High-cobalt martensitic stainless steel submerged-arc welding flux-cored wire, BD roller and preparation method thereof

Abstract

The invention discloses a high-cobalt martensitic stainless steel submerged-arc welding flux-cored wire, a BD roller and a preparation method thereof, and relates to the technical field of welding materials and surfacing. The flux-cored wire comprises, by mass, 50%-80% of a welding wire skin and 20%-50% of a flux core. The flux core comprises the following components: graphite, JCr99-A, cobalt powder, metal nickel, FeMo55-A, tungsten powder, FeV75-A, FNb-3, FTD1 or FTD2, high aluminum, FeB22C0.1, metal titanium, CaF2, rutile and the balance of atomized iron powder. The invention further discloses the BD roller and a preparation method. According to the flux-cored wire, the red hardness and the wear resistance of the roller can be remarkably improved, the stability of the high-temperature strength, the high-temperature hardness and the cold and hot fatigue resistance of the roller can be kept and guaranteed, and the flux-cored wire has higher heat resistance and is more suitable for surfacing of the hot-rolled pass roller.

Description

Technical Field

[0001] This invention relates to the field of welding materials and surfacing technology, specifically to a high-cobalt martensitic stainless steel submerged arc welding flux-cored wire and BD roll and their preparation method. Background Technology

[0002] Steel mills use two rolling mills, BD1 and BD2, for rolling various types of steel rails, H-beams, I-beams, and Z-beams. Both are two-roll reversible arch mills, using BD1 rolls (made of 60CrMnMo forged steel or 60CrNiMo cast steel) and BD2 rolls (made of alloy cast iron). The roll surfaces have 3-6 grooves to meet the rolling requirements of different products, with the most stringent quality requirements for 100-meter heavy rails. The billets used for rolling are continuously cast steel billets, heated to approximately 1150℃ before being rolled. Rolling involves significant water cooling, making the operating conditions extremely harsh. Under normal circumstances, a set of new BD rolls can be used 6-7 times from its maximum size to its minimum size (i.e., scrap size). After each use, the rolls are re-rolled according to the groove dimensions, resulting in a huge workload for preparing spare rolls and significant capital and cost. During rolling, the billet first passes through the various passes of the BD1 rolls in sequence to perform initial processing, forming the "blank" of the desired product. Then, the "blank" passes through the various passes of the BD2 rolls in sequence for further processing, forming the "prototype" of the rolled product for subsequent processes. It is evident that the rolling conditions of the BD2 rolls are more demanding than those of the BD1 rolls in transforming the billet into the product's "prototype." Firstly, the pass patterns are more complex, the deformation is greater, and the asymmetry of the pass patterns is stronger. Secondly, the temperature of the workpiece decreases, requiring greater rolling force, leading to faster failure and scrapping, and greater consumption. Thirdly, the length of the workpiece passing through the BD2 rolls is longer than that passing through the BD1 rolls, and the time spent through each pass of the BD2 rolls is also longer, meaning the continuous rolling force and high temperature exposure last for a longer period. The main failure modes of BD rolls include "steel sticking," roll surface cracking, severe uneven wear, and the formation of various sizes and shapes of nodules due to plastic deformation at the steel passage. When the rolling quality requirements are not met within a rolling cycle, manual grinding is required; otherwise, the rolls are removed from the mill. These factors lead to low single-run rolling capacity, frequent roll replacements, high roll consumption, and high cost per ton of steel. To extend the service life or rolling capacity of this type of roll, and with advancements in materials, equipment, and processes, some domestic units have achieved satisfactory results by using submerged arc welding to strengthen or remanufacture BD1 rolls with Cr5% medium- or high-carbon high-alloy steel or Cr5% martensitic steel. On BD1 rolls used for rolling 38kg / m, 60kg / m, and 75kg / m heavy rails, the welded rolls (hereinafter referred to as welded rolls) achieve a rolling capacity of three times or more per run than the original rolls. This technology offers significant advantages such as reduced cost per ton of steel and high overall cost-effectiveness. To address the problem of BD2 alloy cast iron rolls being unable to be repaired by welding, the material of the BD2 rolls was changed to the same material as the BD1 rolls. However, this welding strengthening method did not meet expectations during use; the "steel sticking" problem of the BD2 rolls was more severe than that of the BD1 rolls.The main problems with all BD rolls are as follows: First, "steel sticking" is a common problem in the rolling process. Once it occurs, it will scratch the surface of the rolled piece to varying degrees. Excessive steel sticking will severely scratch the surface of the rolled piece. Even after subsequent rolling and finishing rolling, the surface of the rolled piece will still have defects such as "rolling scars" and "lines", and the product will not meet the quality requirements, resulting in a large number of scraps. Second, the wear of each pass and each working surface of the pass is extremely uneven. Some parts are severely worn, causing the pass size and the size of the rolled piece to change accordingly, failing to meet the requirements of the finished product size. Third, the thermal resistance of the weld metal of medium and high carbon high alloy steel or martensitic steel containing 5% Cr is insufficient, which does not meet the high temperature strength and high temperature hardness required by BD2 rolls during rolling. This high temperature causes the hardened structure of the weld metal to temper or the stable structure to soften, or the hardness to change due to phase transformation. Fourth, the weld metal is medium-carbon high-alloy steel with a high coefficient of friction. The heat generated by friction raises the temperature of the steel passage, causing microscopic welding between the metal at the steel passage and the surface of the rolled piece. That is, under the high temperature and higher rolling pressure of the BD2 rolls, uncontrollable adhesion or welding occurs between the roll surface and the surface of the rolled piece (steel billet), exacerbating the steel sticking phenomenon. To address this problem, the inventors and their team proposed a new alloy system and the optimal configuration of each alloying element to produce a new flux-cored welding wire for submerged arc welding. This wire can be used simultaneously for weld reinforcement or cyclic preparation of BD1 and BD2 rolls for rolling heavy rails and BD rolls for other rolled products, eliminating problems such as "steel sticking," uneven wear, and abnormal failures during use, thereby reducing roll consumption, controlling costs, and enhancing competitiveness. Summary of the Invention

[0003] To solve the above-mentioned technical problems, the purpose of this invention is to provide a high-cobalt martensitic stainless steel submerged arc welding flux-cored wire and BD rolls and their preparation method. The flux-cored wire of this invention can significantly improve the red hardness and wear resistance of the rolls, and maintain and guarantee the stability of the rolls' high-temperature strength, high-temperature hardness, and resistance to thermal fatigue. It has higher thermal resistance and is more suitable for the surfacing of hot-rolled rolls.

[0004] The technical solution of the present invention to solve the above-mentioned technical problems is as follows: a high-cobalt martensitic stainless steel submerged arc welding flux-cored wire is provided, comprising a wire sheath of 50-80% by mass and a flux core of 20-50% by mass; The flux-cored wire comprises the following components by weight percentage: graphite 0.2-0.5%, JCr99-A 1.2-3.0%, cobalt powder 16.4-41.0%, metallic nickel 6.4-16%, FeMo55-A 10.9-27.3%, tungsten powder 0.32-0.8%, FeV75-A 0.06-0.16%, FNb-3 0.02-0.05%, FTD1 or FTD2 0.1-0.25%, high alumina 1.2-3.0%, FeB22C 0.1% 0.04-0.1%, metallic titanium 0.01-0.03%, CaF2 0.4-1.0%, and rutile 0.25-0.63%, with the balance being atomized iron powder.

[0005] The beneficial effects of adopting the technical solution of this invention are: (1) The welding wire of this invention, after surfacing, produces a low-carbon, high-cobalt martensitic stainless steel with a C content of less than 0.30%, a Cr content in the range of 12-18%, and a hardness in the range of HRC45-58. It is mainly used in applications requiring high hardness and strength. Its main characteristics are: good crack resistance, high hardness, certain wear resistance, and easy machining; good ductility, able to withstand moderate impact; small coefficient of linear expansion, and less tendency to crack and deform. Martensitic stainless steel itself has good medium-temperature strength of 300-600℃, and also has high thermal fatigue performance, that is, the ability to resist surface cracking in the working environment of alternating hot and cold temperatures; at the same time, the addition of a certain amount of cobalt to martensitic stainless steel helps it to maintain good mechanical properties and dimensional stability at high temperatures, and high hardness and excellent wear resistance can be obtained through heat treatment.

[0006] (2) The chromium powder and graphite used provide sufficient chromium and carbon elements to the weld metal. The weld metal contains more than 12% Cr and is a high-chromium martensitic steel with high heat resistance, high thermal strength, good corrosion resistance, high wear resistance and certain impact resistance. Inert and strong oxide film can easily form on the surface of chromium alloy steel with more than 10% Cr, which can prevent the phenomenon of oxide scale adhesion. Its high oxidation resistance is mainly attributed to the high chromium content in the alloy, which is why chromium forms a dense oxide film on the surface. Cr is the core element of martensitic stainless steel and mainly plays a role in improving the hardenability of steel, thereby affecting the depth of the hardened layer of the roll body. C (graphite) and C diluted into the weld by the roll matrix form medium and high strength carbide hard phases with Cr and elements such as Mo, V, Nb, Ti, and W in the flux core component, ensuring the hardness and strength of the working layer metal. In this invention, graphite can only be added to prevent the dislocation density of the weld metal from decreasing due to coarse structure; to prevent the precipitation of M 26Harmful carbides of types C6 and M6C cannot maintain the high-temperature strength and hardness of the weld metal when in contact with rolled parts at 1050~1100℃, thus reducing wear resistance. Meanwhile, the addition of graphite enables the weld metal to form a graphitized structure, improving lubrication, reducing friction at the contact surfaces during rolling, and decreasing wear.

[0007] (3) Molybdenum is the main element in the formation of high-strength stainless steel, which improves the hot strength of steel, strengthens grain boundaries, improves the durability and plasticity of steel, and has a favorable effect on weldability. In dispersion-strengthened steel, molybdenum has the strongest effect as a dispersion strengthener. During surfacing, it has a stronger ability to form carbides than Cr, and has precipitation strengthening and solid solution strengthening effects. It has good high-temperature hardness and high-temperature strength retention, increases the red hardness of the surfacing metal, eliminates brittleness, and significantly improves its impact toughness and tempering resistance. Mo forms Fe2Mo intermetallic compound with Fe, which improves yield strength. Mo also plays the role of Mo passive surface film. The passive surface film is flat and smooth, thereby reducing the friction between the die and the workpiece, improving the anti-scraping ability of the die through the steel surface, and preventing steel sticking.

[0008] Mo and W are elements in the same group, and W is similar to Mo in many ways; W is a strong carbide-forming element, with dispersion strengthening and grain refinement effects. The two can be substituted for each other, which can reduce the amount of alloying elements added and reduce the uneven distribution of carbides in the matrix structure.

[0009] (4) Ni and Co work together to obtain a high-strength and high-toughness martensitic structure in the weld metal, maintain the high density of dislocations in the martensitic laths, and improve toughness and strength; adding a certain amount of Ni to the welding wire can improve the thermal sensitivity of the weld metal.

[0010] (5) The amount of cobalt (Co) added to martensitic stainless steel is small, generally less than 1.00%, and sometimes it is not added at all. This invention requires that Co in the weld metal be no less than 8%, and the specific amount should be determined according to the application target. By increasing the Co content in the welding wire, cobalt reacts synergistically with Mo, W, Ti, Ni, and Fe, which is conducive to the formation of appropriate amounts of alloys such as Co-Mo-Cr, Co-Cr-W, and Ni-Co-Mo-Cr, as well as intermetallic compounds such as Ni3Mo and Fe2Mo. These alloys have high hardness and good anti-adhesion properties, and are mainly used to manufacture components that work at 700~900℃. They can meet the actual situation that the roll surface temperature of BD rolls is around 800℃ during rolling, and can maintain a high hardness of HRC44 or above. These alloys will not cause tempering of the hardened structure of the weld metal or softening of the stable structure due to high temperature, or change in hardness and brittleness due to phase transformation. This high hardness characteristic mainly comes from the solid solution strengthening effect of cobalt and strengthening elements such as chromium and tungsten, which can ensure the high hardness and impact toughness required by the roll during rolling, giving the roll a good wear resistance and resisting the impact load during rolling. At the same time, its good anti-adhesion performance indicates that the weld metal has a strong anti-galling ability and a strong anti-scratch ability, thus achieving non-stick steel during use.

[0011] (6) The addition of other trace Nb, V, Ti, Al, B and Cu alloys can better match the trace alloys already contained in each component, promote dispersion strengthening of the weld metal, improve the hot strength of the steel, and have a significant comprehensive effect. These alloying elements are beneficial to the weld metal becoming a high-strength martensitic heat-resistant stainless steel with trace and diversified alloys during welding.

[0012] Nitrogen (Nb) has a stronger carbon-extraction ability than titanium (V), allowing more V to dissolve in the matrix and play a role in precipitation strengthening. Both V and Ti carbides help reduce segregation and form refractory carbides, refining grains and improving strength and toughness. Adding trace amounts of Ti to the welding wire can reduce droplet size and suppress porosity formation, contributing to high-quality welds.

[0013] Al is one of the main strengthening elements, which can improve strength by aging and can also be used as a deoxidizer to control the oxygen content of the weld. Under the synergistic effect of Co, Al coherently precipitates β-NiAl phase with Ni, which greatly improves the hot strength of the weld metal and enables the roll to maintain high strength during steel rolling. In addition, Al can easily form a dense and well-adhesive oxide film at high temperature, which meets the requirements of the roll in the steel passing part during steel rolling and improves the alloy's resistance to oxidation, carburization and Cl oxidation.

[0014] Trace amounts of boron (B) can purify and strengthen grain boundaries, improving the overall properties of weld metals. This effect is more significant than adding elements such as Zr, Hf, Mg, La, and Ce in enhancing the hot strength of weld metals. Because B segregates at grain boundaries, it reduces grain boundary defects, increases grain boundary strength, and strongly alters grain boundary shape. This influences the precipitation and growth of grain boundary carbides and intermetallic compounds, improving their dense and uneven distribution and forming a spherical, uniform distribution. It also prevents the precipitation of lamellar and cellular phases at grain boundaries, thus improving the alloy's creep rupture life.

[0015] The addition of trace amounts of Cu to martensitic stainless steel mainly serves as precipitation hardening, increasing strength and providing good processing properties such as surface smoothness and dimensional stability.

[0016] The high-cobalt martensitic stainless steel flux core composition of this invention also includes a balance of Fe, which is the main matrix element, ensuring that the sum of the contents of all components reaches 100%. Fe can form a martensitic matrix together with elements such as Ni, Co, Cr, and C, and can also form Laves phases with elements such as Mo and W to enhance the strength of the stainless steel alloy through precipitation strengthening. It is worth noting that this invention can produce welding wire YD-506 with a fixed composition for the cyclic preparation of BD1 and BD2 rolls; alternatively, while maintaining the Cr content and related trace alloys, the C, Ni, Co, and Mo alloy contents can be appropriately adjusted to produce welding wires YD506-1, YD506-2, and YD506-3, with a hardness within the HRC range of 49 to 54. Welding wires with higher hardness are used for the highest rolling pressure pass on the same set of rolls, those with medium hardness are used for passes with medium rolling pressure, and those with low hardness are used for passes with relatively low rolling pressure, better meeting the uniform wear requirements of each pass and truly conforming to the usage conditions of the rolls.

[0017] Furthermore, the outer sheath of the welding wire is made of stainless steel strip.

[0018] The outer sheath of the welding wire is made of 1Cr 17 High-quality cold-rolled steel strip is selected with strict control over its S and P content. This ensures the steel strip provides sufficient Mn and Si alloys for the weld metal after surfacing, allowing each element to perform its intended function.

[0019] Furthermore, the particle size of each component in the core is 80-150 mesh.

[0020] Furthermore, the cross-section of the flux-cored welding wire is O-type or E-type, and the wire diameter is 2.5~4.0 mm.

[0021] Ore powder and rutile are commercially available standard products. Adding a small amount of ore powder to the welding wire primarily acts as a strong diluent, facilitating the escape of gases from the weld and aiding in sulfur removal and reducing the tendency for white spots. Adding a small amount of rutile provides benefits such as weak oxidizing properties, good hot slag removal, arc stability, a fine mist-like transition in the metal, good directional weldability, good slag coverage, and aesthetically pleasing weld formation. Excessive use of ore powder will disrupt arc stability and increase the risk of weld cracking. After welding with this flux-cored wire, the deposited metal will contain trace amounts of unavoidable impurities such as sulfur and phosphorus, but these are within permissible limits.

[0022] This invention also provides a method for preparing the above-mentioned high-cobalt martensitic stainless steel submerged arc welding flux-cored wire, comprising the following steps: (1) Mix the core material according to the formula to obtain the mixture; (2) Roll the pretreated welding wire sheath into a “U” shape, then fill the mixture into the “U” shaped welding wire sheath, and then roll it into shape.

[0023] The present invention also provides a BD roll, wherein a working layer is welded on the surface of the roll, and the working layer is obtained by overlay welding of the above-mentioned submerged arc welding flux-cored wire.

[0024] The present invention also provides a method for preparing the above-mentioned BD roll, comprising the following steps: (1) Pretreatment: A welding forming space is set at the part of the roll surface where steel passes through, and the effective thickness of all steel passing surfaces is 7~15mm; (2) Preheating before welding: After the rolls processed in step (1) are inspected for flaws, they are preheated. The flaw inspection is dye penetrant testing and / or ultrasonic testing. (3) Welding working layer: The preheated roll is hoisted onto the welding machine and installed. The roll body is in a special heat preservation cover. The two ends of the roll are assembled with the headstock and tailstock respectively. Then, submerged arc automatic welding is performed to form a welding layer and leave machining allowance. (4) Heat treatment after welding: In order to ensure the plasticity and toughness of the weld metal, the roll after welding is subjected to heat treatment after welding and subsequent turning to obtain BD roll.

[0025] Further, in step (1), the steel grade of the roll core: BD1 roll is made of medium carbon alloy forged steel or cast steel such as 60CrNiMo or 60CrMnMo, or can be obtained from old rolls with no defects in the original matrix; in order to save costs, BD2 roll can be made of medium carbon alloy forged steel or cast steel such as 45CrNiMo, 42CrMo, or 50CrNiMo, and the steel grades with better mechanical properties than ductile iron are used as the roll core for making the roll.

[0026] Further, in step (1), the effective thickness is taken as the upper limit for BD rolls with relatively symmetrical pass shapes, and as the lower limit for BD rolls with asymmetrical pass shapes, depending on the specific needs.

[0027] Furthermore, the specific process of the preheating treatment in step (2) is as follows: the roll is heated to 440~460℃ at a heating rate of ≤60℃ / h and then kept at that temperature. The holding time is calculated by dividing the maximum roll diameter (unit: mm) by 100, and the unit is hours.

[0028] Furthermore, in step (3), the parameters for submerged arc automatic welding include: Power supply polarity: DC reverse connection; Welding current: 300~450 A; Welding voltage: 28~32 V; Welding speed: 300~450 mm / min; Dry elongation: 20~30 mm; Eccentricity: 25~40 mm; Overlap: 55~60%, overlapping between adjacent weld passes; Interlayer temperature: 320~360℃.

[0029] Further, in step (4), the heat treatment specifically includes the following steps: first, the temperature is raised to 440~460℃ at a heating rate of ≤60℃ / h and then held for 3~5h; then the temperature is raised to 560℃ at a heating rate of ≤40℃ / h and held for 5~7h; then the temperature is raised to 595~600℃ at a heating rate of ≤20℃ / h and held, the holding time is calculated by dividing the maximum roller diameter by 100, the unit of the maximum roller diameter is millimeters, and the unit of the holding time is hours; after the holding is completed, the temperature is lowered to 500℃ at a cooling rate of ≤20℃ / h, the furnace is cooled to 50℃ and then the furnace is removed, and then cooled to room temperature for machining.

[0030] Furthermore, in step (4), the hardness of the BD roll after preparation is 50~53 HRC.

[0031] The present invention has the following beneficial effects: 1. When the submerged arc welding flux-cored wire produced by this invention is used with the corresponding flux for surfacing BD rolls (including BD1 and BD2 rolls), the surfacing metal exhibits good high-temperature hardness retention, high red hardness and wear resistance. The specific working layer does not stick to the steel on the die surface during use. The single rolling capacity of BD1 rolls is 6.5 times or more than that of the original 60CrMnMo forged steel or 60CrNiMo cast steel rolls; the single rolling capacity of BD2 rolls is 3 times or more than that of the original BD2 ductile iron rolls, completely changing the current domestic and international method of manufacturing using ductile iron alloys.

[0032] 2. The welding wire provided by this invention can directly deposit the working layer. Within the effective diameter range of the roll, it directly eliminates the need for an insulation and depositing transition layer, shortening manufacturing time and reducing the labor intensity of workers. For scrap rolls with good substrate, domestically available HRC30 welding wire with appropriate flux can be used to restore the dimensions and reshape the roll core to the required dimensions before depositing reinforcement, thus saving the high cost and manufacturing time of manufacturing new roll cores.

[0033] 3. The overlay welding preparation method provided by this invention is easy to master, simple to operate, and produces high-quality newly manufactured BD rolls.

[0034] 4. Since the customized roll core and the old roll core are made of medium carbon alloy steel, they have good weldability. After they are removed from the machine, it can be ensured that there are no defects in the base material that would affect the welding or use. After the fatigue layer is removed by machining, multiple welding can be carried out. This allows for the recycling and remanufacturing of this type of roll, which can better reduce roll consumption and reduce production costs. Attached Figure Description

[0035] Figure 1 Schematic diagram of BD1 roll configuration for rolling 60kg / m; Figure 2 Schematic diagram of roll configuration for BD2 rolling mill of 310 Z-beams; Figure 3 A schematic diagram of the roll configuration for BD2 rolls used to roll 60kg / m. Detailed Implementation

[0036] The principles and features of this invention are described below. The examples given are for illustrative purposes only and are not intended to limit the scope of the invention. Unless otherwise specified in the examples, conventional conditions or conditions recommended by the manufacturer should be followed. Reagents or instruments whose manufacturers are not specified are all commercially available products.

[0037] Example 1 1Cr with a thickness of 0.3-0.8 mm and a width of 8.0-16.0 mm 17 Cold-rolled high-quality steel strip is used as the outer sheath material for flux-cored welding wire. The flux core components include graphite, metallic chromium, cobalt powder, electrolytic nickel powder, ferromolybdenum, tungsten powder, ferrovanadium, metallurgical niobium powder, electrolytic copper powder, ferroaluminum powder, ferroboron, atomized titanium powder, fluorite powder, rutile, and pure iron. The process involves cutting the steel strip, degreasing and drying, and then preparing and drying the flux core. The steel strip is rolled into a "U" shape, filled with welding flux, rolled into an "O" shape, and then drawn into an "O"-shaped cross-section flux-cored welding wire with a diameter of Φ2.5~4.0mm. The composition of the flux core is shown in Table 1. By adjusting the proportions of the components in the sheath and the flux core, flux-cored welding wires with the composition shown in Table 1 can be produced.

[0038] Table 1 Composition of Flux-Cored Welding Wire

[0039] Regarding the outer sheath of this flux-cored welding wire, 1Cr is preferred for the outer sheath. 17 Cold-rolled high-quality steel strip, with low sulfur and phosphorus content, meets the requirements. Regarding the flux core, high-quality varieties with extremely low sulfur, phosphorus, and impurities are preferred. The descriptions of each component are as follows: (1) Use graphite and metallic chromium: to ensure that the weld has enough chromium and carbon, especially when the amount of powder is large.

[0040] (2) Adding niobium, titanium, vanadium, boron and aluminum: First, medium and high strength carbide alloying elements such as V, Nb and Ti form fine and dispersed strengthening carbides, generate corresponding hard phases, and have dispersion strengthening, solid solution strengthening and precipitation strengthening effects, ensuring the requirements of high temperature rolling; Second, trace amounts of B can purify and strengthen grain boundaries, improve the comprehensive performance of weld metal, and significantly improve the hot strength of weld metal than adding elements such as Zr, Hf, Mg, La and Ce; B segregates at grain boundaries, which can reduce grain boundary defects, improve grain boundary strength, and strongly change the shape of grain boundaries, affecting the precipitation and growth of grain boundary carbides and intermetallic compounds, improving their dense and uneven distribution, forming a spherical uniform distribution, preventing the precipitation of grain boundary lamellar and cellular phases, and improving the alloy's creep life. Third, the addition of titanium helps reduce segregation and generate refractory carbides, refines grains, and improves strength and toughness. Fourth, aluminum can work with the steel strip's own Mn and Si alloys to deoxidize during welding, which is beneficial for clean weld metal and better performance of the weld metal. Al and Co can coherently precipitate β-NiAl intermetallic compounds with Ni, which greatly improves the thermal strength of the weld metal. Al's characteristic of easily forming a dense and well-adhesive oxide film at high temperatures ensures that the roll surface metal and the rolled workpiece metal do not weld together during steel rolling, preventing steel sticking.

[0041] (3) Molybdenum and Tungsten: Mo and W are elements in the same group, and W is similar to Mo in many ways; both Mo and W are strong carbide-forming elements, with dispersion strengthening and grain refinement effects. Adding Mo can reduce the amount of W alloy added, and since tungsten is more expensive, it can save costs. Molybdenum is the main element in forming high-strength stainless steel, which improves the hot strength of steel, strengthens grain boundaries, improves the long-term plasticity of steel, and has a favorable effect on weldability; in dispersion-strengthened steel, molybdenum has the strongest effect as a dispersion strengthening agent. During surfacing, Mo and W have a stronger ability to form carbides than Cr, with precipitation strengthening and solid solution strengthening effects, good high-temperature hardness and high-temperature strength retention, increased red hardness of surfacing metal, elimination of brittleness, and significantly improved impact toughness and tempering resistance. Mo forms Fe2Mo intermetallic compound with Fe, which improves yield strength; Mo also plays the role of Mo passive surface film, which is flat and smooth, thereby reducing friction between the die and the workpiece, improving the anti-scraping ability of the die through the steel surface, and preventing steel sticking.

[0042] (4) Cobalt powder: The amount of metallic cobalt (Co) added to martensitic stainless steel is small, generally less than 1.00%. By increasing the Co content in the welding wire, cobalt reacts synergistically with Mo, W, Ti, Ni, and Fe, which is conducive to the formation of appropriate amounts of alloys such as Co-Mo-Cr, Co-Cr-W, and Ni-Co-Mo-Cr, as well as intermetallic compounds such as Ni3Mo and Fe2Mo. These alloys have high hardness and good anti-adhesion properties, and are mainly used to manufacture components that work at 700~900℃. This meets the actual situation that the roll surface temperature of BD rolls is around 800℃ during rolling, and can maintain a high hardness of HRc44 or higher. This high hardness characteristic mainly comes from the solid solution strengthening effect of cobalt and strengthening elements such as chromium and tungsten, which can ensure the high hardness and impact toughness required by the rolls during rolling, so that the rolls have good wear resistance and resist the impact loads during rolling. At the same time, its good anti-adhesion properties indicate that the weld metal has strong anti-galling ability and strong anti-scratch ability, thus achieving non-sticking of the rolls during use.

[0043] (5) Electrolytic nickel powder: Nickel in welds can improve toughness and strength, and enhance corrosion resistance. Adding an appropriate amount of Ni to general high-strength steel can improve its thermal sensitivity.

[0044] (6) The outer skin of this application is 1Cr 17 Cold-rolled high-quality steel strip contains chromium, carbon, manganese, and silicon. The steel strip, along with molybdenum, vanadium from ferrovanadium, nickel, niobium, atomized iron powder, titanium, aluminum, copper, boron, and iron contained in the outer sheath and core, all contribute to the alloy chemical composition of the weld metal after welding. Of course, the weld metal produced by these flux-cored wires will contain trace amounts of unavoidable impurities such as sulfur (S) and phosphorus (P), but these are within permissible limits. The purpose of adding activators is twofold: first, as a diluent to facilitate the escape of gases from the weld; and second, for desulfurization, as sulfur combines with hydrogen and volatilizes, reducing the tendency for white spots. Furthermore, if fluorite is used as an activator, too much should be added, as the fluorine in fluorite is a highly ionizing element that can affect arc stability.

[0045] Therefore, the high-cobalt submerged arc welding flux-cored wire YD506 (Ф3.2mm) prepared in Example 1, combined with a corresponding flux, is typically used with stainless steel welding wire and a flux with low oxidizing properties. This invention selects HJ107 neutral flux or a corresponding sintered flux. The resulting welded metal exhibits excellent properties, including high-temperature strength and hardness, and meets the stringent rolling conditions of BD rolls. It does not stick to the steel during rolling and maintains good roll pass dimensions, demonstrating significant technical advantages.

[0046] Example 2 A method for manufacturing BD1 rolls, with a schematic diagram of the roll configuration as shown below. Figure 1 As shown, its preparation method includes the following steps: (1) Preparation of roller core The roll core can be prepared using BD1 rolls within the effective size range of rolling, made of 60CrNiMo cast steel. Before welding, the die size is shifted by 8mm based on the finished die size, while the other dimensions and surface finish remain unchanged.

[0047] (2) Pretreatment A working area is set at the part of the roller surface where steel passes through. The effective thickness of the working area is 7mm, which serves as the forming space for the working layer of the roller surface formed by overlay welding. Specifically, a turning method is adopted. During turning, the roller is moved in the vertical direction of the inner arc of the hole to ensure the effective thickness of the working layer. (3) Flaw detection inspection All newly manufactured roll cores must undergo flaw detection before surfacing. The flaw detection methods used are magnetic particle testing and / or ultrasonic testing. Magnetic particle testing checks for surface defects, while ultrasonic testing checks for internal defects, ensuring that there are no defects on the base surface or inside the roll that would affect its use or the surfacing process. If severe internal damage is detected in the roll core (including the roll diameter and roll surface), surfacing manufacturing is not permitted. (4) Preheating before welding The flaw-detected roller cores are preheated in a dedicated electric furnace. The preheating temperature is increased to 440℃ at a rate of 20℃ / h, and then held at that temperature for 12 hours.

[0048] (5) Welding working layer After preheating, the rolls are hoisted onto the welding device and installed. Then, submerged arc welding is performed inside the insulation cover to form a weld overlay layer as the working layer on the roll surface. During the welding process of the working layer, machining allowance is reserved. Specifically, flux-cored welding wire (Ф3.2mm) prepared in Example 1 is used with HJ107 flux for welding. The welding process specifications are as follows: Power supply polarity: DC reverse connection; Welding current: 350~420A; Welding voltage: 28~32V; Welding speed: 300~450mm / min; Dry elongation: 20~30mm; Eccentricity: 25~40mm; Overlap: 55~60% (overlap of adjacent welds); Interlayer temperature: 320~360℃; (6) Heat treatment after welding The weld-overlay rolls are subjected to post-weld heat treatment in a bogie-type tempering furnace to eliminate welding stress, adjust the microstructure and hardness of the working layer metal, and achieve a hardness of 52 HRC after heat treatment. The heat treatment process involves first heating to 440℃ at a rate of 50℃ / h and holding for 2 hours; then heating to 560℃ at a rate of ≤40℃ / h and holding for 3 hours; then heating to 595~600℃ at a rate of ≤20℃ / h and holding for 12 hours; after holding, cooling to 500℃ at a rate of ≤20℃ / h and furnace cooling to 50℃ before unloading and cooling to room temperature for machining.

[0049] (7) Finished product turning The heat-treated BD1 rolls are machined to make the dimensions corresponding to the weld overlay layer meet the rolling requirements for the roll pass size.

[0050] (8) Flaw detection and hardness testing The BD1 rolls that have passed the finished product processing shall be inspected for defects according to the drawings and deemed qualified, with a hardness of 50.5~53.1HRc. After passing the inspection, they shall be oiled, packaged, or put into use. If they are deemed unqualified, the defects shall be removed and the rolls shall be repaired according to the original process until they are qualified.

[0051] Example 3 A BD2 rolling mill roll for rolling 310 Z-shaped steel, the roll configuration diagram is as follows: Figure 2 As shown, its preparation method includes the following steps: (1) Preparation of roller core The roll core can be made using a 310 Z-shaped steel BD2 roll, which is made of 60CrMnMo forged steel. The new roll is usually made by a professional roll factory according to the design drawings. The roll surface diameter is the maximum diameter after the roll is made. The die size is shifted by 12mm based on the finished die size, while the other dimensions and surface finish remain unchanged.

[0052] (2) Pretreatment A working area is set at the part of the roller surface where steel passes through. The effective thickness of the working area is 12mm. It serves as the forming space for the working layer of the roller surface formed by overlay welding. Specifically, a turning method is adopted. During turning, the roller is moved in the vertical direction of the inner arc of the die to ensure the effective thickness of the working layer.

[0053] (3) Flaw detection inspection All newly manufactured roll cores must undergo flaw detection before welding. The detection methods include magnetic particle testing and / or ultrasonic testing. Magnetic particle testing checks for surface defects, while ultrasonic testing checks for internal defects, ensuring that the base surface and interior of the roll are free of defects that would affect its use or the welding process. If severe internal damage is detected in the roll core (including the roll diameter and roll surface), welding is not permitted. (4) Preheating before welding The flaw-detected roller cores are preheated in a dedicated electric furnace. The preheating temperature is increased to 440℃ at a rate of 20℃ / h, and then held at that temperature for 12 hours. (5) Welding working layer After preheating, the rolls are hoisted onto the welding device and installed. Then, submerged arc welding is performed inside the insulation cover to form a weld overlay layer as the working layer on the roll surface. During the welding process of the working layer, machining allowance is reserved. Specifically, flux-cored welding wire (Ф3.2mm) prepared in Example 1 is used with HJ107 flux for welding. The welding process specifications are as follows: Power supply polarity: DC reverse connection; Welding current: 400~420A; Welding voltage: 30~32V; Welding speed: 400~450mm / min; Dry elongation: 20~30mm; Eccentricity: 35~40mm; Overlap: 55~60% (overlap of adjacent welds); Interlayer temperature: 320~360℃; (6) Heat treatment after welding The weld-overlay rolls are subjected to post-weld heat treatment in a bogie-type tempering furnace to eliminate welding stress, adjust the microstructure and hardness of the working layer metal, and achieve a hardness of 52 HRC after heat treatment. The heat treatment process involves first heating to 450℃ at a rate of 20℃ / h, then heating to 460℃ at a rate of ≤60℃ / h and holding for 5 hours; then heating to 560℃ at a rate of ≤30℃ / h and holding for 7 hours; then heating to 595~600℃ at a rate of ≤20℃ / h and holding for 13.5 hours; after holding, cooling to 500℃ at a rate of ≤20℃ / h and furnace cooling to 50℃ before being removed from the furnace and cooled to room temperature for machining.

[0054] (7) Finished product turning The heat-treated rolls are machined to make the dimensions corresponding to the weld overlay layer the required roll profile size, thus becoming a new type of BD2 roll for replacement. (8) Flaw detection and hardness testing The BD2 rolls that have passed the finished product processing shall be inspected for defects according to the drawings and deemed qualified, with a hardness of 51.1 to 52.9 HRc. After passing the inspection, they shall be oiled, packaged, or put into use. If they are deemed unqualified, the defects shall be removed and the rolls shall be repaired according to the original process until they are qualified.

[0055] Example 4 A BD2 rolling mill roll for rolling 60 kg / m is shown in the following diagram. Figure 3 As shown, its preparation method includes the following steps: (1) Preparation of roller core The roll core can be manufactured using a medium-carbon alloy forged steel, 42CrMo, which meets the required strength and toughness. New roll cores are typically manufactured by specialized roll mills according to design drawings. The roll surface diameter (D) of the new roll core should ideally be between the maximum diameter of the new roll and the scrap diameter, preferably around the midpoint. This is primarily because 42CrMo forged steel is less expensive than 50CrNiMo or 60CrMnMo forged steel; and setting the roll surface diameter of the new roll core around the midpoint between the maximum diameter of the new roll and the scrap diameter reduces weight and costs, facilitates roll adjustment and use, and is economical.

[0056] (2) Pretreatment A welding work area is set at the part of the roller surface where steel passes through. The effective thickness of the work area is 8mm, which is usually 8mm. It serves as the forming space for the working layer of the roller surface formed by welding. Specifically, it is made by turning. During turning, it is translated along the vertical direction of the inner arc of the hole to ensure the effective thickness of the working layer.

[0057] (3) Flaw detection inspection All newly manufactured roll cores must undergo flaw detection before welding. The detection methods include magnetic particle testing and / or ultrasonic testing. Magnetic particle testing checks for surface defects, while ultrasonic testing checks for internal defects, ensuring that the base surface and interior of the roll are free of defects that would affect its use or the welding process. If severe internal damage is detected in the roll core (including the roll diameter and roll surface), welding is not permitted. (4) Preheating before welding The flaw-detected roller cores are preheated in a dedicated electric furnace. The preheating temperature is increased to 450℃ at a rate of 10℃ / h, and then held at that temperature for 12 hours.

[0058] (5) Welding working layer After preheating, the rolls are hoisted onto the welding device and installed. Then, submerged arc welding is performed inside the insulation cover to form a weld overlay layer as the working layer on the roll surface. During the welding process of the working layer, machining allowance is reserved. Specifically, flux-cored welding wire (Ф3.2mm) prepared in Example 1 is used with HJ107 flux for welding. The welding process specifications are as follows: Power supply polarity: DC reverse connection; Welding current: 350~420A; Welding voltage: 28~32V; Welding speed: 300~450mm / min; Dry elongation: 20~30mm; Eccentricity: 25~35mm; Overlap: 55~60% (overlap of adjacent welds); Interlayer temperature: 320~360℃.

[0059] (6) Heat treatment after welding The weld-overlay rolls were subjected to post-weld heat treatment in a bogie-type tempering furnace to eliminate welding stress and adjust the microstructure and hardness of the working layer metal. The hardness of the weld overlay layer after heat treatment was 52.3 HRC. The heat treatment process was as follows: first, the temperature was increased to 450℃ at a rate of 60℃ / h, then increased to 450℃ at a rate of ≤30℃ / h and held for 4 hours; then the temperature was increased to 560℃ at a rate of ≤20℃ / h and held for 6 hours; then the temperature was increased to 595~600℃ at a rate of ≤15℃ / h and held for 12 hours; after the holding period, the temperature was reduced to 500℃ at a rate of ≤20℃ / h and furnace cooled to 50℃ before being removed from the furnace and cooled to room temperature for machining.

[0060] (7) Finished product turning The heat-treated rolls are machined to make the dimensions corresponding to the weld overlay layer the required roll profile size, thus becoming a new type of BD2 roll for replacement.

[0061] (8) Flaw detection and hardness testing The BD2 rolls that have passed the finished product processing shall be inspected for defects according to the drawings and deemed qualified, with a hardness of 50.1~52.8HRc. After passing the inspection, they shall be oiled, packaged, or put into use. If they are deemed unqualified, the defects shall be removed and the rolls shall be repaired according to the original process until they are qualified.

[0062] Experimental Example 1 The YD-506 submerged arc welding flux-cored wire prepared in Example 1, combined with HJ107 flux, exhibits excellent metallic properties after welding. The generated hard phase ensures the working layer metal possesses good high-temperature strength and hardness, stable microstructure, and does not stick to steel during use. Furthermore, it maintains good die shape, resulting in stable product quality and high single-pass rolling yield. The main advantages are as follows: (1) Taking the BD1 and BD2 rolls for rolling 60kg / m and the BD2 roll for 310 Z-shaped steel prepared in Examples 2 to 4 as examples, the relevant data test results are shown in Table 2.

[0063] Table 2. Roll Inspection Data Results

[0064] Note: The original BD2 ductile iron rolling mill roll was a roll for the straight rolling process with 5 roll passes. Now the roll has been changed to a roll for the skew rolling process with 6 roll passes.

[0065] (2) The newly manufactured BD1 and BD2 rolls repaired or repaired by overlay welding in this invention have a hardness of HRC45 or higher at 1100℃, with a small hardness drop, thus ensuring high-temperature hardness during steel rolling and exhibiting excellent wear resistance. The tensile strength of the overlay metal after heat treatment at 595~600℃ reaches 1850MPa at room temperature; the tensile strength of the overlay metal at 850℃ can reach 1380MPa, reaching the tensile strength of 1280~1380 MPa of various heavy rails at room temperature, thus ensuring high-temperature strength during steel rolling. This overcomes the shortcomings of the prior art.

[0066] (3) The original BD2 ductile iron rolls cannot be repaired by welding after they are off the machine. The technology of this invention can be implemented by changing the material of the roll core. The new BD1 and BD2 rolls made by this invention can be repaired or remanufactured repeatedly after they are off the machine, which greatly reduces roll consumption, helps to reduce rolling costs and enhance competitiveness.

[0067] (4) The wear amount and grinding amount of the new BD1 and BD2 rolls repaired by welding or newly manufactured according to the present invention are less than 6mm in the radial direction. In the next repair or remanufacturing, as long as the substrate has no defects that affect the welding repair and rolling, the same roll diameter can be repaired or reinforced.

[0068] (5) In use, the rolls repaired or newly manufactured using the present invention form fibroids later, with smaller and fewer fibroids, resulting in shorter repair time; the single machine use time is longer, and the number of roll replacements is greatly reduced. Therefore, the workload and intensity of operators are reduced, and less material is needed for roll replacement.

[0069] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions or improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A flux-cored welding wire for submerged arc welding of high-cobalt martensitic stainless steel, characterized in that, It includes a welding wire sheath comprising 50-80% by weight and a flux core comprising 20-50% by weight; The flux core comprises the following components by total mass percentage: graphite 0.2-0.5%, JCr99-A 1.2-3.0%, cobalt powder 16.4-41.0%, metallic nickel 6.4-16%, FeMo55-A 10.9-27.3%, tungsten powder 0.32-0.8%, FeV75-A 0.06-0.16%, FNb-3 0.02-0.05%, FTD1 or FTD2 0.1-0.25%, high alumina 1.2-3.0%, FeB22C 0.1% 0.04-0.1%, metallic titanium 0.01-0.03%, CaF2 0.4-1.0%, and rutile 0.25-0.63%, with the balance being atomized iron powder.

2. The high-cobalt martensitic stainless steel submerged arc welding flux-cored wire as described in claim 1, characterized in that, The outer sheath of the welding wire is made of stainless steel strip.

3. The high-cobalt martensitic stainless steel submerged arc welding flux-cored wire as described in claim 1, characterized in that, The particle size of each component in the core is 80-150 mesh.

4. The high-cobalt martensitic stainless steel submerged arc welding flux-cored wire as described in claim 1, characterized in that, The cross-section of the flux-cored welding wire is O-shaped or E-shaped, and the diameter of the welding wire is 2.5~4.0 mm.

5. The method for preparing the high-cobalt martensitic stainless steel submerged arc welding flux-cored wire according to claim 1, characterized in that, Includes the following steps: (1) Mix the core material according to the formula to obtain the mixture; (2) Roll the pretreated welding wire sheath into a "U" shape, then fill the mixture into the "U" shaped welding wire sheath, and then roll it into shape.

6. A BD roll, characterized in that, The surface of the roll is welded with a working layer, which is obtained by welding with the submerged arc welding flux-cored wire as described in any one of claims 1 to 4.

7. The method for preparing the BD roll according to claim 6, characterized in that, Includes the following steps: (1) Pretreatment: A welding forming space is set at the part of the roll surface where steel passes through, and the effective thickness of all steel passing surfaces is 7~15mm; (2) Preheating before welding: After the rolls processed in step (1) are inspected for flaws, they are preheated. The flaw inspection is dye penetrant testing and / or ultrasonic testing. (3) Welding working layer: The preheated roll is hoisted onto the welding machine and installed. The roll body is in a special heat preservation cover. The two ends of the roll are assembled with the headstock and tailstock respectively. Then, submerged arc automatic welding is performed to form a welding layer and leave machining allowance. (4) Heat treatment after welding: In order to ensure the plasticity and toughness of the weld metal, the roll after welding is subjected to heat treatment after welding and subsequent turning to obtain BD roll.

8. The method for preparing BD rolls as described in claim 7, characterized in that, In step (3), the parameters for the submerged arc automatic welding include: Power supply polarity: DC reverse connection; Welding current: 300~450 A; Welding voltage: 28~32 V; Welding speed: 300~450 mm / min; Dry elongation: 20~30 mm; Eccentricity: 25~40 mm; Overlap: 55~60%, overlapping between adjacent weld passes; Interlayer temperature: 320~360℃.

9. The method for preparing BD rolls as described in claim 7, characterized in that, In step (4), the heat treatment specifically includes the following steps: first, heating to 440~460℃ at a heating rate of ≤60℃ / h and holding for 3~5h; then heating to 560℃ at a heating rate of ≤40℃ / h and holding for 5~7h; then heating to 595~600℃ at a heating rate of ≤20℃ / h and holding for the time, the holding time is calculated by dividing the maximum roller diameter by 100, the unit of the maximum roller diameter is millimeters, and the unit of the holding time is hours; after the holding is completed, cooling to 500℃ at a cooling rate of ≤20℃ / h, furnace cooling to 50℃ and then cooling to room temperature for machining.

10. The method for preparing BD rolls as described in claim 7, characterized in that, In step (4), the hardness of the BD roll after preparation is 50~53 HRC.

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