鋼坯在輕壓下形成的內(nèi)裂紋外文翻譯（英文）

1、 Journal of University of Science and Technology Beijing Volume 15, Number 2, April 2008, Page 114 MetallurgyCorresponding author: Wenjun Wang, E-mail: wangustb@yahoo.com Also available onli

2、ne at www.sciencedirect.com © 2008 University of Science and Technology Beijing. All rights reserved. Formation of internal cracks in steel billets during soft reduction Wenjun Wang1, 2), Linxin Ning2), Raimund B

3、52;lte1), and Wolfgang Bleck1) 1) Institute of Ferrous Metallurgy, RWTH Aachen University, Aachen 52072, Germany 2) Shougang Group Research Institute, Beijing 100041, China (Received 2007-04-20) Abstract: To investigate

4、 the formation of internal cracks in steel billets during soft reduction, fully coupled thermo-mechanical finite element models were developed using the commercial software ABAQUS, also casting and soft reduction tests w

5、ere carried out in a laboratory strand casting machine. With the finite element models, the temperature distribution, the stress and strain states in the bil-let were calculated. The relation between internal cracks and

6、equivalent plastic strain, as well as maximal principal stress was ana-lyzed. The results indicate that tensile stresses can develop in the mushy zone during soft reduction and the equivalent strain nearby the zero ducti

7、lity temperature (ZDT) increases with decreasing solid fraction. Internal cracks can be initiated when the accumulated strain exceeds the critical strain or the applied tensile stress exceeds the critical fracture stress

8、 during solidification. © 2008 University of Science and Technology Beijing. All rights reserved. Key words: steel; soft reduction; finite element model; internal cracks [This work was financially supported by the D

9、eutsche Forschungsgemeinschaft (DFG) within the Collaborative Research Centre (SFB) 289.] 1. Introduction In the continuous casting of steels, centerline seg- regation and center porosity are common problems. The soft r

10、eduction technique can be applied to mini- mize segregation and porosity by reducing the strand near the final solidification region to compensate for solidification shrinkage. To investigate the effect of soft reduct

11、ion on the internal quality of high carbon steel billets, experimental trials were carried out at dif- ferent solidified shell thicknesses in the laboratory strand casting machine at the Institute of Ferrous Met- allur

12、gy of RWTH Aachen University. The results in- dicated that the internal quality of the billet has been improved by soft reduction with liquid core. The quantity and dimension of the center segregation area and center

13、porosity decreased and the width of the equiaxed crystal zone increased. However, in some billets with soft reduction, internal cracks were ob- served. These cracks are located midway between the surface and centerlin

14、e of the billets and perpendicular to the casting direction. According to Refs. [1-5], the formation of these cracks has a close relation with the strength and ductility of steel in the mushy zone be- tween the zero du

15、ctility temperature (ZDT) and the zero strength temperature (ZST). Strength and ductil- ity have small values in the mushy zone because of the existence of interdendritic liquid films at the grain boundaries. Therefor

16、e, a tensile deformation in this temperature range can cause the separation of den- drites and internal cracks will form when the strain exceeds a critical value [2]. The objective of this study is to detect the reaso

17、n for the internal crack formation in the billets with soft reduction. For this purpose, casting and soft reduction tests were carried out in a laboratory strand casting machine. In addition, 3D coupled thermo-mechan

18、ical finite element models were developed to calculate the temperature, stress and strain in the billet during soft reduction. The calculations were carried out using the commercial finite element software ABAQUS. Wi

19、th these calculations the correlation between internal cracks and deformation of the billet in the mushy zone 116 J. Univ. Sci. Technol. Beijing, Vol.15, No.2, Apr 2008 process, one quarter of the billet with an initi

20、al length of 240 mm was modelled. The geometry and mesh of the rolling model are shown in Fig. 2. In this model, the friction coefficient between the billet and the rolls was assumed to be 0.27, which is in agreement

21、 with the friction coefficient during hot rolling of high car- bon steels at the hot rolling temperature of 900-1200°C [7]. During soft reduction, the roll tem- perature changed approximately from 50°C at the

22、 en- try to 300°C at the exit of the roll gap. Therefore, in the rolling model, an average value of 160°C was chosen as the temperature boundary condition of the rolls during soft reduction. Fig. 2. Geomet

23、ry and mesh of the finite element model. The thermo-physical properties of the billet in the rolling model, such as density, Young’s modulus, thermal conductivity, specific heat capacity, thermal expansion coefficient

24、, change with temperature and are taken from Refs. [8-9]. The flow stress of billets from 600°C to ZDT was determined by hot compres- sive tests. At the temperatures exceeding ZDT, the material model proposed by

25、Lee and Kim was used to obtain the flow stress in the mushy zone [10]. In the roll gap, the billets are cooled down by the cold rolls through heat conduction, and a sharp tem- perature drop appears at the surface of t

26、he billets. The heat transfer coefficient between the billet and the rolls depends on the billet and roll temperature, the rolling speed, the thickness reduction, and the surface roughness of the billet and the rolls

27、. In the rolling model, the heat transfer coefficient was assumed to be 7000 W/(m2?K), which is in agreement with the values in Refs. [11-12]. In the area outside the roll gap, the heat is led away to the atmosphere

28、by radiation and convection from the billet surface. Therefore the approach proposed by Thomas [13] was used as follows to describe the heat flux at the billet surface. 4 4 1.33 b b ( ) 1.24( ) q T T T T σε ∞ ∞ = ? +

29、?qσ the Stefan-Boltzmann constant (5.67× 10?8 W/(m2?K?4)); and ε the emissivity of the billet, which was assumed to be 0.8 [14]. In addition, in the normal direction of all the sym- metry planes, there is neithe

30、r displacement nor heat transfer. The mechanical boundary condition of the rolls allows only a rotation around their axes. 4. Results and discussion To verify the validity of the finite element models, the calculated

31、 temperatures in the mould and at the surface of the billets are compared with the measured values in Figs. 3 and 4, respectively. In Fig. 3, differ- ent depths in the legend stand for different distances from the ext

32、ernal surface toward the internal surface of the mould along its radius. At these positions, the temperatures were measured with thermocouples and calculated with ABAQUS. The results indicate that the calculated and

33、measured mould temperatures agree well with each other. The maximum difference be- tween the measured and calculated mould tempera- tures is 6°C, which is 1.0% of the measured tempera- tures. The calculated surface

34、 temperatures of the bil- lets in Fig. 4 show a good agreement with the meas- ured values. Therefore the finite element models in this article can be used to calculate the solid fraction in the core of billets during s