Structure change of 430 stainless steel in the heating process

Journal of University of Science and Technology BeijlngMaterialsVolume 15, Number 1, February 2008, Page 34EL SEVIERStructure change of 430 stainless steel in the heating processXinzhong Liu, Jingtao Han, Wanhua Yu, and Shifeng DaiSchool of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 00083, China(Received 2007-02-22)Abstract: The microstructure analysis was employed for the femitic stainless steel (SUS430) with the carbon content from 0.029wt%to 0.100wt% under the simulated heating process condition. The higher carbon sample (430H) contains the duplex phase micro-tructure at the temperature of 1150*C; on the other hand, the lower carbon content sample (430L) does not touch two phase areaeven at the temperature of 1450°C and has the single phase ferritic microstructure. The carbon content need be well controlled for the430 frritic stainless steel since it can significantly affect the heating process curve, and the heating process may not be done in thetwo phase area due to the uncotolled carbon content. With the low carbon content and the proper soaking time, the grain size is notsensitive to the heating process temperature and the soaking time. In the present heat treatment experiments, the soaking time isabout 10 min, and the processing parameters can be chosen according to the requirement of the gross energy, the efficiency and thecontinual forming,2008 University of Science and Technology Beijing. All rights reserved.Key words: frritic stainless stel; carbon content; microstructure; heating curve1. Introductionproducts.In this paper, according to the real dimensions of aWith the continually increased consumption fornickel and the intense resources supply, high cost hasheating process furmace used in the company, the in-been the main problem of nickel-based stainless steel.fluence of carbon content on the microstructure evo-The ferrite stainless steel without nickel has been alution during the heating process was investigated formore promising candidate due to its low cost. The ap-430 ferrite stainless steel under the simulated heatingplication range is unceasingly expanding and the usedprocess condition. In order to determine the effect ofamount is continuously increased [1]. 430 stainlessboth heating processing temperature and time on thesteel is one representative ferrite stainless steel andmicrostructure, the samples are first heated to 1 150-has been widely used in industries, including con-1280°C, and then quenched to room temperature; thestruction, decoration, food machinery, and bouseholdmicrostructures of samples were analyzed by SEM.electrical appliances, etc.2. ExperimentalSome recent studies [2-5] indicate that the grain offerrite stainless steel will increase when the steel isThe simulated heating process curve of 430 stain-overheated, which will reduce its ductility and thusless steel was carried out in Gleeble-1500. In the ex-cause cracking. It also shows that the content ofperiment, the process parameters are selected accord-spread varies with the temperature, and thus the heating to the situation of heating process fumace used inprocessing temperature is limited within a small rangesome factories and the stainless steel trial production[3]. Therefore, it is very important to investigate therelated process parameters, which are also referring tomicrostructure evolution of 430 stainless steel at aboutthe domestic and foreign hot rolling stainless sheet1250°C in order to create an optimized heating proc-strip F中国煤化工，ess curve, which can be used as a guideline in practice_ituation, the coldand can also be employed to improve the quality ofrollingYHC N M H Gthe Vacum state,Correspondng aothor: Xinzhong Liu, E-mail: liuxzhyx@ sohu.comAlbo available online at ww.wcenecedireet.com。2008 University of Science and Tectnology Beijing. All rights reserved.X,Z Liu et al, Structure change of 430 stainless steel in the heating process35and then skins forged, and finally machined to differ-After the carbon content adjustment, the new steelsent samples. Since the carbon content is very low inwere noted as 430H and 430L, respectively. The actu-the raw material, more carbon need be added duringal chemical composition was measured.the process of vacuum remelting in order to study theTable 1 shows the chemical composition of the 430influence of carbon content on the heating processingstainless steel used in this study.of ferrite stainless steel.Table 1. Cbemical composition of the 430 stainless steelwt%_SteelCr_Ni_Mn _SPFe430H0.10016.100.1300.0900.5500.0220.004Bal430L0.029. Bal_In the experiments, the specimens were first reheat-immediately in a few seconds with the cooling rateed to 1150- 1250°C at the heating rate of 10°C/s. Afterhigher than 300°C/s. The simulated heating processheating to the specific temperature, the specimenscurves are shown in Fig. 1.were soaked for 10 min, and then water quenched↑(间()1250Cx 10 min, 20 min1200°Cx10 min, 20 min司>300C/s>300"C/s10C/s! 10C/sHeating-up time/sFig. 1. Heating curve of the heat simulation examination: (a) 1200C; (b) 12500C.The tested samples were cut along the axis by a periods are shown in Figs. 2 and 3, respectively.wire cutting machine. Longitudinal sections of sam-From the above figures, it is can be seen that 430Lples for SEM were ground, polished, and etched. Thehas a single ferrite phase microstructure, and the grainmicrostructure evolution in heating was investigatedsize of ferrite increases with the increase of tempera-by using SEM. The test scheme for 430H is the sameture, while the grain size significantly increases atas that for 430L. Table 2 shows the processing1250°C. With the soaking time ranging from 10 to 20parameters of the samples.min, the grain size of ferrite also increases. The 430LTable 2. Experimental parameters for 430 austeniticsteel is in the a phase area of the equilibrium phasestainless steel heating curve used in the heat simulation ex-diagram of iron-chromium binary system, shown inaminationFig. 4 [6].Heating temperature/°CThe 430H specimen has the microstructure of fer-Soaking12001250rite -austenite duplex phase in the temperature rangetime 1 minfrom 1150 to 1200°C. With the heat processing tem-10a)(cc)perature rising from 1150 to 1200°C, both the initial0b)b)_(dd)grain中国煤化工T the second phaseincreaI be observed with:MYHC N M H Gan be concluded3. Results and discussionthat the steel is in the ferrite austenite duplex phaseThe typical microstructures of 430L andarea in the temperature range of 1150 to 1200°C. Itsamples at different temperatures and soaking timehas been reported that [7-12] carbon and nitrogen36J. Univ. Sci TechnoL Beijing, VoL15, No.1, Feb 2008elements in 430 ferrite stainless steel will cause thehigh temperature, the steel with 17wt% Cr is in thea+γ zone to shift towards the higher chromium com-a+Y zone. During water quenching， the austeniteposition area. Hence, with the increase of carbon con-phase will transfer to the martensite phase, and thustent in the simulation test, the zone of a+Y will becauses the ferrite and martensite duplex phase micro-closer to the higher chromium composition area. Atstructure in the 430L steel used in the experiment.100 um100um(CdFig. 2. SEM micrographs of 430L: (a) after 10 min at 1200°C; (b) after 20 min at 1200C; (2) after 10 min at 1250C; (d)after 20 min at 1250°C.b)，((d) |中国煤化工Fig, 3. SEM micrographs of 430H: (旧) after 10 min at 1200°C; (b) afteEnin at 1250°C; (d)MYHCNMHGFor the sample of 430H at 1250°C, the matrix be-phase is dissolved), including some martensite in thecomes the new grain (the strip of the original ferriteteel. The steel will transform into the single ferriteX.Z. Liu et al, Structure change of 430 stainless steel in the heating process37phase again in the temperature range of 1200 tocontent becomes duplex phase even when heated at1250°C.low temperature. In contrast to the former steel, 430Lwith low carbon content is still in the single ferriteCr/ wt%zone instead of the duplex phase.1900 - 20.6080100(2) The content of carbon in the 430 ferrite stainless1700 tsteel has significant influence on the heating process15001394curve, and therefore, it is important to control the1300content of carbon in steels; otherwise, the steel con-1100-tains the duplex phase microstructure inevitably dur-(a-Fe, 8He)830tcing the heating process.(C700(3) If the carbon content is maintained at a low500level, the grain size is essentially not very sensitive tothe soaking temperature and time. The appropriate300420406080 100soaking time in the simulation experiment is about 10Cr/ at%miFig4. Binary phase diagram of the Fe-Cr.Fig.5 shows the phase boundary of γ+a/a in theReferencesphase equilibrium diagram of the binary system in the[1] S.F. Chen, A cllction of selected specimens of 9th inter-430H [3]; the heavy line in the picture shows the re-national stainless steel forum, China Met. Bull. (in Chi-nese), 27(2005), p.12.sults of the test in the literature, which indicates that2] X.N. Ye, Quality control and process study of SUS430 hotthe trend of the phase boundary of γ+a/a in the phaserolled strip, Baosteel Technol (in Chinese), 4(2005), p.32.equilibrium diagram of the binary system moves to-[3] s.Y. Lu, Y.K. Zhang, X.F. Kang, et al, Stainess Steel (inward the higher chromium composition area; the esti-Chinese), A-energy Publishing Company, Beijing, 1995,mated phase boundary of y+a/a for the 430H is shownp.161.as the thin line, where the boundary moves back, and4] C.R. Brooks and JP. Zhou, Microstructural analysis of anthe thin vertical line represents the content of chromi-embritled 422 stainless steel stud bolt after approximately30 years service in a fossil power plant, Metallography,um in 430H.23(1989), No.1, p.27.5] C.R. Brooks and F. Bogni, Metallographic examination ofcrack-path propagation in embritled 12% Cr steel, Mater.00WI%N1400 t0.02wI%N .Charact, 38(1997), No.2. p.103.[6] T.B. Massalski, H. Okamoto, P.R. Subramanian, et al,Binary Alloy Phase Diagrams, Materials Park, ASM In-ζtemational, Ohio, 1990, p.1273.1200 t7] E.A. Tillo and L.E. Mur, Effects of carbon content, de-formation, and interfacial energetics on carbide precipita-Estimated phasetion and corrosion sensitization in 304 stainless steel, Acta.boundaryr Results oftheMater., 47(1998), No.1, p.235.1000test in the[8] A. Goecmen, R. Steins, C. Solenthaler, et al, Precipitationliteraturebehaviour and stability of nitides in high nitrogen marten-sitic 9% and 12% chromium steels, ISLJ Int., 36(1996),800No.7, p.768.30[9] F. Vanderschaeve, R. Taillard, and J. Foct, Effeet of heatCr/Wt%treatment on the microstructure of a high nitrogen 12%Fig. 5. Schematic graph of the phase boundary for thechromium martensitic steel, Steel Res, 64(1993), No.4,binary pbase diagram of Fe-Cr in the tape 430H.p.221.Therefore, in the current work the 430H has the a+y[10] D.H. Jack and K.H. Jack, Carbides and nitrides in steel,Mater. Sci. Eng.. 11(1973), No.1, p.1.duplex phase microstructure when heated to 1150-[11] V.G. Gaviljuk and H Berns, Precipitates in tempered1200°C, and it will transform back to the single astainless martensitic steels alloved with nitrogen, carbonphase with the higher heating process temperature中国煤化工X1999), p.71.(above 1250°C).[12]二ntogen alloying ofYHC N M H GA, 201996 No.2,4. Conclusionsp.159.(1) The microstructure of 430H with high carbon