EP0051511B1 - Cast roll for cold rolling, and method for its production - Google Patents

Abstract

The composite bimetallic roll for cold rolling obtained by casting comprises a heart metal constituted by a nodular or lamellar cast iron and a case metal constituted by a steel having a chromium content of 8 to 16% and a carbon content of 0.65 to 0.95%. The chromium/carbon ratio is between 11 and 16 and the structure of the case metal is martensitic with a content of residual austenite of less than 10%.

Description

The present invention relates to working rolls for cold rolling of ferrous and non-ferrous metals, which are of the composite type and obtained by casting.

Previously used for the manufacture of such working rolls for cold rolling, forged steel rolls quenched with water in carbon grades from 0.8 to 0.9% and chromium from 1.8 to 3 %. These forged cylinders must have perfect internal health to withstand the considerable tensions induced during martensitic quenching by brutal cooling with water.

In some cases, especially when looking for very deep quenching depths, the internal tensions induced by the water quenching treatment are such that it may be necessary to resort to remelting under slag, in order to obtain a perfect crystallization. The chromium content of these cylinders can then reach 5 to 7%.

A competition has started between blacksmiths to increase the quenching depth, in order to increase the usable part of the cylinder in the rolling mill, without having to re-soak it. Thus, from 30 mm a decade ago, we went to a usable part of 70 mm on the diameter. This competition has not, however, brought any appreciable improvement as regards the resistance to softening which, to a certain extent, characterizes resistance to rolling incidents.

More recently, the manufacturers of molded cylinders have introduced onto the market a new type of cylinder consisting of an outer layer of high chromium content cast iron and a core of lamellar or nodular cast iron. These cylinders, which are characterized both by a high resistance to softening, and therefore to incidents, and by a high possibility of use on the diameter (> 70 mm), are taking an increasingly important place on the market. more important, thus coming to compete with the forged cylinders produced until then.

Thus FR-A-2 081 561 describes such a cylinder in which the metal of the external layer is a cast iron with a high chromium content and the core is in lamellar or nodular cast iron.

• - Ils présentent une grande difficulté de rectification.
  En effet, la durée de rectification de ces cylindres entre deux montages de laminage atteint, pour une même retouche, un temps au moins double de celui d'un cylindre en acierforgé.
• - Ils sont plus difficiles à grenailler que les cylindres forgés.
  On peut même noter qu'il est parfois impossible, notamment lorsque les cylindres en acier moulé sont très durs, d'obtenir de fortes rugosités.

These cast iron cylinders with high chromium content, the hardness of which reaches that of cylinders in forged steel quenched with water, have drawbacks which hinder its use in rolling mills:

• - They present a great difficulty of rectification.
  In fact, the duration of rectification of these cylinders between two rolling assemblies reaches, for the same touch-up, a time at least twice that of a forged steel cylinder.
• - They are more difficult to blast than forged cylinders.
  It may even be noted that it is sometimes impossible, particularly when the cast steel cylinders are very hard, to obtain high roughness.

The present invention aims to remedy these two important drawbacks, while retaining the qualities inherent in this type of cylinder obtained by molding, the essential qualities of which are good resistance to softening and a great depth of the usable part over the diameter. .

The present invention thus relates to a bimetallic composite cylinder for cold rolling obtained by molding, and consisting of a ferrous alloy casing and a core of lamellar or nodular cast iron, characterized in that the casing metal consists of a steel having a chromium content of 8 to 16% and a carbon content of 0.65 to 0.95%, the chromium / carbon ratio being between 11 and 16 and the structure of the shell metal being martensitic with a residual austenite content less than 10% and a hardness greater than 700 HV.

The structure of the metal cylinder, the composition of which is defined above, is preferably obtained by a heat treatment process which consists first of all in subjecting the cylinder to an austenitization treatment at a temperature above 900 ° C. for a time from 8 to 24 hours, then after quenching in air or in the blown himid air stopped at a temperature of 450 to 550 ° C, the temperature is maintained at 500-550 ° C for 8 to 24 hours, then cooled the cylinder by air quenching to room temperature and finally the cylinder is subjected to an income for reactivation of the austenite at a temperature between 400 and 450 ° C for a time of 8 to 24 hours.

The austenitization treatment is preferably carried out at a temperature between 1000 and 1050 ° C., for a time of 8 to 24 hours. The cylinder is then subjected to quenching by blowing ambient air or himid air or any other equivalent known means, which is preferably stopped when the cylinder temperature reaches 500 to 520 ° C. The cylinder is then maintained for 8 to 24 hours, in an enclosure brought to the temperature indicated above from 500 to 550 ° C, in order to balance the temperatures between heart and skin. The cylinder temperature is then brought back to the value of the ambient temperature by a new quenching in air. The cylinder is finally subjected to a tempering treatment at a temperature between 400 and 450 ° C, for 8 to 24 hours.

• 1. Utilisation de millimètres utiles jusqu'à 100 sur le diamètre.
• 2. Résistance à l'adoucissement à chaud exceptionnelle au moins jusqu'à 450° C.
• 3. Haute dureté pour éviter les marques et augmenter la tenue du grenaillage.
• 4. Facilité de rectification égale à celle des cylindres en acierforgé.
• 5. Facilité de grenaillage avec obtention aisée de fortes rugosités.

The cylinders defined above, having the compositions and obtained following the heat treatments defined above have the following five essential properties:

• 1. Use of useful millimeters up to 100 on the diameter.
• 2. Exceptional heat softening resistance at least up to 450 ° C.
• 3. High hardness to avoid marks and increase the resistance of shot blasting.
• 4. Ease of rectification equal to that of forged steel cylinders.
• 5. Ease of blasting with easy obtaining of roughness.

These five properties combined on a single type of cylinder constitute significant progress compared to the current state of the art, which until now has not made it possible to reconcile all these advantages. The composite cylinder according to the present invention comprises a layer of envelope metal having a preferred thickness of 30 to 70 mm, which is preferably produced by centrifugation.

This shell metal is a steel with a high chromium and carbon content, preferably having the following composition:

The core metal is preferably a nodular cast iron having the following composition:

The composition and heat treatments defined above make it possible to explain the obtaining of the five properties set out above for the following reasons.

1. Strong useful layer (up to 100 mm on the diameter)

This is obtained using a martensitic transformation over the entire useful thickness and, thanks to induced internal tensions maintained at the lowest level. The martensitic transformation is obtained by air cooling and the quenching effect is limited by the bonding of the working layer of the shell metal with a core metal giving a pearlitic transformation during air cooling.

The low-alloyed core metal therefore consists of a nodular or lamellar cast iron which makes it possible to obtain both good mechanical characteristics and good cirstallization, these two properties being essential to withstand the tensile stresses induced by the martensitic quenching of the envelope metal.

2. Resistance to hot softening

The analysis of the envelope metal is chosen with a composition close to that of the matrix of cold working cylinders made of high chromium content cast iron previously used in the art. The income used on the highly alloyed steel grade according to the present invention does not lead to a transformation of the austenite into bainite, but to a reactivation of the residual austenite which is transformed, upon cooling, into martensite, at l exclusion of any bainite training.

To obtain the reactivation of the residual austenite, it is necessary to go up to temperatures of more than 400 ° C., without the quenching martensite being strongly softened.

3. High hardness

After air quenching, the new composite cylinder according to the present invention achieves a hardness of 700 HV (Vickers hardness) with a residual austenite content of 30 to 40%.

After tempering, the reactivation of this austenite makes it possible to obtain hardnesses from 760 to 800 HV. These high hardness levels are those usually obtained in cold rolling with traditional forged steel cylinders.

4. Ease of rectification

This property is obtained by eliminating the presence of ledeburitic carbides of type M7C3 which were previously present in the cylinders molded according to the prior art.

Indeed, one can explain the singular behavior in the rectification and shot peening of these cylinders according to the prior art, as being the consequence of the presence in the structure of the cast iron of a large quantity of chromium carbides of type M7C3 whose individual hardness can reach 1700 HV. These carbides, firmly embedded in an equally hard martensitic matrix (700 HV), obviously oppose the abrasion of the grinding wheel, as well as the deformation of the shot, which does not reach, far from it, their hardness (the hardest shot reaches 900 to 940 HV). The study of the behavior in service of these cast iron cylinders with high chromium content, according to the prior art, made it possible to conclude that the phase of carbides of type M7C3 did not play an important role during rolling, but caused, by against, the drawbacks noted during grinding and shot peening.

The composition of the new alloy according to the present invention is calculated, in particular with regard to the carbon and chromium content, to achieve a slightly hypereutectoid composition, in order to avoid the presence of these harmful ledeburitic carbides.

5. Ease of peening, especially for heavy roughness

There, also the absence of massive carbides of very hard M7C3 type allows a better work of plastic deformation of the projected shot, as has been explained above.

The study of the composition of the cylinder shell alloy according to the present invention, of which carbon and chromium constitute the two main elements, allows the following conclusions to be drawn.

Chromium can be considered to have an equivalent carbon coefficient of 0.05.

It thus appears that the slightly hypereutectoid composition of the alloy is therefore equivalent, as regards the appearance of ledeburite to a steel of composition 0.7 + 11 x 0.05 = 1.25% carbon for the content maximum in chrome.

It is thus verified that this content is at the lower limit of the appearance of a network of ledeburitic carbides.

, qui détermine avec la température d'austénitisation la teneur en chrome de la matrice, a été déterminé expérimentalement sur toute une série d'alliages carbone-chrome, et doit être de préférence compris entre 11 et 16.Furthermore, to obtain the properties of resistance to hot softening, it is necessary to have a sufficient chromium content in the matrix. The best report

, which determines the chromium content of the matrix with the austenitization temperature, has been determined experimentally on a whole series of carbon-chromium alloys, and should preferably be between 11 and 16.

The other elements, namely Si and Mn, are found in the usual ranges for analyzes of cast steels.

Nickel and manganese are deliberately limited to 0.7% to avoid their stabilizing effect on the residual austenite.

Molybdenum and vanadium improve the hardening resistance of austenite and are found in the usual grades of cold working chrome steels (classes 80 and 90 of the American classification).

New molded bimetallic composite cylinders are thus obtained which exhibit exceptional softening resistance by tempering the matrix.

By way of example, a cylinder according to the present invention has been produced using an envelope metal and a core metal having the following compositions.

The cylinder thus obtained is subjected to an austenitization treatment for 24 hours at 1000 ° C., then to a quenching with blown air which is stopped at a temperature of 520 ° C. The temperature of the cylinder is then maintained at a value of 520 ° C. for 20 hours, then the cylinder is cooled to ambient temperature by air quenching.

It has a hardness after quenching of 692 HV and an austenite content of 40.6%.

This cylinder is then subjected to tempering at a temperature above 400 ° C., for 20 hours, which makes it possible to obtain, at the end of the treatment, a hardness of 780 HV over 50 mm on the radius.

This cylinder was used for the cold rolling of thin sheets and confirmed the advantages stated previously in the course of the description.

Claims (6)

1. A cast bimetallic composite roll for cold rolling, and constituted of a case made of an iron base alloy and a heart made of lamellar or nodular cast iron, characterized in that the case metal is constituted by a steel having a chromium content of 8 to 16% and a carbon content of 0,65 to 0,95%, the ratio between the chromium and carbon being between 11 and 16 and the structure of the case metal being martensitic with a residual austenite content of less than 10% and a hardness exceeding 700 HV.

2. A roll according to claim 1, characterized in that the case metal has a thickness of about 30 to 70 mm.

3. A roll according to claim 1 or 2, characterized in that the case metal has the following composition:

4. A roll according to claim 1 or 2, characterized in that the heart metal is a nodular cast iron having the following composition:

5. A method for producing a roll according to any one of the preceding claims, comprising first centrifugally casting a layer of case metal of steel having a chromium content of 8 to 16%, a carbon content of 0,65 to 0,95% and a chromium/carbon ratio between 11 and 16, then a core of the heart metal of nodular or lamellar cast iron, and subjecting the roll to an austenitisation treatment at a temperature higher than 900° C for a period of 8 to 24 hours, then, after hardening in air or blown damp air stopped at a temperature of 450 to 550° C, maintaining the temperature at 500 to 550° C for a period of 8 to 24 hours, cooling the roll by immersion in blown air to room temperature, and subjecting the roll to a tempering treatment so as to reactivate the austenite at a temperature between 400 and 450° C during a period of 8 to 24 hours.

6. A method according to claim 5, characterized in that the austenitisation treatment is carried out at a temperature of 1000 to 1500° C and the hardening is stopped at a temperature of between 500 and 520° C.