Research on segregation control for round billet of anti-sulfur steel

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1)Research Institute,Baoshan Iron & Steel Co.,Ltd.,Shanghai 201999,China;2)Tube,Pipe and Bar Business Unit,Baoshan Iron & Steel Co.,Ltd.,Shanghai 200940,China;3)School of Metallurgy,Northeastern University,Shenyang 110819,Liaoning,China

Abstract: The carbon segregation that occurs in a round billet leads to instability in the anti-sulfur steel pipe.The maximum difference in the C content of these billets can reach 0.08%,and the equiaxed grain ratio is about 37.0%.In this paper,reasonable casting and mixing parameters were obtained by a study of the casting process,mold electromagnetic stirring,and the final electromagnetic stirring process.First,a mathematical model was established for the solidification and heat transfer of round-billet continuous casting using the characteristics of the continuous-casting process for sulfur-resistant steel pipes.The relationship between the casting speed,cooling-water ratio,and thickness of the shell at the final stirring position was analyzed.Then,the electromagnetic force and the liquid steel flow velocity were simulated and used to obtain reasonable parameters for the mold and final electromagnetic stirring.Through optimization of the casting and electromagnetic stirring technologies,the equiaxed grain ratio of the continuous-casting round billet increased to 53.4% and the maximum difference in the C content of the billet reduced to 0.031%.

Key words: round billet; segregation; electromagnetic stirring

1 Introduction

As oilfield exploitation has expanded into the oceans and harsher areas,API-standard oil casing pipes are unable to meet the requirements of these new oilfields.Therefore,to further develop the API-standard oil casing pipes,non-API-standard oil casing pipes of various steel grades have been successively developed to meet the oilfield require-ments.The BG110S developed at Baosteel is a non-API anti-sulfur product,but the anti-sulfur perform-ance of the resulting pipe is unstable.Analyses show that the carbon segregation in the BG110S round billets is very large.Electromagnetic stirring(EMS) is an effective method for improving the segregation in round billets.The essence of EMS is to change the flow of the molten steel in the solidification process to ultimately improve the quality of the product.In the actual production process,the metallurgical effect of EMS is influenced by many factors,including superheating,casting speed,stirring position,stirring strength,and steel grade,which make this a system problem closely related to the equipment and manu-facturing process[1].In the present work,research on EMS was performed by numerical simulation to analyze the heat transfer and solidification of the billet,and to simultaneously calculate the magnetic induction intensity in the billet to obtain the parameters of the stirring process[2-3].

For this study,a BG110S anti-sulfur round billet was produced using the original casting process and the EMS process was found to have a C segregation of 0.08% and an equiaxed grain ratio of 37.0%,as shown in Figs.1 and 2.The distribution of the C content in the radial direction of the round billet is shown in Fig.1,where the highest C content in the cross section of the round billet occurs at the junction of the equiaxed and columnar grains.The C content is the second highest in the center of the slab and the lowest at the edge of the slab.There-fore,reducing the C content at the boundary be-tween the equiaxed and columnar grains is the key to reducing the C segregation in the round billet.

Fig.1Carbondistributiononthecrosssectionoftheroundbillet

Baosteel usesφ178 round billet to produce BG110S anti-sulfur pipes,but soft reduction technol-ogy cannot reduce the segregation problem in the round billet.In the round-billet continuous-casting process,low-temperature casting,a small section,high cooling strength,and EMS are generally used to reduce the segregation of slab elements.Of these,mold electromagnetic stirring(MEMS) and the final electro-magnetic stirring(FEMS) can change the melt flow condition of the dendrites by controlling the liquid steel flow,and thereby alleviate or eliminate internal segregation.The appropriate EMS method can facili-tate the uniform distribution of the liquid steel components,which makes selective crystallization more difficult and less likely to occur.Moreover,EMS can reduce element segregation by increasing the equiaxed grain ratio of the round billet and reducing the width of the columnar grains region[4-6].Subject to equipment and capacity limitations,EMS is the best method for reducing segregation in the round billet in the current casting process.In this paper,the heat-transfer process in round-billet continuous casting was first studied to obtain suitable casting-process par-ameters for EMS,and then the EMS parameters were optimized based on the electromagnetic strength of MEMS and the FEMS and steel flow velocity.Finally,the C segregation in the round billet was reduced by the optimized EMS process.

2 Heat-transfer analysis of the round-billet casting

In the continuous-casting process of the round billet,the transformation from liquid steel in the mold to a steel billet is a heat-transfer process.Using numerical simulation analysis,a mathematical model was established for the solidification and heat transfer of round-billet continuous casting according to the characteristics of the continuous-casting pro-cess.The effects of casting speed and the secondary cooling-water flow ratio on the thickness and final position of the solidification end of the round billet were studied.Of the casting parameters,the casting speed has the greatest influence on the thickness of the round billet.Fig.3 shows the variation in the thickness of the round billet with the meniscus dis-tance at different casting speeds.The results reveal that as the casting speed increases,the thickness of the round billet decreases,and its influence on the thickness of the round-billet shell is very obvious.For each 0.1 m/min increase in the casting speed,the thickness of the round-billet shell at the FEMS is reduced by 3.5 mm.As the thickness of the shell is reduced,the position of the final solidification end also shifts back.The position of the final solidi-fication end increases by about 0.35 m for each 0.1 m/min increase in the casting speed.

Differences in the secondary cooling-water flow ratio also affect the thickness of the round-billet shell and the position of the final solidification end.As shown in Fig.4,the effect of the secondary cool-ing-water flow ratio on the thickness of the round-billet shell does not change much in the early stage of the secondary cooling zone,but is more obvious after the fourth stage of the secondary cooling zone because the caster has a higher temperature after the mold,which weakens the effect of the secondary cooling-water flow ratio on the surface temperature of the round billet;thus the variation in the thick-ness of the shell is small.In the later stage of the secondary cooling zone,the return temperature is low,and the influence of the change in the secondary cooling-water flow ratio on the thickness of the shell is more obvious.As the secondary cooling-water flow ratio of the secondary cooling zone increases,the final solidification end moves for-ward by about 0.15 m for each 0.05 L/kg increase in the secondary cooling-water flow ratio.

To ensure effective stirring in the FEMS,the solid fraction of the round billet at the FEMS position must be <90%,and the diameter of the liquid core must be 50 mm.Through numerical simulation analysis and optimization of the casting parameters based on the metallurgical criteria,the desired solid fraction of the round billet at the FEMS position can be reached.Specifically,when the casting speeds were 1.9 and 2.0 m/min,and the secondary cool-ing-water flow ratio was 0.35 L/kg,solid fractions of the round billet of 87.8% and 85.1%,and liquid core diameters of 60.4 and 73.0 mm were obtained,which met the FEMS requirements.

3 Research on MEMS of round billet

To be able to determine the optimum stirring parameters for MEMS,a numerical simulation of MEMS was performed.Fig.5 shows the axial distribution of the magnetic induction intensity at the center of the MEMS.The horizontal axis is the distance from the top of the mold on the center line of the MEMS,and the longitudinal axis is the mag-netic induction value at each point of the center line of the MEMS.The maximum value of the magnetic induction is at 0.63 m from the top of the mold.

The fluidity of the liquid steel in a round billet derived by EMS is directly related to the segre-gation that occurs in the round billet.In Fig.6,it can be seen that the flow velocity of the liquid steel in the mold increases with the increases in the current.When the current is 270 or 300 A,the flow velocity of the liquid steel decreases with increases in the frequency.When the current is 350 A,change in the flow velocity with frequency is not obvious.When the current is 400 A,the flow velocity of liquid steel increases first and then decreases.

To ensure the desired effect of MEMS,the flow velocity of liquid steel should preferably be between 0.5 and 1.0 m/s.Based on the relationship between the equiaxed grain ratio and the stirring strength,the stirring strength should be as high as possible with-out causing excessive stirring,which contributes to the improvement of the central equiaxed grain ratio.It can be seen from Fig.6 that the flow velocity of the liquid steel in the mold is between 0.8 and 0.9 m/s when the current is 300 A and the fre-quency is 4 Hz,which is within a reasonable and suf-ficient range.As such,a current of 300 A and fre-quency of 4 Hz are reasonable parameters for MEMS.

4 Research on the FEMS of the round billet

The use of FEMS has a good effect on the segregation of the center component of the round billet.To analyze the influence of the thickness of the shell on the effect of the FEMS,the thickness of the shell and the stirring parameters were nu-merically simulated(as shown in Table 1).

Table 1 Parameters of FEMS

Fig.7 shows a radial distribution of the electro-magnetic force at the FEMS position with a current of 600 A at frequencies of 9,12,and 30 Hz,re-spectively.It can be seen from the figure that the electromagnetic force has a region where it decays relatively quickly from the edge to the inside.The width of the attenuation region is about 5 mm,and the attenuation gradient of this decrease in the electromagnetic force is large.The electromagnetic force has a higher value in the diameter range of 60 to 70 mm.Therefore,the liquid steel moves under electromagnetic force at the outermost portion of the liquid core,and then the inner liquid core moves as the liquid steel rotates.

Next,the flow velocities of liquid steel with diameters of 70 and 60 mm at the FEMS position were analyzed.When the diameter of the liquid core was 70 mm,the flow velocity curves of the liquid steel at the FEMS position at different currents with frequency are shown in Fig.8.It can be seen from the figure that the flow velocity of the liquid steel increases with increases in the current,and first increases and then decreases with increases in frequency.When the frequency is about 9 Hz,the flow velocity reaches the maximum.According to the empirical results,the flow velocity of liquid steel at the FEMS must be higher than 0.3 m/s to achieve effective stirring.Therefore,when the diameter of the liquid core is 70 mm,the frequency of the FEMS should be maintained at 9-12 Hz,and the current should be higher than 550 A.

When the diameter of the liquid core is 60 mm,the flow velocity curves of the liquid steel at the FEMS position with frequency at different currents are shown in Fig.9.It can be seen from the figure that the flow velocity of the liquid steel increases with increases in the current,and first increases and then decreases with increases in frequency.The flow velocity also reaches the maximum when the frequency is about 9 Hz.To ensure that the flow velocity of the liquid steel is higher than 0.3 m/s when the diameter of the liquid core is 60 mm,the frequency of the FEMS should be kept at 9-12 Hz and the current should be higher than 600 A.

The FEMS simulation results reveal that the max-imum stirring strength is about 9 Hz.When the liquid steel core diameter is 70 mm,the current should be higher than 550 A,and when the liquid steel core diameter is 60 mm,the current should be higher than 600 A.

5 Optimization of the EMS parameters

To determine the validity of the EMS process optimi-zation,a test was conducted of the continuous casting of aφ178 round billet of anti-sulfur pipe steel.Table 2 shows the main parameters of the continuous-casting process.

Table 2 Parameters of casting and EMS

Using the parameters listed in Table 2 for the casting tests,the results show that reducing the frequency during the MEMS and FEMS can signifi-cantly enhance the stirring of the liquid steel inside the round billet,thereby facilitating a uniform distribution of the C element and preventing segregation in the round billet.The maximum difference in the distribution of the C element is greatly reduced.Fig.10 shows the distribution of the C element in a section of the anti-sulfur steel round billet when using the optimized continuous-casting parameters.It can be seen from the figure that the C element distribution in the section of the round billet is relatively uniform,with a maximum difference in the C element of 0.031%.The corresponding central equiaxed grain ratio is 53.4%,as shown in Fig.11.

Fig.10Carbondistributiononthecross-sectionofroundbillet

6 Conclusions

(1) Through the mathematical simulation of the heat transferred during solidification in the round-billet continuous-casting process,it was found that the final solidification end shifted about 0.35 m for each 0.1 m/min increase in the casting speed,and the solidification end moved forward 0.15 m for each 0.05 L/kg increase in the secondary cooling-water flow ratio.

(2) When the casting speeds were 1.9 and 2.0 m/min,and the secondary cooling-water flow ratio was 0.35 L/kg,the solid fractions of the round billet at the FEMS position were 87.8% and 85.1%,respec-tively,and the liquid core diameters were 60.4 and 73.0 mm,respectively,which met the FEMS requi-rements.

(3) The simulation result showed that the optimal current was 300 A and the optimal frequency was 4 Hz for the MEMS,and the optimal frequency was about 9 Hz and optimal current was higher than 600 A for the FEMS.

(4) Using the optimized casting and EMS parameters,the equiaxed grain ratio of the anti-sulfur steel round billet increased to 53.4%,and the maximum difference in the distribution of the C element reduced to 0.031%.