CAPPARINI MAURIZIO (IT)
FATA HUNTER DIVISION OF FATA S (IT)
LUISETTO GIAMPAOLO (IT)
CAPPARINI MAURIZIO (IT)
| GB1025680A | 1966-04-14 | |||
| GB1335739A | 1973-10-31 | |||
| US4409027A | 1983-10-11 | |||
| US4772451A | 1988-09-20 | |||
| GB1302445A | 1973-01-10 | |||
| JPH0693373A | 1994-04-05 | |||
| JPS5782458A | 1982-05-22 | |||
| US4802528A | 1989-02-07 | |||
| US5531659A | 1996-07-02 |
| CLAIMS
1) Steel, advantageously but not exclusively for making rolling mills and/or continuous casting rolls, characterized in that it comprises a variable percentage - in weight - alloyed with iron Fe: C from 0.48 % to 0.60%, Mn from 0.45% to 0.60%, Si from 0.30% to 0.60%, Ni from 0.40% to 0.60%,
Cr from 1.80% to 2.50%, Mo from 0.80% to 1.10%, V from 0.30% to 0.50%, W from 0.30% to 0.70%.
2) Steel according to claim 1), characterized in that it comprises a variable percentage - in weight - alloyed with iron Fe: carbon C from 0.52% to 0.57%, manganese Mn from 0.45% to 0.60%, silicium Si from 0.30% to
0.40%, nickel Ni from 0.45% to 0.60%, chromium Cr from 1.90% to 2.40%, molybdenum Mo from 0.85% to 1.0%, vanadium V from 0.33% to 0.37%, tungsten W from 0.30% to 0.40%.
3) Steel according to claim 1), characterized in that it comprises a percentage - in weight - alloyed with iron Fe: 0.52% of carbon C, 0.48% of manganese Mn, 0.33% of silicium Si, 0.46% of nickel Ni, 2.16% of chromium Cr, 0.94% of molybdenum Mo, 0.35% of vanadium V and 0.31% of tungsten W.
4) Steel according to claim 1) or 2) or 3), characterized in that it also comprises impurities in a percentage, in weight, that substantially does not exceed 0.60%.
5) Steel according to claim 4), characterized in that said impurities substantially comprise a percentage, in weight: phosphorus P not exceeding 0.010% and/or sulphur S not exceeding 0.010% and/or titanium Ti not exceeding 0.010% and/or tin Sn not exceeding 0.018% and/or aluminium
Al not exceeding 0.018% and/or copper Cu not exceeding 0.25% and/or arsenic As not exceeding 0.007%.
6) Steel according to claim 4) or 5), characterized in that said impurities also comprise hydrogen H2 up to a maximum of approximately 1.8 ppm. 7) Steel according to claim 4) or 5), characterized in that said impurities substantially comprise a percentage, in weight: 0.008% of phosphorus P and/or 0.004% of sulphur S and/or 0.005% of tin Sn and/or 0.012% of aluminium Al and/or 0.25% of copper Cu and/or arsenic As in a percentage not exceeding 0.007%. 8) Steel according to claim 7), characterized in that said impurities comprise hydrogen H 2 in a quantity substantially equal to 1.6 ppm.
9) Steel according to any of the preceding claims, characterized in that its toughness is included between 19 and 36 joule.
10) Steel according to any of the preceding claims, characterized in that its hardness is included between 380 and 450 HB.
11) Steel according to any of the preceding claims, characterized in that it also comprises a percentage of cobalt Co variable in weight from 0.15% to 0.40%.
12) Steel according to claim 11), characterized in that it also comprises a percentage of cobalt Co variable in weight from 0.20 % to 0.30%.
13) Steel according to claim 11), characterized in that it also comprises a percentage of cobalt Co substantially equal to 0.25%.
14) Heat treatment comprising at least one hardening process and at least one tempering process, characterized in that said at least one tempering process is carried out at a temperature included between 550 0 C and 660 0 C.
15) Heat treatment, characterized in that it comprises:
- a hardening treatment at a temperature included between 870 0 C and 1000 0 C;
- a tempering process at a temperature included between 550 0 C and 660 0 C.
16) Treatment according to claim 14) or 15), characterized in that said hardening is carried out with water-quenching solution.
17) Treatment according to claim 14) or 15) or 16), characterized in that said hardening is carried out at a temperature substantially equal to 950 0 C. 18) Treatment according to claim 14) or 15) or 16) or 17), characterized in that said tempering is carried out at a temperature substantially equal to 600 0 C.
19) Steel obtained by subjecting an alloy to a heat treatment, characterized in that said alloy is produced according to any of the claims from 1) to 8) and in that said heat treatment is carried out according to claim 14) or 15) or
16) or 17) or 18).
20) Steel obtained by subjecting an alloy to a heat treatment, characterized in that said alloy is produced according to claim 11) or 12) or 13) and in that said heat treatment is carried out according to claim 14) or 15) or 16) or 17) or 18). 21) Steel according to claim 20), characterized in that the hardening temperature of said heat treatment is substantially included between 980 0 C and 990° C.
22) Rolling mill (1), characterized in that at least the surface (2) of said rolling mill (1) is made with a steel produced according to any of the claims from 1) to 13).
23) Rolling mill (1), characterized in that at least the surface (2) of said rolling mill (1) is made with a steel produced according to claim 19).
24) Rolling mill (1), characterized in that at least the surface (2) of said rolling mill (1) is made with a steel whose characteristics are in accordance with the contents of claim 20) or 21).
25) Shell for rolling mills, characterized in that at least the working surface (2) of said shell is made with a steel produced according to any of the claims from 1) to 13). 26) Shell for rolling mills, characterized in that at least the working surface
(2) of said shell is made with a steel produced according to claims 19). 27) Shell for rolling mills, characterized in that at least the working surface (2) of said shell is made with a steel produced according to claim 20) or 21). |
STEEL PREFERABLY SUITABLE FOR MAKING SHELLS OF CASTER ROLLS FOR ALUMINIUM AND ITS ALLOYS AND RELEVANT HEAT TREATMENT.
The invention concerns a steel particularly suitable for making shells for rolling mills used in continuous casting systems, as well as a improved heat treatment. As already known, according to one of the techniques for manufacturing sheets, foils and strips of metal like, for example, aluminium, steel etc., the molten material coming from a continuous casting passes between two opposite and properly spaced revolving rolls. Each roll comprises a central element, called "core", and an external element, called "shell", which defines the surface directly in contact with the molten metal. The core comprises a cylindrical body provided with channels for the passage of a cooling fluid and equipped with mechanical members for coupling with a corresponding powered shaft. The external element, called shell, consists of a cylindrical body made of steel provided with an axial through hole suited to house the core through an interference fitting process.
According to the process for producing the metal foil or strip, also called "twin- roll casting", the molten metal coming from the casting is conveyed towards the pair of rolls and uniformly distributed on the surface of the lower roll that causes it to solidify owing to its low surface temperature. The rolls, which rotate in the direction of advance of the molten metal, roll the solidified metal layer and produce a thin sheet. When the roll shell comes into contact with the molten aluminium, its surface temperature rapidly increases. Inside, on the other hand, the temperature is much lower due to the cooling effect resulting from the heat exchange with the cooling liquid of the core. When the portion of surface metal of the shell in contact with the liquid metal moves away from the metal itself, its temperature rapidly decreases and causes a volume contraction. These high temperature gradients, with which high deformation gradients are associated, produce high stress in the material; if the stress exceeds the yield strength of the material, it causes a surface plastic deformation of the shell that may equal and/or exceed the ultimate tensile strength. In addition to the phenomena described above, it is necessary to consider that the high working temperature at the level of the area in contact with the molten metal
- l -
determines a reduction in the yield strength of the material itself.
Due to the above mentioned stress and its cyclic nature, the shell suffers cracks due to thermal stress that reach critical dimensions after a given number of working hours. Once created, these cracks propagate in depth as processing continues, so that they negatively affect both the quality of the final product and the fatigue resistance of the shell itself, thus making it necessary to replace the shell or to grind the surface of the rolls.
After a given number of grinding operations, however, it is necessary to replace the shell, which becomes unusable due to its reduced thickness deriving from the processing and from the previous grinding operations. Generally, the shell is replaced when its thickness is half the original thickness and the following operations are required: removal of the shell from the core, grinding of the core surface and thermal fitting of a new shell. The material of which the shell is made, therefore, is of fundamental importance for the duration of the shell, in terms of operating hours and tons produced, and also in view of the need to obtain a shell with good surface finishing and suitable characteristics.
The shell, in fact, besides extracting heat from the molten metal in order to cause it to solidify and produce good quality rolled sections, must have suitable mechanical characteristics to resist the tensile thermal stress due to the fitting operations and to compression-rotation during use.
In particular, the shell must feature considerable thermal fatigue resistance, low thermal expansion coefficient, high thermal conductivity, high yield point at high temperatures, as well as high ductility and toughness at high temperatures, the latter being very useful for delaying propagation of cracks due to thermal fatigue once they have appeared.
The known technique has proposed various materials that aimed to improve one of these characteristics but worsened at least one of the remaining characteristics. In particular, some solutions pay special attention to the high resistance of the material, reducing on the other hand the toughness and ductility of the same.
A drawback of these solutions lies in that they do not allow optimization of resistance to the propagation of cracks once they have appeared.
Other solutions emphasize the toughness and ductility of the material, but reduce the ultimate strength and the yield strength.
A drawback posed by these solutions is constituted by the fact that they present lower ultimate strength and yield strength values, thus facilitating the appearance of fatigue cracks.
The object of the present invention is to overcome the drawbacks described above.
In particular, it is a first object of the invention to produce a steel particularly suitable for making external elements or shells for continuous casting rolls that has improved characteristics compared to the steels of known type. It is a further object of the invention to develop a steel with excellent resistance to thermal fatigue and thermal shock.
It is a further object of the invention to develop a material with a low coefficient of thermal expansion, high thermal conductivity, high yield point at high temperatures, as well as high ductility and toughness at high temperatures. It is a further object of the invention to develop a steel with higher yield point at operating temperature than the steels of known type.
It is a further object of the invention to develop a steel and a shell with longer useful life or higher operating efficiency - in terms of hours of operation or tons produced in the rolling processes described above - than the known steels and shells in the same operating conditions. It is another object of the invention to develop a steel and shell that, compared to the steels and shells of known type - in rolling processes and in the same operating conditions - offer higher resistance to the propagation of cracks after their appearance. It is a further object of the invention to develop a steel and a shell that offer a good compromise between a high yield point and satisfying toughness.
It is a further object to obtain a steel and a shell that, compared to the steels and shells of known type, are characterized by higher resistance to operating mechanical stress. It is a further object of the invention to develop a steel and a shell that allow the number of grinding operations of the shell surface per unit of time to be reduced compared to the known solutions.
It is a further object to propose a heat treatment that makes it possible to improve the mechanical characteristics also of steels carried out according to the known formulations. Last but not least, it is a further object of the invention to propose an economic
steel that is also easy to produce.
The objects mentioned above are achieved by a steel advantageously but not exclusively for rolling mills and/or continuous casting rolls that, according to the contents of the main claim, is characterized in that it comprises a variable percentage - in weight - alloyed with iron Fe: C from 0.48% to 0.60%, Mn from
0.45% to 0.60%, Si from 0.30% to 0.60%, Ni from 0.40% to 0.60%, Cr from
1.80% to 2.50%, Mo from 0.80% to 1.10%, V from 0.30% to 0.50%, W from
0.30% to 0.70%.
According to a preferred embodiment of the invention, the proposed steel has elements that generate impurities up to a maximum of approx 0.010% per element.
The objects described above are also achieved through a heat treatment for steels in accordance with the contents of claims 14 or 15.
The objects described are achieved also by a steel in accordance with the contents of claims 19 or 20.
The objects described are achieved also through a rolling mill in accordance with the contents of claims 22, 23 or 24.
The objects described above are achieved also through a shell for rolling mills in accordance with the contents of claims 25, 26 or 27. The dependent claims illustrate further advantageous characteristics of the main embodiment.
The proposed solution advantageously makes it possible to obtain a material with particular characteristics as to resistance and stability to high temperatures, also thanks to the introduction of tungsten contents (W) as will be described in greater detail below.
Still to advantage, the alloy obtained presents an increased yield point at high temperatures compared to the steels of known type.
To further increase said yield point, according to a preferred embodiment of the invention the alloy contains cobalt Co. Again advantageously, the solution proposed allows satisfying toughness values to be maintained.
Still to advantage, the proposed alloy presents greater resistance to tempering due to high operating temperatures compared to the known steels.
Still advantageously, the proposed alloy ensures a good compromise between resistance and toughness.
The objects and advantages described above will be highlighted in greater detail in the descriptions of some preferred embodiments of the invention, supplied as examples without limitation, also with reference to the attached drawings, wherein: - Figure 1 shows an axonometric view of a continuous casting roll that is also the subject of the invention;
Figure 2 shows an axonometric exploded view of one of the rolls of Figure l;
Figure 3 shows a schematic side view of a pair of rolls for continuous casting during operation.
The special steel alloy proposed, though having being developed preferably with the purpose to improve the combination of the characteristics of the rolls and/or shells for continuous casting rolls, can in any way be used to make also other types of tools, devices or objects. According to the invention, the proposed steel substantially comprises, a variable percentage - in weight - alloyed with iron Fe: carbon C from 0.48% to 0.60%, manganese Mn from 0.45% to 0.60%, silicon Si from 0.30% to 0.60%, nickel Ni from 0.40% to 0.60%, chromium Cr from 1.80% to 2.50%, molybdenum Mo from 0.80% to 1.10%, vanadium V from 0.30% to 0.50%, tungsten W from 0.30% to 0.70%.
Within the percentage weight values indicated above, a group of preferred steels has been identified, which comprise a variable percentage - in weight - alloyed with iron Fe: carbon C from 0.52% to 0.57%, manganese Mn from 0.45% to 0.60%, silicium Si from 0.30% to 0.40%, nickel Ni from 0.45% to 0.60%, chromium Cr from 1.90% to 2.40%, molybdenum Mo from 0.85% to 1.0%, vanadium V from 0.33% to 0.37%, tungsten W from 0.30% to 0.40%. Within the percentage weight values indicated above, a preferred combination of elements has been identified, which presents an excellent and advantageous combination of mechanical properties. This steel comprises a percentage - in weight - alloyed with iron Fe: 0.52% of carbon C, 0.48% of manganese Mn, 0.33% of silicium Si, 0.46% of nickel Ni, 2.16% of chromium Cr, 0.94% of molybdenum Mo, 0.35% of vanadium V and 0.31% of tungsten W. According to the invention, tungsten W must be introduced in the steel, being characterized by good solubility in ferrite and a marked tendency to form highly stable carbon, in significant percentages.
In this regard it must be observed that in the known technique it is held that in steel alloys intended preferably for the production of shells for continuous casting rolls the presence if tungsten W in the alloy causes an undesired and disadvantageous reduction in the toughness of the shell. According to the known technique, in fact, tungsten (W) cannot be present in the alloy in a quantity exceeding 0.2% in weight.
On the contrary, the applicant believes that, advantageously, the introduction of tungsten W in the alloy, together with the other elements and in particular with molybdenum and vanadium, in the quantities indicated above, enhances the multiplying effect of the alloyed elements and improves the mechanical characteristics of the steel obtained.
The advantageous multiplying effect achieved with the proposed solution makes it possible to amplify the improved metallurgic effect of some chemical elements, each having the effect to improve some characteristics of the alloy. More particularly, in this specific case this effect makes it possible to increase the tempering temperature so that it approaches the operating temperature, introducing the above mentioned elements in the alloy in quantities that are smaller than those that should be introduced if, to obtain the same effect, they were added individually. In other words, the improvement of some characteristics, obtained with a single element in high percentages, in the proposed solution is obtained more effectively through the addition of more elements, in lower percentages than those used for individual elements. More particularly, in the example described herein, the addition of tungsten W and molybdenum Mo increases the tempering temperature so that it approaches the operating temperature.
Always according to the invention, the total impurities present in the alloy preferably do not exceed 0.60% in percentage weight.
Impurities are all those elements introduced in the alloy that produce undesired effects on the characteristics of the product. These impurities include the following elements: phosphorus P in percentages not exceeding 0.010%, sulphur S in percentages not exceeding 0.010%, titanium Ti in percentages not exceeding 0.010%, tin Sn in percentages not exceeding 0.018%, aluminium Al in percentages not exceeding 0.018%, copper Cu in percentages not exceeding 0.25%, hydrogen H 2 up to a maximum of approximately 1.8 ppm, arsenic As in percentages not exceeding 0.007%.
Preferably, said impurities are present in the following percentage values: phosphorus P 0.008%, sulphur S 0.004%, titanium Ti 0.0 %, tin Sn 0.005%, aluminium Al 0.009%, copper Cu 0.25%, hydrogen H 2 1.64 ppm, arsenic As in percentages not exceeding 0.007%. Always according to the invention, to further improve the characteristics of the proposed steel, a heat process has been identified, which makes it possible to improve the mechanical characteristics both of the steel having the proposed composition and of the alloys and steels having known compositions. In particular, the heat process of the invention includes, in addition to hardening, a tempering process at a temperature included between 550 0 C and 660 0 C and preferably at 600 0 C.
Hardening is preferably carried out at a temperature included between 870 0 C and 990 0 C and more particularly between 870 0 C and 970 0 C and preferably at 950 0 C. Advantageous results have been achieved by carrying out hardening with water quenching solution, that is, using mixtures of water and oil in suitable percentages. The compositions of a series of castings carried out according to the invention are listed in TABLE 1, where they are identified by letters Bl, B2 and B3, compared with an example of alloy of known type, indicated by Xl, X2, Yl, Y2, Y3 and Z. TABLE 2 to be found here below indicates the mechanical characteristics, at a high temperature, of the above mentioned castings. In particular, table 2 shows, for each casting, test temperature in 0 C, ultimate strength (Rm) expressed in Mpa, yield strength 0.2% (Rp 0.2) expressed in Mpa, percentage elongation after breakage (A%), percentage contraction after breakage (Z%).
TABLE 1 -
-TABLE 2 - Mechanical characteristics at high temperatures
* Steels in compliance with the present invention
It can be observed that the steel obtained according to the proposed solution, as can be seen from the data in Table 2, has yield strength values and ultimate strength values that exceed those of the known steels, though still maintaining good toughness values (toughness ISO-V). With reference to the data in Table 2, it can be observed that the action of tungsten W in the alloys obtained according to the invention (castings B1-B2-B3) can be deduced from the yield strength values at 650 0 C, which are clearly higher compared to steels with known composition as listed in the same table.
The proposed solution thus advantageously allows a material with particular resistance and stability to high temperatures to be obtained.
According to a further and advantageous embodiment of the invention, the iron alloy comprises also a percentage of cobalt, variable in weight between 0.15 and
0.40 and in particular included between 0.15 and 0.30 and preferably equal to
0.25. The addition of cobalt Co contributes to increasing the resistance of the material to high temperatures, acting differently from tungsten, since it is completely soluble in the matrix.
The cobalt present in the alloy advantageously makes it possible to increase resistance to softening at high temperatures, thus contributing, together with tungsten, to increasing the tempering temperature and improving the resistance of the shell to high operating temperatures.
It must be observed, furthermore, that in this last preferred embodiment the hardening temperature is preferably included between 870 0 C and 1000 0 C and more particularly between 980 0 C and 990 0 C. An example of embodiment of a caster roll that is also the subject of the invention is represented in Figures 1, 2 and 3, where it is indicated as a whole by
1.
According to the invention, the roll 1 has a working surface 2 made with steel produced according to the invention. In this example of embodiment, the roll 1 comprises, as visible in detail in Figure
2, a core 3, and an external element, also called shell, 4, which defines the working surface 2.
More precisely, the core 3 comprises a substantially cylindrical body provided with channels for the passage of a cooling fluid, only the inlet hole 31 thereof being visible, and is provided with equipment for coupling with a corresponding
powered shaft.
The shell 4 comprises, in this case, a cylindrical body 41, visible also in Figure 2, completely made with a steel produced according to the invention, and provided with an axial through hole 5 suited to house the core 3 via an interference fitting process.
Figure 3 schematically shows a process for producing a metal strip, where the molten metal 10 coming from the casting is conveyed towards the pair of rolls 1 that rotate in a direction corresponding to the advance direction of the molten metal 10, thus making it solidify. In fact, the molten metal 10 in contact with the surfaces 2 of each roll 1 solidifies forming two separate parts that are joined at the level of the bottleneck 11 defined by the rolls 1 themselves and forming a foil 12 between the two rolls 1 in the area 11. The above description clearly shows that the steel alloy, the heat treatment and the roll and/or shell proposed achieve the set objects.
In fact, experimental tests have shown that the proposed steel has greater resistance to tempering caused by high operating temperatures, due also to the presence of suitable percentages of tungsten W that forms highly stable carbon insensitive to the coalescence effect. Still to advantage, the introduction of tungsten W makes it possible to improve the hardenability of steel and to increase its stability to tempering, increasing in particular tempering temperature compatibly with the achievement of optimal values for the other characteristics. Said increase in tempering temperature is particularly advantageous when the steel is intended for making shells for rolling aluminium from continuous casting. In fact, in this case the tempering temperature obtained is near to the operating temperature reached by the shell when in contact with molten aluminium. This advantageously makes it possible to reduce the structural variations of steel, at the same time maintaining important values as to yield strength and toughness. In particular, the proposed alloys have tempering temperatures that even exceed 600 0 C, thus also considerably improving resistance to crack propagation (Kic). In fact, experimental tests have also shown that the proposed steel has good toughness to the operating temperature of the shells and this ensures higher resistance to the propagation of cracks after their inevitable appearance over time. Experimental tests have also shown that the proposed alloys advantageously
offer a good compromise between resistance (Rm - Rp0.2 - HB) and toughness (KV) or resilience.
Still to advantage, the alloy obtained presents an increase in the yield point at high temperatures compared to the reference steels. This makes it possible to obtain a higher deformation percentage due to the rapid temperature change in the elastic section of the effort/deformation curve, with less deformation in the plastic section and therefore higher resistance to the appearance of cracks. In the case of the rolls for continuous casting and/or of the shells for said rolls, made with the proposed steel, it has been possible to verify experimentally that their yield point at operating temperature is higher than that of known steels, which advantageously delays the appearance of cracks. This advantageously ensures a longer average life of said rolls and/or shells compared to that of the known rolls and/or shells. This makes it also possible, to advantage, to reduce the manufacturing cost of rolled products, and also to obtain better surface quality of the same.
Still to advantage, the introduction of cobalt Co in the alloy has allowed the characteristics of the proposed steel to be improved. Finally, the heat treatment proposed has made it possible to produce steels with better mechanical characteristics compared to those of the steels obtained with treatments of known type, and this is valid both for steels having formulations of known type and for steels having formulations in accordance with the invention. The above clearly shows that the composition of the steel and the heat treatment proposed allow a ferrous steel to be obtained that is particularly suitable for making rolling mills for continuous casting systems. It is clear, however, that the invention can in any case be used to make any type of steel tools, devices and objects. Even though the invention has been described making reference to some embodiments, in the production stage changes can be made that shall all be considered protected by the present patent, provided that they fall within the scope of the inventive concept expressed in the following claims.