HK1150997A1 - A process for the production of rolling mill cast rolls and a rolling mill cast roll - Google Patents

Abstract

The invention relates to a cast-rolling roller of a rolling mill with high binding resistance and high crack-diffusing resistance, and a process for manufacturing the same. The process comprises the following steps of: (1), putting a waste molten metal (a), an alloying element (b) and a plurality of chips (c) into a melting furnace; (2), melting the loaded materials at the temperature of between 1,200 and 1,500 DEG C; (3), performing chemical pre-analysis on a sample (d) of the loaded materials; (4) regulating the chemical components (by adding an iron alloy or a pure metal (e)); (5), adding sulfur (f) into the loaded materials; (6) performing another chemical pre-analysis on a new sample (g) of the loaded materials; (7) regulating the chemical components (h); (8), adding at least one rear earth metal (k) into a casting ladle (j); (9), transporting the molten metal from the melting furnace to the casting ladle (j); (10), transporting materials in the casting ladle to casting equipment; and (11) forming a half-finished product of the roller.

Description

Production process of casting roller of rolling mill and casting roller of rolling mill

Technical Field

The invention relates to a production process of a high-aging-resistance casting roll of a rolling mill and a roll produced by the process. More particularly, the invention relates to the introduction of sulfur, as a connecting element, into high chromium white cast iron and multi-component white cast iron to form sulfide phase particles in a microstructure in a controlled manner to enable the production of high performance rolling mill rolls from such materials.

Background

As known to those skilled in the art, a rolling mill roll is a tool used in the rolling process by means of which flat and long products of metallic material, mainly steel, are shaped. Like any forming tool, it is the rolling mill rolls that directly contact those formed products, resulting in an increasing ageing of their surface during use and the subsequent need to restore the initial condition of the surface after a certain period of use. This restoration is achieved by means of mechanical machining to remove the aged surface layer, which requires the rolling mill to be shut down in order to extract the aged roll and replace it with a new one that has been repaired.

The quality and productivity of the rolling process are therefore closely linked to the performance characteristics of the rolling mill rolls, since:

(a) the quality of the rolled product is mainly determined by the accuracy and repeatability of its shape, which directly reflects the geometry and the state of the surface of the roll;

(b) the productivity of a rolling mill is determined in part by the number of contingent runs that the rolling mill rolls can undergo, so that the quality of the rolled product remains above the minimum standard level (minimum number of stoppages to replace the rolls), which is made possible by the highly resistant aging rolls, meaning that the productivity of the production line is directly increased.

In the final stage of the process of hot-rolling strip, a set of roller assemblies ("roller stands"), also called finishing train, is used, arranged in line. The number of roll stands may vary depending on the function of the design and the meaning of the mill itself.

Rolls of different materials were also used because of the difference in aging coefficients between the leading and trailing roll stands.

The aging of rolling mill rolls is a process characterized by several distinct loss modes acting simultaneously: wear, oxidation, adhesion and thermal fatigue. However, we know that one or both of these loss modes in each roll stand of a rolling mill train are significant: in the first roll stand, in which the forming is carried out at a temperature of about 1,000 ℃, the phenomenon of thermal fatigue oxidation acts more strongly; in the last roll stand, it was formed at a temperature of around 700 ℃, and the friction and adhesion phenomena were significant.

Another drawback of the finishing train with conventional rolls consists in the unexpected damage caused by operational accidents of the finishing train, such as sticking ("sticking" or "welding") of the rolled strip on the rolls, and also unstable surface diffusion or breaking of the last surface layer of the rolls, simultaneously with the progressive ageing of the surfaces of the rolls of the rolling mill, causing the rolling mill to be out of operation in order to remove the rolls. Two operating characteristics of the last roll stand of the finishing train make the rolls thereon more susceptible to such damage:

(a) low rolling temperature (determines low kinetics of the oxide structure on the roll surface which compromises the performance of the roll due to anti-sticking);

(b) lower thickness of the rolled strip.

The rolling mill rolls designed as finishing rolling mills are, in most cases, bimetallic castings consisting of an "outer protective layer" made of a wear-resistant alloy and a "core layer" made of nodular cast iron or grey cast iron. The processes typically used for the production of such bimetallic rolls are centrifugal casting in a metal mold: pouring a protective layer material into the mold and, due to the centrifugal force, being uniformly distributed to the inner surface of the mold, forming an outer layer (or protective layer) having a thickness between 40mm and 120 mm; after the protective layer is cured, the core material is poured into the same mold and filled by continued rotation. When in contact with the inner surface of the protective layer which has solidified, the core material recalls a small volume of the protective layer (about 10mm along its entire inner surface), creating a metallurgical bond between the protective layer and the core, called the interface.

Rolls with a working layer (protective layer) consisting of high-chromium white cast iron and multi-component white cast iron, also called high-speed steel, for which the oxidation and thermal fatigue coefficients are significant; such rolls therefore have at least about twice the performance characteristics of rolls made of chilled iron, which are normally used in the last roll stand, where wear and sticking coefficients prevail and rolling strip sticking phenomena are frequent.

Therefore, another disadvantage of the conventional finishing train of rolls (the last roll stand) is that the pace at which the first and last roll stands need to be replaced is inconsistent, thereby sacrificing the extension of possible working time, and it is apparent that if the inconsistency can be smoothly eliminated, the number of roll replacements for the last roll stand will be reduced and the productivity of the rolling mill will be increased.

While the alloys used for the roll protection layer of the first roll stand of a finishing mill group have developed rapidly over the past 20 years, the Fe-Cr-Si-Ni-C alloy, also called chilled iron, in the roll protection layer of the last roll stand has been used for over 40 years. The microstructure of the material has a tempered martensitic die (temperedemantensite die) with secondary carbides M₃C and eutectic carbide M₃The interdendritic network of C (about 25% by volume) and the blocky or spheroidal graphites are interdendritic (about 3% by volume). The microstructure is the result of a balance of the properties of the alloying elements of their chemical composition: silicon and nickel are graphite-producing components, chromium is a strong carbide-producing component and, in addition, nickel also determines hardenability, and it is necessary to prevent the formation of pearlite (perlite) in the cooling after casting.

The long service life of chilled iron suitable for the rolls of the finishing rolling stand is due to the fact that, until now, its microstructure represents the best compromise between the wear and adhesion resistance caused by the dies and the unstable propagation of the micro-cracks caused by the graphite. It is understood that graphite acts as a lubricant (reduces sticking) in the roller or rolling strip contact surface and reduces stress concentrations at the end of the microcracks, thus reducing their diffusion rate.

Some significant developments have been implemented in the line/process of the rolls of the last stand of a hot strip rolling mill. Based on the same concept of materials, most typically chilled iron, we can use those materials that include additions of alloying elements for forming hard secondary phase particles that include only dispersed fine carbides. The consolidation process is also modified to achieve finer structures.

The manufacturers strive to meet the demand of users of such rolls for their higher wear resistance, while maintaining other performance characterizing parameters, based on the use of hard alloy elements forming eutectic and/or secondary carbides with a hardness greater than that of the carbide M₃C, e.g. M₇C₃And MC type carbides.

However, it has been found that it is counterproductive to promote the reduction and inhibition of the graphite structure and, therefore, to reduce the adhesion resistance and the unstable propagation of microcracks; that is, the resulting compromise does not indicate that the mill train improves productivity.

Patent PCT/US1996/09181 discloses a production method for chilled iron with the addition of between 0.3 and 6% by weight of niobium and with the use properties of replacement chilled iron. The presence of niobium encourages the formation of free particles of the primary carbide NbC, whose hardness increases without affecting the graphite structure, thus increasing the wear resistance and maintaining the resistance to adhesion and to the unstable propagation of microcracks (the structure formation of the carbide NbC occurs at a higher temperature than the formation of the graphite structure and therefore there is no competition between them).

However, the addition of niobium is in fact around 0.6%, and therefore, a higher content means that when the roll is cast, the casting temperature needs to be increased, thus making the process impractical. At this addition amount, the increase in wear resistance is limited because the volume ratio of the eutectic carbide NbC in the microstructure is limited to less than 1%, and further the precipitation of the carbide NbC is lacking in the die.

In addition to this purpose, only a few rolling mills use multi-component white cast iron in the final roll stand, which is obtained by centrifugal casting or CPC ("continuous cast cladding" process, or, in general, "by continuous casting coating"). However, this rolling mill suffers from the problem that the composition of this type of alloy does not contain graphite, thus impairing the adhesion resistance.

Disclosure of Invention

In order to overcome the disadvantages of the state of the art, the object of the process and the product of the invention has been disclosed.

It is therefore an object of the present invention to provide a process for casting rolled products in a rolling mill to prevent sticking of the rolled strip, to resist wear and to prevent unstable propagation of cracks.

In order to achieve the above object, a method is provided for using a multi-component white cast iron or an alloy of a high-chromium white cast iron in the rolls of a roll stand (finishing roll stand), so that the problem of lack of graphite (lubricating element) can be eliminated.

The embodiment of the invention provides a process for producing a casting roll of a rolling mill, which comprises the following steps:

-introduction (1) of the metal in waste liquid state (a), the alloying elements (b) and a number of scrap (c) into the melting furnace;

-melting (2) of the charge;

-addition (5) of sulphur (f) in the charge;

-adding (8) at least one rare earth metal (k) to the ladle (j);

-transferring (9) the liquid metal (i) from the melting furnace to a ladle (j);

-transferring the ladle charge to a casting plant (10) so as to form a semifinished roll (11).

Wherein the liquid metal (a) comprises carbon, chromium, molybdenum, tungsten, vanadium, manganese and silicon.

From 0.1 to 2.0% by weight of manganese, from 0.1 to 1.0% by weight of sulphur, the sulphur being proportional to manganese in a ratio of 2: 1 (Mn: S).

The rare earth metal is present in an amount of 50% by weight of the liquid metal in the ladle, preferably cerium, in an amount of 0.2%.

The embodiment of the invention also provides a casting roll of a rolling mill manufactured by the process, which is used in a finish rolling roll frame of a rolling process.

Drawings

FIG. 1 is a schematic diagram of the process for producing the casting rolls of a rolling mill according to an embodiment of the present invention.

Detailed Description

The invention provides a process for casting roll products of a rolling mill, which is used for preventing a rolled belt from being adhered, resisting abrasion and preventing unstable crack propagation.

In order to achieve the above object, a method is provided for using a multi-component white cast iron or an alloy of a high-chromium white cast iron in the rolls of a roll stand (finishing roll stand), so that the problem of lack of graphite (lubricating element) can be eliminated.

The conventional steps of a process for producing rolling mill rolls from high-chromium white cast iron or multi-component white cast iron are:

a) introducing the waste liquid metal, the alloy components and the plurality of fragments into a melting furnace;

b) melting the charge;

c) injecting the resulting mixture into a ladle;

d) the material in the ladle is transferred to the casting equipment, thus forming a semifinished roll.

The invention comprises an innovative method, as shown in fig. 1, comprising the steps of:

-introducing (1) the metal in a spent liquid state (a), the alloying element (b) and the plurality of fragments (c) into a melting furnace;

-melting (2) the charge;

-chemical pre-analysis (3) of the charge sample (d) in order to adjust (4) the chemical composition (by adding (e) ferroalloy or pure metal);

-adding (5) sulphur (f) to the charge;

-chemical pre-analysis (6) of a new sample (g) of the charge to adjust (7) the chemical composition (h);

-adding (8) at least one rare earth metal (k) to the ladle (j);

-transferring (9) the liquid metal (i) from the melting furnace to a ladle (j);

-transferring the material in the ladle to a casting plant (10), thus forming a semifinished roll (11).

The addition of sulphur in the process, in combination with at least one rare earth metal, makes it possible to form controlled particles of sulphide phases in the microstructure, thus unexpectedly obtaining a better resistance to wear, and a better resistance to adhesion, and a better resistance to the propagation of unstable cracks. This is due to the presence of vanadium, chromium, molybdenum and tungsten components in the chemical composition of the multi-component white cast iron, and the presence of a large amount of them in the high-chromium white cast iron, resulting in the formation of eutectic and secondary carbides with increased hardness between 2000HV and 2800HV (where "HV" denotes the hardness unit, i.e. "vickers hardness"). The higher wear resistance of the roll is thus promoted by the die and the eutectic carbides, the sulphide phase acting as a graphite substitute, acting as a lubricant on the roll, rolling strip contact surface, thus reducing the possibility of rolling strip sticking and acting in this way reducing the stress concentration at the ends of the crack, thus impairing its diffusion rate.

In this composition, the sulfur component is proportional to the manganese component (ratio Mn: S = 2: 1).

The chemical composition of the initial liquid metal may include, but is not limited to, 1.0% to 3.0% carbon, up to 18% chromium, 8.0% molybdenum, 8.0% tungsten, 10% vanadium, 2.0% manganese, and 2.0% silicon. The final composition of the alloy of the roll may also include up to 1.0% nickel and 0.08% phosphorus, and in addition, the sulfur addition during production may be from 0.1 to 1.0% by weight.

The rare earth metal has a composition of 50% by weight of 0.2% of the liquid metal in the ladle. Preferably, the rare earth metal is cerium.

The process temperature value depends on the value of the initial charge and may vary between about 1.200 ℃ and 1.500 ℃.

The present invention thus provides the advantage of adding sulfur to the cast roll product of a rolling mill in a process that results in high surface aging resistance of the rolling mill rolls during use.

The addition of sulphur thus makes it possible to use a smaller number of rolls and the need to return to the same initial conditions is reduced, thus prolonging the operation of the rolling mill with less stoppage in the rolling process and providing a better surface finish to the final product.

The table shows the chemical composition of the material used in the preferred embodiment for the protective layer of the rolling mill casting rolls of the present invention having a higher resistance to ageing, as follows:

wherein

Alloy a = multi-component white cast iron, also known as high speed steel;

alloy B = high chromium white cast iron; and

q.s.p. = sufficient amount.

= manganese component (Mn) is proportional to the sulfur content in the formation of the sulfide phase.

While the preferred components of this embodiment have been described and illustrated, it is noted that other embodiments may be practiced without departing from the scope of the invention. We have examples of variations when the rare earth metal mixture is added, preferably at the bottom of the ladle (j), in order to achieve better results, but also directly by metal injection, i.e. when cast, or to improve the composition of the casting equipment.

Another variant of the invention is that, in addition to centrifugal casting, the same material (product) disclosed can be used for the manufacture of rollers by CPC (continuous casting clad), which has been used previously.

The embodiment of the invention also provides a casting roll of a rolling mill manufactured by the process, and the casting roll of the rolling mill is used in a finish rolling roll frame of a rolling process.

While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. The scope of the claims should, therefore, be accorded a broad interpretation so as to encompass such similar variations and modifications.

Claims (5)

1. A rolling mill casting roll comprising a protective layer comprising high chromium white cast iron comprising 2.0% to 3.0% carbon, 10% to 18% chromium, 0.1% to 0.8% sulfur, less than 3% molybdenum, less than 1.0% tungsten, less than 1.0% vanadium, less than 2.0% manganese, less than 1.0% silicon, less than 1.0% nickel, less than 0.08% phosphorus, 0.1% cerium and a sufficient amount of iron, and wherein said manganese and said sulfur are present in a ratio of 2: 1 (Mn: S) is proportional.

2. A rolling mill casting roll comprising a protective layer comprising a multi-component white cast iron comprising 1.5% to 2.5% carbon, 3.0% to 10% chromium, 2.0% to 8.0% molybdenum, 2.0% to 8.0% tungsten, 2.0% to 10% vanadium, 0.1% to 0.8% sulfur, less than 2.0% manganese, less than 1.0% silicon, less than 1.0% nickel, less than 0.08% phosphorus, 0.1% cerium and a sufficient amount of iron, and wherein said manganese and said sulfur are present in a ratio of 2: 1 (Mn: S) is proportional.

3. A process for producing the casting rolls of a rolling mill according to claim 1 or 2, characterized by comprising the steps of:

-introduction (1) of the metal in a spent state (a), the alloying element (b) and a number of scrap (c) into a melting furnace to form a charge;

-melting (2) of the charge;

-addition (5) of sulphur (f) in the charge;

-adding (8) at least one rare earth metal (k) to the ladle (j);

-transferring (9) the liquid metal (i) from the melting furnace to a ladle (j);

-transferring the ladle material to the casting plant (10) so as to form a protective layer for the semifinished roll (11).

4. The process of claim 3, wherein the rare earth metal is cerium.

5. Use of a rolling mill casting roll according to claim 1 or 2 in a finishing roll stand of a rolling process.