Assessment of Cleanliness in Interstitial Free Steel T.K.Roy and T. Venugopalan Tata Steel
Requirements of IF steel Typical composition of IF steel C (ppm) Mn S P Si Al N (ppm) Ti 20 0.06 0.008 0.011 0.005 0.035 23 0.06 All are in wt%, unless mentioned otherwise Two important requirements of IF and LC Al-Killed Steels: Stringent Surface Quality Excellent formability Non-metallic inclusions impair both surface quality and formability
Steel cleanliness requirement for IF steel Steel product IF Steel (automotive & deepdrawing sheet) Max impurity fraction, ppm C<= 30 N<= 40 T.O <= 40 Max inclusion size 100 Micron Ref: Zhang et.al, 85 Steelmak Conf, 2002, p.431-52
Steel Product Requirements Japanese Plant: T.O <30 ppm direct shipment, no special inspection T.O - 30 55 ppm critical inspection T.O > 55 ppm downgraded Ref: Zhang et.al, 85 Steelmak Conf, 2002, p.431-52
Sliver in IF coil originated from non-metallic inclusions Clean Steel Low Inclusions How to measure?? Several Direct and Indirect Methods for Steel Cleanliness Measurement
Al and dissolved oxygen in RH Box Plot of RH_out Al Box Plot of Oxygen Celox, ppm 0.070 5.5 0.065 5.0 0.060 4.5 % RH_out_Al 0.055 0.050 0.045 Ox y gen Celox, ppm 4.0 3.5 3.0 0.040 0.035 0.030 Al Median = 0.0445 25%-75% = (0.0395, 0.0495) Non-Outlier Range = (0.034, 0.064) Outliers Extremes 2.5 2.0 1.5 Celox-3 Median = 3 25%-75% = (3, 4) Non-Outlier Range = (2, 5) Outliers Extremes Total Oxygen is generally accepted as a measure for steel cleanliness Steel cleanliness can also be assessed using ultrasonic C scan image analysis technique
Steel Cleanliness TO Approach
Total Oxygen sampling from liquid steel Two types of samplers used: 1.Special Cylindrical Sampler designed by NSC, Japan 2.Normal Dual Thickness Lolly-pop Sampler
Comparison of TO analysed using different samplers TO in Normal Sample (ppm) 50 40 30 20 10 1:1 line 0 0 10 20 30 40 50 TO in Special Sample(ppm) TO in tundish using two types of sampler TO in RH using two types of sampler TO in Normal Sample (ppm) 50 40 30 20 10 0 1:1 line 0 10 20 30 40 50 TO in Special Sample(ppm)
Total oxygen in steel at RH after treatment & at tundish
Effect of FeO and MnO in RH slag on TO in tundish
Effect of Conditioning on FeO and MnO in Slag
Total Oxygen and nitrogen pick-up in tundish TO in Tundish (ppm) 60 50 40 30 20 10 0 0 1 2 3 4 5 N Pick-up in Tundish (ppm)
Nitrogen pick-up and aluminium loss in tundish Al Fading in Tundish (ppm) 120 100 80 60 40 20 0 0 1 2 3 4 5 N Pick-up in Tundish(ppm)
Effect of aluminium loss at tundish on TO TO in Tundish (ppm) 60 50 40 30 20 10 0 0 10 20 30 40 50 60 70 80 90 100 110 Al Fading in Tundish (ppm)
TO in steel and sliver in cold rolled coil Sliver in CR coil (nos/km) 8 7 6 5 4 3 2 1 0 0 10 20 30 40 50 60 TO in steel (ppm)
Assessment of Cleanliness Using Ultrasonic C Scan Image Analysis Technique 10 MHz, 15 mm diameter focused beam probe with amplitude threshold 1.1 to 1.2 a.u.
Correlation between TO in tundish samples and ultrasonic counts 700 Ultrasonic Counts 650 600 550 500 450 400 25 30 35 40 45 50 Total Oxygen, ppm
Ultrasonic Counts vs Cleanliness Severity Level in RH and Tundish Samples
Relation between ultrasonic counts and TO for inclusion size 0.04-0.12 mm 2 Ultrasonic Counts 600 500 400 300 200 100 0 25 27 29 31 33 35 37 39 Total Oxygen, ppm Not much variation in population of smaller inclusions
Relation between ultrasonic counts and TO for the inclusion size range 0.12-0.20 mm 2 Ultrasonic Counts 140 120 100 80 60 40 20 0 25 27 29 31 33 35 37 39 Total Oxygen, ppm Increased level of TO is an indication of large size inclusions
Relation between ultrasonic counts and TO for inclusion size 0.20-0.28mm 2 100 Ultrasonic Counts 80 60 40 20 0 25 27 29 31 33 35 37 39 Total Oxygen, ppm Increased level of TO is an indication of large size inclusions
Some macroinclusions observed under optical microscope
Typical (a) SEM image and (b) EDS spectrum of a globular aluminosilicate inclusion
SEM-EDX results of typical inclusions observed in the samples SampID 97588 97593 <50 microns yes yes Types of inclusion Alumina, Alumino silicate Alumina, Alumino silicate Inclusion size Types of inclusion Calcium yes alumino silicate Calcium yes alumino silicate 50-100 microns >100 microns Types of inclusion Nil - Nil -l Alumina, yes yes Alumina Nil -l Alumino silicate 97596 97586 yes Complex NMI Nil Nil Nil -l 97972 yes Alumina Nil Nil Nil -l 97569 yes Alumina, Calcium aluminate Nil Nil Nil -l
CONCLUSIONS 1. Two types of samplers used for TO measurement. Normal dual thickness sampler showed marginally higher TO values compared to special sampler. This indicates that if required normal sampler can be used for TO measurement with reasonable accuracy which will be effective with regard to time and cost 2. TO observed to be varied with processing stages. The study revealed an increasing trend of TO in tundish with increased TO in steel at RH Degasser 3. A correlation observed between TO in steel in tundish and occurrence of sliver in cold rolled coils
4. Within scatter some trend between reducible oxides such as FeO and MnO in RH slag and TO in tundish was observed 5. The relation between Al losses in tundish with TO indicated reoxidation during casting. The reoxidation due to air entrainment was also evident from nitrogen pick-up. 6. Ultrasonic method can serve as a useful tool in determining steel cleanliness. The ultrasonic evaluation revealed deterioration of steel quality with increased TO with regard to the extent and size of inclusion in steel
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