eccc54微偏析在連鑄zst(零強度)和zdt(零延展)的解決效果

1、Solidification, segregation and high temperature behaviour Session 20 1Düsseldorf, 27 June – 1 July 2011The Effect of Solute Microsegregation on ZST and ZDT of Continuous Casting Steel LUO Sen, ZHU Miao-yong, JI C

2、heng (School of Materials and Metallurgy, Northeastern University, Shenyang 110004, Liaoning, China) Abstract: Accurate prediction of the effect of solute microsegregation on zero strength temperature (ZST) and zero du

3、ctility temperature (ZDT) is highly important for the optimization of continuous casting process. The characteristic temperatures, ZST and ZDT, were investigated by a mathematical model of solute microsegregation, in w

4、hich both the delta-ferrite/gamma-austenite phase transformation and MnS inclusion precipitation during the continuous casting process of steel were taken into consideration. According to the extensive comparison betwee

5、n the calculated results and the experimental data of ZST and ZDT, it is reasonable to conclude that the ZST and ZDT of continuous casting steel can be predicated by calculated temperature at solid fraction of 0.75 and

6、 1.0, respectively. MnS inclusion precipitation should be considered for the accurate prediction of ZST and ZDT. With the increase of C, the γ phase formation will accelerate the segregation of P and S, which will depr

7、ess the ZST and ZDT and increase the possibility of hot crack. 1 Introduction Continuous casting of steel, as the metal manufacture process with high efficient and energy saving, has a wide application in steel ind

8、ustry. To achieve the high quality of steel product, it is very important for the defects of stand occurring in the continuous casting process to be minimized. Furthermore, the technology of hot charging and the

9、 process of thin slab casting and direct rolling, make more strict for the quality control of continuously casting strand. During continuous casting of steels, both facial and internal cracks tend to occur in brittl

10、e temperature ranges due to the thermal and mechanical deformation [1, 2]. To prevent the occurrence of cracks, the better understanding on the mechanical properties of mushy zone of continuous casting steel is vital

11、. Zero strength temperature (ZST) and zero ductility temperature (ZDT), as the typical characteristic temperatures, have been widely studied. Shin et al[3] found that fs, the solid fraction, at ZST and ZDT are about

12、0.6-0.8 and near 1.0 for the in-situ solidifying Fe-C-1%Mn alloys, respectively. However, a different view was raised by Mizukami et al[4] on C-Cr steel, that fs for ZDT is 0.8, nearly the same as that for ZST. Moreo

13、ver, the ZST and ZDT have also been experimentally found to correspond to the temperature at fs=0.75 and 0.98 by Nakagawa[5], respectively. In a word, different researchers have different opinions, and at

14、present there is no common criterion for the determination of ZST and ZDT. In this paper, the regular dendritic structure developed by Ueshima et al[5,6] was adopted to simulate the solute microsegregation with the

15、consideration of delta-ferrite/gamma-austenite phase transformation and MnS inclusion precipitation. The ZST and ZDT were related to the solid fractions estimated by the proposed model at various steel compositions

16、 and cooling rate conditions. 2 Solute Microsegration Model 2.1 Solute Diffusion o P(a)L γ δ XN o n mi 1PM(b)Figure 1: Schematic of (a) the dendrite array morphology and (b) the transverse cross section assumed in the

17、 finite-difference simulation The microsegregations of solute elements were assessed by using the direct finite difference method proposed by the researchers of Nippon Steel[5,6]. The morphology of the growing den

18、drite array is schematically shown in Fig.1(a), and the transverse cross section of dendrites is assumed to be a regular hexagon approximately, one sixth of which is divided into 100 parts for calculation of interd

19、endritic solute diffusion as schematically shown in Fig.1(b). The following one-dimensional solute diffusion equation is solved in the triangular domain. ? ?, , , s i s i s i C C D T t x x? ? ? ? ? ? ? ? ? ? ? ? ?(1)

20、Neglecting interdendritic liquid motion, the mass conservation in control volume can be expressed as Solidification, segregation and high temperature behaviour Session 20 3Düsseldorf, 27 June – 1 July 2011Table 2 Ac

21、tivity interaction parameters at 1873K[11] j i e (j) C Si Mn P S Cr Mn -0.0538 -0.0327 0 -0.06 -0.048 0.0036 S 0.112 0.063 -0.026 0.29 -0.028 -0.011 3 Results and Analysis 3.1 ZST and ZDT Prediction D

22、uring the solidification of steel, the temperatures where the steel commencing affording tension and deformation are ZST and ZDT, respectively. Between ZST and ZDT, the steel can afford a certain tension, but the int

23、erdendritic liquid film is detrimental to the ductility of steel, so the inevitable thermal stress due to the variable heat transfer within the mould, temperature gradients within strand and effects of cooling water

24、sprays, phase transformation stress, and mechanical stress are prone to make crack initiate and propagate. In order to predict the ZST and ZDT, a solute microsegregation model with consideration of MnS inclusion prec

25、ipitation during the continuous casting of steel was proposed as above section described. Many studied steel are chosen to compare with the calculation by proposed solute microsegregation model, where the steel compo

26、sition and cooling rate in literatures are used as input data. The calculated temperature at the solid fraction of 0.75 and 1.0 are compared with the reported ZST and ZDT as shown in Fig.3 and Fig.4, respectively. Fi

27、g.3 obviously shows that the estimated temperate at the solid fraction of 0.75 matches the measurement ZST very well. Meanwhile, although ZDT data scatters on both side of line, most of estimated temperatures match t

28、he measurement very much as shown in Fig.4. So it is also reasonable to predict ZDT with estimated temperature at the solid fraction of 1.0. 1300 1350 1400 1450 1500 1550 130013501400145015001550Measurement:ZSTNakagaw

29、a [5] Suzuki [16]Schmidtmann [12] Shin [3]Adams [14] Measurement/ 0CEstimated temperature at fs=0.75/ 0CFigure 3: Comparison between temperature calculated by the present model at fs=0.75 and Zero Strength T

30、emperature (ZST) measured in different literatures 1250 1300 1350 1400 1450 1500 1550 1250130013501400145015001550Measurment:ZDTNakagawa [5] Seol [15]Schmidtmann [12] Suzuki [16]Weinberg [13] Bleck [17]Adams

31、 [14] Shin [3]Mesurement/ 0CEstimated temperature at fs=1.0/ 0CFigure 4: Comparison between temperature calculated by the present mode at fs=1.0 and Zero Ductility Temperature (ZDT) measured in different litera

32、tures 3.2 Influence of MnS Inclusion precipitation The effect of S on the liquidus, ZST and ZDT and the content of MnS inclusion precipitation during the solidification of steel are shown in Fig.5 and Fig.6, respect

33、ively. It is clear that as the content of S is blow 0.03%, whether the solute microsegregation model considers MnS inclusion precipitation or not, the estimated liquidus and ZST are almost the same. Whereas, the diff

34、erence of ZDT predicted by the solute microsegregation model is prominent, and the effect of MnS inclusion precipitation is not clear. The ZDT predicted by the solute microsegregation model without considering of the

35、 MnS inclusion precipitation is much lower than that predicted by the solute microsegregation model considering the MnS inclusion precipitation. From Fig.6, it is obvious that there is not MnS inclusion precipi

36、tation, because the concentration product of Mn and S in molten steel is below the equilibrium concentration product during the solidification of steel. So the segregation of S plays a crucial role in depressing

37、the ZDT. However, when the content of S is above 0.007%, the content of MnS inclusion precipitation increases with the increase of nominal content of S, so the segregation of S in interdendritic liquid is restrained

38、by MnS inclusion precipitation during the solidification of steel, and the ZDT drops slowly with the increase of nominal content of S. But the ZDT estimated by the solute microsegregation model without considering of

39、 the MnS inclusion precipitation drops dramatically with increase of nominal content of S because of the severe segregation of S in interdendritic liquid at the final stage of solidification. It is necessary to consi