ES2938132B2 - Method for optimizing the roughness of a rolling cylinder using high-speed thermal spraying - Google Patents

Description

DESCRIPTION

Method for optimizing the roughness of a rolling cylinder using high-speed thermal spraying

TECHNIQUE SECTOR

The present invention relates to a specially designed method for creating and optimizing roughness on coated work rolls using high-speed thermal spray technology.

These work rolls can be used for the production of sheet metal or coils in hot or cold rolling mills.

BACKGROUND OF THE INVENTION

In the hot and cold rolling plant, improving the service life of the working rolls is one of the main ways to reduce operating costs for steel manufacturers.

To increase the service life of the working cylinders, compared to a traditional solution such as chrome plating, it is necessary to increase the hardness of the coating. This can be done thanks to the coating that includes carbide particles within a metal matrix.

These coatings are deposited on the surface of the working cylinders by high-speed thermal spraying. The type of hard particles projected and the thickness of the coating greatly affect the life of the working cylinder in service.

According to standard working cylinders, due to process limitations and to avoid web defects, the roughness of the cylinders must be uniform along the length of the cylinder and must meet the requirements in terms of mean roughness and standard deviation.

Because high-speed thermal spray technology has not been developed for managing roughness, it is a challenge to reliably manage both:

• the thickness of the coating to ensure the service life of the working cylinders

• as the target roughness.

Regarding the importance of roughness on the cylinder surface, the following should be highlighted:

• Roll roughness affects the coefficient of friction between the strip and rolls during rolling. The friction coefficient increases as the roughness increases and at the same roughness the friction coefficient increases as the number of peaks increases [1]

• If the coefficient of friction is too low, slipping/skidding of the belt on the cylinder may occur and create defects or limit the thickness reduction in the lamination [2]. On the contrary, if the coefficient of friction is too high, the abrasion of the cylinder on the belt increases and contamination of the belt with iron fines increases [1].

• This is the case of cracks on the edges of the belt, which are normally seen in reversible reduction trains. In that case, it is well known that edge cracks are sensitive to the friction coefficient: the defect increases when the friction increases [3].

• Belt roughness affects the properties, quality and performance of the belt itself. As in the case of Skin-pass mills, to ensure good behavior during stamping, the roughness of the belt must be high enough to avoid cracks in the material.

• The roughness of the belt is transferred from the roughness of the working cylinder. The higher the roughness of the cylinders, the higher the roughness of the laminated strip. Rolling force and belt tension affect how roughness is transferred. Thus, knowing the roughness requested on the band, the roughness of the working cylinders is defined.

• Depending on the conditions of the rolling process and the target roughness of the strip, steel manufacturers define the roughness of the rolls.

This patent presents solutions and methodology to obtain uniform roughness that responds to customer requirements.

In this sense, the nomenclature used in the present invention is introduced.

Ra is defined as the arithmetic mean deviation of the roughness profile. The calculation is made in accordance with the ISO 4287 standard with a cut-off of 0.8 mm.

RPC is defined as the number of peaks per unit of length in centimeters. The calculation is made according to the ISO 4287 standard with a bandwidth of 1 micrometer.

Regarding the cylinders used in the cold rolling process, it should be noted that the work cylinders used in cold rolling mills have different types of surface finish.

Typical values of the texture of working cylinders are shown in Table-1.

Table 1

Regarding the evolution of Roughness during lamination, the following is worth highlighting:

• During the rolling campaign, the roughness of the working cylinders decreases due to the wear produced by the friction between the band and the cylinder. The use of coatings, such as chrome plating, delays this phenomenon. See Fig.2

• The decrease in roughness also depends on the type of texture used in the cylinders. See Fig.3

• As the initial roughness of the rolls is slightly higher than the roughness at the end of the rolling campaign, steel manufacturers regularly control the roughness of the rolls to prevent it from deviating from the range necessary to ensure quality on the surface of the roll. band. The cylinders are changed when the roughness is too low.

For its part, and with regard to texturing technologies, the following should be highlighted:

Currently, there are several texturing technologies to create roughness on the surface of work cylinders.

Yo. Shot Blasting (SBT):

A steel or cast iron shot, with a specific granulometry, is projected onto the surface of the working cylinder. The kinetic energy of the particles is sufficient to produce plastic deformation of the surface. The roughness obtained is a function of the mass and size of the shot, the speed of the shot, the hardness of the base material of the cylinder, the number of passes along the cylinder and the rotation speed of the cylinder.

ii. Electrical discharge texturing (EDT):

The voltage applied between the cylinder (cathode) and the copper electrode (anode), separated by a dielectric medium, produces an electrical discharge capable of creating craters on the surface of the cylinder. Roughness is a function of frequency, the voltage applied between the electrodes and the level of capacitance in the electronics. This technology is rarely used in cold rolling with a high reduction index since it is very sensitive to cylinder wear. Currently this texture is the reference for the Skin-pass and Temper trains.

iii. Laser Texturing (LT):

A laser beam hits the surface of the cylinder, melting the material and expelling the material out of the crater created through the assistance of a gas (O ₂ , CO ₂ or Ar). The final texture of the cylinder corresponds to a uniform distribution of craters along a helical pitch over the circumference of the working cylinder. The axial distance of the craters is controlled by the speed of the longitudinal movement of the cylinder. In the tangential direction, the distance of the craters is determined by both the speed of the cylinder and the speed of the mechanical shutter. The depth of the crater is determined by the power of the laser. This technology is not very commonly used today.

iv. Electron Beam Texturing (EBT):

This technology consists of bombarding a beam of electrons on the surface of the cylinder. During a single shot, the lenses focus the beam to preheat the cylinder material, then bombard the surface with a first shot to create a crater, and then heat the rim surrounding the crater. This cycle can be done two or three times in the same location to create a deeper crater. During the firing cycle, the beam is deflected to compensate for the continuous movement of the cylinder surface (displacement and rotation). This technology is only occasionally used for Skin-Pass trains, but is not very commonly used today.

These different technologies are described in Fig.1.

Regarding coatings using high-speed thermal projection, the following should be highlighted:

• Thermal spraying techniques are coating processes in which molten materials are projected onto a surface. A mixture of gases burns in a combustion chamber, heating and accelerating a powder to deposit it on a substrate. If the oxidizing gas is oxygen, the thermal projection is called HVOF. If the oxidizer is air, the thermal spray is called HVAF.

• To increase the service life, a rolling cylinder is produced with a liner of tungsten carbide alloys where the liner is usually a single layer, with a thickness between 0.003 mm and 0.020 mm, affecting 100% of the work surface. The alloy is preferably selected from: WC-CoCr, WC-NiCr, WC-Co, WC-Ni or WC-CrC-Ni. Preferably, the permeability of the coating is in a range between 0% and 0.1%.

• The coating layer has a final hardness between 1000 Hv and 1600 Hv.

• Patent WO2021148690 describes, in the case of HVAF technology, the use of tungsten carbide coatings for cold rolling rolls.

EXPLANATION OF THE INVENTION

The method that the invention proposes solves the previously mentioned problem in a fully satisfactory manner.

To this end, and more specifically, the method of the invention consists of a method of coating a cylinder by thermal spraying of a powder by means of a spray column to form an isotropic roughness (Ra) on the surface of said cylinder. , in which the cylinder rotates at a speed (Vr) around its longitudinal axis and the projection column moves in translation at a speed (Vt), parallel to the axis of the cylinder to deposit the material according to a helical figure, so that in said method the following operational phases are established:

a) establish a granulometry (G) of the dust to be projected,

b) establish a target roughness (Ra) and a target thickness (t) of the coating,

c) find the corresponding feed flow (Fr) of the powder in an empirical table that presents the target roughness (Ra) as a function of the feed flow (Fr) and the granulometry (G) according to the formula:

where n is the efficiency of the process that depends on the type of equipment to be used and A (G) and B (G) are functions of the granulometry of the powder (G),

d) define the rotation speed (Vr) and the translation speed (Vt) from an equation that relates the target coating thickness (t) as a function of the defined feed flow (Fr), the translation speed ( Vt) and the rotation speed (Vr), according to the formula:

where N is the revolutions per minute of the cylinder and p is the density of the spray powder, while the ratio between the width of the spray cone (d) and the pitch per revolution (p) of the propeller is greater than one.

The thermal projection method may be HVOF or HVAF projection.

For its part, the spray powder will contain hard particles with dimensions less than 1 ^m and in which the final objective roughness (Ra) depends on the average granulometry of the powder (G).

In parallel, the number of peaks (RPc) of the coating surface must not exceed a value related to the roughness (Ra).

BRIEF DESCRIPTION OF THE DRAWINGS

To complement the description that will be made below and in order to help a better understanding of the characteristics of the invention, in accordance with a preferred example of its practical implementation, a set of figures is attached as an integral part of said description. where, for illustrative and non-limiting purposes, the following has been represented:

Figure 1.- Shows a diagram of the different texturing technologies that currently exist, where:

(1)= Textured by shot blasting (SBT). Stochastic overlay.

(2-2')= Electrical discharge texturing (EDT). Stochastic overlay.

(3) = Laser Texturing (LT). Deterministic coating.

(4) = Electron beam texturing (EBT). Deterministic coating.

(5) = Texturing by chrome deposition (TST). Nodular coating.

(6) = HVAF thermal spray coating. Stochastic overlay.

Figure 2.- Shows a graph relating to the evolution of roughness during the lamination process and how the use of coatings such as chrome plating delays the wear produced by friction between the band and the cylinder, where the ordinate axis represents the surface roughness in microns, and the x-axis the strip length in kilometers, and where the lower curve represents the behavior of a forged steel cylinder with 5% chromium (uncoated cylinder), while the curve The top represents the behavior of a cylinder with a chrome or chrome plated electroplating.

Figure 3.- Shows a graph that represents the decrease in roughness as a function of the rolled tons of steel sheet, depending on the type of texture used in the cylinders, specifically for four different textures.

Figure 4.- Shows a schematic representation of the method of the invention, in which the cylinder (7) rotates at a controlled speed around its longitudinal axis (8) and the projection cone moves in translation, parallel to the axis of the cylinder to deposit the material according to a helical figure (10).

Figure 5.- Shows a graph that represents the evolution of the arithmetic mean deviation of the roughness profile as a function of the number of peaks per unit of length in centimeters, where the upper curve corresponds to the maximum ratio of peaks and the curve lower than the minimum peak ratio.

Figure 6.- Shows a graph similar to that of Figure 5, but corresponding to a comparison between the curves in a process without additional treatment and the curve corresponding to the additional treatment to reduce the peaks in roughness less than 2 microns.

Figures 6.1 and 6.2.- Show a profile cut of the coating to measure the number of peaks and their relief made using a tool specifically for this purpose. The ordinate axis reflects the size of the peaks in microns (both crests and valleys being considered peaks) while the abscissa axis represents the length in microns of the profile. Figure 6.1 shows the profile without additional treatment and figure 6.2 an extreme case where all the crests of the peaks above 0.25 microns of the coating thickness pursued for a specific case have been removed.

DETAILED EMBODIMENT OF THE INVENTION

According to the method of the invention, the following has been provided for roughness management:

- For Thermal Spray Coating, the thickness (t) is closely related to the powder feed flow (Fr), as well as to the tangential speed of the piece (Vr) and the transverse speed of the gun (Vt) according to with the following formula:

Being: t= thickness of the coating

N= cylinder revolutions per minute

p= Efficiency of the process according to the type of projection equipment

Fr= Powder feed flow

Vt= Gun transverse speed

p= Dust density

Vr= tangential velocity of the cylinder

- To ensure the uniformity of the isotropic roughness and the thickness of the coating, it is necessary to optimize the overlap of the projection cone between two turns of rotation of the cylinder. To achieve this, the ratio between the width of the projection cone (d) and the length of the step (p) between two turns of rotation must be greater than 1 (see Fig.4).

- The roughness depends on the feed flow and granulometry of the spray powder, according to the simplified empirical formula described below:

Being: ^ = Process efficiency

Fr= Powder feed flow

A(G) and B(G) are functions of the powder granulometry (G)

It is more convenient to use the Tables described below:

Table-2: Ra (^m) as a function of the Feed Flow and Granulometry of the powder-process

HVAF.

Table-3: Ra (^m) as a function of the Feed Flow and Granulometry of the powder-process

HVOF.

As a clarification, powder contains fine, hard particles (like WC) and a binder (usually a softer metal). This means that the granulometry of powder is larger than the sizes of hard particles. A grain of dust may contain more than one hard particle.

Regarding the stages to manage roughness, these are the following:

a) Define the Granulometry of the powder.

b) Knowing the granulometry of the powder and the target roughness, the powder feed flow is defined according to Table-2 or Table-3.

c) Knowing the powder feed flow and the target thickness, the values of Vr and Vt are defined taking into account Equation-1 and respecting d/p >1.

Table-4 describes the thermal projection according to our invention compared to the standard roughness.

Table-4: Comparison between thermal projection roughness and standard stochastic roughness.

Regarding the management of the granulometry of the dust and the size of its hard particles:

- Previously, the steps necessary to manage roughness for a fixed size of the spray powder were explained.

- To access all requested roughness levels, the powder size must be adapted, according to Table-5 to address different roughness ranges.

Table-5: Required powder granulometry

- It is known that the size of the hard particles can affect the final roughness of the high-speed spray coating. As, for example, patent JP09300008 advises adapting the size of the hard particle between 1 and 20 ^m so that the roughness obtained is between 0.3 and 3 ^m. For example, hard particle size between 1 and 5^m to obtain a roughness of about 0.3^m.

- As the useful life of the cylinders increases, the duration of the rolling campaign increases. If the size of the hard particles is too large, the roughness of the cylinder increases again as the rolling progresses and this is due to the "wear" of the metallic binder. To avoid this phenomenon, the size of hard particles should be less than 1 ^m.

For its part, and with regard to managing the number of roughness peaks, the following should be highlighted:

- For uncoated cylinders, or chrome plated cylinders, used in tandem or reversible rolling mills, as mentioned above, steel manufacturers used to set the roughness to ensure the quality of the strip (no contamination, edge cracks, etc. ...) but no specific request is made for the number of spikes.

- The HVAF or HVOF coating that contains hard particles considerably increases the service life of the cylinders. This means a large increase in the duration of the rolling campaign. For the standard duration of the rolling campaign (uncoated cylinders or chrome-plated cylinders), roughness management is sufficient to avoid defects in the rolled strip. In case of coatings with hardness greater than 1000 Hv, the tests indicated that it is important to limit the level of the number of peaks in addition to the roughness. According to [2]% of the flat area affects friction. One way to increase the contact surface is to decrease the number of spikes and/or round the spikes.

- In order to increase the service life of the rolling campaign by 1.5 times compared to chrome cylinders or 2 times compared to cylinders uncoated without any quality problem (edge cracks, contamination, etc...), the maximum number of peaks (RPc) must follow the formula:

- In the case of coatings using high-speed thermal spraying, the number of peaks evolves with roughness as shown in Fig-5. This evolution is typical of the roughness created by high-speed thermal spraying and for spray powder particle sizes less than 50 μm.

- Fig.6. It shows that high-speed thermal spray coatings satisfy Equation-3. For roughness greater than 1.5-2 μm. For a lower roughness it is necessary to add a subsequent treatment operation of the coated surface.

This surface treatment can be mechanical (shot blasting, polishing), chemical, electrochemical or thermal (laser). Through these treatments, the roughness peaks are eroded. At the same time, the roughness and the total number of peaks are reduced (see Fig. 6, Fig. 6.1 and Fig.6.2). The way in which the peaks and roughness decrease depends on the type of final treatment to be performed.

At the same time, for roughness greater than 5 μm it is necessary to pre-treat the cylinder by shot peening.

The references used in this application are the following:

[1] WORK ROLL ROUGHNESS TOPOGRAPHY AND STRIP CLEANLINESS DURING COLD ROLLING AUTOMOTIVE SHEET - Claude Gaspard, Daniel Cavalier, Stefan Wahlund - Technical contribution to the 11th International Rolling Conference, part of the ABM Week 2019, October 1st-3rd, 2019, Sao Paulo , SP, Brazil.

[2] RELATIONS BETWEEN FRICTION COEFFICIENT AND ROLL SURFACE PROFILES, ROLLED SHEET CHARACTERISTICS IN COLD ROLLING OF STEEL SHEETS - Hiroyasu YAMAMOTO, Mansaku SASAKI and Takahiro

KITAMURA - Tetsu-to-Hagané Vol. 95 (2009) No. 5.

[3] THE RESEARCH ON EDGE CARCK OF COLD ROLLED THIN STRIP - Haibo Xie -2011 - Thesis of university of Wollongong.

[4] TEXTURING METHODS FOR COLD MILL WORK ROLLS - Bilal QOLAK*, Fatih BA§OGLU+, Naci KURGAN - UDCS'19 Fourth International Iron and Steel Symposium, 4-6 April, Karabuk.

[5] EFFECT OF WORK ROLL TECHNOLOGY ON COLD MATERIALS ROLLING AND PROGRESS OF MANUFACTURING FUTURE DEVELOPMENTS IN JAPAN -Mitsuo HASHIMOTO, Taku TANAKA, Tsuyoshi INOUE,1) Masayuki YAMASHITA,2) Ryurou KURAHASHI3) and Ryozi TERAKADO ₄ ) - ISIJ International, Vol 42 (2002), No. 9, pp. 982-989.

[6] Patent WO 2021148690.

[7] Patent JP 09300008.

Claims (6)

1a.- Method for optimizing the roughness of a rolling cylinder by means of high-speed thermal spraying, more specifically thermal spraying of a powder by means of a spraying column to form an isotropic roughness (Ra) on the surface of said cylinder, in which the cylinder rotates at a speed (Vr) around its longitudinal axis and the projection column moves in translation at a speed (Vt), parallel to the axis of the cylinder to deposit the material according to a helical figure, in where the following operational phases are established: a) establish a granulometry (G) of the dust to be projected, b) establish a target roughness (Ra) and a target thickness (t) of the coating, c) find the corresponding feed flow (Fr) of the powder in an empirical table that presents the target roughness (Ra) as a function of the feed flow (Fr) and the granulometry (G) according to the formula:

where n is the efficiency of the process that depends on the type of equipment to be used and A (G) and B (G) are functions of the granulometry of the powder (G), d) define the rotation speed (Vr) and the translation speed (Vt) from an equation that relates the target coating thickness (t) as a function of the defined feed flow (Fr), the translation speed ( Vt) and the rotation speed (Vr), according to the formula:

where N is the revolutions per minute of the cylinder and p is the density of the spray powder, while the ratio between the width of the spray cone (d) and the pitch per revolution (p) of the propeller is greater than one. 2a.- Method for optimizing the roughness of a rolling cylinder by means of high-speed thermal spraying, according to claim 1a, characterized in that an HVAF thermal spraying method is used, being the empirical table for calculating the desired roughness (Ra) depending on the powder granulometry (G) and the powder feed flow (Fr):

3a.- Method for optimizing the roughness of a rolling cylinder by means of high-speed thermal spraying, according to claim 1a, characterized in that an HVOF thermal spraying method is used, being the empirical table for calculating the desired roughness (Ra) depending on the powder granulometry (G) and the powder feed flow (Fr):

4a.- Method for optimizing the roughness of a rolling cylinder by means of high-speed thermal spraying, according to claim 1a, characterized in that the spraying powder contains hard particles with dimensions less than 1 ^m and in which the final objective roughness ( Ra) depends on the average granulometry of the powder (G) according to the following rule:

5a.- Method for optimizing the roughness of a rolling cylinder by means of high-speed thermal spraying, according to claim 1a, characterized in that the number of peaks (RPc) of the coating surface does not exceed a value related to the roughness (Ra ) according to the following formula:

6a.- Method for optimizing the roughness of a rolling cylinder by means of high-speed thermal spraying, according to claims ia and 3a, characterized in that the number of peaks (RPc) is obtained through an additional surface treatment step that consists of reducing the height and number of peaks by mechanical, thermal, chemical, or electrochemical ablation/removal, for roughnesses less than 2 ^m.