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Contribution to mechanical metallurgy behaviour of steel during continuous casting
Journal of Materials Processing Technology 78 (1998) 90 – 94
Contribution to mechanical metallurgy behaviour of steel during continuous casting Z. Jons' ta a,*, A. Hernas b, K. Mazanec a a
VS& B-Technical Uni6ersity of Ostra6a, 17.listopadu 15, Ostra6a-Poruba, Czech Republic b Silesian Uni6ersity of Technology, Krasin´skiego 8, Katowice, Poland
Abstract The paper is devoted to the study of the mechanical metallurgy characteristics of the mushy zone. The basic parameters of the brittle temperature range (TB), the zero strength (ZST), the zero ductility temperature (ZDT) and the liquid impenetrable temperature (LIT) are used for explaining the possibility of cracking. This temperature range was defined in the following form: ZDT BTB BLIT. The strains induced in steel during continuous casting process are evaluated. The influence of sulphur content on strain induced in carbon steel is analysed. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Mushy zone; Brittle temperature range; Zero ductility temperature; Liquid impenetrable temperature
1. Introduction The mechanical properties of steels at high temperatures play a very important role in crack formation during continuous casting (CC). Especially, understanding the mechanical metallurgy behaviour of steel in the critical zone, the mushy zone, during the process of continuous casting (CC) forms the basis for achieving the high quality of CC products. The majority of cracks found in CC steels is known to be initiated in the mushy zone, which is characterized by very low plastic properties. A typical example for ductility – temperature curve (in situ solidified) is presented in Fig. 1. As shown in this figure, we can find two regions of low ductility in the dependence shown between reduction of area and the deformation temperature. The brittleness, detected at the highest testing temperature and laying near to the ‘solidus’ temperature, is responsible for the formation of cracks in CC products [1].
cal composition of steels, a different mechanism of embrittlement can be observed. Brimacombe [2] described in detail the temperature zones of reduced ductility of steel related to embrittling mechanisms. The microsegregation, given by the enrichment of interdendritic retained melt with phosphorus and sulphur and other harmful elements, is a cause of the minimum plastic properties determined at testing temperatures near to the solidus temperature. The enrichments reduce the local solidus temperature. The second minimum of reduction in area, found in the temperature range 500–1200°C, can be attributed to the precipita-
2. General characterization of brittleness parameters at high temperatures In relation to the temperature region and the chemi-
* Corresponding author. 0924-0136/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S0924-0136(97)00468-8
Fig. 1. A typical example for the ductility temperature curve in-situ solidified.
Z. Jons' ta et al. / Journal of Materials Processing Technology 78 (1998) 90–94
Fig. 2. The effect of the thermal history of the sample on the ductility curves [3].
tion processes of sulphides, phosphides, nitrides, carbides or carbonitrides in the austenitic grain boundaries. Among these precipitates the sulphides precipitate at the highest temperatures (1000 – 1200°C). By contrast, the precipitation of nitrides, carbides and/or carbonitrides is realized at lower temperatures. In this low temperature minimum, we can also range the formation of proeutectoid ferrite in the austenite grain boundaries. In some examples, it is necessary to take into consideration the effect of the phase transformation of d-ferrite into austenite and its superposing effect on the development of high temperature brittleness [2]. The thermal history of the test specimen can influence the mechanical properties achieved at the highest temperatures laying near to the solidus temperature. This may be because the microsegregation processes are dependent on the material (thermal) history. Fig. 2 shows the values of the reduction of area of steel with 0.35% C as a function of the deformation temperature. Four variants of the measuring technique were applied. One variant corresponds to the conventional heating of specimens at the test temperature. The other three variant were performed at the test temperatures which were attained after cooling to the given test temperature after different preliminary heating. The applied temperature of 1504°C is situated 12°C above the melting temperature of the steel investigated. In this case the specimens solidify in situ in the test machine. In the first case mentioned, the test specimens broke on heating. In the other three cases, the test specimens were broken on cooling from temperatures of 1440, 1476 and 1504°C, respectively [3]. On the basis of results presented, we can conclude that the specimens heated at 1440°C and cooled down from this temperature attain the values of reduction of area corresponding to the values of this ductility parameter determined in the case of ‘direct’ heating (the first variant). The interdendritic segregations can be partially reduced during reheating and cooling. The lowest temper-
91
ature of zero ductility was found in the case of specimens heated at 1504°C and solidifying in the test machine. We propose that the best fitting of ductility– temperature curves to the CC process may be obtained after application of this test technique [1]. For this reason, it is very important to analyse the relationship between stress and strain at the highest temperatures corresponding to the solidification of steel during the CC process and to the behaviour of the mushy zone and/or of the solidifying shell. Many cracks observed in CC steel are known to be formed in the mushy zone of low ductility. They originate and propagate along the interdendrites in the mushy zone, except for the transverse cracks [4]. The ductility loss in material in mushy zone is associated with the microsegregation of solute elements at solidifying dendrite interfaces. The solidus temperature is reduced and the steel matrix is susceptible to brittle fracturing. The object of this study is to contribute to the explanation of the deformation behaviour of the mushy zone, based on the analysis of the physical and/or mechanical metallurgy parameters influencing the brittleness of this zone.
3. Physical metallurgy behaviour of solidifying shell The mechanical properties achieved in the high temperature zone of reduced ductility and the corresponding presentation of the solid–liquid interface during casting are summarized in Fig. 3. The interdendritic weakness is a cause of the separation of dendrites under a tensile strain applied to the mushy zone [4]. As follows from the analysis presented in [5], a very important parameter influencing the level of mechanical properties achieved in the solidifying shell (in the mushy zone) is the volume fraction of the solid matrix in comparison with the volume fraction of the liquid phase. In the case of a sufficient volume fraction of
Fig. 3. The mechanical properties of carbon steel in the high temperature range.
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D Fig. 4. The relationship of f A s and f s to temperature (0.13 wt.% C steel [5]).
surrounding liquid, the possibility of refilling the crack generated in the solid matrix is increased. The limit for effective crack refilling is 90% volume fraction of solid matrix—0.9 fs [4]. Under these conditions, the liquid reaches an impenetrable stage. The dendritic arms are compacted closely together to resist feeding the liquid and the generated cracks in the solid matrix are not refilled. The corresponding temperature is called the ‘liquid impenetrable temperature’ (LIT). When the steel is fully solidified ( fs =1), the dendrite liquid film is removed and the susceptibility to crack formation is reduced. The zero ductility temperature (ZDT), presented in Fig. 3, corresponds to fs = 1 [4]. Therefore the brittle temperature range TB may be defined as ZDTB TB BLIT. In addition to this analysis, it is necessary to take into account the effect of the chemical composition and the development of the segregation processes in the solidifying matrix. These processes influence the solidus temperature and simultaneously modify the volume fraction of the solid matrix dependent on temperature. Beside this effect, the mutual relationship between the D fraction of austenite ( f A s ) and d-ferrite ( f s ) play a very important role in the solidifying processes. The value of fs expresses the solid weight fraction in the solid plus liquid phase. The applied fractions are defined in the D A A following form [5]: fs =f D +f A; f D s =f /fs and f s = f / D A fs, where f is the d-ferrite fraction and f is the austenite fraction in the solid plus liquid phase. and f A The relationship between f D s s is important especially if the phase transformation of d-ferrite into austenite occurs in the brittle temperature range. In this connection, we find very interesting the relationship A between f D s and f s on one side and the value of fs on the other side at a given temperature [5]. The d-ferrite transformation into austenite is completed at f A s = 1. Fig. 4 shows an example of the computed relationships D of the three values mentioned above ( fs; f A s ; f s ) as was determined for a carbon steel with 0.13 wt.% C under non-equilibrium conditions. For comparison, Fig. 5
depicts the same relationships which were found in the case of a carbon steel with 0.27 wt.% C. In the steel with 0.13 wt.% C, the transformation of d-ferrite into austenite occurred at the final stage of solidification. In the steel with 0.06 wt.% C, the phase transformation occurred after solidification was completed [5]. In the steel with 0.27 wt.% C, the phase transformation occurred during the solidification due to a peritectic reacD tion (Fig. 5). Knowing f A s and f s are important in the investigation of the thermomechanical properties of carbon steel because d-ferrite is about 4.4 times weaker than austenite [6]. On the basis of the results presented, we can conclude that at low carbon concentration (CB 0.13 wt.%) the phase transformation d-ferrite–austenite takes place after full solidification of the steel. At a carbon concentration higher than approximately 0.13 wt.%, when the peritectic reaction occurs, the phase transformation (d-ferrite–austenite) takes place during solidification. The solid weight fraction fs and the fractions f D s and f A s in the solid phase were calculated as a function of temperature for the applied carbon content at a cooling rate of 0.17 K s − 1 and a dendritic arm spacing of 700 mm [5,7]. In summary, we can use the parameters ZDT and LIT as decisive characteristics determining the susceptibility to the brittleness of the mushy zone. The zero strength temperature (ZST) corresponding approximately to 0.7 fs is less important from the point of view of cracking (at 0.7 fs the volume fraction of the liquid is sufficiently high and the possibility of crack refilling with liquid is real) as can be seen from Fig. 3 [4].
4. Influence of strain induced in mushy zone on crack formation Strain induced in the brittle temperature range during the CC process of steel could be a very important parameter of crack generation. The total strain ot may
D Fig. 5. The relationship of f A s and f s to temperature (0.27 wt.% C steel [5]).
Z. Jons' ta et al. / Journal of Materials Processing Technology 78 (1998) 90–94
Fig. 6. The relationship of induced strains on the carbon content.
be expressed as the sum of two components, o and o’. The first component o is the strain induced in the brittle temperature range by variation of temperature and the second component o’ is the strain induced by external operating factors [4]. In order to study the effect of the steel composition on the crack formation only o must be taken into account. This strain component can be expressed as the sum of the strain generated during the cooling process and during the phase transformation of d-ferrite into austenite. The diagram of the relationship between the induced strain and the carbon content in steel is presented in schematic form in Fig. 6. In the case of the carbon content laying between the zero and A, the induced strain o1 is only a function of the thermal contraction because the d-ferrite – austenite phase transformation occurs after full solidification of the steel. In the case of a carbon content limited to the points A and B (Fig. 6), the phase transformation occurs in the brittle temperature range corresponding to the position between ZDT and LIT (approximately 0.9 fs). In this case, the strain is expressed as the sum of o1 and o2. The induced strain is only a function of the thermal contraction if the carbon content lies between the points B and C. The high temperature phase transformation (d-ferrite– austenite) is completed before reaching the stage of solidification corresponding to 0.9 fs (LIT). The positions of the both the characteristic temperatures ZDT and LIT and the temperature of transformation of d-ferrite into austenite are dependent on the content of solute elements in the steel and on their equilibrium distribution coefficient k between d-ferrite and austenite. The elements such as silicon, phosphorus and sulphur, for which k B1, are predominantly redistributed from austenite to d-ferrite, while carbon and manganese (with k \ 1) enrich the austenite matrix [8]. The effect of this redistribution of solute elements on the position of the brittle temperature range is important because the influence of the solute elements on the
93
particular critical temperatures during the solidification process is different. For example, in the case of different sulphur content, it was observed that its effect on LIT and ZST (approximately 0.7 fs) is not significant. On the contrary, the effect of the sulphur content on ZDT is significant. The effect of the sulphur content is seen to increase with the increase of content of this element in steel. In 0.3 wt.% carbon steel, the difference between the LIT and ZDT (brittle temperature range) increased for the applied sulphur content (0.01–0.08 wt.%) from approximately 25°C (0.01 wt.% S) up to 80°C (0.08 wt.% S). Simultaneously, the LIT decreased only approximately 15°C over the above mentioned range of sulphur contents. The relationships for LIT and ZDT are presented in Fig. 7. The brittle temperature range extends to the lower temperature and the possibility of cracking is expected to increase with increasing sulphur content [7]. The strain induced in the brittle temperature range increases with increasing sulphur content because the ZDT is drastically reduced at a higher sulphur content in steel due to the segregation of sulphur at the final stage of the solidification process. This gives rise to an increase in the induced thermal strain [4]. In contrast, the effect of the sulphur content on the high temperature transformation (d-ferrite–austenite transformation) is not significant. The maximum strain generated in the brittle temperature range is shifted to a lower carbon content and the values of the strain peak occupy a larger range of sulphur content [4]. All the above mentioned effects result in a greater possibility of cracking in the brittle temperature zone corresponding to the mushy zone of the solidifying steel.
5. Conclusions This paper summarizes the mechanical metallurgy principles acting on the possible crack formation in carbon steels during the CC process. The mechanical metallurgy characteristics of the brittle temperature
Fig. 7. The ZDT and LIT of 0.3 wt.% carbon steel [4].
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range are explained on the basis of the behaviour of the mushy zone. The relationship between the critical temperatures of ZDT and of LIT plays a very important role from the point of view of cracking during the CC process. The strain generated in the brittle temperature range was evaluated (the strain induced by temperature variation and the strain connected with the d-ferrite– austenite phase transformation). Simultaneously, the superposed effects of the sulphur content on the modification of the strain generated in the brittle temperature range and on the susceptibility to cracking were evaluated. Although the solution presented has a preferential bearing upon the formation of longitudinal surface cracks, some of the above discussed mechanical metallurgy principles have a general validity from the point of view of influencing crack formation during the CC process of carbon steels.
.
Acknowledgements The authors are grateful to GAC& R for financial support (project 106/96/K 032).
References [1] K. Schwerdtfeger, Rissanfa¨lligkeit von Sta¨hlen beim Stranggiessen und Warmumformen, Verlag Stahleisen, Du¨sseldorf, 1994. [2] J.R. Brimacombe, Metall. Trans. 24B (1993) 917. [3] R.H. Phillip, M.L. Jordan, Metal. Technol. 4 (1977) 396. [4] K. Kim, T. Yeo, K.H. Oh, D.N. Lee, ISI Jpn. Int. 36 (1996) 284. [5] K. Kim, K.H. Oh, D.N. Lee, Scr. Mater. 34 (1996) 301. [6] P.J. Wray, Metall. Trans. 7A (1976) 1621. [7] G. Shin, T.K. Kajitani, T. Suzuki, T. Umeda, Tetsu to Hagane 78 (1992) 587. [8] Y. Ueshima, S. Mizoguchi, T. Matsumiya, H. Kajioka, Metall. Trans. 17B (1986) 845.
Contribution to mechanical metallurgy behaviour of steel during continuous casting Z. Jons' ta a,*, A. Hernas b, K. Mazanec a a
VS& B-Technical Uni6ersity of Ostra6a, 17.listopadu 15, Ostra6a-Poruba, Czech Republic b Silesian Uni6ersity of Technology, Krasin´skiego 8, Katowice, Poland
Abstract The paper is devoted to the study of the mechanical metallurgy characteristics of the mushy zone. The basic parameters of the brittle temperature range (TB), the zero strength (ZST), the zero ductility temperature (ZDT) and the liquid impenetrable temperature (LIT) are used for explaining the possibility of cracking. This temperature range was defined in the following form: ZDT BTB BLIT. The strains induced in steel during continuous casting process are evaluated. The influence of sulphur content on strain induced in carbon steel is analysed. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Mushy zone; Brittle temperature range; Zero ductility temperature; Liquid impenetrable temperature
1. Introduction The mechanical properties of steels at high temperatures play a very important role in crack formation during continuous casting (CC). Especially, understanding the mechanical metallurgy behaviour of steel in the critical zone, the mushy zone, during the process of continuous casting (CC) forms the basis for achieving the high quality of CC products. The majority of cracks found in CC steels is known to be initiated in the mushy zone, which is characterized by very low plastic properties. A typical example for ductility – temperature curve (in situ solidified) is presented in Fig. 1. As shown in this figure, we can find two regions of low ductility in the dependence shown between reduction of area and the deformation temperature. The brittleness, detected at the highest testing temperature and laying near to the ‘solidus’ temperature, is responsible for the formation of cracks in CC products [1].
cal composition of steels, a different mechanism of embrittlement can be observed. Brimacombe [2] described in detail the temperature zones of reduced ductility of steel related to embrittling mechanisms. The microsegregation, given by the enrichment of interdendritic retained melt with phosphorus and sulphur and other harmful elements, is a cause of the minimum plastic properties determined at testing temperatures near to the solidus temperature. The enrichments reduce the local solidus temperature. The second minimum of reduction in area, found in the temperature range 500–1200°C, can be attributed to the precipita-
2. General characterization of brittleness parameters at high temperatures In relation to the temperature region and the chemi-
* Corresponding author. 0924-0136/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S0924-0136(97)00468-8
Fig. 1. A typical example for the ductility temperature curve in-situ solidified.
Z. Jons' ta et al. / Journal of Materials Processing Technology 78 (1998) 90–94
Fig. 2. The effect of the thermal history of the sample on the ductility curves [3].
tion processes of sulphides, phosphides, nitrides, carbides or carbonitrides in the austenitic grain boundaries. Among these precipitates the sulphides precipitate at the highest temperatures (1000 – 1200°C). By contrast, the precipitation of nitrides, carbides and/or carbonitrides is realized at lower temperatures. In this low temperature minimum, we can also range the formation of proeutectoid ferrite in the austenite grain boundaries. In some examples, it is necessary to take into consideration the effect of the phase transformation of d-ferrite into austenite and its superposing effect on the development of high temperature brittleness [2]. The thermal history of the test specimen can influence the mechanical properties achieved at the highest temperatures laying near to the solidus temperature. This may be because the microsegregation processes are dependent on the material (thermal) history. Fig. 2 shows the values of the reduction of area of steel with 0.35% C as a function of the deformation temperature. Four variants of the measuring technique were applied. One variant corresponds to the conventional heating of specimens at the test temperature. The other three variant were performed at the test temperatures which were attained after cooling to the given test temperature after different preliminary heating. The applied temperature of 1504°C is situated 12°C above the melting temperature of the steel investigated. In this case the specimens solidify in situ in the test machine. In the first case mentioned, the test specimens broke on heating. In the other three cases, the test specimens were broken on cooling from temperatures of 1440, 1476 and 1504°C, respectively [3]. On the basis of results presented, we can conclude that the specimens heated at 1440°C and cooled down from this temperature attain the values of reduction of area corresponding to the values of this ductility parameter determined in the case of ‘direct’ heating (the first variant). The interdendritic segregations can be partially reduced during reheating and cooling. The lowest temper-
91
ature of zero ductility was found in the case of specimens heated at 1504°C and solidifying in the test machine. We propose that the best fitting of ductility– temperature curves to the CC process may be obtained after application of this test technique [1]. For this reason, it is very important to analyse the relationship between stress and strain at the highest temperatures corresponding to the solidification of steel during the CC process and to the behaviour of the mushy zone and/or of the solidifying shell. Many cracks observed in CC steel are known to be formed in the mushy zone of low ductility. They originate and propagate along the interdendrites in the mushy zone, except for the transverse cracks [4]. The ductility loss in material in mushy zone is associated with the microsegregation of solute elements at solidifying dendrite interfaces. The solidus temperature is reduced and the steel matrix is susceptible to brittle fracturing. The object of this study is to contribute to the explanation of the deformation behaviour of the mushy zone, based on the analysis of the physical and/or mechanical metallurgy parameters influencing the brittleness of this zone.
3. Physical metallurgy behaviour of solidifying shell The mechanical properties achieved in the high temperature zone of reduced ductility and the corresponding presentation of the solid–liquid interface during casting are summarized in Fig. 3. The interdendritic weakness is a cause of the separation of dendrites under a tensile strain applied to the mushy zone [4]. As follows from the analysis presented in [5], a very important parameter influencing the level of mechanical properties achieved in the solidifying shell (in the mushy zone) is the volume fraction of the solid matrix in comparison with the volume fraction of the liquid phase. In the case of a sufficient volume fraction of
Fig. 3. The mechanical properties of carbon steel in the high temperature range.
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D Fig. 4. The relationship of f A s and f s to temperature (0.13 wt.% C steel [5]).
surrounding liquid, the possibility of refilling the crack generated in the solid matrix is increased. The limit for effective crack refilling is 90% volume fraction of solid matrix—0.9 fs [4]. Under these conditions, the liquid reaches an impenetrable stage. The dendritic arms are compacted closely together to resist feeding the liquid and the generated cracks in the solid matrix are not refilled. The corresponding temperature is called the ‘liquid impenetrable temperature’ (LIT). When the steel is fully solidified ( fs =1), the dendrite liquid film is removed and the susceptibility to crack formation is reduced. The zero ductility temperature (ZDT), presented in Fig. 3, corresponds to fs = 1 [4]. Therefore the brittle temperature range TB may be defined as ZDTB TB BLIT. In addition to this analysis, it is necessary to take into account the effect of the chemical composition and the development of the segregation processes in the solidifying matrix. These processes influence the solidus temperature and simultaneously modify the volume fraction of the solid matrix dependent on temperature. Beside this effect, the mutual relationship between the D fraction of austenite ( f A s ) and d-ferrite ( f s ) play a very important role in the solidifying processes. The value of fs expresses the solid weight fraction in the solid plus liquid phase. The applied fractions are defined in the D A A following form [5]: fs =f D +f A; f D s =f /fs and f s = f / D A fs, where f is the d-ferrite fraction and f is the austenite fraction in the solid plus liquid phase. and f A The relationship between f D s s is important especially if the phase transformation of d-ferrite into austenite occurs in the brittle temperature range. In this connection, we find very interesting the relationship A between f D s and f s on one side and the value of fs on the other side at a given temperature [5]. The d-ferrite transformation into austenite is completed at f A s = 1. Fig. 4 shows an example of the computed relationships D of the three values mentioned above ( fs; f A s ; f s ) as was determined for a carbon steel with 0.13 wt.% C under non-equilibrium conditions. For comparison, Fig. 5
depicts the same relationships which were found in the case of a carbon steel with 0.27 wt.% C. In the steel with 0.13 wt.% C, the transformation of d-ferrite into austenite occurred at the final stage of solidification. In the steel with 0.06 wt.% C, the phase transformation occurred after solidification was completed [5]. In the steel with 0.27 wt.% C, the phase transformation occurred during the solidification due to a peritectic reacD tion (Fig. 5). Knowing f A s and f s are important in the investigation of the thermomechanical properties of carbon steel because d-ferrite is about 4.4 times weaker than austenite [6]. On the basis of the results presented, we can conclude that at low carbon concentration (CB 0.13 wt.%) the phase transformation d-ferrite–austenite takes place after full solidification of the steel. At a carbon concentration higher than approximately 0.13 wt.%, when the peritectic reaction occurs, the phase transformation (d-ferrite–austenite) takes place during solidification. The solid weight fraction fs and the fractions f D s and f A s in the solid phase were calculated as a function of temperature for the applied carbon content at a cooling rate of 0.17 K s − 1 and a dendritic arm spacing of 700 mm [5,7]. In summary, we can use the parameters ZDT and LIT as decisive characteristics determining the susceptibility to the brittleness of the mushy zone. The zero strength temperature (ZST) corresponding approximately to 0.7 fs is less important from the point of view of cracking (at 0.7 fs the volume fraction of the liquid is sufficiently high and the possibility of crack refilling with liquid is real) as can be seen from Fig. 3 [4].
4. Influence of strain induced in mushy zone on crack formation Strain induced in the brittle temperature range during the CC process of steel could be a very important parameter of crack generation. The total strain ot may
D Fig. 5. The relationship of f A s and f s to temperature (0.27 wt.% C steel [5]).
Z. Jons' ta et al. / Journal of Materials Processing Technology 78 (1998) 90–94
Fig. 6. The relationship of induced strains on the carbon content.
be expressed as the sum of two components, o and o’. The first component o is the strain induced in the brittle temperature range by variation of temperature and the second component o’ is the strain induced by external operating factors [4]. In order to study the effect of the steel composition on the crack formation only o must be taken into account. This strain component can be expressed as the sum of the strain generated during the cooling process and during the phase transformation of d-ferrite into austenite. The diagram of the relationship between the induced strain and the carbon content in steel is presented in schematic form in Fig. 6. In the case of the carbon content laying between the zero and A, the induced strain o1 is only a function of the thermal contraction because the d-ferrite – austenite phase transformation occurs after full solidification of the steel. In the case of a carbon content limited to the points A and B (Fig. 6), the phase transformation occurs in the brittle temperature range corresponding to the position between ZDT and LIT (approximately 0.9 fs). In this case, the strain is expressed as the sum of o1 and o2. The induced strain is only a function of the thermal contraction if the carbon content lies between the points B and C. The high temperature phase transformation (d-ferrite– austenite) is completed before reaching the stage of solidification corresponding to 0.9 fs (LIT). The positions of the both the characteristic temperatures ZDT and LIT and the temperature of transformation of d-ferrite into austenite are dependent on the content of solute elements in the steel and on their equilibrium distribution coefficient k between d-ferrite and austenite. The elements such as silicon, phosphorus and sulphur, for which k B1, are predominantly redistributed from austenite to d-ferrite, while carbon and manganese (with k \ 1) enrich the austenite matrix [8]. The effect of this redistribution of solute elements on the position of the brittle temperature range is important because the influence of the solute elements on the
93
particular critical temperatures during the solidification process is different. For example, in the case of different sulphur content, it was observed that its effect on LIT and ZST (approximately 0.7 fs) is not significant. On the contrary, the effect of the sulphur content on ZDT is significant. The effect of the sulphur content is seen to increase with the increase of content of this element in steel. In 0.3 wt.% carbon steel, the difference between the LIT and ZDT (brittle temperature range) increased for the applied sulphur content (0.01–0.08 wt.%) from approximately 25°C (0.01 wt.% S) up to 80°C (0.08 wt.% S). Simultaneously, the LIT decreased only approximately 15°C over the above mentioned range of sulphur contents. The relationships for LIT and ZDT are presented in Fig. 7. The brittle temperature range extends to the lower temperature and the possibility of cracking is expected to increase with increasing sulphur content [7]. The strain induced in the brittle temperature range increases with increasing sulphur content because the ZDT is drastically reduced at a higher sulphur content in steel due to the segregation of sulphur at the final stage of the solidification process. This gives rise to an increase in the induced thermal strain [4]. In contrast, the effect of the sulphur content on the high temperature transformation (d-ferrite–austenite transformation) is not significant. The maximum strain generated in the brittle temperature range is shifted to a lower carbon content and the values of the strain peak occupy a larger range of sulphur content [4]. All the above mentioned effects result in a greater possibility of cracking in the brittle temperature zone corresponding to the mushy zone of the solidifying steel.
5. Conclusions This paper summarizes the mechanical metallurgy principles acting on the possible crack formation in carbon steels during the CC process. The mechanical metallurgy characteristics of the brittle temperature
Fig. 7. The ZDT and LIT of 0.3 wt.% carbon steel [4].
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Z. Jons' ta et al. / Journal of Materials Processing Technology 78 (1998) 90–94
range are explained on the basis of the behaviour of the mushy zone. The relationship between the critical temperatures of ZDT and of LIT plays a very important role from the point of view of cracking during the CC process. The strain generated in the brittle temperature range was evaluated (the strain induced by temperature variation and the strain connected with the d-ferrite– austenite phase transformation). Simultaneously, the superposed effects of the sulphur content on the modification of the strain generated in the brittle temperature range and on the susceptibility to cracking were evaluated. Although the solution presented has a preferential bearing upon the formation of longitudinal surface cracks, some of the above discussed mechanical metallurgy principles have a general validity from the point of view of influencing crack formation during the CC process of carbon steels.
.
Acknowledgements The authors are grateful to GAC& R for financial support (project 106/96/K 032).
References [1] K. Schwerdtfeger, Rissanfa¨lligkeit von Sta¨hlen beim Stranggiessen und Warmumformen, Verlag Stahleisen, Du¨sseldorf, 1994. [2] J.R. Brimacombe, Metall. Trans. 24B (1993) 917. [3] R.H. Phillip, M.L. Jordan, Metal. Technol. 4 (1977) 396. [4] K. Kim, T. Yeo, K.H. Oh, D.N. Lee, ISI Jpn. Int. 36 (1996) 284. [5] K. Kim, K.H. Oh, D.N. Lee, Scr. Mater. 34 (1996) 301. [6] P.J. Wray, Metall. Trans. 7A (1976) 1621. [7] G. Shin, T.K. Kajitani, T. Suzuki, T. Umeda, Tetsu to Hagane 78 (1992) 587. [8] Y. Ueshima, S. Mizoguchi, T. Matsumiya, H. Kajioka, Metall. Trans. 17B (1986) 845.