METHOD FOR OPTIMISING THE ROUGHNESS OF A ROLLING MILL ROLL BY MEANS OF HIGH-SPEED THERMAL SPRAYING

Description

TECHNICAL SECTOR

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

These work mill 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 work mill rolls is one of the main ways to reduce operating costs for steel manufacturers.

To increase the service life of the work mill rolls, 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 work mill rolls by high-speed thermal spraying. The type of hard particles sprayed and the thickness of the coating greatly affect the service life of the work mill roll.

According to standard work mill rolls, due to limitations of the process and to avoid defects in the strip, the roughness of the mill rolls must be uniform along the length of the mill roll and must meet the requirements in terms of mean roughness and standard deviation.

Due to the fact that high-speed thermal spraying technology has not been developed to manage roughness, it is a challenge to reliably manage both:

• • the thickness of the coating to ensure the service life of the work mill rolls
  • and the target roughness.

Regarding the importance of roughness on the surface of the mill roll, the following is worth highlighting:

• • the roughness of the mill roll affects the coefficient of friction between the strip and the mill rolls during rolling. The coefficient of friction increases as the roughness increases and at the same roughness the coefficient of friction increases as the number of peaks increases [1].
  • If the coefficient of friction is too low, the strip may slip/skid on the mill roll and create defects or limit the thickness reduction in the rolling [2]. On the contrary, if the coefficient of friction is too high, it increases the abrasion of the mill roll on the strip and increases the contamination of the strip with iron fines [1].
  • This is the case of cracks on the edges of the strip, normally seen on reversing reduction mills. In this case, it is well known that cracks on the edges are sensitive to the coefficient of friction: the defect increases with increasing friction [3].
  • The roughness of the strip affects the properties, quality and performance of the strip itself. As in the case of Skin-pass mills, to ensure good behaviour during stamping, the roughness of the strip must be high enough to avoid cracks in the material.
  • The roughness of the strip is transferred from the roughness of the work mill roll. The higher the roughness of the mill rolls, the greater the roughness of the laminated strip. The rolling force and strip tension affect how roughness is transferred. Thus, knowing the roughness required in the strip, the roughness of the work mill rolls is defined.
  • Depending on the conditions of the rolling process and the target roughness of the strip, steel manufacturers define the roughness of the mill 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 standard 4287 with a cut-off of 0.8 mm.

RPC is defined as the number of peaks per unit of length in centimetres. The calculation is made according to the ISO standard 4287 with a strip width of 1 micrometre.

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

Typical values of the texture of work mill rolls are shown in Table-1.

TABLE 1
		Tandem	Tandem	Skin-
	Reversing	Mill 1st	Mill Last	Pass	Temper
	Mills	Box	Box	Mills	Mill
Roughness,	0.2-0.8	0.2-0.8	0.3-3.0	1.0-6.0	1.0-6.0
Ra (μm)

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

• • During the rolling campaign, the roughness of the work mill rolls decreases due to the wear produced by the friction between the strip and the mill roll. The use of coatings, such as chrome, delays this phenomenon. See FIG. 2.
  • The decrease in roughness also depends on the type of texture used on the mill rolls. See FIG. 3.
  • As the initial roughness of the mill rolls is slightly higher than the roughness at the end of the rolling campaign, steel manufacturers regularly monitor the roughness of the mill rolls to prevent them from deviating from the range necessary to ensure quality on the surface of the strip. The mill rolls are changed when the roughness is too low.

In turn, and with regard to texturing technologies, the following is worth highlighting:

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

• • i. Shot blasting (SBT):
    • A steel or cast iron shot, with a specific granulometry, is sprayed onto the surface of the work mill roll. 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, of the speed of the shot, of the hardness of the base material of the mill roll, of the number of passes along the mill roll and the speed of rotation of the mill roll.
  • ii. Electro-discharge texturing (EDT):
    • The voltage applied between the mill roll (cathode) and the copper electrode (anode), separated by a dielectric medium, produces an electrical discharge capable of creating craters on the surface of the mill roll. Roughness is a function of the frequency, of 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 the wear of the mill roll. Currently this texture is the reference for the Skin-pass and Temper mills.
  • iii. Laser Texturing (LT):
    • A laser beam strikes the surface of the mill roll, melting the material and expelling the material out of the crater created through the assistance of a gas (O₂, CO₂ or Ar). The final texture of the mill roll corresponds to a uniform distribution of craters along a helical pitch over the circumference of the work mill roll. The axial distance of the craters is controlled by the speed of the longitudinal movement of the mill roll. In the tangential direction, the distance of the craters is determined both by the speed of the mill roll and by 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 currently.
  • iv. Electron Beam Texturing (EBT):
    • This technology consists of bombarding a beam of electrons on the surface of the mill roll. During a single firing, the lenses focus the beam to preheat the material of the mill roll, they then bombard the surface with a first firing to create a crater and then heat the rim surrounding the crater. This cycle can be performed 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 surface of the mill roll (displacement and rotation). This technology is only occasionally used for Skin-Pass mills, but it is not very commonly used today.

These different technologies are described in FIG. 1.

Regarding the coatings using high speed thermal spraying, the following is worth highlighting:

• • Thermal spraying techniques are coating processes in which molten materials are sprayed onto a surface. A mixture of gases is burnt in a combustion chamber, heating and accelerating a powder to deposit it on a substrate. If the combustion gas is oxygen, thermal spraying is called HVOF. If the combustion gas is air, thermal spraying is called HVAF.
  • To increase the useful life, a rolling mill roll is produced with a coating of tungsten carbide alloys where the coating is usually 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 ranges from 0% to 0.1%.
  • The coating layer has a final hardness comprised between 1000 Hv and 1600 Hv.
  • Patent WO2021148690 describes, in the case of the HVAF technique, the use of tungsten carbide coatings for cold rolling mill rolls.

DESCRIPTION OF THE INVENTION

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

To that end, and more specifically, the method of the invention consists of a method of coating a mill roll by means of thermal spraying of a powder by means of a spraying column to form an isotropic roughness (Ra) on the surface of said mill roll, wherein the mill roll rotates at a speed (Vr) about the longitudinal axis thereof and the spraying column moves translationally at a speed (Vt), parallel to the axis of the mill roll to deposit the material according to a helical figure, such that in said method the following operational phases are established:

• • a) establishing a granulometry (G) of powder to be sprayed,
  • b) establishing an objective roughness (Ra) and an objective thickness (t) of the coating,
  • c) finding the corresponding feed flow (Fr) of powder in an empirical table which shows the objective roughness (Ra) on the basis of the feed flow (Fr) and the granulometry (G) according to the formula:

Ra = η · ( A ⁡ ( G ) · Fr + B ⁡ ( G ) )

• • where η 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) defining rotational speed (Vr) and the translational speed (Vt) are defined from an equation that relates the objective coating thickness (t) as a function of the defined feed flow (Fr), the translational speed (Vt) and the rotational speed (Vr), according to the formula:

t = η · Fr · N Vr · Vt · ρ

• • where N are the revolutions per minute of the mill roll and p is the density of the spraying powder, while the ratio between the width of the spray cone (d) and the pitch per turn (p) of the screw is greater than one. The thermal spraying method may be HVOF or HVAF spraying.

In turn, the spray powder will contain hard particles with dimensions less than 1 μm and in which the objective final roughness (Ra) depends on the average granulometry of the powder (G).

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

BRIEF DESCRIPTION OF THE FIGURES

To complement the description that will be made below and in order to help a better understanding of the features of the invention, according to a preferred practical embodiment thereof, a set of figures is attached as an integral part of said description wherein the following has been depicted with an illustrative and non-limiting character:

FIG. 1 shows a diagram of the different texturing technologies that currently exist, wherein:

• • (1) Shot blasting (SBT). Stochastic coating.
  • (2-2′) Electro-discharge texturing (EDT). Stochastic coating.
  • (3) Laser Texturing (LT). Deterministic coating.
  • (4) Electron beam texturing (EBT). Deterministic coating.
  • (5) Chrome deposition texturing (TST). Nodular coating.
  • (6) HVAF thermal spray coating. Stochastic coating.

FIG. 2 shows a graph relating to the evolution of roughness during the rolling process and how the use of coatings such as chrome plating delays the wear caused by friction between the strip and the mill roll, where the y-axis represents the surface roughness in microns, and the x-axis is the length of the strip in kilometres, and where the bottom curve represents the behaviour of a forged steel mill roll with 5% chromium (uncoated mill roll), while the top curve represents the behaviour of a mill roll with a chrome or silver chrome electrolytic coating.

FIG. 3 shows a graph representing the decrease in roughness as a function of the laminated tons of steel sheet, depending on the type of texture used on the mill rolls, specifically for four different textures.

FIG. 4 shows a schematic representation of the method of the invention, wherein the mill roll (7) rotates at a controlled speed around the longitudinal axis (8) thereof and the spray cone moves translationally, parallel to the axis of the mill roll to deposit the material according to a helical FIG. 10).

FIG. 5 shows a graph representing the evolution of the arithmetic mean deviation of the roughness profile as a function of the number of peaks per unit length in centimetres, wherein the top curve corresponds to the maximum ratio of peaks and the bottom curve to the minimum ratio of peaks.

FIG. 6 shows a graph similar to that of FIG. 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.

FIGS. 6.1 and 6.2. show a profile cut of the coating to measure the number of peaks and the relief thereof made using a tool specifically for this purpose. The y-axis reflects the size of the peaks in microns (both crests and valleys being considered peaks) while the x-axis represents the length in microns of the profile. FIG. 6.1 shows the profile without additional treatment and FIG. 6.2 shows 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 method of the invention, the following has been provided for the management of roughness:

• • 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 the following formula:

t = η · Fr · N Vr · Vt · ρ Equation - 1

• • Being: t=thickness of the coating
    • N=revolutions per minute of the mill roll
    • η=Efficiency of the process depending on the type of projection equipment
    • Fr=Powder feed flow
    • Vt=Transverse speed of the gun
    • =Density of the powder
    • Vr=Tangential speed of the mill roll
  • To ensure uniformity of isotropic roughness and thickness of the coating, it is necessary to optimise the overlap of the spray cone between two turns of rotation of the mill roll. To achieve this, the ratio between the width of the spray cone (d) and the length of the pitch (p) between two tums 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:

Ra = η · ( A ⁡ ( G ) · Fr + B ⁡ ( G ) ) Equation - 2

• • Where: η=Efficiency of the process
    • Fr=Powder feed flow
    • A (G) and B (G) are functions of the granulometry of the powder (G) It is more convenient to use the Tables described below:

TABLE 2
Ra (μm) as a function of Feed Flow and
Granulometry of the powder-HVAF process.

Average granulometry of the Powder (μm)

	10	15	20	25	30	35	40

Feed		0.8	1.4	1.9	2.2	2.5	2.7	2.7
flow		1.1	1.7	2.2	2.5	2.8	3.0	3.0
(kg/h)	6	1.4	2.0	2.5	2.8	3.1	3.3	3.3
	8	1.7	2.3	2.8	3.1	3.4	3.6	3.6
	10	2.0	2.6	3.1	3.4	3.7	3.9	3.9
	12	2.3	2.9	3.4	3.7	4.0	4.2	4.2
	14	2.6	3.2	3.7	4.0	4.3	4.5	4.5
	16	2.9	3.5	4.0	4.3	4.6	4.8	4.8
	18	3.2	3.8	4.3	4.6	4.9	5.1	5.1
	20	3.5	4.1	4.6	4.9	5.2	5.4	5.5
	22	3.8	4.4	4.9	5.2	5.5	5.7	5.8

TABLE 3
Ra (μm) as a function of Feed Flow and
Granulometry of the powder-HVOF process.

Average granulometry of the Powder (μm)

	25	30	35	40	45	50

Feed	8	3.7	4.1	4.3	4.4	4.4	4.4
flow	10	4.1	4.4	4.6	4.8	4.8	4.8
(kg/h)	12	4.5	4.8	5.0	5.1	5.1	5.1
	14	4.8	5.1	5.4	5.5	5.5	5.5
	16	5.2	5.5	5.7	5.8	5.8	5.8
	18	5.5	5.9	6.1	6.2	6.2	6.2
	20	5.9	6.2	6.4	6.5	6.6	6.6
	22	6.3	6.6	6.8	6.9	6.9	7.0

For clarification, the powder contains fine, hard particles (such as 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 powder may contain more than one hard particle.

Regarding the steps to manage roughness, they are the following:

• • a) Defining the Granulometry of the powder.
  • b) Knowing the granulometry of the powder and the objective roughness, the powder feed flow is defined as per Table-2 or Table-3.
  • c) Knowing the powder feed flow and objective thickness, the values of Vr and Vt are defined taking into account Equation-1 and respecting d/p>1. Table-4 describes the thermal spraying according to our invention compared to the standard roughness.

TABLE 4
Comparison between thermal spraying roughness and standard
stochastic roughness.

			Thermal
	SBT	EDT	Spraying
Topography	Stochastic	Stochastic	Stochastic
Ra, (μm)	1.5-6	0.5-10	0.5-10
RPc, (cm−1)	<70	50-150	20-120
Roughness transfer	Low	Average	Average
capacity
Duration of the	Very low	High	Very high
roughness layer

Regarding the management of the granulometry of the powder and the size of the hard particles thereof:

• • The steps necessary to manage roughness for a fixed size of the spray powder were explained above.
  • To access all requested roughness levels, the size of the powder must adapt, in accordance with Table-5 to address different roughness ranges.

TABLE 5
Required granulometry of the powder

	Roughness-Ra	Average granulometry
	(μm)	of the Powder-G (μm)
	Ra ≤ 1 μm	G < 20 μm
	1 μm ≤ Ra ≤ 4 μm	G < 30 μm
	4 μm ≤ Ra	G < 50 μm

• • It is known that the size of the hard particles can affect the final roughness of the high-speed spray coating. Such as, for example, patent JP09300008 advises adapting the hard particle size between 1 and 20 μm so that the roughness obtained is between 0.3 and 3 μm. For example, the hard particle size between 1 and 5 μm to obtain a roughness of around 0.3 μm.
  • As the useful life of the mill rolls increases, the duration of the rolling campaign increases. If the size of the hard particles is too large, the roughness of the mill roll again increases as the rolling progresses and this is due to the “wear” of the metal binder. To avoid this phenomenon, the size of hard particles should be less than 1 μm.

In turn, and in regards to managing the number of peaks of the roughness, the following is worth highlighting:

• • For uncoated mill rolls, or chrome-plated mill rolls, used in tandem or reversing rolling mills, as previously mentioned, steel manufacturers used to set the roughness to ensure the quality of the strip (no contamination, cracks on the edges, etc. . . . ) but no specific request is made for the number of peaks.
  • HVAF or HVOF coating containing hard particles, considerably increases the service life of the mill rolls. This means a large increase in the duration of the rolling campaign. For the standard duration of the rolling campaign (uncoated mill rolls or chrome-plated mill rolls), the management of roughness is sufficient to avoid defects in the laminated strip. In case of coatings with a 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 peaks and/or round the peaks.
  • To be able to increase the service life of the rolling campaign by 1.5 times compared to chrome-plated mill rolls or 2 times compared to uncoated mill rolls without any quality problems (cracks at the edges, contamination, etc.), the maximum number of peaks (RPc) must follow the formula:

RPc ≤ 88 + 47 · Ln ⁡ ( Ra ) Equation - 3

• • In the case of coatings using high speed thermal spraying, the number of peaks evolves with the roughness as shown in FIG. 5. This evolution is typical of the roughness created by high-speed thermal spraying and for granulometries of the spray powder below 50 μm.
  • FIG. 6. shows that high-speed thermal spray coatings fulfil 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, etc.), chemical, electrochemical or thermal (laser, etc.). The peaks of the roughness are eroded by means of these treatments. 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 mill roll by shot blasting.

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, São 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 ÇOLAK*, Fatih BAŞOĞLU+, 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 TERAKADO4)—ISIJ International, Vol. 42 (2002), No. 9, pp. 982-989.
  • [6] Patent WO 2021148690.
  • [7] Patent JP 09300008.