Wire rod for 80-grade cord thread and manufacturing method thereof

Abstract

The invention relates to a wire rod for an 80-grade cord thread and a manufacturing method of the wire rod, and belongs to the field of iron-based alloys. The hot-rolled wire rod comprises the following chemical components in percentage by mass: 0.75%-0.85% of C, 0.10%-0.40% of Si, 0.30%-0.50% of Mn, less than or equal to 0.030% of P, less than or equal to 0.030% of S, 0.03%-0.15% of Cr, 0.03%-0.15% of Ni, 0.03%-0.15% of Cu, 0.01%-0.10% of Mo, less than or equal to 0.01% of N and the balance of Fe and inevitable impurities, Mn + Cr + Cu + Ni + Mo is more than 0.40% and less than 0.70%, Mn / 6 + (Cr + Mo) / 5 + (Cu + Ni) / 15 is more than 0.06% and less than 0.12%, Cr + Cu + Ni + Mo is more than 0.10% and less than 0.25% is more than 0.10% According to the wire rod, electric furnace smelting is achieved on the basis that the high-quality cord thread tensile strength and the low drawing wire breakage rate are guaranteed, the adding proportion of waste steel exceeds 70% of the total loading amount, and the production and manufacturing cost is reduced.

Description

Technical Field

[0001] The present invention relates to an iron-based special wire, in particular to a novel wire for grade 80 cord and a manufacturing method thereof. Background Art

[0002] The cord industry is vigorously promoting the use of short-process electric furnaces (EAFs) to produce cord steel, replacing the traditional long-process converter (BOF) process, thereby reducing overall carbon emissions. Currently, most cord steel on the market is produced using the BOF process, which features low levels of residual chemical elements, stable mechanical properties, and low wire breakage rates. To achieve low-carbon, green manufacturing in the cord industry, the use of electric furnaces to produce raw wire is a future development trend. Compared to EAF processes, the main reason for higher carbon emissions from BOFs is the low amount of scrap added. Therefore, to achieve green manufacturing, the production of cord using the EAF process requires a certain scrap ratio. However, due to the uncontrolled scrap composition, cord steel produced using the EAF process has a relatively high level of residual chemical elements, significantly impacting the heat treatment and mechanical properties of the cord, and failing to meet the requirements for high-quality cord steel. Therefore, there is an urgent need to improve the quality of cord steel produced using the EAF process to meet market demand.

[0003] Patent No. CN 116657062 A discloses a deep-drawing wire rod and its preparation method, belonging to the field of steel material preparation technology. This cord wire rod is smelted in an electric furnace using all-scrap steel. While this approach aims to reduce carbon emissions and production costs, the high content of alloying elements such as Cr, Ni, Cu, and B in its chemical composition significantly impacts downstream heat treatment and drawing properties.

[0004] Patent No. CN 116904863 A discloses a method for producing high-purity, high-carbon steel with low carbon emissions, belonging to the field of steelmaking technology. This high-carbon steel wire rod is produced using an electric furnace-RH process, which takes carbon emissions into account. However, the process does not pass through a refining furnace, which is not conducive to controlling chemical composition and inclusions, and cannot achieve high-quality cord steel production.

[0005] Patent No. CN 115058641 A discloses a method for smelting low-sulfur, low-aluminum, high-carbon wire rod steel in an all-scrap electric furnace, belonging to the field of metallurgy. While this invention considers controlling the S and Al content in the chemical composition under acidic slag smelting conditions, it does not consider the effects of alloying elements such as Cr, Ni, Cu, and Mo on the performance of the wire cord.

[0006] Patent No. CN 113430323 A discloses a method for controlling sulfur in the smelting of grade 70 cord steel at a high scrap ratio, pertaining to the field of iron and steel metallurgy manufacturing. This cord steel wire rod is produced using a converter process, resulting in a low scrap ratio, which is inconsistent with the industry's green and low-carbon development trends.

[0007] Patent No. CN 113403442 A discloses a method for controlling sulfur in the smelting of 80-grade cord steel at a high scrap ratio, pertaining to the field of iron and steel metallurgy manufacturing. The cord steel wire rod is produced using a converter process, resulting in a low scrap ratio, which is inconsistent with the industry's green and low-carbon development trends.

[0008] Patent No. CN 113403522 A discloses a smelting method for alloyed ultra-high-strength and ultra-high-strength high-carbon hard wire rods, belonging to the field of steelmaking technology. The cord steel wire rods are produced using a converter process, resulting in a low scrap ratio, which is inconsistent with the industry's green and low-carbon development trends. Furthermore, the addition of the microalloying element V increases production costs. Summary of the Invention

[0009] The technical problem to be solved by the present invention is to provide an 80-grade wire rod for cord and a manufacturing method thereof in response to the above-mentioned existing technology. External scrap steel is used entirely to reduce the use of molten iron and reduce carbon emissions from cord steel production. Through innovative chemical composition design, alloy elements such as Cr, Ni, Cu, and Mo are added to high-carbon steel to improve the hardenability of the steel and the tensile strength of the cord. The target value of the Mn content is lowered, and the total amount of Mn+Cr+Ni+Cu+Mo is controlled to compensate for the strength loss caused by the reduction in the Mn content, so that the cord steel wire rod has improved strength while reducing the use of manganese alloy, reducing production costs, and improving product market competitiveness.

[0010] The above technical objectives of the present invention are achieved through the following technical solutions: a new type of 80 grade cord wire and a manufacturing method thereof, the chemical composition of the wire is as follows by mass percentage: C: 0.75%-0.85%, Si: 0.10%-0.40%, Mn: 0.30%-0.50%, P≤0.030%, S≤0.030%, Cr: 0.03%-0.15%, Ni: 0.03%-0.15%, Cu: 0.03%-0.15%, Mo: 0.01%-0.10%, N≤0.01%, and the rest is Fe and unavoidable impurities.

[0011] The chemical element action mechanism of the cord steel wire material of the present invention is as follows: C is the main strengthening element in steel. It can significantly improve the strength of steel through solid solution strengthening and precipitation strengthening. With the increase of C content, the strength shows a clear upward trend. At the same time, the wire is significantly hardened after cold drawing, which helps to achieve the strength of the finished steel wire. With the increase of C content, the proportion of sorbite in the wire structure obtained by Stelmor cooling is high. However, with the increase of C content in hypereutectoid steel, the hardenability decreases. Therefore, the C content is controlled at 0.75%-0.85%.

[0012] Si is added to steel as a deoxidizer, reacting with FeO in the molten steel to form silicate, which is removed and improves the purity of the molten steel. In addition, silicate inclusions in the wire are easily extended and deformed along the drawing direction, and will not cause the hazard of wire breakage during drawing. The solid solution strengthening of Si element increases the work hardening rate of steel, which can significantly deteriorate the cold working performance of steel, and at the same time promotes the grain boundary segregation of elements P and S. Therefore, the weight percentage of Si is 0.10%-0.40%.

[0013] Mn is added to steel as a strong deoxidizer. Since Mn can form low-melting-point MnS with S, it can eliminate the harmful effects of S to a certain extent. Mn can improve the strength and hardenability of steel, but as the Mn content increases, it is prone to obvious temper brittleness. Mn also promotes grain growth, so the weight percentage of Mn is 0.30%-0.50%.

[0014] The primary function of Cr is to increase steel strength. Cr and Fe form a continuous solid solution, reducing the austenite phase. Cr reduces the carbon concentration in pearlite and the limiting solubility of carbon in austenite, slowing the decomposition of austenite and significantly improving the hardenability of steel. However, excessive Cr content can easily form dendritic segregation, reducing the steel's plasticity, hindering Stelmor cooling control, and easily forming low-temperature phase transformation structures, hindering subsequent drawing processes. Therefore, the Cr content should be limited to 0.03-0.15% by weight.

[0015] The role of the nickel element is to improve the strength and toughness of steel and reduce its brittleness. Ni and Fe can be infinitely dissolved in solid without forming carbides. Ni expands the austenite region of Fe, lowers the critical transition temperature, reduces the diffusion rate of various elements in the steel, and improves hardenability. At the same time, it reduces the carbon content of eutectoid pearlite. Ni can promote the phase transformation of iron and steel, making its grains finer, making the steel have better plasticity and toughness, strengthening ferrite and refining and increasing pearlite, improving the strength of steel, and having little effect on the plasticity of steel. However, due to the scarcity of nickel and its status as an important strategic material, the production cost is extremely high. Therefore, the weight percentage of Ni is 0.03-0.15%.

[0016] The effect of Cu element is mainly to improve the strength of steel, because Cu can improve the cold working hardening ability of steel, promote grain boundary retardation and dispersed precipitation phase formation, make the tensile strength of steel significantly improve, while maintaining the good plasticity and toughness of steel. In addition, Cu has a certain solid solution effect on the grain boundaries and precipitation phases of steel, can hinder grain boundary migration and phase transformation process, refine the grains and phase interface of steel, and improve the microstructure and mechanical properties of steel. In addition, Cu can also affect the phase transformation temperature and phase transformation kinetics of steel, making the thermal treatment process of steel more controllable and optimized. However, Cu content is too high, easily increases production cost, is unfavorable for cooling process adjustment, and therefore the weight percentage of Cu is 0.03-0.15%.

[0017] The primary function of the element Mo is to improve hardenability and thermal strength. Molybdenum significantly enhances the hardenability of steel, imparting high hardness and strength at room temperature, and significantly increases its high-temperature strength. Molybdenum has a solid-solution strengthening effect on ferrite and also improves the stability of carbides, thereby increasing the strength of the steel. Molybdenum refines the grain size of steel, improving hardenability and thermal strength. However, excessive Mo content can easily lead to supercooling and the formation of low-temperature phase transformation structures. Therefore, the Mo content should be between 0.01% and 0.10% by weight.

[0018] Phosphorus (P) is a harmful element in cord steel, significantly reducing the steel's plasticity and toughness, deteriorating its performance. Phosphorus also tends to segregate within the crystal during crystallization, leading to localized high phosphorus concentrations and an increase in the cold-brittle transition temperature, resulting in cold brittleness. To prevent a decrease in the material's plasticity and toughness, this product is designed with a P content of ≤0.030%.

[0019] Sulfur is also a detrimental element in steel cord, reducing its ductility and toughness. In its solid state, sulfur has extremely low solubility in iron and exists in the steel as FeS. Due to the poor plasticity of FeS, steels containing high levels of sulfur are more brittle. More seriously, FeS and Fe form a eutectic with a low melting point (985°C) that is distributed along the grain boundaries of austenite. When steel is heated to approximately 1200°C for hot pressing, the eutectic at the grain boundaries melts, disrupting the bonds between the grains and causing cracking along the grain boundaries during processing, a phenomenon known as hot brittleness. Furthermore, sulfide-like non-metallic inclusions formed by sulfur segregation can form a fibrous structure during rolling, reducing the mechanical properties of high-strength steel products. Therefore, the sulfur content must be strictly controlled to ≤ 0.030%.

[0020] The above-mentioned 80-grade cord wire with a scrap steel ratio of ≥70% and the manufacturing method thereof, the steps are as follows: (1) Prepare materials according to the designed composition and adopt electric furnace smelting. Scrap steel is classified and managed in advance. The proportion of scrap steel added exceeds 70% of the total charge. Strictly control the tapping temperature to be greater than 1600℃ and the end point C% to be greater than 0.05% to prevent the molten steel from being over-oxidized. Then transfer to the refining furnace for refining. Low basicity synthetic slag is used for refining to ensure the plasticization of inclusions. At the same time, it is precisely controlled according to the target composition. After all the components meet the internal control requirements, transfer to VD for soft argon blowing to ensure the floating of inclusions and the uniformity of molten steel composition.

[0021] (2) Continuous casting adopts low superheat ≤ 30℃ casting, casting speed 0.5m / min-1.0m / min, weak cooling of secondary cooling water, rectangular billets with specifications of 300mm*340mm and above, secondary cooling water 0.10-0.25L / Kg, to prevent corner cracks in the billet, light pressing adopts displacement mode, and each roller presses down according to the set value, and makes appropriate adjustments based on the pressure difference between each flow. The system monitors the thickness of the continuous casting billet in real time, and dynamically adjusts the pressure between each roller according to the thickness of the continuous casting billet. In combination with the end electromagnetic stirring technology, it ensures that the continuous casting billet has no obvious pressing cracks, central shrinkage cavity and V-shaped segregation, and ensures the uniformity of the structure and composition of the continuous casting billet.

[0022] (3) The surface of the continuous casting billet is ground, and then the billet is heated to above 1200℃, kept warm for more than 3 hours, and then taken out of the furnace and cut into 200mm*200mm intermediate billets.

[0023] (4) The billet is surface treated to ensure that the surface is free of defects, and then heated to above 1100℃, kept warm for more than 1 hour, and then taken out of the furnace. After descaling with high-pressure water, it is rolled: the starting rolling temperature is 950-1000℃, the finishing rolling temperature is ≥930℃, and the module temperature is ≥900℃. The rolling process is divided into 39 rolling passes, and the rolling speed is set at 100m / s-108m / s. Finally, it is rolled into φ5.5mm-φ6.5mm wire.

[0024] (5) The spinning temperature is 850-900℃.

[0025] (6) Stelmor cooling line air cooling, the air cooling process realizes the transformation of troostite, and the fan opening degree is adjusted online so that the cooling rate between the spinning temperature and 600℃ is greater than 25℃ / s. Due to the addition of Cr, Ni, Cu, and Mo alloying elements, the cooling rate of the wire rod is greatly improved, thereby effectively suppressing the precipitation of network carbides. When the intermediate point is cooled to about 600℃, slow cooling begins to ensure that the troostite is fully transformed. At the same time, it is necessary to avoid the formation of low-temperature phase transformation structures such as troostite and bainite. Adjust the roller speed on the Stelmor cooling line to 0.85-0.95m / s, the ambient temperature to 10-20℃, and set the opening degree of fans 1-7: No. 1: 70-90%, No. 2: 70-90%, No. 3: 70-90%, No. 4: 50-70%, No. 5: 30-50%, No. 6: 30-50%, No. 7: 10-30%.

[0026] Compared with the existing technology, the advantages of the present invention are: based on the innovative chemical composition design and green low-carbon electric furnace process, the obtained φ5.5mm hot-rolled wire rod meets the tensile strength of 1000-1200MPa and the area reduction is more than 35%, so that the wire can reduce the production cost while ensuring high-quality cord tensile strength and low drawing breakage rate. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 Schematic diagram of the macrostructure of the electric furnace cord steel wire of the present invention; Figure 2 Schematic diagram of the troostite of the electric furnace cord steel wire according to the present invention. DETAILED DESCRIPTION

[0028] The present invention is further described in detail below with reference to the examples. The examples are illustrative and intended to explain the present invention, but are not to be construed as limiting the present invention.

[0029] A 100t electric furnace is used for smelting, 80 tons of scrap steel is added, the tapping temperature is controlled at 1620℃, and the end point C% is ≥ 0.08%. Then the steel is transferred to a refining furnace for refining. Low-basicity synthetic slag is used for refining to ensure sufficient deoxidation and removal of inclusions. At the same time, the target composition is precisely controlled. After refining, VD soft argon blowing is used to further remove inclusions and ensure the uniformity of the molten steel. The molten steel is then transferred to continuous casting with a casting speed of 0.7m / min, secondary cooling water of 0.25L / Kg, a total reduction of 20mm under light pressure, and end-stage electromagnetic stirring of 1.5HZ / 300A. The steel is then cast into continuous casting billets with a cross-sectional size of 300mm*340mm.

[0030] The continuous casting billet is surface treated to ensure that surface defects are completely removed. The billet is heated to above 1200℃, the soaking period is more than 3 hours, and the billet is opened into 200 square intermediate billets. The intermediate billet is then heated to above 1100℃, the residual oxygen content in the furnace is controlled below 4%, and it is kept warm for 1 hour before being taken out of the furnace. After descaling with high-pressure water, it is rolled: the starting rolling temperature is 1080℃, the finishing rolling temperature is 930℃, the module temperature is 900℃, and the rolling process is divided into 39 rolling passes. The rolling speed is set to 100m / s, and the wire drawing temperature is controlled at 900℃. After rolling, the wire rod is air-cooled and slowly cooled on the Stelmor cooling line. In order to achieve a balanced cooling rate on the cooling roller, the air volume of fans 1-7 on the cooling line needs to be adjusted according to the seasonal ambient temperature. The initial roller speed is 0.8 5m / s, increasing in increments of 0.03m / s, ambient temperature 15°C, set fan opening degrees 1-7: fan 1: 80%, fan 2: 80%, fan 3: 80%; fan 4: 60%, fan 5: 40%, fan 6: 40%, fan 6: 20%. Ensure that the cooling rate is greater than 25°C / s between the spinning temperature of 900°C and 600°C to suppress the precipitation of network cementite. After fan 4, the wire rod is slowly cooled to ensure that the sorbite transformation time is long enough to avoid the formation of low-temperature phase transformation structure. After the wire rod passes through the above-mentioned air-cooling roller, a uniform ferrite + sorbite structure is formed. After the wire rod is off the line, the samples are aged and tested for mechanical properties and metallographic structure.

[0031] The chemical composition of the embodiment is shown in Table 1.

[0032] Table 1

[0033] The test results of hot rolled wire rod are as follows: Figure 2 .

[0034] Table 2

Claims

1. A wire material for grade 80 cord, characterized by: The chemical composition by mass percentage is C: 0.75%-0.85%, Si: 0.10%-0.40%, Mn: 0.30%-0.50%, P≤0.030%, S≤0.030%, Cr: 0.03%-0.15%, Ni: 0.03%-0.15%, Cu: 0.03%-0.15%, Mo: 0.01%-0.10%, N≤0.01%, and the rest are Fe and unavoidable impurities.

2. The grade 80 cord wire according to claim 1, characterized in that: The contents of Mn, Cr, Cu, Ni and Mo satisfy the following requirements: 0.40%<Mn+Cr+Cu+Ni+Mo<0.70%.

3. The grade 80 cord wire according to claim 1, characterized in that: The contents of Mn, Cr, Mo, Cu and Ni satisfy the following requirements: 0.06%<Mn / 6+(Cr+Mo) / 5+(Cu+Ni) / 15<0.12%.

4. The grade 80 cord wire according to claim 1, characterized in that: The content of Cr, Cu, Ni and Mo is: 0.10%<Cr+Cu+Ni+Mo<0.25%.

5. The grade 80 cord wire according to claim 1, characterized in that: The tensile strength of φ5.5~φ6.5mm hot-rolled wire rod is 1000-1200MPa, and the area reduction is ≥35%.

6. A method for manufacturing the grade 80 cord wire according to claim 1, characterized in that: include, Molten steel smelting: Prepare materials according to the designed composition, with scrap steel added in an amount exceeding 70% of the total charge. Smelt in a converter, control the tapping temperature to be >1600°C and the endpoint C% to be >0.05% to prevent overoxidation of the molten steel. Then transfer to a refining furnace for refining. Use low-basicity synthetic slag to ensure plasticization of inclusions. Adjust the composition precisely according to the target composition. Once all compositions meet internal control requirements, transfer to a VD furnace for soft argon blowing to ensure inclusions float and uniformity of the molten steel composition. Continuous casting: Use large square billets with a cross-section of 300mm*340mm or above to produce billets; Billeting: The continuous casting billet is heated to a temperature exceeding 1200°C after high-temperature diffusion and kept warm for more than 3 hours before being taken out of the furnace and billeted into an intermediate billet of 200mm*200mm; Rolling: The billet is heated to above 1100℃, kept warm for more than 1 hour and then taken out of the furnace. After descaling with high-pressure water, rolling is carried out: the starting rolling temperature is 950-1000℃, the finishing rolling temperature is ≥930℃, the die entering temperature is ≥900℃, the rolling process is divided into 39 rolling passes, the rolling speed is set at 100m / s-108m / s, and the final rolled wire is φ5.5mm-φ6.5mm; Silk spinning: The spinning temperature is 850-900℃; Cooling: Stelmor cooling line is used for air cooling. The air cooling process realizes the transformation of sorbite. The fan opening degree is adjusted online so that the cooling rate is ≥25℃ / s between the spinning temperature and 600℃. Due to the addition of Cr, Ni, Cu and Mo alloy elements, slow cooling begins when the temperature of the middle point of the wire rod coil cools to 600℃.

7. The method according to claim 6, characterized in that: Continuous casting adopts low superheat pouring ≤30℃, casting speed 0.5m / min-1.0m / min, weak cooling of secondary cooling water, and rectangular billets with specifications of 300mm*340mm or above. Secondary cooling water is 0.10-0.25L / Kg to prevent corner cracks in the billet. Light pressing adopts displacement mode. Each roller of light pressing is pressed down according to the set value, and appropriate adjustments are made based on the pressure difference between each flow. The system monitors the thickness of the continuous casting billet in real time and dynamically adjusts the pressure between each roller according to the thickness of the continuous casting billet.

8. The method according to claim 6, wherein: During Stelmor cooling, adjust the roller speed on the Stelmor cooling line to 0.85-0.95m / s, the ambient temperature to 10-20℃, and set the opening degree of fans 1-7: No. 1: 70-90%, No. 2: 70-90%, No. 3: 70-90%, No. 4: 50-70%, No. 5: 30-50%, No. 6: 30-50%, No. 7: 10-30%.

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