Technologies for Casting Crack-prone Steel Grades

- P. Müller, Salzgitter Flachstahl GmbHH.-G. Wobker, D. Kolbeck, G. Hugenschütt,

H. D. Piwowar, L. Schmitz, KM Europa Metal AG

Technologies for Casting Crack-prone Steel Grades

he present paper reports

on the different measures T to be taken in order to

optimize the casting of slabs of

steel grades that are susceptible

to cracking. These measures

centre on the constructional

design of the mould as the

heart of continuous casting

machines.

Based on numerical

computations on the cooling

behaviour of the steel melt

inside the mould different

models have been generated to

determine the influence that

mould materials with different

thermal conductivities have on

the melt and slab temperature.

These studies have shown that

the use of mould materials with

medium conductivity can help

to ensure favourable slab

cooling curves.

Furthermore the paper

discusses some studies on the

influence of different types of

coatings and coating systems,

and the effect of specifically

produced surface structures, on

mould heat transfer and mould

wear as well as cooling

behaviour of the slab.

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Background

As described in the literature, the behaviour and surface condition of the shell during the initial solidification process in continuous casting have an important influence on the final quality. Most of the quality problems, like cracks and surface defects, are believed to be initiated already in the meniscus area of the mould by thermal and mechanical stresses, and they propagate as the strand progresses through the caster and during downstream processing.

It is well known that especially peritectic steels, with a carbon content in the range of 0.08 - 0.14 %, are particularly vulnerable to cracking, because hot ductility has a

Fig. 1 : Influence of Carbon Content [7]

minimum in that carbon range, and these steel grades are the most difficult to produce with respect to the surface quality.

Most common surface cracks on cast slabs, which develop during the casting process and limit the further processing of the slab, are longitudinal and transverse facial cracks as well as longitudinal corner cracks. The frequency of these facial cracks reaches its maximum above mentioned with carbon contents in that range. The dependence on the carbon content is due to the changes in microstructure during solidification.

Furthermore it has been noted that the solidifying shell grows unevenly because of the peritectic change. This uneven shell growth leads to weak regions or dangerous "hot spots", i.e. shell areas thinner and hotter than the surrounding region, with low mechanical resistance.

Also other steel grades are affected significantly by the initial solidification phase together with transformation. In these steels such as low carbon, hyperperitectic and silicon-alloyed grades, defects may occur in the form of "surface irregularities".

But even for these grades advanced techniques are available that allow the steel

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mill to produce them. However certain defects show a presence that is random and difficult to relate to processing conditions. [1]

Fig. 2 : Casting Machine at Salzgitter Flachstahl Gmbh

Therefore one of the critical challenges in today's continuous casting process is the production of crack-free sections without time and cost-intensive trial and error approaches or post treatment, i.e. flame scarfing, to optimize steel quality and to minimize or eliminate crack formation. [1, 2, 3, 4, 5, 6, 7, 8, 9]

Salzgitter Flachstahl GmbH

At their integrated metallurgical plant in Salzgitter the Salzgitter Flachstahl GmbH is producing hot-rolled strip, thin sheet, surface-processed products as well as primary material for thick plate and sectional steel. The product range comprises high-quality steel grades with narrowest tolerances regarding their dimensions and mechanical properties.

Fig. 3: Influence of Steel Analysis on Longitudinal Cracking

On two curved and one vertical bending casting machines a range of 400 different steel grades is produced:

l HSLA Steels

l Micro-alloyed and Low Alloyed Steels

l Chromium Alloyed Steels

l Molybdenum Alloyed Steels

l Nickel Alloyed Steels

l Carbon Steels (C > 0.25 %)

l Thin Sheet Steels

l IF Steels

l Special Steels

The complete conditioned liquid metal from the steel works is continuously cast for its subsequent processing in the hot-, cold-, profile and thick-plate rolling mill. To ensure optimal purity of the steel melt and a premium surface quality of the slabs is a basic target of the production. To achieve this, the mould is of crucial importance for the final product quality. Furthermore the shell growth, microstructure development and the precipitation of non-metallic inclusions in the region of the mould is influenced by the liquid metal distribution.

Measures to Minimize Cracking Formation

For reducing mould initiated surface and subsurface cracks several countermeasures and optimisations are recommended (some listed below):

l Optimisation of Steel Analysis

l Superheat

l Liquid Metal Distribution in the Mould (SEN Design)

l Casting Parameters:

- Casting Speed

- Taper of Mould Walls

- Mould Level Control

- Oscillation

- Use of EMBR

l Mould Powder Optimisation

l Hot-top Techniques

- Reduction of Initial Heat Flux / Transfer

l Mould Powder

l Mould and Coating Material

l Casting Surface Texture

- Soft Mould Cooling

l Profiled Mould Wall

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l

l Cooling Water Flow Rate

l Uniform Cooling

l Mould

l Secondary Cooling Adjustment

The positive influence on the quality improvement of the above mentioned measures has been proven in several industrial applications. Nevertheless, depending on the casting machine and the surrounding infrastructure not all of these measures can be realised easily. Furthermore the exact effect of each single action implemented in the casting process of a certain steel plant is hard to predict, because most of them are also influenced by other factors.

In this study hot-top mould techniques for improving slab quality are described, which were tested on continuous slab casting machines of SZAG. These techniques have been implemented without costly and laborious modifications of the casting machine / infrastructure or of the entire casting process.

Mould Material

One suitable way for to ensure a predictable and controlled variation of the heat transfer in the mould, and here especially in the meniscus area, is to choose a mould material with a defined thermal conductivity.

For many steel casting applications, the main target is to extract as much heat as possible from the steel in the mould. Therefore mould materials with very high thermal conductivity like CuAg are used to obtain a solid strand shell which withstands the ferrostatic pressure

of the liquid core at the exit of the mould. Nowadays cold worked silver bearing copper is the standard mould material in applications involving high levels of thermal loading.

However when exposed to high temperatures (T > 370°C), as needed in casting crack-prone steel grades, copper silver alloys have got certain limits due to their tendency to soften.

Precipitation hardened Elbrodur® G material based on CuCrZr offers a distinctly higher strength and hardness together with a good thermal conductivity. This material shows much better resistance to recrystallisation even in the case of long-time exposure to high temperatures. In Elbrodur® G, in particular, a detectable major drop in hardness only occurs after long-time exposure to temperatures above 500 °C.

Elbrodur® NIB is an age-hardenable material based on CuNiBe, which has been developed specifically for use in moulds with very high thermal and mechanical loads. Its outstanding characteristics are very high strength along with medium thermal conductivity, and it has a special resistance to cracking when exposed to thermal stresses. Besides its main fields of application in near-net-shape casting and casting rolls, its use in conventional slab casting is becoming common practice. [11, 12, 13]

Design of Cooling Geometry

Fig. 4 : Influence of Mould Material on Hot Face Temperature

At Salzgitter the first attempts at, and i m p r o v e m e n t s i n , r e d u c i n g t h e development of cracks in peri tectic steel grades were made by adjusting the heat transfer by way of u s i n g p o w d e r lubricants of higher basicity. However, when used with mould plates of CuAg such mould powders tended to form scabs at the meniscus as a result of insufficient melting due to too low a temperature of the casting surface. This problem has been totally overcome by the introduction of mould plates of Elbrodur® NIB. Thanks to that material's lower thermal conductivity the hot face temperature at the meniscus was raised to a level which ensures optimal melting of the powder lubricant.

As a result of these actions, surface quality problems of cast slabs of crack-sensitive steel grades were reduced significantly (Fig. 5). The percentage of slabs without defects increased, and the percentage of major surface defects decreased. In addition to the improvement in product quality it has been possible to achieve reduced wear and thus longer mould plate life.

Mould Coatings

Copper materials possess relatively low hardness and thus low resistance to abrasive wear due to mechanical interaction from the solidified strand shell. It was in particular in order to improve the service life of moulds that anti-wear coatings like nickel and nickel alloys, chromium and metal-ceramic coatings were introduced in the past. But when casting crack-sensitive steel grades, in particular, a standard technique of reducing the heat transfer at the meniscus consists in the application of an additional coating material with low thermal conductivity. [9, 15]

Salzgitter started a trial using this hot-top technique in 2001. Just the upper half of a conventional slab mould was coated with 1 mm nickel, to smooth the heat transfer in the meniscus area and thereby to diminish the cracking. Indeed heavy transversal and longitudinal cracks were eliminated, but star cracks, resulting from copper particles picked up from the mould wall at the bottom end, showed up after a short time and the test was broken off for quality reasons.

To prevent this star crack phenomenon, full face nickel coatings on the casting surface were introduced, which are nowadays standard at Salzgitter.

Mould Design

Another important factor contributing to good product quality is uniform temperature distribution in the mould, especially in the meniscus area. Besides the influence of the liquid steel flow pattern, which depends mainly on the SEN design, and the mould geometry, in particular, for near-net-shape casting (beam blanks), it is the cooling design of the mould itself which has the strongest impact on the surface temperature distribution.

If there is no additional cooling the mounting lands between the cooling slots of wide-face and narrow-face mould plates give rise to non-uniform temperatures on the hot face. To obtain more uniform mould

Fig. 5 : Frequency of Slab Surface Defects

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Fig. 6 : Uniform Mould Cooling [11]

temperatures, additional cooling slots or holes can be provided at the mounting lands, depending on the actual design of the mould, available space and the available amount of cooling water.

Apart from lowering the mould peak temperature in front of the mounting lands - whereby the risk of softening and cracking of the mould material is significantly reduced - uniform hot face temperatures form a better basis for uniform consumption of the powder lubricant. This again makes the adaptation of optimized mould powder properties possible, such as the slag melting point. All this is resulting in an improved cast product quality.

Texturing

Besides the aforementioned techniques the surface of a mould plays an important role in heat transfer. Generation of a specified "roughness" of the casting surface is a common technique for reducing the peak heat flux.

Especially in rotating moulds, like casting rolls for strip casting and casting wheels for rod casting applications, the surface texture defines the heat transfer rate and improves the product quality fundamentally. [16, 17, 18]

B y i n c r e a s i n g t h e uniformity of solidification appropriate texturing prevents

surface cracking of the steel shell. The initial solidification occurs on the tops of rugosity or surface perturbations and depending on the degree of roughness, liquid steel does not penetrate between the peaks.

The structure and effect of a 'slitted' mould surface, with longitudinal grooves mechanically manufactured on a nickel coating is described in [8], [10], and [15]. As a result the heat extraction rate was reported to be reduced by about 10 % and it was noted that the frequency of cracks was reduced by more than 40 % compared with casts through conventional moulds. To obtain an appropriate texture on the casting surface methods like machining with knurling tools, photo-resist chemical etching, electric discharge texturing, laser ablation techniques or blasting with steel balls or sand can be used. Such grooved surfaces have been used in trials at Salzgitter aimed at improving the product surface quality. The tests have not been finished so far. Therefore a final evaluation of the impact on product quality is not possible yet. As a positive side-effect to be observed during the tests the wear resistance of this special surface

Fig. 7 : Surface Texture from Shot Blasting

structure has been improved significantly compared with the conventional smooth casting surface. Even in areas at the bottom end of the plate where the nickel coating had already been eroded through abrasive wear, the pattern of the grooves was still visible in the copper surface (Fig. 9).

But on the other hand, depending on the design and workmanship of the applied texture, sharp 'valleys' on the casting surface might also act as starting points of cracks which propagate into the plate material.

Conclusion

For crack-prone steel grades the intensity of heat flux has an important effect on the product's surface quality. Besides other possibilities of influencing the initial heat transfer in the meniscus area it is especially the mould materials and surface coatings which offer several ways and means of ensuring an appropriate level of heat extraction. It has been shown that the above mentioned measures in combination with properly adjusted casting parameters can provide stable process conditions for the casting of these special grades of steel.

References[1] Normanton, A.S.; Ludlow, V.; Smith, A.W.; Gotti, A.; Thiemann, M.; Landa, S.;

Wans, J; Improving surface quality of continuously cast semis by an understanding of shell development and growth, 2003

[2] Thomas, B.G.; Zhu, H.; Thermal distortion of solidifying shell near meniscus, JIM / TMS Solidification Science and Processing Conference, 1995, pp. 197-208

[3] Moitra, A.; Thomas, B.G.; Storkmann, W.; Thermo-mechanical model of steel shell behavior in the continuous casting mold; TMS Annual Meeting, 1992

[4] Wang, B.; Walker, B.N.; Samarasekera, I.V.; Shell growth, surface quality and mould taper design for high-speed casting of stainless steel billets; Canadian Metallurgical Quarterly, Vol. 39, No. 4 pp 441-454, 2000

[5] Samarasekera, I.V.; Ingenuity and Innovation - The Hallmarks of Brimacombe's Pioneering Contributions to Process Engineering; Metallurgical and Materials Transactions B, Vol. 33b, 2002

[6] Mazumdar, S.; Ray, S.K.; Solidification control in continuous casting of steel; Sadhana, Vol. 26 pp 179-198, 2001

[7] Kivelä, E.; Improvement of the surface quality of slabs by mould heat transfer optimization when casting crack-sensitive grades; ATS Steelmaking days, 1992

[8] Fujiyama, T.; Miyagawa, S.; Deshimaru, S.; Mizota, H.; Production of continuous casting slabs free from surface cracks

[9] Billany, T.J.H.; Normanton, A.S.; et al.; Surface cracking in continuous cast products; Ironmaking and Steelmaking, Vol. 18, 1991, pp. 403-410

[10] Guyot, V; Martin, J.V.; Ruelle, A.; d'Anselme, A.; Radot, J.P.; Bobadilla, M.; Lamant, J.Y.; Pontoire, J.N.; Control of surface quality of 0.08 % < C < 0.12 % steel slabs in continuous casting;ISIJ International, Vol. 36, 1996, pp. 227-230

[11] KM Europa Metal AG; Special Products: AMT® - Advanced Mould Technology; Brochure

[12] KM Europa Metal AG; Special Products: Copper and Copper Alloys - Moulds for Continuous Casting of Steel; Brochure

[13] Hugenschütt, G.; Kolbeck, D.; Wobker, H.G.; Copper Alloys for Structural Components in Remelting Furnaces; VIM/VAR/ESR Workshop, 2003

[14] Wobker, H.G; Hugenschütt, G.; Kolbeck, D.; Use of FEM Simulation to optimize continuous casting moulds; AISE Conference, 2001

[15] Wolf, M.; Strand surface quality of austenitic stainless steels: Part 1 macroscopic shell growth and ferrite distribution; Ironmaking and Steelmaking, Vol. 13, 1986, pp. 248-257

[16] Blejde, W.; Mahapatra, R.; Fukase, H.; Application of Fundamental Research at Project 'M'; Belton Memorial Symposium, 2000

[17] Moon, H.K.; et al.; The Status of Twin Roll Strip Casting Process for Steel Strip in POSCO/RIST; Electric Furnace Conference, 2002

[18] Fukase, H.; Osada, S.; Otsuka, H.; Campbell, P.; Solidification Model for Steel Strip Casting; L&SM, 2003

Fig. 8 : Surface with Grooves [8, 10, 15]

Fig. 9 : Broadface with Ni Coating and Grooves after Use