C, thencooled to the coiling temperature at the cooling rate of 25 �C/s between fin-ish-rolling and coiling temperatures, coiling temperatures are 560 �C,580 �C and 600 �C, and finally cooled to the room temperature with aircooling.

Hot-rolling. The hot-rolled dual-phase weathering steel strip was pre-pared by an experiment roller. The rolling procedure is shown below. Ini-tial thickness: 60 mm; final thickness: 6 mm; number of passes: 5; reheatingtemperature: 1200 �C; starting rolling temperature: 1150 �C; finish-rollingtemperature: 870 �C; cooling rate between finishing and coiling tempera-ture: 30 �C/s; coiling temperature: 580 �C; and cooling rate: 30 �C/h.

Metallography. A combination of optical microscopy and transmissionelectron microscopy (TEM) was used to determine the microstructures ofthe specimens. Specimens for metallographic examination were etchedwith Leperal solution. For TEM observation, thin foils were firstlymechanically thinned from 300 mm thick discs to about 50 mm and thenelectro-polished by a twin-jet electropolisher in a solution of 10% per-chloric acid and 90% acetic acid. Thin foil samples were observed underan H-800 TEM. Second phase volume fractions were determined by theliner intercept method.

Table 1Chemical composition of experiment steels (wt%)

Steel C Si Mn S P

09CuPCrNi 0.06 0.48 0.45 0.096 0.014A 0.07 0.49 0.40 0.015 0.10B 0.11 0.53 0.49 0.017 0.098C 0.07 0.53 0.49 0.016 0.11D 0.07 0.50 0.43 0.016 0.098E 0.07 0.53 0.35 0.013 0.063

3. Results and discussion

3.1. CCT diagram of weathering steel 09CuPCrNi

The hot-rolled dual-phase steel concept is based on alow carbon, low alloy steel that exhibits special CCT char-acteristics which permit the steel to be processed on a con-ventional, high production hot-strip mill to produce thedesired ferrite–martensite microstructure in the hot-rolledcoiled sheet. Special characteristics [9] that are needed inthe CCT diagram include (1) an elongated ferrite C-curve,i.e., the ability to form very large amounts of polygonal fer-rite over a reasonably wide range of cooling rates on therunout table, (2) a suppressed (delayed) pearlite nose toensure avoidance of pearlite formation during cooling tothe coiling temperature, (3) a high pearlite finish tempera-ture to avoid pearlite formation after coiling temperaturesup to 620 �C, and (4) a gap between the polygonal ferriteand the bainitic ferrite regions to provide a temperaturerange of at least 75 �C within which no further transforma-tion occurs, permitting the steel to be coiled with little orno sensitivity to the normal variations in coiling tempera-ture that occur in commercial production.

The CCT diagram of weathering steel 09CuPCrNi isshown in Fig. 1. The diagram has three regions, namelyferrite, pearlite and bainite transformation range, andthe three regions are superposed, without separating fromeach other. Under the experiment cooling rates, there is

Cr Ni Cu Mo Nb V Ti

0.51 0.21 0.290.56 0.22 0.25 0.330.60 0.20 0.27 0.410.53 0.22 0.29 0.42 0.030.55 0.21 0.28 0.41 0.030.53 0.21 0.29 0.41 0.04

1762 C. Zhang et al. / Materials and Design 28 (2007) 1760–1766

no martensite transformation happened. Compared withspecial CCT characteristics of hot-rolled dual-phase steel,the CCT diagram of weathering steel 09CuPCrNi cannotsatisfy the necessary of hot-rolled dual-phase treatment.First, pearlite transformation is not delayed, so duringcooling to the coiling temperature, pearlite formation can-

1 10 100 1000 10000 1000000

200

400

600

800

1000

0.050.170.330.51235102050Vs( )

Ms=443

Ac1=715

Ac3=970 a

P

B

F

Tem

pera

ture

,

Time, s

1 10 100 1000 10000 1000000

200

400

600

800

1000

0.0280.056

0.140.280.971.94

3.889.719.432.397Vs( )

Ms=440

Ac1=750

Ac3=990 c

P

F

B

Tem

pera

ture

,

Time, s

1 10 1000

200

400

600

800

1000

5102050Vs( )

Ms=473

Ac1=800

Ac3=960

F

B

Tem

pera

ture

,

Time,

Fig. 2. CCT diagrams of experiment steels. (a) Steel A,

not avoid; Then, pearlite finish temperature is low, aftercoiling temperature, austenite may transform to pearlite;Finally, and the most important is that between thepolygonal ferrite and the bainite regions, there is no meta-stable austenite transformation region providing as ‘‘coil-ing window’’.

1 10 100 1000 10000 1000000

200

400

600

800

1000

0.0280.0560.14

0.280.941.88

3.769.418.831.394Vs( )

b

Ms=425

Ac1=730

Ac3=960

PF

B

Tem

pera

ture

,

Time, s

1 10 100 1000 10000 1000000

200

400

600

800

1000

0.10.5125102050Vs( )

Ms=468

Ac1=700

Ac3=920

d

PF

B

Tem

pera

ture

,

Time, s

1000 10000 100000

e

0.10.512

P

s

(b) steel B, (c) steel C, (d) steel D, and (e) steel E.

1 10 100 1000 10000 1000000

200

400

600

800

1000

BMs=425

a

0.050.10.20.512510203060Vs( )

P

F

MTem

pera

ture

,

Time, s

1000

1 10 100 1000 10000 1000000

200

400

600

800

b

0.050.10.20.512510203060Vs( )

Ms=425

P

F

B

Tem

pera

ture

,

Time, s

Fig. 3. The CCT diagrams of steel B, C with deformation. (a) Steel B, and(b) steel C.

C. Zhang et al. / Materials and Design 28 (2007) 1760–1766 1763

3.2. Alloying of 09CuPCrNi weathering steel

In order to develop hot-rolled dual-phase weatheringsteel, the chemical composition basing on 09CuPCrNiweathering steel must be redesigned.

Based on the special CCT characteristics of hot-rolleddual-phase steel, a suitable alloying element to be addedto hot-rolled dual-phase steel must have the followingfeatures: (1) it should retard both the bainite-start andbainite-finish, thus assisting the austenite to martensitetransformation, and (2) it should retard the pearlite-start,and heighten the pearlite-finish temperature to avoid pear-lite transforming [9]. Judging from these considerations,Mo is a favorable alloying element.

Considering the alloy element effects on weatheringproperty, weldibility and refinement, Mo, Nb, V, Tiare chosen to alloy weathering steel 09CuPCrNi. Thechemical composition of these experiment steels are shownin Table 1.

3.3. CCT diagrams of Cu–P–Cr–Ni–Mo weathering steels

The CCT diagrams of the five Cu–P–Cr–Ni–Mo steelsare shown in Fig. 2. Mo has strong effects on the shapeof the CCT diagram, namely retarding both the bainite-start and bainite-finish, retarding the pearlite-start, andheightening the pearlite-finish temperature. For steel A,the addition of 0.33% Mo induces the bainite transforma-tion region partly separated from that of ferrite, and thepearlite transformation region retarded strongly comparedwith the CCT diagram of commercial weathering steel09CuPCrNi (Fig. 2a). When the addition of Mo isincreased to 0.41%, the ferrite-pearlite transformationregion and the bainite transformation region are separatedcompletely, and a metastable austenite gap appearsbetween the two regions, which can provide ‘‘coiling win-dow’’ after rolling. In addition, the transformation rangesof ferrite and pearlite are separated (Fig. 2b). For steel C,because of adding of 0.03% Nb, the bainite transformationtemperature is heightened, so the bainite transformationregion and the ferrite transformation region superposeagain, but the gap between the ferrite region and bainiteregion is still exist (Fig. 2c). The CCT diagrams of steelD, E are similar as that of steel A, namely bainite transfor-mation region partly separated from the ferrite transforma-tion region, pearlite transformation region and ferritetransformation region still superposed (Fig. 2 d, and e).

From the CCT diagrams of the five steels, we can foundthat bainite transformation region and pearlite transforma-tion region are separated completely, that is to say, whenexperiment steel cools from the austenitization temperatureat some cooling rate, austenite can transform to neitherbainite nor pearlite, thus it can first transform to ferritepartly, then the left austenite transforms to martensite,which is called dual-phase microstructure. It must be notedthat the metastable austenite region of steel B is completelyopen, however the regions of the other four steels are half-

open. So for hot-rolling procedures, steel B can provide an‘‘open coiling window’’, but the other four steels can pro-vide ‘‘half-open coiling windows’’ only. In addition , the‘‘coiling windows’’ of steel A and D are narrow, and the‘‘coiling window’’ temperature of steel E is low.

As described above, for steel B and C, they are possibleto obtain dual-phase microstructure directly by hot-rollingand appropriate cooling with middle temperature coiling.

3.4. Dynamic CCT diagrams of steel B and C

With deforming affects on the shape of CCT diagram, inorder to constitute correct hot-rolling procedures, the CCTdiagrams of steel B and C with deformation must beconstructed.

The dynamic CCT diagrams of steel B, C are shown inFig. 3. For steel B, compared with the static CCT diagram,the gestation period of ferrite transformation is shortenedand the ferrite-start temperature is heightened, so the ferritetransformation region is extended strongly. In addition, thegestation period of pearlite transformation is lengthenedand the pearlite transformation temperature is lowered.

1 10 100 1000 10000 100000200

400

600

800

1000

1200

B

PF

M

Tem

pera

ture

,

Time, s

Platen Cooling (30 /s)

1150

Finish Rolling

Simulated Coil Cooling

(30 /h)

Coiling window

Fig. 4. Schematic representation of simulation of hot-rolled processing forsteel B.

Table 2Hardness and microstructure of steel B with different hot-rolled dual-phase processing treatment

Processing treatment number 1 2 3 4 5

Finish-rolling temperature (�C) 850 850 850 890 890Coiling temperature (�C) 560 580 600 600 580Volume fraction of second

phase (%)28 30 17 21 25

Hardness (HRB) 95.8 95.9 97.8 96.4 97.6

1764 C. Zhang et al. / Materials and Design 28 (2007) 1760–1766

The bainite transformation region is lowered about 40 �C(Fig. 3a). The ferrite transformation region turning leftand up is caused by deformation inducing ferrite transfor-

Fig. 5. Optical micrographs of steel B after hot-rolled dual-phase treat

mation. Since the ferrite transformation is extended, largeamount of austenite transforms to ferrite, the left austeniteis more stable, then the pearlite transformation region turnsright and down and the bainite transformation region turnsdown. For steel C, compared with the static CCT diagram,the ferrite transformation region turns left and up, thepearlite-start is advanced and the pearlite transformationtemperature is lowered, and the bainite transformationregion is lowered (Fig. 3b). So the ‘‘coiling window’’ tem-perature provided by steel C is lowered too.

As described above, only steel B is possible to obtain adual-phase microstructure directly by hot-rolling andappropriate cooling.

3.5. Simulating of Hot-rolled dual-phase steel B

For steel B, the experimental coiling temperature isdeduced from the dynamic CCT diagram. The schematicdiagram of the technical process for hot-rolled dual-phasetreatment is shown in Fig. 4.

Hardness and microstructure for steel B with differenthot-rolled dual-phase treatment procedures are shown inTable 2. The microstructures of all the specimens with dif-ferent hot-rolled treatment procedures are shown in Fig. 5.It must be noted that the microstructures are all dual-phase, namely composed of ferrite and MA or MAC con-stituent. MA or MAC constituents with island-shaped areirregularly distributed in the matrix of equiaxed ferritegrains.

Transmission electron bright-field micrographs of steelB after hot-rolled dual-phase treatment (processing treat-ment number 2) are shown in Fig. 6. Similar to previousresults published for intercritically quenched steels [6,7], it

ment. (a) Process 1, (b) process 2, (c) process 3, and (d) process 5.

Fig. 6. Transmission electron micrographs of steel B after hot-rolled dual-phase treatment. (a) Martensite island surrounded by ferrite with high densitydislocations. (b) Lath martensite with dislocations. (c) Martensite with microtwins.

Table 3Mechanical properties of dual-phase weathering steel B and commercial weathering steel 09CuPCrNi

Steel Microstructure rb (MPa) r0.2 (MPa) d5 (%) n rb/r0.2

09CuPCrNi Ferrite + Pearlite 396.7 517.6 36.3 0.14 0.766B Ferrite + Martensite 466.1 801.5 22.7 0.20 0.581

C. Zhang et al. / Materials and Design 28 (2007) 1760–1766 1765

is frequently observed that the dislocation density in theinterior regions of ferrite grains is lower than that in theferrite regions directly adjacent to the martensite phase(Fig. 6a). The higher dislocation density around martensitephase may be caused by the volume expansion accompany-ing the phase transformation occurring from austenite tomartensite. The martensite phase is essentially lath type(Fig. 6b), although some microtwins can be also observed(Fig. 6c).

As a result of this preliminary investigation, it isexpected that strips adequately cooled after rolling to forman appropriate amount of polygonal ferrite and then heldin the coiling window, namely between 620 �C and550 �C to retain an appropriate amount of enriched austen-ite, will finally generate a dual-phase steel with interestingmicrostructures.

3.6. Hot-rolling dual-phase steel B

The mechanical properties of the hot-rolled dual-phaseweathering steel B and the commercial weathering steel09CuPCrNi are shown in Table 3. It can be clearly seenthat the yield strength (0.2 % offset) and the tensilestrength increase 17.5% and 43.5%, respectively comparedwith those of weathering steel 09CuPCrNi. At the sametime, total-elongation decreases from 26.3% to 22.7%,the decreasing extent is about 21.5%, but the elongationstill higher than the industry standard of weathering steel09CuPCrNi in China. The yield-to-tensile strength ratiodecreases about 24.1% while the work hardening expo-nent increases 42.8%. Formability of steels can be evalu-ated by their total-elongation (d), yield-to-tensile strength

ratio (r0.2/rb) and work hardening exponent (n). Com-pared with weathering steel 09CuPCrNi, hot-rolleddual-phase weathering steel B should have much betterformability.

All the results above presented indicate that a successfulhot-rolled dual-phase weathering steel B with excellentcomprehensive mechanical properties and formability canbe obtained through suitable control rolling and controlcooling.

4. Conclusion

1. The CCT diagram of weathering steel 09CuPCrNi wasconstructed. Since its ferrite, bainite and pearlite trans-formation were superposed, it cannot satisfy the specialnecessary of hot-rolled dual-phase treatment.

2. Based on the special CCT characteristics of hot-rolleddual-phase steel, five Cu–P–Cr–Ni–Mo weathering steelswere designed, and their CCT diagrams were con-structed. Steel B is possible to obtain a dual-phase micro-structure directly by hot-rolling and appropriate coolingwith middle coiling temperature for it can provide a wide‘‘velocity window’’ and a suitable ‘‘coiling window’’.

3. The hot-rolled dual-phase treatment procedures werededuced from the dynamic CCT diagram of steel B.The results of hot-rolled dual-phase treatment simulat-ing showed that the microstructures were dual-phaseof ferrite and martensite.

4. Hot-rolled dual-phase weathering steel B has moreexcellent comprehensive mechanical properties andformability.Compared with commercial weatheringsteel 09CuPCrNi, the yield strength (0.2% offset) and

1766 C. Zhang et al. / Materials and Design 28 (2007) 1760–1766

the tensile strength increase 17.5% and 43.5%, respec-tively, total-elongation decreases about 21.5%, theyield-to-tensile strength ratio decreases about 24.1%while the work hardening exponent increases 42.8%.

Acknowledgements

This work was supported by Science and TechnologyStudy and Development Project of Hebei Province (No.012121175D).

References

[1] De.orian F, Rossi S. Premature corrosion failure of structural highwaycomponents made from weathering steel. Eng Fail Anal 2002;9:541–51.

[2] Konyukhov AD, Ruvinskaya EM. Bridge spans from a weatheringsteel. Prot Met 2002;38(1):83–8.

[3] Ishikawa T, Kumagai M, Yasukawa A. Influences of metal ions on theformation of c-FeOOH and magnetite rusts. Corros Sci 2002(44):1073–86.

[4] Yamashita M, Uchida H. Recent research and development in solvingatmospheric corrosion problems of steel industries in Japan. HyperfineInteract 2002;139/140:153–66.

[5] Zhang Chunling, Cai Dayong, Liao Bo, Zhao Tianchen, FanYunchang. Study on the dual-phase treatment of weathering steel09CuPCrNi. Mater Lett 2004;58:1524–9.

[6] Zhang Chunling, Cai Dayong, Liao Bo, Zhao Tianchen, FanYunchang. Microstructure and mechanical properties of dual-phaseweathering steel 09CuPTiRe. J Mater Sci 2004;39:4393–6.

[7] Dai Yakang, Du Linxiu, Qi Zhengfeng, Zhang Zhilu, Zhang Zhichen.Dual-phase treatment of weathering steel-dual-phase weathering steel.Mater Mech Eng 1992;16:38–41.

[8] Coldren AP, Tither G. Development of a Mn–Si–Cr–Mo as-rolleddual-phase steel. J Met 1978;4:6–9.

[9] Greday T, Mathy H, Messien P. About different ways to obtainmultiphase steels. In: Kot RA, Morris JW, editors. Structure andproperties of dual-phase steels. New York: Metallurgical Society ofAIME; 1979. p. 260–80.