EP0209437B1 - Lightly alloyed, forged steel cylinder for cold rolling - Google Patents

Abstract

The roll is made from a low alloy steel having the following composition by weight : C: 0.76 to 0.92; Mn: 0.70 to 1.40; Si: 0.70 to 1.40; S</=0.020; P</=0.025; Ni</=0.60; Cr: 1.50 to 2.20; Mo: 0.15 to 0.55; V: 0.08 to 0.25; Cu</=0.50; the remainder being iron and accidental impurities.

Description

The present invention relates to forged rolls for cold rolling and more particularly to working rolls intended for the rolling of iron and steels, non-ferrous metals and their alloys at temperatures less than or equal to 100 °. and, optionally, support rolls used in multi-cylinder rolling mills.

• 1. Une dureté superficielle élevée comprise entre 90 et 105 Shore C selon les produits à laminer.
• 2. Une forte épaisseur de la couche trempée qui permettra de limiter ou même d'éliminer les retraitements éventuellement nécessaires au maintien de la dureté désirée sur toute l'épaisseur donnée d'utilisation du cylindre.
• 3. Une grande résistance à l'usure par l'abrasion.
• 4. Une teneur contrôlée de l'austénite résiduelle de la couche trempée ; des teneurs trop élevées en austénite résiduelle étant nuisibles en favorisant la fissuration sous contrainte en service.
• 5. Une structure dendritique des couches superficielles suffisamment homogène pour éviter un phénomène de gravage extrêmement fin du feuillard auquel les professionnels donnent le nom de « peau de crapaud •, « peau d'orange •,

To ensure excellent service performance at the lowest cost, the working cylinders must have a certain number of characteristics in the state of use:

• 1. A high surface hardness between 90 and 105 Shore C depending on the products to be laminated.
• 2. A large thickness of the hardened layer which will limit or even eliminate any reprocessing that may be necessary to maintain the desired hardness over the entire given thickness of use of the cylinder.
• 3. High resistance to abrasion wear.
• 4. A controlled content of the residual austenite from the hardened layer; excessively high contents of residual austenite being harmful by favoring cracking under stress in service.
• 5. A dendritic structure of the surface layers sufficiently homogeneous to avoid an extremely fine etching phenomenon of the strip to which the professionals give the name "toad skin", "orange skin",

A large number of these characteristics can be adjusted by a judicious choice of the conditions of manufacture of the cylinders of cold rolling mills and very particularly of the heat treatment operations: tempering allowing the hardness of the table to be adjusted, conventional quenching mode with heating at a temperature> AC3 of the entire cylinder during austenitization, surface quenching after heating to a temperature> AC3 only of a layer of relatively small thickness, cooling conditions more or less well adjusted.

However, the choice of shade remains essential in order to allow the optimization of the characteristics required at the lowest cost.

The grades currently used for cold rolled working rolls in forged water-hardened steel contain 0.8 to 0.9% carbon, 1.8 to 3.0% chromium and others elements of alloys and are illustrated by the conventional grade 83 CDV7 which has a sufficiently high carbon content to allow obtaining the high levels of hardness required, the contents of Cr, Mo, V are sufficient to have a correct hardenability and the formation of numerous carbides ensuring good resistance to wear. With conventional heating treatments followed by energetic water quenching, it is therefore easy to obtain a surface hardness of 103 Shore C, a hardened layer depth of ≥ 85 Shore C hardness of 15 mm, on cylinders with a diameter from 550 to 650 mm.

With a surface hardening after induction heating at the frequency of 50 Hz, similar surface hardnesses are obtained with, however, a hardened layer of greater depth about 22 mm.

However, to fully exploit the useful thickness of the table, such quenching depths require at least two reprocessings.

These reprocessings are expensive, so many manufacturers have sought to improve the hardenability of the steel in order to obtain hardened layers thick about 30 mm, thus limiting the number of reprocessings to a single operation.

In order to increase this thickness, we turned to more highly alloyed steels with Cr contents up to 3% and Mo contents up to 0.5%. In addition to the fact that these alloying elements are expensive, the increase in their content has the serious disadvantage of generating an undesirable residual austenite level after the martensitic quenching.

High residual austenite levels can be remedied by a treatment after quenching consisting of immersing the cylinder in liquid nitrogen (subzero treatment), but these treatments are delicate and expensive.

Finally, the increase in the content of Cr, Mo, V alloy elements leads to the formation of a band structure and a dendritic structure which are harmful to the surface quality of the rolled products.

The present invention aims to remedy these drawbacks, while making it possible to obtain forged cylinders having a large thickness of hardened layer.

• C : 0,76 à 0,92 ; Mn : 0,70 à 1,40 ; Si : 0,70 à 1,40 ; S s 0,020 ; P _% 0,025 ; Ni ≤ 0,60 ; Cr : 1,50 à 2,20 ; Mo: 0,15 à 0,55 ; V: 0,08 à 0,25 ; Cu ≤ 0,50 ; le reste étant du fer et des impuretés accidentelles.

It also relates to a forged cylinder for cold rolling made of low-alloy steel, characterized in that it has the following weight composition:

• C: 0.76 to 0.92; Mn: 0.70 to 1.40; If: 0.70 to 1.40; S s 0.020; P _% 0.025; Ni ≤ 0.60; Cr: 1.50 to 2.20; Mo: 0.15 to 0.55; V: 0.08 to 0.25; Cu ≤ 0.50; the remainder being iron and accidental impurities.

The essential characteristic of the invention resides in the Si content which causes, in association with Mn, a synergistic effect on the quenchability for a steel with a low content of alloying element and in particular Mo.

• Multiplying factors of the calculation of Hardenability of Hypereutectoid steels Hardened from 1 700° F.
• C. F. Jatezack and D. J. Girardi transaction of ASM 1959 - 51 p 335 ; et Hardenability of high carbon steel C. F. Jatezack and D. J. Girardi Metallurgical transaction - vol 4 oct 73 p 2267 ; décrivent l'influence des éléments d'alliages sur la trempabilité des aciers hypereutectoïdes, et caractérisent la trempabilité des différentes nuances par la distance à l'extrémité trempée du point Jominy où la dureté est 63 HRC, sur éprouvettes Jominy austénitisées à des températures comprises entre Acm + 50 et Acm + 100.

The work of Jatezack and Gérardi in the following articles:

• Multiplying factors of the calculation of Hardenability of Hypereutectoid steels Hardened from 1,700 ° F.
• CF Jatezack and DJ Girardi transaction of ASM 1959 - 51 p 335; and Hardenability of high carbon steel CF Jatezack and DJ Girardi Metallurgical transaction - vol 4 Oct 73 p 2267; describe the influence of alloying elements on the hardenability of hypereutectoid steels, and characterize the hardenability of the different grades by the distance from the hardened end of the Jominy point where the hardness is 63 HRC, on austenitized Jominy test pieces at temperatures between Acm + 50 and Acm + 100.

The structure corresponding to the hardness of 63 HRC is almost completely martensitic with a maximum of 10% bainite, so that the criterion adopted is entirely representative of the conditions of use of the cylinders.

These studies show that the increase in hardenability can be obtained by using higher contents of conventional alloying elements such as Mn, Ni, Cr, V, Si and above all Mo as indicated by the graphs of Fig la and Ib illustrating the multiplicative factor F over the distance from the hardened end as a function of the content of various elements indicated, for an initial structure respectively normalized and annealed.

It clearly appears on these graphs that Mo exerts the strongest action and in particular superior to Si alone or even combined and to Mn.

However, the Applicant has discovered that, contrary to the teachings of this work, Mo exerts an influence on the hardenability, presenting a maximum for relatively low contents.

These results are given in FIG. 2 graphically representing the influence of the addition elements Mo, Mn and Si on the hardenability of a steel 85 CDV7 having undergone an austenization treatment ACm + 60 ° C. On this graph, the Jominy distance is plotted on the ordinate, that is to say the distance in mm at the end of a standardized test piece (25 mm in diameter) for which the Rockwell C hardness (HRC) is greater than or equal to 60.

Furthermore, it appears that Si has a synergistic effect on Mo and especially on Mn.

For comparison, Fig. 3 gives a representation (Hardness as a function of the distance D at the hardened end) of the Jominy curves for a conventional grade which is an 85 CDV7 steel whose Mn contents are 0.25 and Si of 0.42 and for a range of steel grades according to the invention.

The increase in hardness at 70 mm from 45 HRC to 63 HRC is particularly significant.

In addition, the presence of silicon tends to favor the formation of carbides, which is favorable for wear resistance as shown by the various laboratory tests used.

On the other hand, there is a slight decrease in the carbon content of the steel matrix and consequently in the maximum level of hardness that can be obtained; this is not a drawback because it suffices to play on the conditions of tempering after quenching between 100 and 200 ° C.

Silicon also increases resistance to tempering. Its action can therefore only be beneficial during small rolling incidents leading to an increase in the surface temperature of the rolls.

The absence of significant influence of the additions of Mn and Si on the rate of residual austenite after treatment and on the toughness of the metal treated at the level of 64 HRC has been verified in the field of the selected contents. the same is true for the dendritic structure on the surface of the tables. The combined addition of manganese and silicon has proven to be beneficial for the service life of the cylinder.

The examples below are given by way of illustration of the invention.

Example 1

A working cylinder with a table diameter of 325 mm and a table length of 1324 mm is produced at a table hardness of 760 Vickers, ie 92 Shore C, intended for the cold rolling of silicon steel coils.

• C 0,83 - Mn 1,12-Si 0,89 - S 0,009 - P 0,012 - Ni 0,33 - Cr 1,82 - Mo 0,25 - V 0,11.

This cylinder is machined from a blank forged in a steel ingot having the following analysis:

• C 0.83 - Mn 1.12-Si 0.89 - S 0.009 - P 0.012 - Ni 0.33 - Cr 1.82 - Mo 0.25 - V 0.11.

The final treatment of the table is carried out by low frequency surface heating (50 Hz) and quenching with water.

This gives a thickness of the hardened layer of 28.5 mm.

• C 0,83 - Mn 0,29 - Si 0,33 - S 0,007 - P 0,014 - Ni 0,27 - Cr 1,77 - Mo 0,24 - V 0,11.

By way of comparison, an analogous cylinder is produced in the classic shade:

• C 0.83 - Mn 0.29 - Si 0.33 - S 0.007 - P 0.014 - Ni 0.27 - Cr 1.77 - Mo 0.24 - V 0.11.

This cylinder gives, after low frequency surface quenching, a thickness of quenched layer of 20.5 mm.

The invention therefore provides a 40% increase in the thickness of the hardened layer, in a less costly shade, both in terms of the constituent elements and of the manufacturing process.

The cylinders in the steel grade according to the present invention used in a reversible quarto rolling mill made it possible to laminate 3,690 tonnes instead of 3,100 tonnes for the comparison grade, a gain of 19%.

Example 2

• diamètre de table 535 mm
• longueur de table 1 676 mm
• dureté de table visée 830 HV.

A working cylinder is made for cold rolling of automobile body sheet having the following characteristics:

• table diameter 535 mm
• table length 1,676 mm
• target table hardness 830 HV.

Metal analysis:

C 0.86 - Mn 0.96 - Si 1.19 - S 0.004 - P 0.012 - Ni 0.175 - Cr 1.66 - Mo 0.22 - V 0.096.

The final processing of the table is carried out as in Example 1.

After stress relieving and before adjusting the hardness, the surface hardness is 875 HV.

The hardened depth corresponding to a hardness of 700 HV, or substantially 85 Shore C, is 29.6 mm.

The useful thickness of the cylinders being 27 mm, it is possible to use all of this thickness before disposal without reprocessing by re-tempering the cylinder.

With cylinders in conventional grade 83 CDV 7, similar to that of Example 1, the thickness of the hardened layer, after low frequency surface hardening, measured under the same conditions is 22 mm. This requires reprocessing to consume the entire useful thickness of the cylinder.

It is obvious that for a cylinder with the same geometrical characteristics as previously, but, the table diameter of which has been increased to 581 mm and the useful thickness of 50 mm, the grade according to the present invention limits the total use to reprocessing. of this thickness, while it is necessary to carry out two reprocessings with the nuance of comparison.

Claims (1)

1. A forged cylinder for cold rolling, made from a low-alloy steel, characterized in that it has the following composition by weight: C: 0.76 to 0.92; Mn : 0.70 to 1.40 ; Si : 0.70 to 1.40 ; S ≤ 0.020; P ≤ 0.025; Ni ≤ 0.60; Cr: 1.50 to 2.20 ; Mo: 0.15 to 0.55; V : 0.08 to 0.25 ; Cu ≤ 0.50, the residue being iron and adventitious impurities.

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