Cord steel wire rod suitable for deep drawing and its manufacturing method

Abstract

The present invention relates to a cord steel wire rod suitable for deep drawing and its manufacturing method. The wire rod has a metallographic structure of sorbite, pearlite, and ferrite, with the sorbite ratio controlled to 60-70%, the pearlite ratio 30-40%, and a small amount of ferrite present, with a ratio of ≦10%. The pearlite lamellae have a thickness of 0.10-0.35 mm, and the pearlite clusters have a size of 10-20 μm. This pearlite characteristic corresponds to a lower dislocation density. The central carbon segregation level, as evaluated according to YB / T 4413 "Method for evaluating the metal phase of central carbon segregation in high-carbon steel wire rods," is ≦Level 1, and the reticulated cementite level in the structure is ≦Level 1. The tensile strength σ is (103762*Ceq-114606*Ceq) MPa, and the areal shrinkage is ≧38%, making it suitable for deep drawing.

Description

[Technical Field]

[0001] The present invention relates to the field of metallurgy, and more particularly to a cord steel wire rod and a method for manufacturing the same. [Background technology]

[0002] With the rapid development of lightweight automobiles and the rapid rise of new energy vehicles, the strength requirements for steel cords for automobile tire carcasses are gradually increasing. The strength level of steel cords has also gradually evolved from the original normal strength of the standard tension (NT) level to the current ultra-high strength of the super tension (ST) level and the ultra-ultra-high strength of the ultra tension (UT) level.

[0003] As the strength of steel wire increases, the diameter after drawing becomes smaller and the structure becomes more complex. Therefore, for cord steel wire, rational control of inclusions in the steel, mainly MnS and deformable inclusions such as the 40%SiO2+15%Al2O3+20%CaO composite inclusion, contributes to improving the drawability of the wire. The proportion of sorbite in the wire structure also affects the drawability of the wire. Generally, isothermal transformation occurs when the wire is cooled to a temperature range of 600–650°C, and the transformed structure is sorbite. For high-carbon steel wire, the sorbite proportion is generally 85% or higher. If the sorbite proportion is too high and the pearlite lamellae become finer, the tensile strength of the wire becomes excessively high, and dislocation density and drawing hardening increase, making it unsuitable for deep drawing. If the sorbite proportion is too low, the wire strength decreases, plasticity deteriorates, and the wire's drawing deformation capacity decreases, making it unsuitable for deep drawing. Therefore, unless the ratio of sorbite in the wire rod and the tensile strength are controlled within appropriate ranges, it will be impossible to satisfy the requirements for deep drawing processability and low breakage rate of the wire rod and to further promote the development of high-strength, precision steel cords.

[0004] By solving the above technical problems, it will be possible to realize the widespread production of steel cords and steel wire cutting wire rods with a UT level of 4000 MPa or more. Summary of the Invention

[0005] An object of the present invention is to provide a cord steel wire rod suitable for deep drawing, in which the ratio of sorbite in the wire rod structure is reduced. The metallographic structure of the wire rod is sorbite + pearlite + ferrite, in which the ratio of sorbite is controlled to 60 to 70%, the ratio of pearlite is 30 to 40%, and a small amount of ferrite is present, the ratio of which is ≦10%. The ratio of each structure in the wire rod refers to the area occupied by each structure in a metallographic diagram of the wire rod (e.g., an enlarged SEM image).

[0006] Furthermore, the present invention intends to obtain a pearlite structure with thicker lamellae, and the thickness of the pearlite lamella is 0.10 to 0.35. μ m, perlite block The size of these dislocations is 10 to 20 μm, and the reduction in dislocation density in the metal structure prevents the tensile strength from becoming too high and suppresses drawing hardening.

[0007] Furthermore, the present application is intended to control the central carbon segregation level of the wire rod, and the central carbon segregation level of the wire rod of the present invention, as evaluated in accordance with YB / T 4413 "Method for evaluating metal phases of central carbon segregation in high carbon steel wire rod," is ≦Level 1, and the precipitation of reticulate cementite during the cooling process of the wire rod can be effectively suppressed, with the level of reticulate cementite in the structure being ≦Level 1, while the present application is Level 0.

[0008] Furthermore, the wire surface Oxide layer The thickness of the oxide layer is 13 to 18 μm, which is 5 μm thicker than the average thickness of the conventional oxide layer. Oxide layer Among them, FeO / Fe3O4=2~2.5:1, and FeO / Fe3O4 is even lower, which easily achieves an ideal (mechanical) scale removal effect, which helps to significantly reduce the wire drawing breakage rate.

[0009] The tensile strength σ of the wire of the present invention is (103762*Ceq~114606*Ceq) / 100MPa, where the carbon equivalent is Ceq = C + Mn / 6 + Cr / 5, where the element symbols represent the weight percentage content of the elements in the steel. The strength is adequate, making it more suitable for deep drawing and significantly reducing the rate of wire breakage. The tensile strength range of wire rods produced by conventional technology is σ = (120387 * Ceq ~ 131231 * Ceq). / 100 The tensile strength is MPa, and is determined mainly by two factors. The first is the chemical composition, i.e., carbon equivalent. The second is the cooling strength during cooling and the transformation temperature of the wire rod. If the cooling strength is small and the transformation temperature is high, the wire rod strength will be low, and if the cooling strength is large and the transformation temperature is low, the wire rod strength will be high. In this application, the cooling strength is mainly controlled to prevent the tensile strength from becoming too high.

[0010] The chemical composition of the wire rod is calculated by mass percentage as follows: C: 0.70-0.99%, Si: 0.15-0.30%, Mn: 0.15-0.60%, Cr: 0.01-0.50%, and the remainder is Fe and unavoidable impurities. The strength is related to the code steel of levels 72, 82, 86, and 92.

[0011] The principle of action of the chemical elements in the wire of the present invention is as follows.

[0012] C is a major strengthening element in high-carbon hard steel wire, increasing the strength of the steel through solid solution strengthening and precipitation strengthening. As the carbon content increases, the strength of the wire increases, and the strength of the drawn steel wire also increases accordingly. However, in hypereutectoid steel, the probability of network cementite precipitation in the core of the wire increases with increasing carbon content. Therefore, the C content range in the present invention is set to 0.78 to 0.99%.

[0013] Si is the main deoxidizing element in cord steel, and Si deoxidization produces harmless SiO2 inclusions. However, Si also strengthens ferrite, and an excessive Si content in cord steel reduces the plasticity of the ferrite phase in pearlite, resulting in poor ductility during steel wire drawing. Therefore, the Si content in the present invention is controlled to 0.15 to 0.30%.

[0014] Mn is an element that can deoxidize cord steel and form deformable MnS inclusions, and also primarily increases strength in steel. However, Mn is also an element that is prone to segregation. Too much Mn can worsen segregation in steel, improve the hardenability of steel, increase the degree of supercooling during austenite cooling, refine pearlite lamellae, improve strength, and deteriorate plasticity. Therefore, in the present invention, the Mn content is controlled to 0.15 to 0.60%.

[0015] Cr can promote a downward shift of the C curve in cord steel, delay the sorbite transformation time, lower the transformation temperature, and refine the spacing of pearlite lamellae, thereby significantly improving the plasticity index and drawing performance of the steel, enabling large drawing deformation, reducing intermediate heat treatment, and improving the final strength of the drawn steel wire. However, if the Cr content in cord steel is too high, supercooled structures such as bainite and martensite are likely to form during the cooling process, which will deteriorate plastic deformation and affect the drawing performance of the steel. Therefore, in the present invention, it is preferable to control the Cr content to 0.35% or less.

[0016] The present application also provides a method for manufacturing a cord steel wire rod, the specific production process of which includes, in this order, KR molten iron pretreatment, converter smelting, LF refining, square billet continuous casting, square billet hot rolling, and wire rod cooling.

[0017] Among them, Molten steel consistent with the specified composition is smelted and continuously cast into small square billets using a continuous casting process. The tundish superheat is controlled at 15-30°C, an electromagnetic stirrer is installed, and dynamic soft reduction equipment is used at the solidification end. The pressure rollers in each zone of the tension leveler employ a displacement reduction mode. Specifically, the displacement reduction amounts of the 1st to 6th rollers are 2mm, 2mm, 3mm, 4mm, 4mm, and 4mm, respectively. While conventional continuous casting billet carbon segregation control primarily relies on soft reduction technology, this dynamic reduction employs a combined "soft reduction + heavy reduction" technique, employing a soft reduction in the early stage of billet solidification and a heavy reduction in the later stage of solidification. The carbon segregation index of the continuous casting billet is ≦1.05, where the carbon segregation index is the ratio of the core C% of the continuous casting billet to the dissolved C%.

[0018] An appropriate heating temperature is selected for billet rolling. Specifically, the high-temperature zone in the furnace is heated to 1180°C or higher before rolling. The total heating time is 120 minutes or more, and the high-temperature time is 60 minutes or more, ensuring sufficient temperature and time for billet diffusion. High-temperature diffusion further suppresses carbon segregation in the core.

[0019] The finishing rolling temperature of the wire rod is controlled to 800-900°C, and the rolling speed is set to 95-120m / s. spinning thread The temperature is set to 850 to 950°C.

[0020] After rolling, the wire coil is cooled in an air-cooled roll pass. The roll pass speed ranges from 0.95 to 1.05 m / s, gradually increasing from 0.95 m / s. 1 to 3# fans are activated to accelerate the initial cooling rate of the wire. spinning threadThe wire is rapidly cooled from the initial temperature to below 700°C, promoting the refinement of pearlite blocks and suppressing the precipitation of reticulated carbon. A small amount of ferrite precipitates during this stage. The cooling rate during the quenching stage is preferably controlled between 15 and 20°C / s, and the fan speed is adjusted based on the ambient temperature. Specifically, when the ambient temperature is above 20°C, the fan speeds for fans 1 to 3 are 90%, 90%, and 70%, respectively. Specifically, when the ambient temperature is below 20°C, the fan speeds for fans 1 to 3 are 90%, 90%, and 50%, respectively. All subsequent fans (those remaining after fan 3) are turned off. This is primarily to allow sufficient slow cooling, allowing the wire to enter the transformation region, transforming from austenite to sorbite and pearlite. The sorbite transformation temperature rises, achieving sorbite transformation at around 650°C, while extending the transformation time to achieve isothermal transformation. The wire temperature recovers temporarily due to the release of latent heat during the transformation process, but then drops again as the transformation is completed. When the wire temperature drops below 570°C, the transformation is almost complete. After that, a heat-retaining cover is placed on the wire coil and it is slowly cooled, which promotes the thickening of the oxide layer on the wire surface. The FeO in the oxide layer further converts to Fe3O4, and the FeO / Fe3O4 ratio decreases.

[0021] Regarding the installation of heat insulating covers on the air-cooled roll passes, the 1st to 11th heat insulating covers correspond to the quenching and transformation zones. No heat insulating covers are installed on the air-cooled roll passes corresponding to these two cooling zones. All heat insulating covers from 12th onwards are closed, so that the wire rod is sufficiently kept at around 570°C and sufficiently cooled slowly. Oxide layer The thickness of the wire is increased, and the composition of the oxide layer is adjusted to make it more advantageous to remove the scale of the wire, thereby reducing the wire breakage rate during drawing.

[0022] Compared with the prior art, the advantages of the present invention are as follows:

[0023] (1) The wire rod produced by the present invention has coarse pearlite lamellae and blockThe small size of the wire allows it to be used in deep drawing of wire rods, reducing the tendency of steel wires to crack under drawing conditions with large area reduction rates, and effectively reducing the wire breakage rate, which is 30% lower than that of conventional processes.

[0024] (2) Wire rod containing 0.70-0.99% C belongs to high-carbon steel wire rod, and the sorbite ratio in high-carbon steel wire rod is generally 85% or more. However, the wire rod manufacturing method of the present application, particularly the controlled cooling process, controls the sorbite ratio in the wire rod to 60-70% and the coarse lamellar pearlite ratio to 30-40%. Coarsening the lamellae effectively reduces the tensile strength of the wire rod, thereby reducing grinding wheel wear and heat generation on the steel wire surface during the rough drawing stage, thereby reducing the risk of surface defects and the wire breakage rate during drawing. On the other hand, in deep drawing, the pearlite lamellae are too fine, reducing the true strain during the downward areal contraction of the steel wire during the drawing process. This reduces the amount of drawing area reduction that the steel wire can withstand while maintaining stable plasticity. This, in turn, reduces the deep drawing performance of the steel wire.

[0025] (3) The wire rod produced by the present invention has an oxide layer thickness of 13 to 18 μm, which is 5 μm thicker than that of the conventional process, and the composition of the oxide layer is FeO / Fe3O4=(2 to 2.5) / 1, which is more advantageous for mechanical descaling of the wire rod and reduces the effect of surface drawing loss caused by insufficient descaling of the wire rod on the wire drawing breakage rate. [Brief explanation of the drawings]

[0026] [Figure 1] FIG. 1 is a microscopic structure diagram of a wire rod in Example 1 of the present invention. [Figure 2] FIG. 2 is a schematic diagram of an oxide layer of a wire in Example 2 of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0027] The present invention will be described in more detail below in conjunction with specific examples.

[0028] Taking 100 tons of molten steel as an example to produce cord steel wire rod, the following processes are adopted: molten steel pretreatment + converter smelting + LF refining + square billet continuous casting to produce billets with the following compositions in each example: The carbon segregation index of the continuously cast billet must be ≦1.05.

[0029] The continuously cast square billet is heated in a furnace, ensuring that the soaking temperature is above 1180°C and that the high temperature is maintained at 1180°C for at least 4 hours to ensure sufficient thermal diffusion of the billet and further reduce carbon segregation. The continuously cast square billet is continuously rolled into wire rod in the austenite phase region, with the wire rod finish rolling temperature controlled at 800-900°C and the rolling speed set at 95-120m / s. spinning thread The temperature is controlled at 900-950°C. spinning thread The temperature is increased to increase the stability of austenite, and the coil is cooled after rolling using a rapid cooling, slow cooling, and heat retention method. The roll pass speed where the coil is located is set to 0.95 m / s, and gradually increased to 1.05 m / s to widen the coil pitch. Fans 1 to 3 are activated to achieve rapid cooling, accelerating pearlite nucleation and inhibiting pearlite growth. The fan openings are 90%, 90%, and 30%. The fans are turned off from #3 onwards for slow cooling. The wire enters the transformation region, where it transforms under the conditions of ambient temperature and its own latent heat release. When the transformation temperature rises to around 650°C, the sorbite transformation time is extended, and the wire temperature begins to drop. The transformation is considered complete when it drops below 570°C. The insulation covers from #12 onwards are then closed to insulate the transformed wire, promoting the conversion of the FeO layer on the wire surface to Fe3O4. This setting generally allows the FeO thickness / Fe3O4 thickness on the wire surface to be adjusted to (2~2.5) / 1, which improves the effect of removing scale from the wire and reduces the rate of surface defects and wire breakage during drawing.

[0030] The elemental compositions and billet segregation indices of the wire rods of Examples 1 to 5 are shown in Table 1.

[0031] JPEG0007733736000001.jpg65146

[0032] Specific process parameters for Examples 1 to 5 and two comparative examples are as shown in Table 2.

[0033] JPEG0007733736000002.jpg89167

[0034] The measured performance of the final wires of Examples 1 to 5 and the two comparative examples is shown in Table 3.

[0035] JPEG0007733736000003.jpg88164

[0036] A comparison between the above examples and comparative examples proves that the controlled cooling method of the present invention, particularly slow cooling during the controlled cooling process to carry out transformation and subsequent cover slow cooling, can significantly change the sorbite structure content and pearlite lamellar spacing, thereby controlling the tensile strength of the wire rod and improving the drawing performance.

[0037] Although the preferred embodiments of the present invention have been described in detail above, it should be clear to those skilled in the art that the present invention is susceptible to various modifications and variations. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall fall within the scope of protection of the present invention.

Claims

1. A cord steel wire rod suitable for deep drawing has a metal structure of sorbite + pearlite + ferrite, the ratio of sorbite being controlled to 60-70%, the ratio of pearlite being 30-40%, and a small amount of ferrite being present in a structure whose ratio is ≦10%; The chemical composition of the wire rod is calculated by mass percentage as follows: C: 0.70 to 0.99%, Si: 0.15 to 0.30%, Mn: 0.15 to 0.60%, Cr: 0.01 to 0.50%, and the balance is Fe and unavoidable impurity elements; The thickness of the pearlite lamella is 0.10 to 0.35 μm, the size of the pearlite block is 10 to 20 μm, and the pearlite characteristics correspond to a lower dislocation density. Cord steel wire suitable for deep drawing.

2. 2. The cord steel wire rod suitable for deep drawing according to claim 1, wherein the central carbon segregation level of the wire rod is evaluated as level ≦1 based on YB / T 4413 "Method for evaluating metal phases of central carbon segregation in high carbon steel wire rods," and the reticulate cementite level in the structure is level ≦1.

3. 2. The cord steel wire suitable for deep drawing according to claim 1, characterized in that the tensile strength σ of the wire is (103762*Ceq to 114606*Ceq) / 100 MPa, the carbon equivalent in the formula is Ceq = C + Mn / 6 + Cr / 5, and the element symbols in the formula represent the weight percentage contents of the elements in the wire.

4. The thickness of the oxide layer on the wire surface is 13 to 18 μm, and the thickness ratio of the oxide layer is FeO thickness / Fe ３ O ４ 2. The cord steel wire rod suitable for deep drawing according to claim 1, characterized in that the thickness is 2 to 2.5 mm and the cord steel wire rod has a higher scale removal effect.

5. 2. The cord steel wire suitable for deep drawing according to claim 1, characterized in that the areal shrinkage of the wire is ≥ 38%.

6. A method for manufacturing a cord steel wire rod suitable for deep drawing according to claim 1, The molten steel after smelting is cast into a square billet, and the carbon segregation index of the square billet is ≦1.05, and the carbon segregation index is the core C% of the square billet / smelting C%; The square billet is heated to a high temperature zone temperature of 1180°C or higher, and the holding time in the high temperature zone is 60 minutes or longer. Thereafter, the square billet is rolled into a continuous wire rod at a finish rolling temperature of 800 to 900°C and a spinning temperature of 900 to 950°C. After the rolling is completed, the wire coil is cooled in an air-cooled roll path under controlled conditions. The fan is turned on to rapidly cool the wire from the temperature of the wire to 700°C or less at a cooling rate of 15-20°C / s. At this stage, a small amount of ferrite is generated and the number of pearlite nuclei is promoted. Then, the wire enters the transformation region. When the fan is turned off, the wire transforms from austenite to sorbite and pearlite under room temperature conditions. After the temperature of the wire drops to 570°C or less, a heat insulating cover is installed for the wire coil. The wire that has completed the sorbite transformation is cooled slowly in the heat insulating cover to promote the thickening of the oxide layer, and the FeO in the oxide layer is further converted into Fe. ３ O ４ A manufacturing method characterized by transforming a metal oxide into a metal oxide.

7. 7. The manufacturing method according to claim 6, characterized in that a continuous casting process is adopted to cast the molten steel into a square billet, the tundish superheat is controlled to 15-30°C, an electromagnetic stirring device is provided, a dynamic soft reduction device is adopted at the solidification end, and soft reduction is adopted at the early stage of billet solidification and heavy reduction is adopted at the later stage of solidification.

8. The manufacturing method according to claim 7, characterized in that the dynamic soft reduction device refers to a tension leveler, and a displacement reduction mode using pressure rollers in each zone is adopted, and the displacement reduction amounts of pressure rollers 1# to 6# are 2mm, 2mm, 3mm, 4mm, 4mm, and 4mm, respectively.

9. 7. The manufacturing method according to claim 6, wherein the speed of the air-cooled roll pass is 0.95 to 1.05 m / s, and is gradually increased from 0.95 m / s to gradually increase the wire coil pitch.

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