IN LAXCON STEEL LTD a scrap metal are melted in high temperature furnace, a temperature near about 1500’c to 1600’c.After this melted metal bring out to Continuous Casting Machine through ladder ,then this mould of metal poured into tundish car machine. There are stages in machine like approl, intermediate section, withdrawal, auto cutter machine. But quality problems such as Cracks, Rhomboidity, Porosity, Lapping, seems Crack, Piping, Centre Looseness, Ovality, Blow holes, Pin holes and Breakouts are often observed in continuously cast steel billets.
These defects create a problem in metal which is cast. On the one hand fundamental knowledge about heat transfer, solidification and mechanical behavior of steel was applied to identify the origin of a quality problem in the process.
We finally study about Cracks and effect of cracks and also find solution to reduce Transverse Cracks.
TABLE OF CONTENT
CHAPTER NO. TITLE PAGE NO.
ABSTRACT iii
LIST OF TABLE xvi
LIST OF FIGURES xviii
LIST OF SYMBOLS xxvii
1. Company introduction 1
1.1 Company Introduction 1
1.2 Product list 1
2. literature Review 7
2.1 Previous studies on the microstructure of steel 7
2.2 Previous work on detecting the defects 10
2.3 Previous research on the cooling 11
2.4 A review on previous numerical modeling on cooling cast steel 13
3. Problem Summary 15
3.1 Introduction 15
3.2 Defects in detail 15
3.3 Objective of project 16
4. Steel Grade 17
5. Methodology to determine transverse crack 18
5.1 Testing facilities and quality control in laxcon steel ltd. 18
5.2 Methodology to determine the origin of defects 19
5.3 Metallographic observations 19
5.4 Defect analyzed 20
5.4.1 Transverse Cracks 20
5.4.2 Appearance in Billet 20
5.4.3 Causes and Solutions 21
6. Strategies for dealing with Transverse Cracks. 22
7. Techniques for Crack Minimization 23
7.1. Control of Composition 23
7.1.1 Relevance of hot ductility to transverse cracking 23
7.1.2 Composition Effects 24
7.2 Mould Heat Transfer 25
7.2.1 Mold Powder Composition and Properties 26
7.2.2 Mathematical Treatment of Steel Solidification in the Mould 26
7.2.3 Results and Discussion 30
7.2.4 Conclusions 32
7.3 Mould oscillation 33
7.4 Secondary cooling 34
7.4.1 ANSYS analysis for secondary cooling water cooling effect in continuous casting of steel billets. 35
7.4.1.1 Mathematical model 36
7.4.1.2 Thermal stress model 39
7.4.1.3 Calculation region and finite element discretization 40
7.4.1.4 Calculation and results analysis. 41
7.4.1.5 Verification and analysis of heat transfer model. 42
7.4.1.6 Discussion on thermal stress 45
7.4.1.7 Summary 46
8. Result and Discussion 47
9. Conclusion 49
10. Future Enhancement 50
Appendix 1 51
Bibliography 53
LIST OF TABLES
Mechanical properties of steel”””””””””””.17
Chemical properties of steel. ”””””””””””..17
Steps to determine defects. ”””””””””””’19
Different techniques for the study of the defects and the information. ”””20
Mold powder composition and properties””””””””’..26
Parameters used in the calculations and their variation values””””’…27
Main process parameters of CCM’..”””””””””…36
Verification of surface temperature of billet for all covered situation”””…43
LIST OF DIAGRAM
Temperature profile of the key point ( all covered ). ”””””””42
Temperature profile of the key point ( 90% covered ). ””””””…42
Temperature profile of the key point ( 80% covered )””””””’..43
Comparison of temperature profile in the 3 situation”””””””44
LIST OF FIGURES
Bright bars”””””””””””””””.1
Precision quality shaft bars”””””””””””’…2
Square and hexagonal bright bars. ””””””””””…2
Equal angles”””””””””””’……………………..3
Forged and turned round bars”””””””””””..4
Hot rolled round bars”””””””””””’…………4
Continuous casting of steel billets. ””””””””””..5
Ingots ”””””””””””’……………………………..5
Transverse crack in steel billet”””””””””””15
Transverse cracks on observed by magnifying glass””””””’..20
Transverse cracks on observed after etching with HCL””””””21
Mold heat transfer ”””””””””””””.25
Temperature calculation grid in the vertical symmetrical half section of the steel casting copper mould. ”””””””””””””’…27
Calculated solid steel thickness along the mould walls at mould thermal conductivities: (a) = 50, (b) = 150, (c) = 250, (d) =350 (W/m-K). ”””””””31
Calculated solid steel thickness along the mould walls at steel thermal conductivities: (a) = 25, (b) = 30, (c) = 35, (d) = 40 (W/m-K). ””””””””..32
Mould Oscillation”””””””””””””…33
Secondary cooling zone””””””””””””.35
the finite element meshes and key point of the calculation region. ”””’..40
Scheme of the secondary cooling water covering all the surface of bloom””..41
scheme of the secondary cooling water covering part of the surface of bloom”’41
the stress field at the exit of FSSCZ ”””””””””’45
the stress field at the exit of FSSCZ”””””””””’.46
CHAPTER 1 COMPANY INTRODUCTION
1.1. COMPANY INTRODUCTION
LAXCON STEELS LTD.
Ever since its foray into the steel trade in 1978,Gopal Group has established itself as name to reckon with, in the the steel.over the years, through meticulous quality control and outstanding customer care, it has built a reputation for quality, reliability and integrity.
What started off as a small steel plat with a capacity of 2400 MT/year, Gopal Group has now grown into an organization with manufacturing facility at multiple locations with a combined production capacity of 120000 MT/year, strong value of family run enterprise backed by an assembly of industry’s most talented professionals has given the group its current forms & stature.
Laxcon Steels LTD. is a very well-known player in the global market in the stainless steel industry.Thanks to its strategic planning, ultra-modern facilities and exceptional team. The Group companies work hand in hand to produce the finest stainless steel products, with a focus to fit each client’s requirements.Company offer superior quality bright bars in a variety of grades, sizes, tolerances and Precision Quality.
1.2 PRODUCT LIST
Bright bars
Figure 1: Bright bars
Size range -5mm ‘ 10mm
Supply conditions
Length- 3 to 6.5 meters
Cold drawn, centerless ground, peeled and polished, smooth turned bars.
Precision quality shaft bars
Figure 2: Precision quality shaft bars
Pump shaft
Bearing shaft
Valve shaft
Cylinder shaft
Piston shaft
Size range -6mm ‘ 75mm
Supply conditions
Length- 3 to 6 meters
Heat treatment- Soft annealing, solution annealing, quenched and tempered.
Square and hexagonal bright bars.
Figure 3: Square and hexagonal bright bars
Size range -16mm ‘ 45mm
Supply conditions
Length- 3 to 6 meters
Heat treatment- soft annealed, solution annealed, oil and water tempered.
Equal angles
Figure 4: Equal angles
Size- 25*25, 30*30, 40*40, 50*50, 60*60, 65*65.
Supply conditions
Length- upto 6 meters
Hot rolled, annealed pickled, shot blasted.
Forged and turned round bars
Figure 5: Forged and turned round bars
Size range-130mm-350mm
Supply conditions
Length- 3 to 6 meters
Heat treatment- soft annealed, solution annealed, oil and water tempered.
Hot rolled round bars
Figure 6: Hot rolled round bars
Size range- 16mm ‘ 120mm
Supply conditions
Length- up to 8 meters
Hot rolled surface
Continuous casting of steel billets.
Figure 7: Continuous casting of steel billets.
Size range-
100*100, 120*120, 200*200, 220*220, 280*280, 300*300.
Supply conditions
Length- up to 8 meters
Suitable for rerolling, forging and ring rolling.
Ingots
Figure 8: Ingots
Square, fluted, round.
Supply conditions
Spot ground, free of surface cracks
Smooth edge.
CHAPTER 2
Literature review
This section provides a review on previous research work on the topics related to this research work to understand the extensive work done on the topics related to this research work. These literature reviews are partly with focus on the research studies on soundness and other anomalies generated during solidification whose effects determine the critical cooling rate of hot solidified strands of steel. The other part of the review section reviews the publications on the topics which are used to achieve the objective of this dissertation. Hence, the following sections presents literature review in studies on the microstructure defects and the source of the defects, monitoring of microstructure by the NDT method, effect of inclusion, on the microstructure, experimental study on cooling rate and numeral simulation and modeling methods on the study of phase transformation.
2.1 Previous studies on the microstructure of steel
R. Kiessling, J. Harza, D. Caffarrelli, S. Ramalingma
Studies on MnS inclusions have shown that they are responsible for chip fracture during machining by initiating cracks as a result of increasing stress in the machining shear zone [1,2].
D. B. Evteev, Berg Huettenmann
some research work shows these additive elements may also degrade steel quality. Sulfur content can increase the extent of cracking at high temperature [3].
M. A. Shtremel
The susceptibility of steel to overheating depends on alloying and micro-additions [4].
C. J. Adams, H. F. Hall, A Fuchs
Segregation of sulfur and phosphorous from the liquid film at grain boundaries into the interdendritic region creates a low ductility and strength region and, consequently, forms cracks in the microstructure of steel between the solidus and 2420 degrees Fahrenheit [5-7]
K. Matsubara, H. Mori
The ratio of precipitation of MnS in the matrix and precipitation of AlN along grain boundaries generates cracks in steel during solidification by forming different low ductility regions at different temperature ranges [8,9].
Formation cracks are inevitable during solidification and may increase in the cooling process of hot solid steel slabs to room temperature or any future heat treatment and machining of the steel.
Brimacomb J, Sorimachi K
Cracks and microcracks are more serious than other defects occurring during solidification. Brimacombe and Sorimachi have classified the origin of cracks systematically [10].
Grill, Brimacombe, Weinberg
Internal stresses during surface reheating create cracks because of zero ductility of steel at solidification temperature [11].
]. J. K. Brimacombe
During solidification, internal and surface cracks degrade the steel quality. Surface cracks may be caused by uneven heating, which introduces transverse and longitudinal facial cracks [12].
L. I. Motozenskii
Internal cracks formed during solidification can be classified into five groups of halfway and midway cracks, start or centerline cracks, diagonal cracks, bending cracks and pinch roll cracks. The work of Morozenskii et al. [13], shows that near solidification elongation to rupture in steel with carbon content of 0.17 to 0.2% is minimum.
This sudden change can be caused by an unbalanced rate between radiation and convection cooling along secondary cooling zones or different boundary conditions such as below the mold where the hot surface is cooled suddenly by water spray. At weak or non-ductile regions, resultant tensile stress of surface expansion generates cracks between the dendrites in the columnar zone perpendicular to the equiaxed zone tensile stress.
Likewise, centerline cracking is due to tensile stresses raised by abrupt change in temperature of the centerline with completion of solidification because of bad machine conditions at the final solidification area or too low secondary water-cooling [13].
International Iron and Steel Institute; Committee on Technology; Brussels; 1986.
Longitudinal cracks can be formed from stress at the meniscus during solidification by high mold wear and poor mold surface as well as insufficient strand support below the mold, including caster or mold misalignment and thermal stresses due to non-uniform cooling [14].
Transverse cracks in steel may be generated by too large mold taper, poor oscillation conditions, a low surface temperature at straightening and abrupt cast speed changes.
Uneven cooling of the surface also causes local type star cracks [14].
G. van Drunen, J. K. Brimacombe, F Weinerg
Too high or too low cooling of slab’s side surfaces as well as bad equipment conditions can introduce triple point cracks in the slab [15].
H. Mori, Tetsu to Hagane, L Schmit, H Fredriksson
Tensile stress raised by a high, different, temperature at surfaces making an obtuse corner creates diagonal cracks at right angle to the strain [16,17]
Y. Aketa, et al.: Tetsu-To-Hagan
Billet dimensions control the effect of corner radius on longitudinal cracks such that more cracks are formed in larger billets [18]
A. A. Skvortsov
Experiments on tundish temperature show that after a critical temperature, the length of cracks increases as the temperature of the tundish increases [19].
2.2 Previous work on detecting the defects
The concept of cleanliness and soundness is relevant to the characterization of anomalies. Because of the significance of the presence of anomalies in increasing the defect density in the macrostructure during cooling of the slab, it is important to have an economically feasible and analytically reliable method to detect and recognize the anomalies.
Malkiewicz T, Rudnik
Experimental workers identify non-metallic inclusions by metallographic, chemical and petrographic methods [20].
B. G. Zhang, L. Thomas
Zhang and Thomas [21,22] discussed more than 30 methods for detecting the anomalies under two main categories of direct and indirect methods. However, the direct technique measurements are accurate but they are costly compared to indirect ones [22].
Debiesme et. al
Debiesme [23] explained the Hydrogen Induced Crack test and Magnetoscopy for sheet tests. detected the strains with a resolution of about 25 x 10-6 using a non-contact laser speckle technique, in a thermal fatigue test performed by cyclic induction heating and internal cooling test rods.
Furuya et al. [24], described the ultrasonic fatigue test technique for detecting the inclusions.
Drury J. C, http://www.ndt-ed.prg/, 2006 ,http://www.matter.org.uk/steelmatter/casting, 2000.
More background about NDT and its applications can be found in many websites, books, and literature ( [25-27]
Belchenko G. I
However, five sources of stress fields developed during cooling slabs, explained previously, affect the steel matrix in the same way. Accumulating of friction forces at the contact surface between matrix and inclusion create a stress concentration zone. This zone is believed to be responsible for changing the shape of the inclusion [28].
Waudby P. E, Baker T. J., Charles J., Gove KB, Charles JA
For either type, characteristics of the stress field around the inclusion are generated by distribution, concentration and rate of change of the stress field. Some of the main effective factors in inclusion deformation [29-31], include morphology, strength of inclusion and matrix, composition, inclusion-matrix interface, temperature, strain and strain rate, particle size, stress state, second phase particles and frictional forces at interfaces.
N. Nagayama, T. Abe and S. Nagaki 1989
Nagayama and his co-workers [32] used a Von Mises yield criterion in their FEM analyses to model the deformation of rigid-plastic inclusions in an inhomogeneous material with different yield stresses.
Pietzyk, M. Kusiak J., Kusiak K., Grosman , Milenin A. A
Work on large plastic deformation of an inclusion-matrix system for different inclusion shapes were subsequently published [33,34]
2.3 Previous research on the cooling
Varde A. S., Maniruzzaman M., Rundensteiner E. A., Sisson R. D
Parameters affecting the cooling rate include: quenchant type, quenchant temperature, agitation velocity, viscosity, agitation type, aging, polymer foaming, polymer degradation, part material, geometry, part area, part volume, density, specific heat, oxide layer, surface roughness, suspension, carbon content, grain nature, grain size, plastic deformation and number of other issues [35].
Parameters, which shall be considered in the quenching process of a bulk material, are desired suspension, cooling rate, cooling nature, heat transfer coefficient, residual stress, desired hardness and other material properties, distortion tendency, and cracking potential [35].
MacKenzie D. S., Lambert D
Quenching is accelerating cooling with a quenchant such as air, water, oil, salt, natural or vegetable base quenchant, polymers and other chemical compounds [36].Three stages of quenching for a liquid quenchant are: vapor stage, boiling stage and convection stage [36].
Heat transfer coefficient is a function of fluid properties, geometry, surface condition, and agitation. Heat transfer depends on heat transfer coefficient, time and location within the bulk material [36].
North Carolina State University, College of Engineering (NCSU).
First, any participation may vary the shape of the TTT diagram and, secondly, as the cooling rate increases, the residual stresses stored in the bulk material increases as well [37].
MacKenzie D. S
There are many publications on the quenching topic, which provides the information about different quenchants and the thermodynamic and chemical properties [38].
Heat Treating and Surface Engineering, Indiana
Quenching is very common in the manufacturing process, which is known as quench hardening. Factors, which are considered in quenching hardening, are residual stresses and distortion, prevention of crack initiation, microstructural evolution necessary to improve properties (wear resistance and toughness), and meeting the desirable hardness [39].
Baker T. J., Gove K. B. Charles J. A.;
Application of quenching in steel manufacturing is done to obtain the desired mechanical properties such as, hardness, yield strength, and ductility [40].
Bhadeshia H. K. D. H
Bhadeshia [41] has suggested some modes to decrease the temperature of solid-solid phase transformations. Driven force for polymorphic austenite to ferrite transformation is greater than grain growth by two orders.
Kobasko N. I
Kobasko’s [42] experimental results showed that the tendency for crack formation is higher with a range of cooling rates and it decreases for the cooling rate higher or lower than this critical cooling rate range as a function of quenching rate. The factors governing the behavior of material at high temperature are time, temperature, stress, and environment or atmosphere.
Prince J. C., Marono R., Leon F
At low temperature mechanical behavior like fatigue damage, is controlled by the level of the mean stress, amplitude of stress fluctuation and number of cycles [43].
Kametani H
Kametani [44] analyzed the length of longitudinal cracks on the surface of continuously cast steel slabs and gave crack frequency as a function of crack length using fractal distribution methods.
Berns H., Broeckmann C., Weichert D, Gross-Weege A., Weichert D, Broeckmann C
Tensile test experiments on tool steel showed that cracks initiate (or exist) in the carbide band growth straight forward in the matrix and after kink type dislocation in the carbide rich region split in the same direction of the region then jump to the next carbide band and grow in the same manner and so on [45,46]
Donnay B., Jeraman J. C., Leroy V., Lotter U., Grossterlinden R., Pircher H,Parker K. V
Computer simulations have been deployed in the last decade to lower the cost of microstructural experiments. Much work on modeling of thermo-mechanical treatments of steel has provided the capability to predict the final microstructure [47-48]
2.4 A review on previous numerical modeling on cooling cast steel
Smoljan B
Smolijan [49] predicted the strain and residual stress evolution within a geometrically complex specimen (e.g. cylinder, cones, spheres, etc) dealing with estimation of microstructure and hardness distribution after quenching using a mathematical method based on the finite volume method and Jominy tests results. He did not consider in his simulations any existence or formation of anomalies or defects before or after quenching.
Mishnaevsky L., Lippmann N., Schmauder S
Mishnaevsky [50] et al. discussed the crack propagation under dynamic load in real and quasi-real idealized two-phase microstructure of carbide in the steel matrix microstructure model for tool steel material.
Beer, F.P., Johnston, Jr., E.R. (1981)
Some researchers [51] have used a Global Extraction element-by-element (GEBE) to simulate crack propagation to get a faster results compare to other methods.
McClintock F. A, Lay B., Brunet M., Boivin M
Other researchers modeled the nucleation and propagation of voids under tensile forces [52,53]
Ueda Y., Tanigawa M. , Murakawa
Macro crack propagation simulation with size effect properties in some work [54] was presented to model ship collision, which poses a huge numerical model.
Yu H. J.; Brechnung von Abkuhlungs, Leblond j. B., Mottet G, Devaux J. C
Different researchers [55,56] investigated the effect of phase transformation on the residual stresses within quenched bodies. These residual stresses have an important role in changing the soundness of the steel during cooling.
Wittmann F. H
Wittmann [57] used the Fictitious Crack Model (FCM), to predict the crack formation in composite material.
CHAPTER 3 PROBLEM SUMMARY
3.1 Introduction
During our industrial training we found a quality problem in steel billets, which is cast by cone casting machine.
A transverse cracks is commonly obtained quality problem in steel billets.
3.2 Defects in Detail
Figure 9: Transverse crack in steel billet
The surface crack of CC slab is an important problem, which affected the continuous caster yield and the quality of the slab. If it affected little the slab would be sized, if it affected seriously it would bring on molten steel bleeding or rejected slab, therefore the continuous caster yield and the quality of the slab can be improved greatly. In the producing of the hull structur, the transverse corner crack in the slab occurred more, which made difficult to plan the producing and affected the order of structural steel finished punctually.
During continuous casting of steels, longitudinal and transverse cracks tend to occur in brittle temperature ranges by thermal and mechanical deformation. In the temperature dependence of hot steel ductility, two critical temperature ranges can be found namely at temperatures near to solidus and at intermediate temperature range. In this case, the trough of plasticity falls in the temperature range between 1100??C and 700??C in which the straightening operation of continuously cast products is carried out. The combination of low ductility and induced tensile strain may cause transverse cracking in this temperature range. To prevent the occurrence of cracks, the better understanding on physical mechanical, metallurgy properties of continuously cast steel in the course of its cooling is a necessary condition.
3.3 Objective of project
Primary work to know the defects occur in steel billets. Based on the work result find out the defects.To minimize the defects with improving quality of steel billets.Referring the entire process the sample have been evaluated to check the possibilities on our various study and analysis for providing the best quality and reducing the defects in steel billets.
CHAPTER 4 STEEL GRADE
Generally a many types of steel grades are used in billet production, according to the customer demand.
We consider a 20CrMnTiH steel grade.
Table -1: Mechanical properties
20CrMnTiH
Treatment Thermal ref.
Tensil strength >615 N/mm2
Yield point .395 N/mm2
Enlongation 17%
Reduction of area 15%
Absorption energy (j) >47
Table-2: Chemical properties
C 0.17-0.23
Si 0.17-0.37
Mn 0.80-1.10
Cr 1.00-1.30
Ni –
Mo –
V –
Other Tr 0.04-0.10
(SOURCE- LAXCON STEELS LTD.)
CHAPTER 5 METHODOLOGY TO DETERMINE TRANSVERSE CRACK
5.1 Testing Facilities and Quality Control in LAXCON STEEL LTD.
Optical Emission Spectrometers
Leco Gas Analyzer for H2, O2, N
Mobile Handheld Spectrometers
State of the Art Chemical Testing Laboratory
Ultrasonic Testing
Radio Activity Testing
Surface Testing
Brinell Hardness Testing
Rockwell Hardness Testing
Poldy Hardness Tester
Jominy Hardenability Testing
Magnetic Particle Inspection
Impact Testing Machine (Assited with Notch, Broaching & Profile Projector)
Optical Microscope for Testing Microstructure / Grain Size / Defect Depth / Delta Ferrite Measurement, Non-Metallic Inclusion Rating, Decarburization
Universal Testing Machine for Testing Tensile Strength / % Elongation / % Reduction in C/S Area
Proof Test by Electronic Extensometer
Optical Pyrometer for Temperature Measurement
5.2 Methodology to determine the origin of defects
The determination of the origin of defects includes the recording and analysis of general information. It is important to know the frequency of defect, position in same corner or face of the billet, position in bar or wire rod. It is also relevant an exhaustive study of the general appearance of the defect by naked eye or with magnifying glass. After that, the observation of polished samples, generally transverse cuts of the rolled products or of the billet and microscopic study at higher magnification, etching with reagents give insight defects features. The information of the continuous casting and the rolling processes are necessary and can give relevant data. Sometimes it could be important to design the follow up of heats in the metal shop and rolling mill processes.
Table- 3: Steps to determine defects
Tasks Information
General information
Frequency, position, in one of the strands or in one of the faces of the billet, position in wire rod, influence of some steel grade, etc.
Microscopic study
With the necked eye or helped with magnifying glass, polished sample observation, transverse cuts in rolled products & billets, etching with reagents.
Process information
Routine data from Rolling Mill & Steel Melting Shop(CCM)
Own Background
Search for similar defects in own reports and from other plants.
Physical simulation
Generally it has only academic interest or for preventing the problem from repetition in the future.
5.3 Metallographic Observations
The metallographic observation of various samples gives key information when the origin of a defect needs to be determined .often; it is not possible to find out the origin of defect with just observation of transverse cuts under microscope, without an idea of the general aspect of the defect. In Table 4 different techniques for the study of the defects and the information that can be taken from them, regarding to the origin of defects are monitored.
Table- 4: Different techniques for the study of the defects and the information
Observation type
Information obtained
With naked eye or with magnifying glass
Oxygen Penetration (pin holes) due to internal oxidation after cutting billet samples, location on billet corner and mid way cracks.
As polished
Micro inclusions, Irregular solidification structure
Etching by HCL
Decarburization, segregation
5.4 Defect analyzed
5.4.1 Transverse cracks
Transverse cracks, although not always detect in the inspection of billets, may also give place to serious defect in the rolled products.
5.4.2 Appearance in billet
Figure 10 and Figure 11 shows the aspect of transverse cracks in continuous casted billets observed by magnifying glass and by HCL etching closed to a surface, the cracks are looks as if it has been formed at high temperature, probably in the mould.
Figure 10: Transverse cracks on observed by magnifying glass
Figure 11: Transverse cracks on observed after etching with HCL
5.4.3 Causes and Solutions
Transverse cracks can form in the mould or during strengthening. When they are located in any corner, they are likely to be formed due to tensile effort related to sticking, this can be worsen by deep oscillation marks When the cracks are present only in the corner belonging to inner radius, they could be formed by tensile efforts during strengthening. This is common when corner temperature is within low ductility range. A sound approach to solve the problem is to set proper secondary cooling to avoid the dangerous temperature range in the corners during strengthening.
CHAPTER 6
STRATEGIES FOR DEALING WITH TRANSVERSE CRACKS.
Control of Composition
Composition strongly influence transverse cracking by their influence on hot ductility. Choose C and alloy additions to avoid peritectic solidification, and particularly avoid 0.1-0.13%C.
Mould Heat Transfer
The degree of steel superheat has a noticeable; Effect on solidified steel thickness.Surface structure can be influenced by heat transfer in the mould. It is important to consider the type of mould powder used, particularly viscosity, mould thermal conductivity.
Mould Oscillation
Depth of these oscillation marks can be reduced by increasing the mould oscillation frequency and/or decreasing the stroke to reduce the heal time.
Secondary Cooling
Slab straightening is carried out within this ductility trough, transverse cracking can result.A steep temperature gradient through the slab thickness is also desirable using this cooling strategy, to minimize the penetration of surface cracks which may form in cold spots.
Mechanical Stresses
Mechanical stresses can be introduced by poor alignment of the components of the machine, and from many other sources, but of most relevance to transverse cracking is the straightening operation. Straightening may be carried out over a single point, or by multipoint straightening. The effect of these two types of straightening operation on transverse cracking are not clear, but there are reasons to suppose that multi-point bending will not improve hot ductility: strain rate will be reduced, which will reduce hot ductility; more time will be allowed for dynamic precipitation; and at least for Nb containing steels, stress relaxation between each bending point seems to be minimal.
CHAPTER 7 TECHNIQUES FOR CRACK MINIMIZATION
7.1. Control of Composition
It is evident from the above discussion that composition, and particularly the use of micro alloying elements, can strongly influence transverse cracking by their influence on hot ductility. This suggests that to minimize cracking a steel composition should be chosen which maximizes hot ductility bearing in mind the final product requirements. The following guidelines should help to maximize hot ductility and minimize transverse cracking: Choose C and alloy additions to avoid peritectic solidification, and particularly avoid 0.1-0.13%C
Minimize Nb
Use V or V/N combinations to replace Nb
Minimize Al
Minimize N
Make V additions to Nb steels
Consider Ti additions
7.1.1 Relevance of Hot Ductility to Transverse Cracking
In qualitative terms, hot ductility as measured in laboratory tests appears to bear some relationship to the observations of transverse cracking; the detrimental effects of Nb, N and Al on transverse cracking are all reflected in reductions in hot ductility. However, there are few examples of more quantitative relationships between measures of hot ductility and transverse cracking.A relationship between the number of transverse cracks per slab and the reduction of area in a hot tensile test. Above a reduction of area value of 75%, no cracked slabs were observed. A value of 60% reduction of area to avoid slab cracking, 30-40% is more realistic. It can therefore be seen that there is considerable discrepancy between the suggested values, and it is likely that such a value can only be ascribed to specific tensile test conditions and slab assessment methods.
As well as the depth of the ductility trough, the temperature at which it occurs is also significant; if slab straightening can be carried out outside the temperature range of low ductility, then transverse cracking may be avoided. Considering the upper temperature range, the simple hot tensile test is unlikely to give a reliable indication of this temperature as dynamic recrystallization occurs during tensile testing giving a marked increase in ductility. However, the strains encountered at the surface of continuously cast slab are only ~1-2%, which is insufficient for recrystallization to occur. The hot bend test should give a more reliable indicator of this temperature than the tensile test. Considering the lower end of the ductility trough, recovery of ductility is associated with the completion of a certain proportion of transformation, and therefore the hot tensile test should give a more reliable indication of this temperature. However, the higher strains encountered during the tensile test may again give misleading temperatures, due to the influence of strain in transformation, and the bend test is again more likely to give accurate indications.
7.1.2 Composition Effects
V Steels
There have been several studies of the influence of V on hot ductility using hot tensile tests. The results from all the various studies are consistent in that V additions of up to 0.1% at low N contents (<0.005%) have only a very slight detrimental effect on hot ductility by broadening the ductility trough. At higher N levels, the effect of V additions becomes more marked, and the ductility trough. Becomes deeper and broader. In fact, a good relationship can be constructed between the product VxN, and the depth and breadth of the ductility trough, it should also be noted that in this example it is only at the highest VxN product, 0.1%Vx0.01%N, that hot ductility approaches that of a 0.028%Nb steel.
Nb Steel
There have been a very large number of studies of the effect of Nb on hot ductility, and the activity in this field is probably related to the perceived detrimental effect of Nb on slab surface quality. These results can be summarised by saying that Nb additions deepen and broaden the ductility trough to extend to higher temperatures. Nb additions of as little as 0.017% had a detrimental effect, and ductility continues to deteriorate up to at least 0.074%.
As with Ti, the effect of S on the hot ductility of Nb steels also depends upon the thermal cycle used in the test. For steels reheated to a solution temperature prior to testing, S has little effect on hot ductility. However, for in-situ melted test pieces increasing S levels have a detrimental effect on hot ductility, as more S is taken into solution to precipitate on grain boundaries.
Ti Steel
When examining the influence of Ti on hot ductility, it is important to consider the austenite grain size. In some reports, the apparent benefit to hot ductility of Ti additions is due to a refinement of grain size. It is most appropriate to evaluate the influence of Ti on hot ductility using samples melted in-situ, as this technique produces approximately similar grain sizes for Ti and Ti free steels.
There are relatively few reports looking at the influence of Ti additions to C-Mn-Al steels after in-situ melting, and relationship between Ti and hot ductility appears to be complex. In situations where large TiN precipitates can form, such as at slow cooling rates or high values of TixN, hot ductility may be slightly improved by Ti additions, but for conditions which generate large volume fractions of fine TiN particles, such as a stoichiometric ratio of Ti:N in low N steels, hot ductility can deteriorate with Ti additions.
(Source :laxcon steel ltd.)
7.2 Mould Heat Transfer
Figure 12: Mold heat transfer
7.2.1 Mold powder composition and properties
Table -5: Mold powder composition and properties
Chemical
Composition Basicity SiO2 CaO MgO Al2O3 TiO2 Fe2O3 MnO2 P2O5 Na2O K2O F B2O3 Li2O
1.10 38.60 42.28 0.89 6.34 0.19 0.36 0.03 0.03 3.64 0.12 7.14 0 0.37
Properties Solidif. Temperature
(??C) Softening Temperature
(??C) Melting Temperature
(??C) Viscosity at 1300 ??C
Properties Poise
1149 1170 1180 3.12
7.2.2 Mathematical treatment of steel solidification in the mould
Table -6: Parameters used in the calculations and their variation values.
Parameter Units Varied
Values Constant value Remarks
Thermal
conductivity of
copper mould W/m-??k 50, 150,
250, 350
Thermal
conductivity of
steel W/m-??k 25, 30,
35, 40
Super heat Degrees 5, 10, 15,
20
Mould wall thikness Mm 12, 16,
20, 24
Convective heat
transfer coefficient
(calculated) W/m2-??k 250 Constant
Prandtl’s number Pr 7 Assumed
Figure 13: Temperature calculation grid in the vertical symmetrical half section of the steel casting copper mould
Mathematical model
Where the parameters T, Q^m, ??k, ??, t are temperature ( ‘ ), heat rate of heat generation/consumption (W/m^3) thermal conductivity (W/m-??k), time (s) and diffusivity (m/s^2), respectively. Based on the above assumptions, (1) can be simplified to
(d^2 T)/(dx^2 )+(d^2 T)/(dy^2 )+(d^2 T)/(dz^2 )+Q^m/k=1/’ dT/dt (1)
This equation represents transient heat transfer by conduction in one direction, and coupled with appropriate boundary conditions evolved in the cooling process, it can be solved to give temperature distributions in the calculation domain. The method adopted in solving this equation is by the use of finite difference technique and solving the resulting equations explicitly. Referring to the nodal distribution both in the steel and mould wall areas shown in Figureure 13
(d^2 T)/(dx^2 )=1/’ dT/dt (2)
This equation represents transient heat transfer by conduction in one direction, and coupled with appropriate boundary conditions evolved in the cooling process, it can be solved to give temperature distributions in the calculation domain. The method adopted in solving this equation is by the use of finite difference technique and solving the resulting equations explicitly. Referring to the nodal distribution both in the steel and mould wall areas shown in Figureure 13
(d^2 T)/(dx^2 )=(T_(n+1)+T_(n-1)-2T_n)/(‘x^2 ) (3)
Where, ‘x^2 is space interval along the horizontal direction, n denotes a node and (n+1, n-1) represent the next and previous nodes in the same horizontal direction, respectively.Similarly, discretizing the right term in time gives
‘ (dT/dt)’_n=(T_n^(p+1)-T_n^p)/’t (4)
Where ‘t is the time interval chosen in dividing the total time the steel remains in the mould into small time intervals, n and p indicates the previous time step. In this case the total time of cooling is given as
t=m’t (5)
Using (4) and (5), (3) becomes
(T_(n+1)^p+T_(n-1)^p-‘2T’_n^p)/(‘x^2 )=1/’ (T_n^(p+1)-T_n^p)/’t (6)
Rearranging
T_n^(p+1)=F_o (T_(n+1)^p+T_(n-1)^p )+(1-‘2F’_o)T_n^p (7)
Where, the first term indicated the temperature at node n at the current time (p+1) which is calculated from temperatures of the previous time step (p). F_o in (7) is known as Fourier’s number which is given as
F_o=(”t)/'(‘x)’^2 (8)
Equation (7) is used to calculate temperatures within the steel and mould wall, however, for calculation of the temperatures at the external nodes, i.e., at the shared interface between the mould surface and cooling water, the following equation is used
‘ T’_N^(p+1)=’2F’_o [T_(n-1)^p+B_i T_w ]+(1-‘2F’_o-2B_i F_o)T_N^p (9)
Where, B_i is known as the dimensionless Biot’s number expressed as (6)
‘ B’_i=h’x/k (10)
The coefficient h is known as the convective heat transfer coefficient and is given by
h=N_u k/D (11)
N_u is the dimensionless Nusslt’s number given as
N_u=((f’8)R_e P_r)/(1.07+12.7′(f’8)(P_r^(2’3)-1)) (12)
Where R_e and P_r are the dimensionless Reynold’s and Prandtl’s numbers and f is the friction factor, respectively .The friction factor can be expressed as
Equations (7) and (9) are solved with the appropriate parameters shown in Table 1 using MS Excel worksheet by which temperatures at each node in the steel and mould wall were solved at accumulative time intervals covering the whole residence time of the steel in the mould. The total residence time of steel in the mould was determined from knowledge of mould height and volumetric steel casting speed. This time period was divided into a number of calculation time intervals satisfying the stability requirements of (7) and (9).
7.2.3 Results and discussion
Calculation of the increasing solidified steel shell thickness as the steel moves down the mould were performed at different conditions.
When pouring steel into the mould, the amount of superheat which it exhibits (the temperature above the solidifying temperatures) is of paramount importance on how fast the steel starts to solidify when it enters the mould. Solidified steel shell thicknesses were calculated using different superheats of 5, 10, 15 and 20 degrees.
It can be seen from the Figure that the degree of steel superheat has a big effect on the thickness of solidified steel. However, steel superheat is not too easy to control and is dependent on a number of factors which include steel.
Mould thermal properties such as its thermal conductivity controls the rate of heat transfer from the steel to the cooling water, i.e., the cooling and solidification processes.This factor has been varied in the calculations and the results of these variations are shown in Figure 14
Figure 14: Calculated solid steel thickness along the mould walls at mould thermal conductivities: (a) = 50, (b) = 150, (c) = 250, (d) =350 ( W/m-??k )
A factor in steel casting which is of great importance is changing steel chemical composition. Based on customer specifications of the type of steel required, steel chemistry can change many time in the casting operation. Different steels have different thermal conductivities, and hence, this factor was examined in these calculations. As can be seen from Figure 15
Figure 15: Calculated solid steel thickness along the mould walls at steel thermal conductivities: (a) = 25, (b) = 30, (c) = 35, (d) = 40 ( W/m-??k )
Changing steel thermal conductivity resulted. in great variations in the calculated solidified steel shell thickness. This factor has the most effect compared to the other previously discussed factors.
7.2.4 Conclusions
Calculations of solidified steel shell thicknesses within the casting copper mould during casting of steel billets showed that
The degree of steel superheat has a noticeable effect on solidified steel thickness, and hence, this factor should be well controlled in the casting operation.
Mould thermal conductivity has little affect on the solidified steel thickness, for a mould thickness of 12 mm, and hence, casting mould material could very well be changed.
Thermal conductivity of molten steel is of great importance in the casting operation, and hence, great care should be taken when changing steel grades of differing thermal conductivities.
7.3 Mould Oscillation
Figure 16: Mould Oscillation
Oscillation marks are the most recognizable feature of continuous casting and can be related to the subsurface defects that can be found on product rolled from continuous cast slabs.The physical surface defect of the oscillation marks themselves does not necessarily pose a significant problem in many grades. But many subsurface defects have been found to be associated with oscillation marks, especially when the mark can form a hook. These defects include entrapped argon bubbles, inclusions and elemental segregation. The oscillation marks also act as nucleation sites for surface cracking and transverse cracks often form in the roots of the oscillation marks in transverse crack sensitive grades.
The formation mechanism of the oscillation mark is the increased solidification rate in the meniscus area of the mold due to movement of the meniscus towards the mold wall during the negative strip time of the mold. This indicates that it is possible to affect meniscus mark formation by either changing the heat transfer rate in the meniscus area of the caster or by changing the position of the liquid steel meniscus, or by affecting both issues. This leads to the potential for some completely novel approaches to either eliminating or minimizing the oscillation marks that are found on cast slab surfaces.
7.4 Secondary Cooling
There is a wide ductility trough associated with microalloyed steels, and if slab straightening is carried out within this ductility trough, transverse cracking can result. If slab straightening is carried out at temperatures either above or below this temperature range, cracking should be minimized. Both these different cooling strategies (‘soft’ cooling and ‘hard’ cooling) have been used on various machines around the world, with some success in reducing transverse cracking.
Figure 17: Secondary cooling zone
A variant on the use of different cooling strategies which has been used to minimize crack formation during the rolling of hot charged slabs is the use of slab quenching. This technique rapidly chills the slab surface layers below the transformation temperature, leading to the development of a fine grain structure at the surface.This fine grain structure then
restricts the formation and propagation of cracks.
Non-uniform secondary cooling can promote thermal stresses, and hence lead to cracking.This requires good nozzle design and maintenance, and preferably the use of air-mist cooling.
7.4.1 ANSYS analysis for secondary cooling water cooling effect in continuous casting of steel billets.
Foreword
Billet quality, one of the crucial problems in steel industry, is mainly determined by the secondary cooling water distribution, thus the key to achieve improvement of it is to establish rational cooling system. Concerning the formation of external and internal cracks, scholars from different countries have established the solidification heat transfer model and stress model. On the condition of the same specific water flow rate, the spraying effect of secondary cooling water can be evaluated by to way in the productive process of bloom, which cover all thye bloom surface or part of the surface. with the help of finite element analysis software of ansys, the model of solidification and thermo-mechanical analysis have been developed.
7.4.1.1 Mathematical model
20CrMnTiH bloom is choosen as object.maine process parameters of this CCM are as below.
Table 7: Main process parameter
ITEM PROCESS PERAMETER
Machine type Curve type
Section of billet 200mm*200mm
Casting speed 0.70 m/min
Length of 1st segment of sec. Cooling zone 1.19 m
Length of 2nd segment of sec. Cooling zone 1.49 m
Length of 3rd segment of sec. Cooling zone 2.30 m
Length of 4th segment of sec. Cooling zone 2.30 m
Length of 5th segment of sec. Cooling zone 1.20 m
Distance from meniscus to straighning point 14.70 m
Basic hypothesis of thermo-mechanical mathematical model
Into the heat transfer and mechanics of billet during solidification process from meniscus to air cooling zone a two dimension unsteady heat transfer model has been developed on the basis of the two dimensional slice method in moving coordinate system for simplification, several assumptions are made as below
The steel meets the requirements of the small deformation theory, and billet deformation is tiny compared with its cross section.
The billet cross section is assumed to be in the state of plane stress.
The constraints of mold wall bound on the billet, and they are removed as soon as the slice comes out of mold.
The impact of longitudinal thermal stress is ignored.
Heat transfer analytic model
The two dimensional unsteady heat transfer equation is
??c (‘T )/’??=’/’x (?? ‘T/’x)+’/’y (?? ‘T/’y) (1)
Where, ??, c and ?? are steel density (kg/m^3) specific heat (kJ/kg??C) and thermal conductivity (W/ m^2??C) respectively. For billet, variation of enthalpy is considered as the effect of latent heat in solidification process. The Galerkin method of weighted residuals is applied to solve equation (1), and a discrete finite element equation is:
[C] {T}+[??]{T}={F} (2)
Where ,[c] and [??] are heat capacity matrix and heat conduction matrix of the system respectively,{F} is heat load array equation (2) is a series of first order differential equation about time, and the temperature distribution of node [T] [C] can be obtained after time domain is dispersed.
Initial conditions
T=T_(c ) (x’0,y’0,z=0,t=0 (3)
The temperature distribution of molten steel at mold inlet is defined as the initial calculation condition t=0
Where (‘ T’_(c )) casting temperature,
Boundary conditions
Billet center
The heat transfer of billet has axisymmetric feature, and the diffusion boundary of the center symmetry axis can be considered as adiabatic condition.
(?? ‘T/’x),(x=0,’??0)= (?? ‘T/’y),(y=0,’??0)=0 (4)
Billet surface
-(?? ‘T/’x),(x=D_1/2,’??0)=q_s^n (5)
-(?? ‘T/’y),(y=D_2/2,’??0)=q_s^w
Where D_1,D_2 length of side of billet cross section (m), q_s^n is heat flux of narrow surface, q_s^w is heat flux of wide surface q_s^n and ‘ q’_s^w have the same expressions in different cooling regions, and their expressions are as below.
In the mold q_s^m=A-B’T (6)
In the secondary cooling zone q_s^c=h^c (T_b-T_w) (7)
Heat flux of billet surface is calculated separately when the secondary cooling water is covered on the surface of billet. And radiation and air convection are considered as boundary conditions for the uncovered are of corner of billet when the secondary cooling water is not sprayed on all surface of billet.
Radiation q_s^r=[?? ??(T_b+273)^4 – (T_0+273)^4] (8)
Air convection q_s^r=h^a (T_b-T_o) (9)
In the air cooling zone-radiation and air convection are considered as boundary condition for all the surface of billet. Where (q_s^m) heat flux of the billet surface in the mold J/m^2s, ( q_s^c ) heat flux of the billet surface when air convection is considered as boundary condition J/m^2s.A, B test constant, ( h_c) heat transfer coefficient between billet and cooling water W/ m^2??C, ( h^a ) heat transfer coefficient between billet and air convection.( T_b) billet surface temperature, ( T_w ) cooling water temperature ??C ,( T_0) ambient temperature, ( ‘ ) blackness of billet surface (0.7-0.8).
?? = 5.670373(21) ??10’8 W m’2 K’4 (10)
7.4.1.2 Thermal stress model
Thermal stress of the billet can be calculated by obtaining the changes of the temperature field, and thus the constitutive equation is built-
{d??}={d??_e}+{d??_(e,T)}+{d??_T} (11)
In the equation (11)
Total strain increment, {d??}
Elastic strain increment, {d??_e}
Strain increment changed with temperature, {d??_(e,T)}
Thermal strain increment, {d??_T}
The boundary condition of solidified shell is as follows.
Zero displacement constraints are applied to the symmetry boundary.
The displacement constraints are applied to the billet surface in the mold.
Molten steel possesses the fluidity, and its stress impact upon the solidified shell is ignored.
Physical parameters of steel (20CrMnTiH)
Density -The volume of the billet varies in the solidification process, and the density of billet is related with steel grades, temperature and phase transition, however it has little change. In this ??_s =7600 kg/m^3, ??_1=7200 kg/m^3, ??_s^1 = 7400 kg/m^3
Thermal conductivity -In solid region, ??_s is 29.7 W/ m^2??C, while in mushy region and liquid region, the equivalent thermal conductivity is used to analyze the effects of convection.
??^*=m??_s T’T_L
(??_s+m??)/2 T_L ‘T’T_s
(12)
Where (??^* ) equivalent thermal conductivity, W/m^2??C, (m) empirical constant, generally is 4-7 times value of ??_s and 4 is chosen in this analysis.
Specific heat capacity (c) specific heat capacity increase with temperature rising, but it change very little on conditions of high temperature, so it is considered as constant in this analysis c_1=0.88 kJ/kg ??C
Latent heat of solidification (L_f) the solidification of billet is a nonlinear thermal transfer process, thus the latent heat need to be taken into account, and the latent heat of 20CrMnTiH is 272000 J/Kg in this analysis. In the calculation, the latent heat is defined as the enthalpy of steel, which varies as temperature change.
7.4.1.3 Calculation region and finite element discretization
A quarter of billet cross section is selected as the calculation region, and the distribution of temperature and stress field from meniscus to air cooling zone can be calculated by a finite element analysis model of ANSYS. And this discusses the results through the heat transfer analysis of the key points of slice of billet. The finite element meshes and the key point of the calculation region shown in Figure-18
Figure 18: the finite element meshes and key point of the calculation region.
7.4.1.4 Calculation and results analysis
The actual production condition of plant as follows.-superheat of liquid steel 30 ??C, and casting speed is 0.7 m/mm. On the condition of the some specific water flow rate (0.27 l/kg), the focuses on the research of changing the coverage rate of cooling water on billet in secondary cooling zone shown in Figure 19 and 20 below.
Figure 19: scheme of the secondary cooling water covering all the surface of billet
Figure 20: scheme of the secondary cooling water covering part of the surface of billet
Three situations are selected for research in the analysis.
Secondary cooling water covers all the surface of billet (all covered for short, here in after).
Secondary cooling water covers 90% area of the surface of billet.
Secondary cooling water covers 80% area of the surface of billet.
7.4.1.5 Verification and analysis of heat transfer model
The temperature behavior of key points for three situations is shown in graph -1 using calculation model
Graph 1: temperature profile of the key point (all covered )
Graph 2: temperature profile of the key point (90% covered )
Graph 3: temperature profile of the key point (80% covered)
Temperature measuring experiments were carried out to verify the model by infrared measurement of temperature. And the result are shown in Table 8
Table 8: Verification of surface temperature of billet for all covered situation
Distance from meniscus, m 4.09 9.14 14.1
Measured temperature, ??C 1096 1003 986
Calculated temperature, ??C 1105 1011 1004
Absolute error, C 9 8 18
Relative error, % 0.82 0.80 1.81
It can be seen from Table that the calculated temperature values are almost the same as the measured ones, and their relative errors are within 2.0%
From graph-2 it is known that the temperature changes of center point on wide side surface have basically kept the same trend as of wide side surface graph-3.Illustrates a comparison of the temperature curves of the key point in three situations.
Graph 4: comparison of temperature profile in the 3 situation
Discussions on the result from graph 4
With the reduction of coverage are of secondary cooling water, the corner reheating of the billet is intensified in the first segment of the secondary cooling zones, which are 57 ??C, 81??C, 117 ??C for all covered, 90% covered and 80 % covered situation respectively. In the last two cases, the corner of billet is not covered by cooling water in the FSSCZ, where cooling condition are the radiation and air convection, and the relative intensity of cooling falls, therefore, the reheating of billet, corner is enhanced as the coverage area of secondary cooling water is reduced. However, cooling condition of billet corner keeps stable when strands pass the FSSCZ, so the temperature profile are smoother than that in all covered situation, and reheating phenomena of billet obviously decreased after the strands leave the FSSCZ.
With the reduction of spraying area of secondary cooling water, temperature of center point of wide side surface decreases slightly, and the value of 80% covered situation is 20 ??C less than that of all covered one. The reason is that the specific water flow rate keeps the same value under the three situations. Accordingly, cooling water flow rate of billet surface covered by water changes more with the reduction of area coverage of secondary cooling water, hence the cooling intensity of strand is increased.
With the reduction of spraying area of secondary cooling water, temperature difference between the middle and corner of the billet surface decreases a little, the value of 90% covered situation is 35 ??C less than that of all covered one, so the temperature field of 90% covered situation become more homogeneous than that all covered situation.
In short, the temperature field of the billet becomes more homogeneous than that of all covered situation when secondary cooling water covers part of the billet surface. The degree of billet reheating decreased in other secondary cooling zones and is greatly improved after strands experience a strong reheating in the FSSCZ. Therefore, selecting an appropriate coverage rate of secondary cooling water is helpful to obtain a more homogeneous temperature field of billet if the casting steel can withstand within the extent of reheating in the FSSCZ, so the billet quality should be improved. Transverse crack of 20CrMnTiH billet almost disappeared combined with the optimization of the secondary cooling water distribution.
7.4.1.6 Discussion on thermal stress
Compared with water coverage areas of billet in ahead segment, that in the FSSCZ begins to change, the calculation of stress field are shown in Figure- 21 & 22, maximum equivalent stress is presented in the billet corner, and the stress field distribution is similar to that of different water coverage area of the billet, meanwhile, maximum equivalent stress of all covered situation is 25.3 MPa.
Figure 21 the stress field at the exit of FSSCZ
Figure 22: the stress field at the exit of FSSCZ
As the different water spraying coverage rate of billet is adopted, the maximum stress on billet decreases more than 8%, and the incidence rate of transverse cracks is reduced effectively.
7.4.1.7 Summary
A strong reheating of billet corner does happen when secondary cooling water covers part of the billet surface. And with the reduction of coverage area of water, the extent of billet reheating is increased.
Analysis indicates that, under the 90%-covered situation, the cooling effect of billet is better than that of all-covered situation.
According to the actual production selecting an appropriate coverage rate of secondary cooling water would realize a more homogeneous temperature field of billet, and its cooling effect is better than that of all-covered situation, thus the billet transverse crack could be reduced.
In the process of continuous casting, billet surface suffers the greatest stress, and it’s easy to cause transverse cracks, so it’s useful to improve product quality through selecting the appropriate coverage rate of secondary cooling water.
CHAPTER 8 RESULT AND DISCUSSION
The share of the bloom, continuously cast in the company, represents approx 45-60%, the balance being billets for another company. The material defects at the steel continuous casting appear during the solidification and cooling of the continuously cast semi-finished products, often leading to important material losses. To prevent these losses, the purpose of metallurgical technologies and constructive solutions is to detect the causes of occurrence, prevention and removal.
literature reviews are partly with focus on the research studies on soundness and other anomalies generated during solidification whose effects determine the critical cooling rate of hot solidified strands of steel.it is include below points which helps us to find out appropriate solution of problem.
Previous studies on the microstructure of steel.
Previous work on detecting the defects.
Previous research on the cooling.
A review on previous numerical modeling on cooling cast steel.
During continuous casting of steels, transverse cracks tend to occur in brittle temperature ranges by thermal and mechanical deformation in the temperature dependence of hot steel ductility. Methodology is important to know the frequency of defect, position in same corner or face of the billet, position in bar or wire rod. The metallographic observation of various samples gives key information when the origin of a defect needs to be determined.
Strategies for dealing with transverse cracks
Control of composition
Mould heat transfer
Mould oscillation
Secondary cooling
Mechanical stresses
Techniques for crack minimization.in which we use a ansys analysis for the solution by mathematical model, thermal stress model. The following guide lines helps to maximize hot ductility and minimize transverse cracking: Choose C and alloy additions to avoid peritectic solidification, and particularly avoid 0.1-0.13%C
Minimize Nb
Use V or V/N combinations to replace Nb
Minimize Al
Minimize N
Make V additions to Nb steels
Consider Ti addition
When pouring steel into the mould, the amount of superheat which it exhibits (the temperature above the solidifying temperatures) is of paramount importance on how fast the steel starts to solidify when it enters the mould. Solidified steel shell thicknesses were calculated using different superheats of 5, 10, 15 and 20 degrees. Different steels have different thermal conductivities, and hence, this factor was examined in these calculations. Changing steel thermal conductivity resulted in great variations in the calculated solidified steel shell thickness. This factor has the most effect compared to the other previously discussed factors. Thermal conductivity also affects the hot ductility of steel which directly related with transverse crack of steel billet.
This indicates that it is possible to affect meniscus mark formation by either changing the heat transfer rate in the meniscus area of the caster or by changing the position of the liquid steel meniscus, or by affecting both issues.
Analysis indicates that, under the 90%-covered situation, the cooling effect of billet is better than that of all-covered situation.
In the process of continuous casting, billet surface suffers the greatest stress, and it’s easy to cause transverse cracks, so it’s useful to improve product quality through selecting the appropriate coverage rate of secondary cooling water.
CHAPTER 9 CONCLUSION
After the study and observation of process of continuous casting of steel billets we found the major defect like transverse crack in product. And then we have done ANSYS analysis on
Mould thickness, Secondary cooling zone,
We also conduct study other techniques which effect on transverse crack in steel billet, like.
Composition effect,
Mould oscillation
Mould heat transfer
Add the micro alloying element like Nb, V, Ti for improvement of mechanical property of material.Finally we found that the defect of transverse crack can be reduced.
CHAPTER 10 FUTURE ENHANCEMENT
After done this work, apply this strategy for future improvements in continues casting of steel billet. By do more work on this report and changing the PLC coding and operation technique, more good quality steel billet can be obtained. Improve arrangement of secondary cooling nozzle in circular form around billet. Improve micro alloying element and composition of material. Improve SCZ and withdrawal mechanism.
APPENDIX: LIST OF SYMBOLS
?? Density of steel, kg/m^3
v Casting speed, m/min
c Specific heat capacity of steel, kJ/kg ??C
?? Thermal conductivity coefficient, W/ m^2??C
??^* Equivalent thermal conductivity, W/m^2??C
T Temperature, ??C
t time, s
q_s^w Heat flux of wide surface, J/m^2s
q_s^m Heat flux of billet surface in the mold, J/m^2s
q_s^r Heat flux of billet surface when radiation is considered on boundary condition J/m^2s
q_s Heat flux, J/m^2s
C_w Specific heat of water, kJ/kg ??C
Q_m Water flow rate of mold
T_c Casting temperature, ??C
‘ Specific water flow rate, l/kg
h Heat transfer coefficient, W/m^2??C
s Stefan-Boltzmann constant
e Radiation coefficient
C_1 Specific heat of liquid steel, kJ/kg ??C
C_s Specific heat of solid steel, kJ/kg ??C
L_f Latent heat, kJ/kg
T_w Cooling water temperature, ??C
q_s^n Heat flux of narrow surface, J/m^2 s
q_s^c Heat flux of billet surface covered secondary cooling water, J/m^2 s
q_s^a Heat flux of billet surface when air convection is considered as boundary condition J/m^2 s
h_a Heat transfer coefficient between billet and air convection, W/m^2 s
?? Elastic strain increment
d?? Total strain increment
d??_(e,T) Strain increment changed with temperature changes
d??_T Thermal strain increment
T_o Ambient temperature, ??C
BIBLIOGRAPHY
[1]. R. Kiessling, " Nonmetallic Inclusions in Steel ( Part III)", ISI Publication 115, 1968. [2]. J. Harza, D. Caffarrelli, S. Ramalingma, " Free Machining Steels- the behavior of type I MnS inclusions in Machining" No. 73-WA/Prod-10, Transactional ASME, 1973.
[3]. D. B. Evteev; berg Huettenmann; Montash., 1966, vol. 11, p, 238 HB #6973. [4]. M. A. Shtremel; "Problems of the Metallurgical Quality of Steel (Non-metallic Inclusion)"; Moscow Institute of Steel and Alloys (Translated from Metallovedenie i Termicheskaya Obrabtka Metallov); No. 8 pp 2-6; 1980. [5]. C. J. Adams: NOH-BOS Conference Proceeding, Pittsburgh, 1971, vol. 54, pp. 290-302. [6]. H. F. Hall: "Second Report of Steel Castings. Research Committee", The Iron Inst., London, 1936, p. 65. [7]. A Fuchs; ESTEL- Berichte aus Forschung und Entwicklung unserer Werke, 1975, no. 3; pp. 127-135. [8]. K. Matsubara; Trans. Iron steel Inst; Japan, 1966, vol. 6, pp. 139-148. [9]. H. Mori; Tetsu-to Hagane; 1974, vol. 60(7), pp. 784-806. [10]. Brimacomb J., sorimachi K., "Crack formation in the continuous cast products"; Continuous Casting, Volume 9, The Iron &Steel Society, 1997. [11]. Grill, Brimacombe; Weinberg; presented at the 104th AIME Meeting; Feb 1975. [12]. J. K. Brimacombe; "Defect Problem in Continuous casting"; 33rd electric furnace conference; Houston, texas, 1975. [13]. L. I. Motozenskii; O. A. Mitenev; V. K. Krutikov; Stal in English; 1965; no. 4, pp. 272-276. [14]. Continuous Casting of Steel 1985-A Second Study; International Iron and Steel Institute; Committee on Technology; Brussels; 1986. [15]. G. van Drunen, J. K. Brimacombe, F Weinerg; Iron Steelmaking; 1975, vol. 2(2), pp. 125-133. [16]. H. Mori; Tetsu to Hagane; 1972,vol. 58(10), pp. 1511-1525 [Henry Brutcher translation N. 9000-I]. [17]. L Schmit, H Fredriksson; Iron Steelmaking, 1975, vol. 2(1) [18]. Y. Aketa, et al.: Tetsu-To-Hagane, 1959, vol. 45, p. 1341, HB #5223. [19]. A. A. Skvortsov, et al; "Continuous Casting of Steel"; p. 126, Metallurgizdat Press, Moscow, 1970, HB #7594. [20]. Malkiewicz T, Rudnik; " Deformation of Non-metallic Inclusions During Rolling of Steel"; S. J Iron Steel Inst 1963: 201: 33-38 [21]. Zhang, L. Thomas B. G.; ISIJ Inter., 2003, 43(3), 271. [22]. Zhang, L. Thomas B. G.; "Inclusions in continues casting of steel"; XXIV National steelmaking Symposium, Morelia, Mich, Mexico, 26-28, pp 138-183, Nov 2003. [23]. Debiesme et. al.; Revue de Metallurgie- CIT; 1993, 90(3), 387. [24]. Furuya Y. et. al.; Tetsu-to Hagane, 2002, 88(10), 643. [25]. Drury J. C.; "Ultrasonic Flaw Detection for Technicians"; 3rd ed., chapter 2 [26]. Non-destructive Testing Research Center, http://www.ndt-ed.prg/, 2006. [27]. Entrapment of non-metallic inclusions, http://www.matter.org.uk/steelmatter/casting, 2000. [28]. Belchenko G. I.; Gubenko S. I.; "Deformation of nonmetallic inclusion during steel rolling"; Russian Metallurgy (4); 1983, 66-69. [29]. Waudby P. E.; "Factor controlling the plasticity of silicate inclusion"; Steel Times Annual Review; 1972, pp. 147-152. [30]. Baker T. J., Charles J. A. ; "Deformation of MnS inclusions in steel"; J Iron Steel Inst 1972: 210: 680-690. [31]. Gove KB, Charles JA. Metals Technol 1974: 1: 425-431. [32]. N. Nagayama, T. Abe and S. Nagaki 1989 ;Plastic deformation of inhomogeneous materials with elliptic inclusions; Computational Mechanics. 4:433-441 [33]. Pietzyk, M. Kusiak J., Kusiak K., Grosman F.; "Fields of strain around the inclusion of second phase in a uniform matrix undergoing plastic deformation"; Steel Research, 62, 1991, 507-511. [34]. Milenin A. A.; "Theoretical analysis of the stress strain state in the region of the inclusion of second phase under plastic deformation of two phases materials"; Russian Metallurgy, 2, 1995, 97-103. [35]. Varde A. S., Maniruzzaman M., Rundensteiner E. A., Sisson R. D.; "The QuenchMinerTM Expert System for Quenching and Distotion Control"; Heat Treating and Surface Engineering-Chemistry of Quenching, Indiana, USA, Sep 2003, ISBN: 0-87170-797-7, pp:174-183. [36]. MacKenzie D. S., Lambert D.; "Effect of Quenching Variables on Distortion and Residual Stresses"; Heat Treating and Surface Engineering, Indiana, USA, Sep 2003, ISBN: 0-87170-797-7, pp:184-191. [37]. US EPA Air Pollution Training Institute developed in collaboration with North Carolina State University, College of Engineering (NCSU). [38]. MacKenzie D. S.; "Advances in Quenching- A Discussion of Present and Future Technoligies"; Heat Treating and Surface Engineering, Indiana, USA, Sep 2003, ISBN: 0-87170-797-7, pp:184-191. [39]. Heat Treating and Surface Engineering, Indiana, USA, Sep 2003, ISBN: 0-87170-797-7. [40]. Baker T. J., Gove K. B. Charles J. A.; "Inclusion deformation and toughness anisotropy in hot-rolled steels"; Metals Technology; 3; 1976; 1183-1193. [41]. Bhadeshia H. K. D. H.; "Bainite in steels", 1st ed., 1992, London, the institute of Materials. [42]. Kobasko N. I.; Met. Term. Ob. Metallov, 2, 1964, pp: 53-54. [43]. Prince J. C., Marono R., Leon F, ;"Thermo-mechanical analysis of a piercing mandrel for the production of seamless steel tubes"; Proc Instn. Mech. Enhrs Vol. 217 Part E: Process Mechanical Engineering. [44]. Kametani H.; "Fractal Analysis of the Surface Cracks on Continuously Cast Steel Slabs"; Metallurgical and Materials Transactions, V 29B, December 1998-1261. [45]. Berns H., Broeckmann C., Weichert D.; Eng. Fract. Mech., 58, 4, pp:311-325 [46]. Gross-Weege A., Weichert D., Broeckmann C.; Comp. Mat. Sci., 7, pp:123-130. [47]. Donnay B., Jeraman J. C., Leroy V., Lotter U., Grossterlinden R., Pircher H.; Proc. Int. Conf. on "Modeling of metal rolling processes", London, UK, 1996, The Institute of Materials. [48]. Parker K. V.; "Modeling of phase transformations in hot rolled steels"; PhD thesis, University of Cambridge, UK, 1997. [49]. Smoljan B.; "Numerical Simulation of Steel Quenching"; ASM International, JMEPEG Volume 11(1) February 2002 11:75-79. [50]. Mishnaevsky L., Lippmann N., Schmauder S.; "Mesomechanical simulation of crack propagation in real and quasi- real idealized microstructures of tool steels"; ECF 13–13th European Conference on Fracture; San Sebastian; Spain; 6-9 Sept. 2000. 251 pp. 2000. [51]. Beer, F.P., Johnston, Jr., E.R. (1981), Mechanics of Materials, McGraw-Hill, Inc. (New York). [52]. McClintock F. A.; "A criterion for ductile fracture by the growth of holes", Journal of Applied Mechanics, 35, 1968, 363-371. [53]. Lay B., Brunet M., Boivin M.; "Modeling of void nucleation and fiber debonding in a composite material"; Journal of Materials Processing Technology, 77, 1998, 254-59. [54]. Ueda Y., Tanigawa M. , Murakawa H. ; "An evaluation method for ductile crack propagation in pre-strained plates"; Journal of the Society of Naval Architects of Japan (J. Soc. Nav. Archit. Jpn.) ISSN 0514-8499 , 1996, vol. 179, pp. 399-406. [55]. Yu H. J.; Brechnung von Abkuhlungs, Umwandlungs, Schqweib, sowie Verformungseignspannungen mit Hilfe der Methode der Finiten Elenmente, Karlsruhe 1977,(Dr, -ing, thesis) [56]. Leblond j. B., Mottet G, Devaux J. C., ; Mater. Sci. Technol., 1985, pp:815-822. [57]. Wittmann F. H.; "Crack formation and fracture energy of normal and high strength concrete"; Sadhana Vol. 27, Part 4, August 2002, pp. 413-423.