Improvement of inclusion control technology in 304 stainless steel

Transcription

1 SEAISI 2017 Conference 1 Improvement of inclusion control technology in 304 stainless steel Masaki Ando Production Engineering Headquarters. Aichi Steel Corporation.

2 The TOYOTA Group 2 Towa Real Estate Co.,Ltd. Toyota Central RDL Inc. Kanto Auto Works Ltd Toyoda Gosei Co.,Ltd 1949 Toyoda Boshoku Corporation 1950 Denso Corporation 1949 Toyota Industries Corporation 1926 Toyota Motor Corporation 1937 Aichi Steel Corporation 1940 Aishin Seiki Co.,Ltd. Toyota Tsusho Corporation Toyota Auto Body Co.,Ltd Toyoda Machine Works,Ltd. 1941

3 Plant and Sales Office 3 Head office & Plant Sales offices Head Office Chita Plant Forging Plant Nagoya Kariya Plant Tokai Kariya Toyota Hiroshima Nagoya Tokyo Higasiura Plant Fukuoka Osaka Fig. Domestic Bases of manufacturing & Sales offices

4 Steel-Making Process and Products 4 140t BL/CC Line 60t BT/CC Line 140tEAF LF/RH BL/CC 60tEAF AOD/LF BT/CC Specialty steel Stainless steel Total 100,000 t/m Stainless Steel 10,000 t/m Stainless Steel 12,000 t/m Ferrite, Martensite 13Cr, 16Cr 10% Etc. 5% Stractual Steel 90,000 t/m Case hardening Free cutting Bearing Spring etc. Austenite 85% 18Cr-8Ni, 16Cr-12Ni-2Mo

5 Outline 5 1. Introduction 2. Improvements of Secondary Refining 2-1) General Grade 2-2) Cleanliness Grade 3. Conclusion

6 Outline 6 1. Introduction 2. Improvements of Secondary Refining 2-1) General Grade 2-2) Cleanliness Grade 3. Conclusion

7 1-1) Conventional inclusions (SUS304) 7 SiO 2 Silicate type inclusion 実線 1600 液相線 5μm Spinel type inclusion 5μm MgO Al 2 O 3 Fig.1 Conventional composition of inclusions (SUS304). 1) Variation of inclusions composition is very large. 2) Inclusion control technology are important because inclusions in steel cause surface defects and internal defects.

8 1-2) Setting STEEL GRADE for SUS304 8 We set STEEL GRADE in accordance with their use and required characteristics. STEEL GRADE General Cleanliness Free-Cutting Table.1 STEEL GRADE of Stainless Steel Required PROBLEM TARGET Characteristics Contaminetionof Prevent of spinel formation. - huge inclusions Control to low M.P. area. caused by spinel Leak resistance Reduction of Improve cleanliness. (Used under diameter / Number (Low oxygenation) high pressure) of inclusions Free-cutting - - performance We tried to improve the procedure of secondary refining for the purpose of controlling composition of inclusions.

9 Outline 9 1. Introduction 2. Improvements of Secondary Refining 2-1) General Grade 2-2) Cleanliness Grade 3. Conclusion

10 2-1-1) General Grade stainless steel 10 General grade SiO 2 Liquidus line at 1873K Problem Contamination of huge inclusion caused by spinel Conventional Target of inclusion composition To prevent of spinel formation. To control to low melting point area. MgO Al 2 O 3 MgO Al 2 O 3 Fig2. Target composition of inclusions for general grade

11 2-1-2) Mechanism of contamination 11 Spinel were observed in the nozzle after casting. Fig.3 Actual SEM photographs and schematic image of contamination (1) Accumulating of spinel in the immersion nozzle. (2) Falling into the mold due to the flow of the molten steel. (3) Trapped in the shell.

12 2-1-3) Formation mechanism of spinel 12 There are many report on the spinel formation mechanism in SUS304 Slag Molten Steel Al in alloy Al (SiO 2 ) Al 2 O 3 Al 2 O 3 O O MgO Mg O Al Spinel (MgO-Al 2 O 3 ) Refractory MgO Al 2 O 3 Reaction 1 (Elution of Al, and Mg) As the free oxygen content decreases Al 2 O 3 = 2Al + 3O MgO (s) = Mg + O Reaction 2 (SiO 2 is reduced by Al) 4Al + 3SiO 2 (s) = 3Si + 2Al 2 O 3 (s) Fig.4 formation mechanism of spinel In order to prevent spinel formation 3Mg + 4Al 2 O 3 (s) = 3MgO Al 2 O 3 (s)+2al (3) Controlling of free oxygen content in molten steel (To prevent Al and Mg elution)

13 2-1-4) Countermeasures 13 Table.2 Countermeasures for preventing spinel formation OBJECTIVE Prevention of spinel formation COUNTERMEASURES Ladle refractory (Al 2 O 3 -MgO-C MgO-CaO) Fe-Si (%Al:1.5% 0.1%) Controlling of free oxygen content by slag basicity control DONE TODAY

14 2-1-5) Phase stability diagram 14 Verified mechanism with thermodynamic analyses O(ppm) MgO Al 2 O 3 Fig.5 Phase stability diagram and iso-oxygen contour lines (Tetsu- to-hagane, 84 (1998),85). As free oxygen content in molten steel decrease 20ppm Spinel stability region is expanded

15 2-1-6) Slag basicity and free oxygen content Examined effect of slag basicity on free oxygen content Cr-8Ni-0.5Si-1.5Mn Deoxidizer : Si Si + 2O = SiO 2 (s) O (ppm) logk=log a SiO2 log a Si - 2log a O =30,400/T Slag Basicity O 20ppm Spinel stability region Fig.6 Relationship between free oxygen contents and slag basicity (SUS304) [Thermodynamic analyses] Slag basicity 1.7 is spinel stability region.

16 2-1-7) Results of actual survey % SiO % MgO % Al2O Slag Basicity Fig.7 Relationship between composition of inclusion and slag basicity [Actual survey] Slag basicity 1.9 is formation area of spinel. (This value is nearly same with thermodynamic analyses)

17 2-1-8) Current slag basicity 17 Number of heat n=111 X=1.62 σ=0.16 Target 1.6 Formation area of spinel (%CaO)/(%SiO2) Fig.8 Current slag basicity Variation of slag basicity is very large. Some slags are existing within formation area of spinel. We have to control slag basicity more tightly.

18 2-1-9) Change of operations in each process Tried to analyze and control slag basicity during operation 18 Operation (Conventional) EAF 電気炉 TIME (min) (Improved) Variations in slag at EAF certain amount of slag Analyzing slag Controlling slag basicity Variations in slag basicity Reduction of variations in slag basicity Fig.9 Change of operations in each process

19 2-1-10) Results 19 Number of heat n=111 X=1.62 σ=0.16 n=44 X=1.62 σ=0.05 Conventional Improved Formation area of spinel Liquidus line at 1873K SiO 2 Conventional conventional Improved general grade (%CaO)/(%SiO2) MgO Al 2 O 3 Fig.10 Change of slag basicity Fig.11 Change of inclusion composition Variation was greatly decrease. We could avoid slag basicity 1.9. By controlling slag basicity, it was possible to prevent spinel formation Contamination of huge inclusions into billet could also be prevented.

20 Outline Introduction 2. Improvements of Secondary Refining 2-1) General Grade 2-2) Cleanliness Grade 3. Conclusion

21 2-2-1) Cleanliness grade stainless steel 21 General grade SiO 2 Liquidus line at 1873K Required characteristic Leak resistance Problem Reduction of diameter / number of inclusions MgO Conventional Al 2 O 3 MgO Al 2 O 3 Cleanliness grade Targets Improvement of cleanliness (low oxygenation) Controlling the composition of inclusions into MgO Fig12. Target composition of inclusions for cleanliness grade

22 2-2-2) Operation factors 22 Table.3 Operation factors for low total oxygen in secondary refining Process Operation factor low[t.o] : Effective factor Insol. sol. : More effective factor Carbon reduction efficiency AOD Slag composition Flow rate of tuyere gas Treatment time LF Flow rate of bottom blowing gas Slag composition Deoxidation was specialized in LF treatment. We investigated the influence of more effective factors by exchanging the conditions for operation.

23 2-2-3) Slag basicity and gas flow quantity 23 [mass ppm T.O] LFend Conventional Target Single porous(conventional) Double porous SUS304 Treatment time 56~75min (%CaO) / (%SiO2) Fig13. Effect of LF slag basicity and gas flow quantity for total oxygen contents With higher basicity, total oxygen contents decreased. But basicity 4.0, total oxygen contents tended to up. Target of slag basicity : 3.0~3.5

24 2-2-4) Risk of spinel formation Target basicity of General grade Target basicity of Cleanliness grade % SiO % MgO % Al2O Slag Basicity Fig.14 Relationship between composition of inclusion and slag basicity Target basicity of cleanliness grade is over 3.0, so there is the risk of spinel formation.

25 2-2-5) Control of inclusion compositon 25 We tried to control the composition of inclusions into MgO, which is said to be difficult to agglomerate In order to form MgO, inhibiting elution of Al and promoting elution of Mg were carried out. Table.4 Methods for removing a source of Al in refining Source of aluminum Ladle Slag line Refractory Wall, Bottom Fe-Si alloy Al contents (%Al 2 O 3 ) AODtap Slag Removal before LF (%Al 2 O 3 ) LFend Conventional MgO-C Al 2 O 3 -MgO-C 1.00% 17~23 Not enforcement 7~10 Improved MgO-CaO 0.01% 5 Enforcement 2

26 2-2-6) Results 26 Fig15. Change of inclusions composition Some spinel inclusions were also confirmed, But, almost inclusions could be reformed into MgO by Al source less operation.

27 2-2-7) Results (estimated area 100mm 2 ) Reduced variate Conventional Improved [mass ppm T.O] AREA max / μm 0 Conventional Improved Fig16 Estimation of inclusion size distribution by extreme statistic Fig17 Comparison for total oxygen contents The maximum inclusion diameter can be reduced by 66%. Oxygen value can be reduced by 54%.

28 Outline Introduction 2. Improvements of Secondary Refining 2-1) General Grade 2-2) Cleanliness Grade 3. Conclusion

29 3) Conclusion 29 For SUS304 stainless steel, we set steel grades in accordance with their use and required characteristics and carried out operational improvement for control of inclusions composition. (1) Improvement of Secondary Refining for General Grade In order to prevent the formation of spinel, we tried to control slag basicity more tightly. As a result, prevention of spinel formation became possible. (2) Improvement of Secondary Refining for Cleanliness Grade For improvement of cleanliness, we optimized slag basicity, bottom gas flow rate, and we controlled composition of inclusions into MgO by Al source less operation. As a result, the maximum inclusion diameter can be reduced by 66%, and the oxygen value can be reduced by 54% as compared with the conventional

30 30 Thank you

31 31

32 32 鍋下 [S] 分析値 ( 10-3 %) y = x x R 2 = [S] 狙い上限 スラグ塩基度

33 33 スラグ塩基度 = %CaO %SiO 2 石灰 (=CaO) の投入 これまで調整方策なし ケイ砂の概観 SiO 2 源としてケイ砂を投入 10mm 成分 SiO2 Al2O3 Fe2O3 その他 割合 (%) 図 18. ケイ砂の概観と成分値 スラグ組成制御ソフト 1 分析試験室よりデータ受信 2 狙い塩基度を入力 3 必要投入量を瞬時に算出

34 34 AR 実機にてスラグ塩基度調整後 10 分置きに溶鋼サンプリング ケイ砂投入 介在物中の MgO 比率 (mass%) 塩基度 =2.4 塩基度 = 時間 ( 分 )

35 2-9) Result 1 : Variation of slag basicity 35 Number of heat n=111 X=1.62 σ=0.16 n=44 X=1.62 σ=0.05 Conventional Improved Formation area of spinel (%CaO)/(%SiO2) Fig.10 Change of slag basicity Average value : No change Variation : Greatly decrease. We can avoid slag basicity 1.9.