Scale Exfoliation in Continuous Casting Molds - Operational and Safety Issues. Presented by: Christopher Hulings

Transcription

1 Scale Exfoliation in Continuous Casting Molds - Operational and Safety Issues Presented by: Christopher Hulings 0

2 Continuous Caster Basics Continuous Casting Mold provides the initial solidification for producing a steel section commonly referred to as a strand, slab, bloom, round, square, beam or billet Solidification occurs by passing large volumes of water through a mold which is effectively a heat exchanger compromised of carbon steel and copper components Water contacting liquid steel is a serious explosion hazard Although molten steel and water are in close proximity throughput a steel mill within the continuous casting mold the contact is quite intimate. The thickness of the copper that separates the cooling water from the solidifying strand can be less ¼ thick. 1

3 Continuous Caster Mold Water Entry Two primary ways water can enter the mold cavity: Hot face crack in the mold copper liner O-ring leak between copper and water jacket Both of the above issues are more likely to occur in a mold that is over heated. 2

4 Water Flow Rate Interruption Inhibiting the designed water flow rate can cause quality issues In more severe cases the mold coppers can be over heated allowing water to enter the mold due to a resulting hot face crack or mold o-ring leak In recent years the most common reason for inhibited water flow and over heating of mold coppers has been scale Partial blockages can inhibit flow enough to allow boiling to occur which can result in deposits on the cold side of the coppers which further inhibits heat removal causing the mold coppers to over heat In order to operate a caster safely and produce a quality product, preventing scale blockage issues and providing reasonable mold water quality is necessary 3

5 Carbon steel water jackets built more than 10 years ago have begun to experience what is reffered to as "Exfoliation Corrosion" 4

6 Steel pipe illustrating lifting of the surface grains in layers SD² Waterbox B1 7/10/08 5

7 When exfoliation occurs inside a mold waterbox the scale dislodges and is washed into the copper cooling channel inlets where it typically becomes lodged in the holes that feed water into the copper or can enter the copper blocking off vital water flow 6

8 Examples of Scale Issues Many customers with carbon steel water jackets are experiencing issues with scale blockages Minor scale blockages can cause intermittent quality issues due to biased water flow and Major blockages are a quality and safety issue 7

9 Thermal Effect of a Blockage 8

10 This is what the steel industry is facing as idled molds start running again: 9

11 A B C 10 D

12 The longer a waterbox sits without running the worse the exfoliation. This mold sat nearly a year and a half before disassembly 11

13 This mold sat unused for one year awaiting a trial and exfoliated scale was falling off the walls 12

14 Blockages in High Speed Molds Even minor scale blockages can result in caster folds (LFCs) and boiling and creation of problematic cold face deposits Less tolerant to scale issues than conventional or medium thickness slab casters High-speed casters are especially vulernable to overheating due to elevated operating temperatures Fillerbar Copper Rifle-Drilled Copper 13

15 High Iron Levels High ppm levels of iron in the mold water system can lead to cold face deposits, especially a problem for high speed casters Those molds which sit on the sidelines for weeks to months partially filled with water can not only become a scale problem but can also hit your mold water system with a punch to gut, biological growth and concentrated iron oxide solution 14

16 Calcium Deposit on Cold Face of Copper 15

17 Iron Deposit on Cold Face of Copper 16

18 Biofilm and Cupric Oxide Deposits are Black 17

19 Hot Face Leak into Mold Cavity 18

20 Scale Removal Using Acid Cleaning Acid cleaning works well on surface corrosion and small diameter piping i but acid cleaning to remove large areas of exfoliating scale can cause tremendous operational issues. In most situations where acid cleaning of the entire mold water system is completed we have seen the most dramatic cases of scale exfoliation becoming lodged within the molds for a 3 to 4 month period following the acid cleaning process. Before After 19

21 Grit Blasting and Powder Coating Solution Cleanouts added and internal grit blasting followed by powder coating eliminates the creation of scale. Re-blasting and recoating of the interior of the water jackets must occur approximately every 3 years. Before After Powder Coating 20

22 Engineering for Clean Outs and Strainers Additional clean outs are needed to allow for proper blasting and scale removal from all chambers in the water jackets Nearly all water jackets have a need for additional cleanout access in order to allow for proper cleaning and maintenance of water jacket internal cavities 21

23 Copper Inlet Strainers In 2005 began installing custom manufactured strainers across the bottom plenum chamber of CSP molds Strainers and added clean outs do not address the root cause of the scale but are cheap insurance against scale blockages from causing a dangerous water leak or slab quality issues Strainers prevent large scale that might not pass through the mold copper cooling channels from entering the copper 22

24 Convenitional and Medium Slab Caster Many conventional casters have a series of small feed holes drilled across the bottom of the plenum so a different approach was needed for these casters 23

25 Proposed Strainers for Slab Molds 24

26 Mini Strainer Baskets for Small Connecting Holes Installed SS mini strainers in bottom connecting holes Phase 1 25

27 Mini Strainer Baskets for Small Connecting Holes Larger scale exfoliation blinded strainers in Phase 1 Installation 26

28 Mini Strainer Baskets for Small Connecting Holes Phase 2 installation utilizes curved strainers to gain height 27

29 Front Mounted Large Strainer Baskets Installed SS basket strainers in plenum chambers and machined water jackets for additional clean out covers Internally blasted water jackets to remove loose scale and powder coated water jacket internals. 28

30 Quality and Safety Improved Casters implementing strainers have seen Reduced unexplainable breakout frequency Improved product quality reduced longitudinal cracks Improved copper life due to decreased meniscus cracking Any large scale exfoliation issue is contained before safety becomes an issue 29

31 Ultimate Solution Strainers are an immediate solution to prevent current problems related to scale but do not address the root cause of the scale formation Internal Blasting and Powder Coating can be used as a long term solution to stop scale exfoliation 30

32 External Corrosion Speciality Coatings External corrosion of water jackets and cassette backing plates caused from acidic conditions in containment can be reduced SMS Millcraft has three robotic spray areas on site with the ability to coat items up to 20 feet long and 6 feet wide The processes were originally developed to reduce wear on mold coppers and have been in place since the 1990 s 31

33 Caster Segment & Secondary Cooling Sprays Caster Segment Water System Main Functions Effective cooling of caster roll solid and split bearings Increase life and efficiency of bearings Cooling of caster segment frames Maintain safe operating temperature of segment frame structures t to minimize i i deformation and damage Secondary slab cooling Secondary sprays provide additional required cooling of the cast product which directly affect product quality 32

34 Caster Segment & Secondary Cooling Sprays Caster Segment Acid Flushing Setup 33

35 Caster Segment & Secondary Cooling Sprays Caster Segment Water System Maintenance ChemTreat has trained SMS Millcraft employees to use citric acid in a three step process for cleaning the internal machine water system of caster segments and this practice is in use at several SMS Millcraft facilities. The process above cleans the internal piping and water passages of bearing housings. The citric acid cleaning process is strong enough to remove e iron oxide and mineral deposits s without causing severe damage to the segment structure and welded joints on bearing housings. Mechanical cleaning of secondary cooling sprays is imperative to the operation. Both hydraulic and atomized spray systems can be greatly impacted by debris small enough to plug an orifice less than 1MM in diameter. 34