RU1804353C - Method of restoring backup rolls of rolling mill stands - Google Patents

Abstract

Usage: in metallurgy and engineering. Improving the service life of reconditioned rolls. Essence: the method includes heating the rolls before surfacing, the surfacing itself and heat treatment. The roll is restored to its original diameter in several stages, by a series of surfacing with rolling of the rolls directly in the mill stand during strip rolling in a mode regulated by mathematical dependence. 1 tablet, 4 ill.

Description

The invention relates to the field of metallurgy, and more particularly, to methods for restoring steel rolling rolls by surfacing.

The purpose of the invention is to increase the operational stability of the rolls,

This goal is achieved by the fact that in the method of restoring backup rolls of rolling mill stands, including heating the rolls, multilayer surfacing on their surface with alternating layers of steels with different levels of wear resistance and heat treatment, restoration is carried out in the indicated sequence of operations in at least two stages. in this case, between the steps, the rolls are operated in the mill stand, and the thickness of the total deposited layer at each stage is determined according to the dependence:

N, (0.1 - 0.3) P PMax. - D |) + K

/ LJwaKC. where Hi is the thickness of the total guided layer at the i-th recovery stage:

Pi is the metal pressure on the rolls during their operation in the stand after the i-ro recovery stage:

Omax maximum permissible roll diameter;

DJ - the initial diameter of the roll restored at the i-th stage;

K - 14-18 - coefficient taking into account the number of layers deposited at the i-th stage to their thickness.

The essence of the proposed method is as follows.

Restoring the back-up roll from minimum to maximum diameter is carried out in several steps.

I stage. A worn to a minimum diameter roll is machined on a lathe for surfacing. Surfacing is done with tape. The layer directed at stage I consists of three layers deposited by a tape of

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00

about

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ICO Xia

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fe co

various grades of steel (Fig. 1). The first layer is welded with mild steel and serves to firmly bond the roll material (9XF 60XH and others) to the working layer. The second layer is surfaced with steel alloyed with chromium, manganese, and molybdenum. The third working layer is surfaced with a tape of high-strength wear-resistant steel. The total thickness of the deposited layer is calculated by

the formula:

Hi (0.1 -0.3) x

P | (Pmax. Dl) Omax.

+ K

-

where HI is the thickness of the total deposited layer at the i-th recovery stage;

Pi is the metal pressure on the rolls during their operation in the stand, after the i-ro recovery stage;

Omax - the maximum allowable diameter of the roll;

DI is the initial diameter of the roll restored at the i-th stage;

To 14-18 coefficient taking into account the number of layers deposited at the i-th stage and their thickness

After surfacing the layer, the roll is subjected to heat treatment, prepared for filling and dumped into the mill stand, where the roll operates one campaign. After rolling out the roll, it is machined for surfacing, and the thickness of the metal removal along the radius should be 2-4 mm. Surfacing is carried out in the same way as in stage I. Thus, the surfacing of the roll alternates with its filling into the mill and is carried out until the roll is restored to the initial (maximum) diameter.

The results of studies of the structure and texture of layers hardened after rolling-in deformation show that shearing occurs in the metal, accompanied by an increase in the hardness and strength of the surface layers of the roll and the appearance of residual stresses in them

Research institutes.

In addition, the effect of oriented hardening arises both due to the optimal orientation of internal stresses and due to the formation of the tempering martensite and the ordering of carbide precipitates along grain boundaries. The total effect that increases the resistance of the roll is achieved by surfacing layers of a certain thickness and regulated during the layers rolling of the roll before subsequent surfacing, taking into account turning

The nature of the distribution of stresses in a once deposited layer along the radius

5

10

twenty

25

30 35 40 5

50 55

the roll with subsequent running is represented by the diagram in figure 2. The back-up roll deposited at one time is subjected to stresses with an asymmetric cycle on the side of the work roll. When tilted, it forms zone I by compressive stress; zone II - maximum tangential tensile (shear) stresses; zone III - tensile stresses due to the different texture of the layers. The depth of zone II, depending on the diameter of the roll and the pressure experienced by the roll during rolling, is calculated using the Hertz-Belev theory of compression of two cylinders. The roll chipping occurs as a result of the formation of stresses in zone II above the maximum allowable or peeling of the deposited layer in zone III as a result of tensile stresses in the zone of various textures. Repeated deposition and running-in taking into account the turning of the surface layer will lead to a superposition of internal stress diagrams (Fig. 3). As can be seen from Fig. 3, the compressive and tensile stresses cancel each other out, as a result, an optimal distribution of the largest in magnitude predominantly compressive stresses is created over the roll section (Fig. 4).

As a result of the run-in of the deposited roll, deposited strain stresses are created, leading to the transformation of the cast metal structure into a close to forged one, while the cubic crystallographic orientation of the α phase (ferrite martensite) along the normal to the surface of the roll barrel is converted to the octahedral orientation Ill-compression texture Bcc metals. An increase in the number of fragments of a metal substructure with an octahedral orientation along the roll radius gives greater hardness and strength due to the natural anisotropy of the mechanical properties of the α-phase crystal lattice. During the heat treatment of the welded structure of the deposited layer, the carbide network also undergoes transformation: the metal is corrected by dissolving the heterogeneous precipitated carbide particles, dispersing them and orienting them along the normal to the surface of the secondary roll roll barrel. All of these factors (internal stresses, base texture, carbide precipitates) contribute to the increase in hardness, the development of residual compressive stresses in the deposited metal, and increase the resistance to fatigue failure.

Thus, only the totality of the proposed technological methods

(restoration of the roll in at least 2 stages, surfacing of a thickness-normalized layer consisting of three layers, intermediate operation in the cage) allows achieving high operational stability of the restored rolls.

The restoration of the roll at a time and correspondingly greater than the thickness of the deposited layer than is proposed will cause the layer structure to be heterogeneous: on the surface it is close to forged, in the middle there is a cast structure, the roll itself is forged. In this case, various kinds of stresses are formed at the boundary of the structures, which during the operation of the rolls do not anihilate with opposite stresses in sign. In this case, roll chipping occurs during the rolling of the first 100 thousand tons of metal.

When choosing the optimal thickness of the layer deposited at one time, we proceeded from the calculation of the stress distribution over the cross section of the roll, as mentioned above, a once deposited layer consists of three layers of steel of different grades. A layer of low-carbon steel with a thickness of 8 mm serves for stronger bonding of hard-to-digest steels directly from the roll (9ХФ, 60ХН, etc.) with deposited wear-resistant layers. The layer of steel that is directly in contact with the work roll during operation (working layer) is fused from high-strength and wear-resistant steel in order to obtain the smallest wear of the roll during the rolling process and, accordingly, to obtain the least amount of stress concentrates. The thickness of this layer is chosen so that the zone of maximum tensile stresses generated during operation (zone II of Fig. 2) is at a depth of 2/3 of the thickness of the working layer. In this case, when preparing the roll for the second surfacing (groove on the lathe), zone II will be brought to the surface and will be compensated by compressive stresses during re-run after re-surfacing. Calculations carried out using the Hertz-Belev theory show that for rolls of broadband mills operating at metal pressures of up to 3000 tons, the optimum thickness of the working layer, taking into account the groove and grinding under the profile, should be 6-8 mm. The total roll layer is calculated according to the formula indicated above and is selected from the calculation: so that the tensile stress zones formed during the run-in fall from the compressive stress zone during subsequent roll operation. The middle layer is deposited from carbon steels alloyed with chromium, manganese, and molybdenum, which have a somewhat lower wear resistance than steel of the working layer, but which have high ductility. The proportionality coefficient 0.2-0.4 takes into account the brand composition and operating conditions of the backup rolls.

Why it is not possible to surf at a time

the layer is thicker than proposed, explained above. If the once deposited layer is thinner than proposed, then in the case of repeated operation the stress diagram will shift down and instead

Angulation of boring stresses will sum them. As a result of the formation of stresses of a magnitude higher than the maximum permissible ones, the deposited layer will be chipped.

the roll will drop sharply.

The proposed recovery technology, including complete roll restoration in at least two stages; rationing once deposited

layer, the intermediate operation of the roll after surfacing of each layer, allows to increase the operational stability of the restored rolls due to the optimal distribution of stresses generated

during operation, and obtaining a structure similar in nature to forged. Only the combination of the above three features allows to achieve high operational stability of the restored

rolls.

PRI me R 1. The backup roll of the broadband mill 2000 had an initial diameter of 1600 mm After working out a complete

The resource had a diameter of 1460 mm. After grooving on a lathe, the diameter of the mill roll is 1450 mm. Before surfacing, the roll was heated in a chamber furnace to 450 ° C at a speed of 20 ° C / h;

at a temperature of 4 hours, it was installed on a surfacing machine, a layer was deposited with an NL-08KP surfacing tape, a welding current of 650-900 A, a surfacing speed of 30-35 m / h under the flux layer, a layer thickness of 8 mm.

After a single surfacing, the roll was supposed to use the 6th mill stand. The average metal pressure on the rolls in this stand is 2850 tons, the thickness of the high-strength layer, taking into account the processing before

8 mm was assumed by filling in the mill; the total layer thickness was calculated as

2850 x (1600-14501 and 2x 1600 D J

mm

(K 16 coefficient is selected from the calculation: surfacing is performed in three layers, two of which are 08KP and 25x5FMS, each 8 mm thick). The thickness of the alloy steel layer shall be:

43-16 27 mm.

The layer was deposited with a 27 mm thick NL-ZOHGSMA tape, then the layer was deposited with an 8 mm thick NL-25KHFMS tape. After surfacing, the roll was subjected to heat treatment: heating to a temperature of 520-550 ° C at a rate of 40 ° C / h, cooling with an oven to 120 ° C and cooling in air. After cooling, the rolls were machined on a roll and roll grinding machines to obtain the mold necessary for cage filling. The diameter of the roll is 1530 mm. The roll was inundated in stand No. 6 of mill 2000, operated until the next transshipment, and rolled 160 thousand tons of metal. After dumping, the roll was machined for surfacing to a diameter of 1522 mm. The roll was heated to 450 ° C at a speed of 20 ° C / h for 4 hours. It was mounted on a surfacing machine and melted as in the first stage of two layers 25x5FMS and ZOKHGSMA. After surfacing, the roll was supposed to be used in the 7th mill stand. The average value of the metal pressure on the rolls in this stand is 2800 and. The total thickness of the layer was calculated:

2800x1166 1522} 2x1600

The diameter of the roll after surfacing was 1531 mm.

In the second and subsequent surfacing, it is possible not to use a 08kp steel sublayer, but in the calculation it must be taken into account and due to it the thickness of the other two layers should be increased. In this example, the thickness of the layer of steel 25x5FM was set to 12 mm, the thickness of the layer of steel ZOKHGSMA - 22 mm.

Comparative data on the resistance of backup rolls deposited according to the accepted and proposed technology are given in the table.

In the proposed method, an increase in the service life of the rolls is achieved by 5

10 15 20 25 30

increasing the mechanical properties of the deposited layer and improving the adhesion of various materials (surfacing material and roll material) in the fusion zone.

The technical and economic advantages of the proposed method lie in the fact that its implementation significantly increases the operational stability of the reconditioned rolls operating under running pressure of more than 0.8 t / mm. The specified multi-stage method of restoring backup rolls dramatically reduces the tendency of the deposited layer to crack, accumulate fatigue phenomena leading to spalling and delamination of the deposited layer,

Claims (1)

SUMMARY OF THE INVENTION A method of restoring backup rolls of rolling mill stands, including heating the rolls, multilayer surfacing on their surface with alternating layers of steels with different levels of wear resistance and heat treatment, characterized in that, in order to increase the operational stability of the rolls, restoration is carried out in the indicated sequence of operations at least in two stages, while between the stages the rolls are operated in the mill stand, and the thickness of the total layer deposited at each stage is determined removed according to the relationship: 35 Hi (0.1 -0.3} P | (Pmax. Pi) Omax + K where HI is the thickness of the total deposited layer at the i-th recovery stage; Pi is the metal pressure on the rolls during their operation in the stand, after the i-ro recovery stage; Omax is the maximum allowable roll diameter; DI is the initial diameter of the roll restored at the i-th stage; K 14-18 is a coefficient taking into account the number of layers deposited at the i-th stage and their thickness. The operational stability of the backup rolls of the mill 2000 LPC-2 CherMK deposited by various technologies. Fi8.1 3 ™ a / UP / O / layers ФЦГ2 Fig.Z