Effect of deformation temperature on transformation-induced plasticity effect of lean dupl
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Baosteel Technical Research Volume 8,Number3,September2014,Page 42 Effect of deformation temperature on transformation-induced plasticity effect of lean duplex stainless steel ZHANG Wei .ZHUANG Wei and HU Jincheng ’ 1)Research Institute,Baoshan Iron&Steel Co.,Ltd.,Shanghai 200431,China 2)Special Steel Company,Baosteel Group,Shanghai 20043 1,China Abstract:Efiects of deformation temperature on the mechanica1 properties and microstructure of Iean duplex stainless steels B2102 and¥32101 have been investigated.It was found that the strength decreased continuously with increases in temperature from一60℃to l00℃.The strength of¥32101 was higher than that of B2102 owing to its higher nitrogen content.P1asticity of B2102 increased with an increase in deformation temperature from~60℃and reached the optimal elongation ratio of 49%一54%after deformation at 20—50 oc.Martensite transformation was observed during deformation due to the transformation—induced plasticity eflfect.The optimal elongation was achieved at deformation temperatures close to the Md(30/501 temperatures of 62℃and 6℃for B2102 and¥3210l, respectively. Key words:lean duplex stainless steel;transformation—induced plasticity;martensite transformation;deformation temperature doi:10.3969/j.issn.1674—3458.2014.03.006
1 Intr0duction Duplex stainless steel(DSS)is characterized by a dual—phase structure with approximately equal volume fractions of ferrite and austenite,offering an attractive combination of corrosion resistance and mechanical properties -3 j.Among the duplex stainless steels,¥32101.with the typical composition Fe一21.5Cr一5Mn—1.5Ni一0.22N—O.3Mo一 0.3Cu.is known as a“lean”DSS due to its low Ni content.DSS¥32101 is widely used in nuclear plants,vessel equipment and building structures due tO its higher yield strength and better pitting resistance L4 1.However,the elongation ratio of ¥32101 is around 37%.obviously lower than that of 304(55%).This 1imits the application of lean DSS in many fields. Recently,Choi designed a series of lean DSSs, with the composition Fe一2OCr一5Mn一0.2Ni一 0.2Si—xN.associated with the transformation— induced plasticity(TRIP)effect 5j.In these steels. a sequential martensite transformation of — £—}仪 was observed during deformation which was a result of the TRIP effect.Consequently.the elongation ratio increased to as high as 58%.obviously higher than that of¥32101.This was helpful in expanding Corresponding author:ZHANG Wei:E・mail:zwatt@163.com the application of lean DSS to various fields.Herrera also reported L O J a ductile Iean DSS with the composition Fe一19.9Cr一0.42Ni一0.16N一 4.79Mn一0.11C一0.46Cu一0.35Si.which had an ultimate tensile strength of 1 GPa and an elongation ratio to fracture of above 60%.Zhang Wei H found that£一martensite formed in the austenite.and subsequently. ot martensite nucleated at the intersections of the £.martensite bands during deformation,causing the TRIP effect. The present work investigated the effect of deformation temperature on the mechanica1 properties and TRIP effect of 1ean DSS B2102.The results showed that the elongation ratio reached 49% 一54%in B2l02 after deformation at 20—50℃. which was higher than that of typical lean DSS ¥32101(37%).The martensite phase was observed after deformation.contributing to the TRIP effect. The optima1 elongation ratio was achieved at the deformation temperature slightly below the Md(3(∥5()) temperature of the austenite phase.such as 62℃for B2102. 2 Experimental materials and procedures Experimental steels were prepared by induction melting in a ZG一50 vacuum induction furnace,
ZHANG Wei,et aL Effect of deformation temperature on transformation.induced plasticity effect of lean duplex-・・43 using high—purity iron, manganese, silicon, chromium,nickel,molybdenum,and Fe(CrN)alloy。 Chemical composition of the steels is listed in Table 1.After being homogenized at 1 200℃for 2 hours,the ingot was hot forged to a thickness of 48 min at 1 200℃.and then hot rolled to a thickness of 3 irlm.The as—rolled plates were annealed at 1 050 oC for 6 minutes;next,these plates were subjected to cold rolling to a thickness of 1 inii1 and then annealed at 1 050 oC for 2 minutes. Table 1 Chemical composition of the as・received B2102 and¥32101 %
The tensile test was carried temperatures.from一60℃tO 100 8 10 tensile machine.Volume magnetic phase were tested by out at different ℃.using an MTS fractions of the a FERITSCoPE MP30E—S ferrite detector.To observe the microstructure.the specimens were electrochemically etched by 40 g K0H+50 ml H,O solution and visualized under a CARL ZEISS optical microscope. To observe the martensite transformation in detai1. some specimens that were stretched to different strain levels were observed by a JEOL JEM 200CX transmission electron microscope(TEM),operating at 200 kV. 3 Reslllts and discussion Fig.1 shows the optical microstructure of the B2102 specimen annealed at 1 050 oc.The bright and dark areas were the austenite and the ferrite phases, respectively.The austenite phase was obviously elongated along the rolling direction.Volume fractions of the ferrite phase.tested by a ferrite detector.were 40.3%.consistent with the theoretical valHe calculated by the Thermo—Calc software(as shown in Fig.2).