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

Abstract

The invention relates to a method for 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), depositing the material in a helicoidal way. After establishing a granulometry (G) of powder to be sprayed, an objective roughness (Ra) and an objective thickness (t) of the coating, the corresponding feed flow (Fr) of powder is determined in an empirical table which shows the objective roughness (Ra) on the basis of the feed flow (Fr) and the granulometry (G) according to a pre-established formula, after which the 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).

Claims

1. A method for optimising the roughness of a rolling mill roll 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 mill roll, wherein the mill roll rotates at a rotational speed (Vr) about the longitudinal axis thereof and the spraying column moves translationally at a translational speed (Vt), parallel to the axis of the mill roll to deposit the material according to a helical figure, wherein 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: $\mathrm{Ra}=.Math.\left(A\left(G\right).Math.\mathrm{Fr}+\left(G\right)\right)$ where 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 the rotational speed (Vr) and the translational speed (Vt) 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: $t=.Math.\mathrm{Fr}.Math.\frac{N}{\mathrm{Vr}.Math.\mathrm{Vt}.Math.}$ 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.

2. The method for optimising the roughness of a rolling mill roll by means of high-speed thermal spraying according to claim 1, wherein a HVAF thermal spraying method is used, and wherein the empirical table for calculating the desired roughness (Ra) based on the granulometry of the powder (G) and the powder feed flow (Fr) being: TABLE-US-00006 Average granulometry of the Powder (m) 10 15 20 25 30 35 40 Feed 2 0.8 1.4 1.9 2.2 2.5 2.7 2.7 flow 4 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

3. The method for optimising the roughness of a rolling mill roll by means of high-speed thermal spraying according to claim 1, wherein a HVOF thermal spraying method is used, and wherein the empirical table for calculating the desired roughness (Ra) based on the granulometry of the powder (G) and the powder feed flow (Fr) being: TABLE-US-00007 Average granulometry of the Powder (m) 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

4. The method for optimising the roughness of a rolling mill roll by means of high-speed thermal spraying according to claim 1, wherein the spray powder contains hard particles with dimensions less than 1 m and wherein the objective final roughness (Ra) depends on the average granulometry of the powder (G) according to the following rule: TABLE-US-00008 Roughness-Ra Average granulometry of (m) the Powder-G (m) Ra 1 m G < 20 m 1 m < Ra 4 m G < 30 m 4 m < Ra G < 50 m.

5. The method for optimising the roughness of a rolling mill roll by means of high-speed thermal spraying according to claim 1, wherein the number of peaks (RPc) of the coating surface does not exceed a value related to the roughness (Ra) according to the following formula: $\mathrm{RPc}88+47.Math.\mathrm{Ln}\left(\mathrm{Ra}\right)$

6. The method for optimising the roughness of a rolling mill roll by means of high-speed thermal spraying according to claim 1, wherein the number of peaks (RPc) is obtained by an additional surface treatment step consisting of reducing the height and number of peaks by mechanical, thermal, chemical or electrochemical ablation/elimination, for roughness less than 2 m.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0053] 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:

[0054] FIG. 1 shows a diagram of the different texturing technologies that currently exist, wherein: [0055] (1) Shot blasting (SBT). Stochastic coating. [0056] (2-2) Electro-discharge texturing (EDT). Stochastic coating. [0057] (3) Laser Texturing (LT). Deterministic coating. [0058] (4) Electron beam texturing (EBT). Deterministic coating. [0059] (5) Chrome deposition texturing (TST). Nodular coating. [0060] (6) HVAF thermal spray coating. Stochastic coating.

[0061] 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.

[0062] 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.

[0063] 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).

[0064] 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.

[0065] 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.

[0066] 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

[0067] According to method of the invention, the following has been provided for the management of roughness: [0068] 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:

[00003]$\begin{array}{cc}t=.Math.\mathrm{Fr}.Math.\frac{N}{\mathrm{Vr}.Math.\mathrm{Vt}.Math.}& \mathrm{Equation}-1\end{array}$ [0069] Being: t=thickness of the coating [0070] N=revolutions per minute of the mill roll [0071] =Efficiency of the process depending on the type of projection equipment [0072] Fr=Powder feed flow [0073] Vt=Transverse speed of the gun [0074] =Density of the powder [0075] Vr=Tangential speed of the mill roll [0076] 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). [0077] The roughness depends on the feed flow and granulometry of the spray powder, according to the simplified empirical formula described below:

[00004]$\begin{array}{cc}\mathrm{Ra}=.Math.\left(A\left(G\right).Math.\mathrm{Fr}+B\left(G\right)\right)& \mathrm{Equation}-2\end{array}$ [0078] Where: =Efficiency of the process [0079] Fr=Powder feed flow [0080] 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-US-00002 TABLE 2 Ra (m) as a function of Feed Flow and Granulometry of the powder-HVAF process. Average granulometry of the Powder (m) 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-US-00003 TABLE 3 Ra (m) as a function of Feed Flow and Granulometry of the powder-HVOF process. Average granulometry of the Powder (m) 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

[0081] 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.

[0082] Regarding the steps to manage roughness, they are the following: [0083] a) Defining the Granulometry of the powder. [0084] b) Knowing the granulometry of the powder and the objective roughness, the powder feed flow is defined as per Table-2 or Table-3. [0085] 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-US-00004 TABLE 4 Comparison between thermal spraying roughness and standard stochastic roughness. Thermal SBT EDT Spraying Topography Stochastic Stochastic Stochastic Ra, (m) 1.5-6 0.5-10 0.5-10 RPc, (cm.sup.1) <70 50-150 20-120 Roughness transfer Low Average Average capacity Duration of the Very low High Very high roughness layer

[0086] Regarding the management of the granulometry of the powder and the size of the hard particles thereof: [0087] The steps necessary to manage roughness for a fixed size of the spray powder were explained above. [0088] To access all requested roughness levels, the size of the powder must adapt, in accordance with Table-5 to address different roughness ranges.

TABLE-US-00005 TABLE 5 Required granulometry of the powder Roughness-Ra Average granulometry (m) of the Powder-G (m) Ra 1 m G < 20 m 1 m Ra 4 m G < 30 m 4 m Ra G < 50 m [0089] 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 m so that the roughness obtained is between 0.3 and 3 m. For example, the hard particle size between 1 and 5 m to obtain a roughness of around 0.3 m. [0090] 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 m.

[0091] In turn, and in regards to managing the number of peaks of the roughness, the following is worth highlighting: [0092] 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. [0093] 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. [0094] 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:

[00005]$\begin{array}{cc}\mathrm{RPc}88+47.Math.\mathrm{Ln}\left(\mathrm{Ra}\right)& \mathrm{Equation}-3\end{array}$ [0095] 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 m. [0096] FIG. 6. shows that high-speed thermal spray coatings fulfil Equation-3, for roughness greater than 1.5-2 m. For a lower roughness it is necessary to add a subsequent treatment operation of the coated surface.

[0097] 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 m, it is necessary to pre-treat the mill roll by shot blasting.

[0098] The references used in this application are the following: [0099] [1] WORK ROLL ROUGHNESS TOPOGRAPHY AND STRIP CLEANLINESS DURING COLD ROLLING AUTOMOTIVE SHEETClaude Gaspard, Daniel Cavalier, Stefan WahlundTechnical contribution to the 11th International Rolling Conference, part of the ABM Week 2019 October 1st-3rd, 2019, So Paulo, SP, Brazil. [0100] [2] RELATIONS BETWEEN FRICTION COEFFICIENT AND ROLL SURFACE PROFILES, ROLLED SHEET CHARACTERISTICS IN COLD ROLLING OF STEEL SHEETSHiroyasu YAMAMOTO, Mansaku SASAKI and Takahiro KITAMURATetsu-to-Hagan Vol. 95 (2009) No. 5. [0101] [3] THE RESEARCH ON EDGE CARCK OF COLD ROLLED THIN STRIPHaibo Xie2011Thesis of university of Wollongong. [0102] [4] TEXTURING METHODS FOR COLD MILL WORK ROLLSBilal OLAK*, Fatih BAOLU+, Naci KURGANUDCS19 Fourth International Iron and Steel Symposium, 4-6 April, Karabuk. [0103] [5] EFFECT OF WORK ROLL TECHNOLOGY ON COLD MATERIALS ROLLING AND PROGRESS OF MANUFACTURING FUTURE DEVELOPMENTS IN JAPANMitsuo HASHIMOTO, Taku TANAKA, Tsuyoshi INOUE, 1) Masayuki YAMASHITA, 2) Ryurou KURAHASHI3) and Ryozi TERAKADO4)ISIJ International, Vol. 42 (2002), No. 9, pp. 982-989. [0104] [6] Patent WO 2021148690. [0105] [7] Patent JP 09300008.