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High Nitrogen Austenitic Stainless Steels Manufactured by Nitrogen Gas Alloying and Adding Nitrided Ferroalloys
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ScienceDirect JOURNAL OF IRON AND STEEL RESEARCH, INTERNATIONAL. 2007, 14(3): 63-68
High Nitrogen Austenitic Stainless Steels Manufactured by Nitrogen Gas Alloying and Adding Nitrided Ferroalloys LI Hua-bing ,
JIANG Zhou-hua,
SHEN Ming-hui,
YOU Xiang-mi
(School of Materials and Metallurgy, Northeastern University, Shenyang 110004, Liaoning , China)
Abstract: A simple and feasible method for the production of high nitrogen austenitic stainless steels involves nitrogen gas alloying and adding nitrided ferroalloys under normal atmospheric conditions. Alloying by nitrogen gas bubbling in FeCr-Mn-Mo series alloys was carried out in MoSiz resistance furnace and air induction furnace under normal atmospheric conditions. T h e results showed that nitrogen alloying could be accelerated by increasing nitrogen gas flow rate, prolonging residence time of bubbles, increasing gas/molten steel interfaces, and decreasing the sulphur and oxygen contents in molten steel. Nitrogen content of 0. 69% in 18Cr18Mn was obtained using air induction furnace by bubbling of nitrogen gas from porous plug. In addition, the nickel-free, high nitrogen austenitic stainless steels with sound and compact macrostructure had been produced in the laboratory using vacuum induction furnace and electroslag remelting furnace under nitrogen atmosphere by the addition of nitrided alloy with the maximum nitrogen content of 0.81 %. Pores were observed in the ingots obtained by melting and casting in vacuum induction furnace with the addition of nitrided ferroalloys and under nitrogen atmosphere. After electroslag remelting of the cast ingots, they were all sound and were free of pores. The yield of nitrogen increased with the decrease of melting rate in the ESR process. Due to electroslag remelting under nitrogen atmosphere and the consequential addition of aluminum as deoxidizer to the slag, the loss of manganese decreased obviously. There existed mainly irregular A l , 0 3 inclusions and MnS inclusions in ESR ingots, and the size of most of the inclusions was less than 5 pn. After homogenization of the hot rolled plate at 1 150 *C X 1 h followed by water quenching, the microstructure consisted of homogeneous austenite. Key words: nitrogen gas alloying; nitrided ferroalloy; high nitrogen austenitic stainless steel; vacuum induction melting; electroslag remelting
High nitrogen stainless steels exhibit excellent mechanical properties a s well as corrosion resistance, so considerable efforts have been made to develop high nitrogen stainless steels over the past decades"-33. High nitrogen stainless steels can be produced in the liquid stater4] by induction furnace, electric arc furnace, gas bubbling in liquid steels, Argon Oxygen Decarburization ( AOD 1 refining , pressure electroslag remelting (PESR) , plasma arc melting, arc-slag melting, etc. PESR process is the most widely used process for the commercial pioduction of high nitrogen stainless steels. However, the equipment used for PESR is complicated and expensive. Double or triple melting is carried out in order to make ingot homogeneity. When nitrided silicon is
used to improve nitrogen distribution in t h e ingot, silicon pick up might be unacceptable for certain grades. It is a simple and feasible method that manufactures high nitrogen stainless steel by nitrogen gas alloying and adding nitrided ferroalloys under normal atmosphere. In this study, nitrogen gas alloying in Fe-CrMn-Mo series alloys was carried out in MoSi, resistance furnace and air induction furnace under normal atmospheric conditions. Nickel free, high nitrogen austenitic stainless steels were manufactured using vacuum induction furnace by adding nitrided ferroalloy under nitrogen atmosphere and using the eletroslag furnace for remelting under nitrogen atmosphere. T h e microstructure and inclusions of the ESIi
Foundation Item: Item Sponsored by National Natural Science Foundation of China (50534010) Biography:I.I Hua-bing(1978-), Male, Doctor Candidate; E-mail: huabing- IiB163. cam5
Revised Date: January 1 1 , 2007
*
64
ingots and the hot rolled plate were examined.
1 1.1
Vol. 1 4
Journal of Iron and Steel Research, International
*
Nitrogen Gas Alloying Experiments A magnesia crucible containing approximately 1 kg
of alloy was placed in the zone maintained at a constant temperature in an electric resistance furnace and heated under the argon atmosphere using argon flowing from the bottom of the furnace. When the temperature reached 1 550 ' C , the first sample of steel was taken. 2 g of aluminum chip was then added to molten steel as deoxidizer. After 5 min, the slag formed on the liquid steel was removed. T h e argon flowing from the bottom of the furnace was changed to nitrogen. The alumina tube was dipped into liquid steel for gas blowing, and the tube outlet was kept at a height of 10 mm from the bottom of the crucible, after the partial pressure of nitrogen reached 1 MPa. During the whole experimental process, the gas flow rate was controlled at 0. 15 L/min, and 3 min, 6 mint 9 min, 12 min, 15 min, 20 min, 30 min, 40 min, 50 min, 60 min, 75 min, and 90 min were selected as the time intervals to take steel samples from the furnace to analyze nitrogen content using a LECO TC-436 apparatus. An additional experiment was carried out by bubbling nitrogen from the porous plug located at the bottom of an air induction furnace. T h e gas flow
rate was controlled at 5 L/min. Before blowing nitrogen, the argon was blown from the porous plug. T h e slag consisting of 60% of CaO, 30% of Alz03, 5 % of MgO, and 5 % of C a F z w a s added into t h e molten steel. During t h e experiment, aluminum a s deoxidizer w a s continuously added into t h e slag.
Results and discussion T h e chemical composition of alloys is shown in Table 1. T h e variation of nitrogen content is shown in Fig. 1. The nitrogen content of 18Cr18Mn, 18Cr18Mn2M0, 22Cr15Mn, and 18Cr8Mn in electric resistance furnace reached equilibrium after 60 min. T h e nitrogen content of 18Cr18Mn in induction furnace reached equilibrium after 50 min. A new thermodynamic model for calculation of the nitrogen solubility in liquid stainless steels over a wide range of alloy concentrations, temperatures, and pressures had been developed as shown in Eqn. (1) and Eqn. ( 2 ) in previous workc5'. A new parameter s', was obtained, which reflects the effect of nitrogen pressure on the activity coefficient of nitrogen. 1 I g C d N)] =21g(PNz /Po) f1gKN - 1gfN (1) 1.2
%
Table 1 Chemical composition of alloys during experiment C
Si
Mn
P
S
Cr
18Cr18Mn (in MoSiz furnace)
0.15
0.42
18.58
0.016
0.012
20.45
-
18Cr18Mn2Mo (in MoSiz furnace)
0. 16
0. 38
19. 69
0.020
0.018
2. 82
22C.rl5Mn (in MoSiz furnace)
0.18
0.40
14.94
0.015
0.008
18. 88 22.84
18Cr8Mn (in MoSiz furnace) 18Cr18Mn ( i n induction furnace)
0.12 0. 1.2
0.35 0. 35
8. 75 19. 50
0.019 0.015
0.017 0.004
19. 63 18. 96
Steel grade
<
-..-..-.-. -. -
-
-
- --
- - - -T
(in MoSifurnace)
- A - 1Wr18MnZMo (in MoSiz fumare) 22CrlSMn (in MoSi, furnace) 18Cr8Mn (m MoSL~finace) * 18Cr18Mn (in induction furnace)
'I
0
20
40
Timdmin
60
80
100
Fig. 1 Variation of nitrogen content in different alloys
Mo
-
When PN2 /Po 1 . 0 , s', = 0. When PN2 /Po> 1. 0 , &=O. 06. Table 2 shows a comparison of the nitrogen concentrations calculated using the model and the measured values in MoSin furnace and induction furnace at 1 550 ' C , which indicated that there existed a larger negative deviation. A lot of studies on nitrogen gas alloying showed that nitrogen alloying could be accelerated by increasing nitrogen gas flow rate, prolonging residence time of bubbles, increasing gas/ molten steel interfaces, and decreasing the sulphur and oxygen contents in molten steel. The lower nit&-
No. 3
High Nitrogen Austenitic Stainless Steels Manufactured by Nitrogen Gas Alloying
slightly less than the calculated values as shown in Table 2. T h e gas/molten steel interfaces were increased by lots of very small nitrogen bubbles generated from the modified porous plugs at the bottom of the furnace into molten steel. Increased nitrogen flow rate, deeper molten steel increasing bubble residence time, efficient stirring, and lower sulphur and oxygen contents were better for nitrogen absorption.
gen contents in the molten steels obtained in MoSiz furnace was mainly caused by lower gas flow rate, less depth of molten steel leading to short bubble residence time, higher surface-active sulphur content in molten steel preventing nitrogen mass transfer, and inefficient stirring. During experiment in the 100 kg air induction furnace, the measured values of 18Cr18Mn were Table 2
Comparison of calculated nitrogen concentrations with measured ones
Steel grade
Partial nitrogen pressure/MPa
Calculated value/ %
0. 1
0. 81
0. 62
18Cr18Mn (in MoSiz furnace) 18Crl8MnZMo (in MoSiz furnace)
0. 1
0. 83
0. 64
0. 1
0. 80
0. 65
18Cr8Mn (in MoSiz furnace)
0. 1
0. 49
0. 41
0.078
0. 71
0. 69
Experiments T h e Fe-Cr-Mn-Mo series of nickel free high nitrogen austenitic stainless steels were obtained using 25 kg and 100 kg vacuum induction furnace (VIF) under nitrogen atmosphere with additions of nitrided ferrochromium (nitrogen of 5 % ) . The melting temperature was controlled in 1 530-1 550 'C , and the nitrided ferrochromium was added steadily and continuously. The melt was then held for a shorter time and cast when temperature ranged in 1 460- 1 480 'C. The cast ingots were examined by radiography for their soundness, and analyzed for chemical composition. T h e cast ingots were kept at 1 200 'C for 2 h , forged into electrodes of 75 mm (100 kg VIF) and 50 mm (25 kg VIF) in diameter. T h e final forging temperature must be higher than 1 050 "C. T h e electrodes were then subject to electroslag remelting under nitrogen atmosphere. T h e composition of the slag was as follows: ( A ) ANF-6; ( B ) 0. 63CaFZ-0. 17Ca0-0. 15A1203-0.02SiOZ-0.03Mg0.
2.1
Table 3 Steel grade
Mean Mean voltage/V current/A
%
During ESR, the aluminum as deoxidizer was continuously added to the slag. T h e process parameters of ESR are shown in Table 3. T h e electroslag ingots were then analyzed for their chemical composition and examined by radiography. The hot acid etch was carried out for determining the macrostructure. T h e typical composition of the inclusion was examined using SEM. The No. 2 ESR ingot was hot rolled into 6 mm plate, and then typical inclusions were examined using SEM. The microstructure of the hot rolled plate after homogenization at 1 150 'CX 1 h followed by water quenching was examined using optical microscopy.
Nitrogen Alloying by Nitrided Ferroalloys
No.
Measured value/
22Cr15Mn (in MoSiz furnace) 18Cr18Mn (in induction furnace)
2
65
Results and discussion T h e radiography analysis of the cast ingot made in VIF under nitrogen atmosphere showed extensive porosity, as typically shown in Fig. 2. During solidification, there was formation of the low nitrogen solubility bferrite region in Fe-Cr-Mn-Mo steel, which rejected nitrogen to the interdendritic space. Nitrogen nucleation may occur at this heterogeneous
2.2
Process parameters of electroslag remelting
-
Mold diameter/rnrn
Electrode diameter/mm
Melting rate/ Slag (kg min-' )
0. 95
0
No
2
10
Yes
2
10
Yes
2
10
Yes
2
10
Yes
2
10
Yes
3. 5
70
Yes
3
18Cr18Mn
44
1750
80
18Cr18MnZMo
44
1 750
80
50
0. 74
5
18Cr18Mn2Mo
44
1750
80
50
0. 79
6
18Cr14Mn2Mo
44
1800
80
50
0. 84
B B B B
7
18Crl8MnZMo
47
2 200
130
75
1. 22
B
51
2 000
130
44
1600
80
0. 78
0. 80
Protection by nitrogen
3
4
18Cr18Mn 18Cr18Mn
Quantity of deoxidizer/g
A B
75 50 50
1
2
Quantity of slag/kg
66
-
Journal of Iron and Steel Research, International
Comparison of measured nitrogen concentrations in
interface between dendrite and molten metal. All the ESR ingots were sound and free of pores, as shown in Fig. 3. It was less likely that gas bubble nucleation would occur, due to rapid cooling speed and short time of local solidification during electroslag remelting.
Fig. 2
Vol. 14
VIF with the calculated ones is shown in Table 4. T h e cast ingots with high nitrogen concentrations that were close to the calculated values were obtained using VIF by addition of nitrided ferrochromium under nitrogen atmosphere. T h e maximum nitrogen content was 0.83%. T h e chemical compositions of ingots before and after ESR are shown in Table 5. The yield of nitrogen was high during ESR due to nitrogen atmosphere and low melting rate. T h e maximum nitrogen content was 0. 81 %. Homogeneous nitrogen distribution in No. 2 ESR ingots is shown in Fig. 4. chromium and manganese were also homogeneously distributed, as shown in Fig. 4 and Table 5. T h e yield of nitrogen increased with the decrease of melting rate as shown in Fig. 5, This could be probably due to the fact that when the melting rate was low and the pool was shallow, progressive solidification could rapidly trap nitrogen during the ingot solidification. In addition, the nitrogen solubility in FeCr-Mn-Mo series austenitic steels was higher in
Cast steel with porosity examined using radiography
Fig. 3 ljpical pore-free ESR ingots examined using radiography
Table 4 Comparison of measured nitrogen concentration in VIF with calculated ones ~
~~
No.
___
~~
Smelting temperature/%
1
0. 1
1535
0. 69
0. 69
2
0. 1
1550
0. 73
0. 82
3
0. 1
1530
0. 83
0. 85
4
0. 1
1543
0. 77
0. 82
5
0. 1
1550
0. 7 1
0. 8 1
6
0. 1
1570
0. 59
0. 69
7
0. 1
1550
0. 70
0. 84
Table 5
Measured nitrogen concentration/
% Calculated nitrogen concentration/ %
Nitrogen partial pressure/MPa
%
Chemical compositions of cast ingots and ESR ingots
Yield of nitrogen/ %
No.
ESR
C
Si
Mn
P
S
Cr
N
0
Mo
1
Before After
0.055 0. 047
0.26 0. 24
17. 7 15. 6
0.015
0.015
0. 002 5 0. 004 6
__
(0.03
0.009
17.6 17. 5
0. 69
0.015
-
03
Before 2
After
0. 56
Al
0.057
0.36
19.20
0.022
0.014
19.78
0.73
0.008 4
-
03
A
0.053
0.24
19.10
0.020
0.007
19.78
0.71
0.003 6
03
B
0.052
0.22
18.95
0.020
0.008
19.82
0.70
0.003 8
-
03 03 03
3
Before After
0.058 0. 052
0. 20 0. 17
18. 53 18. 30
0.021 0. 020
0. 015
19.93
0. 83
20. 0 1
0. 8 1
0.003 6 0. 002 5
-
0. 006
-
4
Before After
0.055 0.057
0.42 0.38
18.84 18.83
0.019 0.019
0.015 0.007
19.07 19.15
0.77 0.77
0.007 5 0.005 2
2.20 2.25
<0.03 <0.03
5
Before After
0.048 0..045
0.45 0. 40
18.19
0.020 0. 020
0.015 0. 007
19.10 18. 59
0.71 0. 69
0.007 1 0. 003 6
2.23 2. 03
<0.03
17. 54
6
Before After
0.052 0.045
0.49 0.47
15.12 14.83
0,023 0.022
0.012 0.006
19.55 19.47
0.59 0.56
0.008 2 0.005 0
2.28 2.26
<0.03
7
Before After
0.048 0.042
0.23 0.18
18. 95 18.24
0.018 0.017
0.015 0.006
19.55 19.20
0.70 0.65
0.009 0 0.004 0
2.06 2.25
<0.03 <0.03
03
<0.03
81. 2
97. 3
97. 5 100
97. 2 94. 9 92. 9
High Nitrogen Austenitic Stainless Steels Manufactured by Nitrogen Gas Alloying
No. 3
67
Fig. 7 showed the typical macrostructure of longitudinal and transversal direction of ESR ingot. The ingot is free of all defects. The results of analysis of SEM indicated that there existed multiple irregular Al,O, inclusions and a limited number of MnS inclusions, as typically shown in Fig. 8. The size of most of the inclusions was less than 5 pm. Fig. 9 showed the typical inclusions of 6 mm in thickness that existed in hot rolled plate. Because the MnS inclusion is prone to deformation, the strip MnS inclusion of approximate 10 pm in size exists along the direction of rolling. After homogenization of the hot rolled plate at 1 150 'C X 1 h followed b y water quenching, the microstructure consisted of homogeneous austenite, as shown in Fig. 10. Fig. 4
Homogeneous nitrogen distribution in No. 2 ESR ingot
the solid states than in the molten steel, so that during the rapid solidification, more nitrogen was partitioned in the solidified ingots. Due to electroslag remelting under the condition of nitrogen atmosphere and continuous addition of aluminum as deoxidizer to the slag, oxygen potential in the slag decreased, and the loss of manganese decreased obviously, as shown in Fig. 6.
3
Conclusions
( 1) Nitrogen alloying could be accelerated by increasing the flow rate of nitrogen gas, prolonging the
(a) Longitudinal direction;
Fig. 7
0.74
0.76
0.80
0.78
0.82
(b) Transversal direction
Macrostructure of ESR ingot
0.84
Melting rate/(kg.min-')
Fig. 5 Influence of melting rate on yield of nitrogen during ESR Fig. 8 Typical inclusions in ESR ingot analyzed by SEM 100
Li?
o
5
8060
sE
Q4
40
0
3 F
20
o 1
2
3
4
6
6
7
No.
Fig. 6
Yield of manganese during ESR
Fig. 9
Typical inclusions in hot rolled plate analyzed by SEM
68
Journal of Iron and Steel Research, International
Vol. 14
Fig. 10 Microstructure of hot rolled plate after homogenization at 1 150 "c X 1 h followed by water quenching
residence time of bubbles, increasing gas/molten steel interfaces, and decreasing the sulphur and oxygen contents in molten steel. Nitrogen content of 0.69% in 18Cr18Mn was obtained using air induction furnace by nitrogen gas bubbling from porous plug. ( 2 ) Sound ingots of nickel-free, high nitrogen stainless steel could be manufactured using vacuum induction furnace with the addition of nitrided ferrochromium and electroslag remelting furnace under nitrogen atmosphere. T h e yield of nitrogen could be increased by lowering the melting rate. ( 3 ) Due to electroslag remelting under the condition of nitrogen atmosphere and the continuous addition of aluminum as deoxidizer to the slag, the loss of manganese decreased obviously. (4) There existed mainly irregular A l z 0 3 inclusions and MnS inclusions in ESR ingots, and the
size of most of the inclusions was less than 5 pm. ( 5 ) After homogenization of the hot rolled plate at 1 150 "C X 1 h followed by water quenching, the microstructure consisted of homogeneous austenite. References :
c11 c21 131
C4l C51
Simmons J W. High-Nitrogen Alloying of Stainiess Steels [J]. Materials Science and Engineering, 1996, 207A: 159-169. Paton B E , Saenko V Y , Pomarin Y M , et al. Arc Slag Remelting for High Strength Steel and Various Alloys [J]. Journal of Materials Science, 2004, 39(24) : 7269-7274. Stein G , Huchlenbroich I. Manufacturing and Application of High Nitrogen Steels [J]. Materials and Manufacturing Processes, 2004, 1 9 ( 1 ) : 7-17. Mudali U K , Raj B. High Nitrogen Steels and Stainless Steels [MI. Pangbourne: Alpha Science International Ltd, 2004. JIANG Zhou-hua, LI Hua-bing, CHEN Zhao-ping, et al. The Nitrogen Solubility in Molten Stainless Steel [J]. Steel Research International, 2005, 76(10) : 730-735.
ScienceDirect JOURNAL OF IRON AND STEEL RESEARCH, INTERNATIONAL. 2007, 14(3): 63-68
High Nitrogen Austenitic Stainless Steels Manufactured by Nitrogen Gas Alloying and Adding Nitrided Ferroalloys LI Hua-bing ,
JIANG Zhou-hua,
SHEN Ming-hui,
YOU Xiang-mi
(School of Materials and Metallurgy, Northeastern University, Shenyang 110004, Liaoning , China)
Abstract: A simple and feasible method for the production of high nitrogen austenitic stainless steels involves nitrogen gas alloying and adding nitrided ferroalloys under normal atmospheric conditions. Alloying by nitrogen gas bubbling in FeCr-Mn-Mo series alloys was carried out in MoSiz resistance furnace and air induction furnace under normal atmospheric conditions. T h e results showed that nitrogen alloying could be accelerated by increasing nitrogen gas flow rate, prolonging residence time of bubbles, increasing gas/molten steel interfaces, and decreasing the sulphur and oxygen contents in molten steel. Nitrogen content of 0. 69% in 18Cr18Mn was obtained using air induction furnace by bubbling of nitrogen gas from porous plug. In addition, the nickel-free, high nitrogen austenitic stainless steels with sound and compact macrostructure had been produced in the laboratory using vacuum induction furnace and electroslag remelting furnace under nitrogen atmosphere by the addition of nitrided alloy with the maximum nitrogen content of 0.81 %. Pores were observed in the ingots obtained by melting and casting in vacuum induction furnace with the addition of nitrided ferroalloys and under nitrogen atmosphere. After electroslag remelting of the cast ingots, they were all sound and were free of pores. The yield of nitrogen increased with the decrease of melting rate in the ESR process. Due to electroslag remelting under nitrogen atmosphere and the consequential addition of aluminum as deoxidizer to the slag, the loss of manganese decreased obviously. There existed mainly irregular A l , 0 3 inclusions and MnS inclusions in ESR ingots, and the size of most of the inclusions was less than 5 pn. After homogenization of the hot rolled plate at 1 150 *C X 1 h followed by water quenching, the microstructure consisted of homogeneous austenite. Key words: nitrogen gas alloying; nitrided ferroalloy; high nitrogen austenitic stainless steel; vacuum induction melting; electroslag remelting
High nitrogen stainless steels exhibit excellent mechanical properties a s well as corrosion resistance, so considerable efforts have been made to develop high nitrogen stainless steels over the past decades"-33. High nitrogen stainless steels can be produced in the liquid stater4] by induction furnace, electric arc furnace, gas bubbling in liquid steels, Argon Oxygen Decarburization ( AOD 1 refining , pressure electroslag remelting (PESR) , plasma arc melting, arc-slag melting, etc. PESR process is the most widely used process for the commercial pioduction of high nitrogen stainless steels. However, the equipment used for PESR is complicated and expensive. Double or triple melting is carried out in order to make ingot homogeneity. When nitrided silicon is
used to improve nitrogen distribution in t h e ingot, silicon pick up might be unacceptable for certain grades. It is a simple and feasible method that manufactures high nitrogen stainless steel by nitrogen gas alloying and adding nitrided ferroalloys under normal atmosphere. In this study, nitrogen gas alloying in Fe-CrMn-Mo series alloys was carried out in MoSi, resistance furnace and air induction furnace under normal atmospheric conditions. Nickel free, high nitrogen austenitic stainless steels were manufactured using vacuum induction furnace by adding nitrided ferroalloy under nitrogen atmosphere and using the eletroslag furnace for remelting under nitrogen atmosphere. T h e microstructure and inclusions of the ESIi
Foundation Item: Item Sponsored by National Natural Science Foundation of China (50534010) Biography:I.I Hua-bing(1978-), Male, Doctor Candidate; E-mail: huabing- IiB163. cam5
Revised Date: January 1 1 , 2007
*
64
ingots and the hot rolled plate were examined.
1 1.1
Vol. 1 4
Journal of Iron and Steel Research, International
*
Nitrogen Gas Alloying Experiments A magnesia crucible containing approximately 1 kg
of alloy was placed in the zone maintained at a constant temperature in an electric resistance furnace and heated under the argon atmosphere using argon flowing from the bottom of the furnace. When the temperature reached 1 550 ' C , the first sample of steel was taken. 2 g of aluminum chip was then added to molten steel as deoxidizer. After 5 min, the slag formed on the liquid steel was removed. T h e argon flowing from the bottom of the furnace was changed to nitrogen. The alumina tube was dipped into liquid steel for gas blowing, and the tube outlet was kept at a height of 10 mm from the bottom of the crucible, after the partial pressure of nitrogen reached 1 MPa. During the whole experimental process, the gas flow rate was controlled at 0. 15 L/min, and 3 min, 6 mint 9 min, 12 min, 15 min, 20 min, 30 min, 40 min, 50 min, 60 min, 75 min, and 90 min were selected as the time intervals to take steel samples from the furnace to analyze nitrogen content using a LECO TC-436 apparatus. An additional experiment was carried out by bubbling nitrogen from the porous plug located at the bottom of an air induction furnace. T h e gas flow
rate was controlled at 5 L/min. Before blowing nitrogen, the argon was blown from the porous plug. T h e slag consisting of 60% of CaO, 30% of Alz03, 5 % of MgO, and 5 % of C a F z w a s added into t h e molten steel. During t h e experiment, aluminum a s deoxidizer w a s continuously added into t h e slag.
Results and discussion T h e chemical composition of alloys is shown in Table 1. T h e variation of nitrogen content is shown in Fig. 1. The nitrogen content of 18Cr18Mn, 18Cr18Mn2M0, 22Cr15Mn, and 18Cr8Mn in electric resistance furnace reached equilibrium after 60 min. T h e nitrogen content of 18Cr18Mn in induction furnace reached equilibrium after 50 min. A new thermodynamic model for calculation of the nitrogen solubility in liquid stainless steels over a wide range of alloy concentrations, temperatures, and pressures had been developed as shown in Eqn. (1) and Eqn. ( 2 ) in previous workc5'. A new parameter s', was obtained, which reflects the effect of nitrogen pressure on the activity coefficient of nitrogen. 1 I g C d N)] =21g(PNz /Po) f1gKN - 1gfN (1) 1.2
%
Table 1 Chemical composition of alloys during experiment C
Si
Mn
P
S
Cr
18Cr18Mn (in MoSiz furnace)
0.15
0.42
18.58
0.016
0.012
20.45
-
18Cr18Mn2Mo (in MoSiz furnace)
0. 16
0. 38
19. 69
0.020
0.018
2. 82
22C.rl5Mn (in MoSiz furnace)
0.18
0.40
14.94
0.015
0.008
18. 88 22.84
18Cr8Mn (in MoSiz furnace) 18Cr18Mn ( i n induction furnace)
0.12 0. 1.2
0.35 0. 35
8. 75 19. 50
0.019 0.015
0.017 0.004
19. 63 18. 96
Steel grade
<
-..-..-.-. -. -
-
-
- --
- - - -T
(in MoSifurnace)
- A - 1Wr18MnZMo (in MoSiz fumare) 22CrlSMn (in MoSi, furnace) 18Cr8Mn (m MoSL~finace) * 18Cr18Mn (in induction furnace)
'I
0
20
40
Timdmin
60
80
100
Fig. 1 Variation of nitrogen content in different alloys
Mo
-
When PN2 /Po 1 . 0 , s', = 0. When PN2 /Po> 1. 0 , &=O. 06. Table 2 shows a comparison of the nitrogen concentrations calculated using the model and the measured values in MoSin furnace and induction furnace at 1 550 ' C , which indicated that there existed a larger negative deviation. A lot of studies on nitrogen gas alloying showed that nitrogen alloying could be accelerated by increasing nitrogen gas flow rate, prolonging residence time of bubbles, increasing gas/ molten steel interfaces, and decreasing the sulphur and oxygen contents in molten steel. The lower nit&-
No. 3
High Nitrogen Austenitic Stainless Steels Manufactured by Nitrogen Gas Alloying
slightly less than the calculated values as shown in Table 2. T h e gas/molten steel interfaces were increased by lots of very small nitrogen bubbles generated from the modified porous plugs at the bottom of the furnace into molten steel. Increased nitrogen flow rate, deeper molten steel increasing bubble residence time, efficient stirring, and lower sulphur and oxygen contents were better for nitrogen absorption.
gen contents in the molten steels obtained in MoSiz furnace was mainly caused by lower gas flow rate, less depth of molten steel leading to short bubble residence time, higher surface-active sulphur content in molten steel preventing nitrogen mass transfer, and inefficient stirring. During experiment in the 100 kg air induction furnace, the measured values of 18Cr18Mn were Table 2
Comparison of calculated nitrogen concentrations with measured ones
Steel grade
Partial nitrogen pressure/MPa
Calculated value/ %
0. 1
0. 81
0. 62
18Cr18Mn (in MoSiz furnace) 18Crl8MnZMo (in MoSiz furnace)
0. 1
0. 83
0. 64
0. 1
0. 80
0. 65
18Cr8Mn (in MoSiz furnace)
0. 1
0. 49
0. 41
0.078
0. 71
0. 69
Experiments T h e Fe-Cr-Mn-Mo series of nickel free high nitrogen austenitic stainless steels were obtained using 25 kg and 100 kg vacuum induction furnace (VIF) under nitrogen atmosphere with additions of nitrided ferrochromium (nitrogen of 5 % ) . The melting temperature was controlled in 1 530-1 550 'C , and the nitrided ferrochromium was added steadily and continuously. The melt was then held for a shorter time and cast when temperature ranged in 1 460- 1 480 'C. The cast ingots were examined by radiography for their soundness, and analyzed for chemical composition. T h e cast ingots were kept at 1 200 'C for 2 h , forged into electrodes of 75 mm (100 kg VIF) and 50 mm (25 kg VIF) in diameter. T h e final forging temperature must be higher than 1 050 "C. T h e electrodes were then subject to electroslag remelting under nitrogen atmosphere. T h e composition of the slag was as follows: ( A ) ANF-6; ( B ) 0. 63CaFZ-0. 17Ca0-0. 15A1203-0.02SiOZ-0.03Mg0.
2.1
Table 3 Steel grade
Mean Mean voltage/V current/A
%
During ESR, the aluminum as deoxidizer was continuously added to the slag. T h e process parameters of ESR are shown in Table 3. T h e electroslag ingots were then analyzed for their chemical composition and examined by radiography. The hot acid etch was carried out for determining the macrostructure. T h e typical composition of the inclusion was examined using SEM. The No. 2 ESR ingot was hot rolled into 6 mm plate, and then typical inclusions were examined using SEM. The microstructure of the hot rolled plate after homogenization at 1 150 'CX 1 h followed by water quenching was examined using optical microscopy.
Nitrogen Alloying by Nitrided Ferroalloys
No.
Measured value/
22Cr15Mn (in MoSiz furnace) 18Cr18Mn (in induction furnace)
2
65
Results and discussion T h e radiography analysis of the cast ingot made in VIF under nitrogen atmosphere showed extensive porosity, as typically shown in Fig. 2. During solidification, there was formation of the low nitrogen solubility bferrite region in Fe-Cr-Mn-Mo steel, which rejected nitrogen to the interdendritic space. Nitrogen nucleation may occur at this heterogeneous
2.2
Process parameters of electroslag remelting
-
Mold diameter/rnrn
Electrode diameter/mm
Melting rate/ Slag (kg min-' )
0. 95
0
No
2
10
Yes
2
10
Yes
2
10
Yes
2
10
Yes
2
10
Yes
3. 5
70
Yes
3
18Cr18Mn
44
1750
80
18Cr18MnZMo
44
1 750
80
50
0. 74
5
18Cr18Mn2Mo
44
1750
80
50
0. 79
6
18Cr14Mn2Mo
44
1800
80
50
0. 84
B B B B
7
18Crl8MnZMo
47
2 200
130
75
1. 22
B
51
2 000
130
44
1600
80
0. 78
0. 80
Protection by nitrogen
3
4
18Cr18Mn 18Cr18Mn
Quantity of deoxidizer/g
A B
75 50 50
1
2
Quantity of slag/kg
66
-
Journal of Iron and Steel Research, International
Comparison of measured nitrogen concentrations in
interface between dendrite and molten metal. All the ESR ingots were sound and free of pores, as shown in Fig. 3. It was less likely that gas bubble nucleation would occur, due to rapid cooling speed and short time of local solidification during electroslag remelting.
Fig. 2
Vol. 14
VIF with the calculated ones is shown in Table 4. T h e cast ingots with high nitrogen concentrations that were close to the calculated values were obtained using VIF by addition of nitrided ferrochromium under nitrogen atmosphere. T h e maximum nitrogen content was 0.83%. T h e chemical compositions of ingots before and after ESR are shown in Table 5. The yield of nitrogen was high during ESR due to nitrogen atmosphere and low melting rate. T h e maximum nitrogen content was 0. 81 %. Homogeneous nitrogen distribution in No. 2 ESR ingots is shown in Fig. 4. chromium and manganese were also homogeneously distributed, as shown in Fig. 4 and Table 5. T h e yield of nitrogen increased with the decrease of melting rate as shown in Fig. 5, This could be probably due to the fact that when the melting rate was low and the pool was shallow, progressive solidification could rapidly trap nitrogen during the ingot solidification. In addition, the nitrogen solubility in FeCr-Mn-Mo series austenitic steels was higher in
Cast steel with porosity examined using radiography
Fig. 3 ljpical pore-free ESR ingots examined using radiography
Table 4 Comparison of measured nitrogen concentration in VIF with calculated ones ~
~~
No.
___
~~
Smelting temperature/%
1
0. 1
1535
0. 69
0. 69
2
0. 1
1550
0. 73
0. 82
3
0. 1
1530
0. 83
0. 85
4
0. 1
1543
0. 77
0. 82
5
0. 1
1550
0. 7 1
0. 8 1
6
0. 1
1570
0. 59
0. 69
7
0. 1
1550
0. 70
0. 84
Table 5
Measured nitrogen concentration/
% Calculated nitrogen concentration/ %
Nitrogen partial pressure/MPa
%
Chemical compositions of cast ingots and ESR ingots
Yield of nitrogen/ %
No.
ESR
C
Si
Mn
P
S
Cr
N
0
Mo
1
Before After
0.055 0. 047
0.26 0. 24
17. 7 15. 6
0.015
0.015
0. 002 5 0. 004 6
__
(0.03
0.009
17.6 17. 5
0. 69
0.015
-
03
Before 2
After
0. 56
Al
0.057
0.36
19.20
0.022
0.014
19.78
0.73
0.008 4
-
03
A
0.053
0.24
19.10
0.020
0.007
19.78
0.71
0.003 6
03
B
0.052
0.22
18.95
0.020
0.008
19.82
0.70
0.003 8
-
03 03 03
3
Before After
0.058 0. 052
0. 20 0. 17
18. 53 18. 30
0.021 0. 020
0. 015
19.93
0. 83
20. 0 1
0. 8 1
0.003 6 0. 002 5
-
0. 006
-
4
Before After
0.055 0.057
0.42 0.38
18.84 18.83
0.019 0.019
0.015 0.007
19.07 19.15
0.77 0.77
0.007 5 0.005 2
2.20 2.25
<0.03 <0.03
5
Before After
0.048 0..045
0.45 0. 40
18.19
0.020 0. 020
0.015 0. 007
19.10 18. 59
0.71 0. 69
0.007 1 0. 003 6
2.23 2. 03
<0.03
17. 54
6
Before After
0.052 0.045
0.49 0.47
15.12 14.83
0,023 0.022
0.012 0.006
19.55 19.47
0.59 0.56
0.008 2 0.005 0
2.28 2.26
<0.03
7
Before After
0.048 0.042
0.23 0.18
18. 95 18.24
0.018 0.017
0.015 0.006
19.55 19.20
0.70 0.65
0.009 0 0.004 0
2.06 2.25
<0.03 <0.03
03
<0.03
81. 2
97. 3
97. 5 100
97. 2 94. 9 92. 9
High Nitrogen Austenitic Stainless Steels Manufactured by Nitrogen Gas Alloying
No. 3
67
Fig. 7 showed the typical macrostructure of longitudinal and transversal direction of ESR ingot. The ingot is free of all defects. The results of analysis of SEM indicated that there existed multiple irregular Al,O, inclusions and a limited number of MnS inclusions, as typically shown in Fig. 8. The size of most of the inclusions was less than 5 pm. Fig. 9 showed the typical inclusions of 6 mm in thickness that existed in hot rolled plate. Because the MnS inclusion is prone to deformation, the strip MnS inclusion of approximate 10 pm in size exists along the direction of rolling. After homogenization of the hot rolled plate at 1 150 'C X 1 h followed b y water quenching, the microstructure consisted of homogeneous austenite, as shown in Fig. 10. Fig. 4
Homogeneous nitrogen distribution in No. 2 ESR ingot
the solid states than in the molten steel, so that during the rapid solidification, more nitrogen was partitioned in the solidified ingots. Due to electroslag remelting under the condition of nitrogen atmosphere and continuous addition of aluminum as deoxidizer to the slag, oxygen potential in the slag decreased, and the loss of manganese decreased obviously, as shown in Fig. 6.
3
Conclusions
( 1) Nitrogen alloying could be accelerated by increasing the flow rate of nitrogen gas, prolonging the
(a) Longitudinal direction;
Fig. 7
0.74
0.76
0.80
0.78
0.82
(b) Transversal direction
Macrostructure of ESR ingot
0.84
Melting rate/(kg.min-')
Fig. 5 Influence of melting rate on yield of nitrogen during ESR Fig. 8 Typical inclusions in ESR ingot analyzed by SEM 100
Li?
o
5
8060
sE
Q4
40
0
3 F
20
o 1
2
3
4
6
6
7
No.
Fig. 6
Yield of manganese during ESR
Fig. 9
Typical inclusions in hot rolled plate analyzed by SEM
68
Journal of Iron and Steel Research, International
Vol. 14
Fig. 10 Microstructure of hot rolled plate after homogenization at 1 150 "c X 1 h followed by water quenching
residence time of bubbles, increasing gas/molten steel interfaces, and decreasing the sulphur and oxygen contents in molten steel. Nitrogen content of 0.69% in 18Cr18Mn was obtained using air induction furnace by nitrogen gas bubbling from porous plug. ( 2 ) Sound ingots of nickel-free, high nitrogen stainless steel could be manufactured using vacuum induction furnace with the addition of nitrided ferrochromium and electroslag remelting furnace under nitrogen atmosphere. T h e yield of nitrogen could be increased by lowering the melting rate. ( 3 ) Due to electroslag remelting under the condition of nitrogen atmosphere and the continuous addition of aluminum as deoxidizer to the slag, the loss of manganese decreased obviously. (4) There existed mainly irregular A l z 0 3 inclusions and MnS inclusions in ESR ingots, and the
size of most of the inclusions was less than 5 pm. ( 5 ) After homogenization of the hot rolled plate at 1 150 "C X 1 h followed by water quenching, the microstructure consisted of homogeneous austenite. References :
c11 c21 131
C4l C51
Simmons J W. High-Nitrogen Alloying of Stainiess Steels [J]. Materials Science and Engineering, 1996, 207A: 159-169. Paton B E , Saenko V Y , Pomarin Y M , et al. Arc Slag Remelting for High Strength Steel and Various Alloys [J]. Journal of Materials Science, 2004, 39(24) : 7269-7274. Stein G , Huchlenbroich I. Manufacturing and Application of High Nitrogen Steels [J]. Materials and Manufacturing Processes, 2004, 1 9 ( 1 ) : 7-17. Mudali U K , Raj B. High Nitrogen Steels and Stainless Steels [MI. Pangbourne: Alpha Science International Ltd, 2004. JIANG Zhou-hua, LI Hua-bing, CHEN Zhao-ping, et al. The Nitrogen Solubility in Molten Stainless Steel [J]. Steel Research International, 2005, 76(10) : 730-735.