A 60kg grade wire rod for bainite structure gas shielded welding wire and a preparation method thereof

Abstract

This application relates to a 60kg-grade bainitic gas-shielded welding wire rod and its preparation method, belonging to the field of wire rod production technology. The chemical composition of the wire rod, by mass fraction, is: C: 0.06%~0.09%, Si: 0.45%~0.65%, Mn: 1.35%~1.55%, P≤0.015%, S≤0.012%, Cr: 0.30%~0.50%, Mo: 0.20%~0.35%, Ti: 0.04%~0.08%, with the remainder being Fe and unavoidable impurities; the microstructure of the wire rod is bainitic. The phase transformation re-temperature phenomenon is controlled by optimizing the Stellmore cooling process. A segmented accelerated roller conveyor strategy is adopted, and high cooling rate control is implemented at the overlap point at the outlet of the insulation hood, promoting the completion of the bainitic phase transformation under rapid cooling conditions, effectively suppressing the temperature rise caused by phase transformation heat; simultaneously, for non-overlapping areas, high cooling rate conditions are applied simultaneously within the insulation hood to eliminate the risk of localized re-temperature. This solution significantly reduces the microhardness of the hard phase in the wire rod structure, solving the problem of poor drawability caused by the hard phase structure, and providing a wire rod substrate with uniform structure and excellent processing performance for high-strength welding wire.

Description

Technical Field

[0001] This application relates to the field of wire rod production technology, and in particular to a 60kg grade bainitic gas shielded welding wire wire and its preparation method. Background Technology

[0002] Gas-shielded welding wire rod is the core raw material for manufacturing gas-shielded welding wire. The production process of this rod encompasses the entire process of smelting, refining, continuous casting, high-speed wire rod rolling, and Stellmore controlled cooling. The rod undergoes multiple drawing stages, resulting in a total reduction in surface area of ​​92% to 98%, to produce welding wire with a diameter of 0.8 mm to 1.6 mm. Therefore, this places extremely high demands on the rod's plasticity and drawing properties. Especially for 60 kg grade bainitic rods, while ensuring strength, it is also necessary to achieve uniform microstructure to meet the requirements of high-speed drawing and welding stability.

[0003] In current bainitic wire rod production, a low wire drawing temperature combined with a slow cooling process using an insulation cover is typically employed to improve plasticity. However, on the Steyrmo cooling line, the different thicknesses of the coiled sections at the lap and non-lap points lead to uneven cooling rates, resulting in microstructure and property differentiation. Under slow cooling conditions, the lap area easily forms a high-hardness mixed martensite and austenite hard phase microstructure with a microhardness greater than 400 HV, causing an abnormally high local strength in the wire rod, with performance fluctuations exceeding 70 MPa within the same coil. The appearance of this hard phase microstructure and the deterioration of performance within the same coil severely impair the wire rod's drawability, leading to an increased wire breakage rate, which has become a core bottleneck restricting the improvement of high-strength welding wire quality. Summary of the Invention

[0004] This application provides a 60kg grade bainitic gas shielded welding wire rod and its preparation method to solve the following technical problem: how to improve the drawing performance of the 60kg grade bainitic gas shielded welding wire rod.

[0005] In a first aspect, embodiments of this application provide a 60kg grade bainitic gas shielded welding wire rod, characterized in that, by mass fraction, the chemical composition of the wire rod is: C: 0.06%~0.09%, Si: 0.45%~0.65%, Mn: 1.35%~1.55%, P≤0.015%, S≤0.012%, Cr: 0.30%~0.50%, Mo: 0.20%~0.35%, Ti: 0.04%~0.08%, with the remainder being Fe and unavoidable impurities; The microstructure of the wire rod is bainite.

[0006] Optionally, the microhardness of the M / A hard phase in the bainitic structure is ≤350HV.

[0007] Optionally, the wire rod meets the following performance requirements: tensile strength of 700MPa~750MPa, and same-coil performance ≤35MPa.

[0008] Secondly, embodiments of this application provide a method for preparing the wire rod described in the first aspect, the method comprising: A billet with the following chemical composition was obtained: C: 0.06%~0.09%, Si: 0.45%~0.65%, Mn: 1.35%~1.55%, P≤0.015%, S≤0.012%, Cr: 0.30%~0.50%, Mo: 0.20%~0.35%, Ti: 0.04%~0.08%, with the remainder being Fe and unavoidable impurities; The blank is then spun into loose coils; The loose coils are sequentially subjected to Stellmore cooling, coiling, and cooling to obtain wire rods.

[0009] Optionally, the spinning temperature is 950℃~980℃.

[0010] Optionally, the temperature of the insulation cover at the overlap point of the Stellmore cooling system is ≥650°C.

[0011] Optionally, the temperature of the non-overlapping point insulation cover of the Stellmore cooling system is ≤550℃.

[0012] Optionally, the cooling rate inside the non-overlapping insulation cover of the Stellmore cooling system is 1.0℃ / s to 2.0℃ / s.

[0013] Optionally, the cooling rate of the Stellmore cooling overlap point after exiting the insulation cover and before winding is 1.0℃ / s to 2.0℃ / s.

[0014] Optionally, the overlap temperature of the Stellmore cooling coil is ≤500°C.

[0015] Optionally, the segmented roller conveyor speed of the Stellmore cooling system satisfies: The roller conveyor speeds of sections 1 to 10 range from 0.30 m / s to 0.39 m / s, and increase arithmetically by 0.01 m / s. The roller conveyor speed of section 11 is 0.62m / s to 0.72m / s; The roller conveyor speed of section 12 is 0.78m / s to 0.88m / s; The roller conveyor speed of section 13 is 0.95m / s to 1.05m / s.

[0016] The technical solutions provided in this application have the following advantages compared with the prior art: This application provides a 60kg grade bainitic gas-shielded welding wire rod. The chemical composition of the rod, by mass fraction, is: C: 0.06%~0.09%, Si: 0.45%~0.65%, Mn: 1.35%~1.55%, P≤0.015%, S≤0.012%, Cr: 0.30%~0.50%, Mo: 0.20%~0.35%, Ti: 0.04%~0.08%, with the remainder being Fe and unavoidable impurities. The microstructure of the rod is bainitic. Through synergistic effects of composition design and innovative cooling processes, the drawing performance of the bainitic wire rod is fundamentally improved. The core principle lies in overcoming the limitations of traditional slow cooling processes and implementing precise control over the phase transformation reheating phenomenon. Traditional theory suggests that reducing the cooling rate can decrease the hardness of the bainitic structure; therefore, a slow cooling strategy of low roller speed and closed insulation cover is adopted. However, in practice, it has been found that during the slow cooling process inside the insulation cover, the phase transformation heat released by the bainitic phase transformation can trigger localized reheating, causing carbon atoms to accumulate in untransformed austenite, significantly improving its stability. This leads to a lag in the bainitic phase transformation process, with some supercooled austenite transforming into high-carbon martensite under subsequent low-temperature conditions, forming a hard phase structure with excessive hardness. This non-uniformity of the structure not only causes fluctuations in strength within the same ring, but also significantly deteriorates the drawing processability of the wire rod due to the brittleness of the hard phase.

[0017] This innovative solution employs a dynamic accelerated cooling process to overcome the backheating problem: by increasing the roller speed to implement zoned controlled cooling, the overlapping area completes the bainitic phase transformation at a high cooling rate after leaving the insulation cover, allowing the phase transformation heat to dissipate rapidly and thus preventing backheating. Simultaneously, non-overlapping areas are forced to maintain high-intensity cooling within the insulation cover to suppress localized temperature rise. This synergistic cooling mechanism ensures that austenite completes the bainitic transformation within a predetermined temperature range, effectively blocking the abnormal migration of carbon atoms and the formation path of martensite. Combined with a specific composition system to improve hardenability, a uniform low-carbon bainitic microstructure is ultimately obtained across the entire cross-section, significantly reducing the hardness of the hard phase, optimizing the properties of the same coil, and fundamentally solving the wire breakage problem during high-reduction-rate drawing. Attached Figure Description

[0018] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application.

[0019] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the accompanying drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, those skilled in the art can obtain other drawings based on these drawings without any creative effort.

[0020] Figure 1A flowchart illustrating a method for preparing 60kg grade bainitic gas shielded welding wire rods, as provided in this application embodiment; Figure 2 A CCT curve of a 60kg grade bainitic gas shielded welding wire rod provided for an embodiment of this application; Figure 3 An expansion curve of a 60kg grade bainitic gas shielded welding wire rod during phase transformation reheat is provided for an embodiment of this application. Figure 4 Microstructure diagram of a 60kg grade bainitic gas shielded welding wire rod provided in Embodiment 1 of this application; Figure 5 The microstructure image provided for Comparative Example 1 of this application. Detailed Implementation

[0021] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions in the embodiments of this application will be clearly and completely described below. Obviously, the described embodiments are only some, not all, of the embodiments of this application. All other embodiments obtained by those skilled in the art based on the embodiments of this application without creative effort are within the scope of protection of this application.

[0022] The range descriptions used herein, such as numerical ranges and proportional ranges, include all possible sub-ranges and single numerical values ​​within that range. For example, the range descriptions of "1 to 6" or "1~6" cover all sub-ranges between 1 and 6 (such as 1 to 3, 2 to 5, etc.) and single numbers (such as 1, 2, 3, 4, 5, 6). Unless otherwise specified, the terms "including" and "contains" used herein mean "including but not limited to"; relational terms such as "first" and "second" are used only to distinguish different entities or operations and do not imply an actual order or relationship. "And / or" indicates that multiple situations can exist individually or simultaneously. Expressions such as "at least one," "multiple," and "at least one" refer to any combination of the corresponding objects, including combinations of single or multiple objects. The proportional relationships mentioned herein, such as mass ratios and molar ratios, should be understood as the correspondence between the first and second terms of a proportional formula, according to the order of description. The raw materials, reagents, instruments, and equipment used herein can all be obtained through commercial purchase or prepared using existing methods.

[0023] In a first aspect, embodiments of this application provide a 60kg grade bainitic gas shielded welding wire rod, characterized in that, by mass fraction, the chemical composition of the wire rod is: C: 0.06%~0.09%, Si: 0.45%~0.65%, Mn: 1.35%~1.55%, P≤0.015%, S≤0.012%, Cr: 0.30%~0.50%, Mo: 0.20%~0.35%, Ti: 0.04%~0.08%, with the remainder being Fe and unavoidable impurities; The positive effects of limiting the carbon (C) mass fraction to 0.06%~0.09% are as follows: Controlling the C mass fraction within the low-carbon range significantly reduces the risk of martensitic phase transformation while ensuring the necessary hardenability and strength of the wire rod. This design weakens the carbon-rich tendency of austenite from the source, effectively blocking the hard phase formation path caused by carbon atom migration in traditional slow cooling processes, thus laying the compositional foundation for achieving a uniform bainitic structure in subsequent accelerated cooling processes. For example, the C mass fraction can be 0.06%, 0.07%, 0.08%, 0.09%, etc.

[0024] The positive effects of limiting the Si mass fraction to 0.45%~0.65% include: While Si strengthens the structure through deoxidation, its key role is to inhibit premature carbide precipitation and enhance austenite stability. This characteristic effectively slows down the phase transformation process, allowing the bainite transformation temperature range to precisely match the accelerated cooling process window. This prevents carbon atoms from accumulating in untransformed austenite during the reheating process, thereby eliminating the conditions for the formation of high-carbon martensite hard phases and fundamentally improving microstructure uniformity and workability. For example, the Si mass fraction can be 0.45%, 0.55%, 0.65%, etc.

[0025] The positive effects of limiting the Mn mass fraction to 1.35%~1.55% include: Mn significantly improves the hardenability of wire rod, ensuring complete bainitic phase transformation even in low-to-medium carbon compositions. The core value lies in broadening the bainite formation temperature range, allowing the wire rod to achieve sufficient bainite structure even under accelerated cooling conditions, avoiding the retention of non-equilibrium hard phases due to increased cooling rates, thus providing the necessary microstructure and plasticity for high-reduction-area drawing. For example, the Mn mass fraction can be 1.35%, 1.45%, 1.55%, etc.

[0026] The positive effects of limiting the mass fraction of phosphorus (P) to ≤0.015%: By controlling the mass fraction of P at an ultra-low level, the tendency for cold brittleness caused by P segregation at grain boundaries can be effectively suppressed, and stress concentration cracks can be avoided around the hard phase structure during drawing. For example, the mass fraction of P can be 0.005%, 0.010%, 0.015%, etc.

[0027] The positive effect of limiting the mass fraction of sulfur (S) to ≤0.012% is that by controlling the S mass fraction to an extremely low level, the formation pathway of manganese sulfide inclusions can be fundamentally blocked. Manganese sulfide inclusions easily become stress concentration sources in the hard phase structure during drawing deformation, inducing microcracks and accelerating wire breakage. For example, the S mass fraction can be 0.004%, 0.008%, 0.012%, etc.

[0028] The positive effects of limiting the Cr mass fraction to 0.30%~0.50% include: Cr synergistically improves the hardenability of wire rod and the driving force of bainitic phase transformation, ensuring sufficient bainitic microstructure even under accelerated cooling conditions. The solid solution strengthening effect of Cr maintains necessary strength while avoiding dependence on the hard phase microstructure of traditional high-carbon steel, providing a compositional basis for reducing the proportion of mixed martensite and austenite hard phases, thus fundamentally supporting the improvement of drawability. For example, the Cr mass fraction can be 0.30%, 0.40%, 0.50%, etc.

[0029] The positive effects of limiting the Mo mass fraction to 0.20%~0.35% include: Mo's strong hardenability significantly widens the bainitic transformation window and inhibits the formation of proprecipitated ferrite. This characteristic enables wire rods to achieve complete bainitic transformation during rapid cooling processes, effectively blocking the abnormal enrichment path of carbon atoms during reheating, thereby eliminating the conditions for the formation of high-hardness martensite phases and providing a reliable guarantee for high-reduction-ratio drawing with uniform microstructure and good toughness. For example, the Mo mass fraction can be 0.20%, 0.25%, 0.30%, 0.35%, etc.

[0030] The positive effects of limiting the Ti mass fraction to 0.04%~0.08% include: Ti refines the original austenite grains by forming stable nitrides, thereby simultaneously improving the strength, toughness, and hardenability of the wire rod. The refined grain structure not only effectively suppresses the localized temperature rise caused by phase transformation heat but also hinders the long-range migration of carbon atoms by increasing the grain boundary area, thus unlocking the formation mechanism of the hard phase structure at the microscopic level. This allows the bainitic structure obtained through accelerated cooling to possess both low hard phase content and high deformation compatibility. For example, the Ti mass fraction can be 0.04%, 0.05%, 0.06%, 0.07%, 0.08%, etc.

[0031] Fe is a matrix element, and the specific content / range of Fe can be obtained through the upper and lower limit formulas of the component, that is: The sum of the percentages of all components in a composition should equal 100%, and the content ranges of several components should meet the following conditions: the upper limit of a certain component + the lower limit of other components ≤ 100; the lower limit of a certain component + the upper limit of other components ≥ 100. Furthermore, the specific content of Fe is made up to 100% by the actual detected values ​​of the other chemical components mentioned above, together with any unlisted active elements and / or impurity elements, and Fe must constitute the absolute proportion as a matrix element.

[0032] The microstructure of the wire rod is bainite.

[0033] The microstructure of the wire rod is bainitic, eliminating the mixed hard phase of martensite and austenite formed due to uneven cooling in traditional processes, thus removing stress concentration sources caused by localized ultra-high hardness. The uniform bainitic matrix, with its inherent dislocation substructure characteristics, achieves uniform stress distribution and dislocation slip coordination during drawing deformation, significantly improving deformation tolerance during high-area-ratio processing. By accelerating the cooling process to block abnormal carbon atom enrichment pathways, the hardness of residual hard phases in the bainitic microstructure is significantly reduced, ultimately overcoming the core bottleneck of wire breakage during high-strength wire rod drawing.

[0034] In some embodiments, the microhardness of the M / A hard phase in the bainitic structure is ≤350HV.

[0035] The M / A hard phase refers to the mixed island-like structure of martensite and austenite remaining during the bainitic transformation. A microhardness of the M / A hard phase in bainitic microstructure ≤350 HV indicates that the accelerated cooling process successfully suppressed the abnormal migration of carbon atoms, allowing the retained austenite to fully transform into ductile bainite or stabilized austenite, thereby significantly reducing the martensite content. For example, the microhardness of the M / A hard phase in bainitic microstructure can be 200 HV, 250 HV, 300 HV, 350 HV, etc.

[0036] In some embodiments, the wire rod meets the following properties: tensile strength of 700MPa~750MPa, and same-coil strength ≤35MPa.

[0037] Tensile strength: Reflects the upper limit of the wire rod's ability to resist tensile fracture. Excessive tensile strength will exacerbate stress concentration during drawing, while insufficient tensile strength will fail to meet the strength requirements of 60kg welding wire. A tensile strength between 700MPa and 750MPa ensures the welding wire's load-bearing capacity while avoiding the strength exceeding limits caused by the hard phase in traditional high-carbon steel. This creates a strength gradient between the bainitic matrix and the low-hardness M / A phase, ensuring uniform stress transfer during high-reduction-rate drawing and eliminating the risk of localized overload fracture. For example, tensile strengths can be 700MPa, 710MPa, 720MPa, 730MPa, 740MPa, and 750MPa. Coil performance: Characterizes the strength difference between the head and tail of the same coil. Coil performance ≤35MPa demonstrates that the accelerated cooling process has resolved the microstructural differentiation between lap and non-lap points. For example, the performance of the same coil can be 30MPa, 31MPa, 32MPa, 33MPa, 34MPa, 35MPa, etc.

[0038] Figure 1 A flowchart illustrating a method for preparing 60kg-grade bainitic gas-shielded welding wire rods, as provided in this application embodiment.

[0039] Please see Figure 1 Secondly, this application provides a method for preparing the wire rod described in the first aspect, the method comprising: S1. Obtain a billet with the following chemical composition: C: 0.06%~0.09%, Si: 0.45%~0.65%, Mn: 1.35%~1.55%, P≤0.015%, S≤0.012%, Cr: 0.30%~0.50%, Mo: 0.20%~0.35%, Ti: 0.04%~0.08%, with the remainder being Fe and unavoidable impurities; S2. The blank is spun into loose coils; S3. The loose coil is subjected to Stellmore cooling, coiling and cooling in sequence to obtain wire rod.

[0040] In some embodiments, the spinning temperature is 950°C to 980°C.

[0041] The spinning temperature is between 950℃ and 980℃. High-temperature spinning provides sufficient heat capacity to the overlapping area, ensuring it remains at a relatively high temperature after exiting the insulation hood. This provides a necessary reaction window for the accelerated cooling stage to complete the bainitic phase transformation. Simultaneously, this temperature range ensures that the non-overlapping areas have sufficient phase transformation driving force within the insulation hood, preventing transformation stagnation due to premature cooling. This effectively blocks the formation of hard phase structures at the overlapping points due to delayed cooling, while ensuring that the non-overlapping areas complete the full bainitic transformation in a controlled environment. For example, the spinning temperature can be 950℃, 960℃, 970℃, 980℃, etc.

[0042] In some embodiments, the temperature of the insulation cover at the overlap of the Stellmore cooling system is ≥650°C.

[0043] The Stellmore cooling system features an outlet temperature of ≥650℃ at the lap joint of the insulation cover. Utilizing the slow heat dissipation at the lap joint, the phase transformation process is delayed until the accelerated cooling environment after exiting the cover. This allows for the completion of the bainitic transformation under controlled high cooling rates, rapidly dissipating the phase transformation heat and blocking the carbon atom enrichment and subsequent martensite formation pathways caused by backheating, thus eliminating the hard phase structure at the lap joint from its source. For example, the outlet temperature of the Stellmore cooling system at the lap joint of the insulation cover can be 650℃, 660℃, 675℃, 685℃, 700℃, etc.

[0044] In some embodiments, the temperature of the non-overlapping point insulation cover of the Stellmore cooling system is ≤550°C.

[0045] The temperature of the non-overlapping points of the Stellmore cooling system at the insulation cover is ≤550℃, ensuring that austenite fully transforms into uniform bainite, avoiding the risk of residual microstructure caused by phase transformation hysteresis, and achieving zero hard phase defects at the non-overlapping points. For example, the temperature of the non-overlapping points of the Stellmore cooling system at the insulation cover can be 400℃, 510℃, 525℃, 540℃, 550℃, etc.

[0046] In some embodiments, the cooling rate within the non-overlapping insulation cover of the Stellmore cooling system is 1.0°C / s to 2.0°C / s.

[0047] The cooling rate within the non-lap joint insulation hood of the Stellmore cooling system is between 1.0℃ / s and 2.0℃ / s. This avoids the formation of coarse M / A hard phases in granular bainite caused by traditional slow cooling (<1.0℃ / s), which exacerbates stress concentration during drawing due to the excessive size of the hard phases. It also avoids the risk of excessive lath bainite strength caused by high-speed cooling (>2.0℃ / s). In the isothermal field controlled by the insulation hood, the cooling rate of 1.0℃ / s to 2.0℃ / s promotes the smooth transformation of austenite into fine-grained bainite, ensuring the fine dispersion of the M / A hard phase and controlled hardness, providing a uniform plastic deformation matrix for high-reduction-rate drawing. For example, the cooling rate within the non-lap joint insulation hood of the Stellmore cooling system can be 1.0℃ / s, 1.2℃ / s, 1.4℃ / s, 1.6℃ / s, 1.8℃ / s, 2.0℃ / s, etc.

[0048] In some embodiments, the cooling rate of the Stellmore cooling overlap point after exiting the insulation cover and before winding is 1.0℃ / s to 2.0℃ / s.

[0049] The cooling rate at the overlap point of the Stellmore-cooled coil after exiting the insulation cover and before winding is between 1.0℃ / s and 2.0℃ / s, ensuring that the delayed phase transformation overlap point achieves equivalent microstructure control to the non-overlapping points inside the cover even in an open environment. This utilizes both the ambient temperature difference to accelerate heat dissipation and suppress backheating, and dynamic adjustment of the roll speed to ensure stable cooling rate. The synergistic effect of dual-zone cooling rates effectively blocks the martensite formation path, causing the M / A hard phase size and distribution characteristics in the bainite microstructure at the overlap point to converge with those at the non-overlapping points, thereby eliminating performance fluctuations within the same coil caused by cooling differences. For example, the cooling rate at the overlap point of the Stellmore-cooled coil after exiting the insulation cover and before winding can be 1.0℃ / s, 1.2℃ / s, 1.4℃ / s, 1.6℃ / s, 1.8℃ / s, 2.0℃ / s, etc.

[0050] In some embodiments, the overlap temperature of the Stellmore cooling coil is ≤500°C.

[0051] The overlap temperature of the coiling at the Stellmore cooling point is ≤500℃ to ensure that the bainitic phase transformation occurs and completes at the overlap point after exiting the insulation cover. If the coiling temperature is higher than the bainitic phase transformation end point, the phase transformation process will be delayed until after the coiling of the wire rod. The phase transformation heat generated by the accumulated coiled wire rod is prone to causing backheating, which is not conducive to the hardness control of the final microstructure. For example, the overlap temperature of the coiling at the Stellmore cooling point can be 400℃, 420℃, 440℃, 460℃, 480℃, 500℃, etc.

[0052] In some embodiments, the segmented roller conveyor speed of the Steilmo cooling system satisfies: The roller conveyor speeds of sections 1 to 10 range from 0.30 m / s to 0.39 m / s, and increase arithmetically by 0.01 m / s. The roller conveyor speed of section 11 is 0.62m / s to 0.72m / s; The roller conveyor speed of section 12 is 0.78m / s to 0.88m / s; The roller conveyor speed of section 13 is 0.95m / s to 1.05m / s.

[0053] The segmented roller speed control of the Stellmore cooling system achieves precise adaptation between the cooling process and the bainitic phase transformation kinetics through dynamic gradient design. Segments 1 to 10 ensure that the bainitic phase transformation completes at non-overlapping points; segments 11 to 13 enable forced convection heat transfer at the overlapping points after exiting the shroud. This design transforms the inherent defect of uneven cooling into the advantage of segmented control. Non-overlapping points achieve a stable microstructure within the shroud, while overlapping points utilize an open environment to suppress hard phase formation, ultimately achieving homogenization of the entire roll's microstructure and fundamentally eliminating the wire breakage defect during drawing.

[0054] The product prepared by the method of preparing the wire rod is the aforementioned wire rod. Since the method of preparing the wire rod adopts some or all of the technical solutions of the wire rod embodiments, it has at least all the beneficial effects brought about by the technical solutions of the aforementioned embodiments, which will not be elaborated here.

[0055] The present application is further illustrated below with reference to specific embodiments. Experimental methods in the following embodiments that do not specify specific conditions are generally determined according to national standards / industry standards / the disclosure herein; if there are no corresponding national standards / industry standards / the disclosure herein, they are performed according to generally accepted international standards, conventional conditions, or conditions recommended by the manufacturer.

[0056] The chemical composition (mass percentage / %) of the examples and comparative examples is shown in Table 1.

[0057] Table 1

[0058] Example 1 The blank with the chemical composition described in Example 1 in Table 1 was obtained; The billet is spun into loose coils; the spun temperature is 950℃. The loose coils are then subjected to Stellmore cooling. On the Stellmore cooling line, the first three insulation covers are removed, while the remaining insulation covers are closed, and all fans are turned off. At this point, the temperature at the overlap point of the insulation cover is 660℃, and the temperature at the non-overlap point is 550℃. The cooling rate inside the non-overlap point insulation cover is 1.7℃ / s. The roller speeds of the first to thirteenth sections of the Stellmore cooling system are 0.3m / s, 0.31m / s, 0.32m / s, 0.33m / s, and 0.34m / s, respectively. The speeds are 0.35 m / s, 0.36 m / s, 0.37 m / s, 0.38 m / s, 0.39 m / s, 0.67 m / s, 0.83 m / s, and 1.0 m / s; the coiling temperature at the overlap point is 480℃, and the cooling rate after the overlap point exits the insulation cover and before coiling is 1.4℃ / s; subsequently, the loose coils cooled in Stellmor are coiled and cooled sequentially to obtain wire rods; the microhardness of the M / A hard phase in the bainitic structure of the wire rod is 350 HV.

[0059] Example 2 The blank with the chemical composition described in Example 2 of Table 1 was obtained; The billet is spun into loose coils; the spun temperature is 970℃. The loose coils are then subjected to Stellmore cooling. On the Stellmore cooling line, the first four insulation covers are removed, while the remaining insulation covers are closed, and all fans are turned off. At this point, the temperature at the overlap point of the insulation cover is 650℃, and the temperature at the non-overlap point is 540℃. The cooling rate inside the non-overlap point insulation cover is 1.6℃ / s. The roller speeds of the first to thirteenth sections of the Stellmore cooling system are 0.3m / s, 0.31m / s, 0.32m / s, 0.33m / s, and 0.34m / s, respectively. The speeds are 0.35 m / s, 0.36 m / s, 0.37 m / s, 0.38 m / s, 0.39 m / s, 0.67 m / s, 0.83 m / s, and 1.0 m / s; the coiling temperature at the overlap point is 460℃, and the cooling rate after the overlap point exits the insulation cover and before coiling is 1.5℃ / s; subsequently, the loose coils cooled in Stellmor are coiled and cooled sequentially to obtain wire rods; the microhardness of the M / A hard phase in the bainitic structure of the wire rod is 340 HV.

[0060] Example 3 The blank with the chemical composition described in Example 3 of Table 1 was obtained; The billet is spun into loose coils; the spun temperature is 960℃. The loose coils are then subjected to Stellmore cooling. On the Stellmore cooling line, the first two insulation covers are removed, while the remaining insulation covers are closed, and all fans are turned off. At this point, the temperature at the overlap point of the insulation cover is 670℃, and the temperature at the non-overlap point is 530℃. The cooling rate inside the non-overlap point insulation cover is 1.8℃ / s. The roller speeds of the first to thirteenth sections of the Stellmore cooling system are 0.3m / s, 0.31m / s, 0.32m / s, 0.33m / s, and 0.34m / s, respectively. The speeds are 0.35 m / s, 0.36 m / s, 0.37 m / s, 0.38 m / s, 0.39 m / s, 0.67 m / s, 0.83 m / s, and 1.0 m / s; the coiling temperature at the overlap point is 470℃, and the cooling rate after the overlap point exits the insulation cover and before coiling is 1.6℃ / s; subsequently, the loose coils cooled in Stellmore are coiled and cooled sequentially to obtain wire rods; the microhardness of the M / A hard phase in the bainitic structure of the wire rod is 330 HV.

[0061] Example 4 The blank with the chemical composition described in Example 4 of Table 1 was obtained; The billet is spun into loose coils; the spun temperature is 980℃. The loose coils are then subjected to Stellmore cooling. On the Stellmore cooling line, the first two insulation covers are removed, while the remaining insulation covers are closed, and all fans are turned off. At this point, the temperature at the overlap point of the insulation cover is 680℃, and the temperature at the non-overlap point is 520℃. The cooling rate inside the non-overlap point insulation cover is 1.5℃ / s. The roller speeds of the first to thirteenth sections of the Stellmore cooling system are 0.3m / s, 0.31m / s, 0.32m / s, 0.33m / s, and 0.34m / s, respectively. The speeds are 0.35 m / s, 0.36 m / s, 0.37 m / s, 0.38 m / s, 0.39 m / s, 0.67 m / s, 0.83 m / s, and 1.0 m / s; the coiling temperature at the overlap point is 450℃, and the cooling rate after the overlap point exits the insulation cover and before coiling is 1.2℃ / s; subsequently, the loose coils cooled in Stellmore are coiled and cooled sequentially to obtain wire rods; the microhardness of the M / A hard phase in the bainitic structure of the wire rod is 320 HV.

[0062] Comparative Example 1 A blank with the chemical composition described in Comparative Example 1 in Table 1 was obtained; The billet is spun into loose coils; the spun temperature is 830℃. The loose coils are subjected to Stellmore cooling with the lowest roller speed on the cooling line (0.1 m / s for the first section) and all insulation covers are closed. At this point, the temperature at the overlap point exiting the insulation cover is 580°C, and the temperature at the non-overlap point exiting the insulation cover is 450°C. The cooling rate inside the non-overlap point insulation cover is 0.6°C / s, the coiling temperature at the overlap point is 450°C, and the cooling rate after exiting the insulation cover and before coiling is 1.1°C / s. Subsequently, the loose coils after Stellmore cooling are coiled and cooled sequentially to obtain wire rod. The microhardness of the M / A hard phase in the bainitic structure of the wire rod is 430 HV.

[0063] The tensile strength and in-circuit properties of the examples and comparative examples are shown in Table 2.

[0064] Table 2

[0065] The data tables above provide a clear comparison of the differences between various embodiments and comparative examples. The following conclusions can be drawn: As can be seen from the data in Table 2, the wire rod provided in this application embodiment has a tensile strength of 730MPa~750MPa at the lap joint, a tensile strength of 710MPa~720MPa at the non-lap joint, and a coil performance of 20MPa~35MPa.

[0066] As can be seen from Examples 1-4 and Comparative Example 1, Examples 1-4, by employing high-temperature spinning, segmented arithmetic incremental roller speed, and differentiated cooling control, achieved coordinated regulation of the hood exit temperature at the overlap point ≥650℃ and the hood exit temperature at the non-overlap point ≤550℃, resulting in uniform bainitic wire rods with a microhardness of 320HV~350HV for the M / A hard phase, a tensile strength of 730MPa~750MPa, and a performance difference of 20MPa~35MPa within the same coil. In contrast, Comparative Example 1, employing low-temperature spinning, a single low-speed roller conveyor of 0.1m / s, and a fully slow cooling process, caused premature phase transformation and reheating at the overlap point, generating a high-hardness M / A phase of 430HV, resulting in a severe deterioration of the tensile strength to 850MPa and a performance difference of 100MPa within the same coil. Therefore, it has been confirmed that the accelerated cooling process completely eliminates the high-hardness martensitic phase by suppressing the return temperature, providing a wire rod substrate with both low hard phase content and high uniformity for 60kg-grade welding wire, thus overcoming the technical bottleneck of wire breakage during high surface area reduction drawing.

[0067] Appendix Figures 2-5 Detailed explanation: Figure 2 A CCT curve of a 60kg-grade bainitic gas-shielded welding wire rod provided for an embodiment of this application. According to... Figure 2As can be seen from the wire rods provided in this application, the stable range of the bainitic phase transformation falls precisely within the temperature range of 550℃ to 650℃, which is highly compatible with the differentiated cooling control strategy adopted in this application. This application controls the non-overlapping points to complete the bainitic phase transformation within this temperature range inside the insulation cover, while the overlapping points are delayed until they exit the insulation cover before entering this range to complete the phase transformation. This arrangement perfectly conforms to the phase transformation kinetics of this composition system, ensuring that both regions complete the transformation within the bainitic phase transformation window. This effectively avoids the formation of high-hardness martensite from untransformed austenite from a thermodynamic perspective, providing core theoretical support for obtaining a uniform bainitic microstructure.

[0068] Figure 3 This is a curve showing the expansion of a 60kg-grade bainitic gas-shielded welding wire rod during phase transformation reheat, provided as an embodiment of this application. According to... Figure 3 It can be seen that after the reheating process ends, the phase transformation stops, and when cooled to a lower temperature, the expansion curve shows an inflection point of martensitic phase transformation. This curve characteristic confirms the intrinsic mechanism of high-hardness martensite formation induced by phase transformation reheating in traditional slow cooling processes: the reheating process promotes abnormal enrichment of carbon atoms into untransformed austenite, increases the stability of austenite, and causes the bainitic phase transformation to stop prematurely. The remaining supercooled austenite can only undergo martensitic phase transformation at a lower temperature, ultimately forming a hard phase structure that harms drawing performance. This result also verifies the necessity and rationality of the phase transformation kinetics design of this application, which suppresses phase transformation reheating and controls bainitic phase transformation in a territorial manner.

[0069] Figure 4 Microstructure diagram of a 60kg grade bainitic gas-shielded welding wire rod provided in Embodiment 1 of this application. According to... Figure 4 It can be seen that after the composition design and differential cooling process control of this application, the wire rod obtained a uniform and dense bainitic matrix structure. The large blocky high-hardness M / A hard phase that is prone to occur in the traditional process is effectively suppressed. The residual M / A phase is small in size and dispersed in the bainitic matrix. There is no large area of ​​high-hardness brittle phase aggregation. The overall structure has excellent uniformity, which fully proves that the technical solution of this application can eliminate the structural defects that harm the drawing performance at the micro level and obtain a uniform bainitic structure that meets the requirements of high reduction ratio drawing.

[0070] Figure 5 The image provided is a microstructure diagram of Comparative Example 1 of this application. According to... Figure 5It can be seen that the uniformity of the microstructure in Comparative Example 1 is far worse than that of the wire rod microstructure obtained in the embodiments of this application. A large number of large-sized blocky hard phases are visible in the field of view. The gray blocks in the figure are martensite microstructure with a microhardness of 430 HV. This large-area aggregate of high-hardness brittle martensite phase will form significant stress concentration during the drawing deformation process, becoming the core initiation factor for crack initiation. This directly verifies the conclusion that the traditional process does not control the phase transformation reheat, which will induce the generation of a large number of high-hardness hard phases, ultimately leading to the deterioration of wire rod drawing performance.

[0071] One or more technical solutions in the embodiments of the present invention have at least the following technical effects or advantages: This invention provides a novel process for 60kg-grade bainitic gas-shielded welding wire rod, employing a higher wire drawing temperature and increased roll speed to suppress phase transformation heat reversion. The uniformity of the microstructure is controlled through segmented phase transformation. On one hand, the non-lap joints are controlled to complete the bainitic phase transformation at a higher cooling rate within the insulation hood; on the other hand, the lap joints are controlled to complete the bainitic phase transformation after exiting the insulation hood and before coiling. This solution fundamentally solves the technical problem of high martensite content and excessive hardness in the hard phase microstructure caused by phase transformation heat reversion in 60kg-grade bainitic gas-shielded welding wire steel under traditional slow cooling processes, thus affecting its drawing performance. The resulting gas-shielded welding wire rod has a stable tensile strength of 700MPa~750MPa, with a performance fluctuation of ≤35MPa within the same coil. This effectively reduces the strength difference between lap and non-lap joints, and the overall performance of the rod meets the requirements for anneal-free processing during the subsequent gas-shielded welding wire drawing process.

[0072] The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed in this application.

Claims

1. A 60kg grade bainitic gas-shielded welding wire rod, characterized in that, The chemical composition of the wire rod, by mass fraction, is as follows: C: 0.06%~0.09%, Si: 0.45%~0.65%, Mn: 1.35%~1.55%, P≤0.015%, S≤0.012%, Cr: 0.30%~0.50%, Mo: 0.20%~0.35%, Ti: 0.04%~0.08%, with the remainder being Fe and unavoidable impurities; The microstructure of the wire rod is bainite.

2. The wire rod according to claim 1, characterized in that, The microhardness of the M / A hard phase in the bainitic structure is ≤350HV.

3. The wire rod according to claim 1, characterized in that, The wire rod meets the following performance requirements: tensile strength of 700MPa~750MPa, and same-coil performance ≤35MPa.

4. A method for preparing wire rod according to any one of claims 1 to 3, characterized in that, The method includes: Obtain a blank having the chemical composition described in any one of claims 1 to 3; The blank is then spun into loose coils; The loose coils are sequentially subjected to Stellmore cooling, coiling, and cooling to obtain wire rods.

5. The method according to claim 4, characterized in that, The temperature at which the silk is spun is 950℃~980℃.

6. The method according to claim 4, characterized in that, The temperature at the overlap point of the Stellmore cooling system is ≥650℃.

7. The method according to claim 4, characterized in that, The temperature of the non-overlapping point of the Stellmore cooling insulation cover is ≤550℃.

8. The method according to claim 4, characterized in that, The cooling rate inside the non-overlapping insulation cover of the Stellmore Cooling system is 1.0℃ / s to 2.0℃ / s; The cooling rate of the Stellmore cooling overlap point before winding after exiting the insulation cover and before being rolled up is 1.0℃ / s to 2.0℃ / s.

9. The method according to claim 4, characterized in that, The overlap temperature of the Stelmo cooling coil is ≤500℃.

10. The method according to claim 4, characterized in that, The segmented roller conveyor speed of the Steelmo cooling system satisfies: The roller conveyor speeds of sections 1 to 10 range from 0.30 m / s to 0.39 m / s, and increase arithmetically by 0.01 m / s. The roller conveyor speed of section 11 is 0.62m / s to 0.72m / s; The roller conveyor speed of section 12 is 0.78m / s to 0.88m / s; The roller conveyor speed of section 13 is 0.95m / s to 1.05m / s.

Images