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					Design and Process Improvements to Increase Performance and Maintenance Efficiencies on
                            Continuous Caster Equipment


                       Jeffrey Brower/General Manager of Technology and Quality Assurance
                                           Joseph Didwall/Vice President
                             Voest-Alpine Services & Technologies/Pittsburgh, PA, USA
                             Kurt Engel/Head of Design Management Continuous Casting
                                   Voest-Alpine Industrieanlagenbau/Linz, Austria



                                                INTRODUCTION

Within the steel industry today, manufacturers are more aware than in any previous time of the influences that
maintenance processes and practices have on the overall cost of steel production. Only through a planned approach
between a maintenance service company and the steel producer can maintenance optimization be achieved. The
foundation of such an approach is the systems required to measure and monitor the equipments’ performance. As
enhancements are implemented, reliability and component lifetimes are expected to improve. With the systems in place,
it is then possible to effectively evaluate potential enhancements with a high degree of probability for what the
enhancement will yield in terms of cost benefits.

As a primary vendor to the U.S. and Canadian steel industry, Voest-Alpine Services and Technologies, has extrapolated
maintenance and design issues during the reconditioning of Continuous Casting Machinery from all equipment
manufacturers. In the United States and abroad, the conventional theory is for steel mill operations to focus on
production. With this philosophy, a majority of the mold, zone / bender, and segment reconditioning services are
outsourced to vendors such as VAST. Though different in design, appearance, and style, the principles for continuous
casting machinery from various OEM sources are very similar. By breaking down the primary components, we have
derived known and identified Maintenance and Operations issues for conventional and thin slab casters of various
designs. By understanding the variables and through positive customer interaction, VAST has initiated and implemented
engineering changes, process recommendations, as well as modified maintenance procedures. VAST’s intentions are to
introduce Maintenance, Operations, and Engineering personnel to modifications of continuous caster equipment and
maintenance practices while identifying the measurable techniques we utilize to define product performance variables.


                                             MAINTENANCE COST

Through ongoing improvement in the areas of design, maintenance practices, processes, and operational improvements,
maintenance cost can be significantly reduced. Two examples of long term maintenance cost reduction include: one for
a conventional slab caster Figure 1, and one for a high speed thin slab caster shown in Figure 2. Each of the three
improvement areas cited above can have a significant impact on the reduction of cost.
                                              120%

                                              100%                            100%




                          Percent Reduction
                                                                                                               CPT %
                                              80%

                                              60%

                                              40%
                                                                                                28%
                                              20%                                                                 16%
                                                                                                                                        12%                  12%
                                               0%
                                                                           1990            1993                1997               2000                   2004
                                                                                                                                                      Expectation
                                                                                                               Years



Figure 1. Long Term Maintenance Cost Reduction/ISG Sparrows Point Original Caster Design/Conventional Slab Caster

                                                                    120%

                                                                    100%          100%
                                                Percent Reduction




                                                                    80%
                                                                                                                             CPT %

                                                                    60%

                                                                    40%
                                                                                            28%
                                                                    20%                                  16%
                                                                                                                       12%        12%            9%             9%       7%
                                                                     0%
                                                                             1997        1998         1999       2000          2001           2002         2003      2004
                                                                                                                                                                     Expectation
                                                                                                                        Year

               Figure 2: Long Term Maintenance Cost Reduction – NSBHP High Speed Thin Slab Caster

In terms of what drives maintenance cost for a continuous slab caster, there are three main components:

a) Molds
b) Benders / Zones
c) Segments

While there are some differences between the percent of cost to maintain a conventional slab caster Figure 3, versus a
thin slab caster, Figure 4, the order of overall magnitude remains the same.


                                                                                                                                  Molds
                                                                                                                                   26%




                                              Segments
                                                57%                                                                                      Benders / Zones
                                                                                                                                              17%




                  Figure 3. Maintenance Cost by Component/;2002 ISG SP Conventional Slab Caster

                                              Benders / Zones
                                                   19%

                                                                                                                                                 Segments
                                                                                                                                                   48%




                                                                      Molds
                                                                       33%



                Figure 4. Maintenance Cost by Component/2003 NSBHP High Speed Thin Slab Caster
                    MONITORING AND OPTIMIZATION OF COMPONENT VARIABLES

The maintenance supplier should be able to independently identify key performance variables. Through management,
quality and production personnel, a quality system that incorporates inspection documentation and the necessary
tracking mechanisms provides the foundation to adequately define a process, analyze a process, and promote future
product trial and equipment modifications. The best tool to verify these elements is customer provided product
performance information. Customer feedback for the primary equipment is compiled in a database that is representative
of a historical summary for cost, tonnage, and reason for removal. This information provides a feasible and tangible
measurement device to extrapolate trends and to analyze cost and performance in a proactive environment.

Additional information such as dates in the shop, labor, and purchased goods expenses are inclusive in the historical
database. It is also possible to deduce the primary and supplementary reasons for equipment extractions. Targeting the
major reasons for equipment removal from a maintenance aspect is the basis for equipment improvements.

For rolls and coppers, the development of a tracking database is established for all customers.The database provides
detailed information from each service campaign providing a good measurement device for tonnage, wear, stock lost,
removal per campaign, and the monitoring of trial coatings and overlays. Upon further review, patterns of performance
can be observed.

As an example, we can utilize actual customer performance figures to justify the application of Hiper-Coat plating to
100% of the narrowface plates at ISG-Sparrows Point (Figures 5 and 6). Upon reviewing the database, we extracted
pertinent comparison data for like products and began to see identifiable comparison figures. This eliminated the
human perception of performance attributes and introduced a factual based justification to assist in the decision making
process.

                                                          Average Heats

                            120%

                                              100%
                            100%


                             80%
                                                                                 68%

                             60%


                             40%


                             20%


                              0%
                                            Hiper-Coat                          Nickel


                         Figure 5. Hiper-Coat Average Heats vs. Nickel (Breakouts Included)
                                                         Average Wear in MM's

                              4.0
                                                                                   3.53
                              3.5

                              3.0

                              2.5

                                                1.93
                              2.0

                              1.5

                              1.0

                              0.5

                              0.0
                                            HIPERCOAT                             Nickel




                         Figure 6. Hiper-Coat Average Wear vs. Nickel Per Casting Campaign
When comparing the average heats, we can accurately determine for this application that Hiper-Coat outperforms
nickel by 39%. Though the heat averages are lower for nickel, the wear rate is 45% higher than that of Hiper-Coat.
Based on these factors, if we consider average mold reconditioning costs coupled with average copper expenses, we
can determine the true cost savings from applying Hiper-Coat to 100% of the narrowface plates.



                          Cost projection:
                          HIPERCOAT full reconditioning cost:                $2550 ea.
                          Nickel Sulfamate full reconditioning cost:         $2357 ea.
                          Average mold reconditioning cost:                  $28,200     per mold
                          7000 heat per year average:

                          Stand alone HIPERCOAT recognozed savings:
                          HIPERCOAT cost is 7.5% greater than Nickel Sulphamate. However, applying HC
                          to 100% of the narrow coppers would recognize an overall savings of:

                          7000 heats / 388 HC heat average = 18 molds per year:               $2550 x 36 =   $91,800 OA
                          7000 heats / 262 NS heat average = 26.7 molds per year:             $2357 x 53.4 = $125,863 OA
                                                                                                    Savings: $34,063 OA

                          Mold recognized savings:

                          7000 heats / 388 heats avg mold life = 18 MPY:                   $28,220 x 18.0 =    $507,600 OA
                          7000 heats / 262 heats avg mold life = 26.7 MPY:                 $28,220 x 26.7 =    $753,474 OA
                                                                                                      Savings: $245,874 OA

                          $279,937 USD Overall recognized savings per calender year upon conversion to HC.



                    Figure 7: Cost Comparison Using Known Data to Justify Return on Investment

As described in Figure 7, Hiper-Coat is slightly more expensive to apply. However, the average heats of 388 vs. 262
for nickel would result in 18 less copper plates reconditioned per year and 9 less molds for this operation. The result is
a stand-alone Hiper-Coat savings of 34,063 USD. When considering the average mold reconditioning of 28,220 USD,
the overall yearly maintenance savings for increased mold life is projected to be 279,937 USD. This figure does not
include steel mill operational savings and reduced down time resulting from lower equipment change outs.


                                             MOLD MAINTENANCE APPROACH

Within VAST, customer issues that affect equipment performance and longevity are addressed and rectified through
technical expertise. This philosophy is openly shared and communicated with our customers. Dependent upon customer
expectations, our mold based services range from single source mold reconditioning programs to refurbishing copper
plates for all types of OEM continuous casting machines. Our customers often define their objectives as the following:

•   Increase Production Efficiencies
•   Increase Equipment Performance
•   Reduce Costs

We define our customer/maintenance focus as the following:

•   Maintain Positive Customer Relations
•   Provide the latest technology
•   Reduce Maintenance Costs
•   Strive to Increase Equipment Longevity
•   Reduce our own internal process variation

Whether performing internal mold maintenance or relying on outsourcing, a basic understanding of ladle capacity (heat
size), tundish temperature range, casting speed, cast width, range, start-up practices, product origin and mix (scrap or
ore), and finally casting temperatures must be understood. For the companies that concentrate on in-house maintenance,
the limiting maintenance factors include spare parts availability, accessibility to specialists, and resources to perform
routine repairs. These issues result in a majority of maintenance delays. Equipment repairs performed in-house do not
always get the attention they deserve due to the multi-tasking efforts of mechanics (The operations priority can
supersede the maintenance priority).
During the mold maintenance phase, there are often many assumptions. The most predominant is that disassembling the
mold and returning it to print specification is acceptable. This practice may meet the customer’s current expectations,
however there is little allowance for equipment improvements. We have found that simple variations in mold copper
wear patterns, footroll corrosion and wear can be the result of operational practice changes or deviations. More often
than not, operations and product quality dictate the maintenance program regardless of mold life. Open
communications and promoting the latest mold maintenance technologies is often the only opportunity for maintenance
cost reductions.


                             MOLD DESIGN COMPARISON AND IMPROVEMENTS

Though all continuous casting molds are designed with similar capabilities, their appearance, cost, and functionality
differ. Simple things such as plate thickness on mold frames and water jackets can have a significant impact in
operations. Some manufacturers use thin material in the mold frames. It is perceived this promotes a lower up front
cost, but the down stream expenses are not realized until the commissioning phase. Once standard performance
variables are met, the OEM proceeds to another project. It is at this time when the true cost of the mold is recognized.
We have identified the following items as critical to a mold operation and have proposed the associated
recommendations and or improvements:

                                  MOLD CLAMPING CYLINDER DISC SPRINGS

Some current OEM mold designs incorporate a hard clamping system that relies on hydraulic pressure to override
resultant spring force. Others rely on individual mold clamping cylinders to counteract the ferrostatic force. As
supplied, mold maintenance procedures typically do not allow for any variation in spring pack calibration. Therefore,
once hydraulic pressure is applied there is no way to control the mold unclamp gap or width. The process assumes that
all disc springs are identical. In investigations, we have identified collapsed spring packs that produce clamping forces
thousands of pounds less than OEM specification. The result is extreme difficulty with on-line width changes coupled
with the potential for excessive broadface copper gouging and wear. This was confirmed as individual mold spring
packs were removed from clamping cylinders and their overall height verified against the OEM specified height.
Further test using a calibrated load cell bolstered the results.

By design, disc springs have a built in height tolerance of +/- 5%. For larger springs where up to 24-30 are used for an
application, deviations of several hundred to a thousand pounds are not uncommon, even with new springs (Figure 8).
To eliminate any doubt that disc spring packs meet the proper OEM spring forces, VAST developed a system to
calibrate individual spring packs using a calibrated load cell (Figure 9). As four spring packs are typical on a mold, it is
common that each has it’s own pre-load dimensional values. The result is a consistent clamping force typically set to
within +/- 100 pounds of the desired specification forces. The desired operational benefit is the elimination of finning
and bleeders due to excessive corner gaps. Our experiences dictate that even if springs are acceptable for reuse after
multiple campaigns, they should be discarded and replaced after only two casting sequences. In any event, the
calibration procedure discussed during each repair campaign will identify all collapsed or damaged springs.
 Figure 8: Taking the +/- 5% tolerance into account, clamping force variables of 7109# to 7858# can be derived even
             though the design parameters say the proper force would be achieved just by setting the preload to a set
                                                           dimension.

                                                                                    FREE LOAD HGT

                                                                                   PRE-LOAD HGT

                                                                                                        Load Cell
                                                                                                              End Cap



                      Tie-Rod




                         Hollow Ram Hydraulic Cyl.

                                                     Disc Spring #: 60 OD x 25.5 ID x 3 THK
                                                                                                    To Load Cell Display



  Figure 9: Spring pre-load calibration device using load cell to set desired force. Dimensional value between top and
                                                      bottom spring is recorded.


                                                     OPEN GAP SHIM

Some OEM designed molds were supplied with carbon steel “Open” or “Un-Clamp” Gap Shims. It was observed that
during normal operations, these shim plates would corrode and fuse to mating parts causing them to stick in place. If
stuck in the up position, the possibility of a breakout is greatly increased.

Upon this discovery, the shims were replaced with Stainless Steel and the handles were extended. The result was the
shim could be moved with ease. The extended handles also alerted operators when they had been left in the up position
by hitting the mold cover. The cost to convert all molds to this configuration is 8,560 USD over a (1) year period. The
overall cost benefit is estimated to be 81,514 USD per year.
                                       COPPER BASE ALLOY TECHNOLOGY

The primary points of slab contact within any caster are the mold copper plates. The typical base materials are Silver
Bearing Alloy (CuAg) and Chrome Zirconium Copper (CuCrZr). For applications where low speed and lower
temperatures are part of the steel making process, CuAg is adequate. However, the recrystallization temperature is 48%
less than that of CuCrZr. In addition, the tensile strength in N/mm2 is 33% less. These factors are not always conducive
to today’s high speed / high temp casters. As shown in Figure 10, vertical striations in the meniscus region of a nickel-
plated copper represent cracking that propagates through the nickel and into the base alloy. This situation is not always
induced by high temperature alone, but by inconsistencies within the expansion factors of the base material and the
outer coating. Such cracks can propagate into the base copper causing significant heat transfer issues between plates.




           Figure 10: Meniscus Region Narrowface Cracks Shown on CuAg Plate With Standard Ni Plating.

A major factor in the initiation and propagation is the presence of Zinc, a by-product of galvanized scrap often used in
the melting process. At elevated levels during casting, Zinc has been proven to cause hydrogen embrittlement by
attacking the nickel substrate and destroying the grain boundaries. This attack proceeds into the nickel / copper
interface and ultimately into the base alloy. During the copper-reconditioning phase, it is critical to remove every sign
of this condition by means of machining and non-destructive testing. Failure to do so may leave cracks that are not
visible without the use of special equipment. After plating and re-installation into the caster, the base alloy cracks will
split under temperature and progress deeper into the plate. When this situation is present on an ongoing basis it can be
the primary inhibiting factor of overall copper plate life. In fact, we found that new copper plates would often be
scrapped after a single casting campaign. When a steel mill spends 750,000 USD to 1,5000,000 USD per year on
coppers, there is huge potential for cost savings.

It should be noted that research into the optimal base alloy material that is capable of extreme variations of
temperatures and mechanical effects is ongoing. At this time we have had excellent success with a refined CuCrZr
material. Upon the development of this material through a VAST/Supplier relationship, we sought the most extreme
trial for this product. This high temperature / high-speed application incorporates scrap into the melt. The result was
narrowface coppers that averaged 5.75 campaigns due to minimized cracking as opposed to the previous 2.06
campaigns per copper, see Figure 11.
                                      A v e r a g e C a m p a ig n s p e r N a r r o w f a c e C o p p e r
                                  7


                                                5 .7 5 c a m p a ig n s
                                  6
                                                                                           R e f in e d C u C r Z r
                                  5
                                                                                           B a s ic C u C r Z r
                      Campaigns




                                  4



                                  3


                                                                              2 .0 6 C a m p a ig n s
                                  2




                                  1



                                  0
                                                                          1
 Figure 11. Refined Base Alloy vs. Basic Alloy Resulted in 3.69 Additional Casting Campaigns for Narrowface Plates

                                      COPPER MAINTENANCE PRACTICES

Vertical and horizontal gouging (Figure 12) between the narrowface and broadface copper plates can be a limiting
factor in mold life. This condition is often observed after the first heat and seldom gets better during the mold
campaign.


                                          Top of Copper

                                                                Meniscus




               Figure 12. Vertical striations and horizontal marks on broadface surface in NF width range.

Though a detailed analysis of both the mold operational and maintenance practices is the only way to develop a root
cause, there are many factors to research. Operationally, any of the following can affect this phenomenon:

1.   Excessive or uneven clamping as a result of incorrect clamped of un-clamped gap specification.
2.   Incorrect clamping force as a result of improper spring force specification, collapsed springs or out of calibration
     clamping springs at reconditioning. For hydraulic systems, the ideal setting would be a clamping force that
     provides continual feedback and clamping pressure to counterbalance the ferrostatic head. Some simple systems
     only provide a single hydraulic pressure setting to counteract the slab. The theoretical design values do not often
     match the actual forces.
3.   Poor mold packing procedure at start-up
4.   Poor mold cleaning practices
5.   Mold powder build-up on NF back-up plate behind the copper edge.

From a maintenance standpoint, we have identified the following elements as possible contributing factors:

1.  Friction generated by similar NF and BF metals during width changes. This can be controlled by eliminating
    plating on the BF in the temperature zone or by changing plating on either.
2. NF assemblies out of alignment causing a corner to force against the BF. This can be confirmed by unclamping the
    mold and verifying an equal distance between the edges. Realign as required.
3. Excessive nickel edge plating on NF’s. Nickel restricts heat transfer by roughly 4.15:1 over standard copper.
    Excessive nickel edge thickness has been observed to cause NF thermal expansion in excess of the un-clamped gap
    settings. Our recommendation is to copper plate the edges during refurbishment. However, if nickel is used the
    thickness should be restricted to under 1.0 mm if possible.
4. Excessive deviations in copper thickness between the respective broads and narrows can cause uneven expansion
    and cooling. Our implemented practice is to restrict the thickness deviation between plate sets to 0.75 mm.
 5. When the flatness of broadface plates is out of tolerance it can interfere with NF movement. As NF plates are
    usually set to a specific taper at a certain casting width, this issue would not be known until casting has
    commenced. A maintenance program that provides documented feedback by recording the finished copper
    machining flatness after the plates have been installed into the mold is recommended. In extreme instances where
    the copper-mounting surface within the mold is deformed or outside of flatness tolerances, it is recommended that
    all finish copper machining be performed on the mold frame, water jacket, or cassette (Figure 13).
                    Figure 13. Depiction of BF Copper Machining Set-Up for a U-Frame Style Mold

VAST recommends that all of the previously mentioned should be evaluated to develop an action plan prior to any
major changes in current practice. From there, a systematic approach to eliminate the variables is an excellent course of
action.


                        COPPER PLATING APPLICATIONS AND PROCESS QUALITY

Standard copper alloys in their base form provide little wear resistance for casting applications. The primary benefit is
conductivity for heat removal. For standard nickel and high hardness plating applications, there are typically three
primary applications when coating copper plates:

1.   Step Nickel Plating
2.   Fullface Nickel Plating- with constant or variable thickness
3.   Using either of the first (2) with a flash of chrome.

 To extend copper life, historical maintenance practices incorporated peening or hammering of the NF edges to return
them to their operational width. Today, improvements in plating technology allow for plated edges that do not peel or
separate during extreme casting applications. VAST incorporates the following edge plating processes as standard
procedure:

1.   Copper edge plating- Undercut each of the NF edges until they clean up and plate to the desired thickness.
     Standard copper edge plating is typically consistent with the mechanical properties of CuAg regardless of the base
     alloy. It is important to note that edge plating must be applied and machined prior to any hotface plating.
2.   Nickel Edge plating- Same procedure as above. However, due to the difference in thermal properties, a maximum
     of 0.5-1 mm thickness is recommended.

The control of every detail within the plating process is critical to provide and promote our products. Within VAST or
any other maintenance company, the customer relies on our behind the scenes efforts to provide the services he is
paying for. Plating coppers is a science and relies on critical control mechanisms to reduce variance within the process.
The variables that must be understood are numerous as shown in Figure 14.
                                        Categories that Represent Possible Plating Issues

                                                                   TANK
                                                RECTIFIER       OVERHEATING
                                                 FAILURE            9%
                                                   2%
                                           TOO HARD
                                                                              ADHESION
                                              2%
                                                                                28%
                                            PITTING
                                               9%
                                        MASKING
                                          2%
                                                                                    APPEARANCE
                                    DEVELOPMENT
                                                                                        2%
                                         2%
                                                                                    SETUP
                                                                                     2%
                                            LOW SPOTS                     INSUFFICIENT
                                               24%                         THICKNESS
                                                                              18%




        Figure 14. Identification of Problem Areas Within the Plating Process to Target Reduction of Variation

Within VAST, our focus is to provide the customer with a product that continually meets the intended product
expectations. In the chart above, the overall issues represent roughly 1% of the total plated products in a given year.
Having control over our process by being able to identify and rectify any deficiencies is the foundation for continual
improvement.

How this relates to product performance is simple. Any electrolytic plating process relies on the proper bond of (2)
often, dissimilar metals. The potential for the introduction of foreign or organic materials into the process or plating
bath is constant, which can cause improper plating adhesion and ultimate failure in operation. In addition, the proper
copper plate pre-treatment to remove any organic materials such as oil or sulfur prior to plating is essential for the
desired adhesion. For nickel-plating, an adhesion issue can be detected by reviewing the failed sample. When
separating the nickel from the base copper alloy, a secure bond will pull residual copper away with the sample.


                                         COPPER COATING TECHNOLOGIES

Standard nickel-plating in the elementary form provides only marginal wear resistance over copper. Both are relatively
soft and create friction with the slab. The result is often a mold that is operationally sound with the exception of
excessive wear. For some, this is acceptable. For others, a primary component to increasing mold production and
reducing maintenance expense is the introduction of improved wear resistant products. VAST-Sumitec promotes a
family of nickel boron alloy mold coatings to provide a significant amount of wear resistance over standard products.
This technology is known as Hiper-Coat. Hiper–H3 is the latest Hiper-Coat plating technology available at VAST-
Sumitec. This extremely advanced coating is our hardest and toughest electro-plated nickel-boron alloyed mold copper
coating with hardness ranges upwards of 900 Hv. This Super-Hi-Hard Hiper-Coat plating is precision ground to
achieve desired profile and thickness. The hardness relationships can be seen in Figure 15.
                      1000

                       900
                                        Hardness in Vickers                                           900


                       800
                                                                                            700
                       700

                       600

                       500                                                    450

                       400
                                                                  300
                       300
                                                        200
                       200
                              110         130
                       100

                        0
                             CuAg        CuCrZr       Base Ni   Hard Ni   Hiper-Coat     VH Hiper   Hiper H3



        Figure 15. Typical upper hardness range in Vickers for VAST-Sumitec supplied and marketed products.
The application of Hiper-Coat is very similar to that of standard nickel. However, the introduction of hardening agents
at defined intervals during the plating process allows us to achieve controlled results. Dependent upon the casting
application, we are able to provide a finished product with either a constant or variable hardness over the entire plated
hotface region. For high temperature applications, variable hardness Hiper-Coat allows for a ductile meniscus region
with the standard properties of base nickel (roughly 200 Vickers). The lower half to two-thirds of the plate obtain
hardness within a range of 650-700 Vickers. This is particularly important to prevent abnormal cracking, flaking, or
peeling that could interfere with often brittle high hardness plating. The basis for applying Hiper-Coat is to increase
mold life while significantly reducing wear as demonstrated in Figures 16 and 17.



                                                Sumitomo Mold Average Tonnage (HIPERCOAT vs. Nickel)

                                    90000
                                                   Nickel                  HIPERCOAT                HIPERCOAT
                                                 1998-2000                  2001-2002                  2003
                                    80000
                                                                                                       73,095
                                    70000

                                    60000
                                                                            51,645
                       Tonnage




                                                 46,860
                                    50000

                                    40000

                                    30000

                                    20000

                                    10000

                                          0
                                                   1                           2                         3




            Figure 16: Average mold life before and after application of Hiper-Coat to narrowface coppers.


                                                              Sumitomo Average NF Copper Wear


                                        0.130
                                                                                        0.119
                                        0.120
                                        0.110             HIPERCOAT
                                        0.100
                                                          NICKEL
                                        0.090
                       Wear in inches




                                        0.080
                                        0.070
                                        0.060
                                        0.050
                                        0.040
                                        0.030
                                        0.020
                                                                   0.010
                                        0.010
                                        0.000
                                                                   2001-2003       1    1998-2001

   Figure 17: Though mold lifetime improved, narrowface copper wear was reduced by 92% per copper campaign.

Operationally, Hiper-Coat has similar thermal properties to that of standard nickel. However, due to the improved
wearability, thinner Hiper-Coat layers can be applied. For instance, an application that called for 3 mm of constant
thickness full face nickel can now be Hiper-Coated with a thickness range of 0.5 mm at top, and 1.5 mm at the bottom.
The calculations to determine plating application costs are typically expensed by square inch. As a result, Hiper-Coat
cost is only marginally higher than the cost of standard nickel.

Hiper-Coat is also a common product for broadfaces and extremely suitable for any conventional or high speed casting
application. When compared to the alternative broadface coating applications, Hiper-Coat is the most effective avenue
to reduce maintenance costs. Even in its hardest form, machining can be easily performed using standard machinery.
                                                      CONTINUOUS CASTER ROLL OPTIMIZATION

One of the main cost drivers in continuous caster maintenance are the rolls themselves and the roll related components.
Derived from years of data collection and analysis, we have developed the processes necessary to customize roll design
characteristics to optimize roll performance on an individualized basis.

In striving to optimize roll performance, it is necessary to evaluate from the standpoint of the return on the investments.
The variables to consider in this evaluation are:

Initial (one time) investment
Long-term maintenance investment
Lifetime improvements
Reliability improvements

Also, in most cases, because the component changes occur on an ongoing basis, consideration must be given to the
utilization of existing components in the design change. Not considering or planning for this can result in the creation
of obsolete residual inventory that can more than equally offset any potential gain in return that is expected.


                                                                              WELD OVERLAY

One of the roll attributes that can significantly affect reliability and lifetime with only modest increase in investment, is
the weld overlay deposit that is applied to the work surface of the rolls as well as other surfaces that require accurate
geometric tolerance that maintain reliability i.e. bearing and seal journals and bearing and seal bores.

One example of customizing the overlay deposit for optimal performance that can be given are the driven rolls that
were installed at ISG Sparrows Point on the new wide continuous slab caster in 2000.

Shortly after startup, it was discovered that the wear on the driven rolls was a problem area where prolonged lifetime
would be a significant cost benefit.

The current overlay deposit being used is WELDCLAD 2000, applied with the submerge arc process. It was selected
over the previously used deposit, due to the high level of resistances to abrasive wear and a suitable level of resistance
to corrosion.

The comparative performance results from the two different overlays described above are illustrated in Figure 18.



                                                                    Weld Overlay Comparision on 300mm Drive Roll
                                                                                 After 1.5 Million Ton

                                                                                         0.04



                                                             0.04
                                Loss of Material (inches)




                                                            0.035

                                                             0.03                                    0.015
                                                            0.025

                                                             0.02

                                                            0.015

                                                             0.01

                                                            0.005

                                                               0
                                                                      Original Overlay    1     Weldclad 2000


                                                                      Figure 18. Weld Overlay Comparison
Economically the benefit of this improvement is expected to yield a 500,000 USD savings in maintenance cost over a
duration of (10) years.

This is only one specific example where a change in the weld overlay deposit on a continuous caster roll application
can have a significant impact in overall maintenance cost.


                                           ROLL MATERIAL – SEGMENT

Driven and idler rolls within bow, straightener and horizontal segments of continuous casting machines have been
exposed to increased stresses due to production levels exceeding original machine design. Newer generation machines,
(specifically high-speed, thin and medium slab casters) have been designed more compact in size to minimize floor
space requirements and capital investments. Scaled down roll diameters and increased casting speeds are two of many
factors that result in rotational bending fatigue causing journal failures due to higher number of stress cycles generally
not experienced in conventional slab casters.

When materials are subjected to many cycles of reverse bending stresses, failure may occur, even though the maximum
stress at any cycle is considerably less than the value at which failure may occur if the stress were constant. Corrosion
fatigue and thermal fatigue aid in crack initiation and propagation.

In order to minimize or potentially eliminate the failure problem within high speed casting machines or conventional
machines operating well beyond the original tonnage capacity, various improvements have been introduced.

A change in base material chemistry to a forging with high mechanical properties and elevated thermal fatigue
resistance has proven successful. 21CrMoV511 a DIN specification developed in Germany is the ideal caster roll
forging for journal design rolls. Due to a 0.20% Carbon content weldability is excellent and mechanical properties
exceed those of all other commonly used forging specifications (Figure 19).


                                                   Yield(KSI)         Impact(Ft-lbs)
                               21CrMoV511                100                    110
                                    8620                  45                    90
                                    4130                  75                    60
                                    4140                  90                    65
                                    4340                 100                    70


                                 Figure 19: Yield and Impact Strength for Roll Material


                                     ROLL MATERIALS - TOP ZONE ROLLS

Depending on the OEM design, Top Zones are located either directly below the foot-rolls or if no foot-rolls exist,
directly below the mold. Regardless of the actual location, generally the upper part of a top zone remains exposed to
high levels of corrosion and surface deterioration due to elevated temperatures, secondary cooling and mold powder
residue.

We have received such a top zone roll for evaluation and recommendations for base materials or potential overlays.
The roll body was manufactured of solid 17-4PH material. After 160,000 tons of service the zone required removal due
to the severe deterioration of the roll (Figure 20). The customer’s goal was a zone lifetime of 300,000 tons and the roll
in question was the weak link. The analysis concluded that the primary factor of deterioration was the very corrosive
type of mold powder. Elevated temperatures and secondary cooling accelerated the corrosion.
                                Figure 20: Surface Deterioration After 160,000 Tons

Based on all gathered information, our recommendation was a Nickel based overlay material (Ni content >50%) vs. a
ferrous material. Due to the extremely high cost of Nickel based consumables, the overlay was applied approximately
0.050” thick with the GMAW process, on a 21CrMoV5-11alloy forging. The trial roll was in service for 280,000 tons.
Follow up testing concluded minimal loss of diameter (Figure 21) and surface deterioration (Figure 22). Hardness
levels remained within the as-welded levels.




                                  0.120
                                  0.100
                                  0.080
                                  0.060
                                  0.040
                                  0.020
                                  0.000
                                                     Max. Ø Loss

                                              17-4PH    Nickel Overlay



                                      Figure 21. Loss of Diameter Comparison




                                  Figure 22. Surface Condition After 280,000 Tons
Based on the results, the trial was considered a success, with the exception that a relatively expensive roll turned into a
disposable part after a single service campaign. In order to allow for at least a second use by skim machining, the
overlay thickness was increased to 0.100”. To minimize journal corrosion, a 0.010” thick layer of Nickel spray coating
was applied.


                                   BEARING APPLICATION IMPROVEMENTS

On many thin slab casters, the roll design is such that it does not allow for a significant cross section in the journal
region. While the geometric design is usually adequate to perform without premature failure under normal operating
conditions, it is many times not sufficient to accommodate increased rotational forces (non normal) that are prevalent in
all casting processes. These higher forces applied to the typical journal design almost always result in premature
journal failure.

In some select applications we have installed a toroidal roller bearing. See Figure 23. This type of bearing has many
inherent benefits e.g. it is self-aligning, has unconstrained axial displacement and a very compact cross section allowing
for an increase in journal diameter and flexibility in overall journal geometry.




                                           Figure 23: Toroidal Roller Bearing

When the bearing combined with an enhanced journal design was introduced in several caster roll applications, it
resulted in the elimination of premature journal failures that were previously experienced at the rate of over twenty per
month.


                                              ROLL JOURNAL DESIGN

Another successful enhancement has been to increase shoulder radii. For example, an increase from a 12mm radius to a
20mm radius will decrease stress concentration by 15%. In most instances, roll and associated component design will
allow for such a radius change to be undertaken with little to no difficulty or a need to manufacture new components.

Typically, inboard journals experience the greatest stresses. For example, calculations have shown that depending on
casting width, the load carried by the center bearings can be between 25% and 50% of the total load (ferrostatic
pressure, soft reduction etc.). On the other hand, the outer bearings carry as little as 5%. Interestingly enough, the
majority of roll designs have the smaller transition radii on the inboard journals.

Without exception, all radii changes performed to customers’ rolls have proven successful. In some instances journal
failures have been eliminated entirely and other factors such as bearing and seal life or roll wear became primary
reasons for roll removal.


                                                BENDER ROLL SEALS

Traditionally, zone and bender roll shells utilize a lamerllar or piston ring seal design. This type of seal wears grooves
into the seal bore and eventually allows a significant amount of debris to bypass the seals and enter into the bearing
area. This creates two problems. First, it results in the roll sticking and eventually bearing failure. Secondly, when the
roll is rebuilt, it requires the seal bore to be extensively reconditioned see Figure 24.
                                            Figure 24. Original Seal Design

To eliminate the bearing contamination and grooving, a double lip seal using high temperature material was introduced
(Figure 25). Roll failures due to seized bearings have been reduced to less than 1% and lifetime of the units extended
by 50% from an average of 45 days to 60 days.




                                              Figure 25. New Seal Design


Even though the cost of the lip seal is approximately four (4) times the cost of the lamellar ring, the decrease in unit
rebuilds and respectively increased production levels offset the additional investment by far. In addition, the used
sleeves originally housing the lamellar ring were re-machined to accommodate the lip seal and retaining ring, allowing
the use of existing inventory without the investment for new components.


                          PERIPHERALLY COOLED ROLL PORT ENHANCEMENT

Lower in the machine, “Peripherally Cooled Rolls” are used in order to increase cooling of the entire roller body on
long body roll designs. This additional cooling enhances the rolls resistance to bending under elevated temperatures i.e.
high-speed casting or on conventional machines during strand stoppages and creep speed operation. Often, a long body
roll will permanently bend during strand stoppages.

In general, these rolls are either designed with pipe plugs or removable seal rings closing the peripheral cooling bores
in order to provide access for cleaning the bores during reconditioning. Sealing cannot be assured once the counter-
bores (seal locations) have corroded during service. Up to 0.030” oversized bores have been experienced and
compression on the O-ring was not adequate any more to seal properly.
                                                                  Peripheral
                                                                  Cooling Bore




                                                                    300 Series
                                                                    Stainless
                                                                    material welded
                                                                    into face of roll


                                         Figure 26: Face Weldment Schematics.

A cumbersome machining and weld reconditioning process had to be undertaken in order to return counter-bore
dimensions back to drawing specification. In order to eliminate this corrosion, a procedure to weld a stainless steel
groove in the location of the counter-bores on the roll faces has been implemented during the manufacturing of new
rolls (Figure 26).

The cost for implementation of this procedure was approximately 75% of the counter-bore reconditioning cost. In
addition, the stainless counter-bores will not require any type of repair due to corrosion, reducing the overall repair cost
over the lifetime of the roll, which can possibly be as many as ten (10) reconditioning campaigns.


                                          NEW DESIGN IMPROVEMENTS

In addition to the following packages:
   “DYNAFLEX” oscillator which utilizes spring leaves for Mold guidance
   “HYDROWAM” which hydraulically adjusts mold width
   “SMART segments” which provide remote controlled thickness adjustment in containment,
VAI has developed Segments with integrated feed lines within the frames for consumables like internal cooling, water
and air.




                                         Figure 27: Former Segment Generation
                                          Figure 28. New Generation Segment

To avoid damaging hoses, fittings and rotary joints, these segments have fewer hoses, fewer fittings and protected
rotary joints. The rotary joints are integrated into the covers beside the bearing houses.

Water for internal roller cooling is supplied via the segment frames to the bearing blocks, next to the covers, then to the
rotary joints and finally to the cooling bores. The return is done in the opposite way.

Fewer hoses are used to connect the feed lines from the fixed outer bow segment frames to the moveable inner bow
segment frames, to allow thickness exchange and necessary movement of inner bow segment frames for the casting
process. The average annual savings for the elimination of the hoses for a conventional slab caster is 15,000 USD per
year.




                                      Figure 29. External Rotary Joint With Hoses




                                  Figure 30. View of Rotary Joint Area Without Hoses
                                                     CONCLUSION

Maintenance costs and the technology required for sustained improvement vary according to customer scope of supply,
style of equipment and budget. Our approach to systematically reduce variation within our own process only benefits
the customer and his products. By understanding customers’ equipment and operational parameters allows us to seek
our constant goal of increased production and reduced cost. Often, a comprehensive maintenance approach will see
immediate measurable results. With others, the results are immediate and continue over a long period as demonstrated
in Figures 1 and 2.

Such improvement can only be achieved through an excellent customer supplier relationship. The ability of a
maintenance supplier to develop and quantify new technologies and service-based products prior to mass
implementation should be an expectation. This provides both parties an educated outcome to any trial. Going into the
trial, a detailed understanding of the current practices and performance variable should be known to provide adequate
comparison data. For mold, zone and segment improvements, this is critical.


                                                       OUTLOOK

In order for continued cost reduction in maintenance to be experienced, it is necessary to focus first on the fitness-for-
use type approach to improvements. No longer should the best technical solution be foremost. It is absolutely
necessary for every consideration to be given to the cost and the overall financial impact to all improvement solutions.


                                              ACKNOWLEDGEMENTS

Mr. Sascha Plewka for support and ongoing development of customized maintenance approaches.

Ms. Theresa Gillooly for overall coordination of the metrics used to quantify component and cost performance data
used in this paper.

Mr. William Morton for making the necessary calculation to establish performance benchmarks for measurement of
process and design enhancements.


                                                     REFERENCES

[1] Sascha Plewka, Alfred T. Donet, “Continuous Caster Roll Improvements From Machine Head Through
    Horizontal Section” Proceedings 2003 AISE Iron & Steel Exposition and AISE Annual Convention.

				
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