Sulfur Control in Ultra-Low Sulfur Steel Refined byLadle Furnace-Vacuum Degassing

30/10/2013 2:20pm

Автор: Yang Jing-bo, Yang Shu-feng, Gao Xiang-zhou, Li Jing-she

Категории: steelmaking

Sulfur Control in Ultra-Low Sulfur Steel Refined by Ladle Furnace-Vacuum Degassing

 

Abstract:Sulfur is deleterious in most kinds of steelMany factors influence the sulfur content in steel. We have investigated how ladle furnace (LF) slag deoxidizers, LF dynamic conditions and Ca-treatment after Ruhrstahl-Heraeus treatment affect desulfurization. The (FeO+MnO) content can be controlled below 0.5% after the addition of more than 300kg aluminum particles and sulfur in the steel is reduced from 30 to 6ppm. When the bottom gas flowrate is 500NL/min, sulfur in the steel can be reduced to 6ppm after 10min. Too high or too low an argon flowrate reduces the desulfurization efficiency.Vacuum degassing followed byCa-treatment has no effect on desulfurization, but Ca input helpsmaintain a low sulfur content.
Keywords: Ultra-low sulfur steel; desulfurization; ladle furnace-vacuum degassing refining; Ca-treatment
Sulfur is deleterious in most kinds of steel. Itnot only causes hot brittleness and impacts ductility,toughness and weldability, but can also form MnS inclusions, which become extended and elongated in the rolling process and lead to an increase in mechanical anisotropy. To achieve good resistance to the corrosion by hydrogen sulfide, the sulfur content in steel must be controlled below 0.0020%. Refining by desulfurization of clean steel hasbecome the focus of many studies. High basicity refining of slag and calcium treatment can be used to control the sulfur content and reduce MnS inclusions.


1. Experimental methods

Ultra-low sulfur steel is produced bythe basic oxygen furnace-ladle furnace (LF)-Ruhrstahl-Heraeus-Ca-treatment-continuous casting method. The effect of (FeO+MnO) content in the LF slag, the kinetic conditions for LF refining and vacuum degassing (VD)with late calcium treatment on desulfurization were studied. Five industrial trials were carried outto determine the effect of (FeO+MnO) content in the LF slag on sulfur content. Inthe LF unit process, 100, 200, 250, 300 and 320 kg of aluminum particles were added into the furnace. Twenty five steel samples and 25 slag samples were taken before refining which lasted 10, 20, 30 and 45 min. After determining the optimal amount of aluminum particles required, a furtherfive trials were carried out. Fifteen steel samples and 15 slag samples were taken at the LF refining slagstage withargon inflow at the bottom of the furnacefor 5 and 10 min. The optimum input of aluminum and argon flowin the LF process was determined using these trials.Fourteen steel samples from sevensteel trials were taken before and after VD. The elemental content in the steel samples and chemical composition of the slag samples were analyzed.
The basicity of the LF slag is high and up to 9.9 on average during production. The average massratio of CaO/Al2O3 is 2.0.The composition of molten steel produced by the converter is shown in Table 1.The composition of the LF slag produced by LF refining is shown in Table 2.

Table 1.Molten steel composition (%)


Elements in steel

C

Si

Mn

P

S

Als

Content / %

0.18

0.26

0.54

<0.17

0.0025–0.0035

0.020–0.350

Table 2. LF slagcomposition (%)


Composition

CaO

SiO2

MgO

TFe

Al2O3

MnO

P2O5

S

R

Maximum

57.293

10.285

10.169

4.024

36.252

0.267

0.068

0.300

23.056

Minimum

46.347

2.209

4.831

0.379

24.034

0.010

0.012

0.198

5.176

Average

52.583

6.076

7.247

0.782

28.863

0.041

0.019

0.250

9.899

2. Results and Discussion
2.1 Effect of (FeO+MnO) content in LF slag on desulfurization

The oxygen content in the slag depends on the(FeO+MnO)content in the slagand(FeO+MnO) content alsodesulfurization is limited andincreases thedesulfurization time. The relationship between (FeO+MnO) content and distribution ratio of sulfur (Ls) in the initial slag is shown in Fig. 1.The ability of the slag to be oxidized has an obvious influence on the desulfurization efficiency of molten steel. The lower the (FeO+MnO) content in the initial slag, the higher theLs.To limit desulfurization, the (FeO+MnO) content in the initial slag should be below 1%.
A chemical analysis was conducted on the slag samples taken afterLF refining with different aluminum particle input masses.Results from the analysis of (FeO+MnO) content are shown in Table 3.If the initial (FeO+MnO) content is below 0.5%, the desulfurization rate increases. The addition of a certain amount of deoxidizer in the LF refining process can reduce the FeO content in the slag. In this study,aluminum was selected as the deoxidizer.
Results from the experiments investigatingthe change of sulfur content in molten steel from different aluminum particle input masses are shown in Fig.2.To effect desulfurization and to achieve sufficient reactionbetween the deoxidizer and oxide, aluminum particles were added into the slag on average three times. As shown inFig.2, with the addition of 300kg of aluminum particles, the sulfur content in the molten steel after refining for 45min was reduced from the original 29to 7.9ppm and from 30 to 6 ppm with the addition of 320kg.


Table 3  (FeO+MnO) composition in LF slags with different Al input masses (%)


Al input /kg

100

200

250

300

320

(FeO+MnO) / %

1.01

0.77

0.52

0.39

0.46

Relationship between sulfur in molten steel and argon flow
Fig. 1. Relationship between sulfur ratio and

Fig. 2.Relationship between sulfur in molten steel and aluminum particle input in slag


2.2 Effect of dynamic conditions on desulfurization

Fig. 2 also shows that the sulfur content in is reduced rapidly over 30 minfromthe beginning of LF refining.The sulfur content in the molten steel is reduced slowly between 30 to 45 min. When the sulfur content in molten steel is reduced to a certain concentration, the diffusion of [S] in the molten steel is a restrictive step in the desulfurization reaction .Thekinetic conditionsfordesulfurization need to be improved to obtain further desulfurization.
To study the effect of the dynamic conditions on desulfurization, some steel samples were taken after LF refining slag melting ,argon inflow at the bottom of the furnace for 5 and 10 min to determine the sulfur content in molten steel. Aluminum particles (320 kg) were added in the LF refining process. To study the effect of argon flow on desulfurization during gas stirring, the sulfur content in molten steel was analyzedat different argon flowrates, as shown in Fig.3.
In the trials, temperatures were chosen atwhich the sulfur content in the molten steel differs little before argon stirring . Argon flowrates of 400, 500, 600, 700 and800 NL/min were selected. The sulfur content in the molten steel is reducedrapidly inthe 5 min before argon stirring andis reduced more significantlyduringheatingwhen the initial sulfur content is high.After stirring for some time and with an argon flow for 5 and 10 min,the sulfur content in the molten steel is reduced slowly and decreases to approximately 2 ppm. The results shown in Fig. 3 indicate that when the argon flow is 400 NL/min, the sulfur content in the molten steel is reduced theleast. The results shown in Table4 indicate that when the argon flow is 500 NL/min, the sulfur content in the molten steel is reducedby 62.50% from 16to 6ppm.When the argon flowrate increases,higher quantities of largebubbleswill be produced. An increased argon flow facilitates the formation of abare slag surface inmolten steel, which leads touptake by the molten steel and reduces the desulfurization efficiency. A large flow rate can cause the turbulence of molten steel, slag entrapment,severe erosion of the ladle lining and anincrease in the quantity and size of inclusions. Too high or too lowan argon flowratenegatively affects the desulfurization of molten steel under dynamic conditions. In this study, theargon flowrate is 500 NL/minas the sulfur content in the molten steel can be reduced from 16 to 6ppm.

Table4 Relationship between argon flowrateand desulfurization efficiency


Argon inflow at furnace bottom / NL/min

400

500

600

700

800

Reduction in sulfur content/%

30.71

62.50

50.00

52.94

14.06


Fig.3.Relationship between sulfur in molten steel and argon flow

2.3 Relationship between calcium treatment and sulfur content

After the ultra-lowsulfur steel has been treated by VD, the molten steel is fed with silicon-calcium wire to the calcium treatment stage. At this point, the temperature of the molten steel is approximately 1580°C.The [Ca]-[S] reaction calculated at 1580°C is given below.
There is a balance between the [Ca] and[S] in the molten steel:
[Ca]+[S]=CaS (s); (1)
; (2)


.; (3)

 When,.; (4)
The activity interaction coefficient in literature was used to calculatethe activity coefficient of the [Ca] solution in steel.


        (5)
, , , ,,
                                       (6)
,, ,,
where,  and .When the temperature is 1873K, .Because the value changes little at the steelmaking temperature, it is assumed to be constant.
The [Ca]-[S] balance(Sulfur content in molten steel after Ca-treatment)is shown in Fig.4 at 1580°C.
Sulfur content in molten steel after Ca-treatment"
Fig. 5.Sulfur content in molten steel after Ca-treatment


The seven pointsin Fig.4 show the sulfur content before feeding with silicon-calcium wire and the calcium content after feeding calcium with the molten steel in a different furnace. These points are located below the [Ca]-[S] equilibrium line at 1580°C.This indicates that calcium has no desulfurization ability. Fig.5 shows the sulfur content in molten steel after Ca-treatment and feeding with silicon-calcium wire. The sulfur content in the molten steel is essentially invariant and allare below 10ppm except for the sample from the seventhtrial. Feeding calcium with the molten steel helps retain the calcium content in the steel, inhibitsresulfurization and keeps the sulfur content low.

3. Conclusions

  1. The ability of the slag to be oxidized has an obvious influence on the desulfurization efficiency of molten steel. The lower the (FeO+MnO) content in the initial slag, the higher the Ls. To obtain desulfurization, the (FeO+MnO) content in the initial slag should be below 1%.
  2. When the argon inflow at the bottom of the furnace is 500 NL/min, the sulfur content in the steel is reduced to 6 ppm after 10 min. Too high or too low an argon flowrate will reduce the desulfurization efficiency.
  3. The [Ca] content in the molten steel is under the [Ca]-[S] equilibrium line when the steelmaking temperature is 1580°C. Under these conditions, calcium has no desulfurization ability. Addition of calcium into the molten steel is useful forinhibitingresulfurization and helps maintain the sulfur content at a low level.

Acknowledgments

The authors are grateful for support from the National Science Foundation China(grant no.51304016)

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