Mn15 steel wire rod for welding and preparation method thereof

Abstract

The Mn15 steel wire rod comprises the following chemical components in percentage by mass: less than or equal to 0.60% of C, less than or equal to 0.15% of Si, less than or equal to 15% of Mn, less than or equal to 0.010% of P, less than or equal to 0.015% of S, less than or equal to 5.0% of Cr, less than or equal to 10.0% of Ni, less than or equal to 0.10% of Cu, less than or equal to 2.0% of Mo, less than or equal to 0.02% of V, less than or equal to 0.005% of As and the balance of Fe and inevitable impurities. The method has the advantages that the low-cost nickel-saving type is realized, the method is suitable for preparing a steel raw material for welding the 5Ni steel welding wire, normal production and use of the welding wire are ensured, inclusion, segregation and quenching structures are effectively avoided, and the method has excellent low-temperature toughness.

Description

Technical Field

[0001] The present invention belongs to the technical field of welding materials, and particularly relates to a Mn15 steel wire rod for welding and a preparation method thereof. Background Art

[0002] Due to the excellent low-temperature performance of 5Ni steel at -120°C, it is commonly used in the construction of semi-cooled and semi-pressurized liquid tanks of LEG ships, type B cabins of VLEC ships, etc. The design temperature is -104°C for loading strategic resources such as propylene, propane, and ethylene. At present, there is no dedicated submerged arc welding material for 5Ni steel on the market. In actual production, users still choose 9Ni steel welding materials, but its nickel content exceeds 60%, and it is designed for use at -196°C. In recent years, the nickel price has skyrocketed, resulting in a substantial increase in the construction cost of user products. Summary of the Invention

[0003] The purpose of the present invention is to solve the problem of the lack of supporting submerged arc welding materials for existing 5Ni steel, and to provide a Mn15 steel wire rod for welding, realizing low-cost nickel-saving type, suitable for preparing steel raw materials for welding 5Ni steel wire, ensuring the normal production and use of the wire, effectively avoiding the existence of inclusions, segregation and quenched structure, and having excellent low-temperature toughness.

[0004] In order to achieve the above purpose, the present invention adopts the following technical solutions: A Mn15 steel wire rod for welding, its chemical composition and mass percentage include: C≤0.60%, Si≤0.15%, Mn≤15%, P≤0.010%, S≤0.015%, Cr≤5.0%, Ni≤10.0%, Cu≤0.10%, Mo≤2.0%, V≤0.02%, As≤0.005%, and the balance is Fe and unavoidable impurities.

[0005] Furthermore, the metallographic structure of the wire rod is a single austenite structure.

[0006] The functions and mechanisms of each component in the present invention are as follows: The C element, Mn element, and Ni element in the present invention are all austenite-forming elements, and they act together in the wire rod rolling production, with the austenite phase as the initial solidification phase and remaining until room temperature to form a wire rod with an austenite structure. Refer to the Griffith and Wright stability factor relationship or the modified Post and Eberly formula to calculate the austenite stability coefficient. A negative number indicates a strong tendency for austenite to transform into martensite during severe strain. In the case of multiple component systems, the larger the positive value, the more stable the austenite. Design the C, Mn, and Ni contents under the condition that the formula is greater than zero.

[0007] C is one of the alloying elements with the strongest strengthening effect. With the increase of C content, higher tensile strength and hardness can be obtained, but the plasticity and toughness will be significantly reduced, deteriorating the weldability of subsequent welding materials. Considering comprehensively, C is controlled to be ≤0.60%.

[0008] Mn is a very strong austenite stabilizing element and is inexpensive. By adding Mn element to replace Ni element for the improvement of toughness. Since it contains 0.5%C and considering the strength matching of 5Ni steel, the standard tensile strength of 5Ni steel ≥530MPa, the content of Mn element cannot be too high. Therefore, Mn is controlled to be ≤15%.

[0009] When the content of Cr element is too high, it will sharply reduce the low-temperature toughness of the weld and the fluidity of the molten pool after welding, which is not conducive to the exclusion of gas and inclusions in the weld and affects the metallurgical quality of the weld. At the same time, the addition of Cr element can significantly improve the weld strength. Considering comprehensively, Cr is controlled to be ≤5.0%.

[0010] Ni element can refine the ferrite grains and improve the low-temperature impact toughness of the steel. In this patent, by increasing the content of Mn element to replace Ni element for the improvement of toughness, and the actual use requirement of 5Ni steel is -120°C low-temperature impact, which is more relaxed than the requirement of 9Ni steel. Therefore, the content of Ni element in the welding material can be greatly reduced. Considering comprehensively, Ni is controlled to be ≤10%.

[0011] Mo element can expand the bainite region, increase the content of acicular ferrite in the weld, and improve the strength and toughness of the weld. Therefore, Mo is controlled to be ≤2.0%.

[0012] Cu element will reduce the ductility and toughness of the weld metal, but it can improve the strength and hardness of the weld metal. When Cu atoms dissolve into the ferrite lattice, it will cause lattice distortion, thereby improving the strength and hardness of the steel. Therefore, Cu is controlled to be ≤0.10%.

[0013] S and P: S and Fe will form eutectics with low melting points, resulting in hot brittleness, and at the same time reducing the plasticity and impact toughness of the weld and deteriorating the atmospheric corrosion resistance. The segregation effect of P is very strong. When the content of P is too high, it is extremely easy to cause hot cracking. Phosphides are hard and brittle themselves, which is easy to cause cold brittleness of the steel and reduce the plasticity and toughness of the steel. Therefore, P is controlled to be ≤0.010% and S is controlled to be ≤0.015%.

[0014] In this solution, the content ratios of Si, Mn, Cr, Ni, Cu, Mo and micro-alloying elements are added, and As is controlled to be ≤0.005%. Through the precise range of composition, the action mechanism of impurity elements, the structure and properties of the deposited metal, and the strengthening mechanism of the deposited metal, the properties of the weld metal are optimized to the best, and the ratio of Ni is greatly reduced, filling the gap in the field of submerged arc welding materials for 5Ni steel, with obvious Ni-saving effect and good economic benefits.

[0015] To further achieve the object of the present invention, a preparation method of Mn15 steel wire rod for welding is also provided. Its smelting method is electric furnace + LF furnace + VD furnace + ingot casting + blooming + wire rod rolling, specifically as follows: The electric furnace is charged with scrap steel and alloy auxiliary materials. After melting into molten iron, samples are taken to analyze the alloy element content and the target value is adjusted. The LF furnace is used for composition adjustment and desulfurization treatment. After vacuum degassing, it is poured into an ingot casting, and the ingot is bloomed, and the surface of the billet is completely peeled.

[0016] Furthermore, a walking beam reheating furnace is used for wire rod rolling. The holding time is ≥4h - 4.5h, the starting rolling temperature is 1100 - 1140°C, the final rolling temperature is ≥900°C, the spinning temperature is ≥900°C, and it is rapidly cooled to below 600°C to obtain the wire rod.

[0017] Furthermore, when heating the billet in the walking beam reheating furnace, the heating time t = CB, where t is the heating time in hours, B is the billet thickness in cm, and C is a coefficient. C takes 0.1 - 0.15 for low carbon steel, 0.15 - 0.20 for medium carbon steel, 0.15 - 0.20 for low alloy steel, 0.20 - 0.30 for high carbon steel, and 0.30 - 0.40 for high alloy steel.

[0018] Furthermore, when blooming, the ingot is bloomed into a square billet with a specification of 150*150mm.

[0019] Furthermore, when rolling the wire rod, a wire rod with a specification of φ5.5mm is finally obtained.

[0020] Furthermore, the tensile strength of the wire rod is ≤850MPa, and the reduction of area is ≥50%.

[0021] Furthermore, the wire rod is drawn into a welding wire or electrode. The -120°C KV2 impact energy of its weld metal is above 100J, and the strength matches that of 5Ni steel, which can meet the technical requirements for the strength and low temperature toughness of the welded parts applicable to 5Ni steel.

[0022] Compared with the prior art, the advantages of the technical solution of the present invention are specifically as follows: (1) The Mn15 steel wire rod for welding provided by the present invention is used to produce 5Ni steel submerged arc welding electrodes, taking the lead in realizing industrial production, and has good development and application prospects. The tensile strength of the wire rod is ≤850MPa, the reduction of area is ≥50%, and the metallographic structure of the wire rod is a single austenite structure, which does not affect the production and use of the welding wire and prevents the existence of inclusions, segregation and quenched structure; (2) The Mn15 steel wire rod for welding provided by the present invention adopts a low-cost alloy composition system, enabling the material to have an austenite structure and good performance, with obvious Ni-saving effect and good economic benefits; (3)The Mn15 steel wire rod for welding prepared by the present invention has a reasonable component ratio. After being processed into welding electrodes or welding wires, the molten pool has good fluidity during the welding process, and the weld has excellent metallurgical quality. The -120°C KV2 impact energy of the weld metal is above 100 J, and its strength matches that of 5Ni steel, which can meet the technical requirements for the strength and low-temperature toughness of the welded parts applicable to 5Ni steel. (4)The production process of the Mn15 steel wire rod for welding of the present invention is solidified, and it is easy to realize processes such as smelting and rolling wire rods. With a reasonable component design, billet heating system, and rolling system, large-scale industrial promotion can be achieved. Description of the Drawings

[0023] Figure 1 It is the metallographic structure diagram of the Mn15 steel wire rod for welding in the embodiment of the present invention; Figure 2 It is the thermal conductivity calculation diagram of the Mn15 steel wire rod in the embodiment of the present invention; Figure 3 It is the thermal conductivity calculation diagram of the comparative Q345R steel in the embodiment of the present invention; Figure 4 It is the thermal conductivity calculation diagram of the comparative Mn25 steel in the embodiment of the present invention. Detailed Embodiments Embodiment

[0024] To make the present invention clearer and more understandable, the following further describes a Mn15 steel wire rod for welding and its preparation method of the present invention in conjunction with the drawings. The specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0025] A high-manganese steel wire rod for welding, whose chemical components and mass percentages include: C ≤ 0.60%, Si ≤ 0.15%, Mn ≤ 15%, P ≤ 0.010%, S ≤ 0.015%, Cr ≤ 5.0%, Ni ≤ 10.0%, Cu ≤ 0.10%, Mo ≤ 2.0%, V ≤ 0.02%, As ≤ 0.005%, and the balance is Fe and unavoidable impurities.

[0026] The smelting method is electric furnace + LF furnace + VD furnace + ingot casting + blooming 150*150mm square billet + rolling wire rod.

[0027] The electric furnace is charged with scrap steel and alloy auxiliary materials. After melting into molten iron, samples are taken to analyze the alloy element content and the target value is adjusted. The LF furnace is used for component adjustment and desulfurization treatment, and vacuum degassing is adopted. Finally, it is poured into an ingot casting, and the ingot is bloomed into a 150x150mm square billet, and the surface of the billet is fully peeled.

[0028] Rolled wire rod: Walking beam reheating furnace, holding time ≥ 4h - 4.5h, starting rolling temperature 1100 - 1140 °C, finishing rolling temperature ≥ 900 °C, wire laying temperature ≥ 900 °C, rapidly cooled to below 600 °C to obtain wire rod with a diameter of φ5.5mm. The metallographic structure of the wire rod is a single austenite structure, without inclusions, segregation, and quenched structure that affect the production and use of welding wires.

[0029] The wire rod prepared by the method of this example has excellent mechanical properties, with a tensile strength Rm of 800 MPa and an area reduction Z of 55%. See Figure 1 and Figure 2 , which are the metallographic structure diagram and thermal conductivity calculation diagram of the Mn15 steel wire rod for welding in the example. Two comparative examples are also provided in this example, as shown in Figure 3 and Figure 4 .

[0030] In this example, a walking beam reheating furnace is used for rolling the wire rod, with a holding time ≥ 4h - 4.5h, starting rolling temperature 1100 - 1140 °C, finishing rolling temperature ≥ 900 °C, wire laying temperature ≥ 900 °C, and rapidly cooled to below 600 °C to obtain the wire rod. The mechanism is as follows: The carbides in Mn15 steel generally precipitate at the grain boundaries first because the carbon content at the austenite grain boundaries is relatively high and there are more defects, which facilitates the diffusion process and is conducive to the precipitation of carbides.

[0031] When the temperature is 400 - 450 °C, the austenite can only precipitate needle-shaped carbides with a certain directionality along some crystal planes.

[0032] When the temperature is 450 - 500 °C, the carbon element is fully diffused and begins to aggregate into massive carbides. Carbon depletion occurs in the austenite around the carbides, which reduces the stability of the austenite, resulting in the eutectoid decomposition of the austenite in the carbon-depleted regions.

[0033] When the temperature is 570 - 600 °C, the eutectoid decomposition of austenite is the fastest, which is the result of the combined action of the free energy difference between the old and new phases and the diffusion rate of carbon atoms. The smaller the supercooling degree of the austenite in high manganese steel, the smaller the free energy difference, and the more difficult it is for carbides to precipitate. Therefore, the amount of carbides precipitated from austenite will decrease when the temperature is 600 - 650 °C.

[0034] When the temperature is 800 - 850 °C, almost no carbides precipitate in the austenite matrix. Therefore, when designing the rolling process, it is necessary to ensure that the finishing rolling temperature is above the carbide precipitation temperature to avoid the precipitation of carbides.

[0035] In this embodiment, the heating coefficient C is taken as 0.1 - 0.15 for low-carbon steel, 0.15 - 0.20 for medium-carbon steel, 0.15 - 0.20 for low-alloy steel, 0.20 - 0.30 for high-carbon steel, and 0.30 - 0.40 for high-alloy steel. Different steel grades have different heating temperatures. The lower limit of the heating temperature should ensure the finish rolling temperature so that the steel is in the single-phase austenite region with relatively large plasticity. The most suitable temperature range for carbon steel is the single-phase austenite region. For hypoeutectoid steel, the heating temperature range is between 30 - 50 °C above AC3 and 100 - 150 °C below the solidus line. The maximum heating temperature of hypereutectoid steel is 50 - 100 °C lower than the solidus line. See Figures 2 to 4 , considering the characteristics of Mn15 steel, its thermal conductivity is lower than that of ordinary Q345R steel and low-temperature austenitic high-manganese steel Mn25. It is necessary to ensure a sufficient heating time to achieve through burning of the billet to obtain the best plastic rolling range.

[0036] The tensile strength of the wire rod prepared in this embodiment is ≤850 MPa, and the reduction of area is ≥50%. A low-cost alloy composition system is adopted to make the material have austenite structure and good performance, with obvious Ni-saving effect. The welding wire drawn from the wire rod prepared in this embodiment has good fluidity of the molten pool during the welding process, and the weld has excellent metallurgical quality. The -120 °C KV2 impact energy of its weld metal is above 100 J, with excellent low-temperature toughness, and the strength matches that of 5Ni steel, which can meet the technical requirements for the strength and low-temperature toughness of the welded joints applicable to 5Ni steel.

[0037] In addition to the above embodiments, the present invention may have other implementation manners. All technical solutions formed by equivalent replacement or equivalent transformation fall within the protection scope required by the present invention.

Claims

1. A Mn15 steel wire rod for welding, characterized in that: The chemical composition and mass percentage of the wire rod include: C ≤ 0.60%, Si ≤ 0.15%, Mn ≤ 15%, P ≤ 0.010%, S ≤ 0.015%, Cr ≤ 5.0%, Ni ≤ 10.0%, Cu ≤ 0.10%, Mo ≤ 2.0%, V ≤ 0.02%, As ≤ 0.005%, and the balance is Fe and unavoidable impurities.

2. The Mn15 steel wire rod for welding according to claim 1, characterized in that: The metallographic structure of the wire rod is a single austenite structure.

3. A preparation method of the Mn15 steel wire rod for welding as claimed in claim 1 or 2, the smelting method is electric furnace + LF furnace + VD furnace + ingot casting + blooming + wire rod rolling, characterized in that: The electric furnace is charged with scrap steel and alloy auxiliary materials. After melting into molten iron, the alloy element content is sampled and analyzed and the target value is adjusted. The LF furnace is used for composition adjustment and desulfurization treatment. After vacuum degassing, it is poured into an ingot casting. The ingot is bloomed, and the surface of the steel billet is completely peeled.

4. The preparation method of the Mn15 steel wire rod for welding according to claim 3, characterized in that: When rolling the wire rod, a walking beam reheating furnace is adopted, the holding time is ≥ 4h - 4.5h, the starting rolling temperature is 1100 - 1140 °C, the final rolling temperature is ≥ 900 °C, the laying head temperature is ≥ 900 °C, and it is rapidly cooled to below 600 °C to obtain the wire rod.

5. The preparation method of the Mn15 steel wire rod for welding according to claim 3, characterized in that: When heating the steel billet in the walking beam reheating furnace, the heating time t = CB, where t is the heating time, the unit is h, B is the thickness of the steel billet, the unit is cm, C is a coefficient, C takes 0.1 - 0.15 for low carbon steel, 0.15 - 0.20 for medium carbon steel, 0.15 - 0.20 for low alloy steel, 0.20 - 0.30 for high carbon steel, and 0.30 - 0.40 for high alloy steel.

6. The preparation method of the Mn15 steel wire rod for welding according to claim 3, characterized in that: During blooming, the ingot is bloomed into a square billet with a specification of 150 * 150 mm.

7. The preparation method of the Mn15 steel wire rod for welding according to claim 3, characterized in that: When rolling the wire rod, a wire rod with a specification of φ5.5 mm is finally obtained.

8. The preparation method of the Mn15 steel wire rod for welding according to claim 7, characterized in that: The tensile strength of the wire rod is ≤ 850 MPa, and the reduction of area is ≥ 50%.

9. The preparation method of the Mn15 steel wire rod for welding according to claim 7, characterized in that: The wire rod is drawn into a welding wire or electrode, and the weld metal -120 °C KV2 impact energy is ≥ 100 J.

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