Method for manufacturing hot rolling mill rolls by laser cladding

Abstract

This invention relates to a method for manufacturing hot rolling mill rolls by laser cladding a reusable steel shaft substrate with a rotationally symmetric axis with a metal coating outer layer having a tool steel composition, wherein the composition of the metal coating outer layer includes 0.5%-3.5% C, 2%-18% Cr, 0.5%-7% Mo, 0.5%-8% V, 0.2%-7% W, 0%-5% Nb, 0%-1% Ti, 0.5%-2% Mn, 0.2%-3% Si, and 0%- 3% Ni, the remainder being Fe and unavoidable impurities; characterized in that: - the composition of the outer layer of the metal coating further includes nitrogen in the range of 200-2500 ppm; - the total atomic content (mass%) of the MC carbide-forming elements selected from the group consisting of Ti, Nb and V + 3 / 8 of the total atomic content (mass%) of the M23C6 and / or M2C-forming elements selected from the group consisting of Mo, W and Cr is lower than the total atomic content (mass%) of the interstitial elements C and N + 0.01.

Description

Technical Field

[0001] This invention relates to the field of manufacturing work rolls intended for use in hot rolling mills, and preferably for use in strip rolling mills. More specifically, this invention relates to work rolls obtained by laser cladding methods.

[0002] This application relates to an improvement of patent EP 3 006 124 B1, which is incorporated herein by reference. Background Technology

[0003] To meet their customers' high productivity and surface quality requirements, manufacturers of rolling mill work rolls have been forced to explore different materials subject to increasingly demanding specifications. Under normal hot operating conditions, the work roll table must withstand wear and thermal fatigue caused by the cyclical rapid cooling and reheating of the roll per revolution. Such rolls, which require very tough and wear-resistant surfaces while having a ductile core, can be manufactured using a variety of techniques, including rotary or centrifugal casting (the conventional method), laser cladding, or powder metallurgy by establishing the shell on a steel core through hot isostatic pressing.

[0004] Laser cladding is a surface finishing technique that involves depositing materials with different properties onto a metal substrate using a laser beam. The cladding material, in the form of a metal wire or powder delivered by an inert gas, is injected laterally or coaxially into a molten pool with the laser beam. It is melted and solidified using the laser to cover a portion of the substrate. It is often used to improve mechanical properties, providing higher wear resistance, heat resistance, or higher hardness, or increasing corrosion resistance.

[0005] Work rolls for hot strip mills (HSMs) are typically manufactured using a spin casting method followed by heat treatment. The rolls consist of a cast or forged iron or steel core and a high-alloy steel outer shell, with a composition including high carbon, Mn, Si, and carbide-forming elements such as W, Mo, V, Cr, and Co.

[0006] Document EP 0 070 773 A1 discloses a method for laser cladding HSS powder (i.e., high-speed steel powder) onto rolls made of low-carbon steel. The HSS powder has the following general composition (by mass): 0.5%-2.6% C, 0.2%-1.7% Mn, 0.2%-1.4% Si, ≤0.2% S, 2%-14% Cr, ≤12% Mo, ≤20% W, V: ≤10, ≤16% Co, wherein the sum of W, V, Mo, and Co is ≥3. Additional types of HSS with typical elemental ranges are disclosed in this document, such as Cr-W steel, Cr-Mo steel, and Cr-W-Mo steel. Typical coating thickness is, for example, about 15 mm. Following cladding is a tempering treatment to precipitate carbides (softening the matrix and relieving stress).

[0007] In addition, the literature Scandella F.; “Développement d'un acier rapide pour le revêtementde cylindres de laminage à chaud [Development of high-speed steel for hot roll coating]”, Soudageet Technique Connexes [Welding and related technologies], Mars-Avril 2010, pp. 35-46, and EP 0 533929 A1 also relate to the development of high-speed steel as a coating material for rolls (hot work rolls) and composite rolls for rolling.

[0008] Document EP 3 006 124 B1 discloses a method for manufacturing mill rolls by laser cladding a reusable steel shaft substrate with a metal coating having a working tool steel composition onto a reusable steel shaft substrate having a rotationally symmetric axis. The method includes the following steps:

[0009] - Rotate the reusable substrate around its axis of rotational symmetry;

[0010] - Laser cladding is performed on a rotating substrate by forming a molten pool on the surface of the rotating substrate with a laser beam and fixing the coating by feeding powder material into the laser-induced molten pool;

[0011] - The coated substrate is subjected to heat treatment consisting of tempering, which involves heating to a temperature in the range of 500°C to 650°C and then holding at this temperature for a period of time between 2 and 5 hours in order to soften the martensite and precipitate carbides.

[0012] The outer layer of the metal coating consists of the following components: 0.5%-3.5% C, 2%-18% Cr, 0.5%-7% Mo, 0.5%-8% V, 0.2%-5% W, 0%-5% Nb, 0%-1% Ti, 0.5%-1% Mn, 0.2%-3% Si, and 0%-3% Ni, with the remainder being Fe and unavoidable impurities.

[0013] The substrate is preheated using a coating head that combines induction heating and laser cladding processes;

[0014] The cladding rate ranges from 2.35 kg / h to 18 kg / h; and

[0015] The outer coating consists of multiple additional coating sublayers and has a total thickness between 1 and 30 mm, with the thickness of each individual outer coating sublayer between 0.1 and 2.5 mm.

[0016] The roll coating composition disclosed in EP 3 006 124 B1 is similar to the general alloy composition of Scandella F. (ibid.) with the range of elements C, Si, Mn, Cr, Mo, Nb and V, and there is significant overlap in the range of W and Ti, and it is similar to the composition disclosed in several examples of EP 0 533 929 A1.

[0017] However, the subject of EP 3 006 124 B1 differs from the methods known in the three previous documents in that it proposes induction-assisted laser cladding, that is, using a coating head that combines induction heating and laser cladding processes. This allows for preheating of the substrate, thereby providing a more efficient laser cladding process with higher deposition rates and reduced tempering.

[0018] The literature Brückner F. et al., “Innovations in laser cladding and direct metal deposition”, High Power Laser Materials Processing: Lasers, Beam Delivery, Diagnostics, and Applications, SPIE, 100020th St. Bellingham WA, USA (2012), Vol. 8239, No. 1, pp. 1-6, describes induction-assisted laser cladding with high deposition rates up to 18 kg / h on large cylindrical parts. However, this literature does not specifically mention mill rolls, and only mentions Co-based (Stellite 20) and Ni-based (INCONEL 625) cladding alloys in the case of induction-assisted laser cladding. Common HSS steels, or even cladding alloys of EP 3 006 124 A1, are not disclosed in this literature.

[0019] Purpose of the invention

[0020] The present invention aims to provide a work roll intended for use in a hot rolling mill, having a coating consisting of a cladding layer with very high robustness (in terms of cracking, porosity and oxidation).

[0021] The present invention is particularly intended to provide a method for manufacturing mill rolls in which cladding rate and coating quality (i.e., in terms of microstructure and crack size) are simultaneously improved.

[0022] The present invention also aims to provide a long-duration cladding method for work rolls, and improved mechanical properties of laser cladding layers (e.g., resistance to thermal fatigue and resistance to surface degradation). Summary of the Invention

[0023] This invention relates to a method for manufacturing hot rolling mill rolls by laser cladding a reusable steel shaft substrate with a metal coating having a rotational symmetry axis, the metal coating having a tool steel composition, the method comprising the following steps:

[0024] - Rotate the reusable substrate about its axis of rotational symmetry;

[0025] - Laser cladding is performed on the rotating substrate by forming a molten pool on the surface of the rotating substrate with a laser beam and fixing the coating by feeding powder material into the laser-induced molten pool;

[0026] - subject the coated substrate to heat treatment after cladding;

[0027] The outer layer of the metal coating comprises 0.5%-3.5% C, 2%-18% Cr, 0.5%-7% Mo, 0.5%-8% V, 0.2%-7% W, 0%-5% Nb, 0%-1% Ti, 0.5%-2% Mn, 0.2%-3% Si and 0%-3% Ni, with the remainder being Fe and unavoidable impurities;

[0028] The substrate is preheated using a coating head that combines induction heating and laser cladding processes;

[0029] The cladding rate ranges from 2.35 kg / h to 18 kg / h; and

[0030] The external coating is composed of a plurality of additional coating sublayers and has a total thickness between 1 and 30 mm, wherein the thickness of each individual external coating sublayer is between 0.1 and 2.5 mm, characterized in that the method further comprises:

[0031] - The composition of the outer layer of the metal coating is further selected to include nitrogen in the range of 200-2500 ppm to improve wear resistance, wherein the total atomic content (mass%) of the MC carbide-forming elements selected from the group consisting of Ti, Nb and V is +3 / 8 of the M selected from the group consisting of Mo, W and Cr. 23 The total atomic content (mass%) of C6 and / or M2C forming elements is less than the total atomic content (mass%) of interstitial elements C and N by 0.01, so as to achieve uniform hardness in the case of multilayer coatings;

[0032] - The powder used for cladding is dried or heated, and the ambient humidity is strictly controlled and the cladding process is carried out in a controlled protective atmosphere in order to obtain a final hydrogen content of less than 1 ppm in the outer layer of the metal coating, thereby allowing for the suppression of cracks.

[0033] According to a preferred embodiment of the invention, the method is further limited by one or a suitable combination of the following features:

[0034] - The composition of the outer layer of the metal coating includes nitrogen in the range of 200-400 ppm;

[0035] - The ambient dew point during cladding is between -5°C and +15°C;

[0036] -The controlled protective atmosphere consists of N2 or Ar;

[0037] -Laser power settings are from 10 to 80 W / mm 2 Within the range;

[0038] - The method includes a preliminary step of preparing the reusable substrate by cleaning and / or machining the surface of the reusable substrate;

[0039] - The surface roughness of the substrate prior to cladding is between 1 and 8 μm;

[0040] - Apply surface degreasing before cladding to obtain a concentration of less than 1 mg / m³ 2 Surface organic carbon is used to reduce the generation of fumes during high-temperature cladding and further reduce the oxidation of the coating;

[0041] - The composition of the outer layer of the metal coating further includes nitrogen in the range of 200-1500 ppm;

[0042] - The preheating of the substrate is carried out in the range of 20°C to 500°C, and preferably in the range of 200°C to 300°C;

[0043] - The heat treatment after cladding is controlled cooling or tempering, which involves heating to a temperature in the range of 500°C-650°C and then holding at this temperature for a period of time between 2 and 5 hours in order to soften the martensite and precipitate carbides.

[0044] - The composition of this steel shaft includes 0.2%-0.5% C and 0.5%-5% Cr, 0%-1% Mo, 0%-1% Mn and 0%-0.4% Si, with the remainder being Fe and unavoidable impurities;

[0045] The steel shaft is composed of 0.4% C and 1%-2% Cr. Detailed Implementation

[0046] This invention relates to an improved laser cladding method for coating a roll having a steel shaft (or shaft or spindle) substrate with a "layer" of hot or cold working tool steel, the layer being obtained from a series of sublayers. The tool steel used is similar to HSS work roll grade and / or has a higher carbide content.

[0047] The inventors have extensively explored various options to allow for comprehensive industrial commissioning and improvement of the laser cladding manufacturing method for mill rolls described in EP 3 006 124 B1.

[0048] Many parameters influence the robustness of laser cladding layers, characterized by reduced (or absent) cracks, porosity, and oxidation. Porosity should be lower than that of products obtained through spin casting, as measured by methods such as the DGS (Distance Gain Dimension) method as described in ISO 16811:2012(en) (Non-destructive testing - Ultrasonic testing - Sensitivity and range setting) and ISO 5577:2017(en) (Non-destructive testing - Ultrasonic testing - Vocabulary).

[0049] At the interface between the laser cladding layer and the substrate, defects larger than DGS(AVG) 4 mm are not acceptable. Within the laser cladding layer, defects larger than DGS(AVG) 0.5 mm are not acceptable, and a maximum of 5 defects <4 mm are allowed on a 10 cm × 10 cm surface.

[0050] According to some embodiments of the present invention:

[0051] - The hydrogen content of the laser cladding layer is less than 1 ppm;

[0052] - Due to the initial drying or heating of the powder, the hydrogen content of the powder used for cladding is less than 1 ppm;

[0053] - The ambient dew point during cladding is between -5°C and +15°C;

[0054] - Cooling or post-heating after cladding is controlled;

[0055] -The substrate is preheated;

[0056] -Laser power ranges from 10 to 80 W / mm 2 Within the range.

[0057] In particular, the hydrogen content should be kept as low as possible throughout the process. In applications involving steels that are difficult to weld, hydrogen can weaken grain boundaries and lead to cracking, which is to be avoided in this invention.

[0058] Hydrogen may be present in water traces or in the moisture of the environment and cladding powder. Therefore, according to the present invention, the cladding powder is dried or heated, and the ambient humidity is strictly controlled. The cladding process is further carried out under a controlled protective atmosphere (preferably composed of N2 or Ar).

[0059] Furthermore, some parameters should have values ​​that positively influence the duration of cladding. According to some embodiments of the invention:

[0060] - The surface roughness of the substrate is between 0.2 and 8 μm;

[0061] - Apply surface degreasing before cladding to obtain a concentration of less than 1 mg / m³ 2 Surface organic carbon. Proper degreasing is crucial because the fumes generated during the high-temperature cladding process will lead to harmful oxidation of the coating.

[0062] Finally, many parameters particularly affect the mechanical properties of the laser cladding layer. A key feature of this invention is that the composition of the coated cladding layer should conform to a specific relationship, depending on the ratio of carbon (or more generally interstitial elements) to carbide-forming elements.

[0063] According to some embodiments of the present invention:

[0064] - To achieve uniform hardness in multilayer coatings, a specific relationship (i.e., an inequality) between interstitial elements and carbide-forming elements needs to be satisfied. According to the present invention, the atomic content (mass%) of the MC carbide-forming elements (such as Ti, Nb, V, Ta, etc.) + 3 / 8 of M 23 The atomic content (mass%) of C6 and / or M2C forming elements (such as Mo, W, Cr, etc.) should be lower than the atomic content (%) of interstitial elements (such as C, N, B, etc.) by 0.01.

[0065] - Mn content is between 0% and 2% and W content is between 0.2% and 7%;

[0066] - Improved wear resistance is provided by adding nitrogen in the range of 200-2500 ppm, and preferably in the range of 200-400 ppm.

[0067] For a series of eleven coated alloys obtained by this invention, the above relationships between carbide-forming elements and interstitial elements are illustrated in Table 1. For each case, hardness uniformity was checked and compared with whether the inequality was satisfied. If the deviation was less than 50 HV (EN ISO 6507-1 to 6507-4), the hardness was considered uniform.

[0068]

[0069] The laser cladding method used in this invention is characterized as follows:

[0070] - High metallurgical bonding;

[0071] - The benefits of low or no porosity and high cooling rate result in a very fine microstructure; - Uniform composition;

[0072] - Coating thickness: 0.1-2mm per (sub)layer;

[0073] - A thicker coating is obtained by adding multiple layers, with the total outer layer thickness ranging from 1 to 30 mm, and preferably about 20 mm;

[0074] - Using a special head (combining induction heating and laser cladding processes), the cladding rate ranges from 2.35 kg / h to 18 kg / h;

[0075] - Preheat the substrate if necessary;

[0076] - Post-heat treatment (e.g., tempering).

Claims

1. A method for manufacturing a hot-rolling mill roll by laser cladding of a metal coating outer layer on a reusable steel shaft substrate having an axis of rotational symmetry, said metal coating outer layer having a work tool steel composition, the method comprising the steps of: - rotating the reusable substrate around its axis of rotational symmetry; - performing laser cladding on the rotating substrate by forming a molten pool on the surface of the rotating substrate with a laser beam and fixing a coating by feeding a powder material into the laser-induced molten pool; - subjecting the coated substrate to a post-cladding heat treatment; wherein the composition of the metal coating outer layer comprises 0.5-3.5% C, 2-18% Cr, 0.5-7% Mo, 0.5-8% V, 0.2-7% W, 0-5% Nb, 0-1% Ti, 0.5-2% Mn, 0.2-3% Si and 0-3% Ni, the remainder being Fe and unavoidable impurities; wherein the substrate is preheated by means of a coating head combining induction heating and laser cladding processes; wherein the cladding rate is in the range of 2.35 kg / h to 18 kg / h; and wherein the metal coating outer layer is composed of a plurality of additional coating sub-layers and has a total thickness of between 1 and 30 mm, the thickness of each single outer coating sub-layer being between 0.1 and 2.5 mm, characterized in that the method further comprises the steps of: - the composition of the outer layer of the metal coating further comprises nitrogen in the range of 200-2500 ppm in order to improve wear resistance, and wherein the sum of the atomic content (mass %) of MC carbide forming elements selected from the group consisting of Ti, Nb and V + 3 / 8 of M selected from the group consisting of Mo, W and Cr is below 0.5 23 the sum of the atomic content (mass %) of C6 and / or M2C forming elements is below the sum of the atomic content (mass %) of interstitial elements C and N + 0.01 in order to achieve a uniform hardness in case of a multilayer coating; - drying or heating the powder used for cladding, strictly controlling the environmental humidity and performing the cladding process under a controlled protective atmosphere so as to obtain a final hydrogen content in the metal coating outer layer lower than 1 ppm, allowing to suppress cracks.

2. The method of claim 1, wherein, the selected composition of the metal coating outer layer comprises nitrogen in the range of 200-400 ppm.

3. The method of claim 1, wherein, the environmental dew point during cladding is between -5°C and +15°C.

4. The method of claim 1, wherein, the controlled protective atmosphere is composed of N2 or Ar.

5. The method of claim 1, wherein, the laser power is set in the range of 10 to 80 W / mm2.

6. The method of claim 1, wherein, the method comprises a preliminary step of preparing the reusable substrate by cleaning and / or machining the surface of the reusable substrate.

7. The method of claim 6, wherein, the surface roughness of the substrate before cladding is between 1 and 8 pm.

8. The method of claim 6, wherein, a surface degreasing is applied before cladding to obtain a surface organic carbon lower than 1 mg / m2 to reduce the production of fumes during high temperature cladding and further reduce the oxidation of the coating.

9. The method of claim 1, wherein, the composition of the metal coating outer layer further comprises nitrogen in the range of 200-1500 ppm.

10. The method of claim 1, wherein, the preheating of the substrate is performed in the range of 20°C-500°C.

11. The method of claim 1, wherein, the preheating of the substrate is performed in the range of 200°C-300°C.

12. The method of claim 1, wherein, the post-cladding heat treatment is a controlled cooling, or a tempering treatment comprising heating to a temperature in the range of 500°C-650°C and then maintaining at this temperature during a time between 2 and 5 hours so as to soften the martensite and precipitate carbides.

13. The method of claim 1, wherein, the composition of the steel shaft comprises 0.2-0.5% C and 0.5-5% Cr, 0-1% Mo, 0-1% Mn and 0-0.4% Si, the remainder being Fe and unavoidable impurities.

14. The method of claim 13, wherein, The steel shaft has a composition including 0.4% C and 1-2% Cr.

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